DEPENDABLE THERMOSELLS AND METHOD TO PRODUCE THEMSELVES
IHTJROpyCCIQW This application relates to removable heat seals between two sheets of packaging material and a method of producing these seals, in a form that will provide a clean separation between the sheets when the latter are separated by peeling. More particularly, the invention relates to a clean removable thermosel, between a first olefin sheet linked by centrifugation, fibros, as sold under the trademark TYVEK by DuPont, and a second polymeric thermoplastic sheet. The invention has particular application to the packaging of medical supplies and instruments, because the produced thermoseal creates an interface that exhibits good seal integrity, provides a controlled release resistance, exhibits minimal tearing of fibers of the bound olefin by centrifugation and when open by detachment, results in a visual whitening of the polymeric material in the thermoseal area, thereby providing evidence of good initial seal integrity. Typical applications will involve medical packaging in various forms such as bags, rigid packs or trays with lid materials, or other types of flexible packaging.
BACKGROUND OF THE INVENTION Olefin sheets linked by centrifugation have been used for a number of years for various types of packaging. The sheets are formed of high density polyethylene fibers that are centrifuged in continuous strands and bound together with heat and pressure. Because the sheets formed of this material are highly opaque, it is often convenient to form one side of a container or package of a transparent lamination, such as a polyester and polyethylene composite, which is thermosealed in the spin-dried olefin sheet. . In order to open the sealed package, the transparent sheet is detached from the olefin sheet liquefied by centrifugation. In the packaging of medical articles, it is essential that when opening the package, the seal between the opposite sheets separates cleanly without substantial torn or frayed, and without the production of threads that may affect the sterile condition of the medical articles contained in the package. package. It has been particularly difficult to obtain a seal for satisfactory detachment between the first olefin sheet bound by centrifugation and the second transparent sheet preferably of polymeric thermoplastic thermosetting material. This seal should be easy to open and highly reliable, but should provide an extremely low incidence of tearing and fraying of fibers, and it is highly desirable that the seal, when opened, provide on the second transparent sheet, a clear indication of the initial integrity of the seal. Various attempts have been made to produce a seal that will separate cleanly at the interface between an olefin sheet bound by centrifugation, and the transparent polymer sheet. One effort is set forth in Patent No. 4,125,985, dated November 21, 1978, which attempts to provide a removable seal container or receptacle, with low incidence of tearing or fraying. However, this process involves the pretreatment of the olefin sheet bound by centrifugation by the application of a heated matrix in the form of the desired release seal. This procedure is difficult to control. Heat and pressure must be applied very carefully to obtain the desired effect and even when performed optimally, it still results in tearing of fibers when the package is opened. Other prior methods employed to produce a peelable seal between spin-linked olefin sheets and a sheet of thermoplastic, synthetic polymeric heat-sealable sheet material have involved coating the spin-linked olefin sheet to provide a convenient seal surface for the thermoplastic sheet . Nevertheless, spin-linked olefin material has inhomogeneous surface characteristics that tend to affect the seal quality, and the coating of spin-linked olefin material not only increases the cost, but also changes the porosity characteristics with respect to polyolefin bound by uncoated centrifugation. Another attempt to achieve a satisfactory peelable seal between a spin-linked olefin sheet and a thermoplastic polymeric heat sealable material, such as a oriented polyester, has been to apply primer to the polyester and then coat the oriented and primed polyester with a low polyethylene. density. However, the seal produced is one that has inconsistent loosening characteristics and a narrow thermosel range. It also often results in fiber tearing due to its narrow thermosel range. Various suggestions have been made (see U.S. Patent No. 4,189,519) to produce peelable fabrics, but these have not been applied to a fibrous sheet material such as that formed from centrifuged olefin. These sheets are generally designed to be sealed to each other or very similar materials, not fibrous materials such as spin-bonded polyolefin. Films that seal well with themselves tend to have a narrow range of heat sealing temperatures when sealed to spin-linked polyolefin and do not easily or cleanly peel off when opened. It is well