Acid Bond Concentrate Professional Sulfate-Free Conditioner for Hair Damaged by Redken | Concentrated Repair Conditioner without Sulfates for Damaged Hair. This conditioner is the richest formula of Redken to repair damaged hair. It provides complete and intense conditioning and protection against hair color fading. The acid bond concentrate leave-in has been formulated with citric acid which strengthens the hair fiber, gives shine, and reduces hair breakage. This conditioner balances pH as it has been formulated with an acidic pH to protect hair from the negative effects of coloring and external agents.