Introduction

Plausible fire hazards posed by highly combustible materials such as wood and plastics have made flame retardants (FRs) necessary. The need and demand for increased fire safety has led to numerous innovations in the field of FRs. Flame retardants are chemical substances that are incorporated in the coating system in order to prevent combustion and to retard the spread of fire on the substrate.1 The reduced risk of fire hazards averts the loss of lives and properties.2

Wood, which is extensively used for construction, building purposes and as furniture is prone to burn in a fire. Hence, it is essential to develop a fire protective coating for wood. Epoxy acrylate resins are commonly used as UV-curable wood coatings. Cured epoxy resins are excellent versatile adhesives with superior chemical and corrosion resistance and good mechanical properties but are susceptible to fire and have poor fire resistance.3,4,5 The advantages of UV-cured systems over traditional thermally cured and solvent based resins are the lack of volatile organic compounds (VOCs) and the wide-ranging variability of formulations.6,7,8 To make an epoxy coating system fire protective, flame retardants are either added to it or are chemically bonded to the system.

Though additive flame retardants are more economical, they tend to leach out, have an unfavorable impact on processability and have inferior mechanical and physical properties as compared to reactive. Reactive flame retardants tend to chemically bind themselves to the matrix so their release into the environment is hindered.9,10

Concern regarding environmental and human health issues has led to a decreased use of conventional halogen flame retardants. Many of these are considered as contaminants and are linked to adverse health effects in humans and animals. They produce highly carcinogenic substances during thermal degradation which are toxic and corrosive in nature. These substances affect the endocrine system in humans and pollute the environment. Due to this, research on FRs is being focused on developing nonhalogen flame retardants.11,12,13

This has resulted in the development of a new group of FRs containing phosphorous. Organophosphorous FRs are widely used as they are environmentally friendly, less toxic, produce less smoke and provide good efficiency even at low concentration. A variety of modifications in the structure can also be made to optimize the performance of the FR. Organophosphorous FRs tend to follow a condensed phase flame-retardant mechanism which accounts for the conversion of organic matter into char residue during combustion. Phosphorous increases the char yield by producing carbon rather than CO or CO2. The protective char layer is a barrier that reduces the amount of inflammable volatile gases in the flame zone region. This retards the heat transfer from the flame to the substrate, dropping the temperature and obstructing the release of combustible gases. The presence of silicon enhances the thermal stability of the char, and hence, the system displays a synergistic effect.14,15,16,17,18

This study aims to develop a halogen-free, silicon containing organophosphorous variant of flame retardant. Compounds based on phosphorous and silicon have been synthesized. Mechanical, chemical and thermal properties of the photopolymerized coatings which contain these products have been studied in detail.

Materials and methods

Raw materials and chemicals

All analytical grade chemicals including phosphorous oxychloride, methanol, glycidyl methacrylate (GMA), bisphenol-A (BPA), tetramethylolpropyl triacrylate (TMPTA), Irgacure 184 and triethylamine were purchased from SD Fine Chemicals, Mumbai, and were used as received. Epoxy acrylate resin, Desmolux VPLS2266 was received from Covestro, Mumbai.

Synthesis of PGBPA

A 250-mL three-necked round-bottomed flask equipped with a magnetic stirrer was charged with (0.0673 mol, 6.28 mL) POCl3, to which (0.134 mol, 4.29 g) methanol was added drop-wise with gradual stirring for a period of 2 h, while the temperature of the mixture was maintained at 5°C. After completion of the reaction, BPA (0.0673 mol, 15.36 g) dissolved in 20 mL of acetone was added drop-wise to the flask with the temperature maintained between 5 and 10°C. Once BPA was added, the temperature was raised to 50°C and the reaction was carried out for 20 h. The molar ratio of POCl3/CH3OH/BPA was 1:2:1. After completion of the reaction, the solid brown product, labeled as PBPA, was dissolved in chloroform and subsequently, water washed to separate by-products and unreacted components. The yield of PBPA was found to be 87%.

In the next step, PBPA was reacted with glycidyl methacrylate (GMA) in the ratio of 1:1 in the presence of hydroquinone as inhibitor. The reactants were dissolved in chloroform to maintain homogeneity. The reaction was carried out for 4 h at a temperature of 70°C, and triethylamine was used as a catalyst. After completion of the reaction, the product was washed with 1% sodium hydroxide solution followed by washings with lukewarm water to remove unreacted components. The resultant product was abbreviated as “PGBPA” and analyzed for iodine and hydroxyl numbers.

The reaction scheme for the synthesis of PGBPA is shown in Fig. 1.

