Heat resistant semi-rigid polyurethane foams

Abstract

Flame and heat resistant polyurethane foam compositions for forming semi-rigid foams well suited for use as thermal and electrical insulation for batteries, especially Li-ion batteries, the foams formed therefrom, and the devices and components containing such foams.

Description

1321-331 (SUPDIS225)HEAT RESISTANT SEMI-RIGID POLYURETHANE FOAMSFIELD OF THE INVENTION

[0001] The invention relates to polyurethane foams having suitable flame and heat resistance, as well as suitable mechanical properties, for use as thermo and electrical insulation, as well as cushioning, for batteries and battery cells, including lithium ion batteries and cells.BACKGROUND OF THE INVENTION

[0002] Electrochemical chemical energy devices such as rechargeable batteries. including lithium ion (Li-ion) batteries and housings for these batteries, are becoming very important due to the growth and popularity of electric vehicles (EVs) and other battery intensive technologies. Many times, such as in the case of EVs, multi-cell batteries are used. Such batteries employ multiple electrochemical cells connected in series and parallel arrays in a battery pack. Pouch cells having high energy density are also becoming popular. Unfortunately, such cell systems require thermal management to protect against what is known as thermal runaway, whereby the overheating of one cell can induce a thermal runaway propagation reaction in adjacent cells with the potential to cause a serial cascading effect that can ignite the entire battery.

[0003] In view thereof, systems have been devised to assist with the prevention and / or delaying of thermal runaway in batteries. Silicone foam and high-density polyurethane (HDPU) based foams have been used in these applications due to their advantages in vibration management, crunch protection and ability to accommodate the breathing and swelling of battery structures during charging and discharging of cycles. To meet flame resistance (FR) requirements and to manage thermal runaway conditions, flame retardants are often incorporated with these foams. For example, International Publication Number WO2022 / 155056A1,discloses a thermally insulating multilayer sheet for preventing thermal runaway which includes a nonporous elastomeric barrier layer having a first and second opposed surface, a flexible foam layer disposed on the first surface and a flame retardant disbursed within the flexible foam.

[0004] International Publication Number WO2022 / 141035A1 provides a process for thermally insulating adjacent battery cells with a self- levelling non- syntactic silicone foam composition containing a flame retardant filling the voids between adjacent battery cells.

[0005] International Publication Number W02024 / 073350A1 discloses potting battery electric cells with a polyurethane foam potting composition containing a flame retardant.

[0006] While high density polyurethane (HDPU) flexible foams and silicone foams have been used with battery cells and pouch cells, such as lithium ion cells, to prevent or control thermally initiated events, such as thermal runaway, for various reasons, these materials have not been entirely acceptable to fully protect the cells inside the battery unit or housing from potential thermal runaway situations. For example, flexible HDPU’s can only satisfy some minimal heat restrictive standards, not enough to fully protect cells under potential thermal runaway situations. While silicones are somewhat better in this regard, they are considerably more costly than HDPU’s.

[0007] Accordingly, it is desirable to provide new polyurethane foams and structures made from those foams that overcome drawbacks and inadequacies of the prior art.SUMMARY OF THE INVENTION

[0008] Accordingly, it is one object of the present invention to provide polyurethane foams having improved and exceptional mechanical and thermal insulating properties for use with batteries, particularly Li-ion batteries.

[0009] It is another object of the present invention to provide polyurethane foams having excellent mechanical strength, excellent cushioning properties providing resistance to shock and vibration and providing exceptional heat resistance when used with battery cells contained in battery packs and battery modules.

[0010] A further object of the present invention is to provide polyurethane foams having exceptional heat resistance and flame resistance for use with battery cells, such as Li-ion cells, to prevent or delay thermal runaway situations.

