Polyphenylene Oxide (PPO or PPE) is a high-performance thermoplastic engineering plastic whose main molecular chain is composed of alternating aromatic rings and ether bonds. This unique structure gives it extremely high molecular rigidity, which is manifested in a glass transition temperature (Tg) as high as 211℃ and a melting point of about 268℃, allowing it to maintain excellent dimensional stability under high temperature conditions. Although pure PPO resin has excellent performance, it has inherent processing defects: extremely high melt viscosity, poor fluidity, and easy decomposition reaction when exceeding 330℃, producing corrosive gases.
To solve above problems, commercial PPO usually exists in alloyed form and is modified by blending with other polymers:

 

- PPO/PS alloy (such as Noryl®): Addition of PS significantly improves melt fluidity, making material have both heat resistance and dimensional stability of PPO, easy processing of PS, becoming mainstream material for electronic and electrical housings;
- PPO/PA alloy: Introduction of polyamide (PA66) improves high-temperature chemical stability, which is particularly suitable for oil-resistant parts in automobile engine compartments. Its crystalline structure enhances solvent resistance;
- Reinforced modified type: Adding glass fiber (up to 30%) can reduce shrinkage to 0.2%, and heat deformation temperature (HDT) exceeds 190℃, which is used for high-precision structural parts such as water pump housings.
Table: Effect of components in PPO alloy system on performance

This structural design of "base resin + modified component" enables PPO alloy to expand processing window and application scenarios while maintaining its core advantages, laying foundation for subsequent high-value recycling.

PPO/PPE materials, with their unique molecular structure, show comprehensive and excellent performance, making them occupy an important position in the field of engineering plastics.

- Thermal stability: Glass transition temperature reaches 211℃, long-term use temperature range is -50~120℃, and it can withstand high temperatures of 190℃ in short term without deformation. It can still maintain more than 85% of its original performance after 40,000 hours in a high-temperature working environment.
- Dimensional stability: It has extremely low linear expansion coefficient and water absorption (0.06%), which is much lower than engineering plastics such as PA and PBT. Physical properties are almost not attenuated after long-term immersion in hot water, ensuring dimensional accuracy of precision parts.
- Chemical resistance: It has excellent resistance to inorganic acids, alkalis, and salt solutions, especially in PPO/PA system, can withstand automotive chemicals such as engine oil and coolant. However, it is sensitive to halogenated hydrocarbons (such as chloroform) and ketone solvents, and stress cracking may occur under stress.
- Flame retardant properties: Oxygen index is as high as 29, and it has self-extinguishing properties. Halogen-free flame retardants (such as phosphorus compounds) can easily reach UL94 V0 level, meeting strict flame retardant requirements of charging pile shells.

- Dielectric properties: It performs best among engineering plastics, with extremely low dielectric constant (2.58) and loss factor (0.0004), and is basically unaffected by temperature, humidity and frequency changes. Volume resistivity is as high as 10^17 Ω·cm, which is suitable for high-voltage environments such as transformer skeletons.
- Mechanical properties: Rigid molecular chain gives it high strength and creep resistance, and bending modulus is about 2.5 GPa. After alloying with PA, impact strength is significantly improved, and it is suitable for parts that bear periodic loads such as car wheel covers.

- Density and environmental protection: Density is only 1.06-1.10 g/cm³, which is the lightest of five major engineering plastics and conforms to lightweight trend. Body is non-toxic and FDA-certified, can be used for food contact and medical devices.
- Weather resistance limitations: It is easy to turn yellow when exposed to ultraviolet rays for a long time, and ultraviolet absorbers such as carbon black need to be added to improve outdoor service life.
Table: Key performance indicators and comparison benchmarks of PPO/PPE

PPO/PPE materials have been widely used in many high-end industrial fields due to their unique performance combination, especially in scenes with strict requirements on dimensional stability, electrical insulation and chemical resistance.

 

- Automotive industry: engine peripheral parts (such as water pump housing, thermostat cover) take advantage of its resistance to 130℃ coolant; wheel covers and exterior trims benefit from low expansion coefficient and creep resistance; battery module brackets rely on its electrical insulation and flame retardant properties to ensure safety of power battery systems. PPO/PA alloys gradually replace metals in gearbox parts to achieve lightweight.
- Electronics and electrical: as a material for circuit breaker housing and relay base, its high CTI value (Comparative Tracking Index) can prevent high-voltage arc breakdown; optical fiber connectors need to take advantage of their low dielectric loss to ensure signal integrity; charging pile housings use flame retardant grades (UL94 V0), recycled materials have passed safety certification.
- Water treatment equipment: valve cores, pump bodies and pipe joints take advantage of their ultra-low water absorption (0.06%) and hydrolysis resistance to serve for a long time without failure; smart water meter gearboxes are made of FDA-certified PPO/PS alloys to avoid drinking water pollution.
- New energy field: Photovoltaic junction boxes need to withstand outdoor UV radiation and electric sparks. PPO's weather-resistant modified grades can guarantee a service life of 25 years; battery management system (BMS) insulation partitions use their self-extinguishing properties to improve safety.
- Home and office equipment: Printer gears and copier structural parts rely on their wear resistance and low friction coefficient; TV brackets use 30% glass fiber reinforced grades to ensure strength while reducing costs.
Table: Mapping of core advantages of PPO/PPE materials in various application fields

Processing of PPO/PPE materials requires strict control of process parameters to overcome their high melt viscosity, thermal sensitivity to ensure product performance and appearance quality.

