MXPA96003963A - Polimeri films - Google Patents

Abstract

The present invention relates to a multi-layer polyolefin film, having a core layer of a propylene polymer and an outer polyolefin layer on the core layer, the outer layer of polyolefin includes an amount that reduces the coefficient of friction of a particulate, partially crosslinked polysiloxane. The non-crosslinked polysiloxane in the particulate polysiloxane is released when the particles are subjected to mechanical stress which causes the sliding of the films to increase, when they are placed under mechanical stress they grow

Description

POLIME ICAS FILMS This invention relates to polymer films having good sliding properties. Many proposals have been made to date to improve the sliding properties of polymeric films, and those of polyolefin films in particular. In the case of polypropylene-based films, for example, it has been proposed to incorporate migratory slip additives, such as fatty acid amides in the films, so that over time, the slip additive migrates to the surface of the film. film, thereby improving the sliding properties of the film. A problem encountered with sliding migration additives is that the sliding properties of the films containing them tend to increase with time, because more and more of the slip additive accumulates on the surface of the film. Such an increase in sliding properties can cause the film to perform poorly when used in high-speed packaging machines, because the films have become too slippery. In addition, the accumulation of sliding migration additive on the surface of the film tends to reduce the optical characteristics of the films, for example resulting in increased opacity and reduced brightness. REF: 22950 As an alternative to the use of sliding migration additives, it has been proposed to use silicone oil as a slip agent, the silicone oil is either combined in an outer layer or layers of the films, or impregnated on at least a surface of the films. The silicone oil is apparently essentially non-migratory, since the sliding properties of films that include it in a combined form remain substantially constant over time, that is, the silicone oil inside the film apparently remains there, and the properties of Sliding films are a function of the amount of silicone in or on the surface of the film when it is manufactured. However, it has been found that the silicone that is on a surface of a film is transferred to the opposite surface of the film when the film is rolled, whether or not the silicone present on that surface has been put there by combination or by coating. Thus, when silicone oil is used as a slip agent, it only needs to be impregnated on or combined with a surface of the film, since the transfer in the roll causes the silicone to become present on both surfaces of the film. the movie. Although the use of silicone oil as a slip agent for polyolefin films has advantages over the use of migratory slip additives such as fatty acid amides, it has disadvantages. A major disadvantage is that the treatments of the surfaces of the films to increase their surface energy, for example with the aim of increasing the printability or aptitude for the typographic printing of the films, have not been possible with the surfaces of films having silicone. combined in them or impregnated on them, since other properties of the films, for example the ability to be sealed by heat, can be adversely affected. However, it is possible to increase the printability of such films if the surface being treated, for example by a flame or corona discharge treatment, does not include combined silicone oil, and does not have a surface coating of silicone oil on the surface. same at the time of treatment. The silicone oil on the other surface of the film, whether it is there as a result of coating or combination, can then be transferred to the treated surface of the film when the film is in the roll, and at least some of the benefits of surface treatment remain, without the loss of properties that would have resulted if the silicone oil had been present at the time of treatment. Notwithstanding this ability to avoid the problems encountered when treating surfaces of films having a surface layer of silicone oil, by transferring the silicone oil to the surface after the treatment rather than before, there are problems with such films, notably in that the amount of silicone oil transferred from the surface of a film in which the silicone oil is often combined is insufficient to provide the surface to which the desired slip levels are transferred. Coating a surface of the film with silicone oil, followed by transfer to the treated surface on the roll can reduce this problem, but the amount of silicone oil that has to be deposited on the film for this to occur tends to be sufficient. high so that the effect of the transferred silicone oil on the printability of the surface of the treated film can be unpredictable. For example, the surface that has been treated to increase its surface energy may be more or less receptive to the printing inks, with the result that poor print quality is produced. According to the present invention, there is provided a multilayer polyolefin film, comprising a core layer of a propylene polymer, and an outer polyolefin layer on the core layer, the outer polyolefin layer includes an amount which reduces the coefficient of friction of a particulate, partially crosslinked, polysiloxane. The partially crosslinked polysiloxane is preferably a polydialkylsiloxane, and polydimethylsiloxanes are particularly preferred. It is particularly preferred to use 5 partially crosslinked polysiloxanes which are in the form of elastomeric particles including polysiloxane in the form of silicone oil. The films according to the present invention have shown good sliding properties for the surface including the crosslinked polydialkylsiloxane. In particular, the non-crosslinked polydialkylsiloxane in the partially crosslinked polydialkylsiloxane particles is apparently free to be released from the particles, when the particles are subjected to mechanical stress. The result is a film that has a degree of slip that can be increased when the film itself is subjected to increased mechanical stress. The films according to the present invention have also been subjected to a treatment to increase the surface energy of the outer layer containing the crosslinked polysiloxane, without this resulting in the adverse effects found with the films proposed up to now, having combined silicone oil on the treated surface, or a coating of silicone oil thereon. Therefore, it is possible in accordance with the present invention to have a core layer, with an outer layer of polyolefin on either side of the core layer, each outer layer includes an amount that reduces the coefficient of friction of a particulate polysiloxane , partially reticulated. The degree of crosslinking of the polysiloxane can be varied according to the properties required for the films, but excessively high degrees of crosslinking will result in an insufficient amount of free or uncrosslinked polysiloxane being available to impart the desired degree of desolvation. However, low degrees of crosslinking can result in an inability to form particles, and therefore in an inability to keep the free polysiloxane in the form of silicone oil in the particles, for example before mechanical stress releases it. The latter could result in the treatment of the layer that contains the partially cross-linked polysiloxane, to increase its surface energy, for example by treatment corona discharge or flame treatment, which have the undesirable effects associated to date with the treatment of surfaces that have a layer of silicone oil on them. It is generally preferred