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The protective effects and aging process of the topcoat of intumescent fire-retardant coatings applied to steel structures.

Abstract The protective effects of a topcoat on an intumescent fire-retardant coating were studied under hydrochloric acid solution corrosion, accelerated UV aging and natural weathering. The protective effects were determined to be appreciable under all the aging conditions. During hydrochloric acid corrosion testing, the coating sample without the topcoat exhibited no fire resistance after immersion for 48 h. When a topcoat was used, the acid solution could not directly corrode the fire-retardant coating. However, the solution was observed to seep into the coating layer from the edges, demonstrating that special attention must be paid to the edges and corners to avoid the pollution source corroding the coating and substrate. Under the accelerated UV aging conditions, the fire-resistant properties of the unprotected coating deteriorated significantly after 20 days of aging due to the hydrolysis and photooxidation of the components. However, the coating with a topcoat exhibited better fire-proof properties, even after 40 days of aging. During the natural weathering process, the fire resistance of the coating without a topcoat significantly decreased after being exposed to the high levels of precipitation and UV radiation that occur during summer. The fire-resistant properties were completely lost after 1 year of aging exposure. However, the protected sample exhibited no distinct changes in the duration of fire resistance. The results of this study remind engineering personnel to pay attention to the deterioration of the coating properties and to take protection measures. Despite slightly decreasing the fire resistance performance and increasing the cost, the protective effect of the topcoat was remarkable. Therefore, a compatible topcoat is considered indispensable for fire-retardant coatings used outdoors to ensure the best long-term fire resistance performance.

Keywords Protective effect, Topcoat, Aging property, Fire-retardant coating

Introduction

Intumescent fire-retardant coatings are widely used as a means to provide passive fire protection to steel structures due to their unique advantages, such as the ability to not modify the intrinsic properties of the materials, easy processing and the ability to be applied to several different materials. (1-4) These coatings are usually incapable of withstanding weathering. Fires may happen after 1 year of application, or 10 years or even longer. Therefore, for outdoor applications, a protective topcoat is needed. However, some people are reluctant to use the topcoat, because they think the topcoat may limit the expansion of the fire-retardant coating layer and then affect the fire performance of the coating; additionally, a topcoat adds to the cost of engineering. (5)

The literature on the durability of fire-retardant coatings clearly indicates that most samples used in aging studies do not feature a topcoat. Wang reported an experimental study on the degradation of fire protection performance for two types of intumescent coatings after accelerated hydrothermal aging tests. (6) Jimenez et al. studied the fire performance deterioration of intumescent coatings containing epoxy resin, ammonium polyphosphate, melamine and titanium dioxide under three different accelerated aging conditions [80% moisture at 70[degrees]C for 2 months and a static immersion bath both with and without NaCl (5 g/L) at 20[degrees]C for 1 month]. (7) In our previous study, on the corrosion mechanism of a fire-retardant coating in hydrochloric acid solution (3% in weight), none of the coating samples had protective coatings. (8) However, in some studies, the long-term fire resistance performance of fire-retardant coatings was studied, with the samples featuring a topcoat. Sakumoto et al. conducted accelerated aging tests based on a new testing method for evaluating the durability of intumescent coatings by considering high-temperature and high-humidity weather conditions. (9) Roberts et al. and Wade carried out approximately 15 years of studies on the passive fire protection of different commercial intumescent coatings. (10,11)

In the existing research, with the aim of enhancing the fire resistance and anti-aging performance of fire-retardant coatings, most methods involved adding some specific fillers. For example, in the study of Wang et al., it was proved that the nanometer layer of double hydroxides and nanometer titanium dioxide can improve the antioxidation property of the char structure, and nano-Ti[O.sub.2] can greatly improve anti-aging properties. (12) They also found that, by filling with an appropriate amount of expandable graphite, both the fire-resistant properties and water resistance were improved. (13) Gu et al. prepared a fire-retardant coating with good fire resistance and demonstrated that the JLS-APP had excellent waterproof performance. (14) Li et al. investigated the effects of different types of titanium dioxide on intumescent flame retardants. Their results showed that the sample containing 30 phr rutile-type Ti[O.sub.2] had much a better fire resistance performance than the sample containing 30 phr anatase-type Ti[O.sub.2]. (15) Ullah et al. proved that zirconium silicate was helpful in improving the fire resistance performance of intumescent fire-retardant coatings. (16) In Xue et al.'s study, it was found that the fire protection and char structure of the intumescent fireproof coating was significantly improved by adding 7% expanded vermiculite (by mass) into the coating. (17) Yewa et al. used eggshells as a novel bio-filler for intumescent flame-retardant coatings and reported that the incorporation of Al[(OH).sub.3] into the coating formula slowed down the permeation of water and the migration of fire-retardant ingredients because of its poor solubility in water. (18)

