Thermally removable multilayer compositions bonded with a thermoplastic primer - Patent Application 20070122997

Abstract

The multilayer composition may include a substrate having a surface energy of 35 dynes / cm or greater, one or more thermoplastic primer layers comprising 55% to 100% by weight of a maleic anhydride-grafted chlorinated polyolefin and having a glass transition temperature in the range of 50°C to 120°C, and one or more thermosetting adhesive layers.

Description

[Technical Field]

[0001] Embodiments relate to a thermally removable multi-layer composition bonded with a thermoplastic primer layer and a thermosetting adhesive.

[0002] Introduction Thermoplastic and thermosetting adhesives are utilized in many industrial applications, including flexible packaging and electric vehicle (EV) battery packaging. In most applications, thermosetting adhesives are selected based on bond strength, substrate compatibility, and long-term durability under operating conditions. However, many applications require separation of bonded components for reuse, recyclability, maintenance, or part replacement. Physical removal methods, such as prying, cutting, or laser ablation, are typically used to remove EV battery cells bonded to a thermosetting adhesive layer. Other approaches involve the use of solvents, alkalis, or acids to remove thermosetting adhesives by immersion, which can result in chemical hazards and limited penetration into the adhesive layer. Another concern with common removal methods is their effectiveness in high-surface-area attachments, including those present in EV battery packs, which can lead to damage to the bonded components. Summary of the Invention

[0003] The multilayer compositions disclosed herein may include a substrate having a surface energy of 35 dynes / cm or greater, one or more thermoplastic primer layers comprising 55% to 100% by weight of a maleic anhydride-grafted chlorinated polyolefin and having a glass transition temperature in the range of 50°C to 120°C, and one or more thermosetting adhesive layers. DETAILED DESCRIPTION OF THE INVENTION

[0004] Embodiments relate to thermally removable multilayer compositions having one or more thermoplastic primer layers and one or more thermosetting adhesive layers that allow for removal of high surface energy substrates by heating. The thermoplastic primer layers disclosed herein remain solid and maintain thermosetting adhesive performance throughout the operating temperatures of most applications (e.g., 50°C or less), but do not exceed the glass transition temperature (T g ) or melting point temperature (T m ) (for example, in the range of 50°C to 130°C) or higher, it can soften and / or melt.

[0005] Electric vehicle battery designs often include one or more battery packs containing multiple battery cells individually bonded to various types of substrates. EV batteries may also include numerous external features to protect the battery pack and battery cell array, which may include multiple housings and substrates. The large surface contact area of the battery components (e.g., approximately 0.5 m² of a pack block) can be a significant challenge. 2 From the entire pack 1.5m 2 Thermosetting adhesives are often used to provide bond strength between the battery and the underlying or overlying substrates and structures over a wide operating temperature range (e.g., -20°C to 60°C). However, the bond strength of thermosetting adhesives makes it difficult to remove the battery pack or pack substrate without damaging or deforming the bonded components.

[0006] The multilayer compositions disclosed herein may incorporate one or more thermoplastic primer layers that allow for removal from the substrate upon application of heat, particularly for applications where thermosetting adhesives are typically used, such as mediating adhesion between a substrate (e.g., a battery cell) and a thermal management plate (e.g., a heat or cooling sheet) or another substrate (e.g., a battery pack cover, a battery pack bottom). Thermoplastic maleic anhydride-grafted chlorinated polyolefins (MAH-g-CPOs) that mediate adhesion of a thermosetting adhesive layer to substrates having a surface energy greater than 35 dynes / cm can be incorporated into the multilayer compositions.

[0007] The thermoplastic primer layer and thermosetting adhesive layer disclosed herein can maintain good bond strength compared to the bond between the adhesive layer and the substrate without the primer layer. The multilayer composition disclosed herein can have a lap shear strength at room temperature that is reduced by less than 30% compared to a comparative multilayer composition without the thermoplastic primer layer. The multilayer composition disclosed herein can have a cross tensile strength at room temperature that is reduced by less than 30% compared to a comparative multilayer composition without the thermoplastic primer layer.

