The wood industry has strategically capitalised on the abundance of small-diameter timber and forest thinning residues in US and European forests, leading to the development of oriented strand board (OSB) and lumber (OSL).
Strand-based composite materials offer lightweight, high-performance alternatives for structural applications.
While wood-based composites have a long history, recent industrial advancements have focused on industrial chemicals, biofuels, and nanocellulose. Natural fibre composites (NFCs) have seen significant adoption due to their advantages over synthetic fibres, including lower weight and cost, abundance, reduced energy consumption, and safer handling.
These advantages and interests have resulted in NFCs successfully penetrating many markets including aerospace, medical, automotive, food, electronics, and building materials. Due to their sustainability, reduced carbon dioxide (CO2) emissions, and advantages over synthetic materials, natural fibres, and NFCs have gained significant attention from researchers and industry.
Basis for the Study
Despite wood’s inherent advantages, including superior structural properties, its potential has been underexplored. The current wood product manufacturers produce waste in different forms, which can range from sawdust, wood shavings, to strands, veneers and even lumber pieces.
Though most of these have an established recycling market and process, resinated strand waste is generally treated as landfill, forming the primary focus of the current study.
Mass timber panels, such as cross-laminated timber (CLT), have spurred renewed interest in wood composites. The success of utilising low-quality forest resources for high-value building materials has prompted studies on strand-based corrugated core sandwich panels for walls, floors, and roofs with both functional and structural advantages.
A standard OSB mill has a solid waste of about 1.63 kg/cubic metre, even after ensuring the majority of the waste has been utilised. This number would be higher for start-ups, where strand-based technologies would yield about 5–10 percent of solid waste, which can amount to about 300 kgs/day for a small production facility with an average capacity of about 733 cubic metres per annum, producing about 100 panels daily, each measuring 1.22 m × 1.44 m × 0.009 m, in a single daylight press at 75 percent capacity.
These numbers have been developed through calculations made by the authors for current industrial partners interested in strand-based products. For comparison, a standard OSB manufacturing plant has a capacity of 7.3 million cubic metres per year, which is conventionally measured at 75 percent of the plant’s actual capacity.
This waste, having almost 50–70 percent of usable strands, contains feedstock for almost 10 or more panels, where about 50 percent are resinated strands produced from blending, mat forming, and pre-forming stages.
Wastage could be significantly higher with increased production capacities. Current practices involve using the waste biomass as landfill due to the presence of resin, but the cost is substantial.
Addressing the need to investigate the reusability of waste strands, this study explores recycling and sieving to create new panels, potentially increasing carbon sequestration and enabling zero-waste production.
A review of the existing literature shows a lack of data in understanding the reusability and recyclability of waste resinated strands for use in composite panel products. To understand their reusability, it is critical to understand the performance of the panels made from recycled strands, compared to those made from virgin strands (referred to as control strands in this study).
Addressing the need to investigate the reusability of waste strands, this study explores recycling and sieving to create new panels, potentially increasing carbon sequestration and enabling zero-waste production.
The research compares the performance of panels made from recycled strands to those made from control strands.
It focuses only on recycling resinated waste strands collected during blending, mat-forming, and pre-forming stages in making standard boards in lab-scale facilities, which has also been used as the control for the current study, assessing the impact of the initial adhesive application on panel properties.
The primary objectives are to:
1. Assess the recycling effects of re-resinated strands on panel performance, including bond performance among the strands and mechanical properties.
2. Understand the influence of recycling on the dimensional stability of the panels.
3. Compare the fire performance of the panels made from control and re-resinated strands.
Materials And Fabrication Process
Low-quality Englemann spruce and lodgepole pine (ESLP) lumber, acquired from Idaho Forest Group, with an average density ranging between 0.35 and 0.38 g/cubic centimetre, were used to produce the required wood strands.
A CAE Disc Strander available at Washington State University’s (WSU’s) Composite Materials and Engineering Center (CMEC) was used for processing the strands.
Due to the maximum size limitation of the lab-scale CAE strander, the lumber needed sectioning at 140 mm before being soaked in water for three days to reach a moisture content higher than 50 percent before stranding.
The strander has a rotational speed of 500 revolutions per minute, and the blade gap was adjusted to produce strands that were approximately 140 mm in length, 19 to 38 mm in width, and 0.38 to 0.51 mm in thickness. The strands were initially air-dried and then conditioned to 10–12 percent moisture content for three to four weeks before fabrication.
