The Properties And Molding Processing Parameters of Polyphenylene Ether (PPO)

The Properties And Molding Processing Parameters of Polyphenylene Ether (PPO)

I. Basic Properties of Polyphenylene Ether (PPO)

Polyphenylene Ether (PPO), also known as Poly 2,6-dimethyl-1,4-phenylene ether, is a high-strength engineering plastic that emerged in the 1960s. It is abbreviated as PPO or PPE, and is also referred to as polydiphenyl ether, polyphenylene oxide, or polyphenylene ether. It is a thermoplastic resin with a low density of only 1.06g/cm³. It has excellent comprehensive properties, low moisture absorption, and superior electrical performance, resistance to steam, and dimensional stability. However, it has poor melt flowability, making it difficult to mold. In engineering applications, various modified forms of PPO are used.

1. Mechanical Properties

PPO exhibits excellent mechanical properties (see Table 1-1), with a tensile strength of 80 MPa (at 23°C), surpassing engineering plastics such as polycarbonate, polyacetal, and ABS. The strength and rigidity of PPO only decline slowly with increasing temperature. After being boiled in water for 7200 hours, its tensile strength, elongation, and impact strength show no significant decrease. The creep value of PPO is very low, with a creep value of only 0.5% after 300 hours under a load of 14 MPa at 23°C; moreover, the change in creep value is also minimal with the rise in temperature. It can be continuously used within a temperature range of -160 to -150°C.

The mechanical properties of modified polyphenylene ether (PPO) are close to those of polycarbonate (PC), with high tensile strength, flexural strength, and impact strength. It has great rigidity, excellent creep resistance, and maintains high strength over a wide temperature range, with minimal impact of humidity on its impact strength. Tables 1-2 and 1-3 list some of the properties of modified polyphenylene ether.

2. Thermal Properties

Polyphenylene ether (PPO) has high heat resistance, with a melting point above 300°C, a decomposition temperature above 350°C, a brittleness temperature of -170°C, a Martin heat resistance temperature of 160°C, a long-term use temperature of 120°C, and a glass transition temperature of 205°C. The thermal conductivity is 0.192 W/(m·°C), and the molding shrinkage is 0.7% to 0.9%. Its heat deflection temperature under a load of 1.82 MPa is 174°C, which is superior to other thermoplastic engineering plastics such as polycarbonate, polyacetal, polyamide, and ABS, and is close to thermosetting plastics like phenolic and unsaturated polyester resins.

The melting temperature range of PPO is wide, with the maximum melting temperature reaching 267°C. After the melt cools, it exhibits a completely amorphous structure. The coefficient of linear thermal expansion of PPO is relatively low, at 5.2×10^-5/°C, which is closer to the coefficient of metals compared to other plastics. PPO has a high heat deflection temperature, and its melt rheological properties are nearly Newtonian, meaning that the melt viscosity does not decrease with an increase in shear rate. This results in a high viscosity and poor flowability, necessitating very high processing temperatures (315°C), which can make processing difficult or lead to excessive energy consumption. PPO has good flame retardancy and self-extinguishing properties, and it does not undergo chemical changes after being exposed to air at 150°C for 150 hours. Modified PPO has slightly lower thermal properties than PPO, but it has a wide range of heat deflection temperatures.

3. Electrical Properties

Polyphenylene ether (PPO) does not contain highly polar groups in its molecular structure, which means it does not form dipoles. As a result, its electrical properties are very stable, maintaining excellent performance over a wide range of temperatures and frequencies (see Table 1-4). The volume resistivity of PPO is as high as the order of 10^15 and is essentially unaffected by humidity. The dielectric constant and dielectric loss tangent are the lowest among all engineering plastics and are hardly influenced by temperature and frequency. Modified polyphenylene ether still possesses excellent electrical properties.

4. Chemical Resistance

Polyphenylene ether (PPO) exhibits excellent resistance to water and is generally unaffected by aqueous chemicals such as acids, bases, salt solutions, and detergents, whether at room temperature or under heat. However, it has poor resistance to solvents; halogenated aliphatic hydrocarbons and aromatic hydrocarbons can cause PPO to swell or dissolve. Under stress, it is not resistant to aromatic hydrocarbons, halogenated hydrocarbons, oils, ketones, and esters, and is prone to swelling or stress cracking. In concentrated sulfuric acid at 85°C and under a load of 12 MPa, stress cracking can occur. It also has poor resistance to oxidation. The chemical resistance of polyphenylene ether is shown in Table 1-5.

