Abstract
Material alternatives to wood, such as rape straw, are needed for the production of composite materials. This study performed an analysis of rape straw as a composite material source for production. There were three types of rape straw particle modification: untreated particles as reference variant, boiling in water, and soaking in sodium hydroxide (NaOH) solution. The pH and calorific value were highest for the variant soaked in NaOH. The total elemental content and the elements on the rape straw surface varied between variants. The modification method chosen influenced the pH, calorific value, elemental composition, and contact angle.
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Chemical and Physical Parameters of Different Modifications of Rape Straw (Brassica napus L.)
Tereza Částková,a Štěpán Hýsek,b,* Adam Sikora,c Ondřej Schönfelder,c and Martin Böhm b
Material alternatives to wood, such as rape straw, are needed for the production of composite materials. This study performed an analysis of rape straw as a composite material source for production. There were three types of rape straw particle modification: untreated particles as reference variant, boiling in water, and soaking in sodium hydroxide (NaOH) solution. The pH and calorific value were highest for the variant soaked in NaOH. The total elemental content and the elements on the rape straw surface varied between variants. The modification method chosen influenced the pH, calorific value, elemental composition, and contact angle.
Keywords: Modification of rape straw; Brassica napus; Physical and agrochemical parameters; Scanning electron microscopy
Contact information: a: Department of Agroenvironmental Chemistry and Plant Nutrition, Faculty of Agrobiology, Food and Natural Resources, The Czech University of Life Sciences Prague, Kamycka 129, Prague 165 21, Czech Republic; b: Department of Wood Products and Constructions, Faculty of Forestry and Wood Sciences, The Czech University of Life Sciences Prague, Kamycka 129, Prague 165 21, Czech Republic; c: Department of Wood Processing, Faculty of Forestry and Wood Sciences, The Czech University of Life Sciences Prague, Kamycka 129, Prague 165 21, Czech Republic;
* Corresponding author: hyseks@fld.czu.cz
INTRODUCTION
Recently there has been an effort to develop new composite materials using alternative sources of raw materials. This trend is largely explained by population growth, which contributes directly to a limited supply of natural resources, as well as wood shortage in all wood processing industries (Galor and Weil 2000; Bektas et al. 2005; Seintsch 2011; Lauri et al. 2012). One alternative source of raw materials is post-harvest rapeseed crop residue (Mohanty et al. 2002; Dziurka et al.2015). As a prospective raw material, rapeseed has many advantages, as it is available in large quantities and is not yet used in other products with high added value (Bečka et al. 2007; Dukarska et al. 2017). However, waxy and siliceous substances are present on the surface of winter rape stems, which prevents the formation of quality adhesion between particle and adhesive (Grigoriou 2000). This is the main disadvantage of particles from annual plants compared to wood particles. The chemical composition differs between internal and external areas of the stem because of cuticles and epicuticular waxes (Wiśniewska et al. 2003; Trischler, and Sandberg 2014). The surface has a strong impact on the water contact angle, which deteriorates the wetting of particles by adhesives, since adhesives used in particleboard production are mainly water-based (Wiśniewska et al. 2003). Therefore, it is necessary to pre-treat these particles from annual plants before the production of the composite material itself (Mahlberg et al. 1999; Cao et al. 2017). This modification can be done in various ways (Pelaez-Samaniego et al. 2013; Trischler and Sandberg 2014). Boiling in water is regarded favorably as a treatment due to its ease of implementation (Bekhta et al. 2013). An alternative is alkaline treatment, which breaks ester linkages between wax and lignocellulose, as well as dissolving wax and lignin (Binod et al. 2010; Wan et al. 2011). In this study, winter rape chips were modified by applying hydrothermal and chemical (soaking in sodium hydroxide) processes (Xie et al. 2010; Bekhta et al. 2013). It is assumed that the modification destroys the waxy, siliceous substances on the surface of the chips and consequently improves adhesion in the composite product. This study characterized the physical and chemical properties of raw and modified rapeseed particles to measure the effects of modification on particle properties. This information on the effect of the various particle modifications on particle properties will promote the utilization of this waste material in particleboard production.
