Patent US2838524 (http://l2.espacenet.com/dips/viewer?PN=US2838524&CY=gb&LG=en&DB=EPD)
Patent US2347434 (http://l2.espacenet.com/dips/viewer?PN=US2347434&CY=gb&LG=en&DB=EPD)
other peracidshttp://www.geocities.com/dritte123/PSPF.html (http://www.geocities.com/dritte123/PSPF.html)