Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2001-05-29
2003-09-02
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C428S447000
Reexamination Certificate
active
06613859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention discloses a water-reducible urethane oligomer having dimethylsiloxane segment and ionic group. Emphasis is being placed on the use of the compound as release coating in adhesive-coated products as well as a process for creating adhesive tapes and labels.
2. Description of the Prior Art
A pressure-sensitive adhesive tape is generally manufactured and sold with the tape wound upon itself in convolutions to form a roll of some suitable length of tape. Consequently, when it is necessary to use the tape, it must be possible to unwind the desired length from the roll without excessive force or delaminating of the backing, offsetting of the adhesive, or the like, regardless of the time or conditions under which the tape has remained in roll form prior to use. For these reasons, a coating known as a release coat or backsize is generally provided on the backside of the tape backing member, i.e., the side opposite that on which the adhesive mass is applied. Such a coat, compared to an uncoated backing member, as is its objective, offers relatively low adhesion to the adhesive mass.
Although various release agents and compositions thereof have been developed over the years, none of them, to my knowledge, accomplishes all the objects desired by their use. With some release agents, the release characteristics diminish with time, and particularly at high temperatures, because of some chemical or physical change in the release material per se. Others interact with the adhesive mass so that adhesion to various substrates to which the tape is applied is adversely affected.
Whether a material is suitable or not as a release agent, in particular for pressure-sensitive adhesive tapes, depends upon a number of factors. The lower the interfacial tension between the adhesive mass and the material used as a release coating, naturally the better release provided. However, low interfacial tension alone is not enough. The material, to be useful as a release coat, should also be of a suitable cohesive strength and possess good adhesion with the backing member.
In the 1970's, silicone emulsions in water were introduced. Generically, they were dimethylsiloxane oligomers with Si—H reactive groups. They had to be mixed with a catalyst prior to application to paper or plastic film, which made them inconvenient 2-component systems. Upon evaporation of water, a surface film of the oligomer was deposited, which built up in molecular weight, crosslinked, and a solid coating resulted. This process, called “cure”, needed either high temperatures or long time at moderate temperatures.
The necessity for curing the polymeric material after deposition has placed certain restrictions on the use of silicones. This is because the temperature needed for curing, at least to the extent desired, has often exceeded the temperature, which a substrate on which a release coating is deposited could withstand. This has been particularly true where the substrate is a thermoplastic film or a paper-thermoplastic film laminate, care being needed to avoid melting or distorting the thermoplastic film. In paper substrates, high temperatures result in over-drying of paper.
A further disadvantage associated with silicone release polymers is their relatively poor adhesion to certain plastic films on which they are coated. This poor adhesion is thought to be due to the scarcity of polar groups in the silicone. Consequently, the use of a primer is often needed to obtain better anchorage of the silicone film to the substrate. In addition, the release characteristic in itself is sometimes a problem in that too good a release is provided. Thus, tape rolls may be too easy to unwind and, in some cases, the adhesion between the adhesive mass and release coat may be so low that the roll doesn't remain tightly wound.
U.S. Pat. No. 4,287,109 of Schlak et al. discloses an aqueous resin of a Slicone-Si—O—C-Polyester block copolymer.
B.P. 1,128,642 of Keberle et al. and G.P. DE 37 30 780 A1 of Nagorski et al. disclose water base silicone-modified polyurethanes. Keberle suggests their use for impregnating and coating fabrics, leather, paper, glass, wood, laminates and foamed plastics for anti static finishes and hydrophobic coatings, and as binder, lubricant, mold release agent, cleaning agent, leveling agent and corrosion inhibitor. Nagorski suggests usage as coating for wood, metal, paper, synthetic fabric, flock and leather.
EPA 0 342 826 of Higgins and its corresponding U.S. Pat. No. 5,082,704 disclose a silicone-modified polyurethane dispersion in water which may be used to fabricate a release liner for adhesives, most notably for asphalt roofing materials.
EPA 0 380 236 A2 of Leir discloses a diamine-terminated silicone, the preparation of ionically-modified silicone-polyurea from ingredients having “a high degree of difunctionality with little contamination from monofunctional . . . impurities.” which “inhibit the chain extension reaction and limit the attainment of optimum molecular weight, and thereby optimum tensile strength of the polyurea”, and its use as an elastomer, a pressure sensitive adhesive and a low adhesion backsize.
My own inventions, U.S. Pat. Nos. 5,356,706 and 5,543,171, disclose processes for manufacturing aqueous release coatings for pressure sensitive adhesive tapes by reacting a siloxane oligomer having 2-3 isocyanate-reactive groups and a diol having an acidic group with an excess polyisocyanate, and further reacting, in water, with a tertiary amine to form a salt and a primary or secondary polyamine to extend the prepolymer into a polymeric chain.
U.S. Pat. No. 5,679,754 of Larson et al. disclose fluorinated polyurethane with sulfonate groups as release agent dispersion in water.
U.S. Pat. Nos. 5,750,630 and 6,072,019 of Sengupta disclose a “polyurethane polymer whose chain includes silicone-containing segments and whose polymer chain is end-capped with a single isocyanate-reactive silane group” and a 2.5-30 weight percent solution in water of the polyurethane.
The Encyclopedia of Polymer Science and Engineering (Vol. 10, P.432) reports the definition of an oligomer by the International Union of Pure and Applied Chemistry (IUPAC) as “a substance composed of molecules containing a few of one or more species of atoms (constitutional units) repetitively linked to each other.” A polymer, on the other hand, is characterized by the repetition of a great number constitutional units, or monomers. The average number of monomers per molecule, also referred to as average degree of polymerization (DP), is about 3-50 for an oligomer and about 50-1,000 for polymers. The “number-average molecular weights (M
n
) of commercial polymers usually lie in the range of 10,000-100,000” (Textbook of Polymer Science, Third Edition, Fred W. Billmeyer, Jr., Ed., p. 17) with weight-average molecular weights (M
w
) being 2.5-5 times higher.
Condensation polymers, such as polyurerthanes, are synthesized by stepwise polymerization. In order to obtain good physical properties, such as tensile strength, elasticity and cohesive strength, which prevents transfer to the pressure sensitive adhesive in tape and label applications thus detackifying the adhesive, they are prepared from polyfunctional reactants in close stoichiometric proportions to build long polymeric chains containing monomeric units in excess of 50 and M
w
, in the 20,000-120,000 ranges. The use of monofunctional monomers is avoided since they to produce low molecular weight products, which generally results in poor physical properties. Thus, in the preparation of prior art polyurethanes all the monomers are polyfunctional and they are present in stoichiometric proportions to obtain high DP and M
w
. Sengupta's polyurethane also includes a minor stoichiometric amount, 3-6%, of a monofunctional silane, but “the molecular weight of the polymer is not found to be critical, but polymers having weight average molecular weight in the range of from about 20,000 to about 120,000 are found to be suitable” (U.S. Pat. No.
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