A copolyester-containing plastic sheet, a process for the manufacture of the plastic sheet and mouldings manufactured from the plastic sheet

Abstract: Plastic sheet which contains a copolyester which copolyester contains 45-50 mole % monomer units derived from a first aliphatic diol, 45-50 mole % monomer units derived from a first aromatic dicarboxylic acid and 0.5-5 mole % monomer units derived from a second aliphatic diol and/or a second aromatic dicarboxylic acid, with the plastic sheet comprising a core section which contains the copolyester and there being present on at least one surface of the sheet a surface layer of thickness 2-100 µ which contains the copolyester and 1-15 wt.% of a UV light-absorbing additive, with the exception of plastic sheets whose core section and surface layer contain a copolyester that contains 50 mole % monomer units derived from terephthalic acid, 49.5-45 mole % monomer units derived from ethylene glycol and 0.5-5 mole % monomer units derived from cyclohexane dimethanol. The plastic sheet has a lower rate of crystallization, better surface quality and better resistance to solvents.

Description: The invention relates to a plastic sheet which contains a copolyester which copolyester contains 45-50 mole % monomer units derived from a first aliphatic diol, 45-50 mole % monomer units derived from a first aromatic dicarboxylic acid and 0.5-5 mole % monomer units derived from a second aliphatic diol and/or a second aromatic dicarboxylic acid. Plastic sheets of this type are commonly known. Since the copolyester contains a minor amount of the comonomers, the copolyester exhibits a low crystallization rate in comparison with a polyester that does not contain the comonomers. As a result, it is generally possible to extrude the copolyester to a plastic sheet having high luminous transmittance and low haze. A disadvantage of the known plastic sheet, however, is that, especially if the sheet is thick, the copolyester crystallizes during manufacture of the sheet, resulting in inadequate luminous transmittance and unacceptable haze of the plastic sheet. Furthermore, the plastic sheet can be shaped into end products at elevated temperature only to a limited extent without the copolyester crystallizing. As a result, during the shaping process, the luminous transmittance of the sheet decreases and the haze of the plastic sheet increases. The invention aims to provide a plastic sheet that does not have the aforementioned disadvantage or at least has the aforementioned disadvantage to a lesser degree. Surprisingly, this object is achieved in that the plastic sheet comprises a core section which contains the copolyester whilst there is on at least one surface of the plastic sheet a layer of thickness 2-100 µm which contains the copolyester and 1-15 wt.% of a UV light-absorbing additive. This ensures a lower crystallization rate of the copolyester in the plastic sheet according to the invention than in the known plastic sheet. Excluded are plastic sheets whose core section and surface layer contain a copolyester that contains 50 mole % monomer units derived from terephthalic acid, 49.5-45 mole % monomer units derived from ethylene glycol and 0.5-5 mole % monomer units derived from cyclohexane dimethanol. Such a plastic sheet is disclosed in the European patent application EP-A-595413 which falls within the terms of Art. 54(3) EPC. A further advantage of the plastic sheet of the invention is that the plastic sheet has fewer matt stains on the surface. The known plastic sheet has mostly matt stains on the surface, particularly in the case of small sheet thicknesses, which stains develop during extrusion of the plastic sheet, whereas such matt stains hardly develop, if at all, during the extrusion of the plastic sheet of the invention. Another advantage of the plastic sheet of the invention is that the plastic sheet is more resistant to solvents such as acetone. In addition, the plastic sheet exhibits good weather resistance, good optical and mechanical properties, the plastic sheet is readily recyclable and there exists a good bond between the surface layer and the core section of the plastic sheet. The term sheet as used in this application should be understood to mean a body whose thickness is small in comparison with its length and width and having a thickness of at least 0.3 mm. At lower thickness of 0.3 mm the sheet becomes too flexible. Sheet should also be taken to include twin-wall sheet. The sheet may be flat or curved, for instance corrugated sheet. By surfaces of the sheet is meant the two outside surfaces that are at right angles to the thickness direction. The thickness of the sheet is preferably between 0.5 and 10 mm, more preferably between 0.7 and 5 mm. The surface layer may be present on only one surface of the plastic sheet. Such a plastic sheet, however, may be exposed to the weather only with the surface where the surface layer is present. Preferably, the surface layer is present on both surfaces of the plastic sheet. For the copolyester contained in the core section and the copolyester contained in the surface layer any copolyester from the group mentioned in the preamble may be chosen and they may be chosen independently of one another. The core section and the surface layer preferably contain the same copolyester. This results in an even better bond between the core section and the surface layer and even better recyclability. The best results are obtained if the copolyester contains 47-50 mole % monomer units derived from a first aliphatic diol, 47-50 mole % monomer