Polyurethanes moisture-cured

EVA-based adhesives machine more cleanly and are less expensive. For optimum lay-flat and low temperature performance, moisture-curing polyurethane-... 

Moisture-curing polyurethane paints and bituminous paints, specially formulated for the purpose, are also suited for application to damp substrates other polyurethane paints should not even be applied to dry surfaces if the relative humidity is high. 

Example 11. One-Component, Moisture-Cure Polyurethane Sealant. This example is of a low-hardness, high-elongation, moisture-curable polyurethane sealant. The material is based on a low-%NCO prepolymer made from 4,4 -MDI and a low-unsaturation (low-monol-content) Acclaim polyol from Bayer. It is adapted from (a) J. Lear et al., Adhesives Age, February 1999, pp. 18-23 and (b) B. Lawrey, et al., presented at UTECH 2000, The Hague, The Netherlands, March 30, 2000, Crain Communications London, 2000. 

The grit comprised particles ranging from 1 to 2 mm in diameter and the initial studies were into ways of retaining these in a layer of wet adhesive epoxide and moisture-curing polyurethane adhesives were tried. However, adhesion to the substrate was poor (it was found possible to remove easily the entire film of adhesive and grit). 

The use of primer solutions was investigated—chlorinated polypropylene and chlorinated ethylene-vinyl acetate, both in toluene these gave improvements in adhesion, the second being the superior. Such a primer, in conjunction with a moisture-curing polyurethane adhesive, gave a product where it was possible to remove small particles of grit and adhesive only by prising from the surface with a knife. 

Most surfaces of this kind now are laid on a sub-base of hard core, tarmac, or concrete in which has been installed adequate drainage for surface water over this is spread a shock-absorbing layer of porous rubber crumb, made from shredded tyres bonded together as flexible planar material of thickness 5 to 25 mm and supplied in the form of continuous rolls. The rubber layer may be laid loose or pegged, and sometimes is bonded to the sub-base with a moisture-curing polyurethane adhesive. 

Originally for this work two-part polyurethane adhesives (100% solids) were used, mixed immediately beforehand, but material of this type is sensitive during setting to contact with water and rainfall soon after application was found to cause weakness. In consequence, moisture-curing polyurethane adhesives now are preferred such systems can be applied under most weather conditions, even at temperatures down to 5°C. 

The major synthetic adhesives used for bonding wood include urea, phenol, and melamine formaldehyde resorcinol formaldehyde, phenol resorcinol, and polyvinyl acetate emulsions. More recently one-component, moisture cured polyurethane adhesives have become popular for bonding wood. Natural adhesives such as casein and animal glues are also often used for general-purpose wood bonding. Epoxies have been used for certain specialized wood joining applications such as when wood is bonded to metal substrates. 

Anderson, G. J. and Zimmel, J. M., Thermally Stable Hot Melt Moisture-cure Polyurethane Adhesive Composition, U.S. Patent No. 5,939,499, 1999. 

An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formalde-hyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resorcinol-formal-... 

Uradnr. PSM Resins UK Ltd.] Moisture-cure polyurethane resins, polyisocyanate resins for one and two-cotrqx)-nent systems. 

Moisture-Cured Polyurethanes. Some polyurethane coatings form their protective cured film by reaction with the moisture from the air. The properties of the resulting film are different from two-component polyurethanes, but contain a basic urethane side group that classifies them as polyurethanes. 

Moisture-curing polyurethanes can be formulated to produce a wide range of physical properties, from elastomeric and flexible to hard and rigid materials. Among some noteworthy properties are high abrasion resistance and high impact resistance. 

The large volume applications for moisture-curing polyurethanes are discussed briefly in the applications section below. These adhesive offer superior low temperature, high temperature, and specific adhesion performance in comparison to conventional hot melts. These attributes have allowed them to excel in various product assembly applications, including structures such as windows, doors, furniture, and automotive headlamps and trim. Newer applications include bookbinding, fabric laminating, and assembly of athletic shoes. 

Another area for investigation, which has not yet been explored in detail, is the use of sucrose drying oil esters in two-pack or moisture-curing polyurethanes. These types should provide the superior abrasion and chemical resistance required for high performance application,e.g. coatings for chemical plants,floors, etc. 

In some modem curtain coaters, the top tank is a small volume chamber equipped with a rotating double knife. It may apply a thin film of adhesive, with reduced entrapment of air and an electronic control of thickness. With this closed application system there is no evaporation of solvents, it may work with both solvent based or water based adhesives and even the moisture curing polyurethanes. 

The vast majority of one-part elastic adhesives are moisture-curing polyurethanes, which combine high flexibility and elongation at break with good strength characteristics. Moisture has to permeate the adhesive surface and curing starts from the outside of the adhesive to the inside. 

Figure 54 Schematic representation of cross-linking in a moisture-curing polyurethane adhesive. R and R are organic polymer chains.

One-part, moisture-curing polyurethane adhesives are often used. The curing process is therefore mainly dependent on adhesive layer thickness, temperature... 

Table 3-25. The Abrasion Resistance of Moisture-Curing Polyurethane Coatings ...

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