The present invention relates to multi-layered plastic containers. More particularly the present invention relates to containers having a transparent and colored body.
Plastic containers or other plastic structures are often used to store food products. Polypropylene is a common material used for such containers because of its mechanical and thermal properties. However, polypropylene has inadequate gas barrier properties by itself. Therefore, it is typical for polypropylene to be part of a composite container comprising multiple layers. For example, a container often has one or more layers of polypropylene and one or more layers of a barrier material. Barrier materials include EVOH, nylon, PVDC, or other materials having relatively high barrier properties.
The containers typically include tie layers for adhering the polypropylene layers and barrier material layers together. Maleated polypropylene is a common tie layer.
Often for marketing and general aesthetic purposes, it is desirable for the container to be transparent or colored. However, polypropylene is opaque because of differences in refractive indices between the amorphous phase and the crystalline phase. The clarification of polypropylene may be improved with the use of clarifying agents. However, clarifying agents generally are not effective with maleated polypropylene. Therefore, tie layers often contribute greatly to haze in a container.
Colorants, such as pigments or dyes, are applied to some plastics to provide the plastics with a particular color. Pigments are insoluble while dyes are soluble in polymers. Polypropylene has a chemical structure that does not accept coloring dye well. Furthermore, pigments usually compromise the clarity of polypropylene.
The present invention aims to address one or more of the above problems. The present invention provides a multi-layered container having improved clarity and color. The container defines an interior for storing product, such as food products. The wall of the container includes at least a first layer, a second layer, and a third layer. According to an embodiment of the present invention, the first layer comprises an ethylene-vinyl alcohol resin. The second layer comprises a polypropylene resin and a colorant. The third layer comprises an ethylene-vinyl alcohol resin. It is believed that the first and third layers inhibit the colorant of the second layer from migrating away from the second layer and into the first layer and the third layer.
According to some embodiments, the colorant may be a dye and the polypropylene resin may include a clarifying agent. The second layer may further include a maleated polypropylene to promote bonding between the second layer and the first and third layers.
According to another embodiment of the present invention, the container includes one or more polypropylene layers, one or more ethylene-vinyl alcohol layers, and at least one tie layer. At least one of the polypropylene layers includes a transparent polymeric colorant. The tie layer or layers promote bonding between the polypropylene layers and the ethylene-vinyl alcohol layers. The tie layers may include a mixture of a polypropylene and a maleated polypropylene. For example, the tie layer may include a polypropylene and a maleated polypropylene mixture of approximately one to one. The polypropylene in one or more polypropylene layers or tie layers made include a clarifying agent.
In yet another embodiment, the present invention provides a multi-layered container having a first layer of a polypropylene resin of a clarified grade, a second layer of a polypropylene resin of a clarified grade, and a tie layer of an adhesive mixture for bonding the first layer to the second layer, the mixture including a polypropylene of a clarified grade and a maleated polypropylene. At least one of the layers includes a transparent polymeric colorant. The polypropylene resin of the first layer and the second layer and the polypropylene in the tie layer may be substantially the same. The container may include additional layers of the polypropylene resin of a clarified grade.
According to another embodiment, the multi-layered container includes first and second layers of a polypropylene resin of a clarified grade, a first tie layer and a second tie layer of an adhesive mixture, and a layer of an ethylene-vinyl alcohol resin. The layer of the ethylene-vinyl alcohol resin is bonded to the first layer and the second layer of the polypropylene resin by the first and second tie layers, respectively.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
The present invention provides a container having improved clarity and color. The container includes a multi-layered wall that defines an interior for receiving and storing articles such as food products or beverages. As shown in
As further described below, the wall includes two or more layers. For example, the wall may include one or more layers comprising an ethylene-vinyl alcohol resin, one or more layers comprising a polypropylene resin, and one or more tie layers comprising an adhesive or adhesive mixture. In general, adjacent polypropylene layers and ethylene-vinyl alcohol layers may be adhered together by the tie layers.
One aspect of the present invention is the clarity or transparency of the wall and, thus, the container. In order to improve the clarity of the wall, the polypropylene resin in one or more of the layers includes a clarifying agent. As used herein, a clarifying agent is an additive for increasing the transparency of the polypropylene. In general, clarifying agents are a sub-class of nucleating agents. For example, sorbitol derivatives or sorbitol acetals are clarifying agents. Sorbitol derivatives and other clarifying agents are available from Milliken & Company of Spartanburg, S.C. Also, as used herein a clarified grade of polypropylene is a polypropylene containing a clarifying agent.
