Improvement of graphene oxide/epoxy resin adhesive properties through interface modification

Abstract

The heat-conducting, dielectric, and bonding properties of a composite adhesive have been enhanced using toluene diisocyanate (TDI) to chemically modify the interface between graphene oxide (GO) and epoxy resin (EP), which was characterized using Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The thermal conductivity of TDI₁₀-GO_0.5/EP was 0.624 W·m⁻¹·K⁻¹ (166% of EP and 117% of GO_0.5/EP), with its TDI₁₀-GO_0.5/EP interfacial thermal resistance being reduced by a factor of 36 times GO_0.5/EP. The insulating properties of the adhesive in small resistors and capacitors were improved by the inclusion of GO, with the dielectric strength of the adhesive shown to be 25.96 kV·mm⁻¹ and its volume resistivity found to be 1.50 × 10¹⁴ Ω·m.

Keywords

Epoxy adhesive thermal conductivity interfacial thermal resistance influence performances T-peel strength

Introduction

The rapid development of electronic integration technology has resulted in the need for high thermal insulation adhesives with improved performance properties. Currently, the applications of epoxy resin (EP) adhesives are limited because of their high brittleness, low impact resistances, low peeling strengths, and low thermal conductivities.^1
–3 Epoxy adhesive, liquid rubber,^4,5 polyether,⁶ carbon nanotubes,⁷ and other toughening agents^8,9 have been used to improve the mechanical properties of modified EP. Although, the toughness and mechanical properties of these materials can be improved using these additives,^10,11 their thermal conductivities are not normally improved. High thermal conductivity fillers, such as ceramics of hexagonal boron nitride,¹² silicon carbide,¹³ and other fillers,^14

–17 have been used to promote conductivity, however, the insulation and mechanical properties of these adhesives were only improved marginally.

The thermal properties of epoxy adhesives and the thermal conductivities of epoxy composites can be improved via the inclusion of inorganic heat-conducting particles.^18
–20 Most research into their use has concentrated on improving the thermal properties of composites, with modifications sometimes resulting in improvements in their mechanical and electrical properties.^21
–23 The high thermal conductivities of graphene (GR) and graphene oxide (GO) have resulted in their incorporation as additives for the preparation of thermal conductive polymers.^24
–26 These studies have generally focused on the dispersity regulation properties of GR^27
–29 and modified GO.^{30

–43} However, analysis of their interfacial thermal resistance, thermal conductivity, T-peel strength, and bonding properties that affect their insulating properties has been less well-explored. In this study, the nature of the interface structure between GO and EP has been explored with the aim of providing structural information that can be used to improve the properties and performance of these type of adhesive composites.

Based on the above problems, therefore, in this work we prepared EP composite adhesive with modified GO, which was introduced toluene diisocyanate (TDI) onto GO surface by chemical bonding. Moreover, the thermal, electrical, and mechanical properties of epoxy adhesive were analyzed to explore the feasibility of the epoxy composite adhesive system.

Experimental

Materials

EP of diglycidyl ether of bisphenol-A (EP, E-51, from Nantong Xingchen Resin Factory, Nantong, Jiangsu, China) with epoxy value of 0.48–0.54 mol/100 g was used in this work. GO with multilayer structure was purchased from Suzhou TANFENG Graphene Tech Co.,Ltd (Suzhou, Jiangsu, China). Other chemicals including TDI, N,N-dimethylformamide (DMF), methyl hexahydrophthalic anhydride (MHHPA), and methyl nadic anhydride (MNA) were all analysis pure reagents purchased from the Chemical Reagent Co. of Tianjin Fuyu (Tianjin, China).

Method

GO-modified method

GO (0.150 g) and DMF (30 mL) were added in a 150 mL three-necked flask. After ultrasonic dispersion for 4 h (240 W, 25°C), the solution with GO and DMF was dropped into the mixture of 3 g TDI and 10 mL DMF within 30 min. Finally, TDI-GO was achieved after an ice water bath for 24 h.

