The use of polyisocyanates in a mixture with polyamines or polyols as adhesives is known. Since the polyisocyanates react with the polyamines or polyols at ambient temperature, such adhesive mixtures are not stable in storage at ambient temperature. Therefore they must be prepared and processed within the limited pot life by mixing the separately stored reaction components just before they are to be used.
To prepare adhesive compositions which are stable in storage at ambient temperature, blocked polyisocyanates have also been used in which the isocyanate groups are completely reacted with monofunctional compounds such as phenol, caprolactam, malonic esters or similar compounds. These blocked polyisocyanates separate the blocking agent in a reversible manner at elevated temperature and liberate the polyisocyanate for the subsequent, generally irreversible polyaddition reaction to form the adhesive (c.f. Vieweg/Hochtlen, Kunststoff-Handbuch, Volume 7, Polyurethanes, Carl-Hanser-Verlag, Munich 1966). This process has the disadvantage that the blocking agent remains in the adhesive film in relatively large quantities. Moreover, the deblocking temperature of storage-stable blocked polyisocyanates is generally very high. Furthermore, before the reaction temperature of adhesive systems blocked in this way was attained, the adhesive ran off vertical surfaces due to the reduction in viscosity thus produced in the adhesive. Moreover, deformation and damage can easily occur in plastic due to the heating necessary.
Storage-stable adhesives can also be prepared polyurethane prepolymers containing only a small amount of free isocyanate with a complex of alkali salt and 4,4,'-diaminodiphenylmethane. Upon heating, the diamine bound in the complex is liberated and reacts with the NCO-prepolymer (see European Pat. No. 62,598). A disadvantage of this formulation is in the high alkali salt content which remains in the adhesive after the adhesive sets. This considerably reduces the water resistance of the composition.
German Auslegeschrift No. 1,570,548 describes a one component system which can be stored for a prolonged period and which consists of a mixture of (i) 1 mol of polyester, polyether or polythioether, (ii) at least 1.5 mol of a solid isocyanate containing uretdione groups with a melting point of 100.degree. C. or more and (iii) at least 0.3 mol of a solid chain extender containing OH and/or NH.sub.2 groups with a melting point of 80.degree. C. or more. At least 80% of the solid constituents of the mixture must have particle sizes of 30 .mu.m or less. The stability in storage lasts for a few days to a few weeks at ambient temperature and only a few hours at 50.degree. C. A disadvantage of the process is that of three reactants, at least two must be present in solid form to ensure the necessary stability in storage. This means that very high-viscosity mixtures are generally obtained. Additionally, the viscosity continues to rise slowly because the reactivity of the compounds has not been slowed down. The reaction apparent from the continuous increase in viscosity on the surface of the solid isocyanate particles takes place randomly and slowly and does not therefore retard the reactivity of the polyisocyanates sufficiently for a self-stabilization of the system.
German Auslegeschrift No. 3,112,054 (corresponding to U.S. Pat. No. 4,400,497) describes thermosetting mixtures of polyisocyanate and polyol which are stable in storage at ambient temperature. In these mixtures, the polyisocyanate is present in the form of discrete particles in the polyol, the polyisocyanate particles being deactivated on their surface to 0.5 to 20 equivalent % of the total isocyanate groups present by partial reaction with water, polyols or polyamines. These mixtures are suitable as coating and adhesive agents and as sealants, and in particular as undercoating material for automotive vehicles. When using these mixtures as adhesives, several disadvantages arise. Thus, the setting rate of the glycol-containing systems is too slow for practical purposes even in the presence of catalysts. Additionally, exclusively glycol or hydroxy polyether-containing systems tend to foam markedly during curing in heat even after previous careful dewatering. Another disadvantage of single component polyurethane adhesive compositions, even those based on stabilized polyisocyanates containing exclusively polyether polyols and optionally polyol chain extenders as components reactive towards isocyanates, lies in their restricted storage capacity particularly at elevated storage temperature.
Unstabilized single component mixtures of polyol, glycol, solid polyisocyanate (for example dimeric toluylene diisocyanate) and catalyst can be stored for a few hours or days at ambient temperature depending on the composition. The stability in storage of single component polyurethane reactive compositions based on polyether polyols and glycol chain extenders is improved considerably by stabilization of the solid isocyanate by applying a thin polyurea protective covering on the surface of the solid polyisocyanate particles with reaction of about 0.5 to 20% of the NCO groups with, for example, polyamines according to DE-A No. 3,112,054, but their stability in storage is still limited. The possible storage period is therefore a few days at about 40.degree. C. and only a few hours or days at 50.degree. C.