NO171456B - PROCEDURE FOR MAKING A PORO'S MOVIE - Google Patents

Description

The present invention relates to an improved method for producing a porous film. More specifically, it relates to a method for producing a porous film which comprises mixing a polyolefin resin with barium sulfate as a filler, melting the resulting resin material and forming it into a film which is then stretched at least uniaxially.

There are a large number of well-known processes for making porous films by forming a resin material comprising a polyolefin resin and any of a variety of incompatible fillers into a film which is then stretched. For example, Japanese Laid-Open Publications 47334/82 and 203520/82 describe a method for producing porous films which comprises melting a resin material obtained by mixing a polyolefin resin with a filler and liquid rubber or a hydroxylated poly-saturated hydrocarbon, forming the molten resin material into a sheet or film, and stretching said sheet or film. Furthermore, Japanese Laid-Open No. 15538/8 3 describes a method for producing porous films which comprises melting resin material obtained by mixing a linear LD polyethylene resin with a filler and a liquid or waxy hydrocarbon polymer, forming the melted resin material into a sheet or film , and then stretching this sheet or film. However, the films produced by these methods are disadvantageous by

that they exhibit surface tack due to the above-mentioned component contained in addition to polyolefin resin-

sen and the filler, and in that they can only be used in practice in relatively large thicknesses due to their low mechanical strength.

Furthermore, it is described in Japanese publication publication 149303/83 that such porous films can be used as

a leak-proof sheet in disposable nappies. Porous films for use as the leakproof sheet in a disposable diaper are prepared by mixing 100 parts by weight of a polyolefin resin with 28 to 200 parts by weight of a filler and 10 to

70 parts by weight of a liquid or waxy hydrocarbon polymer, forming the resulting resin material into a film, and then stretching that film at least uniaxially by a factor of 1.2 or greater. However, this method for producing porous films has the disadvantage that some types of fillers give poor stretchability and thus do not give fully even pores, and that the resulting film is prone to causing unacceptable noise. Furthermore, the simultaneous use of a hydrocarbon polymer makes this process unsatisfactory because the hydrocarbon polymer tends to form a coating on the film surface and induce a tacky feel.

Such porous films can also be used as leak-proof sheets in volumes. Conventionally, a sheet of paper which has been made liquid impermeable by means of a synthetic resin such as polyethylene has been used for this purpose. However, the resulting napkins have the disadvantage of causing an unsatisfactory feel during prolonged use due to their lack of permeability to water vapor.

An aim of the present invention is to provide an improved method for producing a porous film.

Another aim of the invention is to provide a method for producing a porous film which has sufficiently high porosity and which thereby exhibits high moisture permeability and gas permeability while maintaining excellent water resistance.

Another object of the invention is to provide a method for producing a porous film which is free from surface tackiness, which has excellent softness and which exhibits little reduction in strength.

A further aim of the invention is to provide a method for producing an improved leak-proof sheet for use in disposable nappies and the like.

According to the invention, there is provided a method for producing a porous film, comprising a method for producing a porous film for use as a leak-proof sheet in disposable diapers and the like, comprising melting a resin material consisting mainly of a polyolefin resin and an incompatible filler of medium particle diameter in the range of 0.1 to 7 micrometers, forming the molten resin material into a film and then stretching the film at least uniaxially by a factor of 1.5 to 7, characterized in that barium sulfate is used as a filler in an amount of 50 to 500, preferably 100 to 400, parts by weight per 100 parts by weight of the polyolefin resin.

