Abstract
Blended species plywood blocks comprising of 24 different veneer configurations of naturally durable white cypress pine and non-durable hoop pine were exposed to the subterranean termite Coptotermes acinaciformis in a field trial in Australia. Three thicknesses of cypress (1.8, 2.8, and 3.0 mm) and hoop pine (1.0, 1.5, and 3.0 mm) veneer were included. Blocks were assessed for termite damage using a visual damage rating and mass loss measurement. Blocks using all hoop pine veneers received substantial damage; however, blocks that had cypress face and back veneers had improved termite resistance, particularly for the 1.0-mm hoop pine core veneers. When cypress longbands were blended with hoop pine crossbands that created alternating layers, minimal damage was sustained in the hoop pine veneers; however, the damage increased with increasing hoop pine veneer thickness. All cypress veneers received essentially no termite damage, and cypress veneer thickness did not influence the severity of hoop pine veneer damage. The trial indicated that the plywood made with hoop pine core veneers, cypress pine face, and back veneers offered some termite resistance if the hoop pine veneer thickness was kept thin. Alternating cypress and hoop pine further improved the termite resistance.
Download PDF
Full Article
Blended Species Plywood (White Cypress Pine and Hoop Pine): Effect of Veneer Thickness on Susceptibility to Attack by the Subterranean Termite Coptotermes acinaciformis
Christopher J. Fitzgerald,* and Robert L. McGavin
Blended species plywood blocks comprising of 24 different veneer configurations of naturally durable white cypress pine and non-durable hoop pine were exposed to the subterranean termite Coptotermes acinaciformis in a field trial in Australia. Three thicknesses of cypress (1.8, 2.8, and 3.0 mm) and hoop pine (1.0, 1.5, and 3.0 mm) veneer were included. Blocks were assessed for termite damage using a visual damage rating and mass loss measurement. Blocks using all hoop pine veneers received substantial damage; however, blocks that had cypress face and back veneers had improved termite resistance, particularly for the 1.0-mm hoop pine core veneers. When cypress longbands were blended with hoop pine crossbands that created alternating layers, minimal damage was sustained in the hoop pine veneers; however, the damage increased with increasing hoop pine veneer thickness. All cypress veneers received essentially no termite damage, and cypress veneer thickness did not influence the severity of hoop pine veneer damage. The trial indicated that the plywood made with hoop pine core veneers, cypress pine face, and back veneers offered some termite resistance if the hoop pine veneer thickness was kept thin. Alternating cypress and hoop pine further improved the termite resistance.
Keywords: Plywood; Hoop pine; White cypress pine; Subterranean termite; Natural durability; Veneer
Contact information: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, 50 Evans Rd., Salisbury, Queensland 4107 Australia;
* Corresponding author: chris.fitzgerald@daf.qld.gov.au
INTRODUCTION
In Australia, the demand for veneer-based engineered wood products (EWPs), including plywood and laminated veneer lumber (LVL), continues to grow for building products for both structural and non-structural applications, and in both interior and weather-exposed situations. Despite the economic downturn, which resulted from the global financial crisis of 2008, there has been little evidence of any slowdown in the global production of either plywood or veneer (Hughes 2015). With ever-improving manufacturing technology and continued advances in building manufacture and design, the use and popularity of EWPs is expected to increase.
Veneer-based EWPs provide an opportunity to improve the utilization of forest resources compared to traditional sawn products. This is coupled with the potential to use currently under-used, small-diameter native forest log resources (with the advent of spindleless rotary veneering technology) to produce useful veneer-based products. McGavin et al. (2018) suggested that a suitable pathway for the use of small-diameter native forest resources would be to blend the rotary veneers recovered from peeling operations with existing commercial plantation softwood veneers such as hoop pine (Araucaria cunninghamii). Blending resources can provide a number of benefits including efficient resource utilisation, compatibility with modern building design, and enhanced product performance.
One component of enhanced product performance is the ability to resist biological degradation (termites and fungi) through heightened natural durability, i.e., without the requirement for chemical preservation. Enhanced product durability can potentially be achieved by blending durable and non-durable timber species in an EWP such as plywood or LVL.
