GB1592222A - Method of forming plastisol gaskets in container closures fabricated from polyolefin resins - Google Patents

Description

(54) METHOD OF FORMING PLASTISOL GASKETS IN CONTAINER CLOSURES FABRICATED FROM POLYOLEFIN RESINS (71) We, THE CONTINENTAL GROUP, INC., a Corporation organized and existing under the laws of the State of New York, United States of Ame'rica, of 633 Third Avenue New York 10017, State of New York, United States of America, do hereby declarethe invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described. in and by the following statement:- The present invention relates to gasket lined plastics closures for containers. More particularly the invention relates to a method for forming vinyl chloride plastisol gaskets in closures formed from polyolefin resins.

With the advent of commercially available plastics, i.e. easily formable thermoplastic synthetic resins, it has become common practice to form various products therefrom due to inexpensive material and production costs. Some of such products include container closures. Heretofore. container closures molded from polyolefin resins - such as polyethyléne and polypropylene, have incorporated a positive locking meansíthereon, such as screw thread or a snap lock, or likewise, co-ordinated with an associated container solely through a friction fit. Although these plastics closures have found wide application in the packaging field, they have been found deficient in sealing integrity for containers useå for the packaging of liquids and fine powders. Such applications require the insertiori of a sealing gasket or a prefabricated composite liner to provide a leak-proof seal between the closure and the container to which it is fitted. Gaskets have not been generally used in plastics closures because the thermoplastic resin such as polyolefin resin from which the closure is moulded is generally not compatible with conventional methods for forming a gasket, as such gasket forming methods employ temperatures for forming the gasket at which the olefin plastics closure undergoes softening, stress relaxation and warpage. For example, in the widely used "spin-lining method" for forming gaskets in closures, a vinyl chloride pólyrner based plastisol composition in an uncured, paste-like condition-is squirted from one or more nozzles into metal closure shells which are inverted On a chuck rotating at high speed. Due to centrifugal force, the plastisol compound assumes the desired configuration and shape. After being thus deposited or "flowed-in", the plastisol compound is fused (fluxed) by baking the closure shell in an oven at temperatures in the order of 160-200 C from 1/2 to 5 minutes, or the "flowed-in" compound may be molded and fused in the closure shell with hot molding punches and platens in a turret arrangement; The fact that polyolefin resins such as polyethylene and polypropylene have softening points of 140"C and 1600C respectively makes them poor candidate materials for closures to be spin-lined with plastisol compounds.

Other methods considered for the delivery of elastomeric gasket material to polyolefin closures include hot melt application equipment. Unfortunately, hot melt equipment for high speed applications of lining materials of uniform thickness, e.g., 0.015"-0.040", is not commercially available. The only equipment presently available to deliver plastisol gasket compounds at the tolerances required for liquid and powder tight gasket seals is the aformentioned spin-lining equipment.

There is therefore a need in the plastics closure art for a method whereby plastisol compounds can be delivered at high speeds and uniform thickness to polyolefin closures, e.g., using spin-lining equipment, and then fluxed within the closure without damage to the dimensional and physical properties of the closure.

It has unexpectedly been determined that if plasticizers having a certain range of dielectric constant and loss factor are incorporated in a vinyl chloride polymer based plastisol gasket composition and the plastisol is fluxed in a polyolefin closure by exposure to a field of alternating electrical energy, the polyolefin closure is not deformed and a gasket material of excellent quality is formed in the closure. This result is especially surprising since the art, namely, Britsh Patent, 1,196,126 teaches that plastisol gasket materials may be fluxed in metal closure shells by incorporating ferromagnetic or electrically conductive particles such as iron or aluminium in the plastisol and then heating with a source of a rapidly alternating magnetic field. The incorporation of such particles (up to 50 parts per 100 resin) in the plastisol causes the resultant cured gasket material to be nonresilient, stiff, and rigid rendering it of little value for sealing purposes.

In accordance with the present invention there is provided a method for forming a gasket in a closure for a container, the closure being fabricated from a polyolefin resin, the method comprising introducing a plastisol comound into the closure, forming the plastisol into a gasket of the desired shape, the closure being formed of an olefin polymer exhibiting substantially no response to heat activation by radio frequency electrical energy and the plastisol being comprised of a vinyl chloride polymer or co-polymer and a plasticizer having a dielectric constant between approximately 6 and approximately 8 and a loss factor between approximately 2 and approximately 25, fluxing the plastisol in the closure by dielectrically heating the plastisol with exposure to a source of radio frequency electrical energy having a frequency in the range of 1 to 200 megahertz and then allowing the fluxed plastisol to cool and form the gasket.

