Polypropylene is a versatile thermoplastic polymer, known for its rigidity and resistance to various chemical solvents, bases, and acids. Generally, it is considered hard in the context of everyday objects, like containers and carpets, due to its resilience and durability. However, the hardness of polypropylene can vary depending on its formulation and the manufacturing process it undergoes. For example, incorporating different polymers or additives can make polypropylene more flexible or tougher, catering to specific application needs. Its melting point, around 130°C to 171°C, also contributes to its hard nature at room temperature. Thus, while inherently hard, polypropylene's physical properties can be adjusted to suit varying demands, making it a favored material in numerous industries.

The drying time for PVC glue, also known as solvent cement, typically depends on the type of glue, the size of the pipe, and the environmental conditions. Most manufacturers recommend waiting at least 15 minutes before handling the joint and 2 hours before subjecting it to any pressure. However, for larger pipes (over 3 inches in diameter) or in cooler, more humid conditions, it's advisable to allow the joint to cure for a longer period, ideally 24 hours, to ensure maximum strength and seal integrity. Always consult the product's instructions for the best results, as formulations may vary.

Titanium dioxide and zinc oxide are commonly viewed as safe. particularly in sunscreens and similar topical items. These physical sunscreens deflect UV rays and are gentler on the skin. without being absorbed into it or causing systemic exposure. Yet. it's important to be cautious when using products like spray sunscreens that contain these minerals. as inhaling them could potentially lead to lung inflammation and damage. As a result. the FDA is continuously researching these ingredients to guarantee their safety.