And Zinc Borate-Based Intumescent Coating Materials Exposed to a Quasi-Real Cellulosic Fire

polymers

Article Eﬀect of the B:Zn:H2O Molar Ratio on the Properties of Poly(Vinyl Acetate) and Zinc Borate-Based Intumescent Coating Materials Exposed to a Quasi-Real Cellulosic Fire

Jakub Łopi ´nski 1, Beata Schmidt 1 , Yongping Bai 2 and Krzysztof Kowalczyk 1,*

1 Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, Piastów Ave. 42, 71-065 Szczecin, Poland; [email protected] (J.Ł.); [email protected] (B.S.) 2 School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150000, China; [email protected] * Correspondence: [email protected]

Received: 25 September 2020; Accepted: 29 October 2020; Published: 30 October 2020

Abstract: In order to investigate an influence of the B:Zn:H2O molar ratio on the fire protection efficiency of poly(vinyl acetate)-based thermoplastic intumescent coating materials (ICs), systems containing ammonium polyphosphate, melamine, pentaerythritol and different types of zinc borates (ZBs) were tested in a vertical position in quasi-real fire conditions. 3ZnO 2B O 6H O (ZB6), · 2 3· 2 2ZnO 3B O 3.5H O (ZB3.5) or 3ZnO 2B O (ZB0) were added in amounts of 1–10 wt. parts/100 wt. · 2 3· 2 · 2 3 parts of the other coating components mixture. Char formation processes and thermal insulation features were investigated using an open-flame furnace heated according to the cellulosic fire curve. Thermogravimetric features (DTG), chemical structures (FTIR) and mechanical strength of the ICs and the chars were analyzed as well. It was revealed that the type and dose of the ZBs significantly affect thermal insulation time (TIT) (up to 450 ◦C of a steel substrate) and sagging (SI) of the fire-heated coatings as well as the compressive strength of the created chars. The highest TIT value (+89%) was noted for the sample with 2.5 wt. parts of ZB3.5 while the lowest SI ( 65%) was observed for the − coatings containing 10 wt. parts of the hydrated borates (i.e., ZB3.5 or ZB6). The best mechanical strength was registered for the sample filled with the anhydrous modifier (3 wt. parts of ZB0). The presented results show that the ICs with the proper ZBs can be used for effective fire protection of vertically positioned steel elements.

Keywords: poly(vinyl acetate); intumescent coating; zinc borate; cellulosic ﬁre; ﬂame retardancy

1. Introduction Many types of active and passive fire protection of structures and buildings are known. They are intended to signal a fire, extinguish it, or—in the case of the passive safety measures—to provide more time for the emergency evacuation of people. The most important kind of the passive fire protection is covering the metallic construction elements of buildings by intumescent coatings (ICs) in order to reduce the destructive effect of a high temperature. Structural steels (depending on their composition) lose their load bearing capacity above the temperatures of 450–500 ◦C[1]. The main purpose of the ICs is prolongation the time needed to reach these critical temperature values by the steel substrate during a fire (i.e., the time before building collapse). Although the intumescent phenomenon has been known since the early 1970s [2], various ICs are still tested and developed. Different thermoplastics and reactive film-forming polymers are used as bases of the ICs, e.g., polyacrylates, phenoplasts, epoxides and polyurethanes. Except for the binder, the ICs consist of three main thermoactive

Polymers 2020, 12, 2542; doi:10.3390/polym12112542 www.mdpi.com/journal/polymers PolymersPolymers 20202020, ,1212, ,x 2542 FOR PEER REVIEW 22 of of 14 14 three main thermoactive components. Ammonium polyphosphate (APP) generates phosphoric acids components. Ammonium polyphosphate (APP) generates phosphoric acids (during its thermal (during its thermal degradation), which react with pentaerythritol (PER) and form an expanded degradation), which react with pentaerythritol (PER) and form an expanded carbon structure at carbon structure at a high temperature. At the temperature above 350 °C, melamine (MEL) releases a high temperature. At the temperature above 350 C, melamine (MEL) releases non-flammable gases non-flammable gases foaming the charred layer◦ [3–6]. The char exhibits barrier and thermal foaming the charred layer [3–6]. The char exhibits barrier and thermal insulation properties, thus it insulation properties, thus it hinders mass (and heat) exchange between the environment and the hinders mass (and heat) exchange between the environment and the covered steel substrate. Hot air covered steel substrate. Hot air masses are also accumulated in pores of the char, hence their size and masses are also accumulated in pores of the char, hence their size and distribution have a crucial distribution have a crucial influence on the thermal protection effectiveness of the ICs [7]. In order to influence on the thermal protection effectiveness of the ICs [7]. In order to improve the fire protection, improve the fire protection, various types of fillers and additives are incorporated into the coating various types of fillers and additives are incorporated into the coating systems as well. For example, systems as well. For example, titanium dioxide increases mechanical strength of the chars [8–12]. titanium dioxide increases mechanical strength of the chars [8–12]. Additionally, researches pay the Additionally, researches pay the attention to systems with nanofillers [1,7,13,14], waste polymers [15] attention to systems with nanofillers [1,7,13,14], waste polymers [15] and bio-fillers [3,16–18]. In a few and bio-fillers [3,16–18]. In a few cases, boron compounds (mainly zinc borate such as cases, boron compounds (mainly zinc borate such as 2ZnO 3B2O3 3.5H2O) have been used as flame 2ZnO·3B2O3·3.5H2O) have been used as flame retardants in ·ICs and· as modifiers of the mechanical retardants in ICs and as modifiers of the mechanical properties of charred layers [19–21]. It was stated properties of charred layers [19–21]. It was stated that hydrated zinc borates dehydrate that hydrated zinc borates dehydrate endothermically, they absorb heat from a polymeric material endothermically, they absorb heat from a polymeric material and the emitted water vapor dilutes and the emitted water vapor dilutes flammable gaseous components [20]. Considering this thesis, flammable gaseous components [20]. Considering this thesis, it could be claimed that anhydrous zinc it could be claimed that anhydrous zinc borates should not be used in the fire protective materials, borates should not be used in the fire protective materials, however, there are no scientific however, there are no scientific publications describing an influence of the water content and the publications describing an influence of the water content and the zinc borate type (i.e., the Zn and B zinc borate type (i.e., the Zn and B concentration) on the char formation process and the mechanical concentration) on the char formation process and the mechanical properties of ICs. Unfortunately, properties of ICs. Unfortunately, the importance of the chemical composition of the hydrated boron the importance of the chemical composition of the hydrated boron compounds has not been compounds has not been determined in conditions even slightly similar to real or standardized fires. determined in conditions even slightly similar to real or standardized fires. Generally, the Bunsen Generally, the Bunsen burner test is used for evaluation of the thermal insulation features of the burner test is used for evaluation of the thermal insulation features of the ICs [20,21]. Nevertheless, ICs [20,21]. Nevertheless, due to a constant distance between the burner flame and a sample (and the due to a constant distance between the burner flame and a sample (and the constant temperature of constant temperature of the flame near to the tested IC), this method does not represent the fire with the flame near to the tested IC), this method does not represent the fire with a determined a determined temperature increment during a flashover period. temperature increment during a flashover period. This article presents investigation results of a poly(vinyl acetate)-based intumescent coating This article presents investigation results of a poly(vinyl acetate)-based intumescent coating material filled with various zinc borates (ZBs) and tested on a steel substrate in quasi-real fire material filled with various zinc borates (ZBs) and tested on a steel substrate in quasi-real fire conditions. Thermal degradation and mechanical features of the systems containing the anhydrous conditions. Thermal degradation and mechanical features of the systems containing the anhydrous or hydrated ZBs (3.5H2O or 6H2O; Figure1)—as well as APP, MEL, and PER mixtures—have been or hydrated ZBs (3.5H2O or 6H2O; Figure 1)—as well as APP, MEL, and PER mixtures - have been discussed in relation to their dose and Zn:B:H2O molar ratio. It is generally known that thermoplastic discussed in relation to their dose and Zn:B:H2O molar ratio. It is generally known that thermoplastic ICs often sag during a fire, thus char formation processes have been realized in a vertical position. ICs often sag during a fire, thus char formation processes have been realized in a vertical position. The tests have revealed that only a proper concentration of a selected zinc borate in the poly(vinyl The tests have revealed that only a proper concentration of a selected zinc borate in the poly(vinyl acetate)-based system results in good thermal and mechanical protection of the steel substrate exposed acetate)-based system results in good thermal and mechanical protection of the steel substrate to a cellulosic fire. exposed to a cellulosic fire.

