Thermal interface material with minimum use of epoxy resins

Abstract

Provided herein is a two-component thermal interface material with minimum to none epoxy resins and the method of making thereof.

Description

THERMAL INTERFACE MATERIAL WITH MINIMUM USE OF EPOXY RESINSBackground of the Invention

[0001] The automotive industry has seen a trend to reduce the weight of vehicles in recent decades. This trend has been driven mainly by regulations to reduce the CO2 emission of the vehicle fleet. In recent years lightweight construction strategies have been further fueled by the increasing number of electrically driven vehicles. The combination of a growing automotive market and a growing market share of electrically driven vehicles leads to a strong growth in the number of electrically driven vehicles. To provide long driving ranges in electrical vehicles, batteries with a high energy density are needed. Several battery strategies are currently followed with differing detailed concepts, but what all long-range durable battery concepts have in common is that a thermal management is needed.

[0002] To thermally connect battery cells or modules to the cooling unit, thermal interface materials are needed. Battery cells produce heat during charging and discharging operations. The cells need to be kept in the right operating temperature (preferably 25-40°C) not to lose efficiency. Furthermore, overheating can start a dangerous thermal runaway reaction. For that reason, active cooling is commonly used. In such systems, cooled water glycol mixtures are pumped through channels that cool the metal bottom plate on which the battery cells / modules are placed. In order not to have an insulating air film between the cells and cooling plate, thermal interface materials are employed. The thermal interface materials (TIMs) need to thermally connect the modules with the cooling plate, meaning they must have a high thermal conductivity of > 2 W / mK. Such elevated thermal conductivities can be achieved by formulating a polymeric matrix, such as epoxy and / or polyurethanes, with high amounts (typically > 50 wt%) of thermally conductive fillers such as aluminum hydroxide, aluminum oxide, as disclosed in WO2014047932A1 .

[0003] In some applications, a TIM with even a higher thermal conductivity of 3 W / mK may be required along with some other requirements such as low press-in force, a relatively high strength of more than 1 MPa and a highelongation at break. Therefore, it is desirable to have a TIM formulation with high strength, and high elongation at break.Summary of the Invention

[0004] It was surprising to find that in a two components polyurethane formulation used as TIM, the high strength and high elongation at break can be achieved by not including or keeping epoxy resins in the minimum in component A of the formulation. Therefore, in one aspect of the present invention, it provides a two-component adhesive formulation having 1) a component A comprising more than 50 wt.% of a thermally conductive filler, 0.5 to 20 wt.% of a blocked polyurethane prepolymer, 0 to 0.5 wt.% of an epoxy resin, 0.1 to 5 wt.% of an epoxy silane, and other optional non-epoxy ingredients, all based on the total weight of component A; and 2) a component B; wherein at least one of the component A and B further comprises a plasticizer.Detailed Description of the Invention

[0005] The present invention provides a two-component thermally conductive polyurethane based adhesive formulation comprising (A) a first component which comprises (a1) a blocked polyurethane prepolymer which is the reaction product of a polyisocyanate with a phenol; (a2) none to very small amount of an epoxy resin; and (a3) an epoxy silane. The formulation also comprises (B) a second component which comprises: (b1) a nucleophilic cross-linker capable of reacting with the blocked polyurethane prepolymer (a1) and the aromatic epoxy resin (a2); (b2) a catalyst capable of promoting the reaction of nucleophile (b1) with the blocked polyurethane prepolymer (a1) and the aromatic epoxy resin (a2). The formulation further comprises in either A component and / or B component a thermally conductive filler, and components (A) and (B) are designed to be blended together prior to use.

