Moisture content determination is a critical quality control test for recycled plastic materials, because excessive moisture causes hydrolytic degradation, processing defects, and loss of mechanical properties. This article provides a comprehensive technical overview of moisture content testing for recycled materials (rPET, rPE, rPP, rPA, rPC, rABS, and other recycled resins), covering applicable standards, sampling requirements, test methods, and typical applications. Whether you are a recycler, compounder, or manufacturer using recycled feedstock, understanding moisture measurement techniques such as Karl Fischer titration and loss-on-drying is essential for ensuring stable production and product quality.
Technical Overview
Recycled materials, also referred to as reclaimed or regenerated plastics, include post-consumer recycled (PCR) resins, post-industrial recycled (PIR) resins, regrind, flakes, and regranulated compounds derived from PET bottles, films, fibers, and durable goods. Compared with virgin resins, recycled materials typically exhibit significantly higher and more variable moisture content. This is attributable to hot- and cold-washing steps in the recycling process, residual contamination such as paper labels, adhesives, and organic residues, higher porosity in regranulated material, and prolonged storage under ambient humidity conditions.
Accurate moisture determination is essential because moisture in plastics exists in two forms: free surface moisture, which can be removed relatively easily by drying, and absorbed (bound) moisture, which penetrates the polymer matrix and requires elevated temperature and low dew point conditions to extract. For hydrolysis-sensitive condensation polymers such as PET, PA (nylon), PC, PBT, and TPU, even trace moisture at the ppm level can trigger hydrolytic chain scission during melt processing, causing a sharp drop in intrinsic viscosity or molecular weight, embrittlement, and reduced mechanical strength. For polyolefins such as PE and PP, which are not hydrolysis-sensitive, moisture primarily causes surface defects such as silver streaks, bubbles, voids, and die drool.
Moisture content is typically reported as a mass percentage (wt%) or in parts per million (ppm, equivalent to mg/kg), where 1 wt% equals 10,000 ppm. International and national standards governing this test include ISO 15512 (Plastics — Determination of water content), ASTM D6869 (coulometric Karl Fischer determination of water in plastics using an oven method), ASTM D4019, GB/T 6283 (Karl Fischer determination of water), and GB/T 6284 (drying method for determination of water content). It is important to distinguish moisture content from water absorption as defined in ISO 62 and GB/T 1034, which describes the equilibrium water uptake of a material immersed in water rather than the as-received moisture level.
Testing Samples
Recycled PET (rPET) Flakes and Pellets
Recycled PET is the most moisture-sensitive commodity recycled resin. Hot-washed bottle flakes and rPET pellets typically require a residual moisture level below 0.005 wt% (50 ppm) before injection molding, extrusion, or reclaim pelletizing, in order to preserve intrinsic viscosity (IV) and avoid hydrolytic degradation. Because of the very low target level, Karl Fischer methods with oven sample introduction are strongly recommended for rPET.
Recycled Polyolefins (rPE, rPP)
Recycled polyethylene and polypropylene, derived from films, bottles, crates, and rigid packaging, are non-hygroscopic but carry substantial surface moisture from washing and friction-washing operations. Typical acceptance limits range from 0.05 wt% to 0.2 wt%. Both loss-on-drying and Karl Fischer methods are applicable.
Recycled Engineering Plastics (rPA, rPC, rABS, rPBT, rPOM, rPMMA)
Recycled nylons, polycarbonate, ABS, and other engineering resins are hygroscopic and absorb moisture into the polymer matrix. Residual moisture before processing is usually controlled between 0.02 wt% and 0.1 wt% depending on the resin family. These materials generally require verification of dryer performance through residual moisture measurement.
Regrind, Films, Fibers, and Recycled Compounds
Regrind, recycled films, textile fibers, and filled or modified recycled compounds often have irregular geometry and high surface area, leading to rapid moisture re-absorption. Special sampling protocols, including sealed double-layer packaging and rapid transfer to the laboratory, are required to obtain representative results.
Testing Items
Core test parameters for moisture content determination of recycled materials include:
- Total moisture content, expressed in wt% or ppm (mg/kg)
- Surface (free) moisture versus absorbed (bound) moisture distribution
- Residual moisture after drying, for validating desiccant dryer or dehumidifying hopper dryer performance
- Loss on drying (total volatiles), as a fast complementary indicator for process control
- Equilibrium water absorption (ISO 62 / GB/T 1034), evaluated separately as a material property rather than an as-received condition
Testing Methods
Karl Fischer Titration (Volumetric and Coulometric)
Karl Fischer (KF) titration is the reference method for selective and accurate water quantification and is specified in ISO 15512, ASTM D6869, and GB/T 6283. Coulometric KF is preferred for low moisture levels down to the ppm range, making it ideal for rPET and engineering resins, while volumetric KF suits higher moisture levels. For recycled materials, the oven technique is recommended: the sample is heated in a sealed oven at a material-specific temperature (typically 120 °C to 200 °C for PET and polyolefins), and the evaporated water is transferred by dry carrier gas into the titration cell. This approach avoids chemical interference from additives, pigments, printing inks, adhesives, and degradation by-products that are common in recycled feedstock and that would otherwise produce false positives if the sample were dissolved directly in the KF cell.
