Curing overcuring

There are many ways to measure these properties and some of them are proprietary. However, most laboratory tests are standardized by American Standard Testing Methods (ASTM). Many of them are interactive to various degrees. The rate and state of vulcanization is especially important to consider for components of heavier and thicker tines. The heat used to vulcanize the tine in a mold under pressure requites time to penetrate from both sides of the giant tine to the innermost portions. Securing a balanced state of cure, ie, the maximizing of physical properties in all the components, results in the innermost components having a faster rate of cure. The peripheral compounds should have a cure system which holds its physical properties well when overcured. 

Zinc oxide and stearic acid are used to activate the curing system as well as to preserve cured properties when overcuring, which is curing beyond the point of time and temperature at which maximum properties are obtained. 

Overcuring is encountered to some degree in all compounds of any thickness because of the slow conductance of heat through mbber. Earthmover tires may have tire shoulder gauges in excess of 0.3 m (- 12 in.). The outside/inside are overcured to effect a satisfactory state-of-cure in the tread mbber. 

The cure data show that Perkalink 900 does not affect scorch time or time to optimum cure (Table 14.51). Also, tensile data following overcure (at 150°C and 170°C) and aging at 100°C indicate a trend of improved strength characteristics (Figure 14.20). 

Perkalink 900 is also active in compounds based on blends of NR with the synthetic elastomers SBR and BR. An evaluation in a tank pad formulation has provided evidence of reduced heat buildup on overcure overcure is a common problem in the manufacture of these relatively bulky components. The control and test formulations, in which two levels of the antireversion agent have been evaluated, are fisted in Table 14.52. Cure characteristics are given in Table 14.53. 

The physical properties of automotive enamels are in large part determined by the crosslink structure developed in the paint films during the baking process. Enamels which are not cured sufficiently (undercured) are generally sensitive to humidity and solvents. In addition, they may be prone to chipping and cold cracking. Faints which have been baked excessively (overcured) exhibit intercoat adhesion failure. That is, subsequent coats... 

A section at each end of the platens of a hydraulic press used for curing belting, where the temperature is lower than that of the remainder of the platens. The object is to reduce overcure at the overlapped areas, which would otherwise receive a double cure. 

Maximum productivity is given by the shortest possible time of cure, provided the selected time and temperature do not produce an unacceptable level of defects or early failures in service. Sufficient cure must be given to prevent porosity, since badly undercured rubber will sponge at pressure release. It is better to risk a slight overcure rather than an undercure, since if the curatives are correctly chosen (in type and quantity) the compound should show a plateau effect, which means that a reasonable overcure will not have any marked effect on the physical properties of the product. 

Prolongation of cure beyond that time which gives the optimum cure. An overcure may be accidental due to variation in curing conditions or deliberate as in laboratory determination of curing range, or with the object of enhancing a particular property of a vulcanisate, e.g., compression set. 

The degree of vulcanisation of a rubber compound is assessed technically by the indefinite terms of undercure, correct cure, optimum cure and overcure. It may be given precision by (a) measurement of stress-strain relationship of a range of cures, (b) measurement of the modulus at 100% elongation, (c) measurement of the volume swelling in benzene, or (d) by the use of instruments such as the oscillating disc rheometer and the moving die rheometer. 

Lead monoxide or litharge is mainly used as an accelerator activator, and acid acceptor, and is especially useful in water resistant compounding. Litharge can also be used in long slow cures for such articles as ship s fenders, bridge bearings, etc., where it enables the inside of the product to fully cure before the outside becomes overcured with the possibility of degradation or reversion of the outer layer of the product. 

Special rotary curing units are designed for continuous vulcanisation of V-belts. This method eliminates any areas of overcure and moulded in stresses, which result from methods using presses vulcanising belts in sections. Belt and tension roller pressure release mechanisms allow for easy unloading and loading of the belts. 

In dissimilar rubber/rubber blends, vulcanization ingredients also preferentially end up in only one of the phases, resulting in overcuring of this phase and under-curing of the other one. An example of this effect is the solubility of insoluble sulfur, which is much lower for NBR than for SBR and EPDM. 

Tearing Bad mould design, low hot tear strength of compound, overcure and careless removal of moulded article. Avoid sharp edges, improve compounding technique, cure at optimum level and use simple devices for removal. 

Ethicon, Inc., (Somerville, NJ) vendors Dermabond Topical Skin Adhesive for closing surface or skin type wounds. This material is not intended for internal use. Closure Medical Corporation manufactures the actual adhesive. Ethicon, Inc. is a Johnson Johnson company. Dermabond is chemically 2-octylcyanoacrylate and it cures on contact with water, ethanol and others (i.e., addition polymerization and catalysis by weak bases). For internal tissues, the excessive moisture can overcure cyanoacrylate by saturating its surface with moisture, cause disbondment from the substrate, and render it useless as an adhesive. The cured polymer possesses a hard and brittle consistency. 

Orange-peel (alligator) skin Overcure Part line Term used to denote a typical surface texture caused by the settling of fillers from a batch during casting. The state of cure beyond the state of optimum cure. The surface at which the various pieces of a mold come together when re-assembled. The part lines of a mold determine the flash lines of the casts produced from that mold. 

In certain cases it is desirable to combine the properties of the previously described natural rubber with those of the halobutyls. With straight butyl rubbers this was impossible due to the prolonged cure time of butyl which meant that the natural rubber would be overcured, resulting in an unusable non-homogeneous mix. 

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