Patent Application
20230323174
The present invention relates to the field of two-component thermal interface materials.
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.
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, with high amounts (typically >50 wt %) of thermally conductive fillers such as aluminum hydroxide, aluminum oxide, as disclosed in WO2014047932A1.
The use of multimodal aluminum trihydroxide (ATH) or aluminum oxide is reported to reach high thermal conductivities.
In most battery applications of TIMs, the TIM serves a dual role of providing thermal conductivity for cooling and also mechanical fixation of the battery modules to protect them and keep them in place. For certain battery designs and OEM requirements, high lap shear strengths and cohesive failure modes are needed, so that long term fatigue tests are passed. This renders the battery system robust even after extensive dynamic vibration exposures. Most two-component polyurethane-based TIMs lack sufficient high lap shear strength and good adhesion to the substrate.
There is a need for TIMs having good thermal conductivity and good adhesive properties and fatigue performance.
In a first aspect, the invention provides a kit for a two-component thermally conductive adhesive formulation comprising:
In a second aspect, the invention provides a cured thermally conductive adhesive, resulting from mixing Parts (A) and (B) and allowing curing to occur.
In a third aspect, the invention provides a battery assembly comprising battery modules fixed in place in the assembly by a cured adhesive composition resulting from mixing Parts (A) and (B), and/or mechanical fastening means, such that the adhesive composition provides thermal conductivity between the battery modules and a cooling substrate.
In a fourth aspect, the invention provides a method for assembling a battery assembly, comprising the steps:
The inventors have surprisingly found that in a thermally conductive adhesive (thermal conductivity of >2 W/mK) the addition of an aromatic epoxy resin and an epoxy silane in combination with a blocked polyurethane prepolymer leads to high cohesive failure mode at lap shear strengths higher than 0.8 MPa, as well as good fatigue performance.
Molecular weights of polymers as reported herein are reported in Daltons (Da) as number or weight average molecular weights, as determined by size exclusion chromatography (SEC).
In one embodiment, the invention provides a kit for a two-component thermally conductive adhesive formulation comprising:
The thermally conductive filler is not particularly limited.
Suitable thermally conductive fillers are those that have a coefficient of thermal conductivity that is greater than 5 W/m° K, greater than 10 W/m° K, or greater than 15 W/m° K. Examples of thermally conductive fillers include alumina, alumina trihydrate or aluminum trihydroxide, silicon carbide, boron nitride, diamond, and graphite, or mixtures thereof. Particularly preferred are aluminium trihydroxide (ATH), and aluminium oxide, with ATH being the most preferred.
In a preferred embodiment, the thermally conductive filler has a broad particle size distribution characterized by a ratio of D90/D50 of at or about 3 or more. Particularly preferably the thermally conductive filler is ATH or aluminium oxide having a broad particle size distribution characterized by a ratio of D90/D50 of at or about 3 or more, most preferably ATH.
Also preferred are thermally conductive fillers having a bimodal particle size distribution. A bimodal distribution is when, for example, the ratio D90/D50 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 D50 of 5 to microns and a D90 of 70 to 90 microns, particularly a D50 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 Na4P2O7×10 H2O, preferably at 1 g/l. Preferred are aluminium oxide and ATH having a bimodal distribution, particularly ATH.
The thermally conductive filler is preferably present in the final adhesive 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 greater than 50 wt %, more preferably greater than 60 wt %, more particularly preferably greater than 70 wt %. 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 is less than 93 wt %, as higher levels can affect the adhesive strength and impact resistance negatively. In a particularly preferred embodiment, the thermally conductive filler is present at 85-90 wt %.
The thermally conductive filler may be present in Part (A), Part (B) or both. In a preferred embodiment it is present in both Part (A) and Part (B), as this reduces the amount of mixing required to properly distribute the thermally conductive filler when Parts (A) and (B) are mixed. Preferably it is present at similar or the same concentration in both Parts (A) and (B). In a particularly preferred embodiment it is present at 85-90 wt % in the final mixture of Parts (A) and (B), based on the total weight of the mixture. Preferably it is present both Parts (A) and (B) at 85 wt %.
In a particularly preferred embodiment, the thermally conductive filler is ATH having a ratio D90/D50 of at or about 8 or more, used at a concentration of 85-89 wt % in both Parts (A) and (B), based on the total weight of Part (A) or Part (B).
Part (A) of the adhesive composition 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.
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 poly(ethylene oxide)diol, poly(propylene oxide)diol, poly(tetramethylene oxide)diol. Particularly preferred is poly(propylene oxide)diol, particularly poly(propylene glycol).
Particularly preferred 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.
Particularly preferred is a polyisocyanate made by reacting TDI with a poly(propylene oxide)diol, in particular when the resulting polyisocyanate has an equivalent weight of at or about 950.
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.
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.
