Low melt viscosity thermoplastic polyurethane hot melt adhesive and method of making

Abstract

The application discloses a low-melt-viscosity polyurethane hot melt adhesive and a preparation method thereof. The thermoplastic polyurethane hot melt adhesive is prepared from raw materials including polyether diol, diisocyanate, polypropylene carbonate diol and chain extender containing furan amide groups. The application adopts a functional group activity control method, and polypropylene carbonate diol and polyether polyol diol are compounded as soft segments. The low-reactivity secondary hydroxyl group of the polypropylene carbonate diol is used to reduce the molecular weight of the polyurethane, and then the low-melt-viscosity polyurethane hot melt adhesive is obtained. The obtained polyurethane hot melt adhesive has high bonding strength, low adhesive application temperature and excellent reusability, and is suitable for good bonding of plastic substrates such as PC, PMMA, PP and ABS.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane hot melt adhesive technology, specifically relating to a low melt viscosity thermoplastic polyurethane hot melt adhesive and its preparation method. Background Technology

[0002] Thermoplastic polyurethane hot melt adhesives are suitable for bonding various materials, possessing comprehensive advantages such as high bond strength, high adhesive layer toughness, excellent low-temperature performance and impact resistance, solvent resistance, and non-toxicity. Traditional thermoplastic polyurethane hot melt adhesives require relatively high application conditions, typically with application temperatures between 120℃ and 180℃. Furthermore, excessively high melt viscosity can make it difficult to extrude the adhesive from the nozzle, sometimes necessitating a longer heat treatment time (over 2 hours) to reduce viscosity for successful application and achieve good bonding results. High application temperatures and long heating times can easily cause thermal stress concentration at lap joints, thus reducing bonding effectiveness. Moreover, for plastic substrates such as PC, PMMA, PP, and ABS, which typically have low softening temperatures, excessively high application temperatures make them unsuitable for application. Therefore, there is a need to develop thermoplastic polyurethane hot melt adhesives with low application temperatures and low melt viscosity to suit the bonding of plastic substrates.

[0003] Adding plasticizers, using low molecular weight tackifying resins, and increasing the amount of chain extenders are common methods used in the industry to reduce the melt viscosity of polyurethane. However, these methods often weaken the bond strength due to the influence of additives on the mechanical properties of the colloid and the interaction force between the adhesive layer and the substrate. The melt viscosity and rheological properties of thermoplastic polyurethane are related to multiple factors such as molecular weight, soft segment length and flexibility, hard segment content, and structure. Generally speaking, the smaller the molecular weight of polyurethane, the lower the melt viscosity, but a decrease in molecular weight often leads to a decrease in bond strength. Therefore, it is necessary to find a simple and effective means to reduce the melt viscosity of polyurethane hot melt adhesives, lower the application temperature, and at the same time ensure its bonding performance.

[0004] In the polyaddition reaction during the synthesis of thermoplastic polyurethane, the molecular weight of the product is affected by the activity of functional groups, the feed ratio, and the reaction time. Due to the high reactivity of isocyanates, and the system changes during the secondary feeding process in the prepolymerization of polyurethane, which consume reactive functional groups, and the influence of reaction environmental conditions, it is difficult to control the molecular weight of polyurethane by adjusting the feed ratio and reaction time. How to control the activity of functional groups to regulate the molecular weight of polyurethane within the original reaction time, temperature, and reaction process, thereby obtaining polyurethane hot melt adhesives with low melt viscosity and no decrease in adhesive properties, is a new research topic. Summary of the Invention

[0005] The present application aims to overcome the deficiencies of the prior art, and provides a low-melt viscosity thermoplastic polyurethane hot melt adhesive and a preparation method thereof. The obtained thermoplastic polyurethane hot melt adhesive has a lower gluing temperature, good bonding strength for PC, PMMA, PP, ABS and the like, and is thermoplastic, and can be heated and debonded and bonded again.

[0006] To achieve the above-mentioned object, the present application adopts the technical solutions as follows:

[0007] A low-melt viscosity thermoplastic polyurethane hot melt adhesive is prepared from raw materials including polyether glycol, diisocyanate, polypropylene carbonate glycol, and chain extender containing furan amide groups. The chain extender containing furan amide groups is one or more of the following compounds of formula I:

[0008]

[0009] The ratio of the total amount of substance of the polyether glycol and the polypropylene carbonate glycol to the amount of substance of the chain extender containing furan amide groups is 1:1-5, preferably 1:2-4; and the ratio of the total amount of substance of the polyether glycol, the polypropylene carbonate, and the chain extender containing furan amide groups to the amount of substance of the diisocyanate is 1:1.

