The Curing System

The selection of chemicals used for crosslinking the rubber has to be made based on the chemical reactivity of the polymer, and the stability and type of crosslink that will perform best in the end product. A number of basic types are used but within these categories the complexity of chemicals in use may be high. 

This type of curing system is used for those rubbers containing unsaturation, which is 

Sulfur-based systems are the most frequently used system for several reasons  

The polymers using this system form the bulk of rubbers in use. 

The system is extremely flexible due to the diversity of materials available for use. This allows considerable scope for engineering the system to the needs of both production and performance requirements. 

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Another cure system consideration is the compound scorch behavior. Prior to vulcanisation, mbber is plastic-like and can be processed iato desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processiag depends largely on the cure system and is referred to as the scorch time. If a compound cures prematurely duriag the processiag step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanisation step is to minimise premature vulcanisation or scorch (Fig. 4). 

Natural mbber usually contains sufficient levels of naturally occurring fatty acids to solubilize the zinc salt. However, if these fatty acids are first extracted by acetone, the resultant "clean" natural mbber exhibits a much lower state of cure. Therefore, to ensure consistent cure rate, fatty acids are usually added. Synthetic mbbers, especially the solution polymers, do not contain fatty acids and requite thein addition to the cure system. 

Other ingredients besides the elastomer and the cure system itself influence cure and scorch behavior. Usually the effect of a material on cure is pH-dependent. Ingredients which are basic in nature tend to accelerate the rate of both scorch and cure, whereas acidic materials exhibit the opposite effect. 

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. 

Sihcone polymer plasticizers have historically been used in many formulations. These plasticizers (qv) are of the same Si—O backbone as the functional polymers but generally are terrninated with trimethyl groups which are unreactive to the cure system. This nonreactivity means that, if improperly used, the plasticizer can migrate from the sealant and stain certain substrates. Staining has been a widely pubHcized flaw of sihcone sealants, but the potential of a formulation to stain a substrate can be minimized or eliminated with proper formulation work. In general, this is accompHshed by not using plasticizers for formulations developed for stain-sensitive substrates. 

Long-term compression set resistance is described in Figure 4. Lower set values are achievable by use of higher viscosity gumstock at comparable cross-link densities. Compression set resistance is also very dependent on the cure system chosen. The bisphenol cure system offers the best compression set resistance available today, as shown in Table 5. 

Whilst the properties of the cross-linked resins depend very greatly on the curing system used and on the type of resin, the most characteristic properties of commercial materials are their toughness, low shrinkage on cure, high adhesion to many substrates, good alkali resistance and versatility in formulation. 

Progressive replacement of amine hardener by a low-viscosity flexibiliser will reduce mix viscosity, increase pot life and reduce the heat distortion temperature of the cured system. Higher impaet strengths are achieved using approximately equivalent amounts of hardener and flexibiliser. 

Other flame retardants and/or smoke suppressants can also be used such as magnesium hydroxide, magnesium carbonate, magnesium-zinc complexes and some tin-zinc compositions. Zinc oxide is a common ingredient in many rubber base formulations used as part of the curing system. At the same time, the action of zinc oxide is similar to that of antimony trioxide, but less effective. 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

The selection of the cure system in these applications is directed by constraints such as location of the adhesive in terms of confined space, speed and depth of cure, etc. The volumes of silicones typically applied are relatively small. In general, the uncured adhesive needs to be dispensed in a well-defined and limited area, and needs to stay in place without flowing during cure. No by-products of the cure reaction are acceptable as they may contaminate other sensitive areas of the devices. These constraints often direct the choice to the platinum-catalyzed hydrosilylation cure system that is relatively expensive. 

For making compatible blends, the polymers should have comparable polarities and viscosities. The oil needs to be selected properly so that its solubility parameter is close to those for blend components. The cure system should be efficient for all constituent rubbers and the filler system needs to be appropriate. Finally, cost consideration should be taken into account to provide a commercially viable product. 

The curing systems used to vulcanize specialty elastomers such as EPDM, CR, HR, and NBR are different than those used to cure NR, SBR, BR, and its blends. The former elastomers are less unsaturated and therefore need high ratio of accelerator to sulfur. 

An NR-rich undertread layer can enhance the adhesion between belt or cap-ply and tread whilst a thicker subtread compound may be included to offer some additional benefits of low hysteresis for car tires and low heat generation for truck tires within the bulk of a thick section. The cure system needs better flexibility and low heat generation. Typically the cure system will be based on CV/SEV. Tread base is generally having a composition as depicted in Table 14.40. 

The cure system is shown in formulation as tabulated in Table 14.45. ... 

Gas chromatography-mass spectroscopy is used to obtain extensive compositional information on rubbers. It can identify minor components of the sample such as the breakdown products of the cure system and the antidegradants. 

The most common polyfunctional alcohol used is pentaerythritol, but a base is also required to complete the cure system, magnesium and calcium oxide giving more controlled cure rates than stronger bases. 

Silica fillers also react with the rubber causing an increase in viscosity and dry and unmanageable processing behaviour. Filler activators need to be added to silica-reinforced compounds to overcome these problems. The usual filler activators used are diethylene glycol, polyethylene glycol and amines such as triethanolamine. Some of these activators not only overcome the problems of processing and accelerator absorption, but depending on the cure system used, will also act as vulcanisation activators. 

Stearic acid is normally added to a rubber compound at a 2 phr level. Reduction of the stearic acid level can cause problems from the naturally occurring constituents of NR which include fatty acids, as these can vary depending on the supply source. If the total fatty acid level is too low then the cure system can become too sensitive to other inherent materials. 

The chemical resistance depends on the hardness, the EPDM/PP ratio and the curing system. 

According to the vendor, system capital costs for the CURE system can range from 35,000 for a small, simple system to 1,000,000 for a large, sophisticated system. Additional vendor cost estimates are listed in Table 1. These estimates do not include the cost of a clarifier for the CURE system or the cost of sludge handling (D14357H). 

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