Formula-to-Pack Compatibility Testing

Compatibility testing asks a narrow question: does this formula, in this pack, in this closure, still behave after storage? It is not the same question as stability, which asks whether the formula survives. A product can be perfectly stable in glass and fail in a airless pack within eight weeks, and the failure is usually visible in the pack before it is visible on the skin.

1. Compatibility is not stability

Stability studies hold the formula in an inert container and watch the product. Compatibility studies hold the real pack and watch both. The two run in parallel and are read together. A compatibility failure with a clean stability result points at the pack, the closure or the interaction between them.

2. Weight loss and the desiccation cell

Water loss through a plastic wall is measurable and predictable. A jar of a water-rich cream stored at 40 degrees can lose 1 to 3 percent of its weight in twelve weeks, and the product at the surface dries first. Record weight at fixed intervals on the filled pack, not on the empty one, and plot the slope rather than the endpoint.

3. Stress cracking

Some surfactants, oils and solvents attack moulded plastic under stress, producing hairline cracks at the base or around the thread. The pack usually fails at the point of highest moulded-in stress. Testing under load, with the closure torqued as in production, finds this far more reliably than testing an empty bottle.

4. Migration and absorption

Ingredients can migrate into the plastic, and plastic additives can migrate out. The visible signs are a fragrance that fades, a colour that dulls, or a pack that becomes hazy. On a laminated tube, the ink and adhesive layers also come into play, which is why a tube often shows a problem a bottle does not.

5. Closure and liner interaction

The liner is frequently the weakest element. A foam liner can absorb fragrance and swell; an incompatible liner can soften and lose torque. Test with the production closure and the production torque setting, and re-check torque after storage. A closure that opens with a quarter turn after eight weeks was never compatible.

6. Test conditions and duration

Standard practice is 40 degrees and 75 percent relative humidity for twelve weeks, with a parallel ambient arm and a freeze-thaw arm where shipping demands it. Twelve weeks at 40 degrees is usually taken as an indicator for two years at ambient, though it is an indicator rather than a guarantee.

7. Reading a failure

Distinguish between a cosmetic failure, a functional failure and a safety failure. A slight label lift is cosmetic. A pump that will not prime is functional. A pack that leaks in transit is functional and commercial. Safety failures, such as a compromised seal on a single-use sachet, stop the project regardless of how good the formula is.

8. Sample size and replicates

Three packs per condition is the usual minimum, and five is better where the pack is expensive or the claim is tight. Include packs filled on the production line, not only lab fills, because fill temperature and headspace differ. A lab-filled sample has passed a different test from the one that ships.

9. What to fix first

If compatibility fails, change the closure or liner before the formula, then the pack material, then the formula. Each step outward costs more and delays the launch further. A liner change costs days; a formula change costs a stability restart, typically three months.

10. Where it fits

Compatibility work applies to every sachet and bottle format. Read packaging notes or browse the range.