World Of Rubber

Elastomeric materials that meet tough challenges Dr. Banja Junhasavasdikul



Elastomeric materials that meet tough challenges --------------------------------------------------------- WORLD OF RUBBER Dr. Banja Junhasavasdikul

WORLD OF RUBBER Author Dr. Banja Junhasavasdikul Co-authors Ms. Jutarat Phanmai Mr. Wittayanipon Chittanakee Dr. Wattana Teppinta Dr. Phattarawadee Nun-anan Edited by Prof. Dr. Suwabun Chirachanchai Prof. Dr. Vernon Platts Designed by Ms. Nuchsarawadee Waed-udom Published no. 1 : August 2022 © Copyright 2022: Innovation Group (Thailand) ISBN: 978-616-92530-4-4 Published by Innovation Group (Thailand) 18 Soi Ramkhamhaeng 30 (Ban Rao), Hua Mak, Bang Kapi, Bangkok www.elastomer-polymer.com

Contents Preface Natural Rubber 6 Introduction 36 80 Chapter 1 103 120 Chapter 2 Synthetic Rubber 128 134 Chapter 3 Vulcanization of Rubber 144 Chapter 4 Rubber Compounding 148 153 Chapter 5 Rubber Shaping and Curing 156 157 Chapter 6 Rubber Sponge 158 159 Chapter 7 Wheel and Tire 161 References - Chapter 1 - Chapter 2 - Chapter 3 - Chapter 4 - Chapter 5 - Chapter 6 - Chapter 7 About the Author

PREFACE Rubber is a miracle elastomeric material for which there are hardly any alternatives because of its elastomeric properties. Natural rubber and synthetic rubbers have been developed to serve man-kind in sealing, transporting, conveying and containing solid, liquid and gas that other materials find difficult to do. How could we live in this world without rubber? How would we drive our cars without rubber? Rubber products are everywhere and offer practical solutions for a wide variety of design challenges. However, before natural rubber and synthetic rubbers become useful, they have to be converted from thermoplastics to thermosets. They have to pass through a long process of mixing, shaping and curing. The process involves polymer and engineering knowledge in product design, formulation design and process design to obtain rubber products with the physical and mechanical properties that are required. Innovation Group has been involved in rubber for over 40 years. Nowadays, our Technical Center provides total rubber and polymer technical solutions to customers and industries. We intend to offer a practical rubber handbook to the public, especially to industrial partners, academic researchers and university students. We also hope that this will serve as an educational tool to bring rubber chemists and technologists to increase knowledge in rubber technology. Technology Center Innovation Group August, 2022

INTRODUCTION WORLD OF ELASTOMER Elastomers are a group of materials with viscoelastic properties which have extremely weak intermolecular forces and low Young’s modulus. Compared to many other materials, elastomers have high failure yield strain. They are amorphous when they are above their glass transition temperature and allow significant motion. They can be stretched 1.5 times their original dimension and return to their original dimension when the force applied to them is released. Elastomers are relatively soft when they are at ambient temperature. Elastomers can be classified into 6 types: 1. Thermoset-elastomers. This is a group of elastomers that has unmatched energy return elastomers. It is the largest group of elastomers that are used in various industries and applications; over 12.7 million tons of natural rubber and 14.5 million tons of synthetic rubbers are consumed every year. 2. Block copolymers. They are formed by both rigid and soft blocks in their structures. Manipulating the types of blocks and relative ratios allows the creation of various types of block copolymers. Advantages of block copolymers that they are recyclable and are comparable with plastic processing equipment; for examples: block copolymers on styrene bases (SIS, SEBS, SBS, and SEPS), block copolymers of polyamide block, and polyether block highly engineered properties of block copolymers make 3D printing possible. 3. Ionic-linked elastomers. These are products from ionic cross-linking and have outstanding dynamic and mechanical properties. An example is the ionic-cross-linking of BR using zinc diacrylate in producing the inner cores of golf balls. 4. Thermoplastic Elastomers (TPE). This is a group of elastomers developed by blending rigid thermoplastic with elastomers i.e., blends of EPDM/PP, and blends of nylon with elastomers. 5. Thermoplastic vulcanizates (TPV). This is the blend of elastomers with thermoplastics and elastomers that have been dynamically vulcanized during the mixing step. Examples of these materials: PP/ EPDM, PP/ NR, and polyester/ acrylic rubber. Properties of the materials depend largely on the structure, content and particle sizes of the vulcanized rubbers. They can be processed like thermoplastic and recyclable elastomers. 6. Thermoplastic urethane elastomers (TPU, TPE-U). It is group of materials which is formed when the isocyanate group reacts with the hydroxyl group of the alcohol. TPU and TPE-U have long chains of soft segments of linear polyester (TPE-AU) or polyether (TPE-EU) and short, hard segments of urethane that are formed of di-isocyanate and small alcohol molecule chain extenders (i.e., butanediol). Thermoplastic urethane elastomers are tough materials with excellent abrasion, tear and chemical resistances. These elastomers are found in our daily life; they are consumed because of their elastomeric property, being soft and can be formed into specific shapes. The low compression set property makes these elastomers suitable for seals and gaskets to retain gases and liquid in systems. Thermoplastic elastomers and the alloys of elastomers are recyclable and able to be processed like thermoplastics becoming green material that is recyclable. Elastomers are miraculous materials that can hardly be replaced with plastic or metal.

NATURAL RUBBER

Natural Rubber 1.1) Early History of Natural Rubber The history of natural rubber began on June 11, 1496, when Christopher Columbus returned from his second voyage to the West Indies, bringing back rubber balls. The people of South and central America used natural rubber to make balls, containers, and shoes. They called the rubber tree ‘Caoutchouc’, or the weeping tree. While the Spanish observed that a rubber ball had a very good bounce property, it was merely a novelty. Natural rubber did not catch the attention of Europeans until 1765, when a French scientist, Charles Marie de la Condamine, went to study the rubber tree in the dense forests of Central America. He brought back a milky liquid that he tapped from the rubber tree and called it “latex”, which in Spanish means “milk.” He converted latex into rubber sheets that became widely used by scientists to cover their scientific instruments during sea voyages to the West Indies. Throughout its history, rubber has been variously known as elastomer, tree gum, Indian rubber, or caoutchouc. In 1770, the British scientist who discovered oxygen, Joseph Priestley, developed an eraser from natural rubber. In 1799, Antoine France de Fourcroy discovered to dissolve natural rubber in turpentine, and later on Samuel Peal applied this technique to coat natural rubber on leather, cloth, and paper. These coated materials were used for soldiers’ rubber boots and raincoats. However, the products had a strong rubber smell and became very sticky in summer. During the seventeenth and eighteenth centuries, Brazil was the major country exporting natural rubber, but not in large quantity until Thomas Hancock began to grind the waste rubber from his rubber process. The waste rubber could be minced up very small and the heat was generated during the grinding process. He found that he could more easily fabricate the rubber into finished products after grinding. This method was known as “mastication” In which the long chain of rubber polymer was ground into a shorter chain. The ground rubber was easily dissolved in naphtha (hydrocarbon solvent). Thomas Hancock built a rubber factory to produce rubber belts, rubber boots, stockings and medical equipment as demand for natural rubber increased rapidly after Hancock’s discovery. 7

1.2) Discovery of Vulcanization led to the Growth of many Industries in the 20th Century The discovery of vulcanization by Charles Goodyear was a landmark in the early history of rubber. By nature, products made from natural rubber were sticky at high temperatures and became rigid at low temperatures. Charles Goodyear was an American hardware merchant who became very interested in natural rubber. In his book, Gum-Elastic and Its Varieties, he called natural rubber a miracle material with many fantastic properties. He did research aiming at modifying the properties of rubber to avoid its temperature defects by mixing natural rubber with magnesia bronze powder or nitric acid, and even boiled it in lime water. But nothing improved the property of natural rubber until his friend, Nathaniel Hayward, told him to try sulphur. In 1841, while he was mixing natural rubber with sulphur and lead oxide powder in his wife’s kitchen, he found that the rubber that had accidentally overheated had very good elastomeric properties and did not harden in winter nor soften in summer. He called this process “vulcanization”, from the name of Greek God, Vulcan. He patented this process; patent no 3633 for “metallic gum elastic composite” in December 6, 1842. From history, we know that many scientific discoveries were discovered accidentally. Archimedes discovered the theory of specific gravity when he immersed himself into the bathtub. He found that the volume of overflow water was equal to the volume of his body. He calculated the specific gravity of a subject by using the weight of the subject (himself) divided by the volume of overflow water. Sir Isaac Newton discovered the gravitational force when an apple dropped on his head as he slept underneath an apple tree. In the case of Charles Goodyear, he accepted that his discovery was accidental. After that discovery, a series of developments in natural rubber science and technology in 1840-1860 led to a range of new materials derived from rubber as well as new uses. Demand for natural rubber grew very fast in 1880 to 1920 in Europe, a few thousand metric tons of natural rubber was brought from Brazil in 1886 and increased to 15,000 metric tons in the next 5 years. During that time, the USA was another country that consumed large quantities of natural rubber because of the development of the automotive industry by Henry Ford. In 1890, 16,000 metric tons of natural rubber were imported into the USA. 8

