GB2328438A

GB2328438A - Polymer compositions having improved barrier properties

Info

Publication number: GB2328438A
Application number: GB9717945A
Authority: GB
Inventors: Mark Graham Branch; Matthew James Buchanan Harte
Original assignee: Courtaulds Packaging Ltd
Current assignee: Albea UK Ltd
Priority date: 1997-08-22
Filing date: 1997-08-22
Publication date: 1999-02-24
Anticipated expiration: 2017-08-22
Also published as: GB2328438B; WO1999010428A1; AU8869898A; GB9717945D0

Abstract

To improve the gas and vapour barrier properties of a polymeric composition containing an inorganic platelet filler a coupling agent is added to the composition, the coupling agent having a first chemical group which is reactive with the polymer and a second chemical group which is reactive with the platelet filler. The coupling agent may be a vinyl silane, preferably vinyl trimethoxysilane and the filler may be talc. A free radical initiator may also be added. Flexible tube containers may be formed from such compositions.

Description

COMPOSITIONS HAVING IMPROVED BARRIER PROPERTIES Field of the Invention This invention concerns thermoplastics compositions and articles, in particular containers or parts of containers made from such materials, and having gas and vapour barrier properties.

Background of the Invention Thermoplastics materials are widely used in packaging because of their low cost and ease of forming into a variety of shapes. However, most thermoplastics materials suffer from the disadvantage of providing only a relatively poor barrier to gases and vapours. Poor gas barrier is a particular disadvantage in packaging oxygen- sensitive materials such as foodstuffs which are to be stored unrefrigerated. Poor vapour barrier properties are a disadvantage when packaging materials which are sensitive to moisture vapour, for example foodstuffs and confectionery which deteriorate when they become damp, and there are also disadvantages when the packaged material includes flavouring components which diffuse through the packaging material with consequent loss of flavour.

Thermoplastics containers which are used for the storage and delivery of, for example, toothpastes, are required to store materials for prolonged periods of time without a substantial loss of flavouring.

A number of attempts have been made at improving the gas barrier properties of thermoplastics materials. GB-A1,136,350, for example, proposes the use of circular platelike fillers with a ratio of diameter to thickness between 20:1 and 300:1 and a diameter of at most 40cm, in polyolefin polymers selected from polyethylene, polypropylene, ethylene-containing copolymers containing at least 50 mole percent of ethylene, and polystyrene, the preferred amount of filler being 0.1 to 50 wt% of the total weight of filled polymer. Such filled compositions are proposed to be used to manufacture films, for example for food packaging.

US-A-3,463,350 is concerned with the production of moulded containers for packaging foodstuffs, the containers being made from mixtures of high density polyethylene (HDPE) and mica particles, for example by compression or injection moulding. Such containers are said to reduce the discoloration of canned corned beef as caused by oxygen, compared with the use of similar containers made of HDPE filled with glass fibre or titanium dioxide instead of mica.

It has also been proposed in US-A-4,528,235 to incorporate platelet filler particles with an average equivalent diameter of from 1 to 8ym, the maximum diameter being 25ssm, and thickness of less than 0.5cm, into HDPE having a melt index of from 0.01 to 1.Og/10 minutes at 1900C as measured by ASTM D-1238, to produce films having a thickness of from 10 to 100cm, with the intention of increasing the oxygen barrier of the films compared with films formed from unfilled HDPE. The document also discloses the use of amino silane coupling agents to improve the filler-polymer compatability. When coupling agents are added to the composition the coupling agent generally reacts with the filler, and the improved compatability evidences itself as an increase in the physical properties of any moulded component.

The coupling agents disclosed in US-A-4,528,235 generally react with sites on the surface of the filler.

The "tail" of the coupling agent is more compatible with the polymer, but does not react therewith.

It has been found that the barrier properties of polymeric compositions can also be increased by the use of coupling agents.