However, most previous works focused on the enhancement of the fire resistance performance of the coating, and the aging conditions were relatively simple, and it could not be proved that the modified coatings had good comprehensive aging properties. In addition, these coatings are still in the research stage and not produced on an industrial scale. In practical applications, a topcoat with a specific thickness is often used to protect the fire-retardant coating layer to improve its durability. But there may be some erroneous understanding and prejudice for the topcoat because it will increase the cost and decrease the fire performance, which leads to the protective effect of the topcoat being overlooked.

In our research, we found that the topcoat slightly decreases the fire resistance time, as long as it is compatible with the fire-retardant coating; however, if the topcoat is not compatible with the fire-retardant coating, it will significantly affect the performance of the fire coating. And the cost of the topcoat is acceptable. According to the fire resistance time and the type of the topcoat, the additional cost for the topcoat is 5-30% (in this study, the cost for the topcoat is about 10%). The relative cost will decrease with the increasing fire resistance time and the thickness of the fire-retardant coating, for the thickness of the topcoat is similar. If the topcoat can prolong the service time of the fire coating, the period before reinvestment and reconstruction will be extended. In the long term, the cost is reduced by use of a topcoat. Moreover, when lives and property are concerned, it is worth appropriately increasing the cost, especially for the crowded and high fire-risk places.

In this study, the fire resistance of fire-retardant coatings both with and without a topcoat were examined under different aging conditions (corrosion by hydrochloric acid solution, UV-accelerated aging and natural weathering) by comparing the surface morphology, the char layer morphology and the fire resistance times of the coatings post-exposure. The study proved the comprehensive effectiveness of the topcoat and reminded engineering personnel to pay attention to the deterioration of the coating properties and to take protection measures. Furthermore, the results of this study may serve as references for the future use of fire-retardant coatings in engineering protection systems for steel constructions.

Experimental

Specimen preparation

The fire-retardant coatings employed in this study were waterborne intumescent coatings. The main components of the fire-retardant coatings were an acrylic emulsion, ammonium polyphosphate (APP), melamine (MEL) and pentaerythritol (PER). When selecting the type of topcoat to use, the following aspects should be considered: strong adhesion; good flexibility; high surface gloss; excellent performance in resisting chemical corrosion, including that due to acid, alkali, water, oil and solvents; excellent weather resistance and aging resistance, matching that of intumescent fire-retardant coatings; and no significant effects on expansion or fire-proof properties. Based on these factors, two-component acrylic polyurethane coatings were used as the topcoat in this study. Additionally, a primer was brushed onto the samples to protect the substrate.

The specimen substrate was a Q235 steel plate measuring 80 mm x 40 mm x 1.2 mm. Before brushing on the coating, the steel substrates were thoroughly cleaned. The interval between the coating layer and the entire sample was established in accordance with the manufacturer's instructions. The thicknesses of the primer, fire-retardant coating and topcoat were 91.5, 1210.4, and 57.3 [micro]m, respectively. The specimens were exposed for 30 days at 20 [+ or -] 5[degrees]C and an RH level of 60% [+ or -] 10% after being coated. In addition, the edges and back sides of the samples used for the acid corrosion experiments were sealed with a mixture of rosin and paraffin (1:1 weight ratio). The sealed specimens were exposed for 24 h.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Figure 1 shows samples with a topcoat and without a topcoat. The surface of the fire-retardant coating with the topcoat is smoother and glossier than that without a topcoat. Therefore, the topcoat provided a certain decorative effect on the fire-retardant coating.