Flat panels with final trimmed dimensions of 1220 mm × 2440 mm × 6.35 mm were produced in a hot-press at WSU’s CMEC. The panels were consolidated to a target density of 640 kg/cubic metre, with strands generally oriented along the major axis or the longer panel dimension.
Polymeric methyl diphenyl diisocyanate (pMDI), at a loading amount of 6 wt. percent of oven dry wood weight, was used as the binder and the panels were hot-pressed at a platen temperature of 171 deg C.
The PressMan software, integrated with the hot press, was used to develop the press schedule where a consolidation time at the final thickness of six min was used.
During the process of blending, forming, and hot-pressing panels, there are generally left-over resinated strands that are usually considered waste and disposed of as landfill.
For the current study, these waste resinated strands, acquired only post blending, mat-forming, and preforming processes, were accumulated and sieved to ensure no fines and dust particles were present.
The sieved strands were then conditioned in a chamber at 65 percent relative humidity and 21.4 deg C for 30+ days to emulate the longest possible interval between initial use and re-use times.
A similar panel consolidation process was then followed to hot press 6.35 mm thick flat panels, which were 2440 mm long and 1220 mm wide; strands were again oriented with respect to the major axis or the long dimension of the panel.
A 6sixpercent pMDI resin content was used to re-resinate the strands. A mat was formed and hot-pressed with the same press schedule and temperature as with virgin strands.
A vein box with a spacing of 76.2 mm between the adjacent veins was used for orienting the strands. It should be noted that studies have shown that the ideal gap between the vein box and the surface of the mat being formed should be maintained at around 76.2 mm during the entire mat forming process.
However, in this study, the vein box was maintained at a fixed distance, thus this gap was not constant. This directly influenced the overall orientation of the strands, with the strands in the lower part of the panel having a more random distribution.
The orientation had a profound influence on the mechanical properties of the panels, especially tensile properties. Thus, the study aims to provide a comparison of all the properties, where the forming technique was constant for fabricating both panel types, with virgin and re-resinated strands.
The influence of reprocessing the re-resinated strands was evaluated by testing specimens from hot-pressed panels for their dimensional stability, mechanical performance, and fire performance.
All the tests were carried out as per ASTM standard specifications. Specimens were conditioned for about three to four weeks before testing at 20 deg C and 65 percent RH to achieve an equilibrium moisture content of 12 percent.
Dimensional Stability And Moisture Interaction
Dimensional stability is crucial for wood and wood-composite panels due to wood’s inherent hydrophilic nature. This limits the applications of wood-based materials, particularly in high-moisture environments.
Understanding the interaction between panel surfaces and water is essential for determining panel wettability and overall performance.
Research has shown that resin content directly influences the hydrophilic nature of the final panel, with higher resin content resulting in increased hydrophobicity.
To assess surface wettability, the surface contact angle and surface tension using the sessile drop method was calculated, following the ASTM D7490-13 standard.
The process involved using a VCA Optima video system. Employing a Hamilton 81020 syringe (100 µL capacity), deionised water was dispersed uniformly on the specimen surface.
Five specimens for each panel type were tested. Eventually, to understand the performance of the entire panel when exposed to a high moisture environment, Method A of ASTM D1037-13 standard was employed to estimate the water absorption (WA) and thickness swell (TS) of the specimens.
Again, five specimens of each panel type were tested, and the results were calculated after 2 h and 24 h of complete immersion in water. Five points on each specimen were pre-marked to measure the thickness swell, before and after water soaking.
Deionised water was again utilised, and the total moisture uptake after 24 h was calculated according to ASTM D4442-20 standard, after oven drying for 24 h at 105 deg C. All the WA, TS, and moisture uptake results were plotted in percentage increase from the conditioned samples.
Mechanical Performance
Vertical density profiles (VDP) of both panel types were analysed using an X-ray vertical density profiling machine at CMEC, WSU. Specimens measuring 50.8 mm by 50.8 mm were tested to examine density variations along the panel thickness. 10 specimens from each panel type were evaluated.
Studies have demonstrated that resin content plays a significant role in determining the mechanical performance of wood-composite panels.
Therefore, following non-destructive testing for panel density through thickness, the same specimens were subjected to internal bond (IB) strength analysis according to ASTM D1037-13 standards.
With a testing speed of 0.508 mm/min, based on the 6.35 mm panel thickness, IB strength was assessed as a critical parameter for ensuring proper curing and bonding of pMDI resin during production. Results were compared between the two panel types to evaluate the impact of strand re-resinating on bond performance.
In addition to IB strength, tensile and bending properties were analysed to assess the load-bearing capacities of the panels for design purposes.
