With the rapid development of new energy vehicles, which are mainly pure electric and hybrid electric vehicles, the core components of new energy vehicles - power batteries are also developing rapidly, and the application of structural glue has gradually been paid attention to.
The development and change of power batteries
In the previous years of the development of electric vehicles, the power battery was assembled into a module by several or dozens of cells (including cylindrical cells, square cells and soft pack cells), and then assembled and fixed in the PACK shell by several modules to form an automobile power battery pack.
Due to the existence of modules, this structure is more convenient for mechanical connection and fixing, and the application of structural adhesive is not very necessary.
In recent years, due to the trend pressure of electric vehicles to reduce weight and long driving range, CTP structures have come into being. The CTP structure increases the volume/weight energy density of the battery pack and further extends the vehicle's driving range.
CTP structure refers to the power battery pack is directly assembled into the PACK shell (celltopack) by the battery cell (especially the square cell), this structure omits or greatly reduces the intermediate module components, greatly reduces the overall quality of the battery pack, and directly contributes to the lightweight and mileage of electric vehicles. This CTP structure of the battery pack, generally require a lot of adhesive to connect the fixed battery cell, and can not use the traditional mechanical connection method.
Requirements for adhesives for CTP structures
The role of adhesive in the power battery CTP structure is very important, the adhesive used for CTP has two major types of practical needs: the first type is to take into account the structural bonding, taking into account a certain thermal conductivity; the second type is to conduct thermal bonding, the purpose of adhesive application is to export the heat generated when the battery cell works to the external heat dissipation parts, to achieve the partial function of thermal management, taking into account the structural bonding requirements.
In this article, we mainly explore the specific performance requirements of the first type of structural bonding. This type of structural adhesive must play a role in reliably connecting and fixing the battery cell and the PACK housing, replacing the mechanical connection of the original module structure.
CTP structure power battery assembly with structural glue
Final performance requirements
1. Strength requirements
Because the reliability and stability of the overall PACK and internal structure come from the bonding connection of the structural glue, instead of the original mechanical connection method, the bonding strength and body strength of the structural glue are required to have relatively high indicators, generally speaking, more than 8MPa, or even higher strength is necessary. Such a high-strength structural adhesive is applied to the side or bottom surface of the square cell, according to the size and shape of the cell, the general bonding area can reach more than 3000mm2, the bonding force of a single cell can reach more than 24000N, and some special shapes of square batteries can make the bonding area reach 17000mm2, and theoretically the adhesion force of a single cell can even reach more than 136000N, which better meets the requirements of the battery cell directly and stably fixed in the PACK housing. The strength of the structural adhesive can be measured by the tensile strength of the butt or the tensile shear strength of the lap.
2. Adaptability of bonding material: PET film, 3003 aluminum alloy
In order to make the battery cell obtain a high fixed bonding force, it is necessary not only to have the high tensile strength of the structural adhesive itself and a larger bonding area, but also to adapt to the bonded material and consider the body strength of the bonded material.
So what materials is the power battery structure glue of CTP structure suitable for? First of all, take a look at the material of the square battery: the shell of the square battery generally uses 3003 aluminum alloy. In order to make the battery cell shell insulated, most battery cell manufacturers use PET film-coated or semi-coated square battery shell (Figure 1); there is also an insulation powder coating (spray powder) to coat the battery cell shell with an insulating coating, see Figure 2.
Figure 1 Square batteries are generally coated with PET blue film
Figure 2 Insulated powder coated cell housing
The surface material of these two batteries and the corresponding PACK housing or insulating film are the objects of structural adhesive bonding: PET (also known as blue film), powder coating (epoxy or other resin) and 3003 aluminum alloy. Structural adhesives must have good adhesion to these materials.
Since the PET blue film generally uses pressure-sensitive adhesive backing to coat the PET blue film on the outside of the battery cell shell, or uses heat shrinkage to cover the battery cell shell, then the bonding of the structural glue to the PET blue film (generally subjected to plasma or flame pretreatment) can ensure that the bonding strength is greater than that of the PET blue film backing adhesive and the battery cell aluminum alloy shell or can achieve the strength of PET blue film destruction.
For the bonding of 3003 aluminum alloy and powder spray coating, it is necessary to achieve cohesion damage at the time of destruction, in other words, the destruction of the adhesive layer, rather than the interface between the adhesive layer and the aluminum alloy or powder spray coating.
3. Elasticity (or flexibility) requirements
Power battery PACK is loaded on the car, driving will inevitably be subject to bumps, shock and vibration, etc., the fixed bonding of the battery cell also has to withstand the test of various forces such as extrusion, peeling, bending and fatigue.
Bonding reliability depends not only on bond strength, but also on the flexibility of the adhesive layer when subjected to shock vibration. The flexibility of structural glue can be examined from two aspects.
