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Preventing polymer degradation during mixing.

Degradation caused by oxyidation

Elastomers, like most organic materials, are subject to atmospheric oxidation, even at moderate temperatures. The ease of susceptibility to degradation depends, to a large degree, upon structure and environmental exposure. For example, saturated polymers are inherently more stable than unsaturated polymers due to their stronger bonds, or lack of double bonds in their backbone. Therefore, it would hold that EPDM and butyl rubber would be more stable than SBR or NR against oxidative degradation.

Since this article deals primarily with the prevention of polymer degradation during mixing, the mechanism and prevention of degradation of vulcanizates by the actions of ozone and stress will not be discussed at any great length.

Oxidation is a complex process involving many reactions, each influenced by prevailing conditions such as singlet oxygen, ozone, mechanical shear, heat, light, metals and fatigue.

Effects of oxygen, ozone and shear

Most elastomers are subject to oxidation and it is known that the addition of only 1-2% combined oxygen will render a rubber article useless.

Oxidation proceeds by two mechanisms:

* Chain scission -- Results from the attack of the polymer backbone which causes softening and weakening. It is the primary mechanism observed for natural rubber and butyl oxidation.

* Crosslinking -- Brittle compounds result due to radical crosslinking reactions, resulting in the formation of new crosslinks and a stiffer material. This reaction occurs predominantly with SBR, neoprene, nitrile and EPDM.

In most cases, both types of attack occur and the one which prevails determines the final compound properties. It has been found that loss of elongation is the most sensitive criterion for aging measurement regardless of mechanism, and it is favored over measure of tensile loss.

Finally, cure system selection plays a role in determining aging resistance. The effect of type of sulfur crosslinks and how they can be varied should be considered, but will not be discussed in this article.

Effects of heat

As expected, heat accelerates oxidation. Therefore, the effects described previously are observed sooner and are more severe as temperature is increased. In order to distinguish the effects of heat from those of oxidation, aging tests need to be carried out in an inert atmosphere. When this is done, in natural rubber for example, formation of more crosslinks occurs initially -- followed by reversion, as both crosslinks and the main polymer chain backbone are broken. For example, at 60[degrees]C, it requires 1.2% combined oxygen to reduce the tensile strength of conventionally cured natural rubber in half. However, at 110[degrees]C, only 0.65% oxygen is required. Had similar agings been done at 110[degrees]C in the absence of oxygen, essentially no tensile loss would have been observed.

Effect of light and weathering

U.V. Light promotes free radical oxidation at the rubber surface which produces a film of oxidized rubber. Heat and humidity then accelerate a crazing or alligatoring effect, and this oxidized layer can be rubbed off -- giving a chalking appearance.

Black compounds are more resistant to U.V. light than are light colored ones, and these lighter compounds require addition of larger amounts of a nonstaining antioxidant to replenish that used up at the surface; i.e. an antioxidant reservoir is required.

The problem is more severe with thin parts since product performance can suffer as well as being merely a cosmetic problem.

Effect of metals

Heavy metal (principally cobalt, copper, manganese, iron) ions are believed to catalyze rubber oxidative reactions by influencing the breakdown of peroxides in such a way as to accelerate further attack by oxygen. The first corrective approach is to eliminate all sources of harmful metals. Compounds of copper and manganese which are directly soluble in rubber (stearates and oleates) are particularly active, since they provide a direct source of heavy metal ions. Even the less soluble forms such as the oxides can cause problems by reactions with fatty acids used in compounding to produce more soluble forms.

Although some antioxidants are active against catalyzed oxidation of rubber, in general, the standard antioxidants do not give protection against the heavy metal ions. Since the activity of the metal depends on its being in an ionic form, it is possible to protect compounds by incorporating substances which react with ionic metals to give stable coordination complexes.

Effect of fatigue

One of the major causes of failure in rubber is the development of cracks at the surface. The growth of these cracks under repeated deformation -- or fatigue -- leads to catastrophic failure. This fatigue failure is initiated at minute flaws where stresses are high and mechanical rupture at such points can lead to the development of cracks. Similarly, attack by ozone can cause cracks to occur at the surface whose rate of growth is directly proportional to the ozone concentration.

Many factors, both chemical and physical, are involved and as would be anticipated, corrective action often consists in re-design and recompounding to minimize excessive stress concentration. Different classes of antioxidants can have different effects and practical experience has led to the recognition that certain products -- known as antiflex cracking agents -- in addition to being antioxidants in a more normal sense, possess particular ability to reduce the rate of crack growth.

