At present, various modified phenolic resin and polyimide resin bond are mostly used for resin bond diamond wheels in China. Phenolic resin, as a widely used resin bond, features low price, superior comprehensive performance, and simple production process. Most of the phenolic resins in abrasives are “2123” in China.
The resin is formed at 150℃-180℃with certain mechanical strength. However, “2123” resin has low heat resistance, the working temperature usually cannot over 170℃, and the long-term working temperature cannot over 120℃. It is not suitable for large feed and deep cutting.
This paper describes the thermal performance of heat-resistant phenolic resin, and did the experiment of mechanical performance and grinding, and made a comparison with “2123”. It analyzed the reasons for the difference in wear resistance between the 2 resins by observing the grinding surface of the diamond wheel.
2.1 Raw materials for the experiment
Synthetic diamond: Model RVD, mesh size 140/170
Tungsten Carbide Tools: Model YG8, place of origin: Zhuzhou
Heat resistant phenolic resin: the place of origin: Wuhan
“2123” Phenolic resin: the place of origin: Zhengzhou
Curing agent: hexamethylenetetramine (HMTA)
2.2 Instruments and experimental procedure
2.2.1 MYS-100 type hot press was used for the grinding wheel according to the corresponding curing process system. The grinding wheel was in an electric drying oven after curing. Resin bonded diamond wheel size: 1A1 100 ×16 ×20×4 ，RVD 140/170 B 75
2.2.2 NETZSCH STA449C model comprehensive thermal analyzer was used in the thermogravimetric experiment, and the thermal stability and thermal decomposition temperature of the material were tested in air and N2atmosphere at the heating rate of 10K/min. Studied on the structure and performance of the material.
2.2.3Mechanical performance sample in a ratio of 1:1, add 220# silicon carbide previously wetted with cresol to the resin, and then put into the mold as required, pressed at 130℃ for 1h and at 140℃ for 1h and at 160℃ for 0.5h.
The temperature after curing is 180 °C for 2h. It was applied to KZOQ-1 automatic bending strength testing machine and NS2000 tensile strength testing machine for testing mechanical performance. It was applied to HR-150A Rockwell hardness tester for testing Rockwell hardness, and the load was 1000N.
2.2.4 Did grinding test on a DHM-3 grinding tester manufactured in Zhengzhou. Diamond grinding wheel speed: 5000 r / min, loading pressure: 5-10N. Constant force loading, swing frequency:20 times/min.
2.2.5 Did the microscopic analysis of the grinding surface of the grinding wheel with XTL-3400 stereo microscope.
3.Results and discussions
3.1 TG-DSC Features of Heat-resistant phenolic resin
Fig.1 is a comprehensive thermal analysis spectrum of phenolic resin in N2 and air atmosphere. It can be seen from the figure that the residual carbon ratio of phenolic resin was 54.6%, and the decomposition process of the resin had 3 stages. The weight loss of the resin in N2 was 2.19% at 200℃-300℃. The DSC curve had a weak endothermic peak at 235.9 °C. This was a weightlessness peak in which the resin was further curing cross-linked or the terminal group such as hydroxymethyl group falling. The weightlessness in this part was due to the volatilization of small molecules such as water
and pyrolysis products in the resin.
The range of 300 °C-600 °C was the main weightlessness interval. The weight loss was 35.66% under N2 atmosphere. The DSC curve had a broad endothermic peak at 465.3 °C, which is mainly the fracture of the methylene bridge between the benzene rings., and the pyrolysis product was a phenolic homolog. In the air atmosphere, the weight loss in this interval was as high as 74.12%, and the DSC endothermic peak was at 393.6 °C.
The weight loss in this stage was decomposition and carbonization of the resin.
This was because in oxygen the methylene group was converted into hydroperoxide, and finally formed ketone structure, and the ketone was prone to radical cleavage. At the same time, the part between 2 phenol rings by intramolecular dehydration can form oxonium rings. The decomposition of ketones and ether bond formed a large number of side products such as water, CO, CO2and, etc.
At 600℃-800℃, there was high-temperature dehydrogenation to carbon reaction. In N2,
the weight loss was 7.02% in this interval. The reaction in the air was still very violent. The DSC curve immediately showed an endothermic peak at 606.7 °C after the endothermic peak at 560.8 °C. It indicated that the pyrolysis process of the methylene group had been completed, and there was a new violent reaction.