First, elongation at break: elongation at break refers to the ratio of the elongation length of the adhesive body when it breaks after stretching and the length before stretching, which is expressed as a percentage. This specification characterizes the ability of the adhesive body to withstand force deformation when subjected to external forces, and the higher elongation at break means that it can withstand greater force deformation. An elongation at break of 100 per cent, or even 150 per cent ≥, is a concrete manifestation of this expectation.
The second is the elastic modulus (energy storage modulus): this is an indicator that reflects the material's ability to resist elastic deformation, which characterizes the ability of the material to store elastic deformation energy, the unit is MPa, which is the ratio of stress to strain, and the greater the value, the greater the stress of the material to undergo certain elastic deformation.
For the cell bonding reliability of power batteries, finite element analysis is often required, and an important input indicator for computer simulation analysis is modulus (or Poisson's ratio). Therefore, modulus is a very important indicator.
The energy storage modulus is tested using a dynamic thermomechanical analyzer (DMA) that is more accurate.
In the engineering applications of CTP structural adhesives, low modulus is a more desirable characteristic. Combined with the strength requirements of the first point, low modulus and high strength have a certain degree of mutual restraint. Some companies and some standard energy storage modulus (room temperature) require less than 800MPa, and even individual structural adhesives require less than 300MPa.
4. Resistant to aging requirements
The basic requirements for the long-term reliable operation of the car, transmitted to the power battery, the aging resistance of the structural adhesive in the CTP structure are equally important. For adhesives, the aging resistance can be examined through the cold and heat cycle and the damp heat aging test. You can refer to the relevant provisions of GB/T7124 for testing.
Cold and heat cycle test different automobile manufacturers or standards have different requirements, aging conditions refer to the commonly used aging test methods in automobiles, can be -40 ~ 80 °C (or 90 °C) high and low temperature alternating cycle, single cycle mode see Figure 3, the number of cycles can be agreed by the battery pack design and adhesive suppliers.
Figure 3 Hot and Cold Cycle Diagram
Resistant to humid heat aging, generally "double 85" (85 ° C, 85% relative humidity) conditions, 1000h aging. The performance indicators after aging resistance are compared with the initial indicators, not less than 70%.
5. Flame retardant performance requirements
Structural adhesive is applied to power batteries, as part of the automotive power battery materials, flame retardancy is the basic requirement. Considering its application environment, the flame retardant test adopts the thickness of the aluminum-glue-rubber-aluminum sandwich structure layer of 0.5mm for testing, and the requirements are V0. Testing can be carried out in accordance with the provisions of GB/T2408.
6. Electrical insulation performance requirements
Structural adhesive is directly applied to the battery cell bonding, insulation is also the basic requirement, dielectric strength is greater than 10kV/mm; volume resistivity is greater than 1×1012Ω.cm. It can refer to the relevant provisions of GB/T1408 for testing.
7. Thermal conductivity
For high-strength and low-modulus structural adhesives, the thermal conductivity of adhesives is generally not examined.
In the power battery PACK system, thermal management is a very important subsystem, the heat of the battery cell needs to be able to be dissipated in time to ensure that the battery is within the optimal working temperature range, then the need for a special thermal conductive material (such as thermal adhesive) to play this role, its main role and the structural glue we discussed earlier is not the same, it focuses on the thermal conductivity. Of course, the structural adhesive has a certain thermal conductivity is better, the structural adhesive as the main appeal of the thermal conductivity can be more than 0.2W / (m ·K), can play a certain thermal conductivity and heat dissipation effect [the thermal conductivity of the air is about 0.0267W / (m·K)].
8. Shaker bench test
For the bonding of CTP structural cells with high strength and low modulus structure glue, whether it meets the normal driving requirements of new energy vehicles, or whether it is necessary to test the vibrating bench of the overall battery pack to meet the requirements of shock and vibration resistance, you can refer to the "GB/T31467.3 Lithium-ion Power Battery Pack and System for Electric Vehicles Part 3: Safety Requirements and Test Methods".
Process performance requirements
Engineering adhesive is an applied science, structural adhesive is applied in the power battery PACK assembly process, the importance of its application process is self-evident.
CTP structure of the power battery PACK is characterized by the size of the cell increased or specific shape, the cancellation of the intermediate module or greatly reduced, the assembly process is directly assembled by the battery cell bonded to the PACK shell: the amount of glue used for bonding is relatively large, the glue line is relatively long; the bonding time varies according to the degree of automation; the assembly of the power battery PACK, due to the large size, the handling is relatively difficult; the automotive industry is characterized by large production batches and fast beats.
Structural adhesives should adapt to the above assembly and structural characteristics, and the specific requirements of its process performance can be reflected in the following aspects:
1. Operational time requirements
Generally speaking, the operability time refers to the time when the adhesive remains sticky after sizing and does not affect the bonding effect. For two-component structural adhesives, A/B components begin to react after mixing, and after a certain period of time, the mixed adhesive will lose its viscosity, and the adhesion will have a potential failure such as poor infiltration, which can be considered as operable time.
