ELASTOMERS 4.39 The contours for flex fatigue life are complex. The test is run such that the specimens are about equally strained; however, there is some question as to whether the tests should be run at equal strain or at equal strain energy. For some cases, where strain is restricted by fabric reinforcement, fatigue test data should be compared at equal strain amplitude. For other applications, where the strain is not limited, the tests should be run at equal strain en- ergy. The contours as presented here can be interpreted in terms of either constant strain or constant strain energy. All points on the chart can be compared at an approximately equal strain per cycle; however, if we interpolate between the flex-life contours but only along a constant modulus contour, we can extract values corresponding to approximately equal strain energy per cycle. By choosing higher modulus contours, we are considering higher strain energies. The low values for fatigue life at low levels of sulfur, but high levels of accelerator, have been attributed to high concentrations of accelerator-terminated appended groups and high concentrations of monosulfidic cross-links. Monosulfidic cross-links are not able to exchange, rearrange, or break to relieve stresses without the breakage of main chains. On the other hand, polysulfidic cross-links are able to rearrange under stress. The rear- rangement of a cross-link occurs in two steps: (1) breaking and (2) reforming. Recent data indicate that only the breaking of the weak polysulfide cross-links is required for the strengthening of the vulcanizate network. It is better to relieve the stress by the breaking of a cross-link than by the breaking of a polymer chain. When even higher concentrations of sulfur are used (with the maintenance of constant modulus), flex life decreases. It is possible that this is due to the large amount of cyclic chain modification associated with high levels of sulfur. As always, there are compro- mises. 4.5.1.7 Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers. Over the years, much of the research on accelerated-sulfur vulcanization was done by using natural FIGURE 4.24 Cross-link types and chain modifications. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS 4.40 CHAPTER 4 rubber as a model substrate. Natural rubber was the first elastomer and, therefore, the search for understanding of vulcanization originated with work on natural rubber. Most of what we have discussed so far vis à vis vulcanization has been related to natural rubber. The chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears to have much in common with the vulcanization of natural rubber. Before the formation of cross-links, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-S x -Ac) to give macromolecular, polysulfidic intermediates (rubber-S x -Ac), which then form cross- links (rubber-S x -rubber). As in the case of natural rubber, the average length of a cross- link (its sulfidic rank, the value of x in the cross-link, rubber-S x -rubber) increases with the ratio of sulfur concentration to accelerator concentration (S/Ac) used in the compounded rubber mix. However, in the case of BR or SBR, the cross-link sulfidic rank is not nearly as sensitive to S/Ac as it is in the case of natural rubber. Model compound studies of the vulcanization of EPDM (e.g., wherein ethylidenenorbornane was used as a model for EPDM) indicate that the polysulfidic rank of the EPDM cross-links probably responds to changes in S/Ac in a natural rubber-like fashion. Reversion (when defined as the loss of cross-links during nonoxidative thermal vulca- nizate aging) is a problem associated mainly with natural rubber or synthetic isoprene polymers. It can occur only under severe conditions in butadiene rubber; in SBR, instead of the softening associated with the nonoxidative aging of natural rubber, one can observe FIGURE 4.25 Vulcanizate properties, ______ 300 percent modulus (MPa); ______, De Mattia flex fatigue life (kHz × 10 –1 ); -O-O-O-O-, percent retention of ultimate elon- gation after two days at 100°C. