Dimensional instability of cement bonded particleboard

This paper records work carried out to determine the behaviour of cement bonded particleboard (CBPB) under constant relative humidity (RH) and single change in RH. The movement was analysed with respect to the structure of CBPB and the nature of the cement paste and wood chips. Exposure of CBPB to a constant RH resulted in considerable increase in mass and decrease in dimensions. The ratio for change in mass:length:thickness=47:1:5. The change, in both mass and dimensions with time, under a single change in RH exposure consisted of 2 distinct stages with a point of “inflection” between them: an initial stage in which change in mass parallels change in dimensions, and a later stage in which this relationship is inverted. Such a relationship indicates the existence of two different controlling mechanisms, respectively, a general moisture reaction, and a carbonation or condensation reaction. The relationships between mass and dimensional changes were linear under constant RH and on desorption, but the nature changed at the “inflection points” under a single change in RH. Models were developed to predict the changes in both mass and dimensions under both a constant and a single change in RH. Both mass and dimensional changes with changes in RH were essentially both Fickian and non-Fickian. Fractional changes in dimensions were very similar for all changes in RH, whilst those for mass were significantly influenced by the level of RH. Received 21 June 1999     

Dimensional instability of cement bonded particleboard

This paper examines the dimensional instability of cement bonded particleboard (CBPB) and discusses the behaviour of CBPB during cyclic changes in relative humidities (RH). The results indicated that the changes of CBPB in both mass and dimensions reflected the changes in RH; in a cycle 90–65–35–65–90%RH the change of per percentage change in RH was 1.0:1.3:1.3:2.5 for mass, 1:2:2:1 for length and 1.3:1.1:1.0:1.0 for thickness. The changes were closely related to the structure of CBPB and the nature of the cement paste and wood chips. Thickness change was double that of the length change between 65 and 35%RH and triple that between 65 and 90%RH. The rate of change in mass was much higher than that in dimensions. The hysteresis loops were dissimilar to those of other wood and wood-based materials, with the loops of mass change closed between 65 and 90%RH, while the loops for dimensional changes were open. A set of the former loops moved upward and the latter moved downward with increasing number of cycles, corresponding to an accumulated increase in mass, but decrease in dimensions. The models developed for CBPB under constant and a single changing RH were successfully applied to changes under cyclic RH.The sorption behaviour and dimensional movement of CBPB has been shown to be influenced not only by RH, but also by its intermediate history, with the maximum values for change within every phase of sorption, in both mass and dimensions, being higher for the cycling between 35–90%RH than the cycling regime 35–65–90%RH. The ratios of dimensional changes to mass change of CBPB were greater under the latter regime than under the former regime.The senior author wishes to thank Professor W.B. Banks of University of Wales, Bangor for his constructive discussions and assistance and the British Council for partly financial support.     

Dimensional instability of cement bonded particleboard: stabilisation treatments

As part of an extensive programme of research aimed at a better understanding of the dimensional instability of cement bonded particleboard (CBPB) and establishing methods of improving its stability, this paper discusses the treatment processes for stabilisation of CBPB. CBPB as a composite of wood chips and reacted Portland cement is unstable in service. One solution to this deficiency is to treat the CBPB product to reduce its magnitude. Three treatment processes, namely, storage of, pre-drying and application of surface coatings on CBPB, were investigated. The results indicated that all three treatments were able to reduce both dimensional and mass change of CBPB, but the efficacy of the treatments was closely related to their schedules. Prolonged storage resulted in a stabilisation of CBPB due to further hydration and carbonation of CBPB. Mass change of CBPB with 12 month storage was only about 2/3–3/4 that with 2 month storage under changing relative humidity (RH). A pre-drying process resulted in a structure alternation of CBPB and stabilised it permanently. A considerably irreversible mass decrease and dimensional shrinkage occurred when re-wetting at 65%RH: about 0.7% and 3% for mass, 0.02% and 0.12% for length, and 0.08% and 0.19% for thickness, respectively for 35%RH and 105°C pre-drying CBPB. However, mass change was independent of the pre-drying schedules after exposure at 90%RH, while irreversible dimensional shrinkage remained. Surface coating had a significant influence on the behaviour of CBPB due to physical and chemical reaction. The ranking in effectiveness of sealers was: proprietary system greater than model systems, and solvent-borne sealers greater than water-borne sealers. Compared with uncoated CBPB, that which is coated with the most effective sealer showed 70–90% reduction in both mass and dimensions over the whole range of RH exposure with the exception of the range from 65–90%RH. All coatings also showed a strong resistance to carbonation of CBPB.     