The phrase "a resin material consisting essentially of a polyolefin resin and; 50 to 500, preferably 100 to 400, parts by weight of barium sulfate per 100 parts by weight of the polyolefin resin" as used herein indicates that the resin material may further contain (1) at least one additive selected from common stabilizers , antioxidants, dyes, UV light absorbers and lubricants and/or (2) in addition to the barium sulfate, other inorganic fillers (such as calcium carbonate and the like) or common inorganic and organic modifiers in an amount that is less than the amount of barium sulfate used (e.g., not more than 20% based on the amount of barium sulfate used), but the addition of liquid rubber, a hydroxylated polysaturated hydrocarbon, or a hydrocarbon polymer as described in the above Japanese Laid-Open Publications 47334/82, 203520/82 , 15538/83 and 149303/83 must be avoided to obtain a porous film free from surface tackiness.

According to the invention, porous films which are free from surface stickiness and which have excellent properties and which have so far not been possible to obtain according to the state of the art, can be produced without the use of any of the above-described additives used according to the state of the art. This can be achieved simply by specifying the type of filler, its amount used and its average particle diameter, preferably using an LD polyethylene resin with a specific melt index and density and preferably a linear LD polyethylene resin (in particular a linear LD polyethylene resin comprising a copolymer of ethylene and hexene and/or octene) which preferably have a specific melt index and density, and stretching the film by a specified factor.

The polyolefin resins that can be used according to the invention include homopolymers such as polypropylene, LD polyethylene, HD polyethylene, linear LD polyethylene, polybutylene, etc; copolymers such as ethylene-propylene copolymer, ethylene-butylene copolymer, ethylene-vinyl acetate copolymer etc. and mixtures thereof.

Among the resins, LD polyethylene resins with a melt index of 0.5 to 7 (0.5 to 8.5 for linear LD polyethylene resins) and a density of 0.915 to 0.9 are preferred. Although it is preferred to use a single resin, any mixture of polyethylene resins of different densities can be used. Preferably, the melt index is in the range of 1 to 5. If the melt index is less than 0.5 or greater than 7 (8.5 for linear LD polyethylene resins), the resin will have significantly reduced formability into a film and may fail to form a film with small and uniform thickness.

If, on the other hand, the density is lower than 0.915 or higher than 0.935, the resin will have reduced stretchability and increased stiffness and may fail to form a soft film.

Particularly preferred polyolefin resins are linear LD polyethylene resins. Linear LD polyethylene resins are copolymers of ethylene and one or more α-olefins, and differ from LD polyethylene resins produced by conventional high-pressure processes. Linear LD polyethylene resins are produced by the low pressure process, and useful α-olefins include butene, hexene, octene, decene and the like. The difference between high pressure manufactured LD polyethylene resins and low pressure manufactured LD polyethylene resins lies in the fact that when viewed from a chemical structure point of view, the former are highly branched polymers while the latter are straight chain polymers. Among such linear LD polyethylene resins are copolymers

of ethylene and hexene and/or octene particularly preferred.

When practicing the invention, barium sulphate is used which has an average particle diameter of 0.1 to 7 micrometres and preferably 0.5 to 5 micrometres. If the average particle diameter is less than 0.1 micrometer, well-defined pores cannot be obtained. If, on the other hand, it is greater than 7 micrometres, the resulting film will have poor stretchability and well-defined pores can therefore be difficult to obtain, as is the case where the average particle diameter is too small.

The barium sulphate is used in an amount of 50 to 500 parts by weight and preferably 100 to 400 parts by weight per 100 parts by weight of the polyolefin resin. If the amount of barium sulfate used is less than 50 parts by weight, sufficiently high porosity cannot be obtained, while if it is greater than 500 parts by weight, the resulting film cannot be fully stretched due to its increased stiffness and will therefore exhibit a reduction in porosity.

It is preferred to subject the barium sulfate to a surface treatment with a fatty acid or a metallic salt thereof, silicone, silane, a resin acid or the like because this treatment is effective in improving its dispersibility in the resin and forming well-defined pores.

Provided that the effect of the film produced according to the present invention is not weakened, other inorganic fillers such as calcium carbonate and similar or common inorganic and organic modifiers can be used in addition to the barium sulfate. However, these additives must be used in an amount not greater than 20% based on the amount of barium sulfate used.