Fig. 1
figure 1

Reaction Schematic of PGBPA

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Synthesis of Si-HEA

Si-HEA was synthesized as reported by WeiYi Xing et al.19 A 250-mL three-necked round-bottomed flask equipped with a magnetic stirrer was charged with (0.083 mol, 10.7 g) of dichloro dimethyl silane (DCDMS). 2-Hydroxy ethyl acrylate (HEA) (0.166 mol, 19.27 g) was added drop-wise with gradual stirring for a period of 2 h, while the temperature was maintained between 10 and 15°C. After complete addition of HEA, the temperature was raised to 60°C and the reaction was carried out for 20 h. The product obtained after purification by repeated water washings was named as “Si-HEA”. The reaction scheme of Si-HEA is shown in Fig. 2.

Fig. 2
figure 2

Reaction schematic of Si-HEA

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Preparation of UV-curable wood coatings

UV-curable formulations based on PGBPA were prepared by mixing epoxy acrylate oligomer with a varying concentration (0–25%) of PGBPA along with a fixed concentration of photoinitiator (PI) (3%) and TMPTA (15%) as reactive diluent (RD) (Table 1). These were named as PGBPA with a suffix indicating the percentage of PGBPA incorporated in the formulation. In case of formulations based on PGBPA and Si-HEA, a fixed weight fraction of epoxy acrylate oligomer was mixed with the phosphorous and silicon-based products at a varied proportion (5–20%); these were labeled as PSi with a suffix depending on % Si-HEA present in pure epoxy oligomer. The concentration of initiator and TMPTA remained fixed as former formulation (Table 2). Each formulation was prepared in a beaker by stirring vigorously to form a homogenous mixture. The prepared formulations were then applied on prepared wood panels (10 × 7 cm) using a bar applicator to maintain a uniform film thickness (100 microns). Free films were prepared by coating the formulations onto Teflon molds. Finally, the wet coatings were hardened onto the substrates by passing the coated substrates through a UV irradiation of a medium-pressure mercury lamp (365 nm) built into a UV machine. The speed of the belt was kept at 20 m/min (exposure time 13 s).

Table 1 Formulation of PGBPA containing UV-curable coatings
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Table 2 Formulation of PSi containing UV-curable coatings
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Characterization

The hydroxyl and iodine values were determined as per ASTM standard method of testing, E222-10 and ASTM D1959-97, respectively.

To estimate the gel content of the polymeric films, dry cured films were accurately weighed (Wbefore) and then dipped into tetrahydrofuran (THF) for 24 h at room temperature. The films were then dried in a vacuum oven at 80°C and were reweighed (Wafter).

Gel content of the UV-cured films was calculated by the given equation:

$${\text{Gel}}\;{\text{content}}\;\left( \% \right) = \left( {{\text{Wafter}}/{\text{Wbefore}}} \right) \times 100$$

The water absorption capacity of the cured films was determined according to ASTM D570. UV-cured films were dried at 80°C in a vacuum oven and then weighed (W1). The films were then immersed in water at room temperature for 24 h. Later they were removed, patted dry with paper napkins and reweighed (W2). The percentage of water absorbed by the films was calculated using the following equation:

$${\text{Water}}\;{\text{absorption }}\left( \% \right) = \left( {W2{-}W1} \right)/W1 \times 100$$

W2: Weight of the film after being exposed to water, W1: Weight of the film before being exposed to water.

The optical property of the coating was determined based on its gloss as per ASTM D523-99 using Rhopoint gloss meter. To estimate the mechanical strength, hardness was determined using pencil hardness test (ASTM D 3363). Pencils of various degrees of hardness were drawn over the coated substrate to determine the grade of pencil which causes a depression on the coating.

The degree of adhesion of the coating onto the wood panel was found by crosscut adhesion test (ASTM D 3359). This test was performed using a crosscut adhesion tester which created 100 squares having a cross-sectional area of 1 mm2 each. The ruled area was evaluated by comparing the number of squares retained onto the substrate to the standard values, after a short treatment with adhesive tape. To evaluate the solvent resistance of the coating, a cloth saturated with methyl ethyl ketone (MEK) and xylene was rubbed over it as per ASTM D5402-93. To check its stain resistance, the cured panel was stained using permanent marker, temporary marker, castor oil and nail enamel. After 48 h, ethanol was used to remove the stains and assess the stain resistance of the coating (ASTM D3023-98).

