Thermally conductive silicone composite

Abstract

The present invention relates to a composition comprising alumina trihydrate, an ester-functionalized trialkoxysilane, and a polysiloxane. The composition of the present invention is useful as thermally conductive composite with desirable viscosity and thermal conductivity properties.

Description

[0001] Thermally Conductive Silicone Composite Background of the Invention The present invention relates to a thermally conductive silicone composite, particularly a composite containing an ester functionalized trialkoxysilane. Thermally conductive silicone composites are widely used as thermal interface materials to transfer heat from a heat-generating electronic component to a heat sink. High loadings of one or more thermally conductive fillers such as zinc oxide and alumina trihydrate (Al2O3.3H2O, ATH) are necessary to achieve the desired thermal conductivity. ATH is particularly attractive because it is non-toxic, halogen-free, chemically inert, and inexpensive. ATH also resists abrasion and has the further advantage of being a flame retardant. A continuing challenge with high filler loaded silicone composites is a concomitant increase in viscosity of the composite, which renders the composites difficult to dispense. Accordingly, it would be an advantage in the field of thermally conductive composites to find a way to reduce viscosity of the ATH-containing composites without adversely affecting other desirable properties. Summary of the Invention The present invention addresses a need in the art by providing a composition comprising, based on the weight of the composition, a) from 50 to 90 weight percent alumina trihydrate; b) up to 30 weight percent zinc oxide c) from 0.1 to 2 weight percent of an ester-functionalized trialkoxysilane of Formula 1 or 2: where R is C1-C16-alkyl or C2-C16-alkenyl; each R1is independently C1-C6-alkyl; R2is H or methyl; and n is 1, 2, or 3; and d) from 10 to 20 weight percent of a polysiloxane of Formula 3: where each R3is independently C1-C6-alkyl or phenyl; each R4is independently C1-C6-alkyl or C2-C6-alkenyl; and x is from 20 to 300. The composition of the present invention is useful as a thermal interface material. Detailed Description of the Invention The present invention is a) from 50 to 90 weight percent alumina trihydrate; b) up to 30 weight percent zinc oxide c) from 0.1 to 2 weight percent of an ester-functionalized trialkoxysilane of Formula 1 or 2: where R is C1-C16-alkyl or C2-C16-alkenyl; each R1is independently C1-C6-alkyl; R2is H or methyl; and n is 1, 2, or 3; and d) from 10 to 20 weight percent of a polysiloxane of Formula 3: Formula 3 where each R3is independently C1-C6-alkyl or phenyl; each R4is independently C1-C6-alkyl or C2-C6-alkenyl; and x is from 20 to 300. The concentration of the alumina trihydrate in the composition is in the range of from 50 or from 55 or from 60 weight percent, to 90 or to 70 or to 68 or 65 weight percent, based on the weight of the composition. The composition may further comprises ZnO at a concentration up to 30 weight percent. Preferably the composition comprises ZnO at a concentration in the range of from 5 or from 15 or from 18 or from 20 weight percent, to 30 or to 27 or to 24 weight percent, based on the weight of the composition. The polysiloxane of Formula 3 is preferably a polydimethylsiloxane that is terminated with two trimethylsilyl groups, or one trimethylsilyl group and one vinyldimethylsilyl group, or two vinyldimethylsilyl groups. Thus, in a preferred embodiment, each R3is methyl and each R4is independently methyl or vinyl; x (also referred to as the degree of polymerization) is from 20 or from 50 or from 100, to 300 or to 200. The concentration of the polysiloxane is from 10 or from 12 weight percent to 20 or to 18 or to 16 weight percent, based on the weight of the composition. The concentration of the ester-functionalized trialkoxysilane is in the range of from 0.1 or from 0.2 or from 0.4 weight percent, to 2 or to 1 or to 0.6 weight percent based on the weight of the composition. R is C1-C16-alkyl or C1-C10-alkyl or C1-C6-alkyl or C2-C16-alkenyl or C2-C10-alkenyl; R is preferably methyl; each R1is independently a C1-C6-alkyl group, preferably methyl; R2is preferably methyl; and n is preferably 1 or 2. The ester-functionalized trialkoxysilane of Formula 1 can be prepared, for example, by the condensation of a salt of a carboxylic acid and a trialkoxysilyl alcohol, as described in WO 2011 / 101278 A1. The compound of Formula 1 can also be prepared by a base catalyzed reaction of a salt of a carboxylic acid and a chloromethyltrialkoxysilane: or by an a The ester-functionalized trialkoxysilane of Formula 2 can be prepared by the reaction of a trichlorosilane with an acrylate or a methacrylate in the presence of a hydrosilylation catalyst such as a platinum catalyst, followed by alkanolysis: The of a trimethylsilyl-trimethoxysilyl treating agent of Formula 4: where each R5is independently C1-C6-alkyl, preferably methyl; Y is O or CH2-CH2, preferably O; and x is from 20 to 200. When used, the concentration of the compound of Formula 4 is in the range of from 0.2 or from 1 to 10 or to 8 weight percent, based on the weight of the composition. The composition may be curable or non-curable. To render the composition curable, each R4is a C2-C6-alkenyl group, preferably vinyl, and the composition further comprises a polysiloxane functionalized with at least two Si-H groups and having a degree of polymerization in the range of from 5 to 300 or to 200 or to 100 or to 50. For a curable composition, the mole-to-mole ratio of the Si-H groups to the alkenyl groups of the polysiloxane of Formula 3 is preferably in the range of from 0.2:1 to 2:1. The composition of the present invention exhibits a lower viscosity when an alkyltrialkoxysilane treating agent is replaced by an ester-functionalized trialkoxysilane treating agent. Examples In the following examples, pbw refers to parts by weight; and TA refers to an ester-functionalized trialkoxysilane treating agent. General Preparation of Composites A vinyl-terminated polydimethylsiloxane of Formula 3 (Vi-PDMS, 14.14 pbw, x = 130) and an ester-functionalized trialkoxysilane treating agent (TA1 – TA6, 0.47 pbw) were added to a MAX100 cup, followed by the addition of Zoco 102 zinc oxide (ZnO, 22.51 pbw, 0.12 µm particle size) and MX-200 alumina trihydrate (ATH, 62.88 pbw, 45 µm particle size). The mixture was placed in a Flacktek Speedmixer and mixed at 2500 rpm for 20 s, then hand-mixed with a spatula, then mixed again at 2500 rpm for 20 s. The mixture was transferred to an aluminum pan and heated to 150 °C in vacuo (23 Torr) for 1 h. Table 1 summarizes the materials and amounts (pbw) used to prepare the examples and the comparative examples. Treating agents TA-1 to TA-6 are illustrated:

[0002] Table 1 – Formulations Component Comp.1 Ex.1 Ex.2 Ex.3. Ex.4 Ex.5 ATH 62.88 62.88 62.88 62.88 62.88 62.88 An Ares G2 rheometer (TA Instruments) was used to measure viscosities using a parallel plate configuration with 25-mm diameter serrated steel plates. Care was taken to avoid air entrapment into the samples as they were being loaded. Excess material was trimmed from edge of the fixture with the flat edge of a spatula. Testing was conducted using a gap of 1.0 mm. The measurements were taken using the standard procedure of 10 rad / s oscillation frequency, sweeping from 0.01% to 300% strain amplitude with 20 sampling points per decade. All experiments were conducted at 25 °C. Complex viscosity η* at 0.1% strain was measured. Thermal Conductivity Measurement Thermal conductivity was measured by ISO 22007-2:2015 (Test Method for Determining Thermal Conductivity) using a Hot Disk Instrument TPS 2500 S Hot Disk Instrument and a C5501 sensor. Each sample was placed into two cups with the planar sensor held between the cups. Analysis conditions: Fine-tuned analysis, Temperature drift compensation and time correction, calculation with selected between points 50-150. Table 2 illustrates the impact of the treating agent on thermal conductivity (TC) and viscosity at 0.1% strain (Viscosity). Table 2 – Thermal Conductivity and Viscosity Data for Samples Sample Viscosity (Pa·s) TC (W / m·K) Comp.1 8207 2.1 The viscosities of the e ionalized trimethoxysilane treating agent exhibited a lower viscosity than the comparative example composition containing n-dodecyl-trimethoxysilane as the treating agent. In all instances, the example compositions exhibited comparable or superior thermal conductivities.

Claims

Claims:

1. A composition comprising, based on the weight of the composition, a) from 50 to 70 weight percent alumina trihydrate; b) up to 30 weight percent zinc oxide c) from 0.1 to 2 weight percent of an ester-functionalized trialkoxysilane of Formula 1 or 2:where R is C1-C16-alkyl or C2-C16-alkenyl; each R1is independently C1-C6-alkyl; R2is H or methyl; and n is 1, 2, or 3; and d) from 10 to 20 weight percent of a polysiloxane of Formula 3:Formula 3 where each R3is independently C1-C6-alkyl or phenyl; each R4is independently C1-C6-alkyl or C2-C6-alkenyl; and x is from 20 to 300.

2. The composition of Claim 1 wherein each R4is independently vinyl or methyl; each R3is methyl; and x is from 50 to 200.

3. The composition of either of Claims 1 or 2 wherein, based on the weight of the composition, the concentration of the alumina trihydrate is in the range of from 55 to 68 weight percent; the concentration of the zinc oxide is in the range of from 5 to 30 weight percent; the concentration of the vinyl-terminated polydimethylsiloxane is in the range of 12 to 18 weight percent; and the concentration of the ester-functionalized trialkoxysilane is in the range of from 0.2 to 1 weight percent; wherein x is from 50 to 200.

4. The composition of Claim 3 wherein, based on the weight of the composition, the concentration of the alumina trihydrate is in the range of from 60 to 68 weight percent; the concentration of the zinc oxide is in the range of from 20 to 24 weight percent; the concentration of the vinyl-terminated polydimethylsiloxane is in the range of 12 to 16 weight percent; the concentration of the ester-functionalized trialkoxysilane is in the range of from 0.2 to 0.6 weight percent; and x is from 100 to 200.

5. The composition of Claim 4 wherein R is C1-C10-alkyl or C2-C10-alkenyl; R1is methyl; R2is H; and n is 1 or 2.

6. The composition of Claim 5 wherein the ester-functionalized trialkoxysilane is represented by a compound selected from the group consisting of:

7. Theof the composition, from 0.2 to 10 weight percent of a trimethylsilyl-trimethoxysilyl treating agent of Formula 4:where each R5is independently C1-C6-alkyl; Y is O or CH2-CH2; x is from 20 to 200; where each R4is vinyl.

8. The composition of Claim 7 which further comprises a polysiloxane functionalized with at least two Si-H groups and having a degree of polymerization in the range of from 5 to 300; wherein the mole-to-mole ratio of the Si-H groups to the vinyl groups in the polysiloxane of Formula 3 is in the range of from 0.2:1 to 2:1.

Images