Лако-красочные материалы — производство

The adhesives industry uses many different products of natural and synthetic origin. These are mainly raw materials that are produced in large quantities and hence at reasonable prices. Raw materials that are used solely or primarily for adhesives are relatively rare or, alternatively, represent special types within fairly broad production programs.

Basic raw materials are natural and synthetic polymers or monomers and prepol­ymers which can form such polymers. The polymers primarily impart the required strength (cohesion) to the adhesive layer. In many cases, however, they have adequate adhesion properties of their own. The adhesion properties may be improved by addition of resins or special adhesion promoters. Plasticizers and resins improve the flexibility of the polymers, increase the tack of the adhesives, or establish other required product properties, e. g., rheological properties. Rheological and other special properties can be influenced by adding fillers. Polymers are often dissolved in water or an organic solvent to achieve the wetting necessary for good adhesion.

The required service properties frequently can be obtained by using different basic raw materials or different kinds of formulations. Accordingly, the raw materials often cannot be assigned unequivocally to specific types of adhesive. Because almost every polymer and resin and also many other substances in principle may be used for the production of adhesives, it is not possible to provide a complete list of every possible raw material. For this reason, only the most important are listed in the following.

Starch [9005-25-8]. Although their significance has decreased, the various types of starch, e. g., mainly potato and wheat starch in Europe, corn starch in North America, and tapioca in Latin America, Africa, and Asia/Pacific region, still represent an important group of raw materials in quantitative terms. Starches are used in native form, in degraded, hot — or cold-soluble form, and as solutions.

In addition, they are used in combination with casein solutions and synthetic resin emulsions. Whereas starch ethers and esters and also aldehyde starches are used to only a limited extent in the adhesives field, the thermal degradation products of starches, the water-soluble dextrins, are still important in quantitative terms as raw materials for adhesives. They are used on their own in aqueous form or as mixtures with synthetic resin emulsions.

Proteins. Whereas vegetable protein is of virtually no significance, milk protein is still important, although increasingly less so, for casein glues. The same applies to collagen from animal skins and bones for glue jellies and glutins.

Cellulose Ethers and Cellulose Esters. Cellulose ethers, primarily methyl cellulose, are used as aqueous solutions (wallpaper paste) and also as thickeners.

Nitrocellulose [9004-70-0] is used as a raw material for solvent adhesives (multi­purpose adhesives, shoe adhesives), although in recent years it has been increasingly overtaken in importance by other polymers.

Natural Rubber [9006-04-6]. Pressure-sensitive adhesives, contact adhesives, rubber solutions, and other solvent adhesives are made from natural rubber (generally first- latex crepe). Solutions of regenerated rubber also have some significance. Before dissolution, natural rubber generally has to be degraded by mastication on rolls or in kneaders and solubilized. Recently, soluble types also have been available. Natural rubber latices (rubber latex) having solids content of from 35 to 70 % are widely used for special latex adhesives (envelope gumming and the like). Compared with synthetic rubber, natural rubber has better tack but is more sensitive to heat and oxidation, particularly in contact with nonferrous metals.

Synthetic Rubber. Many different types of synthetic rubber are suitable raw materials for adhesives and sealing compounds. Particularly significant are polychlo — roprene rubber, styrene — butadiene rubber (SBR), nitrile rubber, and polyisobutylene. Unless these rubbers are available as directly soluble types, they have to be degraded by mastication on rolls or in kneaders and solubilized before dissolution.

Standard contact adhesives and pressure-sensitive adhesives are made from SBR in solution. Styrene-butadiene rubber latices, in some cases containing carboxyl groups, are used for special emulsion-based adhesives. Thermoplastic elastomeric block copol­ymers of styrene with butadiene or isoprene are of increasing significance in contact adhesives and hot-melt adhesives.

Polychloroprene rubbers are the principal raw material for high-quality contact adhesives. Grades with high degrees of crystallinity are used preferably. Polychloro­prene latexes are raw materials for aqueous contact adhesives.

