The Sliding Frictional Properties of Untreated and Extrusion-Exploded Wheat and Rice Straw

Abstract

Knowledge about the frictional characteristics of materials is required for equipment design, either to exploit the friction or to overcome it. This paper deals with studying the coefficients of sliding friction of non-treated and extrusion-exploded wheat straw and rice straw on mild steel and aluminum surfaces and the effects of several factors, including the moisture content (at five levels 35, 45, 55, 65, and 75%), normal pressure (at the following five levels: 2, 4, 6, 8, and 10 kPa), and length (at the following five levels: 30, 60, 90, 120, and 150 mm). Through the use of a statistical analysis method, we found that the moisture content, normal pressure, and interactions between them had statistically significant effects on the coefficient of sliding friction (p < 0.01). The coefficient of sliding friction was much larger than that of rice straw on both contact surfaces, and the coefficient values of both straw samples on the aluminum surface were notably higher than those on mild steel. A significantly higher value was found for the coefficient of sliding friction of extrusion-exploded straw compared with that of untreated straw on both surfaces. As the normal pressure increased from 2 to 10 kPa, the coefficient of sliding friction decreased first, and then increased sharply, finally fluctuating slowly for both kinds of straw and on both tested surfaces, with the maximum value was observed at a normal pressure of 6 kPa. When the length increased from 30 to 150 mm, a slightly linear upward trend was observed for the coefficient of sliding friction of untreated wheat straw, and the coefficient of sliding friction of untreated rice straw initially increased and then decreased.

1. Introduction

In order to relieve the global energy crisis and to achieve the sustainable development of agricultural ecological environments, plenty of agricultural biomass residues are widely used as feedstock, forages, fertilizer, and fuel. Knowledge of friction properties is essential in the design and modification of equipment used for harvesting, handling, conveying, feeding, storing and other processes involving agricultural biomass residue [1], as these properties have a direct impact on operational performance.

The frictional behavior of biomass grinds is described by the following two independent parameters: the coefficient of internal friction and the coefficient of wall friction [2]. The coefficient of wall friction is the magnitude ratio of the sliding friction force existing between the contact surfaces of two bodies moving relative to one another and the normal pressure acting on the contact surface [3]. It can be described as the wall friction property of materials. The effects of a material’s moisture content, physical size, normal load, contact surface style, shear rate, and other factors on the coefficient of wall friction have been extensively studied in experimental research. The coefficient of wall friction at different corn straw grind sizes on a galvanized steel surface has been shown to increase significantly with an increase in the moisture content from 7% to 15% and decrease with an increase in the normal pressure from 10 to 200 kPa [4]. It has been demonstrated that the coefficient of friction of soft red winter wheat on corrugated and smooth galvanized steel surfaces increases with an increase in the shear rate and decreases with an increase in the normal pressure. Larsson [5] reported that the coefficient of kinematic wall friction of reed canary grass powder from a hammer mill screen size of 4.0 mm is negatively correlated with normal stress at low (from 0.52 to 7.52 kPa) and high (from 23 to 275 MPa) normal stress levels. Kalkan [6] found that the dynamic coefficient of friction values of wheat cultivars on frictional surfaces of steel and fiberglass increased with an increase in the moisture content, and they observed that the effect of moisture content was greater at higher moisture levels. Ghorbani [7] reported that the coefficient of friction of alfalfa grind on a polished steel surface slightly increased as the moisture content increased from 8 to 11% (wb) and increased with an increase in the screen size from 2.38 to 4.76 mm at a moisture content of 9.3%.

Straw fiber base film is a completely biodegradable film made from crop straw as the main raw material, which not only has the properties of ordinary plastic film, such as in terms of temperature and moisture protection and the inhibition of weeds, but also can be completely degraded during the crop growth cycle to avoid white pollution. [8]. The D200 model straw fiber machine adopts a pure physical pretreatment method to achieve green, clean, and efficient production of agricultural straw fiber. [9]. The friction properties of the straw are an important reference when evaluating the design of the fiber manufacturing machine and its accompanying raw material supply system [10]. The fiber extraction machine was evaluated through an experimental analysis and required the physical characteristics of a large particle size and a high moisture content (>45%) for the crop straw to be processed [10,11]. However, there have been few studies on the friction characteristics of straws with large particle sizes and high moisture content. Almost all studies were performed at a relatively lower material moisture content (3–35%). Related studies about the determinations of the coefficient of sliding friction at high moisture contents are scarce. Furthermore, limited literatures have investigated the concurrent effects of moisture content and normal pressure on the sliding friction. No studies have been conducted on the sliding friction properties of untreated and extrusion-exploded straws.

