Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-08-09
2003-04-08
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S221000, C525S222000, C525S240000
Reexamination Certificate
active
06545094
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
FIELD OF THE INVENTION
This invention describes resin formulations having a flexural modulus of greater than or equal to 100,000 psi or less than or equal to 30,000 psi, which also yield previously inaccessible high melt strengths at a given melt index. This invention also provides fabricated articles including foams made from these resin formulations.
BACKGROUND OF THE INVENTION
Blends of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) are known in the prior art. For example, Ghijsels et al., in “Melt Strength Behavior of Polyethylene Blends”, Intern. Polymer Processing VII (1992), p. 44-50, exemplifies blends of LDPE and LLDPE, where the LLDPE has a melt index (I2) of 0.1 g/10 min and the final blend densities are approximately 0.92 g/cm
3
, and which show a synergistic improvement in melt strength. A polyethylene resin density of 0.92 g/cc corresponds approximately to a flexural modulus of 40,000 psi. However, Ghijsels neither exemplifies nor gives any indication of the range of the ethylenic blend components in which synergy would be observed for blends having a flexural modulus greater than or equal to 100,000 psi, nor less than or equal to 30,000 psi.
U.S. Pat. Nos. 5,863,665 and 5,582,923 describe an ethylene polymer extrusion composition comprising from about 75 to 95 percent of at least one ethylene/&agr;-olefin interpolymer composition selected from the group consisting of a substantially linear ethylene polymer composition, a homogeneously branched linear 5 ethylene polymer composition and a heterogeneously branched linear ethylene polymer composition, (wherein the ethylene/&agr;-olefin polymer is characterized as having a density in the range of 0.85 g/cc to 0.940 g/cc) and from about 5 to 25 percent of at least one high pressure ethylene polymer characterized as having a melt index, I2, of less than 6.0 g/10 minutes, a density of at least 0.916 g/cc, a melt strength of at least 9 cN as determined using a Gottfert Rheotens unit at 190° C., a Mw/Mn ratio of at least 7.0 and a bimodal molecular weight distribution as determined by gel permeation chromatography, wherein the ethylene polymer extrusion composition has a melt index, I2, of at least 1.0 g/10 minutes. The blends of this composition would have flexural modulus less than about 115,000 psi. In contrast, the blends of the present invention would have flexural modulus ≧120,000 psi at comparable melt strength. Furthermore, this patent does not teach, exemplify or claim foams.
U.S. Pat. No. 4,649,001 describes a process for producing a polyethylene extruded foam, which comprises melting and kneading a composition of a polyethylene-based resin containing a foaming agent followed by extrusion-foaming. A linear low-density polyethylene having a broad molecular weight distribution is used as the polyethylene-based resin. The linear low-density polyethylene used has a density of 0.920 to 0.940 g/cm
3
; a melt flow rate of 0.3 to 10 g/10 min and a relationship between a weight average molecular weight and a number average molecular weight (Mw/Mn) greater than or equal to 4. Low density polyethylene of 0.918 to 0.923 g/cm
3
density may also be blended to make foams. The foams were all extruded. Cross-linked foams were not claimed. The highest flexural modulus of the resins used to make the extruded foams would correspond to about 120,000 psi (at a resin density of 0.940 g/cm
3
), but this would not be a blend. Furthermore, the density of the linear low-density polyethylene was 0.940 g/cm
3
or less.
U.S. Pat. No. 4,226,946 discloses polyethylene blend foams having density from about 3.0 to about 15.0 pounds per cubic foot, substantially closed-cell structure and average compressive strength at 10 percent deformation of from about 7 to about 170 psi, preferably about 7 to about 60 psi, and an improved method and a means for making the same from polyethylene blends and at least one blowing agent using gel-forming extrusion technology. The polyethylene blend comprises from about 35 to about 60 weight percent of low density branched polyethylene (0.910 to 0.930 g/cc density) in admixture with from about 40 to about 65 weight percent of intermediate density linear polyethylene (0.931 to 0.940 g/cc density). The densities of the resulting blends would be less than 0.9365 g/cc (corresponding to a flexural modulus less than about 100,000 psi) and greater than 0.9180 g/cc (corresponding to a flexural modulus greater than about 40,000 psi).
However, there is still a need for resin compositions which, while achieving a required flexural modulus ≧100,000 psi or ≦30,000 psi, can also exhibit high melt strength and/or melt extensibility, at a given melt index. We have surprisingly found that certain compositions exhibit synergistic improvements in melt strength and, in some cases, even more surprisingly, in melt extensibility at this melt strength. Branched resins can't achieve the modulus possible with linear polyethylene resins, and linear resins would have to have much lower melt index than branched resins of comparable melt strength. Furthermore, the melt strength achieved with the blends used in the present invention may exceed, at a given melt index, the melt strength achievable with any branched resin or linear resin at the same melt index and/or density. Consequently, the blends used in the present invention exhibit greatly improved melt strength compared with a linear polyethylene resin of the same density.
The extrusion foam manufacturing process requires a resin of sufficiently high melt strength to allow the bubble structure to maintain its integrity during the expansion process immediately after extrusion from the die. Prior to this invention, the only resins capable of meeting this requirement at the melt index suitable for processing (I2>0.5, preferably >1 g/10min) were branched resins such as LDPE, EVA and the like. Hence, the flexural modulus was limited to that obtainable with these branched resins (i.e. about 80,000 psi or less, equivalent to an LDPE of density less than or equal to approximately 0.930 g/cm
3
). It would be highly desirable to produce a foam using resin of higher modulus (>100,000 psi), as this allows the overall density of the foam to be reduced, while maintaining the compressive strength of higher density foam made from a branched resin (although this resin has the limitation of lower modulus).
The present invention describes blends comprising branched resins (eg LDPE) and linear resins (eg LLDPE prepared by for example Ziegler and/or metallocene catalysts). These blends provide a unique combination of increased melt strength and modulus at a given melt index, I2. Optionally, specific blend formulations may also be selected to provide modulus both higher or lower than that achievable with LDPE alone. Thus this invention can provide blends of flexural modulus greater than or equal to 100,000 psi or less than 30,000 psi, at melt strengths similar to or greater than those associated with LDPE or linear polyethylene having similar melt index, I2.
The blends of the present invention are useful for fabricating high modulus foams, the preparation of which requires high melt strength. The resulting high modulus compositions (i.e., greater than 100,000 psi flexural modulus) are particularly suitable for manufacture of crosslinked and noncrosslinked foams. Thus the foams of the present invention have compressive strength and load bearing capacity similar to that of foams made from branched polyethylenes, but the inventive foams have significantly lower foam density (allowing a significant reduction in the amount of resin, by weight, necessary to produce such foams). In addition, the upper service temperature of the foams of this invention may also be improved, resulting in subsequent improvement in foam dimensional stability. No resin with this combination of properties is currently available.
The high modulus foams of the present invention comprise blends of high melt strength
Chaudhary Bharat I.
Oswald Thomas
Weinhold Jeffrey D.
Nutter Nathan M.
The Dow Chemical Company
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