By the practice of the present invention polyolefin closures can be lined with plastisol gasket materials to uniform thickness using conventional spin-lining equipment and fused without encountering heat distortion and warpage. Energy costs are materially reduced since only the plastisol material is heat activated and the time required for fusing the plastisol compound is substantially reduced, e.g., to about 30-60 seconds.

The use of electromagnetic energy at radio frequencies is known to the art for heating many materials, including some which conduct electric currents very poorly or not at all.

The latter are of a class of materials called dielectrics; the heating process is termed dielectric heating. For dielectric heating, two ranges of radio-frequencies are used, namely frequencies in the range of 1-200 megahertz, referred to in the art as high frequency or radio-frequency heating, and frequencies above 890 megahertz, referred in the art as microwave heating. The practice of the present invention uses radio-frequency dielectric heating sources to effect fusion of the plastisol compound.

In dielectric heating, the material to be heated is placed between two metal plates or electrodes. A generator applies to the plates a high-frequency current of 1 to 200 megahertz that sets up an electric field in and around the material. The material absorbs energy at a rate given by the equation: P = 0.555 f E2E tanb x 10-6 where P = heat generated in watts/cc (dielectric loss), f = frequency in megahertz, E = field strength in V/cm, E = dielectric constant and tanb = loss tangent. For most materials, the dielectric constant and the loss tangent are fairly constant over the dielectric heating frequency range at a fixed temperature. Therefore an optimum frequency is not required and the desired heating rate is obtained by selecting a frequency range and voltage for which it is practicable to build equipment and for which a suitable electrode system can be designed. For the dielectric heating of vinyl chloride polymer plastisols containing plasticizers having dielectric constants in the range of about 6 to about 8 following the practice of the present invention, a frequency of between 10 to 50 megahertz is generally employed, 15 to 35 megahertz being preferred. Any source having sufficient power output, e.g. 1 to 15 kilowatt (kw) may be used, outputs in the range of 2 to 5 kw being preferred.

With such power outputs, fluxing of plastisol compounds prepared in accordance with the practice of the present invention can be accomplished in time periods ranging from 30 seconds to one minute.

The ease with which any material may be dielectrically heated is determined by its dielectric constant and its loss tangent. The product, E x tan5 is referred to in the art as the loss factor and such factor is a convenient index to the relative ease of heating of a material.

Polyethylene which has a dielectric constant of 2.35 and a loss factor of 0.0005 and polyproplene which has a dielectric constant of 2.25 and a loss factor of 0.00035 show little or no response to dielectric heating. It has been determined that plasticizers having loss factors in the range of approximately 2 to approximately 25 when incorporated in vinyl chloride polymers in accordance with the practice of present invention enable the resultant plastisol compound to be fused in less than a minute in polyolefin closures without causing heat distortion of the closure. Polyvinyl chloride which has a dielectric constant of 3.5 and a loss factor of 0.023 also exhibits some but not substantial response to dielectric heating.

The term "polyolefin" as used herein includes within its meaning ethylene polymers and copolymers as well as propylene polymers and copolymers such as medium and high density polyethylene, polypropylene, ethylene-propylene copolymers, ethylene-butene cbpolym- ers, ethylene-hexene copolymers, and polybutylene.

The polyolefin resin should desirably exhibit low moisture gain when exposed to the atmosphere. If the pololefin resin absorbs appreciable moisture, e.g., greater thán 0.1% over a 24 hour period when tested according to the procedure of ASTM D570, then the resin may become responsive to dielectric heating so that its value as a closure inaterial is reduced. Generally, polyolefins such as polyethylene and polypropylene exhibit a moisture gain of about 0.01-0.03% when tested according to ASTM D570.

The polyolefin resin used for closure fabrication may contain electrically non-conductive fillers such as talc, mica, clay and TiO2 to improve the opacity and physical properties of the closure inaterial.

