Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Reexamination Certificate
1999-03-15
2001-06-12
Foelak, Morton (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
C264S053000, C521S079000, C521S142000, C521S143000
Reexamination Certificate
active
06245266
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to polyolefin foams and, more particularly, to extruded and oriented foam sheets comprising polyethylene.
Polyethylene foam sheets and methods for manufacturing polyethylene foam sheets are well known in the art. See, e.g., U.S. Pat. Nos. 5,348,984 (Lee), 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE). While LDPE possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of LDPE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications.
One method that has been proposed for improving the tensile strength, cushioning performance, and flexibility of extruded foams formed from a food product starting material is disclosed in JP 10-70973, which proposes that such foams be stretched immediately after extrusion at a take-off rate that is from 1.5 to 3 times higher than the extrusion rate.
The inventors hereof have found, however, that in the production of polyethylene foam sheets, a take-off rate/extrusion rate ratio (hereinafter referred to as a “draw ratio”) ranging from 1.5 to 3 as taught in JP 10-70973 detrimentally reduces the dimensional uniformity of the resultant foam sheet. Such non-uniformity was most prominently manifested in the thickness dimension of the foam sheet, with a high degree of variation in the thickness of the foam occurring when the draw ratio was increased above 1.5. Such variation presents a poor aesthetic appearance and surface feel and, more significantly, indicates that the mechanical properties of the foam (strength, flexibility, etc.) are non-uniformly distributed throughout the foam. Any polyethylene foam sheets made in accordance with JP 10-70973 would therefore be commercially unacceptable.
Accordingly, a need exists in the art for a process of making polyethylene foam sheet having improved flexural modulus but with a low degree of variation in the thickness of such foam.
SUMMARY OF THE INVENTION
That need is met by the present invention, which provides a method for making an oriented foam, comprising the steps of:
a. blending molten polyethylene and a blowing agent to form a foamable mixture;
b. producing a foam by extruding the mixture at a predetermined rate through a die and into a region of reduced pressure so that the blowing agent expands within the mixture; and
c. orienting the foam by drawing the foam out of the die at a rate that is between 1 and 1.5 times greater than the predetermined rate at which the mixture is extruded.
By orienting polyethylene foam sheet at a draw ratio of less than 1.5 in accordance with the method of the present invention, the resultant foam sheet has been found to exhibit dramatic increases in stiffness with minimal thickness variation, typically less than 5% variation in thickness, which results in commercially acceptable foams that are both aesthetically and structurally superior to both non-oriented foams and foams oriented at a draw ratio greater than 1.5.
In accordance with another aspect of the present invention, an oriented foam sheet is formed from a foamable mixture comprising polyethylene having a density of less than about 0.921 g/cc, the foam sheet having an orientation ratio between 1 and 1.5 and having been foamed at a temperature ranging from about 0.5 to about 2° F. above the crystallization temperature of the foamable mixture. Such foams foam provides excellent stiffness and minimal thickness variation, along with economic and processing advantages associated with the employment of lower density LDPE resin as described in further detail below.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, a method for making an oriented foam comprises the steps of:
a. blending molten polyethylene and a blowing agent to form a foamable mixture;
b. producing a foam by extruding the mixture at a predetermined rate through a die and into a region of reduced pressure so that the blowing agent expands within the mixture; and
c. orienting the foam by drawing the foam out of the die at a rate that is between 1 and 1.5 times greater than the predetermined rate at which the mixture is extruded.
Preferably, the polyethylene comprises at least one member selected from the group consisting of low density polyethylene, high density polyethylene, and ethylene/alpha-olefin copolymer. More preferably, the polyethylene comprises low density polyethylene (LDPE).
Any LDPE may be used in accordance with the present invention. Preferably, the LDPE has a density ranging from about 0.91 to about 0.93 g/cc, a crystallinity ranging from about 10% to about 55%, and a softening point (DSC melt peak) ranging from about 95° C. to about 130° C. More preferably, the density ranges from about 0.91 to about 0.925 g/cc, the crystallinity ranges from about 20% to about 50%, and the softening point ranges from about 104° C. to 115° C. The melt index (MI) of the LDPE may range from 0.1 to over 50 g/10 min. but preferably ranges from about 0.3 to about 10 g/10 min. More preferably, the MI ranges from about 1 to 5 g/10 min.
The ethylene/alpha-olefin copolymer can be heterogeneous (Ziegler-Natta catalyzed) or homogeneous (metallocene (single-site) catalyzed) as desired, and may have a density ranging from about 0.89 to about 0.91 g/cc.
The density of the polyethylene resin is preferably as low as possible. Relative to a higher density resin, a lower density resins is generally easier and less expensive to process in that they require less power from the processing equipment, primarily the extruders, and also require lower processing temperatures. In addition, lower density resins allow for a wider range of processing temperatures (exit die temperatures) within which good, commercial quality foams can be made. Thus, in general, the higher the density of the resin, more difficult and expensive it is to process foams and the narrower the processing temperature window is within which quality foams can be made.
One drawback to using a lower density resin in the manufacture of polyethylene foams is that foam stiffness generally decreases with decreasing resin density. However, the inventors have surprisingly discovered that foams made with lower density polyethylene resins in accordance with the present invention, i.e., formed at a draw ratio ranging from 1 to 1.5, have comparable stiffnesses to foams made with higher density resins. Specifically, in the case of LDPE, while a resin of any density can be used as noted above, the density of the LDPE resin is preferably less than 0.925 g/cc, more preferably less than 0.923, more preferably still less than 0.921, and most preferably less than 0.920 g/cc. As shown in the Examples below, foams made in accordance with the invention with a LDPE resin having a density of 0.918 g/cc have a stiffness that compares favorably with foams made with a LDPE resin having a density of 0.925 g/cc.
Any conventional chemical or physical blowing agents may be used. Preferably, the blowing agent is a physical blowing agent such as nitrogen, argon, carbon dioxide, methane, ethane, propane, butanes (e.g., n-butane or isobutane), pentanes, hexanes, butadiene, acetone, methylene chloride, any of the chlorofluorocarbons, hydrochlorofluorocarbons, halogenated hydrocarbon blowing agents, hydrofluorocarbons, alcohols, ketones, ethers, or aromatic hydrocarbons, as well as mixtures of the foregoing.
The blowing agent may be mixed with the polyethylene in any desired amount to achieve a desired degree of expansion in the resultant foam. Generally, the blowing agent may be added to the polyethylene in an amount ranging from about 0.5 to 80 parts by weight, based on 100 parts by weight of the polyethylene. More preferably, the blowing agent is present in an amount ranging from 1 to 30 and, most preferably, from 2 to 15 parts per 100 parts by weight of the polyethylene.
George Lloyd
Ramesh Natarajan S.
Foelak Morton
Lagaly Thomas C.
Sealed Air Corp. (US)
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