Rubber contact conditions

The essential material property of rubbers is their low elastic modulus, which ensures that the contact deformation remains elastic over a very wide range of contact conditions. The abrasive wear of rubbers is due to either fatigue of the material or tearing by a cutting force from impacts with sharp-edged particles. 

In contrast to plastics, rubbers are rarely used in the packaging of food products. Exceptions to this rule are the use of rubber in flip top seals on beer bottles and the seal that is present in food cans. However, in the processing of food, there are a number of situations where significant contact with rubber products can occur. This is due to the fact that the unique properties of rubber lead to it being used in a wide range of products (see Table 12.1). It is also the case that the range of contact conditions encountered (i.e. food type, contact temperature, time and area) mean that a wide variety of rubber compounds are employed (see section 12.2). 

Variations in contact conditions have an important effect on the potential of chemical species to migrate from the rubber components into the foodstuff. 

Contact conditions add even more difficulty and complexity to an already very complex and difficult analysis of rubber products and tires. Contact conditions are unilateral and need to be constantly checked during the incremental nonlinear analysis. In addition, they are not smooth, thus degrading the performance of nonlinear solvers. A number of numerical regularization parameters need to be introduced to prevent chattering and ensure robustness of a finite element analysis (FEA) with frictional contact. 

Surface friction is a source of rail s advantages in transport energy efficiency. Under similar conditions, steel wheels on steel rail generate only abont 20 to 30 percent of the rolling friction that rubber wheels on pavement generate, both because rails are much smoother than paT. cnicnt and because steel wheels arc much more rigid than rubber tires, so steel wheels deform much less at the point of contact with the gi ound. Each rail wheel has only about 0.3 sq in of surface in contact with the rail, whereas an automobile... 

Substances, which after very limited exposure cause death or major irreversible effects, even if prompt medical treatment is undertaken. Also included in this category are substances that easily go through protective rubber clothing and those that, under normal conditions or in a fire, release extremely dangerous vapour, whether they are toxic or corrosive by inhalation or skin contact. 

Sodium chlorite reacts very violently with organic compounds of divalent sulfur, or with free sulfur (which may ignite), even in presence of water. Contact of the chlorite with rubber vulcanised with sulfur or a divalent sulfur compound should therefore be avoided [1]. Application of factorial design techniques to experimental planning gave specific conditions for the safe oxidation of organic sulfides to sulfoxides using sodium chlorite or calcium hypochlorite [2],... 

Exterior surface corrosion or rusting of pipes occurs by the formation of iron oxides. Painting to an appropriate specification will significantly extend the period to the onset of corrosion, but the durability of the paint finish is largely dependent on the quality of the surface preparation as well as the thickness of the coated film. Improperly installed insulation can provide ideal conditions for corrosion and should be weatherproofed or otherwise protected from moisture and spills to avoid contact of the wet material on equipment surfaces. Application of an impervious coating such as bitumen to the exterior of the pipes is beneficial in some circumstances. Hypalon and neoprene rubber-based anticorrosive coatings admixed with chlorinated rubber are finding use in many installations. 

Generally, in any wear process more than one mechanism is involved although one mechanism may predominate. The mechanism, and hence the rate of wear, can change with change of conditions such as contact pressure, speed and temperature. The most important consideration in practice is that the wear process will be complex and critically dependent on the service conditions. It is, therefore, necessary that any laboratory test must essentially reproduce the service conditions if good correlation is to be obtained. Even a comparison between two rubbers may be invalid if the predominant wear process in the test is different from that in service. It is failure fully to appreciate this which has led to the conclusion that all laboratory abrasion tests are useless except for quality control. 

It is immediately apparent that in many processes involving rubber heat flow across the interface between two surfaces has to be considered. This is true in mixing, moulding, cooling after processing and conditioning of test pieces but, nevertheless, very little attention has been paid to the measurement of the coefficient. The effect of the heat transfer coefficient on net heat flow is greatest with thin articles and where one of the materials is a gas. It is probably reasonable to assume a value of infinity for the transfer coefficient when rubber is pressed into intimate contact with a metal but in other cases it will be finite. 

The method employed is essentially the same as that first proposed by Thiele and Lachman1 but has been modified in several details, the most important change involving the manner of extracting2 the nitramide from aqueous solution. The purity and yields are markedly affected by slight changes in procedure consequently exact directions are given in considerable detail. Potassium nitrocarbamate and nitramide are best made under conditions of low temperature and low humidity. Contact with cork and rubber should be avoided wherever possible. 

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