Process of Producing an Oriented Silicone Resin Coated Polymeric Film

This invention concerns the production of polymeric films and in particular oriented polymeric release films.

Release papers are well known as temporary supports for tacky items such as labels and adhesive tapes prior to removal of the items from the temporary supports and fixing them in a desired final position. Such release papers have a paper or card base with a release layer of a material to which the items adhere lightly by their adhesive coated surfaces. The coating holds the items to the paper or card sufficiently securely for storage and transport but permits their ready release without significantly affecting the tackiness of the tacky adhesive surface. Commonly used materials for forming such release layers are silicone compounds.

Release papers are particularly useful in label dispensing machines in which pre-printed labels having a tacky surface are fixed temporarily to the release surface of a continuous reel of release paper. To dispense the labels, the release paper is delivered to a dispensing station where it is passed around a sharp angle to facilitate removal of the label from the paper by an edge tool, the label then being transferred to the article to which it is to be permanently fixed, for example a can of a foodstuff.

It has been proposed to use synthetic polymeric materials, for example polyesters or polypropylene, as a base support for the release layer in place of the paper or board. Synthetic polymeric films have the advantage over cellulosic materials in being more moisture resistant than paper or card, and therefore less likely to undergo undesirable dimensional changes. Polymeric films can usually also be guided around much sharper angles than can paper or card without the risk of tearing, and removal and transfer of the labels can therefore be accomplished more efficiently.

Synthetic polymeric films, for example of polyesters or polypropylene, are often stretched at a temperature between the second order transition temperature and about 5.degree. C. below the first order transition temperature of the polymer at least partially to orient the molecular structure of the film and provide increased tensile strength and stiffness. The stretching can be uniaxial but often it is effected biaxially in directions at right angles to each other. Biaxial stretching is frequently effected sequentially, and it has been proposed to apply various coating compositions to the film between the stretching operations. Such methods have the economic advantage of providing coated films without the necessity for a separate off-line coating operation and associated heated dryer since the second stretching is effected at elevated temperatures.

Japanese Patent Application Publication 58/171916 (Tokyo Cellophane Paper Company Limited) describes the manufacture of a release film by stretching a polypropylene film in the longitudinal direction, coating one surface of the film with a thermosetting silicone resin, stretching the film in the transverse direction at an elevated temperature to cause curing of the resin, and thereafter heat setting and/or heat treating the film while the surface area of the film is permitted to contract by 15 to 30 percent and whitening of the surface of the film occurs.

In practice, the anchorage of cured silicone resins to synthetic polymeric films which are coated and stretched has been found to be unsatisfactory. If curing of the resin occurs before stretching is complete, differential movement between the base film and the cured resin causes the resin layer to crack and break up. If curing is delayed too long, it may not occur properly and the coating can then be rubbed off during subsequent processing, thus giving rise to an unacceptable release product.

According to the present invention there is provided a process for the production of an oriented polymeric film which process comprises forming a layer of an ambient temperature curable silicone resin composition on a surface of a polymeric film and thereafter stretching the film at an elevated temperature to effect orientation of the film, the silicone resin composition including a volatile inhibitor for inhibiting curing of the composition before stretching is effected.

The volatile inhibitor serves to inhibit curing of the composition before the film is heated in a stretching zone. The surface area of the film is initially relatively small compared with its area after stretching, and the loss of inhibitor by evaporation is relatively slow. On stretching the film, it is rapidly thinned, and its surface area rapidly increases so expanding and thinning the layer of the composition so that rapid evaporation of the inhibitor takes place. When the inhibitor has been substantially removed by evaporation, curing of the composition can occur. Provided the amount of inhibitor in the composition has been correctly selected, curing can be arranged to occur after the stretching operation, and preferably after winding up of the film, resulting in a coherent layer of solid silicone resin secured to the base polymeric film.

Preferred silicones for use in accordance with the invention include dialkyl (e.g. methyl) silicone fluids with hydroxyl functionality and alkyl (e.g. methyl) aryl (e.g. phenyl) silicone fluids with hydroxyl functionality. Curing of such silicones is preferably effected by condensation with a tetra-silicate ester, for example a tetra-alkyl silicate, especially tetra-ethyl silicate. The preferred inhibitors for use with silicones having hydroxyl functionality are alkanols, and more particularly C.sub.(1-4) alkanols, ethanol and more particularly propan-2-ol being particularly preferred.

In general, compositions used in accordance with the invention will include a catalyst for catalyzing the curing of the silicone. Organometallic curing catalysts will usually be used, tin-containing catalysts being preferred to catalyze the curing of silicones having hydroxyl functionality.

Stretching of the film can be effected at temperatures conventionally used to effect orientation of the synthetic polymer used to form the base film. It will, however, usually be from 100.degree. to 180.degree. C.

Polymeric films which can be treated in accordance with the present invention include polyester films and polypropylene films. These latter being particularly preferred by virtue of their flatness and stiffness following orientation, which properties make them especially suitable for producing release films.

When polypropylene films are subjected to a process in accordance with the present invention, it is preferred to effect stretching of the film with the curable composition thereon at a temperature of from 150.degree. to 170.degree. C.

