GB2136709A - Heat-sealing sheet material - Google Patents

Abstract

The material comprises a base sheet bearing a sealant layer, the sealant being a graft copolymer comprising a trunk polymer of a 20-60% saponification product of an ethylene/vinyl acetate copolymer of 15-45% vinyl acetate content and a branch polymer of an unsaturated carboxylic acid in a proportion of 10% or less of the graft copolymer. The material is used in the sealing of glass containers particularly containing a food product. A seal structure of high adhesive strength can be obtained which is practically not lowered even in contact with water. <IMAGE>

Description

1

SPECIFICATION

Process for sealing glass container openings GB 2 136 709 A 1 Background of the invention

This invention relates generally to sealing of openings of vessels and containers and more particularly to a process for affixing cover materials in sealed state onto the opening rims of glass containers and to a sealing or cover material therefor.

It is a general practice to place a powder material such as coffee or a dry dairy product in a glass container such as a jar and to affix a cover material comprising a piece of metal foil and a resin layer formed on a surface thereof onto the rim of the container opening thereby to seal the powder material in the container.

For affixing the cover material in sealed state onto the opening rim of the glass container, the induction sealing process has heretofore been widely used.

This induction sealing process comprises causing each of a plurality of glass containers on the opening rim surface of which a cover material (hereinafter referred to as "cover") has been placed to pass through a 16 region where it is exposed to high-frequency electro-magnetic waves thereby to cause its resin layer to be heated by its self-induced heat, whereby the resin layer is melted and caused to adhere to the opening rim surface. A capping machine carrying out such an induction sealing process is capable of high-speed operation of sealing approximately 500 containers per minute and is therefore being utilized for sealing products such as powdery or solid food products other than food products in liquid form.

However, it has heretofore been impossible to carry out satisfactory sealing of liquid products by this induction sealing process. The reason for this is that, since a liquid product is sterilized by heating and charged into glass containers at high temperature, and the vapor pressure of the food product or the like in the glass container gives an interior pressure of the container at the time of sealing and exerts a force tending to cause the heat-sealed part of the cover to separate from the container rim surface immediately 25 after sealing, thereby giving rise to instability of the adhesive or bonding strength of the sealed parts.

Another process for affixing a cover is the heat-sealing process wherein a cover is heated by means of a heating plate and thereby caused to be affixed by heat adhesion to the opening rim surface of a glass container.

However, in general, the heat capacity of a glass container is great, and the cover cannot be caused to adhere positively to the opening rim surface merely by applying heat to the cover from the heating plate including the heat transmitted to the glass container. Particularly in the case of most glass containers, each container is formed into an integral structure by joining together two halves thereof, and in many cases projections of the order of 0.2 to 0.3 mm exist on the opening rim surface in the vicinity of the joint between the container halves. The presence of these projections impairs the adhesion given by heating and pressure application.

In the case of a cover, such as an intermediate or inner cover of a container, which comprises a thin metal foil of a thickness of the order of 100 microns or less and a sealant layer also of a thickness of the order of 100 microns or less laminated onto the foil, projections of this character on the opening rim surface of the container cannot be absorbed by the cover, whereby portions where the applied force is locally not transmitted are created. This defective state can be corrected by providing a cushioning layer of a material such as rubber of a thickness of the order of 2 to 5 mm on the contacting surface of the heating plate, but a presence of such a cushioning layer, itself, obstructs the transmission of heat from the heating plate to the cover. Therefore, the heat-sealing process is still not a satisfactory process on the points of adhesive strength and work efficiency and has not yet been reduced to effective practice.

Summary of the invention

It is a principal object of this invention to provide a process for heat sealing glass containers which process affords much improved seal strength and work efficiency.

More specifically, an object of this invention is to provide a process for affixing covers on glass containers 50 by which a cover can be affixed in positively sealed state on each glass container even in the case where the container is filled with a liquid product, and by which, moreover, the cover can be sealed by a heat-sealing process.

It is another principal object of this invention to provide a sealing or cover material in sheet form capable of giving, upon heat sealing, a seal structure with excellent adhesive strength even in contact with water and 55 suitable for use in the above described sealing process.

As a result of our studies carried out with the above objects, we have learned that, in a sealing process with a heating plate which has the desirable feature that sealing can be carried out as pressure is applied for sealing a container containing a liquid product, positive seals can be obtained with high work efficiency by an effective combination of heating, heat removal, and pressure application. The process for sealing glass 60 containers with covers of this invention is based on this finding.

