Thermoplastic olefin compositions

Details Thermoplastic olefin compositions Thermoplastic olefin compositions

1998-06-15

2001-06-12

Seidleck, James J. (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...

C525S088000, C525S191000, C526S351000, C526S943000

Reexamination Certificate

active

06245856

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to novel Thermoplastic Olefin compositions comprising polypropylene, and ethylene-alpha olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. The ethylene-propylene copolymer compatabilizer imparts a greater degree of compatibility between the polypropylene and elastomer phases yielding improved physical properties.
Thermoplastic Olefin Compositions (TPOs) are defined as blends of polypropylene, olefinic elastomers and optionally fillers and other compounding ingredients. TPOs are multiphase polymer blends where the polypropylene forms a continuous matrix phase and the elastomer and fillers are the dispersed components. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO while the elastomer imparts flexibility and impact resistance.
Traditionally, ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the elastomeric component in TPOs. Recently, other ethylene-alpha olefin copolymers have been used, especially ethylene-butene and ethylene-octene copolymers.
The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. These parts are generally made using injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35). These high melt flow rate (MFR) resins are difficult to toughen, resulting in products that have low impact strength.
One of the reasons impact modification of high MFR polypropylene resins is difficult is because of the large differences in the melt viscosities between the polypropylene resins employed and the elastomers typically used as impact modifiers. These differences lead to a poor dispersion of the elastomer in the polypropylene matrix, resulting in large dispersed elastomer particle sizes which, in turn, is detrimental to overall impact strength.
One proposed solution to the problem has been to decrease the molecular weight of the elastomer used, in order to reduce the viscosity of the elastomer. While this route produces better dispersion of the elastomer in the polypropylene matrix, the reduced molecular weight of the modifier adversely affects the impact strength of the TPO.
Another proposed solution has been to develop products which behave like a low viscosity plastic during the mixing process, yet function like an elastomer in the molded TPO. These type of polymers are generally referred to as plastomers. To date, however, these plastomer products have not yielded satisfactory impact performance when used with high melt flow polypropylene.
A third area that has been explored is the use of branched elastomers. U.S. Pat. No. 5,681,897 discloses the use of substantially linear ethylene-alpha olefin copolymers having a degree of long chain branching as impact modifiers for polypropylene as well as other thermoplastic resins. While the use of these elastomers appears to lead to an improvement in impact strength, there is still a need for impact strength and flexibility in TPOs made with high MFR polypropylene resins.
SUMMARY OF THE INVENTION
The thermoplastic olefin compositions of the present invention are prepared by blending a high melt flow rate (MFR) polypropylene resin with an ethylene-alpha olefin elastomer and compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. It has been found that the high propylene content copolymer acts as a compatabilizer between the polypropylene phase and the elastomeric phase resulting in excellent dispersion of the elastomer throughout the polypropylene matrix.
The polypropylene phase of the TPO comprises polypropylene homopolymer having a relatively high melt flow rate (MFR). Specifically the MFRs should range from about 20 to 100 with a MFR of 35 to 70 preferred. The polypropylene component will typically comprise 88 to 50 weight percent of the TPO with a polypropylene content of 65 to 75 weight percent preferred and a polypropylene content of 70 weight percent most preferred.
The elastomer component of the TPO comprises either an ethylene-C
3
to C
20
alpha olefin copolymer or an ethylene-alpha olefin-diene terpolymer. The elastomer generally will have a number average molecular weight of from 30,000 to 500,000 with a range of 50,000 to 100,000 preferred. The ethylene content of the elastomer will generally range from 45 to 90 weight percent with 45 to 65 weight percent preferred. The elastomers will have a density of from 0.85 to 0.90 gm/cm
3
. Proportionally, the elastomeric component of the TPO comprises 10 to 50 weight percent with 20 to 40 weight percent preferred.
Lastly, the ethylene propylene copolymer compatabilizer will have a number average molecular weight of from 40,000 to 300,000 with 80,000 to 200,000 being preferred. The compatabilizers will generally have a molecular weight distribution (MWD) of from 1.8 to 4.5 with from 2 to 3 preferred. The propylene content will vary from 80 to 92 weight percent with 83 to 91 weight percent preferred and 85 to 90 weight percent most preferred. The preferred copolymers will exhibit a crystallinity of from about 2 to 65% of that of isotactic homopolypropylene, preferably about 5 to 40%. When used as a compatabilizer in TPOs, the high propylene copolymer will be employed in a range of from 2 to 15 weight percent with 3 to 7 weight percent preferred.
Other materials typically used in the preparation of TPOs, such as fillers and the like, can also be used in the practice of the invention.


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