Abstract:
Disclosed are moisture-curing hotmelt adhesives containing polyurethane prepolymers with an isocyanate group content of 0.16-0.84 moles of isocyanate groups per kilogram of prepolymer, the prepolymers being obtained by reacting crystalline polyester polyols having a mean molecular weight of 2,000-10,000, optionally mixed with liquid and/or amorphous polyester and/or polyether polyols having an isocyanate functionality greater than one, the adhesives also containing diisocyanates trimerized to give isocyanurates. Adhesives of this kind have improved processing characteristics and are easier to use.

Description:
This application is a continuation of application Ser. No. 08/310,008 filed on Sep. 21, 1994 now abandoned which is a continuation of Ser. No. 08/039,109 filed on Apr. 12, 1993 now abandoned, which is a 371 of PCT/EP91/01890 filed Oct. 4, 1991. 
    
    
     SUMMARY OF THE INVENTION 
     This invention relates to a moisture-curing hotmelt adhesive containing: 
     a) polyurethane prepolymers, with a content of NCO groups of 0.16 to 0.84 moles of NCO groups per kg of prepolymer, obtainable by reaction of crystalline polyester polyols having a number average molecular weight in the range from 2000 to 10,000, optionally in admixture with liquid and/or amorphous polyester polyols and/or polyether polyols having a number average molecular weight in the range from 500 to 10,000, with isocyanates having a functionality of more than 1 and 
     b) diisocyanates trimerized to isocyanurates. 
     STATEMENT OF RELATED ART 
     Moisture-curing or moisture-crosslinking hotmelt adhesives based on polyurethanes are normally prepared in one step from polyols and isocyanates having a functionality of more than 1, i.e. containing more than one NCO group per isocyanate molecule, with an excess of NCO groups based on free OH groups in the polyols. The molecular weight of the prepolymers obtained is dependent upon the functionality of the polyols and isocyanates used, an increase in functionality resulting in an increase in molecular weight. Although a relatively high functionality of the polyols and isocyanates leads to relatively high tensile shear strengths of the bonds established with the moisture-curing hotmelt adhesives, the viscosity of the prepolymers increases at the same time exponentially with their molecular weight. Accordingly, there are narrow limits to improvements in tensile shear strength by the use of polyols and isocyanates of relatively high functionality. 
     Another disadvantage of the moisture-curing hotmelt adhesives known from the prior art with an excess of the generally liquid isocyanates lies in the fact that the isocyanates have a substantial vapor pressure at the temperatures in the region of 130° C. at which moisture-curing hotmelt adhesives are normally processed, so that special protective measures have to be taken during processing. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Object of the Invention 
     The problem addressed by the present invention was to improve the processing and performance properties of typical moisture-curing hotmelt adhesives and, more particularly, to increase the mechanical strength of the bond without any adverse effect on the viscosity of the hotmelt adhesive, its open time or its initial strength. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The isocyanurates present in the moisture-curing hotmelt adhesives according to the invention have a such a low vapor pressure, even at typical hotmelt processing temperatures, that the above-mentioned protective measures during processing are no longer necessary or do not to be taken to the same extent as before. 
     The moisture-curing hotmelt adhesives according to the invention may contain typical additives familiar to one skilled in the art. Typical examples of such additives are fillers, tackifiers and plasticizers. They may also contain typical catalysts which accelerate curing of the hotmelt adhesives, for example metal catalysts, such as dibutyl tin dilaurate, amine catalysts, such as dimorpholine diethyl ether and the like. 
     Crystalline polyester polyols in the context of the invention are those which are solid and at least partly crystallized at room temperature, for example solid partly crystalline polyester polyols having a degree of crystallization of more than 30%, as measured with X-rays in accordance with Neff, Grundlagen und Anwendung der Röntgenfeinstruktur [Title in English: Fundamentals and Applications of X-ray Fine Structure], (Verlag R. Oldenbourg, München, 1959), page 320. Typical examples are the commercially available DYNACOLL® types RP 320, 340, 350 and 360. 
     Liquid polyester polyols are, for example, those which are liquid at 20° C. and have a glass temperature below 0° C. Typical examples are the DYNACOLL® types 210, 220, 230 and 250. 
     Amorphous polyester polyols are solid at ambient temperature and have a glass temperature above 0° C., for example the DYNACOLL® types RP 110, 130 and 140. 
     In one preferred embodiment of the invention, the polyurethane prepolymers have a content of NCO groups of 0.22 to 0.72 moles of NCO groups per kg of prepolymer. 
     Polyester polyols or polyether polyols and isocyanates are normally reacted in quantities corresponding to an NCO:OH ratio of 3.5:1 to 1.4:1 and preferably 3:1 to 1.7:1. 
     In another advantageous embodiment of the invention, the crystalline polyester polyols, liquid and/or amorphous polyester polyols and polyether polyols have an OH functionality of 1.8 to 2.7 and, more particularly, 2. The OH functionality is the number average of free OH groups per polyester polyol or polyether polyol molecule. 
