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BACKGROUND OF THE INVENTION
The present invention is concerned with a primer composition for a polymeric film and a primed polymeric film suitable for use in the preparation of an anti-fouling surface.
The pollution of the environment, and in particular air pollution, represents a problem for exterior surfaces exposed to the atmosphere. The materials used in the construction of the exterior surfaces of, for example, buildings, vehicles and
other man-made structures, may become contaminated as a result of pollutants present in the atmosphere. Particulate material present in the atmosphere, such as dust, combustion products and inorganic material (for example clay particles), is deposited
on the outer surfaces of buildings and vehicles. Upon exposure to rainfall, such deposits, as well as particles present in rainwater, flow with the rainwater across these outer surfaces. The deposits may become ingrained in the material of the exterior
surface of the building and the building becomes soiled. Such contamination is particularly problematic for constructions that are in close proximity to vehicular traffic and also for the exterior surfaces of the vehicles themselves. The fouling of the
exterior surfaces of the windows and mirrors of vehicles is a particular problem.
Various means of combating this problem have previously been disclosed. For instance, EP-0903389-A discloses an anti-fouling composition having a hydrophobic component and a hydrophilic component. The hydrophilicity is induced by a
photocatalyst component. In one embodiment, EP-0903389-A discloses a composition comprising a photocatalytic oxide such as titanium dioxide, a silicone resin or silica and a water-repellent fluororesin. The composition may be coated onto the surface of
a substrate, such as metal, ceramic, glass, plastic, wood, stone, cement, concrete, fibres and fabrics, in accordance with known coating techniques for the purpose of imparting an anti-fouling property to the substrate.
For reasons of economy and convenience, it is usually desirable to prepare the anti-fouling composition at a location that is remote from the article to which it is to be applied. For this reason it is desirable to provide a film structure
comprising the anti-fouling composition as a coating on a base film. The coated base film would then be transported to the location of the article or surface to be protected, which may comprise any of the surfaces mentioned above, and the film laminated
or adhered thereto. However, it is difficult to obtain a satisfactory and long-lasting adhesion between the base film and the anti-fouling coating composition, which is often silicone-based. In addition, known films often suffer from poor
weatherability and, over time, may become brittle and/or show deterioration in optical properties and/or colour. Thus, on ageing a film may develop microcracks as a result of weathering which will accelerate film degradation, increase haze, promote
brittleness and cause a deterioration in inter-layer adhesion.
It is an object of this invention to provide a primer composition for coating onto a film substrate for improving the adhesion characteristics between the film substrate and an anti-fouling coating composition. It is a further object of this
invention to provide a film which exhibits improved adhesion to an anti-fouling composition. It is a further object of this invention to provide a film substrate suitable for use as a base film for an anti-fouling composition. It is a further object of
this invention to provide a film which exhibits improved UV stability and weatherability.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a composition comprising: (i) a copolymer of (a) 35 to 40 mole % alkyl acrylate, (b) 35 to 40% alkyl methacrylate, (c) 10 to 15 mole % of a comonomer containing a free
carboxyl group, and (d) 15 to 20 mole % of an aromatic sulphonic acid and/or salt thereof; (ii) an acrylic and/or methacrylic polymeric resin; and (iii) a cross-linking agent.
The composition is suitable for use as a primer coating on a polymeric film substrate, particularly a film substrate for an anti-fouling coating composition. The primer layer performs the function of compatabilising the substrate layer and any
subsequent layers, i.e. the anti-fouling coating or any intermediate layer(s), in order to provide good adhesion. The primed film of the present invention exhibits improved adhesion in relation to known films. It is believed that the primer coating is
more hydrophilic than known coatings and that this increases adhesion to the anti-fouling composition. It is not, however, intended that the invention be limited by this theory. The primed film of the present invention also exhibits excellent
weatherability and UV stability.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred embodiment, the composition of the present invention is suitable for improving adhesion between a polymeric film substrate, particularly a polyester film substrate, and a silicone-based anti-fouling composition, particularly one
having a hydrophilic component therein.
In a preferred embodiment of the invention, the primer composition of the present invention is a dispersion of components (i), (ii) and (iii) in water. In an alternative embodiment, the composition is a solution or suspension of components (i),
(ii) and (iii) in an organic solvent.
