Preparation and Characterization of Melamine–Benzoguanamine–Formaldehyde Resins and Their Flame-Retardant Properties in Impregnated Paper for Low Pressure Laminates

Abstract

In this study, Melamine–formaldehyde (MF) resins were subjected to modification with benzoguanamine (BG) to produce MF-BG resins, followed by a comprehensive analysis of their chemical composition using advanced spectroscopic techniques such as Fourier transform infrared (FTIR), ¹H-NMR, and ¹³C-NMR spectroscopy. The flame-retardant characteristics and thermal decomposition behavior of papers impregnated with MF-BG were examined and contrasted with those treated with standard MF. In particular, the optimized MF-BG5-treated paper exhibited a Limiting Oxygen Index (LOI) value exceeding 30%, and analysis using a cone calorimeter indicated a notable decrease in parameters such as the heat-release rate (P-HRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) when compared to papers impregnated with standard MF. The findings from the scanning electron microscopy (SEM) analysis of the residual char following cone calorimeter experiments revealed that the MF-BG5-treated paper exhibited a denser and more uniform char formation. This phenomenon consequently limits the emission of combustion by-products and impedes the spread of flame. This study provides a feasible method for low-pressure laminates with better flame retardancy by using the BG additive up to a limit in MF resin.

1. Introduction

Surface coating technologies, especially those utilizing low-pressure laminates, have become integral in various industries due to their versatile applications and performance benefits [1,2,3,4]. These laminates, often incorporating melamine–formaldehyde (MF) resin, are one of the most common thermoset resins as their key components offer exceptional properties such as enhanced durability, chemical resistance, aesthetic appeal, and processability. They are extensively used in sectors ranging from automotive and aerospace to furniture and construction, providing protective coatings, decorative finishes, and functional surfaces [5,6,7].

Decor paper is utilized on the upper layer to offer the color and design of the surface. It is predominantly infused with a melamine–formaldehyde (MF) resin, owing to its distinctive surface characteristics such as hardness, chemical stability, and transparency. These properties facilitate a lustrous visual appeal of the surface design and optimal functionality at cost-effective rates [8,9,10]. The production of MF resin comprises two primary phases: methylolation and condensation. In the methylolation process, melamine interacts with formaldehyde to yield methylolmelamines. Subsequently, in the second phase, these compounds either react with each other or with unbound formaldehyde, resulting in the generation of a diverse array of oligomers that encompass methylene and methylene ether linkages [11].

Paper sheets containing amino thermosetting resin are commonly utilized in the furniture and laminate flooring industry to safeguard and embellish medium-density fiberboards (MDFs) and particleboards [12]. The prevailing practice involves the use of a melamine–formaldehyde (MF) resin, hence its widely recognized alias as “melamine finish foil”. Following the impregnation process, the paper is subjected to drying and is subsequently pressed onto the desired substrate, leading to the complete curing of the MF resin [13]. It is widely acknowledged that the manufacturing of impregnated papers follows a two-step procedure: initially, the paper is impregnated with urea formaldehyde (UF) resin, followed by a coating of melamine formaldehyde (MF) resin in the subsequent step. The ultimate saturated paper is then dried to achieve a moisture content ranging between 6% and 9% [14].

Benzoguanamine (BG) is a phenyl-substituted triazine derivative related to melamine. BG may be methoxy-methylated and/or butoxy-methylated to various extents. The average amine functionality is lower than melamine because there are only two NH₂ groups per BG molecule [15,16]. The phasing out of melamine, primarily due to its association with health-related risks, has spurred a reevaluation of laminating materials within various industries. Benzoguanamine is more widely used in surface coatings because it has greater adhesiveness than melamine. The stain resistance of laminates produced using benzoguanamine melamine formaldehyde resin is better than laminates produced using standard melamine–formaldehyde resin [17]. The benzoguanamine–melamine–formaldehyde resin exhibits better flexibility and toughness compared to melamine–formaldehyde resin. Benzoguanamine, a versatile compound, has emerged as a promising candidate, offering a compelling combination of performance attributes and a reduced environmental impact. When paired with formaldehyde resins, benzoguanamine not only provides a viable substitute but also introduces a novel dimension to low-pressure laminates (LPLs). LPLs, characterized by the combination of benzoguanamine and formaldehyde resins, represent a category of compact laminates produced under the influence of low pressure (28–34 kg/cm²) and temperature (190–225 °C). These laminates are easy to maintain and find versatile applications across industrial design, furniture manufacturing, interior surface coatings, and decorative finishes.