[0010] Further, preferably, the mass of the polypropylene carbonate glycol is 40%-100% of the total mass of the polyether glycol and the polypropylene carbonate glycol, preferably 60%-80%.

[0011] The number average molecular weight of the polyether glycol is 1000-5000 g / mol, preferably 2000-3000 g / mol.

[0012] The number average molecular weight of the polypropylene carbonate glycol is 1000-5000 g / mol, preferably 2000-3000 g / mol.

[0013] The polyether glycol is selected from one or more of polyethylene oxide glycol, polypropylene oxide glycol, polyethylene oxide-propylene oxide glycol, and polytetrahydrofuran ether glycol.

[0014] The diisocyanate is one or more of isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), and 4,4'-methylenebis(isocyanate phenyl) (MDI).

[0015] The chain extender containing furan amide groups can be mixed with chain extenders of different carbon chain lengths.

[0016] The application also provides a preparation method of the thermoplastic polyurethane hot melt adhesive, which comprises the following steps: adding a catalyst into raw materials of polyether diol, diisocyanate, polypropylene carbonate diol and chain extender containing furan amide groups to prepare the thermoplastic polyurethane hot melt adhesive; and the catalyst is one or more of dibutyltin dilaurate, dibutyltin dilaurate, di-n-butyltin dilaurate and dibutyltin dilaurate.

[0017] Preferably, the mass amount of the catalyst accounts for 0.01-0.1% of the total mass of the polyether diol, diisocyanate, polypropylene carbonate diol and chain extender containing furan amide groups.

[0018] Further, the preparation method is preferably as follows: under nitrogen protection, the polyether diol, polypropylene carbonate diol, diisocyanate and catalyst are reacted at a temperature of 70-90 ℃ for 2-4 hours to obtain a prepolymer mixture, and then the mixture is heated to 90-120 ℃; then the chain extender containing furan amide groups is added into the prepolymer mixture at 90-120 ℃ to perform a chain extension reaction for 18-30 hours, and the thermoplastic polyurethane hot melt adhesive is obtained.

[0019] Further, the polyether diol and polypropylene carbonate diol are first stirred under vacuum at 120-140 ℃ for 1-4 hours to remove water, then the temperature is reduced to 70-90 ℃, nitrogen is introduced to replace the vacuum, and then the diisocyanate and catalyst are added to perform a reaction under nitrogen protection.

[0020] The chain extender containing furan amide groups can be prepared by the following method.

[0021] The furan-2,5-dicarboxylic acid dimethyl ester is added into a methanol solvent, and HO-(CH2) a -NH2 compound is stirred at 50-60 ℃ for 20-30 hours, the methanol solvent is evaporated, and then the mixture is washed with acetone, filtered and dried to obtain the chain extender containing furan amide groups; and the molar ratio of the furan-2,5-dicarboxylic acid dimethyl ester and the HO-(CH2) a -NH2 compound is 1:2-5, and a=1, 2, 3, 4 or 5.

[0022] The thermoplastic polyurethane hot melt adhesive prepared by the application has a viscous flow transition temperature of 50-90 ℃, and further, the viscous flow transition temperature can reach 50-65 ℃.

[0023] The number average molecular weight of the thermoplastic polyurethane hot melt adhesive prepared by the application is 5000-10000 g / mol.

[0024] The application also provides application of the low-melt viscosity thermoplastic polyurethane hot melt adhesive to bonding plastic substrates, which is particularly suitable for bonding PC, PMMA, PP and ABS at a low adhesive application temperature of 50-90 DEG C, further 50-80 DEG C.

[0025] In the application, the molecules of the polypropylene carbonate diol are composed of a large number of ester groups, and have the hydrolysis resistance and chemical resistance of the polyether soft segment and the excellent bonding performance of the polyester soft segment to the substrate. In view of the bonding performance and the toughness of the adhesive layer required by the polyurethane hot melt adhesive, the mixed soft segment of the polypropylene carbonate and the polyether polyol can be a strategy for imparting synergistic performance. Meanwhile, the secondary hydroxyl group of the polypropylene carbonate end hydroxyl group has a much lower reaction speed with diisocyanate than the common primary hydroxyl group of the polyether and polyester polyol, which can significantly reduce the molecular weight of the polyurethane, thereby reducing the melt viscosity transition temperature of the polyurethane hot melt adhesive and enabling the polyurethane hot melt adhesive to enter a low-melt viscosity state at a lower temperature.

