Heat-resistant overwrap film

Abstract: An overwrap film formed from a resin composition that comprises (A) 98 - 70 parts by weight of a 4-methyl-1-pentene based polymer, (B) 1 - 15 parts by weight of a butene based liquid polymer having a kinematic viscosity of 2 - 5000 cSt at 100°C, and (C) 1 - 15 parts by weight of a butene-1 based solid polymer, with the sum of (A) + (B) + (C) being 100 parts by weight has excellent heat resistance, flexibility, tackiness and transparency.

Description: This invention relates to a heat-resistant overwrap film, more particularly, to an overwrap film that is improved not only in heat resistance but also in flexibility and tackiness. Overwrap films are currently used as wrappers of foods under storage or cooking. Conventional overwrap films are made of thin films of polyvinyl chloride or polyvinylidene chloride, which are capable of withstanding heat at temperatures of about 140°C. EP-A-139,892 and FR-A-2 396 782 disclose compositions comprising 4-methyl-1-pentene polymer and other polyolefins containing 1-butene. As the use of overwrap films has expanded recently, a demand has arisen for products that can withstand use at even higher temperatures, namely those which have higher heat resistance. Polymers based on 4-methyl-1-pentene melt at 220 - 240°C and are known to exhibit high heat resistance. Hence, it is expected that overwrap films having high transparency and heat resistance can be produced by shaping 4-methyl-l-pentene based polymers into films. However, films of 4-methyl-l-pentene based polymers have been unsatisfactory not only in flexibility but also in tackiness, which is an important property in the case where they are to be used as wrappers. The present invention seeks to provide a heat-resistant overwrap film that retains the high heat resistance of the 4-methyl-1-pentene based polymers and which also excels in flexibility, tackiness and transparency. The present inventors have conducted intensive studies and found that a film formed from a resin composition comprising a 4-methyl-1-pentene based polymer, a butene based liquid polymer and a butene-1 based solid polymer in specified proportions exhibited not only high heat resistance but also good flexibility and tackiness and, hence, is advantageous for use as a heat-resistant wrapper. Thus, the present invention provides a heat-resistant overwrap film formed from a resin composition comprising: (A) 98 - 70 parts by weight of a 4-methyl-l-pentene based polymer, (B) 1- 15 parts by weight of a butene based liquid polymer having a kinematic viscosity of 2 to 5000 10 -6 m 2 /s (2 - 5000 cSt) at 100°C, and (C) 1 - 15 parts by weight of a butene-1 based solid polymer, with the sum of (A) + (B) + (C) being 100 parts by weight. A particularly preferred heat-resistant overwrap film is one having a thickness of 5 - 20 µm formed from the resin composition defined above. Fig. 1 is a diagram showing conceptually a method of measuring the tack of films prepared in Examples 1 - 5 and Comparative Examples 1 - 4; and Fig. 2 is a diagram showing the test piece used to measure the heat-resisting temperature of films prepared in Examples 1 - 5 and Comparative Examples 1. The heat-resistant overwrap film of the present invention is described below in more detail. Component (A), or the 4-methyl-1-pentene based polymer, which is the first essential component of the heat-resistant overwrap film of the present invention, is a polymer containing 4-methyl-1-pentene as a principal ingredient. Examples of such a polymer are a homopolymer of 4-methyl-1-pentene, as well as copolymers of 4-methyl-1-pentene and other α-olefins. Examples of α-olefins other than 4-methyl-1-pentene are α-olefins having 2 - 20 carbon atoms including ethylene, propylene, 1-butene, 1-heptene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicocene. These α-olefins may be contained in the 4-methyl-1-pentene based polymer either singly or in admixture. When these α-olefins are to be contained in the 4-methyl-1-pentene based polymer, their content is typically in the range of from about 1 to 10 wt%. The intrinsic viscosity [η] of the 4-methyl-1-pentene base polymer as measured in the solvent decalin at 135°C is typically in the range of from 1.0 to 3.0, preferably