Urethane adhesives formulations

The adhesion behaviour of urethanes depends not only on their composition but also on the contribution of physico-chemical factors, in particular on the surface properties of urethanes. Two specific aspects of surface behaviour are considered in this chapter, which is arranged in three sections the first concerns the ability of some polymers, including PU to adopt a variety of surface structures or orientations, as dictated by preparation conditions and the medium in contact with the urethane polymer. The ability to undergo surface restructuring is a factor in the well documented difference between surface and bulk properties of all polymers. A second aspect of surface behavior, exerting major influence on the adhesion of urethanes to a variety of plastics or glass, is the acid-base interaction between the adhesive polymer and the substrate. Acid-base interactions are a subject of discussion in the second section of this chapter. Finally, the role of silane modifiers in affecting the adhesion behavior of defined urethane adhesive formulations is considered. 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

Each of these product types will be discussed, including the method of application, the type of substrates that are bonded by each approach, and one or more typical adhesive formulations. In order to understand the adhesive cure mechanism, a brief review of the common urethane reactions is needed. 

The majority of adhesive laminations in multilayer flexible packaging are manufactured using the dry-bond process. In this technique, a liquid adhesive is applied to one substrate. The adhesive is then dried using hot air. This dried surface can be adhered to a second substrate using heat and pressure at a nip point. The adhesive formulations themselves represent a reactive chemistry (typically urethanes or acrylics) that is chosen to withstand the processing and storage/distribution environment of the filled product. The adhesives polymerise and/or cross-link during production of the laminated product. 

A typical formulation of a two-component epoxy urethane adhesive would include 100 parts of a water dispersed polyurethane resin with carboxylic... 

In the U.S. converting industry, one- and two-component solvent-based adhesives are still the most widely used, accounting for nearly 80% of adhesive formulations. Many of these adhesives are typically polyester-or polyether-based urethanes (see Chapter 4) with isocyanate fimctionaHty that cure by reacting either with atmospheric moisture or a cross-linker. Polyether-based urethanes, while less expensive than polyester-based, do not provide the same strength and do not perform well in some processing environments or end uses. 

The essential ingredients of a free-radical adhesive formulation are an acrylate-terminated prepolymer and a photoinitiator. A wide range of prepolymers can be acrylated, including epoxies, urethanes, polyesters, polyethers, and rubbers. Those most commonly used in adhesive formulations are epoxy and urethane acrylates. Epoxy acrylates have properties similar to those of the parent epoxy resin, with excellent adhesion, chemical resistance, and toughness. Urethane acrylates, on the other hand, are noted for their high reactivity, good adhesion, flexibility, and tear resistance. 

Fillers are used in adhesives to improve physical properties, to control rheology, and to lower cost. The most common polyurethane fillers are calcium carbonate, talc, silica, clay, and carbon black. A more rigorous treatment of this subject can be found in Katz and Milewski [47]. Fumed silicas and carbon blacks are used primarily as thixotropes in application areas that require a nonsagging bead. Calcium carbonates, clays, and talcs are used to improve the economics of an adhesive formulation. A major concern using fillers with urethane prepolymers is the moisture content associated with the fillers. Fillers typically must be dried prior to use with urethane prepolymers or isocyanates. Hygroscopic fillers should be avoided, as moisture introduced by the filler can lead to poor shelf stability of the finished product. 

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. 

A wide variety of epoxy resins are commercially available monofunctional or polyfunctional, aliphatic, cyclic, or aromatic. Brominated epoxies may be useful where flammability is a concern. An oxirane functionality is all that is needed to make an epoxy resin, and structural adhesives are only one of over a dozen different uses for epoxy resins. Many epoxy resins on the market will not necessarily be suitable for adhesives, but their availability does expand the choices available for adhesive formulators. The specialty epoxy resins developed specifically for adhesive use sometimes will be more costly than the DGEBPA resins but may provide the basis for a specialty adhesive that can meet a unique need and therefore command a proportionally higher price. Examples of these are epoxy-fimctional dimer acids, urethanes, and various elastomers. 

