A Discussion on Relationship between Electrical Conduction and Structural Change of Cell Wall due to Moisture Adsorption and Desorption

Although a relationship between DC (direct current) conductivity and moisture content has been discussed by several researchers, it has not been clarified how the swelling and shrinkage of wood cell wall relates to the effect of moisture content on DC conductivity of wood. It is the purpose of this study to discuss the moisture content dependence of DC conductivity and the anisotropic electric behaviour for sugi (Cryptomeria japonica D. Don) wood, and to clarify the relationship between the mechanism of electrical conduction and \u22transient capillaries\u22 which relate intrinsically to the swelling and shrinkage of wood. It is shown through a discussion on the moisture content dependence of electrical conduction that the mechanism of electrical conduction above about five percent moisture content is related to the structure of transient capillaries as well as the mechanism of moisture adsorption in wood. Although the degree of dissociation of adsorbed ions is independent of the structure of transient capillaries, the mobility of ions relates to the structural difference of transient capillaries. Up to about five percent moisture content, it is shown that the dissociation and migration of adsorbed ions are considered not to be important, and tha t transient capillaries are considered to be important. As a result of a discussion on the time dependence of DC conductivity which relates intrinsically to the rate of ionic conduction, it is presumed as follows: I) the swelling is mainly due to the production of new transient capillaries and the enlargement of pre-existing capillaries up to fifteen percent moisture content, and 2) the swelling is mainly due to the increase in number of the transient capillaries being in nearly the saturated state above fifteen percent moisture content

