GB2093054A - Sealing compositions - Google Patents

Abstract

Glass fibre or sand is included as part or all of the filler in a sealing composition suitable for sealing container ends that includes a rubbery polymer such as a styrene butadiene copolymer and optionally also a tackifying resin.

Description

SPECIFICATION Sealing compositions This invention relates to sealing compositions intended for sealing container closures such as top or bottom end closures of cans or replaceable or non-replaceable caps for jars or bottles. The compositions can also be used for other sealing purposes but, for clarity, since they are formulated to meet the particular requirements of can and other container closure seals the invention is described solely in terms of compositions for sealing container closures.

Traditional container end sealing compositions have comprised a liquid medium in which has been dispersed or dissolved rubber or other polymeric material and which includes also fillers, tackifying resin or other additives.

The liquid medium may be aqueous, for instance as in US Patent Specification No.

3,409,567 or British Patent Specification No 1566924, or the liquid medium may be organic, for instance as in British Patent Specification No. 1,340,730. A wide variety of fillers have been proposed for use in the compositions, for instance as is shown by these patent specifications, but only a few have proved to be satisfactory in use.

Typical fillers that have been found satisfactory include kaolin, talc, zinc oxide and calcium carbonate. Generally the amount of filler must not be too high or else the sealing properties are impaired.

The liquid composition is applied to one at least of the mating surfaces of the closure and the sealing face of the container, generally to the closure, and is then dried on the surface. The closure is pressed onto the sealing face of the container so as to grip the container firmly and the composition provides a seal between the container and the closure.

It is necessary that the composition should have appropriate rheological and other physical properties. For instance when applied to can ends it should flow adequately during sealing so as to distribute itself over the mating surfaces, but preferably it does not flow to such an extent that significant extrusion of the composition occurs along the walls of the can. The seal provided by the composition should prevent ingress of bacteria. Generally it should also prevent loss of liquid, vacuum or gas.

It has been our object to provide sealing compositions that satisfactorily seal container closures and that include filler that has not previously been proposed for this purpose. Preferably the filler is such that, compared to use of fillers commonly used at present, either the seal is improved or the amounts of either the rubber or other polymeric material or the tackifying resin, or both, can be reduced without reducing the sealing properties. It has also been our object to provide methods of sealing containers using such compositions, and to provide sealed containers.

A sealed container according to the invention has a closure sealed to it by a seal that includes a gasket fomed of a rubbery polymer in which is dispersed filler including sand of glass fibres.

A novel composition according to the invention comprises a rubbery polymer and filler that is dispersed throughout the composition and that includes sand or glass fibres, and generally also tackifying resin. The composition may be a meltable solid but preferably comprises also a liquid medium in which the polymer is dissolved or dispersed.

The sealed container may be fully sealed, for instance being a jar or a two piece can or a can sealed at both ends, or it may be a can that has a closure sealed to it one end but which is open at the other.

Such a sealed container can be formed from a container and a container closure in conventional manner. Thus the sealing face of the closure is lined with a liquid composition comprising rubbery polymer and a dispersion of the filler, the composition is solidified (generally by drying) to form a gasket, and the sealing face of the closure is compressed around the end of the container thereby sealing the closure to the sealing face with the gasket within the seal.

When the container is a bottle this gasket is trapped between the sealing face of the rim of the bottle and the overlying closure. Preferably however the container is a can in which event the gasket is trapped in the double seam formed in conventional manner by compressing the outer periphery of the container closure around an outwardly extending flange of the side wall and then pressing the flange and the closure periphery against the side wall of the container generally in a single operation.

The preferred compositions in the invention are those in which the filler includes glass fibres.

The fibres may have a diameter of from 1 to 200 microns, preferably 1 to 50 microns and most preferably 5 to 30 microns. They must have a short length, generally less than 500 microns. It may be for instance, 10 to 200 microns, preferably 20 to 1 50 microns. They may be obtained by milling or otherwise chopping continuous lengths of glass fibre.

