Natural rubber degradation resistance

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Neoprene is the generic name for polychloroprene rubber. It has been produced commercially since 1931 and had rapid and wide acceptance because it is much superior to natural rubber for heat and oil resistance. Heat resistance is far better than NR, BR or SBR. but less than EPDM. When heated in the absence of air, neoprene withstands degradation better than other elastomers which are normally considered more heat resistant, and retains its properties fifteen times longer than in the presence of air. Compression set at higher temperature is better than natural rubber and 100°C is typically the test temperature rather than 70°C. Abrasion resistance is not as good as natural rubber but generally better than most heat resistant and oil resistant rubbers. This is also true for tear strength and flex resistance. 

Fair to good aging Cured products often will outperform natural rubber products, but uncured products made from emulsion polymers can degrade rather quickly if not compounded for UV, ozone, and oxygen resistance. Block copolymers are typically better in the uncured state than standard SBR, but still require protectant additives for extended exposure conditions. 

Chain scission The polymer chain breaks, causing a softening of the compound and decreased abrasion resistance. Natural rubber tends to show such degradation. 

Limitations Does not bond well to natural rubber or butyl rubber Strength characteristics poor tendency to creep, lack of tack requires a tackifier for use in adhesives Poor resistance to hydrolytic degradation (reversion), even in the polyether type... 

The more serious cause of deterioration in rubbers is its reaction with atmospheric oxygen. This is possible because rubber is a diene polymer and some, such as natural rubber, EPDM, SBR, nitrile rubber, and butyl rubber, have olefinic double bonds in their structure. Much research work is being done on the oxidative degradation of unvulcanized rubbers, but this is not relevant to the resistance of vulcanized rubbers in storage or in service as their aging behaviors differ widely. Unvulcanized rubber compound has to be vulcanized in order to produce usable products. The nature of the cross-link produced varies considerably, and this can affect the balance of chemical and particularly of physical properties of the vulcanizates. 

These fillers are added to natural rubber to slightly improve the resistance to traces of oil and to stabilize the matrix against thermal degradation by friction. It typically has durometer hardness 55 Shore hardness A. Its temperature limit is 82°C (180°F). The tip speed of the impeller can be a maximum of 28 m/s (5500 ft/sec). 

Air aging for 48 hours at 100 C. does not greatly affect the room temperature properties of the TMO-AGE vulcanizate (Table VII). These conditions would seriously degrade the properties of natural rubber and thus clearly show that TTW elastomers, like PO elastomers, are superior in heat aging to natural rubber. Unfortunately we did not make an extended air aging study on the TMO vulcanizate to determine its long term oxidation resistance. One... 

Expansion joints for a central heating system made from a natural rubber/polychloroprene blend severely degraded due to metal ions catalysing thermo-oxidation. It is well known that many rubbers are susceptible to this type of degradation and it is a distinct possibility that a water supply will contain metal oxides. The material had been chosen on the basis of cost and resistance to water at 85 °C and there had been no direct contact between the supplier and end customer. 

Styrene-butadiene rubber, similar to natural rubber, is only slightly resistant to light attack and in particular to photo-oxidative degradation. In contrast to natural rubber, crosslinking is dominant [32]. 

In elastomers both temperature alone (e.g., in natural rubber) and temperature in combination with oxygen can lead to degradation (depending on the type of rubber). In particular for elastomers in engineering applications this is often even more important than their ozone resistance, see Section 5.2.2.13. 

Natural rubber is very resistant to water, acids, and alkalis, but is attacked by oxidants and solvents. Some metallic salts accelerate oxidative degradation [32]. 

Most pressure-sensitive masscoats contain a blend of elastomers—natural rubber, reclaim and SBR—with tackifiers of low or medium molecular weight, antioxidants, etc. These are applied to the web-tape or label backing from solutions but the newer thermoplastic elastomers —block copolymers of styrene with is-oprene or butadiene—can be applied from melt. Where excellent color and resistance to light and oxidation are needed, the higher priced acrylic ester copolymers are preferred. Polyisobutylene, also resistant to ultraviolet degradation, is utilized for removable labels. 

Table 15.4 hsts properties and applications of common elastomers these properties are typical and depend on the degree of vulcanization and on whether any reinforcement is used. Natural rubber is stiU used to a large degree because it has an outstanding combination of desirable properties. However, the most important synthetic elastomer is SBR, which is used predominantly in automobile tires, reinforced with carbon black. NBR, which is highly resistant to degradation and swelling, is another common synthetic elastomer. 

Although natural rubber has satisfactory properties in many applications there are now many synthetic alternatives available. Natural rubber is rather prone to chemical degradation by ozone and has poor resistance to solvents. Synthetic rubbers are usually better in these respects. The most important use of rubbers is in motor vehicle tyres in which styrene-butadiene rubber (a random copolymer, SBR), is mainly used along with a variety of additives such as carbon black. This reinforces the rubber and improves the strength, stiffness and abrasion resistance. 

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