The present method for producing a porous film will be specifically described in the following.

As required, at least one additive selected from stabilizers, antioxidants, colorants, UV light absorbers and lubricants is added to a polyolefin resin and barium sulfate. These ingredients are mixed with a Henschel mixer, super mixer or drum mixer. Using a conventional single-screw or twin-screw extruder, the resulting mixture is then blended and pelletized. Using an inflation extruder or T-die extruder, these pellets are then melted (alone or mixed with polyolefin resin pellets) at a temperature higher than the melting point of the polyolefin resin (preferably 20°C or more above it) and lower than the cleavage temperature, and formed into a film . In some cases, the above-mentioned mixture can be formed directly into a film with an extruder instead of being pelletized. The film is then stretched at least uniaxially by a factor of 1.5 to 7 according to conventional techniques such as roller stretching, frame stretching or the like. This stretching can be done in steps and/or in two or more directions. When it comes to biaxial stretching, however, it is advantageous to stretch the film simultaneously in both directions. To increase the morphological stability of pores, the stretched film can be hardened by heating.

The porosity is determined by the amount of barium sulphate used, the stretch factor and the like. If the stretch factor is less than 1.5, sufficiently high porosity cannot be obtained, while if it is more than 7, a porous film cannot be uniformly produced due to its frequent breakage during the stretching process.

Porous films produced by the method according to the invention are characterized by high porosity, excellent softness and little reduction in strength.

Furthermore, the good affinity between the polyolefin resin and the barium sulfate provides good stretchability, not only good workability but also an even distribution of pores can be achieved, and a porous film can therefore be produced stably. In particular, when a linear LD polyethylene resin is used as the base resin, the resulting film will show very little reduction in strength. Thus, it is possible to produce porous films that are thinner (e.g. approx. 10 micrometers in thickness) than those produced by known processes.

Furthermore, since the resin material does not contain liquid rubber, hydroxylated poly-n»ated hydrocarbon and hydrocarbon polymers such as are used in the known processes, the resulting porous films are free from surface stickiness.

Thus, the porous films produced according to the invention have sufficiently high porosity and thus exhibit good moisture permeability and gas permeability while retaining excellent water resistance so that they can be used in the clothing and sanitary area. In addition, they can also be used as a filter medium due to their even distribution of pores.

The invention is further illustrated in the following examples.

In the examples, the melt index (MI) was determined according to ASTM D-12 38 and the density was determined according to

ASTM D-1505.

In the examples, the film properties were determined according to the following procedures:

(1) Strength and elongation

Using a Tensilon testing machine, a 25 mm x 100 mm white film was tested at a tensile rate of 200 mm/min. Its strength and elongation at break were determined with respect to the machine direction (MD) and the opposite direction (TD).

(2) Moisture permeability

The moisture permeability was tested according to

ASTM E96 (Method D).

(3) Softness

The softness was assessed emotionally and graded according to the following criteria:

A = very soft and smooth

B = relatively soft and smooth

C = hard and rough

Examples 1 - 19 and Comparative examples 1 - 9

Each of the fillers listed in Table 1 was added

to the corresponding base resin in the amount indicated in Table 1, and mixed with this using a Henschel mixer. Using a twin screw mixer, the resulting mixture was then intimately mixed and formed into pellets. Using a T-die extruder, these pellets were then melted at a temperature of 80°C higher than the melting point of the base resin and formed into a film. This film was stretched uniaxially or biaxially (Example 3) by the factor indicated in Table 1, forming a porous film of the thickness indicated in Table 1. However, the film was not stretched in Comparative Example 1 and could not stretched to a porous film in comparative examples 2 and 9. In comparative examples 5 and 8 the film could only be stretched by a factor up to 2. In comparative examples 3 and 7 no testing was possible due to frequent breakage during the stretching process.

