Polyethylene terephthalate, widely known as PET, is one of the most processed thermoplastics in the packaging and sheet industries, yet it is also one of the most unforgiving when it comes to moisture. The single most common cause of off-specification preforms, brittle bottles, and unstable extrusion is not a faulty screw or a worn barrel but a breakdown in the hydrolysis control chain that begins at the dryer and ends at the vacuum vent. When PET is melted in the presence of water, the ester bonds that give the polymer its chain length begin to cleave, and the intrinsic viscosity (IV) falls. Because IV is the property that determines whether a preform can be stretch-blow molded into a strong, clear bottle, every maintenance decision on a PET line should be judged by one question: does this action protect the molecular weight of the resin?
This guide is written for plant engineers, maintenance leads, and production supervisors who run PET on injection molding machines, stretch blow molding lines, sheet extrusion lines, and PET recycling and pelletizing systems. We will walk through the chemistry of hydrolysis, the drying parameters that prevent it, the maintenance routines that keep a desiccant dehumidifying dryer performing, the conveying and surge bin practices that stop dry resin from re-absorbing humidity, the venting and melt temperature controls on the extruder or injection unit, the test methods that confirm whether degradation occurred, the defect signatures that reveal a failing link, the special handling of rPET and regrind, and the shutdown and restart procedure that protects the screw during idle periods. Wanplas, with its network of specialized factories, supplies PET blow molding machines through its YuDa factory and PET washing and recycling systems through its Polyretec factory, and the maintenance principles below apply across all of these PET processing routes.
1. Understanding PET Hydrolysis Degradation: The Chemistry of IV Loss
Hydrolysis is a chemical reaction in which water breaks the ester linkage (–CO–O–) of the PET chain into two shorter chains, each terminated by a new carboxyl end group. In the solid state below the melt, this reaction is negligible because molecular mobility is low and free water is essentially absent in properly dried resin. The danger appears once the polymer crosses its melting point and enters the molten phase inside the barrel or injection unit, where water that was not removed during drying becomes mobile and reacts directly with the melt. The result is a reduction in average molecular weight, which is measured and reported as a drop in intrinsic viscosity, expressed in deciliters per gram (dL/g).
The relationship between moisture and IV loss is not linear and is strongly influenced by residence time and melt temperature. A useful engineering rule is that every doubling of residual moisture roughly doubles the extent of chain scission for a given thermal history, but temperature and time act as multipliers. At a melt temperature of 275 degrees Celsius, resin that enters the screw at 50 ppm residual moisture may lose only a few hundredths of a dL/g, while the same resin at 300 ppm can lose far more because the water has many more reactive sites and a much longer effective reaction window as it travels through the metering zone, die, and any stagnant flow channels. This is why a dryer that is only slightly out of specification can quietly cost an entire shift of product.
Beyond IV, two secondary indicators rise when hydrolysis occurs. The first is acetaldehyde (AA), a low-molecular-weight by-product of PET degradation that is of particular concern in beverage bottles because it imparts a fruity off-taste and aroma. The second is the carboxyl end group (CEG) concentration, measured in equivalents per ton (mol/t or meq/kg). As ester bonds break, CEG increases, and a rising CEG value is one of the clearest fingerprints that hydrolysis, rather than simple thermal oxidation, is responsible for a quality problem. A maintenance team that tracks IV, AA, and CEG together can usually tell whether a defect came from moisture, from excessive barrel temperature, or from contamination.
The table below translates the chemistry into production reality. It shows how residual moisture at the feed throat maps to expected IV loss and to the kind of defect an operator is likely to see on the line. These are representative engineering ranges for bottle-grade PET around an incoming IV of 0.80 dL/g; absolute numbers shift with grade, colorant, and residence time, but the trend is robust and should be used as an early-warning map.