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. Preferably the reaction is carried out with a tin catalyst.
Molecular Weight data of the polyurethane prepolymers 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 μm) was used as a column, and MALVERN Viscotek TDA was used as a detector.
The blocked polyurethane prepolymer (a1) is preferably present in Part (A) at a concentration of 0.5 to 5 wt %, more preferably at 1 to 3 wt %, particularly preferably at 1.5 to 2.2 wt %, based on the total weight of Part (A).
In a preferred embodiment, the blocked polyurethane prepolymer is made by reacting TDI with a poly(propylene oxide)diol, in particular when the resulting polyisocyanate has an equivalent weight of at or about 950, and capping with cardanol, at 1.5 to 2.2 wt %, preferably at or about 2 wt %, based on the total weight of Part (A).
In use, Parts (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 can be calculated from the proportions of Parts (A) and (B) used to make the final mixed adhesive. In a preferred embodiment, Parts (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 Part (A).
Part (A) comprises an aromatic epoxy resin. The aromatic epoxy resin is preferably a reaction product of a diphenol with epichlorohydrin. Examples of suitable diphenols include bisphenol A, bisphenol F, with bisphenol A being particularly preferred.
In a particularly preferred embodiment, the aromatic epoxy resin is a reaction product of epichlorohydrin and bisphenol A, having the following characteristics:
The aromatic epoxy resin is preferably present in Part (A) at a concentration of 0.3 to 2 wt %, more preferably at 0.6 to 1.5 wt %, particularly preferably at 1 to 1.2 wt %, based on the total weight of Part (A).
In a preferred embodiment, the aromatic epoxy resin is a reaction product of epichlorohydrin and bisphenol A, having the following characteristics:
at 1 to 1.2 wt %, based on the total weight of Part (A).
In use, Parts (A) and (B) are mixed prior to or simultaneously with application to a substrate. The concentration of the aromatic epoxy resin in the final, mixed adhesive can be calculated from the proportions of Parts (A) and (B) used to make the final mixed adhesive. In a preferred embodiment, Parts (A) and (B) are mixed in a 1:1 ratio by volume, in which case the concentration of the aromatic epoxy resin in the final adhesive will be half the value in Part (A).
Part (A) comprises 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, R2 and R3 are independently selected from C1-C3 alkyl, and R4 is a divalent organic radical.
In preferred embodiments, R1, R2 and R3 are independently selected from ethyl and methyl, with methyl being preferred, particularly when R1, R2 and R3 are methyl.
R4 is preferably selected from alkylene, preferably C2-C12 alkylene, more preferably C2-C6 alkylene, particularly preferably propylene.
In a particularly preferred embodiment, the epoxy silane is gamma-glycidoxypropyltrimethoxysilane.
The epoxy silane is preferably present in Part (A) at 0.2 to 0.75 wt %, more preferably 0.25 to 0.6 wt %, particularly preferably at or about 0.5 wt %, based on the total weight of Part (A).
In a particularly preferred embodiment, the epoxy silane is gamma-glycidoxypropyltrimethoxysilane at 0.2 to 0.75 wt %, more preferably 0.25 to 0.6 wt %, particularly preferably at or about 0.5 wt %, based on the total weight of Part (A).
In use, Parts (A) and (B) are mixed prior to or simultaneously with application to a substrate. The concentration of the epoxy silane in the final, mixed adhesive can be calculated from the proportions of Parts (A) and (B) used to make the final mixed adhesive. In a preferred embodiment, Parts (A) and (B) are mixed in a 1:1 ratio by volume, in which case the concentration of the epoxy silane in the final adhesive will be half the value in Part (A).
Part (B) comprises a nucleophilic cross-linker capable of reacting with the blocked polyurethane prepolymer (a1) and the aromatic epoxy resin (a2).
The nucleophilic cross-linker is preferably a di- or tri-amine, with triamines being preferred. The amine groups may be independently secondary or primary, with primary being preferred.
The nucleophilic cross-linker preferably has a molecular weight of 1,500 to 4,000 Da, more preferably 2,000 to 3,500 Da, with at or about 3,000 Da being particularly preferred.
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 having primary amines for greater than 90% of amine groups, a molecular weight of at or about 3,000 Da, and a backbone based on a polyether of propylene glycol.
More particularly preferably, the nucleophilic cross-linker is a trifunctional polyether amine of approximately 3000 molecular weight,
having the following characteristics:
The nucleophilic cross-linker is preferably present in Part (B) at a concentration of 0.1 to 10 wt %, 1 to 5 wt %, more preferably 1.5 to 3.3 wt %, particularly preferably at 3 to 3.2 wt %, based on the total weight of Part (B).
In a particularly preferred embodiment, the nucleophilic cross-linker is a trifunctional polyether amine of approximately 3000 molecular weight,
having the following characteristics:
at 3-3.2 wt %, based on the total weight of Part (B).