- Although PPO has a low water absorption rate (0.06%), trace amounts of moisture can still cause silver streaks or bubbles on the surface of product. It needs to be dried at 100℃ for 2 to 4 hours, and material layer thickness should not exceed 25mm. Avoid exceeding 150℃ to prevent resin from yellowing.
- Recycled materials (PCR-PPO) should be dried for an additional hour because they may contain higher residual moisture. It is recommended to use a dehumidifying dryer for crushed materials with a dew point of ≤-40℃.

- Segmented barrel temperature control: 240-260℃ in the rear zone, 270-290℃ in the middle zone, and 280-300℃ in the front zone. Nozzle temperature must be 10℃ lower than front zone to prevent drooling. Temperatures below 260℃ will cause filling difficulties, and temperatures above 320℃ will cause thermal decomposition (producing phenolic gases).
- Injection pressure: usually set at 600-1500 bar (about 60-150MPa), high speed and high pressure (≥1200 bar) are required for thin-walled parts. Holding pressure can be reduced to 60-80% of injection pressure. Too long holding pressure is prone to internal stress.

- Mold temperature control: 100±10℃ is ideal range, and oil temperature controller is used for precise control. When mold temperature is lower than 80℃, thin-walled areas are prone to stratification; higher than 120℃, cooling time is extended and dents may occur.
- Casting system: Taper of main runner is ≥3°, and runner length is as short as possible. Gate is preferably a fan gate or a direct gate, and diameter of pinpoint gate must be ≥1.5mm. Hot runner system should use a closed nozzle to prevent material retention and degradation.
- Cooling time: It takes about 20-40s for a product with a wall thickness of 2mm. Too short will cause ejection deformation, and too long will reduce efficiency. It is recommended to use corrosion-resistant S136D or H13 mold steel to resist possible acidic decomposition products.

- Annealing treatment: To eliminate internal stress, product needs to be annealed in a hot air cycle at 120℃ for 2-4 hours, especially for mixed systems containing PCR materials.
- Molding shrinkage compensation: Shrinkage rate of pure PPO is 0.3-0.8%, which is reduced to 0.2-0.4% after glass fiber reinforcement. Mold design needs to accurately adjust shrinkage parameters according to material formula. For every 20% increase in PCR content, shrinkage rate increases by about 0.1%.

With tightening of environmental protection regulations, physical recycling (Physical Recycling) and chemical closed-loop recycling (Chemical Recycling) technologies of PPO/PPE have developed rapidly, significantly improving its circular economic value.

 

- Industrial scraps can be directly mixed with new materials after crushing and cleaning. When addition ratio is ≤30%, impact on mechanical properties is controllable (strength reduction <10%). It is especially suitable for water treatment equipment parts (such as valve bodies and pump housings) because it has low color requirements and hydrolysis resistance of PPO itself can still be maintained after multiple processing.
- Electronic waste containing flame retardants needs to be finely sorted: PPO flame retardant grades (V0 grade) are separated from other plastics through electrostatic sorting and density separation technology, and resulting recycled materials are successfully used in charging pile shells, meeting requirements of RTI (relative temperature index) ≥105℃.

- Acid solution closed-loop technology developed by Jilin University has achieved a major breakthrough: cross-linking bonds are dissociated by acidic solutions (such as trifluoroacetic acid), so that material is depolymerized under mild conditions (80℃) into amino functionalized polymer monomers and flame retardant small molecules, with a recovery rate of over 90%. After purification, depolymerization product can be repolymerized into recycled PPO with performance comparable to that of new materials.
- Core of this technology lies in dynamic covalent bond design: flame retardant small molecule B (aldehyde functionalized phosphorus oxychloride) is connected to imine bond of rigid chain polymer A, and can be controlled to break in an acidic environment. Tensile strength retention rate of repolymerized material is >95%, and flame retardant grade is maintained at V0.

- Automotive interior parts: 30% PCR-PPO/PA alloy is used to manufacture door panel frames, meeting odor standard VDA 270≤3 level.
- Medical device trays: Pure PPO recycled in a closed loop has passed ISO 10993 biocompatibility certification, and cost is 40% lower than that of new materials.
- Building flame-retardant formwork: Physically recycled flame-retardant PPO mixture has passed UL94 V1 certification, replacing traditional wood formwork.