WO2008007919A1

WO2008007919A1 - Porous film

Info

Publication number: WO2008007919A1
Application number: PCT/KR2007/003397
Authority: WO
Inventors: Seung Ho Jeon
Original assignee: Polyscientech Inc.
Priority date: 2006-07-14
Filing date: 2007-07-13
Publication date: 2008-01-17
Also published as: KR100712041B1

Abstract

The present invention provides a porous film characteristically prepared by the following steps; preparing a sheet by melt -extruding the mixture of two different biodegradable thermoplastic resins with different glass transition temperatures having the margin of at least 20°C; and stretching the sheet at the temperature ranging from at least the lower glass transition temperature of the two to under the higher glass transition temperature at the area stretching ratio of 1.5 - 10 fold. The porous film of the present invention has excellent physical properties and high moisture vapor permeability as well as economic efficiency owing to thinning in addition to excellent biodegradability which favors environment. Therefore, the porous film of the invention can be effectively used for not only protective absorbent articles such as a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing, medical apparel, etc, but also construction materials including a house wrap, etc.

Description

[DESCRIPTION]

[invention Title] POROUS FILM

[Technical Field]

The present invention relates to a biodegradable porous film that has excellent physical properties and high-moisture permeability in addition to biodegradability and is highly economical owing to thinning.

[Background Art]

The porous film has micro-pores in its inside so that only gas but not liquid can penetrate through. Moisture vapor permeability is measured to evaluate the film capacity using water as test liquid. To obtain a satisfactory level of capacity, moisture vapor permeability has to be at least 500 g/m² • day • atm and more preferably at least 4,000 g/m² • day* atm. Such porous film has been used in a variety of fields to produce protective absorbent articles such as a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing and medical apparel, and to produce dew- proof construction materials such as a house wrap, etc. Various methods have been proposed to produce a porous film. The most applicable and economical method is as follows; an inorganic filler such as calcium carbonate is added to a polyolefin based resin such as polyethylene and polypropylene at a required ratio to give a composition, followed by melt-extrusion. The raw plate sheet obtained from the melt-extrusion of the composition is uniaxial- stretched or biaxial-stretched to make micro-pores in between the polyolefin based resin and the inorganic filler, resulting in a porous film (Japanese Laid-Open Patent Publication Nos . S57-203520, S58-15538, S58-149303, S60- 12940, S60-185803, S60-199037, etc) .

Recently, studies have been actively undergoing to develop a thin- film type porous film with improved economic efficiency. The thickness of such porous film varies from the purpose of use. For example, to produce a porous film applicable for a diaper which is in greater steady demand, a composition containing a polyolefin based resin and calcium carbonate at approximately 50 weight% each is used. This porous film having the weight per unit area of 40 g/m² and the thickness of approximately 70 μm can be used as it is as a back sheet or a very thin polyolefin based porous film having the weight of 20 g/m² and the thickness of approximately 35 μm and a combined product with a polyolefin based non-woven fabric prepared from polypropylene to increase physical strength of the thin film and to enhance tactile properties are widely being used as a back sheet. There are repeated attempts to reduce price per unit area by thinning, which is making a film thinner. However, excessive dose of the filler such as calcium carbonate reduces physical properties of a raw plate sheet and the reduced physical properties continue to decrease after stretching owing to the formed pores. So, the film is easily broken during thinning. Therefore, an alternative method is strongly requested to overcome the problem of being thinner. The porous film is mostly used for disposable products including a diaper, which is once it is used, it is discarded in nature. The problem is that this film is not easily decomposed and thus causes a social problem such as environmental pollution. Therefore, a solution has to be figured out soon. As an effort to solve the above problem, some methods have been proposed, in which a composition comprising a thermoplastic resin such as biodegradable aliphatic polyester and an inorganic filler is melt- extruded and the resultant raw plate sheet is uniaxial- stretched or biaxial-stretched to form micro-porous in between the biodegradable thermoplastic resin and the inorganic filler, resulting in a biodegradable porous film

(Korean Patent Publication Nos . 1995-14215, 1995-18271) .

