Maleated nanolignin as a new coupling agent for wood-plastic composites

Abstract

In this work, the influence of maleic anhydride-treated nanolignin as a coupling agent on the various properties of wood fiber-polypropylene composites was investigated. For this purpose, extracted lignin from Bagasse Soda black liquor was modified with maleic anhydride and then different ratios of unmodified and modified nanolignin (1, 3 and 5 wt%) were added to the wood fiber-polypropylene mixture. The composites were then manufactured by injection molding after mixing mechanically to the wood flour either virgin or maleated nanolignin and polypropylene. Structural and thermal properties of nanolignin after modification by maleic anhydride along with physical and mechanical properties of the manufactured composites were determined according to standard methods. The FTIR analysis indicated nanolignin rearrangements showing that some bonds changed or formed after its modification with maleic anhydride. The glass transition temperature of the nanolignin decreased by esterification from 130°C to 100°C. The panel testing results indicated that all physical and mechanical properties of the composites were continuously improved by increasing the maleated nanolignin content from 1 to 5wt%. Thus, the composites with maleated nanolignin presented higher dimensional stability and mechanical strength compared to those prepared with unmodified nanolignin. Scanning electron microscopy (SEM) showed that treatment with maleic anhydride improves the uniform dispersion of nanolignin in the polymer matrix. The manufactured WPCs can be used as interior and exterior building materials such as decks, sidings and etc.

Keywords

Coupling agent composites nanolignin maleic anhydride physical and mechanical properties

Introduction

In recent years, the applications of natural fiber reinforced polymer-based composites (WPCs) are growing due to their special properties.^1,2 For this reason, to improve the performance of wood-based composites different methods have been proposed.^3,4 One of the best methods proposed is the use of new coupling agents of higher quality than common coupling agents such as maleic anhydride grafted polymer (MAH-g) in the composite.^5,6 However, although MAH-g can improve the most properties of WPCs these types of coupling agent have high cost.⁵ Hence, finding a new coupling agent having a better performance and less expensive than MAH-g is topical to increase the performance of WPCs.

Conversely, several million tons lignin is produced by the pulp and paper industry at year which are unfortunately almost all used only as a fuel. One of the foremost applications of lignin is to use it as a coupling agent in WPCs.^7,8 However, the previous research has shown that WPCs with lignin as coupling agent had lower mechanical strength than those prepared with MAH-g. The main challenge about the applications of lignin as coupling agent is related to its complicated and changeable macromolecular structure.⁹ One way to deal with this problem is to convert unordered raw lignin into uniform nanoparticles. Younesi-Kordkheili and Pizzi⁸ indicated that WPCs with nanolignin as coupling agent presented comparable physical properties as the control samples prepared using maleic anhydride-polypropylene (MAPP). However, the strength of the composites made from nanolignin was still weaker than those prepared with MAPP as coupling agent.

So far various types of grafted coupling agents are widely used to strengthen composites containing fillers and fiber reinforcements. Grafting maleic anhydride to the lignin molecules is one of the best methods to use the advantages of both nanolignin and maleic anhydride. Younesi-Kordkheili⁸ showed that maleated lignin improves the properties of the wood-based panels made from thermoset resins. Hence, the aim of the research work presented here is to investigate the performance of maleated nanolignin as a new coupling agent on the properties of WPCs compared to MAH-g as a common coupling agent.

So far the effect of adding maleic anhydride grafted polymer as coupling agent on the composites has been studied by several research groups.^10,11 There are also some research works on the influence of lignin and nanolignin on the properties of WPCs.^7,12 However, there is no information in the literature on the effect of maleated nanolignin as a coupling agent on the properties of natural fiber-polymer composites.

Materials and methods

Materials

Wood flour as waste from wood industries was sieved to path mesh 60/40 and then oven dried at 103°C for 24 h. Polypropylene (PP) with a grade of SI-080 and a melt flow index (MFI) of 9 g/10 min was prepared from Tabriz Petrochemistry. Bagasse Soda black liquor at pH = 13.5 and 70% solid content to be used as a lignin source of nanolignin was obtained from the Pars Company (Haft Tepe, Iran). Lignin was extracted by sulfuric acid. For that purpose, soda lignin was precipitated from black liquor by adding very slowly concentrated (36N) sulfuric acid, until the pH of the resultant solution was close to 2. The precipitate was then thoroughly washed with water until the pH of the rinse was close to 6. Finally, the obtained lignin was dried in an oven at 60°C for 24 h, after which a dark-brown powder was recovered. Maleated Polypropylene (MAPP) was obtained from the Kimia Javid Factory (Isfahan, Iran) with a melt flow index (MFI) of 100 g/10 min (T = 230°C, load = 2.16 kg).

