Hans Jörg Fehling

Anna Krotkova

Claude Saint-Ruf

Harald Von Boehmer

Hans Jorg Fehling

Claude Salnt-Ruft

Johannes M Van Noort

Arianne C Van Sechel

Jeffrey J Bajramovic

Mustapha El Ouagmiri

Chris H Polmant

Hans Lassmannt

Rivka Ravid

ment resumed. In this sense, it is important to note that after the zeugopodal defect, limb development does proceed in the double-mutant mouse, albeit with some dysmorphology. But clearly the autopod is less affected than the zeugopod. The autopodal phenotypes distal to the major zeugopodal defects are likely to be the result of a coupling between proximodistal and anteroposterior patterning 24 -28 . This coupling may be genetic, with group II genes directly influencing the timing of activation of group 12 and group 13 genes. Alternatively, morphological coupling may account for these defects. The elimination of both hoxa-11 and hoxd-11 gene products may reduce the overall mesenchymal cell population available to form the zeugopod and posterior carpals. A deficiency of cells may also alter the timing ('heterochrony') of formation of the rest of the limb. The second crisis would therefore be revealed at the end of autopod formation, when cartilaginous elements are competing to recruit the few remaining cells. This heterochrony would explain the delay in digit condensation seen during embryogenesis of the double mutant and account for the more severe phalangeal defects seen in digits II and V, which are the last elements formed in the last digits specified.

It is interesting that in the double mutants the forelimbs are more affected than the hindlimbs. Also, whereas (aa; Dd) and (Aa; dd) mice show an intermediate forelimb phenotype, the presence of a third mutant allele does not alter the hindlimb phenotype beyond what is seen in (aa; DD) or (AA; dd) mice. These differences may be a consequence of a third paralogue, hoxc-11, which is expressed more strongly in the hindlimbs 29 and could provide a compensatory function. This hypothesis would predict that in (hoxa -11, hoxc-11, hoxd-11) triple mutants, the tibia and fibula would be completely eliminated and show quantitative dependence on the dosage of mutant alleles. Also, the last vestige of the forelimb radius and ulna observed in the double mutant would be predicted to be absent.

Our model is supported by the hoxd-13 mutant mouse in which the mutant limb phenotype is restricted to the autopod 13 , and predicts that in a (hoxa-13, hoxd-13) double mutant the forepaw should be absent. hoxd-12 has no paralogue in the Hox A family and may therefore act on its own (or in conjunction with hoxc-12) to influence formation of the carpals and tarsals. Alternatively, hoxd-12 may recruit its closest Hox A paralogue (hoxa-11 or hoxa-13) to help specify this region. Single-mutant mice for hoxa-10, hoxc-10 and hoxd-10 should show subtle defects in the humerus and femur, but double and triple mutants should affect strongly the entire stylopod and initiate a cascade of less severe phenotypes downstream through the zeugopod and autopod. Consistent with the model, individual hoxd-10 and hoxc-10 mutant homozygotes show mild malformations of the humerus and femur (E. M. Carpenter, J. M. Goddard and M.R.C., unpublished results; S. L. Hostikka and M.R.C., unpublished results). From this model, the appendicular ground state would be achieved in an animal lacking all of the Hox 10-13 cognate genes. This should be a limbless mouse.

IN T -cell precursors, the T -cell-receptor p chain is expressed before the T-cell-receptor a chain 1 ' 2 and is sufficient to advance T-cell development in the absence ofT -cell receptor a chains>-- 7 • In immature T cells, the T -cell-receptor p protein can form disulphidelinked heterodimers with the pre-T -cell-receptor a chain 8 ' 9 and associate with signal-transducing CD3 molecules 5 • The recently cloned pre-T -cell-receptor a gene encodes a transmembrane protein that is expressed in immature but not mature T cells 9

' 10

• Here we show that ap, but not yo, cell development is severely hampered in pre-T -cell-receptor a-gene-deficient mice, which establishes a crucial role for the pre-T -cell receptor in early thymocyte development.

Intrathymic T-cell development proceeds from CD4_8_ precursors through CD4+8+ intermediates into CD4+s-and CD4-8+ mature thymocytes • Productive T-cell-receptor (TCR)-~ transgenes can partly relieve the developmental block, allowing the accumulation of immature CD4+8+ thymocytes but not mature CD4+sand CD4-8+ T cells 3 -7 that require positive selection by TCRa~ (ref. 13). In T-cell precursors the TCR-~ chain forms disulphidelinked heterodimers with the pre-TCRa (pTa) chain, and can associate with signal-transducing CD3 molecules 5 Here we report on the role of the TCR~-pTa heterodimer in development, based on experiments with pTa-deficient mice. These animals were generated by gene targeting in embryonic stem (ES) cells using a deletion-type targeting vector. On homologous recombination, this construct eliminated exons 3 and 4 of the pTa gene encoding the connecting peptide, which contains the cysteine required for heterodimer formation, the transmembrane region, the cytoplasmic tail and most of the 3' untranslated region (Fig. 1). Homologous recombination in ES cells and the absence of the deleted gene segment in pTa-!-mice was verified by Southern blotting with appropriate probes (Fig. 1, and results not shown). Offspring from intercrosses of pTa+!- mice were killed, thymus and bone marrow removed, and single cell suspensions prepared and analysed by cytofluorometry. There were no significant differences between numbers of marrow cells from age-matched mice and thymocytes from pTa+!+ and pTa+!-age-matched mice, whereas the number of thymocytes in pTa-deficient animals was reduced to less than 10%.

Figure 2 shows subsets from thymus of pTa+ and pTa-;mice only, because there was no difference in subsets of bone marrow and because lymphoid organs from pTa+;-and pTa+!+ mice did not differ. Both pTa+ and pTa-;-mice contain CD4-s-, CD4+s+ and single-positive CD4+s-and CD4-s+ thymocytes, but cells with CD4 and CD8 co-receptors are proportionally under-represented in pTa-;-mice whereas the proportion of CD4-S-25+ cells is drastically increased (Fig. 2). Both types of mice contain CD4 and CDS co-receptor expressing cells with low, intermediate and high levels of TCR-~ chain on Rl pTa gene ----I I Percentages were calculated from the same litter as described in Fig. 2 and Table 1.

