Multimodal copolymer of ethylene and at least two alpha-olefin comonomers and final articles made thereof
The present invention is directed to a multimodal polyethylene copolymer comprising a first and a second copolymer of ethylene and at least two alpha-olefin comonomers. Such multimodal copolymers are highly suitable for conversion processes that require a high Draw Down Ratio, like the production of thin films. Such multimodal polyethylene copolymers provide a good impact strength in the sense of a high Dart drop impact strength (DDI) and good isotropy of the films produces thereof. The invention further presents final articles such as films made therefrom.
1. A multimodal polymer composition said polymer composition having
a) a density of from 906 to 925 kg/m 3 determined according to ISO 1183,
b) an MFR 21 of 10-200 g/10 min determined according to ISO 1133,
wherein the multimodal polymer composition comprises
c) 35 wt % to 50 wt % of a first copolymer comprising
i) a prepolymerisation fraction
ii) a first fraction comprising a copolymer of ethylene and a first alpha-olefin comonomer having 4 to 10 carbon atoms and
iii) a second fraction comprising a copolymer of ethylene and a first alpha-olefin comonomer having 4 to 10 carbon atoms
wherein the first copolymer has a higher MFR 2 than the MFR 2 of the prepolymerisation fraction and first fraction combined, and wherein the weight ratio of the first fraction to second fraction is 2:1 to 1:2; and
wherein the prepolymerisation fraction is from 1 to 5 wt % of the multimodal polymer composition, the first fraction is at least 13 wt % of the multimodal polymer composition, and the second fraction is at least 13 wt % of the multimodal polymer composition;
d) 50 wt % to 65 wt % of a second copolymer of ethylene
i) comprising a second alpha-olefin comonomer having 6 to 10 carbon atoms, and having a density of below 900 kg/m 3 when calculated according to Equation 5:
ρ b =w 1 ·ρ 1 +w 2 ·ρ 2
wherein ρ b , ρ 1 and ρ 2 refer respectively to the densities of the overall mixture b, first copolymer and second copolymer, based on values determined according to ISO 1183; and wherein w 1 and w 2 refer respectively to the weight fractions of the first and second copolymers;
wherein the copolymers of said multimodal polymer composition comprise at least two alpha-olefin-comonomers in total.
2. A multimodal composition as claimed in claim 1 wherein the first copolymer mixture further comprises a prepolymerization fraction.
3. A multimodal polymer composition as claimed in claim 1 wherein the copolymer has a DDI 20 of 400 g or more when being converted into a film.
4. A multimodal polymer composition as claimed in claim 1 wherein the first alpha-olefin comonomer in the first and second fractions comprises 1-butene and the second alpha-olefin comonomer in the second copolymer comprises 1-hexene.