LIU Yunxue, KANG Xiaotian, FAN Zhaorong, GU Yaxin, LIU Peng

(School of Materials Science and Engineering, Shenyang Jianzhu University, Shenyang 110168, China)

Abstract: A novel graphene oxide (GO) modified polyurethane thermal conductive insulating adhesive with small addition and excellent insulation properties was prepared by in-situ polymerization using GO as thermal conductive filler. The effects of GO content on the mechanical performance, thermal conductivity, thermal stability and insulation properties of the modified polyurethane adhesive were studied. The results showed that the tensile strength and elongation at break of polyurethane adhesive increased at first and then decreased with the increase of GO content. The thermal conductivity and thermal decomposition temperature of GO/PU composite adhesive can be effectively improved by adding appropriate amount of GO. The tensile strength, thermal conductivity and thermal decomposition temperature of polyurethane adhesive reached the maximum when GO content was 1.5 wt%. The novel GO-modified polyurethane adhesive exhibited good insulation property. The development of GO/PU thermal conductive adhesive will provide a facile method for effectively solving the “trade-off” problem between low filling and high thermal conductivity.

Key words: graphene oxide; polyurethane; thermal conductive insulating adhesive; performance

With the miniaturization of microelectronic packaging and integrated circuits, more and more heat is generated per unit volume of electronic products.Overheating will affect the service life of electronic products. However, due to poor thermal conductivity,ordinary adhesives could not meet the heat dissipation requirements of high-frequency insulation occasions.Thermal conductive insulating adhesive could release a lot of heat in time during the use of electronic devices. It can not only extend the service life of polymer materials, but also prevent the occurrence of safety accidents. Therefore, thermal conductive insulating adhesives play an important role in the production of electronic components, the assembly of components and printed circuit boards, micro packaging technology and household appliances. With the rapid development of electronic industry, the demand for novel thermal conductive materials with high thermal conductivity,insulation resistance and excellent strength is gradually increasing[1-3].

Polymer thermal conductive insulating materials are mainly made of thermoplastic or thermosetting resins and thermal conductive fillers[4]. Polyurethane (PU)thermal conductive insulating adhesive is a new type of polymer thermal conductive material, which is synthesized by using carbamate bond and/or isocyanate group in macromolecular chain. Polyurethane adhesive can achieve good bonding on the metal, plastic, rubber and other substrates, and has a very broad application prospect. However, due to the poor thermal conductivity of polyurethane resin itself, it must be modified to be used as thermal conductive adhesive[5,6].

At present, the main method for preparing modified thermal conductive polyurethane insulating adhesive is to add thermal conductive fillers, such as aluminum nitride, aluminum oxide, zinc oxide,etc, into the polyurethane matrix[7-10]. The dosage of these traditional thermal conductive fillers usually needs to be more than 40%-50% to obtain better thermal conductivity. However, the adhesive will lose some mechanical properties at high filling degree. Therefore, it is very important to prepare high thermal conductive insulating adhesive with high thermal conductivity filler under low filling.

Graphene oxide (GO) is an oxidation product of graphene. Compared with other fillers, the dispersion of GO in water and some polar organic solvents and the compatibility with matrix have been greatly improved[11]. In addition, there are many oxygen-containing functional groups on the surface and edge of GO, such as -OH, -NCO-, -CO-,etc, which can react with certain substances to achieve chemical modification, thus expanding the application range of GO[12-14].Liet alincorporated the isocyanate-modified GO into the rigid polyurethane foams[15]. They found that NCO groups of modified-GO could participate into the PU foaming reaction and dramatically improve the mechanical properties of nanocomposite foams. Zhanget alused the GO to improve the mechanical properties and negative ions releasing performances of polyurethane/tourmaline nanocomposite fiber, which provide valuable insights into the development of the wearable energy products[16]. Zahidet alused the thermoplastic polyurethane and reduced graphene oxide was fabricated to increase electromagnetic interference shielding.However, there are few reports about GO-modified polyurethane thermal conductive insulating adhesive[17].

The purpose of this study is to develop a polyurethane thermal conductive insulating adhesive with low filling and excellent performance. The effects of GO content on mechanical performance, thermal conductivity and insulation properties of the GO/PU composite materials were studied. The development of GO/PU thermal conductive adhesive will provide a facile and promising method for effectively solving the insulation and heat dissipation problem of high-power integrated circuits when the long-term service temperature is less than 100 ℃.

Fig.1 Preparation process of GO/PU composite adhesive by in-situ polymerization

2.1 Main experimental materials

Acetone (analytical pure) was purchased from Sino-pharm Group Chemical Reagent Co., Ltd. Graphene oxide (GO) was provided by Shenyang Metal Research Institute. 4,4’-diphenylmethane diisocyanate (MDI, analytical pure) was bought from Jining Tainuo Chemical Co., Ltd. Polypropylene glycol (PPG, molecular weight 2 000, industrial grade) was purchased from Tianjin Zhonghe Shengtai Chemical Co., Ltd. Dibutyltin dilaurate (industrial grade) was purchased from Tianjin Chemical Reagent Factory.

