MXPA99004084A - Highly thermally conductive yet highly conformable alumina filled composition and method of making the same - Google Patents

Abstract

A highly thermally conductive but yet highly conformable composition (1) is made from gel filled with alumina. The use of&agr;-alumina in which at least 10 weight%of the&agr;-alumina particles have a particle size of at least 74&mgr;m makes possible the high filler levels needed to attain high thermal conductivity, without causing the decrease in elongation and softness normally associated with high filler levels. Further improvements are observed if the&agr;-alumina and the gel (or precursor thereof) are mixed with a specific energy input of at least 10 Joule/g. The input of such specific energy has the effect making the resulting composition more conformable than it otherwise would be. The composition may be internally supported by a flexible matrix such as an open-celled foam (2) or a fabric (3).

Description

FULL COMPOSITION OF ALUMINA HIGHLY AND THERMALLY CONDUCTOR AND HIGHLY COMFORTABLE AND METHOD FOR DO THE SAME TECHNICAL FIELD OF THE INVENTION This invention relates to highly formable and highly conformable high alumina filled compositions and methods for making them.

BACKGROUND OF THE INVENTION It may be important in an electrical or electronic device to conduct heat away from modules, enclosures, circuit boards, integrated circuit chips, and other components and to a metal plate or other heat collecting element, for effective heat dissipation generated during the operation. Fats and pastes filled with thermally conductive fillers have been used for such purposes. However, they tend to migrate to adjacent spaces over time, particularly at elevated temperatures, contaminating other areas of the device and causing a loss of the desired thermal conductivity. It can also be difficult to handle, particularly when the device is reinserted for repair or replacement, since the surfaces on which they have been placed are difficult to clean. The alternatives for fats and pastes are thermally conductive gels or pressure sensitive adhesives, such as those described by Dittmer et al., USA. 4,852,646 (1989); Mercer et al, WO 96/23007 (1996); and Chiotis et al., WO 96/05602 (1996). Since they are high elongation materials, low modulus (soft), the gels are highly conformable, allowing them to establish an excellent thermal contact with irregular surfaces. Gels offer the advantage of an easy re-insertion capacity: they generally have a cohesive energy greater than their binding energy to the surface on which they have been applied, allowing them to disengage cleanly. Many gels are made of interlaced polymer systems, so they will not migrate, unlike fats and pastes. (Gels made from a thermoplastic base polymer are also known, said gels will also not migrate as long as the operating temperature is maintained below the melting temperature of the base polymer). A thermally conductive gel-based composition is made by filling the gel with a thermally conductive filler such as a particulate silicon nitride, aluminum nitride, boron nitride, or alumina (aluminum oxide). Nitrides, especially aluminum nitride, are desirable for their high specific thermal conductivities that allow them to be used in relatively small amounts, while still achieving desired high thermal conductivity in the resulting composition. However, aluminum and boron nitrides are more expensive than alumina by approximately two orders of magnitude, limiting their commercial utility. Also, it has been found that aluminum nitride is hydrolytically unstable and will react with ambient humidity. The hydrolysis of aluminum nitride can cause degradation in composite properties. In addition, the ammonia generated in the hydrolysis of aluminum nitride can accelerate corrosion and cause degradation of materials if it comes in contact with and can interfere with the entanglement of gel systems such as organopolysiloxane interlaced through hydrosilation chemistry.

The lowest specific thermal conductivity of alumina (compared to nitrides) means that a larger amount must be used in order to achieve the same final thermal conductivity in a full gel. However, the advantages of storage cost and stability of alumina make it attractive as a thermally conductive filler. When the thermal conductivity required in the final gel is relatively low, the need to employ larger quantities of alumina is not a serious limitation. However, when the final gel needs to be high and thermally conductive (meaning, for the purposes of this specification, a thermal conductivity of at least 1.3 watt / m- ° C), the large amount of alumina required (usually in excess 60 percent by weight) adversely affects the elongation and smoothness of the final product, compromising its conformable capacity.

