Rubber degradation and stabilization.

Oxidative degradation of organic materials is a natural and continuous process. It occurs in varying degrees and at various rates depending upon the environmental stresses placed upon a material. Natural and synthetic polymers are particularly susceptible to degradative changes resulting from interaction with molecular oxygen. These changes can manifest themselves as drastic swings in tensile strength, hardness, elongation, tack and adhesion values in a specific polymer system. Surprisingly small amounts of oxygen can cause these changes. Detrimental reactions lead to scission of polymer chains as well as crosslinking in various proportions. It also results in the introduction of various peroxidic and oxy functional groups at points along the polymer chain. Obviously, one well placed polymer scission can result in reduction of the polymer's molecular weight by one-half, while one single crosslink could produce a doubling of molecular weight leading to aggregation and gel formation. Such changes affect the viscosity and strength properties and ultimately the processability and performance of a product even though the bulk of the material is relatively unchanged (ref. 1).

The need for stabilization therefore is an essential one since these materials will be exposed to oxygen throughout various stages of their useful lifetime. Basically there are three distinct areas of concern for the stabilization of a polymer. These are the polymer production stage, the fabrication stage and the final product or application stage. The discussions below will be limited to the production stage and the stabilization of raw polymer.

Typically, antioxidants are added to the polymer just prior to isolation and before it is exposed to oxygen. Such systems are expected to survive coagulation, stripping of unreacted monomer, and extruder or apron drying. In addition, the stabilizer is expected to maintain polymer properties and to suppress changes in gel formation or Mooney viscosity. throughout the isolation process the antioxidant is also expected to retard the deleterious effects of oxidation. This protection is expected to continue through the storage period prior to utilization in a fabrication step. While one would not make an omelet out of spoiled eggs or orange juice out of rotten oranges, it is incredible that compounders expect to make quality vulcanizates out of rubbers that many times do not meet the above quality standards.

Mechanism of polymer degradation

The degradation of polymer proceeds by a free radical chain reaction mechanism. Initiation of the process occurs by exposure to heat, light or mechanical stress and is often catalyzed by the presence of certain metallic impurities. The oxidation of hydrocarbon or related polymers by oxygen is autocatalytic with the primary product of the process being hydroperoxide (ref. 2).

Our understanding of the autoxidation mechanism and the products of the oxidation is based on the detailed studies conducted by the Natural Rubber Producer's Association on simple hydrocarbons (ref. 3). It is composed of three phases - an initiation, a propagation and a termination step - and has the general form shown in figure 1.

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Although the mechanism of autoxidation was studied exclusively for hydrocarbons, it appears that the fundamental reactions are common to all polymer autoxidation reactions. The uninhibited hydrocarbon oxidation model, however, ignores several factors such as relative reaction rate differences between polymers, oxygen permeability in the solid state and the diverse mixture of polymer structures. In addition, and probably most importantly, it does not account for the fact that in various polymers the nature and character of the propagating radical R[center dot] is not the same. It is important to remember that for various polymers, R1[center dot] [not equal to] R2[center dot] [not equal to] R3[center dot], etc.

In a liquid hydrocarbon system, at pressures of oxygen equal to or greater than atmospheric, the rate of reaction of R[center dot] with [O.sub.2] is very fast. The oxygen is assumed to be saturated throughout the hydrocarbon. Therefore the concentration of R[center dot] is much less than ROO[center dot]. In polymer, on the other hand, the amount of oxygen present in the polymer is limited by diffusion and the oxygen permeability rate of the polymer (ref. 4). Modulus studies on differential sections of polymer have shown that the oxidation is essentially a surface phenomenon (ref. 5). Thus the concentration of R[center dot] in polymer will be higher and it can undergo other reactions which are unique to its own relative stability. The following are among some of the reactions which R[center dot] can undergo (ref. 6). R[center dot] can:

* dimerize (crosslink);

* disproportionate (self oxidation-reduction);

* abstract H (chain transfer);

* add to double bonds (polymerize);

* cleave (retropolymerize);

* rearrange;

* react with oxygen.

The nature and reactivity of the R[center dot] radical is realted to structural features present in the polymer. These can be classified from three different perspectives; first there is the radical itself, second there is the structural relationship the radical has to its nearest neighboring mer units, and third there is the relationship that the radical has to the bulk of the polymer.

