Transverse compression of wood can result in only the surface(s) of the wood material achieving a higher density, a process referred to as surface densification. Increased density generally leads to improved mechanical properties. For low-density wood species, this can increase value and open up new areas of use. By enhancing strength and hardness, surface-densified wood is positioned as a sustainable alternative to conventional materials such as plastic, metal, and tropical hardwoods, which are often selected for their high density. Despite decades of research and industrial development, the commercial spread of densified wood remains limited, which is considered to be due to high production costs, significant variations in product properties, and the complexity of scaling up the process for industrial use.

This doctoral thesis addresses several key technical barriers to the industrial production of surface-densified wood, primarily intended for flooring, focusing on a continuous process and the optimisation of material properties. The aims have been to (1) evaluate the feasibility of continuous densification, (2) investigate how knots and the orientation of growth rings in the wood's cross-section affect the process and the resulting product properties, and (3) attempt to establish methods for characterising and optimising the density profile of the compressed wood with regard to the desired mechanical properties of the compressed surface. Achieving repeatability in the process has also been a key focus.

Experimental studies have been conducted in a full-scale belt press specially developed for this project, which is also suitable for industrial use. The results from the studies conducted in this project demonstrate the potential of continuous densification, where desired density profiles can be achieved while avoiding non-value-adding production steps. The heat transfer to the wood material before and during compression was identified as crucial for compressing the wood’s cell structure in areas that result in the desired surface properties being obtained. The orientation of the growth rings in the wood’s cross-section was found to significantly impact how deformations vary within the wood during compression and how the wood surface springs back after compression. The results highlight the importance of adjusting material selection and process parameters to achieve the desired final product result. This is particularly relevant when fast-growing wood and so-called "low-quality" wood are used for densification, as such wood often exhibits strong variations in growth ring orientation and knot size.

Characterisation of the mechanical properties of densified wood showed a strong correlation between the density profile and the surface hardness of the wood. The indentation depth in traditional mechanical hardness tests is significantly affected by the density profile. This results in limitations of traditional hardness testing methods, such as the Brinell test, which can lead to misleading hardness values for a wood surface depending on the density profile. To avoid misjudging the hardness of a wood surface for surface-densified wood, the proposed approach involves continuously adjusting process parameters and the density profile to meet specific end requirements. The density profile of, for example, conventional wood-based panel materials is usually determined using X-ray densitometry. However, this method can be challenging to implement in a continuous industrial densification process. A practically applicable method for process control and design is proposed, where machine learning models are used to predict density profiles from photographic images of the cross-section of the densified wood. The method is considered applicable for industrial use and can be used in real-time for quality control and process management. This integration of digital tools supports data-driven manufacturing of densified wood.

This doctoral work bridges scientific principles and industrial practice, contributing to the potential commercialisation of surface-densified solid wood products. By addressing critical challenges in process efficiency and material characterisation, this work lays some of the foundational stones needed for large-scale industrial production of densified wood, such as for use in flooring.

Komprimering av trä kan resultera i att enbart trämaterialets yta(or) får en förhöjd densitet, en process som benämns ytdensifiering. Förhöjd densitet innebär i det allmänna fallet också förbättrade mekaniska egenskaper. För träslag med låg densitet kan detta resultera i ett ökat värde och öppna upp för nya användningsområden. Genom att förbättra egenskaper som styrka och hårdhet positioneras ytdensifierat trä som ett hållbart alternativ till konventionella material som plast, metall och tropiska träslag som idag väljs på grund av deras höga densitet. Trots decennier av forskning och industriell utveckling är den kommersiella spridningen av densifierat trä begränsad vilket bedöms vara en följd av höga produktionskostnader, stora variationer i produktegenskaper och komplexiteten i att skala upp processen för industriellt bruk.

