WO2003033577A1

WO2003033577A1 - Coated film

Info

Publication number: WO2003033577A1
Application number: PCT/EP2002/011245
Authority: WO
Inventors: Richard Southward; Neill Dutton
Original assignee: Ucb, S.A.
Priority date: 2001-10-11
Filing date: 2002-10-08
Publication date: 2003-04-24
Also published as: GB0124397D0

Abstract

There is described is provided a printable, opaque coated sheet, suitable for use in contact and/or association with foodstuffs, the coating comprising by dry weight of the coat: a) from about 30% to about 70% of a copolymer of an ethylenically unsaturated acid; b) from about 30% to about 70% of an inert particulate filler;where the coating is substantially free from any reactive binder and/or cross-linking compound.

Description

COATED FILM

The present invention relates to improved printable coated films for use in many applications in contact with or associated with foodstuffs (i.e. used in proximity thereto) for example as packaging, labels, synthetic paper and/or substrates for printing.

Printable packaging films with a matt appearance and "paper-like" feel are prepared using a coating with an inorganic filler. Currently such coatings use a reactive binder system to ensure that the filler is sufficiently adhered to the film. Reactive binder systems commonly use cross-linking agents such as isocyanates, aziridines, carbodiimides that are not food approved for coatings on plastic materials.

Such film coatings can suffer from process difficulties during manufacture due to low pot-life stability which may lead to variable surface roughness on the film. A rough film has problems with the readability of the printing thereon which is a particular issue for machine readable information such as barcodes. If the amount of reactive binder is reduced to compensate for these issues the filler adheres less well to the film. This can cause a dusting problem where the silica particles come loose and block flexographic or gravure print plates leading to printing difficulties.

Preferred packaging films also contain a heat sealable surface layer on one or both sides of the film. On heating, preferably at low temperature, such films can be made to self seal during to packaging process to form a pack. Known low temperature heat sealable coatings (LTSC) comprise for example ethylene acrylic acid (EAA) or ethylene methacrylic acid (EMAA) copolymers.

It is desirable to provide an aqueous based food contact approved matt effect coating where a film is to be used in direct contact with foodstuffs (e.g. packaging) and/or is associated with foodstuffs (e.g. labels). It is also desirable that such films are writeable and/or printable by a range of known print methods such as flexography, gravure, lithography, thermal transfer and/or toner-based technologies (for example for digital printing). It is further desired that the coating should absorb sufficiently to produce a clear window on the film upon wetting with a transparent printing lacquer and is heat sealble preferably at low temperature. For applications such as packaging a useful additional feature would be if the film coating was readily heat sealable. For applications such as labels a useful additional feature would be if the film coating was readily compatible with the various adhesives used to apply labels.

Various prior art films containing mineral fillers and EAA coatings are known. US 5776604 (Mobil) describes coatings for biaxially oriented polypropylene film which allows lithographic printing thereon. The coating comprises a binder of a blend of acrylic polymer and ethylene-acrylic acid copolymer, at least one particulate filler (such as silica, calcium carbonate, clay, talc, titanium dioxide and mixtures thereof), a surfactant and, optionally, an ultraviolet stabilizer.

EP 0789281 (Mobil) describes film coatings containing a relatively inert particulate filler additive, (specifically fumed silica or calcium carbonate) which comprise agglomerations of smaller particles and which have an average particle size from 2 to 9 microns, in an amount from 0.1 % to 80 %, by weight of the coating.

EP 0546675 (Mobil) discloses computer printable coated oriented thermoplastic film comprising a base layer of resin comprising a various olefinic polymers. To improve printability the film is coated with an aqueous pigmented polymer dispersion comprising (a) a carboxylic acid and/or its alkali metal salt-functionalized olefinic copolymer and (b) inorganic pigments. The coating is not neutralised with ammonia.

EP 0492907 (3M) relates to a polymeric road marking tape. The surface layer of the tape comprises ductile thermoplastic polymers such a ethylene acrylic acid (EAA) or ethylene methacrylic acid (EMAA) copolymers. Optionally the tape has 15 to 50% by volume of non reinforcing mineral particulate fillers of at least one micron size embedded in the surface layer to improve skid resistance and reflectivity.

EP 0892008 (UCB) describes conventional coated films containing EAA and silica plus cross- linkers. These are not suitable for use in food packaging because of the present of reactive cross- linkers.

US 5,891 ,552 (Mobil) describes a film coating with EAA for thermal transfer printing.

US 6,228,486 (Mobil) and WO 99/19773 (Indigo) both describes various film coatings with EAA for liquid-toner printing.

