Processable polyethylene/EPDM thermoplastic vulcanizates

Details Processable polyethylene/EPDM thermoplastic vulcanizates Processable polyethylene/EPDM thermoplastic vulcanizates

2000-12-29

2003-12-23

Mulcahy, Peter D. (Department: 1713)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S114000, C524S310000, C524S313000, C524S525000

Reexamination Certificate

active

06667364

ABSTRACT:

FIELD OF THE INVENTION
“Thermoplastic vulcanizates” or “TPVs” (also referred to in the past as “thermoplastic elastomers” or “TPEs”) are made by dynamic vulcanization of a blend of an olefin rubber and a crystalline polyolefin (“PO”) which becomes the continuous phase in which microscopic rubber particles are held. A processable TPV is formulated in which polyethylene (“PE”) constitutes a major proportion by weight of the continuous PO phase (referred to herein as a “PE-rich TPV”), with melt viscosity reducers used in a defined range. TPVs formulated with a specified melt reducer in the stated range may contain PE in an amount greater than 25% by weight, based on all the polymer (PO and rubber) in the TPV.
BACKGROUND OF THE INVENTION
The term “elastomer” is used in the broad sense, in that the cured blend is processable as a TPV, and is re-processable, unlike a thermoset resin. By “processable” is meant that a dynamically vulcanized blend can be thermoformed, typically injection molded, extruded, vacuum-formed or blow-molded in a commercially available machine. Such extruders and injection molding machines for TPVs provide internal mixing at a temperature in the range from about 180° C. to 240° C. with a residence time less than 5 min, preferably in the range from 30 sec to 2 min. In practice, an attempt to make a useful and marketable TPV by substituting PE for polypropylene (“PP”) present in an amount greater than 25% based on the weight of polymer (PO and EPDM) present, in a randomly chosen EPDM results in a blend which is not processable. In processable TPVs which are “self-cured” and not physical blends, their combination of desirable elastic and thermoplastic properties depends on the respective amounts of “hard” and “soft” phases provided by each component, and the properties of each component. In most cases, the prior art fails to recognize the unusually high melt viscosity of PE-based TPVs and routinely disclose that PP in PP/EPDM TPVs may be substituted with PE or any other polyolefin without the benefit of an enabling illustrative example.
Commercially available TPVs generally consist of micron-sized (1-10 &mgr;m) crosslinked EPDM rubber particles embedded in a continuous phase mainly of PP having various crystallinities. EPDM is a copolymer of ethylene, propylene, and a diene providing a cure site monomer, most commonly ethylidenenorbornene (“ENB”). Such TPVs are produced by the dynamic vulcanization of blends of EPDM rubber in molten PP, the rubber being selectively cured during intense mixing. (see “Thermoplastic Elastomers” by G. Holden et al, ed. Chap 7, Hansen Publishers, 2nd ed., 1996).
For PP/EPDM TPVs, an increase in blend melt viscosity during TPV formation results presumably from the increased rubber/plastic contact area generated by the micron-sized particles in a continuous plastic phase. The viscous drag of the molten plastic over the rubber particles is a major contributor to melt viscosity of the TPV, with additional contributions due to deformation within and interactions between the rubber particles. Though melt viscosity of PP/EPDM TPVs allows their processability, a PE-rich TPV is not “processable” because of its unusually high melt viscosity.
In most known, usable TPVs, PP is the continuous hard phase and the EPDM is the soft phase present as discrete particles. In the novel TPV, PE is in the continuous “hard” phase, and the “soft” phase is chosen from (i) a copolymer of ethylene-propylene-5-vinyl-2-norbornene, an EPDM rubber containing pendent vinyl unsaturation (hereafter either is referred to as “EP(VNB)” or “pEPDM” to connote the particular olefinic rubbers), and (ii) butyl rubber having a pendent vinyl cure site (referred to as “pButylR”) By varying the ratios of the components including the amount of processing oil, within limits beyond which the TPV is unusable, one is expected to be able to provide desired hardness/softness, oil and temperature resistance, oxidation resistance, and processability, inter alia.