Aging scenarios

All the samples were aged under three conditions: corrosion in a hydrochloric acid solution, exposure to UV light and natural conditions. The aging processes were conducted in parallel and not superimposed. The aging cycles were determined based on the sample type and applied aging scenario. It was expected that the samples without the topcoat would likely lose their intumescent properties completely after the longest aging period. Additionally, before the fire resistance performance testing, the aged samples were dried at room temperature.

With the advent of industrialization, acidic atmospheric conditions have become one of society's most important concerns, especially for petrochemical enterprises. The effects of acid corrosion are significant due to the emission of large amounts of acidic gases. During summer in Jacksonville, Florida, a port city in the USA, the pH of rain ranges from 3.5 to 5.5. (19,20) Acidic corrosion may be one of the most important factors that decreases the fire resistance of fire-retardant coatings. To investigate the protective effects of a topcoat, we submitted specimens to acid corrosion by immersing two-thirds of the specimens vertically in a hydrochloric acid solution (3 wt%). (21)

For UV-accelerated aging exposure, UVB-313 fluorescent lamps were used as the light source. The distribution of the relative spectral power complies with the requirements shown in Table 1. (22) One exposure cycle lasted 12 h and included an 8-h UV exposure period at 60 [+ or -] 3[degrees]C black panel temperature and a 4-h condensation exposure period at 50 [+ or -] 3[degrees]C black panel temperature. The irradiance of the lamps was 0.65 W/[m.sup.2] nm. The relative humidity during the irradiance period was 15%, and that during the condensation period was 100%.

[FIGURE 3 OMITTED]

For natural weathering, the samples were fixed on a plate and placed facing south. The plate was positioned on a bracket 50 cm from the ground at an angle of 45[degrees] with respect to the ground. The studies were conducted in Langfang City in Hebei Province, China. The city is located at 39.6[degrees]N, 116.7[degrees]E, between Beijing and Tianjin. The area is characterized by a warm temperate continental monsoon climate. It should be noted that, for other weathering conditions, the aging period may be different from this study. From this work, we think the main aspects that cause the deterioration of the fire performance of the coating, are precipitation and UV radiation. So, in the region with high precipitation and UV radiation, the fire-retardant coating will lose its fire property in a short period and vice versa. However, the post-weathering measurements used in this paper could be valid in any climate.

Testing equipment and conditions

The fire resistances of the aged samples were tested using a small-scale fire-proof testing furnace for intumescent coatings. (23) Figure 2 shows cross-sections of the furnace. The furnace chamber size was 8 cm x 8 cm x 18.5 cm, the thickness of the insulation material around the furnace was 5 cm, and the thickness of the insulation material in the furnace bottom and top was 15 cm. During testing, the uncoated sides of two specimens were attached to the lower end of both sides of a cooling sheet, which was made of steel or a heat-resistant alloy. Wire was used to combine the cooling chip and the two samples. The cooling sheet and the two samples were placed vertically on the bracket within the furnace. One thermocouple was set in the groove of the cooling sheet to measure the backside temperature of the specimens. Another thermocouple was placed in the furnace to probe the temperature of the furnace. The time-temperature curve in the furnace is in accordance with the ISO 834 standard curve. The time required for the back-side temperature of the samples to reach 538[degrees]C is defined as the fire resistance time.

Experiment results

The protective effect of the topcoat for the acid corrosion

The samples without a topcoat were placed in a hydrochloric acid solution for 0, 16, 24, 36 or 48 h. The samples were then dried, and the mixture layer of resin and paraffin was removed. Fire resistance performance tests were subsequently conducted. Table 2 shows the test results. For the corroded samples, the average thicknesses of the untreated and corroded parts were determined for five points. Based on these data, it was concluded that long corrosion times corresponded to large amounts of sample deterioration. Compared to the initial sample, when the samples were exposed for 48 h, the resistance time degradation reached 43.4%, the fire-resistance time was only 30 min and 48 s (30'48"), the treated layer barely expanded, the thickness of the char was 7.7 mm and the expansion factor was only 4.9. Additionally, the thickness of the char layer on untreated part decreased remarkably. The resistance time degradation shown in Table 2 is the decrease in the fire resistance time of the coating specimens after acid corrosion exposure. This relationship is formulated as follows:

[FIGURE 4 OMITTED]

Resistance time degradation =

Fire resistance time of the untreated sample - Fire resistance time of aged sample/ Fire resistance time of the untreated sample (1)

The expansion factor can be calculated as follows:

Expansion factor = Thickness of the char layer - Initial thickness of the coating/ Initial thickness of the coating (2)

Figure 3 shows the surface morphology of the specimens after 16 h and 48 h of hydrochloric acid corrosion. The corroded part of the coating was thinner than the untreated part; this discrepancy is attributed to the fact that certain components of the coating were lost due to dissolution, acid-catalyzed hydrolysis and chemical reactions with the acid; that is, the PER dissolved in the water, APP had an acid-catalyzed hydrolysis and MEL reacted with the hydrochloric acid which produced with ammelid, ammeline and cyanuric acids. All these processes resulted in the deterioration of the fire-resistant properties of the coating. (8) As the corrosion exposure time increased, the layer gradually thinned.

After the fire resistance time test, the morphology was also observed to vary dramatically. Figure 4 shows the char layer morphologies of the samples after exposure to hydrochloric acid corrosion for different durations. The char on the initial sample was spongy, expanded evenly and compactly, and did not exhibit any large pores or cracks. However, the char on the corroded specimens possessed a hard shell on the corroded portions, which was not obvious on the untreated part. The boundary line between the corroded and untreated parts was not clear on the char layer for the short-exposure corrosion samples. The thickness of the char layer decreased from the corroded side to the untreated side, and the shape of the section was similar to that of a slope. However, for the 48-h corrosion samples, the boundary line was much clearer and the thickness of the char changed significantly.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The samples coated with a topcoat were immersed in hydrochloric acid solution under the same conditions for 0, 10, 15, 20 or 30 days. The fire resistance times and the thickness of the char layer of the samples for the different corrosion times are listed in Table 3. Because there was almost no difference between the corroded and untreated parts, the char thickness was not separated into the corrosion layer and the untreated layer in Table 2. The char layers on the samples shown in this table were the thickest. For the initial samples, the resistance time of the samples with a topcoat was 3'44" shorter than that for the samples without a topcoat. This difference was observed because the topcoat formed a relatively hard shell on the char surface when heated, which could limit the expansion of the char layer, resulting in a smaller char thickness and expansion factor. However, with respect to acid corrosion resistance, the specimens coated with a topcoat performed remarkably better than those without a topcoat. For the samples without a topcoat, the fire-retardant performance was nearly completely lost after 48 h (2 days) of corrosion exposure. However, for the samples with a topcoat, after 10 days of corrosion exposure it was only 6.9%, and the fire resistance time was relatively long even after 30 days of corrosion exposure. Figure 5 shows the expansion factor as a function of corrosion time for the topcoat and non-topcoated specimens. It can be seen that, in the acid corrosion condition, the topcoat can protect the fire-retardant coating fairly well.

[FIGURE 7 OMITTED]

Figure 6 shows the surface morphology of the topcoat samples after different acid corrosion times. As shown, there is no boundary line between the corroded and uncorroded parts, unlike in the samples without a topcoat. The thickness of the coating did not decrease overall, but the edges of the coated samples exhibit the thinning and material loss phenomena. As the corrosion time increased, the affected coating layer area increased. No bubbling or thinning of the layer was observed at the center of the samples. The reason for this phenomenon is that the sealing material [a 1:1 mixture (by weight) of rosin and paraffin] cracked after long exposure to acid corrosion, meaning that the acid solution could infiltrate and corrode the coating layer from the edge. Therefore, if we do not consider the limitations of the experimental conditions, the protective effect of the topcoat for the fire-retardant coating is much better than the measured result. In addition, based on this phenomenon, fire-retardant coating construction personnel should be reminded that special attention must be paid to the edges and corners to ensure a high-quality seal and avoid corroding the coating and substrate along any exposed edges.