White cypress pine (Callitris glaucophylla) (from here on referred to as CYP) is a softwood that is widely distributed within Australia’s inland native forests (McGavin and Leggate 2019). The heartwood of this species is known to be resistant to termite and fungal attack due predominantly to the presence of extractives (natural preservatives) in the heartwood, though this resistance does not extend to the sapwood. These extractives include thujaplicin, nootkatin, dolabrin, thujaplicinol, and pygmaein. The extractives have been investigated as potential natural preservative treatments (as alternatives to chemical preservatives) for other non-durable timbers to prevent termite attack. The extractives can be either toxic or repellent to termites (Evans et al. 2000).
Previous studies (Behr and Wittrup 1969; Kamden and Sean 1994; Evans et al. 1997; Evans et al. 2000; Kartal and Green III 2003) looking at blends of durable (e.g., CYP) and non-durable (e.g., radiata pine, Pinus radiata, or hoop pine) in either particleboard or medium-density fibreboard (MDF) have shown enhanced resistance to termite attack when compared to those composed entirely of a non-durable species.
Faraji et al. (2009) demonstrated that the greater the ratio of durable to non-durable veneers in a plywood panel, the more enhanced the termite resistance. The improved durability was also found to be influenced by the number of veneers, veneer thickness, and the veneer lay-up strategy (i.e., the veneer positioning within the panel). Similarly, Nzokou et al. (2005) reported that LVL made from blending durable black locust (Robinia pseudoacacia) and non-durable red maple (Acer rubrum) species demonstrated enhanced durability when the face and back veneer and at least one core veneer were from the durable species.
The study reported by Faraji et al. (2009) included plywood made from blends of the durable heartwood of cypress pine (Cupressus sempervirens) and the non-durable sapwood of Scots pine (Pinus sylvestris), beech (Fagus sylvatica), and poplar (Populus sp.), which were evaluated against the subterranean termite Reticulitermes santonensis in laboratory trials. Plywood blocks included both 5-ply and 9-ply configurations and consisted of a mix of 2.6-mm and 1.3-mm-thick veneers for various blends of durable and non-durable species, as well as single species controls. Resistance to termite attack in a blended plywood was only achieved where the face and back veneers were cypress pine heartwood. Of the four panels that were deemed termite resistant, three of them consisted of 60% durable plies with an integration of durable and non-durable plies in the core of the plywood block as well.
The percentage of mass loss in the 5-ply configurations was always higher than for the 9-ply configurations (where all the veneers in both configurations were of non-durable species). The authors suggested this could be related in part to veneer thickness. The 5-ply configurations comprised only 2.6-mm veneers while the 9-ply configurations consisted of eight 1.3-mm veneers and a centre veneer of 2.6 mm. Termites indiscriminately attacked the thicker veneers in both configurations but preferentially only the outermost 1.5-mm veneers in the 9-ply configuration. The remaining six 1.5 mm veneers were not attacked. The test block dimensions were 50 × 25 × 15 mm3 and were exposed to 250 termite workers in a laboratory trial.
Trials assessing resistance against basidiomycete fungi, in addition to termites, reported by Faraji et al. (2008) showed that the ratio of exposed durable surfaces vs. non-durable surfaces in plywood is the determiner of resistance rather than the volume of durable vs. non-durable veneers.
Nzokou et al. (2005) assessed LVL manufactured using veneers from decay-resistant black locust (Robinia pseudoacacia) and decay-susceptible red maple (Acer rubrum) to determine the durability impact of the LVL manufacturing process, and to test if the blending of decay-resistant and decay-susceptible species can improve resistance against biological degradation. A laboratory soil block test (against fungi) and a field test (against termites – species unknown) were conducted. The study concluded that durability against decay was shown to improve when the two faces and at least one core veneer were from decay-resistant species. However, the blended LVL was vulnerable to termite attack, and it was concluded that the termites were able to selectively colonize the non-durable red maple veneers even if positioned in the core of the LVL.