The plastisol composition will usually be a semi-liquid paste composition containing a normally liquid plasticizer and a vinyl chloride polymer resin which is paste forming with the plasticizer at a temperature below the fluxing temperature of the resin-plasticizer components. When the mixture is heated, the plastisol compound, which is originally opaque and paste-like, undergoes a series of physical changes and with increasing temperatures increases in tensile strength and gradually loses its opacity. The point at which the plastisol forms a brittle friable films referred to in the art as the "gel point". The point at which opacity is lost is referred to in the art as the temperature of clear point fusion". At temperatures of 1600 to 2000C, the plastisol reaches its maximum tensile- strength, elongation and clarity. The plastisol compound should desirably have as low a gel point and clear point fusion temperatures as possible. Mixtures of plasticizers having dielelectric constants of approximately 6 to approximately 8 and loss factors of approximately 2 to approximately 25 and vinyl chloride polymers having a relative viscosity range between 1.80 and 2.60 when measured in accordance with ASTM D-1243-60 Method A or a number average molecular weight of 45,000 to 75,000 have been found to have gel points in the range of 75"-85"C and clear fusion points in the range of 95"C to 1500C and are preferred.

The.vinyl chloride polymer resins which may be used for the plastisol compound include homopolymers, i.e., polyvinyl chloride as well as copolymers with a minor amount of copolymerizable ethylenically unsaturated monomer. Generally; the copolymerizable monomer is used in an amount of 20% or less, and preferably 10% or less, e:g. 5%. As illustrative copolymerizable materials, there can be used vinyl acetate, vinylidene chloride, acrylonitrile, trichloroethylene, maleic anhydride and diethyl maleate. Polyvinyl chloride is the vinyl chloride polymer resin which is preferred.

Plasticizer compositions having dielectric constants in the range of approximately 6 to approximately 8 and loss factors in the range of approximately 2 to approximately 25 are known to the art. For-example,in an article entitled "Dielectric Constants Of Plasticizers As Predictors of Compatibility with Polyvinyl Chloride", which appeared ifl Polymer Engineering and Science, October 1967 pages 295-300, there are listed over 100 plasticizers for polyvinyl chloride, their dielectric constants- and loss factors. Plasticizers which are especially suitable are listed below: Dielectric Loss Constant Factor Plasticizer (@1Kc) (@1Kc) Ethyl hexyl diphenyl phoshate 7.52 25.20 Butyl phthalyl butyl glycolate 6.86 11.10 Butyl benzyl phthalate 6.45 8.45 Acetyl tributyl citrate 6.05 1.95 Dipropylene glycol dibenzoate 7.52 12.10 Diethylene glycol dibenzoate 7.16 12.40 Tricresyl phosphate 7.25 7.03 Dibutyl phthalate 6.45 5.43 It has been found that for each 100 parts of vinyl chloride polymer resin, the plastisol compound should desirably include 40 to 100 parts of the plasticizer, 50 to 80 parts being preferred.

Other materials such as pigments, lubricants and stabilizers may be included in the plastisol compound as desired. Generally, pigments will be included in the plastisol compound at a concentration of 1 to 3 parts per hundred parts of vinyl chloride polymer resin (PHR), lubricants at 1 to 10 PHR, and stabilizers at 1 to 2 PHR.

Pigments which can be used for the plastisol compound include carbon black, titanium dioxide and zinc oxide. The pigments are included in the plastisol compound for opacity and colour.

Lubricants are normally included in the plastisol compound in order to impart suitable torque values to lined closures of the type that have to be rotated, (e.g. lug or threaded caps) for removal. Suitable lubricants include fatty acids such as stearic and oleic acid, fatty acid amides, silicone oils such as dimethyl polysiloxane and methyl hydrogen polysiloxane and paraffinic waxes.

Stabilizers are included in the plastisol in order to improve the resistance of the plastisol to the deleterious effects of light, oxygen and heat. Suitable stabilizer materials are the so-called "acid acceptor" compounds which are capable of reacting with and neutralizing any hydrogen chloride which might split off from the vinyl chloride polymer resin during fusion. Examples of stabilizers which can be employed are epoxidized oils, such as soybean and linseed, calcium stearate, zinc stearate, magnesium stearate, aluminium stearate, calcium ricinoleate, zinc ricinoleate, calcium laurate, dibutyl tin dilaurate and other fatty acid soaps of these metals.