The present invention also provides a tie layer formulated to minimize haze within the wall. For example, one or more tie layers may include an adhesive mixture of polypropylene and maleated polypropylene. The polypropylene may be of a clarified grade. The mixture may be formed by dry blending a polypropylene resin with a maleated polypropylene resin. It is believed that the maleated polypropylene provides an adhesive quality for encouraging the bonding between adjacent layers, while the mixture with the clarified polypropylene minimizes the negative contribution to the haze of the wall that is normally associated with conventional tie layers.
A tie layer according to an embodiment of the present invention was tested against a conventional tie layer. More specifically, a first sheet comprising a first layer of a polypropylene resin, an intermediate tie layer, and a second layer of a polypropylene resin was tested against a second sheet comprising a first layer of a polypropylene resin, an intermediate tie layer, and a second layer of a polypropylene resin. Each sheet had an overall thickness of 100 mils, wherein each first and second layer had a thickness of 48 mils and each intermediate layer had a thickness of 4 mils. In the first sheet, the intermediate tie layer included a maleated polypropylene. In the second sheet, the intermediate tie layer included a mixture of a polypropylene and a maleated polypropylene. The mixture was formed by dry blending the two together at a ratio of one to one by weight. For testing purposes, the polypropylene resin used in the layers, including the tie layer of the second sheet, was a clarified grade of a copolymer having a melt index of 2.2, density of 0.903 g/cc and tensile strength of 4500 psi. The maleated polypropylene used in both the tie layer of the first and second sheets was a copolymer having a melt index of 5.7, density of 0.89 g/cc, a melting point of 140° C., a 60D-shore hardness, and a stress at yield of 2600 psi. The clarity of each sheet was measured with the following results:
The clarities were measured as a percent of light transmission by using BYK Gardner Haze-Gard Plus equipment. In general, the results indicate a 30% improvement in light transmission by blending maleated polypropylene and clarified polypropylene.
Another aspect of the present invention is providing the wall of the container with a color quality. One or more of the layers may include a colorant. The colorant may be a dye, a pigment, or other agent used to impart a color or shading to a material, such as polypropylene. Colorants are known in the art and available through companies such as Milliken & Company of Spartanburg, S.C.; ColorMatrix Corporation of Berea, Ohio; Polyone Corporation of Avon Lake, Ohio; Ampacet Corporation of Tarrytown, Nev.; and others.
According to another embodiment of the present invention, the wall includes at least three layers. For example and as shown in
The layer of polypropylene 22 may further include an adhesive, such as maleated polypropylene, for encouraging the second layer 22 to bond with the first and third layers 20, 24 of the ethylene-vinyl copolymer resin. Or the wall 12 may further include an adhesive between the layers 20, 22, 24 to encourage bonding. The polypropylene resin may be of a clarified grade.
According to yet another embodiment, the wall includes one or more polypropylene layers, one or more ethylene-vinyl alcohol layers, and at least one tie layer. The tie layer or layers are between adjacent polypropylene and ethylene-vinyl alcohol layers. The tie layers are for encouraging the adjacent polypropylene and ethylene-vinyl alcohol layers to bond together.
For example and as shown in
The polypropylene resin characteristics may vary. Another example of a polypropylene resin is a clarified grade of metallocene homopolymer having approximately a melt index of 1.5, density of 0.9 g/cc, and a tensile strength of 4400 psi.
As shown in
One or more of the polypropylene layers, including a tie layer, may include a colorant. For example, the colorant may be a transparent polymeric colorant provided by Milliken & Company sold under the trademark of ClearTint® colorant.
The seven-layer embodiment illustrated in
Three types of sheets were tested based on the colorant added or lack thereof to the first and fourth polypropylene layers 50, 62. In the clear sheets, no colorant was added to any of the layers. In the red sheets, the ClearTint® colorant in the shade of red was added to the first and fourth polypropylene layers 50, 62. In the blue sheets, the ClearTint® colorant in the shade of blue was added to the first and fourth polypropylene layers 50, 62. As stated, each sheet was thermoformed into a container and each container was tested for clarity and haze using a BYK Gardner Haze-gard Plus and an Illuminant CIE-C meter. The results are tabulated below, with the number after the color representing the overall measured thickness in mils of the seven-layer wall.
One in the art would appreciate that a heat stabilizer, process stabilizer, optical brightener, light stabilizer, internal lubricant, external lubricant, or any other property modifier may be added to any of the above layers.
Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.