EFFECT OF MODIFIED ADHESIVES ON THE PHYSICAL-MECHANICAL PROPERTIES OF REED PARTICLEBOARDS

ABSTRACT

This study developed a method for producing reed particleboards using modified urea-formaldehyde resin (UFR) synthesized via a three-stage process (alkaline, weakly acidic, alkaline) with polyvinyl alcohol and carboxymethylcellulose additives. The resin’s physicochemical properties were optimized, and boards were produced by hot pressing reed particles at 155–165°C and 14–15 MPa for 6 minutes. The boards exhibited density (646–754 kg/m³), bending strength, and screw withdrawal resistance, suitable for construction and furniture applications. Scanning electron microscopy (SEM) revealed uniform resin distribution and low porosity, enhancing mechanical properties. The method reduces formaldehyde emissions, aligning with E1 standards, and utilizes renewable reed resources. Further research should explore water absorption and durability.

АННОТАЦИЯ

В данном исследовании разработан метод производства камышо-стружечных плит с использованием модифицированного карбамидоформальдегидного клея, синтезированного в три этапа (щелочная, слабо кислая, щелочная среда) с добавлением поливинилового спирта и карбоксиметилцеллюлозы. Физико-химические свойства клея оптимизированы, плиты изготовлены горячим прессованием при 155–165°C и 14–15 МПа в течение 6 минут. Плиты показали плотность (646–754 кг/м³), прочность на изгиб и сопротивление выдергиванию шурупов, пригодные для строительства и мебели. Сканирующая электронная микроскопия (СЭМ) выявила равномерное распределение клея и низкую пористость, улучшающие механические свойства. Метод снижает эмиссию формальдегида (стандарт E1) и использует возобновляемый камыш. Рекомендуются исследования водопоглощения и долговечности.

Keywords: urea-formaldehyde resin, reed-shavings boards, physical and mechanical properties, SEM analysis, composite materials, environmental sustainability.

Ключевые слова: карбамидоформальдегидная смола, тростниково-стружечные плиты, физико-механические свойства, СЭМ-анализ, композиционные материалы, экологическая устойчивость.

1. Introduction

Urea-formaldehyde resins (UFR) are widely used adhesive materials in the production of particleboards and other composite materials due to their low cost, high adhesion capability, and chemical stability. These resins are prepared through a three-stage synthesis process (alkaline, weakly acidic, and again alkaline medium), which allows for optimization of their polycondensation degree and reduction of free formaldehyde content [1, 2]. In recent years, renewable biomass resources, particularly reeds and other agricultural residues, have gained attention as environmentally friendly and economically viable alternatives in the construction and furniture industries [3, 4]. In the production of reed-based particleboards, the physicochemical properties of the resin, pressing conditions, and the fraction size of the raw material significantly influence the mechanical strength, density, and water resistance of the final product [5]. In this study, urea-formaldehyde resin was synthesized using a three-stage method and applied as an adhesive in the production of reed-based particleboards. The physical-mechanical properties of the boards (density, bending strength, screw withdrawal resistance) and their surface morphology were investigated using a scanning electron microscope (SEM). The primary objective of this study is to determine the potential for producing environmentally friendly composite materials using renewable resources such as reeds and to evaluate the effectiveness of UFR as an adhesive.

Research on urea-formaldehyde resins has been extensively studied in the field, with their adhesive properties and methods for reducing formaldehyde emissions being the primary focus. Pizzi (2006) analyzed the application of UFR in wood-based composites and methods for optimizing their chemical structure, emphasizing the importance of the three-stage synthesis process in enhancing polycondensation efficiency [1]. Dunky (1998) investigated the mechanical and chemical properties of UFR in wood board production, demonstrating that quality can be improved by adjusting viscosity and gelation time [2]. In studies on composite materials based on reeds and other agricultural residues, El Mansouri and Salvadó (2007) analyzed the chemical composition of lignin and its interaction with resin, determining that the cellulose and lignin content of reeds affects adhesion quality [3]. Rowell (2012) summarized the chemical and mechanical properties of wood and biomass-based composites, highlighting the advantages of materials like reeds, such as low density and high moisture resistance [4].

In the production of reed-based particleboards, pressing conditions and the fraction size of raw materials are of significant importance. Hashim et al. (2011) studied kenaf-based boards, showing that combining coarse and fine fractions can enhance board strength [6]. Additionally, Myers (1984) examined the impact of formaldehyde emissions on board quality and environmental safety, emphasizing the need to optimize the synthesis process in alkaline and acidic media [5]. Regarding SEM analysis, Li et al. (2014) investigated the morphology of adhesive interfaces in wood-based composites, finding that the distribution of resin across particles significantly affects the mechanical properties of the boards [7]. Saheb and Jog (1999) analyzed the advantages of natural fiber polymer composites, highlighting that biomass materials like reeds are environmentally friendly and economically efficient [8]. While these studies have enriched the knowledge base on UFR and reed-based boards, gaps remain in the in-depth investigation of the impact of reed fraction sizes and pressing conditions on board properties. This study aims to address these gaps by determining the physical-mechanical properties and surface morphology of reed-based particleboards through SEM analysis.

2. Materials and methods

2.1. Materials

The following reagents were used for the synthesis of urea-formaldehyde resin:

KFK-85 (formaldehyde 59.8%, urea 24.7%): 2700 g, as the primary raw material.

Technical urea: 1087 g (first stage), 500 g and 492 g (subsequent stages).

Water: 1786 g, as a solvent and for creating the reaction medium.

Polyvinyl alcohol (PVA): To increase viscosity and enhance binding properties.

Carboxymethylcellulose (CMC): 4 g, as a stabilizer.

