Stock material or miscellaneous articles – Structurally defined web or sheet – Including stitching and discrete fastener – coating or bond
Patent
1994-09-02
1997-06-03
Ahmad, Nasser
Stock material or miscellaneous articles
Structurally defined web or sheet
Including stitching and discrete fastener, coating or bond
264 453, 264 464, 4283099, 4283144, 4283197, 442315, 442326, 442370, B32B 326
Patent
active
056352719
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a shaped article comprising a thermoformed composite sheet having an outer surface layer of thermoplastic polymer attached to a surface of a conformable fabric layer. In particular, the invention concerns such a shaped article in which the other surface of the fabric layer of the composite sheet is attached to a layer of rigid foam.
2. Description of the Prior Art
Conventional thermoforming processes are described by Lubin, "Handbook of Composites", Van Nostrand Reinhold Company, 1982, pages 366-367. Such processes generally involve sequential steps of (a) heating a thermoplastic sheet until it softens, (b) forming the softened sheet in a mold and (c) allowing the formed sheet to cool and harden. When thermoforming is used to mold a thermoplastic sheet into a large article, such as the interior surface of a bathtub, sink, canoe hull, or the like, the sheet undergoes a large increase in area, often by a factor of three or more, along with a corresponding decrease in thickness. However, during thermoforming, regions of the thermoplastic sheet that are subjected to large changes in area and/or contour (e.g., in corners and edges of the shaped article) often suffer from excessive and nonuniform thinning, which in turn leads to lower impact resistance and greater fragility. To compensate for such undesired thinning thinning, extra reinforcement to such regions is provided by using a much thicker starting thermoplastic sheet for the thermoforming. This results in a heavier and more expensive molded article.
Composite sheets comprising thermoplastic sheets laminated to a fabric layer are known for use in thermoforming processes. Although such materials may have been useful in thermoforming, improvements in the formed product are desired, particularly in minimizing thinning of the formed walls and in increasing the impact resistance of the exposed plastic surface.
To alleviate the wall-thinning problems associated with thermoforming of large articles, the present inventor found that considerable improvements in wall uniformity can be achieved by using a thermoformable composite sheet having a layer of thermoplastic polymer adhered to a relatively light, conformable, nonwoven fibrous layer stitchbonded with an elastic thread, which fabric amounts to no more than 20% of the total weight of the composite sheet. Although these composite sheets reduce undesired thinning during thermoforming, the utility of shaped articles formed therewith would be further enhanced if the articles also had improved resistance to cracking under impact. Accordingly, it is an object of this invention to provide increased impact resistance to such thermoformed shaped articles.
SUMMARY OF THE INVENTION
The present invention provides an improved thermoformed shaped article. The article is of the type that includes a thermoformed composite sheet that has an outer surface of thermoplastic polymer attached to a first surface of a relatively lightweight conformable fabric layer. In accordance with the improvement of the present invention, the composite sheet has a thermoformed expansion factor of at least 1.5, preferably greater than 2, and most preferably in the range 3 to 7, the conformable fabric amounts to no more than 20%, preferably 5 to 15%, of the weight of the thermoplastic layer and the surface of the fabric opposite the first surface is attached to a layer of rigid foam having a specific volume in the range of 3 to 30 cm.sup.3 /gram and a thickness of at least 2.5 cm. Preferably, the composite sheet is made with a fabric layer having a specific volume of at least 5 cm.sup.3 /gram, a weight in the range of 100 to 500 g/m.sup.2 and a conformability at 180.degree. C. (defined hereinafter) in the range of 2 to 9, most preferably 4 to 7. Preferred thermoplastic layers are of methylmethacrylate homopolymers and copolymers. Preferred fabric layers are selected from nonwoven fabrics stitchbonded with elastic threads, needled felts and knitted fabrics. Preferred
REFERENCES:
patent: 4130614 (1978-12-01), Saidla
patent: 4379103 (1983-04-01), Doerfling
patent: 4753837 (1988-06-01), Hanusa
patent: 5075142 (1991-12-01), Zafiroglu
Database WPI Week 8738, Derwent Publication AN87-267016 & JRA-62 184 815 (Okamura Seisakusho).
Ahmad Nasser
E. I. Du Pont de Nemours and Company
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