Thermoplastic lid for thermoplastic container

Abstract: The invention is a thermoplastic lid (10) which utilizes a bellows action to prevent paneling in the thermoplastic container to which it is attached. The lid comprises a flange for attachment to the container and a recessed center portion (16) which has a thinner flex area comprising a raised ridge (14) adjacent to the outer ridge of the center portion (16). The stiffness of the lid and the thickness of the flex area (12) are no greater than the maximum which will allow the center portion of the lid to flex inwardly sufficiently to provide the minimum displacement of head space necessary to prevent paneling.

Description: This invention relates to a thermoplastic container sealed with a lid. More particularly, the invention relates to a thermoplastic container which can be filled with heated products and sealed with a lid, without unacceptable deformation on cooling. It is known from U.K. Specification No. 1,367,338 to make cupped articles from a sheet of a thermoplastic material such as polypropylene. Such articles can also be made from billets of the same materials. The method comprises introducing a thin sheet or a billet of the thermoplastic material in the solid phase state, i.e. below the crystalline melting point, between a forming plug and a hollow mold having an end opening, moving the forming plug into the mold opening with the sheet to carry the stretched portion of the sheet into the mold, and introducing a pressure fluid into the stretched portion of the sheet to form the article in the mold. The above method is particularly suitable for making cupped articles of polypropylene and has met with considerable commercial success. It is known that after filling and sealing a yieldable body such as a thermoplastic container made from such a cupped article, there is a tendency for the sidewalls thereof to deform or panel inwardly under certain conditions. This deformation of the sidewalls results from the pressure differential between the inside and the outside of the container and these deviations may be brought about by various physical or chemical conditions. In the packaging of food, various materials including fluids such as juices, syrups, salad oils and the like are oftentimes brought to elevated temperatures before introduction into thermoplastic containers. It has been found that there is a marked tendency for such containers to distort inwardly as described above. For example, when hot-filled containers are allowed to cool, the internal pressure will gradually decrease whereby the external atmospheric pressure causes their sidewalls to indent, panel or otherwise partially collapse. This condition renders the containers unacceptable to the ultimate consumer. Aside from the problems associated with such hot-filled containers, there are other related packaging situations where chemical reactions cause noticeable reductions in the pressure differentials of the container resulting in the paneling or deformation of the sidewalls. For example, when lubrication or motor oil is packaged in a plastic container and sealed, chemical reactions take place between the various hydrocarbon constituents and any residual oxygen, e.g. air, causing the total pressure within the container to decrease. With this drop in pressure, there results an inward paneling of the sidewalls in order to equalize or compensate for the decrease in internal pressure. Here again, as with the hot-filled container, the containers are unacceptable to the ultimate consumer. Aside from an undesirable appearance, the container itself loses column strength and sidewall symmetry which presents a problem in stacking them for storage, display and the like. Since the reduction in pressure cannot always be practically avoided, the present invention provides a container/lid configuration wherein a portion of the lid of the container provides sufficient flexibility to prevent deformation on cooling, whilst at the same time providing an effective oxygen barrier to protect the container contents from the atmosphere. The invention is concerned with a container sealed with lid, each being formed from a laminate of propylene/ethylene copolymer and, as oxygen barrier material, ethylene/vinyl alcohol copolymer, said lid comprising a flange attached to said container and a recessed centre portion incorporating a flex area provided with a raised ridge close to the outer edge of the centre portion, characterised in that the thickness of the flex area is less than that of the rest of the lid and is at most 15 x 10⁻⁶ m, whereby a pressure differential between container content and the atmosphere of 4.82 kPa will cause flexing of the lid centre portion sufficient to displace at least 6 x 10⁻³l of head space within the container. Preferred embodiments of this invention are shown in figures 1 to 4. Figure 1 is a side view of the lid of the present invention which illustrates how the lid flexes to eliminate paneling. Figure 2 illustrates the position of the center portion of the lid both before and after cooling of the liquid within the container. Figure 3 illustrates a lid made according to the present invention which incorporates a spout. Figure 4 illustrates a lid made according to the present invention which incorporates a different kind of a spout. There are three major features of a lid that