to use crosslinked silicones that include up to 60, or possibly 70% by weight of free silicone oil, the lower limit usually being determined by the ability to release silicone oil in sufficient quantities to affect the sliding properties Of the movies. As will also be appreciated, the viscosity of the free silicone oil will also have an effect on the sliding characteristics of the films. It is generally preferred that the polysiloxane particles have an average particle size of not more than 6 lim. However, it is generally preferred that the particles have an average particle size of at least 0.5 Jum, and preferably at least 2 jum. The use of partially crosslinked polysiloxane particles of such average particle size can also serve to provide films of the present invention with anti-blocking properties (which prevent blockage), thereby allowing slip and antiblock properties to be provided by a single additive. . The amount of partially crosslinked polysiloxane present in the outer layer can be varied according to the degree of slip desired to be imparted to the films. However, it is generally preferred that the outer layer includes at least 1000 ppm of the partially crosslinked polysiloxane, although amounts greater than 5000 ppm are not generally necessary to achieve the desired slip properties. Although the films according to the present invention may have only one outer layer including a particulate, partially crosslinked polysiloxane, it is generally preferred that both outer layers include such a polysiloxane. In addition to the crosslinked silicone, it is generally preferred to include at least one anti-blocking agent in the outer polymeric layer. The antiblocking agent can be organic, for example a polyamide or a polyester, or inorganic, for example silica, talcum, barium sulfate or zinc sulphide, silica being the preferred antiblocking agent. The amount of anti-blocking agent in the outer polymeric layer containing the cross-linked silicone is preferably at least 500 ppm, based on the weight of the layer, and preferably not more than 5000 ppm. The antiblocking agent preferably has an average particle size from 1 to 6 m. The gliding characteristics of the films of the present invention can, if desired, be modified by the inclusion of one or more migration additives in the core layer. The additives suitable for the purpose are known in the art, glyceryl monomers of fatty acids being preferred, for example glyceryl monostearate and glyceryl monobehenate. It is generally preferred to use at least 1000 ppm of a glyceryl mono-ester of a fatty acid, based on the weight of the core layer, but generally no greater than 6000 ppm are preferred since more such additives they usually do not contribute to providing an increase in sliding properties, while generally reducing the optical properties of the films, for example by increasing their opacity. The films of the present invention can include at least one antistatic agent, for example as known in the art for use in polypropylene films. Preferred antistatic agents for use in the films of the present invention are the bis-alkoxy-side derivatives of long-chain amines, with a particularly preferred antistatic agent being bis- (2-hydroxyethyl) -seal amine. As will be appreciated by those skilled in the art, it is generally preferred not to use a migration additive when the films of the present invention are going to be metallized on a different surface of the specified polymeric layer containing the crosslinked silicone. However, if such additives are used in the core layer of films to be metallized, it is preferred that they should not be present in an amount that significantly adversely affects the adhesion of the deposited metal layers under vacuum. The core layer of the films according to the present invention is of a propylene polymer, and preferably of a propylene homopolymer. The polyolefin forming the outer layer is preferably a copolymer containing units derived from two or more alkenes, for example from ethylene, propylene, or 1-butene, or a mixture of two or more such copolymers. The outer layer is preferably a heat-sealable polymer or polymer blend (with heat sealing capability). In addition to the core layer and the polyolefin outer layer containing the partially crosslinked particulate polysiloxane, with or without an additional outer layer of polyolefin containing such a polysiloxane, the films according to the present invention may include one or more intermediate layers. between the core layer and the outer layer (s). Any of all layers may, if desired, include additives and / or fillers known in the art for use in polyolefin films. However, as will be appreciated, it is generally preferred that the outer layer (s) containing the crosslinked polysiloxane should be free of combined silicone oil separately, and should not have a coating of silicone oil. silicone oil applied to it (s), except when it may result from the free polysiloxane in the crosslinked particles. The films according to the present invention can therefore be pigmented, include at least one layer in which micro-spaces are present, or include a layer of a material that imparts barrier properties to the films. At least one surface of the films according to the present invention is preferably treated for increment- ¬ li Its surface energy, for example by a flame or corona discharge treatment, may be used, and this may be the surface of an outer layer containing the partially cross-linked particulate polysiloxane. However, such surface treatments including the crosslinked silicone may have a detrimental effect on the sliding properties of the films of the present invention, but the presence of a slip migration additive in the core layer may serve to reduce such harmful effects. Additionally, the transfer of silicone from the other surface of the film to the treated surface can also serve to reduce this deleterious effect, if such silicone is present on the other surface of the film in such an amount that a significant transfer can take place. The films according to the present invention can be produced by known methods, for example by melt coextrusion of the appropriate polymers for the respective layers, plus the desired additives, to form a polymer network. It is particularly preferred to form the melts using masterbatches of the various additives, the crosslinked silicone is preferably added as a masterbatch of up to 7% by weight of the silicone, in a polymer to be used in the outer layer. The polymeric network can then be formed into films of the present invention using known techniques, for example by biaxial stretching of the network, either simultaneously, using the process with bubbles, or sequentially, using heated rollers in the direction of extrusion, or followed by transverse stretching, using a stenter oven. The following Example is given by way of illustration only. Example 1 A three layer polymeric film was produced by initially coextruding a polymer network consisting of a core layer of propylene homopolymer containing 1000 ppm of bis-ethoxylated amine with outer layers of a propylene / ethylene copolymer (6). % by weight of ethylene) containing 0.25% by weight of particulate, partially crosslinked polydimethylsiloxane (BY29-110 - ex Toray Silicones, with a particle size in the range of 2-5 a) on either of the two surfaces of the core layer. The coextruded network was cooled on a cooling roller, and then stretched 4.5 times in the extrusion direction, passing it on heated rollers with different peripheral speeds, and thereafter, 9 times in the transverse direction, using a stenter oven. The resulting film was cooled, one side was subjected to a corona discharge treatment, to increase its surface energy to 42 dynes, and the film was wound up.