| Residual Moisture at Feed (ppm) | Typical IV Loss from 0.80 dL/g | AA and CEG Trend | Likely Product Defect |
|---|---|---|---|
| 30 to 50 ppm | 0.80 to 0.78 dL/g (minimal) | AA low, CEG stable | None; good clarity and strength |
| 100 ppm | 0.80 to 0.75 to 0.76 dL/g | AA rising, CEG up slightly | Slight haze, mild injection pressure drift |
| 200 ppm | 0.80 to 0.74 dL/g | AA clearly elevated, CEG up | Yellowing tendency, brittle wall spots |
| 300 ppm | 0.80 to 0.72 dL/g or below | AA high, CEG high | Brittle bottles, burst on stretch blow, silver streaks |
The practical conclusion is simple: moisture control is the master variable. An injection molding machine with a perfectly tuned screw and a brand-new barrel will still produce degraded PET if the resin arrives wet, because no downstream equipment can re-form the ester bonds that water already broke. This is why the drying system deserves the most disciplined maintenance on the entire PET line, and why the next sections treat it as the foundation of hydrolysis prevention rather than as a peripheral utility.
2. The Drying System as the First Line of Defense
For PET, ambient or hot-air drying is not sufficient because the surrounding air itself carries too much moisture to reach the low equilibrium levels the polymer requires. The correct machine is a desiccant dehumidifying dryer, which passes process air through a bed of molecular sieve or activated alumina to strip humidity down to a very low dew point before the air is heated and blown through the resin bed in the drying hopper. The single most important number on this equipment is the dew point of the process air, and for PET the target is a dew point at or below minus 40 degrees Celsius (minus 40 degrees C). Many operators watch the drying temperature and ignore the dew point, but dew point sets the floor on achievable moisture; if the dew point is poor, no amount of extra temperature will get the resin dry.
The drying temperature for PET normally sits in the range of 160 to 180 degrees Celsius. Going below this range slows diffusion so much that the required residence time becomes impractical, while exceeding it risks thermal oxidation and localized sticking. Within the hopper, the resin must remain long enough for moisture to diffuse from the pellet core to the surface and be carried away by the dry air; typical residence time is four to six hours for virgin bottle grade, and longer for flake or regrind. The airflow rate is another parameter that is easy to neglect: the drying hopper should see roughly 0.06 to 0.1 cubic meters per minute per kilogram per hour of throughput (0.06 to 0.1 m3/min per kg/h). Too little air and the surface stays saturated; too much and the hopper can fluidize or channel, creating wet pockets that pass straight into the machine.
Because PET absorbs moisture quickly from the air and because amorphous PET pellets tend to stick together and agglomerate at drying temperatures, most PET lines use a crystallizer ahead of the dryer. The crystallizer raises the resin to a temperature band of about 165 to 175 degrees Celsius, which induces crystallization and prevents the pellets from clustering into clumps that would block airflow in the drying hopper. Crystallization is not the same as drying, but it is the enabling step that makes stable, channel-free drying possible. On lines processing PET flakes from a washing and recycling system, crystallization is even more important because flake geometry and bulk density differ from pellet, and agglomeration can occur earlier and more severely.
The table below consolidates the drying window into a single reference that maintenance and setup staff can post at the machine. These are the normal operating limits for bottle-grade and sheet-grade PET; food-grade or high-IV grades may prefer the tighter end of each range, and recycled feedstocks may require the longer residence time. Always confirm against the resin supplier data sheet, because some copolymer PET grades and some color masterbatches tolerate a narrower band.
| Parameter | Target / Window | Why It Matters | Failure Symptom |
|---|---|---|---|
| Dew point of process air | minus 40 degrees C or lower (target minus 40) | Sets the minimum achievable moisture | Resin never reaches ppm target despite long residence |
| Drying temperature | 160 to 180 degrees C | Drives diffusion rate of moisture out of pellet | Below: under-dried; above: oxidation, sticking |
| Residence time in hopper | 4 to 6 hours (longer for flake/regrind) | Time needed for core moisture to diffuse out | Wet core, fluctuating IV shot to shot |
| Hopper airflow | 0.06 to 0.1 m3/min per kg/h | Carries released vapor away from resin bed | Channeling, saturated surface, wet pockets |
| Crystallizer temperature | 165 to 175 degrees C | Prevents agglomeration before drying | Clumped pellets block hopper, uneven drying |
A note on instrumentation: the dew point transmitter is the most overlooked sensor on a PET drying loop. It should be mounted in the process air stream after the desiccant bed and before the heater, and it should be checked against a calibrated reference at least once a year. A drifting transmitter will report minus 40 degrees C while the real air is far wetter, giving the operator false confidence. Treat the dew point reading as a primary process variable, not as a diagnostic afterthought.
