Fluorocarbon Co., a Laguna Niguel manufacturer of rubber and plastics components, sold $25 million of convertible subordinated debentures Tuesday. The 25-year debentures pay 8% interest annually and are convertible into Fluorocarbon common stock at a price of $20.156 per share. Fluorocarbon closed at $16.13 per share Tuesday in over-the-counter trading. Proceeds of the debenture sale, estimated at $24 million, will be used for repayment of $12.4 million in debts, for working capital and to finance acquisitions. Smith Barney, Harris Upham & Co. Inc. underwrote the offering.