Rate of cure

The rate of cure is the rate at which the cross linking and development of stiffness of the compound occurs after the scorch point. As the compound is heated past the scorch point, the properties of the stock change from a soft plastic to a tough elastic material. The rate of cure is an important parameter of vulcanization, since it, in part, determines the time the compound must be cured which is known as the cure time. 

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Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

A continuous extmsion process, as weU as mol ding techniques, can be used as the thermoforming method. A more rapid rate of cure is then necessary to ensure the cure of the mbber before the ceUular stmcture coUapses. The stock is ordinarily extmded at a temperature high enough to produce some curing and expansion and then oven-heated to complete the expansion and 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. 

To assist in control of the onset of vulcanization, a retarder or prevulcanization iuhibiter (PVI) is used. Retardation of the onset of cure does not mean that the rate of cure is slowed, in fact cure rate may actually be increased. Rather, there is an induction period prior to cure. 

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. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a hardener (accelerator). This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate. A very large number of such latent acid catalysts have been described in the literature, of which some of the more prominent are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate. 

Control tests on the moulding powder include measurement of water content, flow, powder density and rate of cure. 

The cross-linking of the resin is, of course, not carried out until it is in situ in the finished product. This will take place by heating the resin at elevated temperatures with a catalyst, several of which are described in the literature, e.g. triethanolamine and metal octoates. The selection of the type and amount of resin has a critical Influence on the rate of cure and on the properties of the finished resin. 

Even the earliest reports discuss the use of components such as polymer syrups bearing carboxylic acid functionality as a minor component to improve adhesion [21]. Later, methacrylic acid was specifically added to adhesive compositions to increase the rate of cure [22]. Maleic acid (or dibasic acids capable of cyclic tautomerism) have also been reported to increase both cure rate and bond strength [23]. Maleic acid has also been reported to improve adhesion to polymeric substrates such as Nylon and epoxies [24]. Adducts of 2-hydroxyethyl methacrylate and various anhydrides (such as phthalic) have also been reported as acid-bearing monomers [25]. Organic acids have a specific role in the cure of some blocked organoboranes, as will be discussed later. 

The polymer plays several roles in this composition. First, it reduces shrinkage. Second, it increases the viscosity of the adhesive to the point where it can be easily applied. It also speeds the rate of cure. As will be discussed in the section on initiators, the boron compound reacts with atmospheric oxygen to form free radicals. 

Assuming maximum corrosion resistance is required, then an anticorrosive primer will be needed, with best protection coming from a crosslinked epoxy stoving primer. Most other properties are dominated by the finish, which will be based on a high molecular weight-polymer, either linear or (more usually) crosslinked. The precise selection of the polymer depends on the balance of properties required, but will be constrained by the type and rate of curing necessary. 

Hot strength Fast rate of cure (hot), moldings easily removed from die. Containers, trays, housings. 

Degree of cure and rate of cure for thermoset and UV-cured resins and similar are both properties that can be measured and monitored readily by a number of... 

An accelerator which permits processing of rubber compounds to be carried out with less risk of scorching but which does not slow down the rate of cure at normal vulcanisation temperatures. Demoulding... 

The process, now obsolescent, of permitting the coagulum of natural rubber to mature in the wet state before washing or smoking. Due to bacterial activity various natural accelerators are produced and the resulting rubber has a faster rate of cure than that prepared by the conventional method. MBI... 

A compounding ingredient which retards the rate of cure of a rubber compound, thus increasing processing safety. 

Commercial grades of HR (butyl rubber) are prepared by copolymerising small amounts of isoprene with polyisobutylene. The isoprene content of the copolymer is normally quoted as the mole percent unsaturation , and it influences the rate of cure with sulphur, and the resistance of the copolymer to attack by oxygen, ozone and UV light. The polyisobutylene, being saturated, however, naturally confers on the polymer an increased level of resistance to these agencies when compared to natural rubber. Commercial butyl rubbers typically contain 0.5-3.0% mole unsaturation. 

Figure 4. Relative Rates of Curing of MFAs Compared with NVP in admixture (1/1, v/v) with Urethane Acrylate and 0 (1% w/v) using UV Initiation (20).

Polymer products Adhesives, adhesive tapes, sealants, latex emulsions, rubber materials, plastic fabrication, etc. Composition monitoring Rate of cure monitoring Product QC... 

Differential Scanning Calorimetry (DSC) This is by far the widest utilized technique to obtain the degree and reaction rate of cure as well as the specific heat of thermosetting resins. It is based on the measurement of the differential voltage (converted into heat flow) necessary to obtain the thermal equilibrium between a sample (resin) and an inert reference, both placed into a calorimeter [143,144], As a result, a thermogram, as shown in Figure 2.7, is obtained [145]. In this curve, the area under the whole curve represents the total heat of reaction, AHR, and the shadowed area represents the enthalpy at a specific time. From Equations 2.5 and 2.6, the degree and rate of cure can be calculated. The DSC can operate under isothermal or non-isothermal conditions [146]. In the former mode, two different methods can be used [1] ... 

Han et al. [191] found that the rate of cure of a resin is greatly influenced by the presence of fibers and the type of fibers employed. The rate of reaction for resin-fiber system can be 60 percent different from that of neat resin, after a 10-min cure. A similar conclusion was presented by Mijovic and Wang [192] for graphite-epoxy composites based on TGDDM/DDS (33phr). They verified large differences (see Table 2.5) in the kinetic parameters when considering an autocatalytic model. 

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