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Adhesive Formulations

One-Part Adhesives

There is a general industrial move away from application of solvent-borne PU adhesives, but they retain high utility, particularly in the shoe sole market. The one-part adhesive is essentially a thermoplastic PU (TPU) dissolved in a solvent. The solvated TPU is applied to the prepared surface as a contact cement, or is applied, allowed to dry, and then adhesion generated by

Test

Standard description

ASTM D4896

Standard test for performance of adhesion test in a single lap-shear geometry—one of the most common tests and also one of the most difficult to perform for unambiguous data

ASTM D1876

Standard test for adhesion in a T-peel geometry. Primarily used for flexible substrates requiring flexible adhesive such as PU. See also D2919.

ASTM Dl 144

Standard test for determining strength development of adhesive bonds. This test is particularly relevant to PU-reactive adhesives that require days to weeks to develop full adhesion and adhesive strength.

ASTM D905

Standard test method for strength properties of adhesive bonds in shear by compressive loading. Applies specifically to wood substrates for which PU adhesives are highly employed. Standard is very specific about all spects of test.

ASTM D950

Standard test method for impact strength of adhesive bonds. Uses a pendulum impactor on substrates in a shear geometry with a standard velocity of 3.4m/s (7.6 mph).

ASTM D897

Standard test method for tensile properties of adhesive bonds. Utilizes metal substrates and a strain rate of 0.05 in/mm.

ASTM D1151

Standard practice for effect of moisture and temperature on adhesive bonds Continuous exposure test for samples exposed to conditions from -57 to 316 °C and from 50% humidity to saturated by soaking. Samples at high-temperature conditions are aged in oven with uncontrolled humidity.

ASTM D7105

Standard test method for determining the adhesive and cohesive strength between materials in roofing or waterproofing membranes and systems. These are applications for which PU adhesives are commonly applied.

ASTM D896

Standard practice for resistance of adhesive bonds to chemical reagents. Quantification of adhesive performance following chemical exposure is a requirement for many applications.

ASTM D4501

Standard test method for shear strength of adhesive bonds between rigid substrates by the block shear method. Specific to substrates with higher modulus that the adhesive and where machining of the substrate is impractical such as with glass bonding—a common substrate for PU adhesives for windshield adhesive/sealants.

ASTM D3930

Standard specification for adhesives for wood-based materials for construction of manufactured homes. Comprehensive standard for application commonly employing polyurethane adhesives.

ASTM D1337

Standard practice for storage life of adhesives by viscosity and bond strength. A common qualifying attribute for all adhesives including polyurethanes.

ASTM C1523

Standard test method for determining modulus, tear, and adhesion properties of procured elastomeric joint sealants

ASTM D3433

Standard test method for fracture strength in cleavage of adhesives in bonded metal joints. Specifies metal but is applicable to all stiff adherends using structural adhesives not prone to elastic deformation. Employs a dual cantilever beam geometry.

ASTM D 4497

Standard test method for determining the open time of hotmelt adhesives. Very simple manual test.

ASTM D 4498

Standard test method for heat-fail temperature in shear of hotmelt adhesives. A common test for hotmelt adhesive performance.

TABLE 10.3 Example formulation and properties of a one-part solvent-borne polyurethane adhesive

Example formulation and properties of a one-part solvent-borne polyurethane adhesive

contacting the adherends with heat. The adhesive designer will design the system to meet the performance and process requirements of the application [19].

Although polyether soft segments dominate the adhesive application, many one-component solvent-borne PU adhesives will employ a polyester soft segment such as polybutylene adipate for the simple reason that its ability to crystallize can increase the initial adhesion between adherends [20]. A typical formulation is suggested in Table 10.3. The TPU would normally be polymerized in a separate operation and then dissolved in the adhesive solvent. It is noticeable that the TPU has a relatively low hard segment fraction, about 25%, and that the hard segment length (BDO/PBA) is approximately 1. This assures that the resulting adhesive will have a low glass transition temperature for good performance at low temperatures, and also that the adhesive will possess very good flexibility. If the designer determines that the process requires more rapid buildup of adhesion properties, hexanediol could be substituted for butanediol in the soft segment. It is known that for systems that use one-part solvent-borne PU adhesives, pretreatment of the surface can be critical to performance [21]. This can involve extensive roughening of the surfaces to be joined, and also the use of a primer coating that has preferential adhesion to the substrate and the TPU, but does not possess sufficient properties as a sole adhesive layer to function in the application [22]. Typical primer polymers are substances such as chlorinated polyethylene at high dilution in an aromatic solvent such as toluene. For these systems, the solvent can also be a major participant in the growth of adhesion by swelling both the substrate and the adhesive polymers to promote polymer chain entanglement or physical interlocking of larger polymer segments that form as the adhesive laminate swells and then contracts [23, 24].

A two-part solvent-borne version of PU adhesives is typically an isocyanate-tipped prepolymer with relatively low isocyanate content (5-12%) on the one side, and chain extender (diol or diamine), catalyst, and often a cross-linker for the second component. A two-part solvent-borne system has the benefit of allowing the opportunity to tune adhesive modulus and improve solvent resistance via the cross-linking capability. However; beyond the obvious problem with VOCs, this format suffers from the complication of properties being dependent on accurate weighing of the adhesive parts, and also the unavoidable limitations on open time that come from a reacting system. In addition, PU side reactions, particularly with ambient water, are capable of producing C02 gas. In the limit of high humidity, this can have the very undesirable effect of foaming the adhesive layer. A merge of one-part and two-part adhesive technologies is achieved by combining all PU components into a single solvent system but employing a blocked isocyanate. Blocked isocyanates are standard polyisocyanates prereacted with weakly bonded active hydrogen groups. When heated, the weakly reacted components decouple releasing the original poly-isocyanate and the blocking agent. This technology of course requires that the bonded substrates be heated under compression to effect the desired polymerization and bonding chemistry (see Section 10.3.1.5).

 
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