Except for heat,24 enzymatic,32 and special forms of maleated polyolefin (MA-PO),33, 34 and stearic acid,35 treatment, fiber-based methods usually rely on modification of the fiber in organic or aqueous solvents. Of course, this is problematic because such methods are relatively elaborate, and the usage of large amounts of particularly organic solvents is inappropriate for both economic and ecological considerations. Most common are silane treatments,22, 27, 34, 36–46 followed by maleated polyolefins (MA-PE, MA-PP), which can be applied as an organic solution,17, 39, 47, 48 as a melt, employing a thermokinetic mixer33 or a roll mill,34 or as an emulsion.38, 49 Mercerization, or treatment with NaOH, is sometimes employed to activate the fiber surface for subsequent modification steps,11, 36, 42, 44, 46 but can also act as the modification itself.23, 36, 40, 44, 48, 50, 51 Other fiber surface treatments included are acetylation26, 52, 53 and maleic anhydride (MA) treatment.39, 48, 54 Several other treatments have been investigated by various researchers, and are covered in “Other Fiber-Based Strategies” section.
Silane Treatment of Fibers/Fillers
Silanes have been used as coupling agents in glass fiber-reinforced polymers for many years,5, 55, 56 which might explain why they are among the most common fiber-based compatibilization strategies. Initially, silane coupling agents have been developed in the 1940s, and a large variety of types for different applications is available today.57 All those molecules share the same basic structure (Figure 4).
Figure 4. Silane basic structure.58 [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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The R′ group is usually hydrolyzed during the fiber pretreatment. The OH-groups formed thereby are meant to interact with those on the cellulose of the fiber/filler, either via ether-bonds, or hydrogen bridges. Furthermore, the silane molecules themselves can crosslink through such interactions, forming a network on the fiber/filler surface. The functional group R is the part of the coupling agent which is meant to interact with the matrix. By choosing a silane with the right R group, compatibility with different sorts of polymers can be tuned. By those mechanisms, composite mechanical properties should be improved by enhanced fiber–matrix interaction, and possibly also better dispersion of the fiber/filler throughout the matrix.
The fiber/filler pretreatment conditions are, of course, also adapted to silane chemical structure. Depending on this, application is performed in organic-based solvents, such as acetone/acetic acid,44 carbon tetrachloride,34 methanol/water,36, 42 ethanol/water,25, 37, 46 or ethanol,41 and water-based solvents,22, 38–40, 45 respectively. A NaOH pretreatment of the fiber/filler can be done prior to the silane one.36, 42, 46 Furthermore, initiators such as benzoyl chloride34 or dicumyl peroxide (DCP)36 are sometimes added. In any case, the solvents have to be removed after the treatment, and the fibers/fillers dried. Grubbstrom et al., in contrast to others, use a silane-peroxide solution which is added in a direct extrusion process.27 Usually, a heat treatment, which can coincide with a drying step, is performed after the silane application, to promote chemical coupling. Mohanty et al. use a water-based silane/MA-PP emulsion in combination with a powder PP-based compression molding process.38 Two groups combine the fiber-based silane approach with a matrix-based one, employing maleated styrene-ethylene-butylene-styrene22 and MA-PP, respectively.25 Furthermore, a study conducted by Arbelaiz et al. has shown that simple addition of a silane to the matrix rather than a fiber pretreatment can be sufficient for improving interfacial adhesion in a PP-30% flax system (“Other Matrix-Based Strategies” section).48
Most of the groups working with silane coupling agents achieve an increase in tensile and flexural strength, respectively, ranging up to above 100%,27 flexural, LDPE-50% wood,34 tensile, HDPE-40% wood]. Four groups find reductions of approximately 10% in flexural38, 39 and tensile strength,43, 44 respectively, as a result of the fiber pretreatments applied. The effects of the treatment on stiffness are generally smaller, with a maximum increase in Young's modulus of ∼ 40%,37 (PP-30% wood) and in flexural modulus of 74%,45 (PP-20% kenaf). The silane solution/MA-PP emulsion increases flexural strength by ∼ 30%, and flexural modulus by ∼ 60%,38 PP-40% kenaf. The influence of silane pretreatments on impact strength (IS) is ambiguous. Most researchers observe modest effects between approximately −20% and +25% for notched Charpy or Izod measurements.22, 40, 42, 45 Colom et al., on the other hand, find unnotched Izod IS increased by 290% by their silane pretreatment,34 (HDPE-40% wood). Water absorption (WA) was measured by two groups, only. While Bettini et al. report a minimal increase of approximately 3% for their treatment after 1 year of submersion,25 (PP-20% wood), Lee et al. find WA reduced by 40% for a 24 hr test,42 (PP-30% and 50% bamboo, respectively].
