Improvements in Fire Resistance, Decay Resistance, Anti-Mold Property and Bonding Performance in Plywood Treated with Manganese Chloride, Phosphoric Acid, Boric Acid and Ammonium Chloride

Abstract

(1) A compound protectant was prepared using manganese chloride, phosphoric acid, boric acid and ammonium chloride, and then a veneer was immersed in the prepared protectant to prepare plywood in this paper. Great attention was paid to discussing influences of such protectant on fire resistance, decay resistance, anti-mold property and bonding performance of plywood. Results demonstrated that after protectant treatment, the plywood showed not only good fire resistance and smoke inhibition, but also strong char-formation ability, slow flame spreading, long time to ignition, small fire risk and high safety level. (2) The mass loss rates of plywood with protectant treatment after infection and erosion in wood-destroying Coriolus versicolor and Gloeophyllum trabeum were 19.73% and 17.27%, reaching the II-level corrosion grade. (3) There is not a significant difference with Aspergillus niger V.; however, it was possible to observe a strong difference with Trichoderma viride Pers. ex Fr., indicating that the protectant acted as a good anti-mold product for plywood. (4) The protectant influenced the bonding interface of wood and bonding conditions of the adhesive. The bonding strength of plywood was weakened, but it still met the requirements on bonding strength of GB/T 9846-2015. (5) The protectant changed the thermal decomposition and thermal degradation of plywood, inhibiting the generation of inflammable goods, blocking transmission of heats and lowering the thermal decomposition temperature of plywood. These promoted dehydrations and charring of wood and the generated carbon had a high thermal stability. (6) Compared with untreated plywood, the prepared protectant treatment significantly enhanced the fire resistance of plywood, reduced its biodegradability by wood-decaying fungi and showed good mold resistance.

1. Introduction

With the increasing requirements of people for a quality of life, wooden furniture, doors, floors and wooden indoor decorating materials are highly appreciated by the public, and demands for wooden buildings are also increasing year by year. However, wood is a kind of natural organic material composed of cellulose, hemicellulose and lignin, which determine the potential fire risks. Wood is not only flammable but can also release a lot of heat at combustion. The average heat release of wood is 18 kJ/g, thus accelerating flame spreading significantly [1,2]. As the common material in indoor decoration, wooden materials are one of the conditions for the occurrence and spreading of fire disasters. As a result, fire-retardant treatment of wooden materials is one of the effective pathways to decrease fire hazards at present [3,4,5,6].

Fire retardants which contain P, N and B have been the research hotspot in fire retardants for wood for their characteristics of being toxic-free, smoke-inhibiting, cheap and having good fire resistance. Nitrogen–phosphorus fire retardant provides good flame retardation effects to wood by thermal decomposition into non-flammable gases, lowering the thermal decomposition temperature and increasing the char-formation rate [7,8,9,10,11]. Borides are covered onto the wood surface after swelling and melting upon the contact of heat to isolate oxygen, thus realizing the goal of retarding flames by stopping combustion of wood [12]. Moreover, borides have functions of anti-corrosion and insect prevention [13,14,15]. The combination of nitrogen–phosphorus fire retardant and borides into a P-N-B system has a synergistic effect of flame retardation and realizes the good flame retardation effect.

