Thermoset adhesives chemistry

Thermoplastic Adhesives. The polymer adhesives described in the following sections are not used as extensively with wood as the thermosetting adhesives discussed earlier. However, they do illustrate many interesting principles of the chemistry of adhesion through organic polymers. 

Fluid Polyamide Resin Chemistry Thermoset Coatings Thermoset Adhesives... 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

The following sections discuss the chemistry of some major classes of thermosetting, structural adhesives. 

Some of the more interesting and innovative work has occurred in areas combining aspects of more than one chemistry type. For instance, moisture-curable thermoplastic adhesives have received much attention. Hot melt adhesives have been developed that contain active, moisture-curable isocyanate groups. The compositions provide rapid processing on assembly lines because a reasonable bond is formed as soon as the thermoplastic adhesive cools from the melt. However, bond strength and performance improve with time as the composition is slowly crosslinked to a thermoset by reaction of the isocyanates with atmospheric moisture.96,97... 

Van der Klashorst, G. H Cameron, F. A Pizzi, A. The Utilization of Soda Bagasse Lignin in the Manufacture of Thermosetting Wood Adhesives. Paper presented at the International Symposium on Wood and Pulping Chemistry, April 1987. 

Dr. Riew has presented more than 50 technical papers and holds more than 25 patents on emulsion polymers, hydrophilic polymers, synthesis and application of telechelic polymers, and toughened plastics for adhesives and composites. His latest research is in the synthesis, characterization, and performance evaluation of impact modifiers for thermosets and engineering thermoplastics. His research interests include correlating polymer chemistry and physics, morphology, engineering, and static and dynamic thermomechanical properties to the failure mechanisms of toughened plastics. 

This chapter will deal with the chemistry and applications of epoxies, phenolics, urethanes, and a variety of current vogue high-temperature polymers. Applications in fiber-reinforced plastics will be discussed in the individual sections on resin chemistry where appropriate. Separate sections will deal with adhesives and sealants. Adhesives are most important because, as early history demonstrates, they led the way to the application of resins in aerospace. A section is also included on silicone and polysulfide sealants. Although these materials are elastomers rather than resins, no discussion of aerospace polymers would be complete without some mention. Some major thermosetting polymers have been omitted from this review. Among these are the unsaturated polyesters, melamines, ureas, and the vinyl esters. Although these products do find their way into aerospace applications, the uses are so small that a detailed discussion is not warranted. 

The large number of manufacturing processes reflects the variety of applications of thermosetting polymers in structural and functional materials. They can be deposited on substrates for adhesives, paintings, and coating, and they can be used as foams or as matrices for composites. In all these applications, chemistry and processing are strongly interrelated. 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

In Chapter 4, toughening of thermoset resins was discussed in a general way. Among the commercially available thermosetting resins, epoxy resins have been the most extensively studied, and toughening technology has been exploited in the field of adhesive and fibre-reinforced composites. This is due to the inherent ductility of the cured epoxy resins and versatile epoxy resin chemistry. In this chapter, the toughening of epoxy resin will be discussed specifically. 

Chapter 5 Thermosetting formaldehyde based adhesives, by Dr A. Pizzi, a detailed study of UF, PF, RF and MF adhesives and glues, their chemistry and their uses in woodworking, panels and furniture (this chapter will be followed in later volumes by a chapter on Wood Based Panels and a chapter on Isocyanate Based Adhesives ). 

As with the other thermosets in this chapter, these processes will not be discussed in this section. We will nevertheless give a few hints of the reaction engineering of the production of polymers and macromonomers based on this chemistry other relevant uses are adhesives, binders, coatings, thermoplastic elastomers, and fibers. 

However, the majority of sandwich panels now utilise a honeycomb core rather than either balsa wood or plastic foam the adhesive can be based on either thermosetting or thermoplastic chemistries. The three basic components used in honeycomb sandwich construction are discussed below. 

The polymers used to manufacture the film or tape adhesives are either thermoplastic materials including acrylic, polyester, and fully imidized polyimides or thermosetting resins based on epoxy and polyimide chemistries. The tapes are often made with partially cured (B-staged) epoxies similar to those used in the fabrication of the PCBs. Some commercially available films are also... 

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