High-Density Polyethylene (HDPE) is not biodegradable. HDPE is a type of plastic that is known for its durability and resistance to degradation in natural environments. It is often used for items like plastic bottles, containers, and various other products because of its robust properties. However, these same properties that make HDPE plastic so useful also mean that it does not readily break down in the environment.

To clarify, biodegradable materials are substances that can be broken down by natural processes into non-toxic components, such as water, carbon dioxide, and biomass, with the help of microorganisms. HDPE does not meet this criteria, and it can persist in the environment for a very long time if not properly managed through recycling or disposal in a controlled manner.

If you're looking for more eco-friendly alternatives, you might want to consider materials like PLA (Polylactic Acid) or PHA (Polyhydroxyalkanoates), which are biodegradable plastics. However, it's important to note that even biodegradable plastics may not break down in all environments and may require specific conditions to degrade effectively.

HDPE can be recycled (as many thermoplastics can) but once it's in a matrix of other stuff (say calcium) it get's really difficult to do so. Also HDPE is not biodegradable. It can break up into microplastic, but I wouldn't call that exactly enviromentally friendly. Maybe if there are some added enzymes or something…

But you can just use unbleached, recycled paper. Paper is easy to recycle, doen't require a lot of chemicals (unless you bleach it) and is already one of the most recycled resources.