Cured physical properties

Certain chemical linkages are susceptible to hydrolytic attack and, if present in an adhesive or sealant, are potential sites for irreversible reaction with water that has diffused into thejoint. Such hydrolytic (chemical) degradation causes a permanent reduction in the cured physical properties. The functional groups present in the chains are hydrolyzed, resulting in both chain breaking and loss of crosslinking. 

Cured properties of elastomers with a very low number of cure functional sites such as butyl and EPDM rubbers are independent of curative (accelerators) levels used. However, varying the levels of unsaturation during polymerisation can alter cured physical properties often without processing difficulties. For butyl and EPDM rubbers only a small fraction of the monomer units in a chain take part in the crosslinking reactions. For example, if an average chain contains 10,000 monomer units only about 50-250 would normally be involved in the crosslinking. 

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

These are used in concentrations of less than 5phr and have little effect upon the cured physical properties, but there may be more significant changes in the crosslinked products chemical and solvent resistance. Obviously, as the concentration of non-reactive diluent is increased, the effect upon the cured properties becomes more pronounced. Care also has to be taken with thermally cured systems that the curing temperature does not cause volatilisation of the diluent, as this can result in the formation of voids and blisters in the cured material. 

Special fine-particle kaolin clays are commonly used in rubber compounding as economic diluents, mainly to reduce the cost and improve processability. Clays that are used by the rubber industry require further processing to remove larger particles and impurities, which prevents the decrease of the rubber product s cured physical properties. 

Other diols can be used to cure MDI-terminated prepolymers however, they might be more expensive and impart different cured physical properties. 

The use of fillers in the rubber industry is both essential and universal. Typically, a commercial rubber compound will contain between 20 to 50% filler. These fillers are many times necessary to impart the required cured physical property improvements to the commercial compound. Also, fillers are typically less expensive than the base elastomer, and they significantly reduce the pound-volume cost of the rubber product. In addition, many fillers improve the rubber compound s processability. 

Other rubber accelerators can be used in place of MBT however, a different cure profile may result with different cured physical properties. These substitutions could also generate higher levels of nitrosamine gases, which could pose a health risk to factory workers. 

Other dithiocarbamates may substitute, but will impart different scorch and cured physical properties that must be evaluated. 

Other guanidine accelerators may substitute for DPG, but they also have toxicity issues. They will also impart different cured physical properties that must be evaluated. The strong toxicity concerns with the use of DPG are driving an effort to find new substitutes for it. 

Other antioxidants may be substituted, but will impart different cured physical properties and oxidative protection characteristics that must be evaluated on a case-... 

ATH is a relatively inexpensive flame retardant and flller used by the rubber industry. ATH possesses bound water, which is released at higher temperatures to promote a degree of flame retardancy. It can release as much as 34% of its weight as water at elevated temperatures. To be effective, ATH must be used at signiflcantly high concentrations in a rubber compound. ATH grades used in rubber must have sufficiently fine particle size, so as not to degrade the cured physical properties of the rubber compounds. 

Tables 1-4 list selected commercially available anaerobic adhesive products, illustrating the variety of uncured and cured physical properties which results when formulations are tailored to meet specific application needs. Many products are color-coded for easy recognition and on-part identification.

Rheometer at 171°C Scorch time, tsi, minutes Cure time, tC9o, minutes Press cured physical properties 100% modulus, MPa Tensile strength, MPa Elongation, %... 

Cure time, fcgo, minutes Press cured physical properties 7.5 17.3 8.5... 

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