## 2 Materials and Methods  
Single-layer as well as three-layer particleboards (PBs) were produced using cup-plant stalk particles (_Silphim perfolitatum_ L.). Stalks were 1.8 m long and square-sized with \(25\times 25\) mm\({}^{2}\). As a control, single-layer and three-layers PBs were also produced using spruce particles (_Picea abies [L.] Karst_). The material was first chipped in a Klockner chipper 120 \(\times\) 400 HzW.T (Klockner Maschinenfabrik, Lauenburg, Germany), at a cutting speed of 725 rpm, and a feeding speed of 1 m/s. The obtained chips approximately sized \(20\times 10\times 5\) mm\({}^{3}\) were then milled in a Condux-Werk HS 350 (Condux Maschinenbau GmbH & Co. KG, Hanau--Wolfgang, Germany) hammer mill. Afterwards, the particles were screened in the cascade vertical drum screener Allgaier D7336 (Allgaier-Werke GmbH, Uhingen, Germany). The sieve screens had mesh size openings of 5.0 mm, 3.15 mm, 1.24 mm, and 0.60 mm, respectively. Particles used to manufacture PBs were taken from the sieves with openings between >3.15 mm and <5 mm, which were then manually mixed at a weight ratio of 50:50. Afterwards, the particles were oven-dried at 74 \({}^{\circ}\)C for 4 days, reaching moisture contents between 5% and 7%. For the three-layer PBs, spruce particles at dimensions <1.24 mm were used for the surface layers, while cup-plant particles formed the core layer (3 LCP). Same procedure was done for the three-layer PB, with both the core layer and the surface layers made from spruce (3 LSP). For both three-layer PB types, the shelling ratio, which is the ratio of the surface layer thickness to the total thickness of the panel, was set at 0.3. All PBs were produced with the target density of 600 kg/m\({}^{3}\), and a panel thickness of 12 mm, and bonded with polymeric methylene diphenyl diisocyanate (pMDI) resin (Huntsman I-BOND\({}^{\circledR}\) PM4390, Huntsman GmbH, Hamburg, Germany). Two resin dosages were applied--pMDI was applied in amounts of 4% (MDI4), and 6% (MDI6), respectively. Particles were resinated in a drum blender for 5 min, using a pneumatic spraying nozzle. Consequently, the resonated particles were manually distributed in a wooden forming box (550 \(\times\) 550 mm\({}^{2}\)), and pre-pressed. The pre-pressed mat was then hot-pressed at 200 \({}^{\circ}\)C, at 3.2 MPa for 100 s, using a hydraulic Siempelkamp press (Siempelkamp Ma-schinen und Anlagenbau GmbH, Krefeld, Germany). The target thickness of the panels was checked at random positions. In total, one PB per type was manufactured (Figure 1). For scanning electron microscopy (SEM), a Tescan S8000 (Tescan Brno, s.r.o., Brno, Czech Republic) was used to study the surface morphology of the various PB types. Likewise, morphology and interactions between cup-plant particles and wood particles were observed as well. The ultra-high-resolution mode was used, by means of an Everhart Thornley secondary electron detector. Low accelerating voltages (between 500 V--1 kV) were used to avoid surface charging. Sample surface was cut with a sliding microtome [34]. The multifunctional focused-ion beam SEM, TESCAN LYRA3 (Tescan Brno, s.r.o., Brno, Czech Republic), was used, which also had an integrated compact time of flight secondary ion mass analyser (C-TOF-SIMS) (Tofferk AG, Thun, Switzerland). An area of interest (AOI) of \(50\times 50\) um\({}^{2}\) was scanned with a focused Ga\({}^{+}\) ion beam (4092 pA, 30,000 V), while time-of-flight of secondary ions, and their clusters (molecules) were continuously analysed by C-TOF-SIMS. C-TOF was operated at 10 us dwell time, which provided the mass range of 0-170 \(m/Q\). In parallel with the AOI scanning using focused ion beam  
Figure 1: Cross-sectional views of the produced particleboards. 3 LCP—three-layer particleboard (PB) with a cup-plant core layer, and spruce surface layers; 3 LSP—three-layer spruce PB, 1 LCP—single layer cup-plant PB, and 1 LSP—single layer spruce PB.