The present invention relates to a bag and a method of making it.

Bags of various types and materials are previous¬ ly known. Bags were originally made mainly of woven textile material, such as jute, but are nowadays usual¬ ly made of paper or polymer material. Bags of polymer material usually consist of a single layer of poly¬ ethylene. Bags of polymer material are superior to bags made of paper, for example with regard to their resistance to water and other liquids, their impermea¬ bility to oxygen and other gases, etc. Even if the strength usually is adequate, there is need for fur¬ ther improvement.

The present invention aims at satisfying this need and providing a bag of improved strength.

The higher strength of the bag according to the invention brings the advantage that a stronger bag is obtained with the same wall thickness as for known bags. The increased strength can also be utilised for providing a bag which is as strong as known bags, but has a reduced wall thickness, whereby material is saved.

The object of the invention is achieved by means of a bag which consists of a multilayered polymer material consisting of at least one polyolefin layer together with at least one layer of a crosslinked polymer obtained by crosslinking of an olefin copo¬ lymer with hydrolysable silane groups under the ac¬ tion of water and a silanol condensation catalyst. The bag according to the invention is produced by coextruding a multilayered polymer material con¬ sisting of at least one layer of an olefin copolymer with hydrolysable silane groups and at least one poly¬ olefin layer, a silanol condensation catalyst for

crosslinking of the silane group-containing olefin copolymer being provided in a layer which is separate from the silane group-containing olefin copolymer.

Further features of the invention will appear from the following description and the appended claims. As has been mentioned above, the bag according to the invention consists of a multilayered polymer material which includes at least one polyolefin layer and at least one layer of crosslinked polymer. The polyolefin in the polyolefin layer can be selected among different olefin polymers, such as olefin homopolymers, for example polyethylene, poly¬ propylene, polybutene and the like, and olefin copo- lymers, for example poly(ethylene/vinyl acetate), poly(ethylene/butyl acrylate) and the like. At present, it is preferred that the polyolefin consists of poly¬ ethylene (PE) which can be selected among LD poly¬ ethylene (low-density polyethylene, LDPE) and HD poly¬ ethylene (high-density polyethylene, HDPE) , linear LD polyethylene, polyethylene of very low density (VLDPE), polyethylene of ultra-low density (ULDPE) and ethylene copolymers (for example EVA and EBA) . As has been mentioned before, the crosslinked polymer in the multilayered polymer material of the bag according to the invention has been obtained by crosslinking of an olefin copolymer with hydrolysable silane groups. However, the production of the cross¬ linked polymer may cause difficulties, especially when it is in the form of a thin layer, as is the case in the present invention. By thin layer is here meant a thickness corresponding to film and foil, i.e. up to about 2 mm, preferably about 1 mm at most, and more preferred about 0.6 mm at most.

In the production of a multilayered material, for example by extrusion, in which at least one layer is crosslinked, it is important that crosslinking occurs only after the mixture has left the extruder because

premature crosslinking or precuring in the extruder interferes with the rate of production and causes the finished product to deteriorate in quality. Incipient crosslinking or precuring already in the extruder (or similar equipment) causes gel formation and adhesion of polymer gel to the equipment surfaces with the en¬ suing risk of clogging. To prevent this, the equipment must be cleaned of adhering polymer gel, and for each cleaning operation the equipment must be shut down, which means a decline in production.

A further disadvantage is that any gel lumps not clogging the equipment will be discharged and show up in the product as disfiguring undesired lumps which, if they occur in thin layers, such as films and foils, are unacceptable and usually make the product useless.

Undesired precuring may be prevented by incorporat¬ ing in the polymer composition substances counteracting precuring, so-called precuring retarders. For polymers whose crosslinking is moisture dependent, for example the above-mentioned silanes, such precuring retarders may be in the form of drying agents. However, the addi¬ tion of precuring retarders implies that there is intro¬ duced into the polymer composition a further component, which makes the composition more expensive and, besides, may be undesirable, for example in packages in contact with food products. It therefore is an advantage if the addition of such further components as precuring retar¬ ders can be avoided.

