Two-component thermally conductive adhesive

Abstract

Two-component thermally conductive adhesives made from aliphatic polyurethane prepolymers and methods of making the same.

Description

Background Technology

[0001] Over the past decade, the automotive industry has seen a trend towards reducing vehicle weight. This trend is driven by regulations aimed at reducing fleet CO2 emissions. In recent years, the increasing number of electric vehicles has further fueled lightweight construction strategies. To provide longer driving ranges, batteries with high energy density are required. All long-life battery cells require adequate thermal management. Thermal interface materials or thermally conductive adhesives are needed to thermally connect the battery cells or the modules housing them to cooling units.

[0002] Battery cells generate heat during charging and discharging. The cell should be maintained at an appropriate operating temperature (preferably 25°C–40°C) to avoid efficiency loss, overheating, and dangerous thermal runaway reactions. For this reason, active cooling is typically used. Effective active cooling methods involve pumping a cooling water-glycol mixture through channels to cool the metal bottom of a cooling plate on which the battery cells or modules are mounted. The battery cells or cell arrays can be directly bonded to the cooling plate using thermally conductive adhesives. This provides good mechanical fixation and thermal bonding.

[0003] A key requirement for thermally conductive adhesives is a thermal conductivity greater than 1.5 W / mK. Additionally, an overlap shear strength >2.5 MPa is required. Cooling plates and battery cells / modules are typically made of aluminum coated with polyethylene terephthalate (PET) or similar polymer materials. Therefore, good adhesion between the thermally conductive adhesive and PET is necessary. Various potential materials exist that can be used to formulate thermally conductive adhesives. Two-component polyurethane adhesives stand out among other candidates due to their good mechanical properties and elongation at break, as well as good curing kinetics. However, commonly available two-component polyurethane adhesives often exhibit poor adhesion to untreated aluminum or other metal substrates. Furthermore, polyurethane adhesives contain toxic isocyanates, and residual diisocyanate monomers that contribute to the H351 (carcinogenic) label are frequently present in the final composition. Another issue with polyurethane adhesives is that the isocyanate component formulated with aromatic isocyanate-based prepolymers often exhibits high viscosity and poor storage stability.

[0004] The desired outcome is a two-component polyurethane thermally conductive adhesive formulation with low residual diisocyanate monomers in the final composition (which avoids undesirable toxicity labels) and improved shelf-life characteristics compared to currently available polyurethane-based materials. Summary of the Invention

[0005] This invention relates to a two-component thermally conductive adhesive formulation in which the isocyanate component comprises an aliphatic prepolymer used at a high filler content. The aliphatic prepolymer has a low monomeric diisocyanate content, thus, when fully formulated, the adhesive will have a total diisocyanate residual level of less than 0.1 wt.% of the adhesive formulation. This invention also provides improved shelf-life characteristics resulting from the low reactivity between the aliphatic prepolymer and any moisture on the surface or elsewhere of the conductive filler used in the formulation. Detailed Implementation

[0006] To achieve a thermal conductivity greater than 1.5 W / mK, both components of a two-component adhesive formulation typically require at least 50% to 70% thermally conductive filler by weight. In one embodiment, each component of the adhesive formulation independently comprises 50% to 95% thermally conductive filler by weight. In another embodiment, each component of the adhesive formulation independently comprises 60% to 90% thermally conductive filler by weight. In a preferred embodiment, each component of the adhesive formulation independently comprises 65% to 85% thermally conductive filler by weight. In another preferred embodiment, each component of the adhesive formulation independently comprises 70% to 80% thermally conductive filler by weight.

[0007] A variety of thermally conductive fillers can be used. Examples include aluminum hydroxide (also known as aluminum trihydride (ATH)), alumina (such as spherical alumina), and any combination thereof. ATH can be a unimodal ATH powder or an ATH powder with a multimodal particle size distribution (e.g., bimodal, trimodal, etc.). When using unimodal ATH powder, the average particle size can be 5-100 µm. When using multimodal ATH powder, the average particle size of the smallest particles can be less than about 10 µm, while the average particle size of the largest particles can be greater than about 50 µm. Additionally, ATH powder can be surface-treated with silanes, titanates, carboxylates, etc.

