Cured rubber product

Scheme 2.5 Cured rubber products. After Chu [73]. Reproduced by permission of Huthig GmbH...

Rubber-based products permeate our lives, forming part of the many materials used for personal, domestic and industrial purposes. Rubber may be natural, synthetic or a mixture of the two. Since the vast majority of rubberized materials are unlabeled, it is difficult to determine whether a product contains natural or synthetic rubber. The overlap between rubber and plastic further complicates the matter, especially since plastics contain many of the same catalysts, stabilizers, antioxidants and pig-ments/dyes that are present in rubber products. Fregert (1981) listed a number of naphthylamines, substituted para-phenylenediamines, alkylphenols and hydroquinone derivatives, which are utilized in the manufacturing of both rubber and plastic. Although completely cured plastics are rare sensitizers, fully cured rubber products produce allergic reactions as the sensitizers in rubber can leach out or bloom over time. 

The rubber s polymer network allows elasticity and flexibility to be combined with crystallization-induced strength and toughness when stretched. The elastic nature of this network also accoimts for the exceptional resilience of cured rubber products. This resilience means less kinetic energy is lost as heat during repeated stress deformation. Products made from natural rubber are less likely than most other elastomers to fail from excessive heat buildup or fatigue when exposed to severe dynamic conditions. This has secured the place of natural rubber as the preferred sidewall elastomer in radial tires. 

Cure Characteristics. Methods of natural rubber production and raw material properties vary from factory to factory and area to area. Consequentiy, the cure characteristics of natural mbber can vary, even within a particular grade. Factors such as maturation, method and pH of coagulation, preservatives, dry mbber content and viscosity-stabilizing agents, eg, hydroxylamine-neutral sulfate, influence the cure characteristics of natural mbber. Therefore the consistency of cure for different grades of mbber is determined from compounds mixed to the ACSl formulation (27). The ACSl formulation is as follows natural mbber, 100 stearic acid, 0.5 zinc oxide, 6.0 sulfur, 3.5 and 2-mercaptobenzothiazole (MBT), 0.5. 

In the vulcanisation of rubber products, aftercure is the amount of cure received after the termination of the cure proper, e.g., the cure effect resulting from the heat remaining in the product after removal from the mould or autoclave. The term is also applied to the continuation of the curing effect that results from exposure of the article to heat in use, or from accelerated ageing. See Post Cure. 

In the moulding of rubber products, the operation of opening and closing the press rapidly in the early stages of the cure to drive out any trapped air. 

Any method of vulcanising rubber products which proceeds without interruption from start to finish as compared to the method of vulcanising separate batches of products or sections of a product. Continuous vulcanisation processes include the cold curing of proofed cloth, the vulcanisation of belting and flooring, of cables and certain extruded products by either the Liquid Curing Medium, Fluid Bed, Microwave, or Hot Air techniques. 

Any specially shaped device on which a rubber product may be assembled, on which a product may be fitted to bring it nearer the cured shape or on which an article may be produced by dipping Formic Acid... 

The amount of sulphur which has not combined with the rubber during the vulcanisation reaction. The determination of the free sulphur content is widely used in assessing the state of cure of a rubber product. The addition of the free sulphur curve to the modulus and tensile/time curves of the particular compound enables the state of cure of the vulcanised product to be estimated. 

The method of vulcanising those rubber products which do not require the precision of shape given by a mould. The heat for curing is supplied by steam circulating openly in an autoclave, the products being supported on mandrels or embedded in chalk. See Moulding. 

The best cure for a rubber product is always a compromise, but optimum cure may be defined as that time of cure necessary to bring a preselected property of the vulcanisate to near maximum (or minimum) value, at the same time ensuring that the other properties are satisfactory. All the physical properties of a rubber do not reach their optimum values at the same time of cure, and therefore the time must be selected so that the properties are near their optima, the most weight being given to the property considered most important. 

Water in the gaseous state it is a convenient source of heat widely used in the curing of rubber products. See Open-Steam Curing, Press Curing. Steam is really a colourless gas and what is usually termed steam (issuing from an autoclave, a press, thermometer bleed pipe, etc.) is really droplets of water which result from the steam meeting the cooler air of the atmosphere. Steam Trap... 

Classed as ultra accelerators, xanthates are among the fastest of accelerators available to the rubber compounder. Their speed is such that they find only limited application in solid rubber product manufacture but they are used in low temperature curing of latex articles. 

Residues of fatty acids from emulsion polymerisation and from cure activation provide sites for bacterial attack when the rubber product is exposed to warm moist conditions. The addition of a biocide/fimgicide will give excellent fungal growth protection. 

Migration of nitrosamines into consumer products can occur via direct contact of materials such as waxed containers, elastic and rubber etc.81. Morpholine is used extensively as an industrial solvent for wax formulations. The wax formulations are used for coating fruits and vegetables to prevent moisture loss and increase shelf-life of the products. Paper and cardboard packed with morpholine was also found to give rise to NDMA, as these packaging materials were found to be contaminated with NDMA as well. Besides this, rubber products also provided a migratory source for both nitrosamines and nitros-able amine precursors, as trace levels of NDEA and TV-nitrosodibutylamine (NDBA) have been reported in cured meats with amine-based accelerators in the rubber nettings82. 

Polymer products Adhesives, adhesive tapes, sealants, latex emulsions, rubber materials, plastic fabrication, etc. Composition monitoring Rate of cure monitoring Product QC... 

In later stages, the technology developed and rubbers were compounded, mixed in mills, calendered in calenders, extruded in extruders, molded in hydraulic presses, cured in autoclaves and many hand operations of forming and curing became possible. State-of the art techniques of compounding were developed to produce rubber products of any shape and dimensions for many requirements. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

All rubber products exhibit shrinkage after cure, mainly due to the thermal expansion which occurs at vulcanization temperature. Moulded rubber goods are never as big as the moulds in which they are cured. The difference between the dimensions at room temperature of the finished goods and of the mould expressed as a percentage is called the shrinkage from mould dimensions. 

Processing variables can affect to a very great extent the results obtained on the rubber product or test piece and, in fact, a great number of physical tests are carried out in order to detect the result of these variables, for example state of cure and dispersion. In a great many cases, tests are made on the factory prepared mix or the final product as it is received but, where the experiment involves the laboratory preparation of compounds and their moulding, it is sensible to have standard procedures to help reduce as far a possible sources of variability. Such procedures are provided by ISO 2393 which covers both mills and internal mixers of the Banbury or Intermix type, and also procedures for compression moulding. 

The various direct methods of estimating dispersion are essentially dimensional measurements on more or less a microscopic scale and this is just one example of the value of microscopy for fault diagnosis in rubber products. Dispersion measurements are normally made on cured rubber although it is possible to prepare test pieces from some uncured materials. 

NDPHA, A-nitrosoatrazine, A-nitrosobenzylphenylamine, A-nitrosocarbazole, and A-nitroso-carbaryl in foods. In another study, Sen et al. (67) used a normal-phase system for the determination of NDBZA in cured pork products packaged in elastic rubber nettings (Fig. 4). As can be seen from the figure, appropriate solvent programming allowed the simultaneous determination of six nonpolar and three polar NOC. 

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