A method for selecting an adhesive for bonding plastic to metal

Abstract

The present application belongs to the technical field of bonding, and discloses a selection method of adhesive for connecting plastic and metal. The selection method comprises calculating the temperature difference in the metal production process and use process, the difference in thermal expansion coefficient between the metal and the plastic, the length difference between the metal and the plastic caused by the temperature difference, the change rate of the adhesive layer thickness, and determining the correctness of the selected adhesive by comparing the change rate of the adhesive layer thickness with the maximum allowable long-term elongation of the adhesive. Only when gamma is less than beta, the selected adhesive is correct, otherwise the adhesive needs to be reselected and the size of gamma and beta is compared again. The present application compares and analyzes the change rate of the adhesive layer thickness caused by the temperature difference with the maximum allowable long-term elongation of the adhesive, determines the correctness of the selected adhesive by comparing the change rate of the adhesive layer thickness with the maximum allowable long-term elongation of the adhesive, and makes the adhesive layer not easy to crack.

Description

Technical Field

[0001] This invention belongs to the field of adhesive technology, and specifically relates to a method for selecting adhesives for connecting plastics and metals. Background Technology

[0002] The connection between different materials or even the same materials often requires the use of adhesives. In particular, different materials have different coefficients of thermal expansion, which can lead to the adhesive layer cracking after a period of time.

[0003] For example, in the connection between the driver's cab cover and the car body of rail transit vehicles, the cover is often made of plastic, such as fiberglass, while the car body is generally made of alloy, such as aluminum alloy. If an adhesive is chosen arbitrarily, the adhesive layer at the connection between the cover and the car body is prone to cracking. In existing technologies, the selection of adhesives for connecting the cover and the car body often relies on experience, which makes the adhesive layer at the connection prone to cracking.

[0004] Therefore, there is an urgent need for a new method for selecting adhesives. This method can analyze and select adhesives that can effectively bond plastics and metals, greatly reducing or avoiding cracking of the adhesive layer between plastics and metals. Summary of the Invention

[0005] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a method for selecting an adhesive for connecting plastic and metal. The selection method of this invention can select a suitable adhesive to fill the gap between the plastic and the metal, thereby connecting them. Furthermore, the adhesive layer formed in the gap is not easily cracked, thus providing a very stable connection.

[0006] The inventive concept of this invention is as follows: This invention utilizes a comparative analysis of the rate of change in adhesive layer thickness caused by temperature difference and the maximum permissible long-term elongation of the adhesive. The correctness of the selected adhesive is determined by comparing the rate of change in adhesive layer thickness (γ) and the maximum permissible long-term elongation of the adhesive (β). Only when γ < β is the selected adhesive considered correct; otherwise, a new adhesive needs to be selected and the comparative analysis repeated, thereby effectively determining the rationality of the selected adhesive. Furthermore, the selection method described in this invention also provides an effective method for accurately obtaining the maximum permissible long-term elongation of the adhesive.

[0007] A first aspect of the present invention provides a method for selecting an adhesive for joining plastic and metal.

[0008] Specifically, a method for selecting an adhesive for joining plastic and metal is characterized by comprising the following steps:

[0009] (1) Calculate the temperature difference ΔT1 during the metal-plastic bonding process (i.e., bonded joint) and the temperature difference ΔT2 during the cooling process, using the temperature range t1 to t2 of the environment during the metal-plastic bonding process (bonded joint) and the temperature range t3 to t4 of the environment during the use process. ΔT1 = t4 - t1, ΔT2 = t2 - t3. Compare ΔT1 and ΔT2, and ΔT = max(ΔT1, ΔT2).

[0010] (2) The coefficient of thermal expansion of the metal is α1, the coefficient of thermal expansion of the plastic is α2, and the difference in the coefficients of thermal expansion between the metal and the plastic is Δα = α1 - α2;

[0011] (3) Measure the length l of the gap between the metal and the plastic and the thickness d of the adhesive layer, where the thickness of the adhesive layer is equal to the width of the gap;

[0012] (4) Calculate the length difference Δl between the metal and the plastic caused by the temperature difference, where Δl=l / 2*ΔT*Δα;

[0013] (5) Calculate the rate of change γ of the adhesive layer thickness. In the formula, s is the safety factor, and s is not less than 2;

[0014] (6) Evaluate the rationality of the selected adhesive. The correctness of the selected adhesive is determined by comparing the rate of change of adhesive layer thickness γ and the maximum allowable long-term elongation β of the adhesive. Only when γ < β is the selected adhesive correct. Otherwise, the adhesive needs to be reselected and the magnitudes of γ and β need to be compared again.

