World Of Rubber

Figure 3.9 Type I coagents [3] (II) Type II: addition reactions Because type II coagent have less polar molecules, they generate less scorch than type I and reactions take longer. These co-agents improve the cross-link density but do not increase the cure rate [27]. Due to their low polarity, these co-agents have good compatibility with many elastomers and work as good processing aids. Examples of Type II coagents are high-vinyl 1,2-polybutadiene, divinylbenzene, allyl esters of cyanurates, isocyanurates and allyl phosphate. Figure 3.10 Type II co-agents 99

Type II co-agents are less reactive (more stable) and primarily increase the elastomer’s state of cure. The co-agents generate bridges between rubber molecules, increasing cross-linking efficiency by generating extra cross-links. Further, they have a major affinity for radicals, which acids in the reduction of chain scission and disproportionation reactions. When co-agents are added to a peroxide curing system, the following benefits are obtained [25, 28-30]: ¾ Improved peroxide efficiency ¾ Improved mechanical properties such as modulus, tensile strength and hardness ¾ Enhance compression set (i.e., lower compression set) ¾ Improved resilience ¾ Lower viscosity of rubber compound ¾ Good oil and fuels resistance ¾ Good heat ageing ¾ Improved the adhesion properties of rubber coating to metal (in the case of zinc salts) ¾ Enhanced dynamic properties 3.3.3 Mechanism of coagent reaction The general cure mechanism of peroxide/coagents cure is as below: Figure 3.11 Peroxide cross-link of rubber molecule [28] 100

However, the nature and quality of the cross-link can be tailor-made, depending on the elastomer and the nature of the coagents. Figure 3.12 Cross-linked network structure of rubber matrix cured with peroxide in the presence of co-agent; a) No coagent only peroxide; b) Use of coagent compatible with the matrix; C) Use of coagent not fully compatible with the matrix, formation of filler like domains [27] Type I coagents can homo-polymerize and/or graft onto macroradicals, forming effective cross-links (or higher cross-link density) via radical addition reactions as seen in Figure 3.12 [31]. Figure 3.13 Reaction mechanism of certain Type I co-agents with elastomers [31] Type II coagents, containing extractable allylic hydrogens participate in intramolecular cyclization reactions as well as intermolecular propagation reactions [32, 33]. Trifunctional coagents (TAC and TAIC) may form cross-links through the cyclo-polymerization products as well as grafting through pendant allyl groups [32]. The polymeric co-agents with high vinyl microstructure enhance the concentration of reactive pendant unsaturation, resulting in encouraging cross-linking reactions [32]. The mechanism shown below (Figure 3.14) explains how an allyl containing co-agent is incorporated into the rubber molecules [34]. 101

Figure 3.14 Possible reaction mechanism of Type II co-agents suggested by Endstra [34] Figure 3.15 shows cure characteristics of standard peroxide cure (A), that using type I coagents (B) and type II coagents (C) [35]. In a typical peroxide curing system, the reactive Type I coagents reduce scorch time (ts2*) but increase cure time (t90*). Furthermore, type II coagents provide the scorch safety. Also, for both type I and II coagents, the extent of cure (S*) is frequently increased as more effective cross-links are created [35]. Figure 3.15 Cure characteristics of standard peroxide cure (A), that using type I coagents (B) and type II coagents (C) [35] 102

RUBBER COMPOUNDING

Rubber Compounding 4.1) Rubber Compounding Natural rubber (NR) and synthetic rubber are not used in their pure form as polymers. To obtain a rubber product, rubbers have to pass through various processing steps; compounding, shaping and vulcanization. Starting with rubber compounding, the objective of rubber compounding is to mix rubbers with fillers, plasticizers and chemicals to achieve the highest dispersion and distribution of fillers and chemicals into the rubber matrix with the highest throughput and lowest scrape and rejects. In the rubber industry, internal mixers are used to mix rubbers with fillers and chemicals. The dispersive mixing takes place in the region of high shear inside the tangential internal mixer such as at the tips of the mixing rotor flights with the side wall of the chamber. In the case of intermeshing internal mixers, high shear takes place between two rotors in the mixer. Over a decade, manufacturers of tangential mixers have focused on increasing the number of wings of mixing rotors from two to six wings to improve the volume of material undergoing more intensive shearing at the wing tips and give better distribution of materials flowing along the rotor axis. Meanwhile, HF Group has developed intermeshing internal mixers with high shear mixing of rubber compounds between two big rotors. Intermeshing mixers give better heat transfer because the area of high intensive shear is larger and does not cause a high hot spot as the tangential mixers do. Both types of internal mixers are widely used in the rubber industry. In the modern rubber compounding industry, fully automated process lines have been developed to increase productivity and reduce wastes or rejects caused by human error. Digital controls are applied to the whole process line. Fills and oils are fed into the mixer automatically. Hence, rubber compounding involves science and engineering, machines, and rubber chemicals and additives in definite proportions to obtain a uniform mixture that will have the desired properties and performance to meet processing at low cost and end product performance. Rubber technologists must not only have thorough knowledge of rubbers and chemicals, but they must also have good knowledge of engineering, processing equipment, production optimization, process control and many other engineering aspects to produce rubber products at the highest outputs and lowest cost. 104

There are 6 major ingredients in a rubber compound formula and each is measured in parts per hundred rubber (phr) 1. Rubbers 2. Fillers 3. Plasticizers 4. Protective agents 5. Processing aids 6. Curing system When starting to develop a rubber compound, it is necessary that the rubber technologists know what the application of the end product is and its desired physical, mechanical and functional properties, its service conditions and media to be contacted as well as the service temperatures in order to select the suitable rubber(s). Next, after rubber selection, fillers and plasticizers need to be chosen; again they are very dependent on the desired properties of the end product rubber. The fourth part of a rubber compound is protective agents such as antioxidants, antiozonants or wax that help create a physical barrier to prevent rubber products being degraded. Sometimes, the protectant can be mixtures of other rubbers that have physical properties that protect the degradation of the original rubber product, e.g., adding ethylene propylene diene rubber (EPDM) into NR to prevent the degradation of natural rubber by sun light in static applications. The last part of the formulation, which is considered as the most important, is the vulcanization system. Then, from formulation design, rubber technologists must know how to operate the mixing equipment: internal mixers, two roll mills, stock blenders, batch machines and packing machines. 4.2) How to begin the Formulation and Rubber Compounding Formulation development as the path to the optimal final functional properties requires a profound knowledge of polymers, fillers, plasticizers and chemicals in all the diversities. The chemical knowledge and experience to design a starting formulation are acquired through many years of professional practice and interdisciplinary cooperation, which amazes all those in the development process. From product application, service conditions and environment, a rubber technologist selects a type of polymer rubber to start his formulation design. All ingredients are determined by type and quantity in order to reach the designed finished product properties. 105

The selection of polymer rubber under the respective boundary conditions and requirements is important based on: 1. Service conditions of rubber final product in its service environmental such as light, ozone and weathering conditions, upper and lower temperatures of service, contact with what types of media. 2. Mechanical and dynamic properties required. 3. Compounding processing technology. 4. Processing method and equipment to produce rubber parts. 5. Final product inspection and testing 4.2.1 Formulation design Let’s start with rubber selection. As previously work, all rubber technologists are familiar with the heat-oil resistance chart or ASTM D 2000 [1]. Figure 4.1 Elastomers positioned by their resistance to heat aging and swelling in IRM 903 oil, according to ASTM D2000 [1] Rubber technologists must have full knowledge of rubbers and their chemical structures to determine rubber properties. From the above table, a group of hydrocarbon rubbers at the left hand corner of the chart i.e., NR, styrene butadiene rubber (SBR), butadiene rubber (BR), and butyl rubber (IIR) are a group of hydrocarbon rubbers with diene in their structures. These rubbers have poor resistance to oil and ozone. Service temperatures of these rubbers must be low (below 60ºC). These hydrocarbon rubbers are used mainly in the tire and footwear industries, which together consume almost 70% of the total rubber produced. 106

