Volatile cure products

Polymer systems are now available which may be cured by reaction of epoxy resin compounds with the phenolic hydroxyl groups. Such reactions do not evolve volatile by-products. These materials are showing promise in the area of heat-resisting electrical insulation laminates. 

Nearly all polymeric materials (including adhesives and sealants) shrink during solidification. Sometimes they shrink because of escaping solvent or volatile by-products, leaving less mass in the bond line. Even 100 percent reactive adhesives, such as epoxies, with no formation of by-products during cure experience some shrinkage because their sohd polymerized mass occupies less volume than the liquid reactants. 

Acetylene terminated (AT) resins are being studied as a new technology to form the matrix of high performance structural composites (1). This class of resin cures by an addition reaction mechanism. Because of the absence of volatile by-product during cure, these resins are easy to process to yield void-free components. These resins also have the advantage over epoxy resins in terms of resistance to deterioration of properties at elevated temperature due to exposure to humid environments (2). 

Selected reactive diluents are employed to reduce the viscosity of Uvimer M formulations and to Impart a greater flexibility to the cured product. The choice of a monomer or reactive diluent for a Uvimer ig based on a number of factors, including compatibility with the base resin, odor, volatility, toxicity, and its effect on specific properties such as adhesion. Changing the diluent monomer does not drastically alter the basic properties of the resin so that it is possible to supply a given UvimerTM with a choice of diluents in order to comply with particular user requirements. 

In the early 1960s, a new class of addition polymer and addition poly(imide)s was developed by the Rhone Poulenc company. The most important of these were /jw-maleimides (BMI) [12, 13], which could be crosslinked or copolymerized to form thermosets with outstanding thermal and chemical resistance. These materials cure without volatile by-products, thereby, minimizing the formation of voids, and have high glass transition temperatures and low moisture absorption. The major uses of this class of resin is in advanced composites and printed circuit boards. 

Thermogravimetric analysis was used to follow the curing of polymer films. This technique is predicated on the volatility of the unreacted components. In the present study the volatile components have been determined to be EEA, HDODA, TMPTA, Irgacure 651, and their volatile reaction products. The Epocryl 370 resin and the VTBN rubber were found not to contribute to the volatiles. Decomposition of the mixtures generally took place about 310-350°C in all cases, including the rubber modified samples. 

Since the cross-linking of polyester-styrene system occurs by a free-radical chain-reaction mechanism across the double bonds in the polyesters with styrene providing the cross-links, the curing reaction does not give rise to volatile by-products (unlike phenolic and amino resins) and it is thus possible to cure without applying pressure. This fact as well as that room temperature cures are also possible makes unsaturated polyesters most useful in the manufacture of large structures such as boats and car bodies. 

The bismaleimides can be reacted with a variety of bifimctional compounds to form polymers by rearrangement reactions. These include amines, mercaptans, and aldoximes (Figure 4.22). If the reaction is carried out with a deficiency of the bifunctional compound, the polymer will have terminal double bonds to serve as a cure site for the formation of a cross-linked polymer via a double bond polymerization mechanism during molding. The cross-linked in this case occurs without the formation of any volatile by-products. 

The polyimlde molding compound prepared by any of the above techniques Is processed In a preheated mold at 550-600 F, followed by post cure to 625 F for 2-8 hours for development of maximum physical properties. Absence of volatile by-products permits facile production of sound moldings having thick sections, while the modest (1300) molecular weight of the prepolymer permits flow at reasonable temperatures. 

Thermogravimetric analysis (TGA) is a technique whereby weight changes in a sample can be continuously monitored with respect to temperature. TGA can be used to look at weight losses associated with the polymerization of an adhesive product, or to determine percentage volatiles released from a cured product during thermal cycling. 

Adhesives and Sealants. Polyurethanes can be conveniently applied in liquid or paste form and then polymerized/cured in place without evolution of volatile by-products, a very convenient feature in making enclosed adhesive bonds. Their mechanical strength, flexibility, adhesion, and chemical resistance make them attractive in many applications. Typical applications of polyurethane sealants are in expansion joints, aerospace, architectural, electronic, and marine products. 

For this reason, HMTA cured novolac networks are generally brittle in nature and have low fracture energies and impact strengths, probably due to the voids combined with the high crossliiik densities. Curing novolac resins with epoxy resins can produce networks without volatile by products. These can be reinforced with glass or carbon fibers to yield tough, void-fiee composites, which also retain much of the flame-retardant properties of phenolics [39],... 

---