Preferably, component (i) is present in amounts from about 0.1 to about 15%, and preferably from about 2 to about 6%, by weight of the total solids present in the composition. Preferably, component (ii) is present in amounts of about 0.1 to
about 10%, and preferably about 1 to about 5%, by weight of the total solids present in the composition. Preferably, component (iii) is present in amounts of about 0.01 to about 3%, and preferably about 0.2 to about 1%, by weight of the total solids
present in the composition.
Preferred copolymers of component (i) are as follows.
The alkyl group of monomer (a) is preferably a lower alkyl group, for example an alkyl group containing up to six carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, terbutyl, and hexyl. Ethyl acrylate is a particularly
preferred monomer (a).
The alkyl group of monomer (b) is preferably a lower allyl group, for example an alkyl group containing up to six carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl, terbutyl, and hexyl. Methyl methacrylate is a particularly
preferred monomer (b).
Monomer (c) containing a free carboxyl group, i.e. a carboxyl group other than those involved in the polymerisation reaction by which the copolymer is formed, suitably comprises a copolymerisable unsaturated carboxylic acid, and is preferably
selected from acrylic acid, methacrylic acid, maleic acid, and/or itaconic acid; with acrylic acid and itaconic acid being particularly preferred.
The aromatic sulphonic acid monomer (d) may be present in the copolymer as the free acid and/or a salt thereof, for example as the ammonium, substituted ammonium or alkali metal (such as lithium, sodium or potassium) salt. The sulphonate group
does not participate in the polymerisation reaction by which the copolymer is formed. The use of non-aromatic sulphonate-containing monomers can result in poor antistatic properties of the coated film, and also in destabilisation of the copolymer
emulsion during polymerisation, rendering the water sensitivity of the copolymer unacceptable. The aromatic sulphonic acid polymer is preferably p-styrene sulphonic acid and/or a salt thereof.
Component (i) is preferably a copolymer comprising comonomers (a) 35 to 37.5 mole % alkyl acrylate, (b) 35 to 37.5 mole % allcyl methacrylate, (c) 10 to 15 mole % of a monomer containing a free carboxyl group, and (d) 15 to 20 mole % of an
aromatic sulphonic acid and/or a salt thereof. A particuLlarly preferred copolymer comprises ethyl acrylate/ruethyl methacrylate/itaconic acid/p-styrene sulphonic acid and/or a salt thereof in a ratio of 35/35/15/15 mole %, and especially in a ratio of
37.5/37.5/10/15 mole %.
The weight average molecular weight of the copolymer may vary over a wide range, but is preferably from 10,000 to 10,000,000, and more preferably of the order of 1,000,000.
Component (ii) of the composition is an acrylic and/or methacrylic polymeric resin. Suitable polymers comprise at least one monomer derived from an ester of acrylic acid, preferably an alkyl ester wherein the alkyl group is a C.sub.1-10 allyl
group, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, hexyl, 2-ethylhexyl, heptyl and n-octyl, and more preferably ethyl and butyl. Polymers comprising alkyl acrylate monomer units and further comprising alkyl methacrylate
monomer units are particularly preferred. In a particularly preferred embodiment, the polymer comprises ethyl acrylate and alkyl methacrylate. Preferably, the alkyl methacrylate is methyl methacrylate. In a preferred embodiment, the alkyl acrylate
monomer units are present in a proportion in the range of from about 30 to about 65 mole % and the alkyl methacrylate monomer units are present in a proportion in the range of from about 20 to about 60 mole %.
Other monomer units which may be present in the polymeric resin of component (ii) include acrylonitrile, methacrylonitrile, halo-substituted acrylonitrile, halo-substituted methacrylonitrile, acrylamide, methacrylamide, N-methylol acrylamide,
N-ethanol acrylamide, N-propanol acrylamide, N-methacrylamide, N-ethanol methacrylamide, N-methylacrylamide, N-tertiary butyl acrylamide, hydroxyethyl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylamino ethyl methacrylate, itaconic
acid, itoconic anhydride and half ester of itaconic acid.
