What is the molecular structure and characteristics of polyaramid materials?

Polyaramid is a class of synthetic polymer materials known for its high strength, heat resistance and flame retardancy. Its most prominent feature is its unique molecular structure, which is polymerized from specific monomer units. The monomer units are linked together by amide (–CONH–) groups. The amide bond is between the amine group (–NH2) of one monomer and the acyl chloride (–COCl) of another monomer. The by-product hydrochloric acid (HCl) is eliminated during the polymerization process. , aromatic rings and amide bonds are arranged alternately to form polymer chains, which form crystalline regions through highly ordered arrangement and alignment. In addition to crystalline regions, polyaramid also has amorphous regions. In addition, the amide groups in the polyaramid chain can interact with adjacent chains with hydrogen bonds. These hydrogen bonds help to improve the overall strength and stability of polyaramid materials. The combination of aromatic rings, amide bonds, crystalline regions and amorphous regions in the molecular structure of polyaramid mentioned above gives it unique properties, including high strength, heat resistance, flame retardancy and chemical resistance.

What are the characteristics of polyaramid:

1. Polyaramid has excellent strength, which is due to the presence of aromatic rings in the monomer units of aromatic polyamide. Because aromatic rings are ring structures composed of carbon atoms, it is these aromatic rings that provide rigidity and structural stability to the polymer chain, and have strong tensile strength. The tensile strength coefficient symbol σ₄₅/σ₂₅ is used to measure its tensile strength changes with temperature, such as the tensile strength at a specific high temperature (usually 45°C) compared with the tensile strength at a reference temperature (usually 25°C). Generally speaking, the tensile strength coefficient of polyaramid is between 0.5 and 0.8, which means that at 45°C, the tensile strength of polyaramid is usually 50% to 80% of the strength at 25°C, which will vary slightly depending on the material formula used. In this regard, its tensile strength is significantly higher than that of steel on the basis of weight and strength. This feature makes it more suitable for applications that require high-strength materials.

2. Polyaramid has excellent thermal stability. It can usually withstand continuous use in the temperature range of about 150°C (302°F) to 250°C (482°F), and can maintain its mechanical properties and structural integrity. In short-term or intermittent exposure, polyaramid can withstand higher temperatures up to 400°C (752°F). Just be aware that long-term exposure to the upper temperature of this range may cause some degradation. Therefore, polyaramid has an extremely high melting point and plays an important role in some heat-resistant industrial applications.

3. Polyaramid is known for its excellent flame retardancy. When exposed to flames, it will burn instead of melting or dripping, and form a protective barrier to help prevent the fire from spreading further. This property makes polyaramid widely used in protective clothing industries, such as fire-fighting equipment and bulletproof vests for firefighters.

4. In addition, polyaramid has good resistance to a variety of chemicals, such as weak acids and weak bases with a pH range of about 2 to 12, and most organic solvents, including acids and weak acids in common household or industrial environments, acetic acid (vinegar) or dilute sulfuric acid, sodium hydroxide (lye) or ammonia, but it may degrade, weaken or react chemically in contact with strong or concentrated acids, and long-term contact with strong or concentrated alkaline solutions.

5. Polyaramid has a relatively low density of about 1.38 to 1.45 grams per cubic centimeter (g/cm³) or 1380 to 1450 kilograms per cubic meter (kg/m³). This low-density and lightweight feature makes it an ideal choice for applications where high strength-to-weight ratio is critical.

In addition to the above characteristics, polyaramid also has excellent electrical insulation properties and good dimensional stability. Based on its excellent performance, polyaramid can be used in various industries, including aerospace, automobile, defense, electronics and sports equipment, protective equipment, bulletproof vests, reinforced composite materials, electrical insulation materials, gaskets and seals and other products.