[0006] Many different thermally conductive fillers can be used in the present invention. Preferred thermally conductive fillers are those that have acoefficient of thermal conductivity that is greater than 5 W / m°K, greater than 10 W / m°K, or greater than 15 W / m°K. In some preferred embodiments, thermally conductive fillers have a coefficient of thermal conductivity at or more than about 35 W / m°K. Examples of the preferred thermally conductive fillers include alumina, alumina trihydrate or aluminum trihydroxide, silicon carbide, boron nitride, diamond, and graphite, or mixtures thereof. Particularly preferred are aluminum trihydroxide (ATH) in combination with aluminum oxide. In one preferred embodiment, the thermally conductive filler has a broad particle size distribution characterized by a ratio of D90 I D50 of at or about 3 or more. Also preferred are thermally conductive fillers having a bimodal particle size distribution. A bimodal distribution is when the ratio D901 Dso is at or about 3 or more, more preferably at or about 5 or more, more particularly preferably at or about 9 or more. For example, particles having a Dso of 5 to 20 microns and a D90 of 70 to 90 microns, particularly a Dso of 7-9 microns and a D90 of 78-82 microns. Particle size can be determined using laser diffraction. For ATH a suitable solvent is deionized water containing a dispersion aid, such as Na4P2O? x 10 H2O, preferably at 1 g / l. Preferred are aluminum oxide and ATH having a bimodal distribution, particularly ATH.

[0007] The thermally conductive filler is preferably present in the final adhesive formulation at a concentration that gives a thermal conductivity of at or about 2.0 W / mK or more, preferably at or about 2.5 or more, more preferably at or about 2.8 or more, even more preferably at or about 2.9 or more, and most preferably at or about 3.0 or more. For example, this generally requires a concentration of thermally conductive filler of between 50 to 95 wt.%, preferably between 70 to 95 wt.%, more preferably between 80 to 95 wt.%, and most preferably between 85 to 92 wt.%, all based on the total weight of the component that contains the thermally conductive filler. In a particularly preferred embodiment, the thermally conductive filler is present at greater than 80 wt.%. Preferably the thermally conductive filler content in the final adhesive formulation is less than 93 wt.%, as higher levels can affect the adhesive strength and impact resistance negatively.

[0008] The thermally conductive fillers may be present in A component, B component, or both. In a preferred embodiment it is present in both components, as this reduces the amount of mixing required to properlydistribute the thermally conductive fillers when two components are mixed. Preferably the fillers are present at similar or the same concentration in both component A and B. In a particularly preferred embodiment, the filler is present at 85-90 wt.% in the final mixture of the components, based on the total weight of the component mixture. In another embodiment, the filler is present in both component A and B at about 85 wt.%, based on the weight of the component.

[0009] In one preferred embodiment, the thermally conductive filler is ATH having a ratio D901 D50 of at or about 8 or more, used at a concentration of 85- 89 wt.% in both component A and component B, based on the total weight of each component containing the ATH. In another preferred embodiment, the thermal conductive fillers are a mixture of ATH and aluminum oxide.Component ABlocked polyurethane prepolymer (a1)

[0010] Component A of the adhesive formulation comprises a blocked polyurethane prepolymer which is the reaction product of a polyisocyanate with a polyol, capped with a phenol, preferably 70-85 wt% aromatic polyisocyanate with 15-25 wt% phenol. Preferably the reaction is carried out with a tin catalyst. The polyisocyanate may be aliphatic, aromatic, or a mixture, with aromatic polyisocyanates being preferred. Examples of aromatic polyisocyanates include methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), p-phenylene diisocyanate (PPDI), and naphthalene diisocyanate (NDI), all of which can be reacted with a polyol. Particularly preferred is toluene diisocyanate (TDI), reacted with a polyol.

[0011] The polyol preferably is a polyether polyol. The polyol may have two or more OH groups. Examples of polyether polyols include poly(alkylene oxide)diols, wherein the alkylene group is C2-C6, particularly preferably the alkylene group is C2-C4. Examples of suitable polyols include polyethylene oxide)diol, polypropylene oxide)diol, poly(tetramethylene oxide)diol. Particularly preferred is polypropylene oxide)diol, particularly polypropylene glycol).

[0012] Particularly preferred blocked prepolymer is the reaction product of an aromatic diisocyanate with a polyether polyol, in particular those listed above, and then capping with a phenol. The phenol used for capping is preferably a phenol of the following formula:where R is a saturated or unsaturated C15 chain, particularly preferably R is a saturated C15 chain.

