Introduction
Phenol-formaldehyde (PF) resins are thermosetting resin systems used in moulding compounds, laminates, composites, and other industrial material systems. Because moisture uptake can be relevant to dimensional, electrical, or mechanical property evaluation in plastics, water absorption is a practical consideration when assessing PF-based materials under defined service or test conditions. However, there is no single water-absorption value that characterises all phenol-formaldehyde resin materials. A meaningful answer requires clear definition of the material state, formulation, specimen geometry, conditioning, exposure conditions, and test method.
Short Answer
There is no single water-absorption value that applies universally to all phenol-formaldehyde resin materials. A reported water-absorption result should identify the test method, specimen, conditioning, exposure medium, temperature, and duration. For PF-based materials, the material state and formulation should also be identified so that data from neat resin, moulding compounds, laminates, or composites are not treated as interchangeable. Equally important, water absorption and water content are distinct concepts that require different test methods and answer different questions.
What Material Is Actually Being Tested?
For water-absorption interpretation, PF-related materials may need to be distinguished by material state and formulation. Relevant test objects can include:
- Uncured liquid resin or resin solution
- Solid resin, including flakes or other supplied solid forms
- Powder resin
- Partially cured material
- Fully cured neat-resin specimen
- Phenolic moulding compound
- Laminate
- Filled formulation
- Fibre-reinforced composite
- Finished component
These material states and formulations are not interchangeable test objects. A water-absorption value obtained from one material state does not automatically predict the behaviour of another. A result obtained for one material state should not be assumed to represent a formulated compound, composite, laminate, or finished component without an appropriate comparison basis.
Water Absorption Is Not the Same as Water Content
This distinction is fundamental to interpreting PF resin data correctly.
Water absorption refers to the process by which a material takes up water from its environment under defined exposure conditions. It is measured on solid specimens using procedures such as ASTM D570 or ISO 62.
Water content refers to the amount of water already present in a material at the time of testing. ISO 15512:2019 specifies methods for determining water content in plastics in the form of powder, granules, and finished articles, and explicitly states that these methods do not test for water absorption kinetics and equilibrium as measured by ISO 62.
For powder or other applicable material forms, water content may be specified or measured as a separate material parameter. It should not be substituted for the water absorption of a cured specimen. Conversely, a water-absorption result from a cured specimen does not describe the water content of the raw material before processing.
Why a Universal PF Water-Absorption Number Is Not Appropriate
Several factors affect water-absorption behaviour and should be identified where relevant to interpretation or comparison:
- Material form: neat cured resin, moulding compound, laminate, and composite are different test objects.
- Resin formulation: neat resin, filled compounds, laminates, and reinforced composites are distinct material systems, so results should be attributed to the tested formulation.
- Cure state: the cure state should be identified when reporting or comparing specimens because differently prepared specimens should not be assumed to represent the same test object.
- Specimen geometry: thickness and exposed surface area affect the rate and apparent extent of absorption.
- Specimen preparation: preparation, dimensions, and conditioning should be identified where specified by the selected test method.
- Conditioning and exposure: temperature, medium (liquid water versus humid air), and duration are all relevant.
- Test method: ASTM D570 and ISO 62 differ in specimen specification, procedure, and reporting basis.
A water-absorption result is more interpretable when the relevant material, specimen, conditioning, and exposure information is available. Without such information, a water-absorption figure has limited comparative value.
Novolac and Resole: Classification Does Not Determine the Test Result
Novolacs and resoles are distinct classes within phenolic resins. Novolacs are generally thermoplastic phenolic prepolymers that are commonly combined with a curing system such as hexamethylenetetramine (HMTA) to form a thermoset, whereas resoles are heat-reactive phenolic resins capable of further curing.
However, this classification alone does not determine the water absorption of a cured moulding compound, laminate, composite, or finished part. Direct comparison of novolac-based and resole-based materials requires specimens prepared, cured, and tested under equivalent conditions. In the absence of such controlled comparison, no general ranking can be asserted.
ASTM D570-22
ASTM D570-22 covers the determination of the relative rate of water absorption by plastics when immersed. Its scope includes cast, hot-moulded, and cold-moulded resinous products, homogeneous and laminated plastics in rod and tube form, and sheets 0.13 mm or greater in thickness.
The standard notes that time to saturation is strongly dependent on specimen thickness, and that the relationship between moisture content and properties such as electrical insulation resistance, dielectric losses, mechanical strength, and dimensions depends on the type of exposure, part shape, and inherent material properties. ASTM D570 is a test method rather than a PF-specific product specification. Whether a D570 result is required for a particular material should be established by the applicable procurement or product specification.
ISO 62:2008
ISO 62:2008 describes procedures for determining moisture absorption in the through-thickness direction of solid plastics and for determining the amount of water absorbed by defined-dimension specimens immersed in water or exposed to humid air. It is applicable to solid plastics but explicitly excludes cellular plastics, granulates, and powders.
ISO 62 provides controlled procedures for comparing water absorption of defined solid-plastic specimens under specified exposure conditions. ISO 62 defines a plastics water-absorption test framework; its applicability to a particular PF material should be established from the material form and the relevant specification.
ASTM D570 vs. ISO 62
Both ASTM D570 and ISO 62 address water absorption of solid plastics, but they differ in scope, specimen specification, and procedural details. ASTM D570-22 states technical equivalence with ISO 62 when the specimen described in ASTM D570 Section 6.2 is used. This statement should not be interpreted as automatic equivalence of every result generated under the two standards. The applicable procedure and reporting requirements of the selected standard must be followed.