[0008] In one embodiment, a combination of ATH and alumina, such as spherical alumina, can be used in the isocyanate component, polyol component, or both. When using such a combination, the ratio of ATH to alumina can vary and, for example, can range from 0.1:99.9 to 99.9:0.1. In one embodiment, the ratio of ATH to aluminum hydroxide ranges from 80:20 to 20:80, and is preferably from 60:40 to 40:60.

[0009] I. Isocyanate component (also known as the first component of a two-component polyurethane adhesive formulation)

[0010] A. Aliphatic polyurethane prepolymer

[0011] The isocyanate component of the two-component polyurethane adhesive formulation of the present invention comprises an aliphatic polyurethane prepolymer. This aliphatic polyurethane prepolymer typically has less than 0.5 wt.%, preferably less than 0.3 wt.%, more preferably less than 0.2 wt.%, and most preferably less than 0.1 wt.%, of a low monomeric diisocyanate, all based on the total weight of the prepolymer.

[0012] Examples of prepolymers used in this invention include any polyurethane prepolymer having an isocyanate containing an aliphatic isocyanate group, such as hexamethylene-1,6-diisocyanate (“HDI”), isophorone diisocyanate (“IPDI”), or other similar isocyanate compounds. The aliphatic isocyanate can also be a homopolymer having an aliphatic diisocyanate group, such as an HDI dimer, an HDI trimer (e.g., biuret, isocyanurate), or a mixture thereof. The prepolymer is prepared by reacting an aliphatic isocyanate with a polyol based on a polyether or polyester. The resulting prepolymer is a polyol terminated with an aliphatic isocyanate.

[0013] In one embodiment, the isocyanate component comprises 1% to 25% polyurethane prepolymer by weight of the component. In another embodiment, the isocyanate component comprises 5% to 20% polyurethane prepolymer by weight of the component. In a preferred embodiment, the isocyanate component comprises 5% to 15% polyurethane prepolymer by weight of the component. In a more preferred embodiment, the isocyanate component comprises 8% to 12% polyurethane prepolymer by weight of the component. In another more preferred embodiment, the isocyanate component comprises 10% to 12% polyurethane prepolymer by weight of the component.

[0014] B. Organofunctional silanes

[0015] The isocyanate component may further comprise at least a first and / or a second organofunctional silane. The isocyanate component may comprise 0.1% to 5% by weight of the organofunctional silane. The first organofunctional silane may be the same as or different from the second organofunctional silane. In one embodiment, the two silanes are identical. The organofunctional silane may act as a surface modifier for the thermally conductive filler. For example, the organofunctional silane may partially or completely cover the surface of the conductive filler particles. The conductive filler may comprise surface M-OH groups, where M is a metal atom, and the organofunctional silane may comprise functional groups that react with the M-OH groups to form a direct or indirect bond between the surface modifier and M.

[0016] The surface of the conductive filler can be hydrophobized with organofunctional silanes. It should be understood that organofunctional silanes can be added to the filler before or after mixing the conductive filler with the matrix phase material. For example, the conductive filler can be coated with or reacted with an organofunctional silane before mixing it with the matrix phase material. As another example, the organofunctional silane can be mixed with the matrix phase material to form a premix, and then the premix can be combined with the conductive filler. As yet another example, the conductive filler can be mixed with the matrix phase material, and then an organofunctional silane can be added to the mixture.

[0017] In one embodiment, the isocyanate component comprises 0.1% to 5% by weight of an organofunctional silane. In another embodiment, the isocyanate component comprises 0.5% to 3% by weight of an organofunctional silane. In yet another embodiment, the isocyanate component comprises 1% to 2% by weight of an organofunctional silane.

