present state of wood waste recycling and a new … wood on waste quantity. 2.1 material ﬂow of...

Materials Transactions, Vol. 43, No. 3 (2002) pp. 332 to 339Special Issue on Environmentally Benign Manufacturing and Material Processing Toward Dematerializationc©2002 The Japan Institute of Metals

Present State of Wood Waste Recycling and a New Process forConverting Wood Waste into Reusable Wood Materials

Yasushi Hiramatsu ∗, Yuko Tsunetsugu∗, Masahiko Karube,Mario Tonosaki and Tsuyoshi Fujii

Forestry and Forest Products Research Institute, Inashiki 305-8687, Japan

The amounts of new wood used for housing construction and wood waste from demolished buildings was calculated using variousscenarios of wood recycling. Diverse utilization of waste and increase in reuse rate was found to be effective to reduce the total amounts offinal waste and the use of new wood materials. For recycling wood waste, the water vapor explosion (WVE) process was developed. In thisprocess, wood materials were exploded from within by the force of water vapor generated by compression under high pressure and temperatureand evaporation of the internal moisture, and they were separated into small wood elements. The WVE process was directly applied to theseparation of Sugi (Cryptomeria Japonica) and Karamatsu (Laryx leptolepis) sawn timbers and slabs. The WVE of them produced small woodelements including fiber bundles, strands, and chips at certain experimental conditions (pressure, temperature, and compressing time). Fiberbundles were long, narrow and flexible elements, and it appeared that they could not be obtained by the conventional processes.

(Received October 29, 2001; Accepted December 14, 2001)

Keywords: wood waste, Construction and Demolition (C&D) waste, wood recycling, reusing, water vapor explosion, wood element, fiberbundle, strand, chip

1. Introduction

1.1 Present states of wastes in JapanWaste disposal has been a serious social issue in Japan.

More than 430 thousand tons of industrial wastes were es-timated to be illegally dumped in 1999, of which constructionwastes accounted for approximately 70%, and wood wasteamounted to 25%.1) Based on such backgrounds, the Ministryof Land, Infrastructure and Transport established the “Con-struction Recycling Law,” which aims to recycle 95% of con-crete, wood, and asphalt wastes by 2010.

1.2 What wood should doTo attain this percentage of recycling, the methods for uti-

lizing wood as well as the methods for recycling and dispos-ing of wood waste should be considered. Wood is the onlynatural-circulating material among principal construction ma-terials, and is characterized by the storage of carbon dioxidethat it absorbed when it was a growing tree. Therefore, tolengthen the period between cutting and disposal is importantto reduce the amount of carbon dioxide emitted by human-induced activities. Long periods until wood is disposed oflead to the reduction of wastes and efficient use of anothercharacteristic of wood, which is natural decomposition.

One of the principal factors that make it difficult to recyclewood waste is that construction wastes are usually disposedof as mixtures of materials, which are difficult to separate.2)

From such wastes, only small amounts of recyclable materi-als can be obtained, and the way to recycle them is limited tochipping now. The labor, costs, and energy needed to classifya mixture into components increase as there are more compo-nents in the mixture. To solve this problem and to promoterecycling, materials should be produced and buildings shouldbe constructed considering the ease of disassembling and re-

∗Corresponding author, E-mail address: [email protected],[email protected]

cycling. This method will increase the amount of wood thatcan be recycled and enable multilateral reuse of wood.

In this paper, the effects of various scenarios of wood uti-lization on the amount of wood demolition waste are dis-cussed. Also, the development of a new process to recyclewood waste will be reported.

2. Present State of Wood Waste Recycling and ModelSimulation of Recycling

Several studies have been done on the quality and quantityof wood waste. One of the largest-scale surveys was con-ducted in 1992 to 1994.2) In this study, wood wastes fromwood industry, demolition and construction activities and dis-posed pallets were estimated by questionnaire survey. But fewquantitative analyses have been done on how to reduce thesewood wastes. This study aims to document the life cycle ofwood products in Japan today, identify the opportunities ofreducing wood waste, and estimate the effects of recycling orreusing wood on waste quantity.

