Physico-mechanical properties and decay resistance of Cupressus spp. cement-bonded particleboards

Abstract

Control and hot water treated particles of cypress (Cupressus spp.) were used to manufacture cement-bonded particleboards (CBPB). Three replications were carried out for each treatment, totaling six single layered boards. Physical and mechanical properties of the boards were evaluated according to ASTM D 1037-96a [Annual Book of ASTM Standards, vols. 04–09. Philadelphia: American Society for Testing and Materials––ASTM, ASTM D 1037-96a, 1998] standard. The wood of cypress showed suitability as raw material for the manufacture of CBPB. All the properties of cypress CBPB were higher than the Bison HZ type building boards used as reference. The light color and easy wood processing of cypress wood are favorable parameters for panels manufacturing. The results corroborate those reported for CBPB using pine, rubberwood, acacia, babassu, and eucalyptus particles. The CBPB were also submitted to fungi decay resistance test according to ASTM D 2017-81 [Annual Book of ASTM Standards, vols. 04–09. Philadelphia: American Society for Testing and Materials––ASTM, 1994-e1. ASTM D 2017-81, p. 324–8] standard and exposed to the attack of Gloeophyllum trabeum (Persoon ex Fries) Murr. and Trametes versicolor (Linnaeus ex Fries) Pilát. Twelve specimens were tested for each treatment and the average weight loss was determined after 12 weeks of exposure. Laboratory tests showed that there was no measurable wood degradation (weight loss). Fungi mycelium did not even cover completely the surface of the specimens. CBPB when exposed to both brown- and white-rot fungi, rather than weight loss, showed weight gains ranging from 2% to 4%. Therefore, CBPB is technically suitable for exterior use where both moisture and favorable conditions for fungi development are present.

Introduction

The gymnosperms, also known as evergreens, are characterized by marked features other than floral and seed peculiarities. Timber of cypress, mostly Cupressus lusitanica, occupies the leading position in the Kenyan timber market and is one of the few conifers widely planted outside of its natural habitat. It is used as a source of fuel and lumber in Guatemala. Colombia established a program to improve cypress timber quality of end products. Cypress is also used as a source of pulp in Venezuela and is one of the most commonly planted species in the Highlands of Costa Rica. In Brazil cypress plantations has shown good adaptation in the southern region, as a source of pulp and raw material for the panels industry.

According to studies of Haslett [3], the wood of Cupressus macrocarpa, C. lusitanica, Chamaecyparis lawsoniana and Cupressocyparis leylandii are similar. The timber is lightweight, stiff, and moderately strong with higher bending and compression properties than radiata pine (Pinus radiata). Due to their yellow-brown color, fine even texture, and luster, they are frequently compared to kauri (Agathis australis). Cypress wood has a characteristic odour, is graded as medium to low density and is highly suitable for use in exterior joinery, weatherboards, and boat building.

Cement-bonded particleboard (CBPB) has been manufactured using conventional technology, where the boards are kept under pressure for consolidation for several hours. A long pressing time is necessary because the hydration of ordinary Portland cement is a very slow process. Typically, hydration takes at least 8 h in the first stage (setting) where the reaction releases heat. The second stage (hardening) is considered completed in 28 days, although it may take years for the boards to reach maximum strength [4].

The characteristics that make CBPB desirable to construction applications are high fire and decay resistance, low water absorption, and good dimensional stability. It performs well in interior or exterior uses without any superficial treatment.

Lee [5] manufactured cement bonded cypress excelsior boards using excelsior of Taxodium distichum and concluded that cypress is a suitable species for cement-bonded boards. The values of mechanical properties were 924 MPa for modulus of elasticity (MOE) and 2.5 MPa for modulus of rupture (MOR), with board density of 0.512 g/cm³, probably used for ceiling. The manufactured cement-bonded cypress excelsior boards showed the same bending properties as compared to commercial southern pine excelsior boards, according to a t-test.

Sandermann et al. [6] suggested a method to determine the initial hydration of cement by measuring the rate of temperature rise in a Dewar flask. Weatherwax and Tarkow [7], [8] described a method to quantify differences among species by using the inhibitory index, which compares the extension of wood–cement inhibition based upon the percentage increase in the setting time. Hofstrand et al. [9] incorporated maximum temperature and the maximum slope of the temperature curve of the wood–cement mixture and neat cement, respectively, into the inhibitory index calculation (I).

Several studies in the past revealed that sugars available in wood have been identified as the most critical component in the production of CBPB [10], [11]. For producing good quality boards, the amount of sugars in wood should not exceed 0.6% as stipulated by Weber [12] and Bever [13]. It makes some wood species not suitable to be mixed with cement and additional treatments of the wood particles are necessary.

Hardwoods and softwoods are commonly attacked by both brown- and white-rot fungi when in contact with ground. Investigation of CBPB degradation by fungi is not very common. Tests conducted in the UK Building Research Establishment (BRE) suggested that conventionally made CBPB are very resistant to the attack of the white-rot fungus Pleurotus ostreatus and to the brown-rot fungus Coniophora puteana. In fact, CBPB samples gained 5% in mass when tested against C. puteana, which is most likely due to a certain amount of carbonation of the cement matrix [14]. A similar statement was made by Pirie et al. [15] that cement-bonded particleboard composite during natural weathering undergoes changes such as continued hydration and carbonation. When conventional and CO₂ cement-bonded particleboard of Populus sp. were tested against the white-rot fungus, Trametes versicolor, and the brown-rot fungus, Postia placenta, there was no measurable wood degradation. On the contrary, substantial weight gain was also observed in this study [16].

The general purpose of this research was to manufacture wood CBPB using particles of cypress and to evaluate the physical and mechanical properties of the boards according to ASTM D 1037-96a standard [1]. An attempt has been made to evaluate whether hot water extraction of the particles would have any influence on the properties of the boards. An additional objective of this study was to evaluate the decay resistance of the conventional cypress CBPB under laboratory conditions when exposed to white- and brown-rot fungi according to ASTM D 2017-81 [2].