World Of Rubber

Table 2.4 Grades of emulsion styrene-butadiene rubber (E-SBR) Name Production site Styrene ML Type of Oil Stabilization Physical (%) (1+4) oil (phr) form Buna@ SE 1500 Caxias (BR) 23.5 52 none - staining in bales Buna@ SE 1502 H Triunfo (BR) 23.5 53 none - non-staining in bales Buna@ SE 1502 L Triunfo (BR) 23.5 49 none - non-staining in bales Buna@ SE 1723 Caxias (BR) 23.5 50 TDAE 37.5 staining in bales Buna@ SE 1739 Caxias (BR) 40.0 53 TDAE 37.5 staining in bales Buna@ SE 1783 Caxias (BR) 23.5 49 RAE 37.5 non-staining in bales Buna@ SE 1793 Caxias (BR) 23.5 51 TRAE 37.5 staining in bales Buna@ SE 1799 Caxias (BR) 40.0 55 TRAE 37.5 staining in bales Table 2.5 Grades of solution styrene butadiene polymer (S-SBR) Name Production site Styrene ML Type of Oil Tg (ºC) Physical (%) (1+4) oil (phr) form Buna@ VSL 4526-2 Pt.Je ́ro m̂ e (FR) 26.0 44.5 TDAE 37.5 -30 in bales Buna@ VSL 4526-2 HM Pt.Je ŕ o ̂me (FR) 26.0 44.5 TDAE 37.5 -30 in bales Buna@ VSL 2538-2 Pt.Je ́ro m̂ e (FR) 38.0 25 TDAE 37.5 -31 in bales Buna@ VSL 2438-2 HM Pt.Je ́ro ̂me (FR) 38.0 24 TDAE 37.5 -32 in bales Buna@ VSL 3038-2 HM Pt.Je ŕ o m̂ e (FR) 38.0 30 TDAE 37.5 -26 in bales 2.5.4 Compounding and vulcanized properties SBR compounding is similar to NR compounding but does not require mastication as the molecular weight is designed to be not too high for mixing and processing. The same curing ingredients can be used as for NR. However, SBR curing is slower than that for NR so more accelerator or more active accelerator is required [11]. Also, SBR cannot crystallize on stretching like NR, therefore it needs reinforcing filler. Even though most properties of SBR are comparable to NR, some properties are lower like gum tensile strength, elongation at break, tack, hysteresis and resilience. However, reinforcing fillers and designed compound formulation can improve these properties. The better properties of SBR over NR are processability, slightly better abrasion and aging resistance together with less scorch problems. 2.5.5 Applications The main applications of SBR are tires and the rest are hoses, belts, adhesives, footwear, rollers and molded rubber goods. SBR can be used in many applications as a replacement of NR but not in severe dynamic applications and very low heat build-up. This is one of the reasons why SBR cannot be substitute for NR in tire manufacturing. However, blending SBR with NR or BR can improve some properties. 49

2.5.6 Other SBR related polymers Ricon 100, a low molecular weight, liquid copolymer of butadiene and styrene can be used as polymeric plasticizer having a high vinyl content. Table 2.6 Typical physical and chemical properties [13] Property Test method Value Unit Molecular weight (Mn) QCS-651/ Q 34.03 P019 4,500 g/mol 1,2 Vinyl content* QCS-642/ Q 34.03 P040 70 wt.% Styrene content QCS-642 20 wt.% Viscosity @45ºC QCS-630/ Q 34.03 P035 40,000 Cps Tg QCS-681 -15 ºC Specific gravity @25ºC QCS-649 0.90 - 50

2.6) Acrylonitrile Butadiene Rubber (NBR Rubber) Acrylonitrile butadiene rubber, also known as nitrile rubber (NBR), is a random copolymers of butadiene and acrylonitrile, as shown in Figure 2.6 [14]. It was developed in 1931 at BASF and Bayer laboratory. Then, IG Farben commercialized NBR rubber in 1935. The NBR rubber has excellent oil, fuel and acid resistances, this is because of high polarity of acrylonitrile containing in rubber molecule. Butadiene is the polymer back bone. Because of unsaturated in butadiene portion, NBR is able to be cured by sulphur and sensitive to UV and ozone [15]. At higher acrylonitrile (ACN) content, NBR has higher oil resistance and higher glass transition temperature (Tg). This type of NBR is widely used as oil seals and O-ring applications. Figure 2.6 Chemical structure of NBR [14] There are two processes in producing NBR rubber are hot and cold polymerization. • In hot polymerization, acrylonitrile and various butadiene monomers (1,3-butadiene and 1,2-butadiene) are reacted in the emulsion (soap phase). The acrylonitrile and butadiene ratio varies depending on the specific requirements of oil and fuel resistance and low temperature. For special grade of NBR, it contains a third monomer such as divinyl benzene, methacrylic acid for providing specific properties. The mixture was reacted at 70ºC for several hours in the polymerization tank. Dimethyldithipo-carbamate is used to shortstop reaction. The unreacted monomers are removed through the steam in a slurry stripper. After that, NBR latex is transferred through a series of filtration and further mixed with antioxidant. Then, obtained NBR latex is coagulated by calcium nitrate or aluminum sulfate. Finally, the NBR coagulum is subsequentially washed and dried into crumb rubber. • In cold polymerization, the process of cold NBR is very similar to hot polymerization. For cold reaction, it is reacted in the polymerization tanks at 5ºC-15ºC, there are less branching is generated on cold NBR form. 2.6.1 Properties of NBR The ACN content, or the ratio of acrylonitrile groups to butadiene groups in main chain molecules, is a significant property of NBR. NBR has lower Tg at lower ACN content, while at higher ACN content offers the polymer with improved resistance to nonpolar solvents [4]. Most applications that require both solvent resistance and low temperature flexibility require the ACN content approximately 33%. The general properties of NBR is summarized in Table 2.7 [16]. 51

Table 2.7 General properties of NBR [16] Properties Oil, fuel and grease resistance + Good processing characteristics + Variety of curing systems + Good hot air resistance • Long term: 90ºC • 40 days: 120ºC • 3 days: 150ºC Low permanent set + Good abrasion resistance + Low gas permeability + Moderate to good low temperature flexibility - Moderate ozone resistance (except NBR/PVC) - Moderate tack - Compatibility with polar thermoplastics (i.e., PVC, phenolics) + As a consequence of NBR’s special properties, it is widely used in the automotive and aerospace industries to produce seal, grommets, fuel and oil handling hoses, and self-sealing fuel tanks, among other things. It is also utilized in the nuclear industry to produce protective gloves. Because of its operating temperature stability across a wide temperature range of -40°C to 108°C (-40°F to 226°F), NBR is a desirable material for aerospace applications. Moreover, NBR is applied in a variety of applications such as sealants, sponges, footwear, adhesives, expanded foams, and floor mats. 2.6.2 General types of NBR ‹ Cold NBR The current generation of cold NBRs is available in a wide range of compositions. The ACN content ranges from 15% to 51%, resulting in a wide range of Mooney viscosity for raw NBR material ranging from 25 unit to 110 units. A large variety of ingredients, including emulsifier systems, coagulants, stabilizers, molecular weight modifiers, and chemical compositions are used in cold polymerization. Third monomers are added to the main chain of a polymer to improve performance. Each variety serves the unique function. Cold NBR polymerization also uses continuous, semi-continuous and batch polymerization. The procedure to polymerize cold polymers [9] uses temperatures ranging from 5°C to 15°C, depending on the required balance of linear-to-branched structure. Lower polymerization temperatures result in more linear polymer chains. ‹ Hot NBR For Hot NBR polymerization, temperatures ranging from 30°C to 40°C are used to polymerize [9]. This method produces highly branched polymers. Its branching structure 52

provide strong tack and binding in adhesive applications. This type’s physically entangled structure gives greater tear strength than cold NBR polymerization. Furthermore, due of their natural flow resistance, they are great candidates for compression molding and sponge processes. Other uses include thin-walled or complicated process (i.e., extrusion) that need shape retention. ‹ Cross-linked hot NBR Cross-linked hot NBR is branching polymers that has been further cross-linked by the addition of a di-functional monomer. This product is frequently used in molded parts to offer sufficient back pressure or molding forces and to remove trapped air during rubber processing. Another strategy is to improve shape retention (or dimensional stability) in extruded and calendering goods. This results in more successful extruding and vulcanization of intricate shaped parts, as well as release better from calender-rolls. Furthermore, this form of NBR improve shape retention, impact resistance, and flexibility for modified PVC. ‹ Carboxylated nitrile butadiene rubber (XNBR) Carboxylated nitrile butadiene rubber (XNBR) is similar to nitrile rubber, but the polymer backbone contains a terpolymer composed of nitrile, butadiene and monomers which contains carboxyl groups such as methacrylic acids, which drastically accelerates cure characteristic [17, 18]. This material offers higher tensile and tear strengths, as well as better abrasion, as compared to normal NBR grade [17]. As a result, XNBR rubber is typically used in dynamic parts such as seals and rod wipers [19]. ‹ Bound antioxidant NBR The network-bound antioxidant NBR grade is available with an antioxidant polymerized with butadiene and acrylonitrile [16, 19]. This NBR grade’s objective to improve the aging resistance of normal NBR grade by modifying the polymer itself [17]. Furthermore, the polymer bound with antioxidant improves the water and oil resistance for the NBR vulcanizate [21]. ‹ Hydrogenated nitrile butadiene rubber (HNBR) Hydrogenated nitrile butadiene rubber (HNBR) is produced by hydrogenation of conventional NBR polymer to remove the olefinic groups that are susceptible to chemical degradation. The degree of hydrogenation determines the kind of vulcanization that can be applied to the polymer [22]. After hydrogenation of NBR, the structure of HNBR is primarily composed of three types of functional groups, as shown in Figure 2.7. The original NBR polymer is produced during the first phase of the hydrogenation process. After that, the NBR polymer is coagulated and dried. The resultant NBR polymer is then dissolved in a suitable solvent, followed by hydrogenation to produce HNBR [23]. Zhanber (Lianda corporation), Therban (Arlanxeo), and Zetpol (Zeon chemical) are examples of trade names. Figure 2.7 Chemical structure of hydrogenated acrylonitrile butadiene rubber (HNBR) 53

