MXPA98002102A - Polimeri films - Google Patents

Abstract

Biaxially oriented polyolefin films having a polypropylene-based resin layer containing microvoids, produced by orienting the beta form of the polypropylene, and at least one outer layer of olefin copolymer thereon, the film having been biaxially removed are disclosed. at a temperature lower than the melting point of the olefin copolymer of said outer layer, said films have shown a high brightness

Description

POLYMERIC FILMS DESCRIPTIVE MEMORY This invention relates to polyolefin films having a suitable gloss. Adequate gloss is a feature of many polyolefin films, and especially for biaxially oriented polypropylene films. In fact, the brightness of such films has contributed to its widespread acceptance by many end users. However, the brightness tends to be higher for biaxially oriented and unfilled propylene homopolymers, providing outer layers of olefin copolymer, for example, to increase the printability, and fillers within the polypropylene layer to create icrockets in the layer during orientation, which serves to reduce the brightness of said films. Proposals have been made to improve the brightness of polypropylene films having core layers containing micro-voids induced by the presence of filler particles of different phase such as clay, such proposal being to provide a layer of unfilled propylene homopolymer between the layer core with holes and an outer layer of olefin copolymer. Although this approach to the problem has been successful, it reduces advantages for the purpose of using a central layer with holes because the density of the film is increased, and therefore the elasticity of the film decreases. Moreover, since the opacity of voided films is not generally as broad as desired, the reductions in the effective amount of voids within the films by adding gapless gauges to the films serves to reduce opacity even more. The use of intermediate layers of unfilled propylene homopolymer between the core layers of propylene homopolymer with holes and the outer layers of olefin copolymers can improve the brightness of the films, but after the biaxial orientation the copolymers themselves usually have a gloss lower than propylene homopolymers. According to the present invention, there is provided a biaxially oriented polyolefin film comprising a polypropylene-based resin layer containing produced microvoids oriented to the beta form of the polypropylene, and at least one outer layer of olefin copolymer thereon. , the film having been biaxially stretched below the melting point of the olefin copolymer of said outer layer. As a result of the orientation of the films at a temperature below the melting point of the olefin copolymer layer, the copolymer layer generally acquires a higher brightness than if it had been stretched at a temperature above its point of fusion. Although stretching at such temperatures could in theory be used to be used to effect the orientation of other polypropylene films, in practice it is generally not possible since at these temperatures the force required to stretch the alpha form that normally occurs from the homopolymers of propylene is very high, and the presence of voiding agents in organic or inorganic particles to effect voiding of said layers does not significantly improve the position. Unlike the alpha form of polypropylene, stretching at these temperatures is possible with a crystallized strip with a high level of the beta form of the polypropylene, since the force required to do so tends to be considerably lower. Not only do the films according to the present invention generally have adequate gloss, it has been found that the outer layer of olefin copolymer is substantially non-heat sealable, even if it is formed from a copolymer known as sealable by heat after processing under conditions normally used to produce oriented polypropylene films. However, the copolymer outer layers of the films of the present invention are usually printable, particularly after having been subjected to a treatment to increase its surface energy. 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, the production of films using polypropylene containing high concentrations of the beta form of polypropylene is known 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 it is melted and subsequently cooled. An example of such processing is described in EU4386129, in which a variety of so-called beta nucleators are dispersed in the polypropylene, after which films are produced therefrom by melting and subsequent cooling, controlling the crystallinity of the cast films resulting by proper adjustment of 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 mixtures * components of beta nucleating agents to produce icropore films by mono- or biaxial stretching of cast polypropylene strips containing a high concentration of the beta form of the 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 the porosity during the stretching process is accompanied by a significant reduction in the 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 material from a mixture of polypropylene and the nucleating agent to form a film strip which is allowed to crystallize at a temperature of 15 to 140 ° C to form a solid strip containing the beta form of the polypropylene, after which the strip is mono- or biaxially stretched at a temperature of more than 20 ° C but less than the melting point of the crystals of the beta form in the strip. It is mentioned that the resulting stretched films have a high whiteness and hiding power in combination with printing and writing capacity. The formation of microvoids during the plastic deformation of the beta form of polypropylene is also described in the publication POLYMER (Vol. 35, No. 16, pp. 3442-5, 1995; and Vol. 36, No. 13, pp. 2523-30, 1995). It is indicated that the porosity increases with higher crystallization and lower stretching temperatures, and all samples containing the beta form apparently become opaque when stretched at temperatures below 120-130oC. Opacity due to the formation of microvoids also occurs when polypropylene homopolymers are blended with a variety of particulate materials and then biaxially stretched, numerous examples of which are known in the prior art. Typically, different phase organic or inorganic materials with a particle size of 1 to 10 μm are used for that purpose. The polypropylene-based resin layer is preferably formed from a propylene homopolymer or from a random or propylene block copolymer containing a higher proportion of propylene-derived units and having a crystallinity of more than 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-naphthalene dicarboxamide. 