Methods

Preparation of nanolignin

1.4 g lignin was dissolved in 50 mL ethylene glycol, the insoluble impurities were removed by filtration after 4 h stirring, and then hydrochloric acid (4.00 cm³, 0.025 mol L⁻¹) was added to the lignin filtrate solution at the rate of four drops per min. The solution was dialyzed in a 3 L beaker for 3 days with water changed three times a day. The lignin nanoparticles were recovered after precipitating in dilute HCl at pH 2, followed by centrifugation and ultrasonic cleaning to achieve neutral pH.¹³ The specific surface area and average pore diameter of lignin and nanolignin were measured at 200°C by BET method.

Modification of nanolignin

Maleated nanolignin was obtained according to the method of Younesi-Kordkheili et al.¹⁰ First, 15 g constant weight nanolignin was dissolved with DMSO (50 mL) in a four-necked flask fitted with a mechanical stirrer and a reflux condenser. As a catalyst for the esterification reaction 200 μL 1-methylimidazole was added dropwise to the nanolignin solution, and 20 g maleic anhydride was immediately introduced into the reaction system. Subsequently, the suspension was heated to 80°C and stirred continuously for 3 h. After that, the solution was cooled down and precipitated at pH three in order to recover the maleated nanolignin. The solid was continuously washed with water to remove the unreacted maleic anhydride. Finally, the residual solids were dried in an oven at 60°C.

Fourier transform infrared spectrometry

The changes in the chemical structure of nanolignin after modification with maleic anhydride have been analyzed by Fourier Transform Infrared spectrometry (FTIR) (Shimadzu FTIR 8400S, Japan). FTIR spectra were obtained from KBr pellets with 1 mass% of the powdered lignin at wave numbers in the 400 and 4000 cm⁻¹ range.¹⁴

Differential scanning calorimetry analysis

The changes in glass transition temperature (T_g) of oven dried nanolignin before and after of modification by maleic anhydride were determined by DSC using a NETZSCH DSC 200 F3Modelthermal analyzer according to the method of Younesi-Kordkheili et al.⁷ The DSC scans were recorded at a heating rate of 10°C/min under nitrogen atmosphere with a flow rate of 60 mL/min. To determine the T_g, about 5 mg of freeze-dried sample was added to the aluminum pan. The samples were then heated from ambient temperature (25°C) to 200°C under a nitrogen atmosphere. The DSC cups were closed during the whole temperature range investigated.

Composite preparation

Wood-plastic composites were manufactured according to the method of Younesi-Kordkheili et al.² Polypropylene (PP), unmodified and maleated nanolignin (at ratio of 25:1 wt%) or MAPP as a coupling agent, and wood flour were compounded in a mixer (HBI System 90, USA) according to Table 1. The scheme of the composite formation is showed in Figure 1. The mixing was carried out at 180°C and 40 revolutions per minute (RPM) for 10 min with a HAAKE mixer (HBI System 90, USA). The compounded materials were then ground using a pilot scale grinder (WIESER, WGLS 200/200 Model). From the compounds which had been granulated, specimens were injection molded (Imen machine, Iran) at a molding temperature of 180°C, and an injection pressure of 3 MPa. The specimens were stored under controlled conditions (50% relative humidity and 23°C) for at least 40 h prior to testing.

Table 1.

The different components of prepared composites.

Code	PP (%)	MAPP (%)	Nanolignin (%)	Maleated nanolignin (%)	Wood flour (%)
WPC	40	0	0	0	60
WPCM	38	2	0	0	60
WPCL1	39	0	1	0	60
WPCL3	37	0	3	0	60
WPCL5	35	0	5	0	60
WPCnL1	39	0	0	1	60
WPCnL3	37	0	0	3	60
WPCnL5	35	0	0	5	60

Water absorption and thickness swelling of the samples

Water absorption and thickness swelling of the panels were measured according to the ASTM D-7031-04 standard. Five specimens from each type of panels were taken and dried in an oven for 24 h at a temperature of 100 ± 3°C. The weight and thickness of dried specimens were measured with an accuracy of 0.001 g and 0.001 mm, respectively. The specimens were then immersed in distilled water for 24 h and kept at a temperature of 20 ± 2°C. The weight and thicknesses of the specimens were then measured after wiping the excess of water from their surface. The values of the water absorption in percentage were calculated using the following equation:

W A (t) = \frac{W (t) - W_{0}}{W_{0}} \times 100

(1)

where WA (t) is the water absorption (%) at time t, W_O is the oven dried weight and W (t) is the weight of specimen at a given immersion time t.

