GB2210375A - Polyolefin composition - Google Patents

Abstract

Printable films are made of compositions comprising (A) 35 - 92% of a homo- or co-polymer of an olefin, e.g. ethylene, propylene or butene-1, or other polymer prepared from C and H only, e.g. polystyrene, optionally with up to 50% of another thermoplastic polymer, and (B) 8 - 65% of a mixture of (1) 100 parts of a heavy calcium carbonate, optionally up to 50% being replaced by other inorganic powder of specific surface area 100 m<2>/g or less and particle size of 10 mu m or less, and (2) 5 - 150 parts of magnesium hydroxide or carbonate of specific surface area 100 m<2>/g or less. Preferably the (B2) powder is supported on (B1) before mixing with (A). The composition is melt kneaded and extruded, and preferably uniaxially stretched, into a synthetic film. The powder allows the surface to receive printing ink or pencil, and wet ink is not offset onto the next sheet. The film is useful for packaging, posters or the like.

Description

A PRINTABLE FILM OF A POLYOLEF I N COMPOSITION This invention relates to a synthetic resin film having excellent printability, and more particularly to a polyolefinic film having satisfactory printability in gravure printing or offset printing.

Gravure printing is widespread as a technique for printing on a zolgolefinic film having no hydrophilic groups.

It is known that printability of such film can be improved by incorporating an inorganic fine powder into the polymer and that a further improvement in printability can be achieved by stretching the inorganic fine powder-compounded film, as disclosed in JP-B-46-4678, JP-B-46-34896, JP-B-4640794, JP-B-48-3902 and JP-B-60-36173 ("JP-B" refers to an examined Japanese patent publication).

The films containing the inorganic fine powder, and especially the stretched films, have an advantage of satisfactory printability in not only gravure printing but multi-color offset printing, The stretched films are utilized as synthetic papers for posters or stickers.

In continuously conducting multi-color offset printing on these synthetic papers, it has been desirable to shorten the drying time of the printing ink. If a printing ink is not completely dried in continuous printing, there is the defect that the printed wet ink on one sheet is transferred to the back of the next sheet (i.e. offset), or there is the disadvantage that the printed sheet cannot be forwarded to the subsequent step, such as overprinting, bookbinding or bag making.

Accordingly, an object of this invention is to provide a stretched or unstretched film of a polyolefinic polymer containing an inorganic fine powder, which exhibits improved dryness of printing ink.

As a result of extensive studies, it has now been found that an improvement of ink fixing properties can be attained by incorporating in such polymer an inorganic fine powder comprising a heavy calcium powder and magnesium hydroxide and/or magnesium carbonate having a specific surface area of not more than 100 m2/g.

Thus, the present invention relates to a synthetic resin film having excellent printability obtained by filmforming a composition comprising (A) from 35 to 92% by weight of a polyolefin, and (B) from 8 to 65% by weight of an inorganic fine powder comprising a mixture of (b1 ) 100 parts by weight of a heavy calcium carbonate and (b2 ) from 5 to 150 parts by weight of magnesium hydroxide and/or magnesium carbonate having a specific surface area of 10 m2/g or less.

The polyolefin-based resin which can be used in the present invention includes homo- or copolymers of an oleo in (e.g. ethylene, propylene or butene-1 ) or polystyrene.

When, in particular, resistance to chemicals and strength are required, a polypropylene or a polyethylene having a density of from 0.940 to 0.970 g/cm2 are preferred.

These polyolefins can be used either alone or in combination of two or more thereof. If desired, thero- plastic polymers other than the polyolefin-based polymers, such as a polyamide, polyethylene terephthlate, a vinyl chloride-vinylidene chloride co-polymer or a polycarbonate may be used in combination in a proportion up to 50% by weight based on the weight of the total polymer components.

The term "polyolefin" or "polyolefin resin" is used herein to refer to such polymers prepared using carbon and hydrogen or compounds thereof as starting materials, and thus includes, e.g. polystyrene.

The inorganic fine powder which can be used in the present invention is a mixture comprising (bl) 100 parts by weight of heavy calcium carbonate and (b2) from 5 to 150 parts by weight, preferably from 20 to 130 parts by weight, of magnesium hydroxide and/or magnesium carbonate.