The results (Figure 5) do not allow an interpretation as to which silane types are the most effective ones. In general, most of the silane pretreatments applied lead to improvements in material tensile and flexural strength, while for stiffness and impact strength, the situation is not so clear. For the latter, interpretations are hindered by the large variety of methods for determination (Charpy, Izod, notched/unnotched, instrumented, etc.). For water absorption, the results existing are too few to draw a conclusion. The application of silanes is relatively elaborate, but the fact that water-based solutions are sufficient for some types is encouraging. Generally, silane coupling agents, especially the water-soluble ones, do definitely have some potential as modifiers for the fiber–matrix interaction in NFCs and WPCs.
Maleic Anhydride Grafted Polyolefins (Applied to Fiber)
MA-POs are another well-known class of coupling agents which have proven efficient compatibilizers in conventional composites, like PP-glass fiber compounds, already.59, 60 Maleic anhydrid can be grafted onto polyolefins by radical initiated grafting. This can be performed either in the molten state61, 62 (usually as reactive extrusion), in solid state63–65 or in solution,66, 67 of which only the first two processes are of commercial relevance. In any case, initiators, usually peroxides, are needed to start the grafting reaction. In melt and solution processes, tertiary carbon atoms in the PO chain are the preferred acceptors of MA moieties. However, functionalization does also occur on secondary carbons in (CH2)m sequences with m > 3. In longer propylene sequences, chain scission can lead to a structure where the anhydride is attached to the chain terminus via a double bond (Figure 6). Side reactions, like homopolymerization of the MA monomer, chain scission in PP, and crosslinking in PE, respectively, are inevitable.62, 68 In the case of MA-PE, an actual copolymerization approach (in contrast to grafting), resulting in a product showing superior properties, was presented 2009 by Decodts from Dupont.69 To our knowledge, no attempts to copolymerize maleic anhydride with propylene have been published yet.
Figure 6. Structures that can arise in radical initiated grafting of maleic anhydride onto polyolefins. I, MA oligomers as found in HDPE and LDPE; II and III, MA grafted to tertiary carbons in PP and copolymers; IV, Terminal structure resulting from PP chain scission [Reprinted with permission from Ref.68 by the courtesy of ACS Publications].
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Particularly in reactive extrusion, the side reactions lead to a tradeoff between two parameters: For MA-PP, increasing the graft level usually leads to a reduction in molecular weight of the base polymer.61 For MA-PE, on the other hand, achieving high MA contents is often connected to high gel contents resulting from crosslinking.66 Solid-state grafting, on the other hand, allows for a widely mutually independent adjustment of graft level and molecular weight.63, 70 However, at least for PP, there are still crosslinking and degradation reactions, respectively, depending on the process temperature.65 Although increased graft level should increase interaction with a hydrophilic fiber/filler, decreased molecular weight might decrease interaction of the coupling agent polymer backbone with the matrix polymer. Therefore, the right balance between maleic anhydride content and backbone polymer chain length is supposed to be very important for MA-POs to be efficient compatibilizers.71, 72
The actual mechanism of chemical interaction at the interface that MA-POs promote has been investigated by several researchers. In most cases, Fourier transformed infrared spectroscopy (FTIR) analysis is applied to natural fibers or fillers, which have either been pretreated with coupling agents or re-extracted from composites, usually with xylene. Biagiotti et al. investigated MA-PP-treated flax fibers by FTIR, and report a novel peak at 1740 cm−1 representing an ester group.39 Kazayawoko et al. modified bleached Kraft pulp and thermomechanical pulp (TMP) fibers with MA-PP. The group detected ester linkages by FTIR for the first, but not the second fiber type.33 Employing the extraction approach, Lu et al. found signals between 1650 and 1800 cm−1 indicating the existence of ester bonds between wood surface OH-groups and MA-PE.73 These findings are backed by electron spectroscopy for chemical analysis results. Nourbakhsh et al. reported signals in the same wavelength range in PP/wood composites compatibilized with MA-PP, but did not interpret them as a result of covalent bonding at the interface.74 Paunikallio et al. also report detection of ester linkages between viscose and MA-PP on fibers extracted from the composites prepared.75 Wang et al. investigated the influence of several PE-based compatibilizers on