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS ELASTOMERS 4.41 hardening (the so-called marching modulus) during extensive overcure. In natural rubber and synthetic isoprene-polymer rubbers, the cross-links tend to be more polysulfidic than in the case of BR or SBR. The highly polysulfidic cross-links are more heat-labile than their lower rank cousins in BR and SBR; they are more likely to break and then form cy- clic chain modifications. The effect of zinc is much greater in the vulcanization of isoprene rubbers than it is in the vulcanization of BR and SBR. Again, the reason for the difference is not known, but a strong speculation is that this difference is also related to the presence of methyl groups only in the case of the isoprene rubbers. Curing-System Recipes for Accelerated-Sulfur Vulcanization. Recipes for only the curing-system part of formulations are given in Table 4.6. 4.5.1.8 Vulcanization by Phenolic Curatives, Benzoquinone Derivatives or Bismaleim- ides. Diene rubbers such as natural rubber, SBR, and BR can be vulcanized by the action of phenolic compounds, which are (usually di-substituted by -CH2-X groups where X is an -OH group or a halogen atom substituent. A high-diene rubber can also be vulcanized by the action of a dinitrosobenzene which forms in situ by the oxidation of a quino- nedioxime, which had been incorporated into the rubber along with the oxidizing agent, lead peroxide. The attack upon rubber molecules by the vulcanization system can be visualized in a way similar to that which was postulated for the sulfurization of the rubber molecules by TABLE 4.6 Recipes for Accelerated Sulfur Vulcanization Systems * *.Concentrations in phr. Nitrile (NBR) NR SBR 1 2 Butyl (IIR) EPDM Zinc oxide 5.00 5.00 3.00 2.00 3.00 5.00 Stearic acid 2.00 2.00 0.50 0.50 2.00 1.00 Sulfur 2.50 1.80 0.50 0.25 2.00 1.50 DTDM † †.DTDM, 4.4´-dithiodimorpholine; TBBS, N-t-butylbenzothiazole-2-sulfenamide; MBTS, 2,2´- dithiobisbenzothiazole (2-benzothiazole disulfide); MBT, 2-mercaptobenzothiazole; TMTD tetrame- thylthiuram disulfide. Note: conditions change depending on other aspects of the compositions. – – – 1.00 – – TBBS † 0.60 1.20 – – – – MBTS † – – 2.00 – 0.50 – MBT † – – – – – 0.50 TMTD † – – 1.00 1.00 1.00 1.50 Vulcanization temp., °C 148 153 140 140 153 160 Time, minutes 25 30 60 60 20 20 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS 4.42 CHAPTER 4 the action of accelerated-sulfur vulcanization systems. Reaction schemes for these two types of vulcanization can be written as shown in Schemes 10 and 11. As shown, the chemical structural requirements for these types of vulcanization are that the elastomer molecules contain allylic hydrogen atoms. The attacking species from the vulcanization system must contain sites for proton acceptance and electron acceptance in proper steric relationship. This will then permit the rearrangement shown in Scheme 12, where A is the proton acceptor site and B is the electron acceptor site. This is an explanation for the fact that this type of vulcanization is not enabled by dou- ble bonds per se, without allylic hydrogens in the elastomer molecules. (It should be pointed out that the phenolic curative can also act by a slightly different mechanism to give cross-links that contain chromane structural moieties, the allylic hydrogens still being required.) SCHEME 10 Vulcanization by phenolic curatives. SCHEME 11 Vulcanization by benzoquino- nedioxime. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS ELASTOMERS 4.43 Another vulcanizing agent for high-diene rubbers is m-phenylenebismaleimide. A cat- alytic free-radical source such as dicumyl peroxide or benzothiazyl disulfide (MBTS) is usually used to initiate a free-radical reaction. Although a free-radical source is frequently used with a maleimide vulcanizing agent, at high enough vulcanization temperatures, the maleimides react with the rubber without the need for a free-radical source. This could oc- cur as shown in Scheme 13. This is similar to the reaction written for the attack of rubber molecules by phenolic cur- atives or the in situ formed nitroso derivative of the quinoid (e.g., benzoquinonedioxime) vulcanization system. It is also closely related to the sulfurization scheme written for accel- erated-sulfur vulcanization. Comparisons between accelerated sulfur, phenolic, quinoid, and maleimide vulcanization can then be visualized as shown in Scheme 14. Selected recipes for vulcanization by phenolic curatives, benzoquinone-dioxime, or m- phenylenebismaleimide are given by Table 4.7. Vulcanizates based on these types of cura- tives are particularly useful in cases where thermal stability is required. 