COPOLYMERS IN SILOXAN BLOCKS FOR COVERING OF PRINTED RELEASE
BACKGROUND OF THE INVENTION Technical Field The present invention relates to printable release coatings. More particularly, the invention relates to a release coating that includes a block copolymer of siloxane synthesized from an organic diisocyanate and certain polydimethylsiloxanes. Description of the Related Art Pressure sensitive adhesives in the form of labels, for example, are usually coated with a release liner on the back side of the label, (ie, on the opposite side of the adhesive). This release liner is largely responsible for making these adhesive materials that can be removed without the need for excessive force and without causing delamination of the backing or poor adhesion of the adhesive. To be useful as a release liner, a material generally must have low adhesion work with the adhesive. Useful release coating materials also have adequate strength and possess good adhesion to the backing of the adhesive material. Finally, useful release coating materials prevent inadvertent transfer of the adhesive to the back surface (ie, non-adhesive) of the adhesive material. The release coating makes it possible, for example, for pressure-sensitive labels that can be repositioned or removed. Silicones are well known as sticky materials for release coatings, e.g. as pressure sensitive adhesives for backing members. Silicones are composed mainly of alternating silicon and oxygen atoms in the polymer structure. Any 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 the one in which the methyl groups are bonded to the silicon atom. The physical nature of the polysiloxanes depends on the degree of polymerization and on the nature of the end groups in the polymer, which can vary in activity from chemical to relatively inert. A disadvantage of silicone coatings is that they tend to be expensive. In addition, the force to separate the pressure-sensitive adhesive from a silicone-treated surface is normally in the range of 10 to 40 grams per 2.54 cm. in width, too low a force 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. Therefore the silicone release characteristics can present problems due to the release properties that are too good. For example, among other disadvantages, this can result in a roll of tape that does not remain tightly wound. Also, the use of a primer layer between the silicone film and the substrate is often necessary to help secure the silicone film to the substrate. As a result, attempts have been made to modify the silicone polymers by mixing them or by reacting them with other, less effective release materials in order to obtain higher release strength values. For example, it is known that siloxane block copolymers can be obtained by combining very specific polydimethylsiloxanes with organic diisocyanates. The U.S. Patent No. 4,002,794 to Schwarcz discloses a release coating made of a copolymeric material which is the reaction product of a thio- or mercapto-terminated polyorganosiloxane and a difunctional diisocyanate organic compound. Another example of a siloxane polymer or copolymer that has been used as a release material is the fluorinated polyorganosiloxane. The U.S. Patent No. 4No. 810,749 to Pinchuk discloses copolymers with a polymeric structure having repeating isocyanate groups and repeating fluoroalkylsiloxane. However, the modified silicone polymers suffer from certain disadvantages. Some modified silicones contaminate a pressure sensitive adhesive so that it loses its tack. Other modified silicones react fully with a pressure sensitive adhesive, which makes it possible for them not to separate by aging. Some compositions are difficult to reproduce firmly, eg. the release properties change as the silicone gradually migrates to the surface. Other compositions require such high curing temperatures that the substrate on which they are coated can 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 printable release coatings than others. For example, the inventor of the present has discovered that a fluorinated polydimethylsiloxane would have a lower energy surface than a non-fluorinated polydimethylsiloxane and would thus be less printable. As a result, a minor amount of a fluorinated polydimethylsiloxane would be used in the construction of the block copolymer. This in turn would result in loss of flexibility in the final polymer, leading to a higher release force with a noisier release, is known in the art as a more "noisy" release. In addition, it is advantageous that it is capable of being printed on said labels, e.g. with a thermal transfer printer, where the resulting print is not removed by rubbing or becomes blurred. Due to the disadvantage mentioned above, fluorinated polydimethylsiloxanes would not be suitable for said applications. In contrast to the prior art, which required extreme 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 in this way avoids the attendant disadvantages of said end groups. The inventor has found, in fact, that the composition of the present invention works best on the polydimethylsiloxane polymer without the aforementioned end groups. The inventor of the present has found that a siloxane block copolymer formed by the reaction of certain polydimethylsiloxanes with an organic diisocyanate is surprisingly successful as a release coating for pressure sensitive labels. Description of the Invention In order to achieve these and other advantages, and in accordance with the purpose of the invention as are modalized and widely described herein, the present invention is directed to a printable release liner. The release coating includes a block copolymer of siloxane, this 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 combined reagents in step (c) and maintaining a sufficient temperature for a sufficient time to allow the chain extender to react. Instead of the mixing materials having 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 segments (urethane and urea) provide printability. The foregoing and other advantages and aspects of this invention will be made apparent by reviewing the following detailed description. BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a printable release coating that includes a block copolymer of siloxane which 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, diphenylmethylene diisocyanate, toluene diisocyanate, dicyclohexylmethyl diisocyanate and hexamethylene diisocyanate. The materials used for the synthesis of the siloxane block copolymer can also include chain extenders, catalysts and solvents. A chain extender extends the distance between the 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 the printing capacity. Second, the chain extender imparts flexibility to the block copolymer. This reduces the "noisy" release (noise) as well as a total release force. "Noise" is the sound of a buzz often made when tapes or labels are unwound from a roll. The chain extender material used in the present invention can be selected from diols, thiols, polyols and their sulfur equivalents. Nitrogen analogues, e.g. diamines, triamines, polyamines, can 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 that 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 dibutyltin dilaurate. The catalysts serve to raise the speed of the reaction and allow the reaction to take place at lower temperatures. The solvents are used in the present invention to make easier the mixing of the ingredients so that they react and help to preserve the final product of the solidification, that is, they keep their viscosity low enough to make them 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 to 80 parts by weight, preferably in an amount of about 50 to 80 parts by weight. The release coatings of the present invention can be used to make pressure sensitive labels. The release composition can be slightly altered to provide the desired release properties for the different adhesives and the ability to be printed with printers. Pressure sensitive adhesives, for example, can be either relocatable, removable or permanent. The 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 for 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 combined reagents 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 the following example. It will be understood that the invention is not limited to the specific conditions or details set forth in this example except that such limitations are specified in the appended claims. Example 1 Polydimethylsiloxane (alpha, omega-amine crowned, 25.0 g
Goldschmidt, MW = 11500) Polyethylene glycol (Dow Chemical, MW = 8000) 12.5 g Isopropyl diisocyanate (Olin Chemical) 18.13 g
1, 4-butanediol 6.66 g
Dibutyltin dilaurate 0.2 g. Toluene 180 ml. They were dissolved in a 500 ml flask. at 60 ° C, 12.5 g. of polyethylene glycol in 50 ml. of toluene Then the polydimethylsiloxane was mixed with 50 ml. of toluene and the mixture was added to the polyethylene glycol solution. 0.2 g was mixed. of dibutyltin dilaurate, 18.13 g. of isophorone diisocyanate and 50 ml. of toluene. This mixture was then added to the flask containing the polydimethylsiloxane solution. The temperature was maintained at 60-80 ° C for 30 minutes and the solutions were allowed to react. Then 6.66 g were mixed. of 1,4-butane diol with 30 ml. of toluene remaining 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 in a 10% solution with tetrahydrofuran and coated on thermal transfer paper (Consolidated Paper) with a Meyer # 20 bar. The release coating of siloxane block copolymer can be used with a pressure sensitive adhesive, e.g. Swift 84425 permanent fuse with heat, and it 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 in the composition and method of the invention may be made 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 arise within the scope of the appended claims and their equivalents.