Description of Related Art Pressure-sensitive adhesives in the form of labels, for example, are typically coated with a release coating on the back side of the label, (i. e., on the side opposite the adhesive). This release coating is in large part responsible for making these adhesive materials removable without the need for excessive force and without causing delamination of the backing or offsetting of the adhesive.

To be useful as a release coating, a material generally should have low work of adhesion with the adhesive. Useful release coating materials also have suitable strength and possess good adhesion with the backing of the adhesive material. Finally, useful release coating materials prevent inadvertent transfer of the adhesive to the back (i. e., non-adhesive) surface of the adhesive material. Release coatings make it possible, for example, for pressure-sensitive labels to be repositionable or removable.

Silicones are well known as release coatings for tacky materials, e. g., as backing members for pressure-sensitive adhesives. Silicones are composed primarily of alternating silicon and oxygen atoms in the polymeric backbone. Any one of a number of organic groups can be attached to the silicon atoms to create polymers that are referred to as organopolysiloxanes.

The simplest polysiloxane, polydimethylsiloxane, is one in which methyl groups are attached to the silicon atom. The physical nature of polysiloxanes depends upon the degree of polymerization and upon the nature of the end groups in the polymer, which can range from chemically active to relatively inert.

One disadvantage of silicone coatings is that they tend to be expensive. In addition, the force to separate a pressure-sensitive adhesive from a surface treated with silicones is typically in the range of 10 to 40 grams per inch of width, a force much too low for the silicone to be effectively used as a low-adhesion backing. This poor adhesion may be due to the scarcity of polar groups in the organosiloxane polymer. The release characteristics of silicone, therefore, may present problems because the release properties are too good. For example, this can result in a roll of tape not remaining tightly wound, among other disadvantages. Also, the use of a primer layer between the silicone film and the substrate is often needed to help anchor the silicone film to the substrate.

As a result, attempts have been made to modify silicone polymers by blending or reacting them with other less effective release materials in order to obtain higher release force values. For example, it is known that siloxane block copolymers may be obtained by combining very specific polydimethylsiloxanes with organic diisocyanates. U. S. Patent No. 4,002,794 to Schwarcz discloses a release coating made of a copolymeric material that is the reaction product of a thio-or mercapto-terminated polyorganosiloxane and a diisocyanate difunctional organic compound.

Another example of a siloxane polymer or copolymer that has been used as a release material is fluorine-terminated polyorganosiloxane. U. S.

Patent No. 4,810,749 to Pinchuk discloses copolymers with a polymeric backbone having repeating isocyanate groups and repeating fluoroalkylsiloxane.

However, modified silicone polymers suffer certain disadvantages.

Some modified silicones contaminate a pressure-sensitive adhesive so that it loses its tackiness. Other modified silicones slowly react with a pressure- sensitive adhesive, causing inseparability upon aging. Some compositions are hard to reproduce consistently, e. g., the release properties change as the silicone gradually migrates to the surface. Other compositions require such high cure temperatures that the substrate on which they are coated may be melted or destroyed. Finally, some compositions are only effective when used in combination with a specific adhesive.

Certain substituents create more disadvantages and difficulties with the printable release coatings than others. For instance, the present inventor has discovered that a fluorinated polydimethylsiloxane would have a lower surface energy than a non-fluorinated polydimethylsiloxane and would thus be less printable. As a result, a lesser amount of a fluorinated polydimethysiloxane would be used in constructing the block copolymer. This would, in turn, result in a loss of flexibility in the final polymer, leading to a higher release force with a noisier release, known in the art as a more "chattery"peel. Further, it is advantageous to be able to print on such labels, e. g., with a thermal transfer printer, where the resulting print does not easily rub off or become smudged. Because of the aforementioned disadvantage, fluorinated polydimethylsiloxanes would not be suitable for such applications.

In contrast with the prior art, which required end groups on the polydimethylsiloxanes, e. g., fluorine, thio or mercapto groups, the present invention does not require such end groups on the polydimethylsiloxanes in order to obtain effective printable release coatings. The present invention thus avoids the accompanying disadvantages of such end groups. The inventor has found, in fact, that the composition of the present invention works better without the above-mentioned end groups on the polydimethylsiloxane polymer.

The present inventor has found that a siloxane block copolymerformed by the reaction of certain polydimethylsiloxanes with an organic diisocyanate is surprisingly successful as a release coating for pressure-sensitive labels.

Disclosure of the Invention To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described herein, the present invention is directed to a printable release coating. The release coating includes a siloxane block copolymer that is the reaction product of (a) a polydimethylsiloxane free of thio, mercapto, and fluorine groups and (b) an organic diisocyanate.

The present invention further relates to a siloxane block copolymer for use as a printable release coating. The copolymer is the reaction product of (a) a polydimethylsiloxane free of thio, mercapto, and fluorine groups and (b) an organic diisocyanate.

The present invention also relates to a method of making a siloxane block copolymer for use as a printable release coating. The method includes the steps of (a) mixing a polydimethylsiloxane with a solvent; (b) mixing a diisocyanate with a solvent; (c) combining the mixture of (a) with the mixture of (b) at a temperature and for a time sufficient to allow the polydimethylsiloxane to react with the diisocyanate; (d) adding a chain extender to the reactants combined in step (c) and maintaining a sufficient temperature for a sufficient time to allow the chain extender to react.

Instead of blending materials having either good printability or good release properties, the inventor has synthesized siloxane block copolymers having both properties by reacting a polydimethylsiloxane with an organic diisocyanate. The siloxane segments of the block copolymer provide good release properties, while the polar (urethane and urea) segments provide printability.