[0018] A variety of organofunctional silanes can be used. In one embodiment, the organofunctional silane is an alkylsilane. In another embodiment, the organofunctional silane has the following structure:

[0019] ,

[0020] Where n is an integer in the range of 0 to 24, and R 3 -R 7 Independently, it is hydrogen or optionally substituted C1-C4 alkyl. In another embodiment, R 3 -R 7 Independently, it is hydrogen or an unsubstituted C1-C4 alkyl group. In other embodiments, n is an integer ranging from 1 to 15, for example, 2-14, 5-14, 8-14, or 10-14. Specific non-limiting examples include trimethoxy(methyl)silane, ethyltrimethoxysilane, trimethoxy(propyl)silane, butyltrimethoxysilane, trimethoxy(pentyl)silane, hexyltrimethoxysilane, heptyltrimethoxysilane, trimethoxy(octyl)silane, trimethoxy(nonyl)silane, decyltrimethoxysilane, trimethoxy(undecyl)silane, dodecyltrimethoxysilane, trimethoxy(tridecyl)silane, trimethoxy(tetradecyl)silane, trimethoxy(pentadecanyl)silane, hexadecyltrimethoxysilane, or combinations thereof.

[0021] While the organofunctional silane is preferably present in the isocyanate component, it should be understood that it may also be present in the polyol component or both. The weight percentage of this organofunctional silane should be adjusted accordingly so that the total weight percentage present is less than 5 wt.% based on the total weight of the adhesive formulation. A typical range for this organofunctional silane should be 0.1 to 5 wt.%, preferably 0.2 to 3 wt.%, more preferably 0.3 to 2 wt.%, and most preferably 0.5 to 1.5 wt.% based on the total weight of the adhesive formulation.

[0022] II. Polyol component (also known as the second component of two-component polyurethane adhesive formulations)

[0023] The second component of a two-component polyurethane adhesive formulation is a polyol component and typically comprises i) a polyol having a molecular weight of at least 400 g / mol; ii) a diol serving as a chain extender having a molecular weight of 200 g / mol or less; and iii) a catalyst capable of promoting the reaction between the polyol and the polyurethane prepolymer in the isocyanate component. Although in some embodiments both the polyol and the diol may have two hydroxyl groups / molecules, the difference lies in the fact that the diol has a lower molecular weight than the polyol.

[0024] A. Polyols

[0025] In some embodiments, the polyol component comprises 5% to 50% polyol by weight of the component. In other embodiments, the polyol component comprises 10% to 40% polyol by weight of the component. In a preferred embodiment, the polyol component comprises 15% to 30% polyol by weight of the component.

[0026] The molecular weight of polyols can vary. Typically, polyols have a molecular weight ranging from 100 g / mol to 5000 g / mol, with some preferred embodiments having a narrower range of such molecular weights. In some more preferred embodiments of the invention, the polyols have a molecular weight between 400 g / mol and 5000 g / mol. In some even more preferred embodiments of the invention, the polyols have a molecular weight between 400 g / mol and 3000 g / mol.

[0027] Typically, the polyol can be any polyol used with polyurethane technology. For example, the polyol can be any glycerol-initiated propoxylated or ethoxylated polyol or any propoxylated or ethoxylated polyol prepared from alternative trifunctional and bifunctional starting materials. In some embodiments, the polyol can be a polyether polyol or a mixture of polyether polyols. In other embodiments, the polyol can be a homopolymer or copolymer of propylene oxide, or a copolymer of propylene oxide with 70 wt.% to 99 wt.% propylene oxide and 1 wt.% to 30 wt.% ethylene oxide. If a single polyether polyol is present, such a copolymer of propylene oxide and ethylene oxide is preferred. If two or more polyether polyols are present, it is preferred that at least one of the polyols is such a copolymer of propylene oxide and ethylene oxide. In the case of copolymers, propylene oxide and ethylene oxide can be random copolymers, block copolymers, or both. In some embodiments, 50% or more of the hydroxyl groups in the polyether polyol or mixture thereof are primary hydroxyl groups, and the remainder of the hydroxyl groups are secondary hydroxyl groups. In another embodiment, 70% or more of the hydroxyl groups in the polyether polyol or mixture thereof may be primary hydroxyl groups.