EXPERIMENTAL
Materials
Chipped rape straw (Brassica napus L.) particles were treated by one of two processes. In the chemical treatment, the particles were soaked in 2% sodium hydroxide solution (NaOH) at 20 °C for 45 min. Alternatively, a hydrothermal treatment was carried out by boiling in water for 45 min. After both modifications, particles were carefully flushed with water and then oven-dried to 6% moisture content. A portion of each sample was dried at 40 °C and milled for determination of the total element content, calorific value, and percentage of volatile solids. Representation of the individual fractions in chopped rape straw is stated in Table 1.
Table 1. Representation of the Individual Fractions in Chopped Rape Straw
Methods
The goal of this study was to determine the effects of different particle pre-treatment methods on particle properties. In order to estimate this effect and to determine what happened and changed in the particle after modification, the following characteristics were observed: equilibrium moisture content, pH value, calorific value, total element content, elemental composition of the surface, water contact angle, and microstructure of the particle surface.
Chemical analysis of rape straw
Moisture content was determined by measurement at 105 °C by a ML-50 Moisture Analyzer (A&D Company, Elk Grove Village, IL, USA). The pH was measured in samples mixed with deionized water (1:5 w/v wet basis) using a WTW pH 340 I device (GeoTech, Denver, CO, USA), according to BSI EN 15933 (2012). Total element contents were determined through decomposition in an enclosed microwave, using an Ethos 1 system (MLS GmbH, Leutkirch im Allgäu, Germany). Elemental concentrations were determined using inductively coupled plasma optical emission spectrometry (ICP-OES; Varian VistaPro, Mulgrave, Australia) with axial plasma configuration. The calorific value of the material and the percentage of volatile solids were determined using the Laget MS 10A dry calorimeter (Laget, Staufen im Breisgau, Germany). The sample was burned in a 100% oxygen atmosphere, according to ČSN ISO 1928 (2010).
Scanning electron microscopy (SEM) and elemental analysis
The surface of the rape straw particles was observed with a MIRA 3 electron microscope (Tescan Orsay Holding, Brno, Czech Republic) with a secondary electron detector operated at 15 kV acceleration voltage. When rape straw is washed with water, the elements can be changed on the surface, as well as when straw soaked in NaOH. The elemental compositions of the surface were examined by an energy dispersive spectroscopy system (Bruker XFlash X-ray detector, Karlsruhe, Germany, and ESPRIT 2 software).
Contact angle
Contact angle analysis is widely used to determine the wettability of solid materials (Walinder and Ström 2001; Aydin 2004). To determine wetting characteristics of modified particles, the contact angle of the water and particle surface was measured using a DSA 30E goniometer (Krüss GmbH, Hamburg, Germany). Straw particles have both interior and exterior surfaces. However, due to a pith on the interior of the particles, the contact angle of the water droplet could not be measured on the inside, and therefore only the exterior contact angle was measured. Static contact angle was measured for 30 measurements for each straw modification. In order to minimize the influence of variable of straw surface, a new particle was used for each droplet. The volume of distilled each water droplet was 5 µL with the measurement taken 5 s after the application. Contact angle was measured using image analysis software.
Statistical Analysis
All results from the agrochemical analysis represent the mean values of three replicates. Analysis of normality and homogeneity and one-way analysis of variance (ANOVA) were performed with STATISTICA 12 software (StatSoft, Tulsa, OK, USA). ANOVA was based on a 95% confidence level in accordance with Tukey’s range HSD (honest significant difference) test.
RESULTS AND DISCUSSION
In each modification, the pH value of the rape straw was different. The lowest pH value was in the sample modified through maceration in H2O (6.58), and conversely, the highest pH value was in the sample modified through maceration in NaOH (9.44) (Table 2). This result suggests that the straw was not well washed after soaking in the NaOH solution as there was the highest total content of Na present. Both in this variant and the reference variant, there was a high content of calcium (Ca) on the surface of the straw, which was seen in the elemental analysis (Fig. 2a, c). The equilibrium moisture content was very low in all samples, with no statistically significant difference. The highest moisture content was in the sample modified with H2O 7.7% (Table 2).
