Thermal interface material with minimal epoxy

Abstract

This article provides a two-component thermal interface material with minimal or no epoxy resin and its preparation method.

Description

Background Technology

[0001] In recent decades, the automotive industry has seen a trend towards reducing vehicle weight. This trend is primarily driven by regulations aimed at reducing CO2 emissions from vehicle fleets. The increasing number of electric vehicles in recent years has further fueled lightweight construction strategies. The combination of a growing automotive market and a growing market share for electric vehicles has resulted in a robust increase in the number of electric vehicles. To provide long driving ranges for electric vehicles, batteries with high energy density are required. Currently, several battery strategies follow different detailed concepts, but a common thread among all long-range, durable battery concepts is the need for thermal management.

[0002] Thermal interface materials are required to thermally connect battery cells or modules to a cooling unit. Battery cells generate heat during charging and discharging operations. These cells need to be maintained at a suitable operating temperature (preferably 25°C–40°C) to avoid efficiency loss. Furthermore, overheating can trigger dangerous thermal runaway reactions. For this reason, active cooling is typically used. In such systems, a cooling water-glycol mixture is pumped through channels cooling a metal base plate on which the battery cells / modules are placed. Thermal interface materials are used to prevent the presence of an insulating air film between these cells and the cooling plate. Thermal interface materials (TIMs) need to thermally connect the module to the cooling plate, meaning they must have a high thermal conductivity > 2 W / mK. Such high thermal conductivity can be achieved by formulating polymer matrices such as epoxy resins and / or polyurethanes with a large amount (typically > 50 wt%) of thermally conductive fillers such as aluminum hydroxide, alumina, etc., as disclosed in WO 2014047932 A1.

[0003] In some applications, TIMs with even higher thermal conductivity, such as 3 W / mK, may be required, along with other requirements such as low inductance, relatively high strength (greater than 1 MPa), and high elongation at break. Therefore, TIM formulations with high strength and high elongation at break are desirable. Summary of the Invention

[0004] Surprisingly, it has been found that high strength and high elongation at break in two-component polyurethane formulations used as TIMs can be achieved by excluding or minimizing epoxy resin in component A of the formulation. Therefore, in one aspect of the invention, a two-component adhesive formulation is provided, comprising 1) component A, which comprises greater than 50 wt.% thermally conductive filler, 0.5 wt.% to 20 wt.% end-capped polyurethane prepolymer, 0 wt.% to 0.5 wt.% epoxy resin, 0.1 wt.% to 5 wt.% epoxy silane, and other optional non-epoxy components, all based on the total weight of component A; and 2) component B; wherein at least one of components A and B further comprises a plasticizer. Detailed Implementation

[0005] This invention provides a two-component thermally conductive polyurethane-based adhesive formulation comprising (A) a first component comprising (a1) a capped polyurethane prepolymer, which is a reaction product of a polyisocyanate and a phenol; (a2) an epoxy resin (without or in very small amounts); and (a3) ​​an epoxy silane. The formulation further comprises (B) a second component comprising: (b1) a nucleophilic crosslinking agent capable of reacting with the capped polyurethane prepolymer (a1) and the aromatic epoxy resin (a2); and (b2) a catalyst capable of promoting the reaction of the nucleophile (b1) with the capped polyurethane prepolymer (a1) and the aromatic epoxy resin (a2). The formulation further comprises a thermally conductive filler in component A and / or component B, and components (A) and (B) are designed to be blended together before use.

[0006] Many different thermally conductive fillers can be used in this invention. Preferred thermally conductive fillers are those having a thermal conductivity greater than 5 W / m°K, greater than 10 W / m°K, or greater than 15 W / m°K. In some preferred embodiments, the thermally conductive filler has a thermal conductivity equal to or greater than about 35 W / m°K. Examples of preferred thermally conductive fillers include alumina, alumina trihydrate or aluminum hydroxide trihydrate, silicon carbide, boron nitride, diamond and graphite, or mixtures thereof. A combination of aluminum hydroxide trihydrate (ATH) and alumina is particularly preferred. In a preferred embodiment, the thermally conductive filler is characterized by a wide particle size distribution with a D90 / D50 ratio of 3 or about 3 or greater. Also preferred are thermally conductive fillers with a bimodal particle size distribution. A bimodal distribution is when the ratio D90 / D50 is... 90 / D 50 When the value is 3 or about 3 or greater, more preferably 5 or about 5 or greater, and even more particularly preferably 9 or about 9 or greater. For example, the particles have a D of 5 to 20 micrometers. 50 and 70 to 90 micrometers of D 90 Especially 7-9 micrometer D 50 and 78-82 micrometers of D 90 Particle size can be determined using laser diffraction. For ATH, a suitable solvent is deionized water containing a dispersant (preferably 1 g / L) such as Na₄P₂O₇ x 10H₂O. Alumina and ATH with a bimodal distribution are preferred, especially ATH.

