Primer formulation for battery bonding and debonding

Abstract

Provided herein is a novel primer formulation used in a bonded assembly, wherein the bonded assembly comprises A) a substrate; B) a primer layer formed from the primer formulation; and C) an adhesive layer, wherein the primer layer is placed between the substrate and the adhesive layer, and wherein the primer formulation comprises i) at least one film-forming resin selected from polyolefin resins, polyester, copolyester resins, and mixtures of these; and ii) a solvent.

Description

[0001] Primer Formulation for Battery Bonding and Debonding

[0002] Battery cells, for example, for electric vehicles, are usually bonded to the battery module or directly to the battery housing of the battery pack using adhesives. Debonding of the adhesive system in the battery packs is needed for repair or recycling. The bonded cells are difficult to remove from the battery module or battery housing since the adhesive that bonds the cells typically has high adhesive strength and the bonding areas are large.

[0003] It is known to attempt to address this by using a solvent mixture to soak the bonded area to soften the adhesive bonding system. However, the time needed for soaking usually takes several days or more to lower the adhesive strength to a desired level, and solvents are usually dangerous when in direct contact with the battery cells.

[0004] Therefore, there is an ongoing need for adhesive systems which show good adhesive strength but which can be debonded easily when desired to permit repair and / or recycling.

[0005] The inventors have found that by using a primer formulation layer with a softening temperature between 60 and 100°C, in conjunction with a two-component polyurethane adhesive, adhesive bonds can be created that have a strength of greater than 5 MPa at room temperature (25°C). The adhesive strength is diminished or reduced to less than or equal to 1 .5 MPa when the bonded assembly is heated to 60 to 100°C. This reduced adhesion is desired when an adhered battery assembly needs to be disassembled or debonded (e.g. for repairs) without damaging the battery cells and the cooling plate. The primer layer is typically placed between a substrate, e.g. cooling plate in the battery assembly, and adhesives to enable the debonding process when needed. When battery repair is done, fresh adhesives can be applied to bond battery with the cooling plate again. Summary of the Invention

[0006] The present invention provides a thermoplastic primer formulation that has all the desired properties in a debonding process. Specifically, the present invention provides a bonded assembly comprising A) a substrate; B) a primer layer formed from a primer formulation; and C) an adhesive layer; wherein the primer layer is placed between the substrate and the adhesive layer; and wherein the primer formulation comprises i) at least one film-forming resin selected from polyolefin resins, polyester, copolyester resins, and mixtures of these; and ii) a solvent. The primer layer shows a softening temperature between 60 and 100°C which enables a proper debonding process of the bonded assembly.

[0007] Detailed Description of the Invention

[0008] Equivalent and molecular weights are measured by gel permeation chromatography (GPC) with a Malvern Viscothek GPC max equipment. Tetrahydrofuran (THF) was used as an eluent, PL GEL MIXED D (Agilent , 300*7.5 mm, 5 pm ) was used as a column, and MALVERN Viscotek TDA (integrated refractive index viscometer and light scattering) was used as a detector.

[0009] Film-forming resin

[0010] The present invention involves the application of a primer formulation to the substrate on which the adhesive will be applied. The primer comprises at least one filmforming resin that has a softening temperature of between 50 and 120°C, more preferably between 55 and 110°C, more particularly preferably between 60 and 100°C. Softening temperature is preferably measured by differential scanning calorimetry (DSC), particularly preferably according to ISO 11357-3:2018(E).

[0011] The film-forming resin is not particularly limited, but preferably be selected from polyolefin resins, polyester and copolyester resins and mixtures of these. In some preferred embodiments, the film-forming resin is selected from copolyesters, polycaprolactone-copolyesters and polycaprolactone-copolyester polyurethanes or a mixture thereof.

[0012] Suitable polyolefins are, for example, homo- and co-polymers of C2-C8 olefins.

[0013] Preferred polyolefins are those having a molecular weight of from 40,000 to 200,000 Da, particularly preferably from 50,000 to 150,000 Da, more particularly preferably 50,000 to 100,000 Da, even more particularly preferably 5,000 to 10,000 Da. Preferred polyolefins are modified, for example with maleic anhydride. In a preferred embodiment, the film-forming resin is a homo- or co-polymer of C2-C8 olefins, having a molecular weight of from 40,000 to 200,000 Da, particularly preferably from 50,000 to 150,000 Da, more particularly preferably 50,000 to 100,000 Da. In another preferred embodiment, the film-forming resin is a homo- or co-polymer of C2-C8 olefins, having a molecular weight of from 40,000 to 200,000 Da, particularly preferably from 50,000 to 150,000 Da, more particularly preferably 50,000 to 100,000 Da, modified with maleic anhydride. In a particularly preferred embodiment, the film-forming resin is a polypropylene modified with maleic anhydride and having a molecular weight of from 50,000 to 150,000 Da, more preferably 50,000 to 100,000 Da.