In summary, studies on the frictional properties of straw exposed to high moisture content conditions and the sliding frictional properties of the straw fiber after squeeze blasting are lacking. The objective of this study was to investigate the sliding friction characteristics of untreated and extrusion-exploded wheat straw and rice straw on mild steel and aluminum surfaces and the factors influencing the friction characteristics.

2. Materials and Methods

2.1. Materials

Two kinds of agricultural biomass straw (wheat and rice straw) were used in the experiments. The raw materials were harvested during the summer of 2019 from Xiangfang farm and sent to the Biomass Materials Laboratory of the Northeast Agricultural University (Changjiang road 600, Xiangfang District, Harbin, China) where all of the experiments were conducted. The experimental design involved determining the coefficient of sliding friction for untreated and extrusion-exploded straws.

2.1.1. Crushing Pretreatment

For the experimental setup for untreated straws, the raw material moisture content, raw material length, and normal pressure were varied. The straw samples were obtained in two batches, one of which was chopped using a 9QR20-60 Straw hammer crusher (Harbin Longmu Machinery Equipment co. LTD, Harbin, China) to investigate the effects of the moisture content and normal pressure on the coefficient of sliding friction. The straw crusher was modified with the moving blades removed in order to alleviate shearing and strengthen the rubbing action. This facilitated the fibrillation of straw fiber in the subsequent extrusion process.

2.1.2. Extrusion Pretreatment

For the experimental setup for extrusion-exploded straw, the factors varied were the raw material moisture content and normal pressure. Extrusion-exploded straw samples were obtained using a single screw extruder (D200 single screw straw-based fiber extruder, engineering college, Northeast Agricultural University, as shown in Figure 1) [12], which had an average barrel length to screw diameter ratio (l/d) of 5:1. The extruder consisted of feeding, compression, and exploding zones, corresponding to three different screws. The screw in the compression zone had a compression ratio of 3:1. Each zone was equipped with a temperature control and cooling system connected with a barrel in order to avoid the straw becoming charred due to the high temperature in the extrusion process. Water was supplied as a cooling agent along the length of the barrel. The gap in the die at the end of the exploding section was adjustable to allow the acquisition of fibers with different particle sizes [13,14]. The single screw extruder was fitted to a 75-kW adjustable-speed motor, which had a provision to adjust the screw speed from 0 to 200 rpm. The barrel temperature and screw speed were controlled by a controller cabinet connected to the extruder. In the process of extrusion, the exploding phenomenon happened due to a sudden reduction in air pressure when materials were discharged. Once the exploding effect had stabilized, the straw samples were extruded under the conditions of a screw speed of 100 r/min, a temperature of 96 ℃, and a die gap of 1.5 mm. Consequently, the untreated straws were processed to the fiber magnitude level, i.e., the desired extrusion-exploded straws were formed [15,16].

2.1.3. Moisture Content

The chopped straws were cleaned manually to remove foreign matters such as dirt, stones, and other impurities. The initial moisture content of straw was determined as per the procedures presented in [10], according to the recommended national standard (GB/T462-2003, Paper and board-Determination of moisture content). The weights of the samples were recorded on an analytical balance (accuracy 0.001 g). Then, these samples were dried in a forced air oven (DGG-9070AD electrothermal constant temperature blast air drying oven (Shanghai SenXin Experimental Instrument Co., LTD., Shanghai, China) at 105 ± 2 °C for 24 h. The initial moisture content of straw was calculated as M_i. then, in order to further achieve the desired moisture content level, a certain amount of distilled water predetermined using the following Equation (1) was added to the samples. After thorough mixing, the moistened samples were sealed in polyethylene bags and kept in a refrigerator at 5 °C for 24 h to enable the moisture to be distributed uniformly throughout the sample.

$$\mathrm{Q}=\frac{m_i(M_f-M_i)}{1-M_f}$$

(1)

where Q is the mass of water to be added (g); m_i is the initial mass of the sample (g); M_i is the initial moisture content of the samples (% w.b), and M_f is the final content of the samples (% w.b).

For all tests, prior to starting the trials, the necessary sample quantities were removed from the refrigerator to allow them to reach equilibrium with the room temperature.

2.2. Methods

The coefficient of sliding friction was determined using a friction measuring apparatus, shown in detail in Figure 2, according to Equation (2). The test was conducted on mild steel and an aluminum surface.

$${\mu }_{\mathrm{S}}=F_S/N$$

(2)

where μ_s is the = coefficient of sliding friction; F_s is the = force of friction (N); N is the = normal load (N).

