WO2025132340A1

WO2025132340A1 - Battery housing with heat insulation properties

Info

Publication number: WO2025132340A1
Application number: PCT/EP2024/086771
Authority: WO
Inventors: Ann-Kathrin HARTMANN; Maike Dahle; Chokri Ben Hammouda; Nabarun Roy; Martin Linnenbrink
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2023-12-22
Filing date: 2024-12-17
Publication date: 2025-06-26
Anticipated expiration: 2026-06-22

Abstract

The present invention relates to a battery housing comprising an structural layer and an inner layer wherein the inner layer comprises a polyurethane coating, the polyurethane coating is obtainable by mixing (a) one or more organic polyisocyanates, (b) one or more compounds having at least two isocyanate-reactive hydrogen atoms, comprising polyetherpolyol (b1), (c) one or more catalysts, (d) 30 to 90 wt.-% based on the total weight of components a) to e), of solid flame retardant, and optionally fillers and/or polyurethane additives, to give a reaction mixture and allow the reaction mixture to cure. The present invention further relates to a method for the production of a battery housing according to the invention and a battery comprising a battery housing according to the present invention.

Description

Battery housing with heat insulation properties

There is a very fast transition in the automotive industry to go from combustion engines to electrified vehicles. In the event of defects or accidents, battery fires can occur in electric vehicles. Battery fires quickly lead to very high temperatures, which are highly dangerous for the stability of the vehicle and for the occupants. Thermal insulation of the battery cells tries to maintain the stability of the vehicle for longer in the event of a battery fire and to give the vehicle occupants sufficient time to leave the vehicle.

Thermally insulated battery housings are known. For example WO2022128839 discloses an intumescent coating comprising at least one liquid epoxide resin, at least one special amine and at least one salt which releases phosphoric acid upon heating. The intumescent coating according to W02020128839 can be used as coatings in battery boxes of electric vehicles to protect vehicle as well as occupants from heat in case of a battery fire.

DE102020134277 discloses a battery housing for a traction battery of an electric vehicle comprising an intumescent coating. In a preferred embodiment of DE102020134277 the intumescent coating comprises expandable graphite.

A significant disadvantage of thermally insulating layers from the prior art is that the insulating effect is caused by a large increase in volume of the insulating layer. Such an increase in volume is only possible if sufficient space is left in the battery housing, which significantly increases the size of this component. Since the traction batteries in electric vehicles are already very large and the space for the occupants will be further limited due to further enlargement of the battery, this is undesirable. Since the insulating layer must be attached to the inside of the battery housing, contact with electrical conductors can lead to a short circuit, especially when using conductive fillers such as expanded graphite. Therefore, it is an object of the present invention to provide a thermally insulated battery housing that does not have these disadvantages and which in particular has a low electrical conductivity and requires only little space.

The object of the present invention has been solved by a battery housing comprising an structural layer and an inner layer wherein the inner layer comprises a polyurethane coating, the polyurethane coating is obtainable by mixing(a) one or more organic polyisocyanates, (b) one or more compounds having at least two isocyanate-reactive hydrogen atoms, comprising polyetherpolyol (b1), (c) one or more catalysts, (d) 30 to 90 wt.-% based on the total weight of components a) to e), of solid flame retardant, and optionally fillers and/or polyurethane additives, to give a reaction mixture and allow the reaction mixture to cure. The object of the present invention is further solved by a method for the production of a battery housing according to the invention and the use of such a battery housing for the production of a battery, preferably of a traction battery.

A battery housing according to the present invention is at least partially, preferably fully surrounding a number of battery cells and other components such as wires or connections. The battery housing may also be configured to provide protection from moisture, heat, cold, or any other potential factors that may cause damage to the electric cell. In a preferred embodiment the housing comprises a bottom part, a top part and a wall, extending between the bottom and the top. The top part may comprise an opening, covered by a cover. A battery housing according to the invention also comprises any elements protecting the surrounding of the battery from battery runaway reactions.

According to the invention the battery housing comprises at least partially an inner layer. The inner layer is connected to a structural layer. The structural layer may be construed of a metal or a polymeric compound, for example a fiber reinforced polymeric compound for example based on polyurethanes, epoxy resin or acrylate resin. The structural layer may also be construed of different materials, for example in the lower parts of the battery housing of a polymeric material and in the upper parts by a metal. Preferably the structural layer comprises a metal, preferably steel or aluminum. The structural layer might be at least partially covered by an additional material as a metal coating layer or a paint layer. In a preferred embodiment the structural layer of the battery housing is not in direct contact with the battery cells.

In a preferred embodiment at least the upper part of the battery housing, for example the cover of the battery housing, comprises the inner layer. In an especially preferred embodiment, the inner surface of the structural layer is at least 50 %, more preferably at least 80 % and especially preferred at least 100 % covered by the inner layer. There might be a material between inner and the structural layer, for example a conventional metal coating already applied on the structural layer, preferably the inner layer is in direct contact to the structural layer. In a preferred embodiment the thickness of the structural layer is from 0.1 to 10 mm, more preferred 0.3 to 5 mm and especially preferred 0.5 to 2.0 mm.

The inner layer comprises a polyurethane coating, preferably consists of a polyurethane coating, the polyurethane coating is obtainable by mixing (a) one or more organic polyisocyanates, (b) one or more compounds having at least two isocyanate-reactive hydrogen atoms, comprising polyetherpolyol (b1), (c) one or more catalysts, (d) 30 to 90 wt.-% based on the total weight of components a) to e), of solid flame retardant, and optionally fillers and/or polyurethane additives (e), to give a reaction mixture and allow the reaction mixture to cure. A polyurethane coating according to the present invention consists of a continuous polyurethane matrix material which might contain further materials.

