Patent Publication Number: US-5296535-A

Title: Remoistenable adhesive composition

Description:
This application is a continuation-in-part of application Ser. No. 439,750 filed Nov. 21, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a liquid remoistenable adhesive composition having superior non-blocking properties at high temperature and high humidity and comprises water and a selected combination of partially hydrolyzed polyvinyl alcohol and a chemically inert, non-thermoplastic, cold water insoluble, particulate additive which is compatible and dispersible with an aqueous solution of the polyvinyl alcohol. 
     Remoistenable adhesives are commonly utilized in the production of envelopes, stamps, wallpaper, tapes, labels and similar products wherein it is desired to provide a dry adhesive film which on being moistened with water will become tacky and thereby allow for the ready adhesion to a chosen substrate. In the manufacture of remoistenable adhesive products, typically an aqueous solution or dispersion, or an organic solvent solution of the dry adhesive material is applied as a wet film to a substrate. After the water or organic solvent is removed by drying and evaporation, the resulting dry adhesive film will, on being moistened, produce the desired tacky adhesive surface. 
     The ability to provide an adhesive which is remoistenable is not in itself difficult and in fact there are a number of known and commercially available products that are used in various applications requiring a remoistenable adhesive. Some of the known remoistenable adhesive formulations include dextrin solutions (dextrin gums); a dextrin solution plus a water based emulsion of polymers such as polyvinyl acetate or ethylene vinyl acetate as disclosed in U.S. Pat. Nos. 4,575,525 issued Mar. 11, 1986 and 4,678,824 issued Jul. 7, 1987; and a polyvinyl alcohol solution combined with an emulsion of polymers such as polyvinyl acetate or ethylene vinyl acetate as shown in U.S. Pat. No. 2,850,468 issued Sep. 2, 1958. 
     The usefulness of a particular composition as a remoistenable adhesive in a specific application will depend on its ability to meet the many physical and chemical property requirements and to best satisfy the processing conditions to which it is subjected. Accordingly, a remoistenable adhesive composition in addition to providing good adhesion and remoistenability, typically is expected to provide a lay flat or non-curl product, satisfy processing conditions of different manufacturing equipment, provide adequate drying speed, resist hygroscopic blocking and satisfy various other requirements depending on the application involved. 
     The needs of the various applications requiring the use of a remoistenable adhesive have generally been met by different known formulations, such as noted above, as well as various modifications thereof. However, changing operating equipment and processes and increased customer requirements dictate the need for an improved remoistening adhesive which has desired properties at both high temperature and high humidity. Such a product is especially needed in the manufacture of stamps and envelopes where resistance to hygroscopic blocking and thermoplastic blocking is required under high temperature and high humidity conditions. 
     SUMMARY OF THE INVENTION 
     This invention relates to a liquid remoistenable adhesive composition having suitable properties and characteristics at high temperature and high humidity comprising water, a partially hydrolyzed polyvinyl alcohol and a chemically inert, non-thermoplastic, cold water insoluble, particulate additive which is compatible and dispersible with the polyvinyl alcohol in aqueous solution, the alcohol and additive being present in amounts of from about 1:3 to 1:0.5 parts by weight of alcohol to additive, the composition having a viscosity of about 500 to 15,000 cps at 72° F. (22.2° C.) and a solids content of about 30 to 80% and provides resistance to hygroscopic blocking and thermoplastic blocking. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The composition of this invention comprising a selected combination of a partially hydrolyzed polyvinyl alcohol and a chemically inert, non-thermoplastic, cold water insoluble, particulate additive provides good adhesion and remoistening ability, has lay flat characteristics and has suitable rheology and other properties to allow it to be processed under conditions wherein satisfactory drying speeds are attained. More significantly, this composition additionally provides resistance to hygroscopic blocking and thermoplastic blocking under conditions of high temperature and high humidity that can often exist during the manufacture, storage, shipping and other handling of products such as stamps and envelopes. 
