Impact of Sugar Beet Pulp Share on Selected Physical and Mechanical Properties of Particleboards

Abstract

The aim of this study was to investigate the applicability of sugar beet pulp (SBP) in particleboard production as a substitute for wood material. Two variants of board density, 650 kg/m³ and 550 kg/m³, containing 0%, 25% and 50% of SPB, were evaluated. During the study the following features were measured: modulus of rupture (MOR), modulus of elasticity (MOE), thickness swelling (TS) and water absorption (WA). The results showed that boards containing up to 25% of SBP meet standards for boards for general-purpose used in dry conditions. Further increases in SBP content deteriorated some of the investigated properties of the boards.

1. Introduction

Global production of wood-based materials has been following a constant upward trend for years, and in 2020 it reached 102 million cubic meters [1]. Such high value is determined by the constantly growing demand for this type of board. Their mechanical and physical properties as well as their relatively low price make them widely applied in such industries as furniture, building, construction and packaging [2].

High production binds with high demand for the feedstock, and the main raw materials in this case are wood particles. Due to the finite possibilities of obtaining wood raw material from forests, more and more attention is being devoted to attempts to manufacture innovative particleboards using alternative feedstock. Recent research has focused on boards made with post-waste paper [3], cement-particle boards [4,5], basalt-particle boards [6] and with the use of post-consumer thermoplastic materials [7]. However, it seems that the most promising direction for the production of particleboards may be obtaining raw material from agricultural lingo-cellulosic waste [8,9,10].

In recent years a large number of scientific works describing use of alternative lignocellulosic sources in the production of particleboards have been released. The studies included among others, such materials as: vine prunings [11,12], eggplant stalks [13], kenaf stalks [14,15], straw [16], tomato stalks [17], sunflower stalks [18], apple prunings [19], corn cobs [20], oil camellia [21,22] and others. One such agricultural waste is sugar beet pulp (SBP), which is a by-product of sugar production. It is mainly composed of arabinians, hemicelluloses, pectins, and cellulose microfibrils [23,24,25]. SBP is commonly used as animal feed. The low pectin content, high drying costs and large availability of this material led to attempts to use it as an alternative feedstock for different purposes. Studies conducted so far have been focused on the use of SBP as, e.g., a source of cellulosic microfibrils [26], a source of polyols for the production of urethanes and polyurethanes [27], a source of fiber in biodegradable composites [28,29,30] or in paper manufacture [31], and in the production of bioethanol [32].

SBP was also tested as a raw material in the process of particleboard production. Borysiuk et al. [33] demonstrated the possibility of producing particleboards with up to 30% SBP in the core layer without significant deterioration of their mechanical properties. However, the authors did not discuss the potential impact of panel density within the research. Present studies concentrate on the prospect of producing low-density particleboards [34,35]; however, the density is an important factor determining the properties of wood-based panels [2]. The aim of the present study was to determine the influence of the proportion of SBP and the density of particleboards on selected mechanical and physical properties.

2. Materials and Methods

2.1. Material

SBP used in the experiment was provided by a Polish sugar producer (Krajowa Grupa Spożywcza S.A. w Toruniu, Oddział Cukrownia Krasnystaw, Poland). The sugar beet pulp consisted of flakes with dimensions of approximately 5–7 mm in diameter and 1–2 mm in thickness. The SBP used in the particleboard manufacturing process was characterized by a moisture content of 4%.

The industrial wood particles were applied in the face and core layers. The pine wood particles were delivered by a Polish factory producing particleboards. Particles for the face layers were characterized by a moisture content of 7%, and for the core layer of 4%.

The adhesive used was based on urea formaldehyde resin (UF) (Silekol 120, Silekol Sp, z o.o., Kędzierzyn-Koźle, Poland) with a dry matter content of 67%, a relative density of 1.30 g/cm³ and a dynamic viscosity of about 500 mPas. The hardener was a 10% aqueous solution of ammonium sulfate. The unit composition of the adhesive was 50:15.5:1.5 UF resin, water, and hardener, respectively.

