The chemistry of peroxide vulcanization.
In order to more fully understand the peroxide vulcanization process, we should first discuss the chemistry involved. A peroxide is defined as a chemical compound that contains oxygen-oxygen bonds (figure 1). This is a unique type of chemical functionality because this bond breaks on heating to yield free-radical fragments. This type of bond breaking is known as homolytic cleavage, since the two electrons involved in the bond are separated. The free-radical fragments are highly reactive and are the force that initiates the formation of crosslinks in polymers.
The stability of the peroxide bond depends on the chemical constituents that surround it (designated in figure 1 as R and R'). These groups may be inorganic in nature (e.g., sodium peroxide) or they might be organic (e.g., benzoyl peroxide). Most of the peroxides that are commonly used in rubber formulations are very stable and require high temperatures for decomposition to occur.
The extent of peroxide decomposition depends on its thermal history which includes both a time and temperature factor. The same extent of decomposition can occur in a sample that has been exposed to a low temperature for a long period of time or a sample that has been heated for a shorter period of time. A good example is the thermal history that is necessary to decompose 10% of a sample of dicumyl peroxide. At room temperature (25 degrees] C) storage, it would require approximately 500 years to achieve a 10% loss of peroxide, while even under hot warehouse conditions (40 [degrees] C) it would take nearly 24 years. However, at temperatures used for vulcanization ([is greater than] 150 [degrees] C), it decomposes within minutes to initiate the crosslinking process.
A relative measure of peroxide stability is known as the ten-hour half life temperature (10hr HLT). This is the temperature that is required to decompose 50% of a peroxide sample in ten hours. The ten-hour half life varies greatly among peroxides. At any temperature, peroxides that have low 10hr HLTs decompose at a faster rate than ones with higher values. This measure of stability is important to rubber formulators since relatively stable peroxides (high 10hr HLT) will provide greater scorch resistance, but generally require slower processing rates. Peroxides with lower 10hr HLT values will allow for faster processing, but may be more scorchy.
This trade-off is often encountered when selecting the optimum peroxide for an elastomer formulation.
Five of the main classes of peroxides are shown in figure 2. These contain different chemical functionality around the peroxide linkage and have different relative stabilities. This variability of 10hr HLT is even broad within each class, which shows the sensitivity of the peroxide linkage to small chemical differences in the surrounding groups.
The chemical mechanism underlying the peroxide vulcanization of elastomers is relatively simple (figure 3). It requires only three steps:
* 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.
Figure 3 - the three-step peroxide vulcanization reaction ROOR [right arrow] 2 RO* RO* + P - H [right arrow] P* + ROH 2P* [right arrow] P - P
The formation of a carbon-carbon bond as the crosslink is an important feature of peroxide vulcanization. Neither the peroxide nor by-products from the vulcanization process are part of the crosslink. Therefore, the inherent stability of the polymer is not reduced by crosslinking with peroxides.
The rate determining step in this three step reaction is always the homolytic cleavage of the peroxide molecule. This generally requires seconds to minutes, and thermal diffusion is often an important factor. After the highly reactive radicals are formed, the subsequent steps occur in seconds or less.
If a compound formulation consisted only of peroxide and polymer, then the result of vulcanization would be easily predicted. Unfortunately, rubber formulations require many other ingredients to yield the desired physical properties, longevity and to meet cost constraints. Many of these additional ingredients, although relatively inert to sulfur curing, may have a significant effect on peroxide curing chemistry. In order to discuss these potential interferences more fully, we should consider the three steps in the peroxide cure mechanism and how each may be affected by common compound ingredients.
Homolytic cleavage of the peroxide
The first step in the crosslinking reaction is the homolytic cleavage of a peroxide molecule to form two radicals (figure 4). This is a first-order reaction, which is very predictable for any specific peroxide. 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.
Figure 4 - reaction step 1: thermal decomposition of the peroxide ROOR [right arrow] 2RO* Radicals are unstable and react rapidly Half-life is the time required for half of the peroxide to decompose Half-life is related to temperature
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 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. After five half-lives, about 97% of the peroxide has been decomposed, while after seven half-lives, [is greater than] 99% has decomposed. Peroxide half-life is related to temperature (figure 5). For typical peroxides used in the rubber industry, the half-life drops to about one third of its value for each 10 [degrees] increase of temperature.
