WO2025024067A1

WO2025024067A1 - Two-component thermally conductive adhesive

Info

Publication number: WO2025024067A1
Application number: PCT/US2024/034511
Authority: WO
Inventors: Sergio Grunder; Joel KUNZ; Stefan Schmatloch
Original assignee: DDP Specialty Electronic Materials US LLC
Current assignee: DDP Specialty Electronic Materials US LLC
Priority date: 2023-07-27
Filing date: 2024-06-18
Publication date: 2025-01-30
Anticipated expiration: 2026-01-27

Abstract

Two-component thermally conductive adhesive made from aliphatic polyurethane prepolymer and the method of making thereof.

Description

TWO-COMPONENT THERMALLY CONDUCTIVE ADHESIVE

BACKGROUND

[0001] There is a trend in the automotive industry to reduce the weight of the vehicles in the past decade. This trend has been driven by regulations to reduce CO2 emission of vehicle fleets. In recent years, lightweight construction strategies have been further fueled by the increasing number of electrically driven vehicles. To provide for longer driving range, batteries with a high energy density are needed. All long-range durable battery cells need adequate thermal management. To thermally connect battery cells or modules that house the battery cells to a cooling unit, thermal interface materials or thermally conductive adhesives are needed.

[0002] Battery cells produce heat during charge and discharge. The cells should be kept in the right operating temperature (preferably 25-40°C) to avoid efficiency losses, overheating, and dangerous runaway thermal reactions. For this reason, active cooling is commonly used. An efficient active cooling method involves pumping cooled water-glycol mixtures through channels that cool a metal bottom of a cooling plate on which the battery ceils or modules are installed. Battery cells or arrays of cells can be bonded with thermal conductive adhesives directly to the cooling plate. This provides for good mechanical fixation and a thermal connection.

[0003] A key requirement of the thermal conductive adhesives is a thermal conductivity of higher than 1.5 W/mK. Further, a lap shear strength of > 2.5 MPa is required. The cooling plate and the battery cell/modules are often made of aluminum covered with polyethylene terephthalate (PET) or similar polymer materials. Therefore, a good adhesion between the thermal conductive adhesive and PET is required. There is a variety of potential materials available that can be employed to formulate thermal conductive adhesives. Two-component polyurethane adhesives stand out among other candidates in drat they have good mechanical properties and elongation at break and good curing kinetics. However, the commonly available two-component polyurethane adhesives usually have poor adhesion to untreated aluminum or other metal substrates. Furthermore, polyurethane adhesives comprise isocyanates which are toxic, and often there are residual diisocyanate monomers in the final composition that lead to a H351 (cancerogenic) label. An additional problem with polyurethane adhesives is that the isocyanate component formulated with aromatic isocyanate based prepolymers usually shows high viscosity and poor shelf-stability.

[0004] What is desired is a two-component polyurethane thermal conductive adhesive formulation with low residual diisocyanate monomers in the final composition which can avoid an undesirable toxicity label and an improved shelf-life characteristics compared to currently available polyurethane based materials.

SUMMARY OF THE INVENTION

[0005] The present invention relates to a two-component thermally conductive adhesive formulation wherein the isocyanate component has an aliphatic prepolymer used with high filler content. The aliphatic prepolymer has a low monomeric diisocyanate content so when fully formulated, the adhesive will have an overall diisocyanate residual level of less than 0.1 wt.% of the adhesive formulation. The present invention also provides a better shelf-life characteristics resulting from the low reactivity between aliphatic prepolymer and any moistures that was attached to the surface or other places of the conductive fillers used in the formulation.

DETAILED DESCRIPTION OF THE INVENTION

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

[0007] A variety of thermally conductive fi llers can be used. Examples include aluminum hydroxide, also known as aluminum trihydroxide (ATH), aluminum oxide, such as spherical aluminum oxide, and any combination thereof. ATH can be monomodal ATH powders or ATH powders having a multi-modal particle size distribution (e.g., bi-modal, tri-modal, and the like). When monomodal ATH powders are used, the average particle size can be 5-100 pm. When multi-modal ATH powders are used, the average particle size of the smallest particles can be less than about 10 pm, while the average particle size of the largest particles can be greater than about 50 pm. Additionally, the ATH powders can be surface treated with silane, titanate, carboxylates, etc.

