Heterophasic copolymers

Details Heterophasic copolymers Heterophasic copolymers

2002-06-10

2004-04-20

Boykin, Terressa M. (Department: 1711)

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

Synthetic resins

Treating polymer containing material or treating a solid...

Reexamination Certificate

active

06723829

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to heterophasic copolymers, more particularly heterophasic copolymers of polypropylene, PP.
BACKGROUND OF THE INVENTION
PP heterophasic copolymers, also referred to as PP block copolymers, comprise a polymer matrix with a dispersed rubbery copolymer phase. The matrix is a homopolymer or random copolymer matrix. The rubbery copolymer phase is a reactor blend of an amorphous rubber, a rubber-like polymer which is normally an ethylene-propylene copolymer (rubber), and a semicrystalline ethylene copolymer.
The heterophasic copolymers are produced in two or more reactors. The matrix homopolymer or random copolymer may be produced by standard polymerisation with Ziegler-Natta catalyst system in one or more slurry or bulk (loop) reactors or gas phase reactors or combinations of both. In a second stage, the polymerisation is continued and the rubbery copolymer phase is produced in the matrix polymer using one or more gas phase reactors.
The composition of the rubbery phase is controlled in the second stage by the ethylene/propylene ratio and the amount of hydrogen. The comonomer ratio (CR) ethylene/propylene in mol/mol, which is otherwise expressed as
CR
=C
2
/C
3
determines the rubbery copolymer composition. When CR is equal to or higher than 9, essentially ethylene copolymers are produced, and when CR is lower than around 0.1, essentially PP random copolymers are produced. Generally, the lower the CR, the less ethylene copolymers are present in the rubbery phase.
The amorphous rubber content is generally assessed by dissolving the polymer in xylene. The amount of xylene solubles, XS (weight-%) at room temperature (RT), corresponds to the amount of rubber. The rubber composition is defined by the ethylene content by weight-% in the xylene solubles, C
2
of AM, where AM is the with acetone precipitated amorphous rubber content in the xylene soluble fraction, at RT.
The hydrogen in the second stage controls the molecular weight of the rubber, generally measured as the intrinsic viscosity (IV) of the with acetone precipitated amorphous rubber fraction (AM) of the xylene soluble fraction. Intrinsic viscosity is measured in decaline at 135° C.
There is a continuing need for PP heterophasic copolymers with improved properties, notably materials with good flow and impact characteristics, especially for moulding, injection moulding, thin wall packaging and engineering applications.
PP heterophasic copolymers with a high melt flow rate, e.g. MFR>40 g/10 min, and with medium or high impact strength are difficult to produce directly by polymerisation with a Ziegler-Natta catalyst system. The difficulty arises because the matrix polymer of a heterophasic copolymer needs to have an MFR which is 50 to 100% higher than the MFR of the final copolymer. In turn this requirement means that very high hydrogen concentrations have to be used in the polymerisation of the matrix polymer. In many cases that is not possible. In addition a matrix with a very high MFR is very brittle which then affects the whole copolymer.
Heterophasic copolymers with very high amounts of rubber (XS>20 wt-%), so called super high impact copolymers or reactor-made thermoplastic olefin copolymers (rTPOs), are even more difficult to produce with high MFR.
Another known route to a high MFR product involves chemical treatment, i.e., visbreaking (peroxide treatment) of a molten PP heterophasic copolymer. The visbroken copolymer, also called a controlled rheology polymer, generally has low impact properties.
By visbreaking PP with heat or at more controlled conditions with organic peroxides the molar mass distribution, MWD, will be narrower because the long molecular chains are more easily broken up or scissored and the molar mass M
w
will decrease, corresponding to an MFR increase. The MFR increases with increase in the amount of peroxide which is used.
Because the molar mass distribution is narrower and the molar mass is decreased by visbreaking, the flowability will be improved (=controlled rheology). The narrower MWD also changes the mechanical properties of a polymer. For example, visbroken PP homopolymers and random copolymers have lower stiffness (tensile modulus, flexural modulus) and slightly higher impact properties than a standard PP homopolymer or random copolymer with the same MFR.
During visbreaking, ethylene polymers and copolymers are cross-linked by the peroxide. That means that the molar mass of the ethylene polymer will increase corresponding to a great drop in MFR. This is seen as gel formation and lack of flowability.
Visbreaking of PP heterophasic copolymer, the block copolymer, is more complicated because of the complex blend composition of homopolymer or random copolymer matrix, amorphous rubber and semicrystalline ethylene copolymer. Both the matrix polymer and the rubber decrease in molar mass, giving higher MFR, but the ethylene copolymers are cross-linked and then will cause problems in the PP copolymer.
This negative reaction results in flow problems, or gels. The mechanical properties, both stiffness and impact, drop. The visbreaking of a heterophasic copolymer to a certain MFR consumes more organic peroxide than visbreaking of a corresponding homopolymer to the same MFR, because of the negative reactions with the ethylene copolymer. The consumption of the expensive peroxide increases the more rubbery copolymer there is in the heterophasic copolymer, and the more PE rich is the rubbery copolymer.
The present invention addresses the need for PP heterophasic copolymers with good flow and impact characteristics.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method of preparing a polypropylene heterophasic copolymer with good flow and impact properties, which involves visbreaking a polypropylene heterophasic copolymer with an optimised rubber composition. The amorphous rubber has an intrinsic viscosity of 2 dl/g or more and the ethylene content in the rubber, determined as C
2
of AM is from 20 wt-% to 45 wt-%. The rubber should have a higher molecular mass than the matrix,
IV
rubber
>IV
matrix
.
Thus, we have found that a heterophasic copolymer produced with an optimized rubber composition can be visbroken to high MFR materials without a significant loss of impact properties. The rubber needs to be produced with a low comonomer ratio and a high enough molecular weight.
Because the amorphous rubber decreases in molar mass on visbreaking, the starting heterophasic copolymer should be produced with a high enough rubber molar mass. The IV of AM should preferably be higher than 2 dl/g, more preferably higher than 2.5 dl/g so that the visbroken copolymers fulfill the criteria for a good impact copolymer.
Moreover, a suitably amorphous rubber is needed, which can be produced with CR>0.25.
More particularly, good impact properties at low temperatures in a visbroken PP heterophasic copolymer are determined by the amount of rubber (XS and AM in wt-%) and the rubber copolymer composition. In particular, this invention involves visbreaking of PP heterophasic copolymer where the IV of the AM≧2 dl/g and C
2
of AM>20 wt-% (corresponding to a CR>0.25 mol/mol.), and the C
2
of AM<40 wt-% (corresponding to a CR<0.7 mol/mol).
DETAILED DESCRIPTION OF THE INVENTION
In this invention we define an optimized rubber composition of a heterophasic polymer for visbreaking to give good impact properties.
By “starting polymer” is meant the heterophasic polypropylene polymer which comprises a polymer matrix with a dispersed rubbery phase. The matrix in the starting propylene polymer can be a homopolymer or a random copolymer. The rubbery copolymer phase is typically a reactor blend of an amorphous rubber, a rubber-like polymer which is normally an ethylene-propylene copolymer (rubber), and a semicrystalline ethylene copolymer.
The starting polymer is produced by any convenient route with which it is possible to prepare a polymer that fulfills the intrinsic viscosity and ethylene content criteria discussed be

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