Borate treatment of CLT Panels Using Vacuum: A Proof of Concept

doi:10.21203/rs.3.rs-1986827/v1

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Borate treatment of CLT Panels Using Vacuum: A Proof of Concept

https://doi.org/10.21203/rs.3.rs-1986827/v1

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Cross-laminated timber (CLT) is an increasingly popular wood-based alternative for large building applications. Preservatives are often applied to wood products to protect them from decay and extend their service life. However, CLT panels are typically not treated due in part to a perceived lack of efficient and effective methods for treating large panels and a lack of information on what impact treatment processes may have on the panels’ mechanical properties. We propose that treating CLT panels with borate solution, applied under vacuum in flexible bags, could provide a practical and effective option for providing preservative protection for interior applications. Samples were cut from commercially produced CLT panels and treated with borate solution using vacuum. The samples were then evaluated for preservative retention, swelling and degradation of mechanical properties. Initial treatments resulted in a wide range of preservative retentions and property impacts among the products tested. In subsequent ‘optimizations’, the treatment parameters were adjusted to provide consistent and sufficient retention amongst the products. The vacuum treatment method effectively penetrated cracks in the lumber and the bond lines. Swelling and impacts on mechanical properties were minimal in the optimized samples. In addition to these samples that were treated in a rigid pressure vessel, a larger sample was successfully treated in a flexible plastic bag. Data from this study supports the concept that vacuum treatment of CLT panels with borate can provide sufficient levels of preservative retention, can be adjusted to the material being treated and has minimal impacts on mechanical properties.

Cross-laminated timber

preservatives

protection

durability

rolling shear

Cross-laminated timber (CLT) is emerging as an environmentally friendly alternative to concrete and steel in large buildings. CLT is among the mass timber family of products (with glulam, large timbers, etc.) that can provide flexibility in construction, allows for rapid construction, has good fire and seismic performance, and reduces holistic environmental impact. As with all building materials, CLT is liable to wetting during construction and in-service, which increases susceptibility to biological degradation. However, currently most CLT are not treated with preservatives and the strategy for durability is restricted to keeping the wood dry. CLT treatment options are needed to address the inevitable need for supplemental protection (Udele et al., 2021; Wang et al., 2018).

Research is underway on the efficacy of common wood preservative when applied to CLT panels (Mankowski et al., 2018, 2020, 2022). However, the best way to apply such preservatives requires investigation. Traditional vacuum pressure treatment is not possible for panels that are too large to fit in currently available commercial wood treatment cylinders. Dip treating mass timber elements could result in limited penetration and retention levels because of the low surface area to volume ratios(Adnan et al., 2021; Bagheri et al., 2022). It may be possible to pre-treat the lamellae with preservatives (Lim et al., 2020), but this would require the redrying of the lumber and the generation of treated wood waste when the lamellae are planed before bonding. Failure to plane the treated lamellae likely would reduce bond performance (Stirling & Morris, 2017). One potential option is to treat by vacuum, a method that has been successfully used for millwork treatments with light organic solvent preservatives for decades and was demonstrated in millwork with borates (Jermer & Lloyd, 2000). Vacuum bags are commonly used commercially to press laminations during gluing, and to impregnate fiberglass materials with resins. Such bags are readily available in custom sizes suitable for CLT panels, are reusable and are relatively inexpensive. However, both the potential to penetrate CLT via vacuum treatment using a flexible plastic bag setup and the potential impacts on the panels’ mechanical properties are unknown.

Given that most CLT panels will be used in interior applications and mostly protected from wetting, borates are a good potential preservative. Borates are relatively safe, odorless, colorless and inexpensive, all while protecting against a wide range of insects and fungi (Lloyd, 1997; Williams, 1996). Borates are also the only preservative available that is fully diffusible, providing increasing penetration with increasing moisture (Cabrera & Morrell, 2009; de Groot et al., 2000; Schoeman et al., 1998; Smith & Williams, 1969).

