Hot melt adhesive for bonding dissimilar materials, method for bonding dissimilar materials and method for producing dissimilar material joined body

Abstract

To provide a hot melt adhesive for bonding dissimilar materials that is used for bonding dissimilar materials, which can reduce thermal deformation in a process of cooling the adhesive to a normal temperature after heating and melting, a method for bonding dissimilar materials, and a method for producing a dissimilar material joined body.SOLUTION: A hot melt adhesive for bonding dissimilar materials contains a crystalline polyester, wherein the crystalline polyester has a melting point of 90°C or higher, the melting point being determined by differential scanning calorimetry under heating temperature conditions of 10°C / min and 20°C / min, and has a crystallization temperature of 70°C or lower, the crystallization temperature being determined by differential scanning calorimetry under cooling temperature conditions of 10°C / min and 20°C / min.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a hot melt adhesive for bonding dissimilar materials, a method for bonding dissimilar materials, and a method for manufacturing a dissimilar material joined body.

Background Art

[0002] In industries such as the automotive and aircraft industries, the adoption of multi-material structures is being promoted for weight reduction. When manufacturing products using combinations of dissimilar materials, bonding by adhesion is effective. Among these, those that cure by heat treatment can be adhered quickly and obtain high strength, so they are often used for structural adhesion. However, when bonding dissimilar materials, in the process of cooling to the normal temperature after heating, thermal deformation caused by the temperature difference and the difference in the linear expansion coefficient of the materials becomes a problem. An adhesive and a bonding method that can absorb this are desired.

[0003] There is a weld bond joining method (Patent Document 1) that can withstand deformation caused by thermal stress by using adhesion and spot welding in combination, and an adhesion technique that blends a rubber component (Patent Document 2), a low elastic component (Patent Document 3), or a plasticizer (Patent Document 4) into the adhesive to relieve thermal stress and deformation.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, well-bonding involves a combination of two types of bonding processes and is complex. Also, methods of softening the adhesive to relieve thermal stress and deformation can cause thermal deformation and residual stress within the adhesive, leading to a decrease in adhesive strength and durability. Therefore, it is essentially necessary to reduce thermal deformation and residual stress.

[0006] On the other hand, there are various types and classifications of adhesives. One classification based on the form of curing is reactive adhesives. Reactive adhesives cure liquid raw materials through a polymerization reaction. This can be used for structural adhesives that require high-strength and high-durability bonding performance. Also, for structural adhesives, a hot-melt type that heats and melts a thermoplastic resin, adheres it to a substrate, and then cools and solidifies it for bonding is also used.

[0007] In the case of reactive adhesives, there is a type that initiates the curing reaction by mixing two liquids. Since it cures even at room temperature, it has the advantage of not having to consider thermal stress and thermal deformation. On the other hand, the two-component room-temperature curing type has a low glass transition temperature and tends to be difficult to achieve high strength. Also, since post-curing occurs by heating, the heating process can cause thermal deformation and the generation of thermal stress. Moreover, as mentioned earlier, the reaction takes time. Exceptionally, there are those that cure quickly, but these have limitations in terms of storage stability before curing and the properties of the cured product.

[0008] The principle of curing of the hot-melt type is phase transition. Two types of phase transitions, namely the flow-non-flow transition around the glass transition temperature and the flow-non-flow transition around the crystal transition temperature, are utilized. In the hot-melt type, the solidification temperature depends on these transition temperatures. If one selects and uses a material with a low transition temperature, naturally, thermal deformation and thermal stress can be suppressed. However, in that case, usually, the softening point also decreases, so heat resistance cannot be obtained.

[0009] In view of such circumstances, the present invention provides a hot-melt adhesive for bonding dissimilar materials, a method for bonding dissimilar materials, and a method for manufacturing a bonded body of dissimilar materials, which can reduce thermal deformation during the process of cooling to a normal temperature after heating and melting. [Means for Solving the Problems]

[0010] The present inventors have found that a crystalline polyester having a melting point of 90°C or higher and a crystallization temperature of 70°C or lower determined by differential scanning calorimetry under a cooling temperature condition of 10°C / min or higher can achieve contradictory properties of a low solidification temperature and a high melting temperature.

