Stock material or miscellaneous articles – Hollow or container type article – Nonself-supporting tubular film or bag
Reexamination Certificate
1997-08-04
2002-10-15
Nolan, Sandra M. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Nonself-supporting tubular film or bag
C106S415000, C264S115000, C264S328100, C264S328180, C264S330000, C264S331110, C264S331150, C428S036400, C428S036600, C428S036700, C428S036900, C428S323000, C428S331000, C524S451000, C524S584000, C524S586000
Reexamination Certificate
active
06465064
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns thermoplastics compositions are articles made therefrom having gas and vapour barrier properties.
BACKGROUND OF THE INVENTION
Thermoplastic materials are widely used in packaging because of their low cost and ease of forming into a variety of shapes. However, most thermoplastics materials suffer from the disadvantage of providing only a relatively poor barrier to gases and vapours. Poor gas barrier is a particular disadvantage in packaging oxygen sensitive materials such as foodstuffs which are to be stored unrefrigerated. Poor vapour barrier properties are a disadvantage when packaging materials which are sensitive to moisture vapour, for example foodstuffs and confectionery which lose condition when they become damp, and they are also a disadvantage when the packaged material includes flavouring components which diffuse through the packaging material with consequent loss of flavour.
A number of attempts have been made at improving the gas barrier properties of thermoplastics materials. GB-A-1136350, for example, proposes the use of circular platelike fillers with a ratio of diameter to thickness between 20:1 and 300:1 and a diameter of at most 40 &mgr;m in polyolefin polymers selected from polyethylene, polypropylene, ethylene-containing copolymers containing at least 50 moles percent of ethylene, and polystyrene, the preferred amount of filler being 0.1 to 50 wt % of the total weight of filled polymer. Such filled polymer compositions are proposed to be used to manufacture films, for example for food packaging.
U.S. Pat. No. 3,463,350 is concerned with the production of moulded containers for packaging foodstuffs, the containers being made from mixtures of high density polyethylene (HDPE) and mica particles, for example by compression or injection moulding. Such containers are said to reduce the discolouration of so-called canned cornbeef as caused by oxygen compared with the use of similar containers made of HDPE filled with glass fibre or titanium dioxide instead of mica.
It has also been proposed in U.S. Pat. No. 4,528,235 to incorporate platelet filler particles with an average equivalent diameter of from 1 to 8 &mgr;m, the maximum diameter being 25 &mgr;m, and thickness of less than 0.5 &mgr;m, into HDPE having a melt index of from 0.01 to 1.0 g/10 minutes at 190° C. as measured by ASTM D-1238 to produce films having a thickness of from 10 to 100 &mgr;m, with the intention of increasing the oxygen barrier of the films compared with films formed from unfilled HDPE.
Despite the apparent improvements in oxygen barrier resulting from these various hitherto proposed methods using lamellar fillers to impart oxygen barrier properties to polyolefins, even higher oxygen barrier properties have been sought. Thus U.S. Pat. No. 4,536,425 proposes increasing the gas barrier properties of polar thermoplastics resins, for example alkylene terephthalates, by blending the resins with mica flakes having a plurality of platelets using a shear force which effects delamination of the mica platelets and thereby a substantial increase in the aspect ratio of the flakes. Delamination of the mica flakes is said to occur as a result of shear forces generated during mixing of the resin with the mica being transferred to the mica flakes due to chemical bonding of silanol on the surface of the flakes to hydroxyl and carboxyl groups formed in the resin as a result of hydrolysis of ester groups in the resin at the temperature involved. This document goes on to state that the continuous delamination effect with polar thermoplastic resins is not encountered with non-polar resins such as unmodified polyethylene or polypropylene resins since non-polar resins do not normally form any appreciable adhesive covalent bonds with the mica platelet surfaces.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of making a moulding composition for forming an article having increased barrier to gases and/or vapours, the method comprising the step of mixing together a substantially non-polar thermoplastic resin together with a laminar filler, the laminar filler being capable of delaminating when the composition is subjected to high shear to increase the aspect ratio of the filler as it breaks down into platelets.
Compositions in accordance with the present invention have been found to provide a good barrier not only to oxygen but also to flavour molecules. A particularly preferred use of compositions in accordance with the present invention is as shoulders for toothpaste tubes. Whereas it is relatively easy to form the tubular part of such tubes from a rigid or flexible polymer laminate having barrier properties by the use of multi-layer structures including a layer of a polymer having barrier properties, for example ethylene/vinyl alcohol copolymers, this is not possible for the shoulder portion of such tubes. The result has been that the shoulders have had to be moulded from expensive thermoplastic resins or inserts of a barrier resin have had to be made and located within a shoulder having a low inherent barrier to flavour molecules, again leading to increased costs.
The non-polar thermoplastic resin is preferably a polyolefin resin, for example a polymer derived from one or more aliphatic or aromatic alkene, eg a polymer containing units derived from at least one of ethylene, propylene, but-1-ene and styrene. Examples of specific polyolefin resins which can be used include polyethylene, polypropylene, ethylene/propylene copolymers, ethylene, propylene/but-1-ene terpolymers, polyethylenes being particularly preferred by virtue of their good injection moulding characteristics. The polyethylene can be low density polyethylene (density 0.910 to 0.925 g.cm
−3
), medium density polyethylene (density 0.925 to 0.950 g.cm
−3
) or high density polyethylene (density 0.950 to 0.980 g.cm
−3
). High density polyethylene is particularly preferred by virtue of its higher inherent barrier properties compared with lower density polyethylenes.
The platelet filler can be any of a variety of lamellar fillers provided the platelets delaminate under shear as is encountered when the filler is blended with the non-polar resin before injection moulding and more particularly when the mixture of filler and resin is subjected to injection moulding. Lamellar fillers include clays, mica, graphite, montmorillonite and talc. Talc is particularly preferred by virtue of its ease of delamination during shear.
The filler should have a structure consisting of platelets both before and after being subjected to high shear. As will be appreciated, in addition to delamination, subjecting such fillers to high shear also tends to reduce their effective diameter. However, despite a reduction in the effective diameter of the filler particles, high shear generally results in an increase in the aspect ratio of the individual filler particles.
Talc, being a naturally occurring hydrated magnesium silicate, is available in a variety of grades of greater or lesser purity. It has surprisingly been found that the ease of increasing the aspect ratio of talc when it is subjected to high shear in a non-polar thermoplastic resin appears to increase as the level of impurities within the talc decreases. Thus not only does it appear easier to delaminate the platelets of the talc, but the platelets themselves apparently resist fracture.
According to another aspect of this invention there is provided a composition for forming an article having increased barrier to gases and/or vapours, the composition comprising a substantially non-polar thermoplastic resin filled with platelets of talc having an aspect ratio of at least 5 and an average aspect ratio of between 16-30 and a CIE whiteness index of at least 40.
Whatever the mechanism by which the aspect ratio of some talcs is increased to a particularly high degree when they are subjected to high shear in non-polar thermoplastic resins, it has surprisingly been found that talcs which result in compositions in accordance with th
Betts UK Limited
Howson and Howson
Nolan Sandra M.
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