HDPE-wood composites and did also find evidence for ester bonds between the MA moieties of the coupling agents and the re-extracted filler.76 In a similar study, Lai et al. identified increased absorption bands in the carbonyl region that might indicate the formation of covalent linkages at the interface.77 Both the pretreatment and the re-extraction approach are representative of the interfacial features of a real composite only to a limited degree. For the former, despite giving similar improvements in mechanical properties in composites as matrix-based MA-PO deployment,17 it is not likely that the chemical structures formed at the fiber/filler surface during solution treatment remain unchanged in the melt during compounding. For the latter, it is just the other way round: the chemical structures formed in the melt during compounding will most likely be altered upon extraction of the fiber/filler from the compound in hot xylene (or another solvent).
Harper employed another approach for investigating chemical structures at the interface in his dissertation. He subjected 40-μm thin hot stage film specimens to FTIR microscopy. Based on this method, Harper could not confirm covalent bonding between MA-PP and wood. This was because the absorption bands that would result from potential ester bonds largely overlap with those from aliphatic esters in lignin at 1745 cm−1, thus impeding spectra interpretation.72 Of course, this affects all FTIR-based methods and thus hinders conclusive verification of the formation of covalent linkages based on such results. Summing up these findings, it can be said that most FTIR data available suggests the existence of covalent linkages in the form of ester bonds between the MA moieties of grafted polyolefin coupling agents and the OH-groups on the surface of lignocellulosic fibers/fillers. Furthermore, the formation of those bonds seems to be independent of the way of deployment of the coupling agent (fiber/filler pretreatment or incorporation as additive during compounding, see “Maleic Anhydride Grafted Polyolefins as Matrix Additives” section). However, the fact that the absorption bands of the suggested ester linkages would partly overlap with those of existing aliphatic lignin esters demands scrutiny when dealing with FTIR investigations of the interface. Besides the possible covalent linkages, hydrogen bridges might also contribute to interaction (Figure 7).25 The polymer backbone is meant to interact with the matrix by chain entanglement and possibly cocrystallization, which, however, might be hindered by high MA graft levels.61, 71, 72, 76, 78 By the mechanisms summarized above, MA-POs should increase composite mechanical properties by improving the interfacial shear strength and possibly also fiber/filler dispersion.
Figure 7. Assumed coupling reactions of MA-PP with OH-groups of cellulose at the fiber/filler surface. The MA moiety undergoes esterification with OH-groups on the surface, or interacts via hydrogen bridges [Reprinted with permission from Ref.25 by the courtesy of John Wiley and Sons].
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Pretreatment of fibers/fillers is often performed employing an organic solution of the MA-PO coupling agent. The solvent can be either boiling xylene17, 39, 48 or hot toluene.47 In any case, it has to be removed again from the fibers/fillers before composite processing. Furthermore, melt-based pretreatments can be performed in a roll-mill34 or in a thermokinetic mixer.33 Two groups use MA-PO emulsions for fiber pretreatment.38, 49
All groups found increases in tensile strength upon fiber/filler MA-PO pretreatment, reaching from ∼ 15%34 (HDPE-40% wood) to 40%48 (PP-30% flax). For flexural strength, Biagiotti et al. detected a minimal negative effect of their MA-PP pretreatment in a PP-30% flax system.39 All other groups found flexural strength enhanced upon their MA-PO pretreatments, with increases ranging from 15%38 (PP-40% kenaf) to ∼ 70%47 (PP-30% jute). For tensile modulus, two groups report modest effects between zero and ∼ 6% increase.33, 34 On the other hand, Biagiotti et al. and Arbelaiz et al. report 17% and 37% increase, respectively, upon MA-PP pretreatments of the fiber in a PP-30% flax system.17, 39 As regards flexural modulus, the effects of MA-PO pretreatments found range from approximately −10%79 (PP-30% flax) to approximately +10%17 (PP-30% flax), while the silane solution/MA-PP emulsion pretreatment of Mohanty et al. increases this property by ∼ 60%38 (PP-40% kenaf). The effect of MA-PO fiber pretreatments on Izod impact strength was investigated by two groups. While Colom et al. found unnotched IS in their HDPE-40% wood system unchanged, Mohanty et al. report an increase of notched IS by ∼ 30% for their PP-30% jute system. This group also found water absorption in a 24 hr immersion test reduced by ∼ 60% as a result of their MA-PP fiber pretreatment.