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 to 1 being preferred. % in weigh. The outer layer of copolymer may be in direct contact with the base layer of polypropylene with holes, or may be an outer layer on one or more intermediate layers on the base layer. Examples of olefin copolymers that can be used for the outer layer include polymers containing units derived from one or more of higher ethylene, propylene, butene-1 and aliphatic alpha olefins, and mixtures thereof. Polymers that can be used for intermediate layers, when present, include the olefin polymers used for the outer layer that according to the present invention have to be present, and extended polyolefins with unsaturated carboxylic acids and derivatives thereof, by example, ionomers and acid anhydrides. Although one surface of the hollowed layer of the films of the present invention will have an outer layer of an olefin copolymer thereon, the other surface of the base layer need not have additional layers thereon. However, this outer surface of the hollowed layer may have one or more polymeric layers thereon, for example an additional layer, e.g., a layer of a normally heat-sealable polymer, which may be at the same time equal or different from the outer layer of copolymer on the other side of the voided layer, the preferred materials being selected for this additional layer from polyolefins that can be used for the outer layer of copolymer on the other side of the voided layer . Films according to the present invention may include one or more additives used in the polyolefin art, for example slip agents, antistatic agents, antiblocking agents, stabilizers, UV absorbers or pigments, said additives preferably being present in amounts that do not significantly adversely affect the ability of polypropylene to crystallize in its beta form. When the additives are present, they can be added to one or more of the layers that make up the film. Films according to the present invention can be produced using known methods, for example by coextrusion through a slot die of polymer melts of the two specified layers, or other desired methods, to form a polymer strip that it is cooled and then biaxially stretched sequentially. As will be appreciated from the prior art mentioned above, it is generally desirable to carry out certain processing steps, particularly the initial crystallization of the polypropylene from a molten material containing the nucleating agent of the beta form, under conditions that will cause the formation of the desired film structure. Most 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 holes can r 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 polypropylene-based resin layer of the films of the present invention before being stretched, should be at least 20 C, but lower than the melting point 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 it is commonly preferred to use still higher temperatures, for example 90 ° C or plus. However, the cooling temperature is preferably not more than 140 ° C, and for practical considerations it is preferably below the temperature at which the film adheres to the surfaces used to cool it. The cooling of the molten material can be carried out in air of a suitable temperature, but it is generally preferred to carry out the cooling by making contact between the extruded strip and a cooling surface, for example an ice-cold roller. The subsequent biaxial stretching of the cooled strip will generally be carried out sequentially, making it possible for the conditions used in the two directions to be selected independently of one another. Stretching in the extrusion direction (the machine direction) will normally be carried out before stretching in the transverse direction. It has been found that the conditions used to carry out the stretching in the machine direction have a substantial effect on the formation of microvoids, usually bringing the lower drawing temperatures and the lower drawing ratios to an increased hole formation. A preferred temperature scale for stretching in the machine direction is from 70 to 110 ° C, and most preferably from 80 to 95 ° C, and the draw ratio used in the machine direction will normally be at least 3: 1, with a scale of 3.5: 1 to 8: 1 being preferred. The subsequent stretching of the films in the transverse direction will generally be carried out at lower temperatures than are normally used for the transverse stretch of the polypropylene films, for example from 100 to 145 ° C, as long as they are smaller. than the melting point of the outer layer. This serves to produce films with adequate gloss on the outer surface formed from the copolymer, for example a gloss at 20 ° of at least 25, and also have low densities. The draw ratio used in the transverse direction is preferably from 3: 1 to 10: 1. Either or both surfaces of the films of the present invention can, if desired, be subjected to a treatment to increase their surface energy, for example using flame treatment or corona discharge. The following examples are shown by way of illustration only: EXAMPLES 1 TO 6 A series of six polymer strips was produced by coextruding through a die die melt materials of a propylene homopolymer containing 0.1% by weight of a nucleating agent of the beta form (N, N'-dicyclohexyl-2,6- naphthalenedicarboxamide; NJ-Star NU-100, ex New Japan Chemical Co., Ltd.), in all but one (example 6) with a layer of a propylene / ethylene copolymer (4% by weight of ethylene) on the surface of the homopolymer layer. The homopolymer layer of each strip was cooled by contacting it with a quenching roller having a surface temperature of 100 ° C, and cooling a copolymer layer when present in ambient air. The DSC measurements indicated that the homopolymer layer contained a high level of the polypropylene beta form with a T of 153 ° C. Each of the cooled strips was then stretched 4.5: 1 in the extrusion direction by contacting it with hot rollers at 90 ° C having different peripheral speeds to produce monoaxially drawn opaque strips, showing the DSC that almost all the beta-polypropylene had been in each case converted to the alpha form. The monoaxially stretched strips were then stretched 8.0: 1 in the transverse direction using an oven at a variety of temperatures shown in Table 1 to produce a series of biaxially stretched films. The thicknesses, densities, optical densities, heat sealing thresholds (HST) and brightness values at 20 ° total of the films produced are also shown in Table 1.