Studies on Electrical Conduction and its Field Dependece of Wood

はじめに １．研究の背景 ２．実験方法と実験 ２．１．直流コンダクタンスと交流コンダクタンスの関孫 ２．２．電気伝導の測定法 ２．２．１．交流域 ２．２．２．直流域 ２．２．３．電気伝導率$sigma$の算出法 ２．２．４．電極 ２．３．供試した試験片 ２．４．測定条件 ２．４．１．試験片に与えた電圧 ２．４．２．含水率と温度の調整法 ３．電場の周波数が電気伝導にもたらす影響 ３．１．電気伝導率の周波数依存性への含水率の影響 ３．２．電気伝導率の周波数依存性への温度の影響 ３．３．要約 ４．電場の強さが電気伝導にもたらす影響 ４．１．電圧印加後の時間経過に伴う電気伝導率の減少への電圧の影響 ４．２．直流電気伝導率への電圧の影響 ４．３．要約 ５．電場の方向が電気伝導にもたらす影響 ５．１．電気伝導の異方性への印加電圧の影響 ５．２．電気伝導の異方性への周波数の影響 ５．３．電気伝導の異方性への含水率の影響 ５．４．要約 ６．水分吸着と細胞壁構造との相互作用と電気伝導との関係 ６．１．電気伝導のメカニズムと含水率 ６．２．電気伝導のメカニズムと電気的挙動とのかかわり ６．３．水分吸着による細胞壁の構造変化が電気伝導に与える影響 ６．４．要約 ７．結論 引用文献木材細胞壁中の微細空げきは，水分や薬液の細胞壁内通導の通路であり，さらに，水分吸着によって引き起こされる膨潤・乾縮とも重要な関係を持っている．したがって，木材の性質・挙動と細胞壁の構造との有機的な関係づけが極めて大切な研究課題である．ところで，電気伝導の主要な通路は細胞壁中に形成される水分の吸着層であり，しかも含水率が電気伝導率に与える影響は極めて大きいことがよく知られている．そこで，この研究では，木材の吸・脱湿に伴う含水率および細胞壁構造の変化を考え併せながら，異なった電場条件下に置かれた木材の電気的刺激に対する応答性，すなわち①電気伝導の周波数依存性，②電気伝導に与える印加電圧の影響，③電気伝導に与える電場方向の影響を検討し，ひいては電気伝導のメカニズムと細胞壁中での水分の存在状態および細胞壁構造とのかかわりを考察した．高周波（1MHz）から極めて低い周波数（0．0001Hz）にわたる範囲で，含水率と温度のそれぞれが低いほど，周波数の低下に伴う電気伝導率の減少がより低い周波数に至るまで見られた．常温下でかつ約5％以上の含水率範囲では，低い周波数の側での電気伝導率の周波数依存性は印加電圧によって影響を受け，イオン伝導に基づく非可逆的な分極によるものと推定した．この電気伝導率および電気伝導の挙動に与える電圧の影響は，含水率が増加するほど大きくなり，吸着イオンが吸着点から脱着される量と脱着後の移動速さが電圧によって異なることに基づくと考えた．他方，電気伝導率の周波数依存性に関係する分極現象は含水率5％以下で可逆的であり，しかも電圧による影響を認めなかった．したがって，水分が単分子層で吸着されている低含水率の範囲では，吸着イオンの脱着および解離とその移動が，電気伝導にかかわりを持たないことを推定した．電気伝導の異方性は，電圧および周波数によって影響を受け，印加電圧が大きくかつ周波数が低いほど，軸方向と横方向との間の電気伝導の異方度が大きくなる傾向を認めた．これは，細胞壁中での電気伝導の異方性が電荷担体である伝導イオンの移動距離と関係し，細胞壁内の空げき構造がイオンの移動度に与える影響とも関係することを示唆している．ちなみに，含水率が電気伝導率およびその異方度に与える影響は，含水率約8％を境にしてその上下の範囲で異なった挙動が得られ，とくに水分吸着に伴う細胞壁中の構造変化と密接なかかわりを持つことを推定できた．低周波から底流領域で電場条件（周波数，印加電圧，電場の方向）を変えたとき，電気的挙動とその機構は含水率の範囲によって異なっている．これは水分子と木材実質との間の吸着力がイオンの解離度および移動度に与える影響，そして細胞壁構造がイオンの移動度に与える支配的な影響に起因すると推定した。このように，含水率の変化に伴って生じる細胞壁構造と水分との相互的な作用が電気伝導のメカニズムと密接に関係するために，木材の電気伝導を論じるには水分の量だけでなく，細胞壁中の空げき構造を含めた水分の存在状態の影響を十分に考慮しなければならない．The transient micro-capillaries, as formed in the wood cell wall with moisture uptake by wood, make the conducting path of various liquids in the wood cell walls and are also associated with a mechanism of swelling/shrinkage of wood. Therefore an importance of studying the relationship of physical properties and behaviours with the cell-wall structure should be pointed out. The mechanism of electrical conduction in wood, of which dominant current path is formed by the water-molecule layers in the cell walls as revealed by a remarkable effect of moisture content on the electric conductivity, must be dependent on the cell wall structure as well as moisture content. The purpose of the paper is to study the mechanism of response under the influence of electric stimulus through the dependence of conduction on frequency, applied voltage, and field direction, with relation to moisture content by using sugi (Clyptomeria japonica D. Don), and to discuss the dependence of the mechanism of electrical conduction on the mechanism of moisture adsorption includes the structural change of the cell walls with moisture content. The decrease of conductivity with decreasing frequency was extended to lower frequency range as moisture content and temperature were decreased, in the range from high (1 MHz) down to very low frequency (0.0001 Hz). The frequency dependence of conductivity in lower frequency range was affected by the applied voltage in the moisture・content range above 5% at 20℃, and it was therefore thought to be due to the irreversible polarization owing to the ionic conduction. The voltage effect on conductivity and conductive behaviour were developed with moisture content increased over 5%, and that were assumed to be due to the difference in quantity and mobility of ions forced to break from the bonding sites with applied voltage. On the other hand the behaviour of polarization worked for the decreasing conductivity with frequency below the moisture content of 5% was reversible, and it was not affected by the applied voltage. Thus the electrical conduction in the wood cell walls where the bound water molecules make just the mono-layer of water molecule does not accompany the ionic breakage from the bonding sites following the dissociation and migration of ions. The electrical anisotropy expressed as the ratio of longitudinal to transverse conductivity tended to increase with decreasing frequency and increasing voltage at air-dried condition which were supposed to affect the ionic mobility and the number of conducting ions. respectively. The anisotropy was also affected by the moisture content; it was increased with moisture content above 8% and was decreased below 8%. These anisotropic behaviours suggested to relate to the effect of intra-cell wall structure on mobility of conducting ions, or to the structural change of cell-wall structure with moisture content. especially of the inter-connecting structure of transient intra-cell wall capillaries. The electrical behaviours and those mechanism resulting from the conditional change of the field with frequency, intensity, and direction were different among moisture-content ranges, and it was therefore confimed that not only the quantity but the nature of bound water surrounded by the cell walls should be considered as far as the electrical conduction in wood is discussed. Thus the mechanism of electrical conduction in wood should be closely related to the interdependence of moisture and cell wall structure with the change of moisture content, through the effect of the attractive force induced among water molecules or between the water molecules and the wood substance on the degree of dissociation or the mobility of adsorbed ions, and through the dominant effect of intra-cell wall structure on the ionic mobility