Initially, the objectives of this study were to determine if the post-fabrication vacuum treatment of commercially produced CLT can achieve useful levels of borate preservative penetration and retention, and whether that treatment impacts mechanical properties. Based on the initial results (Shahan et al., 2021), the subsequent objectives were to determine if treatment parameters could be adjusted to provide similar and acceptable treatments to the variety of panels considered and to see if treatment could be conducted in a flexible plastic bag.

We obtained commercially produced CLT panels that were composed of a variety of species and layup types. The panels evaluated in this test included Douglas-fir (Pseudotsuga menziesii) from the United States, spruce (Picea spp.) from Austria, and radiata pine (Pinus radiata) from Chile. The Douglas-fir and radiata pine panels were each made from three lamellae, resulting in a total thickness of approximately 102 mm. The spruce panels were made from five lamellae with a total thickness of approximately 114 mm). The production process parameters, including the adhesives used, were not reported by the manufacturers.

Sample Treatment

Samples with dimensions of 229 mm x ~ 102 mm (panel thickness) x 305 mm were cut from the panels. Each sample was completely edge sealed, leaving only the panel faces exposed, using a neoprene coating (1300L, 3M St. Paul MN). The initial weight and dimensions of the samples were recorded.

Samples were vacuum treated with DOT/Glycol[1] solution (Bora-care, Nisus Corporation) in a small, rigid pressure/vacuum vessel using the following sequence (see Table 1 for specific parameters):

1. Initial vacuum was applied to the CLT samples in an empty vessel

2. The treating solution, at ambient laboratory temperature, was introduced using the residual vacuum (the solution completely submerged the samples, which were weighted down to prevent floating)

3. Samples were allowed to soak in the solution under ambient pressure

4. Vessel was emptied using gravity

5. Final vacuum was applied to remove excess treating solution

Post-treatment, the samples were wiped with paper towels and immediately weighed and measured to assess solution uptake and dimensional change. The treating solution was sampled and analyzed for borate content by titration (AWPA, 2022). Preservative retention was calculated as the product of solution uptake and preservative concentration. Swelling was calculated using the dimensional changes in each direction.

The samples were then conditioned to weight equilibrium at 20°C and 65% relative humidity (~ 12%MC) prior to mechanical testing. Some of these samples were cut in half along the 305 mm axis and sprayed with curcumin reagent to indicate the presence of borate above about 0.01% SBX inside the sample (Smith & Williams, 1969).

Initial treatments applied the same parameters to all the product types (Table 1). Subsequently, parameters were adjusted, and treatments repeated in order to find combinations that resulted in preservative retentions of approximately 1.0 kg/m³ SBX (Inorganic boron measured B₂O₃ as per AWPA Standard U1 (AWPA, 2022)) for each product type. This target was chosen because 1 kg/m³ is known to be effective for controlling wood destroying beetles and decay fungi as determined under testing according to EN 599 (DIN, 2014; Lloyd, 1997). These ‘optimized’ treatment parameters are also listed in Table 1. Note that an ambient pressure dip treatment was sufficient for the radiata pine. All retentions were ‘gauge retention” based on the solution uptake and the solution concentration. No attempt was made to measure penetration or retention in an assay zone, given that initial trials indicated that penetration was mostly along bond lines and cracks and, as a laminated product, the more permeable sapwood sections are dispersed throughout the CLT panels.

Table 1

Treatment parameters
Species		Target solution concentration	Initial vacuum		Soak		Final vacuum
Species		% SBX	kPa	mins	kPa	min	kPa	mins
Initial treatment	All species	6	13	30	101	30	13	30
Optimized treatment	Spruce	6	13	30	101	5	13	30
	Douglas-fir	11	13	30	101	60	13	30
	Radiata pine	6	Dip treatment – no vacuum		101	1.5	Dip treatment – no vacuum

Mechanical Testing

Treated samples conditioned to a constant moisture content for each panel type were tested in rolling shear in accordance with EN 16351 and EN 789 (European Committee for Standardization (CEN), 1996, 2021)(21; 22)^{21, 22}(European Committee for Standardization (CEN), 1996, 2021)(21, 22)(European Committee for Standardization (CEN) 1996, European Committee for Standardization (CEN) 2021)(European Committee for Standardization (CEN) 1996, European Committee for Standardization (CEN) 2021)(CEN, 1996, 2021). Rolling shear was selected for evaluation in this study since it captures many aspects of the panel that could potentially be affected by the preservative treatment including the wood and bond lines among multiple boards. Rolling shear is also an important property for the bending of CLT with short spans. In addition, the relatively small size of the sample needed to perform the tests allowed for more replicates with the same amount of material resulting in more statistically robust observations.

