Abstract: (1) A compound protectant was prepared using manganese chloride, phosphoric acid, boricacid and ammonium chloride, and then a veneer was immersed in the prepared protectant to prepareplywood in this paper. Great attention was paid to discussing influences of such protectant on fireresistance, decay resistance, anti-mold property and bonding performance of plywood. Resultsdemonstrated that after protectant treatment, the plywood showed not only good fire resistance andsmoke 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 treatmentafter infection and erosion in wood-destroying Coriolus versicolor and Gloeophyllum trabeum were19.73% and 17.27%, reaching the II-level corrosion grade. (3) There is not a significant difference withAspergillus niger V.; however, it was possible to observe a strong difference with Trichoderma viridePers. ex Fr., indicating that the protectant acted as a good anti-mold product for plywood. (4) Theprotectant influenced the bonding interface of wood and bonding conditions of the adhesive. Thebonding strength of plywood was weakened, but it still met the requirements on bonding strength ofGB/T 9846-2015. (5) The protectant changed the thermal decomposition and thermal degradationof plywood, inhibiting the generation of inflammable goods, blocking transmission of heats andlowering the thermal decomposition temperature of plywood. These promoted dehydrations andcharring of wood and the generated carbon had a high thermal stability. (6) Compared with untreatedplywood, the prepared protectant treatment significantly enhanced the fire resistance of plywood,reduced its biodegradability by wood-decaying fungi and showed good mold resistance.
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 akind of natural organic material composed of cellulose, hemicellulose and lignin, whichdetermine the potential fire risks. Wood is not only flammable but can also release a lotof heat at combustion. The average heat release of wood is 18 kJ/g, thus acceleratingflame spreading significantly [1,2]. As the common material in indoor decoration, woodenmaterials are one of the conditions for the occurrence and spreading of fire disasters. Asa result, fire-retardant treatment of wooden materials is one of the effective pathways todecrease fire hazards at present [3–6].
Fire retardants which contain P, N and B have been the research hotspot in fire retar-dants for wood for their characteristics of being toxic-free, smoke-inhibiting, cheap andhaving good fire resistance. Nitrogen–phosphorus fire retardant provides good flameretardation effects to wood by thermal decomposition into non-flammable gases, loweringthe thermal decomposition temperature and increasing the char-formation rate [7–11].Borides are covered onto the wood surface after swelling and melting upon the contactof heat to isolate oxygen, thus realizing the goal of retarding flames by stopping com-bustion of wood [12]. Moreover, borides have functions of anti-corrosion and insectprevention [13–15]. The combination of nitrogen–phosphorus fire retardant and boridesinto a P-N-B system has a synergistic effect of flame retardation and realizes the good flameretardation effect.
Since the 1950s, scholars have carried out a series of studies on the modification offire retardants for wood, which are composed of N, P and B. Biasi immersed cedarwoodin boric acid and found that boric acid treatment decreased the activation energy andreaction rate of pyrolytic reaction of wood [16]. The wood after boric acid treatmenthad good flame retardation. Baysal processed Douglas fir with boric acid and borax andfound that the temperature of processed wood during the combustion and mass loss ofprocessed wood after combustion decreased to some extent [17]. Branca carried out avacuum pressure treatment of timbers with 5% diammonium hydrogen phosphate andammonium sulfate [18]. They found that the time to ignition of modified wood was longerthan that of unprocessed wood, while heat release and the generated volatile combustibleproducts were decreased to some extent. A P-N-B FRW fire retardant prepared by theNortheast Forestry University of China has good flame retardation, corrosion preventionand 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, findingthat 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 boraxin flame retardation and smoke inhibition [20,21]. According to Winandy’s studies, thebonding performance of flame-retardant treatment would be decreased. It can be concludedthat there are two reasons for this; one reason is the strength loss of the wood itself, whilethe other one is the decrease of wettability of the wood surface [22–24]. If the flameretardant has excellent flame retardance but poor smoke suppression, when it is actuallyused, the damage caused by the smoke does not weaken. The single-function protectantfor wood cannot meet people’s demand anymore. Developing a multi-function protectantwhich has corrosion prevention, anti-mold and flame retardation is the main research inwood modification at present. One-dose and multi-effect protectant has become a researchkey of wood modification. It is pointed out that protectants with manganese (Mn) havemore significant flame retardation and smoke inhibition [25]. The fire retardant preparedmainly with ammonium chloride has good flame-retardant efficiency and smoke inhibition,accompanied with some bacterial inhibition [18]. Phosphoric acid and phosphate havegood flame-retardant efficiency and corrosion prevention [26,27]. Boric acid and borateare equipped with flame-retardant efficiency, insect prevention, corrosion prevention andanti-mold [28,29]. In this study, a compound protectant was prepared by manganesechloride, ammonium chloride, phosphoric acid and boric acid. Moreover, fire resistance,decay resistance, anti-mold and bonding performance of plywood after treatment by thecompound protectant were studied and will lay a foundation for the improvement andapplication of the prepared protectant. P-, N- and B-based compound protectant wasprepared in this paper; the difference from other published treatments was introducing theMn compound, and the purpose was to prepare a protectant that not only had excellentflame retardant but also good smoke suppression. The novelty of this work is discussingP, N, B and Mn compounds as the protectant and on the properties of Pinus massonianaplywood from artificial forest, and the main concern is the overall performance of theprepared plywood.