Natural rubber was an important material used in the twentieth century. After the invention of tires made of canvas bonded with liquid rubber by John Boyd Dunlop in 1888, natural rubber was used in making automotive tires and aircraft tires were first marketed in 1910. The first rubberized bitumen was laid in the Rue Ferrier in Geneva in 1947. Rubber became an important material used in machinery, highway and bridge construction, and manufacturing in the industrial development period in Europe. Rubber-metal laminated bridge bearings were used in 1957 on the Pelham Bridge in Lincoln, UK. Because of its nonslip and its elastomeric properties, it was practical for seals and O-rings in machinery components. Rubber conveyor belts were widely used to convey coal from the mines. In the USA, consumption of natural rubber grew rapidly during 1900-1930 because of the growing automotive industry developed by Henry Ford. To replace the American conventional transport, the horse and wagon, Ford offered Americans a new transportation that they could afford, the Ford Model T. Before the First World War, Ford produced 15 million Model T a year which consumed a large amount of natural rubber from Brazil. As the imported volume increased very fast. Ford established a rubber plantation and a rubber industrial complex, called Fordlandia, at Tapajos in Brazil, to secure the supply of natural rubber. Although he used his successful Detroit management concept to operate the rubber plantation in Brazil, the project was not successful. 1.3) The Industrial Revolution The Industrial Revolution marked a major turning point in the history of mankind for two phases. The first happened during the period of 1700 to 1840 when and was the transition of the manufacturing process from hand-made productions. The second phase happened during 1840 to 1940, when more manufacturing processes were upgraded through the use of processing lines and management systems to increase productivity. The Industrial Revolution started in England after the invention by James Watt of the steam engine which came to use in the cotton and metal industries. The change of cotton spinning from manpower to steam engine consequently created a demand of metal parts in the machinery, as well as machining tools along with batch process for small production. Coal became the major source of energy because it produces more heat of combustion than wood does. The need for increased chemical production was also an important development during the first phase of the Industrial Revolution as evidenced from, the large-scale production of sulphur, alkaline, and hydrochloric acid to support the textile, soap, glass, steel industries, etc. Additionally, the transportation that changed from small boats along canals and rivers to steamboats enable the travel to faraway lands to be faster and easier. The invention of the internal combustion engine by Carl Benz, led to the development of the passenger car in the second phase of the Industrial Revolution, whereas Henry Ford car production was advanced through the concept of the assembly line for higher productivity and cheaper costs. Growth in the automotive industry led to the need for supporting industries. In USA, upstream and downstream petrochemical industries were developed along the Gulf of Mexico, while steel industries were arise along Lake Michigan and rubber industries were grown in Ohio. 9

Rubber was one of the important materials needed for the Industrial Revolution. Without rubber and the discovery of vulcanization by Charles Goodyear, it would have been impossible for James Watt to build the steam engine. Engines driving the movement of machines in factories and cars needed rubber seals and belts; rubber seals stopped the leakage of steam or oil in the engines, and belts were used to transfer the energy generated by engines to drive machines. The development of the pneumatic tire by John Boyd Dunlop led to the production of the passenger-car tire, which made riding in cars more comfortable, thus increasing demand for Henry Ford’s Model T and created a high demand for rubber, plastic, and steel. During 1900 to 1910, people rushed to Santarem, South America, to seek their fortune from rubber. Revenue from exporting rubber from Rio Negro was as high as 14 million pounds sterling in 1906. People believed that there were millions of rubber trees in the dense forests of South America, but those forests became a killing zone i.e., the powerful individuals hired workers, including natives of the Amazon, to tap rubber latex and forced these people to work for them by using firearms and might kill them when tried to run away. From 1880 to 1930 was an important period in the economic and social history of Brazil and the Amazonian regions. The extraction and commercialization of natural rubber caused a rubber boom resulting from the high demand for natural rubber Industrial Revolution in Europe and America. A large workforce was needed for the latex tapping and rubber sheets production on the rubber plantations. Many people moved to Brazil and Peru but they were struggled with many unforeseen problems. Finally, Ford left Brazil with a vast empty land behind. 1.4) Natural Rubber Processing After Charles Goodyear developed the process of vulcanization, natural rubber became an important material in industrial development during the nineteenth century. Prices of natural rubber rose quickly, and South America was then the only source of natural rubber. Trade was well protected and exporting seeds from Brazil became a capital offense. In 1876, Henry Wickham smuggled 70,000 para rubber seeds from Brazil and delivered them to Kew Garden, England, then seeds were sent to India, Ceylon, and Singapore which were part of British Empire. Before the Second World War, para trees were planted in Malaysia, Indonesia, and the southern part of Thailand. The commercial production of rubber in Malaysia and Singapore was heavily promoted by Sir Henry Nicholas Ridley who served as Scientific Director of the Singapore Botanic Garden from 1886 to 1911, and Malaysia became the largest producer of natural rubber. However, after 1985, the Malaysian government decided that palm oil had more commercial value than natural rubber. Many natural rubber plantations in Malaysia were converted to palm oil plantations since then. In 1900, Phraya Ratsadanupradit Mahison Phakdi, the Governor of Trang (Thailand) brought 22 rubber trees from Malaysia to plant at Trang. After that those rubber trees were planted in 14 provinces in southern of Thailand. 10

Thailand is the world’s largest natural rubber producer, with the production at 4.6 million metric tons per year. Indonesia is the second largest producer, producing 3 million metric tons per year, followed by Malaysia, India, Vietnam, and China [1]. The total worldwide production of natural rubber is around 13 million tons with 75% produced in three countries i.e., Thailand, Indonesia, and Malaysia [2]. Agricultural researchers in Malaysia and Thailand have worked hard to develop a rubber tree with highly resistant to the “Toura” fungus that spreads easily in rubber trees. They have also studied how to get rubber trees to produce high yields of rubber latex. They even went back to the Amazon to collect seeds from para rubber trees to bring back to their research laboratories. The average lifespan of a rubber tree is about 35 years. It takes 6-7 years before a tree can be tapped for latex. After about 25 years, the trees produce less and less latex, and they are then cut down and new trees are planted. The wood from para rubber trees is useful and can be treated with chemicals and used for furniture. In the early morning when the internal pressure of the tree is high, the farmers extract latex from rubber trees by using sharp knives to cut away small pieces of bark. Trees are usually tapped on alternate days, but during the summer when the leaves fall, farmers do not tap latex for 2 months. Latex, which contains 25-30% dry rubber, slowly drips from the tapping cut for 3-4 hours and is collected in a small container placed underneath the cut. The collected latex is transferred to a larger container and carried back to the farmer’s home where it is cleaned by filtering through a mesh screen and diluted with mineral-free water to 15% solid content in a coagulation tank. Formic acid is added to the latex while stirring, until the pH of the solution reaches 4-5. At this acidity, the latex coagulates and is left in the tank for another 4-5 hours. Then water is squeezed out from the coagulant by using a two roll mill to form 5 millimeters wet rubber sheets thick which are then dried in the sun for a few days before being sent to the rubber smoking house where rubber sheets are dried at 60°C for 3 days. Hot air in the smoke house comes from burning dry wood, and then hot air is pumped into the smoking room. Dried smoked rubber sheets are pressed into 102 kilograms/bale. The product from this process is called “ribbed smoke sheet (RSS)” which is widely used in tire manufacturing [3]. The complete manufacturing process of RSS is shown in Figure 1.1 11

Figure 1.1 Manufacturing process of ribbed smoke sheet (RSS). There are other processes to produce clean dry rubber. Instead of using hot air from burning wood, hot air from burning liquefied petroleum gas (LPG) gives clean “air dried sheet.” Block rubber is another type of dry rubber that is used to make colored rubber products; small pieces of chopped wet rubber from the coagulation tank are dried in an oven for 24 hours at 70°C as recommended by the Rubber Research Institute of Thailand (RRIT) [4]. Besides dry rubber, natural rubber latex is used to produce rubber tubes, sheeting, film, and dipped goods such as rubber gloves and condoms. Products from natural rubber latex tend to be clean and have the excellent physical properties in terms of elongation, tear resistance, and recovery. Natural latex is normally supplied to latex product factories as 60% dry rubber content (%DRC). The process for concentrating latex is simple as illustrated in Figure 1.2. Fresh latex is centrifuged to obtain a 60% concentration which is known as concentrated latex. Ammonia is added to the concentrated latex to adjust pH to higher than 10, and sometimes chemicals, such as methyl tuods (TMTD) and zinc oxide (ZnO), are also added as preservatives [5]. Then, the concentrated latex is shipped in liquid form to factories, where it is used for dipping, coating, and other products. Today, people who are routinely exposed to rubber products, such as healthcare workers and medical doctors, often develop an allergic reaction after the routine contacts with the products. Therefore, surgical and examination gloves are alternatively made from synthetic latex such as chloroprene latex and nitrile butadiene rubber latex (NBL) and are more acceptable as allergic-free products. 12

Figure 1.2 Concentrated latex supply chain By nature, natural rubber has a very high molecular weight and contains small amounts of proteins and enzymes. During storage, the protein and phospholipid contents of the rubber, which makes the rubber harder [6, 7]. Therefore, to soften the rubber, it is necessary to shorten the polymer chain at the beginning of the mixing process by “mastication,” the mechanical shearing of the natural rubber between rollers inside the mixer [8]. This process results in the reduction of the molecular weight and Mooney viscosity (a measurement named after American physicist, Melvin Mooney) of the rubber, allowing compounding ingredients to be easily mixed into the rubber. Sometimes special chemicals (peptides) such as aromatic mercaptans (i.e., sulphur containing compounds) are added to natural rubber at the beginning of the mixing process to reduce mastication time and to achieve a constant Mooney viscosity. In order to manufacture natural rubber with consistent quality, a small amount of the mercaptan additive is added during the coagulation step to produce natural rubber with constant Mooney viscosity. Because of the four electrons in the cis-1,4-isoprene double bonds in natural rubber, chemical reactions are easily activated. This is the reason why natural rubber can be vulcanized with sulphur at the positions of the double bonds. However, these double bonds can also be activated by UV radiation, other chemicals, and heat, thus causing natural rubber to have poor resistance to deterioration from those sources. Scientists have improved the quality of natural rubber by creating epoxidized natural rubber. The epoxidized natural rubber, with different percentages of epoxide group is commercially available with more higher price than the natural rubber. 1.5) Natural Rubber: Structure and Function In 1963, Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry for the development for their eponymous catalyst for the production of stereoregular polymers from propylene. The catalyst of organoaluminum compounds coupled with a transition metal. This led to the development of synthetic rubber with a structure to that of natural rubber. But the structure of natural rubber, which is known to be provenance of nature’s enzymatic control, could not be duplicated by synthetic pathway because of the unique structure of natural rubber. 13