The measuring device is shown in Figure 2a. The test scene is shown in Figure 2b. When testing, a sample box with a diameter of 160 mm connected to a load cell was placed on a friction surface and filled with the straws. The straw was placed randomly. The desired load was applied to the base plate at top of the straw samples and kept there for 30 s. The friction surface was horizontally moved by a driving unit at a constant velocity of 24 mm/min. It should be noticed that the sample box could not contact the surface so as not to affect the accuracy of the measurement data. The friction force was sensed by a load cell connected to the sample box, and the data were recorded on a PC through a data acquisition system. Those data that changed steadily in a certain period were accepted, and data that fluctuated greatly were excluded. For each replication, the samples were emptied and refilled with a different sample. For the friction tests, were conducted in three replicates were used.

2.3. Experimental Design

The experimental design involved determining the coefficient of sliding friction of untreated and extrusion-exploded straw. For the determination of the sliding friction coefficient of untreated straw, we used a double-factor factorial design. Regarding the double-factor factorial design, the moisture content at five levels (35, 45, 55, 65, and 75% w.b) and the normal pressure at five levels (2, 4, 6, 8, and 10 kPa) were used as the independent variables, and the coefficients of sliding friction of wheat and rice straw acquired by machine on mild steel and an aluminum surface were the dependent variables. As per the single-factor factorial design, the straw samples chopped by hand were selected. The length at five levels (30, 60, 90, 120, and 150 mm) was the independent variable and the coefficients of sliding friction of untreated wheat and rice straw on mild steel and an aluminum surface were the dependent variables. For the determination of the coefficient of sliding friction of extruded exploded straw fibers, a double-factor factorial design, similar to that used with untreated straw, was employed, where the moisture content at five levels (35, 45, 55, 65, and 75% w.b) and the normal pressure at five levels (2, 4, 6, 8, and 10 kPa) were the independent variables, and the coefficients of sliding friction of wheat and rice straw on mild steel and an aluminum surface were the dependent variables. All the results represent the average of three replicates.

2.4. Statistical Analysis

To study the changes in the experimental data measured in the friction tests, the statistical description parameters, namely, the mean and standard deviation were used. The Analysis of Variance (ANOVA) method was applied at the 0.01 level of significance to investigate the influences of the applied factors on the dependent variables using SPSS (version 23). Additionally, the Tukey multiple comparison and means comparison were used to compare the difference between any two treatment means at the 0.01 level of significance. The fitting equations were developed for the coefficient of sliding friction with length being the independent variable using the least squares regression method. All the results are presented in table form and plotted in figures.

3. Results

The factors and levels used in the coefficient of sliding friction characteristic tests are listed in

Table 1. The factors and levels of coefficient used in the sliding friction characteristic test.

Level	Straw Variety	Contact Surface Material	Moisture Content (%)	Normal Pressure (kPa)	Length (mm)
1	Untreated wheat straw	Mild steel	35	2	30
2	Untreated rice straw	Aluminum	45	4	60
3	Extrusion-exploded wheat straw		55	6	90
4	Extrusion-exploded rice straw		65	8	120
5			75	10	150

. The coefficients of sliding friction of the untreated and extrusion-exploded wheat straw and rice straw against mild steel surface and an aluminum surface at different moisture contents and normal pressure values are presented in

Table 2. Coefficient of sliding friction of wheat straw on mild steel and an aluminum surface with different levels of moisture content and normal pressure levels.

Wheat Straw	Moisture Content (%)	Contact Surface
		Mild Steel										Aluminum
		Normal Pressure (kPa)										Normal Pressure (kPa)
		2		4		6		8		10		2		4		6		8		10
Untreated	35	0.560 a 0.002 b	ac A	0.563 0.002	a C	0.688 0.001	d D	0.643 0.001	c C	0.615 0.001	b B	0.651 0.002	c B	0.612 0.002	a B	0.702 0.002	e C	0.660 0.001	d AB	0.635 0.001	b C
	45	0.569 0.003	b AB	0.560 0.001	a C	0.658 0.001	e C	0.641 0.001	d C	0.616 0.000	c B	0.595 0.002	a A	0.606 0.002	b B	0.673 0.001	e A	0.661 0.001	d B	0.636 0.000	c C
	55	0.558 0.003	b A	0.537 0.002	a A	0.633 0.001	e A	0.613 0.001	d A	0.596 0.001	c A	0.650 0.003	b B	0.612 0.003	a B	0.685 0.001	d B	0.667 0.001	c C	0.615 0.001	a AB
	65	0.581 0.003	b B	0.554 0.002	a B	0.644 0.001	d B	0.620 0.001	c B	0.615 0.001	c B	0.600 0.003	b A	0.569 0.002	a A	0.686 0.001	e B	0.673 0.002	d D	0.616 0.000	c B
	75	0.562 0.003	a A	0.562 0.002	a C	0.635 0.001	d A	0.611 0.001	c A	0.598 0.001	b A	0.601 0.004	a A	0.606 0.001	ab B	0.681 0.001	d B	0.655 0.001	c A	0.612 0.001	b A
Extrusion-exploded	35	0.694 0.002	b B	0.674 0.001	a A	0.788 0.001	e D	0.759 0.001	d D	0.718 0.001	c D	0.792 0.003	d C	0.760 0.002	b E	0.850 0.001	e E	0.770 0.001	c D	0.728 0.001	a D
	45	0.697 0.002	b B	0.681 0.001	a B	0.788 0.001	d D	0.749 0.001	c C	0.701 0.001	b C	0.733 0.002	b B	0.728 0.001	a D	0.821 0.001	d D	0.771 0.000	c D	0.734 0.001	b E
	55	0.708 0.003	c C	0.677 0.002	a AB	0.773 0.001	e B	0.725 0.001	d B	0.694 0.002	b B	0.728 0.002	c B	0.704 0.002	a C	0.775 0.002	d C	0.730 0.001	c C	0.717 0.001	b C
	65	0.722 0.002	c D	0.687 0.001	a C	0.780 0.002	e C	0.746 0.001	d C	0.704 0.001	b C	0.723 0.003	c B	0.667 0.002	a B	0.753 0.001	d B	0.706 0.001	b B	0.667 0.001	a B
	75	0.679 0.002	ab A	0.675 0.002	a A	0.737 0.002	d A	0.719 0.001	c A	0.685 0.001	b A	0.682 0.002	d A	0.625 0.002	b A	0.702 0.002	e A	0.656 0.001	c A	0.610 0.001	a A

