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The influence of ionic liquid pretreatment on the physicomechanical properties of polymer biocomposites: A mini-review

  • Ahmad Adlie Shamsuri EMAIL logo , Siti Nurul Ain Md. Jamil and Khalina Abdan
Published/Copyright: October 27, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

Increasing concern for the environment has led researchers to pay more attention to the fabrication of polymer biocomposites for many different applications. Polymer biocomposites have generally been fabricated utilizing synthetic or natural polymers with natural fillers. Recently, ionic liquids have been used for the pretreatment of natural fillers prior to the fabrication of polymer biocomposites. In this mini-review, four types of ionic liquids used for the pretreatment of natural filler are classified, specifically chloride-, diethyl phosphate-, acetate-, and bistriflimide-based ionic liquids. In addition, the pretreatment processes of natural fillers with ionic liquids are described in this review. Furthermore, the influence of ionic liquid pretreatment on the physicomechanical properties of polymer biocomposites is succinctly reviewed. Besides, the information presented in this review contributes to a clearer understanding of the process of ionic liquid pretreatment and the vital physicomechanical properties of polymer biocomposites. In summary, most ionic liquid pretreatments can improve almost all physicomechanical properties of polymer biocomposites.

Keywords: ionic liquid; physicomechanical properties; polymer biocomposite; pretreatment; natural filler

1 Introduction

Polymer biocomposites are polymer composites in which natural fillers are incorporated, regardless of the type of polymer matrix; thermoplastic or thermoset (1) and natural fillers may be either organic or inorganic materials (2). Lately, the advancement of polymer biocomposites in the composite industry has expanded promptly because they are biodegradable. Besides, polymer biocomposites are excellent substitutes for typical polymer composites that are not biodegradable; this can indirectly reduce environmental pollution when they are disposed of in landfills. Table 1 shows examples of polymer matrices and natural fillers that have been utilized for the fabrication of polymer biocomposites. It can be observed that synthetic polymers, such as polyethylene (PE) (3,4) and polypropylene (PP) (5,6), are commonly utilized for this purpose. The reason is that PE and PP are economical, easy to obtain, have good impact strength, and require moderate processing temperature. In addition, the world’s third-most widely produced synthetic polymer after PE and PP, specifically polyvinyl chloride (PVC), can be utilized as a polymer matrix for the fabrication of polymer biocomposites (7). Moreover, natural and synthetic biodegradable polymers, such as corn starch (CS) (8) and polylactic acid (PLA) (9). have been utilized as polymer matrices for polymer biocomposites. Figure 1 exhibits the chemical structures of PE, PP, PVC, CS, and PLA.

Table 1

Examples of polymer matrices and natural fillers utilized for the fabrication of polymer biocomposites

Polymer matrix Abbreviation Natural filler References
Corn starch CS Oil palm fronds (8,14,15,16,17,18,19)
High-density polyethylene HDPE Cellulose nanofibers (20)
High-density polyethylene HDPE Kenaf core fibers (21)
High-density polyethylene HDPE Wood flour (3,4)
Isotactic polypropylene iPP Pine wood (5,6)
Low-density polyethylene LDPE CS (22)
Low-density polyethylene LDPE Kenaf core fibers (23)
Polylactic acid PLA Rice husks (9)
Polypropylene PP Bagasse powder (24)
Polypropylene PP Bamboo (25)
Polypropylene PP Rice husks (12)
Polyvinyl chloride PVC Bamboo (7)
Recycled high-density polyethylene rHDPE Bamboo (26,27)
Figure 1 Chemical structures of (a) PE, (b) PP, (c) PVC, (d) CS, and (e) PLA.
Figure 1

Chemical structures of (a) PE, (b) PP, (c) PVC, (d) CS, and (e) PLA.

The utilization of natural fillers for the fabrication of polymer biocomposites has received considerable attention from industries because the production cost of polymer composites can be reduced by employing low-priced natural fillers. It is also an approach for decreasing the consumption of synthetic fillers or synthetic polymers (2) and producing environmentally friendly and cost-effective composite products. In addition, natural fillers are abundantly available, renewable, and non-abrasive. They are also having low-density and high specific strength. It can be seen in Table 1 that natural fillers from non-wood- and wood-based materials have been successfully utilized for the fabrication of polymer biocomposites. Furthermore, non-wood-based materials, such as oil palm fronds, cellulose nanofibers, kenaf core fibers, CS, rice husks, bagasse powder, and bamboo, are frequently utilized as natural fillers compared to wood-based materials like wood flour and pine wood. Nevertheless, the incompatibility between natural fillers and polymer matrices is a critical challenge in polymer biocomposites. This affects their performance, especially the physicomechanical properties, such as thermal, crystalline, morphological, and mechanical properties. Therefore, the treatment of natural fillers with different types of chemicals has been carried out (10,11,12,13) to improve the compatibility between natural fillers and polymer matrices.

