Enhanced crosslinking terpolymer

Details Enhanced crosslinking terpolymer Enhanced crosslinking terpolymer

1999-03-22

2001-08-28

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S284000, C526S336000, C526S339000, C526S340300, C526S348500, C526S348600, C526S916000

Reexamination Certificate

active

06281316

ABSTRACT:

TECHNICAL FIELD
This invention relates to a terpolymer, which is easily processed and contains stabilized unsaturation, which enhances crosslinking.
BACKGROUND INFORMATION
In a gas-phase polymerization process, traditional Ziegler-Natta catalysts have been shown to readily polymerize ethylene with one or more higher alpha-olefin comonomers such as propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene and 3,5,5-trimethyl-hexene, to produce a linear low-density polyethylene (LLDPE) with minimal if any long-chain branching (LCB). The increasing development of metallocene catalyzed olefin polymers has resulted in the ability to produce similar polymers with a more well-defined molecular structure than can be achieved with conventional Ziegler-Natta catalysts . Metallocene linear low density polyethylenes made according to U.S. Pat. Nos. 5,420,220 and 5,324,800, for example, possess narrow comonomer and molecular weight distributions. U.S. Pat. Nos. 5,527,752 describes further a family of metallocene catalyst precursors which are useful, when combined with a cocatalyst or catalyst activator, in the manufacture of polyolefins.
The various linear low density polyethylenes referred to above are known to be useful, among a wide variety of applications, in those applications in which a crosslinkable polymer is requited.
In this vein, industry has been seeking polymers, which have improved crosslinking properties without sacrificing processability.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a polymer, which is easily processed and contains stabilized double bonds useful in applications where crosslinking is of substantial importance.
According to the present invention, such a polymer has been discovered. The polymer of the invention is comprised of the comonomers ethylene, one or more alpha-olefins having 3 to 20 carbon atoms, and one or more cyclic dienes having up to 30 carbon atoms, said polymer having a density of at least 0.890 gram per cubic centimeter; long chain branching; a plurality of double bonds; an Mw/Mn ratio (PDI) of at least 2.5; a flow activation energy of greater than about 6.5 kcal/mol; and a Relaxation Spectrum Index (RSI), PDI, and Melt Index (MI), such that RSI·MI
a
>2.7 and RSI·MI
a
·PDI
b
is in the range of about 0.8 to about 60, when a and b are about 0.6 and minus 1.2, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The addition of a cyclic diene comonomer (CDC) with ethylene and a higher alpha-olefin comonomer in a gas-phase reactor provides a mechanism by which long-chain branching can be introduced into the polymer. In norbornadiene (NBD), for example, both double bonds in its cyclic structure are strained due to the molecular geometry causing them to be very reactive during the gas-phase polymerization process. For this reason, NBD is readily incorporated into a growing polymer chain at one of the double bonds. Subsequent re-incorporation of the second double bond into a growing polymer chain occurs somewhat less frequently, though when it does occur, the result is that a 4-arm star or similar LCB structure is formed as long as the concentration of the cyclic diene is kept to some low level in the process (see structures 2 and 4). If the second double bond is not re-incorporated, the double bond remains as residual unsaturation when the polymer is continuously removed from the reactor (see structures 1 and 3).
The long chain branches are described as being at least about 250 carbon atoms in length. One of the characteristics of long chain branches is that they become entangled in the melt state, so they can also be described as being at least as long as the entanglement molecular weight of about 3800 Daltons since that corresponds to the minimum chain length required to be recognized by the melt rheological properties of polyethylene (See Ferry,
Viscoelastic Properties of Polymers
, John Wiley & Sons, 1980, pages 243 and 378).
Structures 1, 2, 3, and 4 are as follows (when the cyclic diene comonomer is NBD):
Stabilization of that residual unsaturation provides preferred structures for subsequent cross-linking (e.g., by a wire or cabling manufacturer) such that less cross-linking agent, such as an organic peroxide, needs to be used relative to cross-linking of a polymer with less residual unsaturation. 4-arm star LCB or similar moieties provide enhanced rheological behavior that will lead to easier extrusion and superior melt strength relative to polymers without those structures.
Examples of typical cyclic dienes are as follows:
Vinyl norbornene (VNB) has one double bond within the cyclic portion of its structure that is strained on an atomic level, and a second double bond as part of a pendant vinyl structure. The strained nature of the former double bond makes it far more reactive than the latter, such that incorporation of VNB into a growing polymer chain will occur via the strained location. In fact, the pendant vinyl group may be less reactive in the gas-phase reactor than the alpha-olefin, therefore the probability of cross-linking is even lower than that with NBD. It will, however, remain readily available for cross-linking.
Ethylidene norbornene (ENB) has a structure that is similar to that of VNB, though the double bond that is not within the cyclic comonomer structure is not as readily available for re-incorporation as it is in the pendant vinyl structure of VNB. As in the case of VNB, incorporation of ENB into a growing polymer chain will occur at the strained double bond that is part of the cyclic portion of its structure. The probability for re-incorporation of the second double bond is lower than that in both NBD and VNB, but it will still be readily available for cross-linking.
This invention describes polyolefin products made in a gas-phase reactor by the polymerization of ethylene with at least one higher alpha-olefin comonomer and at least one CDC such that some of the unsaturation that results from the initial incorporation of the CDC is re-incorporated into another growing chain to form LCB and some of the unsaturation remains following the reaction.
Polymers of this invention include copolymers of ethylene with at least one linear or branched higher alpha-olefin containing 3 to about 20 carbon atoms and at least one cyclic diene comonomer that may be produced in the gas phase in a mechanically stirred or gas-fluidized bed reactor using equipment and procedures well known in the art. The densities range from 0.890 to 0.965 gram per cubic centimeter with melt indices from about 0.1 to about 200 grams per 10 minutes, in accordance with ASTM D1238, condition E, at 190 ° C. Suitable higher alpha-olefins, linear and branched, include, for example, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene and 3,5,5-trimethyl-hexene. Suitable cyclic diene comonomers include, but are not limited to, norbornadiene, vinyl norbornene, and ethylidene norbornene. A cyclic diene comonomer can have a structure that is mono-cyclic, bi-cyclic, or otherwise multi-cyclic. Further, a cyclic diene comonomer can have a structure in which both double bonds are internal to the ring structure, one of the double bonds can be exocyclic, or both double bonds can be exocyclic. Further, when one of the double bonds is exocyclic, the cyclic diene comonomer can have, but is not limited to, the following molecular structure:
where n=0→∞. In a preferred embodiment, n=0→20.
The polymers of this invention have polydispersity indices greater than 2.5, preferably greater than about 3.0. The polydispersity index (PDI) of a polymer is defined as the ratio of the weight average molecular weight of the polymer to the number average molecular weight of the polymer (Mw/Mn). PDI, uncorrected for long chain branching, is determined using size exclusion chromatography (SEC) with a WATERS™ 150 degrees C. GPC instrument operating at 140 degrees C. with 1,2,4-trichlorobenzene at a flow rate of 1 milliliter per minute. The pore size range of the column set provides for a mol

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