[0028] In another embodiment, the polyol may be a polyether polyol or a polyester polyol. Other suitable polyols that can be used as a polypropylene-based diol may include VORANOL 1010L with a molecular weight of 500 g / mol, VORANOL 2000L with a molecular weight of 1,000 g / mol, VORANOL CP4610, a glycerol-initiated ethylene oxide-based propoxylated triol with an average molecular weight of 4,700 g / mol, and mixtures thereof. All of these are commercially available from Dow Chemical Company.

[0029] In some specific embodiments, the polyol may be a glycerol-initiated ethylene oxide-based propoxylated triol having a molecular weight of 1,500 g / mol to 1,700 g / mol, which may be present in 15% to 75%, 20% to 70%, 25% to 65%, 30% to 60%, 35% to 55%, 40% to 50%, or 45% to 50% by weight of the polyol component.

[0030] B. Low molecular weight diols

[0031] Diols with a molecular weight of 200 g / mol or less act as chain extenders. The polyol component may contain 0.1% to 10% diol by weight of the component. In some embodiments, the polyol component may contain 0.5% to 8% diol by weight of the component. In other embodiments, the polyol component may contain 0.5% to 5% diol by weight of the component.

[0032] Typically, a diol may have at least two carbon atoms, may be branched, linear, or functionalized, and may have at least two hydroxyl groups / molecules. In some embodiments, the diol may be a linear or branched aliphatic diol having 2-20 carbons (e.g., 2-18 carbons, 2-16 carbons, 2-14 carbons, 2-12 carbons, 2-10 carbons, 2-8 carbons, or 2-6 carbons). In various embodiments, the diol has a molecular weight of 20-200 g / mol (e.g., 20-150 g / mol, 40-150 g / mol, 50-130 g / mol, or 60-120 g / mol).

[0033] In some embodiments, the diol has the formula C x H y O z Where x is an integer ranging from 2 to 20, y is an integer equal to x + m (where m is an integer ranging from 4 to 12), and z is an integer equal to xn (where n ranges from 0 to 6). Non-limiting examples include monoethylene glycol (MEG), diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2,3-dimethyl-1,3-propanediol, dipropylene glycol, tripropylene glycol, 1,4-butanediol, or 1,6-hexanediol. In one embodiment, the diol may be monoethylene glycol, 1,4-butanediol, or a mixture thereof.

[0034] C. Catalyst

[0035] The polyol component may contain a catalyst capable of catalyzing the reaction of hydroxyl groups with isocyanate groups. The catalyst may be present in the isocyanate component, the polyol component, or both. In some preferred embodiments, the catalyst is present in the polyol component. The polyol component may contain 0.001% to 0.5% of the catalyst by weight of the component. In some embodiments, the polyol component may contain 0.002% to 0.2% of the catalyst by weight of the component. In other embodiments, the polyol component may contain 0.01% to 0.1% of the catalyst by weight of the component.

[0036] Examples of such catalysts include tertiary amine catalysts, organometallic catalysts such as bismuth catalysts, alkyltin carboxylates, alkyltin oxides, and tin thiols.

[0037] Specific examples of tertiary amine catalysts include N-methylmorpholine, N-methylimidazolium, triethylenediamine, bis-(2-dimethylaminoethyl)-ether, 1,4-diazabicyclo[2.2.2]octane (DABCO), dimethylcyclohexylamine, dimethylethanolamine, 2,2-dimorpholino-diethyl ether (DMDEE), N,N,N-dimethylaminopropylhexahydrotriazine, dimethyltetrahydropyrimidine, tetramethylethylenediamine, dimethylcyclohexylamine, 2,2-N,N-benzyldimethylamine, and dimethylethanolamine. Dimethylaminopropylamine, penta-dimethyldiethylenetriamine, N,N,N',N'-tetramethyl-1,6-hexanediamine, N,N',N'-trimethylaminoethylpiperazine, 1,1'-[[3-(dimethylamino)propyl]imino]bisprop-2-ol, 1,3,5-tris[3-(dimethylamino)propyl]hexahydro-1,3,5-triazine, NN-dimethyldipropylenetriamine, N,N,N'-trimethylaminoethylethanolamine, of which DMDEE is particularly preferred.