     In another advantageous embodiment of the invention, the isocyanates have an NCO functionality of 1.8 to 2.7 and, more particularly, 2. Isocyanates such as these are commercially available compounds. 
     In another advantageous embodiment of the invention, the number average molecular weights of the crystalline polyester polyols are in the range from 3000 to 7000, the number average molecular weights of the liquid and/or amorphous polyester polyols are in the range from 1500 to 5000 and the number average molecular weights of the polyether polyols are in the range from 1500 to 5000. 
     In another advantageous embodiment of the invention, the crystalline polyester polyols are condensation products of glycols selected from the group consisting of ethylene glycol, butylene glycol, hexamethylene glycol and decamethylene glycol and dicarboxylic acids selected from the group consisting of oxalic acid, suberic acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid and hexahydrophthalic acid (cis and trans forms). 
     Crystalline polyester polyols in the form of condensation products of hexamethylene glycol or decamethylene glycol and adipic acid are particularly preferred. 
     In another advantageous embodiment of the invention, the liquid and/or amorphous polyester polyols are condensation products of polyols from the group consisting of ethylene glycol; polyethylene glycol, for example diethylene glycol, triethylene glycol, tetraethylene glycol and the like; propylene glycol; polypropylene glycol, for example dipropylene glycol, tripropylene glycol and the like; butylene glycol; neopentyl glycol; hexamethylene glycol; decamethylene glycol; bisphenol A; reaction products of bisphenol A with ethylene oxide and/or propylene oxide; glycerol and pentaerythritol with dicarboxylic acids from the group consisting of adipic acid, maleic acid, phthalic acid and isophthalic acid. Liquid and/or amorphous polyester polyols obtainable by condensation of mixtures of the above-mentioned polyols and dicarboxylic acids are also suitable. Liquid and/or amorphous polyester polyols, which are condensation products of glycols from the group consisting of ethylene glycol, diethylene glycol and neopentyl glycol with the dicarboxylic acids mentioned, are particularly preferred. 
     In another advantageous embodiment of the invention, the polyether polyols are selected from the group consisting of polyethylene glycol, polypropylene glycol and polytetramethylene glycol, including copolymers thereof. Physical mixtures of the above-mentioned glycols may also be used. 
     In another advantageous embodiment of the invention, the isocyanates having a functionality of more than 1 are selected from the group consisting of diphenylmethane-4,4′-diisocyanate,diphenylmethane-2,4′-diisocyanate,hydrogenation products of the above-mentioned diisocyanates, tolylene-2,4-diisocyanate, tolylene-2,6-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate and naphthylene-1,5-diisocyanate, including technical mixtures thereof. 
     In another advantageous embodiment of the invention, the diisocyanates trimerized to isocyanurates are derived from diisocyanates selected from the group consisting of diphenylmethane-4,4′-diisocyanate, diphenylmethane-2,4′-diisocyanate, hydrogenation products of the above-mentioned diisocyanates, tolylene-2,4-diisocyanate, tolylene-2,6-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate and naphthylene-1,5-diisocyanate, including technical mixtures thereof. Trimerized isophorone diisocyanate (IPDI isocyanurate) is particularly preferred. 
     In another advantageous embodiment of the invention, the polyurethane prepolymers are obtainable by reaction of: 
     94 to 40% by weight of crystalline polyester polyols, 
     0 to 40% by weight of liquid and/or amorphous polyester polyols or polyether polyols and 
     6 to 20% by weight of isocyanates having a functionality of more than 1, the sum of the percentages lying within these ranges being 100% by weight. 
     Polyurethane prepolymers obtainable by reaction of: 
     90 to 50% by weight of crystalline polyester polyols, 
     4 to 30% by weight of liquid and/or amorphous polyester polyols and 
     6 to 20% by weight isocyanates having a functionality of more than 1, the sum of the percentages lying within these ranges being 100% by weight, is are particularly preferred. 
     A particularly preferred moisture-curing hotmelt adhesive according to the present invention has the following composition: 
     80 to 99.8% by weight of polyurethane prepolymers and 
     20 to 0.2% by weight of diisocyanates trimerized to isocyanurates, the additives mentioned above optionally being incorporated in the usual quantities. 
     The moisture-curing hotmelt adhesives according to the invention may be obtained by initially introducing the crystalline or liquid and/or amorphous polyester polyols and, optionally, the polyether polyols and, after the careful removal of water at elevated temperatures, reacting them in vacuo with the isocyanates having a functionality of more than 1 in the absence of moisture and optionally in an inert gas atmosphere until the required NCO content of the prepolymer has been reached. The diisocyanates trimerized to isocyanurates are incorporated in the prepolymers thus obtained at elevated temperature in the desired quantity ratios, optionally together with the usual additives. 
     The present invention also relates to the use of diisocyanates trimerized to isocyanurates in the production of moisture-curing hotmelt adhesives. 
     The invention will be illustrated in more detail by the following Example of a preferred embodiment. 
    
