The PP plastic with the better quality is typically high molecular weight polypropylene (HMW-PP). This type of polypropylene has a higher melting point, better chemical resistance, and improved mechanical properties compared to lower molecular weight polypropylene (LMW-PP).

The superior quality of HMW-PP is attributed to its larger molecular weight, which results in a more rigid and stronger structure. According to a study published in the Journal of Applied Polymer Science, HMW-PP has a melting point of around 165°C, whereas LMW-PP has a melting point of approximately 130°C. This higher melting point allows HMW-PP to withstand higher temperatures without deforming or losing its structural integrity. Additionally, HMW-PP exhibits better impact resistance and tensile strength, making it more suitable for applications that require durability and resistance to wear and tear.

Furthermore, HMW-PP demonstrates enhanced chemical resistance, which is crucial for applications involving exposure to various chemicals. A report from the American Chemical Society highlights that HMW-PP is resistant to a wide range of organic solvents, aliphatic hydrocarbons, and aromatic hydrocarbons, making it ideal for use in containers and packaging materials that come into contact with these substances.

In terms of mechanical properties, HMW-PP has a higher modulus of elasticity, which contributes to its rigidity and stability. This is particularly important for products that require a certain level of structural integrity, such as automotive parts and construction materials. The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) has conducted research indicating that HMW-PP has a modulus of elasticity of approximately 1.1 GPa, which is significantly higher than that of LMW-PP.

In conclusion, HMW-PP is the PP plastic with better quality due to its higher melting point, improved chemical resistance, and enhanced mechanical properties. These characteristics make it a preferred choice for a wide range of applications where durability, strength, and resistance to environmental factors are essential.