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2. Materials and Methods2.1. Materials
Phosphoric acid (with a purity of 85%), boric acid (with a purity of 99.5%), ammoniumchloride (with a purity of 99.8%) and manganese chloride (with a purity of 99%) werepurchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All otherchemicals 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 was100:80) and then adding 0.5% ammonium chloride for the preparation of plywood, waspurchased from S.A. WOOD CHEMICALS SND. BHD. (Shah alam, Malaysia. Aspergillusniger V. Tiegh (AV mold), Trichoderma viride Pers. ex Fr. (TV-mold), Coriolus versicolor (CVfungus) and Gloeophyllum trabeum (GT fungus) were applied. Pinus massoniana wood witha size of 2200 mm (length) × 130 mm (width) × 2.5 mm (thickness) and moisture content10–14% was purchased from Rongjiang Guizhou, China, and after drying, knotless andnormally 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 protectantwas mixed with 3% (w/w) Phosphoric acid, 2% (w/w) boric acid, 6% (w/w) ammoniumchlorideand and 4% (w/w) manganese chloride.
Pinus massoniana veneers were dried in an oven (101-1AB electric blast drying oven) at60 ± 5 ◦C until reaching constant weights and then moved out and stored in a glass dryerto 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 pilesof wood samples under vacuum conditions (−0.09 MPa, 60 min). Next, the samples weretaken out and the surface liquids of each wood sample were removed gently by a piece offilter paper. The wood samples were kept in an indoor environment for about one weekand 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 atroom temperature for 15–20 min. The assembled veneers were then exposed to single-layerhot press unit (XLB type) at Shanghai Rubber Machinery Plant and pressed with a pressureof 1.5 MPa at 100 ◦C for 15 min to obtain a plywood panel. The plywood panel wasconditioned 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 ChineseNational Standard (GB/T 9846-2015). A mechanical testing machine (model WDS-50KN)was used to determine the bonding strength of the plywood specimens. The bondingstrength 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 behaviorsof plastics, the oxygen index was tested by a TTech-GBT2406-2 intelligent oxygen indexanalyzer.
Cone calorimeter tests were performed according to the procedures indicated in theISO 5660-1-2016 standard using a Fire Testing Technology cone calorimeter FTT2000 (Lon-don, UK). The plywood panel was conditioned in the laboratory at 20 ± 2 ◦C and relativehumidity of 65 ± 5% for 1 day, and then, they were cut into specimens with dimensionsof 100 mm (length) × 100 mm (width) × 10 mm (thickness) prior to testing. The fire sce-nario comprised four steps: ignition, growth, fully developed and decay. The tests wereconducted with 50 kW/m2 of heat flux, which corresponded to the fully developed step.
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2.6. Testing of Decay Resistance
Decay resistance of plywood was tested with references to Chinese Forestry IndustrialStandard (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 appliedwith wood-rotting fungi and then was conditioned at 28 ◦C and 80% RH for 2 weeks. Testsamples were sterilized and then kept on the feeding wood for another 12 weeks afterwood-rotting fungi covered the whole the culture medium. Next, samples were taken outand surface impurities were removed. The corrosion strength was evaluated as belongingto 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 aculture dish and mold was input into the culture medium after it was cooled, which wasconditioned at 28 ◦C and 80% RH for 1 week. After mold covered the whole mediumsurface, two pieces of glass rods with a diameter of 3 mm were put on the surface of culturemedium. Meanwhile, plywood was on the glass rods after being sterilized and cultured foranother 4 weeks. Plywood was taken out to observe mold infection. The anti-mold effectwas determined according to infection area.