2.2 Preparation of GO/PU composite adhesive

The GO/PU composite thermal conductive insulating adhesives were fabricated by employing thein-situpolymerization method. The preparation process of GO/PU composites byin-situpolymerization was shown in Fig.1.

Table 1 Formulation design of GO/PU composite adhesives

A brief preparation method description of GO/PU composites was described as follows: Firstly, a specified amount of GO (0-2.0 wt%) was added into a 250 mL conical flask containing 50 mL anhydrous acetone. After ultrasonic oscillation for 4 hours, a certain amount of MDI was added into the flask. The specific mole ratio of monomers and GO content was shown in Table 1. After reflux operation for 3.0 h in a water bath pan, the metered PPG (isocyanate indexR=2.5) and a small amount of catalyst (Dibutyltin dilaurate, 1‰-2‰)after vacuum drying at 110-120 ℃ were added. And then the solution was poured into the mold and placed in a vacuum (80-85 ℃) drying oven for 1.0 h. Finally,GO/PU composite thermal conductive insulating adhesive was obtained by curing in an oven at 100 ℃ for 2.0 h. The performance tests of the samples need to be placed at room temperature for more than 7 days.

2.3 Performance test of GO/PU composites

2.3.1 Mechanical properties test

The tensile strength and elongation at break of GO/PU composites were tested using the TCS-2 000 Electronic Universal Testing Machine of High-speed Rail Technology Co., Ltd. The tensile speed was 20 mm/min. The peel strength of GO/PU composites was tested according to GB/T 2790-1995 of PR China by 180opeel strength test method. The test piece was made of steel, and the test piece specification was 25 mm wide, 200 mm long and 1.5 mm thick. The stretching speed was set to 100 mm/min. Each group of experimental samples were stretched at least three times.

2.3.2 Thermal conductivity test

The GO/PU composite samples were made into a square sample of 40 mm×40 mm. The thermal conductivity tests were conducted by TPS-2500S Thermal Conductivity Tester (Hot Disk Company, Sweden).

2.3.3 Thermo-gravimetry test

The thermo-gravimetry properties were tested by Thermo-gravimetric analyzer (STA4493F3, Netzsch,Germany). The samples were pre-dried and heated from room temperature to 600 ℃ in nitrogen atmosphere at a heating rate of 10 ℃/min.

2.3.4 Volume resistivity test

The volume resistivity of GO/PU composites was measured by Volume Surface Resistance Measuring Instrument (GEST-121 type, Beijing Crown Testing Instrument Co., Ltd). The average value of the measurement results was tested and recorded at least three times.

2.4 Structure characterization of GO/PU composites

The morphology structures for each sample were observed under the SU8010 Scanning Electron Microscope (SEM, Hitachi Co., Ltd., Japan). Each sample surface was sprayed with gold prior to characterization.Infrared test was carried out by the IS5 Fourier transform infrared spectrometer (IR, Thermo Fisher technology, USA).

3.1 Effect of GO content on tensile properties of GO/PU composites

The application of thermal conductive insulating adhesive determines that it needs to have certain mechanical properties. The better the mechanical properties, the better the quality of thermal conductive insulating adhesive. The effect of GO content on the tensile properties of GO/PU composites was shown in Fig.2.

Fig.2 Effect of GO content on the mechanical properties of GO/PU composite adhesive

As shown in Fig.2, with the increase of GO content from 0 wt% to 2 wt%, the tensile strength of the composite adhesive exhibited a trend of obvious increase at first and then decrease continuously. When the addition amount of GO was 1.5 wt%, the maximum tensile strength was 9.3 MPa, which is 18.3% higher than that of pure polyurethane adhesive. When GO was added to 2 wt%, the tensile strength of composite adhesive decreased to 8.7 MPa, but it was still higher than that of the pure polyurethane adhesive. The elongation at break of the composite adhesive also improved with the increase of GO content in a certain range. When the GO content was 1.0 wt%, the elongation reached the maximum (635%), which is 36.5% higher than that of pure polyurethane adhesive. However, when the content of GO reached 2.0 wt%, the elongation at break of the composite adhesive was lower than that of pure polyurethane. This may because the excessive GO could increase the number of aggregates and stress concentration points in polyurethane, which will reduce the resistance to external damage[18,19]. After comprehensive consideration, the optimum dosage of GO was 1.0 wt%-1.5 wt%.

