The present invention relates to substantially anhydrous binders based on polyisocyanates which are modified by the addition of an organic sulfonic acid such that they have a self-releasing effect. The invention is also directed to the use of such binders in the production of shaped articles such as panels by the hot pressing of a wide variety of organic and/or inorganic materials such as substances containing lignocellulose.
Pressed materials such as chip boards, composite panels and other shaped articles are usually produced by hot pressing the inorganic or organic raw material such as a composition of wood shavings, wood fibers or another material containing lignocellulose, with binders such as aqueous dispersions or solutions of urea/formaldehyde or phenol/formaldehyde resins. It is also known to use isocyanates or isocyanate solutions as binders for pressed panels instead of formaldehyde resins (German Auslegeschrift No. 1,271,984; German Offenlegungsschriften Nos. 1,492,507; 1,653,177 and 2,109,686). The use of polyisocyanates as binders improves the stability and the moisture-resistance of the products and improves their mechanical properties. In addition, polyisocyanates have a wide range of processing advantages as binders as disclosed in German Offenlegungsschrift No. 2,109,686.
The large scale production of materials which are bonded with polyisocyanates, in particular materials containing lignocellulose such as chip boards, fiber boards or plywood is, however, impaired by the marked tackiness of the polyisocyanates. This tackiness, after the hot pressing treatment, causes the molded articles to adhere strongly to metal articles, in particular the steel or aluminum surfaces of the press. Such adherence thus makes it more difficult to remove the molded articles from the mold.
Previously proposed methods of solving this mold release problem suffer from significant disadvantages. Release agents which have been developed especially for isocyanates frequently have a good release action but, in industrial applications, they are neither reliable nor economical enough and, additionally, may cause faulty bonding or difficulties in coating during the subsequent processing of the plates.
It has been proposed in German Offenlegungsschrift No. 1,653,178 that, during the production of panels or shaped articles by the hot pressing of mixtures of materials containing lignocellulose and polyisocyanates, the surfaces of the press or pressing molds be treated with polyhydroxyl compounds such as glycerin, ethylene glycol or polyester and polyether polyols before the pressing operation. A disadvantage of this process is that a separate operation is required to apply this release agent and, in addition, a proportion of the polyisocyanate is consumed by the reaction with the release agent. According to German Offenlegungsschrift No. 2,325,926, another possible way of improving the release behavior of the shaped articles involves using as release agents those compounds which act as catalyst with isocyanates to form isocyanate. However, a disadvantage of this approach is that the catalysts have a destabilizing effect on the isocyanate and thus prevent the formation of a suitable isocyanate binder.
An object of the present invention is to overcome the above-mentioned disadvantages in the production of shaped articles using polyisocyanates by providing binders based on isocyanates which may be stored and which ensure that the pressed articles may be removed from the mold without difficulty. It has surprisingly been found that this object may be achieved if the isocyanate is used in combination with an organic sulfonic acid.