Preparation and properties of oriented sorghum-thermoplastic composites using flat hot-pressing technology

Abstract

In order to increase the value of sorghum for bioethanol products, a new method of fabricating oriented sorghum fiber and high-density polyethylene composites (OFPC) using flat hot-pressing technology is presented in this study. The effect of high-density polyethylene (HDPE) loading (0, 10, 20, 30, 40 wt %) on the physical and mechanical properties of OFPC was analyzed. In addition, the mat compression properties, vertical density profile, and microscopic structure of the resulting OFPC were examined. Improvement in mechanical and physical properties of OFPC was observed when 10% HDPE was included with sorghum fiber in the composites. HDPE loading greater than 10% did not further enhance the mechanical properties of OFPC, but improved their water resistance ability. Lower consolidation pressure was required to condense the composite mats with higher HDPE loadings during the hot pressing process. The density distribution of OFPC was greatly improved by the addition of HDPE, but larger density fluctuation was observed at higher HDPE loading.

Keywords

Sorghum fiber high-density polyethylene hot pressing wood-plastic composites mechanical properties

Introduction

Sweet sorghum (Sorghum bicolor (L.) Moench) has a history in the Southeastern USA, where it has been traditionally used as a syrup and sugar crop.¹ It is an annual crop with a typical growing season of 3–5 months, and possesses favorable characteristics including higher tolerance to temperature fluctuations, salinity, alkalinity, and drought.² During recent years, sweet sorghum has been quite widely recognized for its potential as a green energy source. Sweet sorghum stalk has very similar chemical components compared with wood, making it a potential source of fiber for natural fiber composites.^3–6 Juice extracted from sweet sorghum stalk can be easily fermented to ethanol due to its high concentration of sugar.^7,8 Almodares and Hadi⁹ thoroughly reviewed bioethanol produced from sweet sorghum. Though sweet sorghum is a promising resource for bioethanol production, a notable challenge is that the large-scale utilization of bagasse after the sugars are removed from the sorghum.¹⁰

The bagasse produced during extraction of juice has been used in a variety of applications, including forage, silage, combustion energy, synthetic gas, methane, soil amendments, and paper.^11–14 With a worldwide increase in fiber demand, sweet sorghum bagasse has become a novel, potential raw material for production of natural fiber composites. Belayachi and Delmas¹⁵ used sweet sorghum bagasse as a raw material to manufacture chemical paper pulp. Yu et al.¹⁰ fabricated sweet sorghum bagasse reinforced polylactic acid composites by extrusion technology.

Natural fiber reinforced plastic composites (NFPCs) are typically produced by extrusion and injection molding technologies. Their panel dimensions are particularly limited by width; a considerable drawback considering large-dimension NFPCs may be especially suitable for construction and general-purpose applications.¹⁶ Satyanarayana et al.¹⁷ concluded that compression molding is the preferred method for the production of natural fiber reinforced composites, and Benthien and Thoemen¹⁶ further proved that flat press technology is preferable for the manufacturing of large-dimension NFPCs. Natural fiber-reinforced thermoplastic composites have been fabricated previously by researchers using hot press molding; however, both the fiber and thermoplastics in these studies were in powder form.^18–20 Natural fibers that are long and uniform are expected to improve the mechanical properties of the resulting NFPCs.²⁰ Unlike extrusion or injection molding, in hot-pressing, there is a specific challenge regarding the creation of a homogenous mixture of natural fiber and thermoplastic powder – especially if the natural fiber is long and in the form of large fiber bundles. Zampaloni et al.²¹ observed during their process that the thermoplastic powder fell through to the bottom of the mixer, and the fibers were not distributed evenly in the resulting composites. An efficient and cost-effective way to solve this problem is the use of a thermoplastic film instead of powder.²¹ Fang et al.²² used high density polyethylene (HDPE) film as a binder for plywood fabrication. However, there are few reports about using thermoplastic film as matrix to fabricate oriented natural fiber composite products.

