CA1179799A

CA1179799A - Sealing compositions

Info

Publication number: CA1179799A
Application number: CA000396239A
Authority: CA
Inventors: David N. Samuel; Kenneth M. Sinnott
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co Conn
Priority date: 1981-02-14
Filing date: 1982-02-15
Publication date: 1984-12-18
Also published as: GB2105360A; WO1982002721A1; JPS57190068A; EP0072810A1; GB2105360B

Abstract

ABSTRACT OF THE DISCLOSURE
Metal particles are included as part or all of the filler in a sealing composition suitable for sealing container ends and that includes a rubbery polymer such as a styrene butadiene copolymer, and optionally also a tackifying resin.

Description

7~ c W. R. GRACE & CO.

SEALING COI'~POSlTIONS
This invention relates to sealing compositions -intended for sealing container closures such as top or bottom end closures of cans or replaceable or non-replace-able 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 require-ments of can and other container closure seals the invention is described solely in terms of compositions for seal;ng 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 and other additives.
The liquid medium may be aqueous, for instance as in US Patent Specificat;on No. 3,409,567 or British Paten~ 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 satis-factory 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 closurc and the '7~

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sealing face of the conta;ner, generally to the closure, and is then ~ried on the surface. The closure is pressed onto thc sea]ing face oI the container so as to grip the container firmly and the composition provides a S 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 2~ 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 formed of a rubbery polymer in which is dispersed filler including crush resistant metal part;cles having a particle size of less than 200 microns.
A novel composition according to the invention comprises a rubbery polymer and filler that is clispersed throughout the composition and that includes the particles 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 .t ~,' ,.

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3 , instance b~ing a jar or a one 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 wh;ch is open at the other.
Such a sealed container can be formed from 5 a container and a container closure in conventional manner. Thus the sea]ing face of the closure is lined with a liquid composit;on comprising rubbery polymer and a dispersion of the filler, the composition is solidified (generally by drying) to form a gasket, and the sealing ace 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 i$
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 I 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 particles must be crush resistant, that i~
to say they must have sufficient strength to resist any risk of crushing during the sealing use to which the composition is to be subjected. Thus in a can end sealing composition the beads must have sufficient strength that they will not crush in the seal. In order that they are crush resistant the particles must be formeclof a hard metal. Thus a soft metal such as tin is unsatisfactory but hard metals such as stainless steel, nickel alloy or chromium are satisfactory.
The particles may have a rough surface or a smooth surface and may have a regular shape or an irregular shape. Preferably however any roughness or irregularity is not too extensive since otherwise this may increase the adhesion of the rubbery polymer to the surface of the particles to such an extent to deminish the advantages i, ~ ..

that are otherwise obtained. Preferably the particles are substantially smooth.
The particle size range of the beads is generally between 1 and 100 microns, preferably 10 to 75 microns, The maximum particle size most preferably is not more than 60 microns. The average particle size is generally from 5 to 100 microns, most preferably 10 to 50 microns, with best results generally being achieved with an average size of 20 to 50 microns.
The metal particles 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.
lS Throughout this specification amounts of components of the composition, including amounts of metal particles and other fillers, are expressed as amounts by volume based on the volume of rubbery polymer, unless otherwise specified. For instance 10~ particies means 10 volumes particles per 100 volumes rubbery polymer.
The amount of the particles in the composition should be at least 1%, since lower amounts tend to give inadequate improvement. Generally the amount is below 150~o ~ 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 lOO~o ~ most preferably 10 to 50~.
The filler may consist substantially only of the particles, 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~, ', ,, ", " s, ~L~7~9 Good results are also obtained when the filler does include other particulate inorganic material and this is generally preferred. The material other than the particles may be present in an amount of 0 to 150~o (based on the volume of rubbery polymer), generally 10 to 100% and preferably 15 to 100~. Preferably the composition includes 0.05 to 2 parts, most preferably 0.1 to 1 part, by volume particles per part by volume other inorganic particulate filler.
Although the total volume of filler, including the particles, 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 satisfactory sealing properties. For instance the total amount of filler, including the particles, is usually at least 20~ ~by volume based on the volume of rubbery polymer) and can be up to 175~, for ins.ance 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 .. ..