2.1 Material flow of woodWe first documented the material flow of wood by integrate

statistics and references and conducting interview surveys tounderstand the present state of the wood use and wood waste(Fig. 1).3, 4)

In 1998, approximately 15 million cubic meters of remain-der materials were produced at timber factories, over 90% ofwhich was recycled as pulp, materials for producing woodbased panels, and fuel.4) On the other hand, most of the 17million cubic meters of construction, demolition and pack-age wastes were not recycled but incinerated, landfilled, ordumped illegally.4) The precise situation is still not clear, andthese values contain estimates.

Present State of Wood Waste Recycling and a New Process for Converting Wood Waste into Reusable Wood Materials 333

2.2 Scenarios for wood recyclingWe compared the amount of demolition waste in one year

and use of new wood for construction in the next year (theamount of new materials to be used for construction minusthe amount of reused and recycled wood), for various meth-ods for recycling/reusing wood waste from demolished struc-tures. Three typical scenarios were established and mutuallycompared. We then investigated the effects of ease of demol-ishing.2.2.1 Assumptions and conditions

The following assumptions were made for our investiga-tion:

1) 11.2 million cubic meters (the amount from demolishedbuildings in 1998) of demolition wood waste are pro-duced from wooden houses.4) The same amount of wood(11.2 million cubic meters) is to be used in the next yearto build the same number of new houses as the numberof demolished houses.

2) Of newly built houses, 81.2% were built using theJapanese traditional post and beam construction meth-ods, and 18.8% were built using wood-frame construc-tion or panel construction methods.5) The unit amountsof timber used in each construction method are listed inthe Table 1.

3) Five kinds of timber are used for housing construc-tion: large lumber, medium lumber, small lumber, ply-wood, and composite panel. The same amounts of large,medium, and small lumber are used for structural mem-bers. Fixtures are made of small lumber alone. Althoughcomposite panels are not widely used in the wood-frameor panel construction method, the use of composite pan-els will likely increase, such as for the lining of walls.Therefore, we assumed that 5/6 of the boards used inthe wood-frame and panel construction method are com-posite panels. The conditions and assumptions about thetimber used for and produced as demolition wood fromhouses are summarized in Table 2.

4) The following constrains are applied for recycling wood

chips and reusing lumber (Fig. 2).5) The chips, which are produced from the wood waste of

demolished buildings, are to be used to produce pulp andwood based panels and as fuel. We assumed that onlylumber could be reused.

6) We assumed two cases for the demand for wood wastechips as shown in Table 3.

Case D1 is the status quo. Case D2 was estimated by as-suming that recycled pulp would account for 5% of the total

Input Wood Industry Utilization

Products(Domestic)Lumber, Plywood,

Glulam, LVL, Flooring24,666

Products(Imported)12,300

Industrial waste15,258

Wood chip(Domestic)9,384

Composite panel(Domestic)

2,001

Paper, Pulp(Domestic)

29,885

Utilization 39,607Construction

Furniture, FixturesCivil engineeringPallets, Crates

Other

Waste Fuel

Waste

Demolition waste11,200

Construction waste 3,680

Crates, Pallets2,144

Fuel2,707

LandfilledIncinerated

13,619Wood chip(Imported)

26,094

Paper, Pulp(Imported)

4,365( 1000t)

Composite panel(Imported)

815

Rough lumber

Domestic19,316

Imported20,088

CompostOther5396

LandfilledIncinerated

1.053

Unit: 1000m 3/yr

Fig. 1 Material flow of wood in Japan, 1998.

Table 3 Estimated demand for wood waste chips.

(V /Mm3)

Pulp Wood composite panel Fuel

D1 0.36 0.90 2.70

D2 1.80 1.35 2.70

Table 2 Timbers included in demolition wastes and input for construction.

Percentage(%) Volume, V /Mm3

Large lumber 23 2.58

Medium lumber 23 2.58

Small lumber 46 5.16

Plywood 5 0.56

Wood composite panel 3 0.32

Table 1 Amounts of wood used per floor area in a house.