COMPENDIUM OF THE INVENTION In this way, the problem addressed and solved by this invention is how to make a gel filled with high thermally conductive alumina, which retains a high degree of formability. First, it has been found that such a result can be obtained by employing an α-alumina, wherein at least 10 percent by weight of the α-alumina has a particle size exceeding a prescribed minimum size. (In contradiction, the prior art has taught that particle size and orientation, not particle size, are key parameters for determining thermal conductivity in a particle-filled system, see, for example, Hansen et al., Polymer. Engineering and Science, Vol. 15, No. 5, pp. 353-356 (May 1975)). Secondly, it has been found that the result of high thermal conductivity while retaining a high conformability is more effectively achieved if the combination of α-alumina and gel material (precursor or its precursor components) is mixed with a minimum prescribed amount of energy mechanical that is being introduced to the combination. Accordingly, in a first embodiment, there is provided a method for making a thermally conductive and conformable composition, comprising the steps of: (a) providing 100 parts by weight of a gel; (b) combining the gel with between 150 and 400 parts by weight of α-alumina, wherein at least 10 percent by weight of the α-alumina has a particle size of at least 74 μm, to form a combination of the gel and a-alumina; and (c) mixing the combination to make the composition thermally conductive and conformable. In a second embodiment, a method for making a thermally conductive and conformable composition is provided, comprising the steps of: (a) providing 100 parts by weight of a gel curable precursor material; (b) combining the precursor material with between 150 and 400 parts by weight of an α-alumina, wherein at least 10 weight percent of the α-alumina has a particle size of at least 74 μm, to form a combination of the precursor material and the α-alumina; and (c) curing the precursor material to a gel to form the conformable, thermally conductive composition. In a third embodiment, there is provided a method for making a thermally conductive and conformable composition, comprising the steps of: (a) separately providing first and second precursor components, which when mixed together, are cured to form a gel, the combined amounts of the first and second precursor components totaling 100 parts by weight; (b) separately combining α-alumina with each of the first and second precursor components to form first and second respective combinations comprising α-alumina and a first or second precursor component respectively, wherein at least 10 percent by weight of the α-alumina has a particle size of at least 74 μm and the total amount of α-alumina is between 150 and 400 parts by weight; and (c) mixing the first and second combinations and curing to form a conformable, thermally conductive composition. It is preferred that, after the α-alumina has been combined with the gel, that the precursor material, or the first and second components (whichever is the case), the combination is mixed with a specific energy input of minus 10 Joules / g. In a fourth embodiment, a conformable and thermally conductive composition is provided, comprising: (a) 100 parts by weight of a gel; and (b) between 150 and 400 parts by weight of α-alumina, wherein at least 10% by weight of the α-alumina has a particle size of at least 74 μm. In a fifth embodiment, an article is provided comprising '(I) a conformable and thermally conductive composition comprising: (a) 100 parts by weight of a gel; and (b) between 150 and 400 parts by weight of α-alumina, wherein at least 10 percent by weight of the α-alumina has a particle size of at least 74 μm; and (II) a flexible matrix internally supporting the composition.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1a and 1b show the composition of this invention supported by an open cell foam matrix and a fabric matrix, respectively.

DETAILED DESCRIPTION OF THE INVENTION The compositions of this invention have a high thermal conductivity, while retaining a high degree of formability, that is, they remain soft and elastic. Preferably, they have a thermal conductivity of at least 1.3 watt / m ° C. a hardness less than 1,000 g. and an elongation at break of at least 50%. They comprise a gel and an α-alumina as a particulate filler to impart thermal conductivity.

The gels can be seen as substantially diluted, extended fluid polymer systems, which do not exhibit a steady state flux. As discussed by Ferry, "Viscoelastic Properties of Polimers," 3rd ed., P. 529 (J. Wiley &Sons, New York 1980), a polymer gel is an interlaced solution through chemical bonds, crystallites, or some other type of bond. The absence of steady-state flux is the key definition of solid-type properties, while substantial dilution is necessary to give the gels the relatively low modulus. The nature of the solid is achieved through a continuous network structure formed in the material generally through the interlacing of the polymer chains. The entanglement can be physical or chemical as long as the entanglement sites are maintained at the conditions of gel use. In this manner, gels suitable for use in this invention can be classified as chemically crosslinkable, wherein a precursor material is cured to form the gel material, and thermoplastics, wherein the entanglements are physical. Suitable chemically interlaced gel materials include systems based on polyurethanes, polyureas, silicones (also known as polysiloxanes or organopolysiloxanes), polymers containing anhydride, and the like. Illustrative descriptions include Dubrow et al., USA. 4,595,635 (1986); Debbaut USA 4 600,261 (1986), Dubrow et al., USA. 4,777,063 (1988); Dubrow et al., USA. 5,079,300 (1992); Rinde et al., USA 5,104,930 (1992); Chiotis et al., WO 96/10608 (1996); and Mercer et al., WO 96/23007 (1996); the descriptions of which are incorporated herein by reference in their entirety. Preferably, the gel is a silicone gel based on polydimethylsiloxane (PDMS) and prepared through the platinum catalyzed reaction between a PDMS functionalized with vinyl and a PDMS functionalized with hydride. Such gels can be formed in a number of ways. One method synthesizes the entangled polymer in the presence of a non-reactive extension fluid, for example, trimethylsiloxy-terminated PDMS. An alternative method manufactures the silicone gel by reacting a stoichiometric excess of a silicon substituted with multifunctional vinyl with a silicon substituted with multifunctional hydride in such a way that a smooth, fluid extended system is obtained. In the last aspect, a fraction of sol rich in vinyl is obtained. Of course, combination systems are possible. Suitable examples of any of these that gel systems are described among others by Debbaut, USA. 4,600,261 (1986); Debbaut, USA 4,634,207 (1987); Debbaut, USA 5,357,057 (1994); Dubrow et al., USA. 5,079,300 (1992); Dubrow et al., USA. 4,777,063 (1988); and Nelson, USA 3,020,260 (1962): the descriptions of which are incorporated herein by reference. Silicone gel systems based on alternative healing techniques such as peroxide, UV light, and high energy radiation can also be used.