[0038] If an organometallic catalyst is used, it is any organometallic catalyst capable of catalyzing the reaction of isocyanates with a functional group having at least one reactive hydrogen. Examples include bismuth catalysts, metal carboxylates such as tin carboxylate and zinc carboxylate. Metal alkanoates include stannous octoate, bismuth octoate, or bismuth neodecanoate. Preferably, the at least one organometallic catalyst is a bismuth catalyst or an organotin catalyst. Examples include dibutyltin dilaurate, dimethyltin dinedecanoate, dimethyltin thiol, dimethyltin carboxylate, dimethyltin dioleate, dimethyltin dithioglycolate, dibutyltin thiol, bis(2-ethylhexyl thioglycolate) dibutyltin, dibutyltin sulfide, dioctyltin dithioglycolate, dioctyltin thiol, dioctyltin dioctanoate, dioctyltin dinedecanoate, and dioctyltin dilaurate. In a particularly preferred embodiment, it is a tin catalyst, particularly preferably dioctyltin thiol.

[0039] D. Rheology fillers / additives

[0040] The polyol component may also contain rheology modifiers or fillers to prevent sedimentation. Optionally, such modifiers may also be present in the isocyanate component. The rheology filler may be a particulate filler. The particulate filler may be a solid material at room temperature and is insoluble in the polyol component or other components of the isocyanate component. The filler may be a material that does not melt, volatilize, or degrade under the conditions of the curing reaction between the polyol and the isocyanate component. The fillers can be, for example, inorganic fillers such as glass, silica (e.g., fumed silica), boron oxide, boron nitride, titanium oxide, titanium nitride, fly ash, calcium carbonate, and various aluminosilicates (including clays such as wollastonite and kaolin); metal particles such as iron, titanium, aluminum, copper, brass, bronze, etc.; thermosetting polymer particles such as polyurethane, epoxy resin, cured particles of phenol-formaldehyde or cresol-formaldehyde resin, cross-linked polystyrene, etc.; thermoplastic plastics such as polystyrene, styrene-acrylonitrile copolymer, polyimide, polyamide-imide, polyetherketone, polyetheretherketone, polyethyleneimine, poly(p-phenylene sulfide), polyoxymethylene, polycarbonate, etc.; and various types of carbon such as activated carbon, graphite, molecular sieves, carbon black, etc.; and mixtures thereof.

[0041] In one embodiment, the particulate filler may be in the form of particles having a size of 50 nanometers (nm) to 100 micrometers (µm). In other embodiments, the filler may have a particle size (d50) of 250 nm or larger in one embodiment, 500 nm or larger in another embodiment, and 1 µm or larger in yet another embodiment. In one embodiment, the filler may have a particle size (d50) of 50 µm or smaller, 25 µm or smaller, or 10 µm or smaller. The particle size of particles having a size less than 100 nm can be conveniently measured using dynamic light scattering or laser diffraction.

[0042] In some embodiments, the particulate filler particles may have an aspect ratio of up to 5, up to 2, or up to 1.5. In other embodiments, some or all of the filler particles may be grafted onto one or more polyether polyols of the polyol component.

[0043] Typically, when rheology fillers are present in a polyol component, the polyol component may contain 0.1% to 5% rheology filler by weight of the component. In some embodiments, the polyol component may contain 0.3% to 3% rheology filler by weight of the component. In other embodiments, the polyol component may contain 0.5% to 2% rheology filler by weight of the component.