Conventional stretch ratios can be used, for example from 8:1 to 10:1 for polypropylene films.

Following stretching of the film, it can be heat set by allowing the film to contract by up to 10, and preferably up to 5, percent of its fully stretched dimension in the direction of stretch while holding the film at an elevated temperature.

In a preferred embodiment of the invention, polypropylene film is stretched in the longitudinal direction over sets of rollers, the film is coated with the desired curable silicone resin composition containing a curing inhibitor, and it is then stretched in the transverse direction using a stenter. Heat setting is preferably effected in a heated zone within the stenter following the stretching zone. The heat curable composition can conveniently be applied to the lower surface of the polypropylene film immediately before the film enters the stenter using an off-set smooth roller coater.

It is especially preferred to effect stretching in the longitudinal, or machine, direction to relatively high ratios, typically at least 4.5:1, preferably 5.5 to 6.5:1, as the resultant films tend to have reduced extensibility. This is important with certain methods of dispensing labels. In general, films produced in accordance with the present invention will be at least 20 microns thick, preferably from 40 to 60 microns thick. A reduction in extensibility can be achieved by the use of relatively thick films in particular within this preferred thickness range, for example with a thickness of at least 45 microns.

The present invention can be used to produce voided or non-voided oriented synthetic polymeric release film. Voided film having a density below that of the polymer from which it is produced can be formed by the inclusion of inorganic particles, for example calcium carbonate, or polymeric particles into a melt of the polymer used to form the film prior to extrusion and subsequent stretching. U.S. Pat. No. 3,154,461 describes the formation of voids by this technique. Voids can also be formed by including in the film a particulate blowing agent which is activated under the conditions of extrusion of the film.

The preferred amount of particles used to effect voiding will depend on the size and density of the particles used. When calcium carbonate is used as a voiding agent, preferred amounts are from 4 to 8 percent by weight with a mean particle size of about 3 microns. As will be clear to those skilled in the art these figures can be adjusted accordingly for other particles.

Voided films are preferred for use in the present invention since dispensing of labels from such films treated in accordance with the invention is often better than from similarly treated unvoided films. Furthermore, the cured composition has been found to be particularly even when applied to voided films as the composition tends to be more evenly distributed over the film surface after the film has been wound into a roll and before curing of the silicone composition is complete.

Voided films generally have an opaque silvery appearance which forms a suitable background in release applications where the position of items such as labels attached to the surface of the release film can be detected by photoelectric means.

The synthetic polymeric film can be a single layer, for example of polypropylene hompolymer, or it can consist of two or more polymeric layers. Examples of other layers which can be used include polymers of propylene containing units derived from ethylene and/or but-1-ene. When present, such other layers can be used, for example, to improve anchorage of the cured silicone resin composition to the base film or they can be used to impart heat seal properties to the film on the uncoated face. When the film consists of two or more layers, they can be formed by coextrusion.

The surface of the polymeric film to which the curable silicone resin composition is applied can be pretreated with corona discharge or a flame to improve the anchorage of the cured composition to the film. Such treatments are preferably effected immediately before application of the curable composition.

After stretching, preferably in a stenter, the film will usually be wound into a roll, curing of the composition preferably being delayed by the inhibitor so that most of the curing occurs while the film is rolled up. Subsequently the roll can be unwound and the cured resin layer can have a layer of adhesive applied thereto, for example as is commonly used for self adhesive labels and tapes, e.g. a polyacrylate composition. Continuous label stock or a base tape film can then be applied to the adhesive to which they bind permanently. The combination of release film and temporarily adhered tacky items can then be wound into a roll.

A freshly coextruded film of polypropylene homopolymer containing 6 percent by weight of calcium carbonate having an average particle size of 3 microns and having outer layers on each side of a propylene/ethylene copolymer containing 4 percent by weight of copolymerised ethylene was quenched by a chill roll. The film was then heated and stretched longitudinally in the direction of extrusion (machine direction) by passing it over two sets of rollers, the second set having a higher peripheral speed than that of the first set, and the rollers being heated to bring the film to a temperature of 110.degree. C. The difference in peripheral speeds of the two sets of rollers was adjusted to give a stretch ratio of 4.5:1.

The uniaxially stretched film was reduced in temperature by passage through the air, and it was then coated on its under surface with an ambient temperature curable silicone resin composition using an off-set smooth roller coater. The composition consisted of:

The composition readily wetted the film and it formed a thin layer on its under surface.

The coated film was then stretched transversely at a ratio of 10:1 in a stenter at a temperature of 158.degree. C. The surface area of the film increased rapidly, and the propan-2-ol evaporated at a rapidly increasing rate until it had been totally expelled when curing of the composition started. The film was then heat set by relaxing its width by 7 percent before leaving the heated zone of the stenter. The edges of the film were trimmed, and the film was wound into a roll.

The concentration of propan-2-ol in the curable composition was adjusted to ensure that curing of the composition was only effected after the stretching and subsequently after the film had been wound into a roll.

The cured silicone resin layer on the film was smooth and substantially free from blemishes. Adhesive coated films and label stock adhered lightly to the coated surface and they could be readily removed without damage to the tacky coatings.