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Article

Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers

by
Yutao Yan
1,2,*,
Jinhui Wang
2,
Zhou Shen
3,
Haiming Bi
3 and
Baoqing Shentu
1,*
1
State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China
2
College of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, China
3
Research Institute of TREEZO New Material Technology Group Co., Ltd., Hangzhou 311100, China
*
Authors to whom correspondence should be addressed.
Forests 2023, 14(4), 741; https://doi.org/10.3390/f14040741
Submission received: 1 March 2023 / Revised: 31 March 2023 / Accepted: 3 April 2023 / Published: 4 April 2023
(This article belongs to the Special Issue Advances in Preparation and Modification of Wood-Based Materials)
Browse Figures
Review Reports Versions Notes

Abstract

In this study, fire-retardant plywood was fabricated using a simple guanidine phosphate (GP) impregnation treatment of the veneers, and the influence of the treatment on the flame resistance and bonding strength of the plywood was fully investigated. The results showed that GP modification could effectively endow the plywood with excellent fire resistance and smoke-suppression effect. When 10% GP solution was applied, the limiting oxygen index (LOI) of the impregnated wood was up to 37%, which was almost twice of unmodified plywood. The heat-release rate (HRR) and total smoke production (TSP) were also greatly decreased from the pristine 94.14 MJ/m2 and 0.77 m2 to that of modified 43.94 MJ/m2 and 0.08 m2, respectively. The excellent fireproof performance was mainly due to the thermal decomposition of GP to phosphoric acid and guanidine during combustion, which could promote the catalytic carbonization of wood and release of incombustible CO2 and NH3 to dilute and decrease the combustible gases, thus collectively preventing the wood form burning. However, the guanidine phosphate modification could seriously damage the bond performance of plywood, especially the UF resin adhesive-bonded plywood. When 10% guanidine phosphate was applied, the dry and wet bonding strength of the UF resin adhesive-bonded plywood were decreased to only 0.7 MPa and 0.12 MPa, respectively, which may be due to the blocking effect of GP in wood pores and the hygroscopic and soluble properties of GP itself in water, thus decreasing the effective bonding between wood veneers. What’s worse, the poor water resistance of the UF resin adhesive was also adverse to the bonding strength of plywood. Surprisingly, the PF resin adhesive was proved to be suitable for gluing the GP-modified wood without obviously decreasing the bonding strength, which could be used to prepare plywood with both high bonding strength and flame resistance.