     &#34;Blocking&#34; is the undesirable adhesion that can occur between touching layers of a material under certain conditions. Hygroscopic blocking resistance is the ability of the adhesive to resist reactivating due to humidity. In a situation where substrates, such as envelopes or sheets of stamps, are coated with an adhesive which is non-resistant to hygroscopic blocking and are stored and exposed to varying degrees of humidity, there is a tendency for adjacent surfaces of the stacked substrates to adhere and stick to one another. 
     Thermoplastic blocking is the adhesion that can occur when adhesive coated substrates are exposed to varying amounts of temperature and pressure during processing, storage and other handling operations. Pressure conditions to create blocking can exist, for example, during storage or stacking of products while processing, during the printing of products such as the laserjet printing of papers and envelopes and in perforating sheets of stamps and other products. Depending on the particular conditions used, the amount of pressure that is created can cause blocking in non-resistant adhesive formulations. 
     Hygroscopic blocking has been more particularly defined by a standard test method, ASTM D 1146-53, wherein hygroscopic blocking is measured on a test specimen at 50 percent relative humidity at 38° C. for 24 hours, and if there is no blocking (free) at increments of successively higher humidities until blocking occurs (critical humidity) or until a suitably high humidity is reached. Thermoplastic blocking has been defined and measured under the same ASTM D 1146-53 procedure at 38° C. for 24 hours and if there is no blocking (free) at successively higher temperatures (increments of 5° C.) until blocking occurs (critical temperature) or until a suitably high temperature is reached. Especially suitable high humidity and high temperature conditions for determining hygroscopic and thermoplastic blocking are 95 percent relative humidity and 90° C. 
     Another problem often found in the use of remoistenable adhesives is the tendency of coated substrates to curl and buckle. This curling results from the unequal rates of expansion and contraction on the part of the paper backing in relation to the adhesive film as conditions vary, particularly humidity. Excessive curl can cause equipment jam-ups or other handling difficulties. While some adhesive materials have good lay flat or non-curl properties, others require various techniques such as the addition of humectants to overcome or minimize this curling tendency. While these techniques often alleviate the curling problem, other properties such as hygroscopic blocking resistance can be unfavorably affected. 
     The remoistenable adhesive composition of this invention exhibits a very desirable combination of properties including hygroscopic blocking resistance, thermoplastic blocking resistance and lay flat or non-curl characteristic along with its remoistening capability. 
     The polyvinyl alcohol component used in this adhesive composition is partially hydrolyzed. Polyvinyl alcohol is a well known, commercially available product prepared by the hydrolysis of polyvinyl acetate. It is typically available in grades defined by the degree of hydrolysis with the &#34;super hydrolyzed&#34; grades having a % hydrolysis (i.e. mole %) of greater than about 99.0, &#34;fully hydrolyzed&#34; grades being about 97.5 to 98.9, &#34;intermediate hydrolyzed&#34; grades from about 95 to 97.5 and &#34;partially hydrolyzed&#34; grades being less than about 95. Partially hydrolyzed polyvinyl alcohol as used in this invention will be a product having a degree of hydrolysis of from about 70 to 92, preferably from about 80 to 90, and more preferably from about 85 to 90%. The viscosity of the polyvinyl alcohol also varies with a useful range being a viscosity in centipoise of a 4% aqueous solution at 20° C. of from about 2 to 50. More particularly, the viscosity of the polyvinyl alcohol will be from about 2 to 30 cps and preferably in the lower range of from about 2 to 8 cps. 
     While polyvinyl alcohol is a well known adhesive material and has been used in different formulations, the particular composition of this invention where it is specifically combined with a selected additive provides a remoistenable adhesive having especially desirable high temperature and high humidity properties, heretofore not present in such formulations. The additive useful in this composition is a chemically inert, non-thermoplastic, cold water insoluble, particulate material which is compatible and dispersible with an aqueous solution of the polyvinyl alcohol. This additive material must additionally be non-toxic, be combinable with the alcohol in a manner which will provide the composition with a sufficient solids content and viscosity to allow suitable processing conditions and have a sufficiently small particle size so as not to interfere with the processing and application equipment. Most important, this selected additive in combination with the polyvinyl alcohol must enhance the humidity blocking resistance and thermoplastic blocking resistance and provide a lay flat or non-curl characteristic. 