[0008] The thermoplastic primer layer disclosed herein can be removably bonded to the thermosetting adhesive layer and / or substrate such that the primer layer maintains bond strength at temperatures below 50°C, but melts or softens at elevated temperatures to facilitate removal. For example, at elevated temperatures (e.g., above 50°C), the thermoplastic primer layer(s) melts or softens, thereby facilitating removal between the thermoplastic primer layer and the substrate and / or thermosetting adhesive layer. The thermoplastic primer layer disclosed herein has a glass transition temperature (T) in the range of 50°C to 130°C, 50°C to 120°C, 55°C to 120°C, 55°C to 115°C, 55°C to 100°C, or 60°C to 80°C. g ).

[0009] The multilayer composition can include a high surface energy substrate onto which one or more thermoplastic primer layers and / or thermosetting adhesive layers are formed, coated, or deposited. High surface energy substrates can include, for example, metal, epoxy, or polyacrylate surfaces used in EV battery packs, thermal management plates, and other industrial applications.

[0010] As used herein, "high surface energy substrate" refers to a substrate having a surface energy of 35 dynes / cm or greater, and excludes substrates having a surface free energy of less than 35 dynes / cm. High surface energy substrates disclosed herein include metals such as aluminum, steel or alloys, zinc, non-metals including glass, and polar polymers such as epoxies, polyurethanes, and polyesters.

[0011] Table 1 includes additional examples of suitable high surface energy substrates.

[0012] [Table 1]

[0013] In some cases, a high surface energy substrate may be placed in contact with a multi-layer composition containing one or more thermoplastic primer layers and / or thermosetting adhesive layers assembled on a second substrate or surface. In some cases, the multi-layer composition may contain a thermoplastic primer layer in contact with the high surface energy substrate and a thermosetting adhesive in contact with the thermoplastic primer layer. The thermosetting adhesive may also mediate adhesion to a second substrate, such as a thermal management plate.

[0014] In another example, a multilayer composition may include a first thermoplastic primer layer in contact with a high surface energy substrate and a thermosetting adhesive layer in contact with the first thermoplastic primer layer, and a second thermoplastic primer layer may be in contact with the thermosetting adhesive layer and may mediate adhesion to a second substrate.

[0015] The multilayer composition may include one or more thermosetting adhesive layers, which may include one or more polyurethanes, epoxies, polyacrylates, polyesters, their cross-linked derivatives, and the like. Thermosetting adhesives include polar thermosetting adhesives having a surface energy of 35 dynes / cm or greater. In one example, the thermosetting adhesive layer may include a methylene diphenyl diisocyanate (MDI)-based 2K polyurethane structural adhesive. The thermosetting adhesives disclosed herein may be water-based, solvent-based, or solvent-free.

[0016] The multilayer compositions disclosed herein may contain one or more thermoplastic primer layers comprising maleic anhydride-grafted chlorinated polyolefin (MAH-g-CPO), alone or in combination with an additional thermoplastic polymer. The maleic anhydride-grafted chlorinated polyolefins disclosed herein may have a maleic anhydride grafting degree of greater than 1% and a chlorination degree content of greater than 10%. In some cases, the maleic anhydride-grafted chlorinated polyolefins may have a chlorination degree of 10% to 30% and / or at least one maleic anhydride degree of 1% to 5%. The maleic anhydride-grafted chlorinated polyolefins disclosed herein may have a weight average molecular weight of 50,000 Da or greater, 60,000 Da or greater, or 100,000 Da or greater. The maleic anhydride grafted chlorinated polyolefin may have a weight average molecular weight in the range of 40,000 Da to 100,000 Da, 50,000 Da to 90,000 Da, or 50,000 Da to 80,000 Da.

[0017] The thermoplastic primer layer may contain MAH-g-CPO in combination with one or more additional thermoplastic polymers, including thermoplastic polar polymers such as polyurethane or polyester, or thermoplastic low-polarity polymers such as maleic anhydride grafted polyolefin (MAH-g-POE), maleic anhydride grafted ethylene-vinyl acetate (MAH-g-EVA), ethylene-vinyl acetate (EVA), maleic anhydride grafted styrene-ethylene-butylene-styrene (MAH-g-SEBS), etc. The thermoplastic primer layer may also comprise a polymer blend including MAH-g-CPO in combination with one or more additional polymers (e.g., more or less polar polymers) that modify the overall polarity of the blended layer.