ASTM D1037-13 was used to determine tensile and flexural properties in both longitudinal and transverse directions.
Five replicates of each panel type were tested for each property. Dog bone specimens were cut from panels using a cutting jig on a router machine, with dimensions of 254 mm in length and 50.8 mm in width, featuring a reduced cross-section and a gauge length of 50.8 mm with 70 mm grip lengths on each side.
A universal testing machine (UTM) was employed at a crosshead speed of 4 mm/min to calculate tensile strength and Young’s modulus using standard equations from ASTM D1037-13.
The same standard was applied to determine the modulus of elasticity (MOE) and modulus of rupture (MOR) through midpoint bending tests.
Specimen spans were set at 48 times the panel thickness, with a width of 50.8 mm and a loading rate of 12.2 mm/min, utilising the same UTM for testing. The nail withdrawal properties of the panels were also examined to evaluate fastener holding capabilities and mounting performance.
This analysis followed ASTM D1037-13 standards using an 8d common nail driven completely through panel samples measuring 76.2 mm by 152.4 mm, with seven replicates per panel type.
The nail depth corresponded to the panel thickness, and withdrawal testing was conducted at a crosshead speed of 1.5 mm/min using the UTM. To contextualise these results, equivalent specific gravity (ESG) values were calculated for comparison with established engineered wood products.
ESG values were derived using empirical equations from the Wood Handbook and National Design Specification, correlating specific gravity with mechanical properties to benchmark the new material against traditional timber products.
The calculated analytical value would also help establish the functionality of the panels, compared to standard strand-based panels in the market, like OSBs.
These analyses provide comprehensive insights into the mechanical performance of wood-composite panels, including their bonding quality, load-bearing capacity, and fastener retention properties, enabling practitioners to evaluate their suitability for various applications while comparing them against established materials through ESG calculations.
Fire Performance
The fire reaction properties of the panels were analysed to evaluate how strand recycling and resin reapplication affected thermal stability and combustion behaviour.
Thermogravimetric Analysis (TGA) and Derivative Thermogravimetric Analysis (DTGA) were initially conducted following ASTM E1131-20.
The specimens underwent controlled heating at 10 K/min in an inert atmosphere, with three replicates per panel type tested to track normalised mass loss and decomposition rates as functions of temperature. Results revealed compositional variations between recycled and virgin materials through distinct thermal degradation patterns.
Mass-loss calorimetry tests, following ASTM E2102-17 and ISO 17554:2014 standards and using GBH International Corp equipment at the University of Idaho, were conducted.
Five replicates of each panel type were exposed to 50 kW/sqm heat flux at 25 mm from a cone heater, with a 3 mm spark gap positioned 13 mm above the samples.
Heat release rate (HRR) profiles were recorded for 120 s post-flameout to capture sustained combustion behaviour, ensuring stable data for comparison.
The tests were conducted using a calibrated mass loss calorimeter, with a calibrated load scale, ensuring the accuracy and repeatability of the results of how recycling influenced flammability characteristics through mass loss changes and HRR trends.
The distance between the specimen and the cone heater was maintained at 25 mm, and a heat flux of 50 kW/sqm was used. A spark plug was used with a 3 mm spark gap, located 13 mm above the centre of the sample.
Results And Analysis
The experimental results and analysis are most effectively organised based on the three distinct characterisation methods that evaluate the feasibility of recycling waste strands generated during the fabrication of strand-based composite panels.
By comparing these findings with the control, valuable insights can be gained into the impact of recycling re-resinated strands on the final properties of the composite panels.
To investigate the hydrophobicity of the panels and assess the impact of recycling re-resinated strands, water interaction with the panels was analysed using contact angle measurements.
Panels fabricated from re-resinated strands exhibited a higher average contact angle (100.15° ± 4.1°), representing a four percent increase compared to panels made from control strands (96.28° ± 3.1°). However, a single-factor ANOVA analysis (α = 0.05) revealed that this variation was not statistically significant, with a p-value of 0.18.
Similarly, surface tension measurements showed a 12 percent difference between the panels, which was also statistically insignificant.
Panels made from re-resinated strands demonstrated lower surface tension compared to control panels, agreeing with findings from previous studies comparing hydrophilic and hydrophobic surfaces.
These results suggest that while recycling may slightly influence surface properties, the observed differences are not substantial enough to be statistically significant.
The water absorption (WA) and thickness swell (TS) results after 24 h are presented. These findings demonstrate that recycling the strands significantly improves the dimensional stability of the panels, with approximately a 51 percent reduction in WA and a 58 percent reduction in TS.
