"Chemical analyses by the laboratories were consistent and showed that the dark brown splatter material was consistent with a
polyurethane elastomer." -- 16.4.6 Examination of Brown Splatter Material --
http://www.ntsb.gov/doclib/reports/2000/AAR0003.pdf
"A shaped charge casing 1 designed to be nested to reduce overall volume, which benefits storage or carriage. ... The lid and body portions may be formed of polymeric material. Preferably the lid and body of the shaped charge will be comprised of structural
polyurethane foam. This is a lightweight material that is easy to manufacture at low cost. This material is considered non-fragmenting which allows a very low weapon danger area when compared with other commonly used shaped charge casing materials." --
http://www.google.com/patents/US20140060369