(m3/m2)

Post and beam6) Wood frame7)

construction construction

Structural lumber 0.138(69.1%) 0.152(73.5%)

Fixture lumber 0.052(26.2%) 0.018(8.8%)

Plywood 0.010(4.8%) 0.037(17.7%)

Wood composite panel — —

Total 0.200(100%) 0.207(100%)

334 Y. Hiramatsu, Y. Tsunetsugu, M. Karube, M. Tonosaki and T. Fujii

Lumber

Plywood

Compositepanel

Pulp

Compositepanel

Fuel

Demolitionwood

Wood ChipsUses

Quality

LargelumberMediumlumberSmalllumber

MediumlumberSmalllumber

Chip

Demolitionwood Reused lumber

Chipping

Quality

Chipping

Fig. 2 Constraints of recycling chips and reusing lumber.

0 4 8 12

S3

S2

S1

Volume, V /Mm3

WasteInput

Fig. 3 The amounts of waste and input of new wood for each scenario.Reuse rate: 0.6 Demand: case D1.

Figure 3 shows the amounts of waste and new materials foreach scenario.

Comparison between S1 and S2 suggests that use of woodchips for various purposes reduces the amount of waste butdoes not reduce the amount of new materials. This is at-tributable to low percentages of wood composite boards usedfor building houses. S3 showed reductions in both the amountof waste and the input of new materials. Since demand forlumber is high, lumber wastes are highly reused thus reduc-ing the amounts of both waste and new materials.(2) Effects of the facility and method of disassembling

Figures 4 and 5 show the amounts of waste and new mate-rials for each combination of conditions.

For a constant demand, an increase in the reuse rate causedlinear drops in both the amount of waste and input of new

0.4 0.6 0.8

D2

D10

2

4

6

8

10

Vol

ume,

V/M

m3

Reuse Rate

(S3)

Fig. 4 The effect of reuse rate and demand for wood waste chips on theamounts of waste.

0.4 0.6 0.8

D2

D10

2

4

6

8

10

Vol

ume,

V/M

m3

Reuse Rate

(S3)

Fig. 5 The effect of reuse rate and demand for wood waste chips on theamounts of input of new wood.

buildings and are of poor quality.S2: Wood construction waste are converted into chips. Thechips, from good to inferior qualities, are used as pulp materi-als, wood composite panel materials, and fuel. This scenarioassumes that buildings are dismantled by classifying wastesbut do not produce lumber that can be reused. This most re-sembles the present state.S3: Of wood construction waste, lumber is reused. The reuserate (the ratio of the remaining volume of reused lumber to theoriginal volume) is 0.6 for both large lumber to medium lum-ber and medium lumber to small lumber. The rest of the lum-ber, plywood, and panels are converted into chips and usedfor the three purposes.2.2.3 Effects of the facility and method of disassembling

To efficiently reuse lumber, buildings should be con-structed so that they are easy to disassemble and be carefullydisassembled. This will change the reuse rate and demand forwood waste. To investigate the effects of these changes, weestimated the amounts of wood waste and new materials usedunder scenario S3 and the following conditions:Reuse rate: 0.8,0.6 and 0.4Demands: D1 and D2 shown in Table 3.These conditions give six combinations, one of which is S3studied in B.2.2.4 Results and discussion(1) Differences in utilization methods of wood waste

pulp used in 1998 and wood based panels would consist of50% recycled chips (which were 1% and 33% in 1998, re-spectively). Demand for fuel was set to be 1.1 times of caseD1, according to the government target, which is aiming tomake the energy supplied by black liquor and wood waste 1.1times of the level of 1998 by 2010.8, 9)

2.2.2 Effects of differences in utilization methods ofwood waste

We devised and compared three typical scenarios of recy-cling.S1: Wood construction waste is converted into chips and usedas fuel. Those that are not used are disposed of. This scenarioassumes that the wastes are not classified while dismantling


of construction materials, and do not affect the input of newmaterials much.

2.3 SummerySo far, we investigated the recycling of wood waste from

demolished wooden houses using models.In our model:

1) Production of good-quality wood chips that can be usedfor various purposes decreased the amount of waste.

