Mechanical Performances of Phenolic Modified Epoxy Resins at Room and High Temperatures

Abstract

Epoxy is an important resin matrix and has been widely applied in laminated composites as a coating or adhesive material. In this article, the phenolic was applied to modify the mechanical properties of epoxy resin. The phenolic modified epoxy resins with various phenolic content were prepared via a polytetrafluoroethylene mould, and the phenolic modified epoxy-based plain woven laminated composites (PWLCs) were manufactured via vacuum assisted resin transfer method for further study of phenolic modified epoxy resins’ mechanical properties. The compression tests were performed perpendicularly to thickness at 2 mm/min to investigate the mechanical performances of phenolic modified epoxy resins and epoxy-based PWLCs. The results showed that the addition of phenolic into epoxy could improve the mechanical performances of epoxy resins and epoxy-based composites at room temperature, and the phenolic influenced epoxy-based PWLC more than epoxy matrix at room temperature. However, at high temperatures, the addition of phenolic decreased the mechanical performances of epoxy resins and epoxy-based composites, and the adverse effect of phenolic became more serious with the increase of phenolic content at high temperature. In addition, the thermogravimetric analyses were also conducted from 30 °C to 800 °C on phenolic modified epoxy resins and the results showed that the phenolic modified epoxy resin had an earlier loss in weight than unmodified epoxy resin. The earlier loss in weight meant that the addition of phenolic into epoxy resin led to the formation of unstable molecules at high temperature.

1. Introduction

Epoxy resins own good mechanical properties and chemical resistance and are often used for coating and bonding. Much research has been conducted on epoxy and epoxy-based composites [1,2,3,4]. Epoxy-based composites have been applied in aerospace, auto parts, marine engineering and other fields due to their superior performances [5,6,7].

Epoxy resins also have some shortcomings such as poor heat resistance and poor toughness. Therefore, many studies [8,9] have been performed to overcome these weaknesses or further improve the whole performance of epoxy resin. Kim et al. [10] performed a chemical reaction between diisocyanate and fatty acid-modified epoxy polyols to synthesize polyurethane which was designed to increase the toughness of epoxy compositions. Yung [11] synthesized amine-functionalized graphene nanosheets to reinforce epoxy resin. Cui et al. [12] employed 1,7-bis(aminophenylene)meta-carborane as a curing agent to prepare heat-resistant epoxy adhesives. Kim et al. [13] used zirconia-impregnated halloysite nanotubes to improve the mechanical and thermal properties of epoxy composites. Kong et al. [14] used lignin to improve the heat-resistance and shear strength of epoxy. Zeng et al. [15] used a bis-silane prepolymer to modify epoxy resin with the purpose of enhancing the corrosion resistance.

Epoxy resin is often used as an adhesive material to constrain or fix fibers in advanced composite materials, such as carbon fiber reinforced epoxy-based composites [16,17]. The mechanical properties of carbon fibers are not sensitive to temperature [18] and the mechanical properties of the epoxy matrix are sensitive to temperature, which causes the sensitivity of epoxy-based composites to temperature [19,20]. In addition, the mechanical properties of epoxy resin are very poor compared with reinforcements [21,22], and the poor mechanical properties matrix will constrain the performance of carbon fiber reinforced composites.

Efforts have been made to enhance the mechanical properties of epoxy resin and the improvements of epoxy’s mechanical properties are mostly by dispersing second phase material in epoxy resin systems [23]. Liu et al. [24] introduced biscitraconimide resin to enhance the cryogenic tensile strength, bending strength, impact strength and fracture toughness of epoxy. Via molecular dynamics simulations, Zhu et al. [25] found that the helical graphenes modified epoxy resin had higher Young’s modulus and yield strength than pure epoxy resin. Korkmaz et al. [26] improved the mechanical properties of epoxy after hydrothermal ageing by incorporating boron nanoparticles. Shen et al. [27] used AgO nanoparticles as fillers to improve the tensile strength and bending strength of epoxy.

