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Copolymerization of Lignin and Glyoxal as New
Plasticizer for Improving the Properties of Bagasse Fiber- based Composites
Hamed Younesi- Kordkheili
Department of Wood and Paper Sciences and Technology, Faculty of Natural Resources, Semnan University, Semnan, Iran
Email: [email protected]
Abstract
Improving the physical and mechanical properties of experimental cement type panels made by reinforcing them with bagasse fiber and by addition of the graft copolymerization product of lignin and glyoxal was the objective of this investigation. Lignin was modified with glyoxal and then the different ratios of glyoxalated lignosulphonate (0, 10, 15 and 20 wt %) were added to the mix of bagasse fiber and cement. For comparison, the same panels were made with unmodified lignin at the same ratios. FTIR analysis indicated that some main chemical bonds were changed by modification of lignin with glyoxal. The water absorption and thickness swelling of the composites decreased continuously with increasing amounts in the panels of both virgin and modified lignin from 10 to 20 %. From the results obtained, it appears that the composites with glyoxalated lignin present improved dimensional stability and mechanical strength (MOE and MOR) than the panels made from unmodified lignin.
Higher panels mechanical strength could be achieved by increasing the amount of lignin in the composites.
Keywords: Bagasse fiber-cement composites, lignin, bending properties
Introduction
Nowadays, the use of natural fibers as a reinforcing agent in cement- based composites is growing.
Compared to synthetic fibers most natural fibers have specific properties such as low cost and density, lower pollution during production resulting in minimal health hazards and are environment friendly.
However, previous studies have shown that natural fiber-cement composites usually exhibit lower dimensional stability, mechanical and thermal properties than those made with synthetic fibers (Younesi-Kordkheili et al. 2012). In recent years, there have been considerable efforts to decrease defects and develop natural fiber reinforced cement-based composites for production of affordable structural units (Blankehorn et al. 2001; Qi et al. 2005). For this reason, many modification studies have been carried out in order to improve the composites performance. One of the best methods proposed is the use of plastisizers such as sulfonated acetone-formaldehyde (SAF) resins, and sulfonated phenolic resins (SPF) superplasticizers. So far several researchers have indicated that physical, mechanical and performance of natural fiber- cement composites can be improved by addition of plastisizers (Hospodarova et al. 2018; Hospodarova et al. 2017). However, not only most plastisizers are expensive but they are also toxic due to the presence of formaldehyde. For this reason, researchers try to find a new green, high quality plastisizer for natural fiber-cement composites.
Conversely, more than 100 million tons of black liquor (as the main source of lignin) are produced yearly at the world pulp and paper mills. Traditionally, lignins were only used as a fuel to heat the pulping boilers. Recently this by-product is used for higher value industrial applications such as
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biomaterials, bioacids, adhesives, health products etc. in the developed countries. One of the best applications of black liquor is to be used as additive for concrete admixtures (Chang and Chan 1995;
Kamoun et al. 2003; Nadif et al. 2002). Previous studies have shown that lignosulfonate which is obtained from the sulfite pulping method exhibited the best results among all industrial lignins (Kraft, Soda, Organosolv and etc.) as a plasticizer in the cement-based composites (Arel and Aydin 2017).
Because of its structure, with specific functional groups such as SO32-, OH-, and O-, lignosulfonate has a high level of reactivity and has been widely used for producing water-reducing admixtures (Yousuf et al. 1995). However, because of the lower mechanical properties of the composites made from lignin compared to those with chemical plasticizers, lignosulfonate cannot completely meet the requirements for modern composites. The methods of modification used at present to develop lignin as plasticizer in wood-cement composites consists of increasing the number of sulfonic groups, increasing the molecular weight, and copolymerization with other (Areskogh et al. 2010; Ouyang and Qiu 2006; Pei et al. 2008). For this reason, many modification studies have been carried out to improve lignin performance for this application (Younesi-Kordkheili et al. 2015). Younesi-Kordkheili et al. (2015) have shown that graft copolymerization of lignin and glyoxal (as a nontoxic di-aldehyde) is one the best methods to increase the molecular weight and the reactivity of lignin. Results of previous studies have shown that phenol and formaldehyde (as a much more toxic aldehyde) -based plasticizers can be used in cementitious composites. Thus, the research presented here focused on the effect of glyoxalated lignin (GL) as a green plasticizer (thus nontoxic) on physical and mechanical properties of natural fiber-cement composites.