The plastisol compounds of the present invention may usually be prepared by simply blending the ingredients together in the desired proportions.

If desired a chemical blowing agent may be incorporated in the plastisol compound to give the final gasket a foamed structure. Such a blowing agent should have a decomposition temperature above the gel point temperature of the vinyl plastisol. A blowing agent possessing a decomposition temperature above the gel temperature of the plastisol and within a temperature range of 100" to 1500C is preferred. Typical blowing agents which may be employed include nitrogen evolving agents, such as e.g. p, p'-oxybis (benzenesulfonyl hydrazide), and N, N'- dimethyl - N, N'- dinitroso terephthalamide. The blowing agents are incorporated in the plastisol in amounts ranging from 0.5 to 15 parts of the blowing agent per 100 parts of the vinyl chloride polymer resin. Amounts of from 0.5 to 3 parts per hundred parts of resin have been found to be particularly useful.

The following Example illustrates the invention.

Example A series of vinyl chloride polymer plastisol compositions containing plasticizers having varying dielectric constants and loss factors were prepared having the following composition: Component .Parts Polyvinyl chloride emulsion grade resin 75 Polyvinyl chloride suspension grade resin 25 Plasticizer 60 Zinc stearate 1 The emulsion grade polyvinyl chloride was composed of finely divided, spray dried particles, having an average particle size of 4 microns, a molecular weight of 75,000, and a bulk density of 17 Ibs./cu. ft. The suspension grade polyvinyl chloride had an average particle size of 30 microns, a bulk density of 40 Ibs/cu. ft., and a molecular weight of 55,000.

The mixture of polyvinyl chloride and plasticizer had gel points ranging from 79"-80"C and clear fusion points of 100" to 140"C. The gel points and clear fusion points were determined using a method whereby 1/4" wide x .015" thick strips of plastisol were applied to a preheated gradient plate (50 to 2600C. range). Two minutes were allowed for fusion to occur. Near the end of the two minute period, the gradient plate indicator was superimposed over the juncture of the transparent and opaque areas of the plastisol strip and the indicated temperature of "clear point fusion was recorded. At two minutes, the hot end of the plastisol strip was lifted at a 900 angle to the plate moving from the hot to cool end in one continuous sweep until the strip broke. The gradient plate indicator was superimposed over the break point and the indicated temperature of "gel point" was recorded. Each plastisol was run in triplicate.

Closures, 28.0 mm. in diameter, having a depending skirt portion of 6.35 mm. length were injection molded from polypropylene and 0.25 grams of the deaerated plastisol composition was spin-lined into the interior closure shell to a thickness of 0.015 inches. The plastisol compounds were then heated and fused by placement between the electrodes of a commercial dielectric heating unit. The electrodes of the unit were spaced 1.0 inch apart.

The unit operated at 27.12 megahertz and had an output of 12 kw. The time for clear point fusion to be reached by the plastisol was 0.5 to 1 minute.

After fusion of the plastisol in the closure, the assembly was allowed to cool and the resultant gasket evaluated for commercial utility.

If the gasket has resiliency and good elastic or spring-back:properties approximating gasket materials of the type spin-lined and fused in metal closures using conventional hot punch and/or fusing techniques, the integrity of the gasket was rated Excellent. If the gasket material had some resiliency but no spring-back the gasket was rated Fair. If the gasket had no resiliency and was inelastic, it was rated Poor. In order to be acceptable for commercial use, the gasket, by this test, must be rated Excellent.

The sealing and chemical resistance properties of gasket lined closures were evaluated by filling 8 ounce glass bottles with ethylene glycol, methyl alcohol, and mineral spirits and then screwing the gasket lined closures on the open ends of the bottles to seal the container.

The containers were turned upside down for 720 hours and.then examined for leakage and chemical resistance properties.

The results of the gasket integrity and leakage and chemical resistance tests are summarised in Tables I and II below. For purposes of comparison, plasticizers having dielectric constants and loss factors outside the scope of the present invention were substituted in the plastisol compositions. The results of these comparative tests designated by the symbol "C" also recorded in the Table.