The present invention obviates, the above-mentioned disadvantages by providing the silanol condensation catalyst in a layer separate from the silane group- containing olefin copolymer, i.e. either in the layer or layers consisting of polyolefin or in a different separate layer, such as a layer which is provided between the silane group-containing polymer and the polyolefin and which contains the silanol condensation

catalyst. The last-mentioned layer may consist of, for example, a so-called master batch layer of the silanol condensation catalyst.

The silane group-containing olefin copolymer is crosslinked by subjecting it to the action of water and by causing the silanol condensation catalyst to diffuse into the layer of the silane group-containing polymer.

The silane group-containing polymer is restricted to silane group-containing olefin copolymers. The rea¬ son for this is that it was found, when the invention was in progress, that the aim of the invention cannot be achieved with all silane-containing olefin polymers. Thus, the desired result is not obtained with silane- containing graft polymers, even if the silanol conden¬ sation catalyst is incorporated in another layer free from crosslinkable silane. Although the silanol con¬ densation catalyst is originally provided in another layer, and undesired precuring thus should be precluded, such precuring still occurs and imparts to the film a grainy, unacceptable appearance. The cause of this must presumably be attributed to peroxide residues from the production of the graft polymer which initiate precur¬ ing of the polymer. The use of silane-containing graft polymers also leads to free monomer residues in the final product, which result in an obnoxious smell and may constitute a health hazard, for example if the ma¬ terial is used in food packagings. It was therefore found necessary, in the context of this invention, to utilise for the crosslinkable polymer a silane group- containing olefin copolymer and to provide the silanol condensation -catalyst in a layer separate from this polymer. The present invention thus is characterised by the combination of these two requirements. As has been mentioned, the crosslinkable polymer material according to the invention is a silane-con-

taining copolymer by which is meant an olefin copolymer, preferably an ethylene bipolymer or terpolymer contain¬ ing crosslinkable silane groups provided in the polymer by copolymerisation. The manner in which the crosslink- able silane groups are attached to the polymer chain thus is critical; according to the invention, for example unsaturated silane compounds can be copoly- merised with olefins, or amino silane compounds can react with acrylate esters, whereas the invention does not include graft polymers in which peroxides are de¬ composed and graft unsaturated silane compounds on the finished polymer by direct reaction with the polymer chain.

The silane-containing polymer has preferably been obtained by copolymerisation of an olefin, preferably ethylene, and an unsaturated silane compound which is represented by the formula

in which R is an ethylenically unsaturated hydrocarbyl or hydrocarbyloxy group, R' is an aliphatic saturated hydrocarbyl group, Y is a hydrolysable group, and n is 0, 1 or 2. If there is more than one Y group, these need not be identical.

Specific examples of the unsaturated silane com- pound are those in which R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl, or gamma-(meth)aeryloxy propyl,

Y is methoxy, ethoxy, methoxyethoxy, formyloxy, acetoxy, propionyloxy, chloride or an alkyl or arylamino group, and R' is a methyl, ethyl, propyl, decyl or phenyl group.

An especially preferred unsaturated silane compound is respresented by the formula

CH ₂ =CHSi(OA) in which each A which is the same or different, is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon atoms.

The most preferred compounds are vinyl trimethoxy silane, vinyl trismethoxyethoxy silane, vinyl triethoxy silane, gamma-(meth)aeryloxypropyltrimethoxy silane, gamma-(meth)acryloxypropyltriethoxy silane and vinyl triacetoxy silane.

