Improvement on the properties of gypsum particleboard by adding cement*

Transcription

1 J Wood Sc (1998) 44: The Japan Wood Research Socety 1998 Yuhe Deng Takesh Furuno Tohru Uehara Improvement on the propertes of gypsum partcleboard by addng cement* Receved: February 26, 1997 / Accepted: October 13, 1997 Abstract Gypsum partcleboard (GPB) has hgh thckness swellng (TS), hgh water absorpton (WA), and low mechancal propertes compared wth cement-bonded partcleboard. The propertes of GPB were mproved by addng cement. The expermental results showed that GPB wth the added cement had good physcal and mechancal propertes compared wth those of gypsum partcleboard wth no added cement. The TS and WA of gypsum partcleboard wth added cement were reduced by 10%. The mechancal propertes of GPB, such as nternal bond strength (IB), modulus of rupture (MOR), and modulus of elastcty (MOE), ncreased when the GPB was made wth added cement. The propertes of GPB mproved relatve to the quantty of cement added. Wth an ncrease of cement content from 5% to 10%, the TS and WA were reduced, and the IB, MOR, and MOE were ncreased. In contrast, the TS and WA ncreased and the IB, MOE, and MOR decreased when the cement content was ncreased from 15% to 30%. Thus the physcal and mechancal propertes of GPB were successfully mproved when the added cement content was 10%. Key words Gypsum partcleboard Cement Thckness swellng Internal bond strength - Intal curng Y. Deng T. Furuno ([]) - T. Uehara Faculty of Scence and Engneerng, Shmane Unversty, Matsue , Japan Tel ; Fax e-mal:t-furuno@rko.shmane-u.a.jp Y. Deng College of Wood Scence and Technology, Nanjng Forestry Unversty, Nanjng, Chna * An outlne of ths paper was presented at the 47th Annual Meetng of the Japan Wood Research Socety n Koch, Aprl 1997 Introducton Gypsum partcleboard (GPB) s a wood-based panel developed n recent years n Germany. A good buldng constructon materal, z t has the features of hgh resstance to fre and nonformaldehyde emsson. GPB shows lower frablty than gypsum board. The effect of tannn on gypsum curng s low n some wood speces. 2 As a consequence, gypsum partcleboard can be produced usng most wood speces. 3 Natural and chemcal gypsum can be used to produce gypsum partcleboard. 4 Gypsum resources are rch n the world. 5 The chemcal formula of gypsum s CaSO4 H20. When water s added to gypsum, t makes CaSO 4 H20 --) CaSO4 2H20. The formula reverts to CaSO 4 H20 when the water evaporates. Due to weak resstance aganst water, gypsum partcleboard shows low mechancal propertes and hgh thckness swellng when absorbng water. We nvestgated the effects of addng cement on the physcal and mechancal propertes of GPB. Materals and methods Test materals The test materals were commercal gypsum used for a plaster cast and whte Portland cement (Chchbu Onoda Cement, Tokyo, Japan); partcles were obtaned from partcleboard manufacture (Dantan, Kta-Kyushu, Japan). The proportons of screen analyss (S) of partcles (openng) were 9.2% n S < 0.71ram, 10% n 0.71mm < S < 1.00ram, 16.6% n 1.00ram < S < 1.40mm, 34.2% n 1.40mm < S < 2.00ram, and 30% n 2.00mm < S, respectvely. Producton method and condtons of GPB Each materal was weghed at a wood/gypsum rato of 0.25 and a water/gypsum rato of 5 n the laboratory. 6'7 The partcles were then put n a blender, and water and the mxture of gypsum and cement were added. After beng

2 99 blended, sample boards of 10mm 40cm x 40cm were formed. Sxteen sample boards possessng a target densty of 1.2 g/cm 3 and eght levels of cement content (5 %-30 % based on the weght of the gypsum, usng 100% gypsum and 100% cement) were produced. For decreasng the effect of densty on the physcal and mechancal propretes of GPB, the average GPB densty was controlled wthn g/cm 3. To measure the effect of densty on nternal bond strength (IB) and the thckness swellng (TS), 12 sample boards were produced based on denstes of 1.18,1.20,1.24, and 1.28 g/cm 3 at each cement addton of 10%, 20%, and 25%. All mats were pressed at 3MPa n a cold press for 4h. The