was determined since the density properties are not reasonably measurable on the loose and bulky furnish material and not required from the raw material. The properties of the cured fibre mats (Fmat, Chapter IV–1.1) with nominal area densities of 𝜌A,nom= 1 kg m⁄ 2 and 𝜌A,nom= 2 kg m⁄ 2 obviously depend on the individual forming and densification. The MC values, how-ever, can be combined to one Fmat mean due to simultaneous manufacturing of the specimens.
Likewise, the cured particle mats (SLmat and CLmat) reveal a structural impact. Furthermore, resin-unblended SL particles were conditioned on a moist (20/83) and dry (OD) level in addition to the standard conditions. Consequently, MC significantly differs from 20/65 level, where, how-ever, 𝑀𝐶 = 2 % represents the effective EMC of the OD but hygroscopic material during the, nev-ertheless, rapidly performed X-ray measure-ments. Accordingly, the results are found to vary more considerably with 𝐶𝑉 = 5.7 % (of the meas-uring values, i. e., 0.11 % MC). Moreover, the shown 𝜌 values and ranges of 𝑡 and 𝜌A represent the results of manual mat forming (Figure IV-1), which was exclusively performed by means of varyingly conditioned SL particles. Finally, the properties of the utilised MDF of industrial origin (indMDF) are listed according to the nominal panel thickness and the values are found to range in common orders. Regardless of the typ-ical raw density differences between the individ-ual MDF (or HDF) types, MC values are similar with a rather low variation of 𝐶𝑉(mean) = 1.7 %.
The range of variation of all determined material properties in Table IV-5 is found to be in a typical order and CV tendentially more or less de-creases with increasing panel thickness. Moreo-ver, CV of the thickness is expectedly low due to calibration sanding of the panels except the cured furnish mats owing to the compressible (Fmat) and partly coarse (SLmat and CLmat) structure of the specimens, where the variations are rather attributed to measurement. All differ-ences from typical CV ranges, e. g. in the case of labMDF400-11.7, were double-checked and are considered to be attributed to panel pro-cessing. The like applies to 𝜌 and 𝜌A, where the industrially obtained homogeneity (indMDF)
de-pends on the panel type (application) and man-ufacturer and the lab-made materials (labMDF and all furnish mats) reveal the human impact despite all the care taken. Accordingly, the mean CVs of labMDF exceed the indMDF values.
Obviously, the actual properties of lab-made samples individually differ from their target val-ues (Table IV-1, Table IV-2). Differences of the panel and particularly the mat thickness 𝑡 corre-spond to the unpredictable springback after con-solidation as well as the swelling and shrinkage behaviour of the final specimens. However, the setpoint for panel calibration sanding was rather kept beyond target thickness not to yield too thin panels. In consequence of thickness calibration, the density values partly fall below the target (particularly in the case of labMDF1056) alt-hough an appropriate sanding allowance of 1.5 mm was considered in the gravimetrical fur-nish dosing. However, 𝜌A differences from the target may further be attributed to not double-checking the resulting MC of the furnish after blending, where, in turn, the actual dry forming mass is affected by the very same. Furthermore, a defined forming allowance was not applied and lateral expansion during hot-pressing of the lab-MDF was just considered via fibre distribution (edge overmetering). Note here, the cured fur-nish mats were formed and consolidated in steel cylinders. Eventually, the aforementioned hu-man impact particularly regarding hu-manual mat forming variations is unavoidable in the case of lab-made material.
Beyond sample manufacturing, the human influ-ence affects also the specimen measurement, where both systematic and random errors may occur during gravimetrical raw density determi-nation. FREYBURGER et al. (2009) compute the relative error Δ𝜌 𝜌⁄ = 0.1 … 0.15 % depending on specimen size, where questionable single errors are assumed corresponding to the division (dis-played digits) of the utilised calliper and scale.
Nevertheless, KORTÜM, RIEGEL (2017) point out the potentially high range of variation of the measuring device and explore the gauge capa-bility in woodworking. Moreover, own un-published capability investigations reveal meas-uring related deviations particularly due to
oper-84 1 Material Section IV
ator influence considering the mechanical di-mension determination of the specimen with fur-ther dependence on the material structure. The results determined via repeat measurements (𝑛 = 100) on common MDF and PB specimens clarify the practically achievable accuracies by means of a calliper in the order of 𝐶𝑉 = 0.03 … 0.08 %, where typically right-skewed di-mension distributions occur. Therefore, trun-cated volumes are mechanically measured.