This study focuses on synthesizing and characterizing melamine–benzoguanamine–formaldehyde resins (MF-BG), with melamine replaced with benzoguanamine in different molar ratios. Furthermore, another important part of this study is investigating the flame retardancy of impregnated paper by MF-BG resin produced for LPLs. Benzoguanamine, known for its versatility and reduced environmental footprint compared to melamine, presents a promising avenue for advancing the field of surface coatings. By integrating benzoguanamine into melamine–formaldehyde resins, we not only aimed to comply with regulatory standards but also enhance the performance and safety profiles of LPLs.

2. Materials and Methods

2.1. Materials

Formaldehyde (37 wt% solution), melamine, diethylene glycol (DEG), and sodium hydroxide 10 wt% along with resin additives such as an acid catalyst, release agent, and surfactant, as well as the decor paper for impregnation (Barok decor paper of 65 g/m²), were provided by Kastamonu Entegre Ağaç Sanayi ve Tic. A.Ş. (Gebze, Turkiye). Furthermore, Kastamonu Entegre Ağaç Sanayi ve Tic. A.Ş. (Gebze, Turkiye) also supplied the commercial melamine–formaldehyde resin. Benzoguanamine (BG) was acquired from Sigma Aldrich (St. Louis, MO, USA).

2.2. Synthesis of Resins

Samples were formulated within a four-necked round-bottom flask that was furnished with a mechanical stirrer and a thermometer submerged in a thermostatic oil bath arrangement to uphold the reaction temperature.

After preliminary studies were carried out and the addition step was clarified, the synthesis of MF-BG resin containing 5%, 10%, and 15% BG was carried out as follows. Samples were labeled as MF-BGx, where x represents the present BG loading in percent molar ratios (5%, 10%, and 15%). Formaldehyde (26% formaldehyde solution) was loaded into a 2000 mL round-bottom flask and the pH was adjusted to 8.42. Afterward, DEG and melamine were loaded. The heating was turned on and the reaction temperature was increased to 90 °C. After 20 min, the reaction temperature reached 90 °C and the pH value was measured as 8.45. Then, nausea on the ice was monitored. The initial nausea appeared on the ice after 25 min and then the desired amount of benzoguanamine was added immediately. Approximately 1 h after adding the benzoguanamine, cooling was started at 22 water tolerance. The reaction’s endpoint was known as the hydrophobicity or the water tolerance point. The latter gives the percentage of water or mass of liquid on the reaction mixture that the MF resin can withstand before precipitating out and is a direct indicator of the extent of condensation of the resin [18]. The chemical structure of the MF-BG resins and their curing reactions are shown in Figure 1.

2.3. Physical Properties of the Resin

The viscosity of the resins was measured at 20 °C and 200 rpm using a Brookfield CAP 2000+ (Toronto, ON, Canada) viscometer with each measurement derived from the mean of three replicates. The pH level of resins was determined at 20 °C through the utilization of a Mettler Toledo (Columbus, OH, USA) pH meter. The surface tension of resins was measured in a Sigma 702 (Biolin Scientific Instruments, Shanghai, China) force tensiometer using the Ring method.

The solid contents of each resin sample, specifically MF and MF-BGs, were quantified through the process of heating 2 g of the sample in an aluminum pan within an oven set at 120 °C for a duration of two hours until a consistent weight was achieved. The water tolerance of the resins was assessed through the transfer of a 5 mL portion of each resin into a test tube maintained at 25 °C. The quantity of distilled water added, leading to cloudiness in the mixture, was employed in the subsequent calculation.