The properties of hydrogenated nitrile rubber (HNBR) depend upon the acrylonitrile content and the degree of hydrogenation of the butadiene copolymer. HNBR rubber has greater tensile strength and very good low temperature properties in terms of both brittle point and stiffness when compared to NBR rubber [22]. Furthermore, HNBR material is much more resistant to oil and chemical [23]. This lead to apply in O-rings, seals, hoses and belts that are used in automotive industry [24, 25]. Other applications include bladders, heat-shielding materials, pipes and valve linings [24, 25]. Depending on filler selection and loading, HNBR compounds typically have tensile strengths of 20-31 MPa at 23°C. Compounding techniques allow for HNBR to be used over a broad temperature range, -40°C to 165°C, with minimal degradation over long periods of time. For low-temperature performance, low ACN grades should be used; high-temperature performance can be obtained by using highly saturated HNBR grades with white fillers. As a group, HNBR elastomers have excellent resistance to common automotive fluids (i.e., engine oil, coolant, fuel, etc.). ‹ Plasticizer-modified NBR Plasticizer-modified NBR grades, also known as oil-extended grades, are also commercially available to reduce long time compounding and compounding cost [26]. The addition of ester and ether plasticizers to NBR during polymerization stages is done to decrease the compounding time of rubber compound [17]. This is especially true when large content of plasticizer are used. Plasticizers are commonly used in rubber to improve processability during rubber processing, such as mixing, extruding, and calendaring [22]. In the case of NBR, Phthalates are often used to promote rubber processing and cost effectiveness [27]. Furthermore, di-2-ethylhexyl phthalate (DEHP) or dioctyl phthalate (DOP) is phthalate ester that is widely used as a general purpose primary plasticizer in the industry [28]. Because of its characteristics, it is suitable for a wide range of applications in the flexible vinyl industry. Furthermore, DEHP has good gelation characteristics, good softening action, and suitable viscosity properties in PVC [28]. 2.6.3 Applications of NBR Most standard NBR compounds are based on a 33 to 36 ACN polymers. These give the most versatile compound performance for oil swell and low temperature properties. Higher ACN content, polymers are important where seal performance cannot compensate for large volume swell and compound is fully immersed in fuel or oils. Low ACN polymers are used to improve low temperature properties. In addition, XNBR give good physical, mechanical properties (i.e., modulus, tear and tensile strengths) and abrasion resistance of NBR rubber. They find their applications in printing rollers, conveyor belts, hose covers and down hole seals. Furthermore, HNBR has excellent oil and fuel resistance as well as resistance to oxidation and ozone, as well as superior physical properties and good oxidation and ozone resistance. Low ACN-HNBR gives low temperature performance. It is widely used in aeronautic industry and North Pole and deep-sea oil explorations. 54

2.7) Ethylene Propylene Rubber (EPM) and Ethylene Propylene Diene Rubber (EPDM) EPDM rubber is a terpolymer of ethylene, propylene and a non-conjugated diene. Ethylene contents in EPDM varying from 45-80 wt% of polypropylene have been copolymerized to reduce the formation of the polyethylene crystallinity. EPDM is a fully saturated elastomer in the main chain with a small quantity of unsaturated diene in the side chain and classified as M-class rubber under ASTM standard D-1418. Therefore, EPDM has excellent weathering, ozone, oxygen and heat resistances. EPDM is also “good in” aqueous systems, polar media and a broad range of acidic and alkaline chemicals. EPDM is also used in outdoor applications at elevated temperatures. In early 1950, Ziegler-Natta catalysts based on transition metals, such as vanadium and titanium, were developed for the commercial production of polyethylene (PE) and isotactic polypropylene (PP) and soft and rubbery EPM copolymer was produced. Development extended to the random copolymerization of ethylene and propylene, yielding EPM copolymers with soft and rubbery properties. Several years later, the terpolymers based on ethylene, propylene and non-conjugated third monomer were introduced into the market as EPDM. Diene present in EPDM ranged from 3-10%. dicyclopentadiene (DCPD), 5-ethylidene-2- norbornene (ENB) or 1,4- hexadiene are three dienes used, but ENB is the most common EPDM diene. Figure 2.8 Chemical structure of EPDM Polymerization by Ziegler-Natta catalyst system yielded a broad molecular weight distribution (MWD) of EPDM, which sometimes affected the mechanical properties of rubber product produced. In 1960’s ExxonMobil introduced a patent of bimodal MWD that offered the balance of processability and mechanical properties of EPDM. Recently ExxonMobil developed new technology of polymerization using metallocene catalyst systems based on zirconium, titanium or hafnium which can produce narrow MWD and low density of EPDM with density of less than 0.91g/cc. DuPont/Dow Elastomer, a joint venture between DuPont and Dow Chemical, has also started to produce EPDM by a solution process using Dow’s Insite™ metallocene catalyst. 55

Figure 2.9 Different catalyst technologies to produce EPDM rubber [29] Some EPDM producers are still using vanadium catalyst Ziegler-Natta (ZN) catalyst technology which gives linear polymer with low branching from cation coupling, but has low monomer conversion and low production rate. Newer EPDM plants apply metallocene catalysts to lower the amount of catalyst while producing EPDM with high molecular weight, high efficiency production, narrow MWD and low gelation. The advantages of EPDM produced by metallocene catalysis over the traditional ZN catalysis are: 1. Higher catalyst activity resulting in the lower amounts of catalyst used. 2. Higher reaction-temperature operating window, resulting in less energy for the deep-cooling of monomer/solvent reactor feed, the recycling of unreacted monomers and the solvent stripping. Commercial EPDM rubbers have average MM ranging from 10 to 600 kg/mol. Liquid EPDMs have MM of 10 kg/mol and EPDMs extended with oil have MM up to 125 kg/mol. The demand of EPDMs with very high molar mass extended with oil to enable both production and mixing with maximum oil content of up to 100 phr oil is increasing as more filler incorporation is possible. 2.7.1 Effect of molecular weight of EPDM on processability In the molecular weight study, it was found that the average molecular weight of the EPDM rubber dominated the compound Mooney viscosity. Low molecular weight EPDM can give efficiency of incorporation and wetting with materials during the compound mixing process. Bimodal EPDM provides more efficient mixing and dispersion than single broad MMD polymers, because of the balance between shear force and wetting with materials while compound mixing, which can reduce the mixing time. In a branching study, it was observed that EPDM with a medium level of long chain branching made it easier to incorporate fillers and oil into the compound matrix than the EPDM with the higher level of long chain branching. 56

2.7.2 Effect of ethylene content in EPDM on processability In an ethylene content study, high ethylene content and highly crystalline EPDM rubber did not soften until the compound temperature rose above its melting temperature, resulting in the filler and oil not being easily incorporated and mixed. This affects the cycle time of the mixing process. However, highly crystalline EPDM produces rubber product with better physical properties than low ethylene content EPDM products. Also, the higher crystalline EPDM compound provides the better extrusion processability. 2.7.3 EPDM and its applications EPDM has excellent weathering, heat, oxidation and ozone resistances, as well as excellent electrical insulation, low compression set and low temperature resistance down to -40ºC. It can resist many polar fluids and hot water up to 200ºC. However, it is not resistant to hydrocarbon fuels, solvents and mineral or synthetic ester lubricant and also has very poor flame resistance. From its good properties of weathering resistance, EPDM is useful in many seal applications, in refrigerators, window seals and automotive sealing systems (solid and sponge seal parts), coolant hoses, grommets, transmission belts and gaskets. EPDM is also used as an engine oil additive for insulation foam in TPV and construction industry. Figure 2.10 EPDM application 2.7.4 How to select EPDM Nordel™ EPDM of Dow’s are produced by Dow’s proprietary advanced molecular catalyst (AMC) technology in solution process [30]. 57

Table 2.8 Product selection guide of Nordel EPDM [30] Product Grade Ethylene ENB content Mooney viscosity MWD characteristics content (%) (%) (ASTMD 1646) (DOW test method) NORDEL 3430 42 0.7 27 Narrow NORDEL 3460 55 1.8 40 Medium NORDEL 3720P 70 0.6 20 Board NORDEL 3722P 71 0.5 18 Medium NORDEL 3745P 70 0.5 45 Narrow NORDEL 3760P 67 2.2 63 Medium NORDEL 3765 XFL 67 2.5 65 Board NORDEL 4520 50 4.9 20 Medium NORDEL 4570 50 4.9 70 Medium NORDEL 4571 XFM 47 4.9 70 Board NORDEL 4640 55 4.9 40 Medium NORDEL 4725P 70 4.9 25 Board NORDEL 4760P 67 4.9 60 Medium NORDEL 4770P 70 4.9 70 Medium NORDEL 4771 XFL 71 4.9 70 Board NORDEL 4785HM 68 4.9 85 Medium NORDEL 4820P 85 4.9 20 Narrow NORDEL 5565 50 7.5 65 Medium NORDEL 6530 XFC 55 8.5 30 Board NORDEL 6555 OE 53 8.5 55 Medium NORDEL 6565 XFC 55 8.5 65 Board 58

Table 2.9 Typical properties and applications of Nordel EPDM [30] Product grade Density Crystallinity Tg (ºC) Form Application (g/cc) (mass, %) NORDEL™ 3430 0.86 <1 - Bale Oil and lubricant modification, peroxide- cured parts NORDEL™ 3640 0.86 4 -10 Bale Blends with butyl rubber in inner tubes, peroxide-cured belts, molded goods, conveyor belts NORDEL™ 3720P 0.88 14 43 Pallet Thermoplastic modification, electrical insulation, molded connectors, belts, rolls (peroxide-cured) NORDEL™ 3722P 0.88 15 46 Pallet Thermoplastic modification, electrical insulation, molded connectors, belts, NORDEL™ 3745P rolls (peroxide-cured) 0.88 12 34 Pallet Thermoplastic modification, cable bedding, sound insulation, molded foam, belts NORDEL™ 3760P 0.88 12 18 Pallet Roofing, belts NORDEL™ 3765 XFL 0.87 12 18 Pallet Coolant hoses, belts,membranes NORDEL™ 4520 0.86 <1 - Bale Molded seals, brake diaphragms, gaskets, sealants, weatherstrip corner molding NORDEL™ 4570 0.86 <1 - Bale Extrusions, automotive and general purpose hose, profile gaskets, weatherstripping NORDEL™ 4571 XFM 0.86 <1 - Bale Automotive extruded profiles, coolant hoses, building profiles, general 0.86 4 purpose moldings NORDEL™ 4640 -10 Bale Molded automotive and industrial parts, hose and tubing, weatherstripping, belts NORDEL™ 4725P 0.88 12 36 Pallet Rolls, high hardness compounds, gaskets, extruded profiles NORDEL™ 4760P 0.88 10 35 Pallet Extrusions, automotive and general purpose hose, profile gaskets, weatherstripping NORDEL™ 4770P 0.88 13 34 Pallet Automotive and general purpose hose, extruded profiles, glass run channel, NORDEL™ 4771 XFL 0.87 14 low voltage wire and cable jacketing, thermoplastic vulcanizates (TPV) 34 Pallet High filler loading automotive extruded 8 profiles, hoses (including radiator, industrial, garden, and appliance), TPV, low voltage wire and cable jacketing NORDEL™ 4785HM 0.88 29 Pallet Weatherstripping, extrusions, profiles, TPV NORDEL™ 4820P 0.91 28 79 Pallet Property modification of thermoplastic polyolefin and thermoset rubber formulations-high hardness, weather- stripping, molded goods NORDEL™ 5565 0.86 <1 - Bale Weatherstripping, extrusions, profiles, metal carriers NORDEL™ 6530 XFC 0.86 <1.5 - Pallet Extra fast cure molding, high hardness rubber parts NORDEL™ 6555 OE 0.86 <1.5 - Pallet Weatherstripping sponge profiles NORDEL™ 6565 XFC 0.86 <1.5 - Pallet Extra fast cure, weatherstripping, dense, micro-dense and sponge profiles 59

