Compounding Crosslinked Elastomers

Handbook for the Chemical Analysis of Plastic and Polymer Additives 

Internal mixers must be ran in a full or nearly full condition, so a batch recipe is calculated to provide an appropriate volume. If not filled, the ingredients will not be properly sheared and heat transfer will be compromised. Typical commercial mixers have a batch size of at least 100 pounds of compound. A mixer of this size will have a drive motor of no less than 75 horsepower. Proper dispersive mixing is a balance between proper shear, sequence of addition of ingredients, and thermal stability. Mixers have extensive monitoring instrumentation that provides continuous feedback about thermal conditions, rotor torque, and rotor speed. Once a mixing process has been developed, a standard protocol is followed for preparation of the compound. 

A two-roll mill consists of two large diameter counter-rotating cylinders, with a variable gap between them. The rolls are maintained at a fixed temperature through internal fluid heating and an external heat exchanger. The elastomer compound forms a sheet on the roll surface. This is continuously cut into wide strips by the operator, folded over, and placed back on the rolls. This process is repeated for perhaps 15 minutes to assure that the compound is fully homogenized. 

In some cases, the curing agent is added during the milling operation, generally in the form of a master batch. This practice is used for peroxides that decompose at a lower temperature, to reduce the risk of accidental curing in the internal mixer, when localized temperature can be very high. 

With a closed mixer, control of distributive mixing is inherently good, since all materials are added in a single batch and mixing occurs in two stages. Failure to add an ingredient or to improperly treat raw materials may lead to dispersion and homogeneity problems. 

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This chapter addresses three basic classes of polymers and the approaches for processing them into compounds. These classes include thermoplastic polymers, and two types of elastomers -crosslinked elastomers, and thermoplastic elastomers. Compounds prepared from each class have a range of achievable properties, and each category of compounds may have overlapping properties. Each category is prepared by different technical approaches with varying controls, energy requirements, and limitations. A brief definition of each class follows. Also included, later in the chapter, is a detailed description of how additives influence the production process. 

The swelhng method may not determine solubility parameters effectively if the rigid crosslinked samples tend to chip or split on swelling. The method can be used for polymers with a physical network that contains knots formed by polymer crystalhtes. Samples of unplasticized PVC do not swell in ester plasticizers, but if the same polymer is hot-compounded with dioctyl phthalate and cooled, it can swell to equilibrium as does the amorphous crosslinked elastomer. Note that such swelling of PVC depends on the thermal treatment history of samples. 

Fig. 13 (a) Stracture of the polyacrylate-based side-chain LC compound, (b) Compaiisrai of the asymptotic diffraction intensity tails from the crosslinked elastomer circles slope —2.40) and the corresponding uncrosslinked homopolymta- triangles slope -1.85) [138]... 

A large volume of literature exists for PO blends with easily crosslinkable elastomers, but the information on radiation processing and degradation of HTPB is scarce. For dimensionally recoverable applications a blend of LCP with PVDF or PE was radiation crosslinked. Similarly, PSF or PES was blended with water-soluble PVP and crosslinked to make it water insoluble for medical or food applications. For the use in cable jackets and heat-shrinkable applications PVDF or a copolymer of tetrafluoroethylene and ethylene (ETFE) could be compounded with a thermoplastic elastomer, formed and radiation crosslinked. Adjusting composition and irradiation dose produced a series of materials with good balance of tensile strength and elongation. ... 

The influence of stearic acid methyl ester on crosslinking of elastomer compounds and elastomer properties has been studied. The purpose of the research is to recognise the possibility of application of stearic acid esters which have been obtained according to Polish technology from waste technical fats as the crosslinking activator of diene elastomers. 7 refs. 

API has launched Apifive, a new line of crosslinked, expanded EVAs that, unlike conventional EVAs, have a matt finish. API has developed these new products by introducing a series of elastomers into the EVAs, it is briefly reported. The compounds are especially suitable for insoles, where the requirements are for matt, very soft and dimensionally constant parts, and for quality soles for mounting or milling. 

The synthesis of PU can be carried out by the reaction described in Eq. (2.24). If the functionality of the hydroxy-containing compounds or the isocyanate is increased beyond 2, branched and possibly crosslinked polymers are produced. Because the nature of the polyol (polyether, polyester, polybutadiene, etc.) and isocyanate components can vary widely, PU are among the most versatile polymers, producing a wide variety of materials such as elastomers, foams, coatings, adhesives, or fibers. 

Raman and IR spectroscopic studies dealing with the qualitative and/or quantitative determination of rubber compounding ingredients, i.e., the elastomer itself [22, 26-31], fillers [32, 33], vulcanisation chemicals and other additives [34-37], are not included here. The same applies to studies dealing with the crosslinking of elastomers by means of chemicals other than sulfur or peroxide [38-41], self-crosslinking of elastomers blends [42-44], crystallisation (strain-induced) [45-48] and oxidation/ageing [49-53]. 

Co-agents are multi-unsaturated compounds, which are used in the peroxide-curing of elastomers. When classical co-agents, such as triallylcyanurate (TAC), trimethylolpropanetrimethacrylate (TRIM) or diallylterephthalate (DATP), are added, the crosslinking efficiency is enhanced [98-102]. Various mechanisms for the increase of the crosslinking efficiency have been proposed. In all cases a fast reaction between the... 

Apart from the effect on the crosslinking efficiency, the use of co-agents in peroxidecuring also imparts the molecular structure of crosslinks. It has been reported that coagents with two or more unsaturated moieties can be incorporated as individual molecules between two elastomer strands to form crosslinks [103-109]. In this way the crosslink structure of peroxide-cured elastomers can be altered. Thus, apart from the expected benefits, such as improved crosslinking efficiency, decreased compound viscosity and faster cure, the use of co-agents may also provide a tool for manipulating mechanical properties. 

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