Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1996-04-15
2002-10-01
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S348100, C526S352000, C526S158000, C526S129000, C526S348200, C526S348300, C526S348400, C526S348500, C526S348600
Reexamination Certificate
active
06458910
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the production of linear low density polyethylene (LLDPE) films. The LLDPE films are characterized by superior impact strength and MD tear resistance. The process of the invention comprises high stalk extrusion of LLDPE composition having molecular weight distributions, determined as M
z
/M
w
of greater than 3.5. The film products of high stalk extrusion exhibit Dart Drop impact values and MD tear resistance which are superior to those same values for films, of the same composition at the same thickness and density, but produced by methods other than high stalk extrusion.
SUMMARY OF THE INVENTION
The invention relates to the production of blown films of linear low density polyethylene. It also relates to polymers of linear low density polyethylene which exhibit molecular weight distributions as measured by M
z
/M
w
>3.5 which are formed in one reactor. Films of these polymeric products and the particular film production technique of the invention, high stalk extrusion, results in enhancing impact strengths and MD tear resistance.
That molecular weight distribution, as measured by M
z
/M
w
in the LLDPE appears to be attributable to the use of dimethyl aluminum chloride as a cocatalyst, combined with a catalyst precursor, in the production of the LLDPE.
The subject molecular weight distribution, M
z
/M
w
greater than 3.5, appears to be attributable to a high molecular weight fraction in the LLDPE which, in turn, appears to enhance the melt strength of the LLDPE, and to make it eminently useful in the process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention, LLDPE films are produced which are characterized by superior impact strength and MD tear resistance. Herein, impact strength is determined by dart impact (ASTM-1709). The dart impact resistance, measured in grams, of films of the invention ranges from 50 to >800, preferably 200 to >800, and most preferably 350 to >800 for example 70 to 1,000 or 800 to 1000. MD tear resistance of films of the invention are determined by ASTM-D1922, measured in grams/mil, ranges from 50 to 500, preferably 200 to 500, and most preferably 250 to 500.
The copolymer products used in the invention contain 70 to 350 ppm of dimethylaluminum chloride (DMAC) activator.
They are low density products characterized by a density ranging from 0.915 to 0.940 g/cm
3
. They exhibit a melt flow ratio range of 25 to 45.
As noted above, the copolymers used in the invention exhibit rather broad molecular weight distribution (MWD), as characterized by a M
z
/M
w
of greater than 3.5, and are not unimodal. The ratio M
z
/M
w
is a measure of the skewness of the molecular weight distribution towards the HMW part of the distribution. The individual moments are defined as follows:
M
w
=
Weight
Average
Molecular
Weight
=
Σ
M
i
w
i
Σ
w
i
M
z
=
“
Z
”
Average
Molecular
Weight
=
Σ
M
i
2
W
i
Σ
M
i
xw
i
where w
i
=weight fraction of the polymer with molecular weight between M
i
and M
i
+M.
For the invention polymers with a significant hump on the HMW side, the higher moment M
z
is significantly higher for the same M
w
as a normal LLDPE. Thus resulting in a higher M
z
/M
w
.
The invention LLDPE is typically broader in molecular weight distribution as measured by GPC or flow properties (MFR). The GPC curve is characterized by a significantly higher amount of high molecular weight species compared to a normal LLDPE. The GPC analysis was performed on a Waters 150C instrument with a set of 4 columns (1E6, 1E6, 1E4, 1E3 angstrom) at 140° C. All the analyses were performed with a 0.1% solution in 1-2-4 trichlorobenzene. The most-consistent way of characterizing this difference is by the ratio Mz/Mw which measures the skewness of the distribution on the HMW side. We find that the DMAC cocatalyzed LLDPEs have Mz/Mw that is consistently higher than 4 while normal LLDPEs tend to be around 3. The presence of the HMW species may also provide an explanation of the observed improvement in the optical properties of the invention LLDPE. It may be argued that the presence of the HMW species gives rise to significantly higher stresses prior to the onset of crystallization, retarding crystal growth. This may be of even greater significance at the surface of the polymer film. The DMAC LLDPE films tend to have smoother surfaces compared to normal LLDPEs and consequently have better opticals, i.e. lower haze and higher gloss.
In our prior patents, U.S. Pat. Nos. 5,210,167 and 5,258,449, we described films of LLDPE which exhibited excellent optical properties and excellent dart impact (ASTM D-1709). By excellent optical properties we mean that a haze of less than 5 (as determined by ASTM D-1003), preferably less than 10 and gloss of greater than 50, preferably greater than 70 (as determined by ASTM D-2457); at the time conventional LLDPE yielded films with poor optical properties with haze greater than 15 and gloss of less than 50. The dart impact (ASTM-D1709) of the films in our patents exceeded those of conventionally commercially produced films LLDPE by about 20-30%. These unobvious properties were attributed at least in part to the novel copolymer described in the patents; the novel copolymers were believed to be the result of catalytic effects of a dimethylaluminum chloride (DMAC) as a cocatalyst for a Ziegler type catalyst or precursor.
We now can produce films which exhibit dart impact (ASTM D-1709) which is superior to those described in those prior patents, U.S. Pat. Nos. 5,210,167 and 5,258,449. These new films are produced by the process described below under the heading “Film Production.” and from the same copolymers of ethylene described in U.S. Pat. Nos. 5,210,167 and 5,258,449 in catalysis employing DMAC as a cocatalyst. The improvement in dart impact properties of films of the invention is at least 25% greater than those, preferably 25 to 500% and most preferably 25 to 300%. It is noted that the improvement in dart impact properties of films of the invention is not accompanied by improved optical properties. The optical properties of the films of the invention include values of haze of greater than 15 (as determined by ASTM D-1003) and gloss of less than 50 (as determined by ASTM D-2457).
Polymerization
Olefins are polymerized with the catalysts prepared according to the present invention by any suitable process. Such processes include polymerizations carried out in suspension, in solution or in the gas phase. Gas phase polymerization reactions are preferred, e.g., those taking place in stirred bed reactors and, especially, fluidized bed reactors.
The linear polyethylene polymers prepared in accordance with the present invention are homopolymers of ethylene or copolymers of ethylene with one or more C
3
-C
10
alpha-olefins. Thus, copolymers having two monomeric units are possible as well as terpolymers having three monomeric units. Particular examples of such polymers include ethylene/1-butene copolymers, ethylene/1-hexene copolymers, ethylene/1-octene copolymers, ethylene/4-methyl/1-pentene copolymers, ethylene/1-butene/1-hexene terpolymers, ethylene/propylene/1-hexene terpolymers and ethylene/propylene/1-butene terpolymers. When propylene is employed as a comonomer, the resulting linear low density polyethylene polymer preferably has at least one other alpha-olefin comonomer having at least four carbon atoms in an amount of at least 1 percent by weight of the polymer. Accordingly, ethylene/propylene copolymers are possible, but not preferred. The most preferred comonomer is 1-hexene.
The linear low density polyethylene polymers produced in accordance with the present invention preferably contain at least about 80 percent by weight of ethylene units.
The molecular weight of the polymer may be controlled in a known manner,
Firdaus Vaseem
Shirodkar Pradeep P.
Choi Ling-Siu
ExxonMobil Oil Corporation
Prodnuk Stephen D.
Wu David W.
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