Moisture Content Determination Test for Recycled Materials

Moisture content determination is a fundamental quality control experiment for recycled materials such as recycled plastic pellets, flakes, and regrind. Because recycled feedstocks undergo washing, sorting, and reprocessing, they typically carry higher and more variable moisture levels than virgin resins, which can trigger hydrolytic degradation and processing defects during extrusion or injection molding. This article explains the testing principles, sample requirements, experimental methods, and relevant standards—including ISO 15512 and ASTM D6980—used by accredited third-party laboratories to accurately measure moisture in recycled materials.

Technical Overview

Moisture content determination quantifies the free and bound water present in a recycled material, expressed either on a wet basis (water mass as a percentage of the as-received sample mass) or on a dry basis (water mass as a percentage of the oven-dry sample mass). For hygroscopic and hydrolysis-sensitive recycled polymers, results are frequently reported in parts per million (ppm) to reflect the very low thresholds required for safe melt processing.

Recycled materials are particularly susceptible to elevated moisture levels for several reasons. Mechanical recycling lines include hot washing, flotation, and centrifugal drying steps that leave residual surface moisture. Regrind, flakes, and irregular particles present a much higher surface area and contain microvoids that trap water. Hygroscopic polymers such as recycled PET, PA, PC, ABS, and PBT readily reabsorb atmospheric humidity during storage and transport, even after drying.

Excess moisture causes well-documented processing and performance problems, including hydrolytic cleavage of ester and amide bonds, reduced molecular weight (for example, intrinsic viscosity loss in rPET), silver streaks and splay marks, bubbles, voids, poor surface finish, dimensional instability, and reduced impact strength. Typical purchase specifications limit moisture to approximately 50 ppm (0.005%) for rPET, 0.1% for PA, 0.02% for PC, and 0.05% to 0.1% for polyolefins, which is why the moisture content determination experiment is a mandatory incoming inspection item for most recycled feedstock buyers.

Testing Samples

Recycled Polyolefin Pellets and Regrind (PE, PP)

Polyolefins are non-hygroscopic, but recycled PE and PP streams retain surface washing water and condensation in flakes and regrind. A minimum of 100 grams of representative material should be submitted. Regrind with irregular geometry requires particular care because trapped water in microvoids may not be fully released at low drying temperatures.

Recycled PET Flakes and Pellets (rPET)

rPET is the most moisture-critical recycled stream because hydrolysis rapidly degrades intrinsic viscosity above the glass transition. Both hot-washed flake and re-pelletized material can be tested; amorphous and crystallized pellets should be identified separately since amorphous material absorbs moisture faster. Samples must be sealed immediately at the production site, typically in aluminum foil laminate bags or airtight glass jars with desiccant.

Recycled Engineering Plastics (PA, PC, ABS, PBT)

Recycled nylons, polycarbonate, and polybutylene terephthalate are strongly hygroscopic and can reach equilibrium moisture levels several times higher than processing limits within hours of exposure to humid air. Testing should be performed as soon as possible after receipt, and the container should only be opened inside the laboratory immediately before weighing.

Recycled Films, Fibers, and Powdered Materials

Fine powders, chopped fibers, and film flakes have very high specific surface areas and exchange moisture with the environment within minutes. These samples should be double-bagged with desiccant, and the ambient relative humidity during sampling should be recorded and reported alongside the result.

Testing Items

• Moisture content, wet basis or dry basis, expressed in percent (%) or parts per million (ppm)
• Residual moisture after drying, used to validate desiccant dryer performance and drying residence time
• Moisture uptake kinetics under defined humidity conditions, for storage and packaging studies
• Volatile matter content as a complementary indicator, since oven methods cannot chemically distinguish water from other volatiles
• Water absorption characteristics under immersion or humid conditions per ISO 62 or ASTM D570, which is a related but distinct property from the as-received moisture content

Testing Methods

Oven Drying (Loss on Drying) Method

This gravimetric approach follows the principle of ASTM D6980 and the general loss-on-drying practice of GB/T 6284. A clean weighing bottle is dried and weighed (m0), charged with 3 to 10 grams of sample and reweighed (m1), then dried in a ventilated or vacuum oven at 105 ± 2 °C, or at a polymer-specific temperature between 90 °C and 135 °C, until constant mass is reached (two consecutive weighings differing by no more than 0.5 mg). The bottle is cooled in a desiccator for approximately 30 minutes before each reweighing (m2). Moisture content is calculated as (m1 − m2) / (m1 − m0) × 100% on a wet basis. The method is simple, inexpensive, and suitable for moisture levels above approximately 0.1%, but it lacks sensitivity at ppm levels and may include other volatiles.