the cell surface. The fraction of CD4 -s-thymocytes with TCRo chains on the cell surface was more prominent in pTa-;-mice, and both TCR~ and TCR8 chains were stoichiometrically associated with CD3 molecules. The data also show that the TCR~ chains were associated with TCRa chains, as revealed by doublestaining with TCR~ and TCR Va2, 3.2, S and 11 antibodies (Fig. 2, lower right). From triple stainings with various antibodies listed in Fig. 2 and thymocyte numbers, we calculated the absolute number of cells belonging to various thymocyte subsets as shown in Table I; the data show that the pre-TCR is not required to generate normal numbers of CD4 -s-3'ow25+ precursors of a~ T cells. In fact, their number is increased in pTa-;mice, probably because of the developmental arrest in these mice (see below). There is likewise no decrease in the number of CD4_S_ or CD4 and CDS co-receptor expressing yo T cells (Table 1). Table 2 shows that among CD4-s-3'ow cells, pTa-!mice have normal or increased numbers of CD44+25-, CD44+25+ and CD44-25+ cells, but no CD44-25-cells, which represent 45% in pTa+ mice. As most thymocyte expansion occurs in this subset'', it is not surprising that the absolute number of CD4+S+ cells as well as mature TCR a~h;gh CD4+s-and CD4-S' cells is much lower in pTa-;-than in pTa+ mice. Also, the proportion of TCR-a~-positive cells among CD4+s+ cells is lower in pTa-;-than in pTa+ mice. Lymph-node cells from 2Sday-old pTa-;-mice contain mature single positive CD4 and CDS cells representing about 5% of numbers found in pTa+ littermates (results not shown). -----------2.9 kb----FIG. 1 Disruption of the pTa gene by homologous recombination: partial organization of the pTa locus 22 and structure of the targeting vector. The pTa gene was cloned from a 129/0ia-derived genomic library (kind gift of A. Berns, Amsterdam). The targeting vector was constructed by replacing approximately 2.6 kilobases (kb) of the pTa sequence between the unique Xhol site in intron 2 and a Bg/11 site -60 base pairs (bp) upstream of the AATAAATAAA polyadenylation site with a 1.2kb Xhoi-BamHI fragment of pMC1neopA (Stratagene) carrying the neomycin resistance gene (neo). The isogenic targeting construct was electroporated into E14.1 embryonic stem cells as described 18 . ES colonies surviving G418 selection were analysed by the polymerase chain reaction (PCR) in pools of 12 using primers specific for the neo cassette within the tk promotor (ATICGCCAATGACAAGACGCTGC) and for the pTa gene just upstream of the EcoRV site (GTIGGATGTIATIGGTIAC-TACTCCTGA), respectively. Colonies within positive pools were rescreened individually by PCR and eventually by Southern analysis using 796 EcoRI-digested DNA and probe I (a 470-bp PCR fragment specific for pTa sequences outside the targeting construct, ~1.3 kb upstream of exon 2 (primers: TAGGTTIGAACTCAGAT; TGATTICTCTCTGTAGC)). Out of ~1,600 colonies screened, 3 had undergone homologous recombination, and one of these clones (pT355) gave rise to chimaeric mice. Chimaeric males were backcrossed with (C57BL/6 x DBA/2)F1 females and heterozygous offspring carrying a mutant pTa allele were intercrossed to obtain mice deficient in pTa. The absence of the deleted pTa sequences in homozygous knockout mice was confirmed by Southern blotting of EcoRI-digested genomic tail DNA and hybridization with a 310-bp ApaljBspEI complementary DNA fragment spanning the region of exon 4 that encodes the transmembrane portion, the cytoplasmic tail and 120 nucleotides of 3' untranslated sequence. Abbreviations for restriction sites: Rl, EcoRI; RV, EcoRV; X, XHol; H, Hindlll; Xb,Xbal.

The above data indicate that pTa has no role in the development of most yo T cells for one of three reasons: (I) because it is not expressed in the yo lineage; (2) it cannot pair with they chain; or (3) because there is no need for a putative TCRy-pTa heterodimer in yo T-cell development. Although this issue requires further investigation, we have been unable to detect RNA for pTa in thymocytes of TCRo surface-positive thymocytes 10 • The increase in the number of yo cells could depend on the availability of space, and/or the possibility that ongoing yo rearrangement is not terminated by the pre-TCR. The fact that a few TCRa~-positive CD4/8-co-receptorexpressing thymocytes can be generated in the absence of the pre-TCR is consistent with an earlier finding of TCRa rearrange- This view is consistent with data showing that TCR-positive thymocytes can induce the development of CD4+8+ cells when injected into rearrangement-deficient mice 14 ' 15 and explains the absence of CD4 "8+ thymocytes in rearrangement-deficient 6 ' 7 ,

TCR-negative 6 as well as CD3-negative mice (B. Malissen, personal communication). Differentiation through the CD4-8-25subset requires cell-autonomous signals delivered by the pre-TCR, and for that reason in normal mice almost all CD4+8+ T cells contain productive TCR-~ genes 16 , whereas in the absence of the pre-TCR, only ~2% of TCRa~-positive CD4+8+ precursors are generated by some aberrant differentiation. Nevertheless, positive selection will operate on these cells and generate mature a~ T cells whose number is regulated by homeostasis independent of the pre-TCR The small heat-shock protein aB-crystallin as candidate autoantigen in multiple sclerosis THE identification of key antigens in human autoimmune diseases is a crucial step towards the development of specific intervention. The autoantigen(s) relevant to multiple sclerosis (MS) probably reside in myelin of the central nervous system, the target of the disease 1 • Here we examine proliferative responses of human peripheral blood T cells to the complete collection of myelin proteins fractionated by reversed-phase high-performance liquid chromatography. Myelin isolated from MS-affected brain contained a single protein fraction to which T cells from MS patients and from healthy controls showed dominant responses. This highly immunogenic protein was identified as aB-crystallin, a small heat-shock protein. Immunohistochemical examination of MS lesions revealed the presence of oligodendrocytes and astrocytes with raised aDcrystallin expression, which were not found in unaffected myelin. Our findings indicate that aD-crystallin serves as immunodominant myelin antigen to human T cells when expressed at the elevated levels found in active MS lesions.

To examine responses of human T cells to the complete collection of myelin proteins from the central nervous system (CNS), short-term cultures were raised in vitro by priming peripheral blood mononuclear cells (PBMC) from 24 HLA-typed donors with total myelin protein. Proteins used for this purpose were prepared from either healthy control myelin or from myelin purified from MS-affected white matter. Five PBMC donors were clinically defined MS patients and the other nineteen were healthy control subjects. After one cycle of restimulation with total myelin protein, T cells were collected and tested for proliferative responses to each of the forty reversed-phase high-performance liquid chromatography (RP-HPLC) fractions prepared from the same protein preparation as that used for priming. Figure Ia shows the RP-HPLC profile of fractionated total myelin protein from MS-affected brain, which is virtually identical to that from control material. Data from the proliferation assays were essentially the same in all cases, irrespective of HLA-type or clinical status of the donor. A representative set of data from five donors is shown in Fig. lb.

The data in Fig. I illustrate two points. First, bulk T-cell responses after priming with all CNS myelin proteins were always directed primarily at the various minor myelin proteins contained in HPLC fractions 7-20. In contrast, responses to the major components proteolipid protein or myelin basic protein were barely significant. These findings extend our previous observations that the relative immunogenicity of individual CNS myelin proteins to human T cells bears no apparent relationship to their relative abundance in myelin 2 . Second, the data in Fig. I reveal a distinct difference between responses to proteins derived from either healthy myelin or MS-affected tissue. In the latter case, a consistently predominant response was detected against the contents of RP-HPLC fraction 8. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed that this fraction contained only one detectable protein, with an apparent relative molecular mass (Mr) of about 23K (Fig. 2). The same protein was detected as the single component in the corresponding fraction derived from the control preparation but in smaller amounts.

