Background of the Invention The present invention concerns silicone coated release films which are particularly useful as a temporary support substrate for an adhesive coated article or an interiayer in thermoset pressing operations. Examples of such support substrates are adhesive labels, adhesive shelf paper, removable lidding for microwavable food containers, and peel-off labeling stickers. Thermoset pressing operations require a release film to separate the product from the mold and thermoset layers from each other. In particular, the present invention relates to a temporary substrate made of biaxially oriented PENBB copolyester film which has been in-line coated with an aqueous silicone resin system. More particularly, the present invention deals with coa- ting the copolyester film with a blend of an aqueous silicone resin system and glycidoxy silane. Moreover, the present invention provides a release film with improved modulus and heat resistance, which allow longer re-use and/or higher temperatures in industrial applications such as thermoset pressing.

Silicone coated release substrates are known, particularly in applica- tions where the substrate is paper. It is important that the silicone coated release substrate permit an adhesive backed article or thermoset article to minimally adhere to it, thereby serving as a temporary support or covering. On the other hand, the release substrate must also release the adhesive backed or thermoset article with a minimum of effort, and yet not transfer silicone coating to the adhesive (called migration), thereby preventing it from adhering to the final substrate. Also in the case of thermoset release ap¬ plications, the silicone should not transfer, as this prevents re-use of the release film. Silicone coated release substrates are most frequently produced by coating the substrate with a solvent or solventless based silicone resin composition. As the silicone resin composition dries, it adheres to the sub¬ strate.

It is known to both off-line and in-line coat polyethylene terephthalate (PET) film. Off-line coating occurs when the film is coated after the heat- setting stage and is usually not performed by the film manufacturer. In-line coating is done during the film manufacturing stage, typically before or after one stretching operation, and before heat-setting the film.

U.S. Patent 4,851 ,166 to Kendall discloses a polymeric film (polypro¬ pylene film) which has been in-line coated with a heat curable, non-aqueous silicone resin composition. This reference also discloses that the polymeric film may be PET film and that the film is coated prior to the stretch orienta- tion stage.

Japanese patent 58/171916 to Kori also teaches a polypropylene film which is in-line coated, prior to orientation of the film, with a heat curable non-aqueous silicone resin composition.

Canadian patent 1 ,120,176 to Hockemeyer discloses an aqueous based silicone resin system which can be coated upon plastic foil substrates for release film applications (referred to in this patent as adhesive repellant coatings). This patent discloses an adhesive repellant coating composition which is applied to the surface of a substrate as an aqueous emulsion consi¬ sting of: 1 ) diorganopolysiloxanes having silicon bonded vinyl groups in their terminal units; 2) from 20 to 50% by weight of organopolysiloxanes having at least three silicon bonded hydrogen atoms per molecule based upon the weight of the diprganopolysiloxane; and 3) a catalytic amount of catalyst (such as platinum) which promotes the addition of silicon bonded hydrogen to vinyl groups. Preparation of the aqueous based silicone resin system is adequately explained in the Canadian reference and this reference is incorpo¬ rated herein by specific reference.

While the aqueous based silicone resin composition disclosed in the Canadian patent does provide some desirable properties in silicone release applications, it does not provide adequate protection against rub-off and migration, and provide good release results on polymeric film. Protection against migration and obtaining good release were previously discussed. Adequate protection against rub-off is desired when further processing or re-

use of the film is contemplated, particularly where the film will be in contact with rollers and winders.