Karl Fischer Titration Method

Karl Fischer (KF) titration is the reference technique for low-moisture recycled resins and is covered by ISO 15512, ASTM D6869, and ASTM E203. In the volumetric variant, the sample is dissolved or suspended in an anhydrous solvent and titrated directly, which suits moderately moist samples. In the oven-coulometric variant, the specimen is heated in a sealed oven, typically between 120 °C and 180 °C depending on the polymer, and a dry carrier gas transports the evolved water into a coulometric titration cell. Because the KF reaction is chemically specific to water, interference from other volatiles is eliminated, and detection limits reach the 10 microgram water level, enabling reliable quantification down to single-digit ppm in rPET, PA, and PC. The system is verified using certified water standards or sodium tartrate dihydrate.

Rapid Moisture Analyzer Method

Halogen-lamp moisture analyzers combine an integrated heating source with a precision balance to complete a loss-on-drying determination in 5 to 15 minutes. This approach is widely used for in-process screening on recycling production lines and in receiving inspection. Its accuracy at very low moisture levels is limited by ambient humidity drift and volatilization of non-water components, so results must be correlated against the oven method or Karl Fischer method before being used for lot release decisions.

Regardless of the method chosen, good experimental practice requires duplicate determinations with an acceptance criterion of no more than 0.02% absolute or 10% relative difference (whichever is greater), an analytical balance readable to 0.1 mg, blank determinations for KF work, and testing performed promptly after the container is opened to prevent moisture exchange with laboratory air.

Applications

• Incoming quality control of recycled pellets, flakes, and regrind by injection molders, extruders, and fiber producers
• Validation of desiccant dryer settings, including drying temperature, residence time, and dew point, before processing hydrolysis-sensitive recycled resins
• Lot release and contractual compliance verification against buyer moisture specifications in domestic and international trade
• Technical documentation supporting recycled-content certification programs and customer audits
• Research and development studies correlating residual moisture with intrinsic viscosity retention, melt flow index stability, and mechanical performance
• Failure analysis of processing defects such as silver streaks, bubbles, and hydrolytic embrittlement traced back to wet feedstock

FAQ

Why is moisture testing more critical for recycled materials than for virgin resins?

Recycled materials pass through washing steps, have higher and more irregular surface areas, and often have undefined storage histories, all of which lead to higher and less predictable moisture levels. For hydrolysis-sensitive polymers such as rPET and PA, even small amounts of water during melting cause irreversible molecular weight loss.

Which method is the most accurate for low-moisture recycled resins?

Oven-coupled coulometric Karl Fischer titration per ISO 15512 is the most accurate approach for ppm-level moisture in rPET, PA, PC, and PBT, because it is chemically specific to water and offers the lowest detection limits. Oven drying remains appropriate for routine screening of polyolefins at higher moisture levels.

What moisture limit should recycled PET meet before processing?

Most processing guidelines require rPET to be dried to 50 ppm (0.005%) or lower before extrusion or injection molding in order to protect intrinsic viscosity; some applications tolerate up to 100 ppm. Exact limits should be agreed between supplier and buyer based on the end product.

How should samples be packaged and submitted for testing?

Samples should be sealed at the point of production in airtight foil laminate bags or glass jars with minimal headspace and, where appropriate, desiccant. A minimum of 100 grams is recommended, and testing should be completed as soon as possible after receipt, ideally within 24 hours.

How long does the experiment take?

A loss-on-drying determination typically requires 2 to 4 hours of drying plus cooling and weighing time, while a Karl Fischer determination is usually completed in 10 to 30 minutes per specimen once the instrument is conditioned. Complete reports from accredited third-party laboratories, including duplicate testing and data review, are generally issued within one to three working days.