3.2 Effect of GO content on the peel strength of GO/PU composites

The use temperature of the thermal conductive insulating adhesive should be higher than the ambient temperature. In order to ensure that the thermal adhesive does not break away from the substrate at high temperature and ensure the service life of thermal adhesive, it is usually required that the thermal adhesive has a high peel strength at high heat conductivity.

Fig.3 Effect of GO contents on the peel strength of GO/PU composite adhesive

The effect of GO content on the peel strength of GO/PU composites was shown in Fig.3. According to Fig.3, with the increase of GO content from 0 wt%to 2.0 wt%, the peel strength of GO/PU composites increased first from 7.22 to 8.61 N·mm-1and then decreased to 7.43 N·mm-1. When the amount of GO was 1.5 wt%, the maximum peel strength was 8.61 N·mm-1,which is 19.3% higher than that of unmodified PU. The main reasons for these phenomena may be attributed to the influence of the layered structure of GO[20-22]. When GO was uniformly distributed in the PU matrix, the external load of the matrix material can be transferred,the stress concentration may be reduced, so the peel strength of GO/PU composites was improved. However, when the amount of GO was greater than 1.5 wt%,the aggregation of GO may cause the increase of stress concentration point, which easily leaded to the cohesive failure of the adhesive, and then leaded to the decrease of the peel strength.

3.3 Effect of GO content on thermal conductivity of GO/PU composites

Thermal conductivity is an important index to measure the thermal conductivity of a material. Fig.4 shows the curve of the thermal conductivity of GO/PU composites changing with the amount of GO. It can be seen from Fig.4 that the thermal conductivity of GO/PU composites increased obviously first from 0.17 to 1.27 W/m·K and then decreased when GO content increased from 0 wt% to 1.5 wt%. When the GO content was 1.5 wt%, the maximum thermal conductivity of composites was 1.27 W/m·K. The reason for this may be that the thermal conductivity of GO is much higher than that of polyurethane elastomer. With the increase of GO content, the thermal conductivity channel formed by GO became more perfect. In addition, the covalent bond formed by the chemical reaction between the oxygen-containing functional groups of GO and polyurethane could reduce the interfacial thermal resistance of GO and PU[23,24]. It is the combined effect of the two that makes the thermal conductivity and thermal conductivity of GO/PU composites increase. However, when the GO content exceeded 1.5 wt%, excessive GO addition reduced the compactness of composite adhesive and produces voids of different sizes, which damaged the heat conduction channel in the composite adhesive to a certain extent, so that the thermal conductivity of GO/PU composite adhesive decreased.

3.4 Thermal stability of GO/PU composite adhesive

Table 2 shows the temperature corresponding to 5% and 50% weight loss of GO/PU composite adhesive and the residual amount at 600 ℃. The temperature of 5% weight loss is usually referred to the thermal decomposition temperature. According to Table 2, thethermal decomposition temperature of GO/PU composite adhesive significantly increased first and then decreased with the increase of GO content. When the GO content reached 1.5 wt%, the maximum thermal decomposition temperature of the composite adhesive was about 333 ℃, which is 22.5 ℃ higher than that of the pure PU adhesive (310.7 ℃). When the weight loss was 50%, it had the same rule. The residual amount at 600 ℃ increased with the increase of GO content. The results indicated that the thermal stability of polyurethane could be improved by the chemical modification of GO. The increase of thermal stability of GO/PU composite adhesive was mainly related to the participation of GO in the chemical reaction of polyurethane.The introduction of GO into the PU chain restricted the motion of the chain segment, and inhibited the thermal decomposition of the PU macromolecular chain to a certain extent[25]. At the same time, the thermal conductivity of GO was much higher than that of PU.The combined effect of the two results makes the thermal stability of GO/PU composite adhesive enhanced.However, the heat resistance of GO/PU composite adhesive could not reach the heat resistance temperature of the thermal conductive silica gel, and it is recommended to use it below 100 ℃.

Table 2 Temperature corresponding to 5% and 50% weight loss of GO/PU composite film and the residual amount at 600 °C

3.5 Structure morphologies of GO/PU composite adhesive

Considering that the dispersion of GO in polyurethane matrix and the interfacial force between them may affect the properties of the modified composite adhesive. SEM pictures were used to observe the structures of composite adhesives to obtain the dispersion of GO in the main body of PU and the interaction between the interfaces.

Fig.5 exhibits the microstructure morphologies of composite adhesives with different GO content. It can be seen from Fig.5 (a-e) that the dispersion of GO in the PU matrix was related to the amount of GO. When the content of GO was less than 1.5 wt%, GO was well dispersed in the PU matrix, indicating that the interface between GO and PU had a good binding effect. However, when the GO exceeded 1.5 wt%, the surface of the cross section began to agglomerate. The reason for this phenomenon may be that some oxygen-containing groups on the surface of GO could greatly improve the binding ability of GO with PU molecular chain.When the content of GO exceeded 1.5 wt%, due to the high specific surface area and surface energy of GO nanoparticles, there was an opportunity for agglomeration between the particles, which eventually led to voids in the GO/PU composite adhesive section. The roughness of tensile section of the modified GO/PU composite adhesives increased with the increase of GO content. Meanwhile, there were a few pores in the cross-section of composite adhesive, which was mainly caused by the volatilization of acetone and other small molecules from the composite during vacuum drying.