2.4. Paper Impregnation

Decorative paper sheets of 21.0 × 29.7 cm (A4 size) were impregnated with a commercial MF resin and all MF-BG resin formulations prepared in this study by using small amounts of additives such as acid catalyst (0.5–1 w/w%), release (0.25–0.50 w/w%), anti-block (0.10–0.15 w/w%), anti-dust (0.10–0.15 w/w%), and wetting agents (0.30–0.50 w/w%). All papers underwent a two-stage process within a laboratory impregnation system. During the laboratory-scale paper impregnation, the decor paper was immersed in MF-BG resin for 10 s. Subsequently, the impregnated paper passed through dual rollers to eliminate surplus resin. This impregnation procedure was duplicated twice for each test specimen. Following resin removal, the moist paper was affixed to a drying frame using magnets and dried for 10 s at 120 °C in a Mathis LTE lab Dryer. The dried paper was then re-immersed in MF-BG resin for 60 s, after which it underwent the same roller process to eliminate excess resin. The moist paper was then dried for 155 s at 120 °C in the Mathis LTE (Niederhasli, Switzerland) lab Dryer. All papers exhibited a resin content ranging from 53%–55% and moisture content between 5% and 6% post-drying. To simulate industrial drying on a laboratory scale, a Mathis LTE lab Dryer was utilized with an inline infrared sensor to assess the surface film temperature. The oven maintained a constant temperature of 120 °C, with the fan operating at 1500 rotations per minute.

2.5. Low-Pressure Laminates Production

The medium-density fiberboard (MDF) with a thickness of 8 mm was covered with the resin-impregnated papers on both sides and then cured under a compression molding press (34 kg/cm²) in the temperature range of 140–160 °C for 70 s. The preparation of the MF-BG resin and low-pressure laminates is shown in Figure 2.

2.6. Low-Pressure Laminate Characterization

A Taber abraser (model 5155, New York, NY, USA) was utilized for the purpose of investigating the resistance to abrasion in accordance with the EN438-2 standard [19]. Observations of the specimens were made subsequent to every 100 revolutions, with a replacement of the abrasive papers occurring after every 200 revolutions until the initial wear was identified [11].

The scratch resistance of the samples was analyzed according to the EN 14323 standard, and the results were expressed as numerical ratings [20]. Two square-shaped specimens of each laminate were subjected to scratching using an Erichsen Scratch Tester Model 413 (Hemer, Germany).

The stain resistance of the samples was determined according to EN 14323. Three samples of each laminate were prepared with dimensions of 70 mm × 70 mm. Different stain agents were applied to the surface of the samples and kept for a specific time. At the end of this period, the surfaces of the samples were cleaned, and it was determined whether there was a color change on the surfaces. The categorization of the surface was conducted utilizing a scale rating ranging from 1 to 5, whereby a rating of 5 denoted no discernible alterations.

The determination of the acid value in low-pressure laminates serves as an indicator of the chemical resilience of the surface. This value was ascertained through the treatment of a specified surface area with concentrated hydrochloric acid for a duration of 15 min, followed by a microscopic classification of the surface based on an arbitrary scale varying from very good or “5” (indicating no acid-induced damage) to very poor or “1” (representing complete surface destruction) in increments of 0.5 units. Acid values below 3 indicate minimal susceptibility to hydrochloric acid attack and affirm the high quality of the film [21].

The water vapor resistance of low-pressure laminates was analyzed according to EN 14323. For a period of 1 h, the laminate was positioned atop a conical flask with the paper side exposed to boiling water vapor. Subsequently, the sample was extracted and left to equilibrate to room temperature over a 24 h duration. Visual inspections to identify alterations on the surface of the laminates, such as color transformations, swelling, or delamination, were conducted, with outcomes expressed utilizing a standard scale from 1 to 5.

The dry heat resistance of the low-pressure laminates was examined according to EN 14323. The exposure of laminate samples’ surfaces to dry heat involved 20-min contact with a metal block previously heated to 160 °C within an oven. Following this interval, the removal of the metal block allowed the laminate sample to cool off for 45 min. The surface was evaluated and rated on a scale ranging from 1 to 5, with a score of 5 indicating the absence of visible alterations [11].