[0011] The present invention has been completed based on these findings and includes the following aspects. [1] A hot melt adhesive for bonding dissimilar materials containing a crystalline polyester, wherein the crystalline polyester has a melting point of 90°C or higher determined by differential scanning calorimetry under a heating temperature condition of 10°C / min and 20°C / min, and a crystallization temperature of 70°C or lower determined by differential scanning calorimetry of the crystalline polyester under a cooling temperature condition of 10°C / min and 20°C / min, a hot melt adhesive for bonding dissimilar materials. [2] A method for bonding dissimilar materials, in which the dissimilar materials are joined through the hot melt adhesive for bonding dissimilar materials according to [1], and cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, wherein at least the cooling rate at the crystallization temperature is 10°C / min or higher. [3] A method for manufacturing a dissimilar material joined body, in which the dissimilar materials are joined through the hot melt adhesive for bonding dissimilar materials according to [1], and cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, wherein at least the cooling rate at the crystallization temperature is 10°C / min or higher. [Advantages of the Invention]

[0012] According to the present invention, there are provided a hot-melt adhesive used for joining dissimilar materials, a hot-melt adhesive for joining dissimilar materials capable of reducing thermal deformation in the process of cooling to a normal temperature after heating and melting, a method for joining dissimilar materials, and a method for manufacturing a joined body of dissimilar materials.

Brief Description of Drawings

[0013]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0014] [Hot-melt adhesive for joining dissimilar materials] The hot-melt adhesive for joining dissimilar materials according to the present embodiment contains a crystalline polyester, and the melting point of the crystalline polyester determined by differential scanning calorimetry under the temperature rising conditions of 10 °C / min and 20 °C / min is 90 °C or higher, and the crystallization temperature of the crystalline polyester determined by differential scanning calorimetry under the temperature cooling conditions of 10 °C / min and 20 °C / min is 70 °C or lower. In this specification, dissimilar materials refer to materials having different coefficients of thermal expansion due to differences in material quality, composition, etc. As dissimilar materials, an aluminum alloy plate, a cold-rolled steel plate, a stainless steel plate, carbon fiber reinforced plastics, etc. can be used.

[0015] Generally, there are two types of polymer materials that can be used in hot melt type: crystalline plastics and amorphous plastics. When both plastics are in a molten state, they have a random molecular arrangement. During the process of cooling and solidifying from the molten state, some molecular chains of crystalline plastics are regularly arranged to form crystal parts. On the other hand, the other molecular chains that have lost their freedom cannot form crystal parts and thus form amorphous parts. The ratio of the crystal parts in the total volume is defined as the crystallinity. Even for the same crystalline plastic, there are differences in this crystallinity depending on the type. The generation of crystals during the cooling process is related to the cooling rate. If the cooling rate is fast, it solidifies before sufficient crystals are generated, so the ratio of the amorphous part increases. When the ratio of the amorphous part reaches 100%, it becomes an amorphous plastic.

[0016] Among these, there are also amorphous plastics such as polystyrene and polymethyl methacrylate that do not have crystallinity in the first place regardless of the cooling rate within the range of real time. The temperature at which these amorphous plastics solidify from the molten state is only the glass transition temperature.

[0017] Glass transition is a transition to a non-equilibrium state and is rate-dependent. There are various methods for measuring the glass transition temperature of plastics, such as TMA, DMA method, DTA method, and DSC method. According to the Polymer Handbook (Wiley), the DSC method is the most common method for measuring the glass transition temperature. JIS K 7121 describes the standards for measurement methods using DSC and DMA. The heating rate during measurement is set at 20°C / min. This standard also describes the determination conditions for the melting point and crystallization temperature. The measurement of the melting point is to perform heating at a rate of 10°C / min. For the determination of the crystallization temperature, conditions of -10°C / min or -5°C / min are to be used.