[0011] These and other objects are achieved herein by providing a semi-rigid polyurethane foam forming composition (and the foams formed therefrom) comprising: a) at least one polyol having a molecular weight of at least about 1000; b) at least one polyisocyanate wherein the polyol and isocyanate are proportioned to form a foam having an isocyanate index of at least about 150; c) at least one blowing agent; d) at least one catalyst for the polyurethane-forming reaction; e) optionally, at least one flame retardant comprising expandable graphite.BRIEF DESCRIPTION OF THE DRAWING

[0012] For a fuller understanding of the invention, reference is had to the following description, taken in connection with the accompanying drawing, in which:

[0013] Fig. 1 is a burn measurement graph of temperature with time of foam samples illustrating the advantages of the present invention.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present disclosure may be understood more readily by reference to the following detailed description. It is to be understood that this disclosure is not limited to thespecific materials, methods, conditions or parameters described and / or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed disclosure.

[0015] Also, as used in the specification and including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” or “approximately” one particular value and / or to “about” or “approximately” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment.

[0016] In accordance with the present invention it has been surprisingly discovered that reacting a (i) a high molecular weight polyol component, generally associated with making flexible polyurethane PUR foams, with (ii) an isocyanate component wherein the isocyanate component and polyol are proportioned in amounts generally associated with PIR formulations for making rigid PIR polyurethane foams, will lead to the production of semi-rigid (semiflexible) polyurethane foams which provide thermal insulation and are thermally stable and suitable for use in preventing or controlling thermal runaway situations which may occur with battery cells, such as Li-ion cells, as well as providing excellent cushioning between the cells. Moreover, it has been surprisingly discovered that adding an expandable graphite flame retardant to the semi-rigid foam composition significantly increases the heat resistance of the polyurethane foam and extends the time before the foams become heated to critical temperatures, such as which occur during thermal runaway situations.

[0017] The general method steps for making polyurethane foams are well known in the art. Process for making polyurethane foam are described, for example, in U.S. Pat. Nos.8,058, 322 and 10,730,995, the contents of which are incorporated herein in their entirety. Generally, the production of flexible or rigid polyurethane foams involves basic chemical reactions between isocyanate groups and hydroxyl groups. However, each type of foam presents different issues to the manufacturer. Many of these problems can be addressed, at least partially, by appropriate choice of auxiliary agents, for example catalysts, surfactants, foam stabilizers and the like.Polyol Component

[0018] The semi-rigid foams in accordance with the present invention are preferably formed with polyols commonly used in the production of flexible polyurethane foams such as polyether polyols, polyester polyols and polymer polyols. Such polyols are typically long-chain polyols compared with the short-chain polyols used in rigid PIR formulations. For the purposes of this invention, these polyols preferably have a number average molecular weight of from about 1000 to about 10,000, preferably from about 1000 to about 5000, and more preferably from about 2000 to about 5000, preferably about 3000.

[0019] Examples of polyether polyols for the purposes of the present invention can include those with a hydroxyl value of from about 25 to about 70 KOH mg / g, which arc obtained by the random or block addition of alkylene oxides such as ethylene oxide and propylene oxide to polyfunctional polyols, amine compounds, and the like. Examples of polyfunctional polyols include glycols such as ethylene glycol and propylene glycol; triols such as glycerol and trimethylolpropane; polyols such as pentaerythritol, sorbitol and sucrose. Examples of aminecompounds include ammonia, triethanolamine, ethylene diamine, diethylene triamine, aminoethyl piperazine and aniline.

[0020] Polyester polyols are compounds having terminal hydroxyl groups obtained by the polycondensation of polyfunctional carboxylic acids and polyfunctional hydroxyl compounds or the ring-opening self-condensation polymerizations of a lactone. Examples of polyfunctional carboxylic acids include adipic acid, phthalic acid, succinic acid, azelaic acid and sebacic acid. Examples of polyfunctional hydroxy compounds include glycols such as ethylene glycol, propylene glycol, butanediol and diethylene glycol, and polyhydric alcohols such as glycerol, trimethylol propane and pentaerythritol. Examples of lactones include gamma-butyrolactone and epsilon-caprolactone.

