Plastic materials can be categorized based on their characteristics and applications. The following provides a detailed introduction to thermosetting plastics, thermoplastic plastics, and engineering plastics.
1. Classification and Characteristics of Plastics
- Thermosetting Plastics
Thermosetting plastics, such as epoxy resin and polyurethane (PU), have a network molecular structure and cannot undergo reversible reactions between liquid and solid states. As a result, they cannot be recycled. These plastics harden upon heating and exhibit high rigidity. - Thermoplastic Plastics
Thermoplastic plastics have a linear molecular structure and can undergo reversible reactions between liquid and solid states, making them recyclable. Common general-purpose thermoplastics include PVC, ABS, PP, and PMMA. - Engineering Plastics
Engineering plastics are a category of thermoplastics with higher heat resistance and strength. They are further divided into:- General-purpose engineering plastics: Such as POM, PCT, PC, and PBT, with heat resistance ranging from 100°C to 140°C.
- Super engineering plastics: Such as PPS and LCP, with heat resistance exceeding 150°C.
2. Key Terms in Plastic Properties
- Tensile Strength
The ability of a material to resist stretching, defined as the maximum stress it can withstand when being pulled apart. - Flexural Strength
The strength of a material when bent until it breaks or reaches a strain value of 5% at its outermost edge. - Flexural Modulus
The slope of the linear portion of the stress-strain curve, indicating the rigidity of the material. Higher modulus values signify stronger rigidity. - Additives
Additives or compositional changes can significantly alter material properties. For example, adding glass fibers increases tensile strength, density, and the usable temperature range but reduces elongation, thermal expansion, and shrinkage. - Flammability
The flammability of a product depends on factors such as part size, ignition difficulty, burning rate, and the degree of oxidation. These tests are not entirely accurate and depend on specific conditions. - Secondary Material Additive Ratio
The proportion of secondary materials added to plastics is generally kept below 30%.
3. Comparison of Common Plastic Materials
The table below summarizes the advantages and disadvantages of several common plastic materials:
| Material | Advantages | Disadvantages |
|---|---|---|
| ABS | Good impact resistance, low cost | Poor resistance to organic solvents, lacks long-term heat aging |
| ACETAL | Good flowability and chemical resistance, low cost | Degrades in acidic or alkaline environments, poor flame retardance |
| PBT | Good flowability and smoothness, excellent electrical and chemical resistance, good thermal properties | High shrinkage, prone to heat under load, poor temperature resistance, unsuitable for SMT processes |
| PET | Reduced warping compared to PBT, better heat resistance | Poor flowability compared to PBT, sensitive to humidity |
| PCT | Excellent heat resistance, good flowability and chemical resistance | Brittle, narrow processing window |
| PC | Naturally flame-retardant, good impact resistance, stable dimensions, transparency, excellent thermal properties | Poor chemical resistance, prone to splitting under pressure, poor flowability |
| PPO Blends | Good thermal properties, naturally flame-retardant, moderate chemical resistance | Poor flowability, limited colorability |
| Nylon | Good flowability, heat resistance, impact resistance | Poor moisture resistance, unstable dimensions |
| PPS | Good flowability, heat resistance, chemical resistance | Brittle, reflective, limited colorability |
| PEX | Excellent thermal properties, good environmental fatigue resistance, naturally flame-retardant | Slow crystallization, high cost |
| PEI | Excellent thermal properties, naturally flame-retardant | High cost |
| LCP | Excellent flowability, thermal properties, flame retardance, electrical and mechanical properties | Poor weaving strength, limited colorability, high cost |
By understanding the characteristics and trade-offs of these materials, you can select the most suitable plastic to meet the requirements of various applications.