[0026] Compared with the prior art, the application has the following beneficial effects: the soft segment is compounded by the polypropylene carbonate and the polyether diol, the secondary hydroxyl group of the polypropylene carbonate diol end hydroxyl group has a much lower reaction speed with diisocyanate than the common polyether and polyester polyol, the molecular weight of the prepared polyurethane hot melt adhesive is significantly reduced, which enables the polyurethane hot melt adhesive to enter a low-melt viscosity state at a lower temperature, thereby achieving good wetting and adhesion to the substrate. The addition of the polypropylene carbonate increases the content of the free carbonyl group C=O in the polyurethane molecular chain, the carbonyl group has a stronger polarity than the ether group, the free carbonyl group C=O can form hydrogen bonds, coordination bonds and other non-covalent interactions with the substrate, thereby strengthening the bonding performance of the hot melt adhesive. The application uses the synergistic effect of the mixed soft segment of the polypropylene carbonate and the polyether polyol to improve the bonding performance and the toughness of the adhesive layer required by the polyurethane hot melt adhesive. The low-melt viscosity polyurethane hot melt adhesive provided by the application has high bonding strength, low adhesive application temperature and excellent reusability, and is suitable for good bonding of plastic substrates such as PC, PMMA, PP and ABS at a low adhesive application temperature. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 Fig. 1 is an infrared spectrum line graph of the polyurethane hot melt adhesive in Examples 1-4 and Comparative Examples 1-2 of the application.

[0028] Figure 2 Fig. 2 is a curve graph of the storage modulus (G') and the loss modulus (G'') of the polyurethane hot melt adhesive in Examples 1-4 and Comparative Examples 1-2 and of the application with respect to temperature.

[0029] Figure 3 Fig. 3 is a curve graph of the complex viscosity of the polyurethane hot melt adhesive in Examples 1-4 and Comparative Examples 1-2 of the application with respect to temperature.

[0030] Figure 4is a single lap shear strength change diagram of the polyurethane hot melt adhesive of the present application embodiments 1-4 and comparative examples 1-2 at different gluing temperatures. DETAILED DESCRIPTION

[0031] To better illustrate the technical solutions and advantages of the present application, the present application will be further described below in conjunction with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0032] The raw materials used in the experiments in the present application embodiments and comparative examples are as follows, but are not limited to the following raw materials, and the present application only takes the following raw materials as specific examples to further specifically illustrate the effect of the low-melt viscosity thermoplastic polyurethane hot melt adhesive described in the present application.

[0033] Polytetrahydrofuran ether diol: PTMG-2000, number average molecular weight 2000 g / mol, Shanghai Maikelin Biotechnology Co., Ltd.; polypropylene carbonate diol: PPC-2000, number average molecular weight 2000 g / mol, Shanghai Maikelin Biotechnology Co., Ltd.; diisocyanate: isophorone diisocyanate IPDI, Araldite Reagent Co., Ltd.; catalyst: dibutyltin dilaurate DBTDL, Araldite Reagent Co., Ltd.; furan-2,5-dicarboxylic acid dimethyl ester (FDE), Araldite Reagent Co., Ltd.; ethanolamine, Araldite Reagent Co., Ltd.

[0034] Infrared spectroscopy (FT-IR) tests were performed on a Paragon 1000 spectrometer (PerkinElmer), and the polyurethane hot melt adhesive was tested at 4000 cm -1 -500 cm -1 -1 cm-1.

[0035] The rheological behavior of the polymer was tested by an ARES-G2 rheometer. A 25 mm parallel plate fixture was used for temperature sweep testing, with a strain size of 0.1%, an angular frequency of 1 rad / s, and a heating rate of 5°C / min.

[0036] Single lap shear strength test, polyurethane hot melt adhesive was pressed into a film with a thickness of 0.2 mm at 90°C by using a hot press. The hot melt adhesive film was cut into a rectangle (25 mm x 12.5 mm) and placed between two substrates (100 mm x 25 mm x 1.5 mm) to form an overlapping area of 25 mm x 12.5 mm. The substrates were fixed with two paper clips and placed in an oven set at a certain temperature for 10 min of heat treatment. Before testing, the sample needs to be cured at 25°C for 24 h. The lap shear strength test was performed on a universal tensile machine Sun Technology Stock Co. Ltd equipped with a 5000N sensor, and each sample was tested at least 3 times.

[0037] Embodiments 1-4 and Comparative Examples 1-3 of the present application are provided.

