Primary Disadvantages of Plastics

Although plastics are extremely useful materials, some shortcomings inevitably exist, particularly when compared to some metals. Plastics major deficiencies are:

• Lack of Strength: Most plastics are much weaker than metals and mild steel has approximately six times the strength of nylon. However, mild steel is six times the weight of nylon. So, they are comparable on a strength/weight ratio.
• Low Stiffness: Plastics have a very inferior value of Young’s Modulus compared with the common metals.
• Low Impact Strength: Many plastics have poor impact strength, but there are a few exceptions, such as with certain polycarbonates.
• Poor Dimensional Stability: This is mainly due to high values of thermal coefficient of expansion.
• Poor High-Temperature Capability: Metals are generally capable of retaining reasonable strength at much higher temperatures than the plastics. The long-term maximum operating temperature for the better plastics is not usually above 250ºC. High-temperature metals can operate for long periods well in excess of 800ºC.
• Moisture Absorption: Many types of plastic absorb moisture, which can result in a significant loss of strength in a humid environment.
• Ultra Violet Light: Some plastics deteriorate when exposed to UV light for long periods. Increased brittleness and loss of strength can occur.