1. Introduction

Plywood, as the largest type of wood-based panel products consumed in China, has been widely used in construction, furniture, flooring, interior decoration, aircraft, packaging, and many other fields due to its high wood-utilization rate, high strength, little deformation, low shrinkage rate, and high tolerance to damage [1,2,3]. However, it burns easily, can cause fire, and contains lignocellulosic composite materials, which can bring about property damage and life risk. To reduce the fire hazard, the mandatory Chinese National Standard GB 20286-2006, “Combustion performance requirements and identifications of products and components in public places”, was issued by the Fire Bureau of the Ministry of Public Security and enforced in July 2008. According to the standard, ordinary wooden floors, furniture, and wood-based panels without fire-retardant properties are not allowed to be used for decoration in public places. Therefore, preparing flame-retardant plywood is of great importance, which could promote improving the added value of plywood and broaden its application fields.
At present, there are mainly three approaches to preparing flame-retardant plywood. The first one is to deposit a flame-retardant coating on the prepared plywood surface, and the coatings mainly compose intumescent fire retardants or polymers with inorganic salts, oxides, hydroxides, or other inorganic compounds [4]. By spreading the fire-retardant UF resin adhesive modified by expanded vermiculite (EVMT) onto the plywood surface, Wang [5] successfully manufactured flame-retardant plywood with high LOI, which was mainly due to the thermal insulation effect of EVMT incorporated. Using ammonium polyphosphate-chitosan/boron nitride (APP-CS/BN) as flame retardants, Chen [6] deposited a coating on poplar plywood by layer self-assembly method, giving the plywood the excellent flame retardant property. Though depositing flame-retardant coating is a convenient and effective way to endow plywood with fire resistance, the coating can be easily destroyed by mechanical abrasion or impact. The second method is to incorporate flame retardants into the adhesive that is used to glue the veneers and then endow the plywood with flame resistance. The fire retardants used are similar to that used in flame-retardant coatings. Lv [7] mixed NP-type flame retardant (NPR) with UF resin adhesive and applied it to prepare flame-retardant plywood; when 8% NPR was incorporated, the fire resistance of the plywood could meet the requirement of the Chinese National Standard GB 8624-2012 for the B1-grade materials. However, the incorporation of flame retardants into the adhesives could inevitably affect the viscosity and curing behavior of the adhesive, which would decrease the bonding strength of the plywood. In addition, the fire-retardant performance was usually not satisfied with this method. Impregnating modification is the most commonly used method, which means impregnating fire retardants into wood veneers and then gluing them to prepare fireproof plywood, and the fire retardants used are usually nitrogen, phosphorus, or boron-based compounds. This approach is much more efficient in endowing plywood with excellent fire-retardant performance than the abovementioned methods. Wang [8] impregnated veneers into ammonium polyphosphate (APP) aqueous solution with the addition of 3A zeolite at different levels and then glued by UF resin adhesive to produce fire-resistant plywood, and the results showed that the application of APP could significantly decrease the peak heat release rate values and total heat release of the modified plywood, and the synergistic effect of 3A zeolite could further increase the flame-retardant properties of the poplar plywood. In addition, boron compounds were also applied in preparing fire-retardant plywood [9,10]. Though these nitrogen, phosphorus, and boron-based compounds had the disadvantages of hygroscopicity and poor water resistance, which could leach out [11,12,13] and damage the bonding strength of the plywood [14,15], the excellent fireproof properties of these fire retardants still make them the most commonly used flame retardants for plywood by impregnating modification.
Guanidine phosphate (GP) as a P–N compound possesses both an acid source and a gas source, which could foam and expand to achieve a dose of multi-effect function during the combustion process. Moreover, the decomposition of guanidine phosphate at high temperatures produces ammonia, carbon dioxide, and other gases, which do not cause harm to the human body and the environment [16,17]. Therefore, GP is one of the ideal flame retardants and has been proven to be efficient in fire-retardant modification for lignocellulosic materials such as paper, cotton, wood, and natural fibers [18,19,20,21]. However, nearly no research was reported about GP modification in fire-retardant plywood preparation, let alone the effects of GP modification on the combustion behavior and bonding performance of the prepared plywood. In this study, different concentrations of GP were applied to impregnate the veneers and then heat-pressed to fabricate fire-retardant plywood with the gluing of different thermosetting adhesives. The thermal stability, combustion behavior and fire-retardant mechanism, water resistance, and bonding performance of the GP-modified plywood were detailedly investigated. The research of this study will provide valuable references for fabricating fire-retardant plywood with excellent bonding properties and reveal the flame-retardant mechanism of GP in plywood modification.

2. Materials and Methods

2.1. Materials

The wood veneer of Chinese Poplar (Populus tomentosa Carr.) with a size of 400 mm × 400 mm × 1.7 mm and moisture content of 10% was provided by the TREEZO New Material Technology Group Co., LTD. The guanidine phosphate (GU) (purity of 98%) was purchased from Aladdin Reagent (Shanghai) Co., LTD. The urea formaldehyde resin (UF) adhesive (solid content 60%, 330 mPa·s at 25 °C) and phenol formaldehyde resin (PF) adhesive (solid content 48%, 650 mPa·s at 25 °C) were supplied by Taier Adhesive (Guangdong) Co., Ltd.

2.2. Sample Preparation

First, different concentrations of GU solutions (2.5%, 5%, 10%) were prepared by directly adding the GP powder into deionized water and stirring until the GP was completely dissolved. Then, the completely dried wood veneers were immersed into the prepared GP solution and vacuum impregnation for 5 min at room temperature. After impregnation, the veneers’ surfaces were rinsed with deionized water and dried in a fan blow-type electric drying oven under 103 °C until the moisture content was about 10%. To calculate the weight percentage gain (WPG), the weight of the veneer before and after GP impregnation needed to be completely dried in the oven under 103 °C until the weight did not change, and the calculation method was referred to Yan [22]. Then, PF adhesive and UF adhesive were brushed on the middle veneers of the plywood with a spreading rate of 280 g/m2 and 320 g/m2 on both sides, respectively. Finally, three-layer plywood was prepared with the grain directions of two adjacent veneers perpendicular to each other, followed by hot-pressing at 120 °C for 10 min with a pressure of 1.2 MPa for UF resin adhesive-bonded plywood, while for PF resin adhesive-bonded plywood the hot-pressing temperature, time, and pressure were 150 °C, 8 min, and 1.2 MPa, respectively.