Further monomer units which may be present in the polymeric resin of component (ii) include vinyl esters such as vinyl acetate, vinyl chloracetate and vinyl benzoate, vinyl pyridine, vinyl chloride, vinylidene chloride, maleic acid, maleic
anhydride, styrene and derivatives of styrene such as chlorostyrene, hydroxystyrene and alkylated styrenes wherein the alkyl group is a C.sub.1-10 alkyl group.
In a preferred embodiment, the polymer of component (ii) comprises about 35 to 60 mole % ethyl acrylate, about 30 to 55 mole % methyl methacrylate and about 2 to 20 mole % methacrylamide.
Preferably, the molecular weight of the polymer of component (ii) is from about 40,000 to about 300,000, and more preferably from about 50,000 to about 200,000.
The cross-linking agent of component (iii) functions to cross-link the composition to improve adhesion to a substrate and should also be capable of internal cross-linking within the composition. Suitable cross-linking agents include
epoxy-resins, alkyd resins, amine derivatives (such as hexamethoxymethylmelamine) and condensation products of an amine (such as melamine, diazine, urea, cyclic ethylene urea, cyclic propylene urea, thiourea, cyclic ethylene thiourea, alkyl melamines,
aryl melamines, benzo guanamines, guanamines, alkyl guanamines and aryl guanamines) with an aldehyde, for example formaldehyde. A preferred cross-linking agent is a condensation product of melamine with formaldehyde. The condensation product is
optionally alkoxylated, for example methoxylated or ethoxylated. Particularly preferred cross-linking agents are selected from oxazoline-modified addition polymers containing pendant oxazoline groups, such as those disclosed in, for example, U.S. Pat.
Nos. 4,474,923 and 4,508,869 and commercially available as EPOCROS.TM. cross-linking agents (Nippon Sholcubai Co. Ltd).
In a preferred embodiment, the composition further comprises a cross-linking catalyst to facilitate the cross-linking of the cross-linking agent. Preferred catalysts for the embodiment of the invention wherein the cross-linker comprises melamine
formaldehyde or a derivative thereof include ammonium chloride, ammonium nitrate, ammonium thiocyanate, ammonium dihydrogen phosphate, ammonium sulphate, diammonium hydrogen phosphate, para-toluene sulphonic acid, maleic acid stabilised by reaction with
a base, and morpholinium para-toluene sulphonate.
According to a second aspect of the invention, there is provided a film comprising a substrate layer and a primer layer, said primer layer being a composition according to the first aspect of the invention. The film is suitable for use as a base
layer onto which an anti-fouling composition may be coated.
The substrate may be formed from any suitable film-forming material. Thermoplastic polymeric materials are preferred. Such materials include a homopolymer or copolymer of a 1-olefin, such as ethylene, propylene and but-1-ene, a polyamide, a
polycarbonate, PVC, PVA, polyacrylates, celluloses and particularly a synthetic linear polyester.
The synthetic linear polyesters useful as the substrate may be obtained by condensing one or more dicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters, eg terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or
2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid, hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane (optionally with a monocarboxylic acid, such as pivalic acid) with one
or more glycols, particularly an aliphatic or cycloaliphatic glycol, eg ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexanedimethanol. An aliphatic glycol is preferred.
In a preferred embodiment, the polyester is selected from polyethylene terephthalate and polyethylene naphthalate. Polyethylene terephthalate (PET) is particularly preferred.
In a further preferred embodiment the polyester is a copolyester of terephthalic acid (TPA) and isophthalic acid (IPA) with one or more diols selected from the group consisting of aliphatic and cycloaliphatic diols. In this embodiment, the
preferred molar ratios of the isophthalate polyester units to the terephthalate polyester units are from 0.1 to 40 mol % isophthalate and from 99.9 to 60 mol % terephthalate, preferably from 15 to 20 mol % isophthalate and from 85 to 80 mol %
tereplithalate. The terephthalic acid/isophthalic acid copolyester may be obtained by condensation polymerisation of terephthalic acid and isophthalic acid with the diol. Suitable aliphatic diols include ethylene glycol, 1,3-propane diol, 1,3-butane
diol, 1,4-butane diol, 1,5-pentane diol, neopentyl glycol and 1,6-hexane diol. Suitable cycloaliphatic diols include 1,4-cyclohexanedimethanol and 1,4-cyclohexane diol. It is preferred that the copolyester is derived from ethylene glycol. Thus, in
this embodiment, the preferred polyester is a copolyester of ethylene isophthalate and ethylene terephthalate (referred to herein as PE-TPA/IPA), preferably comprising from 0.1 to 40 mol % ethylene isophthalate and from 99.9 to 60 mol % ethylene
terephthalate and more preferably from 15 to 20 mol % ethylene isophthalate and 85 to 80 mol % ethylene terephthalate. In a particularly preferred embodiment the second layer comprises a copolyester comprising substantially 18 mol % ethylene
isophthalate and 82 mol % ethylene terephthalate.