After the tire and footwear industries, the automotive industry consumes the next largest quantity of rubbers. EPDM rubber, at the upper left hand corner of the chart, is a hydrocarbon rubber having diene on the side chain and has excellent weathering resistance properties but poor oil resistance. Its largest uses are mainly in automotive weathering seals and automotive parts which do not contact oil and chemicals. Moving toward the right-hand side of the chart, there are rubbers that contain Cl, N, O, F, Si elements in their structures. These elements have much higher molecular weights than hydrogen, resulting in higher temperature service of these polymers than the hydrogen carbon rubbers. Rubbers with functional groups with Cl, N, O become polar rubbers which are resistant to oil and chemicals. Polarities of rubbers increase by increasing the content of functional group of Cl, N, O in the structures, (Cl content in case of chloroprene and N content of acrylonitrile in case of nitrile rubber (NBR) and hydrogenated nitrile butadiene rubber (HNBR), acrylic and ethylene acrylic contents in acrylic and ethylene acrylic rubbers. In the case of silicone rubber, the bond strength of carbon to the siloxane group is much higher than the carbon-hydrogen bond, resulting in the silicone rubber product allowing very high service temperatures. Similarly with fluoro-elastomers (FKM), their structures have fluorine atoms replacing hydrogen atoms, hence, fluoro-elastomers can be used at very high service temperatures and are highly resistant to oil and chemicals. From the above heat versus oil resistance chart, rubber technologists will select polymers to use for the specific applications. Following that, rubber technologists have to select fillers and plasticizers, and decides about processing aids and curing system from physical properties and mechanical requirements. 4.2.2 Fillers Fillers are necessary in rubbers to improve the processing and service properties, especially the mechanical properties. Besides improving mechanical performance, fillers also improve tear and abrasion resistance of rubber products. Particle sizes and structures of fillers determine the degree of reinforcement. Typical types and levels of filler such as commercial carbon black, silica, calcium carbonate and clay available in the market have their published details and properties easily accessible. In general, fillers increase the hardness of rubber; smaller particle sizes of filler give higher degree of reinforcement, and higher hardness. Structures of fillers and active surfaces have effects on the rubber compounding process and final product properties. Replacing carbon black with silica in passenger car tire in mid-1990s by Michelin and by Degussa/Evinik was a set-up change in improving rolling resistance performance. The rolling resistance was reduced by 20%, the ice traction was improved by 8% and the wet traction was improved by 5%, while the abrasion resistance and noise were maintained at comparative level. The passenger car tire tread compounds contain S-SBR and silica provide in lowest tan δ at 0ºC. 107

4.2.3 Plasticizers Plasticizers are important additives in terms of reducing the process viscosity of a rubber compound, improving the absorption and dispersion of fillers in the rubber compounding step, balancing hardness of rubber compounds, reducing glass transition temperature (Tg) of the compound and cost reduction. Microscopically, plasticizers increase the mobility of polymer chains. They work in two ways; a. In primary plasticizers, the plasticizer molecules have good compatibility with polymer chains, so the polymer chains dissolve in plasticizer on a molecular scale. This results in an improvement in compound flexibility by reducing Tg (This also applies to the case of polar rubbers with polar plasticizers). b. In secondary plasticizers, the compatibility of the plasticizer molecules with the polymer chain is low, so the molecules are not thermodynamically miscible. The plasticizer molecules form separate domains in the polymer matrix, which also improves the low-temperature flexibility of compounds. There are two main types of plasticizers, mineral oil plasticizers and synthetic plasticizers. Mineral oil plasticizers or ‘processing oil’ are obtained from the refining of crude oil residue in the refinery, and are classified according to the content of paraffinic, naphthenic and aromatic structural units. The composition can be determined by the viscosity density constant (VDC). As a general rule, paraffinic oils are used for the non-polar rubbers, such as EPDM, BR, IIR and polyisoprene (IR), while naphthenic oils are used with most of the rubbers with higher polarity. The aromatic oils which have a higher density and viscosity compared to paraffinic and naphthenic oils were used for tread tires to reduce the hardness of compounds, but currently, aromatic plasticizers which contain polycyclic aromatics are classified as carcinogenic and have been withdrawn from the market; treated distilled aromatic extracts (TDAE) oils have been introduced. Synthetic plasticizers, can be classified into two types, liquid and polymer synthetic plasticizers. Acid esters of adipic, sebacic, and phthalic are widely used in polar rubbers, such as NBR and chloroprene rubber (CR) to improve low-temperature properties of rubber products. Ethers, thioethers, and ether-thioether are also used for rubber products to achieve low-temperature properties. Phosphoric acid esters are used as flame retardant alternatives to improve flame resistance of rubber compounds. Liquid plasticizers are commonly used in accelerating filler incorporation during the mixing step, since plasticizers reduce the hardness of compounds. It has frequently been found that liquid plasticizers are incorporated into rubber compounds in large quantities (at high mixing temperatures, liquid plasticizers are incorporated into rubber compounds easily). The excess liquid plasticizer bleeds out from rubber products after a period in store; this is called ‘oil bleed’. Polymer plasticizers are used for rubber products at high operating temperatures. There are only a few liquid rubbers such as liquid butadiene available to improve processability of hard compounds. Polymer plasticizers are cross-linked in the final cross-linking process. Final products have better physical and mechanical properties than using liquid plasticizers, and products can be used at high operation temperatures. 108

4.2.4 The cross-linking systems Chemical cross-linking provides a three-dimensional network of the rubber polymer chains, so that the finished products will maintain their shapes and achieve the mechanical and functional properties required. The common cross-linking systems are sulphur cross-linking, peroxide cross-linking, phenolic resin cross-linking and metal oxide cross-linking. Nowadays, radiation-cure has become widely used in the cross-linking of thin rubber products. Sulphur cross-linking systems are more widely used because of their flexibility and being more economical than other systems. However, not all rubbers can be cross-linked with sulphur cross-linking systems. Only unsaturated rubbers with diene in the structures such as NR, IR, BR, SBR, NBR, IIR and EPDM, are cross-linked with sulphur cross-linking systems whereas peroxide cross-linking systems are suitable particularly for rubbers without diene in the structures such as HNBR, ethylene propylene rubber (EPM), ethylene vinyl acetate rubber (EVA), vinyl methyl siloxane (VMQ) and FKM. Peroxide cross-linking proceeds via a free-radical mechanism to form short and stable C-C cross-links between polymer chains of polymers. Products from peroxide cross-linking have better aging resistance and compression-set properties than products from sulphur cross-linking, but have inferior elongation and dynamic properties (see more detail in Chapter 3). 4.2.5 Antioxidants and antiozonants For centuries, natural rubber was applied as elastomer, but rubber products made from natural rubber would soon become soft and tacky, and would no longer be serviceable. This was determined as the degradation and premature failure of rubber due to the reaction with oxygen in the atmosphere. Initially, products such as waxes, coal tar and creosote were used to coat those rubber products, but these protective coatings would be scuffed or worn off and the unproduced rubber products would soon fail. It was found that derivatives of phenols, hydroxylamine and secondary aromatic amine derivatives were useful in retarding the degradation effects of oxygen. These chemicals can be added into the rubber compounds during mixing. It was also discovered that rubber products, especially tires, stored for several years, failed quickly when put into use. Chemists found that those products which contained antioxidants to protect them against attacks from oxygen, failed because of ‘static storage’ as a result of ozone attack. ‹ Theory of oxidation in rubber products During the compound mixing process and at the service conditions of rubber products, high temperatures occur the unsaturated double bonds of hydrocarbon rubbers, especially, natural rubbers, isoprene rubber, styrene butadiene rubbers butadiene rubber and bottle rubbers, contain free peroxide and hydroperoxide radicals in the main chain of the polymers. These free radicals are the causes of degradation of rubbers. Antioxidants are needed to react with the hydrocarbon free radicals when they are formed and eliminate peroxide and hydroperoxide radicals before damaging to the polymer main chains. They also react with hydrocarbon radicals to shortstop the formation of oxy-radicals; one of the key performances of antioxidants is their solubility in rubbers. 109