According to thi s invention, briefly summarized, there is provided a pyocess for affixing covers onto glass containers which comprises the steps of: (a) providing a glass container having an opening end; (b) providing a cover comprising a base sheet having a shape suitable for covering the opening end and a sealant layer of a thermoplastic resin laminated onto one surface of the base sheet; (c) preheating the 65 2 GB 2 136 709 A opening end; (d) placing the cover on the opening end so that the sealant layer contacts the opening end; (e) pressing the cover, as it is heated, against the opening end by means of a heating plate thereby to cause the cover to adhere to the opening end; and (f) pressing the cover against the opening end by means of a pressing plate at a temperature lower than that of the heating plate in step (e) thereby to assure positive sealing of the opening end by the cover.

In the practice of this invention, since the container opening end is heated beforehand, the heat-seal surface of the cover to contact the container opening end, the heating of which heat-seal surface is considered to be necessary, is heated preferentially at the time of heating and pressing of the cover by means of a heating plate, and the heating step as a whole becomes rapid. Furthermore, since the supplying lo of heat by the heating plate and the subsequent pressing action by the pressing plate are separated, the heating and cooling in the sealing process become rapid as almost all of this process is carried out while pressure is being applied, and a positive seal is quickly obtained.

In the case where there are projections on the opening rim surface of the conaiher as mentioned hereinbefore, also, such projections can be easily absorbed by providing cushioning layers on the heating plate and the pressing plate. That is, since only the supplying of heat need be considered in the case of the 15 heating plate, it is not necessary to make the thermal inertia of the heating plate very small (i.e., to make the thermal response of the heating plate very good) as in the case where positive sealing is obtained by cooling the heating plate while pressure is being applied by the heating plate. Accordingly, there is not a very great adverse effect in providing a cushioning layer of a thickness of a certain extent on the heating plate, and, if necessary, by narrowing the object to supplying heat, it is also possible to omit the cushioning layer.

On the other hand, in the case of the pressing plate, a cushioning layer of ample thickness can be provided since there is no problem of supplying heat. Thus, by the practice of the proceiss of this invention, a high work efficiency of the entire process and high seal strength stability can be obtained.

In accordance with this invention, there is also provided a heat-sealing sheet material suitable for use in the process for sealing glass containers with covers described herein. More particularly, the heat-sealing 25 sheet material of this invention comprises a base sheet and a sealant layer formed on one side of the base sheet, said sealant being a graft copolymer comprising a trunk polymerof a 20 to 60% saponification product of an ethylenelvinyl acetate copolymer (EVA) of a vinyl acetate content of 15 to 45% by weight and a branch polymer of an unsaturated carboxylic acid in a proportion of 10% by weight or less of the graft copolymer.

Using a graft copolymer of the above-mentioned particular composition, the heat-sealing sheet material of 30 the present invention not only excels in adhesiveness with respect to glass containers but also maintains strong adhesive strength even in contact with water and furthermore has low toxicity.

The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description, including a specific example of practice, when read in conjunction with the accompanying drawing briefly described below.

Brief description of the drawing

In the drawing:

Figure 1 is a perspective view of one example of a glass containerto be sealed bythe process of this 40 invention; Figures 2 and 3 are sections taken along planes in the thickness direction respectively showing examples of covers used in accordance with the invention; and Figures 4(a) through 4(e) are elevational views, in vertical section, showing the region around the opening of a glass conainer for a description of steps in the process of the invention.

Detailed description of the invention

One example of a glass container to which this invention can be applied is a wide-mouth bottle or jar as shown in Figure 1. This container 1 has an opening 2 encompassed by a rim surface 3. Particularly in the case where this container 1 has been formed by joining together two halves of the bottle bead-like projections of a height of approximately 0.2 to 0.3 mm tend to be formed on this rim surface 3 in te vicinity of the joint seam 4 where the bottle halves are joined.

The opening rim surface 3 of this glass container 1 is sealed according to this invention by a sealing sheet or cover material 7 as shown in Figure 2, which comprises a sealant layer 6 of a thermoplastic resin formed on one surface of a base sheet 5.