The use of chemicals for the treatment of natural fillers is effective; however, some chemical treatments require process conditions at very high temperatures, extreme pressure, and excessive time. Furthermore, many chemical treatments consume toxic, hazardous, and highly corrosive chemicals that can degrade certain natural fillers and demand special equipment for processing (3). Most recently, the pretreatment of natural fillers using green solvents like ionic liquids has become an alternative way to enhance the compatibility between natural fillers and polymer matrices in polymer biocomposites (14,24). Ionic liquids are organic salts that have low melting points (typically below 100°C). They consist entirely of ions and are regarded as green solvents because they are non-volatile, non-flammable, and recyclable (6). In addition, ionic liquids have intriguing physicochemical properties, such as high polarity, good chemical stability, high thermal stability, and high ionic conductivity. Moreover, their chemical structures can be designed and exploited for specific and challenging applications where organic liquids cannot be applied (28). In addition, ionic liquids have broadly been used to prepare and modify natural and synthetic biodegradable polymer solutions (29). Therefore, the use of ionic liquids in the pretreatment of natural fillers is a promising technique because they have good dissolution ability and low vapor pressure and are reusable and non-corrosive. Besides, it can prevent the use of highly corrosive chemicals, such as strong bases or acids, which need massive volumes of water for the dilution process (17).

Various types of ionic liquids have been used for the pretreatment of natural fillers for polymer biocomposites. Table 2 displays examples of ionic liquids that have been used for the pretreatment of natural fillers. It can be noticed that ionic liquids with ammonium and imidazolium cations are regularly used for the pretreatment of natural fillers. Moreover, there are four types of ionic liquids that can be classified, specifically chloride-, diethyl phosphate-, acetate-, and bistriflimide-based ionic liquids. 1-Butyl-3-methylimidazolium chloride (BmimCl) and 1-ethyl-3-methylimidazolium diethyl phosphate (EmimDep) are the most used ionic liquids; this is due to their availability and reasonable cost. Besides, choline acetate (ChOAc), 1-ethyl-3-methylimidazolium acetate (EmimOAc), and 1-carboxymethyl-3-dodecylimidazolium bis(trifluoromethylsulfonyl)imide (CmdimNTf2) ionic liquids are usually used for the pretreatment of natural fillers. Figure 2 indicates the chemical structures of BmimCl, EmimDep, ChOAc, EmimOAc, and CmdimNTf2. So far, to the best of the authors’ knowledge, no mini-review has been created focusing on the physicomechanical properties of polymer biocomposites pretreated with ionic liquids. That is the main objective of this ordered review, although narrow and not broad, is still relevant to other recent studies.

Table 2

Examples of ionic liquids used for the pretreatment of natural fillers

Ionic liquid Abbreviation References
Benzyltriethylammonium acetate BzteaOAc (25)
1-Butyl-3-methylimidazolium chloride BmimCl (8,14,16,17,18,19)
1-Carboxymethyl-3-dodecylimidazolium bis(trifluoromethylsulfonyl)imide CmdimNTf2 (5)
1-Carboxymethyl-3-tetradecylimidazolium bis(trifluoromethylsulfonyl)imide CmtimNTf2 (5)
Choline acetate ChOAc (24,25)
Didecyldimethylammonium bis(trifluoromethylsulfonyl)imide DddmaNTf2 (6)
N-Dodecyl-N-carboxymethyl-N,N-dimethylammonium bis(trifluoromethylsulfonyl)imide DcmdaNTf2 (5)
1-Ethyl-3-methylimidazolium acetate EmimOAc (9,16)
1-Ethyl-3-methylimidazolium chloride EmimCl (22)
1-Ethyl-3-methylimidazolium diethyl phosphate EmimDep (8,14,15,17,18,19)
1-Hexyl-3-methylimidazolium tetrafluoroborate HmimBF4 (20)
N-Tetradecyl-N-carboxymethyl-N,N-dimethylammonium bis(trifluoromethylsulfonyl)imide TcmdaNTf2 (5)
Triethylammonium hydrogen sulphate TeaHSO4 (25)
1-(3-Trimethoxysilylpropyl)-3-methylimidazolium chloride TopmimCl (3,4)
1-(3-Trimethoxysilylpropyl)-3-methylimidazolium thiocyanate TopmimSCN (3,4)
Figure 2 Chemical structures of (a) BmimCl, (b) EmimDep, (c) ChOAc, (d) EmimOAc, and (e) CmdimNTf2.
Figure 2

Chemical structures of (a) BmimCl, (b) EmimDep, (c) ChOAc, (d) EmimOAc, and (e) CmdimNTf2.

2 Synthesis of ionic liquids and pretreatment of natural fillers

2.1 Synthesis of ionic liquids

2.1.1 Chloride-based ionic liquids

The synthesis of chloride-based ionic liquids can be done via an alkylation reaction by reacting N-alkylimidazole with an alkylating agent, such as alkyl chloride. Figure 3 demonstrates the schematic representation of the alkylation reaction of N-alkylimidazole with alkyl chloride to synthesize the N-alkyl-N-alkylimidazolium chloride ionic liquid. The alkylation reaction of N-alkylimidazole is normally performed under a reflux condition overnight at an elevated temperature with stirring (30). A polar aprotic solvent, for example, acetonitrile, can be employed as a medium for the reaction. It can be easily separated from the synthesized ionic liquid by distillation under vacuum pressure.

