Same As Engineering Plastics, Why PPO Can't Compete with PA66 in Automotive Applications?

Same As Engineering Plastics, Why PPO Can't Compete with PA66 in Automotive Applications?

Among the five major engineering plastics, PPO has a relatively low profile. Despite its low density, hydrolysis resistance, self-extinguishing properties, excellent mechanical and dimensional stability, PPO is not commonly seen in automotive applications, which have been gaining popularity in recent years. What is the reason behind this? We have selected PA66, a material with similar overall properties to PPO from the engineering plastics category, and compared their chemical structure, thermal properties, mechanical properties, chemical resistance, environmental resistance, and processing characteristics.

PPO, also known as PPE, is an amorphous thermoplastic with excellent dimensional stability, hydrolytic stability, and high heat resistance. On the other hand, PA66 is a semi-crystalline polyamide with outstanding tensile strength, toughness, and wear resistance.

I. Molecular Structure

PA66 is characterized by its linear structure and amide bonds, which facilitate the formation of hydrogen bonds between polymer chains (Figure 1). This chemical structure enhances its tensile strength, toughness, and chemical resistance. PA66 is also semi-crystalline, featuring ordered crystalline regions interspersed with amorphous regions.

PPO (Figure 2) is a miscible blend of polyphenylene ether and high-impact polystyrene (HIPS). As an amorphous material, PPO is known for its rigid molecular structure composed of benzene rings and methyl groups. This structure imparts a high glass transition temperature, heat resistance, good electrical properties, excellent hydrolytic stability, and the ability to use halogen-free flame retardants. When blended with polystyrene (PS), the resulting material exhibits tunable thermal and mechanical properties.

The comparison between these two materials represents a classic battle between amorphous and semi-crystalline polymers. However, PA66's unique chemical structure gives it a toughness advantage that other semi-crystalline materials do not possess. Meanwhile, PPO materials combine the advantages of amorphous materials with a more customizable combination of properties.

II. Thermal Properties

PA66 is a semi-crystalline polymer with a melting point (Tm) of approximately 260°C. Its glass transition temperature (Tg) ranges from 55°C (dry) to -37°C (100% RH). The Tg of PA66 is affected by humidity, as water acts as a plasticizer for the polymer backbone.

A comparison of the modulus versus temperature of PA66 and PPO reveals significant differences in the thermal behavior of these two amorphous and semi-crystalline materials (Figure 3). The glass transition temperature of conditioned PA66 is lower than that of dry PA66. Above this glass transition temperature, PA66 maintains modulus at much higher temperatures than PPO materials. If your primary goal is to meet the long-term high-temperature performance required for automotive under-the-hood applications, then PA66 is the better choice.

PPE has a Tg of up to 215°C, offering excellent heat resistance and dimensional stability at elevated temperatures. Blending PPE with other polymers affects its thermal transition temperature, resulting in a glass transition temperature range from 110°C for blends with higher PS content to 175°C for blends with lower PS content.

The thermal behavior of PPO is marked by stability, with a high modulus from ambient temperature to glass transition temperature. In contrast, the modulus of polyamide materials decreases above the glass transition temperature and is highly dependent on the moisture content of the material. Therefore, polyamide materials can be relied upon to maintain similar properties over their effective temperature range.

PA66 boasts good toughness, as well as impact, creep, and fatigue resistance. Its mechanical properties are influenced by humidity. higher humidity leads to lower strength and stiffness but higher toughness. However, stiffness and strength can be adjusted by adding glass fiber reinforcement.

In the conditioned state, the tensile modulus of PA66 is lower, but its tensile strength is comparable to that of PPO materials. Additionally, the toughness of PA66 is higher and almost on par with that of PPO materials. This indicates that although the properties of polyamide materials can change due to humidity, they still provide sufficient strength and toughness for most applications. Furthermore, PA66 exhibits excellent repeated impact toughness, a low coefficient of friction, and low wear resistance, making it suitable for a wide range of applications.

Where PPO holds significant advantages over PA66 is in mechanical and thermal properties. Even at high temperatures, it maintains consistently high stiffness and tensile/flexural strength. As the temperature rises, the modulus only gradually decreases—a valuable characteristic for applications requiring performance stability under severe conditions. Moreover, PPO's tensile strength is largely unaffected by humidity and remains predictable over a wide temperature range. PPO offers greater reliability than PA66, which may lose strength under similar environmental conditions. These characteristics make PPO ideal for applications demanding durability and performance consistency.

