PET Preform and Bottle Masterbatch: Colour, Clarity and IV

PET is the least forgiving of the common packaging polymers, and almost every difficulty traces back to one property: intrinsic viscosity. IV is a measure of molecular weight, and in PET it determines whether a preform can be stretch-blown into a bottle that holds pressure without stress-cracking.

Anything that reduces IV – moisture, excess heat, long residence time, contamination – reduces the bottle. Colouring PET means adding something to the melt without paying for it in IV.

Why PET is different

It hydrolyses. PET is hygroscopic and, at melt temperature, water chemically cleaves the polymer chain. This is not a cosmetic problem like the silver streaking moisture causes in other polymers – it is permanent chain scission. The polymer is shortened and cannot be recovered by drying afterwards.

Resin must be dried to very low moisture before processing. The same applies to anything else entering the melt, including the colour concentrate. An undried masterbatch on a hygroscopic carrier carries water directly into the process, and the IV loss it causes will be blamed on the dryer.

It generates acetaldehyde. Thermal degradation of PET produces acetaldehyde, which migrates into the bottle contents and imparts a detectable off-taste – a serious problem for water in particular, where there is no flavour to hide it. AA generation rises with melt temperature and residence time, so anything requiring higher processing temperature or longer residence carries a cost.

It is clear, and clarity is unforgiving. In an opaque part, a small agglomerate is invisible. In a clear preform wall it is a visible defect, and in the stretched bottle wall it is a stress concentration.

It is stretch-blown. The preform is reheated and biaxially stretched. Anything that changes how the preform absorbs infrared during reheat changes how it stretches – which is both a problem and, as below, an opportunity.

Carrier: the central question

In polyolefins, the carrier is chosen to match the host polymer. In PET, all the obvious options have drawbacks.

A PET carrier is compatible and dissolves cleanly into the melt. It must be dried exactly as the resin is, and it needs to be made from a suitable IV grade.

A polyolefin carrier is a foreign phase in a clear polymer. It does not dissolve, it can cause haze, and at any meaningful loading it compromises clarity. It also complicates recycling, because polyolefin contamination is precisely what bottle-to-bottle recyclers work to remove.

Non-carrier and low-carrier systems – liquid colour, or highly concentrated systems using compatible dispersants – minimise the foreign material entering the melt. Liquid colour is common in PET for exactly this reason, dosed directly into the throat.

The right answer depends on whether the bottle must be clear, and on where it goes at end of life. For a clear bottle destined for a bottle-to-bottle stream, minimising non-PET material is not just a clarity question – it is a recyclability question, and increasingly a regulatory one under EPR recycled content obligations.

Whichever route you take, the carrier and its moisture handling belong in writing on the specification. This is a specific case of the general rule in how to write a masterbatch specification.

The four things PET concentrates actually do

### 1. Colour

Amber and green for UV and light protection, blue and other shades for brand identity, and white for opaque containers such as dairy.

The constraint is that PET is processed hot. The pigment must be thermally stable at PET processing temperature – considerably above polyolefin range – without shifting shade or degrading. A pigment perfectly good in polypropylene may not survive PET.

### 2. Blue toner

PET has a natural yellowish cast, and recycled PET has considerably more. A very small quantity of blue or violet toner shifts the perceived hue back toward neutral, so the bottle reads as water-clear rather than faintly yellow.

The mechanism is the same as optical brightening but achieved subtractively: the toner absorbs slightly in the yellow region, removing yellowness at the cost of a very small loss in overall lightness. It is a trade the eye strongly prefers – a marginally darker neutral looks cleaner than a brighter yellow. Related reading: optical brighteners in plastics.

Toner dosage is tiny and precision matters. Too much and the bottle reads visibly blue.

### 3. UV blocking

PET transmits ultraviolet, and many bottle contents degrade under it – edible oils, vitamins, some beverages, pharmaceuticals. A UV blocker in the preform cuts transmission below a specified wavelength.

Two routes: an amber or green pigmentation, which is visually obvious and traditional, or a transparent UV absorber that leaves the bottle clear while blocking UV. The second commands a premium and is what brands choose when they want the product visible.

Specify the requirement as percentage transmission at a stated wavelength through the finished bottle wall, not through the preform. Wall thickness changes after stretching, and it is the bottle that has to protect the contents.

### 4. Reheat additives

This one is specific to PET and worth understanding because it pays for itself.