known in the art that at high levels such as 25% polyolefin, these materials are sealed together with such a small opening force that it is unacceptable to pack even the lightest articles. COMPENDIUM pg THE INVENTION In accordance with this invention, a package is provided which includes a first olefin sheet bonded by spin and a second sheet of thermoplastic polymeric heat sealable material. It is preferable that the thermoplastic material be transparent. The package includes a peelable seal between the first and second sheets by means formed by heat and pressure. The second preferred sheet is a lamination that can be achieved by various means, one of which is to apply a primer to a polyester layer, then laminate an extrusion of that layer to a coextruded sealer using low density polyethylene (homopolymer). A coextruded layer of the seal preferably is a low density polyethylene (homopolymer) and the other co-extruded layer of the sealant is preferably a mixture of a polyethylene ethylene vinyl acetate copolymer (preferably in the order of 75% of the mixture) and a polybutylene resin (preferably in the order of about 25% of the mixture). The ethylene vinyl acetate copolymer is preferably a 2% vinyl acetate composition and the mixture forms the outer surface of a co-extruded sealant, ie the surface to be thermosellated to the spin-linked olefin sheet. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a heat-sealed portion of a package made in accordance with this invention, showing the various heat-sealing layers between the first and second package sheets. Figure 2 is a cross-sectional view of a package showing thermosets between the two sheets on the sides and a product such as a sterile medical instrument disposed within the package. DETAILED DESCRIPTION OF THE PREFERRED MODALITY Figure 1 shows a portion of a package constructed in accordance with this invention, wherein there is a first sheet 10 formed of an olefin bound by centrifugation, formed by centrifuging continuous strands of very fine interconnected fibers and then joining them together. in conjunction with heat and pressure. The olefin bound by centrifugation is a material as sold by DuPont under the trademark TYVEK. The second sheet 24 is a lamination of polymeric thermoplastic materials which are heat sealable at least on their inner surface and preferably transparent. The outer layer 12 is preferably orientated polyester film of size forty-three to forty-eight. It is contemplated that this layer 12 can be constructed of other materials such as polyethylene, polypropylene, nylon polyamide or other such materials.
According to this invention, the outer layer 12 is treated on its inner surface with a convenient primer 14 such as a PEI primer, and then a low density polyethylene (homopolymer) 16 is used for extrusion laminating the outer layer 12 to a 18 coextruded sealant with an approximate thickness of .381 mm (1.5 mils) with approximately .033 mm (1.3 mils) of that film, consisting of a layer of low density polyethylene (homopolymer) 20 and a second layer 22, approximately thickness of .00508 mm (0.2 mils) that forms an interior surface to heat seal the olefin sheet bound by centrifugation. This second layer 22 of sealant 18 is made of a mixture of a polybutylene resin and a copolymer of ethylene and vinyl acetate, with suitable quantities of anti-block and sliding processing aids, aggregates to give the proper characteristics of sealing, as is well known in the art. It is preferred that the copolymer of layer 22 be a 2% polyethylene ethylene vinyl acetate copolymer form approximately seventy-five parts of the mixture, with the polybutylene resin constituting the remaining twenty-five parts of the mixture constituting the inner surface layer about .00508 mm (0.2 mil) 22 of the co-extruded sealant 18. Layers 20 and 22 may also be based on polyethylene homopolymer or use any of the copolymers well known in the art, such as for example ethylene-acrylic acid (EAA ), ionomers such as Surlyn from DuPont, ethylene-α-olefin copolymer, ethylene methyl acrylate (EMA) or ethylene-methacrylic acid copolymers (EMAA). In addition to other copolymers and homopolymers, other polymeric materials may be employed as will be readily apparent to those skilled in the art. In the patent of the U.S.A. No. 4,189,519 there is a description of a process by which the outer surface layer 22 of the co-extruded sealant 18 can be prepared. The co-extruded sealant 18 can be produced in a circular die-blown film extruder. A convenient process for laminating transparent polyethylene sealant 18 to polyester sheet 12 is described in U.S. Pat. No. 4,421,823. After the extrusion lamination of the primed polymer sheet 12 with the co-extruded sealant 18, the second pouch wall 24 is brought into contact with the first spin-linked olefin sheet 10 around the perimeter of the package with the co-extruded layer of approx. 