MXPA00012073A - Soft propylene polymer blend with high melt strength - Google Patents

Abstract

A polymer blend having a high melt strength and a low Young's modulus after irradiation of the polymers contains (1) a propylene homopolymer or copolymer having an isotactic index of greater than 90 and (2) a propylene homopolymer or copolymer having a crystallinity of<24%made using a metallocene catalyst. The blend can be used for extrusion coating (including fabric coating), foam extrusion, blow molding, and thermoforming applications.

Description

SOFT MIX OF PROPYLENE POLYMER WITH HIGH RESUSCITATION TO FUSION - *!% > Field of the Invention This invention relates to a mixture of propylene polymers.

BACKGROUND OF THE INVENTION Propylene polymers are known from High melt strength, for example, those described in the U.S. Pat. 4,916,198. However, these materials tend to be fragile. Several methods have been tried to overcome this problem, such as mixing a mild polymer material with a polymer material of high melt strength or irradiate a smooth 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 mixture of (1) a propylene polymer selected from the group consisting of (a) a homopolymer of propylene and (b) a copolymer of propylene and ethylene or an alpha-olefin of C 4-8, wherein the content of polymerized ethylene or alpha-olefin polymerized in 10% or less, the propylene polymer having a higher isotactic index 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 an alpha-olefin of C 4 -8 wherein the polymerized ethylene or polymerized alpha-olefin content 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 greater than 7 centiNewtons (c) at 200SC and a Young's modulus of less than 1000 MPa. In another embodiment, an irradiated mixture (a) is prepared by preparing a mixture comprising (1) and (2) above, (2) by irradiating the mixture in an environment in which the concentration of active oxygen is established and maintained at less than 15% by volume with the high energy ionization 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) keeping the irradiated material in said environment for a period of up to two hours; and (4) treating the irradiated material while in said environment to substantially deactivate all free radicals present in the irradiated material, whereby the irradiated mixture has a melting tension greater than 7 centiNewtons at 200SC and a Young's modulus of less than 1000 MPa.

Detailed Description of the Invention The component (1) of the mixture 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 an alpha- olefma of C 4-8, wherein the polymerized ethylene or polymerized alpha-olefin content is 10% or less. The propylene polymer has an isotactic index greater than 90, preferably greater than 94. The component (2) of the mixture 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 an alpha-olefin of C 4-8 wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the propylene polymer having a crystallinity of less of 24%, measured from £ .- '; I the heat of crystallization. An atactic polymer or copolymer, that is, 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. These catalysts are well known in the art. One of these metallocene catalysts is the reaction product of an organic Ti compound, Zr or Hf, e.g., dichloride of di ethyl- or dibutylsilandiylbis (f luorenyl) zirconium, and an alumoxane. The preparation of atactic polypropylene and any suitable catalyst are described, for example, in the U.S. Patent. 5,596,052, which is incorporated herein by reference. The polymer blend may also contain conventional additives for polyolefins such as, for example, antioxidants, UV light stabilizers, and antacids. A process for preparing the polymer blend comprises: (1) preparing a mixture comprising: (a) a propylene polymer selected from the group consisting of (i) a propylene homopolymer and (ii) a copolymer of propylene and ethylene or an alpha-olefin of C 4-8, 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 an alpha-olefin of C 4-8 wherein the content of polymerized ethylene or The 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 mixture in an environment in which the concentration of active oxygen is established and maintained less than 15% by volume with high energy ionization radiation at a radiation dosage of 3 to 12 Mrad, preferably 6 to 9 Mrad, for a sufficient period of time for a substantial amount of radical formation to occur, but insufficient to cause the gelation of the material; (3) keep the irradiated material in that environment for a period of up to two hours, AND (4) treat the irradiated material while in said environment to substantially deactivate all free radicals present in the irradiated material, whereby the irradiated mixture has a melting tension greater than 7 centiNewtons at 200eC and a Young's modulus less than 1000 MPa. The polymer mixture can be prepared prior to irradiation by mixing the polymers in solution, mechanically mixing the two previously formed polymers, or by making the mixture in a polymerization reactor to prepare the two polymers in sequence using different catalysts for each component. Alternatively, each component can be irradiated separately and then mixed with the other component. Irradiation after mixing of the two components is preferred. The term "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 using a vacuum or by replacing some or all of the air in the environment with 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 amount of ionization radiation that results in the absorption of 100 ergs of energy per gram of irradiated material, regardless of the source of radiation. The absorption of ionization radiation energy is measured by the well-known conventional dosimeter, a measuring device in which a strip of fabric containing a radiation-sensitive dye is the element of perception of energy absorption. The term "rad" means that amount of ionization radiation that results in the absorption of the equivalent of 100 ergs of energy per gram of the fabric of a dosimeter placed on the surface of the propylene polymer that is being irradiated, either in the form of a bed or layer of particles, or a film or sheet. The ionization radiation can be of any kind, but the most practical classes are electrons and gamma rays. Electrons fired 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 deactivation of free radial or rapid cooling step, can be performed by applying heat or by adding an additive that functions as a free radical trap, such as, for example, methyl mercaptan . The process for irradiating the propylene polymer mixture is described in greater detail in the U.S. Patent. 4,916,198, which is incorporated herein by preference. The irradiated mixture or mixture of irradiated polymers has a melting tension greater than 7 cN at 200eC, preferably greater than 12 cN, more preferably greater than 17 cN, and a Young's modulus of less than 1000 MPa, preferably less of 900 MPa, more preferably less than 750 MPa. The amount of each component necessary to provide these properties in the final product varies with the radiation dosage and the molecular weight of the starting materials. It is well known in the art that the higher the molecular weight of the starting material, the higher the melting stress after irradiation. The selection of the radiation dosage and the molecular weight of the starting materials can be easily determined by those skilled in the art. Typically, the weight average molecular weight of component (1) will be > 260,000 and the number average molecular weight of the component (2) will be >75,000. The blends of this invention are useful in applications, such as, for example, extrusion coating (including fabric coating), thermoforming, foam extrusion and blow molding. In the following examples, the melting tension, which provides an indication of the melting strength of the material, was measured on a Goettfert Rheotens apparatus at 200eC. The Rheotens device consists of two wheels against swivel mounted on a beam of sensitive balance. A strand of fusion is extruded from the capillary die and pulled between the rotating wheels until the strand breaks. Initially, the traction speed is constant to establish a baseline of force. Then a constant acceleration is applied. The maximum force measured during the test is taken as the voltage melts. The extension capacity of the fusion is represented by the speed at break.