According to a preferred embodiment of the present invention the inner layer is essentially free of expendable graphite. Essentially free of expendable graphite means that the inner layer comprises less than 0.1 % by weight, preferably less than 0.01 % by weight, based on the total weight of the inner layer of expandable graphene and especially preferred does not comprise any expandable graphene. Preferably the inner layer comprises less than 5 %, more preferably less than 1 %, of flame retardants comprising halogene atoms, each based on the total weight of compounds (a) to (e), and especially preferred is free of flame retardants comprising halogene atoms. Examples flame retardants comprising halogene atoms are chlorinated phosphates such as tris-(2-chloroethyl)-^,phosphate, tris-(2-chloropropyl)phosphate (TCPP), tris(1 ,3- dichloropropyl)phosphate, tricresyl phosphate, tris-(2,3-dibromopropyl)phosphate and tetrakis- (2-chloroethyl)-ethylene diphosphate In a preferred embodiment the thickness of the inner layer is from 0.1 to 3.5 mm, more preferred 0.5 to 2.0 mm and especially preferred 0.7 to 1.5 mm. Preferably the inner layer has electric resistance of more than 1 * 10⁶ Ohm*cm, more preferred more than1*10⁹ Ohm*cm and especially preferred more than 1*10¹° Ohm*cm measured according to EN 62631 -3-1. In a preferred embodiment the inner layer, after flame treatment at 600 to 1400 °C for 5 minutes has a thickness of 0.1 to 30 mm, more preferred 0.5 to 20 mm and especially preferred 1 to 10 mm.

In a preferred embodiment the polyurethane is a solid polyurethane having a density of preferably more than 850 kg/m³, preferably 1200 kg/m³ to 2500 kg/m³ and particularly preferably 1500 kg/m³ to 2200 kg/m³. A solid polyurethane is obtained without addition of a blowing agent. Small amounts of blowing agent, for example water, present in the polyols as a consequence of production are not to be understood in the present invention as constituting blowing agent addi- tion. The reaction mixture for producing the compact polyurethane preferably comprises less than 1 % by weight, particularly preferably less than 0.4% by weight and in particular less than 0.03% by weight, of water.

According to the present invention, the polyisocyanate components (a) used for the production of the polyurethanes of the invention comprise any of the polyisocyanates known for the production of polyurethanes. These comprise the aliphatic, cycloaliphatic, and aromatic difunctional or polyfunctional isocyanates known from the prior art, and also any desired mixtures thereof. Examples are diphenylmethane 2, 2’-, 2,4’-, and 4,4’-diisocyanate, the mixtures of monomeric diphenylmethane diisocyanates with diphenylmethane diisocyanate homologs having a larger number of rings (polymer MDI), isophorone diisocyanate (IPDI) and its oligomers, tolylene 2,4- and 2,6-diisocyanate (TDI), and mixtures of these, tetramethylene diisocyanate and its oligomers, hexamethylene diisocyanate (HDI) and its oligomers, naphthylene diisocyanate (NDI), and mixtures thereof.

In one preferred embodiment of the present invention the isocyanate (a) comprises tolylene 2,4- and/or 2,6-diisocynate (TDI) or a mixture thereof, monomeric diphenylmethane diisocyanates, and/or diphenylmethane diisocyanate homologs (polymer MDI), and mixtures of these. Other possible isocyanates are mentioned by way of example in "Polyurethanes Handbook”, Carl Hanser Verlag, 2^rd edition 1994, chapter 3.2 and 3.3.2.

In an especially preferred embodiment, the polyisocyanates (a) comprise at least one Isocyanate selected from the group consisting of monomeric MDI, polymeric MDI, MDI based prepolymers or mixtures of at least two of these. At least 80 %, preferably at least 90 %, and more preferred 100 % by weight of the isocyanates (a) consist of monomeric MDI, polymeric MDI, MDI based prepolymers or mixtures of at least two of these.

Polyisocyanate component (a) used can be used in form of polyisocyanate prepolymers. These polyisocyanates prepolymers are obtainable by reacting the polyisocyanates described above (constituent (a-1)) in excess, for example at temperatures of from 30 to 100°C, preferably at about 80°C, with polymeric compounds (b) (constituent (a-2)), having groups reactive toward isocyanates, and/or with chain extenders (c) (constituent (a-3)) to give the isocyanate prepolymer.

Polymeric compounds (a-2) having groups reactive toward isocyanates are known to the person skilled in the art and are described by way of example in "Polyurethanes Handbook", Carl Hanser Verlag, 2^rd edition 1994, chapter 3.1 : by way of example, it is also possible to use, as polymeric compounds (a-2) having groups reactive toward isocyanates, the polymeric compounds described under (b) having groups reactive toward isocyanates. Preferably the NCO- content of the isocyanate prepolymers is 5 to 30, more preferably 10 to 25.

In a more preferred embodiment of the present invention the isocyanate (a) comprises at least one aliphatic isocyanate or its isocyanate groups containing derivatives as prepolymers biu- rettes or isocyanurates. Examples are isophorone diisocyanate (IPDI) and its derivatives, hexamethylenediisocyanate and its derivatives and H12-MDI and its derivatives. Especially preferred are hexamethylenediisocyanates comprising isocyanurate groups. These hexamethylene diisocyanates which comprise isocyanurate groups preferably has an isocyanate content of 15 to 30 % by weight, more preferred 18 to 26 % by weight and especially preferred 20 to 24 % by weight. The content of monomeric hexamethylene diisocyanate preferably is below 5 % by weight, more preferred less than 1 % by weight and especially preferred less than 0.1 % by weight, each based on the wight of the hexamethylene diisocyanates which comprise isocyanurate groups.

Employable compounds having isocyanate-reactive hydrogen atoms (b) include all compounds having at least two isocyanate-reactive hydrogen atoms commonly used in the production of polyurethanes, for example those having a functionality of 1 to 8 and a number-average molecular weight of 62 to 15 000 g/mol, wherein the average functionality averaged over all polymeric compounds having isocyanate-reactive groups is at least 2. It is accordingly possible for example to employ compounds selected from the group of polyether polyols, polyester polyols, also referred to as polyether polyol or polyether alcohols or polyester polyol or polyester alcohols, chain extenders, cross linkers or mixtures thereof.