Natural fibers are hydrophilic, where most thermoplastics are hydrophobic. Polar natural fibers possess inherently low compatibility with non-polar thermoplastics, which prevents the formation of a durable interface between them and leads to failure during the stress transfer from one phase to another.²⁰ Polymeric diphenylmethane diisocyanate (pMDI) has been commonly used as adhesive to enhance the bonding between natural fibers. pMDI come in liquid form, are easily manageable, can be chemically linked to cellulose matrices through strong covalent bonds, and are reported as an effective adhesive for natural fibers by previous researches.^23–25 Addition of pMDI in natural fiber resulted in formation of urethane (–HN-COO–) linkage due to the reaction between hydroxyl (–OH) group of sorghum fiber and isocyanate (–NCO) of pMDI. Therefore, in this study, the pMDI is selected as a potential adhesive or coupling agent for sorghum fiber-reinforced composites.

The objective of this study was to manufacture oriented natural fiber and thermoplastic composites by flat hot-pressing technology, using large-dimension sweet sorghum fiber bundles and HDPE film as raw materials. The effect of HDPE loading on the mechanical and physical properties of oriented sorghum fiber and HDPE composites (OFPC) was investigated, and their vertical density profile and microscopic structure were analyzed, as well.

Materials and methods

Materials

Extracted sweet sorghum bagasse was obtained from ChloroFill LLC in San Diego, California. Bagasse was produced by crushing sweet sorghum stalks by rollers to extract majority of its water-soluble sugars with approximately 10% retained in the bagasse. The crashed sweet sorghum bagasse, referred as sorghum fiber, is depicted in Figure 1(a). Chemical components and thermal properties of the crush sweet sorghum were described by our previous works.^26,27 Distribution of the sorghum fiber length used in this study is shown in Table 1. The sorghum fiber was sorted using screens into three lengths and dried for removing the moisture using drum dryer until the moisture content was less than around 3%.

Table 1.

Size distribution of sorghum fibers.

Fiber type	Length (mm)	Width (mm)	Thickness (mm)	Loading (%)
Long fiber	Above 45	5∼10^a	1∼2^a	25
Median fiber	19∼45			48
Small fiber	13∼19			27

The value is the same for all fiber types.

HDPE films were purchased from Tee Group Films, USA. The thickness of the film was 0.1 mm, with a melting point of 130℃, and melt flow index of 20 g/10 min at 190℃/2.16 kg. HDPE loading was 0%, 10%, 20%, 30% and 40% based on the target dry weight of the final OFPC.

Adhesive, polymeric diphenylmethane diisocyanate (pMDI, MONDUR ®541) was obtained from Bayer Material Science, Pittsburgh, PA. Two percent pMDI, based on the dry weight of the sorghum fiber, was used. Table 2 shows the all formulations of OFPC produced and evaluated in this study.

Table 2.

Formulation and experimental design of oriented sorghum fiber and HDPE composites.

HDPE loading^a (%)	Sorghum fiber loading^a (%)	pMDI loading^b (%)	Target density (kg/m³)	Moisture content of sorghum fiber (%)
0	98.0	2^c	800^c	<3^c
10	88.2
20	78.4
30	68.6
40	58.8

HDPE and sorghum fiber loading were calculated based on the dry weight of composites.

pMDI loading was calculated based on dry weight of sorghum fiber.

The value is the same for all formulations.

Oriented sorghum fiber and HDPE composites fabrication

The manufacturing process of OFPC is shown in Figure 2. The sorghum fiber was first oven-dried to approximately 3% moisture content and then sprayed with pMDI (2% by weight of fiber) in a drum blender. Fiber resinated with pMDI was then evenly divided into five portions for forming a mat. The HDPE films for each formulation were also divided into six portions (10%, 20%, 20%, 20%, 20% and 10%). A forming box with parallel vanes spaced at 25.4 mm (Fig. 1(b)) were used to achieve a preferred longitudinal orientation (parallel to the longer dimension of the panel) of sorghum fibers during mat formation. First portion of HDPE film (10%) was first placed on the bottom of forming box, followed by forming a portion of sorghum fiber on the HDPE film, with random overlapping of fibers. After the first portion of sorghum fiber was formed, the second portion of HDPE film (20%) was layered on the fibers, which was then followed by forming another portion of sorghum fibers. This procedure was followed till the entire mat was formed with 10% HDPE layer on top of completely formed mat.

Except for control mat samples (the formulation without HDPE film), all mat samples will be composed of HDPE films align-shuffled with the layers of sorghum fiber and also placed at the top and bottom surface of the mat. Ten percent layers of the HDPE films (based on the total weight of HDPE film of each formulation, as shown in Table 2) were placed on the top and bottom of each mat, whereas 20% HDPE layers were located between the five oriented sorghum fiber layers. Therefore, all mat samples with HDPE layers were of 11 layers composing of six layers of HDPE films and five layers of sorghum fibers. Double side-silicone release papers were placed on each side of the top surfaces of the prepared mats in order to prevent sticking of the molten HDPE on the metal plates during hot-pressing. The mat structure after forming is shown in Figure 1(c).

