GB2476083A - Manufacturing a breathable polymer film - Google Patents

Abstract

A method for manufacturing a breathable polymer film comprises the steps of reacting gypsum (CaSO4) with carbon dioxide in the presence of water to provide calcium carbonate (CaCO3) isolating the calcium carbonate, mixing the calcium carbonate with a polymer resin, forming the mixture into a precursor film and stretching the precursor film to form a breathable polymer film 10. Stretching the precursor film causes the particles of calcium carbonate filler to break, creating pores in the film. The reaction between the gypsum and carbon dioxide may take place in the presence of an ammonia containing catalyst. The mixture of calcium carbonate and polymer resin can further contain one or more additional inorganic fillers, such as barium sulfate, and/or one or more additional organic components, such as castor oil. In another aspect the breathable polymer film is incorporated into a disposable article 100, preferably as a backsheet 120 thereof. The disposable article maybe diapers, incontinence guards, medical dressings etc.

Description

METHOD FOR MAKING POLYMER FILM

TECHNICAL FIELD

The present invention relates to an environmentally-friendly method for manufacturing breathable polymer films. Such films find application in absorbent articles, in particular as backsheets for such articles.

BACKGROUND OF THE INVENTION

Polymer films which allow air and vapour to pass through them by virtue of their pores are known. The pores in such films can be introduced in a number of ways, including mechanical means or the application of heat or an electric discharge.

Another way to introduce pores into polymer films is by mixing particles of a brittle filler material in the polymer resin. A precursor film is formed from the resin, and then the precursor film is stretched, causing the particles of the filler material to break. This in turn creates fine pores in the film, which extend from one face of the film to the other, making it breathable. The fine pores in the breathable polymer film also discourage the passage of liquid, making such films suitable in situations where liquid-impermeable, yet breathable films are required (e.g. backsheets of absorbent articles). Methods for forming breathable films in this way are described inter alia in GB 2 068 968 and EP 0 779 325.

A typical filler material used in creating pores is calcium carbonate (CaCO3), which is corn monly mined as limestone. Grinding limestone directly to calcium carbonate particles does not usually produce calcium carbonate particles of high quality, e.g. sufficient for inclusion into personal hygiene articles. Instead, limestone is usually heated at high temperature in an oven to provide calcium hydroxide. The calcium hydroxide is then reacted with carbon dioxide to form calcium carbonate.

This roundabout process is by no means environmentally-friendly, requiring large amounts of energy to decompose limestone and producing corrosive calcium hydroxide as an intermediate. In addition, this process releases large amounts of carbon dioxide (C02), which is a well-known greenhouse gas.

It would therefore be beneficial to provide a method for manufacturing breathable polymer films comprising calcium carbonate which had a lower environmental impact than current methods. In particular, methods in which calcium carbonate is produced in a more efficient manner, preferably from waste materials from other processes, and avoiding corrosive intermediates, would be advantageous.

SUMMARY OF THE INVENTION

The present invention therefore provides a method for manufacturing a breathable polymer film. The method comprises the steps of: a. reacting gypsum (CaSO4) with carbon dioxide (GO2) in the presence of water to provide calcium carbonate (CaCO3) b. isolating the calcium carbonate obtained in step a.

c. mixing the calcium carbonate from step b. with a polymer resin d. forming the mixture obtained in step c. into a precursor film e. stretching the precursor film to form a breathable polymer film.

Suitably, step a. takes place in the presence of an ammonia-containing catalyst. The polymer resin may be present in an amount of 25-70 wt%, preferably 30-60 wt%. The calcium carbonate may be present in an amount of 75-30 wt%, preferably 70-40 wt%.

The mixture in step d. may comprise one or more additional inorganic fillers, such as e.g. barium sulfate, calcium sulfate, barium carbonate, magnesium hydroxide, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, silica and talc, The mixture in step d. may additionally or alternatively comprise one or more additional organic components, such as e.g. castor oil, or saturated or unsaturated fatty acid esters.

The invention also provides a method for manufacturing a disposable absorbent article.

The method comprises manufacturing a breathable polymer film as described above, and incorporating the breathable polymer film into a disposable absorbent article, preferably as a backsheet thereof.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in more detail with reference to the enclosed schematic figures, in which: Figure 1 shows an absorbent article comprising the film of the invention.