According to the present invention the compounds having isocyanate-reactive hydrogen atoms (b) comprises polyetherpolyol (b1). Polyetherols are by way of example produced from epoxides; for example, propylene oxide and/or ethylene oxide, or from tetrahydrofuran with starter compounds exhibiting hydrogen-activity containing 1 to 8, preferably 2 to 6 and more preferably 2 to 4 reactive hydrogen atoms bound, or a starter molecule mixture which contains 1 .5 to 8, preferably 1.8 to 6 and more preferably 1.9 to 3.5 reactive hydrogen atoms bound in the presence of catalysts. As starter molecules for example aliphatic alcohols, phenols, amines, carboxylic acids, water, or compounds based on natural substances, for example sucrose, sorbitol or mannitol can be applied. If mixtures of starter molecules with different functionalities are used, fractional functionalities can be obtained. Influences on the functionality, for example through side reactions, are not considered in the nominal functionality. Examples for suitable catalysts are basic catalysts and double-metal cyanide catalysts, as described by way of example in PCT/EP2005/010124, EP 90444, or WO 05/090440. Polyethers (b1) are preferably free of ester groups.

The polyether polyol (b1) is preferably obtainable by reacting at least one a starter molecule having a functionality of 2 to 4, more preferred 2 to 3 with alkylene oxides. In one preferred embodiment the polyetherpolyol (b1) is comprises a polyetherpolyol (b1 a) obtainable by reacting at least one starter molecule having a functionality of 3, with alkylene oxide wherein the alkylene oxides comprise preferably at least 50 mol-%, more preferred at least 85 mol-% ethylene oxide, and having a hydroxyl value of preferably more than 170 to 900 mg KOH/g, more preferred 200 to 700 mg KOH/g and especially preferred 400 to 650 mg KOH/g. In a more preferred embodiment, the polyetherpolyol (b1) comprises in addition to the polyetherpolyol (b1a) a polyetherpolyol (bi b) obtainable by reacting at least one starter molecule having a functionality of 3 with alkylene oxide wherein the alkylene oxides comprise preferably at least 50 mol-%, more preferred at least 70 mol-% and especially at least 80 mol-% propylene oxide, and having a hydroxyl value of preferably 20 to less than 170 mg KOH/g, more preferred 25 to 100 mg KOH/g and especially preferred 30 to 50 mg KOH/g. Preferably polyetherols (b1a) and (bi b) are used in a mass ratio of 5:1 to 1 :40, more preferred 1 :1 to 1 :12 and especially preferred 1 :3 to 1 :8.

Furthermore, component (b) may comprise chain extenders and/or crosslinking agents (b2), for the purpose, for example, of modifying the mechanical properties, such as the hardness or for modifying the viscosity of component (b). Chain extenders and/or crosslinking agents (b2) used are diols and/or triols and also amino alcohols having molecular weights of less than 280 g/mol, preferably 62 to 250 g/mol, more preferably 62 to 200 g/mol, more preferably still 62 to 150 g/mol, and more particularly from 60 to 130 g/mol. Examples contemplated include aliphatic, cycloaliphatic and/or araliphatic diols having 2 to 8, preferably 2 to 6, carbon atoms, such as ethylene glycol, 1 ,2-propylene glycol, diethylene glycol, dipropylene glycol, 1 ,3-propanediol, 1 ,4- butanediol, 1 ,6-hexanediol, o-, m-, and p-dihydroxycyclohexane, bis(2- hydroxyethyl)hydroquinone. Also contemplated are aliphatic and cycloaliphatic triols such as glycerol, trimethylolpropane, and 1 ,2,4- and 1 ,3,5-trihydroxycyclohexane.

Chain extenders and crosslinking agents are generally used to adapt the hardness of the polyurethane which is well known to a person skilled in the art. Preferably the hardness of the polyurethane coating according to the present invention is between shore A 60 and shore D 90, more preferred between shore D40 and shore D 80. Where chain extenders, crosslinking agents or mixtures thereof (b2) are employed in producing the rigid polyurethane foams, they are used usefully in an amount of 0 to 15 wt.-%, preferably of 1 to 5 wt.-%, based on the total weight of component (b) wherein the use of glycerol ond/or trimethylolpropane are most preferred. Other possible polyols are listed by way of example in "Kunststoffhandbuch, Band 7, Polyurethane” [Plastics Handbook, volume 7, Polyurethanes], Carl Hanser Verlag, 3rd edition 1993, chapter 3.1.

In a preferred embodiment of the present invention the compounds having at least two isocyanate-reactive hydrogen atoms (b) comprise less than 20 % by weight, more preferred less than 10 % by weight, even more preferred less than 5 % by weight and especially preferred 0 % by weight, each based on the total weight of the compounds (b), of compounds having ester structures. Compounds having ester structures comprise polyesters obtainable from the reaction of an acid component having at least 2 carboxyl groups and a polyalcohol having at least 2 alcohol products as well as the reaction product of esters such as polyesters or fatty esters such as triglycerides and epoxides.

In a preferred embodiment the viscosity at 25 °C of the compounds having isocyanate-reactive hydrogen atoms (b), is less than 400 Pas, more preferred from 5 to 250 Pas and especially preferred from 10 to 150 Pas, measured at shear rates of 10/s.

Catalysts (c) greatly accelerate the reaction of the polyols (b) with the polyisocyanates (a). As catalysts (c) any catalyst known in the field of polyurethane catalysts may be used. These comprise basic amine catalysts and metal-based catalysts. In a preferred embodiment the catalysts comprise incorporable amine catalysts. In a further preferred embodiment the catalysts comprise delayed action catalysts. Delayed action catalysts are well known in the art and provide a long open time of the reaction mixture at room temperature and a fast curing at elevated temperatures.

Incorporable amine catalysts have at least one, preferably from 1 to 8, and particularly preferably from 1 to 2, groups reactive toward isocyanates, for example primary amine groups, secondary amine groups, hydroxy groups, amides, or urea groups, preferably primary amine groups, secondary amine groups, or hydroxy groups. Incorporable amine catalysts are used mostly for the production of low-emission polyurethanes which are in particular used in the automobile-interior sector. These catalysts are known and are described by way of example in EP1888664. These comprise compounds which preferably comprise, alongside the group(s) reactive toward isocyanates, one or more tertiary amino groups. It is preferable that at least one tertiary amino groups of the incorporable catalysts bear at least two aliphatic hydrocarbon moie- ties, preferably having from 1 to 10 carbon atoms per moiety, particularly preferably having from 1 to 6 carbon atoms per moiety. It is particularly preferable that the tertiary amino groups bear two moieties selected mutually independently from methyl and ethyl moiety, and bear another organic moiety. Examples of incorporable catalysts that can be used are bisdimethylaminopropylurea, bis(N,N-dimethylaminoethoxyethyl) carbamate, dimethylaminopropylurea, N,N,N-trimethyl-N-hydroxyethylbis(aminopropyl ether), N,N,N-trimethyl-N- hydroxyethylbis(aminoethyl ether), diethylethanolamine, bis(N,N-dimethyl-3-aminopropyl)amine, dimethylaminopropylamine, 3-dimethyaminopropyl-N,N-dimethylpropane-1 ,3-diamine, dimethyl- 2-(2-aminoethoxyethanol), and (1 ,3-bis(dimethylamino)propan-2-ol), N,N-bis(3- dimethylaminopropyl)-N-isopropanolamine, bis(dimethylaminopropyl)-2-hydroxyethylamine, N,N,N-trimethyl-N-(3 aminopropyl)bis(aminoethyl ether), 3- dimethylaminoisopropyldiisopropanolamine, and mixtures thereof.