WO 00/63309 (Paramelt) describes certain EAA copolymers which are used a heat sealable coatings. However current films do not have a proper balance between each of the different properties of sealability, good printability, adhesion of filler and suitability for use near food (i.e. food contact approval). It is an object of the invention to provide improved films which solve some or all of the preceding problems in the prior art.

The applicant has found that conventional LTSC coats are sufficiently wet to accept high percentages of inert particulate filler to achieve a desired matt look and printability. Yet surprisingly at these high loadings the applicant has further found that the filler still adheres sufficiently well to the coating to remove the need for a reactive binder to cross-link the coat. Such coatings can be prepared solely from use food contact approved ingredients and thus are suitable for use on packaging films for foodstuffs.

Therefore broadly in accordance with the current invention there is provided a printable, opaque coated sheet, suitable for use in contact and/or association with foodstuffs, the coating comprising by dry weight of the coat: a) from about 30% to about 70% of a copolymer of an ethylenically unsaturated acid; b) from about 30% to about 70% of an inert particulate filler; where the coating is substantially free from any reactive binder and/or cross-linking compound.

Preferably the copolymer of an ethylenically unsaturated acid is present in the coat in an amount by weight of the dry coating of: from about 35% to about 65 %; more preferably from about 40% to about 60%, most preferably from about 45% to about 55%, for example about 50%.

Preferably the ethylenically unsaturated acid is neutralised by ammonia and not metal cations.

Preferably the ethylenically unsaturated acid comprises and/or is obtained and/or obtainable by polymerising monomers selected from acrylic acid, ethyl acrylate and methyl methacrylate. Other ethylenically unsaturated acid copolymers suitable for use in the present invention are those described in WO 00/63309 (Paramelt) the contents of which are hereby incorporated by reference.

Preferably the inert filler is present in the coat in an amount by weight of the dry coating of: from about 35% to about 65 %; more preferably from about 40% to about 60%, most preferably from about 45% to about 55%, for example about 50%.

Suitable inert fillers used in the present invention may comprise any of the following which are suitable: silica, calcium carbonate, diatomaceous earth, calcium silicate, bentonite, aluminium silicates, clay, talc and pulp kaolins, such as kaolinite, dickite, nacrite; halloysite (7 ) and/or halloysite (10 ); serpentines, such as lizardite, chrysotile, antigorite, carlosturanite, forsterite, asbestos, amestite, cronstedite, chamosite, berthierine, and/or gamierite; talcs, pyrophyllites, and/or ferropyrophyllites; smectites and/or montmorillonites; such as bentonite; beidellite, nontronite, hectrorite, fluorohectorite, saponite, sauconite; volkhonskoite, medmontite, pimelite, stevensite and/or stephanite; illites and/or micas such as muscovite, bravaisite, degraded mica, hydromica, hydromuscovite, hydrous illite, hydrous mica, K-mica, micaecous, clay and/or sericite; glauconites such as Caledonia; chlorites and/or vermiculites such as clinochlore, chamosite, nimite, baileychlore, donbassite, cookite, sudoite, franklinfurnaceite and/or corrensite; palygorskites and/or sepiolites such as attapulgite; mixed layer minerals which may comprise two three or more mineral components in random and/or regular order, examples of such mixed layer minerals comprising pillared clays, illite- montmorillonite, smectite-illite, illite-chlorite-smectite and/or illite-smectite-vermiculite; amorphous clays such as allophane and/or imogolite, and/or high alumina clays such as diaspore, boehmite, gibbsite, cliachite, bauxite, bauxitic kaolin and/or bauxitic clays; other clays such as fire clay, flint clay, nodular clay, burley clay, abrasive clay, loess and/or abobe. and/or any suitable mixtures and combinations thereof.

Without wishing to be bound by any mechanism it is believed that the ethylenically unsaturated acid acts as a low temperature sealable component and the filler acts as an opacifier and also to aid printability to provide a key to receive inks and open up the film structure.

Optionally a hardening ingredient may be added to the coating such as any suitable non reactive polymer. Preferably the harder comprises an acrylic polymer with a T_g which is above room temperature. Advantageously the non-reactive polymer is a polymethyl methacrylate (PMMA) latex for example that available commercially from UCB Chemicals under the trade name Ucecryl H.

The hardener may be present in an amount from about 1% to about 10%, more preferably from about 3% to about 8%, by dry weight of the coating. Further advantages of the coated films of the invention or preferred embodiments thereof are given below.

Preparing the coating from an aqueous based dispersion or solution of the copolymers allows improved wetting of the inert filler particles to attain a higher loading of filler without the need for cross-linking agents. Without wishing to be bound by any mechanism it is believed that this is because high acid functionality on for example the EAA type copolymers could readily bind to hydroxy groups present on the for example OH functional silica particles.

