Influencing factors of flame retardant HIPS relative tracking index

By Kevin Wong posted 07-06-2020 04:21

Influencing factors of flame retardant HIPS relative tracking index

Abstract: The effects of resin types, flame retardants, fillers and processing technology on the relative tracking index of flame retardant high impact strength polystyrene (ene HIPS) were studied. The results show that in order to increase the relative tracking index, it is necessary to consider both the raw material and the process.

Key words: High impact polystyrene; Flame retardancy; Comparative tracking index

High impact strength polystyrene (HIPS) is polystyrene modified with polybutadiene rubber. Because of its easy injection and extrusion molding, as well as relatively low price and ideal application performance, it has a wide range of applications in household appliances, information products, food packaging, toys, stationery, etc. It is the most commonly used plastic raw material in the thermoplastics market one.

Under the influence of the humidity and impurity environment, HIPS electrical products may cause leakage (reduced insulation performance) between live parts of electrodes of different polarities or between live parts and grounded metal. The resulting arc is very likely to cause breakdown short circuit to HIPS electrical products Or the electrical corrosion of the material due to discharge may even cause a fire and fire. Testing the relative tracking index (CTI) is actually a destructive test for insulating materials that simulates the above conditions to measure and assess the relative resistance to leakage of the insulator when it is subjected to an electric field and water containing impurities at a specified voltage Traceability, this performance is an indicator of the electrical performance of electrical and electronic products, solid electrical insulation materials for household appliances and their products.

There are many factors that affect CTI, especially for flame retardant HIPS [1], because a large amount of flame retardant is added to the material, which will reduce the CTI grade of the material [2], this experiment studied flame retardants and resins , Fillers and processing technology on the flame retardant HIPS CTI.

1 Experimental part

1.1 Raw materials

Tris(tribromophenyl) cyanurate, FR-245, Israel Dead Sea Bromine Company;

Decabromodiphenylethane, SAYTEX8010, particle size 4 μm, Yabao, USA;

HIPS, STYRON A-TECH 1300, Dow Chemical Company;

HIPS, MA5210, Chevron Phillips; ultrafine talc powder, 8 000 mesh, commercially available.

1.2 Main equipment

Co-rotating twin-screw extruder, TSE-35A, Nanjing Ruiya Company; injection machine, CJ80M3V, Zhende Injection Machine Factory.

1.3 Sample preparation

The HIPS resin and various additives are mixed into the twin-screw extruder after being mixed by a high-speed mixer according to the formula, and melted, blended, extruded and granulated at a certain temperature. After the obtained pellets were dried in a blast oven, they were injection molded into standard test strips using an injection machine.

1.4 Determination of relative tracking index

According to the UL 746A-2010 standard, the spline was measured at 23°C and 50% relative humidity for 48 h.

2 Results and discussion Influencing factors of flame retardant HIPS relative tracking index

2.1 Damage mechanism of tracking resistance index

When the surface of the polymer insulating material product is polluted by moisture and pollutants with positive and negative ions, the leakage current on the surface is relatively large under the action of an applied voltage. This leakage current will generate heat, evaporate moisture contaminants, and make the surface of the insulating material in an uneven dry state, resulting in the formation of local drying points or drying belts on the surface of the material. 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, and the resistance of the carbide is small, so that the strength of the electric field formed by the tip of the electrode to which the voltage is applied increases, so flashover discharge is more likely to occur. Such a vicious cycle, until the surface insulation between the electrodes caused by the applied voltage is destroyed, a conductive path is formed, and tracking of the leakage occurs. Once the insulation material has traces of leakage, there are three kinds of deterioration phenomena. One is the appearance of carbonized black dendritic conductive channels. After several consecutive discharges, the conductive channels gradually increase. When the two electrodes are bridged together, the material will occur. Breakdown damage; second, the material is ignited and damaged under the action of multiple discharges; third, there are some pits in the material. When the discharge continues, the pits deepen, causing electrical corrosion, and sometimes breakdown damage.

2.2 The effect of resin on CTI

The difference in CTI levels of HIPS produced by different manufacturers is large.

1 is the CTI grade of different grades of resin.