Examples for delayed action catalysts are carboxylic salt used of a conventional basic amine catalyst. The carboxylic salts of the basic amine catalysts for example are obtained here by mixing the amine catalysts with carboxylic acids, optionally in presence of an alcohol as ethylene glycol. If an alcohol which falls under the definition of a chain extender or a crosslinker the amount is considered when calculating the amount of crosslinker and chain extender in the reaction mixture.

Basic amine catalysts suitable for the production of delayed action catalysts are described by way of example in "Polyurethane Handbook”, Carl Hanser Verlag, 2^nd edition 1994, chapter 3.4.1. Examples of these are amidines, such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tertiary amines, such as triethylamine, tributylamine, dimethylbenzylamine, N-methyl-, N-ethyl-, N- cyclohexylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'- tetramethylbutanediamine, N,N,N',N'-tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, bis(dimethylaminopropyl)urea, N,N-bis(3- dimethylaminopropyl)-N-isopropanolamine, dimethylpiperazine, 1 ,2-dimethylimidazole, 1 - azabicyclo[3.3.0]octane and preferably 1 ,4-diazabicyclo[2.2.2]octane and alkanolamine compounds, such as triethanolamine, triisopropanolamine, N-methyl- and N-ethyldiethanolamine N,N-bis(3-dimethylaminopropyl)-N-isopropanolamine, and dimethylethanolamine. Basic amine catalysts which have at least one, preferably precisely one, group reactive toward isocyanates are in particular used here, an example being N,N-bis(3-dimethylaminopropyl)-N- isopropanolamine. The catalysts can be used individually or in the form of mixtures.

Carboxylic acids used are preferably those whose molar mass is smaller than 300 g/mol. It is particularly preferable here to use saturated and unsaturated aliphatic monocarboxylic acids having from 1 to 18 carbon atoms, e.g. formic acid, acetic acid, cyanoacetic acid, or 2- ethylhexanoic acid, aromatic carboxylic acids, aliphatic, saturated and unsaturated dicarboxylic acids having from 2 to 16 carbon atoms, or tricarboxylic acids, or a mixture thereof. Derivatives of the abovementioned carboxylic acids can also be used. Other preferred carboxylic acids used are dicarboxylic acids of the general formula HOOC-(CH₂)n-COOH, where n is a whole number from 2 to 14. Dicarboxylic acids of this type are generally less corrosive. In particular, the carboxylic acid used comprises adipic acid.

The ratio of acid and amine catalyst here is selected in such a way that the number of equivalents of acid groups of a carboxylic acid comprised is from 0.5 to 1.5, preferably from 0.7 to 1.3, particularly preferably from 0.90 to 1.10, and in particular from 0.95 to 1.05 equivalents, based on one equivalent of amine of the amine catalyst.

An example of a concentration that can be used of the carboxylic salts of an amine catalyst (c) is from 0.001 to 10% by weight, preferably from 0.05 to 5% by weight, and particularly preferably from 0.05 to 2% by weight, based on the weight of components (b) to (e).

Conventional, non-incorporable amine catalysts may comprise amidines, such as 2,3-dimethyl- 3,4,5,6-tetrahydropyrimidine, tertiary amines, such as triethylamine, tributylamine, dimethylbenzylamine, N-methyl-, N-ethyl-, and N-cyclohexylmorpholine, N,N,N',N'- tetramethylethylenediamine, N,N,N',N'-tetramethylbutanediamine, N,N,N',N'- tetramethylhexanediamine, pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether, bis(dimethylaminopropyl)urea, dimethylpiperazine, 1 ,2-dimethylimidazole, 1-azabicyclo[3.3.0]octane, and preferably 1 ,4-diazabicyclo[2.2.2]octane, and alkanolamine compounds, such as triethanolamine, triisopropanolamine, N-methyl- and N-ethyldiethanolamine, and dimethylethanolamine.

Suitable metal based catalysts comprise organometallic compounds, preferably organotin compounds, such as tin(ll) salts of organic carboxylic acids, e.g. tin(ll) acetate, tin(ll) octoate, tin(ll) ethylhexoate, and tin(ll) laurate, and the dialkyltin(IV) salts of organic carboxylic acids, e.g. dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate, and dioctyltin diacetate, and also bismuth carboxylates, such as bismuth(lll) neodecanoate, bismuth 2-ethylhexanoate, and bismuth octanoate, or a mixture thereof. The organometallic compounds can be used alone or preferably in combination with strongly basic amines. In a particularly preferred embodiment, catalysts (e) used comprise or consist of delayed action catalysts and especially preferred incorporable delayed action catalysts. If catalysts (c) are used, these can by way of example be used at a concentration of from 0.001 to 5% by weight, in particular from 0.05 to 2% by weight, as catalyst or, respectively, catalyst combination, based on the weight of component (b).