To identify the 23K protein in fraction 8, the protein was purified from MS-affected white matter by RP-HPLC. Direct amino-acid sequencing of the purified preparation was unproductive, indicative of anN-terminal modification. Thus, the 23K protein was trypsinized and a selection of the resulting fragments was purified by RP-HPLC and sequenced. As a result, four internal sequences were identified that were identical to sequences of human aB-crystallin. The amino-acid composition of the purified 23K protein was consistent with the full sequence of human aB-crystallin. In line with this the behaviour of purified human 23K protein and bovine aB-crystallin was identical in RP-HPLC and in SDS-PAGE. Furthermore, polyclonal rabbit antibodies

  -----5.6 kb-

FIG. 2

 2 FIG. 2 Thymocyte subsets from pTa+ and pTa-;-mice. Staining of thymocytes from 5-day-old (top two rows) and 2B-day-old (bottom two rows) litters with CD4, CDB, TCR~. TCRil and TCR-V" antibodies. All stainings were three-colour stainings with the following antibody-conjugates: anti-CD3-biotin and anti-CD3-FITC conjugates (500 A2 (ref. 19)), anti-TCR~-biotin and anti-TCR~-FITC (H57 (ref. 20)), anti-CD4-phycoerythrin (H129.19 Gibco), anti-COB-biotin and anti-COB-phycoerythrin (53-6.7 Pharmingen, San Diego), Anti-CD25-biotin (Pharmingen, San Diego), anti-CD44-phycoerythrin (ATCC collection KMB1), anti-HSAbiotin (M1169 (ref. 21)), and anti-TCRil--FITC (G3, Pharmingen, San NATURE • VOL 375 • 29 JUNE 1995

TABLE 1 Absolute

 1

	CD4_8_3Iow25+	25.6	54.0
		22.0	40.4
	CD4-8 8'	12.4	55.1
		14.6	32.0
	CD4'8'8+	10.0	6.5
		14.5	11.0
	CD4+8+	34.54	1.19
		34.62	1.13
	CD4 +8+TCRWnt.	14.50	0.24
		13.75	0.23
	CD4 +8-TCRp"'g"	3.51	0.13
		3.05	0.13
	CD4 -8+TCRp"'g"	0.81	0.09
		1.39	0.13
	Numbers were obtained from two different mice of each genotype
	from a 5-day-old litter.

number of thymocytes with different phenotypes Phenotypes pTa+ (x10-6 ) pTa ;-(x10 6 )

TABLE 2

 2

		Subsets among CD4 -8-3'ow thymocytes
	Phenotypes	pTa+(%)	pTa 1 (%)
	CD44+25	12.1	11.0
	CD44'25"	2.20	2.30
	CD4425"	40.7	86.6
	CD44-25-	45.0	0.10

supported by an NSF predoctoral fellowship and an NIH genetics training grant. We thank K. Saalfeld, T. Smith and C. Lewis for technical support,


Giacomo Goli

Andreja Kutnar

Dennis Jones

Dick Sandberg

Bertrand Marcon
email: bertrand.marcon@ensam.eu

Louis Denaud

Kenji Umemura

Joseph Gril

Shuichi Kawai

"Understanding wood modification through an integrated scientific and environmental

Keywords: Poplar Wood Modification, Heat Treatment, Time-temperature Equivalency, Energy of Activation, Kinetic Analysis, Mechanical Properties, Hygroscopicity, Wood Colour

The kinetics of several poplar (Populus alba L.) wood properties during thermal modification conducted in conventional oven with air recirculation were analysed and modelled in this paper. A wide range of properties was assessed, such as: equilibrium moisture content, sorption diagram, shrinkage coefficients, specific shrinkage coefficients, mass loss, modulus of elasticity, strength and colour. The tests were executed at different temperatures ranging from 90 °C to 180 °C and with different durations. The time-temperature equivalency was checked and property modifications over time analysed through master curves in order to obtain a general model connecting together properties, treatment temperature and duration. Different activation energies arising from each property evolution with treatment temperature and duration are provided showing that every modification could occur with different kinetics.

Introduction

The heat treatment of wood involves chemical and physical changes that improve some properties such as dimensional stability, decay resistance, and aesthetics, while simultaneously leading to cell wall degradation [START_REF] Matsuo | Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species[END_REF]. The effects of heat treatments can be beneficial, and classified as "thermal modification", or detrimental, then classified as "thermal degradation". Since thermal modification induces wood degradation, the treatment conditions must be optimized to enhance the positive effects while mitigating the negative ones [START_REF] Matsuo | Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species[END_REF]). Because heat treatment involves a large number of variables such as temperature, time, and type of atmosphere and treatment se-quence, a large number of methods were developed over the years, and a considerable research is still on-going to get a better understanding of the reactions involved. As a consequence, many treatment technologies are actually available on the market [START_REF] Rapp | Review on heat treatment of wood[END_REF][START_REF] Johansson | Strenght and colour response of solid wood to heat treatment[END_REF], and the general principles of wood thermal degradation are well documented in textbooks such as [START_REF] Navi | Thermo-hydro-mechanical processing of wood[END_REF] and [START_REF] Hill | Wood modification[END_REF]. The whole field of thermo-hydro processing of wood was subjected to large review papers such as [START_REF] Esteves | Wood modification by heat treatment: a review[END_REF] and [START_REF] Sandberg | Thermo-hydro and thermo-hydro-mechanical wood processing: an opportunity for future environmentally friendly wood products[END_REF]; those studies make partially the point on the different effects of thermal modification on properties.

A promising field of application for ther-mal modification is to consider the artificial aging of wood for special applications in cultural heritage. Many questions about the behaviour of wooden material during aging are still unanswered; wood researchers are more and more investigating cultural heritage problems [START_REF] Marcon | Hygromécanique des panneaux en bois et conservation du patrimoine: des pathologies aux outils pour la conservation[END_REF][START_REF] Dureisseix | A partitioning strategy for the coupled hygromechanical analysis with application to wood structures of cultural heritage[END_REF]). Indeed, because it is often impossible to perform mechanical tests on wood from the cultural heritage, it is practically very difficult to describe the effect of aging on wood. Moreover, the previous conditions of any given specimen of historical wood is usually poorly documented and can only be guessed. Actually, it is well known that aged wood and freshly cut wood have significant hygro-mechanical differences [START_REF] Esteves | Wood modification by heat treatment: a review[END_REF][START_REF] Bf ; Enayati | Study of dimensional stability and EMC of poplar wood (Populus alba) treated at different heat-temperature and initial moisture content[END_REF]. The only alternative to destructive tests on cultural heritage artefacts, to get information about aged wood, is to make artificial aging via thermal modification [START_REF] Obataya | Effects of high temperature kiln drying on the practical performances of Japanese cedar wood (Cryptomeria japonica). II: Changes in mechanical properties due to heating[END_REF]. Previous works conducted in Japan (RISH Institute and Tsukuba University) have shown similarities in colour properties between naturally aged wood and thermally modified wood [START_REF] Matsuo | Aging of wood: analysis of color changes during natural aging and heat treatment[END_REF][START_REF] Matsuo | Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species[END_REF]. These studies were mainly carried out on Japanese softwoods traditionally used in construction. Other studies consider thermal modification as very close to natural aging (Froidevaux et al. 2012, Ganne-Chédeville et al. 2012). As a first coarse approximation thermal modification in conventional oven seems to produce similar results than natural aging; however, the physical properties of aged wood depends also on its hygroscopic history [START_REF] Endo | Effects of heating humidity on the physical properties of hydrothermally treated spruce wood[END_REF]. Strictly speaking, oven-heated wood is partially different from naturally-aged wood because natural ageing proceeds at moderate relative humidity and not at a dry state. In addition, chemical reactions induced by heating above 150 °C seems to be qualitatively different from those below 150 °C, according to [START_REF] Ganne-Chédeville | Natural and artificial ageing of spruce wood as observed by FTIR-ATR and UVRR spectroscopy[END_REF].

