Analysis of the properties of alkaline-treated rape straw and stalk polyvinyl chloride composites

Abstract

Wood-plastic composites are being widely used more and more, and the bonding strength between fillers and matrix is an important factor affecting the properties of wood-plastic composites. In this study, rapeseed straw and rapeseed stalk were pretreated with sodium hydroxide and then filled with polyvinyl chloride matrix to prepare composites. The chemical composition, thermal stability, microstructure, physical properties, mechanical properties, and abrasive wear resistance of the composites were characterized, and the effects of different concentrations of alkaline treatment on rapeseed straw composite and rapeseed stalk composite were revealed. The results show that alkaline treatment has no significant effect on the chemical composition of the composites but has an obvious effect on other properties. Among them, after 1% alkaline treatment, rapeseed straw composite has the highest hardness, thermal stability, and impact strength, rapeseed stalk composite has the highest impact strength and bending strength. After 3% alkaline treatment, rapeseed straw composite has more wear resistance, rapeseed stalk composite has the highest density, the highest tensile strength, and more wear resistance. The rapeseed straw composite with 5% alkaline treatment has the highest density, and the rapeseed stalk composite with 7% alkaline treatment has the highest thermal stability. The hardness of rapeseed stalk composite and the tensile strength and bending strength of rapeseed straw composite decreased with the increase of alkaline treatment concentration.

Keywords

Rape plant fiber alkaline treatment properties composites abrasive wear polyvinyl chloride

With the increase in application of wood-plastic composites (WPCs) and shortage of forest resources, the choice of composite filler material has gradually shifted to crop straws, various forest residues, and waste residue after extracting nutrients to prepare biomass fibers^1,2 since they have advantages of low cost, low technical barriers, and sustainable development.³ Natural fibers reinforced with matrix for composite production have already been used for many applications, such as packaging, building, aerospace, and automobile industries where high load bearing capacities are not required.⁴ There have also been huge opportunities to manufacture superior materials for recreational, aerospace, and automotive applications.⁵

Plant natural fibers contain cellulose, hemicellulose, lignin, pectin, waxes, and other water-soluble extractives.⁶ However, their use in green composite materials has many challenges and limitations that need to be solved or minimized. To mention a few limitations and variable fiber properties, there is sensitivity to moisture, the inherent incompatibility between hydrophilic fibers and hydrophobic matrix,^7,8 the type of fiber, nature of the matrix and fiber-matrix interaction,⁹ high moisture absorption and weak adhesion with the matrix,¹⁰ inadequate fiber distribution and, subsequently, poor stress transfer from the matrix to the fibers.¹¹

Different treatment strategies have been developed to overcome limitations regarding use of bio-composite materials which include, but are not limited to; surface modification of the fibers by removing constituents which are not likely to affect the properties of the composites, or changing the fiber structural composition in order to improve adhesion, and other ways include the use of compatibilizers or coupling agents to improve stress transfer from matrix to fiber.^12,13 Several approaches have been studied regarding modification of natural fibers for composite development and the ideas have been applied in these studies.¹⁴ However, physical methods modify the surface properties, thereby enhancing fiber-matrix adhesion.^15,16 Chemical methods are the most widely developed and result in structural changes of fibers, enabling fiber-matrix interfacial bonding enhancements,¹⁷ and biological methods that employ bacterial action on the fiber surface, resulting in significant improvement in interfacial adhesion with polymeric matrices.¹⁸ Alkaline treatment of the natural fibers diminishes the moisture-related hydroxyl groups and thus weakens the hydrophilic nature of the fibers. This treatment also withdraws a small fraction of hemicellulose, lignin, pectin, wax, and oil coverings from the surface thereby increasing interfacial bonding of fiber. However, a high alkaline concentration level may cause excess elimination of covering materials from the cellulose surface and strongly remove lignin, which can negatively affect the strength of the fiber.¹⁹ Shah et al.²⁰ reported that the properties of kapok, sisal, jute, and hemp fibers were significantly improved by alkaline treatment, which helps in an increase of fiber resin adhesion, due to which interfacial energy increased. Sunny et al.²¹ observed that the tensile strength and modulus of hemp fiber was improved by alkaline treatment with a concentration of 5%.