Figure 2 is a cross-sectional view along line Il-Il in Figure 1.

DEFINITIONS

The term "breathable" is used to describe a film which allows the passage of air and s vapour. It typically comprises pores through which air or vapour can pass. Various methods for measuring the breathability of films have been developed. Breathability according to the present invention can be measured using ASTM E96-00.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a method for manufacturing a breathable film (10) as set out in claim 1.

The first step in the method is reacting gypsum (CaSO4) with carbon dioxide (GO2) in the presence of water to provide calcium carbonate (CaCO3).

Gypsum is the common name for calcium sulfate, and is a common mineral, with large deposits found in sedimentary rocks in Europe, Asia and North America. Gypsum is also a by-product of the scrubbing process in which sulfuric oxide gases are removed from the waste flues of power stations (a process known as "flue gas desulfurization"). The product of flue gas desulfurization is usually an aqueous slurry of calcium sulfate.

Gypsum is most commonly found in its dihydrate form, i.e. CaSO4.2H20.

Carbon dioxide is freely available in the atmosphere, and is a common by-product of the combustion of fuels such as natural gas, bio-fuels such as ethanol or oil. CO2 is thus also readily available from the waste flues of power stations. In the present invention, 002 may be in gaseous, liquid or solid form, although liquid and solid forms are preferred due to their improved handling properties compared to gaseous 002.

The gypsum and carbon dioxide are reacted together in the presence of water. The chemical reaction is: CaSO4 + CO2 + H2O -CaCO + H2S04 The water required may be present in the gypsum slurry, or may be added separately.

The 002 may be bubbled through the aqueous mixture, or added in solid or liquid form.

The reaction is described in more detail in WO 96/09248 and in Sulphur, no. 85, Nov/Dec 1969, p.29.

The above reaction between gypsum and carbon dioxide may take place in the presence of a catalyst, in particular an ammonia-containing catalyst. In this case, the sulfuric acid product combines with ammonia to form ammonium sulfate, yet can be recovered as set out in WO 96/09248.

In the second step of the method, the calcium carbonate is isolated. As calcium carbonate is generally poorly-soluble in water, it tends to precipitate out of the above reaction, and can be collected in high yield and high purity by simple filtration of the above reaction mixture. The precipitation of calcium carbonate also pushes the above reaction towards completion. Other suitable methods for isolating the calcium carbonate exist, e.g. centrifugation.

The by-product of the reaction is an aqueous solution of sulfuric acid, which itself is a useful chemical for industrial processes.

The calcium carbonate is isolated in particulate form (as a powder). The average particle diameter of the calcium carbonate is preferably less than 20im, more preferably less than 10l.m, most preferably in the range 0.5 5pm. Depending on the reaction conditions (e.g. temperature, concentration, agitation) the calcium carbonate produced may have the required particle size. Otherwise, it may be ground to provide finer particles.

The calcium carbonate may be treated in various ways to improve its surface properties.

For example, a hydrophobic treatment may comprise higher fatty acids (e.g. stearic or lauric acids) and salts thereof.

In the third step of the method, the calcium carbonate obtained above is mixed with a polymer resin. Suitable polymer resins are polyolefin resins, polyester resins, polyamide resins and the like. The polyolefin resin which can be used in the present invention, for example, include rnonoolefin polymers of ethylene, propylene, butene or the like, or copolymers thereof as a main component. Typical examples of the polyolefin resin include polyethylene resins such as low-density polyethylene, linear low-density polyethylene (ethylene-alpha -olefin copolymer), middle-density polyethylene and high-density polyethylene; polypropylene resins such as polypropylene and ethylene-polypropylene copolymer; poly(4-methylpentene); polybutene; ethylene-vinyl acetate copolymer; and mixtures thereof. These polyolefin resins may be obtained by the use of a Ziegler catalyst, or obtained by the use of a single site catalyst such as a metallocene catalyst. Above all, polyethylene resins are preferable, and linear low-density polyethylene (ethylene-alpha -olefin copolymer) and low-density polyethylene are most preferable. Mixtures of the above-mentioned polymer resins are also suitable.

Suitably, the polymer resin is present in an amount of 25-70 wt%, preferably 30-60 wt%.