The phenolic resin has a high concentration of aromatic cycles which could provide good thermal properties at high temperatures [28]. Phenolic resin contains aldehyde groups and the aldehyde groups can react with epoxy groups, which will increase intermolecular cross-linking of epoxy resin. So, in this article, the phenolic resin was employed to modify the epoxy resin for the purpose of enhancing the mechanical properties of the epoxy matrix, which could provide a reference for the improvement of epoxy’s mechanical properties, and the mechanical properties of phenolic modified epoxy were also investigated at high environmental temperatures. The phenolic or phenolic resin mentioned in the following text means a fluid phenolic resin. Various proportions of phenolic resin were introduced into epoxy resin. The compression tests were performed to investigate the influences of phenolic on the mechanical properties of epoxy resin at room and high temperatures. For further study of the phenolic modified epoxy resins’ mechanical properties, the epoxy-based laminated composites were also prepared and tested.

2. Materials and Methods

2.1. Materials

The epoxy resin used in this study was purchased from Changshujiafa (Changshu, China). The epoxy resin was composed of epoxy (JC-02A) and hardener (JC-02B) with a weight ratio of 10:8. The epoxy had an epoxide number of 0.5–0.53 eq/100 g with a viscosity of 1000–3000 mpa·s. Phenolic used in this study was purchased from Kechuangsuhua (Guangzhou, China) and the free phenol was below 14%. Plain woven used in this study was formed by carbon fiber tows (T700-3k).

2.2. Preparation of Phenolic Modified Epoxy

Phenolic modified epoxy was prepared on the condition that the weight ratio of epoxy and phenolic to hardener was 10:8. Phenolic modified epoxy resins with various weight ratios of phenolic to epoxy were prepared.

The curing process is presented in Figure 1. The resin was obtained by mixing epoxy, hardener and phenolic, and then the mixed resin was vacuumed at 60 °C to remove the air bubbles. The vacuum degassed resin was then poured into a mould containing rectangular grooves (as shown in Figure 1). The mould was made of polytetrafluoroethylene and was coated a layer of release agent before pouring the resin. After pouring the resin, the mould was put into an oven to cure the resin. After curing, the resin was cooled down naturally to room temperature and taken out from the mould, and then the cured resin was cut into resin blocks with a size of 20 mm (height) × 20 mm (width) × 10 mm (thickness). The curing temperatures were 90 °C for 2 h, 110 °C for 1 h and then 130 °C for 4 h.

During the preparation of epoxy, we found that the viscosity of uncured epoxy fluid increased with the increase of phenolic content. When the weight ratio of phenolic to epoxy exceeded 2:8, the fluidity of the mixture of epoxy and phenolic was very poor and the curing time was shortened. The shortened curing time meant the allowed time for the resin to transfer was shortened. The shortened curing time was not good for resin transfer during the molding process of the composite.

In addition, water molecules will be formed during the curing process of the phenolic. The curing temperature for the epoxy is above the boiling point of water, so too many water molecules will cause bubbles in phenolic modified epoxy resin. The bubbles will lead to stress concentration and weaken the mechanical properties of the samples. So, the weight ratio of phenolic to epoxy should not be too high.

For a high weight ratio of phenolic to epoxy, the cured resin matrix contained lots of bubbles as presented in Figure 2. Surface A is the surface in contact with the mould as shown in Figure 1 and surface B is the surface in contact with air. Surface A of the cured resin formed many bubbles. So high weight ratios of phenolic to epoxy were not used.

2.3. Preparation of Laminated Composite

Plain woven laminated composite (PWLC, as shown in Figure 3) was also prepared via vacuum assisted resin transfer method (VATRM) to further study the effect of phenolic on mechanical properties of epoxy-based composites at different temperatures, which can reflect the interlayer bond performances. The resin was obtained by mixing epoxy, hardener and phenolic, and then the resin was vacuumed at 60 °C to remove the air bubbles before being injected into the laminated plain woven fabrics. The cured composite was cut into blocks with a size of 12 mm (height) × 12 mm (width) × 6 mm (thickness).

The PWLC contained 24 layers of plain woven fabrics which were formed by carbon fiber tows (T700-3k). The carbon fibers were coated by phenolic modified epoxy and then glued together. Considering the shortened curing time and fluidity of resin, the M0, M05 and M10 were used as the resin matrix of PWLCs. The types of samples are listed in

Table 1. Types of samples.