The TS/WA ratio, further highlights this trend, as a lower ratio indicates improved dimensional stability. Panels made from re-resinated strands exhibited a reduced average TS/WA ratio compared to those made from control strands; however, statistical analysis revealed that this variation was not significant, with a p-value of 0.19.
The density of the panels had negligible variations, with a slightly higher average value for the panels made from re-resinated strands.
The use of re-resinated strands also resulted in reduced variability in density values. The density values of the panels were illustrated, with the control panels having a density of 683.6 ± 123.1 kg/cubic metre, compared to 734.97 ± 93.24 kg/cubic metre for the panels made from re-resinated strands.
The ANOVA, however, confirmed that the variation between the values is not statistically significant, with a p-value of 0.68. Therefore, it can be concluded that the variations in the following mechanical properties, if any, will not be influenced by the variations in the panel densities.
Additionally, the average densities of the panel types were further normalised to prove that the statistically significant variations were not driven by increased density in the panels made from the re-resinated strands.
The IB study demonstrated significantly improved strength in panels made with re-resinated strands compared to controls. This improvement is directly linked to residual pMDI resin retained in the strands even after 3–4 weeks of conditioning.
The higher resin content facilitated stronger interfacial bonding during curing, likely due to increased adhesive distribution per unit cross-sectional area of the strands. Further chemical analysis of the strands might help establish the claim.
Re-resinated strand panels exhibited a 107 percent increase in the IB strength relative to the control panels. ANOVA confirmed these differences as statistically significant with an extremely low p-value (approximately 0).
These results support the value of recycling the waste strands during panel processing by re-resinating and reusing the strands for composite panels. This approach offers a sustainable pathway for optimising composite panel production without compromising quality. Equivalent IB strength in the control panels was 0.81 ± 0.16 MPa, which was still significantly lower than those obtained from the re-resinated panels.
The influence of tensile strength was analysed to assess the effect of recycling re-resinated strands on the tensile properties of the panels.
Although the random orientation of strands in the lower mat limits the representativeness of absolute tensile strength and stiffness values, the study provides valuable insights.
Panels made from re-resinated strands exhibited higher average tensile strength compared to control panels, with the strength increasing approximately by 40 percent along the major axis and 11 percent perpendicular to it.
Equivalent average tensile strengths of the control panels were also calculated to be 30 percent and three percent lower than those of the re-resinated panels parallel and perpendicular to the major axis, respectively.
Similarly, modulus values were about 12 percent and 30 percent higher (5 percent and 23 percent higher than the equivalent moduli) for re-resinated strand panels in the respective directions. However, a single-factor ANOVA revealed that these variations were not statistically significant, with p-values of 0.12 and 0.54 for the tests along the major and perpendicular axes, respectively.
A cross-comparison between the tensile strength of recycled panels perpendicular to the major axis and control panels along the major axis also showed no statistical significance (p = 0.72). Higher variability in control panel data was attributed to density fluctuations. Importantly, it was assumed that the strand length was not reduced during recycling, resulting in no statistically significant variations in the tensile properties, as the strand length is known to significantly influence panel properties.
Overall, these findings indicate that recycling and re-resinating strands had no statistically significant impact on the tensile properties of the panels in any direction.
Nail withdrawal tests were conducted to evaluate the withdrawal strength of the panels, a critical property for applications in the built environment.
The result indicate that panels made from re-resinated strands exhibited a screw withdrawal strength of 35.4 ± 8.6 N/mm, compared to 27.4 ± 6.4 N/mm for the control panels and 29.46 ± 6.9 N/mm for equivalent strength in the control panels. Despite this improvement, a single-factor ANOVA revealed that the variation was not statistically significant (p = 0.11). The ESG values were also calculated for both panel types.
Panels made from recycled strands showed an ESG of 0.48 ± 0.05, approximately 11 percent higher than the control panels, which had an ESG of 0.44 ± 0.04, with negligible change in the average ESG value when equivalent strength of the control panels.
These values are comparable to or slightly higher than those of similar strand-based structural panels; for example, OSBs typically have an ESG of around 0.4, while OSLs exhibit a slightly higher ESG of approximately 0.51.
It must be noted that all the tests were conducted only in the face direction. These findings demonstrate that panels made from recycled strands achieve high-quality performance and are potentially suitable for structural applications in the built environment.
Furthermore, the overall mechanical performance comparisons confirm that waste strands can be effectively reused to manufacture panels without negatively impacting their mechanical properties, where, in some cases, properties such as internal bond strength, withdrawal strength, and ESG can even be improved.