Nitrile rubber is used preferably for contact adhesives with improved plasticizer resistance. Polyisobutylene is used in pressure-sensitive adhesives. Butyl rubber is sometimes added to pressure-sensitive and hot-melt adhesives, although it is mainly used in sealing compounds. Epoxy resins and reactive (meth)acrylate adhesives (reac­tive adhesives) are modified with polychloroprene, butyl, and nitrile rubber. Chlo­rinated rubber is added in small quantities to contact adhesives and also to rubber — to-metal bonding agents for improving the adhesion properties.

Polyethylene [9002-88-4]. Low molecular mass polyethylene, vinyl-modified poly­ethylene and polyethylene waxes are used for hot-melt adhesives.

Chlorosulfonated polyethylene can used in reactive adhesives in combination with (meth)acrylate monomers and also in rubber-to-metal bonding agents.

Ethylene — vinyl acetate copolymers are the most important polymer raw materials for hot-melt adhesives. They contain 18 — 45 % vinyl acetate and have melt flow indices of from less than 1 to 2500. The quantitatively most important copolymers contain ca. 30% vinyl acetate and having melt flow indices of 40-400. Ethylene — vinyl acetate
copolymer emulsions containing ca. 20% ethylene are used in emulsion-based adhe­sives.

Polypropylene [9003-07-0] and various copolymers are used in atactic form for standard hot-melt adhesives.

Poly(vinyl esters) are one of the most important groups of raw materials for adhesives. Poly(vinyl acetates) are important in solvent adhesives. Synthetic resin emulsions, based on poly(vinyl esters) with a solids content of 50-70% are the principal raw materials for emulsion-based adhesives.

Besides poly(vinyl acetate) emulsions, emulsions of vinyl acetate copolymers with ethylene, (meth)acrylates, vinyl chloride, maleic esters and vinyl laurate are of consid­erable importance as internally plasticized synthetic resin emulsions.

Poly(vinyl acetate) emulsions for adhesives contain preferably polyfvinyl alcohol) as a protective colloid. Spray-dried polyfvinyl acetate) emulsions, known as redispersion powders, are used in combination with cellulose ethers as binders in floor-leveling compositions and for increasing solids content in emulsion-based adhesives. Polyfvinyl acetates) are used as solid polymers for formulating solvent adhesives.

Polyfvinyl chloride) [9002-86-2] homopolymers are processed primarily into plas — tisol adhesives and special solvent adhesives. Chlorinated PVCs (ca. 65 % Cl) are more readily soluble and are used for certain solvent adhesives.

Copolymers of vinyl chloride are used both as solid resins, for example, in solvent adhesives (vinyl chloride-vinyl acetate copolymers), and as additives in plastisol adhesives, but especially as synthetic resin emulsions for heat-sealing adhesives. Im­portant copolymers are those of vinyl chloride with vinyl acetate, acrylates, or maleates, as well as copolymers of vinylidene dichloride with acrylates or acrylonitrile.

Polyfvinyl alcohol) [9002-89-5] is used primarily as a protective colloid for polyfvinyl acetate) emulsions. Relatively small quantities are added to modify emulsion-based adhesives. Aqueous solutions of poly(vinyl alcohol), in some cases combined with fillers and emulsions, are used as paper glues and as labeling adhesives.

Polyfvinyl acetals). Poly(vinyl formal) and poly(vinyl butyral) are used in solution as primers, in combination with phenolic resins for specialty reactive adhesives, and as films for the manufacture of laminated glass.

Acrylate and Methacrylate Polymers. Polyfethyl acrylate) and polyfbutyl acrylate) solutions and emulsions are important raw materials for pressure-sensitive adhesives. Copolymers of various esters, which give films of tailor-made hardness and which may additionally contain functional groups (carboxyl, amide, amino, methylol, hydroxyl), are used for pressure-sensitive adhesives to improve the adhesion properties or to enable the adhesive layer to be cross-linked to a limited extent.

Methyl acrylates are used as comonomers for increasing the hardness of the adhesive layer. Acrylate — styrene copolymers are used as emulsions for coating paper.