However, the products produced from the above method are hardly selected in actual industry. The reason is that the price of a biodegradable resin is too high, even though a cheap inorganic filler can be used as a counter part, which is approximately several fold higher than the price of a polyolefin based porous film. Various attempts have been made to reduce the price, for example it has been tried to co-use a comparatively low-price biodegradable resin such as starch or wooden powder. But, the result has not been satisfactory with exhibiting significant decrease of physical properties. As an alternative, it has been tried to reduce the unit price via thinning by the same manner as tried for the polyolefin based porous film above. However, in this attempt, the film was apt to be broken during the stretching process. Therefore, a novel method is in need.

[Disclosure]

[Technical Problem]

To overcome the above problems, the present inventors made various attempts and finally completed this invention.

It is an object of the present invention to provide a highly economical pro-environmental biodegradable porous film via thinning process that has excellent physical properties and high moisture vapor permeability and is produced without an inorganic or an organic filler, unlike the conventional films, by mixing two kinds of biodegradable thermoplastic resins having different glass transition temperatures and by stretching the mixture at the interim temperature between two different glass transition temperatures of the biodegradable thermoplastic resins .

[Technical Solution]

To achieve the above object, the present invention provides a biodegradable porous film which is characteristically produced by the following procedure; 20 - 250 parts by weight of the biodegradable thermoplastic resin (D) having the glass transition temperature [T_g(D)] of at least T₉(C) +20^°C is added to 100 parts by weight of the biodegradable thermoplastic resin (C) having the glass transition temperature of T₉(C); this mixture is mixed and dispersed to make the biodegradable thermoplastic resin C as a continuous phase (matrix) and to make the biodegradable thermoplastic resin D as a disperse phase of

0.1 - 10 μm in domain size; the resultant biodegradable thermoplastic resin composition is melt-extruded to give a sheet; the obtained sheet is stretched at the temperature ranging from at least T_g(C) to under T_g(D) at the area stretching ratio of 1.5 - 10 fold to give the final biodegradable porous film.

According to the present invention, a filler can be selectively added by 1 - 50 parts by weight for 100 parts by weight of the biodegradable thermoplastic resin composition of the invention.

The present inventors confirmed that the continuous phase biodegradable thermoplastic resin (C) was deformed by heat during the stretching process but the disperse phase biodegradable thermoplastic resin (D) was not deformed by heat and an organic filler was well maintained as its particle form was, indicating that the micro-pore formation was maximized in between the continuous phase biodegradable thermoplastic resin and the disperse phase biodegradable thermoplastic resin. And thus the present inventors completed this invention by further confirming that the porous film of the invention has pro-environmental biodegradability and high porosity and moisture vapor permeability owing to the above explained maximized micropore formation.

Thus, the present invention is characterized by producing a porous film in which an inorganic filler is not used, unlike the conventional inventions, but a biodegradable thermoplastic resin having high glass transition temperature playing a role as an organic filler is used.

In the present invention, 'glass transition temperature' indicates the temperature where plasticity is generated, that is, when a thermoplastic resin gets heat and reaches a certain temperature range, its glass-like solid phase is deformed by micro-Brownian motion of its high molecular molecules .

The biodegradable thermoplastic resin C or D of the present invention is exemplified by a biodegradable polyester based resin, a polyurethane based resin, a polyamide based resin, a cellulose based resin, and a modified starch based resin, and more specifically polylactide (Cargill Dow polymers, brand name: Natural Works PLA) , polybutylene succinate adipate copolymer (Showa Highpolymer, brand name: Bionolle) , terpolymer of terephthalic acid, adipic acid and 1, 4-butanediol (BASF, brand name: Ecoflex, Eastman, brand name: Eastar) , polyhydroxyalkanoate (Was Monsanto, brand name: Biopol) , polycaprolactone (UCC, brand name: Tone polymer), polyesterurethane (Bayer, brand name: MHP 9029, Sejung C&M, brand name: Enfresin) , polyesteramide (Bayer, brand name: BAK), cellulose acetate (Mazzucchelli, brand name: Bioceta) , a modified starch based resin (Novamont, brand name: Master Bi, Biop, brand name: Biopar) , etc, and these compounds can be used independently or combined.