One-part urethane adhesives have been used for many years as high performance sealants. In this capacity they provide a useful combination of strength, flexibility, and elastic recovery. As adhesives, these systems have limited use im-less formulated to overcome their inherent disadvantages. One-part polyurethane adhesives are typically moisture-cured and rely on a multistep reaction sequence as follows isocyanate reacts with water to form carbamic acid, the unstable car-bamic acid loses carbon dioxide and generates an amine, the amine reacts with additional isocyanate to form a urea, and the urea reacts with additional isocyanate to form a biuret, which includes a cross-link. Unless it diffuses out of the system, the CO2 can cause foaming. Formulators learn to minimize the isocyanate content (%NCO) of a system in order to balance cure speed with foam control. Cure speeds—and foaming rates—of these systems decrease from the outside in and vary with the amount of atmospheric moisture in the air, which changes hourly and seasonally. 

Acetone and methyl ethyl ketone are components of solvent blends in urethane, nitrile rubber, and neoprene industrial adhesives. Acetone is the primary solvent in resin-type adhesives and pressure sensitive chlorinated rubber adhesives. Methyl isobutyl ketone is a solvent component for nitrile rubber and acrylic adhesives as well as in polyvinyl chloride and polyvinyl chloride-polyvinyl acetate copolymer bonding adhesives. Again, the desired evaporation rate of the adhesive formulation will often determine the ketone selected. 

Methylene chloride is a widely used chemical solvent with a diverse number of applications. It was introduced as a replacement for more flammable solvents over 60 years ago. Methylene chloride is commonly used in paint removers and industrial adhesive formulations. It also is employed in the production of flexible urethane foams, pharmaceutical products, and plastics, as a cleaning agent for fabricated metal parts, and as an extraction solvent. 

The main reasons for the use of methyl chloroform in formulations for urethane and neoprene/phenolic contact adhesives, mastics, sealants, and natural mbber tire repair cements are its ability to substantially reduce flammability, its non-photochemical reactivity, and the favorable characteristics of the resulting adhesive formulation. 

Thus, a urethane adhesive can be based on a wide range of polymers as long as they can be reacted to form a urethane linkage. In practice, polymers can contain aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea and allophanate groups. This leads to a very wide range of raw materials being available, and formulations tend to differ in the type of polymer used rather than on the variations in additives, as seen with many other adhesives. 

NeoRez R-1400 is an aliphatic polyester urethane polymer dispersion developed for heat-activated adhesive formulation. NeoRez R-1400 offers the combined properties of low temperature activation, good early green strength, and high temperature resistance when crosslinked with poly-dispersible isocyanate. 

Method D appears to be possibly the most important type of isocyanate-based adhesive system. It is similar to Method B in that a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly until the full bond has formed. Method D differs from Method B in that its vehicle polymer is a polyurethane. A further difference is that the inherent adhesive character and strength of the polyurethane vehicle frequently enables its use without added di- or poiyisocyanate. This strength may be realized in essentially amorphous compositions such as the thermoplastic polyurethane elastomers or millable gums. Or it may be achieved with crystallizing urethane adhesive polymers. 

Rigid Urethane Adhesives Evaluation Formulations, Technical Data Bulletin S-10, Revised 3/1/65, Mobay Chemical Co., Pittsburgh, Penna. 

Aminoamides Polyaminoamides are used to improve bonding of PVC plastisols to metal surfaces. Used at 3-5 phr, generally dissolved in solvent or plasticizer, such products are available under the trade names Euretek (Schering) and Versamid (Henkel = Cognis). The bond formed is to the surface metal oxide layer. Combinations with urethane adhesion promoters should not be used, since the aminoamides are strong urethane catalysts. A topcoat (without adhesion promoter) is usually desirable it should be formulated to resist amine stain. Phenolic antioxidants used with aminoamides (or other amine additives) should have all ortho and para positions blocked to prevent color development on aging. Similarly, aliphatic phosphites are the best choice. 

J. F. Regan, "Urethane Formulations," Proceedings of Caulks and Sealants Short Course, The Adhesive and Sealant Council, Dallas, Tex., 1990. 

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