From the first perspective, the formation of a radical in a polymer requires abstraction of a hydrogen atom from the same point in the polymer. Bond dissociation energies can be helpful in assigning relative reactivities for this reaction for various polymers. Thermoloysis of the polymer itself is beyond the temperature range and conditions that one normally sees for autoxidation at normal temperatures. The initiation of oxidation then occurs by some other process and is generally attributed to the decomposition of latent hydroperoxide or by direct oxidation of the contained stabilizer. Once the free radical process is initiated, abstraction of the most labile hydrogen atom in the polymer by alkylperoxy radical predominates in the propagation step (ref. 7).

ROO[center dot] + weakest R-H [right arrow] ROOH + R[center dot]

R[center dot] +[O.sub.2] [right arrow] ROO[center dot]

Therefore, the bond dissociation energies are quite important since they suggest a reaction path for the oxidation. A high value indicates a strong bond and a lesser value indicates a weaker more reactive bond (ref. 8).

Reactivity toward abstraction increases as the bond dissociation energy decreases. A comparison of these energies for primary verses secondary verses tertiary verses allyl or benzylic hydrogens is in figure 2. With some practice a technically trained person should be able to look at a polymer and predict which hydrogen should be easily removable. Predictions of how the new formed radical would participate in other reactions in the polymer or how this radical once reacted with oxygen to form the peroxy radical would continue the chain autoxidation process should also be possible.

Figure 2

Strongest bond                          [right arrow] Weakest bond
Primary      Secondary      Tertiary    Allylic/benzylic
(98 kcal)    (94.5 kcal)    (91 kcal)   (86 kcal)
R-CH2-H      R2-C-H         R3C-H       R2C=CR-CR2-H

This is why the second aspect, the structural relationship of the nearest neighboring mer units becomes important. The significance of this type of analysis is best described by example. If one compares the autoxidation of polyethylene and polypropylene this will become clearer. Polyethylene is a linear hydrocarbon wherein all the hydrogens in the polymer chain save for the end groups are secondary hydrogens. As a saturated polymer it is among the most stable and is naturally quite oxidation resistant. Removal of a hydrogen atom would produce a polymer radical which would react with oxygen to produce the corresponding peroxy radical. The peroxy radical could remove hydrogen atoms from itself or from adjacent polymer and the chain process would continue. The rate of removal would be relatively slow since once again these would be less reactive secondary hydrogens.

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Polypropylene on the other hand has tertiary hydrogens in its polymeric structure. These are more easily removed than are the hydrogen atoms in polyethylene. Once the tertiary radical forms, it also reacts quickly with oxygen to give a peroxy radical. This peroxy radical has an unusual structural constraint in that it is ideally set up to abstract tertiary hydrogen from the mer unit that is directly adjacent. If unchecked, catastrophic oxidation will occur in a very limited region the polymer (ref. 9).

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The third aspect, the relationship the radical has to the bulk of the polymer, can be demonstrated by comparing polyisoprene to polybutadiene under anaerobic conditions of peroxide cure. When polyisoprene is peroxide cured, the number of crosslinks that result are directly proportional to the amount of peroxide used. One molecule of peroxide causes one crosslink. When polybutadiene is peroxide cured, one molecule of peroxide can cause up to 40 crosslinks (ref. 10). There are two reasons for this difference and both have to do with the presence of the methyl group on the polymer chain. First the methyl group protects the double bond preventing the addition of free radicals. Secondly, the methyl group on polyisoprene protects the allyl radical and only allows crosslinking to occur between two isoprene radicals.

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The allyl radical in polybutadiene does not have such protection and basically undergoes a free radical polymerization where the contained double bonds serve as the polymerization site. It is this reaction in aged butadiene that causes gel formation and Mooney viscosity changes for stored polybutadiene polymer.

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A comparison of these two polymers under oxidizing conditions is also interesting. The oxidation of polybutadiene polymer results in polymer hardening while polyisoprene softens under the same conditions. The oxidation of polybutadiene produces an unhindered polymeric peroxy radical which can either abstract hydrogen or add to an additional double bond. Either reaction produces the type of peroxide curing noted above and it occurs simultaneously to the oxidation. The net result is polymer hardening.

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The tertiary allylic radical in polyisoprene undergoes completely different chemistry. Reaction with oxygen at the tertiary center leads to an alkylperoxy radical which is ideally situated to add to a neighboring double bond. There is considerable driving force for this closure since the product is a stable cycloperoxy tertiary radical. Interestingly, this is an example of a ROO[center dot] radical being converted to a R[center dot] radical by addition to olefin rather than by hydrogen abstraction. It is this cycloperoxy intermediate which leads to chain scission and softening of the polyisoprene on oxidation (refs. 11 and 12).