The data summarized in this study will be useful for modelling and simulating poplar wood modification process by heat treatment. Until now, several scientific works used general energies of activation to model the kinetic of mass-loss during drying process [START_REF] Chaouch | Effect of heat treatment intensity on some conferred properties of different European softwood and hardwood species[END_REF][START_REF] Pétrissans | Wood thermodegradation: experimental analysis and modeling of mass loss kinetics[END_REF][START_REF] Candelier | Utilization of temperature kinetics as a method to predict treatment intensity and corresponding treated wood quality: durability and mechanical properties of thermally modified wood[END_REF]. The mass loss is widely considered as a good indicator of mechanical performances [START_REF] Pétrissans | Wood thermodegradation: experimental analysis and modeling of mass loss kinetics[END_REF]) and durability against insects [START_REF] Candelier | Control of wood thermal treatment and its effects on decay resistance: a review[END_REF][START_REF] Gérardin | New alternatives for wood preservation based on thermal and chemical modification of wood a review[END_REF]) and fungal attack [START_REF] Kamdem | Durability of heat-treated wood[END_REF]. Nevertheless, considering all properties changes at the same moment with the same rate is not satisfying; the present work will enhance the understanding of wood properties evolution during heat treatment using objective descriptors already exposed in a previous work [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF]) on mass loss, but here extended to other physical and mechanical properties.

Material and methods

Specimens preparation and conditioning

The wood used for this study was Italian Populus alba L. with a density (ρ) of about 0.34 ± 0.02 g cm -3 at standard environmental conditions, 20 °C of temperature (T) and 65% of relative humidity (RH). The annual ring width was 10 ± 2 mm and the estimated age of the tree 30 years. In order to investigate the moisture relations, physical and mechanical properties of unmodified and thermally modified wood material, different types of specimens were cut from heartwood following the anatomical orientations: longitudinal (L), radial (R), tangential (T). A series named L-samples measuring 120 (L) by 20 (R) by 2 (T) mm 3 were used to assess the colour variation in LR plane and the bending stiffness and strength in L direction. A series named R-samples measuring 2 (L) by 60 (R) by 10 (T) mm 3 were used to assess the bending stiffness in R direction. Finally a series named RT-samples measuring 3 (L) by 40 (R) by 40 (T) mm 3 were used to assess the R and T shrinkage. A detailed description of the specimens is given in Tab. 1.

Out of the total amount of specimens prepared (480 L-samples, 600 R-samples, and 175 RT-samples), a selection was done by removing from the experiment the specimens with a specific modulus of elasticity (sMOE) above or below the value of sMOE ± σ, ending to reduce the number of samples to 288 L-samples, 314 R-samples, and 110 RT-samples.

Thermal modification

The specimens were kept for 3 weeks at environmental standard conditions (20 ± 2 °C and 63 ± 2% RH with NaBr salt powder in excess in distilled water). To determine their dry mass before the thermal modification they were dried at 60 °C in a vacuum oven with P2O5 salts for 24 to 48 hours depending on their dimensions. The dried specimens were then thermally modified in a ventilated oven at four different temperatures: 90, 120, 150, and 180 °C. The 90 °C thermal modification treatment is a very mild treatment that can be very helpful to understand if other reactions take part at higher temperatures or if the model can be extended at lower temperatures. [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF] studied the effect of air exchange rate (AER) on the modifications kinetic (mass loss); the heat treatments presented in this study had been performed at the same air exchange rate, AER = 220 AE day -1 . The treatments schedule is presented in Tab. 2 and it was developed on the base of a time/temperature equivalence assumption, where a 10 °C increase in temperature results in dividing the treatment time by two as largely reported in literature [START_REF] Stamm | Thermal degradation of wood cellulose[END_REF][START_REF] Dlouhá | On the time-temperature equivalency in green wood: characterisation of viscoelastic properties in longitudinal direction[END_REF].

At 90 °C a treatment period up to 7.5 months was scheduled while at 180 °C only 168 hours (1 week) were necessary, showing the large sensitivity of the process to temperature. It is important to notice that most of the properties given in the following sections are measured at oven dry condition. Only equilibrium moisture content (EMC) and shrinkage coefficient involved tests at different moisture contents.

Colour measurement

At each treatment step the colour was measured on the oven-dry samples using a Konica Minolta CM-2600d device and expressed with CIE-L*a*b* system. The parameters L*, a*, b* and ΔEab* under a D65 light source and an observation angle of 10°a re reported (see Fig. 1c). The measuring area was a circle of 8 mm diameter. The colour measurement was done only for Lsamples on the tangential face. On each specimen three measurements were done in three different locations and averaged. The variation of ΔL* , Δa*, Δb*, and the to-Tab. 1 -Main properties of the unmodified material at oven dry conditions (0% MC).

iForest -Biogeosciences and Forestry

Kinetic analysis of thermal modified poplar wood tal colour variation ΔEab* were calculated using the following formulas [START_REF] Bekhta | Effect of high temperature on the change in color, dimensional stability and mechanical properties of Spruce wood[END_REF], Matsuo et al. 2016 -eqn. 1, eqn. 2):

(

(2)

where L* is the lightness, a* and b* are the colour coordinates and "0" (in subscript) indicates the reference values obtained as the average of the 288 untreated specimens before heat treatment and in oven dry state.

Physical properties assessment

Samples dimensions in oven dry state were measured before and after the thermal modification by a self-designed apparatus implying Mitutoyo digital gauges with 0.001 mm of accuracy and dedicated spindle lifting cables to enhance test repeatability. The mass was determined by an analytical balance with 0.001 g accuracy. The dimensional variations during the treatment in the different anatomic directions was measured at dry state, while the dry mass losses determined as a relative mass loss from the initial value. The variation of density was also computed from mass and dimensions measurements.