Alternatively, the gel may be a thermoplastic gel, based on a thermoplastic polymer such as a block copolymer of styrene (ethylene-butylene) -styrene (SEBS) or a block copolymer of styrene- (ethylene-propylene) -styrene ( SEP), and the like, spread with a hydrocarbon oil extension oil with a naphthenic or non-aromatic content, or a low aromatic content. Illustrative descriptions include Chen, USA. 4,369,284 (1983); Gamarra et al., USA 4,716,183 (1987); Gamarra, USA 4,942,270 (1990); Suterland et al., WO 90/05166; and Hammond et al., WO 93/23472 (1993); the descriptions of which are incorporated herein by reference. Gels produced from SEBS or SEPS and paraffinic oils comprise vitreous styrenic microphases interconnected through an elastomeric extended fluid phase. The styrenic domains separated by microfase serve as the junction points in the systems. These gels are examples of thermoplastic systems. Conversely, silicone gels are thermo-fixation gels, chemically entangled through a multifunctional entanglement agent. The preferred form of alumina is α-alumina, since it is more thermally conductive than other forms such as gamma and beta forms. Most preferably, α-alumina is calcined α-alumina. In the calcination, the α-alumina is subjected to a high temperature treatment (typically between 1,100 and 1,200 ° C). said temperature making it insufficient to fuse the individual particles together (although some agglomeration may occur). Calcined alumina is to be distinguished from other forms of alumina, such as concreted, fused or tabular alumina, which are heated to a higher temperature such as 1,650 ° C where melting occurs. The α-alumina contains at least 10, and preferably at least 20, by weight particles having a particle size of at least 74 μm, the weight percentage based on the total weight of the alumina. Preferably, substantially all of the particles in the α-alumina have a size below 700 μm, most preferably below 176 μm. In certain applications, compositions of this invention are used as thin pads. Obviously, in such applications the particle size must be maintained below the thickness of the pad. The particle size is measured through a light diffusion technique, according to ASTM C1070-86 (Reapproved 1992). For particle sizes greater than 176 μm, mechanical or manual screening is recommended in accordance with ASTM C92-88. Also preferably, α-alumina is a powder having an overall density of less than 1.0 g / cm 3, as measured in accordance with ASTM D1895-69 (Reapproved 1979), Method A. The α-alumina with the particle size distribution Here taught can be obtained by selecting the appropriate commercially available grade of a-alumina or by combining different commercial grades. Suitable grades of α-alumina include Alean C-75 (unground), Alean C-751 (a purer grade of C-75), Alean C-76 (unground), Alcoa A-12 (unground), individually or mixed with each other or mixed with some other α-alumina such as Alean C-75 (-325 mesh) and Alcoa A-12 (-325 mesh). Additives conventionally used in the art may be added, including antioxidants, UV stabilizers, flame retardants (both halogenated and non-halogenated), acid scavengers, and pigments. To improve the mixing of the α-alumina with the gel material, a coupling agent can be added. When high levels of inorganic fillers are mixed in organic or polymeric materials, coupling agents are sometimes added to aid mixing and to modify the interaction of the filler with the matrix. The filler can be treated with a coupling agent before its addition to the organic phase, or the coupling agent can be added to the organic phase, followed by the filler. Two common types of coupling agents are organosilanes and organic titanates. The organosilanes contain hydrolysable groups (typically halogen, alkoxy, acyloxy, or amino), which, after hydrolysis, form a reactive silanol group, which can be condensed with the α-alumina and also a non-hydrolysable organic radical possessing a functionality to impart desired characteristics. Most widely used organosilanes have one of these organic radicals. Typical examples of useful silane coupling agents include trimethoxysilanes, triethoxysilanes, methyldiethoxysilanes, and methyldimethoxysilanes containing one of the following substituents: phenyl, vinyl, alkyl of 1 to 10 carbon atoms, 3-phenylpropyl, 2-phenyl Methyl, 3-methacryloxypropyl, 3-acryloxypropyl, allyl, aminophenyl, aminopropyl, benzyl, chloromethyl, chloromethylphenyl, vinylfyl, 2-cyanoethyl, and the like. Organic titanates are typically titanium alkyl esters, which are rapidly hydrolyzed and can also be used in the surface modification of inorganic fillers. Typical examples of organic titanate coupling agents include alkyl titanates of 1 to 10 carbon atoms, such as tetraisopropyl titanate, tetra-n-butyl titanate. and tetrakis (2-ethylhexyl) titanate, and titanate chelators. such as titanium acetyl acetonate and titanium ethyl acetonate. It has been found that it is preferred to subject the combination of the α-alumina and the gel (or its precursors, whichever is the case) to mechanical mixing wherein at least 10 Joules / g of the specific energy is imparted thereto. (hereinafter "the high specific energy regime"). The specific energy is the mechanical energy per unit mass, which is imparted to the combination by the mixer. The units for specific energy are energy per mass, for example, Joules / g. The energy imparted can be directly measured in many mixing instruments by determining the amount of powder used to mix the combination and multiplying by the mixing time. In turn, the amount of powder used can be determined by subtracting the powder consumed in the activation of the mixing elements of the mixing instrument when the instrument is operated without the present combination of the powder consumed in the activation of the mixed elements when the combination is present Examples of mixing instruments that can be used to impart the above mixing energy include a Brabender Plasti-Corder® mixer, a Cowles mixer, a two-roll mill, a Versamixer, a single screw or twin screw extruder, a mixer of Sigma blade, a propeller mixer such as a Meyer mixer, and the like. It has been observed that, with the specific high energy regime, the combination tends to be much more fluid, still pourable, and the final product is a much softer, more conformable composition, whereas without it the combination is a coarse paste or it has the consistency of a wet powder and the final product is almost not so conformable. Generally, mechanical mixing is required to introduce that amount of energy. Manual mixing, with a spatula or mixing rod, imparts a relatively low amount of energy per unit time. Manual mixing for time scales of less than 10 minutes results in a mixing process that is relatively low in specific energy, which is less than about 2 Joules / g. The practical upper limit of specific energy will be determined more by economic and convenience considerations. The compositions can be kept at the energy levels described here for days or weeks without damage to the materials, but it is not practical to do this. These specific mixing energies can be obtained by blending at low energy for long times or higher energy for shorter times. It is preferred to use a specific energy of at least 0.001 W / g, preferably at least 0.003 W / g. Typical mixing times are from about 20 to about 60 minutes. If the specific energy is too high, heating and degradation of the material may occur. If the specific energy is too low, it takes too much time. One skilled in the art will appreciate that, therefore, as a matter of practice, either the end of the specific energy should be avoided., and at that time, the temperature, and mixing speed needed to obtain the desired specific energy will vary with the type of equipment and the type of composition. When the gel is a thermoplastic gel, such as SEPS or SEBS base, the α-alumina can simply be combined with the polymer and spread the fluid and, if desired, subject to the high specific energy regime. When the gel is a thermal cure gel (cure), such as polydimethylsiloxane base, several preparative options are available. In a thermofixing system, the gel is made from the first and second components, which are mutually reactive when mixed, plus an extension fluid. In one variation, the α-alumina is combined with the first and second components and optionally subjected to the high specific energy regime. Although typically the α-alumina is substantially and finally distributed between the first and second components (on a weight / weight basis), the uneven distribution is allowed (and including the addition of all the α-alumina only to one of the components) . In the case where there is an uneven distribution and the specific high energy regime is applied, it must be applied to the combinations where the weight ratio of the α-alumina to the first or second component (whatever the case) is equal to or greater than 3: 2. Then the combinations are mixed and allowed to cure to form the gel. In an alternative embodiment, the first and second components are combined, followed by the α-alumina, and then the optional