In this study, a termite exposure trial was established to investigate the effect of veneer thickness (of both durable CYP and non-durable hoop pine) on enhancing subterranean termite resistance in blended-species plywood panels all consisting of a CYP face and back veneer but half with a full hoop pine core and the remainder having a CYP longband integrated with a hoop crossband. The study aimed to determine in what plywood panel lay-up configurations can the durable CYP enhance the protection of the non-durable hoop pine from subterranean termite attack. Subterranaean termites are social insects and cause significant damage to timber-in-service in Australia. Colonies can have up to hundreds of thousands of individuals and can be wholly subterranean (no above-ground mound) or be associated with visible ground mounds, tree or arboreal structures or in dead or dying limbs (Hadlington 1996). Coptotermes acinaciformis is a subterranean termite found in all States of Australia (except Tasmania) and is the most widely distributed and most destructive pest termite species within the country (Evans 2010). The capacity to damage wood and other cellulose based materials is higher than for other Coptotermes species. C. acinaciformis was identified from the exposure site used in this trial.
EXPERIMENTAL
Materials
Veneer source and test sample matrix
The CYP and hoop pine were the two species included in the study. The CYP represents a mid-high density (basic density 580 kg/m3), durable softwood that is sourced from sustainably managed native forests, while hoop pine (basic density 450 kg/m3) represents a plantation softwood resource and is non-durable (Bootle 2010; DAF 2018). Both of these species are commercially available to the Australian timber industry.
The CYP veneers were sourced from small-diameter (< 25 cm) native forest logs that were processed using a spindleless rotary veneering system. The hoop pine veneers were recovered from approximately eight logs peeled by a commercial veneer producer during standard commercial operations. There were three dry-veneer thicknesses of CYP (1.8, 2.8, and 3.0 mm) and hoop pine (1.0, 1.5, and 3.0 mm). A previous DAF (Department of Agriculture and Fisheries) trial investigated a blended species plywood (exposed to C. acinaciformis) using just one thickness of CYP veneer (3.0 mm) and one thickness of hoop pine veneer (1.5 mm) comprising five plywood combinations. The hoop pine veneers were damaged by C. acinaciformis in all instances other than when incorporated with a CYP face and back veneer and a CYP longband veneer. The intention of this trial was to study the effects of additional thicknesses of CYP and hoop pine veneer and also build on the work done by Faraji et al. 2009 which looked at varying thicknesses of durable and non-durable veneer but only in a laboratory trial.
Four different groups of 7-ply plywood were manufactured with different thickness variations represented within each group. This resulted in a total of 24 plywood configurations (Table 1).
Table 1. Eighteen Blended Plywood Configurations and Six Same Species Configurations that were Manufactured andTested
⃰ These configurations had only 8 test blocks due to limited availability of veneers
- 1 to 9 – CYP face / back and hoop core;
- 10 to 18 – CYP face / back / longband and hoop crossband;
- 19 to 21 – Full hoop pine;
- 22 to 24 – Full CYP
Sample Preparation
The CYP and hoop pine veneers were conditioned to 6% moisture content (MC) and then reduced to sheets measuring 300 × 300 mm2 using a panel saw. The resultant sheets and a phenol formaldehyde adhesive (Jowat Universal Adhesives Australia Pty. Ltd., Ingleburn, NSW, Australia) were used to manufacture the 7-ply plywood panels. This adhesive is moisture and ultraviolet (UV) resistant, and it is an approved adhesive for external, weather exposed, and structural applications in accordance with AS/NZS 2754.1 (2016).
The adhesive was applied to each face of the veneers targeting a total spread rate of 200 gsm (grams per square metre) per glue line. The assembly stage included an open assembly time of approximately 20 min or until the adhesive was tacky. Pre-pressing was undertaken at 1.2 MPa (approx. 174.0 psi) for 15 min followed by a hot press, at the same pressure, for 12 min at 135 ℃ in a laboratory press (Enerpac Australasia, Regents Park, NSW, Australia). The heat and pressure applied during the hot press enabled the glueline to cure and bonded the assembled veneers and adhesive into a plywood panel. The panels were then stored for at least 24 h before cutting into test blocks.