FigureFigure 1. 1. StructuresStructures of of the the tested zinc borates: 3ZnO3ZnO·2B2B2O3·6H6H2OO (ZB6), (ZB6), 2ZnO·3B 2ZnO 3B2OO3·3.5H3.5H2OO (ZB3.5) (ZB3.5) · 2 3· 2 · 2 3· 2 andand 3ZnO·2B 3ZnO 2B2OO3 (ZB0).(ZB0). · 2 3 2.2. Materials Materials and and Methods Methods

2.1. Materials 2.1. Materials The intumescent paints and coatings were prepared using the technical grade components: The intumescent paints and coatings were prepared using the technical grade components: - - Poly(vinylPoly(vinyl acetate) acetate) (PVAc) (PVAc) with with an an average average molecular molecular weight weight of of 176,000 176,000 g/mol, g/mol, glass glass transition transition temperaturetemperature ca. ca. 42 42 °C,◦C, softening softening temperature temperature 15 1500 °C◦C and and solid solid content content >99%>99% (M50; (M50; Synthomer, Synthomer, Essex,Essex, UK); UK);

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- Ammonium polyphosphate (type II) (APP), a powder with an average particles diameter of 18 µm (FR Cross 484; Budenheim, Germany); - Melamine (MEL), a powder with a particles size 40 µm (Melaﬁne; OCI Nitrogen, Geleen, ≤ The Netherlands); - Pentaerythritol (PER), a powder with a particles size 15 µm (Charmor PM15; Perstorp Specialty ≤ Chemicals, Perstorp, Sweden);

- Titanium dioxide (rutile type TiO2) with an average particles size of 290 nm (Tytanpol R-001; GA Z.Ch. Police, Police, Poland); - A wetting/dispersing additive based on a hydroxyl-functional carboxylic acid ester (Disperbyk 108; BYK-Chemie, Wesel, Germany); - A silicone defoamer (Byk-066N; BYK-Chemie); - Hydrated zinc borates: 3ZnO 2B O 6H O (ZB6; POCh, Poland) and 2ZnO 3B O 3.5H O (ZB3.5; · 2 3· 2 · 2 3· 2 Firebrake ZB, U.S. Borax, Chicago, IL, USA) with a particles size of ca. 20 µm; - Anhydrous zinc borate (3ZnO 2B O ) with a particles size of ca. 20 µm (ZB0, ZUT, · 2 3 Szczecin, Poland); - n-Butyl acetate (Chempur, Piekary Sl´ ˛askie,Poland).

2.2. Samples Preparation PVAc was dissolved in n-butyl acetate and mixed (500 rpm, 5 min) with the auxiliary additives using the laboratory dissolver with a heavy-duty dispersion impeller (VMA Getzmann GmbH, Reichshof, Germany). Then, APP, MEL, PER, TiO2, and (optionally) the zinc borate were added to the system and homogenized at 3000 rpm for 1 h. The composition of the reference paint (IC-0) is speciﬁed in Table1. The zinc borates (ZBs) were added in the amounts of 1–10 wt. parts /100 wt. parts of the reference paint solids (Table2). The coating compositions (65% of solids) were applied by means of an adjustable gap applicator (Ascott, Tamworth, UK) onto a low-carbon steel substrate with the dimensions of 102 mm 152 mm 0.81 mm (Q-Panels, Q-Lab Europe, Bolton, UK). The coatings × × were dried at RT for 24 h and at 45 ◦C for 48 h. Subsequently, the samples were cut for panels with the dimensions of 100 mm 100 mm; the thickness of the dry intumescent layers was 1 0.03 mm. × ± Samples of PVAc/ZB mixtures (for thermogravimetric tests) were prepared by mechanical mixing of the PVAc solution with ZB0 (37 wt. parts; the sample abbreviated to PVAc/ZB0), ZB3.5 (28 wt. parts; PVAc/ZB3.5) or ZB6 (48 wt. parts/100 wt. parts of PVAc; PVAc/ZB6). Then, the compositions were dried at RT (24 h) and at 45 ◦C for 48 h.

Table 1. The reference intumescent system composition.

Component The Component Dose (wt. parts) PVAc 16.0 1 APP 40.9 MEL 20.4 PER 13.6 TiO2 5.0 Disperbyk 108 4.0 Byk 066N 0.1 1 Solid content of a solution in n-butyl acetate (wt. parts). Polymers 2020, 12, 2542 4 of 14

Polymers 2020,Table 12, x FOR 2. Results PEER REVIEW of the furnace test of the intumescent coatings with various zinc borates. 4 of 14