Figure 3 Schematic representation of the alkylation reaction of N-alkylimidazole with alkyl chloride to synthesize the N-alkyl-N-alkylimidazolium chloride ionic liquid.
Figure 3

Schematic representation of the alkylation reaction of N-alkylimidazole with alkyl chloride to synthesize the N-alkyl-N-alkylimidazolium chloride ionic liquid.

2.1.2 Diethyl phosphate-based ionic liquids

The synthesis of diethyl phosphate-based ionic liquids can be carried out through an ethylation reaction by reacting N-alkylimidazole with a trialkyl phosphate, particularly triethyl phosphate. Figure 4 shows the schematic representation of the ethylation reaction of N-alkylimidazole with triethyl phosphate to synthesize the N-ethyl-N-alkylimidazolium diethyl phosphate ionic liquid. The ethylation reaction of N-alkylimidazole is typically made under a reflux condition overnight at an elevated temperature with stirring. A polar aprotic solvent like ethyl acetate can be used as a wash solvent for removing excess triethyl phosphate (31). In addition, impurities can be removed from an ionic liquid by vacuum distillation.

Figure 4 Schematic representation of the ethylation reaction of N-alkylimidazole with triethyl phosphate to synthesize the N-ethyl-N-alkylimidazolium diethyl phosphate ionic liquid.
Figure 4

Schematic representation of the ethylation reaction of N-alkylimidazole with triethyl phosphate to synthesize the N-ethyl-N-alkylimidazolium diethyl phosphate ionic liquid.

2.1.3 Acetate-based ionic liquids

The synthesis of acetate-based ionic liquids can be conducted via a metathesis reaction by replacing the chloride anion of quaternary ammonium with an acetate anion using sodium acetate. Figure 5 exhibits the schematic representation of the metathesis reaction of quaternary ammonium chloride with sodium acetate to synthesize the quaternary ammonium acetate ionic liquid. The metathesis reaction of quaternary ammonium chloride is generally done at a slightly elevated temperature with stirring (25). A polar aprotic solvent, such as acetone, can be employed as a medium for such a reaction. Sodium chloride can also be produced from the reaction and can be simply separated since it is insoluble in acetone.

Figure 5 Schematic representation of the metathesis reaction of quaternary ammonium chloride with sodium acetate to synthesize the quaternary ammonium acetate ionic liquid.
Figure 5

Schematic representation of the metathesis reaction of quaternary ammonium chloride with sodium acetate to synthesize the quaternary ammonium acetate ionic liquid.

2.1.4 Bistriflimide-based ionic liquids

The synthesis of bistriflimide-based ionic liquids can also be performed through a metathesis reaction by exchanging the chloride anion of N-alkyl-N-alkylimidazolium with a bistriflimide anion using lithium bis(trifluoromethylsulfonyl)imide. Figure 6 indicates the schematic representation of the metathesis reaction of N-alkyl-N-alkylimidazolium chloride with lithium bis(trifluoromethylsulfonyl)imide to synthesize the N-alkyl-N-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid. The metathesis reaction of N-alkyl-N-alkylimidazolium chloride is mainly made at room temperature under stirring (5). A polar protic solvent like water can be used as a medium for the reaction. Lithium chloride can also be produced and removed from an ionic liquid together with the aqueous phase by decantation.

Figure 6 Schematic representation of the metathesis reaction of N-alkyl-N-alkylimidazolium chloride with lithium bis(trifluoromethylsulfonyl)imide to synthesize the N-alkyl-N-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid.
Figure 6

Schematic representation of the metathesis reaction of N-alkyl-N-alkylimidazolium chloride with lithium bis(trifluoromethylsulfonyl)imide to synthesize the N-alkyl-N-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid.

2.2 Pretreatment of natural fillers

Table 3 displays the natural fillers, ionic liquids, co-solvents, weight ratios, temperatures, and times that have been employed for the pretreatment processes. It can be perceived that most of the ionic liquids used for the pretreatment of the natural fillers require a co-solvent to support the process. It is not easy to pretreat natural fillers using only ionic liquids because of their high viscosity unless they are room temperature ionic liquids. Therefore, a polar aprotic solvent like dimethyl sulfoxide (DMSO), polar protic solvents, such as ethanol (EtOH) and water, and a nonpolar solvent, e.g., toluene (Tol), are usually used as co-solvents for ionic liquid pretreatment. Figure 7 shows the chemical structures of DMSO, EtOH, and Tol. A previous study has revealed that the addition of co-solvents has decreased the viscosity of ionic liquids and increased their volume (32). Moreover, the presence of a co-solvent can improve the interaction between the ionic liquid and the natural filler without joining in the interaction. Hence, co-solvents play an essential role in the pretreatment of natural fillers for the fabrication of polymer biocomposites. In addition, the weight ratios of ionic liquid and co-solvent are commonly the same or lower than that of the co-solvent. This subsequently can reduce the cost of pretreatment of the natural filler since only a minimal amount of the ionic liquid is needed without affecting the ability of the ionic liquid in the pretreatment process.

