Effects of Size of Zinc Borate on the Flame Retardant Properties of Intumescent Coatings
This paper is aimed at assessing the fire retardancy and thermal stability of intumescent flame retardant (IFR) containing ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (MEL). Zinc borate (ZB) was added at the loading of 2%, 4%, 6%, 8%, 10%, and 12% by weight of IFR. The sizes of investigated ZB fall in 3 ranges: 1-2 μm, 2-5 μm, and 5-10 μm. The performance of APP/PER/MEL was investigated by using thermogravimetry analysis (TGA), cone calorimeter test, Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy, and energy-dispersive spectrometry. The results obtained from the above experiments show that the incorporation of ZB can improve the fire protection performance. A 77% decrease in total smoke production and 84.6% decrease in total heat release were achieved for the addition of 2 wt% ZB (2-5 μm) in the IFR coating. TGA results indicate an increased amount of char residue. Compared to the control IFR coating, the char residue of IFR containing 2 wt% ZB (2-5 μm) has increased approximately 1.5-fold, 10-fold, and 25-fold, at 600°C, 700°C, and 800°C, respectively. The effective char formation results in excellent smoke suppression. Regarding smoke suppression performance, the order for smoke density is IFR/ZB (2-5 μm) < IFR/ZB (5-10 μm) < IFR/ZB (1-2 μm), regardless of investigated loading levels. The decline of smoke suppression performance for IFR/ZB (5-10 μm) and IFR/ZB (1-2 μm) is believed to be due to the poor char formation, as a result of a weak interaction of APP, PER, MEL, and ZB. This weak interaction is caused by the decrease in the specific surface area and agglomeration of ZB particles for IFR/ZB (5-10 μm) and IFR/ZB (1-2 μm), respectively.
Flame retardant coatings are widely used to enhance the flame retardancy of various materials including flammable materials (e.g., polymers and wood) and nonflammable materials (e.g., steel) . Flame retardant coatings may be categorised into two groups: intumescent fire retardant coating and nonintumescent coating. Intumescent flame retardant (IFR) coatings swell in a fire to form a porous char, leading to delay the decomposition and combustion of base materials under high temperature radiation. The intumescent coatings are composed of char formation agents: ammonium polyphosphate-pentaerythritol-melamine (APP-PER-MEL). It is noted that these agents, together with resins in IFR, produce dense smoke and toxic gas during the combustion. Analysis of the death in fire disasters shows that around 85% of the casualty is a result of production of smoke and toxic gases . Therefore, the research of smoke suppression of IFR systems has become a hot topic.
Jiao et al.  studied smoke suppression effects between ferrous powder and ammonium polyphosphate (APP) in the flame retardant thermoplastic polyurethane (TPU). The results showed that the addition of iron powder significantly reduce the smoke generation rate and total smoke release rate. Effects of iron powder on smoke suppression effects depend on the loading levels. The same authors  also tried to reduce smoke release of flame retardants by chemically modifying APP with ETA. When 12.5 wt% of APP/ETA was incorporated into TPU composites, the total smoke release of the sample was reduced by 41.7%. Chen et al.  reported that ferrite yellow (FeOOH) can be used as an effective smoke suppression agent and a good synergism with ammonium polyphosphate (APP) in TPU composites. Yan et al.  investigated the effects of nanosilica on the flame retardancy and smoke suppression properties of IFR coatings. They found that the total smoke release is reduced by 53.1% compared to the control coating, when 3 wt% of nanosilica was added into the IFR coatings. Xu et al.  produced an IFR polypropylene (PP) composite by using SCTCFA-ZnO combined with ammonium polyphosphate (APP). When the mass ratio of APP to SCTCFA-ZnO is 2 : 1, the IFR system exhibits the best flame retardancy and smoke suppression.