Dimensional instability of cement bonded particleboard: Modelling CBPB as a composite of two materials

Previous papers have quantitatively indicated that the total movement of cement bonded particleboard (CBPB) is equal to the sum of the movement of its components. This paper examined the efficacy of the law of mixtures when applied to the movement of a wood-cement composite under internal swelling or shrinkage stresses. Abundant data generated in companion papers were first manipulated to develop the universal formulae for predicting the movement of components. In conjunction with previous numerical results from image analysis of the structure of CBPB, and the orientated elasticity and stress algorithms, the models for theoretically predicting mass and dimensional changes of CBPB were derived. Validation studies were conducted and these demonstrated an excellent agreement of the theoretical predictions with experimental data for both mass and dimensional changes of CBPB due to internal swelling or shrinkage stresses during adsorption and desorption. The success also implied that CBPB can be treated as a composite and its properties can be well derived by the law of mixtures even though CBPB is an unusual type of composite having a very high volume fraction of wood chips, but a very high mass fraction of cement paste.Notation E_RT Mean transverse modulus of elasticity of wood - E_L Longitudinal modulus of elasticity of wood - E_p Modulus of elasticity of cement paste - E_wa Modulus of elasticity of embedded wood chips at angle - E Modulus of elasticity of wood chips at direction - E Modulus of elasticity of wood chips at direction - G_LRT Mean transverse shear modulus of wood - L(T)_cp Length/width (thickness) change of CBPB at angle - L(T)_p Length (thickness) change of cement paste - m_pf Mass fraction of cement paste in unit mass of CBPB - m_wf Mass fraction of wood chips in unit mass of CBPB - M_cpj Mass change of CBPB at the various conditions tested - M_pj Mass change of cement paste at corresponding conditions - M_wj Mass change of wood chips at corresponding conditions - M(L; T)_w/P Mass, length or thickness changes of wood chips or cement paste at various conditions - t Duration of exposure - _LRT Mean transverse Poissons ratio of wood - V_pf Volume fraction of cement paste in unit mass of CBPB - V_wf Volume fraction of wood chip in unit mass of CBPB - _cp Density of CBPB - _k Density of wood chip or cement paste - _cp Overall stresses of CBPB at angle - _L Stress in the longitudinal direction of wood - _RT Mean stress in the transverse direction of wood - _p Stress of cement paste - _w Stress of the wood chips at angle - Stress of the wood chips at direction - Stress of the chip at direction - _cp Strain in CBPB - _p Strain of cement paste - _WL Strain in the length of wood chips - _WT Strain in the thickness of wood chips - _w Strain in wood chips - Angle between the longitudinal direction of wood chips and surfaces or edges of CBPB - Angle between wood chips and edges (length direction) of CBPB - Angle between wood chip and vertical coordinate - A, B, C Coefficients related to the feature of materials and exposure conditions The senior author wishes to thank Professor W.B. Banks of University of Wales, Bangor for his constructive discussions and assistance and the British Council for partly financial support.     

生产优质酚醛刨花板新途径—双重压制

采用双重压制新工艺压制出８种２４张优质酚醛刨花板。经常规压制的刨花板，再在１７０℃下进行二次热压１０ｍｉｎ或２０ｍｉｎ。据对板材各种物理性质的测定结果表明：刨花板经１０ｍｉｎ的处理即可显著降低吸水厚度膨胀率和不可逆厚度膨胀率，其原因为机械吸附蠕变和碱，热塑化作用，促使木－胶系统内锁的应力释放。     

豆胶杨木/麦秸复合刨花板制造工艺

研究利用无醛豆胶生产杨木/麦秸复合刨花板的制造工艺。采用正交试验设计方法,探讨了施胶量、杨木/麦秸刨花质量比例、热压温度、热压时间等工艺因素对刨花板的静曲强度、弹性模量、内结合强度、吸水厚度膨胀率等性能的影响。试验结果表明:利用无醛豆胶生产杨木/麦秸复合刨花板是可行的,厚度11 mm复合刨花板的优化工艺参数为施胶量14%、杨木/麦秸刨花质量比70/30、热压时间10 min、热压温度170℃。     

水泥刨花板砂光粉尘回收的研究

1试验材料与方法1·1试验材料杨木刨花:长10~30 mm,宽1~6 mm,厚0·2~0·4 mm。先用旋切机将杨木段旋出0·2~0·4 mm厚的单板带,然后用锤式再碎机进行宽度方向的加工,最后筛选成所需的规格刨花[1]。水泥:32·5级普通硅酸盐水泥。添加剂:氯化钙(CaCl2),纯度96%;硅酸钠(Na2SiO3)为     