In stretch blow moulding the preform is reheated by infrared lamps. A reheat additive – a very finely divided infrared-absorbing material, typically carbon-based or an inorganic such as iron oxide – increases IR absorption so the preform heats faster.

The benefits are direct: shorter oven residence, faster cycle, lower energy consumption per bottle, and more even heating through the wall.

The cost is optical. Any IR absorber also absorbs some visible light, so the bottle loses a little clarity and gains a slight grey or amber cast. The whole art is in loading: enough to gain the cycle, little enough that the haze stays below what the customer will accept.

This is a genuine engineering trade-off with a real financial answer, and it is worth calculating rather than guessing. A high-volume water bottler and a premium spirits bottler will land in different places.

Recycled PET

Bottle-to-bottle rPET is now central to Indian packaging strategy, and it changes the colour problem in three ways.

The base is yellower and greyer. More toner is needed, and there is a limit beyond which you are darkening rather than neutralising.

IV is lower and variable. Mechanical recycling costs IV; solid-state polycondensation recovers some. Either way, the feedstock varies more than virgin, and a formulation must tolerate that variation.

Contamination is present. Residual PVC, polyolefin caps, adhesive and colourant from previously coloured bottles all survive into the flake at some level. This is the strongest argument against coloured PET bottles: a green or blue bottle contaminates the clear stream and reduces the value of everyone’s recyclate. Several markets now price that in.

Our approach to variable recycled feedstock is set out in masterbatch for recycled and PCR plastic and in our recycling masterbatches range.

Specification checklist

– Resin IV and grade, and whether virgin, rPET or a blend – Application – water, CSD, hot fill, edible oil, pharma – since each sets its own AA and barrier requirements – Clarity requirement, stated as haze rather than as an adjective – Colour standard with illuminant and tolerance, per Delta E – UV blocking as percent transmission at a wavelength, through the bottle wall – Reheat requirement, and the haze you will accept in exchange – Carrier system and its moisture specification – Food contact and regulatory status – Recyclability constraints, if the bottle enters a bottle-to-bottle stream

Talk to us

We supply PET masterbatches for preform and bottle production – colour, toner, UV blocking and reheat – manufactured under ISO 9001:2015.

Send us your resin IV, your application, your clarity target and your cycle time and we will formulate against them. If you are moving to rPET and the bottle has started reading yellow, tell us the recyclate source too, because the correction depends on it.

Related: packaging, pharma, cosmetics, sustainability.

FAQs

Why does PET need special masterbatch? Because PET is hygroscopic and hydrolyses at melt temperature, is processed far hotter than polyolefins, must often stay clear, and depends on intrinsic viscosity for bottle performance. A standard polyolefin-carrier masterbatch introduces moisture risk, a foreign phase and haze.

What is intrinsic viscosity in PET? A measure of molecular weight. Higher IV means longer chains and a preform that can be stretch-blown into a bottle able to hold pressure without stress cracking. Moisture, excess heat and long residence time all reduce it permanently.

What is a reheat additive in PET preforms? A finely divided infrared-absorbing material that makes the preform heat faster under the blowing oven lamps, shortening cycle time and reducing energy per bottle. The trade-off is a small loss of clarity and a slight grey or amber cast, so loading has to be optimised.

Why do PET bottles have a blue toner? Because PET has a natural yellowish cast, and recycled PET considerably more. A very small amount of blue or violet absorbs in the yellow region and shifts the perceived hue toward neutral. The bottle loses a fraction of lightness and looks distinctly cleaner.

What is acetaldehyde in PET and why does it matter? A degradation product formed when PET is thermally stressed. It migrates into the contents and gives a detectable off-taste, which is most damaging in bottled water where no other flavour masks it. Generation rises with melt temperature and residence time.

Can coloured PET bottles be recycled? Technically yes, but they reduce the value of the recyclate stream, because colour cannot be removed and a coloured flake contaminates clear rPET. Several markets now discourage or penalise coloured PET bottles for this reason.

Does masterbatch need drying before use in PET? If the carrier is hygroscopic, yes, to the same standard as the resin. Water entering the melt causes hydrolytic chain scission and permanent IV loss. This is a common hidden cause of IV loss that gets blamed on the resin dryer.

How should UV protection for a PET bottle be specified? As a maximum percentage transmission at a stated wavelength, measured through the finished bottle wall rather than the preform. Wall thickness changes during stretching, and it is the bottle that has to protect the contents.