According to the present invention, it is preferred to select two different resins, that is a continuous phase biodegradable thermoplastic resin and a disperse phase biodegradable thermoplastic resin, but it is also possible to select two resins among same type resins because even if they are in the same biodegradable thermoplastic resin family, their compatibilities and glass transition temperatures can be different according to the raw material structure and compositions. For example, it is possible to use same type biodegradable polyester based resins for the continuous phase resin and the disperse phase resin respectively as long as their compatibilities and glass transition temperatures can meet the requirement of the invention, because their compatibilities and glass transition temperatures can be different according to constituents and compositions, as explained above. When not a single resin but a biodegradable thermoplastic resin mixture is used as a continuous phase and a disperse phase respectively, the glass transition temperature of the mixture used for each phase has to be checked to judge whether these resins are suitable for the film of the invention or not.

The filler used in the present invention is an inorganic or an organic filler. The inorganic filler of the invention is exemplified by calcium carbonate, talc, clay, kaolin, silica, diatomite, magnesium carbonate, calcium chloride, calcium sulfate, aluminum hydroxide, zinc oxide, magnesium hydroxide, titanium oxide, alumina, mica, asbestos, zeolite, silicate terra alba, etc. The organic filler of the present invention can be selected from the group consisting of starch, wooden powder and pulp, and these compounds can be used independently as a single compound or combined as a mixture . The form of the inorganic filler is board type, needle type or sphere type, and among these sphere type is preferred. The preferable mean diameter for the inorganic filler is 0.01 - 5 IM₁ more preferably 0.5 - 3 μm. The surface is preferably coated or treated with a dispersant such as higher fatty acid or wax to improve dispersibility . In the meantime, the mean diameter of the organic filler is preferably 0.05 - 10 μm and more preferably 1 - 5 μm. There is no need to treat any dispersant to the organic filler because the organic filler has excellent compatibility for the biodegradable thermoplastic resin, compared with the inorganic filler.

The core of the technique adapted for the present invention to produce a porous film having excellent physical properties, high moisture vapor permeability and economic efficiency based on thinning is to introduce a composition with a novel structure composed of a continuous phase biodegradable thermoplastic resin, instead of a conventional inorganic filler which is not desirable for physical properties, and a disperse phase structured composition in similar form with the biodegradable thermoplastic resin organic filler which has appropriate compatibility to the continuous phase biodegradable thermoplastic resin and excellent physical properties. Another key factor of the technique is to introduce a novel conception of forming micro-pores by stretching the disperse phase biodegradable thermoplastic resin mixture that has higher glass transition temperature than the continuous phase biodegradable thermoplastic resin at the temperature range from at least the glass transition temperature of the continuous biodegradable thermoplastic resin to less than the glass transition temperature of the disperse phase biodegradable thermoplastic resin.

First, instead of using a conventional inorganic filler, the present inventors prepared a disperse phase biodegradable thermoplastic resin composition having similar form as an organic filler and excellent physical properties along with appropriate compatibility to a continuous phase biodegradable thermoplastic resin. This structure was confirmed by the observation under an electron microscope. The obtained composition was melt- extruded to give a raw plate sheet, which was then stretched to be a thin film of under 35 μm in thickness, which is the general and most representative thickness of the conventional porous film, by stretching at different temperatures and stretching ratios. As a result, the prepared thin film was confirmed to have excellent physical properties and porosity to some degree. In particular, when the size of the domain of the disperse phase biodegradable thermoplastic resin was 0.1 - 10 μm, the resultant thin porous film exhibited better properties. Nevertheless, the present inventors failed in increasing porosity to the target level, which the inventors believe was because that the disperse phase biodegradable thermoplastic resin could not stand as an organic filler in particle form and was stretched with the continuous phase biodegradable thermoplastic resin as they were in incomplete form and the micro-pore formation was unsuccessful between the continuous phase biodegradable thermoplastic resin and the disperse phase biodegradable thermoplastic resin.

