Process for making foam articles from high melt strength propylene polymer materials

Abstract: A foam article is made by (1) irradiating a propylene polymer material selected from the group consisting of (a) a terpolymer of propylene, ethylene, and butene-1, (b) a polyolefin composition that includes (i) about 31% to about 39 % of a copolymer of propylene and ethylene, and (ii) about 58% to about 72% of a terpolymer of propylene, ethylene, and butene-1, and (c) a polyolefin composition that includes (i) about 30% to about 65% of a copolymer of propylene and butene-1, and (ii) about 35% to about 70% of a copolymer of propylene and ethylene, and (2) extruding the irradiated propylene polymer material through a die in the presence of a physical expanding agent and a cell nucleating agent, whereby a structure is formed whose density is at least 10X less than the initial density of the propylene polymer material. The foam articles exhibit improved flexibility and low temperature toughness compared to conventional propylene polymer materials.

Description: Field of the Invention This invention relates to a process for making foam articles from a high melt strength propylene polymer material. Background of the Invention Foamable polymers with improved flexibility and low temperature toughness would be desirable for such applications as (1) flexible pipe insulation for high service temperature environments, (2) automotive interior components where soft touch properties are important, e.g., thermoplastic olefin skin/foam systems for instrument panels and/or door panels, and interior roof liners and trunk liners where foam and carpet are laminated and thermoformed to produce a composite structure, and (3) extruded bead applications where impact cushioning properties are desired, e.g. automotive energy absorbers (exterior bumpers, interior pillar cushions and side impact panels), packaging of sensitive electronics, and flotation devices. Low density polyethylene resins dominate the flexible pipe insulation market at the present time, but lack the thermal stability required for use in industrial and automotive insulation applications. Propylene-based polymers are attractive because of their high melting point and chemical inertness. High melt strength propylene polymer materials made by irradiating conventional propylene polymers have been used to make foam articles. For example, the use of irradiated, high melt strength propylene polymer materials to make foam articles by expanding the molten polymer with a foaming agent is disclosed in U.S. 4,916,198. Foams made by extruding an irradiated, high melt strength polyolefin composition through a die in the presence of a chlorofluorocarbon blowing agent and a nucleating agent are described in U.S. 5,414,027. A process for making foamed propylene polymer articles by subjecting pre-foamed beads made from an irradiated, high melt strength propylene polymer material to thermoforming by sintering is disclosed in U.S. Patents 5,324,753 and U.S. 5,338,764. However, the properties of the foamed, irradiated propylene polymer materials made by these processes, particularly toughness, are not adequate at temperatures as low as -20°C. Summary of the Invention Foam articles are made by a process comprising: (1) irradiating a propylene polymer material selected from the group consisting of (a) a terpolymer of propylene, ethylene, and butene-1 comprising 85% by weight to 96% by weight propylene, 1.5% by weight to 5% by weight ethylene, and 2.5% by weight to 10% by weight butene-1, wherein the total comonomer concentration with propylene is 4.0% by weight to 15% by weight, (b) a polyolefin composition comprising (i) 31% by weight to 39% by weight of a copolymer of propylene and ethylene having an ethylene content of 1.5% by weight to 10% by weight, (ii) 61% by weight to 69% by weight of a terpolymer of propylene, ethylene, and butene-1 having a propylene content of 85% by weight to 96% by weight, an ethylene content of 1.5% by weight to 6% by weight, and a butene-1 content of 2.5% by weight to 12% by weight, wherein the total comonomer concentration with propylene is 4.0% by weight to 18% by weight, and (c) a polyolefin composition comprising (i) 30% by weight to 65% by weight of a copolymer of propylene and butene-1 having a propylene content of 80% by weight to 98% by weight, and (ii) 35% by weight to 70% by weight of a copolymer of propylene and ethylene having an ethylene content of 2% by weight to 10% by weight, and (2) extruding the irradiated propylene polymer material through a die in the presence of a physical expanding agent and a cell nucleating agent, whereby a foam article is formed whose density is at least 10X less than the initial density of the