4.5.1.9 Vulcanization by the Action of Metal Oxides. Chlorobutadiene, i.e., chloro- prene rubbers (CR), also called neoprene rubbers, are generally vulcanized by the action of metal oxides. CR can be represented by the following structure: SCHEME 12 SCHEME 13 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS 4.44 CHAPTER 4 TABLE 4.7 Recipes for Vulcanization by Phenolic Curatives, Quinone Derivatives, or Maleimides * IIR SBR 1 2 1 2 NBR Zinc oxide 5.00 5.00 – – – Lead peroxide (Pb 3 O 4 ) – 10.00 – – – Stearic acid 1.00 – – – – Phenolic curative (SP-1056) † 12.00 – – – – Benzoquinonedioxime (GMF) – 2.00 – – – m-Phenylenebismaleamide (HVA-2) ‡ – – 0.85 0.85 3.00 2-Benzothiazyl disulfide (MBTS) – – 2.00 – – Dicumyl peroxide – – – 0.30 0.30 Vulcanization condition ** Temperature, °C 180 153 153 153 153 Time, min 30 20 25 25 30 *.Concentrations in phr. †.Schenectady Chemicals. ‡.Du Pont. **.Conditions change depending on other aspects of the compositions. SCHEME 14 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS ELASTOMERS 4.45 Zinc oxide is the usual cross-linking agent. It is used along with magnesium oxide. The magnesium oxide is used for scorch resistance. The reaction is thought to involve the al- lylic chlorine atom, which is the result of the small amount of 1,2-polymerization: A mechanism that has been written for the vulcanization of CR by the action of zinc oxide and magnesium oxide is shown in Scheme 15. Most accelerators for accelerated-sulfur vulcanization do not work for the metal oxide vulcanization of neoprene rubbers. An exception to this is in the use of the so-called mixed curing system for CR, in which metal oxide vulcanization is combined with accelerated- sulfur vulcanization. In this case, along with the metal oxides, accelerators such as tetram- ethylthiuram disulfide(TMTD) or N,N´-di-o-tolylguanidine (DOTG) are used with sulfur. This may be desirable for high resilience or for good dimensional stability. The accelerator that has been most widely used with metal oxide cures is ethylenethio- urea (ETU), N,N´-diphenylthiourea or 2-mercaptoimidazoline. The use of ETU in the vul- SCHEME 15 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS 4.46 CHAPTER 4 canization of CR is somewhat in doubt since it is a suspected carcinogen. A mechanism for ETU acceleration is shown in Scheme 16. Examples of recipes for metal oxide vulcanization are given in Table 4.8. It should be noted that, in one case, calcium stearate was used instead of magnesium oxide to obtain better aging characteristics. 4.5.1.10 Vulcanization by the Action of Organic Peroxides. Peroxides are vulcanizing agents for elastomers, which contain no sites for attack by other types of vulcanizing agents. They are useful for ethylene-propylene rubber (EPR), ethylene-vinylacetate copol- ymers (EAM), certain millable urethane rubbers, and silicone rubbers. They are not gener- ally useful for vulcanizing butyl rubber, poly(isobutylene-co-isoprene). Elastomers derived from isoprene and butadiene are readily cross-linked by peroxides, but many of SCHEME 16 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS ELASTOMERS 4.47 the vulcanizate properties are inferior to those of accelerated-sulfur vulcanizates. How- ever, peroxide vulcanizates of these diene rubbers may be desirable in applications where improved thermal ageing and compression set resistance are required. Peroxide Vulcanization of Unsaturated Hydrocarbon Elastomers. The initiation step in peroxide-induced vulcanization is the decomposition of the peroxide to give free radicals. If the elastomer is derived from butadiene or isoprene, the next step is either the abstraction of a hydrogen atom from an allylic position on the polymer molecule or the ad- dition of the peroxide-derived radical to a double bond of the polymer molecule. In either case, polymeric free radicals are the result (Scheme 17). For isoprene rubber, the abstraction route predominates over radical addition. Two polymeric free radicals then unite to give a cross-link. Cross-links could also form by a chain reaction that involves the addition of polymeric free radicals to double bonds. In this case, cross-linking occurs without the loss of a free radical, so