As the base sheet 5, papers or sheets of synthetic resins such as oriented polyolefin, polyester, oriented 55 nylon and polyvinyl chloride can be used, but metal foils, for example, of aluminum, having excellent gas barrier properties are preferable. The thickness of the base sheet 5 is not particularly limited, but, when used as an inside coverfor glass containers for food products, the base sheet should preferably have a thickness of 20 to 100 p. One more preferred example meeting this purpose is a heat- sealing sheet material shown in Figure 3 comprising a laminate as a base sheet, consisting of an aluminum foil 5a of 50 [L thickness and a 60 polyethylene film 5b of 20 g thickness laminated on one side of the aluminum foil 5a, and a sealant layer 6 formed on the polyethylene film 5b. The presence of this polyethylene film 5a is desirable because it increases adhesive strength between the aluminum foil 5a and the sealant layer 6 and hence overall sealing strength. The polyethylene film can be formed on the aluminum foil by a method such as extrusion lamination. At that time, applying an anchor coat such as that of an isocyanate type beforehand on the 65 Z1 1 el 3 GB 2 136 709 A 3 aluminum foil is effective in order to obtain a stronger adhesive strength. As the thermoplastic resin for forming the sealant layer 6 according to the process of this invention, any resin known as a so-called hot-melt type adhesive, such as polyethylene, an ethylene/vinyl acetate copolymer (EVA) or a partially saponified EVA can be used. In order to provide a seal structure having high adhesive strength while being capable of withstanding filling under heating of the container with a water-containing content, however, it is 5 especially preferable to use a graft copolymer having a partially saponified product of an ethylene/vinyl acetate copolymer (EVA) as a trunk polymer and a polymer of an unsaturated carboxylic acid as a branch polymer grafted onto the trunk polymer.

The content of vinyl acetate in the ethylene/vinyl acetate copolymer as the starting material of the trunk polymer is preferably 15 to 45% by weight and especially about 30% by weight. When the vinyl acetate content is less than 15% by weight, sufficient adhesive strength cannot be obtained because of shortage of polar groups, and when it is higher than 45% by weight, the sealant layer becomes too tacky, resulting in undesirable blocking tendency and poor heat resistance as a film. 1 The trunk polymer comprises a partially saponified product of the abovedefined ethylene/vinyl acetate copolymer. The saponification degree of the polymer is preferably 20 to 60% and especially about 50%.

When the saponification degree is less than 20%, the hydroxyi groups introduced become insufficient, resulting in insufficient adhesive strength, and, when it is over 60%, adhesive strength retained in contact with water or steam and elongation at breakage decrease with respect to the resulting sealant.

The sealant used in the present invention can be obtained by graftpolymerizing onto the above trunk polymer an unsaturated carboxylic acid of 10% by weight or less of the trunk polymer. As the unsaturated carboxylic acid, acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid and others are used. Of these acids, maleic acid is most preferable. the graft polymerization is conducted by graft-polymerizing onto the above trunk polymer a specific quantity of the unsaturated carboxylic acid in the presence of a peroxide catalyst according to the conventional method. The polymerization may be carried out in an emulsion or solution state.

The sealant used in the present invention can also be obtained by firstly graft-polymerizing an unsaturated carboxylic acid onto an ethylene/vinyl acetate copolymer and then saponifying the vinyl acetate portion of the thus obtained graft copolymer.

By the above graft polymerization of a trunk polymer with an unsaturated carboxylic acid, the heat-sealing strength of the sealant obtained is increased and moreover the sealing strength is maintained even in contact with water. The effect of the grafting of the unsaturated carboxylic acid is attained when the content of the acid in the sealant is at least 0.1% by weight. When the content exceeds 10% by weight, side reactions such as crosslinking reaction of the base resin concur, resulting in reduction of processability and physical properties such as melt index.

In orderto obtain improved adhesive strength even when the sealant is in contact with water, it is 35 particularly preferable that the content of the unsaturated carboxylic acid be in the range of 2 to 4% by weight.

It is preferred that the sealant layer 6 be formed on the base sheet generally in the thickness of 10 to 100 li.

When the thickness is less than 10 L, the sealing strength becomes insufficient, and, above 100 g, a greater quantity of heat is required for heat sealing, sealing workability becoming worse, whereby stable adhesion 40 cannot be obtained. For the heat-sealing material thus obtained, an appropriate heat-sealing temperature is 230 to 350'C.

In the process of this invention, the opening rim surface 3 to become the adhesion surface on the side of the glass container 1 is preheated to a temperature of 40 to 90'C, preferably 70'C:L 5oC by means such as a flame 8, heated air stream (not shown), or a heating plate (not shown) as indicated in Figure 4(a). This preheating is particularly effective when a heated liquid material has been charged into the container since it eliminates dewing of vapor thereby to enhance the adhesion with a cover. In the case where a heating plate is to be used for this preheating, it is preferable to use an additional heating plate separate from that used for heat sealing, as described hereinafter, thereby to shorten the entire process cycle.