Fig. 1. Electron micrographs of rape straw surfaces (magnification 10000x). (a) Reference variant, (b) variant modified by cooking in H2O, and (c) variant modified in NaOH solution
Table 2. pH and Moisture Content in Modified Rape Straw
Values are the means ± SD (n = 3). Different letters (in superscript) in a column indicate significant differences (Tukey’s HSD test, P < 0.05).
The percentage of volatile solids present in all modifications was relatively high. The highest value of volatile solids was found in the modification with H2O (96.97%) (Table 3). It appeared that the H2O dissolved in the compounds into an improved combustible form. This variant had the highest value of volatile solids, but also the lowest calorific value. In contrast, the variant with NaOH had the highest calorific value of 18.4 MJ.kg-1 but the lowest value of volatile solids. The calorific value can be influenced by the content of carbon (C) or sulfur (S) present (Erol et al. 2010). As the elemental analysis showed, the highest C content was found in the variant with NaOH (Fig. 2c), explaining the high calorific value of this variant. Total S content was the highest in the reference variant and the lowest in variant with H2O (Table 4), again explaining its calorific value.
Table 3. Volatile Solids and the Calorific Value of Modified Rape Straw
Table 4. Elemental Analysis of Modified Milled Rape Straw
Values are the means ± SD (n = 3). Different letters (in superscript) in a column indicate significant differences (Tukey’s HSD test, P < 0.05).
An extensive analysis of element concentrations was performed, including an analysis of the total element content (Table 4) and an analysis of particle surfaces using SEM (Fig. 1a-c; Fig. 2a-c). The total content of arsenic (As) and lead (Pb) were below the detection limit. The total contents of cadmium (Cd), chromium (Cr), and nickel (Ni) were very low and therefore were not statistically different. None of the above elements were found on the surface of the rape straw.
The total contents of copper (Cu), zinc (Zn), manganese (Mn), boron (B), iron (Fe), and aluminum (Al) were low but statistically significant. Specifically, the total content of Mn and Cu was the highest in the reference variant, almost twice as high as in the variant with NaOH (Table 4). Significant differences were not found in the total content of Fe. None of these elements were detectable using SEM (Fig. 2a-c).
Fig. 2. Elemental content of rape straw surface taken by SEM. (a) Reference variant, (b) variant modified by cooking in H2O, and (c) variant modified in NaOH solution
The total phosphorus (P) content was over 200 mg/kg in all variants, and no significant differences were found. However, a significant difference was found in the total content of Na. The total content of Na was 17 times higher (4187.8 mg/kg) for the variant with NaOH compared with the reference variant (245.5 mg/kg). As Na was only found on the exterior surface of this variant, it can be assumed that the large amount of Na present was probably caused by poor washing of the NaOH solution.
The total S content of the reference variant was four times higher than in variant with NaOH, which further confirming the calorific value of this variants. Potassium (K) was found only on the surface of the reference variant and the variant with NaOH, as shown in pictures taken by SEM (Fig. 2a-c). In the variant with H2O, K could have been washed away from the surface. In the variant with NaOH, K could have precipitated on the surface. Total K content was the highest in the reference variant (5037 mg/kg) and was almost 7 times higher in the reference variant than in the variant with NaOH (733 mg/kg).
The lowest magnesium (Mg) content was found in variant with H2O (739 mg/kg). However, Mg on the surface of the rape straw was found only in the variant with H2O. The total Ca content corresponds to the content found on the rape straw surface by SEM analysis. The hydroxide precipitated Ca on the surface of the variant with NaOH (Fig. 2c). This Ca deposit created crystals that were observed also by SEM, as can be seen in Fig. 1c. There were no Ca crystals on the surface of reference particles or the particles modified by cooking. Only dust was found on the surface of these particles (Fig. 1a, b). The pith of the rape straw particles was observed also. Figure 3a shows the undisturbed pith of the reference sample. Both hydrothermal and alkaline modification destroyed the pith, which is shown in Fig. 3b and Fig. 3c, respectively.