Symbol of Matrix	Symbol of Composite	Weight Ratio of Phenolic to Epoxy
M0	P0	0:10
M05	P05	0.5:9.5
M10	P10	1:9
M15	/	1.5:8.5
M20	/	2:8

.

2.4. Experiment

The compression tests were performed on INSTRON Testing Systems (Model 5696, America) with environmental chamber (as shown in Figure 4) to investigate the mechanical properties of samples at room and high temperatures. Loading rate was 2 mm/min. The testing temperatures were room temperature (20 °C), 80 °C and 150 °C. The test was repeated three times at each condition.

Thermogravimetric (TG) analyses were conducted on phenolic modified epoxy resins through a NETZSCH simultaneous thermal analyzer (Model STA 449 F5, Selb, Germany) to investigate the thermostabilization. The testing temperature range was from 30 °C to 800 °C.

2.5. Statistical Analysis

Standard deviation was used for the statistical analysis of the sample’s compressive peak stress and the computational formula is as follows:

$$\mathrm{S}=\sqrt{\frac{1}{\mathrm{N}-1}{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{N}}{\left({\mathrm{X}}_{\mathrm{i}}-\overline{\mathrm{X}}\right)}^2}$$

(1)

S is the standard deviation, N is the number of repeated test, X is the value of peak stress and $\overline{\mathrm{X}}$ is the average value of peak stress.

3. Results and Discussion

3.1. Compressive Properties of Phenolic Modified Epoxy

Figure 5 shows the compression stress-strain curves of phenolic modified epoxy at different temperatures. At the same temperature, the shapes of stress-strain curves had no significant change with the increase of phenolic content. At room temperature (RT) and 80 °C, the shapes of stress-strain curves exhibited elastoplasticity, whereas the shapes of compressive stress-strain curves of phenolic modified epoxy exhibited a characteristic of high elasticity at 150 °C. Moreover, at 150 °C, the failure strain of phenolic modified epoxy decreased obviously with the increase of phenolic content, and M20 had the smallest failure strain with obvious elastic-brittle characteristic. At 150 °C, the addition of phenolic increased the brittleness of epoxy. This was due to the increase of intermolecular cross-linking with the increase of phenolic content.

Figure 6 shows the peak stress of phenolic modified epoxy. From the view of peak stress, the addition of phenolic increased the mechanical properties of epoxy at room temperature, but weakened the mechanical properties of epoxy at 80 °C and 150 °C.

Figure 7 shows the amplitude of variation of peak stress compared with M0, and Figure 8 shows the percent change of peak stress compared with M0. The amplitude of variation in Figure 7 is the peak stress difference between the observed specimen and M0. At room temperature, phenolic could increase the peak compression stress of epoxy by about 5 MPa and 5%. However, the mechanical properties of epoxy decreased with the increase of phenolic content at 80 °C and 150 °C, and the peak stress of M20 was the lowest. At 80 °C for M20, phenolic decreased the peak stress of epoxy by 21.8 MPa and 29.1%. At 150 °C for M20, phenolic decreased the peak stress of epoxy by 37.4 MPa and 93.1%. Obviously, for the resin matrix, high temperature could magnify the adverse effect of phenolic on epoxy.

3.2. Compressive Properties of Phenolic Modified Epoxy-Based Composites

In order to exemplify the interlayer bond performances, the phenolic modified epoxy based PWLCs were compressed along the in-plane direction at room and high temperatures. PWLCs were formed by carbon fibers. Carbon fibers are insensitive to temperature. So, the influences of temperature on PWLCs’ compressive properties were mainly due to the epoxy matrix and the interface between fiber and epoxy.

Figure 9 shows the compression stress-strain curves of PWLCs. Similarly, the addition of phenolic into epoxy had no significant influence on the shape of stress-strain curve of PWLC. The temperature had a slight influence on the shape of stress-strain curves of PWLC. At room temperature and 80 °C, the compressive stress dropped right after the peak stress, exhibiting obvious brittleness. At 150 °C, the compressive stress dropped gradually after the peak stress which meant the brittleness of PWLC was weakened.

