Vinyl acetal-phenolic adhesives

PVAc-based commercial wood adhesives are evaluated using standard tests for non-structural applications, as reported in EN 205 [8], and they are classified in agreement with the standard EN-204 [9]. This standard allows to classify wood adhesives in 4 categories from D1 to D4. D1 adhesives show a good resistance only in dry conditions D2 adhesives should withstand a rather low water presence, such as in occasional exposure in kitchens and bathrooms D3 adhesives are suitable to come in contact with cold water, such as for outside windows and doors, kitchen and bathrooms furniture D4 adhesives are suitable to be used in extreme conditions (resistance to hot water). Vinyl acetate homopolymer can be used to formulate D1 or D2 adhesives. Vinyl acetate based adhesives cross-Unked with hardeners and urea-formaldehyde (UF) adhesives belong to class D3. Only the phenol-formaldehyde (PE), resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF) adhesives, some special 2-component polyurethanes (PUs), and cross-linking vinyl adhesives belong to class D4. 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Compared with the phenolics and polyesters the resins have better heat resistance, better chemical resistance, particularly to alkalis, greater hardness and better water resistance. In these respects they are similar to, and often slightly superior to, the epoxide resins. Unlike the epoxides they have a poor adhesion to wood and metal, this being somewhat improved by incorporating plasticisers such as poly(vinyl acetate) and poly(vinyl formal) but with a consequent reduction in chemical resistance. The cured resins are black in colour. 

The PVF is made by acidic reaction between poly(vinyl alcohol) (PVA) and formaldehyde. The poly(vinyl alcohol) is, in turn, made by hydrolysis of poly(vinyl acetate) or transesterification of poly(vinyl acetate). Thus, residual alcohol and ester functionality is usually present. Cure reportedly occurs through reaction of phenolic polymer hydroxyls with the residual hydroxyls of the PVA [199]. The ester residues are observed to reduce bond strength in PVF-based systems [199]. This does not necessarily extend to PVF-P adhesives. PVF is stable in strong alkali, so participation of the acetals in curing is probably unimportant in most situations involving resoles. PVF is physically compatible with many phenolic resins. 

Although the manufacture and sale of adhesives is a worldwide enterprise, the adhesives business can be characterized as a fragmented industry. The 1987 Census of Manufacturers obtained reports from 712 companies in the United States, each of which considers itself to be in the adhesives or sealants business only 275 of these companies had more than 20 employees. Phenolics, poly(vinyl acetate) adhesives, rubber cements, and hot-mclt adhesives arc the leading products in terms of monetary value. These products are used primarily in llie wood, paper, and packaging industries. The annual growth rate of the adhesives market is 2.3%, and individual segments of the market are expected to grow faster than this rate. 

A new approach was proposed for making effective helmets which could replace the former British army steel helmet. Essentially the new helmet used modified phenolic resins reinforced with nylon, and the crown cap inside was thermoformed from polyethylene. Formerly the crown cap was attached to the steel by rivets—not an appropriate method for fixing polyethylene to reinforced plastics. Instead a method was developed with a hot-melt adhesive based on ethylene-vinyl acetate copolymers cast as film on release paper. For assembly, the cast film is cut in advance to match the intricate shape required and activated by heat to bond under light pressure subsequently, a further heat activation is employed to fix the crown cap in place (Figure 52 illustrates this). 

Tackifiers are used to increase the tackiness and the setting speed of adhesives. They increase tackiness by softening the poly(vinyl acetate) polymer in the wet and the dry adhesive film. Tackifiers are usually rosin or its derivatives or phenolic resins. Other additives frequently needed for specific application and service conditions are antifoams, biocides, wetting agents, and humectants. 

Poly(vinyl alcohol) is utilized as a component of starch-based adhesives.11121114 Other patents report the use of partially oxidized starch,1115 dextrins,1116 dextrins and urea,1117 borax,1118 boric acid,1119 and vinyl methyl ether-maleic acid copolymers.1120 Other patents indicate the use of poly (vinyl alcohol) with partially hydrolyzed poly(vinyl acetate),1121 nonhy-drolyzed poly(vinyl acetate),1122 and poly(vinyl chloride).1123 A few patents have reported such poly acrylic additives as poly (acrylic acid)1124 and its salts,1125 poly(acrylamide),1126 1127 A-methylacrylamide or poly(A-acryl-amide),1128 and polyethyleneimine.1129 Polystyrene has also been used,1130 as well as more complex copolymers such as a maleic acid monobutyl ester-methyl vinyl ether copolymer, together with dextrin and polyacrylamide),1131 carboxylated ethyl acrylate-styrene zinc salt copolymer,1132 ethylene-methyl acrylate-vinyl acetate copolymer,1133 vinyl acetate-vinyl pyr-rolidone copolymer,1134 and ethylene-vinyl acetate copolymer.1135 Some adhesives are compounded with SBR latex1136 1138 and phenol-formaldehyde resins.1139... 