a: means of 3 replicates, b: standard deviation, c: Tukey HSD multiple comparison test at the 1% level of significance at the same moisture content for different normal pressures (a, b, c, d, and e) and for the same normal pressure at different moisture contents (A, B, C, D, and E).

and

Table 3. Coefficient of sliding friction of rice straw on mild steel and an aluminum surface with different levels of moisture content and normal pressure levels.

Rice Straw	Moisture Content (%)	Contact Surface
		Mild Steel										Aluminum
		Normal Pressure (kPa)										Normal Pressure (kPa)
		2		4		6		8		10		2		4		6		8		10
Untreated	35	0.487 0.002	a D	0.490 0.001	a E	0.552 0.001	d B	0.539 0.001	c C	0.512 0.001	b B	0.510 0.002	a A	0.535 0.002	b B	0.634 0.002	e A	0.599 0.001	d A	0.589 0.001	c A
	45	0.455 0.002	a B	0.450 0.002	a B	0.549 0.001	d B	0.537 0.001	c C	0.518 0.001	b C	0.573 0.003	a BC	0.574 0.001	a C	0.677 0.002	d D	0.646 0.000	c C	0.614 0.001	b C
	55	0.474 0.002	a C	0.471 0.002	a D	0.577 0.001	d C	0.540 0.001	c C	0.521 0.001	b C	0.515 0.003	a A	0.515 0.001	a A	0.649 0.002	d B	0.597 0.001	c A	0.585 0.001	b A
	65	0.419 0.003	a A	0.438 0.002	b A	0.516 0.001	d A	0.516 0.001	d A	0.505 0.001	c A	0.581 0.003	b C	0.570 0.001	a C	0.657 0.002	d C	0.600 0.001	c A	0.596 0.001	c B
	75	0.465 0.003	a BC	0.463 0.002	a C	0.553 0.001	c B	0.527 0.001	b B	0.522 0.001	b C	0.567 0.002	b B	0.538 0.002	a B	0.631 0.002	d A	0.609 0.001	c B	0.626 0.001	d D
Extrusion-exploded	35	0.682 0.002	b B	0.662 0.001	a C	0.761 0.002	d C	0.721 0.001	c D	0.682 0.001	b E	0.800 0.003	d E	0.719 0.002	b E	0.827 0.001	e D	0.729 0.001	c C	0.694 0.001	a C
	45	0.671 0.002	b A	0.623 0.002	a A	0.759 0.001	e C	0.706 0.001	d C	0.678 0.001	c D	0.727 0.003	b C	0.660 0.002	a C	0.801 0.002	c C	0.757 0.001	d E	0.724 0.001	b E
	55	0.703 0.002	c D	0.645 0.001	a B	0.759 0.002	e C	0.720 0.000	d D	0.673 0.001	b C	0.774 0.002	c D	0.704 0.002	a D	0.805 0.002	d C	0.747 0.002	b D	0.699 0.001	a D
	65	0.709 0.002	d D	0.621 0.002	b A	0.710 0.001	d B	0.648 0.000	c B	0.594 0.001	a B	0.682 0.002	b B	0.631 0.002	a B	0.749 0.002	c B	0.657 0.000	b B	0.630 0.001	a B
	75	0.694 0.002	e C	0.622 0.002	c A	0.652 0.002	d A	0.546 0.000	b A	0.522 0.001	a A	0.608 0.003	c A	0.581 0.001	b A	0.679 0.001	e A	0.631 0.001	d A	0.569 0.001	a A

a: mean of 3 replicates, b: standard deviation, c: Tukey HSD multiple comparison test at the 1% level of significance at the same moisture content for different normal pressure values (a, b, c, d, and e) and for the same normal pressure at different moisture contents (A, B, C, D, and E).