The strength, elongation, moisture permeability and softness of the porous films thus obtained were determined according to the procedures described above, and the results are given in Table 1.

Porous films produced by the method according to the invention have high porosity and excellent softness and furthermore show little reduction in strength, so that these are very suitable for use as a leak-proof sheet in disposable nappies. Although such a leak-proof sheet is generally used as the outermost layer of a disposable diaper, a material (such as a conventional die-cut film or sterically imprinted sheet) which does not impair its permeability can be placed on top of the outer side of the leak-proof sheet to provide a cloth-like look.

In such disposable diapers, any of the usual liquid absorbents can be used, including down composed of pulp fibers such as down material wrapped in absorbent paper or the like, polymeric absorbent materials that have a high water-absorbing ability and the like.

As the liquid-permeable sheet which will be in direct contact with the skin, a non-woven fabric material composed of polyester fibres, nylon fibres, poly-olefin fibers or the like can preferably be used.

In addition, pressure-sensitive tape can be used to secure the nappy, and elastic pieces (such as pieces of rubber) provided along the side edges to prevent leakage.

Disposable diapers are produced by placing a liquid-absorbing material on the above-mentioned leak-proof sheet, after which a liquid-permeable sheet is placed thereon.

In disposable diapers where the porous film produced according to the invention is used as the leak-proof sheet, the outermost leak-proof sheet has a large number of pores. As these pores allow water vapor to pass while small water droplets are retained, the child's skin is not wet, but kept in a dry state, resulting in a slight tendency to develop nappy rash. Furthermore, it also has the advantage that they can hardly be torn apart due to their high strength and are furthermore sufficiently strong not to cause unwanted noise.

The following examples illustrate the use of the porous film produced according to the invention as a leak-proof sheet in disposable nappies.

Examples 20 - 22 and Comparative examples 10 - 14

Barium sulfate with an average particle diameter of 5.5 micrometers or each of the other fillers listed in Table 2 was added to 100 parts by weight of LD polyethylene (LDPE) with a melt index (MB) of 3 (Examples 20 and 21) or linear LD polyethylene (L- LDPE) with a melt index (MI) of 5 (Example 22 and Comparative Examples 10-14) in the amount indicated in Table 2, and were mixed using a Henschel mixer. Using a twin-screw mixer, the resulting mixture then intimately mixed and formed into pellets.Using a T die extruder, these pellets were then melted at 130°C and formed into a film.This film was uniaxially stretched between a heat roll heated to 50°C and a stretch roll with a factor as indicated in Table 2, while forming a porous film with a thickness of 50 micrometers. The properties of the thus obtained porous film were determined and the results are shown in Table 2. Disposable diapers were produced by placing a filling of the down pulp and a non-woven polyester cloth on each of the porous films obtained in Examples 20-22 and Comparative Examples 10, 13 and 14, which were then fitted with pressure-sensitive tape and rubber parts.

The disposable diapers according to examples 20 - 22 were superior

in strength, moisture permeability and softness compared to those of Comparative Examples 10, 13 and 14, so that they produced only a faint crackling sound and felt pleasant to the touch. When these disposable diapers were tested in practical use on children, they did not cause any rashes on the skin according to examples 20-22. In contrast, disposable nappies according to comparative examples 10, 13 and 14 caused severe rash or minor rash (comparative example 13).

Example 23

Using a biaxial stretching machine heated to 70°C, the stretched film formed in Example 20 was simultaneously stretched both in the direction of the machine and in the opposite direction (by factors of 2 x 2) to form a porous film with a thickness of 50 micrometers. The properties of this porous film were determined and the results are shown in Table 2. Disposable diapers using this porous film as the leakproof sheet exhibited good performance such as those of Example 20.

Example 24

The same resin material used in Example 22 was formed into a film. Using a roll heated to 50°C, this film was uniaxially stretched by a factor of 4 to form a porous film with a thickness of 15 microns. The properties of this porous film were determined and the results are shown in Table 2.

