Hello and welcome to the Part 2C of our High Performance Thermoplastics selection blog series. Today we discuss Polyphenylene Ether (PPE) and PPE blends, their chemistry and production processes, their main properties, processing methods, and applications.
Overview - 6 major high performance thermoplastics families (“the magnificent six”)
In this blog post series we discuss six major high performance thermoplastics families (“the magnificent six”) which are outlined in the following enumeration
1. Introduction to High Performance Polymers
2. Short profile of the "magnificent six" families:
-Part 2C: Polyether (PPE, PAEK, PEEK, PEKK)
-Part 2D: Liquid Crystal Polymers (LCP) and High-performance Polyesters (Polycyclohexylene terephthalate - PCT)
-Part 2E: Semi- and Fully Aromatic Polyamides (PARA, PPA, Aramid)
-Part 2F: Polyhalogenolefins (PTFE, PCTFE, FEP, PVDF, ECTFE)
Polyphenylene Ether (PPE) and its Blends with Polystyrene (PS) and Polyamide (PA)
Polyphenylene ether (PPE), also known as polyphenylene oxide (PPO), is a high-performance thermoplastic polymer renowned for its excellent thermal, mechanical, and electrical properties. While pure PPE exhibits some processing challenges due to its high melt viscosity, blending it with polystyrene (PS) significantly improves its processability while retaining many of its desirable characteristics. These PPE/PS blends have become commercially significant engineering thermoplastics.
Jack Welch's team at General Electric faced a challenge with Polyphenylene Oxide (PPO): its extremely high glass transition temperature (208°C) made it difficult to process without degrading the material (the methyl groups of PPE are expected to undergo autoxidation). An important key to future success was the research team around Dan Fox, Allan S. Hay, and E. M. Boldebuck, who found out the miscibility of PPO with PS first. To overcome this, they decided to blend PPO with polystyrene (PS). This clever solution allowed them to maintain many of PPO's desirable properties, lower the glass transition temperature, while also making the material easier to process at lower temperatures and more cost-effective.
Chemistry and Production Process
1. Chemistry of Polyphenylene Ether (PPE)
The base polymer, PPE, is typically synthesized through an oxidative coupling polymerization of substituted phenols, most commonly 2,6-dimethylphenol (also known as 2,6-xylenol). This reaction is catalyzed by a copper-amine complex in the presence of oxygen. The general chemical structure of PPE can be represented as (Figure 1):
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| Figure 1: Chemical structure of the polymer Polyphenylene Ether (PPE) [1]. |
The ether linkages (-O-) in the polymer backbone, along with the aromatic rings and the methyl substituents, contribute to PPE's stiffness, thermal stability, and chemical resistance.
- Raw Material Preparation: PPE and PS resins are typically received in pellet form. They may be dried to remove any moisture before blending.
- Melt Blending: The PPE and PS pellets are fed into an extruder, where they are heated and mechanically mixed. The screw design and processing conditions (temperature, screw speed) are crucial for achieving a homogeneous blend. Compatibilizers, such as styrene-butadiene-styrene (SBS) or styrene-ethylene/butylene-styrene (SEBS) block copolymers, are often added to improve the compatibility between the relatively non-polar PS and the more polar PPE. These compatibilizers help to reduce interfacial tension and prevent phase separation, leading to enhanced mechanical properties.
- Pelletizing: The molten blend exiting the extruder is then cooled and cut into pellets, which are the final product form for subsequent processing by manufacturers.
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| Figure 2: Changing the glass transition temperature of PPE by changing the ratio of PPE and PS [1]. |
- Excellent Thermal Stability: PPE inherently possesses high glass transition temperatures (Tg ) and heat deflection temperatures (HDT). Blending with PS can reduce these values compared to pure PPE, however the blends still offer good high-temperature performance compared to many other engineering thermoplastics.
- Figure 3 shows the DMA of PPE+PS blend in comparison to High Impact Polystyrene (HIPS) and Polycarbonate (PC). PS has a glass transition temperature of about 100°C and up to this temperature PPE+PS can match PC in thermal performance (PC drops sharply at Tg of 147°C). There is no sharp drop in modulus observed with PPE+PS.
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| Figure 3: Dynamic Mechanical Analysis (DMA) of PPE+PS vs HIPS vs PC. |
- Good Mechanical Strength and Stiffness: PPE contributes to the blend's rigidity and strength. The impact strength can be tailored depending on the blend ratio and the use of impact modifiers.
- Excellent Electrical Insulation Properties: PPE is an excellent electrical insulator with a low dielectric constant and high dielectric strength, which are largely retained in the blends.