2.8) Acrylic Rubber (ACM) Acrylic rubber or polyacrylate rubber (ACM) is a copolymer or terpolymer of ethyl acrylate and other acrylates [31], with a small amount of active cross-linking comonomers. Polyacrylate elastomers are based on various monomers, such as ethyl acrylate (EA), butyl acrylate (BA), methoxyethyl acrylate (MEA) and ethoxy-ethyl acrylate (EEA). These monomers are coupled with active cross-linking comonomers, typically 2-chlorovinyl ether, vinyl chloroacetate, allyl glycidyl ether and acrylic acid. ACM can be produced by emulsion polymerization with radically initiated or suspension polymerization. The various copolymer modifications can improve the properties of acrylic rubbers. The copolymer modifications include other backbone monomers and the incorporation of reactive site groups (1-5%) for subsequent cross-linking [2]. Figure 2.11 Chemical structure of acrylic rubber [2, 31] Because ACM is a saturated rubber, it is impossible for it to be vulcanized through traditional sulphur vulcanizing systems like unsaturated rubbers. However, acrylic rubbers can be cross-linked by diamines, fatty acid soaps and peroxides. The selections of the cure-site monomers and the corresponding curing agents are a critical aspects in the influencing the characteristics of the acrylic rubbers [31]. Monomers containing active cure-sites (i.e., epoxy, chlorine, and carboxyl groups) have traditionally been the most favored for industrial applications. These with carboxylic and epoxy cure-sites are relatively safe, but chlorinated monomers cause serious problems of toxicity and corrosion issues [31]. The self-contained reactive sites are not activated until the high temperature of vulcanization, 170ºC or higher. In self-cross-linked elastomers the scorch and vulcanizing rate can be accelerated by the addition of an acidic material such as phthalic anhydride during the compounding. 60

Table 2.10 Species of cure-site monomers and their corresponding curing systems of acrylate rubbers Cure-site monomers Curing system Halogen groups - Soap-sulphur or peroxide Carboxyl groups - Trithiocyanuric acid and calcium hydroxide - Aliphatic polyamines Epoxide groups - Quaternary ammonium salts - Hexamethylenediamine carbamate and Unsaturated double bonds Carboxyl groups N,N’-diortholylguanidine coagent Chlorine groups - Quaternary ammonium salts - An isocyanuric and quaternary ammonium salts Carboxyl and epoxide groups - UV Aliphatic diamine carboxylate - Sulphur and peroxide - Quaternary ammonium salts - Hexamethylenediamine carbamate - Non-alkali metal oxy compound and quaternary ammonium salts, tertiary amine or guanidine - A combination of Sodium stearate and tetramethylthiuram disulphide - Quaternary ammonium salts - Guanidine compound and diamine compound Polyvalent amine 2.8.1 Characteristics of acrylic rubber The combination of a saturated backbone and polar side chain make acrylic rubber outstanding in resistance to high heat aging, environmental oxidation and hydraulic and mineral oils. Acrylic rubber also good resistance with ozone and weathering resistance than nitrile rubber. On the other hand, acrylic rubber has poor resistance to acids, alkalis, water and moisture. Although acrylic rubber can resist high temperature, but for low temperature applications are usually limited to approximately -14°C [31]. it will lose the flexibility and compression set. Ethylene acrylate rubber (AEM) is known under the trade name “VAMAC” from DuPont has characteristics like ACM, however a better rigidity, heat resistance, but worse mineral oil resistance. 2.8.2 Ethylene acrylic elastomers (AEM) Ethylene acrylate copolymers are synthetic elastomers composed of ethylene and methyl acrylate. The ethylene repeating units are outstanding in good low temperature properties, and the acrylic component helps to improve swelling resistance in non-polar oils. Due to the saturated backbone with polar side groups like ACM, AEM is also better than ACM in resistance to heat, ozone, weathering and many chemicals [32]. AEM is also outstanding in vibration damping and abrasion in a wide range of temperature. 61

Figure 2.12 Chemical structure of ethylene acrylate copolymer (AEM) [31] AEM rubber is mostly use in the application that required ozone, heat and some mineral oils at moderate temperatures below 150ºC [31, 33]. However, AEM rubbers should not be exposed to aromatic hydrocarbons, gasoline, brake fluids and phosphate esters. 2.8.3 Applications of acrylic rubber (ACM/AEM) Acrylic rubbers are commonly used in automotive transmission, power steering seals and O-rings that have to be resistant to transmission fluids and many other common automotive lubricants and hydraulic fluids. Other applications are diaphragms, plumbing seals, boots, hoses, vibration mounts, pads, isolators and custom molded rubber goods and parts. Previously, almost all applications required a service temperature range from -30ºC to 150ºC. Furthermore, some have the potential to reach 175ºC for a short period of time. Table 2.11 DuPont Vamac® grade [34] Grade ML 1+4 Tg by DSC Key feature (100ºC) (ºC) General purpose Vamac® G 16.5 -30 Dynamic fatigue resistance Low oil swell Vamac® GXF 17.5 -30 High viscosity Intermediate viscosity Vamac® GLS 18.5 -23 Improved performance grade for molding & extrusion Vamac® HVG 26.0 -30 High temperature High temperature / Oil resistance Vamac® Ultra XF 23.0 -30 High viscosity / Low oil swell Peroxide curable dipolymer Vamac® Ultra IP 29.0 -30 Improved processing peroxide curable dipolymer Low temperature Vamac® Ultra HT 29.0 -30 Vamac® Ultra HT-OR 31.0 -24 Vamac® Ultra LS 33.0 -23 Vamac® Ultra DP 22.0 -27 Vamac® Ultra DX 28.0 -29 VMX4017 11.0 -41 62

Table 2.12 DuPont Vamac® grade [34] Grade ML 1+4 Tg by DSC (ºC)1 Key feature (100ºC) VMX5015 67.0 -23 Compression molding pre-compound2 VMX5020 53.0 -30 Injection molding pre-compound2 1 Tg of compounds with Vamac® may be extended typically -10ºC lower with addition of plasticizer 2 Not suitable for steam autoclave cure. 63

2.9) Silicone Rubber Silicone rubber is an elastomer composed of silicon and oxygen atoms that covalently bonded in a molecular chain of inorganic siloxane (Figure 2.13) [35, 36]. It is resistance to ozone, UV, heat and chemicals which is very stable in extreme environments. It also has excellent dielectric strength (at high voltage) fire resistance and good mechanical properties at extreme temperatures [36, 37]. Therefore, it is a material selected by food, medical, electric and wire and cable industries which require good chemical and environmental resistant material and can retain the initial shape and mechanical strength under heavy thermal stress or sub-zero temperatures. It also finds its useful applications in high voltage line insulators; automotive wire harness applications; electronics; sealants for aviation applications. However, silicone rubber has low tensile strength, poor wear and tear wear properties [36, 38, 39]. Figure 2.13 Chemical structure of silicone rubber [35] 2.9.1 Structure Polysiloxanes structure in silicone rubber differs from other hydrocarbon polymers that in the back bones consist of Si-O-Si unites. Because of the bond energy of Si-O is much higher than C-C bond of hydrocarbon rubber [38], therefore, silicone rubber is more stable to environment and UV than the hydrocarbon rubbers. Polysiloxane is very flexible due to the large bond angles and bond length when compared to C-C back bone of other hydrocarbon polymers. Polymer segment of silicone rubber can move farther and change conformation easily. Polysiloxanes is stable and less chemical active and it is more stable than hydrocarbon rubber because of higher charge and mass of silicone with 14 protons and 14 neutrons and add layer of electrons which has screening effect changes the electronegativity (Carbon atom of hydrocarbon rubber has only 6 protons and 6 neutrons). 2.9.2 History of silicone rubber Swedish chemist, Jons Jackob Berzelius heated silicon in his laboratory with chlorine, after a vigorous blast, he found silicon tetrachloride which was one of raw material used to produce silicone today. Not further development was carried on significantly until the beginning of 1930 Dr. J. Franklin Hyde, a chemist at Dow Corning Corporation in Midland who worked on corning glass works, researched on element silicone research and developed into silicones. His work had been developed by Dr. Frederick Stanley Kipping who achieved the synthesis of silicone compounds [40]. One of the first uses of silicones was in toys. Silicone rubber is a very soft material that has viscoelastic properties that could bounce. In 1969, Neil Armstrong took his first step 64

on the moon. Rubber outsoles of his boots were made of silicone. Nowadays, silicones are widely used in automotive, aerospace, electrical industry and medical devices and molding [36]. 2.9.3 Curing The un-vulcanized silicone rubber must be cured, vulcanized or catalyzed in order to cross-link the polymer chains. This is normally used in a two-stage processes at the point of manufacturing into the desired product. Any one of the following curing systems is used to cure the un-vulcanized material. ‹ Peroxide cure system Peroxide curing is normally used for curing silicone rubber [38]. The curing process leaves behind by-products which usually need a post-curing process to reduce the peroxide breakdown products. Dicumyl peroxide is commonly used. ‹ Addition cure system This curing system is also known as the platinum-based cross-linking system. In this curing system, a hybrid- and a vinyl-functional siloxane polymer reacts with a platinum complex catalyst, forming an ethyl bridge between the functional groups. There is no by-product of the reaction. The reaction occurs quickly, but if sulphur element or any amine compounds are present, the rate of curing is prevented [37]. ‹ Condensation cure system Condensation curing systems can be one-part or two-part system [37]. In one-part or room-temperature vulcanizing (RTV) system. The crosslinker exposed to the ambient humidity cased a hydrolysis step. The silanol condenses further with another hydrolysable group on the polymer or crosslinker and reactions continue until the system is fully cured. In contrast to the platinum-based addition cure system, such a curing system will cure at room temperature and is not easily countered by contact with other ingredients. Two-part condensation system package, the crosslinker and condensation catalyst are in the first part while the polymer and other ingredients (i.e., filler and pigment) are in the second part. The curing process is started by combining two parts. Typically, sealants, thermal insulation ablative material and aerospace materials are among the applications for this type of curing system [38]. 2.9.4 Classified of silicone rubber Silicone rubber is classified into four groups by polymer type and performance characteristic, as shown below [41].  Polydimethyl siloxane elastomer (MQ) contains only methyl groups on the molecular chains [41]. 65