[0008] In one embodiment, a combination of ATH and aluminum oxide such as spherical aluminum oxide can be used in the isocyanate component, the polyol component, or both. When this combination is used, the ratio of ATH to aluminum oxide can vary and can range for example 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 preferably 60:40 to 40:60.

I. The Isocyanate Component ( also referred to as the first component of the two- component polyurethane adhesive formulation)

A. Aliphatic Polyurethane Prepolymer

[0009] The isocyanate component of the two-component polyurethane adhesive formulation of the present invention comprises an aliphatic polyurethane prepolymer. Such aliphatic polyurethane prepolymer typically has a low monomeric diisocyanate of 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.%, all based upon the total weight of the prepolymer.

[0010] Examples of prepolymers used in the present invention include any polyurethane prepolymers that have isocyanate having an aliphatic isocyanate group such as hexamethylene- 1,6-diisocyanate (‘HDI”), isophorone diisocyanate(“IPDT’), or other similar isocyanate compounds. Aliphatic isocyanate can also be homo-polymers having aliphatic diisocyanate groups such as HDI dimer, HDI trimer (e.g. biuret, isocyanurate) or a mixture thereof. The prepolymer is made from the reaction between the aliphatic isocyanate and a polyol which is based on polyether or polyester. The resulting prepolymer is the polyol capped with aliphatic isocyanate. [0011] In one embodiment, the isocyanate component comprises 1% to 25% of the polyurethane prepolymer by weight of the component. In another embodiment, the isocyanate component comprises 5?^zo to 20% of the polyurethane prepolymer by weight of the component. In a preferred embodiment, the isocyanate component comprises 5% to 15% of the polyurethane prepolymer by weight of the component. In a more preferred embodiment, the isocyanate component comprises 8% to 12% of the polyurethane prepolymer by weight of the component. In another more preferred embodiment, the isocyanate component comprises 10% to 12% of the polyurethane prepolymer by weight of the component.

B. Organofunctional Silane

[0012] The isocyanate component can also comprise at least a first and/or a second organo functional silane. The isocyanate component can comprise 0.1% to 5% of the or gano functional silane by weight of the component. Tire first organo functional silane can be the same or different from the second organofunctional silane. In one embodiment, both silanes are the same. The organofunctional silane can function as a surface modifier for the thermally conductive filler. For example, the organofunctional silane can partially or completely cover the surface of the conductive filler particle. The conductive filler can include surface M-OH groups, where M is a metal atom, and the organofunctional silane can include a functional group that reacts with the M-OH group to form a direct or indirect bond between the surface modifier and M.

[0013] The surface of the conductive filler can be hydrophobized with the organofunctional silane. It will be appreciated that the organofunctional silane 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 or reacted with organofunctional silane prior to mixing the conductive filler and the matrix phase material. As another example, the organofunctional silane can be mixed with the matrix phase material to form a premix which is then combined with the conductive filler. As another example, the conductive filler and the matrix phase material can be mixed and then organofunctional silane can be added to the mixture.

[0014] In one embodiment, the isocyanate component comprises 0.1% to 5% of the organofunctional silane by weight of the component. In a further embodiment, the isocyanate component comprises 0.5% to 3% of the organofunctional silane by weight of the component. In a further embodiment, the isocyanate component comprises 1% to 2% of the organofunctional silane by weight of the component.

[0015] A variety of organofunctional silanes can be used. In one embodiment, the organofunctional silane is an alkyl silane. In a further embodiment, the organofunctional silane has the following structure:

wherein n is an integer ranging from 0 to 24 and R’-R⁷ are independently hydrogen or optionally substituted C1-C4 alkyl. In a further embodiment, R³-R⁷ are independently hydrogen or unsubstituted C 1-C4 alkyl. In a further embodiment, n is an integer ranging from 1 to 15, e.g., 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(pentadecyl)silane, hexadecyltrimethoxysilane, or a combination thereof.