Each of the treated samples was cut in half to produce two mechanical testing samples with a nominal size of 95 mm x ~ 102 mm x 305 mm. An equal number of untreated controls cut from the same parent panels were also tested. The spruce samples were cut down in thickness to ~ 102 mm with a bandsaw before testing (but after treatment) to provide correct dimensions for the test. Due to an error in the testing process, seven of the ‘optimized’ radiata pine sample were cut to 250 mm long; strength and stiffness calculations were based on the actual length when tested. Samples were placed in a specially designed and fabricated rolling shear testing rig in a uniaxial testing machine (Fig. 1). Applied load was measured with a load cell with a tolerance of less than 0.5%. Deformation of the panel was measured with two linear variable differential transformers (LVDTs) that were attached to opposite sides of the sample using aluminum brackets and hanger bolts. The measurements from the two LVDTs were averaged to obtain the shear deformation of the sample. Load was applied in displacement control at a rate of 2.54 mm/min until a decisive rolling shear or glue bond failure occurred. For some samples, the ends failed in compression prior to rolling shear failure. For these samples, thin steel plates were attached to the outer faces using adhesive and screws to better distribute the applied loads and the samples were retested. The screws were short and did not penetrate the bond line.

An example of the applied load vs. shear deformation response for one of the Douglas-fir samples from the initial treatment is shown in Fig. 2. The red circle indicates the maximum load obtained during testing which was used to determine the rolling shear strength as:

$${f_r}=\frac{{{F_{max}}}}{{lb}}$$

where f_r is the rolling shear strength, F_max is the maximum load obtained during testing, b is the width of the sample, and l is the length of the sample.

The red dashed line is a secant between 10% and 40% of F_max. The slope of this line was used to determine the rolling shear stiffness as:

$${G_r}=\frac{{\left( {{F_2} - {F_1}} \right)}}{{\left( {{u_2} - {u_1}} \right)}}\frac{t}{{lb}}$$

where G_r is the rolling shear stiffness; F₁ = 0.1F_max; F₂ = 0.4F_max; u₁ and u₂ are the shear deformations at F₁ and F₂, respectively; and t is the gage of the measurement (i.e., initial distance between hangar bolts supporting the LVDTs).

Results from the two mechanical testing samples cut from the same treatment sample were averaged.

Vacuum bag treatment demonstration

One test treatment of a Douglas-fir (same stock as above) panel 762 mm x 470 mm x 102mm thick was performed using a polyurethane bag (Vacuum Press Technologies, Brunswick ME) (Fig. 3).

The non-edge sealed sample was weighed and placed in the bag, a 30-minute vacuum was pulled (approx. 20 kPa) and the vacuum was used to draw in the solution (15.67% disodium octaborate tetrahydtate (DOT) and 2.3% didecyl dimethyl ammonium chloride (DDAC); Boracare + Moldcare, Nisus Corporation, Rockford TN). DDAC is used in combination with borate in this product because it mitigates mold fungi (Micales-Glaeser et al., 2004). As a powerful surfactant, the DDAC may also improve surface wetting by the treatment solution and thus aid penetration. This potential effect was not studied here. The panel was allowed to soak for 30 minutes (no vacuum), the solution was drained, and a final vacuum pulled for 30 minutes. After removal from the bag, the panel was reweighed, and preservative retention calculated by weight gain with adjustment for solution concentration. After drying for a week, the sample was cut open and the cut surfaces sprayed with borate indicator, as above.

