Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-03-20
2004-02-24
Cole, Elizabeth M. (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C521S084100, C521S097000, C528S308200, C264S235800, C264S290200, C264S041000
Reexamination Certificate
active
06696146
ABSTRACT:
BACKGROUND OF THE INVENTION
Polyester resin films are widely used in various technological fields by virtue of their excellent mechanical, electrical and chemical-resistance properties.
In particular, biaxially-stretched films of polyethylene terephthalate are superior to other films both in terms of dimensional stability and in terms of tensile properties, particularly in view of their high modulus of elasticity.
However, polyester films have drawbacks, mainly due to their very high relative density and to the fact that applications in the field of information technology, such as for example for electronic whiteboards and similar devices require them to be highly loaded with white pigments in order to be sufficiently opaque. Various methods for producing foamed films or sheets of polyester resin are known.
Thick low-density foamed materials made of polyester resin, due to their high thermal insulation properties, which prevent effective cooling of the internal parts of the materials as they exit from the extruders, have a relatively high crystallinity which is difficult to reduce.
No solution has been found so far to the problem of being able to mono- or biaxially stretch foamed sheets made of polyester resin, which have a density of less than 600-700 kg/m
3
and have crystallinity or are crystallizable.
The main difficulty encountered in the mono- and biaxial stretching of said low-density foamed sheets consists in the possibility of their breaking during stretching.
It is known from WO 97/33948 to produce labels from polyester foamed films which may also be mono or biaxially stretched when obtained from amorphous resins such as poly(1,4-dimethylolcyclohexile)terephthate or amorphous copolyethylene terephthate-isophthalate.
The possibility to have mono- and biaxially-stretched polyester-resin foamed sheets or films having low apparent density and sufficiently high crystallinity might offer considerable advantage, especially in the view of the improved mechanical properties that said sheets and films might have.
BRIEF DESCRIPTION OF THE INVENTION
It has now been found unexpectedly that it is possible to mono- and biaxially stretch, without rupture problems or other drawbacks, foamed aromatic polyester resin sheets and films having a bulk density of less than 700 kg/m
3
, preferably less than 400 kg/m
3
, in which the resin has a crystallization rate such that by heating for 10 minutes at 120° C. the crystallinity can reach values as high as 30-35%, and to obtain stretched sheets or films having a relatively low apparent density which have high mechanical properties, particularly in terms of high modulus and high impact resistance and good opacity or translucence associated with sparkling reflectance properties.
DETAILED DESCRIPTION OF THE INVENTION
Preferably, the crystallinity that can be developed by heating at 120° C. for 10 minutes is from 5 to 35%.
The high impact resistance of the resulting stretched sheets or films is surprising being considerably higher than that of the sheets and films before stretching.
It has been found that the mono- and biaxial stretching of foamed sheets having the above indicated thickness, crystallinity and density characteristics is feasible if said sheets are obtained from polyester resin having sufficiently high melt strength and melt viscosity values which are higher than certain given limit values.
The melt strength of the usable resin is at least 1 centinewton at 280° C. and melt viscosity is at least 1500 Pa.s at 280° C. with a shear rate which tends to zero.
Melt strengths of 10 to 150 or more centinewtons and melt viscosities of 2,000-20,000 Pa.s can be used conveniently.
The melt strength measured on the resin forming the foamed sheets or films presents value lower than those of the resin used in for preparing the sheets and films.
The intrinsic viscosity is generally between 0.8 and 1.5 dl/g.
The above specified rheological properties refer to the resin before it is subjected to the extrusion-foaming process, but they can be acquired during said process.
The aromatic polyester resins usable to obtain the resins having the above specified rheological properties are prepared by polycondensation, according to known methods, of dicarboxylic aromatic acids with diols containing 2 to 12 carbon atoms or by transesterification of lower alkyl esters of dicarboxylic acids with diols having 2-12 carbon atoms and subsequent polycondensation of the diol esters.
Terephthalic acid, isophthalic acid and naphthalene dicarboxylic acids are preferred aromatic acids.
Polyethylene terephthalate and copolymers thereof in which 1 and up to 20-25 and preferably 1-25 monomeric units derived from terephthalic acid are substituted with units derived from isophthalic acid and/or naphthalene dicarboxylic acids are preferred resins.
The polyester resins having the above specified rheological properties can preferably be obtained by solid-state polycondensation (SSP) of polyester resins having an intrinsic viscosity of less than about 0.7 dl/g added with a dianhydride of a preferably aromatic tetracarboxylic acid, particularly pyromellitic dianhydride, in an amount of 0.05 to 2% by weight, working under such temperature conditions and with such durations as to increase the melt strength and the melt viscosity of the resin to the chosen values.
The intrinsic viscosity of the resin after SSP is generally increased to values of more than 0.8 dl/g.
The above indicated solid-state polycondensation is performed according to known methods.
A particularly suitable method is described in U.S. Pat. No. 5,243,000, whose description is included herein by reference.
Other methods suitable to obtain the melt strength and melt viscosity values according to the invention are disclosed in U.S. Pat. Nos. 5,288,764 and 5,229,432, whose description also is included by reference.
The polyester resins can be used in mixture with other thermoplastic polymers, particularly with polyamide resins used in an amount from approximately 2 to 50% by weight. Mixtures or alloys of this type are described in WO 94/09069, whose preparation method is included herein by reference.
A polyamide which is particularly suitable especially when one wishes to give improved gas-barrier properties (oxygen and CO
2
) is poly-m-xylilene adipamide.
This polyamide is mixed while melted with the polyester resin, which is premixed, also while melted, with a dianhydride of a tetracarboxylic aromatic acid, particularly pyromellitic dianhydride, used in an amount from 0.05 to 2% by weight on the polyester resin.
Other polymers that can be used are aliphatic polyester resins obtainable from aliphatic dicarboxylic acids and from diols or from aliphatic hydroxides-acids or from the corresponding lactones or lactides.
Poly-epsilon-propiolactone is a representative resin.
These resins are added in amounts of up to 40% by weight and give biodegradability properties to the resin thus mixed.
Another aspect of the invention is the finding that the addition of amounts between 0.5 and 10% by weight to the polyester resin of an aliphatic or aromatic polyamide with a high or low molecular mass allows to significantly reduce the amount of the unreacted pyromellitic dianhydride present in the stretched and foamed sheets and films and the amount of acetaldehyde.
The foamed sheets suitable for being mono- and biaxially stretched have a bulk density of about 50 to 700 kg/m
3
. Thickness is generally form 0.5 to 5 mm.
To produce the monoaxially-stretched foamed films with a thickness reduced to approximately 30 microns, the thickness of the starting foamed sheets is from about 0.6 to 2 mm; when instead one wishes to obtain biaxially-stretched sheets, one begins with thicker sheets (2-5 mm).
In the case of biaxial stretching, the bulk density after stretching is increased considerably (even fourfold for 4:1 stretching).
However, when a hydrocarbon is used as foaming agent, the residual hydrocarbon enclosed within the cells expands due to the heating required to bring the sheet or film to the temperature suitable for stretching. It is t
Al Ghatta Hussein
Cobror Sandro
Severini Tonino
Cole Elizabeth M.
Josif Albert
Modiano Guido
O'Byrne Daniel
Sinco Ricerche S.p.A.
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