Polyurethane hot-melt adhesive

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-... 

Reactive Polyurethane Hot-Melt Adhesives (Solvent-Free)... 

Water-based dispersions or emulsions such as polyvinyl acetate, acrylics, polyvinyl chloride and polyvinyl alcohol with plasticizers and tackifiers. In addition, this range can include urea formaldehyde and phenolic adhesives, resins, natural adhesives produced from starch, dextrin, casein, animal glues (see Polyvinyl alcohol in adhesives, Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs. Animal glues and technical gelatins) and rubber latex (see Emulsion and dispersion adhesives). Solvent-free 100% solids such as polyurethane. Hot melt adhesives include Ethylene-vinyl acetate copolymers, polyolefins, polyamides, polyesters with tackifiers and waxes. More recent additions include cross-linkable systems. 

Reactive polyurethane hot-melt adhesives, one-pack, setting under the influence of atmospheric moisture. Uses bookbinding (high-quality perfect bindings), wood gluing, shoe manufacturing. 

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. 

EVA hot-melt adhesives have heat resistance up to 85 T, polyolefin and polyamide hot-melt adhesives up to 130 °C, and reactive polyurethane hot-melt adhesives up to 150 °C. The cold resistance lies between -20 and -30 °C in the case of polyurethane-based hot-melt adhesives, it is even lower than - 30 °C. 

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. 

In the interior of the vehicle, prefabricated roofs consist of outer decorative layers that are bonded to the layers imparting mechanical rigidity (emulsions, contact adhesives, polyurethane hot-melt adhesives). Contact adhesives, polyurethane adhesives, and high-frequency welding auxiliaries (resin solutions or PVC-based emulsions) are used for attaching textiles or plastic films to the supports made of molded fiberboard or plastics for door inner panels. Emulsions, contact or polyurethane hot-melt adhesives are used for bonding textiles to molded fiberboard or plastics for parcel shelves. 

On the exterior of the vehicles, rubber profiles around windows and doors and the trunk lid are "lap bonded with cyanoacrylate adhesives because continuously extruded profiles cannot be used for sharp comers. To obtain effective sealing and better sliding in the case of sliding windows, these profiles are flocked by means of polyurethane-based flocking adhesives. The lenses of headlamps are joined to the housing by polyamide or polyurethane hot-melt adhesives and epoxy resins. The layers of laminated safety glass are bonded with a film based on poly(vinyl butyral). 

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. 

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. 

This type of adhesive is generally useful in the temperature range where the material is either leathery or mbbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mosdy uncompounded state. 

Pentaerythritol in rosin ester form is used in hot-melt adhesive formulations, especially ethylene—vinyl acetate (EVA) copolymers, as a tackifier. Polyethers of pentaerythritol or trim ethyl ol eth an e are also used in EVA and polyurethane adhesives, which exhibit excellent bond strength and water resistance. The adhesives maybe available as EVA melts or dispersions (90,91) or as thixotropic, one-package, curable polyurethanes (92). Pentaerythritol spko ortho esters have been used in epoxy resin adhesives (93). The EVA adhesives are especially suitable for cellulose (paper, etc) bonding. 

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. 

Some grades of polyurethane and polyester copolymers are used as hot-melt adhesives. AppHcations include shoe manufacture and as an adhesive interlayer in coextmsion. 

Whilst approximately twice the raw material cost of TPO- and S-B-S-type polymers, thermoplastic polyurethane elastomers find applications where abrasion resistance and toughness are particular requirements. Uses include gears, timing and drive belts, footwear (including ski boots) and tyre chains. Polyether-based materials have also achieved a number of significant medical applications. There is also some minor use as hot melt adhesives, particularly for the footwear industry. 

Analytical studies of the PVC profile surface indicated that the reasons for failure of the adhesive bond between the PVC film and PVC profile in decorative film laminated structures, was finely dispersed lubricant and wax particles on the PVC profile. These contaminants prevented the build-up of bond forces at the interface between the adhesive and PVC profile. A biodegradable alkaline cleaning agent was developed to remove the contaminants, which, when combined with flame treatment and a solvent free polyurethane hot melt... 

According to the chemical structure of the hot-melt adhesive polymers (polyamide resins, saturated polyester, ethylene vinyl acetate copolymers, polyurethanes), the processing temperatures range between 120 and 240 °C. 

As previously noted, the primary use for this elastomer has been as solvent-based adhesives and sealants. Solvent-based products are losing market share to water-based poly-chloroprenes, to other polymer types such as acrylics and polyurethanes, and to hot melt adhesives. However, where the processing facility is able to contain the vapor emissions, a solvent-based adhesive or sealant is preferred because of better wetting of surfaces, faster drying, and higher performance of the cured or dried product. Many rubber bonding... 

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