Dynamic shear tests, ie, low shear rate, apparent polydispersity, and tapping, were conducted at 200eC on a Rheometrics Mechanical Spectrometer, Model 605, with a cone and plate geometry. The properties of the irradiated mixtures were measured by the following methods: Young Module ASTM D1708-96 Performance Effort ASTM D1708-96 Fatigue Effort ASTM D1708-96 Melt Flow Regime, 350SC, 2.16 kg ASTM 1238 Molecular Weight was determined by gel permeation chromatography. The percentage of crystallinity (Xc) can be determined by differential scanning calorimetry according to the equation Xc =? H /? H0, in where? H is the enthalpy change experimentally observed or fusion and? H ° is the enthalpy change during fusion d? 100% crystalline material The isotactic index is defined as the percent d? olefin polymer insoluble in xylene. The weight percent olefin polymer soluble in xylene at room temperature is determined by dissolving 2.5 g of the polymer in 250 ml of xylene at room temperature in a vessel equipped with a stirrer, and heating at 135 ° C with stirring for 20 minutes. The solution is cooled to 25aC while stirring is continued, and then allowed to stand without agitation for 30 minutes so that the solids can settle. The solids are filtered with filter paper, the remaining solution is evaporated by treating it with a stream of nitrogen, and the solid residue is dried under vacuum at 80 ° C until a constant weight is reached. The weight percent polymer insoluble in xylene at room temperature is the isotactic index of the polymer. The value obtained in this way corresponds substantially to the isotactic index determined through extraction with boiling n-heptane, which by definition constitutes the isotactic index of the polymer. In this specification, all parts and percentages are by weight unless otherwise noted, Example 1 This example illustrates the Theological properties of mixtures of various amounts of isotactic propylene homopolymer (i-PP) made using a Ziegler-Natta catalyst and an atactic propylene homopolymer made using a metallocene catalyst (m-aPP) and mixtures of 50% of i-PP and 50% of m-aPP at various radiation dosages. The m-aPP used in this and the following example was prepared by adding 2.1 1 of propylene to a 3.785 1 (1 gallon) coated stainless steel autoclave equipped with a stirrer and a thermocouple connected to a thermostat for temperature control. The reactor was then heated to 50 ° C. A metallocene catalyst was prepared by dissolving 3 mg of di-n-buty-silanediyl-bis- (9-f luorenyl) ZrCl2 in a solution of methyl alu-oxane in isopar (Al / Zr = 5000). After 10 minutes of stirring at room temperature, the mixture was injected into a reactor at 50 ° C. The polymerization was carried out at a constant temperature for one hour. The catalyst activity was approximately 49.1 kg of polymer / gram of catalyst / hour. The crystallinity of the polymer was too low to be measured by DSC. M "was 110,000. Xylene (1200-1400 ml), 30 g of I-PP, 12.5 g of the m-aPP prepared as described above, and 0.0425 g of Irganox B225 antioxidant were added to a 2 liter reaction flask which was equipped With a condenser, a thermometer and a mechanical stirrer, the i-PP had an MFR of 3 g / 10 min, an isotactic index of 95.4%, and a H. of ~ 700,000 and is commercially available from Montell USA Inc. The antioxidant Irganox B225 is a mixture of 1 part of Irganox 1010 tetrakis [methylene (3, 5-di-tert-butyl-4-hydroxyhydro-cinnamate)] -methane and 1 part of Irgafos 168 tis (2,4-di-t- butyl phenyl) phosphite and is commercially available from Ciba Specialty Chemicals Corporation. The mixture was purged with nitrogen for 30 minutes and then heated to 130 ° C until all of the polymer dissolved in the xylene. The xylene solution was then slowly poured into cold methanol (-78SC)White polymer was precipitated. The polymer mixture was then filtered, washed with cold methanol, and dried in a vacuum oven (60SC / 10 mm Hg) for 12 hours. The white polymer mixture was compression molded into a 40 ml thick film which was then cut into small strips of 1 cm x 0.5 cm for irradiation. The strips were placed in a reaction tube and the tube was purged with nitrogen for 1 hour to ensure that the polymer was in an inert atmosphere for radiation treatment. After purging, the reaction tube was closed and the polymer mixture was irradiated under an electron beam. After irradiation, the tube was placed in an oven set at 80 ° C for 1.5 hours and then placed in a second oven at 140 ° C for another 1.5 hours. After cooling to room temperature, the polymer mixture was ready for evaluation. The rheological properties of mixtures containing various amounts of i-PP and m-aPP after irradiation at 6 Mrads are shown in Table 1.

