Use and production of polypropylene

Details Use and production of polypropylene Use and production of polypropylene

2001-08-24

2004-04-27

Rabago, Roberto (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S160000, C526S351000, C526S943000

Reexamination Certificate

active

06727332

ABSTRACT:

TECHNICAL FIELD AND SUMMARY OF THE INVENTION
The present invention relates to the use of homopolymers and copolymers of propylene prepared with a metallocene catalyst component in applications which require a low melting temperature, a high crystallisation temperature and a high transparency. The present invention also relates to a process for producing isotactic polypropylene homopolymers.
BACKGROUND
It is known in the art to obtain polypropylene with low melting temperature by inserting comonomers in the polymer chain during polymerisation. With Ziegler-Natta catalysts, the addition of ethylene (or other) comonomers in the growing chains of polypropylene during polymerisation gives rise to a random propylene copolymer that is characterised by a lower melting point, a lower flexural modulus, lower rigidity and higher transparency that the homopolymers of propylene. The comonomers generate defects in the polymer chain which impede the growth of thick crystalline structures and reduce the degree of crystallinity of the overall polymer. The comonomers are not evenly distributed in the polymer chains. Among the many comonomers that can be used in the copolymerisation process, ethylene and butene have been most frequently utilised. It has been observed that the melting temperature of the propylene copolymers is reduced by about 6° C. per wt % of inserted ethylene in the copolymer chain or by about 3° C. per wt % of inserted butene.
The addition of comonomer in industrial polymerisation processes has however other impacts than just decreasing the melting temperature of the polypropylene; it has both economical and technical impacts.
These known random propylene copolymers also suffer from the technical problem that the crystallisation temperature is relatively low. This is technically disadvantageous when the polypropylene is being processed since the low crystallisation temperature increases the cycle time of any process where the polypropylene is being solidified from the melt. With the lower crystallisation temperatures, the period for solidification is longer, thereby increasing the cycle time for injection moulding, injection blow moulding and extrusion blow moulding and decreasing the production line speed in film, tube, profile or pipe extrusion.
EP-A-0881236 in the name of Fina Research S.A. and EP-A-0537130 in the name of Fina Technology, Inc. each disclose a metallocene catalyst component for use in producing isotactic polypropylene.
EP-A-0870779 in the name of Fina Technology, Inc. discloses metallocene catalysts for producing a blend of iso-and syndiotactic polypropylene.
EP-A-0742227 in the name of Fina Technology, Inc. discloses a metallocene compound for producing hemisotactic polypropylene.
EP-A-0905173 discloses the production of biaxially oriented metallocene-based polypropylene films.
Despite the disclosures of these prior Fina patent specifications, there is a need in the art for polypropylene having not only relatively low melting temperature but also relatively high crystallisation temperature which enables the polypropylene to be used more readily in processes requiring the polypropylene to be solidified from the melt using shorter cycle times or higher film speeds.
Accordingly, the present invention provides use of isotactic polypropylene homopolymers or copolymers in processes in which the polypropylene solidifies from a melt.
DETAILED DESCRIPTION
In a preferred embodiment, the present invention is directed to the use of isotactic polypropylene homopolymers or copolymers in processes in which the polypropylene solidifies from a melt, wherein for enhanced speed of solidification of the polypropylene the polypropylene has a melt temperature and a crystallisation temperature not more than 50° C. less than the melt temperature resulting from the polypropylene having been produced using a metallocene catalyst component having the general formula:
R″(C
p
R
1
R
2
R
3
)(C
p
′R
n
′)MQ
2
  (I)
wherein C
p
is a substituted cyclopentadienyl ring; C
p
′ is a substituted or unsubstituted fluorenyl ring; R″ is a structural bridge imparting stereorigidity to the component; R
1
is a substituent on the cyclopentadienyl ring which is distal to the bridge, which distal substituent comprises a bulky group of the formula XR*
a
in which X is chosen from Group IVA, and when a=3 each R* is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms, or when a=2 one R* is chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms and the other different R* is chosen from a substituted or unsubstituted cycloalkyl where X is a carbon atom in the cycloalkyl ring, R
2
is a substituent on the cyclopentadienyl ring which is proximal to the bridge and positioned non-vicinal to the distal substituent and is hydrogen or of the formula YR#
3
in which Y is chosen from Group IVA, and each R# is the same or different and chosen from hydrogen or hydrocarbyl of 1 to 7 carbon atoms, R
3
is a substituent on the cyclopentadienyl ring which is proximal to the bridge and is a hydrogen atom or is of the formula ZR$
3
in which Z is chosen from Group IVA, and each R$ is the same or different and chosen from hydrogen or hydrocarbyl of 1 to 7 carbon atoms, each R′
n
is the same or different and is hydrocarbyl having 1 to 20 carbon atoms in which 0≦n≦8; M is a Group IVB transition metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or is a halogen.
The metallocene catalyst component may be employed either alone or in a mixture of one or more metallocene catalyst components.
In accordance with the invention, it has now been found that homopolymers and copolymers of propylene obtained using these metallocene catalysts have characteristics which are similar to and better than those exhibited by random polypropylene (PP) copolymers produced using Ziegler-Natta catalysts. These desired characteristics are not only a low melting temperature and a high transparency, but also a high crystallisation temperature. The polymers usable in accordance with the invention can also tend to have higher rigidity as compared to random PP copolymers produced using Ziegler-Natta catalysts and higher flexibility than homopolymers produced using Ziegler-Natta catalysts. As well as having high transparency, the preferred polymers for use in the invention also have low haze.
The present invention is predicated on the discovery by the inventor that the use of particular metallocene catalysts enables polypropylene homopolymers, or polypropylene copolymers with a small amount of comonomer, to have a combination of relatively low melting temperature and relatively high crystallisation temperature which reduces the cycle time for processing the polymer from the melt, for example in injection moulding and injection or extrusion blow moulding. The amount of comonomer in the copolymers is not more than 25 wt %, typically less than 10 wt %, more preferably less than 5 wt % and yet more preferably less than 3 wt %. Typical comonomers are ethylene and butene, but other alpha-olefins may be employed. For extrusion processes, such as pipe, tube or profile extrusion and for film production, the higher crystallisation temperature permits higher line speeds. The film may be produced by casting, a tenter frame process or blowing. Other polyolefin processing methods exist for which the present invention has utility in which the polypropylene is processed from the melt. The combination of a low melting temperature and a high crystallisation temperature provides a reduced temperature “window” between those two temperatures, which enables the polypropylene to be more easily and more quickly processed when the polypropylene is processed in the melt and then solidified from the melt.
Thus the present invention further provides a process for producing an isotactic homopolymer of propylene having a melt temperature of from 139 to 144° C. and a difference between the melt temperature and the crystallisation temperature of

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