Vibrational properties of heat-treated green wood

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Y o s h i t a k a K u b o j i m a 9 T a k e s h i O k a n o 9 M a s a m i t s u O h t a

Vibrational properties of heat-treated green wood

Received: September 16, 1998 / Accepted: May 11, 1999

Abstract To investigate the influence of w a t e r on h e a t t r e a t m e n t , g r e e n w o o d was h e a t - t r e a t e d . Sitka spruce (Picea sitchensis Carr.) with a b o u t 60% m o i s t u r e c o n t e n t ( M C ) was used. Y o u n g ' s m o d u l u s and loss t a n g e n t were m e a s u r e d by the f r e e - f r e e flexural v i b r a t i o n test. T h e speci- m e n s were h e a t e d in n i t r o g e n at 160~ for 0.5 h. T h e results w e r e as follows. (1) R e c o g n i z i n g that the effects of h e a t t r e a t m e n t are mild and that the s a m e specimens c a n n o t b e used for b o t h h e a t t r e a t m e n t and as controls, it was neces- sary to investigate the effects of the h e a t t r e a t m e n t b a s e d on the variations of p r o p e r t i e s in the w h o l e of the test lumber. (2) Y o u n g ' s m o d u l u s i n c r e a s e d and the loss t a n g e n t de- c r e a s e d due to h e a t t r e a t m e n t . W h e n the v i b r a t i o n a l p r o p - erties w e r e m e a s u r e d at various MCs, the MCs at the m a x i m u m value of Y o u n g ' s m o d u l u s and the m i n i m u m value of the loss t a n g e n t were lower in h e a t - t r e a t e d speci- m e n s t h a n in controls. T h e effects of h e a t t r e a t m e n t in g r e e n w o o d w e r e similar to those in a i r - d r i e d wood. (3) T h e loss tangents of h e a t - t r e a t e d specimens were s m a l l e r than those of controls at a b o u t 0% M C but w e r e l a r g e r than t h o s e of controls at a b o u t 10% MC. W e t h o u g h t that this r e s u l t e d f r o m the d e c r e a s e d M C at the m i n i m u m loss tan-

Y. Kubojima ( ~ )

Forestry and Forest Products Research Institute, Ministry of Agriculture, Forestry and Fisheries, PO Box 16, Tsukuba Norin Kenkyu Danchi-nai, Ibaraki 305-8687, Japan

Tel. +81-298-73-3211; Fax +81-298-73-3798 e-mail: kubojima@ffpri.affrc.go.jp

T. Okano ~ 9 M. Ohta

Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan

Present address:

Wood Product Information Desk, Wood Information Hall, Woodyland Tokyo, Tokyo 135-0052, Japan

This study was presented in part at the 46th annual meeting of the Japan Wood Research Society, Kumamoto, April 3 5, 1996; and at the 47th annual meeting of the Japan Wood Research Society, Kochi, April 3-5, 1997

gent after the h e a t t r e a t m e n t m e n t i o n e d above. (4) The p r o p e r t i e s m e a s u r e d at several M C s w e r e m o r e useful t h a n those at only one m o i s t u r e c o n t e n t for investigating the effects of h e a t t r e a t m e n t .

K e y words G r e e n w o o d 9 H e a t t r e a t m e n t - V i b r a t i o n a l p r o p e r t i e s 9 V a r i a t i o n in w o o d 9 M o i s t u r e c o n t e n t

Introduction

In a previous p a p e r ~ we e x a m i n e d the v i b r a t i o n a l p r o p e r t i e s of sitka spruce h e a t - t r e a t e d in nitrogen gas or in air. T h e density d e c r e a s e d at high t e m p e r a t u r e and with a long heat- ing time. Roughly, the specific Y o u n g ' s modulus, specific shear modulus, crystallinity index, and crystalline width in- c r e a s e d with time at the initial stage and t h e n d e c r e a s e d later. T h e loss t a n g e n t in the l o n g i t e d i n a l ( L ) - d i r e c t i o n in- c r e a s e d at all condition, w h e r e a s that in the r a d i a l ( R ) - direction d e c r e a s e d .

It is well k n o w n that various d a m a g e , such as collapse and checking of w o o d l u m b e r , is caused w h e n the l u m b e r is d r i e d at high speed. O n the o t h e r hand, study of the structural changes o f cellulose molecules in a noncrystalline r e g i o n by h y d r o t h e r m a l t r e a t m e n t s h o w e d that the pres- ence of w a t e r m a r k e d l y e n h a n c e d d e p o l y m e r i z a t i o n and crystallization to cellulose IV. 2

W e t h o u g h t that the effects of h e a t t r e a t m e n t on w o o d p r o p e r t i e s are influenced by its m o i s t u r e c o n t e n t b e f o r e the h e a t t r e a t m e n t , as well as b y o x i d a t i o n in air or t e m p e r a - tures during the heating. T h e r e f o r e , we i n v e s t i g a t e d the influence of w a t e r on h e a t t r e a t m e n t .