The advantage of using an ammonia stabilised EAA solution to prepare these coatings means that on drying NH₃ evaporates and acid groups become less moisture sensitive compared to metal cation stabilised EAA dispersions.

As the coating comprises a single component non-reactive binder system there are no problems during the process with pot-life due to reaction of the components.

The coating used in the present invention can be prepared from ingredients (such as ethylene acrylic acid and silica) which have full food contact approval.

Use of the combination of an associative polymer (e.g. EAA) with a filler (e.g. silica) provides a film coating with high resistance and a matt effect yet which is food approved as the coating has no need for cross-linkers.

Sheets of the invention may comprise any suitable well known film substrate and may be multi-layer or single layer. Suitable substrates include polymeric films, such as synthetic thermoplastic films and/or biopolymer films, which may or may not be oriented as desired. Examples of thermoplastic films include polyolefinic films (for example polyethylene and polypropylene films (e.g. BOPP)). Examples of biopolymer films include films comprising cellulose, cellulose derivatives and/or polylactic acid.

Yet further aspects of the invention and preferred features thereof are given in the claims herein.

Films of the present invention are illustrated by reference to the accompanying drawings in which Figure 1 is a plot of readability versus surface roughness of a film; and Figures 2 to 5 are photographs taken of various examples herein after the tape test. The Figures are described more fully below. The invention is further illustrated by reference to the following non-limiting Examples, which were tested for various properties. In the Examples the following ingredients were used:

EEA1 and EAA2 denote an aqueous dispersions of ethylene-co-acrylic acid with respectively 20% and 15% acid by weight both having a melt flow index of 30 g /(1 Omin).

Acematt TS 100 and Sipemat 500 SL are trade names of precipitated silicas which are available commercially from Degussa Huls. Both Sipernat 500 SL and Acematt TS100 have a mean particle size of 8 microns. Of the two Acematt TS 100 s regarded as a harder particle and is considered to disperse better as it forms less agglomerates in its solid powdered form.

Ucecryl H is a trade name of a non reactive polymethylmethacrylate (PMMA) latex used as an optional non reactive hardener and is available commercially from UCB Chemicals.

The films coated in the all the examples herein were coated in a conventional manner on a base BOPP film primed with a primer of mica and polyethylene imine.

The coatings used in Examples 1 to 8 are given in Table 1 below:

Table 1

Notes a Examples 1 to 7 were prepared with EEA1 and Example 8 was prepared with EAA2.

Comp A is a known non-food contact approved coated BOPP film similar to those of the present invention in that the coating contains both silica (Sipernat 500SL) and an acrylic polymer (Plextol BV 595). However in addition these known coatings contain a reactive isocyanate cross-linker (Bayhydur 3100) to bind the silica to the film, which is why the film is not approved for applications such a packaging of foodstuffs where it would be in contact with food.

The coating composition of Comp A is as follows (all % by dry weight):

46.1 % of the acrylic latex available commercially from Rohm & Haas under the trade name Plextol

BV595

24.8 % of the reactive isocyanate crosslinker available commercially from Bayer under the trade name Bayhydur 3100.

22.5 % of the precipitated silica of mean particle size 8 microns that is available commercially from

Degussa Huls under the trade name Sipernat 500 SL.

5.6 % of the precipitated silica of mean particle size 4 microns that is available commercially from

Degussa Huls under the trade name Sipernat 50 S.

1.0 % of the silicone free anti-foam surfactant designed for use with synthetic latex emulsions that is available commercially from Cognis under the trade name Foamaster NS-1

The chemical and mechanical resistance properties of the non cross-linked coated films of the invention (Examples 1 to 7) were obtained and compared in a conventional manner to those properties of the similar film (Comp A) where the coating is cross-linked. The results obtained are given in Table 2 below:

Table 2

Example 8 was also tested similarly and it was found that increasing the silica concentration from 40% to 50 % improved the ink key significantly. The UV ink key for this coating (for both screen and flexographic inks) was also found to be excellent. The various properties of the films in Table 2 were tested in a conventional manner.

Generally the rougher the surface of the film the less readable is printing thereon. This is illustrated in Figure 1 which plots bar code readability in % (ordinate) and surface roughness in cm³/m² (abscissa). It can be seen that a lower roughness is desirable and Examples 1 to 7 are less rough then Comp A.

In the tape test a piece of Scotch tape is applied to the Example and pulled off at an angle of 90° rapidly. This will reflect the adhesion of the coating to the film surface. The percentage area of the coating that is removed is recorded, and a loss of 10% or less is considered acceptable.

