Plastic Loss on Drying Testing: Determination of Moisture and Volatile Content in Plastic Materials

Plastic loss on drying testing is a gravimetric analytical method used to determine the moisture and volatile matter content of plastic resins, pellets, and finished polymer products. Accurate moisture measurement is critical for plastics processing, because excessive water content causes hydrolytic degradation, surface defects, bubbles, and reduced mechanical properties in materials such as PET, PA, PC, and PBT. This article explains the technical principles, applicable standards, sample requirements, testing procedures, and typical applications of loss on drying analysis for plastic materials.

Technical Overview

Loss on drying (LOD) testing, also referred to as dry mass loss determination, measures the amount of mass a plastic sample loses when heated under controlled temperature and time conditions. The mass loss primarily represents absorbed moisture, but it may also include residual monomers, solvents, plasticizers, and other volatile substances that evaporate during the drying process. The result is expressed as a percentage of the original sample mass.

The testing principle is straightforward: a weighed sample is placed in a drying oven or rapid moisture analyzer at a specified temperature. After drying to constant weight, the sample is reweighed, and the difference in mass before and after drying is calculated as the loss on drying. This method is widely used in the plastics industry for incoming material inspection, drying process validation, and quality control, and it is generally governed by national and international standards such as GB/T 6284, ASTM D6980, ASTM D3030, and ISO 15512.

Moisture control is particularly important for hygroscopic engineering plastics. For example, polyamide (PA) can absorb several percent of water from ambient air, while PET and PBT are highly susceptible to hydrolytic degradation during melt processing when moisture exceeds recommended limits. Even non-hygroscopic resins such as PE and PP can carry surface condensation or process water that affects product quality. Loss on drying testing therefore serves as a fast, reliable indicator of whether a material is suitable for injection molding, extrusion, or other melt-processing operations.

Testing Samples

Thermoplastic Resins and Pellets

Common commodity plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS, and PVC are routinely tested to verify surface moisture and volatile content before processing.

Hygroscopic Engineering Plastics

Materials such as polyamide (PA6, PA66), polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyoxymethylene (POM), and polymethyl methacrylate (PMMA) absorb significant moisture from the environment and require strict moisture verification prior to molding.

High-Performance and Specialty Polymers

PEEK, PPS, PI, PPO, and liquid crystal polymers are tested to confirm that drying operations have achieved the extremely low moisture levels required for high-temperature processing.

Recycled and Regrind Materials

Recycled pellets and regrind flakes often carry elevated and inconsistent moisture levels, making loss on drying testing essential for incoming quality control of recycled feedstock.

Powders, Compounds, and Masterbatches

Polymer powders, color masterbatches, filler compounds, and additive concentrates may also be analyzed, since volatiles in these materials can cause defects or foaming in downstream products.

Sample Requirements

• Samples should be taken from representative locations of the batch and sealed in moisture-proof bags or containers immediately to prevent moisture exchange with ambient air.
• A minimum of 5 to 10 grams of sample per determination is typically required, with duplicate or triplicate testing recommended for improved reliability.
• Sample identity, resin grade, lot number, storage conditions, and drying history should be documented before submission to the laboratory.

Testing Items

The core testing items covered under plastic loss on drying analysis include:

• Loss on drying (moisture plus volatile content) at a specified temperature and time, expressed as a mass percentage.
• Moisture content determination as part of drying process verification for injection molding and extrusion.
• Volatile matter content of plastic resins and compounds at elevated temperature.
• Constant-weight verification to confirm complete removal of free moisture.
• Moisture regain or hygroscopicity evaluation for storage and packaging studies.
• Complementary water content determination by Karl Fischer titration when trace-level water quantification below 0.05 percent is required.
• Related water absorption testing per ISO 62 or GB/T 1034 for long-term immersion studies.

Testing Methods

Oven Drying Gravimetric Method

The classical method follows the general procedure of GB/T 6284 and ASTM D3030. A clean weighing bottle or aluminum dish is dried to constant weight and accurately weighed on an analytical balance with 0.1 mg readability. A test portion of the plastic sample is added, the total mass is recorded, and the open container is placed in a forced-convection oven at the specified temperature, commonly 105 °C for 2 to 4 hours for general thermoplastics. After drying, the container is transferred to a desiccator, cooled to room temperature, and reweighed. The drying-cooling-weighing cycle is repeated until the change in mass between consecutive weighings is negligible, typically within 0.3 mg.

