MXPA98002074A - Polimeri films - Google Patents

Abstract

Biaxially oriented polyolefin films having a thickness of not more than 50% having a layer of polypropylene-based resin with micro-void therein, the microvoids having been formed by stretching a ribbon containing the polypropylene form at a ratio of stretch area of at least 15: 1, these films of similar thickness to those that have formed holes using clay as the hue forming agent

Description

POLYMERIC FILMS DESCRIPTIVE MEMORY This invention relates to biaxially oriented polyolefin films containing microvoids, and in particular to polypropylene films with voids. Polypropylene films that include microvoids induced by the stretching of biaxial films from fillers containing fillers have found wide acceptance for many end uses such as packaging and labeling. Said films have an inherent opacity as a result of the presence of the microvoids, and this is usually combined with an increased film yield since the voids result in a film with a density considerably lower than that of the polymer and filler. which movie is made Despite these desirable properties, said films have no disadvantages. In particular, the opacity and other optical properties of said films may be less than those desired for particular end uses, their opacity is often uneven on the surface of the film, these undesirable properties leading to be greater with the films, for example, less than about 30 μ, and thin films with voids may be difficult to produce as a result of the presence of the particles used to make the voids. According to the present invention, a biaxially oriented polyolefin film having a thickness of not more than 50 μ is provided which comprises a polypropylene-based resin layer having microvoids thereon, the microvoids having been formed by stretching a ribbon that contains the beta form of polypropylene at a stretch area ratio of at least 15: 1. Films in accordance with the present invention have shown particularly uniform opacity on their surfaces when compared to films with voids of similar thickness in which the gap has been produced using an inorganic particulate material such as chalk. In addition, they can be produced with combinations of thickness and density that can not be produced using clay as a hole-forming agent or can only be produced with difficulty, for example, 40 μm thick films with densities of 0.55 g / cm3 and weight unit of 22 g / m2, and films of 25 μ of thickness with densities of 0.70 g / cm3 and unit weight of 17.5 g / m2. 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 baths are extruded and then cooled to form a polymeric film which can then 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 melts and subsequently cools. 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, the crystallinity of the resulting cast films being controlled 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 the mono- or biaxial stretching of cast polypropylene tapes 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 from the beta form to the alpha form during the stretching procedure, the alpha form having a greater 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, in EP0632095 it has been proposed 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 ribbon of film that is allowed to crystallize at a temperature of 15 to 140 ° C to form a solid tape containing the beta form of polypropylene, after which the tape 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 tape. It is said that the resulting stretched films have high whiteness and hiding power in combination with the ability to print and write. 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 evidently become opaque when stretched at temperatures below 120-130 ° C.

The polypropylene base resin of the base layer is preferably formed from a propylene homopolymer or from a random or block copolymer containing a significant proportion of units derived from propylene 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 as 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. to 1% by weight. The films according to the invention may consist of a single layer of polypropylene-based resin containing microvoids, but will often include one or more layers, for example, to impart particular properties such as gloss, printability and sealing capacity. . For example, the films may include an outer layer that confers those properties, and this layer may be in direct contact with the polypropylene layer with voids. Additional layers are preferably formed from polyolefins; examples of polymers that may be used for the purpose include polymers containing units derived from one or more of ethylene, propylene, butene-1 and higher alpha-olefins, and mixtures thereof. Other heat-sealable polymers, for example acrylic polymers, and polyolefins extended with unsaturated carboxylic acids and derivatives thereof, e.g., ionomers and acid anhydrides. Although only one surface of the hollow polypropylene layer may have an additional layer thereon, the other surface of the base layer may also have some additional layers, for example, a heat sealable layer which may be the same or different of the optional layer on the other surface of the films, the preferred materials for this layer being selected from heat-sealable polymers referred to above. The 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 which 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.