Natural rubber is a long chain polymer that contains repeated units of isoprene as a result of a series of biochemical reactions starting form isopentenyl pyrophosphate within the tree [9]. Natural rubber has very high molecular weight which results in less chain ends and more entanglement than an equal weight of synthetic rubber [10]. The chain ends are weak points at the molecular scale, because they do not transmit the strength of covalent bonds in the molecular chain. Therefore, tensile strength is high for high molecular weight polymers like natural rubber. In general, microstructural characteristic related to the branching lead to a decrease of glass transition temperature (Tg) of a polymer. Natural rubber has these large bulky groups of branched chains, thus causing the Tg of natural rubber to be as low as -72ºC. As the microstructure of natural rubber, consists almost entirely of cis-1,4 polyisoprene (Figure 1.3) [11], it is very stereoregular and confers many of the mechanical properties. Crystallinity of natural rubber is a characteristic provided by this microstructure. If the units of polymer chain are in a regular enough for spatial arrangement, the interactions between units i.e., hydrogen bond, hydrophilic-hydrophilic or hydrophobic-hydrophobic, including ionic-interaction among functional groups will lead to the crystalline structures which stiffen the polymer. Because of stereo regularity, natural rubber form crystals upon storage (rubber becomes harder after a period of storage) causing some processing difficulties and upon stretching. The reversible crystallization upon stretching, ‘strain induced crystallization’, which is caused by intermolecular forces in the polymer, provides many unique properties, especially in excellent green strength, tensile strength, building tack, cut resistance, tear resistance, cut and crack growth [12, 13]. These properties of find natural rubber are very useful in tire applications. Figure 1.3 Chemical structure of cis-1,4-polyisoprene from natural rubber [11] 1.6) Strain Induced Crystallization of Natural Rubber Low-temperature performance of rubber is determined not only by the presence of Tg, but also the presence of crystallinity. Polymers may show crystallization upon cooling of a homogeneous melt, if the polymer chain (partially) aligns in an exothermic process. Melting is an exothermic process occurring at the characteristic melting temperature with the enthalpy released being a direct measure of the degree of crystallization. For crystallization formation, the loss in entropy from the increased order has to be overcome by a sufficiently large gain in the enthalpy. The present of crystallization in polymer results in a much higher modulus and rigidity and a lower toughness, i.e., in the less rubbery behavior. In NR, isoprene rubber (IR), butadiene rubber (BR), nitrile rubber (NBR), butyl rubber (IIR) and chloroprene rubber (CR), the crystallization occurs significantly. 14

Strain-induced crystallization (SIC) may occur for highly stereoregular rubber with melting temperature below room temperature, such as IR, IIR, CR, and especially NR [14]. At high strain levels, the polymer chain aligns, which facilitates crystallization [15]. Stretching the rubber chain results in a shift of melting temperature, to temperature above the room temperature. The crystalline regions formed result in self-reinforcement of the rubber as evidenced from the higher tensile strength and elongation at break. Natural rubber is renowned for its high degree of SIC and its excellent ultimate properties [15, 16]. Because of the resilience of natural rubber after cross-linking upon curing, the elasticity and flexibility, combined with crystallization induced toughness when stretched. This mean less kinetic energy is lost during repeated stress deformation. In tires, natural rubber is used extensively to provide low heat build-up. For instance, the shoulder temperature of heavy-duty truck tires may rise up to 100ºC, and the heat of this magnitude increased the risk of a blow-out or other delamination related to crock growth. Tremendous stresses also occur on the tread and sidewall of the truck tire if it backs up over the curb, causing strain in the sidewall. Besides hydrocarbons, natural rubber contains approximately 6% non-rubber components (i.e., 2.2% protein and 3.4% lipid and other substances). It was found that these small amounts of non-rubber also exhibit crystallization effects and affect associated properties [17, 18]. In conclusion, it is difficult to replace natural rubber with synthetic rubbers in tire applications. 1.7) Composition of Natural Rubber Latex Although many species of plants are found to exude NR latex (NRL) on tapping, only ‘Hevea Brasiliensis’ plant is of commercial importance and accounts for 99% of World’s NR production. NRL is mainly consists of rubber molecules as the major fraction and other fraction called as non-rubber components such as protein, lipid, carbohydrate, etc. as summarized in Table 1.1 [19, 20]. These rubber and non-rubber components may vary due to various factors such as season, weather, soil and especially rubber clone [19, 21]. Some of these components are suspended and dissolved in the aqueous phase of the latex, while the others are adsorbed on the rubber surface of rubber particles. It is noted that 5% of non-rubber components can be removed or degraded during dry rubber processing [20]. 15

Table 1.1 Composition of NRL [20] Composition %w/v fresh latexa Latex %w/w dry matter of latexb Rubber hydrocarbon 35.0 84.0 Lipids 1.3 3.2 Protein 1.5 3.7 Carbohydrate 1.5 3.7 Organic substances 0.5 1.1 Inorganic substances 0.5 1.2 aAveraged from data published by Wititsuwannakul, D.; Wititsuwannakul, R. In Biopolymers. Vol. 2:Polyisoprenoids; Koyama, T., Steinbüchel, A., Eds.; Wiley-VCH: Weinheim, 2001; pp 151–201.25 bCalculated. 1.8) Rubber and Mastication The very high molecular weight of natural rubber results in less chain ends and more entanglement than equal weight of synthetic rubber. Its Mooney viscosity varies from type of rubber tree, fertilizer used, season of rubber tapping and especially staging time. Its viscosity affects rubber breakdown during plasticization which in turn affects the mold filling condition in finished product production. In tire production, natural rubber has to be masticated to obtain the required Mooney viscosity before the rubber compounding step. Mastication is a polymer chain breakdown process which is related to mechanical breakdown at low temperature and a thermo-oxidative effect at high temperature [8, 22, 23] as shown in Figure 1.4. Figure 1.4 The effect of mastication in variation of temperature [22] 16

The first effect occurs during low temperature shearing in the internal mixer (<80ºC). The long-chain branched networks do not have time to relax and break by the reaction of stresses, therefore, the shorter chain molecules are formed and the viscosity decreases [23, 24]. The rate of emission of heat depends upon the distribution of both shear and elongation stresses, as well as the nature of the polymer and the temperature, the natural of polymer, and the temperature. When the temperature increases, the polymer chains are more mobile resulting in the decrease in relaxation time. The higher temperature, the lower the effect of mechanical breakdown. The second effect is thermo-oxidation breakdown. This chemical oxidative reaction happens when the temperature of mastication increases beyond 80ºC. As consequence, the radicals species (R*) are form [25]. These free radicals react with oxygen to form proxy radicals (ROO*) followed by transforming to be a cyclic diperoxide or hydroperoxide group (ROOH). As a consequence of the hydrogen atom abstraction, the free radicals are formed along the chains to propagate the chain rupture (Figure 1.5), as described in the previous work [23]. Figure 1.5 The reactions for the mastication of natural rubber [23] The use of peptizing agents can accelerate the breakdown of natural rubber polymer chains. The peptizing agents act by the mechanochemical and thermo-oxidative breakdown of natural rubber as radical acceptor at low temperatures and as oxidation catalysts at high temperature [23]. Peptizing agents usually are compounds of thiophenol or aromatic disulfides combined with metal complexes of Fe, Cu, and Co as the catalysts for the oxidative breakdown. 1.9) Latex Compounding Technology Latexes (or latex compounds) are complex colloid systems containing polymer molecules as the major fraction. The polymer may be a homo-polymer or co-polymer (random/ block/ graft) having stereo regularity with complexity related to the polymer chains i.e., linear, branched and molecular weight distribution of the latex depend on the grade of selected. Latex is rubbery or resinous in nature while the rubber molecules in the latex can be cross-linked, plasticized and oil extended. The type of latex lead to the different mechanical properties and temperature limits of serviceability [26]. The rubber particles is generally oval with the particle size less than 5 micron under a typical distribution. The aqueous phase consists of dissolved and suspended matter and the information about the concentration and pH are essential for successful latex compounding. Latex products are subject to degradation, therefore, an adequate antioxidant protection is necessary. 17

1.9.1 Preservation of NR latex (I) High ammonia NR latex (HANR latex) NR Latex causes coagulation within a few hours due to acidity through micro-organisms, and the pH of latex decreases to roughly 5.0. Ammonia at a concentration of 0.7-1.0% is mostly used for long term preservation [26], but it also serves as a latex bactericide and sequestering agent for Mg2+ and PO43- ions. The weak points of using ammonia are from its strong odor, the additional cost, including the thickening when compounded with ZnO which, interferes the gelation of latex foam while further compounded with sodium silicofluoride (SSF). Therefore, the excess ammonia has to be driven off before compounding. (II) Low ammonia NR latex (LANR latex)  Centrifugation is used to generate low ammonia NR latex (or LA-TZ) which is then preserved with low ammonia coupling with other preservatives such as tetramethylthiuram disulphide (TMTD) and zinc oxide (ZnO). With 0.025% TMTD/ZnO and 0.05% of ammonium laurate on latex, the ammonia level is less than 0.29% [26]. The LA-TZ latex is commonly used in all dipped products application.  Zinc dialkyl dithiocarbamates i.e., zinc dimethyldithiocarbarmate (ZDMC) and zinc diethyldithiocarbarmate (ZDEC), at 0.1-0.2 % concentration along with 0.2% ammonia and 0.2% lauric acid enhance the stability of NR latex to be in the similar level as 0.7% ammonia preserved latex without any significant effect on vulcanization [26]. However, the latex may discolor badly on aging, in some cases, the color develops from the trace amount of copper contamination, so-called (copper staining).  For other types of low ammonia latex, the latex is preserved with a low ammonia content (about 0.2%) and 0.2% boric acid coupling with 0.05% lauric acid, which is a common LA-preservative system [26]. The benefits of LA-TZ latex are its ease of use, low cost, low toxicity and free from discoloration. However, the un-vulcanized deposits tend to soften more quickly.  Low ammonia latex can also be preserved by coupling 0.2% sodium pentachlorophenate with 0.2% ammonia as a stabilizer [26] although this method is not widely used due to toxicity of the stabilizer which is linked to pentachlorothiophenol. 18

1.9.2 Destabilization of latex or gelation Chemical methods (acidification, addition of salts of polyvalent metals, higher concentration of salt) and mechanical methods (mechanical agitation and dehydration) can both destabilize NR latex. Destabilization produces that generate homogenous destabilization, also known as gelation, of a three-dimensional aggregate of rubber particles. The NR latex has good gel strength that most suitable for latex applications and foamed products, though excessive stabilization results in weaker gels and slower gelation rate. Gels do not have a consistent composition (or structure), and the film shrinks due to aqueous phase exudation, which is retained in the interstices of the gel. Three methods of Gelation [26] (I) Organic acid/acids or acid liberating substances (II) Use of salts of multivalent cations (III) Application of heat (I) Gelation By Acids Rubber particles are stabilized as a result of the net negative charge (from both lipids and protein structure on the surface of rubber particles) which creates repulsive forces between rubber particles. The pH of latex is dropped once acid is added, resulting in a decrease in the ionization of adsorbed anions and a decrease in repulsive forces between rubber particles, resulting in gelation. (II) Gelation by Salts Normally, calcium nitrate is used in the dipping process to create gels of the compounded latex on the ‘formers’. Calcium ions destabilize the latex by forming insoluble salts with all fatty acid soaps and protein solution. Calcium ions also produce a high concentration of ions, reducing the colloidal stability of NR latex. (III) Heat sensitized gelation Immersing a hot former in to a suitably heat sensitized latex compound is knowns as heat sensitized gelation. Zinc amine ions and hydrophilic polymers can both help to heat sensitize NR latex [27]. Zinc oxide-ammonium salt process (ZOA) facilities the role of ammonia solutions to increase the solubility of zinc ions [26]. The addition of an ammonium salt also increases the ionic strength of the aqueous phase and contribute to destabilization. Because the gelation occurs at room temperature in a short period, the ZOA process is truly heat sensitive. However, heat accelerates the process. The gelling time or the thickness of dipped product can be used to determine the degree of heat sensitization. The thickness of the dry film is influenced by a number of processing parameters such as temperature of the former, rate of immersion, dwell time and heat capacity of the former including the compounding latex characteristics [27]. 19

