Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2002-02-22
2004-08-03
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S081000, C521S139000
Reexamination Certificate
active
06770683
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to polyolefin foams and, more particularly, to extruded foams comprising a blend of ethylene/styrene interpolymer and low density polyethylene.
Polyolefin foams, particularly polyethylene foams, and methods for manufacturing such foams are well known in the art. See, e.g., U.S. Pat. No. 5,348,984 (Lee), U.S. Pat. No. 5,462,974 (Lee), and U.S. Pat. No. 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 polyethylene (PE) possesses a number of beneficial physical and chemical properties when used to make foam, it also has a number of disadvantages. For example, the compression strength of foam made from PE is less than would otherwise be desired for certain applications, such as when foam is used for packaging, i.e., to serve as a cushion to absorb impacts, prevent movement, and otherwise protect packaged articles during shipment and storage. Increased compressive strength is desired in such applications because the foam is better able to withstand large impacts and bear the weight of relatively heavy objects, such as machine parts, equipment, furniture, etc., without collapsing.
Another disadvantage associated with the use of PE to make foams is that PE, particularly, LDPE, foams over a very narrow temperature range, resulting in a foaming temperature that must be tightly controlled to within +/−0.5° F. If the foaming temperature is too high, the foam collapses; if too low, the polymer either will not foam or the foam will have regions of solid, non-foamed polymer. As will be appreciated by those skilled in the art of foam production, it is quite a challenge to maintain the proper foaming temperature within such a tight tolerance when producing PE foam on a commercial scale, i.e., at flow rates in excess of 800 pounds/hour.
Further, an ongoing challenge in the production of PE foams is achieving the highest possible degree of uniformity of the cell structure within the foam, both in terms of the shape of the individual cells and also the distribution of the cells within the foam. Generally, the higher the degree of cell uniformity, the better will be the physical/mechanical properties of the foam.
Accordingly, a need exists in the art for an improved foam that overcomes the foregoing disadvantages and challenges.
SUMMARY OF THE INVENTION
The inventors hereof have determined that the compression strength of PE foams can be improved and the foaming temperature tolerance increased to +/−1° F. by blending ethylene/styrene interpolymer (“ESI”) with the PE. In addition, the inventors found that the addition of ESI to PE provides a higher degree of cell uniformity.
Surprisingly, however, the inventors discovered that if more than 5 wt. % ESI is added to PE, the improvements in compression strength cease to exist and, perhaps more surprisingly, the compressive strength of the resultant foam deteriorates to such an extent that it is less than that of foam made from PE alone (i.e., with no added ESI).
Moreover, the inventors found that an ESI content above 5% caused the foam to exhibit more than 10% “creep.” Creep, which may be determined in accordance with ASTM D3575-93, suffix BB, provides a measure of a foam's tendency to “flow” away from a region of pressure applied to the foam, thereby reducing the thickness of the foam in the area where pressure is applied. Creep is particularly problematic when foam is used in load-bearing packaging applications because the amount of cushioning provided by the foam diminishes over time as creep progresses. In general, a foam that exhibits more than 10% creep is commercially unacceptable for commercial packaging applications. When less than 5 wt. % ESI is blended with PE, the resultant foam was found to exhibit less than 10% creep.
Further, PE/ESI foams with less than 5% ESI were found to have more uniform cell distribution and a lower percentage of open cells than foams having more than 5% ESI. As noted above, more uniformity in the distribution of cells results in better mechanical properties. In addition, for packaging purposes, open cells are undesirable as they decrease the cushioning performance of the foam. Thus, a lower percentage of open cells is beneficial.
Accordingly, the present invention is directed to a foam, comprising a blend of
(a) from 0.1 to 4.9 weight percent ethylene/styrene interpolymer; and
(b) from 95.1 to 99.9 weight percent polyethylene homopolymer or copolymer, said weight percentages being based on the total amount of (a) and (b) in the blend.
Another aspect of the invention is directed toward a method for making a foam, comprising
(a) blending (1) from 0.1 to 4.9 weight percent ethylene/styrene interpolymer; and (2) from 95.1 to 99.9 weight percent polyethylene homopolymer or copolymer, said weight percentages being based on the total amount of (1) and (2) in the blend;
(b) mixing a blowing agent with the blend of step (a); and
(c) causing the blowing agent to expand within the mixture of step (b), thereby forming a foam.
DETAILED DESCRIPTION OF THE INVENTION
The ethylene/styrene interpolymer (ESI) used in accordance with the present invention belongs to a class of substantially random interpolymers that comprise polymer units derived from one or more &agr;-olefin monomers with one or more vinyl or vinylidene aromatic monomers and/or a hindered aliphatic or cycloaliphatic vinyl or vinylidene monomers, and optionally, one or more other polymerizable ethylenically unsaturated monomer(s).
The term “substantially random interpolymers” as used herein means that the distribution of the monomers of said interpolymer can be described by the Bernoulli statistical model or by a first or second order Markovian statistical model, as described by J. C. Randall in POLYMER SEQUENCE DETERMINATION, Carbon-13 NMR Method, Academic Press New York, 1977, pp. 71-78. Preferably, substantially random interpolymers do not contain more than 15 percent of the total amount of vinyl or vinylidene aromatic monomer in blocks of more than 3 units. More preferably, the interpolymer is not characterized by a high degree of either isotacticity or syndiotacticity. This means that in the carbon
−13
NMR spectrum of the substantially random interpolymer, the peak areas corresponding to the main chain methylene and methine carbons, representing either meso diad sequences or racemic diad sequences, should not exceed 75 percent of the total peak area of the main chain methylene and methine carbons.
Suitable &agr;-olefins include, for example, &agr;-olefins containing from 2 to about 20, preferably from 2 to about 12, more preferably from 2 to about 8 carbon atoms. Particularly suitable are ethylene, propylene, butene-1, pentene-1,4-methyl-1-pentene, hexene-1 or octene-1 or ethylene in combination with one or more of propylene, butene-1,4-methyl-1-pentene, hexene-1 or octene-1. These &agr;-olefins do not contain an aromatic moiety.
Suitable vinyl or vinylidene aromatic monomers, which can be employed to prepare the interpolymers, include, for example, those represented by the following formula:
wherein R
1
is selected from the group of radicals consisting of hydrogen and alkyl radicals containing from 1 to about 4 carbon atoms, preferably hydrogen or methyl; each R
2
is independently selected from the group of radicals consisting of hydrogen and alkyl radicals containing from 1 to about 4 carbon atoms, preferably hydrogen or methyl; Ar is a phenyl group or a phenyl group substituted with from 1 to 5 substituents selected from the group consisting of halo, C
1-4
-alkyl, and C
1-4
-haloalkyl; and n has a value from zero to about 4, preferably from zero to 2, most preferably zero. Exemplary vinyl or vinylidene aromatic monomers include styrene, vinyl toluene, &agr;-methyl styrene, t-butyl styrene, chlorostyrene, including all isomers of these compounds, and the like. Particularly suitable such monomers include styrene and lower alkyl- or halo
Greco Thomas
Ramesh Natarajan S.
Foelak Morton
Lagaly Thomas C.
Sealed Air Corporation (US)
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