The calcium carbonate is suitably present in an amount of 75-30 wt%, preferably 70-40 wt%.

The mixture of calcium carbonate and polymer resin may comprise one or more additional inorganic fillers, such as e.g. barium sulfate, calcium sulfate, barium carbonate, magnesium hydroxide, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, silica and talc.

The mixture of calcium carbonate and polymer resin may also comprise one or more additional organic components. These organic components may regulate the physical properties of the film, such as its flexibility, feel, strength and hydrophobicity/hydrophilicity.

Examples of additional organic components are castor oil and saturated or unsaturated fatty acid esters Additional organic components may function as stabilisers, antioxidants, colorants and ultraviolet light absorbers and the like.

Mixing may be carried out by any suitable method, and a number of mixing apparatus of suitable size and type are commercially available. For example, mixing may be carried out using a Henschel mixer, a super mixer or a tumbler mixer. If desired, the mixture may be pelletized, for example using a single screw extruder or a twin-screw extruder.

Following mixing, the mixture is formed into a precursor film 20. Forming is normally carried out at an elevated temperature, suitably at or above the melting point of the polymer resin. Suitable molding machines and dies for film forming are commercially available, and will be known to the skilled person.

The precursor film 20 is then stretched to form a breathable polymer film 10. Stretching may be uniaxial (in one direction only), or biaxial (in two directions). Stretching may occur in one step, or a number of steps. The precursor film is suitably stretched 1.2 to 5 times, more preferably 1.2 to 4 times its original dimensions in any given direction. Stretching can be carried out mechanically, but also by inflating the precursor film with air.

Upon stretching, the calcium carbonate (and any other inorganic filler which might be present) undergoes interlacial separation. Pores are thus formed.

After the stretching, a heat setting treatment may be carried out in order to stabilize the shape of obtained pores. The heat setting treatment is, for example, a heat setting treatment at a temperature of from the softening point of the resin to less than the melting point of the resin for a period of 0.1 to 100 seconds The breathable polymer film formed from the method of the invention has a thickness in the range 5-100.tm, suitably 10-65gm. The film has a breathability of 500-l0000glm2/24h, as measured by ASTM E96-00 Water method The method of the present invention, and the breathable polymer film produced thereby, involves the recycling of a number of waste products from industrial processes, namely gypsum slurry and carbon dioxide. It also avoids the wasteful processes involved in converting limestone into commercial-grade calcium carbonate.

The present invention provides a breathable polymer film 10, which has lower environmental impact in its manufacturing than other such films.

The breathable polymer film 10 is suitable for use in any application in which the passage of air or vapour through the film is required, yet in which the liquid barrier properties of the film are also important. Such applications include absorbent articles (e.g. sanitary napkins, diapers, incontinence guards, panty liners and the like), medical dressings, clothing (especially outdoor clothing), outdoor equipment, buiding materials, packaging materials and the like.

The term absorbent article" refers to products that are placed against the skin of the wearer to absorb and contain body exudates, like urine, faeces and menstrual fluid. The invention mainly relates to disposable absorbent articles, which means articles that are not intended to be laundered or otherwise restored or reused as an absorbent article after use. Typically, absorbent articles comprise a topsheet 110 which is designed to contact the skin of the wearer, and is liquid-permeable, a backsheet 120 which is designed to contact the garments of the wearer, and is suitably liquid-impermeable, and an absorbent core 130 located between said topsheet 110 and said backsheet 120. An absorbent article 100 in the form of an open diaper is illustrated schematically in Figure 1. The components of the absorbent article 100 are more clearly illustrated in the cross-section in Figure 2.

The topsheet 110 can consist of a nonwoven material, e g spunbond, meitblown, carded, hydroentangled, wetlaid etc. Suitable nonwoven materials can be composed of natural fibers, such as woodpulp or cotton fibres, manmade fibres, such as polyester, polyethylene, polypropylene, viscose etc. or from a mixture of natural and manmade fibres. The topsheet 110 may further be composed of tow fibres, which may be bonded to each other in a bonding pattern, as e.g. disclosed in EP-A-1 035 818. Further examples of materials suitable for topsheet 110 are porous foams, apertured plastic films etc. The materials suited as topsheet 110 should be soft and non-irritating to the skin and intended to be readily penetrated by body fluid, e.g. urine or menstrual fluid. The topsheet 110 may be different in different parts of the absorbent article.