The proposed alternative to overcome the above problem is to maximize micro-pore formation by stretching a mixture of the biodegradable thermoplastic resin forming continuous phase and the biodegradable thermoplastic resin forming disperse phase at a specific temperature range set between the glass transition temperatures of the resins. According to this method, the porosity of the biodegradable thin film was remarkably increased.

This is based on the theory that when stretching is performed at the temperature between two different glass transition temperatures of the resins, the biodegradable thermoplastic resin forming continuous phase is deformed by heat but the thermoplastic resin forming disperse phase is not, and thus the organic filler can keep its particle form best, resulting in the maximization of the micro-pore formation. Therefore, the method using the difference of glass transition temperature enables producing a film with pro-environmental biodegradability and high porosity or moisture vapor permeability.

According to the present invention, the biodegradable thermoplastic resin D forming disperse phase (domain) is preferably added by 20 - 250 parts by weight, more preferably 50 -200 parts by weight, most preferably 70 - 150 parts by weight to 100 parts by weight of the biodegradable thermoplastic resin C forming continuous phase (matrix) . If the content of the resin D is less than 20 parts by weight, the degree of porosity will be unsatisfactory although physical properties will be excellent. If the content is more than 250 parts by weight, porosity will be greatly improved but physical properties might be reduced. According to the present invention, the size of the domain of the disperse phase (matrix) formed in the biodegradable thermoplastic resin composition is preferably 0.1 - 10 μm, more preferably 0.5 - 5 μm and most preferably 1 - 3 μm. If the size of the domain is less than 0.1 μm, satisfactory physical properties will be achieved with keeping the desirable form for best compatibility but porosity will not be good. On the contrary, if the size is more than 10 μm, porosity will be very excellent but physical properties will be reduced. To express the disperse phase (domain) of the above size effectively, it is important to select and co-use an appropriate compatabilizer according to the types and characteristics of a biodegradable thermoplastic resin.

Another key factor of the present invention is the glass transition temperature of the biodegradable thermoplastic resin forming the composition of the invention. The glass transition temperature of the biodegradable thermoplastic resin forming disperse phase

(domain) is preferably 20^°C higher, more preferably 30^°C higher and most preferably 40^°C higher than the glass transition temperature of the biodegradable thermoplastic resin forming continuous phase. If the difference of the glass transition temperature is less than 20^°C, it is very difficult for the biodegradable thermoplastic resin forming disperse phase to keep its form as an organic additive, so that porosity generated in the interface of disperse phase and continuous phase will be in doubt.

According to the present invention, a filler is preferably added by 1 - 50 parts by weight, more preferably 1 - 40 parts by weight and most preferably 1 - 30 parts by weight to 100 parts by weight of the biodegradable thermoplastic resin composition. And the resultant composition is melt-extruded to give a raw plate sheet, which is stretched at the temperature selected in the range from the temperature over the glass transition temperature of the biodegradable thermoplastic resin forming continuous phase and the temperature under the glass transition temperature of the biodegradable thermoplastic resin forming disperse phase at the area stretching ratio of 1.5 - 10 fold and more preferably 2 - 7 fold, resulting in the porous film of the invention. Even though a filler is added, the price of the filler is inexpensive, so the price competitiveness of the product can be excellent. But if the filler is added more than 50 parts by weight, physical properties will be damaged significantly. So, it is not encouraged to use the filler a lot.

The biodegradable thermoplastic resin composition for porous film of the invention can additionally include a general additive such as a lubricant, an antioxidant, a processing aid, a white enhancer, a drying agent, a heat stabilizer, and a fluorescence material, as long as this additive does not affect the purpose or object of the invention.

The biodegradable thermoplastic resin composition for porous film of the invention and the preparation method for the porous film are described in detail hereinafter.