Statements of the Invention According to the present invention there is provided a method of improving the barrier properties of a polymeric composition which includes an inorganic platelet filler, wherein a coupling agent is added to the composition, said coupling agent having a first chemical group that is reactive with the polymer, and a second chemical group that is reactive with the platelet filler.

In an alternative aspect, the invention provides a composition for forming an article having increased barrier to gases and/or vapour transmission and which comprises at least one polymeric thermoplastic resin, an inorganic platelet filler and a coupling agent having a first chemical group reactive with the polymeric material and a second chemical group which is reactive with the platelet filler.

The invention also extends to a flexible tube container having a component formed from a composition as defined above. The tube component may be a moulded tube shoulder or closure, or a tube body including a body formed from laminate, said composition forming at least one layer of the laminate.

The platelet filler can be any of a variety of lamellar fillers, preferably one in which the platelets delaminate under shear when the filler is blended with a thermoplastics resin before processing, and more particularly when the mixture of filler and thermoplastics resin is subjected to extrusion. Suitable lamellar fillers include clays, mica, graphite, montmorillonite and talc.

Talc is a particularly preferred lamellar filler by virtue of its ease of delamination during shear. Talc, being a naturally occurring hydrated magnesium silicate, is available in a variety of grades of greater or lesser purity. It has been found that the ease of increasing the aspect ratio of talc when it is subjected to high shear in a non-polar thermoplastics resin appears to increase as the level of impurities within the talc decreases. Thus not only does it appear easier to delaminate the platelets of the talc, but the platelets themselves apparently resist fracture. Thus purer grades of talc are generally preferred since they lead to compositions which not only have good barrier properties but also have a high degree of whiteness without the necessity to include a white pigment such as titanium dioxide.

The preferred polymer in the composition is a non-polar thermoplastic resin. Preferably, the non-polar thermoplastics resin is a polyolefin resin, for example a polymer derived from one or more aliphatic or aromatic alkylenes, e.g. a polymer containing units derived from at least one of ethylene, propylene, butylene, styrene, hexene and octene. The non-polar resin may also comprise a compound of one or more polymers. Examples of specific polyolefin resins which can be used include polyethylene, polypropylene, ethylene/propylene copolymers, and ethylene/propylene/butylene terpolymers. Polyethylenes are particularly preferred by virtue of their good processing and welding characteristics. The polyethylene can be low density polyethylene (density of from 0.910 to 0.925 g.cm~3), linear low density polyethylene, medium density polyethylene, linear medium density polyethylene, or high density polyethylene (density of from 0.950 to 0.980 g.cm3) High density polyethylene, or a compound of high density polyethylene and linear low density polyethylene, is particularly preferred by virtue of its higher inherent barrier properties compared with lower density polyethylenes.

The preferred HDPE resin has a density of at least 0.953 g. cm-3 and a melt flow index of 4-10 g/10 min, preferably about 7-10 g/10 min, (2160 g load at 1900C) measured to ASTM D-1238. A suitable material is available from DSM grade 7108.

Particularly preferred grades of talc for use in the present invention are sold by Richard Baker Harrison Group, England under the Trade Mark MAGSIL, and an especially preferred grade is "Magsil Osmanthus" which delaminates during processing to form platelets having an average aspect ratio of from 16-30 and a minimum aspect ratio of 5.

Since the purity of talc is related to its whiteness, the preferred talc forms a moulded composition, as described below, having a CIE whiteness index of at least 40. These CIE (Commission International d'Eclairage) whiteness index values are determined for compositions containing 15 percent by weight of talc in high density polyethylene with no other filler present, the determination being in reflectance mode with W light included and specular reflection excluded, the observer angle being 100C and the samples being backed by a white tile.

The talc is blended with the polyethylene in the weight ratio of 15 parts to 85 parts of polymer using a twin screw extruder or a Banbury type mixer, with a temperature profile ranging from 150 0C to 2200C, the mixture being subjected to high shear during mixing, and then being extruded and cut into pellets. The pellets are then compression moulded to form plaques at a temperature of 1500C and pressure of 0.39 tonnes for 5 minutes.