    
     EXAMPLE 
     A basic formulation for a moisture-curing polyurethane hotmelt adhesive was initially prepared from the following components: 
     57 parts by weight of hexamethylene glycol adipate (crystalline polyester glycol; commercially available; hydroxyl value 26.7; equivalent molecular weight 2100 g/mole; OH functionality 2); 
     29 parts by weight of commercially available ethylene glycol/diethylene glycol adipate (hydroxyl value 42; equivalent molecular weight 1333 g/mole; OH functionality 2); 
     14 parts by weight of diphenylmethane-4,4′-diisocyanate; commercially available quality (NCO content 33.6%). 
     To prepare the basic formulation, the polyester glycols mentioned above were initially put into liquid form and freed from water in an oil pump vacuum at 100° C. After addition of the diphenyl methane-4,4′-diisocyanate, the mixture was left to react for about 30 to 60 minutes at 120° C. in an oil pump vacuum. The prepolymer obtained had an NCO content of about 2.5% by weight and an equivalent molecular weight, based on the NCO group, of 1680 g. 
     Various moisture-curing hotmelt adhesives of which the composition is shown in Table 1 were prepared from the basic formulation thus obtained with addition of dibutyl tin dilaurate and isophorone diisocyanate isocyanurate or (for comparison purposes) a commercially available liquid diphenylmethane-4,4′-diisocyanate (NCO content 30-32%; equivalent molecular weight 140 to 131 g/mole; Brookfield viscosity at 23° C. of 200±50 mPa.s; average functionality of 2.3; Trade name DESMODUR® VKS). 
     Formulation No. 1 contains only the basic formulation apart from the dibutyl tin dilaurate and is intended for comparison purposes. The same applies to formulations Nos. 6 and 9 which contain commercially available diphenylmethane-4,4′-diisocyanate. Formulations Nos. 2 to 5 contain varying quantities of isophorone diisocyanate isocyanurate and are moisture-curing hotmelt adhesives according to the present invention. 
     