2) Reuse of lumber as well as recycling of wood waste toproduce chips was effective to reduce waste.

3) An increase in the demand for chips reduced the amountof waste but did not greatly affect the input of new mate-rials into wooden houses. This was because wood basedpanels were used little in houses.

4) The combination of reuse rate of 0.8 and demand of D2reduced the waste by 78% from that of 0.6 and D1.

3. Development of a Wood Waste Recycling Process

In the first chapter, it was clarified that improving diverseutilization of wood waste was effective to reduce the quan-tity of waste. One way to recycle wood waste is separatingthem into small wood elements like chips, and to producewood based panels by adhesion, lamination, and compositeformation. For more use of wood based panels from waste inhouses, it will be necessary to improve the efficiency of theconventional processes for producing wood elements such ashammermilling, shaving, etc., and to improve the quality ofwood elements, since they are closely related to the propertiesof the wood based panels such as fiberboard, particleboard,and oriented strand board.

It is thought that new wood recycling processes, which sep-arates wood waste into small wood elements with propertiesand shapes different from those obtained by the conventionalprocesses, need to be developed to expand the use of woodmaterials from waste, not only for structural use in houses butalso for non-structural use such as exterior, interior, furniture,shock absorber, and sound absorber.

materials.For a constant percentage of reuse, the use of D2 instead of

D1 reduced the amount of waste by half but did not affect theamount of new materials much. This was because changes inthe demand for chips only affect the amount of new materi-als used to produce wood based panels, which are little usedcompared to the other materials.

We also discussed the composite effects of reuse rate anddemand, since different demolishing methods produce wastesof different qualities, both the demand and reuse rate arelikely to change simultaneously. The effect of a combinationwas the sum of the two effects. For a reuse rate of 0.8 and de-mand of D2, the amount of waste was about 22% of that forS3 (60% and D1). The input of new materials little decreasedcompared to the reduction in the amount of waste. This isbecause even if high percentages of lumber are to be reused,used lumber accounts for only a small percentage in the to-tal amount of materials. Also, wood based panels are usedlittle, accounting for a small percentage in the total amount

3.1 Water vapor explosion processThe water vapor explosion (WVE) process10) that was de-

veloped in this study separates wood materials into smallwood elements including fiber bundles, strands, and chips.In this process, wood materials are exploded from withinby the force of water vapor generated by compression un-der high pressure and temperature and evaporation of theinternal moisture (Fig. 6), which is significantly differentfrom conventional explosion (usually known as “steam explo-sion”11–13)). Unlike the steam explosion process, the WVEprocess does not depend on external steam of high pressureand temperature; instead, it uses water vapor generated byevaporation of the internal moisture to separate wood mate-rials. And the aims of steam explosion and WVE for woodmaterials will be different. Steam explosion of wood materi-als aims at pulping, wood saccharification, production of cat-tle feed, and total utilization of wood components (cellulose,hemicellulose, and lignin). WVE of wood materials aims atreproducing wood materials from the wood elements obtainedby this process.

In this study, the WVE process was directly applied toseparation of Sugi (Cryptomeria Japonica) and Karamatsu(Laryx leptolepis) sawn timbers and slabs with the differentmoisture content (MC) of air-dried and water-soaked condi-tions, and the appropriate conditions (pressure, temperature,

Fig. 6 Schematic illustration of water vapor explosion of wood materials.


and compressing time) of WVE were investigated for the sep-aration of them. The yield of wood elements obtained bywater-soaked Sugi sawn timbers was measured.