mosture contents of GPB mats were reduced to about 2%-3% at 45 C n a dryer. After beng removed from the dryer, the mats were stored at room temperature for 1 week. There were four specmens per treatment. Measurement of physcal and mechancal propertes of GPB The physcal and mechancal propertes were measured accordng to the JIS (Japanese Industral Standard) A The specmens for measurng IB to determne the effect of addng cement were dvded nto two groups. The IBs for one group were measured drectly. The specmens for another group were soaked n water (21 C) for 24h and the thckness due to swellng and water absorpton (WA) then measured. The specmens were dred to a mosture content of about 2%-3% at 45 C n the dryer. Fnally, the IBs were measured] The reducton rato (K) of the IB of specmens soaked n water compared wth the specmens wthout beng soaked n water was calculated usng the followng formula. K = 1 - ; 100 (%) where IB~ s the nternal bond strength wthout beng soaked n water and IB2 s the nternal bond strength after beng soaked n water. All the data for the physcal (TS, WA) and mechancal (IB, MOE, MOR) propertes are shown as average values. Results and dscusson Effect of addng cement on TS and WA Fgure 1 shows the effects of addng cement on thckness swellng and water absorpton of GPB after specmens were soaked n water (21 C) for 24h. The results ndcate that the TS and WA were markedly reduced n cement contents of 5%-15%. The reducton ratos of TS and WA were hghest at a cement content of 10% (Fg. 1). It s well known that cement has a hgh resstance to water and produces a hgh bondng strength between partcles when t cures n the approprate condton? When the GPB was produced by addng an approprate amount of cement, the cement acted to ncrease the bondng strength of GPB, so the strength of the board was hgh. When the specmens were soaked n water, the hgh bondng strength ressted absorpton of water nto the boards. For ths reason, the GPB showed the lowest TS and WA values at 10% added cement. An excessve amount of cement caused the TS and WA of GPB to ncrease. Because the cementbonded partcleboard at a cement content of 100% was produced n the same way as GPB, t exhbted lower mechancal propertes and hgher TS and WA values, as shown n Table 1, resultng from the fact that the cement could not acheve good bondng strength between partcles when t was cured at hgher cement contents. Consequently, when the GPBs were produced by addng cement at 20%-30%, the mxture of gypsum and cement as an adhesve could not produce a good bondng strength between partcles at hgher cement contents. The GPB easly absorbed water and produced hgher TS and WA values when the bondng strength of the board was low. Therefore the ncreases and decreases n TS and WA at hgher and approprate cement contents can be consdered to cause the decrease and ncrease of IB of board. Effect of addng cement on IB Fgure 2 shows the effect of addng cement on the IB of GPB under condtons of water-soakng and no soakng. Measurement of the ntal curng tme, exothermc temperature, and exothermc tme The ntal curng tme (IC), the tme untl gypsum started to cure after water was placed n the gypsum, was measured accordng to DIN (Deutsches Insttute ft~r Normung) The partcles, water, cement, and gypsum were treated for 1 day under condtons of constant temperature and humdty. The materals were mxed at the same wood/gypsum rato and the same water/gypsum rato as descrbed above. They were then placed n a hermetcally sealed vacuum flask. Two thermometers were postoned n the mddle of the mxture. The hghest temperature and tme acheved n the mxture were measured as the exothermc temperature (ET) and the exothermc tme (Et), respectvely ~'2.5 0o ~ Fg. 1. Relatons between thckness swellng (TS), water absorpton (WA), and cement content

