Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Reexamination Certificate
1998-01-26
2001-02-13
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
C428S038000, C428S096000, C428S213000, C428S042100
Reexamination Certificate
active
06187422
ABSTRACT:
The invention relates to an amorphous, transparent sheet of a crystallizable thermoplastic whose thickness is in the range from 1 to 20 mm. The sheet is distinguished by very good optical and mechanical properties. The invention furthermore relates to a process for the production of this sheet, and to the use of the sheet.
Amorphous, transparent sheets having a thickness of from 1 to 20 mm are well known. These two-dimensional structures comprise amorphous, non-crystallizable thermoplastics. Typical examples of such thermoplastics which can be converted into sheets are, for example, polyvinyl chloride (PVC), polycarbonate (PC) and polymethyl methacrylate (PMMA). These semifinished products are produced on so-called extrusion lines (cf. Polymer Werkstoffe, Volume Is, Technologie 1, p. 136, Georg Thieme Verlag, Stuttgart, 1984). The pulverulent or granular raw material is melted in an extruder. After extrusion, the amorphous thermoplastics can easily be shaped via polishing stacks or other shaping tools as a consequence of their viscosity, which continuously increases with decreasing temperature. After shaping, amorphous thermoplastics then have adequate stability, i.e. a high viscosity, in order to be self-supporting in the calibration die. However, they are still sufficiently soft to be shaped by the die. The melt viscosity and inherent rigidity of amorphous thermoplastics is so high in the calibration die that the semifinished product does not collapse in the calibration die before cooling. In the case of easily decomposed materials, for example PVC, special processing aids, for example processing stabilizers against decomposition and lubricants against excessive internal friction and thus uncontrollable warming, are necessary during extrusion. External lubricants are necessary to prevent sticking to walls and rolls.
The processing of PMMA is carried out using, for example, a vented extruder to enable removal of moisture.
The production of transparent sheets from amorphous thermoplastics sometimes requires expensive additives, which in some cases migrate and can cause production problems owing to evaporations and surface coatings on the semifinished product. PVC sheets can be recycled only with difficulty or using special neutralization or electrolysis processes. PC and PMMA sheets can likewise only be recycled with difficulty and only with loss or extreme impairment of the mechanical properties.
In addition to these disadvantages, PMMA sheets also have extremely poor impact strengths and splinter on fracture or mechanical loading. Furthermore, PMMA sheets are readily combustible, which means that they cannot be employed, for example, for internal applications and in exhibitions.
PMMA and PC sheets furthermore cannot be shaped when cold; PMMA sheets disintegrate to form dangerous splinters, while PVC sheets undergo hairline cracking and stress whitening.
EP-A-0 471 528 describes a process for shaping an article from a polyethylene terephthalate (PET) sheet. The PET sheet is heat-treated on both sides in the thermofilming mold in a temperature range between the glass transition temperature and the melting point. The shaped PET sheet is removed from the mold when the degree of crystallization of the shaped PET sheet is in the region of 25 to 50%. The PET sheets disclosed in EP-A-0 471 528 have a thickness of from 1 to 10 mm. Since the thermoformed molding produced from the PET sheet is partially crystalline and thus no longer transparent and the surface properties of the molding are determined by the thermoforming process and by the thermoforming temperature and mold, it is unimportant what optical properties (for example gloss, haze and light transmission) the PET sheets employed have. In general, the optical properties of these sheets are poor and in need of optimization.
U.S. Pat. No. 3,496,143 describes the vacuum thermoforming of a 3 mm thick PET sheet whose degree of crystallization is said to be in the range from 5 to 25%. However, the crystallinity of the thermoformed molding is greater than 25%. Again, no requirements regarding optical properties are made of these PET sheets. Since the crystallinity of the sheets employed is already between 5 and 25%, these sheets are hazy and non-transparent.
The object of the present invention is to provide an amorphous, transparent sheet having a thickness of from 1 to 20 mm which has both good mechanical properties and good optical properties.
The good optical properties include, for example, high light transmission, high surface gloss, extremely low haze and high clarity.
The good mechanical properties include, inter alia, high impact strength and high breaking strength.
In addition, the novel sheet should be recyclable, in particular without loss of the mechanical properties, and have low combustibility so that it can also be employed, for example, for internal applications and in exhibitions.
This object is achieved by a transparent, amorphous sheet having a thickness in the range from 1 to 20 mm which contains, as principal constituent, a crystallizable thermoplastic, where the surface gloss, measured in accordance with DIN 67530 (measurement angle 20°), is greater than 130, preferably greater than 140, the light transmission, measured in accordance with ASTM D 1003, is greater than 84%, preferably greater than 86%, and the haze of the sheet, measured in accordance with ASTM D 1003, is less than 15%, preferably less than 11%.
The transparent, amorphous sheet contains, as principal constituent, a crystallizable thermoplastic. Suitable crystallizable or partially crystalline thermoplastics are, for example, polyethylene terephthalate, polybutylene terephthalate, cycloolefin polymers and cycloolefin copolymers, particular preference being given to polyethylene terephthalate.
For the purposes of the invention, the term crystallizable thermoplastic is taken to mean
crystallizable homopolymers,
crystallizable copolymers,
crystallizable compounds,
crystallizable recyclate and
other variations of crystallizable thermoplastic.
For the purposes of the present invention, the term amorphous sheet is taken to mean a sheet which is noncrystalline, although the crystallizable thermoplastic employed preferably has a crystallinity of from 25 to 65%. Non-crystalline, i.e. essentially amorphous, means that the degree of crystallinity is generally less than 5%, preferably less than 2%, particularly preferably 0%.
In the case of polyethylene terephthalate, measurement of the Charpy impact strength a
n
(measured in accordance with ISO 179/1D) of the sheet is preferably not accompanied by a fracture. In addition, the Izod notched impact strength a
k
(measured in accordance with ISO 180/1A) is preferably in the range from 2.0 to 8.0 kJ/m
2
, particularly preferably in the range from 4.0 to 6.0 kJ/m
2
.
The clarity of the sheet measured at an angle of less than 2.5° (ASTM D 1003) is preferably greater than 96%, particularly preferably greater than 97%.
Polyethylene terephthalate polymers having a crystalline melting point T
m
, as measured by DSC (differential scanning calorimetry) at a heating rate of 10° C./min, of from 220° to 280° C., preferably from 250° C. to 270° C., a crystallization temperature range T
c
from 75° C. to 280° C., a glass transition temperature T
g
of from 65° C. to 90° C. and a density, measured in accordance with DIN 53479, of from 1.30 to 1.45 and a crystallinity of from 5% to 65%, preferably from 25% to 65%, are preferred polymers as starting materials for production of the sheet.
The standard viscosity SV (DCA) of the polyethylene terephthalate, measured in dichloroacetic acid in accordance with DIN 53728, is from 800 to less than 1800, in particular from 800 to 1400, preferably from 950 to 1250, particularly preferably from 1000 to 1200.
The intrinsic viscosity IV (DCA) is calculated from the standard viscosity SV (DCA) as follows:
IV (DCA)=6.67·10
−4
SV (DCA)+0.118
The bulk density, measured in accordance with DIN 53466, is preferably from 0.75 to 1.0 kg/dm
3
, particularly preferably from 0.80 to
Brunow Rainer
Gawrisch Wolfgang
Murschall Ursula
Dixon Merrick
Frommer Lawrence & Haug
Hostaglas LTD
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