MXPA06005995A - Method of forming thermoplastic foams using nano-particles to control cellmorphology - Google Patents

Abstract

A process for making closed-cell, alkenyl aromatic polymer foams using nano-particle nucleation agents to control the cell morphology of the resulting foam includes forming a polymer melt at a temperature above the polymer glass transition temperature (for crystal polymers) or the polymer melt point (for amorphous polymers);incorporating selected nano-particles into the polymer melt;incorporating blowing agents into the polymer melt at an elevated pressure;optionally incorporating other additives, such as flame retardants, into the polymer melt;and extruding the polymer melt under conditions sufficient to produce a foam product having a desired cell morphology, characterized by parameters such as reduced average cell size range and/or increased asymmetry of the cells.

Description

METHODS FOR FORMING THERMOPLASTIC FOAMS USING NANO-PARTICLES TO CONTROL THE MORPHOLOGY OF THE CELLS BACKGROUND OF THE INVENTION This invention relates to a process for producing rigid alkenyl aromatic polymer foams, which have a wide range of cell morphologies by using nanoparticles as nucleating agents. These rigid foams are useful for forming rigid insulating foam boards, suitable in many conventional thermal insulation applications. The physical properties of rigid polymer foam boards, such as their compressive strength, thermal conductivity, dimensional stability, absorption rate in water, depend in large part on the micro-structure of the material forming the boards, ie , the morphology of the foam cells. However, it can be difficult to control the polymer foaming to the extent necessary for a consistent production of a suitable cell morphology, which will tend to optimize the total foam process, or to improve a specific property, such as the thermal insulation value. of the foam. Attempts by the prior art to produce foam micro-structures having suitable cell morphology have included the use of nucleating agents such as powders, formed from inorganic oxides, various organic materials and metals. Among these nucleating agents, inorganic oxides, such as talc, titanium dioxide and kaolin, are the most commonly used. The size, shape, particle distribution and surface treatment of the nucleating agent (s) used to form a foam will all affect the nucleation efficiency and consequently the cell size morphology and the resulting foam distribution. Conventional methods for controlling the cell morphology, however, tend to be limited by difficulties in uniformly distributed particles of the nucleating agent through the polymer and / or to suppress the coagulation of the dispersed particles. Certain structural defects in the resulting foams are generally attributed, at least in part, to dimensional differences between the particles of the nucleating agents -which may be in the range of several microns, particularly in situations where there has been a certain degree of coagulation- and the desired cell micro-structures - which may have a target cell wall thickness of one or less - for low density commercial insulating foams.

This difference in size between the nucleating agent particles and the cell wall thickness can also result in relatively weak interactions between the nucleating agent and polymer on a large scale, thereby weakening the total foam structure. Similarly, cell defects can also be attributed, at least in part, to the hydrophilic surface of most conventional inorganic nucleating agents, which makes them difficult to disperse uniformly in a polymer.

