Adhesives which harden without chemical reaction
Adhesives can harden without a chemical reaction, using water-soluble materials, or through the loss of solvents. Environmental pressures are leading to the development of water-based adhesives to replace solvent-based ones. Various adhesives like neoprene adhesives and starch-based pastes offer different characteristics and applications based on their compositions. Understanding these types of adhesives is crucial for selecting the right one for specific bonding needs.
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Adhesives which harden without chemical reaction
Water soluble There is now much pressure from environmental, and health and safety regulators to reduce or eliminate the use of solvents in adhesives, and the industry is responding by developing water- based systems to replace them. There are, however, fundamental problems, one being the low rate at which water evaporates because of its high enthalpy of vaporization, which is compared with values for some common solvents in Table 5.1. A second is that water-soluble materials are essential to stabilize emulsions, and these remain in the adhesive after drying, so increasing water absorption and the sensitivity of joints to water. The water soluble materials can be ionic or non-ionic surfactants, or water soluble polymers.
ADHESIVES WHICH HARDEN BY LOSS OF SOLVENT Contact adhesives are probably the best known solvent-based adhesives. These are solutions of a polymer in organic solvents, which are applied to both surfaces to be bonded. Some time is allowed for the solvents to evaporate and the surfaces are then pressed together, at which point some interdiffusion of polymer chains will occur. The surfaces can also be heated to increase tack.
Clear solvent-based adhesives, which are sold to the public in tubes, are often solutions of nitrile rubber (a copolymer or butadiene and acrylonitrile) in organic solvents. Prominent contact adhesives are based on neoprene (polychloro-prene, poly-2-chlorobutadiene). The diene units can be incorporated into polymer chains as four different isomers, which are shown in structural formulae 5.1. The ratios of these isomers affect the ability to crystallize. The trans-1,4 unit is the most common and AC and AD grades are made up of about 90% of them. The W grade is composed of 85% of such units and crystallizes more slowly.
Neoprene adhesives have good tack, rapid development of bond strength and are resistant to oils and chemicals. The formulation of a typical neoprene contact adhesive is shown in Table 5.2. Without stabilization, polychloroprene degrades to liberate HCl, which attacks the adhesive and is obviously a threat to metallic adherends. The metal oxides act as acid acceptors; they are more effective in combination than singly. They can also act as crosslinking agents. Oxygen initiates the dehydrochlorination of polychloroprene, which is a free-radical process, and the purpose of butylated hydroxytoluene (BHT) is to scavenge radicals. Uses of polychloroprene adhesives include Do-it-Yourself contact adhesive, shoe soling, rubber dinghies and rubber-to- metal bonding.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Water Solutions and Pastes Starch is cheap and plentiful, maize and corn being the main sources for adhesive use. It consists of glucose units and has linear and crosslinked components, which are termed amylose and amylopectin, respectively (structures 5.2 and 5.3). For use in adhesives it is modified by lowering the molecular weight by the following processes: hydrolysis in dilute acid; alkaline chlorination to give an anionic product; dry heat in the presence of an acid to produce dextrin; alkaline treatment in the presence of tertiary or quaternary ammonium salts, or epoxides, gives a cationic starch.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Additives: Additives can include up to 10% borax (sodium tetraborate) to increase viscosity and tack, urea as a plasticizer, and urea-formaldehyde, melamine- formaldehyde or resorcinol-formaldehyde condensates to improve water-resistance. Clay (e.g. bentonite) can be used as a filler. Applications: The main uses are for bonding paper, board and textiles. Applications include corrugated board, paper bags, tube winding, wallpaper paste and remoistenable adhesives.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Water-moistenable adhesives include poly(vinyl alcohol) (PVOH), which is used on postage stamps, natural gums (e.g. acacia and dextrins), and poly(vinyl acetate) (PVA) latices with a large amount of PVOH stabilizer (e.g. 15%). PVOH is the only common polymer that is not made from its monomer.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Aqueous Emulsions The ingredients for an emulsion polymerization are water, monomer(s), stabilizer and initiator. The stabilizer can be a surfactant or water- soluble polymer, the use of these being referred to as surfactant and colloid stabilization. Anioinic surfactants include sodium and potassium salts of long chain fatty acids (stearate, laurate and palmitate) and sulfates and sulfonates with a long alkyl chain (e.g. sodium dodecyl sulfate and sodium dodecylbenzene sulfonate). Colloid stabilizers include polyethylene oxide, PVOH and hydroxyethylcellulose. Amounts used are 0.2-3.0 wt% based on water for ionic surfactants and 2-10% for water soluble polymers.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Aqueous Emulsions The product of emulsion polymerization is a latex of polymer particles with adsorbed stabilizer. The particle diameters are of the order of 1 m and the water content is normally 50-55%. Polymer latices are best known as emulsion paints that are based on PVA. Whether used as surface coatings or adhesives, they are spread on surfaces and a continuous film is formed as the water evaporates. The lowest temperature at which a continuous film can be formed is the minimum film-forming temperature (MFT), which is close to the glass transition temperature. Latex adhesives can be plasticized internally by incorporating a suitable comonomer or externally by adding conventional liquid plasticizers (e.g. phthalates) to the hot latex.