from 2.0 to 2.5. Component (B), or the butene based liquid polymer, which is the second essential component of the heat-resistant overwrap film of the present invention, is a polymer containing isobutylene as a principal ingredient. Examples of such a polymer are a homopolymer of isobutylene, as well as copolymers that contain a major amount of isobutylene and a minor amount, say, no more than 40 mol%, of other α-olefins. Examples of α-olefins other than isobutylene include 1-butene, 2-butene and butadiene. These α-olefins may be contained in the butene based liquid polymer either singly or in admixture. The butene based liquid polymer has a kinematic viscosity of 2 to 5000 10 -6 m 2 /s (2 - 5000 cSt) at 100°C; preferably, this polymer has a kinematic viscosity of 50 to 1000 10 -6 m 2 /s (50 - 1000 cSt) since it contributes to the production of a heat-resistant overwrap film having excellent moldability and quality. Component (C), or the butene-1 based solid polymer, which is the third essential component of the heat-resistant overwrap film of the present invention, is a crystalline polymer containing butene-1 as a principal ingredient. Examples of such a polymer are a homopolymer of butene-1, as well as copolymers of butene-1 and other α-olefin. Examples of α-olefins other than butene-1 include ethylene and propylene. These α-olefins may be contained in the butene-1 based solid polymer either singly or in admixture. When these α-olefins are to be contained in the butene-1 based solid polymer, their content is typically less than 30 mol%. A content of 20 mol% or less is particularly preferred. The butene-1 based solid polymer as component (C) can be prepared by polymerizing butene-1 either alone or in combination with other α-olefins in the presence of a Ziegler catalyst. The butene-1 based solid polymer (C) typically has 20 - 60% crystallinity, preferably from 30 to 50% with to ensure substantial effectiveness in providing improved flexibility. The butene-1 based solid polymer (C) generally has a melt flow rate of 0.01 - 50 g/10 min, preferably 0.05 - 20 g/10 min, in to provide an overwrap film having satisfactory mechanical strength, and producing a highly transparent overwrap film that features good compatibility between the butene-1 based solid polymer (C) and the 4-methyl-1-pentene based polymer (A). The melt flow rate specified in the present invention is measured in accordance with ASTM D 1238E. The heat-resistant overwrap film of the present invention is produced by molding a resin composition which comprises the above-described polymers (A), (B) and (C). In order to ensure the production of a heat-resistant overwrap film which excels in heat resistance, flexibility, tackiness and transparency and which is substantially free from the bleeding of the butene based liquid polymer (B), the relative proportions of the three components (A), (B) and (C) in the resin composition are such that each of the components (B) and (C) is contained in an amount of 1 - 15 parts by weight for 98 - 70 parts by weight of the component (A). In view of ensuring the production of a heat-resistant overwrap film that has a good balance between tackiness and transparency, each of the components (B) and (C) is contained in an amount of 3 - 8 parts by weight for 94 - 84 parts by weight of the component (A). In whichever proportions, the sum of (A) + (B) + (C) is 100 parts by weight. The resin composition of interest may optionally contain one or more compounding ingredients selected from antistats colorants, UV absorbers, inorganic fillers, antistats, antihaze agents and heat stabilizers. The method of producing the heat-resistant overwrap film of the present invention is in no way limited. In a typical case, the essential components (A), (B) and (C) of the resin composition, as well as any of the compounding ingredients listed above which may be added as required,are supplied separately into an extruder in predetermined proportions; alternatively, the individual components and ingredients are preliminarily mixed to prepare the resin composition, which is then supplied into the extruder; in the extruder, the feed is melted and kneaded, followed by extrusion through a suitable molding die such as a T-die or tube die for shaping into