1.9.3 Film formation and structure Formation of film involves immobilization of free-moving polymer particles when brought into contact. In most latex-goods manufacturing processes, the contact between polymer particles is achieved by the ‘gelation’ process as a consequence of pH dropping with ammonia stabilizer during the initial stages of drying. Note that if the pH is maintained by using a fixed alkali stabilizer like KOH, this process will not proceed. Gelation does not affect the rate of drying of the film or any other characteristics. Before drying, the film is leached with clean water to remove water-soluble chemical in compounding residues from the compounding process, as well as residual coacervant (i.e., aqueous calcium nitrate) and other surface-active substances [26]. This improves the film’s feel, eliminates porosity defects and makes it resistance to water absorption and aging. The film thickness of dipped goods varies between 0.1 mm to 0.2 mm depending on the viscosity of the latex compound, a multi-dip process is used to achieve the necessary film thickness. 1.9.4 Chemical modification of NR latex (I) Pre-vulcanized natural rubber (PVNR) latex PVNR latex is a chemically modified NR latex, which on drying gives a vulcanized film, and can be produced latex stage with concentrated latex (or HANR latex). During the maturation process, the PVNR latex compounds containing ZnO and ultra-fast accelerators (i.e., ZDBC or ZDEC) and other ingredients usually achieve some degree of pre-vulcanization. The formulation of produced PVNR latex is prepared as follows [26]: Table 1.2 Compounding ingredients of PVNR latex Compounding Ingredients Dry Wet (by weight) (by weight) 60%HANR latex 100 167 50% ZnO dispersion 1 2 50% ZDEC dispersion 1 2 50% Sulphur dispersion 2 4 10% KOH Solution 0.3 3 10% Sodium Caseinate 0.2 2 In the jacked mixing tank, the HANR latex and ingredients are mixed with stirring. When the desired degree of cross-linking has been achieved, the latex is heated up to 50°C-60°C by flowing hot water through the jacket for 3-5 hr or when QC the QC tests confirmed the as-desired of cross-linking. The latex compound is cooled to room temperature, then filtered or collected. The degree of cross-linking of latex compounds is determined by swelling method, combined sulphur analysis or tensile property evaluation [28]. PVNR latex is popular in the medium/ 20

small sector dipped product because is does not necessary or is limited to incorporate the pigments such as balloons, medical product and feeding bottle teats. The cross-linking can be achieved by reaction with sulphur, sulphur donors (i.e., dithiodimorpholine (DTDM), tetraethylthiuram disulfate (TETD) and tetramethylthiuram disulfate (TMTD)) or by gamma radiation. The compounding formulations can be varied to be suitable for the application. ZnO is not necessary if ZDEC/ZDBC is present. ZnO reduces the film clarity but it be substituted by ZnCO3 to improve the clarity. For peroxide cross-linking, tert-butyl hydroperoxide and tetraethylene pentamine are used. Maximum film clarity is obtained by using ZDBC alone. The cross-links found in the pre-vulcanized latex are predominantly polysulphidic (except for sulphurless / sulphur donor cures). (II) Heveaplus MG graft polymers Poly(methyl methacrylate)-grafted-natural rubber (Heveaplus MG) grafted side chains of polymethyl-methacrylate on NR molecule [29] as shown in Figure 1.6. Figure 1.6 Poly(methyl methacrylate)-grafted-natural rubber (Heveaplus MG) [30] The polymethyl-methacrylate used are 15% (MG15), 30% (MG30) and 49% (MG49) [31]. As the amount of polymethyl methacrylate in MG increases, their capacity to form declines. This form of latex can be blended in any proportions with unmodified latex. The modulus, tensile strength and tear strength of such blends are significantly improved. When methyl acrylate is partially substituted with butyl methacrylate in the grafting reaction, the film forming properties improve [29, 31]. 21

(III) Hydroxylamine modified latex (HRH latex) Because of the cross-linking process, the viscosity of the concentrated latex increases during storage. During the first 20-30 days, the rate of storage hardening is faster (and over 100 days the process is completed). When hydroxylamine is added for 0.15 at the latex production stage; the storage hardening effect is inhibited [32]. The vulcanizates show slightly low modulus (resulting in less volume shrinkage of latex foam), whereas other properties are unchanged. This type of latex is used to make latex foam and latex adhesives for use in footwear [26]. 1.9.5 Latex compounding ingredients The transformation of wet NR latex into a final product is accomplished using various processes, which are based on the criteria of minimum energy consumption during various stages (High energy consumption during drying has been a major concern). In all the processes, a stable colloidal system is maintained for a desired time after which the system is made unstable to convert the same to a solid product. Maintaining the balance of stability is the major challenge. The latex compounds contain four or more distinct dispersed phases and are highly polydispersed with several different surfactants. The aqueous phase of high ionic strength gives the NR latex compounds a relatively low colloid stability but facilitate conversion to solid products. For dipped goods, the latex compounds used must produce continuous films on the former and maintain film integrity during drying and vulcanizing stages. Table 1.3 Latex compounding ingredients No. Function Ingredients 1. Vulcanizing agents Sulphur, Sulphur donors & others. 2. Accelerators Dithiocarbamates, Thiazoles, Thiurams, Xanthates 3. Antioxidants Amine derivatives, Phenolic derivatives. 4. Fillers & pigments Inorganic, Organic. 5. Surface-active agents Anionic, Cationic, Amphoteric, Non-ionogenic. 6. Viscosity modifiers Plant hydrocolloids, Proteins, Polyvinyl alcohols, Cellulose derivatives, Starches, Polyacrylates, Carboxylate polymers, Colloid clays, etc.) 7. Other ingredients: Mineral oils, Waxes, Resins, Antifoaming agents, Antiwebbing agents, Corrosion inhibitors, etc. 22

1.10) GAP/GMP in Natural Rubber Natural rubber is a product produced by photo-synthesis of carbon dioxide and water inside the rubber tree. Its quality varies from type of rubber tree, fertilizer used, season in tapping and method of tapping and process in making rubber sheet. Therefore, the quality of natural rubber varies widely from various factors. The total process of natural rubber is very labor-intensive. It is different from synthetic rubber that is produced from a continuous operating process in petrochemical plants that have high processing control. However, natural rubber has its unique properties that are hardly replaced by other materials. That is why natural rubber is 42% of total rubber consumed every year. Currently, natural rubber is grown in many countries and its supply is over demand. Consumers buy natural rubber firstly because of price and quality. Large tire companies will select natural rubber suppliers based on price, consistent supply, and quality management in the total supply chain. Thailand is the largest natural rubber producer. Total area of the rubber plantation is 20.58 million rai. (8.13 million acres.). Over 1.2 million people are involved in natural rubber plantations. Yield of production is 234 kilograms/rai/year is lower than what it should be of 300 kilograms/ rai / year. Meanwhile the quality of rubber sheets varies from time to time. It passes through many processes such as latex tapping, latex collection and rubber sheet process in the factory. Without good knowledge, farmers produce natural latex using hazardous chemicals and wrong process in latex tapping, rubber coagulation and drying. Natural rubber produced in the conventional method normally has lower yield and contamination by foreign materials and chemicals, so the rubber sheets are classified as substandard grades. This reflects incomes of rubber farmers which have been at a low level for years. They need financial support from the government from time to time. Ms. Preprame Tassanakul, Director of Rubber Testing and Certification Center, Southern region, Rubber Authority of Thailand, analyzed those problems. She found that method of tapping latex, contamination in the latex during tapping, and processing method in making rubber sheets, etc. are not well managed. Main reasons behind this are: 23

1. Thai rubber farmers do not have sufficient knowledge in rubber plantation. 2. They lack knowledge and good practice in tapping latex and how to collect clean latex to deliver to rubber drying plants. 3. In the rubber processing plant, is operating in fully manual process and there is no quality control system etc. Ms. Preprame set up standards and educated these farmers how to produce good latex to supply to rubber plants so they could produce consistent quality rubber sheets supply to the market. She wrote a handbook of ‘How to produce rubber smoked sheet’ and enhance it to the agricultural standard (TAS 5906-2013) [33] and a handbook in producing crepe rubber in the following five years (TAS 5907-2018) [34]. Those handbooks’ contents are related to the process of Good Agricultural Practice (GAP) and Good Manufacturing Practice (GMP) to encourage good practice by rubber farmers in their farms, latex tapping, collecting and delivery to rubber plants. In the rubber processing plants, GMP guides them how to have good manufacturing practices to produce consistent rubber sheets. She also provides standards in the process of producing good quality latex (TAS 5908-2019) [35], cup-lump rubber to supply to the rubber plants (TAS 5910-2020) [36] and excellent latex collection centers (TAS 5911-2021) [37]. Innovation Group realizes the value that Ms. Preprame Tassanakul produces by GAP and GMP programs and gives strong support to GAP and GMP programs. 1.10.1 What is GAP? Good agricultural practice (or GAP) is good practice in rubber plantation [35]. 1. Land to plant rubber trees: Owner must have the legal right on those lands (not from the deforested area). The suitable planting area is the tropical monsoon area with average rainfall 1,250 mm per year, pH in soil in the range of 4.5-5.5 2. Hazardous chemicals and herbicides are not allowed to be used. 3. Starting from clones of rubber tree that are recommended by Rubber Authority of Thailand. Rubber trees to tap latex must be mature, rubber trees must have a minimum circumference of 50 cm and height not less than 1.5 meter from the ground level. 4. Tapping latex by barking half of the circumference of a rubber tree with an angle of 30-35 degrees. Tapping should be done 2 days and stop 1 day or in a dry period or the dry areas, tapping should be done 1 day and stop 1 day (alternate days). This should be done after the mid-night till 6 o’clock in the morning. 5. Collection of latex should be done and delivered to the process plant, not over 8 hours after tapping. 6. Equipment to use; latex cups and containers for collecting latex have to be kept clean and avoid any contamination from unwanted material. 7. Filter the latex from latex cups into the stainless steel container with 7 mesh-filters. 8. Record quantity of latex collected every time as a production statistic. 9. Clean rubber cups and place upside down, every time after collecting the latex, to prevent contamination of foreign material. 24