A TPA/IPA copolyester tends to reduce the crystallinity of the film in relation to a film comprising, for example PET, which is an important factor when it is desired to improve the weatherability or UV stability, particularly with regard to the
brittleness, of the film.
The substrate may also comprise a polyarylether or thio analogue thereof, particularly a polyaryletherketone, polyarylethersulphone, polyaryletheretherkeetone, polyaaryletherethersulphone, or a copolymer or thioanalogue thereof. Examples of
these polymers are disclosed in EP-A-001879, EP-A-0184458 and U.S. Pat. No.4,008,203 and include polymers sold as STABAR.RTM. (ICI plc). Blends of such polymers may also be employed.
The substrate may comprise one or more discrete layers of the above film-forming materials. For instance, the substrate may comprise one, two, three, four or five or more layers. The polymeric materials of the respective layers may be the same
or different. In a preferred embodiment, the film may comprise a multilayer substrate comprising two or three, preferably two, different types of layer. Typical multilayer structures may be of the AB, ABA, ABC, ABABA or ABCBA type.
Where the substrate comprises more than one layer, preferably at least one of the layers comprises a polymer selected from polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and a TPA/IPA copolyester as described above.
For example, where the substrate comprises a two layer AB structure or a three layer ABA structure or a five layer ABABA structure, the substrate may comprise the following combinations of layers: A=PET, B=PET; A=PET, and B=PE-TPA/IPA;
A=PE-TPA/IPA and B=PET; A=PE-TPA/IPA and B=PE-TPA/IPA; and the corresponding combinations wherein PET is replaced by PEN. Where the substrate comprises a three layer ABC structure or a five layer ABCBA structure, the substrate may comprise combinations
of layers wherein A and B are as defined above and C is PE-TPA/IPA or polyethylene isophthalate.
Formation of the substrate may be effected by conventional techniques well-known in the art. Conveniently, formation of the substrate is effected by extrusion, in accordance with the procedure described below. In general terms the process
comprises the steps of extruding a layer of molten polymer, quenching the extrudate and orienting the quenched extrudate in at least one direction.
The substrate may be uniaxially oriented, but is preferably biaxially oriented by drawing in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties. Orientation
may be effected by any process known in the art for producing an oriented film, for example a tubular or flat film process.
In a tubular process, simultaneous biaxial orientation may be effected by extruding a thermoplastics polyester tube which is subsequently quenched, reheated and then expanded by internal gas pressure to induce transverse orientation, and
withdrawn at a rate which will induce longitudinal orientation.
In the preferred flat film process, the substrate-forming polyester is extruded through a slot die and rapidly quenched upon a chilled casting drum to ensure that the polyester is quenched to the amorphous state. Orientation is then effected by
stretching the quenched extrudate in at least one direction at a temperature above the glass transition temperature of the polyester. Sequential orientation may be effected by stretching a flat, quenched extrudate firstly in one direction, usually the
longitudinal direction, i.e. the forward direction through the film stretching machine, and then in the transverse direction. Forward stretching of the extrudate is conveniently effected over a set of rotating rolls or between two pairs of nip rolls,
transverse stretching then being effected in a stenter apparatus. Stretching is effected to an extent determined by the nature of the polyester, for example polyethylene terephthalate is usually stretched so that the dimension of the oriented film is
from 2 to 5, more preferably 2.5 to 4.5 times its original dimension in the or each direction of stretching. Typically, stretching is effected at temperatures in the range of 70 to 125.degree. C. Greater draw ratios (for example, up to about 8 times)
may be used if orientation in only one direction is required. It is not necessary to stretch equally in the machine and transverse directions although this is preferred if balanced properties are desired.