Figure 7 shows the char morphology of samples with a topcoat after different acid corrosion times. The morphology of the sample corroded for 10 days was similar to that of the initial sample. The char layer expanded uniformly, the char shell generated on the topcoat was white and the low expansion factor appeared only in a few corners of the layer. After a long period of corrosion, the color of the char shell changed to black and no longer covered the entire char layer. It appeared that the char shell was cracked by the expanding char layer. The edge of the immersed part of the coating did not expand, and the area of the unexpanded coating grew as the corrosion time increased.

The topcoat components decomposed during the acid corrosion process. The composition of the thermal decomposition products was different from the initial composition, which led to the change in the color of the char shell. Additionally, the thickness and toughness of the char shell also changed. The char shell was easier to crack due to the expanding char layer, and the restriction on char layer expansion was reduced. Therefore, the largest thickness of the sample submitted to long corrosion exposure was slightly larger than that for the sample exposed to corrosion for a short period. However, for the larger unexpanded area of the sample exposed for a long corrosion period, the resistance time was shorter than that for the sample exposed for a short period.

The protective effect of the topcoat for accelerated UV aging

The samples without a topcoat were UV-aged for 0, 5, 10, 15 or 20 days, and the samples with a topcoat were UV-aged for 0, 10, 20, 30 or 40 days. Table 4 lists the fire resistance times and the char thicknesses of the samples submitted to different aging times. It can be observed that increasing the aging time decreased the fire resistance times, with the resistance time degradations of the first three samples increasing by approximately 13%. The fire resistance times of the sample aged for 15 days was 34 min, and the resistance time degradation was 40.8%. Late in the aging process, the acceleration of the resistance time degradation decreased. The fire resistance time of the sample aged for 20 days was 32T3", and the resistance time degradation was 43.9%, an increase of only 3.1% over that of the sample aged for 15 days. The thickness of the char layer on this sample was also the greatest, and the thickness and the expansion factor decreased with increasing aging time.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Figure 8 shows the surface morphology of the samples without a topcoat after different accelerated UV aging times. With increasing aging time, the surface of the coating became rough and yellow, with bulging parcels and cracks observed for some of the longest aging times. Figure 9 shows the char morphology of the samples without a topcoat after different accelerated UV aging times. The char layer of specimen aged for 5 days was relatively intact, but the hard shell that formed on the char surface limited the expansion of the internal char, which led to a decrease in both the thickness and fire resistance time relative to those of the initial sample. Additionally, some parts of the char on the edges of the sample aged for 15 days did not expand, and the thickness of the intumescent char decreased. Although the greatest thickness of the sample aged for 20 days was similar to that of the sample aged for 15 days, the area of the unexpanded parts increased, the char layer was loose, the intensity and density were poor and the fire resistance time was significantly reduced.

In some studies, it has been postulated that accelerated weathering aging conditions cause the hydrophilic components in a coating to migrate from the inside to the outside of the coating due to the surrounding moisture, which affects the fire protection performance of the coating. (13,24-26) Additionally, the photooxidation of the acrylic resin is another aspect that enhances the deterioration of the fireproof properties of the coating. (27) In our study, we determined that other components, such as APP and MEL, also experienced photooxidation; that is, the bonds of PO and the bonds of C-N, which were included in APP and MEL respectively, were broken. The details about the degradation mechanism during UV accelerated weathering will be reported on in another paper; herein, we focus only on the protective effects of the topcoat.

Data regarding the fire resistance times and char thicknesses of the samples with a topcoat after different UV accelerated aging times are listed in Table 5. The tendency of the fire resistance time to decrease with an increase in the aging time were again observed. Compared with those of the specimens without a topcoat, the anti-aging properties are excellent. For the same condition of aging for 20 days, the fire resistance time and resistance time degradation of the sample without a topcoat were 32T3" and 43.9%, respectively, and 44T2" and 17.6%, respectively, for the sample with a topcoat. Among all the samples aged for long periods (20, 30 and 40 days), the fire resistance times and resistance time degradations showed few differences. Figure 10 shows the expansion factor as a function of aging time for the specimens with and without a topcoat.

