Adhesive resins, manufacturing steps

Adhesives of the aminoplastic (see Step polymerization) and phenol formaldehyde (see Phenolic adhesives single-stage resoles and Phenolic adhesives two-stage novolacs) types are most widely used. Although basically similar, an adhesive for plywood manufacture will require a different formulation to one for particle board, or medium-density fibre board (MDF) since methods of application and processing differ. Thus, in plywood, large sheets of veneer must be uniformly coated with adhesive, usually by a roller or curtain coater in particle board, chips or wafers must be coated with very fine adhesive droplets, while small bundles of wet fibres must be sprayed with adhesive in the manufacture of MDF. Hence, formulation and production of resins has become a mixture of art and science, with resin manufacturers able to produce resins tailored for use in a particular board-manufacturing plant, or with a particular species of timber. 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

The need to paint the blends has also resulted in new painting systems. Recently, a nonpolar color coat based on a hydrogenated polybutadiene diol and melamine resin for TPO bumper fascia was invented. The breakthrough technology allows the elimination of the TPO pretreatment step such as adhesion promoter, flame, or plasma during manufacturing. The paintability of two different types of E-plastomers was evaluated. The olefinic white paint was found to provide excellent paint adhesion for both types of metallocene plastomers. Paint peeling was not observed in any of the test... 

Simple resoles are based on phenol and catalysed with NaOH or a caustic/anunonia combination. They generally utilize phenol to formaldehyde ratios (P F ratios) in the region of 1 1.4 to 1 1.6, which appear to be optimum for structural adhesives ratios higher than 1 1.6 produce resins, which, for example, are used in plywood manufacture, where excellent moisture resistance is required. Phenolic resoles are produced by condensation reactions (Step polymerization), and a typical schematic is as follows ... 

Resins based on para-substituted phenols can be either one-step or two-step, but they cannot cure to a thermoset state. In the manufacture of phenolic resins, smaller quantities of acetaldehyde and furfuraldehyde are used in addition to formaldehyde. Furthermore, resorcinol, bisphenol A, and p-alkylphenols are employed, in addition to phenol, when special properties are desired. Formaldehyde concentrations of 37-50 weight % in aqueous solutions are most commonly employed. The catalysts most frequently used are acids such as oxalic, hydrochloric, sulfuric, p-toluenesul-fonic, and phosphoric and bases such as sodium, calcium, and barium hydroxide. In the weakly acidic range metal carboxylates are employed. Thermoset phenolic resins are employed as structural adhesives for laminating and bonding applications. Para-alkyl-substituted resins are employed as tackifiers in contact adhesives, pressure-sensitive adhesives, and hot-melt adhesives. 

The general process used to synthesise aromatic polybenzimidazoles (PBIs) is presented in Section 4.3.3. More detailed information can be found in previously published books [87,88]. During 1960-1970 a number of publications, comparable to those on polyimides, reported the synthesis and properties of all aromatic and aryl-aliphatic polybenzimidazoles. Most of these polymers were prepared by the two-step process illustrated in Fig. 14 with the reaction of 1,3-benzenedicar-boxylic acid diphenyl ester 27 and [l,l -biphenyl]-3,3, 4,4 -tetramine 11 yielding ultimately PBI 29. All the applications - laminates and filament winding resins, adhesives, fibres and foams - used polymer 29, which was produced in semicommercial quantities by the Whittaker Corporation (Narmco Division) under the generic trade mark Imidite . Currently, forty years later, this polymer is manufactured by Hoechst-Celanese and its only commercial success is in the area of heat resistant fibres and fabrics. It is, however, worth noting the adhesive properties of this polymer and the reasons explaining the major obstacles to the development of PBIs as heat-resistant adhesives. 

An example of the manufacturing and characterization of a nanostructured composite material is now reported. A complex step is the definition of a procedure aimed at the realization of a homogeneous dispersion of a nanometric powder in epoxy resin. Moreover, adhesion problems related to the interfacial activity of the resin, the powder and the nanotubes must be solved. The materials employed to manufacture the samples were ... 

Melamine, C3N3(NH2)3, is used in adhesives and resins. It is manufactured in a two-step process in which urea, CO(NH2)2, is the sole starting material, isocyanic acid (HNCO) is an intermediate, and ammonia and carbon dioxide gases are by-products. 

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