Isocyanates laminating adhesives

The laminating adhesive for Samples 1-3 was a 100 25 mix of an isocyanate terminated polyurethane prepolymer based upon polypropylene glycol and MDI (amount of free monomer approximately 25%) and a tri-functional polypropylene glycol with a molecular weight of approximately 450. 

Bisphenol A (chemically similar to MDA but having -OH groups rather than amine groups) was identified in the water and 3% acetic extracts from Sample 2. Bisphenol A was not detected in the extracts from Sample 1. Infrared transmission spectra recorded through Samples 1 and 2 showed that the MDl isocyanate was slower to react in Sample 2 and so the bisphenol A could possibly relate to the laminating adhesive. As a result, LC-MS tests were undertaken later in the research, on extracts from Sample 3 (laminated with the same adhesive) specifically looking for bisphenol A - none was detected. Tri-ethyl phosphate, thought to be used as a cure catalyst, was identified in the extracts from both Sample 1 and Sample 2. 

FDA regulations only allow the use of aliphatic isocyanates in laminating adhesives for high-temperature applications. (Cured aromatic systems can decompose to regenerate isocyanate and thus form aromatic amines at retortable or boiling water temperatures.) Discussions with a number of UK packaging producers revealed that they are aware of the need to use adhesive systems... 

NovocoU . [IKeice Stevens] Isocyanate-terminated polyester urethane adhesive laminating adhesive for transparent pkg. fiOims, foils, metallized sur-... 

As discussed in the Introduction, in addition to hot-melt adhesives, one of the most important forms of adhesives for the future is the waterborne system. In general, an advantage of the waterborne system is that it is free from the solvent-pollution problem. For this reason, we shall describe several new waterborne systems in detail acrylics, modified ethylene-vinyl acetate, polyurethanes, phenolics, epoxies, isocyanates, and tackifiers. Applications of these systems range from PSA s contact adhesives, and laminating adhesives to wood adhesives. 

Isocyanate-based adhesives are being studied as partial or complete replacements for the traditional formaldehyde-hardened adhesives, such as urea-formaldehyde and phenol-formaldehyde resins. These latter adhesives are being used for the production of wood laminates, bonded wood joints, and as binders for composition board. 

Performance can be enhanced with the addition of an isocyanate crosslinker, which can lead to an overall cost reduction due to the lessened amount of crosslinker required to equal the performance of competitive urethane systems. NeoRez P-900 also features high performance and maintains excellent bond strengths under retorting conditions, which makes it ideal as a flexible packaging lamination adhesive and an alternative to commercial crosslinked, solvent-based urethanes. NeoRez P-900 is free of aromatic isocyanate and amine additives in its composition and is compliant with the U.S. Food and Drug Administration s food contact requirements. 

When UV-curable resins are used as laminated adhesives to protect three-dimensional objects having complex shapes, it may happen that some remote areas will not be accessible to UV radiation and, therefore, remain uncured. To address the issue of such shadowed areas, dual-cure systems combining UV irradiation and thermal treatment have been developed. They contain two types of reactive functions a UV-curable functional group (usually an acrylate double bond) and a thermally curable functional group (usually an isocyanate... 

Parallam, or laminated strand lumber (LSL) is a beam made by a continuous manufacturing process composed of bigger-size wood needles (very elongated wood particles) reassembled with a structural exterior grade adhesive, the favorite adhesive being isocyanates (pMDI) when heat-curing and PRFs when cold-curing. 

Adhesives based on isocyanate (especially PMDl, polymethylene diisocyanate, more exactly polymeric 4,4 -diphenylmethane diisocyanate) have been used for more than 25 years in the wood-based panel industry [88], but still have a low market value in the wood-working industry compared to systems based on UF-, MUF- or PF-resins. The main application is the production of waterproof panels, but also the production of panels from raw materials that are difficult to glue, like straw, bagasse, rice shells or sugar cane bagasse. They can be used as adhesives for wood-based products like particleboard, oriented strandboard (OSB), laminated strand lumber (LSL), medium-density fiberboard (MDF) or... 

Because the reaction between isocyanates and polyols continues for many days after lamination (i.e. the adhesive continues to cure), it is crucial to test laminates for residual isocyanates and for PAA release in a realistic manner. This must not be sooner or later than food packaging would conunence. One must also use realistic pre-test storage conditions for the film (especially with respect to temperature and humidity). Indeed, many laminators routinely determine the extent of the cure of the adhesive over time. Only when a predefined level of cure has been reached can a laminated film be considered to be ready for packaging food. 

