Stabilized film based on titanium-catalyzed polyesters

Details Stabilized film based on titanium-catalyzed polyesters Stabilized film based on titanium-catalyzed polyesters

2002-09-12

2004-08-17

Acquah, Samuel A. (Department: 1711)

Stock material or miscellaneous articles

Composite

Of polyester

C528S272000, C528S287000, C528S293000, C528S298000, C528S302000, C528S307000, C528S308000, C528S308600, C524S081000, C524S115000, C524S186000, C524S323000, C264S176100, C264S210700, C264S211120, C264S212000, C264S141000, C430S032000

Reexamination Certificate

active

06777099

ABSTRACT:

The invention relates to a single- or multilayer, biaxially oriented film based on preferably titanium-catalyzed polyesters and comprising at least one primary stabilizer for inhibiting oxidative degradation. The invention further relates to the use of a film, and to a process for its production.
BACKGROUND OF THE INVENTION
The catalysts used industrially for the polycondensation of polyesters are antimony compounds, titanium compounds, and germanium compounds. Of this group, germanium-containing catalysts have the highest catalytic activity, followed by titanium catalysts. Antimony catalysts have the lowest relative activity. However, the vast majority of the polyesters used in the world market are prepared with the aid of antimony catalysts. The reason for this is that germanium catalysts are generally uneconomic due to their extremely high price, and titanium catalysts can cause undesirable yellowing and reduced heat resistance in the resultant film. This is the result of side reactions which lead to an increased number of undesirable gel particles and also to a marked reduction in the stability of films of this type during processing.
Nevertheless, there is a high level of interest in markedly reducing the concentration of antimony in polyester products, one reason being that there are new EU directives which prescribe limits for antimony migration, and compliance with these is particularly difficult in low-crystallinity PET (polyethylene terephthalate).
Films based on polyesters, in particular based on polyethylene terephthalates, are known. Polyester films which comprise additives for inhibiting oxidative degradation have also been described.
Polyester films are generally produced from polymer pellets melted in an extruder. The resultant polymer melt is molded by way of a slot die to give what is known as a prefilm. The prefilm is then applied to a take-off roll and chill roll, and then stretched longitudinally and transversely, and finally wound up. These films are intended to have a low number of gel particles and other defects which impair appearance or further processing. Gel particles in the film reduce stability during processing in film production, i.e. they cause undesirable break-off. In addition, gel particles block the extrusion screens, causing further economic losses.
It is therefore an object to find a polyester film which does not have the disadvantages known from the prior art, e.g. high antimony content and gel particles in the film. In particular, the film should have fewer specks and fewer defects caused by gel particles. In addition, stability during processing should be increased, i.e. no film break-offs or similar disruptions should occur during production. The screen service times should be prolonged, and it should also be possible for even relatively large proportions of the film regrind to be reusable. Furthermore, these improved properties should permit optimization of throughput and production speed.
BRIEF DESCRIPTION OF THE INVENTION
The object is achieved by way of a film based preferably on a polyester which uses titanium-based catalysis and comprises a stabilizer at least some of which has been covalently incorporated into the polyester. This covalent incorporation of the stabilizer is preferably achieved by adding the stabilizer to the polyester polymers before polycondensation begins.
The present invention therefore provides a single- or multilayer, biaxially oriented film, preferably based on a Ti-catalyzed (Ti=titanium) polyester, and comprising at least one (primary) stabilizer for inhibiting oxidative degradation, wherein at least some of the (primary) stabilizer present in the polyester is in covalently bonded form.
DETAILED DESCRIPTION OF THE INVENTION
The proportion of the stabilizer in the polyester of the invention is preferably from 100 to 10 000 ppm, in particular from 150 to 3 000 ppm, particularly preferably from 200 to 1 000 ppm. ppm are parts by weight based on the total weight of the polyester comprising the stabilizer. It is also possible to use mixtures of various stabilizers.
This “primary” stabilizer is preferably a constituent of the main polymer chain of the polyester. It therefore contains groups capable of polycondensation during formation of the polyester, preferably carboxy and/or ester groups, particularly preferably ester groups. Particularly preferred primary stabilizers are phenolic compounds, in particular sterically hindered phenols, which have additional ester groups and/or carboxy groups which are capable of participating in the polycondensation reaction. Examples of these sterically hindered phenols are obtainable with the name Irganox® from Ciba Specialty Chemicals. Other suitable stabilizers are thiobisphenols, alkylidenebisphenols, alkylphenols, hydroxybenzyl compounds, acylaminophenols, and hydroxyphenylpropionates. Aromatic compounds having 2 or more secondary amino groups are also suitable. Theme primary stabilizers are described by way of example in the monograph by Gächtar and Müller “
Kunststoffadditive
” [Plastics additives], 2nd edition, Carl Hanser Verlag, and in the monogragh by Dr. Hans Zweifel, “
Plastics Additives Handbook
”, 5th edition, Carl Hanser Verlag.
Where appropriate, these primary stabilizers are used in combination with secondary stabilizers which support, or else intensify, the action of the primary stabilizers. Particularly suitable secondary stabilizers are thioethers, and also zinc dibutyldithiocarbamates. The secondary stabilizer is usually an additive, i.e. is not an integral (covalently bonded) constituent of the polyester. Its proportion is preferably 0.01 to 1.0% by weight, in particular from 0.05 to 0.5% by weight, based in each case on the weight of the polyester comprising the stabilizer.
The polyester of the invention having the covalently incorporated primary stabilizer (also termed “modified polyester” below) preferably has an SV in the range from 450 to 1 100, in particular from 700 to 900.
If this polyester is prepared by the dimethyl terephthalate (DMT) process, for example, it is advantageous for the primary stabilizer to be added prior to the transesterification process, or after the transesterification process and directly prior to the polycondensation process, in the form of a solution or dispersion in glycol. The melt resistivity of the resultant modified polyester is in the range from 1·10
7
to 120·10
7
ohm/cm, and is therefore not significantly different from that of an unmodified polyester. Where appropriate, the melt resistivity may be adjusted by adding ionic additives until the desired value has been reached. Examples of these additives are magnesium stearate and potassium acetate, etc.
The modified polyester preferably contains units of ethylene glycol and terephthalic acid and/or units of isophthalic acid, naphthalene-2,6-dicarboxylic acid, and/or biphenyldicarboxylic acid (=bibenzoic acid), and/or units of aliphatic or cycloaliphatic diols having from 2 to 8 carbon atoms, in particular of propylene glycol, butylene glycol, and/or cyclohexanedimethanol. The materials are therefore preferably modified polyethylene terephthalates, polybutylene terephthalates, poly(1,4-cyclohexanedimethylene terephthalate), polyethylene naphthalene-2,6-dicarboxylate, polyethylene naphthalene-1,5-dicarboxylate, or polyethylene naphthalate/bibenzoate. Among these, preference is given to modified polyethylene terephthalate (PET) and modified polyethylene naphthalate (PEN), and also to mixtures of these. In another preferred embodiment, the polyester also contains up to 5% by weight isophthalic acid.
The term modified polyethylene terephthalates or polyethylene naphthalates here is intended to mean homopolymers, compounded materials, copolymers, recycled materials, and other variants.
An example of a method for preparing the polyesters of the invention is from dimethyl terephthalate, using known processes, e.g. using the transesterification process (DMT process). Suitable transesterification catalysts used here are salts of zinc, of magnesium,

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