Table 1 MT VB n * @ 0.1 Ap Tañó (cN) (cm / sec) rad / sec PI @ 0, 1 (Pa-sec) rad-seg í-PP m-aPP 6Mrad. { % in (% by weight) weight) 100 0 15.3 6.7 15840 6, 05 2.01 85 15 12.9 11.6 13680 5, 37 2.16 75 25 7.3 18.1 10010 4 2.68 50 50 14.4 6.8 17090 4, 67 2 05 0 100 18 9 2.1 19460 i: > 1.57 The rheological properties of a mixture of 505 by weight of iPP and 50% by weight of m-aPP after various irradiation doses are shown in Table 2.

Table 2 i-PP / m- • aPP MT VB n * @ 0.1 Ap Tañó @ (50/50) (cN) (cm / sec) rad / sec PI 0.1 Mrad (Pa-sec) rad / sec 3 9.1 13.9 25910 3.29 2.13 4. 5 11.8 8.9 23010 4.25 1.98 6 14, 4 6.8 17090 4.67 2.05 9 28.7 3.1 15850 6, 36 1, 87 In the tables, MT is the melting stress in centiNe tons (cN), VB is the speed at break, n * is the viscosity at low shear stress, Ap PI is the apparent polydispersity index, and tañó = G "/ G ', where G "is the loss module and G' is the storage module. A low tacho is an indication of high fusion elasticity. The data in Table 1 show at a radiation dose of 6 Mrad the melting tension increased as the amount of m-aPP increased when the mixture contained 25% or more of m-aPP. The data in Table 2 show that for a 50/50 mixture of i-PP and m-aPP, the melt tension increased and the low shear viscosity decreased with increasing radiation dose.

