GB2191728A - Moulding container closure sealing gaskets - Google Patents

Abstract

A method is described of forming microcellular gaskets from vinyl chloride plastisol in container closures made from high melting olefin polymers especially polypropylene. The liquid plastisol sealing compound within the container closure is heated both by conventional thermal means and also by microwave energy. Thermal heating is used to attain a temperature between 100 DEG and 130 DEG C in the container closure, whilst allowing the microwave energy to heat the plastisol to a temperature sufficient both to decompose the blowing agent within the plastisol and to fuse or flux the gasket, generally between approximately 170 DEG and 195 DEG C. The microwave heating of the plastisol is carefully controlled so that the transfer of heat from the gasket to the polyolefin closure does not cause the closure to soften or distort.

Description

SPECIFICATION Container closure sealing gaskets This invention relates to container closure sealing gaskets, and specifically to a method offorming microcellular sealing gaskets in container closures made of thermoplastic polyolefin polymers.

Container closures, for example closures for jars and bottles, are generally provided with a sealing gasket in orderto seal the contents of the container from the atmosphere. The sealing gaskets are commonlu made by introducing into the closure a liquid, or semi-liquid, material and distributing it within the closure to achieve the desired shape of gasket, and then causing the so shaped material to solidify to form the gasket. A particularly convenient and satisfactory gasket4orming material of this kind is a dispersion of vinyl polymer in plasticiser, known as a plastisol.The plastisol in its semi-liquid state is most conveniently shaped by spinning the cap, to produce a gasket thickest at the periphery of the cap, and subsequently heating it so that the vinyl polymer absorbs the plasticiserto form what is generally regarded as a solid solution, which then produces a solid gasket on cooling.

With the use of a metal closure the plastisol is normally fluxed or fused by heating in a conventional hot air oven, commonly for 1 -2 mins with an airtemperature of 1 900-2500C. This is necessary in order to achieve the critical fluxing or fusion tempsrature of the plastisol gasket, generally a minimum of about 180 C.

It is obvious that using a high melting olefin polymer such as polypropylene having a typical melting point of approx 165 obtaining the minimum temperature of 180 forthefusion of the plastisol gasket is not possible using a conventional hot air oven. A method for fluxing such plastisol gaskets in polyolefin closures was described in B.P. 2,051,660 B, and consists of heating the closure containing the liquid plastisol in a hot air oven to a temperature from 6 to 35 C or preferably 541 6C below the melting point of the polyolefin cap or closure (approx 165 C) and then while maintaining the ambienttemperaturefurther heating the plastisol by means of microwave energy until the fusion or fluxing temperature is reached.The closure is then cooled or allowed to reach normal room temperature when it will contain the gasket of solidified plastisol.

The hardness of such a gasket can be varied to accommodate the various finishes of the glass or plastic containers which the caps are intended to close, generally by means of varying the filler and/or the plasticiser contentofthe liquid plastisol. But a favoured method of producing a softer gasket is to make a "puffed" or microcellulargasket.

A microcellular gasket consists of a fused or fluxed plastisol containing various amounts of largely spherical non connecting or discrete voids filled with gas.

The effect of the presence of such voids in a gasket is a) To reduce the hardness ofthe gasket, which under certain conditions such as the sealing of carbonated beverages, enhances the sealing efficiency.

b) Produces a gasket of larger volume for a given weight of plastisol i.e. reduces the final specific gravity of the gasket.

c) May enable gaskets of acceptable sealing efficiency to be produced at a lower gasket weight thus reduc ing the cost ofthe seal.

Such gaskets are commonly termed "puffed" or "blown", and are produced by the incorporation of a "blowing agent" within the liquid plastisol. A commonly used and favoured blowing agent is azodicarbonamide (oraxobisformamide) sold undertrade names such as GENITRON AC from FBC Industrial Chemicals. This is commonly referred to as ADC.

The chemical formula may be expressed thus:

When heated to a pproxi m ately 200C it decomposes into principally gaseous products, nitrogen 65%, carbon monoxide 32% and other gases, including a trace of ammonia, 3%. The actual mechanism ofthe breakdown of ADC into the various gases is not completely understood.

The decomposition temperature may be reduced by the use of various accelerators or "kickers" in the plastisol formulation. Zinc compounds are commonly used for this purpose, zinc oxide or zinc soaps being particularly useful.

The decomposition of ADC in a plastisol is affected by a) Thetemperatureattained in the plastisol.

b) The period of heating i.e., time taken to reach the fusion or fluxing temperature of the plastisol.

c) The nature of and amount of "kicker" used.

d) The particle size ofthe ADC.

The ideal combination of these factors is usually obtained by empirical means.

