In a market where formulations are becoming increasingly complex and pressure on raw material costs remains high, coating measurement is becoming a process management tool. Near-infrared (NIR) spectroscopy demonstrates how this technology is being applied to the development of barrier papers and packaging structures
The search for products that are easier to recycle, together with the growing use of bio-based and biodegradable materials, is bringing coatings, barriers, binders and raw materials with increasingly complex formulations to the market. Accurately measuring the quantities of material used or the composition of a multicomponent structure is therefore becoming an important tool for keeping both product quality and the associated costs under control, while maintaining production line performance.
Valmet is addressing these challenges with a technology presented at the Aticelca 2026 Congress by Henri Lahtinen, Program Manager at the company.
New measurement challenges
Valmet has developed two technologies to meet different application requirements: Valmet IQ Reflection Spectrum Measurement (RSM), based on reflection spectroscopy, and Valmet IQ Transmission Spectrum Measurement (TSM), which uses transmission spectroscopy instead.
“These are spectroscopy-based sensors for in-line quality control of coatings,” explains Lahtinen. The transition towards materials that can replace plastics introduces new measurement challenges, particularly “when working with barrier coatings or very thin films, where even small variations in the applied coating weight can have an economic impact”.
In-line measurement therefore makes it possible to optimise raw material and energy consumption, reduce waste and limit downtime caused by quality issues or web breaks, helping to maintain process control.
The new technology is based on near-infrared spectroscopy (NIR). One of its most interesting features, Lahtinen explains, is the ability to perform measurements directly on the production line without contact. The sensor therefore “analyses the material as the product passes beneath the measuring head, without physically contacting the surface”.
Another important feature of this technology is the absence of ionising radiation. As the manager points out, “there is no need for nuclear sources for these measurements”. This is particularly relevant for applications in which the material must be monitored continuously along the paper machine.
The RSM sensor
The first of Valmet’s two configurations is Reflection Spectrum Measurement (RSM). The system uses a single measuring unit positioned on the same side of the product.
“The reflection geometry was developed primarily to measure coatings and barriers applied to the surface of paper and board.” Applications include polymer-based barrier coatings and coatings formulated with other materials, extrusion coatings and various special structures. The range of applications also includes primers, adhesives, stearates and other materials used in paper and board converting.
“Another application is the measurement of surface moisture, which can also provide useful information for monitoring dryer performance.”
The operating principle is based on the interaction between infrared radiation and the material being analysed. “The sensor head illuminates the surface of the moving paper or board. Some of the light penetrates the coating and reaches the underlying layers, interacting with the different components present. The radiation is then reflected back towards the sensor, which directs it to the spectrometer.”
In particular, “absorbance measures the attenuation of light: an absorbance of 1 corresponds to a tenfold attenuation. The measurement is based on different light attenuation mechanisms at different wavelengths.” Reading the entire spectrum therefore makes it possible to obtain information that would not be available by considering just a single wavelength.
From a technical standpoint, Lahtinen explains, “the RSM sensor operates in an infrared range of 1,200 to 2,450 nm. The measuring distance can vary from 0 to 100 mm, while the measurement spot is approximately 5 mm in diameter. This allows us to achieve a high degree of measurement accuracy.”
The TSM sensor
The second configuration is Transmission Spectrum Measurement (TSM). In this case, the system uses two measuring units positioned on opposite sides of the material.
“The light passes through the sample, and the transmitted radiation is collected by the second part of the system and directed to the spectrometer. The aim is to determine the total amount of a material present in the sample,” explains Lahtinen.
Applications identified by Valmet include tissue, nonwovens, specialty papers and certain barrier structures. In nonwovens, for example, the system can be used to analyse fibres, plastics and biomaterials, as well as binders and additives.
“Measuring total moisture is another application.” In tissue production, contactless measurement is particularly interesting because it makes it possible to determine the ratio of fibres to water without physically contacting a relatively delicate material.
Another application concerns plastic films directly. “PE, PP, PET and EVOH can also be analysed when they form part of multilayer structures. The ability to distinguish between the different components makes it possible to determine parameters such as basis weight or layer thickness without using radioactive sources.”
Technically, “the specified measuring distance for TSM ranges from 6 to 25 mm, while the spectral range is similar to that of the RSM sensor. In this case too, the radiation is generated by a halogen source, passes through the sample and is collected by the optical system on the opposite side.”
The two configurations are therefore not simply different versions of the same sensor. Their different geometries allow them to address different measurement challenges. Reflection is particularly suitable for determining the properties of surface coatings, whereas transmission is appropriate when radiation can pass through the sample and provide information about the overall composition of the structure.