[0133] In a preferred embodiment, polyisocyanate is made by reacting TDI with a polypropylene oxide)diol, in particular when the resulting polyisocyanate has an equivalent weight of at or about 950.

[0144] The phenol-containing compound typically has a linear hydrocarbon attached to the phenol group to provide some aliphatic characteristics to the compound. The linear hydrocarbon preferably includes 3 or more carbon atoms, more preferably 5 or more carbon atoms, even more preferably 8 or more carbon atoms, and most preferably 10 or more carbon atoms. The linear hydrocarbon preferably includes at or about 50 or less carbon atoms, at or about 30 or less carbon atoms, at or about 24 or less carbon atoms, or at or about 18 or less carbon atoms. A particularly preferred phenol is cardanol.

[0015] In a particularly preferred embodiment, the blocked polyurethane prepolymer is made by reacting toluene diisocyanate with a polyether polyol, having an NCO content of at or about 4 - 5% and an equivalent weight of at or about 500 - 1500 g / eq.

[0016] In another preferred embodiment, the blocked polyurethane prepolymer is made by reacting an aromatic polyisocyanate, based on toluene diisocyanate, with cardanol, preferably 70-85 wt.% TDI-based polyisocyanate with 15-25 wt.% cardanol, based on the total weight of the blocked prepolymer.

[0177] The blocked polyurethane prepolymer (a1 ) is typically present in component A at a concentration of 0.5 to 20 wt.%, preferably at 1 to 10 wt.%,more preferably at 2 to 10 wt.%, and most preferably 5-10 wt.%, based on the total weight of component A.

[0018] In use, components A and B are mixed prior to or simultaneously with application to a substrate. The concentration of the blocked polyurethane prepolymer in the final, mixed adhesive formulation can be calculated from the proportions of components A and B used to make the final mixed adhesive formulation. In a preferred embodiment, components A and B are mixed in a 1 :1 ratio by volume, in which case the concentration of the blocked polyurethane prepolymer in the final adhesive will be half the value in Component A.Epoxy resin (a2)

[0019] As one unique feature of the present invention, component A comprises none to very small or de minimis amounts of epoxy. In some embodiments of the present invention, the epoxy resin is present at less than 0.5 wt.%, preferably less than 0.3 wt.%, more preferably less than 0.1 wt.%, and most preferably 0.0 wt.%, all based on the total weight of the component A.Epoxy silane (a3)

[0020] Component A also comprises an adhesion promoter, preferably an epoxy silane. An epoxy silane is any molecule that bears a di- or trialkoxy silane moiety bonded to an epoxy moiety. Suitable epoxy silanes are of the formula:where R1, R2and R3are independently selected from C1-C3 alkyl, and R4is a divalent organic radical.

[0021] In preferred embodiments, R1, R2and R3are independently selected from ethyl and methyl, with methyl being preferred, particularly when R1, R2and R3are methyl. R4is preferably selected from alkylene, preferably C2-C12alkylene, more preferably C2-C6 alkylene, particularly preferably propylene. In a particularly preferred embodiment, the epoxy silane is gamma- glycidoxypropyltrimethoxysilane.

[0022] The epoxy silane is typically present in component A at 0.1 to 5 wt.%, preferably 0.2 to 1 wt.%, more preferably 0.3 to 0.9 wt.%, and most preferably 0.5 to 0.8 wt.%, based on the total weight of component A.Component BNucleophilic cross-linker (b1)

[0023] Component B comprises a nucleophilic cross-linker capable of reacting with the blocked polyurethane prepolymer (a1) and the epoxy resin (a2), if present. Many polyamines can be used as the nucleophilic crosslinker, with a di- or tri-amine being preferred. The amine groups may be independently secondary or primary, with primary being preferred. One of the unique features of the present invention is to include a combination of different molecular weights of polyamines in component B. Molecular weight used in the present application refers to the number average molecular weight (Mn). At least two polyamines, e.g. polyetheramines with functionality of 3, with different molecular weights should be used in the present invention. The polyamine with higher Mn will typically have a Mn of more than 2000, preferably more than 3000, more preferably more than 4000, and most preferably about 5000. The polyamine with lower Mn will typically have a Mn of less than 1900; preferably less than 1500, more preferably less than 1000, and most preferably about 400. In a preferred embodiment, the total amount of polyamines is usually a fixed value. By adjusting the ratio between the two polyamines, one may control the amount of active NH groups available for reacting with the blocked polyurethane prepolymer and the epoxy resin, if present.

