Urethane adhesives durability

In order to address these issues, a brief discussion of thermal, oxidative, and hydrolytic stability of urethanes will be offered, so as to aid the adhesion scientist in designing a urethane adhesive with the desired durability. 

Radiation curable adhesives can be based on raw materials that are not greatly different from those used in acrylic adhesives (first, second, and third generations), epoxy adhesives, urethane adhesives, and anaerobic adhesives. The formulation approach, however, is somewhat different. Very important is that no solvents are used to thin the resins a monomer reactive diluent is used instead. The monomer must be matched with the resin to give the desired set of properties with respect to adhesion, substrate, flexibility or stiffness, cure behavior, and durability of the cured product. Most of the radiation curable adhesives have an acrylate (or methacrylate) basis many of the acrylics are modified. 

As already mentioned, photolytic decomposition is not a major problem with urethane adhesives. However, thermal oxidation should be considered when the durability of polyurethane bonds is of concern. Aromatic isocyanates are typically more resistant towards this type of oxidation than are aliphatic isocyanates. XDI or HMDI polyester polyurethanes are reported to lose 60-70% of their initial tensile strength after 23 days at 80 C. Similar materials derived from TDI or MDI actually gain 30-40% of their tensile strength after this time. This increase is most likely due to an increase in crosslink density. 

Performance and durability of wood products bonded with isocyanate and urethane adhesives have been the subject of several investigations. The strength of composition board is generally determined by its modulus of rupture (MOR), which is a flexure to break test its modulus of elasticity (MOE), or stiffness and its internal bond strength (IB), or tensile strength. These tests are run on dry and on water-soaked or wet samples, and are described in ASTM test method D 1037. As compared to phenol-formaldehyde adhesives, composition boards prepared with isocyanate or urethane adhesives are reported to display similar or superior initial strengths. Moisture resistance of isocyanate-bound par-... 

Commonly used adhesives include epoxy, rubber, acrylic or vinyl emulsions, and urethanes. Epoxies are high-priced, but they have better chemical resistance and durability than the others, and they have dominated the market in outdoor applications. Significant advantages of the epoxy-based adhesives are that they have no solvents and, therefore, exhibit little shrinkage. They cure relatively fast and, therefore, are not as exposed to inclement... 

All of the commercial epoxy adhesives presented in App. B bond well to aluminum and to a wide variety of other materials. Sell22 has ranked a number of aluminum adhesives in order of decreasing durability as follows nitrile-phenolics, high-temperature epoxies, elevated-temperature curing epoxies, elevated-temperature curing rubber-modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes. 

The durability and solvent resistance of nitrocellulose-urethane finishes have prompted studies of these coatings for metal substrates. Excellent adhesion to steel and aluminum has been demonstrated. Pigmented systems dry rapidly and have high initial gloss and scratch resistance. 

Chem. Descrip. Aliphatic urethane diaciylate Uses Urethane-aciylic in exterior coatings for wood, rigid and flexible plastics and metal, low gloss coatings Features Good flexibility, light fastness, exterior durability, adhesion, and abrasion resist. 

Uses Urethane-actylic in silk-screen inks tor plastics, laminating adhesives, clear coating for paper, plastics, and wood Features Good exterior durability, flexibility, toughness and adhesion,... 

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]... 

Adhesives (128) play an important role in the construction of a solar parabolic trough reflector. The adhesives should be compatible with silvered glass mirrors and should be able to carry structural loads. Moreover the adhesives should be durable for at least twenty years without losing their properties. Adhesives screened for solar collectors were epoxies, urethanes, and acrylics. Three types of stress conditions were identified as the most likely causes of failure in the trough modules ... 

CHEMISTRY, DURABILITY OF STRUCTURAL ADHESIVE BONDS Urethane Based Adhesives... 

Fig, 13. Sustained Load Durability Results for Room Temperature Cured Urethane Based Paste Adhesives on FPL Etched 5052T-4 Bare Aluminum... 

When using two-part urethane structural adhesives, priming of the substrates is often required in order to obtain optimum bond durability, particularly when bonding metals. Primers are generally developed for use with a particular combination of adhesive and adherends. There are various types of primers for polyurethane adhesives, including one-part heat cure epoxies, two-part heat and room temperature cure epoxies, and one-part urethanes. 

Table VII. Durability of Several Commercial Urethane Structural Adhesive OLS Bonds (psi)...

The ample in-service durability of urethane structural bonds has been demonstrated by their use in the trucking and RV industries for nearly 20 years. Extensive laboratory testing of these adhesives has also illustrated their very good durability as shown in Table VII. 

---