Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-10-10
2003-05-27
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C264S145000, C264S171100, C264S171230, C264S210100, C264S210600, C264S288800, C523S200000, C523S205000, C523S220000
Reexamination Certificate
active
06569527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a particulate alkaline earth metal carbonate, eg calcium carbonate, for use with a polymer composition for producing a polymer based end product, ie a thermoplastic film product, which may have a high mineral film loading. In particular, the present invention relates to a carbonate, for use in preparing a film composition, and breathable films thereof with an unusually low surface moisture level, and the processing and use of this carbonate.
2. Description of Related Art
Alkaline earth metal carbonates, particularly calcium carbonates are used as a filler in end products comprising compositions incorporating thermoplastic polymers, such as film products. Such films, porous or non-porous, are manufactured for a number of consumer products such as garbage bags, backing materials or outer covers on diapers, bandages, training pants, sanitary napkins, surgical drapes, and surgical gowns. The compositions from which these films are made may include two basic components, the first being a thermoplastic polymer, usually a predominantly linear polyolefin polymer such as a linear low density polyethylene and the second being an inorganic particulate filler such as calcium carbonate. A third component, namely a bonding or tackifying agent may often be present. These components are mixed and compounded together to form a compound or concentrate which is formed (usually in a subsequent process) into a film layer using any one of a variety of film-producing processes known to those of ordinary skill in the film making art including casting, or blowing, or may be laid down on a substrate such as paper or board in a process known as extrusion coating.
After the film is fabricated into its desired form, and if the film is to be a porous breathable film, the film can then be stretched, uniaxially or biaxially, by any of the well-known techniques in the art including by hydraulics, by pinch rolls moving at different rates, or by tentering.
Filler loading levels determine to a great extent how far the precursor film must be stretched to attain a given degree of overall porosity. Below a lower end of the loading range, the pores are less numerous and less interconnected, and therefore, the film is less permeable at a given draw ratio than when a higher filler loading is employed. Above a higher end of the loading range, either the materials will not blend uniformly or the sheet made from the composition will not stretch. The preferred loading in some applications, such as that in manufacturing the microporous film of U.S. Pat. Nos. 5,008,296 and 5,011,698, is very high, eg 60% to 75% by weight of the composition, with the filler preferably being a calcium carbonate.
U.S. Pat. No. 4,698,372 discloses a microporous polymeric film having good water vapour transmission rates and hydrostatic resistance to water penetration thereof. The film has a filler loading of 25-35 volume % of inorganic fillers such as calcium carbonate, among others, and uses an “antagonizer” such as stearic acid in order to reduce the effective surface tension of the filler to the approximate level of that of the matrix polymer.
U.S. Pat. No. 3,903,234 discloses gas permeable biaxially oriented film prepared from compositions of polyolefins containing 26% to 50% by weight of inorganic filler particles.
U.S. Pat. No. 4,176,148 discloses microporous oriented films composed of polybutene containing 3% to 80% by weight of inorganic fillers.
U.S. Pat. Nos. 5,376,445, 5,695,868, and 5,733,628 disclose breathable film or film laminates or composites which may or may not consist of fillers.
If the moisture level within the film forming composition becomes significant, it may be difficult to prepare a film compound and/or to form a film from the compound with a smooth surface which is free from voids. A smooth surface which is free from voids is necessary to enable the film to be stretched uniformly, a process which is generally employed in the production of breathable and other films.
The aforesaid U.S. Pat. Nos. 5,008,296 and 5,011,698 teach a method of maintaining the moisture level of a melt blended composition below 700 parts per million (ppm) and preferably below 300 ppm by cooling the extruded strands and/or pellets composed of polymer plus filler, which are used in the film composition using flowing air or employing vacuum-drying. However this additional processing step is time consuming and costly.
Conventionally in the film making art, usage levels of a mineral filler, such as a ground calcium carbonate, in a host material have been less than 20% loading. In certain applications requiring higher loadings greater than 20%, we have found that conventional filler materials, such as ground calcium carbonate, are unlikely to provide a successful product if made in a conventional manner. Stated in another way, conventional filler products, eg of calcium carbonate, have not been designed in the prior art, to have a surface moisture level which facilitates moisture minimisation when used in thermoplastic polymer composition for film applications, especially for breathable films and for making compositions for preparing such films.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a mineral filler comprising an alkaline earth metal carbonate, especially calcium carbonate, which has properties allowing a film end product comprising a thermoplastic polymeric material together with the filler to be easily and successfully produced with filler loading of at least 10 percent by weight, preferably at least 20 percent by weight, and, surprisingly, in some cases at least 40 percent by weight and in some cases even up to 75 percent by weight. Fillers, especially calcium carbonates, produced in a conventional manner would in many cases be unsuccessful in producing such film products without the application of special time consuming and costly additional processing steps during or after the formation of the composition (compound, masterbatch or blend) of filler plus thermoplastic polymer to be employed to produce the film.
We have found that the production and use of a carbonate particulate mineral filler having a reduced moisture level and a low susceptibility to pick up surface moisture can surprisingly be achieved and improves the quality of the filler and the intermediate and final product in which it is used thereby enhancing the preparation of these products. Less processing is required or processing is facilitated in manufacturing the composition comprising the thermoplastic material plus filler and the final product, especially when a breathable film of high filler solids content is to be produced therefrom. We have found that a moisture content above but not below a minimum level (which we have determined and is specified later) associated with the carbonate mineral filler used in the composition for manufacturing a polymer film product can result in unwanted macroscopic size voids or holes (ie several mm in length) forming in the film as a result of steam generation whilst the thermoplastic polymer of the film is in the plastic melt phase. We have shown that in order to avoid such undesirable voids or holes the moisture content of the filler should desirably be limited to or preferably below the specified minimum level at all times before use in producing the composition with the polymer. Although reducing the moisture content of particulate materials has been described previously, eg in JP-A-61-97363 for minimising silver marking in filled plastics moulded bodies, minimisation of adhered surface moisture of a filler for use in producing a high filler loaded composition for fabrication into a film, especially a breathable film, has not previously been suggested in the film making art.
According to the present invention in a first aspect there is provided an inorganic particulate material comprising an alkaline earth metal carbonate suitable for use as a mineral filler in the manufacture of thermoplastic film products which i
Ansari Deeba Marjan
Calhoun Allison Anne
Shurling Dickey S.
Skelhorn David A.
Acquah Samuel A.
Finnegan Henderson Farabow Garrett & Dunner LLP
Imerys Minerals Limited
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