[0018] The introduction of additional polymers into the thermoplastic primer layer can modify the polarity and compatibility of the primer layer with the thermosetting adhesive layer or substrate, which can increase or decrease the corresponding interlayer bond strength at operating temperatures. For example, increasing the polarity of the thermoplastic primer layer can increase compatibility with the polar thermosetting adhesive layer and increase bond strength at low temperatures. Conversely, blending MAH-g-CPO with a less polar polymer in the thermoplastic primer layer decreases compatibility with the polar thermosetting adhesive layer, decreasing bond strength and increasing release properties at high temperatures.

[0019] Thermoplastic primer layers incorporating multiple polymers may include an MAH-g-CPO component at a weight percent (wt%) of 55 wt% or more, 60 wt% or more, or 70 wt% or more. Thermoplastic primer layers containing a blend of resins may include MAH-g-CPO and one or more thermoplastic polymer components in a weight percent (wt%) ranging from 0 wt% to 45 wt%, 0 wt% to 40 wt%, or 0 wt% to 30 wt%, with the balance being MAH-g-CPO and / or additives.

[0020] The thermoplastic primer layer can be applied to the adhesive layer and / or substrate layer as a solid or thin film (e.g., 100% solids by weight). The thermoplastic primer layer can be produced by solvating the MAH-g-CPO resin, optionally with one or more additional thermoplastic polymers, in a suitable solvent and depositing the resulting solvated resin composition onto a substrate or surface. The solvent is then evaporated as the thermoplastic primer layer is formed. Suitable solvents may vary depending on the solubility of the selected primer resin or resin mixture and may include aqueous or organic solvents. Mixtures of nonpolar and polar organic solvents may also be used. Nonpolar solvents may include cycloalkyl or aromatic species, such as methylcyclohexane, toluene, etc. Polar organic solvents may include methyl ethyl ketone, ethyl acetate, butyl acetate, etc.

[0021] The organic solvent-salted resin composition may contain one or more primer resins (e.g., solids) at a weight percent (wt%) ranging from 1 wt% to 35 wt%, 1 wt% to 20 wt%, or 3 wt% to 20 wt%. The organic solvent-salted resin composition may contain a non-polar organic solvent at a weight percent (wt%) ranging from 60 wt% to 99 wt%, 65 wt% to 99 wt%, or 70 wt% to 99 wt%. The organic solvent-salted resin composition may contain a polar organic solvent at a weight percent (wt%) ranging from 1 wt% to 40 wt%, 1 wt% to 45 wt%, or 1 wt% to 30 wt%.

[0022] The aqueous solvated resin composition may include one or more primer resins in a weight percent (wt%) range of 15 wt% to 65 wt%, 20 wt% to 60 wt%, or 30 wt% to 55 wt%. The aqueous solvated resin composition may include an aqueous fluid in a weight percent (wt%) range of 35 wt% to 85 wt%, 40 wt% to 80 wt%, or 45 wt% to 70 wt%.

[0023] Methods for preparing the removable multilayer compositions disclosed herein can include providing a substrate surface, one or more thermoplastic primer layers, and one or more thermosetting adhesive layers. The thermoplastic primer layer and thermosetting adhesive layer can be produced by solid deposition or by coating from a solvent composition using known methods such as roller coating, flow coating, dip coating, spin coating, spray coating, knife coating, and die coating. The multilayer compositions disclosed herein can include a thermoplastic primer layer having a thickness ranging from 3 μm to 150 μm, 5 μm to 100 μm, or 8 μm to 90 μm. The layer thickness can be measured by any suitable method, including a PROGAGE™ thickness tester available from Thwing-ALBERT Instrument Company, where the layer thickness is determined after drying at room temperature for one day.

[0024] A method for removing a battery pack adhered to a substrate by a multi-layer composition can include heating the multi-layer composition to a "removal temperature" above 50°C to induce softening or melting of the thermoplastic primer layer, followed by separating one or more layers of the multi-layer composition to remove the battery pack from the substrate. The removal method can also include mechanically separating the primer and / or adhesive layer from the substrate layer by suitable techniques such as prying, wedging, and / or impact. In some cases, gravity or other "passive" techniques can also be used to separate one or more layers of the multi-layer composition.