Fig. 3. Electron micrographs of rape straw pith (magnification 1000x). (a) Reference variant, (b) variant modified by cooking in H2O, and (c) variant modified in NaOH solution
Figure 4 depicts the analysis of contact angle between water droplet and straw particle surface. The vertical columns depict 95% confidence intervals. Both modifications caused the desired decrease in water contact angle, while the contact angle was different for each modification. The largest contact angle was found in the reference variant, with a mean of 94.1° and the lowest mean, 82.7°, was seen in the modification with H2O. A smaller contact angle is better for the strength of a glued joint (Banea and Silva 2008; Moghadamzadeh et al. 2011). The variant with H2O exhibited a significant difference from the others, but there was no significant difference between the reference and NaOH variants. The alkaline modification resulted in the lowest variability of water contact angle (Fig. 4).
Fig. 4. Influence of modification technique on contact angle: 1-reference variant, 2-variant modified by cooking in H2O, 3-variant modified in NaOH solution
CONCLUSIONS
- The elemental composition of surface and the total element content of rape straw are highly influenced by the type of modification.
- A variant modified with H2O exhibited the highest value of volatile solids and the lowest calorific value, but a variant modified with NaOH had the highest calorific value and the lowest value of volatile solids.
- A disadvantage of soaking rape seed straw particles in NaOH is that it is not possible to rinse out all of the hydroxide. Therefore, the variant modified with NaOH had the highest pH value.
- Contact angle is also dependent on the type of modification. The variant with the best strength for a glued joint was modified with H2O.
- Moisture content is not dependent on the type of modification, as there were no significant differences.
ACKNOWLEDGMENTS
This work was supported by the Czech University of Life Sciences Prague under the CIGA project No. 20174306.
REFERENCES CITED
Aydin, I. (2004). “Activation of wood surface for glue bonds by mechanical pretreatment and its effects on some properties of veneer surfaces and plywood panels,” Applied surface Science233(1-4), 268-274. DOI: 10.1016/j.apsusc.2004.03.230
Banea, M. D., and Silva da, L. F. M. (2008). “Adhesively bonded joints in composite materials: An overview,” Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications 233(1), 1-18. DOI: 10.1243/14644207JMDA219
Bečka, D., Vašák, J., Zukalová, H., and Mikšík, V. (2007). “Řepka ozimá-pěstitelský rádce (Winter rapeseed-growing advisor),” FAPPZ, Prague, Czech Republic.
Bekhta, P., Korkut, S., and Hiziroglu, S. (2013). “Effect of pretreatment of raw material on properties of particleboard panels made from wheat straw,” BioResources 8(3), 4766-4774. DOI: 10.15376/biores.8.3.4766-4774
Bektas, I., Guler, C., Kalaycıoglu, H., Mengenoglu, F., and Nacar, M. (2005). “The manufacture of particleboards using sunflower stalks (Helianthus annuus L.) and poplar wood (Populus albaL.),” Journal of Composite Materials 39(5), 467-473. DOI: 10.1177/0021998305047098
Binod, P., Sindhu, R., Singhania, R. R., Vikram, S., Devi, L., Nagalakshmi, S., Kurien, N., Sukumaran, R. K., and Pandey, A. (2010). “Bioethanol production from rice straw: An overview,” Bioresource Technology, 101(13), 4767-4774. DOI: 10.1016/j.biortech.2009.10.079
BSI EN 15933 (2012). “Sludge, treated biowaste and soil. Determination of pH,” Czech Office for Standards, Metrology and Testing, Prague, Czech Republic.
ČSN ISO 1928 (441352). (2010). “Solid mineral fuels – Determination of gross calorific value by the bomb calorimetric method, and calculation of net calorific value,” Czech Office for Standards, Metrology and Testing, Prague, Czech Republic.