Rapeseed (Brassica napus) is a yellow-flowered plant cultivated for its seeds and yields edible oil. Due to the fact that rapeseed plant is a renewable source, and can be grown locally, its application has potential in wood composites.²² Compared to other types of straw, rape straw contains a high content of cellulose and relatively low amount of wax on its surface.²³ Rape straw is a widely available agricultural biomass resource. China, in particular, has an annual rape straw planting area of over 100 m ha, and a large amount of the crop residues is either directly discarded or burned, leading to severe environmental pollution and waste of potential resources.²⁴ Composites containing rape straw can be utilized in many ways and may provide a wide range of mechanical, thermal, and hydric properties.²⁵ As a fast-growing plant, rapeseed plant provides a promising opportunity to radically decrease the carbon footprint of synthesis materials.^26
–29

The aim of this study is to analyze the properties of two different parts of alkaline-treated rapeseed plant (straw and stalk) behavior with polyvinyl chloride (PVC) for composite applications. However, existing studies mainly examine alkaline-treated rape straw behavior with polymer matrix alone. In this study, the chemical composition of the composites was characterized to confirm the specific content using Fourier transform infrared (FTIR). The molecular structure has been investigated using X-rays and thermal degradation, the physical and mechanical behavior of the two composites were characterized to evaluate their strength for possible adoption in composite applications. The surface topography of electron interactions with atoms of the composites using scanning electron microscopy (SEM), relative color change of the composite materials, composite water absorption test, and the abrasive wear performance of the composites have been examined.

Materials and methods

Materials

Rape straw and rape stalk are illustrated in Figure 1, and were collected in Lanzhou City, Gansu Province, China. PVC was purchased from Dongguan Ante Plastic Raw Material Co. Ltd (Dongguan, China), maleic anhydride graft coupling agent was purchased from Dongguan Zhuoyi Chemical Co. Ltd (Dongguan, China), calcium-zinc compound stabilizer was purchased from Dongguan Qiancheng Plasticizing Raw Material Co. Ltd (Dongguan, China).

Preparation of composite materials

About 3–5 cm of air-dried chopped pieces of rape straw and rape stalk were prepared and soaked with 1%, 3%, 5%, and 7% sodium hydroxide solutions separately at room temperature for about 12 h,³¹ and then were rinsed with distilled water several times until around pH = 7 level.^32,33 The wet samples were air-dried and then dried by an electronic drying machine at 80°C for about 12 h, samples were weighed with an electronic weighing balance and then recorded until the moisture content was less than 3%.^34,35 The dried samples were then crushed into ≤150 ± 6.6 μm sized powder (filler).

Fillers (rape straw and rape stalk), PVC, coupling agent, and stabilizer were mixed separately using a SYH three-dimensional motion mixer (Changzhou Beicheng drying equipment Engineering Co. Ltd, Changzhou, China) for about 10 min with the formulation shown in Table 1. The mixture was then extruded with a SXT-I single screw extruder (Harbin Harp Electric Technology Co. Ltd, Harbin, China). The temperatures from the hopper to the die zones were set to be 150°C, 155°C, 160°C, and 165°C, with the screw speed of 20 rpm.

Table 1.

Composite formulation

Straw/stalk fiber (wt. %)	PVC (wt. %)	Stabilizer (wt. %)	Coupling agent (wt. %)
30	59	8	3

PVC: polyvinyl chloride.

Performance testing

The Agilent Cary660 (Agilent Technologies, Inc., USA) model FTIR spectrometer was used to determine the infrared spectrum of both rape straw composite (SWC) and rape stalk composite (SKC) with a resolution of 4 cm⁻¹ and a scanning range of 4000∼400 cm⁻¹.