[0018] The melting point and crystallization temperature are higher than the glass transition temperature. Therefore, the temperature at which the crystalline polyester melts is directly related to the melting point. Also, when solidifying by cooling from the molten state, the crystallization temperature appears first, so the crystallization temperature has the most influence on thermal deformation and residual stress. The glass transition temperature is rate-dependent, but as can be seen from the fact that the measurement condition of measuring the glass transition temperature at a fast cooling rate is set, even if the temperature is changed relatively rapidly, a large decrease in the transition temperature cannot be expected. It is known that the crystallization of crystalline polymers is delayed depending on the cooling rate due to the low mobility of the polymers. For example, polyethylene terephthalate, which is a type of polyester, has a high melting point of 260 °C, but it is known to vitrify without crystallizing when quenched. Crystallization hardly proceeds below 70 °C, which is the glass transition temperature, but above the glass transition temperature, crystallization proceeds even below the melting point. If this delay in crystallization can be increased, the solidification temperature can be lowered. Specifically, the target heat resistance temperature is as follows.

[0019] It is said that the temperature rises to nearly 80 °C near the dashboard in summer inside a car. Considering such a possibility, as the heat resistance required in the normal use range in the short term, it is desirable that the melting point of the crystalline polyester used in the hot melt adhesive for bonding dissimilar materials, determined by differential scanning calorimetry under the heating temperature conditions of 10 °C / min and 20 °C / min, is 90 °C or higher.

[0020] On the one hand, to reduce thermal deformation, it is desirable to have a crystallization temperature as close as possible to room temperature. Here, the room temperature assumed is 20 - 25°C. Based on this, the standard temperatures during testing are defined in JIS Z 8703 as 20, 23, and 25°C. In ISO 554, the recommended temperature is 23°C. It is assumed that the normal operating temperature of products made by adhesive bonding is generally within this range. As rapid cooling methods, there are natural air cooling, forced convection (blowing) cooling, contact with a heat sink, etc. In any case, as the temperature difference from room temperature decreases, the cooling rate decreases, making it difficult to delay the progress of crystallization. In the crystalline polyester used in hot melt adhesives for bonding dissimilar materials, the upper limit of the crystallization temperature determined by differential scanning calorimetry under the cooling temperature conditions of 10°C / min and 20°C / min is set at 70°C as the realistically achievable crystallization temperature.

[0021] To find a hot melt adhesive that meets the above conditions, the rate dependence of the transition temperatures of several polymer materials was investigated. The results are shown in the examples and reference examples. For any polymer material, there is no significant rate dependence on the melting point. On the other hand, it can be seen that the crystallization temperature decreases as the cooling rate increases and is lower than the melting point. In particular, the polyester-based resin (poly(1,4-butylene succinate), extended with 1,6-diisocyanatohexane) represented by the following formula shows a large rate dependence.

[0022]

Chemical formula

[0023] Actually, for this polyester-based resin, the crystallization temperature during cooling at 20°C / min was as low as 57°C. Due to this large rate dependence, it is expected that if the cooling rate is further increased, the crystallization temperature will decrease even more. On the other hand, for polymers such as polyethylene and polypropylene, the crystallization temperature exceeds 90°C at any cooling rate, which does not meet the current purpose.

[0024] In the case of an amorphous material that does not crystallize, it solidifies by undergoing glass transition. Since the glass transition appears at a lower temperature, only low heat resistance can be obtained. From the perspective of reducing thermal deformation, it is desired to delay crystallization, but from the perspective of heat resistance, crystallization is indispensable. Therefore, it is necessary for crystallization to proceed slowly during the cooling process or after cooling. If glass transition occurs, crystallization will not proceed, so it is desirable for the glass transition temperature to be as low as possible. In the above polyester-based material system (poly(1,4-butylene succinate), extended with 1,6-diisocyanatohexane), no glass transition temperature is observed between the melting point and room temperature. Examples of the crystalline polyester include aliphatic polyesters extended with polymethylene isocyanates such as hexamethylene diisocyanate.

[0025] Poly(ethylene succinate) is the same polyester-based polymer, and its melting point appears at 90 °C or higher. On the other hand, during cooling, no crystallization temperature is observed between room temperature, so crystallization is slow and it seems like an ideal polymer. However, when the temperature is raised again, there is a glass transition temperature around 50 °C, and then crystallization and melting occur. In the case of such a polymer, it becomes a glass solid after cooling, and the progress of crystallization cannot be expected, so heat resistance is not guaranteed.

