Polymer mixture based on a polybutylene terephthalate ester and a S-MA copolymer and films made thereof

Disclosed are polymer mixtures comprising a polybutylene terephthalate ester and a rubber-modified styrene-maleic anhydride copolymer. The polymer mixtures are suitable for the manufacture of films which can be readily bonded to polystyrene substrates.

The invention relates to a polymer mixture based on a polybutylene 
terephthalate ester and styrene-maleic anhydride copolymer and to films 
made thereof. 
Polymer mixtures comprising a polybutylene terephthalate and a vinyl 
aromatic polymer are known from U.S. Pat. No. 3,644,574. The vinyl 
aromatic polymer is added to the polybutylene terephthalate in order to 
increase the impact strength and/or stiffness and/or heat distortion 
temperature and/or tensile strength of the polybutylene terephthalate. By 
way of example of a vinyl aromatic polymer one may use according to U.S. 
Pat. No. 3,644,574 a poly (50/50 styrene/maleic anhydride). 
It has now been found that by using a special type of styrene-maleic 
anhydride copolymer in polybutylene terephthalate based polymer mixtures 
one obtains a mixture from which one can make a film which readily adheres 
to a polystyrene film. Even without using a special bonding agent one can 
obtain a good adhesion by merely applying heat and pressure between the 
polystyrene film and the film made out of the polymer mixture according to 
the invention. 
The use of a special type of styrene-maleic anhydride copolymer in the 
polymer mixture according to the invention moreover results in a mixture 
having an optimal combination of properties in particular an optimal, 
combination of adhesive properties, melt processability and mechanical 
properties. 
The polymer mixture according to the invention is characterized in that it 
comprises a rubber modified styrene-maleic anhydride copolymer with a 
rubber content of 3-20% by weight, with 3-20 mol % maleic anhydride and 
80-97 mol % styrene and with a number average molecular weight as 
determined by gel permeation chromatography (against a polystyrene 
standard) of 50,000-400,000. 
The polymer mixture preferably comprises 1-50% by weight of the 
styrene-maleic anhydride copolymer and 99-50% by weight of the 
polybutylene terephthalate. 
GB-A-2076832 describes the addition of styrene-maleic anhydride copolymers 
to polybutylene terephthalate esters in order to improve the adhesion 
properties thereof. As styrene-maleic anhydride copolymers those having a 
molecular weight from about 2,000 to about 20,000 are according to 
GB-A-2076832 the preferred ones. Said copolymers are exemplified by 
DYLARK.RTM. 232, which is a non rubber modified styrene-maleic copolymer. 
It has now been found that the special styrene-maleic anhydride as defined 
above in polybutylene terephthalate results in a better combination of 
properties as compared to the use of the low molecular weight non-rubber 
modified copolymers as referred to in GB-A-2076832. 
The polymer mixture according to the invention comprises at least the 
following constituents: 
A. a polybutylene terephthalate ester and 
B. a rubber modified styrene-maleic anhydride copolymer. 
A. Polybutylene terephthalate ester 
Polybutylene terephthalate esters (PBT) are generally known polymers. They 
are usually prepared by the condensation reaction of 1,4-butanediol and 
terephtalic acid. 
In the PBT esters according to the invention up to 20 mol % of the 
butanediol component may have been replaced by one or more suitable diol- 
or polyol compounds. It is also possible to use PBT esters in which up to 
20 mol % of the terephthalate acid component has been replaced by other 
dicarboxylic acids or polycarboxylic acids. When both the 1,4-butanediol 
and the terephthalate acid have been partly replaced the sum total of the 
1,4-butanediol and the terephthalate acid component in the PBT ester 
should be at least 70 mol %. 
The polymer mixture according to the invention preferably has an intrinsic 
viscosity from 0.7 to 2.0, more preferably from 0.7-1.5 dl per gram 
(measured in a 60/40 mixture of phenol/tetrachloroethane at 25.degree. 
C.). 
B. Rubber modified styrene-maleic anhydride copolymers 
Rubber modified styrene-maleic anhydride copolymers of the types used in 
the polymer mixture according to the invention are generally known. They 
are commercially available from Arco under the trademark designation 
DYLARK.RTM.. 
In order to obtain an optimal balance in mechanical and adhesive properties 
and in melt processability the copolymer should have a rubber content 
between 3-20% by weight, a maleic anhydride content of 3-20 mol % and a 
styrene content of 80-97 mol %. The number average molecular weight as 
determined by gel permeation chromatography while using a polystyrene 
standard, should lie between 50,000 and 400,000, preferably between 
100,000 and 400,000. 
In addition to the above-mentioned constituents the polymer mixture 
according to the invention may comprise further constituents generally 
used in mixtures based on polyalkylenephthalate esters. Examples of such 
further constituents are agents to improve the impact strength, inert 
fillers, reinforcing fibres, agents to improve the flame-retarding 
properties, pigments and dyes, mould-release agents, stabilisers, 
processing aids, plasticisers. 
The polymer mixture according to the invention can be obtained according to 
the conventional methods of preparing polymer mixtures based on 
thermoplastics. The polymer mixture according to the invention can be 
prepared by the collective extrusion of the individual constituents in an 
extruder. The resulting extrudate is then chopped to form pellets. It is 
possible to manufacture a layer or a film from the resulting pellets by 
melt extrusion. This layer may be bonded to a polystyrene layer. It is 
also possible to co-extrude the pellets to form a laminate in a machine 
which is suitable for coextrusion. 
It is also possible to transfer the individual constituents of the polymer 
mixture according to the invention to an extruder or coextruder which has 
a gapshaped extrusion head. In this case, a film or foil of the polymer 
mixture according to the invention is directly obtained.

