The main objective of the KonTre project was to implement a process analytical technology (PAT) system in the new coating plant of the Norwegian wood cladding manufacturer Gausdal Bruvoll SA. The system enables automated monitoring, controlling, and documentation of process properties, and provides input data for multivariate analysis and prediction models; this allows to control quality and to reduce waste and costs. KonTre was the first attempt to apply the PAT concept to industrial coating processes of wooden cladding. Its implementation consequently required substantial R&D work on defining critical quality attributes (CQA) and process parameters (CPP), finding applicable measurement tools, and developing feasible solutions for data processing, analysis and modelling.
The background is that wood cladding has been the most common façade material in Scandinavia for centuries. During the last years, industrial coating application has been strongly increasing, accounting for approximately 80% of the cladding marked today. Process and product technology are evolving rapidly, and industrial quality control is a key aspect in this development.
Initially, the consortium focused on the characterization of CQA and CPP. Most attributes and parameters were known beforehand, but specific aspects required a more detailed clarification to establish a sound basis for investments in measurement equipment. The aim was to use equipment that is established in the wood industry, such as in-line capacitance moisture meters, but many tools had to be adapted, developed, and tested on laboratory and industrial scale. One subject of interest was the control of the wood temperature before coating, which is of special importance considering the cold Scandinavian winters when frozen boards from the outdoor storage are fed in the coating line. Modelling the temperature development throughout the cross-section of boards revealed that it takes several minutes on a cold winter day before sub-surface layers, which may affect film formation of the water-based coatings, reach a sufficient level. FTIR-transmission measurements indicated that IR-thermometers have a sufficient measuring depth in wood and, thus, are applicable as in-line tools. In contrast, an automated control of the color stability and coating failures with camera systems was found not to be feasible. This applies especially to transparent coatings as software algorithms cannot distinguish between the surface texture of the wood and coating defects. The consortium therefore focused on methods for monitoring the coating application. A laboratory study on thermal-optic equipment, which is known to accurately measure the coating film thickness on metal and plastic, revealed a significant influence of the wood moisture content on the accuracy of the equipment. Since the moisture content in cladding boards varies by several percent, the idea of controlling the application rate was favored and an in-line weighing system was developed. Laboratory trials with acoustic sensors mounted on spraying nozzles gave promising results in the automated control of coating application failures such as clogging or wear of nozzles. Due to the corona crisis, scheduled industry trials at Gausdal Bruvoll's coating plant could not be carried out. Thus, the technology's feasibility for process implementation is pending. Another emphasis was on assessing the influence of microfoam on the durability of coatings. Microfoam is air inclusions in the coating typically occurring in spraying application. Tensile strength test of free films, finite element analysis (FEA) and accelerated weathering testing of coated wood revealed that microfoam reduces the durability of a coating film and should therefore be avoided. Preliminary investigations indicated that pulse thermography in the near IR-range may detect microfoam in coatings; however, the technology has not advanced far enough to be used in the coating plant. Therefore, Gausdal Bruvoll has established a routine of taking samples that are visually examined for microfoam using microscopy.
Based on the work on CQA, CPP and sensor technology, Gausdal Bruvoll's plant was instrumented and a database was developed that registers at 54 points wood moisture content, surface temperature, application rate, climate data and other CPPs of importance for process and product quality. The database links the CPPs to production data on single-board level using RFID-technology. Control charts allow an easy statistical process monitoring and contribute to a significantly improved process understanding.
Future tasks are to assess the industrial viability of acoustic chemometrics for monitoring of the spraying process in Gausdal Bruvoll's coating plant. In addition, the work with multivariate analysis will continue based on a continuously increasing data set that allows improved modelling and prediction of process and product properties.
Overflatebehandlet trekledning er det mest vanlige fasematerialet for boliger i Norge. De siste ti årene har industriell overflatebehandling tatt store markedsandeler og utgjør nå ca. 70 % av totalt produsert volum. Denne utviklingen gjenspeiles i de betraktelige investeringene i malingslinjer i den norske treindustrien. Teknologi og produktutvikling er i rask endring, og de fleste kledningsprodusentene investerer regelmessig i nytt utstyr for å forbedre prosesser, og dermed 1) forbedre konkuranseevnen i det internasjonale markedet, 2) sikre at trevirket fremdeles er det mest brukte bærekraftige, og miljøvennlige, fasademateriale for boliger, og 3) gjøre bruk av tre som fasademateriale i urban arkitektur mer attraktiv. Innovasjonen i KonTre ligger i utviklingen av et avansert prosesskontrollsystem hos den norske kledningsprodusenten Gausdal Bruvoll. Gausdal Bruvolls produksjonsanlegg vil instrumenteres med moderne måleinstrumenter som 1) muliggjør automatisert overvåkning og dokumentasjon av prosess- og produktegenskaper, og 2) gir input-data til multivariate analyser og prediksjonsmodeller som skal bidra til prosessforbedring og produktoptimalisering. Et instrumentert overflatebehandlingsanlegg for behandling av trekledning er enestående i Skandinavia. De forskningsmessige utfordringene i KonTre omfatter utvikling av egnede målesystemer som vil danne grunnlaget for prosesskontroll- og optimalisering. Avanserte industristatistiske verktøy skal benyttes til å gjennomføre prosesskontroll. Denne typen statistiske verktøy har ikke tidligere vært i bruk i treindustrien, og heller ikke i norsk prosessindustri. Prosjektresultatene skal føre til mer lønnsom og miljøvennlig produksjon, dette vil oppnås ved å optimalisere utnyttelse av ressurser (trelast, malingsprodukter og energi). Bedre produkter vil bidra til forlenget levetid for malt kledning, og til lengre vedlikeholdsintervaller. Dette vil gjøre produktene mer bærekraftige og forenkle vedlikehold av bygningsmasse.