Besides the above smoke suppression agents, zinc borate (2ZnO·3B2O3·3.5H2O) has also been extensively used in a flame retardant as a smoke suppression agent. Fang et al.  developed flame retardant wood-polymer composites (WPCs) by utilizing zinc borate (ZB) as synergist with APP. During combustion, ZB and APP can produce carbonaceous foam protecting the underlying material. He et al.  investigated the effects of APP and nano-ZB (nZB) on flame retardation and smoke suppression properties of wood. They found that the total smoke production (TSP) of treated wood with APP/nZB depends on the loading levels of nZB. Compared with wood treated with APP along, the TSP and mean yield of CO of wood treated with APP/nZB were reduced by 78.43 and 71.43%, respectively, when the loading level of nZB was 15 wt%. Zhang et al.  investigated the smoke suppression of ZB and diantimony trioxide (Sb2O3) in epoxy-based IFR coatings. Results show that the values of heat release rate, smoke production rate, smoke factor, and fire growth index all decreased significantly with the addition of ZB and Sb2O3. Tai et al.  investigated the effects of organically modified iron-montmorillonite and ZB on the thermal degradation behaviors and flame retardancy of melamine polyphosphate (MPP) flame-retarded glass fiber-reinforced polyamide 6 (GFPA6), and the results show that replacing a certain proportion of MPP with ZnB or Fe-OMT could improve the UL-94 rating of GFPA6/MPP composites to V0. Recently, Feng et al.  added ZB into an APP/CNCA-DA polypropylene/IFR system. The flame retardancy and thermal stability of the PP/IFR system were investigated. They suggested that the addition of ZB helps to form a compact and homogeneous char layer during combustion. As a result, the flame retardancy of PP/IFR is significantly improved. They also measured the TSP of PP/IFR and reported the minimum value at the loading level of 1 wt%. However, the effects of ZB on the suppression of toxic gases (e.g., CO and CO2) have not been investigated in this study. Wu et al.  investigated the effects of ZB on both smoke suppression and toxicity reduction of an APP/PER polyethylene/IFR system. When ZB was introduced into the polyethylene/IFR system, the TSP value and carbon monoxide concentration were reduced by 47% and 59%, respectively. However, the loading level (of ZB) has not been considered in this study. A latest study  investigated the flame retardancy of APP/PER/MEL systems containing various loading levels of ZB. The loading level has been found to have a significant influence on the formation of the char layers. Doğan and Bayramlı  also identified the effect of a loading level on the performance of IFR/ZB. In their study, ZB was used to improve the flame retardancy of the PP/nanoclay/intumescent system composed of APP/PER. They reported that 3 wt% ZnB containing composite shows the highest rating (V0) according to the UL-94 test. It is noted that the ZB with different particle size was used in the previous research [13–15]. The smaller ZB particles exhibit larger specific surface area which can absorb more smoke and dust during combustion, as compared to larger ZB particles. Therefore, the particle size has influence on the flame retardancy of a coating system containing ZB. However, effects of particle size (of ZB) on the performance of an APP/PER coating have not been reported in the literatures.
In this study, ZB with different particle size was thus added into an APP/PER/MEL coating which was then applied to the surface of the wood. The IFR coatings with 2 wt%, 4 wt%, 6 wt%, 8 wt%, and 10 wt% of ZB were prepared by an orthogonal matrix design, which was similar to the study of Liu et al. . Effects of ZB on the thermal degradation, smoke suppression, and toxicity reduction of the IFR coating have been investigated by thermogravimetry analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, cone calorimeter test (CCT), and infrared smoke analysis.
2. Experimental Programs
To prepare the IFR systems, melamine (MEL), acrylic resin, and pentaerythritol (PER) were purchased from Chengdu Kelong Chemical Reagent Factory. Ammonium polyphosphate (APP) was supplied by Shanghai Yuanye Biotechnology Co. Ltd. Zinc borate, which was used as the additive for smoke suppression, was supplied by Ji’nan Shangshan Fine Chemical Co. Ltd.
2.2. Sample Proportions
The IFR system mainly contains of about 30 wt% of APP, 17 wt% of MEL, 13 wt% of PER, and 14 wt% of acrylic resin. The remaining ingredients are water and additives. ZB was one of the additives used in this study. It was used in three size ranges: 1-2 μm, 2-5 μm, and 5-10 μm. Zinc borate was added into the IFR at six designed contents of 2%, 4%, 6%, 8%, 10%, and 12% by weight. The particle size and loading level for each specimen are summarized in Table 1. The control coating contains no ZB.