The formed mat was flat-pressed at a temperature of 160℃. The mat thickness during hot press linearly decreased from 127 mm to 20 mm in 40 s, followed by a further linear decrease to the target thickness of 15 mm in 300 s (Figure 3). At the given final target thickness (15 mm), the introduced pressure was kept constantly for 10 min, and after that the coolant water (20℃) was then released to pass through the cooling channel of the hot press plates. The gradually cooling and solidifying of the composite mat was taken place in the hot press. Until the temperature of the metal plate reached for 35℃, the solidified composite mat was removed from the hot press. In the case for the mat samples without HDPE films (neat sorghum fiber mat), no water coolant process is introduced for the metal cooling plates. At least three replicates of each formulation were manufactured. Pressed OFPC were trimmed (at least 50 mm off all edges) to a final panel size of 660 × 800 × 15 mm³ with a target density of 800 kg/m³ (Figure 1(d)).

Characterization of composites

Mat compression behavior

Consolidation of mat was thickness controlled. Applied pressure during the hot pressing process was detected by hydraulic transducer, and mat thickness was measured by displacement sensor. Their values were recorded by computer program over pressing time for each formulation.

Mechanical and physical properties

For mechanical properties of OFPC, measurements of flexural properties (three-point loading mode) in terms of modulus of rupture (MOR) and elastic (MOE), Tensile properties in terms of tensile strength (TS) and Young's modulus, internal bond strength (IB), thickness swelling (THS), water absorption (WA) and screw withdrawals (SW, Number 10 Type AB screw) of OFPC were carried out in concordance with ASTM D1037. Their mechanical properties were tested using an Instron machine at crosshead speeds of 6 mm/min for the static bending, 4 mm/min for tension, 1 mm/min for IB, and 1.5 mm/min for SW tests. Nine specimens for each formulation were tested for their mechanical and physical properties, except for IB where the number of replications was 12 for each formulation. A single continuous 24-h submersion in water was employed for WA and THS tests. All specimens were placed in a conditioning room at a relative humidity of 65 ± 5% and a temperature of 20 ± 2℃ for at least two weeks to obtain equilibrium moisture content before testing.

Vertical density profile measurement

The vertical density profile (VDP) of OFPC was analyzed using an X-ray density profiler (QMS, Model QDP-01X) with a specimen size of 50 × 50 mm². Three replicates were taken from each panel for each formulation. Density data were measured at 0.06 mm increments by an X-ray beam passing through the entire thickness of the specimen. Prior to VDP scanning, all specimens were conditioned in 20 ± 2℃ and 65 ± 5% relative humidity for at least two weeks.

Morphological examination

Morphology of the panels was examined using a Quanta FEI 200F scanning electron microscope (SEM). Three specimens for each formulation were mounted on aluminum holders using double-side electrical-conducting carbon adhesive, and sputtered-coated with gold for 1 min for SEM observations. A high voltage, 20 kV, was used to create the micrographs. SEM micrographs were used to investigate the interface between the sorghum fiber and HDPE.

Data analysis

Because density has strong influence on the physical and mechanical properties of composites, a normalization procedure was undertaken to analyze the testing data. An F-test of each data series representing one formulation was applied to ensure a linear regression (least squares method) between density and measured values, based on a significance level of 99%. Property values were normalized to target density (800 kg/m³) based on linear regression equations if the test was significant; however, the values were not normalized for further analysis if a linear relationship was not assumed. A similar normalization procedure was reported by Benthien and Thoemen.¹⁶

Results and discussion

Mat compression behavior

Mat compression behavior provides an understanding of magnitudes of peak pressures that can be expected during the hot pressing process of OFPC to bring surfaces together for subsequent resin penetration, wetting, and curing. Figure 3 shows the relationship between mat pressure and thickness over hot-pressing time. A loose mat structure was formed and consolidated under heat and pressure to fabricate OFPC. Applied compression load for consolidation to final thickness was significantly affected by the percentage of HDPE in the mat. Peak pressures decreased significantly with increasing HDPE levels in the mat. Consolidation pressure along with other parameters (press closing time, moisture distribution, platen temperature, fiber properties, resin type) will influence the formation of vertical density distribution²⁸ in the end panel product.

