Soft segment adhesive

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

The role of crystallizable soft segments in urethane adhesives... 

The most common non-crystallizing soft segment in urethane adhesives is based on poly(oxypropylene) polyols, shown in Table 2. Most non-crystalline soft... 

The soft segments made from asymmetrical (amorphous) polyols are important for two-component structural adhesives and one-component moisture-curing adhesives. These materials are applied and usually cured at room temperature. 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

Merrill and Salzman have developed PEO soft segment polyurethanes and indeed have demonstrated minimal adsorption of blood proteins and minimal platelet adhesion on such surfaces121). Nagaoka et al. have studied various methacrylate copolymers with PEO side-chains of varying lengths and showed a direct correlation between minimization of platelet adhesion with increased PEO side-chain length and surface... 

The polyester type polyols used in polyurethane laminating adhesives are produced by the direct esterification of polyfunctional carboxylic acids and glycols. Polyester polyols provide the soft segment in polyurethane products giving the adhesive flexibility. Ester groups of the polyol also contribute to adhesion. Polyester polyols provide limited wetting and adhesion of olefinic surfaces with amide slip additives (in contrast to polyether polyols). Typical examples include adipic acid, caprolactone, maleic acid and isophthalic based polyester polyols. 

Lower molecular weight polyether in the soft segment increases the glass transition temperature and therefore decreases the low temperature limit of elastomeric character of the adhesive. 

The soft segment transitions of 73-15, 73-18, and 73-19 are very similar to those shown for 73-14 in Fig. 1. Table III lists these transitions for each adhesive. The presence of tetrafunctional alcohols in the 73-18 and 73-19 polymers raises the soft segment glass transition temperature of these adhesives slightly as compared to 73-14 and 73-15. 

Figure L The low-temperature dynamic mechanical spectrum of Halthane 73-14 is typical of the 73-series polyurethane adhesives. Two secondary relaxations, Tp and Ty, are shown as peaks in the loss modulus at —100° and —150°C. The soft segment glass transition, Tg(SS), occurs at about —50°C. The frequency of oscillation was held constant during the measurement at 0.1 Hz.

C)8 Pa as compared to 10 Pa) over the temperature range between hard and soft segment glass transitions. Table IV lists the transition temperatures for 87- and 88-series adhesives. 

By proper choice of either the isocyanate or the polyol, actual chemical cross-links can be introduced in either the hard or soft segments that may be beneficial to some properties. The effectiveness of these cross-links is offset by a disruption of the hydrogen bonding between polymer chains. Highly cross-linked polyurethanes are essentially amorphous in character exhibiting high modulus, hardness, and few elastomeric properties. Many adhesives fall into this category. 

Uses Component of thermoplastic urethane elastomers for inj. molding, extrusion, thermoset PU, castable prepolymers, spandex fibers, metal and textile coatings/linings, adhesives, elastomeric polyesters, as polyester modifiers soft segment in elastomers food pkg. adhesives, poly-... 

Uses Soft segment In formulation of elastomers such as thermoplastic urethanes, coatings, fibers, adhesives and sealants, cast urethanes, and copolymers... 

Uses Soft segment or extender in formation of elastomers such as thermoplastic urethanes, polyether esters, polyether amides, coatings, adhesives and sealants, casting resins, and urethane foams Features Resist, to microbial attack, low temp, flexibility and elasticity Properties APHA 50 max. liq. sol. in most org. solvs. and water m.w. 250 sp.gr. 0.991 (40 C) acid no. 0.05 hyd. no. 408.2-498.4 flash pt. 180C 0.03% water... 

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