2.3. Characterization and Measurements

Field-emission scanning electron microscopy (FE-SEM) (Model SU8010, Hitachi, Tokyo, Japan) was utilized to characterize the morphology of modified or unmodified veneers.
Fourier transform infrared spectroscopy (FT-IR) was performed to analyze the reaction between wood and GP. The treated and untreated wood specimens were milled to 120 mesh meal, embedded in potassium bromide (KBr) pellets with a ratio of 1:70, and then analyzed by Nicolet 6700 spectrometer (Thermo-Nicolet, Tokyo, Japan) with the scanning region of 4000–400 cm−1 at a 4 cm−1 resolution for 32 scans. All the FT-IR spectra obtained were treated by automatic baseline correction before further analysis.
Thermogravimetric (TG) analysis of different wood samples was performed on a TGA-Q500 apparatus (TA Co., New Castle, Delaware, USA) from 30 to 600 °C at a heating rate of 10 °C/min under air atmosphere; for each measurement, 8–10 mg sample was weighed. Before measurement, all the samples need to be pre-dried under 103 °C for 8 h to remove the moisture in the meals, which were 120 mesh scraped from the surface of the wood.
The limited oxygen index (LOI) test was conducted according to the GB/T2406.2-2009 with the JF-3 oxygen index apparatus (Jiangning Analysis Instrument Company, Jiangning, China). The combustion test was carried out by cone calorimeter (Fire Testing Technology Ltd., East Grinstead, UK) under an external heat flux of 50 kW/m2 (750 °C approximately).
Fourier transform infrared spectroscopy-thermogravimetric analysis (FTIR-TGA) was carried out on a Netzsch TG 209 (Selb, Germany) thermal analyzer connected with a Nicolet iS10 FT-IR (Tokyo, Japan) spectrometer via a Teflon tube, and the samples were tested under a nitrogen atmosphere from 40 to 700 °C at a heating rate of 20 °C/min while the scanning range was from 500 to 4000 cm−1 with a 4 cm−1 resolution.
The contact angle was tested by a contact angle measuring instrument (OCA 20, DataPhysics Instruments GmbH, Stuttgart, Germany) at an interval of 0.0375 s for a total duration of 60 s, and each sample was tested six times to calculate the average value.
The leaching experiment of GP in modified wood was conducted by immersing the sample in water for 24 h, and the WPGs of the treated wood before and after water immersing were calculated according to the above-mentioned method. The bonding strength of GP-modified plywood was measured according to GB/T 17657-2013.

3. Results and Discussion

3.1. Morphology Characterization of Impregnated Wood Veneer

Figure 1 shows the tangential section SEM images of wood veneers, both modified and unmodified. It is obvious that both the pits and ray cells on the tangential section of unmodified wood were clearly visible and hollow, showing the typical morphologies and structures of the wood tangential section. However, when the wood was modified by GP impregnation, almost all the pits and ray cells were blocked and filled with solid compounds, proving the substantial introduction of GP into the wood. What’s more, the excellent compatibility between GP and wood was also proved by the tight connections between GP compounds and pits or ray cell walls without any cracks, as shown in Figure 1d,f. However, whether chemical reactions between GP and wood components happened is still needed to investigate further.

3.2. FT-IR Analysis

The possible interaction between wood and GP was analyzed by FT-IR characterization, and the spectra were presented in Figure 2. It is obvious that the absorption peak at 3343 cm−1 and 2899 cm−1 are assigned to O–H bond-stretching vibration and symmetric stretching of the C–H bond of pure wood, respectively, while the peak at 1732 cm−1 was mainly attributed to the stretching vibration of C=O bond. However, when treated by GP, the peaks at 1665 cm−1 and 1394 cm−1 assigned to the stretching vibration of C=N and C–N bond [23], respectively, of GP, were significantly increased in the GP-modified wood spectra and further increased as the increase GP concentration was applied. What is more, the absorption peak at 1245 cm−1 corresponding to the stretching vibration of P=O of GP [15,16] also appeared in the FT-IR spectra of the modified wood sample, indicating that GP was incorporated into the wood. However, no new absorption peaks were found in the spectra of the modified wood samples, indicating that GP had no reactions with wood components.