Since the 1950s, scholars have carried out a series of studies on the modification of fire retardants for wood, which are composed of N, P and B. Biasi immersed cedarwood in boric acid and found that boric acid treatment decreased the activation energy and reaction rate of pyrolytic reaction of wood [16]. The wood after boric acid treatment had good flame retardation. Baysal processed Douglas fir with boric acid and borax and found that the temperature of processed wood during the combustion and mass loss of processed wood after combustion decreased to some extent [17]. Branca carried out a vacuum pressure treatment of timbers with 5% diammonium hydrogen phosphate and ammonium sulfate [18]. They found that the time to ignition of modified wood was longer than that of unprocessed wood, while heat release and the generated volatile combustible products were decreased to some extent. A P-N-B FRW fire retardant prepared by the Northeast Forestry University of China has good flame retardation, corrosion prevention and insect prevention, and it can increase the flame retardation of wood to B1 level [19]. Yang impregnated veneers with ammonium polyphosphate, boric acid and borax, finding that the total heat release and total smoke output of plywood were decreased significantly, thus proving the good synergistic effect of ammonium polyphosphate, boric acid and borax in flame retardation and smoke inhibition [20,21]. According to Winandy’s studies, the bonding performance of flame-retardant treatment would be decreased. It can be concluded that there are two reasons for this; one reason is the strength loss of the wood itself, while the other one is the decrease of wettability of the wood surface [22,23,24]. If the flame retardant has excellent flame retardance but poor smoke suppression, when it is actually used, the damage caused by the smoke does not weaken. The single-function protectant for wood cannot meet people’s demand anymore. Developing a multi-function protectant which has corrosion prevention, anti-mold and flame retardation is the main research in wood modification at present. One-dose and multi-effect protectant has become a research key of wood modification. It is pointed out that protectants with manganese (Mn) have more significant flame retardation and smoke inhibition [25]. The fire retardant prepared mainly with ammonium chloride has good flame-retardant efficiency and smoke inhibition, accompanied with some bacterial inhibition [18]. Phosphoric acid and phosphate have good flame-retardant efficiency and corrosion prevention [26,27]. Boric acid and borate are equipped with flame-retardant efficiency, insect prevention, corrosion prevention and anti-mold [28,29]. In this study, a compound protectant was prepared by manganese chloride, ammonium chloride, phosphoric acid and boric acid. Moreover, fire resistance, decay resistance, anti-mold and bonding performance of plywood after treatment by the compound protectant were studied and will lay a foundation for the improvement and application of the prepared protectant. P-, N- and B-based compound protectant was prepared in this paper; the difference from other published treatments was introducing the Mn compound, and the purpose was to prepare a protectant that not only had excellent flame retardant but also good smoke suppression. The novelty of this work is discussing P, N, B and Mn compounds as the protectant and on the properties of Pinus massoniana plywood from artificial forest, and the main concern is the overall performance of the prepared plywood.

2. Materials and Methods

2.1. Materials

Phosphoric acid (with a purity of 85%), boric acid (with a purity of 99.5%), ammonium chloride (with a purity of 99.8%) and manganese chloride (with a purity of 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals mentioned in this work were reagent grade. Powdery urea formaldehyde resin (C360), which was used by mixing with water (mass ratio of resin power to water was 100:80) and then adding 0.5% ammonium chloride for the preparation of plywood, was purchased from S.A. WOOD CHEMICALS SND. BHD. (Shah alam, Malaysia. Aspergillus niger V. Tiegh (AV mold), Trichoderma viride Pers. ex Fr. (TV-mold), Coriolus versicolor (CV fungus) and Gloeophyllum trabeum (GT fungus) were applied. Pinus massoniana wood with a size of 2200 mm (length) × 130 mm (width) × 2.5 mm (thickness) and moisture content 10–14% was purchased from Rongjiang Guizhou, China, and after drying, knotless and normally grown sapwood (without reaction wood, decay or insect or fungal damages) materials were selected.

2.2. Treatments of Pinus Massoniana Veneers with Protectant

Based on many tests of different proportions and adding sequences, the protectant was mixed with 3% (w/w) Phosphoric acid, 2% (w/w) boric acid, 6% (w/w) ammonium chlorideand and 4% (w/w) manganese chloride.

Pinus massoniana veneers were dried in an oven (101-1AB electric blast drying oven) at 60 ± 5 °C until reaching constant weights and then moved out and stored in a glass dryer to cool down to the room temperature, weighted and then kept in the vacuum chamber. The protectant was poured into the chamber until 5 cm higher than the surface of the piles of wood samples under vacuum conditions (−0.09 MPa, 60 min). Next, the samples were taken out and the surface liquids of each wood sample were removed gently by a piece of filter paper. The wood samples were kept in an indoor environment for about one week and then dried in an oven at 90 ± 5 °C until the moisture content was at 5~9%.