.

The ANOVA results for the coefficient of sliding friction versus the moisture content and normal pressure are tabulated in

Table 4. The ANOVA results for the coefficient of sliding friction versus the moisture content and normal pressure.

Dependent Variable			Source	Type III Sum of Squares	Df	Mean Square	F
Pretreatment Method	Sample Variety	Contact Surface
Untreated	Wheat straw	Mild steel	Moisture content(MC)	0.007	4	0.002	448.605 **
			Normal pressure(NP)	0.099	4	0.025	6124.979 **
			MC * NP	0.006	16	0.000	85.963 **
			Error	0.000	50	4.03 × 10−6
			Total	0.112	74
		Aluminum	Moisture content(MC)	0.006	4	0.002	338.85 **
			Normal pressure(NP)	0.072	4	0.018	4092.656 **
			MC ∗ NP	0.011	16	0.001	159.051 **
			Error	0.000	50	4.43 × 10−6
			Total	0.090	74
	Rice straw	Mild steel	Moisture content(MC)	0.014	4	0.004	1151.702 **
			Normal pressure(NP)	0.099	4	0.025	8094.475 **
			MC ∗ NP	0.006	16	0.000	124.901 **
			Error	0.000	50	3.06 × 10−6
			Total	0.119	74
		Aluminum	Moisture content(MC)	0.021	4	0.005	1339.903 **
			Normal pressure(NP)	0.116	4	0.029	7211.612 **
			MC ∗ NP	0.013	16	0.001	196.076 **
			Error	0.000	50	4.00 × 10−6
			Total	0.150	74
Extrusion-exploded	Wheat straw	Mild steel	Moisture content(MC)	0.008	4	0.002	726.172 **
			Normal pressure(NP)	0.085	4	0.021	7322.999 **
			MC ∗ NP	0.006	16	0.000	121.415 **
			Error	0.000	50	2.91 × 10−6
			Total	0.099	74
		Aluminum	Moisture content(MC)	0.142	4	0.035	9886.966 **
			Normal pressure(NP)	0.076	4	0.019	5283.518 **
			MC ∗ NP	0.011	16	0.001	186.817 **
			Error	0.000	50	3.58 × 10−6
			Total	0.228	74
	Rice straw	Mild steel	Moisture content(MC)	0.095	4	0.024	9548.013 **
			Normal pressure(NP)	0.100	4	0.025	10089.428 **
			MC ∗ NP	0.066	16	0.004	1651.347 **
			Error	0.000	50	2.48 × 10−6
			Total	0.260	74
		Aluminum	Moisture content(MC)	0.216	4	0.054	12511.669 **
			Normal pressure(NP)	0.128	4	0.032	7419.379 **
			MC ∗ NP	0.019	16	0.001	281.899 **
			Error	0.000	50	4.32 × 10−6
			Total	0.364	74

** Significant at p < 0.01.

. As expressed in the table, the moisture content, normal pressure, and the moisture content x normal pressure interaction were shown to have statistically significant effects on the coefficient of sliding friction at p < 0.01. Significant effects were observed for untreated and extrusion-exploded wheat straw and rice straw on both mild steel and aluminum surfaces.

Our results reveal that the coefficient of sliding friction of untreated wheat straw against mild steel and aluminum surfaces ranged from 0.537 to 0.688 and from 0.569 to 0.702, respectively, in the moisture content range from 35 to 75% (w.b) along with a normal pressure range from 2 to 10 kPa. In contrast, the coefficient of sliding friction of untreated rice straw against mild steel and aluminum surfaces varied between 0.419 and 0.577 and between 0.510 and 0.677, respectively, under the same conditions.

With respect to extrusion-exploded wheat straw and rice straw, it was stated that the coefficient of sliding friction on mild steel and aluminum surfaces varied from 0.685 to 0.788 and from 0.610 to 0.850. Respectively, with a concurrent increase in the moisture content from 35 to 75% (w.b) and normal pressure from 2 to 10 kPa. As a comparison, the coefficient of sliding friction of extrusion-exploded rice straw on mild steel and aluminum surfaces varied from 0.522 to 0.761 and from 0.569 to 0.827, respectively, in the same conditions.