A. Effect of heat Temperatures is a key factor of degradation of rubbers because of oxidation (heat oxidation). Generally, there are two conditions that will elevate the temperature of rubbers and rubber products and generate rubber degradation. (I) Antioxidants of phosphate derivatives are designed to protect rubbers during the mixing step and curing step. They are burned up during the high temperature mixing and vulcanization periods. (II) Stress applied to rubber products during the intended use. Antioxidants containing amines and amine derivatives are designed to provide anti-flex cracking properties to rubber products B. Effect of UV light Degradation of rubbers can be triggered by UV light. This degradation can be retarded with UV stabilizers. The hindered amine light stabilisers (HALS) are chemical compounds containing an amine functional group that are used as stabilizers in polymer [2]. Furthermore, phenols are commonly used in combination with secondary antioxidants and UV light stabilizers. Because they are photostable [3]. C. Effect of oxidation on polymers Upon oxidation, rubber products will become soft and tacky or hard and brittle. This is the cause of polymer chain scissions or cross-linking hardening. This can happen in isoprene rubber, natural rubber butyl rubber or even the polar rubber ‘G’ type of neoprene. NBR and SBR rubbers and some types of polymer become hardened or undergo cross-linking. Non-polar saturated polymers such as EPDM or EPM and silicone are used as anti-degradants to protect rubber products from oxidation. Example, blending 25-30 phr of EPDM to natural rubber or a system of isoprene rubbers, will significantly retard the oxidation of the rubber products. EPDM also retards UV degradation of natural rubber and isoprene rubber. ‹ Ozone degradation Rubber can be degraded by ozone attack, because ozone directly attacks the carbon-carbon double bonds of the rubbers. Only polymers having backbone unsaturation were found to be fractured by ozone [4]. Unlike oxidation, ozone degradation cannot be accelerated by increasing the temperature. It is produced when polymer stretches, bur cracks do not form if the underlying double carbon-carbon boundary is not exposed to ozone. Ozone cracking is a physicochemical phenomenon that occurs when polymer chains are attack, resulting in chain scission and the formation of decomposition product [5, 6]. The formation of a relatively unstable ozonide, which cleaves to form an aldehyde or ketone and a carbonyl group, is the initial step in the process [5, 6]. Following that, the aldehyde and carbonyl groups recombine to form a second ozonide. Furthermore, due to the attack of the carbonyl groups generated by primary ozonide cleavage on the rubber carbon-carbon double bonds, cross-linking and chain scission may form during rubber ozonation, especially in IR, IIR and SBR [5, 6]. These rubbers (i.e., IR, IIR and SBR) are more prone to produce chain scission product or crack at the deactivated double carbon-carbon bonds. 110

Antiozonant should have two functions: they should reduce the rate of crack growth in the rubber and decrease the critical stress value (i.e., the stress at which crack growth occurs) [4, 7] 1) Effective antiozonant provides an effective barrier against the penetration of ozone at the rubber surface. 2) Antiozonant must be very reactive with ozone. 3) Since ozone attack is at the surface of rubber, antiozonant should have adequate solubility and diffusivity with the rubber and must migrate to the surface of the rubber to prevent ozone attack. Poor solubility in rubber may result in excessive bloom of antiozonants. 4) Antiozonant must not have adverse effects to the rubber processing and physical and mechanical properties of rubbers. 5) Antiozonants must have low toxicity and not discolor or stain the rubber products. Hydrocarbon waxes and blends of paraffins with micro-waxes are common types of physical antiozonants; p-phenylenediamine derivatives are the prevalent chemical antiozonants. Waxes migrate to the rubber surface and form a protective barrier which remains stable at temperatures from -10ºC to above 50ºC. 1-2 phr of microcrystalline waxes are recommended for applications where rubber products are used. In conditions which involve continuous flexing, p-phenylenediamines (N,N alkyl-aryl derivatives) are recommended. These chemicals scavenge the ozone before it attacks the rubbers, forming an ionized products that protects both the rubber and antiozonants from further attack. However, the p-phenylenediamines are staining chemicals to the rubber compounds. Whenever color is a concern, blends of saturated elastomers at a high level of 30 phr are commonly seen to provide sufficient effectiveness. Furthermore, for long periods of static and dynamic stresses, the combination of antiozonants in rubber compounds total 1.5-3.0 phr of waxes and chemical antiozonants. 4.2.6 Processing aids Processing aids are resins used to improve compatibility of dissimilar elastomers and improve mixing, processing and surface track. Homogenizers are used when compounding dissimilar rubber polymer, such as mixing natural rubber and halobutyl rubber (HIIR) in side-wall compounds of tire processing. Various grades of homogenizers also improve surface appearance and filler incorporation resulting in reduction of compound viscosity and energy consumption in mixing [7]. 111

4.3) Compounding Equipment Previously, two roll mills were used to mix rubber, filler and other ingredients. However, this is extremely time consuming and not viable in today’s commercial manufacturing. When a large quantity needs to be mixed efficiently, internal mixers are used. 4.3.1 Two roll mill Mixed compounds from the internal mixers will drop on to the two roll mill underneath, which will enhance the distribution of fillers and chemicals into the compound matrix. Stock blenders are sometimes installed to speed up mixing time at the two roll mill and give better distribution of fillers and chemicals in the compounds. Compound from this mixing step, is called rubber compound A. It is the rubber compound before curing. Figure 4.2 Two roll mill [13] ‹ Compound B Mixing Compound A is cooled down and transferred to the other mixing line. A Kneader mixer is commonly used because it gives a low shear force which does not generate high mixing heat. Sulphur and accelerators are mixed in the Kneader. A Kneader is different from a Banbury and an intermeshing mixer, as the mixed compound is discharged through a side discharge door. Mixed compound will be further mixed in a two roll mill to give excellent distribution of chemicals into the final rubber compound which is called rubber compound B. This compound will be ready to be delivered to the rubber part manufacturing unit. 112

4.3.2 Internal mixer ‹ Types of internal mixer 1. Dispersion mixer or Kneader, kneader is a small internal mixer that provides low shear force and needs long mixing time. It is suitable for mixing rubber compounds for a sulphur curing system. 2. Tangential mixer, Banbury mixer is still the first choice for diverse applications in the tire industry because it is ideally suitable for the specific requirements of multi-step mixing applications. 3. Intermeshing mixer is an internal mixer that provides better cooling control and is suitable for industrial rubber product mixing in some applications that the Banbury mixers cannot achieve. ‹ Internal mixers and rotors Rubber compounding or mixing is the process of putting together various materials into a rubber matrix to get a final compound that has properties substantially suitable for the final product process with the product properties required. During the mixing cycle, there are four basic physical operations occurring. ƒ Grinding: reduction in particle size of filler agglomerates to its ultimate particle size and disperse into the rubber matrix. ƒ Incorporation: wetting of solid particles by the polymer. ƒ Plasticization: modifying the rheological properties of the mix by reducing viscosity. ƒ Distribution: uniformly distributing all the particles already dispersed in order to obtain homogenous compound. Open mill or two roll mills have been used in rubber mixing since the beginning of rubber development. When the demand for rubber increased because of the growth of the automotive and tire industries, industry required a higher productivity process. After the discovery of different types of rubbers, fillers etc. the rubber industry demanded higher performance mixing equipment to give higher productivity and better rubber compound performance. Farrel’s Banbury and Shaw’s Internmixers were introduced to the rubber industry in 1916 and 1931 respectively. These two types of internal mixers have been accepted as effective compounding devices. On-going development works have resulted in changes and improvements of both types in compounding efficiency. Basically both mixers have various components almost the same, including a hopper assembly for loading materials that contains a ram for forcing materials into the mixing chamber, a mixing chamber with two counter-rotating rotors for mixing the products, rotor, and plates that seal the two ends of the mixing chamber. Dust stoppers seal the areas between the mixing rotor and stationary rotor end plate, discharging door and a drive system which turns the rotors to accomplish the mixing process. The differences between these two mixers are the mixing rotors. Current standard design of 4-6 wing-tangential Farrell’s rotors consists of two rotors with one big 113

