Urethane adhesives properties

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

The specialty class of polyols includes poly(butadiene) and polycarbonate polyols. The poly(butadiene) polyols most commonly used in urethane adhesives have functionalities from 1.8 to 2.3 and contain the three isomers (x, y and z) shown in Table 2. Newer variants of poly(butadiene) polyols include a 90% 1,2 product, as well as hydrogenated versions, which produce a saturated hydrocarbon chain [28]. Poly(butadiene) polyols have an all-hydrocarbon backbone, producing a relatively low surface energy material, outstanding moisture resistance, and low vapor transmission values. Aromatic polycarbonate polyols are solids at room temperature. Aliphatic polycarbonate polyols are viscous liquids and are used to obtain adhesion to polar substrates, yet these polyols have better hydrolysis properties than do most polyesters. 

In order to understand the widely varying physical properties and adhesion properties of urethanes, it is important to understand the different classes of urethane raw materials and their functions in urethane adhesives. In order to... 

It is difficult to generalize regarding adhesion properties and physical characteristics of urethanes, because both the adhesion properties and physical properties can vary widely. One two-part urethane adhesive may be an outstanding adhesive for bonding SMC/SMC. Another urethane adhesive may fail miserably using the same SMC, due to delamination at the interface or adhesive failure. 

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

It would not be possible to discuss the infinite variations of two-component urethane adhesives in this writing. In order to demonstrate the extremes possible with two-component urethanes, two simplified adhesives are shown below with widely differing properties ... 

The two-component waterborne urethanes are similar in nature to the one-component waterborne urethanes. In fact, many one-component PUD s may benefit from the addition of a crosslinker. The two-component urethanes may have higher levels of carboxylic acid salt stabilizer built into the backbone than is actually needed to stabilize the urethane in water. As a result, if these two-component urethane dispersions were to be used as one-component adhesives by themselves (without crosslinker), they would show very poor moisture resistance. When these two-component urethane dispersions are used in conjunction with the crosslinkers listed in Fig. 8, the crosslinkers will react with the carboxylic pendant groups built into the urethane, as previously shown in the one-component waterborne urethane section. This accomplishes two tasks at the same time (1) when the crosslinker reacts with the carboxylic acid salt, it eliminates much of the hydrophilicity associated with urethane dispersion, and (2) it crosslinks the dispersion, which imparts solvent and moisture resistance to the urethane adhesive (see phase V in Fig. 5). As a result of crosslinking, the physical properties may be modified. For example, the results may be an increase in tensile properties and a decrease in elongation. Depending upon the level of crosslinking, the dispersion may lose the ability to be repositionable. (Many of the one-component PUD s may... 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

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,... 

Goel, A. B., Sag Resistant Urethane Adhesives with Improved Antifoaming Property, U.S. Patent No. 4,728,710,1988. 

Adhesive properties are listed in Table IX. Shear strength, wood failure, and swelling data for two wood species, southern pine and hard maple, illustrate excellent performance properties of lignin-based urethane products. 

Hydroxyl groups formed at the p-carbon atom of the urethane moiety increase adhesion properties. Plurality of intra- and intermolecular hydrogen bonds... 

The LRT data is summarized in Table II. The only change of note is with Urethane C and since that change indicates improved slope error, it cannot be related to adhesive failure. The other small changes are not considered significant enough to warrant any statement about a change in adhesive properties that might cause them. 

The more viscous, mastic-type cements include some of the epoxies, urethanes and sflicones. Epoxies adhere well to both thermosets and thermoplastics. But epoxies are not recommended for most polyolefin bonding. Urethane adhesives have made inroads into flexible packaging, the shoe industry, and vinyl bonding. Polyester-based polyurethanes are often preferred over polyether systems because of their higher cohesive and adhesive properties. Sflicones are especially recommended where both bonding and sealing are desired. 

Polyester polyols are used widely in urethane adhesives because of their excellent adhesive and cohesive properties. Compared to polyether-based polyols, polyester-based polyol adhesives have higher tensile strengths and improved heat resistance. These benefits come at the sacrifice of hydrolytic resistance, low-temperature performance, and chemical resistance. One of the more important application areas for these products is in the solvent-borne thermoplastic adhesives used in shoe sole binding. These products are typically made from adipic acid and various glycols (see Fig. 15). 

The second major classification of common polyurethane adhesives is the two-component system. Two-component polyurethane adhesives are widely used where fast cure speeds are critical, as on OEM (original equipment manufacturers) assembly lines that require quick fixture of parts, especially at ambient or low bake temperatures. Two-component urethanes are required in laminating applications where no substrate moisture is available or where moisture cannot penetrate through to the adhesive bond. Two-component urethanes are also useful where CO2 (generated by a one-component moisture cure) or a volatile blocking agent would interfere with the adhesive properties. 

Uses Defoamer for fiexo inks, adhesives, elastomeric coatings, aq. urethane coatings Properties Liq. 

Chem. Descrip. Acrylic resin Uses Modifier, tlexibilizer tor other acrylics Features Exc. adhesion to plastic, steel, and metallic substrates when formulated with aliphatic polyisocyanates top urethane durability Properties Dens. 8.6 Ib/gal 60% solids G-Cure 869 [Cognis/Coatings Inks]... 

Chem. Descrip. Benzophenone 8 CAS 131-53-3 EINECS/ELINCS 205-026-8 Uses UV absorber/stabilizer for PVC and polyester, oil-based paints, urethanes, pressure-sensitive acrylic and vinyl adhesives Properties Lt. yel. free-flowing powd. insol. in water 20% in methanol, 55% in acetone, 55% in MEK, 30% in xylene, and 2% in hexane mass dens. 1.37 g/cm m.p. 68-70 C 100% act. 

Uses Filler for coatings (interior/exterior architectural, protective, maintenance, marine), silicone and urethane rubbers, adhesives Properties Wh. fine powd. trace thru 325 mesh 2.5 p median particle size sp.gr. 2.65 dens. 22.07 Ib/gal surf, area 1,6 mVg oil absorp. 29.9 brightness 86 m.p. 1610 C ref. index 1.54-1.55 pH 6.3 hardness (Mohs) 6.5... 

Structural panels with outer facings, skins, fire retardants, and cores with foam are glued with EPI adhesives [1,4]. Well formulated EPI adhesives have shown very good wetting and adhesion properties to metals and due to the good processing properties EPI adhesives have taken over some of the epoxy, urethane and cross-linked PVAc markets [1], especially in the USA. 

The adhesion behaviour of urethanes depends not only on their composition but also on the contribution of physico-chemical factors, in particular on the surface properties of urethanes. Two specific aspects of surface behaviour are considered in this chapter, which is arranged in three sections the first concerns the ability of some polymers, including PU to adopt a variety of surface structures or orientations, as dictated by preparation conditions and the medium in contact with the urethane polymer. The ability to undergo surface restructuring is a factor in the well documented difference between surface and bulk properties of all polymers. A second aspect of surface behavior, exerting major influence on the adhesion of urethanes to a variety of plastics or glass, is the acid-base interaction between the adhesive polymer and the substrate. Acid-base interactions are a subject of discussion in the second section of this chapter. Finally, the role of silane modifiers in affecting the adhesion behavior of defined urethane adhesive formulations is considered. 

---