units derived from a first aromatic dicarboxylic acid and 1-3 mole % monomer units derived from a second aliphatic diol and/or a second aromatic dicarboxylic acid. Ethylene glycol is preferably chosen as first aliphatic diol and terephthalic acid is preferably chosen as first aromatic dicarboxylic acid. Isophthalic acid is preferably chosen as second aromatic dicarboxylic acid. In principle, any UV light-absorbing additive suited for use in thermoplastic polyesters or copolyesters may be used in the plastic sheet of the invention, for example those mentioned in DE-A-2853631, EP-A-247480, EP-A-320632, US-4859726 and Plastics Additives Handbook, 3rd edition, Hanser Publishers of Munich, (1990), page 176 et seq. As UV light-absorbing additive use is preferably made of a benzotriazole or a benzophenone. Use may be made of for example the 2-Hydroxyphenylbenzotriazoles according to general formula 1, which is given on the sheet of formulae. Here, R1 is an alkyl group having 1-18 C atoms or an arylalkyl group having 18 C atoms. R2 is a hydrogen or chlorine atom or an alkyl group having 1-18 C atoms or an arylalkyl group having 7-18 C atoms. Examples of benzophenones that are suited for use in the sheet according to the invention are represented by general formula 2, which is given on the sheet of formulae. Here, R is a hydrogen atom or an alkyl group having 1-12 C atoms. Preferably, a benzotriazole having 2 benzotriazole groups in the molecule, for example those represented by general formula 3 on the sheet of formulae, is used in the sheet according to the invention. Here, R1 and R2 are the same or different and are a hydrogen atom or a halogen atom, an alkyl group having 1-10 C atoms, a cycloalkyl group having 5-10 C atoms, an arylalkyl group having 7-13 C atoms, an aryl group having 6-14 C atoms, OR7 or -COOR7. R7 is a hydrogen atom or an alkyl group having 1-10 C atoms, a cylcoalkyl group having 5-10 C atoms, an arylalkyl group having 7-13 C atoms or an aryl group having 6-14 C atoms. R3 and R4 are the same or different and are a hydrogen atom, an alkyl group having 1-15 C atoms, a cycloalkyl group having 5 or 6 C atoms, an arylalkyl group having 7-20 C atoms or an aryl group having 6-14 C atoms. R5 and R6 are the same or different and are an alkyl group having 2-15 C atoms, a cycloalkyl group having 5-10 C atoms, an arylalkyl group having 7-20 C atoms, an aryl group having 6-14 C atoms, -OR7 or -COOR7. a, b are the same or different and are 1, 2, 3 or 4. c, d are the same or different and are 1, 2 or 3. The benzotriazoles with 2 benzotriazole groups in the molecule possess a high molecular weight and, in consequence, evaporate or migrate less rapidly from the plastic sheet than do the UV light-absorbing additives having a lower molecular weight. The plastic sheet may be manufactured by extruding or calendering a plastic sheet which contains the copolyester and pressing onto it just before it is cooled down a film of thickness 2-100 µm which contains the copolyester and the UV light-absorbing additive. The plastic sheet is preferably manufactured by coextruding the layer or the layers which contain the copolyester and the UV light-absorbing additive and the remaining part of the sheet in one step. Such a plastic sheet possesses even better optical properties and there exists an even better bond between the surface layer and the core section of the sheet. The plastic sheet may contain the usual additives such as lubricants, antioxidants, optically active additives, antistatic agents and colouring agents. The plastic sheet can be processed into shaped articles by, for example, working the plastic sheet at elevated temperature, by bending or folding it at elevated temperature or at ambient temperature or possibly by machining, for instance cutting and sawing, it at ambient temperature. The invention also relates to shaped articles wholly or partially manufactured from the plastic sheet according to the invention like covers, skylights, glazing for greenhouses and buildings, advertising signs and displays. Example I, comparative example A Preparation of the samples: A compound X was prepared by hot-melt-compounding and mixing a mixture of 92-wt.% PET copolymer containing 3 mol.% isophthalic acid and having an IV (intrinsic viscosity) of 0.84 dl/g and 8 wt.% of the UV light-absorbing additive bis(2-hydroxy-5-t-octyl-3-(benzotriazole-2-yl)phenyl)-methane in a twin-screw extruder in the usual manner. Plastic sheets I, 2 mm thick and 1270 mm wide, were produced by means of a coextrusion process in the usual manner. The plastic sheets were made up of a core section and two surface layers. The core section consisted of the aforementioned PET copolymer whilst the surface layers consisted of the aforementioned compound X. The surface layers were 40 µm thick. Plastic sheets A, having the same dimensions as the sheets I, were produced in the usual manner using the same installation as that in which the plastic sheets I had been produced, except that only the main extruder was used so that the plastic sheets A did not contain the surface layer but consisted solely of the core section. The plastic sheets A were produced by extruding a mixture of 96 wt.% of the PET copolymer and 4 wt.% of the compound X. This means that the plastic sheets were 99.7 wt.% PET copolymer and 0.3 wt.