PROCESS FOR THE MANUFACTURE OF MICROPOROUS FILM

Abstract: A microporous film suitable for diaper back sheets and other disposable applications is produced by abrading an unoriented film by passing it through at least one set of at least two cooperating nip rollers, each nip roller having an abrasive grit surface.

Description: Porous films have been produced in the past by a number of processes. One group of such processes comprises forming a film of a thermoplastic resin composition containing a noncompatible filler and then stretching the film in one or more directions to impart a microporous structure to the film. Examples of these techniques are disclosed in US-A-4,472,328 and GB-A-2,151,538. Generally, the physical properties, especially the strength and the hydrostatic pressure resistance, of the films so produced are not as good as would be desired. Other processes have been disclosed for manufacture of porous films, which processes comprise forming an unfilled thermoplastic resin film and then passing the film through the nip of two rollers, one of these having a smooth, often heated, surface and the other having either projections in its surface as in US-A-3,073,304 or an abrasive surface coating as in US-A-3,718,059. These latter processes have not been entirely successful because of the manufacturing cost of the rollers having surface projections and also because of operational problems causing undesired large variations in the film pore diameters. In addition the improvements in the moisture vapor transmission rates have been less than desired using the smooth pressure roller techniques. US-A-4,364,985 discloses a process for preparing a porous, filled thermoplastic film, which process requires the particles of the filler to extend from one film surface to the other. The filled film is passed over a set of serially positioned abrading rollers for the purpose of rubbing off the surfaces of the film that overlie the outermost portions of the filler particles. A drawback of this method is that the average filler particle size has to be about equal to the film thickness, which obviously limits the process to the production of thin films only. The present invention provides a process for the production of a microporous film having an improved moisture vapor transmission rate and an acceptable hydrostatic pressure resistance. In accordance with the present invention there is provided a process for the production of an unoriented microporous film which comprises passing an unoriented film prepared from a high molecular weight filled thermoplastic resin composition through at least one set of two cooperating nip rollers, each nip roller being provided with an abrasive surface. The high molecular weight thermoplastic resin can be any alphaolefin based component from which films and sheets which are foldable, soft and relatively strong can be produced. Suitable resin components are high molecular weight polyethylenes, linear low polyethylenes, polypropylene, polybutylene and ethylene-propylene copolymers. Mixtures of such resin components can be used. For diaper linings and other clothing and sanitary applications, a particularly suitable resin comprises 65 to 95 wt % of a low density polyethylene having a density from 0.910 to 0.935 cm³, and at least 5 wt % of at least one resin component selected from high density polyethylene having a density exceeding 0.940 g/cm³, a linear low density polyethylene of a density of from 0.915 to 0.945 g/cm³ and high molecular weight polybutylene and mixtures thereof. The linear low density polyethylene is an interpolymer of ethylene and one or more C₄ - C₁₂ alpha olefins. Especially preferred are the copolymers of ethylene with hexene-1 and/or octene-1. The melt index of the total resin should be between 0.5 and 10 g/10 min. The resin composition also contains a filler, such as calcium carbonate, barium sulfate, mica, polytetrafluoroethylene and titanium dioxide. The concentration of the filler should be between 5 and 25 parts by weight per 100 parts by weight of the resin. Preferably, the filler is calcium carbonate having an average particle size between 3 and 30 µm. For appearance, the resin composition usually contains a whitener such as titanium dioxide pigment when titanium dioxide is not the principal filler. The concentration thereof is generally maintained between 3 and 16 parts by weight per 100 parts by weight of the resin. Other additives such as antioxidants, lubricants and stabilizers may also be added for their known purposes and in conventional quantities. The film can be produced by any known commercial process, e.g. by casting or blown film techniques. The film thickness should range between 23 and 150 µm (0.9 and 6 mil). Prior to treatment according to the process of this invention, the film may be subjected to an embossment or other treatment to impart a decorative pattern onto the surface of the film. The nip rollers have an abrasive surface which is usually provided in the form of coating with an abrasive material such as sand paper, emery cloth, garnet paper or aluminum oxide. The abrasive typically has a grit size ranging between 80 and 250 grain. It is not necessary that the entire surface of each of the rollers be covered with the abrasive. The pressure at the nip of the rollers can vary between 6.9 and 69 kPa gauge (10 psig and 100 psig). The temperature is maintained at ambient conditions. It is not necessary that a set of the cooperating nip rollers have equal roller diameters, and one roller can have a larger diameter than the other. It is also possible to have one larger roller in cooperative relationship with more than one other abrasive roller of smaller diameter. For instance, the equivalence of five serially positioned nips can be accomplished by passing the film over an abrasive nip roller partially surrounded by and in cooperative relation with five abrasive nip rollers of smaller diameter. The films produced by the process of this invention have excellent properties making them especially useful in disposable film applications, such as diaper back sheets. Specifically, the films exhibit excellent moisture vapor transmission rates while having an acceptable hydrostatic pressure resistance. Because no stretching or orientation is required to provide the desired porosity, the physical properties are virtually unaffected and remain at desired high levels. Also, the appearance of the film products of the invention, as measured by opacity, softness and gloss, is very good. Reference is now had to the Figure which schematically depicts an embodiment of the process of the invention. Film 10 is fed from a supply roll (not shown) through the nip between abrading rollers 11 and 12 at least partially covered with a suitable abrasive. By means of the rotations of the rollers 11 and 12 in opposite directions from each other the abraded film is carried to windup onto rewind roller 13. It is to be understood that various takeoff rollers may be present in the assembly to facilitate the movement of the untreated film to the treated film wind-up station. Also, the invention covers the use of additional sets of cooperating abrading rollers (not shown) in the process, to provide further improvement in porosity as measured by moisture vapor transmission rate (MVTR). For a better understanding reference is had to the following example. EXAMPLE 1 A blend was prepared from 60.5 wt % low density polyethylene (0.924 g/cm³ density, 0.8 MI), 22.3 wt % high density polyethylene (0.967 g/cm³ density, 5.0 MI), 2.0 wt % high molecular weight polybutylene and 8.2 wt. % calcium carbonate (particle size of 3 µm) and 7.0 wt % titanium dioxide whitener. A 25 µm (1 mil) cast film was produced from this blend and subsequently embossed to produce a patterned film having a thickness of about 56 µm (2.2 mil). The embossed film was subjected to abrasion without any stretching or orientation in any direction prior to the treatment. The abrasion treatment was carried out by passing the film 15 times through the nip of two rollers covered uniformly with 80 grit sand paper at a nip pressure of 0.3 MPa (40 psi). Each of the rollers had a length of 41 cm (16 inches) and a diameter of 7.6 cm (3 inches). The speed of each of the rollers was 122 m (400 feet) per minute and the film passed through the nip at the same rate. The abraded film had a MVTR of about 1000 g/m²/24h, a 50 fold increase over that of the embossed film before the abrasion treatment. Table 1 below shows the physical properties of the porous film product of this example:

Claims: A process for the production of an unoriented microporous film which comprises passing an unoriented film prepared from a high molecular weight filled thermoplastic resin composition through at least one set of at least two cooperative nip rollers, each nip roller having an abrasive grit surface. A process according to claim 1 wherein the pressure at the nip of the rollers is between 6.9 and 69 KPa gauge (10 and 100 psig). A process according to claim 1 or 2 wherein the abrasive has a grit size between 80 and 250 grain. A process according to claim 1, 2 or 3 wherein the thermoplastic resin composition comprises at least one of polyethylene, linear low polyethylene, polypropylene, polybutylene and ethylene-propylene copolymers. A process according to claim 4 wherein the resin comprises from 65 to 95 wt % of a low density polyethylene having a density of 0.910 to 0.935 g/cm³ and at least 5 wt % of at least one resin component selected from high density polyethylene having a density exceeding 0.940 g/cm³, a linear low density polyethylene having a density of 0.915 to 0.945 g/cm³ and a high molecular weight polybutylene. A process according to claim 5 wherein the resin is a blend of the low density polyethylene, the high density polyethylene and the polybutylene. A process according to any one of the preceding claims wherein the thermoplastic resin composition comprises as filler at least one of calcium carbonate, barium sulfate, mica, polytetrafluoroethylene and titanium dioxide in an amount between 5 and 25 parts by weight per 100 parts by weight of the resin. A process according to claim 7 wherein the average particle size of the filler is from 3 to 30 µm. A process according to any one of the preceding claims wherein the resin composition contains a whitener pigment in an amount from 3 to 16 parts by weight per 100 parts by weight of resin. A process according to claim 9 wherein the thermoplastic resin composition contains from 5 to 25 parts calcium carbonate filler having an average particle size from 3 to 30 µm and from 3 to 10 parts titanium dioxide whitener per 100 parts resin.

Inventors: DOUGLAS, NOEL LAWRENCE
Applicants: REXENE PRODUCTS COMPANY (A DELAWARE CORPORATION)
Application year: 1987
Main classification: B29C67
IPC codes: B24B19, B26F1, C08J9