2.3. IFR Preparation
The preparation of IFR systems includes the following procedures: grinding, sieving, mixing, and discharging. Firstly, fire retardant components, such as APP, MEL, and PER, were ground to powder. Then, the powders were sieved to allow them pass 200 mesh sieve. The sieved powders were mixed until the uniformity was achieved. The acrylate resin, liquid assistant, and proper amount of water were mixed at a speed of 1800 r/min. This slurry was mixed with the fire retardant components, together with ZB in a mechanical mixer at a speed of 1500 r/min for 30 mins. Then, the mixing rate was adjusted to 750 r/min for defoaming and discharging. The final IFR system was collected and stored in a fridge at 25°C.
2.4. Preparation of Treated Wood
Five-layer plywood was used for coating substrate. Before the coating procedures, plywood was cut in pieces. The smooth surface of the plywood plates showed no scars and obvious defects. The IFR coating used per square meter was . IFR slurry was coated on the surface of substrate repeatedly until the uniform coating was achieved. Two coatings were carried out at a 24-hour interval. During this period, the coated substrates were dried at a constant temperature of and relative humidity of . The mass changing due to drying action was not more than 0.5%. Three samples were prepared for each coating formulation.
2.5. Cone Calorimeter Test
The parameters such as time to ignite (TTI), total heat release (THR), heat release rate (HRR), total smoke production (TSP), and smoke production rate (SPR) of samples were measured by the cone calorimeter (FTT 0007), following the procedures given by ISO 5660-1 . The sample was placed horizontally on the stainless steel sample shelf which was equipped with asbestos net at the bottom. According to ISO 5660-1, the irradiance level (for cone calorimetry) can be adjusted in the range of 10-100 kw·m-2. Carosio et al.  investigated the effects of heat flux on the combustion properties of APP/acrylic systems. They found the char-forming efficiency is higher at 25 kw·m-2 than that at 35 kw·m-2. 25 kw·m-2 was proposed to conduct cone calorimetry testing for acrylic fabrics . It is noted that this irradiance level is higher than the heat flux level required for flashover which normally occurs at an incident heat flux (at floor level) of 20 kw·m-2 . Therefore, in the current study, the samples were exposed to an incident flux of radiant heat of 25 kw·m-2 in the first stage. Then, the cone calorimetry tests for samples Control IFR, ZBS2%, ZBM2%, and ZBL2% were also conducted at the heat flux of 50 kw·m-2.
2.6. Infrared Smoke Analysis
MGA5 infrared smoke analyzer was used to measure the concentration of CO in the smoke which was produced by burning the specimens. The specimen having a size of was placed on the metal net in the smoke box. The gas pressure of propane was adjusted to 276 kPa. The sample was directly ignited with the propane burner, and the smoke produced was completely collected in the test box.
2.7. Thermogravimetric-Differential Thermal Analysis
TGA and DTA were carried out on a TGA/DSC 3+ thermogravimetric analyzer at a heating rate of 10°C/min. 2–4 mg of the sample was examined under both air atmosphere and nitrogen atmosphere. The specimens were heated up to 900°C.
2.8. Fourier-Infrared Spectroscopy Experiments
The residual char of IFR systems was collected after a cone calorimeter test. These char samples were examined by a Nicolet iS10 FT-IR Spectrometer. To prepare pellets, the ratio of KBr to residual char was 200 : 1, and the mixture was ground uniformly in an agate mortar. The grounded powders were subjected to a compressive pressure of 20 MPa for 1 min to form KBr pellets.
2.9. Scanning Electron Microscopy and EDS Analysis
The microstructure of residual char was observed with a scanning electron microscope (Hitachi S3700N). The residual chars were coated with gold palladium. The elemental composition of samples was observed using EDAX (energy-dispersive X-ray) spectrometry.
3. Results and Discussion
3.1. Combustion Performance
Cone calorimeter testing was carried out to study the effects of ZB on the flammability of IFR systems. The measured properties include TTI, THR, HRR, SPR, TSP, and mass loss. The values of these measured properties are summarized in Table 2.