2.2. Particleboard Manufacturing

Three-layer particleboards with a thickness of 16 mm were produced in two variants of density: 550 kg/m³ and 650 kg/m³. The face layers constituted 35% of the panel. The content of the binder in these layers was 12% and in the core layer 10%. The core layer of board was varied in the beet pulp content as follows: 0% (control variant), 25%, 50%, and the face layers were made with 100% wood particles.

Table 1. Variants of manufactured boards.

Variant	Core Layer SBP Share (%)	Density (kg/m3)
A	0	650
B	0	550
C	25	650
D	25	550
E	50	650
F	50	550

shows variants of particleboards used in the experiment.

The boards were produced using a ZUP-NYSA PH-1LP25 hydraulic press at a unit pressing pressure of 2.5 MPa, a temperature of 180 °C and pressing time of 288 s (pressing factor 18 s/mm). Four boards with dimensions of 320 × 320 mm were produced for each variant. The manufactured particleboards were conditioned under normal conditions (20 ± 2 °C, 65 ± 5% air humidity) for at least seven days.

2.3. Experimental

The modulus of rupture (MOR) and modulus of elasticity (MOE) were determined according to EN 310 [36]. Internal bonding was determined through the tensile test perpendicular to the surface of the board, according to EN 319 [37]. All mechanical tests were conducted via a laboratory-testing machine custom-made by the Research & Development Center for Wood-Based Panels Sp. z.o.o. in Czarna Woda, Poland. At least 10 samples were used to determine each property.

The density of particleboards was measured according to EN 323 [38]. Thickness swelling (TS) after 2 h and 24 h of soaking in water were measured according to EN 317 [39]. Briefly, samples were completely immersed in water at room temperature for 2 h and 24 h in such a way that their possible dimensional changes were not limited. Water absorption (WA) was measured during TS and calculated as follows:

$$WA=\frac{{{\displaystyle m}}_2-{{\displaystyle m}}_1}{{{\displaystyle m}}_1}\cdot 100,$$

(1)

where WA is the water absorption (%), m₁ is the sample weight before soaking (g) and m₂ is the sample weight after soaking (g).

2.4. Statistical Analysis

Statistical analysis of the results was carried out in Statistica (data analysis software system), Version 13 (TIBCO Software Inc., Palo Alto, CA, USA). Analysis of variance (MANOVA) were used to test (significant level α = 0.05) for significant differences between factors. A comparison of the means was performed by Tukey test, with a 0.05 significance level.

3. Results and Discussion

The particleboards produced in the study had an average density in the range of 644–663 kg/m³ for variants A, C, E and 553–562 kg/m³ for variants B, C, F (

Table 2. Average density of manufactured particleboards.

Variant	Particleboards Density (kg/m3)
	Average	Std. Dev.
A	663 a	32
B	553 b	27
C	648 a	34
D	566 b	22
E	644 a	34
F	562 b	31

^a,b—homogenous group by Tukey test (α = 0.05).

). It should be noted that the difference in the particleboards’ density in relation to their assumed value did not exceed 3%. In addition, statistical analysis did not reveal statistically significant differences in density values between board variants with the same assumed density.

Particleboards with 25% of SBP content and the density of 650 kg/m³ revealed a significant decline in modulus of rupture (about 40% lower) compared to the boards with no SBP (Figure 1). The same parameter was somewhat lower in the case of boards with the density of 550 kg/m³ and 25% SPB compared to the boards of the same density and no SBP additive.

The technical requirements of particleboards, depending on their intended use, are compiled and described in the EN 312 standard [40]. The minimum value of modulus of rupture for particleboards intended for general purpose and use in dry conditions (P2 type boards) is 10 N/mm². When analyzing the results presented in Figure 1, it can be concluded that none of the particleboards made with SBP met these requirements. However, it should be noted that the variants with a 25% SBP share were characterized by a strength of at least 9.4 N/mm² regardless of the density. Similar dependencies were noted by Borysiuk et al. [33].

An analysis of the MOE results demonstrates that the manufactured boards with 25% SBP share met the minimum requirements of the EN 312 standard (1600 N/mm²) [40]. A significant decrease in the MOE value for particleboards with 25% SBP share compared to boards without SBP was observed only for boards with a density of 650 kg/m³ (Figure 2). Interestingly, the decrease in the parameter occurs much more smoothly along with the increase in the SBP share in the case of boards with the lower density. This may be related to pectins and sugars in the beet pulp, which may behave as additional adhesives due to pressing. In addition, the proportion of SBP may influence how the board is compacted during pressing. The analysis of variance showed a significant influence of the board density and SBP share on the properties of the manufactured particleboards (

Table 3. MANOVA results of mechanical properties of particleboards.