The biodegradable thermoplastic resin forming continuous phase generally in pellet form, the biodegradable thermoplastic resin forming disperse phase, a filler if necessary, and a generally acceptable additive were all mixed. The mixture were properly mixed in a conventional apparatus such as single screw extruder, twin screw extruder, mixing roll, bambury mixer or kneader to prepare the biodegradable thermoplastic resin composition pellet forming disperse phase (domain) structure of 0.1 -

10 μm in domain size on the biodegradable thermoplastic resin forming continuous phase. The obtained biodegradable thermoplastic resin composition pellet was placed in a hopper, followed by molding of a raw plate sheet by the conventional film forming apparatus based on extrusion. At this time, combination of inflation molding by round die and extrusion by T-die is preferred. The obtained raw plate sheet was stretched at the temperature selected from the range from the temperature over the glass transition temperature of the biodegradable thermoplastic resin forming continuous phase and the temperature under the glass transition temperature of the biodegradable thermoplastic resin forming disperse phase by uniaxial stretching or biaxial stretching, at the area stretching ratio of 1.5 - 10 fold, more preferably 2 - 7 fold, resulting in the porous film of the invention. The area stretching ratio is necessarily regulated considering the characteristics of a resin and the target level of porosity. If the area stretching ratio is less than 1.5 fold, porosity is not secured. On the contrary, if the area stretching ratio is excessively high (more than 10 fold) , fracture phenomenon, occurs frequently during stretching, suggesting that a film producing process is not stable. After stretching, a film might need heat-set to improve dimensional stability.

To improve tactile sensation and appearance of the porous film of the invention, embossing can be performed and this film can be used as a laminate laminated with a thermoplastic non-woven fabric made by a polypropylene based resin or polylactide by heat treatment or hot-melt.

The porous film prepared by the method of the invention can be applied to various products including protective absorbent articles such as a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing and medical apparel, and to produce dew-proof construction materials such as a house wrap, etc.

As explained hereinbefore, to produce the porous film of the invention, a novel biodegradable thermoplastic resin composition having the structure wherein a biodegradable thermoplastic resin forms a continuous phase (matrix) and another biodegradable thermoplastic resin having the glass transition temperature which is at least 20 ^"C higher than that of the first resin at a regular size is melt-extruded, resulting in a raw plate sheet. Then, the sheet is stretched at the temperature selected from the temperature range from the temperature higher than the glass transition temperature of the biodegradable thermoplastic resin forming continuous phase to the temperature lower than the glass transition temperature of the biodegradable thermoplastic resin forming disperse phase, at the area stretching ratio of 1.5 - 10 fold, more preferably 2 - 7 fold. As a result, the invention not only provides a porous film with excellent physical properties even after thinning but also brings an economic effect by reducing the material cost. During stretching, the biodegradable thermoplastic resin forming continuous phase is deformed by heat, but the biodegradable thermoplastic resin forming disperse phase is not deformed by heat and instead, its particle form as an organic filler is well maintained, resulting in the maximization of the micro-pore formation between the biodegradable thermoplastic resins. Therefore, the present invention proudly provides a porous film that has pro- environmental biodegradability and high porosity or moisture vapor permeability. [Best Mode]

Hereinafter, the present invention is described in detail with preferred embodiments. These embodiments are examples to illustrate the present invention and to help understanding of the invention and thus cannot limit the spirit or scope of the present invention.

The average size of the domain of the biodegradable thermoplastic resin composition prepared according to the following examples and comparative examples and the thickness, tensile property and moisture vapor permeability of the porous film were measured as follows.

(Average size of the domain) A picture of the raw plate J sheet obtained from

I extrusion of the biodegradable thermoplastic resin pellet was taken by SEM and then the average size ! of the domain particle was measured by Image Analyzer (AX70 1W/ACC, Olympus Optical Co.) .

(Thickness)

The thickness of a sample film was measured by film gauge meter (Toyoseiki Co.) . (Tensile property)

Based on ASTM D882, the fracture strength (kg/in²) and elongation (%) of a sample were measured in the machine direction (MD) and in the transverse direction (TD, perpendicular to the MD) (sample size: 100 mm x 25 mm, tensile speed: 500 mm/min, temperature: 23±1^°C).