Symbol Zinc Borate Type Zinc Borate Content (wt. parts) 1 TIT 2 (min) INI 3 (a.u.) SI 4 (%) NINI 5 (a.u.) IC-ZB0/5 ZB0 5 18.7 23 22 18 IC-0 - 0 15.7 27 28 19 IC-ZB0IC-ZB0/7.5/1 ZB0ZB0 1 7.5 20.0 20.6 3213 3520 10 21 IC-ZB0IC-ZB0/10/2 ZB0ZB0 2 10 19.7 17.0 262 3017 2 18 IC-ZB0IC-ZB3.5/1/2.5 ZB0ZB3.5 2.5 1 21.1 24.9 3039 2355 18 23 IC-ZB0IC-ZB3.5/2/3 ZB0ZB3.5 3 2 22.3 25.2 2625 2124 19 21 IC-ZB0/4 ZB0 4 21.9 25 19 20 IC-ZB3.5/2.5IC-ZB0/5 ZB0ZB3.5 5 2.5 18.7 29.6 2326 2219 21 18 IC-ZB0IC-ZB3.5/3/7.5 ZB0ZB3.5 7.5 3 20.6 29.0 1324 20 19 10 IC-ZB0IC-ZB3.5/4/10 ZB0ZB3.5 10 4 17.0 28.3 226 1713 23 2 IC-ZB3.5/1 ZB3.5 1 24.9 39 55 18 IC-ZB3.5IC-ZB3.5/5/2 ZB3.5ZB3.5 2 5 25.2 27.1 2523 2412 20 19 IC-ZB3.5IC-ZB3.5/7.5/2.5 ZB3.5ZB3.5 2.5 7.5 29.6 24.4 2618 1910 16 21 IC-ZB3.5IC-ZB3.5/10/3 ZB3.5ZB3.5 3 10 29.0 21.8 2412 2014 10 19 IC-ZB3.5/4 ZB3.5 4 28.3 26 13 23 IC-ZB6/1 ZB6 1 20.5 36 48 19 IC-ZB3.5/5 ZB3.5 5 27.1 23 12 20 IC-ZB3.5IC-ZB6/2/7.5 ZB3.5ZB6 7.5 2 24.4 24.2 1825 1021 20 16 IC-ZB3.5IC-ZB6/2.5/10 ZB3.5ZB6 10 2.5 21.8 26.3 1227 1417 22 10 IC-ZB6IC-ZB6/3/1 ZB6ZB6 1 3 20.5 28.4 3629 4816 24 19 IC-ZB6/2 ZB6 2 24.2 25 21 20 IC-ZB6IC-ZB6/4/2.5 ZB6ZB6 2.5 4 26.3 25.1 2726 1718 21 22 IC-ZB6IC-ZB6/5/3 ZB6ZB6 3 5 28.4 24.8 2927 16 23 24 IC-ZB6IC-ZB6/7.5/4 ZB6ZB6 4 7.5 25.1 23.1 2616 1810 14 21 IC-ZB6/5 ZB6 5 24.8 27 16 23 IC-ZB6/10 ZB6 10 18.1 4 15 3 IC-ZB6/7.5 ZB6 7.5 23.1 16 10 14 IC-ZB61 wt./10 parts of aZB6 zinc borate/100 wt. parts 10 of the reference paint 18.1 solids; 2 thermal 4 insulation 15 time; 33 intumescent1 wt. parts of index; a zinc borate 4 sagging/100 wt. index; parts 5 of normalized the reference intumescent paint solids; 2 index.thermal insulation time; 3 intumescent index; 4 sagging index; 5 normalized intumescent index. 2.3. Methods 2.3. Methods The thickness of the dry intumescent coatings was measured with the electronic film gauge (Byko-testThe thickness8500; BYK-Gardner, of the dry intumescentGeretsried, Germany). coatings was Fire measured characteristics with theof the electronic coatings film (applied gauge onto(Byko-test the steel 8500; plates) BYK-Gardner, were investigated Geretsried, by Germany). means of Firea programmable characteristics laboratory of the coatings furnace (applied directly onto heatedthe steel by plates) an open were flame investigated of propane by gas means (Figure of a programmable2). The temperature laboratory in a furnace furnace chamber directly heated(TF) was by automaticallyan open flame regulated of propane (acc. gas to (Figure the standard2). The temperaturecellulosic fire in curve a furnace described chamber in PN-EN ( TF) was 1991-1-2:2006) automatically byregulated a control (acc. unit to of the a gas standard burner: cellulosic fire curve described in PN-EN 1991-1-2:2006) by a control unit of a gas burner: =++°() () TtCF 345 log 8 1 20 (1) TF = 345 log(8t + 1) + 20(◦C) (1) where: t—time of the test [min]. where: t—time of the test [min].

FigureFigure 2. 2. AA scheme scheme of of the the propane propane gas gas flame-heated ﬂame-heated programmable programmable furnace furnace used used for for the the thermal thermal insulationinsulation tests tests of of the the coatings: coatings: (1) (1) the the furnace furnace ch chamber,amber, (2) (2) a asteel steel substr substrateate with with the the intumescent intumescent coating,coating, (3) (3) three three thermocouples thermocouples measuring measuring temperat temperatureure of of the the steel steel substrate, substrate, (4) (4) a athermocouple thermocouple measuringmeasuring temperature temperature of of the the furnace furnace chamber, chamber, (5) (5) a apropane propane gas gas burner burner and and (6) (6) a atemperature temperature controlcontrol unit. unit.

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The TF and temperature of the steel substrates were monitored using K-type thermocouples and a PC unit. Thermal insulation time (TIT) was presented as a time needed to reach the temperature of 450 ◦C (the critical temperature value) measured on a backside of the tested sample. The thickness of charred samples (i.e., after the furnace test) was determined using a dial gauge with a stand (ﬁve measurements for the each sample). The intumescent index (INI) for the coatings was calculated according to the equation [15]:

L L INI = 1 − (a.u.) (2) Lo where: L is the thickness of the steel substrate (i.e., 0.81 mm), L0 is a thickness of a dry coating before the furnace test (mm) and L1 is a thickness of a char after the test (mm). The sagging index (SI) represents sagging of the coatings during the furnace test (i.e., percentage of the steel substrate not covered by the charred layer) and was determined by a digital analysis of the sample image. Thermal analysis of the intumescent coatings and their components was carried out using the thermogravimetric analyzer (50 ◦C–900 ◦C, a heating rate of 100 ◦C/min, air atmosphere; TG Q5000, TA Instruments, New Castle, DE, USA). Selected chars were also milled and investigated using the Fourier Transform Infrared Spectroscope (FTIR) with attenuated total reﬂectance (ATR) accessories (Thermo Nicolet Nexus FT-IR, Thermo Electron, Waltham, MA, USA). The compressive strength of the charred intumescent coatings was monitored using the Z010 machine (Zwick/Roell, Ulm, Germany) equipped with a ﬂat steel indenter with a diameter of 20 mm (indentation speed of 10 mm/min). Compressive tension values at the strain of 5%, 15%, and 25% were calculated by means of the TestXpert II software (Zwick/Roell).