[0025] The application of heat to the multilayer composition can involve the use of an external heat source, such as an electric heating platform, electric heating pad, electric heating sheet, or electric heating blanket, or an internal heat source, such as a heat management plate, heat management pad, or embedded heating element. Heat management plates are often used to maintain EV batteries within a steady temperature range (e.g., −20°C to 50°C). During operation, heat management plates operate to dissipate or supply heat to the EV battery by passive (e.g., heat sink) or active (e.g., using flowing fluids or gases) heat transfer. The methods disclosed herein may utilize a heat management plate or other methods to directly heat the primer layer and adhesive layer to a release temperature (i.e., a temperature above the transition temperature of the thermoplastic primer layer) while minimizing heat transfer from the heat source to the battery pack and subsequent battery damage.

[0026] Removing the substrate from the thermoplastic primer layer and the thermosetting adhesive layer can include heating the multi-layer composition to a temperature of 50° C. or greater and removing at least one of the layers of the multi-layer composition. Methods of delivering heat can include a heat management plate, an oven, a heat gun, steam, microwave radiation, resistive heating, induction heating, and the like.

[0027] Solvent-based primers were prepared by solubilizing primer resin particles in an 85:5 mixture of methyl cyclohexane (MCH) and methyl ethyl ketone (MEK). Resin particles were added to a flask at a designated solids content (e.g., 1 wt%, 3.5 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt%) and heated to 80°C with stirring until dissolved. After cooling to room temperature, the solvent-based primers were sealed and applied for testing.

[0028] In the test, the solvent-based primer is coated on substrates of different types and sizes, and the thickness of the primer layer is controlled by the solid content of the primer or by the coating time. After drying, the surface-treated substrate is subjected to lap shear strength, cross tensile strength, or laboratory-established removal tests.

[0029] Lap shear strength tests were performed in accordance with GB / T 7124. The tested substrates included epoxy-coated Type-3 aluminum (3003), Type-3 aluminum (3003), and Type-5 aluminum (5754) with dimensions of 25 mm x 100 mm x 1.5 mm. The substrate surfaces were cleaned with ethanol, brush-coated with the primer layer sample solution, and allowed to dry. A thermosetting adhesive was applied by mixing the isocyanate component and the isocyanate-reactive component under vacuum, and 0.5 g to 1.5 g of adhesive was applied to the substrate. A second substrate with the same bonding area was applied along the length of the adhesive layer. Pressure was applied, and the multilayer composition was cured at 23°C for 7 days. Lap shear strength was tested using an Instron testing system.

[0030] Cross-tensile strength tests were performed in accordance with GB / T 6329. The tested substrates included epoxy-coated Type 3 aluminum (3003) with a cylinder 60 mm high and 15 mm in diameter. Prior to treatment, the cylinder head surface was cleaned, a thermoplastic primer composition was applied, and the surface was allowed to dry. A control sample without primer treatment was also prepared. An adhesive composition was then prepared, and 0.5 g to 1.0 g was applied to the substrate. A second substrate was applied, and the flat surfaces of the substrates were bonded together by curing at 23°C for 7 days. Cross-tensile strength was measured using an Instron tensile tester.

[0031] The removal test was carried out using a qualitative method established in the laboratory, with the following process. 1) The substrate surface (100 mm x 150 mm x 2 mm) is cleaned by wiping with ethanol. 2) The substrate was then coated with primer using a brush and allowed to dry. A comparison sample was also prepared without primer treatment. 3) A thermoset adhesive layer was prepared by combining Part A and Part B of the 2K PU adhesive in a speed mixer at 1,000 rpm for 1 minute. 4) Apply approximately 10 g to 15 g of adhesive to the substrate using two copper wires with a diameter of 1 mm to control the layer thickness. 5) A second substrate was then laminated lengthwise. The bonding surfaces of the substrates were then brought into contact and held in place with clips. The bonded substrates were then cured at 23°C for 7 days. 6) Before the removal test, the samples were heated in an oven at 80°C for 10 minutes. 7) The sample was then removed, secured in a bending clamp, and the substrate was manually pried apart using a screwdriver. Removal in less than one minute was considered a "success." "Failure" was indicated by two modes: cohesive failure (adhesive on both sides after removal) and adhesive failure (adhesive on one side after removal). [Example]

[0032] The following examples are presented to illustrate embodiments of the present invention, but are not intended to limit its scope. All parts and percentages are by weight unless otherwise indicated. Table 2 lists the materials used in the following examples.

