Formulation flexibilizers

Flexibilized epoxy resins are important structural adhesives [69]. Liquid functionally terminated nitrile rubbers are excellent flexibilizing agents for epoxy resins. This liquid nitrile rubber can be reacted into the epoxy matrix if it contains carboxylated terminated functionalities or by adding an amine terminated rubber. The main effects produced by addition of liquid nitrile rubber in epoxy formulations is the increase in T-peel strength and in low-temperature lap shear strength, without reducing the elevated temperature lap shear. 

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... 

One problem with early epoxy formulations is that they cured to a relatively brittle material. By using reactive flexibilizers, such as polysulfides, epoxy adhesive formulators have obtained the flexibility required for many applications in this industry. Polyamides and even coal tars have also been used to provide flexibility to epoxy base resins. 

Plasticizers and flexibilizers are incorporated into an adhesive formulation to provide it with flexibility and/or elongation. Plasticizers may also reduce the melt viscosity of hot melt adhesives or lower the elastic modulus of a solidified adhesive. Similar to diluents, plasticizers are nonvolatile solvents for the base resin, and by being incorporated into the formulation, they separate the polymer chains and enable their deformation to be more easily accomplished. Plasticizers generally affect the viscoelastic properties of the base resin whereas diluents simply reduce the viscosity of the system. Whereas diluents result in brittle, hard adhesive systems, plasticizers result in increased flexibility and lower modulus. The temperature at which polymers exhibit rubbery properties (i.e., the glass transition temperature) can also be modified by incorporating plasticizers. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Elastic adhesives deform when exposed to such internal stress and are less affected by shrinkage. Thus, flexibilizers or tougheners may be added to the formulation to distribute the stresses over a larger area (Chap. 8). 

It should also be emphasized that the epoxy resins are only building blocks in the development of epoxy-based adhesives. The formulation will ultimately involve one or more epoxy resins curing agents modifiers, such as plasticizers, flexibilizers, accelerators, stabilizers, and flow control agents and fillers. In general, flexibility can be controlled by the parameters shown in Table 3.8. 

Future chapters describe the other raw materials that contribute to the epoxy adhesive formulation (curing agents and catalysts, Chap. 5 solvents and diluents, Chap. 6 hybrid resins, Chap. 7 flexibilizers and tougheners, Chap. 8 fillers, Chap. 9 and adhesion promoters, Chap. 10). Complete adhesive formulations are then discussed in subsequent chapters. 

TABLE 4.4 Properties of an Epoxy Resin Formulation Based on a Blend of Polyglycol Diepoxy Flexibilized Resin and Standard DGEBA Resin, Cured with Methylene Dianiline5... 

Diluents are higher-MW components than solvents that are also added to the epoxy adhesive formulation to lower the viscosity and modify processing conditions. The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to make it easier to compound with fillers, to improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin is illustrated in Fig. 6.3. 

Since many of the difunctional epoxy diluents are added to epoxy adhesive formulations as flexibilizers as well as viscosity reducers, they are discussed in greater detail in Chap. 8 with other flexibilizing additives. 

By adding nonepoxy flexibilizing resins to the formulation to create an epoxy-hybrid adhesive system... 

Di- or polyfunctional epoxy resins may be considered reactive diluents that provide a degree of resiliency, yet most preserve the inherent properties of the base epoxy resin. When added to an epoxy formulation, they will not reduce functionality, and in some cases an actual increase in crosslink density is noticed. Typical resins of these types are shown in Fig. 8.4. As flexibilizers, these diluents are often blended with the more reactive epoxy resins. They, of course, can also be used as epoxy resins in themselves. 

FIGURE 11.5 Effect of a flexibilized epoxy on tensile shear and peel strength of epoxy adhesive formulation.19... 

Flexibilized epoxy adhesives have moderate strength on flame and corona treated polyolefin substrates. Elevated cure temperature results in better adhesion because of more efficient wetting of the substrate surface. Table 16.13 shows a starting formulation for an epoxy adhesive that develops high peel strength to many difficult-to-bond substrates such as polyethylene, thermoplastic rubber, and polyester film. 

Chapters 4 through 10 describe the basic raw materials that are commonly employed in formulating epoxy adhesives. These include the epoxy resins, curing agents and catalysts, solvents and diluents, resinous modifiers, flexibilizers and tougheners, fillers, and adhesion promoters. 

Early researchers who worked on flexible vinyl formulations were convinced that the technique of external plasticizing was doomed because of inadequate "permanence" in the composition. Although good initial properties were attainable, it was felt these could not be maintained. Consequently, a great amount of study was devoted to flexibilizing by "internal plasticization" through copolyraerization. Resins so developed, however, are usually found unable to meet the wide range of property demands and therefore in turn require additional external plasticization. 

Blends of polyolefins (e.g., HPDE/LDPE, LDPE/ ethylene copolymers, PP/EPDM, PP/HDPE/EPDM, HDPE/butyl rubber) have been commercial since the late 1960 s and early 1970 s. Specific film formulations were commonly based on polyolefin blends to achieve the proper balance of processing, environmental stress crack resistance, modulus, toughness, cling, transparency, filler acceptance, printability, tear resistance, shrinkage characteristics, and permeability. Ethylene-propylene mbber (EPR, EPDM) was commonly incorporated into polypropylene as an impact modifier at moderate levels and as a flexibilizer at high levels. One of... 

To a lesser extent, amine-terminated polybutadiene/acrylo-nitrile (ATBN) liquids are also used by epoxy formulators. This polymeric disecondary amine is employed in admixture with amines, amldoamlnes or fatty polyamides of choice (10, 11). Thus, one admixes chemical and polymeric amines to create a novel toughening or flexibilizing hardener package. 

A second approach to reducing modulus and flexibilizing an adhesive is to add a plasticizer to the formulation. Since most plasticizers are of relatively low-molecular weight and do not co-react with the resin, there is a risk of separation, leaching, or outgassing during thermal or thermal-vacuum environments. 

Substrates such as diamond-copper combine excellent thermal conductivity and closely matched thermal-expansions but at a significantly higher cost. On the other hand, copper that has high conductivity and is low in cost, presents an expansion-coefficient mismatch with the LED materials. These problems may be resolved by formulating adhesives with modified epoxies, silicones, or flexibilizers to relieve stresses (see Chapter 2). 

Uses Binder in formulation of highly flexible, water-fhinnable coatings for wood, metal, plastics, and min. substrates binder, flexibilizer for wafer-thinnable coatings... 

Chem. Descrip. Linear polymer with ether and urethane groups, contg. crosslinking blocked isocyanate groups Uses Raw material, flexibilizer for epoxy resins, PL) systems, formulation of flexible coatings and sealants Features Solv.-free... 

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