3. Results and Discussion

3.1. Furnace Test Results Results of the fire resistance test of the poly(vinyl acetate)-based intumescent coatings are presented in Figure3, while digital images of the samples (after the furnace test) are shown in Figure4. In detail, the curves represent a relation between thermal insulation time values (TIT) and the content of the zinc borates in the ICs. As can be seen, the incorporation of 1–10 wt. parts of the anhydrous (ZB0) or hydrated borates (ZB3.5 or ZB6) into the coating systems caused positive changes of TIT; even the lowest concentration of the ZBs (1 wt. part) resulted in a significant improvement of the analyzed parameter. In the case of the IC-ZB0/1 and IC-ZB6/1 samples, TIT was increased by 27% (20.0 min) and 30% (20.5 min; Table2) while IC-ZB3.5 /1 reached ca. 58% higher TIT value (24.9 min) in comparison to the reference coating without borates (15.7 min; IC-0). Nevertheless, a further TIT improvement was recorded for the systems with the higher ZB content as well. Interestingly, the highest TIT values for all the ZB-modified systems were reached at the similar contents of the borates, i.e., at 3 wt. parts of ZB0 (IC-ZB0/3), 2.5 wt. parts of ZB3.5 (IC-ZB3.5/2.5), and 3 wt. parts of ZB6 (IC-ZB6/3), but the recorded TIT values for these samples were markedly different (22.3 min for IC-ZB0/3, 29.6 min for IC-ZB3.5/2.5, and 28.4 min for IC-ZB6/3). The observed results for the hydrated ZBs-based coatings (i.e., the higher TIT values than for IC-ZB0/3 and IC-0) could be explained by an endothermic dehydration reaction of ZB3.5 [22] and ZB6 during the fire test; it is generally known that release of the crystalline water causes reduction of the heated system temperature. It is noteworthy that the thermogravimetric analyses of the both borates revealed a higher dehydration process temperature of ZB3.5 (maximal weight loss rate at 437 ◦C) in comparison to ZB6 (185 ◦C; Figure5). Additionally, the former borate contains only 14.6 wt% of the water while its concentration in ZB6 is much higher (ca. 20.8 wt%). Considering shapes of the thermal insulation curves for IC-ZB3.5/2.5 and IC-ZB6/3—registered at the beginning of the furnace test of the ICs (Figure6)—it can be claimed that the latter sample exhibited better thermal insulation features than IC-ZB3.5/2.5 and the other systems. In detail, after 5 min of their heating (i.e., at the temperature of the furnace ca. 580 ◦C), the steel substrate protected by IC-ZB6/3 reached Polymers 2020, 12, 2542 6 of 14

lower temperature (176 ◦C) in relation to IC-ZB3.5/2.5 (185 ◦C), IC-ZB0/3 (187 ◦C), and the reference coating (247 ◦C; IC-0). It was caused by the mentioned low-temperature dehydrating process of ZB6 in the IC-ZB6/3 sample. Nevertheless, during the further heating of the samples (>8 min of the ﬁre Polymerstest), the 2020 temperature, 12, x FOR PEER of REVIEW the steel plate with IC-ZB3.5/2.5 was markedly decreased in comparison6 of 14 Polymers 2020, 12, x FOR PEER REVIEW 6 of 14 with IC-ZB6/3; it was resulted by the dehydration process of ZB3.5, which begins at the much higher dehydration process of ZB3.5, which begins at the much higher temperature than for ZB6 (Figure 5). temperaturedehydration than process for ZB6of ZB3.5, (Figure which5). Finally, begins IC-ZB3.5 at the much/2.5 reached higher temperature the higher TIT than value for (ZB6+1.2 (Figure min) than 5). Finally, IC-ZB3.5/2.5 reached the higher TIT value (+1.2 min) than IC-ZB6/3 and the other samples. It IC-ZB6Finally,/3 IC-ZB3.5/2.5 and the other reached samples. the It hi showsgher TIT that value the dehydration (+1.2 min) than process IC-ZB6/3 temperature and the is other more samples. important It shows that the dehydration process temperature is more important than the crystalline water content thanshows the that crystalline the dehydration water content process in temperature ZB3.5 and ZB6, is more and important it crucially than aﬀects the thecrystalline TIT parameter water content of the in ZB3.5 and ZB6, and it crucially affects the TIT parameter of the tested ICs with these borates. testedin ZB3.5 ICs and with ZB6, these and borates. it crucially affects the TIT parameter of the tested ICs with these borates.

Figure 3. Thermal insulation time for the intumescent coatings as a function of zinc borate content. Figure 3. Thermal insulation time for the intumescent coatings as a function of zinc borate content. Figure 3. Thermal insulation time for the intumescent coatings as a function of zinc borate content.

Figure 4. Digital images of the charred intumescent coatings (after the furnace test) with various Figure 4. Digital images of the charred intumescent coatings (after the furnace test) with various zinc zincFigure borates. 4. Digital images of the charred intumescent coatings (after the furnace test) with various zinc borates. borates.

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FigureFigureFigure 5. 5.5. Thermogravimetric Thermogravimetric curves curves for for the the hydrated hydrated zinc zinczinc borates. borates.borates.

FigureFigure 6. 6.6. Thermal Thermal insulation insulation curves curves for for the the selected selected intu intumescentintumescentmescent coatings coatingscoatings with withwith various variousvarious zinc zinczinc borates. borates.borates.

Interestingly,Interestingly,Interestingly, the thethe TIT TITTIT values valuesvalues for forfor the thethe ZB0-based ZB0-basedZB0-based coatings coatingscoatings were werewere also alsoalso significantly significantlysignificantly higher higherhigher in inin relation relationrelation tototo the thethe reference referencereference sample, sample,sample, however, however,however, this thisthis borate borateborate does doesdoes not notnot contain containcontain the thethe crystalline crystalline water. water.water. It ItIt should shouldshould be bebe pointedpointedpointed out outout that thatthat the thethe higher higherhigher thermal thermalthermal insulation insulationinsulation effi eeffifficiencyciencyciency of ofof all allall the thethe compositions compositionscompositions with withwith the thethe anhydrous anhydrousanhydrous (or(or(or hydrated) hydrated)hydrated) ZBs ZBsZBs could couldcould be bebe affected aaffectedffected by byby a aa crosslinki crosslinkingcrosslinkingng process processprocess of ofof the thethe polymeric polymericpolymeric coating coatingcoating binder binderbinder by byby the thethe boratesboratesborates [7]. [[7].7]. Arguably, Arguably, the the PVAc-ZB PVAc-ZB interactions interactionsinteractions increase increaseincrease viscosity viscosityviscosity of ofof the thethe coatings coatings (during (during the the fire firefire test) test) andandand affect aaffectffect formation formationformation processes processesprocesses of ofof foamed foamedfoamed layers. layers.layers. Thus, Thus, different didifferentfferent shapes shapesshapes of ofof the thethe charred charredcharred samples samplessamples after afterafter thethethe furnace furnacefurnace test testtest have havehave been beenbeen observed observed (Figure (Figure 4 4).).4). Indeed,Indeed,Indeed, thethethe carbonaceous carbonaceouscarbonaceous layers layerslayers exhibited exhibitedexhibited varied variedvaried intumescentintumescentintumescent index indexindex (INI), (INI),(INI), but butbut also alsoalso different didifferentfferent saggin saggingsaggingg index indexindex values valuesvalues (SI; (SI;(SI; Table TableTable 22). ).2). The The latter latter parameterparameter parameter describesdescribesdescribes denudation denudationdenudation efficiency eefficiencyfficiency of ofof a aa steel steelsteel plate plateplate cove coveredcoveredred by byby a aa coating coatingcoating (i.e., (i.e.,(i.e., its its thermoplastic thermoplasticthermoplastic behavior) behavior) duringduringduring its itsits intensive intensiveintensive fire firefire heating. heating.heating. In In the the case case of of a a high high SI SI value, value, the the tested tested steel steel sample sample is is only only partially partially protectedprotectedprotected by byby a aa char, char,char, thus thus its its averaged averaged TIT TIT value value is is relatively relatively low. low. In InIn authors’ authors’authors’ opinion, opinion,opinion, the thethe sagging saggingsagging tendencytendency of of the the ICs ICs during during a a fire fire is is a a crucial crucial fe featureature of of thermoplastic thermoplastic binder-based binder-based systems systems utilized utilized atat verticalvertical positions.positions. Therefore,Therefore, takingtaking intointo coconsiderationnsideration thethe INIINI calculcalculationation methodology,methodology, thisthis