A stretched film may be, and preferably is, dimensionally stabilised by heat-setting under dimensional restraint at a temperature above the glass transition temperature of the polyester but below the melting temperature thereof, to induce
crystallisation of the polyester. In applications where film shrinkage is not of significant concern, the film may be heat set at relatively low temperatures or not at all. On the other hand, as the temperature at which the film is heat set is
increased, the tear resistance of the film may change. Thus, the actual heat set temperature and time will vary depending on the composition of the film and its intended application but should not be selected so as to substantially degrade the tear
resistant properties of the film. Within these constraints, a heat set temperature of about 135.degree. to 250.degree. C. is generally desirable, as described in GB-A-838708.
Where the substrate comprises more than one layer, preparation of the substrate is conveniently effected by coextrusion, either by simultaneous coextrusion of the respective film-forming layers through independent orifices of a multi-orifice die,
and thereafter uniting the still molten layers, or, preferably, by single-channel coextrusion in which molten streams of the respective polymers are first united within a channel leading to a die manifold, and thereafter extruded together from the die
orifice under conditions of streamline flow without intermixing thereby to produce a multilayer polymeric film, which may be oriented and heat-set as hereinbefore described. Formation of a multilayer substrate may also be effected by conventional
lamination techniques, for example by laminating together a preformed first layer and a preformed second layer, or by casting, for example, the first layer onto a preformed second layer.
The substrate is suitably of a thickness between about 12 and 300 .mu.m, preferably from 20 to about 150 .mu.m and particularly from about 30 to about 70 .mu.m.
The ratio of substrate to primer layer thickness may vary within a wide range, although the thickness of the primer layer preferably should not be less than 0.004% nor greater than 10% of that of the substrate. In practice, the thickness of the
primer layer is desirably from about 0.01 .mu.m to about 10 .mu.m, and preferably from about 0.1 .mu.m to about 5 .mu.m. Generally the primer layer is not more than about 1.0 .mu.m.
The substrate and/or the primer composition preferably comprise one or more UV absorbers in order to improve the ageing characteristics of the film, and in particular to reduce the tendency of the film to become brittle over time. In principle,
any organic or inorganic UV absorber, particularly one which is suitable for use with polyester, may be employed in the present invention. Suitable examples include the organic UV absorbers disclosed in Encyclopaedia of Chemical Technology, Kirk-Othmer,
Third Edition, John Wiley & Sons, Volume 23, Pages 615 to 627. Particular examples of UV absorbers include benzophenones, benzotriazoles (U.S. Pat. Nos. 4,684,679, 4,812,498 and 4,681,905), benzoxazinones (U.S. Pat. Nos. 4,446,262, 5,251,064 and
5,264,539), triazines (U.S. Pat. Nos. 3,244,708, 3,843,371, 4,619,956, 5,288,778 and WO 94/05645) and hindered amines. The teaching of the aforementioned documents is incorporated herein by reference.
Preferably, the UV absorber is non-volatile and does not cause excessive yellowing of the product.
In a preferred embodiment of the invention the UV absorber comprises one or more triazines, more preferably hydroxyphenyltriazines, and particularly hydroxyphenyltriazine compounds of Formula 1: ##STR1##
wherein R is hydrogen, C.sub.1 -C.sub.18 alkyl, C.sub.2 -C.sub.6 alkyl substituted by halogen or by C.sub.1 -C.sub.12 alkoxy, or is benzyl and R.sup.1 is hydrogen or methyl. R is preferably C.sub.1 -C.sub.12 alkyl or benzyl, more preferably
C.sub.3 -C.sub.6 alkyl, and particularly hexyl. R.sup.1 is preferably hydrogen. An especially preferred UV absorber is 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxy-phenol, which is commercially available as Tinuvin.TM. 1577 FF from Ciba-Additives.
Further examples of a preferred UV absorber are benzylidene malonate ester (commercially available as Sanduvor.TM. PR-25 from Sandoz), and benzoxazinone (commercially available as Cyasorb.TM. 3638 from Cytec).