% UV light-absorbing additive. Tests conducted on the plastic sheets 1) Crystallization behaviour Test coupons of 5x5 cm were taken from the extruded plastic sheets. The test coupons were placed in a circulating-air oven at a number of different temperatures. For each temperature the residence time in the furnace until crystallization was seen to begin, as evidenced by the coupons beginning to show some haziness, was determined. The residence time until crystallization was complete was also determined. This is the residence time needed for crystallization to progress to the point where light transmission of the coupon is almost completely lost and no further change is observed. The results are given in Table 1. 2) Environmental stress cracking The susceptibility to environmental stress cracking was determined by bending test coupons from plastic sheets I and A at room temperature through an angle of 180° and then immersing them in acetone. The results are given in Table 2. 3) Weather resistance Test coupons were taken from the plastic sheets A and I. The test coupons were subjected to accelerated ageing in a QUV Accelerated Weathering Tester (TM) supplied by Q-Panel Company of the USA and equipped with QUV-B313 lamps. The ageing cycle consisted of 4 hours UV irradiation at 60°C alternated with 4 hours condensation at 50°C. After the coupons had been subjected to accelerated ageing for 0, 100 and 250 hours, the Yellowness Index (YI) to ASTM D 1925, the luminous transmittance (LT) and the impact strength measured with the aid of an instrumented falling dart energy meter were determined. The falling dart test was conducted using an Impact (TM) II apparatus supplied by Messrs J&B of the Netherlands. The dart had a round tip 10 mm in diameter and the samples were clamped in two ring-shaped clamps with an inside diameter of 40 mm. The weight of the dart was 23 kg and the dropping height was 1 m. This test was conducted so as to determine the Maximum Load Energy, which is the amount of energy absorbed by the sample at the point where the recorded force reaches its maximum value. The results are given in Table 3. Temperature Time to beginning of crystallization Time to complete crystallization I A I A 160°C 57 s 44 s 1 min 43 s 1 min 22 s 150°C 1 min 12 s 53 s 2 min 06 s 1 min 50 s 140°C 1 min 31 s 1 min 18 s 2 min 53 s 2 min 25 s 130°C 2 min 1 min 39 s 3 min 15 s 2 min 37 s 120°C 3 min 25 s 3 min 10 s 5 min 11 s 4 min 41 s Table 1 shows that plastic sheet I of the invention crystallizes more slowly than the known plastic sheet A. Furthermore, plastic sheet A contains very many matt stains on the surface whereas plastic sheet I of the invention contains hardly any discernible matt stains. Submersion of cold-folded test sample in acetone I A Observation after 5 min no fracture fracture Observation after 24 h no fracture fracture Table 2 shows that plastic sheet I of the invention is well resistant to acetone and that the known sheet A is not. Plastic sheet Ageing 0 hours 100 hours 250 hours YI I 2.5 2.6 3 A 2.8 4.0 4.3 LT % I 87.4 87.0 86.8 A 87.5 85.9 85.1 MLE (J) I 10 10 10 A 10 2 2 Table 3 shows that sheet I of the invention yellows less rapidly, loses its luminous transmittance less rapidly and retains its impact strength.

Claims: Plastic sheet with a thickness of at least 0.3 mm which contains a copolyester which copolyester contains 45-50 mole % monomer units derived from a first aliphatic diol, 45-50 mole % monomer units derived from a first aromatic dicarboxylic acid and 0.5-5 mole % monomer units derived from a second aliphatic diol and or a second aromatic dicarboxylic acid, characterized in that the plastic sheet comprises a core section which contains the copolyester whilst there is on at least one surface of the sheet a surface layer of thickness 2-100 µm which contains the copolyester and 1-15 wt.% of a UV light-absorbing additive, with the exception of plastic sheets whose core section and surface layer contain a copolyester that contains 50 mole % monomer units derived from terephthalic acid, 49.5-45 mole % monomer units derived from ethylene glycol and 0.5-5 mole % monomer units derived from cyclohexane dimethanol. Plastic sheet according to Claim 1, characterized in that the surface layer is present on both surfaces of the plastic sheet. Plastic sheet according to either Claim 1 or 2, characterized in that the core section and the surface layer contain the same copolyester. Plastic sheet according to any one of Claims 1-3, characterized in that the copolyester contains 47-50 mole % monomer units derived from a first aliphatic diol, 47-50 mole % monomer units derived from a first aromatic dicarboxylic acid and 1-3 mole % monomer units derived from a second aliphatic diol and/or a second aromatic dicarboxylic acid. Plastic sheet according to any one of Claims 1-4, characterized in that ethylene glycol is chosen as first aliphatic diol and terephthalic acid is chosen as first aromatic dicarboxylic acid. Plastic sheet according to any one of Claims 1-5, characterized in that isophthalic acid is chosen as second aromatic dicarboxylic acid. Plastic sheet according to any one of Claims 1-6, characterized in that a benzotriazole or a benzophenone is chosen as UV light-absorbing additive. Plastic sheet according to Claim 7, characterized in that the benzotriazole contains two benzotriazole groups in each molecule. Process for the manufacture of a plastic sheet according to any one of Claims 1-8, characterized in that the plastic sheet is manufactured with the aid of a coextrusion process. Mouldings wholly or partially manufactured from the plastic sheet according to any one of Claims 1-9.

Inventors: VENEMA GERTJAN, VENEMA, GERTJAN
Applicants: AXXIS NV, AXXIS N.V.
Application year: 1995
Main classification: B32B27
IPC codes: C08K5, B32B27, C08G63