Testing & Analysis
FTIR Binder Identification: Understanding Existing Coating Systems
FTIR laboratory analysis can help identify and classify the binder chemistry of existing coating systems when maintenance history is uncertain.
Introduction
When an existing structure is being assessed for maintenance, one of the most important questions can also be one of the hardest to answer: what coating is already on the steel? Original coating records may be incomplete, unavailable or no longer reliable after decades of maintenance. A structure may have received several coating campaigns, local repairs or additional coats, resulting in a system containing multiple layers of different materials. This information can be important when deciding whether an existing coating can be retained, overcoated or requires more extensive removal. Fourier Transform Infrared Spectroscopy — commonly referred to as FTIR — is a laboratory analytical technique that can assist with the identification and classification of coating binder chemistry. Used alongside a properly planned coating condition survey, FTIR can provide valuable evidence about an existing coating system and support more informed maintenance decisions.
What Is FTIR?
FTIR stands for Fourier Transform Infrared Spectroscopy. The technique analyses how a sample interacts with infrared radiation across a range of wavelengths. Chemical bonds within a material absorb infrared radiation at characteristic frequencies. The resulting infrared spectrum contains absorption features associated with the chemical structure of the material being analysed. By evaluating these features and, where appropriate, comparing the spectrum with reference information, an experienced laboratory can obtain information about the chemistry present within a coating sample. For protective coatings, this can assist with classification or identification of the generic binder chemistry.
Why Identify the Binder?
The binder or resin is a fundamental component of a protective coating. Understanding its generic chemistry can help provide information about the nature of an existing coating system when historical records are unavailable or uncertain. Before specifying an additional coating over an existing system, consideration needs to be given to the nature and condition of the existing material. Applying a new coating simply because the existing surface appears sound does not, by itself, establish that the proposed materials will be suitable together. Binder identification can provide another piece of evidence for the engineer, coating specialist or asset owner when developing a maintenance strategy.
Where Can FTIR Help?
FTIR analysis may be useful where the original coating specification is unavailable; historical maintenance records are incomplete; the generic coating type is uncertain; an asset has undergone multiple maintenance campaigns; different areas appear to contain different coating systems; compatibility requires further investigation; a coating failure is being investigated; laboratory evidence is required to support a condition survey; or maintenance and overcoating options are being developed. It can be particularly valuable on older infrastructure where the coating history may extend over many years.
Coating Systems May Contain Multiple Layers
Existing protective coating systems are not always a single material. During the life of a structure, additional maintenance coats, local repairs and complete or partial recoating campaigns may have been undertaken. A sample removed from an asset may therefore contain several distinct coating layers. If the objective is to understand the historical coating system, analysing only the outermost layer may not provide the complete picture. Depending on the investigation, laboratory examination of individual layers or selected portions of the coating system may be required. This is why the sampling strategy should be considered before samples are removed rather than simply collecting convenient pieces of paint.
Sampling Is Critical
Laboratory analysis is only as useful as the samples submitted for examination. Samples should be selected so they are representative of the question being investigated. Consider different structural areas, apparently different coating systems, original and repaired areas, sound and defective coating, different colours or visible layers, sheltered and exposed locations, and areas representing different maintenance histories. Each sample should be traceable back to its location on the structure. Photographs, sample references, drawings or survey location records can help maintain this traceability. Without appropriate location information, even good laboratory data can lose much of its value to the maintenance investigation.
What Can FTIR Tell Us?
FTIR can provide information that assists with the classification and identification of chemical groups and binder types present within coating samples. ASTM guidance on infrared examination of paints recognises FTIR as a technique that can be used for the classification of paint binder types and pigments and for comparison of spectra. Depending on the sample and analytical interpretation, FTIR may help distinguish between different generic coating chemistries. The laboratory result should, however, be interpreted in the context of the coating sample and the wider investigation. FTIR is an analytical tool — not a complete coating condition assessment on its own.
What FTIR Does Not Automatically Tell Us
An FTIR result should not automatically be interpreted as identifying the exact coating manufacturer or commercial product, the precise age or original application date of the coating, remaining service life, original dry film thickness, adhesion strength, the presence or concentration of every pigment or hazardous constituent, or whether a proposed maintenance coating is guaranteed to be compatible. Some coatings can contain mixtures of resins, pigments, fillers, additives, degradation products or contamination that complicate interpretation. Weathering and ageing may also affect the material being examined. Results need competent interpretation rather than being treated as a simple product-identification printout.