(Moisture vapor permeability)

Moisture vapor permeability g/m^{2 •} 24hr ^• atm was measured according to ASTM E96-63 (23±1^°C, 90% RH).

[Example 1]

As a biodegradable thermoplastic resin forming continuous phase, a well-dried pellet type biodegradable polyester-urethane (Sejung C&M, brand name: Enfresin, resin A2; density: 1.18g/cm³, melting point: 105 ^°C, glass transition temperature: -40^°C) was prepared. And as a biodegradable thermoplastic resin forming disperse phase, a well-dried pellet type biodegradable polylactide (Cargill Dow polymers, brand name: NaturalWorks PLA, resin B2 ; density: 1.24g/cm³, melting point: 210 ^°C , glass transition temperature: 65^°C) was prepared. The resin A2 and the resin B2 were mixed at the ratio of 100:100 (parts by weight) . The mixture was placed in a twin screw extruder (screw diameter: 120 mm, L/D 32) , followed by melt-extruding to give a biodegradable thermoplastic composition pellet. The pellet was put in a hopper, followed by molding in a T-die extruder to give a raw plate sheet . The molded raw plate sheet was uniaxial-stretched (stretching temperature: 55^°C, area stretching ratio: 3.0 fold), resulting in a thin biodegradable porous film of 20 μm in thickness. The properties of the film are shown in Table 1.

[Example 2] As a biodegradable thermoplastic resin forming continuous phase, a well-dried pellet type biodegradable polyester-urethane (Sejung C&M, brand name: Enfresin, resin A2; density: 1.18g/cm³, melting point: 105 ^°C, glass transition temperature: -40^°C) was prepared. And as a biodegradable thermoplastic resin forming disperse phase, a well-dried pellet type biodegradable polylactide (Cargill Dow polymers, brand name: NaturalWorks PLA, resin B2 ; density: 1.24g/cm³, melting point: 210^°C, glass transition temperature: 65 ^"C) was prepared. The resin A2 and the resin B2 were mixed at the ratio of 100:70 (parts by weight). A porous film was prepared by the same manner as described in example 1 except the stretching was performed at 4.0 fold of the area stretching ratio by uniaxial stretching. The properties of the prepared thin biodegradable porous film of 15 μm in thickness are shown in Table 1.

[Example 3]

As a biodegradable thermoplastic resin forming continuous phase, a well-dried pellet type biodegradable polyester-urethane (Sejung C&M, brand name: Enfresin, resin

A2; density: 1.18g/cm³, melting point: 105 ^°C, glass transition temperature: -40^°C) was prepared. And as a biodegradable thermoplastic resin forming disperse phase, a well-dried pellet type biodegradable polylactide (Cargill Dow polymers, brand name: NaturalWorks PLA, resin B2; density:

1.24g/cm³, melting point: 210^°C, glass transition temperature: 65"C) was prepared.

The resin A2 and the resin B2 were mixed at the ratio of 100:40 (parts by weight) . A porous film was prepared by the same manner as described in example 1 except the stretching was performed at 2.5 fold of the area stretching ratio by biaxial stretching. The properties of the prepared thin biodegradable porous film of 10 /zm in thickness are shown in Table 1.

[Example 4]

