Cure systems, cross-link density

A typical resin has an initial molecular weight of 150 to perhaps 1500. For systems of unsubstituted phenols, the final cross-link density is 150—300 atomic mass units (amu) per cross-link. In other words, 25—75% of the ring-joining reactions occur during the cure phase. 

Three different covalent cure systems are commonly used sulfur-based or sulfur donor, peroxide, and maleimide. These systems rely on a cross-linking agent and one or more accelerators to develop high cross-link density. 

In the maleimide cure, the cross-linking agent is -y -phenjlenedimaleimide [3006-93-7] HVA-2. This system has two significant advantages Htharge is not required for high cross-link density and low compression set may be obtained. The accelerators are weak bases, ie,... 

Peroxide curing systems are generally the same for CSM as for other elastomers but large amounts of acid acceptor must be present to complete the cure. A small amount of a polyfunctional alcohol, ie, pentaerythritol (PER) in the compound significantly reduces the amount of base required by acting as a solubiHzer. TriaHyl cyanurate [101-37-17 is an additional cure promoter and leads to higher cross-link density. 

Ring S. In O-ring appHcations, the primary consideration is resistance to compression set. A fluorocarbon elastomer gum is chosen for O-ring apphcations based on its gum viscosity, cross-link density, cure system, and chemical resistance so that the best combination of processibiUty and use performance is obtained. Sample formulations for such uses are given in Table 4. 

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. 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

For most rubber systems the network is formed after the polymer is compounded and molded into the desired final shape. Once cross-linked, the material no longer can be processed. If cross-linking occurs prior to compounding or molding, the material is referred to as gelled, and it cannot be used. Most rubber is used in a compounded and cured form. There is an optimum cross-link density for many failure properties such as tensile strength and tear which will be discussed in the next section. 

The polyols can have a functionality of two or greater. Higher functionality results in increased viscosities (or reduced molecular weights) and increased cross-link density. Likewise, the isocyanate can have a functionality of two or greater. Epoxy systems have been incorporated into the polyol component to offer dual curing mechanisms and increased product-resistance properties. 

Including 4-bromophenol in the phenol-formaldehyde resol system impacts the cross-link density of the cured product. In a systematic study of this copolymer, a comparison was made among the polymers obtained using phenol only, a 9 1 mole ratio of phenol to 4-bromophenol and a 1 1 mole ratio of phenol to 4-bromophenol. Comparisons included measurement of interlaminar shear strength and cone calorimetry tests of composites prepared using these phenolic resins and S2-glass fiber plain weave. 

The effect of adding a mono-epoxy diluent to reduce the cross-linking density is shown in Figure I, middle curve. The reactive diluent NC-513 has been added to the NC-540-cured system and this allows the cure to proceed to where only 11% unreacted epoxy is present after 7 days of reaction. Surprisingly, the NC-540/NC-513 system has an extended pot life even though the maximum cure is achieved faster. 

Cure characteristics of EPDM/PP blends were investigated by Sengupta and Konar (27). They calculated the state of cure in blends containing conventional sulfur curing system under variable time-temperature conditions. They found that the activation energy for the cross-linking is almost similar for the virgin EPDM and EPDM/PP mixtures. Cross-link densities in TPVs can be analyzed by swollen-state... 

The nature of the photopolymerizable components such as chemical stmcture, molecular weight, and functionality plays a significant role in the efficiency of the polymerization reaction and physical, chemical, photochemical, mechanical, and stmctural properties of the cured system. When monomers or multifunctional monomers are combined with oligomers, such as in the case of carbonate, carbamate, or oxazolidone acrylates, they become highly reactive, exhibit low residual saturation and good flexibility. " " The mechanical properties sharply increase with functionality of the monomer with an increase in the cross-linking density, but they become more brittle. Solubility, biocompatibility, thermal resistance. 

The possible influence of cross-link density on the microbial deterioration of vulcanized NR has been obscured by the microbiocidal activity of the curing agent. In the curing system of CBS-sulphur, however, it was observed that the resistance of the vulcanizates is in good correlation with the cross-link density, regardless of the content of sulphtir or CBS (Figure 2) [42]. 

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