Polymer mixture for slush molding

Details Polymer mixture for slush molding Polymer mixture for slush molding

1999-05-14

2001-03-20

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...

C525S232000, C525S240000

Reexamination Certificate

active

06204330

ABSTRACT:

BACKGROUND OF THE INVENTION
This application is a U.S. national stage of international application PCT/EP99/06329, filed Nov. 13, 1997.
The present invention concerns a mixture of thermoplastic and elastomeric polyolefin compositions; the mixture is partially cross-linked, and has in spite of this a good melt flow rate.
The mixtures of the present invention are particularly suitable for the production of laminated articles produced by way of the slush molding process. In particular, the mixtures are used for the production of synthetic leather to be used, for example, in the automotive industry to cover some of automobile parts.
Synthetic leathers produced from thermoplastic polyolefins are already known in the art. For example, published European patent application EP-A-633 289, on behalf of Himont Incorporated, describes the use of partially cross-linked thermoplastic and elastomeric polyolefin compositions to be used in a slush molding process for the production of synthetic leathers. Said compositions have good mechanical properties, such as high elongation at break values, therefore the laminates obtained from them are adequate for use in thermoforming processes. However, these compositions present the problem of having a low melt flow rate, and consequently a number of pinholes form in the laminates produced by slush molding.
Moreover, published European patent application EP-A-637 610, on behalf of Himont Incorporated, describes thermoplastic polyolefin compositions suitable for the production of very soft synthetic leathers using a slush molding process. The leathers produced from these compositions do not have high tensile strength and elastic recovery values.
Patent application WO 95/35344 (Reydel) also describes compositions for the production of synthetic leather by way of slush molding. These compositions comprise a heterophasic matrix made of a crystalline propylene polymer and a propylene/ethylene rubber with a small content of ethylene. In order to facilitate the removal of the leathers from the mold and prevent the surfacing of low molecular weight rubbery particles that cause problems related to sticking as well as soiling the mold, said compositions also contain a resin comprising a rubber, EPR or EPDM, either cross-linked or not cross-linked. However, in order to achieve the desired properties, such as specific levels of softness of the finished product and melt flow rate, all the compositions cited in said patent application contain extender oil, as is proven by our analyses and demonstrated in the comparative examples.
Moreover, the exemplified compositions of the above mentioned patent application WO 95/35344, according to measurements we carried out which are demonstrated in the comparative examples, present tensile strength and elongation at break values that are not completely satisfactory. Good values related to said properties are important in order to prevent lacerations of the laminate when it is being removed from the mold.
BRIEF SUMMARY OF THE INVENTION
Now it has been found that it is possible to produce, by using the slush molding process, laminated articles with a good degree of softness although they do not contain extender oil. Without the extender oil, the quantity of low molecular weight material that is exuded by the laminate over time, which causes fouling and fogging, is considerably reduced, and also eliminated is the undesirable shiny and oily appearance of the laminate that is caused by the surfacing of said oil.
It has also been found that by using the mixtures of the present invention it is possible to produce, by way of slush molding, laminates with a reduced number of pinholes and reduced size of said pinholes, both on the surface, particularly the surface that is in contact with the mold, and in the inner part of said laminates. In order to achieve said result one needs a good melt flow rate of the polymer. In fact, a good melt flow rate, together with a good flowability of the polymer powder that is typical in slush molding processes, allows for a homogeneous and fast distribution of the polymer in the mold; therefore, the creation of voids that cause the formation of pinholes of various sizes in the laminate is reduced considerably, thus improving the appearance of the laminate, which would otherwise be marred by the presence of a number of big pinholes on the surface. Moreover, said pinholes are even more undesirable since usually they form also on the inside of the laminate thus reducing the mechanical properties of said laminate such as a decreased tensile strength.
In order to avoid said inconveniences a laminate has been produced from powders of a mixture of elastomeric thermoplastic compositions, said compositions containing polymer fractions having a viscosity that is sufficiently low to allow, in spite of the presence of partially cross-linked rubber, a good flow of the melt mixture during the filling of the slush-molding mold. Due to its melt flow, the mixture spreads more homogeneously, thus reducing first of all the size and number of voids and consequently of the pinholes in the laminate.
Although the mixtures of the present invention have low viscosity values, they do cause some limited fogging and stickiness to occur.
An additional advantage is the fact that the laminates obtained from said mixtures have good mechanical properties; in particular, they have tensile strength and elongation at break values that confer to the laminates good tensile strength as well as making them deformation proof. Said characteristics make it possible to remove the laminates from the molds without damaging them.
Therefore, object of the present invention is a mixture of elastomeric thermoplastic compositions comprising (weight percentage):
A) from 40 to 90%, preferably from 60 to 80%, of a heterophasic polyolefin composition (A) having a melt flow rate (measured according to ASTM-D 1238, condition L) ranging from 20 to 100 g/10 min, preferably from 30 to 60 g/10 min; and
B) from 10 to 60%, preferably from 20 to 40%, of a partially dynamically cross-linked heterophasic polyolefin composition (B).
BRIEF SUMMARY OF THE INVENTION
Heterophasic polyolefin composition refers to a composition comprising polymers of the CH
2
═CHR olefins, where R is hydrogen or a C
1
-C
8
alkyl radical; the composition comprises both crystalline and amorphous elastomeric polymers.
By definition “partially cross-linked” it is meant that to the degree of cross-linking in terms of the content of gels with respect to the weight of the fraction of elastomeric copolymer soluble in xylene at ambient temperature (i.e. about 25° C.) before cross-linking, is preferably less than or equal to 70%, more preferably less than 50%, from 3% to 45% for example. The gels content corresponds to the fraction of elastomeric copolymer that is rendered insoluble by cross-linking.
Preferably heterophasic compositions (A) and (B) are obtained from a basic composition comprising the following polymer fractions (parts and percentages by weight):
a) 10-40 parts, preferably 20-40, of an isotactic propylene homopolymer having an isotactic index greater than 80, preferably greater than 90, or a propylene random copolymer with ethylene and/or a C
4
-C
10
&agr;-olefin of formula CH
2
═CHR, where R is a C
2
-C
8
alkyl radical; said copolymer containing greater than 80% of propylene and having an isotactic index in boiling heptane greater than 80;
b) 0-20 parts of a copolymer fraction containing ethylene, insoluble in xylene at ambient temperature; and
c) 40-95 parts, preferably 60-80, of a copolymer fraction of ethylene with propylene and/or a C
4
-C
10
&agr;-olefin of formula CH
2
═CHR, where R is a C
2
-C
8
alkyl radical, and optionally with a small quantity of diene; said fraction, soluble in xylene at ambient temperature, contains ethylene in quantities less than or equal to 35%, preferably from 15 to 30%.
Examples of the above mentioned basic composition are described in European patent application EP-A-0 472 946 published on behalf of Himont Incorporated.
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