Abstract:
Adhesive compositions comprising a mixture of polyvinyl acetate and an alcohol soluble resol phenolic resin and about 2 to 20 parts by weight of lower polyhydric alcohol per 100 parts by weight of the resin solids have enhanced binding strengths on curing even after a prolonged open assembly time.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a polyvinyl acetate-modified phenolic resin composition having improved adhesion. 
     Polyvinyl acetate-modified phenolic resin is a thermosetting resin obtained by admixing an alcohol-soluble phenolic resin of the resol type with a polyvinyl acetate resin, and is characterized by properties different from those of alcohol-soluble resol type phenolic resin and of the vinyl acetate resin. Alcohol soluble resol resins (typified by resol resins soluble in lower monohydric alcohols such as methanol) are more particularly described in D. F. Gould &#34;Phenolic Resins&#34; Reinhold Publishing Corporation 1959, Chapter 3, the pertinent disclosure of which is incorporated herein by reference. Mixtures of phenolic resins and polyvinyl acetate are disclosed by Gould op cit page 95, lines 26-29. More specifically the polyvinyl-acetate modified phenolic resin is an advantageous adhesive in that it can meet the higher adhesive durability standard required in the specification of Type I plywood of JAS (Japanese Agricultural Standards) which is difficult to achieve with vinyl acetate resins. It is also advantageous in that the applicators and related equipment used with the resin mixture are washable with water, which advantage is not realizable in the case of alcohol-soluble resol type phenolic resins. 
     Adhesive compositions are prepared from such resins by the addition, directly before use, of a strongly acidic hardener such as benzenesulfonic acid or para-toluenesulfonic acid (used in sufficient proportion in the adhesive composition to impart a pH of about 0.1 to about 1.0 to the adhesive composition) which is applied onto the adherend. The resin-containing adherends are pressed under a pressure of about 2-15 kg/cm 2  for about 5-20  hours at 15°-40° C. to cure the resin, thereby completing the adhesion. However, in comparison with alcohol-soluble resol type phenolic resins or vinyl acetate resins, the polyvinyl acetate-modified phenolic resins generally have a serious drawback in showing a significant loss in bonding strength when the socalled open assembly time, or the period between the coating of adhesive composition on the adherend and curing of the resin is long i.e. about 10 minutes or longer. The necessity of limiting open assembly time in using conventional polyvinyl acetate-modified phenolic resin compositions inevitably necessitates rapid processing in wood working plants, and eventually leads to defective adhesion of the resin product, particularly at high ambient temperatures. 
     This phenomenon can be ascribed to an insufficient flow of the adhesive composition when compressed after a prolonged open assembly time, and an insufficient bonding strength resulting therefrom due to the faster hardening speed of such an adhesive composition in comparison with that of ordinary alcohol-soluble resol type phenolic resins or of vinyl acetate resins, as the adhesive composition of polyvinyl acetate-modified phenolic resin containing a hardener already starts hardening on the adherend at normally encountered temperatures above 15° C., and as the vinyl acetate resin used as its modifier is originally provided with a relatively fast drying speed. 
     SUMMARY OF THE INVENTION 
     It is now found that a sufficiently high bonding strength can be obtained even after a prolonged open assembly time by the addition of a saturated aliphatic polyhydric alcohol of 2 to 6 carbon atoms and 2 to 6 alcoholic hydroxy groups per molecule in an amount of about 2 to about 20 parts by weight to 100 parts by weight of polyvinyl acetate-modified phenolic resin solid. This may be ascribed to a suitable flow being retained by the plasticizing effect of polyhydric alcohol even after the adhesive composition of polyvinyl acetate-modified phenolic resin coated on the adherend hardens when compressed as explained above. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The amount of polyhydric alcohol is preferably within a range of about 2 to about 20 parts by weight with respect to 100 parts by weight of polyvinyl acetate-modified phenolic resin solid. An addition in excess of this range results in reduced bonding strength due to an excessive plasticizing effect. 
     Generally the mixture of polyvinyl acetate and resol-type phenolic resin contains a weight ratio of vinyl acetate polymer to phenolic resin of from about 9:1 to about 1:1. 
     Lower polyhydric aliphatic alcohols for use in the present invention are exemplified by ethylene glycol, propylene glycol, glycerine, diglycerine, sorbitol, etc. Mixtures of these and equivalent polyhydric alcohols can also be used. The present invention is further illustrated by the following examples, which shall not be construed as limiting the scope of this invention. In these examples parts percentages and proportions are by weight unless otherwise specified. 
    
    
     EXAMPLE 1 
     (1) addition to ethylene glycol to polyvinyl acetate-modified phenolic resin 
     Adhesive compositions were prepared by the addition, to 100 parts of a polyvinyl acetate-modified phenolic resin (a mixture of an aqueous emulsion of vinyl acetate homopolymer and methanolic resol-type phenol-formaldehyde resin having a resin solids content of 60%), of 0, 2, 6, 10, 14 and 18 parts of ethylene glycol and then 5 parts of hardener (50% aqueous benzenesulfonic acid solution). 
     (2) Adhesion of wood 
     Wood samples were bonded with the above-mentioned adhesive compositions under the following conditions. The compressive shear bonding strengths obtained are summarized in the following Table 1. 
     