[0014] Suitable polyesters are those resulting from polymerization of one or more diols with one or more diacids. Preferred polyesters are based on aliphatic diols and aliphatic diacids. Preferred polyesters have a molecular weight of between 2,500 and 4,500 Da, more preferably between 3,000 and 4,000 Da, particularly preferably 3,500 Da. Preferred polyesters have a softening point of from 50 and 110°C, more preferably between 55 and 105°C, more particularly preferably between 60 and 100°C.

[0015] In a particularly preferred embodiment, the at least one film-forming resin that has a softening temperature of between 60 and 100°C and it contains a linear, saturated, semi crystalline copolyester. One example of such copolyesters can be the product under the tradename DANPOL commercially available from Danquinsa GMBH. The primer formulation of the present invention typically contains 1 to 50 wt.%, preferably 7 to 30 wt.%, more preferably 15 to 25 wt.%, and most preferably about 20 wt.%, of at least one film-forming resin, all based on the total weight of the primer formulation. Solvent

[0016] The primer formulation of the invention also comprises a solvent. The solvent is not particularly limited, provided the film-forming resin is soluble in it, and sufficiently volatile that it readily evaporates. Preferably it has a vapor pressure at 20°C of greater than 500 Pa. Examples of suitable solvents include water, benzene, alkyl benzenes (e.g. toluene, methoxybenzene or Anisol, commercially available from KHBoddin GmbH or Merck ethylbenzene), dialkyl benzenes (e.g. xylenes), n-butanone, ethyl acetate, and Solvesso 150 (predominately of C9-C11 aromatic hydrocarbons, predominately C10). The solvent is preferably used in the primer formulation at 50 to 99 wt.%, preferably 70 to 95 wt.%, more preferably 75 to 90 wt.%, and most preferably about 80 wt.%, all based on the total weight of the primer formulation. In a preferred embodiment, the solvent is methoxybenzene.

[0017] Optional ingredients in the primer

[0018] The primer may additionally comprise other ingredients, such as adhesion promoters, UV indicator, and rheology additive.

[0019] Preferred adhesion promoters are polyisocyanates, isocyanates, silanes and mixtures thereof. Preferred silanes include trialkoxy alkyl silanes, more particularly trimethoxy alkyl silanes. In a preferred embodiment, the adhesion promoter is a trialkoxy alkyl silane in which the alkyl group bears an epoxy, amine or mercapto group, particularly preferably an epoxy group. In a particularly preferred embodiment, the adhesion promoter is gamma-glycidoxypropyltrimethoxysilane. If used, the adhesion promoter is preferably used at 0.1 to 1 .5 wt.%, more preferably 0.25 to1 wt.%, based on the total weight of the primer formulation. In a particularly preferred embodiment, the primer comprises gamma-glycidoxypropyltrimethoxysilane at 0.1 to 1.5 wt.%, more preferably 0.25 to1 wt.%, based on the total weight of the primer formulation.

[0020] The primer of the present invention may further comprise UV indication agent or rheology additives. Any commercially available UV indication agents or rheology additives may be used in the present primer. To prepare the present primer formulation in a preferred embodiment, the selected resin is first dissolved in the selected solvent using a pneumatic primer shaker for about 60 minutes at room temperature.

[0021] Application of the primer formulation

[0022] The primer formulation can be applied to a substrate, typically at room temperature, using any method. Preferred methods include brush, spray or spreading. After application, the primer is allowed to harden or dry. For primers with water as solvent, evaporation of the solvent may be accelerated by heating and / or applying a current of air and / or applying a vacuum. For primers with organic solvents such as xylene, evaporation is usually fast enough at room temperature, although it may be accelerated by heating and / or applying a current of air and / or applying a vacuum. The dry film thickness of the primer is preferably less than or equal to 30 microns.

[0023] Adhesives

[0024] The adhesive for use in the invention is preferably a two-component polyurethane adhesive. This includes adhesives with a polyol component and a polyisocyanate component with a typical 1 :1 volumetrix mix ratio during application. Adhesives can be applied directly over the primer layer form as taught above.

[0025] Debonding

[0026] The present novel primer formulation is particularly useful in a debonding process. The steps of debonding include (1 ) heating the adhesively bonded assembly to a temperature between 60 and 100°C for a time sufficient to soften the adhesive bond between the two substrates; and (2) disassembling the two substrates. The heating step may be carried out using any heating method, for example an oven, IR, radio-frequency heating (including microwave). The disassembly step involves separating the adhered substrates by pulling the substrates apart. The primer and adhesive provide a lap shear strength of greater than or equal to 5 MPa at 25°C, preferably greater than 6 MPa, and in certain preferred embodiments greater than 10 MPa at 25°C, when measured according to ASTM D 1002. When heated, the primer and adhesive provide a lap shear strength of less than or equal to 1 .0 MPa at temperatures greater than 60°C, more preferably greater than 80°C, when measured according to ASTM D 1002.

