Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-04-27
2001-03-13
Yoon, Tae (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S334000, C524S045000, C524S378000, C524S832000, C528S499000, C427S208400
Reexamination Certificate
active
06201042
ABSTRACT:
BACKGROUND OF THE INVENTION
There has been considerable development over the last twenty years in the design and manufacture of repositionable adhesives, specifically in methods of making inherently tacky, elastomeric, solvent insoluble, solvent dispersible polymeric microspheres. These microspheres, either solid or hollow, are typically combined with appropriate binders and thickening agents to form adhesives which are coated by various methods on backing materials such as papers, films and foils.
Inherently tacky acrylic microspheres utilized in these adhesive compositions are produced by suspension polymerization methods which have become quite common. In all of these techniques surfactants are used to stabilize the microspheres in the suspension polymerization process. Following the polymerization process, additional wetting agents or surfactants are often added to increase the wetting ability of the microsphere composition on support surfaces. For example, Tsujimoto Patent WO97/14510 describes a transfer coatable water based microsphere adhesive composition for sheet-to-sheet coating processes. It teaches that a surfactant, in addition to surfactant within the polymeric microsphere composition itself and its binders and thickening agents, is added during the manufacture of the adhesive composition to enhance wetting on a silicon surface. This is a well known practice.
However, the addition of wetting agents or surfactants to microsphere compositions in adhesive manufacture is not without disadvantages. The wetting agent or surfactant may migrate from the finished product if present in excessive amounts. In transfer coating processes, the tack and peel of the adhesive microsphere compositions can be diminished by wetting agents or surfactants in too great an amount. Also, typical wetting agents such as fluorosurfactants and specialty diols are quite expensive.
It is the purpose of the present invention to improve upon methods of making repositionable pressure sensitive adhesives in a manner such that surfactants are not added to the microsphere compositions as in the Tsujimoto reference and those surfactants present in the microsphere composition or its binders or thickening agents are significantly reduced. By achieving these objects, migration of wetting agent from the finished products is greatly lessened if not eliminated, the inherent tack and peel of the adhesive microsphere compositions are increased and use of expensive added wetting agents such as fluorosurfactants and specialty diols is avoided.
SUMMARY OF THE INVENTION
The improvement of the invention is in the manufacture of repositionable pressure sensitive adhesive of tacky elastomeric microspheres wherein the microspheres are polymerized and form in an aqueous suspension which includes a surfactant. The improvement comprises removing a certain amount of surfactant from the aqueous suspension after completion of polymerization of the microspheres so that a predetermined reduced amount of surfactant remains. Thereafter a thickening agent is added to the aqueous suspension. As a consequence of this removal of surfactant, migration of the surfactant from the adhesive is at least substantially reduced.
The aqueous suspension in which the polymerized microspheres are formed preferably is partly of solids. In a specific form of the method of the invention a wash liquid is added to the aqueous suspension so that some of the surfactant transfers to the wash liquid. The wash liquid is removed along with the transferred surfactant. to leave an aqueous suspension of a greater amount of solids and a lesser amount of the surfactant.
This addition and removal of the wash liquid may be repeated until the desired predetermined reduced amount of surfactant remains in the suspension.
A preferred form of the surfactant is ethoxylated oleyl alcohol. The predetermined reduced amount of this preferred surfactant in the aqueous suspension is 0.028 percent to 0.133 percent by weight on a mass basis of both solids and liquid before addition of the thickening agent and 0.020 percent to 0.093 percent by weight on a mass basis of both solids and liquid after addition of the thickening agent.
The repositionable pressure sensitive adhesive made in accordance with the invention may be coated on a release sheet which is then laminated to paper stock to form labelstock. The microspheres are preferably of an average particle diameter of 20 to 60 microns and they may be either solid or hollow. The thickening agent may be a sodium carboxy methyl cellulose based slurry.
DESCRIPTION OF PREFERRED EMBODIMENT
Key to the method of the invention is that a substantial amount of the surfactant present during the polymerization of the microspheres is removed after polymerization is completed. As noted, this removal of surfactant is accomplished by separation of the microsphere suspension followed by decantation to increase the percentage of solids in the suspension. This is then repeated with additional dilution of the microsphere suspension and further decantation until a desired predetermined reduced surfactant level is achieved. The resulting adhesive composition may be coated on a release sheet which is laminated to paper stock to form lablestock or it can be used in a sheet-to-sheet coating process as described in the aforementioned patent to Tsujimoto.
In a preferred form of the method of the invention the elastomeric microspheres, either solid or hollow, have an average particle diameter of 20 to 60 microns. The preferred surfactant is ethoxylated oleyl alcohol. From one to six washings each followed by phase separation reduces the amount of surfactant in the microspheres to the desired level. That level is from 0.028 percent to 0.133 percent by weight on a mass basis of both solids and liquid before the final addition of a thickening agent and 0.020 percent to 0.093 percent by weight on a mass basis of both solids and liquid after addition of a thickening agent. A preferred form of thickening agent is an aqueous suspension of sodium carboxy methyl cellulose. The addition of such a thickening agent increases the bulk viscosity of the suspension to between 100 and 2,000 centipoise.
REFERENCES:
patent: 4495318 (1985-01-01), Howard
patent: 4598120 (1986-07-01), Thoma et al.
patent: 5877252 (1999-03-01), Tsujimoto et al.
patent: 97/14510 (1997-04-01), None
Holbrook David K.
Kaye Howard B.
Vogler Brian M.
Brown, Esq. Charles J.
Yoon Tae
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