3. Drying System Maintenance Checklist
A desiccant dehumidifying dryer is not a fit-and-forget utility. Its performance degrades gradually through desiccant aging, heater decay, filter blockage, and air leaks, and because the degradation is slow the line can drift out of spec for weeks before a quality complaint appears. The maintenance routine below is built around the components that actually determine dew point and airflow, and it is organized so that a technician can complete most of it during a planned stop.
The molecular sieve bed is the heart of the dryer. Over repeated regeneration cycles the desiccant loses capacity through fracture, dusting, and contamination by volatiles desorbed from the resin, and its ability to reach minus 40 degrees C dew point declines. Under normal PET duty the bed typically lasts three to five years, but the decisive criterion is performance, not calendar age: when the regenerated bed can no longer hold a stable minus 40 degrees C dew point at design airflow, it should be changed. Some operations choose to change one of the twin beds on a staggered schedule so that total desiccant capacity never drops at once. After a bed change, the dew point must be re-verified across the full airflow range, because a partially packed or improperly sealed bed will channel air and defeat the purpose of the replacement.
The regeneration heater is the next common failure point. Electric heaters lose power output as elements age or as contactors pit, and a heater that delivers only 80 percent of its rated power extends the regeneration time and leaves the bed partially saturated. Measured heater power should be checked against the nameplate, and any decay beyond the manufacturer tolerance is a reason to service the heater bank. Return air filters are equally important: the air leaving the drying hopper carries fine PET dust, and if the return filter blinds, the pressure drop forces the process blower to work harder while actually reducing the volume of air that reaches the resin. Filters should be inspected on a fixed interval and changed when they show loading, not when the line already complains.
Insulation and air tightness are the silent killers of drying performance. The drying hopper and the process air duct should be insulated so that the heated, dried air does not cool and re-condense moisture onto the metal before it reaches the resin. Any breach in the insulation layer, or any leak in the duct joints, lets humid plant air intrude and destroys the dew point. Compressed air leaks in the dryer’s valving and the crystallizer’s pneumatics waste energy and can also draw humid air into the wrong stream. A simple soap-bubble or ultrasonic leak check of the air system during the planned stop catches most of these issues before they reach the product.
The drying hopper itself deserves attention to geometry and level. The hopper cone angle and the level control strategy determine whether resin flows evenly or forms stagnant zones where material sits far longer than the design residence time and then dumps into the machine in unpredictable batches. A level probe that drifts can let the hopper run nearly empty, exposing the feed throat to ambient humidity, or overfill, causing bridging. Both conditions produce the classic symptom of hydrolysis: an IV that wanders shot to shot even though the dryer setpoints look correct.
| Component | Maintenance Interval | Replacement / Action Criterion | Cost Tier |
|---|---|---|---|
| Molecular sieve desiccant bed | Inspect annually; full service 3 to 5 years | Dew point cannot hold minus 40 degrees C at design airflow | Medium |
| Dew point transmitter | Calibrate once per year | Reading drift beyond tolerance vs reference | Low |
| Return air filter | Inspect monthly; change when loaded | Visible dust loading or rising pressure drop | Low |
| Regeneration heater bank | Power check every 12 months | Output below nameplate tolerance | Medium |
| Hopper and duct insulation | Visual check quarterly | Cracked, wet, or missing insulation section | Low |
| Hopper level probe and cone | Check every 6 months | Erratic level, bridging, stagnant zones | Medium |
| Compressed air leaks in valves | Leak check every 6 months | Audible or ultrasonic leak at joints | Low |
Wanplas, as the main brand coordinating its specialized factories, recommends that PET lines keep a written drying log that records dew point, drying temperature, airflow, and residence time for every shift, with the annual desiccant service date flagged. When a quality issue later appears, this log is what lets the team separate a dryer problem from a machine problem in minutes rather than days. The shared after-sales program across the Wanplas factories includes spare parts support so that desiccant beds, heaters, and filters can be serviced on schedule without waiting for a breakdown.
