Elastomer Compounding

Raw elastomeric polymers typically contain an antioxidant that is added at the time of manufacture to provide thermal stability for compounding, milling, and subsequent operations to produce beads, bales, or other forms where the polymer is first heated, then cooled into an intermediate form. The as-provided antioxidant may provide sufficient stability to protect an elastomeric compound through final forming and curing. 

External antioxidants are often added to provide additional protection for elastomer compounds, especially during mixing and curing operations where temperatures will be highest. Antioxidants are available in many forms and they vary in functionality, according to their composition and structure. Typical antioxidants for elastomers include a range of phenolics, amines, and bisphenol derivatives. 

Elastomers must be crosslinked to hold their final form. The crosslinking reaction takes place through generation of free radicals that promote bonding at sites of unsaturation. The most common crosslinking agents for this include reactive peroxides, such as dicumyl peroxide, diacetyl peroxide, di-tert butyl peroxide, and others. Since each has a different temperature at which thermal decomposition initiates, curing conditions vary with the peroxide type. 

For some products, elemental sulfur is added to the compound as a curing agent. Sulfur reacts to form C-S-C bonds. This is an inexpensive reaction scheme and it is useful for production of mbber compounds across a very wide hardness range. Residual sulfur compounds lead to odors and acid products that are not well suited to applications where clean materials are desired. 

The crosslinking reaction rate may be too slow for some commercial processes and the reaction may exceed the oxidation resistance time for the elastomer compound. In such cases, curing accelerators are used with the sulfur-curing process. Zinc oxide is a commonly used accelerator, however thioureas, hexamethylenetetramine, and others are effective. For organic peroxides, the cure rate can be greatly increased with an increase in applied temperature, though oxides of zinc, 

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Vulcani2ation is a chemical process for improving an elastomer compound s performance. However, in most cases not all of the desired properties reach their optimum levels simultaneously. One of the mbber compounder s key responsibiHties is to achieve a balance of the most important property requirements by the proper selection of cure system (chemical) and time—temperature cure cycle (physical). 

Fig. 2. Tensile strength retention, continuous service, for fluorocarbon elastomers. Compound I (see Table 4).

Chemical Resistance. Fluorocarbon elastomer compounds show excellent resistance to automotive fuels and oils, hydrocarbon solvents, aircraft fuels and oils, hydrauHc fluids, and certain chlorinated solvents, and may be used without reservation. 

Compounding. Owing to the number of ingredients required in a conventional mbber recipe, fluorocarbon elastomer compounding seems simple compared to typical hydrocarbon elastomer recipes. However, the apparent simplicity of such formulations makes a selection of appropriate... 

Upaeglis A. and O Shea F.X., Thermoplastic elastomer compounds from sulfonated EPDM ionomers. Rubber Chem. TechnoL, 61, 223, 1988. 

Previous studies have demonstrated that QDI improves the formation of bound rubber. From the previous discussion of free-radical chemistry, the formation of bound mbber in butadiene elastomer compounds would be expected to occur at a higher rate than in NR. 

Process and extender oils function as physical lubricants in elastomer compounds. The oils used in elastomer compounding serve three main purposes [26] ... 

Leyden and Rabb [68] have illustrated the rubber formula reconstruction process. Table 2.8 shows some of the difficulties encountered in the reconstruction of a complex wire and cable jacket compound. Forrest [38] has illustrated the reverse engineering of the reasonably complex highly flame retardant elastomer compound of... 

Products made from fluorocarbon elastomers are cured (vulcanized) typically at temperatures from 170 to 220°C (338 to 428°F). However, to achieve optimum properties, postcure is required. Standard postcure conditions are 18 to 24 hours at 220 to 250°C (428 to 482°F).39 Figure 5.339 illustrates the effects of postcure at different temperatures on tensile strength and compression set of a carbon black-filled fluorocarbon elastomer compound.39... 

Commercial fluorosilicone elastomer compounds are made from high-molecular-weight PMTFPS (MW is typically 0.8 to 2.0 million) and are cross-linked by organic... 

The thermal characterisation of elastomers has recently been reviewed by Sircar [28] from which it appears that DSC followed by TG/DTG are the most popular thermal analysis techniques for elastomer applications. The TG/differential thermal gravimetry (DTG) method remains the method of choice for compositional analysis of uncured and cured elastomer compounds. Sircar s comprehensive review [28] was based on single thermal methods (TG, DSC, differential thermal analysis (DTA), thermomechanical analysis (TMA), DMA) and excluded combined (TG-DSC, TG-DTA) and simultaneous (TG-fourier transform infrared (TG-FTIR), TG-mass spectroscopy (TG-MS)) techniques. In this chapter the emphasis is on those multiple and hyphenated thermogravimetric analysis techniques which have had an impact on the characterisation of elastomers. The review is based mainly on Chemical Abstracts records corresponding to the keywords elastomers, thermogravimetry, differential scanning calorimetry, differential thermal analysis, infrared and mass spectrometry over the period 1979-1999. Table 1.1 contains the references to the various combined techniques. 

For comparison, tests were also run with the same equipment using a single frequency sinusoidal input, repeated at different frequencies over the range of interest. All comparative tests were run at 25°C. To further check the final results, the properties of the elastomer compounds were also measured using a Rheovibron. 

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. 

Elastomer compounds can be plasticized by addition of organic compounds. Elastomer compounds are inherently flexible and selection of a base polymer on the basis of molecular weight characteristics, chemical composition, and degree of crystallinity serves as the basis for the properties of the compound from which an elastomer is made. Oils are the most common plasticizer for elastomers. Oils of paraffinic structure or aromatic structure can be used with elastomers in which they are compatible. Paraffin wax would also be included in this category. Other plasticizers include phthalic acid esters and adipic acid esters. Fatty acids can be used as plasticizers but these contribute to an increase in surface tack of elastomer compounds. Examples include stearic and palmitic acid. Plasticizer addition has the added benefit of aiding with incorporation of inorganic materials. 

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. 

Prior to crosslinking, softer grades of elastomer compounds flow slowly, so these cannot be produced in a form that is unconstrained. A common intermediate form of such compounds is long, continuous ribbons that are dusted with a release agent and coiled. Sheets or other forms can also be hot-pressed, dusted, and shipped with release packaging that helps to separate the layers for subsequent processing. In still other cases, the elastomer compound is wrapped into polyethylene... 

Santoflex 77PD is used as an antiozonant in natural and synthetic elastomer compounds that can tolerate discoloration (it will discolor compounds and cause severe contact and migration staining). It also protects against catalytic degradation by copper and other heavy metals. 

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