EXAMPLE 2 This example shows the tensile properties of two mixtures of i-PP and m-aPP compared to a propylene homopolymer of high melt strength alone. The high melt strength propylene homopolymer (HMS PP) was made irradiating the i-PP described in Example 1 at a radiation dose of 9 Mrad in an inert atmosphere, and is commercially available from Montell USA Inc., i-PP and m-aPP in the mixtures were described in Example 1 Both mixtures were irradiated at a radiation dose of 9 Mrads. The polymer mixture was compression molded into 0.5 mm plates using the following molding conditions: 4 minutes at 200eC without pressure, then 3 minutes at 200SC at 140.60 kg / cm2 gauge (2000 psig) The plates were then cooled under ambient conditions during 15 minutes. Microwave tension bars were cut using the Dewes-Gumb manual ejection press equipped with a micro tension die according to ASTM D1708-95. The bars were tested on an Instron Model 42202 test machine in accordance with ASTM D1708-96. The results of the tests are given in Table 3.

Table 3 Polymer Stress Fatigue Stress Module Fusion Young Performance at (cN) (MPa) (MPa) Performance (%) HMS PP llll 29 30 i-PP / m-aPP (75/25) 351 13 16 26 i-PP / m-aPP (50/50) 86 5 34 The data shows that the blends have high melt strength similar to isotactic polypropylene with high melt strength alone, but the blends are much softer, ie the Young's modulus is much smaller. Other features, advantages and embodiments of the invention described herein will be readily apparent to those who exercise ordinary experience after reading the foregoing expositions. In this regard, even when specific embodiments of the invention have been described in considerable detail, variations and modifications may be made without departing from the spirit and scope of the invention as described and claimed.

Claims (6)

1. - A composition comprising a mixture of (1) a propylene polymer selected from the group consisting of (a) a propylene homopolymer and (b) a copolymer of propylene and ethylene or an alpha-olefin of C 4-8 , wherein the polymerized ethylene or polymerized alpha-olefin content is 10% or less, the propylene polymer having an isotactic index greater than 90, and (2) a propylene polymer made with a metallocene catalyst and selected from the group consisting of () a homopolymer of propylene and (b) a copolymer of propylene and ethylene or an alpha-olefin of C 4-8 wherein the content of polymerized ethylene or polymerized alpha-olefin is 10% or less, the polymer of propylene having a crystallinity of less than 24% measured from the heat of crystallization, the mixture having a melting tension greater than 7 centiNewtons at 200eC and a Young's modulus of less than 1000 MPa.

2. The composition according to claim 1, wherein (1) and (2) are both homopolymers of propylene.

3. The composition according to claim 1, wherein the melting tension of the mixture is greater than 12 cN at 200eC.

4. The composition according to claim 1, wherein the Young's modulus of the mixture is less than 900 MPa.

5. - A process for making a mixture of irradiated polymer comprising (1) preparing a mixture comprising: (a) a propylene polymer selected from the group consisting of (i) a propylene homopolymer and (ii) a propylene-ethylene copolymer or a C 4-8 alpha-olefin, wherein the polymerized ethylene or polymerized alpha-olefin content is 10% or less, the propylene polymer having an isotactic index of more 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 an alpha olefin of C 4-8. wherein the polymerized ethylene or polymerized alpha-olefin content is 10% or less, the propylene polymer having a crystallinity of less than 24% measured from the heat of crystallization, (2) irradiating the mixture in an environment in which the Active oxygen concentration is established and maintained at less than 15% by volume with high energy ionization radiation at a radiation dose of 3 to 12 Mrad for a sufficient period of time for a substantial amount of radical formation to occur, but insufficient to cause gelation of the material; (3) keep the material irradiated in said environment for a period of up to two hours; and (4) treating the irradiated material while in said environment to substantially deactivate all free radicals present in the irradiated material, whereby the irradiated mixture has a melting tension greater than 7 centiNewtons at 200SC and a Young's Modulus of less than 1000 MPa.

6. The process according to claim 5, wherein both (a) and (b) in (1) are propylene homopolymers. 7, - The process according to claim 5, wherein the radiation dose in (2) is about 6 to about 9 Mrads. 8. The process according to claim 5, wherein the oxygen concentration is less than 1% by volume. 9. The process according to claim 5, wherein the melting tension of the mixture is greater than 12 cN to 200SC. 10. The process according to claim 5, wherein the Young's modulus of the mixture is less than 900 MPa.