[0016] While the organofunctional silane is preferably present in the isocyanate component, it is understood that it can also be present in the polyol component or in both. The weight percentage of such organofunctional silane should be adjusted accordingly so that the total weight percentage present will be less than 5 wt. % based on the total weight of the adhesive formulation. A typical range of such 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. IL Polyol Component (also referred to as the second component of the two- component polyurethane adhesive formulation)

[0017] The second component of the two-component polyurethane adhesive formulation is the polyol component and in general comprises i) a polyol, the polyol having a molecular weight of at least 400 g/mol; ii) a diol, used as chain extender, the diol 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 the polyol and diol may both have two hydroxyl groups per molecule, in some embodiments, they are differentiated in that the diol has a lower molecular weight than the polyol.

A. Polyol

[0018] In some embodiments, the polyol component comprises 5% to 50% of the polyol by weight of the component. In other embodiments, the polyol component comprises 10% to 40% of the polyol by weight of the component. In a preferred embodiment, the polyol component comprises 15% to 30% of the polyol by weight of the component.

[0019] The molecular weight of the polyol can vary. In general, the polyol has a molecular weight of 100 g/mol to 5000 g/mol with some preferred embodiments having a narrower range of such polyol molecular weight ranges. In some more preferred embodiments of the present invention, the polyol has a molecular weight betw een 400 g/mol to 5000 g/mol. hr some even more preferred embodiments of the present invention, the polyol has a molecular weight between 400 g/mol to 3000 g/mol.

[0020] In general, the polyol can be any polyol used with polyurethane technology. For example, the polyol can be any glycerin initiated propoxylated or ethoxylated polyol or any propoxylated or ethoxylated polyol prepared from alternative tri-functional and bis-functional starters. In some embodiments, the polyol can be a polyether polyol or 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 from 1 wt.% to 30 wt.% ethylene oxide. Such a copolymer of propylene oxide and ethylene oxide can be preferred if a single polyether polyol is present. If two or more polyether polyols are present, it can be preferred that at least one of the polyols is such a copolymer of propylene oxide and ethylene oxide. In the case of a copolymer, the propy lene oxide and ethylene oxide can be randomly copolymerized, block copolymerized, or both. In some embodiments, 50 % or more of the hydroxyl groups of the polyether polyol or mixture of polyether polyols are primary hydroxyl, with the remainder of the hydroxyl groups being secondary hydroxyl groups. In another embodiment, 70% or more of the hydroxyl groups in the polyether polyol or mixture thereof can be primary hydroxyl groups.

[0021] In additional embodiments, the polyol can be a poly ether polyol or a polyester polyol. Other suitable polyols useful in the polyol component can include polypropylene based diols such as VORANOL 1010L with a molecular weight of 500 g/mol, VORANOL 2000L with a molecular weight of 1,000 g/mol, glycerin-initiated ethylene oxide based propoxylated triol VORANOL CP4610 with an average molecular weight of 4700 g/mol; and mixtures thereof. All these are commercially available from the Dow Chemical Company.

[0022] In certain specific embodiments, the polyol can be a glycerin-initiated ethylene oxide based propoxylated triol having a molecular weight of 1,500 g/mol to 1,700 g/mol, which can be present at 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.

B. Lo w Molecular Weight Diol

[0023] The diol having a molecular weight of 200 g/mol or less functions as a chain extender. Tire polyol component can comprise 0.1% to 10% of the diol by weight the component. In some embodiments, the polyol component can comprise 0.5% to 8% of the diol by weight the component. In other embodiments, the polyol component can comprise 0.5% to 5% of the diol by weight the component.

[0024] In general, the diol can have at least two carbon atoms, can be branched, linear, or functionalized, and can have at least two hydroxyl groups per molecule. In some embodiments, the diol can be a linear or branched aliphatic diol having from 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. [0025] In some embodiments, the diol has the formula C_xH_yO_z, where x is an integer ranging from 2 to 20, y is an integer equal to x + m (m being an integer ranging from 4 to 12), and z is an integer equal to x-n (n ranging from 0 to 6). Non-limiting examples include monoethylene glycol (MEG), diethylene glycol, triethylene glycol. 1,2-propane diol, 1,3-propane diol, 2,3-dimethyl- 1,3-propane diol, dipropylene glycol, tripropylene glycol, 1,4-butane diol, or 1,6-hexane diol. In one embodiment, the diol can be monoethylene glycol, 1,4-butanediol, or a mixture thereof.

