A review of the fundamentals of crosslinking with peroxides.Crosslinking elastomers with peroxides has been known for more than 50 years. In the early years, peroxide vulcanization vulcanization (vŭl'kənəzā`shən), treatment of rubber to give it certain qualities, e.g., strength, elasticity, and resistance to solvents, and to render it impervious to moderate heat and cold. was not considered to have commercial utility. A popular organic chemistry text published in 1944 dismissed peroxide vulcanization with the comment: "Peroxides produce vulcanization of a sort" (ref. 1). Serious interest in vulcanization with peroxides began with the commercial introduction of dicumyl peroxide in the late 1950s. The major feature of this peroxide was its higher curing temperature, which provided scorch time needed for many rubber manufacturing systems. Also, dicumyl peroxide was more storage-stable than other available peroxides and consequently was less hazardous. Peroxides are widely accepted today and are used in many areas of the rubber industry. However, in some applications, workers are hesitant to use peroxides because of concerns about safety and unfounded fear of contamination of traditional sulfur vulcanization. This subject was reviewed in 1995 (ref. 2). Peroxide vulcanization of elastomers is relatively simple. It requires only three steps (ref. 3): * The peroxide undergoes homolytic cleavage to form two alkoxy radicals. * An alkoxy radical abstracts a hydrogen atom from a polymer chain. * Two radicals on adjacent polymer chains couple to form a carbon-carbon bond A carbon-carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond – a bond composed of two electrons, one from each of the two atoms. . The formation of a carbon-carbon bond as the crosslink is an important feature of peroxide vulcanization. Neither the peroxide nor byproducts from the vulcanization process are part of the crosslink. Therefore, the inherent stability of the polymer is not reduced by crosslinking with peroxides. Peroxide vulcanization mechanism Step 1: Homolytic cleavage of the peroxide The first step in the crosslinking reaction is the homolytic cleavage of a peroxide molecule to form two radicals (refs. 4 and 5). This is a first-order reaction, which is very predictable. The cleavage rate of the peroxide molecule is proportional only to the concentration of the peroxide at any time. The rate is also controlled by the energy required for homolytic cleavage. Heat supplies the energy, so the rate of radical formation depends on the temperature of the system. The cure time and cure temperature in a typical application are related to the half-life of the peroxide. The half-life is the time required for half of the peroxide to decompose de·com·pose v. de·com·posed, de·com·pos·ing, de·com·pos·es v.tr. 1. To separate into components or basic elements. 2. To cause to rot. v.intr. 1. at the reaction temperature. Therefore, the half-life is related to the cure temperature. A peroxide that has a short half-life can initiate cure at lower temperatures. During vulcanization, the amount of peroxide remaining at any time is the initial concentration of peroxide multiplied by [(1/2).sup.n], where n is the number of half-lives. Therefore, one half of the peroxide remains after one half-life, one quarter remains after two half-lives, and one eighth remains after three half-lives. The peroxide consumption can be easily seen in a bar graph (figure 1). After five half-lives, about 97% of the peroxide has been decomposed de·com·pose v. de·com·posed, de·com·pos·ing, de·com·pos·es v.tr. 1. To separate into components or basic elements. 2. To cause to rot. v.intr. 1. , while after seven half-lives, [is greater than] 99% has decomposed. [Figure 1 ILLUSTRATION OMITTED] Peroxide half-life is related to temperature (figure 2). For typical peroxides used in the rubber industry, the half-life drops to about one third of its value for each 10 [degrees]C increase of temperature (refs. 4 and 5). [Figure 2 ILLUSTRATION OMITTED] For maximum crosslinking efficiency, the peroxide group must quantitatively cleave cleat, cleave claw of any cloven-footed animal. to form two alkoxy radicals. Crosslinking efficiency is reduced if acidic ingredients are present. Acids cause heterolytic or ionic decomposition of the peroxide molecule (ROOR [right arrow] [R.sup.+] + [ROO roo Noun pl roos Austral informal a kangaroo .sup.