The CIE whiteness index is measured using a Macbeth Spectrophotometer 2020+.

Preferably about 1-3% by weight of the coupling agent based on the weight of talc is added to the talc prior to mixing with the polymer.

The coupling agent may have the general formula:

where: (a) M is one of Si, Ti, Al, P, Zr, B, Sn, Ge, Sb, or alternatively M is a mixed metal system, e.g. a zircoaluminate or a zircotitanate.

(b) R1 is a group capable of bonding to an inorganic platelet filler through at least one of covalent bonding, ionic bonding or hydrogen bonding.

Ri may comprise: -OR - where R = alkyl, aryl, phenyl, or benzyl -OH -COOH -COO-M+ where M is sodium, potassium, lithium, calcium, or magnesium Preferably R1 is a methoxy or ethoxy group.

(c) R2 is a group capable of bonding to the polymer by ionic, covalent, or hydrogen bonding.

R2 can be the same as R1, or preferably R2 is a group that reacts selectively with the polymer and may comprise a group selected from, acylchlorides, ammonium salts, esters, acid halides, acid anhydrides, amides, sulphamic acid, sulphamic acid salts, isocycanates, sulphates, phosphates, halides, carbodiimides, glycidyl and epoxy groups, sulphides, mercapto groups, azido groups, azo groups, vinyl, allylic, acrylic and methacrylic groups, together with a free radical initiator.

(d) R3 can be a group that is inert or capable of bonding to one of the filler or polymer. Preferably R3 is -OR where R is an alkyl, aryl, benzyl, or phenyl group.

(e) R4 is present only if M has a valency greater than three. R4 preferably is the same as R3.

Preferably R2 is a vinyl, allylic, or acrylic group which bonds to the polymer through a free radical reaction initiated by the use of a peroxide catalyst, or by use of an azo compound such azobisbutyronitrile.

The preferred coupling agent includes a methoxy or ethoxy group for bonding to the talc and a vinyl group for bonding with the polyethylene and is preferably vinyltrimethoxysilane.

The peroxide catalyst may be benzyl peroxide which is added to the talc after addition of the coupling agent preferably in an amount of 0.1-0.2W by weight based on the weight of talc.

It is thought that the methoxy group reacts with the talc and that the vinyl group takes part in a free radical reaction with the peroxide and the polyethylene chain.

Coupling agents may also be used with compositions including polar polymers such as nylon, aliphatic polyketones, polyesters, and ethylene vinyl alcohol.

For example a polyester resin such as polyethylene teraphthalate can reactively couple with a coupling agent having an epoxy group. Nylons and ethylene vinyl alcohol can be reactively coupled to a coupling agent having a carboxylic acid group or an acid anhydride.

Preferably the composition includes 15% by weight of talc, and about 2 by weight of coupling agent based on the weight of talc; preferably a vinyl silane coupling agent.

Description of Drawings The invention will now be described in more detail by way of example only, and with reference to the accompanying drawings in which: Fig 1 is a cross-section through a moulded tube shoulder as used for the vapour transmission tests, and Fig 2-4 are graphs of loss of weight of particular flavourings over time, for moulded shoulders of different composition.

Production of Test Samplers A number of different samples of an injection moulded tube shoulder of the type shown in Figure 1 were made as follows: (1) 1 kg of talc Magsil Osmanthus was spray coated in a Kenwood food mixer at a steady rate of 10 ml/min up to a desired percentage weight of coupling agent based on weight of talc. The coupling agent was vinyl trimethoxy silane available from Hauls, which was made up as a 50% weight/weight solution in isopropyl alcohol. 0.15% of benzyl peroxide powder (available from Akzo Nobel) (based on the weight of talc) was then added to the mixture, which was mixed for a further 2 minutes.