C. Catalyst

[0026] The polyol component can include a catalyst capable of catalyzing the reaction of a hydroxyl group with an isocyanate group. Catalyst can be present in the isocyanate component, the polyol component, or both. In some preferred examples, the catalyst is present in the polyol component. The poly ol component can comprise 0.001% to 0.5% of the catalyst by weight the component. In some embodiments, the polyol component can comprise 0.002% to 0.2% of the catalyst by weight the component. In other embodiments, the polyol component can comprise 0.01% to 0.1% of the catalyst by weight the component

[0027] Examples of such catalysts include tertiary amine catalysts, organometallic catalysts. Such as bismuth catalysts, alkyl tin carboxylates, oxides and tin mercaptides.

[0028] Specific examples of tertiary amine catalysts include N-niethyl morpholine, N-methyl imidazole, triethylenediamine, bis-(2-dimethylaminoethyl)-ether, 1 ,4-diazabicyclo[2.2.2]octane (DABCO), dimethylcyclohexylamine, dimethylethanolamine, 2,2-dimorpholinyl-diethylether (DMDEE), N,N,N-dimethylaminopropyl hexahydrotriazine, dimethyltetrahydropyrimidine, tetramethylethylenediamine, dimethylcyclohexylamine, 2,2-N,N benzyldimethylamine, dimethylethanol amine, dimethylaminopropyl amine, Penta-dimethyl diethylene triamine, N, N,N',N' -tetramethyl- J ,6-hexanediamine, N,N',N'-trimethylaminoethylpiperazine, 1 , 1 ’-[[3- (dimethylaniino)propyl]imino]bispropan-2-ol, l,3,5-tris[3-(dimethylamino)propyl]hexahydro- 1 ,3,5-triazine, N-N-dimethyldipropylene triamine, N,N,N'-trimethylaminoethylethanolamine, with DMDEE being particularly preferred.

[0029] If an organometallic catalyst is used, it is any organometallic catalyst capable of catalyzing the reaction of isocyanate 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, dimethyl tin dineodecanoate, dimethyltin mercaptide, dimethyltin carboxylate, dimethyltin dioleate, dimethyltin dithioglycolate, dibutyltin mercaptide, dibutyltin bis(2 -ethylhexyl thioglycolate), dibutyltin sulfide, dioctyltin dithioglycolate, dioctyltin mercaptide, dioctyltin dioctoate, dioctyltin dineodecanoate, dioctyltin dilaurate. In a particularly preferred embodiment, it is a tin catalyst, particularly preferably dioctyltin mercaptide.

D. Rheology Filler/Addiiive

[0030] The polyol component may also contain rheology additive or filler to prevent sedimentation. Optionally, such additive may also be present in the isocyanate component. The rheology filler can be a particulate filler. Tire particulate filler can be a solid material at room temperature, and is not soluble in the other ingredients of the polyol component or the isocyanate component. The filler can be a material that does not melt, volatilize, or degrade under the conditions of the curing reaction between the polyol and isocyanate components. The filler can be, for example, an inorganic filler such as glass, silica (e.g., filmed silica), boron oxide, boron nitride, titanium oxide, titanium nitride, fly ash, calcium carbonate, and various alumina-silicates including clays such as wollastonite and kaolin, and the like; metal particles such as iron, titanium, aluminum, copper, brass, bronze and the like; thermoset polymer particles such as polyurethane, cured particles of an epoxy, phenol-formaldehyde, or cresol-formaldehyde resin, crosslinked polystyrene, and the like; thermoplastics such as polystyrene, styrene acrylonitrile copolymers, polyimide, polyamide-imide, polyether ketone, polyether-ether ketone, polyethyleneimine, poly(p-phenylene sulfide), polyoxymethylene, polycarbonate and the like; and various types of carbon such as activated carbon, graphite, molecular sieve, carbon black and the like; and mixtures thereof.

