Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-05-10
2002-04-16
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S210000, C525S232000, C525S240000, C525S241000
Reexamination Certificate
active
06372847
ABSTRACT:
SUMMARY
This invention is related to blends of propylene-based polymers and branched ethylene-based elastomers.
BACKGROUND
Thermoplastic olefin elastomers blends (TPO's) are defined as blends of polypropylene with olefinic copolymer elastomers and optionally fillers and other compounding ingredients. TPO's are multiphase polymer systems where the polypropylene forms a continuous matrix and the elastomer and filler are the dispersed phase. The polypropylene matrix imparts to the TPO tensile strength, rigidity and chemical resistance, while the elastomer, sometimes referred to as the “modifiers”, provides flexibility, resilience, toughness, especially at low temperature, and, in some cases, paintability.
TPO's are particularly well suited for producing flexible structures such as body parts for automotive applications. The principal automotive applications include bumper fascia, air dams and other trim, dashboards, and airbag covers. Various TPO formulations are available having physical properties highly desirable in such applications. The ability of TPO's to be injection molded makes them particularly attractive for the high volume production necessary in automotive applications.
Traditionally, ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the elastomeric component in TPO blends. The low temperature impact strength of polypropylene modified with EP/EPDM increases as the ethylene content of the elastomer decreases down to about 50 percent by weight and/or, at constant morphology, as molecular weight (as measured by Mooney viscosity) of the elastomer increases.
The blending process involves the melt mixing of the polymers and blend pelletization. Such compounding is typically effected by Banbury mixing of elastomer bales with polypropylene pellets followed by extruder pelletization of the blend. In recent years, the preferred method of compounding has been by extrusion blending of the olefinic elastomer with the polypropylene and on-line blend pelletization of the final blend. This latter process is more economical but requires the use of olefinic elastomers in a stable pellet form to feed the mixing extruder. This requirement has limited the choice of olefinic elastomer modifiers to those that can be produced and stored in pellet form without agglomerating.
Two solutions have been developed and used to address this requirement. The first involves the use of pelletized elastomer masterbatches prepared by blending amorphous olefinic elastomers with a crystalline polyolefin (generally ethylene or propylene polymers). A disadvantage of this solution is that it significantly increases production costs of the modifier.
The second solution involves the use of olefinic elastomer modifiers having an ethylene content sufficiently high to develop a level of crystallinity that allows their production in a non-agglomerating pellet form. Ethylene elastomers suitable for the production of storage-stable pellets have a measurable crystallinity at or above storage temperature. In EPDM, the crystallinity is function of the ethylene content of the polymer, the monomer sequence distribution, and the compositional distribution. For additional information, see G. Ver Strate, “Ethylene-Propylene Elastomers,”
Encyclopedia of Polymer and Engineering Science,
vol. 6, (1986).
For example, in Ziegler-Natta, single-site catalyzed ethylene-propylene elastomers, the required minimum ethylene level to exhibit sufficient crystallinity for pellet stability is about 74 mole %. However, the use of a surface stabilizer dusting agent like HDPE powder or talc may still prove necessary to completely prevent pellet agglomeration. This minimum ethylene content can vary somewhat depending on the type of catalyst used for the synthesis of the polymer, which can in turn affect the monomer sequence distribution and compositional distribution (see Ver Strate, cited earlier).
The crystallinity of ethylene based elastomers is conveniently measured by Differential Scanning Calorimetry (DSC), and is strongly related to the thermal history of the polymer. Semi-crystalline EP/EPDM elastomers have single or multiple crystalline melting points above 23° C. The temperature of these melting peaks depends on the ethylene content of the polymer, on the ethylene sequence and compositional distribution, and on the thermal history of the polymer. They typically range from about 30° C. up to about 90° C. in polymers of very high ethylene content. When a polymer sample is melted (typically at 150° C. or above) and allowed to cool down, these crystalline melting peaks develop slowly with time. Therefore, their relative quantification requires rigorous sample preparation protocols. We prefer therefore characterizing the crystallinity of EP elastomers by their total crystallization enthalpy measured by DSC after sample annealing at 150° C. or above. Also, we prefer using a modulated DSC technique which permits clear identification of the onset and the end of the crystallization exotherms and the fusion endotherms. Accordingly, ethylene based elastomers suitable for the production of non-agglomerating pellets generally have a crystallization enthalpy of at least 12 joules per gram (J/g).
Recently, other ethylene-alpha olefin elastomers have been used as such modifiers, especially ethylene-butene and ethylene-octene copolymers. These materials, especially the ethylene-octene copolymers, develop crystallinity at a lower ethylene weight content than EP/EPDM's. This permits the production pellet-stable ethylene-octene modifiers exhibiting good performance as polypropylene impact modifier. However, these materials also increase the overall production costs of the modifier, and thus the TPO, due to the higher raw material cost of octene versus propylene.
Dharmarajan and Kaufman, “High flow TPO compounds containing branched EP(D)M modifiers,”
Rubber Chem. Tech.,
71(4) (1998) pp. 778-794, discloses compounding polypropylene with branched EPDM's. EP(D)M as used herein is intended to mean that the diene monomer is optional, as indicated by the parentheses, and although EP technically represents ethylene-propylene elastomer, “EP,” as used herein, is intended to include elastomeric copolymers of ethylene and any alpha-olefin capable of forming such a polymer. The introduction of long chain branching in the EPDM structure was shown to result in improved TPO impact strength over linear EP/EPDM. However, the TPO's described still showed a strong dependence of their low temperature impact strength on the ethylene content of the modifier, and the modifiers described still had too low an ethylene content to be produced in non-agglomerating pellet form.
The use of branched EPDM in combination with a compatibilizer is also described in WO 98/27154. Again, the EPDM described has too low an ethylene content to be produced in storage stable pellets.
A paper published in
Journal of Additive Technology,
vol. 2 (1998), pp. 235-239, describes the effect of EPM long chain branching on TPO properties and indicates that branching results in lower impact strength. The blends described comprise a branched EPM of too low ethylene content to be produced in stable pellet form. The authors report that the poor results could be explained on the basis of a poor morphology, and did not highlight the benefits brought by long chain branching to the low temperature impact strength of TPO's.
U.S. Pat. No. 5,688,866 describes TPO blends based on polypropylene and about 30 weight percent of substantially linear ethylene interpolymers. The authors use a melt index ratio I
10
/I
2
to quantify the shear thinning which is said to result from the presence of branching in the described polymers, and claim a I
10
/I
2
ratio measured at 190° C. of greater than or equal to 5.83. This melt flow rate ratio is however not well adapted to EP(D)M's since most EP(D)M's show very low flow under the I
2
conditions (typical MFI under 2.16 Kg @ 190° C. are below 0.1 g/10 min.). As will b
Exxon Mobil Chemical Patents Inc.
Nutter Nathan M.
Reid Frank E.
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