- Good Chemical Resistance: PPE offers good resistance to many chemicals, including acids, bases, and detergents. The chemical resistance of the blend is generally good but can be influenced by the PS content, which is more susceptible to certain solvents.
- High Stability towards Hydrolysis: hydrolysis resistance of PPE is superior when compared to other engineering plastics such as PBT and PA.
- Improved Processability: The addition of PS significantly lowers the melt viscosity of PPE, making the blends easier to process using conventional methods like injection molding and extrusion.
- Dimensional Stability: PPE contributes to low water absorption and excellent dimensional stability, which is important for applications requiring tight tolerances.
- Flame Retardancy: PPE is inherently flame retardant. Blends often exhibit good flame retardant properties, especially when combined with flame retardant additives.
- Combines Key Strengths: Blends the excellent dimensional stability, low water absorption, and heat resistance of PPE with the superior chemical resistance and flow of PA.
- Enhanced Performance: The resulting material is exceptionally chemically resistant and boasts the stiffness, impact resistance, and heat performance needed for demanding applications like on-line painting.
- Significant Weight Savings: Unfilled PPE/PA blends can offer part-weight reductions of up to 25% compared to glass or mineral-filled resins, thanks to their low density.
- Low specific gravity: 1.1 g/cm3. Most engineering polymers such as PC, PBT, and POM have a specific density of 1.2 g/cm3 and more.
- High self-extinguishing property: PPE has a high oxygen index (27-29) and it is easy to add flame resistancy.
- Excellent dielectric properties: PPE has a dielectric constant of 2.8 and a dielectric tangent of 6x1E-3. The dielectric breakdown strength (110 MV/m @ 0.5 mm thickness) is the highest among engineering plastics.
- High dimensional accuracy: PPE has one of the lowest coefficient of linear thermal expansion among engineering polymers (5.8 x 10E-5 1/°C).
- Injection Moulding: This is the most common method for producing complex shaped parts from PPE/PS blends, leveraging their improved flow properties compared to pure PPE.
- Extrusion: These blends can be extruded into profiles, sheets, and films for various applications.
- Blow Moulding: Certain grades of PPE/PS blends can be blow molded to produce hollow parts.
- Thermoforming: Sheets extruded from PPE/PS blends can be thermoformed into various shapes.
- Construction and Plumbing: Applications include pump housings, impellers, and other components requiring good mechanical and chemical resistance.
- Automotive Industry: Interior and exterior components such as instrument panels, door panels, wheel covers, and electrical connectors benefit from the blends' thermal stability, dimensional stability, and impact resistance.
- Electrical and Electronics: Housings for electrical connectors, circuit breakers, switchgear, and other electrical components utilize the excellent electrical insulation properties and flame retardancy of PPE/PS blends.
- Household Appliances: Components for washing machines, dishwashers, microwave ovens, and other appliances benefit from the blends' heat resistance, chemical resistance, and good mechanical properties.
- Business Equipment: Housings and internal components for computers, printers, and other office equipment utilize the blends' dimensional stability, electrical properties, and aesthetic appeal.
- Healthcare: Certain grades of PPE/PS blends can be used in medical devices and equipment due to their sterilizability and chemical resistance.
- Noryl™: Formerly a trademark of GE Plastics, now owned by SABIC Innovative Plastics. This is one of the most widely recognized families of PPE-based resins, including various PPE/PS blends and other modifications.
- XYRON™ from Japanese chemical company Asahi Kasei.
- Various manufacturers such as Global Acetal (Lemalloy PPE) produce generic PPE/PS blends with specific property profiles.
- Cost Factors: The cost of PPE/PS blends is influenced by the price of the base polymers (PPE and PS), the concentration of PPE in the blend, the type and amount of compatibilizers and additives used, and the manufacturing process. Generally, higher PPE content leads to higher costs.
- Market Trends: The trend towards lightweighting in the automotive industry and miniaturization in electronics continues to drive the demand for materials like PPE/PS blends that offer a balance of performance and processability. Growing environmental concerns are also pushing for more sustainable material solutions, which may influence the development of new PPE-based blends with recycled content or improved recyclability.
- Regional Variations: The demand and market dynamics for PPE/PS blends can vary across different geographical regions based on industrial activity and specific application needs.
Literature:
[1] Allan S. Hay: Polymerization by Oxidative Coupling: Discovery and Commercialization of PPO and Noryl Resins
[2] https://www.sabic.com/en/products/specialties/noryl-resins
[3] https://www.gpac.co.jp/en/product/lemalloy/
[4] https://www.findoutaboutplastics.com/2016/09/jack-welch-and-his-uprise-in-ge.html
[5] https://www.findoutaboutplastics.com/2024/08/high-performance-thermoplastic.html
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