[0015] Preferably, a method for selecting an adhesive for joining plastic and metal includes the following steps:

[0016] (1) Calculate the temperature difference ΔT1 during the heating stage and the temperature difference ΔT2 during the cooling stage using the temperature range t1 to t2 of the environment during the metal production process and the temperature range t3 to t4 of the environment during the metal use process, respectively. Where ΔT1 = t4 - t1 and ΔT2 = t2 - t3. Compare ΔT1 and ΔT2, and take the maximum value between the two, that is, ΔT = max(ΔT1, ΔT2).

[0017] (2) The coefficient of thermal expansion of the metal is α1, the coefficient of thermal expansion of the plastic is α2, and the difference in the coefficients of thermal expansion between the metal and the plastic is Δα = α1 - α2;

[0018] (3) Measure the length l of the gap between the metal and the plastic and the thickness d of the adhesive layer, where the thickness of the adhesive layer is equal to the width of the gap;

[0019] (4) Calculate the length difference Δl between the metal and plastic caused by the temperature difference, where Δl=l / 2*ΔT*Δα. In the formula, l / 2 is because both shrinkage and expansion are carried out simultaneously in two directions and the two directions are consistent. Therefore, we only need to take the change in one direction to facilitate the subsequent calculation of the change rate of the adhesive layer thickness.

[0020] (5) Calculate the rate of change γ of the adhesive layer thickness. In the formula, s is the safety factor, and s is not less than 2;

[0021] (6) Evaluate the rationality of the selected adhesive. The correctness of the selected adhesive is determined by comparing the rate of change of adhesive layer thickness γ and the maximum allowable long-term elongation β of the adhesive. Only when γ < β is the selected adhesive correct. Otherwise, the adhesive needs to be reselected and the magnitudes of γ and β need to be compared again.

[0022] Preferably, in step (1), the metal is an aluminum alloy, such as aluminum alloy A6N01S-T5.

[0023] Preferably, in step (1), the value of t1 is in the range of 10-18℃, preferably 12-15℃.

[0024] Preferably, in step (1), the value of t2 is in the range of 30-40℃, preferably 30-35℃.

[0025] Preferably, in step (1), the value of t3 is in the range of -30℃ to -20℃, and more preferably -28℃ to -25℃.

[0026] Preferably, in step (1), the value of t4 is in the range of 38-42℃, preferably 38-40℃.

[0027] Preferably, in step (2), the plastic is fiberglass.

[0028] Preferably, in step (2), α1 is (2.0-2.5)×10 -5 2.4×10 -5 .

[0029] Preferably, in step (2), α2 is (2.6-2.9)×10 -6 2.7×10 is preferred. -6 .

[0030] Preferably, in step (5), the type of adhesive selected for γ≤5% is different from that selected for γ>5%.

[0031] Preferably, in step (5), a high-modulus, low-elasticity adhesive is selected when γ ≤ 5%; and a low-modulus, high-elasticity adhesive is selected when γ > 5%. When γ ≤ 5%, a high-modulus, low-elasticity adhesive can be used for grouting, resulting in a smooth and non-deformable adhesive layer surface. For example, a high-modulus, low-elasticity adhesive can be used for grouting the plastic and metal surfaces of the vehicle floor because the interior environment of the vehicle is moderate and does not change much during vehicle operation, and is less affected by thermal loads. When γ > 5%, a low-modulus, high-elasticity adhesive should be used for grouting to avoid cracking.

[0032] In existing technologies, there is no standard method in the industry to determine the maximum permissible long-term elongation β of adhesives, so the value of β is usually determined by experience. However, this invention proposes a method for accurately measuring β, which is an inventive point of this invention.

[0033] To further obtain a precise value of the maximum permissible long-term elongation β of the adhesive, the present invention also provides a method for obtaining β.

[0034] Preferably, in step (6), the method for obtaining the maximum permissible long-term elongation β of the selected adhesive includes the following steps in sequence:

[0035] Step (S1), Sample making: Select n sets of dumbbell-shaped test fixtures, n is greater than 3, and make n sets of samples using the selected adhesive and test fixtures. The sample includes adhesive joints at both ends and a substrate in the middle. The adhesive joint size of each set of samples is a×b×L. The size of a, b, and L must be selected to ensure that the strength of the substrate in the middle is less than the strength of the adhesive joints at both ends.