U.S. Patent 3,427,270 to Northrup discloses a non-aqueous (i.e., solvent based) silicone resin composition useful as a release agent and adhesion promoting silanes. The non-aqueous silicone resin composition states that two silane ingredients are necessary for adequate adhesion of the silicone coating on a cellulosic substrate (the only substrate mentioned). The two components consist of an epoxysilane and an alkylsilane which is mono- valent hydrocarbon radical having no more than three carbon atoms. As disclosed by this reference, the epoxysilanes by themselves do not give reproducible results when employed as adhesion promoters for silicone release agents. To overcome this problem, a second component of alkyl silane is needed. Presumably, rub-off resistance and adhesion are synony¬ mous. U.S. Patent No. 3,008,934 discloses copolyesters containing as acid derived units 4,4'-bibenzoate and a host of other dicarboxylates including 2,6-naphthalic dicarboxylate. It also discloses oriented fibers and films prepa¬ red from these copolyesters, however, biaxially oriented PENBB films are not disclosed or envisioned. In particular, those films with improved stiffness (tensile modulus) and tensile strength in both MD and TD as well as thermo- stability, UV stability, hydrophobicity, dimensional stability and impermeability toward gases in comparison to PET film are not disclosed in U.S. Patent No. 3,008,934.

There continues to be a need for silicone coated release films with improved physical properties of the substrate film, e.g. improved modulus

(stiffness) and tensile strength, excellent heat stability, lower moisture ab- sorbtion etc., whereby the silicone release coating can be applied from an aqueous system and has excellent adhesion to the substrate film, providing excellent rub-off resistance, protection against migration, and good release such that further handling of the film does not disrupt the silicone coated release composition.

Summary of the Invention

It has now been found that a primer coated copolyester film, wherein the copolyester is PENBB provides excellent heat stability, lower moisture absorption, improved tensile strength and stiffness (tensile modulus), and wherein the primer coating contains an aqueous based emulsion coating composition which greatly improves rub-off and migration resistance and also is excellent in silicone release applications.

In the broadest sense, the present invention relates to a silicone release film including a biaxially oriented PENBB film having a coating thereon in an amount sufficient to act as a silicone coated release film, said coating com¬ prising glycidoxysilane intimately and uniformly mixed with an aqueous based silicone release composition.

In the broadest sense of the present invention, a process is also con- templated for coating a film comprising the steps of extruding an amorphous melt of PENBB onto a chilled casting drum; biaxially orienting the film; coating said film before or after said biaxial orientation with a coating sufficient to perform as a release coating; and heat setting said coated film, wherein said coating consists of a uniform mixture of a glycidoxysilane and an aqueous based silicone release composition.

Description of the Preferred Embodiments of the Invention

The copolyester film of the present invention is a copolyester contai¬ ning as acid-derived unit at least 5 mole % of a radical of the formula

(bibenzoate, BB)

In the case that more than 10 mole-% of terephthalic acid derived radicals are present in the copolymer, the content of bibenzoate derived units is at least 25 mole-%. Films of these copolyesters are mentioned in the unpublished German Patent Application P 4224161.8, which is incorporated

herein by reference. Preferably PENBB is a copolyester wherein at least 80 mole-% of the acid derived unts (NBB) consist of bibenzoate (20 to 80 mole- %, preferably 40 to 60 mole-%) and naphthalate (80 to 20 mole-%, prefera¬ bly 60 to 40 mole-%). The remaining 20 or less mole-% may consist of other acid derived units, which e.g. affect the melting point or the crystallization kinetics. Preferably at least 80 mole-% of the diol-derived units consist of - O-(CH ₂ ) ₂ -O- units. The remaining 20 or less mole-% consist of other diol- derived units, which e.g. may also affect the melting point or the crystalliza¬ tion kinetics. It may also be desirable to replace minor amounts of the acid - and/or diol-derived units with hydroxycarboxylic-acid-derived units, e.g. such derived from p-hydroxybe'nzoic acid.

In order to achieve the desired mechanical properties in the biaxially oriented PENBB film it is recommended that the IV value (inherent viscosity, as measured in a 1 : 1 weight-ratio mixture of pentafluorophenol and hexafiuoroisopropanol at a concentration of 0.2 g/dl and a temperature of 25 ^β C) of the PENBB polymer after extrusion be > 0.5 dl/g and preferably > 0.55 dl/g.

To produce the copolyester film, the polymer melt is extruded through a slot die onto a chill roll where it solidifies, is then biaxially oriented, heat set, optionally post treated and wound on a roll. The solidified film as ex¬ truded on the chill roll should be obtained in an essentially amorphous state. To achieve this, the melt film must be pinned to the chill roll by a known method such as electrostatic pinning or vacuum, air knife or the like.