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

[0008] The thermally conductive filler can be present in component A, component B, or both. In a preferred embodiment, the thermally conductive filler is present in both components because this reduces the amount of filler required to properly distribute when the two components are mixed. Preferably, the filler is present in both component A and component B at similar or identical concentrations. In a particularly preferred embodiment, the filler is present in the final mixture of components at 85 wt.%–90 wt.% based on the total weight of the component mixture. In another embodiment, the filler is present in both component A and component B at approximately 85 wt.% based on the weight of the components.

[0009] In a preferred embodiment, the thermally conductive filler has a Di of 8 or about 8 or greater. 90 / D 50 The ATH is used in both component A and component B at a concentration of 85 wt.%-89 wt.% based on the total weight of the individual components containing ATH. In another preferred embodiment, the thermally conductive filler is a mixture of ATH and alumina.

[0010] Component A

[0011] End-capped polyurethane prepolymer (a1)

[0012] Component A of the adhesive formulation comprises a capped polyurethane prepolymer, which is the reaction product of a polyisocyanate and a polyol (capped with phenol), preferably 70 wt%-85 wt% of an aromatic polyisocyanate and 15 wt%-25 wt% of a phenol reaction product. Preferably, the reaction is carried out using a tin catalyst. The polyisocyanate can be an aliphatic polyisocyanate, an aromatic polyisocyanate, or a mixture thereof, wherein aromatic polyisocyanates are 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 react with polyols. Toluene diisocyanate (TDI) is particularly preferred, as it reacts with polyols.

[0013] The polyol is preferably a polyether polyol. The polyol may have two or more OH groups. Examples of polyether polyols include poly(epoxide) glycols, wherein the alkylene groups are C2-C6, and particularly preferably C2-C4. Suitable examples of polyols include poly(ethylene oxide) glycols, poly(propylene oxide) glycols, and poly(tetrahydrofuran) glycols. Poly(propylene oxide) glycols are particularly preferred, especially poly(propylene glycol).

[0014] Particularly preferred end-capping prepolymers are reaction products of aromatic diisocyanates and polyether polyols (especially those listed above), followed by end-capping with a phenol. The phenol used for end-capping is preferably a phenol having the following formula:

[0015]

[0016] Where R is saturated or unsaturated C. 15 The chain, particularly preferably R, is saturated C. 15 chain.

[0017] In a preferred embodiment, the polyisocyanate is prepared by reacting TDI with poly(propylene oxide) glycol, particularly when the resulting polyisocyanate has an equivalent weight of 950 or about 950.

[0018] Phenolic compounds typically have a straight-chain hydrocarbon attached to the phenolic group to impart some aliphatic properties to the compound. The straight-chain hydrocarbon preferably contains 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 straight-chain hydrocarbon preferably contains 50 or about 50 or fewer carbon atoms, 30 or about 30 or fewer carbon atoms, 24 or about 24 or fewer carbon atoms, or 18 or about 18 or fewer carbon atoms. A particularly preferred phenol is cashew phenol.

[0019] In a particularly preferred embodiment, the end-capped polyurethane prepolymer is prepared by reacting toluene diisocyanate with a polyether polyol, having an NCO content of 4%-5% or about 4%-5% and an equivalent weight of 500-1500 g / eq or about 500-1500 g / eq.

[0020] In another preferred embodiment, the end-capped polyurethane prepolymer is prepared by reacting a toluene diisocyanate-based aromatic polyisocyanate with cashew phenol, preferably 70 wt.%-85 wt.% of the total weight of the end-capped prepolymer, a TDI-based polyisocyanate, and 15 wt.%-25 wt.% of cashew phenol.

[0021] Based on the total weight of component A, the end-capped polyurethane prepolymer (a1) is typically present in component A at a concentration of 0.5 to 20 wt.%, preferably 1 to 10 wt.%, more preferably 2 to 10 wt.%, and most preferably 5 to 10 wt.%.

[0022] In use, components A and B are mixed before or simultaneously with application to the substrate. The concentration of the end-capped polyurethane prepolymer in the final mixed adhesive formulation can be calculated by the ratio of components A and B used to manufacture the final mixed adhesive formulation. In a preferred embodiment, components A and B are mixed in a 1:1 volume ratio, in which case the concentration of the end-capped polyurethane prepolymer in the final adhesive will be half the value in component A.

