Formula Compatibility: What the Product Does to the Pack
A pack is chosen for what it holds. pH, solvent and actives decide the resin, the seal and whether a metal finish survives the first month.
The Three Questions We Ask First
- pH. Below 4 and above 9, the pack is a chemical decision before it is a design decision.
- Solvent. Alcohol, glycol ethers, essential oils and acetone all attack different plastics.
- Actives. Retinol, vitamin C, salicylic acid and benzoyl peroxide are the four that cause the most packaging complaints.
Material by Formula
| Resin | Handles | Avoid |
|---|---|---|
| PET | Water-based, neutral to mildly acidic, alcohol up to about 20 % | Strong solvents, hot fill above ~60 °C |
| PETG | Thick-wall clarity, most emulsions, moderate alcohol | Strong solvents, high heat, some essential oils |
| HDPE | Acids, alkalis, bleach, most surfactants | Not for clarity; some oils permeate |
| PP | Broad chemical resistance, hot fill, closures | Poor clarity, lower gloss |
| ABS + plating | Decorative caps and bases | Acid peels, high alcohol, long contact with fragrance |
The Plated Part Is the One That Fails
A champagne-gold ABS cap is a cosmetic surface over a plastic substrate. An acid peel or a high-alcohol formula reaches the substrate through a pinhole, lifts the plating and stains the product. By the time it shows, the batch is packed.
Two ways out: keep the plated part out of product contact — a PP liner or an inner cup — or move the finish to a part that never touches the formula. On a multi-part pack both are usually possible without changing the look.
Compatibility Testing, in Order of Cost
- Data sheet check — a day, free, catches the obvious mismatches.
- Sample soak — fill a stock bottle with the formula, 40 °C for 4 weeks, inspect weekly.
- Migration test — for food and cosmetic contact, per EU 10/2011 or the customer's market.
- Full stability — the brand's own lab, on the finished pack, before launch.
We do the first two. The third is done by an accredited lab and we supply the declaration. The fourth is the brand's responsibility, and any supplier who claims otherwise is guessing.
What We Need to Answer Properly
The formula's pH, its solvent list, the fill temperature and the expected shelf life. Not the full formula — the four lines that decide compatibility.
Send the pH and the Solvent
We come back with a resin, a seal material and the finish that survives it.