high specific energy regime is applied. In such a mode, the components and the catalyst (if any) must be selected so that the cure rate is slower than the time necessary to complete the specific high-energy regime. The compositions of this invention and the gels used therein (or their precursors) can be characterized by various analytical techniques, described below. Hardness, stress relaxation, and adhesions are measured with a Voland-Stevens texture analyzer model LFRA, Texture Technologies Texture Analyzer TA-XT2, or similar machines, using a load cell of 5kg to measure force, a trigger of 5 grams, and a 6.35 mm stainless steel ball probe as described by Dubrow et al., USA. 5,079,300 (1992), the description of which is incorporated herein by reference. For example, a 20 mL glass vial containing approximately 12.5 g of analyte (gel or other material to be analyzed) is placed in the TA-XT2 analyzer and the probe is forced into the analyte at the rate of 0.2 mm / second to a penetration depth of 4.0 mm. Alternatively to use the analyte contained in the jars, the analyte may be in the form of a stack of slices with a thickness of 5.08 centimeters by 5.08 centimeters by 0.317 centimeters or a stack of four slices with a diameter of 2.54-0.158 centimeters, and a thickness of 0.635 centimeters maintained between the two halves of copper pipe with a diameter of 2.54 centimeters. The analyte hardness is the force in grams required to force the probe to penetrate or deform the analyte surface for the specified distance of 4.0 mm. Higher numbers mean harder materials. The TA-XT2 analyzer data is recorded and analyzed on an IBM PC or similar computer, running the software Microsystems Ltd, XT.RA Dimension, Version 3.76. Adhesion and relaxation by tension are read from the voltage curve generated by the XT.RA software automatically tracks the force curve against the time experienced by the load cell when the penetration speed is 2.0 mm / second and the probe is forced into the analyte at a preset penetration distance of about 4.0 mm. The probe is maintained at that penetration for 1 minute and withdrawn at a speed of 2.0 mm / second. Tension relaxation is the ratio of the initial force (F,) which resists the probe at the pre-set penetration depth minus the force resisting the probe (Ff) after 1 minute, divided by F, and expressed as a percentage. That is, the percentage of relaxation by tension is given by: (F, - Ff) x 100% "F," the relaxation by tension is a measure of the ability of the analyte to relax any induced compression placed on it. Adhesion is the force in grams that the probe resists as it is pulled out of the analyte when the probe is removed at a speed of 2.0 mm / second from the pre-set penetration depth. The tensile strength, final elongation and stiffness are determined in an Instron model 4501 tester with a 10.21 kg load cell. The microtension test specimens were cut according to the dimensions described in ASTM 1708-93, where the total length is 3.81 centimeters and the thickness of these analyte sheets is 0.3175 centimeters. Two parallel fine lines with a separation of 0.25 (marks) are drawn with a fine-tipped pen on the thin section of the test specimen, perpendicular to the direction of elongation. The Instron tester is used to apply a constant tension speed of 12.7 centimeters / minutes. The distance between the two parallel lines is continuously verified until the fault. The final elongation is calculated by subtracting the original mark separation from the mark separation to the break, dividing by the original mark separation and expressing the result as a percentage. The software (Series IX, version 5.30) provided with the instron tester verifies the voltage created in the sample during the constant voltage velocity experiment. The software integrates the area under the formation tension curve to determine the rigidity of the sample. In all cases, at least seven specimens of "dog bone" type were tested for each material. An alternative way to characterize the materials is through cone penetration (CP) values in accordance with ASTM D-217-82 as taught by Debbaut, USA. UU 4,600,261 (1986); Debbaut USA UU 4,634,207 (1987); Debbaut, USA UU 5,140,746 (1992); and Debbaut, EE. UU 5,357,057 (1994). The CP values range from about 70 (10"1 mm) to about 400 (10 ~ 1 mm.) The harder gels are preferably from about 70 (10" 1 mm) to about 120 (10 ~ 1 mm). Softer gels, preferably used to seal terminals, cable splices, and the like, are from about 200 (10 ~ 1 mm) to 400 (10"1 mm) with a particularly preferred scale of about 250 (10" 1 mm) at approximately 375 (10"1 mm) For a particular material system, a relationship between CP values and hardness can be developed as taught by Dittmer et al., U.S. 4,852,646 (1989). cited in this paragraph are incorporated herein by reference.The thermal conductivity is measured using a modification of the assured hot flow measurement method of ASTM E1530-93, in an Anter Model 2021 thermal conductivity instrument. The modification involves the inclusion of a separator between the upper and lower plates to ensure that the dimension of a sample is exactly measured and remains constant throughout the test.The separator is machined from Teflon PTFE and is designed to have an Minimal influence on conductivity measurement, while containing a consistent gap between the upper and lower plates. The samples have a diameter of 5.08 centimeters and a thickness of 2540 to 3048 microns. The thermal transfer compound (Anter 2021-075) is applied to the upper and lower plates before insertion of the sample. A pressure of 2,109 kg / cm2 is applied to compress the mixture. For soft materials, the samples are compressed until the top plate rests on the separator with a thickness of 2286 microns. After 15 minutes under load at room temperature, a thickness measurement is taken. A calibrated test method is used to ensure that the heat flow transducer is calibrated at the temperature and at the sample resistance scales. When the test is started, heat flows from the upper plate through the sample to the lower plate, creating an axial temperature gradient. The heat flow through the sample is measured with a heat flux transducer located just below the sample. By measuring the temperature difference across the sample together with the output from the heat flow transducer, the thermal conductivity of the sample can be determined when the thickness is known. At a thermal equilibrium, the Fourier thermal flux equation applied to the sample is: Rs = [(Tu - Tm) / Q] - R? Nt where Rs = thermal resistance of the test sample Tu = surface temperature of the upper plate Tm = surface temperature of the lower plate Q = heat flow through the test sample R, nt = total abutting surface resistance between the sample and the surface plates. The thermal conductivity is obtained by dividing the thickness of the sample between the thermal resistance of the sample. To improve handling ability, the compositions of this invention can be internally supported or reinforced by a flexible matrix such as a polymer, glass fiber, or metal mesh or fabric, or an open cell foam. The gel penetrates the interstitial space in the matrix and becomes more manageable in this way. The matrix serves to improve the mechanical properties of the composition, such as tensile strength and modulus of elasticity. A fabric matrix can be an individual layer or a plurality of layers and can be woven or non-woven. When electrical conductivity is undesirable (for example, to avoid short-circuit electrical components), a metal mesh or cloth should not be used. A preferred material for the flexible matrix is polyurethane. The impregnated gel matrices, their applications and their variants are described by Uken, USA UU 4,865,905 (1989), the description of which is incorporated herein by reference. Figure 1a shows a composition 1 of this invention supported by an open cell foam matrix 2. Figure 2a shows the same composition 1 supported by a fabric matrix 3. The high and thermally conductive and conformable compositions of the invention can be used to seal gaps and at the same time conduct heat away from circuit boards and electrical components to a metal plate, cabinet or cover, cooling device, or other heat sink or heat sink element. The composition is especially suitable for thermal control and / or encapsulation of components and regularly configured such as transistors printed circuit boards, integrated circuits, capacitors, resistors, diodes, power amplifiers, transformers and other electrical or electronic components, which generate heat during the service. The composition is also suitable for thermal control in light valve modules on screens, for example, liquid crystal projection screens where a liquid crystal material is combined with an active matrix (e.g., CMOS). In these screens, heat is generated not only from the power of the electrical energy emitted, but also by the absorption of light from the projecting lamp. The practice of the invention can be further understood by reference to the following examples, which are provided by way of illustration and not limitation.

