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
2000-03-30
2002-05-28
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
Reexamination Certificate
active
06395831
ABSTRACT:
The present invention relates to crystalline propylene copolymer compositions useful in the preparation of heat-sealable films, sheets and films thereof and to a process for preparing said compositions.
Crystalline copolymers of propylene with other olefins (mainly ethylene, 1-butene or both), or mixtures of such copolymers with other olefin polymers are known in the prior art as heat-sealable materials.
These crystalline copolymers are obtained by polymerizing propylene with minor amounts of other olefin comonomers in the presence of coordination catalysts.
The polymerized comonomer units are statistically distributed in the resulting copolymer and the melting point of said copolymers results to be lower than the melting point of crystalline propylene homopolymers. Also the seal initiation temperature (as later defined in detail) of the said copolymers results to be favorably low.
However, the introduction of the comonomer units adversely affects the crystal structure of the polymer, resulting in relatively large quantities of a polymer fraction soluble in organic solvents, so that the copolymers having a particularly low seal initiation temperature cannot be used in the field of food packaging.
Many technical solutions are disclosed in the prior art in order to find a good balance between heat-sealability (as demonstrated by low seal initiation temperatures) and solubility. In particular, published European patent application 483523 discloses compositions prepared directly in a polymerization process, having a low seal initiation temperature and a low content of a fraction soluble in xylene at room temperature or in n-hexane at 50° C. These compositions comprise (by weight):
30-60% of a copolymer of propylene and a C
4
-C
8
alpha-olefin, containing 80-98% of propylene;
35-70% of a copolymer of propylene with ethylene and optionally 1-10% of a C
4
-C
8
alpha-olefin, wherein the content of ethylene is 5-10% when the C
4
-C
8
alpha-olefin is not present, or 0.5-5% when the C
4
-C
8
alpha-olefin is present.
Published European patent application 674991 discloses other compositions prepared directly in a polymerization process, having a good ink adhesion in addition to a low seal initiation temperature and low content of a polymer fraction soluble in organic solvents. These compositions comprise (by weight):
20-60% of a copolymer of propylene with ethylene, containing 1 to 5% of ethylene;
40-80% of a copolymer of propylene with ethylene and a C
4
-C
8
alpha-olefin, the ethylene content being 1-5% and the C
4
-C
8
alpha-olefin content being 6-15%;
the total content of ethylene in the compositions being 1-5% and the total content of C
4
-C
8
alpha-olefin in the compositions being 2.4-12%.
Other heat-sealable compositions, comprising two different kinds of copolymers of propylene with higher alpha-olefins, are disclosed in the published European patent application 560326.
On the other hand, it is known that, in general, the Melt Flow Rate (MFR) of an olefin polymer can be adjusted by degradation, in particular by treatment at elevated temperatures in the presence of an initiator of free radicals (“visbreaking” treatment). In the field of heat-sealable olefin copolymers it is also known that the said degradation treatment can improve the heat-seal, optical and solubility properties, as explained in particular in U.S. Pat. No. 5,246,769 and in the Japanese published patent application Sho 59-117506.
However, in these two documents the degradation treatment is applied to single copolymers of propylene with ethylene and/or higher alpha-olefins, resulting into a still high seal initiation temperature.
An attempt to apply the degradation treatment to heat-sealable compositions comprising two copolymers of propylene with ethylene and/or higher alpha-olefins is disclosed in the published European patent application 203727. In this case a copolymer of propylene with 25-45% by weight of butene-1 is first subjected to the degradation treatment and then blended with another copolymer of propylene with ethylene and/or butene-1. However, while the seal initiation temperature and the haze values are brought to satisfactory levels, the content of fraction soluble in organic solvents results to be high (25° C. xylene-soluble fraction contents of around 30% are considered to be satisfactory).
It has now surprisingly been found that a particularly valuable balance of heat-sealability, low content of a fraction soluble in organic solvents and optical properties (in particular a very low Haze) is obtained by subjecting specific crystalline propylene copolymer compositions to a degradation treatment. Therefore the present invention provides crystalline propylene copolymer compositions having MFR L values from 2 to 15 g/10 min., preferably from 3 to 15 g/10 min., more preferably from 4 to 10 g/10 min., and comprising (percent by weight):
A) 20-80%, preferably 20-60%, more preferably 30-50%, of one or more propylene copolymers selected from the group consisting of (A 1) propylene/ethylene copolymers containing 1-7% of ethylene; (A 2) copolymers of propylene with one or more C
4
-C
8
alpha-olefins, containing 2-10% of the C
4
-C
8
alpha-olefins; (A 3) copolymers of propylene with ethylene and one or more C
4
-C
8
alpha-olefins, containing 0.5-4.5% of ethylene and 2-6% of C
4
-C
8
alpha-olefins, provided that the total content of ethylene and C
4
-C
8
alpha-olefins in (A 3) be equal to or lower than 6.5%;
B) 20-80%, preferably 40-80%, more preferably 50-70%, of one or more propylene copolymers selected from the group consisting of (B 1) copolymers of propylene with one or more C
4
-C
8
alpha-olefins, containing from more than 10% to 30% of C
4
-C
8
alpha-olefins; (B 2) copolymers of propylene with ethylene and one or more C
4
-C
8
alpha-olefins, containing 1-7% of ethylene and 6-15% of C
4
-C
8
alpha-olefins;
the said MFR L values (MFR L (2)) being obtained by subjecting to degradation a precursor composition comprising the same components A) and B) in the above said proportions, but having MFR L values (MFR L (1)) from 0.3 to 5 g/10 min., preferably from 0.5 to 3 g/10 min., with a ratio MFR L (2) to MFR L (1) of from 2 to 20, preferably from 3 to 15.
From the above definitions it is evident that the term “copolymer” includes polymers containing more than one kind of comonomers.
As previously said, the compositions of the present invention have low seal initiation temperatures (preferably lower than 100° C.), a low content of a fraction soluble or extractable in organic solvents (preferably equal to or lower than 20% by weight in xylene at 25° C. and equal to or lower than 5% by weight in n-hexane at 50° C.), and very low haze values (preferably lower than 1%, more preferably equal to or lower than 0.5%, measured on films according to the method described in the examples).
The melting temperature of said composition is preferably from about 126 to 147° C.
Moreover, the compositions of the present invention can be obtained by an efficient and inexpensive process (constituting a further object of the present invention), comprising the following stages:
1) preparing the previously said precursor composition by polymerizing the monomers in at least two sequential steps, wherein components A) and B) are prepared in separate subsequent steps, operating in each step in the presence of the polymer formed and the catalyst used in the preceding step, and dosing the molecular weight regulator (preferably hydrogen) in such amounts as to obtain a MFR L (1) value for the precursor composition of from 0.3 to 5 g/10 min., preferably from 0.5 to 3 g/10 min.;
2) subjecting the precursor composition obtained in 1) to a degradation treatment in order to obtain MFR L (2) values for the final composition from 3 to 15 g/10 min. preferably from 4 to 10 g/10 min., with a degradation ratio, in terms of ratio MFR L (2) to MFR L (1), of from 2 to 20, preferably from 3 to 15.
Such a preferred process is extremely convenient, as it avoids the separate preparation of the components of the precursor composition and separate
Ferrari Paolo
Pelliconi Anteo
Baselltech USA Inc.
Nutter Nathan M.
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