Adhesives, aromatic isocyanates

Aromatic Isocyanates. In North America, aromatic isocyanates ate heavily used as monomers for addition and condensation polymers. The principal appflcafions include both flexible and rigid polyurethane foam and nonceUulat appflcations, such as coatings, adhesives, elastomers, and fibers. 

R is an organic moiety, which can be aliphatic or aromatic. Most commonly used in adhesives are the aromatic isocyanates, e.g., methylene diphenylisocyanate (MDI) and toluene diisocyanate (TDI). These polyisocyanates and others will be discussed in Section 3. 

Polyurethanes produced with aromatic isocyanates tend to yellow rather badly when exposed to the UV rays in sunlight. Internal pigmentation is not always successful in parts that are exposed to the sun. Polyurethanes pigmented with colored pigments (especially white) still change color drastically. The two main problems in obtaining good adhesion of paint to the surface of the part are ... 

Dozens of isocyanate functional compounds have been synthesized, but only a few find much use in urethane structural adhesives. The choices are largely dictated by a combination of performance, price, and safety considerations. Most of the materials used in adhesives are derived from the aromatic isocyanates, toluene diisocyanate (TDI) and 4,4 -diphenyl-methane diisocyanate (MDI). 

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 ... 

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. 

Because of their carcinogenicity, amines have to be monitored. For example, primary aromatic amines (PAAs) are substances that can be transferred from food packaging materials into foodstuffs. In the production of multilayer plastic materials it is common to use reactive adhesive mixtures containing aromatic isocyanate monomers. In cases of incomplete curing, residues of the aromatic isocyanates react with water to produce PAAs. Some of these amines, including... 

Hydroxybenzamines of the general formula have been claimed to improve the adhesion of a wide range of coatings to zinc and cadmium and other metallic substrates when used as either pretreatment primers or additives [63]. Ethylenically unsaturated hydroxy-functional amines have been claimed to improve the adhesion of water-based systems [64], and amines have been examined as adhesion promoters for aromatic isocyanate cured adhesives on glass and other substrates [65]. Primary aliphatic amines are claimed to improve the bondability of polyolefines [66] and an oxyethylated polyethylene... 

Aliphatic isocyanates are used whenever resistance to ultraviolet light is a critical concern. Examples of aliphatic isocyanates are hexamethylene diisocyanate, hydrogenated MDI, isophorone diisocyanate, and tetramethylxylene diisocyanate. Structures for these molecules are shown in Fig. 23. The aliphatic isocyanates are usually more expensive than aromatic isocyanates and find limited use in adhesive applications. Resistance to ultraviolet light is usually not a critical concern in adhesives because the substrate shields the adhesive from sunlight. 

Furthermore, prepolymers of these types can be polymerized with aromatic isocyanate or with aliphatic ones. To formulate the adhesive layer required, both types were tested. The best results were obtained with combinations of aromatic and aliphatic isocyanates. 

Light cured compositions that contain acrylated polyurethanes have a general reputation of possessing good abrasion resistance, toughness, and flexibility. Furthermore, these properties can be varied by picking either aliphatic polyisocyanates, or aromatic ones, depending upon need. Usually, prepolymers derived from aromatic isocyanates yield better adhesion to the substrate than do the aliphatic ones. 

Using heat-reactive p-alkylphenolic resins or ortho-ortho resole resins in aromatic solvent and a polymeric isocyanate in the presence of a small amount of tertiary amine, a rapidly curing adhesive system of good overlap shear for both wood and aluminum can be obtained. The best metal and wood adhesive is based on p-cresol or a straight phenolic ortho-ortho resole in combination with a polymeric aromatic isocyanate such as PAPI or Mondur MRS." ... 

As already mentioned, photolytic decomposition is not a major problem with urethane adhesives. However, thermal oxidation should be considered when the durability of polyurethane bonds is of concern. Aromatic isocyanates are typically more resistant towards this type of oxidation than are aliphatic isocyanates. XDI or HMDI polyester polyurethanes are reported to lose 60-70% of their initial tensile strength after 23 days at 80 C. Similar materials derived from TDI or MDI actually gain 30-40% of their tensile strength after this time. This increase is most likely due to an increase in crosslink density. 