The glass fibres may have been given a surface coating of a variety of materials provided the surface coating does not interact with other components in the composition in such a way as to reduce significantly the sealing properties of the composition. Preferably it is not coated with a mercapto silane or other silane that increases the adhesion of the fibres to the rubber since this seems to impair sealing properties.

Throughout this specification amounts of components of the composition, including amounts of fibres and other fillers, are expressed as amounts by volume based on the volume of rubbery polymer, unless otherwise specified. For instance 10% fibre means 10 volumes fibre per 100 volumes rubbery polymer.

The amount of glass fibre in the composition should be at least 1%, since lower amounts tend to give inadequate improvement. Generally the amount is below 150%, and normally below 100%, since greater amounts tend not to give significant further improvement. Generally the amount is at least 3%, and normally, at least 5%. Preferably the amount is at least 10%.

Generally the amount is up to 50%. Typically the amount may be from 5 to 100%, most preferably 10 to 50%.

The filler preferably consists substantially only of glass fibre, with the result that the composition may contain no significant amounts of other fillers, although it may include fillers that are present primarily for their pigmentary purposes, for instance titanium dioxide which may be present in amounts of up to 10 or 15%.

The filler may however include other particulate inorganic material. The material other than glass fibre may be present in an amount of 0 to 150% (based on the volume of rubbery polymer), generally 10 to 100% and preferably 1 5 to 100%. The composition may include 0.05 to 2 parts, preferably 0.1 to 1 part, by volume glass fibre per part by volume other inorganic particulate filler.

Although the total volume of filler, including flass fibre can be similar to that conventionally used in commercial sealing compositions, for instance 10 to 45%, a particular advantage of the invention is that larger amounts of total filler may be used while still obtaining satisfactorily sealing properties. For instance the toatal amount of filler, including glass fibre, is usually at least 20% (by volume based on the volume of rubbery polymer) and can be up to 175%, for instance 50 to 125%.

Titanium dioxide or other pigmentary filler (for instance carbon black) generally has a particle size below 5 microns but other particulate inorganic fillers that may be used in the invention generally have a particle size of from 1 to 50 microns. The filler should be substantially nonabrasive, so that it does not cause wear to the machinery by which the composition is mixed and lined onto the can or other end.

The preferred other filler, is kaolin or china clay or zinc oxide but other fillers include synthetic colloidal silica ant other silicic fillers, synthetic silicate, calcium carbonate or sulphate, aluminium hydroxide, talc, dolomite, barium sulphate, or magnesium oxide or carbonate or silicate.

Such fillers may have been surface treated, for instance in conventional manner.

Instead of modifying the colour of the composition by including particulate pigment some other colouring material, for instance a soluble dye, may be included.

The invention also includes the use of sand as filler. Its particle size is generally from 1 to 200 microns, preferably 10 to 75 microns with its average particle size generally being from 5 to 100 microns preferably 10 to 50 microns. Amounts, surface treatments, and mixtures with other fillers may all be described above with reference to glass fibres.

The composition is formed from a rubbery polymer, that is to say a polymer that, when dried, forms a gasket that is sufficiently flexible and resistant to be capable of serving as a seal. It should have the conventional properties of rubbery polymers, i.e. it should be capable of being subjected to substantial reversible deformation. Rubbery polymers suitable for forming seals are well known. Generally the Mooney viscosity (ML1l 4c) of the rubbery polymer is from 20 to 200, preferably 40 to 160.

The rubbery polymer may be a natural polymer for instance natural rubber, or may be a synthetic polymer. Suitable synthetic rubbery polymers include butyl rubber, polycloroprene, butadiene acrylonitrile copolymers, ethylene propylene copolymers, ethylene-propylene-diene terpolymers, styrene isoprene block copolymers, polybutadiene, styrene acrylic copolymers, polyvinylidene chloride, polyvinylidene chloride copolymers, plasticised polyvinyl chloride, polyvinyl chloride copolymers, plasiticised polyvinyl propionate or acetate, polyvinyl propionate or acetate copolymers, polyacrylic acid copolymers, polymethylacrylic acid copolymers, acrylic ester copolymers, methacrylic ester copolymers, plasticised polystyrene, vinly acetate copolymers with for instance ethylene, styrene butadiene block copolymers, stryrene butadiene rubbers solution polymerised or emulsion polymerised, and carboxylated styrene butadiene copolymers.