4. Conveying and Surge Bin Management: Preventing Secondary Moisture Pickup
Even a perfectly tuned dryer can be defeated after the fact if the dried resin re-absorbs humidity during conveying and storage. PET is hygroscopic, and dry PET at 160 to 180 degrees C exiting the hopper will pull moisture from the surrounding air almost immediately once it cools in an open or leaky system. The maintenance focus in this part of the line is therefore to keep dried material sealed, warm where possible, and moving, and to prevent any unnecessary idle time in an open hopper.
Central conveying systems are efficient for large plants but introduce a specific risk: the resin travels through long pipe runs and intermediate surge bins where, if any seal is compromised, humid plant air is drawn in. A vacuum loader with a worn or cracked seal will not just lose suction; it will inhale moist air around the dried pellets, undoing the dryer’s work before the material reaches the machine. The seal rings and flap valves on vacuum loaders should be on a replacement schedule, not a run-to-failure schedule, because the failure mode is silent until the IV drops. Similarly, pipe elbows wear from abrasive PET flake and regrind; a thinned elbow eventually develops a pinhole that leaks air, and the leak path is exactly where dried material is most exposed. Elbows on flake and regrind lines deserve more frequent wall-thickness checks than those on virgin pellet lines.
The open machine hopper is the most common site of secondary moisture pickup. A dried pellet that drops into an open hopper and sits there for more than about twenty minutes can absorb enough surface moisture to matter, especially in a humid climate or during a rainy season. The countermeasure is to keep the machine hopper small and to feed it frequently from the dried side, or to fit a dried-air blanket (a small flow of minus 40 degrees C dew point air) over the machine hopper so that the exposed surface never sees plant humidity. Some operations also keep the machine hopper lightly heated or insulated to slow condensation. The rule of thumb to post at the line is simple: dry material should not linger in an open hopper longer than twenty minutes.
Surge bins between the dryer and the machine need the same discipline. If a bin is used to buffer production, it should be a closed, dried-air-purged vessel, and the residence time inside it should be counted as part of the total drying residence, not as free storage. A common mistake is to dry for five hours, then park the resin in a room-temperature surge bin for several hours before use; the resin equilibrates toward the bin environment and the effective moisture climbs back up. Where buffering is unavoidable, the bin should be purged with dried air and kept warm, and the material should be used on a first-in-first-out basis so that nothing sits long enough to re-wet.
For plants running PET through a washing and recycling system before re-extrusion, the conveyance from the crystallizer and dryer to the pelletizing line or extruder is especially sensitive. Flake has a larger surface area than pellet and re-wets faster, so the dried flake should move directly into the feed throat of the extruder or into a sealed, purged buffer. Wanplas’s Polyretec factory, which builds PET washing and recycling systems, designs these lines so that the drying and crystallizing steps feed the extrusion step with minimal open residence, because the economic value of the recycled pellet depends entirely on the IV it retains through the process.
5. Extrusion and Injection Equipment: Venting and Melt Temperature Control
Once dried PET reaches the screw, the equipment side of hydrolysis control is about two things: keeping the melt temperature in the safe window and removing any residual vapor or degradation by-product through venting. The dryer does the heavy lifting, but the screw, barrel, and vent are the last line of defense, and their condition determines whether a small moisture remainder becomes a major defect.
For PET, the melt temperature window is typically 265 to 285 degrees Celsius. Below this range the material may not be fully plasticized or may carry excessive viscosity that stresses the screw; above it, thermal oxidation accelerates and the margin against degradation narrows. Because hydrolysis rate climbs steeply with temperature, every degree above the necessary minimum is a risk, so the barrel profile should be set to the lowest temperature that still gives a clean, homogeneous melt and a stable cycle. A well-maintained temperature control system with calibrated sensors is essential; a sensor that reads 10 degrees C low lets the actual melt run 10 degrees C hot without the operator knowing, and that gap is often the difference between good and degraded product.
