1 Introduction

Phenolic molding compounds are the oldest thermosetting plastics with the largest output, and are widely used in telecommunications, electrical machinery, electrical appliances, automobiles, instrumentation, office appliances, and household utensils. It has excellent electrical insulation and product dimensional stability, good mechanical strength and heat resistance. However, when phenolic molding compounds are used in low-voltage electrical products, their leakage-resistant trace properties are inferior to other thermosetting plastics such as polyesters and aminos. Phenolic molding plastics have a lower Comparative Tracking Index (CTI) value, usually belonging to Insulating Group IIIb (≤175V), but only after surface treatment such as impregnation can increase the CTI value to 175V. ~275V. The amino molding compound is generally an insulation group IIIa (CTI is 175V-350V), but when asbestos is used as a filler, the CTI value can be greatly increased to reach the insulation group I (CTI ≥ 600V). The CTI value of the unsaturated polyester molding compound generally reaches the insulation group I class 1. This article will initially discuss the methods for improving the resistance to leakage traces of phenolic molding compounds and apply them to the company's new product development.

2. Tracking of Polymer Insulation Material and Its Theoretical Analysis 12

2.1 Leakage tracking of polymer insulation material

When the surface of the polymer insulating material is contaminated by moisture and positive and negative ion contaminants, the leakage current at the surface under applied voltage is much larger than that of the clean surface. The leakage current will generate heat, evaporate moisture contaminants, cause the surface of the insulating material to be in a non-uniformly dry state, resulting in the formation of a localized drying point or drying zone on the insulating surface. The dry area increases the surface resistance, so that the electric field becomes non-uniform and flashover discharge occurs. Under the combined action of the electric field and heat, the surface of the insulating material is carbonized, the resistance of the carbide is small, and the strength of the electric field formed by the tip of the electrode that applies the voltage is increased, so that flashover discharge is more likely to occur. Such a vicious circle, until the insulation between the electrodes that causes the applied voltage breaks down, forms a conductive path, and generates tracking marks.

Once the polymer insulation material leaks and traces, there are three kinds of deteriorating phenomena. First, carbonized black dendritic conductive channels appear. After continuous multiple discharges, the conductive channels gradually increase. When the two electrodes are bridged, the material Breakdown occurs; second, under the action of multiple discharges, the material catches fire and is destroyed; third, some pits appear in the material. When the discharge continues to continue, the pits deepen, causing electric corrosion, sometimes causing breakdown damage. Sometimes it is not broken down.

2.2 Theoretical analysis of creepage tracking of polymer insulation materials

The flashover discharge of a polymer insulating material during sparking generates a spark, and the spark serves as a heat source, causing the temperature of the surface of the insulating material to rise locally, causing gradual changes in chemical composition or structure, such as hydrogen, low molecular hydrocarbons, and others. The escape of gas; chain breaking or cross-linking occurs; the degree of branching and crystallinity of the molecular chain changes, and the isomers of the molecule convert to each other. Therefore, the tracking performance of insulation materials is mainly determined by the chemical structure of the polymer material and the composition of the composite. In the leakage-resistance test, if the material has flammable volatiles, it will easily cause combustion and damage; if the material is not susceptible to residual carbon traces and is not prone to volatiles during the flashover discharge process, its leakage resistance must be better.

Judging from the chemical reaction mechanism, the mechanism of the trace scarification reaction of polymer materials is similar to the mechanism of thermal decomposition reaction. The heat generated by the discharge is sufficient to thermally decompose the polymer insulating material. When the thermal energy generated after the thermal decomposition reaches the interatomic bond energy, the bond with a small bond energy naturally breaks. For example, the weaker C—H bond (84 Kcal/mol) in the phenolic resin breaks first, and the stronger H—O bond (110 Kcal/mol) breaks, leaving both carbon residues, and thus tends to produce traces. In polyester resins, the ester group (—COO—) is thermally decomposed to a non-flammable CO 2 gas, and the p-electron of O on the C—O bond in —COO— and the double bond in the carbonyl group (O=C—) The electrons produce a p-quinone conjugate effect, so that the bond length of the C—O bond is shortened correspondingly, and its bond energy is enhanced, so that it is not easy to generate trace marks.

