Transparent articles of polyester resin

Details Transparent articles of polyester resin Transparent articles of polyester resin

2000-01-20

2001-07-10

Short, Patricia A. (Department: 1712)

Stock material or miscellaneous articles

Structurally defined web or sheet

Physical dimension specified

C525S444000, C528S305000

Reexamination Certificate

active

06258452

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to thick transparent articles obtained from polyalkylene terephthalate resins having characteristics suitable for the preparation of the said articles.
Thick transparent sheets obtained from amorphous thermoplastic polymers such as PVC, polymethylmethacrylate, polycarbonate and polystyrene are known.
Such sheets have various disadvantages such as, for example, the presence of plasticizers in PVC, which tend to migrate to the surface or the difficulty recycling PMMA and PC sheets, and the inflammability and fragility of PMMA sheets.
Relatively thin transparent sheets obtained from crystallizable modified PET having a cold crystallization temperature between 120°-160° C. are known.
It is known from U.S. Pat. No. 2,965,613 that copolyethylene terephthalate resins containing 5-15% isophthalic acid units having an intrinsic viscosity of 0.5-0.7 dl/g (obtained using known methods by esterification/polycondensation of mixtures of terephthalic and isophthalic acid or by transesterification with ethylene glycol of mixtures of terephthalate and dimethyl isophthalate and polycondensation of the glycol esters obtained) are crystallizable although at very much lower rate than that of polyethylene terephthalate homopolymer. Only when the unit content of isophthalic acid is very high (25-85%) are the resins no longer crystallizable. In this case Tg is very low and makes the resin unsuitable for the preparation of products having sufficiently good mechanical properties.
In U.S. Pat. No. 4,234,708 copolyethylene terephthalate/isophthalate copolymers containing 5-15% of units deriving from isophthalic acid are described having very high intrinsic viscosity and melt strength, suitable for the preparation by blow-extrusion of thin wall transparent containers.
The copolymers are prepared by polycondensation of mixtures of terephthalic and isophthalic acid and ethylene glycol or by transesterification of mixtures of dimethyl terephthalate and isophthalate and subsequent polycondensation of the esters, in the presence of a branching agent such as, for example, trimethylolpropane and pentaerythitol and a chain terminator (benzoic acid and the like).
The intrinsic viscosity of the resin that is obtained (which is less than 0.7 dl/g) is brought to values greater than 1 dl/g by solid state polycondensation (SSP).
After SSP, the resins have a rather higher viscosity in the molten state (10
4
-10
5
Pa·s at temperatures from 265° C. to 300° C. in the absence of shear forces) which falls drastically, under the action of shear forces, to values of 10
2
-10
3
Pa·s.
Such a high sensitivity to shear forces, although on the one hand suitable for blow moulding operations, is not suitable for the preparation by extrusion of thick products where it is necessary that the melt maintains a sufficiently high viscosity even under the action of shear forces.
Heat formed articles having a thickness of 1-20 mm obtained from sheets of crystallizable polyalkylene terephlthalate having a crystallization temperature of between 120-160° C. are known from WO-A-97/12750.
In the examples, there are heat formed sheets with the thickness of 5 mm at most.
The possibility of preparing thick transparent articles and sheets having good mechanical properties starting from polyalkylene terephthalate resins has been considered, but the problem until now has not been resolved.
SUMMARY OF THE INVENTION AND DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
It has now been unexpectedly found that is possible to prepare transparent sheets and articles having thickness greater than 5 mm starting from copolyalkene terephthalate/isophthalate resins containing 5-20% of units of isophthalic acid, preferably 7-15%, having the following properties:
an intrinsic viscosity greater than 0.8 dl/g;
a melt strength greater than 1 cN at 280° C.; and
a viscosity in the molten state at 280° C. greater than 2000 Pa·s for shear rates tending to 0 and not less than 200 Pa·s for shear rates of 1000 sec
−1
.
The crystallinity of the articles is in general less than 5%.
Resins containing units of isophthalic acid greater than 7-8% do not have exothermic transitions in the cold crystallization DSC curves. The articles have good mechanical properties, in particular high impact resistance even at low temperatures, and optimum optical properties.
Copolyalkylene terephthalate resins having the properties indicated above are prepared according to known methods of esterification of mixtures of terephthalic and isophthalic acid with alkylene glycols with 2-8 carbon atoms, such as for example ethylene glycol, 1,4 butanediol, and polycondensation of the associated glycol esters or from mixtures of dimethyl esters by transesterification/polycondensation and by poly addition in the solid state of the resins thus obtained, operating in the presence of a dianhydride of a tetracarboxylic acid, preferably aromatic, to obtain an increase in the viscosity of the starting resin of at least 0.1 unit dl/g and melt strength values greater than 1 cN at 280° C.
The intrinsic viscosity after SSP preferably lies between 0.9-1.2 dl/g and the melt strength is preferably greater than 2 cN and in general lies between 5-20 cN, always at 280° C.
The viscosity in the molten state between 2000-3500 Pa·s at 280° C. for shear rates tending to 0 are generally suitable; Higher viscosity, however, can be utilised. The viscosity in the molten state at 280° C. corresponding with shear rates of 1000 sec
−1
is in general greater than 200 Pa·s.
The polyaddition reaction in the solid state is conducted using known methods operating at temperatures less than the melting point of the resin and greater than the Tg thereof, in general between 130°-200° C.
Before being subjected to the reaction in the solid state, the first resin is subjected to a crystallization treatment for the purpose of avoiding the phenomena of sticking in the polycondensation reactor. The working temperature lies in general between 130° and 180° C. utilizing, for example, a fluid bed.
A polyethylene terephthalate containing 8-15% of units of isophthalic acid is the preferred resin.
Beyond the statistical copolyethylene terephthalate/isophthalate copolymers prepared as indicated above, block copolymers of various types can also be used, always having an isophthalic acid unit content of 5-20%, and are obtained, for example, by extrusion of mixtures of copolyethylene terephthalate/isophthalate having a different content of isophthalic acid units, (for example, 2% and 15% of isophthalic acid) added of pyromellitic anhydride and subsequent solid state poly addition to obtain the desired rheological characteristics.
The block copolymers thus obtained have a crystallization temperature generally lying between 130° and 180° C. depending on the isophthalic acid content.
Copolyalkylene terephthalate/isophthalate resins can be used in mixtures with compatible amorphous polymers such as, for example, &egr;-polycaprolactone, used in such quantities so as not to compromise the transparency characteristics of the articles.
The transparency is determined by UV/VIS spectrophotometric measurements. The transmission of light is greater than 50% in the wavelength interval from 1100 to 450 nm.
The dianhydride of the tetracarboxlic acid is mixed with the resin in quantities from 0.05 to 2% by weight, preferably 0.1-0.6%, at the output of the molten state polycondensation reactor or added in the extrusion phase of the resin.
The mixing and residence times in the extruder are relatively short (up to several minutes).
Pyromellitic dianhydride is preferred.
Examples of other dianhydrides are the dianhydrides of 1,2, 3,4-cyclobutane tetracarboxylic acid, 3,3′-, 4,4′-benzophenone tetracarboxylic acid, 2,2-bis (3,4-dicarboxyphenyl) propane, bis (3,4-dicarboxyphenyl) ether.
The transparent articles of the present invention are prepared using known moulding, extrusion, injection, thermoforming techniques or other technologies.
The articles are usable for both indoor and outdoor

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