[0033] [Table 2]

[0034] Example 1: Effect of primer resin type on performance In this example, sample multilayer compositions were prepared on epoxy-coated Type 3 aluminum substrates, which were first coated with different thermoplastic primer compositions. The substrates were then bonded with a thermosetting 2K PU structural adhesive, aged, and tested for cross-tensile strength and removability. For comparison, an unprimed epoxy-coated Type 3 substrate was also tested.

[0035] The results are summarized in Table 3. Data in parentheses represent exemplary percent strength loss compared to a control sample without a thermoplastic primer layer at 23°C.

[0036] [Table 3]

[0037] Example 1 shows that the polarity of the primer resin affects adhesion and removability properties, in particular, decreasing the polarity character of the primer increases removability at higher temperatures (e.g., >50°C) but decreases bond strength at room temperature, while increasing the polarity character of the primer resin (e.g., TPU, PET) maintains room temperature bonding and does not improve removability at high temperatures.

[0038] For CE1, when bonded with a thermosetting adhesive without priming, the cross-tensile strength of the specimen decreases from 22 MPa to 3.7 MPa as the temperature increases from 23°C to 90°C. However, the removal test results indicate inseparability and a cohesive failure mode within the thermosetting adhesive layer.

[0039] Samples CE3-CE8, bonded with less polar thermoplastic primers and thermosetting adhesives, exhibited reduced cross-tensile strength at 23°C or 90°C compared to CE1, potentially indicating that the lower polarity of the primer layer composition adversely affects bonding performance with polar thermosetting adhesives. The reduction in cross-tensile strength at 23°C exceeded 30% for the comparative samples, indicating that the thermoplastic primer layer with reduced polarity is poorly compatible with the thermosetting adhesive, reducing overall bond strength to unacceptable levels. In practice, this manifests as adhesive failure at the substrate / primer layer and / or primer layer / adhesive layer interfaces, while the adhesive layer remains intact.

[0040] For CE9-CE12, samples bonded with a polar thermoplastic primer and a polar thermosetting adhesive are shown in Table 3 to have cross tensile strengths at 23°C and 90°C equivalent to the unprimed CE1. CE9-CE12 were characterized by cohesive failure, indicating that the polar thermoplastic primer layer did not enhance removability at elevated temperatures.

[0041] Sample IE1, which contained an MAH-g-CPO-based primer, showed a slight decrease in cross-tensile strength of 8.2% at 23 °C compared to sample CE1, which did not contain a thermoplastic primer layer. However, at 90 °C, IE1 exhibited a cross-tensile strength of 2.3 MPa (a decrease of approximately 37% compared to CE1) and was successfully removed. The decrease in cross-tensile strength at high temperatures also correlated with the adhesive failure mode. Thus, the thermoplastic primer layer incorporating MAH-g-CPO exhibits a polar balance with epoxy-coated Al and 2K PU thermosetting adhesives, characterized by adequate bonding properties at low temperatures and removability at high temperatures.

[0042] Example 2: Effect of Thermoplastic Resin on MAH-g-CPO Primer Performance In the following examples, primer layer formulations containing MAH-g-CPO and blends of polymers of various polarities were tested for release performance on epoxy-coated Type 3 Al substrates. Samples were prepared essentially as described for Example 1. Data in parentheses represent exemplary percent strength loss compared to a control sample without a primer layer at 23°C.

[0043] [Table 4]

[0044] When different dosages of hydrogenated rosin were introduced into the MAH-g-CPO-based primer, IE2 (4.5 wt% MAH-g-CPO and 0.5 wt% hydrogenated rosin) and IE3 (3.5 wt% MAH-g-CPO and 1.5 wt% hydrogenated rosin) indicate that inclusion of 0.5 wt% to 1.5 wt% hydrogenated rosin in the primer formulation (less than 50 wt% based on the solids content of the primer) resulted in acceptable cross-tensile strength at room temperature and high-temperature removal performance. Similarly, good adhesion and removal properties were obtained using a 3.5:1.5 mixture of MAH-g-CPO and TPU (IE4) and a 3.5:1.5 mixture of MAH-g-CPO and MAH-g-EVA (IE5).