1.10.2 What is GMP? Good manufacturing practice (or GMP) is the instructions on good manufacturing practice in rubber plants [37]. 1. Condition of rubber plant: it should have a proper design and construction according to standards of processing plant with good utilities. 2. Stainless steel is recommended to be used in the latex receiving station and the coagulation tank. 3. Every arriving latex batch is required to go through standard quality control. 4. Arriving latex must pass through filtration with 10 mesh-filters. 5. Take a sample of each arriving lot and check dry rubber content (DRC) to calculate weight of formic acid to use. Only formic acid is used in rubber coagulation (adding 4% by weight of formic acid to 100 parts DRC). 6. Check volatile fatty acid number (VFA no.) of latex accordingly to ISO 506 7. Stainless steel coagulation tank is recommended. 8. Squeeze water out from the sponge sheets by a clean two roll mill. Dry those sheets under the shade before sending to the smoking room. 9. Smoke rubber sheets at a controlled temperature of 55-60ºC for 3 days. 10. Pack the sheet in plastic bags of 25 or 30 kilograms/ pack (according to the requirement of customers) 1.10.3 Benefits of GAP/ GMP 1. Good properties control: dirt, ash content, volatile materials and has a good initial Mooney viscosity range. 2. Contamination free 3. Traceability of rubber sheet, right from original plantation. 4. Easy to handle (a 30 kilograms/bale instead of a 110 kilograms/bale) and power free. 5. Non-hazardous chemicals. 6. The most important are higher yield, better product, and farmers can sell at a higher price. GAP and GMP are good standard practices in producing consistent quality of natural rubber. However, this needs cooperation from rubber farmers in practicing GAP/GMP standards. 25

1.11) Value Chain of Natural Rubber Natural rubber is one of the strategic agricultural products of the country. Thailand is the largest rubber producer harvesting 4.2 million tons each year involving 1 million families in the upstream industry of natural rubber. However, natural rubber has a very long value chain. In order to develop a sustainable growth of natural rubber, we have to consider how to create values to this total value chain. The total value chain of rubber can be divided into three sectors: 1. Upstream industries involve the growers and tappers. This has to add value to primary production of rubber latex or basic dried rubber sheet, such as cup lump, scrap raw sheet and crepe rubber. Starting from diagram 1, there are 1 million families or approximately 6 million of the Thai population living in rubber plantations and working in the upstream industries. 2. Intermediate or midstream industries may be called rubber processors. They produce rubber on plantations and convert it into semi-finished products, such as ribbed smoked sheet rubber (RSS), standard Thai rubber (STR) (or block rubber) and concentrated latex 3. Downstream product industries produce various rubber products and latex dipped goods Over 85% of Thai intermediate rubber (dried rubbers and concentrated latex) is exported. The main importing countries of Thai intermediate rubber are China (58%), Malaysia (15%) Japan, and Korea. The main use of rubber sheet is for the production of tires (about 49%) followed by latex dipped goods (13%). Natural rubber supply is largely impacted by market pricing, which is mainly set up by buyers, China, Singapore and Japan. Because the price setting by buyer markets is done irrespective of production costs, artificially low prices disincentivize producers from sustainable production and replanting, further compromising the supply of natural rubber. Meanwhile two international organizations, Forest Stewardship Council (FSC) and Global Platform for Sustainable Natural Rubber (GPSNR) are becoming influencers to the manufacturing and marketing of natural rubber. In order to sustain growth of natural rubber, the Rubber Authority of Thailand should support upstream and midstream of natural rubber to qualify for FSC and GPSNR certification. Figure 1.7 Total supply chain of natural rubber (NR) [38] 26

1.11.1 Global platform for sustainable natural rubber (GPSNR) The GPSNR is an international, multi-stakeholder, voluntary membership driven platform for improvements in socioeconomic and environmental performance of the natural rubber value chain by defining and implementing industry wide standards on fairness, equity and environmental sustainability. The development of the GPSNR was initiated by the CEO’s of the World Business Council for Sustainable Development (WBCSD) and Tire Industry Project (TIP) in 2018. The members of the platform include tire manufacturers, rubber suppliers, rubber processors and vehicle makers. The GPSNR policy frame works are composed of 8 sustainable natural rubber principles, [39]: Commitment to legal compliance, forest sustainability, respect human rights, community livelihoods, increase production efficiency, traceability and management, systems and processes to drive effective implementation of policy, and reporting [40]. 1.11.2 Forest stewardship council (FSC) FSC is an independent, not for profit, non-governmental organization established to support environmentally appropriate, socially beneficial, and economical management of the world’s forest. To achieve the mission of FSC, the FSC has developed a set of 10 principles as follows [41]: Commitment to legal compliance, respect worker and human rights, enhancing the social and economic well-being of local communities, indigenous peoples’ rights, benefits from the forest, environmental values and impacts, management planning, monitoring and assessment, forest conservation, implementation of management activities [41]. 1.11.3 How to apply the FSC trademark in the finished goods Chain of custody certification (COC) of any company which is the process to ensure that all the processes or transformations of FSC-certified products have been checked at every processing stage (Traceability system) [42]. Remark: FSC certification applies to plantation owners. COC certification applies to manufacturers, processors and traders of FSC-certified forest products. 27

1.12) Rubber Gloves In 1894, William Stewart Halsted, the first chief of surgery at Johns Hopkins Hospital, invented surgical gloves for his wife as he noticed her hands were affected by the daily operations she performed and in order to prevent medical staff from developing dermatitis from surgical chemicals [43]. In 1965, Ansell Rubber Co. Pty. Ltd. developed the first disposal medical gloves. Nowadays, over 300 billion pairs of rubber gloves are consumed every year and demand remains high for medical gloves that protect against virus [44]. USA, Europe and Japan consume 60% of the rubber gloves produced. Malaysia is the largest glove manufacturer, accounting for 63% of the total world production. Although, Thailand has the main source of raw materials, Thailand produces only 18% of the world’s glove market. China is the third highest producer at 10%, while others such as Indonesia, Belgium and Vietnam share 9% [44]. The major materials in producing rubber gloves are natural rubber (NR) latex, acrylonitrile latex, neoprene latex, isoprene latex and polyvinyl chorine latex. The main reason why people use rubber gloves is to protect their hands from contacting fluid when they are performing an operation; for example: household gloves are used by housewives in their daily works, washing, cooking and gardening. Household gloves protect their hands from detergents, contact with food and soil during their operations and NR latex or chloroprene gloves are widely used. Industrial gloves such as latex gloves protect workers from contact with chemicals, but NBR (nitrile) or chloroprene gloves, which have oil and chemical resistances, are the preferred gloves. Surgical gloves need good resistance to fluids and have good permeability resistance; therefore, chloroprene gloves are preferred. Medical and examination gloves, which are consumed in very large quantities are typically made the areas of nitrile and NR latex. However, the highest demand for disposal gloves; they are vital goods in the healthcare environment. They not only protect healthcare providers and patients from potentially harmful microorganisms, but they also assist set a standard for hygiene and care in the business. Different materials and design choices make certain products better suited for different medical environments. Nitrile, NR and vinyl latex are most common material used in disposable gloves. For decades, NR latex gloves have been used as the medical disposable gloves worldwide, because they had been recommenced for protection since 1980 against blood borne pathogens like HIV. Disposable gloves have to be comfort, able have a high degree of touch sensitivity and be relatively cheap. NR latex gloves were most popular until it was found that many people were allergic to the NR latex. Vinyl gloves are prepared by using PVC liquid, as a petroleum-based material. The advantage of vinyl gloves is their low cost, but they are less durable than NR latex gloves and have limited protection against 28

biomedical exposure. Nitrile gloves came into prominence in the market in the 1990s as the leading rubber gloves, being the ideal choice for disposable gloves because they have exceptional puncture-resistance and are not allergenic to human skin. Figure 1.8 Market demand and trend–overall NR vs NBR gloves [44-46] 1.12.1 Overview of the global glove market In 2020, Thailand emerged as the world’s second largest rubber gloves manufacturer and exporter behind Malaysia. Thailand exported over 20.5 billion pairs of rubber gloves, an increase of approximately 22% from the same period the previous year due to the Covid-19 outbreak [44]. According to EIC, Thailand earned approximately 700 million USD per year from glove exports, while Malaysia’s earned more over 1 billion USD per year from rubber glove product exports [39]. Malaysia imports concentrated latex from Thailand in order to manufacture rubber products (i.e., glove, condom, glue and pharmacy products). However, Malaysia produces rubber products for domestic consumption as well as export to other countries on the international market [47]. Malaysia’s gain could be viewed as Thailand’s economic loss, as Thailand lack the ability to transform concentrated latex into a valued-added product [44]. 29

Figure 1.9 Market demand and trend – by Segmentation [48] Table 1.4 Thailand’s and Malaysia’s rubber gloves industries comparison [44, 47] Rubber glove Thailand Malaysia Strategic adjustment industry of Thailand Product (1) Latex glove (1) Nitrile glove (I) enhancing the concen trated latex (2) Nitrile glove (2) Latex glove producers into the rubber glove market (II) improve the drawback of NR product (i.e., reducing-proteins) Manufacturers Not many Various The support from Thai government and manufacturers (i.e., manufacturers (i.e., the related rubber industry only 1 manufacturer 4 manufacturers among 5 largest among 5 largest manufacturers 2021) manufacturers 2021) Efficiency 6-7 million gloves/ 20 million gloves/ Increased advanced technology for (glove/month) month (type on month (based on faster and higher production capacity different number different number of factories) of factories) Export market Mostly exported to Mostly exported to Extend distribution to new markets (i.e., USA, an approximate USA, and approximate Asia Pacific, South-East, Middle East rate 36% rate 33% and Africa), due to lower price of latex glove and lower number of proteins- allergic cases 30