The time to ignition was used as a measurement of ignitibility of materials. The ignition time of the samples is shown in Figure 1. Some samples were not ignited during the experiment; the exposure time of these samples was recorded as the maximum TTI of 1200 s. The ignition time of substrates coated with the control coating (no ZB incorporated) is 657 s. The incorporation of ZB with a large particle size (5-10 μm) has a little influence on TTI. On the other hand, the incorporation of ZB with a medium particle size (2-5 μm) increases TTI up to 1200 s. These trends are independent on the concentration of ZB in IFR coatings. The incorporation of ZB with a small particle size (1-2 μm) increases TTI up to 1200 s at the loading levels of 2 wt% and 4 wt%. TTI decreases as the ZBS concentration further increases.
THR results are presented in Figure 2. Figures 2(a), 2(b), and 2(c) represent the total heat release curves of IFR coatings incorporation of ZB with particle sizes of 1-2 μm, 2-5 μm, and 5-10 μm, respectively. The THR value of the control coating is 10.06 MJ·m-2. When small ZB (1-2 μm) was added into the IFR coating, the THR value decreases at the loading levels of 2 wt%, 4 wt%, and 6 wt%. The minimum value of 4.754 MJ·m-2 was recorded when the loading level is 4 wt%. When medium ZB (2-5 μm) was added into the IFR coating, the THR value decreases all investigated loading levels. The minimum value of 2.318 MJ·m-2 was recorded at a loading level of 2 wt%. When large ZB (5-10 μm) was added, the THR value of modified IFR coating is higher than that of control IFR coating, regardless of the investigated loading levels. When ZB with different particle size was added, the HRR curves of the IFR coating at an optimum loading level (corresponding to a minimum value of THR) are presented in Figure 3. The HRR curves of the control IFR coating are also presented in Figure 3. The HRR values of all samples are presented in Table 2. The HRR value is influenced by the size of ZB and the loading level in a similar way as the THR value.
(a) ZBS samples
(b) ZBM samples
(c) ZBL samples
(a) ZBS samples
(b) ZBM samples
(c) ZBL samples
These results show that both the THR value and the HRR value are significantly reduced when medium ZB was added into the IFR coating. When the particle size is small, ZB tends to agglomerate during mixing, leading to a poor dispersion. This may lead to the damage of formation of the char layer and thus increases the THR value and the HRR value, when large amount of small ZB particles was added into the IFR coating. When the particle size is large, ZB may have poor compatibility with the fire retardant components due to the reduced surface area. This may be also unfavorable for the formation of the char layer, leading to the increase in the THR value and the HRR value.
3.2. Smoke Suppression Performance
Smoke is the most common cause of death in a fire disaster. Therefore, it is very important to study the smoke suppression performance of IFR coatings with ZB . The smoke produce rates measured by a cone calorimeter for coated specimens are shown in Figure 4. The control coating has the largest SPR peak of 0.0212 m2/s at 140 s. When small ZB was added into the IFR coating at the loading level of 2 wt%, the largest SPR peak at 115 seconds is 0.02 m2/s. At the same loading level, when medium ZB was added into the IFR coating, the SPR peak at 120 seconds is 0.0057 m2/s. This is the lowest SPR value for all specimens. Effects of ZB on SPR may be explained by several reasons: (1) ZB can absorb dispersed solid (in a smoke) in the early stage of the combustion and (2) Zn generated by the decomposition of ZB can promote formation of char and enhance the quality of char layer; the enhanced char layer can hold back large amount of combustible gas and prevent oxidation reaction.
(a) ZBS samples
(b) ZBM samples
(c) ZBL samples
The total smoke production of wood treated with IFR systems containing of ZB with a particle size range of 1-2 μm, 2-5 μm, and 5-10 μm is shown in Figures 5(a), 5(b), and 5(c), respectively. As it can be shown in Figure 5, the total smoke of the specimen with control coating is 2.8354 m2/m2 and the TSP value of ZBS2% is less than that of the control coating. When a medium size was used, the addition of ZB decreases TSP values. This is independent on the loading of ZB. When a large size was used, the addition of ZB decreases TSP values at the loading levels of 2 wt%, 8 wt%, and 12 wt%. The lowest TSP value was observed on ZBM2%, in which, the TSP value was reduced by 77.02%, as compared to the control coating.