It is thought that the use of zinc oxide as the "kicker" for ADC may reduce the decomposition temperature by as much as 20into approx 180 C.

Another commonly used blowing agent is oxybisbenzenesulphonhydrazide, available underthetrade name FICEL OB from FBC Industrial Chemicals. This has a decomposition temperature of approximately 150.1600 which is generally too Iowfor use in plastisols having a fusion orfluxing temperature in the region of 1800c. However, it is sometimes employed in a small proportion in the order of 10% with ADC as a blowing initiator or nucleator.

It is evident then that liquid plastisol gaskets must be heated to a minimum temperature of 180 Capp- roximately in orderto effectthe decomposition of the ADC which releases the gases to form the voids necessary in a "puffed" or "blown" gasket, and at the same time fuse orfluxthe PVC plastisol. Failure to com pletelyfluxthe plastisol may lead to "cutthrough", a term supplied to a gasket insufficiently hard to resistthe pressure applied onto the container, leading to possible leakage ofthe contents or of pressure loss in the case of carbonated beverages. Another consequence of an insufficiently fluxed gasket is the leaching out of plasticiser into the pack which is obviously undesirable.

The art or practice of producing such "puffed" gaskets lies in the carefu control ofthe rate and duration of heating and final temperature obtained during the fluxing of the gasket and the concomitant decomposition ofthe blowing agent. If the blowing agent decomposes before the plastisol is sufficiently solid to contain the gases, the gases will tend to come through the surface of the gasket and escape into the atmosphere and to form large voids or blisters. If on the other hand the plastisol is fluxed completely at a temperature belowthat atwhich the blowing agent decomposes (and is not further heated to the decomposition temperature ofthe blowing agent) insufficient gas will be produced to give the desired quantity of voids.

There is also the dangerthatifthe plastisol fluxes at a lowertemperature than the decompositiontemperature of the blowing agent used and is then further heated to achieve this temperature then the vinyl polymer may degrade.

Because of these constraints in the production of "puffed" gaskets, and the softening pointofthefavoured polypropylene materialfortheclosure being in the order of 165"C, the production of puffed gaskets using ADC as the blowing agent was not envisaged under BP 2,051,660 B.

Now it has been found, surprisingly, that by careful control ofthe ambienttemperature in both the pre- heating zone and the microwave zone ofthe oven, so that the polypropylene closure is not heated above 1200.1 300Cthetemperature ofthe plastisol may be allowed to rise under the influence of the microwave field sufficiently to cause the decomposition of the blowing agent ADC and to produce puffed gaskets having controllable void contents. It must be emphasiedthattheduration of heating the plastisol in the microwave field and the thermal heating ofthe polymeric closure must be carefully controlled so that the heat loss from the gasket at the interface between the gasket and closure is not sufficient to cause distortion or softening of the closure.It is of course still necessary to heat the gasket and closure by conventional thermal heating upto a minimum of approximately 1 OO"C. If no thermal heating is used or the temperature attained by the gasket and closure by thermal means is under approximately 1 000C during the heating ofthe gasket by microwave energy then the loss of heat from the interfaces of the gasket between the gasket and closure, and the gasket and air, will cause eitherthe gasket to be incompletely fluxed at its surfaces or if complete fluxing is achieved atthe surfaces degradation of the vinyl polymer in the interior of the gasket.

Thus by means ofthis invention it is now possible to produce puffed or microcellular PVC gaskets in a polyolefin closure, the preferred polyolefin being polypropylene.

The procedure will normally involve 1) Pre-heating the closure containing the plastisol by normal means (i.e., by other than microwave heating) to a temperature between 110 - 130 C, and then while maintaining the ambient or atmospherictemperature at 110- 1300C heating the plastisol gasket to a temperature of approximately 175-1 950C by means of microwave energy in order to completelyflux the plastisol and decompose the blowing agent or 2) Heating the closure containing the gasket in a microwave oven in which the atmospheric temperature is maintained at 110 -130 Cuntil the PVC gasket is completelyfluxed and the blowing agentdecomposed.

The closure containing the fluxed microcellular plastisol gasket can be cooled in any convenient mannerto a suitable temperature for subsequent handling without damage to the closure.

The invention is of particular applicability to closures made of polypropylene which typically has a softening point of about 165 C. Atthe present time polypropylene is the preferred material for the closures involved in the application of this invention, butthe invention is equally applicable to closures made of other polymers substantially transparentto microwave radiation.

