U.S. patent application 20190101337 (applicant SABIC Global Technologies B.V., Bergen op Zoom, Netherlands):
"A thermal energy storage system utilizes a phase change material with an encapsulating material surrounding the phase change material. The encapsulating material fully contains the phase change material to prevent passage of the phase change material out of the encapsulating material and/or prevent direct contact of the phase change material with external objects. The encapsulating material is a high density polyethylene (HDPE), which may be a non-crosslinked or crosslinked HDPE. In a method of forming a thermal energy storage system a phase change material is surrounded with the encapsulating material."