Moisture-cure hot melt adhesives

Most moisture-curing hot-melt adhesives utilize a crystallizable backbone and are based almost exclusively on monomeric MDI at NCO/OH ratios of 1.5 to 2.2. Poly(hexamethylene adipate) polyol is the workhorse of the curing hot-melt adhesives. 

Some recent literature has been published dealing with moisture-reactive hot-melt polyurethane adhesives (Frisch Jr 2002). Most moisture-curing hot-melt adhesives are obtained by reacting a crystallizable polyol such as poly(hexamethylene adipate) and monomeric MDl (NCO/OH ratios = 1.5-2.2). A catalyst such as dimorpholinediethyl ether is also necessary. The polyester polyol plays a key role because open time, viscosity, and glass transition temperature can be adequately tailored. A mixture of hydroxyl-terminated polyesters having different characteristics allows to control the adhesion and hardening time of the adhesive. 

Moisture-curing hot melts are a small but fast growing segment of the urethane adhesive market. They are used mostly in construction and furniture assembly applications. Recent applications include RV sidewall assembly and other OEM automotive applications. Smaller applications include bookbinding and footwear. A typical adhesive is shown below ... 

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. 

Effect of moisture cure on properties of a urethane-based hot melt adhesive... 

Curing hot melts usually have less fixturing strength than do standard hot-melt adhesives but eventually develop structural strength. Once the moisture cure is complete, the curing hot melt has good structural properties and has better elevated heat strength than does a traditional hot melt such as EVA. 

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-... 

Collano, for example, has developed a series of moisture-curing PU hot-melt adhesives for textiles (Collano, 2013). These provide high initial strength due to their hot-melt mechanism, but they cross-link to a heat- and moisture-resistant adhesive on exposure to ambient moisture once the joint is made. As a result, these adhesives have excellent resistance to dry-cleaning, washing detergents, and even sterilization processes. In addition, PU adhesives have low processing temperatures (90—140 °C) and are ideal for heat-sensitive substrates. 

The most significant volume trends, however, are the increasing use of hot-melt adhesives and moisture-curing PUs as replacements for either solvent- or water-based systems. 

Chemically reactive polyurethanes include both one- and two-component systems. One-component systems are usually based on a polyether polyol treated with a polyisocyanate to give an isocyanate-terminated polymer. A one-component system cures when exposed to moisture at room temperature. One-component polyurethane hot-melt adhesives are also cured by moisture after application. Two-component systems result from the reaction of low molecular mass polyols and isocyanates or from isocyanate-terminated prepolymers with either polyols or polyamines. Two-component systems cure at room and/or elevated temperatures. 

Manual application of the adhesives with toothed spatulas, blades, casting knives, or brushes is the simplest but least uniform method of application. Pasty adhesives can be applied from cartridges which are squeezed by hand or mechanical devices. This method is also used for moisture-curing polyurethane hot-melt adhesives with heated cartridges. 

The porous coat system (Nordson) allows the application of discrete, random, and open patterns of hot-melt adhesive to substrates such as films, papers, fabrics, and nonwovens. With the control coat system, hot melt adhesives are applied continuously or intermittently by air-controlled nozzles without contact to the substrate. This technique is used as well for reactive hot-melt adhesives such as moisture-curing polyurethane hot-melt adhesives. 

With moisture curing polyurethane hot-melt adhesives a new innovative technology has been introduced in the sole-bonding process. It can be used as a one-way bonding system or as a two-way contact adhesive, applied either to the sole or to the upper. In this process, an adhesive coat must be apphed on the other substrate. 

This subject has already been discussed in connection with structural metal and wood adhesives. It is intimately connected with whether or not the adhesive is supplied as a two-part material mixed before use or whether any chemical reactivity is latent within the formulation and is released by rise in temperature. Another release mechanism used with adhesives, though not with structural materials, is the moisture curing property of acetoxy siloxanes. Moisture may also limit the life of the isocyanate part of a two-part polyurethane or an isocyanate cured polyester which is, of course, a form of polyurethane. Hot melt adhesives held in reservoirs at elevated temperatures are prone to oxidation and, although actual time limits are difficult to quote, continually recharging the reservoir without emptying and cleaning is a bad habit even when the reservoir is held under a blanket of an inert gas. 

Factors other than pot life can affect the working time of an adhesive or sealant. Some silicones and isocyanates are cured by moisture which causes a skin to form on the surface, the thickness of which increases in proportion to the square root of time. Solvent-based adhesives can suffer from rapid evaporation, which limits their ability to wet the substrates. Hot-melt adhesives are prone to oxidative degradation at temperatures of application. 

Currently, waterborne adhesives are being introduced into the shoe industry. Their performance is quite similar to that of the solvent-borne adhesives, so it can be estimated that for several years they will be used in shoe industry. However, the future seems to be directed through the use of moisture-curing holt-melt urethane and thermoplastic urethane adhesives as they are 100% solid reactive systems and evaporation of solvents is not necessary. Although hot-melt urethanes could replace solvent-borne adhesives, this could take longer to occur because of the vastly different equipment requirements and the change in bonding concept by the shoe manufacturers. 

Reactive hot melts are single-component adhesives that are applied in the molten form (like conventional hot-melt adhesives) but have an additional secondary cure with atmospheric moisture to give a crosslinked polymer. Reactive hot melts will give... 

Current inventions show improvements in solvent utilization, as follows. PVC-based adhesive for PVC pipes, typically containing solution of PVC in tetrahydrofuran, was replaced by solution of chlorinated PVC in 1,3-dioxolane and/or its derivatives whieh are far less toxic than THF. Ethanol is used in polyimide adhesive and dental adhesive. Monomer solvent mixture is used in crosslinkable acrylate adhesive, which permits formulation of VOC-free composition. Polyurethane hot-melt adhesive produced from polyacrylates and polyesters does not need solvents for its production and cure which occurs under the effect of moisture. Similar observations can be made for sealants. For example, sealing agent for semiconductor light emitting elements have been made from acrylic monomers without application of solvent. Material for production of printed wiring board was produced and cured without solvent from polymethacrylate. It is clear from these examples that new processes are consciously directed towards less toxic solutions. 

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