WO2010097466A1

WO2010097466A1 - Polymer granulation process and polymer granulates

Info

Publication number: WO2010097466A1
Application number: PCT/EP2010/052496
Authority: WO
Inventors: Joseph Maria Henri Janssen; Leonardus Martinus Cals
Original assignee: Dsm Ip Assets B.V.
Priority date: 2009-02-27
Filing date: 2010-02-26
Publication date: 2010-09-02

Abstract

The invention relates to a process for the production of a polymer granulate from a thermally conductive thermoplastic polymer composition comprising a thermoplastic polymer and a heat conductive material dispersed in the thermoplastic polymer, comprising steps of (a) providing an extruder with heating zones, an extrusion die comprising a hot plate, optionally being in thermally conductive contact with the heating zones, and a cutting plate in contact with a cooling liquid, the extrusion die comprising multiple bores passing through the hot plate and the cutting plate, and a cutting knife submerged in the cooling liquid positioned adjacent to the cutting plate, (b) preparing a melt of the thermally conductive thermoplastic polymer composition in the extruder, (c) extruding the melt through the bores to form an extrudate, and (d) cutting the extrudate with the cutting knife to form polymer granules.

Description

POLYMER GRANULATION PROCESS AND POLYMER GRANULATES

The invention relates to a process for the production of a polymer granulate from a thermally conductive thermoplastic polymer composition comprising a thermoplastic polymer and a heat conductive filler.

Thermally conductive thermoplastic polymer compositions have been introduced in the market to replace metal are being used in particular for heat sinks and other E&E applications requiring a good dissipation of the heat produced by the electronic components. In the last years these materials have gained tremendously in volume. Thermoplastic polymers are typically thermally isolators, even though these polymers might transfer some heat, but only in a very limited extend, which is in fact neglectable compared to good thermal conductors such as metals. To make a polymer composition thermally conductive, it is known that thermally conductive materials that can be dispersed in the thermoplastic material and that improve the thermal conductivity of plastic composition. Compounding can be done, for example, by polymer extrusion.

Polymer granulates of thermoplastic polymer compositions, which can be used in a molding process for producing injection molded or extrusion molded parts, are generally produced by melt compounding and extrusion, through a die plate or extrusion die, into strands and cutting of the strands. A polymer granulate prepared by such a process consists of many individual granules. Preferably the granules have a regular shape and are of about the same size to provide for smooth flow and transport during the injection molding or extrusion molding process. In that respect fragmentation of the granules and dust formation is highly undesirable. It has been observed by the inventors that production of a polymer granulate from a thermally conductive thermoplastic polymer composition can be very difficult. It was also observed that in a melt-extrusion process, the extruded material cools must faster than regular thermoplastic materials that are not thermally non- conductive, and due to that fast cooling it appeared that the thermally conductive thermoplastic polymer composition becomes relative brittle, complicating the cutting of the extruded strands. In particular when the thermoplastic polymer composition comprises a high load of thermally conductive filler and/or has high thermal conductivity, the compositions show a very poor melt strength and result in granulates of very poor quality if producible at all. Production at large scale requires a smooth process, and also the processing for the different applications requires a trouble free product. Therefore there is a need for granulate materials made of thermally conductive thermoplastic polymer composition, that does not show the problems indicated above. The aim of the present invention is to provide a process that allows production of a polymer granulate from a thermally conductive thermoplastic polymer composition suitable for use in heat sinks with good granulate quality.

This aim has been achieved with the process according to the invention, comprising steps of: a. providing an extruder with heating zones, an extrusion die comprising a hot plate, optionally being in thermally conductive contact with the heating zones, a cutting plate in contact with a cooling liquid, the extrusion die comprising multiple bores passing through the hot plate and the cutting plate, and a cutting knife submerged in the cooling liquid positioned adjacent to the cutting plate, b. preparing a melt of the thermally conductive thermoplastic polymer composition in the extruder, c. extruding the melt through the bores thereby forming an extrudate, and d. cutting the extrudate with the cutting knife thereby forming polymer granules.

The effect of the process according to the invention comprising the steps as described hereabove, is that a polymer granulate consisting of regularly shaped granules is obtained. This result is obtained despite the fast cooling of the extrudate in the cooling liquid.

A polymer granulate is herein understood a material consisting of small particles or granules. With the term "thermally conductive" in thermally conductive thermoplastic polymer composition is herein understood that the composition has such a thermal conductivity that the composition is suitable for use in heat sink applications. Typically such a composition has an in-plane parallel thermal conductivity of at least 2 W/m-K and/or a through-plane thermal conductivity of at least 0.5 W/m-K, and wherein the thermal conductivity is measured according to ASTM E1461-01.