-]). No radicals are formed by heterolytic cleavage so crosslinking does not occur, and peroxide efficiency is reduced. An acidic filler, such as an untreated clay, will reduce peroxide efficiency. For example, crosslinking will not take place if ethylenepropylene-diene monomer monomer (mŏn`əmər): see polymer. monomer Molecule of any of a class of mostly organic compounds that can react with other molecules of the same or other compounds to form very large molecules (polymers). (EPDM EPDM Ethylene-Propylene-Diene-Monomer EPDM Enterprise Product Data Management EPDM Ethylene Propylene Dimonomer (industrial/commercial piping/plumbing components) EPDM Engineering Product Data Management ) is filled with 110 phr of untreated clay. However, a satisfactory cure will occur if the filler is 110 phr of a silane-treated clay (figure 3). [Figure 3 ILLUSTRATION OMITTED] Step 2: Formation of the polymer chain radical The second step in peroxide vulcanization of elastomers is the abstraction of a hydrogen atom from a polymer chain by an alkoxy radical. The radical is transferred to the polymer molecule. The reaction is bimolecular bi·mo·lec·u·lar adj. Relating to, consisting of, or affecting two molecules. bi  mo·lec because an alkoxy radical must react with a
polymer chain. However, the reaction is not second order. A large number
of hydrogen atoms are available on the polymer chain, so the
concentration of hydrogen atoms is not significantly reduced as the
reaction proceeds. Therefore, the kinetics of the reaction are
determined only by the concentration of alkoxy radicals, and the process
follows first-order kinetics. The rate-determining step The rate-determining step (RDS) is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which of the
consecutive reactions, peroxide [right arrow] alkoxy radical [right
arrow] polymer chain radical, is the homolytic cleavage of the peroxide.
The alkoxy radical is a high-energy species, and once the radicals are
formed, they react very rapidly. Therefore, the concentration of
radicals at any time is very low. This means, the amount of crosslinking
that occurs is related to the amount of peroxide consumed. If we accept
that crosslinking is represented by the delta torque measured by an
oscillating os·cil·late intr.v. os·cil·lat·ed, os·cil·lat·ing, os·cil·lates 1. To swing back and forth with a steady, uninterrupted rhythm. 2. disk rheometer rhe·om·e·ter n. An instrument for measuring the flow of viscous liquids, such as blood. (ODR ODR Online Dispute Resolution ODR On-Demand Routing ODR One-Definition Rule (C++) ODR Octal Data Rate (high speed memory interface transfers 8 bits of data per clock cycle) ODR Office of Dispute Resolution ), then the diagram in figure 4 displays the relationship between peroxide consumed and crosslinking. Peroxide crosslinking is very predictable, so it is easy to match delta torque to meet a specific objective. The delta torque units per weight of peroxide can be calculated from an ODR plot. Using this relationship, we can adjust peroxide concentration to match a target value for delta torque. If the first adjustment is not close enough, a second adjustment is usually successful. [Figure 4 ILLUSTRATION OMITTED] A radical can abstract a hydrogen atom from any available source in the composition, not only from the polymer. If hydrogen is transferred from a nonpolymeric ingredient, crosslinking efficiency of the peroxide is usually reduced. The nonpolymeric radical that is formed will couple, graft or decompose without yielding a crosslink. The ease of hydrogen abstraction by an alkoxy radical is in the order expected from hydrogen lability lability /la·bil·i·ty/ (lah-bil´i-te) 1. the quality of being labile. 2. in psychiatry, emotional instability. lability the quality of being labile. (figure 5). In order of decreasing ease of hydrogen abstraction are phenolic phe·no·lic adj. Of, relating to, containing, or derived from phenol. n. Any of various synthetic thermosetting resins, obtained by the reaction of phenols with simple aldehydes and used as adhesives. [is greater than] benzylic [is greater than] allylic al·lyl n. The univalent, unsaturated organic radical C3H5. [Latin allium, garlic + -yl (so called because it was first obtained from garlic). [is greater than] tertiary [is greater than] secondary [is greater than] primary (ref. 6). Concentration is obviously very important in the competition among various hydrogen donors. For example, a low concentration of phenolic hydrogen will not greatly reduce peroxide efficiency. However, at normal use levels, an aromatic oil, which can contain a significant concentration of benzylic and allylic hydrogen atoms, will have a significant effect. The comparative effects of paraffinic, naphthenic and aromatic oils can be demonstrated in an EPDM: clay:oil formulation (figure 6). Paraffinic oil has the least effect on peroxide efficiency, based on delta torque. Naphthenic oil reduces the delta torque obtained with the paraffinic oil by one third, while the aromatic oil reduces the delta torque by two thirds. Therefore, paraffinic oils are preferred when compounding for peroxide vulcanization. Aromatic oil should be avoided to prevent a reduction in crosslinking efficiency. [Figures 5-6 ILLUSTRATION OMITTED] The alkoxy radical can also initiate polymerization polymerization Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. of ethylenic unsaturation un·sat·u·rat·ed adj. 1. Of or relating to an organic compound, especially a fatty acid, containing one or more double or triple bonds between the carbon atoms. 