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
1999-04-19
2001-05-01
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...
C525S201000, C525S240000
Reexamination Certificate
active
06225411
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a blend of propylene polymers.
BACKGROUND OF THE INVENTION
High melt strength propylene polymers are known, for example, those described in U.S. Pat. No. 4,916,198. However, these materials tend to be brittle. Various methods have been tried to overcome this problem, such as blending a soft polymer material with a high melt strength polymer material or irradiating a soft polymer starting material, but with limited success. There is still a need for a propylene polymer material that exhibits both high melt strength and softness.
SUMMARY OF THE INVENTION
The composition of this invention comprises a blend of (1) a propylene polymer selected from the group consisting of (a) a propylene homopolymer and (b) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having an isotactic index of greater than 90, and (2) a propylene polymer made using a metallocene catalyst and selected from the group consisting of (a) a propylene homopolymer and (b) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having a crystallinity of less than 24% measured from the heat of crystallization. The blend has a melt tension of greater than 7 centiNewtons (cN) at 200° C. and a Young's modulus of less than 1000 MPa.
In another embodiment, an irradiated blend is prepared by (1) preparing a blend comprising (1) and (2) described above, (2) irradiating the blend in an environment in which the active oxygen concentration is established and maintained at less than 15% by volume with high energy ionizing radiation at a radiation dose of 3-12 Mrad, for a period of time sufficient for a substantial amount of radical formation to occur, but insufficient to cause gelation of the material; (3) maintaining the irradiated material in such an environment for a period of up to two hours; and (4) treating the irradiated material while in such an environment to deactivate substantially all of the free radicals present in the irradiated material, whereby the irradiated blend has a melt tension of greater than 7 centiNewtons at 200° C. and a Young's modulus of less than 1000 MPa.
DETAILED DESCRIPTION OF THE INVENTION
Component (1) of the blend of this invention is a propylene polymer selected from the group consisting of (a) a propylene homopolymer and (b) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin, wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less. The propylene polymer has an isotactic index greater than 90, preferably greater than 94.
Component (2) of the blend is a propylene polymer made with a metallocene catalyst, selected from the group consisting of (a) a propylene homopolymer and (b) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having a crystallinity of less than 24%, measured from the heat of crystallization. An atactic polymer or copolymer, i.e., one having little or no crystallinity, is preferred. Any metallocene catalyst that is capable of producing propylene polymers with the low crystallinity specified above can be used. Such catalysts are well known in the art. One such metallocene catalyst is the reaction product of an organic compound of Ti, Zr, or Hf, e.g., dimethyl- or dibutylsilanediylbis(fluorenyl)zirconium dichloride, and an alumoxane. Preparation of atactic polypropylene and a suitable catalyst are described, for example, in U.S. Pat. No. 5,596,052, which is incorporated herein by reference.
The polymer blend can also contain conventional additives for polyolefins such as, for example, antioxidants, UV light stabilizers, and antacids.
One process for preparing the polymer blend comprises:
(1) preparing a blend comprising:
(a) a propylene polymer selected from the group consisting of (i) a propylene homopolymer and (ii) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin, wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having an isotactic index greater than 90, and
(b) a propylene polymer made using a metallocene catalyst and selected from the group consisting of (i) a propylene homopolymer and (ii) a copolymer of propylene and ethylene or a 4-8 C alpha-olefin wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having a crystallinity of less than 24% measured from the heat of crystallization,
(2) irradiating the blend in an environment in which the active oxygen concentration is established and maintained at less than 15% by volume with high energy ionizing radiation at a radiation dose of 3 to 12 Mrad, preferably 6 to 9 Mrad, for a period of time sufficient for a substantial amount of radical formation to occur, but insufficient to cause gelation of the material;
(3) maintaining the irradiated material in such an environment for a period of up to two hours; and
(4) treating the irradiated material while in such an environment to deactivate substantially all of the free radicals present in the irradiated material, whereby the irradiated blend has a melt tension of greater than 7 centiNewtons at 200° C. and a Young's modulus of less than 1000 MPa.
The polymer blend can be prepared before irradiation by mixing the polymers in solution, mechanically blending the two preformed polymers, or by making the blend in a polymerization reactor by preparing the two polymers sequentially using different catalysts for each component. Alternatively, each component can be irradiated separately and then blended with the other component. Irradiation after blending of the two components is preferred.
The expression “active oxygen” means oxygen in a form that will react with the irradiated material and more particularly the free radicals in the material. The active oxygen content requirement of the process of this invention can be achieved by use of vacuum or by replacing part or all of the air in the environment by an inert gas such as, for example, nitrogen. The active oxygen concentration of the environment is preferably less than 5% by volume, and more preferably less than 1% by volume. The most preferred concentration of active oxygen is about 0.004% by volume.
The term “rad” is usually defined as that quantity of ionizing radiation that results in the absorption of 100 ergs of energy per gram of irradiated material, regardless of the source of radiation. Energy absorption from ionizing radiation is measured by the well known conventional dosimeter, a measuring device in which a strip of fabric containing a radiation sensitive dye is the energy absorption sensing means. The term “rad” means that quantity of ionizing radiation resulting in the absorption of the equivalent of 100 ergs of energy per gram of the fabric of a dosimeter placed at the surface of the propylene polymer being irradiated, whether in the form of a bed or layer of particles, or a film or sheet.
The ionizing radiation can be of any kind, but the most practical kinds are electrons and gamma rays. Electrons beamed from an electron generator are preferred.
The third step of the process is performed in a period of time generally in the range of about one minute to about two hours, and preferably about 2-90 minutes. The final step of the process, which is the free radical deactivation or quenching step, can be performed by the application of heat or by the addition of an additive that functions as a free radical trap, such as, for example, methyl mercaptan.
The process for irradiating the blend of propylene polymers is described in more detail in U.S. Pat. No. 4,916,198, which is incorporated herein by reference.
The irradiated blend or blend of irradiated polymers has a melt tension of greater than 7 cN at 200° C., preferably greater than 12 cN,
Dang Vu A.
Dong Dinshong
Montell Technology Company BV
Nutter Nathan M.
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