(2) The coated talc was then tumbled with high density polyethylene (HDPE) pellets in a 40:60 mixture by weight and fed to the hopper of a twin screw extruder operating at standard conditions for polyethylene compounding. The mixture was subjected to high shear during mixing, and was then extruded and cut into pellets. This was then treated as talc-filled master batch.

(3) The talc-filled master batch was then tumble blended with natural polyethylene pellets, in a ratio of 100 parts master batch:150 parts of HDPE by weight, and colorant if desired, to produce a mixture containing about 15% talc.

The final talc-filled composition was then injection moulded in a Klockner Ferromatic 240 tonne injection moulding machine at a temperature of about 220-2300C to manufacture the shoulders shown in Figure l.

In a scaled-up method using commercial quantities of material the procedure prior to injection moulding was as follows: (1) 1 tonne of talc was placed in a Z-blade mixer and the solution of vinyl trimethoxy silane in isopropyl alcohol was added over a twenty minute mixing period.

For 2% w/w addition of trimethoxysilane this requires a rate of addition of 2 litre/minute of solution. The benzoyl peroxide powder was then added and the mixture further mixed for about 15 minutes.

(2) The coated talc was then added to HDPE pellets and mixed in an internal mixer, e.g. a Banbury mixer and the mixture extruded and pelletised.

(3) The tube shoulders (Fig 1) were injection moulded as previously described.

In practice it was found that there was no essential difference between shoulders produced by either route described above.

The physical properties of the various compositions were measured using moulded test sheet samples under test conditions as specified in ASTM D638, D256 and D790 for tensile, impact and flexural properties respectively.

The results are shown below in Table 1 for moulding compounds containing 15% talc and 1-3% of coupling agent.

The values for natural HDPE, and HDPE containing 15% talc filler, without coupling agent, are given for comparison purposes.

Physical Properties Table 1

Physical Property Mixture Flexural Flexural Tensile Tensile I m p a c Strength Modules Modules Strength Strength (MPa) (GPa) (SPa) kMPa) HDPE Natural 21.23 0.78 .43 2.92 10.76 .iDPE+15WTalc (Untreated) 26.84 1.32 0.71 23.28 5.79 HDPE +15% Talc (1% VTMS) 26.60 1.36 0.75 24.53 5.61 HDPE +15% Tale 2t VTMS) 28.47 1.23 0.74 24.68 5.03 HDPE li58 Talc (3% VTMS) 25.58 1.26 0.85 24.03 5.55 It can be seen that the inclusion of the coupling agent provides a marginal increase in the flexural and tensile properties as compared with the untreated talc-filled material. It is noted that the impact strength is marginally reduced.

Barrier Properties The vapour barrier properties were tested by using samples of tube shoulders of the type shown in Fig 1, into which different amounts of different flavourings were sealed using impermeable aluminium foil.

The tube shoulder shown in Fig 1 is a 35 mm diameter tube shoulder having an overall height of about 20 mm, with a 12 mm diameter nozzle with a height of about 10 mm.

Shoulders of this general shape and dimension are sold as part of commercially available flexible tube containers sold by Courtaulds Packaging Ltd.

Sample tube shoulders were weighed, sealed at the base with aluminium foil 50 m thick, filled with 0.5g of either cineole or menthone or with 1.0g of limonene. The larger weight of limonene used is due to its faster transmission.

The sample shoulders were then secured at the nozzle, again with aluminium foil (50 m). Each shoulder was re-weighed and 5 then stored in air at 230C and 60% relative humidity and weighed at intervals over a 45 day period for limonene and 120 day period for the others.

Table 2 below shows the weight loss of flavouring due to body absorption and transmission over the designated time 10 period.

% weight loss due to transmission and absorption.

Table 2

Materials Flavouring mix Cineole o Menthone O Limonene * Absorption Transmission Absorption Transmission Absorption mfmioion Natural 50% 18S 65% 10% 9t 56% KDPE HDPE 18k 98 67% 11% 8% 49% +15% talc HDPE 16t 4t 68S 5t 8t 38% +15% talc + 2% VTMS these flavourings were observed over 120 day period.