The composition of the plastisol used in the invention will normally consist of a vinyl chloride polymer ora vinyl chloride co-polymer with vinyl acetate. This vinyl chloride polymer (either homopolymer orcopolymer) is commonly either wholly or in part is "paste grade" form ("paste grade" is a term of the art used to denote a resin of fine particle size made by emulsion polymerisation). With co-polymers of vinyl acetate, the proportion of vinyl acetate is preferably not greaterthan 5%, because of associated problems of poorviscosity stability in the resulting plastisol. The paste grade of homopolymer of co-polymer may be partially substituted by a suspension polymer or "filler" resin. This suspension polymer is of larger particle size than emulsion or paste grade polymers and does not absorb plasticiserso readily. Because of this, filler resins can tolerate highervinyl acetate contents, for example up to 14% by weight.

The plastisols may contain plasticisers in any conventional amount, typically from 60 to 85 weight parts per one hundred parts of vinyl resin. Commonly available plasticisers may be used, such as di-octyl phthalate and di-isoctyl phthalate, either alone or in conjunction with such plasticisers as butyl benzyl phthalate, acetyl tributyl citrate, ethyl diphenyl phosphate, or di-isodecyl phthalate.

The plastisolswill normally also contain other conventional ingredients, for example a filler, pigment, a heat stabilizer (to assist in stabilizing the vinyl polymer against thermal decomposition), a blowing agent, a kickerforthe blowing agent, and a "slip agent" (i.e., an additive to lower the removal torque of the closure). It should be explained that threaded closures are sometimes difficult to unscrew because of the friction between the gasket in the closure and the neck of the container. It is therefore desirable to include a removaltorque reducing agent to reduce this friction. Many such agents are well known, such as parafin wax and microcrystalline wax.

The filler content may vary from, for example, 10 to 200 parts by weight per 100 parts of vinyl polymer according to the specific gravity and oil absorption of the filler. Normally this will not exceed 50 parts by weight.

The placement of the plastisol sealant into the closure, a procedure known in the art or practice of producing gasket in closure as "lining" the closure, is normally and conveniently carried out by injecting the liquid orsemi-liquid plastisol into the closure while the closure is rotating on its longitudinal axis in a chuck or similar holding device in a "lining" machine. Such lining machines are made and supplied for example by W.R. Grace Ltd in London.

The rotation or spinning of the closure causes the plastisol to flow outwards by centrifugal force to produce a gasket of a "dished" section. Another method of obtaining a dished shape of gasket is by moulding the plastisol within the closure. A much favoured device applicable to this invention is to use a bottle cap having an inner skirt portion in its center so that the plastisol may be injected into the channel formed by the inner skirt and the outer wall of the cap. However there are no restrictions on the shape or construction ofthe closure and gasket. The invention is available for use on a wide variety of closures for containers. The invention is of particular use in providing gaskets in threaded caps and closures in which the threads have been preformed in moulding the cap, and also for snap-on caps. Pilfer-proof devices may be incorporated in either type of cap.It is envisaged also that caps or closures having lugs on the inside ofthe cap skirt or wall which engage with multi-start coarse threads on the container may be used. These are commonly known as lug caps. Caps will normally have an internal diameter between 25-32 mm for bottles and for wider mouth bottles or jars between 50 and 100 mm.

The order of operations before the microwave heating may be varied. The closure may be pre-heated in an ordinary thermal oven before the injection of plastisol into the heated closure, the temperature of the plast isoi is normally between 20' and 50'. The closure is then spun or rotated in orderto distribute the plastisol into the required shape and passed without delay, to minimise heat loss, into the microwave oven.

Another and more usual procedure is to inject the plastisol at a temperature between 20' and 50"C into the closure and obtain the desired gasket shape by spinning, the closure is then passed into a pre-heating oven and heated to between 100 and 130 C before passing into a microwave oven having an airtemperature between 100 and 1 30"C. Although the temperature of the plastisol when injected into the closure is usually between 20 - 50"C it is possible that temperatures up to 700 may be used.

However, a pre-heating oven is not necessary in this process, the thermal heating to 100-130 and the microwave heating may be carried out simultaneously. Alternatively, especially for experimental purposes, the closure containing the unfluxed plastisol may be preheated in a closed vessel to 100-130 C and then rapidly transferred to a microwave oven with a similar atmospherictemperature.

The microwave frequency used is dictated by government regulations, and must be either 91 and 2450 megaHerz, in the U.K although theoretically any frequency in the range 300-300,000 megaHerz might be employed. The microwave field in the microwave oven is scattered by any convenient mode stirer.

Avariety of plastisol formulations may be used in the present invention. Examples oftheircompositions are given in the following Table, wherein the figures are parts by weight. These are for use in polypropylene closures.