Blends may be used. Compositions based on vulcanisable polymers may include vulcanising agent.

Naturally the rubbery polymer will be chosen having regard to, for instance, the type of composition that is being used for forming the seal. The preferred polymers are styrene butadiene rubbers having a styrene content of 1 5 to 60% preferably 18 to 45% by weight.

They have been made by any convenient polymerisation method, and thus may have been made by hot or cold polymerisation techniques.

Tackifier resins are generally included in can sealing compositions and they may be included in the compositions used in the invention. However because of the good sealing properties obtained by the use of novel filler satisfactory results can often be obtained without a tackifier resin in the invention. Instead of using a tackifier resin a liquid plasticiser, such as white oil or other hydrocarbon oil, that softens the polymer may be used in amounts of for instance 1 to 60%, preferably 5 to 40%.

Best results are generally obtained when tackifier resin is included. Suitable materials are well known and are generally selected from synthetic hydrocarbon or petroleum resins, polyterpene resins, phenolic resin modified with natural resins such as rosin or terpene, xylene formaldehyde resin and modified products thereof, and esterified rosins or other rosin type resins such as rosin, hydrogenated rosin, or hardened rosin. The amount of tackifier is generally at least 10% (by volume of rubbery polymer) but less than 250% and preferably less than 200%. Generally the amount is at least 15%.

The compositions may include minor amounts, e.g. up to 1% or at most up to 5% of other additives that are known to those skilled in the art and that are conventional in filled sealing compositions, such as viscosity increasing agents (for instance ammonium alginate, bentonite or gum karaya or high molecular weight polyacrylic acid), bactericides, corrosion inhibitors, surfactants, anti-oxidants (for instance phenolic or amino anti-oxidants) and pH adjusters (for instance ammonia, primarily amine, sodium hydroxide or sodium carbonate).

The composition preferably is liquid at room temperature and thus preferably includes a liquid medium that serves as a carrier for the rubbery polymer and the filler.

The amount of the liquid medium will be chosen having regard to the maximum total solids concentration obtainable in the final composition consistant with solubility or dispersibility of the polymer in the liquid medium, ease in preparing the composition, storage stability of the composition, and application of the composition to the can end using high speed automatic lining equipment. In general, the amount of liquid medium is such as to yield a composition having a solids content of from 20% to 85% by weight.

Preferably the liquid medium is aqueous. The aqueous composition will generally contain at least one stabiliser for stabilising the dispersion. This stabiliser may be selected from any of the materials conventionally used for stabilising aqueous sealing compositions based on filler and rubbery polymer. Such stabilisers include styrene maleic an hydroxide or other styrene copolymers, methyl cellulose, polyacrylamide, ethoxylated condensates, polyvinyl pyrrolidone ammonium oleate, and casein. Such stabilisers may be used in admixture, for instance with other materials.

The aqueous composition is preferably a latex obtained by dispersing the specified filler and tackifier (if present) into a latex of the chosen rubbery polymer, for instance as formed by emulsion polymerisation. The composition may be made simply by mixing into the chosen latex (optionally after dilution) the tackifying resin, the selected filler or fillers, and any other additives, all in conventional manner. Naturally care must be taken to ensure that the latex does not coagulate and that a uniform dispersion is obtained. For instance it may be desirable to form a dispersion of the filler or fillers, and optionally also tackifier, and add this stable dispersion to the latex. The total solids content of the composition is generally from 20 to 85% by weight, preferably 30 to 80%.