propylene polymer material. The foam articles exhibit improved low temperature toughness compared to irradiated propylene polymer materials that do not contain butene as a comonomer. Brief Description of the Drawings Figure 1 is a plot of melt tension (cN) against beam dose (Mrad) for an ethylene/propyleno/butene-1 terpolymer (C 2 C 3 C 4 ), a blend of an ethylene/propylene copolymer and a propylene/butene-1 copolymer (OPC 2), and a blend of an ethylene/propylene copolymer and an ethylene/propylene/butene-1 terpolymer (OPC 3). Figure 2 is a plot of foam density in pounds per cubic foot (pcf) against beam dose (Mrad) for a C 2 C 3 C 4 terpolymer, OPC 2, and OPC 3. Figure 3 is a plot of impact energy (Nm/cm) (ft.lb/in) at 23°C and at -20°C for a 0.173 cm (0.068 inch) foam sheet, which is an indication of the toughness of the sheet. The foam sheets tested were made from an irradiated propylene homopolymer, an irradiated ethylene/propylene copolymer containing 3.3% ethylene (C 2 C 3 ), OPC 2, and OPC 3. Detailed Description of the Invention The first step of the process of this invention is the irradiation of a propylene polymer material selected from the group consisting of (a) a terpolymer of propylene, ethylene, and butene-1, (b) a polyolefin composition comprising (i) a copolymer of propylene and ethylene and (ii) a terpolymer of propylene, ethylene, and butene-1, and (c) a polyolefin composition comprising (i) a copolymer of propylene and butene-1 and (ii) a copolymer of propylene and ethylene. The terpolymer of propylene, ethylene, and butene-1 comprises 85% to 96%, preferably 90% to 95%, propylene; 1.5% to 5.0%, preferably 2% to 3%, ethylene, and 2.5% to 10%, preferably 4% to 6%, butene-1, wherein the total comonomer concentration with propylene is 4.0% to 15.0%. Polyolefin composition (b) comprises (i) 31% to 39%, preferably 33% to 37%, of a copolymer of propylene and ethylene having an ethylene content of 1.5% to 10%, preferably 2% to 6%, and (ii) 61% to 69%, of a terpolymer of propylene, ethylene, and butene-1 having a propylene content of 85% to 96%, preferably 90% to 95%; an ethylene content of 1.5% to 6%, preferably 2% to 4%, and a butene-1 content of 2.5% to 12%, preferably 6% to 10%, wherein the total comonomer concentration with propylene is 4.0% to 18.0%. Polyolefin composition (c) comprises (i) 30% to 65%, preferably 35% to 65%, more preferably 45% to 65% of a copolymer of propylene and butene-1 having a propylene content of 80% to 98%, preferably 85% to 95%, and (ii) 35% to 70%, preferably 35% to 65%, more preferably 35% to 55%, of a copolymer of propylene and ethylene having an ethylene content of 2% to 10%, preferably 7% to 9%. The propylene polymer materials described above can be prepared according to the polymerization process and using the catalyst disclosed in U.S.Patents 5,296,548 and 5,486,419. The terpolymer and the olefin polymer compositions are generally prepared by sequential polymerization of monomers in the presence of stereospecific Ziegler-Natta catalysts. Such catalysts contain as an essential element, a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond and an electron donor compound, both supported on a magnesium halide in active form. Alternatively, the terpolymer can be made using a single stage reactor, and the olefin polymer compositions can be prepared by physical blending of the components. In order to obtain the desired low temperature toughness in the foam product, it is important that the propylene polymer materials of this invention contain butene-1 as a comonomer. It has been found that the melt strength and foamability of the materials of this invention increase with increasing beam dose, while propylene polymers having ethylene rather than butene-1 as the comonomer exhibit a tendency to crosslink in the presence of the amount of electron energy typically employed in the process of this invention. The magnitude of the increase in melt strength with increased energy level in the polymers containing butene-1 is usually indicative of chain branching and tends to loosely predict foamability. The butene-containing polymers of this invention also exhibit better low temperature toughness compared to irradiated propylene polymer materials that do not contain butene-1 as the comonomer. The process for irradiating the propylene polymer materials of this invention comprises (1) irradiating the propylene polymer material 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 dose rate of about 1 to about 1x10 4 megarads per minute for a period of time sufficient for a substantial amount of chain scission of the propylene polymer to occur, but imfficient to cause gelation of the material, (2) maintaining the irradiated material in such an environment