Table 3

Natural fillers, ionic liquids, co-solvents, weight ratios, temperatures, and times employed for the pretreatment processes

Natural filler Ionic liquid Co-solvent Weight ratio Temp. (°C) Time (h) References
Bagasse powder ChOAc DMSO 1:1:1 110 16 (24)
Bamboo BzteaOAc EtOH U:0.2:1 70 3 (25)
Bamboo ChOAc EtOH U:0.2:1 70 3 (25)
Bamboo TeaHSO4 EtOH U:0.2:1 70 3 (25)
Cellulose nanofibers HmimBF4 None 1:10 25 and 35 24 (20)
Oil palm fronds BmimCl DMSO 15:50:50 90 1 (8,18)
Oil palm fronds BmimCl DMSO 15:50:50 90 3 (14,17)
Oil palm fronds BmimCl None 1:1 90 3 (16)
Oil palm fronds BmimCl DMSO 15:50:50 130 3 (19)
Oil palm fronds EmimDep DMSO 15:50:50 90 1 (8,18)
Oil palm fronds EmimDep DMSO 15:50:50 90 3 (14,17)
Oil palm fronds EmimDep None 1:1 90 3 (15)
Oil palm fronds EmimDep DMSO 15:50:50 130 3 (19)
Oil palm fronds EmimOAc None 1:1 90 3 (16)
Pine wood CmdimNTf2 None 1:8 100 6 (5)
Pine wood CmtimNTf2 None 1:8 100 6 (5)
Pine wood DcmdaNTf2 None 1:8 100 6 (5)
Pine wood DddmaNTf2 EtOH U:0.2:1 R 3 (6)
Pine wood TcmdaNTf2 None 1:8 100 6 (5)
Rice husks EmimOAc Water U 80 4 (9)
Wood flour TopmimCl Tol 50:11:259.5 RT 4 (3,4)
Wood flour TopmimSCN Tol 50:11:259.5 RT 4 (3,4)

DMSO – dimethyl sulfoxide, EtOH – ethanol, Tol – toluene, U – unstated, R – room temperature, and RT – reflux temperature.

Figure 7 Chemical structures of (a) DMSO, (b) EtOH, and (c) Tol.
Figure 7

Chemical structures of (a) DMSO, (b) EtOH, and (c) Tol.

In Table 3, it can also be seen that elevated temperatures are typically applied in the ionic liquid pretreatment. High temperatures can also decrease the viscosity of ionic liquids. Furthermore, the pretreatment kinetics speed up with an increase in temperature. Nonetheless, the maximum pretreatment temperature was 130°C (19), which may avoid the degradation of the natural fillers. Besides, the pretreatment times are around 3–6 h. This demonstrates that pretreatment with an ionic liquid does not take too long to carry out. Nevertheless, the pretreatment time also hinges on the type and weight ratio of the natural fillers in the ionic liquid. Thus, the pretreatment time of the natural fillers constantly decreased with lessening the weight of the natural fillers. Finally, after the pretreatment process, the pretreated natural fillers are often suspended in an anti-solvent, filtered, washed, and dried to be utilized for the fabrication of polymer biocomposites. In addition, ionic liquids or ionic liquid/co-solvent mixtures can be recovered from the supernatants by evaporating the anti-solvent and followed by the solid separation via vacuum filtration (16,19). No reducing sugar was formed in the recycled ionic liquids, which implies that the natural filler was not degraded to the small-molecule level during pretreatment in an ionic liquid (31). Besides, recycled ionic liquids or ionic liquid/co-solvent mixtures can be reused for the pretreatment of natural fillers (8).

3 Physicomechanical properties of ionic liquid-pretreated natural filler/polymer biocomposites

3.1 Influence of chloride- and diethyl phosphate-based ionic liquid pretreatments

Table 4 exhibits the physicomechanical properties of polymer biocomposites, and their natural fillers were pretreated with chloride- and diethyl phosphate-based ionic liquids. The oil palm frond/corn starch (OPF/CS) biocomposites were fabricated by Mahmood et al. using BmimCl as a pretreatment agent (18). The physicomechanical properties, e.g., thermal, morphological, and mechanical properties, of the fabricated biocomposites were characterized using a thermal gravimetric analyzer, a scanning electron microscope, and a universal testing machine. The thermal property, such as the thermal degradation of the BmimCl-pretreated OPF/CS biocomposites, was improved compared to the unpretreated OPF/CS biocomposites. This was due to the alterations in the chemical composition of OPF, which increased the thermal stability of OPF and its biocomposites. Moreover, the morphological property, for example, the surface roughness of the pretreated OPF, decreased in comparison to the unpretreated OPF. This was caused by most of the lignin and other non-cellulosic impurities on the cell wall were solubilized and eliminated by the BmimCl pretreatment (18). In addition, the mechanical properties, for instance the flexural strength and flexural modulus of the pretreated OPF/CS biocomposites, increased by up to 70% and 75%, respectively, compared to the unpretreated OPF/CS biocomposites. This was attributed to some of the lignocellulosic fractions that were eventually dissolved and removed, which gave a more reachable cellulose surface area for CS flow and interaction (18). Therefore, it can be concluded that the BmimCl pretreatment provides OPF/CS biocomposites with high thermal stability and high flexural properties.

