Engineering · Formula Compatibility

Formula Compatibility: Matching the Container to What Goes Inside It

What water-based, acidic, alkaline, alcoholic, essential-oil and hot-fill products do to a polymer — and why the answer to every one of them has to be tested rather than assumed.

Quick answer

Compatibility is a system question, and no material is unconditionally safe for any formula

Plastic packaging fails on chemistry far more often than it fails on mechanics. The failures are slow: a bottle that passes sampling and cracks in a warehouse three months after the factory has been paid. The reason is that compatibility is not a property of the polymer alone. It is the result of the formulation, the concentration, the contact and storage temperature, the time in contact and the mechanical load the pack carries — stress cracking in particular needs both chemistry and stress to happen.

The rule that governs this page

Actual material/formulation compatibility must be validated against the real formula, concentration, temperature, storage conditions and intended use.

This is why nothing here is written as a guarantee. What follows names the mechanism each formula family uses, the candidates worth evaluating, and the questions a test has to answer.

Variables that decide it

Materials molded

Test routes: soak and stress

±2%

Weight tolerance

ISO 9001

Quality system

Assembled

Closure test basis

Mechanism

Why a polymer fails: the five failure modes

Almost every compatibility complaint we see is one of five mechanisms. Knowing which one is at work tells you which material is even worth evaluating, and which test will reproduce it.

Failure mode	What is happening	What it looks like in the field
Environmental stress cracking	Chemical attack and mechanical stress act together, opening and growing micro-cracks	Vertical cracks or crazing that appear weeks or months after filling, often starting at the base or shoulder
Chemical attack	The formulation reacts with the polymer itself, breaking chains or dissolving additives	Softening, bloating, loss of gloss, a sticky surface, or a collapse in stiffness
Permeation and weight loss	The product migrates through the wall, or the wall absorbs it	Fill level dropping in storage, shrinkage, panelling, or a change in product concentration
Additive extraction and discolouration	Plasticiser, stabiliser or colourant is drawn out of the polymer	Yellowing, colour shift, or a haze that deepens over time
Closure and dispenser attack	The elastomer or the pump components are attacked, not the bottle	A pump that stiffens, weeps or stops priming — which is a compatibility failure that looks like a dispensing failure

The fifth one is worth emphasising. A compatibility problem is often reported against the bottle when the part that failed is the closure, the gasket or the pump chamber. That is why we test the assembled unit: bottle, closure and dispenser together, filled and held.

Formula families

The formula families we are asked about most

For each family: what it attacks, which materials are candidates worth evaluating, and what the test has to settle. Nothing below is a safety statement about your specific product.

Family 01

Water-based formulas

What it attacks

Generally the mildest family. The risks are additive extraction over long storage, and loss of actives through the wall if the formula is sensitive.

Candidates worth evaluating

All five primary materials — PET, HDPE, PE, PETG and PP — are candidates. This is the one family where appearance, not chemistry, normally decides.

Consider when

You want clarity, in which case PET or PETG; or you want a lower unit cost and more chemical headroom, in which case HDPE, PE or PP.

Depends on

Preservative system, surfactant load, pH and the storage temperature. Water-based is not the same as inert.

Family 02

Acidic formulas (roughly pH 3–5)

What it attacks

Acids attack PET more readily than polyolefins, and acidic products with a high active load can attack the polymer while leaving colour and gloss apparently unchanged.

Candidates worth evaluating

HDPE, PE and PP are the usual candidates. PET and PETG remain candidates at low acid load and short contact time.

Consider when

The product is a peel, an AHA/BHA treatment or a vitamin C serum at the low end of the pH range, where the acid load and the packaging are usually evaluated together.

Depends on

Which acid, at what concentration, at what pH — two formulations at the same pH can behave completely differently.

Family 03

Alkaline formulas

What it attacks

Strong alkali hydrolyses PET and can attack PETG. This is the clearest material split in the whole table: sodium hydroxide is the reason a clear PET bottle is often the wrong pack for a soap.

Candidates worth evaluating

HDPE, PE and PP are candidates. PET is not a candidate without a test at the specific alkali load.