However, specimen mass determination is less error-prone by operator impact. Consequently, gravimetrically determined raw densities are em-pirically assumed to exceed the actual values of the solid body in general. Here, final accuracies of gravimetric raw density determination are computed (via Gaussian propagation of uncer-tainties) in the order of 𝐶𝑉 = 0.07 … 0.15 %, which is just the potential error of the manual measuring process regardless of the material variations. In the case of rather soft WBC mate-rial such as insulation boards, the compressibil-ity comes in addition and results in apparently di-minished volumes and increased raw densities, where the measuring error is considered to ex-ceed above figures. Therefore, DIN EN 12085 (2013) defines the maximum measuring pres-sure of the applied gauge. In contradiction, teared out fibres or particles in the edge region reduce the specimen mass resulting in appar-ently decreased raw densities, where the influ-ence decreases with increasing specimen size.
Hence, gravimetrical raw density determination with contacting dimension measurement is in-herently erroneous regarding reasons such as - operator influence (handling of the
measur-ing device),
- material influence (compressibility), and - material preparation influence (rectangular
cutting and teared-out particles),
which has rarely been pointed out yet. The actual impact is, finally, hardly quantifiable and de-pends on the individual material and measuring conditions including operator skills.
2 Material characterisation
2.1 True density and porosity
2.1.1 Sampling and method
Purposing both general material characterisation and particular true density (i. e. solid matter den-sity) determination of the wood-particle-resin-matrix as actually radiation attenuating matter, gas pycnometry was carried out in accordance with DIN 66137-2 (2004). Measurements were exclusively performed on labMDF (𝑡𝑛𝑜𝑚= 19 mm, all raw density levels) and MDF-19 as well as PB-19 (both from round robin test) as in-dustrial comparison samples. Specimens were prepared following the recommendations of ZAUER et al. (2013). Accordingly, thin slices in the tangential-radial plane, i. e. 𝑡long< 3 mm, are required to facilitate free access of displacement gas to all pore spaces, which can here addition-ally be inhibited by the applied adhesive resin.
Since MDF and PB are composed of randomly in-plane-aligned TMP fibres or particles, respec-tively, considerably less wood fibres are as-sumed to be cut by preparing respective cross-sectional slices compared to described solid wood slices. Nevertheless, specimens were cut from two distant areas of the respective panels with nominal dimensions of 2 … 3 × 10 × 19 mm3 (𝑙 × 𝑤 × 𝑡panel). One sample comprises three random specimens from each panel area (6 pieces in total). Additionally, raw TMP fibres (resin-unblended) of labMDF were investigated to evaluate both adhesive resin and hot-pressing influence. Therefor, cluster sampling was per-formed directly from the bag at representative positions and the obtained material was remixed again. For size reduction of fibrous material and cutting of its cell lumina, the sample was milled by means of a laboratory rotor mill (Ultra Centrif-ugal Mill ZM 200, RETSCH GmbH, Haan, Ger-many) with a 12-tooth rotor at 𝑛 = 14’000 min−1 utilising a ring sieve with 0.5 mm trapezoid holes.
Fibre morphology characterisation in Chapter
23 True density determinations were performed at the Institute of Wood and Paper Technology, Technische Universität Dres-den, Germany, with outstanding support by the local staff. Their obliging and straightforward cooperation allowing own per-formance of the analyses is highly appreciated.
IV–2.2 reveals that milling evidently was efficient by cutting wood cells as intended, which ob-tained crucially reduced fibre lengths with values below 1200 µm as upper quartile Q3 (< 2400 µm, 97.5 % quantile). Note, milled fibres actually orig-inate from bulk sample for elemental analysis (Chapter IV–2.4.1). Here, five samples were withdrawn again via cluster sampling from the milled material with individual masses around 0.5…0.8 g (OD). After sampling was performed at EMC in consequence of common conditioning at 20 °C and 65 % RH (20/65), all samples were oven-dried to constant mass and individually vacuum-sealed for transport. Measurements on 𝑛 = 3(5) samples as repeat determination with maximum ten iterations each were carried out by means of the device ULTRAPYCNOMETER 1000T, QUANTACHROME GmbH & Co. KG, Odelzhausen, Germany with Helium (He) as dis-placement gas. For illustrated specimen prepa-ration, sampling, and measuring equipment, re-fer to Appendix VII–2.1. Device was calibrated for bulk volumes around 1.9 cm³. After appropri-ate conditioning at room temperature, samples were unpacked and weighed (𝑚OD) immediately prior to measurement and positioned vertically preferably free-standing within measuring cell of the pycnometer to facilitate gas flow around.