Instrumentation

Fourier-transform infrared (FTIR) analysis was conducted on Shimadzu (8400s, Japan) instrument using attenuated total reflection technique and the spectrum obtained in the wavelength range of 4000–600 cm−1. 1H NMR analysis was performed on Mercury Plus NMR spectrometer (400 MHz, Varian, USA). CDCl3 was used as solvent and tetramethylsilane as an internal standard. The inflammability of photopolymerized film and efficiency of developed products as FRs were studied by determining the limiting oxygen index (LOI). It is the minimum concentration of oxygen present in flowing mixture of oxygen and nitrogen that is just sufficient for combustion of a material. Thermal analysis was further carried out by UL-94 vertical burning test as per ASTM D1356-2005. Test samples with dimensions 127 × 12 × 2 mm were suspended vertically and ignited, by using LPG Bunsen burner. The end of the sample bar was ignited; ignition was carried out for 10 s. VTM ratings were assigned to samples according to the results of UL-94 vertical burning test. Table 3 summarizes the connotations of VTM ratings.20,21,22

Table 3 Definitions of VTM ratings
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Results and discussions

The iodine value of the synthesized products was estimated via Wijs method and was found to be close to the theoretical iodine values as observed in Table 4.2,13 The practical hydroxyl value of PBPA was found to be 148.2 mg of KOH per gram of sample against the theoretical value of 166.9 mg of KOH per gram of sample which confirmed the formation of the desired product.23

Table 4 Iodine value of synthesized products- PGBPA and Si-HEA
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FTIR analysis

The FTIR spectrum of PGBPA as shown in Fig. 3, exhibited prominent peaks at 1718 cm−1, 1635 cm−1 and 3456 cm−1 corresponding to −C=O13 and –C=C-2 and –OH13, respectively. Incorporation of phosphorous was confirmed as a peak at 1237 cm−1 was observed which represents –P=O bond.24 Reaction of POCl3 with CH3OH and further with bisphenol-A was established as peaks were found at 826 cm−1, which corresponds to aliphatic –P–O–C stretching13 and at 941 cm−1 which indicates the presence of –P–O–Ph.25 In case of Si–HEA, as observed in Fig. 4, peaks at 1734 cm−1 and 1628 cm−1 indicated the presence of –C=O– and −C=C-19 in the compound due to incorporation of HEA. Successful reaction between DCDMS with HEA was confirmed as peaks at 1196 cm−1 and 1022 cm−1 corresponding to Si–O–C19 were detected.

Fig. 3
figure 3

FTIR spectrum of PGBPA

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Fig. 4
figure 4

FTIR spectrum of Si-HEA

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1H NMR analysis of PGBPA

Figure 5 displays the 1H NMR for the developed phosphorous containing product—PGBPA. The singlet observed at 3.48 ppm corresponds to protons of the methyl group due to the incorporation of methanol in the product. The chemical shift observed at 1.03–0.95 ppm which correspond to the –CH3 protons, can be assigned to the methyl groups of bisphenol-A. Successful reaction of GMA with PBPA via ring opening can be proven by the chemical shift at 2.11–2.14 ppm corresponding to the proton of hydroxyl group. The presence of acrylic proton representing –CH=CH is identified by the peak at 5.36 ppm. Successful synthesis of PGBPA is confirmed by observing different peaks at 7.09 ppm (i) and 7.11 ppm (h) corresponding to the protons of the aromatic rings of bisphenol-A.

Fig. 5
figure 5

1H NMR of PGBPA

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Coating properties

The formulations were applied on wood panels, cured and investigated for their performance properties. The properties exhibited by a coating are a feature of its structure, reactive groups and chemical composition. Thus, the effects of varying percentages of synthesized compounds in the formulation on the coating properties were studied.

Mechanical properties

Each formulation was applied to a wood panel, cured and evaluated for its mechanical properties. Gel content represents the amount of unsaturation present in the coating after photopolymerization by solvent extraction. The cured coatings exhibited excellent gel content behavior (> 96%), irrespective of the concentration of PGBPA and Si-HEA used in the formulations.

Usually, absorbed water has a detrimental effect on the quality of coating and hence durability of the product. Thus, water absorption behavior of a coating is studied to determine its end use application. Figure 6 shows a linear increase in the amount of absorbed water with an increase in concentration of PGBPA in the formulation. A probable explanation for the observed trend would be the presence of hydroxyl group in PGBPA. Hydroxyl group renders the cured film polar. The polar film exhibits affinity toward water molecules via hydrogen bonding. A minimal increase in water absorption capacity is observed with an increasing concentration of PGBPA due to the hydrophobicity of benzene rings. In case of PSi coatings, decrease in weight fraction of Si-HEA resulted in an increase in water absorption as shown in Fig. 7. Si-HEA having two unsaturations displays a functionality of four which increases its tendency to undergo crosslinking when exposed to UV radiation. Thus, higher content of Si-HEA resulted in a highly crosslinked network filling the voids wherein water would penetrate.