1.12.2 Types of rubber gloves Rubber gloves are protective items to cover the hands of the wearer and provide physical protection for the wearer. There are two main categories of rubber gloves by material; namely, (I) latex glove and (II) synthetic gloves (i.e., nitrile and vinyl gloves). In addition, rubber gloves can be classified into two types: (I) medical and (II) non-medical gloves. The market is divided into end-user industries such as healthcare, food and beverage, automotive, machinery, and others. The key information for the two major categories of rubber glove by materials are illustrated in the figure below: Figure 1.10 Comparison of latex glove and synthetic gloves (i.e., nitrile and vinyl gloves) 1.12.3 Rubber glove manufacturing processes Rubber glove manufacturing processes are normally comprised of seven steps: (I) raw material testing, (II) compounding, (III) dipping, (IV) leaching and vulcanizing, (V) stripping, (VI) quality control, and (VII) packing [49-51]. These process steps are shown in the figure below: 31

Figure 1.11 Rubber glove manufacturing process model [52] ‹ Raw material testing Before the latex compounding process, raw materials are tested in the factory’s laboratory, where materials are subjected to various detailed and stringent quality tests (i.e., latex and chemical properties testing). ‹ Latex compounding Ingredients and their functionalities for rubber glove manufacturing processes are summarized in Table 1.5 [51]. All ingredients are generally mixed with latex (i.e., concentrated latex, HANR latex) such as surfactant or stabilizer, activator, accelerators, antioxidants, and vulcanizing agent (i.e., sulphur) based on the specified formulation. Before being fed to the production line, the compounded latex is tested to ensure that it meets the specification requirements. 32

Table 1.5 Rubber formulation of rubber glove (surgical glove) [50, 51] Ingredients phr 60% HANR latex 100.0 10% KOH 0.30 20% K-laurate 0.20 50% ZnO 0.40 50% Wing stay L 0.75 50% ZDC 0.75 40% SDBE 0.50 50% Sulphur 0.50 Total 103.40 ‹ Dipping Before forming the gloves on glove molds (or formers), cleaning and dipping processes of the rubber glove molds are required. Firstly, the rubber glove molds are cleaned fwoiltlhowaecdidb(yi.ele.,aHch2SinOg4 or HCl) or alkali (i.e., NaOH or KOH) to remove contaminant and dust, with clean water, and then dried at 50ºC. After that, the glove molds are dipped into the coagulant tank (i.e., the mixed solution of CaNO3 or CaCl2 and CaCO3), which contains a processed chemical. Next, the glove molds are then dipped into the latex dipping tank to coat them with a thin layer of latex. Coagulant and compound latex tanks are both controlled for properties and conditions such as total solid content (%TSC) and temperature, and so on. ‹ Leaching and vulcanizing The gel films on rubber glove molds are beaded, further dried, and then leached in the pre-leached tank before they are further vulcanized to provide good physical properties and reduce moisture content. In the leaching step, all gloves are moved through a water bath to leach excess additives from previous stages (i.e., coagulant and other ingredients) and reduce protein content. The effectiveness of this process is dependent on three major factors: (I) the temperature of the water, (II) the duration of the process, and (III) the rate of water exchange. In the vulcanizing step, the latex film is vulcanized by the combination of sulphur and accelerator generating cross-linking of the rubber molecules while being heated (i.e., 100- 120ºC); this provides high elasticity and good tensile strength to the rubber film. ‹ Stripping In this step, the leached gloves are dipped into a closely controlled wet-slurry tank to remove any protein buildup, including, bacterial and other contaminants which remain on the rubber glove. Finally, auto-stripping lines are used to remove the gloves from the formers. 33

‹ Quality control The quality control process is carried out by random sampling after all the batch of products has been finished. Several methods are used to inspect the products. The first method is called “inspection”. In this method, air blowers are used in this method to investigate for pin holes on the glove’s surface; if any are found, these gloves are rejected. For this method, air is blown into the gloves for about 1 h. Furthermore, the watertight test is the second quality control method to which the gloves are subjected. This method is similar to the air blower inspection test, except that water is poured inside the gloves to investigate any product defects. The third method of quality control is a visual inspection to check for stain marks on the gloves and/or misshaped gloves. Gloves with defects are rejected. Finally, size, thickness and aesthetic appeal are all inspected to ensure that the gloves produced is in accordance with specifications. ‹ Packaging The rubber glove packing area is under a tightly controlled dust-free environment by using a hygienic filtered air system. Before packaging, packing operators must perform one final visual inspection and remove any defective products before packing the gloves. Lot continuing a hundred pairs of a specific size are first weighed and, if within specification are packed into small boxes. Finally, these boxes are loaded into larger cardboard boxes ready for delivery to customers. ‹ Standard specification of rubber gloves The specifications for rubber surgical gloves (in terms of physical requirements) are summarized in Table 1.6. Table 1.6 Physical requirements of surgical gloves Before aging After aging Type 500% Tensile Elongation at Tensile Elongation at modulus (MPa) strength (MPa) break (%) strength (MPa) break (%) I 750 560 II 5.5 24 18 650 490 7 7 12 Note: Type 1 is compounded primarily from natural rubber latex and Type 2 is compounded from a rubber cement or from synthetic rubber latex. 34

1.12.4 Future & market trend of rubber glove Although demand for rubber gloves varies by product, continued growth in demand is possible against a background of rising hygiene and safety consciousness among consumers. The following significant shifts in disposable gloves demand have occurred in recent years [53]: ƒ High demand for synthetic gloves due to the risk of type I hypersensitivity in latex glove ƒ More light weight or less thickness of rubber glove (especially in NBR glove) ƒ Ease of working while wearing gloves ƒ More sustainable production of degradable gloves COVID-19 health workers use approximately 80 million gloves per month, but the synthetic material from which they are primarily made is disposed of in landfill for 100 years. However, latex gloves can biodegrade 100 times faster than the synthetic glove and produce 30% less waste during production [54]. This has resulted in more sustainable and green ways of life; slowly but steadily, everything is shifting towards biodegradable products. Previous works has reported the potential of biodegradable natural rubber latex gloves for commercialization. It was found if a new green additive was included in the NRL gloves formulation to accelerate the biodegradation process, the biodegradable gloves could be decomposed in soil in four weeks [55]. Therefore, a special formulation of latex glove (more biodegradable material) is an ideal future composition for sustainable manufacturing, providing a greener future to benefit everyone. 35

SYNTHETIC RUBBER

Synthetic Rubber 2.1) Synthetic Rubber Throughout the nineteenth century, many scientists tried to determine the structure of natural rubber. The English scientist Michal Faraday (1791-1867) found that natural rubber has the chemical formula C5H8. Charles Williams (1829-1910) was another English scientist who analyzed hurl rubber by destructive distillation and obtained a large quantity of light oil that he called isoprene. In 1879, Gustave Bouchardat, a French chemist, heated isoprene with hydrochloric acid, so was the first person to obtain a synthetic rubber-like product. The first truly synthetic rubber was made by William Tilden 3 years later, when he heated turpentine with a catalyst and obtained isoprene. Incidentally, Carl D Harris and Francis Matthews along with E. Halford Strange discovered that isoprene could be polymerized more rapidly by sodium. In the early 1900s when prices and demand for natural rubber increased readily, Carl Duisberg, CEO of Bayer in Germany, realized the opportunity and the benefit of synthetic rubber as a substitute for natural rubber, he then formed a research group headed by Fritz Hofmann, the chief chemist in the pharmaceutical division of Farbenfabriken. In 1909, his research team developed a synthetic rubber from the polymerization process of pure isoprene using p-cresol, a component from coal tar, as the starting material. However, the cost of isoprene production was very high. The research team applied dimethyl butadiene (CH3-CH2-CH=CH3) as the starting material and became successful in 1910 to name it ‘K Rubber’. This synthetic rubber was relatively hard and easily degraded. In 1918, Germany was cut off from natural rubber supply, so Bayer produced 2400 metric tons of K Rubber for the German Army. After the First World War, demand for rubber increased significantly because of the sharp rise in the automotive industry. BASF, Bayer, Agfa Farwerke, Hoechst and Chemische Fabrik Kalk decided to resume research on synthetic rubber. Eduard Tschunkur and Walter Bock were assigned to lead this research, which managed to polymerize styrene and butadiene in the aqueous emulsion. In June 21, 1929, IG Farben obtained a patent on the invention of butadiene-styrene copolymers (SBR). They named it Buna S. The name Buna came from the first two letters ‘butadiene’ and ‘natrium’ which meant sodium. In 1930, the team of Helmut Kleiner, Erich Konrad and Eduard Tschunkur succeeded in developing acrylonitrile- butadiene rubber (NBR or Buna N) Buna N has high oil and petroleum resistance and is widely used in petrochemicals and automotive industries in oil seals and O-rings. 37

On the other side of the world Neoprene was discovered by Dr. Wallace Carothers of DuPont in 1930. While Carothers started his research on polymerization, Father Arthur Nieuwland of the University of Notre Dame carried out on a research on acetylene chemistry and produced divinyl dichloride, an elastomeric compound. DuPont decided to purchase the patent rights from the university and continued to work on divinyl dichloride. In April 1930, Collins discovered an elastomeric material which has excellent elastic and chemical resistance properties; DuPont called it DuPrene but it was changed to Neoprene later on. Buna S, Buna N and Neoprene were three synthetic rubbers that had been discovered before the Second World War. After the war, the automotive industry had been well developed in the USA, consumption of rubber grew very fast and many medium and high performance rubbers were developed to serve requirements in automotive, petroleum, petrochemical, construction industries. 2.2) Development of Synthetic Rubber in the United States of America On the other side of the world, President Franklin D. Roosevelt was well aware of the vulnerability threatened supply of natural rubber in USA. In June 1940, he declared rubber as a strategic material for the country and formed the Rubber Reserve Company (RRC) to stockpile and conserve the use of natural rubber. He encouraged research and development in synthetic rubber. During the Second World War, the USA was cut off from the supply from Southeast Asia. The RRC called for the production of synthetic rubber in the country, and on December 19, 1941, Jersey Standard, Firestone, Goodrich, Goodyear, and United States Rubber Company signed a patent and information sharing agreement under the auspices of the RRC. Before that time, the giant oil company of Standard Oil of New Jersey, who had a working relationship with IG Farben, had go through a transatlantic transfer of synthetic rubber technology. In the 1930s, chemists at Jersey Standard Oil started the research and development on the production of butadiene, the basic material for producing synthetic rubbers, from petroleum. Their research allowed large-scale production. During the Second World War, the natural rubber supply situation became more serious. Roosevelt appointed a Rubber Survey Committee to speed up the production of synthetic rubber. The technology chosen for synthetic rubber production was based on Buna S research because Buna S could be mixed with natural rubber and milled on the same machinery as natural rubber. Research and development to produce this all-purpose substitute for natural rubber was dominated by the big four rubber companies. Fifty-one plants were constructed to produce the monomers and polymers needed for the manufacture of synthetic rubber (GR-S, or government rubber-styrene rubber) using a recipe that consisted of 75% butadiene and 25% styrene with potassium per-sulfate as a catalyst. Right after the Second World War, the United States was the largest producer of synthetic rubber at 920,000 metric tons per year. 38