Consider when

The fill is a soap, an ammonia-based cleaner or a high-pH gel — the family where the polyolefins are chosen for chemistry rather than for cost.

Depends on

Alkali type, concentration, contact time and temperature. Ammonia adds a permeation question on top of the chemical one.

Family 04

Alcohol-containing formulas

What it attacks

Ethanol and other alcohols soften PETG and can trigger stress cracking in it under load. PET is more tolerant but not immune at high alcohol content and long contact.

Candidates worth evaluating

HDPE, PE and PP are the candidates. PET is a candidate at moderate alcohol content; PETG needs a specific test before it is specified.

Consider when

The fill is a sanitiser, a toner or an aftershave at a significant ethanol percentage — and the pack carries a load, for example a full bottle stacked in a carton.

Depends on

Alcohol type and percentage, the presence of water, the closure elastomer, and how much stress the pack carries.

Family 05

Essential oils and fragrance concentrates

What it attacks

Terpenes and fragrance actives are solvents. They attack PETG most readily of the five primary materials, and they can soften PET, extract additives and migrate into the closure elastomer.

Candidates worth evaluating

HDPE, PE and PP are candidates. PET is a candidate for diluted fragrance; PETG needs a specific soak test before it is specified.

Consider when

The product is an undiluted essential oil, a fragrance concentrate, or a high-load fragrance in a pack that will stand for months.

Depends on

Terpene content and load, the diluent, and the storage temperature. This is the family where "it looked fine at sampling" causes the most expensive failures.

Family 06

Hot-fill formulas

What it attacks

Hot fill is a thermal problem more than a chemical one. Above the material’s fill window the container distorts, and as the contents cool the internal pressure drop pulls the side walls in unless the geometry has vacuum compensation panels.

Candidates worth evaluating

HDPE and PP for heat resistance; PET only with a dedicated heat-set grade, heat-set tooling and a modified preform; PETG to roughly 70–75 °C.

Consider when

The fill temperature is above roughly 70 °C, or the pack has to be filled hot for microbiological reasons and stay sealed.

Depends on

Fill temperature, cooling profile, headspace and the closure. A hot fill is a change to the specification and the tooling, not a resin swap.

Family 07

Cleaning formulations

What it attacks

Surfactants, solvents, bleach and ammonia together. Surfactants are the classic cause of environmental stress cracking in PE and HDPE, which is why the material has to be chosen for stress-crack resistance rather than for stiffness alone.

Candidates worth evaluating

HDPE and PE are the candidates, with PP for components. PET is not a candidate for alkaline or solvent-heavy cleaners.

Consider when

The fill is a household cleaner, a degreaser or a detergent — the family where an opaque polyolefin pack is chosen because it is the right chemistry, not because it is cheap.

Depends on

Surfactant type and load, solvent content, alkali or bleach concentration, and whether the pack sits under load in a stacked carton.

Family 08

Personal care formulas

What it attacks

The widest family and the least predictable, because it spans water-based lotions, acidic treatments, alcohol toners and essential-oil blends under one label.

Candidates worth evaluating

All five primary materials are candidates, and the choice is normally made on appearance and cost after the chemistry has been narrowed.

Consider when

The pack has to be seen, in which case PET or PETG; or the formula is at the aggressive end, in which case HDPE, PE or PP.

Depends on

Which sub-family the product actually belongs to. "Personal care" describes a shelf, not a chemistry — the formulation has to be classified before the material can be.

Notice what the eight families have in common: in every case the answer ends at depends on, and the thing it depends on is a number that only your formula has.

Testing

How compatibility is actually validated

There are two test routes and most projects need both. Neither is a paper exercise, and neither can be replaced by a material data sheet — a data sheet describes the polymer, not your product.

Classify the formula

Identify pH, surfactant type and load, solvent and alcohol content, essential-oil load, oxidising agents and the fill temperature. This is a desk exercise and it narrows the candidates before anything is molded.

Soak test at temperature

Fill sample containers with the real formula, seal them with the real closure, and hold them at ambient and at an elevated temperature to accelerate the chemistry. The control is the same bottle filled with water.

Measure, do not just look

Weight loss or gain, dimension and volume change, fill level, stiffness, colour and haze, and closure function — measured against the control rather than judged by eye.