Denna doktorsavhandling behandlar några viktiga tekniska hinder för industriell tillverkning av ytdensifierat trä avsett för i första hand golv, med fokus på en kontinuerlig process och optimering av materialegenskaper. Syftet har varit att: (1) utvärdera genomförbarheten av kontinuerlig densifiering, (2) undersöka hur kvistar och årsringarnas orientering i virkets tvärsnitt påverkar processen och de resulterande produktegenskaperna, samt (3) att försöka etablera metoder för karakterisering och optimering av det komprimerade virkets densitetsprofil med avseende på den komprimerade ytans mekaniska egenskaperna. Att uppnå repeterbarhet i processen har också varit i fokus.

Experimentella studier har bland annat genomförts i en fullskalig bandpress speciellt framtagen för detta projekt, men där det också är möjligt att använda den industriellt. Resultaten av i projektet genomförda studier visar på möjligheter med kontinuerlig densifiering, där eftersträvade densitetsprofiler kan uppnås samtidigt som icke värdeskapande aktiviteter kan undvikas. Värmeöverföringen till trämaterialet innan och under komprimeringen identifierades som avgörande för att komprimera träets cellstruktur i de områden som medför att eftersträvade ytegenskaper erhålls. Årsringarnas orientering i virkets tvärsnitt visade sig ha en stor inverkan på hur deformationer varierar inom virket under komprimeringen och hur träytan återfjädrar efter komprimeringen (spring-back). Resultaten visar på vikten av att anpassa materialval och processparametrar för att uppnå önskat slutresultat för produkten. Detta är särskilt relevant när snabbvuxet virke och virke av så kallad ”låg kvalitet” används för densifiering, eftersom sådant virke ofta uppvisar stora variationer i årsringsorientering och kviststorlek.

Karakterisering av mekaniska egenskaper för densifierat trä visade på ett starkt samband mellan den densitetsprofilens form och träytans hårdhet. Intrycksdjupet vid traditionella mekaniska hårdhetstest påverkas betydligt av densitetsprofilen. Detta visar på begränsningar i traditionella testmetoder för hårdhet, till exempel Brinell test, vilket kan resultera i missvisande hårdhetsvärden för en träyta beroende på densitetsprofilen. För att undvika missbedömning av en träytas hårdhet för ytdensifierat trä föreslås i stället för traditionella hårdhetstest, ett tillvägagångssätt där processparametrar och densitetsprofil kontinuerligt anpassas i processen för att uppfylla specifika slutkrav. Densitetsprofilen för till exempel konventionella träbaserade skivmaterial kan bestämmas med röntgendensitometri, men metoden kan vara svår att implementera i en kontinuerlig industriell process för densifiering. En praktiskt tillämpbar metod för processkontroll och design föreslås där modeller för maskininlärning används för att förutsäga densitetsprofiler från fotografiska bilder av det densifierade träets tvärsnitt. Metoden bedöms kunna tillämpas industriellt och kunna användas i realtid för kvalitetskontroll och processtyrning. Denna integration av digitala verktyg främjar datadriven tillverkning densifierat trä.

Detta doktorandarbete bygger en bro mellan vetenskapliga principer och industriell praktik, och bidrar till möjlig kommersialisering av ytdensifierade produkter av massivt trä. Genom att ta sig an kritiska utmaningar inom processeffektivitet och materialkarakterisering, har detta arbete med några grundstenar som behövs för en storskalig industriell produktion av densifierat trä för användning som till exempel golv.

The sawmill industry generates substantial waste in the form of wood chips, shavings, and sawdust, which can be repurposed to manufacture particleboards. Conventional particleboards rely on formaldehyde-based adhesives, posing health risks due to formaldehyde emissions. Seeking alternatives, this study explored a bio-based adhesive system composed of citric acid and sorbitol (10 wt%) combined with 0–20 wt% fire retardants (imidazolium-based ionic liquid or ammonium dihydrogen phosphate) for particleboards made from residual sawmill processing. The objective was to assess the efficacy of the adhesive system in enhancing fire retardancy, moisture resistance, and mechanical properties of particleboards. Results indicate that incorporating ammonium dihydrogen phosphate significantly improves fire retardancy, evidenced by limiting oxygen index values of 50–78% and a thickness swelling after water immersion of 9.7%. However, with an internal bonding strength of max 0.24 MPa and modulus of rupture of max 4.3 MPa, the bio-based boards fell short of meeting standard requirements. Future research should focus on optimising the general citric acid and sorbitol-based adhesive formulations to overcome this limitation. Achieving sustainability and safety standards in particleboard production remains a critical objective for future research and industry implementation.