[0024] The nucleophilic cross-linker preferably has a backbone based on poly(alkylene oxide)diols, particularly C2-C6 alkylene, more particularly C2-C4 alkylene, with C3 alkylene being most preferred. Particularly preferably the backbone is based on a polyether of propylene glycol. In a particularly preferred embodiment, the nucleophilic cross-linker is a triamine havingprimary amines for greater than 90 % of amine groups, and a backbone based on a polyether of propylene glycol.

[0025] The nucleophilic cross-linker is typically present in component B at a concentration of 0.1 to 20 wt.%, preferably 1 to 15 wt.%, more preferably 2 to 10 wt.%, most preferably at 5 to 10 wt.%, based on the total weight of component B.Catalyst (b2)

[0026] Component B further comprises a catalyst capable of promoting the reaction of nucleophile (b1) with the blocked polyurethane prepolymer (a1) and the epoxy resin (a2), if present. The catalyst is preferably selected from Lewis bases and Lewis acids. Preferred are tertiary amines, including diazabicyclo[2.2.2]octane, 2,4,6-tris((dimethylamino)methyl)phenol, DM DEE (2,2'-Dimorpholinodiethylether), imidazoles, such as 4-methylimidazole), triethanolamine, polyethyleneimine. Also suitable are organotin compounds, such as dioctyltindineodecanoate, and other metal catalysts such as tetrabutyltitanate, zirconium acetylacetonate, and bismuthneodecanoate. Particularly preferred catalyst in one embodiment is diazabicyclo[2.2.2]octane.

[0027] The catalyst is preferably used at 0.01 to 3 wt.%, more preferably 0.01 to 1 wt.%, most preferably 0.01 to 0.5 wt.%, based on the total weight of component B.Other ingredients

[0028] Components A and B may each or both further optionally comprise, depending on the needs of applications, other ingredients which are typically non-epoxy based chemicals, such as: i) plasticizers, such as esters of unsaturated fatty acids, in particular C -Cis fatty acids, in particular methyl esters, tris(2-ethylhexyl)phosphate; ii) stabilizers, such as polycapralactone; ill) dyes and colorants; iv) other fillers in additional to the thermally conductive filler, such as carbon black, calcium carbonate, glass fibres, wollastonite; polyester, urea, fumed silica etc. and v) viscosity reducers, such as hexadecyltrimethoxysilane. The amounts of these optional ingredients can vary based on different applications.

[0029] While the amount of the components useful in making the reaction product constituting the adhesive formulation can vary, once component A and component B are formulated (separately and individually) and the two components are ready for combining to form the reaction product adhesive formulation, components A and B can be mixed at a volume ratio ranging from 2:1 to 1 :2. In one preferred embodiment, such volume ratio between component A and B is about 1 :1.

[0030] The following examples further illustrate the present invention. The scope of the invention and claims is not limited by the scope of the following examples.EXAMPLES

[0031] Above is Table 1 listing the main ingredients used in the present examples and their respective chemical names and functions. Additional details of these ingredients are provided below.

[0032] GF200, the blocked polyurethane prepolymer, is the reaction product of Aromatic polyisocyanate A and Cardanol. Reaction procedure: Cardanol (22.1 wt%) and Aromatic polyisocyanate A (77.85 wt%) were heated in a reactor to 60°C. Dibutyltin dilaurate catalyst (0.05 wt%) was then added. The reaction mixture was stirred for 45 min at 80°C under an atmosphere of nitrogen and then for 10 min under vacuum. The colourless reaction product was then cooled to room temperature and transferred into a container.