2. Capable of dissolving more of a solute at a given temperature. . In this reaction, the end group of the ethylenic polymer will be the alkoxy group In chemistry, the alkoxyl group is an alkyl group linked to oxygen thus: R-O. The range of alkoxy groups is as great, the simplest being methoxy (-OCH3). An ethoxy substituent is found in the organic compound phenetol, C6H5OCH2CH from the peroxide (figure 7). Polymerizable multifunctional monomers, known as coagents, are often added to peroxide-curing formulations to alter curing characteristics and performance of the composition. The polymerization of ethylenic bonds is a possible path for the response of coagents in the system. Examples of useful coagents (ref. 7) are shown in figure 8. [Figure 7-8 ILLUSTRATION OMITTED] There are two possible mechanisms for coagent cures. An interpenetrating network can be produced if the coagent homopolymerizes during polymer crosslinking. Also, the coagent can form a grafted chain on the polymer backbone. This will occur if the radical on a polymer chain initiates polymerization of the coagent, or the original alkoxy radical initiates polymerization of the coagent and the terminal radical of the polymerized coagent couples with a polymer radical. Performance changes caused by coagents are a function of the coagent concentration. Elastomers crosslinked with coagents in the formulation generally have a higher modulus than the unmodified crosslinked polymer. This is also reflected in a higher delta torque. The effects of the coagents are to increase hardness and modulus and to decrease elongation and compression set. Changes in tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its are often small because the modulus is increased as the elongation is decreased. Mooney viscosity is reduced, as is Mooney scorch time (ref. 7). Although coagents reduce scorch time, paradoxically, they can be beneficial in formulations designed to increase the scorch time of peroxide-cured compositions. Once peroxides are cleaved cleaved (klevd) split or separated, as by cutting. to alkoxy radicals, they provide rapid crosslinking. The only way to increase scorch time is to intercept the radicals. To do this, a radical scavenger can be added. The scavenger reacts with the radicals as they are generated, until the scavenger itself is consumed. This delays crosslinking. When the scavenger has been depleted de·plete tr.v. de·plet·ed, de·plet·ing, de·pletes To decrease the fullness of; use up or empty out. [Latin d , the remaining peroxide crosslinks the polymer (ref. 8). Consumption of the peroxide by a radical scavenger reduces the final cure state. The cure state can be recovered by either increasing the starting concentration of peroxide or adding coagents to the system (figure 9). The coagent will recover the modulus lost by using a radical scavenger, but it may affect end-use properties. Both peroxides and coagents can be purchased as scorch-retarding grades. Scorch-retarding coagents contain radical scavengers, and scorch-retarding peroxides contain both radical scavengers and coagents (ref. 8). [Figure 9 ILLUSTRATION OMITTED] Step 3: Coupling pf polymer chain radicals The final step in peroxide vulcanization of elastomers is the coupling of two radicals on adjacent polymer chains to form a crosslink. The peroxide is not part of the crosslink and does not affect polymer stability by introducing a foreign structure. The crosslink is a carbon-carbon bond between chains. This bond is rigid and strong compared with the sulfur crosslink. The sulfur crosslink is more flexible than the carbon-carbon bond, but is less thermally stable. Therefore, a peroxide crosslink gives lower compression set and improved heat resistance, while the sulfur crosslink gives higher tear strength and abrasion resistance. An example of sulfur vs. peroxide crosslinking of an EPDM formulation is presented in table 1. As indicated, the original properties of the three compounds are very similar. However, the compression set after 70 hours at 150 [degrees]C is only about 20 for the peroxide cures, while it is close to 80 for the sulfur cure. Also, the aged physical properties of the peroxide cures are greatly improved compared with sulfur-cured EPDM (ref. 9). Table 1 - peroxide vs. sulfur cure