2.8. Thermogravimetric Analysis
A thermogravimetric (TG) analyzer (NETZSCH, Bavaria, Germany) was used toevaluate the thermal resistance of the samples under nitrogen atmosphere at a heating rateof 10 ◦C/min from room temperature up to 600 ◦C.
3. Results and Discussion3.1. 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 ofwood is generally measured by LOI, which refers to the lowest concentration of oxygenneeded 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 ofunprocessed plywood was 26.28%, indicating the plywood was combustible. The LOIof plywood after protectant treatment was 50.50%, and it was far higher compared tothat of unprocessed plywood, indicating that protectant could increase the fire resistanceof 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 by90.04% to 6.89 MJ/m2 for plywood after the protectant treatment, indicating that plywoodafter protectant treatment had a relatively small THR in the combustion process andshowed very evident fire resistance. Protectant treatment decelerated the temperaturerise surrounding the ignition point in the fire accident significantly and decreased the firespreading time and speed effectively.
Potential fire risks of plywood can be evaluated comprehensively by the FPI andFGI. The former one reflects the fire spreading capability when plywood is exposed to ahigh-heat environment. The FPI is positively related with fire risks. The latter one reflectsthe tendency of combustion of plywood. A lower FPI indicates a smaller fire risk. The FGIof unprocessed plywood was 6.148 kW·m−2·s−1, and it was only 3.646 kW·m−2·s−1 afterprotectant 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 was0.106 s·m2·kW−1 for unprocessed plywood and 0.145 s·m2·kW−1 for protectant-processedplywood, indicating that protectant could prolong the time to ignition of plywood andincrease the time for escape. In a word, plywood treated with such protectant had a higherFPI and lower FGI, showing the high safety level.
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Figure 1. Results of plywood combustion via a cone calorimeter test.
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 ply‐
wood. 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 protect‐
ant‐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 pro‐
tectant 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 py‐
rolysis 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 sub‐
stances effectively. The combination of these reagents and supplement develops a mu‐
tual, 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 re‐
sistance of plywood was improved significantly (Figure 2), and no less than with similar
flame retardants [7−9].
Figure 1. Results of plywood combustion via a cone calorimeter test.
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 acceleratethe char formation of plywood, inhibit flame spreading, produce heat and decrease theoutput of toxic gases. Phosphoric acids in protectants can generate metaphosphoric acidsin the thermal decomposition process, accelerate dehydration during wood pyrolysis andpromote 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 oxygento prevent combustion. The melting substances formed by boric acid during pyrolysis arecovered onto plywood, which isolates the spreading of oxygen and heat and promotes thegeneration of char. MnCl2 can inhibit smoke and decrease volatile substances effectively.The combination of these reagents and supplement develops a mutual, synergistic effectto accelerate the char-forming rate, thus inhibiting flame spreading and heat productionand decreasing the output of toxic gases. As a result, the fire resistance of plywood wasimproved significantly (Figure 2), and no less than with similar flame retardants [7–9].
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Figure 2. The state of plywood after combustion.
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 pro‐
tectant 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.
Figure 3. Decay resistance of plywood.
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. 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,
Figure 2. The state of plywood after combustion.
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3.2. Decay Resistance Analysis
Mass loss rate can be used to evaluate wood damages by decay fungi. The results ofmass loss rates of two plywood pieces after erosion by GT fungus and CV fungus are shownin Figure 3. The mass loss rates of unprocessed plywood and protectant-processed plywoodafter erosion of CV fungus were 26.14% and 17.27%, and the mass loss rates after erosionby GT fungus were 29.47% and 19.73%, respectively. This indicated that this protectantcould control wood-destroying fungus, especially for CV fungus, and the plywood reachedII-level corrosion resistance (>11%, <20%). This was because the protectant containedboric acid and borate, and it had high toxic effects to wood biology and destroyed theappropriate environment for the survival of decay fungi.
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Figure 2. The state of plywood after combustion.
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 pro‐
tectant 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.
Figure 3. Decay resistance of plywood.
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. 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,
Figure 3. Decay resistance of plywood.