Cao, Y., Song, W., Yang, Z., Chen, Z., and Zhang, S. (2017). “The Properties of particleboard made from alkaline-treated wheat straw and methylene diphenyl diisocyanate binder,” BioResources 12(2), 3265-3276. DOI: 10.15376/biores.12.2.3265-3276
Dukarska, D., Czarnecki, R., Dziurka, D., and Mirski, R. (2017). “Construction particleboards made from rapeseed straw glued with hybrid pMDI/PF resin,” European Journal of Wood and Wood Products, 72(2), 175-184. DOI: 10.1007/s00107-016-1143-x
Dziurka, D., Mirski, R., Dukarska, D., and Derkowski, A. (2015). “Possibility of using the expanded polystyrene and rape straw to the manufacture of lightweight particleboards,” Maderas. Ciencia y Tecnología 17(3), 647-656. DOI: 10.4067/S0718-221X2015005000057
Erol, M., Haykiri-Acma, H., and Küçükbayrak, S. (2010). “Calorific value estimation of biomass from their proximate analyses data, “Renewable Energy 35(1), 170-173. DOI: 10.1016/j.renene.2009.05.008
Galor, O., and Weil, D. (2000). “Population, technology, and Growth: From Malthusian stagnation to the demographic transition and beyond,” American Economic Review 90(4), 806-828. DOI: 10.1257/aer.90.4.806
Grigoriou, A. (2000). “Straw-wood composites bonded with various adhesive systems,” Wood Science and Technology 34(4), 355. DOI: 10.1007/s002260000055
Lauri, P., Kallio, A. M. I., and Schneider, U. A. (2012). “Price of CO2 emissions and use of wood in Europe,” Forest Policy and Economics 15, 123-131. DOI: 10.1016/j.forpol.2011.10.003
Mahlberg, R., Niemi, H. E., Denes, F. S., and Rowell, R. M. (1999). “Application of AFM on the adhesion studies of oxygen-plasma-treated polypropylene and lignocellulosics,” Langmuir 15(8), 2985-2992.
Moghadamzadeh, H., Rahimi, H, Asadollahzadeh, M., and Hemmati, A. R. (2011). “Surface treatment of wood polymer composites for adhesive bonding,” International Journal of Adhesion & Adhesives 31(8), 816-821. DOI: 10.1016/j.ijadhadh.2011.08.001
Mohanty, A. K., Misra, M., and Drzal, L. T. (2002). “Sustainable bio-composites from renewable resources: Opportunities and challenges in the green materials world,” Journal of Polymers and the Environment 10(1), 19-26. DOI: 10.1023/A:1021013921916
Pelaez-Samaniego, M. R., Yadama, V., Lowell, E., and Espinoza-Herrera, R. (2013). “A review of wood thermal pretreatments to improve wood composite properties,” Wood Science and Technology 47(6), 1285-1319. DOI: 10.1007/s00226-013-0574-3
Seintsch, B. (2011). Stellung der Holzrohstoffe in der Kostenstruktur des Holz- und Papiergewerbes in Deutschland, University Hamburg, Hamburg.
Trischler, J., and Sandberg, D. (2014). “Monocotyledons in particleboard production: adhesives, additives, and surface modification of reed canary grass,” BioResources 9(3), 3919-3938. DOI: 10.15376/biores.9.3.3919-3938
Walinder, M. E., and Ström, G. (2001). “Measurement of wood wettability by the Wilhelmy method. Part 2. Determination of apparent contact angles,” Holzforschung 55(1), 33-41. DOI: 10.1515/HF.2001.006
Wan, C., Zhou, Y., and Li, Y. (2011). “Liquid hot water and alkaline pretreatment of soybean straw for improving cellulose digestibility,” Bioresource Technology 102(10), 6254-6259. DOI: 10.1016/j.biortech.2011.02.075
Wiśniewska, S. K., Nalaskowski, J., Witka-Jeżewska, E., Hupka, J., and Miller, J. D. (2003). “Surface properties of barley straw,” Colloids and Surfaces B: Biointerfaces 29(2), 131-142.
Xie, Y.; Xiao, Z., Gruneberg, T., Militz, H., Hill, C. A. S., Steuernagel, L., and Mai, C. (2010). “Effects of chemical modification of wood particles with glutaraldehyde and 1,3-dimethylol-4,5-dihydroxyethyleneurea on properties of the resulting polypropylene composites,” Composites Science and Technology 70(13), 2003-2011. DOI: 10.1016/j.compscitech.2010.07.024
Article submitted: August 25, 2017; Peer review completed: October 8, 2017; Revised version received and accepted: November 1, 2017; Published: November 6, 2017.
DOI: 10.15376/biores.13.1.104-114