After accelerated UV aging, with the exception of the glossiness decreasing, there were no distinct changes in the morphology of the topcoat. Some samples showed cracked topcoat surfaces after long aging exposure times, as shown in Fig. 11. However, based on the fire-testing results, the cracks in the coating surface did not have a significant effect on the fire resistance performance. Figure 12 shows the char morphology of a coating with a topcoat after different UV-aging times. All of the char layers on the aged samples were intact and uniform with no unexpanded parts. The color of the topcoat char was black after aging, and the area of the black shell increased with the aging time. When the aging time reached 20 days, the black shell covered almost the entire char layer on the coating.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

During the aging process, the topcoat experienced an oxidative decomposition reaction during UV radiation under high-temperature and high-humidity conditions. (21) The molecular structures changed, and the compatibility with the internal coating deteriorated when the coating was exposed to heat. As a result, the aged topcoat limited the expansion of the coating, reduced the thickness of the char layer and decreased the fire resistance time. At the same time, during the UV aging, the components of the topcoat may produce aromatic hydrocarbon, which would enhance the amount of residual carbon, and the color of the topcoat layer was black. When the topcoat was aged to a certain extent, it remained unchanged, and the fire resistance times for the long-exposure samples exhibited little differences.

The protective effect of the topcoat for natural weathering

Samples with and without topcoats were aged under the above-mentioned natural weathering condition from November 25, 2013 onward (for analysis convenience, the aging start time was dated from December 2013). The aging periods were 6, 9 and 12 months. The fire resistance time and the char thickness of each aged sample are listed in Table 6. By contrast, when the samples were aged for 6 months (the total precipitation was 71.4 mm, the total radiation was 1849.2 MJ/[m.sup.2]), the fire performance was similar between the two types of samples, with those featuring a topcoat exhibiting slightly better performance than those without. However, when the aging time was extended to 9 months (the total precipitation was 335.8 mm, the total radiation was 3642.4 MJ/[m.sup.2]), the durability of the specimen with the topcoat was remarkably better than the sample without the topcoat. The fire resistance time for the 12-month-aged sample without a topcoat (the total precipitation was 441.2 mm, the total radiation was 4846.0 MJ/[m.sup.2]) was only 21'18", the resistance time degradation reached 62.9% and the expansion and heat insulation effects were completely lost. However, the fire resistance time for the sample with a topcoat was 42'04", which was almost twice that of the sample without a topcoat. In terms of expansion, the sample without a topcoat completely lost its expansion abilities after 12 months of aging, whereas the expansion factor was 18.4 for the sample with a topcoat. Figure 13 shows the expansion factor as a function of natural weathering aging time for the specimens with and without topcoat.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

Figure 14 shows the surface morphologies of the samples without a topcoat after different natural weathering times. The 6-month-aged sample (tested from November 2013 to May 2014) experienced winter and spring outdoors. Due to the climatic characteristics of this period, the amount of sand exposure was relatively large, the average temperature was low and the average precipitation and UV radiation exposure were low. Therefore, the surface of the sample absorbed a significant amount of sand and developed several cracks, the depths of which were shallow. The samples aged for 9 months experienced an additional 3 months of exposure (from June to August). The average temperature, precipitation and the UV radiation were highest during this period. The dust that adhered to the coating surface was washed off by the rainfall, and the surface was much clearer. However, both the number and depth of the cracks on the surface increased. The sample aged for an additional 3 months (September-November) experienced a significantly greater amount of cracking, and the cracks were both wider and deeper. Table 7 lists the relevant meteorological data gathered from December 2013 to November 2014. All these data were obtained from the local meteorological bureau. The type of the ultraviolet radiometer employed in this study was an HSC-FZAB1. For this instrument, the photoelectric detector is used to receive the electrical signal of the ultraviolet light. The wavelength range of the filter is from 280 to 400 nm, and the output voltage range is from 0 to 200 mV.

[FIGURE 15 OMITTED]

Figure 15 shows the char layer morphology of the samples without a topcoat after different natural weathering aging times. The char layer of the 6-month-aged sample was dense and expanded evenly. However, a large pit appeared in the middle of the char layer, and a hard char shell formed on the edge of the sample. The height of the char layer on the 9-month-aged sample was not uniform, the char texture was loose and not intact and some of the edge parts did not expand. The 12-month-aged coating did not expand at all and only generated a hard black char layer. These phenomena were related to the different climatic conditions experienced by the samples. From December 2013 to May 2014, the amounts of precipitation and UV radiation the samples were exposed to were small, and these factors had little negative effect. However, in the following months, from June to October, both the precipitation and UV radiation levels were relatively high, and the components degraded heavily, resulting in a lack of expansion and the deterioration of the fire resistance properties.