So far the effect of lignosulfanate addition on cement-based composites has been studied by several researchers (Ouyang and Qiu 2006). There are also some research works on the influence of phenol and formaldehyde-based plasticizers on natural fiber-cement composites (Chen et al. 1999). However, there is no information in the literature on the effect of glyoxalated lignin as a green copolymer on the properties of natural fiber-cement composites. Thus, the aim of the research work presented here was to investigate the influence of the addition of glyoxalated lignosulfonate as a plasticizer on some main properties of the natural fiber- cement composites.
2. Materials and Methods 2.1 Materials:
Cement: Commercially manufactured Portland cement type 2 was supplied by Kiasar Cement Industrial Company, Sari, Iran.
Bagasse fibers:Bagasse fibers were supplied from local company which used the fibers in paper pulp preparation. Fibers were initially soaked into water for two days to enhance overall disintegration quality of the material before they were dried in a laboratory type oven at a temperature of 80 °C for three days. Then this fibers were used in the composite panel production.
Lignin:Calcium Lignosulfanate powder was prepared by Negin Tejarat Payam Co, Iran.
2.2 Methods:
Modification of lignin: Lignin glyoxalation was performed according to the method of Navarrete et al. (2012). Lignin powder (295 parts by mass, 96% solid) was slowly added to 476.5 parts water, and sodium hydroxide solution (30%) was added from time to time to keep the pH of the solution between 12 and 12.5 for better dissolution of the lignin powder; this was also facilitated by vigorous stirring. A total of 141 parts by mass of sodium hydroxide solution (30%) was added, which resulted in a final pH close to 12.5. A 250 ml flat bottom flask equipped with a condenser, thermometer and magnetic stirrer bar was charged with the previous solution and heated to 58°C. Glyoxal (175 parts by mass, 40% in water) was added, and the lignin solution was then continuously stirred with a magnetic stirrer/hot plate for 8 h. The solids content for all glyoxalated lignins (GL) was around 31%.
Fourier Transform Infrared spectrometry (FTIR): The changes in the structure of lignin molecules before and after modification with glyoxalation have been analyzed by Fourier Transform Infrared
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spectrometry (FTIR) analysis (Shimadzu FTIR 8400S, Japan). FTIR spectra were obtained from KBr pellets with 1 mass% of the powdered resins at wave numbers in the 400 and 4000 cm-1 range.
Composite preparation: Portland cement, water, CaCl2, bagasse fiber and unmodified/ glyoxalated lignin at three levels (10%, 15%, and 20 wt% by mass based on dry cement weight) were mixed to obtain the composites. A rotating drum was employed to convert into a homogeneous compound for 5 min. Hence six types of panels with different ratios of raw materials were prepared as displayed in Table 1. Amount of CaCl2 and bagasse fiber were fixed for all types of the samples. Also the panel without any lignin was also manufactured as control sample. The ratio of cement to water was 2:1.
Water of mixture was eliminated by applying vacuum to form the mats .After mat preparation; the mixtures were compressed at a pressure of 30 kg/cm2 at room temperature for 24 hr to composite manufacturing. Later the samples were kept for 24 days in an ambient temperature before they were dried at temperature of 75 °C for 10 hr in a laboratory type of oven to cure them.
Table 1- The different components of prepared composites
Water Absorption and Thickness Swelling of the Samples: Water absorption and thickness swelling tests of the panels were performed according to ASTM C 67-03a standard.
Mechanical Tests: Three-point bending of the panels was carried out according to procedure stated in ASTM C 67-03a standard by employing an Instron 1186 with load cell 20 KN. The impact strength of the samples was carried out using equipment Model Zwick 5102.
3. Results and Discussion
FTIR Analysis of lignin: The FTIR spectra of unmodified and modified lignin are shown in Figure 1.