3.2 Materials and methods3.2.1 Experimental equipment of WVE

For WVE of wood materials, an explosion device consist-ing of heating plates and side blocks was designed (Fig. 7).Heating plates were used for compressing and heating woodmaterials, and side blocks were used for stopping the lateralexpansion of wood materials. Compressing pressure, heatingtemperature and compressing time were all controlled withthe control panel. The maximum size of wood materials thatcould be examined was 150 mm in width (W ), 150 mm inthickness (T ), and 900 mm in length (L). Wood materials lo-cated in the space between the closed side blocks of the explo-sion device were compressed and heated by the heating platesunder a certain pressure for a few minutes. When the heat-ing plates and side blocks opened simultaneously to releasethe external pressure, the wood materials exploded from in-side due to the force of water vapor generated within them,and they separated into small wood elements with differentshapes.3.2.2 Wood materials

Sugi sawn timbers included air-dried (MC = 10–20%)and water-soaked (MC = 100–200%) conditions, with di-mensions of 150 mm (W ), 23 mm (T ), and 900 mm (L).Karamatsu sawn timbers included air-dried (MC = 10–20%)and water-soaked (MC = about 60%) conditions, with150 mm (W ), 25 mm (T ), and 900 mm (L). Sugi slabs (water-soaked, MC = about 200%) were less than 150 mm (W ) by900 mm (L).3.2.3 Conditions of WVE

Sawn timbers and slabs were examined under the followingexplosion conditions.

The WVE conditions of Sugi sawn timbers (air-dried) werepressures; 2.5–10.0 MPa, temperature; 250◦C, time: 120–420 s, and for Karamatsu, they were 5.0–10.0 MPa, 250◦C,150–300 s, respectively.

The WVE conditions of Sugi and Karamatsu sawn timbers(water-soaked) were 2.0–10.0 MPa, 200–300◦C, 60–720 s,

densities of wood elements in the air-dried condition obtainedby WVE of water-soaked Sugi sawn timbers were measured.Bulk density was calculated from the weight of wood ele-ments put in the container (100 mm × 100 mm × 500 mm)without applying any pressure.

3.3 Results and discussion3.3.1 WVE of wood materials

Sawn timbers and slabs were partially exploded, and sepa-rated into small wood elements by WVE. The remaining partswere unexploded or only cracked. Shapes and sizes of woodelements obtained by WVE were different according to theexperiment conditions (pressure, temperature, and compress-ing time), MC of wood materials, and wood species.

The explosion of air-dried sawn timbers mainly producedstrands and chips. The explosion of water-soaked sawn tim-bers and slabs produced fiber bundles, strands, and chips.Fiber bundles were long, narrow, and flexible wood elements,and they were the most characteristic wood element obtainedby WVE of water-soaked sawn timbers and slabs. Water-soaked wood materials were softened and compacted withmoisture and heat when they were compressed. Pressure ofwater vapor generated in sawn timbers and slabs increased.Softening of lignins and degradation of hemicelluloses oc-curred, and the internal bonding of wood fibers became weak.Thus, by releasing the external pressure of the explosion de-vice and destroying the balance of the external pressure and

and 2.5–5.0 MPa, 300◦C, 180–270 s, respectively.The WVE conditions of Sugi slabs (water-soaked, 3–4

slabs) were 2.5–7.5 MPa, 300◦C, 90–300 s.3.2.4 Measurement of weights, sizes, and bulk densities

of wood elementsShapes and sizes of wood elements obtained by WVE were

nonuniform (Fig. 8), and it is difficult to classify them. Inthis study, these wood elements were divided into fiber bun-dles, strands, chips, and powders according to their shapesand sizes (Fig. 9). Fiber bundles and strands were furtherdivided into three size classes. The interconnected fiber bun-dles were unraveled by hand. Chips were classified into twosize classes as well. Powders were the smallest elements thatpassed through the 3 mm mesh. The weights, sizes and bulk

Fig. 7 Experimental equipment for water vapor explosion.


Fig. 8 Water vapor explosion of Sugi sawn timber (water-soaked).

Fig. 9 Wood elements obtained by water vapor explosion of Sugi sawn timbers (water-soaked).(a) Fiber bundles, (b) Strands, (c) Powders and (d) Chips


soaked Sugi sawn timbers is shown in Fig. 10. The yield ofwood elements, including fiber bundles, strands, chips, andpowders, was 30–50% in weight. This would be due to thelow airtightness of the explosion device. Water vapor gener-ated in sawn timbers escaped from the narrow space betweenthe heating plates and side blocks, so the pressure of water va-por required for the explosion was not maintained in the sawntimbers. Especially, water vapor in the edges of the wood ma-terials easily escaped; the edges had lower MC and so werehard to be exploded. In order to increase the yield of woodelements, particularly fiber bundles, an improved explosiondevice that is closed up tightly and holds the water vapor inthe wood materials should be developed. The yield of fiberbundles that were particularly obtained by WVE was less thanthose of strands and chips, but the yield of fiber bundles wouldbe expected to be increased.