EXAMPLE 7 A three layer polymer strip was produced by coextruding through a slot die a middle layer made of a melt material of a mixture of a propylene homopolymer containing 5% by weight of clay (average particle size 3 μm), with molten materials of the copolymer used in Examples 1 to 6, and the resulting strip was cooled on a cooling roller having a surface temperature of 20 ° C. Subsequently, the strip was stretched 4.5: 1 in the machine direction by passing it on hot rollers with different peripheral speeds and at a surface temperature of 110 ° C, followed by 8: 1 in the transverse direction in a 160 ° C oven.

The thicknesses, densities, optical densities, heat sealing thresholds (HST) and brightness values at 20 ° total of the films produced are also shown in Table 1.

TABLE 1 Ex. Temp. Thickness Density Density HST Extra optical brightness (ßC) (20 °) (μm) 1 1x1 136 55.0 0.58 0.85 - 30.8 2 lxl 128 60.0 0.58 0.83 - 40.1 3 1x1 120 60.5 0.61 0.775 - 44.5 4 * lxl 158 38.2 0.88 0.36 122 27.2 * lxl 144 54.0 0.59 0.87 122 11.9 6 * _ 128 54.0 0.58 0.78 - ÁÁ 7 * 2X1 160 50.0 0.75 0.55 122 16.00 * Examples of comparison. Examples 1 to 3 are in accordance with the present invention. The films had high gloss and were not sealable by heat. Examples 4 to 7 were not in accordance with the invention, and the films of examples 4 and 5 were drawn in the transverse direction at a temperature above the melting point of the respective copolymer layers. The film of Example 6 did not include an outer layer of copolymer, and the film of Example 7 was a heat sealable film of the prior art with hollows made of clay.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A biaxially oriented polyolefin film comprising a polypropylene-based resin layer containing microvoids produced by orienting the beta form of the polypropylene, and at least one outer layer of olefin copolymer thereon, the film is biaxially stretched to a lower temperature than the melting point of the olefin copolymer of said outer layer.

2. A film according to claim 1, further characterized in that the biaxial stretching is carried out at a temperature of at least 5 ° C below the melting point of the olefin copolymer of said outer layer. 3.- A film in accordance with the claim 1 or claim 2, further characterized in that the film is drawn in the machine direction at a temperature of 70 to 110 ° C. 4. A film according to claim 3, further characterized in that the film is stretched in the machine direction at a temperature of 80 to 95 ° C. 5. A film according to any of the preceding claims, further characterized in that the film is stretched in the transverse direction at a temperature of from 100 to 145 ° C. 6. A film according to any of the preceding claims, further characterized in that the surface formed by the copolymer layer has a gloss at 20 ° of at least 25. 7. A film according to any of the preceding claims , characterized in that a surface of the same is treated to increase the surface energy of the same. 8. A film according to any of the preceding claims, further characterized in that an intermediate polymeric layer is present between the voided layer and the outer layer. 9. A film according to any of the preceding claims, which includes a slip agent, an antistatic agent, an antiblocking agent, a stabilizer, a UV absorber or a pigment.