Table 4

Physicomechanical properties of polymer biocomposites pretreated with chloride- and diethyl phosphate-based ionic liquids

Ionic liquid Polymer biocomposite Physicomechanical properties References
Thermal Morphological Mechanical
BmimCl Oil palm frond/CS T d↑ SR↓ FS↑, FM↑ (18)
EmimDep Oil palm frond/CS T d↑ SS↑ FS↑, FM↑ (19)

T d – thermal degradation, SR – surface roughness, FS – flexural strength, FM – flexural modulus, and SS – surface smoothness. The symbol ‘↑’ corresponds to an increase in the properties, and the symbol ‘↓’ corresponds to a decrease in the properties.

In addition, the OPF/CS biocomposites were fabricated by Mahmood et al. using EmimDep as a pretreatment agent (19). The physicomechanical properties, e.g., thermal, morphological, and mechanical properties, of the fabricated biocomposites were characterized using a thermal gravimetric analyzer, a scanning electron microscope, and a universal testing machine. The thermal property, such as the thermal degradation of the EmimDep-pretreated OPF/CS biocomposites, was also increased in comparison to the unpretreated OPF/CS biocomposites. This was ascribed to the enhancement in adhesion between the OPF filler and the CS matrix, which offered high thermal stability to the biocomposites. Furthermore, the morphological property, for example, the surface smoothness of the cross-section of the pretreated OPF/CS biocomposites, was better than that of the unpretreated OPF/CS biocomposites. This was because the interfacial bonding of filler with matrix became comparatively stronger (19). Besides, the mechanical properties, for instance the flexural strength and flexural modulus of the pretreated biocomposites, improved by up to 69% and 94%, respectively, in comparison to the unpretreated biocomposites. This was owing to the efficiency of stress transfer from the CS matrix to the pretreated OPF and the reduction in the diameter of the pretreated filler, which increased the aspect ratio (19). Hence, it can be inferred that the EmimDep pretreatment also gives OPF/CS biocomposites with high thermal stability and high flexural properties.

3.2 Influence of acetate-based ionic liquid pretreatments

Table 5 shows the physicomechanical properties of polymer biocomposites, and their natural fillers were pretreated with acetate-based ionic liquids. The bamboo/PP biocomposites were fabricated by Mudzakir et al. using BzteaOAc as a pretreatment agent (25). The physicomechanical properties, e.g., crystalline, morphological, and mechanical properties, of the fabricated biocomposites were characterized using an X-ray diffractometer, a scanning electron microscope, and a strength tester. The crystalline property, such as the degree of crystallinity of the BzteaOAc-pretreated bamboo/PP biocomposites, was decreased compared to the unpretreated bamboo/PP biocomposites. This was due to the influence of BzteaOAc interaction on bamboo cellulose, which induced the mobility of cellulose chains, and created the intra- and intermolecular hydrogen interactions in the cellulose region. Moreover, the morphological property, for example, the surface flatness of the pretreated bamboo/PP biocomposites, was observed in comparison to the unpretreated bamboo/PP biocomposites. This was caused by the presence of the interaction of PP with pretreated bamboo (25). In addition, the mechanical properties, for instance the tensile strength and elongation at break of the pretreated biocomposites, increased by up to 181% and 12%, respectively, compared to the unpretreated biocomposites. This was attributed to the increased interfacial adhesion between the pretreated bamboo filler and the PP matrix, which increased the wettability of the bamboo filler (25). Thus, it can be deduced that the BzteaOAc pretreatment yields bamboo/PP biocomposites with low crystallinity and high tensile properties.

Table 5

Physicomechanical properties of polymer biocomposites pretreated with acetate-based ionic liquids

Ionic liquid Polymer biocomposite Physicomechanical properties References
Crystalline Morphological Mechanical
BzteaOAc Bamboo/PP X c↓ SF↑ TS↑, EB↑ (25)
ChOAc Bagasse powder/PP CrI↕ NV↓ TS↑, EM↑ (24)
EmimOAc Oil palm frond/CS CrI↓ SR↓ FS↑, FM↑ (16)
EmimOAc Rice husk/PLA n/a MV↓ TS↑, TM↑ (9)

X c – degree of crystallinity, SF – surface flatness, TS – tensile strength, EB – elongation at break, CrI – crystallinity index, NV – number of voids, EM – elastic modulus, SR – surface roughness, FS – flexural strength, FM – flexural modulus, MV – micro voids, TM – tensile modulus, and n/a – not available. The symbol ‘↑’ corresponds to an increase in the properties and the symbol ‘↓’ corresponds to a decrease in the properties, while ‘↕’ describes unchanged.