2.3. Preparation of Five-Layer Plywood

The treated veneers with a double-sided adhesive loading of 220 g/m² were rested at room temperature for 15–20 min. The assembled veneers were then exposed to single-layer hot press unit (XLB type) at Shanghai Rubber Machinery Plant and pressed with a pressure of 1.5 MPa at 100 °C for 15 min to obtain a plywood panel. The plywood panel was conditioned in the laboratory at 20 ± 2 °C and at a relative humidity of 65 ± 5% for 1 day.

2.4. Bonding Strength

The plywood was cut into specimens with dimensions of 100 mm (length) × 25 mm (width). The bond strength of the plywood specimens was tested according to the Chinese National Standard (GB/T 9846-2015). A mechanical testing machine (model WDS-50KN) was used to determine the bonding strength of the plywood specimens. The bonding strength is the mean of 8–10 specimens.

2.5. Fire Resistance Tests

With references to GB/T 2406.2-2009 Oxygen index test of combustion behaviors of plastics, the oxygen index was tested by a TTech-GBT2406-2 intelligent oxygen index analyzer.

Cone calorimeter tests were performed according to the procedures indicated in the ISO 5660-1-2016 standard using a Fire Testing Technology cone calorimeter FTT2000 (London, UK). The plywood panel was conditioned in the laboratory at 20 ± 2 °C and relative humidity of 65 ± 5% for 1 day, and then, they were cut into specimens with dimensions of 100 mm (length) × 100 mm (width) × 10 mm (thickness) prior to testing. The fire scenario comprised four steps: ignition, growth, fully developed and decay. The tests were conducted with 50 kW/m² of heat flux, which corresponded to the fully developed step.

2.6. Testing of Decay Resistance

Decay resistance of plywood was tested with references to Chinese Forestry Industrial Standard (LY/T 1283-2011). Specifically, the culture media were prepared with river sand, saw dust and maltose. Poplar wood was put on the surface as the feeding wood and applied with wood-rotting fungi and then was conditioned at 28 °C and 80% RH for 2 weeks. Test samples were sterilized and then kept on the feeding wood for another 12 weeks after wood-rotting fungi covered the whole the culture medium. Next, samples were taken out and surface impurities were removed. The corrosion strength was evaluated as belonging to the I-level if the mass loss rate was within 0–10%, II-level in 11–24%, III-level in 24–44% and zero decay resistance in >45%.

2.7. Testing of Anti-Mold Property

Anti-mold property was tested according to Chinese National Standard (GB/T 18261-2013). The details are introduced as follows. Potato agar medium was poured into a culture dish and mold was input into the culture medium after it was cooled, which was conditioned at 28 °C and 80% RH for 1 week. After mold covered the whole medium surface, two pieces of glass rods with a diameter of 3 mm were put on the surface of culture medium. Meanwhile, plywood was on the glass rods after being sterilized and cultured for another 4 weeks. Plywood was taken out to observe mold infection. The anti-mold effect was determined according to infection area.

2.8. Thermogravimetric Analysis

A thermogravimetric (TG) analyzer (NETZSCH, Bavaria, Germany) was used to evaluate the thermal resistance of the samples under nitrogen atmosphere at a heating rate of 10 °C/min from room temperature up to 600 °C.

