Polyethylene compositions having improved mechanical...

Details Polyethylene compositions having improved mechanical... Polyethylene compositions having improved mechanical...

2001-01-05

2002-08-20

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...

C525S240000, C428S500000, C428S515000, C428S516000

Reexamination Certificate

active

06437047

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 improved balance of mechanical properties and good 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 weight-average molecular weight ({overscore (M)}
w
) and the number-average molecular weight ({overscore (M)}
n
), that is in terms of {overscore (M)}
w
/{overscore (M)}
n
, which is particularly narrow (corresponding to values of {overscore (M)}
w
/{overscore (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 published patent applications We 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 {overscore (M)}
w
/{overscore (M)}
n
typically greater than or equal to 4.
According to U.S. Pat. No. 4,871,813, it is possible to prepare LLDPE copolymers having MW/Mh 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 {overscore (M)}
w
/{overscore (M)}
n
value of 4 is used.
Also in the US patent cited 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 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 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 mechanical 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 impact resistance (Dart Test), to those typical of an LLDPE copolymer obtained with metallocene catalysts.
The LLDPE copolymers obtained with metallocene catalysts in general show Dart Test values (measured on blown film of thickness 25 &mgr;m by the method described in the examples) higher than about 300 g.
Corresponding to the said high Dart Test values, such LLDPE copolymers also have satisfactory tear resistance values (Elmendorf), which generally vary in inverse proportion, i.e. they increase with decreasing Dart Test values, and excellent optical properties.
However, the LLDPE copolymers obtained with metallocene catalysts display unsatisfactory processability in the melted state, attributable to the narrow distribution of molecular weights that characterizes them.
Hence, it would be particularly desirable to obtain polyolefin compositions having the aforesaid high levels of impact resistance, with the best balance possible of impact resistance and tear resistance, improved optical properties, and good processability in the melted state.
In 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 mole % of butene and a copolymer containing 3.4 mole % of ethylene and 1.6 mole % of butene is 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 obtaining excellent balances of impact resistance and tear resistance is not considered.
Polyethylene compositions have now been produced 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 present invention provides 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), {overscore (M)}
w
/{overscore (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 values of melt flow rate F/E ratio from 10 to 20, preferably from 12 to 20 (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 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
, with the proviso 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)), with respect 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 Dart Test values greater than or equal to 300 g, preferably greater than or equal to 350 g, in particular between 300 and 800 g, preferably between 350 and 800 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 particularly high tear resistance values (Elmendorf), both as such and compared to the Dart Test values. Generally, such values are greater than or

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