2.7. Characterization

The determination of the limiting oxygen index (LOI) of melamine-impregnated paper was conducted using the ProWhite limiting oxygen index analyzer (Prowhite, Istanbul, Türkiye) in accordance with the TS EN ISO 4589-2 standard [22]. Samples were sized at 150 mm × 55 mm. The flammability assessment was carried out utilizing a cone calorimeter (Fire Testing Technology ICone, East Grinstead, UK) with a heat flux of 50 kW/m², and the distance between the heater and the sample was maintained at 60 mm.

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Alpha-P spectrometer (Billerica, MA, USA) in Attenuated Total Reflectance (ATR) in the range of 4000–650 cm⁻¹. ¹H and ¹³C spectra were recorded on a Varian 500 MHz (500 MHz for ¹H, 125 MHz for ¹³C, Palo Alto, CA, USA) spectrometer at room temperature; the chemical shifts (δ) were measured in ppm with respect to the solvent residual proton chemical shift (CDCl₃, ¹H: δ = 7.26 ppm, ¹³C: δ = 77.2 ppm; [D6] DMSO, ¹H: δ = 2.50 ppm, ¹³C: δ = 39.5 ppm) or with TMS as internal standard. The thermal characteristics of MF- and MF-BGx-impregnated paper samples were recorded with the Mettler Toledo TGA 851 instrument at a heating rate of 10 °C/min and differential scanning calorimeter DSC 822 at a heating rate of 10 °C/min under N₂ flow of 50 mL/min between 25 °C and 600 °C for TGA and 25 °C and 250 °C for DSC, respectively. Images of the impregnated paper samples were obtained via scanning electron microscopy (SEM) using a Philips XL30 SFEG at 15 kV (Amsterdam, The Netherlands).

3. Results and Discussion

3.1. Characterization of Modified MF Resin

The physical properties of all modified MF resins are summarized in

Table 1. The physical properties of modified MF resins.

Properties	Standard MF	MF-BG5	MF-BG10	MF-BG15
Solid Content (%)	54.00	54.55	54.70	54.85
Viscosity (cPs)	30	48	43	45
Gel Time (sec)	45	43	48	51
Density (g/cm3)	1230	1228	1225	1228
pH	9.30	9.30	9.40	9.31
Water Tolerance (g/100 g)	10–20	10–13.5	10–15	10–14
Surface Tension(mN/m)	54.60	57.18	58.10	57.16

. The addition of BG did not change the gel time and surface tension, which affected the paper impregnation parameters significantly. Parameters like density, pH, and solid content also remained largely unaltered compared to the commercial MF resin. The water tolerance was the sole property impacted by the introduction of BG due to the presence of the benzene ring originating from BG in the final resin compositions.

The physical properties of the low-pressure laminates are outlined in

Table 2. Physical properties of low-pressure laminates with modified MF resin.

Test	Test Method	Standard Limits		Units	Standard MF	MF-BG 5	MF-BG 10	MF-BG 15
Resistance to Abrasion	TS EN 438-2	Class 1 Class 2 Class 3A Class 3B Class 4	<50 ≥50 ≥150 ≥250 ≥350	Revolution	50	50	50	50
Resistance to Scratch	TS EN 14323	Min. ≥ 4 N		Newton	5	4.5	4.5	4
Resistance to stain	TS EN 14323	Min. 3		-	5	5	5	5
Acid Value	Keas Special	Min. 4		-	5	2	2	2
Resistance to Dry Heat	TS EN 14323	Min. 4		-	5	5	5	5
Resistance to Water Vapour	TS EN 14323	Min. 4		-	5	5	5	5

. Low-pressure laminates developed using conventional resin exhibited better scratch resistance compared to those prepared with MF-BG resins. The acid value of low-pressure laminates made with MF-BG resins was lower than that of laminates made with commercial MF resin under the same pressing conditions. Despite these differences, the low-pressure laminates obtained with MF-BG resin still met the standard requirements for abrasion resistance, water vapor resistance, and dry heat resistance.