X-ray diffraction of the SWC and SKC materials was taken using the X-ray polycrystal diffractometer (XD3, Beijing Purkinje General Instrument Co. Ltd, Beijing, China) model by filling the sample holder with the composites which were exposed to Cu Kα radiation and then scanned at 5°/min scanning rate with 10° < 2θ < 80° in a continuous mode.

The STA449F5 TG-DSC thermal analyzer (NETZSCH Scientific Instrument Trading Shanghai Co. Ltd, Shanghai, China) was used to determine the thermal stability of the composite in which 20 mg of the composite samples was placed in an alumina crucible with a heating range of 30∼800°C and a heating rate of 20°C/min. An empty crucible was used as a blank control and then the flow rate of protective gas and scanning gas were calibrated to be 50 ml/min.

After calibrating the XHR-150 Plastic Rockwell Hardness tester (Shanghai Luer Test Equipment Co. Ltd, Shanghai China) in accordance with the GB/T33981-2008 standard, the hardness of the composites was determined with indenter diameter of 12.7 mm, 60 kg loaded and maintained for about 5 s, and the loading and unloading times were 15 s respectively, while the mean value of five tests was taken.

The density of the composite measuring 80 mm ×10 mm × 4 mm was determined in accordance with GB/T 1463-2005 as in Equation 1.

ρ = \frac{m}{v}

(1)

where ρ is the density, m is the mass of sample, and

v

is the volume of the sample.

Flexural and tensile properties of the SWC and SKC were determined with the E43.104 microcomputer-controlled electronic universal testing machine (Meters Industrial Systems Co. Ltd, Shanghai, China) in accordance with GB/T9341-2008 and GB/T1040.1-2006 standards, respectively. The flexural and tensile sample sizes were measured (80 mm ×10 mm × 4 mm), and span of (64 mm) with a loading speed (2 mm/min), and test results were taken as the average of five tests.

According to the GB/T1043.1-2008 standard, the impact testing of composites was carried out by using the G-P01 simply supported beam impact testing machine (Crowney (Beijing) Instruments Co. Ltd, Beijing, China). The size of each impact specimen was 80 mm × 10 mm × 4 mm, pendulum energy was 2 J, and test results were taken as the average of five tests.

The micro-morphology of impact section of the composite after sputtering gold spraying was observed using a scanning electron microscope (S-3400N, Hitachi Corporation, Tokyo, Japan).

The CS-200 precision colorimeter (Hangzhou Color Spectrum Technology Co. Ltd, Hangzhou, China) was used to record SWC and SKC color change according to CIE 1976 L*a*b* color space, taking chromaticity of the untreated straw composite as the initial value, and the color difference ( $Δ E$ ) of the composite was measured and calculated according to Equation 2.

Δ E = \sqrt{Δ L^{2} + Δ a^{2} + Δ b^{2}}

(2)

where

Δ L

indicates difference in color lightness,

Δ a

indicates difference in color red (

Δ a

> 0)/green (

Δ a

< 0), and

Δ b

indicates difference in color yellow (

Δ b

> 0)/blue (

Δ b

< 0).

Using the GB/T1034-2008 standard of test, fiber/PVC composite water absorption was determined by immersing dried composite samples in distilled water for 60 days at room temperature and the mass of the sample was recorded after every 24 h period from the initial date. Water absorption ( $c$ ) was calculated according to Equation 3, and average values of three groups of samples under the same exposure conditions were taken in each group.

c = \frac{m - m_{0}}{m_{0}} \times 100 %

(3)

where

m

represents sample mass after soaking (g);

m_{0}

represents sample mass before soaking and drying (g).