Considering that sorghum fiber is a porous material, the total void volume of a loose mat can be classified as between- and within-fiber voids. During the closure stage (0–340 s), the mat thickness decreased, and the voids began to shrink with the consolidation of sorghum fiber and initiation of its plasticization, melting of HDPE, and initiation of pMDI resin polymerization (some of these changes can be seen in the SEM photos taken before and after compression as shown in Figure 4). After reaching target thickness, mat stress decreased with an increase in press time because of melting and flowing of the HDPE films into the voids under heat and pressure, and the viscoelastic nature of hemicelluloses and lignin in sorghum fiber that required less pressure to condense at higher temperatures.²⁹

Figure 3 also demonstrates various compression behaviors of the mat for different HDPE loading. Mat pressure reduces to balance the mat stress when the HDPE loading is high. Lower HDPE loading indicates a higher sorghum fiber concentration in the mat, which results in a thicker mat because HDPE density (940 kg/m³) is much higher than sorghum fiber bulk density (approximately 150 kg/m³). Higher pressure was required to condense the loose sorghum fiber to target density and thickness with higher sorghum fiber loading levels.

Mechanical and physical properties

Mechanical properties

Figure 5 shows the results of mechanical properties of OFPC. Improvements in modulus of rupture (MOR), tensile strength (TS) and internal bond strength (IB) by 35.9%, 44.5% and 110.2 %, respectively, were observed when 10% HDPE was added in the composites. MOR in the parallel direction increased by 35.9%, TS by 44.5%, and IB by 110.2%. No significant increase in these properties was observed when HDPE loading increased from 10% to 20%; however, with further addition of HDPE (at 20%, 30% and 40% loading levels), all properties including modulus in flexure and tension reduced. Comparing 10% HDPE to zero HDPE, a notable increase in mechanical properties can be explained by the uniform vertical density profile (Figure 6(b)) and perhaps increase in bonding between fibers due to increased mechanical interlocking with initial inclusion of HDPE.

The HDPE film melted and flowed into between-sorghum voids under high temperature and pressure during hot pressing, contributing to mechanical interlocking and better water resistance. It is surprising to see an increase in strength even with 10% HDPE as Bakeer et al.’s⁶ research show that the tensile strength of sorghum rind is approximately 280 MPa, and the tensile strength of neat HDPE is about 30 MPa.³⁰ As per composite theory, the strength of the composites is affected by fiber strength, matrix strength, and interfacial bonding between both the materials. Results of our study indicate that inclusion of smaller percentage of HDPE (10%) assists in increasing the interfacial bond between the fibers, but as expected the higher loadings of HDPE (above 10%) shear stress transfer capacity and decrease the mechanical properties (Figure 5); however, moisture absorption into the voids and moisture uptake by cell wall is significantly reduced because of the barrier effect with increasing HDPE loading.

The typical load–deflection curves of a three-point bending test are shown in Figure 7, reflecting the effects of HDPE loading on the structure of composites. The load–extension curves of the composites containing HDPE surprisingly had higher slopes than the composites without HDPE. These results show an interaction between HDPE and pMDI resin resulting in increasing the stiffness slightly (but not statistically significant), especially with inclusion of lower quantities of HDPE. Introducing HDPE in the form of thin films instead of particulate form may have also played a role and needs further investigation.

Screw withdrawal (Figure 5(e)) is a crucial property that influences the material selection for many applications, such as furniture construction and decorative paneling. In general, screw withdrawal strength increased with inclusion of 10% HDPE and then plateaued out with any further increase in HDPE content. A direct correlation between screw withdrawal and bond strength between the fibers has been reported in literature³¹; these results support the prior argument of increase in bond strength between fibers with increase in HDPE up to 10%. Screw withdrawal force from the face of the panel was greater than that through the parallel (longer dimension) and perpendicular (panel ends) edges; no significant difference was found between the parallel and perpendicular edge screw withdrawal strengths.

Minimal research currently exists regarding flat-pressed natural fiber-reinforced HDPE composites in a hot press, especially regarding long fiber reinforcements and high fiber loading. Performance of OFPC fabricated in this study is more favorable than those compared to medium density fiberboard, particleboard, and wood powder reinforced thermoplastics^32–34 (Table 3). These increases in mechanical properties demonstrate tremendous potential for applications in furniture and construction purposes.