[0031] The particulate filler can be in the form of particles having a size of from 50 nanometers (nm) to 100 micrometers (iim) in one embodiment. In other embodiments, the fillers may have a particle size (d50) of 250 nm or greater in one embodiment, 500 nm or greater in another embodiment and 1 pm or greater in still another embodiment. In other embodiments, the fillers can have a particle size (d50) of 50 pm or less, 25 pm or less, or 10 pm or less. Particles sizes are conveniently measured using dynamic light scattering methods, or laser diffraction methods for particles having a size below 100 nm.

[0032] In some embodiments, particulate filler particles can have an aspect ratio of up to 5, an aspect ratio of up to 2, or an aspect ratio of up to 1.5. bi other embodiments, a portion or all of the filler particles can be grafted onto one or more of the polyether polyol(s) of the polyol component.

[0032] In general, when a rheology filler is present in the polyol component, the polyol component can comprise 0.1% to 5% of the rheology fil ler by weight the component. In some embodiments, the polyol component can comprise 0.3% to 3% of the rheology filler by weight the component. In other embodiments, the polyol component can comprise 0.5% to 2% of the rheology filler by weight the component.

[0033] In some embodiments, rheology filler can also be present in the isocyanate component, or in both the isocyanate and the polyol component.

III. Methods and Applications

[0034] In one embodiment, the process for preparing the thermal conductive adhesive formulation of the present invention includes providing the isocyanate component and the polyol components. When provided as a kit, each adhesive component can be co-packaged or packaged separately. When the adhesive is ready to be used, the components can be mixed, admixed, or blended together which results in a reaction product when the combination of components are cured. One or more additional optional components can be added to the formulation as desired.

[0035] While the amount of the components useful in making 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 for combining to form the reaction product adhesive formulation, the isocyanate component and the polyol component can be mixed at a volume ratio ranging from 2:1 to 1 :2. In one preferred embodiment, such volume ratio between the isocyanate component and the polyol component is about 1 :1. [0036] In making the components separately and individually, the ingredient raw materials can be mixed together in the desired concentrations discussed above at a temperature of from 5 °C to 80° C, e.g., 15°C to 50°C, or room temperature for example. In one embodiment, the mixing of the ingredients can be carried out under vacuum and or with a planetary mixer or on a dual asymmetri c centrifuge. The order of mixing is not criti cal and two or more compounds can be mixed together followed by addition of the remaining ingredients.

[0037] The adhesive formulation of the present invention may also be premixed before using. In doing so, all adhesive formulation ingredients that make up the components can be mixed together by any known mixing process and equipment. In mixing various raw material ingredient, it is preferred that the liquid phases are mixed first before the addition of solid ingredients. The entire mixture should be mixed for approximately 30 to 45 minutes to form the final formulation before being packed into cartridges, pails, drums or other commonly known packages.

[0038] The adhesive formulation of the present invention can be used in a variety of applications. Various articles comprising the adhesive are contemplated. In on embodiment, the article comprises a battery module that is formed of at least one battery cell and a cooling plate, wherein the battery module is affixed to the cooling plate via the cured thermal conductive adhesive of the present invention, hi a further embodiment, the battery module is part of an electrically driven vehicle.

EXAMPLE

[0039] The following examples further illustrate the present in vention. The scope of the invention and claims is not limited by the scope of the following examples.

I. Raw Material Ingredients and Inventive Example

[0040] The following tables list an Inventive Example of the present invention and a prior art Comparative Example. All weight percent is based on the total weight of the respective components.