Composite produced method

The composite adhesives were prepared as follows: the mixture solution of TDI-GO was placed in an oil bath. E-51 (30 g) was dropped into the suspension. After that, the mixture of TDI-GO and E-51 was degassed in a vacuum oven to remove the solvent. The curing agents MHHPA and MNA were dispersed into the epoxy mixture uniformly and cured in an oven at 90°C for 1 h, 110°C for 2 h, and 170°C for 3 h. A reference sample with neat epoxy was prepared following the same procedure.

For convenience, the content of each component was written as GO_X/EP and TDI₁₀-GO_X/EP throughout this work, in which the X (the value of X was 0.3, 0.5, 0.7, and 1.0) represented the percentage of GO for the EP. The TDI₁₀-GO_0.5/EP means that the percentage of TDI was 10 wt% and GO was 0.5 wt% for the EP of the composite material.

Characterization

The chemical structures of the GO, TDI-GO, and the composite adhesive were characterized by Fourier-transform infrared (FTIR) spectroscopy on an AVATER-360 analysis system (Nicolet, Madison, Wisconsin, USA) and X-ray photoelectron spectroscopy (XPS) employing a Thermo Fisher Scientific (Waltham, MA, USA) Multilab-2000 XPS spectrometer. The microstructures of GO and TDI-GO were characterized by a transmission electron microscope (TEM; FEI, Holland). Sealing and drying methods were used to store TDI-GO before testing.

The thermal conductivity (K _c) and the interfacial thermal resistance (R _B) of the composites with different contents of GO were calculated based on the GR/polymer²⁴ thermal model with a Netzsch LFA 467 (Netzsch, Bavaria, Germany) light flash apparatus by standard method at 30°C. The dielectric strengths of composites were tested using the machine HT-100 (Guilin, China) at the oil bath environment. The volume resistivities of epoxy composites were tested according to GB 1410-78 using the machine ZC-36 (Shanghai, China) at room temperature. The dielectric constants of cured epoxy composites were characterized by German Alpha-A within the frequency range of 10¹–10⁷ Hz and the Nyquist diagrams within the frequency range of 10⁰–10⁴ Hz. The tensile strength was measured based on GB/T 7124-2008 at room temperature under the speed of 10 mm·min⁻¹ on the sample. The T-peel strength was detected following the same procedure except that the specimens were prepared based on ASTM D1876: 2015. The impact testing was carried out according to GB/T 2571-1995 at room temperature without notch. The three-point bending flexural test was conducted according to GB/T 9341-2006 at room temperature under the speed of 5 mm·min⁻¹ with a span of 60 mm.

Results and discussion

Characterization of GO and TDI-GO

The thermal, electrical, and mechanical properties of EP composite adhesives prepared by the treatment of modified GO with TDI have been analyzed. These GO and TDI-GO composites were characterized using FTIR (Figure 1(a)), XPS (Figure 1(b) to (d)), and TEM (Figure 1(e) and (f)) analyses.

The FTIR results revealed the presence of absorption peaks for NCO and aryl groups, with no C=O absorption peaks for carboxyl and hydroxyl groups of GO, with absorptions for C–H bonds shifted from 3420 cm⁻¹ to 3285 cm⁻¹, that were lower than for pure GO due to the conjugation effect of the benzene ring.^44,45 As illustrated in Figure 1(b), the characteristic peaks for the O atoms at 530 eV were weaker due to the reaction of TDI with GO, while the characteristic peaks for N⁻³ were present at 400 eV,⁴⁶ with an enhanced peak for C atoms at 290 eV due to the presence of new carbon groups from grafted TDI. The characteristic peaks for O atoms were also reduced, with C atomic absorption peaks increased and new atomic absorption peaks for N being present. The C 1s and N 1s absorptions for TDI-GO were shown in Figure 1(c) and (d), which reveal that the C atoms of TDI-GO were present in six environments,^47,48 that were assigned to O–C=O absorptions of –COOH; C=N absorptions from unreacted –NCO groups of TDI; and C=O bonds formed from the reaction of the NCO groups of TDI with the –OH groups of GO.⁴⁹ These results confirm that TDI was grafted onto the surface of TDI with surplus NCO groups also being present, with layers of GO sheets exhibiting a transparent clean surface (Figure 1(e)) and the occurrence of black spots formed due to the presence of grafted TDI (Figure 1(f)).