a film. The heating temperature in the extruder is typically from 240 to 300°C. While all heat-resistant overwrap films of the present invention may be produced in the manner described above, those films which have a post-aging tack of 1 - 7 g/8 cm and a thickness of 8 - 15µm are characterized by a good handling property in practice. The term "tack" as used herein means the 180° peel strength of two films. This factor is specifically expressed in terms of the value that is obtained by the method of tack measurement described hereinafter. The heat-resistant overwrap film of the present invention excels in heat resistance, flexibility, tackiness and transparency; hence, it can advantageously be used in high-frequency heating, particularly in those applications where very high heat resistance is required, as exemplified by the reheating and cooking of oil-containing foods. The following Examples and Comparative Examples further illustrate the present invention. Example 1 Ninety parts by weight of poly4-methyl-1-pentene (MX021 of Mitsui Petrochemical Industries, Ltd.), 5 parts by weight of liquid polybutene (B) (HV300 of Nippon Petrochemicals Co., Ltd.; kinematic viscosity, 630 10 -6 m 2 /s (630 cSt) at 100°C) and 5 parts by weight of polybutene-1 (C-1) (Beaulon M2181 of Mitsui Petrochemical Industries, Ltd.; melt flow rate, 1.0 g/10 min; density, 0.900 g/cm 3 ; m.p. 71°C) were mixed in a Henschel mixer to prepare a resin composition. The resin composition was then supplied into an extruder, in which it was melted and kneaded at a molding temperature of 290°C. The melt was extruded through a T-die to yield an overwrap film 12 µm thick and 300 mm wide. The yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature of the overwrap film thus produced were measured by the methods described below and the results are shown in Table 1. Yield strengths breaking strength and elongation Measured in accordance with ASTM D 882. Tear strength Measured in accordance with ASTM D 1992 (Elmendorf tear test). Haze Measured in accordance with ASTM D 1003. Gloss Measured at 60° in accordance with ASTM D 523. Tack Two films 8 cm wide by 10 cm long that had been aged at 40°C for 1 week after molding were placed in superposition and passed between press rolls at a nip pressure of 0.2 kg/cm 2 , thereby preparing a test piece in which the two films were held in intimate contact with each other in all areas except for a width of 2 cm from one side of the film. Then, the test piece was set up as shown in Fig. 1 in which the test piece is indicated by 1; and end portion 2 of one film 1a on the side of the test piece 1 where the two films were not held in intimate contact with each other was secured by means of double-coated tape to an end portion 5 of support 4 fixed On a table 3. Furthermore, a load 7 was put on an end portion 6 of the other film 1b on the same side of the test piece 1 (where the two films were not held in intimate contact with each other) in an atmosphere of 23°C x 50% RH. The load was successively replaced by heavier ones until the superposed films 1a and 1b of the test piece 1 separated from each other, whereupon the load 7 to started to move. The weight (in grams/8 cm) of the load applied at that time was measured as the tack of the film. Heat-resisting temperature As shown in Fig. 2, a heat-resistant overwrap film 11 measuring 3 cm wide by 14 cm long was provided and sheets of flat-grained paper 13a and 13b and each measuring 3 cm wide by 2.5 cm long were superposed on an upper end 12a and a lower end 12b, respectively, of the overwrap film 11 and the corresponding members were bonded together with double-coated adhesive tape to prepare a test piece. The upper end 12a of this test piece was secured to a jig and a load weighing 10 g was applied to the lower end 12b. Immediately after this setup operation, the test piece was put into an air oven and heated with the temperature raised in 5°C increments at 1-h intervals until the sample broke. The maximum temperature which the sample could withstand without break was designated as the "heat-resisting temperature" of the sample. Example 2 A heat-resistant overwrap film was prepared by repeating the procedure of Example 1, except that poly4-methyl-l-pentene, liquid polybutene