水泥刨花板齐边废料回收的研究

水泥刨花板生产中齐边废料的量较大,如果不采取措施回收再利用将占用很大的堆放空间。笔者通过试验发现,齐边废料经过回收处理后可以再利用于水泥刨花板生产,齐边废料回收量为5%时,生产出来的水泥刨花板性能可以达到水泥木屑板标准,其生产过程适宜的胶木比为3∶1、水胶比为0.5∶1。     

杨木水泥刨花板工艺的研究

对杨木水泥刨花板的物理力学性能与主要工艺参数——密度、木灰比、水灰比、添加剂及添加量的关系作了研究。结果表明,密度(D)对板材性能影响最大,木灰比(W/C)次之,水灰比(W_1/C)最小;板材的主要性能如MOR、MOE、IB同密度、木灰比、水灰比的关系,可用方程Y=a_0+a_1D+a_2(W/C)+a_3(W/C)~2+a_4(W_1/C)+a_5(W_1/C)~2表示;在各种添加剂中,以氯化钙的效果最好。     

Improvement on the properties of gypsum particleboard by adding cement

Gypsum particleboard (GPB) has high thickness swelling (TS), high water absorption (WA), and low mechanical properties compared with cement-bonded particleboard. The properties of GPB were improved by adding cement. The experimental results showed that GPB with the added cement had good physical and mechanical properties compared with those of gypsum particleboard with no added cement. The TS and WA of gypsum particleboard with added cement were reduced by 10%. The mechanical properties of GPB, such as internal bond strength (IB), modulus of rupture (MOR), and modulus of elasticity (MOE), increased when the GPB was made with added cement. The properties of GPB improved relative to the quantity of cement added. With an increase of cement content from 5% to 10%, the TS and WA were reduced, and the IB, MOR, and MOE were increased. In contrast, the TS and WA increased and the IB, MOE, and MOR decreased when the cement content was increased from 15% to 30%. Thus the physical and mechanical properties of GPB were successfully improved when the added cement content was 10%.An outline of this paper was presented at the 47th Annual Meeting of the Japan Wood Research Society in Kochi, April 1997     

Dimensional stability and strength properties of particleboard produced by a closed-press system

The purpose of this study was to reveal the effects of various levels of mat-moisture content (m.m.c.) and the closed-press system for making single- or three-layer particleboard on the density profile, thickness swelling, specific moduli of elasticity (MOE) and rupture (MOR) and internal bond strength. Internal gas pressure was measured in an enclosed frame; and the larger the m.m.c., the higher the internal gas pressure became. When rising water vapor (steam) struck particles, it plasticized them and cured the adhesive, resulting in improved interparticle contact. The vertical density gradient in the three-layer board was larger than that in the single-layer board. As for thickness swelling by cold-water soaking, the single-layer boards were less affected than the three-layer boards and showed good dimensional stability with increased m.m.c. The open-system boards swelled more than the closed-system boards. The closed-system single-layer board made at high m.m.c. returned nearly to the prime thickness by air-drying after cold-water soaking. Specific MOE and MOR were larger at 15% or 10% m.m.c. than those at other m.m.c. Considerable reductions of specific MOR and MOE of the closed-system three-layer board were observed at 20% or 25% m.m.c.Part of this report was presented at the 45th annual meeting of the Japan Wood Research Society, Tokyo, April 1995 and at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

Effect of curing heat,alkali and cement set in cement particleboard on the visco-elastic properties of chip mattresses

Summary The aim of this study was to investigate the influence of temperature, alkali and cement set on the visco-elastic properties of chip mattresses made in cement and calcium hydroxide, respectively. Two temperatures were used, room temperature and 78°C, believed to represent the limits of the range of temperatures experienced industrially in the manufacture of cement composites. Calcium hydroxide and cement were used to find the effect of alkali and the effect of cement set. The temperature affected the visco-elastic properties by increasing the unrecovered strain, and alkali also increased the unrecoverable strain. The effect of heat and alkali increased when the two were used in combination. In cement chip composites, the effect of cement set was the most dominant that the effect of heat and alkali could not be observed.I am indebted to Dr. A. J. Bolton of the University College of North Wales, Bangor, for his assistance. This study was carried out at this university as an Honours Project     

MUF共缩聚树脂的制备及其在防潮型刨花板上的应用

介绍了三聚氰胺改脲醛树脂的主要制备工艺并对该树脂在防潮型刨花板中的使用条件如热压参数、施胶量、板坯含水率等方面进行了探讨。试验表明，采用三聚氰胺改性脲醛树脂压制的刨花板物理力学性能达到或超过相关标准。     

Blowout conditions and properties of isocyanate resin bonded particleboard manufactured from high-moisture particles using an air-injection press

