Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Reexamination Certificate
1998-08-26
2001-12-04
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
C264S054000, C264S138000, C264S176100, C264S320000, C264S328100
Reexamination Certificate
active
06325956
ABSTRACT:
This invention relates to crosslinking of polyolefins, more specifically crosslinking of polyolefins using insertion into carbon hydrogen (C—H) bonds.
As used herein, the term “crosslinking” means forming bonds between polymer chains such that gels insoluble in xylene are formed to an extent of at least about 10 percent as measured according to ASTM 2765-84.
Polyolefins are frequently crosslinked using nonselective chemistries involving free radicals generated for instance using peroxides or high energy radiation. However, chemistries involving free radical generation at elevated temperatures also degrade the molecular weight, especially in polymers containing tertiary hydrogen such as polystyrene, polypropylene, polyethylene copolymers etc.
Other methods of crosslinking are also known.
The teachings of U.S. Pat. Nos. 3,058,944; 3,336,268; and 3,530,108 include the reaction of certain poly(sulfonyl azide) compounds with certain polyolefins.
It would be desirable to have a method of crosslinking, especially for polyolefins especially for those prepared using single site catalysts, more particularly for ethylene and its copolymers, most preferably of narrow molecular weight distribution (MWD), which avoids the chain scission effects of free radicals and more desirably also avoids the need for functional groups on polymers for reactive sites for crosslinking agents. Desirably the crosslinking me hod would be initiated by heat, more desirably the temperatures normally encountered in polymer processes within the skill in the art such as extrusion and other industrial heat processes for instance as disclosed by J. A. Brydson, Rubbery Materials and their Compounds, page 348, Chapter 18, Elsevier Applied Science, New York, 1988. (E.g. the decomposition temperature of the crosslinking agent is advantageously matched with melt temperature of polymer, such that the decomposition temperature is sufficiently high to be melt mixed and in a shaping step with precuring, and the decomposition temperature is advantageously sufficiently low to have a desirably short curing time in a curing step).
Which polymers, in the case of high density polyethylene especially when the polymer produced by single site catalysts, and preferably density greater than 0.945 g/ml) (hereinafter HDPE produced using single site catalysts), would desirably have a higher toughness, tensile and/or elongation than a crosslinked high density polyethylene prepared using Ziegler Natta catalyst (which would generally be of broader molecular weight distribution) the HDPE produced using single site catalysts crosslinked using the same equivalents of a free radical crosslinking agent. Advantageously, compositions would have less undesirable odor than the same starting material crosslinked using the same chemical equivalents of free radical generating agents. Preferably, a process of the invention would result in more consistent crosslinking than methods of crosslinking involving free radicals, that is use of the same reactants, amounts and conditions would result in consistent amounts of crosslinking or consistent (reproducible) property changes, especially consistent amounts of gel formation. Preferably, a process would be less subject to effects from the presence of oxygen than would a crosslinking involving agents which generate free radicals. Further, for a comparatively high melting polymer like HDPE (about 140° C.) it is desirable to avoid premature crosslinking during melt mixing process. That is, desirably, a process would provide a broader processing window for crosslinking than is observed with peroxides.
In the case of, medium and lower density polyethylene (that is polymers having a density of from about 0.94 g/cc to about 0.90 g/cc), also produced using single site catalysts, which are advantageously copolymers of ethylene in which the percent comonomer is preferably about 0.5 to 5 mole percent comonomer based on total polymer as determined by ASTM 5017, the polymers would desirably show a combination of high upper service temperature performance (measured by the thermal mechanical analysis described hereinafter)
and creep resistance (measured according to ASTM-D-2990-77) as compared to non crosslinked polymer.
In the case of elastomeric polymers containing ethylene repeating units in which the preferred density less than about 0.89 g/mL and more preferably with a comonomer content greater than about 5, most preferably about 5-25 mole percent as determined by ASTM 5017, also produced using single site catalysts it would be desirable to have a better mechanical properties such as elongation and tensile, strength, and lower compression set than would be achieved by crosslinking using the same chemical equivalents of free radical generating agent like a peroxide. Desirably, the crosslinked material would have better organoleptic qualities, especially less foul odor, than the same starting material crosslinked using peroxides.
SUMMARY OF THE INVENTION
It has been discovered that crosslinking polyolefins using poly(sulfonyl azide) produces polymer products having surprisingly useful properties when the polyolefin has been prepared using a single site catalyst, such as a vanadium catalyst, a metallocene catalyst or a constrained geometry catalyst.
The invention includes a process comprising (a) forming a polymeric admixture including at least one polyolefin which has been prepared using a single site catalyst and at least a crosslinking amount of at least one poly(sulfonyl azide) crosslinking agent; (b) shaping the resulting admixture; and (c) heating the resulting shaped admixture to a temperature at least the decomposition temperature of the crosslinking agent. The steps take place in any sequence and optionally include substeps. Preferably in step (b) the polymeric admixture is in a softened or melted condition for shaping; or step (b) comprises thermoforming, compression molding, injection molding, extrusion, casting, blow molding, blowing, profile extrusion, spinning, other molding or combination thereof; or step (c) comprises foaming; or step (a) includes forming a foamable melt polymer material by admixing and heating a decomposable chemical blowing agent and other components of the polymeric admixture; and step (b) includes extruding the foamable melt polymer material through a die; or step (a) comprises the substeps of (i) suspending discrete polyolefin particles in a liquid medium in which they are insoluble, (ii) impregnating the particles with a crosslinking amount of poly(sulfonyl azide) crosslinking agent and a blowing agent at a superatmospheric pressure and temperature above the softening point of the polymer; and step (b), (c) or a combination thereof includes (iii) rapidly discharging the particles into a pressure less than that in substep (ii) to form foam beads or (iv) cooling the particles and subsequently expanding them with at least one heated gas; or step (a) includes admixing at least one polyolefin, a crosslinking amount of a poly(sulfonyl azide) crosslinking agent, and a chemical blowing agent to form an admixture; step (b) comprises a first substep forming a slab of the admixture; step (c) includes heating the admixture in a mold such that the crosslinking agent crosslinks the polymer material and the blowing agent decomposes; and either step (b), step (c) or a combination thereof includes expanding the slab formed in the first substep of step (b) by releasing pressure in the mold; or step (b) comprises a first substep of forming a sheet of the polymeric admixture containing a crosslinking amount of poly(sulfonyl azide) crosslinking agent; step (c) comprises heating the sheet sufficiently to result in crosslinking; step (b) further includes a second substep of impregnating the sheet with N
2
at a temperature above the softening point of the polymer and at a pressure and a third substep of releasing the pressure to result in nucleation of bubbles and some expansion in the sheet or a combination thereof. More preferably, the crosslinking agent is introduced into the polymeric admixture in melt
Babb David A.
Chaudhary Bharat I.
Cummins Clark H.
Ho Thoi H.
Kao Che-I.
Kuhns Allan R.
The Dow Chemical Company
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