Laminates containing melamine-formaldehyde resin

Resins for this purpose generally use melamine-formaldehyde ratios of 1 2.2 to 1 3. Where electrical grade laminates are required the condensing catalyst employed is triethanolamine instead of sodium carbonate. 

Decorative laminates have a core or base of Kraft paper impregnated with a phenolic resin. A printed pattern layer impregnated with a melamine-formaldehyde or urea-thiourea-formaldehyde resin is then laid on the core and on top of this a melamine resin-impregnated protective translucent outer sheet. The assembly is then cured at 125-150°C in multi-daylight presses in the usual way. 

Decorative laminates have achieved remarkable success because of their heat resistance, scratch resistance and solvent resistance. Their availability in a wide range of colours has led to their well-known applications in table tops and as a wall-cladding in public buildings and public transport vehicles. 

Glass-reinforced melamine-formaldehyde laminates are valuable because of their good heat resistance (they can be used at temperatures up to 200°C) coupled with good electrical insulation properties including resistance to tracking. 

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At one time thiourea-urea-formaldehyde resins were of importance for moulding powders and laminating resins because of their improved water resistance. They have now been almost completely superseded by melamine-formaldehyde resins with their superior water resistance. It is, however, understood that a small amount of thiourea-containing resin is still used in the manufacture of decorative laminates. 

Thermosetting polymers are usually liquids at room temperature. They are eonverted to solids by combining with other liquid reactants in the presence of liquid or paste catalysts. Heat may be applied to hasten the setting reaction. Unsaturated polyesters constitute the largest volume, followed by polyurethanes and polyureas. Phenol-, urea- and melamine-formaldehyde resins are used in large volumes in laminates, but these contain little or no mineral filler. Epoxies are a relatively low volume market for fillers. 

Amino Rosins. Amino resins are the reaction product of an aldehyde with a compound containing an amino (—NH2) group (see Amino Resins). Both urea and melamine react with formaldehyde, first by addition to form methy-lol compounds, and then by condensation to form cross-linked polymers through methylene bridge and methylene ether formation. The cross-linking reactions are exothermic and produce water and formaldehyde as volatile products in reactions similar to resole phenolics, illustrated in equation 3. Urea-formaldehyde and melamine-formaldehyde account for the bulk of the amino resins. Their characteristics include water solubility and unlimited colorability with dyes and pigments. Applications include decorative high pressure phenolic laminates, adhesives in the laminated wood and furniture industries, and as an additive to textile fabrics to impart wash and wear properties. 

The joining of wood by adhesives is widely used in many fields of application. This comprises structural joints, glued-laminated timber, and coatings. In industrial processes, wood is structurally connected by formaldehyde-containing melamine or phenol resorcin resins which cure by polycondensation. Due to the very thin adhesive layers and the minimized formaldehyde content, the expected incorporation into the surrounding air is considerably less than the specified thresholds. In manually operated applications, dispersions of acrylates and polyvinyl acetate as well as urethanes are usually used to bond wood (Richter and Steiger 2005). 

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylo1 melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple deterrnination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Collective term for resins that are capable of being cross-Unked. Terms, aminoplast and amino plastic, are also used. The materials are based on compounds containing NH2 group and formaldehyde. The representatives are urea-, melamine-, and dicyanodiamide-based resins used for laminating and molding. 

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