[0033] The epoxy resin (a2), if present, is a reaction product of epichlorohydrin with bisphenol A commercially available from Olin as D.E.R. 330.

[0034] Table 2 summarized the formulation details of components A and B, some physical properties thereof and some testing results of the adhesive formulation after combining the two components.Table 2. Comparative Examples vs. Inventive ExamplePreparation of components A and B

[0035] The inventive and comparative examples were prepared by mixing the ingredients listed in Table 2 on a planetary mixer or on a dual asymmetric centrifuge. In a first phase the liquid phases were mixed before the solid material is added to the formulation. The formulation was mixed for ca 30 min under vacuum before being filled into cartridges, pails, or drums.Formulation of the adhesive

[0036] The A and B components of the adhesive were mixed 1 : 1 by volume with a static mixer and applied from a manual cartridge system.Test methods

[0037] Press-in force was measured with a tensiometer (Zwick). The adhesive material was placed on a metal surface. An aluminium piston with 40 mm diameter is placed on top and the material is compressed to 5 mm (initial position). The material was then compressed to 0.3 mm with 1 mm / s velocity and force deflection curve was recorded. The force (N) at 0.5 mm thickness is then reported in Table 2 and considered as the press-in force.

[0038] Thermal conductivity was measured according to ASTM 5470-12 on a thermal interface material tester from ZFW Stuttgart. The tests were performed in Spaltplus mode at a thickness of between 1 .8 - 1 .2 mm. The described thermal interface material was considered as Type I (viscous liquids) as described in ASTM 5470-12. The upper contact was heated to ca 40°C and the lower contact to ca 10°C, resulting in a sample temperature of ca 25°C. The A and B components of the adhesive were mixed with a static mixer when applied from a manual cartridge system. The results are contained in Table 2.

[0039] Molecular Weight data were measured by gel permeation chromatography (GPC) with a Malvern Viscothek GPC max equipment. EMSURE - THF (ACS , Reag. Ph EUR for analysis) was used as an eluent, PL GEL MIXED D ( Agilent , 300*7.5 mm, 5 pm ) was used as a column, and MALVERN Viscotek TDA was used as a detector.

[0040] Lap shear strength was measured according to according to DIN EN 1465:2009. e-coated steel substrates (140 x 25 mm, 0.8 mm thick) were used. The substrates were cleaned with isopropanol before use. Parts (A) and (B) were mixed 1 : 1 by volume, and the resulting adhesive was applied on one substrate, before the second substrate was joined within 5 minutes. The thickness was adjusted to 1 .4 mm, the overlap area was 25 mm x 25 mm. The material was allowed to cure and rested for 7 days at 23°C, 50 % relative humidity before the lap shear tests were performed. The lap shear samples were then mounted in a tensiometer and the lap shear tests were performed in the conventional way, using a pull speed of 1 mm / min. The force deflection curve was monitored and the strength at break is reported as lap shear strength in Table 2.

[0041] Rheology measurements, viscosity, were performed on an Anton Paar MC 302 rheometer with a parallel plate geometry. 25 mm diameter plates were used, the gap was fixed at 0.5 mm. The formulation was brought between the two plates and a shear rate test was performed from 0.001 to 20 1 / s. The viscosity at 10 1 / s is reported in Table 2.Test results

[0042] As shown in Table 2, the present invention provides an adhesive formulation, used also as thermal interface materials, that can meet high performance requirements for battery application including an increased elongation. The present formulation is based on a blocked polyurethane technology comprising a blocked polyurethane prepolymer in component A and polyetheramines as hardeners in component B. While prior art has taught the use of epoxy resins added to the formulation to increase the strength and adhesion to certain substrates like aluminum. For many battery designs, a significant swelling of the cells is observed leading to a displacement of the thermal interface material or thermal conductiveadhesive, respectively. The swelling is particularly taking place during charging and discharging cycles in fast-charging applications, and / or after running the battery for many years. To reach a thermal conductivity of 3 W / mK, high filler loadings are needed. As shown in Table 2, large amounts of combined ATH biomodal and multimodal aluminum oxide are used.