[0021] Polymer polyols can be obtained by mixing a polyether polyol and an ethylenically unsaturated monomer, and, when necessary, adding chain transfer agents, dispersion stabilizers, and the like, to bring about the radical polymerization of the ethylenically unsaturated monomer in the presence of a radical initiator. Examples of ethylenically unsaturated monomers include monomers containing the cyano group such as acrylonitrile and methacrylonitrile; (meth)acrylic esters such as methyl (meth)acrylate, butyl (meth)acrylate, stearyl (meth)acrylate, hydroxyethyl ( meth Jacry late, dimethylaminoethyl (meth)acrylate and dimethylaminopropyl (meth)acrylate; monomers containing carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid; acid anhydride monomers such as maleic anhydride and itaconic anhydride; hydrocarbon compounds such as butadiene, isoprene and 1 ,4-pentadiene; aromatic hydrocarbon compounds such as styrene, alpha-methyl styrene, phenylstyrene and chlorostyrene; halogen-containing monomers such as vinyl chloride and vinylidene chloride; vinyl ethers such as vinyl ethyl ether and vinyl butyl ether; vinyl ketonessuch 25 as vinyl ethyl ketone;- vinyl esters such as vinyl acetate; acrylamides such as acrylamide, N,N-dimethylacrylamide, N-isopropylamide, N,N-dimethylaminopropyl acrylamide and methylene bisacrylarnide; and methacrylamides such as N,N-dimethyl methacrylamide.Such ethylenically unsaturated monomers can be used alone or in combinations of two or more.

[0022] The aforementioned polyol components can be used alone or in combinations of two or more depending on the properties required of the flexible polyurethane foam that is to be prepared.Isocyanate Component

[0023] The polyisocyantes used to prepare the semi-rigid foams of the present invention are those generally used to make rigid PIR (isocyanurate) polyurethane foam. These isocyanates may be any of the known aliphatic, alicyclic and aromatic types, as well as mixtures of at least two of these types, and these isocyanates may be used singly or two or more of any them can be combined, whether of the same or different types.

[0024] Non-limiting examples of suitable isocyanates for use in this invention include aromatic diisocyanates such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, crude tolylene diisocyanate, diphenylmethane diisocyanate, and crude diphenylmethane diisocyanate; aromatic triisocyanates such as 4,4',4"-triphenylmethane triisocyanate and 2,2',6-tolylene triisocyanate; aromatic tetraisocyanates such as 4,4'- dimethyldiphenylmethane-2,2',5,5'- tetraisocyanate; aliphatic isocyanates such as hexamethylene- 1,6-diisocyanate; alicyclic isocyanates such as hydrogenated diphenylmethane diisocyanate; and other diisocyanates such as m-phenylene diisocyanate, naphthylene- 1. diisocyanate, l-methoxyphenyl-2,4-diisocyanate, 4,4'- biphenyl diisocyanate, 3, 3'-diniethoxy-4, 4' -biphenyl diisocyanate, and 3,3'- - dimethyldiphenylmethane-4,4'-diisocyanate. Among the various isocyanates which can beused, preferred are 2,4-tolylene diilsocyanate, 2,6- tolylene diisocyanate, crude tolyene diisocyanate, diphenylmethane diisocyanate, crude diphenylmethane diisocyanate, hexamethylene- 1,6-diisocyanate, and hydrogenated diphenylmethane diisocyanate.

[0025] In a most preferred embodiment of the present invention, the isocyanates for use in this invention arc diphenylmethane diisocyanate products. These isocyanates are commonly referred to as MDI-type isocyanates, and are commercially available with functionalities of from 2.0 to 3.2. Methylene diphenyl diisocyanate and polymeric methylene diphenyl diisocyanate are particularly preferred.

[0026] In one embodiment of the present invention, the isocyatate and polyol- components are. proportioned to form an isocyanate index in the range of about 150 to about 400. In another embodiment of the present invention, the polyol and isocyanate components are proportioned to form an isocyanate index in the range of about 175 to about 350. In yet another embodiment of the present invention, the polyol and organic isocyanate components are proportioned to form an isocyanate index in the range of about 200 to about 300.