These effects tend to result in processing difficulties, such as corrugation of the resulting foam board, when adding nucleating agents at levels greater than about 2 weight percent or the average cell size of the resulting foam, is less than about 120 microns. Attempts of the prior art to avoid corrugation effects in the foam structure, have used agents for enlarging the cell size, such as the waxy compositions described in US Pat. No. 4,229,396, and the non-waxy compositions described in U.S. Patent No. 5,489,407. Another effort directed towards foam structures having bi-modal cell morphology (Kanelite Super EIII, Kane, Japan) includes the use of immiscible blowing agents such as water and hydrocarbons. This combination, however, tends to result in processing difficulties due to the low solubility of water in the polymer and the reaction of water with the pyro-retardant, such as hexabromocyclododecane (HBCD) at the elevated temperatures typically employed during the extrusion process. SUMMARY OF THE INVENTION The present invention provides a process for producing closed cell alkenyl aromatic polymer foams, wherein nanoparticle nucleating agents are used to control cell morphology. The exemplary process comprises: 1) heating an aromatic alkenyl polymer to the temperature over the vitreous transition temperature of the polymer (for crystal polymer), or melting point of the polymer (for amorphous polymer) to form a polymer melt; 2) incorporating an appropriate amount of selected nano-particles in the polymer melt, to alter the polymer property and the behavior of the process, such as rhgy, melting strength; 3) incorporating blowing agents into the polymer melt at elevated pressure; 4) incorporate other additives, such as pyro-retardants in the polymer melt; and 5) extruding and forming a foam board under atmospheric or sub-atmospheric pressure (partial vacuum) to produce a desired cell morphology, characterized by parameters such as range and distribution of cell sizes, cell orientation and wall thickness. cells Further in accordance with the present invention, the nano-particles are typically particles with at least one dimension less than 100 nm and can be incorporated into the polymers as surface-modified nano-particles, nano-particles having ecanochemical bonds, to a particle of core size microns, nano-particle compounds in combination with polymers, such as master batch compositions, and / or liquid blowing agents. In addition, the nanoparticle polymer composites can be interleaved nanolayers, such as compounds formed simply by mixing nano-montmorillonite (MMT) or expanded graphite with a polymer, or exfoliated nano-layers, such as compounds formed by in situ polymerization of the precursor. of polymer in the presence of nano-MMT or other surface-modified graphite or inorganic particles. A first exemplary embodiment of the present invention provides a process for producing a rigid polymer foam having a relatively small median cell size of about 60 microns by using hydrophobic surface modified nano-MMT particles. Conventional foams in comparison tend to have a median cell size greater than 150 microns produced by using conventional inorganic nucleating agents such as hydrophilic talc. The rigid foams prepared in accordance with this embodiment of the invention exhibit no detectable corrugation and an improvement in compression strength of about 30%. A second exemplary embodiment of the present invention provides a process for producing rigid foams, which have an increased cell orientation of at least about 1.4, compared to a conventional cell orientation of about 1.0, is observed by adding nano-particles in the form of needle, for example, of calcium carbonate, in addition to a conventional nucleating agent, for example talc. A third exemplary embodiment of the present invention provides a process for forming an improved foam structure utilizing a carbon dioxide blowing agent, in combination with a nano-scale nucleating agent, such as nano-MMT, to produce a rigid foam. which has a reduced median cell size and thinner cell walls, both to improve the mechanical strength and to decrease the thermal conductivity (thereby increasing the insulation value) of the resulting foam. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an SEM image of the cellular wall structure of a typical XPS foam. Figure 2 shows an SEM image of the cellular column structure of a typical extruded polystyrene foam ("XPS"). Figure 3 shows an SEM image of an XPS foam, with average cell size of approximately 81 microns, produced with approximately 0.5% of a nano-clay nucleating agent. Figure 4 shows an optical microscope image of cell size, cell size distribution, and cell orientation (x / z) of an XPS foam with 2% nano-calcium carbonate. Figure 5 shows an optical microscope image of the cell size, cell size distribution, and cell orientation (x / z) of an XPS foam with 3.3% of a nano-expanded graphite nucleating agent. Figure 6 shows a SEM cell morphology image of an XPS foam sample prepared using 5% nano-MMT as a nucleating agent and C02 at 6% as a blowing agent. DETAILED DESCRIPTION OF EXEMPLARY MODALITIES The morphology of the cells includes parameters such as average cell size, anisotropic ratio of cells or cell orientation, cell density, and cell size distribution, cell wall thickness, effective cell column diameter, proportion of open / closed cells, cell-shaped, such as pentagonal dodecahedron, rhombic dodecahedron, tetra-dodecahedron (with curved surface), and other cell models such as bi-cell and cell-encelda models. Within these cell morphology parameters, average cell size, cell wall thickness, effective cell column diameter and cell orientation are the key parameters for determining the physical properties of closed cell foams. Figures 1 and 2 show the SEM images of the cell wall and column structure of a typical XPS foam. If a polymer foam is ideally illustrated as a closed wall of pentagonal dodecahedral cells of uniform size, the cell wall thickness and the effective column diameter, then depend primarily on the density of the foam, and the cell size.

