Poly(vinyl acetate), PVAc, adhesives are commonly used for wood bonding; however, they are fossil-based and the final products usually do not have a sufficient water resistance for more durable applications. In this study we prepared an adhesive formulation by grafting VAc from chitosan using emulsion polymerization, chitosan-graft-PVAc. Thereby, we could decrease the fossil-based content of the adhesive and at the same time significantly improve the water resistance. Chitosan by itself has very good bonding properties as a wood adhesive, especially regarding water resistance; however, very low solid contents of the adhesive formulation can be achieved due to a very high viscosity of chitosan adhesives. In our chitosan-graft-PVAc adhesives, we explored two chitosan samples with different molecular weights, by using as-received chitosan and hydrolyzing it to a lower molecular weight. The chitosan fractions in the adhesives prepared with a higher molecular weight chitosan were 15, 20 and 25 wt%. However, due to the high viscosity, a solid content higher than 17 wt% could not be achieved for these adhesives. Sufficient bond strengths were achieved when the adhesive was applied in 122 g/m2 solid spread rate. In order to decrease the viscosity, we used hydrolyzed chitosan, with a lower molecular weight, to allow for a higher adhesive solid content, 34 wt%, and for a higher chitosan fraction, 40 wt%. In the adhesive with 40 wt% chitosan and 17 wt% solid content, all VAc was grafted from chitosan. This decreased the molecular mobility of the chains, leading to a lower susceptibility to plastic creep in the adhesive which contributes to the final bond strength. The dry and wet strengths of the specimens bonded with adhesives containing chitosan were higher than the strength of the specimens bonded with the reference PVAc adhesive.

Trees play a crucial role in a time of urgent need for a green transition. Forest-based materials have the potential not only to replace fossil products but also to drive innovation across most technological fields. This was showcased in an exhibition at KTH Library in 2024.

The exhibition was organised by KTH Library in collaboration with Wallenberg Wood Science Center (WWSC). At this research center, researchers explore the amazing building blocks of trees to develop new materials with groundbreaking properties. Transparent wood, the world’s strongest bio-based materials, and membranes for energy storage are just a few examples of how we can create new types of sustainable materials from the forest. 

In addition to showcasing forest-based materials, the exhibition also features interviews with KTH researchers discussing the forest's role in the development, potential, and challenges of new wood-based materials. These interviews were conducted by KTH Library in August and September 2024 and were included in the exhibition. 

In connection with the exhibithion there was a book discussion and a popular science lecture.

Locust bean gum, derived from the carob tree, was evaluated as a biobased adhesive in particleboard manufacturing, investigating the effect of adhesive amount, mat moisture content, and process parameters such as temperature and time. Single-layer particleboards prepared with locust bean gum showed that effective hydration of the polymer chains is necessary to achieve satisfactory interactions with wood and thus yield a sufficient particleboard strength. A mat moisture content below 30 % resulted in weak particleboards, which easily broke immediately after pressing. With increasing mat moisture content, while keeping the adhesive amount constant, the internal bond strength was increased. Moreover, with constant mat moisture contents (40 %), the internal bond strength increased when the adhesive amount was increased, even though not proportionally. With an increase from 9 % to 18 % adhesive, the internal bond strength was increased by more than 100 %. However, with a further increase in adhesive content from 18 % to 36 %, the increase in internal bond strength was statistically insignificant. Even with high mat moisture contents (35–45 %), larger lab-scale particleboards had internal bond strength that fulfilled standard requirements for P2 boards, commonly used for furniture in dry conditions (SS EN 312), when the pressing time was long enough (75 s/mm) to allow for water and vapors to be removed before releasing the pressure. Using biopolymers as adhesives, without extensive chemical modification and hazardous crosslinkers, could lead to a more benign and sustainable particleboard production. Since the chemistry and setting/curing processes of biopolymer-based adhesives differ from those of the fossil-based adhesives used today, increased understanding of how production parameters affect the properties of the particleboards prepared with biopolymers may pave the way for their better utilization in this field.