The influence of thermal stability due to recycling and re-resinating the strands was established using the TGA study.
TGA was used to generate mass loss curves, revealing that the re-resinated samples exhibited higher mass retention (25 percent) and lower overall mass loss compared to the control panels, which showed a mass retention of 22 percent.
This improved thermal stability can be attributed to the higher pMDI content present in the strands during the panel-forming process. The DTGA study further showed that the reference temperature or peak mass loss temperature (TR) was 354.17 deg C for the control panels compared to 349.67 deg C for the panels made from re-resinated strands.
However, this variation of about 1.4 percent was not statistically significant. Therefore, the TGA and DTGA studies helped prove that recycling of the strands had no significant influence on the thermal stability of the final panels.
The increase in mass retention, however, would have a direct influence on the fire reaction properties of the samples, which were observed to be significantly better in many aspects tested. The primary reaction that helps the pMDI adhesive bond the strands together to form a panel is the one between the adhesive and moisture.
Besides, the time to flameout was also observed to be significantly higher for the panels with re-resinated strands. The control panels had a time to flameout of 306.67 ± 17.8 s, resulting in a burning time of 289 ± 17.7 s. The panels from recycled strands had a flameout time of 472.33 ± 18.6 s, resulting in a burning time of 430 ± 18.2 s.
Higher burning time further proves that the char formation was higher in the panels with re-resinated strands, which suppressed the heat for a longer time and resulted in a comparatively more fire-safe structure.
The ability to sustain an induced flame for longer times helps produce panels that can maintain their structural integrity longer, and if applied in the built environment, can be denoted as a safer structure where inhabitants would have more time to evacuate in any kind of fire scenario. Thus, a higher amount of pMDI helps make structures comparatively safer under fire.
Total heat evolved (THE), which can also be called the peak total heat released, is known to directly influence the surrounding temperature and conditions of the structure under fire.
This factor is critical during evacuation, as higher heat might cause detrimental effects to human evacuation activities. The results showed that the total heat evolved was almost similar for both samples, with a slightly higher value for the panels made from re-resinated strands.
It can be safely concluded that the panels made from re-resinated samples had lower THE compared to the control at any time of the burning phase. The overall fire performance analysis revealed that recycling the strands and subsequent re-resination significantly enhanced the panels’ fire reaction properties.
This improvement can be directly attributed to the increased availability of urea, resulting in higher, thicker, and more stable char, due to the higher pMDI content in the re-resinated panels.
Future Prospect
This study comprehensively evaluated the physical, mechanical, and fire performance of panels incorporating recycled pMDI-resinated strands, a material often discarded as waste during strand-based panel production.
Notably, even after three to four weeks of conditioning, recycled strands retained residual pMDI resin, and when combined with re-resination, this resulted in panels with elevated adhesive content.
This enhancement led to substantial improvements in water absorption (reduced by 51 percent) and thickness swell (reduced by 58 percent) compared to control panels.
While improvements in other dimensional stability metrics (such as contact angle, surface tension, and TS-to-WA ratios) were observed, these differences were not statistically significant.
Importantly, panels made from re-resinated strands demonstrated markedly superior internal bond strength, indicating improved inter-strand adhesion.
Flexural properties in the transverse direction (parallel to the major strand orientation) significantly improved, with flexural strength and modulus increasing by 44 percent and 56 percent, respectively.
Although the overall mechanical property variations were not statistically significant, the consistently higher average performance suggests that panes made through re-resinating recycled strands hold promise as products that can not only be used in equivalent markets but also in better premium markets.
In terms of fire performance, recycled- and re-resinated-strand panels exhibited notable advantages. Despite no significant differences in thermal stability, re-resinated panels showed approximately 12 percent lower mass loss.
More critically, fire performance metrics—including time to ignition, peak heat release rate, total heat evolved, and time to flame-out—all pointed to superior fire resistance. This indicates the potential for using recycled-strand panels in fire-safe construction applications, adding a valuable safety dimension to their reuse.
Overall, these findings highlight that panels made from recycled strands not only match but, in some respects, exceed the quality and performance of panels made from virgin material.
Therefore, not only can these panels be used for the same market, but the study also opens promising opportunities for manufacturers to market these panels as a premium alternative, potentially offsetting the costs of re-resination.
To fully realise this potential, future research should focus on structural applications, long-term performance assessments, and detailed techno-economic analyses to validate the industrial viability of recycled-strand panels. Additionally, optimising the resin content during secondary resination could help balance product quality with cost efficiency—a critical factor for large-scale commercial adoption.