     The additive material used in this invention is a chemically inert, non-thermoplastic, cold water insoluble, particulate material that is compatible and dispersible with polyvinyl alcohol. Materials of this type include clay, talc, calcium carbonate, gypsum, mica and ungelatinized granular starch with clay and more particularly, kaolin clay, being the preferred additive. The particle size of the additive must be sufficiently small so as not to detrimentally effect the processing and application of the formulation. Generally, a suitable particle size will be less then about 30 microns (μ), preferably less than about 20μ and more preferably less than about 10μ. 
     In order to achieve the desired property characteristic, the adhesive composition must have a defined combination of partially hydrolyzed polyvinyl alcohol and additive. More particularly, the polyvinyl alcohol and additive will be present in amounts of from about 1:3 to 1:0.5 parts by weight of alcohol to additive and preferably from about 1:2 to 1:1. The adhesive formulation will have a viscosity range of about 500 to 15,000 cps and preferably about 1000 to 12,000 cps and a solids range of about 30 to 80%, preferably about 40 to 60%. 
     The viscosity of the adhesive formulation is a Brookfield viscosity determined using Brookfield viscometer model RVT at 20 rpm and 72° F. (22.2° C.). 
     Generally, the remoistenable adhesive composition will, while maintaining the relationships noted above, comprise about 10 to 50% by weight of partially hydrolyzed polyvinyl alcohol, from about 5 to 40% by weight of selected additive and from about 20 to 85% by weight of water. Preferably from about 12 to 30% by weight of polyvinyl alcohol, from about 20 to 40% by weight of additive and from about 40 to 60% by weight of water will be used. 
     Some optional ingredients, such as defoamers, plasticizers, lay flat additives etc., may be added to the remoistening adhesive composition to modify certain characteristics thereof, as long as they do not detrimentally affect the hygroscopic and thermoplastic blocking properties. Such ingredients will generally be used in small amounts of less than about 15% by weight and usually in minor amounts of less than about 3% by weight. 
    
    
     The following examples are further illustrative of this invention and are not intended to be construed as limitations thereof. In the examples, all parts and percentages are by weight and all temperatures in degrees Celsius, unless otherwise noted. The viscosity of the adhesive formulation is a Brookfield viscosity, as described previously, determined using Brookfield viscometer model RVT at 20 rpm and 72° F. (22.2° C.). The viscosity of polyvinyl alcohol is measured in a 4% aqueous solution at 20° C. 
     EXAMPLE I 
     A remoistenable adhesive composition was prepared by adding 25 parts of partially hydrolyzed polyvinyl alcohol (87.0-89.0% hydrolysis) having a viscosity of 3-4 cps, 25 parts of kaolin clay and 50 parts of water and agitated and heated to obtain a uniform formulation having a solids content of 50.3% and a viscosity of 10,000 cps. 
     The composition was applied to a paper substrate and an air dried film was obtained in 2 minutes 30 seconds. 
     The coated sample was tested and found suitable for remoistenability and adhesion and was further tested for hygroscopic blocking and thermoplastic blocking using the procedure of ASTM D 1146-53. Hygroscopic blocking was determined by storing the test specimen for 24 hrs at 95 percent relative humidity at 38° C. (100.4° F.). The sample was free of blocking indicating its resistance to hygroscopic blocking. Thermoplastic blocking was tested by exposing a sample at varying temperatures and a pressure of 65 psi at low humidity for 24 hrs. This sample was free of thermoplastic blocking at a temperature up to 249° C. (475° F.). 
     EXAMPLE II 
     Samples having varying amounts of partially hydrolyzed polyvinyl alcohol (87.0-89.0% hydrolysis) and kaolin clay additive were prepared, tested and compared with a sample standard remoistening adhesive product comprising 80% by weight of polyvinyl acetate emulsion and 15% by weight of polyvinyl alcohol. As indicated in Table 1, all samples provided suitable adhesion and hygroscopic blocking, however, while the polyvinyl/alcohol/clay additive combinations did not exhibit thermoplastic blocking until temperatures of at least about 149° C. (300° F.) at 65 psi, the comparative polyvinyl alcohol/polyvinyl acetate product exhibited thermoplastic blocking at about 38° C. and a pressure of 40 psi. 