3.3. TG Analysis

Thermogravimetric analysis was used to evaluate the thermal stability of control wood and GP-modified wood under air atmosphere, and the TG and DTG curves are shown in Figure 3. From Figure 3a, it can be seen that there are three main thermal degradation stages for both control wood and GP-modified wood. The first stage is the volatilization stage of water and volatile matter in wood. At this stage, the TG curve is relatively gentle, and the material quality loss is lower, especially when the wood sample was pre-dried before the test. The second stage is the pyrolysis stage of wood’s main components, such as hemi-cellulose, cellulose, and lignin. During this stage, both the TG curves of GP-modified and unmodified wood are sharply dropped, and the mass loss is great. What is different is that the TG curves of GP-modified wood are shifted to a lower temperature direction and are consistent with the peak position changes of maximum thermal degradation temperature (Tmax) from the pristine 328.53 °C to 276.10 °C of 10% GP-modified wood shown in Figure 3b, indicating that the GP modification could accelerate the degradation of wood and lower the decomposition temperatures of treated wood, which could protect the inner wood matrix. What is more, the Tmax was decreased with the increase in the GP concentration. The last stage is the carbonization stage of wood, in which the residual carbon is formed. It could be seen from Figure 3a and Table 1 that a very low residual char ratio (0.84%) was obtained for control wood after TG testing, while all the GP-modified wood had a higher residual char ratio. Furthermore, when 10% GP was applied in wood modification, the residual char ratio of modified wood was up to 24.67%, far higher than that of control wood, proving that GP possesses the excellent catalytic char-forming ability to wood during the thermal degradation process. On the one hand, the more the char formed, the less combustible volatiles would generate; on the other hand, the formed char was heat and oxygen insulation. Therefore, the GP could be regarded as an effective fire retardant for wood fireproof modification. However, the char yield in this study at 10% GP modification was much lower than that of Gao’s research; in their study, the char yield was up to 34.9% at the same GP concentration of wood modification, which may be due to the pre-treatment of wood by boiling water and the longer immersion time of wood by GP solution; thus, much GP was incorporated into the wood [20]. Therefore, in order to obtain a much higher char yield and further improve the fireproof performance of GP-modified wood, similar treatments could be applied in future study.

3.4. Combustion Properties Analysis

Combustion properties of control wood and GP-modified wood were evaluated by cone calorimetry and LOI testing. The heat release rate (HRR) and total heat release (THR) curves of control wood and GP-modified wood are presented in Figure 4, and the related data are listed in Table 2. HRR is an important performance parameter to characterize fire intensity, and it is the rate of heat release per unit area after the material is ignited. The larger the HRR, the larger the heat of the material produced, and the more harmful the fire will be. From Figure 4a, it can be seen that after GP modification, the HRR obviously decreased as the increase in GP concentration, indicating that GP could effectively reduce the combustion intensity of modified wood. The reason might be due to the excellent char-forming ability of GP during combustion, which could cover the surface of the wood matrix and insulate heat and oxygen, thus preventing the flame from spreading into the matrix and decreasing the combustion intensity. When 10% GP was applied to modify wood, the HRR significantly decreased from the pristine 94.14 kW/m2 to the modified 43.94 kW/m2, which showed a decrease of 53.32% compared with that of control wood (Table 2). As with HRR, the THR in Figure 4b showed similar change trends; after GP modification, the THR was also significantly decreased. In addition, as shown in Table 2, the total smoke produced (TSP) also noticeably changed when modified by 10% concentration GP; the TSP reduced to only 1/7 of that unmodified wood, indicating that GP also possessed excellent smoke suppression properties. For unmodified wood, the smoke was mainly coming from the thermal decomposition and combustion of wood; however, when treated by GP, a lot of carbon was formed on the wood surface by the catalysis of produced phosphates and prevented the decomposition and combustion of the inner wood, and the smoke was mainly coming from the incomplete combustion of the formed char; therefore, the smoke production was decreased [24]. In addition, the LOI of GP-modified wood also distinctly improved, nearly twice that of control wood from the original 19% to 37%, further proving the excellent fire resistance of GP in wood modification.