EPDM
Typical formulation
Normal levels of crosslinkers
Sulfur Di-Cup Vul-Cup
Original properties Similar
Compression set,
70 hr. 302 [degrees] F 77 21 19
Change on aging,
70 hr. 302 [degrees] F
[M.sub.100], % +144 -8 +9
TB, % -16 +2 -7
EB, % -45 +7 -7
H, pts. +10 -4 0
The expected reaction when curing with peroxides is the coupling of two polymer chains. However, in the presence of oxygen, the radical on a polymer chain can couple with an oxygen molecule to form a hydroperoxy radical. This leads to polymer degradation Polymer degradation is a change in the properties - tensile strength, colour, shape, etc - of a polymer or polymer based product under the influence of one or more environmental factors such as heat, light or chemicals. . If oxygen is not excluded from a peroxide vulcanization, the surface of the rubber part will be sticky and partially cured. Mold flash from press-cured peroxide systems will also show this effect. Certain polymer chain radicals can undergo another type of unfavorable decomposition. Polypropylene, polyisobutylene and butyl rubber butyl rubber: see rubber. can undergo [Beta]-cleavage along the polymer chain, which degrades the polymer. Therefore, these elastomers cannot be successfully crosslinked with peroxides. Selection of peroxides for compounding studies The structures of typical peroxides are presented in figure 10. The left column contains monoperoxides; the right, bisperoxides. Not all of these peroxides are practical choices for crosslinking. They help illustrate the basis for selecting a specific peroxide. [Figure 10 ILLUSTRATION OMITTED] All peroxides produce the identical C-C C-C Carbon-Carbon C-C Carotid-Cavernous (relating to the carotid artery and the sinuses) crosslink, but the byproducts are different. This may be a secondary consideration in selecting a peroxide. For example, one of the byproducts from dicumyl peroxide has a sweet odor, which some workers find objectionable (ref. 10). Under some conditions, other peroxides may produce a visible bloom (ref. 11). The principal reason for selecting a peroxide is its reactivity and reaction temperature. The cure temperature of a peroxide must be suitable for the temperature needed to mix the rubber compound. The lower-temperature peroxides, such as benzoyl peroxide benzoyl peroxide n. A flammable white granular solid used as a bleaching agent for flour, fats, waxes, and oils, and in pharmaceuticals. benzoyl peroxide, n 1. , 2,4-dichlorobenzoyl peroxide and the peroxy ketal at the bottom of the right column, may not provide sufficient scorch time. Di(t-butyl)peroxide requires a high cure temperature, but is too volatile for general use. Replacing a methyl group Noun 1. methyl group - the univalent radical CH3- derived from methane methyl, methyl radical alkyl, alkyl group, alkyl radical - any of a series of univalent groups of the general formula CnH2n+1 derived from aliphatic hydrocarbons of di(t-butyl)peroxide with a phenyl group In chemistry, the phenyl group or phenyl ring (often abbreviated as -Ph) is the functional group with the formula
where the six carbon atoms are arranged in a cyclic ring structure. It is in the Aryl group. gives t-butyl cumyl peroxide. This peroxide initiates cure at a lower temperature than di(t-butyl)peroxide. Replacing a second methyl group with a phenyl phenyl (fĕn`əl), C6H5, organic free radical or alkyl group derived from benzene by removing one hydrogen atom. on the opposite t-butyl group gives dicumyl peroxide. Dicumyl peroxide cures at a lower temperature than t-butyl cumyl peroxide. The general rule for cure temperature, in descending temperature, is dialkyl [is greater than] alkyl-aralkyl [is greater than] diaralkyl [is greater than] alkyl-ketal [is greater than] diaroyl. Another consideration in selecting a peroxide is its physical form. Peroxides can be liquids or solids. Some liquids may be more volatile, and peroxide can be lost during compounding. The solids have different melting points, which will affect ease of dispersion. The equivalent weight of a peroxide will indicate the quantity needed to reach a target cure state. The amount of byproducts formed is directly related to the amount of peroxide used in the compound. Peroxides are available in refined or technical grades, on solid supports or predispersed in a carrier. Although peroxides recommended for use in the rubber industry are all relatively safe, the supported and dispersed forms are less hazardous (refs. 10 and 