(a) ZBS samples
(b) ZBM samples
(c) ZBL samples
The smoke density (SD) is an important parameter to evaluate the performance of smoke suppression. The low SD value is associated with high performance of smoke suppression. Figure 6 shows the SD results of wood treated with IFR/ZB coating. For ZBS, the SD value increases with the loading level. For ZBM, as loading level increases, the density of smoke increases at a low ZB loading level and then decreases at a high ZB loading level. For ZBL, the loading level has a little influence on the DR value. ZBM2% shows the lowest SDR value of 13.72%, which is significantly lower than that (25.32%) of the control coating. This result is consistent with TSR results obtained by using a cone calorimeter.
3.3. Effects of Particle Size on Combustion Performance and Smoke Suppression Performance at High Irradiance Level
It can be seen from the results presented in Section 3.1 and 3.2 that the particle size has significant effects on the flame retardancy of IFR/ZB coatings. To further investigate size effects, the loading level of 2 wt% was selected for IFR/ZB coatings with different particle size. It is noted that all IFR/ZBM coatings and some IFR/ZBS coating were not ignited at fixed at an irradiance level of 25 kw·m-2. Therefore, an irradiance level of 50 kw·m-2 was selected to carry out cone calorimetry tests. At a high irradiance level, woods coated with Control IFR, ZBS2%, ZBM2%, and ZBL2% were all ignited, and the trend was similar with that at a low irradiance level. It can be seen from Figure 7 that the TTI of substrates coated with ZBM2% is the longest among all samples.
The THR and HRR curves of wood treated with Control IFR, ZBS2%, ZBM2%, and ZBL2% at the irradiance level of 50 kw·m-2 are presented in Figures (8) and (9), respectively. At Figure (8), Control IFR has the largest THR value of 42.39 kw·m-2·kg-1. The addition of ZBS and ZBM reduces the THR value to 17.98 kw·m-2·kg-1 and 11.81 kw·m-2·kg-1, respectively, while the addition of ZBM shows a little effect of the THR value. In Figure (9), the peak value of Control IFR is 284.2 kw·m-2. The addition of ZB can bring down the peak HRR value. This is independent on a particle size. Among all samples, ZBM2% shows the lowest peak HRR value of 76.01 kw·m-2. According to the results of THR and HRR, the trend of samples Control IFR, ZBS2%, ZBM2%, and ZBL2% at a high irradiance level (50 kw·m-2) is consistent with the results at a low irradiance level (25 kw·m-2).
Figures 10 and 11 show the SPR curve and the TSP curve of IFR/ZB coatings, measured at an irradiance level of 50 kw·m-2. Again, results presented in these figures show a similar trend to those obtained at an irradiance level of 25 kw·m-2. The Control IFR has the largest peak SPR value of 0.16 m2·s-1, and the peak SPR value is reduced significantly to 0.045 m2·s-1 by the addition of ZBM2%. The TSP value of Control IFR is 29.33 m2·m-2, and the TSP value is reduced to 6.12 m2·m-2 after addition of ZBM2%.
Based on the results presented in this section, the best flame retardancy is achieved when medium ZB was introduced into the IFR coating.
3.4. Toxicity Reduction
In the process of the combustion of polymer, smoke released into the atmosphere may contain many toxic and noxious products such as CO, CO2, and nitrogen. It is believed that the death in a fire disaster is mainly due to the production of CO . Therefore, the production of CO was selected as one of the parameters to evaluate the effects of smoke suppression.
The variation in concentration of CO released from each sample is shown in Figure 12; the results showed that specimens (which were not ignited) have the lower CO concentration in the experiments. The CO concentration of ZBM samples was lower than that of ZBS and ZBL, regardless of loading levels. At all investigated loading levels, ZBL samples exhibit higher values of CO concentration compared to the control coating. As far as ZBS is considered, the CO concentration of ZBS is lower than that of the control coating at loading levels of 2 wt% and 4 wt% but higher than that of the control coating at 8% and 10%. Similar to the trend demonstrated by THR and TSP results, ZBM2% is an optimum coating which could effectively reduce the produced CO and favor the escape of people from a fire disaster.