The loss on drying is calculated as: LOD (%) = (mass before drying − mass after drying) ÷ mass before drying × 100.

Rapid Moisture Analyzer Method

Halogen or infrared rapid moisture analyzers, as referenced in ASTM D6980, combine a heating unit and an integrated precision balance. A small sample is heated while being continuously weighed, and the instrument automatically reports the loss on drying when the mass stabilizes. This method provides results within 10 to 30 minutes and is well suited to high-throughput production control, although parameters should be validated against the oven method.

Vacuum Oven Drying Method

Heat-sensitive or easily oxidized plastics, such as certain polyformaldehyde and vinyl polymers, may be dried under reduced pressure at 60 to 80 °C. The vacuum environment lowers the evaporation temperature of water, minimizing thermal degradation of the specimen while achieving complete moisture removal.

Karl Fischer Titration

For engineering resins such as PET and PC, where processing specifications may require moisture below 50 ppm, loss on drying may lack sufficient sensitivity. Karl Fischer titration, aligned with ISO 15512, selectively quantifies water molecules and is recommended as a complementary technique for trace moisture determination.

Instrumentation and Conditions

• Analytical balance with 0.1 mg readability and regular calibration.
• Forced-convection drying oven with temperature stability of ±2 °C, or vacuum oven for sensitive materials.
• Desiccator with fresh desiccant for cooling samples without reabsorbing atmospheric moisture.
• Aluminum dishes, glass weighing bottles, and appropriate handling tools cleaned and pre-dried.
• Controlled laboratory environment to limit ambient humidity interference during weighing.

Applications

Plastic loss on drying testing supports quality assurance and process optimization across the entire plastics value chain:

• Incoming material inspection to verify that purchased resins, pellets, and masterbatches meet specified moisture limits before production.
• Drying process validation for dehumidifying dryers, hopper dryers, and vacuum dryers used prior to injection molding or extrusion.
• Prevention of hydrolytic degradation in condensation polymers such as PET, PBT, PC, and PA, where excess moisture causes molecular weight loss and embrittlement.
• Elimination of processing defects including silver streaks, splay marks, bubbles, voids, and poor surface finish caused by residual moisture.
• Quality control and lot release testing for resin producers, compounders, and converters, with testing performed by accredited third-party laboratories when independent certification is required.
• Research and development support for new formulations, recycled content evaluation, packaging design, and shelf-life studies.
• Trade and regulatory documentation for export shipments where material conformance reports are requested by customers or authorities.

FAQ

What is the difference between loss on drying and water content?

Loss on drying measures the total mass lost upon heating, which includes water plus any volatile substances such as residual monomers, solvents, or low-molecular-weight additives. Water content methods, such as Karl Fischer titration, respond selectively to water molecules only. For most process control purposes the two values are close, but for volatile-rich materials or trace moisture specifications, Karl Fischer titration provides a more specific and more sensitive result.

What drying temperature and time are typically used?

A common general condition is 105 °C for 2 to 4 hours, or drying to constant weight, for stable thermoplastics. Hygroscopic engineering plastics and heat-sensitive polymers may require method-specific conditions, including vacuum drying at 60 to 80 °C, to avoid thermal degradation while ensuring complete moisture removal. The exact parameters should follow the relevant product standard or the agreed test protocol.

What moisture levels are acceptable for processing?

Typical processing guidance suggests moisture below 0.005 percent for PET, below 0.02 percent for PC and PBT, below 0.02 to 0.1 percent for PPO and PPS, and below 0.1 to 0.2 percent for polyamide resins, although exact limits depend on the grade, application, and customer specification. Loss on drying testing confirms whether these targets have been achieved.

How much sample is required for a test?

A single determination usually requires 3 to 10 grams of material, and laboratories typically request 30 to 100 grams in total to allow duplicate or triplicate determinations, sample homogenization, and possible repeat testing if results are inconsistent.

Why must the sample be weighed to constant weight?

Weighing to constant weight confirms that all free moisture and volatiles have been removed. If mass continues to decrease between consecutive drying cycles, the determination is incomplete and the calculated loss on drying would understate the true moisture content.

How long does testing take and when is a report issued?

A single oven drying determination with constant-weight verification typically takes 4 to 24 hours depending on the material and specification. Rapid moisture analyzer testing can deliver results within an hour. Accredited third-party laboratories generally issue formal test reports within 3 to 5 working days after sample acceptance, including test conditions, standards applied, individual and mean results, and instrument calibration information.