As per Figure 3 and Table 2, for extrusion-exploded straw, the phenomenon whereby the coefficient of sliding friction of wheat straw was greater than that of rice straw was observed. The difference was significant on the surface of mild steel but not on aluminum. Furthermore, once again, it was noted that the coefficient of sliding friction of samples was greater on aluminum compared with mild steel; however, the difference was not significant.

Table 5. The ANOVA results for the coefficient of sliding friction regarding length.

Dependent Variable			Sum of Squares	df	Mean Square	F
Straw Variety	Contact Surface
Wheat straw	Mild steel	Between Groups	0.005	4	0.001	415.318 **
		Within Groups	0.000	10	0.000
		Total	0.005	14
	Aluminum	Between Groups	0.003	4	0.001	177.250 **
		Within Groups	0.000	10	0.000
		Total	0.003	14
Rice straw	Mild steel	Between Groups	0.018	4	0.005	1041.861 **
		Within Groups	0.000	10	0.000
		Total	0.019	14
	Aluminum	Between Groups	0.072	4	0.018	4306.853 **
		Within Groups	0.000	10	0.000
		Total	0.072	14

** Significant at p < 0.01.

presents the ANOVA results for the coefficient of sliding friction of untreated straws regarding the length of mild steel and aluminum. The results indicate that the coefficient of sliding friction length had a statistically significant difference at p < 0.01 at various lengths.

The coefficient of sliding friction of untreated straw on mild steel and aluminum surfaces versus the length at a moisture content of 75% (w.b) and normal pressure of 4 kPa was exhibited in

Table 6. Coefficient of sliding friction for different lengths of straw on mild steel and aluminum surfaces.

Sample Variety	Contact Surface	Length (mm)
		30		60		90		120		150
Wheat straw	Mild steel	0.457 a 0.001 b	aDY c,d	0.473 0.002	bcDY	0.479 0.002	cDY	0.472 0.001	bDY	0.510 0.002	dDY
	Aluminum	0.548 0.002	aEY	0.565 0.002	bEY	0.571 0.002	bcEX	0.574 0.002	cEX	0.590 0.002	dEY
Rice straw	Mild steel	0.354 0.002	aDX	0.420 0.002	bDX	0.446 0.001	cDX	0.454 0.002	dDX	0.421 0.002	bDX
	Aluminum	0.383 0.002	aEX	0.525 0.002	bEX	0.569 0.002	dEX	0.567 0.002	dEX	0.547 0.001	cEX

^a: means of 3 replicates; ^b: standard deviation; ^c: Tukey HSD multiple comparison test at the 1% level of significance for a given contact surface at various lengths (a, b, c, and d); ^d Means comparison at the 1% level of significance for a given length and variety on different contact surfaces (D and E) and for the same length and contact surface but for different sample varieties (X and Y).

and Figure 4.

According to the results presented, previously, the coefficient of sliding friction of both test samples on the aluminum surface was markedly higher than on mild steel. The results of the means comparison showed that the difference was significant (p < 0.01) in the length range from 30 to 150 mm for both varieties of samples. Furthermore, the coefficient of sliding friction of untreated wheat straw was greater than that of rice straw on both contact surfaces. A significant difference (p < 0.01) was noted by referring to the comparison of means with only two exceptions.

4. Discussion

4.1. The Effects of the Normal Pressure and Moisture Content on the Coefficient of Sliding Friction

A comparison of the means of the coefficient of sliding friction between untreated and extrusion-exploded straw, wheat straw and rice straw, and mild steel and aluminum surfaces is presented in Figure 3 and

Table 7. Comparison of the coefficients of sliding friction between untreated and extrusion-exploded straw, wheat straw and rice straw, and mild steel and aluminum surfaces.

Straw Variety	Treatment Method	Contact Surface
		Mild Steel		Aluminum
Wheat straw	Untreated	0.602 a 0.039 b	aEX c	0.639 0.036	bEX
	Extrusion-exploded	0.718 0.036	aEY	0.725 0.055	aDY
Rice straw	Untreated	0.504 0.040	aDX	0.592 0.045	bDX
	Extrusion-exploded	0.670 0.059	aDY	0.703 0.070	aDY

^a: means of 3 replicates; ^b: standard deviation; ^c: means comparison at the 1% level of significance for a given straw variety and treatment method but on various contact surfaces (a and b); for the same treatment method and contact surface but for different straw varieties (D and E); for the same straw variety and contact surface but for different treatment methods (X and Y).

. It is demonstrated that the coefficient of sliding friction is greater for wheat straw as compared to rice straw, and the difference is significant at the 0.01 probability level. In addition, the comparison between the coefficient of sliding friction of samples on the aluminum and mild steel surfaces indicated that the former value is higher than the latter with no exceptions, and a significant difference was found at a 99% confidence interval. This could be attributed to the difference in material surface roughness for mild steel and aluminum surfaces, thus leading to a difference in sliding friction. For extrusion-exploded straw, the difference was less apparent.