The comparative FTIR spectra of MF and MF-BG resins are illustrated in Figure 3. The broad band at 3340–3220 cm⁻¹ is attributed to N-H and O-H stretching vibrations available on the melamine formaldehyde structures. The band at 2960 cm⁻¹ corresponds to the C–H stretching vibration in the C-H methylol group (OCH₂) [15], while that at 1167 cm⁻¹ corresponds to the C–O stretching vibration of aliphatic ether groups. The peaks at 1500–1380 cm⁻¹ and 1050–1030 cm⁻¹ are associated with C-H stretching vibration in CH₂ and CH₃ and C-N stretching of methylene linkages (NCH₂N), respectively. In addition, the characteristic absorption bands associated with triazine ring bending vibrations of melamine are visible at 808 cm⁻¹, while the absorption of –C=N stretching of secondary amines in the triazine ring is noted at 1550 cm⁻¹ [16,23,24]. MF-BG resins exhibit very similar spectra to the MF resin with the same absorption bands, while showing a band at 705 cm⁻¹, indicating the presence of a monosubstituted benzene ring that can be attributed to the presence of the benzoguanamine structure.

Figure 4 shows the ¹H-NMR spectra of three MF-BG resin formulations alongside a reference commercial MF for comparison. The intensity of the phenyl ring proton resonances at δ = 8.2 to 8.3 ppm arising from the BG increases with the increasing BG ratio in the MF-BG formulations. The protons of DEG used in the preparation of MF-BG resins are observed at δ = ~3.4–3.5 ppm in each resin. The chemical shifts in the protons in methylol groups on secondary and tertiary nitrogen atoms, M-NH-CH₂-OH and M-N(CH₂-OH)₂, are observed at δ = 4.75–5.00 ppm, while the peaks corresponding to the primary amino groups (M-NH₂) appear at δ = 6.1–6.4 ppm (Figure 4). The chemical shifts for the secondary amino groups (M-NH-R) are observed at δ = 7.00–7.30 ppm (Figure 4).

To further investigate the structural properties of the developed resin formulations, we also used ¹³C NMR (Figure 5). The carbon resonances between δ = 166.0 and 168.0 ppm were associated with substituted and unsubstituted BG and triazine rings. Resonances from δ = 129.0 to 137.0 ppm are ascribed to the carbon atoms present in the benzene ring of BG. The characteristic signals of methylene–ether linkages appear at δ = 67.0 to 70.0 ppm, and those of methylol groups are found at δ = 64.2 to 63.5 ppm. It should be noted that all four resin formulations exhibit similar chemical shifts due to the presence of the same functional groups. An exception to this is observed in the standard MF resin, which does not show carbon resonances between δ = 129.0 and 138.0 ppm, indicating that the inclusion of BG was successful in the modified MF-BG resins. This confirms that BG spacers were successfully integrated into the MF resins [25,26].

3.2. Thermal Analysis of MF-BG Resin Formulations

TGA is one of the most widely used techniques to evaluate the thermal stability and thermal decomposition of various materials. The initial decomposition temperature is defined as the point at which a 5 wt% weight loss occurs, while Tmax represents the temperature corresponding to the highest weight loss rate [27]. The TGA and DTG profiles of both MF and MF-BG resins under an N₂ atmosphere are depicted in Figure 6, alongside detailed data presented in

Table 3. Summary of selected thermal properties of MF-BG-impregnated papers under N₂.

Sample	T5%/°C	T10%/°C	Tmax (°C)
MF	160	269	340
MF-BG5	163	285	340
MF-BG10	149	251	340
MF-BG15	156	274	340

.