[0027] The densities of the flexible and semi-rigid foams in accordance with the invention can range from about 14 to 180, preferably about 16 to 80, and more preferably about 20 to 40 kg / m .Blowing Agent

[0028] As the blowing agent in the semi-rigid polyurethane foam forming compositions of the present invention, known blowing agents heretofore used in such compositions are suitably selected according to the properties required of the foamed product. Water is a preferred and typical example of such a blowing agent. However, other examples include methylene chloride, n-butane, isobutane, n-pentane, isopentane, dimethyl ether, acetone, carbon dioxide andthe like. Depending on the desired density and other properties of the foamed product, these and other blowing agents can be used alone or in combination. The amount of blowing agent will generally depend upon the desired density and isocyanate index of the foam which ordinarily range from about 0.1 to about 40 parts by weight per 100 parts by weight of the polyol component.Catalysts

[0029] Polyurethane foam-forming compositions in accordance with the invention can preferably contain catalysts and combinations of catalysts, heretofore known or used for the production of polyurethane foams. Examples of useful catalysts include sodium hydroxide, sodium acetate, tertiary amines or materials which generate tertiary amines such as trimethylamine, triethylene diamine, N-methyl morpholine, N,N-dimethyl cyclohexylamine, and N,N-dimethyl aminoethanol. Also applicable are metal compounds such as hydrocarbon tin alkyl carboxylates, dibutyl tin diacetate, dibutyl tin dioctoate dibutyl tin dilaurate and stannous octoate; as well as other compounds intended to promote trimerization of the polyisocyanate such as, 2,4,6-tris(N,N-dimethylamino-methyl)phenol, l,3,5-tris(N,N-dimethyl-3-aminopropyl)- S-hexahydrotriazine, potassium octoate, potassium acetate and catalysts such as Dabco TMR® and Polycat 43®, Pel-Cat 9540A®, Pel-Cat 9650®, and / or Polycat 5 / PMDETA®. Polycat 43®, Pel-Cat 9540A®, Pel-Cat 9650® are preferred for purposes of this invention.

[0030] Many other kinds of catalysts can be substituted for those listed above, if desired. The amount of catalyst used can advantageously range from 0.05 to 5 weight percent or more based on the total weight of polyol in the foam-forming composition.Expandable Graphite

[0031] As disclosed herein, the addition of a non-halogenated expandable graphite flameretardant filler to the semi-rigid polyurethane foam compositions of the present invention substantially increases the thermal heat resistance of the foam product. Non-halogenated expandable graphites are commercially available, for example, from Nyacol Nano Technologies, Inc. A particularly preferred expandable graphite for the purposes of the present invention is available from Nyacol Nano Technologies, Inc. and is known as Nyagraph FP.[00321 In the practice of the present invention from about 15 to about 25 parts, preferably about 20 parts expandable graphite based on 100 parts of the foam composition can be used. Surfactants

[0033] Surfactants, including organic surfactants and silicone based surfactants, may be added to serve as cell stabilizers. Some representative materials are sold under the designations SF-1109, L-520, L-521 and DC- 193, which are, generally, polysiloxane polyoxylalkylene block copolymers. Also included are organic surfactants containing polyoxy-ethylene- polyoxybutylene block copolymers. It is particularly desirable to employ a small amount of a surfactant to stabilize the foaming reaction mixture until it cures. Other surfactants that may be useful herein are polyethylene glycol ethers of long-chain alcohols, tertiary amine or alkanolamine salts of long-chain allyl acid sulfate esters, alkylsulfonic esters, alkyl arylsulfonic acids, and combinations thereof. Such surfactants are employed in amounts sufficient to stabilize the foaming reaction against collapse and the formation of large uneven cells.

[0034] Typically, a surfactant total amount from about 0.2 to about 3 wt %, based on the formulation as a whole, is sufficient for this purpose. However, it may be in some embodiments desirable to include some surfactants, e.g., DABCO DC-5598, available from Air Products and Chemicals, Inc., in a higher amount. In view of this a surfactant may be included in the inventive formulations in any amount ranging from 0 to 6 wt %, based on the polyol component.