3.5. Flame-Retardant Mechanism Analysis

3.5.1. Residual Carbon Characterization

Figure 5 represents the surface morphology of treated wood after combustion. As can be seen for the pure wood, after combustion, the primary wood structure was destroyed, and the loose burning residue was found (Figure 5a). However, after being treated with a 2.5% concentration of GP, the wood char formed after combustion was not destroyed, and the pristine wood structure was clearly preserved. With the increase in GP concentration, the surface morphology of the modified wood after burning was significantly changed (Figure 5c); a lot of polymer-like substances with some bubble shapes were displayed on the surface, which might be due to the polyphosphoric or phosphoric acid substances produced by the degradation of GP and remained in the residual char. The bubble shapes may be due to the generation of non-flammable gases, such as NH3 and CO2, from the prepared guanidine during combustion [25], which had a blowing effect on the formed char. When the GP concentration applied in wood modification further increased, much more bubble shapes appeared on the surface of the residual char. These special char structures formed on the burned wood surface could effectively isolate the oxygen and heat from the underlying wood matrix, thus preventing the burning of the wood.

3.5.2. TG-IR Analysis

TG-IR was applied to analyze the evolved gaseous products of control wood and GP-treated wood samples during the thermal degradation process. The three-dimensional images of these gases are shown in Figure 6a,c, while the IR spectra of the gases released at 100, 200, 250, 300, 350, 400, and 500 °C are shown in Figure 6b,d. As can be seen, no distinct differences were found in the absorption peaks of the main evolved gaseous products for the control wood and GP-modified wood IR spectra, except for the intensity of these peaks. The absorption band at 3800–3500 cm−1 attributed to the stretching of O–H bonds is from the H2O released; the absorption band at 3100–2800 cm−1 in the spectrum indicate the existence of CH4, and the peaks at 2358, 2310, and 669 cm−1 are corresponding to CO2, while the peaks at 2239 and 2203 cm−1 are assigned for CO. The C=O stretching absorbance peaks at the band of 1800–1600 cm−1 are representative of aldehyde or ketone compounds; the absorption peak at 1500 cm−1 is related to C–O–C bend stretching for the groups of ethers; absorption bands at 1050–1185 cm−1 are corresponding to the C–OH-containing alcohols [26,27,28,29]. For the GP-modified wood, it is evident that when the pyrolysis temperature was higher than 300 °C, a great deal of non-combustible gas (CO2) was released while less combustible gases (CH4, C=O-containing compounds, C–OH-containing alcohols) were produced compared with the control wood. What is more, the pyrolysis time and temperature for producing the gases were brought forward and decreased compared with the control wood, suggesting that GP-treated wood degraded earlier than control wood, which was similar to most of the nitrogen and phosphorus-based fire retardants in wood modification [19,26]. All the above-mentioned may be due to the earlier degradation of GP at a lower temperature to produce phosphoric acid and guanidine, and the guanidine was further converted to CO2 and NH3, which was proved by the absorption peaks at 930 cm−1 and 965 cm−1 [30,31]; thus, GP modification could effectively dilute and decrease the combustible gases and prevent the wood from burning.

3.6. Water Resistance of GP-Modified Wood

3.6.1. Contact Angle Analysis

In order to find out the water resistance of GP-modified wood, the wetting behavior of modified wood was first investigated by contact angle testing. The droplet morphology and contact angle of different wood samples with time are shown in Figure 7. As can be seen, after GP modification, the contact angle of modified wood significantly decreased with the increase in the incorporated GP concentration. The original 117.9° decreased to 85.9° of 5% GP-modified wood, indicating that the incorporated GP was hydrophilic, which could change the wetting ability of the treated wood and make the water droplet gradually spread out on the wood surface; thus, the morphology of the water droplet changed, and the size became smaller (Figure 7a). However, when the GP concentration was further increased to 10%, the contact angle didn’t change evidently (80.6°), which may be due to the fully filled wood pores (pits and ray cells) shown in Figure 1d,f that prevented the further infiltration of water, thus slowing down the changing trend of the contact angle as the GP concentration applied in wood modification. On the other hand, the contact angle also greatly changed with time, as shown in Figure 7b. After 60 s, the contact angle of control wood was decreased to 59.9°, while the 2.5% GP, 5% GP, and 10% GP-modified wood were decreased to 36.4°, 29.0°, and 18.0°, respectively, indicating that as time went by, water could slowly penetrate into the wood through wood pores. The much smaller contact angle of GP-modified wood compared with the control wood after 60 s was mainly due to the hydrophilic of GP, which could absorb water over time and make the water droplet much smaller. The dynamic change rules of the contact angles are shown in Figure 8. It is evident that in both the control wood and the GP-modified wood, all the contact angles were significantly decreased during the first 10 s, then slowly decreased, and finally, leveled off, indicating that the water spreading on the wood surface is a slow and complex process, which is determined by the surface chemical composition, structure, and morphology. Because of the hydrophilic properties of GP and the GP-blocked wood structure, the finally obtained contact angle after 60 s was also smaller than that of the control wood. Therefore, the contact angle analysis of this part indicated that GP modification could increase the hydrophilic property of wood, and it would increase with the GP concentration applied.