Another factor to be considered with flexing is the effect of sulfur concentration. The rate of oxidation is proportional to the amount of combined sulfur, and with lower sulfur levels, we obtain better aging. It would appear, therefore, that it is better to compound with low sulfur. However, these low sulfur cures yield poor fatigue resistance in natural rubber, but little fatigue loss upon aging is experienced. SBR, on the other hand, exhibits both good aging and good fatigue resistance with low sulfur cures (ref. 1).

Polymer degradation

Polymeric degradation typically occurs via a free radical process. Chemical bonds, whether they are within the main chain of the polymer or in side groups, can be dissociated by energy resulting from heat, mechanical shearing or radiation to create a free radical, [R.sup.*]. The formation of ([R.sup.*]) can occur in any one of the various phases of a polymer's life cycle: polymerization, processing and end use, and is called the initiation reaction.

Propagation, the second stage of the degradation process, occurs when atmospheric oxygen reacts with ([R.sup.*]) to form a peroxy radical ([ROO.sup.*]), as shown in equation 1. The propagation stage is usually quite rapid compared to initiation. This peroxy radical can further react with labile hydrogens of the polymer to yield unstable hydroperoxides. These hydroperoxides decompose to alkoxy and hydroxy radicals which, in turn, abstract more hydrogens generating more polymer radicals and the cycle becomes autocatalytic, as shown in equations 2 through 5, which illustrate some, but not all, of the reactions which can occur during the propagation stage.

(1) [R.sup.*] + [O.sup.2] [right arrow] ROO

(2) [ROO.sup.*] + RH [right arrow] ROOH + [R.sup.*]

(3) ROOH [right arrow] [RO.sup.*] + [sup.*]OH +

(4) [RO.sup.*] + RH [right arrow] ROH [R.sup.*]

(5) [HO.sup.*] + RH [right arrow] HOH [R.sup.*]

Autoxidation will progress until termination results from the formation of stable products. Eventually, propagating radicals combine or disproportionate to form inert products and the process is terminated, as shown in equations 6 through 10.

(6) 2 R* [right arrow] R-R

(7) [Mathematical Expression Omitted]

(8) 2 [R.sub.3] COO* [right arrow] [R.sub.3]COOC[R.sub.3] + [O.sub.2]

(9) 2 RO* [right arrow] ROOR

(10) ROO* + *OH [right arrow] ROH + [O.sub.2]

Equations 6, 8 and 9 represents crosslinking, and increase the molecular weight of the polymer; this type of degradation manifests itself as brittleness, gelation and decreased elongation. Chain scission, equations (7) and (10), results in a decrease in molecular weight leading to increased melt flow and reduced tensile strength.

The manner in which various common elastomers degrade is listed in table 1.
Table 1 - degradation of elastomers
Natural rubber Scission (softens)
Polyisoprene Scission (softens)
Polychloroprene Crosslinking and scission (hardens)
SBR Crosslinking and scission (hardens)
NBR Crosslinking (hardens)
BR Crosslinking (hardens)
IIR Scission (softens)
EPM Crosslinking and scission (hardens)
EPDM Crosslinking and scission (hardens)


Inhibition of degradation

Antioxidants do not completely eliminate oxidative degradation, but they markedly retard the rate of autoxidation by interfering with radical propagation. Depending on the types and combinations used, antioxidants can provide suitable polymer protection during the phases of its life cycle.

Two general classifications can be used to categorize antioxidants -- primary (chain terminating) and secondary (peroxide decomposing).

Primary antioxidants

Hindered phenols and secondary aryl amines act as primary antioxidants by donating their reactive hydrogen (N-H, O-H) to free radicals, particularly peroxy radicals, as shown below:

[ROO.sup.*] + AH ROOH + [A.sup.*]

In order to sufficiently terminate the oxidative process, the antioxidant radical ([A.sup.*]) must be rendered stable so as not to continue propagation of new radicals. These radicals, in most cases, are stabilized via their electron delocalization, or resonance.

Phenolics

Hindered phenolics, because of their non-staining qualities, are the most preferred type of primary antioxidant for light colored applications such as footwear. This group can be further categorized into: simple phenolics (1), bisphenolics (2). polyphenolics (3), and thiobisphenolics (4) (figure 1).

Amines

Secondary aryl amines function by hydrogen donation, similar to the phenols; however, at higher temperatures. they are also capable of decomposing peroxides. This single feature, which will be discussed later, eliminates. most times. the need for a secondary antioxidant in combination with an amine antioxidant, which is more so the case with phenolic

Although the amine class of primary antioxidants is usually more effective than the phenols, because of their ability to act both as chain terminators and peroxide decomposers, their use is generally limited to those applications where their discoloring characteristic can be tolerated or masked. The amines are perhaps most used in unsaturated polymers containing carbon black.