Polymers 2020, 12, 2542 8 of 14 tendency of the ICs during a fire is a crucial feature of thermoplastic binder-based systems utilized at vertical positions. Therefore, taking into consideration the INI calculation methodology, this parameter (i.e., an increment of the coating thickness) is not sufficient to reliable description of intumescent efficiency of the tested coatings. For a better presentation of the ICs foamability (an increment of layer volume), the new parameter called normalized intumescent index (NINI) should be used. It was calculated according to the following equation:

INI (100 SI) NINI = · − (a.u.) (3) 100 As can be seen, the reference coating (IC-0) exhibited the relatively high SI value (28%; Table2) and it noticeably increased up to 35, 55 or 48% after the addition of 1 wt. part of ZB0, ZB3.5 or ZB6, respectively. The INI values were also increased (from 27 to 32, 39 or 36 a.u., respectively) while NINI was relatively unchanged (18–21 a.u.). Nevertheless, higher doses of the borates (2–5 wt. parts; mainly the hydrated forms) caused significant limitation of the SI parameter (up to 12%) and only a slight increment of NINI (18–24 a.u.). It means that the ZBs addition (up to 5 wt. parts) reduces sagging of the coatings but does not markedly change volume of the charred layers. On the other hand, at the higher doses of the ZBs (>5 wt. parts), the values of the mentioned parameters (INI, SI, NINI) were diminished. Considering the recorded SI variations (resulted by the ZB type and content), it could be claimed that this phenomenon is simultaneously affected by the supposed crosslinking reaction of the PVAc-ZB system and—optionally for the ICs with ZB3.5 or ZB6—by the dehydration processes of the applied borates. Thus, it seems that the chemical composition of the tested borates is a crucial parameter affecting the analyzed features of the ICs. As mentioned above, ZB0 contains no water, therefore, the B and Zn content in this borate (11.3 wt% and 51.1 wt%, respectively) is higher in relation to ZB6. Nevertheless, the B:Zn molar ratio is the same for ZB0 and ZB6 (1:0.75; Table3). On the other side, ZB6 consists the higher amount of the crystalline water and Zn (39.9 wt%) as well as much less B (8.8 wt%) in comparison with ZB3.5 (30.1 wt% of Zn, 14.9 wt% of B). As can be seen in Figure4 (Table2), the charred samples with ZB6 ( >1 wt. part) exhibited generally lower sagging index values (21–15%) than the coatings filled with the anhydrous form of this additive (ZB0; 30–17%). This shows that the SI limitation was not directly caused by the B (or Zn) concentration in the coating, because ZB6 contains the lower amounts of these elements than ZB0. Additionally, it should be noted that the samples containing almost theoretical stoichiometric ratios of PVAc and these ZBs (needed to complete crosslinking of the polymer by boron [7]) reached also significantly different values of SI, i.e., 22% for IC-ZB0/5 and 10% for IC-ZB6/7.5 (Table2). Thus, it can indicate that the water (released via the dehydration process of ZB6) positively affects the PVAc crosslinking phenomenon, e.g., by the facile hydrolysis of acetate groups and generation of poly(vinyl alcohol) (PVAl) or polyenes [23], boric acid (or other borate hydroxides) as well as zinc acetate dihydrate (as a basic form of this compound) [7,15] (Figure7). According to the literature, boric acid needs cations (zinc) to be able to crosslink the PVAl matrix [24]. Perhaps their creation is also more effective in the ICs containing the crystalline water released from the hydrated zinc borates. On the other hand, it should be explained why the anhydrous borate (ZB0) reduces sagging of the PVAc-based coatings. In authors’ opinion, the water generated during dehydroxylation of pentaerythritol acts similar to the crystalline water generated by the hydrated borates. Due to the relatively high temperature of the former process (ca. 320 ◦C[7]), the sagging effect of the ZB0-based systems was relatively more effective in relation to the coatings filled with ZB6. Other explanation of the SI reduction observed for the ICs-ZB0-type coatings is a direct interchange reaction between PVAc and ZB0 (transesterification reaction, Scheme1); in this case, the water would not be needed in the crosslinking process of the polymeric binder. Polymers 2020, 12, 2542 9 of 14

Table 3. Characteristics of the zinc borates.

Chemical Composition (wt%) Zinc Borate Molecular B:Zn:H2O PVAc:ZB 1 Symbol Weight (g/mol) B Zn H2O Molar Ratio Weight Ratio ZB0 383.5 11.3 51.1 0 1:0.75:0 100:37 ZB3.5 434.7 14.9 30.1 14.5 1:0.33:0.58 100:28 ZB6 491.5 8.8 39.9 21.9 1:0.75:1.5 100:48 1 Polymers 2020, 12, xin FOR relation PEER to REVIEW the stoichiometric ratio of VAc: B needed to complete crosslinking of PVAc. 10 of 14

FigureFigure 7.7. ThermogravimetricThermogravimetric curves curves (the (the first ﬁrst derivati derivative;ve; DTG) DTG) for forPVAc, PVAc, PVAc/ZB0, PVAc/ZB0, PVAc/ZB3.5 PVAc/ZB3.5 and PolymersandPVAc/ZB6. 2020 PVAc, 12/,ZB6. x FOR PEER REVIEW 9 of 14

Interestingly, the NINI values calculated for the coatings modified with the anhydrous (ZB0) or hydrated zinc borates (ZB3.5 and ZB6) are quite comparable and vary in the range of 2–23 a.u. for IC- ZB0, 10–23 a.u. for IC-ZB3.5, and 3–24 a.u. for IC-ZB6 (Table 2). Especially, the samples containing the ZBs with the same Zn-B ratios (i.e., the IC-ZB0 and IC-ZB6-type systems) exhibit the similar ranges of the SI parameter values. It may confirm that the crystalline water (after its release from the hydrated borate) only slightly increases foamability of the coatings. Arguably, a major part of the water evaporates from the samples before the final char forming process. A scheme of charred coatings creation from IC-0, IC-ZB0/3, IC-ZB3.5/2.5, and IC-ZB6/3 systems (considering the crystalline water evaporation process and the registered SI and NINI parameters values) is presented in Figure 9.

Scheme 1. A scheme of PVAc crosslinking processprocess byby hydratedhydrated andand anhydrousanhydrous zinczinc borates.borates.