3.3. Anti-Mold Properties Analysis
Mildew does not generally influence the mechanical properties of wood, but it cancause surface color changes. The results of anti-mold properties of two plywood piecesare shown in Figure 4 and Table 1. The areas of both plywood pieces accounted for nearly50% after AV mold erosion for one week, and the plywood pieces were completely erodedin the second week. This did not represent a significant difference with AV mold, butit was a strong difference with TV, indicating that protectant acted as a good anti-moldfor plywood.
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but it was a strong difference with TV, indicating that protectant acted as a good an‐
ti‐mold for plywood.
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 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%, respec‐
tively. 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 in‐
hibit TV mold. This was also attributed to boric acid and borate in the protectant which
destroyed the survival environment for TV mold.
Figure 4. Preservative effect of anti‐mold properties of plywood.
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 de‐
creased 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
Figure 4. Preservative effect of anti-mold properties of plywood.
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Table 1. Results of anti-mold properties of plywood.
Mold PlywoodInfection Rate/%
1st Week 2nd Week 3rd Week 4th Week
AVControl group 40.73 100.00 100.00 100.00
Experimental group 46.08 100.00 100.00 100.00
TVControl group 0.00 5.21 74.55 79.27
Experimental group 2.08 2.60 4.17 46.19
The unprocessed and protectant-processed plywood had different performances basedon 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. Inthe 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 plywoodpieces by TV mold increased continuously as time went on. However, plywood treatedwith protectant showed a better protection effect in the first three weeks (infection ratewas lower than 5%). Therefore, the protectant could strongly inhibit TV mold. This wasalso attributed to boric acid and borate in the protectant which destroyed the survivalenvironment for TV mold.
3.4. Bonding Performance Analysis
The results of bonding performance of plywood are shown in Figure 5. As shownin Figure 5, the bonding strength was 2.2 MPa for the unprocessed plywood, and it de-creased by 50% to 1.1 MPa for the protectant-processed plywood, which still could meetthe requirements of bonding strength in GB/T 9846-2015 (≥0.7 MPa) and could be mainlyused indoors. The decreased bonding strength of plywood after protectant treatment wascaused by the following: (1) Strength loss of the wood itself with due to treatments of woodwith 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 reasonfor this was the acidic material of the protectant, such as phosphoric acid, boric acid andammonium chloride (acid from hydrolysis). Another reason was the degradation of wood.Low pH would cause early solidification of adhesives, especially formaldehyde-basedadhesives, such as urea-formaldehyde resin adhesive, melamine formaldehyde resin ad-hesive, melamine-urea-formaldehyde resin adhesive and so on. (4) During treatment onveneers, some protectant would be precipitated and retained on the surface to influencewetting and penetration of adhesive, thus influencing bonding performance. (5) Chloridesin the protectant could protect plywood from corrosion to some extent and even from insectdamage. However, chlorides had a prominent disadvantage of high moisture absorptionand hygroscopy. After protectant treatment, the hygroscopicity of plywood was strength-ened, thus increasing the initial moisture content of plywood. On one hand, it was easy tocause excessive penetration of adhesive to wood inside, thus causing lack of adhesive andlowering the bonding strength. On the other hand, water diffusion to the outside duringthermal compression could decrease the cross-linking of the adhesive and also destroy theinterface, thus influencing the bonding performances. Nevertheless, the most importantreason for the decrease of bonding strength is the influence of bonding interface, whichcan be improved by changing other adhesives, such as phenol formaldehyde resin andisocyanate resin to meet the requirements for outdoors.
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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 disad‐
vantage of high moisture absorption and hygroscopy. After protectant treatment, the
hygroscopicity of plywood was strengthened, thus increasing the initial moisture con‐
tent 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.
Figure 5. Bonding performance of plywood.
3.5. Thermal Performance Analysis
In pyrolysis, plywood firstly experiences heat dissipation from water under the ac‐
tion of heat, with temperature ranging from 30 to 150 °C. With the increase of tempera‐
ture, 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,
Figure 5. Bonding performance of plywood.