Selecting suitable fillers has a significant effect on improving the tracking resistance of polymer insulation materials. The tracking of the material is determined by two relative processes, namely the formation of carbon and the volatilization of carbon. When the current is faster than the latter, the tracking occurs. The oxidation process is most likely to produce volatile carbon (CO or CO2). If oxygen-enriched compounds such as hydrated alumina (Al2O3.3H2O) or aluminum hydroxide are added to the material, its tracking resistance can be obtained. To improve, the chemical reaction is:

Al2O3.3H2O + 3C --→ CO↑ + Al2O3 + 3H2 ↑

Aluminum hydroxide is an effective inorganic flame retardant, therefore, many of the flame retardant plastics with aluminum hydroxide added have higher CTI values than the original. Asbestos as a filler also improves the tracking resistance of insulating materials. When the polymer compound on the surface of the material is destroyed, due to the excellent high temperature resistance of the asbestos, the further development of the carbonization channel is hindered and further deterioration of the material is prevented.

3. Test

3.1 The main test material

a. Acid phenolic resin (homemade)

b. Wood pulp, short lint and other alpha cellulose (industrial products)

c. Compound coupling agent (homemade)

d. Other raw materials are all industrial products

3.2 The main test equipment

a. SK250 open type plastic machine (manufactured by Wuxi Rubber Machinery Factory)

b. RTT type low-voltage leakage tracking tester (manufactured by Wuhan Qiyi Research Institute)

c. 45t flat vulcanizer (manufactured by Shanghai Great China Rubber Machinery Co., Ltd.)

d. 3mm thick Ф100 wafer mold (homemade)

3.3 Preparation of phenolic molding compound and its resistance to tracking performance test

After mixing all kinds of raw materials (some of them are pretreated), they are uniformly rolled on an open mill and rolled into sheets according to the rolling process of a normal phenolic molding compound, cooled and crushed, pressed into a crucible 100 wafer 3, adjusted by state 4, and then pressed according to GB4207. -84 Standard 5 Tested on RTT Low Voltage Leakage Track Tester.

4. Test results and discussion

4.1 Effect of Low Molecular Weight Phenolic Resins on CTI of Phenolic Molding Compounds

Phenolic resins have low content of low-molecular substances such as free phenols, and the leakage resistance of phenolic molding compounds has been improved (see Table 1). Because the phenolic resin is the main resin of the molding compound, the content of the low-molecular-weight organic material is low, and the molded plastic is not likely to generate flammable volatiles in the leakage-resistance test.

4.2 Effect of Surface Treatment of Alpha Cellulose on CTI of Phenolic Molding Compounds

The natural α-cellulose surface such as wood flour and short lint is rich in low-molecular-weight organics such as oils and fats. If the surface is properly treated, both the degreasing effect and the coupling with the phenolic resin can be achieved. The CTI value of phenolic molding compounds is likely to increase (see Table 2).

4.3 Effect of the Addition of Aluminium Hydroxide on CTI of Phenolic Molding Compounds

Aluminum hydroxide as a commonly used inorganic flame retardant is widely used in plastics, and because it is rich in oxygen, it can form volatilizable carbon and reduce the residual carbon when the traces are formed. Thus, the value of CTI for phenolic molding compound is Improve the benefits (see Table 3). However, the amount of aluminum hydroxide in the phenolic molding compound is limited due to the large specific gravity of aluminum hydroxide and the greater wear of the production equipment and the molding processing equipment.

4.4 Application

In combination with the above improved method, our company is applying to the development of EA-5555J and EA-5558J phenolic molding compounds. These two products have been approved by the UL company in the United States. Their CTIs meet the UL (Performance Level Categories) of UL746A standard 6. Level 3 (175V to 250V) meets the technical requirements of PF2836 (225V/250V) and PF85 (125V/150V) of similar products from Germany Bakelite.

5 Conclusion

a. Controlling the content of organic low molecular substances in phenolic resin, surface treatment of natural α cellulose, and adding oxygen-rich inorganic compounds such as Al(OH)3, etc., may improve the tracking index of phenolic plastics (CTI) ) The proven method.

b. The above method was applied to the company's new products EA-5555J and EA-5558J so that these two products reached the technological level of similar foreign products.