Blowouts of particleboards were artificially induced by increasing the vapor pressure inside the boards. Isocyanate resin bonded boards were manufactured from high-moisture particles, and the blowouts and board properties were analyzed. Boards with a high resin content of 5 % showed high bonding strength and did not blow out when pressed at 190 °C, but blew out at a raised temperature of 210 °C to increase vapor pressure inside the boards, thereby showing that blowout occurred when vapor pressure inside the boards exceeded the bonding strength of isocyanate resin. Boards with a low resin content of 2.5 % had low bonding strength and blew out when manufactured without air injection, but were successfully manufactured with air injection that prevents blowouts. However, the injection of high-pressure air reduced the strength properties of the board and increased the coefficient of variation, likely due to the discharge of isocyanate resin from the boards. Therefore, very small local blowouts occurred inside the boards, which lowered the strength properties of some specimens and led to a large coefficient of variation. When the pressure of injected air was lowered, the strength properties increased and the coefficient of variation decreased. This was possibly because the low-pressure air allowed isocyanate resin to remain in the boards, resulting in virtually no parts showing very low-strength properties.     

A METHOD TO IMPROVE DIMENSIONAL STABILITY OF PF PARTICLEBOARD

A method of alkaline pretreatment of particles was designed to produce threetypes of chips.The pretreatmentet of particles in 1% water solution of NaOH for 30minutes offered a possibility of producing PF bonded particleboards with minimal irrecover-able swell and a reduction in thickness swelling.The method used in this work could beused to produce more stable PF particeboard.     

Formation of the density profile and its effects on the properties of particleboard

Summary Two types of particleboards bonded with an isocyanate resin, one with uniform vertical density profile (homo-profile), and the other with conventional U-shaped profile, were fabricated to various density levels using lauan (Shorea spp.) particles. The fundamental relationships between the density profile and the board properties were determined, and the results are summarized as follows: 1. In homo-profile boards, the moduli of rupture (MOR) and elasticity (MOE), internal bond (IB) strength, and screw withdrawal resistance (SWR), are highly correlated to the board mean density. 2. The bottom limit of the board density is estimated to be ca. 0.25 g/cm³, based on the correlation regressions between mechanical properties and mean density. 3. At equal mean density level, the MOR and MOE of the conventional particleboards are higher than the homo-profile boards, due to the higher density near the faces. However, the reverse is true for IB, owing to the presence of the low density core in the former. 4. The net impact of peak density on MOR and MOE is greater at higher mean density level while raising the core density results in more pronounced improvement in IB at lower density. 5. In addition to the compaction ratio, the dimensional stability of the board is also affected by the peak area and mat moisture content. Received 9 January 1997     

几种树种木材制造石膏刨花板的适应性

本文论述了挪威云杉、山杨、白桦和柞木及山杨、白桦、柞木的混合刨花制造石膏刨花板的适应性。结果表明,在工艺参数相同的情况下,木材密度越大的树种制成的板材强度越低,挪威云杉板强度最大,山杨板次之,柞木板极低。挪威云杉和山杨是制造石膏刨花板的优质树种。改变木膏比、刨花体积和对刨花进行水抽提均没有使白桦和柞木制成合格的石膏刨花板。混合刨花干扰石膏刨花板的凝固时间,致使板子强度低劣,在生产中应予避免。     

Feasibility of Cordia trichotoma (Vell.) wood and its by-products for particleboard manufacturing

The wood panel industry uses a large volume of wood and it is essential to research the use of new tree species for these types of products. As such, the aim of this study was to evaluate the feasibility of Cordia trichotoma (Vell.) wood, both on its own and as a mixture with Pinus spp., as well as to understand the effect on particleboard quality when adding the species’ bark and shavings. The experimental design included nine treatments that assessed various mixture proportations of C. trichotoma and Pinus; for example, 10% bark, and 10% shavings of C. trichotoma with Pinus spp. and C. trichotoma particles, as well as mixtures of these two species in proportions of 25/75%, 50%/50%, and 75/25%. The results indicate that the Cordia trichotoma specie is technically feasible when manufacturing particleboards in pure form. The incorporation of bark (at 10% of the mixture) is also technically feasible. It is also feasible to use the shavings of C. trichotoma. In general, all treatments produced outputs that showed dimensional stability and mechanical strength statistically equal or higher than control treatments (Pinus spp.); thus, these products may be used for non-structural applications in dry and/or moist conditions.     

Heat Post-Treatment to Reduce Thickness Swelling of Particleboard from Fast-Growing Poplars

IntroductionInrecentyears,particleboardhasbeenusedmoreandmorewidely,Itisnotonlyusedforfurnituremanufac-turingandinteriordecoratingbutalsoaseXteriorbuild-ingmaterial.Becauseofthechangesofthesurroundingatmosphere,thevariationinmoisturecontentofparti-'cleboa…