pushing wing which joins with 3 or 5 smaller counter pushing wings. Each wing has a narrow tip. Two rotors are fixed in the mixer and the space between rotors and chamber wall has a narrow fixed clearance. Functions of the pushing wing are to disperse and distribute fillers and chemicals into the rubber. Small wings help in turning the mixing compounds from side to side and to the other chamber (distribution). Shearing happens in the area of rotors with the wall of the chamber (Figures 4.3 and 4.4). The basic physical operation in tangential mixers in mixing rubber compounds is to incorporate polymer with filler and chemicals and distribute the additives evenly in the whole compound (dispersion and distribution). In a tangential rotors (Figure 4.5) of internal mixer, the high shear zone is the small area between the chamber wall and the rotating rotor’s tip. The design of intermeshing rotors (Figure 4.5) consists of two big rotors on each of which are one long uninterrupted wing (long wing) and two small wings (called islands). Mixing or dispersion happens between the narrow gap of the two big wings. Regarding the distribution of mixing compound, the long wings push the mixing material in the axial direction which the small islands push the mixing material into the other chamber (see Figures 4.3 and 4.4). Figure 4.3 Basic physical operation of tangential mixer in mixing rubber compounds [8] 114

Figure 4.4 Filler incorporation, dispersion and distribution in Intermeshing mixers [8] Figure 4.5 Intermeshing and tangential rotors of internal mixer [9, 10] In the tire industry, tangential mixers have been used because of higher loading factors than the intermeshing mixers. However, intermeshing mixers are gaining more acceptance in mixing industrial compounds which need higher performance rubber compounds in producing precision rubber parts. Today’s tread compounds are based on S-SBR mixed with highly disposable precipitated silica; to achieve the good compound properties, intermeshing mixers are more preferable 115

‹ Banbury mixer Banbury mixer which was named after Fernley H. Banbury, is a high strength interstitial mixer for rubber and plastic. Banbury mixer has three important parts that affect the rubber compounding process; they are the chamber, two rotors and ram cylinder. The chamber is a steel cylinder containing two rotors with rotor blades and small wings, rotating in opposite directions inside the chamber. Ram cylinder is operated pneumatically a rising and lowering the floating weight in vertical movement and is used to control pressure and mixer volume during the mixing process. The other main parts of a Banbury mixer are the feeding door and discharge door, which are located near the top and at the bottom of the machine, respectively. Materials are filled into the chamber through the feeding door and discharged out at the discharge door. During the mixing process, both doors must completely close to prevent leak of materials. The feeding door is closed by a ram. Mixer volume is one of the important factors of rubber compounding, because it is used to calculate the suitable volume or weight of materials for each batch of mixing. When the Banbury mixer is in operation, the loading and discharging doors are closed. Materials from the feeding port are forced into the roll gap in the chamber by the rotors compression and sheared by rotors inside the chamber. Two rotors rotate relative to each other in opposite directions. The rotors motion causes shear of the mixture held between the rotors, tips and the chamber, thus creating distribution of materials. During the flow of materials around the rotors, fillers and chemicals are sheared into small particles and dispersed into the polymers. The small wings of the rotors push the mixture rotating inside the chamber to give good distribution of fillers and chemicals into the rubber compound matrix. The shearing friction everywhere inside the chamber cause the temperature of the compound to rise sharply and the viscosity of the compound to decrease. This causes a wetting of the rubber on the surface of the compound and ensures good contact between them. Mixed compound is discharged through the bottom drop door into the mixing rolls underneath. 116

Figure 4.6 Banbury mixer [11, 12] ‹ How to use the Banbury mixer Fill factor, ram pressure, rotor speed, coolant temperature, and design of rubber mixing (i.e., fill factor and mixing sequence, time together with the number of passes through the mixer) are all machine-related parameters that influence the properties of rubber compounds. These related parameters have a directly impact on the level of carbon black dispersion of the rubber compound at the final stage of rubber process. • Ram pressure: the pressure applied to the ram during rubber mixing must be regulated to ensure that rubber and other ingredients in the mixer engage rapidly with the rotors. This will also prevent any subsequent up thrust of the batch. • Rotor speed and design: the rotor speed can be adjusted to achieve good dispersion quickly. The sequence of mixing step is related to the design of the rotors in order to maximize mixing time and quality. • Fill factor: the fill factor can adjust to meet a particular mixture’s specifications. If the fill factor is too high, parts of the batch may avoid shear mixing, resulting in non-homogeneity of the material and ingredients or poor dispersion in the rubber compound. Conversely, if the fill factor is too low, voids will occur in the rubber compound behind the rotor wing. • Coolant temperature: the mixing temperature must be controlled to a specific level depending on type of rubber compound. If the mixing chamber is not appropriately cooled during mixing, there may rise rapidly, causing scorch problem for the rubber compound. Furthermore, as the temperature increased, the rubber chains to break down into segmental moieties. This could have a negative impact on the mechanical properties of the rubber compound. 117

• Mixing sequence: another important factor is the order in which new ingredients are added to the mixture. Depending on type of rubber and ingredients such as filler and oil including other ingredients used, the sequence should be adjusted. • Number of passes: the level of carbon black dispersion is affected by the number of times a rubber compound repeats the mixing process. A three-stage mixing process is typically used for compounds containing a high reinforcing filler loading. When a single stage is used in the mixing process with high filler loading, the carbon black dispersion is poor. As a result, the mixing stages influence the level of carbon black dispersion at the final stage of mixing.  To start the mixing operation, rubber technologists begin by calculating the amounts of rubbers, fillers, oils and chemicals in accordance with the volumetric capacity of the mixing chamber. The suggested initial filling factor is 65% of the chamber’s volume before increasing to maximize the productivity. 1) After the mixer is preheated, let it stabilize for a period. Meanwhile, prepare the rubber and chemicals in line with the sequence of mixing. 2) As a general rule, high Mooney rubber will be loaded first, especially natural rubber, which needs a mastication to reduce its Mooney viscosity, before loading the lower viscosity rubber. 3) In the process of mastication, the important factors are rotor speed, ram pressure and time of mixing. These three factors are related to the shear of materials in mixing. 4) Sometimes, small amounts of carbon black and hard fillers are added into the shearing rubber to provide higher shearing force between rubber and filler. 5) In the case of diene rubber, antioxidant is necessary and is added at the early mixing step. 6) After rubber is filled into the mixing chamber, increase the rotor speed and close the chamber by operating the pneumatically controlled ram. The ram pressure will affect the mixing pressure, temperature of compound being mixed and dispersion of fillers, and chemicals in the compound matrix. Recently, Banbury has much better functional automation control, such as chamber temperature controller, rotors speed adjustment and pressure stability control to maximize the efficiency of compound mixing. Mixing torque, ram pressure and mixing temperature are displayed in the monitoring control board. 7) After the viscosity of rubber has been reduced, carbon black or white fillers and antioxidants are added. During the dispersive mixing step, carbon black agglomerates are broken down to less than 1 micron size. Oils or plasticizers are slowly fed continuously into the mixer. Dispersion of carbon black into the compounding matrix largely depends on the shear stress of mixing in breaking down the agglomerates. Temperature of the mixer will rise, being controlled at 160ºC. In the case of high filler loading, additional fillers and plasticizers are added as an additional mixing step. 118

