Last updated: 3 September 2026.
Quick answer: Packaging is downstream of formulation — the formula narrows your options before you have looked at a single bottle. Four properties do most of the work: pH, alcohol content, viscosity, and whether the blend carries essential oils, active ingredients or oxidisers. The most expensive failures are the slow ones: a bottle that looks perfect at day three and crazes at week six because an ingredient attacked stressed plastic. Test the real formula on the real bottle, not a water substitute.
Packaging is downstream of formulation
Brands usually treat packaging as a downstream purchase: formula locked, then find a bottle. That sequence works — but only if the formula’s constraints are carried forward into the packaging specification. Too often they are not, and the packaging team discovers the constraints during filling trials.
The mechanism behind most of these failures is environmental stress cracking. It is worth understanding because it explains why the failures look the way they do.
Every molded bottle carries some residual internal stress from the molding process, concentrated at thin sections, sharp corners and the gate area. On its own that stress is harmless — the bottle sits on a shelf indefinitely. Add a compatible chemical agent and the same stress becomes a crack: the agent penetrates the surface, locally softens the polymer, and the stored stress opens it into a fine craze that then propagates.
Three consequences follow, and they are the reason this topic deserves its own page:
- It is slow. Crazing typically appears weeks after filling, not during.
- It is invisible until it is structural. The pack looks fine, then it leaks or fails.
- It is specific. The same bottle survives one formula and fails another, which is why a supplier’s track record with “shampoo bottles” tells you nothing about your blend.
The four properties that decide everything
| Property | Why it matters | Where the risk starts |
|---|---|---|
| pH | Drives hydrolysis and stress-cracking rate in PET | Below pH 3, and above pH 9 |
| Alcohol content | Acts as a stress-cracking agent and raises permeation | Above roughly 20% |
| Viscosity | Decides whether a closure can dispense the product at all | Above ~10,000 cps for standard pumps |
| Actives and oils | Light and oxygen sensitivity; essential oils as cracking agents | Any meaningful essential oil load; retinoids, vitamin C, UV filters |
Most formulas sit comfortably in the middle of all four. The value of the table is in telling you quickly whether yours does — because if it does not, the material and closure choice is no longer open.
Ingredient × plastic: what actually happens
| Ingredient | What it does | Which plastics are most affected | What to do |
|---|---|---|---|
| AHA, BHA, high-acid vitamin C (pH < 3) | Accelerates hydrolysis and stress cracking over shelf life | PET more than PP or PETG | More margin on wall thickness, larger radii, consider PETG or PP; validate on the real formula |
| Ethanol above ~20% | Stress-cracking agent; raises permeation and weight loss | PET, especially thin walls with sharp corners | Thicker wall, generous corner radii, sealed liner, 30-day weight-loss check |
| Essential and citrus oils | Potent stress-cracking agents | HDPE and PP more susceptible than PET | Run an ESCR test on the actual blend; do not substitute a simulant |
| Surfactant systems | Stress cracking at high concentration and temperature | HDPE depending on grade and wall stress | Confirm with the supplier; avoid stressed thin sections |
| Mineral and vegetable oils | Slow permeation and swelling | HDPE and PP can swell; PET generally better | PET where clarity allows; verify permeation over 30 days |
| Hypochlorite and peroxides | Gas generation; attacks many elastomers and liners | Closure components more than the bottle | Vented closure, HDPE body, verify the liner not just the bottle |
| Retinoids, vitamin C, UV filters | Degrade with light and oxygen | Any — this is a formula problem, not a resin problem | Opaque or amber, UV-inhibiting decoration, airless where justified |
| High salt or high solids | Abrasive to pumps; can crystallise at the orifice | Closure more than bottle | Specify a closure tested for the formulation, not just the neck size |
If you want the material-level comparison behind that table, see our ranges of PET, HDPE, PP and PE and PETG bottles.
The pattern across the table: the closure fails more often than the bottle. Liners, gaskets and pump internals are elastomers and foams, and they are frequently the first thing a formula attacks. When you run a compatibility test, test the assembled pack — bottle, closure, liner — filled with the real product, stored the way it will actually be stored.
The failures that take weeks to appear
If you only remember one section, make it this one.