As to the effects of MA-PO pretreatments on material properties (Figure 8), the situation is similar to silane pretreatments: tensile and flexural strength can definitely be improved via this approach, while effects on stiffness are smaller. As regards impact strength and water absorption, too few results exist to draw conclusions. The same is true for the method of application of the MA-POs: solution-based, emulsion-based, and melt-based pretreatments can be effective, but the data available to date do not allow a rating as to which method brings about the best properties.
Mercerization of Fibers/Fillers
Mercerization or treatment with strong alkali bases was developed as a method for cotton fiber modification by John Mercer in 1850. It causes swelling of the cell walls, along with longitudinal shrinkage and an increase of the amount of cellulose-II at the expense of cellulose-I. Furthermore, it renders treated fibers a more circular cross section.80 Applied to lignocellulosic natural fibers, mercerization also reduces the lignin- and hemicellulosics-content, which in turn can improve tensile properties.81 For flax and jute fibers, improvements of 15%–40% in Young's modulus and tensile strength as a result of mercerization have been reported.82 Goda et al. applied mercerization to ramie fibers. They report an increase in tensile strength of the fibers of 4%–18%, along with a twofold to threefold increase in strain at break. This was accompanied by a reduction of tensile modulus.83 Tripathy et al., on the other hand, found a reduction in tensile strength of ∼ 25% upon NaOH treatment of jute fibers. By making cellulose at the fiber surface more easily accessible, mercerization can be applied as a form of activation for subsequent modification steps,36, 42, 46 or in combination with matrix-based compatibilization strategies.23, 46
Fibrillation, meaning a splitting of fiber bundles into filaments, increases the surface available for fiber–matrix interaction, and also the aspect ratio, thus potentially improving the material properties of composites (Figure 9).11, 82 Mercerization also tends to increase the fiber surface roughness. This in turn improves the potential for stress transfer through mechanical interlocking at the fiber–matrix interface.36, 85, 86 Summing up all these mechanisms, it can be said that fiber/filler mercerization can potentially influence composite mechanical properties not only via fiber–matrix interaction but also via changing the fiber properties themselves.
Figure 9. SEM images of sisal fibers: (a) untreated, (b) mercerized, (c) mercerized under tension. Fibrillation as a result of the treatment can be seen in (b) and (c) [Reprinted with permission from Ref.84 by the courtesy of Elsevier].
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Mercerization is usually performed applying aqueous solutions of NaOH, at reaction times of 30 min up to 3 hr. Theoretically, other alkali types can be used as well, but sodium atoms have been shown to provide the optimal diameter for cellulose swelling, meaning that the treatment with NaOH is most efficient.31 In any case, excess alkali has to be removed by washing the modified fibers/fillers subsequently, followed by a drying step.
