Polypropylene (PP) is regarded as a semi-rigid and highly flexible polymer. Its flexibility can be adjusted through copolymerization and the addition of plasticizers. Typically, polypropylene exhibits excellent resistance to fatigue, making it ideal for hinges and flexible parts in design. However, its rigidity and flexibility are highly dependent on the molecular weight and the proportion of comonomers (like ethylene) in copolymers. PP's versatility is showcased in its wide range of applications, from rigid containers to flexible packaging films and automotive parts. Though not as flexible as some elastomers or specific plastics like PVC, its balance between stiffness, toughness, and moldability makes it a popular choice in various industries. Adjustments in its polymer structure can further enhance or reduce its flexibility, demonstrating polypropylene's adaptability to specific requirements.

Tree roots can indeed penetrate PVC pipes, particularly if the pipes have pre-existing cracks or weaknesses. PVC, being a form of plastic, is not inherently susceptible to root intrusion. However, roots naturally seek out moisture and nutrients, leading them toward water lines. If a PVC pipe has a crack, even a minor one, roots can infiltrate and eventually widen the crack as they grow, causing blockages or further damage to the plumbing system. Preventative measures include regular inspection of pipes for damage, installing root barriers, and choosing less invasive tree species to plant near piping systems. It's crucial to address any root intrusions early to prevent extensive pipe damage and costly repairs.

The COF between rubber and polypropylene is crucial for optimizing performance in applications where these materials interact. It depends on various factors, such as material properties, surface conditions, and environment. Rubber's high friction is due to its flexibility and deformability, while polypropylene's smoother surface results in lower friction.

COF values may range from 0.2 to 0.4 under normal conditions, but can vary depending on factors like rubber compound and type of polypropylene. To obtain accurate measurements, it is important to conduct tests in real-world conditions. In cases where low friction is needed, surface treatment or lubrication may be considered for optimal performance.