The sizes and bulk densities of fiber bundles and chipsobtained by WVE of Sugi sawn timbers (water-soaked) areshown in Table 4. Fiber bundles and chips could be classifiedusing screens, because their shapes, sizes, and bulk densitieswere different. The shapes of fiber bundles were various, soclassifying them into three size classes using screens wouldbe difficult. The bulk densities of fiber bundles were almostthe same, so classifying them by their weights would be alsodifficult. Chips could be divided into two size classes usingscreens.

In this study wood elements obtained by WVE were clas-

5.0MPa, 300 ,120-240s

3.0MPa, 300 ,150-180s

2.5MPa, 300 ,150-270s

2.0MPa, 300 ,180-240s

Exp

erim

enta

lCon

ditio

nsof

WV

E

Fiber bundles Strands Chips Powders Unexploded

Fig. 10 Average yield of the wood elements obtained by water vapor ex-plosion of Sugi sawn timbers (water-soaked).

Table 4 Average sizes and bulk densities of fiber bundles and chips ob-tained by water vapor explosion of Sugi sawn timbers (water-soaked).

Fiber bundles Chips

Size classes A B C A B

Length, l/mm 229 123 136 58.7 42.1

(61)∗ (35) (53) (17.7) (14.2)

Diameter, D/mm 3.89 3.23 1.04 3.10 1.93

(1.40) (1.04) (0.49) (1.24) (0.80)

Bulk density, d/kg·m−3 13.1 13.6 10.1 29.8 35.3

(2.32) (1.24) (2.61) (5.80) (6.02)

∗Standard deviations are given in parentheses.

conditions must be appropriately adjusted to soften wood ma-terials and create high-pressure water vapor.

The appropriate conditions of WVE for Karamatsu sawntimber (water-soaked) were considered to be 2.5 MPa, 300◦C,and 210–270 s. The quantity of fiber bundles obtained byWVE of Karamatsu was less than that produced by WVE ofSugi, and fiber bundles of Karamatsu had less flexibility thanthose of Sugi. The reasons are attributed to the spiral grain,high density, and hardness of Karamatsu. Another reason wasthat MC (= about 60%) was not enough to produce a suffi-ciently high pressure of water vapor for explosion.

The appropriate conditions of WVE for Sugi slabs (water-soaked) appeared to be 3.0–3.5 MPa, 300◦C, and 210–270 s.Shapes and dimensions of slabs were irregular, so spaces ex-isted among slabs when they were located in the experimentalequipment. In order to produce WVE of slabs, it was neces-sary to collapse the slabs and fill those spaces first, and then toraise the pressure of water vapor generated in the slabs enoughto explode them from inside. Therefore, a little higher exter-nal pressure was needed for WVE of slabs than for WVE ofsawn timbers that were flat and without space. Over the pres-sure of 4.0 MPa, midway explosions occurred frequently be-fore releasing the pressure of the explosion device, and fiberbundles were not effectively produced.3.3.3 Weights, sizes and bulk densities of wood elements

obtained by WVEThe yield of wood elements obtained by WVE of water-

the internal pressure, sawn timbers and slabs exploded amongthe intercellular layers, and separated into small wood ele-ments along fiber directions. Therefore fiber bundles wereeffectively produced by WVE of water-soaked wood materi-als.3.3.2 Appropriate conditions of WVE

The quality and quantity of wood elements obtained byWVE varied according to the experiment conditions, MC ofwood materials, and wood species. In order to produce fiberbundles effectively by WVE and to maximize the yield ofwood elements, it was necessary to adjust MC and determinethe appropriate explosion conditions for different wood mate-rials.