[0035] Other additives may be included in the foam composition described herein. Such additives include, without limitations, adhesion promoters, antioxidants, anti-static agents, antimicrobials, dyes, pigments, heat stabilizers, light stabilizers, plasticizers, preservatives, and ultraviolet stabilizers in the customary amounts. Flame retardants, such as phosphorous-based flame retardants can also be included.

[0036] The semi-rigid polyurethane foam forming compositions of the present invention are suitable for use as potting compositions for potting a plurality of cells in a battery module and after curing provide excellent thermal, flame retardance and mechanical stability. In other embodiments, the cured semi-rigid foams of the present invention may be employed as layers or slabs between or around adjacent cells in the battery module.

[0037] The following examples of foam formulations are provided for illustration only, and is not intended to be interpreted as limiting. The amounts are presented as parts by weight.ExamplesFoam Sample Preparation

[0038] Semi-rigid foam samples designated Formulation 26-2 and Formulation 26-3 were prepared from the ingredients listed in Table 1 below. The polyol, water, amine catalyst, stabilizer, and expandable graphite (when included) were first mixed together in a container. Formulation 26-2 does not contain expandable graphite, while formulation 26-3 includes expandable graphite flame retardant. The isocyanate was added to the mixture, with continued stirring, and the mixture was quickly poured into a new container and permitted to foam and cure. The reaction proceeds at a temperature of about 25° C and cures in about 24 hours. After the foam fully cured, it was cut into dimension of 130x100x10mm.Table 1

[0039] Regarding the materials used in the above Formulations 26-2 and 26-3, Voranol8136 is a glycerine -initiated heteropolymer triol, with a nominal molecular weight of 3100, available from Dow Chemical.

[0040] Wannate® PM-700 is a polymeric methylene diphenyl diisocyanate (MDI) with high functionality. It is available from Yantai Wanhua America Co., LTD.

[0041] Rubinate® M polymeric MDI available from Huntsman Corporation.

[0042] Pel-Cat 9540A (potassium 2-ethylhexanoate, potassium octoate), Pel-Cat 9650(potassium acetate), are trimerization catalyst available from Ele Corp, and Polycat 5 / PMDETA amine catalyst available from Evonik.

[0043] Tegostab B84508 is a surfactant available from Evonik Corp.

[0044] Nyagraph FP is expandable graphite available from Nyacol Corp.

[0045] Before testing the heat resistance of the semi-rigid foams of Formulations 26-2 and 26-3, a base-line flexible PU foam sample was prepared for comparative purposes. 80 parts clear polyol (Voranol-8136) and 20 parts co-polymer polyol (Voraguard), 1.5 parts water, 0.2 parts Dabco 33LV and 1.22 parts triethanolamine. Dabco 33LV, available from Evonik. is a strong urethane reaction (gelation) catalyst for multipurpose use. The final foam had density of 150kg / m3with minimal shrinkage. Polymeric MDI was used as the isocyanate to form a high- density formulation, due to its high equivalent weight, which will help to achieve higher density under the same index.Hot Plate Thermal Runaway Simulation Test

[0046] The following test protocol was used to test the heat resistance of Formulations 26-2 and 26-3 and to compare those Formulations to the flexible base-line PU foam in a simulation of a thermal runaway event. A 130x100x10mm slab of each foam sample was placed on top of a quartz hot plate set at 1000 °F (538°C). A 0.5-inch-thick piece wood wrapped with aluminum foil was placed on top of the foam sample. A hole had been drilled through the center of the wood, and a thermocouple surface probe was mounted in the hole. A temperature data logger was used to record the temperature of the hot plate, as well as the temperature of the top surface of the foam sample, as the hotplate was heated to 1000 °F.

[0047] The comparison foam sample was subjected to the hot plate test described above. The temperature at the top surface of the baseline comparison foam sample exceeded 150 °C (deemed the critical temp) in only 34 seconds. This is considered to be a baseline for the hot plate test. The temperature recording had to stop at 60s due to loss of the entire comparison foam sample.