Table 3.

Properties comparison of oriented sorghum fiber and HDPE composites with other wood-based composites.

Type	MOR in parallel (MPa)	MOE in parallel (GPa)	Screw withdrawal (N)		Internal bond (MPa)	Thickness swelling (%)	Reference
Type	MOR in parallel (MPa)	MOE in parallel (GPa)	Surface	Edges	Internal bond (MPa)	Thickness swelling (%)	Reference
OFPC without HDPE	27.4 ± 3.0	3.8 ± 0.3	917 ± 104	763 ± 78	0.7 ± 0.1	5.5 ± 0.9	This study
OFPC with 10% HDPE	37.2 ± 3.3	4.7 ± 0.5	1652 ± 143	1133 ± 118	1.4 ± 0.2	2.8 ± 0.7	This study
MDF	14.0 ∼ 31.0	1.4 ∼ 3.1	778–1557	672–1335	0.3–1.1	<10.0	ANSI A208.2³³
Particleboard	11.0–23.5	1.7–3.1	1001–2002	899–1326	0.4–1.0	–	ANSI A208.1³⁴
Wood fiber and PP (50:50) composites	25.9 ± 1.3	2.7 ± 0.1	1010 ± 58	–	1.0 ± 0.1	6.5 ± 0.5	Ayrilmis et al.³²

Thickness swelling and water absorption

As reported by previous researchers,^20,32 the addition of thermoplastics improves the thickness swelling and water absorption of composites. The thickness swelling of the OFPC decreased by 50.9% when 10% HDPE was added, and WA decreased by 44.4% (Figure 5(f)). These values continuously improved with increases in HDPE loading up to 40%. THS was below 3%, and WA was below 20% when over 10% of HDPE was added. Compared to similar wood-based composites (Table 3), OFPC fabricated in this study have superior THS and WA properties, demonstrating their potential suitability for applications in humid environments.

The thickness swelling and water absorption in composites occur mainly due to the presence of lumens, fine pores, and hydrogen bonding sites in the fiber, plus gaps and flaws at the interfaces and microcracks in the plastic matrix.³⁵ With increases in hydrophobic HDPE loading, access to hydrophilic sorghum fiber with free hydroxyl and polar groups is decreased leading to fewer hydrogen bonds formed with water, which results in lower weight gain and volume expansion. The hydrophobic HDPE distributed throughout the surface and interior of the composites provides a barrier and prevents the water from penetrating into the composite interstitial spaces and easily accesses the fiber cell wall. The addition of the pMDI also improved THS and WA in OFPC, due to improved adhesion between the sorghum fibers and inherent moisture resistance of the chemical bonds between the hydroxyl parts of sorghum fiber and the binder system.

Vertical density profile

Density is an important and fundamental property which affects both the mechanical and physical properties of composites.^36,37 Typical variation in vertical density profiles in the OFPC at different HDPE loadings is shown in Figure 6. For sorghum fiber composites without HDPE addition, the transient temperature and moisture gradients inside the mat result in non-uniform and changing compression properties of the natural fiber component. When the compaction pressure applied to the mat is coupled with the changing compression properties, a density gradient develops.²⁹ The VDP of sorghum fiber composites without HDPE resembled a “U” shape, and the density at the core of composites was much lower than the near the surfaces (Figure 6(a)), resulting in weak internal bond.

With the addition of 10% HDPE, the density fluctuation decreased, with relatively small variations, through the entire thickness (Figure 6(b)), due to melting and flowing of HDPE into the interstitial spaces between the sorghum fibers. This uniform density has positive effect on internal bond strength as shown in Figure 5(d). With HDPE loading greater than 10%, besides filling the voids between the sorghum fibers, HDPE also forms layers in OFPC, and these layers are thicker at higher HDPE loadings similar to what was reported in previous publication by the authors.²⁶ As the HDPE density (940 kg/m³) is higher than the target density of composites (800 kg/m³), density peaks can be seen in the vertical density profiles anywhere one of these thick HDPE layers was present at higher HDPE loadings (Figure 6(c) to (e)).

