Adhesive, brittle resin

Figure 1. Tensile stress-strain behaviour. Indicative Gc values from tests on cast brittle resin, and on tough epoxy as adhesive on steel substrates.

Poly(vinyl ethers) formed from methyl ethyl ether or isobutyl ether are used as soft plasticizing resins. Solubility and compatibility depend on the alkyl group. In the paint sector, poly(vinyl ethers) are used mainly as plasticizing and in some cases as adhesion-improving resins for chlorine-containing binders, styrene polymers, nitrocellulose, and brittle resins. An example of a commercial product is Lutonal (BASF). 

Thermoplastic rubber block copolymers, with completely new adhesive performance, were developed in 1965 [21]. The first commercial product was Shell Chemical s Kraton 101, of styrene polybutadiene-styrene composition. This development led to the carboxy-terminated nitrile (CTBN) rubber modifiers used to flexibilize epoxy and other brittle resin adhesives in the late 1960s. Today, the thermoplastic rubber block copolymer adhesives are used in hot melt-, solvent- and water-based adhesives, and as hot melt- and solvent-based sealants. Major applications are as pressure-sensitive adhesives, construction adhesives and sealants, and general assembly adhesives. 

Microcrystalline quartz is obtained by pulverizing quartz sands and is a hard solid (7 Mohs). It increases the thermal shock resistance in brittle resins - some filled thermosetting resins are cracked by relatively few thermal cycles between, say, ambient temperature and 100°C - when added at high concentrations (typically 100-200 parts per hundred by weight). It can be surface treated with an aminosilane to enhance adhesion, when used in epoxy compositions to improve flexural modulus, electrical insulation or thermal properties, and in the case of unsaturated polyesters, it can be treated with a methacrylic silane. 

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

Nitrile rubber/phenolic resin blends. Blends of equal parts by weight of a nitrile rubber and a phenolic resin in methyl ethyl ketone (at a 20-30 wt% total solids content) is suitable for many adhesive purposes. The more phenolic resin in the formulation, the greater the bond strength and brittleness of the NBR adhesive [67]. Table 10 shows the effect of phenolic resin on nitrile rubber properties. On the other hand, the higher the acrylonitrile content in the rubber. 

The single filament pull out test, sometimes called the microdebond test, has received attention for some years as a way to assess the adhesion between fibers and matrices in fiber composite [90,91]. It provides a direct measure of interfacial adhesion and can be used with both brittle and ductile matrix resins. 

Rosin, a brittle solid, mp 80 °C, is obtained from the gum of trees and tree stumps as a residue after steam distillation of the turpentine. It is made of 90% resin acids and 10% neutral matter. Resin acids are tricyclic monocarboxylic acids of formula C20H30O2. The common isomer is 1-abietic acid. About 38% of rosin is used as paper size (its sodium salt), in synthetic rubber as an emulsifier in polymerization (13%), in adhesives (12%), coatings (8%), and inks (8%). 

Evaluation of Adhesive Bond Strength. Since the cured product of LP-A was brittle, probably due to the low level of formaldehyde charged, LP-B and LP-C were selected as adhesive resins. In order to increase their cure rate, the pH s of LP-B and LP-C resins were adjusted to around 11.2 and 12.0, respectively, prior to adhesive formulation. For the purpose of comparison, the bond strength of the methylolated SEL resin adhesive was examined as well. 

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. 

Amines are one of the most important curing agents for epoxy resins. They provide fast cures with a relatively high crosslink density. Unmodified amine cured epoxy resins are generally too brittle for adhesive applications, and so there are many derivatives that have been developed. More information on amine curing agents can be found in Chap. 5. 

Their use in adhesive systems is minimal because they are relatively brittle and higher in cost than aromatic resins. However, cycloaliphatic epoxy resins are used in cationi-cally cured epoxy adhesive formulations. These are cured via uv or electron beam (EB) radiation. 

The number of possible hybrid systems that can be manufactured with epoxy resins is nearly infinite, and many adhesive formulations have been attempted in a quest to improve the main disadvantages of a cured epoxy brittleness and rigidity. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Temperature-resistant two-part, elevated-temperature curing epoxy adhesives can be formulated with aromatic amines, such as metaphenylenediamine (MPDA), methylene dianiline (MDA), or a eutectic blend of the two. These adhesives will provide relatively high temperature strength, but they are generally brittle. When mixed with epoxy resin at concentrations of about 15 pph for MPDA and 26 pph for MDA, they provide complete cure in about 30 min at 175°C. The aromatic amines also provide a working life of several hours at room temperature. Starting formulations for aromatic amine cured epoxy adhesives are shown in Table 12.4. 

A wide range of epoxy resins as well as a wide range of curing agents and catalysts are available for formulating solid epoxy adhesives. Resins with different viscosities, amounts of reactive groups, and structures are available. Additives that change the uncured resin viscosity, reduce brittleness, or impart some other property are also available. 

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