The biaxial orientation of the film is achieved by stretching the film at elevated temperature in the machine (MD) and transverse direction (TD). This stretching can be either simultaneous or sequential. In the case of sequential stretching the first stretching step can be in either MD or TD, followed by stretching in the other direction. The orientation in MD can also be achieved in several steps, either one after another prior to stretching in TD, or before and after the TD stretching. Preferred temperatures for stretching lie bet¬ ween the glass transition temperature (T _g ) and about 30 ^β C above the cold crystallization temperature (T _cc ) of the PENBB copolymer composition in use

(both temperatures can easily be measured on amorphous films by DSC). The total stretch ratios (Λ) in MD and TD lie between 1 : 2 and 1 : 10, prefe¬ rably between 1 : 2.5 and 1 : 5. The product of the total stretch ratios should be between 1 to 30 preferably between 5 to 20. Biaxial drawing is performed such that the birefringeance is < 0.2, preferably < 0.1 to ensure adequately isotropic properties. Birefringeance as mentioned herein is the absolute value of the difference between the maximum and minimum refracti¬ ve indices in the plane of the film, as measured on common instruments such as Abbe refractometer, optical bench or compensators. In order to optimize properties, relaxation steps can be included in the orientation and heat setting processes.

The heat setting takes place at a temperature between the cold cry¬ stallization temperature and the melt temperature of the PENBB copolymer composition. In some cases, a surface treatment such as a corona, plasma or flame treatment should be employed before coating the PENBB film.

For biaxially oriented film, the surface treatment, such as corona, plasma or flame treatment, followed by the coating may occur during the in¬ line manufacturing process either before stretch orientation, or between the machine draw and the transverse draw of biaxial stretch orientation, or after stretch orientation. Again, if the surface treatment and coating step occur after stretch orientation is complete, it is preferred that the film be completely dry before winding. Moreover, the biaxially oriented PENBB film must be heat set to lock in the physical properties and this is generally sufficient to dry the film before winding. If the coating and optional previous surface treatment occurs before orientation, or between draws during orientation, the latter orientation steps are generally sufficient to drive off the water from the coating. Preferably, the optional surface treatment and subsequent coating occur between draws during the stretch orientation stage. The copolyester sheet of the present invention is coated on the optio¬ nally e.g. electric corona discharge treated surface with the coating whose composition will be described below. The coating composition may conven-

iently be applied as an aqueous emulsion using any of the well known coating techniques. For example, the film may be coated by roller coating, spray coating, gravure coating, reverse gravure coating, or slot coating. The heat applied to the film during the subsequent preheating, stretching and heat- setting stages is generally sufficient to evaporate the water, cure and bind the coating to the biaxially oriented PENBB film.

The biaxially oriented PENBB film, is generally heat-set at a temperature ranging from 190 ^* C to 270 "C, preferably from 230 ^* C to 265 "C. The coated biaxially oriented copolyester film is then wound into a roll for further processing or shipping.

The coating is generally prepared by hydrolyzing the glycidoxy silane in deionized water and blending with an aqueous silicone resin emulsion and its corresponding crosslinker. Generally, the aqueous silicone resin compositions are platinum catalyzed. However, condensation type siloxanes may be employed and the emulsion may be catalyzed with a tin catalyst. The cross¬ linker employed is that recommended by the particular silicone resin composi¬ tion manufacturer for the specific aqueous silicone resin composition.

Some suitable aqueous based silicone resin compositions are:

1 ) Wacker Silicones' aqueous based 400E silicone resin composi- tion with the V20 crosslinking system, (Adrian, Michigan);

2) Dow Coming's X2-7720 aqueous silicone resin composition with the X2-7721 crosslinking system, (Midland, Michigan);

3) PCL's PC-105 aqueous based silicone resin composition with the catalyst component of PC-95, (Rhone-Poulenc Inc., Rock Hill, S.C.); 4) PCL's PC-107 aqueous based silicone resin composition with the above-identified PC-95 crosslinker; and

5) PCL's PC-188 aqueous based silicone resin composition with the above-identified PC-95 crosslinker.