EXAMPLE 1 A one part gel precursor was made by adding 7.8 g of VDT-131 (1000 cSt 1% methylvinylsiloxane, polydimethylsiloxane, Gelest, Inc.). 23.4 g of PS445 (10,000 cSt of vinyl-terminated polydimethylsiloxane, United Chemical Technologies, Inc.), 93.6 g of 1000 cSt of polydimethylsiloxane fluid (Shin-Etsu), 0.62 g of peroxybenzoate t-butyl (Aldrich Chemical), and 0.62 g of the coupling agent of vinyltriethoxysilane (Gelest, Inc.), to a 250 mL beaker and mixed with an overhead stirrer for 5 minutes at room temperature. In a 1 liter Pyrex dish, 120.94 g of the precursor gel and 330.0 g of α-alumina were added. The material was mixed manually (a specific low energy process) using a bronze spatula with a width of 5.08 centimeters for 5 to 6 minutes before the mixture had a homogeneous appearance. A third of the mixture was retained and the rest was placed in a two-roll mill with a diameter of 3.62 centimeters with a gear ratio of 6: 5. The gap between the two rollers was adjusted so that the material could rotate on top. The material was constantly scraped from the lower half of the rollers with a bronze spatula and placed on the upper half to improve production. The material was mixed at room temperature and 36 rpm for 20 minutes in a mill. Half of the material was removed and the remaining half mixed for another 40 minutes in the mill. The viscosity of the material dropped as the mixing continued. The material mixed manually had the consistency of a coarse paste (or, in the case of 100% C-75 (unground), the consistency of a wet powder), while the material after mixing in the two-roll mill It was a pourable fluid. Heat conductivity samples were prepared through compression molding of 18.5 g of the a-alumina / gel composite in aluminum rings to form disks with a diameter of 6.35 cm, and a thickness of 0.254 cm. The samples were degassed in a vacuum chamber for 2 to 3 minutes, then cured for 30 minutes in a press at 160 ° C under a ramp pressure of 703 kg / cm2. The samples were transferred to a press cooled with water and kept for 5 minutes at a ramp pressure of 703 kg / cm2 in order to cool the samples at room temperature. Discs with a diameter of 5.08 cm were cut from the center of the 6.35 cm discs using a skin punch. The samples were tested for thermal conductivity through the test described above. Samples were prepared for hardness measurements through compression molding of 84 g of the composition in aluminum paint frames, of 7.62 by 7.62 cm by a thickness of 0.635 cm. The samples were degassed in a vacuum chamber for 2 to 3 minutes, then cured for 30 minutes in a press at 160 ° C under a ramp pressure of 703 kg / cm2. The samples were transferred to a press cooled with water and kept for 5 minutes at a ramp pressure of 703 kg / cm2 in order to cool the samples at room temperature. Four discs with a diameter of 2.54 kg-0.1587 centimeters were cut from the slice using a skin punch. The hardness of this cell was measured on a TA-XT2 analyzer as described above. Compositions made from 4 different types of α-alumina (all from Alean Chemicals) were prepared using the above procedure: (a) Fine C-75 (-325 mesh, approximately 44 μm), (b) a mixture of 30 % by weight of unmilled C-75 and 70% by weight of fine C-75, (c) to a mixture of 60% by weight of unmilled C-75 and 40% by weight of fine C-75 and (d) ) C-75 not ground. The results are summarized in table 1 below.