for a period of up to one hour, and (3) treating the irradiated material while in such an environment to deactivate substantially all of the free radicals present in the irradiated material. The melt tension of the irradiated polymer is typically 10 cN or greater at 200°C. Foam articles with the desired cellular structure are typically obtained at an absorbed surface beam dose of 4.5 to 6 megarads, depending on the starting material used. The lower the ethylene content, the higher the beam dose that can be used. The irradiation process is described in more detail in U.S. Patents 4,916,198 and 5,414,027. In the second step of the process of this invention, the irradiated propylene polymer material is extruded through a die in the presence of a physical expanding agent and a cell nucleating agent. Low density foamed articles with a uniform cellular structure and an absence of irregular voids are obtained. In order to prepare the foamed articles, one can use commercially available extruders, including single screw extruders. The foaming agent is preferably injected into the melted polymer mass inside the extruder, from a point downstream from the point at which the solid polymer is fed into the extruder, so that the distance between will allow the polymer to have reached the form of a melted, homogeneous mass. In the section of the extruder where the foaming agent is introduced, the temperature preferably ranges from 190°C to 250°C. The temperature maintained at the extruder outlet, equipped with a die, is adequate to obtain the foaming of the polymer, and preferably ranges from 119°C to 125°C. Suitable expanding agents include, for example, pentane,, n-butane, isobutane, n-pentane, i-pentane, HCHC-22 (CHF 2 Cl), HCFC-142b (CF 2 ClCH 3 ), HFC-152a (CHF 2 CH 3 ), HCFC-123 (CHCl 2 CF 3 ), HCFC-123a (CHFClCF 2 Cl), HCFC-124 (CHFCICF 3 ), HFC-134a (CH 2 FCF 3 ), HFC-143a (CH 3 CF 3 ) CFC-11 (CFCI 3 ), CFC-12 (CF 2 Cl 2 ), CFC-113 (CFCl 2 CF 2 Cl), CFC-114 (CF 2 ClCF 2 Cl), CFC-115 (CF 2 ClCF 3 ), CH 3 Cl, CH 2 Cl 2 , CO 2 , N 2 , and O 2 . The quantity of expanding agent that is added to the polymer is 1% to 30%, preferably 2% to 15%, by weight based on the weight of the polymer. One or more cell nucleating agents are added before or during extrusion in quantities of 0.1% to 1.0% by weight of the active ingredient, based on the weight of the polymer. The preferred concentration depends upon whether the nucleating agent is a chemical or a physical type nucleating agent, the efficiency of the nucleating agent, the die design, the operating pressure, and the expanding agent selected. The preferred concentration for chemical nucleating agents is typically 0.2% by weight active, and the preferred concentration for physical nucleating agents is typically 0.4 % by weight. Suitable physical nucleating agents are in the form of finely divided particles having a particle size of <10 µm and include, for example, talc, colloidal silica, calcium carbonate, and carbon black. Suitable chemical nucleating agents include, for example, sodium bicarbonate or its blends with citric acid, and azo derivatives such as azodicarbonamide. Other additives, dyes, or fillers that may optionally be required can be added before or during extrusion. The foam article that is produced has a density at least 10X less than the initial density of the propylene polymer material and exhibits improved toughness at temperatures as low as -20°C, preferably > 0.53 Nm/cm (1 ft.lb/in) at -20°C. In the following examples, the melt tension values were obtained using a Gottfert Rheotens melt tension instrument model 2001. The tensile strength of a strand of molten polymer was measured in cN (centiNewtons) operating at a specific stretch velocity. The polymer to be tested was extruded at 200°C through a die with a capillary hole 22 mm long and 1 mm in diameter. The exiting strand was then stretched by using a system of traction pulleys at a constant acceleration of 0.012 cm/sec 2 , measuring the tension until the breaking point. The apparatus registers the tension values of the strand (resistance in cN) as a function of the stretching. The maximum tension value corresponds to the melt strength. The melt flow rate of the polymers was measured at 230°C and 2.16 kg according to ASTM D-1238. The toughness of the foam sheet was measured using the instrumented dart drop test, ASTM D4272-90. The % xylene solubles was determined by dissolving 2.5 g of polymer in 250 ml of o-xylene at 135°C, cooling in a constant temperature bath to 25°C, and filtering through fast filter paper. An aliquot of the filtrate was evaporated to dryness, the residue was weighed, and the weight % soluble fraction was calculated. In this specification, all parts and