What Is Moisture?
Moisture determination is done with two basic approaches. In the loss-on-drying method, the sample is heated and mass loss is measured; this loss covers everything volatile, not just water. In Karl Fischer titration, the amount of water is determined directly and chemically, and other volatiles do not enter the result.
Purpose of Moisture
The purpose is to verify that the raw material is sufficiently dried before processing. Chain scission occurs when hygroscopic polymers (PA, PET, PC) are processed while moist; even if the part looks normal, its mechanical strength is permanently reduced.
Which Products Is Moisture Applied To?
- Polyamide, PET and polycarbonate granules
- Hygroscopic engineering plastics
- Masterbatch and additives
- Recycled granules
- Drying oven efficiency check
In Which Sectors Is Moisture Used?
Standards Related to Moisture
- ISO 15512 (determination of water content in plastics)
- ISO 760 (determination of water by the Karl Fischer method)
- ASTM D6980 (moisture in plastics — mass loss)
- ASTM D789 (moisture and viscosity in polyamides)
Moisture Conditions
- Method
- Karl Fischer (direct water) or mass loss on drying (total volatile)
- Temperature
- The temperature defined in the standard at which the polymer will not decompose
- sample transport
- The sample should be collected in a closed and moisture-proof container.
- Measurement timing
- The sample should be measured as soon as possible after collection.
Equipment for Moisture
- Karl Fischer titrator and oven unit
- Moisture analyser (thermogravimetric)
- Analytical balance
- Moisture-proof sample containers
- Desiccator
Sample Requirements for Moisture
- The sample must be taken so as to represent the batch or production run.
- Sample description, material type, and production direction if any, must be stated.
- The standard to be applied and any customer specification must be provided together with the sample.
- The test may be destructive; the required number of samples must be confirmed in advance.
- Surfaces that will be tested must be protected during transport.
Duration of Moisture
Karl Fischer measurement takes minutes; the loss-on-drying method depends on the time to reach constant mass.
Moisture Procedure
- 1The sample is taken in a moisture-proof container and delivered to the laboratory quickly.
- 2The method is selected and the instrument verified.
- 3The sample is weighed.
- 4In the Karl Fischer method, the sample is heated in the oven unit and the released water is titrated.
- 5In the loss-on-drying method, the sample is dried at the defined temperature.
- 6The mass difference or titration result is calculated.
- 7The measurement is repeated and averaged.
Moisture Acceptance Criteria
The method standard defines how the test is performed, not what the result must be. The acceptance limit comes from the material or product standard, the customer specification, or the relevant regulation; this page therefore does not give a numerical limit.
Evaluation of Results
The result is compared against the maximum moisture value given by the raw material manufacturer for pre-processing. This value varies by polymer and comes from the technical data sheet; the method standard does not define a limit.
Moisture Report
- Sample description, material/product information, and batch number if any
- Sample preparation method and conditioning conditions
- Standard and test conditions applied
- Instrument used and calibration status
- Measurement results and units
- Measurement uncertainty (if requested)
- A statement that the result applies only to the specimen submitted for testing
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- Harmonised standards applied according to the product family
- EU 765/2008 (market surveillance and accreditation framework)
- Relevant product regulations and customer specifications
Plastics Testing — What Does This Testing Field Cover?
The section below describes the field that the Moisture test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Plastics testing determines the mechanical, thermal and burning behaviour of a polymer material. Unlike metals, plastics are VISCOELASTIC: their behaviour depends on loading rate, temperature and moisture. The same material therefore gives different results under different conditions, and the test conditions are an inseparable part of the result.
The practical consequence of viscoelasticity is this: a plastic part continues to deform over time under a constant load (creep) and the stress it carries at constant deformation falls over time (stress relaxation). A screwed joint loosens, a clamp loses its clamping force, a shelf sags over time. A short-term tensile test does not show this behaviour; a long-term test or an accelerated model is needed.
Thermal behaviour is the second main axis. Melt flow rate indicates the processability of the material and lot-to-lot consistency; heat deflection temperature and the Vicat softening point give a rough limit for service temperature. Thermal analysis (DSC, TGA, DMA) reveals the identity of the material: glass transition temperature, melting behaviour, crystallinity, filler content and decomposition temperature.
Burning behaviour is the third axis and ties directly into product standards. The UL 94 class and the limiting oxygen index are written into the material data sheet; a particular class is required for the housing of an electrical appliance, while a different classification system applies to a construction product.
Ageing is the most frequently skipped part of the field. Plastics degrade under UV, heat, moisture and oxygen: colour changes, the surface cracks, impact strength falls. For a part intended for outdoor use, what matters is not its values on day one but its values after ageing. Xenon arc and fluorescent UV equipment accelerate this degradation and allow comparison.
The output of this family is usually written into the material data sheet, and product standards refer to that sheet. But there can be a difference between the data sheet value and the behaviour of the actual part: orientation formed during moulding, weld lines and internal stresses make the part behave differently from the specimen.
The use of recycled content has added a new question to the field. Recycled resin varies from lot to lot; mechanical properties, colour and burning behaviour can shift between batches. For a manufacturer using recycled material, incoming material control is therefore not a choice but a necessity.
Method groups in this field
Mechanical
Tension, compression, bending, Charpy and Izod impact, hardness (Shore), creep and environmental stress cracking.
Thermal
Melt flow index (MFI), heat deflection temperature (HDT), Vicat softening temperature, linear thermal expansion.
Thermal analysis
Transition temperatures and crystallinity by DSC, composition and decomposition by TGA, dynamic mechanical behaviour by DMA.
Burning
UL 94 classification, limiting oxygen index (LOI), glow wire and needle flame tests.
Ageing
Accelerated ageing by xenon arc and fluorescent UV, hot air ageing, humidity and condensation.
Physical
Density, water absorption, ash content, gas and water vapour permeability of films and sheets.
Commonly confused in this field
- A plastics result without a stated test rate and temperature cannot be interpreted; viscoelastic behaviour depends on these parameters.
- A specimen result does not transfer directly to the actual part; moulding orientation and weld lines are decisive.
- HDT and Vicat values are NOT service temperatures; they are a rough indication of an upper limit and do not apply under load.
- The UL 94 class depends on thickness; a different wall thickness requires re-verification.
- For moisture-absorbing polymers (such as polyamide), results are misleading unless conditioning is carried out.
- ASTM D638 and ISO 527 results are not directly equivalent; the specimen geometries and rate definitions differ.
Which decisions these results support
- Material selection and supplier approval
- Meeting the flammability and thermal requirements of the product standard
- Verifying the use of recycled content and lot control
- Moulding process control and lot consistency
- Demonstrating ageing resistance for outdoor use
Related Tests
Frequently Asked Questions About Moisture
Why do loss on drying and Karl Fischer give different results?
Loss on drying measures all volatiles besides water: residual monomer, plasticiser and lubricant also contribute to the loss. Karl Fischer measures only water.
Why must the sample be measured immediately?
Hygroscopic polymers absorb moisture from open air within minutes. The result of a sample left exposed reflects the laboratory air, not the actual process moisture.