8) Viscosity of the compound can be demonstrated from the electrical current-torque curve. From the curve, the technician can follow observe the mixing pattern of the compound inside the chamber, because the torque will show a rise when the rubber starts to be mixed; it will continue rising until it becomes constant. At this point, materials are completely dispersed into the compound matrix. 9) Discharge door opens when the compound mixing is complete. The speed of the rotors slows down while the ram is still in the down position to prevent the compound reverting to the feeding door.  Mixing problems and reasons Problem Reasons 1) Poor dispersion • Batch size not optimized, mixing time is too low 2) Batch to batch variations • Filler and additive time not correct, insufficient ram pressure 3) Poor processability • Poor temperature control, excess moisture contentment in the rubbers and fillers. • Variation of start temperature, variation in dump time and/or processing temperature, Variation in mixing sequence, processing temperature, variation in mixing sequence, polymers and chemical changes • Compound viscosity not within controlled limit. • Under or over mastication of NR • Poor dispersion, too high loading 119

ARNUDB CBUERRINSGH A P I N G

Rubber Shaping and Curing 5.1) Rubber Processing Rubber processing consists of 4 basic steps 1) Mastication 2) Mixing or compounding 3) Shaping 4) Curing  Mastication: long chain polymers are broken down to make them receptive to chemicals and fillers.  Mixing or compounding is the step in which polymers are mixed with fillers, plasticizers, and chemicals homogeneously (good dispersion and distribution). Mixed compound is ready for shaping or forming into rubber products.  Shaping or forming is the step where the mixed rubber compound is transformed into the desired forms.  Curing is the final step of rubber processing. Product from the shaping step is heated up either under pressure or without pressure. In this step, uncured rubber is transformed into thermoset rubber. 121

Shaping and curing are the last two steps; usually these two steps happen simultaneously. In the case of compression molding and injection molding; curing happens inside the molds during the shaping step. In the extrusion process, curing occurs after the rubber extrusion. The rubber profiles pass through the heating tunnel or curing bath to cure the profile. In the calendaring process, rubber sheet, after passing through the calendar process is fed to heated rolls to cure. Compression, transfer molding and injection are the three main rubber shaping processes. Extrusion and calendaring are processes for the production of rubber profiles and sheets. 5.2) Compression Molding Compression molding is the original production method for molding rubber. It is ideal for low to medium production volumes and is a particular useful process for molding gaskets, seals, O-rings and large, bulky parts. It is a widely used, efficient and economical production method for low production volumes of medium to large parts and higher cost materials. There are 3 steps in compression molding; Pre-shape Compression Curing 5.2.1 Step 1: Pre-shape Rubber compound with curing agents from the final mixing process has to pass through the pre-shape process to prepare small pieces of rubber of a controlled dimension, thickness and weight prior to the molding process. Small two roll mills are commonly used for shaping rubber compound into small pieces with the required sizes, dimensions, thicknesses and weights. In this step, air trapped in the compounds is squeezed out, a process which is necessary to avoid air bubbles in finished products. Sometimes small extruders are also used in preparing pre-shaped rubber to an exact shape and weight. Figure 5.1 Pre-shape rubber in small two rolls mill 122

Figure 5.2 Extruder used in preparing pre-shape compound 5.2.2 Step 2: Molding There are three types of rubber molding: Hydraulic compression molding, Transfer molding and Injection molding ‹ Hydraulic compression molding: This is the most popular rubber molding method because it is the simplest and requires low investment. In the compression molding process, shaping and curing happen simultaneously in the molds. Different from plastic, molding of rubber is a “hot molding” whereas plastic undergoes “cold molding”. In the rubber molding process, firstly, the rubber mold has to be heated up (by heating rods) to 140-200°C (depending on types of rubber being molded). Hydraulic compression machines for molding have ‘top plate’ and a ‘bottom plate’. The top plate can be moved up and down by the hydraulic machine. The rubber mold is placed on the bottom plate. Pre-shaped compound is then placed onto the mold manually. The top plate is moved down by hydraulic pressure. Under heat and pressure, the rubber compound becomes soft and flows into cavities of the mold and is cured under the time, temperature, and pressure conditions designed. 123

Figure 5.3 Hydraulic compression molding machine [1] ‹ Transfer molding This process is suitable to mold rubber parts that are more complicated than compression molding can handle. It is a combination of injection and compression molding. During the process, rubber compound is placed in the heating transfer-pot to warm and soften. Soft rubber from the transfer-pot is pushed down from the transfer-pot through the sprues into the cavities of the mold. Compound is cured under the heat, pressure and time as required. Rubber products are removed from the molds ready for the finishing process. The disadvantage of this process is the time wasted in cleaning the upper part of the molding after operation to remove rubber scrap. Figure 5.4 Transfer molding [2] 124

‹ Injection molding Rubber compounds are produced by feeding rubber as a continuous ribbon. Rubber in the ribbon is continuously pulled into the feeding chamber by the screw. In the heating chamber, rubber is heated and softened. It is forced into the injection chamber and injected into the mold. Temperatures of screw unit, injection unit and nozzle are controlled at temperatures below T10 of rubber compound to prevent rubber scorch. Soft rubber compound is pushed into the cavities of the mold, where rubber is cured under the preset temperature, pressure and time. Advantages of injection molding over the compression molding are the better dimensions and shorter cycle times. Injection molding can also be used for thermoplastic elastomers. Figure 5.5 Rubber injection molding machine [3] Cylinder and screw are the main parts of an injection machine. Rubber compound is conveyed into the cylinder through the throat by the driving screw. Rubber is softened inside the heated cylinder while the screw drives the soft rubber compound slowly down the cylinder. Then it is injected into the rubber mold beneath. Temperature control of the compound at the cylinder is crucial to prevent rubber scorch. 125

5.3) Mold Technology Shaping plastic is a ‘cold molding’ process whereas in the rubber industry, the shaping is done by ‘hot molding’. In the rubber processing, there are scraps and wastes that are thermosets which cannot be recovered. Therefore shaping and vulcanization are crucial steps in rubber processing. To get the right products with less waste or defects, the mold becomes an important part in the rubber process. In injection molding, rubber is warmed up to become soft and flow along the channels paths of the mold. Soft rubber is continuously fed into the mold through the machine nozzle, which sits in the sprue bushing of the mold, and through the runners and gate into the cavities of the mold. In compression molding, soft rubber is heated up inside the mold allowing it to flow through the runners and gate into the cavities. In designing a mold, the molder has to work with the mold designer to try to produce as little waste as possible. This means sprue diameters and runners should be kept as small and short as possible, but they need to be large enough to fully supply the cavities. From the mold nozzle, the sprue leads into the runner. The melted rubber flows into the mold cavity through the gate. A small gate is designed to squeeze the soft rubber and force it into the cavities. Gate size is determined by filling speed and component thickness. The gate is normally smaller than the component wall thickness, but too small a gate can induce defects and incomplete filling of rubber into the cavity. 5.3.1 Hot runner Hot runner is needed in compression molding and transfer molding. In injection molding mold designs are more complicated than for compression molding. In the hot runner system, the top and bottom plates of the mold are heated up by the heat up before feeding the system. Generally, in molding and curing rubber, a particular time and extreme heat from the mold are preferred. In the hot runner process, rubber remains in the molten state until it flows into the cavity. 5.3.2 Cold runner A cold runner is designed for the injection molding. In cold running mold, the runner and mold are maintained at the same temperature, so the mold has two or three plates. In order to ensure the cavity is not under-filled, the runner diameter must be larger than the runner of a hot runner system. Because the cold runner has an unheated channel to convey molten rubber into the mold cavity. Rubber from the sprue runs through another layer of cooling plate (cooled by water) to the sprue that injects molten rubber into the cavity. 126