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fillers that may be used in the invention generally have a particle size of from 1 to 50 microns. The -filler should be subs~antially non-abrasive, so that it does not cause wear to the machinery by which the composition is mixed and li~ed onto the can or other end.
The preferred other filler is kaolin or china clay or zinc oxide but other fi]lers inc]ude co]loidal silica and other silicic fil]ers, synthetic silicate, calcium carbonate or sulph~te, a3uminium hydroxide, talc, dolomite, barium s~ phate, or magnesium oxide or carbonate or silicate. Such fillers may have been surface treated, for instance in conventional manner. ..
Instead of modi~ying the colour of the composition by including particulate pigment some other 15 colouring material, for instance a soluble dye, may be included.
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 20 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 (ML104 C) 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.
30 Suitable synthetic rubbery polymers include butyl rubber, polychloroprene, butadiene acrylonitrile copolymers, ethylene propylene copolymers, ethylene-propy3ene-diene terpolymers, styrene isoprene block copolymers, poly-butadiene, styrene acrylic copolymers, polyvinyl;dene 35 chloride, polyvinylidene chloride copolymers, plasticised polyvinyl chloride, polyvinyl chloride copolymers, plasticised polyvinyl propionate or acetate~ polyvinyl propionate or acetate copolymers, polyacrylic acid copolymers, polymethylacrylic acid copolymers, acrylic ~ t~3 ester copoly~pel-s, methacr~rlic ester copol~ers, plasticised pol~styrene, vinyl acetate copol~mers ;ith for instance ethylene, styr~ne butadiene block copol~-mers, styr~ne butadiene rubbers solùtion polymer-ised or emulsion polymerised, and carboxylated st~renebutadiene 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 15 to 60~ preferably 18 to 45~ by weight.
They may 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 ~he compositions used in the invention. However ~0 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 hydro-carbon oil, that softens the polymer may be used in amountsof for instance 1 to 60%, preferably 5 to 40~
Best results are generally obtained when tackifier resin is included. Suitab~e 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 otller rosin type resins such as rosin, hydrogenated rosin, or hardened rosin. The amount of tac~ifier 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~.

:L'"~ 3 8 l The compositions may include minor amounts, e.g. up to 1~ or at most up to 5-0 of other additives that are ~nown to those skilled in the art and that are conventional in filled sealing compositions, such as viscosity increasing agents (for instance aJ~onium alginate, bentonite or gum karaya or high Molecular ~eight polyacrylic acid), bactericides, corrosion ; inhibitors, surfactants, anti-oxidants (for instance phenolic or amino anti-oxidants) and p~-l adjusters (for instance ammonia~ primary amine, sodium hydroxide or sodium carbonate~.
The composition preferably is liquid at room temperature and thus preferably includes a liqùid.
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 consistent 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 anhydride 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.
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The aque~us composition is pre~er~bly a late~ obtained by dispersing the specified fil]er and tac~ifier (if present) into a latex of the chosen rubbery polymer, for instance as formed by emulsion polymerisation, ~he composition may be made simply ; by mixing into the chosen latex ~optionally after dilution) the tackifying resin, the se]ected fil]er or fillers, and any other additives, all in convc-ntional 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 iller or fillers, and optionally also tac~ifier, and add this stable dispersion to the .
latex. The total solids content of the composition is generally from 20 to 85~ by weight, prëferably 30 to 80~.
Instead of formulating the aqueous composition as a latex it may be a dispersion obtained by dispersing solid rubbery pol~mer and filler into the aqueous medium.
The aqueous dispersion may be made by mill ing the solid rubber with filler and other optional additives, including also generally any tackifying resin, using an internal mixer, for instance a ~anbury mixer, so as to form a rubbeT 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 35 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 , ~;hich the liquid medium comprises organic solvent in ~hich some or all o~ the rubbery polymer will dissol~e, any remaining polymer going into dispersion.
Suitable organic liquids ~Ihich 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 elastomers.
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 j thus obtained 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 ~iller, 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 find that the inclusion oE the metal particles does, as a ~enerality, result in improved sealing properties compared to the same composition in wh;ch an equivalent volume of other filler (such as kaolin) is used in place of the metal particles.
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 red~1ced. In some instances there ~'7~'7~

may be no improvement in the results of some sealing tests but improvements in other tests wi]l sho\~ that, despite this, there is a useful practical impro~ement.
Metal partieles also have the advantage that they can have inert surface characteristics and so can avoid some of the handling and other difficulties that may be encountered 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 necessary to take particular precautions to prevent coagulation of any latex or dispersion that , is present.
The most consistently satisfactory sealing properties and the greatest tendency for a significant improvement in sealing propertie~ is obtainable in the invention when the metal particles are included in aqueous compositions, and especially in latex compositions, and so these are preferred.
Some non-limiting examples of the in-veJl-tion are now given.
In these sealing properties are identified by two sets of quantitative values which are referred to - 25 as "biological 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 composition 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 97C, leaving a small headspace. The test closures are double seamed onto these filled cans whilst simultaneously injecting steam into the headspace. The closed cans 7~

are th~n sterilised, ~ypically at ~21C for 30 mi~utes, and after sterili~at;on are immediatel~ cooled in .ater containing ~as-producing, non-pathogenic micro-organisms capable of growth in the aforementioned nutrient. After cooling and whilst still wet with the cooling ~:ater, 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 thol-~and in this biological seal test is of course ~ery much greater than that which would occur with commercially - ¦
packed cans sealed with these compositions. -"Ster isation 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 ara filled with water at typically 70C
to leave no headspace and test closures are double scamed onto these filled cans. The closed cans are then sterilised typically at 130C 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 !
.