The heavy calcium carbonate as the component (bl) may be partly replaced with other inorganic fine powders having a specific surface area of 100 m2/g or less and a particle size of 10 iim or less, such as diatomaceous earth, calcined clay, titanium oxide or bariu;n sulfate, in a proportion of up to 50% by weight. The above-specified inorganic fine powder functions to impart writability with a pencil and offset ink printability to the synthetic resin film. In addition, the inorganic fine powder serves as nuclei forming a number of fine voids in the film resin matrix, to thereby make the synthetic film lightweight and paper-like after the film is stretched.

In the present invention, the heavy calcium carbonate (bl) is used in combination with 5 to 150 parts by weight of magnesium carbonate and/or magnesium hydroxide (b2) having a specific surface area of 100 m2/g or less per 100 parts by weight of the heavy calcium carbonate. The component (b2) has a- great effect to improve ink drying properties.

If the proportion of the component (b2) is less than 5 parts by weight, the effect to improve dryness of printing ink is insufficient. On the other hand, if it exceeds 150 parts by weight, the resin composition shows a poor bite into an extruder screw on melt-kneauing and pelletization. Moreover, a printing ink spread on the resulting synthetic resin film undergoes a so-called color glooming phenomenon, i.e., poor color development.

The specific surface area of the inorganic fine powder as component (B) ::st not exceed 100 m2/g. If it exceeds 100 m2/g, not only the bite of the resin composition into an extruder screw becomes deteriorated, but the above-described color glooming phenomenon would occur.

The proportion of the inorganic fine powder as component (B) in the film-forming resin composition greatly depends on the desired degree of transparency (transparent, semi-transparent or opaque) of the film.

In general, when transparency or semi-transparency is requested as in tracing films, the inorganic fine powder is incorporated in an amount of from 8 to 50% by weight, preferably from 8 to 42% by weight, based on the weight of the resin composition. When opaqueness is requested as in posters or stickers, it is incorporated in an amount of from 35 to 65% by weight. Since the degree of transparency of synthetic resin films also depends on the stretching temperature and stretching ratio in addition to the content of the inorganic fine powder, the proportion of the inorganic fine powder should be determined taking all these factors into consideration together.

In preparing the resin composition comprising the polyolefin resin (A) and the inorganic fine powder (B), the components (A), (bl), and (b2) may be mixed all at once, but it is more effective for improving dryness of printing ink that the component (B) is first prepared by supporting the component (b2) on the component (bl) and the component (B) is then mixed with the component (A).

Supporting of magnesium carbonate and/or magnesium hydroxide (b2) on heavy calcium carbonate (bl) can be carried out, in the most convenient way, by a high-speed stirring technique, in which 100 parts by weight of the component (bl) (specific surface area: 0.5 to 5 m2/g) and 2.5 parts by weight of the component (b2) (specific surface area: 5 to 100 m2/g) are charged in a supermixer ("SMV-20" manufactured by Kawata Cho., Ltd.) and stirred for 1 minute, and 2.5 parts by weight of the component (b2) is then fed thereto, followed by stirring for 1 minute. A higher concentration of the component (b2), if desired, can be achieved by repeating this operation.

If desired, the film-forming resin composition may further contain additives, such as heat stabilizers, ultraviolet absorbents, antioxidants, lubricants, dispersing agents, and the like.

The resin composition comprising the abovedescribed components is melt-kneaded in an extruder and molded into a film by blown-film extrusion or T-die extrusion.

In a preferred embodiment of the present invention, the resulting film is stretched at least uniaxially at a temperature lower than the melting point of the polyolefin-based resin, to thereby providing a synthetic resin film having an excellent printability.

The stretching can be carried out at a temperature of from 140 to 1620cur preferably from 150 to 1600C, in case of using polypropylene; from 120 to 1330C, preferably from 125 to 1300C, in case of using high-density polyethylene; from 90 to l100C, preferably from 95 to l050C, in case of using low-density polyethylene; and from 120 to 1400C in case of using polystyrene.

In cases where two or more kinds of polyolefinbased resins are used in combination, the stretching is conducted at a temperature lower than the melting point of at least one of the polyolefin resins.

The stretching is conducted uniaxially in the longitudinal or transverse direction or biaxially in the longitudinal and transverse directions by means of a tenter, a mandrel, or a set of rolls. The stretching ratio - is selected depending on the required film quality, the stretching process employed, and the kind of the material and is, usually, at least 4 times in case of using a tenter; at least 1.3 times in case of using a mandrel; and at least 2.5 times in case of using rolls. As the stretching temperature is lowered and the stretching ratio is increased, the resulting film synthetic paper becomes more opaque.