The above and other advantages and features of this invention will become apparent upon review of the following detailed description.

Best Mode for Carrying Out the Invention The present invention relates to a printable release coating including a siloxane block copolymer that is the reaction product of (a) a polydimethylsiloxane free of thio, mercapto, and fluorine groups and (b) an organic diisocyanate.

The present invention further relates to a siloxane block copolymer for use as a printable release coating. The copolymer is the reaction product of (a) a polydimethylsiloxane free of thio, mercapto, and fluorine groups and (b) an organic diisocyanate.

Polydimethylsiloxanes useful in the present invention include, for example, amine-terminated oligomers and hydroxyl-terminated oligomers.

Examples of useful diisocyanates include, but are not limited to, isophorone diisocyanate, diphenylmethyl diisocyanate, toluene diisocyanate, dicyclohexylmethyl diisocyanate, and hexamethylene diisocyanate.

The materials used for synthesis of the siloxane block copolymer may also include chain extenders, catalysts, and solvents. A chain extender lengthens the distance between silicone segments in the siloxane block copolymer. In doing so, the chain extender imparts two properties to the block copolymer. First, the chain extender raises the surface energy of the block copolymer. This improves printability. Second, the chain extender imparts flexibility into the block copolymer. This reduces"chattery" (noisy) release as well as release force overall."Chatter"is the zipping sound often made when tapes or labels are unwound from a roll.

The chain extender material used in the present invention may be selected from diols, triols, poly-ols, and their sulfur equivalents. Nitrogen analogues, e. g., diamines, triamines, polyamines, could also be used if a stoichiometric excess of diisocyanate is maintained. Preferably, the chain extender is polyethylene glycol, 1,4-butanediol or a mixture thereof. If the chain extender is polyethylene glycol, it is preferably present in the amount of about 0-50 parts by weight. If the chain extender is 1,4-butanediol, it is preferably present in the amount of about 0-20 parts by weight.

Catalysts useful in the present invention include, for example, ferric acetyl acetonate, tributyl tin diacetate, tin octoate, and dibutyl tin dilaurate.

Preferably, the catalyst used in the present invention is dibutyl tin dilaurate.

The catalyst serves to speed up the reaction and allow the reaction to take place at lower temperatures.

The solvents are used in the present invention to make it easier to mix the ingredients for reaction and aid in keeping the final product from solidifying, i. e., keeping its viscosity sufficiently low to make it pourable and coatable. Solvents useful in the present invention include tetrahydrofuran, xylene, and toluene. The preferred solvent is toluene. The solvent is preferably present in the amount of about 0-80 parts by weight, preferably in an amount of about 50-80 parts by weight.

The release coatings of the present invention may be used to make pressure-sensitive labels. The release composition may be slightly altered to provide the desired release properties and printability for different adhesives and printers. Pressure-sensitive adhesives, for example, could be either repositionable, removable, or permanent. Labels made with the release coatings of the present invention can be printed with a thermal transfer printer.

The present invention also relates to a method of making a siloxane block copolymer for use as a printable release coating. The method includes the steps of (a) mixing a polydimethylsiloxane with a solvent; (b) mixing a diisocyanate with a solvent; (c) combining the mixture of (a) with the mixture of (b) at a temperature and for a time sufficient to allow the polydimethylsiloxane to react with the diisocyanate; (d) adding a chain extender to the reactants combined in step (c) and maintaining a sufficient temperature for a sufficient time to allow the chain extender to react.

Preferably, the polydimethylsiloxane and the diisocyanate are reacted at a temperature of 60-80°C for 20-40 minutes. More preferably, the reaction time for the polydimethylsiloxane and the diisocyanate is 30 minutes. The chain extender is preferably reacted with the polydimethylsiloxane and the diisocyanate at a temperature of 60-80°C for 20-40 minutes, and more preferably for 30 minutes.

The present invention is illustrated by way of the following example. It is to be understood that the invention is not to be limited to the specific conditions or details set forth in this example except as such limitations are specified in the appended claims.

Example 1 Polydimethylsiloxane (a, (ß-amine capped, 25.0 g Goldschmidt, MW = 11500) Polyethylene glycol (Dow Chemical, MW = 8000) 12.5 g Isophorone diisocyanate (Olin Chemical) 18.13 g 1,4-butanediol 6.66 g Dibutyltin dilaurate 0.2 g Toluene 180 ml 12.5 g of polyethylene glycol was dissolve in 50 ml toluene in a 500 ml flask at 60°C. Next the polydimethylsiloxane was mixed with 50 ml toluene and the mixture was added into the polyethylene glycol solution. 0.2 g dibutyl tin dilaurate, 18.13 g isophorone diisocyanate and 50 ml toluene were mixed.

This mixture was then added into the flask containing the polydimethylsiloxane solution. The temperature was maintained at 60-80°C for 30 minutes and the solutions allowed to react. Then 6.66 g of 1,4- butanediol was mixed with the remaining 30 ml of toluene and the solution was added to the flask. The temperature was maintained at 60-80°C for another 30 minutes.

The siloxane block copolymer formed in the flask was diluted into a 10% solution with tetrahydrofuran and coated on thermal transfer paper (Consolidated Paper) with a #20 Meyer rod. The siloxane block copolymer release coating may be used with a pressure-sensitive adhesive, e. g., Swift 84425 hot-melt permanent, and is printable with a thermal transfer printer (Zebra 140) and a ribbon (Coding Products TTR 7981).

It will be apparent to those skilled in the art that various modifications and variations can be made in the composition and method of the invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.