[FIGURE 16 OMITTED]

[FIGURE 17 OMITTED]

Compared to the initial sample, the samples experienced a different degree of sand dust adhesion. The glossiness of the topcoat decreased, but the integrity of the topcoat was better than that of the sample without a topcoat. Additionally, no cracks appeared, as shown in Fig. 16. Figure 17 shows the char morphology of the topcoat specimens after different natural weathering exposure times. The area of the black char shell formed by the aged topcoat during the heating condition increased with the aging period. The expansion effect was limited by the hard black shell, and the expansion factor decreased, especially for the 12-month-aged sample. However, the morphology of the internal char layer exhibited almost no changes and was still fluffy, dense and compact due to the protection of the topcoat. Therefore, the fire performance of the coating was not severely affected.

Conclusion

In this study, the protective effect of a topcoat was examined under three aging conditions: hydrochloric acid solution corrosion, accelerated UV aging and natural weathering. For the un-aged samples, the use of a topcoat may have reduced the fire resistance time due to limitations on coating expansion. However, the restriction was acceptable if the topcoat was matched to the fire-retardant coating. The protective effect of the topcoat was observed to be excellent with respect to long-time aging performance.

During the hydrochloric acid corrosion test, the coating sample without a topcoat exhibited almost no fire resistance after immersion for 48 h. This result was observed because certain components in the coating were lost due to dissolution, acid-catalyzed hydrolysis and chemical reactions with the acid. With the protection of the topcoat, the acid solution did not corrode the fire-retardant coating directly, and the coating could be well protected. However, the solution could seep into the coating layer from the nonsealed edges, demonstrating that the edges and the corners must be paid special attention to prevent a pollution source from corroding the coating and the substrate along the exposed edges. Under the accelerated UV aging conditions, the fire resistant properties of the coating not protected by a topcoat deteriorated significantly after 20 days of aging due to hydrolysis and photooxidation of the components. However, the coating with a topcoat exhibited better fireproofing, even after 40 days of aging. During natural weathering tests, the fire resistance of the coating without a topcoat was significantly reduced due to the high levels of precipitation and UV radiation that occur during summer, and the fire-resistant properties were completely lost after 1 year of aging. However, the sample protected with a topcoat exhibited no distinct changes in the fire resistance time.

Aging may cause fire-retardant coatings to degrade and lose their fire protection performance. They should be protected by the topcoat for improving its durability. And the topcoat hardly decreases the fire resistance performance if it is compatible with the fire-retardant coating. Though the topcoat will increase the cost from 5% to 30%, in the long-term, the cost is reduced for the long service time of the coating. Moreover, when the lives and property are concerned, it is worth appropriately increasing the cost, especially for crowded and high fire-risk places. Therefore, a compatible topcoat is indispensable to ensuring long-term fire resistance performance, especially when used outdoors.

DOI: 10.1007/S11998-015-9733-9

J. Wang ([mail])

Fire Protection Engineering Department, Chinese People's Armed Police Forces Academy, Xichang Road 220#, Anci District, Langfang 065000, Hebei, China

e-mail: wanghutty@163.com

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(15.) Li, Hongfei, Hua, Zhongwu, Zhang, Sheng, et al., "Effects of Titanium Dioxide on the Flammability and Char Formation of Water-Based Coatings Containing Intumescent Flame Retardants." Prog. Org. Coat., 78 318-324 (2015)

(16.) Ullah, Sami, Ahmad, Faiz, "Effects of Zirconium Silicate Reinforcement on Expandable Graphite Based Intumescent Fire Retardant Coating." Polym. Degrad. Stab., 103 49-62 (2014)

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=====
Table 1: Relative spectral power distribution specification
for fluorescent UVB-313 lamps

Spectral pass band
wavelength (nm)                              Minimum (%)   Maximum (%)