Sugi and Karamatsu sawn timbers (air-dried) exploded ex-tremely under conditions of 5.0–10.0 MPa, 250◦C, and 180–300 s, and mainly produced strands and chips. Almost nofiber bundles were obtained. Strands and chips of Karamatsuwere harder than those of Sugi.

Fiber bundles were obtained by WVE of water-soakedsawn timbers and slabs.

For water-soaked Sugi sawn timbers, the appropriate condi-tions of WVE were 2.0 MPa, 300◦C, 180–240 s, and 2.5 MPa,300◦C, 150–270 s. The highest yield of fiber bundles ob-tained under these explosion conditions. Over the pressure of3.0 MPa, midway explosions often occurred before releasingthe pressure, and fiber bundles were not effectively produced.Especially over the pressure of 5.0 MPa, only a few fiber bun-dles were obtained. It was considered that under the pressureof 5.0 MPa, sawn timbers were not broken by the high pres-sure of water vapor generated in sawn timbers, but collapseddue to the pressure of the explosion device. In order to in-crease the yield of fiber bundles, high pressure that breakswood materials from outside is not required. The explosion


ments, fiber bundles that became interconnected were unrav-eled by hand. But in order to divide the interconnected fiberbundles effectively, mechanical methods to separate them ormethods that could classify them without unraveling werenecessary.

3.4 ConclusionsIn this study, the WVE process was applied to the separa-

tion of wood materials, and the appropriate conditions (pres-sure, temperature, and compressing time) for producing WVEand fiber bundles were measured. The results obtained can besummarized as follows.

Compressing wood materials under high pressure and tem-perature for a short time, and instantaneously releasing thepressure caused WVE in the wood materials from withinand they were separated into small wood elements along thefiber directions. Various shapes and sizes of wood elementswere obtained by WVE according to the explosion conditions,MC of wood materials, wood species, etc. The explosion ofair-dried wood materials mainly produced strands and chips,whereas that of wood materials soaked in water producedmore fiber bundles that were long, narrow, and flexible. It ap-peared that fiber bundles could not be obtained by other pro-cesses, and fiber bundles were the most characteristic woodelement obtained by WVE.

In order to effectively obtain fiber bundles by WVE, thewood materials required excessive water inside of them, andappropriate conditions for WVE were determined. Sugi andKaramatsu sawn timbers (air-dried) exploded extremely atcertain pressures, temperatures, and times, but fiber bundlescould not be obtained. By WVE of water-soaked Sugi sawntimbers, fiber bundles were effectively produced under con-ditions of 2.0 MPa, 300◦C, 180–240 s, and under 2.5 MPa,300◦C, 150–270 s. WVE of Sugi slabs (water-soaked) ef-fectively produced fiber bundles under 3.0–3.5 MPa, 300◦C,210–270 s. By WVE of Karamatsu sawn timbers (water-soaked), fiber bundles were obtained under 2.5 MPa, 300◦C,210–270 s, but fiber bundles of Karamatsu were less flexiblethan those of Sugi.

sified according to their shapes and sizes. This classifica-tion method was subjective, and the variation in the classifiedwood elements would become large. But the data collectedin this study (sizes and bulk densities of the wood elements)were considered to be useful to develop an objective classi-fication method in future. In the classification of wood ele-

In this chapter, the WVE process was introduced as amethod of wood waste recycling. A feature of WVE processis using no cutting edge. In the conventional processes ofseparating wood materials into small elements, cutting edgesare generally used, and they are often damaged by metals,cement, earth and sands contained in wood waste. In theWVE process, no cutting edge is used; so wood waste willbe more easily separated into small elements than the conven-tional process. The properties and shapes of the fiber bundlesobtained by the WVE will allow new wood materials to bedeveloped, and expand the use of wood materials from waste.

Acknowledgements

The authors wish to thank Prof. B. Tomita, Dr. T. Hayashi,Y. M. Wei, A. Miyatake, and M. Harada for help with the ex-periments and valuable discussion. This research was finan-cially supported in part by Development of technologies foreco-system establishment based on recycling in rural commu-nities for the 21st century from the Ministry of Agriculture,Forestry, and Fisheries of Japan.

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