Next, as indicated in Figure 4(b), a cover 7 as described hereinbefore is placed on the glass container 50 opening so that its sealant layer 6 contacts the opening rim surface 3.

Then, as indicated in Figure 4(c), the cover 7 is pressed against the rim surface 3 by means of a heating plate 9. At the same time, the cover 7 is heated to the optimum temperature for heat adhesion according to the material characteristic of the sealant layer 6 to cause it to adhere under heating to the rim surface 3, thereby carrying out heat sealing. More specifically, a heating plate with a temperature of, for example, 230 55 to 350'C is used. In the practice of this invention, the principal function of the heating plate 9 is to supply heat to the sealant layer 6, and the completion of the sealing procedure is carried out in the succeeding step.

Accordingly, the upper limit of the temperature of the heating plate 9 need not be set very strictly, and it is possible to use a temperature exceeding the optimum temperature range. - Since the opening rim surface 3 of the glass container 1 has been preheated in the heat-sealing process of 60 this invention, there is no possibility of inadequate heating by the heating plate 9, and good adhesion of the cover 7 to the rim surface 3 can be achieved. Furthermore, since the sealing is accomplished as the cover 7 is pressed against the rim surface 3, even when the content within the glass container 1 is a fluid substance and is in a highly pressurized state after high-temperature sterilization, heat adhesion can be accomplished as turning up of the peripheral lip of the cover 7 is suppressed, whereby a good seal can be obtained.

4 GB 2 136 709 A 4 Furthermore, by providing a thin elastic layer 10 made of a material such as a rubber of a thickness of the order of 2 to 10 mm underneath the lower surface of the heating plate 9 as shown in Fig ure4(d) and pressing and heating the cover 7 over this elastic layer 10 interposed. therebetween, the aforementioned projections of a height of the order of 0.3 mm on the opening rim surface 3 of the glass container 1 are absorbed, whereby the cover 7 can be caused to adhere intimately and firmly against the rim surface 3, and an even more positive heat sealing can be achieved. Since the material forming this elastic layer 10 generally has a poor heat-transmitting characteristic, it is desirably made as thin as possible.

Finally, as indicated in Figure 4(e), the cover 7 in the heat-sealed state is pressed against the opening rim surface 3 by means of a pressing plate 11 to assure the adhesion between the sealant layer 6 and the rim surface 3. This pressing plate 11 has the function of holding the sealant layer 6 until it acquires ample adhesive strength as it is cooled by the pressing plate 11. The pressing plate 11 is preferably made entirely of a rubber-like material or provided at least as a surface layer with a rubber-like cushion layer of a thickness of at least 0.5 mm which is ample for absorbing the aforementioned projections on the rim surface 3 of the glass container 1.

The pressing of the cover 7 with this pressing plate 11 can be carried out effectively with a pressing force of 15 1.0 to 5.0 kg/cM2, with a surface temperature of the pressing plate 11 of room temperature to WC, which is 140oC or more lower than that of the heating plate 9, and with a pressing time of 0.5 to 4.0 seconds.

By this pressing step, the cover 7 is caused to adhere to the opening rim surface 3 with uniform adhesive strength, whereby the adhesive strength after heat sealing can be maintained at a high level for a long time.

Furthermore, even if the preceding heat-sealing step has been inadequate and a portion of the cover 7 has 20 separated from the rim surface 3, the sealing of the cover 7 can be positively accomplished by this pressing step.

In order to indicate more fully the nature and utility of this invention, the following specific examples of practice of the sealing process according to this invention and related results are set forth, it being understood that these examples are presented as illustrative only and are not intended to limit the scope of 25 the invention.

Example 1

Aglass containerwas sealed bythe process describedwith referenceto Figures 4(a)through 4(b). Forthe cover 7, a laminated sheet material of the structure shown in Figure 3 comprising an upper aluminum foil 5a 30 of 50-micron thickness, a lower sealant layer 6 of 70-micron thickness of a sealant resin prepared by graft polymerizing maleic acid onto a 50-percent saponified EVA (vinyl acetate content being 30% by weight), the quantity of the maleic acid being 3% by weight of the partially saponified EVA, and a polyethylene layer 5b of 15-micron thickness interposed between the foil 5a and the layer 5b was used.