[0045] Example 3: Effect of substrate on removal performance In the following example, a different substrate (epoxy-coated Al, Type-3 Al) was coated with the thermoplastic primer composition used in IE3 (3.5 wt% MAH-g-CPO + 1.5 wt% hydrogenated rosin) and bonded with a 2K PU thermosetting adhesive. Samples were prepared essentially as described for Example 1. Lap shear strength, cross tensile strength, and removal tests were performed. The results are summarized in Table 5. Cross tensile strength, lap shear strength, and removal test results show removal on Type-3 Al, with ease of removal being Type-3 Al > epoxy-coated Al.

[0046] [Table 5]

[0047] Example 4: Effect of primer layer thickness on performance In this example, samples were assayed having various thicknesses of solid thermoplastic primer layers on epoxy-coated Al substrates, then bonded with a thermosetting adhesive. The thickness of the thermoplastic primer layer was controlled by varying the solids content and / or using multiple layers of thermoplastic primer. Samples were prepared essentially as described for Example 1. The results are summarized in Table 6.

[0048] [Table 6]

[0049] Example 4 shows that, compared to the unprimed CE1, both the lap shear strength and cross tensile strength of CE16 and CE17 decreased when the dried thermoplastic primer layer thickness was less than 10 μm. However, when the dried thermoplastic primer layer thickness exceeded 10 μm, the performance of IE6-IE9 met the target adhesion and removal performance. However, selected multilayer compositions exhibit an upper limit of effectiveness at higher layer thicknesses. For example, when the dried solid primer layer thickness was approximately 80 μm, the CE18 data showed a 76.1% decrease in room temperature cross tensile strength compared to CE-1 without a thermoplastic primer layer.

[0050] While the foregoing is directed to exemplary embodiments, other and further embodiments may be devised without departing from the basic scope thereof, which scope is determined by the claims that follow.

Claims

1. 1. A multi-layer composition comprising: a substrate having a surface energy of 35 dynes / cm or greater; A multi-layer composition comprising one or more thermoplastic primer layers comprising 55% to 100% by weight of a maleic anhydride-grafted chlorinated polyolefin and having a glass transition temperature in the range of 50°C to 120°C, and one or more thermosetting adhesive layers.

2. 10. The multilayer composition of claim 1, wherein the maleic anhydride grafted chlorinated polyolefin has a maleic anhydride grafting degree greater than 1% and a chlorination degree content greater than 10%.

3. 10. The multi-layer composition of claim 1, wherein the maleic anhydride grafted chlorinated polyolefin has a glass transition temperature of 60.degree. C. to 80.degree. C.

4. 10. The multilayer composition of claim 1, wherein the one or more thermoplastic primer layers further comprise one or more additional thermoplastic polymers at a weight percent (wt%) of up to 45 wt%, and the one or more thermoplastic primer layers have a glass transition temperature of 50°C to 100°C.

5. 10. The multilayer composition of claim 1, wherein the one or more thermoplastic primer layers further comprise one or more additional thermoplastic polymers selected from maleic anhydride grafted polyolefin (MAH-g-POE), polyolefin (POE), maleic anhydride grafted ethylene-vinyl acetate (MAH-g-EVA), ethylene-vinyl acetate (EVA), maleic anhydride grafted styrene-ethylene-butylene-styrene (MAH-g-SEBS), hydrogenated rosin, thermoplastic polyurethane (TPU), or polyester (PET).

6. 10. The multilayer composition of claim 1, wherein the cross tensile strength of the multilayer composition is reduced by less than 30% at 23°C compared to a comparative multilayer composition that does not include the one or more thermoplastic primer layers.

7. 10. A method of removing the multi-layer composition of claim 1, comprising heating the multi-layer composition to a temperature greater than 50°C and removing at least one of the layers of the multi-layer composition.

8. An electric vehicle battery assembly comprising the multilayer composition of claim 1.

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