Most notable amine antioxidants are those derived from diphenylamine and p-phenylenediamines. Some of the alkylated diphenylamines are less discoloring than the phenylenediamines and find application in plastics. For example, 4,4'bis (alpha, alpha-dimethylbenzyl)diphenylamine is widely used in flexible polyurethane foams and polyamide hot melt adhesives. The p-phenylenediamines are more recognized for their activity as antiozonants.

Secondary antioxidants

This class of antioxidants consists of various trivalent phosphorus and divalent sulfur containing compounds, most notable of which are organo phosphites and thioesters. These antioxidants are also termed preventive stabilizers, because they prevent the proliferation of alkoxy and hydroxy radicals by destroying hydroperoxides.

Phosphites

Phosphites function by reducing hydroperoxides to alcohols, and thus, converting themselves to phosphates.

[[Mathematical Expression Omitted]

Not only are phosphites non-discoloring, but they are color stabilizing, in that they inhibit the formation of the discoloring quinoidal structures of phenolic antioxidants.

The most popular stabilizer is tris(nonylphenyl) phosphite (TNPP). The introduction of TNPP revolutionized the area of non-discoloring rubber stabilization.

A serious drawback of phosphites is their sensitivity to hydrolysis. The commercially available phosphites vary significantly in their resistance to hydrolysis. Several phosphites are available with various additives present to decrease susceptibility to hydrolysis. Hydrolysis of phosphites can ultimately had to the formation of phosphorous acid which can cause corrosion of processing equipment.

Phosphite stabilizers behave synergistically with hindered phenolics and provide good processing protection, and some cases, enhance stability during ultraviolet exposure. They are the recommended secondary antioxidants for use in combination with the primary phenolic antioxidants

Thioesters

Aliphatic esters of B-thiodipropionic acid are highly effective peroxide decomposers for long term heat exposure application when used in combination with phenolics. They are more widely used in thermoplastic polymers, where sulfur will not interfere in a vulcanization process. The major thioester stabilizers are dilauryl thiodiproprionate (DLTDT) and distearyl thiodiproprionate (DSTDP) (ref. 2).

Polymer degradation during mixing

Natural rubber

When we speak of natural rubber, we are concerned with an elastomer in which nature has provided naturally occuring antioxidants or stabilizers. These stabilizers protect the uncured hydrocarbon very effectively. The presence of oxidation inhibitors in the natural polymer has been demonstrated. Early investigators extracted natural rubber with acetone and found that the rate of oxygen taken up in the extracted rubber was measurably increased. Reincorporation of the acetone extract into the extracted polymer restored the oxygen absorption rate to the original low level (figure 2). Similar tests were made with vulcanized rubber (figure3). Removal of acetone extractable material from the vulcanizate lowered the resistance to oxidation. Adding the original acetone extract to the extracted vulcanizate and also to the unextracted vulcanizate, improved the resistance to oxidation considerably, but not as greatly as hydroquinone, a man made antioxidant. It is evident that the natural inhibitor in the acetone extract is a good deal more effective in the raw polymer than in the vulcanizate. This phenomenon is generally recognized, and the addition of more powerful antioxidants for vulcanizate protection is common practice.

Synthetic rubber

In contrast to natural rubber, synthetic polymers require the addition of antioxidants to guard against oxidative degradation. It is a general practice to add the antioxidant stabilizer during the manufacturing process and prior to the drying operation. This practice ensures that the polymer is adequately protected from the heat of the drying equipment. In the case of styrene butadiene rubber (SBR), incidences are known where improperly stabilized polymer has actually ignited in the drying oven.

Simple oven aging tests reveal the effectiveness of oxidation inhibitors in SBR polymer. Temperatures in the range of 100-130[degrees]C are ordinarily used in the aging tests. Within as little as 30 minutes at 130[degrees]C, inadequately protected polymer will show the effects of oxidation by the formation of a melting surface film. Upon cooling, the melted layer resinifies into a hard surface film which breaks easily when the polymer sample is stretched slightly. The time required to form a fracturable film is taken as the "resinification time." Effective antioxidants usually inhibit resinification for several hours at 130[degrees]C, the time depending on the effectiveness of the inhibitor. The initial melting and subsequent surface hardening indicate that both chain scission and cross-linking reactions occur as the polymer is oxidized. Mooney viscosity determinations on polymers heated for varying periods of time reveal some interesting differences in the behavior of natural rubber and SBR.