[0027] EXAMPLES

[0028] Some comparative and inventive examples are prepared and tested for their Lap Shear Strength values using the testing method of DIN EN 1465. To prepare the testing examples, e-coated steel substrates with a Cathoguard 800 coating (manufactured by BASF) were used. The substrates were cleaned with isopropanol before use. The adhesive is applied on the substrate primed with debonding primer, if used, before the second substrate is joined within 3 minutes. The thickness is adjusted to 1 .5 mm, the overlap area is 25 mm x 15 mm. The prepared samples were cured and rested for 7 days at 23 °C, 50 % relative humidity before the lap shear tests were performed. The lap shear samples were then mounted in a tensiometer, and the lap shear tests were performed as known by somebody skilled in the art, 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. The failure mode of the adhesive system is reported.

[0029] Three primer formulations were prepared as shown in Table 1 with the raw materials and their sources provided below. The solvents (anisol, ethyle acetate, toluene) can be from any commercial producers of these common chemical agents.

[0030] Table 2 shows the tested lap shear strength values and failure mode for various samples prepared as discussed above. Table 2 also included an example (Comparative Example or CE I) where no primer was used in the sample. The Inventive Example (IE) used Primer 1 and a two-component polyurethane (“2K PU”) adhesive. Similarly, CE II and III used Primer 2 and Primer 3 respectively with 2K PU adhesives. The 2K PU samples used in the present examples are BETAFORCE product available from DuPont. Other 2KPU adhesives, such as those disclosed in WO2023 / 196368, incorporated herein by reference in its entirety, may also be used.

[0031] Table 1 . Formulation details of the three primer formulation samples (numerical values are in wt.% of the total weight of the formulation examples)

[0032] Table 2. Lap shear strength values and failure modes

[0033] As shown in Table 2, the Comparative Example I applied 2kPU adhesives without any primers. The lap shear strength drops slightly at higher temperatures and the failure mode remains 100% cohesive failure at all temperatures. The lap shear strengths at high temperatures are too high (5 MPa at 100°C) for any debonding steps.

[0034] Comparative Example II used a 2K PU adhesive after a primer layer formed from Primer 2. The debonding primer here does not provide a good adhesion to the substrate and the lap shear value shows adhesive failure modes as well as low strength when tested at 23°C and 60°C. Such an adhesive system is not suitable for debonding applications because it does not fulfill the adhesive strength requirement: lap shear > 5 MPa in the working temperature range -40°C to 60°C.

[0035] Comparative Example III used a 2K PU adhesive after a primer layer formed from Primer 3 formulation. Here, the primer layer does not melt or soften enough at elevated temperatures. Even at 100 °C, the lap shear strength is still above 3.5 MPa. This is undesirable for any debonding operations.

[0036] The Invention Example used a 2K PU adhesive after a primer layer formed from Prime 1 formulation. Here, the resulting performance data indicated desirable conditions for a successful debonding operation. The lap shear strength at 23°C and 60°C is above 5MPa and failure mode 100% cohesive failure. When the temperature is elevated to 80°C and 100°C, the primer layer melts resulting a 100% adhesive failure mode and the lap shear strengths is low enough and ideal for debonding.

Claims

Claims1 . A bonded assembly comprising A) a substrate; B) a primer layer formed from a primer formulation; and C) an adhesive layer; wherein the primer layer is placed between the substrate and the adhesive layer; and wherein the primer formulation comprises i) at least one film-forming resin selected from polyolefin resins, polyester, copolyester resins, and mixtures of these; and ii) a solvent.

2. The bonded assembly of Claim 1 wherein the film-forming resin comprises linear, saturated, semi-crystalline thermoplastic resins.

3. The bonded assembly of any one of the preceding claims where in the filmforming resin is selected from copolyesters, polycaprolactone-copolyesters and polycaprolactone-copolyester polyurethanes.

4. The bonded assembly of any one of the preceding claims wherein the primer formulation comprises 1 to 50 wt.% of the at least one film-forming resin, based on the total weight of the primer formulation.

5. The bonded assembly of Claim 4 wherein the primer formulation comprises 7 to 30 wt.% of the at least one film-forming resin, based on the total weight of the primer formulation.

6. The bonded assembly of Claim 5 wherein the primer formulation comprises about 20 wt.% of the at least one film-forming resin, based on the total weight of the primer formulation.

7. The bonded assembly of any one of the preceding claims wherein the at least one film-forming resin has a softening temperature of between 60 and 100°C.

8. The bonded assembly of Claim 1 , wherein the solvent is one or more of water, benzene, alkyl or dialkyl benzenes, n-butanone, ethyl acetate, or a mixture thereof.

9. The bonded assembly of Claim 8, wherein the solvent comprises methoxybenzene.

10. The bonded assembly of any one of the preceding claims wherein the primer formulation comprised 50 to 99 wt.% of the solvent.11 . The bonded assembly of Claim 10 wherein the primer formulation comprised 75 to 90 wt.% of the solvent.

12. The bonded assembly of any one of the preceding claims wherein the adhesive layer is formed from a two-component polyurethane adhesive.

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