Diluent, Nonreactive

An ingredient added to an adhesive to reduce the concentration of base resin or binder is called a diluent. Diluents are principally used to lower the viscosity and modify the processing conditions of some adhesives. The degree of viscosity reduction caused by various diluent additions to a conventional epoxy adhesive is shown in Fig. 1.5. Diluents do not evaporate as does a solvent, but they become part of the final adhesive. Reactive diluents react with the resin base during cure, so that the final adhesive characteristics are determined by the reaction product of the binder and diluent. Nonreactive diluents do not react with the resin or curing agent and, therefore, more seriously weaken the final properties. Coal and pine tar are common nonreactive diluents. 

In the category of diluent, nonreactive adhesives, one example is model airplane cement (often cellulose nitrate in a mixture of ketones and aromatic solvents). As a coating we would call it a lacquer. Aqueous solutions of natural and synthetic gums are used in library paste. Some of the popular white glues for paper and wood are simply poly(vinyl acetate) emulsions with a small amount of plasticizer. All of these materials solidify after contacting the surface as a liquid by loss of solvent or diluent. Evaporation or diffusion into a porous substrate may be involved. 

Depending on the characteristics and performance requirements, adhesives systems are frequendy modified with diluents (reactive and nonreactive) and polyfunctional high performance resins, as weU as with fillers of various types. 

Nonreactive compatibilization, of polymer blends, 20 324-325 Nonreactive diluents, 10 429—430 Nonreactive silicones, in fiber finishing, 22 593... 

Solvents used as nonreactive diluents include acetone, cellosolve, methyl ethyl ketone, methyl isobutyl ketone, methylene chloride, 1,1,1-trichloroethane, toluene, and xylene. Skin and eye irritation and, in higher concentrations, CNS depression and respiratory irritation may result ftom exposure to these solvents as diluents for epoxy resin... 

Most epoxy formulations contain diluents, fillers or reinforcement materials, and toughening agents. Diluents may be reactive (mono- and diepoxides) or nonreactive (di-n-butyl phthalate). Toughening (flexibilizing) agents such as low-molecular-weight polyesters or... 

Fig. 11.3 Variation in the quantum yield of anthracene disappearance in presence of nonreactive diluent, bromobenzene.

Diluents Diluents reduce viscosity. Reactive diluents have epoxy groups, and nonreactive diluents have no epoxy groups available for reaction. 

Of course, in the case of both curing agents and catalysts, suitable adjustments will have to be made for the presence of nonreactive fillers and modifiers. Such ingredients can be liquids such as a solvent, a hydrocarbon resin, or a plasticizer. Since they do not contribute any epoxide functionality, they should not be considered when one is determining stoichiometry. However, if the additives have epoxy functionality, such as in the case of reactive diluents, the stoichiometric calculations will have to take these materials into consideration, by calculating ratios similarly as with an epoxy resin. 

Diluents have much lower vapor pressures and generally do not evaporate at ambient conditions. However, they do have a finite vapor pressure, and given the right set of conditions (time, temperature, and pressure) they will vaporize. Two distinct classes of diluents are used with epoxy resins nonreactive diluents and reactive diluents. Reactive diluents will enter into the crosslinking reaction with the primary resin, and nonreactive diluents will not. Nonreactive diluents primarily act as low-molecular-weight plasticizers for the epoxy composition. 

Diluents will also affect the performance properties of the adhesive. Diluents generally lower the degree of crosslinking and degrade the physical properties of the cured epoxy. This reduction in crosslink density increases the resiliency of the adhesive, but it also reduces tensile strength as well as heat and chemical resistance. These effects are more pronounced at elevated temperatures than at room temperature. The degree of these effects will depend on whether the diluent has epoxy functionality (reactive diluents) or whether the diluent is incapable of reacting with the epoxy system (nonreactive diluents). 

Both reactive and nonreactive diluents should be used sparingly if the properties of the cured system are to be preserved. An amount of 5 to 10 pph is best and generally provides a sharp reduction in viscosity. Concentrations greater than 20 pph are seldom used in adhesive formulations. 

High-boiling-point solvents such as xylene can be used as a nonreactive diluent however, this is not normally done because of the high vapor pressure of the solvent and the probability that solvent remaining after cure would degrade the physical properties and... 

Coal and pine tar are examples of common nonreactive diluents from natural substances. These are interesting nonreactive diluents because of their relatively low cost. They are often used as extenders in epoxy systems to reduce the cost. Coal tar is widely used because of its excellent compatibility with epoxy resins and relatively small sacrifice in cured properties. Nonyl phenol, furfural alcohol, and dibutyl phthalate are also common nonreactive diluents for epoxy systems. Dibutyl phthalate is also used as a plasticizer in many thermoplastics, such as polyvinyl chloride. 

Since nonreactive diluents do not enter into the crosslinking reaction, they can be lost due to volatilization, especially when exposed to the elevated temperatures of the exotherm or curing cycle. If vaporization does occur, shrinkage of the adhesive film can result in internal stresses being generated within the joint. These internal stresses reduce the degree of adhesion that is realized on final cure. 

Dibutyl phthalate (Fig. 6.4) is a commonly used nonreactive diluent because it does not exhibit migratory tendencies on aging. It is generally incorporated into the DGEBA epoxy with heating. When it is used at about 17 pph, the viscosity of the resin can be reduced from... 

Some nonreactive diluents have been used to impart special properties on the cured epoxy in addition to lowering the viscosity of the uncured system. For example, chlorinated diluents have been used with antimony oxide to impart flame resistance to cured epoxy systems. A typical formulation of this type based on DGEBA employs about 15 pph chlorinated... 

The monofunctional epoxy diluents are essentially chain stoppers since they inhibit crosslinks from forming. The extent to which the cured properties are affected is directly dependent on the concentration of the diluent added to the epoxy resin. The general effect is to reduce viscosity and improve the impact and thermal shock resistance while slightly reducing the thermal resistance. The thermal expansion of the cured resin is increased, as it is also with nonreactive diluents. This can lead to internal stress on the bond line depending on the thermal expansion of the substrate material. 

Reactive diluents, on the other hand, have one or two reactive epoxy groups that allow the modifier to be compatible and nonmigrating. Thus, they are more effective than nonreactive diluents for adhesive systems. 

The viscosity of a DGEBA resin is dependent primarily on molecular weight. Even at low molecular weight, viscosity is typically in excess of 6,000 cP, while at EEW 190 viscosity is usually around 12,000 cP. For applications requiring low viscosity it is thus necessary to include other types of epoxy resin or to use reactive or nonreactive diluents to achieve the desired viscosity. 

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