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Plastics can be classified into three categories as on performance. These are general-use plastics, High-performance plastics, and Engineering plastics.
Engineering plastics are a class of polymer materials that offer significantly better mechanical, thermal, chemical, and electrical properties than commodity plastics, making them suitable for structural and high-performance engineering applications.
High-performance plastics are valued for their excellent properties and are used in many demanding applications. Among the high-performance plastics, there are fluoropolymers, sulfur-containing aromatic polymers, aromatic polyaryl ether and polyketones, and Liquid crystal polymers.
Compared to the general use of plastics and engineering plastics, high-performance plastics differ in their:
The most popular high-performance plastics are Poly-ether-ether-ketone (PEEK) and Poly-tetrafluoro-ethylene (PTFE). PTFE is also known as “Teflon”.
The chemical structural formula of poly-ether-ether-ketone (PEEK). PEEK is a semi-crystalline thermoplastic. PEEK is synthesized by step-growth polymerization by the dialkylation of bis-phenolate salts.
PTFE is a fluorocarbon solid, consisting wholly of carbon and fluorine. Due to the Fluorine, PTFE has high flame resistance. PTFE is hydrophobic: neither water nor water-containing substances wet PTFE. PTFE has one of the lowest coefficients of friction of any solid.
Over the last 30 years, plastics have been developed to the point where they have started replacing many traditional materials such as wood and metal. Traditional materials have been replaced by engineering plastics for many reasons.
The main benefit of using engineering plastics is the low cost. It is more economical than using steel or wood. The other advantages are low weight, aesthetics (attractive appearance), functional design, reduced maintenance, corrosion resistance, and chemical resistance.
One of the primary drivers for adopting engineering plastics is economics. While the raw material cost per kilogram can sometimes be higher than steel, the final fabricated part is often significantly cheaper due to lower weight and easier processing.
| Property | Steel rim | Nylon rim (Reinforced) | Analysis |
| Density | 7.8 g/cm3 | 1.15 g/cm3 | |
| Tensile strength | 400 MPa | 200 MPa | |
| Cost | 2 USD/kg | 4 USD/kg | |
| Volume of the rim | V | 2V | |
| Thickness | 10 mm | 20 mm | To keep the same strength as steel, 200MPa (Nylon) × thickness = 400 MPa (Steel) × 10mm Thickness of nylon rim = (400 MPa × 10mm) / 200MPa The thickness of the nylon rim = 20mm |
| Weight of the rim | 7.8 g/cm3 x V | 1.15 g/cm3 x 2V | |
| 7.8V g | 2.3V g | ||
| Cost of the rim | 2 USD x 7.8V / 1000 | 4 USD x 2.3V / 1000 | |
| 15.6 V / 1000 USD | 9.2 V / 1000 USD |
Cost of the Nylon rim < Cost of the Steel rim
Conclusion: For the same strength, the reinforced nylon rim is over 3 times lighter and approximately 41% cheaper than the steel rim.
As the analysis shows, even though the nylon material is more expensive per kilogram, the massive weight savings make the final product significantly more economical.
Beyond cost, engineering plastics offer numerous advantages:
| Property | Nylon | Bronze | Steel | Aluminum |
| Density (g/ cm3) | 1.15 | 8.8 | 7.84 | 2.7 |
| Tensile strength (psi) | 12000 | 22000 | 36000 | 30000 |
| Elasticity modulus (psi) | 0.4 x 106 | 16 x 106 | 30 x 106 | 10 x 106 |
Engineering plastics have good strength, temperature resistance, and good dimensional stability at a wide range of temperatures.
Due to the high thermal stability, thermal applications of engineering plastics include:
A variety of engineering thermoplastics are available today. Those are,
Normally, engineering plastics contain a large number of polar groups. These polar groups undergo hydrolysis reactions in the presence of moisture. Engineering plastics such as Nylon, ABS, and PET have a high amount of polar groups.
Nylons absorb moisture in equilibrium with the relative humidity of their immediate surroundings. Nylon 6 absorbs 9% and Nylon 6.6 absorbs 7% of moisture. This results in a measurable dimensional change of 2% and 3.5%. Moisture absorption also happens due to the presence of polar groups.
The tensile strength of nylon 6 and 6.6 drops significantly with increasing moisture content. In the case of nylon 6, the drop in tensile strength from dry (100 %) to saturated state is over 80 %. For nylon 6.6, the drop is over 60 %. With increased moisture content, the modulus of elasticity and other stiffness properties are also reduced.
Engineering plastics normally have a high melting point. As a result, they also have high processing temperatures. As an example, the melting point of PET is 270 ℃, and it has a processing temperature of around 270-280 ℃. At this high temperature, if Oxygen is present, degradation of the polymer can happen. If there is moisture, hydrolysis can happen.
Another disadvantage of the high melting point is the difficulty of obtaining clear products. Cooling from about 280 ℃ to room temperature is not easy. The cooling process takes time. So, the slow cooling causes the crystallization of the polymer. Crystallization products will not be clear (transparent) products.
Due to the high melting point, the polymer needs high energy to melt. So, the energy consumption is high. To avoid hydrolysis at the processing stage, a polymer can be pre-dried to remove moisture content. In order to avoid oxidation of the polymer, contact of molten plastic with oxygen should be avoided.
Increasing the molecular weight of the polymer, and the mechanical properties will increase. But flowability will decrease. Engineering plastics have very low flowability. Traditional methods like injection molding and extrusion blow molding cannot be applied to making products out of high molecular weight engineering plastics.
Thanks to their versatility, engineering plastics are found across numerous sectors:
Margolis, J. Engineering Plastics Handbook; McGraw-Hill Professional, 2006.
Cover Image was designed using an Image by cherylt23 from Pixabay