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Experiment

Specimen

Sitka spruce

(Picea sitchensis

Carr.) with 60% moisture content (MC) were used. L-direction and R-direction specimens for the vibration tests were carefully cut to the dimensions of 180mm (L) • 25mm (R) • 8mm (T) and 110mm (R) • 25ram (L) • 8ram (T), respectively.

The specimens were matched in the L-direction as shown in Fig. 1. The specimens with odd numbers were used for the heat treatment and those with even numbers for the control in both L-direction and R-direction specimens. The oven-dried weight was measured after all the vibration tests.

Vibration tests

To obtain Young's modulus and the loss tangent, the free- free flexural vibration test was conducted by the same method as described in our previous paper)

Heat treatment

As in our previous paper, ~ the specimens were encapsulated in a pressure-resistant stainless steel potable reactor. The gases in the potable reactors were not refreshed during the heat treatment. The reactor was then heated in a constant-temperature (160~ oven for 0.5 h. After the heat treatment, the specimens were vacuum-dried at room tem- perature. The moisture content of the specimens was then increased by leaving the specimens at 20~ and 11%, 33 %, 65%, 75%, and 98% relative humidity (RH). When the weight of the specimens became constant at each relative humidity, they were tested.

Results and discussion

Figure 2 shows the variation in Young's moduius. Young's modulus in the R-direction (ER) decreased with the increase in specimen number because the angle between the thick- ness direction and the tangential direction ~annual ring angle) increased from 0 c to 10 ~ with the increase in speci- men number.

It is believed that the effect of heat treatment on ER is not clear by this variation of E R because the effects of heat treatment are mild and different specimens must be used for controls and for the heat treatment. Therefore. we tried to eliminate the variation as follows.

It is possible that all the values of Ea of heat-treated specimens are larger than those of controls when the match- ing pairs are (no. 1. no. 2), Ino. 3. no. 4) . . . . On the other hand. those of the heat-treated specimens may be smaller than those of controls when the matching pairs are (no. 2. no. 3), (no. 4. no. 5) . . . . Here. variations of wood properties are usually thought to be gradual in the L-direction. We then noted that E a was changed by the heat trea tment when the ER of the heat-treated specimens was not the original variation of ER in the lumber, which is expressed by the controls in this study. The same method was adopted for the EL in Fig. 2. specific Young's modulus in Fig. 3. and loss tangent in Fig. 4,

In regard to the variation of Young's modulus in the L- direction. {EL) was increased by the heat treatment at nos. 6-10. whereas there were no clear effects of the heal treat- ment at nos. 1-5. In the region of nos. 1-5. the ER was increased by the heat treatment, whereas there were no clear effect of the heat treatment at nos. 6-14. The specific Young's modulus behaved similarly to Young's modulus, as shown in Fig. 3. The Young's modulus and specific Young's

Fig. 1. Preparation of specimens

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S p e c i m e n N o .

Fig. 2. Variation of Young's modulus. Filled circles, heat-treated, measured in a vacuum-dried state, open circles, control, measured in a vacuum- dried state; filled triangles, heat-treated, measured at 20~ and 65% RH; open triangles, control, measured at 20~ and 65% RH

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modulus were measured in an oven-dried state or air-dried state, and the tendencies of their changes by the heat treat- ment were similar.

Figure 4 shows the variation of loss tangent. In terms of the variation of loss tangent in the L-direction (tangk) of vacuum-dried specimens, the t a n ~ was decreased by heat treatment at nos. 6-10, whereas there were no clear effects

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Fig. 5. Changes in Young's modulus at various moisture contents.

Filled circles, values for L-direction of heat-treated specimens; open circles, vaiues for L-direction of controls; .filled triangles, values for R- direction of heat-treated specimens; open triangles, values for R- direction of controls

Figure 5 shows the change in Y o u n g ' s m o d u l u s at various moisture contents. T h e values in Figs. 5-7 are averages of the heat-treated specimens or controls. Both EL a n d ER had their peak values during the initial stage, decreasing later with an increase in moisture c o n t e n t as previously re- ported. 3 C o m p a r i n g the curves of h e a t - t r e a t e d specimens a n d controls, it was f o u n d that the moisture contents at the peak value were lower in the h e a t - t r e a t e d specimens t h a n in

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Fig. 6. Changes in specific Young's modulus at various moisture con- tents. Symbols: refer to Fig. 5

controls. Suzuki a n d Nakato 4 have reported that the mois- ture content at the m a x i m u m Young's m o d u l u s decreased toward a lower moisture c o n t e n t with an increase in the heating temperatures.

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Fig. 7. Changes in loss tangent at various moisture contents. Symbols: refer to Fig. 5

Figure 7 shows the change in loss tangent at various moisture contents. Both tang~ and t a n ~ had their minimum values during the initial stage, increasing later with an in- crease in moisture content as previously reported. 5 Compar- ing the curves of the heat-treated specimens and the controls, it was found that the moisture content at the mini- mum value of the loss tangent was lower in heat-treated specimens than in controls. This finding has also been re- ported by Suzuki and NakatoJ

The shift in moisture content at the maximum Young's modulus and the minimum loss tangent to lower values were related to a decrease in absorption points by the heat treatment. 4 Hence, it is thought that the decrease of absorp- tion points was caused by the heat treatment of green wood (Figs. 5-7).