Instead of formulating the aqueous composition as a latex it may be a dispersion obtained by dispersing solid rubbery polymer and filler into the aqueous medium. The aqueous dispersion may be made by milling the solid rubber with filler and other optional additives, including also generally any tackifying resin, using an internal mixer, for instance a Banbury mixer, so as to form a rubber stock. This rubber stock is then dispersed in water in conventional manner, for instance using a Z-blade type of mixer. Additional components, for instance thickening agent and more water, may be added to the dispersion to alter its consistency. Instead of including all the major additives in the rubber stock some may be added to the dispersion. For instance the rubber may be milled with some of the additives and then dispersed in water and other major additives introduced at this stage.For instance the novel filler of the invention may be added to the aqueous dispersion obtained by dispersing solid rubber and optionally some of the filler.

Although aqueous compositions are preferred the compositions of the invention can be organic, in which the liquid medium comprises organic solvent in which some or all of the rubbery polymer will dissolve, any remaining polymer going into dispersion.

Suitable organic liquids which may be employed in preparing organic compositions include aliphatic and aromatic hydrocarbons, for example 3-methylheptane, hexane, heptane, xylene and toluene; chlorinated hydrocarbons, such as dichloropentane; ketones; ethers, ether-alcohols, and mixtures of these and other volatile organic liquids which together form media as known in the art for the selected elatomers.

The organic compositions are generally made by blending the solid rubber with filler and optional additives such as anti-oxidants in an internal mixer, for instance a Banbury mixer. The solid rubber stock is comminuted and dissolved in the chosen solvent or solvent mixture in conventional manner. If tackifying resin is to be introduced it may be added to the solvent or it may be blended into the solid rubber stock.

The composition may be an organosol of the rubbery polymer, filler plasticiser, organic solvent and other optional additives.

The composition may be a melt consisting of the rubbery polymer and filler, and other optional additives. For instance the novel filler of the invention, and optionally other filler, may be mixed into the polymer while it is soft and the mass then fully melted before application to the closure.

We have found that the inclusion of the chopped glass fibres does, as a generality, result in improved sealing properties compared to the same composition in which an equivalent volume of other filler (such as kaolin) is used in place of the chopped glass fibres. The use of sand also gives an improvement.

A number of sealing tests are used in the industry and are recognised as being meaningful and by saying that the sealing properties are improved we mean that the number of cans that fail a meaningful sealing test will be reduced. In some instances there may be no improvement in the results of some sealing tests but improvements in other tests will show that, despite this, there is a useful practical improvement.

The novel fillers also have the advantages that they can have inert surface characteristics and so can avoid some of the handling and other difficulties that may be encoutered with other fillers. For instance fillers such as calcium carbonate can lead to a risk of coagulation of a polymer latex into which such a filler is incorporated. Accordingly when such fillers are used it is neccessary to take particular precautions to prevent coagulation of any latex or dispersion that is present.

A number of sealing tests are used in the industry and are recognised as being meaningful and by saying that the sealing properties are improved we mean that the number of cans that fail a meaningful sealing test will be reduced. Because there is no absolute test of sealing properties various tests are used and a judgement is made on the basis of the results of all these tests. For instance if one test shows an improvement in properties and another shows a variation or possible a deterioration in sealing properties the composition may well be assessed as giving improved sealing properties because of the improvement show in the first test.

It is recognised in the industry that extrusion of composition around the rim of a can, so as to give visible beads of composition on the outside of the can after sealing, is undesirable. Indeed for some purposes cans containing a noticeable amount of extruded composition around the rim may be rejected, irrespective of the properties recorded in any other sealing test. The inclusion of uncoated fibre tends to reduce the tendancy for such extrusion to occur, especially during hot sterilisation that may follow sealing. The extent of extrusion is recorded in the examples below by the "sterilisation extrusion" test that is described below. In this test improved extrusion properties are manifested by less extrusion occurring and thus by a lower quantitative value being recorded.

The most consistently satisfactory sealing properties and the greatest tendency for a significant improvement in sealing properties, especially in the extrusion properites, is obtainable in the invention when the compositions are aqueous, and especially in latex compositions, and so these are preferred. Useful results are, however, obtained in the other types of compositions described above, especially in the described organic compositions.

Some non-limiting examples of the invention are now given.