The MLS-225 rubber wheel three-body sand wear tester (Zhangjiakou Taihua Machinery Co. Ltd, Zhangjiakou, China) was used to carry out a three-body abrasive wear test of the composites. Abrasives used were sand particles collected from the Lanzhou section of the Yellow River, washed, air-dried, and screened with 36 mesh (0.5 mm aperture), the Shore hardness of the rimmed rubber was A-70 degree, 0.559 m perimeter of hub, 225 N pressure, 300 r/min speed of the hub, linear speed of 2.8 m/s and each sample run for 20 min. Anhydrous ethanol was used to clean each sample surface before recording to ensure accurate results and three repeated tests were carried out in each group. The specific wear rate ( $W_{S}$ ) was calculated according to Equation 4.

W_{S} = \frac{Δ M}{L F ρ}

(4)

In the formula,

Δ M

is weight loss (mg),

L

is sliding distance (m),

F

is applied load (N), and

ρ

is sample density (g · cm⁻³).

Results and discussion

Chemical compositions of the composites

As shown in Figure 2, the FTIR spectra of composites from SWC and SKC are illustrated. Changes of bands observed were related to lignin, hemicellulose, and cellulose group characteristics.³⁶ The band at 3350 cm⁻¹ was due to the O-H group available in both fibers, and the band at 2918 cm⁻¹ and 2850 cm⁻¹ was due to stretching vibration of the hydrocarbon chain C-H group.³⁷ The band at 1422 cm⁻¹ corresponded to vibration of the aromatic skeleton caused by C-O stretching in lignin, as these bending strengths were reduced due to the removal of lignin after alkaline treatment. However, a band at 1739 cm⁻¹ was attributed to stretching vibration of the carbonyl C=O group, the peak of band observed around 1510 cm⁻¹ was due to C=C vibrations of aromatic rings³⁸ and the band around 1035 cm⁻¹ which corresponds to stretching vibration of C-O and C-C was due to deformation in fiber and PVC.³⁹ It can be observed that both treated and untreated (SWC and SKC) have similar spectra curves which indicate no effect of alkaline treatment in the molecules of the fibers and is attributed to the nature of the fibers.⁴⁰

The X-ray diffraction (XRD) patterns of SWC and SKC are shown in Figure 3. From the diffraction patterns, diffraction peaks of 2θ at about 16° and 23° were observed in all composites, which corresponds to typical cellulose-I structure.⁴¹ There was no obvious disappearance, shift, or superposition of diffraction peaks in both SWC and SKC. Moreover, the trends of peaks observed are similar in untreated and alkaline-treated composites, indicating the crystal structure of each component has not been destroyed in the composite fabrication process. However, the reason for the results obtained are attributed to the non-effect of alkaline treatment on the fiber, and could be due to unchanged hydrogen bonding of the filler molecules after alkaline treatment.⁴²

Thermal degradation properties of the composites

The thermogravimetry (TG) and derivative thermal gravity (DTG) curve and pyrolysis characteristic data of each SWC and SKC are represented in Figure 4 and Table 2, respectively. From the curves and data, it can be observed that weight loss of the composites can be divided into two stages. The first stage occurs between 152∼400°C, during which weight loss rate was fastest and largest. This was mainly due to thermal degradation of lignin, cellulose, and hemicellulose in the main components of the composite and thermal decomposition of some PVC molecules.⁴³ However, weight loss in the second stage occurred between 400∼633°C, which was due to breakage and recombination of the molecular structure of PVC, resulting in volatile components.^44,45 The 1% alkaline-treated SWC was observed to have better thermal stability, and the trend of the other treated SWC changes was according to their thermal rates, although the untreated one was the poorest. However, SKC behaved better when 7% alkaline-treated and this kept decreasing as concentration decreased with untreated as intermediate.

Table 2.

