Pressure sensitive adhesive

A pressure sensitive adhesive is disclosed that is tack-free to the touch, but forms a permanent bond under moderate pressure. The pressure sensitive adhesive contains a tacky adhesive latex, a non-tacky latex or polymer, and an inert filler. The tacky adhesive latex provides the sealing properties, the non-tacky latex or polymer provides blocking properties and the filler absorbs oil that may adversely affect the sealing properties. The adhesive is tack-free to the touch yet becomes activated to form a permanent bond under moderate sealing pressure.

BACKGROUND OF THE INVENTION 
The present invention relates to a pressure sensitive adhesive composition 
and its application. 
The majority of pressure sensitive adhesives disclosed by the prior art are 
tacky to the touch and present difficulties in storage and handling 
because they tend to exhibit "blocking" properties, i.e., they tend to 
adhere to themselves when stored on a reel. Thus, there exists a need for 
a pressure sensitive adhesive that is initially tack-free, but forms a 
permanent bond upon moderate sealing pressure. 
U.S. Pat. No. 4,889,884 to Dust et al. discloses an adhesive that is 
capable of forming a seal to itself with application of pressure, yet 
resists blocking. However, Dust et al. does not disclose the presence of a 
filler. As a result, silicone oil, often used in the nip for cut sheet 
forms to protect the fused roll, would be expected to migrate to the 
surface of the adhesive and interfere with the sealing operation. Thus, 
there is an additional need for a pressure sensitive adhesive whose 
adhesive properties will not be adversely affected by contact with 
silicone oil. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to overcome these and 
other disadvantages of the prior art. 
It is another object of the invention to provide a pressure sensitive 
adhesive that is tack-free to the touch, but forms a permanent bond under 
moderate bonding pressure. 
It is another object of the invention to provide a pressure sensitive 
adhesive that will not be adversely affected by contact with silicone or 
other oils. 
These and other objects are achieved by the pressure sensitive adhesive of 
the invention which comprises a tacky adhesive latex, a non-tacky latex or 
polymer, and an inert filler. The adhesives prepared according to this 
invention are tack-free to the touch yet become activated to form a 
permanent bond under moderate sealing pressure. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be apparent from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention will be realized and attained by 
means of the compounds, compositions and processes particularly pointed 
out in the written description and appended claims. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The first component of the adhesive of the present invention, a tacky 
adhesive latex, is generally a permanent acrylic-type adhesive latex. 
Acrylic emulsions such as Robond PS-60, Robond PS-61, and Rhoplex N-619 
(marketed by Rohm & Haas Co., Philadelphia, Pa.) and Gelva RA 2397, RA 
2404, and RA 2373 (marketed by Monsanto Co., St. Louis, Mo.) are examples 
of the tacky adhesive latex that may be used in accordance with the 
invention. Generally, these adhesives are copolymers of (1) a flexible 
alkyl acrylate or methacrylate having about 4 to about 12 carbon atoms, 
(2) a hard alkyl acrylate or methacrylate having 1 to about 4 carbon 
atoms, and (3) a functional monomer such as acrylic acid, methacrylic acid 
or vinyl pyrrolidone, or multifunctional monomers such as difunctional or 
trifunctional acrylates or methacrylates. The tacky material is the main 
ingredient, incorporated to produce a fiber tearing bond when sealing 
pressure is applied. 
The second component of the composition of the invention may be either a 
non-tacky latex or a non-tacky polymer, or combinations thereof. 
Appropriate examples of non-tacky latexes that may be used include 
acrylate latexes, such as Rhoplex P-376 and B-85 (marketed by Rohm & Haas 
Co., Philadelphia, Pa.) and styrene-butadiene latexes such as Dow latex 
620, 640 and 650 (marketed by Dow Chemical, Midland, Mich.). Appropriate 
examples of non-tacky polymers that may be used include 
adhesive-compatible, water-soluble polymers such as polyvinyl alcohol, 
starch and cellulose derivatives. The non-tacky material is incorporated 
to provide the anti-block characteristics of the adhesives during form 
handling and storage. Too much of this non-tacky material will reduce the 
sealing properties, while too little will result in blocking of the forms 
together. 
The final component of the composition is an inert filler. Examples of 
appropriate fillers that may be used include high capacity oil-absorbing 
silicate powder, silica, titanium dioxide, and clays. The inert filler is 
incorporated to absorb oil. Generally, when silicone oil is used in the 
nip for sheet cut forms to protect the fuser roll, migration of the 
material to the adhesive surface will interfere with the sealing 
operation. The high oil-absorbing filler is incorporated to absorb this 
oil. The adhesive strength is greatly decreased if the filler is 
overdosed. However, the oil absorbing capability will be insufficient if 
it is underdosed. 
A small amount of pigment or dye may also be added to make the 
adhesive-coated substrate more appealing to the eye. 
The composition of the present invention preferably contains about 54 to 
about 84 parts of tacky adhesive latex, about 2 to about 25 parts of 
non-tacky latex or polymer and about 8 to about 35 parts of inert filler 
based upon 100 total parts, on a weight basis. 
The composition according to the invention generally has a solids content 
of about 25 to about 40%, a pH of about 7 to about 10 and a viscosity of 
about 200 to about 2000 cps. 
The composition of the invention can be coated on a substrate such as a 
sheet of about 12 to about 28 # bond paper at about 3.5 to about 15 
microns thickness. Lab wire wound coating rod or production roll coaters 
can be adapted to perform the coating. Spraying, dipping or printing 
methods can also be employed. Other substrates may be coated besides 
paper, such as metal foil, polymer fibers such as cellophane, polyethylene 
terephthalate, olefin polymers and the like. After the adhesive mixture 
has been drawn onto the substrate, the sheet is allowed to dry or is 
passed through a heated oven to obtain a tack-free sheet. 
The following examples are illustrative. Unless otherwise indicated, 
amounts shown are in parts by weight. The following testing procedures 
were used to determine % seal and % blocking. 
Sealing Test Procedure 
1. Coated sheets were cut into 3" by 8" rectangles. 
2. The coated sheets were positioned adhesive-to-adhesive --10 sets were 
tested. 
3. Coated sheets were sealed on a lab sealer at the conditions specified in 
the examples. 
4. The sealed samples were slowly peeled apart and the % of samples with 
fiber tearing seals was recorded. This number was the % seal. 
BLOCKING TEST PROCEDURE 
This test measures paper to pressure-seal adhesive blocking. The test 
requires one 4.times.5 inch (1/4 inch thick) piece of stainless steel for 
a base, one 1.times.3 inch (1 inch thick) piece of aluminum for the top, 
and a 6 pound weight. 
1. 1.times.3 inch samples were cut from identical areas from 20 identical 
samples of sheets coated with adhesive. 
2. The sample strips were stacked and aligned. 
3. The aligned stack was placed on the stainless steel base and placed into 
the preheated oven (220.degree. F.). 
4. The top piece of aluminum was carefully placed onto the stack. 
5. The 6-pound weight was placed on top of the stack and the oven door was 
closed. 
6. After 30 minutes, the samples were removed from the oven and checked for 
any blocking (sticking together). The % of samples with blocking was 
recorded. This number was the % blocking.