The copolymerisation of the olefin (ethylene) and the unsaturated silane compound may be carried out under any suitable conditions causing copolymeri¬ sation of the two monomers. Furthermore, polymerisation may be carried out in the presence of one or more further comonomers copo- lymerisable with the two monomers. Examples of such comonomers are: (a) vinyl carboxylate esters, such as vinyl acetate and vinyl pivalate; (b) (meth)acry- lates, such as methyl(meth)acr late, ethyl(meth)acrylate, and butyl(meth)acrylate; (c) olefinically unsaturated carboxylic acids, such as (meth)acrylic acid, maleic acid and fumaric acid; (d) (meth)acrylic acid deriva¬ tives, such as (meth)acrylonitrile and (meth)aer 1amide; and (e) vinyl ethers, such as vinyl methyl ether and vinyl phenyl ether. Of these comonomers, vinyl esters of monocarboxylic acids having 1-4 carbon atoms are preferred, such as vinyl acetate, and (meth)acrylates of alcohols having 1-4 carbon atoms, such as methyl(meth)- acrylate. An especially preferred comonomer is butyl acrylate. Two or more such olefinically unsaturated compounds may be used in combination. The expression " (meth)acrylic acid" is here intended to comprise both acrylic acid and methacrylic acid. The comonomer content in the copolymer may amount to about 40% by weight, preferably about 0.5-35% by weight, and most preferred about 1-25% by weight of the copolymer. The silane-containing polymer of the present invention contains the silane compound in a content of 0.001-15% by weight, preferably 0.01-5% by weight, and most preferred 0.1-3% by weight.

Crosslinking of the polymer is carried out by so-called moisture hardening which means that the silane group, under the action of water, is hydrolysed and splits off alcohol to form silanol. The silanol groups are then crosslinked under the action of a so-called silanol condensation catalyst by a conden¬ sation reaction during which water is split off.

Generally, all silanol condensation catalysts may be used for the present invention. More particu- larly, they are selected among carboxylates of metals, such as tin, zinc, iron, lead and cobalt, organic bases, inorganic acids and organic acids.

Specific examples of silanol condensation catalysts are dibutyl tin dilaurate, dibutyl tin diacetate, dioctyl tin dilaurate, stannoacetate, stannocaprylate, lead naph¬ thenate, zinc caprylate, cobalt naphthenate, ethyl amines, dibutyl amine, hexyl amines, pyridine, inorganic acids, such as sulphuric acid and hydrochloric acid, and orga¬ nic acids, such as toluene sulphonic acid, acetic acid, stearic acid, and maleic acid. Especially preferred catalyst compounds are the tin carboxylates.

The amount of silanol condensation catalyst employed usually is of the order 0.001-10% by weight, preferably 0.01-5% by weight, especially 0.03-3% by weight, relative to the amount of silane-containing polymer in the com¬ position.

The crosslinkable polymer may contain different addi¬ tives, as is usually the case in polymer compositions. Examples of such additives are miscible thermoplastics, stabilisers, lubricants, fillers, colourants and foaming agents.

Among additives in the form of miscible thermo¬ plastics, mention may be made of miscible polyolefins, such as polyethylene of low density, medium density and high density, polypropylene, chlorinated polyethy¬ lene, and various copoly ers including ethylene and one or more other monomers (such as vinyl acetate,

methyl acrylate, propylene, butene, hexene and the like). The above-mentioned polyolefin may be used alone or as a mixture of several polyolefins.

As examples of fillers, mention may be made of inorganic fillers, such as silicates, for example kaolin, talc, montmorillonite, zeolite, mica, silica, calcium silicate, asbestos, glass powder, glass fiber, calcium carbonate, gypsum, magnesium carbonate, magne¬ sium hydroxide, carbon black, titanium oxide and the like.

It should be pointed out that what has been said above concerns the composition of the preferred cross¬ linkable polymer of the multilayered polymer material according to the invention. o make the bag according to the invention, com¬ positions of the above-mentioned polyolefin polymers and the olefin copolymers with hydrolysable silane groups are coextruded to form a multilayered sheet material which is wound into a roll which then con- stitutes the starting material for the manufacture of bags in per se known manner. When the sheet material has been wound into a roll, it is protected against external influence, especially against moisture, so that the roll of sheet material can be stored for longer or shorter periods of time without any risk of crosslinking of the silane group-containing olefin co¬ polymer. It will be appreciated that this is an impor¬ tant practical advantage. During bag manufacture, the sheet material is unwound from the supply roll, and bags are manufactured in per se known manner from the sheet material. After manufacture, the finished bags are stored so that the silane group-containing olefin copolymer in the sheet material is crosslinked under the action of ambient moisture. This moisture may be the natural moisture of the ambient atmosphere, or the moisture content can be increased intentionally, for example by supplying water or water vapour.