3 100 Table L Effect of addng cement on propertes of gypsum partcleboard Quantty Propertes of added cement (%) Den T8 TS m WA MOE MOE m MOR MOR m ET Et (g/cm 3) (%) (%) (MPa) (MPa) ( C) (mn) Den, densty; TS, thckness swellng; TSm, mean square devaton of TS; WA, water absorpton; MOE, modulus of elastcty; MOEm, mean square devaton of MOE; MOR, modulus of rupture; MORm, mean square devaton of MOR; ET, exothermc temperature; Et, exothermc tme ~ Non-soakng ~ 0.2 -, Water-soakng \ Fg. 2. Relatons between nternal bond strength (IB) and cement content Regardless of the soakng or no soakng n water, the boards exhbted the hghest IB when the cement content of 10% was added to the boards. Compared wth a cement content of 10%, boards produced wth other cement contents had lower IB values. The IB of GPB was reduced at hgher cement contents because the excessve cement affected the curng of gypsum. For ths reason, IBs of boards at cement contents of more than 20% were lower, and the IB mprovement was nsgnfcant. An approprate amount of cement could ncrease the bondng strength of GPB, workng as a renforcng materal. That s, the IBs of GPB showed hgh values when addng cement of 5%-10%. The reducton ratos (K) of IB of the specmens soaked n water (21 C) for 24h were vared, dependng on the cement content, as shown n Table 2. The IBs for no soakng and water-soakng condtons were and 0.447MPa, respectvely, at a cement content of 10%, gvng a reducton rato of 4.9%; the IBs wthout added cement were 8 and 0.29 MPa for no soakng and water-soakng condtons, respectvely, gvng a reducton rato of 23.7%. As ndcated above, when GPB was produced by addng an approprate amount of cement, the cement acted to ncrease the bondng strength of boards and to resst the absorpton of water nto partcles and gypsum; hence the reducton rato of IB was smallest when cement of 10% was added. Excessve cement and the lack of cement resulted n weak bondng strength between partcles when the specmens were soaked n water because the partcles had to absorb much water to reduce the IB of GPB. Table 2. Effect of addng cement on nternal bond strength of gypsum partcleboard Quantty Propertes of added cement (%) IB1 Densty IBlm IB 2 Densty IB2m K (MPa) (g/cm 3) (MPa) (g/cm ~) (%) IBm, nternal bond strength (no soakng); IB2, nternal bond strength (water-soakng); IBs, mean square devaton of IBx; IB2m, mean square devaton of IB2; K, [(IB~ - IB2)/IB1] 100.

4 Effect of addng cement on MOE and MOR The effects of addng 5%-30% cement on the modulus of elastcty (MOE) and modulus of rupture (MOR) are sgnfcant, as shown n Fg. 3. The results ndcated that the MOE and MOR of GPBs ncreased abruptly when the cement content was ncreased from 5% to 10%. The MOR and MOE of boards had a tendency to decrease at a cement content of more than 15%, but they were hgher than n the boards wthout the added cement. GPB s a well known renforcng composte materal. For ths reason, the MOR and MOE of GPB are mportant not only to the bondng strength of board but also to the partcle sze and the qualty of gypsum tself. When the bondng strength of boards was wthn a certan range, the nfluence of the partcle sze on the MOR and MOE was prmary. 3 In ths study the sze of the partcles was not changed so the effects on the MOR and MOE of addng cement were not obvous at hgher cement contents. Effect of board densty on IB and TS The effect of the densty of boards on IB and TS was marked. The IB and TS of boards ncreased wth ncreased 101 cement, but they were not related to the cement contents (Fgs. 5, 6). The ICs of GPB wth and wthout the addton of cement were about 10 and 17mn, respectvely. The IC was reduced by the addton of cement. That cement works as a promoter ~1 helped accelerate the curng speed of the soluton of CaSO4 - H20. The IC of GPB was thus shortened when cement was added to gypsum. Accordng to the report on GPB produced by cold pressng, 12 gypsum generated heat when t cured wth the addton of water. Hence the curng of gypsum was an ~0.4 densty, as shown n Fg. 4: the hgher the densty of the board, the more partcles and gypsum were seen n a unt area. s Hence there was hgher bondng strength between the partcles and gypsum, enhancng the mechancal proper- 0.6 tes of GPB. For ths reason, the hgher the densty