Foamable composition using high density polyethylene

Details Foamable composition using high density polyethylene Foamable composition using high density polyethylene

2000-01-11

2001-05-01

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...

C521S134000, C521S139000, C525S240000

Reexamination Certificate

active

06225363

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a polymeric composition to be used in producing foam. Specifically, the polymeric composition is comprised of a high density polyethylene, an alkenyl aromatic polymer and a resiliency modifier resin.
BACKGROUND OF THE INVENTION
Low density foam, such as polystyrene foam, is commonly made by combining a physical blowing agent with a molten polymeric mixture under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
From about the 1950's to the present, physical blowing agents of choice have included halocarbons, hydrocarbons or combinations thereof. Examples of these include commercially available halocarbon compositions such as dichlorodifluoromethane, trichlorofluoromethane and mixtures thereof, and the C
2
-C
6
hydrocarbons. During the 1980's, the worldwide scientific community presented sufficient evidence linking chlorofluorocarbons (CFCs) with atmospheric ozone depletion and sought governments to regulate CFCs. As of a result of such regulations, hydrocarbons are generally the choice of physical blowing agents.
There are two foams that are commonly produced. The first foam is made from polystyrene and the second foam is made from low density polyethylenes (LDPEs). Pure polystyrene foam is too brittle for some applications such as protective packaging which require protection from multiple impacts.
LDPE foams are generally considered to be resilient and non-brittle, which are desirable properties. The LDPE foams, however, have disadvantages such as adding a stability control agent (also referred to as a permeation modifier) to the polymeric composition so as to produce a commercially acceptable foam (e.g., a foam that does not change its dimensions significantly over time).
The amount of total residual blowing agent in the LDPE foam immediately after its manufacture is typically in the range of from about 5 to about 10 weight percent of the polymeric composition. This amount is dependent upon factors such as is the desired density of the foam and the selected blowing agent. This amount of total residual blowing agent generally produces a potentially flammable condition if the foam is located in a confined area. Typically, the aging process for an LDPE foam containing a stability control agent takes from about 14 to about 30 days. The aging process is dependent upon a number of factors including, but not limited to, the density of the foam, the selected blowing agent and storage temperature of the foam.
Accordingly, a need exists for foams that overcome the above-noted shortcomings associated with existing foams.
SUMMARY OF THE INVENTION
The polymeric composition to be used in producing foam of the present invention comprises from about 5 to 45 weight percent of a high density polyethylene (HDPE), from about 3 to about 45 weight percent alkenyl aromatic polymer, and from about 10 to about 85 of a resiliency modifier resin. The HDPE resin has a z-average molecular weight, M
z
, greater than about 1,000,000. The foam of the present invention is produced with a stability control agent, generally in amounts less than traditional LDPE only foams.
According to one process for preparing a polymeric foam of the present invention, a high density polyethylene, an alkenyl aromatic polymer and a resiliency modifier resin are melted to form a polymeric composition. The polymeric composition comprises from about 5 to 45 weight percent of high density polyethylene, from about 3 to about 45 weight percent of alkenyl aromatic polymer and from about 10 to about 85 weight percent of the resiliency modifier resin. The high density polyethylene has a z-average molecular weight, M
z
, greater than about 1,000,000. A stability control agent is added to the polymeric composition. An effective amount of blowing agent is dissolved to form a mixture. The mixture is transferred to an expansion zone and is permitted to expand in the expansion zone to produce the polymeric foam.
A polymeric foam of the present invention may be prepared by the above described steps. The polymeric foam has a cross-machine direction tensile toughness greater than about 33 KJ/m
3
.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The polymeric composition to be used in producing foam of the present invention comprises high density polyethylene (HDPE) from about 5 to 45 weight percent, alkenyl aromatic polymer from about 3 to about 45 weight percent, and a resiliency modifier resin from about 10 to about 85 weight percent. The preferred polymeric composition comprises HDPE from about 15 to about 40 weight percent, alkenyl aromatic polymer from about 10 to about 25 weight percent, and a resiliency modifier resin from about 60 to about 85 weight percent.
The most preferred polymeric composition comprises HDPE from about 15 to about 30 weight percent, alkenyl aromatic polymer from about 10 to about 20 weight percent, and a resiliency modifier resin from about 65 to about 75 weight percent. It is contemplated that more than one HDPE, alkenyl aromatic polymer and/or resiliency modifier resin can comprise the respective HDPE, alkenyl aromatic polymer and resiliency modifier resin weight percents of the polymeric composition. For example, two HDPE resins (each 15 weight percent) can be blended to comprise 30 weight percent HDPE of the polymeric composition.
HDPEs
The high density polyethylene (HDPE) of the present invention has a specific gravity of from about 940 to about 970 kg/m
3
, and a z-average molecular weight, M
z
, greater than about 1,000,000. The M
z
preferably is greater than about 1,200,000 and most preferably is greater than about 1,400,000. The z-average molecular weight (M
z
) is characterized by a concentration of extremely high molecular weight polymer chains (i.e., those near an upper end of the molecular weight distribution).
The HDPE of the present invention generally has a melt flow index (MI) in the range of from about 0.05 to about 2.8 dg/min. as measured by ASTM D1238 (nominal flow rate at 190° C. and 198.2 kPA). In general, the high density ethylene polymer should have a melt flow index of less than about 10 dg/min., and preferably less than about 3 dg/min.
The preferred HDPE is uncrosslinked and has a specific gravity of from about 943 to about 951 kg/M
3
, a melt flow index in the range of from about 0.18 to about 0.28 dg/min., a weight average molecular weight, M
w
, in the range of from about 223,000 to about 233,000, a number average molecular weight, M
n
, in the range of from about 12,500 to about 16,500, and a polydispersity index, D=M
w
/M
n
, from about 12 to about 20. The polydispersity index that is most preferred is from about 14 to about 18.
The HDPE of the present invention may be obtained by blending two or more HDPEs. For instance, an HDPE having an M
z
of 1,100,000 may be blended with a second HDPE having an M
z
of 1,500,000. It is contemplated that the HDPE of the present invention may include an HDPE having an M
z
of 800,000 blended with a second HDPE having an M
z
of 1,600,000 as long as the composite M
z
is greater than about 1,000,000. The most preferred HDPE has a bimodal distribution of molecular weight.
It is contemplated that the HDPE of the present invention may comprise a copolymer of at least 50 mole percent of a ethylene unit and a minor (i.e., less than 50%) proportion of a monomer copolymerizable with the ethylene unit. It is contemplated that the term HDPE of the present invention may also include physical blends of two or more different homopolymers that are classified as HDPEs or physical blends of at least 50 percent by weight of an ethylene homopolymer with another predominately high density polyethylenic copolymer. The physical blends are combined in a dry form after the blend components have previously been polymerized.
Alkenyl Aromatic Polymer
The term “alkenyl aromatic polymer” as used herein includes polymers of aromatic hydrocarbon molecules that contain an aryl group joined to an olefinic group with only double bonds

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