that the process can be repeated until termination by radical coupling. Coupling can be between two TABLE 4.8 Vulcanization Systems for Chloroprene Rubber * ZnO 5.00 5.00 5.00 MgO 4.00 – 4.00 Calcium stearate – 5.50 – Stearic acid – – 1.00 TMTM – – 1.00 DOTG – – 1.00 ETU 0.5 0.5 – Sulfur – – 1.0 Vulcanization † Temperature, °C 153 153 153 Time, min 15 15 15 *.Concentrations in parts by weight per 100 parts of neoprene W. †.Conditions change depending on other aspects of the compositions. SCHEME 17 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS 4.48 CHAPTER 4 polymeric radicals to form a cross-link or by unproductive processes. A polymeric radi- cal can unite with a radical derived from the peroxide. Also, if a polymeric radical de- composes to give a vinyl group and a new polymeric radical, a scission of the polymer chain is the result. Few monomeric radicals are lost by coupling with polymeric radicals when dialkyl peroxides are used as the curative. Also, if the elastomer is properly chosen, the scission reaction is not excessive. For dicumyl peroxide in natural rubber, the cross-linking effi- ciency has been estimated at about 1.0. One “mole” of cross-links is formed for each mole of peroxide; cross-linking is mainly by the coupling of two polymeric radicals. One perox- ide moiety gives two monomeric free radicals that react with rubber to give two polymeric radicals, which couple to form one cross-link. In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if all antioxidant materials are removed. A chain reaction is indicated here. One might expect that nitrile rubber would also be vulcanized with efficiencies greater than 1.0; however, though the double bonds in nitrile rubber are highly accessible, the cross-linking effi- ciency is somewhat less than 1.0. Peroxide Vulcanization of Saturated Hydrocarbon Elastomers. Saturated hydro- carbon polymers are also cross-linked by the action of organic peroxides, though the effi- ciency is reduced by branching. Polyethylene is cross-linked by dicumyl peroxide at an efficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubber cannot be cured at all. For polyethylene, the reaction scheme is similar to that of the unsat- urated elastomers. However, branched polymers undergo other reactions. Though the per- oxide is depleted, no cross-links may be formed between polymer chains, and the average molecular weight of the polymer can even been reduced by scission. Sulfur or the so- called coagents can be used to suppress scission. Examples of coagents are m-phenyleneb- ismaleimide, high-1,2 (high-vinyl) polybutadiene, triallyl cyanurate, diallyl phthalate, eth- ylene diacrylate, and others. Peroxide Vulcanization of Silicone Rubbers. Silicone rubbers (high-molecular- weigh polydimethylsiloxanes) can be represented by where R can be methyl, phenyl, vinyl, trifluoropropyl, or 2-cyanoethyl. Silicone rubbers that contain vinyl groups can be cured by dialkyl peroxides such as dicumyl peroxide. Sat- urated silicone rubbers require diacyl peroxides such as bis-(2,4-dichlorobenzoyl)perox- ide. In the case of saturated siloxane rubbers, the mechanism is hydrogen atom abstraction followed by polymeric radical coupling to give cross-links. The incorporation of vinyl groups in the rubber molecule improves the cross-linking efficiency. Vulcanization is frequently done in two steps. After a preliminary vulcanization in a mold, a high-temperature (e.g., 180°C) postcure is carried out in air. The high-temperature postcure removes acidic materials that can catalyze hydrolytic decomposition of the vulca- nizate. Also, the high temperature enables the formation of additional cross-links of the following type: Peroxide Vulcanization of Urethane Elastomers. Urethane elastomers suitable for peroxide vulcanization are typically prepared from an hydroxyl-group-terminated oligo- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. ELASTOMERS [...]