木材の応力～ひずみ関係の挙動をとらえる一つの考えかた : 繊維方向の荷重と関連させて

This study deals with the mechanical response of the cell-wall to the compressive load applied parallel to the grain. The results from the analysis of sugi-wood (Cryptomeria japonica) were used to introduce a concept on stress-strain behaviour of wood. The differences in the mechanical behaviour between earlywood and latewood were remarkable at air-dried condition, while the water-saturated wood had much less difference in stress/strain behaviour between the earlywood and the latewood than the air-dried wood. The mechanical properties and behaviour of timber are affected by the cell-wall characteristics of tracheids. The difference between the latewood and the earlywood in the stress induced increased considerably with increased stress level. It is apparent that the latewood performs the function of mechanical support against the load applied.木材は数種類の細胞で構成され,それぞれの細胞には異なった役割りが負わされている.いわゆる繊維は樹木に強さを与え,たとえば針葉樹材の主要構成要素である仮道管は,典型的な強固組織と見なすことができる.一方,木材の力学的性質に関連して,弾性定数や破壊現象が実験的成果として得られてきた.ところが,木材の破壊現象や荷重を受けたときの変形現象を,強固組織の力学的挙動と関連させた研究は極めて少なく,十分な知見が得られていない.以上のようなことを踏まえ,この研究ではスギ(Cryptomeria japonica D.Don) 材を使って,繊維方向に荷重を与えたときの木材の変形とその挙動,仮道管の変形とその挙動を実験的に検討した.その結果,つぎのことが明らかになった.(1)気乾材では,負荷時と除荷時のいずれでも,早材部と晩材部の間で著しく異なる挙動を認めた.すなわち,早材部仮道管壁が低い応力でも塑性的な挙動を示すのに反して,晩材部仮道管壁では木材試験片の破壊直前まで弾性的な挙動が認められた.(2) 飽水材では,早材部と晩材部のいずれの仮道管壁でも,荷重と変形の関係に気乾材ほどの著しい相違を認めなかった

曲げ強度性能と細胞壁変化へのたわみ速さの影響

This study deals with the mechanical behaviour in bending and the anatomical changes under several deflection rates. Sugi (Cryptomeria japonica D. Don) wood was used in the test, and the four-point bending method was adopted. The sizes and dimensions of test specimen were 2cm(R)×2cm(T)×32cm(L). Microscopic observations were carried out with the filmy replica method and the SEM (Scanning electron microscopy). The mechanical properties of wood are affected by variation of the deflection rate. The MOE, modulus of elasticity, and the value ofσ_p/MOR, proportional limit by modulus of rupture, increased linearly with logarithm of deflection rate. With increasing deflection rate, the microscopic failures in tracheids after ultimate decreased. In water-saturated wood, more microscopic compression creases observed than air-dried wood. It is believed that the variation of mechanical properties due to deflection rates are closely related to the variation of microscopic compression creases of wood.破壊までの静的応力を受けた木材の組織上の変化, 特にたわみ速さ変化に対する組織変化を調べるために, 試験片に異なるたわみ速さで荷重を加えて, 各たわみ速さ条件で荷重を受けているときと, 破壊後に認められる試験片表面の変化を調べた. すなわち, 荷重を受けているときには試験片の圧縮面と引張面の表面からのアセチルセルロースフィルムのレプリカを使って, 破壊後には実材による木材細胞壁の変化を観察し, たわみ速さの影響を検討した. また, 気乾材と飽水材を区別して, たわみ速さ変化で木材細胞壁に生じたmicroscopicな破壊程度の変異と, 木材の力学的挙動の関係について検討を行い, 次の結果を得た. 木材の力学的性質は, 含水率が低いほど, またたわみ速さが速いほど向上した. たわみ速さ変化による組織変化の程度に, 差が認められた. また, 破壊までの荷重に対して, 全体的に気乾材より飽水材表面に組織変化が多かった. 気乾材の場合, 比例限の曲げ荷重に対して, たわみ速さの変化による細胞壁内のシワの発生が認められた. すなわち, たわみ速さが遅いときにはわずかながらシワの発生が認められたが, 速いたわみ速さではシワの発生はほとんど認められなかった. 破壊までの荷重を受けた場合, 気乾材と飽水材ともに, たわみ速さ変化による組織変化の程度に差が認められた. すなわち, 速い荷重を受けた試験片表面では, 遅い荷重を受けた試験片よりもシワの発生が少なかった. このことから, たわみ速さ変化による弾性率の変化には, 木材細胞壁の損傷をも考慮する必要性を認めた