Pyrolysis behavior of rape straw composite (SWC) and rape stalk composite (SKC)

Pyrolysis behavior		First stage		Second stage		Residual mass (%)
Filler	Alkaline concentration (%)	T₁ (°C)	Mass loss (%)	T₂ (°C)	Mass loss (%)	Residual mass (%)
SW	Untreated	190	56.95	575	20.9	17.95
	1%	168	52.62	578	19.81	23.93
	3%	159	53.77	573	20.27	21.82
	5%	168	55.70	573	21.46	19.54
	7%	179	53.93	572	20.62	20.97
SK	Untreated	152	55.86	571	19.51	21.31
	1%	153	55.76	633	20.84	20.34
	3%	159	53.77	572	21.91	13.87
	5%	153	52.56	570	19.46	23.98
	7%	158	51.01	620	19.63	26.52

SK: rape stalk; SW: rape straw.

Note: T₁ is the beginning temperature of pyrolysis; T₂ is the end temperature of the second stage pyrolysis.

Physical properties of the composites

The hardness of SWC and SKC is shown in Figure 5. The hardness levels of untreated SWC and 1% alkaline-treated SKC were the highest and the trend of the rest of the composites decreases as the alkaline-treated concentrations increase, with untreated SWC slightly higher than SKC 1% alkaline-treated by 3.95%, and was attributed to the effect of alkaline treatment on hemicellulose and impacts on pectin, wax, and other organic compounds from the fiber surface.⁴⁶ As a result, the bonding strength between matrix and fillers was distorted as the increase in alkaline concentration made the internal structure of the composite less durable with an inability to resist exerted forces.^47,48 From the trends of the composite density shown in Figure 5(b), it is indicated that increase in density was due to less porosity effect. An increase in water absorption causes a higher mass of composites which shows that the hydrophobic character of filler and PVC was affected by alkaline treatment that leads to minimal pore spaces between fillers and PVC interface, causing the internal structure of the composite to compact, thereby increasing composite density.⁴⁹ The density of 5% and 3% alkaline-treated forms of both SWC and SKC are the highest respectively, and these were attributed to the effect of alkaline treatment on fiber and the matrix by removal of low-density components such as hemicellulose, pectin, wax, and other compounds.⁵⁰ The density of SWC 5% alkaline-treated is slightly higher than SKC 3% alkaline-treated by 0.05%.

Mechanical properties of the composites

Figure 6 represents the mechanical properties of SWC and SKC by analyzing flexural, tensile, and impact strength properties of the composites. Higher strength from untreated or low alkaline-treated composites could be a result of weak effects of alkaline treatment on the fiber, as also observed from the previous literature and that leads to agglomeration of reinforcement and stress concentrations.⁵⁰ Furthermore, alkaline treatment on fibers during stress transfer allows composites to carry a load to a higher strain limit, although a strong interface enables crack propagation, which ultimately reduces toughness and strength.⁵¹ Flexural strength of SWC untreated and 1% alkaline-treated were the highest and increased by 1.39% higher than that of SWC 3% alkaline-treated. However, the flexural strength of SKC 3% alkaline-treated is the highest, higher than SKC 1% alkaline-treated by 1.37% and the flexural strength of SKC 3% alkaline-treated increased by 1.37% higher than the SWC untreated and 1% alkaline-treated. The tensile strength trend of SWC decreases as the alkaline treatment concentrations increases from SWC untreated, and the SWC untreated is the highest. However, the tensile strength of SKC 1% alkaline-treated is the highest, higher than SKC untreated by 7.19%. The tensile strength of SKC 1% alkaline-treated increased by 6.16% higher than SWC untreated. The impact strength of SWC 1% alkaline-treated is the highest and increased by 15.01% higher than SWC untreated. Moreover, the impact strength of SKC 1% alkaline-treated is also the highest, higher than SKC untreated by 24.32% and SWC 1% alkaline-treated is higher than SKC 1% alkaline-treated by 13.45%.

