Cure reactions, chemistry

RTV Silicone Chemistry. There are two basic cure chemistries used by RTV sihcones the acetoxy-based and the alkoxy-based cure systems. Acetoxy-based RTV sihcones were first commercialized in the early 1960s (422,423). The general chemical reactions of these first-generation products are shown in Figure 8. 

The effective development of blocked isocyanate based coatings requires a complete understanding of the cure chemistry. Many materials have been identified which will improve cure response. Often these are reported as deblocking catalysts even though no direct evidence exists to support this claim. Most of these materials are well known catalysts of the cure reaction between hydroxyl and isocyanate and may be improving cure response solely by catalyzing this reaction. Effective development of catalyst systems requires a better understanding of the effect of catalysts on cure. 

In conclusion to the short analysis on curing chemistry of epoxy-aromatic amine networks (for more detailed analysis see papers of K. Dusek and B. Rozenberg in this volume), one can say that the chemical structure of the polymers under consideration is mainly determined by the curing reaction in Eq. (I). Equation II becomes important only for polymers with an excess of epoxy groups at T 150 °C. This rather simple situation makes the analyzed polymers very suitable for basic investigations. 

Exact definition of the cure chemistry is difficult since formulations of this type may also contain auxiliary catalysts (Lewis Acids) which enhance the homopolymerization reaction. 

The most important inference is that Chemisorption is a direct response to carboxyl group concentration indicated by the XPS photopeak component at 288.7 eV. It seems likely that weak add functionality is of minor import to applications for surface treatments, while interfacial phenomena such as practical adhesion may be sensitive to small concentrations of very high site energies. Interphase modification in epoxy resins, for example, can occur by direct reaction of epoxide groups with surface carboxyls (17), or by accelerated cure chemistry near the surface (39). Carboxyl groups on carbon surfaces may interact with basic moieties in polymers such as polycarbonate or poly(ethylene)oxide (40=42), or promote interfacial crystallinity that improves impact strength and other aspects of composite performance (43, M)-... 

In the mid-1970s, polysulfide polymer chemistry was advanced when Products Research and Chemical Corporation introduced a polyoxypropylene urethane backbone with mercaptan terminal groups [7]. Molecular weight regulation and minimization of side reactions are important features. The backbone is significantly different from that of conventional polysulfide polymers, yet the curing chemistry is the same and the eured product is a polysulfide rubber. A typical structure of this type of resin is shown below. 

Acrylic adhesives cure by addition polymerization reactions. These chain reactions are initiated by the formation of free radicals that result in the adhesive curing by way of a very rapid polymer chain growth. This cure chemistry is significantly more rapid than a typical cure curve (i.e., condensation type) found in epoxy and urethane adhesives. A comparison of the cure profile of condensation (epoxy and urethane) versus addition... 

The tendency of acrylic monomers to spontaneously and quickly polymerize makes it necessary for adhesive formulators to include various types of chemical stabilizers as part of the formulation to assure good shelf stability. These stabilizers are generally complex organic compounds that have a strong ability to react with free radicals (the cure chemistry most commonly employed with acrylics). These stabilizers stop unwanted side reactions and assure good shelf life of formulated adhesive products. 

One of the more common two-part cure chemistries is based on the addition reaction of Si—H cross-links with vinyl functional polymers using platinum catalysts. This chemistry is shown below. One advantage of this addition chemistry is that it produces no cure by-products. Another common two-part chemistry involves condensation cure with alkoxysilane cross-linkers using Sn(IV) catalysts. 

In contrast to epoxy-amines, water really participates in the cure chemistry of the epoxy-anhydride system and alters the living polymerisation mechanism because of interfering termination reactions [34]. A less dense network structure with altered properties results [35]. 

Figure 1. Schematic of polyarylene curing chemistry via a Diels-Alder cycloaddition reaction.

In fact, RIM was the first plastic to be approved for bumper fascia in North America to meet the low-temperature crashworthiness demanded by some OEMs. Unfortunately, due to process complexity, RIM could not be fabricated at a high enough rate to meet the demands of large car platforms. Faster cycle time for part production is delimited by the speed of curing chemistry and the ability of the resin to flow into molds. Furthermore, the automotive industry wanted to move away from the use of thermoset polymer systems that cannot be remelted. This attribute of thermosets was labeled by the industry as non-recyclable, and the industry made room for other thermoplastics such as TPO that can be remelted. Reaction-injection-molded parts are made from urethane epoxies, polyesters, and polyamides. A study of cost versus performance reported that RIM gives the best cost-performance characteristic for composite materials and is competitive with steel. ... 

---