The film had a general thickness of 30 μm, with each outer layer being 0.8 μm thick. The static and dynamic sliding properties of the film were then measured, the coefficient of dynamic friction (CFD) being measured using a Davenport slip test apparatus with a bar from 700 g to 80 cm / minute, the film was fixed to the base plate using double-sided tape. As can be seen from Table 1, the coefficient of dynamic friction is significantly less than the coefficient of static friction for the same type of surface-to-surface contact. The difference between the static and dynamic values of the coefficient of friction is such as to indicate that the free polydialkylsiloxane found in the crosslinked particles is being released from the particles, to give a positive contribution to the dynamic coefficient of measured friction. The surface treated in corona of the film was both, heat-sealable and capable of being printed. An analogous film that included combined silicone oil was treated with a corona discharge, and the treated surface showed very low print adhesion. Example 2 A three layer film was produced by the method described in Example 1except that the two outer layers each contained 0.25% by weight of the crosslinked polydimethylsiloxane plus 1000 ppm of a synthetic silica with an average particle size of 2 p. The film had an opacity value of 1.0, and the eigenvalues of its coefficient of friction are given in Table 1. Example 3 A three-layer film was produced by the method described in Example 1, except that the two layers The exteriors each contained 0.5% by weight of the re-ticked polydimethylsiloxane plus 1000 ppm of a synthetic silica with an average particle size of 2 jra. In addition, the core layer contained 0.11% by weight of glyceryl mono-stearate. The film had an opacity value of 0.9, and the measurements of its coefficient of friction are given in Table 1. Table 1 Friction Coefficient Treated / Treated Untreated / Not treated Static Example Dynamic Static Dynamic 1 0.44 0.25 0.56 0.24 2 0.32 0.27 0.35 0.24 3 0.43 0.31 0.39 0.27 It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following:

Claims (8)

1. CLAIMS 1. A multi-layer polyolefin film, characterized in that it comprises a core layer of a propylene polymer and an outer layer of polyolefin on the core layer, the outer layer of polyolefin includes an amount that reduces the coefficient of friction of a particulate, partially crosslinked polysiloxane.
2. 2. A film according to claim 1, characterized in that the partially cross-linked polysiloxane is a polydialkylsiloxane.
3. 3. The film according to claim 2, characterized in that the partially crosslinked polydialkylsiloxane is a polydimethylsiloxane.
4. 4. The film according to any of the preceding claims, characterized in that the poly-dialkylsiloxane has an average particle size of not more than 6 μm.
5. 5. The film according to any of the preceding claims, characterized in that the poly-dialkylsiloxane is present in an amount of up to 0.5% by weight of the outer layer.
6. 6. The film according to any of the preceding claims, characterized in that the polydialkylsiloxane particles include up to 60% by weight of the polydialkylsiloxane that is not crosslinked.
7. 7. The film according to any of the preceding claims, characterized in that an outer layer of polyolefin, including an amount that reduces the coefficient of friction of a partially crosslinked polydialkylsiloxane, is present on both sides of the core layer.
8. 8. The film according to any of the preceding claims, de-iced because at least one surface thereof has been treated to increase its surface energy.