Polyurethane adhesives curing

Since these one-component polyurethane adhesives cure through reaction with water, one talks of moisture curing one-component polyurethane adhesives. Their characteristic feature is their excellent adhesiveness. Apart from adhesive applications, these formulations are applied to a great extent as sealing compounds, for example, as fitting foam glue or PU-foam (Section 4.9). 

Figure 7.5 Effect of ageing at 50 °C, 50% RH on lap-shear adhesion of polyurethane/ PVC (a) and polyurethane/ABS (o) joints. Polyurethane adhesive cured according to...

Small die castings are conveniently bonded with polymethylmethacrylate dissolved in its monomer together with a peroxide initiator. Metal naphthenates are to be avoided as accelerators and hence the so-called second generation or toughened acrylics are not recommended for use with zinc. Other acrylics may be used but are slow curing after evaporation of solvent. Polyester adhesives cured with styrene monomer frequently contain some free phthalic acid as an impurity and this militates against their use. Polyurethane adhesives cured with isocyanate should behave quite satisfactorily. 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Initially, the water slowly reacts with the isocyanate. However, the reaction can be catalyzed with an appropriate catalyst, such as dibutyltin dilaurate or a morpholine tertiary amine catalyst. The isocyanate will react with water to form a carbamic acid, which is unstable and splits off carbon dioxide, to produce a terminal amine end group (see p. 76 in [6]). This amine then reacts with more isocyanate-terminated prepolymer, as shown above, to form a polyurea. This process repeats itself, building up molecular weight and curing to become a polyurea-polyurethane adhesive. 

As it had been shown that silanes were effective as pretreatments for a variety of coatings and particularly so when used as additives, selected silanes were examined as pretreatments and additives in conjunction with a two pack polyamide cured epoxide adhesive (Epikote 828/Versamid 115, 1/1) and a structural polyurethane adhesive based on diphenylmethanediisocyanate and a polyester resin. 

E. and Salthammer, T. (2003) Time course of isocyanate emission from curing polyurethane adhesives. Atmospheric Environment,... 

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. 

Figure 1. The dynamic viscosity (G"/w in pascal seconds) of Halthane 73-18 segmented polyurethane adhesive increases with time until the chemorheology of cure is complete. Cure curves for four different temperatures show a positive temperature coefficient which should be proportional to the overall reaction rate.

First-generation solventless polyurethane adhesives are one-component isocyanate terminated prepolymers formed by the reaction of MDI (4,4 methylene bis (phenyl isocyanate)), or other isocyanates with polyether and/ or polyester polyols. One-component 100% solids adhesives rely on moisture from the air or substrates or from induced moisture misting during the converting process, to cure the adhesive via an isocyanate/water reaction and subsequent polyurea-polyurethane polymer formation. Typically the high viscosity of the adhesive is such as to require adhesive delivery equipment and application rollers heated from 65-80 °C for use. They have a high level... 

The water adsorbed on the adherends and present in the ambient air, which is available as a reactant for the moisture-curing one-component polyurethane adhesives (Section 4.2.2). This reaction is also typical for sealing compounds on polyurethane basis (Section 4.9) used, for example, in construction engineering for the sealing of joints between window and door frames and brickwork. 

Figure 4.2 Moisture curing of one-component polyurethane adhesives.

Regarding their curing reaction these adhesive systems differ considerably from the described epoxy and polyurethane adhesives that are characterized by the principle of polyaddition. 

Consequently, this kind of reaction is not based on two monomer molecules A and B of completely different structure, as in the case of epoxy resin and polyurethane adhesives, but on monomers of the same kind or, at least as far as the C=C double bonds are concerned, of similar monomers. The double bond is therefore the precondition for the curing process of acrylate adhesives. 

At this point, an important difference compared to moisture curing, one-component polyurethane adhesives described in Section 4.2.2 has to be mentioned. While for cyanoacrylates already small traces of moisture are sufficient for a quick polymerization, polyurethanes require a considerably higher moislure amount for complete curing, because in such bonds, water in chemically bound form becomes a component of the adhesive layer. In the case of cyanoacrylates, water is merely the starter of the curing reaction. 

Seven polyurethane adhesives have been developed at Lawrence Livermore National Laboratory (LLNL). These adhesives, designated Halthanes were synthesized because of OSHA restrictions on the use of the curing agent methylene bis(2-chloroani1ine). Four of the Halthanes were made fromLLNL-developed 4,4 -methylene bis(phenylisocyanate) terminated prepolymers cured with a blend of polyols three were made from an LLNL-developed prepolymer terminated with Hylene W and cured with aromatic diamines. In this paper we report the dynamic mechanical and thermal behavior of these seven segmented polyurethanes. 

STRUCTU RE 4.3 Schematic curing agent. (Reprinted from O. Figovsky and L. Shapovalov, Cyclocarbonate-Based Polymers Including Non-Isocyanate Polyurethane Adhesives and Coatings, in Encyclopedia of Surface and Colloid Science, ed. P. Somasundaran, vol. 3, 1633-1653. New York Taylor Francis, 2006. With permission.)... 

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