Moisture control before PET melt processing may be one of the most underestimated steps in the entire spinning process.
It is easy to think of drying as simply putting PET chips into a hopper and blowing hot air through them for several hours.
In reality, however, drying effectiveness directly affects IV retention, spinnability, and fiber strength. At the molecular level, water can break polyester chains through hydrolysis. This means that moisture levels measured in only a few dozen ppm can ultimately influence the grade and performance of the final product.
The Essence of Drying: It Is a Chemical Reaction Problem
Ester bonds connect PET molecular chains. At elevated temperatures, these ester bonds can hydrolyze in the presence of water, causing chain scission and reducing molecular weight.
The hydrolytic degradation reaction can be represented as follows:
—COOC₂H₄OOC— + H₂O → —COOH + HOC₂H₄OOC—
The cleavage of an ester bond generates one carboxyl end group and one hydroxyl end group.
More importantly, PET hydrolysis is autocatalytic.
The carboxyl end groups generated during hydrolysis can further promote degradation. The process can begin at temperatures of around 100°C. Within the 100–120°C range, the hydrolysis rate can be about 10,000 times faster than thermal degradation.
The figure below illustrates this molecular-level chain-scission mechanism:
A water molecule attacks an ester bond, generating carboxyl and hydroxyl end groups.
Within the normal PET processing temperature range of approximately 280–290°C, hydrolytic degradation can proceed much faster than thermal degradation.
If insufficiently dried PET enters the extruder, its molecular weight may drop dramatically in just a few minutes.
The Quantitative Scale: Every PPM Can Be Calculated
The Melt Behavior chapter provides a quantitative equation for predicting the effect of moisture on PET molecular weight:
Equation 1: Molecular-weight reduction caused by hydrolytic degradation
Where Mn is the initial number-average molecular weight, M’n is the number-average molecular weight after reaction with water, and x is the water content (wt%).
Assume an industrial-yarn-grade PET with an initial molecular weight of approximately 25,000, corresponding to an IV of roughly 0.95.
The theoretical degradation results at several typical moisture levels are shown below:
A moisture level of 3,000 ppm is close to the environmental moisture PET chips can absorb under normal storage conditions (approximately 0.3–0.4 wt%).
In other words, if PET chips are taken directly from storage and melt-extruded without proper drying, the molecular weight may fall from around 25,000 to approximately 10,000.
A material originally suitable for industrial yarn production could therefore be degraded to a much lower-viscosity polymer that may no longer be suitable even for ordinary textile applications.
The figure below illustrates this effect.
At 280°C, PET melt containing only 3 ppm moisture shows almost no IV change after two minutes, while PET containing 3,000 ppm moisture can see its IV fall from approximately 0.60 to below 0.30 within the same period.
Change in PET melt IV with extruder residence time at different initial moisture levels, 280°C
High-IV PET Has a Tighter Drying Window
Industrial-yarn-grade PET with an IV of approximately 0.9–1.0 is generally more demanding in terms of moisture control than conventional textile-grade PET with an IV of approximately 0.6–0.7.
One reason can be seen from the relationship between melt viscosity, IV, and temperature:
Equation 2: Relationship between PET melt viscosity, IV, and temperature
Where η0 is the zero-shear melt viscosity (poise), and T is the absolute temperature (K).
The effect of IV on melt viscosity follows approximately a power-law relationship with an exponent of 5.35.
When IV increases from 0.65 to 0.95, melt viscosity increases by roughly one order of magnitude.
In a higher-viscosity melt, moisture removal becomes more difficult. At the same time, degradation of a high-IV resin can result in a greater absolute loss of molecular weight and processing performance.
For example, the pressure required for polymer flow may be approximately 10 times higher.
High IV therefore means:
- higher processing pressure,
- higher shear heating,
- and greater sensitivity to residual moisture.
Together, these factors make the drying process window for industrial-yarn-grade PET considerably narrower than that for conventional textile-grade PET.
Engineering Drying Practice
Target Moisture Level
For satisfactory processing performance, keep PET moisture content below 15 ppm.
Reducing moisture from typical environmental absorption levels of about 0.3–0.4 wt% (3,000–4,000 ppm) to below 15 ppm requires roughly two orders of magnitude of moisture removal.
That is why PET drying isn’t simply a matter of heating the material for several hours. Temperature, drying time, airflow, air dew point, crystallinity, and material characteristics all affect the final moisture level.
Temperature Control
Under hot-air drying conditions, the temperature should generally not exceed 180°C to minimize the risk of thermal-oxidative degradation.
