Innovation Chem Packaging Deep-Dive Series
1. Polymer Chemistry in Plain English
| Resin | Typical pH Window | Molecular Backbone | Key Risk if Mis-used |
|---|---|---|---|
| PET / rPET | 9 – 12 (alkaline) | Ester links + benzene rings | Hydrolysis & stress cracking at very high pH |
| HDPE | 1 – 8 (acidic / neutral) | Saturated –CH2– chain | Swelling / permeation with strong solvents |
2. PET & rPET — Ideal for Alkaline, Water-Based Cleaners
- Hydrolysis resistance: Stable up to about pH 12, giving “good” compatibility in most degreasers.
- Glass-clear aesthetics: High transparency showcases bright dyes; oxygen-barrier blends slow colour fade.
- Tax & sustainability: ≥30 % rPET avoids UK Plastic Packaging Tax and boosts eco-credentials.
Limitations: Above pH 12 or with hot caustic CIP fluids, PET can craze; always moderate free alkali or switch resins.
3. HDPE — First Choice for Acidic Descalers & Aggressive Detergents
- Acid resistance: Excellent with hydrochloric, phosphoric and citric acids.
- Tough walls: Survive drop tests & hot-fill conditions.
- Colour flexibility: Masterbatch pigments disperse easily without hurting chemical resistance.
Limitations: Aromatic solvents can swell HDPE—specify a fluorinated or co-extrusion barrier if needed; natural HDPE is translucent, so clarity-focused brands may prefer PET.
4. Testing: The 12-Week Accelerated Study
Run a 40 °C / 75 % RH, 12-week compatibility programme tracking:
- Weight change (permeation)
- Visual defects (crazing, panel sink)
- pH drift (ion exchange with polymer)
- Label adhesion / print bleed
Record data at 0, 4, 8 and 12 weeks—and repeat for every colour variant.
5. Five-Step Resin Selection Checklist
- Measure pH & solvent content at operating temperature.
- Balance shelf-appeal (clarity) vs. opacity needs.
- Factor in recycled content to minimise tax.
- Match the closure seal material (EPDM vs Viton).
- Document results for ISO or BRCGS audits.
6. Quick Reference Table
| Formula Type | Typical pH | Recommended Resin | Common Bottle Size | Notes |
|---|---|---|---|---|
| Kitchen Degreaser | 10 | PET / rPET | 500 ml Boston | Showcase colour; monitor stress cracking > pH 12 |
| Wheel-Acid Cleaner | 2 | HDPE | 1 L Cylindrical | Add fluorination if solvent boosters present |
| Industrial Soap | 7 | Either | 5 L HDPE Jerry | Choose PET only when clarity is critical |
| Concentrated Bleach | 12.5 – 13 | HDPE | 25 L UN Can | PET hydrolyses; HDPE delivers ESCR |
7. Take-Away
Use PET/rPET where clarity and alkaline stability (≤ pH 12) are vital; switch to HDPE for acids, aggressive solvents or UN hazardous-goods rules. Validate every formula with a 12-week, 40 °C / 75 % RH compatibility test before scaling.
Need help running resin trials or sourcing PCR-rich bottles? Contact the Innovation Chem packaging team—avoid costly mistakes by getting your resin choice right from day one.