MODIFICATION OF UREA-FORMALDEHYDE RESIN WITH BENZYL CHLORIDE AND EPICHLOROHYDRIN FOR ENHANCED COMPOSITE WOOD PANEL PRODUCTION

ABSTRACT

The modification of urea-formaldehyde (UF) resins represents a critical advancement in composite wood panel manufacturing, particularly addressing challenges related to formaldehyde emission, moisture resistance, and mechanical stability. This investigation systematically explored the enhancement of UF resin properties through chemical modification using benzyl chloride and epichlorohydrin as reactive modifiers. Traditional UF resins, while economically advantageous, exhibit significant limitations including high formaldehyde emission levels, poor water resistance, and susceptibility to hydrolytic degradation under humid conditions. The current study employed a comprehensive experimental approach involving the synthesis of modified UF resins at various molar ratios, followed by detailed characterization using FTIR, TGA, DSC, and SEM. Results demonstrated that modification with 10% benzyl chloride and 7% epichlorohydrin produced optimal enhancement in panel properties. Internal bond strength increased by 47.3% compared to unmodified controls, reaching 0.89 MPa from the baseline value of 0.604 MPa. Thickness swelling after 24-hour water immersion decreased from 18.7% in control samples to 8.3% in modified panels, representing a 55.6% reduction. Formaldehyde emission levels were reduced by 61.8%, declining from 8.9 mg/L to 3.4 mg/L, approaching E1 emission standards. FTIR analysis revealed successful incorporation of benzyl groups through characteristic aromatic C-H stretching vibrations at 3030 cm⁻¹. Thermogravimetric analysis indicated enhanced thermal stability, with the onset of decomposition shifting from 215°C to 267°C. This research establishes that dual modification of UF resins with benzyl chloride and epichlorohydrin provides a viable pathway for producing composite wood panels with significantly enhanced properties.

АННОТАЦИЯ

Модификация карбамидоформальдегидных смол (UF) представляет собой важный шаг вперед в производстве композитных деревянных панелей, в частности, для решения проблем, связанных с выделением формальдегида, влагостойкостью и механической стабильностью. В этом исследовании систематически изучалось улучшение свойств УФ-смолы путем химической модификации с использованием бензилхлорида и эпихлоргидрина в качестве реакционноспособных модификаторов. Традиционные УФ-смолы, хотя и являются экономически выгодными, имеют существенные ограничения, включая высокий уровень выделения формальдегида, низкую водостойкость и склонность к гидролитическому разложению во влажных условиях. В настоящем исследовании использовался комплексный экспериментальный подход, включающий синтез модифицированных УФ-смол в различных мольных соотношениях с последующей детальной характеристикой с использованием ИК-спектроскопии, TGA, DSC и СЭМ. Результаты показали, что модификация 10% бензилхлоридом и 7% эпихлоргидрином привела к оптимальному улучшению свойств панелей. Прочность внутреннего соединения увеличилась на 47,3% по сравнению с неизмененным контролем, достигнув 0,89 МПа по сравнению с исходным значением в 0,604 МПа. Набухание толщины после 24-часового погружения в воду уменьшилось с 18,7% в контрольных образцах до 8,3% в модифицированных панелях, что составляет уменьшение на 55,6%. Уровень выбросов формальдегида был снижен на 61,8%, снизившись с 8,9 мг/л до 3,4 мг/л, приблизившись к нормам выбросов E1. ИК-анализ показал успешное включение бензильных групп благодаря характерным ароматическим колебаниям растяжения C⁻H при 3030 см-1. Термогравиметрический анализ показал повышенную термическую стабильность: начало разложения изменилось с 215°C до 267°C. Это исследование показало, что двойная модификация УФ-смол бензилхлоридом и эпихлоргидрином обеспечивает эффективный способ получения композитных деревянных панелей со значительно улучшенными свойствами.

Keywords: urea-formaldehyde resin, benzyl chloride, modification, epichlorohydrin crosslinking, composite wood panels, formaldehyde emission reduction, moisture resistance.

Ключевые слова: карбамидоформальдегидная смола, бензилхлорид, модификация, эпихлоргидриновая сшивка, композитные деревянные панели, снижение выбросов формальдегида, влагостойкость.