Figure 2.14 Chemical structure of molydimethyl siloxane elastomer [41]  Vinyl methyl siloxane (VMQ) is similar to polydimethylsiloxane (MQ), but it has methyl and vinyl groups which known as methyl vinyl silicone rubber [41]. Figure 2.15 Chemical structure of vinyl methyl siloxane elastomer [41]  Polymethyl/vinyl/phenol siloxane elastomers (PVMQ) contains methyl, phenyl and vinyl groups on the polymer chains [41]. It is excellent in low temperature performance [42]. Figure 2.16 Chemical structure of polymethyl/vinyl/phenol siloxane elastomer [41]  Poly-Ƴ-trifluoropropyl methyl/ vinyl methyl siloxane elastomer (FVMQ) contains fluoro, vinyl and methyl groups on the molecular chains [41]. This silicone rubber is extremely resistant to oil, chemicals and other fluids, with improved heat resistance [41]. Figure 2.17 Chemical structure of poly-Ƴ-trifluoropropyl methyl/ vinyl methyl siloxane elastomer [41] 66

2.9.5 Silicone material classification Silicone material rubber can be classified into three forms including, solid silicone rubber (HCR), liquid silicone rubber (LCR) and room temperature vulcanized (RTV) ‹ Solid silicone rubber (HCR) Solid silicone rubber (HCR) or high-temperature vulcanized (HTV) is a silicone rubber with high molecular weight and long chain. They are un-vulcanized rubber or raw rubber [38]. This silicone is suitable for compounding and molding processes. Because of the high viscosity of silicone rubber, it may be mixed and process in the same way as other rubber such as NR, EPDM and others elastomers. They are cure by peroxide or platinum catalyst and require post-cure to ventilate organic peroxide by-products in order to maximize and stabilize rubber properties. It can enhance physical properties of silicone rubber while also reducing the odor of small organic or acidic materials. ‹ Liquid silicone rubber (LSR) Liquid silicone rubber (LSR) has a lower molecular weight and shorter chains length than HTV. Because of its low molecular weight, it has superior flow properties and is suitable for specially low injection pressure and low pressure extrusion processes [36]. This material increase productivity by reducing cycle time, minimizing material waste and allowing for the use of smaller machines [43]. The vulcanization of LSR is exclusively carried out with a platinum-catalyzed hydrosilylation. ‹ Room temperature vulcanized (RTV) Room temperature vulcanized (RTV) is a kind of silicone rubber produced from one-part or two-component systems; condensation cross-linked materials and addition cross-linked polymers [36]. RTV is designed to be cured in a room temperature environment. RTV is also widely used for a wide range of applications because to its ability to flow to soft pastes, good thermal resistance, great adhesion and particularly curing without heating temperature [39]. As a results, it is useful for sealants, adhesives and protective coating with metal, plastic and wood for both indoor and outdoor [39]. 2.9.6 Peroxide vulcanization of silicone rubbers It is not all kinds of organic peroxide are suitable for curing silicone rubbers. The dialkyl peroxides such as dicumyl peroxide can cure silicone rubbers, which contain vinyl groups (VMQ). Saturated silicone rubbers (MQ) require diacyl peroxides such as bis-(2,4-dichlorobenzoyl) peroxide to be curing agent. In peroxide vulcanization, it can be divided in to two steps of vulcanization. First step occurs in molding process vary from about 100ºC to 180ºC, called preliminary vulcanization. The second step occurs at the post cure stage with high temperature around 180ºC in the ventilation oven. In this high temperature post cured stage, the acidic materials that came out behaved as a catalyze hydrolytic decomposition of the vulcanizate and form an additional cross-linked product [44]. 67

Figure 2.18 Peroxide vulcanization for silicone rubber [37] 2.9.7 Additives and fillers for silicone rubber In order to enhance its properties, silicone rubbers are normally produced in rubber compounds by adding various additives such as filler, flame retardant and pigment.  Reinforcing fillers such as silica and carbon black are used as reinforcing fillers in silicone compounding to improve mechanical and conductivity properties of silicone products [45, 46].  Heat stabilizer is mainly applied in silicone rubber to improve thermal resistance and also enhance mechanical properties for silicone rubber [47].  Flame retardant is used in low content to improve flame resistance of end product [48].  Pigment such as titanium dioxide and other organometallic compounds as pigments can be added into silicone rubber that give transparency for silicone products even needed in some application. For titanium dioxide, it forms aggregates easily and probably is aggregated in the silicone rubber in view of the high loadings that are possible [49]. 2.9.8 Silicone rubber applications Silicone rubber has been considered safe in the United States and Canada for applications in consumer cookware and medical products since the FDA approved it in 1979. However, the European Union has labelled chemicals D4, D5, and D6, used in the production of silicone rubbers, due to, they concern that some of chemicals can leach from silicone products. The majority of silicone rubber are beneficial in industry, both on their own and when combine with other elastomers or materials.  Silicone rubber is frequently used in applications for insulating tape, sealant, varnish, lubricants, keyboards, and housings due to its high purify or low toxicity.  Silicone rubber is widely applied in seals, tooling materials, spacesuit fabrics and gaskets for aerospace and aircraft parts, due to its wide temperature service range (-100ºC to 300ºC),  Silicone rubber is used in construction as adhesive, sealant, and coating due to its weathering resistance properties and ability to bond to metal.  Silicone rubber can be used for heat, oil, and fuel resistance in automotive applications in addition to using coatings and varnishes.  Silicone rubber can be used in medical parts as tubing, adhesives, and defoamers.  Food containers, utensils, toys, and even silicone rubber band bracelets may be produced from silicone rubber. 68

2.10) Fluoroelastomers (FKM) Fluoroelastomers (FKM) are remarkable elastomers that are used in harsh environments and when other elastomers would fail to resist heat and chemicals [50]. The original FKM were developed by the Du Pont company in 1957 [51]. Nowadays, FKM are produced by many companies, including Daikin (DAI-El), 3M (Dyneon), Solvey S.A. (TECHNOFLON) and some local manufacturers also exist in China and Russia [50, 51]. There are presently several applications for fluoroelastomers due to their crucial role in resolving critical issues in the aerospace, automotive, chemical and petroleum industries [51]. Other key benefits include excellent resistance to aging and ozone, very low gas permeability and being self-extinguishing material [52]. Additionally, the fluorine content has an impact on a variety of properties of fluoroelastomers, including fluid resistance and low temperature properties. 2.10.1 Types of FKMs Based on the monomer composition, fluorocarbon elastomers are classified in ASTM D 1418 as follows [53-55]:  Type 1: The copolymer of FKM is comprised of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), which is a popular kind of fluoroelastomer [54]. These copolymers are cured by bisphenol to give a strong overall performance and are used in general-purpose applications [54]. They contain around 66 percent fluorine by weight.  Type 2: These terpolymers of FKM composed of VDF, HFP and tetrafluoroethylene (TFE) have a higher fluorine content than other copolymers (typically between 68-69.5% weight percent fluorine). This improves heat resistance, and chemical resistance. On the other hand, it may have a negative impact on compression set and low temperature [55].  Type 3: A ternary copolymer containing, VDF, TFE and perfluoromethyl vinyl ether (PMVE) is one type of FKM that offers better low temperature flexibility. Fluorine content in type 3 FKM varies from 62-68 %wt [53, 56].  Type 4: The other kind of FKM terpolymer is made of VDF, propylene, and TFE. It contains fluorine approximately 67 weight percent content. In comparison to FKM, this kind of other FKM has improved low temperature performance, better electrical properties and steam resistance.  Type 5: This kind of FKM, composed of VDF, HFP, TFE, PMVE and ethylene, especially provides better low-temperature performance, low swelling in hydrocarbon and greater chemical resistance (i.e., base). 2.10.2 Chemical structures If FKMs are generated from hexafluoropropylene (HFP) and vinylidene fluoride (VF2), the polymers are referred to as dipolymer. Approximately 66%wt of fluorine is found in the dipolymer [57] 69

Figure 2.19 Vinylidene fluoride-co-hexafluoropropylene [58] If FKMs come from VF2, HFP and tetrafluoro-ethylene (TFE), the polymers are called terpolymers and will contain 68%wt fluorine content [57]. Figure 2.20 Tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride [58] Scientists developed a terpolymer consisting of VF2, HFP and perfluoroalkyl vinyl ether (PTFE) to enhance the performance of FKM at low temperarture. The fluorine content is 64%-67% providing good low temperature flexibility [54]. Figure 2.21 Vinylidene fluoride-tetrafluoroethylene-perfluoroalkyl vinyl ether [58] DuPont Dow Elastomers L.L.C produces Viton as advanced polymer architectures (APAs) that have much better performance in low temperature and chemical resistance. This APA polymer is used in oil and gas applications [59]. Figure 2.22 Fluorene content on solvent and low temperature resistance [55] 70