According to the invention, the reaction mixture comprises 30 to 90 wt.-%, preferably 45 to 80 wt.-% and especially preferred 60 to 75 wt.-% of solid flame retardant. Solid flame retardants (d) are solid at 25 °C. Examples of solid flame retardants are inorganic or organic solid flame retardants such as red phosphorous, red phosphorous containing compositions, antimony trioxide, aluminum trihydroxide (ATH), layered silicate, boron nitride, arsenic oxide, ammonium polyphosphate, phosphinic acid salts as phosphinic acid diethylaluminum salt, titan dioxide, barium sulphate, calcium sulphate, calcium carbonate, glass hollow spheres, aerogels, mica, layered silicates, or cyanuric acid derivatives as melamine or mixtures from at least two of these solid flame retardants. In one preferred embodiment the solid flame retardant comprises more than 50 % by weight more preferred more than 80 % by weight and especially preferred more than 90 % by weight of aluminum trihydroxide, each based on the total weight of the solid flame retardant (d). Most preferred, the solid flame retardant (d) consists of aluminum trihydroxide.

In another, more preferred embodiment, the solid flame retardants (d) comprise at least two solid flame retardants selected from the group, consisting of aluminum hydroxide, amonium polyphosphate and titan dioxide. A combination comprising aluminum hydroxide and ammonium polyphosphate is preferred. In a more preferred embodiment, the solid flame retardants (d) comprise at least aluminum hydroxide, ammonium polyphosphate and titan dioxide. If aluminum hydroxide, ammonium polyphosphate and titan dioxide are contained in the flame retardant (d), compound (d) comprises preferably 5 to 90% by weight of aluminum hydroxide, 5 to 90% by weight of ammonium polyphosphate and 1 to 40% by weight of titan dioxide, more preferred 20 to 50% by weight of aluminum hydroxide, 20 to 50 % by weight of ammonium polyphosphate and 5 to 20% by weight of titan dioxide, each based on the total weight of aluminum hydroxide, ammonium polyphosphate and titan dioxide. Preferably the total amount of solid aluminum hydroxide, ammonium polyphosphate and titan dioxide, based on the total amount of solid flame retardants (d) is at least 80 % by weight, more preferred at least 90 % by weight and especially preferred compound (d) consists of aluminum hydroxide, ammonium polyphosphate and titan dioxide.

Preferably the solid flame retardant (d) has an average particle size D₅o of less than 150 pm, more preferred of less than 100 pm. In an especially preferred embodiment of the present invention the flame retardant comprises at least one flame retardant having a particle size of less than 1 pm. Preferably the solid flame retardant has a bimodal, trimodal or multimodal particle size distribution. In a preferred embodiment the size distribution of the component (d) is bimodal or trimodal to allow a dense packing of the filler in the binder matrix.

To improve compatibility and dispersibility of the flame retardant (d) in the reaction mixture, the flame retardant (d) might be partly or fully surface modified. The surface modification preferably is obtained by an alkyl-silane treatment. Such surface modified flame retardant, for example surface modified aluminum tri-hydroxide, is known and for example disclosed in WO9932554. Preferably the surface modification can be obtained by reacting a silicon compound and a solid flame retardant, for example aluminum trihydroxide. Preferably the silane content of the solid flame retardant is in the range of 0.01 to 0.5 parts by weight, more preferred 0.05 to 0.4 parts by weight, based on the total weight of the solid flame retardant (d). In a preferred embodiment the silane molecule of the surface modified solid flame retardant does not comprise isocyanate reactive groups, i.e. hydroxyl groups are coordinated and not available for a reaction with isocyanate groups.

The term “particles” in connection with the solid flame retardant (d) of the invention relates to solid flame retardant having a particular particle size DX, based on a particle size distribution where X % of the particles have a diameter less than the DX, value. The D50 particle size is the median value of the particle size distribution. According to the present invention, the D90 value relates to the numerical distribution, where 90 % of the total number of particles has a smaller diameter. Particle sizes, such as D10, D50 and D90 values and particle size distributions of powders and powdery materials can be measured, using a wide variety of measurement methods known per se to the person skilled in the art, for example via sieve analyses according to DIN 66165-2:2016-08, sedimentation or light scattering, e.g. laser diffraction in accordance with DIN ISO 13321 :2004-10. Particle size can be measured by dispersing the powder in a suitable solvent and to perform laser diffraction in accordance with ISO 13320:2009 or dynamic light scattering in accordance with ISO 22412:2008.

The particle size distribution can be reported as intensity distribution, volume distribution, surface distribution or numerical distribution. In the present case, given particle sizes of the fillers are determined by dispersing the powder in 2-isopropanol using laser diffraction in accordance with ISO 13320:2009.

Fillers and/or polyurethane additives (e) used can comprise any of the additives known for producing polyurethanes. Examples that may be mentioned are surfactant substances, release agents, coupling agents, fillers, dyes, pigments, liquid flame retardants, hydrolysis stabilizers, viscosity reducers, water scavengers, antifoaming agents, and substances having fungistatic and bacteriostatic action. Substances of this type are known and are described by way of example in " Polyurethane Handbook, 2^nd edition, Hanser Publishers, 1993, chapter 3.4.4 and 3.4.6 to 3.4.11.

Examples of liquid flame retardants are those commercially used in polyurethane chemistry.

Preferred liquid flame retardants are TCPP, TEP, DEEP, DMPP, DPK, PHT4-Diol™, brominated ethers and tribromo neopenthylalcohol, more preferred TCPP, TEP and PHT4-Diol™ and especially TCPP. In a preferred embodiment of the present invention in addition to the solid flame retardants liquid flame reatrdants are added to the reaction mixture. If added, liquid flame retardants are preferably added in an amount of 0.5 to 10 % by weight, preferably 1 to 10 % by weight and especially preferred 2 to 6 % by weight, each based on the total weight of components (b) to (d).

Examples of additives that can be used for water adsorption are therefore aluminosilicates, selected from the group of the sodium aluminosilicates, potassium aluminosilicates, calcium silicates, cesium aluminosilicates, barium aluminosilicates, magnesium aluminosilicates, strontium aluminosilicates, sodium aluminophosphates, potassium aluminophosphates, calcium alumino- phosphates, and mixtures thereof. It is particularly preferable to use mixtures of sodium aluminosilicates, potassium aluminosilicates, and calcium aluminosilicates in castor oil as carrier substance.

The number-average particle size of the water-absorption additive is preferably not greater than 200 pm, particularly preferably not greater than 150 pm, and in particular not greater than 100 pm. The pore width of the water-absorption additive of the invention is preferably from 2 to 5 Angstroem. If a water-absorption additive is added, the amounts here are preferably greater than one part by weight, particularly preferably in the range from 0.5 to 5 parts by weight, based on the total weight of components (b) to (d).