All DTG curves exhibit one major decomposition peak (Figure 6), revealing that MF- and MF-BG-impregnated papers undergo a single-step decomposition process and that various amounts of BG do not affect the thermal decomposition route of MF. The details of observed data such as T_5%, T_10%, T_max, and LOI are also summarized in Table 3. As seen from the TGA curves of each impregnated paper, weight loss is observed between 30 and 50 °C, caused by volatile gases such as moisture and formaldehyde. With the addition of 5% BG, the lowest weight loss is seen in this range. It is observed that the addition of 10% and 15% BG does not contribute to thermal stability, and even indicates more weight loss compared to MF-impregnated paper. This is thought to be due to the fact that adding more than 5% BG to MF reduces the crosslink density of the MF network polymer. Between 250 and 350 °C, the weight loss is due to the release of formaldehyde as a result of the breaking of ether bridges. As seen, with the addition of BG, the T_5%, T_10%, and T_max values increase on MF-BG5 and then decrease by increasing the BG ratio to 10% and 15%.

3.3. Flame-Retardant Performance Analysis

Since the combustion environment of the cone calorimeter is very similar to that of the real fire, it is frequently used to sufficiently investigate the burning behavior of flame-retardant materials. We therefore used a cone calorimeter to investigate the heat and smoke-release behaviors of MF- and MF-BG-impregnated papers [28]. The heat-release rate (HRR), total heat release (THR), smoke production rate (SPR), and total smoke production rate (TSP) versus time curves of MF-BG-impregnated papers are comparatively shown in Figure 7, and some significant data, such as the time to ignition (TTI), the peak heat-release rate (P-HRR), THR, TSP, Av-CO (average CO yield), and Av-CO₂ (average CO₂ yield), are listed in

Table 4. Cone calorimeter result of MF- and MF-BG-impregnated papers.

Sample	TTI (s)	P-HRR (kW/m²)	THR (MJ/m²)	TSP (m²/m²)	av-CO (kg/kg)	av-CO2 (kg/kg)	LOI/%
MF	4	127.13	0.78	0.29	0.04	0.45	35.2
MF-BG5	6	76.78	0.46	0.19	0.03	0.32	30.7
MF-BG10	7	106.71	0.77	0.47	0.01	0.50	33.4
MF-BG15	6	160.87	1.34	0.32	0.03	0.69	34.2

. The P-HRR is a vital parameter for evaluating performance in fire. A high P-HRR value reflects a low fire resistance [29,30].

As shown in Table 4, MF is easily flammable after ignition, as the PHRR is 127.13 kW/m² at approximately 10 s. In contrast, with the incorporation of BG and MF resin formulations, a drastic reduction in the PHRR can be observed. The PHRR of MF-BG5 showed a relatively lower value (76.78 kW/m²) than MF-BG10- (106.71 kW/m²) and MF-BG15-impregnated papers (160.87 kW/m²). It shows that the addition of 5% BG into the MF resin formulation can promote the formation of a protective char layer on the impregnated paper and thus reduce heat release during combustion. On the other hand, the addition of 10% and 15% BG makes the MF resin more flammable than the standard MF, thus decreasing the fire-retardant features. The reason for this is that adding more than 5% BG into the MF will decrease the crosslink density of the network polymer MF (see Figure 1) and, thus, will negatively affect properties such as surface area and porosity to protect the paper from fire.

The data in Figure 7 also showed that the TSP of the MF-BG5- (0.19 m²/m²) impregnated paper was greatly reduced compared to MF-impregnated paper (0.29 m²/m²). The SPR and TSP results suggest that MF-BG5-impregnated paper suppressed the formation of smoke very well. Furthermore, the Av-CO and Av-CO₂ values of MF-BG5 are also less than those of MF-impregnated paper. These results suggest that MF-BG5-impregnated paper suppressed the formation of smoke very well. On the other hand, it can be seen that the LOI value of MF-BG5-impregnated paper decreased and its LOI value was around 30.7%. This result reveals that MF-BG5 still has a high enough LOI to be considered a self-extinguishing (LOI > 21%) material but comparatively higher fire risk than MF, MF-BG10, and MF-BG15 resin formulations.