[0026] As the target hot melt adhesive, it is effective to use a crystalline polymer with a high softening temperature and a low solidification temperature. Since polymers have a large molecular weight, it takes time for the molecular chains to orient and form a regular ordered structure (crystalline structure). That is, it takes time to reach a thermally stable state. Therefore, as the heating and cooling rates are increased, a difference occurs between the softening temperature and the crystallization temperature. In particular, polyester-based polymers, for which a large crystallization delay effect is expected, are ideal. By utilizing this temperature difference, almost no thermal stress or deformation occurs at normal use temperatures, and at the same time, a heat-resistant adhesive can be realized.

[0027] [Adhesion method for dissimilar materials and manufacturing method for dissimilar material joints] The method for bonding dissimilar materials according to the present embodiment is a method for bonding dissimilar materials, in which the dissimilar materials are joined via the hot-melt adhesive for bonding dissimilar materials according to the above-described embodiment, and are cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, and at least the cooling rate at the crystallization temperature is 10°C / min or higher. The method for manufacturing a dissimilar material joined body according to the present embodiment is a method for manufacturing a dissimilar material joined body, in which the dissimilar materials are joined via the hot-melt adhesive for bonding dissimilar materials according to the above-described embodiment, and are cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, and at least the cooling rate at the crystallization temperature is 10°C / min or higher. By using the hot-melt adhesive for bonding dissimilar materials according to the above-described embodiment, when bonding dissimilar materials, thermal deformation can be reduced in the process of cooling to a normal temperature after heating and melting. Note that, in order to enhance the adhesiveness, a pretreatment of the adherend such as application of a primer layer may be performed.

Example

[0028] Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the examples shown below at all.

[0029] [Example 1] 10 mg of poly(1,4-butylene succinate), 1,6-diisocyanatohexane extension (manufactured by Sigma-Aldrich, 448028) was sealed in an aluminum container and set in a differential scanning calorimeter. Heating and cooling were performed in the range of 25°C to 200°C at a rate of 5°C / min, differential scanning heat was measured, the position of the peak top during heating was defined as the melting point, and the position of the peak top during cooling was defined as the crystallization temperature. Similarly, heating and cooling were performed in the range of 25°C to 200°C at rates of 10 and 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of differential scanning calorimetry analysis are shown in FIG. 1, and the results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 1.

[0030]

Table 1

[0031] (Adhesion test of dissimilar materials) This polymer was heat-melted, sandwiched between flat plates, and then cooled to prepare a hot melt adhesive sheet 1 with a thickness of 0.3 mm. This hot melt adhesive sheet 1 was placed in an electric furnace at 125 °C in the state of a joined body 4 sandwiched between an aluminum alloy plate 2 (A6061, 150 mm × 25 mm × t 2 mm) and a cold-rolled steel plate 3 (150 mm × 25 mm × t 1.5 mm) and heated to melt. Note that the aluminum alloy plate 2 and the cold-rolled steel plate 3 are each urethane-coated to enhance the adhesiveness with the hot melt adhesive sheet 1.

[0032] After holding at 120 °C, the temperature control of the electric furnace was stopped and it was slowly cooled. As a result, due to the linear expansion difference, the joined body 4 began to warp, and the final warpage amount d was 0.9 mm (the warpage amount after slow cooling from 120 °C). Figure 2 is a schematic diagram showing the state when the warpage amount d was measured on Table 5. Next, the warpage amount d was measured when it was the same up to heating and melting, and then it was taken out of the electric furnace and rapidly cooled to room temperature (corresponding to a temperature drop rate of 10 °C / min or more in the range from 120 °C to 60 °C). The result was 0.5 mm (the warpage amount after rapid cooling from 120 °C). In the hot melt adhesive for joining dissimilar materials containing the crystalline polyester of the present invention, by rapidly cooling from a temperature higher than the melting point at a cooling rate of 10 °C / min or more, even when joining dissimilar materials, thermal deformation can be reduced, and by crystallization, strong adhesion becomes possible. In the hot melt adhesive for joining dissimilar materials containing the crystalline polyester of the present invention, even when rapidly cooled under non-crystallizing conditions, it can be crystallized under temperature conditions not lower than the glass transition temperature. Also, when the joined test piece was heated to 100 °C, it warped in the reverse direction while remaining adhered. This indicates that the adhesive has heat resistance up to 100 °C in terms of adhesive performance.