3.5. Mass Loss
When exposed to a fire, the ratio of loss mass to initial mass of all coatings is shown in Figure 13. The low mass loss suggests that large amount of solids (dispersed in a smoke) was captured by the residual char, indicating the good smoke suppression capacity. The results show that ZBL experienced higher mass loss compared to the control coating at ZB loading of 2 wt% and 4 wt%. At the same ZB loading levels, ZBS and ZBM lost their mass slower than the control coating. When the loading level further increases, mass loss of ZBS and ZBM is similar or even higher than the control coating. When the loading level is fixed (e.g., 2%), mass loss of ZBM is much lower than that of the control coating. As irradiance level increases, ZBM shows the lowest mass loss among all samples.
When the coating decomposed and gas formed in the process of being heated, the fire retardant components are dehydrated and expanded to form a dense char layer which captures the dispersed solid in the smoke. At small loading levels, the addition of ZBS or ZBM refines the char layer, leading to an increase in the mass of residual charring due to capturing more solid particles in the smoke. As the loading level increases, the occurrence of agglomeration of zinc borate leads to a reduction in reaction efficiency between ZB and fire retardant components. As a result, the formed char layer may have a relative low density. During the combustion, large amount of solid particles may pass through the coarse char layer, resulting in the increase of mass loss rate. On the other hand, a dense char layer may be formed at a low loading level of ZB. For example, mass loss of ZBM2% is around 27%, which is lower than the control coating with a 32% of mass loss. This sample also shows the lowest value of THR and SPR indicating an excellent flame retardancy. Therefore, the IFRs with a low loading level of ZB were selected for further characterisation. For a purpose of a comparison, the control coating was also examined by TGA, DTA, SEM, EDS, and FTIR.
3.6. Thermogravimetric-Differential Thermal Analysis
The thermal degradation behavior of selected IFR coatings is investigated by thermal analytical techniques. The curves of TGA and DTA are presented in Figures 14 and 15, respectively. The thermal analysis parameters and results are presented in Table 3. The values of degradation temperature and of IFR/ZB are slightly lower than those of the control coating. When mass loss of IFR coatings increases, the degradation temperatures of IFR/ZB are consistently higher than those of the control coating. This tendency is independent on the size of ZB at a loading level of 2%. In comparison with IFR/ZBS and IFR/ZBL, IFR/ZBM exhibits the highest thermal stability. This is demonstrated by its high initial degradation temperature. For example, the addition of ZBM improved the initial degradation temperature of IFR coating from 315°C to 331°C and 540°C to 630°C, corresponding to and , respectively. Table 3 also presents the mass of residual of char at elevated temperatures. At temperatures above 600°C, all IFR/ZB coatings exhibit higher amount of char residue compared to the control coating, indicating that ZB could reduce mass loss caused by char oxidation. Again, IFR/ZBM shows the highest amount of residual mass.
The mass loss of a APP-PER-MEL IFR system may mainly take place in three temperature ranges. At temperatures below 260°C, the mass loss is attributed to the escaping water from the coating. In the temperature range of 260 to 450°C, the mass loss is attributed to the decomposition of fire retardant component and acrylic resin. At temperatures above 450°C, the mass loss is attributed to decomposition of char residue. In Figure 15, small endotherms at around 100°C are associated with the evaporation of water from the control coating. At 190°C and 390°C, the endotherms are attributed to the decomposition of APP and PER, respectively . An endothermic peak at 420°C may be due to the decomposition of acrylic resin. As temperature increases, a distinctive exothermic peak at 630°C is attributed to the oxidation of char. When ZB was introduced into the coating system, the decomposition of acrylic resin takes place at temperatures below 400°C. In the temperature range of 180 to 400°C, APP and PER decompose. Thus, the interaction of APP, PER, and acrylic resin will be promoted, leading to the formation of a good “honeycomb” char structure. The addition of ZB significantly reduces the exothermic peak at 630°C, indicating a better resistance to the oxidation of char. These tendencies are observed on all IFR/ZB coating at a loading level of 2%.