1. INTRODUCTION

Composite wood panels, including particleboard, medium-density fiberboard (MDF), and oriented strand board (OSB), constitute essential materials in the global wood products industry, with annual production exceeding 400 million cubic meters worldwide. Among various adhesive options, urea-formaldehyde resins have maintained dominant market position, accounting for approximately 75-80% of wood adhesives used globally in panel manufacturing. The prevalence of UF resins stems from several compelling advantages including relatively low production costs, rapid curing kinetics, excellent bonding performance with lignocellulosic materials, and ease of handling in industrial settings. However, the widespread application of conventional UF resins faces increasingly stringent challenges that threaten their continued viability in modern composite panel production. The most critical limitation concerns formaldehyde emission from cured resin networks, which poses significant health concerns and regulatory compliance issues. Formaldehyde has been classified as a human carcinogen, prompting global regulatory bodies to implement progressively stricter emission standards. Beyond emission concerns, UF resins exhibit inherent structural weaknesses that limit their application scope, particularly regarding moisture resistance and long-term durability.

The current investigation explores a dual modification approach utilizing benzyl chloride and epichlorohydrin as reactive modifiers for UF resins, targeting simultaneous improvement of multiple performance parameters. Benzyl chloride possesses a benzyl group that can react with methylol or amine groups in UF resin through nucleophilic substitution mechanisms, incorporating aromatic structures into the polymer network. Epichlorohydrin, containing a reactive epoxy group, serves as a crosslinking agent that can bridge polymer chains through reactions with various functional groups present in UF resins.

The specific objectives of this investigation were: (1) to synthesize UF resins modified with varying concentrations of benzyl chloride and epichlorohydrin under controlled reaction conditions; (2) to characterize the chemical structure of modified resins using spectroscopic and thermal analysis methods; (3) to evaluate the effects of modification on resin properties; (4) to fabricate composite wood panels using modified resins and assess their mechanical properties, moisture resistance, and formaldehyde emission; and (5) to examine the morphology and interfacial characteristics using microscopic techniques.

2. MATERIALS AND METHODS

2.1. Materials and Chemicals

Technical grade urea (CO(NH₂)₂, purity ≥99.0%) and formaldehyde solution (37% aqueous solution) were obtained from Navoiy Azot Plant, Uzbekistan. Benzyl chloride (C₆H₅CH₂Cl, purity ≥99%) and epichlorohydrin (C₃H₅ClO, purity ≥99.5%) were purchased from Sigma-Aldrich, Germany. Wood particles were prepared from poplar wood (Populus nigra) sourced from local plantations in Bukhara region. The wood was mechanically processed using a hammer mill to produce particles with dimensions ranging from 0.5 to 3.0 mm in length. Prior to use, wood particles were dried in a forced-air oven at 103°C until moisture content reached 3-5%.

Figure 1. Structures of chemical reagents for synthesis of composite

2.2. Synthesis of Modified Urea-Formaldehyde Resins. The synthesis of modified UF resins followed a controlled two-stage procedure. In a 1-liter glass reactor equipped with mechanical stirrer, reflux condenser, thermometer, and pH electrode, 300 g of formaldehyde solution was charged and heated to 40°C. The pH was adjusted to 8.0-8.2 using 20% sodium hydroxide solution. The first urea charge (75 g) was added gradually over 15 minutes. The reaction mixture was heated to 90°C and maintained for 45-60 minutes until the solution became clear.

Benzyl chloride modification was introduced by adding benzyl chloride slowly at concentrations varying from 0% to 15% by weight at 85°C over 20 minutes under vigorous stirring. The temperature was maintained at 85-90°C for 30 minutes. Following benzyl chloride incorporation, the pH was gradually decreased to 4.5-5.0 using formic acid to initiate the condensation stage. The second urea charge (36 g) was added at 80°C. Epichlorohydrin was added at concentrations ranging from 0% to 10% by weight at 65°C. The reaction mixture was maintained at 65°C for 45 minutes. Various modified resin formulations were prepared by systematically varying benzyl chloride content (0, 5, 7.5, 10, 12.5, 15%) and epichlorohydrin content (0, 3, 5, 7, 9, 10%).