TABLE 1 Thermal Conductivity (W / M- ° C) For Compositions of a- Alumina / Gel Time of 100% of 30% or not 60% not 100% or of C- mixed of two C-75 fine ground / 70% > ground / 40%) 75 no rolls, minutes fine fine ground 0 1.63 2.43 3.29 2.96 20 1.48 2.07 2.61 2.42 60 1.44 1.91 2.19 2.00 TABLE 2 The data show that by increasing the relative amount of the unground material (ie, larger particle size) to the same total weight load, the thermal conductivity of the composite materials was raised, even where high mechanical energy was not introduced. . The data also shows that the hardness of the composite material is reduced by mixing the material at a relatively high shear stress in a two-roll mill, under a specific energy can rate. It is desired that the materials of the present invention have a low hardness and a high thermal conductivity. The thermal conductivity is reduced as the samples are subjected to higher energy mixing, but the change in thermal conductivity is small, although at the same time, the hardness is significantly reduced. These samples also had a final elongation greater than 50%. The combination of thermal conductivity and hardness for the samples containing combinations of unground and fine alumina is particularly desirable.

EXAMPLE 2 Into a 400 mL beaker were added 69.73 g of part A of McGhan-Nusil 8170 silicone gel, and 190.27 g of a mixture of α-alumina (30%) by weight of unground C-77 alumina / 70 % by weight of fine C-75 alumina). The material was mixed manually using a stainless steel spatula with a width of 2.54 cm for 5 to 6 minutes until the mixture had a homogeneous appearance. The same procedure was used to prepare Part B filled with alumina. The manual mixing procedure is a specific low energy mixing process.