The peak assignments of the FTIR spectra of lignin are summarized in Table 2. After glyoxalation, the 3440 cm-1 band representing the hydroxyl groups (-OH) in the phenolic and aliphatic parts of the lignin increased compared to unmodified lignin (Figure 1). Increasing the proportion of hydroxyl groups in glyoxalated lignin increases the molecular weight of lignin and results in an improvement in lignin reactivity of lignin. Figure 1 also indicates that the content of C=O groups represented by the 1710 cm-1 band significantly increased in glyoxalated lignin compared to untreated lignin. This is probably due to the formation of chemical bonds between glyoxal and lignin as indicated also by previous studies on the glyoxalation of lignin (Navarrete et al. 2013; Navarrete et al. 2012).
CaCl2)%(
Glyoxalated Lignin (%)*
Lignin )%(
Bagasse fiber
(%) Cement )%(
Code
3 0 -
15 82
CB
3 10 -
15 82
CB10L
3 15 -
15 82
CB15L
3 20 -
15 82
CB20L
3 10
15 - 82
CB10GL
3 15
15 - 82
CB15GL
3 20
15 - 82
CB20GL
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Figure 1. FTIR analysis of lignosulfanate before and after modification by glyoxal
Finally it can be noted that according to the FTIR analysis results, lignosulfanate can form chemical bonds with glyoxal and thereby change to a bioplastisizer of high molecular weight and good reactivity for cement-based composites. So far several researchers have already shown that the reactivity of lignin can be improved by glyoxalation (Younesi-Kordkheili et al. 2015; Younesi- Kordkheili and Pizzi 2017; El-Mansouri et al. 2011).
Table 2- Absorption bond Assignment of FTIR spectra of lignin
Absorption bond location cm-1 Type of vibration
3100-3500 Stretching vibrations of alcoholic and phenolic OH groups involved in hydrogen bonds
1710-1715 Stretching vibrations of C=O bonds at ϐ location and in COOH group
1655 Stretching vibrations of C=O bonds at α and δ location
1461 C-H in methylene groups
1425- 1665 Aromatic ring vibrations
1330-1340 Vibration of syryngyl rings and stretching vibrations of C-O bonds
1220-1272 Vibration of guaiacyl rings and stretching vibrations of C-O bonds
1140-1150 Deformation vibrations of C-H bonds in guaiacyl rings 1125 Deformation vibrations of C-H bonds in syryngyl rings 1085 Deformation vibrations of C-O bonds in secondary alcohols
and aliphatic ethers
1035-1130 Deformation vibrations of C-H bonds in the aromatic rings and deformation vibrations of C-O bonds in primary alcohols 780- 945 Deformation vibrations of C-H bonds in associated to
aromatic rings
68 78 88 98 108 118
500 1000
1500 2000
2500 3000
3500 4000
T (%)
Wave length (cm-1) Virgin lignin
Glyoxalated lignin
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Water absorption and thickness swelling: Figures 2 and 3 illustrate the water absorption and thickness swelling of the composites, respectively. Water absorption and thickness swelling of the composites decreased continuously with increasing lignin content from 10 to 20 wt%. This result can be explained by the hydrophobic character of lignin which acts as a water repellent and the existence of non-polar alyphatic chains and aromatic rings in the lignin molecule. So far several researchers have indicated that the addition of modified lignin can increase the dimensional stability of the composites (Younesi-Kordkheili et al. 2015; Behrooz et al. 2012).
Conversely, Figures 2 and 3 show that glyoxalation of lignin further slightly reduced water absorption and thickness swelling of the composites when compared to unmodified lignin. The water absorption of the composites with 10, 15 and 20 wt% modified lignin was 8, 22 and 34% lower than those with unmodified lignin. Lower water absorption and thickness swelling of the composites containing GL can be related to a higher number of reactive sites and to the molecular weight of the modified lignin compared to unmodified lignin. Increasing the reactivity of lignin by glyoxalation improves bonding of lignin to bagasse fiber and cement thus improving the dimensional stability of the panels. Based on the finding of this research work, the lowest water absorption was observed for the panels made with 20% modified lignin while the highest absorption value belongs to those made without lignin. As regards the hydrophilic behaviour this is a limiting factor to develop the applications of the composites, hence the addition of GL as a new plastisizer is one the methods that can be used to overcome to this problem.