To further investigate the flame retardancy properties, a digital image of the remaining impregnated paper specimens was taken after the cone calorimeter experiments (Figure 8). It was established that the presence of a carbon layer provides efficient shielding for the underlying material, thereby impeding the spread of fire [31]. As seen in Figure 8, commercial MF-impregnated paper almost completely burned during the cone calorimeter test whilst only a small amount of residual char remained on the surface. In contrast, a swelling char layer was clearly observed on the surface of the MF-BG-impregnated paper samples. In fact, the amount of char residue increases by increasing the BG loading (5%–10%) in the MF resin formulation. However, MF-BG5- and MF-BG10-impregnated paper samples had a thick and consistent swelling char layer on the surface after the cone calorimeter test whilst MF-BG15 could not form a similar residual carbonized layer and thus could not prevent heat transfer.

The increase in the carbon monoxide (CO) ratio during combustion means that the efficiency of oxygen and carbon interaction during combustion is low. This may explain the oxygen level required for combustion in the MF-BG15 sample. In MF-BG5, although the oxygen level required for combustion was low, carbon monoxide levels were high. This may be an indication of inefficient combustion for MF-BG 5. Low smoke formation is an important criterion in combustion resistance. MF-BG resins showed an improvement in an important criterion demanded in flame-retardant surfaces in the wood-based panel industry by reducing the rate of smoke formation.

The investigation of the surface morphology of char residues following cone calorimeter analysis was also conducted using SEM analysis, as illustrated in Figure 9. The MF-impregnated paper (Figure 9(1a)) displays big holes and a fluffier surface layer, which may enhance fire spread and oxygen permeation, resulting in complete combustion (Figure 9(2a)). In contrast, the MF-BG5 and MF-BG10 formulations show a denser and more uniform char layer with fewer visible pores, suggesting improved flame retardancy by reducing combustion by-products and slowing fire progression [32]. However, the MF-BG15 formulation shows large holes, similar to MF-impregnated paper, likely due to reduced crosslink density from the higher BG content (Figure 9(1d,2d)). These observations indicate that the char layer plays a crucial role in enhancing the flame retardancy of the impregnated papers.

4. Conclusions

In summary, various amounts (5, 10, and 15%) of BG were introduced as a component of the MF resin formulation and successfully used in impregnated papers for low-pressure laminates. The chemical composition of the MF-BG formulations was analyzed by FTIR, ¹H-NMR, and ¹³C-NMR. The thermal stability of the resulting impregnated papers was compared, and it was found that the addition of BG at 5% to the MF resin resulted in an improvement, whilst the addition of a higher molar percentage (10% and 15%) negatively affected the thermal stability. SEM imaging demonstrates the formation of a dense char layer on the surface of impregnated paper after burning due to the addition of 5% BG to the MF formulation, whilst the increased BG amount does not show a similar feature. The residual char layer generated serves as a hindrance against heat and mass transfer, effectively isolating the oxygen supply in the environment during combustion processes. Therefore, the more compact char layer formed on the MF-BG5-impregnated paper led to a significant reduction in P-HRR, THR, and TSP values.

In the examination of the surface tension of MF and MF-BG resins, a rise in surface tension values was observed following the incorporation of BG. This escalation could potentially be attributed to the presence of the benzene ring within the BG structure. It is plausible to assert that the benzene ring contributes to the augmentation of surface energy by elevating the dispersive index of the overall surface tension. Likewise, an upsurge in viscosity values was observed upon the introduction of BG.

Moreover, in addition to the improved properties outliened above, an improvement in the scratch resistance in low-pressure laminates fabricated using the MF-BG resins was noted in comparison to the reference MF resin. The improvement in scratch resistance can be attributed to the presence of the benzene ring causing a deceleration in the curing process through the generation of steric hindrance, leading to a softer structure of the paper. The low-pressure laminates fabricated using MF-BG resins demonstrate a performance equivalent to those derived from commercial MF resins with regard to the resistance against abrasion, water vapor, dry heat, and staining. Hence, this study provides a novel approach to producing high-performance impregnated paper LPLs with better flame retardancy and improved scratch resistance via the addition of 5% BG into the MF resin formulation.