2.3) Rubber Structure and Related Properties Natural rubber and synthetic rubbers before cross-linking show properties of thermoplastics, but after being cross-linked the chemical structures will change and thus offer different characteristics and performance. Rubber chemists have to fully understand the background for the structure/properties relationship of each rubber in order to predict the final properties of the product to be produced. The chemical structure determines the chemical reactivity toward the design of cross-linking. It also affects the mixing behaviour, processability and the properties of products. Rubber has an organic hydrocarbon structure as the backbone of the polymer which then forms a long chain structure in the polymerization process. Natural rubber has its single unit of isoprene, though by photosynthesis, isoprene monomers are polymerized to polyisoprene structure in the colloidal form of latex. Man-made rubber or synthetic rubbers all start from small units of monomer which through the process of polymerization, form long chain polymers that can have many combinations of monomers. These polymerized monomers can be the same monomers or different kinds of monomers, either only two kinds of monomer or more than two kinds of monomers. Some monomers have only hydrocarbon structure while other monomers may contain heteroatoms of N, Cl, O, F or Si. Each chemical structure of a monomer will exhibit different physical and chemical properties in the final polymer’s properties and performance. Therefore, rubber chemists have to know the chemical structures of polymers to predict the properties of final products. First, we have to start to study the skeleton of polymer, starting from its backbone of polymer which is the main C-C chain of polymers. Atoms of carbon have 8 electrons at the outer layer. In order to form a bond between C-C, two carbon atoms share two outer electrons to form a covalent bond which is a stable bond. Polymers with the single C-C bond are more stable than polymers which share four outer electrons to form a double bond, or C=C bond. Four electrons that are sharing are active, so when polymers are exposed to heat, radiation or oxidizing agents, one electron will leave the sharing bond to become a negative charge at the site of the polymer. This negative charge of free radical is very active in promoting further reactions, which cause the breakdown of the polymer main chain, or easily react with positive charge chemicals to form a side chain. Therefore, polymers with double bonds (unsaturated bonds) are prone to be active and not stable when they are exposed to ozone, oxygen sunlight, heat, UV and oxidizing chemicals. Natural rubber is one of the examples, the main chain of natural rubber has double bonds which cause natural rubber to be unsuitable when exposed to sunlight and heat. However, the double bond is essential for sulphur curing. 39

Secondly, rubber chemists have to study the chemical structures of each type of polymer. The type and content of heteroatoms, such as N, O, F, Si and Cl atoms affect the heat resistance, polarity and density of rubber. The polarity in turn affects the compatibility of rubber with other rubbers, polymers and compounding ingredients, and also its resistance against liquid media, especially oil. Thirdly, the low temperature performance, flexibility and elasticity of rubber are governed by the glass transition temperature of the polymers which depends on the stereoregularity of the structure of polymers. The presence of some crystallinity and especially the occupancy of strain-induced crystallinity are beneficial to the strength and abrasion resistance. Fourthly, other structural features of the rubber polymer, such as molecular weight, molar mass distribution, number of branching chains and size of branched chains, all affect the production conditions of rubber processing and the final performance of both rubber compounds and vulcanized rubber. Molecular weight affects the Mooney viscosity and strength of vulcanizates. The flexibility of a polymer chain affects the entanglement density, which in turn determines the rubber compound viscosity and the acceptance of fillers and plasticizers by the polymer and the modulus and hardness of the vulcanizates. Fifthly, rubbers are polymers with a glass transition temperature (Tg), below their application temperatures, having no or hardly any crystallinity. They are soft and flexible with modulus in the MPa range at their application temperature. Dynamic mechanical properties of a polymer are characterized by the storage modulus (G’) and the loss modulus (G”). Sixthly, rubber polymer is different from thermoplastic polymer, which has two thermal transition temperatures, Tg, and melting temperature (Tm) whereas rubber polymer has only Tg, does not melt, but at temperatures above Tg, becomes soft and flows. Mooney viscosity of rubber is the measurement of rubber polymer at a certain temperature. At temperatures below Tg, molecules inside the thermoplastic polymer form crystalline structures which become more rigid and dense. If high strength properties are needed, thermoplastic with high Tg is used at room temperature and at service temperatures of the plastic product up until reaching its heat distortion temperature. In case the properties of flexibility and softness of thermoplastic, are required, thermoplastic with very low Tg is the choice. Rubber polymers are used because of the flexibility as their Tg is below the service temperature. Sevenly, molar mass (MM) and molar mass distribution (MMD) and long chain branching (LCB) have to be put into the selections of polymers. MM, MMD and LCB which are governed by monomer structures, types of catalysts and polymerization processes, have their effects on the mixing, processing and vulcanization as well as the final properties of the products. MM increases with the Mooney viscosity of the polymers. MM here with is the mass-average molar mass. In rubber industry, the Mooney Viscosity (MV) correlates with MM but is also affected by MMD and LCB. Selection of MV of a rubber polymer for a particular rubber application is often a compromise in rubber compounding mixing, processing and the final properties of the products. Lower rubber MM and MV gives a faster wetting of solid filler particles, resulting in a shorter mixing cycle and is easier to process and shaping. The higher rubber MM and MV gives benefit in the higher physical and mechanical properties of 40

the final rubber product, compression set and improves solvent resistance. Higher MV rubber gives higher shear force during compound mixing. Therefore, the selection of MM and MV of rubber in an application, one has to identify the right balance between compound mixing, process, vulcanizate performance and total cost of product. Eighty, strain-induced crystallization may occur for highly stereo-regular rubber with melting temperature below room temperature, such as IR, IIR, CR, and especially NR. At high strain levels, the polymer chain aligns, which facilitates crystallization. Stretching the rubber chain results in a shift of melting temperature, to temperature above the room temperature and forms crystalline region. The crystalline regions formed result in self-reinforcement of the rubber, resulting in a higher tensile strength and elongation at break. Natural rubber is renowned for its high degree of strain-induced crystallization and its excellent ultimate properties. 41

2.4) Neoprene or Chloroprene Rubber Neoprene is a trade name of DuPont for this excellent weather, ozone, chemical corrosion and oil resistant synthetic rubber. In 1934, DuPont announced its success in developing chloroprene rubber through the emulsion polymerization process. DuPont called it neoprene [1]. The construction of a 2000 tons/year plant was completed in 1940, providing the first synthetic rubber used by the US military during the Second World War. In 1957, Bayer produced Perbunan C (chloroprene rubber) by a continuous production process. The first commercial production of Denka chloroprene with a capacity of 200 tons/ month was established in 1962. On November 4, 2005, Denka announced its acquisition of DuPont chloroprene Rubber, which made Denka the largest chloroprene supplier. Chloroprene is the common name for 2-chloro-1,3-butadiene with the formula CH2=CCl-CH=CH2 [2]. Acetylene was used as a feed stock in the older manufacturing process, while the modern processes use butadiene. By 1980, most of the chloroprene was produced from butadiene in the USA and Western Europe, while in Japan, Denki-Kagaku still used the acetylene route. Acetylene came from the production of calcium carbide and its dimerization to form mono-vinylacetylene in the aqueous hydrochloric acid solution of CuCl2 and NH4Cl at 80ºC in the reactor tower. Then chloroprene was formed by a direct hydrochlorination step, a process that needed high investment and was very energy intensive. The more recent chloroprene process is based on butadiene, a less expensive feedstock. The initial step is a gas-phase free-radical chlorination with chlorine at 250ºC and 1-7 bar to give a mixture of 3,4-dichlorobutene and 1,4-dichlorobutene. Step 2 is the isomerization and de-hydro chlorination of 3,4-dichlorobutene-1, followed by treatment with base to induce dehydrochlorination to 2-chloro-1,3-butadiene in the last step of the process [3]. The dehydrochlorination entails loss of a hydrogen atom in the carbon 3 position and chlorine atom in the carbon 4 position, therefore a double bond between carbon 3 and 4 is formed. Currently, the polymerization of chloroprene is a free-radical polymerization in an emulsion process which allows good temperature control of the reaction and is a stable process [3]. This leads to better batch-to-batch control of variations in structure and rheological properties of grades of chloroprene produced. 2.4.1 Structures and properties Chloroprene has four different chemical structures, 1,4-trans, 1,4-cis 1,2 and 3,4 in the polymer chain as shown in Figure 2.1 [2, 4]. The composition of these types of polymer which determines the grade of chloroprene is dependent on the reaction temperature. Lower polymerization temperature, produces a higher yield of 1,4-trans incorporation as resulting in higher crystallization rate of the polymer. Grades of chloroprene from low temperature polymerization are needed for adhesive applications benefiting from strain-induced crystallization. More grades of 1,4-cis incorporation are produced at higher temperatures for various grades elastomeric properties. 42

Figure 2.1 Chemical structure of chloroprene rubber [2, 4] The high chlorine content with its high density of 1.23 g/cc [4] and mid-range polarity makes chloroprene inherently fire resistant and gives moderate fluid swelling resistance. Although chloroprene has an unsaturated polymer backbone, the chlorine atom which is attached to the tertiary carbon atom of the double bond, withdraws electrons from the unsaturation in the backbone. This chlorine atom reduces the reactivity in oxidation reactions of chloroprene, therefore chloroprene, in spite of the double bond in its structure, offers a reasonable resistance to weathering, ozone and heat. The high amorphous grade of chloroprene has very low glass transition temperature of -40ºC to be an excellent rubber for low temperature applications [4]. Chloroprene also has high green strengths, high tear resistance and overall good mechanical and excellent dynamic properties [4]. Generally, the grades of chloroprene offered by DuPont, (acquired by Denka) Arlanxeo, and Denka, can be separated into three groups [5]. 1. Adhesive grades which have high crystalline structure are offered in different Mooney viscosities. As a consequence of crystallization, the adhesive strength of the adhesive film is considerably greater than the amorphous grades of chloroprene. 2. Chloroprene grades which have more 1,4-cis give amorphous structures; various Mooney viscosities are offered. For extrusion applications, a small quantity of pre-crossing agent is added to the polymer to provide good extrusion processing [5]. 3. Chloroprene grades with sulphur from the peptisation process contain polysulphide bridges, giving unique properties [5]. They have high initial viscosity allowing faster and better filler dispersion. These compounds are good for the extrusion and calendaring process. They also promote good bonding between rubber and substances such as fabric. 43

