Poly phenylene sulfide, abbreviated as PPS, is a new type of engineering plastic, which is mainly divided into two categories: one is a branched thermoplastic polymer with a high viscosity, and the other is a thermosetting polymer which has a linear molecular structure before curing, and can be softened to a certain extent if sufficiently heated after curing. In the following, we will mainly introduce the thermosetting PPS.
The Properties of PPS
- Physical Properties
PPS is a white, highly crystalline polymer with a density of 1.34. It is characterized by excellent mechanical properties, with tensile strength and flexural strength superior to that of PA, PC, and PBT, etc. It possesses extremely high rigidity and resistance to creep, but high brittleness and low notched impact strength that is even lower than that of PA, PC and PBT, but higher than that of POM. After being reinforced by glass fiber, better mechanical properties can be obtained. PPS is an inert and non-toxic substance.
- Thermal Properties
Since PPS is a crystalline polymer, the highest crystallinity can reach up to 65%, its crystallization temperature is 127°C, melting point is 286°C, and the heat distortion temperature is 260°C. With a thermal stability far exceeding that of such engineering plastics as PA, PBT and POM, it only decomposes when the air temperature reaches above 430-460°C, so its long-term application temperature is the highest among all thermoplastics, reaching up to 220-240°C. PPS also has great thermal insulation and flame retardancy performances. Its critical oxygen index is equivalent to that of PVC, up to 47%, so there is no need to add flame retardant, because PPS is able to reach the UL94 V-0 flammability rating.
- Electrical Properties
PPS has a symmetrical molecular structure, no polarity and a low water absorption rate, so it boasts an excellent electrical insulation performance. Compared with other engineering plastics, its dielectric constant is low, and its arc resistance is equivalent to that of thermosetting plastics. Under such conditions as high temperature, high humidity and frequency conversion, PPS can still maintain outstanding electrical insulation. Conductive PPS composites can be obtained by adding conductive fillers for antistatic and electromagnetic shielding effects.
- Chemical Resistance
It is insoluble in any organic solvent below 200°C. It is able to withstand the erosion of all kinds of acids, alkalis and salts, except the strong oxidizing acid. It still maintains a high strength after long-term immersion in various chemicals under high temperature conditions. PPS also boasts great weatherability and resistance to radiation.
PPS Injection Molding Processability
1). Water Absorption
PPS features a low water absorption of only 0.02%.
With great fluidity, PPS can be used to make thin-walled products. However, if the temperature is too high or the material stays in the barrel for too long, the material will partially crosslink, resulting in a low fluidity.
PPS is a crystalline polymer, of which the crystallinity varies depending on the cooling temperature and rate during the plastic injection molding process. The faster the cooling rate, the lower the crystallinity. The degree of crystallinity has a great influence on its strength, thermal resistance, weatherability and dimensional stability. As the degree of crystallinity increases, the heat deflection temperature (HDT) of the product rises, with increased rigidity, surface glossiness, surface shrinkage, and dimensional stability.
4). Thermal Stability
PPS will undergo partial oxidation and crosslinking reaction if exposed to high temperature conditions for a long time, resulting in a decreased fluidity and a deeper color of the material, which will affect the quality and performance of the product. In addition, PPS is strongly adhesive to metals, so it is necessary to prevent the material from solidifying in the barrel.
For the crystalline plastics, the crystallinity increases as the mold temperature increases during the plastic injection molding process, while the shrinkage rate increases with the increase of crystallinity. Therefore, the shrinkage increases with the rise of mold temperature. Generally speaking, the shrinkage rate of PPS is low, but the shrinkage perpendicular to the flow direction is 2-4 times higher than that in the flow direction. Product thickness, shape, and injection speed also the factors that influence the shrinkage rate.
6). Secondary Processing
PPS products can be processed by machining, ultrasonic welding, adhesive bonding, etc., such as cutting and tapping.
Molding Preparation: Conventional injection molding machines can all be used for PPS processing. It is recommended to use wear-resistant barrels, screws and molds that are suitable for such fillers as fiberglass and minerals.
Drying Preparation: Although PPS absorbs very little water in a humid environment, drying is necessary if you want to get high quality products. Please refer to the following drying conditions:
120℃: 4-6 hours
130℃: 3-5 hours
140℃: 2-3 hours
If the drying temperature is too high or the drying time is too long, the color or fluidity of the raw material may change. The melting point of PPS is 280°C, so the barrel temperature of 300-340°C is the most commonly used for most applications.
The mold surface temperature should be no lower than 120°C. Usually, 130-150°C is recommended. Higher mold temperatures ensure a high degree of crystallization, a smooth product surface and reduced shrinkage after molding. If a lower mold temperature is required in special circumstances, you should try to avoid the temperature range of 80-100°C, which may cause the vitrification of PPS, thus making it hard for ejection.
To prevent melt cooling / curing during the injection process, a higher injection speed is necessary. The optimal product appearance is usually obtained at an appropriate speed. The filling time is usually 0.5-1.5 seconds, but excessively high speed may cause warpage and burn marks.
Since a higher injection speed is recommended, a higher injection pressure is often adopted to keep the pressure at 50MPa or higher.
Screw Speed & Back Pressure
The plasticization process requires a low to medium screw speed of 40-150rpm, usually with a low back pressure of 1-2MPa. If the back pressure is not stable, 3-4MPa is recommended. If both the screw speed and the back pressure are too high, the glass fiber will be destroyed, which decreases the physical properties of the final products.