The venting system is where residual moisture and volatile by-products leave the melt. A vented barrel combined with a vacuum pump is the standard configuration: the melt passes a vent port where the reduced pressure pulls vapor and low-boiling species out of the polymer before it is pumped to the die or injection nozzle. The target vacuum is typically at or below minus 0.09 MPa (gauge), and the vacuum pump must be maintained so that it actually reaches that level at the vent. A vacuum line that has lost 20 percent of its capability will still look like it is working on the gauge at the pump but will be far weaker at the vent port, especially if the line is long or has leaks. The vent port itself must be kept clear; a clogged or flooded vent (where melt backs up into the port) both stops devolatilization and can cause material to spew from the opening.
Screw design matters for both mixing and residence time. PET is commonly run on a two-stage vented screw, where the first stage compresses and melts the resin and the second stage, after the vent, re-compresses and meters it to the die. A compression ratio in the range of 2.0 to 2.5 is typical for PET; too low and the melt is poorly worked and poorly degassed, too high and shear heating pushes the temperature past the safe window. Residence time must be controlled as well: long, stagnant areas in the screw or at the dead spots of the die let the melt sit hot and wet longer than intended. Regular inspection of the screw and barrel for buildup (distinct from general screw wear) and confirmation that the screw geometry matches the PET grade being run are part of keeping the residence time predictable.
The melt filtration and screen changer are the final safeguard against contaminants that could otherwise cause localized degradation or pressure instability. A continuous screen changer keeps pressure stable while removing gel, char, and foreign particles, and a stable melt pressure is what allows a consistent injection profile or extrusion rate. Pressure drift from a blinded screen looks very much like hydrolysis-induced viscosity change on the control panel, so the screen changer sequence and pressure trend should be reviewed whenever IV appears to wander. Keeping the filtration system in good order protects both product quality and the ability to diagnose true hydrolysis problems quickly.
For injection molding of PET preforms, the same principles apply at the injection unit: dried material, a melt temperature in the safe window, and where the machine is equipped with a vented barrel or a decompression and degassing option, proper use of that feature. The injection pressure stability that results from good drying and venting is itself a useful diagnostic: when dried, well-vented PET is run, injection pressure should be steady cycle to cycle, and a drifting pressure is a signal to check moisture, dew point, or venting before assuming a screw problem.
6. Testing and Verification: Measuring IV, Moisture, and Acetaldehyde
Maintenance without measurement is guesswork. A PET line that wants to avoid hydrolysis degradation needs a small, disciplined testing routine that confirms the chain is working at three points: the incoming resin, the dried resin at the machine throat, and the finished part or pellet. The tests below are the industry-standard methods, and the cost of running them is Low compared with the cost of a recalled batch or a lost customer.
Intrinsic viscosity is the headline test. It is most often measured by solution viscometry following ASTM D4603 or ISO 1628-5, in which a precisely weighed sample is dissolved in a solvent and the flow time relative to the pure solvent gives the IV in dL/g. Running IV on the incoming resin establishes the baseline, and running it on the dried material at the throat and on the final product shows whether chain scission occurred in your process. A drop of more than a few hundredths of a dL/g between incoming and finished part is the clearest evidence of hydrolysis, and the size of the drop points to how much moisture or how much excess thermal history was involved.
Moisture is measured directly on the dried resin, and the reference methods are Karl Fischer titration and ASTM D6869. Karl Fischer is the most common laboratory method and gives a precise ppm value that can be compared against the drying target. The best practice is to sample the resin as it enters the machine hopper, not at the dryer outlet, because that sample reflects the true condition after conveying. If the Karl Fischer result is above 50 ppm, the dryer or the conveyance system is the suspect, not the screw. A handheld or inline moisture meter is a useful supplementary check on the floor, but it should be periodically validated against a laboratory Karl Fischer result so that nobody trusts a meter that has drifted.