Specimens of the samples were handled without skin contact very rapidly to avoid moisture ab-sorption. For solid matter volume 𝑉S [cm3] deter-mination, standard procedure of the laboratory23 with established device settings was carried out per sample with ≤ 10 iterations aiming at repeat deviation < 0.1 %. At this, 𝑉S is derived from cor-respondingly measured pressure differences based on ideal gas equation at isothermal condi-tions (Boyle-Mariotte law). For methodical de-tails, reference is made to DIN 66137-2 (2004) and particularly to ZAUER et al. (2013) regarding measurements on wood. Special procedures for applications on, e. g., carbonaceous materials or plastics are standardised in DIN 51913 (2013)
86 2 Material characterisation Section IV
and DIN EN ISO 1183-3 (2000), respectively.
Both state requirements for repeatability in the order of 0.1…0.2 % likewise predefined here.
Finally, true density 𝜌t [kg m⁄ 3] is computed via 𝜌t=𝑚OD
𝑉S (IV-4)
per sample. Furthermore, porosity Φ [%] is cal-culated according to
Φ = (1 −𝜌OD
𝜌t ) ∙ 100 [%] (IV-5) with global mean oven-dry raw density 𝜌OD [kg m⁄ 3] of the respective material. Local (oven-dry) raw density and porosity based on bulk volume measurements via calliper on each single (tiny) specimen were not determined be-cause a valid volume measurement cannot be ensured regarding to partly soft and fibrous, thus compressible, material showing holes and non-parallel surfaces from simple cutting. Alterna-tively, respective global values were taken from Table IV-5 in Chapter IV–1.5.
In order to complete furnish and WBC material, cured adhesive resin was analogously investi-gated, i. e. exclusively preparation type UF2-CH according to Table IV-4. To this end, crushed material was further milled by rotor mill as above.
The powder enables enhanced displacement gas accessibility to potential pores occurred from evaporating water during curing. Assuming a certain hygroscopicity of solid UF resin, a sample of milled solid resin was dried due to requirement of DIN 66137-2 (2004) and further related stand-ards. To avoid over drying in the oven, which is associated with post-curing of adhesive resin, emission of free formaldehyde, and, on the other hand, thermally-induced hydrolysis with cracking of the macromolecules, process control is re-quired. Thus, drying at approximately 103 °C and simultaneous MC determination was carried out by means of a rapid lab device (CM easy plus, C.M. Instruments, Oerlinghausen, Germany) in-cluding a balance (0.0001 g displayed digits) be-sides radiative heat source. Here, controlled dry-ing stops when constant mass of the weighed portion is reached. However, considerable mass loss exceeding potential MC was observed sup-posedly owing to aforementioned effects during drying. Thus, besides oven-dry material
(UF2-CH-OD), two samples conditioned at 20/65 (UF2-CH) were prepared and individually vac-uum-sealed again. Pycnometric 𝑉S measure-ments with weighed portions of nominally 1.33 g corresponding to powder bulk volume of approx-imately 1.9 cm³ were performed as repeat deter-mination with 𝑛 = 2 samples each. Subsequent 𝜌t calculation according to eq. (IV-4) yields den-sity of the cured resin which appears as solid non-porous matter. Hence, Φ computation ac-cording to eq. (IV-5) is dispensable.
In addition to gas pycnometry, exploratory meas-urements on dry adhesive resin as solid body were performed by immersion method. Follow-ing DIN EN ISO 1183-1 (2013), density determi-nation was carried out via Archimedes’ principle (i. e. buoyancy method) within purified water (without wetting agent) as immersion liquid (IL).
To this end, analytical balance XS205DU, Met-tler-Toledo GmbH, Giessen, Germany, equipped with a convenient density kit was utilised at un-controlled surrounding conditions with 𝜗RT= 25.7 ℃. Therefore, actual density of the immer-sion liquid H2O determined via an appropriate sinker yielded 𝜌IL= 997.177 kg m⁄ 3, which is in good agreement with tabulated value 𝜌H2O,25.7= 996.89 kg m⁄ 3 (cf. DIN CEN/TS 15405 (DIN SPEC 1152) (2010)). For further performance description, reference is made to DIN EN ISO 1183-1 (2013). Accordingly, specimen density 𝜌S [kg m⁄ 3] at 𝜗RT is calculated via
𝜌S= 𝑚S,A 𝑚S,A− 𝑚S,IL
∙ 𝜌IL (IV-6)
with apparent specimen mass in air 𝑚S,A [mg]
and within immersion liquid 𝑚S,IL [mg], respec-tively. The specimens with irregular shapes but without undercuts and visually free of pores were fragments with 𝑚S,A= 297 … 1613 mg of the solid resin blocks prepared according to Chapter IV–1.4. Coarsely crushed material was utilised without further milling. Here, density measure-ments were again exclusively carried out on UF2 where samples were taken from preparation types UF2-D and UF2-DH according to Table IV-4. Re-conditioning and interim storage was carried out at 20/65. Considering potentially bound water, part of UF2-DH sample was