Also the values of the thickness swelling in percentage were calculated using equation (2).

T S (t) = \frac{T (t) - T_{0}}{T_{0}} \times 100

(2)

where TS (t) is the thickness swelling (%) at time t, To is the initial thickness of specimens, and T (t) is the thickness at time t.

Mechanical tests

The bending and tensile tests of the composites were done according to the ASTM D790-03 and ASTM D 638 specifications respectively, using an Instron universal testing machine (Model 1186, UK). Impact tests were carried out according to ASTM-D256 by using an IZOD testing machine with Model Zwick 5102.

Scanning electron microscopy

The morphology of the composites was examined using a scanning electron microscope (XL30) supplied by the Philips Company Limited (the Netherlands). The fracture surfaces of the specimens after the bending test were sputter-coated with gold before analysis. All images were taken at an accelerating voltage of 17 kV.

Results and discussion

FTIR analysis

The infrared spectra of the nanolignin before and after modification with maleic anhydride are showed in Figure 2. A comparison between the FTIR spectra of the two types of nanolignins showed that some main peaks varied when incorporating maleic anhydride into a nanolignin. A comparison between the IR spectra of nanolignin and maleated nanolignin indicated that the intensity of the 1700 cm⁻¹ which is related to the -COOH group increased in the maleated nanolignin, a clear indication of the anhydride ring opening to form maleic acid.⁸ The intensity of the 2800 cm⁻¹ band related to the C-O groups and 1200 cm⁻¹ band assigned to the C = C bonds also increase in maleated nanolignin, this again being due to the addition of maleic anhydride. Furthermore, the nanolignin modified with the maleic anhydride showed a smaller peak at 3420 cm ⁻¹ (the hydroxyl group) than the virgin nanolignin. The decreasing in intensity of –OH groups is probably due to the esterification of the free –OHs groups of lignin by the carboxylic acid functions of the maleic acid generated by the hydrolysis of maleic anhydride.¹⁵ This confirms that chemical reactions have occurred and that maleate groups or carbonyl groups replace the hydroxyl groups in the nanolignin structure. The possible reactions between nanolignin and maleic anhydride are shown in Figure 3. Diels-Alder condensation reactions between nanolignin units are also in principle possible (Figure 3) but the conditions of reaction used indicate that these are much less likely to occur, or not at all, and no clear evidence for them is observed in the FTIR spectra.

The introduction of maleic anhydride in nanoloignin and in lignin can and does occur according to both mechanisms shown in Figure 3. While it is difficult to determine which is in the majority it must be pointed out that once the anhydride has reacted in both manners with the lignin the final results is almost the same. This is so, because the product obtained in the first reaction by Diels-Alder reaction will open the anhydride ring to esterify other lignin units. Thus, while the routes are different the final result isnot too different for route one to what shown in the second route in Figure 3.

DSC analysis

Differential scanning calorimetry is widely used to measure the glass transition temperature (T_g) of polymers, this indicating the temperature of chain segment motion. Figure 4 shows the changes in heat flow of the nanolignin alone and of the maleated nanolignin. The difference in heat flow, which is lower than the one of nanolignin (130°C) compared to that of the maleic anhydride-treated (100°C) cases is probably due to a number of different reasons. The T_g of lignin is affected by several factors, such as the presence of low molecular weight contaminants (including water and solvents), molecular weight, thermal history, amount of crosslinking, degree of stiffness etc.¹⁶ Even the flexibility of the molecule and the interactions between molecules affect T_g directly. The strong hydrogen bonds between esterified nanolignin molecules contributed to the stiffness of the molecules, resulting in higher T_g than that of unmodified nanolignin. The previous work has indicated that the reaction of maleic anhydride on lignin causes an increase in its glass transition temperature.¹⁷