Acetaldehyde is measured by headspace gas chromatography, where a sample is sealed, heated to release volatile species into the headspace, and the AA concentration is quantified chromatographically. For beverage bottles, AA is both a quality and a regulatory concern, and a rising AA alongside a falling IV is a near-certain hydrolysis signature. The b* color value from a spectrophotometer is a simpler, faster indicator: as PET degrades, the yellowness index (b*) rises, so a trend of increasing b* on otherwise clear parts is an early warning that the degradation chain is active even before mechanical failure appears. Tracking b* on every production lot is an inexpensive way to catch a drifting dryer weeks earlier than visual inspection would.
| Test | Standard / Method | What It Reveals | Sampling Point |
|---|---|---|---|
| Intrinsic viscosity | ASTM D4603 / ISO 1628-5 | Chain scission, molecular weight loss | Incoming resin, throat, finished part |
| Moisture content | Karl Fischer / ASTM D6869 | Residual ppm at the machine | At machine hopper throat |
| Acetaldehyde | Headspace gas chromatography | Degradation by-product level | Finished bottle or preform |
| Color b* value | Spectrophotometry (yellowness) | Early yellowing trend from degradation | Finished part, per lot |
| Carboxyl end group | Titration (CEG, mol/t) | Confirms hydrolysis vs oxidation | Incoming and finished resin |
The discipline that turns these tests into prevention is trend tracking. A single IV or moisture reading is a snapshot; a logged trend across shifts is what reveals a dryer slowly losing dew point or a vacuum pump slowly losing suction. Wanplas’s YuDa factory, which builds PET bottle blow molding machines, and the broader Wanplas group both advise customers to keep these records as part of a quality system, because the records are what let a plant prove to a beverage customer that the bottles were made within specification and what let an engineer find the root cause of a defect in one shift instead of one week.
7. Common Defect Mapping: From Yellowing Preforms to Burst Bottles
Hydrolysis rarely announces itself as a number on a panel; it shows up as a defect on the line. The value of understanding the degradation chain is that each defect maps back to a specific failing link, and a good maintenance team can read the defect like a diagnostic code. The table below pairs the common PET defect with its most likely root cause in the hydrolysis chain, the test that confirms it, and the corrective action.
| Defect | Likely Root Cause | Confirmation Test | Corrective Action |
|---|---|---|---|
| Yellowing preform | High residual moisture plus excess barrel temperature | IV drop, rising b*, CEG up | Verify dew point and drying residence; lower melt temp |
| Brittle bottle wall | IV loss from hydrolysis reducing orientation strength | IV below spec, AA elevated | Improve drying; check vacuum vent performance |
| Burst on stretch blow | Low IV preform cannot sustain biaxial stretch | IV at or below lower limit | Re-dry resin; confirm throat moisture under 50 ppm |
| Melt viscosity fluctuation | Inconsistent moisture batch to batch or venting loss | Karl Fischer variation; pressure trend | Service vacuum pump; seal conveying leaks |
| Injection pressure drift | Wet material changing melt rheology cycle to cycle | Moisture at throat above target | Close open hopper dwell; check dryer dew point |
| Silver streaks or splay | Trapped vapor from residual moisture at gate | Moisture test; visual on short shots | Strengthen drying; improve venting |
Reading this table in the other direction is just as useful: if the maintenance log shows the dew point creeping upward, the team should proactively expect yellowing and burst bottles before they appear, and schedule the desiccant service before the product fails. This is the shift from reactive firefighting to planned prevention, and it is the core message of any serious PET equipment maintenance program. The defects are not random; they are the predictable output of a specific broken link, and the link can almost always be found in the drying or venting chain rather than in the screw or barrel geometry.
8. Handling rPET and Regrind: Extra Drying and IV Compensation
Recycled PET (rPET) and in-house regrind are now standard on many PET lines, driven by both regulation and economics, but they change the hydrolysis equation in two ways. First, recycled material usually carries more residual moisture and more surface area, so it needs more drying, not less. Second, because mechanical recycling itself shortens chains, the starting IV of rPET is typically lower than that of virgin resin, and any hydrolysis on top of that pushes the final IV below the window needed for stretch blow molding. The maintenance and process discipline therefore has to be tighter, not looser, when recycled content is in the mix.