affect the lid's functionality as a closure for a thermoplastic container. These are sealability, oxygen barrier, if that is required for the application, and vacuum responsiveness, i.e. the lid's ability to eliminate paneling. Sealability simply means that the materials of the lid are compatible with and can be sealed, preferably hermetically, to the thermoplastic container. In many applications today, such containers are made of polypropylene. Therefore, the chosen material for the outer layer of the lid is a propylene polymer such as a copolymer of propylene and ethylene because these copolymers have lower flexural modulus (0.91 GPa) than homopolymer polypropylene (1.26 GPa). Many applications require that the container and the lid provide excellent oxygen barrier properties. For many shelf stable food products, the maximum amount of oxygen transmission which is permissible is 10 to 40 parts per million O₂ per year or 0.003 ml per lid per day. Therefore an oxygen barrier materials is present in the lid of the present invention, which is a copolymer of ethylene and vinyl alcohol. As stated above, this invention incorporates a thermoplastic lid which is intended to be used as a closure for containers which contain hot filled products such as juice or apple sauce. In one embodiment, the lid includes a pour spout which is raised above the level of recessed center portion and which has a sealable opening therein to allow access to the contents of the container. If this option is utilized, then less head space can be displaced because the recessed section of the container is smaller than if there were no spout. The vacuum within the container is caused by the condensation of the hot gases in the head space after hot filling. Since the walls of the container are made of a relatively flexible thermoplastic, the vacuum within the container causes the sides of the container to buckle or pannel. Since the internal vacuum in the container is proportional to the head space volume, it can be seen that paneling can be reduced by decreasing head space. In the present invention, the decrease in the amount of head space is accomplished in two ways. First, the center portion of the lid is recessed to reduce the internal head space. Additionally, a flex area of reduced thickness is formed in the lid. The flex area in the lid is less resistant to the forces caused by the internal vacuum than are the sides of the container. Therefore, the flex area of the lid will be pulled inwardly to displace some of the head space in the container before the walls of the container will begin to panel. The lid of the present invention deflects in response to pressure differential so as to eliminate container paneling. The mechanism by which the lid moves is a combination bellows and diaphragm formed into the lid. In Figure 1, it is seen that the lid 10 has a flex area 12 which is comprised of a raised ridge 14 adjacent to the outer edge of the top or center portion 16 of the lid 10. The flex area 12 is thinner than the rest of the lid 10, being no more than 15.10⁻⁶ m in thickness, so that it will be weak enough to allow the top 16 to be pulled inwardly by action of the vacuum within the container and the external pressure so as to displace a certain proportion of the head space, i.e. at least 6.10⁻³ l at a pressure differential of 4.82 kPa. The dotted lines in Figure 1 indicate how the top 16 of the lid 10 will flex away from the flange 18 of the lid 10 due to the action of the vacuum within the container. Figure 2 shows a lid which is sealed onto a cup containing hot liquid and illustrates the positioning of the top 16 of the lid 10 while the liquid is still hot and then after the liquid has been cooled. It can be seen that the head space within the container after the liquid has cooled has been dramatically decreased. Figure 3 illustrates a lid 10 which incorporates a pour spout 22. It can be seen that the ridge 14 extends around the outer edge of the top 16 of the lid 10 except at the spout 22 where it extends around the inside edge thereof. In one embodiment, as shown in Figure 6, the ridge 14 ends at pour spout 22. The bellows action is required herein even though the head space is reduced by recessing the top of the lid. Some head space must be provided within the container so that the liquid contents of the container do no spill prior to sealing. A container filled to the brim will spill over during transport to the seal area of a filling machine. The spillage will contaminate the seal area and destroy seal integrity. The bellows action of the lid of the present invention can, when designed properly, eliminate the paneling effects of the residual head space. In order to eliminate paneling, there must be enough displacement of the head space of the container to reduce the internal vacuum such that the walls of the container will not panel. The amount of displacement attainable is proportional to the ability of the lid to flex inwardly. The flexing ability of the lid is related to the stiffness of the lid itself, which itself is related to the materials chosen and the thickness of the lid, as well as the thickness of the flex area. All of these factors must be combined properly to provide a lid which will provide sufficient displacement of the head space in the container to prevent paneling. The specific numbers mentioned above