3. Results and Discussion

3.1. Fire Resistance Analysis

The lowest oxygen index (LOI), total heat release (THR), fire performance index (FPI) and fire growth index (FGI) of plywood are shown in Figure 1. Fire resistance of wood is generally measured by LOI, which refers to the lowest concentration of oxygen needed to maintain combustion of woods. The wood with the lower LOI is easier to burn; otherwise, the wood with the higher LOI is more difficult to ignite [30,31]. The LOI of unprocessed plywood was 26.28%, indicating the plywood was combustible. The LOI of plywood after protectant treatment was 50.50%, and it was far higher compared to that of unprocessed plywood, indicating that protectant could increase the fire resistance of plywood and plywood after protectant treatment helped it to basically reach a flame-retardant level. The THR of unprocessed plywood was 69.21 MJ/m², and it decreased by 90.04% to 6.89 MJ/m² for plywood after the protectant treatment, indicating that plywood after protectant treatment had a relatively small THR in the combustion process and showed very evident fire resistance. Protectant treatment decelerated the temperature rise surrounding the ignition point in the fire accident significantly and decreased the fire spreading time and speed effectively.

Potential fire risks of plywood can be evaluated comprehensively by the FPI and FGI. The former one reflects the fire spreading capability when plywood is exposed to a high-heat environment. The FPI is positively related with fire risks. The latter one reflects the tendency of combustion of plywood. A lower FPI indicates a smaller fire risk. The FGI of unprocessed plywood was 6.148 kW·m⁻²·s⁻¹, and it was only 3.646 kW·m⁻²·s⁻¹ after protectant treatment, indicating that protectant led to a small peak heat release of plywood. It took a longer time to reach the peak, and the fire spreading was slow. The FPI was 0.106 s·m²·kW⁻¹ for unprocessed plywood and 0.145 s·m²·kW⁻¹ for protectant-processed plywood, indicating that protectant could prolong the time to ignition of plywood and increase the time for escape. In a word, plywood treated with such protectant had a higher FPI and lower FGI, showing the high safety level.

As a char-forming material, wood forms a char layer on the surface at combustion, showing that wood itself can resist fire to some extent. The protectant is to accelerate the char formation of plywood, inhibit flame spreading, produce heat and decrease the output of toxic gases. Phosphoric acids in protectants can generate metaphosphoric acids in the thermal decomposition process, accelerate dehydration during wood pyrolysis and promote the charring reaction and capture active H· or OH· from gas phases to resist fires. Ammonium chloride is decomposed into vapor, ammonia gas, nitrogen and other non-combustible or difficult-to-combust gases and dilutes combustible gases or isolates oxygen to prevent combustion. The melting substances formed by boric acid during pyrolysis are covered onto plywood, which isolates the spreading of oxygen and heat and promotes the generation of char. MnCl₂ can inhibit smoke and decrease volatile substances effectively. The combination of these reagents and supplement develops a mutual, synergistic effect to accelerate the char-forming rate, thus inhibiting flame spreading and heat production and decreasing the output of toxic gases. As a result, the fire resistance of plywood was improved significantly (Figure 2), and no less than with similar flame retardants [7,8,9].

3.2. Decay Resistance Analysis

Mass loss rate can be used to evaluate wood damages by decay fungi. The results of mass loss rates of two plywood pieces after erosion by GT fungus and CV fungus are shown in Figure 3. The mass loss rates of unprocessed plywood and protectant-processed plywood after erosion of CV fungus were 26.14% and 17.27%, and the mass loss rates after erosion by GT fungus were 29.47% and 19.73%, respectively. This indicated that this protectant could control wood-destroying fungus, especially for CV fungus, and the plywood reached II-level corrosion resistance (>11%, <20%). This was because the protectant contained boric acid and borate, and it had high toxic effects to wood biology and destroyed the appropriate environment for the survival of decay fungi.

3.3. Anti-Mold Properties Analysis

Mildew does not generally influence the mechanical properties of wood, but it can cause surface color changes. The results of anti-mold properties of two plywood pieces are shown in Figure 4 and

Table 1. Results of anti-mold properties of plywood.

Mold	Plywood	Infection Rate/%
		1st Week	2nd Week	3rd Week	4th Week
AV	Control group	40.73	100.00	100.00	100.00
	Experimental group	46.08	100.00	100.00	100.00
TV	Control group	0.00	5.21	74.55	79.27
	Experimental group	2.08	2.60	4.17	46.19

. The areas of both plywood pieces accounted for nearly 50% after AV mold erosion for one week, and the plywood pieces were completely eroded in the second week. This did not represent a significant difference with AV mold, but it was a strong difference with TV, indicating that protectant acted as a good anti-mold for plywood.

