3.1 Infrared spectroscopy analysis
D-PF and N-PF have similar molecular structures, and their infrared spectra show roughly the same characteristic peaks. The FTIR spectra of the cured D-PF, N-PF and PF are shown in Fig. 3
The characteristic peak of D-PF at 1631 cm− 1 and the peak of N-PF at 1630 cm− 1 correspond to C = C-C = O group and C = O group40, 42, 44, 46, respectively. We can find these two characteristic peaks at the same position in the infrared spectra of daidzein and naringenin (Fig. 3a). The two peaks at 1608 cm− 1 and 1477 cm− 1 are ascribed to C = C stretching vibration of the benzene ring backbone of D-PF, N-PF and PF. The characteristic peaks around 3422 cm− 1 belongs to -OH group. The absorption band at 1210 cm− 1 is the vibrational peak of C-O-C in the pyran ring and C-O of the phenolic hydroxyl group on the phenol ring. The characteristic peaks of -CH2 appears around 2922 cm− 1. The absorption peaks in the low band (600-900cm− 1) are generally considered to be related to the substitution of hydrogen atoms on the phenol ring. The two peaks at 824 and 755 cm− 1 belong to the characteristic peaks of para and ortho C–H substitutions on the phenolic rings. According to above analyses, the synthesized modified resin successfully introduced the rigid structure of flavonoid phenols (daidzein and naringenin).
3.2 Curing Behaviors
The curing behaviors of D-PF, N-PF and PF were monitored by DSC, and their corresponding DSC exotherms were recorded for analysis. As shown in Fig. 4, the DSC curves of D-PF, N-PF and PF all show the results of a single exothermic peak. The DSC curves of the two flavonoid phenol modified resins D-PF and N-PF have some similar rules. The curing initiation temperature Ti of D-PF, and N-PF is about 120 to 130°C. The curing termination temperature Te is about 190°C to 200°C, while the Ti of PF is 110°C with the curing peak temperature Tp of 139.5 and Te of 160°C. Compared with the low curing peak temperature of ordinary PF, the curing temperature of D-PF, N-PF are greatly increased. Interestingly, the curing peaks of D-PF and N-PF show opposite results. As shown in Fig. 4c and 4d, with the increasing of the proportion of daidzein in the reaction, the curing peak temperature of D-PF shows an overall downward trend, while the curing peak temperature of N-PF shows a trend of overall improvement with the increasing of substitution rate of naringenin. The modified resin exhibits higher Ti, Tp and Te than those of PF.
It is well known that under the same conditions, the peak temperature monitored by DSC can be used to judge the strength of the curing reactivity of phenolic resin. This means that the lower the peak temperature is, the higher the reactivity is. Therefore, the reactivity of the raw material of the synthetic resin is the main reason for this result. Biomass phenols are generally less active than petroleum phenols. Therefore, higher energy and conditions are required during synthesis and curing. Secondly, the reaction site and steric hindrance are also important reasons. The bio-phenols of daidzein and naringenin have large rigid skeletal structures, and the steric hindrance generated by the rigid group slows down the curing reaction to a certain extent, which is the reason why their curing temperature is higher than PF. The difference in reaction sites and steric hindrance and reactivity may be the reason why the curing peak temperatures Tp of D-PF and N-PF proceeds in opposite directions. The reaction activity of naringenin is higher than that of daidzein. With a small amount of naringenin added, the curing peak temperature of N-PF is lower. With the increasing of the addition ratio, the steric hindrance effect caused by the large volume of naringenin makes the polycondensation more difficult, and the curing peak temperature of N-PF is increased accordingly. The activity of daidzein is low, the energy required for curing reaction is high, resulting in the high curing peak temperature. However, with the increasing of the proportion of daidzein, the opportunity for daidzein to participate in the polycondensation reaction is increased, and the curing peak temperature of D-PF is gradually decreased.
3.3 Thermal Stability of the Cured Resins
Thermal performance is an important indicator of thermosetting resins and an important means to evaluate the thermal stability of phenolic resins. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the successfully modified phenolic resins. The TGA and DTG curves of cured N-PF, D-PF and PF are shown in Fig. 5. The thermal degradation characteristic parameters of cured N-PF, D-PF and PF include T5% (temperature at which 5% weight loss occurs), T10% (temperature at which 10% weight loss occurs), Td,max (temperature of maximum degradation rate) and YC800 (residue at 800°C) are recorded in the Table 1 to provide the relevant thermal stability performance analysis.