Because the power battery PACK components and battery cell size are relatively large, the need to glue assembly parts more, after the adhesive is applied to the parts of the bonding assembly time is determined by the assembly process beat and equipment capacity, then the operational time of the structural glue should consider from A / B components after mixing (if it is two-component) to the bonded parts of the closed assembly, tooling fixture positioning time interval, and leave enough safety factor.
The operation time of the corresponding structural glue will be different depending on the beat of the assembly process, the degree of automation is different, and the subsequent curing method is different.
If it is a fast beat assembly, curing after room temperature, the operating time requirements are relatively short, and may require ≥ 5min, or even shorter; if the assembly beat is relatively slow and the assembly line considers the heating curing after gluing assembly, the operable time can be longer, or even ≥ 40min.
2. Initial solidification time (curing speed or time to reach positioning strength) requirements
The curing characteristics of the adhesive are gradually increasing in strength over time (or other measures such as heating). Combined with the power battery PACK bonding assembly process and subsequent process, the bonded battery cell has a certain structural strength after more convenient for the implementation of the subsequent assembly process, so the initial solidification time of the structural glue (curing speed or positioning strength time) is also a very important process performance indicator.
According to the PACK assembly process, tooling and equipment design, the curing process after bonding has two ways: room temperature curing and heating curing.
In order to ensure assembly efficiency, room temperature curing structural adhesives generally require that the positioning strength (about 0.5MPa) that can be handled in the shortest possible time is required, which is contradictory to sufficient operability time. Therefore, the initial fixation time to achieve the positioning intensity is generally 30 to 60 minutes.
The heated curing structural adhesive can take into account the requirements of long operability time and rapid positioning of initial solidification, and the initial solidification time is 15 to 20min under the condition of 70~80 °C, reaching the positioning strength of 0.5MPa. Post-curing can achieve strength of more than 8MPa at 80 °C, 4h or basically complete cure. Heated curing assembly lines need to be equipped with heating equipment.
3. Thixotropy and viscosity requirements
In the process of bonding and assembling the battery PACK battery, the process actions such as the battery cell being taken, positioned and glued, as well as the assembly bonding process action after the glue is completed, are likely to need to be flipped, and the glue position may also be vertical facade. This requires that the structural adhesive applied to the battery cell or PACK housing (or protective layer) cannot flow, and the applied adhesive adhesive line needs to maintain the shape of the application to ensure that the assembly bonding process is reliable and stable. Therefore, good thixotropy is a necessary process performance of structural adhesives to ensure that the shape of the applied adhesive is stable and the façade does not flow.
At the same time, because the power battery PACK size is relatively large, the assembly process of the gluing machine manual operating radius is also large, some up to 2m or more, coupled with the gluing process site, battery cell loading station and the actual distance of the structural rubber barrel positioning station, the length of the structural glue pipeline is relatively long, may reach 5 ~ 10m, so the viscosity of the structural glue should not be too high, in order to facilitate smooth gluing, A / B components after mixing the viscosity should not exceed 60000cps. Otherwise, a very high conveying pressure is required, which may also cause a low glue delivery rate, affecting the glue application efficiency and the overall assembly efficiency. In addition, the structural glue of A/B components, the viscosity of the two components should not be too different, in order to ensure a stable mixing effect.
4. Mixing ratio requirements
The structure of the general power battery is two-component, the adhesive after mixing uniformly can play its performance, for the mixing ratio of A / B components, although not mandatory, but considering the stability of the mixing, the actual engineering application or the proportion of A / B components is not much different, more able to ensure that the mixing is uniform and stable. The mixing ratio of A/B components 1:1 or 2:1 or 4:1 is conducive to the relatively stable and accurate mixing of the applicator, and is also conducive to the stable guarantee of bonding quality.
epilogue
In summary, the structural glue for the bonding of the power battery CTP structure, from the application point of view, the basic performance requirements have good material adaptability, high strength and low modulus, good weather resistance and flame retardant insulation and environmental protection and other basic properties. With the assembly process of the power battery, there are some engineering process performance requirements in terms of operating time, initial fixing time, thixotropy and mixing ratio. H.B. Fuller in-depth power battery bonding application research, the recently launched power battery structure adhesive product AT1241/AT1242, with higher strength and lower modulus, fully fits the application environment of new energy vehicles and assembly process requirements, can be cured according to the assembly process at room temperature or high temperature heat curing, so that CTP structure of the power battery PACK assembly more choices.
Source: AI "Automotive Manufacturing"
Author: Hu Dongsheng Xu Duowen
Work unit: H. H. Fuller (China)
【Important Statement】This article is an original article and may not be reproduced without permission
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