In one embodiment of the invention, a UV absorber may be chemically incorporated in the chain of a layer-forming polyester. Preferred UV-stable polyesters are produced by incorporating benzophenones into the polyester, for example as described
in EP-A-0006686, EP-A-0031202, EP-A-0031203 and EP-A-0076582, the teaching of which is incorporated herein by reference.
In a further embodiment of the invention, a UV absorber may be chemically incorporated into the chain of a polymer contained in the primer composition. For instance, the acrylic and/or methacrylic polymeric resin of component (ii) described
herein may comprise a UV-absorbent functional group such as a benzotriazole. Suitable acrylate resins containing a UV-absorbent functional group are commercially available as SAIVINOL.TM. (Saiden Chemical Industry Co.). Typically, the UV functional
group is present at a level of up to about 45%, and generally about 35%, by weight of the total amount of UV functional group-containing acrylate resin present in the primer composition. Examples of suitable acrylate copolymers include EA/MMA; BA/MMA;
EHA/MMA; EA/BA/MMA; EHA/BA/MMA; BA/MMA/MAA; BA/MMA/AA; BA/MMA; and BA/MMA/2-HEMA, wherein EA=ethylacrylate; MMA-methylmethacrylate; BA=butylacrylate; EHA=2-ethylhexylacrylate; MAA=methacrylic acid; AA=acrylic acid; and 2-HEMA=2-hydroxyethylmethacrylate.
Suitable inorganic UV absorbers include metal oxide particles, such as zinc oxide or titanium dioxide, having a mean crystal size, as determined by electron microscopy, of less than 200 nm, more preferably in the range from 5 to 150 nm,
particularly 10 to 100 nm, and especially 15 to 40 nm. Titanium dioxide particles are particularly preferred.
The titanium dioxide particles may be of anatase or rutile crystal form. The titanium dioxide particles preferably comprise a major portion of rutile, more preferably at least 60% by weight, particularly at least 80%, and especially
approximately 100% by weight of rutile. The particles can be prepared by standard procedures, such as the chloride process or the sulphate process.
In one embodiment of the invention the titanium dioxide particles are coated, preferably with inorganic oxides such as aluminium, silicon, zinc, magnesium or mixtures thereof. Preferably the coating additionally comprises organic compound(s),
such as fatty acids and preferably alkanols, suitably having from 8 to 30, preferably from 12 to 24 carbon atoms. Polydiorganosiloxanes or polyorganohydrogensiloxanes, such as polydimetlhylsiloxane or polymethylhydrogensiloxane are suitable organic
compounds. The coating is suitably applied to the titanium dioxide particles in aqueous suspension. The inorganic oxides are precipitated in aqueous suspension from water-soluble compounds such as sodium aluminate, aluminium sulphate, aluminium
hydroxide, aluminium nitrate, silicic acid or sodium silicate. The coating layer on the titanium dioxide particles is preferably in the range from 1 to 12%, more preferably 2 to 6% of inorganic oxides, and preferably in the range from 0.5 to 3%, and
more preferably 0.7 to 1.5% of organic compound, by weight based upon the weight of titanium dioxide.
Where the substrate comprises more than one layer, the UV absorber(s) may be present in one or more or all layers of the substrate. The respective layers of the substrate may comprise the same or different UV absorber(s).
The amount of UV absorber incorporated into a layer is generally in the range from 0.1% to 10%, more preferably 0.5% to 9%, more preferably 1.2% to 8%, particularly 2% to 6%, and especially 3.2% to 5.5% by weight, relative to the weight of the
polymer of that layer. It should be noted that certain inorganic UV stabilisers such as metal oxides, particularly titanium dioxide, are also suitable for use as particulate fillers, as described herein below, and may therefore be present in amounts
above 10% by weight of the polymer. In situations where such materials are utilised, the amount of, for example, titanium dioxide will generally be determined by the desired appearance of the film (e.g. opaque or optically clear), as described herein
below.
In one embodiment of the invention, both an organic UV absorber, preferably a triazine, and an inorganic UV absorber, preferably titanium dioxide, are present. The ratio, by weight of inorganic to organic UV absorber is preferably in the range
from 0.5 to 10:1, more preferably 1 to 5:1, and particularly 1.5 to 2.5:1.