* this was restricted to 45 days, due to the rapid weight loss.

It can be seen that the inclusion of the 26 coupling agent (vinyltrimethoxysilane) greatly decreased the transmission of the flavouring through the tube shoulder.

The actual weight loss solely due to the transmission of the flavouring through the shoulder as monitored over the full period is shown in Figures 2-3.

The graph shown in Figure 2 relates to cineole and also includes an additional curve for a shoulder made from 10% talc filled HDPE. It can be seen that as the percentage of talc increases the vapour transmission is reduced. The treatment of talc with the coupling agent (2%) reduces the transmission rate even further as compared with the samples containing untreated talc. This trend is also repeated for menthone as is shown in Fig 3. It can be seen that the addition of 15% talc has little effect upon the transmission of menthone, but the inclusion of the coupling agent (2W) reduces the menthone loss rate.

The loss of weight by transmission for limonene is shown in Fig 4. It can be seen that the transmission of limonene is reduced by the inclusion of talc treated with coupling agent as compared with untreated talc and that the rate of transmission loss is reduced when the percentage of coupling agent is increased from 2% to 3%.

Claims

1. A method of improving the barrier properties of a polymeric composition which includes an inorganic platelet filler, wherein a coupling agent is added to the composition, said coupling agent having a first chemical group that is reactive with the polymer, and a second chemical group that is reactive with the platelet filler.

2. A method as claimed in claim 1, wherein the polymer composition includes at least one non-polar thermoplastic polymeric resin.

3. A method as claimed in claim 2, wherein the polymer composition comprises at least one polyolefin, preferably high density polyethylene.

4. A method as claimed in any of claims 1 to 3, wherein the platelet filler is high purity talc which is capable of delamination when the composition is subjected to high shear.

5. A method as claimed in claim 4, wherein the talc has an average particulate size of about 17cm, with 70% of the particles having a size of between 10-254m.

6. A method as claimed in any preceding claim, wherein the coupling agent is a vinyl silane, preferably vinyl trimethoxysilane, in combination with a free radical initiator.

7. A method as claimed in any preceding claim, wherein the coupling agent is blended with the talc prior to mixing with the polymer.

8. A method as claimed in claim 7, wherein a free radical initiator is also added to the talc prior to mixing with the polymer.

9. A method as claimed in any preceding claims, wherein the composition includes up to 3% by weight of coupling agent.

10. A composition for forming an article having improved barrier to gases and/or vapours, the composition being made by a method as claimed in any one of claims 1 to 9.

11. A method of manufacture of a component for a flexible tube container, wherein the component is formed from a composition as claimed in claim 10.

12. A composition for forming an article having increased barrier to gases and/or vapour transmission and which comprises at least one polymeric thermoplastic resin, an inorganic platelet filler and a coupling agent having a first chemical group reactive with the polymeric material and a second chemical group which is reactive with the platelet filler.

13. A composition as claimed in claim 12, wherein the polymeric resin is a non-polar resin, preferably a polyolefinic resin.

14. A composition as claimed in claim 13, wherein the polyolefin resin is a high density polyethylene having a melt flow index (MFI) of about 10.

15. A composition as claimed in any one of claims 12 to 14, wherein the platelet filler is talc, the talc particles having an average size of 2-10Cim after processing.

16. A composition as claimed in claim 15, wherein the composition contains between 5-30W by weight of talc, preferably 15% talc.

17. A composition as claimed in claim 13 or claim 14, or as claimed in claim 15 or claim 16 when dependent upon claim 13 or claim 14, wherein the coupling agent is a vinyl silane.

18. A composition as claimed in claims 17, wherein the coupling agent is vinyl trimethoxysilane.

19. A composition as claimed in claim 17 or claim 18, wherein the composition contains 1-3% of coupling agent based on the weight of talc.

20. A flexible tube container having a component thereof formed from a composition as claimed in any one of claims 12 to 19.