Glues based on urea formaldehyde condensates are nowadays used on a wide scale for veneering and laminating wood and wood products. These glues are normally applied by the following method:
A mixture is prepared from urea formaldehyde resin, hardener, water and optionally organic extenders (e.g. flour or starch) and/or fillers (e.g. chalk or ground up coconut shell). The glue is applied to a wood panel by means of a roller spreading machine. The veneer or laminating material is placed on the supporting plate and hot pressed. One particular disadvantage of gluing systems based on aminoplastic resins is that the formaldehyde splits off during or after pressing. It is therefore difficult to comply with the stringent regulations for the maximum permissible concentration of formaldehyde in the surrounding atmosphere when such glued products are used in enclosed spaces.
Other gluing systems which contain no formaldehyde have therefore also been used. The so-called white glues based on polyvinyl acetate (PVAc) are well known. These glues may be applied either hot or cold but their use is limited by certain disadvantages due to the thermoplastic properties of polyvinyl acetate. Thus they are less water-resistant and heat-resistant then condensation resin products. Furthermore, problems arise when sanding down the surfaces and discoloration occur (in special types of wood such as oak when iron impurities are present) as well as problems due to the formation of steam blisters.
It is known from the publication by W. Hackbarth and W. V. Bockelmann, "Dispersions-Isocyanatsysteme: Neue Klebstoffgeneration fur die Holzindustrie", Adhasion 1982, 11, pages 4-6, that improved PVAC glues are obtained by the addition of polyisocyanates. The advantageous processing characteristics of aqueous PVAc dispersions are combined with the advantages of reactive isocyanate adhesives. More stable glues can therefore be obtained but the problems of sanding down and risk of discoloration remain.
A process for the preparation of aqueous polyurethane glue systems based on low molecular weight or relatively high molecular weight polyols plus di/polyisocyanates is described in DE-A No. 2,403,656. In this process, lignin sulphonate and/or dextrin and/or methylcellulose are stirred in as well as surface-active substances. For economical reasons, however, polyurethane glue systems have become established only in special fields of application.
According to the publication by O. Wittman and H. Lehnert, "Diisocyanate als Bindemittel (Zur Herstellung von Holzwerkstoffen mit Diisocyanat)", Holz-Zentralblatt, May, 1974, pages 913-9, water resistant glues for plywood can be prepared from pure non-aqueous polyisocyanate adhesives. However, the strong tendency of both low viscosity and high viscosity non-aqueous polyisocyanate adhesive systems to bleed out is mentioned as one of the disadvantages. Although this tendency was prevented by the addition of organic extenders, the mixtures could only be used within a very short time and the polyisocyanate reacted rapidly with the extenders. Even the additional use of fillers could not completely prevent the bleeding out of the glues. It was also pointed out that these non-aqueous polyisocyanate adhesive systems could not be applied with conventional glue spreading machines because they soiled the rollers. The organic solvents were also said to cause problems in some cases (difficulty of removal), environmental problems, costs and toxicity).
It is an object of the present invention to develop polyisocyanate containing adhesive systems which have a suitable viscosity as well as a sufficiently long pot life and can be applied by conventional mechanical glue spreading processes. According to the invention, these objects are achieved by emulsifying the polyisocyanate in an aqueous mixture of starch and flour. Bleeding of the polyisocyanate through the veneer can thus be prevented with inexpensive and simple glues.