If the elapsed time between lamination and food contact is too short, imreacted monomeric isocyanate can be available to react with water to form low molecular weight amines. The presence of aromatic amines can be an issue with the use of MDl or toluene diisocyanate (TDl) containing isocyanate systems. Often aromatic isocyanates are selected to obtain good adhesion under moist and low temperature conditions. Aromatic isocyanates are also cheaper than aliphatic systems. The crystallinity of the film can reduce permeation rates of monomeric isocyanates into adjacent film layers. Other factors that can influence the rate/degree of cure (isocyanate reaction) include the following ... 

Migration of unreacted polyol components through the polyethylene for the polyether-based laminate was observed. Although an excess of isocyanate was present, diffusion of polyol components into the polyethylene prior to reaction with isocyanate was postulated to explain the migration. Cyclic oligomers from the polyol starting materials were identified as the main migrants from the polyester-based adhesive. 

For those products laminated using reactive polyurethane based adhesives, suppliers have been aware for many years of the potential migration of amines formed by the reaction of unreacted isocyanate monomer with water and recommend that laminated products are given time to fully cure before they are used in contact with food. Adhesives have been developed that contain low levels of monomeric aromatic isocyanates, in order to reduce any potential migration of aromatic amines into food in critical applications. Such adhesives are not considered to present any notable amine migration hazard unless used in high-temperature applications. 

Wood was soaked or impregnated with chemicals. After that they were set in a hot press and pressed at 160 iC/0.5-2MPa for 5-30min. [Heat-Pressed Treatment]. Comparison specimens were set in a hot oven and dried at 160 °C for 2-6hrs. [Heat-Dried Treatment]. Heat pressed material was put on top of each other or untreated material with emulsion polymer-isocyanate adhesives to make a laminated board. 

A second type of urethane adhesive is currently being introduced to the U.S. construction industry. The adhesive was originally developed in Japan. The system is two component. The isocyanate component is a modified polymeric MDI. The cross-linkable component contains water with water-soluble or water-emulsifiable resins. The adhesive produces strong, water-resistant bonds and has shown promise as a possible lower cost substitute for resorcinol formaldehyde resins in the adhesion of laminated beams. The product is now being used in Japan to bond wood and various other porous substrates at the rate of about 10 million pounds per year. The first U.S. application of the product has been the bonding of wood doors. The opportunities for products of this type should be excellent in view of the ever-increasing governmental pressures on solvents. 

Polyurethanes are produced by the chemical action of di-isocyanate and polyol. The properties can be varied by the type of isocyanate used and the proportion of the two monomers. There are four main groups of classification for the thermoplastic groups of polyurethane, i.e. rigid foam, flexible foam, non-cellular and cellular polymers. Two main isocyanates used are toluene di-isocyanate (TDI) and diphenylmethane diisocyanate (MDI). Polyurethanes have limited application in the pharmaceutical or medical industries. Polyurethane is used as an adhesive for laminations (thermosetting material). Like thermosetting polyurethane, thermoplastic polyurethanes can be found as esters and ethers. 

In present-day high-speed assembly line operations, adhesives are applied robotically. The adhesive bead is apphed quickly and evenly to parts on a conveyor line just prior to being fitted. These operations, especially the need to handle the adhered substrates soon after assembly, demand fast-curing adhesive systems [65]. Two-component adhesives are used to bond metals to plastics in automobiles, to laminate panels in the construction industry, to laminate foams to textiles, to laminate plastic films together, and to bond poly(vinyhdene chloride) films to wood for furniture. A commercial waterborne two-component adhesive is sold by Ashland under the trademark ISOSET. This system is used for exterior sandwich panels by recreational vehicle manufacturers and is composed of a water-emulsiliable isocyanate and a hydroxy-functionalized emulsion latex. 

Over the last four decades there have been a number of attempts to wed the unique benefits of polyurethane adhesives with the benefits of other adhesive systems. These attempts have led to the reporting of a variety of urethane hybrids. Early work focused on simple blends for example, in 1964 Union Carbide blended organic isocyanates with ethylene-vinyl acetate copolymers [66]. These blends were used as an adhesive interlayer in glass laminations, particularly safety glass laminates. Similarly, polylurethane poxy blends for safety glass laminates were reported in 1970 [67]. 

With both types of adhesives, a long-term, slow release of the VOC are expected, due to their possible absorption into the surfaces and hence, sealing of the adhesive. Adhesives used in carpet backings can contain residual formaldehyde or isocyanates and vinyl acetate and various hydrocarbon solvents (from treatments and adhesives used to laminate the backing). From new carpeting, 4-phenylcyclohexane (a byproduct of styrene-butadiene commonly used to bind the backing) is usually emitted which... 

EPI, Emulsion Polymer Isocyanate, MDI, pMDI, wood adhesives, solid wood panel, parquet, window frame, furniture, plywood, finger joint, glulam beam, I-betim, cross-laminated timber... 

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