[0023] Epoxy resin (a2)

[0024] As a unique feature of this invention, component A contains no more than a very small or extremely small amount of epoxy resin. In some embodiments of this invention, epoxy resin is present in amounts of 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 component A.

[0025] Epoxysilane (a3)

[0026] Component A also contains an adhesion promoter, preferably an epoxy silane. An epoxy silane is any molecule carrying a di- or trialkoxysilane moiety bonded to the epoxy moiety. A suitable epoxy silane has the following formula:

[0027]

[0028] Where R 1 R 2 and R 3 Independently selected from C1-C3 alkyl groups, and R 4 It is a divalent organic group.

[0029] In a preferred embodiment, R 1 R 2 and R 3 Independently selected from ethyl and methyl, wherein methyl is preferred, especially when R 1 R 2 and R 3 When it is methyl. R 4 Preferably selected from alkylene groups, preferably C2-C 12 Alkylene, more preferably C2-C6 alkylene, and particularly preferably propylene. In a particularly preferred embodiment, the epoxysilane is γ-glycidyl etheroxypropyltrimethoxysilane.

[0030] Based on the total weight of component A, 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.%.

[0031] Component B

[0032] Nucleophilic crosslinking agent (b1)

[0033] Component B contains a nucleophilic crosslinking agent capable of reacting with the end-capped polyurethane prepolymer (a1) and epoxy resin (a2), if present. Many polyamines can be used as nucleophilic crosslinking agents, with diamines or triamines being preferred. The amine group can be independently a secondary or primary amine group, with primary amine groups being preferred. One of the unique features of this invention is the inclusion of a combination of polyamines with different molecular weights in component B. The molecular weight used in this application is the exponential average molecular weight (Mn). At least two polyamines with different molecular weights should be used in this invention, such as polyetheramines with a functionality of 3. Polyamines with a higher Mn will typically have an Mn greater than 2000, preferably greater than 3000, more preferably greater than 4000, and most preferably about 5000. Polyamines with a lower Mn will typically have an Mn less than 1900; preferably less than 1500, more preferably less than 1000, and most preferably about 400. In preferred embodiments, the total amount of polyamines is typically a fixed value. By adjusting the ratio between the two polyamines, the amount of active NH groups that can be used to react with the end-capped polyurethane prepolymer and epoxy resin (if present) can be controlled.

[0034] The nucleophilic crosslinking agent preferably has a backbone based on poly(epoxide) glycol, particularly C2-C6 alkylene groups, and more particularly C2-C4 alkylene groups, with C3 alkylene groups being the most preferred. Particularly preferably, the backbone is a polyether based on propylene glycol. In a particularly preferred embodiment, the nucleophilic crosslinking agent is a triamine having a primary amine comprising more than 90% of the amine groups and a backbone based on a polyether of propylene glycol.

[0035] Based on the total weight of component B, the nucleophilic crosslinking agent is typically present in component B at a concentration of 0.1 to 20 wt.%, 1 to 15 wt.%, more preferably 2 to 10 wt.%, and most preferably 5 to 10 wt.%.

[0036] Catalyst (b2)

[0037] Component B further comprises a catalyst capable of promoting the reaction of the nucleophile (b1) with the capped polyurethane prepolymer (a1) and 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, DMDEE (2,2'-dimorpholinodiethyl ether), imidazoles (such as 4-methylimidazole), triethanolamine, and polyethyleneimine. Also suitable are organotin compounds such as dioctyltin dineodecanate and other metal catalysts such as tetrabutyl titanate, zirconium acetylacetonate, and bismuth neodecanoate. In one embodiment, a particularly preferred catalyst is diazabicyclo[2.2.2]octane.

[0038] The catalyst is preferably used at a concentration of 0.01 wt.% to 3 wt.% based on the total weight of component B, more preferably 0.01 wt.% to 1 wt.% and most preferably 0.01 wt.% to 0.5 wt.%.

[0039] Other ingredients

[0040] Depending on the application requirements, components A and B may each, or both, optionally further contain other ingredients, typically non-epoxy chemicals, such as: i) plasticizers, such as esters of unsaturated fatty acids, especially C 16 -C 18 ii) Fatty acid esters (especially methyl esters), tris(2-ethylhexyl) phosphates; iii) Stabilizers, such as polycaprolactone; iii) Dyes and colorants; iv) Fillers other than thermally conductive fillers, such as carbon black, calcium carbonate, glass fiber, wollastonite; polyesters, urea, fumed silica, etc.; and v) Viscosity reducers, such as hexadecyltrimethoxysilane. The amounts of these optional components can vary depending on the application.