       
         
               
             
               
               
               
               
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Weighed quantities in parts by weight 
               
             
          
           
               
                   
                 Basic 
                   
                   
                   
               
               
                 No. 
                 formulation 
                 IPDI-I 
                 VKS 
                 DBTDL 
               
               
                   
               
               
                 1 
                 100.00 
                 — 
                 — 
                 0.015 
               
               
                 2 
                 98.87 
                 1.13 
                 — 
                 0.014 
               
               
                 3 
                 97.24 
                 2.76 
                 — 
                 0.014 
               
               
                 4 
                 94.74 
                 5.26 
                 — 
                 0.015 
               
               
                 5 
                 89.94 
                 10.06  
                 — 
                 0.013 
               
               
                 6 
                 98.71 
                 — 
                 1.29 
                 0.015 
               
               
                 7 
                 97.18 
                 — 
                 2.82 
                 0.015 
               
               
                 8 
                 94.73 
                 — 
                 5.27 
                 0.016 
               
               
                 9 
                 89.96 
                 — 
                 10.04  
                 0.016 
               
             
          
           
               
                 IPDI-I: 
                 IPDI isocyanurate 
               
               
                 VKS: 
                 DESMODUR ® VKS 
               
               
                 DBTDL: 
                 dibutyl tin dilaurate 
               
               
                   
               
             
          
         
       
     
     Formulations Nos. 1 to 9 were tested for their viscosity behavior, their tensile shear strengths on test specimens of beechwood and their open time. The results are set out in Table 2. The viscosity measurements were carried out with a Physica LC 10 viscosimeter (insert Z4). 
     As can be seen from the test results in Table 2, the viscosity of the moisture-curing hotmelt adhesive according to the invention remains the same where IPDI isocyanurate is added. The same applies to the open time and initial strength by comparison with a hotmelt adhesive corresponding to the basic formulation with dibutyl tin dilaurate. By contrast, the strength of the cured product increases in dependence upon the quantity of IPDI isocyanurate added. Where the system tested for comparison purposes—containing the basic polymer and the liquid diisocyanate VKS instead of IPDI isocyanurate—is used, an increase in the open time has to be accepted. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Viscosity in P · s 
                 Tensile shear strengths 
                   
               
               
                   
                 at 130° C. 
                 in N/mm 2   
               
             
          
           
               
                   
                 Shear rate in l/s: 
                 Uncured 
                 Cured 
                 Open time 
               
             
          
           
               
                 No. 
                 64.6 
                 129.1 
                 258.2 
                 20° C. 
                 20° C. 
                 90° C. 
                 (%) 
               
               
                   
               
               
                 1 
                 6.91 
                 6.8 
                 6.61 
                 1.02 ± 0.07 
                 6.31 ± 0.18 
                 1.58 ± 0.26 
                 100 
               
               
                 2 
                 7.35 
                 7.20 
                 6.87 
                 1.09 ± 0.11 
                 6.71 ± 0.26 
                 1.60 ± 0.17 
                 100 
               
               
                 3 
                 7.05 
                 6.95 
                 6.72 
                 0.99 ± 0.09 
                 7.31 ± 0.52 
                 1.72 ± 0.19 
                 100 
               
               
                 4 
                 6.84 
                 6.65 
                 6.24 
                 1.11 ± 0.08 
                 8.29 ± 0.28 
                 1.63 ± 0.19 
                 100 
               
               
                 5 
                 5.84 
                 5.65 
                 5.46 
                 1.06 ± 0.05 
                 8.19 ± 0.90 
                 2.43 ± 0.41 
                 105 
               
               
                 6 
                 6.85 
                 6.75 
                 6.60 
                 1.05 ± 0.05 
                 7.56 ± 0.14 
                 1.94 ± 0.23 
                 100 
               
               
                 7 
                 6.45 
                 6.24 
                 6.09 
                 1.03 ± 0.10 
                 6.56 ± 0.59 
                 1.47 ± 0.21 
                 130 
               
               
                 8 
                 5.94 
                 5.84 
                 5.72 
                 1.07 ± 0.06 
                 7.57 ± 0.90 
                 2.24 ± 0.50 
                 140 
               
               
                 9 
                 4.73 
                 4.83 
                 4.76 
                 0.97 ± 0.02 
                 8.96 ± 1.16 
                 2.26 ± 0.49 
                 190 
               
               
                   
               
               
                 Tensile shear strengths: 100 mm × 50 mm × 3.8 mm test specimens of beechwood 20 mm overlap  
               
               
                 Test speed: 50 mm/min.  
               
               
                 Tensile shear strength of the uncured product: determined 20 minutes at most after production  
               
               
                 Open time: time after which a 0.5 mm thick film drawn at 130°0 C. is tack-free.