The unprocessed and protectant-processed plywood had different performances based on TV mold erosion. In the first week, two plywood pieces were eroded by 0% and 2.08%, respectively. In the second week, they were eroded by 5.21% and 2.60%, respectively. In the third week, they were eroded by 74.55% and 4.17%, respectively. In the fourth week, they were eroded by 79.27% and 46.19%, respectively. The infection rate of both plywood pieces by TV mold increased continuously as time went on. However, plywood treated with protectant showed a better protection effect in the first three weeks (infection rate was lower than 5%). Therefore, the protectant could strongly inhibit TV mold. This was also attributed to boric acid and borate in the protectant which destroyed the survival environment for TV mold.

3.4. Bonding Performance Analysis

The results of bonding performance of plywood are shown in Figure 5. As shown in Figure 5, the bonding strength was 2.2 MPa for the unprocessed plywood, and it decreased by 50% to 1.1 MPa for the protectant-processed plywood, which still could meet the requirements of bonding strength in GB/T 9846-2015 (≥0.7 MPa) and could be mainly used indoors. The decreased bonding strength of plywood after protectant treatment was caused by the following: (1) Strength loss of the wood itself with due to treatments of wood with phosphoric acid, boric acid, ammonium chloride from this compound protectant. (2) The degradation of wood components would lead to the roughness of the wood surface, which would affect the penetration of adhesives. (3) Surface pH was too low. One reason for this was the acidic material of the protectant, such as phosphoric acid, boric acid and ammonium chloride (acid from hydrolysis). Another reason was the degradation of wood. Low pH would cause early solidification of adhesives, especially formaldehyde-based adhesives, such as urea-formaldehyde resin adhesive, melamine formaldehyde resin adhesive, melamine-urea-formaldehyde resin adhesive and so on. (4) During treatment on veneers, some protectant would be precipitated and retained on the surface to influence wetting and penetration of adhesive, thus influencing bonding performance. (5) Chlorides in the protectant could protect plywood from corrosion to some extent and even from insect damage. However, chlorides had a prominent disadvantage of high moisture absorption and hygroscopy. After protectant treatment, the hygroscopicity of plywood was strengthened, thus increasing the initial moisture content of plywood. On one hand, it was easy to cause excessive penetration of adhesive to wood inside, thus causing lack of adhesive and lowering the bonding strength. On the other hand, water diffusion to the outside during thermal compression could decrease the cross-linking of the adhesive and also destroy the interface, thus influencing the bonding performances. Nevertheless, the most important reason for the decrease of bonding strength is the influence of bonding interface, which can be improved by changing other adhesives, such as phenol formaldehyde resin and isocyanate resin to meet the requirements for outdoors.

3.5. Thermal Performance Analysis

In pyrolysis, plywood firstly experiences heat dissipation from water under the action of heat, with temperature ranging from 30 to 150 °C. With the increase of temperature, hemicellulose, cellulose and lignin are successively degraded. The decomposition temperatures of hemicellulose, cellulose and lignin are mainly 180–350 °C, 275–350 °C and 250–500 °C [32,33], respectively. Specifically, there are two decomposition pathways of cellulose: one is the dehydration reaction at about 300 °C, thus generating free radicals, carboxyls and non-flammable gases. The other is the fission and dehydration of cellulose, thus generating low-molecular derivatives, such as levoglucose and water.