Fig.5 Microstructure morphologies of GO/PU composite adhesives:(a) 0 wt%; (b) 0.5 wt%; (c) 1.0 wt%; (d) 1.5 wt%; (e) 2.0 wt%

3.6 Infrared analysis of GO/PU composite adhesive

Fig.6 FTIR spectra of the GO and GO/PU composite adhesives: (a) GO; (b) GO/PU composite adhesive (1.5% GO)

Graphene oxide (GO) is the oxidation product of graphene, and its surface and edge contain many oxygen-containing functional groups such as hydroxyl, epoxy, carboxyl,etc.The isocyanate is easy to react with hydroxyl on the surface of GO, and the product after reaction still contains -NCO group. The group can react with polyether polyol to form polyurethane. The infrared spectrums of GO and GO/PU composite adhesive are shown in Fig.6 (a) and Fig.6 (b), respectively. As can be seen from Fig.6 (a), the wider absorption peak of 3 325 cm-1is the stretching vibration peak of -OH on the GO surface. The characteristic absorption peaks at 1 714, 1 571, 1 402 and 1 142 cm-1indicate there are many oxygen-containing functional groups in graphene oxide.

As shown in Fig.6 (b), the weak and wide absorption peak at 3 340 cm-1is the stretching vibration peak of N-H in the structural state, while 2 970 and 2 868 cm-1are symmetrical and asymmetric stretching vibration peaks of methyl and methylene, 1 727 cm-1is stretching vibration peak of carbonyl group (C=O)in carbamate group, 1 091 cm-1is the absorption peak of ether bond in polypropylene glycol, 1 536 cm-1is the combined absorption peak of N-H deformation vibration and C-N stretching vibration in -NH-COO- or amide group, which indicates that carbamate group is formed by the reaction of -NCO with hydroxyl group.Furthermore, the infrared spectrum also showed that the -OH absorption peak on the GO surface disappeared in the spectrum of the composite adhesive, and the stretching vibration peak of N-H appeared at 3 340 cm-1, which indicated that carbamate was formed in the reaction process. In addition, the characteristic absorption peaks of GO such as C-O-C (1 073 cm-1) and C=O (1 732 cm-1) were retained in the spectra of the composite adhesive, indicating that some functional groups on the surface of GO participated in the reaction. Compared with Fig.6 (a), the absorption peak of-OH in Fig.6 (b) is obviously weaker, which indicates that the molecular chains of GO and PU in thein-situpolymerized composites were connected by chemical bonds.

3.7 In fluence of GOcontenton the insulation performance

Fig.7 shows the relationship between volume resistivity and GO content. It can be seen from Fig.7 that the volume resistivity of polyurethane conductive adhesive modified by GO decreased slightly with the increase of the mass fraction of GO. When the content of GO was 2.0 wt%, the lowest resistivity was 5.7×1013Ω.cm. This may be because the resistivity of polyurethane itself was very high, and the amount of GO added was very small, so the resistivity of the modified polyurethane conductive adhesive had little effect. The novel GO-modified polyurethane adhesive still exhibited good insulation property.

Fig.7 Effect of GO content on the volume resistivity of GO/PU composite adhesive

A novel graphene oxide modified polyurethane adhesive was developed byin-situpolymerization using 4,4’-diphenylmethane diisocyanate (MDI) and polypropylene glycol (PPG) as basic raw materials and GO as modifier. The results showed that all the tensile strength, elongation at break and peel strength show a trend of obvious increasing at first and then decreasing continuously with the increase of GO content from 0 wt% to 2 wt%. When the addition amount of GO was 1.5 wt%, the maximum tensile strength was 9.3 MPa,which was 18.3% higher than that of pure PU adhesive.When the GO content was 1.0 wt%, the elongation reached the maximum, which was 36.5% higher than that of pure PU adhesive. The maximum peel strength was 8.61 N·mm-1, which was 19.3% higher than that of unmodified PU. Graphene oxide could improve the thermal conductivity and stability of GO/PU composite adhesive to a certain extent. When the amount of GO was 1.5 wt%, the thermal conductivity increased from 0.17 to 1.27 W/m·K, and the thermal decomposition temperature increased from 310.7 to 333.2 ℃. In the experimental range, the addition of GO had little effect on the volume resistivity of polyurethane adhesive. The development of GO/PU thermal conductive adhesive will provide a facile method for effectively solving the“trade-off” problem between low filling and high thermal conductivity.