[lambda] < 290                                   1.3           5.4
290 [less than or equal to] [lambda] [less      47.8          65.9
  than or equal to] 320
290 < [lambda] [less than or equal to] 360      26.9          43.9
360 < [lambda] [less than or equal to] 400       1.7           7.2

Table 2: The fire resistance times and char thicknesses of the
coatings without a topcoat for different acid corrosion exposures

                                           Untreated layer
               Fire
Corrosion   resistance   Resistance time   Thickness of the   Expansion
time (h)       time      degradation (%)      char (mm)        factor

0             57'24"           --                36.4           27.0
16            50'26"           7.3               32.4           23.9
24            39'27"          27.5               26.1           19.0
36            34'45"          36.1               23.0           16.7
48            30'48"          43.4               21.9           15.8

            Corrosion layer

Corrosion   Thickness of the   Expansion
time (h)       char (mm)        factor

0                  --             --
16                24.9           18.1
24                18.2           13.0
36                11.8            8.1
48                 7.7            4.9

Table 3: The fire resistance time and char thickness of the coating
with a topcoat for different acid corrosion exposures

Corrosion    Fire resistance  Resistance time  Thickness of   Expansion
time (days)       time        degradation (%)  the char (mm)   factor

0                53'40"             --             32.8         23.1
10               49'59"             6.9            28.7         20.1
15               42'04"            21.6            27.1         18.9
20               38'56"            27.5            31.4         22.1
30               35'57"            33.0            30.2         21.2

Table 4: The fire resistance times and char thicknesses of the
samples without a topcoat after different UV-aging exposure times

Aging        Fire resistance  Resistance time  Thickness of   Expansion
time (days)       time        degradation (%)  the char (mm)   factor

0                57'24"             --             36.4         27.0
5                49'26"            13.9            27.5         20.1
10               41'27"            27.8            23.4         17.0
15               34'00"            40.8            21.6         15.6
20               32'13"            43.9            21.2         15.3

Table 5: The fire resistance times and the char thicknesses of the
samples with a topcoat after different UV-aging exposure times

Aging        Fire resistance  Resistance time  Thickness of   Expansion
time (days)       time        degradation (%)  the char (mm)   factor

0                53'40"             --             32.8         23.1
10               47'04"            12.3            26.4         18.4
20               44'12"            17.6            24.6         17.1
30               45'02"            16.1            25.0         17.4
40               43'18"            19.3            23.8         16.5

Table 6: The fire resistance times and the char thicknesses
of the samples exposed to natural weathering conditions
for different times

                                   Resistance time
           Fire resistance time    degradation (%)

Aging      Without      With       Without      With
time (m)   topcoat    topcoat      topcoat    topcoat

0           57'24"     53'40"         --         --
6           50'45"     52'50"        11.6       1.6
9           34'15"     44'42"        40.3       16.7
12          21'18"     42'04"        62.9       21.6

           Thickness of the
           char (mm)               Expansion factor

Aging      Without      With       Without      With
time (m)   topcoat    topcoat      topcoat    topcoat

0            36.4       32.8         27.0       23.1
6            29.3       31.8         21.5       22.4
9            20.8       27.2         19.5       19.0
12            --        26.3          --        18.4

Table 7: The meteorological data during
the natural weathering period

                   Daily average     Daily average
                      maximum           minimum
                    temperature       temperature
Period             ([degrees]C)      ([degrees]C)

December 2013          5.6               -5.3
January 2014           5.1               -5.7
February 2014          4.1               -4
March 2014             16                4.2
April 2014             23.7              10.5
May 2014               28.7              15.5
June 2014              30.9              19.4
July 2014              33.2              23.6
August 2014            31.5              21
September 2014         25.9              16.5
October 2014           19.8              9.4
November 2014          12.4              0.8

                  Precipitation   Total radiation
Period                (mm)        (MJ/[m.sup.2])

December 2013         1.8             228.3
January 2014          0               216.8
February 2014         2.1             249.7
March 2014            0               254.8
April 2014            25.1            407.9
May 2014              42.4            491.7
June 2014             77.9            635.4
July 2014             64.3            602.4
August 2014           122.2           555.4
September 2014        83.9            534.5
October 2014          21.2            379.4
November 2014         0.3             289.7
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Author:Wang, Ji
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jan 1, 2016
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