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While the above examples are meant to be demonstrative of some of the changes that occur during autoxidation they are by no means thorough. To do so would be beyond the scope of this article. However, the discussion should stimulate how we think about antioxidants and stabilizers and how they function within the polymer matrix.

Polymer stabilization

Historically, antioxidants have been classified into two types which have been based on their function as potential disrupters of the autoxidation chain cycle. Materials which act as peroxide decomposers retarding the formation of free radicals in the initiation step are known as preventive antioxidants. Materials of this type include thiosynergists and phosphite antioxidants. Materials which retard radical propagation reactions by nullifying ROO[center dot] and R[center dot] are know as chain breaking antioxidants. Materials of this type include hydrogen donors such as the hindered phenols and aromatic amines (ref. 13).

For stabilized polymer, the termination reactions of the autoxidation chain cycle are replaced by the following competitive reactions for the reactive intermediates of the propagation step.

ROO[center dot] + AH [right carrow] Products + A[center dot]

R[center dot] + A[center dot] [right carrow] Products + AH

From this perspective the antioxidant should no longer be thought of as an additive for polymer but rather that the polymer and its antioxidant define a compatible stabilized system. This interactive view of stabilized oxidation essentially defines a catalytic participation of the antioxidant in the stabilization of a polymer. Several chain breaking hydrogen-donating, hydrogen-abstracting antioxidants have been shown to behave in this catalytic manner (ref. 14). A better representation of how this works is shown in figure 3.

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For those interested in the improvement of polymer stabilization and quality, the above interpretation of polymer protection implies several insights about antioxidant function. First there is a unique relationship between the antioxidant and the polymer matrix in which it is used. This has been known from practical experience in that no one antioxidant will work satisfactorily in all polymers. The catalytic cycle better defines the need for balanced use of the antioxidant. Use of too little or too much can be detrimental. The system should be matched to polymer in terms of optimized concentration of use and also in its ability to compete with the direct neutralization of ROO[center dot] and R[center dot]. The antioxidant should not be destroyed by direct reaction with oxygen nor should it be proxidative by initiating polymer oxidation. Finally, the catalytic cycle implies that it should be possible to make future stabilization improvements since antioxidant choices should be made on the ability to interact preferentially with ROO[center dot] or R[center dot]. Therefore, problem areas such as stabilization of Mooney viscosity or the prevention of gel in polymer should be addressed directly.

Stabilization of synthetic polymer is a relatively new science since the commercial production of these materials only dates back slightly over the past 50 years. Amine stabilizers were first chosen for the stabilization of these polymers and this followed the natural path that had been established historically for stabilization of natural rubber vulcanizates. The desire to utilize these synthetic rubbers in non staining and non discoloring applications necessitated the use of different stabilizers. From earlier work materials such as phenol, cresol and hydroquinone had shown some utility as being useful as antidegradants but these materials alone were insufficient to protect the new synthetic rubbers. Simple alkylated derivatives of phenol and hydroquinone were found to produce much better stabilized product. Many of these early stabilizers are still being used today even though their activity as antioxidants and their ability to control long term Mooney viscosity variation of polymer is poor. Examples include styrenated phenol, alpha methylstyrenated butylated phenol and butylated octylated phenol.

Work in our laboratory has been directed toward understanding the influence that the nature of the hindering groups in phenolic stabilizers has on the intrinsic activity of these antioxidant systems. It has been our general finding that the larger these hindering groups become the poorer the antioxidant activity becomes in a standard polymer system (ref. 15). This poorer activity we believe is due to a restricted ability of the antioxidant to regenerate itself in the hydrogen donorhydrogen abstractor mechanism. In this model, highly hindered phenolic radicals have a harder time abstracting hydrogen atoms from activated radical sites in polymer. To optimize this abstraction, antioxidant systems which have less hindrance have been synthesized and evaluated. Interfering with the R[center dot] part of the chain autoxidative mechanism has significant ramifications for butadiene based polymers since it is the allyl radical in such systems that leads to polymer crosslinking, gel formation and subsequent Mooney viscosity changes (figure 4)(ref. 16).

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We have recently developed and commercialized Wingstay K to address many of these additional stabilization areas. Its use in butadiene containing polymers is resulting in significant polymer quality improvements for these polymers. W-K is an acid catalyzed condensation product of 1-dodecanethiol, formaldehyde and p-nonylphenol. This material shown in figure 5 also contains a secondary antioxidant synergist which is coproduced during the condensation reaction. This synergist is methylene bis(n-dodecylsulfide) (ref. 17).