Mechanical properties

The flexural and tensile modulus of the composites prepared is shown in Table 2. It can be seen that the addition of every types of coupling agents (nanolignin and MAPP) increased the flexural and tensile modulus of WPCs. The results so obtained indicated that WPCs containing 5% maleated nanolignin had the highest flexural modulus (2975 MPa) and tensile modulus (2803 MPa) while those without any coupling agents presented the weakest flexural modulus (1803 MPa) and tensile modulus (1978 MPa). Nanolignin can fill the cracks and the voids in the matrix and wood flour-PP interface which were produced in the process, thus resulting in an improvement in flexural and tensile modulus. The reduction in the particle size to nanoscale also contributes to increase the modulus because of the larger surface area generated.¹⁸ As shown in Table 2, the composites made from maleated nanolignin had higher flexural and tensile modulus compared to the composites with virgin nanolignin at constant level of nanolignin. The differences between the modulus of the composites containing maleated nanolignin and those with unmodified nanolignin can be related to the difference in the chemical reactivity of the two nanolignins. Maleic anhydride (MA) is an α, ϐ unsaturated carbonyl compound, that contains one carbon-carbon double bond. Modified nanolignin with MA containing carbon-carbon double bonds and carboxyl groups can be linked with other monomers by radical polymerization or esterification, thus introducing new material properties. The double bonds of the maleated half-esters can react with nonpolar polymer, resulting in an increase in stiffness of the composites. Conversely, the results obtained show that the addition of MAPP improves the flexural and tensile modulus of the composites (Table 2). The greater stiffness of WPCs by addition of MAPP can be attributed to the efficient stress transfer between wood and polymer and improves the dispersion of wood flour in the polymer matrix. So far several papers reported that a higher flexural modulus value of WPCs can be achieved by addition of MAPP.^10,11 There are not significant differences between the flexural and tensile modulus of the WPCs with 3% maleated nanolignin and those made from 2% MAPP.

Table 2.

The different mechanical properties of the prepared composites.

	Flexural modulus (MPa)	Tensile modulus (MPa)	Flexural strength (MPa)	Tensile strength (MPa)	Impact strength (J/m²)
WPC	1803 ± 112	1978 ± 100	22 ± 3	15 ± 2	800 ± 34
WPCM	2408 ± 143	2503 ± 92	34 ± 6	21 ± 3	1250 ± 66
WPCL1	2023 ± 107	2354 ± 89	25 ± 1	18 ± 2	1000 ± 54
WPCL3	2398 ± 102	2426 ± 100	31 ± 4	20 ± 4	1322 ± 31
WPCL5	2536 ± 97	2672 ± 87	38 ± 2	23 ± 1	1492 ± 60
WPCnL1	2125 ± 90	2451 ± 49	28 ± 2	20 ± 3	1113 ± 72
WPCnL3	2806 ± 67	2657 ± 94	35 ± 3	24 ± 3	1541 ± 43
WPCnL5	2975 ± 77	2803 ± 82	43 ± 2	28 ± 2	1715 ± 20

The flexural and tensile strengths of the prepared composites are shown in Table 2. The composite made from maleated nanolignin exhibited a higher flexural and tensile strengths than those made with unmodified nanolignin. The addition of unmodified/maleated nanolignin up to 5 wt% dramatically enhanced both tensile and flexural strengths of the composites. The strength of the composite is strongly dependent on the bonding strength of the wood surface to the polymer matrix. The degree of compatibility of the wood flour with the polypropylene matrix can also markedly influence the mechanical properties of the composites.¹⁹ Many esterified nanolignins were utilized in blends to get better compatibility with other natural or synthetic polymers, or to compatibilize the phases of polymer blends. In this respect unless surface secondary forces are altered, and these cannot be determined at this stage, it is unlikely that the interaction of the maleated nanolignin is with the polypropylene, this due to its hydrophobic characteristics. The interaction of some of the nanolignin is most likely with the wood constituents as once the anhydride linked to a nanolognin unit is partially open the carboxylic acid group formed by the opening of the anhydride can further react with another nanolignin unit, but also with the wood lignin and the wood carbohydrates, esterifying especially these last due to their abundace of –OH groups presenting an alcohol character. The double bonds introduced by esterification and carbonyl groups also provided reactive sites for further chemical modifications or strong molecular interactions (hydrogen bonds) for blending. In the meantime, the double bonds can react with nonpolar polymers, which may counterbalance its lower compatibility with these polymers. Moreover, due to the maleated nanolignin high reactivity, it cross-links well in the composites and improves the bonding between the various constituents. Contrary to nanolignin in which only a physical mechanism is present, in the malated nanolignin both chemical and physical mechanisms can affect the dimensional stability and mechanical properties. SEM micrographs of the composites prepared also showed that the esterification caused uniform dispersion of the nanolignin into the polymer matrix (Figure 5). The particle distribution has a significant effect on the stress distribution of the polymer matrix.^20–22 As shown, maximum flexural and tensile strengths of the composites were respectively 43 MPa and 28 MPa for composites with 5 wt% maleated nanolignin, while minimum flexural and tensile strengths are approximately 22 MPa and 15 MPa for the control samples, respectively. The results so obtained also showed that adding MAPP improves the strength of the composites.