[0041] Although the amounts of the components that can be used to prepare the reaction product constituting the adhesive formulation can vary, once component A and component B are formulated (separately and individually) and are ready to be combined to form the reaction product adhesive formulation, component A and component B can be mixed in a volume ratio ranging from 2:1 to 1:2. In a preferred embodiment, this volume ratio between component A and component B is about 1:1.

[0042] The following examples further illustrate the invention. The scope of the invention and the claims is not limited to the scope of the following examples. Example

[0043]

[0044] Table 1 above lists the main components used in this example, along with their respective chemical names and functions. Further details about these components are provided below.

[0045] GF200 (terminated polyurethane prepolymer) is the reaction product of aromatic polyisocyanate A and cashew nut shell powder. Reaction procedure: Cashew nut shell powder (22.1 wt%) and aromatic polyisocyanate A (77.85 wt%) were heated to 60°C in a reactor. Then, dibutyltin dilaurate catalyst (0.05 wt%) was added. The reaction mixture was stirred at 80°C under a nitrogen atmosphere for 45 min, and then stirred under vacuum for 10 min. The colorless reaction product was then cooled to room temperature and transferred to a container.

[0046] The epoxy resin (a2) (if present) is the product of the reaction of epichlorohydrin and bisphenol A, and is commercially available as DER330 from Olin.

[0047] Table 2 summarizes the formulation details of components A and B, some of their physical properties, and some test results of the adhesive formulation after combining the two components.

[0048] Table 2. Comparative Examples Compared to Examples of the Invention

[0049]

[0050] Preparation of Component A and Component B

[0051] Examples and comparative examples of the present invention are prepared by mixing the components listed in Table 2 on a planetary mixer or on a dual asymmetric centrifuge. In the first stage, after mixing the liquid phase, the solid material is added to the formulation. After mixing the formulation under vacuum for approximately 30 min, it is filled into a cylinder, barrel, or large container.

[0052] Adhesive formulation

[0053] Mix components A and B of the adhesive by volume in a static mixer at a 1:1 ratio and apply from a manual cartridge system.

[0054] Test methods

[0055] The indentation force was measured using a tension meter (Zwick). The adhesive material was placed on a metal surface. An aluminum piston with a diameter of 40 mm was placed on top, and the material was compressed to 5 mm (initial position). The material was then compressed to 0.3 mm at a speed of 1 mm / s, and the force-deflection curve was recorded. The force (N) at a thickness of 0.5 mm was then reported in Table 2 and considered as the indentation force.

[0056] Thermal conductivity was measured according to ASTM 5470-12 using a thermal interface material tester from ZFW in Stuttgart. Tests were performed in Spaltplus mode at thicknesses between 1.8 and 1.2 mm. The thermal interface material was considered to be Type I (viscous liquid) as described in ASTM 5470-12. The upper contact was heated to approximately 40°C and the lower contact to approximately 10°C, resulting in a sample temperature of approximately 25°C. Components A and B of the adhesive were mixed using a static mixer while being applied from a manual cartridge system. The results are provided in Table 2.

[0057] Molecular weight data were measured by gel permeation chromatography (GPC) using a Malvern Viscothek GPC max instrument. EMSURE-THF (ACS, Reag. Ph EUR for analysis) was used as the eluent, and PL GEL MIXED D (Agilent, 300) was used as the eluent. The column was 7.5 mm (5 µm) and the detector was MALVERN Viscotek TDA.

[0058] The lap shear strength was measured according to DIN EN 1465:2009. Electroplated steel substrates (140 × 25 mm, 0.8 mm thick) were used. The substrates were cleaned with isopropyl alcohol before use. Parts (A) and (B) were mixed in a 1:1 volumetric ratio, and the resulting adhesive was applied to one substrate, followed by bonding the second substrate within 5 minutes. The thickness was adjusted to 1.4 mm, with an overlap of 25 mm × 25 mm. The material was allowed to cure and then allowed to stand at 23°C and 50% relative humidity for 7 days before the lap shear test. The lap shear sample was then mounted in a tension meter and the lap shear test was performed in a standard manner using a pulling speed of 1 mm / min. The force-deflection curve was monitored, and the breaking strength was reported as the lap shear strength in Table 2.