The performance of FKM rubber is affected by fluorine content, as shown in Figure 2.22. It can be seen that with increasing fluorine content, the low temperature resistance (ºC) tested using TR-10 method increased, and the volume swell in methanol for 40 hrs at 70ºC decreased [55]. 2.10.3 Curing of FKMs Amine/Diamine curing has been used for curing FKM since 1950. In the late of 1960, the bisphenol curing system was introduced. Bisphenol curing with accelerators can accelerate the cure rate of FKM. Phosphonium salt, hexamethylenediamine carbamate and N,N’-dicinnamylidene-1,6 hexanediamine are normally used as accelerators. Although diamine cure outperforms bisphenol cure in terms of heat aging and compression set, the bisphenol cured system outperforms diamine cure in terms of metal adhesion and faster curing rate [60]. Since 1950, peroxide with triallyl isocyanurate systems has been used in FKM cross-linked systems. Peroxide cure can improve physical properties, heat aging resistance and chemicals resistance. Additionally, peroxide-cured fluoroelastomers provide greater resistance to steam, acids, and other aqueous solvents while not requiring metal oxide activators as used in bisphenol cure systems [56, 60]. In cross-linking FKM, metal oxides such as Ca(OH)2, CaO, MgO, ZnO, and PbO are necessary to absorb traces of HF generated during the curing process [60]. 2.10.4 Post Cure For most rubbers peroxide cured, “post-cure” is necessary to achieve their optimal physical product properties. The post-cure is performed in an air-circulating oven for 2 to 24 hrs at 150-250°C, depending on the size and thickness of products [61]. Post-cure is a step to eliminate residual volatiles from inside the vulcanizates. Post curing will improve various properties such as tensile strength, and compression set but may provide low elongation at break. Post-cure is most critical for bisphenol and peroxide cures of FKMs to achieve the optimal properties. From figures 2.23(a) and 2.23(b), temperatures of post cure affect to compression set of moulded parts. It can be seen that Viton showed performance improvement of products post cured at temperatures 200ºC and 232ºC with the time of post curing. Higher post cure temperature and longer post cure time improve compression set [61]. 71

Figure 2.23 Compression set of bisphenol AF-cured and peroxide-cured formulations after post-curing at 200ºC (a) and 232ºC (b) for a period of 4-24 hrs [61] Figure 2.24 Tensile strength of bisphenol AF-cured and peroxide-cured formulations after post-curing at 200°C (392°F) for a period of 4-24 hrs [61] 72

Tensile strengths of bisphenol AF-cured and peroxide-cured formulations after post-cure at 200ºC are shown in Figure 2.24. It can be seen that tensile strength of products enhanced after post-curing. All products show higher %improvement in biphenyl cured than peroxide cured at higher temperature and longer post cured time [61]. 2.10.5 Application of FKMs  In automotive industry: Because of fuel resistance, low fluid leaks and high and low temperature resistance, FKMs have been used for many automotive parts for fuel and combustion systems such as fuel hose, seal, gasket and O-ring of the engine.  In chemical plants; FKMs have been used for gaskets and seals because of high resistance to petrochemicals, steam, acids, and bases.  In oil & gas drilling processes; fluids contain petroleum and chemicals so, FKMs have been selected for rubber products in the oil and gas industry.  In aerospace; Polymers that are used in aerospace must have high and low temperature flex resistance, FKMs and perfluoroelastomers are the choice. 2.10.6 Product reference from DuPont Viton® fluoroelastomers are classified as A, B, or F based on their fluid and chemical resistance. Fluid resistance varies with fluorine content in the polymer, which is decided by the types and relative contents of copolymerized monomers that compose the polymer [62]. (HFP). It  Viton A is a dipolymer made mfroomldivnign,yelixdternuesifolunoarniddes(oVlFu2t)ioanndcohaetxianfgl.uFourortphreorpmyloernee, is commonly used in injection this kind of FKM is used for a variety of applications such as O-rings, roll covers, gaskets, fuel hose and tubing, parts with complicated shapes and solution for coatings of tanks and fabrics [62-64]. (HFP),  Viton B is a terpolymer polymerized from vinylidene fnluoot rcidoen(tVaFin2),ahecxuarfilnugoraogperonpt.ylTehnies and tetrafluoroethylene (TFE) monomers and does kind of FKM needs to be cross-linked or cured by a diamine or bisphenol cured system to obtain the final rubber vulcanizate. The higher fluorine content of Viton gives it better fluid resistance than Viton A [62].  Viton F is also aandteterptroalfylumoerroetthhaytleinsep(oTFlyEm) leikreizVeidtofnroBm, buvtingyivlefsluthoeribdees(tVflFu2i)d, hexafluoropropylene (HFP), resistance of all the Viton types. So, Viton F is usually used for applications that require low fluid permeation [62].  Viton GB and Viton GBL are terpolymers that are polymerized from vinyl fluoride (pVeFr2o)x, ihdeexcauflrueotroopprroopvyidleenheig(hHeFrPr)e, sainsdtantecteratfoluaogrgoreetshsyilveeneau(tToFmEo).tiTvheelyubarriecadteinsgigonielsd, to use and acids [62]. steam  Viton GLT is a terpolymer that is designed to resist chemicals and high temperature while improving low temperature flexibility. GLT has a temperature resistance range 8ºC to 12ºC lower than that of general Viton grades [62].  Viton GFLT is similar to Viton GLT in that it is reported to increase the low temperature performance of Viton GF, can resist high temperature and is also resistant to a variety of chemicals [62]. 73

Table 2.13 Polymer fluorine content versus fluid resistance and low temperature flexibility [65] Properties Standard types AB F GLT-S GFLT-S ETP-S Nominal polymer fluorine content, %wt 66 68 70 64 67 67 Percent volume change in Fuel C, 4 3 2 5 2 4 168 hrs at 23ºC (73ºF)* Percent volume change in methanol, 90 50 5 90 5 5 168 hrs at 23ºC (73ºF)* Percent volume change in methyl ethyl >200 >200 >200 >200 >200 19 ketone, 168 hrs at 23ºC (73ºF)* Percent volume change in potassium (Sample too swollen and degraded to test) 14 hydroxide, 168 hrs at 23ºC (73ºF)* Low temperature flexibility, TR-10, ºC* -17 -13 -6 -30 -24 -12 *Nominal values, based on results typical of those obtained from testing a standard, 30 phr MT (N990) carbon black filled, 75 durometer vulcanizate. These are not intended to serve as specification. Table 2.14 Comparison of cure systems used in cross-linking Viton™ [65] Property, processing characteristic Diamine Type of cure system Peroxide Bisphenol Processing safety (scorch) P-F E E Fast cure rate P-F E E Mold release/ Mold fouling P G-E G-E Adhesion to metal inserts E G G Compression set resistance P E E Steam, water, acid resistance F G E Flex fatigue resistance G G G Rate, E= excellent, G=Good, F=Fair, P=Poor Table 2.15 Physical property differences among types/families of Viton products [65] Type of Viton Resistance to General Fluids/ Low temperature Fluoroelastomer compression set* Chemical resistance* flexibility** A 1 3 3 B, GBL-S 2 3 3 F, GF-S 3 2 3 GLT-S 2 3 1 GFLT-S 2 2 2 ETP-S 3 1 3 1=Excellent-Best performance capability of all types, 2= Very Good, 3= Good-Sufficient for all typical fluoroelastomer applications *See Table 4 for a detailed guide to choosing the best type of Viton™ fluoroelastomer relative to specific classes of fluids and chemicals. **Flexibility, as measured by temperature of retraction (TR-10), Gehman torsional modulus, glolawsstetmempepreartuartueresetraalinnsgitcioapna(bTigl)i,tyorofCalavsuhl-cBaneirzgattee.mperature, brittle-point tests are a measure of impact resistance only and do not correlate at all with the 74

Table 2.16 Differences in fluid resistance among types of Viton Fluoroelastomer [65] Type of Viton AB F GAL-S GBL-S GF-S GLT-S GFLT-S ETP-S Cure system Bisphenol Peroxide Hydrocarbon automotive, 1 1 1 1 1 1 1 1 1 aviation fuels NR 2 1 NR 2 1 NR 1 1 Oxygenated automotive fuels (containing MeOH, EtOH, MTBE, 2 1 1 1 1 1 1 1 1 etc.) 3 2 2 Reciprocating engine lubricating 1 1 1 1 1 1 1 1 1 oils (SE-SF grades) 2 2 1 Reciprocating engine lubricating 3 2 2 1 1 2 1 1 oils (SG-SH grades) Aliphatic hydrocarbon process 1 1 1 2 1 1 fluids, chemicals Aromatic hydrocarbon process 1 1 1 1 1 1 fluids, chemicals Aqueous fluid: water, steam, NR NR NR 3 3 3 3 3 1 mineral acid (H2SO4, HNO3, NR NR NR NR NR NR NR NR 1 HCl, etc.) Amines, high pH caustics (KOH, NaOH, etc.) Low molecular weight carbonyls (MTBE, MEK, MINK, etc.) l volume increase, change in physical properties. 2= Very Good-Good serviceability in this class of fluid/chemical, small amounts of volume increase and/or changes in physical properties. 3=Good-Suitable for use in this class of fluid/chemical, acceptable amounts of volume increase and/or changes in physical properties. NR= Not recommended-Excessive volume increase or change in physical properties 2.10.7 Processing of fluoroelastomers The essential principle for compounding fluoroelastomer is the same as for other elastomers. The grade of fluoroelastomer and other ingredients used are decided by the required properties of the final vulcanizate (or finished product) as well as by the rubber compound’s behaviour during rubber processing (i.e., mixing and curing). On the other hand, the slow relaxation rates of fluoroelastomers presents an issue for rubber processing steps including, mixing, extrusion, and injection processes which are generally run at high shear rates [54]. Furthermore, fluoroelastomer compounds cause issues during rubber processing such as sticking to mold surfaces, and of inadequate adhesion to metal inserts [54]. The relatively low production volume of fluoroelastomer parts production requires that equipment used for other high-volume elastomers be adapted to fluoroelastomer processing [54]. ‹ Mixing It has previously been noted that fluoroelastomer compounding is normally performed in small batch size mixing equipment because of high materials costs and limited production quantities. However, most mixing has been transferred from two roll mills to internal mixer as volume has increased and quality control has become more critical [54]. In general, 75

fluoroelastomers should be mixed on as cool as possible two roll mill (i.e., at 23ºC) [66]. Avoiding contamination of fluoroelastomer compounds is a critical issue for a rubber production unit that also handles other elastomers. Strict cleaning processes are necessary to ensure that elastomer, oil, grease, and other contaminants are removed from equipment before and after processing fluoroelastomers. ‹ Compounding ingredients Ingredients for compounding should be stored in sealed containers stored in cool, dry conditions. Metal oxides and hydroxides that may interact with moisture and carbon dioxide in ambient air should be kept carefully. Excessive moisture uptake by elastomer, filler, or other ingredients can lead to unpredictable curing and defects in fabricated parts such as porosity [54]. Some ingredients must be used in special forms to provide good dispersion and curing performance [54]. Curative uniform dispersion is especially challenging in rubber compounds cured with the bisphenol system. Bisphenol AF crosslinker and quaternary phosphonium salt accelerators are high-melting solids that must be micropulverized to fine particles for good dispersion in rubber compounds [51, 54]. Because many fabricators would have problems in attaining the uniform dispersion necessary for reproducible curing, polymer producers offer these curatives pre-mixed with fluoroelastomer in the form of pre-compounded grade, which gives the optimal combination of curative agents (i.e., accelerator and curing agents) [51]. For example, DuPont Dow supplies the VDF/HFP dipolymer Viton® E-60 as a gum polymer to be mixed with curing agent as well as Viton® E-60C as a pre-compound. Bisphenol AF (BPAF) and benzyl triphenyl phosphonium chloride (BTPPC) in the proper amounts are offered for optimized cure characteristic [54]. The concentration of curative VC-30, 50% BPAF in dipolymer, and VC-20, 33% BTPPC, are readily incorporated by fabricators in the amounts chosen for the optimized cure characteristic [54]. Similar curative concentrates are offered by other fluoroelastomer suppliers. DuPont Dow and Dyneon also offer pre-compounds that contain curative in the form of a mixture of BTPP+ BPAF- salt with additional BPAF (weight ratio BPAF/BTPP+ about four) [54]. The isolated mixture which is supplied by DuPont Dow as VC-50 is a low-melting glass that is readily dispersed. As is well known, providers of fluoroelastomer offer a variety of bisphenol curable in a pre-compound form, typically with processing aids, for different purposes. With the guidance of these materials, manufacturers may be sure that the compounding will process the desired cure characteristics and produce a vulcanizate with the finest potential properties [54]. 76