A computerized Brabender Plasti-Corde®, Model DR-2052, was used to provide the mixing. The evaluation of the mixer selected was Semi-Automatic Universal Evaluation, version 4.0. The included measurements were instantaneous torque, temperature, blade speed, and the calculated results included the previous measurements and the total mixing energy. In a 60-bent Brabender mixing vessel equipped with roller blades was added 120 grams of part A filled with alumina. The material was mixed at 40 rpm (gear ratio 3: 2) for one hour at 25 ° C. The torque of the mixing was verified in real time and showed a 69% reduction from 36.2 meter-gram to 11.3 meter-gram. The energy input specified to part A was 19.2 Joules / g. Part B filled with alumina was prepared in a similar manner and showed a 64% reduction from 34.1 gram-meter to 12.3 gram-meter. The specific energy input to part B was 19.2 Joules / g. Again, the manually mixed material had the consistency of a paste, while the material after mixing in the Brabender vessel was a pourable fluid. Samples were prepared from the above process in the same manner described in Example 1 for the measurement of thermal conductivity and hardness of the texture analyzer. The results of the thermal conductivity and the hardness in the texture analyzer are summarized below.

TABLE 3 These results also show that the milder alumina / gel compositions were made without compromising the thermal conductivity. These compositions also had final elongations greater than 50% > .

EXAMPLE 3 An EPDM gel precursor was made from one part by combining 34.13 g of the EPDM solution with 105 g of C-75 alumina, 0.52 g of phenyltriethoxysilane, and 0 35 g of t-butyl peroxybenzoate in a Brabender mixing vessel. The EPDM solution consisted of 12% by weight of Royaltuf 465 (EPDM, Uniroyal), 1% by weight of the Irganox antioxidant of 1076 (Ciba-Geigy), 1% by weight of triallyl isocyanurate, and 86% by weight of mineral oil (Hydrobrite 380, Witco Corp.). A computerized Brabender Plasti-Corder®, Model DR-2051 computer was used to provide mixing, as described above. The EPDM gel precursor filled with alumina was placed in a Brabender 85 cc mixing vessel equipped with cam blades. The material was mixed at 30 rpm (gear ratio 3: 2) for one hour at 35 ° C. For the sample containing unground C-75 alumina, the mixing torque was reduced to 70% from 555 meter-gram to 166 meter-gram. For the sample containing the fine C-75 alumina, the mixing torque was reduced by 35% >; from 325 meter-gram to 212 meter-gram. The mixtures have the consistency of a paste. Samples of thermal conductivity were prepared through compression molding of 22 g of the alumina / gel composite material into aluminum rings with an internal diameter of 6.35 cm by a thickness of 0.254 cm. The samples were cured for 15 minutes in a press at 145 ° C under a ramp pressure of 1406 kg / cm2, then cooled for 5 minutes in a press cooled with water under a ramp pressure of 1406 kg / cm2. Discs with a diameter of 5.08 centimeters were cut from the center using a skin punch. The thermal conductivity was 1.17 W / m ° C for the non-ground C-75 sample and 0.87 W / m ° C for the fine C-75 sample. The thermal conductivities were obtained in a similar manner to those described above, but no separator was used. The advantage of using calcined aluminas of larger particle size to obtain higher thermal conductivity is demonstrated.

EXAMPLE 4 In a 2.5-liter Sigma blade mixer were added 60 g of Septon 2006 (Kuraray), 1050 g of alumina, 15 g of Irganox 1076 (Ciba), 7.5 g of Irganox B220 (Ciba), 7.5 g of Tinuvin 327, and 360 g Fine mineral oil A360B. the components were mixed for one hour at 220 ° C. Samples were removed from the Sigma blade mixer and compressed to plates for testing. Composite materials were prepared from 5 different types of alumina using the above procedure. The 5 types of alumina (all from Alean Chemicals) were (a) 100% > of fine C-75 (-325 mesh), (b) a mixture of 30% by weight of unground C-76 and 70% > by weight of fine C-75, (c) a mixture of 50% by weight of unground C-76 and 50% by weight of fine C-75, (d) a mixture of 70% by weight of C-76 not ground and 30% of C-75 fine, and (e) 100% of C-76 not ground. Table 4 summarizes the results. (The elongation measurements for this example 4 were made in accordance with ASTM D412-92 using rings cut from a gel sheet with a thickness of 0.3175 cm The dimensions of the ring were internal diameter 2.25 cm and external diameter 2.885 cm, corresponding to an internal circumference of 7.08 cm, the extension speed was 10.16 cm / minute).

TABLE 4 Thermoplastic Gel Amina With ductivid ad Thermal Elongation (W / m - ° C) final (%) 100% C-75 fine 0.80 1,191 % > by weight of C-76 not ground and 1.18 867 70% by weight of C-75 fine 50% > by weight of C-76 not molly and 1.36 736 50%) by weight of C-75 fine 70%) by weight of non-ground C-76 and 1.46 564 % or by weight of C-75 fine 100% or C-76 not molda 1.50 311 The data shows that by increasing the relative amount of unground material in the same total weight load, the thermal conductivity of composite materials increases. The module and elongation data shows that the materials are relatively soft and have a high elongation despite the high filler load. The above detailed description of the invention includes passages that briefly or exclusively refer to particular parts or aspects of the invention. It should be understood that this is for clarity and convenience, that a particular aspect may be relevant in more than one passage where it is described, and that the description herein includes all appropriate combinations of information found in the different passages. Similarly although various aspects and descriptions of the present refer to specific embodiments of the invention, it is understood that where a specific aspect is described in the context of a particular Figure, said aspect may also be used, to the appropriate degree, in the context of another Figure, in combination with another aspect, or in the invention in general. In addition, since the present invention has been particularly described in terms of certain preferred embodiments, the invention is not limited to such preferred embodiments. Rather, the scope of the invention is defined by the appended claims.