Wood densification itself does not, in general, improve the fire-retardant properties sufficiently to reach the standard requirements. The object of this study was to enhance the fire-retardant properties of thermo-mechanically densified wood without any loss of moisture stability and hardness. Scots pine sapwood was pretreated before densification by impregnation with a fire retardant (FR) consisting of ammonium dihydrogen phosphate and urea and then cured in-situ by hot pressing at 150 °C or 210 °C. Densified specimens without FR were used as a control. Set-recovery, fire retardancy in an open flame test, and Brinell hardness were determined. The set-recovery was slightly reduced as a result of the FR treatment, but the pressing temperature and time had a much greater influence. In the open flame test, specimens without FR-treated ignited within 15-50s of exposure to the flame, whereas all the FR-treated specimens exhibited ignition resistance over the 10 minutes duration of the test. Water-soaking cycles had no impact on the ignition resistance in these groups, indicating a strong resistance to water leaching of FR after pressing at 210 °C for 60 minutes. The hardness increased due to the presence of FR after pressing at 210 °C, but sharply decreased after water immersion.

Many research papers highlight the positive impact of wood densification on mechanical properties of solid wood as well as wood veneers. Previous experimental studies comparing densified to non-densified wood materials of the same thickness consistently demonstrated higher strength properties in bending for densified materials. However, these studies often overlooked an important comparison: the strength and load-carrying capacity of non-densified wood material to its densified state, in which the thickness is reduced and thus the material’s moment of inertia. The current study, aims to address this gap for plywood made from low-quality, underutilized hardwood species, encompassing both non-densified and densified veneers. Additionally, these plywood panels are compared with high-quality birch plywood. Plywood samples made from common aspen (Populus tremula L.) and black alder (Alnus glutinosa L.) species, incorporating densified veneers, are compared to conventional silver birch (Betula pendula Roth.) plywood. The study also considers material utilization efficiency. Results showed that plywood made from non-densified aspen and black alder veneers exhibited comparable strength to standard birch plywood, positioning them as competitive alternatives to high-quality wood species. While densified black alder veneers increased strength in average from 83.6 MPa to 126.3 MPa, a loss in load-carrying capacity was observed from 1930 N to 1637 N due to reduced thickness and moment of inertia.

Wood, a fundamental material in construction, confronts durability and weathering challenges, notably UV-induced degradation leading to colour changes. This study investigated a novel treatment using citric acid and urea to enhance the UV stability of wood. The reaction between these compounds generates water-soluble fluorescent species and insoluble particles upon thermal treatment which may provide wood with UV protection. Specimens were treated with two different treatment methods and then exposed to 2016 h of accelerated weathering, during which colour was measured regularly. Citric acid and urea were either pressure impregnated into the wood and thermally reacted in situ during heat treatment or pre-reacted in the absence of wood with subsequent implementation into melamine formaldehyde (MF) and water-based surface coatings. The results showed that water-soluble fluorophore compounds were formed with both treatment methods. Accelerated weathering tests revealed significant colour changes over time, where specimens coated with a mixture of MF and fluorescent particles from the reaction between citric acid and urea, exhibiting the least alteration. The lowest colour change ΔE of 5.9 was observed for specimens coated with a MF-based coating containing 1 wt% of citric acid and urea thermally pre-reacted at a temperature of 180 °C, showcasing potential wood protection applications.