Stress test the assembly

Fill, close to the specified torque, stack or load as the distribution will, and hold — that is what converts a chemical attack into environmental stress cracking. A stressed sample fails where an unstressed one looks fine.

Reproduce the failure, then fix the specification

The output is a specification decision: a different resin, a different wall thickness, a barrier layer, or a closure with a more resistant elastomer.

What we need from you to run this: a sample of the actual formula, a sample closure or dispenser if you have one, and the storage, transport and shelf conditions the pack has to survive. Without the formula itself we can test the material but we cannot test the compatibility.

Limits

Why we will not give a verdict on a material alone

A supplier who answers "HDPE is safe for that" on the basis of a formulation summary is not giving you information, they are transferring risk. The gap between a formula summary and a formula is exactly where compatibility failures live.

▸Concentration is not in the summary. "5% surfactant" and "15% surfactant" are different material decisions.
▸Temperature is often left unspecified. Compatibility that holds at 20 °C for two years can fail at 40 °C in six weeks in a warehouse.
▸Contact time is usually understated. A pack that spends eighteen months on a shelf is a different test from one filled and shipped in a month.
▸Mechanical load is invisible in a summary. Environmental stress cracking requires stress, and stress depends on stacking, torque and shape.
▸The closure is a separate material. The elastomer in a pump or liner has its own compatibility profile.

So the honest output of a compatibility review is a shortlist of candidates plus a test protocol, not a single answer. That is also the shape of the deliverable we send: a specification and a test result, both naming the formula they were run against.

Materials in detail

PET →HDPE →PE →PETG →PP →PCR PET →PCR HDPE →

Formula compatibility: buyer questions

What does formula compatibility mean in packaging?+

It means the container, the closure and the dispenser survive contact with the product through storage, transport and use without cracking, softening, discolouring, permeating or losing seal. It is a property of the finished assembly against a specific formula — not a property of the polymer on its own.

Which plastic is safe for an alkaline cleaner?+

HDPE, PE and PP are the candidates worth evaluating. Alkali hydrolyses PET, so PET is not a candidate without a test at the specific alkali load and concentration. The right answer still depends on the surfactant system, the solvent content and the storage temperature, so it has to be confirmed on the real formula.

Can PET hold alcohol-based products?+

PET tolerates moderate alcohol content far better than PETG, which softens and can stress-crack under load when the alcohol level is significant. Both depend on the alcohol percentage, the water content, the storage temperature and how much mechanical load the pack carries. Test the formulation rather than the material class.

Why did my clear bottle crack weeks after filling?+

The usual cause is environmental stress cracking: chemical attack and mechanical stress acting together. It is the failure mode that most often escapes a short sampling cycle, because a stressed sample held for weeks fails where an unstressed one looks perfect. It is reproduced by filling and loading the real assembly and holding it, not by inspecting the bottle.

Do you test compatibility for me?+

We run compatibility and stress tests on request, using your formula and your closure where you can supply them. Tell us the storage and transport conditions the pack has to survive and we will agree the protocol — fill, temperature, load and duration — before we start.

Can an essential oil go in a PETG bottle?+

PETG is the most solvent-sensitive of the five primary materials, and terpenes are solvents, so undiluted essential oils and fragrance concentrates are the highest-risk fill for it. Depends on the terpene load and the diluent: diluted fragrance in PETG is common, undiluted oil in PETG needs a specific soak test first.

What happens to the pack in a hot fill?+

Above the material’s fill window the container distorts, and as the contents cool the pressure drop pulls the walls in unless the geometry has vacuum compensation panels and the resin is a heat-set grade. It is a change to the resin, the tooling and the geometry at the same time.

What do you need from me to assess compatibility?+

A sample of the actual formula, the fill temperature, the storage and transport conditions, the closure or dispenser you intend to use, and the shelf life you have to support. From that we narrow the candidates to a shortlist and propose the test protocol.

Send the formula, not a summary of it

The difference between a formula and a formula summary is exactly where compatibility failures live. Send the product, the fill temperature and the shelf conditions and we will propose the test.

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Part of Packaging Engineering: Bottles, Closures and Decoration | Shijin

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