5. Applications of Polyphenylene Ether (PPO)

Polyphenylene ether is suitable for applications in moist environments with loads, where excellent electrical insulation, mechanical properties, and dimensional stability are required. In the mechanical and electrical industry, it can be used to manufacture gears, bearings, cams, parts for transportation machinery, impeller blades, blower blades, water pump components, chemical industry pipes, valves, and municipal water engineering parts. It can also replace stainless steel for the production of various chemical equipment and components. Due to its good resistance to creep and stress relaxation, as well as its high strength, polyphenylene ether is also suitable for making screws, fasteners, and connecting parts. Furthermore, its superior electrical properties make it suitable for use as motor winding cores, rotors, housings, and electronic equipment parts, as well as high-frequency printed circuit boards. Electrical-grade polyphenylene ether, used in the ultra-high frequency range, can be manufactured into television tuning plates, microwave insulation, coil cores, transformer shielding sleeves, coil frames, tube seats, and television deflection system components. Because polyphenylene ether can withstand steam sterilization, it can replace stainless steel in surgical instruments. Additionally, polyphenylene ether films, due to their high heat resistance and good mechanical strength, have broad application prospects in the electrical, mechanical, electronic, aerospace, and aviation industries.

Currently, the modified polyphenylene ether (PPO) that is widely used in commercial products is made by blending it with high-impact polystyrene. Modified PPO has excellent molding and processing properties, with low molding shrinkage, good dimensional stability, low water absorption, and good electrical and thermal resistance. It does not easily decompose when exposed to hot water, is resistant to acids and bases, has a low density, and can easily meet the UL flame retardancy standards with non-halogen flame retardants. As an important engineering plastic, modified PPO is extensively used in various fields such as televisions, electronic components, automobiles, office machinery, and household appliances.

II. Molding and Processing of Polyphenylene Ether (PPO)

Unlike crystalline plastics such as POM (Polyoxymethylene) and PA (Polyamide), PPO is similar to PC (Polycarbonate) in terms of good dimensional stability. PPO does not undergo a crystallization process during molding, and residual internal stress can lead to stress cracking, so the molding conditions should be carefully considered.

1. Molding and Processing Characteristics

Polyphenylene ether has a high melting point, with the melting temperature of its crystalline portion reaching up to 262–267°C. Moreover, its melt viscosity is quite high below 300°C, which poses difficulties for the molding and processing of PPO. Both PPO and modified PPO melts are non-Newtonian fluids, and the viscosity of the melt is highly dependent on temperature, decreasing linearly with increasing temperature. This characteristic of high viscosity and poor flowability necessitates the use of very high temperatures during processing, making it challenging and energy-intensive.

The molding shrinkage of polyphenylene ether (PPO) and modified PPO is very low, and it remains essentially unchanged under various molding conditions, which is very advantageous for the production of precision molded parts, and ejection problems are rarely encountered.

PPO has low moisture absorption, and the material can usually be molded without prior drying. However, if the material's melt granulation process involves water cooling, or if the material is poorly packaged, it can adsorb a small amount of moisture from the air due to its low density and large surface area, especially in the case of powdered PPO resin, which is more prone to absorbing moisture. If not dried, this can lead to the formation of silver streaks and the creation of bubbles on the surface of the molded products during the molding process. Additionally, drying also serves a preheating function. This is particularly beneficial for the molding of large-area, thin-walled products, as it can improve the surface gloss of the finished products. Drying can be done in simple equipment such as an oven, and for materials with a thickness of 50mm, it is generally sufficient to dry at around 107°C for about 2 hours.

2. Injection Molding

Piston-type or screw-type injection machines can process polyphenylene ether (PPO), with screw-type injection machines generally being preferred. It is required that the length-to-diameter ratio of the screw should be greater than 15, and the compression ratio should be between 1.7 and 4.0 (typically 2.5 to 3.5). A gradually changing screw profile is recommended. The nozzle should be of the straight-through type, as it has less pressure loss compared to an automatic opening and closing nozzle, and is less likely to cause material retention. The mold should be equipped with a heating device, and there should be an insulating board between the mold and the injection machine plate.

The injection molding temperature for PPO is relatively high, and depending on the size and shape of the product, the barrel temperature is generally controlled between 280°C and 340°C. When the injection volume is 20% to 50% of the barrel capacity, the barrel temperature can be as high as 330°C without causing degradation. However, it should not exceed 340°C, as temperatures above 340°C can lead to material degradation and a reduction in performance; temperatures below 280°C may result in excessive material viscosity, making it difficult to process. The nozzle temperature is usually slightly lower than the melting zone temperature of the barrel by 10°C to 20°C to prevent material leakage from the nozzle.

The mold temperature should be determined based on factors such as the thickness of the product and the barrel temperature, generally ranging from 100°C to 150°C. This range can minimize stress, which is beneficial for reducing surface roughness and filling thin-walled sections. Exceeding 150°C can easily cause bubbles and extend the molding cycle; temperatures below 100°C may result in higher residual stress and defects such as underfilling and delamination.

Polyphenylene ether (PPO) should not be kept at high temperatures for extended periods. If the material remains in the barrel for more than 2 hours, discoloration and degradation may occur, and the barrel should be cleaned promptly. Waste material from PPO injection molding can be reused multiple times, typically up to three times, without a significant decrease in mechanical properties. Table 2-1 lists the injection molding process conditions for some grades of Noryl PPO.

