Handbook of Adhesives and Sealants

322 Chapter NineAn ingredient added to an adhesive to reduce the concentration ofbase resin or binder is called a diluent. Diluents are principally usedto lower the viscosity and modify the processing conditions of someadhesives. The degree of viscosity reduction caused by various diluentadditions to a conventional epoxy adhesive is shown in Fig. 9.1. Diluentsdo not evaporate as do solvents, but they become part of thefinal adhesive. Reactive diluents react with the resin base during cure,so that the final adhesive characteristics are determined by the reactionproduct of the binder and diluent. Nonreactive diluents do notreact with the resin or curing agent and, therefore, generally dilutethe final physical properties. Coal and pine tar are common nonreactivediluents.Extenders are substances which usually have some adhesive propertiesand are added to reduce the concentration of other adhesivecomponents and the cost of the overall formulation. Extenders alsohave positive value in modifying the physical properties of the adhesive.Common extenders are flours, soluble lignin, and pulverizedpartly cured synthetic resins.Fillers are relatively non-adhesive substances added to the adhesiveformulation to improve its working properties, strength, permanence,or other qualities. The improvements resulting from the use of commonfillers are listed in Table 9.1. Fillers are also used to reduce materialcost. By selective use of fillers, the properties of an adhesive canFigure 9.1 Reduction of diglycidyl ether of bisphenol A (DGEPA) epoxyresin by reactive diluents. 1