Figure 5.6 Cold runner mold in injection molding [4] 5.4) Rubber Extrusion Process Rubber extrusion is the process of turning rubber materials into long profiles or tubes by using extruder machines. The extruder machine has two main parts: the screws that push the stock while it is being heated in the conveyer channel and the other part is the hopper from where the rubber stock is sent to the conveyor. In the conveyor, rubber stock is softened through heating, shearing and pressurized by the screw rotating. The pressurized stock is pushed into the die which is located at the end of the extruder. As it emerges, it acquires its required shape. Extrusion is a continuous process that can produce finished products of different lengths in a variety of shapes. Some of the products from extrusion process are door and window seals, edge trim profile, hoses and tubes. The conventional simple screw extruder has various zones: ƒ Feeding zone is where the continuing rubber compound is input. ƒ Solid conveying zone is to transport the rubber compound and slightly compress it. ƒ Plasticating zone where rubber stock is softened to become viscous plastomer. Viscous rubber is pushed through the barrel to the die ƒ Die forming where the rubber is formed into the required shape of the extrudate. At the end of the extruder, extrudate is conveyed into the rubber curing system to cure the rubber. Microwave, hot air vulcanization, salt bath, (HAV) and steam cure are four common curing systems used in the rubber extrusion process. Microwave cures the rubber from inside out quickly and evenly. Salt bath has good heat exchange properties and can be used for highly cured products and as a short-length curing unit. HAV system is a continuous process that gives uniform curing at high production speed. While autoclave or steam cure is used in a discontinuous (or batch) process. 127

SPONGE RUBBER

Sponge Rubber 6.1) What is Sponge Rubber? Sponge rubber is a rubber product which contains a large number of tiny foam holes inside the rubber matrix. Sponge rubber is usually soft and provides good cushioning, thermal and noise insulation and has a quick recovery property. It can be either in closed cell or open-cell structures (or the mixture of both). Open-cell sponge rubber contains open interconnected pockets that permit the passage of air, water, gases and chemicals. Most of the cushioning sponge rubbers have open-cell structure, while sponge rubbers used in shock absorption, vibration damping and weathering strips have closed cell structure. Sponge rubber was first produced in 1929 by E.A [1, 2]. Murphy and Eric Owen, researchers at Dunlop Rubber, who whipped latex and isocyanate together to make an open-cell foam for a mattress. In the 1950s, polyether polyurethane-based foams were developed by Charles C. Price [2, 3]. Polyurethane foam is widely used in construction, transportation, home furniture and noise insulation. Neoprene, EPDM, nitrile rubber and ethylene vinyl acetate (EVA) resin are used in closed cell sponge rubber. Neoprene sponge rubber is commonly used in wet-suites because of good weathering and tear resistance properties of Neoprene. EPDM is widely used in automotive weathering strip, construction and industrial insulations because of good weathering resistance of EPDM. EVA foam is used widely as the midsole of shoes because it has low compression set property. Silicon foam sponge rubber has low temperature applications, as low as -65ºF, while fluorosilicone foam can be used in temperature going down to -80ºF. 6.2) How to produce Open and Closed-cell Sponge Rubber? To produce open-cell sponge rubber, sodium bicarbonate is normally used as the source of gas. It is added to the polymer compound which contains other components and a curing system. When the polymer compound is heated up, sodium bicarbonate decomposes and carbon dioxide gas is released, creating open, interconnected cell-foam. In the case of closed-cell sponge rubber, blowing agents are used to produce gas bubbles in the closed cell foams [4]. When polymer compound containing blowing agent and curing system is heated up in the steel mold, the blowing agent decomposes to generate gas bubbles and these tiny bubbles are trapped inside the matrix of the cross-linking rubber. Each of these cells formed in the matrix of polymer is isolated from its neighbors. In the process, two chemical reactions 129

happen simultaneously; gas generated from the decomposition of the blowing agent and a cross-linking reaction also. Controlling the time of decomposition of blowing agent while the cross-linking reaction is happening results in different cell structures (Figure 6.1). Figure 6.1 Sponges from balancing crosslinking and gas releasing time 6.3) How to develop Sponge Rubber? To produce good sponge rubber, rubber chemists select the types and grades of polymer that are suitable to the end-applications. Then they select the grade of blowing agent which will decompose to give gas at the related time with the cross-linking reaction time. • Polymer: generally, select the type of polymer that is suitable for the application. Then select the grade of polymer, usually high Mooney viscosity grades. In the case of EVA, low melt-flow index (MFI) grades are the choices. • Fillers: a small amount of filler is used. Filler in this case is not only used as reinforcing filler, but the fine particles of filler also acted as nucleus in generating bubbles. Silica dioxide is used in case of white product. • Zinc oxide; ZnO is commonly used as ‘kicker’ of the blowing agent and initiator of the cross-linking reaction. • Curing systems; either peroxide curing or sulfur curing system. • Blowing agents: dinitrosopentamethylenetetramine (DPT), azodicarbo-namide (ADA), benzenesulfonylhydrazide (OBSH) and P-toluenesulphonyl hydrazine (TSH) are organic blowing agents commonly used in making closed-cell sponge rubber [5]. The decomposition temperatures of these commercial blowing agents are higher than the process temperatures so it is necessary to use chemical as “a kicker” to reduce the decomposition temperatures 130

down to the process temperatures [5]. Zinc oxide and urea are commonly used as ‘kickers’. In this case, the decomposition temperatures of blowing agents are decreased from ~200ºC to ~ 150ºC which are the process temperatures of cross-linking of polymer. Decomposition of blowing agents gives mixtures of nitrogen gas, carbon monoxide and dioxide gases that generate bubbles in the polymer matrix. Table 6.1 Types of blowing agent [6-9] Chemical name Kicker Structure Decomposition Gas yield References temp (ºC) in ml/g, air, blowing agent Dinitrosopenta- Urea 200-210 240-260 [6] methylenetetramine Zinc oxide (N2, CO, NH3) (DPT) and urea Azodicarbonamide (ADA) 204-213 125 [7, 8] (N2, CO, NH3) [8] [9] Benzenesulfonylhydrazide Zinc oxide 157-160 125 (OBSH) and urea (N2, H2O) P-toluenesulphonyl Zinc oxide 120 115 hydrazine (TSH) (N2) 6.4) Type of Sponge EVA sponge: it is widely used in the footwear industry; 18-22% vinyl acetate (VA) content in ethylene vinyl acetates with low melt index (MI) 1-3 is commonly used to make mid-sole sponge in shoe application. Neoprene G type is used in making sponge rubber for wetsuits, because neoprene has good weathering, chemical resistance and high tear resistance properties. EPDM with medium to high Mooney viscosity is used to produce industrial insulation foam as well as weathering strips for automobiles because EPDM has good weathering and ozone resistance. High Mooney viscosity EPDM can absorb high filler loading to produce low cost sponge rubber. NBR sponge rubber is used in the case where the product has to be in contact with oil or chemicals. NBRs with low to medium acrylonitrile contents are mainly selected. Silicone sponge rubber is used in the food industry and electrical applications, such as electric cable jacketing and high temperature applications, up to 200ºC Compression molding is a common process for making sheet sponge rubbers. Extrusion process is used to produce automotive weathering strips 131