Fire occurs in air only in a very limited time. The combustion is practically performed in an oxygen-depleted environment. Therefore, TGA and DTA were also carried out under nitrogen and the results are presented in Figures 16 and 17, respectively. The thermal analysis parameters and results are presented in Table 4. All IFR coatings (with or without ZB) start to decompose in nitrogen at higher temperature than those in air. This is agreed with results observed on polypropylene . The thermal decomposition of polymers mainly proceeded by the action of heat. In many polymers, the thermal decomposition processes are accelerated by oxidants . Therefore, the minimum decomposition temperatures are lower in air than those in nitrogen. A further examination of the decomposition temperatures shows that the degradation temperatures of IFR/ZB are still consistently higher than those of the control coating. This trend is the same as TGA results under air. A comparison of DTA curves obtained under different atmosphere shows that the exothermic peak at 630°C is not observed in the control coating tested under nitrogen. This is due to the lack of oxidation of char under nitrogen. Based on the results obtained under nitrogen, it can also be concluded that the addition of ZB increases the initial decomposition temperature of fire retardant coating.
3.7. Morphologies of Char Residues
Residual char is an important barrier for intumescent fire retardant coatings to retard the decomposition of combustible materials and prevent heat transfer and oxygen entry . Figure 18 gives the photo for residual char of the control coating and the IFR coatings with ZB at 2 wt% of loading after cone calorimeter tests. All specimens had formed intumescent and compact char layers. A comparison of char layers of different specimens reveals that the char layer of ZBM2% is thick and relatively dense without cracks. This char layer serves as an excellent thermal barrier, holding back the heat and oxygen. As a result, the wood treated with ZBM2% coating exhibits the longest time to ignite during cone calorimeter tests. The intumescing char residue of all these samples is further examined by SEM.
Figure 19 shows the microstructure of intumescing chars at high magnification. As can be seen from the figure, intumescing chars taking from IFR/ZBs show a refine microstructure in comparison with the control coating. At high magnification, the wood substrate can be clearly identified in Figure 19(a). Figures 19(b), 19(c), and 19(d) present the SEM micrographs of intumescent chars obtained from ZBS2%, ZBM2%, and ZBL2%, respectively. The IFR/ZB samples show a continuous and compact char structure. There is no efficient “honeycomb” char structure formed from IFR/ZBS. This may be attributed to the agglomeration of fine ZB particles, leading to the damage of a synergistic effect between flame retardant additives and acrylic resin. This leads to the formation of a loose char structure with small holes. A homogenous char structure without visible cavities is shown in Figure 19(c). The medium ZB particles have a good interaction with fire retardant components, promoting the formation of char. At high temperature, ZB was decomposed into ZnO and B2O3. ZnO may take part in the chemical reaction during combustion to form more crosslinking network which reduces crack and shrinkage. Some ZB particles are identified in Figure 19(d), indicating a poor interaction between ZB and flame retardant additives. The intumescing chars of ZBL2% show cracking due to the nonuniform shrinkage at high temperature.
The chemical compositions of char layers were determined by EDS, as shown in Figure 20 and Table 5. Zn element is identified on ZBM2%, indicating that ZB was decomposed into ZnO and B2O3. ZnO may take part in the chemical reaction during combustion to form more crosslinking network in the char, leading to a dense char layer and thus improves the performance of the coating. Compared to the control coating, ZBM2% shows a relative higher content of P and O element. This is agreed with the result reported in the previous study. Feng et al.  proposed that ZB could promote to remain more P and O in the outer char layer and increase the crosslinking network in the char layer.
(a) Sample Control IFR
(b) Sample ZBM2%
3.8. Infrared Spectrum Analysis
The residual char after CCT was collected and examined by FTIR. The spectrum curves of the control coating and ZBM2% are presented in Figure 21. It can be seen from this figure that the intensity of the absorption peak of –(CH2)n– (720 cm-1) increases when ZB was incorporated in the IFR coating. This implies an increase in the mass of residual char, which is agreed with the TGA result. The absorption peak at 1369 cm-1 is assigned to the bending vibration of C-H [27, 28]. The absorption peaks of –OH (1527 cm-1 and 3742 cm-1) are an indication of decomposition of PER while the absorption peak of C-NH2 (3313 cm-1) is an indication of decomposition of APP . The absorption peak at 2417 cm-1 is associated with CO2  which is produced, as a result of the oxidation of char. The addition of ZB decreases the intensity of this peak. This again confirms that ZBM2% has higher resistance to oxidation of char compared to the control coating. The stretching vibration peaks of C=O (1778 cm-1) bond is attributed to the formation of aldehyde or ketone compounds. They are the intermediate nitrogen-containing production produced by the reaction between acrylic resin and APP. The occurrence of these intermediate products indicates the poor interaction between acrylic resin and APP. ZBM2% shows no stretching vibration peaks of C=O bond. On the other hand, a new peak at 1680 cm-1 is observed on ZBM2%. This peak is attributed to water vapor, as a result of decomposition of ZB into ZnO and B2O3. Zn may be involved in the interaction between APP and acrylic resin, prompting the formation of a char layer.