2.11) Thermoplastic Vulcanizate (TPV) Thermoplastic vulcanizate (TPV) is a kind of thermoplastic elastomer (TPE) which combines the characteristic of elastomeric behaviour and thermoplastic processability. It can be melt-processed and reshaped by using conventional thermoplastic processing equipment such as injection, extrusion and compression machines. Unlike the others in the TPE family, TPV consists of micro-size vulcanized rubber particles encapsulated in thermoplastic matrix [67]. That results in the TPV having elastomeric properties and appearance of traditional thermoset rubber. Ingredients of TPV are usually EPDM and PP. However, engineering TPVs of acrylic rubber and polyester resin have been developed to serve the requirements of high temperature and oil resistant applications. NR-TPV has also been developed to serve the purpose of green resin. TPV is normally produced by a reactive mixing process namely “Dynamic Vulcanization (DV)”. High porting of raw rubber is melt-mixed with thermoplastic under high shear condition and temperature in the presence of cross-linking agent. At the initial stage of mixing, both rubber and plastic phase are elongated in the flow field and preferably form a co-continuous phase. When the cross-linking reaction is activated, the viscosity of the rubber phase increased. The changes in rubber-plastic viscosity ratio and interfacial tension cause phase inversion. Finally, fully cured rubber is presented as a disperse phase in the thermoplastic matrix. On an industrial scale, dynamic vulcanization is done by a twin screw extruder as shown in Figure 2.25. That promotes high productivity, good temperature control and provides high shear rate and stress for breaking the cured elastomer phase. Twin screw extruders for TPV have been developed and patented for many years in various aspects such as screw configuration, L/D ratio, screw element as well as processing parameters and control system. Figure 2.25 Schematic process of TPV production by twin-screw extruder 77

In principle, cross-linking reaction of the elastomer phase could be achieved by adding several chemical reagents. Sulphur is the preferable curing agent in the rubber industry but it is not applied in commercial TPV because the weak S-S linkage leads to low thermal stability of TPV during processing and low weathering resistance of the finished product. Nowadays, the preferred curing systems are peroxide and resole-phenolic resin systems [68]. Peroxide cross-linked systems provide good elastic behaviour in particular compression set, and high temperature resistance caused by the strong C-C linkage. Moreover, there is no discoloration of the final products. The suitable peroxide should be selected on the basis of its decomposition rate at the processing temperature. Other criteria for the selection of the peroxide relate to the compatibility with polymer base and propensity to decompose into smelly by-products. Coagents are popularly added in peroxide cross-linked systems. Coagents are multifunctional monomers that are highly reactive toward free radicals to enhance properties of vulcanizates. Coagents increase cure rate and state of cure, consequently, thermal resistance and mechanical properties of TPV are improved. However, the peroxide systems in TPV production have low selectivity. In some cases, free radicals generated from peroxide decomposition lead to the abstraction of hydrogen atoms from the polymer chain and result in polymer radicals which can combine to form C-C linkages. Sometimes, H-abstraction occurs on the thermoplastic main chain (i.e., polypropylene) causing undesired side reactions such as β-chain scission and the breakdown of polypropylene. Resole-phenolic resins, the poly condensation products of phenols and aldehydes, are called “workhorse” for TPV. Resole-phenolic resin generally consists of reactive methyl-groups and dimethylene-ether units that can react with the unsaturated elastomer phase selectively at the processing temperatures and yields thermally-stable cross-links. The cross-linking mechanism of elastomer by resole-phenolic resin has been widely reported. During the cross-linking reaction, ether linkages are split by SnCl2 as an activator (other halogen containing reagents can also be used), providing mono-phenolic units having benzylic cations. After that, cationic intermediates add to the unsaturated bond and cross-linking elastomer chains occur. In the case of the absence of the unsaturated bond in thermoplastic, resole-phenolic does not react with thermoplastic. That is one of the benefits of the phenolic curing system over the peroxide system by avoiding the degradation of the thermoplastic segment by the chain scission reaction. That results in excellent properties and good processability of the corresponding TPV. TPV can be made from various rubber-plastic pairs depending on their compatibility and required specific property of final product. However, Ethylene-propylene-diene monomer rubber (EPDM) and polypropylene (PP) based TPV, are the popular ones in the TPV market today. EPDM/PP based TPV was intensively studied in the 1980s and was commercially introduced by Monsanto in 1981. Saturated main chains in EPDM rubber, as well as high crystallinity and melting point of PP offer good resistance to heat, oil, oxygen, and ozone. So, it can be used in various applications, especially in the automotive industries, because of its excellent weathering resistance, low density, and relatively low manufacturing cost compared to cross-linked rubber parts. 78

Figure 2.26 TPV finished products in the automotive industry High performance TPV can be formulated by using various high performance polymer blends. NBR/PP TPV, sometimes classified to be mid-engineering TPV, promotes good oil resistance. It is suitable for applications requiring enhanced oil resistance such as oil contact seals. Other high engineering TPVs are composed of acrylate rubber and engineering thermoplastic (i.e., polyamide, polyester, etc.) that provide the superior heat and oil resistant properties. They are applicable for the final product that is utilized in severe conditions such as sealing parts or hoses in engine rooms. In addition, “green” raw material such as natural rubber, bio-based elastomer or recycled plastic can be incorporated into the in TPV formulation. Therefore, TPVs have attracted considerable attention in recent years. They have become the fastest growing elastomers to replace unrecyclable petroleum-based thermoset rubbers because of the requirements of environmental protection and resource saving. 79

VRUUBLCBAERNIZATION OF

Vulcanization of Rubber 3.1) Vulcanization of Rubber Vulcanization refers to a group of processes used to hardening rubber. The terms vulcanization and curing are sometimes used interchangeably; they refer to the formation of cross-links between polymer chains, resulting in improved rigidity and durability, as well as changes in mechanical and physical properties. The word vulcanization is derived from Vulcan, the Roman God of fire and forge. After Columbus brought a rubber ball to Europe from his second voyage to the New World, natural rubber was brought from the forests of the Amazon and used to make containers waterproof. Little more had been done with it, partly because nurture rubber was somewhat unstable, becoming too hard in winter time and too soft under various conditions until the discovery of vulcanization by Charles Goodyear in 1839. Goodyear was working to improve tube tires and tried heating up rubber with some chemicals. One day in 1839, he mixed his rubber with sulphur and white lead but accidently dropped the mixture in his wife’s hot frying pan. To his astonishment, instead of melting further, the rubber became harder. He worked out a consistent system for this rubber hardening, which he called vulcanization, and obtained the patent in the same year. What Goodyear had discovered was the impact of sulphur atoms attacking and binding to the double bonds of carbon atoms in the isoprene. Sulphur atoms can also generate ‘disulphide’ bonds with one other, leading adjacent strands of isoprene to link together, thus causing the permanent state of cure known as ‘cross-linking’. This cross-linking produces a netlike structure that provide more stable elasticity than the purely electrostatic nature of the pre-vulcanization. As is generally known, once cross-linking occurs, vulcanized rubber cannot be easily broken down; hence term’ Thermoset’. Goodyear’s traditional sulphur curing system was just the beginning of what has evolve into a vast selection of curing systems, since when a vast range of synthetic rubbers along with many types of curatives, processing aids, fillers and chemical additives have been developed followed to serve end-product requirements of rubbers. 81

3.2) Sulphur Vulcanization Sulphur vulcanization is a chemical process that converts double bonded natural rubber or synthetic rubbers into materials with varying hardness, elasticity and mechanical durability by heating them with sulphur or sulphur-containing compounds. Sulphur forms cross-linking bridges between a variety of rubber molecules, affecting the polymer’s physical and mechanical properties. It is commonly accomplished by forming a cross-linked network in rubber molecules, as illustrated in Figure 3.1 [1]. Figure 3.1 Cross-links by sulphur vulcanization [1] In the sulphur vulcanization process, sulphur forms bridges between ‘cure sites’ of selective polymer chains; ‘cure sites’ refers here to the allyl groups in the rubber molecule (s-uClpHh=uCrHa-tCoHm2-s)(. dTih-seuslephbirdied)geosr may consist of one sulphur atom (mono-sulphide) or two many sulphide cross-links which form bridges between the chains [2] as stress bearing members contributing to elasticity and strength. In the case of forming cyclic sulphides, accelerator fragments and vicinal cross-links, these groups of sulphides do not contribute to elasticity of vulcanized rubber. Figure 3.2 Model of an “accelerated” NR-sulphur network [2] 82