On the outer side of this porous film was placed an LDPE film with a thickness of 70 micrometers with openings of 1 mm diameter (20/cm 2 ) over the entire surface. Disposable diapers using this composite material as the leakproof sheet also performed as well as those of Examples 20-23.

Samiti equation example 15

120 parts by weight of calcium carbonate with an average particle diameter of 1.2 micrometers and 20 parts by weight of a hydroxylated poly-saturated hydrocarbon (liquid polybutadiene GI-2000) were added to 100 parts by weight of linear LD polyethylene (L-LDPE) with a melt index (MI) of 5 , and were mixed using a Henschel mixer. Using a double scrubber, the resulting mixture was then intimately mixed and formed into pellets. Using a 40 mm inflation extruder, these pellets were formed into a film. The film was roll stretched at 80°C by a factor of 3.0 to form a porous film with a thickness of 50 microns. This porous film varied in moisture permeability according to location and exhibited a slight degree of surface tackiness. Disposable diapers made using this porous film as the leak-proof sheet produced small rashes on the skin.

Comparative example 16

The procedure described in comparative example 15 was repeated with the exception that liquid polybutadiene or rubber EPR was used as hydroxylated poly-saturated hydrocarbon.

Films with a thickness of

50 micrometers. These porous films exhibited surface tackiness and varied in moisture permeability according to location. Disposable diapers made using each of these porous films as the leakproof sheet produced mild skin rashes.

Porous films obtained by the method according to the invention have high porosity and excellent softness, and furthermore show little reduction in strength despite the small thickness, so that they are very suitable for use as a leak-proof sheet in binders. Conventional sanitary napkins are constructed so that liquid absorbent material such as down pulp, cotton, absorbent resin or the like is partially covered with a film of paper which has been made liquid impermeable by treatment with a synthetic resin such as polyethylene or the like, and the resulting structure is then wrapped in a non-woven cloth. In sanitary napkins using the porous film produced according to the invention as the leak-proof sheet, this leak-proof sheet has a large number of pores which enable water vapor to pass. Consequently, these can keep the skin in a dry state and cause no unsatisfactory sensation even during prolonged use.

The following examples illustrate the use of the porous film produced according to the invention as a leak-proof sheet in sanitary napkins.

Examples 25 - 27

Barium sulfate with an average particle diameter of 0.8 micrometers was added to 100 parts by weight of linear LD polyethylene (L-LDPE) having a melt index (MI) of 2.1 in the amount indicated in Table 3, and was mixed using of a Henschél mixer. Using a double scrubber, the resulting mixture was then intimately mixed and formed into pellets. Using a T-die extruder, these pellets were then melted at 230°C and formed into a film. This film was

o uniaxially stretched between a heating roller heated to 80 C

and a stretch roller with a factor as indicated in Table 3, forming a porous film with a thickness of 20 micrometers. The properties of this porous film were determined and the results

are shown in Table 3. Sanitary napkins were prepared by partially covering a downed pulp filler with each of the porous films obtained in Examples 25 - 27, wrapping the resulting structure in a nonwoven fabric material, after which its overlapped portions were heat sealed. When these napkins and commercially available napkins with a liquid-impermeable film of polyethylene-coated paper were comparatively tested by using them practically for longer periods of time, the napkins produced according to the invention did not produce an unsatisfactory tight feeling.

Claims (1)

Process for producing a porous film for use as a leak-proof sheet in disposable diapers and the like, comprising melting a resin material consisting mainly of a polyolefin resin and an incompatible filler having an average particle diameter in the range of 0.1 to 7 micrometers, forming the melted resin material into a film and then stretching the film at least uniaxially by a factor of 1.5 to 7, characterized in that barium sulfate is used as a filler in an amount of 50 to 500, preferably 100 to 400, parts by weight per 100 parts by weight of the polyolefin resin.