It is thought that this shift in moisture content influences the difference in the tendency of the loss tangent shown in Fig. 4. When the moisture content at the minimum Joss tangent of the heat-treated specimens decreased, loss tan- gents at 5%-10% MC consequently become larger than those of controls. Although the loss tangents of heat-treated specimens were smaller than those of controls in the vacuum-dried state, in the region of 5%-10% MC the loss tangents of heat-treated specimens increased more than those of the controls with an increase in moisture content. It is thought that this produced the differences in tendencies of the moisture content of the specimens during the vibra- tion tests, as shown in Fig. 4.

The change in Young's modulus due to the moisture content in the region of 0-10% MC is not as large as that of the loss tangent. Hence, it is thought that the variations of Young's modulus in the test lumber which consist of the heat-treated specimens and the controls (Fig. 2) are inde- pendent on the relative humidity in measuring them.

The shift mentioned above was found by measuring these properties at several relative humidities. Hence, the properties measured at various moisture contents were more useful than those measured at only one moisture con- tent for investigating the effects of heat treatment.

Conclusions

Green wood of sitka spruce was heated in nitrogen at 160~ for 0.5h. Results obtained were as follows: (1) Because the effects of heat treatment are mild and the same specimens cannot be used for both heat treatment and as controls, it is necessary to investigate the effect of the heat treatment by the variations of properties in the entire test lumber. (2) Young's modulus was increased and the loss tangent was decreased by heat treatment. When the vibrational proper- ties were measured at various moisture contents, the mois- ture contents at the maximum value of Young's modulus and the minimum value of the loss tangent were lower in heat-treated specimens than in controls. These effects of heat treatment on green wood were similar to those on air- dried wood. (3) The loss tangents of heat-treated specimens were smaller than those of controls at about 0% MC, whereas they were larger at about 10% MC. We believer this resulted from the decreased moisture content at the minimum loss tangent after the heat treatment mentioned above. (4) The the properties measured at various moisture contents were more useful than those at only one moisture content for investigating the effects of heat treatment.

Acknowledgment We thank Mr. Hisashi Ohsaki, a researcher at Hokkaido Forest Products Research Institute, for his help in conduct- ing our experiments.

References

1. Kubojima Y, Okano T, Ohta M (1998) Vibrational properties of sitka spruce heat treated in nitrogen gas. J Wood Sci 44:73-77 2. Isogai A, Akishima Y, Onabe F, Usuda M (1991) Structural changes

of amorphous cellulose by thermal and hydrothermal treatments. J Soc Fiber Sci Technol Jpn 47:T573-T579

3. Carrington H (1922) The elastic constants of spruce as influenced by moisture. Aeronaut J 26:462-471

4. Suzuki M, Nakato K (1963) Effect of water-sorption on dynamic viscoelastic and dielectric properties of heat-treated wood (in Japanese). Mokuzai Gakkaishi 9:211-226

Figure

Fig. 1. Preparation of specimens

Fig 1.

Preparation of specimens . View in document p.2
Fig. 2. Variation of Young's modulus. Filled circles, heat-treated, measured in a vacuum-dried state, open circles, control, measured in a vacuum- dried state; filled triangles, heat-treated, measured at 20~ and 65% RH; open triangles, control, measured at 20~ and 65% RH

Fig 2.

Variation of Young s modulus Filled circles heat treated measured in a vacuum dried state open circles control measured in a vacuum dried state filled triangles heat treated measured at 20 and 65 RH open triangles control measured at 20 and 65 RH . View in document p.3
Fig. 3. Distribution of specific Young's modulus. Symbols: refer to Fig. 2

Fig 3.

Distribution of specific Young s modulus Symbols refer to Fig 2 . View in document p.3
Fig. 4. Distribution of loss tangent. Symbols: refer to Fig. 2

Fig 4.

Distribution of loss tangent Symbols refer to Fig 2 . View in document p.4
Fig. 5. Changes in Young's modulus at various moisture contents. Filled circles, values for L-direction of heat-treated specimens; open circles, vaiues for L-direction of controls; .filled triangles, values for R- direction of heat-treated specimens; open triangles, values for R- direction of controls

Fig 5.

Changes in Young s modulus at various moisture contents Filled circles values for L direction of heat treated specimens open circles vaiues for L direction of controls filled triangles values for R direction of heat treated specimens open triangles values for R direction of controls . View in document p.4
Fig. 6. Changes in specific Young's modulus at various moisture con- tents. Symbols: refer to Fig

Fig 6.

Changes in specific Young s modulus at various moisture con tents Symbols refer to Fig. View in document p.4
Fig. 7. Changes in loss tangent at various moisture contents. Symbols: refer to Fig. 5

Fig 7.

Changes in loss tangent at various moisture contents Symbols refer to Fig 5 . View in document p.5

References

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