Table 2.1 Grades of Neoprene offered by DuPont [6] Type Co-monomer Mooney Viscosity Crystallization rate Distinguishing Features Adhesive AD none Low to high rapid Fast crystallization AG Sulphur Medium slow Rheological control W polymer W none 42-51 fast General purpose WHV 100 none 90-105 fast General grade medium Mooney viscosity WRT 2,3-dichloro- Medium very slow Low temperature app. WD 1,3-butadiene High very slow High viscosity WRT GRT containing S Medium medium High tear GW Structures of G type and W type are shown in Figure 2.2 and 2.3, respectively. Figure 2.2 G type: contains sulphur in the structure Figure 2.3 W type: contains chlorine atom in the structure Presence of chlorine atoms in the backbone of chloroprene rubber make its properties as follows [4, 7]: 1. Chloroprene is resistant to non-polar oil such as paraffinic and naphthenic oil, but swells in aromatic and engine oil. 2. Chlorine atoms in chloroprene can help to provide flame resistance. 3. Chloroprene is resistant to oxidation and ozone. 4. Chloroprene can be self-scorching at high temperature. This is because chloroprene releases HCl to react with metal oxide in the compound. 5. Chloroprene rubber has poor dielectric resistance. 44

2.4.2 Applications Because of its high crystalline structure in the adhesive grades, chloroprene finds application in solvent adhesives [7, 8]. In the moulded rubber area, chloroprene is well recognized for its high gum vulcanized strength due to the strain-induced crystallization. It has good chemical stability, excellent weathering and ozone resistance [4], and good thermal resistance properties which it can retain at low temperatures. It has excellent dynamic properties and good gas permeability resistance [8]. It is considered for a general-purpose rubber goods produced by molding, extrusion and calendaring. Chloroprene is self-extinguishing hence it has been used in high power-cable jacketing and mining belt applications. Because of its excellent dynamic property, Neoprene GRT is also used in V belts. Neoprene WRT and WD are used in bridge bearing pad, wiper blade and seal joint [8]. Neoprene W finds its applications in general rubber applications, such as industrial hoses, axle boots, wire and cable jacketing, engine mountings and bearing pads. Neoprene GRT and GW types have excellent tear resistance, soft touch feeling and are used in sport wet-suit applications. Neoprene latex finds its application in industrial and surgical glove applications. 2.4.3 Vulcanization of chloroprene Chloroprene is generally vulcanized by metal oxides; the common cross-linking agent is usually zinc oxide, along with magnesium oxide which is necessary to give scorch resistance [9]. The reaction involves the allylic chlorine atom, which is the result of the small amount of 1,2 polymerization, as shown in the chemical reactions below (Figure 2.4): Figure 2.4 Vulcanization of chloroprene rubber by the action of zinc oxide and magnesium oxide [10] Metal oxide vulcanization with accelerated-sulphur vulcanization is common. Tetramethylthiuram disulphide (TMTD), N, N’-di-o-tolyguanidine (DOTG) and sulphur are commonly used for high resilience and good dimensional stability, while metal oxide with ethylenethiourea (ETU) is a curing system being used to avoid carcinogens. 45

2.5) Styrene-butadiene Rubber (SBR) Styrene-butadiene rubber (SBR) is a general purpose synthetic rubber, produced from a copolymerization of styrene and butadiene Figure 2.5 Chemical structure of SBR [2] Exceeding all other synthetic rubber in consumption, SBR is used in large quantities in automotive and truck tires. It is the most consumed type of synthetic rubber, widely used in place of natural rubber because of its good abrasion resistance. However, because of the butadiene content, SBR become swollen and weakened by hydrocarbon oil and is degraded over time by atmospheric oxygen and ozone. 2.5.1 History of styrene butadiene rubber SBR was developed and commercialized in Germany in 1930 with the name of Buna S. It was produced by emulsion polymerization which contained 68-70% butadiene and 30-32% styrene. Around 1942, GR-S (Government rubber styrene) was produced in the United States. The product was designed to be similar to the German Buna S but lower in molecular weight for easier processing. It was also polymerized by emulsion polymerization. Immediately after the Second World War, the United States was the largest producer of synthetic rubber at 920,000 metric tons per year. 2.5.2 Polymerizations of styrene butadiene rubber SBR can be produced either by emulsion polymerization (E-SBR) or solution polymerization (S-SBR) techniques which lead to different properties. Emulsion SBR can be produced by both hot and cold polymerizations. Hot emulsion SBR has more branch chains than the cold emulsion SBR, which makes it better for extrusion, be more stable and have less shrinkage. Cold SBR is more abrasion resistant and has higher tensile strength. The S-SBR was developed with higher molecular weight and a smaller molecular weight distribution. S-SBR has better flexibility and tensile strength with lower rolling resistance than E-SBR. Therefore, S-SBR finds its application in tire manufacturing. In E-SBR, there are two processes, hot polymerization and cold polymerization. The polymerization temperature of E-SBR hot polymerization is 50ºC or higher, while the cold is about 5°C or even lower (-10°C or -18°C). In the cold polymerization process, more active initiators are used. Cold polymerization gives better controlled structures, such as less branching which will give better abrasion resistance and higher tensile strength (E-SBR hot polymerization produces more branching which makes it better for extrusion and less shrinkable). Cold polymerized E-SBR is more popular in the market than hot polymerized E-SBR. 46

Solution styrene butadiene S-SBR is a copolymer of styrene and 1,2-butadiene produce via anionic polymerizations in a solution process. The main chain of S-SBR is highly unsaturated which give high sulphur utilization, but poor ozone, oxygen and heat resistances. But S-SBR has good resistance properties toward polar media, acid and alkaline. The unique nature of the insertion of butadiene on the growth 1,4- and 1,2-addition of butadiene, as well as the formation of the two cis-1,4 and tran-1,4 stereo isomers, implies the presence of a four-monomer units comprising copolymer. The ration of the structural unit content of styrene and the butadiene inserted in 1,4- and 1,2-addition mole along the chain is the important parameter influencing the glass transition temperature of S-SBR. The styrene content of S-SBR typically range of 10 to 40 wt.% with the variation of the vinyl content (1,2-butadiene) for the butadiene part of the copolymer of butadiene part of the copolymer of between 10 and 70 wt.%. Both the styrene content and the vinyl content influence the glass transition temperature (Tg) of the polymer. S-SBR produces narrower molecular weight distribution, higher molecular weight, higher cis-1,4-polybutadiene content and lower non-rubber content. These give improvement in flexibility, dynamic properties and, superior mechanical properties like tensile strength, low rolling resistance but S-SBR more difficult to process than E-SBR. In the early stage of development of S-SBR, it was not very popular. Until 1980s, there were requirements of green tires for lower fuel consumption, so low rolling resistance and good traction of tires were required. Good control of microstructure of butadiene especially vinyl and styrene contents and molecular weight in later developments of S-SBR were able to achieve the requirements of green tires and also balance processing problems and properties. The latest generation of S-SBR can raise Tg by adjusting the vinyl and styrene contents which can help to improve grip and rolling resistance in tire performance. 2.5.3 Grades of SBR The last two numbers of grades indicate staining and non-staining antioxidants being used i.e., 1500 which contains staining antioxidant while 1502 contains non staining antioxidants. Table 2.2 Grades of SBR Type Grade “Hot” emulsion grades “Hot” black masterbatch +oil (≤14 phr) SBR 1000 “Cold” emulsion grades SBR 1100 “Cold” black masterbatch +oil (≤14 phr) SBR 1500 “Cold” oil masterbatch SBR 1600 “Cold” oil black masterbatch +oil (≤14 phr) SBR 1700 Emulsion resin rubber masterbatch SBR 1800 “Hot” lattices SBR 1900 “Cold” lattices SBR 2000 SBR 2100 47

Table 2.3 Microstructure of some SBR grades [11] Grade Microstructure of 100% BR Emulsion Grades Styrene Vinyl content Cis-1,4 trans-1,4 Tg (°C) Oil (%) Krylene 1500 content (%) (%) content (%) content (%) Krylene 1502 -50 0 Krylene 1509 23.5 16 12 72 -50 0 Krynol 1712 -50 0 Krynol 1721 23.5 16 12 72 -50 27 -35 27 23.5 16 12 72 23.5 16 12 72 40.0 16 12 72 1st Gen’ Solution Grades 18.0 10 38 52 -75 0 Buna SL 704 Bayer 24.0 10 38 52 -65 0 Buna SL 705 Bayer 18.0 10 38 52 -75 27 Buna SL 750 Bayer 25.0 10 38 52 -65 27 Buna SL 751 Bayer 18.0 10 38 52 -70 33 Buna SL 754 Bayer 2nd Gen’ Solution Grades 25 33 25 42 -50 0 Buna VSL 1924 S25 Bayer 20 Buna VSL 1939 S20 Bayer 20 50 19 31 -45 0 Buna VSL 1940 S20 Bayer 15 Buna VSL 1944 S15 Bayer 15 50 19 31 -40 27 Buna VSL 1945 S15 Bayer 25 Buna VSL 1950 S25 Bayer 25 53 18 21 -50 0 Buna VSL 1954 S25 Bayer 25 Buna VSL 1955 S25 Bayer 53 18 29 -50 27 67 13 21 -25 27 73 10 17 -20 0 73 10 17 -20 27 High aromatic oil (HA oil) was added to the SBR in the past. HA oils contain high concentrations of polycyclic aromatic hydrocarbons (PAH) which have been identified as carcinogenic. In 2010, European Legislation controlled extended oils in E-SBR and processing oil in tire compounding to contain not more than 1 mg/Kg (0.0001% by weight) of Benzo[a]pyrene and not more than 10 mg/kg (0.001% by weight) of all listed PAH. Currently, treated distilled aromatic extract (TDAE), residue aromatic extract (RAE), and treated residue aromatic extract (TRAE) have been used as substitutes for HA oils [12]. SBR grades do not used HA oils now. 48