Sodium hydroxide (NaOH, 20% solution): To adjust pH (8.5; 6.5; 7.5–8.0).

Ammonium chloride (NH₄Cl, 20% solution): To create a weakly acidic medium.

Coarse (1–5 mm) and fine (5–15 mm) fractions of reed particles were used as raw materials for the production of reed-based particleboards.

2.2. Resin synthesis

The synthesis of urea-formaldehyde resin was carried out in a 25-liter laboratory reactor. To the reactor, 2700 g of KFK-85, 1786 g of water, 4 g of CMC, and PVA were added. The mixture was heated to 40°C, and the pH was adjusted to 8.5 using a 20% NaOH solution. Then, 1087 g of technical urea was added, the temperature was raised to 90°C, and the mixture was stirred for 45 minutes. To lower the pH, a 20% NH₄Cl solution was gradually added over 30 minutes. Subsequently, the pH was raised to 6.5 using a 20% NaOH solution, and 500 g of urea was added. The pH was then adjusted to 7.5–8.0 (using a 20% NaOH solution), 492 g of urea was added, and the mixture was cooled to 30°C.

The properties of the prepared resin were as follows: flowability 42 s, density 1240 g/cm³, gelation time 58 s, dry residue 66.7%, viscosity 435 mPa·s.

2.3. Making reed-shaving plates

The plates were prepared in three different samples (1-, 2-, 3-):

Sample 1: 1200 g of large fraction + 155 g of resin, 300 g of small fraction + 65 g of resin.

Sample 2: 740 g of large fraction + 120 g of resin, 220 g of small fraction + 46 g of resin.

Sample 3: 750 g of large fraction + 125 g of resin, 220 g of small fraction + 47 g of resin.

To form the plates, small fraction reed chips were placed on the surface and large fraction chips were placed in the middle. The plates were hot-pressed at 155–165°C and under a pressure of 14–15 MPa for 6 minutes.

2.4. Analysis methods

Density (kg/m³), flexural strength (MPa) and syrup hardening resistance (N/mm and N) were measured using standard methods. The surface morphology of the plates and the resin-reed chip bonding were studied using a QUORUM Q150 RS scanning electron microscope (approximate data, the exact model needs to be confirmed). Flowability, density, gel time and viscosity were determined using a viscometer and other laboratory instruments.

3. Results and discussion

3.1. Properties of resin

The synthesized UFR exhibited high viscosity (435 mPa·s) and dry residue (66.7%), confirming its effectiveness as an adhesive. The gelation time (58 s) and flowability (42 s) ensured uniform distribution of the resin with reed particles. The three-stage synthesis process (alkaline-acidic-alkaline) optimized the polycondensation degree of the resin and minimized the free formaldehyde content [5].

3.2. Physical and mechanical properties of plates

Table 1.

Physical and mechanical properties of plates

Sample 1 exhibited the highest density (754 ± 10 kg/m³) and good bending strength (12,87 ± 0,5 MPa), which is associated with the larger fraction (1200 g) and higher resin content (220 g). The screw withdrawal resistance was moderate (32 ± 1,5 N/mm), but a high result (646 ± 5 N) was recorded when testing the lateral screw withdrawal resistance. Sample 2 showed the highest screw withdrawal resistance (43,63 ± 1,5 N/mm) and bending strength (13,05 ± 0,5 MPa), but a lower density (646 ± 8 kg/m³). This is explained by the use of less raw material (677 ± 9 g) and resin (166 g). Sample 3 recorded the lowest bending strength (9,3 ± 0,4 MPa) and lateral screw withdrawal resistance (506,7 ± 6 N), but the density (677 ± 9 kg/m³) was at an average level. The slightly higher temperature (162–165°C) may have led to excessive hardening of the resin.

3.3. SEM analysis

SEM images showed the bonding between the reed chips and the resin (the image of glue on the surface of the reed is shown by arrows). In sample 1, the resin uniformly covered the reed chips, resulting in a smooth surface with low porosity. In sample 2, small pores were observed due to the low density, which may explain the high resistance to hardening of the syrup. In sample 3, the high hardening of the resin may have led to some cracking of the surface, which caused a decrease in the bending strength. The distribution of the resin on the reed surface affected the mechanical properties of the plates [7].

3.4. Discussion

The research results indicate that UFR is an effective adhesive for reed-based particleboards. Sample 2 exhibited the best mechanical properties, which is associated with the optimal ratio of raw material and resin. The reduced density increases the board’s lightness, but long-term durability testing is required. The low strength of Sample 3 is attributed to the high temperature and resin hardening. SEM analysis confirmed good bonding between the resin and reed particles, but it was determined that surface porosity and resin distribution affected the board properties. The natural composition of reeds (lignin, cellulose) may have improved the chemical interaction with the resin [8-9].

4. Conclusion

Urea-formaldehyde resin was successfully synthesized through a three-stage process and used as an adhesive in the production of reed-based particleboards. The resulting boards met the requirements for construction and furniture manufacturing in terms of density (646–754 kg/m³), bending strength (9,3–13,05 MPa), and screw withdrawal resistance (32–43,63 N/mm). SEM analysis confirmed high-quality bonding between the resin and reed particles. Sample 2 exhibited the best mechanical properties, which is attributed to the optimal raw material ratio and pressing conditions. It is recommended to further investigate the boards’ water absorption, biodegradability, and long-term durability in future studies.

References:

1. Pizzi, A. (2006). Urea-formaldehyde adhesives. Handbook of Adhesive Technology, 2nd Edition, pp. 345–362.
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3. El Mansouri, N. E., & Salvadó, J. (2006). Structural characterization of technical lignins for the production of adhesives. Industrial Crops and Products, 24(1), 8–16.
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