If necessary, the stretched film may be subjected to surface treatment, such as sizing, coating, corona discharge treatment, etc.

The synthetic resin film according to the present invention can be printed by offset printing and silk screen printing as well as gravure printing, and is useful as coated paper, wrapping paper, bases of levels and stickers, tracing films, decorative paper, back cloth, etc.

The present invention is now illustrated in greater detail by reference to the following Examples and Comparative Examples, wherein parts are by weight.

EXAMPLE 1 In a supermixer ("SMG-100" manufactured by Kawata Co., Ltd.) were charged 75 parts of polypropylene ("Y'-6" produced by Mitsubishi Petrochemical Co., Ltd.; melting point: 1640C), 5 parts of high-density polyethylene ("EY-40" produced by Mitsubishi Petrochemical Co., Ltd.; melting point: 1300C), 16 parts of a heavy calcium carbonate powder ("Softon 1800" produced by Bihoku Funka Kogyo Co., Ltd.; specific surface area: 2 m2/g; average particle size: 1.5 pom), and 4 parts of magnesium carbonate ("GP-20" produced by Konoshima Chemical Kogyo Co., Ltd.; specific surface area: 28 m2/g; average particle size: 0.35 m), and the mixture was stirred for 3 minutes to prepare a resin composition.

The resin composition was melt-kneaded in an extruder, extruded in strands, and cut into pellets.

The pellets were melt kneaded in an extruder, extruded from a die into a sheet at 2400C and, immediately thereafter, cooled with -cooling rolls to 800C to obtain a white opaque film having a thickness of 150 pm and a density of 1.26 g/cm3.

EXAMPLE 2 Pellets were obtained from a resin composition comprising 75 parts of polypropylene MA-6, 5 parts of high-density polyethylene EY-40, 16 parts of heavy calcium carbonate Softon 1800, and 4 parts of magnesium hydroxide ("Kisuma 5B" produced by Kyowa Chemical Co., Ltd.; specific surface area: 8 m2/g; average particle size: 0.70 pom). The pellets were melt-kneaded in an extruder, extruded from a die at 24doC into a sheet and, immediately thereafter, cooled with cooling rolls to 800C to obtain a white opaque film having a thickness of 150 pm and a density of 1.25 g/cm3.

EXAMPLE 3 Polypropylene (MA-6) 80 parts High-density polyethylene (EY-40) 10 parts Heavy calcium carbonate (Softon 1800) 10 parts The above components were melt-kneaded in an extruder and extruded from a die into a sheet at 2000C.

After cooling to about 50 C, the sheet was heated to about 1350C and longitudinally stretched by a set of rolls to a stretching ratio of 5 times.

Separately, 75 parts of MA-6, -5 parts of EY-40, and 4 parts of GP-20 supported on 16 parts of Softon 1800 were stirred in supermixer SMG-100 for 2 minutes.

The resulting blend was melt-kneaded in an extruder at 2400C, extruded into stands, and cut into pellets. The pellets were melt-kneaded in an extruder and extruded from the die in a sheet at 2400C, and the sheet was laminated on both sides of the above-obtained stretched film at 2000C.After once cooling the laminated film to a temperature 200C higher than room temperature, the laminated film was re-heated to about 1500C and transversely stretched by a tenter to a stretching ratio of 8 times, followed by heat setting by passing through an oven at 1600C to obtain a synthetic paper having a three-layered structure composed of a 80 pm thick biaxially stretched film as an intermediate layer and uniaxially stretched films as outer layer (paper-like layers) each having a thickness of 35 pm and a density of 0.80 g/cm3.

EXAMPLES 4 TO 7 AND COMPARATIVE EXAMPLES 1 TO 6 Three-layered synthetic papers were prepared in the same manner as in Example 3, except for using each of resin compositions as shown in Table 1 below as a resin composition for paper-like outer layers.

Each of the synthetic resin- films or papers obtained in Examples 1 to 7 and Comparative Examples 1 to 6 was evaluated for dryness of printing ink as follows. An offset printing ink of an indigo or crimson color ("BEST SP" produced by Toka Shikiso Chemical Industry) was printed on the synthetic resin film or paper by means of an offset printing tester ("RI Tester" manufactured by Akira Seisakusho), and the setting time (the time required for the ink to be dried to such a degree that an offset does not occur to next paper when fingers lightly touched) and the complete drying time (the time required for the ink to be dried to such a degree that an offset does not occur onto the next sheet when upon writing with an HB pencil ) were determined. The results obtained are shown in Table 1.