Cross-links with more than two sulphur atoms are referred to as ‘Sx’ (polysulphidic cross-links). With the action of heat (at as low as 90-100ºC), ‘Sx’ is desulphurated to generate di or mono sulfidic or poly-sulphidic cross-links, depending on the amount of sulphur, accelerators used, temperature, pressure and time in the curing process, As shown in the following examples, the degree of cross-linking has a significant impact on both the physical and mechanical properties of rubber produced: • More cross-linking makes rubber harder • Number of sulphur atoms in the cross-linking chains affects the thermal stability, physical and mechanical properties of rubber produced. • Longer cross-links, with a high number of sulphur atoms, give the rubber improved elongation, but poor weathering resistance and compression set. • High temperature vulcanization with high amounts of sulphur can cause sulphur bloom • Polymer with high level of allyl or unsaturation exhibit higher level vulcanization rate. • In some exceptional cases, terminal vinyl unsaturation as in high-vinyl BR has a low reactivity toward sulphur vulcanization which hardly affects CR because the Cl attaches to the C=C unsaturated site. Halogenation of IIR to XIIR results in an enhanced reactivity towards sulphur vulcanization with BIIR being more reactive than CIIR. 3.2.1 Mechanism of sulphur vulcanization Elementary sulphur has a cyclic eight atom molecule at normal temperature with an average energy of S-S bond of 252 kJ.mol-1 [3]. Naturally, sulphur exists in two forms, soluble and insoluble sulphur. Soluble sulphur is in the rhombic state and can partially dissolve in polymer whereas insoluble sulphur is amorphous. Soluble sulphur can easily cause blooming because after it has dissolved in polymer, it does not form a cross-link reaction and comes out to the polymer surface later as blooming. The cross-link mechanism of sulphur starts from S8 opening its ring to form free radicals, which then react with hydrogen at the secondary or tertiary carbon atom of the polymer chains and form cross-links between two chains of polymers (Figure 3.3) [4]. 83

Figure 3.3 Cross-link mechanism of sulphur [4] Vulcanization of rubber by sulphur is a time-consuming and inefficient process. Sulphur and rubber hydrocarbon react chemically mostly at the C=C (double bonds), with each cross-link requiring 40 to 55 sulphur atoms (in the absence of an accelerator). The process takes approximately 5 hours to complete at 140ºC, which is inefficient by any manufacturing standards [1]. Furthermore, vulcanized rubber products are also susceptible to oxidative degradation and lack of the mechanical properties required for rubber products. Moreover, accelerators, which later became components of rubber compounding formulae and research subjects, were developed to overcome these limitations. Table 3.1 summarizes the classification of vulcanization systems as conventional (CV), semi-efficient (semi-EV), and efficient (EV) system depending on the level of sulphur and accelerator-to-sulphur ratio [1, 4]. For EV vulcanization, it contains more monosulphidic and a few poly- and di-sulphidic cross-links (Table 3.2) are preferred for better heat stability, lower compression set, longer resist reversion, thermal aging and over-cured [1, 4]. In comparison to the polysulphide predominant network (i.e., ~95% polysulphidic cross-links, Table 3.2), a high accelerator/sulphur ratio curing system is required to offer a longer cure time and a product with a high number of monosulphide cross-linking for reversion time. Furthermore, the sulphur concentration to accelerator moiety concentration ratio for semi-EV vulcanization is all in the intermediate (Table 3.1), which is the excellent compromise since they have a strong unaged fatigue life that is maintained after heat aging [1]. 84

Table 3.1 Sulphur vulcanization systems and types of cross-link [4, 5] Vulcanization system Sulfur Accelerator A/S ratio (phr) (phr) Conventional (CV) 2.0-3.5 1.2-0.4 0.1-0.6 Semi-EV 1.0-1.7 2.5-1.2 0.7-2.5 Efficient (EV) 0.4-0.8 5.0-2.0 2.5-12.0 1. The CV system performs poorly in terms of reversion, heat aging and long-term flex resistance. However, the products have high tensile and tear strength as well as fatigue and low temperature resistance. 2. The EV system generates thermally stable products with a network of mono- and disulphidic cross-links. The products exhibit low tensile and tear strength, flex-fatigue life and abrasion resistance because to the short sulphur cross-linking. However, EV systems provide excellent heat aging and low compression set. This curing procedure is utilized for rubber products with thick sections and those that require good static properties in use. 3. The semi-EV curing system is a compromise system between CV and EV cures and is used in curing NR which requires heat ageing and good fatigue life. Table 3.2 Effect of vulcanization system on technological properties [4] Cure system CV Semi EV EV system system system Poly- and di-sulphidic cross-links, % 95 50 20 Monosulphidic cross-links, % 5 50 80 Cyclic sulphides concentration High Medium Low Reversion resistance Low Medium High Heat aging resistance Low Medium High Flex-fatigue resistance High Medium Low Heat build-up High Medium Low Tear resistance High Medium Low Compression set High Medium Low Furthermore, rather of using a lot of accelerators to have EV and semi-EV system, it is frequently better to use a sulphur donor to replace part of the sulphur, such as tetramethylthiuram disulfide (TMTD) and 4,4’-dithiodimorpholine (DTDM), among other (Table 3.3) [4]. This type of curing system has been discovered to have good curing properties, thermal stability and fatigue resistance [6]. There is no elemental sulphur in this vulcanization system since the decomposition of sulphur-donor accelerator generates sulphur for cross-linking. The EV curing system contains sulphur-donor accelerators. At the optimum cure temperature of 143ºC or 183ºC, the sulphur donor cure system offers a stable vulcanization network with 80% mono- and di-sulphidic cross-links [7]. 85

Table 3.3 Sulphur-donor accelerator [7] Material Chemical structure Mol. Melting Active wt. point, ºC sulphur, % N,N-Captolactam 224 120 11.1 disulfide (CLD/DTDC) 2-morthilino-dithio 284 130 11.3 benzothiazole (MBSS) N-oxydiethylene 284 130 12.9 thiocarbamyl-N’-oxydiethylene sulfenamide (OTOS) 240 155 13.3 Tetramethylthiuran disulfide (TMTD) 4,4’-dithiomorpholin (DTDM) 236 135 13.6 Dipentamethylene thiuram tetrasulfide (DPTT) 384 130 16.6 3.2.2 Effect of accelerator in sulphur vulcanization Accelerators have been used since 1906. Before the introduction of accelerators, rubber cross-linking was done with aniline in sulphur vulcanization, as discovered by Oenglager [8]. This method is not used commercial using because of too toxic for use in rubber products. The first accelerators introduced to shorten curing time were thiocarbanilide and guanidine, but in 1919 cabondisulfide and aliphatic amines or dithiocarbamates were used [9]. However, this kind of accelerator made very short scorch time and generated problems to some rubber parts makers. Then, the curing retardant was introduced to extend the period for scorch safety and the molding process [9]. MBT and MBTS were used commercially in 1925 to delay the action of accelerators and were favored in deployment of cord-ply construction in automobile tires. However, efficient delay accelerator chemicals were introduced in 1968, when pre-vulcanization inhibitor (PVI) and N-cyclohexyl thiopthalimide (CTP) were used as vulcanization inhibitors [10]. Before the development of PVI and CTP, acidic retarders such as benzoic acid, acetylsalicylic acid, salicylic acid and phthalic anhydride were used [11]. 86

Sulphur vulcanization with accelerators is the most used method. Zinc oxide (ZnO) and stearic acid are activators for the vulcanization. Both chemicals can combine to generate a salt that forms complexes with accelerators and the reaction products. Table 3.4 summarizes a typical sulphur vulcanization system, which is listed in the table below [1]. Table 3.4 Typical sulphur vulcanization system [1] Ingredients phr Zinc oxide 2.0 – 10.0 Stearic acid 1.0 – 4.0 Accelerator 0.5 – 4.0 Sulphur 0.5 – 3.0 3.2.3 Mechanism of sulphur-accelerator vulcanization The chemistry of accelerated sulphur vulcanization is extremely complex because several chemical reactions proceed at the same time with various reaction speeds at the selected vulcanization temperature. Both radical and ionic process are involved, the consequent effect is significantly reliant on the compounded [4, 12] formulations indicated in Figure 3.4. 87

A. Redical mechanism: B. Ionic mechanism: Figure 3.4 Radical (A) and ionic (B) mechanisms of sulphur vulcanization [4, 12] (I) The creation of ‘Accelerator-polysulphide’ by reaction of ‘Accelerator-polysulphide’ is the first stage in sulphur vulcanization Accelerator + Zinc Oxide + Stearic acid + Sulphur 88

(II) The reactions can be divided into three kinds of complex substances A, B and C as shown below: (III) In the accelerator-polysulphide, Zn may also form a combination with sulphur, as seen below: All of these compounds can sulphurate rubber chains and are classified as active sulphurating agents [13]. The relation between compounded formulation and reaction type is shown in the Table 3.5. 89

Table 3.5 Type of reactions [14-17] Type of mechanism Cure system Radical NR + CBS + Sulphur NR + TMTD + Sulphur NR + TMTD NR + Sulphur Ionic NR + TMTD + Sulphur + ZnO + St. acid NR + TMTD + ZnO Mixed (Radical + Ionic) NR + CBS + Sulphur + ZnO + St. acid During the cross-link reaction, three competing reactions occur simultaneously during vulcanization, including [18]: y Cross-linking y Cross-linking desulphuration y Main chain modifications (i.e., dehydrogenation and cyclic sulphide formation) 3.2.4 Types of Accelerators Each chemical group can give different vulcanization speed, different cross-link density and scorch safety as shown in the Table 3.6. Table 3.6 Classification of accelerators their relative vulcanization rates [12] Accelerators Chemical group Vulcanization speed BA, HMT Aldehyde group Slow DPG, DOTG Guanidine Slow MBT, MBTS, ZMBT Thiazole Semi Ultra fast ZBDP Thiophosphate Ultra fast CBS, TBBS, MBS, DCBS Sulfenamides Fast-Delayed action ETU, DPTU, DBTU Thiourea Ultra fast TMTM, TMTD, DPTT, TBzTD Thiuram Ultra fast ZDMC, ZDEC, ZDBC, ZBEC Dithiocarbamate Ultra fast ZIX Xanthates Ultra fast 90

Figure 3.5 Characteristic of accelerators [17] 3.2.5 Cross-link density and vulcanizate properties The number of molecules of cross-linked units per unit weight of the cross-linked polymer is referred to cross-link density. The cross-linked level is calculated by dividing the number of molecules of cross-linked basic units by the total number of polymer basic units. The cross-link density of rubber vulcanizate has a significant impact on its properties [1]. Various properties such as static modulus, dynamic modulus and hardness increased as cross-link density increased. Furthermore, when cross-link density decrease, fracture properties such as tensile strength and tear strength peak before declining. It should be noted that the attributes shown in Figure 3.6 are influenced not only by the cross-link level, but also affected by the types of polymer, cross-link and filler loading [1]. 91

