Influence of graphene oxide on the thermal properties of phenolic resin

Addition of graphene oxide enhances the elasticity, stiffness, and tensile strength of phenolic resin, and improves its heat stability by nearly 30°C.The popularity of phenolic resin is increasing among plastics manufacturers, owing in part to the advantages of the material,…

Addition of graphene oxide enhances the elasticity, stiffness, and tensile strength of phenolic resin, and improves its heat stability by nearly 30°C.The popularity of phenolic resin is increasing among plastics manufacturers, owing in part to the advantages of the material, such as fine machinability and tolerance to harsh weather. Phenolic resin is thermally stable from –196 to 150°C. This range is limited compared with high-performance resins such as polyimide (PI), which is usually stable at 250°C. However, phenolic resin has a number of other excellent properties, such as fire resistance, less smoke during burning, and a low heat conduction coefficient. Moreover, it is more affordable than PI. Enhancing the thermal endurance properties of phenolic resin would make it possible to substitute it for PI in applications such as high-temperature-resistant containers.Carbon nanofillers have proven effective in obtaining relatively high thermostability,1–4 but are limited by high cost and challenges in processing and dispersion.5 Graphene, a 2D carbon material, shows promise as a nanofiller material in polymer composites owing to its extremely high aspect ratio, unique planar structure, and low manufacturing cost. Here, we describe our investigations of phenolic resins reinforced with graphene oxide (which is particularly compatible with certain kinds of polymers). Using dynamic mechanical analysis (DMA), we studied the mechanical properties of the resins at various temperatures to infer the influence of graphene oxide on thermal performance of the materials.5 We prepared resoles, a ‘prepolymer’ of phenolic resins, (W=85%, 2000–3000mPas) under alkaline conditions. The molar ratio of phenol to formaldehyde was 1:1.5. Benzene sulfonic acid, a curing agent, and natural graphite were supplied by Sinopharm Chemical Reagent Co. We prepared graphene oxide from the graphite using the Hummers method. We added the resoles, graphene oxide, and benzene sulfonic acid into a beaker and stirred for 20min. We also prepared a control sample without graphene oxide. We then loaded the solution into a film mold and placed it in a thermo press at 90°Cfor 1h.Figure 1.Storage modulus and tan δ(ratio of elastic modulus, E ′ , and imaginary—loss—modulus, E ′′ ) vs. temperature. We used a DMA (TA Instruments Q800) three-point bending test at 1Hz to evaluate changes in mechanical properties under the influence of temperature. We obtained thermogravimetry/derivative thermogravimetry (TG/DTG) curves of the film by means of a Shimadzu TGA-50 thermogravimetric instrument. We used a temperature range from 50 to 550°C with a ramp rate of 20°C min−1. We collected differential scanning calorimetry (DSC) data from 30 to 250°C. All analyses were performed under nitrogen atmosphere.Figure 2.Thermogravimetry curves (top) and derivative thermogravimetry (DTG) curves (bottom) of phenolic resin and graphene oxide/phenolic resin composites.Figure 3.Heat flow varied with temperature. We analyzed the results of the DMA for storage modulus and tan δ—defined as the ratio of elastic modulus (E ′ ) and imaginary (loss) modulus (E ′′ )—versus temperature (see Figure 1). The addition of graphene oxide significantly improved the elastic modulus of phenolic resin. For phenolic resin without graphene, the curve falls sharply at about 100°C and reaches bottom near 119°C. Thereafter, post-curing occurs and the curve grows slowly. For the graphene oxide/phenolic resin composite, the curve falls much more slowly and reaches the bottom near 150°C. In comparing the two curves, note that even a small amount (0.5%) of graphene oxide improved the elastic modulus of the phenolic resin.In polymers, damping (tan δ) is usually caused by movement of the molecular chain. In Figure 1, the tan δ of the graphene oxide/phenolic resin composite is much lower than that of phenolic resin alone. This finding suggests that addition of graphene oxide effectively impedes movement of the molecular chain. The peak of tan δ is usually defined as a characteristic temperature of glass transition (Tg), which is an important indicator of thermostability (Tg describes the maximum temperature under which the polymer can be used as a structural material). We observed significant changes in the Tg of the graphene oxide/phenolic resin composite compared with raw phenolic resin. The Tg values of phenolic resin and the graphene oxide/phenolic resin were 119.2 and 149.5°C, respectively (see Figure 1). In other words, the addition of graphene oxide improved the heat resistance of phenolic resin by 30.3°C. We plotted the TG and DTG curves of graphene oxide/phenolic resin composite and the raw material (see Figure 2). Both samples showed a gradual decrease in weight from 50 to 550°C. Addition of graphene oxide improved the residue rate (an indicator of fire retardancy) of phenolic resin by 6.3%. Thermal degradation (material breakdown) was observed on each sample when the temperature was raised to 300°C. Phenolic resin is known to undergo pyrolysis when the temperature exceeds 300°. Weight loss below 300° is mainly attributed to the post-curing process.6As shown in Figure 2, graphene oxide mitigates reduction of mass in phenolic resin, indicating interactions between graphene oxide and phenolic resin to the detriment of interactions among phenolic chains. These effects serve to block the post-curing process. It is clear that graphene oxide does not change the temperature distribution of phenolic pyrolysis but rather significantly reduces the rate. In a DSC curve, the peak of heat flow is also defined as Tg. The peak position of the graphene oxide/phenolic resin composite moved nearly 30°C to the right: i.e., the structural integrity of phenolic resin reinforced by graphene oxide persists to 135°C, whereas that of untreated phenolic resin reaches its limit at 106°C (see Figure 3).In summary, we determined and analyzed the thermal performance of a graphene oxide/phenolic resin composite by comparing it with raw phenolic resin. In general, graphene oxide improved the Tg temperature by nearly 30°C. DMA showed that the addition of graphene oxide—even a small amount (0.5%)—significantly improved the elastic modulus of phenolic resin. In turn, results of thermogravimetric analysis showed that addition of graphene oxide improved the residue rate of phenolic resin by 6.3%, whereas the temperature distribution of phenolic pyrolysis remained unchanged. Both DMA and DSC suggest that graphene oxide improves the Tg temperature of phenolic resin. As a next step, we plan to investigate the use of phenolic-foam-insulated piping under high temperatures for which most polymer foam insulation is unsuited.