Composite micromorphology is illustrated in Figure 7. SWC (Figure 7(a)) and SKC (Figure 7(b)), were observed with full-out voids, and some fiber breakage, which could be due to non-availability of alkaline treatment to increase the bond strength as compared to the treated composites. However, in Figure 7(b) SWC was best, a great filler-matrix interfacial bond better than that of Figure 7(e) SKC of the same 1% alkaline-treated composites and could be attributed the effects of alkaline treatment on the filler.⁵² Moreover, in section of Figure 7(c) SWC indicates a strong bond resulted in fiber damage that could be attributed to the high alkaline concentration throughout the damage process, although the fiber-matrix interface was greatly similar to the SKC counterpart (Figure 7(f)) of the same 7% alkaline-treated.^53
–55 The interfacial improvement observed in Figure 7(b) and 7(e) which are 1% alkaline-treated in both SWC and SKC confirms the result obtained in mechanical properties, and implies that stress can be effectively transferred to the fiber from the matrix with moderate alkaline concentration treatment. Further increases in alkaline concentration can result in poor mechanical properties.

Sample color change

An evaluation of SWC and SKC color change parameters is illustrated in Table 3. The brightness of the color change was effected by alkaline treatment of the filler and showed a considerable amount of increase in all of the color trend of SWC. However, sequential increases in color trend were observed in SKC with some few interruptions in the case of 1% observed from all the composites. These sequential changes in brightness were contributed by alkaline treatment’s effects on all of the treated composite samples.

Table 3.

Color changes of rape straw composite (SWC) and rape stalk composite (SKC)

Filler	Alkaline concentration	ΔL	Δa	Δb	ΔE
SW	Untreated	0	0	0	0
	1%	2.01	−0.50	0.84	2.24
	3%	2.08	0.32	1.90	2.84
	5%	1.32	0.94	2.72	3.17
	7%	1.58	1.37	2.81	3.50
SK	Untreated	5.49	−0.53	−1.91	5.84
	1%	4.52	−0.68	0.29	4.58
	3%	5.29	0.83	2.65	5.97
	5%	3.96	1.97	4.62	6.40
	7%	2.57	2.03	4.98	5.96

SK: rape stalk; SW: rape straw.

Water absorption of the composites

The water absorption of SWC and SKC samples during the 60-day immersion period showed a rapid increase and then a slow rise until saturation as indicated in Figure 8(a) and (b), respectively. It showed that water absorption increased with an increase in alkaline concentration. This phenomenon is due to the hydrophobic characteristics of PVC and specific quality of hydrophilic nature from natural fiber having a better interfacial area between fiber and matrix.⁵⁶ The study shows 5% alkaline-treated SWC and SKC have a rapid increase and maintain a lower trend than 7% alkaline-treated SKC as the alkaline treatment influences water absorption and water absorption rate of the composites. This is because alkaline treatment removes hydrophobic components on the surface of the filler, such as pectin and wax, and increases hydrophilicity of the filler, resulting in a composite with more water absorption after alkaline treatment.^57,58 These behaviors simply mean that alkaline treatment has a negative effect on both composites as the untreated samples absorbed lesser amounts of water among others.

Abrasive wear of the composites

Table 4 shows weight loss and specific wear rate of SWC and SKC. From Table 4, both the composites with 3% alkaline treatment have the highest wear resistance which means that alkaline treatment has a great role in changing the interfacial bond of the composites. However, in the case of straw composites, untreated composite has a poor performance compared to untreated SKC which performed higher but slightly more than the 3% SKC and could be attributed to the enhanced fiber-matrix adhesion for the alkaline-treated composites in the SKC sample.⁵⁹ The alkaline treatment may lead to an increase in surface hardness, and thus increases the wear resistance of the composite.⁶⁰ Perhaps, low alkaline concentration on the composite sample improves surface roughness, strengthens the interfacial bond, thereby resisting abrasive wear as also observed in Figure 9 below.

Table 4.