III. Chemical Resistance

PA66 exhibits excellent chemical resistance, making it a versatile material for various industries. It is extremely resistant to hydrocarbons, oils, and greases, making it suitable for automotive and industrial applications.

PA66 also shows good resistance to a wide range of solvents and weak acids. However, it is less resistant to strong acids, bases, and oxidizers, which can cause its structure to degrade over time. Its performance varies with the temperature and concentration of the chemicals to which it is exposed.

PPO possesses excellent chemical resistance, particularly to acids, bases, and many organic solvents. It is highly resistant to hydrolytic degradation and performs well in humid environments due to its low hygroscopicity. Nevertheless, PPO is less resistant to aromatic hydrocarbons, halogens, and certain ketones. Additionally, it is prone to environmental stress cracking when exposed to organic solvents.

In specific applications, especially in environments exposed to hydrocarbons, oils, and greases, PA66 demonstrates superior chemical resistance to PPO. PA66's ability to withstand prolonged exposure to these substances gives it a significant advantage in automotive and mechanical components, which are often exposed to such environments. In contrast, PPO's susceptibility to hydrocarbons, aromatic solvents, and ketones can limit its performance in similar situations. While PPO can withstand a broad range of chemicals, its sensitivity to these specific substances grants PA66 a considerable advantage in applications requiring durability. This toughness, combined with PA66's mechanical strength, underscores its superior performance in chemically demanding environments.

Although PPO is typically more susceptible to chemical influences due to its amorphous nature, it outperforms PA66 in various environments, including exposure to organic acids, inorganic chlorides, hydrogen peroxide, and antifreeze.

IV. Environmental Resistance

PA66 has excellent environmental resistance, making it a reliable material for demanding applications. It exhibits exceptional resistance to heat aging and retains its mechanical properties for extended periods, even at high temperatures. Special grades, such as glass fiber-reinforced PA66, can withstand temperatures up to 220°C, making them suitable for high-temperature environments.

While PA66 is resistant to a wide range of chemicals, its hydrolytic stability may be compromised by prolonged exposure to water or moisture, resulting in dimensional changes and reduced mechanical properties. However, stabilizers are commercially available to enhance stability under hydrothermal conditions. PA66 itself has limited UV resistance, and prolonged exposure to sunlight can lead to surface degradation and discoloration. To improve its UV resistance, stabilizers or protective coatings are often added to make it more suitable for outdoor applications.

PPO is renowned for its excellent environmental resistance, particularly to moisture and hydrolytic degradation. With low hygroscopicity, PPO maintains its dimensional stability and mechanical properties even in humid environments, making it ideal for applications requiring prolonged water exposure. Additionally, PPO shows excellent resistance to a wide range of acids, alkalis, and many organic solvents, ensuring its durability in certain chemically aggressive environments. Its inherent thermal stability complements its resistance, enabling reliable operation across various environmental conditions.

PPO's hydrolytic stability surpasses that of PA66, and its water absorption is among the lowest of all engineering thermoplastics. This superior resistance to water absorption ensures that PPO maintains its mechanical integrity and minimizes performance degradation or dimensional expansion. In contrast, nylon 66 absorbs water, leading to volume expansion, increased interference stress, and reduced stiffness and strength. PPO's superior stability in wet or humid conditions makes it a more reliable choice for applications where dimensional accuracy and consistent mechanical properties are critical.

V. Processing

PA66, as a semi-crystalline material, offers several advantages in molding processing over PPO. First, PA66 has a lower melt viscosity than PPO, which reduces the pressure required to fill injection molds. This allows for longer runners and the ability to fill thinner wall thicknesses.

PPO excels in dimensional control with low molding shrinkage. It also has a lower specific gravity than PA66, resulting in lighter parts.

VI. Conclusion

The choice between PPO and PA66 depends on the specific application requirements. If your application demands a material that can perform well under various stresses, then PA66 is definitely the better option. This includes automotive applications, industrial tools, and high-temperature sustained use environments. PPO stands out in applications requiring high heat resistance, dimensional stability, and hydrolytic stability.

It is important to note that the choice between PPO and PA66 should be based on a comprehensive consideration of the specific needs of the application. Both materials have their unique advantages and are suitable for different scenarios.

In the field of flame retardancy, YINSU, besides offers a variety of flame retardant solutions for nylon materials, we also have developed flame retardants for PPO and HIPS alloy. FRP-950X, a high-content coated red phosphorus flame retardant, can achieve highly efficient flame retardancy and halogen-free environmental protection when used in PPO and HIPS alloys. It opens up new directions in the flame retardant market.