Table 1 MATERIAL FORMULA NUMBER 1 2 3 4 5 6 7 8 9 10 Paste grade PVC resin 60 60 60 60 60 60 60 Suspension grade PVC resin of higher particle size (filler resin) 40 40 40 40 40 40 40 Filler resin, co-polymer of 5% by weight vinyl accetate,95% by weight vinyl chloride 40 Paste grade co-polymer of 5% by weightcyclohexyl maleiimide and 95% byweight of vinyl chloride 40 Di-isooctyl phthalate 65 66 65 9 9 80 10 35 65 65 Di-iso decyl phthalate 71 71 Acetyl tri-butyl citrate 60 35 Filler eg, talc, clay, barium sulphate 20.0 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 18.0 Titanium dioxide 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Heat stabilizer eg, "Lankro 152", a calcium-zinc epoxydised fatty acid ester 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 MicrocrystallineWax 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 ADC 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Zinc oxide 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 The microcrystalline or parafin wax is normally added to these compositions as a 20% solution in plasticiser. ADC is normally added in the form of a 50% dispersion in plasticiser, the dispersion being effected by passing a 1:1 mixthrough atriple roll mill, a horizontal bead mill orsimilarequipment.

All the exemplified plastisol formulations may be further modified by adding up to 2 parts by weight of white mineral oil to improve their viscosity stability and up to approximately 2 parts ofweight of aviscosity depressant. A commonly used viscosity depressant is a lauryl aicohol-ethylene oxide adduct.

The following Examples illustrate a variety of plastisols which can be used in the present invention. They are for polypropylene caps.

Example 1 Using a plastisol according to Formulation No.1 in Table 1 and where the filler used was barium sulphate, the following results were obtained using a microwave oven at 900 watts power.

Initial plastisol specific gravity 1.60 Gasket weight 0.5 gram No Temperature Time in Final specific Void volume microwave Gravity oven 1 130"C 4mins 0.95 42% 2 120 C 4mins 1.08 33% 3 125 C 3 mins 1.26 21% In all cases the distribution ofthe voids and the diameters of the voids within the gasket was satisfactory.

That is to say, the voids were predominately spherical, discrete, and evenly distributed throughoutthe gasket, the size of the voids was small and uniform, without blisters ie. a satisfactory sealing gasket.

It is clear from the results shown in Table 2 that by suitably adjusting the airtemperature within the pre heater and the microwave oven and the pass time in the microwave oven control ofthe void volume orthe characteristics of the microcellular aspect ofthe gasket can be controlled.

Example2 Similar satisfactory microcellular gaskets were obtained using Formulation No. 4from Table 1. The results are summarised in thefollowing Table.

Initial plastisol specific gravity 1.37 Gasket weight 0.45 grams.

No Temp Time in microwave Final specific Void oven Gravity Volume 1 1 130'C 4mins 0.90 34.5% 2 130 C 3mins 1.05 23.5% 3 120 C 4mins 1.02 25.5% 4 120 C 3mins 1.15 16.0% 5 110 C 5 mins 1.02 25.5% 6 6 100 C 5 mins 1.10 20.0%

Claims (9)

1. CLAIMS 1) A method offorming a microcellular sealing gasket in a container closure made of a high melting polyolefin resin which is substantiallytransparentto microwave radiation, which method comprises:- introducing into the closure a vinyl chloride resin plastisol, and forming in therein into the configuration of a gasket; heating the thus formed plastisol in the closure by a combination ofthermal and microwave energy in orderto raise the closure to a temperature just below its softening point, and to flux the plastisol; and then cooling the plastisol to form the gasket; characterised in that the plastisol also contains a blowing agent, and the temperature ofthe closure is carefully controlled to be from 35to 65 C below the closure's softening point, so that the temperature ofthe gasket may be allowed to rise sufficiently to decompose the blowing agent in the plastisol while the plastisol fluxes, and yet so thatthe loss of heat from the gasket into the closure is not sufficient to bring the temperature of the closure above its softening point.
2. 2) A method as claimed in Claim 1, in which the high melting polyolefin resin is polypropylene, having a typical softening point of 1 65 C.
3. 3) A method as claimed in any of the preceding Claims, in which the vinyl chloride resin plastisol is a 69:40 by weight mixture of emulsion and suspension homopolymers of vinyl chloride.
4. 4) A method as claimed in any of the preceding Claims in which the plastisol is formed into a gasket by spinning.
5. 5) A method as claimed in any ofthe preceding Claims, in which the blowing agent is axodicarbamide (ADC).
6. 6) A method as claimed in Claim Sin which the ADC is used with a zinc-based kicker.
7. 7) A method as claimed in any of the preceding Claims, in which the closure temperature is maintained at from 40 to 50 C below the closure's softening point.
8. 8) A microcellular-sealing-gasket4orming method as claimed in any of the preceding claims and substantially as described hereinbefore.
9. 9) Acontainerclosure made of a high melting point polyolefin resin and having therein a microcellular sealing gasket formed by a method as claimed in any of the preceding Claims.