Figure 2- Water absorption of the prepared composites after 24h immersion time 0
10 20
Water absorption (%)
Content of lignin (%)
CBvL CBGL
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Figure 3- Thickness swelling of the prepared composites after 24h immersion time
Conversely, the hydrophobicity of lignosulfonates can also play an important role in their dispersing performance. In one study on calcium lignosulfonates, it was found that when lignosulfonates were fractionated based on molecular weight; the higher was the molecular weight of the fraction, the lower was the charge density and hydrophilicity of the fraction (Yang et al. 2008). As a result, the increased hydrophobicity allowed for an increased surface activity and decreased surface tension. Chen et al.
(1999) indicated that the addition of phenolic materials such as sulphonated phenolic resin (SPF) can also reduce the water content of concrete.
Flexural Properties: The MOE of each sample is shown in Table 3. The cementitious panels with lignin exhibited the higher MOE compared to those without lignin (Table 3). At constant content of lignin, the composites with GL exhibited higher MOE than those containing unmodified lignin. The flexural moduli of the samples with 10, 15 and 20 wt % modified lignin was 16, 16 and 17 % higher than those made from unmodified lignin, respectively. The MOE in composites is mainly function of the modulus of individual component (Kazemi Najafi and Younesi-Kordkheili (2011). Increased flexural properties for the composites with GL can be attributed to the high stiffness of lignin. It has also been confirmed that the MOE of composites is directly related to their stiffness (Boding and Jayne 1993). Boding and Jayne (1993) research on the elastic behavior of lignin found that lignin is an isotropic materials and that its MOE was approximately 2 GPa.
The MOE of bagasse fiber and lignin is significantly higher compared to neat cement, respectively;
hence their addition improves the MOE of the composites. Additionally to the high elastic constant of lignin, they can help the good dispersion of natural fiber within the cement matrix. Hospodrova et al.
(2018) indicated that the good dispersion of a plasticizer influences significantly the MOE of the cement- based composites.
As can be seen from Table 3, the addition of lignin had a positive effect on the MOR as the composite panels with unmodified/glyoxalated lignin exhibited greater MOR value than the control panel.
Moreover, the results show that increasing the proportion of lignin from 10 to 20 wt% also improved the overall MOR of the cement panels.
The MOR of fiber reinforced cement based composites is affected by the adhesion between matrix and fibers (Younesi-Kordkheili et al. 2012). Contrary to unmodified lignin, glyoxalated lignin can be well distributed in the composites and improving the adhesion between bagasse fiber and cement. Greater composites MOR was achieved when the proportion of glyoxalated lignin was increased in the
0 10
Thickness swelling(%)
Content of lignin (%)
CBvL CBGL
CB 10 15
20
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manufacture of the composites. Makar and Beaudoin (2003) found that strong bonding between the cement paste and the fibers can be developed by the plasticizer and additives. He and Fathi (2015) also indicated that increasing the molecular weight of lignin by modification increased the adsorption of the lignin on cement particles to 6 mg/g lignin dosage. This also increased the fluidity of the cement paste. Hence glyoxalation of lignin has a marked effect on increasing MOR of the composites.
Table 3- The different mechanical properties of the prepared composites
4. Conclusion
In this study, the influence of graft copolymerization of black liquor and glyoxal as a new plasticizer on natural fiber- cement composites was investigated and the following conclusions could be made from the experimental results obtained:
• The FTIR spectrum shows that lignosulfanate and glyoxal can form chemical bonds. This supports and confirms previous results obtained by 13C NMR on the reaction of lignin and glyoxal
• The cement based composites containing glyoxalated lignin presented better dimensional stability and higher mechanical strength compared to those prepared with unmodified lignin.
• Greater dimensional stability and mechanical strength could be achieved by increasing the modified/unmodified lignin content from 10 to 20 wt%.
• Generally calcium lignosulunate can be used as a good plasticizer in natural fiber-cement composites. The low toxicity of modified lignosulfonates further increases their potential for a safe use.
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