The process according to the invention may be performed in different embodiments and variations.

The extruder used to prepare the polymer melt and to supply the polymer melt to the extrusion die can be any type of extruder suitable for preparing polymer granulate from a polymer melt. The extruder may be, for example, a single screw extruder, or a twin screw extruder. The hot plate in the extrusion die may be hot by steam, hot oil, or electricity for maintaining the passing polymer melt at the required temperature.

For the cooling liquid, for example water may be used. The cooling liquid may have a temperature varying over a wide range. Preferably, has cooling liquid, in particular water, has a temperature between 25°C and 95°C, more preferably between 40⁰C and 90⁰C, and very suitably between 60°C and 80⁰C. The cooling liquid might be recycled, after removal of granulate collected therein, into the process. During the recycling the cooling liquid might gradually be hot up by warm or hot freshly extruded polymeric material. The temperature of the cooling liquid might be controlled by passing the cooling liquid through a heat exchanger.

To prevent unwanted heat transfer between hot plate and cutting plate, both plates shall be thermally isolated from each other. Alternatively the surroundings of the bores shall be isolated. This is necessary for preventing solidification of the thermally conductive polymer inside the bores.

For the thermal isolation of the plates, different solutions may be used. The cutting plate may consist of a thermally isolating material, such as ceramic material. Another possibility is to have a separate isolation layer between the hot plate and the cutting plate. A further possibility is to use spacers between the hot plate and the cutting plate, thus creating a space between the two plates. This space might be filled with a gas, for example air, or a thermally isolating material. Meanwhile, the bores shall extend through the space between the hot plate and the cooled plate. In the alternative solution, a number of small cavities can be present around the bore holes in the hot plate and / or cutting plate. The cavities may be filled with gas, such as air, or a thermally isolating material.

The bores in the extrusion die can have dimensions varying over a wide range and also various shapes. Suitably the bores have a circular or an ellipsoid cross section, or a similar shape. Herein the cross section is taken perpendicular to the flow direction of the polymer melt during the extrusion. Preferably, the bores have a cross section with a diameter between

0.8 and 3.5 mm, preferably between 1 and 2.5 mm, and more preferably between 1.2 and 2 mm.

An example of a die plate suited for use in the process according to the invention is described in US patent 5,620,130. - A -

On leaving the bores, or extrusion holes, the polymer extrudate is cut, thereby forming polymer granules. For the cutting, a knife is used which is positioned adjacent to or even in direct contact with the cutting plate. The cutting knife suitably is a rotating knife that during the granulating process is sweeping over the surface of the cutting plate. The cutting knife adjacent to the cutting plate preferably is at a very small distance from the cutting plate, e.g. less than 0.2 mm, or even better less than 0.1 mm. Ultimately, the cutting knife may be in direct contact with the cutting plate.

In the process according to the invention, the extrudate may be cut with an intermitting distance between consecutive cuts over a variable length. Suitably, the intermitting distance is between 0.8 and 5 mm preferably between 1 and 3.5 mm, and more preferably between 1.5 and 2.5 mm. Preferably the bore diameters and the intermitting distance are equal or similar, to obtain granules with the most regular shape.

Depending on the bore diameter and intermitting distance, the volume of the granules obtained with the process according to the invention may vary.

Preferably the granules are neither very small nor very large. Suitably, the granules have a weight average volume in the range between 2 mm³ and 25 mm³, preferably between 4 mm³ and 20 mm³, more preferably between 6 mm³ and 15 mm³. A higher lower limit provides for a better flow, while a lower upper limit provides for better and more accurate processing and molding, in particular when articles with small dimensions have to be produced.

The granules obtained with the process according to the invention often have an appearance that differs from a conventional extrusion and granulation processes. Granules obtained with the process according to the invention often have a sphere like appearance, optionally flattened at one side, or a disc like shape with rounded off edges or cylindrical shape with one rounded off top, and a flattened end at the other side. The shape generally depends on the dimensions of the granules, the diameter of the bores and the intermitting distance between consecutive cuts.

This in contrast with granules of regular performing materials in standard granulation processes, wherein the granules typically have a cylindrical shape, flattened at both ends.

The thermally conductive thermoplastic polymer composition used in the process according to the invention may comprise any thermoplastic polymer and any heat conductive filler that can be used in a melt extrusion process for the preparation of a thermally conductive thermoplastic polymer composition. The thermoplastic polymer may comprise, for example polyamides, polyesters, polyarylene sulfides, polyarylene oxides, polysulfones, polyarylates, polyimides, poly(ether ketone)s, polyetherimides, polycarbonates, copolymers of said polymers among each other and/or with other polymers, including thermoplastic elastomers, and mixtures of said polymers and copolymers. Preferably, the polymer is chosen from the group consisting of polyesters, polyamides, thermoplastic elastomers, and combinations thereof.