As a biodegradable thermoplastic resin forming continuous phase, a well-dried pellet type biodegradable polyester-urethane (Sejung C&M, brand name: Enfresin, resin A2; density: 1.18g/cm³, melting point: 105 "C, glass transition temperature: -40 ^"C) was prepared. And as a biodegradable thermoplastic resin forming disperse phase, a well-dried pellet type biodegradable polylactide (Cargill Dow polymers, brand name: NaturalWorks PLA, resin B2 ; density: 1.24g/cm³, melting point: 210 ^°Q glass transition temperature: 65 TJ was prepared. Non-treated calcium carbonate (mean diameter: 1.7 μm, Omiya, calcium carbonate A) was used as a filler. 100 parts by weight of the resin A2, 50 parts by weight of the resin B2 and 20 parts by weight of calcium carbonate A were mixed in Henschel mixer. The mixture was placed in a twin screw extruder (screw diameter: 120 mm, L/D 32) , followed by melt-extruding to give a biodegradable thermoplastic composition pellet. The pellet was put in a hopper, followed by molding in a T-die extruder to give a raw plate sheet. The molded raw plate sheet was uniaxial- stretched (stretching temperature: 55 "C, area stretching ratio: 3.5 fold), resulting in the thin biodegradable porous film of 20 μm in thickness. The properties of the film are shown in Table 1.

[Comparative Example 1]

As a biodegradable thermoplastic resin forming continuous phase, a well-dried pellet type biodegradable polyester-urethane (Sejung C&M, brand name: Enfresin, resin A2; density: 1.18g/cm³, melting point: 105 ^°Q glass transition temperature: -40 ^°Q was prepared.

100 parts by weight of the resin A2 and 70 parts by weight of the non-treated calcium carbonate (mean diameter: 1.7 μm, Omiya, calcium carbonate A) were well mixed in Henschel mixer. The mixture was placed in a twin screw extruder (screw diameter: 120 mm, L/D 32) , followed by melt- extruding to give a biodegradable thermoplastic composition pellet. The pellet was put in a hopper, followed by molding in a T-die extruder to give a raw plate sheet. The molded raw plate sheet was uniaxial-stretched (stretching temperature: 50 "Q area stretching ratio: 4.0 fold), resulting in the thin biodegradable porous film of 15 μm in thickness. The properties of the film are shown in Table 1.

[Table 1]

As shown in the above examples, the porous film of the present invention is thinner than any other conventional porous films. Nevertheless this film has excellent physical properties and moisture vapor permeability at least 4,000 g/m^{2 •} 24hr ^• atm as well. As mentioned hereinbefore, in this invention, an inorganic filler was not used. Instead, a novel biodegradable thermoplastic resin composition having an organic filler like structure that is able to express a disperse phase structure and having an appropriate compatibility to the biodegradable thermoplastic resin forming continuous phase and excellent physical properties was prepared, which is also characterized by maximized micropore formation between the biodegradable thermoplastic resin forming continuous phase and the biodegradable thermoplastic resin forming disperse phase that was attributed to the principle that biodegradable thermoplastic resin forming continuous phase was easily deformed by heat but the biodegradable thermoplastic resin forming disperse phase was hardly deformed and maintained particle status as an organic filler, by which the inventors presumed the film product of the invention can have excellent physical properties and high moisture vapor permeability.

According to example 1 - example 4, the film keeps its excellent physical properties and moisture vapor permeability under various conditions including different compositions, stretching ratios and thinning conditions. On the other hand, according to comparative example 1, the composition containing an equal amount of the inorganic filler instead of the biodegradable thermoplastic resin forming disperse phase exhibited acceptable moisture vapor permeability but poor physical properties.

[industrial Applicability] The present invention relates to a biodegradable porous film prepared by the following steps: two different biodegradable thermoplastic resins having different glass transition temperatures (the margin is at least 20 TJ are mixed; this mixture is mixed and dispersed; the two biodegradable thermoplastic resins included in the resultant biodegradable thermoplastic resin composition express respectively the continuous phase (matrix) and the disperse phase (domain) structures in which the size of the domain is 0.1 - 10 μm,- this composition is melt-extruded to give a raw plate sheet; and the sheet is stretched at a required temperature at the area stretching ratio of 1.5 - 10 fold. The porous film of the present invention has excellent mechanical properties and high moisture vapor permeability as well as economic efficiency owing to thinning in addition to excellent biodegradability which favors environment. So, the porous film of the invention can be effectively used for not only protective absorbent articles such as a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing, medical apparel, etc, but also construction materials including a house wrap, etc.