The bagasse powder/PP biocomposites were fabricated by Ninomiya et al. using ChOAc as a pretreatment agent (24). The physicomechanical properties, e.g., crystalline, morphological, and mechanical properties, of the fabricated biocomposites were characterized using an X-ray diffractometer, a scanning electron microscope, and a universal testing machine. The crystalline property, such as the crystallinity index of the ChOAc-pretreated bagasse powder (44%), remained almost unchanged, which is about the same as the unpretreated bagasse powder (46%). This indicates that both powders have nearly similar crystalline property. Furthermore, the morphological property, for example, the number of voids on the cross-section of the pretreated bagasse powder/PP biocomposites, was lower than that of the unpretreated bagasse powder/PP biocomposites. This was ascribed to the ChOAc pretreatment lessened the rigid structure of crystalline cellulose in the plant cell walls of the bagasse powder (24). In addition, the mechanical properties, for instance the tensile strength and elastic modulus of the pretreated biocomposites, increased by up to 14% and 30%, respectively, compared to the unpretreated biocomposites. This was because of the improvement in compatibility between the pretreated filler and the PP matrix (24). Therefore, it can be concluded that the ChOAc pretreatment provides bagasse powder/PP biocomposites with unchanged crystallinity and high tensile properties.

The OPF/CS biocomposites were also fabricated by Mahmood et al. using EmimOAc as a pretreatment agent (16). The physicomechanical properties, e.g., crystalline, morphological, and mechanical properties, of the fabricated biocomposites were characterized using an X-ray diffractometer, a scanning electron microscope, and a universal testing machine. The crystalline property, such as the crystallinity index of the EmimOAc-pretreated OPF filler (23.3%), was slightly reduced in comparison to the unpretreated OPF filler (25.9%). This was owing to the gradual swelling of the cellulose in the OPF filler during pretreatment in EmimOAc. In addition, the morphological property, for example, the surface roughness of the pretreated OPF filler, diminished compared to the unpretreated OPF filler. This suggests that the EmimOAc pretreatment altered the fibril structure of the cell wall by removing most of the lignin and non-cellulosic impurities (16). Besides, the mechanical properties, for instance the flexural strength and flexural modulus of the pretreated biocomposites, improved by up to 150% and 108%, respectively, in comparison to the unpretreated biocomposites. This was due to the pretreatment restructured the lignocellulosic OPFs by changing their chemical composition and providing a more active surface area for interacting with CS (16). Hence, it can be inferred that the EmimOAc pretreatment gives OPF/CS biocomposites with low crystallinity and high flexural properties.

The rice husk/PLA biocomposites were fabricated by Islam et al. using EmimOAc as a pretreatment agent (9). The physicomechanical properties, e.g., morphological and mechanical properties, of the fabricated biocomposites were characterized using a scanning electron microscope and a universal testing machine. The morphological property, such as the micro voids of the fractured surface of the EmimOAc-pretreated rice husk/PLA biocomposites, was lower than those of the unpretreated rice husk/PLA biocomposites. This was caused by the change in the properties of the rice husks during pretreatment; the EmimOAc molecules adsorbed on the surface of the rice husks and minimized the polarity gap between the rice husk filler and PLA, resulting in the better distribution of the rice husks in the PLA matrix (9). In addition, the mechanical properties, for example the tensile strength and tensile modulus of the pretreated rice husk/PLA biocomposites, increased by up to 20% and 56%, respectively, compared to the unpretreated rice husk/PLA biocomposites. This was ascribed to the pretreatment considerably reduced the hydrophilic character of the rice husks and improved the attractive force to establish a strong interfacial bond with the PLA matrix (9). Thus, it can be deduced that the EmimOAc pretreatment yields rice husk/PLA biocomposites with enhanced surface morphology and high tensile properties.

3.3 Influence of bistriflimide-based ionic liquid pretreatments

Table 6 displays the physicomechanical properties of polymer biocomposites, and their natural fillers were pretreated with bistriflimide-based ionic liquids. The pine wood/iPP biocomposites were fabricated by Odalanowska et al. using CmdimNTf2 as a pretreatment agent (5). The physicomechanical properties, e.g., crystalline, morphological, and mechanical properties, of the fabricated biocomposites were characterized using a differential scanning calorimeter, a hot stage optical microscope, and a universal testing machine. The crystalline property, such as the crystallization temperature of the CmdimNTf2-pretreated pine wood/iPP biocomposites, was increased in comparison to the unpretreated pine wood/iPP biocomposites. This was attributed to the high nucleation ability of the pretreated pine wood filler, which became the most efficient in the nucleating process of the iPP matrix. In addition, the morphological property, for example, the formation of transcrystalline layers in the pretreated pine wood/iPP biocomposites, is higher than that of the unpretreated pine wood/iPP biocomposites. This confirms a very high nucleation activity of the pretreated pine wood surface, which has a high density of the nucleation centers formed (5). Moreover, the mechanical properties, for instance the tensile strength and Young’s modulus of the pretreated biocomposites, increased by up to 17% and 7%, respectively, compared to the unpretreated biocomposites. This was owing to an increase in the interactions between the pretreated pine wood filler and the iPP matrix, as well as the homogeneous dispersion of the pretreated filler in the polymer matrix (5). Therefore, it can be concluded that the CmdimNTf2 pretreatment provides pine wood/iPP biocomposites with high crystallinity and high tensile properties.