This was, the phenolic nucleus was violently oxidized into ruthenium structure at high temperature and then decomposed to form small molecules such as CO2, CH4, H2O. After 660℃, the endothermic peak quickly dropped, indicating that the reaction finished. In the air, the final residual amount was 8%.
It can be seen from the analysis of the whole TG-DSC curves that the resin has good thermal stability before 250℃. It is good for improving the grinding performance of the resin bond diamond grinding wheel when the heat resistance temperature of the resin is higher.
3.2 Mechanical performance analysis
The strength of the resin bond determines the wear resistance, the productivity of abrasives during the grinding process. It can be seen from Fig.2 that the bending strength of heat-resistant phenolic resin is usually higher than “2123”, and the average bending strength reaches 824N. It indicated that the heat-resistant phenolic resin has good toughness, and the finished diamond grinding wheel does not collapse. The profile keeps better.
It can be seen from Fig.3 that the Rockwell hardness of the heat-resistant phenolic resin is significantly lower than that of “2123”, which is only 44.7HRB, slightly higher than the hardness requirement of the resin bond grinding wheel. The hardness of “2123” phenolic resin is as high as 82HRB. The hardness represents the ability of the grinding wheel to resist the intrusion of external forces. This hardness of the resin is low, which may make the resin difficult to be used for heavy-load grinding wheels, and where the grinding wheel requires good wear resistance and forced dressing. If increasing the hardness of the resin bonded diamond wheel, it can adjust the accessories and process.
Table 1 Tensile strength of heat-resistant phenolic resin and “2123” phenolic resin
|Heat-resistant phenolic resin||41||35||44||39||39.7|
It can be seen from Table 1 of the tensile strength of 2 resins that the average are 39.7 MPa and 39.5 MPa respectively. It indicates that the bonding performance of 2 phenolic resins is similar. Good bonding performance can make therein to better wrap the diamond and the diamond cannot fall off.
3.3 Grinding performance study of resin bonded diamond wheel
3.3.1 Grinding test
In this part, the grinding performance of this kind of heat-resistant phenolic resin(NR) and “2123” phenolic resin which is widely used in China was analyzed, further states that the grinding ability and application value of the heat-resistant phenolic resin. Usually, evaluate the grinding performance by the grinding ratio. This experiment is applied to the grinding ratio as a quality ratio. The formula as follows:
G: grinding ratio, ΔMv: wear quality of the tungsten carbide tool, ΔMs: wear quality of grinding wheel
Grinding efficiency and grinding wheel removal rate is also an important indicator to study the grinding performance of the grinding tool. The ratio of wear quality to grinding time is used in this experiment.
Z w = △ Mw / t .Zs = △Ms / t
Zw: grinding efficiency, Zs: resin bonded diamond wheel removal rate. Table 2 shows the results of the grinding experiment. From the grinding efficiency value, it can be seen that the grinding efficiency of “2123” is slightly lower than that of the NR diamond wheel. This may be due to the high hardness of “2123”, which makes the self-sharpness of the grinding wheel not good. When the hardness of the bond is high, the bond falls off slowly that is relative to the falling of the diamond.
From the grinding wheel removal rate value, it can be found that “2123” is nearly 150% higher than NR. This is because “2123” has poor heat resistance and high heat temperature in the grinding zone makes the bond fall off quickly. Also “2123” has poor toughness, and the diamond grinding wheel often collapses and causes the large blocks to fall off.
Both resins produce burnt smell in the grinding process by observing the grinding phenomenon. However, “2123” burnt smell is very big. It indicates that the burn of “2123” is very serious, and NR has better heat resistance.
Under the same loading pressure, the grinding of the heat resistant phenolic is improved by about 160% compared with “2123” resin diamond grinding wheel. Because it is high temperature locally in the grinding of the wheel, the heat generated by the abrasive grains is quickly transferred to the resin around the abrasive grains.
The ordinary resin is rapidly aged and degraded and loses the holding force of the abrasive grains. Most of the abrasive grains fall off, making the bond of most of the grinding wheel extrudes the workpiece, and the grinding force is very weak.