Figure 5a,b shows the effects of the moisture content and normal pressure on the coefficient of sliding friction of untreated straw. At any level of moisture content, the coefficient of sliding friction was shown to first decrease with an increase in normal pressure from 2 to 4 kPa and then increase sharply when the normal pressure increased from 4 to 6 kPa before finally decreasing gradually as the normal pressure continued to increase from 6 to 10 kPa. The highest coefficient of sliding friction value was found at a normal pressure of 6 kPa, and the lowest value was found at a normal pressure of 2 or 4 kPa. It is commonly found that an increase in the normal pressure leads to a decrease in the wall friction coefficient [17,18]. The data points recorded in this study showed a different trend compared with those presented in the literature mentioned above. This may be related to the contact surface moving too fast.

As shown in Figure 5, the moisture content had different effects on the coefficient of sliding friction based on the various sample varieties, contact surfaces, and normal pressure levels. Generally, the coefficient of sliding friction fluctuated up and down with moisture contents ranging from 35 to 75%. Unlike the specific trends observed by other researchers [19,20], no specific relationship was observed between the coefficient of sliding friction of untreated straw and the moisture content. Probably the contact surface was moving too fast during the test, causing the slip-stick phenomenon to disappear and preventing a specific pattern from being observed. Xueli Chen [21] suggested that the frictional properties of material systems are affected in the following two ways: as the moisture content increases, the friction either decreases because the water acts as a lubricant [22] or increases due to the force of adhesion stemming from the inter-straw capillary force [23]. These two effects coexist and interact, leading to a fluctuation in the coefficient of friction.

Figure 6 presents the contributions of factors to the coefficient of sliding friction, which were calculated in terms of the sum of the squares of the variant. For untreated straw, it could be inferred that the normal pressure plays a predominant role in influencing variation in the coefficient of sliding friction [24]; however, the moisture content and the moisture content x normal pressure interaction showed extremely weak effects [25].

As seen in Figure 3, the coefficient of sliding friction of extrusion-exploded straw was significantly higher than that of untreated straw for both wheat and rice straw on both mild steel and aluminum surfaces. The different letters in Table 7 show significant variation. This phenomenon may be due to the fact that the fibrous external surface of extrusion-exploded straw was rougher compared to that of untreated straw. Additionally, it is possible that, for extrusion-exploded straw, the amount of straw forced to contact the surface increased; in other words, closer contact existed when a normal load was applied. Consequently, this increased the contacting surface area, and then the sliding friction force increased. This could be due to the fact that the structure of straw became looser and more flexible as a result of the removal of partial lignin and hemicellulose, reductions in crystallinity and polymerization, and an increase in porosity through the extrusion process [26].

Figure 7c,d shows that the effect of the normal pressure on the coefficient of sliding friction of extrusion-exploded straw is analogous to that of untreated straw; that is, the coefficient of sliding friction decreased initially and then increased dramatically to reach a peak value, before finally decreasing with an increase in normal pressure of 2–10 kPa. The minimum coefficient of sliding friction value occurred at a normal pressure of 4 or 10 kPa.

The effect of the moisture content on the coefficient of sliding friction varied with different sample varieties, contact surfaces, and normal pressure levels. As shown in Figure 6, compared with untreated straw, relatively large fluctuations existed for the coefficient of sliding friction of extrusion-exploded straw, and no concrete relationship was observed between the coefficient of sliding friction and moisture content. At all levels of normal pressure, the lowest coefficient of sliding friction was observed at a moisture content of 75%.

As shown in Figure 6b, the moisture content had a more striking effect on the coefficient of sliding friction for extrusion-exploded straw compared with untreated straw, which even surpassed that of normal pressure. This indicates that the sliding friction property of extrusion-exploded straw is more susceptible to the effects of the moisture content. Little effect was observed for the moisture content x normal pressure interaction. This may be related to the breaking of straws. When the pressure gradually increases, the straw is broken, and the contact surface of mild steel becomes larger and rougher, and the friction coefficient increases. When the pressure increases to the critical value, the contact surface tends to be flat, and the friction coefficient decreases.

4.2. The Effect of Length on the Coefficient of Sliding Friction of Untreated Straw

As shown in Figure 4, for the coefficient of sliding friction of untreated wheat straw, a predominantly linear upward trend was noted from a minimum value of 0.457 to a maximum value of 0.510 on mild steel and from 0.548 to 0.590 on aluminum when the length was in the ascending range from 30 to 150 mm. The multiple comparison results indicated that the difference in the coefficient of sliding friction was significant between two adjacent length intervals, with a few exceptions.