[0036] Step (S2), sample curing: Fix the 5 sets of samples in step (1) and place all samples at room temperature for curing for more than 7 days;

[0037] Step (S3), high and low temperature cycling test: The sample is placed in a high and low temperature aging test chamber for aging test. The test duration is more than 100 cycles. The conditions for one cycle of the cycling test are: place at 70-80℃ and 85-95% relative humidity for 3-4 hours, cool to -35℃ to -40℃ within 1.5-2 hours, place at -35℃ to -40℃ for 3-4 hours, and heat up to 70-80℃ and 85-95% relative humidity within 1.5-2 hours. After all cycles are completed, place at room temperature for more than 20 hours.

[0038] Step (S4), Sample testing: Fix the sample to be tested on the two clamps of the tensile testing machine and perform tensile and shear tests at a test speed of 190-200 mm / min. After all samples have been tested, check the test curve and record the maximum displacement m when the stress and strain in each group of samples have a linear relationship.

[0039] Step (S5): Calculate the maximum permissible long-term elongation β of the selected adhesive. in L is the average of the maximum displacement of all samples, and L is the length of the sample.

[0040] More preferably, in step (6), the method for obtaining the maximum permissible long-term elongation β of the selected adhesive includes the following steps in sequence:

[0041] Step (S1), Sample fabrication: Select n sets of dumbbell-shaped test fixtures, where n is greater than 3. Use the selected adhesive and test fixtures to fabricate n sets of samples. Each sample includes adhesive joints at both ends and a substrate in the middle. The adhesive joint size of each set of samples is a×b×L. The size of a, b, and L must be selected to ensure that the strength of the substrate in the middle is less than the strength of the adhesive joints at both ends. That is, ensure that the failure mode of the adhesive joint during the tensile-shear test is cohesive failure, so as to ensure that the obtained test curve includes the complete deformation behavior of the adhesive body.

[0042] Step (S2), sample curing: fix the 5 sets of samples in step (1) (for example, by using clips to fix them) to avoid poor adhesion or misalignment of the adhesive joint size during the curing process, and place all samples at room temperature for curing for more than 7 days.

[0043] Step (S3), high and low temperature cycling test: The sample is placed in a high and low temperature aging test chamber for aging test. The test duration is more than 100 cycles. The conditions for one cycle of the cycling test are: place at 70-80℃ and 85-95% relative humidity for 3-4 hours, cool to -35℃ to -40℃ within 1.5-2 hours, place at -35℃ to -40℃ for 3-4 hours, and heat up to 70-80℃ and 85-95% relative humidity within 1.5-2 hours. After all cycles are completed, place at room temperature for more than 20 hours.

[0044] Step (S4), Sample testing: Fix the sample to be tested on the two clamps of the tensile testing machine and perform tensile and shear tests at a test speed of 190-200 mm / min. After all samples have been tested, check the test curve and record the maximum displacement m when the stress and strain are linearly related in each group of samples. That is, the maximum displacement when the slope of the curve is constant. When the stress and strain are linearly related, the deformation behavior of the sample is elastic deformation, which satisfies Hooke's Law σ=E*ε. When the stress is removed, the sample will immediately return to its original state and will not affect its long-term use.

[0045] Step (S5): Calculate the maximum permissible long-term elongation β of the selected adhesive. in L is the average of the maximum displacement of all samples, and L is the length of the sample.

[0046] Preferably, in step (S1), the value of n is 4-8, preferably 5.

[0047] Preferably, in step (S1), the test fixture is made of aluminum alloy or carbon steel.

[0048] Preferably, in step (S1), if the adhesive strength is high, the cohesive failure mode can be obtained by increasing the bonding area.

[0049] Preferably, step (S1), the sample making process also includes cleaning the test fixture and applying an adhesion promoter.

[0050] Preferably, in step (S3), the test duration is 110-120 cycles.

[0051] Preferably, in step (S3), the product is placed at room temperature for more than 24 hours.

[0052] A second aspect of the invention provides an application of a method for selecting an adhesive for joining plastics and metals.

[0053] The above-mentioned method for selecting adhesives to connect plastics and metals is applied to the connection between the fixed cover of the vehicle driver's cab and the gap in the vehicle body.

[0054] Preferably, the vehicle driver's cab cover is made of plastic, and more preferably fiberglass.

[0055] Preferably, the vehicle body is made of metal, and more preferably aluminum alloy.