	MOR		MOE		IB
	p	X	p	X	p	X
Share	0.000	67.68	0.000	74.13	0.000	38.98
Density	0.001	2.51	0.001	2.15	0.212	1.48
Share × Density	0.000	17.66	0.000	13.83	0.009	9.46
Error		12.15		9.89		50.07

p—significant with α = 0.05; X—percentage of contribution.

). The percentage impact coefficient of SBP share was 67.68% for MOR and 74.13% for MOE, while the density values were 2.51% for MOR and 2.15% for MOE. This proves that it is mainly the SBP share that determines the MOR and MOE of the manufactured particleboards, thus evidencing that the SBP share can be expectedly to affect all the other panel properties. If the SBP share predominantly determines the properties of the MOR and MOE, its properties will largely affect the properties of the manufactured particleboards. When comparing the results of the research on the use of agriculture residues, a general trend of a decrease in the mechanical properties of particleboards with an increase in the share of agriculture residues can be observed [19,33]. The observed relationships are consistent with the literature data.

A significant increase in the IB values was observed in the case of boards with 25% SBP compared to reference boards regardless of their density (Figure 3). It should be noted that all the particleboards produced met at least the minimum requirements of the EN 312 standard (0.24 N/mm²) for P2 type boards. The analysis of variance showed a significant influence of the SBP share (X = 38.98%). However, the error value (X = 50.07%) was higher than the X values for SBP content and the density; this indicates that untested factors had a greater influence on IB.

Figure 4 illustrates the results of the physical properties of the manufactured particleboards following soaking. When analyzing the results of thickness swelling and water absorption, it should be noted that no hydrophobic agents were used in the process of manufacturing the particleboards. SBP particleboards generally showed a higher TS value than reference boards. Analyzing Figure 4, it can be seen that the increase in the value of the TS after 24 h of immersion in water is almost proportional to the share of SBP in the particleboard. The analysis of variance showed that both the density of the manufactured particleboards and the share of SBP are statistically significant (

Table 4. MANOVA results of physical properties of particleboards.

	TS2H		TS24H		WA2H		WA24H
	p	X	p	X	p	X	p	X
Share	0.000	55.31	0.000	88.97	0.000	24.90	0.000	44.88
Density	0.000	16.07	0.000	4.52	0.237	1.11	0.216	1.02
Share × Density	0.058	2.86	0.004	1.20	0.000	31.87	0.000	18.71
Error		25.75		5.31		42.11		35.38

p—significant with α = 0.05; X—percentage of contribution.

). However, the analysis of the percentage of factors’ impact showed that the determining factor in the case of TS was the SBP share. On the other hand, the density played a less significant role in this case, and its value of X was lower than that of the errors.

In the case of water absorption, only the SBP share was statistically significant. However, the X value for the SBP share after 2 h of soaking in water was lower than that recorded for the error, which may indicate that in the first 2 h of immersion in water, the absorption is mainly affected by other factors than the tested ones.

Then again, analysis of variance demonstrated that water absorption after 24 h of soaking in water was significantly influenced by the SBP share (X = 44.88%) and other factors untested in the experiment (error X = 35.38%).

4. Conclusions

Based on the conducted research, it can be concluded that the SBP share significantly influenced the mechanical and physical properties of manufactured boards. MOR and MOE properties significantly decreased as the content of SBP increased. However, addition of SBP improved IB. The tests revealed that the SBP share, when not exceeding 25%, enables the production of boards with strength properties which are close to meeting the requirements of the EN 312 standard for boards for general-purpose use in dry conditions, that may find applicability in the construction industry as non-structural filling elements. SBP addition is one of the main factors determining the thickness swelling and water absorption. At the same time, the density in this study did not significantly affect the water absorption. In order to reduce the swelling and water absorption values, the use of hydrophobic additives during the production of boards should be considered.