In the following discussion, only the most widely used adhesive types are described. These are the urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins, phenol-formaldehyde (PF) resins, diisocyanates, polyisocyanates, polymers and copolymers of vinyl acetate, and polyamides. These are all predominantly thermosetting resin systems. 

The most widely used thermoplastic polymer is the ethylene—vinyl acetate copolymer, which is obtainable in a wide range of molecular weights as well as in a variety of compositions. Often flexibilizers or plasticizers are added in order to improve both the mechanical shock resistance and the thermal properties of the adhesive. Polybutenes, phthalates, and tricresyl phosphate have been used as plasticizers. Tackifying agents can also be added. Because hot-melt adhesives are frequendy ethylene-based, they are subject to oxidation if, as in a typical situation, the adhesive sits in an applicator for long periods before use. Thus, antioxidants such as hindered phenols are often used, as are fillers. Fillers are added to opacify or to modify the adhesive s flow characteristics, as well as to reduce cost. Wax is also a very important component. Wax alters surface characteristics by decreasing both the liquid adhesive s surface tension and its viscosity in the melt. Upon solidification, however, the wax acts to increase the strength of the adhesive. Both paraffin and microcrystalline wax are used (see Waxes). 

The coreaction of phenolic resins with alkyds is an "old timer" in which the excellent gloss retention and durability of alkyds are combined with the water and alkali resistance of phenolics. Alkyd resin can be designed to have compatibility with hydroxyl-modified vinyl chloride-vinyl acetate copolymer resins, and this combination of fast drying, adhesion, excellent water resistance, and durability is specified in a number of coatings for military and marine use. 

A great many of outstanding adhesive formulations are based on epoxy resins. A broad spectrum of adhesive formulations with a wide range of available properties have resulted from the use of polymeric hardeners such as polyamides and polyamines, phenolics, isocyanates, alkyds, and combinations of amines with polysulfide elastomers, and the alloying of the epoxy with compatible polymeric film-formers, such as poly(vinyl acetate) and certain elastomers. 

When the aldehyde in this reaction is formaldehyde, the product is poly(vinyl formal). This polymer is, however, made directly from poly(vinyl acetate) and formaldehyde without separating the alcohol. The product with low hydroxyl (5-6%) and acetate (9.5—13%) content (the balance being formal) is used in wire enamel and in structural adhesives (e.g., Redux). In both applications the polymer is used in conjunction with phenolic resins and is heat cured. 

The neutral behavior of poly(vinyl acetate) allows the use of all conventional pigments. Poly(vinyl acetate) is highly compatible with ester-soluble nitrocellulose and improves the adhesion and lightfastness of the latter. Poly(vinyl acetate) can also be readily combined with phenolic resins, ketone resins, and colophony resins. 

Phenolic resin adhesive films (Resinous Products Chemical Co.) Poly(vinyl acetate) adhesives (Carbide Carbon Chemicals Co.) Chlorinated rubber adhesives... 

Adhesive abbreviations are as follows EP, amine-cured epoxy P, polysulfide rubber flexibilizers EP/20P, EP/ 40P, and EP/60P, amine-cured epoxy with 20, 40, and 60 parts polysulfide flexibilizer EPI, emulsion polymer isocyanate ISO, isocyanate MF, melamine-formaldehyde PF, phenol-formaldehyde PF/PVA, phenol-formaldehyde flexibilized with poly(vinyl acetate) PVA, poly(vinyl acetate) PRF, phenol/resordnol-formaldehyde RF, resorcinol-formaldehyde UF, urea-formaldehyde UF/filler, UF with wheat flour UF/MF/fiUer, UF/MF copolymer with wheat flour. 

D nition Contains 5-9% vinyl alcohol, 9-50% vinyl acetate monomers Prq>erties Wh. amoqrhous powd. sol. in phenols, alcohols, aromatic and certain chlorinated soivs. m.w. 24,000-40,000 Uses Phenolic/polyvinylfonnal resins (wire enamels, can coatings) mfg. of paints, adhesives, and films in food-pkg. adhesives in food-con-tact coatings in paper/paperboard in contact with aq./fatty foods Regulatay FDA 21CFR 175.105,175.300,176.170 Manuf./DiM. CarboMer ChissoAm. Fluka Monomer-Polymer Dajac Labs Monsanto... 

Ethylene-vinyl acetate copolymer, terpene-phenol resins, polyethylene oxide, PMMA and some of their blends were solution cast on basic (aluminium oxide) and acidic (hydroxylated glass) substrates. Fourier transform infrared reflection absorption spectroscopy (IRRAS) was used to determine both the nature and the free energy of interfacial adduct formation in the polymer/metal systems. A correlation between IRRAS and adhesive strength may be used to predict both the acid-base work of adhesion and the density of interfacial interacting sites. 14 refs. 

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