The amount of deionized water blended with the aqueous silicone resin composition is dependent upon the coating method and desired amount of solids, by weight, to be coated on the biaxially oriented PENBB film.

The glycidoxy silane may be a glycidoxypropyltrimethoxysilane or generally any glycidoxysilane represented by the formula

1

R

X— Y — S

wherein X is a glycidoxy group,

Y is an alkylene group, such as methylene, ethylene, propylene, etc., R ¹ , R ² and R ³ are hydrolyzable groups, such as methoxy, ethoxy, acetoxy, and the like. These silanes possess water solubility or water dispersibility. The solids level of the coating may be from about 3% to about 30% by weight solids. Preferably, the solids content by weight is from about 5% to about 12%. While it may be possible that a solids concentration below 3% by weight for the coating may be effective, it is believed that such a level would be minimally effective. Additionally, while a solids level greater than 30% by weight may be effective, it is believed that at such a level, a haze may result in the film, or the coating is more expensive but no more effective than a film having a solids level in the preferred range, for example.

The coating consists of the aqueous thermosetting silicone resin composition, including any necessary crosslinkers, etc., and the glycidoxy silane. The minimum amount of glycidoxy silane believed to be effective for an aqueous silicone resin composition in the present invention is about 0.3% by weight. It is believed that a glycidoxy silane level of about 5% by weight of the total weight coating would be an effective upper limit. The preferred level of the glycidoxy silane is about 1 % by weight. The thickness of the coating is from about 25 nm to about 125 nm.

Generally a thickness less than the above amount is not effective as a release coating, while a thickness more than the above amount is not cost effective.

General Experimental Conditions

In the following Examples a -g'ycidoxypropyltrimethoxy- silane is em¬ ployed where indicated in the coating composition. The structure of this glycidoxysilane, which is designated Z-6040, is set forth below.

H 2C \ / CH-CH 2— O- CH 2-CH 2- CH 2-S COCa ^), o

The following tests were conducted on the film as indicated.

1 ) Smear - measured by rubbing the index finger, once across the silicone coated surface and observing for haze; lack of haze indicates sufficient curing of the silicone coating. Smear resulting from tests conducted beyond 7 days after coating may be indicating blooming (to the surface) of other compo¬ nents, rather than insufficient curing.

The subjective rating of the smear test is: 1 = no haze change;

2 = very slight haze change;

3 = slight haze change;

4 = distinct haze change;

5 = very distinct haze change; and 6 = severe haze change.

2) Rub-off - measured by rubbing the index finger back and forth across the silicone coated surface and testing for differential release by applying a piece of 3M 610 tape on both the rubbed and unrubbed areas and peeling the tape from the film. The rub-off test indicates the adhesion of the silicone coating to the biaxially oriented PENBB film.

The subjective rating on the rub-off test with respect to the difference in release employing the 610 tape from the film where rubbed versus the film not rubbed is:

1 = no change in release; 2 = very slight change in release;

3 = slight decrease in release;

4 = distinct decrease in release;

5 = very distinct decrease in release; and

6 = very poor release, indicating the silicone coating rubbed off and the tape was not easily released from the biaxially oriented

PENBB film.

3) Re-adhesion test - Tesa 4154 tape is pressed against the silicone coated surface of the film and then pulled from the surface. Then the adhesi¬ ve surface of the Tesa 41 54 tape is placed against a clean stainless steel surface. The adhesive tape is pulled at a 180 ^° angle from the steel surface and the force required to move it is measured. "Virgin" tape is used as the control. The film passes the test if its re-adhesion force is 70% or more of the adhesion force the control tape. In these experiments, the virgin tape has a peel value of about 1 .4 N/cm.

