PET black masterbatch: gels, high filter pressure and where they come from
PET is the most demanding resin in this category to colour, and the reason is not the pigment. It is that PET is a condensation polymer processing at high temperature with a very low tolerance for water, so anything the masterbatch brings into the melt has a shorter path to becoming a defect than it would in PE or PP.
If you are seeing gels, specks, or a screen pack pressure that climbs through a run, this is the order we work through it — and why the first item on the list is nearly always the answer.
1. Moisture, because PET does not simply hold water — it reacts with it
In PE or PP, moisture is a physical problem: it turns to steam, and steam makes bubbles and streaks. Annoying, visible, and fixable with drying.
PET is different. It is a condensation polymer, which means water attacks the ester bonds themselves. At processing temperature, absorbed moisture hydrolyses the chain: molecular weight falls, and the degraded material that results behaves as gel. That is why PET is dried to a far lower figure than polyolefins before processing — and it is why a drying regime that has always worked on your PE film line tells you nothing useful about your PET line.
The consequence for masterbatch is direct: a masterbatch entering a PET melt has to be at least as dry as the resin it is going into. If your resin is dried on the line but the masterbatch is fed from an open bag, you have introduced the one thing PET cannot tolerate, at the point where it can do the most damage.
The two controls that matter most:
- Dry the masterbatch, not just the resin. Where the grade is hydroscopic in its carrier system, dry it to the TDS figure. Where a grade is supplied dry and sealed, keep it sealed until the hopper.
- Check moisture on receipt, not on the CoA alone. The CoA figure was measured at our gate. After weeks in a container and a humid warehouse, the number that matters is the one you measure.
2. Filtration: what a rising screen pack is telling you
A PET line usually runs a screen pack because PET end uses — fibre, film, sheet, strap — cannot tolerate a gel or a hard particle. That screen is also the most useful diagnostic instrument you have, if you read it as a trend rather than as a pass/fail.
| What you see | Most likely cause | Where to look first |
|---|---|---|
| Pressure rises steadily from the start of the run | Undispersed carbon black agglomerates being screened out continuously | The masterbatch lot's filter pressure value; the let-down ratio; the line's shear |
| Pressure climbs specifically after a masterbatch change or a new bag | Moisture, or a lot-to-lot dispersion difference | Masterbatch drying, then the batch number |
| Pressure is fine but gels appear in the product | Gel passing the screen, or forming downstream of it | Resin drying and residence time; degradation in dead spots of the screw |
| Pressure spikes, then partly recovers | Large discrete contaminants, not a dispersion trend | Handling and conveying — foreign material entering between bag and hopper |
The pattern matters more than any single reading. A screen pack that climbs slowly and predictably across a run is a dispersion conversation. One that spikes is a contamination conversation. They have different answers, and treating them as the same problem wastes a lot of time.
3. Filter pressure value: what the number actually predicts
Filter pressure value (FPV) is the lab-scale version of what your screen pack is doing in production: a defined quantity of masterbatch is extruded through a screen of defined mesh, and the pressure rise is recorded. Done to a stated method — EN 13900-5 is the common one — it gives a number you can compare between lots.
What it predicts, and what it does not:
- It predicts screening behaviour well. A low FPV says the masterbatch will not blind a pack quickly. For PET, this is the single most useful figure on the lot report.
- It does not predict appearance. A masterbatch can screen cleanly and still give a poor surface if the dispersion is coarse in a way the screen does not capture. FPV and visual dispersion answer different questions.
- It is only comparable within one method. FPV figures from different methods, or measured at different let-down ratios, are not comparable numbers. This is why "our FPV is lower than theirs" is an argument to check the method, not to switch supplier.
4. The other three sources, in the order we check them
Contamination between bag and hopper
PET's high screen load means small amounts of foreign material matter more than they would in a polyolefin line. Open bags, dusty conveying systems, a hopper that has held a different grade — these show up as spikes rather than trends. Worth ruling out before blaming the masterbatch, because it is the cheapest thing to fix.