ItIt isis noteworthy noteworthy that that the the thermogravimetric thermogravimetric analysis analysis of the of PVActhe PVAc/ZB6/ZB6 composition composition (i.e., the (i.e., binder the mixedbinder with mixed ZB6) with revealed ZB6) re avealed signiﬁcant a significant weight loss weight of the loss sample of th ate thesample temperature at the temperature range of 140–235 range◦ ofC (the140–235 maximal °C (the weight maximal loss at weight 190 ◦C; loss Figure at 1907). °C; The Figure similar 7). phenomenon The similar wasphenomenon not recorded was for not PVAc recorded/ZB0. Thefor PVAc/ZB0. mentioned The weight mentioned loss value weight correlates loss value with thecorrelates crystalline with water the crystalline content in water ZB6. Itcontent indicates in ZB6. that theIt indicates entire water that evaporates the entire from water IC-ZB6-type evaporates systems from at IC-ZB6-type the relatively systems low temperature at the relatively and before low the maintemperature foaming and process. before Considering the main foaming the hypothesis process.of Considering the acetate groupsthe hypothesis hydrolysis of the and acetate generation groups of poly(vinylhydrolysis alcohol)and generation/polyenes, of boric poly(vinyl acid/boron alcohol) hydoxides,/polyenes, and zincboric acetate acid/bor dihydrateon hydoxides, in the presence and zinc of theacetate released dihydrate crystalline in the water presence (Scheme of the1), released the crosslinking crystalline process water of (Scheme the binder 1), andthe crosslinking dehydration process of zinc acetateof the binder dihydrate and shoulddehydration be realized of zinc below acetate 235 dihydr◦C. Inate this should case, be the realized released below water 235 amount °C. In would this case, be the released water amount would be similar to the weight loss value recorded for the PVAc/ZB6 sample (according to [25], the dehydration process of zinc acetate dihydrate is realized at 103 °C). The presented hypothesis seems so be also right, because—as mentioned above—ICs with ZB6 (>1 wt. part) exhibited significantly lower SI values than IC/ZB0-type samples. It may confirm that the ICs with the hydrated borate (ZB6) were crosslinked at a lower temperature (the lower SI and the higher TIT values) than these with ZB0 (the higher SI and the lower TIT values). Considering the Figure 8. DTG curves for the selected intumescent coatings with various zinc borates. theory of the PVAc-ZB0 transesterification reaction (Scheme 1), the crystalline water would not be needed to realize the crosslinking process of the polymeric binder; finally, the chemical structures created from the ZB0 or ZB6-based systems should be similar. In this case, thermal stability of the systems should be similar as well. As can be seen in Figure 8, the DTG curves for IC-ZB0/3 and IC- ZB6/3 (or PVAc-ZB0 and PVAc-ZB6, Figure 7) are similar except for the peak at ca. 200 °C, which represents evaporation of the crystalline water from the ZB6-based coating. Additionally, the curves differ from these registered for IC-0 and IC-ZB3.5/2.5. It can represent that thermal degradation processes occurring during heating the IC-ZB0/3 and IC-ZB6/3 systems (i.e., the processes causing the weight loss of the samples) are similar and the crystalline water is not necessary for the PVAc-ZB crosslinking reactions (but positively affects them). Unfortunately, the water evaporation from the ZB3.5-based coatings was not revealed by the DTG analysis (Figure 7) due to similar temperatures of the zinc borate dehydration (439 °C, Figure 5) and the PVAc degradation processes. It should be noted that the crystalline water (before its evaporation) arguably plasticizes the binder of the IC-ZB6-type compositions, thus the SI value for the system with the lowest ZB6 dose (i.e., 48% for IC-ZB6/1) was markedly higher in comparison with IC-ZB0/1 (35%) and IC-0 (28%). That phenomenon resulted in a decrement of coating viscosity during the fire heating test (the similar process was observed for IC- ZB3.5/1; Table 2). Nevertheless, the higher doses of ZB6 or ZB3.5 significantly reduce the plasticizing effect (by more effective crosslinking of the binder); finally, it causes marked limitation of the SI parameter.

Polymers 2020, 12, x FOR PEER REVIEW 10 of 14

Polymers 2020, 12, 2542 10 of 14 similar to the weight loss value recorded for the PVAc/ZB6 sample (according to [25], the dehydration process of zinc acetate dihydrate is realized at 103 ◦C). The presented hypothesis seems so be also right, because—as mentioned above—ICs with ZB6 (>1 wt. part) exhibited significantly lower SI values than IC/ZB0-type samples. It may confirm that the ICs with the hydrated borate (ZB6) were crosslinked at a lower temperature (the lower SI and the higher TIT values) than these with ZB0 (the higher SI and the lower TIT values). Considering the theory of the PVAc-ZB0 transesterification reaction (Scheme1), the crystalline water would not be needed to realize the crosslinking process of the polymeric binder; finally, the chemical structures created from the ZB0 or ZB6-based systems should be similar. In this case, thermal stability of the systems should be similar as well. As can be seen in Figure8, the DTG curves for IC-ZB0 /3 and IC-ZB6/3 (or PVAc-ZB0 and PVAc-ZB6, Figure7) are similar except for the peak at ca. 200 ◦C, which represents evaporation of the crystalline water from the ZB6-basedFigure 7. Thermogravimetric coating. Additionally, curves the(the curves first derivati differve; from DTG) these for PVAc, registered PVAc/ZB0, for IC-0 PVAc/ZB3.5 and IC-ZB3.5 and /2.5. It canPVAc/ZB6. represent that thermal degradation processes occurring during heating the IC-ZB0/3 and IC-ZB6/3 systems (i.e., the processes causing the weight loss of the samples) are similar and the crystalline water is notInterestingly, necessary for the the NINI PVAc-ZB values crosslinking calculated for reactions the coatings (but positively modified awithffects the them). anhydrous Unfortunately, (ZB0) or hydratedthe water zinc evaporation borates (ZB3.5 from the and ZB3.5-based ZB6) are quite coatings comparable was not and revealed vary in by the the range DTG of analysis 2–23 a.u. (Figure for IC-7) ZB0,due to 10–23 similar a.u. temperatures for IC-ZB3.5, of and the zinc3–24 borate a.u. for dehydration IC-ZB6 (Table (439 ◦2).C, Especially, Figure5) and the the samples PVAc degradation containing processes.the ZBs with It should the same be notedZn-B thatratios the (i.e., crystalline the IC-ZB0 water and (before IC-ZB6-type its evaporation) systems) arguably exhibit the plasticizes similar rangesthe binder of the of SI the parameter IC-ZB6-type values. compositions, It may confirm thus th theat the SI valuecrystalline for the water system (after with its release the lowest from ZB6 the hydrateddose (i.e., borate) 48% for only IC-ZB6 slightly/1) was increases markedly foamability higher inof comparisonthe coatings. withArguably, IC-ZB0 a/ 1major (35%) part and of IC-0 the (28%).water evaporates That phenomenon from the resulted samples in abefore decrement the final of coating char viscosityforming process. during the A firescheme heating of testcharred (the coatingssimilar process creation was from observed IC-0, IC-ZB0/3, for IC-ZB3.5 IC-ZB3.5/2.5/1; Table2, ).and Nevertheless, IC-ZB6/3 systems the higher (considering doses of the ZB6 crystalline or ZB3.5 watersignificantly evaporation reduce process the plasticizing and the eregisteredffect (by more SI and effective NINI crosslinkingparameters values) of the binder); is presented finally, in it Figure causes 9.marked limitation of the SI parameter.

