How hot does water have to be to melt plastic?

Answered by Bill Hernandez

As an expert sommelier and brewer, I have had my fair share of experiences with hot liquids and the materials they come into contact with. When it comes to plastic, the temperature at which it melts depends on the type of plastic. Let's take a closer look at two commonly used plastics for cups and their respective melting points.

High-density polyethylene (HDPE), also known as recyclable 2, has a melting point of approximately 130 degrees Celsius (266 degrees Fahrenheit). This type of plastic is often used for packaging, including cups, due to its durability and resistance to chemicals. I recall a time when I accidentally left a HDPE bottle in my car on a scorching summer day. To my surprise, the bottle had partially melted and lost its shape, as the temperature inside the car exceeded the melting point of HDPE.

On the other hand, low-density polyethylene (LDPE), or recyclable 4, has a slightly lower melting point of around 120 degrees Celsius (248 degrees Fahrenheit). LDPE is often used for flexible packaging, such as plastic bags and cling film. I remember once using a LDPE bag to store some hot leftovers. After reheating the food in the microwave, the bag started to warp and deform, indicating that it was approaching its melting point.

It's worth noting that the melting points mentioned above are average values and can vary depending on the specific composition and additives of the plastic. Different manufacturers may use different formulations, resulting in slight variations in melting points.

When it comes to cups designed for hot liquids, polypropylene (recyclable 5) is a commonly used plastic. Polypropylene has a higher melting point compared to HDPE and LDPE, typically around 165 degrees Celsius (329 degrees Fahrenheit). This makes it suitable for holding hot beverages without deforming or melting.

In my experience as a brewer, I have used polypropylene containers for hot wort during the process. The high melting point of polypropylene ensures that the containers remain intact even when exposed to boiling temperatures.

To summarize, the temperature at which water needs to be to melt plastic depends on the type of plastic. HDPE (recyclable 2) melts at around 130 degrees Celsius (266 degrees Fahrenheit), LDPE (recyclable 4) at around 120 degrees Celsius (248 degrees Fahrenheit), and polypropylene (recyclable 5) at approximately 165 degrees Celsius (329 degrees Fahrenheit). Understanding these melting points is essential for selecting the appropriate plastic for specific applications, such as cups for hot liquids, to ensure they maintain their integrity and functionality.