MXPA98002075A - Polimeri films - Google Patents

Abstract

Biaxially oriented polyolefin films having a thickness of at least 50æm and consisting of a layer of a polymer resin based on polypropylene having microvoids thereon, the microvoids having been formed by stretching a band containing the beta form of polypropylene at a stretch ratio of at least 15: 1, these films have shown good performance characteristics during the conversion of the film, for example, during printing

Description

POLYMERIC FILMS DESCRIPTIVE MEMORY This invention relates to biaxially oriented polyolefin films, and in particular dense biaxially oriented polypropylene films. Polypropylene films have found wide acceptance for many end uses such as packaging and labeling due to their convenient physical properties, especially after biaxial orientation. In fact, it is important that these films not only have the desired properties for their final use, but have characteristics that allow them to be processed satisfactorily in packaging and labeling machines. More particularly, it is important that the thickness of these films be within narrow tolerance limits to improve their conversion efficiency, for example, by printing them and using them in packaging. The variation in gauge is potentially a problem with biaxially oriented polypropylene films in general, and tends to be a particular problem with relatively dense films having a central layer of a propylene homopolymer, for example, those containing microvoids formed as a result to use a different phase gap formation. Said films are used especially to form labels and, in general, have thicknesses of at least 50μm. In accordance with the present invention, a biaxially oriented polyolefin film having a thickness of at least 50μm is provided which comprises a layer of a polypropylene-based resin having microvoids thereon, the microvoids having been formed by stretching a band. which contains the beta form of polypropylene at an area stretch ratio of at least 15: 1. The films according to the present invention have shown a particularly low thickness variation compared to similar films in which the formation of voids has been produced using chalk. The beta form of polypropylene is relatively unstable as compared to the corresponding alpha form under the conditions normally used to produce polypropylene films. In this way, when the molten polypropylene is extruded and then cooled to form a polymeric film which can be subsequently stretched, the alpha form of the polypropylene tends to predominate. However, it is known how to produce films using polypropylene containing high concentrations of the beta form of polypropylene, by mixing polypropylene containing a high proportion of the alpha form with a suitable nucleating agent which induces the formation of high concentrations of the beta form when melts and cools subsequently. An example of such processing is described in EU4386129, in which a variety of so-called beta nucleators are dispersed in polypropylene, after which films are produced therefrom by melting and subsequent cooling, controlling the crystallinity of the resulting cast films by properly adjusting the cooling conditions. The selective extraction of the beta form of the polypropylene from the films leaving a matrix of the alpha form is then used to impart porosity to the films. Document EU5231126 describes the use of two component mixtures of beta nucleating agents to produce microporous films by mono- or biaxial stretching of cast polypropylene bands containing a high concentration of the beta form of polypropylene resulting from the use of the mixture of nucleating agents . It is believed that the porosity results from voids induced by the change of the beta form to the alpha form during the stretching process, the alpha form having a higher density than the beta form from which it is derived. The development of porosity during the stretching process is accompanied by a significant reduction in apparent film density, and the films become opaque with a high degree of whiteness. More recently, it has been proposed in EP0632095 to use a variety of organic amides as beta nucleating agents in the formation of mono- and biaxially stretched polypropylene films, by casting a molten bath of a polypropylene mixture and the nucleating agent to form a film web which is allowed to crystallize at a temperature of 15 to 140 ° C to form a solid band containing the beta form of the polypropylene, after which the band is mono- or biaxially stretched at a temperature of more than 20 ° C, but lower than the melting point of the crystals of the beta form in the band. It is said that the resulting stretched films have a high whiteness and hiding ability in combination with printing and writing capabilities. The formation of microvoids during the plastic deformation of the polypropylene beta form is also described in the POLYMER journal (Vol. 35, No. 16, pp. 3442-5, 1995; and Vol. 36, No. 13, pp. 2523-30, 1995). It is said that porosity increases with higher crystallization and lower stretching temperatures, and all samples containing the beta form apparently become opaque when stretched at temperatures below 120 to 130 ° C. The polypropylene-based resin is preferably formed from a propylene homopolymer or a block or random copolymer containing a large proportion of propylene-derived units and having a crystallinity of at least 40%. The nucleating agent used to induce the formation of the beta form of the polypropylene of the base layer can be selected from those proposed hitherto for that purpose. However, particularly good results have been achieved using amides such as those proposed in EP0632095, and very particularly N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide. The amount of nucleating agent used to induce the formation of the beta form of the polypropylene can vary, for example, from 0.0001 to 5% by weight of the nucleating agent based on the weight of the polypropylene, as described in EP0632095, with amounts of 0.001 being preferred. 1% by weight Films in accordance with the present invention may consist of a single layer of a polypropylene-based resin containing microvoids, but will often include one or more additional layers, for example, to impart particular properties such as brightness, printability and heat sealing. The films may include, for example, an outer layer and / or intermediate layers conferring said properties. Said additional layers are preferably formed from polyolefins, examples of polymers which may be used for the purpose include polymers containing units derived from one or more of ethylene, propylene, butene-1 and higher alpha aliphatic olefins, and mixtures of said polymers. Additional layers can also be formed from modified polyolefins, for example, polyolefins extended with unsaturated acids and their derivatives, for example, ionomers and anhydrides. Other additional layers that can be used can be formed from acrylic polymers.