of the board, the hgher was the IB. In contrast, due to weak resstance to water, water absorbed at a unt area caused more swellng of the boards when the specmens were soaked. Consequently, hgher-densty board resulted n ~ 0.4 hgher IB and TS. The densty of board thus exhbted a sgnfcant effect on IB and TS % cement OIB TS / Densty (g/cm 3 ) 20% cement OIB J~ ~TS / Effect of addng cement on IC, ET, and Et durng GPB curng The ntal curng tme, exothermc temperature, and exothermc duraton of GPB were nfluenced by the addton of W O o- MOE 0.2 ' ' ~., O ~ ~ O. 2 Densty (g/cm 3 ) 25% cement o I B ~ ~TS [ ~ ' ' ' ' Densty (g/cm 3 ) 0.3 Fg. 3. Relatons between modulus of elastcty (MOE), modulus of rupture (MOR), and cement content Fg. 4. Effect of densty on nternal bond strength and thckness swellng

5 102 A E 14 O F _ Fg. 5. Relatons between ntal curng (IC) - tmes and cement content oq 46 Temperature Tme '~= I ' 46 Fg. 6. Exothermc temperatures and duraton of curng for gypsum partcleboard wth varous cement contents exothermc reacton. The ET and Et of gypsum and cement were 46 C and 55mn and 32 C and ll0mn, respectvely (Table 1). The ET of cement was lower than that of gypsum, so the ET of GPB was reduced by addng cement. Though the Et of cement was longer than that of gypsum, the cement worked as a knd of promoter n ths study and reduced the exothermc duraton of GPB when cement was added to gypsum. Conclusons The physcal and mechancal propertes of GPB were mproved markedly by addng cement. When cement of 5%- 15% was added, t successfully decreased the TS and WA and ncreased the IB of boards. The reducton rato of IB wth the addton of cement was also lower than that after no addton of cement when specmens were soaked n water (21 C) for 24h. The mprovement n MOE and MOR nduced by addng cement was not as marked as that of the IB, TS, and WA. The test not only researched the effect of addng cement on the physcal and mechancal propertes of GPB, t also explored the effect of addng cement on the IC, ET, and Et of GPB. For samples wth no wet treatment, cement content of -<15% had a sgnfcant effect on the physcal and mechancal propertes of GPB, a result dfferent from that reported by L et al. 7 There was no sgnfcant effect nduced by addng cement at ->20%. Though the IC, ET, and Et were nfluenced by the addton of cement, the effects were not related to the cement content. The IB and TS of GPB were related to the densty of the board, ncreasng wth an ncrease n densty. Acknowledgment The authors thank Dantan Corp. for provdng the samples of wood partcles that were used n ths study. References 1. Wang K, Zhang Y (1987) New tectoncs of gypsum partcleboard producton. Chna Forest Prod Ind 2: Zhang Y (1990) Influence of wood extractve on hardenng and propertes of gypsum plaster and gypsum-bonded partcleboard (n Chnese). Wood Ind 4(1): Zhang Y (1990) Compatblty of some wood speces to the manufacture of gypsum bonded partcleboard (n Chnese). Wood Ind 4(4): Deng YH, Zhang Y (1994) Effect of new materals on the propertes of gypsum partcleboard (n Chnese). Buldng Wood Board Panels 4: Uehara T, Furuno T, Jod S (1989) Producton of wood mealgypsum composte board and ts bendng propertes (n Japanese). Bull Fac Agrc Shmane Unv 23: Song XJ, Xu RX, Sh QL (1990) Development of gypsum partcleboard (n Chnese). J Fujan Forest Coll 10: L XM, Chen SY (1989) Prelmnary study on water-resstant propertes of gypsum (n Chnese). Wood Ind 3(4): JIS A 5908 (1994) Japanese Industral Standards 9. Eusebo DA, Yamauch H, Sakak H, Kawa S (1996) Bark-cement compostes. In: Toward the new generaton of bo-based compostes products. Pacfc Rm Bo-based Compostes Symposum, Kyoto, pp Zhang Y (1989) Prelmnary studes on relatonshp between densty, wood-gypsum rato, water-gypsum rato of gypsum-bonded partcleboard (n Chnese). Wood Ind 3(2): Zhang Y (1991) The relatonshp between the propertes of gypsum partcleboard and partcle geometry (n Chnese). Wood Ind 5(1): Takahash RD (1977) Study on wood gypsum board (n Japanese). J Forest Prod Res Inst 10:11-16