3.5. Thermal Performance Analysis
In pyrolysis, plywood firstly experiences heat dissipation from water under the actionof heat, with temperature ranging from 30 to 150 ◦C. With the increase of temperature,hemicellulose, cellulose and lignin are successively degraded. The decomposition tem-peratures of hemicellulose, cellulose and lignin are mainly 180–350 ◦C, 275–350 ◦C and250–500 ◦C [32,33], respectively. Specifically, there are two decomposition pathways ofcellulose: 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 hemicel-lulose, cellulose and lignin of unprocessed plywood occurred during 181–448 ◦C, in which181–400 ◦C mainly were the degradation of hemicellulose and cellulose [34,35]. The peak at280 ◦C was the pyrolysis peak of hemicellulose, and the major peak at 353 ◦C was causedby the pyrolysis peak of cellulose. The mass loss during 400–550 ◦C was mainly attributedto degradation of lignin, and the small mass loss during 550–600 ◦C was caused by thereleasing 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 peaktemperatures 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 massloss 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 protectantcatalyzed the decomposition of wood, which manifested in the quick mass loss before350 ◦C. Moreover, the protectant changed the decomposition and reaction processes ofplywood, and it drove plywood toward the generation of higher-quantity and more stablechar. As a result, the protectant could promote the dehydration and charring performancesof wood and accelerate char-forming rates, and it could inhibit flame spreading and heatgeneration effectively. The TG curve parameters of two plywood pieces are compared inTable 2. The pyrolysis residual weight ratio of plywood after protectant treatment was8.13% and 4.06% higher at 500 ◦C and 600 ◦C, respectively, compared to that of unprocessedplywood. The initial temperature of pyrolysis was 38 ◦C earlier, the end temperature wasdecreased by 100 ◦C and the pyrolysis peak temperature decreased by 25 ◦C. As a result,
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the pyrolysis temperature interval was shortened and moved toward the low temperaturerange generally.
In a word, the components of the protectant had a synergistic effect. On one hand, thisdecelerated the pyrolysis and decreased the thermal degradation time of wood and blockedheat transmission, thus controlling thermal decomposition and thermal degradation. Onthe other hand, it could promote combustible substance yield in the low ignition tempera-ture range and decrease the pyrolysis temperature of plywood. Combustible substanceswere formed and released under a low temperature and then dissipated under the premiseof no ignition. Finally, thermal decomposition and thermal degradation of plywood waschanged. The protectant promoted the dehydration and charring of plywood to control thegeneration of acute combustible substances.
4. Conclusions
Single-function wood protectant cannot meet people’s demands anymore. Devel-opment of multi-functional protectant with corrosion prevention mildew proof and fireresistance has become a major research topic for wood modification at present. One-dosemulti-effect protectant has attracted wide attention in studies on wood modification. Acompound protectant was prepared using manganese chloride, phosphoric acid, boric acidand ammonium chloride, and veneer was immersed in the prepared protectant to prepareplywood in this study. Results showed that:
1. The plywood after protectant treatment showed not only good fire resistance andsmoke inhibition, but also strong char-formation ability, slow flame spreading, longtime to ignition, small fire risk and a high safety level.
Coatings 2021, 11, 399 10 of 11
2. The mass loss rates of protectant-processed plywood after infection and erosion inwood-destroying CV fungus and GT fungus were 19.73% and 17.27%, reaching theII-level corrosion grade.
3. There was no significant difference with AV mold; however, it was possible to observea strong difference with TV, indicating that protectant acted as a good anti-moldproduct for plywood.
4. The protectant influenced the bonding interface of wood and bonding conditions ofthe adhesive. The bonding strength of plywood was weakened, but it still met therequirements 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 low-ering the thermal decomposition temperature of plywood. These promoted dehydrationand charring of wood, and the generated carbon had a high thermal stability.
6. Based on all the results obtained, the applied protectant was highly effective as a fireretardant, while it did not deteriorate the overall performance of the treated plywood.Thus, it is applicable in practice.
Author Contributions: Z.W. contributed to the analysis of the results and the edit of the paper;X.D. and Z.L. contributed to the preparation of the samples and the testing of the fire resistanceand bonding strength; B.Z. and X.X. contributed to the testing and analysis of decay resistance andanti-mold property; L.Y. and L.L. contributed to the design of the experiment and analysis of the TGresults. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by Science-technology Support Foundation of Guizhou Provinceof China (Nos. [2019]2308, ZK [2021]162, [2020]1Y125, and NY [2015]3027), National Natural ScienceFoundation of China (No. 31800481), Forestry Department Foundation of Guizhou Province of China(No. [2018]13), Cultivation Project of Guizhou University of China (No. [2019]37).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: All the data is provided in the manuscript.
Acknowledgments: The authors highly appreciate the program from Science-technology SupportFoundation of Guizhou Province of China (No. [2019]2325). The authors also thank the anonymousreviewers for their invaluable comments and suggestions to improve the quality of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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