A glass container (jar) 1 was then filled with a food-product content (i. e., jam) at WC, and its opening rim 35 surface 3 was heated to a temperature of WC by means of a metal preheating plate.

Next, the cover7 was placed on the opening rim surface 3 and was pressed against and heat sealed to the rim surface 3 under the conditions of a pressing time of 3 seconds and a pressure of 2.0 kg/cM2 by means of a heating plate at a temperature of 28WC.

In addition, the cover 7 was thereafter pressed against the rim surface 3 by means of a pressing plate 11 40 made of a silicone rubber of 2.0 mm thickness and a rubber hardness of 40 Rockwell (Shore A) under the conditions of a pressure of 2.0 kg/cM2, pressing time of 3 seconds, and surface temperature of the pressing plate of WC.

The glass container heat sealed in the above described manner was found to be sealed with stable adhesiveness with an interior degree of vacuum of 40 cmHg, and its pressure withstanding strength was 0.3 45 kg/cM2. Furthermore, this heat-sealed glass container was stored under refrigeration at a temperature of 7'C, and measurements were carried out, whereupon no decrease in adhesive strength in relation to the initial value was observable.

In a related experiment, the above described heat-sealing process was carried out with a solid content product in place of a fluid content. As a result, it was found that the glass container could be amply sealed. 50 Example 2

In order to evaluate the heat sealing characteristics of the heat-sealing sheet material of the present invention, the following tests were conducted.

A base sheet was prepared by extrusion-laminating,a polyethylene film of 20 L thickness on one side of an 55 aluminum foil of 50 K thickness. On three pieces of this polyethylene layer were formed sealant layers respectively comprising (a) an ethylene/vinyl acetate copolymer (EVA) containing 30% of vinyl acetate, (b) a 50%-saponification product of the above EVA, and (c) a graft polymer obtained by graft-polymerizing 3% by weight of maleic acid onto the product (b) to a thickness of 50 [L by extrusion lamination. The three kinds of the heat-sealing sheet materials thus obtained were cut into slips each of 15 mm width. Each slip was placed 60 on a smooth glass plate and was bonded to the glass plate by pressing a heating plate at 28WC against the slip with a pressure of 2 kg/cm'.

The adhesive strengths (peeling at an angle of 90', peeling speed of 300 mm/min) of the three kinds of the laminates thus obtained were measured right after bonding and after 10 days of immersion in water at room temperature.

R 1. k i.

GB 2 136 709 A 5 The results obtained are summarized in the following table.

Heat-sealing strength (gl 15 mm) 5 Rightafter After 10 days Sealant heatsealing immersion in water (a) EVA 800 0 10 (b) Partially saponified 1750 0 product of EVA (c) Product obtained by 15 graft-polymerizing 2000 1950 3% of maleic acid on to the product (b) The results in the above table show that the heat-sealing sheet material according to thff present invention 20 not only has excellent sealing strength but also is excellent in its water resistance, and therefore the sealing strength remains almost the same even after 10 days of immersion in water.

Claims (8)

1. A heat-sealing sheet material which comprises abase sheet and a sealant layer formed on one side of the base sheet, said sealant being a graft copolymer comprising a trunk polymer of a 20 to 60% saponification product of an ethylene/vinyl acetate copolymer (EVA) of a vinyl acetate content of 15 to 45% by weight and a branch polymer of an unsaturated carboxylic acid in a proportion of 10% by weight or less of 30 the graft copolymer.

2. A heat-sealing sheet material according to claim 1 wherein the base sheet comprises a metal foil.

3. A heat-sealing sheet material according to claim 1, wherein the base sheet comprises a laminate of an aluminium foil and a polyethylene film, and the sealant layer is formed on the polyethylene film.

4. A heat-sealing sheet material according to claim 1, 2 or3, wherein the base sheet has a thickness of 20 35 to 100 11.

5. A heat-sealing sheet material according to any of claims 1 to 4, wherein the sealant layer has a thickness of '10 to 10011.

6. A heat-sealing sheet material according to any of claims 1 to 5, wherein the unsaturated carboxylic acid is maleic acid.

7. A sealed glass container which comprises a glass container containing a product and having an 40 opening rim and a heat-sealing sheet material of any of claims 1 to 6 affixed on the opening rim of said containerwith the sealant layer in heat-sealed contact with the opening rim.

8. A sealed glass container according to claim 7, wherein the product is a water-containing food product.

Printed in the UK for HMSO, D8818935, 7184, 7102.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.