Although only one surface of the recessed polypropylene layer may have an additional layer thereon, the other surface of the recessed layer of polypropylene-based resin may also have at least one additional layer thereon, eg, a layer heat sealable which may be the same or different from the optional layer on the other surface of the films, the preferred materials for this layer being selected from polymers referred to above. Films in accordance with the present invention can include one or more additives used in the polyolefin art, for example, slip agents, antistatic agents, antiblocking agents, stabilizers, UV light absorbers or pigments, said additives preferably being present in quantities that do not significantly adversely affect the ability of polypropylene to crystallize in its beta form. When additives are present, they can be added to one or more of the layers that make up the film. The films according to the present invention have thicknesses of at least 50μm, and can be considerably more dense, for example, 80μm or more. Films in accordance with the present invention can be produced using known methods, for example, by extrusion or coextrusion through a slot die of molten baths of the appropriate polymers of the layer or layers that may be desired, to form a web. polymer that is cooled and then biaxially stretched sequentially at an area stretch ratio of at least 15: 1. However, the outer layer or layers can be applied to the recessed resin based on polypropylene, then coating the recessed layer that has been formed. As will be appreciated from the prior art referred to above, it is generally desirable to carry out certain processing steps, particularly the initial crystallization of the polypropylene from a molten bath containing the nucleating agent of the beta form, under conditions that will cause the formation of the desired film structure. More particularly, the cooling must be carried out under conditions that promote the production of the beta form of the polypropylene, and so that the formation of voids may occur during the subsequent stretching of the films. The cooling or crystallization temperature used to induce the formation of the beta form of the polypropylene in the base layer of the films of the present invention before being stretched, must be at least 20 ° C, but lower than the of fusion of the beta form of polypropylene. Although temperatures can be used at the lower end of this scale, for example up to 50 ° C, it is generally preferred to use temperatures of at least 70 ° C, and higher temperatures are still frequently preferred, for example 90 ° C or more . However, the cooling temperature is preferably not higher than 140 ° C, and from practical considerations, the temperature at which the film joins the surfaces used to cool it is preferably lower. The cooling of the molten bath can be effected in air at a suitable temperature, but it is generally preferred to effect cooling by contacting the extruded web with a cooling surface, for example a hardening roller. The subsequent biaxial stretching of the cooled strip is carried out sequentially, for example, under conditions known in the art of polypropylene films, the sequential stretching allowing the conditions used in the two directions to be independently selected from each other. Stretching in the direction of the extrusion (the machine direction) will usually be effected before stretching in the transverse direction. The area stretch ratio of the films of the present invention should be at least 15: 1, and will often be considerably larger, for example, at least 20: 1. Excessively large stretching ratios can result in difficulties in the production of the film, and the area stretching ratio will usually be no greater than about 70: 1. Particularly preferred area stretch ratios are generally in the range of 25: 1 to 50: 1. It has been found that the conditions used to perform the stretch in the machine direction have a substantial effect on the formation of microvoids, the lower stretching temperatures and the lower stretching ratios usually leading to a greater voiding. A preferred temperature scale for stretching in the machine direction is from 70 to 110 ° C, and more preferably from 80 to 90 ° C, so the draw ratio used in the machine direction will usually be at least 3: 1, being a preferred scale of 3.5: 1 to 8: 1. The subsequent stretching of the films in the transverse direction will generally be carried out at lower temperatures than are conventionally used for the transverse stretch of the polypropylene films, for example, from 100 to 160 ° C. However, it is generally preferred to perform the stretch in the transverse direction at temperatures of not more than 150 ° C, and preferably not more than 145 ° C to produce biaxially stretched films having low densities, for example, of less than 0.63 g. / cm3, and high opacity The stretch ratio used in the transverse direction is preferably from 3: 1 to 10: 1, although additional layers may be produced on the recessed layer of polymeric propylene resin by coextrusion of molten baths of the polymeric materials Appropriate biaxial stretching followed by appropriate application to the recessed layer of propylene polymer resin after the latter has been formed can be applied, for example, by off-line coating techniques, either of the surfaces, or both surfaces , of the films according to the present invention can be treated to increase their energies of respective surface, for example, using flame discharge or corona treatment. The following examples are given by way of illustration only.