[0056] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0057] This invention utilizes a comparative analysis of the rate of change in adhesive layer thickness caused by temperature difference and the maximum permissible long-term elongation of the adhesive. The correctness of the selected adhesive is determined by comparing the rate of change in adhesive layer thickness (γ) and the maximum permissible long-term elongation (β) of the adhesive. Only when γ < β is the selected adhesive considered correct; otherwise, a new adhesive must be selected and the comparison analysis repeated, thus effectively determining the rationality of the selected adhesive. Furthermore, the selection method described in this invention also provides an effective method for accurately obtaining the maximum permissible long-term elongation of the adhesive.

[0058] The adhesive selected by the method of the present invention is used to fill the gap between plastic and metal. The resulting adhesive layer is not easy to crack. Therefore, the adhesive selection method of the present invention can be used to fill the gap between the vehicle driver's cab cover and the vehicle body. Even when the vehicle is running, the adhesive layer is not easy to crack. Attached Figure Description

[0059] Figure 1 This is a diagram of the test fixture used in step (S1) of Example 1;

[0060] Figure 2 This is a diagram of the sample inside the test fixture in step (S2) of Example 1;

[0061] Figure 3 This is a diagram showing the cohesive failure of the sample in Example 1;

[0062] Figure 4 The image shows the tensile test curve of the adhesive in step (S4) of Example 1. Detailed Implementation

[0063] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0064] Unless otherwise specified, the raw materials, reagents or devices used in the following examples are available from conventional commercial sources or can be obtained by existing known methods.

[0065] Example 1

[0066] Taking the gap at the connection between the driver's cab fixed cover (made of fiberglass) and the car body (made of aluminum alloy A6N01S-T5) of a high-speed train as an example, the method of this invention is used to analyze and select adhesives. The specific implementation method is as follows:

[0067] A method for selecting an adhesive for connecting the driver's cab mounting cover (made of fiberglass) of a high-speed train to the car body (made of aluminum alloy) includes the following steps:

[0068] (1) According to the existing technical conditions, the ambient temperature range of the bonding process between the car body and the fixed cover of the EMU driver's cab is 15℃-35℃, and the ambient temperature range of the vehicle operation process (after the car body and the fixed cover of the EMU driver's cab are bonded) is -25℃ to 40℃. Therefore, the temperature difference ΔT1 in the heating stage of 5℃-40℃ is 25℃, and the temperature difference ΔT2 in the cooling stage of 35℃ to -25℃ is 60℃. So the maximum temperature difference ΔT is 60℃.

[0069] (2) In this embodiment, the coefficient of thermal expansion α1 of the aluminum alloy body is 2.4 × 10⁻⁶. -5 The coefficient of thermal expansion α2 of fiberglass is 2.7 × 10⁻⁶. -6 The difference in thermal expansion coefficients between the two substrates is Δα = α1 - α2 = 2.13 × 10⁻⁶. -5 ;

[0070] (3) The measured circumference length l of the gap between the driver's cab fixed cover and the vehicle body is 5890mm, and the gap width, i.e. the thickness of the adhesive layer d, is 8mm.

[0071] (4) Calculate the length difference Δl between the fiberglass and aluminum alloy substrates caused by the temperature difference: Δl = l / 2 * ΔT * Δα = 5890 / 2 * 60 * 2.13 * 10 -5 =3.76mm;

[0072] (5) Calculate the rate of change γ of the adhesive layer thickness, and substitute the above values ​​into the formula. In the formula, s is the safety factor, which is taken as 2. It can be seen that under normal vehicle operation conditions, the adhesive at the connection between the driver's cab cover and the vehicle body will have a long-term change rate of 21%. Therefore, a flexible adhesive should be selected (for example, Sika265 single-component moisture-curing polyurethane sealant).

[0073] (6) Further evaluate the rationality of the selected adhesive. The correctness of the selected adhesive is determined by comparing the change rate of adhesive layer thickness γ and the maximum allowable long-term elongation β of the adhesive. As calculated above, γ = 21% < β = 37%, indicating that the change rate of adhesive layer thickness during vehicle operation is less than the maximum allowable long-term elongation of the adhesive itself. The adhesive layer will not crack due to the influence of heat load, etc. Therefore, the selected adhesive is correct.

[0074] Step (6), the method for obtaining the maximum permissible long-term elongation β of the selected adhesive, includes the following steps in sequence:

[0075] Step (S1): Select 5 dumbbell-shaped test fixtures made of aluminum alloy. Use masking tape to protect the two long sides of the test fixtures. At the same time, the masking tape should also be attached to the test workbench.