Coupling agents that can be used comprise silanes, such as isocyanate silanes, epoxysilanes, or aminosilanes. Substances of this type are described by way of example in E. P. Pluedde- mann, Silane Coupling Agents, 2nd ed., Plenum Press, New York, 1991 and in K. L. Mittal, ed., Silanes and Other Coupling Agents, VSP, Utrecht, 1992.

Examples of viscosity reducers that can be used are y-butyrolactone, propylene carbonate, and also reactive diluents, such as dipropylene glycol, diethylene glycol, and tripropylene glycol. In addition, compound (e) may comprise polymers, for example polyacrylates. In a preferred embodiment the content of polymers, based on the total weight of compounds (a) to (e), is less than 30 % by weight, preferably less than 10 % by weight and especially preferred 0 to 3 % by weight.

To produce the polyurethane coating according to the present invention, the components (a) to (e) are mixed to form a reaction mixture. In a preferred embodiment of the present invention an Isocyanate component (A) comprising one or more organic polyisocyanates (a) and solid flame retardant (d) and a polyol component (B) comprising one or more compounds having at least two isocyanate-reactive hydrogen atoms (b), one or more catalysts and of solid flame retardant (d) are formed. Fillers and additives (e) can be added to either component but are preferably added to the polyol component (B). Subsequently, the polyisocyanate component (A) and the polyol component (B) are mixed to produce the reaction mixture. Solid flame retardant (d) can be added to the isocyanate component (A), the polyol component (B) or in a preferred embodiment to the isocyanate component and to the polyol component. In this embodiment the amount of the solid flame retardant in the component (A) and component (B) is chosen that the viscosity of the isocyanate component (A) and the polyol component (B) at 25 °C is in the range of less than 300 Pas, more preferred from 5 to 250 Pas and especially preferred from 10 to 150 Pas, measured at shear rates of 10/s.

This two-component process has proven preferable in practice. In the context of the present invention, a reaction mixture refers to the mixture at reaction conversions of less than 90%, based on the isocyanate groups. Preferably the reaction mixture is sprayed or casted onto the structural layer to produce the inner layer. Preferably the mixing ratio is chosen as such that the isocyanate index is from 80 to 400. More preferred is an isocyanate index in the range of 85 to 130, preferably 90 to 120 and especially preferred from 95 to 110. The isocyanate index is the molar ratio of isocyanate groups to groups reactive with isocyanate groups, multiplied by 100.

It was observed, that the higher the network density of the network density of the polyurethane coating according to the invention is, the higher is its thermal resistance. Network density can for example be increased by use of higher functional polyols, for example polyols having a functionality in the range of 3 to 8, preferably 4 to 6 and crosslinkers. In addition, crosslinking density of the polyurethane coating can be increased by formation of isocyanurate rings. It is well known to a person skilled in the art that Isocyanurate rings may be formed at an excess of isocyanate groups over isocyanate reactive groups and the addition of an isocyanurate reaction catalyzing catalyst as for example sodium or potassium carboxylates. The polyurethane coating of the inner layer may be covered partly or in total by at least one flame protective cover material known for flame protective properties and low thermal conductivity.

This results in a layer structure of the battery housing, with the structural layer in the direction of the battery cells is first followed by the polyurethane coating layer and then the cover layer. Preferably, the polyurethane coating is directly attached to the structural layer and the cover layer directly to the polyurethane coating without the use of an additional adhesive layer. The cover layer in the direction of the battery cells is particularly preferred, the final layer.

Such flame protective cover materials are known in the art and are commercially available. The Thickness of the cover material is not limited. Preferably the thickness is in the range of 0.05 to 10 mm, more preferred 0,1 to 5 mm and especially preferred 0.2 to 2 mm. A preferred example of a cover material of the polyurethane coating is a layer of mica, especially a mica composite. Mica is a group of minerals from the division of phyllosilicates. Preferably mica has a layered structure. Mica material has the advantages of high electrical insulation, large dielectric constant, low loss, high dielectric strength and high chemical stability.

Flame protective mica composite layers can be produced by applying mica particles to a carrier such as a fiber mesh, preferably a glass fiber mash for example by using an adhesive such as a silicon adhesive.

If cover material is added onto the polyurethane coating as long as the polyurethane coating is not fully cured, the polyurethane coating can serve as adhesive for the cover material to adhere the cover material to the structural layer.

By covering the polyurethane coating, the resulting temperature during flame treatment, measured in the backside of the structural layer, can be significantly lowered compared to the polyurethane coating without cover material and adhesion property of the polyurethane coating both to the structural layer and to the cover layer is good before and during/after flame treatment. Thus, thermal and electrical isolation together with very good adhesion forces of the polyurethane coating are combined with the know performance of flame protective cover material. Thereby, high performance even against high energetic cells is achieved.

The battery housing according to the present invention may contain several battery cells as for example in traction batteries as used in e-mobility of vehicles. Such batteries comprising a battery housing according to the invention are a further embodiment of the present invention. In a preferred embodiment the electric cells within the battery housing are potted into a potting material, preferably a polyurethane foam. Such potting is well known and disclosed for example in WO 2019161292, WO 2020044744 or WO 2022188050. When battery housing according to the present invention is at least partially potted into a potting material in the event of battery fires the temperature increase is further delayed allowing additional time to leave the vehicle. In a preferred embodiment of the present invention the backside temperature after flame treatment of 600 to 1400 °C, preferably 800°C, for 5 minutes of a front side coated metal plate, for example a steel plate or aluminum plate, preferably a KTL coated steel plate, having a thickness of 0.5 to 3 mm, preferably 1 mm, and coated with a polyurethane coating according to the present invention at the frontside, wherein the coating has a thickness of 0.3 to 3 mm, preferably 1 .5 mm, is less than 400 °C, preferably less than 300 °C and especially preferred less than 280 °C. In addition, during flame treatment, the polyurethane coating according to the present invention preferably does not melt or delaminates from the structural layer nor produces gases. Further, after heat treatment with a ceramic heat source of 500-800°C, preferably 600 °C, for 15 minutes of the same front side coated metal plate, the coated metal plate preferably shows a backside temperature of less than 270 °C, more preferred of less than 210°C and especially preferred of less than 170°C. In addition, the polyurethane coating according to the present invention preferably is resistant to particle yet over at least 1 min, with 5g/s jet and about 95 pm particle diameter of aluminum oxide particles, no damage on polyurethane side of substrate is visible.