Morphological structure

SEM images of sorghum fiber and the interface between sorghum fiber and HDPE are shown in Figure 4. By comparing the sorghum fiber pith before and after hot press, as demonstrated in Figure 4(a) and (b), respectively, we observed consolidation of the pith and significant reduction in lumen volume. No visible gap can also be seen between the sorghum fiber and HDPE interface (Figure 4(b) to (d)), indicating a reasonably good interface bonding leading to positive contributions to water resistance and thickness swelling.

Conclusions

Oriented sorghum fiber and HDPE composites were manufactured using flat hot-pressing technology. HDPE in a film form, instead of in a powder form (commonly used), was included in the fiber mat during the fabrication of composites. Lower consolidation pressure was required to condense the composite mats with higher HDPE loadings during the hot pressing process. The vertical density profile of OFPC with 10% HDPE was more uniform than those without HDPE; however, higher HDPE loadings resulted in greater variation in VDP. Compared to sweet sorghum composites without HDPE, significant improvements in the mechanical and physical properties of OFPC was observed with inclusion of 10% HDPE with 2% pMDI adhesive. Further physical improvements, including thickness swelling (below 3%) and water absorption (below 20%) were obtained for HDPE loading over 10%, but mechanical properties in these cases did not improve but diminished compared to panels with 10% HDPE loading. Relatively homogeneous density distribution, improved mechanical interlocking, and positive interfacial bonding contributed to the significantly higher mechanical and physical properties of OFPC fabricated in this study compared to other typical wood-based composite panel products (Table 3). Thickness swelling and water absorption properties are greatly improved in particular, demonstrating these composites’ significant potential for applications in high humidity environments.

Footnotes

Conflict of interest

None declared.

Funding

This study was supported by The Fundamental Research Funds for The Central Universities of China (No. BLX2013003). We are grateful to Michael Hurst of ChloroFill LLC for supplying sweet sorghum fiber.

References

Teetor

Duclos

Wittenberg

. Effects of planting date on sugar and ethanol yield of sweet sorghum grown in Arizona. Ind Crop Prod 2011; 34: 1293–1300.

Davila-Gomez

Chuck-Hernandez

Perez-Carrillo

. Evaluation of bioethanol production from five different varieties of sweet and forage sorghums (Sorghum bicolor (L) Moench). Ind Crop Prod 2011; 33: 611–616.

Billa

Koullas

Monties

. Structure and composition of sweet sorghum stalk components. Ind Crop Prod 1997; 6: 297–302.

Richie

McBee

. Structural components in Sorghum stem biomass. Bioresource Technol 1991; 38: 15–22.

Audilakshmi

Mall

Swarnalatha

. Inheritance of sugar concentration in stalk (brix), sucrose content, stalk and juice yield in sorghum. Biomass Bioenergy 2010; 34: 813–820.

Bakeer

Taha

El-Mously

. On the characterisation of structure and properties of sorghum stalks. Ain Shams Eng J 2013; 4: 265–2671.

Laopaiboon

Nuanpeng

Srinophakun

. Ethanol production from sweet sorghum juice using very high gravity technology: Effects of carbon and nitrogen supplementations. Bioresource Technol 2009; 100: 4176–4182.

Liu

Shen

. Impacts of main factors on bioethanol fermentation from stalk juice of sweet sorghum by immobilized Saccharomyces cerevisiae (CICC 1308). Bioresource Technol 2008; 99: 847–854.

Almodares

Hadi

. Production of bioethanol from sweet sorghum: a review. Afr J Agric Res 2009; 4: 772–780.

10.

Zhang

Zhong

. Biorefinery of sweet sorghum stem. Biotechnol Adv 2012; 30: 811–816.

11.

Whitfield

Chinn

Veal

. Processing of materials derived from sweet sorghum for biobased products. Ind Crop Prod 2012; 37: 362–375.

12.

Liang

Tang

Siddaramu

. Lipid production from sweet sorghum bagasse through yeast fermentation. Renew Energ 2012; 40: 130–136.

13.

Negro

Solano

Ciria

. Composting of sweet sorghum bagasse with other wastes. Bioresource Technol 1999; 67: 89–92.

14.

Antonopoulou

Gavala

Skiadas

. Biofuels generation from sweet sorghum: Fermentative hydrogen production and anaerobic digestion of the remaining biomass. Bioresource Technol 2008; 99: 110–119.

15.