Table 1 CTI grades of different brands of HIPS resins

Tab. 1 CTI rating of different HIPS resins

Manufacturer Grade CTI Level

Dow Chemical

HIPS1300 2

HIPS1110 2

HIPS1180 2

Yangtze BASF Corporation 456F 0

Chevron Phillips

MA8000 0

MA5210 1

As can be seen from Table 1, the CTI grades of HIPS produced by Dow Chemical Company are all grade 2, and the HIPS products of BASF and Chevron can reach grade 1 and grade 0, indicating that the selection of HIPS resin is very important. Because the addition of bromine flame retardants will lower the CTI grade, the HIPS produced by Dow Chemical Company can only reach grade 2, so HIPS of BASF and Chevron must be selected as the base resin to achieve grade 1 CTI. At present, HIPS manufacturers mainly use two rubbers with completely different structures for toughening PS. The HIPS produced by Dow Chemical Company all use high-cis polybutadiene rubber to toughen PS. The distance between the molecular chains of high cis polybutadiene is large, the elasticity of rubber

It has good performance and high impact strength of toughened PS, but it is easier to crosslink and carbonize, thus reducing the CTI grade of the material. The HIPS produced by Chevron Phillips and Yangzi BASF all use low-cis polybutadiene to toughen PS. The distance between the molecular chains of low cis polybutadiene is small, the structure of rubber is regular, the elasticity is poor, and the impact performance of toughened PS is poor. It is not suitable for blending modification, but it is not easy to crosslink and carbonize, so it can improve the material's CTI level.

2.3 Effect of sample surface roughness on CTI

Table 2 shows the effect of sample surface roughness on CTI. Table 2 Effect of sample surface roughness on CTI

Tab. 2 Effect of surface roughness of sample on CTI sample CTI value/V CTI grade MA5210 rough surface 400 1 MA5210 fine surface 350 2

It is generally believed that the CTI test results for a smooth test surface will be higher, but from Table 2 it is the opposite, indicating that the smooth surface is more easily carbonized and more likely to form carbon channels. This is because the rough surface only forms a localized carbonization point. The carbonization point does not diffuse and cannot generate carbon channels, so the CTI value and CTI level of the rough sample are higher.

2.4 The impact of flame retardants on CTI

Table 3 shows the effect of flame retardants and fillers on CTI. Table 3 Effect of flame retardants and fillers on CTI

Tab.3 Effect of flame retardants and filler on CTI




Superfine talc powder/%

CTI value/V CTI grade

98 2 400 1 98 2 350 2 80 20 450 1

Brominated flame retardants generally reduce the CTI value of the material. It can be seen from Table 3 that the flame retardant tri(tribromophenyl) cyanurate has a smaller effect on CTI, while decabromodiphenylethane has a greater effect on CTI. It shows that the same is a bromine flame retardant, because the molecular structure is different, the impact on CTI is also different, the decabromodiphenylethane molecule is diphenylethane connecting two pentabromobenzenes, showing a rigid structure; tri(tribromophenyl ) The cyanurate molecule is cyanuric acid linked to three tribromophenoxy groups. Because there are three rotatable ether bonds in the structure, the molecular structure is flexible. A small amount of rigid-structured organic decabromodiphenylethane has poor compatibility with resins. Local bromide accumulation will increase its instability, and it is easy to form carbonization points on the sample surface and generate carbon channels.


Influencing factors of flame retardant HIPS relative tracking index

It can also be seen from Table 3 that the addition of ultra-fine talc powder can greatly improve the CTI of the material. The reason may be that a large amount of talc powder is distributed in the HIPS resin, which prevents the formation of carbon channels in the resin, thereby improving the material's CTI value.

2.5 The impact of extrusion equipment on CTI

Table 4 shows the CTI value and CTI grade of flame retardant HIPS produced by different extruders. It can be seen from Table 4 that the same product has different CTI values ​​on different strength shear screw extruders, indicating that the dispersion of the flame retardant has a greater impact on the CTI value. The more uniformly dispersed the flame retardant in the resin, the higher the CTI value. Because the flame retardant containing bromine is unstable, it is easy to decompose and carbonize. After the flame retardant is evenly dispersed, it is not easy to form a carbonization point locally and generate a carbon channel, which can increase the CTI value. Table 4 CTI of flame retardant HIPS produced by different extruders

Tab.4 CTI values ​​of flame-retardant HIPS prepared by different extruders

Extruder strong shear screw weak shear screw

CTI value/V 400 350 CTI level 1 2

3 Conclusion

(1) The CTI value of HIPS resins from different manufacturers is different and must be

Choose HIPS resin that can achieve high grade CTI.

(2) The CTI test result of the rough surface sample is higher than that of the smooth surface sample.

(3) Different bromine flame retardants have different effects on CTI. Tri(tribromophenyl) cyanurate is superior to decabromodiphenylethane.

(4) Adding a large amount of ultrafine talc powder can improve the CTI grade.

(5) The dispersion ability of the processing equipment has an impact on the CTI grade of the material. A good dispersion ability can increase the CTI value.