The invention will be described in greater detail with reference to the 
ensuing specific examples. 
In the examples, polymer mixtures were used which had been prepared as 
follows: the polybutylene terephthalate ester together with the indicated 
functionalised polystyrene compound was extruded in an extruder and 
chopped to form pellets. The extruder had been adjusted at a temperature 
of approximately 250.degree. C. (250 rpm). 
In the manner described hereinafter in Example I and Example II, laminates 
were manufactured from the resulting pellets of the polymer mixture 
according to the invention. The laminates always consisted of two layers: 
one film of high-impact polystyrene and one film of a pure polybutylene 
terephthalate ester or of a polymer mixture according to the invention. 
EXAMPLE I 
Several different films were manufactured out of polymer mixtures having a 
composition as indicated hereinafter (Table A). The films were prepared by 
compressing the polymer mixtures in a mould (dimension of the compressed 
films: 180.times.50.times.0.65 mm). The mixtures were first heated in the 
mould to just above their softening point and were kept at that 
temperature for 30 seconds. The material was then brought under a pressure 
of 25 kN for two minutes. The obtained films were then cooled in air. 
TABLE A 
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Layer no. 
Composition (% by weight) 
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1 High-impact polystyrene (with 9.5% by weight 
of rubber and 20-25% by weight of gel 
fraction). 
2 Polybutyleneterephthalate (PBT) (intrinsic 
viscosity 1.18 dl/g. 
3 80% by weight of PBT + 20% by weight of 
rubber-modified styrene-maleic anhydride 
copolymer (DYLARK .RTM. 250), with a rubber con- 
tent of about 15% by weight, a maleic 
anhydride content of about 10 mol %, a 
styrene content of about 90 mol % and a 
number average molecular weight as deter- 
mined by gel permeation chromatography 
(against a polystyrene standard) of about 
190,000. 
4 50% by weight of PBT + 50% by weight of 
DYLARK .RTM. 250. 
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Several multilayer structures, each consisting of one film 1 and one of the 
films 2 to 4 were manufactured. 
The structures were manufactured by pressing the films one on top of the 
other under a pressure of 25 kN for 2 minutes at 250.degree. C. 
The mutual bonding strength of the films in the resulting two-layer 
structures was determined as follows. The two layers were separated from 
each other mechanically (by means of a knife) over a length of 40 mm. The 
resulting ends were each bent at an angle of approximately 90.degree. with 
the two-layer structure and clamped in a drawing machine. The layers were 
pulled apart at a drawing rate of 5 mm/minute. The force occurring when 
the two layers detached (fracture) was determined. The values found are 
recorded in Table B. 
TABLE B 
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Structure built up 
Force when the layers 
from layer Nos. 
detached (N) 
______________________________________ 
2 & 1* 4 
3 & 1 12.5 
4 & 1 24 
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*comparative example 
It may be seen from the results of Table B that it is possible with the 
polymer mixtures according to the invention (used in the layers 3 and 4) 
to obtain a film having a good bonding to polystyrene. 
EXAMPLE II 
Two structures, each built up from a film of high-impact polystyrene with a 
rubber content of 8-10% by weight and a gel fraction of 20-25% by weight 
and a layer of polybutylene terephthalate (PBT having an intrinsic 
viscosity of 1.10 parts/g) were manufactured by coextrusion in a 
Reifenhauser extruder. In the first case the layer consisted of pure PBT; 
in the second case it consisted of a mixture of 90% by weight of PBT and 
10% by weight of DYLARK.RTM. 250 (as used in Example I). 
In the coestrusion in a Reifenhauser extruder, the PBT/DYLARK.RTM. mixture 
was prepared directly in the coextruder of the machine. The high-impact 
polystyrene was added in the main extruder. 
A procedure similar to that described in Example I was followed for 
determining the bonding strength between the layers: the layers were 
separated from each other mechanically over a certain length. The end of 
one of the two layers was bent back over an angle of approximately 
180.degree. C. The ends of the detached layers were clamped in a drawing 
machine. The force was determined which occurred when the two layers 
detached (fracture). The found values are recorded in Table C. 
TABLE C 
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Force when the layers 
Structure detached (N) 
______________________________________ 
.smallcircle. High-impact polystyrene/ 
0.3 
PBT* 
.smallcircle. High-impact polystyrene/ 
&gt;35 
PBT + DYLARK .RTM. 250 
______________________________________ 
*Comparative example 
It also appears from Table C that in structures having layers consisting of 
(high-impact) polystyrene and PBT a certain bonding already occurs between 
the layers. This bonding can be considerably improved by using a polymer 
mixture according to the invention.