One or more of the layers of the polymeric film according to the invention may conveniently contain any of the additives conventionally employed in the manufacture of polymeric films. Thus, agents such as dyes, pigments, voiding agents,
lubricants, anti-oxidants, radical scavengers, thermal stabilisers, anti-blocking agents, surface active agents, slip aids, optical brighteners, gloss improvers, prodegradents, viscosity modifiers and dispersion stabilisers may be incorporated in the
substrate and/or primer layer(s) as appropriate. In particular the substrate and/or primer layer, may comprise a particulate filler. The filler may, for example, be a particulate inorganic filler or an incompatible resin filler or a mixture of two or
more such fillers.
By an "incompatible resin" is meant a resin which either does not melt, or which is substantially immiscible with the polymer, at the highest temperature encountered during extrusion and fabrication of the layer. The presence of an incompatible
resin usually results in a voided layer, by which is meant that the layer comprises a cellular structure containing at least a proportion of discrete, closed cells. Suitable incompatible resins include polyamides and olefin polymers, particularly a
homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms in its molecule. Preferred materials include a low or high density olefin homopolymer, particularly polyethylene, polypropylene or poly-4-methylpentene-1, an olefin copolymer,
particularly an ethylene-propylene copolymer, or a mixture of two or more thereof. Random, block or graft copolymers may be employed.
Particulate inorganic fillers include conventional inorganic fillers, and particularly metal or metalloid oxides, such as alumina, silica (especially precipitated or diatomaceous silica and silica gels) and titania, calcined china clay and
alkaline metal salts, such as the carbonates and sulphates of calcium and barium. The particulate inorganic fillers may be of the voiding or non-voiding type. Suitable particulate inorganic fillers may be homogeneous and consist essentially of a single
filler material or compound, such as titanium dioxide or barium sulphate alone. Alternatively, at least a proportion of the filler may be heterogeneous, the primary filler material being associated with an additional modifying component. For example,
the primary filler particle may be treated with a surface modifier, such as a pigment, soap, surfactant coupling agent or other modifier to promote or alter the degree to which the filler is compatible with the substrate layer polyester.
Preferred particulate inorganic fillers include titanium dioxide, such as hereinbefore described, and silica.
The inorganic filler should be finely-divided, and the volume distributed median particle diameter (equivalent spherical diameter corresponding to 50% of the volume of all the particles, read on the cumulative distribution curve relating volume %
to the diameter of the particles--often referred to as the "D(v,0.5)" value) thereof is preferably in the range from 0.01 to 5 .mu.m, more preferably 0.05 to 1.5 .mu.m, and particularly 0.15 to 1.2 .mu.m.
The size distribution of the inorganic filler particles is also an important parameter, for example the presence of excessively large particles can result in the film exhibiting unsightly `speckle`, i.e. where the presence of individual filler
particles in the film can be discerned with the naked eye. It is preferred that none of the inorganic filler particles incorporated into the substrate layer should have an actual particle size exceeding 30 .mu.m. Particles exceeding such a size may be
removed by sieving processes which are known in the art. However, sieving operations are not always totally successful in eliminating all particles greater than a chosen size. In practice, therefore, the size of 99.9% by number of the inorganic filler
particles should not exceed 30 .mu.m, preferably should not exceed 20 .mu.m, and more preferably should not exceed 15 .mu.m. Preferably at least 90%, more preferably at least 95% by volume of the inorganic filler particles are within the range of the
volume distributed median particle diameter .+-.0.8 .mu.m, and particularly .+-.0.5 .mu.m.
Particle size of the filler particles may be measured by electron microscope, coulter counter, sedimentation analysis and static or dynamic light scattering. Techniques based on laser light diffraction are preferred. The median particle size
may be determined by plotting a cumulative distribution curve representing the percentage of particle volume below chosen particle sizes and measuring the 50th percentile.
If employed in the primer layer, the filler particles may comprise from about 20 to about 200%, and more preferably 50 to 150% by weight of the primer layer relative to the weight of the polymer of the primer layer.
The substrate of the polymeric film may also comprise one or more plasticisers, such as glycerol or ethylene glycol or other conventional plasticiser known in the art. The plasticisers may be beneficial in the reduction of the amount of UV
absorber(s) lost from the structure.
The components of the compos | | |