Moisture relations assessment

The moisture relations is studied with two main purposes: to determine sorption isotherms of the modified material as well as the shrinkage coefficients. The sorption isotherms were determined for RT-samples using boxes at constant (20 °C inside a climatic chamber) and four RH levels: 35%, 52%, 63% and 85% obtained by salt dissolution technique as from Young (1967 -see Tab. 3 for details on the salts used).

The dry mass used as reference was obtained after a stage at 60 °C in a vacuum oven with P2O5 salts for 24 to 48 hours depending on the dimensions of the specimens and measured at 20 °C. At the different conditions, once the equilibrium moisture content was reached (this was checked by verifying the stability of the mass), the equilibrium moisture content (EMC) was determined according to eqn. 3:

(3) with m(%RH) and mod are the weight of the samples at the given RH and oven dry state, respectively. The swelling coefficient (β) and the specific shrinkage coefficient (β S ) were determined considering the dimension at the various EMC levels at 20 °C and the dimension at dry state and computed according to eqn. 4 and eqn. 5:

(4) (5)
where dimR %RH is the measured dimensions of the RT-specimens in the given anatomical direction (here R stands for radial) at the equilibrium state with the different relative humidity conditions at 20 °C, dimR od are the same specimens measured at oven dry state, and EMC(RH) is the moisture content determined at the different equilibrium conditions. The isotherms were modelled using the Guggenheim, Anderson and Boer-Dent (GAB) expression. This model is particularly appropriate for food materials and widely used for multi-layer materials such as wood [START_REF] Bratasz | Analysis of water adsorption by wood using the Guggenheim-Anderson-de Boer equation[END_REF]. GAB system general equation is reported in eqn. 6: (6) where EMC is the equilibrium moisture content, RH is the relative humidity, and wm, K, and C are the three free parameters characterizing sorption.

Mechanical assessment

Control (un-treated) and thermally modified samples were mechanically tested using a three-points bending test (environment condition of 24 °C and 50 ± 10% RH) iForest -Biogeosciences and Forestry

Δ E ab * =√ Δ L *2 +Δ a *2 +Δ b *2 EMC ( %RH ) = m ( %RH ) -m od m od ⋅100 β R∨T∨ L = dim R∨T ∨L %RH -dim R∨T ∨L od dim R∨T ∨ L od ⋅100 Δ L * =L * -L 0 * Δ a * =a * -a 0 * Δ b * =b * -b 0 * β R∨T∨ L S = β R∨T ∨L EMC ( RH) EMC ( RH ) =w m C⋅K⋅RH (1-K⋅RH )( 1-KH +C⋅K⋅RH )
with an Instron © material testing machine at oven-dry state. The mass of the tested samples was measured before and after the bending test; the moisture content variation through the test was lower than 2%. The tests were performed with a crosshead speed of 5 mm min -1 . The following properties were determined: the Young's modulus (MOE) and the strength (MOR) in L and R directions, the breaking strain (εm) and the breaking energy (W).

Kinetic analysis

It is widely accepted that the classical Arrhenius law suits very well for the timetemperature equivalence analysis [START_REF] Stamm | Thermal degradation of wood cellulose[END_REF][START_REF] Herrera | A kinetic study on the thermal decomposition of six hardwood species[END_REF][START_REF] Gillen | Time-temperaturedose rate superposition: a methodology for extrapolating accelerated radiation aging data to low dose rate conditions[END_REF][START_REF] Hutchinson | Physical aging of polymers[END_REF][START_REF] Dlouhá | Evidence and modelling of physical aging in green wood[END_REF][START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF]). The Arrhenius law in classical format is reported in eqn. 7: [START_REF] Bf ; Enayati | Study of dimensional stability and EMC of poplar wood (Populus alba) treated at different heat-temperature and initial moisture content[END_REF] where k is the rate constant (dimensionless) of the chemical reaction considered, AT is the pre-exponential factor (dimensionless) with temperature dependency, Ea the apparent activation energy (in kJ mol -1 ), R = 8.314 J mol -1 K -1 is the universal gas constant and T the absolute temperature (in °K) of the reaction. Considering the pre-exponential factor as a constant usually gives quite good results. This assumption leads to a linear variation in the Arrhenius plots as used for example in [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF]. To improve the accuracy of the method some modified Arrhenius law were proposed to express the temperature dependency, as in [START_REF] Brown | The questionable use of the Arrhenius equation to describe cellulose and wood pyrolysis[END_REF] and [START_REF] Herrera | A kinetic study on the thermal decomposition of six hardwood species[END_REF] who showed a bi-linear law instead of the widely used linear one and a constant pre-exponential factor, or in [START_REF] Dlouhá | On the time-temperature equivalency in green wood: characterisation of viscoelastic properties in longitudinal direction[END_REF] who added a softening effect with temperature. A non-constant pre-exponential factor is then introduced by [START_REF] Laidler | A glossary of terms used in chemical kinetics, including reaction dynamics[END_REF] as shown in eqn. 8: (8) where the pre-exponential factor A is here a constant and the temperature power n is an empirical dimensionless constant typically lying in the range -1<n<1. The model is completed by the computation of the horizontal (log time) shift factor (aT) to reach the correspondence between the curve obtained at the given temperature T and the one at the reference temperature Tref as shown in eqn. 9: (9) Some authors discussed the validity of Arrhenius law linearity to explain reaction rates and time-temperature effects [START_REF] Brown | The questionable use of the Arrhenius equation to describe cellulose and wood pyrolysis[END_REF][START_REF] Agrawal | On the use of the Arrhenius equation to describe cellulose and wood pyrolysis[END_REF][START_REF] Hutchinson | Physical aging of polymers[END_REF][START_REF] Stamm | Thermal degradation of wood cellulose[END_REF]. However, in the case of samples free from external loads during the thermal modification and at quite low tem-perature, there is no doubt on the relevance of the law application. Besides, others authors have applied the model to structural modification such as [START_REF] Repellin | Evaluation of heat treated wood swelling by differential scanning calorimetry in relation with chemical composition[END_REF].

Results and discussion

In this section, the Arrhenius law applicability is verified and the evolution of the physical and mechanical properties with treatment temperature and duration is reported; both experimental and modelled data are shown. The parameters used to perform the whole modelling are summarized in Tab. S1 (Supplementary material) on the theoretical basis of [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF] improved with the pre-exponential factor dependency to temperature as presented in the following subsection.