Comp A and Examples 4, 5 and 6 were further examined to see if there was a difference in level of pull off of spherical additive on to the tape. The additives in question were 4 and 8 microns in size and none of these particles were seen on the surfaces of the tape after pull off. This can be confirmed by photographs which were taken of the tapes after the tape test using reflected DIG (see Figures 2 to 5). The samples were not metallised before imaging. In each photo the darker areas are believed to correspond to that part of the tape where no coating or silica adhered (i.e. no coating has been pulled off). Thus the more black in the photo the more adherent the tested coating is to the film. Such a coating is also likely to be more printable as the silica is less removable and thus less prone to dusting which can block printing plates.

Figure 2 is a photo of the tape pull off from the prior art film Comp A Figure 3 is a photo of the tape pull off from Example 4 Figure 4 is a photo of the tape pull off from Example 5 Figure 5 is a photo of the tape pull off from Example 6

The photos show that, contrary to what might have been predicted the non cross-linked coatings of Examples 3, 4 and 5 have comparable or better resistance to tape pull off (similar or greater areas of the black in photo) compared to prior art cross-linked coating of Comp A. Preferred films of the invention contain a non reactive hardener (Figures 4 and 5).

Claims

1 A printable, opaque coated sheet, suitable for use in contact and/or association with foodstuffs, the coating comprising by dry weight of the coat: a) from about 30% to about 70% of a copolymer of an ethylenically unsaturated acid; b) from about 30% to about 70% of an inert particulate filler; where the coating is substantially free from any reactive binder and/or cross-linking compound.

2. A sheet as claimed in claim 1 , where the copolymer of ethylenically unsaturated acid is present in an amount of from about 35% to about 65 % by weight.

3. A sheet as claimed in claim 2, where the copolymer is present in amount of from about 40% to about 60% by weight.

4. A sheet as claimed in claim 3, where the copolymer is present in amount of from about 45% to about 55% by weight.

5. A sheet as claimed in claim 4, where the copolymer is present in amount of about 50% by weight.

6. A sheet as claimed in any preceding claim, where the ethylenically unsaturated acid is neutralised by other than metal cations.

7. A sheet as claimed in any preceding claim, where the copolymer comprises from about 10% to about 20% by weight of acid.

8. A sheet as claimed in claim any preceding claim, where the inert filler is present in the coat in an amount of from about 35% to about 65 % by weight.

9. A sheet as claimed in claim 8, where the inert filler is present in the coat in an amount of from about 40% to about 60 % by weight.

10. A sheet as claimed in claim 9, where the inert filler is present in the coat in an amount of from about 45% to about 45 % by weight.

11. A sheet as claimed in claim 10, where the inert filler is present in the coat in an amount of about 50% by weight.

12. A sheet as claimed in any preceding claim, where the inert filler is selected from the group consisting of: silica, calcium carbonate, diatomaceous earth, calcium silicate, bentonite, aluminium silicates, clay, pulp, kaolin, serpentine, talc, pyrophyllite, ferropyrophyllite; smectite montmorillonite; illite, mica, glauconite, chlorite, vermiculite, palygorskite, sepiolite, mixed layer minerals and any suitable mixtures and combinations thereof.

13. A sheet as claimed in claim 12, where the inert filler comprises silica and/or calcium carbonate.

14. A sheet as claimed in any preceding claim, where the coating further comprises a hardening ingredient

15. A sheet as claimed in claim 14, where the hardener comprises a non reactive polymer.

16. A sheet as claimed in claim 14, where the hardener comprises an acrylic polymer with a T_g which is above room temperature.

17. A sheet as claimed in claim 14, where the hardener comprises a poly methyl methacrylate (PMMA) latex.

18. A sheet as claimed in any of claims 14 to 17, where the hardener is present in an amount from about 1 % to about 10% by dry weight of the coating.

19. A sheet as claimed in claim 18, where the hardener is present in an amount from about 3% to about 8% by dry weight of the coating.

20. A method of coating a sheet comprising applying thereto a coat comprising a) from about 30% to about 70% of a copolymer of an ethylenically unsaturated acid; b) from about 30% to about 70% of an inert particulate filler; where no reactive binder and/or cross-linking compound is applied

21. A coated sheet obtained and/or obtainable by the method as claimed in claim 20.

22. Method of packaging and/or labelling a foodstuff using a film as claimed in any of claims 1 to 19 or 21.

23. A package comprising a film as claimed in any of claims 1 to 19 or 21.

24. A label comprising a film as claimed in any of claims 1 to 19 or 21.

25. Use of film as claimed in claim 1 , in a method of manufacturing packaging and/or labels suitable for use in contact with and/or associated with foodstuffs.