As shown in Fig. 5, N-PF and D-PF with different ratios have similar TGA and DTG curves, which indicates that they have similar thermal decomposition laws. For N-PF and D-PF, the thermal decomposition at low temperature (T5% and T10%) shows a trend of increasing firstly and then decreasing (the detailed temperature values are described in Table 1). The T5% and T10% values of N-PF and D-PF are higher than those of PF-1 and PF-2, indicating that their thermal properties at low temperature are much better than those of PF-1 and PF-2. There is a small gap between the YC800 values of different ratios of N-PF. The carbon yield YC800 for N-PF is approximately 59%, which is slightly higher than that of PF-1 (56%). D-PF resin has a high residual carbon at 800°C, and the highest YC800 is 64.39%, which is higher than that of PF-2 with 58.15%.
Table 1
Thermal stability related data of resins
Resin | T5%(℃) | T10%(℃) | Td, max(℃) | Char Residues at 800℃(wt%) |
PF-1 | 136.76 | 219.24 | 513.59 | 56.85 |
PF-2 | 127.36 | 212.66 | 508.59 | 58.15 |
N-PF-1-20 | 154.00 | 250.64 | 520.59 | 58.89 |
N-PF-1-18 | 149.91 | 251.64 | 522.59 | 58.24 |
N-PF-1-16 | 168.31 | 251.61 | 523.41 | 57.81 |
N-PF-1-14 | 204.77 | 297.61 | 511.29 | 59.81 |
N-PF-1-12 | 203.95 | 284.53 | 519.79 | 59.56 |
N-PF-1-10 | 200.64 | 279.72 | 515.90 | 59.48 |
N-PF-1-8 | 195.04 | 269.66 | 510.00 | 58.18 |
D-PF-1-20 | 187.53 | 332.94 | 519.23 | 61.75 |
D-PF-1-18 | 223.54 | 365.51 | 524.94 | 62.14 |
D-PF-1-16 | 226.42 | 337.93 | 520.71 | 61.03 |
D-PF-1-14 | 202.31 | 350.43 | 533.48 | 61.92 |
D-PF-1-12 | 160.90 | 312.32 | 543.09 | 62.95 |
D-PF-1-10 | 154.52 | 297.82 | 540.50 | 64.39 |
D-PF-1-8 | 152.69 | 269.15 | 535.02 | 59.94 |
The weight loss caused by the thermal decomposition of N-PF, D-PF and PF in the low temperature section (below 300°C) is the volatilization and release of formaldehyde and water in the resin, as well as the possible existence of small molecules released from the condensation reaction. Compared with phenol, the bio-phenols daidzein and naringenin have large and complex structures, and their rigid groups have better heat resistance. It is well known that the higher the degree of crosslinking is, the better the heat resistance is. It can be seen that the appropriate introduction of daidzein and naringenin can improve the degree of crosslinking and improve heat resistance. However, with the increasing of the proportion of daidzein and naringenin, the huge rigid structure can produce inhomogeneous products during the synthesis and curing stages, resulting in the increasing of voids in the three-dimensional space network of the synthetic resin. Therefore, in the thermal decomposition in the middle and low temperature range, the T5% and T10% of N-PF and D-PF are firstly increased and then decreased. The decomposition of the N-PF and D-PF resin at high temperature (400℃-600℃) is mainly due to the degradation of cross-linking, the cleavage and decomposition of ether bonds and methylene groups, which is the reason for the maximum decomposition rate. As shown in the Fig. 5b, d and Table 1, the N-PF, D-PF and PF all have a large decomposition peak at 400°C to 600°C. The Td,max of N-PF is approximately 523℃, while the corresponding Td,max of PF-1 is 513.59℃. The Td,max of D-PF is approximately 543℃, while the Td,max of PF-2 is only 508.59℃. This is attributed to the pyrone ring structure connecting two benzene rings in the structures of daidzein and naringenin, which ensures their higher rigidity and heat resistance. In the final temperature range of 600–800°C, the decomposition weight loss curve tends to be flat, indicating that the proportion of weight loss is small, which is due to the residual resin is pyrolyzed to form an amorphous residual carbon. Overall, the heat resistance of the modified phenolic resin prepared from the rigid structure of daidzein or naringenin is excellent.