Figure 8. DTGDTG curves for the selected intumescent coatings with various zinc borates.

Interestingly, the NINI values calculated for the coatings modified with the anhydrous (ZB0) or hydrated zinc borates (ZB3.5 and ZB6) are quite comparable and vary in the range of 2–23 a.u. for IC-ZB0, 10–23 a.u. for IC-ZB3.5, and 3–24 a.u. for IC-ZB6 (Table2). Especially, the samples containing the ZBs with the same Zn-B ratios (i.e., the IC-ZB0 and IC-ZB6-type systems) exhibit the similar ranges of the SI parameter values. It may confirm that the crystalline water (after its release from the hydrated borate) only slightly increases foamability of the coatings. Arguably, a major part of the water evaporates from the samples before the final char forming process. A scheme of charred coatings Polymers 2020, 12, 2542 11 of 14 creation from IC-0, IC-ZB0/3, IC-ZB3.5/2.5, and IC-ZB6/3 systems (considering the crystalline water evaporation process and the registered SI and NINI parameters values) is presented in Figure9. Polymers 2020, 12, x FOR PEER REVIEW 11 of 14

FigureFigure 9. 9.A A schematic schematic illustration illustration of foaming of foaming process process of the of selected the selected intumescent intumescent coatings coatings with various with zincvarious borates. zinc borates.

3.2.3.2. Mechanical Mechanical Features Features of of the the Chars Chars TheThe compressive compressive strengthstrength values for for the the charred charred intumescent intumescent coatings coatings (at (at the the strain strain of 5, of 15, 5, and 15, and25%) 25%) are arepresented presented in Figure in Figure 10. As10. can As be can seen, be seen, the introduction the introduction of the of anhydrous the anhydrous zinc borate zinc borate (ZB0) (ZB0)resulted resulted in a significantly in a significantly higher higher compression compression resist resistance,ance, especially especially at atthe the higher higher strain strain (12.0 (12.0 kPa kPa at at15% 15% and and 11.5 11.5 kPa kPa at at 25%) 25%) in in comparison comparison with with the reference sample (4.9 andand 8.78.7 kPa,kPa, respectively).respectively). Interestingly,Interestingly, the the latterlatter samplesample reached reached a a higher higher me mechanicalchanical strength strength than than the the other other systems systems (4.6 (4.6 and and6.3 kPa 6.3 kPafor IC-ZB3.5/2.5, for IC-ZB3.5 4.2/2.5, and 4.2 4.9 and kPa 4.9 for kPa IC-ZB6/3). for IC-ZB6 Taking/3). into Taking consideration into consideration the INI, SI, the and INI, NINI SI, andvalues NINI for values the investigated for the investigated samples samples (Table (Table2), it could2), it could be claimed be claimed that thatthese these parameters parameters do donot notcorrelate correlate with with the the recorded recorded mechanical mechanical features. features. Additionally, Additionally, the the compressivecompressive strengthstrength doesdoesnot not dependdepend on on the the boron boron content content in in the the samples samples (the (the highest highest concentration concentration in in IC-ZB3.5 IC-ZB3.5/2.5,/2.5, the the lowest lowest in in IC-ZB6IC-ZB6/3)./3). Nevertheless, Nevertheless, it it seems seems that that the the TIT TIT parameter parameter may may influence influence the the mechanical mechanical resistance resistance of of thethe charred charred layers layers due due to to the the shorter shorter heating heating time time of of the the reference reference sample sample (15.7 (15.7 min) min) and and the the sample sample withwith ZB0 ZB0 (22.3(22.3 min)min) inin comparison with IC-ZB3.5/2.5 IC-ZB3.5/2.5 (29.6 (29.6 min) min) and and IC-ZB6/3 IC-ZB6/ 3(28.4 (28.4 min; min; Table Table 2).2). In Inthis this case, case, the the longer longer heating heating time time to to the the critical critical temperature of thethe coveredcovered steelsteel substratesubstrate (450(450◦ °C)C) negativelynegatively a affectedffected the the compressive compressive strength strength of of the the created created char. char. On On the the other other hand, hand, IC-0 IC-0 (the (the shorter shorter heatingheating time) time) exhibited exhibited markedly markedly lower lower mechanical mechanical resistance resistance than than IC-ZB0 IC-ZB0/3/3 (the longer (the longer heating heating time). Arguably,time). Arguably, the Zn presence the Zn inpresence the latter in samplethe latter may sample be considered may be asconsid an additionalered as an factor additional affecting factor the featureaffecting of thisthe feature material; of IC-ZB0this material;/3 contained IC-ZB0/3 the highercontained amount the higher of Zn (ca.amount 1.5 wt%) of Zn in (ca. relation 1.5 wt%) to the in referencerelation sampleto the reference (no Zn), IC-ZB6sample/3 (no (ca. Zn), 1.2 wt%),IC-ZB6/3 and (ca. IC-ZB3.5 1.2 wt%),/2.5 (ca. and 0.7 IC-ZB3.5/2.5 wt%). As can (ca. be 0.7 observed wt%). inAs Figurecan be 11 observed, the FTIR in spectrum Figure 11, for the the FTIR charred spectrum IC-ZB0 fo/3r layerthe charred is similar IC-ZB0/3 to the IC-ZB6 layer /is3 spectrumsimilar to exceptthe IC- 1 forZB6/3 the bandsspectrum at ca. except 970, 620,for the and bands 560 cm at −ca.(markedly 970, 620, and registered 560 cm− for1 (markedly the former registered system). for The the band former at 1 970system). cm− isThe not band significantly at 970 cm visible−1 is not in the significantly spectra for visible the charred in the reference spectra samplefor the andcharred IC-ZB3.5 reference/2.5. 1 1 Accordingsample and to IC-ZB3.5/2.5. [26], the 623 andAccording 566 cm to− [26],peaks the represent 623 and 566 O-P-O cm−1 bonds peaks whilerepresent 961 cmO-P-O− corresponds bonds while to961 P-O-P; cm−1 theycorresponds refer to the to chemicalP-O-P; they structure refer ofto titaniumthe chemical pyrophosphate structure of (TiP titanium2O7), which pyrophosphate improves mechanical(TiP2O7), which features improves of charred mechanical intumescent features coatings of charred [12]. Itintumescent means that coatings charred IC-ZB0[12]. It /3means contains that muchcharred more IC-ZB0/3 TiP2O7 thancontains the othermuch samples, more TiP thus2O7 itsthan mechanical the other strengthsamples, is thus better. its Probably,mechanical Zn strength catalyzes is better. Probably, Zn catalyzes the TiP2O7 formation process via a reaction of TiO2 and APP. Nevertheless, it seems that the charred IC-ZB0/3 (and IC-ZB6/3) sample contains a zinc phosphate glass (ZnO-P2O5) as well. The characteristic bands for this compound (i.e., ca. 1380 cm−1 and 1220 cm−1 [27]) were markedly observed for the chars due to the relatively highest Zn concentration in these systems.