relate to the standard size 75.10⁻³ m diameter plastic cups which are in common use in industry today. Such containers generally have a wall thickness of no less than 4.10⁻⁵ m. In general, a stiffer container needs less displacement and, in such cases, the flex area could be less flexible, i.e., thicker or made of a less flexible material. For the most part herein, the discussion relates to polypropylene polymer containers which utilize ethylene vinyl alcohol copolymers as a barrier layer. However, polystyrene containers which are comprised of laminates of polystyrene with ethylene vinyl alcohol copolymers and polyethylene may also be used. These laminates are softer than polypropylene because of the softer polyethylene. Thus, the flex area of the lid would have to be thinner to allow the displacement of more headspace at the same pressure. The overall flexural modulus would probably be less than that of the polypropylene laminate (1.40 GPa). Also, polycarbonate laminates could be used. These are much stiffer than polypropylene (10 times). If such materials are used to make the container, then the flex area of the lid could be thicker and still function properly since in this situation because less head space displacement is necessary. Also, a stiffer material than the random copolymers discussed herein could be used in the lid. The material of the lid itself must not be too stiff or else the flex area will be insufficiently flexible to prevent paneling. Materials with a flex modulus of higher than 1.26 GPa are too stiff and will allow containers to panel. Materials which are commonly used in barrier packaging applications are ethylene vinyl alcohol copolymers and polypropylene. The former may have a flexural modulus of about 1.95 GPa and the latter about 1.40 GPa which is also the approximate modulus of a polypropylene/ethylene vinyl alcohol copolymer/polypropylene laminate. Clearly, the combination of these two materials in a laminate would be too stiff for the present application. However, it has been found that ethylene vinyl alcohol copolymers can be used in combination with copolymers of propylene and a minor amount of ethylene which have a flexural modulus in the range of 0.91 GPa. When these materials are combined in a laminate, the flexural modulus of the overall laminate can be 1.26 GPa or less, especially if the amount of the ethylene vinyl alcohol copolymers used in the laminate is from 5% to 20% by weight. The overall thickness of the lid itself, excluding the flex area, is also a major consideration to achieve the advantages of the present invention. If the lid thickness is too great, then the lid will not be flexible enough to deflect inwardly to prevent paneling. Also, if the lid is too thin, it will be unacceptable in oxygen barrier applications. The thickness of the lid is obviously related to the thickness of the sheet from which it is made. For example, if the thermoforming process described above is used, then the thickness of the sheet from which the lid is made can be no more than 48.10⁻⁶ m. The flex area of the lid must then be thinner. The performance of thermoplastic lids was tested as follows. Sheets were formed of a copolymer of propylene and 2% ethylene (MFI 2) on the outside and a layer of an ethylene vinyl alcohol copolymer on the inside wherein the ethylene vinyl alcohol copolymer comprises 10% of the total weight of the sheet. Lids were formed by pressure forming, the polypropylene composition sheet was heated to its melting point and then clamped between a forming cavity and a hollow pressure box. Compressed air was applied to the hollow pressure box and traveled therethrough to stretch the sheet into the forming cavity. The lid deflection was measured by inducing a vacuum on the lid and measuring the displacement of the lid for a variety of pressure differentials. Displacement at a pressure differential of 4.82 kPa for flex area thickness of 38, respectively 48.10⁻⁶ m was 7.8, respectively 6.10⁻³ l. When repeating these tests with lids made of polypropylene homopolymer and ethylene-propylene copolymer comprising 2 %wt ethylene and having a thickness of the flex area of 38.10⁻⁶ m. At a pressure differential of 4.82 kPa the displacement found was 6, respectively 11.5.10⁻³ l.

Claims: Container sealed with a lid (10), each being formed from a laminate of propylene/ethylene copolymer and, as oxygen barrier material, ethylene/vinyl alcohol copolymer, said lid comprising a flange (18) attached to said container (20) and a recessed centre portion incorporating a flex area (12) provided with a raised ridge (14) close to the outer edge of the centre portion, characterised in that the thickness of the flex area is less than that of the rest of the lid and is at most 15 x 10⁻⁶ m, whereby a pressure differential between container content and the atmosphere of 4.82 kPa will cause flexing of the lid centre portion (16) sufficient to displace at least 6 x 10⁻³l of head space within the container. Sealed container as claimed in claim 1 wherein the lid includes a pour spout (20;22) which is raised above the level of the recessed centre portion (16) and which has a sealable opening therein to allow access to the contents of the container.

Inventors: THOMAS JOHN SHERIDAN JR, THOMAS, JOHN SHERIDAN, JR.
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: B65D51
IPC codes: B65D77, B65D79