Solvent adhesives and reactive adhesives are made from homo — and copolymers of methacrylates, generally methyl and ethyl methacrylate and, occasionally, butyl meth­acrylate. Monomeric (meth)acrylates are also used in reactive adhesive systems (po­lymerization adhesives). Poly(ethyIene glycol) dimethacrylates are the basis of anaer­obically curing liquid resins (reactive adhesives). They also are added as adhesion promoters to plastisol adhesives. Acrylate — ethylene copolymers, in some cases with a small content of carboxyl groups, are used instead of ethylene — vinyl acetate copol­ymers as fusible polymers for special hot-melt adhesives. Salts of polyacrylate and acrylate — acrylic acid copolymers are used as thickeners for aqueous adhesive solutions and emulsion-based adhesives.

a-Cyanoacrylic esters (above all methyl, ethyl, butyl, but also allyl and methoxyethyl esters) are used for one-component reactive adhesives.

Poly(vinyl ethers) are used for specialty pressure-sensitive adhesives.

Poly (vinyl pyrrolidone) [9003-39-8] is used in gluesticks and in aqueous pressure — sensitive adhesives.

Polystyrene [9003-53-6] is used as a solution for bonding polystyrene and other plastics.

Polyamides and Polyamidoamines obtained by the condensation of dimerized fatty acids with aliphatic diamines are used in solid form as hot-melt adhesives with particularly pronounced adhesion properties. The low molecular mass condensates, some of which are still liquid and contain free amino groups (amine equivalent ca. 3000-12 000), are used widely as hardeners for epoxy resins.

Copolyamides based on C12 and C14 amino acids or lactams are preferably used as heat-sealing powders for textile applications.

Polyesters. Unsaturated polyesters dissolved in styrene or methacrylates are used as reactive adhesives. Hot-melt adhesives can be made from saturated and fusible poly­esters based on aliphatic and/or aromatic carboxylic acids, in some cases by using dimerized fatty acids. High molecular mass amorphous polyesters and low molecular mass crystalline polyesters are important raw materials for moisture cross-linking polyurethane adhesives in solution or solvent-free systems.

Low molecular mass polyesters and polycaprolactones containing hydroxyl groups are important raw materials for polyurethane adhesives.

Polyurethanes, by virtue of their considerable variability, have become very valuable to the adhesives industry over recent decades. They generally show above-average adhesion and cohesion properties. Low molecular mass reaction products of hydroxyl
compounds and polyisocyanates, which contain cross-linkable hydroxyl or isocyanate groups, are used for various reactive adhesives. The hydroxyl components are aliphatic and aromatic polyesters, polyethers, castor oil, and even simple polyalcohols.

High-polymer polyurethane rubbers with various crystallization properties are used for solvent adhesives, in some cases in combination with resins and other polymers, or for contact adhesives. In many cases, solutions of this type are also used for heat — reactivatable adhesive coats. By adding polyisocyanates, it is possible to improve both the adhesion properties and the bond strength at elevated temperature.

Polyisocyanates. Whereas any standard, generally difunctional, polyisocyanate, but preferably toluene diisocyanate (TDI) and methylenediphenyl diisocyanate (MDI), may be used for the production of polyurethane resins, only polyisocyanates with low vapor pressures at room temperature, such as MDI and its homologues, the reaction product of trimethylol propane with TDI, and triphenylmethane-4,4′,4"-triisocyanate, are rec­ommended as hardeners for adhesives for reasons of industrial hygiene.

Nonaromatic isocyanates such as isophorone diisocyanate (IPDI), hexamethylene diisocyanate prepolymers, and dicyclohexylmethane diisocyanate prepolymers have also gained some importance. Guidelines for the use of isocyanates are given in [27].

Epoxy resins are used primarily in combination with hardeners (polyamines, poly — aminoamides, dicyanodiamide, acid anhydrides) for cold-setting and hot-setting reac­tive adhesives. Liquid epoxy resins of the bisphenol A type with epoxy equivalents of 170 — 300 are used preferably; relatively high molecular mass solid resins are used only occasionally.

In quantitative terms, novolak epoxy resins (epoxy equivalent 120-200) and elas- ticized or otherwise modified bisphenol A (F) resins have relatively little significance.