6.5) Ethylene Vinyl Acetate (EVA) Ethylene vinyl acetate (EVA) is a copolymer of ethylene and vinyl acetate. It is a thermoplastic elastomer that has good clarity and gloss, with low temperature toughness and stress-crack resistance. EVAs are used to produce extrusion film; packaging, surface protection film, green house film and photovoltaic cell encapsulation. EVAs with VA content from 4-15% are used in producing soft plastic products such as soft toys. The copolymers with 18-22% VA content are used as shoe midsole foams (Figure 6.2). EVAs with high VA contents (28-40%) are used as resins in making hot melt glues. Figure 6.2 Major component of shoe [10] ‹ EVA foam in shoe application Midsole foam helps to cushion the foot bed providing arch support and enhance athletic performance. PU and EVA are the two main materials used in making midsole foam [11]. Nike introduced midsole foam into its athletic shoes in the 1970s [12]. At that time; mid-form was shaped by die-cutting. EVA sheets were sliced to make small sheets in the shapes and sizes of shoes with the thickness of about 3 mm. Die-cut EVA foams were produced in big foam blocks through the compression molding process and block-forms were die-cut into small foam sheets. EVA foam sheets were glued in between the uppers of the shoes and the rubber outsoles. EVA 15-18 VA was commonly used. General formulations of EVA die-cut midsole foam were: EVA (15-18% VA content) 100 phr, 10-15 phr of SiO2, 2-4 phr blowing agent, 1 phr stearic acid, 2-4 phr DCP and 1-2 phr of ZnO. Properties of foam were adjusted by varying percentages of blowing agent and cross-linking agent to obtain foam hardiness of 51-55, shore D. Phylon mid-foam was introduced in the 1980s. The concept of Phylon mid-foam is to produce mid-foam soles that have the contour of shoes [11, 13]. Outer hardness of EVA Phylon foam remains at 51-55 shore D, but hardness of the inside is about 35-40 shore D. Phylon foam is produced by compression molding to obtain die-cut sheets that have a hardness of 35-40, shore D. Foam sheets are sliced into small pieces of foam, with the size larger than the size of the shoe. They are placed in the Phylon steel molds that have the same contoured shape as the shoes. Foams are heated up to a temperature around 80ºC by steam for 5 minutes. Then, EVA foam inside the mold starts to melt. After turning off the steam, and chilled water 132

is pumped in. By doing so the outer surfaces of the EVA foams become harder than the inner. Then the Phylon foams are taken out from the molds. These foam in the required shapes have outer surface hardness of 50-55 shore D and inner hardness around 35-40 shore D. In the Phylon process, higher VA contents of EVA (20-22% VA contents) are used in order to obtain die-cut foams at hardness of 35-40 shore D, and higher degree of cross-linking (than the die-cut foam) for better split tear and low compression set. Unfortunately, the Phylon process, produces almost 40-45% of scrap (same as the waste in the die-cut foams), and the process is very labor intensive. Direct injection Phylon process is the latest Phylon EVA foam process that has been developed by using a rotating injection machine to improve the rate of production and yield. In this process, EVA compound in the form of pellets is produced by using a twin screw extruder. EVA of 22% VA content with MFI 5-6 is fed into the twin-screw extruder using the same compound formulation for making Phylon foam. Generally, ratios of blowing agent and cross-linking agent are adjusted to obtain the best final properties of foams. Filler, cross-linking agents, blowing agents and chemicals are also fed into the twin screw extruder through the auto-feeding system; the resulting compound is pelletized. The pellets from the twin screw extruder are fed into the molds of the rotating Phylon molding machine. Inside the molds, EVA compounds are heated up and foams are blown in the steel molds. After cooling with chilled water, the final Phylon foams with the outer hardness of 50-55 shore D are obtained. This is an automatic process improving productivity with very little scrap. 133

WHEEL AND TIRE

Wheel and Tire 7.1) Development of Tire The tire industry consumes 50% of the total rubber production and 60% of the natural rubber production. Currently over 2.4 billion tires are manufactured, with China being the largest tire producer at about 20% of global production, followed by the United States, Japan, South Korea, and Germany. In terms of revenue, Continental Tire is the leader followed by Bridgestone, Michelin, Goodyear, Sumitomo, and Pirelli. The wheel is probably mankind’s most important mechanical invention. In early history, humans used logs, usually many logs, as rollers to move large loads around. The need for faster transportation and the idea of using less material (fewer logs) stimulated the breakthrough in the evolution of the wheel. Around 2000 BC, the Egyptians invented a spoked wooden wheel for transport and the Greeks developed the cross-bar wheel. Wheels made it possible to carry heavy objects from one place to another quicker [1]. Wooden wheels for horse-drawn vehicles usually have a wrought iron tire. This construction was extended to wagons on horse-drawn tramways, rolling on granite setts or cast iron rails. The wheels of some railway engines and older types of rolling stock are fitted with railway tires in order to prevent the need to replace the entirety of a wheel. The tire, usually made of steel, surrounds the wheel and is primarily held in place by interference fit. 135

The history of the rubber tire begins with John Boyd Dunlop when he invented a pneumatic tire for his son’s tricycle in 1887 [2]. Dunlop was a veterinary surgeon born in Dreghorn, North Ayrshire, Scotland. The tire he developed was an inflated tube of rubber sheeting fitted to a wooden disc, which performed very well when tested. He patented his pneumatic tire on December 7, 1888 (although the patent was later declared invalid due to the idea having been patented earlier by Robert William Thomson) [3]. Willie Hume used Dunlop’s tires in winning cycle races, thus showcasing their superiority. The commercial production of pneumatic tires began in 1890 in Belfast, Ireland. The first automobile (three wheels), developed by Carl Benz in 1885, used steel wheels fitted with hard rubber, but after Dunlop’s creation became well known, Benz made the switch to pneumatic tires [4]. On the other side of the world during 1900-1930, the automotive industry became well developed in America by Henry Ford. After starting out manufacturing tires for bicycles and horse-drawn wagons, Goodyear Tire and Rubber (founded by Frank A. Seiberling) and Firestone Rubber (founded by Harvey S. Firestone) converted their factories to provide pneumatic tires to the Ford Motor Company. A technologically innovative company, Goodyear developed rubber tires for airplanes in 1908 as well as an air-sling for the U.S. Navy to use during the First World War [5, 6]. It also developed many rubber products to serve the U.S. military during the Second World War. Goodyear was one of the synthetic rubber (GR-S) producers for the U.S. government during the Second World War, and after the war, it became the largest producer of synthetic rubber as well as tires. For over fifty years after John Boyd Dunlop’s first pneumatic tire, automotive tires were made up of an inner tube that contained compressed air and an outer casing to protect the inner tube and provide traction [2]. Michelin introduced steel-belted radial tires in Europe in 1949 [7]. The radial tires introduced by Michelin had longer tread life, better steering control, and less rolling resistance. Goodyear produced radial, bias-belt tires in 1967 after investing billions of dollars in radial technology [8]. All American new cars came with radial tires by 1983. Currently, Goodyear is one of the largest global radial-tire producers, having 20% of the market share in radial tires. Modern pneumatic tires consist of a tread and body. The tread provides traction while the body provides containment for a quantity of compressed air. The materials used for modern pneumatic tires are synthetic rubber and natural rubber mixed with carbon black and many chemical compounds. Fabric or steel wire is used to strengthen the structure of tires and provide safety in driving. With over 3 billion tires sold around the world each year, tire manufacturing is a major consumer of natural rubber. 136

7.2) Types of Tires There are several types of rubber tires: 7.2.1 Light-duty tires Light-duty tires for passenger vehicles carry loads in the range of 550 to 1,100 pounds (250 to 500 kg) on the drive wheel. Light-to-medium duty trucks and vans carry loads in the range of 1,100 to 3,300 pounds (500 to 1,500 kg) on the drive wheel [9]. They are differentiated by speed rating for different vehicles, including (starting from the lowest speed to the highest): winter tires, light truck tires, entry-level car tires, sedans and vans, sport sedans, and high-performance cars. Apart from road tires, there are special categories [5]  Snow tires are designed for use on snow and ice at temperatures below 7°C (45°F). Some snow tires have metal or ceramic studs that protrude from the tire to increase traction on hard-packed snow or ice. Studs abrade dry pavement, causing dust and create wear in the wheel path. Regulations that require the use of snow tires or permit the use of studs vary by country in Asia and Europe, and by state or province in North America [5].  All-seasons tires are typically rated for mud and snow (M+S). These tires have tread gaps that are smaller than snow tires and larger than conventional tires. They are quieter than snow tires on clear roads, but less capable on snow or ice [10].  All-terrain tires are designed to have adequate traction off-road, yet have designed handling and noise characteristics for highway driving [11]. Such tires are rated better on snow and rain than street tires and “good” on ice, rock and sand [12].  Mud-terrain tires have a deeper more open tread for good grip in mud than all-terrain tires, but perform less well on pavement [13].  High-performance tires are rated for speeds up to 168 miles per hour (270 km/h) and ultra-high-performance tires are rated for speeds up to 186 miles per hour (299 km/h), but have harsher ride characteristics and durability [14].  Electric vehicles have unique demands on tires due to the combination of weight (resulting in a new load index), higher torque and requirements for lower rolling resistance [15]. 137