Table 6

Physicomechanical properties of polymer biocomposites pretreated with bistriflimide-based ionic liquids

Ionic liquid Polymer biocomposite Physicomechanical properties References
Crystalline Morphological Mechanical
CmdimNTf2 Pine wood/iPP T c↑ TL↑ TS↑, YM↑ (5)
DddmaNTf2 Pine wood/iPP T c↓ TL↓ TS↑, YM↑ (6)

T c – crystallization temperature, TL – transcrystalline layer, TS – tensile strength, and YM – Young’s modulus. The symbol ‘↑’ corresponds to an increase in the properties, and the symbol ‘↓’ corresponds to a decrease in the properties.

The pine wood/iPP biocomposites were fabricated by Borysiak et al. using DddmaNTf2 as a pretreatment agent (6). The physicomechanical properties, e.g., crystalline, morphological, and mechanical properties, of the fabricated biocomposites were characterized using a differential scanning calorimeter, a hot stage optical microscope, and a universal testing machine. The crystalline property, such as the crystallization temperature of the DddmaNTf2-pretreated pine wood/iPP biocomposites, was decreased in comparison to the unpretreated pine wood/iPP biocomposites. This was because of the low nucleation ability of the pretreated pine wood filler, which reduced the crystal conversion rate of the iPP matrix. In addition, the morphological property, for example, the formation of transcrystalline layers in the pretreated pine wood/iPP biocomposites, was worse than that of the unpretreated pine wood/iPP biocomposites. This proves that the pretreatment of the pine wood filler diminished the nucleation activity of its surface (6). In contrast, the mechanical properties, for instance the tensile strength and Young’s modulus of the pretreated biocomposites, improved by up to 12% and 9%, respectively, compared to the unpretreated biocomposites. This was due to the increased interfacial bonding between the pretreated pine wood filler and the iPP matrix, which enhanced the stress transfer between the pretreated filler and the polymer matrix (6). Hence, it can be inferred that the DddmaNTf2 pretreatment gives pine wood/iPP biocomposites with low crystallinity and high tensile properties.

3.4 Summary of physicomechanical properties and special advantages

Table 7 shows the summary of physicomechanical properties of polymer biocomposites pretreated with four types of ionic liquids, namely chloride-, diethyl phosphate-, acetate-, and bistriflimide-based ionic liquids. It can be seen that polymer biocomposites pretreated with chloride- and diethyl phosphate-based ionic liquids have high thermal degradation. On the other hand, polymer biocomposites pretreated with acetate-based ionic liquids have a low crystallinity index. However, polymer biocomposites pretreated with bistriflimide-based ionic liquids have high and low crystallization temperatures depending on the type of ionic liquid cations. Besides, polymer biocomposites pretreated with four types of ionic liquids have surface morphologies with low surface roughness, high surface smoothness, low number of voids, and high and low transcrystalline layers.

Table 7

Summary of physicomechanical properties of polymer biocomposites pretreated with four types of ionic liquids

Physicomechanical properties Ionic liquid
Chloride-based Diethyl phosphate-based Acetate-based Bistriflimide-based
Thermal T d↑ T d↑ n/a n/a
Crystalline n/a n/a CrI↓ Tc↑↓
Morphological SR↓ SS↑ NV↓ TL↑↓
Mechanical FS↑70% FS↑69% TS↑181% TS↑17%

T d – thermal degradation, CrI – crystallinity index, T c – crystallization temperature, SR – surface roughness, SS – surface smoothness, NV – number of voids, TL – transcrystalline layer, FS – flexural strength, TS – tensile strength, and n/a – not available. The symbol ‘↑’ corresponds to an increase in the properties, and the symbol ‘↓’ corresponds to a decrease in the properties.

More interestingly, polymer biocomposites pretreated with ionic liquids have high flexural and tensile strength. The highest increase in tensile strength was found in polymer biocomposites pretreated with acetate-based ionic liquids (181%). Conversely, the lowest increase in tensile strength was in polymer biocomposites pretreated with bistriflimide-based ionic liquids (17%). In addition, some special advantages of each type of ionic liquid were discovered in terms of pretreatment conditions. Table 8 exhibits the pretreatment conditions for four types of ionic liquids. It can be observed that chloride- and diethyl phosphate-based ionic liquids can pretreat medium weight ratios of natural fillers at moderate temperatures for short times compared to other ionic liquids. In addition, acetate-based ionic liquids can pretreat high weight ratios of natural fillers at low temperatures for shorter times than other ionic liquids. Finally, bistriflimide-based ionic liquids do not require a co-solvent for the pretreatment process (5).