From a drying standpoint, rPET flakes and regrind should be treated as the more demanding feed. They often benefit from a longer residence time in the crystallizer and dryer, and from staying at the higher, safe end of the drying temperature window, because the flake geometry and any residual contamination slow moisture release. The moisture target at the throat should be held at least as tight as for virgin material, and because recycled feed is more variable, the Karl Fischer check should be run more frequently rather than accepted on trust. Conveying leaks matter even more, because the larger surface area of flake re-wets faster than pellet, so the vacuum loader seals and pipe elbows discussed earlier deserve extra attention on rPET lines.
Because the starting IV is lower, many operations use an IV compensation step to recover molecular weight. The most common approach is a chain extender, a reactive additive that re-links polymer chains during processing and raises the effective IV back into the usable range. The concept is straightforward: the chain extender reacts with the carboxyl and hydroxyl end groups created by degradation and rebuilds chain length. What matters for this article is the maintenance implication: when a chain extender or a compatible coupling agent is used, the dosing and mixing must be consistent, the additive must be kept dry, and the screw residence and temperature must be set so that the reaction completes without over-processing. The precise additive chemistry, brand, and cost are outside the scope of equipment maintenance and should be selected with the material supplier; the point for the engineer is that the IV recovery step is only as reliable as the drying and venting that protect it.
There is also a sorting and contamination dimension. PET that arrives with PVC, polyolefin, or high moisture labels will not only dry poorly but can also release volatiles that load the dryer filters and the vacuum system faster. A washing and recycling line that delivers clean, well-dewatered flake to the dryer makes the downstream hydrolysis control far easier. Wanplas’s Polyretec factory builds PET washing and recycling lines designed to deliver food-grade flake with low contamination, and the cleaner the incoming flake, the lower the maintenance load on the dryer and the venting system downstream. Treating the recycling front end as part of the hydrolysis prevention chain, rather than as a separate department, is what lets a plant run high rPET content without sacrificing bottle quality.
9. Shutdown and Restart SOP
The moments when a PET line is most exposed to hydrolysis are not always during steady running; they are during stops and restarts, when material can sit hot and wet inside the screw or when cold, humid air can enter an open system. A clear, written standard operating procedure for shutdowns removes the guesswork and protects both the equipment and the first batches after restart.
For a short stop of less than fifteen minutes, the rule is to keep the dryer running, keep the barrel at temperature, and keep the screw turning slowly or maintain a gentle purge with dried resin so that material does not stagnate in the metering zone. Because fifteen minutes is short relative to the drying residence time, the resin in the hopper stays dry, and the main risk is simply localized overheating from a stationary screw, which slow rotation prevents. The machine hopper should remain sealed and, where fitted, the dried-air blanket should stay on, so that the brief exposure does not let plant humidity reach the feed.
For a long stop of more than one hour, the procedure changes. The barrel should be cooled below the degradation-risk zone to stop any reaction in trapped material, and the screw and die should be purged with a stable, fully dried purge compound so that no wet or degraded PET is left sitting in the machine. The dryer can be left in a low-energy hold or shut down per the dryer manufacturer guidance, but the key is that the material path is emptied and sealed. On restart, the dryer must be brought back to a minus 40 degrees C dew point and confirmed before resin is fed to the machine; feeding cold resin or resin from a hopper that sat open during the stop is a frequent cause of the first bad batch after a weekend. The first several shots or the first extruded length should be treated as startup scrap and verified by IV or at least by a moisture check before being released to production.
A written SOP should also cover the vacuum system: on long stops the vacuum pump should be isolated and, where the design allows, the vent port protected from air ingress so that condensate does not form inside the line. On restart, the vacuum should be confirmed at or below minus 0.09 MPa at the vent before the line is brought to full rate. These steps take minutes and prevent hours of degraded production. Plants that run multiple PET machines under the Wanplas group umbrella often standardize one shutdown and restart SOP across all sites so that operators moving between lines follow the same routine, and the shared service program supports training on exactly these procedures.
Frequently Asked Questions
What moisture level is safe for processing PET?
For most bottle-grade and sheet-grade PET, the residual moisture content entering the metering zone should be at or below 50 ppm, with 30 ppm being a common target for high-IV bottle preform production. Above 100 ppm the risk of measurable IV loss becomes significant, and above 300 ppm severe degradation, yellowing, and brittle parts are likely. The moisture reading should be taken at the machine throat, not at the dryer outlet, to reflect true process condition.