[0043] Comparative Example 1 and 2 comprise 0.5 and 1 wt.% of epoxy resin, respectively. While the formulation achieved a high thermal conductivity, and high values of lap shear strength, the displacement until break in lap shear tests (measured by the traverse movement) of the Comparative Examples, are, however, only about 0.5 mm (with lap shear samples showing a 2 mm gap). In contrast, the Inventive Example does not comprise any epoxy resin. The lap shear strength of the Inventive Example is about 1 .2 MPa. However, and surprisingly, the displacement until break in lap shear test, also known as elongation test, is significantly higher at about 1.5 mm.

Claims

Claims1 . A two-component adhesive formulation comprising: a component A having more than 50 wt.% of a thermally conductive filler, 0.5 to 20 wt.% of a blocked polyurethane prepolymer, 0 to 0.5 wt.% of an epoxy resin, 0.1 to 5 wt.% of an epoxy silane, and other optional non-epoxy ingredients, all based on the total weight of component A; and a component B; wherein at least one of the component A and B further comprises a plasticizer.

2. The two-component adhesive formulation of claim 1 comprising: a component A having more than 50 wt.% of a thermally conductive filler, 0.5 to 20 wt.% of a blocked polyurethane prepolymer, 0 to 0.3 wt.% of an epoxy resin, 0.1 to 5 wt.% of an epoxy silane, and other optional non-epoxy ingredients, all based on the total weight of component A; and a component B; wherein at least one of component A and B further comprises a plasticizer.

3. The two-component adhesive formulation of any one of the preceding claims comprising: a component A having more than 50 wt.% of a thermally conductive filler, 0.5 to 20 wt.% of a blocked polyurethane prepolymer, 0 to 0.1 wt.% of an epoxy resin, 0.1 to 5 wt.% of an epoxy silane, and other optional non-epoxy ingredients, all based on the total weight of the component A; and a component B; wherein at least one of the component A and B further comprises a plasticizer.

4. The two-component adhesive formulation of any one of the preceding claims comprising:a component A consisting of more than 50 wt.% of a thermally conductive filler, 0.5 to 20 wt.% of a blocked polyurethane prepolymer, 0.1 to 5 wt.% of an epoxy silane, and other optional non-epoxy ingredients, all based on the total weight of the component A; and a component B; wherein at least one of the component A and B further comprises a plasticizer.

5. The two-component adhesive formulation of any one of the preceding claims wherein the thermally conductive filler comprises aluminum trihydroxide.

6. The two-component adhesive formulation of any one of the preceding claims wherein the thermally conductive filler comprises a mixture of aluminum trihydroxide and aluminum oxide.

7. The two-component adhesive formulation of any one of the preceding claims wherein component B comprises a nucleophilic cross-linker and the nucleophilic cross-linker comprises at least one polyamine.

8. The two-component adhesive formulation of claim 7 wherein the polyamine is a polyetheramine with a functionality of 3.

9. The two-component adhesive formulation of any one of claims 7 and 8 wherein the nucleophilic cross-linker is present at a concentration of 0.1 to 20 wt.% based on the total weight of component B.

10. The two-component adhesive formulation of any one of claims 1 to 7 wherein the component B comprises a nucleophilic cross-linker and the nucleophilic cross-linker comprises at least two polyamines with different molecular weights and both having a functionality of 3.11 . The two-component adhesive formulation of any one of the preceding claims wherein the plasticizer comprises esters of unsaturated C16- C fatty acids.

12. The two-component adhesive formulation of any one of the preceding claims wherein the plasticizer is present in component A at about 2.6 wt.% based on the total weight of component A.

13. The two-component adhesive formulation of any one of the preceding claims wherein the plasticizer is present in component B at about 0.5 wt.% based on the total weight of component B.

14. The two-component adhesive formulation of any one of the preceding claims wherein the volume ratio between component A and component B is between 2:1 to 1 :2.

15. The two-component adhesive formulation of any one of the preceding claims wherein the volume ratio between component A and component B is about 1 :1.

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