In these sealing properties are identified by two sets of quantitative values which are referred to as "bioligical seal" and "sterilisation extrusion". These are recorded as follows: "Biological seal". The composition is lined into can closures (often termed can ends) and dried in conventional manner, the amount of the compositions being such as to give the dry film volume generally recommended for the particular size. Cans having a soldered side seam are then filled with a hot liquid nutrient, typically at a temperature of 97"C, leaving a small headspace. The test closures are double seamed onto these filled cans whilst simultaneously injecting steam into the headspace.The closed cans are then steilised, typically at 121 C for 30 minutes, and after sterilisation are in mediately cooled in water containing gas-producing, nonpathogenic micro-organisms capable of growth in the aforementioned nutrient. After cooling and whilst still wet with the cooling water, the cans are subjected to a controlled deformation at the junction of the side seam and the double seam of the test closure. After incubation for six days at an elevated temperature optimum for the growth of the micro-organisms, followed by one day at ambient temperature, the cans are examined visually and the number of swollen cans recorded. The retained vacuum in the remaining cans is measured. Cans having a low retained vacuum and the swollen cans are considered to have reached this condition through failure of the seal in the test closure. The swollen and low vacuum cans are termed failures and the "biological seal" value is the failure rate expressed as the number of such cans per thousand tested. Because of the procedures used the number of failed cans per thousand in this biological seal test is of course very much greater than that which would occur with commercially packed cans sealed with these compositions.

"Sterilisation extrusion". The composition is lined into can closures and dried, in conventional manner, the amount of the composition being such as to give a dry film volume approximately 20% greater than that generally recommended with the particular closure size. Cans are filled with water at typically 70"C to leave no headspace and test closures are double seamed onto these filled cans. The closed cans are then sterilised typically at 1 30 C for one hour and allowed to cool to room temperature before examination. The number of protrusions of compound from the double seam along the outside wall of the can body at the test closure is counted, typically on a sample of 10 cans for each composition. Large protrusions are counted as appropriate multiples of the typical, more commonly occurring, small protrusions.The average number of protrusions per can is recorded as the value for "extrusion". This value should be as low as possible, preferably below 10 under the conditions of the test. However, because of the extreme conditions of the test, greater values than this are commercially tolerable.

Since the extrusion and biological seal results will vary according to, for instance, variable conditions under which the tests are carried out comparisons should, in general, be made only between results within a single example. It is desirable that the "biological seal" and "sterilisation extrusion" values should be as low as possible.

In each of the following examples the chosen rubbery polymer is supplied with minor amounts of conventional additives known to those skilled in the art and is combined with the specified amounts of filler, titanium dioxide pigment, tackifier resin if present, and stabiliser if the composition is aqueous. In each of the examples the amount of water or organic solvent is selected so that the composition has a solids content, and therefore a viscosity, suitable for the appartus being used for depositing the composition on the can closure. Generally the solids content is from 25 to 85% by weight. Thus those compositions in which the filler consists of kaolin have a total solids content of about 60% by volume and comparative compositions in which the filler is replaced by glass fibre or sand will have the solids contents that follows automatically from this substitution.

In each example the compositon is lined onto the can closure, dried and then tested in the described manner.

In each example the filler consists of glass fibre or sand, if present, and the stated inorganic particulate material, if present which generally has a particule size of 1 to 50 microns although titanium dioxide may have a particle size down to 0. 1 micron.

Examples 1 and 2 are of latex based compositions and are formed by mixing a latex of the chosen rubber with 3.2% TiO2, 22% takifying resin and 4.2% stabiliser (all by volume based on the rubber) with the stated amount of filler (by volume based on the rubber).