Why does dew point matter more than drying temperature for PET?
Drying temperature drives the rate of diffusion, but the final equilibrium moisture is set by the dew point of the drying air. A desiccant dehumidifying dryer must reach a dew point of minus 40 degrees Celsius or lower; if the dew point degrades, resin can sit at 160 to 180 degrees Celsius for hours yet never reach the required ppm level because the air itself is too humid. Always treat dew point as a primary process variable.
How often should the molecular sieve in a desiccant dryer be replaced?
Typical molecular sieve service life is three to five years under normal operating conditions. The practical replacement criterion is dew point performance: when the regenerated bed can no longer hold a stable minus 40 degrees Celsius dew point at design airflow, the desiccant bed should be changed regardless of calendar age. Staggered bed changes keep total capacity from dropping at once.
Can vacuum venting fully compensate for poor drying?
No. Vacuum venting removes vapor that enters or forms in the melt, but it cannot repair ester bonds already broken by hydrolysis during residence in the screw and barrel. Venting is a safeguard against residual moisture and degradation by-products, not a substitute for correct upstream drying. A vented barrel with a healthy vacuum pump protects a well-dried feed; it does not rescue a wet one.
What is the best practice for short stops on a PET line?
For stops under 15 minutes, keep the dryer running, maintain barrel temperature, and keep the screw turning slowly or purge with dried resin to avoid stagnation. For stops longer than one hour, cool the barrel below the degradation-risk zone and purge the screw and die with a stable, dried purge compound to prevent localized hydrolysis during the idle period. Always verify dew point before refeeding.
How does rPET or regrind change the drying strategy?
Recycled PET and regrind typically carry higher residual moisture and a lower starting IV, so they need longer residence time in the dryer and often a slightly higher drying temperature within the safe window. Because mechanical recycling further reduces IV, many operations use a chain extender or a compatible coupling agent to recover molecular weight, and they verify IV after drying rather than assuming the virgin curve applies. Moisture checks should be run more frequently.
Which test tells me whether hydrolysis actually happened?
Intrinsic viscosity measurement following ASTM D4603 or ISO 1628-5 is the primary indicator. A drop versus the incoming resin specification confirms chain scission. Supporting checks are moisture by Karl Fischer, acetaldehyde by headspace gas chromatography, and b* color shift, which together separate hydrolysis from thermal oxidation and contamination. Carboxyl end group titration adds confirmation that the mechanism was hydrolysis.
Conclusion
Controlling PET hydrolysis degradation is not a single adjustment but a disciplined chain of equipment care that runs from the desiccant dehumidifying dryer, through the crystallizer and the sealed conveying system, to the vented barrel and the vacuum pump at the screw. The chemistry is unforgiving: water in the melt breaks ester bonds, IV falls, acetaldehyde and carboxyl end groups rise, and the bottle or sheet loses the strength and clarity the customer paid for. The good news is that the failure modes are predictable and the maintenance responses are well established. Keep the dew point at or below minus 40 degrees Celsius, hold the drying temperature at 160 to 180 degrees Celsius with four to six hours of residence, seal the conveying path so dry resin never lingers in open air beyond about twenty minutes, maintain the vacuum vent at or below minus 0.09 MPa, hold the melt temperature in the 265 to 285 degrees Celsius window, and verify the result with IV, moisture, and acetaldehyde testing. When recycled content is used, tighten the drying and add IV compensation rather than relaxing it.
Wanplas, as the main brand behind specialized PET factories such as YuDa for bottle blow molding and Polyretec for washing and recycling, supports these practices through its shared service and spare parts program and through training on the exact shutdown, restart, and drying maintenance routines described here. A PET line that follows this guide will not eliminate every quality issue, but it will remove hydrolysis from the list of mysteries and turn it into a managed, measurable, and preventable variable. The payoff is steadier IV, fewer burst bottles, lower acetaldehyde, and a drying and venting system whose condition is known at every shift rather than discovered after a complaint.