3.6.2. Leachability Analysis of GP-Modified Wood

To further investigate the water resistance of GP-modified wood, weight percentage gain (WPG) and LOI after being immersed in water for different times were measured and calculated, and the results are shown in Figure 9. When the wood was modified by 10% GP, the WPG was 13.33%, and the LOI was 37%, indicating that GP was easily introduced into the wood and endowed it with excellent fire retardance. However, when the modified wood was immersed in water, the WPG sharply decreased after only being immersed in water for 2 h; the WPG of modified wood decreased to 5.37% and decreased by 59.7% compared with the wood unsoaked in water. As immersion time increased, the WPG of GP-modified wood further decreased; in other words, the leachability of GP in modified wood was further increased. After 8 h, the WGP was only 1.25%, and the GP introduced into the wood was almost completely lost, indicating that GP had poor water resistance and was easy to leach. The leachability of the GP inevitably could affect the fire resistance of the GP-modified wood. After 8 h water immersion, the LOI of GP-modified wood was only 20%, which was comparable to that of unmodified wood and decreased by 45.9% compared with that of GP-modified wood without water immersion treatment, indicating that water immersion treatment could seriously decrease the fire resistance of GP-modified wood due to the massive loss of GP.

3.7. Bonding Strength of GP-Modified Wood

To explore whether GP modification had effects on the mechanical properties of the urea–formaldehyde (UF) resin adhesive glued plywood, the dry and wet bonding strength of wood modified and unmodified were measured, and the results are shown in Figure 10. As can be seen, for the unmodified UF resin glued plywood, the dry and wet bonding strengths were 1.22 MPa and 0.70 MPa, which meet the minimum requirements (0.7 MPa) of Chinese national standards GB/T 17657-2013 marked in green line in Figure 10. However, when modified by GP, both the dry and wet bonding strengths of the produced plywood were decreased. What is different is that though the dry bonding strength of GP-modified wood decreased with the increase in the GP concentration, the strength was still higher than 0.70 MPa even when the GP concentration was up to 10%, while the wet strength of the plywood modified by GP was greatly decreased, when the GP concentration was up to 10%, the wet strength was only 0.12 MPa, which nearly had no bonding strength. These results showed that GP modification had obvious bad effects on the bonding strength of the produced plywood, especially the wet bonding strength. The decreased bonding strength was mainly due to the following four reasons: First, the incorporation of GP could significantly block the pore structures in wood veneer, proved by the SEM characterization shown in Figure 1, which was adverse to the formation of glue nails between wood veneers in the preparation of plywood, and the more the GP incorporated into the wood, the more pores would be blocked (Figure 1d,f). Therefore, the effective glue nails contributing to the bonding strength decreased, and so did the bonding strength; Second, due to the strong hygroscopic property of the GP, the GP in the veneer would absorb moisture in the air and increase the surface wettability of the treated veneer, so that the adhesive was easier to spread and penetrate into the veneer during the preparation of plywood, thus resulting in insufficient sizing on the bonding interface and decreasing the bonding strength [32]; Third, the GP is hydrophilic and easy to leach in water which was proved by the previous leaching test research. Therefore, when the GP-modified wood was immersed at 63 °C for 3 h to test the wet bonding strength, most of the GP in wood might leach as water and migrate from the wood interior to the bonding interface, thus further impeding the bonding between wood veneer and UF resin adhesive; Last, the poor water resistance of UF resin adhesive itself was also adverse to the wet bonding strength. In summary, UF resin adhesive is not suitable for the bonding of GP-modified veneers.
However, when the GP-modified wood was glued by phenol-formaldehyde (PF) resin adhesive, the effects of GP on the bonding strength were quite different from that of UF resin adhesive-bonded plywood, and the results are shown in Figure 11. It is obvious that though GP modification could decrease both the dry and wet bonding strength of the PF resin adhesive-bonded plywood, the strength was still much higher than the minimum requirements (0.7 MPa) of Chinese national standards GB/T 17657-2013, marked as the green line in Figure 11, indicating that GP modifications have much less adverse effects on the PF resin adhesive-glued plywood than that of on the UF resin adhesive-glued plywood. The reason for the decrease in the bonding strength was similar to the above-mentioned reasons for UF resin adhesive-bonded plywood; that is to say, the incorporated GP blocked the wood pores and decreased the formation of bonding nails between wood veneers by PF resin adhesive. However, the much higher wet bonding strength that remained for PF resin adhesive-bonded plywood was mainly attributed to the excellent water resistance of the cured PF resin itself; thus, the formed bonds between wood and PF resin were hard to be destroyed by water boiling treatment, and the bonding strength could be reserved. Therefore, according to the above results, PF resin adhesive is considered to be suitable for gluing the GP-modified wood veneers and could be used to fabricate fire-retardant plywood with high bonding strength.