percentages are by weight unless otherwise noted. Example 1 This example shows the effect on the melt flow rate, melt tension, crystallization temperature, and second heat temperature when the starting polymers are irradiated with increasing dosages of electron beam radiation. The second heat temperature is the true melting point of the polymer and is an indication of crystal nucleation. The random ethylene/propylene copolymer (C 2 C 3 ) contains 4.7% ethylene. Olefin polymer composition (OPC) 1 contains 35% propylene homopolymer, 58.1% of an ethylene/propylene copolymer that is xylene soluble at room temperature, and 6.9% of an ethylene/propylene copolymer that is insoluble in xylene at room temperature, and is commercially available from Montell USA Inc. The ethylene/propylene/butene-1 terpolymer (C 2 C 3 C 4 ) contains 2.5% ethylene, 92.3% propylene, and 4.7% butene-1, and is commercially available from Montell USA Inc. OPC 2 is a blend of 48% propylene/butene-1 copolymer (C 3 C 4 ), of which 16% is butene-1, and 52% of an ethylene/propylene copolymer, of which 4.0% is ethylene, and is commercially available from Montell USA Inc. OPC 3 is a blend of 35% ethylene/propylene copolymer, of which 3.3% is ethylene, and 65% of an ethylene/propylene/butene-1 terpolymer, of which 3.3% is ethylene, 10% is butene-1, and 86.7% is propylene, which is commercially available from Montell USA Inc. The properties of the polymers before irradiation are given in Table 1. Polymer MFR (dg/min) Melt Tension @200°C (cN) Crystallization Temp. (°C) 2 nd Heat Temp. (°C) C 2 C 3 4 4 91.9 139 OPC 1 (PP+C 2 C 3 ) 0.75 8 105 167 C 2 C 3 C 4 5 2 84 133 OPC 2 (C 2 C 3 +C 3 C 4 ) 7 1.6 83 134 OPC 3 (C 2 C 3 +C 2 C 3 C 4 ) 5.4 2 81 134 Pellets of the various polymers were passed by conveyor belt through an electron beam generated by a 2 MeV Van de Graff generator operating at an 80 µamp beam current under an atmosphere of pure nitrogen. The conveyor belt speed was adjusted to produce the absorbed surface dose indicated in Table 2. The properties of the polymers after irradiation are given in Table 2. Polymer Beam Dose (Mrad) Pellet MFR (dg/min) Melt Tension @200°C (cN) Crystallization Temp. (°C) 2 nd Heat Temp. (°C) C 2 C 3 1.5; 3; 4.5 9.3; 6.3; 5.3 10; 4; 7 110; 111; 111 145; 144; 144 OPC 1 (PP + C 2 C 3 ) 1.5; 3; 6; 9 0.2; 0.2; 0.5; 0.7 8; 15; 9; 5 117; 118; 118; 118 161; 161; 160; 157 C 2 C 3 C 4 1.5; 3; 5 11; 11; 10 3; 6; 6 100; 101; 101 136; 136; 136 OPC 2 (C 2 C 3 + C 3 C 4 ) 3; 6 9; 7 5; 16 97; 101 135; 137 OPC 3 (C 2 C 3 + C 2 C 3 C 4 ) 1.5; 3; 4.5; 6 9; 9; 10; 8 5; 3; 13; 19 90; 93; 101; 101 133; 135; 138; 137 The data in Table 2 show a significant increase in melt tension with increased energy level for the polymers containing butene-1 as a comonomer. The magnitude of this response is usually indicative of chain branching and tends to loosely predict foamability. Example 2 This example demonstrates the ability of the irradiated polymers described in Example 1 to undergo a thermoplastic expansion while maintaining a continuous cellular structure. Polymers that exhibit satisfactory foamability formed a low density, continuous cellular solid. "Low density" corresponds to at least a 10X reduction (<6 pounds per cubic foot) of initial solid polymer density. If the polymer is designated as "not foamable", a collapsed, irregular voided structure was formed. The formulations used for the foamability test consisted of the irradiated polymer, 0.2 wt. % Hydrocerol CF-20 cell nucleating agent for cell size control, and at least 6 volume % expanding agent. Hydrocerol CF-20 cell size control agent is a mixture of sodium bicarbonate and citric acid encapsulated in a polyethylene carrier resin, and is commercially available from Bohringer Ingelheim. The foam products were produced on a laboratory foam line consisting of a 5.08 cm (2") diameter (D) 16 L/D counter-rotating twin screw extruder coupled to a 1.91 cm (¾") diameter 10:1 L/D Kenex static mixer, which in turn was coupled to a 0.4 cm (5/32") diameter rod die for making 2.54 cm (1") diameter foam rods, or a 2.54 cm (1") diameter adjustable gap annular die for the foam sheets. CFC 114 in an amount of 11 wt. % was used as the expanding agent for the irradiated C 2 C 3 random copolymer, OPC 1, and the C 2 C 3 C 4 terpolymer. The formulation also contained 0.2 wt. % Vertol 360 talc, which has an average particle size of 3-7 µm and is commercially available from Cypress Industries. One inch diameter foam rods were produced from these polymers. The results of the foamability tests are given in Table 3. Polymer Density Reduction Comments C 2 C 3 1.5 Mrad = 12X 3 Mrad = 18X • 3 Mrad gave finest cell structure • 4.5 Mrad - not foamable, 4.4 % insoluble OPC 1 (PP + C 2 C 3 ) Not