[0044] In some embodiments, the rheology filler may also be present in the isocyanate component, or in both the isocyanate component and the polyol component.

[0045] III. Methods and Applications

[0046] In one embodiment, a method for preparing the thermally conductive adhesive formulation of the present invention includes providing an isocyanate component and a polyol component. When provided as a kit, each adhesive component may be co-packaged or packaged separately. When the adhesive is ready for use, the components may be mixed, blended, or co-blended together, and a reaction product is generated when the combination of components cures. One or more additional optional components may be added to the formulation as needed.

[0047] Although the amounts of the components that can be used to form the reaction product constituting the adhesive formulation can vary, once the isocyanate component and the polyol component are formulated (separately and individually) and the two components are ready to be combined to form the reaction product adhesive formulation, the isocyanate component and the polyol component can be mixed in a volume ratio ranging from 2:1 to 1:2. In a preferred embodiment, this volume ratio between the isocyanate component and the polyol component is about 1:1.

[0048] When the components are manufactured separately and individually, the raw materials can be mixed together at the desired concentrations discussed above at temperatures, for example, 5°C to 80°C (e.g., 15°C to 50°C) or at room temperature. In one embodiment, the mixing of the components can be carried out under vacuum and / or using a planetary mixer or on a double asymmetric centrifuge. The order of mixing is not critical, and two or more compounds can be mixed together, followed by the addition of the remaining components.

[0049] The adhesive formulation of the present invention can also be premixed before use. In doing so, all the components of the adhesive formulation can be mixed together using any known mixing method and equipment. When mixing the various raw material components, it is preferable to mix the liquid phase first before adding the solid components. The entire mixture should be mixed for approximately 30 to 45 minutes to form the final formulation, and then packaged into tubes, drums, cylinders, or other commonly known packaging.

[0050] The adhesive formulation of the present invention can be used in a variety of applications. Various articles comprising the adhesive are contemplated. In one embodiment, the article comprises a battery module formed of at least one battery cell and a cooling plate, wherein the battery module is fixed to the cooling plate by a cured thermally conductive adhesive of the present invention. In another embodiment, the battery module is part of an electric vehicle. Example

[0051] 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.

[0052] I. Raw material composition and invention examples

[0053] The table below lists examples of the invention and comparative examples of the prior art. All weight percentages are based on the total weight of the respective components.

[0054] Table 1. Composition of isocyanate components

[0055]

[0056] Table 2. Composition of polyol components

[0057]

[0058] The MDI prepolymer used as a comparative example is prepared by reacting an MDI polyisocyanate with a mixture of different polyols. Details of this method are described in PCT / U52019 / 045071, which is incorporated herein by reference in its entirety. It is generally preferred to combine the various polyols prior to the reaction, although the polyols may react sequentially with the polyisocyanate in sub-combinations or individually. Fillers as described below may be present during the MDI prepolymer formation reaction. The reaction typically continues until the hydroxyl groups of the polyol mixture have been consumed and a target or constant isocyanate content has been obtained. The MDI prepolymer itself is typically characterized by having free isocyanate 20 groups and being a room-temperature solid material that softens at high temperatures, such as 70°C to 130°C, prior to curing. The MDI prepolymer may or may not exhibit a crystallization melting temperature within this temperature range. As a reaction product, the MDI prepolymer primarily contains aromatic isocyanate prepolymers.

[0059] Desmodur TM E30700 and E30600 are aliphatic isocyanate prepolymers commercially available from Covestro. The prepolymer is based on hexamethylene-1,6-diisocyanate (HDI) having a monomeric diisocyanate content of less than 0.3 wt.% in the prepolymer.

[0060] The organofunctional silane used in this example contains hexadecyltrimethoxysilane. This material is commercially available from Evonik.

[0061] The thermally conductive filler used in this example is bimodal aluminum hydroxide (ATH), which is commercially available from Nabaltec.

[0062] The polyol used in this example is an ethylene oxide-terminated polypropylene oxide polyol, commercially available from Dow Chemical Company.