[0076] Wipe the inner surfaces of the two short sides of the test fixture (i.e. the bonding surfaces) with clean, lint-free wiping paper dipped in anhydrous alcohol until there is no grease or dust. Use a pneumatic grinder with 80-grit sandpaper to grind the bonding surfaces of the test fixture until the body color of the aluminum alloy is exposed. Wipe the bonding surfaces of the test fixture again with wiping paper dipped in anhydrous alcohol and let it dry for 15 minutes.

[0077] Wipe the bonding surfaces of the test fixture with an activator (silane coupling agent) using a sponge and let it dry for 20 minutes. Then, use a nano sponge to apply a thin layer of primer (isocyanate homopolymer) to the bonding surfaces and let it dry for 20 minutes. The primer should be applied as thinly as possible to fully cover the bonding surfaces without exposing the substrate. Do not brush back and forth. Before using the primer, shake it at about 45 degrees Celsius until the small balls in the container holding the primer roll freely and make a knocking sound. Continue shaking for 1 minute.

[0078] The test fixture is assembled by passing through the two protected long sides and the two activated and primed short sides, and the four corners are fixed with clamps.

[0079] Using a manual glue gun, apply the above-mentioned single-component moisture-curing polyurethane adhesive into the dumbbell-shaped groove of the test fixture. When applying the adhesive, the nozzle should be inserted deep into the bottom of the dumbbell-shaped groove to ensure that the adhesive overflows from the bottom of the groove. After application, use a scraper to press and smooth the adhesive to ensure that there are no voids inside the groove. The bonding dimensions of all samples are 30mm×10mm×100mm.

[0080] Step (S2): After the adhesive is applied, the test fixture is fixed with clamps and the above 5 sets of samples are placed at room temperature for curing for 7 days. During the curing process, ensure that all samples are well fixed to avoid deviations in the bonding dimensions.

[0081] Step (S3): After curing, the above sample is placed in a high and low temperature aging test chamber for aging test. The test duration is 120 cycles. The conditions for one cycle of the cyclic alternation test are: placed at 80℃ and 95% relative humidity for 4 hours, cooled to -40℃ within 2 hours, placed at -40℃ for 4 hours, and heated to 80℃ and 95% relative humidity within 2 hours. The conditions for other cycles of the cyclic alternation test are the same as those for this one cycle. After all cycles are completed, the sample is placed at room temperature for 24 hours.

[0082] Step (S4): After the aging test, fix the sample to be tested on the two clamps of the tensile testing machine and perform a tensile test at a test speed of 200 mm / min. After all samples have been tested, check the test curve and record the maximum displacement m when the stress and strain of each group of samples are linearly related, as shown in Table 1 below.

[0083] Step (S5): Calculate the maximum permissible long-term elongation β of the selected adhesive, β = m / L * 100% = 37.0 / 100 = 37%, where L is the length of the adhesive strip (sample), i.e., 100 mm.

[0084] Table 1

[0085]

[0086] Figure 1 This is a diagram of the test fixture used in step (S1) of Example 1; Figure 2 This is a diagram of the sample inside the test fixture in step (S2) of Example 1; Figure 3 This is a diagram showing the cohesive failure of the sample in Example 1; Figure 4 This is a graph showing the adhesive tensile test results in step (S4) of Example 1. Figure 4 In this context, σ=Eε is the formula for Hooke's Law.

[0087] Example 2

[0088] After selecting the adhesive using the method in Example 1, apply it by following these steps:

[0089] (1) Before operation, confirm the ambient temperature and humidity. The temperature should be 15℃-35℃ and the relative humidity should be 50%-75%. Apply protective tape around the gap between the driver's cab cover and the vehicle body. The tape should be applied starting 3mm from the outside of the gap to ensure that the sealing surface meets the requirements for subsequent adhesive application.

[0090] (2) Wipe the surface of the fixing cover and the area to be connected on the vehicle body with clean, lint-free wiping paper dipped in anhydrous alcohol until there is no grease or dust. Use a pneumatic grinder with 80-grit sandpaper to grind the surface of the area to be connected until the original color of the fiberglass and aluminum alloy is exposed (Note: Grinding of deeper areas in the gaps should be completed before the fixing cover is installed). Wipe the surface of the fixing cover and the area to be connected on the vehicle body with wiping paper dipped in anhydrous alcohol again and let it dry for 15 minutes.

