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
2001-01-05
2002-10-08
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...
C525S240000, C428S500000, C428S515000, C428S516000
Reexamination Certificate
active
06462134
ABSTRACT:
The present invention concerns polyethylene compositions comprising a linear low-density copolymer of ethylene (LLDPE) having a narrow distribution of molecular weights, and a crystalline copolymer of propylene.
The films obtainable from the said compositions possess an excellent balance of mechanical properties and optical properties.
Moreover, the aforesaid compositions are readily processable in the melted state, since they do not require large expenditures of energy in the machines used for their processing and do not cause high pressures at the head in the machines themselves.
The copolymer of ethylene used for the compositions of the present invention possesses a molecular weight distribution, in terms of the ratio between the average ponderal molecular weight (M
w
) and the average numerical molecular weight (M
n
), that is in terms of M
w
/M
n
, which is particularly narrow (corresponding to values of M
w
/M
n
less than 4) and hence typical of the polyethylenes obtained with metallocene catalysts.
In that respect, the compositions of the present invention differ from the compositions described in the published patent applications WO 93/03078 and WO 95/20009, in which the LLDPE copolymer (which is mixed with a crystalline copolymer of propylene) is prepared with Ziegler-Natta catalysts and hence possesses values of M
w
/M
N
typically greater than or equal to 4.
According to the U.S. Pat. No. 4,871,813, it is possible to prepare LLDPE copolymers having M
w
/M
n
values less than 4 (from 2.5 to 6) even when the catalyst used is of the Ziegler-Natta type, however in the examples only an LLDPE copolymer having a M
w
/M
n
value of 4 is used.
Also in the cited US patent the LLDPE copolymer is mixed with a copolymer of propylene, however the crystallinity of that copolymer of propylene is rather low, as is indicated by the low values of the enthalpy of fusion (less than or equal to 75 J/g) and, in particular, the degree of crystallinity (less than 35%).
The aforesaid documents show that by the addition of the copolymer of propylene to the LLDPE copolymer, polyethylene compositions having improved processability in the melted state, in the aforesaid sense, are obtained.
According to the U.S. Pat. No. 4,871,813, this effect is obtained without substantially changing the optical and mechanical properties of the film, compared to those of a film obtained from the pure LLDPE copolymer.
According to the published patent application WO 95/20009, as well as improving the processability in the melted state, the addition of the crystalline copolymer of propylene is capable of improving the resistance of the polyethylene film to impact and to tearing.
However, the optical properties of the compositions comprising an LLDPE copolymer obtained with Ziegler-Natta catalysts and a copolymer of propylene are inferior, in particular as regards the haze and gloss values, to those typical of an LLDPE copolymer obtained with metallocene catalysts and having relatively high F/E ratio values, i.e. greater than 20.
The aforesaid LLDPE copolymers obtained with metallocene catalysts in general show haze values less than ca. 20% and gloss values greater than ca. 30% (measured on blown film of thickness 25 &mgr;m by the method described in the examples).
Corresponding to the said high haze and gloss values, such LLDPE copolymers also possess satisfactory impact resistance (Dart Test) and tear resistance (Elmendorf) values.
However, the LLDPE copolymers obtained with metallocene catalysts display unsatisfactory processability in the melted state.
Hence, it would be particularly desirable to obtain polyolefin compositions having the aforesaid optical properties, with the best balance possible of impact resistance and tear resistance and good processability in the melted state.
In the U.S. Pat. No. 5,674,945, polyethylene compositions comprising an LLDPE copolymer obtained with metallocene catalysts and a copolymer of propylene having a density greater than or equal to 0.900 g/cm
3
are described.
In particular, in the examples a copolymer containing 0.2 mol % of butene and a copolymer containing 3.4 mol % of ethylene and 1.6 mol % of butene are used.
In both cases, the relative quantity of propylene copolymer in the polyethylene compositions is 10% by weight, and the transparency of the films obtained from such compositions is found to be substantially unchanged compared to the transparency of the films obtained from the corresponding LLDPE copolymers in the pure state.
Moreover, the films obtained from the aforesaid compositions show high tensile modulus values, superior to those of the films obtained from the corresponding LLDPE copolymers in the pure state, and high breaking strain values.
The technical problem consisting in the obtention of excellent balances of optical properties, impact resistance and tear resistance is not considered.
Polyethylene compositions have now been created which fully satisfy the aforesaid requirements, thanks to an unusual and particularly favourable balance of mechanical and optical properties and processability in the melted state.
Hence the object of the present invention is constituted by polyethylene compositions comprising (percentages by weight):
A) from 60 to 95%, preferably from 60 to 90%, more preferably from 70 to 88%, of a copolymer of ethylene with an alpha-olefin CH
2
═CHR, in which R is an alkyl radical containing from 1 to 18 atoms of carbon (LLDPE copolymer), the said copolymer having a density from 0.905 to 0.935 g/cm
3
, preferably from 0.910 to 0.930, more preferably from 0.915 to 0.925 g/cm
3
(measured according to ASTM D 4883), M
w
/M
n
values less than 4, preferably from 1.5 to 3.5, more preferably from 1.5 to 3 (measured by GPC, i.e. gel permeation chromatography) and F/E ratio values greater than 20, preferably from 25 to 70, more preferably from 25 to 50 (measured according to ASTM D 1238);
B) from 5 to 40%, preferably from 10 to 35%, more preferably from 12 to 30%, of one or more crystalline copolymers of propylene selected from among (i) copolymers of propylene with ethylene containing from 3 to 8%, preferably from 4 to 6%, of ethylene; (ii) copolymers of propylene with one or more alpha-olefins CH
2
═CHR
I
, where R
I
is an alkyl radical having from 2 to 8 carbon atoms or an aryl radical, containing from 6 to 25%, preferably from 8 to 20%, of alpha-olefins CH
2
═CHR
I
; (iii) copolymers of propylene with ethylene and one or more alpha-olefins CH
2
═CHR
I
, where R
I
has the aforesaid meaning, containing from 0.1 to 8%, preferably from 0.5 to 5%, more preferably from 1 to 4%, of ethylene, and from 0.1 to 20%, preferably from 1 to 15%, more preferably from 2.5 to 15%, in particular from 2.5 to 10%, of alpha-olefins CH
2
═CHR
I
, on condition that the total content of ethylene and alpha-olefins CH
2
═CHR
I
in the copolymers (iii) is greater than or equal to 5%.
Optionally, in order to improve the optical properties, the compositions of the present invention can contain, in addition to the components A) and B), from 0.5 to 10%, preferably from 1 to 6%, by weight of an LDPE polyethylene (component C), relative to the total weight of A) +B) +C).
As seems clear from the foregoing description, polymers containing two or more types of comonomers are also included in the definition of copolymers.
The aforesaid compositions are generally characterized by haze values less than or equal to 25%, preferably less than or equal to 20%, in particular between 20 and 5%, and gloss values greater than ca. 30%, in particular between 30 and 60% (measured on blown film of thickness 25 &mgr;m by the method described in the examples).
Moreover, the compositions of the present invention are generally characterized by dart test values greater than or equal to 150 g, preferably greater than or equal to 200 g, in particular between 150 and 400 g, preferably between 200 and 400 g (measured on blown film of thickness 25 &mgr;m by the method described in the examples).
Moreover, the compositions of the present invention show part
Anibaldi Remo
Cecchin Giuliano
Ciarafoni Marco
Collina Gianni
Covezzi Massimo
Basell Polyolefine GmbH
Nutter Nathan M.
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