Loss on Drying (Gravimetric Method)
The gravimetric drying method specified in GB/T 6284 determines moisture by weighing the sample before and after heating in a ventilated oven or on a halogen moisture analyzer. Typical conditions are 105 °C to 120 °C until constant mass. The method is simple, fast, and inexpensive, and is widely used for in-house quality control of polyolefins and quick screening. However, it measures total mass loss and therefore overestimates water content when volatile residues, residual solvents, or monomers are present, which is a notable limitation for contaminated recycled materials.
Other Techniques
Alternative and complementary techniques include the calcium hydride reaction method, in which water reacts with calcium hydride to generate hydrogen and a corresponding pressure increase; dew point and chilled-mirror measurements of evolved moisture; and thermogravimetric analysis (TGA), which can distinguish water from other volatiles through programmed heating. In-line capacitive, infrared, and quartz crystal microbalance sensors are increasingly used for real-time moisture monitoring in drying and extrusion lines, although they require calibration against laboratory KF reference measurements.
Sampling and Handling Requirements
Because recycled pellets and flakes re-absorb moisture within minutes of exposure to ambient air, correct sampling is decisive for result validity. Samples must be collected into dry, airtight containers or aluminum-laminate bags, filled as completely as possible to minimize headspace, sealed immediately, and tested promptly. Duplicate or triplicate determinations are recommended, and results should be reported with the mean value, method used, and drying temperature. Accredited third-party laboratories can provide traceable results with documented measurement uncertainty for supplier qualification and trade settlement purposes.
Applications
- Incoming quality control of recycled pellets, flakes, and regrind prior to processing
- Validation and routine verification of desiccant dryer, vacuum dryer, and dehumidifying hopper dryer performance, including residual moisture after drying
- Troubleshooting of processing defects such as silver streaks, bubbles, voids, sink marks, and poor weld-line strength in injection molding and extrusion
- Control of hydrolytic degradation and intrinsic viscosity (IV) retention in rPET and rPA processing, including reclaim pelletizing and solid-state polycondensation feedstock checks
- Support for recycled-content certification, food-contact rPET compliance dossiers, and customer or buyer specification conformity
- Research and development of recycled compounds, blends, and modified formulations, where moisture behavior affects dispersion and properties
- Supplier qualification, batch release, and trade settlement documentation for recycled resin transactions
FAQ
What moisture level is acceptable for recycled plastics?
Acceptable limits depend on the resin family and process. Typical targets are below 0.005 wt% (50 ppm) for rPET in injection and extrusion applications, below 0.02 wt% for rPC, below 0.1 wt% for rPA, and below 0.1 wt% to 0.2 wt% for rPE and rPP. The specific limit should always be confirmed against the resin supplier's processing guidelines and the end product requirements.
Why do recycled materials show higher moisture content than virgin resins?
Recycled materials undergo washing, friction drying, and melt filtration steps that leave residual water and hygroscopic contaminants. Regranulated material also tends to be more porous than virgin pellets, and irregular flake geometry provides higher surface area for moisture adsorption. Storage in non-sealed big bags or silos further increases moisture uptake.
Which method should be chosen: Karl Fischer or loss on drying?
Karl Fischer titration is selective for water and delivers the highest accuracy at low ppm levels, making it the preferred method for hydrolysis-sensitive resins such as rPET and rPA, and for certification purposes. Loss on drying is faster and cheaper and is suitable for routine in-house screening of polyolefins, but it may overestimate moisture in contaminated recycled materials containing volatiles.
How can moisture re-absorption during sampling be prevented?
Use dry, airtight glass bottles or aluminum-laminate pouches, fill them as fully as possible to reduce humid headspace, purge with dry nitrogen if available, seal immediately after sampling, and deliver the sample to the laboratory promptly. Avoid sampling immediately after dryer discharge into open containers, since a few minutes of exposure can change the result by tens of ppm.
How are the test results reported?
Results are reported as moisture content in wt% or ppm (mg/kg), usually as the mean of replicate determinations, together with the test standard, method (coulometric KF with oven, volumetric KF, or loss on drying), oven temperature, and, for accredited tests, the measurement uncertainty. Typical laboratory turnaround is a few working days for standard samples.
Can a halogen moisture analyzer replace Karl Fischer testing?
A halogen moisture analyzer is useful for rapid process monitoring and dryer trending, but it measures total mass loss rather than water specifically. For recycled materials containing inks, adhesives, and volatile residues, and for verification below 0.02 wt%, Karl Fischer determination with oven sample introduction remains the reference technique.