[0091] (3) Use a nano sponge to apply activator (silane coupling agent) to wipe the area to be connected and let it dry for 20 minutes. Use a nano sponge to apply primer (isocyanate homopolymer) to the surface of the area to be connected and brush a thin film layer on it. Let it dry for 20 minutes. The thinner the film layer, the better. It should fully cover the area to be connected without exposing the base layer. Do not brush back and forth.

[0092] (4) Use a pneumatic glue gun to apply the adhesive selected in Example 1 to the gap between the fixed cover and the vehicle body. When applying the adhesive, the nozzle of the pneumatic glue gun should be inserted as deep as possible into the bottom of the gap to ensure that the adhesive overflows from the bottom outward. After the adhesive is applied, use a scraper to press and smooth the adhesive to ensure a seal without voids. After the adhesive is applied, it needs to be cured at room temperature for 50 hours.

[0093] (5) After the adhesive has cured, remove the protective tape and wipe the vehicle body, mounting cover and adhesive surface with clean, lint-free wiping paper dipped in anhydrous alcohol until there is no grease and dust. Use a pneumatic grinder with 80-grit sandpaper to polish the cleaned surface until the mounting cover (fiberglass) and vehicle body (aluminum alloy) are exposed. Wipe the surface again with wiping paper dipped in anhydrous alcohol and let it dry for 15 minutes.

[0094] (6) Cut glass fiber mats with widths of 80mm, 100mm, and 120mm and lengths of 5890mm for later use. Mix epoxy resin with a weight ratio of 3:1 between the main agent (the main agent is a mixture of epoxy resin (50%) and modified toughening resin (40%), the main agent product model is MegaGlue4006 / B20) and the curing agent (the curing agent is modified amine, the product model is MegaGlue HD4006 / B20) and let it stand for about 30 minutes to activate. Use a roller to apply epoxy resin evenly to the car body, the fixing cover, and the surface of the adhesive that needs to be leveled. Apply epoxy resin evenly to the surface of the 80mm wide glass fiber mat in the same way, and lay the glass fiber mat on the car body, the fixing cover, and the surface of the adhesive that needs to be leveled. After completion, use a roller to apply epoxy resin to the surface of the laid glass fiber mat to ensure that the epoxy resin completely wets the glass fiber mat layer. At the same time, remove the bottom air bubbles by rolling.

[0095] (7) After the 80mm wide glass fiber mat (first layer) is laid and bonded, the 100mm wide glass fiber mat (second layer) and the 120mm wide glass fiber mat (third layer) are laid and bonded in the same way, and cured at room temperature for 30 hours.

[0096] (8) After the glass fiber mat layer has cured, use 80-grit sandpaper to sand the surface of the glass fiber mat layer and clean the dust with a tack cloth. Mix the epoxy primer with the following weight ratio: (the main agent is epoxy resin, and the main agent product model is THD06WEP-200) and the curing agent (the main component of the curing agent is polyamide emulsion, and the curing agent product model is WEP-200-G) at 5:1. Use a high-pressure airless spray gun and a wet-on-wet spraying process to spray two coats of epoxy primer onto the surface of the glass fiber mat layer. Each coat needs to be surface dry for about 30 minutes. After the overall spraying is completed, cure at room temperature for 20 hours. The average dry film thickness is controlled at about 60μm.

[0097] (9) Use sandpaper to lightly sand the epoxy primer surface and clean it with a dust cloth. Mix the elastic polyester putty (manufacturer Sifang Weikai, model UP100) at a weight ratio of 100:2.5. Level the areas with larger depressions in advance. After curing for 3 hours, use a soft scraper to apply the elastic polyester putty to the surface. Apply the elastic polyester putty horizontally along the outside of the fixed cover for the first time. Apply the putty vertically along the perimeter of the fixed cover for the second time. For the third and fourth times, the consistency of the elastic polyester putty needs to be slightly reduced (add 2% of thinner in 3 stages. The main component of the thinner is styrene, which improves the fluidity of the putty until the putty can fully fill the pores and small pits on the surface). Apply the putty to the surface using horizontal and vertical scraping methods respectively. The thickness of each scraping should not exceed 1mm, and the total thickness should be 3mm until the surface flatness meets the requirements. Cure each layer at room temperature for 4 hours to form a putty layer.