Figure 3.6 Vulcanizate properties as a function of the extent of vulcanization [11] The tensile strength of the vulcanizate is related to the average molecular weight of the polymer between two adjacent cross-links (Mc). When un-vulcanized rubber is stressed, the rubber molecules detangling (slip). The fracture occurs at low stress by viscous flow at a lower rate without breaking any chemical bonds. A few cross-links increases the molecular weight of the rubber molecules, resulting in a branched molecule with a broader molecular weight distribution (MWD). Detangling the branched chains becomes more difficult as a result, enhancing the tensile strength of the vulcanizate. Rubbers have an optimal cross-link density range for their practical application. The cross-link level must be high enough to prevent viscous flow fracture while remaining low enough to avoid brittleness. As a result, the degree and type of cross-links are the most important factor in achieving the desired vulcanizate properties. Several factors influence the type of cross-link formed, including sulphur level, accelerator type, accelerator-ratio-sulphur and cure time. In general, a higher accelerator/sulphur ratio and a longer cure time promote the formation of monosulphide cross-link formation at the expense of polysulphide cross-links. Because C-S are more stable than S-S bonds, rubber vulcanizates with polysulphide cross-link provide better heat stability, lower compression set and longer reversion time than polysulphide predominant networks. In addition, the rubber vulcanizates containing higher proportions of polysulphide cross-links most provide enhanced tensile and tear strength as well as flex-fatigue resistance, due to the possibility for S-S bonds to break reversibly and so locally release high stresses that could initiate failure. 92

3.2.6 To improve the storage and curing scorch technique Sulfenamide accelerator can be used to delay scorch time of natural rubber cross-links. The three major aspects which decide the activity of a sulfenamide are: - Amine group in sulfenamide with steric hindrance structure delays the scorch time. Basicity of steric hindering of the amine group offers scorch delay and slower cure rates. Amine group, the shorter the scorch time and the faster is the cure rate. - The bond strength of S-N in sulfenamide delays the scorch time. - The presence of MBT, as shown in Figure 3.7 [17], is first generated by the thermal decomposition of sulfenamide (CBS) accelerator [19]. The formed MBT is immediately converted to MBTS. Furthermore, the reaction of the remaining CBS with MBT to create MBTS was observed during the induction period [19]. These reactions give the scorch delay generated by sulfenamide accelerator [20]. Figure 3.7 MBTS accelerated sulphur vulcanization [1, 17] 93

In the absence of PVI, sulfenamide accelerator decomposes into MBT and amine, with the MBT releasing the sulfenamide accelerator via autocatalytic decomposition. Sulphur cross-linking does not occur as long as the sulfenamide accelerator level is not decreased. After forming active sulphurating agent, sulphurated rubber and pendant groups, MBT is converted to MBTS for cross-linking. Sulphur vulcanization can increase the efficiency of adhesion (adhesive strength) between rubber and copper. In tire production, adhesive strength between steel cord and rubber is necessary when manufacturing high performance steel-cord tires. With sulfenamide accelerator, a thin film of copper sulfide is formed and promotes good adhesion and cohesion between copper and rubber. Cohesive strength occurs within the sulfide film and a layer below the sulfide film but the sulfide film must completely form before cross-linking starts. Retarder CTP or DCBS is added to delay the action of the accelerator and cross-linking. Furthermore, benzothiazoles and sulfenamides are commonly applied in the process of copper adhesion. 3.3) Organic Peroxides Vulcanization Organic peroxides can vulcanize most elastomers that contain saturated and unsaturated bonds. They were first used in 1915 by Ostromyslenskij using dibenzoyl peroxide with NR [21]. Nowadays, they are used as vulcanization agents of rubbers if temperature resistance is required, or to vulcanize rubber compounds consisting of a combination of saturated and unsaturated rubbers. The peroxide that is most suitable for cross-linking rubber is the peroxide group that is fixed to a tertiary carbon. Peroxides with the peroxide group fixed to primary or secondary carbon are less stable. Example; since dibenzoyl peroxide has carboxyl groups in the molecule, the rate of its decomposition is increased by the oxygen. This kind of peroxide it is not suitable for rubber compounding because its decomposition temperature is 130ºC, which is too low for normal rubber processing. ‹ Classification of organic peroxides (I) Organic peroxides with carboxylic groups (II) Organic peroxides without carboxylic groups 94

(III) Organic peroxides with mixed structure (IV) Organic peroxides polymeric Vulcanization by peroxide is normally done at temperature ranging of 140ºC to 180ºC. Compared with sulphur curing, rubber products cured by organic peroxide have good resistance to temperature and good electrical properties. But these finished products have some weak physical and mechanical properties i.e., less elastic and worse in dynamic properties, tensile strength, structural resistance and resistance to wear. Dialkyl peroxide with t-butyl perbenzoate gives good performance rubber products, but di-t-butyl peroxide and dicumyl peroxide generate volatile acetophenone with its strong odor during the process. 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclo-hexane and 2,5-dimethyl- 2-5-bis(t-butyl-peroxy) hexane are suggested to be used to avoid an unpleasant odor during the process. 3.3.1 Peroxide vulcanization mechanism The oxygen-oxygen bonds in the chemical structure of an organic peroxide can be easily broken by heat to generate two free radicals (*). These radicals are unstable and very reactive. They react with the weak carbon-hydrogen bond of the polymer to transfer radicals to the hydrogen chain, forming hydrocarbon radicals which are very active [22]. It is dehydrogenation reaction in which two active hydrocarbon radicals share free radical electrons to form a cross-linking chain. In general, the first place to be affected in the rubber molecule is the alpha-methylene carbon atom. Since the covalent bond of carbon-carbon (350 kJ) is stronger than carbon-sulphur (270 kJ) bond, the stronger C-C bond requires higher energy to break the bond. Therefore, peroxide vulcanized rubbers provide better heat resistance and better compression set than rubber products that are vulcanized by sulphur curing systems [3]. The general scheme for organic peroxide cure of elastomer is shown in Figure 3.8 [23]. 95

Figure 3.8 Peroxide cross-linking of elastomer [23, 24] In general, the first place to be affected in rubber molecule is alpha-methylene carbon atom. Example: Peroxide vulcanization systems are relatively more expensive than the sulphur curing systems and have some limitations. Most of the antioxidants used in rubber compounds retard or decrease the vulcanization performance of the peroxide cross-links reaction. Acidic compounds that contain fatty acid, carbon black or silica can catalyze and retard the radical generation in the peroxide curing process. They slow down the half-time of decomposition of peroxide at the vulcanization temperature, which decreases the cross-link density. It should be noted that, peroxide cannot be used for cross-linking butyl rubber, because the tertiary carbon of butyl rubber will generate more chain scission than the cross-linking by peroxide. 96

Table 3.7 Examples of some commercial peroxides Peroxide trade name Chemical name 10 hr. Processing Typical cure Cross-linking half-life temp. (ºC) temp. (ºC) efficiency (%) Luperox 103 2,5-Dimethyl-2, temp. (ºC) Trigonox 145 5-di(t-butylperoxy) 150 195 30 Perhexyne 25B hexyne-3 128 Luperrox 101 2,5-Di-methyl-2, 130 185 41 Perhexa 25B 5-di(t-butylperoxy) 119 Varox DBPH hexane Luperox F 1,3 Bis-(t-butylp- 117 130 180 52 Perkadox 14 eroxy-iso-propyl) VulCup benzene Perkadox BC Di-cumylperoxide 115 120 170 50 DiCup 110 150 21 Varox DCP 1,1-Di-t-butyl- 92 Luperox 231-XL peroxy-3,3,5- Triganox 29 trimethylcyclohexane Perkadox 3M Note: The temperature required to decompose half of a peroxide sample in ten hours. Table 3.8 Advantages and disadvantages of peroxide cure over sulphur cure [3] Advantage Disadvantage • Simple formulation, long term compound storage • Sensitivity to oxygen during cure. stability and possibility of using higher processing • Process oils, antioxidants, resins, acidic clays temperatures. and other acidic materials used in compounding • Rapid cure at high temperature and yet no reversion. can affect peroxide cured products significantly. • Low compression and permanent set, higher • Properties of vulcanized products such as temperature resistance and no extractable constituents. tensile strength, tear strength, flex-fatigue • Non-staining, non-blooming and non-discoloring. resistance, abrasion, etc. are significantly • Co-vulcanization of saturated and unsaturated rubbers. affected. • Selecting suitable co-agents. Properties of rubber • Unpleasant odor occurs during process as well products can improve, such as tensile strength, tear as in the cured products in some cases. strength, flex- fatigue resistance, and abrasion. • Longer cure time and need higher curing temperatures. Post cures are necessary in most cases. • Higher cost of peroxides and vulcanization process. *The temperature required to decompose one half of a peroxide sample in ten hours. 97

3.3.2 Co-agents in peroxide vulcanization Because of drawbacks in physical and mechanical properties from organic peroxide curing, peroxide curing with coagents has been developed to improve those properties. Efficiency of peroxide cross-linking increases by using co-agents such as (meth)acrylates and polyolefin with bi- or multi-functional double bonds like allylic and vinyl or derivatives of maleic acid. These products increase the cross-link density and improve physical and mechanical properties of the final products. These co-agents also act as the plasticizer and improve processing. Co-agents are multi-functional organic molecules which are highly reactive towards free radicals. They are used as reactive additives to boost peroxide vulcanization efficiency. The most commonly used co-agents are organic chemicals with (meth)acrylate groups, maleimide groups or allylic groups and polymeric materials with a high vinyl content (i.e., 1,2-polybutadiene). In the absence of co-agents, the efficiency of peroxide curing system is rather low due to the presence of side reactions that consume radicals. Co-agents can significantly increase peroxide efficiency by preventing inefficient side reactions such as cleavage (or chain scission) and disproportionation. Co-agents will form coagent bridges or bonds between polymer chains as extra cross-links. Those are the reasons why coagents increase cross-linking efficiency and improve physical and mechanical properties of final rubber products. According to their contribution to the vulcanization process, co-agents can be classified into two types: type I and type II [25, 26]. (I) Type I: addition and hydrogen abstraction Type I co-agents are polar molecules with a low molecular weight. Their main characteristic is that they are extremely reactive towards radicals, therefore scorch occurs suddenly. The benefit of using these co-agents is that they not only increase the rate of cure, but also increases cross-link density [27]. Type I co-agents generally have reactive free radicals which increase both the rate of cure and the state of cure of the system. These co-agents are mostly low molecular weight polar molecules which are capable of homo-polymerization as well as grafting i.e., acrylate and methacrylate ester. N,N’-m-phenylenedimaleimide can also react with in-chain unsaturation through an “ene” reaction mechanism. Type I co-agents include methacrylates, acrylates, bismaleimides and zinc salts [3]. 98