11). General compounding recommendations Compounding recommendations for peroxide cures are based on understanding the crosslinking mechanism. Fillers should be neutral or basic products. Acidic fillers will produce ionic decomposition of the peroxide, which will not provide crosslinking. Paraffinic plasticizers plasticizers mostly triaryl phosphates, such as tricresyl, triphenyl phosphates, which are poisonous. See also triorthocresyl phosphate. are preferred because they consume fewer alkoxy radicals than aromatic or napthenic oils. Peroxide-cured compounds can be stabilized by adding 1 phr of TMQ TMQ Terminal-Port Queueing (Cisco) TMQ Talking Message Queue stabilizer stabilizer: see airplane. . TMQ does not scavenge scav·enge v. scav·enged, scav·eng·ing, scav·eng·es v.tr. 1. To search through for salvageable material: scavenged the garbage cans for food scraps. 2. radicals rapidly and provides aging stability to the finished compound. Oxygen should be excluded from peroxide-cure systems to prevent polymer degradation and the appearance of partially cured, sticky surfaces. Industrial use of peroxides Polymers that cannot be crosslinked with peroxides are listed in table 2 (ref. 4). All other elastomers that contain hydrogen atoms can be successfully crosslinked. These are listed in table 3. Peroxides are used commercially to crosslink EPDM, EPR EPR Electron Paramagnetic Resonance EPR Extended Producer Responsibility EPR Electronic Patient Record(s) EPR Emergency Preparedness and Response (US DHS) EPR Endpoint Reference EPR Ethylene-Propylene Rubber , chlorinated chlorinated /chlo·ri·nat·ed/ (klor´i-nat?ed) treated or charged with chlorine. chlorinated charged with chlorine. chlorinated acids some, e.g. polyethylene, nitrile rubber Nitrile rubber, or Buna-N,is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene. , hydrogenated nitrile rubber, fluorocarbon fluorocarbon /flu·o·ro·car·bon/ (floor´o-kahr?b?n) any of the class of organic compounds consisting of carbon and fluorine only. rubber, silicone rubber Noun 1. silicone rubber - made from silicone elastomers; retains flexibility resilience and tensile strength over a wide temperature range synthetic rubber, rubber - any of various synthetic elastic materials whose properties resemble natural rubber and ethylene-vinyl-acetate rubber. Natural rubber, SBR SBR - Spectral Band Replication and polybutadiene can be crosslinked with peroxides, but sulfur-cure systems remain the predominate choice. With further study, it may be possible to improve the peroxide-cure systems for many of the elastomers that are currently sulfur cured. Variables that should be considered are the selection and concentration of peroxides, use of coagents and scorch retarders, and the use of mixed peroxide-sulfur cure systems. Table 2 - polymers that cannot be crosslinked with peroxides * Butyl rubber * Halobutyl rubber * Polyisobutylene rubber * Polyepichlorohydrin rubber * Polypropylene * Polyvinyl chloride Table 3 - polymers that can be crosslinked with peroxides Natural rubber Styrene-butadiene rubber Polybutadiene rubber Polyisoprene rubber Nitrile rubber Ethylene-propylene copolymer Ethylene-propylene terpolymer Acrylonitrile-butadiene-styrene Ethylene-vinyl acetate copolymer Silicone rubber Chlorinated polyethylene Fluorocarbon rubber Chlorosulfonated polyethylene Hydrogenated nitrile rubber Acrylic rubber Polyurethane rubber Polynorbornene Polyethylene References (1.) Fieser and Fieser, Organic Chemistry, D.C. Heath and Company, Boston, Ma, 1994, p. 330. (2.) Jay B. Class, Rubber and Plastics News, October 9, 1995, "Fundamentals of crosslinking with peroxides." (3.) J. Sanchez and T.N. Myers, "Peroxides and compounds (organic)" in J.I. Kroschwitz, ed., Encyclopedia of Chemical Technology 4th Edition, vol. 18, John Wiley John Wiley may refer to:
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , 1996, pp. 230-310. (4.) Hercules technical bulletin ORC-101F, "Fundamentals of crosslinking. " (5.) Elf Atochem product bulletin, "Chemical curing of elastomers and cross-linking of thermoplastics." (6.) Daniel Swern (ed.), Organic Peroxides, Wiley Interscience, New York, 1969. (7.) Hercules technical bulletin ORC-110H, "Effects of coagents on peroxide cures." (8.) Elf Atochem product bulletin, "Scorch resistant peroxide." (9.) Hercules technical bulletin ORC-104F, "Comparison of peroxide vs. sulfur cure of EPDM." (10.) Hercules technical bulletin ORC-2011, "Di-Cup dicumyl peroxide - vulcanizing agent and polymerization catalyst." (11.) Hercules technical bulletin ORC-301J, "Vul-Cup Peroxide - vulcanizing agent and polymerization catalyst."3 |
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