In use, Parts (A) and (B) are mixed prior to or simultaneously with application to a substrate. The concentration of the nucleophilic cross-linker in the final, mixed adhesive can be calculated from the proportions of Parts (A) and (B) used to make the final mixed adhesive. In a preferred embodiment, Parts (A) and (B) are mixed in a 1:1 ratio by volume, in which case the concentration of the nucleophilic cross-linker in the final adhesive will be half the value in Part (A).
Part (B) comprises a catalyst capable of promoting the reaction of nucleophile (b1) with the blocked polyurethane prepolymer (a1) and the aromatic epoxy resin (a2).
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′-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 is diazabicyclo[2.2.2]octane.
The catalyst is preferably used at 0.05 to 0.2 wt %, more preferably 0.075 to 0.15 wt %, more particularly preferably at or about 0.1 wt %, based on the total weight of Part (B).
Parts (A) and (B) may additionally comprise other ingredients such as:
The invention also provides a cured thermally conductive adhesive, resulting from mixing Parts (A) and (B) and allowing curing to occur.
Parts (A) and (B) may be mixed in any proportion. Preferably the final concentrations of the ingredients fall within the following ranges after mixing (A) and (B):
0.3-0.6
0.05-0.15
Parts (A) and (B) are mixed and can be applied to a substrate using known methods, such as a manual application system or in an automated way with a pump system using 20 l pails or 200 l drums or any other preferred container.
The cured adhesive composition is characterized by a thermal conductivity, measured according to ASTM 5470-12 (as described in the Examples), of 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).
The cured adhesive composition preferably has a lap shear strength, according to DIN EN 1465:2009, as measured in the Examples, of greater than 0.7 MPa, more preferably greater than 0.8 or 0.9 MPa.
The two-part composition cures at room temperature (preferably as characterized by an increase in a press-in force of about 100% or more, after aging for 24 hours after mixing).
The invention also provides a battery assembly comprising battery modules fixed in place in the assembly by a cured adhesive composition and/or by mechanical fastening means, resulting from mixing Parts (A) and (B), such that the mixture, when cured, provides thermal conductivity between the cells and the substrate.
Parts (A) and (B) are mixed in the desired ratio, and the mixture is applied, before curing, in a manner to separate the battery cells physically and electrically and to fix the cells in place on a substrate designed to cool the cells, such that the mixture, when cured, provides thermal conductivity between the cells and the substrate.
The thermal conductivity of the adhesive in the assembly, measured according to ASTM 5470-12 (as described in the Examples), is preferably 2.0 W/mK or more, more preferably at or about 2.5 or more, more particularly preferably at or about 2.8 or more.
The following are examples of some preferred embodiments of the adhesive kit of the invention:
Blocked polyurethane prepolymer GF200: GF200 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 RT and transferred into a container.
D.E.R. Epoxy Resin [Aromatic Epoxy Resin (a2)]
Reaction product of epichlorohydrin with bisphenol A, having the following properties:
Diamine a [Nucleophilic Cross-Linker (b1)]
Diamine A is a member of a family of polyamines having repeat oxypropylene units in the backbone. As shown by the above structure, Diamine A is a difunctional primary amine with an average molecular weight of approximately 2000. Its amine groups are located on secondary carbon atoms at the ends of an aliphatic polyether chain.
Triamine a [Nucleophilic Cross-Linker (b1)]
Triamine A is a trifunctional polyether amine of approximately 3000 molecular weight.
It has the following characteristics:
Bimodally distributed ATH was used, having the following characteristics:
The experimental formulations 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.
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.
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.
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 listed in Table 2.
Molecular Weight data of the polyurethane prepolymers 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 μm) was used as a column, and MALVERN Viscotek TDA was used as a detector.
Lap shear strength was measured according to according to DIN EN 1465:2009. e-coated steel substrates (140×25 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×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.
Rheology measurements 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.
The results in Table 2 show that all compositions have acceptable thermal conductivity of greater than 2.5 W/m° K. However, Comparative Examples CE1 and CE2, which lack the aromatic epoxy resin (a2) and the epoxy silane (a3) have low press-in force, unacceptably low lap shear strengths and very high levels of adhesive failure. Comparative Examples CE3 and CE4 which have the aromatic epoxy resin (a2), but lack the epoxy silane (a3) also have unacceptably low lap shear strengths, and CE3 has a high level of adhesive failure. Comparative Example CE5, which lacks the aromatic epoxy resin (a2), but has the epoxy silane (a3) has an unacceptably low lap shear strength, and high level of adhesive failure.
In contrast, Examples 1 and 2, which are representative of the adhesives of the invention have higher thermal conductivities, excellent lap shear strengths, and 100% cohesive failure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/049615 | 9/9/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63077866 | Sep 2020 | US |
