Corrosion Of Rubbers

Many involved in the field of corrosion would not regard the deterioration of rubber as a corrosion process. The author's opinion differs from this viewpoint as "rubber undergoes deterioration by interaction with its environment."

General

Rubber and rubber components form an essential part of food processing equipment—-joint rings on pipelines, gaskets on heat exchangers, and plate evaporators. Although natural rubber was the first material to be used for manufacturing these components, nowadays they are made almost exclusively from one of the synthetic rubbers listed in Table 5.

Unlike metals and alloys, which have a strictly defined composition, the constituents used in the formulation of rubbers are rarely stipulated. More often, they will reflect the views and idiosyncrasies of the formulator on how to achieve the desired end product. The important constituents of a rubber are

• Basic Polymer. Largely determines the general chemical properties of the finished product.

• Reinforcing Fillers. These are added to improve the mechanical properties and will invariably be one of the grades of carbon black—or if a white rubber is required, mineral fillers such as clays or calcium silicate.

• Vulcanizing Agents. These cross-link the basic polymer and impart rubberlike properties that are maintained at elevated temperatures.

• Antioxidants. To stabilize the rubber against oxidative degradation, hardening or softening, after prolonged operating periods at elevated temperatures.

• Processing Aids. Which facilitate the molding of the rubber.

• Plasticizers. To modify the mechanical properties.

A complicating factor to be considered when formulating rubber for food-contact surfaces is the acceptability (or nonacceptability) of the compounding ingredients. Some countries, notably Germany and the USA, have drawn up lists of permitted ingredients (22,23), while other countries regulate the amount of material that can be extracted from the finished article by various test media.

Invoking these regulations may impose limits on the inservice performance of a rubber component, which could be a compromise, exhibiting desirable properties inferior to those achievable were it for a nonfood application. For example, the resistance to high pressure steam of some rubbers can be enhanced by using lead oxide as an ingredient. Obviously, such materials could not be contemplated for any food contact application.

Table 6. Performance of Rubbers in Some Environments Found in the Food and Beverage Industries

Whole milk Beer, wines, spirits Fats, oils, cream Sauces

Salad dressings Fruit drinks and juicesd

SBR Medium nitrile Natural rubber Polychloroprene Butyl EPDM Silicone"

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