Table 1 . Ingredients for the Isocyanate Component

Table 2. Ingredients for the Polyol Component

[0041] MDI Prepolymer, used as an ingredient for the Comparative Example, is prepared by reacting the MDI polyisocyanate with a mixture of different polyols. Detail of the process is described in PCT/U52019/045071, incorporated herein by reference in its entirety. It is generally preferred to combine the various polyols prior to reaction, although the polyols can be reacted sequentially with the polyisocyanate in sub-combinations or individually. A filler as described below may be present during the MDI prepolymer forming reaction. The reaction is generally continued until the hydroxyl groups of the mixture of polyols have been consumed and a target or constant isocyanate content has been obtained. The MDI prepolymer by itself is generally characterized in having free isocyanate 20 groups and in being, prior to curing, a room temperature solid material that heat softens at an elevated temperature of, for example, 70 to 130°C. The MDI prepolymer may or may not exhibit a crystalline melting temperature within that temperature range. As a reaction product, this MDI prepolymer contains mostly aromatic isocyanate prepolymer.

[0042] Desmodur™ E30700 and E30600 are aliphatic isocyanate prepolymers commercially available from Covestro. The prepolymer is based on hexamethylene- 1,6-diisocyanate (HDI) having less than 0.3 wt.% of monomeric diisocyanate content in the prepolymer.

[0043] Organofunctional silane used in the example contains hexadecyltrimethoxysilane. This material is commercially available from Evonik.

[0044] Thermal conductive fillers used in the example is a bimodally distributed aluminum trihydroxide (ATH) commercially available from Nabaltec.

[0045] The polyol used in the present example is an ethyleneoxide capped polypropyleneoxide polyether polyol commercially available from Dow Chemical Company.

I ,4-Butanediol is commercially available from ARCO Chemical.

[0046] The rheology additive used in the present example is hydrophobic filmed silica commercially available from Evonik.

[0047] Catalysts used in the present examples are respectively dioctyl tin dineodecanoate catalyst and dioctyl tin mercaptide catalyst both commercially available from Momentive.

II. Methods

A. Sample preparation

[0048] In each of the Comparative Example and the Inventive Example, all ingredients as listed in Table 1 and Table 2 (first liquid components, then solid components) were added in a planetary mixer or dual asymmetric centrifuge, mixed for about 30 minutes under vacuum then transferred into cartridges, pails or drums for storage. B. Tests

[0049] The test methods described herein include both of those used in the examples shown and for those values included in the detailed description of the present invention.

[0050] Press-in Force; The press-in force is measured with a tensiometer (Zwick). The gap filler material is placed on a metal surface. An aluminum piston with 40 mm diameter is placed on top and the material is compressed to 5 mm (initial position). The material is then compressed to 0.3 mm with 1 mm/s velocity and force deflection curve is recorded. The force (N) at 0.5 nun thickness is then reported in the data table and considered as the press-in force.

[0051] Thermal Conductivity. Thermal conductivity is measured according to ASTM 5470- 12, using a thermal interface material tester from ZFW Stuttgart. The tests are performed with 2 mm thick adhesive plates cured for 7 days at room temperature. The thermal conductivity tests are performed at 1-5 bar pressure and the effective thermal conductivity' is reported at 5 bar pressure. The upper contact is heated to approximately 40°C and the lower contact is heated to approximately 10°C, resulting an overall sample temperature of about 25°C.

[0052] Gel Penneation Chromatography (GPC): 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 x 7.5 mm, 5 m) was used as a column, and MALVERN Viscotek TDA was used as a detector.

[0053] Lap Shear tests: e-coated steel substrates with a cathoguard 800 coating were used. Size of the samples are 100 mm x 25 mm with a thickness of 1.2 mm. The substrates were cleaned with isopropanol before use. The adhesive is applied on one substrate, before the second substrate is joined in less than 3 minutes. The thickness is adjusted to 1.0 nun, the overlap area is 25 mm x 15 mm. The now joined substrates unit is cured and rested for 7 days at 23 °C, 50 % relative humidity before the lap shear tests were performed. The sample units were then mounted in a tensiometer and the lap shear tests were performed using a pull speed of 10 mm/min. The force deflection curve is monitored and the strength at break is reported as lap shear strength. [0054] Viscosity: Rheology measurements were performed on an Anton Paar MC 302 rheometer with a parallel plate geometry⁷25 mm diameter plates were used, the gap was fixed at 0.5 mm. The thermal interface material is brought between the two plates and then a shear rate test was performed from 0.001 to 20 1/s and the viscosity at 10 1/s was reported. [0055] Tensile tests: Tensile tests for values in tensile strength, e-modulus, and elongation at break were performed according to DIN 527-2. Dog-bone samples with 2.0 mm thickness were used. Tensile tests are at performed 10 mm/min.