[0038] First, the chain extender containing furan amide group, N,N-bis(2-hydroxyethyl)-2,5-furan dicarboxamide, was prepared according to the following steps: furan-2,5-dicarboxylic acid dimethyl ester (7.37 g, 40 mmol) was dissolved in 100 mL of methanol at 50°C, then ethanolamine (9.77 g, 160 mmol) was added to the solution, the mixed solution was stirred at 50°C for 24 hours, the solvent methanol was removed by a rotary evaporator, then the product and raw material were washed and filtered repeatedly with acetone by virtue of the solubility difference in acetone, and dried in a vacuum oven to obtain white solid product N,N-bis(2-hydroxyethyl)-2,5-furan dicarboxamide (HO-FDAM-OH). The molecular formula is shown in the following formula:

[0039]

[0040] The preparation of low melt viscosity polyurethane hot melt adhesive was further carried out, and embodiments 1-4 and comparative examples 1-3 of the present application were provided, the equivalent parts of the raw materials are shown in Table 1, and the molecular weight and viscous flow transition temperature information of the prepared polyurethane hot melt adhesive are also listed in Table 1:

[0041] Table 1:

[0042]

[0043]

[0044] The preparation steps of the polyurethane hot melt adhesive of embodiments 1-4 and comparative examples 1-3 are as follows:

[0045] PTMG-2000 and PPC-2000 were added into a 500 mL three-neck flask with mechanical stirring according to the ratio in Table 1, the total weight of PTMG-2000 and PPC-2000 was 200 g, and vacuum dehydration was carried out at 120 ℃ for 2 hours. Then the temperature was reduced to 70 ℃, and nitrogen was introduced to replace the vacuum. The corresponding amount of IPDI and 0.1 g of DBTDL were added into the flask, and the temperature was gradually increased, and the reaction was carried out at 70 ℃, 80 ℃ and 90 ℃ for 1 hour respectively; then the temperature was continuously increased to 110 ℃, and the corresponding amount of HO-FDAM-OH or 1,4-butanediol was added into the prepolymer mixture, and the reaction was carried out at 110 ℃ for 24 hours, and the polyurethane hot melt adhesive PUPPC-x (x is the mass percentage of polypropylene carbonate diol in the total mass of polyether diol and polypropylene carbonate diol) was obtained by discharging.

[0046] The molecular weight information of the polyurethane hot melt adhesives in Examples 1-4 and Comparative Examples 1-3 is shown in Table 1. When furanamide diol is used as a chain extender, the molecular weight of the polyurethane decreases as the proportion of PPC-2000 in the soft segment increases, indicating that the terminal secondary hydroxyl group of polypropylene carbonate diol indeed has a low activity characteristic when reacting with diisocyanate. Lower molecular weight is beneficial to the reduction of the melt viscosity of polyurethane, which will have a positive impact on the application of polyurethane hot melt adhesive.

[0047] The infrared spectrum of the polyurethane hot melt adhesives in Examples 1-4 and Comparative Examples 1-2 is shown in Figure 1 The isocyanate group absorption peak at 2276 cm -1 around does not appear in the FT-IR spectrum of all products, indicating that IPDI has been completely converted. PUPPC-0 which completely uses PTMG-2000 as the soft segment does not show free carbonyl C=O absorption, and the carbonyl absorption is from the hydrogen bonding of amide groups and urethane groups C=O between hard segments. As the amount of PPC-2000 in the soft segment increases, the absorption at 1740 cm -1 -1743 cm -1 increases gradually, indicating that the PPC-2000 soft segment is successfully introduced into the polyurethane molecular chain, and many free C=O also appear.

[0048] The curves of storage modulus (G') and loss modulus (G") of the polyurethane hot melt adhesives in Examples 1-4 and Comparative Examples 1-2 with temperature are shown in Figure 2The temperature sweep was selected in the range of 60-140℃. The G" of the samples with more than 40% PPC-2000 in soft segment was higher than G' when the temperature was higher than 60℃, which indicated that the PUPPC entered into a liquid-like state. The liquid-like transition (G' and G" intersect) of PUPPC-0 and PUPPC-20 occurred at 92.6℃ and 90.8℃, respectively, which was caused by their high molecular weight. This resulted in the failure of PUPPC-0 and PUPPC-20 to adhere to the substrate under the sizing condition below the transition temperature.

[0049] From Figure 3 the complex viscosity-temperature curves, it can be seen that the viscosity of PUPPC-80 and PUPPC-100 decreased to 10 3 Pa.s or below at 100℃, which was beneficial for the sizing and coating on plastic substrates.