Today, the most commonly used adhesives in the wood industry are fossil-based, but the search for new, renewable and less hazardous, resources for adhesives is intensifying. Hemicelluloses show promising bonding performance when used as a component in wood adhesives. Since batch-to-batch variations can affect the use of hemicelluloses in a large-scale production, we have investigated the effect of hemicellulose molecular weight on important adhesive properties, such as viscosity and bond strength, by using locust bean gum as a hemicellulose model and varying the molecular weight through hydrolysis. Results showed that there is a nonlinear proportionality between bond strength and molecular weight. In the molecular weight range used in the study, 70–1460 kDa, anoptimum in the adhesive performance was achieved with intermediate molecular weights, 320 and 530 kDa, especially when considering applicability and bond strength. Adhesives with lower molecular weights, 70 and 150 kDa, did not exhibit sufficient cohesive strength; therefore, the bond strength was lower. The adhesive with higher molecular weight, 1460 kDa, was difficult to apply, especially since its maximum solid content was very low, 5 wt %.

Today, most wood adhesives are prepared from fossil-based polymers and contain hazardous components,e.g., formaldehyde. With the growing environmental concern there is an urge to develop bio-based and harmless substitutes. In this study, the ambition is to explore and valorise hemicelluloses, a biproduct from pulping, as the main component in wood adhesives. Wood adhesives were prepared from different sources: xylan from beech wood, hemicellulose-rich liquids obtained from hydrolysis of hardwood, and ultrafiltered softwood hemicellulose recovered from the process water of a thermomechanical pulp mill. Hemicelluloses themselves do not exhibit sufficient bonding performance, but excellent bond strength and water resistance were obtained in combination with poly(vinyl amine). It was also demonstrated that chitosan can be used as a bio-based amino-functional alternative to synthetic poly(vinyl amine), with similar or superior properties. Hemicelluloses alone show insufficient water resistance, but hemicelluloses in combination with chitosan exhibit exceptionally good bonding performance, especially regarding water resistance. Adhesives prepared from liquids rich in hardwood- and softwood hemicelluloses showed similar bond strength in combination with amino-functional polymers (poly(vinyl amine) and chitosan), regardless of their differences in structure. The current study constitutes an example on how sidestreams from the pulp industry in combination with chitosan can be used to substitute fossil-based materials in the quest for a more sustainable society.

The polymerization of a bio-based terpene-derived monomer, sobrerol methacrylate (SobMA), was evaluated in the design of polymeric nanoparticles (nanolatexes). Their synthesis was accomplished by using emulsion polymerization, either by free-radical polymerization in the presence of a cationic surfactant or a cationic macroRAFT agent by employing RAFT-mediated polymerization-induced self-assembly (PISA). By tuning the length of the hydrophobic polymer, it was possible to control the nanoparticle size between 70 and 110 nm. The average size of the latexes in both wet and dry state were investigated by microscopy imaging and dynamic light scattering (DLS). Additionally, SobMA was successfully copolymerized with butyl methacrylate (BMA) targeting soft-core nanolatexes. The comparison of the kinetic profile of the cationically stabilized nanolatexes highlighted the differences of both processes. The SobMA-based nanolatexes yielded high T-g similar to 120 degrees C, while the copolymer sample exhibited a lower T-g similar to 50 degrees C, as assessed by Differential Scanning Calorimetry (DSC). Thereafter, the nanolatexes were adsorbed onto cellulose (filter paper), where they were annealed at elevated temperatures to result in polymeric coatings. Their morphologies were analysed by Field Emission Scanning Electron Microscopy (FE-SEM) and compared to a commercial sulfate polystyrene latex (PS latex). By microscopic investigation the film formation mechanism could be unravelled. Water contact angle (CA) measurements verified the transition from a hydrophilic to a hydrophobic surface after film formation had occured. The obtained results are promising for the toolbox of bio-based building blocks, focused on sobrerol-based monomers, to be used in emulsion polymerizations either for tailored PISA-latexes or facile conventional latex formation, in order to replace methyl methacrylate or other high T-g-monomers.