4) Migration - is conducted by pressing 3M's 610 adhesive tape against the silicone coated surface. The tape is then removed and the adhesive side of the tape pressed against itself by doubling the tape up. The tape is then pulled apart from itself. If the tape has lost its "adhesiveness", it fails this test. The test is rated subjectively against virgin 610 tape. Failing this test indicates that the silicone release coating has transferred from the coated film to the adhesive tape.

The mechanical properties are measured in a tensile testing machine made by Zwick (Ulm, Germany) on 15 mm wide strips of film. The initial distance between the chucks is 100 mm and the crosshead speed is 100 mm/min for strength and elongation determination and 10 mm/min for the tensile modulus determination.

The UV resistance is tested by measuring the retention of tensile elongation after exposure to UV light in a "Suntest" apparatus manufactured by Heraeus (Hanau, Germany) for 14 days.

EXAMPLE 1 A PENBB copolyester is made of 289 parts by weight of dimethyl 2,6- naphthalene dicarboxylate, 322 parts by weight of dimethyl 4,4'-bibenzoate and 368 parts by weight of ethylene glycol. Pellets of this polymer, having a melting point of 281 °C are melted in a single screw extruder at temperatu¬ res of 280° to 320 ^" C and extruded through a sheet die onto a cooling roll temperature controlled at 20 ^* C. A 120 μm thick film is obtained which is clear and transparent. Its density is 1.31 g/cm ³ . This pre-film is then se¬ quentially biaxially oriented. The longitudinally drawn film is corona treated by a corona discharge apparatus and thereinafter coated with a latex des¬ cribed below by reverse gravure coating. The corona treated longitudinally drawn, coated film is dried at a temperature of about 100"C. Thereafter the film is stretched in the transverse direction. The biaxially drawn film is heat set at a temperature of 250 °C. These heat treatments result in the cross- linking of the primer coating.

The film produced according to the above example is in-line coated during the manufacturing process of the film with a thermosetting aqueous based silicone emulsion supplied by Wacker Silicones having the following formulation: 77 parts deionized water; 20 parts silicone emulsion 400E; 3 parts crosslinker V72 and 1.5 part of the glycidoxy silane Z-6040. The silicone emulsion 400E is a methylpolysiloxane having vinyl groups. It con¬ tains a platinum catalyst and an inhibitor to prevent premature reaction when crosslinker is added. The crosslinker is a methylhydrogenpolysiloxane emul¬ sion which reacts with the double bond in the vinyl group in the methyl- polysiloxane. The smear and rub-off test results are measured after 8 and 30 days and show good results.

Further characteristics are given in Table 1

TABLE 1

As is evident from the above table, tensile strength, elongation at break, stiffness (tensile modulus) and shrinkage are improved as compared to similarly coated biaxially oriented polyethylene terephthalate (PET) films.

EXAMPLE 2 In Example 2, different aqueous thermosetting resin compositions are compared, the amount of glycidoxysilane in each sample being the same, i.e., 1.5% by weight based on the total coating emulsion. The base PENBB film is prepared as described in Example 1.

In this Example, biaxially oriented PENBB films are coated with Wacker Silicone's aqueous thermosetting silicone resin composition 400E with cross¬ linker V20, Dow Coming's X2-7720 with crosslinker X2-7721 , PCL's PC-105 with crosslinker PC-95, PCL's PC-107 with crosslinker PC-95; and PCL's PC-

188 with crosslinker PC-95. Each of the sheets were corona treated prior to coating at the same power level of 27 Watt/m ² min.

A variety of aqueous silicone thermosetting resin compositions are within the scope of the present invention. It is noted that best results for these various aqueous thermosetting silicone resin compositions occur when the coating composition is applied in-line.

The physical characteristics of these films are the same as the one in example 1 (c.f. table 1).

Thus it is apparent that there has been provided, in accordance with the invention, a biaxially oriented PENBB film coated with an aqueous silicone resin composition and a glycidoxy silane, and a process for coating such a film that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodi¬ ments thereof, it is evident that many alternatives, modifications, and varia¬ tions will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the invention.