The backsheet 120 on the garment-facing side of the core is of a liquid impervious material, such as the breathable polymer film of the invention, optionally laminated with one or more other layers, such as layers of nonwoven material. The backsheet 120 in Figure 2 is shown as a laminate of the polymer film 10 of the present invention, with a layer of nonwoven material on the outside (wearer-facing side) of the article.

The absorbent core 130 can be of any conventional kind. Examples of commonly occurring absorbent materials are cellulosic fluff pulp, tissue layers, highly absorbent polymers (so called superabsorbents), absorbent foam materials, absorbent nonwoven rnateriais or the like. It is common to combine cellulosic fluff pulp with superabsorbents in an absorbent body. The thin absorbent cores 130, which are common in for example baby diapers and incontinence guards, often comprise a compressed mixed or layered structure of cellulosic fluff pulp and superabsorbent. The size and absorbent capacity of the absorbent core 130 may be varied to be suited for different uses such as for infants or for incontinent adults.

The absorbent core 130 may comprise one or more layers which may be selected to improve the handling of bodily waste. Such layers are designed to receive a large amount of liquid in a short space of time and distribute it evenly across the absorbent core 130.

They may include so-called transfer, distribution, surge or acquisition layers, and are usually located between the topsheet 110 and the absorbent core 130.

The topsheet 110 and backsheet 120 generally have a similar extension in the X-Y plane, while the absorbent core 130 has an extension which is somewhat smaller. The topsheet and backsheet 120 are joined to one another around the periphery of the absorbent core 130, so that the absorbent core 130 is enclosed within the envelope formed by the topsheet 110 and the backsheet 120. The absorbent core 130 is at least located in the crotch portion of the article 100, and may also extend somewhat into the front and rear portions. The topsheet 110 and backsheet 120 may be joined to one another by any means common in the art, e.g. ultrasonic welding, thermal welding or gluing. Other components of the absorbent article 100, such as e.g. leg elastics, waist elastics, standing gathers or fastening means are known to the skilled person, and can be included in the absorbent article 100 as required.

The invention therefore provides a method for manufacturing an absorbent article 100.

The method comprises first carrying out the method described above, so as to obtain a breathable polymer film 10. Then, the breathable polymer film 10 is incorporated into a disposable absorbent article 100. The breathable polymer film 10 is preferably incorporated into the article 100 as the backsheet 120 thereof.

Although the present invention has been described with reference to a number of embodiments, the scope of protection should not be considered as limited thereto.

Combinations or replacement of features from various embodiments also fall within the scope of the invention. The actual scope of protection sought is to be determined by the appended claims.

Claims (7)

1. CLAIMS1. A method for manufacturing a breathable polymer film (10), said method comprising the steps of: a. reacting gypsum (CaSO4) with carbon dioxide (C02) in the presence of water to provide calcium carbonate (CaCO3) b. isolating the calcium carbonate obtained in step a.c. mixing the calcium carbonate from step b. with a polymer resin d. forming the mixture obtained in step c. into a precursor film (20) e. stretching the precursor film (20) to form a breathable polymer film (10).
2. 2. A method according to claim 1, wherein step a. takes place in the presence of an ammonia-containing catalyst.
3. 3. A method according to any of the preceding claims, wherein the polymer resin is present in an amount of 25-70 wt%, preferably 30-60 wt%.
4. 4. A method according to any of the preceding claims, wherein the calcium carbonate is present in an amount of 75-30 wt%, preferably 70-40 wt%.
5. 5. A method according to any of the preceding claims, wherein the mixture in step d.comprises one or more additional inorganic fillers, such as e.g. barium sulfate, calcium sulfate, barium carbonate, magnesium hydroxide, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, silica and talc.
6. 6. A method according to any of the preceding claims, wherein the mixture in step d.comprises one or more additional organic components, such as e.g. castor oil, or saturated or unsaturated fatty acid esters.
7. 7. A method for manufacturing a disposable absorbent article (100), said method comprising carrying out the method according to of any of claims 1-6 above, and incorporating the breathable polymer film into a disposable absorbent article (100), preferably as a backsheet (120) thereof.