A compatibility failure that shows up in the first 48 hours is cheap. It is caught during sampling, the specification is changed, and nobody outside the project hears about it. A failure that shows up at week six is the expensive kind: the goods are filled, shipped, and possibly already on shelves.
The standard way to compress that risk into your project timeline is an accelerated storage test:
| Test condition | What it simulates | Typical duration |
|---|---|---|
| Elevated temperature and humidity storage | Accelerated ageing; surface and closure effects | 30 days at elevated temperature, plus a room-temperature control |
| Real product, real closure, inverted and upright | Liner and seal performance under product contact | Same period, both orientations |
| Weight-loss measurement | Permeation through the wall and seal | Measured at intervals over the test period |
| Visual and top-load check at intervals | Crazing, swelling, embrittlement | 7, 14 and 30 days |
You do not need a laboratory to run the useful version of this. Fill production-sample bottles with the real formula, store them at elevated temperature and at room temperature, and inspect at intervals against a control. The elevated-temperature set fails first and tells you early; the room-temperature set tells you what will actually happen.
What to tell your supplier before you sample
Four pieces of information, sent with the first enquiry, remove most of the round trips:
- pH and alcohol content — approximate values are enough to rule materials in or out.
- Viscosity at room temperature — decides the closure more than the bottle.
- Any essential oils, solvents or oxidisers — including fragrance, which is frequently the most aggressive component in the blend.
- Storage and use conditions — ambient, bathroom, cold chain, or outdoor.
With those four, a factory can tell you immediately whether your first choice is viable. Without them, the honest answer is “it depends”, and you pay for that uncertainty in sampling rounds.
When a formula change means a packaging change
This is the case that catches growing brands. The formula is reformulated — a preservative system change, a higher active load, a new fragrance — and nobody revisits the packaging specification because the bottle has not changed.
Re-open the compatibility question whenever you change:
- Fragrance or essential oil load — the most common trigger we see
- Solvent or alcohol level by a meaningful margin
- pH across a threshold, particularly into acidic territory
- Viscosity enough to change how the product dispenses
- Preservative system, which can change the pack’s chemical environment
You do not need to re-run everything. A shortened elevated-temperature check on the new blend against the existing bottle will usually tell you what you need to know, and it costs a fraction of a failed shipment.
Frequently asked questions
Does alcohol damage plastic bottles?
High alcohol content raises stress-cracking risk, particularly in thin-wall PET with sharp internal corners. Below about 10% it is rarely a problem; above 20% it becomes a specification driver. The mitigation is usually geometry — more wall thickness and larger radii — rather than a different resin.
Which plastic is safe for an acidic formula?
For pH below 3, PETG or PP generally give more margin than PET over a long shelf life. Whatever the resin, avoid thin walls and sharp shoulders, and validate against the actual formula rather than water.
Do essential oils dissolve plastic bottles?
Not in the everyday sense of dissolving, but many essential and citrus oils act as stress-cracking agents and will craze a stressed bottle over weeks. HDPE and PP tend to be more susceptible than PET. Test the real blend.
How long does a compatibility test take?
A useful screening run is 30 days: production bottles filled with the real product, stored at elevated temperature with a room-temperature control, inspected at 7, 14 and 30 days. That fits inside most development timelines and catches the slow failures that matter.
Should I test the closure as well as the bottle?
Yes — closures fail more often than bottles. Liners, gaskets and pump internals are the first things many formulas attack. Test the assembled pack, filled with real product, in both upright and inverted orientations.
My formula changed slightly. Do I need to retest?
If the change touches fragrance load, alcohol level, pH across a threshold, or viscosity, yes — run a shortened elevated-temperature check. If the change is a minor adjustment within the same profile, a documented review of the specification is usually sufficient.
Next step
Send us your pH, alcohol content, viscosity and any essential oils or actives, and we will tell you which materials are viable before you spend anything on sampling — and if the blend is demanding, we will say so.
Shijin Packaging — factory-direct cosmetic and daily-chemical plastic bottles since 2003, operating 30+ automatic blow-molding lines and 10+ injection-molding machines across a 15,000 m² facility in Huizhou, Guangdong, with daily capacity of approximately 200,000 pieces.
- Website: https://shijinpackaging.com
- Full catalogue: https://shijinpackaging.com/catalog/
- Email: sales@shijinpackaging.com