Effects of natural fiber mercerization on the tensile strength of the resulting NFCs as reported in the literature reach from −4% to +50%44, 51 (PE-20% TMP; LDPE-30% sisal), those on Young's modulus from −27% to +46%51, 87 (PP-20% sisal) (Figure 10). Valadez-Gonzalez et al. investigated HDPE-23% henequen composites. They report tensile strength unchanged for mercerization alone, while in combination with a subsequent xylene-based HDPE solution treatment, this property is increased by 10%. Combining NaOH with silane treatment, tensile strength is enhanced by ∼ 30%.36 Herrera-Franco and Valadez-Gonzalez have investigated the same composite system. For a NaOH/silane pretreatment, they report increases of tensile and flexural strength of +30% and +10%, respectively. Silane pretreatment alone yielded only half the effect on tensile, but twice the effect on flexural strength.11 Combining NaOH fiber pretreatment with matrix modification by MA-PP addition, Soleimani et al. achieved considerable improvements in mechanical properties in their PP-30% flax system. In fact, while tensile and flexural strength are increased by 50% and 20%, respectively, the respective stiffness values are improved by ∼ 20%. Also, tensile impact strength is enhanced by ∼ 50%, and water absorption upon 24 hr immersion is reduced by ∼ 25%. Without the NaOH fiber pretreatment, addition of MA-PP is far less efficient (tensile/flexural strength +30%/+15%, respectively, moduli +10%/+20%, IS none, WA 24 hr −20%).23 Employing a similar compatibilization approach, Farsi achieved improvements in tensile and flexural strength of 20% and 30%, respectively, while notched Izod IS was reduced by ∼ 10%. With MA-PP alone, strengths of the PP-40% wood composite were increased to a lesser degree, while the negative effect on IS was avoided.46
The material property improvements in NFCs and WPCs achieved by fiber/filler mercerization (Figure 10) are far less pronounced than those resulting from silane and MA-PO pretreatments, respectively. However, there is not enough data yet to disqualify this compatibilization strategy for natural fiber/filler-reinforced polyolefins. In any case, NaOH pretreatment might be a useful “activation step” in combination with other fiber- or matrix-based approaches.11, 23, 36, 42, 46
Acetylation of Fibers/Fillers
Acetylation as a method for solid wood modification is known since 1946.88 This treatment can improve wood durability and dimensional stability due to its anti-shrink effect. A partial replacement of OH-groups in the wood by the more bulky and less hydrophilic acetate groups results in a permanently swollen state of the material.89 Of course, for reasons of increased surface/volume ratio, this modification can be applied to wood fibers or particles even more easily. Thus, the permeability of the material for liquids is far less important than is the case for solid wood.90
Acetylation is usually performed using acetic anhydride, acetyl chloride, or thioacetic acid and keten (plus, e.g., suitable solvents, catalysts, and swelling agents), of which only the first approach is being applied in larger scale up to commercial industrial production as yet.91 As a side product of the acetic anhydride reaction with wood OH-groups, acetic acid accumulates, causing unpleasant odor, and potentially cellulose degradation in the modified wood (Figure 11). This problem can only partially be solved by extraction of the byproduct. A more recent acetylation method is based on isopropenyl acetate (Figure 12). This approach allows for selective reactions under mild conditions. Acetone which is formed as a side product can be removed with relative ease.92 A patent covering this procedure has been granted to Wacker Chemie and Wood K plus in 2006.93
Tronc et al. have modified blue agave fibers by acetylation with acetic anhydride in octanoic acid. After extraction of acetic acid with acetone and removal of the solvent, the fibers were characterized employing FTIR and NMR. Peaks at 1750 cm−1 and 172 ppm, respectively, provided indication for a successful modification.52 Bledzki et al. have acetylated flax fibers in a toluene solution of acetic anhydride. Afterwards, chemical characterization was performed, including a determination of the degree of acetylation by saponification.26 Besides applying acetylation directly to fibers or fillers, industrially treated solid wood can be machined (milled) to particles. Grüneberg et al. have used commercially available modified woods (besides in their study. However, this approach leads to limited comparability of the results, because the different treatments affect milling behavior and thus particle size distributions.53
Tronc et al. have produced compression molded HDPE composites with 40% treated and untreated fibers. They report a slight reduction of tensile modulus, along with an increase in unnotched Izod IS of ∼ 15% upon acetylation. Bledzki et al. have investigated injection-molded PP composites with 30% flax fibers. They report tensile and flexural moduli increased by 20% and 5%, respectively, as a result of fiber acetylation. The respective strengths were found increased by 15% and 10%, while notched Charpy IS was reduced by ∼ 10%. In combination with matrix-based compatibilization by MA-PP addition (“Maleic Anhydride Grafted Polyolefins as Matrix Additives” section), tensile and flexural strength could be further improved, while IS was further reduced. Grüneberg et al. have prepared PP-based WPCs (60% wood content) using particles made from commercial treated and untreated wood. They did not find an influence on tensile properties as a result of using acetylated particles, but unnotched Charpy IS was reduced by ∼ 25%. On the other hand, water absorption upon 14 days of immersion was reduced by 50%. Summarizing these results (Figure 13), it has to be said that there is not enough data at the moment to come to a concluding statement on the effectiveness of acetylation for the improvement of NFCs and WPCs. However, the method seems to have some potential and has the benefit of already being applied industrially to solid wood.