[0063] 1,4-Butanediol is commercially available from ARCO Chemical.

[0064] The rheology modifier used in this example is hydrophobic fumed silica, which is commercially available from Evonik.

[0065] The catalysts used in this example are dioctyltin dinedecanoate catalyst and dioctyltin thiol catalyst, both of which are commercially available from Momentive.

[0066] II. method

[0067] A. Sample Preparation

[0068] In each of the comparative and inventive examples, all the components listed in Tables 1 and 2 (first the liquid components, then the solid components) are added to a planetary mixer or a double asymmetric centrifuge, mixed under vacuum for about 30 minutes, and then transferred to a cylinder, barrel, or drum for storage.

[0069] B. Test

[0070] The test methods described herein include both those used in the illustrated examples and those values ​​included in the detailed description of the invention.

[0071] Indentation force: The indentation force was measured using a tension meter (Zwick). The gap filler 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 the datasheet and considered as the indentation force.

[0072] Thermal conductivity: Thermal conductivity was measured according to ASTM 5470-12 using a thermal interface material tester from ZFW, Stuttgart. Tests were performed on 2 mm thick adhesive boards cured at room temperature for 7 days. Thermal conductivity tests were conducted at pressures of 1–5 bar, and the effective thermal conductivity was reported at 5 bar. The upper contact was heated to approximately 40°C. o C, and heat the lower contact to approximately 10 o C, yielding approximately 25 o C represents the overall sample temperature.

[0073] Gel permeation chromatography (GPC): Molecular weight data of the polyurethane prepolymer 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, PL GEL MIXED-D (Agilent Technologies, 300 x 7.5 mm, 5 μm) was used as the column, and MALVERN Viscotek TDA was used as the detector.

[0074] Overlap Shear Test: E-coated steel substrates with Cathoguard 800 coating were used. Sample dimensions were 100 mm x 25 mm with a thickness of 1.2 mm. The substrates were cleaned with isopropyl alcohol before use. Adhesive was applied to one substrate, and then the second substrate was joined within less than 3 minutes. The thickness was adjusted to 1.0 mm, with an overlap of 25 mm x 15 mm. The joined substrate units were cured at 23°C and 50% relative humidity and allowed to stand for 7 days prior to the overlap shear test. The sample units were then mounted in a tension meter, and the overlap shear test was performed using a pulling speed of 10 mm / min. The force-deflection curve was monitored, and the breaking strength was reported as the overlap shear strength.

[0075] Viscosity: Rheological measurements were performed on an Anton Paar MC 302 rheometer with a parallel plate geometry, using plates with a diameter of 25 mm and a fixed gap of 0.5 mm. The thermal interface material was placed between the two plates, and shear rate tests were then performed from 0.001 to 20 1 / s, with the viscosity reported at 10 1 / s.

[0076] Tensile testing: Tensile strength, e-modulus, and elongation at break were tested according to DIN 527-2. A dog bone-shaped sample with a thickness of 2.0 mm was used. The tensile test was performed at 10 mm / min.

[0077] The test results are summarized in Tables 3 and 4.

[0078] Table 3. Viscosity of the isocyanate component

[0079]

[0080] Table 4. Other test data for two-component polyurethane adhesive formulations (1:1 v / v combination of isocyanate and polyol components).

[0081]

[0082] AF stands for "adhesive failure" and CF stands for "cohesive failure", as commonly understood by one of ordinary skill in the art.