The TG–DTG curves of plywood are shown in Figure 6. The degradation of hemicellulose, cellulose and lignin of unprocessed plywood occurred during 181–448 °C, in which 181–400 °C mainly were the degradation of hemicellulose and cellulose [34,35]. The peak at 280 °C was the pyrolysis peak of hemicellulose, and the major peak at 353 °C was caused by the pyrolysis peak of cellulose. The mass loss during 400–550 °C was mainly attributed to degradation of lignin, and the small mass loss during 550–600 °C was caused by the releasing and combustion of residual volatiles in plywood. Degradation of hemicellulose, cellulose and lignin of unprocessed plywood occurred during 143–450 °C. Specifically, degradation of hemicellulose and cellulose was under 143–378 °C, and the pyrolysis peak temperatures of hemicellulose and celluloses were 251 °C and 328 °C, respectively.

The TG curves of two plywood pieces crossed at about 350 °C. Before 350 °C, the mass loss of plywood after protectant treatment was higher than that of unprocessed plywood, while the opposite phenomenon was observed after 350 °C. This showed that the protectant catalyzed the decomposition of wood, which manifested in the quick mass loss before 350 °C. Moreover, the protectant changed the decomposition and reaction processes of plywood, and it drove plywood toward the generation of higher-quantity and more stable char. As a result, the protectant could promote the dehydration and charring performances of wood and accelerate char-forming rates, and it could inhibit flame spreading and heat generation effectively. The TG curve parameters of two plywood pieces are compared in

Table 2. TG parameters of plywood.

Plywood	Initial Temperature /°C	Peak Temperature /°C	End Temperature /°C	Residual Char Yield/%
				150	500	600
Control	181	353	550	96.17	21.28	17.58
Experimental	143	328	500	96.04	29.41	21.64

. The pyrolysis residual weight ratio of plywood after protectant treatment was 8.13% and 4.06% higher at 500 °C and 600 °C, respectively, compared to that of unprocessed plywood. The initial temperature of pyrolysis was 38 °C earlier, the end temperature was decreased by 100 °C and the pyrolysis peak temperature decreased by 25 °C. As a result, the pyrolysis temperature interval was shortened and moved toward the low temperature range generally.

In a word, the components of the protectant had a synergistic effect. On one hand, this decelerated the pyrolysis and decreased the thermal degradation time of wood and blocked heat transmission, thus controlling thermal decomposition and thermal degradation. On the other hand, it could promote combustible substance yield in the low ignition temperature range and decrease the pyrolysis temperature of plywood. Combustible substances were formed and released under a low temperature and then dissipated under the premise of no ignition. Finally, thermal decomposition and thermal degradation of plywood was changed. The protectant promoted the dehydration and charring of plywood to control the generation of acute combustible substances.

4. Conclusions

Single-function wood protectant cannot meet people’s demands anymore. Development of multi-functional protectant with corrosion prevention mildew proof and fire resistance has become a major research topic for wood modification at present. One-dose multi-effect protectant has attracted wide attention in studies on wood modification. A compound protectant was prepared using manganese chloride, phosphoric acid, boric acid and ammonium chloride, and veneer was immersed in the prepared protectant to prepare plywood in this study. Results showed that:

• The plywood after protectant treatment showed not only good fire resistance and smoke inhibition, but also strong char-formation ability, slow flame spreading, long time to ignition, small fire risk and a high safety level.
• The mass loss rates of protectant-processed plywood after infection and erosion in wood-destroying CV fungus and GT fungus were 19.73% and 17.27%, reaching the II-level corrosion grade.
• There was no significant difference with AV mold; however, it was possible to observe a strong difference with TV, indicating that protectant acted as a good anti-mold product for plywood.
• The protectant influenced the bonding interface of wood and bonding conditions of the adhesive. The bonding strength of plywood was weakened, but it still met the requirements on bonding strength of GB/T 9846-2015.
• The protectant changed the thermal decomposition and thermal degradation of plywood, inhibiting the generation of inflammable goods, blocking transmission of heats and lowering the thermal decomposition temperature of plywood. These promoted dehydration and charring of wood, and the generated carbon had a high thermal stability.
• Based on all the results obtained, the applied protectant was highly effective as a fire retardant, while it did not deteriorate the overall performance of the treated plywood. Thus, it is applicable in practice.