3.4 Mechanical Properties
Mechanical properties are one of the most important performance indicators of phenolic resins, and the strength of mechanical properties directly restricts the application fields of phenolic resins. Therefore, in order to avoid the influence of errors, tensile and flexural tests are used to jointly evaluate the mechanical properties of the modified resin. The comparison of tensile and flexural properties between N-PF, D-PF and PF after curing are shown in Fig. 6 and Table 2. The tensile strength and flexural strength of N-PF and D-PF resins are increased firstly and then decreased with the increasing of the proportion of daidzein and naringenin. The maximum tensile and flexural strengths of N-PF are 33.86 MPa and 110.42 MPa, respectively, compared with PF-1 (28.77 and 79.89 MPa). Similarly, the maximum tensile and flexural strengths of D-PF are 35.61 and 103.17 MPa, respectively, while the strengths for PF-2 are 24.48 and 55.79 MPa. However, we found that with the increasing of proportion of daidzein and naringenin, the tensile and flexural strengths of N-PF and D-PF resins are lower than those of unmodified common PF resins. This shows that in the process of modifying PF resin, the substitution rate of phenol is not as high as possible. The strength of the modified resin is lower than that of the unmodified resin due to the excessive proportion of daidzein and naringenin in the reaction system. We have mentioned this insight in the thermal analysis section. The bulky rigid structure can cause steric hindrance and product inhomogeneity during the synthesis and curing stages, resulting in the increased voids in the three-dimensional spatial network of the synthetic resin. This means that the synthetic modified resin has some defects, which is the reason why the strength of the modified resin is lower than that of the unmodified resin. Rockwell hardness results are also recorded in the Table 2. The Rockwell hardness of N-PF and D-PF resins with bio-based rigid structure is not much different from that of ordinary PF resins. The hardness of the modified resin is moderate, which avoids the problem of excessive noise in some applications.
Table 2
Mechanical properties of resins
Resin | Tensile strength/MPa | Flexural strength/MPa | Rockwell hardness/HRR |
PF-1 | 25.86–32.14 | 77.45–83.06 | 121.02 |
PF-2 | 21.24–29.57 | 51.19–61.18 | 122.78 |
N-PF-1-20 | 18.18–26.32 | 93.03-101.38 | 119.9 |
N-PF-1-18 | 31.59–35.93 | 102.30-117.34 | 121.9 |
N-PF-1-16 | 30.17–34.51 | 100.37-111.05 | 122.62 |
N-PF-1-14 | 31.35–35.62 | 93.26-108.83 | 120.4 |
N-PF-1-12 | 29.08–34.39 | 84.57–87.89 | 119.23 |
N-PF-1-10 | 22.35–27.85 | 75.02–84.97 | 121.26 |
N-PF-1-8 | 18.11–21.46 | 63.38–73.39 | 119.0 |
D-PF-1-20 | 17.51–19.23 | 88.57–89.08 | 121.74 |
D-PF-1-18 | 18.71–21.03 | 101.74-107.31 | 119.0 |
D-PF-1-16 | 33.46–38.86 | 88.41–97.75 | 119.78 |
D-PF-1-14 | 31.09–33.65 | 88.03–99.94 | 120.38 |
D-PF-1-12 | 33.72–35.76 | 79.05–90.47 | 119.88 |
D-PF-1-10 | 19.29–23.05 | 51.87–68.19 | 119.34 |
D-PF-1-8 | 20.56–21.16 | 58.78–70.94 | 118.98 |
3.5 SEM Analysis of Resins surface section morphology
(a) N-PF(b)D-PF(c)PF of tensile fracture morphology;(d) N-PF(e)D-PF(f)PF of flexural fracture morphology
SEM images of tensile and flexural fracture surface morphology are used to explain the differences in the mechanical properties of the analytical resins at the microscopic level. The SEM images of tensile fracture sections and flexural fracture sections of N-PF, D-PF and PF resins are shown in Fig. 7. The tensile and flexural sections of PF are smooth, and the crack propagation direction is in a single direction, showing a brittle fracture state. After introducing the rigid structure of daidzein and naringenin, the roughness of tensile fracture section and flexural fracture section of D-PF and N-PF increase significantly. The fracture direction tends to disperse. The rougher fracture surface with dislocations and vacancies indicates that the modified resins N-PF, D-PF have higher toughness, and their corresponding tensile and flexural strengths are also improved. This verifies the analysis results in the mechanical properties analysis section. At the same time, the three-dimensional spatial defects mentioned in thermal analysis and mechanical analysis are confirmed by SEM. In the tensile and flexural fracture section of N-PF, D-PF, we can see some void defects, which is the reason for the decline of the mechanical properties of the resin.