Epoxy resins are also combined with phenolic resins and poly(vinyl acetate) resins and used as adhesion promoters, for example, in plastisol adhesives.

Phenolic Resins [9003-35-4]. Curable phenol-formaldehyde resins of the resol type are of considerable importance as hot-setting reactive adhesives for bonding wood and metals. Suitable types are used also in combination with poly(vinyl acetals) and epoxy resins for special, in some cases filmlike, reactive adhesives for bonding metals.

Standard and modified phenolic resins, alkyl phenol resins, etherified phenolic resins, and phenol — terpene resins also are used as primers and resin components in other types of adhesives, for example, in contact cements, pressure-sensitive adhe­sives, rubber-to-metal bonding agents, and similar adhesive systems.

Resorcinol Resins [24969-11-7]. Resorcinol — formaldehyde resins and resorcinol — phenol-formaldehyde resins are used as cold-setting reactive adhesives for waterproof wood bonds and as binders between tire cord and rubber.

Urea Resins [9011-05-6]. In quantitative terms, urea — formaldehyde resins are one of the most important groups of raw materials for adhesives. They are used in the form of aqueous solutions or spray-dried powders, generally containing add hardeners, as reactive adhesives for bonding wood, particularly for the production of plywood and chipboard.

Modified urea resins are also important for other purposes, for example for labeling adhesives and as cross-linking components.

Melamine-formaldehyde resins [9003-08-1], like urea resins, are used primarily for bonding wood. Because they are not particularly stable in aqueous solution, mela­mine-formaldehyde resins are marketed primarily in powder form.

Nonreactive resins with a variety of compositions are of considerable importance in the formulation of adhesives. In some cases, they are used on their own in solvent adhesives, so-called resin adhesives, but generally in combinations with polymers, in which they perform various functions. Above all, they increase tack, improve adhesion, influence viscosity, fluidity, and sealability, and, in some cases, also act as plasticizers. The most important applications for nonreactive resins are in pressure-sensitive adhe­sives, contact adhesives, hot-melt adhesives, solvent adhesives, and emulsion-based adhesives. In this context, the term resin covers materials differing very widely in their composition:

— Rosin and its derivatives

— Tail-oil rosins and their derivates

— Hydrocarbon resins

— Carbamic ester resins

— Cyclohexanone resins

Plasticizers are used in adhesive formulations, above all for elastification of the polymer films in emulsion-based adhesives and also for increasing wet tack. They are used, e. g., in homopolymer emulsion-based adhesives. Phthalate plasticizers are the most widely used, diisobutyl phthalate being especially important for emulsion-based adhesives. In addition to phthalate and adipate plasticizers epoxy and phenol sulfonic amide plasticizers are used in PVC plastisols. Besides ester plasticizers, mineral oils and also chlorinated low molecular mass polyethylenes and low molecular mass hydrocar­bon resins are used in rubber adhesives.

Other plasticizers, such as benzoates, citric esters, and glycerol triacetate are less important. In hot-melt adhesives, mineral oil and low molecular mass polyolefins are used.

Solvents, primarily for solvent adhesives, but sometimes also added to emulsions in small quantities as temporary plasticizers, are mainly petroleum, toluene, xylene, dichloromethane, trichloroethylene, trichloroethane, ethyl acetate, acetone, 2-butanone,
methyl isobutyl ketone, and cyclohexanone. The use of solvents has been reduced significantly by the introduction of solvent-free adhesives (e. g., polyurethanes) in several market segments. Aromatic solvents, such as toluene and xylene, and chlo­rinated products like trichlorethane and dichloromethane have been nearly eliminated or are used in closed systems.

Fillers are added to adhesives primarily for establishing certain consistencies and occasionally for reducing cost. Preferred fillers are pyrogenic and precipitated silicas, chalks, and light and heavy spar. Fibers and metal powders are used in special cases [28].

Other Additives. Small quantities of standard preservatives, antioxidants, UV/light stabilizers, colored pigments, and defoaming agents are added to many adhesives.