3.9. Effect of Particle Size on Flame Retardancy and Smoke Suppression
Figure 22 presents a schematic illustration for effects of particle size on the performance of IFR coatings. The small particle is associated with a large surface area which may absorb more dispersed solid (in a smoke), as compared to large particles. On the other hand, the reduced surface area (of IFR/ZBL) affects the interaction between ZB and fire retardant components, leading to a coarse char layer. As a result, ZBM2% exhibits better flame retardancy and smoke suppression performance compared to ZBL2%. Compared to ZBM and ZBL, ZBS has a larger surface area which is associated with a large surface force. However, the increase in surface force causes agglomeration of ZB particles in a mixing process. The agglomerated particles may even absorb less dispersed solids, as compared to large particles. Therefore, the IFR/ZBS coatings show worse smoke suppression performance compared to the IFR/ZBL and IFR/ZBM coatings. At a lower loading level, most of small particles are well dispersed. These particles have a good interaction with fire retardant components, promoting the formation of char. The ZBS2% exhibits an increased charred residue compared to ZBL2%. As reflected by THR and HRR values, the flame retardancy of IFR/ZBS coatings is higher than that of IFR/ZBL coatings. However, due to the presentence of agglomerated particles, charred residue of IFR/ZBS coatings shows a relative loose structure with some small holes on the outer char surface, declining flame retardancy compared to IFR/ZBM coatings.
In this paper, the APP-PER-MEL acrylic/IFR coatings were prepared by the incorporation of ZB with different particle size. Effects of particle size and loading level on fire retardancy and smoke suppression of the acrylic/IFR/ZB coating were studied. The DTA result reveals a decrease in exothermic peak at 2417 cm-1, indicating that addition of ZB enhances the resistance of char oxidation of the IFR coating. This is believed to be due to the increased crosslinking network in the char layer. During the combustion, ZB decomposes into B2O3 and ZnO. Zn is hypothesised to be involved in the interaction of APP, PER, and acrylic resin, promoting the reaction for char formation. As a result, a dense “honeycomb” char structure was observed on the acrylic/IFR/ZB coating. The dense and homogeneous char layer holds back heat and oxygen and thus improves fire retardancy of the IFR coating.
The particle size of ZB shows a significant influence on fire retardancy of the IFR coating. The presence of the ZB with a medium particle size increased the time to ignition from 700 to 1200 s. This is independent on the loading level of ZBM. The addition of ZB with a small particle size increased the TTI value at the loading level up to 6 wt%. As the loading level increases, the addition of ZBS shows a little influence on the TTI value. The addition of ZB with a large particle size also shows a little influence on the TTI, regardless of the loading levels. The result of THR and HRR is agreed with the TTI result. At all investigated loading levels, the addition of ZBM decreases the THR and HRR value. The lowest THR and HRR values were observed on acrylic/IFR/ZB at 2 wt% of ZBM. Compared to the control coating, the THR and HRR values of ZBM2% were reduced by 84.6% and 67.4%, respectively.
The smoke suppression performance of IFR coatings is experimental investigated by both cone calorimeter test and infrared smoke analysis. ZBM2% shows the lowest value of TSP (determined by CCT) and smoke density (determined by infrared smoke analysis). The toxicity reduction of IFR coatings is investigated by monitoring CO concentration during the combustion. ZBM2% again exhibits the lowest CO concentration. Compared to the control coating, the TSP value and CO concentration were reduced by 77% and 73.8%, respectively.
The cone calorimeter test of Control IFR, ZBS2%, ZBM2%, and ZBL2% is also conducted at a high irradiance level (50 kw·m-2). The results of TTI, THR, HRR, SPR, and TSP at a high irradiance level are consistent with those at a low level (25 kw·m-2). Compared with the control coating and other IFR/ZB coatings, ZBM2% shows the best fire retardant and smoke suppression performance.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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