00508 mm (0.2 mil) 22 of the sealant 18 (consisting of the mixture of the polyethylene vinyl acetate copolymer and the polybutylene) in contact with the surface of the bound olefin sheet by centrifugation. The two sheets 10 and 24 are then heat sealed at the edges in the usual manner at a temperature in the range between 115.5 and 148.9 ° C (240 and 300 ° F). The temperature of the thermoseal is important because if it is too cold an adequate seal can not be obtained and if it is too hot, the selladpr binds too well to the olefin bound by centrifugation resulting in fiber tearing when opened. For best results, the seal strength should be between 178.7 and 536.2 g / cm (1-3 pounds per inch). a bag or package formed in this manner is illustrated in Figure 2. A product receiving chamber 26 is defined between the sheet 10 and the sheet 24 which are sealed around three edges. A produsto 28 is inserted into the chamber 26 through the open edge. The product can be a medical instrument that requires sterilization or any < ie a wide variety of other medical instruments or products. The final seal is then made on the edge that was left open. Figure 2 illustrates a bag or package wherein the thermal seal is made face-to-face between the layer 22 and the layer 10. It will also be possible to have any of the lower sheet 24 that extends beyond the side edge of the top sheet 10, or having the top sheet 10 extend beyond the bottom sheet 24, and then fold the longer sheet over the top edge of the shorter sheet and create a seal therebetween by means of a pressure sensitive tape in both sides, as is well known in the art or some other adhesive or other seal mechanism. The patent of the U.S.A. No. 4,276,982 demonstrates such a type of closure. A chevron-shaped thermal seal can be formed at the end of the opposite package from the end having the opening through which the product is inserted into the package. The package end having the gallon seal also includes fastening means formed by portions of the layers 10 and 24 which are not sealed together and which are located outwardly from the gallonized thermal seal. The clamping means facilitates the detachment of the layers 10 and 24 from each other, to open the package over the thermal seal that provides the appearance of "whitening" as evidence of seal integrity. It is important to facilitate the opening through a thermally sealed edge as opposed to an adhesively sealed edge. Only in this way can you ensure the appearance of "whitewashing" that demonstrates previous seal integrity. When this bag or package is placed in a sterilization device, the sterilization medium readily permeates the olefin layer bound by centrifugation to perform the sterilization function. The package or pouch of the present invention includes a release seal between the spin-linked olefin layer 10 and the layer 24 of polymeric thermoplastic thermosealing material. The seal has high integrity with minimal transfer of fibers from the olefin bound by centrifugation. The seal formed in the package or bag of the present invention, when opened to remove the product contained in the chamber 26, provides a uniform indication that the package has been previously sealed and the sign indicative of the integrity of the previous seal, due to the appearance of "bleached" on the portion of layer 22 that was previously sealed to layer 10. This sign results, because unlike the approaches of the prior art, polybutylene acts to weaken the structural strength of the layer sealing, not its surface characteristics. When sealing to olefin bound by centrifugation, it is not desired to achieve a fusion seal. To do so would result in fusing the centrifuged olefin that would destroy its strength and other beneficial physical properties. The seal of the package of the present invention causes the portions of the sealing layer 22 to circulate between the fibers of the spin-linked olefin, thereby creating a strong but removable bond. When the seal is detached to expose the contents to be removed, the seal fails internally leaving in place the portions that have circulated between the fibers. This can be confirmed by analytical tests such as surface infrared spectrometry, which is used to confirm the presence of polybutylene and EVA in the seal area of the olefin bound by TYVEK centrifugation after the packages were opened. Spunbonded olefin fibers are stronger than the internal strength of the sealant layer, such that there is no resultant fiber tear. It is this failure of the inner sealing layer that produces the "bleached" appearance indicative in layer 22, when it is peeled off. The present invention also produces a package that has a uniform seal strength so important for medical packaging applications. Various features of the invention have been particularly illustrated and described in connection with the illustrated embodiment of the invention, however it will be understood that these particular assemblies simply illustrate and that the entire interpretation will have to be given to the invention within the terms of the claims annexes.