3. Compression Molding

Compression molding can be used to manufacture polyphenylene ether (PPO) into sheets of various thicknesses using a hydraulic press. When compressing thicker sheets, it is necessary to process them at a temperature about 10°C lower than the use temperature for 24 hours to eliminate internal stress. During molding, it is important to ensure that the demolding temperature is sufficiently high to prevent the sheets from cracking. Additionally, immediately demolding the sheets after they are released from the press can cause them to stick to the mold, and sudden cooling can lead to cracking. Therefore, the sheets should be allowed to cool slowly to a certain temperature before demolding.

The specific compression molding process for PPO glass fabric laminates is as follows:

(1) Adhesive Preparation: Dissolve PPO resin in a benzene solvent to create a solution with a mass fraction of 10% to 18%. Heat and stir at 60°C until the resin is completely dissolved, resulting in a transparent liquid.

(2) Coating: Treat the 0.1mm thick glass fabric with H151 coupling agent and then immerse and dry it in a coating machine. The drying temperatures are 100–110°C for the upper layer, 90–100°C for the middle layer, and 70–80°C for the lower layer. The coating speed is 0.5m/min, with a resin content of 30% to 40% (mass fraction).

(3) Molding: Cut and stack the coated fabric to a thickness of approximately 4–10mm and form it under heat and pressure in the press. Start with the mold at room temperature, then heat to 250°C and hold for 5 minutes, applying pressure to 6MPa in one go; continue to heat to 300°C and maintain for 1 hour; then cool to 180°C, and finally, cool with water to room temperature before demolding.

4. Extrusion Molding

Extrusion molding can process polyphenylene ether (PPO) into rods, pipes, sheets, and wire coatings. The extruder typically uses a vented extruder with a length-to-diameter ratio of the screw usually ranging from 20 to 24, and a compression ratio of 2.5 to 3.5. The screw often adopts an equidistant but uneven depth design, with the metering section having an appropriate depth. The extruder should have a long straight section at the die to accommodate the higher cooling temperature of PPO material during extrusion. During extrusion molding, the barrel temperature is slightly lower than that used in injection molding.

III. Advantages and applications of polyphenylene ether

Polyphenylene ether (PPO) is an engineering plastic with a variety of advantages, the main performance benefits of which include:

1. Excellent thermal stability: PPO can maintain its physical properties at high temperatures, with a wide range of long-term use temperatures, generally between -127°C and 120°C.

2. Good electrical insulation: PPO has outstanding electrical insulation properties, and its dielectric performance ranks first among plastics.

3. High mechanical strength: PPO has high tensile and impact strength, and even after 200 treatments in high-pressure steam at 132°C, there will be no significant change.

4. Water and steam resistance: PPO has good tolerance to water and steam, with a low water absorption rate.

5. Good dimensional stability: PPO exhibits good dimensional stability over long-term use, and is not prone to creep.

6. Chemical resistance: PPO is resistant to corrosion by a variety of chemicals, including inorganic acids, bases, and certain organic solvents.

7. Flame retardancy: PPO is self-extinguishing and is considered a flame-resistant material, making it suitable for applications where fire safety is a concern.

8. Processing performance: PPO is easy to process and can be formed through methods such as extrusion, injection molding, and compression molding.

The application fields of polyphenylene ether (PPO) are very extensive, mainly including:

1. Electronics and electrical industry: Used for manufacturing connectors, coil cores, tube sockets, control shafts, transformer shielding sleeves, etc.

2. Automotive industry: Used for manufacturing dashboards, bumpers, radiator grilles, speaker grilles, etc.

3. Medical devices: Due to its resistance to hydrolysis, steam, and heat, PPO can be used for manufacturing medical devices and sterilization equipment.

4. Aerospace sector: Used for manufacturing lightweight structural components and functional materials.

5. Home appliances: Used for parts of televisions, air conditioners, microwave ovens, and other household appliances.

6. Office equipment: Used for the casings and components of computers, printers, fax machines, and other office devices.

7. Industrial machinery: Used for parts of pumps, blowers, valves, and other industrial machinery.

8. New energy sector: Used as energy storage materials, insulation materials, and battery materials, etc.

Due to its excellent comprehensive properties, PPO has become one of the top five general-purpose engineering plastics and plays an important role in various industries.

IV. Conclusion

In conclusion polyphenylene oxide (PPO) is a high-performance engineering plastic known for its various excellent properties, which makes it highly favored across many industries. PPO's adaptability to various temperatures and flame retardancy make it an ideal choice for applications with high fire safety requirements. In addition to this, PPO can be combined with YINSU Flame Retardant Company's red phosphorus flame retardant FRP-950X for HIPS flame retardant solutions.

YINSU's red phosphorus flame retardant FRP-950X is a high-content microencapsulated, low smoke, halogen-free flame retardant masterbatch. The halogen-free flame retardant for HIPS has a higher addition amount and is more costly. However, with YINSU Flame Retardant Company's PPO+FRP-950X flame retardant solution, it is possible to achieve halogen-free, high efficiency, and low-cost flame retardancy. For more details, feel free to inquire, and we will provide you with a more efficient flame retardant solution.