Short answer: SLS cakes because it is hygroscopic and because stacking pressure and humidity compress it. Keep it sealed, dry and cool, and rotate stock.
Why it happens
- Moisture uptake: SLS absorbs water from humid air, and the dissolved surface re-crystallises into bridges between particles.
- Pressure: pallets stacked high or stored for months compress the lower bags.
- Temperature cycling: warm days and cool nights inside a container or warehouse drive moisture migration.
Prevention checklist
- Keep the inner polyethylene liner intact and re-close bags tightly after each withdrawal.
- Store cool and dry, off the floor on pallets, away from doors and external walls.
- Limit stack height and avoid long-term storage of fully loaded pallets.
- Run FIFO rotation and check the oldest pallets first.
- Inspect liners and bag integrity on arrival — damage in transit is the usual root cause.
If it has already caked
Break lumps mechanically and sieve if your process needs free-flowing material, then test active matter and moisture before dosing, because absorbed water lowers the effective active content. Reserve any material with visible moisture damage for non-critical use or reject it. Related: how to store sodium lauryl sulfate, shelf life and needles vs powder handling.