TABLE I Gasket Integrity Time to Reach Dielectric Loss Clear Clear Point Test Constant Factor Gel Point Point Fusion Gasket No. Plasticizer @ 1Kc @ 1Kc C Fusion C (Minutes) Integrity 1. Ethylhexyl di- 7.52 25.2 78 115 0.5 Excellent phenyl phosphate (EHDP) 2. Buty phthalyl- 6.86 11.10 79 110 0.5 Excellent butyl glycolate (BPBG) 3. Butyl benzyl 6.45 8.45 80 100 0.5 Excellent phthalate (BBP) 4. Acetyl tributyl 6.05 1.95 100 140 1.00 Excellent citrate (ATBC) C1 Di-2-ethylhexyl 5.20 0.587 103 137 1.25 Fair phthalate (DOP) C2 Di-2-ethylhexyl 4.13 0.195 117 149 1.50 Fair adipate (DOA) C3 Di-2-ethylhexyl 3.88 0.071 138 168 72.0 Poor sebacate (DOS) TABLE II Chemical Resistance % Change in % Change in Plasticizer Solvent Leakage % Change in WT. Volume Hardness 1. EHDP Ethylene glycol None - 2.94 - 3.42 0 Mineral spirits None - 5.97 - 4.56 0 Methyl alcohol None 3.25 5.37 0 2. BPBG Ethylene glycol None - 2.17 - 2.54 0 Mineral spirits None - 3.04 - 2.12 0 Methyl alcohol None 1.73 2.48 0 3. BBP Ethylene glycol None - 1.03 - 1.19 0 Mineral spirits None - 2.13 - 1.74 0 Methyl alcohol None 2.06 2.75 0 4. ATBC Ethylene glycol None - 1.85 - 2.30 0 Mineral spirits None - 3.50 - 1.63 0 Methyl alcohol None 3.07 4.72 0 C4 DOP Ethylene glycol None 0.70 0.57 4.00 Mineral spirits Yes - 18.37 - 19.57 33.33 Methyl alcohol None 2.52 3.49 0 C5 DOA Ethylene glycol None 2.13 2.11 0 Mineral spirits Yes - 17.49 - 19.20 38.89 Methyl alcohol None 2.90 3.21 2.78 C6 DOS Ethylene glycol None 2.69 2.51 0 Mineral spirits Yes - 13.59 - 16.83 32.00 Methyl alcohol None 1.69 1.58 0 By reference to the Tables it is immediately apparent that dielectrically fused, spin-lined gaskets formed from plastisols containing plasticizers within the scope of the present invention (Test Nos. 1-4) exhibit substantially superior gasket integrity and chemical resistance properties especially when compared with dielectrically fused gaskets formed from plastisols containing plasticizers outside the scope of the present invention.

Claims (11)

WHAT WE CLAIM IS:

1. A method for forming a gasket in a closure for a container, the closure being fabricated from a polyolefin resin, the method comprising introducing a plastisol compound into the closure, forming the plastisol into a gasket of the desired shape, the closure being formed of an olefin polymer exhibiting substantially no response to heat activation by radio frequency electrical energy and the plastisol being comprised of a vinyl chloride polymer or co-polymer and a plasticizer having a dielectric constant between approximately 6 and approximately 8 and a loss factor between approximately 2 and approximately 25, fluxing the plastisol in the closure by dielectrically heating the plastisol with exposure to a source of radio frequency electrical energy having a frequency in the range of 1 to 200 megahertz and then allowing the fluxed plastisol to cool and form the gasket.

2. A method as claimed in claim 1, wherein the vinyl chloride polymer is polyvinyl chloride.

3. A method as claimed in claim 1, wherein the plasticizer is butyl phthayl butyl glycolate.

4. A method as claimed in claim 1, wherein the plasticizer is butyl benzyl phthalate.

5. A method as claimed in claim 1, wherein the plasticizer is ethyl hexyl diphenyl phosphate.

6. A method as claimed in claim 1, wherein the plasticizer is acetyl tributyl citrate.

7. A method as claimed in claim 1, wherein the olefin polymer is polyproplene.

8. A method as claimed in claim 1, wherein the olefin polymer is polyethylene.

9. A method as claimed in claim 1, wherein the plastisol is comprised of 100 parts of the vinyl chloride polymer and 50 to 80 parts plasticizer.

10. A method of forming a gasket in a closure, substantially as hereinbefore described.

11. A closure provided with a gasket by a method as claimed in any preceding claim.