Wood Properties of Sugi (Cryptomeria japonica D. Don) Cultivars : Preliminary Experiment of 6 Native Clonal Cultivars Grown in Kyushu

スギ品種の材質特性を明らかにするための基礎的な知見を得ることを目的にして, この研究を始めた.まず,九州地方の代表的な在来スギ6品種すなわち,クモトオシ,ヤイチ,オビアカ,メアサ,ヤブクグリ,アヤスギについて,樹幹木部の組織･構造,および力学的性質の予備実験的な検討を行った.晩材率は,年輪幅が広い品種で小さく,年輪幅が狭い品種で大きいことがわかり,肥大生長の速さとのかかわりを認めた.容積密度数は年輪幅が広い品種ほど小さく, 容積密度数の樹幹内分布は樹心部で大きく,樹幹外周部へ向かうに伴って小さくなり,また樹幹の下部から上部へ向かって大きくなる傾向がある.樹幹材積生長量が大きい品種ほど仮道管は長く,また,髄からの年輪番号の増加に伴って生じる仮道管長の増加割合いが大きく,形成層齢の仮道管長への影響が著しかった.仮道管の接線方向の直径･壁厚･壁圧比には,目立った品種間差異を認めなかった.樹幹材積生長量が大きい品種ほど,静的曲げヤング率,静的曲げ比例限応力度,縦圧縮ヤング率,縦圧縮比例限応力度が大きく,静的曲げ破壊仕事率,縦圧縮破壊仕事率,衝撃曲げ吸収エネルギーが小さかった.しかし,静的曲げ強さと縦圧縮強さには,品種間で際立った差を認めなかった.The wood properties of Sugi (Cryptomeria japonica D. Don) were examined to determine the effects of differences in cultivars on the physical and mechanical properties. In this study six cuItivars which have been realised as the native clonal cultivars in Kyushu, the southernmost major island of Japan, were sampled from a 13-year-old stand. The results may be summarised as follows: I) Growth rate data for sample trees from a 13-year-old stand are shown in Table 1. 2) Late wood percentages of the fast-growing cultivars were less than those of the slow-growing cultivars. 3) Basic densities increased inwards and upwards in a stem, and the fastergrowing cultivars showed lower values on whole-stem basic density. 4) Tracheid lenghts of the fast-growing cultivars were longer than those of the slow-growing cuItivars. Tracheid length was highly dependent on the cambial age and this trend is remarkable in the fast-growing cultivars. 5) Six cultivars were comparable in tracheid diameter, wall thickness, and cell waIl percentage and insignificant difference can be measured statistically among cultivars. 6) There were not found remarkable differences in the static bending strength values among the cultivars. Young\u27s modulus and fibre stress at the proportional limit in bending were higher for the fast-growing cultivars than for the slow-glowing cuItivars. On the other hand, work to ultimate load in static bending and absorbed energy in impact bending were lower for the fast-growing cultivars than for the slow-growing cultivars. 7) The results of the longitudinal compressive test showed similar tendencies to the results from static bending test. 8) Fractures occurred during bending are shown in Photo 1