Vacuum drying can usually provide better polymer quality when properly controlled.
The lower temperature limit is determined mainly by drying efficiency, while the upper limit is constrained by thermal and oxidative degradation.
From an engineering perspective, 180°C represents an upper boundary for hot-air drying: beyond this point, the additional drying efficiency may be offset by polymer degradation.
A Practical Example from Our Pre-Crystallized Low-Melting PET
Drying conditions must also be adjusted according to the specific PET grade. Low-melting-point copolyesters, for example, cannot use the same drying conditions as conventional PET.
For our pre-crystallized 180°C low-melting-point polyester grade, the actual recommended processing parameters are:
- Drying temperature: approximately 135°C
- Drying time: approximately 10 hours
- Final moisture content: ≤30 ppm
- Dew point: ≤−80°C
These parameters illustrate an important engineering point: the optimum drying conditions depend on the specific polymer grade rather than on PET chemistry alone.
A low-melting PET grade may require a lower drying temperature and longer residence time to achieve the required moisture level while avoiding sticking, agglomeration, or premature softening.
Drying Equipment
Common industrial PET drying systems include:
- Integrated crystallization and drying hoppers: amorphous chips are first heated to approximately 120–190°C to crystallize and prevent sticking, then transferred to the drying section.
- Dehumidified-air or nitrogen circulation systems: drying gas with a dew point below −40°C is circulated through the material, while molecular-sieve dryers continuously remove moisture from the gas stream.
- Vacuum rotary dryers: commonly used for smaller batches or applications with particularly strict quality requirements. They can provide excellent drying performance, although production capacity may be limited.
The figure below shows a complete industrial PET drying process:
Industrial PET drying system — complete process from amorphous chips to the extruder
A nitrogen purification loop may include filters, a platinum catalyst bed, and a gas dryer, reducing the circulating gas moisture level to a dew point below −40°C.
Drying During the SSP Process
Drying is not only important immediately before melt processing.
Moisture is also a critical process variable during Solid-State Polymerization (SSP).
During crystallization, a large proportion of the moisture and acetaldehyde present in PET chips is removed.
This becomes particularly important before heating the chips to SSP temperatures above 180°C, because residual moisture at elevated temperatures can promote hydrolysis and cause IV loss.
The influence of initial moisture content on the SSP reaction rate
Under the same reaction conditions, increasing the moisture content from 30 ppm to 400 ppm significantly reduces the reaction rate because hydrolysis consumes reactive end groups that would otherwise participate in molecular-chain growth.
What Happens When PET Is Not Dried Properly?
The consequences of insufficient drying are clearly visible in the final product.
Insufficient drying before processing can reduce tensile strength.
Micropores may also be observed on fracture surfaces. These defects can be associated with hydrolytic degradation occurring during melt processing.
The figure below summarizes the complete chain from moisture absorption to fiber defects.
Environmental moisture absorption → hydrolytic degradation → IV reduction → strength loss → filament breakage and unstable spinning
Back to Engineering Reality
- PET drying is not simply a physical moisture-removal process. Its fundamental purpose is to prevent hydrolysis before and during melt processing. At 280°C, even very small amounts of residual water can become highly destructive to the polymer chain.
- A drying target of 15 ppm means reducing moisture from typical environmental levels above 3,000 ppm to about 1/200 of the original level. This is not a process parameter with much room for compromise.
- High-IV industrial-yarn-grade PET requires tighter moisture control than conventional textile-grade PET. The approximately 5.35-power relationship between IV and melt viscosity amplifies the processing consequences of IV loss.
- The 180°C upper limit for conventional hot-air drying balances moisture-removal efficiency with the risk of thermal-oxidative degradation. Increasing the temperature indefinitely does not necessarily improve polymer quality.
- Drying is equally important during SSP. If the moisture level is too high before PET enters the solid-state polymerization stage, hydrolysis can compete with chain extension and reduce IV build-up efficiency.
- Different PET grades require different drying strategies.
Drying is the starting point of every downstream PET processing step. If you don’t properly remove moisture absorbed before processing, everything that follows—from IV control to spinning stability and final fiber strength—can be compromised.
If you are evaluating PET chips for fiber spinning, monofilament, hot-melt applications, or other specialty polyester processes, we are happy to discuss suitable grades and processing requirements.
Hubei Decon Polyester Co., Ltd.
🌐 Website: https://www.polyestermfg.com/category/products/
✉ Email: info@polyestermfg.com
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