and polybutene-1 (C-1) were mixed in relative proportions of 85, 5 and 10 parts by weight, respectively. The film was measured for its yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature. The results are shown in Table 1. Example 3 A heat-resistant overwrap film was prepared by repeating the procedure of Example 1, except that polybutene-1 (C-1) was replaced by polybutene-1 (C-2) (Beaulon M3080 of Mitsui Petrochemical Industries, Ltd.; melt flow rate, 0.2 g/10 min; density, 0.890 g/cm 3 ; m.p. 98°C). The film was measured for its yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature. The results are shown in Table 1. Example 4 A heat-resistant overwrap film was prepared by repeating the procedure of Example 1, except that poly4-methyl-l-pentene, liquid polybutene and polybutene-1 (C-2) which replaced polybutene-1 (C-1) were mixed in relative proportions of 85, 5 and 10 parts by weight, respectively. The film was measured for its yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature. The results are shown in Table 1. Example 5 A heat-resistant overwrap film was prepared by repeating the procedure of Example 1, except that poly4-methyl-l-pentene, liquid polybutene and polybutene-1 (C-2) which replaced polybutene-1 (C-1) were mixed in relative proportions of 85, 7 and 8 parts by weight, respectively. The film was measured for its yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature. The results are shown in Table 1. Comparative Example 1 A heat-resistant overwrap film was prepared by repeating the procedure of Example 1, except that the resin composition from which the film was molded was replaced by a composition consisting of 90 parts by weight of poly4-methyl-l-pentene and 10 parts by weight of liquid polybutene. The film was measured for its yield strength, breaking strength, elongation, modulus, tear strength, haze, gloss, tack and heat-resisting temperature. The results are shown in Table 1. The overwrap film of the present invention excels in heat resistance since it has a high heat-resisting temperature of from 190 to 195 °C, whereas the values for prior art products are in the neighborhood of 140°C. Hence, the overwrap film of this invention can be used in the high-temperature range where conventional overwrap films have been entirely unsuitable for use. Principally for this anticipated expansion of use, the overwrap film of the present invention will offer great benefits in practice.

Claims: A heat-resistant overwrap film formed from a resin composition comprising: (A) 98 - 70 parts by weight of a 4-methyl-1-pentene based polymer, (B) 1 - 15 parts by weight of a butene based liquid polymer having a kinematic viscosity of 2 to 5000 10 -6 m 2 /s (2 - 5000 cSt) at 100°C, and (C) 1 - 15 parts by weight of a butene-1 based solid polymer, with the sum of (A) + (B) + (C) being 100 parts by weight. A film according to claim 1, wherein the film as formed from said resin composition is 5 - 20 µm thick. A film according to claim 1 or 2, wherein the polymer (A) has an intrinsic viscosity [η] of 1.0 to 3.0, measured in decalin at 135°C. A film according to claim 1, 2 or 3, wherein the polymer (B) has a kinematic viscosity of 50 to 1000 10 -6 m 2 /s (50 - 1000 cSt). A film according to any one of the preceding claims wherein the polymer (C) has 20 - 60% crystallinity. A film according to claim 5 wherein the polymer (C) has 30 - 50% crystallinity. A film according to any one of the preceding claims wherein the polymer (C) has a melt flow rate of 0.01 - 50 g/10 min. A film according to any one of the preceding claims, which comprises 94 to 84 parts by weight of (A) and 3 - 8 parts by weight of each of (B) and (C), the sum of (A), (B) and (C) being 100 parts by weight.

Inventors: IMUTA SHINICHI, KATO TATSUO, KOBAYASHI MASAMI, NAGASE YASUSHI, IMUTA, SHINICHI, KATO, TATSUO, KOBAYASHI, MASAMI, NAGASE, YASUSHI, Imuta, Shinichi, c/o Mitsui Petrochem. Ind., Ltd., Kato, Tatsuo, c/o MITSUI PETROCHEM. IND., LTD., Kobayashi, Masami, c/o Chugoku Resin Co., Ltd., Nagase, Yasushi, c/o CHUGOKU RESIN CO., LTD.
Applicants: CHUGOKU RESIN CO LTD, MITSUI PETROCHEMICAL IND, CHUGOKU RESIN CO., LTD., MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1993
Main classification: C08L23
IPC codes: C08L23, C08J5