Kinetic analysis

An observation of the Arrhenius plots of the moisture content evolution is presented in Fig. 2a; only this plot is shown out of all the analyses performed for the sake of brevity. As for the Arrhenius plots present in the extensive works of [START_REF] Matsuo | Aging of wood: analysis of color changes during natural aging and heat treatment[END_REF][START_REF] Matsuo | Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species[END_REF], a simple non-modified Arrhenius law could exhibit some weakness. To be more specific, the non-modified approach is very suitable when it comes to model evolutions in the range of temperatures considered to identify the model (as for the previously cited works) and its energy of activation. For instance, the model accuracy is R² = 0.968 for a non-modified Arrhenius approach (Fig. 2a). Nonetheless, several authors expressed limitations and advices to reconsider the application of the non-modified Arrhenius law in favour to a modified one (Ströfer-Hua 1990, [START_REF] Barbero | Equivalent time temperature model for physical aging and temperature effects on polymer creep and relaxation[END_REF][START_REF] Calvini | On the kinetics of cellulose degradation: looking beyond the pseudo zero order rate equation[END_REF]. Hence, to obtain a predictive model for temperature out of the temperature domain studied (used for the identification), it is absolutely necessary to improve the determination of the kinetic law. To such purpose, the present study introduces a new way to express the pre-exponential temperature dependency, since it seems logical to consider a constant energy of activation, constituting a material property of the wooden species for the considered property. Consequently, the energy of activation is determined as being the slope of the tangent at the chosen reference temperature (here Tref = 180 °C) divided by the universal gas constant (R). Hence, here is considered a pre-exponential factor function of the temperature considered, as define by the eqn. 10. (10) where the pre-exponential factor has to be identified experimentally and probably depends of the wooden species, as suggested by previous works [START_REF] Matsuo | Aging of wood: analysis of color changes during natural aging and heat treatment[END_REF][START_REF] Matsuo | Effect of hygrothermal treatment on wood properties: color changes and kinetic analysis using four softwood and seven hardwood species[END_REF].

Fig. 2b shows the identification obtained using the EMC data and provides the evolution of the pre-exponential factor as (eqn. 11): (11) for Tref = 180 °C. This pre-exponential temperature dependency has been checked on the whole properties considered in this study and results as very accurate for all of them; those other plots are not presented. All the kinetic models proposed for all the properties, summarized in Tab. S1 (Supplementary material), are considering this preexponential dependency to temperature.

Colour modification

The effect of the thermal modification on the samples colour is shown in Fig. 1a. It has the effect to reduce considerably the lightness (L*) by 53% (from 89 to 39), resulting in an important darkening (Fig. 1b).

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k = A (T ) ⋅exp ( -E a RT ) k = A⋅T n ⋅exp ( -E a RT ) a T = t T t T ref =exp [ E a R ⋅ ( 1 T - 1 T ref )] k = A ( T ) ⋅exp ( -E a RT ) A (T ) / A (T ref ) =0.0078⋅T

Kinetic analysis of thermal modified poplar wood

This darkening leads to an increase in Eab*, which is mainly driven by a decrease in L*, since the variations of a* and b* are relatively less important. On the other hand, while L* presents a continuous reduction, the a* and b* chromatic parameters result in more complex behaviours, increasing at a first stage and decreasing in a second stage; this is the result of a first oxidation reaction followed afterward by a degradation of the constitutive hemicelluloses polymers. The hemicelluloses starts to decay at about 120 °C [START_REF] Tjeerdsma | Characterisation of thermally modified wood: molecular reasons for wood performance improvement[END_REF], through a deacetylation process that creates acetic acid acting as a catalyser for subsequent depolymerisation [START_REF] Fengel | Wood -chemistry, ultrastructure, reactions[END_REF][START_REF] Tjeerdsma | Characterisation of thermally modified wood: molecular reasons for wood performance improvement[END_REF]) and a lignin reticulation [START_REF] Stamm | Thermal degradation of wood cellulose[END_REF][START_REF] Tjeerdsma | Characterisation of thermally modified wood: molecular reasons for wood performance improvement[END_REF][START_REF] Repellin | Evaluation of heat treated wood swelling by differential scanning calorimetry in relation with chemical composition[END_REF]. Emsley & Stevens (1994) indicates that the cellulose compound experiences a decomposition process to furfural, then proceeds by a series of rearrangement and elimination reactions as also reported by [START_REF] Shafizadeh | Thermal deterioration of wood[END_REF]. Both a* and b* evolution are the result of those different chemical reactions occurring at different treatment times; they remain in the CIE-L*a*b* reddish and yellowish areas leading to the orange/brown colour.

For a* and b*, a moderate increase of their maximum value can be observed. Several authors [START_REF] Hutchinson | Physical aging of polymers[END_REF][START_REF] Stamm | Thermal degradation of wood cellulose[END_REF][START_REF] Dlouhá | On the time-temperature equivalency in green wood: characterisation of viscoelastic properties in longitudinal direction[END_REF]) observed such effect and applied a vertical shift factor (usually referred as bT) in addition to the more classical horizontal shift factor (in log time axis); this effect is not taken into account in this model, since it has not been fully identified over a wide range of temperatures.

Physical properties modification

The relative reduction (%) of the oven-dry dimensions consecutive to the thermal modification are reported in Fig. 3. This re-duction was measured separately for L, R and T directions. As it can be observed, the trends are very clear and the model follows the experimental results. The reduction is conspicuous for T and R directions, while it is negligible in the L direction. The ratio between T and R reduction is about 2, being the same as the T over R ratio of drying shrinkage coefficients for unmodified wood. These data confirm that hemicelluloses suffer a larger thermal degradation compared to the other wood constituents, considering their dominant contribution to the drying shrinkage in the transverse directions. On the other hand, the longitudinal bonds of wood are less affected since the shortening in this direction is negligible. The relative reduction in volume (%) at dry state shows a trend very similar to T and R directions with an excellent accuracy of the model. Oven-dry mass and oven-dry density show the same behaviour, highlighting how the main physical properties exhibit the same trends and how the model presents a good agreement with the experimental data.

The high accuracy of the model, that fits well with different temperatures ranging from 90 to 180 °C, shows how the ongoing reaction is the same but made faster or slower because of the effect of temperature. In this case, as for the other properties studied, the time/temperature equivalency is effective: different temperatures can result with the same effects in different amount of time.

Moisture relations modification

As shown in Fig. 4, the moisture content at different equilibrium conditions decreases significantly with the thermal modification intensity. The longer the treatment duration, the lower becomes the EMC; and the higher the treatment temperature, the faster the EMC reduction. The EMC reduction is generally ascribed to the degradation of hemicelluloses that leads to a reduction of hydroxyl groups and then to a lower capacity of the cell wall to bond with water. Crosslinking between molecules can also be part of the reason of reduction of water bonding sites. [START_REF] Esteves | Wood modification by heat treatment: a review[END_REF] have also shown an enhanced inaccessibility of cellulose hydroxyl groups to water molecules due to the increase of cellulose crystallinity. [START_REF] Repellin | Evaluation of heat treated wood swelling by differential scanning calorimetry in relation with chemical composition[END_REF] studied the swelling of heat-treated beech wood by differential scanning calorimetry in relation to chemical composition, and concluded that the reduction of wood swelling could not be attributed only to the disappearance of adsorption sites by hemicellulose destruction, but other phenomena such as structural modifications and chemical changes of lignin might also play an important role. Fig. 4 also shows the behaviour of the shrinkage coefficients (β) in R, T, and L directions as determined from eqn. 4. The total shrinkage gradually decreases with the treatment intensity and it decreases faster at higher temperatures, showing a behaviour in line with the expectations and a good predictability by the model. The specific shrinkage coefficients (β s ) as computed from eqn. 5 are also reported in Fig. 4: the T shrinkage coefficient is almost twice as the R shrinkage coefficient, while the L is negligible (this behaviour is in line with the expectations). It is not surprising that the specific shrinkage coefficients does not vary significantly with the treatment temperature and duration while the total shrinkage coefficients (β) decreases, the EMC also decreases. This statement is supported by [START_REF] Hill | Wood modification[END_REF] and [START_REF] Hill | The water vapour sorption properties of thermally modified and densified wood[END_REF] and finds its rationale in the fact that, even if sorption sites are lost during thermal modification, in order to get to the same shrinkage or swelling, the same amount of water must be lost or adsorbed. As a direct consequence, it is important to Fig. 3 -Physical properties, measured at dry state, evolution over treatment duration for different modification temperatures. The markers are the experimental data while the lines are the identified model. The time axis scale is logarithmic.