[0059] Rheological measurements (viscosity) were performed on an Anton Paar MC 302 rheometer with a parallel plate geometry. Plates with a diameter of 25 mm and a fixed gap of 0.5 mm were used. The formulation was placed between the two plates, and shear rate tests were performed from 0.001 to 20 1 / s. The viscosity at 10 1 / s is reported in Table 2.

[0060] Test Results

[0061] As shown in Table 2, this invention provides an adhesive formulation that can also be used as a thermal interface material, meeting the high-performance requirements of battery applications, including increased elongation. This formulation is based on end-capped polyurethane technology, which includes an end-capped polyurethane prepolymer in component A and a polyetheramine as a curing agent in component B. Prior art has taught the use of epoxy resins added to formulations to increase strength and adhesion to certain substrates such as aluminum. For many battery designs, significant swelling of the battery cell has been observed, leading to displacement of either the thermal interface material or the thermally conductive adhesive. Swelling occurs particularly during charge and discharge cycles in fast-charging applications and / or after many years of battery operation. A high filler loading is required to achieve a thermal conductivity of 3 W / mK. As shown in Table 2, a large combination of bimodal ATH and multimodal alumina is used.

[0062] Comparative Examples 1 and 2 contained 0.5 wt.% and 1 wt.% epoxy resin, respectively. However, while the formulations achieved high thermal conductivity and high lap shear strength values, the displacement to fracture (measured by lateral movement) in the lap shear test of the comparative examples was only about 0.5 mm (the lap shear samples showed a 2 mm gap). In contrast, the examples of the present invention did not contain any epoxy resin. The lap shear strength of the examples of the present invention was about 1.2 MPa. However, and surprisingly, the displacement to fracture in the lap shear test (also known as the elongation test) was significantly higher at about 1.5 mm.

Claims

1. A two-component adhesive formulation, the two-component adhesive formulation comprising: Component A, comprising greater than 50 wt.% thermally conductive filler, 0.5 wt.% to 20 wt.% end-capped polyurethane prepolymer, 0 wt.% to 0.5 wt.% epoxy resin, 0.1 wt.% to 5 wt.% epoxy silane, and other optional non-epoxy components, all based on the total weight of Component A; and Component B; The at least one of the components A and B further comprises a plasticizer.

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

3. The two-component adhesive formulation as claimed in any of the preceding claims, wherein the two-component adhesive formulation comprises: Component A, comprising greater than 50 wt.% thermally conductive filler, 0.5 wt.% to 20 wt.% end-capped polyurethane prepolymer, 0 wt.% to 0.1 wt.% epoxy resin, 0.1 wt.% to 5 wt.% epoxy silane, and other optional non-epoxy components, all based on the total weight of said component A; and Component B; The at least one of the components A and B further comprises a plasticizer.

4. A two-component adhesive formulation as claimed in any of the preceding claims, wherein the two-component adhesive formulation comprises: Component A, comprising greater than 50 wt.% thermally conductive filler, 0.5 wt.% to 20 wt.% end-capped polyurethane prepolymer, 0.1 wt.% to 5 wt.% epoxy silane, and other optional non-epoxy components, all based on the total weight of said component A; and Component B; The at least one of the components A and B further comprises a plasticizer.

5. The two-component adhesive formulation as claimed in any of the preceding claims, wherein, The thermally conductive filler contains aluminum trioxide.

6. The two-component adhesive formulation as claimed in any of the preceding claims, wherein, The thermally conductive filler comprises a mixture of aluminum hydroxide and aluminum oxide.

7. The two-component adhesive formulation as claimed in any of the preceding claims, wherein, Component B contains a nucleophilic crosslinking agent, and the nucleophilic crosslinking agent contains at least one polyamine.

8. The two-component adhesive formulation as described in claim 7, wherein, The polyamine is a polyetheramine with a functionality of 3.

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

10. The two-component adhesive formulation according to any one of claims 1 to 7, wherein, Component B contains a nucleophilic crosslinking agent, and the nucleophilic crosslinking agent contains at least two polyamines with different molecular weights and each having a functionality of 3.

11. The two-component adhesive formulation as claimed in any of the preceding claims, wherein, The plasticizer comprises an unsaturated C 16 -C 18 ester of a fatty acid.

12. The two-component adhesive formulation as claimed in any of the preceding claims, wherein, The plasticizer is present in component A at approximately 2.6 wt.% of the total weight of component A.

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

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

2.

15. A two-component adhesive formulation as claimed in any of the preceding claims, wherein, The volume ratio between component A and component B is approximately 1:1.

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