~9 3 occurring, small protrusions The average numl~er 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.
Ho~ever, because of the extreme conditions of the test, greater values than this are commercially tolerable.
Since the extrusion and biological seal resu]ts will vary according to, for instance, variable conditions under which the tests are carried out compari-sons should, in general, be made only between resultswithin a single example. It is desirable that the "biological seal" and "sterilisation extrusion" values should be as low as possible.
In the following examples each composition is made by mixing together a latex of a rubbery polymer and containing minor amounts of conventional additives known to those skilled in the art, stabiliser, filler, titanium dioxide pigment, and tackifier resin. The composition is lined onto the can closure, dried, and tested in the described manner.
The amount of tackifier resin is 22%, the amount of stabiliser is 4.2% and the amount of titanium dioxide is 3.2%, all based on the volume of rubbery polymer in the latex. The amount of filler is given in the examples by volume based on the rubbery polymer.
When the filler is kaolin the total solids content of ~~~~
the composition is about 60% by volume.
The filler consists of metal particles (if present) and the stated inorganic particulate material (if present) which generally has a particle size of 1 to 50 microns although titanium dioxide may have a particle size of down to 0.1 microns.
The latex is a styrene butadiene latex hav;ng a solids content of 66 to 69% by weight and containing 31 to 36% bound styrene and which has been polymerised cold (at 5C) using fatty acid soaps. The polymer in the latex has a Mooney value (as defined above~ of 100 to 130. I~owever, similar results may be obtained using , .

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other styrene butadiene latices that may have' been polymerised hot or cold such as those listed in the following table:
Type Total Solids Bound Mooney Emulsifier ~ ~~~ ~ Styrene ~D Value Cold 63 29 140 Fatty acid Cold 67 34 75 Fatty acid Cold 68 30 150 Fatty acid Hot 45 46 90 Rosin ester }lot 42 50 30 Rosin ester Hot 59 46 75 Rosin ester Hot 50 46 70 Rosin ester The tackifier resin is a polymer of mixed 5-carbon alkenes having a melti~g point of about 100C.' The stabiliser is a styrene-maleic anhydride copolymer.
Similar results are obtainable with other tackifiers and stabilisers.
Exam~le 1 Test Filler ¦Biological Sterilisation l~al- __ Extrusion .
lA 30 Kaolin 250 26.4 lB 30 Spherical Tin Beads240 22.8 (4 to 60 microns~
lC 30 Nickel Alloy spheres15 8.2 (50 microns mean diameter~
lD 30 Nidel spheres having120 0.0 smooth surface (2 to 40 microns) lE 30 Nickel spheres having70 0.0 rough surface (3 to 30 microns~
lF 30 Chromium irregular shaped 95 17.7 beads ~50 to 75 microns) lG 30 Chromium irregular beads 35 0.0 (2 to 40 microns) $ ~

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Example 2 -~ __ ._ __ _ Test Filler B~ogical Sterilisa-Seal tion Extrusion _ ..
2A 30 Kaolin 825 16.0 2B 30 Stainless Steel 285 0.1 spheres ~0 to 60 microns) .__ _ _ , ~.
.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 crush resistant metal particles having a particle size less than 200 microns.

2. A composition according to claim 1 in which the metal particles have a maximum particle size of 75 microns and are of nickel, chromium or steel.

3. A composition according to claim 1 in which the metal particles have an average particle size of 10 to 50 microns.

4. A composition according to claim 1 in which the amount of the particles is from 5 to 100% by volume based on the volume of rubbery polymer.

5. A composition according to claim 3 in which the amount of the particles is from 10 to 50% by volume based on the volume of rubbery polymer.

6. A composition according to claim 1 in which the total amount of filler is from 20 to 175% by volume based on the volume of rubbery polymer.

7. A composition according to claim 1 in which the rubbery polymer is a styrene butadiene copolymer.

8. A composition according to claim 1 additionally including a tackifier resin.

9. A composition according to claim 8 in which the amount of tackifier resin is from 10 to 250% by volume based on the volume of rubbery polymer.

10. A composition according to claim 1 which is a liquid and includes a liquid medium in which the rubbery polymer is dissolved or dispersed.

11. A composition according to claim 10 in which the liquid medium is an aqueous medium in which the rubbery polymer is dispersed.

12. A composition according to claim 11 and which comprises a latex of the rubbery polymer and wherein the filler has been dispersed in the latex.

13. A method of sealing a container closure to a container comprising lining the sealing face of the closure with a composition according to claim 1, 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 container with the gasket within the seal.

14. A method according to claim 13 in which the container is a can and the closure is a top or bottom can end.

15. 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 crush resistant metal particles having a particle size less than 200 microns.