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consider that the observed increase in dimensional stability (largely exposed in literature) is mainly due to a general reduction of the equilibrium moisture content.

As regards the hygrothermal behaviour, the control samples (untreated) studied in this paper exhibit a sorption behaviour that leads to the following GAB coefficients: wm= 5.1%, K = 0.74, and C = 11.0. The fitting performed to identify the GAB coefficients for the different moisture contents, the treatment durations, and the different temperatures highlights that GAB coefficient wm is influenced by the heat modification, while both the GAB coefficients K and C remain unchanged regardless of the temperature and duration of the treatment. In Fig. 5a, the EMC of poplar wood treated at 180 °C for different durations is reported for an isotherm at 24 °C. It clearly shows how modified wood presents a lower EMC than the control and how the treatment duration affects the EMC decrease. The effect is the same for all the tested modification temperatures. This is a very important factor for modified wood because a generally lower EMC results in a better resistance to biological degradation. It can also be noted how the EMC reduction is higher for high RH than for low RH. In Fig. 5b, the evolution of wm with different treatment temperatures and durations is shown. The trends clearly shows how the kinetic analysis can be successfully applied also to GAB coefficients.

Mechanical properties modifications

The changes in mechanical properties due to thermal modification, measured at dry state, are reported in Fig. 6.

As a general rule the mechanical performances are markedly reduced by the thermal modification. The higher the treatment duration, the higher the reduction. The temperature, as for the other processes, accelerates the reaction. This reduction directly depends on the cell wall loss, that has a negative effect on the stiffness (MOE) both in L and R directions. This behaviour is widely described in the literature (e.g., [START_REF] Beall | Thermal degradation of wood components: a review of the literature[END_REF][START_REF] Navi | Thermo-hydro-mechanical processing of wood[END_REF]. Several research works report the fact that thermal modifications improve the stiffness of the material in the initial phase [START_REF] Esteves | Wood modification by heat treatment: a review[END_REF] before dropping drastically afterward. It is worth pointing out that the experimental data shown in Fig. 6 are not exhibiting such behaviour. The above authors believe that this behaviour is a direct consequence of the dry state condition prepared for testing the specimen. The thermal modification results in loss of cell wall material but also in a decreased EMC that could result in an apparent stiffness increase. Thus, the conclusions on the increase of the initial stiffness generally observed by the aforementioned authors has to be mitigated, since the moisture content is reduced as well during the heat treatment. In order to understand the role of cell wall loss and EMC variation on the stiffness at low modification rates, the stiffness measured at dry state was corrected according to the empirical model proposed by [START_REF] Guitard | Mécanique du matériau bois et composites [Mechanic of wood and composite materials[END_REF] and reported in eqn. 12 (see below). The stiffness at 20 °C of temperature and 65% of relative humidity MOEL EMC was then computed according to the EMC levels measured in this work as follows (eqn. 12):

(12) As shown in the Fig. S1 (Supplementary material), the oven dry stiffness does not increase at low modification rates but remains constant before decreasing. On the other hand, the apparent stiffness computed at standard environmental conditions (20 °C of temperature and 65% of relative humidity) increases at low modification rates as a consequence of the moisture content reduction due to the thermal modification. According to [START_REF] Esteves | Wood modification by heat treatment: a review[END_REF], some authors explain the increased stiffness by the increase of cellulose crystallinity. Because this behaviour was not observed in our experimental work at dry state, only the variation of the equilibrium moisture content seems to justify the initial improvement in stiffness. Similar conclusions could be drawn on the strength trend and other mechanical properties when an apparent improvement for low modification rates is reported (Finnish ThermoWood Association 2003).

To summarize, during the thermal modification: (a) for low modification rates (under 5% of mass loss), the stiffness reduction because of the cell wall degradation cannot compensate the stiffness increase due to lessening of moisture content, resulting in an apparent improvement of the stiffness; (b) for higher modification rates (greater than 5% of mass loss), the trend is reversed because the cell wall degradation has a stronger impact than the moisture content reduction on the apparent stiffness. As regards the natural aging of wood, some authors [START_REF] Kohara | Studies on the durability of wood. I: Mechanical properties of old timbers[END_REF][START_REF] Obataya | Caracteristiques du bois ancien et technique traditionnelle japonaise de revêtement pour la protection du bois [Characteristics of ancient wood and traditional Japanese technique of coating for wood protection[END_REF] gave the same conclusions about the improvement of the mechanical properties during the first 300 years of natural aging before a drop of the performances.

The strength (MOR) is lowered severely as well, e.g., by 80% while the MOE had been reduced by about 30% at the end of the 168 hours of heat treatment at 180 °C. MOR can drop until being almost null for very extreme treatments (high temperature and/or long exposure time). Several authors studied the strength modification after heat treatment [START_REF] Winandy | Modeling strength loss in wood by chemical composition. Part I: An individual component model for southern pine[END_REF], Froidevaux et al. 2012) and observed the same effects. The maximal strain at breaking and the energy needed to break the samples in flexural tests are provided in Fig. S2 (Supplementary material); those properties exhibit the same trends than the MOE and the MOR. The kinetic model proposed and fitted to the experimental data follows very accurately the mechani-cal properties over treatment duration and temperatures.