[1] Glycol is known to help borate diffusion over time, especially in dry wood Turner, A., & Lloyd, J. (2008). Penetration depth of borates in historic wooden structures in Virginia City Montana. Paper prepared for the 39th Annual Meeting of the International Research Group on Wood Protection, Istanbul, Turkey. IRG/WP 08-30475

As expected, initial treatments provided highly variable retentions depending on the product type (Douglas-fir, spruce or radiata pine; Table 2). The preservative solution appeared to preferentially penetrate along pre-existing cracks in the wood and along the bond-lines, particularly in the panels made with refractory species (Douglas-fir and spruce; Fig. 4). The distributions of borate shown in Fig. 4 are following treatment and without any diffusion storage or subsequent wetting. If these treated samples were exposed to increased moisture, the yellow (unprotected) areas could be expected to turn red (protected) with time (Cabrera & Morrell, 2009), as long as adequate overall retention was achieved.

Table 2

Summary of preservative retention, swelling and mechanical properties
Species	Treatment	N	SBX Retention (kg/m³)^i,ii	Thickness Swelling (%)^i,ii	Rolling Shear Strength (MPa)^i,ii	Rolling Shear Stiffness (MPa)^i,ii
Douglas-fir	None	10	--	--	1.70 ± 0.11a	195 ± 32ab
	Initial	10	0.25 ± 0.04a	0.40 ± 0.49a	1.59 ± 0.13a	154 ± 20b
	Optimized	10	1.04 ± 0.19b	0.80 ± 0.61b	1.68 ± 0.13a	246 ± 64a
Spruce	None	10	--	--	1.28 ± 0.11ab	172 ± 23a
	Initial	10	2.48 ± 1.04a	0.93 ± 0.59a	1.10 ± 0.07b	131 ± 33ab
	Optimized	18ⁱⁱⁱ	1.12 ± 0.19b	1.63 ± 0.65b	1.29 ± 0.08a	124 ± 18b
Radiata Pine	None	10	--	--	2.06 ± 0.23a	294 ± 59a
	Initial	10	9.11 ± 2.09a	2.77 ± 1.24a	1.18 ± 0.38b	149 ± 51b
	Optimized	6ⁱⁱⁱⁱ	1.80 ± 1.03b	0.51 ± 0.95b	2.68 ± 0.26a	188 ± 53ab
ⁱMean values shown with 90% confidence intervals ⁱⁱValues within a species and measurement group with a different letter are significantly different (p < 0.05, ANOVA with Tukey HSD test for multiple comparisons) ⁱⁱⁱExtra samples of this treatment were tested accidentally ⁱⁱⁱⁱSamples of this treatment were limited due to limited supply of CLT panels

There is currently no standard for preservative treatment of CLT; however, the New Zealand (where subterranean termites do not occur) Standard NZS3640 (New Zealand Standard 3602, 2003) for lumber is approximately 1.0 kg/m³ SBX, as is the ‘biological reference value’ (similar to a ‘toxic threshold’ for various fungi) (Lloyd, 1997) - suggesting that it might be suitable for CLT to prevent beetles, drywood termites and fungi. Interestingly this retention may also mitigate subterranean termite activity over time (Jones, 1991), although it does not prevent damage in small test specimens according to EN 599 (DIN, 2014). The American Wood Protection Association (AWPA) standard for lumber for interior applications (UC2; (AWPA, 2022)) requires much higher retentions (2.7 or 4.5 kg/m³ SBX), but this is intended to protect against subterranean termites (non-Formosan and Formosan, respectively), in addition to decay and other insects. For this study, we targeted the lower biological reference value and the New Zealand Standard, based on the assumptions that CLT buildings are constructed above-grade and that separate subterranean termite control measures will be implemented according to building codes (e.g., soil termiticides). By this reasoning, the main threat to CLT will be non-subterranean insects and fungi, for which 1.0 kg/m³ SBX will provide effective control.

With reference to the 1.0 kg/m³ target, the initial treatments undertreated the Douglas-fir while over-treating the spruce and radiata pine samples. The ‘optimized’ treatments came close to achieving the target, across all the species. This potential for adjustment is noteworthy, given that Douglas-fir and spruce are refractory species, while radiata pine in this test was mostly highly permeable sapwood.