PolymersPolymers 2020 2020, 12, x12 FOR, x FOR PEER PEER REVIEW REVIEW 12 of12 14 of 14

ConsideringConsidering the the DTG DTG curves curves for for the the tested tested coatin coatings gs(Figure (Figure 8), 8),it couldit could be benoted noted that that the the ZBs ZBs significantlysignificantly affect affect their their thermal thermal stability stability at theat the temperature temperature range range of 650of 650 °C–850 °C–850 °C; °C; the the main main changes changes of theof the DTG DTG curves curves (vs. (vs. IC-0) IC-0) were were registered registered for for the the samples samples containing containing ZB0 ZB0 and and ZB6. ZB6. According According to to [7],[7], the the weight weight loss loss at theat the mentioned mentioned temperatures temperatures represents represents thermal thermal destruction destruction processes processes of APPof APP Polymers 2020, 12, 2542 12 of 14 (or(or its itsderivatives). derivatives). It seemsIt seems that that ZB0 ZB0 and and ZB6 ZB6 markedly markedly influence influence that that phenomenon phenomenon (in (ina similar a similar way)way) due due to theto the mentioned mentioned high high Zn Zn content content in thesein these borates. borates. The charred IC-ZB0/3 and IC-ZB6/3 layers consisted of detectable amounts of the zinc phosphate the TiPThe2O charred7 formation IC-ZB0/3 process and via IC-ZB6/3 a reaction layers of TiO consisted2 and APP. of detectable Nevertheless, amounts it seems of the that zinc the phosphate charred glass (ZnO-P2O5), however, that compound did not positively affect the compressive strength of the IC-ZB0glass (ZnO-P/3 (and2 IC-ZB6O5), however,/3) sample that contains compound a zinc did phosphate not positive glassly (ZnO-Paffect the2O 5compressive) as well. The strength characteristic of the samples (the values of the mechanical parameter1 were different1 for IC-ZB0/3 and IC-ZB6/3, and bandssamples for (the this compoundvalues of the (i.e., mechanical ca. 1380 cm −parameterand 1220 were cm− different[27]) were for markedly IC-ZB0/3 observed and IC-ZB6/3, for the chars and similarduesimilar to for the for IC-ZB6/3 relatively IC-ZB6/3 and highest and IC-ZB3.5/2.5; IC-ZB3.5/2.5; Zn concentration Figure Figure 10). in 10). theseIn Inthis systems.this case, case, the the highest highest mechanical mechanical strength strength of of ZB0/3ZB0/3 was was resulted resulted by bythe the highest highest TiP TiP2O72 Ocontent7 content in thisin this charred charred sample. sample.

Figure 10. Compressive stress (at the selected strain) for the charred intumescent coatings with FigureFigure 10.10.Compressive Compressive stress stress (at (at the the selected selected strain) strain) for the for charred the charred intumescent intumescent coatings coatings with various with variouszincvarious borates. zinc zinc borates. borates.

2.52.5 Figure 11. FTIR spectra for the charred intumescent coatings with various zinc borates. FigureFigure 11. 11.FTIR FTIR spectra spectra for forthe the charred charred intumescent intumescent coatings coatings with with various various zinc zinc borates. borates. Considering the DTG curves for the tested coatings (Figure8), it could be noted that the ZBs 4. Conclusions4. Conclusions significantly affect their thermal stability at the temperature range of 650 ◦C–850 ◦C; the main changes of theConsideringConsidering DTG curves the the (vs. presented presented IC-0) were features features registered of theof the forPVAc-b PVAc-b the samplesasedased intumescent containingintumescent coatings ZB0 coatings and (ICs) ZB6. (ICs) modified According modified with to with [ 7], thethethe various weightvarious zinc loss zinc borates at borates the mentioned (ZBs), (ZBs), it canit temperatures can be beclaime claimed represents thatd that the the anhydrous thermal anhydrous destruction (ZB0) (ZB0) and processesand hydrated hydrated of ZBs APP ZBs (i.e., (or (i.e., its derivatives). It seems that ZB0 and ZB6 markedly influence that phenomenon (in a similar way) due to the mentioned high Zn content in these borates. The charred IC-ZB0/3 and IC-ZB6/3 layers consisted of detectable amounts of the zinc phosphate glass (ZnO-P2O5), however, that compound did not positively affect the compressive strength of the Polymers 2020, 12, 2542 13 of 14 samples (the values of the mechanical parameter were different for IC-ZB0/3 and IC-ZB6/3, and similar for IC-ZB6/3 and IC-ZB3.5/2.5; Figure 10). In this case, the highest mechanical strength of ZB0/3 was resulted by the highest TiP2O7 content in this charred sample.

4. Conclusions Considering the presented features of the PVAc-basedintumescent coatings (ICs) modified with the various zinc borates (ZBs), it can be claimed that the anhydrous (ZB0) and hydrated ZBs (i.e., ZB3.5 with 3.5H20, ZB6 with 6H2O/per molecule of the ZBs) positively affect the charring processes and thermal insulation time (TIT) of the fire-protective materials. The longest TIT was recorded for the sample filled with 2.5 wt. parts of ZB3.5/100 wt. parts of the other coating components, i.e., +13.9 min in relation to the reference sample (IC-0). It shows that the borate with the higher dehydration temperature (ZB3.5) more effectively elongates the TIT parameter than the ZB6 modifier (with the higher crystalline water content). The tested ZBs significantly reduce sagging of the formed chars (from 28% for IC-0 to 10% for the samples with 7.5 wt. parts of either ZB3.5 or ZB6), however, the ZB6 borate more effectively decreases this parameter than its anhydrous form (ZB0). It seems that the sagging of the chars is limited by the crosslinking processes of the poly(vinyl acetate) binder during its thermal degradation. The ZBs slightly increase foamability of the chars as well; the highest volumetric increment of the ICs during the fire tests (24 times vs. 19 times for IC-0) was registered for the sample filled with ZB6. That parameter reached 23 a.u. for the coatings with ZB0 or ZB3.5. Moreover, the anhydrous borate (ZB0) markedly increases the compressive strength of the formed chars (from 4.9 kPa to 12.0 kPa for the compressive strain of 15%). It was affected by the relatively higher TiP2O7 content in the ZB0-based charred layers in relation to the reference sample and the samples with the other borates.

Author Contributions: J.Ł.: Conceptualization, Investigation, Visualization, Writing—Review & Editing. B.S.: Conceptualization, Development of methodology, Writing—Review & Editing; Y.B.: Validation, Writing—Review & Editing; K.K.: Conceptualization, Development of methodology, Creation of models, Investigation, Writing—Review & Editing, Visualization, Supervision. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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