The present invention is illustrated below with examples:

The following substances were used to produce the examples:

Polyol 1 : Polyetherester based on castor oil having a functionality of 3.5 and a hydroxy number 173 mg KOH/g, viscosity at 25 °C 3000 mPas

Polyol 2: Polyetherester based on castor oil having a functionality of 2.1 and a hydroxy number 227 mg KOH/g

Polyol 3: polyetherol starting from trimethylolpropane as starter molecule and propylene oxide with hydroxy number 860 mg KOH/g

Polyol 4: polyetherol starting from glycol as starter molecule and ethylene oxide and propylene oxide with hydroxy number 35 mg KOH/g

Polyol 5: polyetherol starting from trimethylolpropane as starter molecule and ethylene oxide with hydroxy number 605 mg KOH/g

Polyol 6: polyetherol starting from glycol as starter molecule and ethylene oxide and propylene oxide with hydroxy number 26 mg KOH/g Polyol 7: polyetherol starting from propylene glycol as starter molecule and propylene oxide with hydroxy number 100 mg KOH/g

Polyol 8: polyetherol starting from glycol as starter molecule and ethylene oxide and propylene oxide with hydroxy number 55 mg KOH/g

Polyol 9: polyetherol starting from glycol as starter molecule and ethylene oxide with hydroxy number 535 mg KOH/g

Polyol 10: polyetherol starting from glycol as starter molecule and ethylene oxide with hydroxy number 805 mg KOH/g

Polyol 11 : mixture of 85.3 parts by weight of polyol 4, 12 parts by weight of polyol 5 and 2.7 parts by weight of chain extender

Polyol 12: polyetherol started from glycol as starter molecule and ethylene oxide and propylene oxide with hydroxy number 170 mg KOH/g

Polyol 13: polyetherol started from glycol as starter molecule and ethylene oxide a with hydroxy number of 570 mg KOH/g

Polyol 14: polyesterol based on adipic acid with a hydroxy number of 55 mg KOH/g

Polyol 15: polyesterol based on adipic acid with a hydroxy number of 60 mg KOH/g

Polyol 16: polyesterol based on therephthalic acid with a hydroxy number of 242 mg KOH/g

Chain extender 1 : 1 ,4-butanediol

Chain extender 2: diethyltoluoldiamine

Crosslinker: glycerol

Liquid flame retardants:

Liquid Flame retardant 1 (LFR1): triethyl phosphate (TEP)

Liquid Flame retardant 1 (LFR2): liquid brominated polyetherpolyol Ixol® B251 obtained from

Solvay

Solid Flame retardants

Solid Flame retardant 1 (SFR1): aluminum trihydroxide, D50=18pm

Solid Flame retardant 2 (SFR2): Ammoniumpolyphosphate, D50=17pm

Solid Flame retardant 3 (SFR3): layered silicate (Mica)

Solid Flame retardant 4 (SFR4): barium sulfate

Solid Flame retardant 5 (SFR5): boron nitride

Solid Flame retardant 6 (SFR6): Hexaboro dizinc undecaoxide

Solid Flame retardant 7 (SFR7): Phosphinic acid, diethyl- aluminium salt

Solid Flame retardant 8 (SFR8): Phosphoric acid-ethylenglycol-polyester

Solid Flame retardant 9 (SFR9): melamine Solid Flame retardant 10 (SFR10): titanium dioxide

Solid Flame retardant 11 (SFR11): Calcium carbonate

Solid Flame retardant 12 (SFR12): glass hollow spheres

Solid Flame retardant 13 (SFR13): aerogel, D50=1-15 urn, hydrophobic Solid Flame retardant 14 (SFR14): aerogel, D50=1-100 urn, hydrophobic

Water adsorbant: K-Ca-Na-zeolite A in castor oil

Drying agent: isocyanate based drying agent for isocyanate component

Surfactant 1 : silicone surfactant, Xiameter ACP 1000 antifoam Dispersant: Byk W 980 (Dispersing agent)

Catalyst 1 : tin catalyst

Catalyst 2: tertiary amine catalyst

Catalyst 3: triethylene diamine

Catalyst 4: Imidazol-based catalyst

Isocyanate:

Isol : Methylendiisocyanate having an NCO-content of 26.2%

Iso2: Hexamethylene diisocyanate isocyanurate having an NCO-content of 22 % by weight.

Production of the coated plates

The polyols as indicated in table 1 were prepared in a premix and were then mixed with the flame retardants. The mixing was done at 1600 rpm for 10 min under vacuum. The A- component and B-component according to table 1 , table 2, and table 3, were mixed at room temperature for 45 sec at 1600 rpm under vacuum. The resulting material was poured onto a metal sheet (25x25x0.1 cm) and was distributed evenly via knife coating technique. The coated metal plates were cured at room temperature over three days before tested in internal burning test. The resulting coating thickness was 0.7-0.8 mm.

Burning test:

The coated metal sheets were tested in a burning test. In this test, the coated metal sheets were placed vertically in the set up. The burner was placed in front of the coated side and the front side temperature was adjusted to 800 °C via the distance between the surface of the coated metal sheet and the burner and the sheet was flame treated for 15 minutes. The temperature was measured in the middle of the sheet, 2mm in front of the coating and at the same location on the backside of the sheet in direct contact with the metal sheet. The backside temperature was recorded over the duration of experiment and the highest temperature was evaluated. To evaluate the melting of the coating grades 1 to 6 were given wherein the grades have the following meaning:

1 : No melting

2: One drop would run down in 15 min from the burning spot. Coating in tact

3: On the whole coating bubbles from delamination due to melting can be observed.

4: Material shows delamination over the whole surface.

5: Material shows delamination over the whole surface and shows sagging.

6: Material running down the substrate, heavy dropping from the substrate, along with delamination and sagging.

To evaluate the flaming of the coating grades 1 to 6 were given wherein the grades have the following meaning:

1 : No flame over the whole test duration

2: Very small but visible flames might occur now and then (<3 sec., <2 cm)

3: Larger flames can be observed at the beginning of the test but reduce with time.