In examples 1 and 2 the latex is a styrene butadiene latex having a solids content of 66 to 69% by weight and containing 31 to 36% bound styrene and which has been polymerised cold (at 5"C) using fatty acid soaps. The polymer in the latex has a Mooney value (as defined above) of 100 to 1 30. However, similar results may be obtained using other styrene butadiene latices that may have been polymerised hot or cold such as those listed in the following table:: Type Total Solids Bound Styrene Mooney Emulsifier Value Cold 63 29 140 Fatty acid Cold 67 34 75 Fatty acid Cold 68 30 1 50 Fatty acid Hot 45 46 90 Rosin ester Hot 42 50 30 Rosin ester Hot 59 46 75 Rosin ester Hot 50 46 70 Rosin ester In the examples 1 and 2 the tackifier resin is a polymer of mixed 5-carbon alkenes havng a melting point of about 1 00 C. The stabiliser is a styrene maleic anydride copolymer. Similar results are obtainable with other tackifiers and stabilisers.

Example 1 Test Filler Biological Sterilisa Seal tion Extrusion 1A 30 Kaolin 315 30.9 1B 30 Glass Fibre (formed 75 5.0 from Type E glass 70 microns long, 1 5 microns diameter) 1C 30 Glass Fibre (formed 180 21.5 from Type E glass 150 microns long, 1 3 microns diameter with a silane surface treatment) 1D 30 sand 85 1.8 Example 2 Test Kaolin Glass Fibre Biological Sterilisation Seal Extrusion 2A 27 3 (as in 1B) 675 10.6 2B 24 6 (as in 1B) 535 5.4 2C 27 3 (as in 1C) 725 2.4 2D 24 6 (as in 1C) 630 8.4 Example 3 This composition is formed by blending 67 parts hot polymerised SBR having 23% bound styrene and a Mooney value of 50-58, 33 parts by volume cold polymerised SBR having 44% bound styrene and containing divinlbenzene as a cross linking agent, 2% by volume TiO2, 45% by volume polyterpene resin tackifier (a polymer of beta-pinene, m.p. approximately 11 5 C) and 22% by volume filler.

Test Kaolin Glass Fibre Biological Sterilisation Seal Extrusion 3A 22 0 370 16.2 3B 11 11 (as in 1B) 245 13.6 3C 0 22(asinlB) 170 29.8 3D 11 11 (as in 1C) 225 27.6 3E 0 22 (as in 1C) 255 14.8

Claims (14)

1. A sealing composition suitable for sealing container closures and that comprises a rubbery polymer and that has a filler dispersed throughout, characterised in that the filler includes material selected from sand and glass fibres.

2. A composition according to claim 1 in which the filler includes glass fibres having a diameter of 1 to 200 microns and a length of less than 500 microns.

3. A composition according to claim 1 in which the filler includes glass fibres having a diameter of 1 to 50 microns and a length of 10 to 200 microns.

4. A composition according to claim 2 in which the amount of glass fibres is from 3 to 100% by volume of rubbery polymer.

5. A composition according to claim 4 in which the amount of glass fibres is from 10 to 50% by volume based on the volume of rubbery polymer.

6. A composition according to claim 2 in which the filler consists substantially only of glass fibres.

7. A composition according to any preceding claim in which the total amount of filler is from 20 to 175% by volume based on the volume of rubbery polymer.

8. A composition according to any preceding claim in which the rubbery polymer is a styrene butadiene copolymer.

9. A composition according to any preceding claim additionally including a tackifier resin.

1 0. A composition according to claim 9 in which the amount of tackifier resin is from 10 to 250% by volume based on the volume of rubbery polymer.

11. A composition according to any preceding claim which is a liquid and includes a liquid medium in which the rubbery polymer is dissolved or dispersed.

12. A composition according to claim 11 in which the liquid medium is an aqueous medium in which the polymer is dispersed.

1 3. A composition according to claim 1 2 and which comprises a latex of the rubbery polymer and the filler has been dispersed in the latex.

14. A method of sealing a container closure to a container comprising lining the sealing face of the closure with a composition according to any preceding claim, solidifying the composition to form a gasket and then compressing the sealing face of the closure around the end of the container and thereby sealing the closure to the conainer with the gasket within the seal.

1 5. A method according to claim 1 4 in which the container is a can and the closure is a top or bottom can end.

1 6. A container having a closure sealed to it by a seal that includes a gasket formed of a rubbery polymer in which is dispersed a filler including sand or glass fibres.