     Similar samples were tested for thermoplastic blocking at different temperatures and 65 psi. As shown in Table 2, the polyvinyl alcohol/clay combinations showed only very slight fiber picks/tears at a temperature of 121° C. (250° F.) and no fiber tear bond until temperatures of at least 176.7° C. (350° F.). The comparative polyvinyl alcohol/polyvinyl acetate adhesive showed a very slight pick at about 80° C. (175° F.) and a fiber tear bond at 93.3° C. (200° F.). 
     
                                           TABLE 1                                 
__________________________________________________________________________
Properties of Remoistening Adhesives                                      
Adhesive     Solid      Hygroscopic                                       
                               Thermoplastic                              
(parts by weight)                                                         
             Content(%)                                                   
                   Adhesion                                               
                        Blocking                                          
                               Blocking                                   
__________________________________________________________________________
PV Alcohol/Clay (1:3)                                                     
             54.7  Fiber                                                  
                        Free   Free at 246° C./                    
                   Tearing     65 psi                                     
                   Bond                                                   
PV Alcohol/Clay (1:2.3)                                                   
             54.4  Fiber                                                  
                        &#34;      Free at 218° C./                    
                   Tearing     65 psi                                     
                   Bond                                                   
PV Alcohol/Clay (1:1.85)                                                  
             51.2  Fiber                                                  
                        &#34;      194.6° C./65 psi                    
                   Tearing                                                
                   Bond                                                   
PV Alcohol/Clay (1:1.5)                                                   
             51.9  Fiber                                                  
                        &#34;      162.8° C./65 psi                    
                   Tearing                                                
                   Bond                                                   
PV Alcohol/Clay (1:1.2)                                                   
             49.5  Fiber                                                  
                        &#34;      162.8° C./65 psi                    
                   Tearing                                                
                   Bond                                                   
PV Alcohol/Clay (1:1)                                                     
             50.3  Fiber                                                  
                        &#34;      148.9° C./65 psi                    
                   Tearing                                                
                   Bond                                                   
PV Alcohol/PV                                                             
             51.2  Fiber                                                  
                        &#34;       37.7° C./40 psi                    
acetate            Tearing                                                
(comparative)      Bond                                                   
__________________________________________________________________________
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
Thermoplastic Blocking at Different Temperatures (65 psi)                 
             Temperature °C. (°F.)                          
             79.4                                                         
                93.3                                                      
                   107.2                                                  
                      121.1                                               
                         135                                              
                            148.9                                         
                               162.8                                      
                                  176.7                                   
                                     190.6                                
                                        204.4                             
                                           218.3                          
                                              232.2                       
                                                 246                      
Adhesive     (175)                                                        
                (200)                                                     
                   (225)                                                  
                      (250)                                               
                         (275)                                            
                            (300)                                         
                               (325)                                      
                                  (350)                                   
                                     (375)                                
                                        (400)                             
                                           (425)                          
                                              (450)                       
                                                 (475)                    
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PV Alcohol/PV Acetate                                                     
             2  5  5  5  5  5  5  5  5  5  5  5  5                        
(comparative)                                                             
PV Alcohol/Clay (1:3)                                                     
             -- -- -- -- -- -- -- -- 1  2  1  2  2                        
PV Alcohol/Clay (1:2.3)                                                   
             -- -- -- 1  1  2  3  3  3  3  3  3  5                        
PV Alcohol/Clay (1:1.85)                                                  
             -- 1  -- 1  1  3  3  3  4  5  5  5  5                        
PV Alcohol/Clay (1:1.5)                                                   
             -- -- 1  2  3  3  4  4  5  5  5  5  5                        
PV Alcohol/Clay (1:1.2)                                                   
             -- -- 1  3  4  4  4  4  5  5  5  5  5                        
PV Alcohol/Clay (1:1)                                                     
             -- 1  1  2  2  4  4  5  5  5  5  5  5                        
__________________________________________________________________________
Code               Best                                                   
1 - Tick - No fiber tear                                                  
                   ↓                                               
2 - Pick - Very slight, spotty fiber picks                                
                   ↓                                               
3 - Sl. Ft. - Slight fiber tear                                           
                   ↓                                               
4 - Sp. Ft. - Spotty fiber tear                                           
                   ↓                                               
5 - FTB - Fiber tear bond                                                 
                   Worst                                                  
 
    
     EXAMPLE III 
     In order to further show the comparison between standard adhesive products and the remoistenable adhesive compositions of this invention in relation to hygroscopic blocking and thermoplastic blocking, several samples were prepared and tested in accordance with ASTM D 1146-53. Results are shown in Tables 3 and 4. 