Water Content in Powder or Other Applicable Material Forms
For PF materials supplied in a form covered by an applicable water-content method, water content is a separate parameter from the water absorption of a cured solid specimen. ISO 15512:2019 covers plastics in the form of powder, granules, and finished articles. It explicitly distinguishes those measurements from water absorption kinetics and equilibrium as measured by ISO 62. It should not be assumed to apply to every liquid PF resin system.
How Formulation and Material State Affect Interpretation
Water-absorption data should be attributed to the complete tested material. A cured neat-resin specimen, a filled moulding compound, a laminate, and a reinforced composite are different test objects. A value measured for one should not be transferred directly to another without representative evidence. A value measured for a raw resin or one defined specimen should not by itself be treated as the water-absorption value of a different formulated or finished material.
How to Compare Water-Absorption Data
Meaningful comparison requires equivalent test conditions or a clear account of relevant differences. The following is a practical comparison framework rather than a complete reporting checklist prescribed by ASTM D570 or ISO 62. Before comparing two water-absorption values, confirm the following:
- Exact material or grade designation
- Material state (neat resin, compound, laminate, composite)
- Resin type where relevant
- Formulation details (fillers, reinforcements, additives)
- Cure state
- Test method and edition (ASTM D570-22, ISO 62:2008, or other)
- Specimen dimensions
- Conditioning procedure
- Exposure medium (liquid water or humid air)
- Temperature
- Duration
- Whether the value represents a specified-duration or equilibrium result
- Calculation basis and units
- Acceptance criteria and their technical basis
Differences in material state, specimen preparation, exposure conditions, method, or reporting basis should be resolved before values are treated as directly comparable.
TDS, CoA, and Test Report
The following distinctions are practical documentation considerations rather than document definitions prescribed by ASTM D570, ISO 62, or ISO 15512.
A Technical Data Sheet (TDS) may present typical, nominal, or specification-range information for a material. A Certificate of Analysis (CoA), where supplied, may report batch- or lot-specific results and may also list relevant acceptance limits. A test report may identify the test method, specimen or sample, conditions, and reported result.
Document format and content vary by supplier, material, and transaction. Buyers should therefore verify whether a reported value represents a typical property, a specification limit, or a batch-specific test result.
Common Interpretation Errors
Error 1: All PF resins have one standard water-absorption value.
Correction: Water absorption is a property of a defined material in a defined state, measured under defined conditions.
Error 2: A powder water-content value equals cured-resin water absorption.
Correction: Water content (ISO 15512) and water absorption (ISO 62) are different measurements.
Error 3: Novolac/resole classification alone predicts water absorption.
Correction: Comparison requires controlled specimens and conditions.
Error 4: ASTM D570 and ISO 62 results are automatically interchangeable.
Correction: ASTM D570 states technical equivalence with ISO 62 only when the specimen described in ASTM D570 Section 6.2 is used.
Error 5: A neat-resin result automatically predicts a filled composite or finished component.
Correction: Neat resin, filled compounds, laminates, composites, and finished components are different material systems or test objects. A result from one should not automatically be transferred to another.
Error 6: A TDS typical value is automatically a batch result.
Correction: TDS, CoA, and test reports serve different functions.
What Buyers Should Specify
When requesting or evaluating PF resin water-absorption data, the following information should be specified where relevant to the application:
- Resin/material form (liquid, solid, powder, flake, etc.)
- Grade designation
- Resin class where relevant (novolac or resole)
- Intended formulation and processing route
- Cure state of the test object
- Test object (neat resin, moulding compound, laminate, composite, finished part)
- Test standard and edition
- Specimen geometry
- Conditioning procedure
- Exposure medium, temperature, and duration
- Acceptance limit and its technical basis
- Documentation required (TDS, CoA, test report)
The test method should be selected according to the material form and the property being evaluated; ASTM D570 and ISO 62 concern water absorption of applicable solid plastics, whereas ISO 15512 addresses water content within its stated scope. Not every procurement scenario requires all of these specifications, but clarity on material state and test conditions is essential for meaningful data comparison.
Conclusion
The water absorption of phenol-formaldehyde resin cannot be represented by a single universal number. It is a property of a specific material in a specific state, measured under defined conditions using a defined method. Reported values are useful for comparison only when the material identity, cure state, specimen preparation, exposure conditions, and test method are known. Water absorption and water content are distinct concepts requiring different test methods. For application-relevant comparison, data from a representative cured formulation, specimen, or finished material may be more informative than a value measured on a materially different test object.
Professional CTA
Customers evaluating phenol-formaldehyde resin for moisture-sensitive applications may provide the resin or material form, intended formulation, test object, applicable water-absorption method or water-content method, exposure or conditioning conditions, and acceptance criteria so that available grade documentation can be reviewed against the stated requirement.
References
[1] ASTM International. (2022). ASTM D570-22, Standard Test Method for Water Absorption of Plastics.https://doi.org/10.1520/D0570-22
[2] International Organization for Standardization. (2008). ISO 62:2008, Plastics - Determination of water absorption (3rd ed.).
https://www.iso.org/standard/41672.html
[3] International Organization for Standardization. (2019). ISO 15512:2019, Plastics - Determination of water content (5th ed.).
https://www.iso.org/standard/73834.html
[4] Pilato, L. A. (Ed.). (2010). Phenolic Resins: A Century of Progress. Springer Berlin, Heidelberg.https://doi.org/10.1007/978-3-642-04714-5
[5] Asim, M., Saba, N., Jawaid, M., Nasir, M., Pervaiz, M., & Alothman, O. Y. (2018). A Review on Phenolic Resin and its Composites. Current Analytical Chemistry, 14(3), 185–197.https://doi.org/10.2174/1573411013666171003154410