The bag can be formed in known manner, and bags of the so-called valve type are especially preferred in the context of this invention. The multilayered polymer material from which the bag is manufactured, may in principle have an unlimited number of layers, but it is preferred that the multilayered polymer material has two or three layers. If the material is a two-layer material, one layer consists of poly¬ olefin, such as polyethylene, and the other layer of crosslinked silane group-containing olefin copolymer, such as poly(ethylene/vinyl trimethoxy silane). In principle, any one of these layers may be facing the bag inside or outside, but when the polyolefin layer consists of polyethylene, especially HD polyethylene, it is preferred to make the HD polyethylene layer face the inside of the bag to facilitate opening of the bag.

When the multilayered polymer material is a three- layer material, these layers may optionally consist of two polyolefin layers and one crosslinked layer of silane group-containing olefin copolymer, or of two layers of crosslinked silane group-containing olefin copolymer and one polyolefin layer. Furthermore, the two layers comprising the same type of polymer need not be identical, and their composition can be varied within the scope of the above-mentioned definitions. Thus, the multilayered polymer material may consist of an outer layer of LD polyethylene, followed by an intermediate layer of crosslinked silane group-contain- ing olefin copolymer, such as pol (ethylene/vinyl tri¬ methoxy silane), and finally a layer of HD polyethylene. The HD polyethylene layer is preferably arranged to face the inside of the bag to facilitate opening of the bag, as mentioned above, while the LD polyethylene layer is arranged to face the outside of the bag to prevent stacked bags from sliding off.

To further illustrate the invention, the following nonrestrictive Examples are given.

In these Examples, the materials, the testing tech¬ niques, the conversion equipment, the extrusion condi¬ tions employed, and the procedure were as follows.

MATERIALS EMPLOYED

SILANE Ethylene vinyl trimethoxy silane copolymer with 1.7% trimethoxy silane. Melt index = 0.7, MW = 200000

CAT.OCTYL 1% dioctyl tin dilaurate in LDPE 1

LDPE 1 High pressure poly¬ Melt index 4.0 g/10 min. ethylene with: 3 Density 922 kg/m

M(w) 123800

LDPE 2 High pressure poly¬ Melt index 0.2 g/10 min. ethylene with: Density 922 kg/m

M(w) 140000

HDPE High density poly¬ Melt index 0.3 g/10 min. ethylene with: Density 958 kg/m

M(w) 170000

TESTING TECHNIQUES

1. Crosslinking degree The film sample is ground and screened to recover that part which passes through a 30 mesh sieve but stays on a 60 mesh sieve. The sample is boiled in xylene, and 1% antioxidant is added to avoid further cross¬ linking.

2. Dart drop : ASTM D 1709

3. Tear strength : ISO R 6383/2

5. Tensile strength: ISO R 1184

6. Elongation : ISO R 1184

7. Drop test of ba : The bags were dropped from a drop table according to ISO/TC 122/SC 2/W3 N10. The bags were dropped from the following heights: 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, 2.50, 3.00, 3.50, 4.00 5.00 to a miximum of 6.00 meters

unless the bag had previously been damaged. The bags are dropped in three different po¬ sitions. On the flat side and the side edge, the bag being turned over on the other side after each drop, and on its end.

The bag is always placed on the same edge. The bag is filled wit 50 kg of a mixture of polyethy¬ lene and titanium dioxide master batch having a bulk density of 950 kg/m3. in the Examples, the bag filling degree was 90%. The drop height is calculated as the mean value of 6 drop heights without damage to the bag.