Fig. 6
figure 6

Water absorption behavior of UV-cured coatings containing various amounts of PGBPA

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Fig. 7
figure 7

Water absorption behavior of UV-cured coatings containing PSi

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Gloss of a UV-cured film is dependent on its flow properties, its ability to level and smoothness of its surface. As shown in Table 5, gloss of cured coatings decreased with an increasing percentage of PGBPA. This trend can be explained by the fact that PGBPA is a bifunctional monomer with a single unsaturation. The reactivity of this monomer is impeded due to steric hindrance. Thus, percent crosslinking during curing reduced as the amount of PGBPA increased. Nevertheless, a high gloss value was retained due to the presence of aromatic rings in PGBPA as well as in epoxy.

Table 5 Mechanical properties of UV-cured coatings
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PSi coatings exhibited excellent gloss irrespective of the composition of PGBPA and Si-HEA in the formulation. The strength of adhesion of cured films to the wood panel was determined by crosscut adhesion, and hardness was gauged by pencil hardness test. As shown in Table 5, a decrease in adhesion and hardness was observed with an increase in weight fraction of PGBPA. PGBPA was incorporated into the system to replace epoxy acrylate, a compound with two double bonds. Due to the additional double bond, epoxy acrylate imparts more rigidity to the coating than PGBPA, which contains only one unsaturation. Therefore, with an increase in the percentage of PGBPA which contributed to a decrease in crosslinking density, hardness and adhesion were reduced. PSi on the other hand, presented a stable adhesion of 5B irrespective of its composition. Furthermore, in the case of pencil hardness, a continuous streak of 4H followed until the PSi5 composition which displayed 5H hardness level.

Chemical properties

All the coatings were analyzed for their solvent and stain resistances. It was noted that every coating was able to withstand > 200 double rubs of MEK and xylene irrespective of their composition. Excellent stain resistance against permanent and temporary marker, nail enamel and castor oil was exhibited by all the formulations as observed in Figs. 8 and 9.

Fig. 8
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Stain resistance of PGBPA-based coatings

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Fig. 9
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Stain resistance of PSi-based coatings

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Flame-retardant properties

The efficacy of PGBPA and PSi-based wood coating was determined by its LOI and UL-94 vertical test rating. A higher LOI corresponds to better flame-retardant properties as it represents the percentage of oxygen (in a free-flowing mixture of nitrogen and oxygen) required to combust a material.

As observed in Table 6, the LOI value for PGBPA-based coating increased with an increasing percentage of PGBPA in the coating formulation. This enhancement in flame retardancy can be attributed to the presence of phosphorous in PGBPA. As the percent composition of PGBPA in the system increases, the amount of phosphorous increases thus improving the flame retardancy of the coating. Phosphorous has the tendency to decompose at a lower temperature as compared to the polymer. This results in a phosphorous-rich char layer which acts as an insulation for heat transfer and reduces the accumulation of combustible gases during fire.26,27,28,29,30,31,32,33 Thus, phosphorous provides a solid-phase mechanism for fire resistance by altering the thermal decomposition process of polymers.29,30,33 LOI value for PSi-based cured films further improved (Table 6); this result can be accredited to the synergistic property of silicon and phosphorous in PSi coatings. Silicon has the tendency to form continuous layers of silica which deters oxidation of char. This enables thermal stability of the produced char at high temperatures.34,35,36,37 Further, it was observed that an increase in PGBPA value in PSi enhanced the LOI value.

Table 6 Flame-retardant properties of UV-cured coatings containing PGBPA and PSi
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As for UL-94 test, an abrupt change from VTM-2 to VTM-1 was observed in PGBPA from PGBPA5 to PGBPA10 which remained stable until PGBPA25. The burning of the specimens stopped within 30 s of ignition, and the drips were inflamed in the cases of PGBPA0 and PGBPA5. The improvement from PGBPA10 to PGBPA25 was caused due to increase in phosphorous content in formulation which enhanced flame retardancy. In PSi coatings, formulation PSi10 resulted in VTM-0 reading where the flame quenched within 10 s with no dripping of char residue. This improvement can be attributed to the synergistic effect displayed by phosphorous and silicon in the system.

Conclusion

Phosphorous (PGBPA) and silicon (Si-HEA)-based flame retardants were successfully synthesized and incorporated into UV-curable formulation along with epoxy oligomer, TMPTA and photoinitiator with different weight fractions. Mechanical properties revealed that up to 15 wt% of PGBPA, coatings exhibited excellent adhesion and pencil hardness. On the other hand, in the case of PSi, irrespective of the concentration of PSi all the coatings exhibited good adhesion and hardness. Moreover, all the coatings formulated with PGBPA and PSi, showed excellent solvent and stain resistance. Further, it was observed that with an increase in PGBPA in the coating formulation, flame-retardant properties of the coating improved. An optimum formulation with 15 wt% PGBPA and 10 wt% Si-HEA resulted in UL-94 rating of VTM-0 and LOI of 24. This enhancement in flame retardancy was attributed to the synergistic effect of phosphorous and silicon.