Stock material or miscellaneous articles – Composite – Of polyester
Reexamination Certificate
2001-02-08
2003-02-18
Szekely, Peter (Department: 1714)
Stock material or miscellaneous articles
Composite
Of polyester
C523S351000, C524S423000, C428S332000, C428S337000
Reexamination Certificate
active
06521351
ABSTRACT:
The invention relates to an opaque, white UV-resistant film with low transparency made from a crystallizable thermoplastic and having a thickness of from 10 to 500 &mgr;m. The film comprises at least barium sulfate, at least one optical brightener, and also a UV stabilizer as light stabilizer, and has good orientability, low transparency, and also very good optical and mechanical properties. The invention further relates to a process for producing this film and to the use of the film.
BACKGROUND OF THE INVENTION
Opaque films with a thickness of from 10 to 500 &mgr;m are well known. None of the known films comprises either UV stabilizers as light stabilizer or optical brighteners, and neither the films nor items produced from them are therefore suitable for outdoor applications. In outdoor applications these films yellow, even after a short period, and their mechanical properties become impaired as a result of photooxidative degradation of the thermoplastic by sunlight.
EP-A-0 620 245 describes films with improved heat resistance. These films comprise antioxidants suitable for scavenging free radicals formed in the film and for degrading any peroxide formed. However, this specification makes no proposal as to how the UV resistance of films of this type may be improved.
DESCRIPTION OF THE INVENTION
The object of the present invention was to provide an opaque, white film with a thickness of from 10 to 500 &mgr;m which, besides having good orientability, good mechanical properties and good optical properties and a low Yellowness Index, above all has high UV resistance and offers a high level of protection from light.
For the purposes of the present invention, high UV resistance means that the films suffer no damage or extremely little damage when exposed to sunlight or other UV radiation, and therefore that the films are suitable for outdoor applications and/or critical indoor applications. In particular, when the films are used outdoors for a number of years they should in particular not yellow, nor become brittle and not have surface-cracking, and also have unimpaired mechanical properties. High UV resistance therefore implies that the film absorbs the UV light and does not transmit light until the visible region has been reached. The good optical properties include uniform, streak-free coloration over the entire length and width of the film, low luminous transmittance/transparency (≦0%), acceptable surface gloss (≧10), and also a low Yellowness Index (dependent on thickness, ≦45 for 250 &mgr;m films and ≦20 for 50 &mgr;m films).
The good mechanical properties include a high modulus of elasticity (EMD≦3300 N/mm2; ETD≧4200 N/mm2), and also good tear strengths (in MD≧120 N/mm2; in TD≧170 N/mm2) and good longitudinal and transverse elongations at break (in MD≧120%; in TD≧50%).
Good orientability includes excellent capabilities of the film for orientation during its production, both longitudinally and transversely, without break-offs.
The novel film should moreover be recyclable, that is to say that any cut material arising during continuous film production can be fed back into the production operation as regrind, in particular without loss of optical or mechanical properties from the film, so that it can still be used for interior applications and in constructing exhibition stands, for example.
This object is achieved by an opaque white film with a thickness of from 10 to 500 &mgr;m whose principal constituent is a crystallizable thermoplastic, wherein the film comprises at least barium sulfate as pigment, at least one UV stabilizer as light stabilizer and at least one optical brightener.
The novel film comprises, as principal constituent, a crystallizable thermoplastic. Examples of suitable crystallizable or semicrystalline thermoplastics are polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate, and preference is given to polyethylene terephthalate.
For the purposes of the present invention, a crystallizable thermoplastic is
a crystallizable homopolymer;
a crystallizable copolymer;
a crystallizable compound;
a crystallizable recycled material, or
another type of crystallizable thermoplastic.
The novel film may have one layer, or else two or more layers, and it may have been coated with various copolyesters or with adhesion promoters.
The novel film comprises at least one UV stabilizer as light stabilizer, and this is appropriately fed by way of what is known as masterbatch technology directly during film production, and the amount of UV stabilizer here is preferably from 0.01 to 5% by weight, based on the weight of the crystallizable thermoplastic.
The novel film comprises at least barium sulfate as pigment, and the amount of pigment here is preferably from 0.2 to 40% by weight, based on the weight of the crystallizable thermoplastic. The barium sulfate is preferably fed to the thermoplastic by way of what is known as masterbatch technology during film production.
The film comprises at least one optical brightener, and the amount of the optical brightener used here is from 10 to 50,000 ppm, in particular from 20 to 30,000 ppm, particularly preferably from 50 to 25,000 ppm, based on the weight of the crystallizable thermoplastic. It is preferable for the optical brightener, too, to be fed to the thermoplastic by way of what is known as masterbatch technology during film production.
Light, in particular the ultraviolet content of solar radiation, i.e. the wavelength region from 280 to 400 nm, induces degradation in thermoplastics, as a result of which their appearance changes due to color change or yellowing, and there is an adverse effect on mechanical/physical properties.
Inhibition of this photooxidative degradation is of considerable industrial and economic importance, since otherwise there are drastic limitations on the applications of many thermoplastics.
The absorption of UV light by polyethylene terephthalates, for example, starts at below 360 nm, increases markedly below 320 nm and is very pronounced at below 300 nm. Maximum absorption occurs at between 280 and 300 nm.
In the presence of oxygen it is mainly chain cleavage which occurs, but there is no crosslinking. The predominant photooxidation products in quantity terms are carbon monoxide, carbon dioxide and carboxylic acids. Besides the direct photolysis of the ester groups, consideration has to be given to oxidation reactions which likewise form carbon dioxide, via peroxide radicals.
In the photooxidation of polyethylene terephthalate there can also be cleavage of hydrogen at the position alpha to the ester groups, giving hydroperoxides and decomposition products of these, and this may be accompanied by chain cleavage (H. Day, D. M. Wiles: J. Appl. Polym. Sci 16, 1972, p. 203).
UV stabilizers, i.e. light stabilizers which are UV absorbers, are chemical compounds which can intervene in the physical and chemical processes of light-induced degradation. Carbon black and other pigments can give some protection from light. However, these substances are unsuitable for opaque white films, since they cause discoloration or color change. The only compounds suitable for opaque white films are those organic or organometallic compounds which produce no, or only extremely slight, color or color change in the thermoplastic to be stabilized. UV stabilizers suitable as light stabilizers are those which absorb at least 70%, preferably at least 80%, particularly preferably at least 90%, of the UV light in the wavelength region from 180 to 380 nm, preferably from 280 to 350 nm. These are particularly suitable if they are thermally stable in the temperature range from 260 to 300° C., that is to say they do not decompose into cleavage products and do not cause release of gases. Examples of UV stabilizers suitable as light stabilizers are 2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickel compounds, salicylic esters, cinnamic ester derivatives, resorcinol monobenzoates, oxanilides, hydroxybenzoic esters, and sterically hindered amines and triazines, and among these pref
Crass Guenther
Kern Ulrich
Murschall Ursula
Oberlaender Klaus
Mitsubishi Polyester Film GmbH
ProPat L.L.C.
Szekely Peter
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