Claims (26)

1. A method for making a thermally conductive and conformable composition, comprising the steps of: (a) providing 100 parts by weight of a gel; (b) combining the gel of between 150 and 400 parts by weight of α-alumina, wherein at least 10 percent by weight of the α-alumina has a particle size of at least 74 μm, to form a combination of gel material and a-alumina; and (c) mg the combination to make the composition thermally conductive and conformable.

2. A method according to claim 1, wherein ge I is a thermoplastic gel made from a block copolymer of styrene- (ethylene-butylene) -styrene or styrene- (ethylene-propylene) -styrene.

3. A method for making a thermally conductive and conformable composition, comprising the steps of: (a) providing 100 parts by weight of a gel curable precursor material: (b) combining the precursor material with 150 to 400 parts by weight of α-alumina, wherein at least 10 weight percent of the α-alumina have a particle size of at least 74 μm. to form a combination of the precursor material and the α-alumina: and (c) cure the precursor material to a gel to form the conformable, thermally conductive composition.

4. A method according to claim 3, wherein the gel is an organopolysiloxane gel.

A method according to any one of the preceding claims, wherein the combination of the gel and the α-alumina is mixed with a specific energy input of at least 10 Joules / g.

6. A method for making a thermally conductive and conformable composition, comprising the steps of: (a) separately providing first and second precursor components, which when mixed together, are cured to form a gel, the combined amounts of the first and second precursor components totaling 100 parts by weight; (b) separately combining α-alumina with each of the first and second precursor components to form first and second respective combinations comprising α-alumina and a first and second precursor component respectively, wherein at least 10 percent by weight of the α-alumina has a particle size of at least 74 μm and the total amount of α-alumina is between 150 and 400 parts by weight; and (c) mg the first and second combinations and curing them to form the thermally conductive and conformable composition.

7. A method according to claim 6, wherein before the step of mg and curing each of the first and second combinations, wherein the weight ratio of α-alumina to the first and second component, respectively, is equal to or greater than 3: 2, is mixed with a specific energy input of at least 10 Joules / g.

8. A method according to claim 6 or 7, wherein the gel is an organopolysiloxane gel.

9. A method according to any of the preceding claims, wherein the α-alumina is calcined α-alumina.

10. A method according to any of the preceding claims, wherein the α-alumina is a powder, which has an overall density of at least 1.0 g / cm3.

A method according to any of the preceding claims, wherein the thermally conductive and conformable composition has a thermal conductivity of at least 1.3 watt / m- ° C, a hardness of at least 1,000 g, and an elongation at the break of at least 50%.

12. A thermally conductive and conformable composition, made through the method of any of the preceding claims.

13. A conformable and thermally conductive composition, comprising: (a) 100 parts by weight of a gel; and (b) between 150 and 400 parts by weight of α-alumina, wherein at least 10% by weight of the α-alumina has a particle size of at least 74 μm.

14. A composition according to claim 13, wherein the gel is an organopolysiloxane gel.

15. A composition according to claim 13, wherein the gel is a thermoplastic gel made from a block copolymer of styrene- (ethylene-butylene) -styrene or styrene- (ethylene-propylene) -styrene.

16. A composition according to any of claims 13-15, wherein the α-alumina is calcined α-alumina.

17. A composition according to any of claims 13-16, wherein the α-alumina is a powder, which has an overall density of at least 1.0 g / cm3.

18. A composition according to any of claims 13-17, having a thermal conductivity of at least 1.3 watt / m- ° C, a hardness of less than 1,000 g, and an elongation at break of at least 50% .

19. An article comprising: (I) a conformable and thermally conductive composition, comprising: (a) 100 parts by weight of a gel; and (b) between 150 and 400 parts by weight of α-alumina, wherein at least 10 weight percent of the α-alumina has a particle size of at least 74 μm; and (II) a flexible matrix internally supporting the composition.

20. An article according to claim 19, wherein the flexible matrix is a fabric.

21. An article according to claim 19, wherein the flexible matrix is an open cell foam.

22. An article according to any of claims 19-21, wherein the gel is an organopolysiloxane gel.

23. An article according to any of claims 19-21, wherein the gel is a thermoplastic gel made from a block copolymer of styrene- (ethylene-butylene) -styrene or styrene- (ethylene-propylene) -styrene.

24. A composition according to any of claims 19-23, wherein the α-alumina is calcined α-alumina.

25. A composition according to any of claims 19-24, wherein the α-alumina is a powder, which has an overall density of at least 1.0 g / cm3.

26. A composition according to any of claims 19-25, having a thermal conductivity of at least 1.3 watt / m- ° C, a hardness of less than 1,000 g, and an elongation at break of at least 50 % > .