4. Conclusions

In this study, fire-retardant plywood was fabricated by impregnating the veneers with GP, and the effects of GP modification on the performances of the prepared plywood were comprehensively investigated and characterized. SEM and FT-IR characterizations indicated that though GP was successfully incorporated into wood veneers and filled with the pits and ray cells, it did not react with wood components. TG analysis showed that GP could promote the wood to decompose into carbon in advance and lead to a high amount of char residue, thus protecting the inner wood matrix and improving the thermal stability and fire resistance of the plywood. Cone and LOI results further proved the excellent fireproof ability of GP in plywood modification, and the LOI of GP-modified plywood was up to 37%, while the HRR was significantly decreased to 43.94 kW/m2, which was 53.32% lower compared with that of the control wood. The flame-retardant mechanism of the GP-modified wood plywood was revealed by the residual char characterization and TG-IR analysis. The results showed that earlier degradation of GP at lower temperature could produce phosphoric acid and guanidine. On the one hand, the produced phosphoric acid had obvious dehydration and carbonization effect on lignocellulose, which could promote the formation of residual char and insulate heat and flame; on the other hand, the produced guanidine would further convert to CO2 and NH3, which could bulk the char layer and dilute the combustible gases, thus preventing the wood from burning. However, contact angle and leachability experiments indicated that GP in the plywood has poor water resistance, which could seriously affect the bonding strength of the plywood, especially the wet bonding strength glued by UF resin adhesive. Luckily, though GP modification could also decrease the bond strength of PF glued plywood, the bonding strength could still meet the requirement of Chinese National Standards GB/T 17657-2013 for plywood. Therefore, a fire-retardant plywood with high bonding strength could be easily fabricated by GP-impregnating modification and PF resin adhesive gluing.

Author Contributions

Conceptualization, Y.Y. and B.S.; methodology, software, and validation, J.W.; formal analysis, investigation, and resources, Z.S.; data curation, J.W.; writing—original draft preparation, Y.Y.; writing—review and editing, Y.Y. and B.S.; visualization, J.W. and Z.S.; supervision, B.S.; project administration, B.S. and H.B.; funding acquisition, Y.Y. and H.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Zhejiang Province, China, (No. LQ19C160014) and the National wood and bamboo industry technology innovation strategic alliance (TIAWBI 2021-06).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We would like to thank other members of our group for helping us prepare the samples. The advanced analysis and testing center of Zhejiang A&F University is acknowledged.

Conflicts of Interest

All the authors (including Zhou Shen and Haiming Bi) declare no conflict of interest.