Figure 10 shows the compressive peak stress of phenolic modified PWLC. From the view of peak stress, the addition of phenolic obviously increased the mechanical properties of epoxy-based PWLCs at room temperature, and P10 had the highest peak stress. At 80 °C and 150 °C, the addition of phenolic weakened the compressive properties of epoxy-based PWLCs.

Figure 11 shows the amplitude of variation of peak stress compared with P0. The amplitude of variation in Figure 11 is the peak stress difference between the observed specimen and P0. From the point of amplitude of variation of peak stress, the PWLC’s peak stress had the largest amplitude of variation at 20 °C due to the addition of phenolic. Compared with P0, the peak stress of P10 increased by 75.3 MPa at room temperature, and decreased by 44.6 MPa at 80 °C and 10.5 MPa at 150 °C. In addition, compared with Figure 7, at room temperature and 80 °C, the phenolic influenced epoxy-based PWLCs much more than epoxy matrix.

Figure 12 shows the percent change of peak stress compared with P0. From the point of percent change of peak stress, the addition of phenolic could increase the peak stress of PWLC by over 20% at room temperature but decreased the peak stress of PWLC by 28.6% at 150 °C.

A finite element analysis (FEA) (as shown in Figure 13) was conducted on PWLC at room temperature through a representative volume element to investigate the influences of resin’s properties on the mechanical properties of PWLC. In the FEA, the interaction between fiber and matrix was assumed to be an ideal interface. The mechanical parameters of pure epoxy and phenolic modified epoxy were obtained from Figure 5. The mechanical parameters of carbon fiber are listed in

Table 2. Mechanical parameters of carbon fiber.

E11 (GPa)	E22 = E33 (GPa)	υ12 = υ13 = υ23	G12 = G13 (GPa)	Strength (MPa)
230	14	0.25	9	4900

. The fiber volume fraction within yarn was assumed as 65% [29] and the properties of yarn were deduced by a bridging model [30]. On the condition of an ideal interface, the curves of FEM-0 and FEM-10 were obtained based on pure epoxy M0 and phenolic modified epoxy M10, respectively.

The FEA results showed that the maximum load of FEM-10 was higher than FEM-0 by 5.25%, which was similar to the experimental results of epoxy resin in Figure 8. However, the peak stress of P10 was higher than P0 by over 20% at room temperature based on the experimental results, which meant the phenolic influenced epoxy-based PWLCs more than epoxy resin at room temperature. Phenolic improved the mechanical properties of resin matrix at room temperature and would enhance the interfacial bond properties between fiber and epoxy at the same time.

3.3. TG Analysis

As regards the different influences on epoxy by phenolic at room and high temperatures, TG analyses were performed on M0, M05, M10, M15 and M20. The results (as shown in Figure 14) showed that the phenolic modified epoxy resins presented an earlier loss in weight at around 200 °C. There was no obvious weight decrease for M0 and M05 until 300 °C. This phenomenon may be due to the uncured or unstable micromocules. During the curing process, the addition of phenolic may lead to the formation of micromocules. At room temperature, the adverse influence of the micromocules on resin matrix was limited, whereas the increased intermolecular cross-linking had a greater influence. When in high temperature fields, the micromocules became unstable and may weaken the intermolecular forces and even lead to phase separation at high temperatures. In addition, thermal stress would form around micromocules. The adverse influences of micromocules became larger at high temperatures. All these would decrease the mechanical properties of the phenolic modified epoxy matrix and its composites.

4. Conclusions

This work has investigated the influence of phenolic on epoxy’s mechanical properties at room and high temperatures. At room temperature, the addition of phenolic could increase the compression strength of epoxy resin by 5% through the increase of intermolecular cross-linking. However, at high temperatures, the phenolic did not improve the mechanical properties of epoxy resin and even decreased the compression strength of epoxy resin by 29.1% at 80 °C and 93.1% at 150 °C. With the increase of ambient temperature, the phenolic modified epoxy resin had an earlier loss in weight than unmodified epoxy resin and this adverse effect of phenolic on epoxy became more serious with the increase of phenolic content. Through the research on the compression properties of carbon fiber PWLCs, we found that the phenolic influenced epoxy-based PWLCs’ mechanical properties more than the epoxy matrix, and the phenolic may also enhance the interfacial properties between epoxy and carbon fibers at room temperature.