The films according to the present invention have thicknesses of less than 50 μ, and can be thinner, for example, of less than 40 μ, or in fact considered thinner, for example of 35 μ or less, eg. , 25 μ or less. The films according to the present invention can be produced using known methods, for example by extrusion or coextrusion through a slotted die of molten baths of the appropriate polymers for the layer or layers that may be desired, to form a polymer ribbon. which is cooled and then biaxially stretched in sequence at an area stretching ratio of at least 15: 1. However, when additional layers are present on the polypropylene-based resin layer with voids, they can be applied to the voided layer by coating after the voided layer has been formed. As will be appreciated from the prior art referenced above, it is generally desirable to carry out certain process 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. Most particularly, the cooling of the polypropylene-based resin 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 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 less than the melting point of the polypropylene beta form. Although temperatures at the lower end of this scale, for example up to 50 ° C, can be used, it is generally preferred to use temperatures of at least 70 ° C, and it is commonly preferred to use even higher temperatures, for example, of 90 ° C or more. 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 bath 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 tape and a cooling surface, for example, a cold roll. The subsequent biaxial stretching of the cooled ribbon is generally carried out sequentially, for example, under conditions known in the polypropylene film art, sequential stretching allowing 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. The area stretch ratio of the films of the present invention should be at least 15: 1 and will often be considerably more, for example, at least 20: 1. Excessively large stretching ratios can give rise to difficulties in film production, and the area stretch ratio will be no greater than 70: 1. The area stretching relationships particularly referred to are generally in the range of 25: 1 to 50: 1. 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, driving the lower stretching temperatures and the lower stretching 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 those which are normally used for the transverse stretching of the polypropylene films, for example, at temperatures of 100 to 160 ° C. However, it is generally preferred to perform the stretch in the transverse direction at temperatures not higher than 155 ° C and preferably not higher than 150 ° C in order to produce biaxially stretched films having low densities. The draw ratio used in the transverse direction is preferably from 3: 1 to 10: 1. The area stretching ratios of the films of the present invention is at least 15: 1, and is preferably at least 20: 1. If desired, area stretching ratios, for example, haeta of 65: 1, can be used, with a preferred scale of area stretching ratios of 30: 1 to 50: 1. Either or both of the surfaces of the films according to the present invention can be subjected to a treatment to increase their surface energy, for example, by using flame treatment or by corona discharge. The following examples are given by way of illustration only: EXAMPLES 1 TO 4 Four coext polymer tapes were produced by running through a slotted die a base layer of propylene homopolymer containing 0.1 wt.% Of a nucleating agent of the beta form (N, N '-dicyclohexyl-2,6-naphthalenedicarboxamide).; NJ-Star NU-100, former New Japan Chemical Co., Ltd.), one (example 3) also including a second layer of a propylene / ethylene copolymer (4% by weight of ethylene). In each case, the homopolymer layer was cooled by contacting it with a cooling roller having a surface temperature of 100 ° C, and cooling a copolymer layer when it was present in the ambient air. The DSC measurements indicated that the homopolymer layer of each tape contained a high level of the beta form of the polypropylene with a Tm of 153 ° C. Each of the cooled tapes was then stretched in the extrusion direction to the stretch ratio shown in Table 1 by contacting them with rollers heated to 90 ° C having different peripheral speeds to produce in each case a monoaxially stretched opaque tape, showing the DSC that almost all the beta-polypropylene had in each case been converted to the alpha form. The monoaxially stretched tapes were then drawn in the direction transverse to the stretching ratios shown in Table 1 using an extension furnace at temperatures also shown in Table 1. The optical densities of the films are also shown in table 1. The film of Example 3 had an outer layer of the copolymer having 1 μ of thickness.

EXAMPLE 5 (COMPARISON) A three layer polyolefin film with voids was produced by coextruding through a slotted die a three layer tape, the middle layer made up of the propylene homopolymer used in Examples 1 to 4, but containing 5% by weight of chalk in particles (average particle size 3 μ), and the two outer layers consisting of the copolymer used to form a second layer of the films of Example 3. The extruded tape was cooled on a cooling roller at 20 ° C before being stretched on the direction of the machine on heated rollers with different peripheral speeds at a temperature of 100 ° C at a stretch ratio of 5: 1 and then in a stretching oven at 160 ° C at a ratio of 8: 1. The film produced had a total thickness of 25 μ with a core layer with holes and two external layers each 1 μ thick.

TABLE 1 Ex, Relationship Ratio Thickness Density Density Estimated weight of the optical unit unitaramiento pelícu (g / cm3) rio in DM in DT Temp. the (μ) (g / m2) (x: l) (y: l) (° C) 1 4.5 8 140 30.0 0.68 0.55 20.4 2 5 7 135 32.0 0.62 0.65 19.8 3 4.5 8 144 25.0 0.60 0.43 15.0 4 4.5 8 144 26.0 0.65 0.44 16.9 * 4.5 8 160 25.0 0.69 0.30 17.3 * Comparison All these films except that of Example 5 had a substantially uniform opacity on their all surfaces while the film of Example 5 was irregular.

Claims (7)

NOVELTY OF THE INVENTION CLAIMS

1. - A biaxially oriented polyolefin film having a thickness of not more than 50 μ comprising a polypropylene-based resin layer having micro-voids thereon, the micro-voids having been formed by stretching a tape containing the polypropylene beta form at an area stretch ratio of at least 15: 1.

2. A film according to claim 1, further characterized in that the stretch ratio is at least 20: 1. 3.- A film in accordance with the claim 1 or claim 2, further characterized in that the stretch ratio is from 25: 1 to 50: 1. 4. A film according to any of the preceding claims, in the form of an on-line. 5.- A film in accordance with the claim 1 or claim 3, which has at least one additional polymer layer thereon. 6. A film according to claim 5, having at least one intermediate layer between the voided layer and the additional polymer layer (at least one). 7. 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.