The process is in particular advantageously applied for thermally conductive compositions comprising a semi-crystalline thermoplastic polymer, in particular semi-crystalline thermoplastic polymer with a melting temperature (Tm) of at least 200⁰C. More preferably the Tm is at least 25O⁰C and most suitably in the range of 275 - 34O⁰C. Suitably, the semi-crystalline thermoplastic polymer with the Tm in any one of these preferred ranges is a semi-crystalline thermoplastic polyester or a semi- crystalline thermoplastic polyamide. For the thermally conductive material in the thermally conductive thermoplastic polymer composition any material that can be dispersed in the thermoplastic polymer and that improves the thermal conductivity of the plastic composition can be used. Suitable thermally conductive materials include, for example, alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, zinc oxide, glass, mica, graphite, ceramic fibers, carbon fibres and the like. Mixtures of such thermally conductive materials are also suitable.

The thermally conductive material may be in the form of granular powder, particles, whiskers, short fibers, or any other suitable form. The particles can have a variety of structures. For example, the particles can have flake, plate, rice, strand, hexagonal, or spherical-like shapes.

The thermally conductive material suitably is a thermally conductive filler or a thermally conductive fibrous material, or a combination thereof. A filler is herein understood to be a material consisting of particles with an aspect ratio of less than 10:1. Suitably, the filler material has an aspect ratio of about 5: 1 or less. For example, boron nitride granular particles having an aspect ratio of about 4:1 can be used. A fiber is herein understood to be a material consisting of particles with an aspect ratio of at least 10:1. More preferably the thermally conductive fibers consisting of particles with an aspect ratio of at least 15:1 , more preferably at least 25:1. For the thermally conductive fibers any fibers that improve the thermal conductivity of the plastic composition can be used. Suitably, the thermally conductive fibers comprise glass fibers, metal fibers and / or carbon fibers. Suitable carbon-fibers, also known as graphite fibers, include PITCH-based carbon fiber and PAN-based carbon fibers. For example, PITCH-based carbon fiber having an aspect ratio of about 50:1 can be used. PITCH-based carbon fibers contribute significantly to the heat conductivity. On the other hand PAN-based carbon fibers have a larger contribution to the mechanical strength.

The choice of thermally conductive material will depend on the further requirements for the thermally conductive thermoplastic polymer composition and the amounts that have to be used depend on the type of thermally conductive material and the level of heat conductivity required. The thermally conductive thermoplastic polymer composition produced with the process according to the invention suitably comprises 30-90 wt% of the thermoplastic polymer and 10-70 wt% of the thermally conductive material, preferably 40-80 wt % of the thermoplastic polymer and 20-60 wt % of the thermally conductive material, wherein the wt% are relative to the total weight of the plastic composition. It is noted that for the amount of 10 wt.% might be sufficient for one type of thermally conductive material to attain a minimum thermal conductivity, such as for specific grades of graphite, whereas for others, such as pitch carbon fibers, boron nitride and in particular glass fibers, much higher wt.% are needed. The amounts necessary to attain the required levels can be determined by the person skilled in the art of making thermally conductive polymer compositions by routine experiments.

The thermally conductive thermoplastic polymer composition used in the process according to the invention suitably has an in-plane parallel thermal conductivity of at least 2 W/m-K and/or a through-plane thermal conductivity of at least 0.5 W/m-K, wherein the thermal conductivity is derived from the thermal diffusivity (D) measured by laser flash technology according to ASTM E1461-01 on injection molded samples of 80x80x2 mm in in-plane respectively through plane direction, the bulk density (p) and the specific heat (Cp), at 20⁰C, using the method described in Polymer Testing (2005, 628-634).

Preferably, the in-plane parallel thermal conductivity is in the range of 5-50 W/m.K, more preferably, 10-30 W/m.K. Also preferably, the through-plane thermal conductivity is in the range of 1-20 W/m.K, preferably 2-10 W/m.K. The person skilled in the art of making thermally conductive thermoplastic polymer compositions, avails of the knowledge needed to select thermally conductive fillers and amounts needed to make a thermally conductivity thermoplastic polymer composition with a certain thermally conductivity and tailor it by routine experiments. The effects of the invention are larger with polymer composition having a larger thermally conductivity.