[0098] (10) Use 80-grit, 120-grit, and 180-grit sandpaper to sand the putty layer step by step and clean it with a dust cloth. Mix polyurethane intermediate paint (polyurethane intermediate paint includes main agent and curing agent, the main agent is mainly acrylic resin emulsion, model TBZ04 WIPU-100, and the curing agent is mainly 1,6-hexamethylene diisocyanate homopolymer, model WIPU-100-G) with an air spray gun and a wet-on-wet spraying process to spray two coats of polyurethane intermediate paint on the surface of the putty layer. Each coat needs to be surface dry for about 30 minutes. After the overall spraying is completed, cure at room temperature for 18 hours. The average dry film thickness is controlled at about 50 μm to form a polyurethane intermediate coating.

[0099] (11) Visually inspect the surface of the polyurethane intermediate coating under light intensity of not less than 400 Lx to check for any obvious uneven areas. If any are found, sand them and then fill them with putty. After that, sand the polyurethane intermediate coating with 240 grit sandpaper and clean it with a dust cloth. Mix the white polyurethane topcoat (which includes the main agent and the curing agent, the main agent being an acrylic copolymer, model TBS04B WPU-400. The curing agent is mainly composed of 1,6-hexamethylene diisocyanate homopolymer, model WPU-400-G) with an air spray gun and a wet-on-wet spraying process. Apply two coats of white polyurethane topcoat to the surface of the polyurethane intermediate coating. Each coat should be surface dry for about 30 minutes. After the overall spraying is completed, allow it to level for 2 hours, dry it at 60°C for 3 hours, and then air dry it at room temperature for 9 hours. The average dry film thickness should be controlled at about 30 μm.

[0100] (12) Visually inspect the surface of the white polyurethane topcoat layer again under light intensity of not less than 400 Lx to check for pits or other defects. If any are found, treat them by sanding and then patching with putty. After completion, sand the polyurethane topcoat layer with 320 grit sandpaper and clean it with a dust cloth. Mix the white polyurethane topcoat in a ratio of 4:1. Use an air spray gun and a wet-on-wet spraying process to spray the white polyurethane topcoat in two coats. Each coat should be surface dry for about 30 minutes. After the topcoat is surface dry for 1 hour, spray the polyurethane clear varnish. The polyurethane clear varnish should be mixed with a main agent to curing agent weight ratio of 3:1 (the main agent is acrylic copolymer, manufactured by Sifang Weikai; the main components of the curing agent are 1,6-hexamethylene diisocyanate homopolymer (80%) and propylene glycol diacetate (20%), manufactured by Sifang Weikai). After the overall spraying is completed, level it for 2 hours, dry it at 60℃ for 2.5 hours, and then air dry it at room temperature for 18 hours. The average dry film thickness should be controlled at about 50 μm.

[0101] Comparative Example 1

[0102] Compared to Example 1, the Nogawa Chemical 309 adhesive used in Comparative Example 1 had a β of 6%, which was less than the inherent change rate γ of 21% of the adhesive layer at the connection between the fixing cover and the vehicle body. The rest of the process was the same as in Example 1. The adhesive selected in Comparative Example 1 was used to fill the gap between the fixing cover (fiberglass) and the vehicle body (aluminum alloy) according to the method of Example 2. As a result, after the vehicle was treated by the method of Comparative Example 1 and left outdoors for 1.5 years, a crack with a length of approximately 250 mm appeared at the connection between the vehicle body and the driver's cab fixing cover.

[0103] Product effectiveness test

[0104] Samples of the bonding joint between the driver's cab fixing cover and the vehicle body (joint size 100mm×12mm×5mm) treated according to the methods of Example 2 and Comparative Example 1 were subjected to a high and low temperature cyclic alternating test for 120 cycles. The conditions for one cycle of the cyclic alternating test were: placed at 80℃ and 95% relative humidity for 4 hours, cooled to -40℃ within 2 hours, placed at -40℃ for 4 hours, and then heated to 80℃ and 95% relative humidity within 2 hours. After all cycles were completed, the samples were conditioned at room temperature for 24 hours. The results are shown in Table 2.

[0105] Table 2

[0106]

[0107] As can be seen from Table 2, the driver's cab fixing cover treated by the method of Embodiment 2 of the present invention is firmly connected to the vehicle body and is less prone to cracking, while the driver's cab fixing cover treated by the method of Comparative Example 1 is not firmly connected to the vehicle body and is prone to cracking. Therefore, it is evident that the driver's cab fixing cover treated by the method of the present invention is firmly connected to the vehicle body and is less prone to cracking.