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Figure 1. SEM images of wood veneers unmodified and modified at tangential section. (a,b) An unmodified wood with different magnifications. (c,d) A 10% GP-impregnated wood-blocked pits with different magnifications. (e,f) A 10% GP-impregnated wood-blocked wood ray cells with different magnifications.
Figure 1. SEM images of wood veneers unmodified and modified at tangential section. (a,b) An unmodified wood with different magnifications. (c,d) A 10% GP-impregnated wood-blocked pits with different magnifications. (e,f) A 10% GP-impregnated wood-blocked wood ray cells with different magnifications.
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Figure 2. FT−IR spectra of wood, unmodified and modified by different GP concentrations.
Figure 2. FT−IR spectra of wood, unmodified and modified by different GP concentrations.
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Figure 3. TG (a) and DTG (b) cures of unmodified wood and GP-modified wood under air atmosphere.
Figure 3. TG (a) and DTG (b) cures of unmodified wood and GP-modified wood under air atmosphere.
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Figure 4. (a) HRR and (b) THR curves of the control wood and GP-modified wood.
Figure 4. (a) HRR and (b) THR curves of the control wood and GP-modified wood.
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Figure 5. SEM images of different wood char after combustion, (a) control; (b) 2.5% GP modified; (c) 5.0% GP modified; (d) 10% GP modified.
Figure 5. SEM images of different wood char after combustion, (a) control; (b) 2.5% GP modified; (c) 5.0% GP modified; (d) 10% GP modified.
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Figure 6. TG−IR spectra of the gas phase during combustion and the corresponding FT−IR spectra under different temperatures; (a,b) pure wood and (c,d) 10% GP-modified wood.
Figure 6. TG−IR spectra of the gas phase during combustion and the corresponding FT−IR spectra under different temperatures; (a,b) pure wood and (c,d) 10% GP-modified wood.
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Figure 7. Water droplet morphology and contact angle on control wood and GP-modified wood surface with time; (a) time = 0 s; (b) time = 60 s.
Figure 7. Water droplet morphology and contact angle on control wood and GP-modified wood surface with time; (a) time = 0 s; (b) time = 60 s.
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Figure 8. Curves of dynamic contact angle with time in 60 s.
Figure 8. Curves of dynamic contact angle with time in 60 s.
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Figure 9. Weight percentage gain and LOI of GP-modified wood after being immersed in water for different times; 10% GP-modified wood.
Figure 9. Weight percentage gain and LOI of GP-modified wood after being immersed in water for different times; 10% GP-modified wood.
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Figure 10. Dry and wet bonding strength of different GP concentration modified wood, glued by UF resin.
Figure 10. Dry and wet bonding strength of different GP concentration modified wood, glued by UF resin.
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Figure 11. Dry and wet bonding strength of control wood and 10% GP concentration modified wood, glued by PF resin adhesive.
Figure 11. Dry and wet bonding strength of control wood and 10% GP concentration modified wood, glued by PF resin adhesive.
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Table 1. Thermal degradation data of control wood and GP-modified wood.
Table 1. Thermal degradation data of control wood and GP-modified wood.
SampleTonset (°C)Tmax (°C)Maximum Mass Loss Rate (°C/%)Char Yield (%)
Control251.73328.532.040.84
2.5% GP244.18291.661.723.99
5.0% GP248.27287.051.8417.96
10% GP245.08276.101.8324.67
Tonset means 10% weight-loss temperature; Tmax means maximum weight-loss temperature.
Table 2. LOI and cone calorimetry data of control wood and GP-modified wood.
Table 2. LOI and cone calorimetry data of control wood and GP-modified wood.
SamplespHRR (kW/m2)HRR (kW/m2)THR (MJ/m2)TSP (m2)Weight Gain (%)LOI (%)
Control381.1394.1455.540.77——19
2.5% GP270.5870.5341.680.234.1025
5.0% GP247.2262.6337.080.087.6828
10% GP190.4143.9425.790.1113.3337
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Yan, Y.; Wang, J.; Shen, Z.; Bi, H.; Shentu, B. Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers. Forests 2023, 14, 741. https://doi.org/10.3390/f14040741

AMA Style

Yan Y, Wang J, Shen Z, Bi H, Shentu B. Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers. Forests. 2023; 14(4):741. https://doi.org/10.3390/f14040741

Chicago/Turabian Style

Yan, Yutao, Jinhui Wang, Zhou Shen, Haiming Bi, and Baoqing Shentu. 2023. "Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers" Forests 14, no. 4: 741. https://doi.org/10.3390/f14040741

APA Style

Yan, Y., Wang, J., Shen, Z., Bi, H., & Shentu, B. (2023). Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers. Forests, 14(4), 741. https://doi.org/10.3390/f14040741

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