Nr

°R—Little effect on the rubber. '(?)—Possibly some degradation.

CNR—Not recommended as significant degradation may occur.

°R—Little effect on the rubber. '(?)—Possibly some degradation.

CNR—Not recommended as significant degradation may occur.

Corrosion by Rubber

The majority of rubbers and formulating ingredients have no effect on stainless steels even under conditions of high temperature and moisture. There are two notable exceptions, namely, polychloroprene and chlorosulfonated polyethylene. Both of these contain chlorine, which under the influence of temperature and moisture, undergo hydrolysis to produce small quantities of hydrochloric acid. In contact with stainless steel, this represents a serious corrosion hazard causing the three main forms of attack.

When specifying a rubber component, it is easy to avoid these two polymers but a fact not often appreciated is that many of the rubber adhesives are produced from one of these polymers. That is the reason why manufacturers of heat exchangers and other equipment, which necessitates sticking the rubber onto metal, specify what type of adhesive should be used. Many DIY adhesives and contact adhesives are formulated from polychloroprene, and it is not unknown for a maintenance engineer having to stick gaskets in a heat exchanger to get the supply of rubber cement from the local hardware shop. The results can be catastrophic, as shown in Figure 23. Similarly, many self adhesive tapes used as a polychloroprene-based adhesive and direct contact of these with stainless steels should be avoided.

Corrosion of Rubber by Environments

From the standpoint of food processing, the environments likely to interact with rubber are classified under the following headings:

• Foodstuffs containing no fat or a low level of fat, eg, milk

• Fatty products: butter, cream, cooking oils, shortenings

• Alkaline detergents

• Acid detergents

• Sanitizing agents

Unlike the corrosion of metals, which is associated with oxidation and loss of metal, rubber deterioration usually takes other forms. When a rubber is immersed in a liquid, it absorbs that liquid or substances present in it to a greater or lesser degree. The amount of absorption determines whether the rubber is compatible with the environment. The absorption will be accompanied by changes in mass, volume, hardness, and tensile strength. For example, immersing and oil resistant rubber in vegetable oil may produce a change in volume of only 2-3%, whereas a non-oil-resistant rubber may swell by 150% or more. Such a volumetric change will be accompanied by a large reduction in the tensile strength and a high degree of softening.

Broadly, speaking, a rubber should not exhibit a volumetric or weight change greater than 10% nor a hardness change of more than 10 degrees (International Rubber Hardness Degrees—IRHD or Shore A) to be classed as compatible. For general guidance, data presented in Tables 6 and 7 (19) indicate the compatibility of rubbers with some food industry environments. But for more information, the reader should refer to one of the national or international test procedures (24-26).

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