All plywood combinations consisted of 7-ply plywood in either a blended (CYP face/back and hoop core; CYP face/back/longband and hoop crossband) or same species (full hoop pine or full CYP) configuration (Fig. 1).
Test blocks measuring 135 × 70 mm by the thickness of the plywood panel, which varied from 7 to 22 mm depending on the veneer thicknesses, were cut from the panels. Eight test blocks were cut from each plywood panel providing a total of 312 (a combination of 16 replicates and eight replicates) test blocks across the 24 different plywood configurations. To attract termite activity towards the test blocks, 350 feeder blocks (135 × 70 × 20 mm3) were cut from low durability softwood (Pinus sp.) sawn timber (predominantly sapwood).
Fig. 1. 7- ply plywood test block configurations (3 CYP thicknesses; 3 hoop thicknesses)
Test Block Arrangement
All test blocks and feeder blocks were weighed to enable mass loss calculations post-exposure to termites. Test block sets were then prepared alternating a feeder block and one test block from each configuration. Corrugated cardboard was used to separate all samples (Fig. 2). The test block sets were then randomly distributed across 24 exposure boxes (opaque plastic boxes).
Fig. 2. Plywood test blocks and pine feeder sapwood blocks positioned in an exposure box
The feeder blocks were included to encourage on-going termite foraging in the exposure box and provide an indicator of termite vigour (based on mass loss of feeder blocks) within each box. The corrugated cardboard was used to provide a series of runways for the termites once they had entered the box and aid the movement of termites throughout the exposure box. Additional feeder blocks and the cardboard were also added to accommodate any free space in the exposure box.
Methods
Field exposure
Several weeks prior to the trial, a dedicated trench was prepared at a field trial site at Esk (27.2333° S, 152.4167° E) in South East Queensland, Australia (Fig. 3). This was in an area where C. acinaciformis are known to be very active. The trench was excavated and filled with termite susceptible feeder material (pine off-cuts) to promote further activity. Concrete blocks were laid on top of the trench and pine feeder stakes were driven into the trench through the holes in the concrete blocks ensuring that they were in contact with the timber materials buried in the trench.
Fig. 3. Timber placed atop the aggregation trench was heavily infested with C. acinaciformis
At this stage non-durable pine stakes were positioned along the length of the concrete blocks as feeder material to ensure termite activity was present when the exposure boxes were placed in the field. The pine stakes were covered with black plastic secured with soil and additional concrete blocks. The black plastic was 100 µm multi-purpose builders film, which helped protect the exposure boxes from the weather and maintain a dark, humid environment beneath the plastic sheeting.
At trial establishment the black plastic was removed to reveal the pine stakes heavily infested with termites (Fig. 3). The exposure boxes were placed upturned on the concrete blocks before the trench was liberally doused with water and the black plastic re-instated to maintain a dark, humid environment conducive to sustained termite foraging (Fig. 4). The boxes were inspected after one month to ensure that termites were active within all the boxes (as observed through the top of the upturned exposure box) and then left un-disturbed for a further 20 weeks culminating in a 24-week exposure period. The trial ran from November 2018 to May 2019 during the hot summer months when the termites were most active.
Fig. 4. The exposure boxes were placed atop the trench and covered with black plastic
Post exposure assessment
After the 24-week exposure period, the boxes were retrieved from the field and returned to the laboratory for assessment. Each test block set was removed from the boxes, the test blocks were separated from the feeder blocks, and any dirt, debris, and termites were removed. Live C. acinaciformis were identified and found in the majority of the 24 exposure boxes at this time (Fig. 5).
Each test and feeder block was visually examined for termite damage. For the test blocks, it was noted whether the face and back veneers and/or the core veneers sustained damage. Each test block was then weighed to determine the mass loss due to termite attack, and subsequently, the percentage of mass loss was calculated to enable further comparison. From the visual assessment and calculated percentage mass loss, each test block was assigned a score based on the following rating system (Table 2) that was adapted from Peters and Creffield (2004).