This invention utilizes nanoparticles and a related extrusion process to control cell size, cell wall thickness, effective column diameter, as well as cell orientation within a relatively broad range. Although conventional polymer foams tend to exhibit an average cell size in the range between about 120 and 280 microns. By using the nano-particle technology according to the present invention, it is possible to manufacture polymer foam structures having an average cell size from several tens of microns to several hundred microns. The nanoparticles used to make polymer foams according to the present invention are preferably included in the polymer melt at a rate between about 0.01 to about 10% by weight or more preferably about 0.05 to about 2.5% by weight of alkenyl aromatic polymer material. The particle size of the present agent for controlling the size of nano-particle cells, typically is not greater than 100 angstroms is at least one dimension and can be an organic and inorganic material with or without surface modification. The primary component of the foam structure is an alkenyl aromatic polymer material. Suitable alkenyl aromatic polymer materials include alkenyl aromatic homopolymers and copolymers of alkenyl aromatic compounds and copolymerizable ethylenically unsaturated comonomers. The alkenyl aromatic polymer material may also include minor proportions of non-alkenyl aromatic polymers. The alkenyl aromatic polymer material may comprise only one or more alkenyl aromatic homopolymers, one or more of each of alkenyl aromatic homopolymers and copolymers or mixtures of any of the foregoing with a non-alkenyl aromatic polymer. Suitable alkenyl aromatic polymers include those derived from alkenyl aromatic compounds such as styrene, alpha-methylstyrene, chlorostyrene, bromostyrene, ethylstyrene, vinyl benzene and vinyl toluene. A preferred alkenyl aromatic polymer is at least 95% polystyrene and can be composed entirely of polystyrene. The present foam structure will also typically include one or more blowing agents selected from 1) organic blowing agents, such as aliphatic hydrocarbons having 1-9 carbon atoms (including, for example, methane, ethanol, ethane, propane, n- butane and isopentane) and total or partially halogenated aliphatic hydrocarbons having 1-4 carbon atoms (fluorocarbons, chlorocarbons and chlorofluorocarbons); 2) inorganic blowing agents such as carbon dioxide, nitrogen and water; 3) Chemical blowing agents, such as azodicarbonamide, p-toluenesulfonyl. Useful blowing agents include 1-chloro-1, 1-diflouroethane (HCFC-142b), HCFC-134a, carbon dioxide, mixtures of HCFC-142b with carbon dioxide, HCFC-134a with carbon dioxide, carbon dioxide with Ethanol, or carbon dioxide with water. The foam composition can also incorporate various additives, such as pyro-retardants, mold release aids, pigments and fillers, intended to improve the processing of the foam or modify one or more properties of the resulting foam. Exemplary embodiments of the polymer foam made in accordance with the present invention may exhibit densities from about 10 to about 500 kg / m3, but more preferably have densities from about 20 to about 60 kg / m3, when measured in accordance with ASTMD -1622 Although polymer foams made in accordance with the present invention may have structures that exhibit both closed cells and open cells, preferred foam compositions will have at least 90% closed cells as measured in accordance with ASTM D2856-A. The following are examples of the present invention, and should not be considered as limiting. Unless otherwise indicated, all percentages, parts or proportions are based on the weight of the total composition. EXAMPLES A series of foam, exemplary and comparative structures were prepared and evaluated to determine the morphology of the cells, that is, cell size, wall thickness of the cells (Figure 1), effective cell column diameter ( Figure 2), proportion of anisotropy of the cells, and certain other properties related to the morphology of the foam cells. The physical properties tested include one or more density, compressive strength, thermal conductivity, aged thermal insulation value, dimensional thermal stability. In connection with these examples, the cell size was measured according to ASTMD3576; the density was measured in accordance with ASTMD1622; the thermal conductivity was measured according to ASTM C518; the compressive strength was measured in accordance with ASTMD1621; and thermal dimensional stability was measured according to ASTM D2126.