ADHESIVES WHICH HARDEN BY LOSS OF WATER Aqueous Emulsions Vinyl acetate is a major constituent of adhesives for bonding wood and paper. The cost of this monomer is low, but because the glass transition temperature of the homopolymer is low (32 "C but it may be lowered by co-monomers, plasticizers or water), there is a tendency to creep, which is the major disadvantage. Such adhesives are usually stabilized with PVOH with about 20% residual acetate groups. Particle sizes are in the range 0.5-2.0 . The use of surfactants is minimized, because the consequent lowering of surface tension permits excessive penetration of the adhesive into the substrate. If the substrate is porous (e.g. ceramic tiles) a filler such as china clay or calcium carbonate can be added to retain the adhesive in the bond-line.
ADHESIVES WHICH HARDEN BY COOLING Hot melt adhesives are one-part systems that are applied to substrates as a hot liquid, and rapidly form an adhesive bond on cooling. Their application is readily automated. They can be used to bond paper and board, many plastics and wood but a problem with bonding metals is that the substrate conducts heat too rapidly, restricting the extent of wetting.
ADHESIVES WHICH HARDEN BY COOLING Ethylene Vinyl Acetate (EVA) Hot Melts EVA random copolymers containing up to 30% vinyl acetate are used, and the effect of adding VA to polyethylene is to reduce crystallinity and increase polarity. Melt viscosity is very dependent on molecular weight. Tackifiers are added to reduce viscosity and improve wetting, and include hydrocarbon (C,-C,) resins, polyterpenes and rosin esters of pentaerythritol and glycerol. Waxes can be added to lower cost and reduce viscosity. Fillers such as calcium carbonate lower cost and increase viscosity. Antioxidants are needed to protect the adhesive during application and service life.
ADHESIVES WHICH HARDEN BY COOLING Ethylene Vinyl Acetate (EVA) Hot Melts Uses include: cardboard boxes, bookbinding, iron-on patches and edge-tapes on chipboard. Because of creep, books develop a memory and tend to open at the same place each time. This can be prevented by crosslinking brought about by adding a peroxide and heating, or by electron beam irradiation.
ADHESIVES WHICH HARDEN BY COOLING Polyamide Hot Melts Polyamide hot melt adhesives have lower melting points than poly-amide plastics, and tend to employ a mixture of monomers. They have better heat resistance than EVAs but cost more; however, they give good tack without needing additives. One or two numbers, which indicate the numbers of carbon atoms in repeat units, are used to specify the structures of polyamides. If there is single number there is just one sequence of carbon atoms in the repeat unit, as in polyamide 6 (nylon 6) which is shown in structure 5.4, but here there are two numbers the first is for the diacid repeat unit and the second for the diamine. Polyamide 6-10 is the example shown in sructure 5.5.
ADHESIVES WHICH HARDEN BY COOLING Polyester Hot Melts Polyester hot melt adhesives are based on terephthalic acid (benzene-1,4-dicarboxylic acid), but other diacids such as isophthalic (benzene-1,3- dicarboxylic acid), adipic (hexane- 1,6-dioic acid), and azelaic (nonane-1,9-dioic acid) are used. The range of diols includes ethane diol, butane- 1,4- diol, hexane- 1,6-diol, diethylene glycol and propane- 1,2-diol. Crystallinity falls as the number of diol carbon atoms increases.