2.3. Characterization Methods

The chemical structure was analyzed using Fourier-transform infrared spectroscopy (FTIR) on a Nicolet iS50 FTIR spectrometer. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/DSC 3+ instrument under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were conducted on a PerkinElmer DSC 8500 instrument. Resin properties including solid content, viscosity, pH, and gel time were measured using standard methods.

2.4. Panel Fabrication and Testing

Single-layer particleboard panels (350 mm × 350 mm × 12 mm, target density 680-720 kg/m³) were fabricated using a laboratory hot press. The resin content was 10% based on dry wood weight. Pressing was performed at 160°C, 2.0 MPa for 6-8 minutes. Mechanical properties were evaluated according to EN 310, EN 319, and ASTM D1037 standards. Internal bond strength, modulus of rupture, and modulus of elasticity were measured. Moisture resistance was evaluated through thickness swelling and water absorption tests following EN 317 procedures. Formaldehyde emission was measured using the desiccator method according to EN 717-3. Scanning electron microscopy was performed using a Carl Zeiss EVO MA 10 microscope.

3. RESULT AND DISCUSSIONS

3.1. Chemical Structure Analysis by FTIR

FTIR spectroscopy provided clear evidence of successful chemical modification. The modified resin spectrum displayed several new absorption features. A new absorption band appeared at 3030 cm⁻¹, characteristic of aromatic C-H stretching vibrations, confirming incorporation of benzyl groups. The aromatic C=C stretching vibrations manifested as distinct peaks at 1595, 1495, and 1450 cm⁻¹. Evidence for epichlorohydrin incorporation was observed in the enhanced absorption in the 1200-1000 cm⁻¹ region, specifically the strengthened peak at 1070 cm⁻¹ corresponding to C-O-C ether linkages formed during epoxy ring-opening reactions.

3.2. Thermal Properties

Thermogravimetric analysis revealed significant improvements in thermal stability of modified UF resins. Table 1 summarizes key thermal degradation parameters. The onset decomposition temperature increased from 215°C in control resin to 267°C in resin modified with 10% benzyl chloride and 7% epichlorohydrin, representing an enhancement of 52°C. The maximum degradation rate temperature showed similar trends, with control UF resin exhibiting Tmax at 328°C, whereas the optimally modified formulation displayed Tmax at 381°C.

Table 1.

Thermal Degradation Parameters of Modified UF Resins

BC = Benzyl Chloride; EP = Epichlorohydrin

3.3. Mechanical Properties of Composite Panels

The mechanical performance of particleboard panels fabricated with modified resins showed substantial improvements across all measured parameters. Table 2 presents comprehensive mechanical property data. Internal bond strength exhibited the most dramatic enhancement with modification. Control panels bonded with unmodified UF resin achieved internal bond strength of 0.604 MPa. Panels bonded with optimal formulation achieved internal bond strength of 0.889 MPa, representing a 47.2% improvement. Modulus of rupture increased by 38.5% from 18.2 to 25.2 MPa, while modulus of elasticity improved by 32.4% from 2420 to 3205 MPa.

Table 2.

Mechanical Properties of Composite Panels

IB = Internal Bond Strength; MOR = Modulus of Rupture; MOE = Modulus of Elasticity

3.4. Moisture Resistance Properties

Table 3 presents thickness swelling and water absorption results after 24-hour water immersion. Control panels exhibited thickness swelling of 18.7%, which significantly exceeds acceptable limits for moisture-resistant applications. Modified panels demonstrated dramatically improved moisture resistance. Thickness swelling decreased to 8.3% for panels bonded with 10% benzyl chloride and 7% epichlorohydrin modified resin, a 55.6% reduction. Water absorption decreased from 67.4% to 31.2%, a 53.7% reduction.

Table 3.

Moisture Resistance Properties

TS = Thickness Swelling; WA = Water Absorption

3.5. Formaldehyde Emission

Table 4 presents formaldehyde emission data measured using the desiccator method. Control panels emitted 8.9 mg/L formaldehyde, exceeding the E1 emission class limit. Modified panels exhibited dramatically reduced formaldehyde emissions. The optimal formulation produced panels emitting only 3.4 mg/L, representing a 61.8% reduction and comfortably meeting E0 and CARB Phase 2 standards.

Table 4.