The impact strength of the composites prepared is reported in Table 2. The addition of both types of coupling agents (nanolignin and MAPP) improves the impact strength of the WPCs prepared. The composites made without coupling agent also exhibited the lowest impact strength (800 J/m²). The highest impact strength (1715 J/m²) relates to the WPCs with 5 wt% maleated nanolignin. It can be observed that the impact strength of the composites with maleated nanolignin was stronger than those made virgin nanolignin. The composites with 1, 3 and 5 wt% maleated nanolignin had 11, 17 and 15% higher impact strength than the composites with unmodified nanolignin, respectively. It can be seen from Table 2 that the impact strength of the composites increases when the proportion of maleated nanolignin increases from 1 to 5 wt%. The increasing impact strength of composites with addition of maleated nanolignin is due to the improvement of the wood–polypropylene interaction. The bridging linkages mechanism of maleated nanolignin prevents the spread of cracks and increases the impact strength of the prepared composites. As can be seen from Table 3, the specific surface area of lignin is 0.05371 m² g⁻¹, which is only about one tenth of the specific surface area of nanolignin (6.1766 m² g⁻¹). The pore volume of nanolignin is 0.0098 cm³ g⁻¹, which is 14 times the pore volume of lignin (0.0007 cm³ g⁻¹). The high specific surface area and porous structure of nanolignin increase the contact between nanolignin and other components, which results in the enhanced reactivity of nanolignin.

Table 3.

Specific surface area, average pore volume, and pore size of lignin and nanolignin.

Sample	Specific surface area (m² g⁻¹)	Average pore volume (cm³ g⁻¹)	Average pore size (nm)
Lignin	0.05371	0.0007	8.85
Nanolignin	6.1766	0.0098	5.99

Water absorption and thickness swelling

The water absorption and thickness swelling of the composites made from maleated and unmodified nanolignin is shown in Figures 6 and 7, respectively. These figures show that the addition of modified/unmodified nanolignin as a coupling agent can improve the composites dimensional stability. Decreasing water absorption and thickness swelling of the WPCs by addition of nanolignin can be explained by the hydrophobic nature of nanolignin which acts as a water repellent.⁸ Moreover, the cracks and voids which formed in the WPCs manufacturing process were filled with nanolignin and resulted in the prevention of the penetration of water into the inner parts of the composites. Figures 6 and 7 also show that modification of nanolignin by maleic anhydride reduced further water absorption of the composites than unmodified nanolignin. The water absorption of the composites with 1, 3 and 5 wt% maleated nanolignin was 16, 17 and 36% lower than those with unmodified nanolignin. Lower water absorption and thickness swelling of the composites containing maleated nanolignin can be related to its higher number of reactive sites in esterified nanolignin than in unmodified nanolignin. The higher reactivity of the maleated nanolignin improves the bonding of nanolignin to wood flour and polypropylene and improves dimensional stability of the composites. Increasing the proportion of maleated nanolignin from 1 to 5 wt% increases the amount of linkages between the hydroxyl groups of wood flour and polypropylene. Based on the finding of this research work, the lowest water absorption was observed for the panels made with the addition of 5% maleated nanolignin while the highest absorption value occurred for those made without coupling agents. So far several researchers have indicated the positive effect of lignin as bio-coupling agents on the dimensional stability of the WPCs.⁷

Finally it can be noted that the T_g of nanolignin can be influenced on the mechanical properties and dimensional stability of the composites prepared. To determine the exact relationship between the T_g behavior of nanolignin and the properties of WPCs does need deeper studies.²³

Conclusion

In this study, the influence of maleated nanolignin as a coupling agent on the various properties of wood flour-polypropylene composites was investigated. FTIR analysis indicated that modification of nanolignin with MA changed the chemical bonds in the structure of nanolignin. DSC analysis showed that the glass transition temperature (T_g) of nanolignin decreased from 130°C to 100°C by esterification with maleic anhydride. The composites containing maleated nanolignin presented better dimensional stability and mechanical strength compared to those prepared with unmodified nanolignin. Greater dimensional stability and mechanical strength could be achieved by increasing the maleated nanolignin content from 1 to 5wt%. Contrary to nanolignin in which only a physical mechanism is present, in the maleated nanolignin both chemical and physical mechanisms can affect dimensional stability and mechanical properties. SEM micrographs of the prepared composites indicated that the MA-treatment caused uniform dispersion of nanolignin in the polymer matrix. The effect of other modification methods of nanolignin on the properties of WPCs will be investigated in the future studies.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hamed Younesi-Kordkheili

Data availability statement

The data that support the findings of this study are available from the corresponding author on reasonable request.

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