The foam structures were made with a co-rotating twin screw extruder, comprising a pair of extruder screws, a heating zone mixer, a blowing agent injector, a coolant, a die and a moulder, in accordance with the operational conditions cited below in Table 1. Unless otherwise indicated, the polymer used to prepare the exemplary foam compositions, was a granular polystyrene of AtoFine, having a weight average molecular weight (Mw) of approximately 250,000 and a melt index of approximately 3.1 gm for 10 minutes. Table 1 Twin Screw Extruder Extruder co-rotating twin screw spindles LMP co-rotary with Leistritz MIC 27 static refrigerant GL / 400 (ft / min) Matrix Space 0.6-0.8 Gap-mm Vaci-kPa (inch 0-3.4 Hg Atmosphere) ( 0 16) Example 1 Polystyrene foams were prepared with both (7347) and without (7346) a 2.5% nanoparticle filler using an LMP extruder. The nano-particles used to prepare this example were an organo-clay, specifically Nano-MMT 20A grade from Southern Clay Products Inc., which was formulated in fusion with a polystyrene polymer, specifically grade CX5197 of AtoFine, to form a polymer of fusion. The nano-particles exhibit an interlaced nano-MMT layer structure when examined using X-ray diffraction. The comparison sample does not include nano-particles, but incorporates 0.8% talc filler as the nucleating agent. The comparison sample exhibited an average cell size of about 186 microns while the exemplary example using the nano-particle foam exhibits a significantly reduced average cell size of about 60 microns. The exemplary example also exhibits a cell wall thickness of about 0.5 mire, and an effective column diameter of about 5 microns. As reflected in Table 2 below, the exemplary foam composition exhibited no corrugation, was processed without undue process difficulty and provided improvements in compression strength of about 30%. Table 2 * Anisotropic cell ratio: K = z / (x and z) 1/3, where x is an average cell size in the longitudinal direction (extrusion), and, cell size in the transverse direction, and z cell size in the thickness direction of the board. Example 2 Sample foams (7349) were produced according to the process set forth in Example 1, but using 0.5% of a nano-MMT interleaved in a polystyrene composition, to produce an exemplary foam having a density of approximately 26.5kg / m3, a thickness of approximately 38 mm and a width of approximately 600 mm. The reduction in the amount of nano-MMT incorporated in the composition resulted in a slightly increased cell size, approximately 83 microns (FIG 3) compared to Example 1, while maintaining improved resistance 329 kPa on the foam compositions comparative Example 3 Foams (7790) were prepared using 2% nano-calcium nanocarbonate carbonate loading of Ampacet, together with 1% talc as an additional nucleating agent and 1% hexabromocyclododecane stabilized as a pyro-retardant agent in a LMP extruder The calcium nanocarbonate particles were typically elongated, with average dimensions of 80 nm, x 2 μm, and were provided in a 50% masterbatch composition in combination with an olefinic eopolymer carrier resin. The rest of the formulation was polystyrene: 80% Nova 1220 (melt index = 1.45) and 16% Nova 3900 (melt index = 34.5). The exemplary foam produced was 28 mm thick, 400 mm wide and an average cell size of 230 microns, with a cell orientation-the ratio of cell dimension in the direction of extrusion to the cell dimension in the direction of thickness (x / z) - as high as 1.54 (see FIG 4). Example 4 Foams (7789) were produced as in example 3, but expanded nano-graphite interspersed with 3.3% of Superior Graphite Company, such as nano-particles. The nano-expanded graphite includes graphite nano-sheets having thicknesses in the range from about 10 to about 100 nm and widths of about 3 μm. The exemplary foam exhibited substantially the same thickness, width and density (49 kg / m3) as Example 3, but had a smaller average cell size of 166 microns and a cell orientation value of 1.21 (see FIG. . The thermal conductivity of this foam is as low as 0.14 K.m2 / for samples after being aged for 20 days. Example 5 Foams (7289, 7291) were prepared using a Leistritz extruder, to produce samples having a thickness of about 10 mm, a width of about 50 mm, a density of about 46 kg / m 3. Both samples with 0.5% talc as a nucleating agent and 10% HCFC142b / 22 as the blowing agent. Some characters of cell morphology are summarized in table 3. Table 3 Example 6 Foams (7293, 7294) were prepared as in Example 5, but using 6% by weight of carbon dioxide as the blowing agent and 0.2% by weight of talc as a conventional nucleating agent. Some characteristics of the resultant cell morphologies (FIG 6) are summarized below in Table 4: Table 4 While exemplary embodiments of the process of the present invention have been described with reference to specific details and parameters, those of ordinary skill in the art will appreciate that the process described encompasses a variety of components and operating conditions that can be customized to produce a range of manufacturing processes and foam composition, which can be tailored to achieve desired foam composition properties or adapted to the particular manufacturer's equipment, without departing from the spirit and scope of the present invention as defined in the following claims.

Claims (21)

