Adhesion Property

The demand for radiation-cured adhesives is largely attributed to the requirement for specific adhesive properties essential in certain end-use applications. Some of the properties being focused on by radiation-curable adhesives manufacturers are  

Standard analysis techniques that are also evaluated for radiation-cured adhesives before they are certified as usable in packaging applications include  

Overview of radiation-curable adhesives in packaging applications 

Adhesives used in food packaging must be safe and reliable as is the case for any other material that is classified as a food-contact material. Even within the adhesive family, the product range is immense covering different chemical compositions and technological properties. One of the most aitical control points in the applicability of an adhesive is the curing time. Curing is essential in order to  

Curing is usually a time-consuming process. Often it requires additional time for storage either at the laminate producer or prior to being used by the food producer. Curing aids are likely to fuel various kinds of toxicological considerations once they are used for food packaging adhesives. 

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The 2-imino-4-thiazolines may be used as ultraviolet-light stabilizers of polyolefin compositions (1026). 2-Aminothiazole improves adhesive properties of wood to wood glue (271). Cbmpound 428 exhibits antioxidant properties (Scheme 242) (1027). Ammonium N-(2-thiazolyl)dithio-carbamate (429) is a bactericide and fungicide used in industrial products such as lumber, paint, plastics, and textiles (1037). Compound 430 is reported (1038) to form an excellent volume of foam coating in aluminum pans when ignited with propane. 

These monomers provide a means for introducing carboxyl groups into copolymers. In copolymers these acids can improve adhesion properties, improve freeze-thaw and mechanical stability of polymer dispersions, provide stability in alkalies (including ammonia), increase resistance to attack by oils, and provide reactive centers for cross-linking by divalent metal ions, diamines, or epoxides. 

Its key characteristics are its excellent optical and adhesive properties. It is used as the interlayer film for safety glass. 

The polymers of the 2-cyanoacryhc esters, more commonly known as the alkyl 2-cyaiioacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their Hquid precursors or monomers are placed between two closely fitting surfaces. The spontaneous polymerisation of these very reactive Hquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesives of great versatiUty. The materials that can be bonded mn the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

Epoxy phenoHc coatings either are made by blending of a soHd epoxy resin with a phenoHc resin or are the products of the precondensation of a mixture of two resins. A three-dimensional stmcture is formed during curing which combines the good adhesion properties of the epoxy resin with the high chemical resistance properties of the phenoHc resin. The balanced properties of epoxy phenoHc coatings have made them almost universal in their appHcation on food cans. 

Polyesters. Polyesters containing carbonate groups have been prepared from this diol (see Polycarbonates) (99). Films of this polymer, formed from an acetone or ethyl acetate solution, exhibit exceUent adhesive properties. 

Gum ghatti is the calcium and magnesium salt of a complex polysaccharide which contains L-arabinose, D-galactose, D-mannose, and D-xylose and D-glucuronic acid (48) and has a molecular weight of approximately 12,000. On dispersion in water, gum ghatti forms viscous solutions of viscosity intermediate between those of gum arabic and gum karaya. These dispersions have emulsification and adhesive properties equivalent to or superior to those described for gum arabic. 

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). 

In the late 1800s, when the demand for coated paper for the halftone printing process increased, casein rapidly replaced glue. Casein forms a hard, tough film when dry, and can be waterproofed easily with formaldehyde (qv). The properties of soy protein are similar to those of casein, and soy protein has been substituted for it in many types of coated papers requiring a casein-type binder (see Soybeans and other oilseeds). Casein, a valuable food product, is seldom used as a paper adhesive, in spite of its excellent adhesive properties. 

Interest in naphtha (nafta) began with the discovery that petroleum could be used as an illuminant and as a supplement to bituminous incendiaries, which were becoming increasingly common in warfare. Greek fire was a naphtha—bitumen (or naphtha—asphalt) mix the naphtha provided the flame and the bitumen (or asphalt) provided the adhesive properties that prolonged the incendiary effect. 

Phenolic resin substantially increases open time and peel strength of the formulation (80). For example, higher methylol and methylene ether contents of the resin improves peel strength and elevated temperature resistance. Adhesive properties are also influenced by the molecular weight distribution of the phenoHc low molecular weight reduces adhesion (82). 

Solders should flow promptly and smoothly over the surfaces of the parts to be joined. This property depends on the surface tension, viscosity, and adhesive properties of the molten solder. Finally, the color of a solder should match that of the metal employed, and its physical properties should be at least as good as those of the metal, in order for the joint not to be a source of weakness (150). 

In addition to transport properties, the adhesive properties are characterized by tensile measurements. For instance, the peel strength is deterrnined by measuring the force required to pull the adhesive from a substrate at a constant speed in a controUed temperature and humidity environment. 

The type of chloroprene polymers used is perhaps best illustrated by the variety of special products, designed for adhesive appHcations, that Du Pont has developed. These are described ia Table 8. Standard polymer grades are also often used, especially to modify adhesive properties and to reduce cost. 

Because there are many other properties that also are important, coatings cannot be selected only on this basis. The mechanical and chemical properties of the coating, change of properties with temperature, dielectric and adhesion properties, and particulady the cost of fabrication are all important parameters. Coatings can also be used to transport heat created away from a component and keep the component functioning as designed, or to protect a component from temperature variations in the environment. 

The outstanding performance characteristics of the resins are conveyed by the bisphenol A moiety (toughness, rigidity, and elevated temperature performance), the ether linkages (chemical resistance), and the hydroxyl and epoxy groups (adhesive properties and formulation latitude, or reactivity with a wide variety of chemical curing agents) (see also Phenolic resins). 

A thickness of up to. 50 microns is possible through one coat of primer and paint. To obtain a greater thickness an additional coat of paint, rather than primer, may be applied after almost curing the first coat. A thickness of primer of more than 30 microns is not considered satisfactory as it may diminish its adhesive properties. To obtain u thickness of up to 100 microns, for instance, each coat (one of primer and two of paint) may be around. 30-35 microns,... 

Many isocyanates have good adhesive properties and one of them, triphenyl-methane-pp /7"-triyl tri-isocyanate, has been successfully used for bonding of rubber. Isocyanates are, however, rather brittle and somewhat limited in application. Somewhat tougher products are obtained from adhesives involving both polyols and isocyanates, i.e. polyurethane-type materials. The major application of these materials to date is in the boot and shoe industry. 

Silicone fluids containing Si—H groups are also used for paper treatment. The paper is immersed in a solution or dilute emulsion of the polymer containing either a zinc salt or organo-tin compound. The paper is then air-dried and heated for two minutes at 80°C to cure the resin. The treated paper has a measure of water repellency and in addition some anti-adhesive properties. 

Treatment of polymer surfaces to improve their wetting, water repulsion, and adhesive properties is now a standard procedure. The treatment is designed to change the chemistry of the outermost groups in the polymer chain without affecting bulk polymer properties. Any study of the effects of treatment therefore requires a technique that is specific mostly to the outer atomic layers this is why SSIMS is extensively used in this area. 

RosaZeiser, A., Weilandt, E., Hild, S. and Marti, O., The simultaneous measurement of elastic, electrostatic and adhesive properties by scanning force microscopy pulsed-force mode operation. Measur. Sci. Techno ., 8(11), 1333-1338 (1997). 

Koberstein et al., J.T, Creating smart polymer surfaces with selective adhesion properties. J. Adhes., 66, 229-245 (1998). 

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