[0033] [Comparative Example 1] Using a one - component reactive epoxy adhesive that cures at 170°C (the adhesive described in THE JOURNAL OF ADHESION, 2021, VOL. 97, NO. 13, 1255 - 1270), the adhesion test of dissimilar materials was similarly carried out. As a result, the amount of warpage d after slow cooling from 120°C was 2 mm. Also, the amount of warpage d after rapid cooling from 120°C was 2 mm. From these results, it was shown that by using crystalline polyester, the thermal deformation can be significantly reduced.

[0034] [Reference Example 1] For 10 mg of polypropylene - graft - maleic anhydride (Mw: 9,100, Mn: 3,900, maleic anhydride content: 8 - 10 wt%, manufactured by Sigma - Aldrich, 427845), in the same manner as in Example 1, the temperature was raised and lowered in the range of 25°C to 200°C at rates of 5, 10, 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 2. Also, the determination of the average molecular weight was calculated by GPC measurement using THF (tetrahydrofuran) as the developing solvent and polystyrene as the standard substance.

[0035]

Table 2

[0036] [Reference Example 2] For 10 mg of polypropylene (manufactured by Prime Polymer Co., Ltd., Prime Polypropylene (registered trademark), J105G), in the same manner as in Example 1, the temperature was raised and lowered in the range of 25°C to 200°C at rates of 5, 10, 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 3.

[0037]

Table 3

[0038] [Reference Example 3] For 10 mg of poly(ethylene succinate) (Mw: 10,000, manufactured by Sigma-Aldrich, 182036), in the same manner as in Example 1, the temperature was raised and lowered in the range of 25°C to 200°C at rates of 5, 10, and 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 4.

[0039]

Table 4

[0040] [Reference Example 4] For 10 mg of ultra-high molecular weight polyethylene (manufactured by Sigma-Aldrich, 4290150), in the same manner as in Example 1, the temperature was raised and lowered in the range of 25°C to 200°C at rates of 5, 10, and 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 5.

[0041]

Table 5

[0042] [Reference Example 5] For 10 mg of ultra-high molecular weight polyethylene surface polar group-introduced powder (average particle diameter: 34 to 50 μm, manufactured by Sigma-Aldrich, 434272), in the same manner as in Example 1, the temperature was raised and lowered in the range of 25°C to 200°C at rates of 5, 10, and 20°C / min, and the melting point during heating and the crystallization temperature during cooling were determined. The results of the transition temperatures of the melting point and the crystallization temperature are shown in Table 6.

[0043]

Table 6

Explanation of Symbols

[0044] 1... Hot melt adhesive sheet, 2... Aluminum alloy plate, 3... Cold-rolled steel plate, 4... Bonded body, 5... Table, d... Amount of warp

Claims

1. A hot-melt adhesive for bonding dissimilar materials containing a crystalline polyester, wherein the melting point of the crystalline polyester determined by differential scanning calorimetry under heating temperature conditions of 10 °C / min and 20 °C / min is 90 °C or higher, and the crystallization temperature of the crystalline polyester determined by differential scanning calorimetry under cooling temperature conditions of 10 °C / min and 20 °C / min is 70 °C or lower, a hot-melt adhesive for bonding dissimilar materials.

2. A method for bonding dissimilar materials, wherein the dissimilar materials are bonded through the hot-melt adhesive for bonding dissimilar materials according to Claim 1 and cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, wherein the cooling rate at least at the crystallization temperature is 10 °C / min or higher.

3. A method for manufacturing a dissimilar material bonded body, wherein the dissimilar materials are bonded through the hot-melt adhesive for bonding dissimilar materials according to Claim 1 and cooled from a temperature equal to or higher than the melting point to a temperature lower than the crystallization temperature, wherein the cooling rate at least at the crystallization temperature is 10 °C / min or higher.

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