From HDB website:
Q:
My housing loan with HDB is fully paid up. Can I use my flat as collateral to obtain a bank loan for personal purposes?
A:
HDB flats can only be mortgaged to banks or financial institutions to finance the purchase of the flat itself.
You are not allowed to use your HDB flat, which has been fully paid for, as collateral to banks to raise credit facilities for private reasons. This is to avoid exposing flat owners like yourself to the risk of losing your flat should you be unable to repay the bank loan.
http://askhdb.hdb.gov.sg/Themes/HDB...You are not allowed to,to repay the bank loan.