State of cure

The state of cure is a term used to indicate the development of a property of the rubber as vulcanization progresses. As the cross 

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Coagents ate often used with peroxides to increase the state of cure. Some coagents, such as polybutadiene or multifimctional methacrylates, are used at high levels to form polymer grafts or interpenetrating networks. Other coagents such as triaHyl cyanurate, triaHyl trimelHtate, and y /i -phenjiene bismaleimide are used at low levels to reduce the tendency of the polymer to degrade by chain scission. 

Fig. 2. Cure curve from oscillating disk rheometer where A represents scorch safety B, cure rate C, state of cure D, optimum cure time and E, reversion.

Permanent set and low hysteresis properties depend on minimizing the viscous or plastic component of modulus. Because cross-linking increases elasticity, a high state of cure typically provides the best compression set and heat buildup properties. 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

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. 

Nonblack fillers such as the precipitated siHcas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between mbber and filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevents this competition thereby maintaining the desired cure characteristics. Neutral fillers such as calcium carbonate (whiting) and clays have Httie or no effect on the cure properties. 

Process Oils, Plasticizers. Petroleum-based mbber process oils generally contain a mixture of paraffinic, naphthenic, and aromatic components. These oils vary in composition from grade to grade, but most contain some unsaturated moieties and this unsaturation can compete with the polymer for curatives. Therefore, state of cure can be decreased. This is not easily detected because oil softens the compound which masks the loss of state of cure. 

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. 

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. 

This is an activator-starved formulation and so is highly sensitive to the presence of nonmbbers that are capable of activating or accelerating vulcanization, and Table 2 illustrates the cure behavior of different grades of SMR (28). Cup lump grades show the highest state of cure and fastest rate of cure, whereas the stabilized grade, SMR CV, shows the lowest state of cure and slowest cure rate. 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

The sulfonyl chloride group is the cure site for CSM and determines the rate and state of cure along with the compound recipe. It is less stable than the Cl groups and therefore often determines the ceiling temperature for processing. The optimum level of sulfonyl chloride to provide a balance of cured properties and processibiUty is about 2 mol % or 1—1.5 wt % sulfur at 35% Cl. It also undergoes normal acid chloride reactions with amines, alcohols, etc, to make useful derivatives (17). 

Mold temperatures vary between 150—200°C, depending on the mol ding methods and part size. Parts can be molded in 1.5—10 min depending on the configuration and thickness of the part, the mold temperature, and the desired state of cure at demolding. Since most ethylene—acryflc parts are postcured, it is sometimes possible to demold partly cured articles and complete vulcanization in the postcuring oven. 

The effect of palm oil fatty acid additive (POEA) on curing characteristics and vulcanizate properties of sdica-filled NR compounds was studied by Ismail [30]. The incorporation of POEA improved the cure rate and state of cure of the compounds. Compared to the control, the incorporation of POEA not only enhanced the vulcanizate properties but also improved the reversion resistance of the silica-filled NR compounds. 

The effective crosslink density is used to predict state of cure in thermoset coatings. Side reactions have been found to play an important role in cure of typical coating mixtures. 

The amount of sulphur which has not combined with the rubber during the vulcanisation reaction. The determination of the free sulphur content is widely used in assessing the state of cure of a rubber product. The addition of the free sulphur curve to the modulus and tensile/time curves of the particular compound enables the state of cure of the vulcanised product to be estimated. 

The time of vulcanisation required by the compound in question to reach a given state of cure compared to the time required by a standard compound to reach the same state of cure. 

A method of obtaining an even state of cure in thick articles. The source of external heat is shut off before cure is complete and the heat permitted to penetrate to the interior of the article. See Drop, Hold, Rise. 

Increase in dimensions of an elastomer due to the absorption of any organic liquid with which it may come in contact or in which it may be immersed. Measurement of the swelling of vulcanised rubber in solvents can be used to determine state of cure and to assess the suitability of elastomers for such uses as chemical plant lining or oil seals. 

One method for determining the state of cure of a mbber compound. A sample of the rubber is stretched, cooled to a temperature of around -70 °C while stretched and then released. As the temperature rises the mbber retracts and the T-50 value is the temperature at which it has lost 50% of its original elongation. The T-50 test is now obsolescent. 

Due to the low level of unsaturation in the main chain, sulphur cures require the more active thiuram and dithiocarbamate accelerators to achieve an adequate state of cure. 

The dicyclopentadiene terpolymer can give higher states of cure with peroxides than the copolymer, although in peroxide curing of both the copolymer and terpolymer it is common practice to add a coagent, to increase the state of cure. Triaryl isocyanurate or sulphur are the most common coagents. 

The saturated main chain of the copolymer confers excellent resistance to oxygen, ozone and light, but means that these materials cannot be crosslinked by sulphur. Peroxides and radiation are the only methods by which crosslinking can be accomplished, and coagents are often required to achieve the required state of cure. 

Ground silica, generally available below 300 mesh in size (5 m2/g), is used as a cheap heat resistant filler for a variety of compounds. There is no effect on the rate or state of cure. 

Bisphthalonitrile monomers were cured neat, with nucleophilic and redox co-reactants, or in combination with a reactive diluent. Dynamic mechanical measurements on the resulting polymers from -150 to +300°C turn up several differences attributable to differences in network structure. Rheovibron results were supplemented with solvent extraction, differential scanning calorimetry (DSC), vapor pressure osmometry, and infrared spectroscopy to characterize the state of cure. 

In this work, bis-phthalonitrile networks (1, 2) were examined by dynamic mechanical and dielectric methods, supplemented with infrared measurements of state of cure, DSC, vapor pressure osmometry, and solvent extraction. For resins cured with 4,4 -methylene dianiline as co-reactant, a simple network model rationalizes the data. 

Solvent absorption measurement has been shown to be a sensitive and useful test method in the manufacture of epoxy powder coatings. A test method was defined and the effects of time and temperature of immersion described. It was shown that solvent absorption is a measure of raw material properties (EEW of the epoxy resin, and CTBN elastomer type and concentration), the homogeneity of the extrudate, as well as the state of cure. The information obtained from solvent absorption measurements has proven to be extremely important not only in quality control analysis but also in providing an insight into the structure function relationships in epoxy resin chemistry. 

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