Owing to high strength index of “2123”, the falling speed of the bond is lower than that of the abrasives. The resin bonded diamond wheel produces burnt smell and even burns the workpiece.
Owing to the high thermal stability of heat resistant phenolic, the bond does not lose the holding force at local high temperature, and more abrasive grains participate in the grinding. At the same time, the design hardness is not high, the abrasive grains fall off and the bond also falls off, ensuring the grinding wheel sharp and improving the grinding efficiency of the grinding wheel.
Table 2 Grinding test of phenolic resin bond diamond wheel
|Item||Tool abrasion quantity（g）||Grinding Wheel
|Heat-resistant phenolic resin||64.2||0.1428||0.1||1200||449.6||0.0535||0.000119|
|Heat-resistant phenolic resin||64.6||0.1466||0.1||1200||440.7||0.0538||0.000122|
The current value represents the pressure of the grinding wheel and the workpiece. It can be seen from the above grinding data that the grinding ratio of the heat-resistant phenolic is much higher than that of the ordinary phenolic.
The current value is small, and the grinding resistance is also small, which proves that the diamond grinding wheel is sharp and wear-resistant. “2123” phenolic has large current value, which proves that the grinding resistance is big, and increases the times of dressing in the actual grinding and reduces the grinding efficiency.
It can be seen from the above that the grinding performance of the heat-resistant resin is good, and the grinding ratio is much higher than that of “2123” phenolic resin.
For these features of the resin, the heat-resistant resin is more suitable for the grinding wheel which requires higher self-sharpness, reducing the dressing interval of the grinding wheel and improving the durability of the resin diamond wheel.
It also can improve the hardness of the grinding wheel by adjusting the formula of the auxiliary material and the pressing technology to make grinding wheels with high wear resistance.
3.3.2 Grinding surface analysis of diamond grinding wheel
The following are figures of the surface worn by 2 kinds of resin bonded diamond wheel in the grinding test, which are magnified under the microscope.
From the microscopic surface of “2123” phenolic resin grinding wheel as shown in Fig.4(a), the abrasive grains of resin bond diamond grinding wheel fall off a lot and see the burnt holes on the surface, and almost no diamond abrasives are seen.
Since the phenolic resin wheel produces a large number of pores in the pressing process, the diamond grains slip easily in grinding, and the grinding heat causes the resin around the abrasive to be carbonized and decomposed, causing the diamond to fall off. From fig.4(b), chipping can also be seen.
There are blocks of the bond falling off at the edge of the grinding wheel, which causes the wheel to be severely worn and broken. This is because “2123” resin has poor toughness, the profile damages in the grinding process for a long time.
From Fig.5, the microscopic condition of the grinding surface of the heat-resistant phenolic resin grinding wheel can be seen. It can be seen the holes of the abrasive falling off on the surface of diamond wheel, but it is less than that of “2123” resin diamond grinding wheel and keeps a lot of the diamond and the combination is tight, which indicates that the heat resistance of the heat resistant phenolic resin is stronger than that of “2123”.
It can be seen some pores on the surface of the heat-resistant phenolic resin wheel, but the number is much less than that of Fig. 4(a). The wheel structure is compact and the surface quality of the workpiece is better. Better heat resistance and toughness of the heat-resistant ensures the smooth grinding, less burnt holes, diamond grains remain intact, and no chipping. So,
The grinding ratio of the heat-resistant resin bond wheel is higher than that of “2123” wheel. It can be seen that the mechanical performance and heat resistant of rein are important factors affecting the grinding performance of resin grinding wheel.
The bending strength of the resin can improve the profile retention of the diamond wheel, increase the wear resistance. High heat resistance makes the resin withstand the high-temperature damage, keeps the bonding effect for the abrasive.
- Experiments show that the heat-resistant phenolic resin is an ideal for diamond wheel bond. And it is excellent for wear resistance of the resin bonded diamond wheel and the surface quality of the workpiece.
- The TG-DSC curve shows that the heat-resistant phenolic resin has good thermal stability before 250 °C, which is good for improving the grinding performance of the wheel.
- Heat-resistant phenolic resin has good toughness and high bending strength, which can make diamond wheel better profile retention and improve wear resistance.