3.6 Tribological properties
In this test, D-PF-1-20 and N-PF-1-20 are selected as modified resins for performance comparison with ordinary PF resins. The dynamic coefficient of friction and the average coefficient friction of resins are recorded in Fig. 8 and Table 3.
Table 3
Tribological properties related data of resins
Resin | Average coefficient of friction | Wear rate(mm3/ Nm) |
N-PF-1-20 | 0.1511 | 9.428*10− 6 |
PF-1 | 0.1550 | 1.862*10− 5 |
D-PF-1-20 | 0.1211 | 1.113*10− 5 |
PF-2 | 0.1528 | 1.068*10− 5 |
From the Fig. 8, we can find that the dynamic friction coefficients of the four resins are very stable in the friction test for up to 0.5h, and fluctuates steadily within a certain range. From the test results of friction properties of these four resins, the dynamic friction coefficient of D-PF-1-20 fluctuates the least with time, indicating that its contact surface is smoother and its friction coefficient is also the smallest. The dynamic friction coefficient of N-PF-1-20 fluctuates widely in the early stage of the test, but it is stable in the later stage. The cause may be uneven contact surface roughness and deep scratches caused by voids inside the resin. The dynamic friction coefficient of ordinary PF resin tends to be normal with time. The average friction coefficients of D-PF-1-20, N-PF-1-20, PF-1 and PF-2 are 0.1211, 0.1511, 0.1550, 0.1528, respectively. Obviously, with the addition of bio-based daidzein and naringenin, the friction coefficient of the resin is decreased, and it is more stable than that of the common PF resin with the same curing process. The sound produced during the friction test is moderate and not sharp. The moderate hardness prevents the resin from making annoying, disgusting noises during using.
The tribological properties of modified resins are reflected by the friction coefficient and wear rate. The width and depth of the worn surface of the four resins (D-PF-1-20, N-PF-1-20, PF-1 and PF-2) were obtained by optical microscope and 3D profilometer to calculate the wear rate. The wear rate is calculated according to the following equation:
and
Where V is the wear volume (mm3), R is the wear scar radius of the worn surface (mm), b is the wear scar width of the worn surface (mm), h is the depth of the wear scar on the worn surface, and K is the wear rate (mm3/ Nm), L is the load applied to the resin (N), and d is the sliding distance (m).
The wear rates of D-PF-1-20, N-PF-1-20, PF-1 and PF-2 are recorded in Table 3. It can be concluded that the wear rate of N-PF-1-20 resin is lower than that of PF-1, while the wear rate of D-PF-1-20 is comparable to that of PF-2. The wear rates of D-PF-1-20, N-PF-1-20, PF-1 and PF-2 are 1.113*10− 5, 9.42*10− 6, 1.862*10− 5, 1.068*10− 5mm3/ Nm, respectively. Although D-PF-1-20 resin has the smallest coefficient of friction, its wear rate is slightly higher than that of PF-2 resin. The N-PF-1-20 resin with a friction coefficient similar to that of PF-1 resin has an excellent wear rate. From the 3D profile, we can observe the changes in the depth and width of the wear scar of the four resins. The normal PF resins with two different curing conditions represents the deepest wear marks and the widest wear mark widths, respectively, while the two modified resins show moderate wear marks depth and wear mark widths. This indicates that the addition of daidzein and naringenin can appropriately reduce the wear marks depth and wear marks width of PF resin. We can also observe the overall condition of the surface of the resin wear part from the Fig. 9. The plough marks on the wear surface of the PF resin are more obvious, and the wear track is rougher and larger. Compared with pure PF resin, the surface of the worn part of D-PF-1-20 and N-PF-1-20 is smoother and more uniform. This is mainly attributed to the rigid structure and excellent three-dimensional network structure of the modified resin which hinders the movement of molecular chains, ensuring its excellent wear and friction properties. At the same time, the purpose of testing the friction properties of the modified resin is to consider the application of the modified resin in tribology and brake materials. Friction materials, brake materials can easily reach a higher temperature of 300 ℃ during using. In the analysis of thermal properties, we have proved that the modified resins D-PF and N-PF have excellent heat resistance in the middle temperature. The result shows that the D-PF and N-PF resins modified and enhanced by daidzein and naringenin have lower friction coefficient and more excellent wear resistance than pure PF. In the preparation of friction braking materials, the two modified resins can be used as polymer matrix with excellent properties.