Abrasive wear resistance of rape straw composite (SWC) and rape stalk composite (SKC)

Filler	Alkaline concentration	Mass loss (mg)	W_S (mm³/Nm)×10^–5
SW	Untreated	105 ± 4	7.1 ± 0.2
	1%	97 ± 7	6.3 ± 0.2
	3%	52 ± 3	3.3 ± 0.4
	5%	53 ± 5	3.4 ± 0.3
	7%	57 ± 3	3.7 ± 0.1
SK	Untreated	30 ± 1	2.2 ± 0.1
	1%	26 ± 2	1.9 ± 0.3
	3%	26 ± 3	1.8 ± 0.2
	5%	59 ± 3	4.2 ± 0.4
	7%	46 ± 4	3.4 ± 0.3

SK: rape stalk; SW: rape straw.

Figure 9 is a micro-morphology of an abrasive wear of SWC and SKC in order of increases in alkaline concentration, SWC (Figure 9(a) untreated, (b) 3% alkaline-treated and (c) 7% alkaline-treated) and SKC (Figure 9(d) untreated, (e) 3% alkaline-treated and (f) 7% alkaline-treated). It was observed that, as the concentration of alkaline increases, the structure of composite surfaces also changes. The furrow cuttings observed in Figure 9(b) (3% alkaline-treated) were associated with weak interfacial bonding, which increased surface roughness of the worn surface and aggravated the wear of the material as a result of an increase in alkaline concentrations from untreated composites Figure 9(a).⁶¹ However, ductile characteristics such as the microlayer and corrugated structure were observed in Figure 9(d) and, additionally, the appearance of voids and gaps in Figure 9(c) and (f) was due to interaction between fillers and matrix which made interfacial adhesion stronger, thus leading to cavitation of PVC particles.^55,62 Moreover, Figure 9(b) and (e) which correspond to 3% alkaline-treated, happened to resist more wear from SWC and SKC, and indicated that low alkaline concentration on SWC and SKC improve resistance to abrasive wear more than higher alkaline concentration treatment.

Conclusions

Alkaline-modified SWC and SKC were subjected to tests for chemical composition, thermal degradation, physical and mechanical properties, SEM, color change, water absorption, and abrasive wear performance of the composites. The results were analyzed and concluded as follows:

FTIR and XRD test results show that alkaline treatment has no obvious effect on the chemical structure of the composites. SEM photos show that alkaline treatment affects the surface roughness of the filler and the interfacial bonding strength between the filler and PVC matrix, thus affecting the properties of the composites. alkaline treatment can significantly affect the color difference of SKC and SWC.

The water absorption of SWC increased with the increase of alkaline treatment concentration, while the tensile strength and bending strength decreased with the increase of alkaline treatment concentration. After 1% alkaline treatment, SWC has the highest thermal stability, highest hardness, and highest impact strength. After 3% alkaline treatment, SWC has the highest density and abrasive wear resistance.

Alkaline treatment can increase the water absorption of SKC, but did not show a clear relationship, and the hardness decreases with the increase of alkaline treatment concentration. After 1% alkaline treatment, SKC has the highest flexural strength and impact strength, 3% alkaline treatment gives the highest density, highest tensile strength and abrasive wear resistance, and SKC has the highest thermal stability after 7% alkaline treatment.

The physical and mechanical properties of the rape straw and rape stalk can be improved with low alkaline concentration. This study can be used to guide further research in modifying specific properties intended for use. Furthermore, future research studies on rape straw and stalk fiber can investigate other treatment methods with additional parameters not covered in this study due to human limitations.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the Fund for Young Supervisor of Gansu Agricultural University, Grant No. GAU-QDFC-2021-07, the Higher Education Industrial Support Plan Project of Gansu Province, Grant No. 2021CYZC-29 and the Science and Technology Projects of Lanzhou, Grant No. 2019-4-55.

ORCID iD

Keping Zhang

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