Acidic materials can interfere with the homolytic cleavage of peroxides by catalyzing an alternative decomposition pathway for the peroxide. This alternative reaction occurs at a lower temperature than homolytic cleavage and is nonproductive since free radicals are not generated for crosslinking. This acid catalyzed decomposition is most pronounced with strong acids such as sulfuric and phosphoric acids. But there are common rubber ingredients that are also acidic enough to cause this decomposition. A good example is clay fillers. Figure 6 shows the effect of Dixie clay and Burgess KE clay on a peroxide cure of an EPDM compound. Dixie clay is more acidic and causes much of the peroxide to decompose via the nonproductive acid-catalyzed reaction. Because of this, only a small portion of the peroxide actually accomplishes the intended crosslinking reaction. For this reason, it is recommended that compound formulators use deactivated or basic fillers.
Hydrogen abstraction -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 (figure 7). The radical is transferred to the polymer molecule. The reaction is bimolecular because an alkoxy radical must react with a polymer chain. However, the reaction is not second order. A large number of hydrogen atoms is 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 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. Since crosslinking is represented by the delta torque measured by an oscillating disk rheometer (ODR), the diagram in figure 8 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 7 - reaction step 2: hydrogen abstraction from polymer by alkoxy radical RO* + P-H [right arrow] P* + ROH First order reaction, if large number of H* available Rate is proportional to the concentration of peroxide
Additives which might interfere with the second step of the peroxide mechanism include chemicals that can offer easily abstractable hydrogens. If a compound additive contains readily abstractable hydrogens, these will be the preferred targets for abstraction by radicals (figure 9). The resulting radicals are relatively stable and will not further the crosslinking of the compound. In this way, peroxides are consumed in a non-productive way.
Figure 9 - competitive sources of easily abstractable hydrogens RO* + A-H [right arrow] A* + ROH Other sources of hydrogen can compete for peroxide H* abstraction from non-polymer source reduces efficiency A* will couple, graft or decompose without yielding a crosslink
The relative ease of hydrogen abstraction follows the order shown in figure 10. Most elastomeric polymers contain mainly hydrogens on secondary and tertiary carbons, which are relatively difficult to abstract. Phenolic hydrogens are among the easiest to abstract and will be a more attractive target for alkoxy-free radicals. In the same way, hydrogens on benzylic and allylic carbons are more easily abstracted than hydrogens found in polymers.
Benzylic and allylic carbons are mainly found in aromatic and naphthenic oils. Although these are less expensive than paraffinic oils, they can have a detrimental effect on the peroxide cure reaction. Figure 11 shows a comparison of the results of a peroxide cure using paraffinic, napthenic and aromatic oils. Of these, the paraffinic oil provides the best results since it does not consume free radicals preferentially over the polymer.
Phenolics are commonly used as antioxidants in rubber compounds to provide heat stability. These sources of easily abstractable hydrogens could have a dramatic effect on peroxide curing. Fortunately, many antioxidants do not have a significant effect on the peroxide crosslinking reaction and can be used in peroxide-cured compounds.
Another potential interference of the hydrogen abstraction step is that caused by coagents. Coagents are chemicals that contain two or more unsaturated sites per molecule. In the presence of radicals, these unsaturated groups may polymerize, forming a plastic-like reinforcement in situ (figure 12). This reaction consumes free radicals, but increases the modulus of a cured compound to a greater extent than peroxide alone. The mechanism of this reaction is not clear. One hypothesis is that, in the presence of radicals, the coagent simply polymerizes into a highly branched plastic reinforcing filler (figure 13). Another hypothesis is that the coagent binds to polymer free radicals and serves as a highly branched crosslink. The actual mechanism is probably a combination of the two hypotheses, since coagents would be expected to interact with any free radicals that are present during vulcanization.
Some typical coagents are shown in figure 14. Coagents vary greatly in their efficiency and cost, so it is always recommended that testing be performed in order to optimize the cost/benefit of a particular formulation. The use of a coagent can be expected to have a large effect on the properties of a compound. Coagents tend to decrease the Mooney viscosity of a compound because they act as plasticizers before the cure. They also increase the hardness and modulus and decrease the elongation, since they make the cured compound more plastic-like.
Coupling of 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 that is nearly identical to the bonds that make up the length of the chains.
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 (figure 15). 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.
In addition to the interference of oxygen, certain polymer chain radicals can undergo another type of unfavorable decomposition. Polypropylene, polyisobutylene and butyl rubber are among the polymers that can undergo cleavage along the polymer chain, which degrades the polymer. Therefore, these elastomers cannot be successfully crosslinked with peroxides.
In summary, rubber formulators should be aware of potential chemical interferences that common ingredients could cause in peroxide-cured systems. This article mentions some of the more important effects that should be taken into consideration in devising a formulation. The proper selection of peroxides, oils, fillers, antioxidants and coagents can lead to optimized processing and performance of the final product.
(This article is based on a paper given at the October, 2000 Rubber Division meeting.)