Dimensional changes in the length and width were negligible, as expected for a cross-laminated product in which the longitudinally oriented lamellae restrain swelling in that direction. Thickness swelling was measurable and mostly positively correlated to the amount of preservative retained (Table 2). The apparent increase in spruce swelling in the optimized samples (with lower retention) is counterintuitive and we believe may be an artifact of a different person making the measurements of those samples. Standard tolerances for thickness variation of CLT panels are 2% (FPInnovations, 2019); the average thickness swell of the samples receiving optimized treatments was below that level, suggesting that the thickness swelling observed here would be acceptable in practice. Delamination was not measured but appeared to be minimal across all samples.

The rolling shear test typically resulted in failure near one or both bond lines, usually crossing over the middle lamella (Fig. 5). Initial treatments that resulted in overtreatment led in some cases to dramatic reductions in mechanical properties (e.g., radiata pine rolling shear strength; Table 2). In contrast, the optimized treatments did not lower the strength values of any species tested. Reductions in rolling shear strength, associated with very high treatment (water-based treatment) retentions, may result from the wetted wood swelling, and the differential swelling across- and along-the grain causing stresses on the bond lines. Stiffness values were less consistent in response to treatment retention levels and poorly correlated to strength values (R² = 0.31, simple linear regression). Guidance in EN 789 (European Committee for Standardization (CEN), 1996) notes that the variability of rolling shear stiffness obtained from planar shear testing is high.

Overall, these results suggest that meaningful retention levels of borate treatment can be achieved using vacuum, that the process can be adjusted to the species being treated, and that the impacts on the mechanical properties can be minimized.

Vacuum bag treatment demonstration

The vacuum bag trial was successful. It provided high retention (1.52 kg/m³ SBX) and good penetration (as indicated by red color between interior lamellae and in internal checks; Fig. 6) in a panel made with a refractory species (Douglas-fir) in which the lamellae were edge-glued in addition to being face-glued. This result suggests that vacuum treatment of full-size panels in plastic bags is practically possible, even in less permeable panels. In our trial, some of the treatment liquid entered the vacuum port which was close to the surface of the panel. Thus, relocating the vacuum port and/or installing a vacuum trap may be necessary to prevent problems with treating solution entering the vacuum pump.

Purpose-built, rigid vacuum treatment tanks may be more practical in commercial settings, but flexible bags may provide useful test chambers for the development of treating processes with reasonably large samples.

An initial test of double vacuum borate treatment of CLT samples achieved a wide range of solution uptakes. In some cases, preservative retention was well above standards for solid sawn lumber. Solution uptake was related to preservative retention, penetration, and volumetric shrinkage. The treatment reduced the rolling shear strength, and the degree of impact was correlated to solution uptake.

Subsequent optimization of treatment parameters yielded samples with consistent levels of preservative retention, with minimal impact on dimensions. or mechanical properties. A demonstration of vacuum treating using a plastic bag as the container likewise provided good treatment results. These data suggest that vacuum treatment of CLT panels is practical and adjustable, while providing effective levels of protection against degradation and maintaining panel integrity.

Work is ongoing to confirm that the preservative treatments provide the expected levels of durability in field testing. Further work could include investigating CLT made with other species and adhesives, as well as the application of other preservatives.

ACKNOWLEDGEMENTS

The United States Department of Agriculture Forest Service Forest Products Laboratory provided funding to the University of Tennessee to conduct this research.

Nisus Corporation contributed treatment products and analytical services.

The hard work of students Andrew Shahan, Alex Moore, Chris Webb and Quintinne Madsen is gratefully acknowledged.

AUTHOR CONTRIBUTIONS

JL provided the concept. MD and AT conducted the research and wrote the main manuscript. GK, MM and JL reviewed the manuscript.

CONFLICT OF INTEREST STATEMENT

The authors have no competing interests to declare that are relevant to the content of this article.

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No competing interests reported.

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Borate treatment of CLT Panels Using Vacuum: A Proof of Concept

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Introduction

Materials And Methods

Sample Treatment

Mechanical Testing

Results And Discussion

Vacuum bag treatment demonstration

Conclusions

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References

Additional Declarations

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