Rheology modification of low density polyethylene

Details Rheology modification of low density polyethylene Rheology modification of low density polyethylene

1998-08-05

2003-02-18

Pyon, Harold (Department: 1772)

Stock material or miscellaneous articles

Hollow or container type article

Polymer or resin containing

C428S035200, C428S035700, C428S516000, C428S517000, C524S194000

Reexamination Certificate

active

06521306

ABSTRACT:

This invention relates to coupling of polyolefins, more specifically coupling of polyolefins to form products suitable for films.
As used herein, the term “rheology modification” means change in melt viscosity of a polymer as determined by dynamic mechanical spectroscopy. Preferably the melt strength increases while maintaining the high shear viscosity (that is viscosity measured at a shear of 100 rad/sec by DMS) so that a polymer exhibits more resistance to stretching during elongation of molten polymer at low shear conditions (that is viscosity measured at a shear of 0.1 rad/sec by DMS) and does not sacrifice the output at high shear conditions. An increase in melt strength is typically observed when long chain branches or similar structures are introduced into a polymer.
Polyolefins are frequently rheology modified 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. The reaction of polypropylene with peroxides and pentaerythritol triacrylate is reported by Wang et al., in Journal of Applied Polymer Science, Vol. 61, 1395-1404 (1996). They teach that rheology modification of isotactic polypropylene can be realized by free radical grafting of di- and tri-vinyl compounds onto polypropylene. However, this approach does not work well in actual practice as the higher rate of chain scission tends to dominate the limited amount of chain coupling that takes place. This occurs because chain scission is an intra-molecular process following first order kinetics, while coupling is an inter-molecular process with kinetics that are minimally second order. Chain scission results in lower molecular weight and higher melt flow rate than would be observed were the branching not accompanied by scission. Because scission is not uniform, molecular weight distribution increases as lower molecular weight polymer chains referred to in the art as “tails” are formed.
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 isotactic polypropylene or other polyolefins by nitrene insertion into C—H bonds. The product reported in U.S. Pat. No. 3,058,944 is crosslinked. The product reported in U.S. Pat. No. 3,530,108 is foamed and cured with cycloalkane-di(sulfonyl azide) of a given formula. In U.S. Pat. No. 3,336,268 the resulting reaction products are referred to as “bridged polymers” because polymer chains are “bridged” with sulfonamide bridges. The disclosed process includes a mixing step such as milling or mixing of the sulfonylazide and polymer in solution or dispersion then a heating step where the temperature is sufficient to decompose the sulfonylazide (100° C. to 225° depending on the azide decomposition temperature). The starting polypropylene polymer for the claimed process has a molecular weight of at least about 275,000. Blends taught in U.S. Pat. No. 3,336,268 have up to about 25 percent ethylene propylene elastomer.
U.S. Pat. No. 3,631,182 taught the use of azido formate for crosslinking polyolefins. U.S. Pat. No. 3,341,418 taught the use of sulfonyl azide and azidoformate compounds to crosslink of thermoplastics material(PP (polypropylene), PS (polystyrene), PVC (poly(vinyl chloride)) and their blends with rubbers (polyisobutene, EPM, etc.).
Similarly, the teachings of Canadian patent 797,917 (family member of NL 6,503,188) include rheology modification using from about 0.001 to 0.075 weight percent poly(sulfonyl azide) to modify homopolymer polyethylene and its blends with, especially polyisobutylene. The polyethylene is referred to as linear polyethylene. Polyethylene having a density of about 0.945 is exemplified. The product is said to be useful for thermoforming.
It would be desirable to have polymers rheology modified rather than crosslinked (that is having less than about 10 percent gel as determined by xylene extraction specifically by ASTM 2765). 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), 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 processability improved over the starting material with retention or improvement of toughness, low heat seal initiation temperature, low haze, high gloss or hot tack properties characteristic of the starting material.
Currently, thick films (films having a thickness greater than about 6 mil, that is 15×10E-02 mmeters) are often formed from low density polyethylene (LDPE) because of its processability in the area of bubble stability and melt strength. Desired higher toughness is obtained by blending the LDPE with linear low density polyethylene (LLDPE), but the LLDPE lowers processability (that is, it raises extruder pressures resulting in the reduction of the output and reduces the melt strength). Melt strength is, however, necessary to form a bubble. It would be desirable to achieve higher toughness than is obtained with LDPE alone, preferably at least the toughness attained using LDPE/LLDPE blends having up to about thirty weight percent LLDPE with desirable bubble stability or melt strength, preferably both, at least as great as that of the starting material LDPE alone. Mechanical properties are Elmendorf tear strength (for instance as measured according to the procedures of ASTM D1922); tensile properties (for instance as measured using the procedures of ASTM D638); and toughness which is conveniently measured by puncture, for instance using a testing frame commercially available from Sintech, Inc. under the trade designation MTS Sintech RE New which is an Instron Model 4201 with Sintech Hardware Upgrade using Sintech Testing Software; and melt strength as indicated by low shear (0.1 rad/sec) viscosity.
Thick polyethylene films are useful for instance as greenhouse films, mulch films, agricultural films and the like.
SUMMARY OF THE INVENTION
The invention includes a process of preparing a coupled polymer characterized by heating an admixture containing (1) at least one ethylene polymer or blend of ethylene polymers having a density of at least about 0.89 g/mL and less than about 0.935 g/mL and a comonomer content between about 0.5 and 50 weight percent of an alpha olefin having greater than 2 and less than 20 carbon atoms per molecule and (2) a coupling amount of at least one poly(sulfonyl azide) to at least the decomposition temperature of the poly(sulfonyl azide) for a period sufficient for decomposition of at least about 80 weight percent of the poly(sulfonyl azide) and sufficient to result in a coupled polymer. The amount of poly(sulfonyl azide) is preferably from about 0.01 to about 5 weight percent of polymers in the admixture. The invention also includes any composition which is the product of any of the processes of the invention and articles made from those compositions, particularly any film of any composition of the invention. Additionally the invention includes a use of any composition of the invention in a process of blowing or calendaring a film. More particularly the invention includes articles of compositions of the invention which are trash bags, agricultural films, construction films, or geomembranes, grocery sacks, sealant layers, tie layers, produce bags, garment bags, shipping sacks, medical films, stretch films, shrink films, agricultural films, construction films, or stretch hooders.
DETAILED DESCRIPTION OF THE INVENTION
Polymers to which the practice of this invention is applicable include homopolymers and copolymers of ethylene (hereinafter ethylene polymers) with narrow and broad (including bimodal) molecular weight distribution. One type of preferred polymers for use i

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