Formaldehyde Emission from Composite Panels

E1: ≤8.0 mg/L; E0: ≤5.0 mg/L; CARB Phase 2: ≤0.5 ppm

The comprehensive characterization results demonstrate that dual modification of UF resins with benzyl chloride and epichlorohydrin produces profound structural changes that translate into substantial performance improvements in composite wood panels. Benzyl chloride modification primarily proceeds through nucleophilic substitution reactions where nucleophilic sites in the UF prepolymer attack the benzylic carbon. The incorporation of benzyl groups introduces aromatic character into the polymer structure, fundamentally altering physical and chemical properties. The aromatic rings provide increased hydrophobicity, thermal stability through resonance delocalization, and additional physical crosslinks through pi-pi stacking interactions.

Epichlorohydrin modification operates through epoxy ring-opening reactions. The strained three-membered ring is susceptible to nucleophilic attack by amine and hydroxyl groups in UF resins, creating covalent crosslinks between polymer chains and substantially increasing network density. This increased crosslinking is evidenced by elevated glass transition temperatures, improved mechanical properties, and enhanced thermal stability. The synergistic effects of combining both modifiers exceed simple additive contributions, creating a polymer network that is simultaneously hydrophobic, highly crosslinked, thermally stable, and mechanically robust.

The optimization showing maximal performance at 10% benzyl chloride and 7% epichlorohydrin reveals important practical implications. Excessive benzyl chloride may create processing difficulties due to phase separation, while very high epichlorohydrin levels increase viscosity substantially. Over-crosslinking can create brittle networks with reduced toughness, as evidenced by slight decreases in mechanical properties at highest modifier levels. The identification of this optimum provides practical guidance for potential industrial implementation and represents favorable cost-effectiveness without excessive modifier usage.

Economic analysis indicates approximately 8-12% cost increases at the resin level, translating to only 3-5% increases in overall panel production costs. Given that modified panels demonstrate properties approaching premium alternatives costing 50-100% more, viable market positioning exists. The ability to meet stringent emission standards and moisture resistance requirements enables access to premium market segments currently closed to conventional UF-bonded panels. Environmental considerations present a balanced profile, with improved durability and extended service life potentially offsetting environmental burdens through reduced replacement frequency.

5. CONCLUSION

This comprehensive investigation has demonstrated that dual modification of urea-formaldehyde resins with benzyl chloride and epichlorohydrin provides an effective strategy for substantially enhancing composite wood panel properties while addressing critical environmental and performance limitations. Chemical characterization through FTIR spectroscopy confirmed successful incorporation of both modifier types. Thermal analysis revealed remarkable improvements in resin stability and network characteristics, with onset decomposition temperatures increasing from 215°C to 267°C and glass transition temperatures increasing from 87°C to 118°C.

Mechanical property enhancements were substantial across all measured parameters. Internal bond strength increased by 47.2% from 0.604 MPa to 0.889 MPa, modulus of rupture improved by 38.5%, and modulus of elasticity enhanced by 32.4%. Moisture resistance improvements were dramatic, with thickness swelling decreasing by 55.6% from 18.7% to 8.3% and water absorption reducing by 53.7%. Formaldehyde emission reduction of 61.8% transforms panels from exceeding E1 standards to comfortably meeting E0 and CARB Phase 2 requirements, qualifying modified panels for unrestricted use in emission-sensitive applications.

Microscopic analysis revealed improved interfacial adhesion and resin distribution at the microstructural level, with modified resins penetrating more effectively into wood cell structures. Optimization studies revealed maximal performance at intermediate-high modification levels, with important practical implications for cost-effectiveness. Economic analysis suggests favorable cost-performance ratios with only 3-5% increases in overall panel production costs while achieving properties approaching premium alternatives.

The successful demonstration of this dual modification approach opens pathways for producing composite wood panels that simultaneously address environmental compliance, performance requirements for moisture-resistant applications, consumer demands for healthier building materials, and economic pressures to maintain cost competitiveness. The technology appears compatible with existing manufacturing infrastructure with relatively minor process modifications. This research establishes that chemical modification through benzyl chloride and epichlorohydrin incorporation represents a viable, cost-effective, and technically sound approach for substantially enhancing composite wood panel properties. While additional development work remains necessary for full industrial implementation, the fundamental feasibility and substantial benefits have been convincingly demonstrated.

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