The melting point of PVC (Polyvinyl Chloride) varies because it's a semi-crystalline plastic, but it generally starts to soften around 80-100°C (176-212°F) and melts between 160-210°C (320-410°F). The amount of heat required to melt PVC depends not just on reaching this temperature range but also on the specific PVC compound, as plasticizers and other additives can alter its thermal properties. To melt a certain volume of PVC safely, it's crucial to provide continuous, even heat to avoid degradation or burning, which occurs above 260°C (500°F). Given heat is measured in joules (J) or calories (cal), the precise amount of energy to melt PVC would depend on the mass to be melted and the specific heat capacity of the PVC used, which can vary. Expressing this in a simple equation isn't straightforward without specific mass and heat capacity values.

In the 1920s, the insulation used in homes and buildings was quite rudimentary by modern standards. The most common type of insulation material was cellulose, fabricated from recycled paper products like newspapers, treated with chemicals to make it fire retardant. Another type was mineral wool, also known as rock wool, made from slag and natural rock. These materials were chosen for their availability, affordability, and basic insulating properties, despite being less efficient and more fire-prone than today's insulation materials. Asbestos was also used for its heat-resistant properties, especially around boilers and pipes, though its health risks were not yet widely recognized. The use of these early insulation materials marked the beginning of a broader understanding and implementation of insulation in building construction, aiming to improve energy efficiency and indoor comfort.

Polypropylene, a type of plastic marked with the recycling symbol #5, is generally considered safe for use at moderate temperatures. It is often used in containers for food storage due to its resistance to chemicals, water, and fatigue. The material can withstand temperatures up to about 120°C (248°F). Beyond this, it may start to soften or deform, potentially leading to the leaching of chemicals into food or drink. However, for everyday uses, such as microwaving or dishwashing, polypropylene is considered safe by the FDA and other health agencies. It's crucial, though, to use products as directed and avoid exposing polypropylene containers to high-temperature situations for extended periods. For high-heat applications, glass or metal may be a safer choice.