Library-Based Calibration (LBC)
The ability to distinguish between different materials depends largely on calibration. This is where Library-Based Calibration (LBC) comes into play.
“The spectrum is like a fingerprint,” says Lahtinen, using an effective analogy to explain how each material has its own spectral characteristics. A reference library allows the system to compare the material being measured with information already available. “Valmet has a library of spectra for numerous raw materials used in the industry; this information provides the basis for identifying the different components present in the product.”
The problem, however, is that the spectral fingerprints are not always sufficiently different from one another. The LBC procedure begins by defining the product structure and the process.
Lahtinen gives the example of a structure consisting of an extruded polyethylene coating applied to board containing latex and kaolin. “Once the spectrum of the analyte, meaning the component to be measured, and the spectra of any potential interferents have been identified in the library, the calibration model determines which variables should be considered and constructs the calibration vector. The next step is to refine the model using actual samples supplied by the customer”.
The advantage of the LBC methodology is that the library can be used as a starting point, after which the model can be adapted to the specific characteristics of the production process.
Tests on barrier coatings
To verify the performance of the new sensors, Valmet technicians carried out several tests on barrier coatings and extrusion coatings. The tests were designed to reproduce conditions in which coating measurement is particularly challenging, both because of the low amount of material applied and because of the presence of components with interfering spectral characteristics.
One case involved a barrier coating typically consisting of a binder and other ingredients, including pigments. “In this situation, the coating signal must be separated from that of the base paper, which is a major source of interference.”
The test was carried out on a production line equipped with a curtain coater for applying the barrier to paper. “Different types of barrier, different application weights and different paper and board substrates were considered”, with the aim of assessing sensor performance under a range of conditions.
One example involved PVA (polyvinyl alcohol), for which “a good correlation was observed between the sensor measurement and the reference value”. However, the test also highlighted a typical difficulty associated with measurements at very low levels: when the amount of material approaches around 1 g/m² or less, even the laboratory reference measurement may be uncertain.
A second example involved a pigmented latex coating, which also showed a good correlation between the sensor signal and the reference value. Overall, “the tests demonstrated that the RSM sensor can be used to measure coating weights in the region of 0.5 g/m²”.
Laboratory and on-line tests conducted by Valmet specialists demonstrated the ability of NIR technology, in both reflection and transmission configurations, to measure low coating weights reliably, including values below 1 g/m², for both conventional coatings and binders and bio-based materials.
Extrusion coatings
A second area of testing involved extrusion barrier coatings. In this case, measurement becomes more complex when the coating consists of several layers.
In the example presented by Lahtinen, the structure comprised board, PE (polyethylene) and EVOH (ethylene vinyl alcohol). “Minimising PE usage was one of the key objectives for the extrusion coating,” explains the manager, while “EVOH, used as a barrier to replace aluminium in certain structures, was another material of interest”.
In these applications, the measuring system must therefore be able to distinguish the contribution of the different materials and determine the amount attributable to the component being monitored.
“Here too, the sensor readings were compared with laboratory reference data. The correlation was very good.”
Furthermore, “the ability to monitor coating quantity continuously makes it possible to link measurement to raw material management and, consequently, to waste reduction”.
The final example involved samples containing different barrier materials, with basis weights ranging from 180 to 320 g/m². Here too, good correlation with the reference values was achieved.
“Calibrations were developed for the different grades. The sensor can therefore be configured to recognise the specific characteristics of individual structures and maximise measurement performance.”
Measurement as a tool for process optimisation
Replacing plastics with latex-based barrier coatings, introducing bio-based binders, using different polymers and the growing adoption of multilayer structures are all changing the measurement challenge.
It is no longer sufficient to know the weight of the product as a whole. In many cases, it is necessary to determine the weight of a specific component and, above all, distinguish it from the other materials present.
NIR spectroscopy provides a solution to this challenge, while calibration is crucial, as are product samples supplied by customers for system development and fine-tuning.
Finally, the ability to measure coating weight in-line can provide useful information for adjusting operating conditions, making measurement an integral part of the process control system.
“An in-line system can help keep the value within the desired range during production, identifying variations both in the machine direction (MD) and in the cross direction (CD).” This is particularly important when the applied material is expensive.
As Lahtinen concludes, measurement accuracy is therefore becoming essential to address the current challenges facing the paper and packaging industries.
Three elements for reliable measurement
Valmet IQ Reflection Spectrum Measurement (RSM) and Valmet IQ Transmission Spectrum Measurement (TSM) sensors combine three main technological elements:
The latest developments in NIR (near-infrared spectroscopy) linear arrays and spectrometers.
An optomechanical design engineered to optimise light illumination and collection.
Computational algorithms and calibration methodologies that adapt the system to the wide variety of raw materials and product structures.