Reactive adhesives diluents

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). 

Classification Nonaromatic ether Definition Glycidyl ether of Cl 2-14 alcohols Properties Dens. 0.890 ref. index 1.4470 Toxicology Skin irritant sensitizer possible mutagen Uses Reactive epoxy diluent modifier for flooring, aggregate bonding, potting, encapsulation, adhesives Manuf./Distrib. Aldrich... 

Uses Reactive epoxy diluent for exposed aggregates, potting, flooring, casting, tooling, laminates, solv.-free coating systems, fiber-reinforced composites stabilizer for chlorinated hydrocarbons adhesion promoter for PVC, acrylic resins, PU Manuf./Distrib. Aldrich Raschig... 

Uses Reactive epoxy diluent functional monomer modifier for coatings, tooling applies., adhesives... 

Uses Reactive epoxy diluent for casting, laminating, tooling, potting, elec., adhesive, civil engineering applies. 

A reactive ingredient added to an adhesive to reduce the concentration of binder is called a diluent. Diluents are principally used to lower viscosity and modify processing conditions of some adhesives. Diluents react with the binder during cure, become part of the product, and do not evaporate as does a solvent. 

Some reactive adhesives are applied with the aid of a volatile diluent. When a phenol-formaldehyde resin is used to impregnate and glue together the layers of wood that make up plywood, water is evaporated from the system. Alkyd resins and oleoresinons varnishes with a volatile diluent can be applied to a surface that is glued to another snrface after the diluent is evaporated. 

Tetrahydrofurfuryl alcohol is used in elastomer production. As a solvent for the polymerization initiator, it finds appHcation in the manufacture of chlorohydrin mbber. Additionally, tetrahydrofurfuryl alcohol is used as a catalyst solvent-activator and reactive diluent in epoxy formulations for a variety of apphcations. Where exceptional moisture resistance is needed, as for outdoor appHcations, furfuryl alcohol is used jointly with tetrahydrofurfuryl alcohol in epoxy adhesive formulations. 

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. 

An EB-curable struetural adhesive formulation usually eonsists of one or more crosslinkable oligomeric resins or prepolymers, along with such additives as reactive diluents, plasticizers, and wetting agents. The oligomer is an important component in terms of the development of mechanical properties. The adhesive and cohesive properties depend on the crosslink density, chemical group substitution, and molecular organization within the polymer matrix. Adhesion is achieved... 

The reactive monomer, besides being a diluent, plays a role similar to that of tackifying resin, namely, affecting the deformability of the adhesive. 

Secondary ingredients in epoxy adhesives include reactive diluents to adjust viscosity mineral fillers to lower cost, adjust viscosity, or modify the coefficient of thermal expansion and fibrous fillers to improve thixotropy and cohesive strength. Epoxy resins are often modified with other resins to enhance certain properties that are necessary for the application. Often these modifications take the form of additions of elastomeric resins to improve toughness or peel strength. 

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. 

The higher-MW semisolid (EEW of 225 to 280) and solid epoxy resins (EEW > 450) may be blended into lower-MW resins to improve flexibility and decrease reactivity. They also improve adhesion due to the higher concentration of hydroxyl groups along the molecular chain. Ten percent of a higher-MW epoxy resin blended into a conventional liquid epoxy resin (EEW of 190) can significantly improve flexibihty, but a reactive diluent frequently needs to be added to the formulation to counteract the increased viscosity caused by the addition. 

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. 

Reactive diluents enter into the polymerization reaction of the epoxy resin and the curing agent. In this way the final adhesive characteristics are determined by the reaction product of the binder and the diluent. The most common reactive diluents used for epoxy adhesive formulations are shown in Table 6.2. 

The primary reactive diluents are monoepoxy low-molecular-weight epoxy resins. These may be used at rather high concentration with little effect on cured properties. Often they are employed to make selective improvements on certain properties such as adhesion, thermal cycling resistance, and impact strength. Some lower-viscosity commercial epoxy resins are already reduced with these diluents. 

Monofunctional epoxy diluents are used primarily with DGEBA epoxy blends. The most common monofunctional diluents are butyl glycidyl ether and phenyl glycidyl ether. The effect of butyl glycidyl ether and other reactive diluents on the viscosity of epoxy resin is shown in Fig. 6.3. Because the monofunctional diluents reduce crosslink density, they are used at relatively low levels to avoid degrading heat and chemical resistance or other properties of the adhesive. 

Of the two groups, the reactive diluents are generally preferred for structural adhesives applications because they do not degrade end properties as much as the unreactive diluents do. Note that diluents, either reactive or unreactive, do not necessarily result in an increase in elongation of the cured epoxy. They may only reduce modulus and hardness. Their enhancement regarding flexibility will depend on the component s functionality and molecular nature. 

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. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

Reactive diluents can be used to reduce the modulus and increase the elongation of the cured waterborne epoxy formulations just as they are often used for 100 percent solids and solvent-borne epoxy adhesives. The reactive diluents become codispersed in the formulation with mechanical and chemical stability similar to that of the base epoxy emulsion. Polyglycidyl ether of caster oil, phenyl glycidyl ether, and diglycidyl ether of neophenyl glycol are examples of mono- and difunctional reactive diluents that have been used to improve flexibility and increase the tack-free time of waterborne epoxy adhesives. 

Starting formulation for a flexible epoxy adhesive containing polyamide curing agent and reactive diluent Starting formulation for a high-peel-strength adhesive Epoxy-polysulfide adhesive formulation Formulation for a flexible epoxy-polysulfide adhesive Epoxy-polysulfide adhesive formulations... 

Various aliphatic and aromatic mono-glycidyl ethers are available that can be used as reactive diluents of many formulated epoxy structural adhesives. They are... 

Epoxidized oils such as soybean oil or linseed oil, which are made by oxidizing the internal unsaturation in the fatty acid chain, find much use as plasticizers, particularly in vinyl resins. An internal epoxy ring is much less reactive than a terminal epoxy ring and usually does not take part in typical epoxy curing reactions under mild conditions. These materials can be used as plasticizers in epoxy adhesives but cannot really be considered reactive diluents. 

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