[0083] C. Discussion of Results

[0084] As shown in Table 1, the comparative example of the isocyanate component has approximately 23 wt.% MDI-based prepolymer, 75 wt.% bimodal ATH content, and 2 wt.% hexadecyltrimethoxysilane. Table 2 shows the initial measured viscosity of the isocyanate component as 533 Pa·s. When this isocyanate component was stored at 40°C for one week, the component became solid and its viscosity could no longer be measured. This demonstrates the poor shelf life of the comparative example, where the MDI prepolymer is present in the isocyanate component. However, surprisingly, when the isocyanate component was made primarily from a mixture of two aliphatic prepolymers based on low-monomer HDI, the initial viscosity of the isocyanate component was approximately 136 Pa·s, significantly lower than the initial viscosity of the component containing the MDI-based prepolymer. After one week of storage at 40°C, the viscosity of the component only increased to 321 Pa·s, which is still within an acceptable range. This demonstrates a significantly improved shelf life for formulations based on low-monomer HDI prepolymers compared to formulations based on aromatic isocyanates.

[0085] Table 4 shows the test results for the two components of the combination of the present invention. The indentation force (also referred to in the art as the extrusion flow value) is 792 N, which is relatively high. After a 5-minute open time (the time between the application of the adhesive and the compression test), the indentation force is much higher, reaching 4164 N. However, for the formulation of the present invention, the initial indentation force value and the indentation force value after a 5-minute open time are much lower and within the preferred and acceptable range, as shown in Table 4. All other test results of the present invention (including lap shear strength and thermal conductivity) are within the acceptable range compared to those of the comparative examples.

Claims

1. A two-component adhesive formulation, the two-component adhesive formulation comprising: The isocyanate component comprises a polyurethane prepolymer having a monomeric diisocyanate content of less than 0.5 wt.% based on the weight of the prepolymer; and Polyol components.

2. The two-component adhesive formulation according to claim 1, wherein, The polyurethane prepolymer has a monomeric diisocyanate content of less than 0.3 wt.% based on the weight of the prepolymer.

3. The two-component adhesive formulation according to any one of the preceding claims, wherein, The polyurethane prepolymer has a monomeric diisocyanate content of less than 0.1 wt.% based on the weight of the prepolymer.

4. The two-component adhesive formulation according to any one of the preceding claims, wherein, The polyurethane prepolymer is an aliphatic prepolymer.

5. A two-component adhesive formulation according to any one of the preceding claims, wherein, The isocyanate component further comprises an organofunctional silane.

6. The two-component adhesive formulation according to any one of the preceding claims, wherein, The polyurethane prepolymer is a reaction product of hexamethylene-1,6-diisocyanate and polyol.

7. The two-component adhesive formulation according to any one of the preceding claims, wherein, The polyol component comprises i) 5 to 50 wt.% of a polyol based on the weight of the polyol component, wherein the polyol has a molecular weight of at least 400 g / mol; ii) 0.1 to 10 wt.% of a diol based on the weight of the polyol component, wherein the diol has a molecular weight of not more than 200 g / mol; and iii) 0.001 to 0.5 wt.% of a catalyst based on the weight of the polyol component.

8. A two-component adhesive formulation according to any one of the preceding claims, wherein, The isocyanate component further comprises 50 wt.% to 95 wt.% of thermally conductive filler based on the weight of the isocyanate component.

9. A two-component adhesive formulation according to any one of the preceding claims, wherein, The polyol component further comprises 50 wt.% to 95 wt.% of thermally conductive filler based on the weight of the polyol component.

10. The two-component adhesive formulation according to any one of claims 8 and 9, wherein, The thermally conductive filler is aluminum hydroxide.

11. The two-component adhesive formulation according to any one of claims 8 to 10, wherein, The isocyanate component comprises approximately 75 wt.% of the thermally conductive filler based on the weight of the isocyanate component.

12. The two-component adhesive formulation according to any one of claims 8 to 11, wherein, The polyol component comprises approximately 75 wt.% of the thermally conductive filler based on the weight of the polyol component.

13. The two-component adhesive formulation according to any one of claims 11 and 12, wherein, The thermally conductive filler is aluminum hydroxide.

14. A two-component adhesive formulation according to any one of the preceding claims, in, The volume ratio between the isocyanate component and the polyol component ranges from 2:1 to 1:

2.

15. The two-component adhesive formulation according to claim 14, wherein, The volume ratio between the isocyanate component and the polyol component is approximately 1:1.

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