Polyurethanes are formed in the reaction of isocyanates with polyols. The most important commercial aromatic isocyanates are toluenediisocyanate (TDI), diphenylmethane diisocyanate (MDI) and naphthalene diisocyanate (NDI), while the important aliphatic isocyanate is hexamethylene diisocyanate (HDI). Cycloaliphatic isocyanates of industrial importance are isophorone diisocyanate (IPDI) and hydrogenated MDI (HMDI). A number of triisocyanates, such as triphenylmethane triisocyanate, are used in coatings and adhesives. 

Results are presented of a studies of the ageing of PU adhesives for leather, the adhesives used being based on hydroxyl-terminated PU prepolymers prepared from suitable adipate polyesters and aromatic isocyanates. Particular attention is paid to the yellowing of white leather in contact with the adhesive. 35 refs. YUGOSLAVIA EASTERN EUROPE... 

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. 

Keywords Additives Aliphatic Aliphatic isocyanates Amine-isocyanate reaction Aromatic isocyanates Blocked isocyanates Chain extenders Construction (application in) Glass transition temperature Hydroxyl-isocyanate reactions Hot melt reactive PU Isocyanates MDl Moisture curing PU One-component PU Packaging (application in) Plastic and composites (application in) Polyester Polyether polyols Polyisocyanates Sealants Silane PU hybrids TDI Testing of PU adhesives testing of PU sealants Testing standards TPU thermoplastic PU Transportation (application in) Two-component PU Water-borne PU... 

Common isocyanates used as building blocks for polyurethane adhesives include aromatic and aliphatic isocyanates. Commercial aromatic isocyanates include principally toluene diisocyanate (TDI), 4,4 -diphenylmethane diisocyanate (MDI), and polymeric MDI. 

In general, the aliphatic isocyanates have better resistance to degradation by ultraviolet light than do the aromatic isocyanates. In spite of their higher cost and lower reactivity, the aliphatic isocyanates do find some use in adhesives where the adhesive is visible and color stability is important in the final assembly. 

In certain niche applications, aliphatic isocyanates, such as isophorone diisocyanate (IPDI), hexamethylene diisoeyanate (HDI), methylene 4,4 -biscyclo-hexylisocyanate (H12MDI), and polymeric versions of these diisocyanates, are used, e.g., in instances where light stability or reduced reactivity is needed. These isocyanates usually cost more than the aromatic diisocyanates. Thus, they are used in adhesive areas that can Justify the higher costs. 

Chlorinated aromatics, including monochlorobenzene (MCB), o-dichloroben-zene (o-DCB), and p-dichlorobenzene (p-DCB), are the major chlorinated aromatic species produced on an industrial scale. MCB is used as both a chemical intermediate and a solvent. As an intermediate, it is used to produce chloroni-trobenzene, pesticides, and pharmaceutical products. In solvent applications, MCB is used in the manufacture of isocyanates. Its high solvency allows it to be used with many types of resins, adhesives, and coatings. The o-dichlorobenzene is used primarily for organic synthesis, especially in the production of 3,4-dichlo-roaniline herbicides. Like MCB, it can be used as a solvent, especially in the production of isocyanates. It is also used in motor oil and paint formulations. The p-dichlorobenzene is used as a moth repellent and for the control of mildew and fungi. It also is used for odor control. It is a chemical intermediate for the manufacture of pharmaceuticals and other organic chemicals. 

Aromatic polyisocyanates Aromatic polyisocyanates are primarily used for a wide variety of PU foamed plastics, elastomers, and adhesives. More than 90% of PUs are produced from aromatic polyisocyanates. The isocyanate group bonded to an aromatic ring is more reactive toward water or hydroxyl groups than that bonded to an aliphatic compound. The most important aromatic diisocyanates are also given in Figure 4.7. 

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