Epoxy-urethane

Epoxy-urethane adhesives provide properties when cured that are similar to those of epoxynylon adhesives except they offer a major improvement in moisture resistance. Isocyanate monomers and prepolymer react with the hydroxyl groups on epoxy resins to give tough, 

The isocyanate reaction is several orders of magnitude faster than the epoxy reaction. Hence, attempts to cure mixtures of epoxy and isocyanate resins must provide for essentially complete consumption of all -NCO groups before any of the epoxy rings can react. 

These reactive hybrid epoxy-urethane adhesives were developed initially for bonding to oily cold-rolled steel, but they have given good results on other substrates as well. It is believed that the oil contaminant on the substrate is adsorbed into the uncured adhesive and acts as a plasticizer. Table 7.9 shows tensile shear values obtained after curing 20 min at 177°C on various untreated substrates. The epoxy-urethane adhesives resist hydrolysis and give useful bond strengths up to about 100°C. 

FIGURE 7.3 Common amine reaction mechanisms for epoxy and polyurethane resins. 

TABLE 7.9 Cured Tensile Shear Strength of Uncleaned Substrates Bonded with Epoxy-Urethane Hybrid System16 

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Methacrylates and acrylates are readily synthesized from low-cost commercially available resins and (meth)acrylate intermediates or (meth)acrylic acid [19]. A wide range of structural backbones are available, including epoxies, urethanes. 

IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

Ammonium pentaborate (NH4)20 5B203 8H20 (64.6%) 120 10.9 Epoxy, urethane, coating... 

The technical success of the UV-drying inks may be attributed to the introduction of a number of types of acrylated oligomer and photo-initiator. The broad categories of acrylated oligomers include polyesters, epoxies, urethanes, ethers, and polyacrylates. Photo-initiators generally are derived from compounds that on exposure to UV light cleave into free radicals or abstract hydrogen to form free-radical species. 

Some more recent examples of epoxy-urethane hybrids in one- and two-component structural adhesives are noted below. These have had success in the automotive industry where high impact strength, fatigue resistance, and high bond strength to a variety of substrates are valued characteristics. 

These adhesives require relatively high cure temperatures (above 130°C). In the past this had not been a problem, because the adhesive cured in the paint bake cycle. However, with newer paint systems and efforts to reduce energy consumption, lower-cure-temperature adhesives are needed, and epoxy-urethane adhesives and other flexibilized adhesives are beginning to replace the plastisols. 

Another common method of flexibilizing epoxy adhesives is by blending the primary epoxy resin with other, more elastic polymers. Epoxy-nylon, epoxy-polysulfide, and to a certain extent epoxy-urethane hybrids use such a mechanism to provide flexibility. These flexibilizers are important additives for epoxy adhesives even though they may reduce certain... 

The two most prominent liquid radiation curable adhesives are free radical polymerization epoxy acrylates and cationic polymerization epoxies. Such adhesives are generally used as polymerizable syrups. A wide range of prepolymers can be acrylated including epoxies, urethanes, polyesters, polyethers, and rubbers. Elastomer-tackifying resin blends are often used in these formulations. 

Next to the epoxy-nylons and epoxy-urethanes, the epoxy-polysulfide adhesives show the greatest lap shear tensile strength at temperatures below 0°C. Bonded etched steel substrates show a tensile shear strength of 2900 psi at room temperature, and this increases to 3400 psi when the temperature is reduced to -156°C. Epoxy-nylon and epoxy-polysulfide are the only adhesives that show an increase in strength as the temperature is significantly reduced. 

Adhesives commonly used on thermosetting materials include epoxies, urethanes, cyanoacrylates, thermosetting acrylics, and a variety of nonstructural adhesive systems. The following discussion includes a very brief description of various thermosetting substrate materials, the properties that are critical relative to epoxy adhesion, and any special processes that should be noted for the particular substrate. 

Alkyd parts are generally very rigid, and the surfaces are hard and stiff. Surface preparation for alkyd parts consists of simple solvent cleaning and mechanical abrasion. Epoxies, urethanes, cyanoacrylates, and thermosetting acrylics are commonly used as structural adhesives. 

Typical surface preparation calls for cleaning with acetone, MEK, or other common solvent. Once clean, the substrate is then mechanically abraded with sand, grit or vapor blast, or steel wool. The surface is again wiped clean with fresh solvent. Typical adhesives that are employed include epoxies, urethanes, and cyanoacrylates. Polysulfides, furanes, and polyester adhesives have also been suggested. 

When solvent welding or thermal welding is not practical or desired, adhesive systems can be used. Adhesive types such as epoxies, urethanes, thermosetting acrylics, nitrile phe-nolics, and cyanoacrylates permit ABS to be bonded to itself and to other substrates. The best adhesives have shown strength greater than that of ABS however, these adhesives provide very rigid bonds. 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

Paste adhesives are supplied as either one- or two-component adhesive systems. They can be used in applications where pressure cannot be applied. Some two-part pastes cure at room temperature after the appropriate proportions are mixed. Epoxy, urethane, and acrylic adhesives are all available as paste adhesives. 

Conventional Solvent-Based Coatings Aliphatic- Aromatic Hydrocarbons Melamine Epoxy Blocked isocyanates Polyester Acrylic Epoxy Urethane 40-60% solvent 30-40% polymer 10-20% crosslinking resin... 

The Chemistry of Structural Adhesives Epoxy, Urethane, and Acrylic Adhesives... 

Adhesives usually are classified as epoxy, urethane, or acrylic, based upon the chemistry used to bring about the polymerization of the liquid adhesive. However, the distinction among the various types of adhesives is not... 

A typical formulation of a two-component epoxy urethane adhesive would include 100 parts of a water dispersed polyurethane resin with carboxylic... 

Another development has been reported by P.A. Lucas, W.E. Stamer and S.G. Musselman of Air Products and Chemicals Inc. (Lucas et al., 1994). An acrylate functional urethane flexibiliser has been used to modify epoxy resin, which optimises reactivity and is more compatible with epoxy. Urethane-acrylate flexibiliser offers very tough hybrid epoxy systems meeting the more demanding requirements of civil engineering applications. 

The mortar may be applied to properly prepared concrete or steel substrates by usual acid-resistant bricklaying methods. An epoxy, urethane asphalt, bi-tumastic, polyester or vinyl ester membrane is required behind the block to ensure corrosion protection of the substrate. Because the mortar joints are rigid, a system of expansion/contraction joints, usually filled with ceramic paper, must be designed to prevent cracks. 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

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