 Other types of light-duty automotive tires include run-flat tires and race car tires:  Run-flat tires obviate the need for a spare tire, because they can be traveled on at a reduced speed in the event of a puncture, using a stiff sidewall to prevent damage to the tire rim [16]. Vehicles without run-flat tires rely on a spare tire, which may be a compact tire, to replace a damaged tire [16].  Race car tires come in three main categories, DOT (street-legal), slick, and rain. They are designed to maximize cornering and acceleration friction at the expense of longevity. Racing slicks have no tread in order to maximize contact with the pavement [10]. 7.2.2 Heavy duty tires Heavy duty tires for large trucks and buses come in a variety of profiles and carry loads in the range of 4,000 to 5,500 pounds (1,800 to 2,500 kg) on the drive wheel [5, 9]. These are typically mounted in tandem on the drive axle [16].  Truck tires come in a variety of profiles that include “low profile” with a section height that is 70 to 45% of the tread width, “wide-base” for heavy vehicles, and a “super-single” tire that has the same total contact pressure as a dual-mounted tire combination [5, 16].  Off-road tires are used on construction vehicles, agricultural and forestry equipment and other applications that take place on soft terrain. The category also includes machinery that travels over hardened surfaces at industrial sites, ports and airports [17]. Tires designed for soft terrain have a deep, wide tread to provide traction in loose dirt, mud, sand, or gravel [5]. 7.2.3 Others Aircraft, semi-pneumatic, airless, bicycle and a variety of industrial applications have distinct design requirements.  Aircraft tires are designed for landing on paved surfaces and rely on their landing gear to absorb the shock of landing. To conserve weight and space required, they are typically small in proportion to the vehicle that they support. Most have radial-ply construction. They are designed for a peak load when the aircraft is stationary, although side loads upon landing are an important factor [18]. Although hydroplaning is a concern for aircraft tires, they typically have radial grooves and no lateral grooves or sipes [5]. Some light aircraft 138

employ large-diameter, low-pressure tundra tires for landing on unprepared surfaces in wilderness areas [19].  Semi-pneumatic tires have a hollow center, but they are not pressurized [5]. They are light-weight, low-cost, puncture proof, and provide cushioning. These tires often come as a complete assembly with the wheel and even integral ball bearings. They are used on lawn mowers, wheelchairs, and wheelbarrows. They can also be rugged, typically used in industrial applications, and are designed to not pull off their rim under use.  An airless tire is a non-pneumatic tire that is not supported by air pressure [20, 21]. They are most commonly used on small vehicles, such as golf carts, and on utility vehicles in situations where the risk of puncture is high, such as on construction equipment. Many tires used in industrial and commercial applications are non-pneumatic, and are manufactured from solid rubber and plastic compounds via molding operations. Solid tires include those used for lawn mowers, skateboards, golf carts, scooters, and many types of light industrial vehicles, carts, and trailers. One of the most common applications for solid tires is for material handling equipment (forklifts). Such tires are installed by means of a hydraulic tire press.  Bicycle tires may be designed for riding on roads or over unimproved terrain and may be mounted on vehicles with more than two wheels. There are three main types: clincher, wired and tubular [22]. Most bicycle tires are clincher and have a bead that presses against the wheel rim. An inner tube provides the air pressure and the contact pressure between bead and wheel rim [23].  Industrial tires support such vehicles as forklifts, tractors, excavators, road rollers, and bucket loaders. Those used on smooth surfaces have a smooth tread, whereas those used on soft surfaces typically have large tread features. Some industrial tires are solid or filled with foam.  Motorcycle tires provide traction, resisting wear, absorbing surface irregularities, and allow the motorcycle to turn via counter steering. The two tires’ contact with the ground affect safety, braking, fuel economy, noise, and rider comfort. 139

7.3) Components of Passenger Tires The main components of a tire are its tread, bead, sidewall, shoulder, ply, and valve stem, and they are described briefly as follows [24, 25]: Figure 7.1 Cross-section of passenger tire [24] 7.3.1 Tread Tread is the thick, patterned rubber (usually a mixture of natural and synthetic rubber) that comes into contact with the road surface. Both the rubber compound formulation and the tread pattern are designed to meet each tire company’s specific product market position. Tread patterns feature lugs, voids and grooves, each with a specific purpose. Tread lugs form the contact surface necessary to provide traction. Tread voids and grooves provide space for the lug to flex and deform and they provide channels for rainwater, mud, and snow to be channeled away from the tread. 7.3.2 Bead The part of the tire that contacts the rim on the wheel. The bead is typically reinforced with steel wire and rubber compounded with high strength, low flexibility rubber [25]. The bead sits tightly against the two rims on the wheel to ensure that a tubeless tire will hold air without leakage [26]. 140

7.3.3 Sidewall The part of the tire that bridges between the tread and the bead. It is largely rubber but reinforced with fabric or steel cords that provide for tensile strength and flexibility [25]. The sidewall contains the air pressure and transmits the torque applied by the drive axle to the tread to create traction. 7.3.4 Shoulder The part of tire at the edge of the tread as it makes transition to the sidewall. The sidewall is mostly comprised of rubber, although it is reinforced with fabric or steel for increased strength and flexibility [27]. 7.3.5 Ply cord Ply cord is the main body of the tire. It is also known as carcass and is composed of layers of fabric called plies [27] with relatively inextensible cords embedded in the rubber to hold its shape by preventing the rubber from stretching in response to the internal pressure [5]. The orientation of the plies influences the performance of the tire and is one of the primary ways tires are classified [5, 28]. 7.3.6 Valve stem The valve stem is made up of metal or rubber and is used to inflate the tire [28]. For tubeless tires, the valve stem mounts directly to the rim. 7.4) Type of Tire Construction Following the 1968 consumer reports announcement of the superiority of the radial design, radial tires began an inexorable climb in market share, reaching 100% of the North America market in the 1980s. Radial tire technology is now the standard design for essentially all automotive tires, but other methods have been used.  Radial tire construction utilizes body ply cords extending from the beads and across the tread so that the cords are laid at approximately right angles to the centerline of the tread, and parallel to each other, as well as stabilizer belts directly beneath the tread. The belts may be cord or steel. The advantages of this construction include longer tread life, better steering control, fewer blowouts, improved fuel economy, and lower rolling resistance. Disadvantages of the radial tire are a harder ride at low speeds on rough roads and in the context of off-roading, decreased “self-cleaning” ability and lower grip ability at low speeds. 141

 Bias tire (or cross ply) construction utilizes body ply cords that extend diagonally from bead to bead, usually at angles in the range of 30 to 40 degrees. Successive plies are laid at opposing angles forming a crisscross pattern to which the tread is applied. The design allows the entire tire body to flex easily, providing the main advantage of this construction, a smooth ride on rough surfaces. This cushioning characteristic also causes the major disadvantages of a bias tire: increased rolling resistance and less control and traction at higher speeds.  A belted bias tire starts with two or more bias plies to which stabilizer belts are bonded directly beneath the tread. This construction provides smoother ride that is similar to the bias tire, while lessening rolling resistance because the belts increase tread stiffness. The design was introduced by Armstrong, while Goodyear made it popular with the “Polyglas” trademark tire featuring a polyester carcass with belts of fiberglass. The “belted” tire starts with two main plies of polyester, rayon, or nylon annealed as in conventional tires, and then circumferential belts are placed on top at different angles that improve performance compared to non-belted bias tires. The belts may be fiberglass or steel. 142

REFERENCES

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