EXAMPLES 1 AND 2 Two polymer bands were produced by extruding through a slot die a layer of propylene homopolymer containing 0.1% by weight of a nucleating agent of beta form (N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide; NJ- Star NU -100, ex New Japan Chemical Co., Ltd.) and, in each case, the band was cooled by contacting it with a temper roller having a surface temperature of 100 ° C. The DSC measurements indicated that the homopolymer layer contained a high level of the polypropylene beta form with Tf of 153 ° C.

The cooled strips were then drawn 3.5: 1 in the extrusion direction by contacting them with rollers heated to 80 ° C and having different peripheral speeds to produce monoaxially stretched opaque strips, DSC showing that virtually all the beta polypropylene had been converted to the form alpha. The monoaxially stretched webs were then stretched 6.0: 1 in the transverse direction using a Stenter oven at the temperatures shown in Table 1 to produce two biaxially stretched films having thicknesses, densities and optical densities as shown in Table 1.

EXAMPLE 3 Following the procedure of Examples 1 and 2, but coextruding the propylene homopolymer with a layer of a propylene / ethylene copolymer (4% by weight of ethylene), a two-layer polymer web was produced, the surface of the web of propylene homopolymer being cooled on a hardening roller having a surface temperature of 90 ° C. Thereafter, the web was stretched 4.5: 1 in the machine direction using heated rollers with a surface temperature of 90 ° C, and then 6.0: 1 in the transverse direction in a Stenter oven at 135 ° C to produce a film biaxially oriented having a thickness of 90.0 μm and a single surface layer of the copolymer that was 1 μm thick. The overall thickness, density and optical density of the film produced are shown in Table 1.

TABLE 1 Temperature Ratio of Stretch Thickness Stretch Stretch Film Density Density HD (X: l) HD (#C) TD (* C) (μi) (g / c? 3) Optical 1 3.5 80 140 95.0 0.53 1.02 2 3.5 80 134 88.5 88.5 0.97 4. 5 90 135 90.0 90.0 0.96 All these films were successfully processed during the hollow rape.

Claims (11)

NOVELTY OF THE INVENTION CLAIMS

1. - A biaxially oriented polyolefin film having a thickness of at least 50 μm and comprising a layer of a polymer resin based on polypropylene having microvoids thereon, the microvoids having been formed by stretching a band containing the form Beta polypropylene at an area stretch ratio of at least 15: 1.

2. A film according to claim 1, characterized in that it is an individual band.

3. A film according to claim 1, characterized in that it has an additional polymeric layer on a surface thereof.

4. A film according to claim 1, characterized in that it has an additional polymer layer on both surfaces thereof.

5. A film according to any of the preceding claims, characterized in that the area stretching ratio is at least 20: 1.

6. A film according to any of the preceding claims, characterized in that the ratio of area stretching is from 30: 1 to 45: 1.

7. A film according to any of the preceding claims, characterized in that it has a thickness of at least 40 μm.

8. A film according to any of the preceding claims, characterized in that it has a thickness of at least 80 μm.

9. A film according to any of the preceding claims, characterized in that at least one surface thereof has been treated to increase its surface energy.

10. A film according to any of claims 3 to 9, characterized in that an intermediate layer is present between the recessed layer and said additional layer.

11. A film according to any of the preceding claims, characterized in that it includes a slip agent, an antistatic agent, an antiblocking agent, a stabilizer, a UV light absorber or a pigment.