In order to evaluate pelletization characteristics of the resin composition constituting the paperlike layer, a bite into an extruder screw and an extrusion output were examined and rated according to the following rating system. The results obtained are shown in Table 1.

Good ... Satisfactory Bad ... Poor bite into an extruder screw and small extrusion output In addition, each of the synthetic resin films or synthetic papers was cut to sizes of 636 mm x 936 mm.

The cut sheet was printed with a printing ink of an indigo or crimson color ("BEST SP" produced by Toka Shikiso Chemical Industry) by means of a printing machine ("Dia II", manufactured by Mitsubishi Heavy Industry Ltd.), and the color glooming of the printed ink (printability) was examined and rated as "good" or "bad". The results obtained are also shown in Table 1.

Notes to the headings of the table are as follows: 1): Polypropylene 2): High-density polyethylene 3): "Siloid 244" produced by Fuji Davidson Co., Ltd.; specific surface area: 300 m2/g 4): Setting time 5): Complete drying time TABLE 1 Pellet- Ink Drying Resin Composition (parts) ization Stretched Characteristics Example PP) HDPE) CaCO3 MgCO3 Mg(OH)2 Charac- or Non- Indigo Ink Crimson Ink Print No. (MA6) (EY40) (Softon) (GP-20) (Kisuma5B) Siloid ) teristics Streched ST4) CDT5) ST4) CDT5) abilit@ (min) (min) (min) (min) Example 1 75 5 16 4 - - good non- 120 165 100 150 good stretched Example 2 75 5 16 - 4 - good " 125 170 105 155 good Example 3 75 5 16 4 - - good streched 110 155 95 150 good Example 4 75 5 16 - 4 - good " 120 165 100 150 good Example 5 75 5 9 11 - - good " 105 155 90 145 good Example 6 50 - 35 15 - - good " 90 145 80 130 good Example 7 50 3 35 6 6 - good " 95 150 85 140 good Comparative 75 5 20 - - - good " 210 280 205 240 good Example 1 comparative 75 5 - 20 - - bad " 105 150 85 145 bad Example 2 Comparative 75 5 - - 20 - bad " 115 150 100 150 bad Example 3 TABLE 1 (cont'd) Pellet- Ink Drying Resin Composition (parts) ization Stretched Characteristics Example PP) HDPE) CaCO3 MgCO3 Mg(OH)2 Charac- or Non- Indigo Ink Crimson Ink Print No. (MA6) (EY40) (Softon) (GP-20) (Kisuma5B) Siloid ) teristics Streched ST4) CDT5) ST4) CDT5) ability (min) (min) (min) (min) Comparative 75 5 16 - - 4 bad streched 110 155 90 140 bad Example 4 Comparative 75 5 7 13 - - bad " 110 150 90 145 good Example 5 Comparative 75 5 19.5 0.5 - - good " 200 260 195 225 good Example 6

Claims (6)

CLAIMS:

1. A synthetic film obtained by film-forming a resin composition comprising (A) from 35 to 92% by weight of a polyolefin resin prepared from carbon and hydrogen only as starting materials, optionally mixed with up to 50% of another thermoplastic polymer, and (B) from 8 to 65% by weight of an inorganic fine powder comprising a mixture of (b1 ) 100 parts by weight of a heavy calcium carbonate and (b2 ) from 5 to 150 parts by weight of magnesium hydroxide and/or magnesium carbonate having a specific surface area of 100 m2/g or less, so as to render the surface receptive to ink or writing.

2. A film as claimed in Claim 1, wherein said composition was prepared by incorporating said inorganic powder in which said magnesium hydroxide and/or magnesium carbonate is su;Jported on said heavy calcium carbonate into said polyolefin resin.

3. A film as claimed in Claim 1 or 2, which is an orientated film having been subjected to at least uniaxial stretching at a temperature lower that the melting point of the polyolefin resin.

4. A film as claimed in Claim 1, 2 or 3, wherein up to 50% of the powder b1 was replaced by another inorganic powder of specific surface area of 100 m2./g 0 less and a particle size of 10 micrometres or less.

5. A film as claimed in Claim 1, 2 or 3, substantially as hereinbefore described in any of Examples 1 to 7.

6. A film as claimed in any preceding claim bearing offset ink printing.