4: After a couple of minutes the material starts to burn in large flames.

5: The major part of the coating is burned in large flames from the beginning of the test.

6: The whole coating is burned in large flames, delaminates and drops from the substrate.

To evaluate the delamination of the coating grades 1 to 6 were given wherein the grades have the following meaning:

1 : No delamination over whole test duration

2: The ash might be brittle and some dust can be observed below the set up. Or some (< 10% of the area) non burned part of the coating is delaminated from the substrate.

3: A small part of the burned area is delaminated (< 10%). Either still intact as a coating or found as brittle material under the test setup.

4: Part of the burned area is delaminated (>50%)

5: Coating is delaminating at more than 30% of the total area.

6: Coating delaminated at more than 50% of the total area

To evaluate the gas formation of the coating grades 1 to 6 were given wherein the grades have the following meaning: 1 : No gas can be observed or smelled.

2: Gas can be noticed to be released from the material at some point in the test, certain odor perceptible.

3: Gas can be seen to be released from the material over test duration. Clearly perceptible smell.

4: Constant gas stream released from the material over test duration. Intense smell.

5: Heavy gas stream released from the material over test duration. Intense smell.

6: Heavy gas emission that can be seen and biting smell. Test needs to be aborted. Some parameters are more essential than others. From most important to less important: Flaming, melting, delamination, expansion.

In table 1 the flame retardant was varied while in table 2 the polyol component was varied. In table 3 the isocyanate was varied. The expansion of the coating was measured in the following way: Measurement of the final coating thickness after burning test.

The amount of catalyst was adjusted to result in a tack free time of less than 15 minutes.

able 1

atalyst 1 0.4 atalyst 2 0.1 olorant 5

so 1 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

Viscosity measured at 60 °C and a shear rate of 20 1/s. able 2

Table 3:

The present examples show that coatings based on polyetherols result in improved insulation properties as shown by a reduced backside temperature. Lower viscous polyesters as fatty es- ter based polyesters show very bad insulation properties while other polyesters have such high viscosities that it is difficult to disperse sufficient amounts of solid flame retardant.

From table 1 it can be observed that especially the combination of at least two solid flame retardants selected from the group, consisting of aluminum hydroxide, ammonium polyphosphate and titan dioxide result in improved performance. In case that all three solid flame retardants are used, the performance is further improved.

Claims

1. Battery housing comprising a structural layer and an inner layer wherein the inner layer comprises a polyurethane coating, the polyurethane coating is obtainable by mixing a) one or more organic polyisocyanates, b) one or more compounds having at least two isocyanate-reactive hydrogen atoms, comprising polyetherpolyol (b1) c) one or more catalysts, d) 30 to 90 wt.-% based on the total weight of components a) to e), of solid flame retardants, and e) optionally fillers and/or polyurethane additives to give a reaction mixture and allow the reaction mixture to cure.

2. Battery housing according to claim 1 , characterized in that the at least one polyether polyol (b1) is obtainable by reacting at least one a starter molecule having a functionality of 2 to 4, more preferred 2 to 3.

3. Battery housing according to claim 1 or 2, characterized in that the polyetherpolyol (b1) comprises a polyetherpolyol (b1a) obtainable by reacting at least one starter molecule having a functionality of 3, with alkylene oxide wherein the alkylene oxides and having a hydroxyl value of more than 150 to 800 mg KOH/g and at least one polyetherpolyol (bib) obtainable by reacting at least one starter molecule having a functionality of 3 with alkylene oxide having a hydroxyl value of 20 to less than 150 mg KOH/g.

4. Battery housing according to any of claims 1 to 3, characterized in that that the solid flame retardants (d) comprise at least two solid flame retardants selected from the group, consisting of aluminum hydroxide, ammonium polyphosphate and titan dioxide.

5. Battery housing according to claim 4, characterized in that the solid flame retardants (d) comprise at least aluminum hydroxide, ammonium polyphosphate and titan dioxide.

6. Battery housing according to claim 5, characterized in that the solid flame retardants (d) comprise 5 to 90% by weight of aluminum hydroxide, 5 to 90% by weight of ammonium polyphosphate and 1 to 40% by weight of titan dioxide, each based on the total weight of aluminum hydroxide, ammonium polyphosphate and titan dioxide.

7. Battery housing according to any of claims 4 to 6, characterized in that the total amount of solid aluminum hydroxide, ammonium polyphosphate and titan dioxide, based on the total amount of solid flame retardants (d) is at least 70 % by weight.

8. Battery housing according to any of claims 1 to 7, characterized in that the polyurethane additives (e) comprise liquid flame retardant.

9. Battery housing according to any of claims 1 to 8, characterized in that the isocyanate (a) comprises hexamethylene diisocyanate wherein the hexamethylene diisocyanate comprises isocyanurate groups.

10. Battery housing according any of claims 1 to 9, characterized in that components (a) to (e) are mixed at an isocyanate index of 80 to 400.

11 . Battery housing according to any of claims 1 to 10, characterized in that the inner layer has a thickness of 0.1 to 3.5 mm.

12. Battery housing according to any of claims 1 to 11 , characterized in that the inner layer has electric resistance of more than 1*10⁶ Ohm*cm.

13. Battery housing according to any of claims 1 to 12, characterized in that the inner layer, after flame treatment at 800 °C for 15 minutes has a thickness of 0.1 to 30 mm.

14. Battery housing according to any of claims 1 to 13, characterized in that the polyurethane material is at least partially covered by at least one flame protective cover material.

15. Battery, comprising a battery housing according to claim 14..

16. Method for the production of a battery housing as claimed in claim 1 to 13, the battery housing comprising a structural layer and an inner layer wherein the inner layer comprises a polyurethane coating, the method comprising the steps of mixing

(a) one or more organic polyisocyanates,

(b) one or more compounds having at least two isocyanate-reactive hydrogen atoms, comprising polyetherpolyol (b1),

(c) one or more catalysts,

(d) 30 to 90 wt.-% based on the total weight of components a) to e), of solid flame retardant, and

(e) optionally fillers and/or polyurethane additives to give a reaction mixture and applying the reaction mixture onto the structural layer and allowing the reaction mixture to cure.