[0056] Tests results are summarized in Tables 3 and 4.

Table 3. Viscosity of the Isocyanate Component

Table 4. Other Testing Data of the Two-Component Polyurethane Adhesive Formulation (with 1 : 1 v/v combination of the Isocyanate and the Polyol Components)

[0057] AF stands for “adhesive failure” and CF stands for “cohesive failure” as one commonly understood by one of ordinary skilled in the art.

C. Discussion of the Results

[0058] As shown in Table 1 , the Comparative Example of the Isocyanate Component has about 23 wt.% of an MDI-based prepolymer with a bimodal ATH content of 75 wt.% and 2 wt.% of hexadecyltrimethoxysilane. Table 2 showed that the initial viscosity measured of the Isocyanate Component is 533 Pa.s. When this Isocyanate Component formulation is stored for 1 week at 40 °C, this component formulation becomes solid and the viscosity can no longer be measured. This demonstrates the poor shelf-life of the Comparative Example where MDI prepolymer is present in the Isocyanate Component. Surprisingly, however, when the Isocyanate Component is largely made of a mixture of two low-monomeric HDI based aliphatic prepolymers, the initial viscosity of the Isocyanate Component is about 136 Pa.s, much lower than that of the MDI based prepolymer containing Component. After storing the Isocyanate Component for 1 week at 40 °C, the viscosity of the component formulation increased only to 321 Pa.s which is still in an acceptable range. This demonstrates the significantly improved shelf-life of low monomeric HDI prepolymer based formulations compared to aromatic isocyanate based formulations.

[0059] Table 4 shows the test results of the combined two Components of the present invention. The Press-in Force, also may be referred to as Squeeze Flow value in the art, is 792 N which is relatively high. The Press-in Force is a lot higher and reached 4164 N after 5 minutes of open time (time between application of adhesive and compression test). With the formulation of the present invention, however, the Press-in Force values initially and after 5 min open time are much lower and in a preferred and acceptable range as shown in Table 4. All other test results of the present invention, including the Lap Shear strength as well as the Thermal Conductivity, are all in an acceptable range compared to those of the Comparative Example.

Claims

CLAIMS What is claimed is:

1 . A two-component adhesive formulation comprising: an isocyanate component comprising a polyurethane prepolymer with less than 0.5 wt.% of monomeric diisocyanate, based on the weight of the prepolymer; and a polyol component.

2. The two-component adhesive formulation according to Claim 1 wherein the polyurethane prepolymer has less than 0.3 wt.% of monomeric diisocyanate, 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 less than 0.1 wt.% of monomeric diisocyanate, 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. The two-component adhesive formulation according to any one of the preceding claims wherein the isocyanate component further comprises an organofonctional 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 a 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, and the polyol having 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, and the diol having a molecular weight of no 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. The two-component adhesive formulation according to any one of the preceding claims wherein the isocyanate component further comprises 50 wt.% to 95 wt.% of a thermal conductive filler, based on the weight of the isocyanate component.

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

10. The two-component adhesive formulation according to any one of Claim 8 and Claim 9, wherein the thermal conductive filler is aluminum trihydroxide.

1 1. The two-component adhesive formulation according to any of Claim 8 to Claim 10, wherein the isocyanate component comprises about 75 wt.%, based on the weight of the isocyanate component, of the thermal conductive filler.

12. The two-component adhesive formulation according to any of Claim 8 to Claim 1 1, wherein the polyol component comprises about 75 wt.%, based on the weight of the polyol component, of the thermal conductive filler.

13. The two-component adhesive formulation according to any one of Claim 11 and Claim 12 wherein the thermal conductive filler is aluminum trihydroxide.

14. The two-component adhesive formulation according to any one of the preceding claims wherein 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, where in the volume ration between the isocyanate component and the polyol component is about 1 to 1.