1. CLAIMS 1. A method for manufacturing a rigid foam, characterized in that it comprises: preparing a polymer melt including a major portion of at least one alkenyl aromatic polymer, selected from a group consisting of alkenyl aromatic homo-polymers, co-polymers of alkenyl aromatic compounds and ethylenically co-polymerizable unsaturated co-monomers; incorporating nano-particles having minimum dimensions of less than about 100 nm in the polymer melt, at a concentration of at least 0.5% by weight, based on the weight of the polymer; incorporating a blowing agent into the polymer melt; extruding the polymer melt under conditions sufficient to allow the polymer melt to expand and form a foam; and cooling the foam to form a foam product having an average cell size of between 60 and 500 μm, an average cell wall thickness of less than 1 μm, an average cell column diameter of 4 μm to 6 μm , a foam density of 20 to 60 kg / m3 as measured by ASTM D-1622; and at least 90% closed cells as measured by ASTM D-2856-A.
2. 2. A method for manufacturing a rigid foam according to claim 1, characterized in that the polymer includes a major portion of at least one alkenyl aromatic polymer selected from a group consisting of the styrene polymerization products, alpha-methylstyrene, chlorostyrene, bromostyrene, ethylstyrene, vinyl benzene and vinyl toluene; and minor portions of a non-alkenyl aromatic polymer.
3. 3. A method for manufacturing a rigid foam according to claim 2, characterized in that the polymer includes at least 80% by weight of polystyrene.
4. 4. A method for manufacturing a rigid foam according to claim 1, characterized in that the blowing agent includes at least one composition selected from a group consisting of aliphatic hydrocarbons having 1 to 9 carbon atoms, halogenated aliphatic hydrocarbons which they have 1 to 4 carbon atoms, azodicarbonamide and p-toluenesulfonyl.
5. A method for manufacturing a rigid foam according to claim 1, characterized in that the blowing agent includes at least one composition selected from a group consisting of carbon dioxide, nitrogen and water.
6. 6. A method for manufacturing a rigid foam according to claim 4, characterized in that the blowing agent includes at least one composition selected from a group consisting of methane, methanol, ethane, ethanol, propane, propanol, n-butane and isopentane, azodicarbonamide, p-toluenesulfonyl, HCFC-142b and HCFC-134a.
7. A method for manufacturing a rigid foam according to claim 1, further characterized in that it comprises incorporating an additive into the polymer melt before forming the foam.
8. A method for manufacturing a rigid foam according to claim 7, characterized in that the additive includes at least one composition selected from a group consisting of pyro-retardants, mold release agents, pigments and fillers.
9. 9. A method for manufacturing a rigid foam according to claim 1, characterized in that the nanoparticles have a minimum dimension, less than about 100 nm and are chosen from a group consisting of calcium carbonate, intercalated clays, graffiti intercalated , exfoliated clays and expanded graphites.
10. A method for manufacturing a rigid foam according to claim 9, characterized in that the nanoparticles are incorporated into the polymer melt at a rate between 0.01 and 10 weight percent, based on the weight of the polymer.
11. 11. A method for manufacturing a rigid foam according to claim 9, characterized in that the nanoparticles are incorporated into the polymer melt at a rate between 0.5 and 5 weight percent based on the weight of the polymer.
12. 12. A method for manufacturing a rigid foam in accordance with claim 9, characterized in that the nano-particles are incorporated into the polymer melt at a rate between 0.5 and 5 weight percent, and a conventional nucleating agent is incorporated at a speed between 0.2 and 1 weight percent, based on the weight of the polymer.
13. A method for manufacturing a rigid foam according to claim 11, characterized in that the nano-particles include a major portion of nano-montmorillonite (MMT); and the polymer includes a major portion of polystyrene (PS), polyethylene (PE), or polymethyl methacrylate (PMMA).
14. A method for manufacturing a rigid foam according to claim 10, characterized in that the nano-particles are formed by a technique selected from a group consisting of intercalation with poly-styrene, in-situ polystyrene polymerization (PS) or polymethyl methacrylate (PMMA) with a surface modified nano-montmorillonite (MMT), and exfoliate expandable graphite particles in a poly-styrene or polymethyl methacrylate matrix.
15. 15. A method for manufacturing a rigid foam according to claim 1, characterized in that the average cell size is from 62 μm to 174; the average cell wall thickness is less than 10 μ; the average column diameter is less than 20 μm; the cell orientation is between 0.5 and 2.0; and the foam density is less than 100 kg / m3.
16. 16. A method for manufacturing a rigid foam according to claim 15, characterized in that the average cell size is between about 60 and about 120 μm; the average cell wall thickness is between about 0.2 and about 1.0 μ; the average column diameter is between about 4 and about 8 μm; the orientation of cells is between approximately 1.0 and approximately 1.5; and the foam density is between about 20 and about 50 kg / m3.
17. 17. A method for manufacturing a rigid foam according to claim 1, characterized in that it further comprises incorporating a conventional nucleating agent into the polymer melt at a rate of 0.2 to 2 weight percent, based on the weight of the polymer.
18. 18. A method for manufacturing a rigid foam according to claim 17, characterized in that the cell size distribution is bimodal with a first peak centered between about 50 μm and 120 μm and a second peak centered on about 200 μm.
19. 19. A rigid foam, characterized in that it comprises: at least about 80% by weight of a polymer matrix including a major portion of at least one alkenyl aromatic polymer selected from a group consisting of alkenyl aromatic homo-polymers, copolymers of compounds alkenyl aromatics and copolymerizable ethylenically unsaturated comonomers; and between 0.5 and 10 weight percent of nano-particles having a minimum dimension of less than about 100 nm; the polymer matrix is further characterized by an average cell size of between about 60 and about 120 μm; an average cell wall thickness between about 0.2 and about 1.0 μm; an average column diameter of between about 4 and about 8 μm; an orientation of cells between approximately 1.0 and approximately 1.5; and a foam density of between about 20 and about 50 kg / m3.
20. 20. A rigid foam according to claim 19, characterized in that the polymer matrix is further characterized by a foam compression strength of at least 300 kPa according to ASTM DI621.
21. 21. A rigid polymer foam according to claim 19, characterized in that the cell orientation is at least 1.2; and also where at least 90% of the cells are closed cells.