GB491882A - Improvements in or relating to the manufacture of composite wall board - Google Patents

Abstract

Composite wall board is made by allowing to set, while in contact on both sides with liners of pulpboard, cardboard, &c., a wet plaster mix containing a water-soluble urea-aldehyde or thiourea-aldehyde condensation product, the formed board being subjected to a moderately elevated temperature to convert the condensation product into resin. The plaster may be mixed with resin syrups containing 30-50 per cent of water. Usually 1-10 per cent of syrup by weight of the final product is adequate but higher proportions, e.g. up to 80 per cent may be used to obtain boards having high wet-strength. The molecular ratio of formaldehyde to urea is advantageously 1,5 : 1, but higher ratios, e.g. 3 : 1 may be employed; above 1,8 : 1 the resin may be condensed by heating the syrup under acid conditions, e.g. at a pH of 6,5, with or without removal of water. Acid catalysts, e.g. sulphuric, hydrochloric and trichloracetic acids, and plaster accelerators and retarders may be added. In examples: (1) 100 parts by weight of plaster is mixed with 8 parts sawdust, 55 parts water and 10 parts of a resin syrup obtained by stirring 60 parts urea into 122 parts of 40 per cent formaldehyde, if necessary with gentle heat; 0,035 parts of sulphuric acid are added during mixing and the mix is fabricated into boards with pulpboard facings in known manner, the board being dried in a continuous drier having an inlet temperature of 100-150 DEG C. and an outlet temperature of 60-70 DEG C.; (2) a resin syrup obtained by refluxing on a water bath for one hour 60 parts urea and 163 parts of 40 per cent formaldehyde, at pH 6,5, is similarly worked up; (3) a resin syrup containing urea and formaldehyde in molecular ratio 1,5 : 1 is made as in example (1) and is mixed with plaster and wood fibre and further worked up. Specification 453,703 is referred to.