Claims

1. A method for selecting an adhesive for joining plastic and metal, characterized in that, Includes the following steps: (1) Calculate the temperature difference ΔT1 during the metal heating stage and the temperature difference ΔT2 during the cooling stage using the temperature range t1 to t2 of the environment during the metal-plastic bonding process and the temperature range t3 to t4 of the environment during the metal-plastic bonding process. Where ΔT1 = t4 - t1, ΔT2 = t2 - t3, and compare ΔT1 and ΔT2. ΔT = max(ΔT1, ΔT2); (2) The coefficient of thermal expansion of the metal is α1, the coefficient of thermal expansion of the plastic is α2, and the difference in the coefficients of thermal expansion between the metal and the plastic is Δα = α1 - α2; (3) Measure the length l of the gap between the metal and the plastic and the thickness d of the adhesive layer, where the thickness of the adhesive layer is equal to the width of the gap; (4) Calculate the length difference Δl between the metal and the plastic caused by the temperature difference, where Δl=l / 2*ΔT*Δα; (5) Calculate the rate of change γ of the adhesive layer thickness. In the formula, s is the safety factor, and s is not less than 2; (6) Evaluate the rationality of the selected adhesive. The correctness of the selected adhesive is determined by comparing the rate of change of adhesive layer thickness γ and the maximum allowable long-term elongation β of the adhesive. Only when γ < β is the selected adhesive correct. Otherwise, the adhesive needs to be reselected and the magnitudes of γ and β need to be compared again.

2. The selection method according to claim 1, characterized in that, In step (1), the metal is an aluminum alloy.

3. The selection method according to claim 1, characterized in that, In step (1), the value of t1 is in the range of 10-18℃; the value of t2 is in the range of 30-40℃.

4. The selection method according to claim 1, characterized in that, In step (1), the value of t3 is in the range of -30℃ to -20℃; the value of t4 is in the range of 38-42℃.

5. The selection method according to claim 1, characterized in that, In step (2), the plastic is fiberglass.

6. The selection method according to claim 1, characterized in that, In step (2), α1 is (2.0-2.5)×10 -5 The α2 is (2.6-2.9)×10 -6 .

7. The selection method according to claim 1, characterized in that, In step (5), the type of adhesive selected for γ≤5% is different from that selected for γ>5%.

8. The selection method according to any one of claims 1-7, characterized in that, In step (6), the method for obtaining the maximum permissible long-term elongation β of the selected adhesive includes the following steps in sequence: Step (S1), Sample making: Select n sets of dumbbell-shaped test fixtures, n is greater than 3, and make n sets of samples using the selected adhesive and test fixtures. The sample includes adhesive joints at both ends and a substrate in the middle. The adhesive joint size of each set of samples is a×b×L. The size of a, b, and L must be selected to ensure that the strength of the substrate in the middle is less than the strength of the adhesive joints at both ends. Step (S2), sample curing: Fix the 5 sets of samples in step (1) and place all samples at room temperature for curing for more than 7 days; Step (S3), high and low temperature cycling test: The sample is placed in a high and low temperature aging test chamber for aging test. The test duration is more than 100 cycles. The conditions for one cycle of the cycling test are: place at 70-80℃ and 85-95% relative humidity for 3-4 hours, cool to -35℃ to -40℃ within 1.5-2 hours, place at -35℃ to -40℃ for 3-4 hours, and heat up to 70-80℃ and 85-95% relative humidity within 1.5-2 hours. After all cycles are completed, place at room temperature for more than 20 hours. Step (S4), Sample testing: Fix the sample to be tested on the two clamps of the tensile testing machine and perform tensile and shear tests at a test speed of 190-200 mm / min. After all samples have been tested, check the test curve and record the maximum displacement m when the stress and strain in each group of samples have a linear relationship. Step (S5): Calculate the maximum permissible long-term elongation β of the selected adhesive. in L is the average of the maximum displacement of all samples, and L is the length of the sample.

9. The selection method according to claim 8, characterized in that, Step (S1), where n is 4-8; and / or, in step (S1), the material of the test fixture is aluminum alloy or carbon steel; and / or, in step (S1), the sample preparation process also includes cleaning and adhesion accelerator treatment of the test fixture; and / or, in step (S3), the test duration is 110-120 cycles; and / or, in step (S3), the sample is placed at room temperature for more than 24 hours.

10. The application of the selection method according to any one of claims 1-9 in the connection between the fixed cover of the vehicle driver's cab and the gap of the vehicle body.

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