[0050] The properties of the polyurethane hot-melt adhesives obtained in Examples 1-4 and Comparative Examples 1-3 were tested, and the results at a sizing temperature of 80℃ are shown in Table 2.

[0051] Table 2:

[0052]

[0053] The single-lap shear strength of the substrate PMMA at different sizing temperatures is shown in Figure 4

[0054] From the comprehensive performance evaluation of Table 2 and Figure 4 it can be seen that Examples 1-4 all exhibited good adhesive properties for PC, PMMA and PP at 80℃. Comparative Examples 1-2 could not adhere to the plastic substrate at 80℃ because they did not have the ability to infiltrate the substrate below the transition temperature. Comparative Example 3 used 1,4-butanediol instead of furan amide diol as a chain extender, although it also had a low transition temperature (see Table 1), but the rigidity of the molecular chain decreased, and the mechanical properties of the prepared polyurethane decreased sharply, and the adhesive properties also decreased significantly.

[0055] From the comprehensive performance evaluation of Table 2 and Figure 4 ​The comprehensive performance evaluation can be seen that at the sizing temperature of 80℃, the bonding performance of PUPPC-80 is the best. With the increase of PPC-2000 content in soft segment, the content of free carbonyl C=O in polyurethane molecular chain increases. Carbonyl has stronger polarity than ether group. Free carbonyl C=O can form hydrogen bond, coordination bond and other non-covalent interactions with the substrate, thereby strengthening the bonding of hot melt adhesive. However, the toughness of the adhesive layer should also be balanced. High PPC-2000 content will make the adhesive layer brittle and reduce the bonding performance. In addition, the low molecular weight characteristics brought by high PPC-2000 content may also affect the strength of the adhesive layer. By adding furan amide diol chain extender and adjusting its content is also a means to maintain good bonding.

Claims

1. A low melt viscosity thermoplastic polyurethane hot melt adhesive, characterized in that: The thermoplastic polyurethane hot melt adhesive is synthesized from raw materials including polyether diol, diisocyanate, polypropylene carbonate diol, and a chain extender containing furanamide groups. The chain extender containing furanamide groups is one or more compounds of the following formula I: ； The ratio of the total amount of the polyether diol and polypropylene carbonate diol to the amount of the chain extender containing furanamide groups is 1:1 to 5; the ratio of the total amount of the polyether diol, polypropylene carbonate, and chain extender containing furanamide groups to the amount of diisocyanate is 1:

1. The number-average molecular weight of the polyether diol is 1000~5000 g / mol; the number-average molecular weight of the polypropylene carbonate diol is 1000~5000 g / mol. The thermoplastic polyurethane hot melt adhesive has a viscosity transition temperature of 50~90℃ and a number average molecular weight of 5000~10000 g / mol. The mass of the polypropylene carbonate diol is 40% to 100% of the total mass of the polyether diol and the polypropylene carbonate diol.

2. The low melt viscosity thermoplastic polyurethane hot melt adhesive as described in claim 1, characterized in that: The polyether diol is selected from one or more of polyethylene oxide diol, polypropylene oxide diol, polyethylene oxide-propylene oxide diol, and polytetrahydrofuran ether diol.

3. The low melt viscosity thermoplastic polyurethane hot melt adhesive as described in claim 1, characterized in that: The diisocyanate is one or more of isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, and 4,4'-methylenebis(phenyl isocyanate).

4. The method for preparing the low melt viscosity thermoplastic polyurethane hot melt adhesive as described in any one of claims 1 to 3, characterized in that, The method involves adding a catalyst to polyether diol, diisocyanate, polypropylene carbonate diol, and a chain extender raw material containing furanamide groups to react and obtain the thermoplastic polyurethane hot melt adhesive; the catalyst is one or more of dibutyltin dilaurate, dibutyltin dilaurate, di-n-butyltin dilaurate, and dibutyltin dilaurate.

5. The method as described in claim 4, characterized in that, The preparation method is as follows: Under nitrogen protection, polyether diol, polypropylene carbonate diol, diisocyanate and catalyst are reacted at 70~90℃ for 2~4 hours to obtain a prepolymer mixture, which is then heated to 90~120℃; then a chain extender containing furanamide groups is added to the prepolymer mixture at 90~120℃, and a chain extension reaction is carried out for 18~30 hours, and the product is discharged to obtain thermoplastic polyurethane hot melt adhesive.

6. The application of the low melt viscosity thermoplastic polyurethane hot melt adhesive as described in any one of claims 1 to 3 in bonding plastic substrates.

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