CONVERSION EQUIPMENT

Reifenhauser 3_-la_y_er_film blowin equipment.

1st extruder: - Screw diameter 70 mm

- Combined LDPE/LLDPE screw, length 25 D, with UC mixing zone and mixing unit, length 3 D 2nd extruder: - Screw diameter 50 mm

- HDPE screw, length 20 D, with mixing unit, length 5 D

3rd extruder: - Screw diameter 50 mm

- Polyamide screw, length 23 D 3-layer revolving die, diameter 200 mm and slot 1 mm

EXTRUSION CONDITIONS

FILM BLOWING: The extruder temperature for layers containing SILANE and HDPE from the feeding zone to the filter during the tests had been set at: for SILANE 130°C, 160°C, 170°C, and for HDPE 180°C, 190°C, 190°C. The filter temperature was maintained at 200 C, as was the temperature in the adapter. During the runs with SILANE and LDPE 2, the corresponding temperatures were: for SILANE 130°C, 160°C, 170°C, and for LDPE 2 170°C,

190°C, 190°C. The temperature in the filter, the adapter and the die was 200 C.

The film was run with a blowing ratio of 1.6 and with a frost line of 950 mm.

The total production rate was maintained at 165 kg film/hour. PROCEDURE

FILM BLOWING: The extruder and die temperatures were set with LDPE 1 in the film blowing line. When constant conditions had been established, the selected layer or layers were charged with the silane polymer and polyolefin. When the silane polymer and the poly¬ olefin had displaced LDPE 1 , the catalyst was charged into the polyolefin layer. In this manner, a film free from defects and without gel formation can be produced. CROSSLINKING: Crosslinking of the bag film begins after the bag film has been unwound and the bags have been welded or glued, and the film comes into contact with the humidity of the air. In the Example, cross¬ linking was measured two weeks after the valve bag had been glued and stored at 20°C and 30% air humidity.

EXAMPLES

Using the above-mentioned materials, equipment and techniques, there were produced by coextrusion multilayer films having the general configuration

Silane polymer/Catalyst-containing layer. More par¬ ticularly, the multilayer films had the following com¬ position: EXAMPLE 1: Outside SILANE 40 μm Interme¬ diate layer SILANE 50 μm

Inside 84% HDPE+16% CAT.OCTYL 40 μm

Total thickness = 130 μm

EXAMPLE 2; Outside SILANE 40 μm

Interme¬ diate layer SILANE 90 μm

Inside 75% LDPE 2+25% CAT.OCTYL 40 μm

Total thickness = 170 μm

STANDARD BAG: Monolayer film 200 μm LDPE 2 Mechanical strength

Crosslinking/% 55 62 Tensile strength/MPa machine direction 23.4 20.1 23 transverse direction 20.1 20.2 20 Yield point/MPa machine direction 15.4 10.2 11 transverse direction 16.4 10.8 11 Elongation/% machine direction 550 510 500 transverse direction 710 670 600 Tear strength/kN/m machine direction 27.7 35.6 33.8 transverse direction 120.9 75.8 87.7 Dart drop/g/50% F 150 500 550 1575 700

Bag drop/m flat side side edge end edge

The essential bag production characteristics are that the bag has a good dart drop value, high yield point and stiffness, good tear strength, and good drop strength. It appears from the above Table that it is possible in Exam¬ ple 2 to obtain with a coextruded film of vinyl silane/

LDPE which is thinner than the film material of a con¬ ventional bag, a strength which is the same or higher than in the conventional bag. This can be utilised to make a bag which is as strong as the conventional bag, but thinner, or one can make a bag which has the same thickness but is stronger than the conven¬ tional bag. Production of a thinner bag means not only a saving of material, but also a saving of the costs for storing and transporting bags and granules. Furthermore, the bag according to the invention has a far higher temperature resistance than conventional bags so that it is possible to package far warmer products in the bag according to the invention than in a conventional bag.