Maleic Anhydride Treatment of Fibers/Fillers
In efforts to increase chemical compatibility between natural fibers/fillers and polyolefins, maleic anhydride (MA) can be applied. The objective of this approach is to produce ester bonds between MA and OH-groups on the fiber/filler surface, thus reducing its hydrophilicity. Fiber/filler pretreatment with MA is usually performed applying an organic solution of the chemical to the reinforcement. The solvent can be acetone39, 48 or xylene,94 for example. By employing FT-IR analysis, Biagiotti et al. found indications for such an esterification reaction upon MA treatment of flax pulps (more intense peak at 1735 cm−1). This group also reports reduced surface polarity of treated flax fibers, as determined by contact angle measurements.39
As to the influence of MA-pretreatments on mechanical properties, relatively modest effects are reported in the literature. For tensile and flexural strength, Biagiotti et al. found results between −8% and +12% for a PP-30% flax system. Tensile and flexural moduli are reported to increase by approximately 10%–15% upon MA-pretreatment of the reinforcement constituent.39 Investigating the same composite system, Arbelaiz et al. have reported effects of MA-pretreatments on tensile and flexural properties in the same range.48 Deploying MA via the matrix rather than via the fiber, significantly higher improvements were achieved (“Other Matrix-Based Strategies”). Effects similar to those reported for PP-30% flax have been detected by Nunez et al. in a PP-50% wood system. For impact strength, the group reports notched Izod IS reduced by 20% as a result of their MA-pretreatment.54
Other Fiber-Based Strategies
Other strategies reported in the literature which are aimed at improving the fiber–matrix interaction employ a paper wet-strength additive,95 stearic acid,35, 43 acrylic acid and benzoyl chloride,35, 46, 50 potassium permanganate and toluene-2,4-diisocyanate (TDI, KMnO4),51, 87 polymethylene (polyphenyl isocyanate) (PMPPIC)87 and polymeric methylene diphenyl diisocyanate (PMDI).96 Furthermore, m-phenylene bismaleimide (mPBM),97 bleaching (sodium hypochlorite treatment),23 octanoyl chloride,98 O-hydroxybenzene diazonium salt (HBDa)99 heat treatment,24 or enzymes32 have been applied.
Geng et al. have investigated the suitability of the paper-wet-strength additive Kymene 557H as a compatibilizer in a HDPE-40% wood system. Various pretreatment regimes were tested. Combining fiber pretreatment with addition of stearic anhydride to the matrix, tensile strength, and stiffness could be improved by 35%.95
In a PP-25% flax system, fiber pretreatment with stearic acid did not influence tensile strength, but reduced both tensile and flexural stiffness by ∼ 10%.43 Similar results were achieved by Danyadi et al. for a PP-20% wood system.35 Acrylic acid (AA) and benzoyl chloride (BC) have been tested as compatibilizers in a PP-40% wood composite by Farsi46 and Gashemi and Farsi50 Only minor changes upon pretreatments were reported by Farsi (Increases in strength between 10% and 20%, decreases in Izod IS around 15%). The latter group, however, found tensile modulus and unnotched Izod IS enhanced by 40% and 25%, respectively, upon AA pretreatment, while the BC pretreatment was less effective. AS a result of filler pretreatment with BC, Danyadi et al. reported tensile properties of a PP-20% wood composite reduced by 20%, while WA upon 500 hr of immersion was reduced by 85%.35
KMnO4 has been employed as a sisal fiber pretreatment agent by Joseph et al., for both LDPE-30% fiber51 and PP-20% fiber87 as composite systems. While for the first system, considerable improvements in tensile properties were achieved (tensile strength/modulus +48%/+85%), the results for the latter system were less encouraging (tensile strength/modulus +10%/−27%).