The invention also relates to the polymer granulate obtainable with an underwater granulation process comprising the steps according to the invention, and any selection or variation or preferred embodiment thereof. The granules differ markedly in shape from granules obtained by a conventional melt extrusion and cutting process. The granules obtained by the underwater granulation process according to the process are more or less spherically or mushroom shaped, while those of the conventional process are more cylindrical. The invention is further illustrated with the following examples and comparative experiments

EXAMPLES

Molding compositions were prepared from polyamide-46 and different heat conductive fillers. For the heat conductive fillers the following materials were used: - graphite powder TIMREX® BNB90 (denoted as BNB), available from TIMCAL Ltd., Bodio, Switzerland, boron nitride PolarTherm® PT100 (denoted as BN), available from Momentive Performance Materials, Inc. (formerly GE Advanced Ceramics), The formulations of the molding compositions are shown in Table 1.

The weight percentages of filler materials are based on the total weight of the polymer composition. The compositions further comprised about 1 % of auxiliary additives, the remainder adding up to 100 % being polyamide-46.

The materials were compounded in a Berstorff ZE25-48D co-rotating twin screw extruder using a standard melt compounding process. The temperature settings of the extruder were such that the melt temperature at the exit of the extruder was typically 320⁰C.

The extruder was initially equipped with a standard die plate and granulation set-up, wherein strands were extruded, where possible guided through a cooling bath and subsequentially through a standard cutter.

The experiment was repeated with the extruder equipped with an underwater granulation set-up with an extrusion die having a hot plated and a cutting plate thermally isolated from each other.

The extrusion and granulation process with the standard set-up resulted in strand breaking in particular with the composition comprising the BNB graphite powder, while with the BN less problems occurred but still resulted in irregular granule particles and severe dusting problems. The extrusion and granulation process with the underwater granulation set-up resulted in granules with much regular shapes and less dusting problems. The largest improvement was seen with the granulate comprising the BNB graphite powder, i.e. the material with the largest thermal conductivity.

Table 1. Compositions, thermal conductivity and extrusion results, for Examples I and Il and Comparative Experiments A and B.

++ Very smooth and regularly shaped particles + smooth and regularly shaped particles +/- irregularly shaped particles

- improper running process, extensive strand breaking, very bad and irregular shaped particles, impossible to obtain decent granulate

Claims

1. Process for the production of a polymer granulate from a thermally conductive thermoplastic polymer composition suitable for use in heat sinks, comprising a thermoplastic polymer and a heat conductive material that improves the thermal conductivity of the polymer composition dispersed in the thermoplastic polymer, comprising steps of: a. providing an extruder with heating zones, an extrusion die comprising a hot plate, and a cutting plate in contact with a cooling liquid, the extrusion die comprising multiple bores passing through the hot plate and the cutting plate, and a cutting knife submerged in the cooling liquid positioned adjacent to the cutting plate, b. preparing a melt of the thermally conductive thermoplastic polymer composition in the extruder, c. extruding the melt through the bores to form an extrudate, and d. cutting the extrudate with the cutting knife to form polymer granules.

2. Process according to claim 1 , wherein the cutting plate is thermally isolated from the hot plate.

3. Process according to claim 1 or 2, wherein the cooling liquid is water.

4. Process according to any one of claims 1-3, wherein the cooling liquid has a temperature between 40⁰C and 90⁰C.

5. Process according to any one of claims 1-4, wherein the bores have a diameter between 1 and 3.5 mm.

6. Process according to any one of claims 1-5, wherein the extrudate is cut with an intermitting distance between consecutive cuts in the range of 1 - 3.5 mm.

7. Process according to any one of claims 1-6, wherein the thermoplastic polymer comprises a polymer chosen from polyesters, polyamides and thermoplastic elastomers.

8. Process according to any one of claims 1-7, wherein the heat conductive filler comprises a filler chosen from the group consisting of alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, zinc oxide, glass, mica, graphite, ceramic fibers, and carbon fibres.

9. Process according to any one of claims 1-8, wherein the thermally conductive thermoplastic polymer composition has an in-plane parallel thermal conductivity of 2 to 50 W/m-K and/or a through-plane thermal conductivity of 0.5 to 10 VWm-K, measured according to ASTM E1461-01 ,

10. Polymer granulate consisting of a thermally conductive thermoplastic polymer composition having an in-plane parallel thermal conductivity of 2 to 50 W/m-K and/or a through-plane thermal conductivity of 0.5 to 10 W/m-K, measured according to ASTM E1461-01 , obtainable by an underwater granulation process comprising the steps according to any of claims 1 - 9.

11. Polymer granulate according to claim 10, consisting of granules having a weight average volume in the range of 2 mm³ and 25 mm³, preferably 5 mm³ and 20 mm³, more preferably between 6 mm³ and 15 mm³.