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W-K has several designed features that account for its activity as an outstanding highly active antioxidant and gel prevention agent. First it has a very high molecular weight which gives the material good persistence in the polymer. It is autosynergistic in so far that it contains sulfide links is its structure which serve as peroxide decomposers. It has an open phenol structure which makes it highly active toward ROO[center dot] neutralization and its phenoxy radical should be highly active toward hydrogen abstraction from allylic R[center dot] species present in a butadiene type polymer. The material is a very powerful antioxidant. Closed system testing by oxygen absorption shows that this material lasts over 960 hrs. to 1.0% oxygen uptake in SBR-1006 verses a control of a styrenated phenol at 125 hrs. or a control of an alpha methylstyrenated butylated phenol at 275 hrs.

As figure 6 shows, W-K performs outstandingly in the protection or raw polymer from variation in Mooney viscosity. SBR oven aged for 35 days at 70[degrees]C no variation in Mooney. Similar results were obtained for BR aged at room temperature for 110 days (figure 7).

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Conclusion

Historically, raw polymers were stabilized with simple alkylated phenolics. These materials were chosen primarily for their ability to interfere with the chain mechanism of oxidation by hydrogen donation. While this approach was sufficient to stabilize polymer against catastrophic oxidative decomposition, it did little to protect the polymer against aging due to crosslinking, gel formation or Mooney viscosity changes. Modern polymer stabilizers now can be designed to do many more important functions than simply neutralize ROO[center dot] in the chain cycle. The following are among some of these additional functions: the stabilizer can be made persistent by increasing its molecular weight, it can serve as its own synergist thereby destroying peroxide species and thus preventing the initiation of new radical chains, it can neutralize R[center dot] in the chain cycle by hydrogen abstraction thereby regenerating itself at the same time it is preventing the formation of gel and Mooney change. All of these functions can now lead to the production of superior grades of rubber. Implicitly it also leads to a new level of quality for the starting materials with which the compounders work.

References

(1.)Shelton, J.R., Rubber Chem. Technol., 45, 359 (1972).

(2.)Scott, G., Atmospheric oxidation and antioxidants, Elsevier, Amsterdam, 1965.

(3.)Cunneen, J.I., Rubber Chem. Technol., 41, 182 (1968).

(4.)Rabek. J.F., Comprehensive chemical kinetics, 41, 425 (1956).

(5.)Gillen, K.T. and Clough, R.L., "Techniques for monitoring heterogeneous oxidation of polymers," In: Handbook of Polymer Science and Technology, Vol. 2. (N.P. Cheremisinoff, Ed.), Marcel Decker, Inc. 1989, p. 167.

(6.)Pryor, W.A., Free Radicals, McGraw Hill Book Co., New York, 1966.

(7.)Hawkins, W.L., Oxidation and combustion reviews, Vol. 1 (C.F.H. Tipper, Ed.), Elsevier, Amsterdam, 1965, p. 170.

(8.)Kerr, J.A., Chem. Rev., 66, 465 (1966).

(9.)Kelen, T., Polymer degradation, Van Nostrand Reinhold Co. New York, 1983.

(10.)Loan, L.D., Rubber Chem. Technol., 40, 149 (1967).

(11.)Bevilacqua, E.M., English, E.S., and Phillip, E.E., J. Org. Chem., 25, 1276 (1960).

(12.)Barnard, D., Cain, M.E., Cunneen, J.I., and Houseman, T.H., Rubber Chem. Technol., 45, 381 (1972).

(13.)Scott, G., Atmospheric oxidation and antioxidants, Elsevier, Amsterdam, 1993, p. 121.

(14.)Scott, G., "Stabilization of polymers by catalytic antioxidants," Macromol. Chem., Macromol. Symp. 27, 1 (1989).

(15.)Kuczkowski, J.A., Dean II. P.R., and Hollingshead, W.S., "The influence of the hindering groups on the mechanism of phenolic antioxidants," presented at the 5th Annu. Int. Conf. on Advances in the Stabilization and Controlled Degradation of Polymers, Zurich, Switzerland, June, 1983.

(16.)Kuczkowski, J.A., "The inhibition of oxidative and ozonic processes in elastomers," In: Oxidation inhibition in organic materials," Vol. 1 (J. Pospisil and P.P. Klemchuk, Eds.), CRC Press, Boca Raton, 1990, p. 247.

(17.)Kuczkowski, J.A., Sturm, B.H., Cottman, K.S., Grimm D.C. and Dillon, J.J., "Gel stabilization of diene based polymers," presented at the 134th Meeting of the Rubber Division, American Chemical Society, Cincinnati, OH, October 18-21, 1988.