Properties relations to mass loss

The property modifications occurring under thermal modification in conventional oven can be also related to mass loss. This is important because mass loss is one of the easiest parameters to be measured during the treatments and one of the best indicators of treatment intensity, as reported by [START_REF] Candelier | Control of wood thermal treatment and its effects on decay resistance: a review[END_REF]. Hence, all relevant properties (those significantly affected by heat treatment) are plotted against mass loss variation in Fig. S2 (Supplementary material). The relation between mass loss and a given property variation can be very different depending on the property considered: monotonous and linear relationships are observed in some cases or much more complex relations in others. First, the modifications are all

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MOE L EMC =MOE L EMC ref ⋅ [ 1-0.015⋅ ( EMC-EMC ref )] MOE R EMC =MOE R EMC ref ⋅ [ 1-0.030⋅( EMC-EMC ref )]
strongly related to the mass loss; second, the heat treatment temperature does not produce significant differences, since all experimental points, whatever the temperature, are overlapping. The trends, being independent on the temperature, clearly confirm the time-temperature equivalency principle. For several property variations, such as oven-dry dimensions, oven-dry volume, and oven-dry density, the trends are very close to linearity up to very large mass loss rates; in those specific cases, a linear approximation is provided in the graphs allowing to compute the property variation, from a measured mass loss, in an easier way than fully applying the kinetic models proposed in this work. For the other properties, the behaviour could be simplified with a linear relation but up to 5% mass loss only. The absence of a linear trend above that level suggest that the properties do not follow the same kinetics and that heat treatments are complex processes to understand and control.

Nevertheless, the kinetic model seems not to be perfectly accurate to predict the variations of few properties such as moisture content, swelling/shrinkage coefficients, and strain at break for mass losses greater than 20%. These unexpected results, concerning a limited number of conditions, are possibly due to experimental issues since an increase of the moisture content appears not plausible.

Conclusions

The present work has made an assessment of the changes of most properties of poplar wood when subjected to thermal modification applied with different temperatures and durations in a conventional atmosphere. The variation of the physical and mechanical properties as well as the wood-moisture relations were modelled with a modified Arrhenius law approach and a pre-exponential dependency to temperature was identified; the model parameters identifications for each properties are summarized in the Tab. S1 (Supplementary material). The proposed models showed a very good ability to describe the experimental data for temperatures ranging from 90 to 180 °C. This approach has shown to work well up to 260 °C [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF]). The kinetic model proposed is evidently accurate for colour, mass loss and hydric shrinkage. However, a noticeable deviation was observed in hygroscopic and mechanical parameters for heating at low temperatures (90 °C). This could be due to different chemical reactions occurring at higher temperatures compared to those at lower temperatures; this statement should be mitigated by the few data obtained to check the model reliability at that low temperature, because of the very long treatment duration (10 months of treatment has allowed only to obtain 4 experimental points).

This kind of model could be profitably used in industry to predict any property variation resulting from a treatment ranging between the experimental maximal and minimal temperature. The good agreement of the data from 90 °C to 180 °C with the model predictions shows how the chemical degradation processes occurring at the different temperatures are not very far the one from the other. However, the present study highlights the fact that properties are modified at different time and with different kinetics during the heat treatment. Moreover, the good fit of very low treatment temperatures with the other data suggests that possibly the thermal modification of wood cannot be considered very far from conventional aging of wood.

The model proposed could help modelling the whole set of properties variation in a given material for any industrial treatment, even though it has some limitations in terms of risk of fire under such conditions. Our results demonstrates that it is possible to target a specific final product quality and to choose the suitable treatment to apply. The direct transposition of the results of this work to actual industrial wood modification processes is currently ongoing under steam environment.

Fig. 1

 1 Fig. 1 -(a) Colour evolutions due to the heat treatments performed measured under a D65 light source and an observe angle of 10°. (b) Colour evolution on samples treated at 180 °C depending on the time of treatment (in hours). (c) 3D representation of the CIE L*a*b* colour space. Markers are the experimental colour measurements and the lines are the application of the model proposed.

Fig. 2

 2 Fig. 2 -(a) Moisture content Arrhenius plot of the horizontal shift factor with different fitting strategies: a global linear regression (dashed grey line) highlights how the linear approach without pre-exponential factor (A(T)/A(T-ref) = 1) can be limited and how the use of a non-linear (purple continuous line) pre-exponential factor (A(T)/A(T-ref) ≠ 1) can be used to improve the fitting; aT is the horizontal shift factor. (b) The perfect fit of the model using the pre-exponential factor as linearly dependent of the treatment temperature is displayed.

Fig. 4 -

 4 Fig. 4 -Moisture relations vs. treatment temperature and duration (time axis is in logarithmic scale). Symbols represent the experimental data, while the lines are the identified model. (a) Equilibrium Moisture Content (EMC) measured at 35%, 52%, 63% and 85% of relative humidity and 20 °C. (b) Values of the shrinkage coefficients (β) and specific shrinkage coefficients (β S ) in the main anatomical directions (R, T and L) measured passing from the equilibrium condition with 85% RH and 20 °C to the oven dry state.

Fig. 6 -

 6 Fig. 6 -Longitudinal (left column) and radial (right column) mechanical properties evolution over modification time (horizontal axis, in logarithmic scale); symbols represent the experimental points and the lines are the identified model.

Properties Variable Series L-samples R-samples RT-samples

			Dimensions (mm, L×R×T)	120×20×2	2×60×10	3×40×40
			No. of samples		288	314	110
	Sample Characteristics	ρo (g cm -3 ) MOEod (GPa) sMOEod (GPa kg m -3 ) MORod (MPa) sMORod (MPa kg m -3 ) L*od a*od b*od β S 0-85%RH_R		0.31 ± 0.03 8.57 ± 0.72 27.30 ± 1.00 93.90 ± 11.40 10.78 ± 0.93 0.31 ± 0.02 0.66 ± 0.06 2.12 ± 0.11 0.30 ± 0.02 35.00 ± 2.00 83.00 ± 1.50 -5.00 ± 0.90 -19.20 ± 1.70 ---	0.29 ± 0.01 -------0.13 ± 0.03
			β S 0-85%RH_T			-	-	0.20 ± 0.02
			β S 0-85%RH_L			-	-	0.03 ± 0.01
			Colour			Yes	No	No
	Measured	Properties	Mechanical tests Dimensions Mass Swelling coefficients Moisture content		Yes Yes Yes No No	Yes Yes Yes No No	No Yes Yes Yes Yes
			Sorption curve		No	No	Yes
	Tab. 2 -Schedule of thermal modifica-tion performed in ventilated oven. (a): about 7.5 months; (b): about 5.5 months; (c): about 1 month; (d): one week.
	-	90	Temperatures (°C) 120 150	180
	Thermal modification duration (h)	311 643 1056 1539 2569 3602 5542 (a) --------	43 66 139 209 338 -643 811 980 1171 1539 2323 3072 3842 (b) -	4 9 20 26 44 60 82 100 130 166 214 334 411 504 600 (c)	0.5 1 2 3 5 7 -13 -23 31 36 51 62 78
		-	-	-	103
		--	--	--	127 168 (d)

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Acknowledgments

The authors acknowledge funding from JSPS (Japan Society for the Promotion of Science) and Professor Shuichi Kawai from the RISH, Ky0to University (Japan) for hosting in his research team. Special thanks to Prof. Luca Uzielli (Dept. GESAAF, Florence University, Italy) who provided the wood material for this study and for the discussion and advises over many years on wood science and cultural heritage wooden artefacts preservation.

Supplementary Material

Tab. S1 -Settings of the modelling performed according to [START_REF] Goli | Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics, physical and mechanical properties[END_REF]. Link: Marcon_2422@suppl001.pdf