In U.S. Pat. Nos. 3,957,919 and 4,059,654 to Von Bodungen, et al. the beneficial effect of PE in the TPV is secured when PE is present in an amount greater than 15% but not more than 25%, with the remainder of 70% to less than 85% by weight divided between the EPDM interpolymer and the monoolefin polymer in the ratio of 90-10 parts by weight of EPDM polymers to 10-90 parts by weight of monoolefin polymers. The PE component may include copolymers of ethylene containing 10% or less copolymerized &agr;-olefins having from 3-16 carbon atoms.
But the '654 disclosure teaches that within the ambit of the proviso with respect to ratios, any PO may be added to a PE and PP combination in any EPDM and worked with any free radical generating agent to provide TPVs with acceptable compression set. This broad disclosure of any EPDM interpolymer reads on a vast array of EPDMs including ethylene-propylene-5-vinyl-2-norbornene; and, of any PO, reads on a vast array including poly-1-butene, and copolymers of ethylene-co-butene and ethylene-co-propene-1-butene, which are peculiar in that they have a melt viscosity lower than that of PP. It is believed that, unlike other poly-C
3
-C
16
-olefins, the common characteristic which makes the aforementioned polymers and some amorphous polymers useful as melt viscosity reducers in a PE-rich TPV is their peculiar morphology in the rubber/plastic contact area, and the resulting low viscous drag of the molten plastic over the rubber particles. The melt viscosity reducer appears to maintain itself as a separate phase in each of the phases of the vulcanizate, making the TPV processable.
EP(VNB) or pEPDM and pButylR are readily blended in the molten state in any proportions with PE. Such high compatibility of molten PE with these rubbers is greater than that of a blend in which PP is substituted for PE. However, when such a substantially “PE only” blend is vulcanized, the TPV has too high a viscosity to be processable; that is, when measured in an automatic capillary rheometer (“ACR”), the viscosity is above 8000 Poise at 200° C. Such high viscosity is too high to allow the components of the TPV to be processable in commercially available equipment. The high viscosity is attributable to the compatibility of PE with EPDM rubber. A TPV of PP/EP(VNB) or PP/pButylR in which either PP or the rubber is present in a larger amount than the other, is deemed processable; but a TPV of PE/EP(VNB), or PE/pButylR in which the rubber is present in a larger amount than the PE, or vice versa, defies extrusion or fabrication (referred to as having “poor fabricability”) in such equipment.
Though the disclosure of the '654 patent teaches that all TPVs containing high density PE in an amount greater than 15% by weight of the total EPDM-PO-PE but in which the PE component does not exceed 25% by weight, are ideally suited for use in the manufacture of flexible hose, EP(VNB) substituted for EPDM is not. Neither is a blend of EP(VNB) with 25% PE processable when cured with a silane curing agent. (see Examples 32, 33 and 34 in Table 8 herebelow).
Because trouble-free processsability in commercially available injection molding machines and extruders is tied to melt viscosity, a blend of rubber and PO only, that is, without any processing aids including oil, is required to have a melt viscosity in the range from about 200 P (Poise) to 4000 P. Above 4000 P the processability diminishes progressively and at 8000 P a blend is deemed unprocessable. At about 8000 P the melt viscosity of a TPV, as measured in a Monsanto ACR Model No. 3501, at 204° C. (400° F.) and 118 kPa constant stress is so high that there is no transfer, or an insufficient amount of transfer of polymer from the melting zone in the tube, to trip a switch in the tube at the end of 4 min, thus starting the measurement.
Blends having a melt viscosity in the range from 4000 P to 8000 P are made processable by adding conventional processing aids.
The Problem
To benefit from the properties of a TPV containing a major proportion of PE relative to polypropylene (“PP”), most pr

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