FTIR and Coating Compatibility
One important reason for investigating an existing binder is maintenance overcoating. Knowing the generic chemistry of an existing coating can help inform decisions about potential maintenance systems. However, binder identification alone does not prove compatibility. Compatibility can also depend on condition and adhesion of existing layers, contamination, surface preparation, coating thickness, ageing and degradation, solvent sensitivity, proposed repair materials, proposed overcoating product and application conditions. Where uncertainty remains, additional investigation or representative compatibility trials may be appropriate before a maintenance system is adopted across an asset. FTIR should form part of an evidence-based decision rather than being treated as a standalone compatibility test.
FTIR as Part of a Pre-Condition Coating Survey
FTIR becomes particularly useful when incorporated into a wider pre-condition coating survey. A survey may combine laboratory binder identification with visual coating condition, corrosion and defect mapping, existing dry film thickness, adhesion testing where appropriate, coating-layer observations, photographic records, substrate condition and maintenance history where available. Combining laboratory analysis with field inspection provides a much stronger understanding of the existing protective coating system than either source of information considered in isolation. This information can support development of an appropriate maintenance strategy.
FTIR and Coating Failure Investigation
FTIR can contribute to coating failure investigations. Where the identity of an existing coating or individual layer is uncertain, laboratory analysis may help establish the generic chemistry present. This information can be considered alongside failure location and interface, adhesion results, coating thickness, surface preparation evidence, environmental exposure, application records, microscopy or other laboratory techniques. FTIR should not be expected to determine the cause of a coating failure by itself. Failure investigation normally requires several sources of evidence to be considered together.
What Should Be Recorded?
Good sample traceability is essential. Records may include asset and project reference, sample identification number, exact sample location, component or structural element, photographs before removal, apparent coating colours and layers, condition of the sampled area, date of sampling, person taking the sample, reason for selecting the location, laboratory reference, analytical result, interpretation and limitations. Where multiple samples are taken, a sample schedule can provide a clear link between the physical asset, survey photographs and laboratory results.
Practical Inspection Note
Do not ask the laboratory simply to “identify the paint” without considering what information is actually required. The question being investigated should influence the sampling strategy. If an asset contains several coating layers or appears to have undergone multiple maintenance campaigns, a single sample from one convenient location may not adequately represent the coating system across the structure. Define the objective, select representative locations, maintain sample traceability and interpret the laboratory result alongside the physical condition of the asset.
Why Independent Inspection Matters
Paint Inspection Ltd can incorporate coating sampling and laboratory analysis into wider condition assessment programmes. The value comes from connecting the laboratory result to what is actually happening on the structure. Rather than treating FTIR as an isolated laboratory exercise, results can be considered alongside coating condition, defects, corrosion, thickness, adhesion and maintenance history. This provides asset owners and project teams with a stronger evidence base when determining future maintenance requirements.
Conclusion
FTIR binder identification can provide valuable information when the history or composition of an existing protective coating system is uncertain. It can help classify binder chemistry, investigate different coating layers and support decisions about further investigation and maintenance. Its greatest value comes when it is used as part of a wider assessment. FTIR can help answer “what type of coating are we dealing with?” — but understanding what should happen next requires the laboratory evidence to be considered alongside the condition of the coating and the requirements of the asset. For ageing infrastructure where coating records may be incomplete, combining field inspection with targeted laboratory analysis can turn uncertainty into evidence that supports a more informed maintenance strategy.
PRACTICAL INSPECTION NOTE
FTIR results are only as useful as the sampling strategy behind them. Establish what information is required, select representative and traceable samples, and interpret laboratory binder identification alongside the condition and history of the coating system. Binder identification alone does not prove compatibility with a proposed maintenance coating.
RELATED STANDARDS
ASTM E1252-98(2021) — Standard Practice for General Techniques for Obtaining Infrared Spectra for Qualitative Analysis.
RELATED EXPERTISE
This technical content is provided as general guidance and does not replace the requirements of the applicable project specification, coating manufacturer or current edition of the referenced standard. Official standards should always be obtained from the relevant standards organisation.
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