     In Table 3, hygroscopic blocking is given for samples tested at 38° C. for 24 hours at varying relative humidities (48 hours for 95% relative humidity). The degree of blocking is shown in accordance with ASTM D 1146-53 with 3rd degree blocking representing a full bond or adhesion with very severe damage. The polyvinyl alcohol/clay composition exhibited no hygroscopic blocking (free) as compared with the various standard adhesive products which exhibited the degrees of blocking shown. 
     In Table 4, thermoplastic blocking is given for samples tested in accordance with ASTM D-1146-53 at different temperatures for 24 hours. The degree of blocking is shown with 3rd degree blocking representing a major failure or major fiber tear. The polyvinyl alcohol/clay samples exhibited no thermoplastic blocking (free) while the other comparative adhesive products all had the degree of blocking shown at 121.1° C. 
     
                       TABLE 3                                                     
______________________________________                                    
Hygroscopic Blocking of Remoistening Adhesives                            
Relative Humidity (%)                                                     
Adhesive                                  95                              
Type     54     62      75    83    91    (48 hrs)                        
______________________________________                                    
Dextrin  Free   Free    1st   2nd   3rd   3rd                             
                        Degree                                            
                              Degree                                      
                                    Degree                                
                                          Degree                          
PV Acetate/                                                               
         Free   Free    Free  1st   3rd   3rd                             
Dextrin                       Degree                                      
                                    Degree                                
                                          Degree                          
PV Acetate                                                                
         Free   1st     1st   1st   1st   1st                             
PV Alcohol      Degree  Degree                                            
                              Degree                                      
                                    Degree                                
                                          Degree                          
with                                                                      
Plasticizer                                                               
PV Acetate/                                                               
         Free   Free    1st   1st   1st   1st                             
PV Alcohol              Degree                                            
                              Degree                                      
                                    Degree                                
                                          Degree                          
w/o                                                                       
Plasticizer                                                               
Dextrin/ Free   Free    1st   2nd   3rd   3rd                             
PV Acetate              Degree                                            
                              Degree                                      
                                    Degree                                
                                          Degree                          
(dry                                                                      
gumming                                                                   
type)                                                                     
PV Alcohol/                                                               
         Free   Free    Free  Free  Free  Free                            
Clay (same as                                                             
Ex I 1:1)                                                                 
______________________________________                                    
 Code:                                                                    
 Free  no blocking                                                        
 1st degree  slight clinging/parted with no evidence of damage            
 2nd degree  some adherence/parted with one surface or other showing some 
 damage.                                                                  
 3rd degree  full bond or adhesion/severe damage.                         
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
Thermoplastic Blocking of Remoistening Adhesives                          
             Temperatures                                                 
Adhesive     29.4° C. (85° F.)                              
                     65.6° C. (150° F.)                     
                              121.1° C. (250° F.)           
__________________________________________________________________________
Dextrin      Free    Free     2nd degree                                  
PV Acetate/Dextrin                                                        
             Free    Free     3rd degree                                  
PV Acetate/PV Alcohol                                                     
             Free    Free     2nd degree                                  
PV Alcohol/Clay                                                           
             Free    Free     Free                                        
(sames as Ex. I, 1:1)                                                     
__________________________________________________________________________
 Code:                                                                    
 Free  no blocking                                                        
 1st degree  slight clinging/parted with no evidence of damage            
 2nd degree  some adherence/parted with one surface or the other showing  
 some damage.                                                             
 3rd degree  full bond or adhesion/severe damage.