foamable at any test dose 19% insoluble C 2 C 3 C 4 1.5 Mrad = 5X 3 Mrad = 10X 4.5 Mrad = 14 X 4.5 Mrad gave finest cell structure The irradiated C 2 C 3 random copolymer and OPC 1, which included a C 2 C 3 copolymer as one of its components, exhibited a tendency to crosslink in the presence of electron energy. The C 2 C 3 copolymer was foamable at 3 Mrad, while not foamable at 6 Mrad, which suggests that there is a finite energy window for the copolymer. OPC 1 was not foamable at any test dose. The melt tension and foamability of the irradiated C 2 C 3 C 4 terpolymer appeared to improve with increased beam dose (see Figures 1 and 2). Isobutane was used as the expanding agent for OPC 2 and OPC 3 and foam sheets were produced from these polymers. The formulation also contained 0.4 wt. % Vertal 360 talc as a cell size control agent. Seven wt. % isobutane blowing agent was injected into the melt and the solution was cooled and formed through the annular die while maintaining a solution temperature between 119°C-124°C (246° - 255°F). The resulting foam tube was oriented and stabilized over a 8.89 cm (3.5") diameter cooling mandrel, and single slit to form a sheet. The foam sheet was 27.9 cm (11") wide and 0.173 cm (0.068") thick and was wound onto a roll and stabilized for five days prior to sheet characterization. The results of the foamability test are given in Table 4. Polymer Density Reduction Comments OPC 2 (C 2 C 3 + C 3 C 4 1.5 Mrad = 4X 3 Mrad = 6X 4.5 Mrad = 11X 6 Mrad = 13X 4.5 and 6 Mrad gave finest cell structure OPC 3 (C 2 C 3 + C 2 C 3 C 4 ) 3 Mrad = 4X 6 Mrad = 15X 6 Mrad gave best results The melt tension and foamability of OPC 2 and 3 improved with increasing beam dose (see Figures 1 and 2). Although the C 2 C 3 C 4 terpolymer and OPC 2 and 3 have a different molecular structure, they share C 4 in the main chain. These data, coupled with the rising melt tension trend, suggest that the presence of C 4 as a comonomer promotes branching. Example 3 This example demonstrates the difference in toughness between polymers with and without butene-1 as a comonomer. The toughness of the foam sheets prepared as described in Example 2 was quantified by means of the dart drop test. Foam sheets made from an irradiated propylene homopolymer (PP) having a melt flow rate of 3 g/10 min, commercially available from Montell USA Inc., and an irradiated ethylene/propylene copolymer containing 3.3% ethylene (C 2 C 3 ), commercially available from Montell USA Inc., were used as a comparison. The results are shown in Figure 3. The low temperature properties of the polymers containing butene-1 as a comonomer were better than those of the irradiated propylene homopolymer and the ethylene/propylene copolymer. A value of > 0.53 Nm/cm (1 ft. lb/in) at -20°C was considered acceptable. The higher the value, the better the low temperature toughness of the material.

Claims: A method for making foam articles comprising: (1) irradiating a propylene polymer material selected from the group consisting of (a) a terpolymer of propylene, ethylene, and butene-1 comprising 85% by weight to 96% by weight propylene, 1.5% by weight to 5% by weight ethylene, and 2.5% by weight to 10% by weight butene-1, wherein the total comonomer concentration with propylene is 4.0% by weight to 15% by weight, (b) a polyolefin composition comprising (i) 31% by weight to 39% by weight of a copolymer of propylene and ethylene having an ethylene content of 1.5% by weight to 10% by weight, (ii) 61% by weight to 69% by weight of a terpolymer of propylene, ethylene, and butene-1 having a propylene content of 85% by weight to 96% by weight, an ethylene content of 1.5% by weight to 6% by weight, and a butene-1 content of 2.5%by weight to 12% by weight, wherein the total comonomer concentration with propylene is 4.0% by weight to 18% by weight, and (c) a polyolefin composition comprising (i) 30% by weight to 65% by weight of a copolymer of propylene and butene-1 having a propylene content of 80% by weight to 98% by weight, and (ii) 35% by weight to 70% by weight of a copolymer of propylene and ethylene having an ethylene content of 2% by weight to 10% by weight, and (2) extruding the irradiated propylene polymer material through a die in the presence of a physical expanding agent and a cell nucleating agent, whereby a foam article is formed whose density is at least 10X less than the initial density of the propylene polymer material. The method of claim 1 wherein the propylene polymer material is irradiated at an electron beam dose of 4.5 to 6 Mrad.

Inventors: BAVARO VINCENT P, BAVARO, VINCENT P.
Applicants: BASELL NORTH AMERICA INC, BASELL NORTH AMERICA INC.
Application year: 1999
Main classification: C08J9
IPC codes: C08J9, C08L23, C08J7