Performance analysis

Thermal properties

The effect of increasing the amount of GO on the thermal conductivity of pure EP, GO_X/EP, and TDI₁₀-GO_X/EP are shown in Figure 2, with only small differences in thermal conductivity observed between GO_X/EP composites and EP.

The thermal conductivity of TDI₁₀-GO_X/EP decreased initially, then increased, and again decreased with increasing GO content. The thermal conductivity of TDI₁₀-GO_0.5/EP was 0.624 W·m⁻¹·K⁻¹, that was 166% of EP and 117% of GO_0.5/EP, respectively. The composite structures of GO and TDI-GO were comprised of a filled island structure covered with EP that was formed by the reaction of EP and NCO groups on the GO surface.^50,51 Increasing amounts of GO result in a reduction in the low speed conduction pathways and the heat capacities of composites, which results in its thermal conductivity being increased.³¹ The effect of changing the interface on the thermal conductivity of the composites was investigated by calculating its interfacial thermal resistance (R_B ) using a GR/polymer thermal model with the following calculation formula

K_{c} = K_{f} \frac{3 K_{m} + 2 V_{f} (K_{p} - K_{m})}{K_{p} (3 - K_{f}) + K_{m} V_{f} + \frac{R_{B} K_{p} V_{f}}{H}}

R_{B} = H (\frac{3 K_{f}}{V_{f} K_{c}} - \frac{3}{V_{f}} + \frac{K_{f}}{V_{f}} - 1)

where K_c and R_B are the thermal conductivity and the interfacial thermal resistance of composites, K_f is the thermal conductivity of GO, V_f is the filling volume of GO, K_p and K_m are the thermal conductivity of continuous phase (EP matrix, K_p and K_m are the same value), and H is the GO thickness, which are detailed in Online Supplementary Table S1.

As shown in Figure 3, the interfacial thermal resistance of GO/EP composites decreased as the amount of GO increased, with 0.5 wt% GO resulting in interfacial thermal resistance decreasing to 38.71 × 10⁻⁴ K·m²·W⁻¹. The interfacial thermal resistance of TDI₁₀-GO_X/EP composites was lower than that of GO/EP composites, exhibiting a minimum value of 1.19 × 10⁻⁴ K·m²·W⁻¹ (3% of GO_X/EP (38.71 × 10⁻⁴ K·m²·W⁻¹)) when the amount of GO present was 0.5 wt%.

The FTIR spectra shown in Figure 4(a) revealed that no chemical reaction between EP and GO occurred without prior modification with TDI, however, treatment with TDI results in reaction between TDI-GO and EP to form an interface. As a result, increased phonon transmission in this interfacial region results in smaller thermal resistance,⁵² with the insulating properties of derived polymers dependent on the percentage of composite present (see interfacial thermal resistance diagrams shown in Figure 4(b)).

As shown in Figures 2 and 3, the thermal conductivities of GO_X/EP and TDI₁₀-GO_X/EP composites decreased when the content of GO was higher than 0.5 wt%. Different mass fractions of DMF were mixed with GO and TDI-GO to study their dispersion stability, with the stability of samples decreasing with increasing GO content (Figure 5), with greater settlement of mixed solution from TDI-modified composites being observed.

This is because the proportion of TDI content decreases as the levels of GO increase, which results in greater cross-linking of GO to form agglomerates. Variation of the interfacial thermal resistance of TDI₁₀-GO_X/EP was similar to that of TDI₁₀-GO_0.5/EP, with the thermal conductivities of adhesives being reduced by the formation of agglomerates and the scanning electron microscopic photos of TDI₁₀-GO_X/EP revealing high levels of when GO content was high (see Figure 6). The thermal conductivity of composites was also reduced when the relative concentration of GO was high due to the formation of a large number of GO-TDI-GO structures.^20,49

Dielectric properties

The dielectric strength and volume resistivity of the EP and TDI₁₀-GO_X/EP composites with different contents of GO were examined, as presented in Figure 7.