[0048] Next, the thermal runaway simulation hot plate test was run on Samples 26-2 and26-3. The results are shown in table 1 above. The first run of Sample 26-2 and both runs of Sample 26-3 are graphed in Fig. 1. For sample 26-2, the temperature at the top surface of the foam slab after 3 minutes (180 seconds) was 127°C and 141°C for runs 1 and 2 respectfully (134 °C average). For sample 26-3, with expandable graphite, the temperature at the top surface of the foam slab after 180 seconds was 81°C and 52° (66.5°C average). For sample 26-2, the time to reach the critical temperature of 150°C for runs 1 and 2 was 203 and 187 seconds respectively (195 average). For sample 26-3 with expandable graphite, the time to reach the critical temperature of 150°C for runs 1 and 2 was 457 and 625 seconds, respectively (541 average).

[0049] Fig. 1 is a graph of the temperature at the top surface of the foam sample during the hotplate test, with the hotplate set to 1000° F. The critical temperature was determined to be 150° C. Acceptable foams should be able to resist reaching this temperature for at least about 180 seconds, and superior foams can last much longer. In Fig.l, the middle curve and the lowest curve correspond to Sample 26-3, runs 1 and 2, respectively. Sample 26-3 took 457 and 625 seconds respectfully to reach 150° C. Furthermore, the temperature at the top surface of the foam slab after three minutes (180 seconds) for Sample 26-3 was below 100° C. The foam sample of Formulation 26-3 which contains expandable graphite was therefore deemed exceptional in terms of heat resistance, survivability, and superior to the same foam without expandable graphite and considered to be well suited to prevent a thermal runaway when used as thermal or electrical insulation or cushioning in the manufacture of battery packs and battery related equipment and devices, such as arrays of battery cells. Both samples 26-2 and 26-3 were superior to the base level comparison flexible foam.Comparative Example

[0050] A high-density flexible foam containing expandable graphite was prepared in accordance with the foam formulation in Table 2.Table 2Foam Formulations

[0051] The flexible high-density foam was evaluated using the same hot plate procedure described above. The longest time to reach critical temperature (150°C) was 327 seconds at 10 mm thickness which is far less than the semi-rigid foams containing expandable graphite of the present invention which reached critical temperatures at 457 sec and 625 sec.

[0052] While the above description contains many specifics, these specifics should not be construed as limitations of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other embodiments within the scope and spirit of the invention as defined by the claims appended hereto.

Claims

CLAIMSWhat is claimed is:

1. A semi-rigid polyurethane foam forming composition comprising: a. at least one polyol having a molecular weight of at least about 1000; b. at least one polyisocyanate wherein the polyol and isocyanate are proportioned to form a foam having an isocyanate index of at least about 150; c. at least one blowing agent; d. at least one catalyst for the polyurethane-forming reaction; and e. optionally at least one flame retardant comprising expandable graphite.

2. The foam forming composition of claim 1, wherein the polyol has a molecular weight of at least 1000.

3. The foam forming composition of claim 1, wherein the polyol has a molecular weight of at least 2000.

4. The foam forming composition of claim 1, wherein the polyol has a molecular weight of at least about 3000 and the isocyanate and polyol are proportioned to form a foam having an index of at least about 250.

5. The foam forming composition of claim 1, wherein the polyol is a heteropolymer triol, with a molecular weight of about 3000.

6. The foam forming composition of claim 1, wherein the isocyanate is polymeric methylene diphenyl diisocyanate.

7. The foam forming composition of claim 1, and comprising about 15 to 30 parts by weight expandable graphite flame retardant.

8. The foam forming composition of claim 1, and comprising about 20 to 25 parts byweight expandable graphite flame retardant.

9. The foam forming composition of claim 1, wherein the composition contains about 90-110 parts by weight polyol, about 45-55 parts by weight isocyanate and about 15-25 parts by weight expandable graphite.

10. A semi-rigid polyurethane foam obtained by curing the foam forming composition of any of claims 1-9.

11. A battery device comprising the foam of claim 10.

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