Table 8

Pretreatment conditions for four types of ionic liquids

Condition Ionic liquid
Chloride-based Diethyl phosphate-based Acetate-based Bistriflimide-based
Co-solvent Yes Yes Yes No
Weight ratio Medium Medium High Low
Temperature Moderate Moderate Low High
Time Short Short Short Long

4 Conclusions

In this article, the examples of polymer matrices and natural fillers, types of ionic liquids, synthesis of ionic liquids, and pretreatment of natural fillers for the fabrication of biocomposites are succinctly reviewed. Moreover, the significant physicomechanical properties, such as the thermal, crystalline, morphological, and mechanical properties of biocomposites, are also covered in this mini-review. The ionic liquids used for the pretreatment of natural fillers are mostly chloride-, diethyl phosphate-, acetate-, and bistriflimide-based ionic liquids. Chloride- and diethyl phosphate-based ionic liquids are frequently synthesized via alkylation and ethylation reactions, respectively, whereas acetate- and bistriflimide-based ionic liquids are generally synthesized through a metathesis reaction. In most studies, the use of ionic liquids as pretreatment agents was found to increase the thermal stability of the biocomposites. In addition, the ionic liquid-pretreated natural filler/polymer biocomposites exhibited low crystallinity. On the other hand, the ionic liquid pretreatments enhanced the surface morphology of the biocomposites. Besides, the mechanical properties of the pretreated biocomposites are typically higher than those of the unpretreated biocomposites. Hence, the physicomechanical properties of polymer biocomposites can be improved by pretreatments with ionic liquids.

Acknowledgments

The authors would like to thank the editors and the reviewers for their constructive and helpful review comments.

  1. Funding information: This mini-review was supported by the Universiti Putra Malaysia under the Grant Putra IPM Scheme (project number: GP-IPM/2021/9697900).

  2. Author contributions: Ahmad Adlie Shamsuri: conceptualization, funding acquisition, investigation, project administration, writing – original draft preparation; Siti Nurul Ain Md. Jamil: data curation, formal analysis, methodology, writing – review and editing; Khalina Abdan: resources, supervision, validation. All authors have read and agreed to the published version of the manuscript.

  3. Conflict of interest: Authors state no conflict of interest.

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Received: 2022-08-07
Revised: 2022-09-08
Accepted: 2022-09-14
Published Online: 2022-10-27

© 2022 Ahmad Adlie Shamsuri et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0074
Keywords for this article
ionic liquid; physicomechanical properties; polymer biocomposite; pretreatment; natural filler
Creative Commons
BY 4.0

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  61. Preparation and evaluation of high-performance modified alkyd resins based on 1,3,5-tris-(2-hydroxyethyl)cyanuric acid and study of their anticorrosive properties for surface coating applications
  62. A novel defect generation model based on two-stage GAN
  63. Thermally conductive h-BN/EHTPB/epoxy composites with enhanced toughness for on-board traction transformers
  64. Conformations and dynamic behaviors of confined wormlike chains in a pressure-driven flow
  65. Mechanical properties of epoxy resin toughened with cornstarch
  66. Optoelectronic investigation and spectroscopic characteristics of polyamide-66 polymer
  67. Novel bridged polysilsesquioxane aerogels with great mechanical properties and hydrophobicity
  68. Zeolitic imidazolate frameworks dispersed in waterborne epoxy resin to improve the anticorrosion performance of the coatings
  69. Fabrication of silver ions aramid fibers and polyethylene composites with excellent antibacterial and mechanical properties
  70. Thermal stability and optical properties of radiation-induced grafting of methyl methacrylate onto low-density polyethylene in a solvent system containing pyridine
  71. Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes
  72. Oxidized hyaluronic acid/adipic acid dihydrazide hydrogel as cell microcarriers for tissue regeneration applications
  73. Study of the phase-transition behavior of (AB)3 type star polystyrene-block-poly(n-butylacrylate) copolymers by the combination of rheology and SAXS
  74. A new insight into the reaction mechanism in preparation of poly(phenylene sulfide)
  75. Modified kaolin hydrogel for Cu2+ adsorption
  76. Thyme/garlic essential oils loaded chitosan–alginate nanocomposite: Characterization and antibacterial activities
  77. Thermal and mechanical properties of poly(lactic acid)/poly(butylene adipate-co-terephthalate)/calcium carbonate composite with single continuous morphology
  78. Review Articles
  79. The use of chitosan as a skin-regeneration agent in burns injuries: A review
  80. State of the art of geopolymers: A review
  81. Mechanical, thermal, and tribological characterization of bio-polymeric composites: A comprehensive review
  82. The influence of ionic liquid pretreatment on the physicomechanical properties of polymer biocomposites: A mini-review
  83. Influence of filler material on properties of fiber-reinforced polymer composites: A review
  84. Rapid Communications
  85. Pressure-induced flow processing behind the superior mechanical properties and heat-resistance performance of poly(butylene succinate)
  86. RAFT polymerization-induced self-assembly of semifluorinated liquid-crystalline block copolymers
  87. RAFT polymerization-induced self-assembly of poly(ionic liquids) in ethanol
  88. Topical Issue: Recent advances in smart polymers and their composites: Fundamentals and applications (Guest Editors: Shaohua Jiang and Chunxin Ma)
  89. Fabrication of PANI-modified PVDF nanofibrous yarn for pH sensor
  90. Shape memory polymer/graphene nanocomposites: State-of-the-art
  91. Recent advances in dynamic covalent bond-based shape memory polymers
  92. Construction of esterase-responsive hyperbranched polyprodrug micelles and their antitumor activity in vitro
  93. Regenerable bacterial killing–releasing ultrathin smart hydrogel surfaces modified with zwitterionic polymer brushes
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