Claims

[CLAIMS]

[Claim l]

A biodegradable porous film, prepared by the following processes; 20 - 250 parts by weight of the biodegradable thermoplastic resin (D) having the glass transition temperature [T₉(D)] of at least T_g(C)+20°Cis added to 100 parts by weight of the biodegradable thermoplastic resin (C) having the glass transition temperature of T₉(C) ; this mixture is mixed and dispersed to make the biodegradable thermoplastic resin C as a continuous phase (matrix) and to make the biodegradable thermoplastic resin D as a disperse phase of 0.1 - 10 μm in domain size; the resultant biodegradable thermoplastic resin composition is melt- extruded to give a sheet; the obtained sheet is stretched at the temperature ranging from at least T₉(C) to under T₉(D) at the area stretching ratio of 1.5 - 10 fold to give the final biodegradable porous film.

[Claim 2] The biodegradable porous film according to claim 1, wherein the biodegradable thermoplastic resin is one or more compounds selected from the group consisting of a biodegradable polyester based resin, a polyurethane based resin, a polyamide based resin, a cellulose based resin, and a modified starch based resin. [Claim 3]

The biodegradable porous film according to claim 2, wherein the biodegradable thermoplastic resin is one or more compounds selected from the group consisting of polylactide, polybutylene succinate adipate copolymer, terpolymer of terephthalic acid, adipic acid and 1,4- butanediol, polyhydroxyalkanoate , polycaprolactone, polyesterurethane, polyesteramide, cellulose acetate, and a modified starch based resin.

[Claim 4]

The biodegradable porous film according to claim 1, wherein a filler is added by 1 - 50 parts by weight for 100 parts by weight of the biodegradable thermoplastic resin composition.

[Claim 5]

The biodegradable porous film according to claim 4, wherein the filler is one or more compounds selected from the group consisting of calcium carbonate, talc, clay, kaolin, silica, diatomite, magnesium carbonate, calcium chloride, calcium sulfate, aluminum hydroxide, zinc oxide, magnesium hydroxide, titanium oxide, alumina, mica, asbestos, zeolite, silicate terra alba, starch, wooden powder and pulp .

[Claim 6]

A laminate prepared by laminating the biodegradable porous film selected from those of claim 1 - claim 5 with a non-woven fabric comprising a thermoplastic resin.

[Claim 7]

A protective absorbent article characteristically containing the biodegradable porous film selected from those of claim 1 - claim 5.

[Claim 8]

A protective absorbent article characteristically containing the laminate of claim 6.

[Claim 9]

The protective absorbent article according to claim 7, wherein the protective article is selected from the group consisting of a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing and medical apparel.

[Claim lθ] The protective absorbent article according to claim 8, wherein the protective article is selected from the group consisting of a diaper, a toilet training inner-pant, a hygienic band, a sanitary pant, an incontinence device, a wound dressing and medical apparel.

[Claim 11]

A construction material characteristically containing the biodegradable porous film selected from those of claim 1 - claim 5.

[Claim 12]

A construction material characteristically containing the laminate of claim 6.

[Claim 13]

The construction material according to claim 11, wherein the construction material is a house wrap.

[Claim 14] The construction material according to claim 12, wherein the construction material is a house wrap.

[Claim 15]

A preparing method of the biodegradable porous film comprising the following steps: (a) preparing a biodegradable thermoplastic resin composition pellet by mixing and dispersing of the mixture of 20 - 250 parts by weight of the biodegradable thermoplastic resin D having the glass transition temperature of T₉(C) +20 ^°C and 100 parts by weight of the biodegradable thermoplastic resin C having the glass transition temperature of T_g(C), wherein the biodegradable thermoplastic resin C forms a continuous phase (matrix) and the biodegradable thermoplastic resin D forms a disperse phase (domain) of 0.1 - 10 μm in domain size;

(b) preparing a sheet by melt-extruding the above biodegradable thermoplastic resin composition pellet; and

(c) preparing a biodegradable porous film by stretching the above sheet at the temperature ranging from at least T_g(C) to under T_g(D) at the area stretching ratio of 1.5 - 10 fold.