Observations on cell wall response and mechanical behaviour in wood subjected to repeated static bending load

静的な単一の曲げ荷重と、くり返し曲げ荷重を受けた木材はりについて、1)荷重を受けているときに生じる木材の細胞壁の変形、2)荷重が取り除かれたあとの細胞壁の変形の回復状態について、走査電子顕微鏡による観察を行った。さらに、3)荷重を受けたとき木材細胞壁に生じたmicroscopicな破壊と、木材の力学的挙動との関係についても検討を加えた。木材が比例限荷重よりもはるかに小さな曲げ荷重を受けたときでも、木材はりの圧縮側縁面の早材部細胞壁にわずかながらシワ状の変形を残し、すでに細胞壁の破壊が始まっていることが認められた。また、この除荷後にも認められる細胞壁のシワは、木材の残留変形と密接に関係することがわかった。Although many papers and reviews on the mechanism of fracture in wood have been reported by a number of researchers, there are still problem areas. Hence, the purpose of this study is to investigate： 1) the microscopic failure in the wood cell wall, which is due to repeated bending load, 2) the effects of loading levels on cell wall damage, and 3) the relationship between the cell all damage caused by loading and the mechanical behaviour of wood. The cell walls of the wood specimens were continuously observed during repeated bending, employing the techniques of scanning electron microscopy with filmy (acetyl cellulose) replica method. The test specimens were prepared from SUGI (Cryptomeria japonica D. Don) -wood and were conditioned to 12% M.C. The results of the electron microscopic observations are summarised in Table 1. The wood subjected to bending loads less than the proportional limit exhibited microscopic crease-like compression failures (Fig. 3). These occurred to a slight degree in the cell walls in earlywood on the compression side of the wooden beams. These microscopic compression creases in cell walls were very closely correlated to the residual deformation of wood

Tracheid Length of Sugi(Cryptomeria japonica) as an lndex of Wood Quality for Forest Tree Breeding

Eleven sugi cultivars in 20-year-old stands were sampled to quantify the tracheid lengths for juvenile wood and mature wood. Tracheid lengths of latewood were estimated from macerated tissue in every ring at the breast height of three to five trees in each cultivar. Tracheid lengths of every cultivars were initially short near the pith and increased outwards within about 15 rings from the pith, as shown in Fig.1. The result was that the tracheid length of the 16th ring was regarded as the representative value of tracheid length in mature wood. There were high correlations between tracheid lengths of juvenile wood and the tracheid length of the 16th ring from the pith, and the decision coefficients for correlations were shown in Fig.2 and Fig.3. However Figs.4 and 5 show that the plots of rings near the pith for cv. obiaka and cv. hinode deviate from the regression lines, and the tracheid lengths of these rings may be unsuitable for predicting the tracheid length in mature wood. In the 8th ring from the pith, there was no cultivar deviated markedly from the regression line of the tracheid length of the 16th ring on tracheid length of a given ring in juvenile wood as shown in Fig.9.スギ11品種の胸高部位円板を用いて, 未成熟材仮道管長から成熟材仮道管長を予測できるか否かを検討した. この検討に先立ち, 未成熟材から成熟材への移行時期を調べたところ, いずれの品種でも, 髄から15年輪目以降を成熟材とみなせた. そこで, 成熟材仮道管長として髄から16年輪目の仮道管長を採用した. 未成熟材の仮道管長と成熟材の仮道管長との聞に, 極めて高い相関関係が存在することが明らかになった. しかし, 髄近くの年輪では, 両者の関係を表す回帰直線から大きくはずれる品種が観察され, 年輪数の増加にともなう仮道管の伸長率に, 品種間で違いがみられた. このことは, 髄付近の年輪から成熟材の仮道管長を予測するとき, 重要な留意点と位置づけられねばならないであろう. なお, 髄からの年輪数が増加し, 8年輪目以降の年輪では, 回帰直線から大きくずれる品種は認められず, 成熟材の仮道管長を精度良く予測できるとみなすことができた