... given at the website R F F F F R R F F R F F 1500 1502 15 07 1509 1516 1 573 170 7 171 2 177 8 1609 1808 1843 86 48–58 61–68 49–55 49–55 49–55 115 40 30–35 30–35 50–52 50–52 NS S S NS S NS NS NS NS NS NS S Color NAPH HAR HAR NAPH HAR NAPH – – – – – – Grade Oil 15,0 47, 5 5,0 37, 5 37, 5 37, 5 – – – – – – PHR N 770 N330 N110 – – – – – – – – – Grade 100 76 40 – – – – – – – – – PHR Carbon black Uses V-belts and... amount of oil can be 100, 200, or more parts per 100 parts of rubber (phr) Properties of Ethylene-Propylene Rubbers Elastic properties of EPR or EPDM are better than those of many synthetic rubbers, but hysteresis is not as good (low) as in the case of NR and BR Resistance to compression set for EPR and EPDM vulcanizates is excellent For EPDM, this is especially true in grades containing larger amounts of. .. consumption of vehicles on the road However, the presence of BR gives rise to poor wet traction, but the presence of about 40 percent (of the polymer) BR gives improvements in ice traction In addition, BR has a great tolerance for high levels of extender oil and carbon black Uses BR was first used largely in the blend of elastomers in tire treads to give improved abrasion resistance Because of the emergence of. .. comparable vulcanizates of E-SBR Also, solution polymers contain less nonrubber material This is because there is absence of emulsifier (e.g., soap) during polymerization During coagulation of the polymerized emulsion to obtain the rubber, fatty acids are formed The presence of such fatty acid, in part, reduces the rate of vulcanization with respect to that of solution SBR compounds The absence of such nonrubber... vulcanizates have lower gas permeability, offer better ozone and weather resistance, and are faster curing than those of CIIR or IIR The properties of CIIR are between those of BIIR and IIR Uses of Chlorobutyl and Bromobutyl Rubbers CIIR and BIIR are used in inner liners of tubeless tires with improved (over IIR) covulcanization (in blends) and adhesion to other components of the tires, in inner tubes for heavy-duty... We note that some of the peroxide decomposers are also accelerators for sulfur vulcanization 4.5.2.3 Degradation by the Action of Ozone The degradation of polydiene rubbers by the action of atmospheric ozone is characterized by the appearance of cracks on the surface of a finished rubber product This degradation is caused by direct ozone attack and reaction with the double bond sites of unsaturation in... Terms of Use as given at the website ELASTOMERS 4.68 CHAPTER 4 Properties of Nitrile Rubbers Used with reinforcing fillers, NBR vulcanizates of excellent mechanical properties are obtained With proper compounding, a wide range of hardness grades are possible, with good resistance to compression set Elastic properties of unplasticized NBRs are somewhat less favorable than those of NR or SBR The use of. .. nevertheless give rise to compositions of good elasticity The presence of reinforcing fillers can give abrasion resistance that is considerably better than that of comparable NR or SBR vulcanizates, and XNBR vulcanizates have extremely good abrasion resistance The heat resistance of NBR vulcanizates is generally better than for vulcanizates of NR of SBR With reduced amounts of oxygen, as in an oil environment,... vulcanized than is CM Polyvalent metal oxides, such as those of lead and magnesium, react in the presence of small amounts of acids (such as stearic) or sulfur vulcanization accelerators, e.g., TMTD or MBT Properties of Chlorosulfonated Rubber Vulcanizates Properties of CSM vulcanizates are very similar to those of CM vulcanizates It has a combination of toughness, resistance against dry heat and weathering,... can prepare polymers with a variety of chemical structures and physical properties by reacting a great variety of low-molecular-weight compounds or oligomers with diisocyanates The introduction of polyurethanes made it possible to produce a range of materials from hard plastics to soft rubbers and polymers with properties between these extremes Before the introduction of polyurethanes, tough, useful polymers . 153 Time, min 15 15 15 *.Concentrations in parts by weight per 100 parts of neoprene W. †.Conditions change depending on other aspects of the compositions. SCHEME 17 Downloaded from Digital Engineering. number of carbon atoms per molecule of wax varies from 18 to 50. Microcrystalline waxes are heavier and less crystalline. They have between 37 and 70 carbon atoms per molecule. The migration rate of. associated with high levels of sulfur. As always, there are compro- mises. 4.5.1 .7 Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers. Over the years, much of the research on accelerated-sulfur