In figure 4, the Mooney viscosities of natural rubber smoked sheet, pale crepe and of an unstabilized sample of SBR are plotted. It is apparent that the predominant effect of heat on natural rubber is in the direction of lower viscosity (chain scission) whereas with SBR, the predominant effect is that of Mooney viscosity increase (cross-linking). The effects of several antioxidants on the Mooney viscosity curves of SBR are shown in figure 5.

The rising Mooney viscosity curves with unprotected SBR is indicative of an oxidative cross-linking reaction and the formation of higher molecular weight polymer. This theory is confirmed by solubility and gel determinations. Gel is defined as the benzene insoluble portion of the polymer. Table 2 shows the effects of 130[degrees]C oven aging on an oil extended SBR polymer. These data were taken from a study comparing the effectiveness of several stabilizers in controlling gel development in SBR.

[TABULAR DATA 2 OMITTED]

An interesting study was performed by Williams and Carlton on the effect of 350[degrees]F internal mixer mastication and gel build-up in SBR. In table 3, the changes in Mooney viscosity and gel are shown after several periods of mastication at 350[degrees]F. The milled samples were subsequently passed three times through a cold mill which is capable of redispersing at least part of the gel produced in the hot mixing treatment. Redispersible gel is referred to as "loose" gel, whereas gel that remains insoluble in benzene after the cold mill treatment is called "tight" gel. It is interesting to note that after six minutes, considerable loose gel was produced; whereas after nine and twelve minutes a great deal of tight gel was found in the polymer.
Table 3 - effect of 350[degrees]F mixer mastication on
gel formation
Time Mooney % gel (remilled - 3 passes
(min.) viscosity % gel through a cold mill)
 3 55 0 0
 6 52 12 0
 9 47 24 20
 12 36 34 30


Table 4 shows the effect of mixing temperature on gel formation. At lower temperatures little gel was formed. Mixing at 325[degrees]F and above produced appreciable gel, and at higher temperatures the gel became the indispersible tight variety. Williams also studies the effect of gel build-up on the physical properties of vulcanizates prepared from these mixer masticated polymers. The data are given in table 5. At lower mastication temperatures where little gel was produced, the physical properties were best, whereas at higher temperatures, the development of gel resulted in deterioration of the physical properties. With increasing temperatures we see an increase in the modulus and decreases in the tensile and elongation. The DeMattia flexing data are especially interesting. It is apparent that gel in the polymer may have a dramatic effect in lowering the flex cracking resistance of SBR (ref. 3).
Table 4 - effect of mastication on gel formation
12' mixer at temperature indicated
Mastication Mooney % gel
temperature, [degrees]F Viscosity % gel (remilled)
 225 43 0 0
 250 44 0 0
 275 39 0 0
 300 45 0 0
 325 48 23 0
 350 54 34 25
 375 42 41 40


[TABULAR DATA 5 OMITTED]

Preventing polymer degradation

First and foremost, the use of a properly stabilized polymer or an elastomer containing an adequate amount of antioxidant will immensely assist the rubber compounder and/or processor because it will:

* Prevent gel build-up -- which is detrimental to the development of good physical properties and of good flexing resistance as has been shown.

* Prevent viscosity increase or decrease -- which interferes with the processing after mixing. The result will be observed in better handling on calenders, extruders, injection molding machines, etc.

* Prevent discoloration and staining -- especially minimizes yellowing of SBR and NBR compounds which are known to yellow when subjected to high heat during mixing, processing and vulcanization.

Secondly, mixing speeds and temperatures must be kept as low as possible since these factors have been shown to contribute to gel build-up in even properly stabilized elastomers.

Conclusions

The selection of the proper antidegradants for elastomers and rubber compounds should be based upon a careful assessment of the elastomers utilized, mixing and processing conditions, the normal service requirements expected from the derived compounds, and the environment in which the vulcanizates are to perform. Only then can one choose the ideal antioxidant or combination of antidegradants. Since the degradation of rubber proceeds via several mechanisms, it must be emphasized that it is essential to choose and utilize the combination of antidegradants which will inhibit all of the described mechanisms of degradation.

[FIGURE 1 TO 5 ILLUSTRATION OMITTED]

References

[1.] Barnhart, R.R., "Antioxidants and antiozonants," Uniroyal Chemical Company, Inc. 1966. [2.] Paolino, P.R., "Antidegradants," Uniroyal Chemical Company, Inc., April 24, 1989, presented at the 26th Annual Akron Rubber Group Lecture Series. [3.] Hunter, B.A., "Chemical protection against degradation of hydrocarbon polymers," Uniroyal Chemical Company, Inc., 1966.
COPYRIGHT 1995 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1995, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Mazzeo, Russell A.
Publication:Rubber World
Date:Feb 1, 1995
Words:3164
Previous Article:Dynamic properties of rubber.
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