Rheology modification of elastomers

Details Rheology modification of elastomers Rheology modification of elastomers

2001-10-29

2003-01-14

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S332700, C525S332800, C525S333900, C525S344000, C525S351000

Reexamination Certificate

active

06506848

ABSTRACT:

This invention relates to coupling of polyolefins, more specifically coupling of elastomeric polyolefins.
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. Rheology modification is particularly important when the polyolefins are elastomers.
The term “elastomer” was first defined in 1940 to mean synthetic thermosetting high polymers having properties similar to those of vulcanized natural rubber, e.g. having the ability to be stretched to at least twice their original length and to retract very rapidly to approximately their original length when released. The elastic recovery (that is recovery of original dimension) of an elastomer is generally at least 40 percent, preferably at least 60 percent, and more preferably at least 70 percent after the sample is elongated 100 percent of an original dimension at 20° C. according to the procedures of ASTM D 4649. Representative of these “high polymers” were styrene-butadiene copolymer, polychloroprene, nitrile butyl rubber and ethylene-propylene polymers (aka EP and EPDM elastomers). The term “elastomer” was later extended to include uncrosslinked thermoplastic polyolefin elastomers, that is, thermoplastic elastomers (TPEs).
ASTM D 1566 defines various physical properties of elastomers, and the test methods for measuring these properties.
A dilemma faced in the production of commercially viable cured elastomers is that a high weight average molecular weight is generally desired to improve physical properties such as tensile strength, toughness, and compression set, in the cured product, but the uncured high molecular weight elastomers are more difficult to process than their lower molecular weight counterparts. In particular, the higher molecular weight uncured elastomers are typically more difficult to isolate from solvents and residual monomer following polymerization of the elastomer. The uncured higher molecular weight elastomers are also typically more difficult to extrude at high rates, since they are generally prone to shear fracture at lower extrusion rates and require more power consumption by polymer processing equipment such as batch mixers, continuous mixers, extruders, etc., and cause increased wear on the parts of such equipment exposed to high shear stresses, such as expensive extruder components. These disadvantages reduce production rates and/or increase the cost of production.
Often, a relatively low molecular weight elastomer produced then is fully crosslinked in a final product to obtain the desired tensile strength, toughness, compression set, etc. The relatively lower molecular weights of elastomer are easiest to produce. Disadvantageously, however, a low molecular weight of an elastomer also in most instances, corresponds to a low “green strength” (i.e., strength prior to crosslinking). The disadvantage is particularly noticeable in applications such as coating wire and cable, continuous extrusion of gaskets, etc., where low green strength results in sags or uneven polymer thickness. Rheology modification of a lower molecular weight elastomer, however, is an advantageous manner to solve the problem.
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 branching 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 branching 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 polysulfonyl azide to modify homopolymer polyethylene and its blend with polyisobutylene.
In the case of elastomeric polymers containing ethylene repeating units in which the preferred comonomer content is about 5-25 mole percent, and preferably a density less than about 0.89 g/mL, it would be desirable to have a better mechanical properties such as elongation and tensile strength than would be achieved in the starting material or by coupling using the same chemical equivalents of free radical generating agent like a peroxide.
SUMMARY OF THE INVENTION
Polymers coupled by reaction with coupling agents according to the practice of the invention advantageously have at least one of these desirable properties and preferably have desirable combinations of these properties.
The present invention includes a process of preparing a coupled polymer comprising heating an admixture containing (1) at least one elastomer comprising ethylene and at least one comonomer which is selected from alpha olefins having at least 3 carbon atoms, dienes and combinations thereof and (2) a coupling amount 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 having a gel content of less than about 2 weight percent. The elastomer preferably comprises ethylene, and alpha olefin of at

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