Carrier compatibility
A carrier that does not melt and mix with the PET melt leaves a separate phase, and a separate phase is something a screen pack will catch. For PET this is a real constraint, not a theoretical one — which is why a PET-specific grade exists rather than a universal one. If you are running a PE-carried grade in a PET line and seeing screen build-up, that is the first thing to question. Our PET grade is compounded for this reason.
Shear, and the residence time that goes with it
Dispersing carbon black takes work, and at PET's high screen load there is less room for a marginal mixing section than in a film line. The related trap is residence time: PET degrades when it sits too long at temperature, so a screw configuration or an output rate that leaves material loitering in a dead spot can generate the gel it is being blamed for not dispersing. If your screen build-up correlates with reduced output or with startup, residence time is worth a look.
How to run the comparison that actually settles it
When a buyer suspects the masterbatch, the test is straightforward — provided the process is held constant:
- Control run. Straight PET, no masterbatch, for a defined period. Record screen pack pressure against time and output rate.
- Test run. Same resin, same drying, same screen, same output, same duration — with the suspect lot at the ratio you actually run.
- Compare the slopes. Not the absolute pressures, the slopes. A masterbatch problem shows as a steeper climb in the test run.
Two rules that make the result mean something. Change one variable. Switching masterbatch and drying condition at the same time tells you nothing, and it is the most common way a useful test is wasted. Record the slopes, not just the endpoint — the rate of change is the signal, and the final figure alone hides it.
If both runs climb equally, the masterbatch is not the problem and changing supplier will not help. That is a useful answer too: it redirects effort to drying, residence time or handling, which is where the next hour should have been spent anyway.
What we put into a PET grade, and what we report
Our PET grade is compounded on a carrier compatible with the PET melt rather than a polyolefin carrier, so there is no separate phase for a screen pack to catch. Every lot reports carbon content, MFI, moisture and filter pressure value, with the method stated — because an FPV figure without its method is not a comparable number.
If you are troubleshooting a PET line, send us the symptoms, the drying figures at both ends, and the screen behaviour. In most of the cases we see, the answer is in the drying or the residence time rather than the lot — and we will tell you that plainly rather than sell you a replacement.
Questions buyers ask
What causes a high filter pressure value with PET black masterbatch?
In practice it comes down to four things: moisture, because PET hydrolyses above a very low water content and degraded polymer forms gel; contamination, including carbon black agglomerates that are not dispersed finely enough to pass the screen pack; a carrier that does not match the PET melt, leaving a separate phase that screens out; and processing shear insufficient to break down the agglomerates carried into the melt. The order matters, moisture first, because it also accelerates the other three.
Why is PET more sensitive to moisture than PE or PP?
PET is a condensation polymer, so water does not just sit in it, it breaks it. At processing temperature the ester bonds hydrolyse and the molecular weight falls, which produces degraded, cross-linked material that shows up as gel and raises filter pressure. Polyolefins do not degrade this way, which is why a drying figure that is comfortable for a PE film line is entirely inadequate for PET. PET is normally dried to a very low moisture figure before processing, and the masterbatch entering that melt has to be at least as dry as the resin it is going into.
Can a bad masterbatch lot cause gels on its own?
It can, and the honest way to test that is to hold the process constant. Run a straight PET control for a defined period with no masterbatch, then the same resin with the suspect lot, at the same drying conditions and the same screen pack. If filter pressure rises on the second run and not the first, the masterbatch is implicated. If both runs climb, the moisture or the contamination is coming from somewhere else in the process and changing supplier will not fix it.
What should a PET-grade black masterbatch contain?
A carrier compatible with the PET melt rather than a PE carrier, a carbon black with controlled agglomerate size, a moisture figure low enough not to introduce water into a condensation polymer, and dispersion validated by filter pressure testing rather than by a visual assessment. Ask for the filter pressure value on the lot, not just the carbon content.
Related pages
- Carrier compatibility by resin system — why a mismatched carrier leaves a separate phase
- PET black masterbatch for fibre, sheet and strap — carrier and loading specifications
- Black masterbatch defects: gels, streaks and specks — the general troubleshooting order
- How to read a black masterbatch CoA — including what a filter pressure value does and does not tell you
- A 30-minute incoming inspection protocol — the receiving checks that catch this early
- All 12 black masterbatch grades — carriers, loadings and data sheets