         ______________________________________Adherend         white firMoisture content 10 - 20 %Coating amount   250 g/m.sup.2 (coated on one side)Open assembly time            20 minutes at 20° CClosed assembly time            1 minute at 20° CCompression      Temperature 20° C            Pressure  8 kg/cm.sup.2            Time  20 hoursConditioning after            24 hours at 20° Cremoval of pressure______________________________________ 
    
     
                       Table 1______________________________________Compressive shear bonding strength(according to JIS-K-6802-1973)______________________________________CompositionPolyvinyl acetate-modified phenolic resin          100    100    100  100  100  100(parts)Ethyelene glycol (parts)          0      2      6    10   14   18Hardener (parts)          5      5      5    5    5    5Amount of ethylene glycolwith respect to 100 partsof polyvinyl acetate-          0      3.3    10   17   23   30modified phenolic resinsolid (parts)Compressive shear bondingstrengthBonding strength innormal state (kg/cm.sup.2)          11.5   49.5   58.2 53.2 46.5 38.5Wood failure in normalstate (%)      0      61     71   85   58   25Bonding strength afterrepeated boiling          peeled 18.3   21.5 23.2 17.3 15.6(kg/cm.sup.2)Wood failure afterrepeated boiling (%)          peeled 29     60   70   25   18______________________________________ 
    
     As shown in Table 1, the adhesive composition containing 3.3, 10 and 17 parts of ethylene glycol with respect to 100 parts of polyvinyl acetate-modified phenolic resin solid showed higher bonding strengths than in the other compositions. 
     EXAMPLE 2 
     (1) addition of sorbitol to polyvinyl acetate-modified phenolic resin 
     Adhesive compositions were prepared by the addition, to 100 parts of polyvinyl acetate-modified phenolic resin (resin solid content 60%), of 0, 4, 8, 12, 16 and 20 parts of 70% aqueous sorbitol solution, and then of 5 parts of hardener (50% aqueous paratoluenesulfonic acid solution). 
     (2) Adhesion of wood 
     The wood samples were bonded with the above-mentioned adhesive composition under the following conditions. The compressive shear bonding strengths of the bonded samples are summarized in the following Table 2. 
     
         ______________________________________Adherend         birchMoisture content 8 - 12 %Coating amount   250 g/m.sup.2 (coated on one side)Open assembly time            20 minutes at 25° CClosed assembly time            1 minute at 25° CCompression      Temperature 25° C            Pressure  12 kg/cm.sup.2            Time  20 hoursConditioning afterremoval of pressure            24 hours at 25° C______________________________________ 
    
     
                       Table 2______________________________________Compressive shear bonding strength(according to JIS-K-6802-1973)______________________________________CompositionPolyvinyl acetate-modified phenolic resin          100    100    100  100  100  100(parts)70% Aqueous sorbitolsolution (parts)          0      4      8    12   16   20Hardener (parts)          5      5      5    5    5    5Amount of sorbitolwith respect to 100parts of polyvinyl          0      4.7    9.3  14   18.7 23.3acetate-modifiedphenolic resinsolid (parts)Compressive shear bondingstrengthBonding strength innormal state (kg/cm.sup.2)          52.0   105    143  153  140  95.3Wood failure in normalstate (%)      0      22     40   38   28   11Bonding strength afterrepeated boiling (kg/cm.sup.2)          peeled 18.2   48.5 52.3 31.6 12.3Wood failure afterrepeated boiling (%)          peeled 11     35   37   15   3______________________________________ 
    
     As shown in Table 2, the adhesive compositions containing 4.7, 9.3, 14 and 18.7 parts of sorbitol with respect to 100 parts of polyvinyl acetate-modified phenolic resin solid showed higher strengths than in the other compositions. 
     EXAMPLE 3 
     (1) Addition of glycerine to polyvinyl acetate-modified phenolic resin 
     Adhesive compositions were prepared by the addition, to 100 parts of polyvinyl acetate-modified phenolic resin (resin solid content 60%), of 0 and 6 parts of glycerine and then of 5 parts of hardener (50% aqueous paratoluenesulfonic acid solution). 
     (2) Adhesion of wood 
     Wood samples were bonded with the above-mentioned adhesive compositions under the following conditions. The compressive shear bonding strength of these bonded samples are summarized in the following Table 3. 
     
         ______________________________________Adherend         Japanese cypressMoisture content 10 - 12 %Coating amount   250 g/m.sup.2 (coated on one side)Open assembly time            5, 10, 20 or 40 minutes at 20° CClosed assembly time            1 minute at 20° CCompression      Temperature 20° C            Pressure  10 kg/cm.sup.2            Time  20 hoursConditioning afterremoval of pressure            24 hours at 20° C______________________________________ 
    
     
                                           Table 3__________________________________________________________________________Compressive shear bonding strength(according to JIS-K-6802-1973)__________________________________________________________________________CompositionPolyvinyl acetate-modified phenolic resin          100               100(parts)Glycerine (parts)          0                 6Hardener (parts)          5                 5Amount of glycerine withrespect to 100 parts ofpolyvinyl acetate-          0                 10modified phenolic resinsolid (parts)          (Without          (With          Polyhydric Alcohol)                            Polyhydric Alcohol)__________________________________________________________________________Open assembly time (minute)          5   10  20   40   5   10  20  40__________________________________________________________________________Compressive shear bondingstrengthBonding strength innormal state (kg/cm.sup.2)          79.3              63.2                  21.5 peeled                            61.5                                68.5                                    75.3                                        69.4Wood failure innormal state (%)          86  28  0    peeled                            63  71  83  75Bonding strength afterrepeated boiling          38.5              28.9                  peeled                       peeled                            35.2                                39.2                                    38.9                                        37.6(kg/cm.sup.2)Wood failure afterrepeated boiling          68  25  peeled                       peeled                            58  67  70  65(%)__________________________________________________________________________ 
    
     As shown in Table 3, the adhesive composition containing 10 parts of glycerine with respect to 100 parts of polyvinyl acetate-modified phenolic resin solid showed higher bonding strengths for an open assembly time of about 10 minutes or longer in comparison with the adhesive composition with no addition of glycerine.