Generation of renewable polymers is a long-standing goal toward reaching a more sustainable society, but building blocks in biomass can be incompatible with desired polymerization type, hampering the full implementation potential of biomaterials. Herein, we show how conceptually simple oxidative transformations can be used to unlock the inherent reactivity of terpene synthons in generating polyesters by two different mechanisms starting from the same alpha-pinene substrate. In the first pathway, alpha-pinene was oxidized into the bicyclic verbanone-based lactone and subsequently polymerized into star-shaped polymers via ring-opening polymerization, resulting in a biobased semicrystalline polyester with tunable glass transition and melting temperatures. In a second pathway, polyesters were synthesized via polycondensation, utilizing the diol 1-(1'-chydroxyethyl)-3-(2'-hydroxyethyl)-2,2-dimethylcyclobutane (HHDC) synthesized by oxidative cleavage of the double bond of alpha-pinene, together with unsaturated biobased diesters such as dimethyl maleate (DMM) and dimethyl itaconate (DMI). The resulting families of terpenebased polyesters were thereafter successfully cross-Iinked by either transetherification, utilizing the terminal hydroxyl groups of the synthesized verbanone-based materials, or by UV irradiation, utilizing the unsaturation provided by the DMM or DMI moieties within the HHDC-based copolymers. This work highlights the potential to apply an oxidative toolbox to valorize inert terpene metabolites enabling generation of biosourced polyesters and coatings thereof by complementary mechanisms.

Significantly increased production of biobased polymers is aprerequisite to replace petroleum-based materials towardsreaching a circular bioeconomy. However, many renewablebuilding blocks from wood and other plant material are notdirectly amenable for polymerization, due to their inert backbonesand/or lack of functional group compatibility with thedesired polymerization type. Based on a retro-biosyntheticanalysis of polyesters, a chemoenzymatic route from (@)-apinenetowards a verbanone-based lactone, which is furtherused in ring-opening polymerization, is presented. Generatedpinene-derived polyesters showed elevated degradation andglass transition temperatures, compared with poly(e-decalactone),which lacks a ring structure in its backbone. Semirationalenzyme engineering of the cyclohexanone monooxygenasefrom Acinetobacter calcoaceticus enabled the biosynthesis ofthe key lactone intermediate for the targeted polyester. As aproof of principle, one enzyme variant identified from screeningin a microtiter plate was used in biocatalytic upscaling,which afforded the bicyclic lactone in 39% conversion in shakeflask scale reactions.

Sobrerol methacrylate (SobMA) was synthesized and subsequently polymerized using different chemical and enzymatic routes. Sobrerol was enzymatically converted from -pinene in a small model scale by a Cytochrome P450 mutant from Bacillus megaterium. Conversion of sobrerol into SobMA was performed using both classical ester synthesis, i.e., acid chloride-reactions in organic solvents, and a more green approach, the benign lipase catalysis. Sobrerol was successfully esterified, leaving the tertiary alcohol and ene to be used for further chemistry. SobMA was polymerized into PSobMA using different radical polymerization techniques, including free radical (FR), controlled procedures (Reversible Addition Fragmentation chain-Transfer polymerization, (RAFT) and Atom Transfer Radical Polymerization (ATRP)) as well as by enzyme catalysis (horseradish peroxidase-mediated free radical polymerization). The resulting polymers showed high glass-transition temperatures (T-g) around 150 degrees C, and a thermal degradation onset above 200 degrees C. It was demonstrated that the T-g could be tailored by copolymerizing SobMa with appropriate methacrylate monomers and that the Flory-Fox equation could be used to predict the T-g. The versatility of PSobMA was further demonstrated by forming crosslinked thin films, either using the ene'-functionality for photochemically initiated thiol-ene'-chemistry, or reacting the tertiary hydroxyl-group with hexamethoxymethylmelamine, as readily used for thermally curing coatings systems.