The same group has also employed TDI as a compatibilizer to improve the properties of the polyolefin-sisal composites mentioned earlier. As for KMnO4, while for the LDPE-based system, considerable improvements were achieved (tensile strength/modulus +33%/+57%), the approach was less successful for the PP-based system (tensile strength/modulus +9%/−74%).
PMPPIC was tested as a fiber-based compatibilizer by Joseph et al., the results being published in the paper cited already.87 Pretreatment of the sisal fibers improved tensile strength of the resulting PP composites by 10%, but reduced the respective modulus value by 40%. Pretreatment of a wood filler with PMDI was performed by Geng et al.96 In the resulting HDPE composites with 40% filler content, tensile properties were found increased by 25%. However, simple addition of the PMDI in the compounding step was even more effective (Isocyanate-Functionalized Polymers as Additives).
Sain and Kokta have pretreated chemithermomechanical pulp (CTMP) with mPBM. PP composites with 35 m% fibers were prepared by melt compounding and compression molding. The authors report an increase in tensile strength and modulus of 77% and 20%, respectively. On the other hand, unnotched Charpy IS was reduced by ∼ 20%.97
Soleimani et al. have, besides mercerization, also investigated bleaching (meaning a sodium hypochlorite treatment) as a method for flax fiber modification. In PP composites containing 30% fibers, significant improvements in material properties were found. Tensile and flexural strength were improved by 50% and 20%, respectively. The corresponding moduli were increased upon the bleaching by 25% and 30%, respectively. Furthermore, IS as determined by a tensile test was increased by 50%, and water absorption upon 24 hr of immersion was reduced by ∼ 25%.23
Zhang et al. have investigated PP-based WPCs with 60% wood content. The wood particles were treated with octanoyl chloride (C8Cl), employing DMF and chloroform as solvents, respectively. For both treatments, flexural properties of compression molded specimens decreased significantly compared with WPCs containing untreated wood. Nevertheless, water absorption upon 24 h of immersion was reduced by 80% and 60%, respectively.98
Islam has employed an o-hydroxybenzene diazonium salt pretreatment to render coir fibers a higher compatibility with PP. In composites with 25% coir fibers, this led to improvements in mechanical properties of up to 15% (tensile strength). WA, on the other hand, could be reduced by 25%.99
Kaboorani et al. have assessed the feasibility of using heat-treated wood for HDPE-based WPC production (25/50% wood content). Wood particles were treated at different temperatures (175°C–205°C) and compounded with HDPE, specimens were prepared by injection molding. While tensile strength could be slightly increased, Young's modulus was reduced (both by ∼ 5%) for the WPCs containing 50% wood particles. In combination with MA-PP, strength could by increased by 50%, while the modulus was further reduced. Heat treatment at 175°C proved most favorably, judged by the properties of the resulting composites.24
Bledzki et al. have investigated two enzyme-based modification methods as applied to abaca fibers. PP-compounds with 30% of the treated or untreated fibers were produced in a heat-cooling mixer system, and specimens prepared by injection molding. The objective of the treatments applied was twofold. First, the fiber bundles should be separated to individual fibers, so that their higher strength might be exploited. Furthermore, such a mainly longitudinal separation would also increase the aspect ratio of the reinforcement. Second, a removal of undesired, loosely bonded substances on the fiber surface (like fats and waxes) should also improve the fiber–matrix interaction. Thus, the treatments would enhance composite mechanical properties both by improving the fiber–matrix interaction and fiber aspect ratio. Two approaches to the desired modification were tested: In the first, fibers were collected from elephant dung, washed and dried. Thus, the natural digestive system (NDS) of the animals was used as a “reactor.” In the second approach, an enzyme mixture called “fungamix” was applied as an aqueous solution. With both treatments, material properties of the PP-30% abaca composites could be improved significantly. For the NDS approach, tensile and flexural strength were increased by ∼ 5%–10%, while notched Charpy IS was enhanced by ∼ 15%. With the fungamix enzyme, tensile, and flexural strength could be improved by 45% and 35%, respectively, along with an increase of notched Charpy IS by ∼ 25%. For both treatments, water absorption upon 90d of exposure to 95% rh was reduced by ∼ 45%.32