Increasing environmental awareness and the carbon-storing capability of wood have amplified its relevance as a building material. The demand for high-quality wood species necessitates exploring alternative, underutilized wood sources due to limited forest areas and premium wood volume. Consequently, the veneer-based industry is considering lower-value hardwood species like grey alder (Alnus Incania), black alder (Alnus glutinosa), and aspen (Populus tremula) as substitutes for high-quality birch (Betula pendula). Initially less appealing due to their lower density and mechanical properties, these species show promise through densification, which enhances their density, strength, and hardness. This study aims to enhance plywood screw withdrawal capacity and surface hardness by densifying low-density wood species and using them in plywood face-veneer layers, or in all layers. The relationship between the wood density, surface hardness, and screw withdrawal capacity of plywood made of low-value species like aspen and black alder is examined. Experimental work with a pilot-scale veneer and plywood production line demonstrates improved surface hardness (65% and 93% for aspen and black alder, respectively) and screw withdrawal capacity (16% and 35% for aspen and black alder, respectively) in densified face veneer plywood. This research highlights the potential of densified low-value wood species to meet construction requirements, expanding their practical applications.

Citric acid can be used as a bio-based binding agent in composites made from lignocellulosic material due to its crosslinking ability with hydroxyl groups via ester formation. Even though plenty of research has investigated diverse manufacturing conditions, with a focus on citric acid content and pressing temperature, an apparent knowledge gap persists regarding their effect on the properties of particleboards made from softwood sawdust. Furthermore, the optimal temperature for particleboards crafted from mostly non-wood lignocellulosic materials was found to be 180–200°C which is lower than that observed for softwood material in a prior study. This study aimed to systematically examine the effects of citric acid content and pressing temperature on particleboards derived from softwood sawdust. Results highlighted a positive correlation of both citric acid content and pressing temperature with mechanical and hygroscopic properties of the particleboards, the best results being achieved at 20 wt% citric acid and 220°C, exhibiting a thickness swelling of 9%, internal bonding strength of 0.77 MPa and a modulus of rupture of 4.3 MPa. This study served as a foundational framework for future investigations into the addition of other chemicals, offering a comprehensive understanding of their interactions with citric acid in particleboard manufacturing.

To improve the resistance of wood to biological decay the Maillard reaction between introduced amines and wood cell-wall polymers can be utilised. However, initial studies in wood modification showed almost complete leaching of bicine and tricine from treated wood and the loss of beneficial effects. The objective of this study was to assess whether possible reactions of bicine or tricine with wood could be further enhanced and reaction products stabilised through the addition of glucose and/or citric acid. Thus, Scots pine sapwood specimens were impregnated with tricine or bicine, with or without glucose and citric acid, and then heated to a temperature of 160°C. The dimensional stability, degree of chemical leaching and mechanical properties were assessed. Overall, it was concluded that neither the presence of glucose nor citric acid did appear to enhance the reactivity of tricine or bicine. Anti-swelling efficiency (ASE) of 50% was observed for combined treatments of bicine/tricine and citric acid but the leaching resistance originated mainly from citric acid and glucose, with no indication for the retention of bicine or tricine. The presence of citric acid led to a strongly reduced modulus of rupture. 

Densification, i.e. the transverse compression of sawn timber has been studied and commercialised for well over 100 years but remains an expensive niche product with low annual production volumes. One reason for this is the reliance on time-consuming batch processes in a hot press. To solve this, a continuous densification process using a belt press, capable of densifying full-sized sideboards was developed. However, there is insufficient knowledge about the effect of knots on the densification outcome. The objective of this study was to assess how different knot parameters affect the densified wood in terms of damage and deformation to the knot itself and the surrounding wood material. Multivariate data analysis methods were applied to a dataset of 171 knots, described by 23 variables. The data showed that it is possible to densify knots in a continuous process without causing damage. Especially sound knots are often unproblematic, even at relatively large sizes, while densifying dead knots often resulted in unacceptable damage to the knot or the surrounding wood. From a material selection standpoint, any knots bleeding into the board edge and dead knots greater than 20 mm in diameter should be avoided altogether.