The dielectric strength and volume resistivity of TDI₁₀-GO_X/EP composites increased and then decreased with increasing amounts of TDI-GO, with a dielectric strength of 25.96 kV·mm⁻¹ and a volume resistivity of 1.50 × 10¹⁴ Ω·m for TDI₁₀-GO_0.5/EP (see Figure 7). The dielectric strength and volume resistivity of TDI₁₀-GO_0.5/EP were 1.07 and 1.13 times larger than those for GO_0.5/EP, as shown in Online Supplementary Figure S1. The changes in dielectric properties of these TDI₁₀-GO_X/EP composites were explored using Nyquist diagrams (see Figure 8), which showed that the fitted curves largely coincided with the experimental data.

It is known that grafted modified GO forms small resistors and capacitors in the solid state,^53
–55 with Figure 8(b) revealing that the dielectric constant of TDI₁₀-GO_X/EP composites initially increased at lower frequencies, and then decreased as the amount of GO present increased. The storage charge capacity of the capacitors and the resistance of electron migration were increased in small resistors,⁵⁶ resulting in better insulation performance, higher breakdown field strength, greater volume resistivity, and a higher dielectric constant for EP and GO_X/EP composites (see Online Supplementary Figure S2). Alternatively, when the interface between GO and EP is modified by the formation of chemical bonds between GO and TDI, a reduction in the number of defects improved the insulation performance of composites containing lower GO content. Overlap of micro capacitors can be caused by excessive GO agglomeration when GO levels were more than 0.5 wt%, leading to current leakage and reduced storage charge ability of the composite materials, which resulted in lower breakdown field strengths, volume resistivities, and dielectric constants.

Mechanical properties

The results obtained for mechanical testing of EP and TDI₁₀-GO_X/EP and GO_X/EP are shown in Figure 9 and Online Supplementary Figure S3, which reveal that their mechanical properties increased initially and then decreased. The best mechanical properties were obtained when the GO amount was 0.5 wt%, with T-peel strength and tensile strength of TDI₁₀-GO_0.5/EP determined as 9.62 N·mm⁻¹ and 4.25 MPa, respectively, while the bending and impact strengths of TDI₁₀-GO_0.5/EP were 768.33 MPa and 20.51 kJ·m⁻², respectively. The presence of TDI altered the dispersion of GO in the matrix resin by improving interfacial bonding between GO and EP,^7,57
–59 with surplus –NCO groups contributing to the formation of GO-TDI-EP structures.^60
–62 In addition, modified GO sheets structure could act as a good buffer to share stress loads within the composite. These three properties increased the ability of the adhesive composites to resist external force damage, which gave better mechanical properties than those measured for EP. Different degrees of aggregation were observed when lower amounts of TDI were present in the grafted TDI-GO composite, which resulted in structural failure when relatively low external forces were applied.

Conclusion

The thermal conductivity of TDI₁₀-GO_0.5/EP was 0.624 W·m⁻¹·K⁻¹, which was due to reduced interfacial thermal resistance caused by the interface structure of GO and EP being changed by the TDI grafting process. The dielectric strength and volume resistivity of TDI₁₀-GO_0.5/EP were 25.96 kV·mm⁻¹ and 1.50 × 10¹⁴ Ω·m, respectively, which was caused by the small resistance and capacitance provided by the GO sheets. The T-peel strength, tensile strength, impact strength, and bending strength of the EP composites containing 0.5 wt% GO were improved by 200%, 35%, 181%, and 55% when compared to the performance levels of EP.

Supplemental material

Supplemental Material, Electronic_Supplementary_Information - Improvement of graphene oxide/epoxy resin adhesive properties through interface modification

Supplemental Material, Electronic_Supplementary_Information for Improvement of graphene oxide/epoxy resin adhesive properties through interface modification by Guangkai Hu, Xiaorui Zhang, Lizhu Liu, and Ling Weng in High Performance Polymers

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51677045).

Supplemental material

Supplementary material for this article is available online.

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