Belayachi

Delmas

. Sweet sorghum bagasse: A raw material for the production of chemical paper pulp.: Effect of depithing. Ind Crop Prod 1997; 6: 229–232.

16.

Benthien

Thoemen

. Effects of raw materials and process parameters on the physical and mechanical properties of flat pressed WPC panels. Compos Part A-Appl Sci Manuf 2012; 43: 570–576.

17.

Satyanarayana

Arizaga

GGC

Wypych

. Biodegradable composites based on lignocellulosic fibers—An overview. Prog Polym Sci 2009; 34: 982–1021.

18.

Huang

Wang

Zhang

. Effect of mechanical activation pretreatment on the properties of sugarcane bagasse/poly(vinyl chloride) composites. Compos Part A-Appl Sci Manuf 2012; 43: 114–120.

19.

Graupner

Herrmann

Müssig

. Natural and man-made cellulose fibre-reinforced poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and application areas. Compos Part A-Appl Sci Manuf 2009; 40: 810–821.

20.

Guo

Liu

. Preparation and properties of cotton stalk bundles and high-density polyethylene composites using hot-press molding. J Reinf Plast Comp 2012; 31: 1017–1024.

21.

Zampaloni

Pourboghrat

Yankovich

. Kenaf natural fiber reinforced polypropylene composites: A discussion on manufacturing problems and solutions. Compos Part A-Appl Sci Manuf 2007; 38: 1569–1580.

22.

Fang

Chang

Guo

W-j

. Preparation and characterization of wood-plastic plywood bonded with high density polyethylene film. Europ J Wood Wood Prod 2013; 71: 739–746.

23.

Shi

Gardner

. Hygroscopic thickness swelling rate of compression molded wood fiberboard and wood fiber/polymer composites. Compos Part A-Appl Sci Manuf 2006; 37: 1276–1285.

24.

Sahoo

Misra

Mohanty

. Enhanced properties of lignin-based biodegradable polymer composites using injection moulding process. Compos Part A-Appl Sci Manuf 2011; 42: 1710–1718.

25.

Nagarajan

Misra

Mohanty

. New engineered biocomposites from poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/poly(butylene adipate-co-terephthalate) (PBAT) blends and switchgrass: Fabrication and performance evaluation. Ind Crop Prod 2013; 42: 461–468.

26.

Yadama

Guo

. Thermal conductivity of sorghum and sorghum–thermoplastic composite panels. Ind Crop Prod 2013; 45: 455–460.

27.

Yadama

Guo

. Thermal stability evaluation of sweet sorghum fiber and degradation simulation during hot pressing of sweet sorghum–thermoplastic composite panels. Ind Crop Prod 2015; 69: 335–343.

28.

Kamke

Wolcott

. Fundamentals of flakeboard manufacture: wood-moisture relationships. Wood Sci Technol 1991; 25: 57–71.

29.

Wolcott

Kamke

Dillard

. Fundamentals of flakeboard manufacture: viscoelastic behavior of the wood component. Wood Fiber Sci 1990; 22: 345–361.

30.

Salleh

Hassan

Yahya

. Effects of extrusion temperature on the rheological, dynamic mechanical and tensile properties of kenaf fiber/HDPE composites. Compos Part B: Eng 2014; 58: 259–266.

31.

Semple

Smith

. Prediction of internal bond strength in particleboard from screw withdrawal resistance models. Wood Fiber Sci 2006; 38: 256–267.

32.

Ayrilmis

Jarusombuti

Fueangvivat

. Effect of thermal-treatment of wood fibres on properties of flat-pressed wood plastic composites. Polym Degrad Stabil 2011; 96: 818–822.

33.

ANSI-A208.2. Medium density fiberboard (MDF) for interior applications, Gaithersburg, MD: Coposite Panel Association, 2002.

34.

ANSI-A208.1. Particleboard, Gaithersburg, MD: Composite Panel Association, 1999.

35.

Stokke

Gardner

. Fundamental aspects of wood as a component of thermoplastic composites. J Vinyl Addit Technol 2003; 9: 96–104.

36.

Wang

Winistorferf

. Fundamentals of vertical density profile formation in wood composites. Part II. Methodology of vertical density formation under dynamic conditions. Wood Fiber Sci 2000; 32: 220–238.

37.

Thoemen

Ruf

. Measuring and simulating the effects of the pressing schedule on the density profile development in wood-based composites. Wood Fiber Sci 2008; 40: 325–338.