By comparison, at an aggregate level, the coefficient of sliding friction was more susceptible to the effect of length for rice straw than for wheat straw. The coefficient of sliding friction of untreated rice straw increased initially and then decreased for both contact surfaces, ranging from 0.354 to 0.454 and from 0.383 to 0.569, respectively, on the surfaces of mild steel and aluminum in the length range from 30 to 150 mm. The critical points of length were 120 and 90 mm for mild steel and aluminum, respectively. The multiple comparison results indicate that the mean coefficient of sliding friction differed significantly in almost all intervals of length. This may be attributed to the fact that the original surfaces of the straw were smooth, but the fracture surfaces were rough. As the length decreased, the proportion of the rough surface increased, and the coefficient of sliding friction of the straw increased [26]. Ling et al. (1997) also reported that both the static and sliding coefficients of friction decreased with an increase in ash particle size.

As expressed in

Table 8. The fitting model of the coefficient of sliding friction based on length.

Sample Variety	Contact Surface	Fitting Parameters
		Fitting Equation	R2	Sy.x
Wheat straw	Mild steel	Y = 0.0003462 ∗ X + 0.4469	0.7179	0.009990
	Aluminum	Y = 0.0003098 ∗ X + 0.5419	0.9268	0.004336
Rice straw	Mild steel	Y = −1.714 × 10−5 ∗ X2 + 0.003644 ∗ X + 0.2605	0.9948	0.004022
	Aluminum	Y = −2.941 × 10−5 ∗ X2 + 0.006524 ∗ X + 0.2223	0.9738	0.017740

, the relationship between length and the coefficient of sliding friction can be depicted by a one-variable linear regression equation and a quadratic polynomial equation using the least-squares regression method for untreated wheat straw and rice straw, respectively. This can provide the basis for the design of the forced feeding device.

5. Conclusions

These results of this study show that the coefficient of sliding friction of untreated wheat straw against mild steel and aluminum surfaces ranged from 0.537 to 0.688 and from 0.569 to 0.702, respectively, in the moisture content range from 35 to 75% (w.b) under a normal pressure range from 2 to 10 kPa. In contrast, the coefficient of untreated rice straw against mild steel and aluminum surfaces was found to vary between 0.419 and 0.577 and between 0.510 and 0.677, respectively. The coefficients of sliding friction of extrusion-exploded wheat straw on mild steel and the surfaces of aluminum ranged from 0.685 to 0.788 and from 0.610 to 0.850, respectively. The coefficients of rice straw on mild steel and aluminum surfaces ranged from 0.522 to 0.761 and from 0.569 to 0.827, respectively.

The coefficient of sliding friction was markedly greater for wheat straw as compared with rice straw on the two tested contact surfaces; The coefficient of sliding friction of both straw samples on the aluminum surface was notably higher than the values on mild steel. A significantly higher value was found for the coefficient of sliding friction of extrusion-exploded straw compared with that of untreated straw on both surfaces.

The moisture content, normal pressure, and the moisture content x normal pressure interaction had statistically significant effects on the coefficient of sliding friction at (p < 0.01). Significant effects were observed for untreated and extrusion-exploded wheat straw and rice straw on both mild steel and aluminum surfaces. At any level of moisture content, the coefficient of sliding friction first decreased and then increased sharply before finally decreasing gradually as the normal pressure increased from 2 to 10 kPa. The maximum coefficient of sliding friction was observed at a normal pressure of 6 kPa.

When the length increased from 30 to 150 mm, a slightly linear upward trend was observed for the coefficient of sliding friction of wheat straw, and the coefficient of sliding friction of rice straw initially increased and then decreased.

The normal pressure, straw moisture content, and straw length all affect the friction coefficient of straw on different metal surfaces. Different moisture and normal pressure values affect the advantages and disadvantages of fiber materials produced by a D200 single-screw straw-based fiber extruder. The structural parameters of the feeding device are determined by the friction characteristic interval of the straw so as to achieve a continuous, stable, and uniform conveying effect. In the process of rice and wheat harvesting, different climatic conditions lead to different moisture contents and different compression ratios of straw. This means that the harvesting machine and the packing machine correspond to different parameters. When straw is used as biomass energy, it is fused and compressed to reach a certain density. At this time, the friction between straw and metal material is also an important parameter. This study provides a reference for the optimal design of the D200 single-screw straw-based fiber extruder and its supporting raw material supply system and provides a basis for the wear prediction and service life evaluation of harvesting machinery, packing machinery, and straw treatment equipment. The results provide a theoretical reference for the optimal design of agricultural machinery.

This study was carried out under a specific temperature and straw length. The experiments were also conducted on representative mild steel and aluminum surfaces, and the frictional properties of straw on rubber and other material surfaces can be explored in future studies. It could also be interesting to study the coefficient of friction of the straw at different speeds.