Lignin was modified by oxidation to prepare lignin-derived polycarboxylic acids (LPCAs), which can be introduced into waterborne polyurethane (WPU) elastomers through the interactions with the soft segments of WPU. The changes of water resistance, thermal and tensile properties of WPU elastomers were observed. After LPCAs were added, the water solubility of WPU elastomers was increased. Besides, the weight loss peak of the soft segments of WPU moved to the lower temperature, indicating that the decomposition of soft segments became easier and the interspace of the soft segments regions was enlarged by LPCAs. Comparing with the blank WPU elastomer, when the LPCA loading was 2.5 wt% (based on PPG-1000), the tensile stress of the LPCA-WPU materials at 100% and 300% strain increased 135.4% and 90.5% separately, and the modulus of elasticity in tension increased 383.6%. Therefore, LPCAs could serve as reinforcing fillers in WPU elastomers, which contributes to develop more functional and eco-friendly WPU materials. Moreover, the usage of lignin is broadened.
Polyurethane is a widely used polymeric material for its performance can be adjusted in a large range by applying different polyols, isocyanates, catalysts, and auxiliary compounds [
Lignin, an important bioresource, is the most abundant aromatic polymer in nature [
In this work, we took advantage of lignin-derived polycarboxylic acids (LPCAs) to modify the properties of WPU elastomers. NaOCl solution, an oxidative reagent which can be operated under mild conditions, was chosen to perform oxidative modification on steam explosion lignin to prepare LPCAs. LPCAs were introduced into WPU in the midst of emulsification. We discussed the modification mechanism of the LPCA-WPU elastomers and observed the changes of the water resistance, thermal and mechanical properties of LPCA-WPU elastomers.
Steam explosion lignin was donated by Jilin KAIYU Biomass Development & Utilization Co. Ltd. Polypropylene glycol-1000 (PPG-1000) was obtained from Jiangsu Haian Petrochemical Works. Tolylene-2,4-diisocyanate (TDI) was supplied by Tianjin Fuchen Chemical Reagents Factory. 2,2-Dimethylolpropionic acid (DMPA) was provided by Chengdu Gracia Chemical Technology Co. Ltd. All the other reagents used in this study were purchased in China and were of analytical grade. The concentration of NaOCl solution was calibrated according to the China National Standard GB 19106-2003.
5.00 g steam explosion lignin and 150 mL distilled water were placed into a 500 mL three-neck round-bottom flask. The mixture in the flask was heated to 30°C in a water bath and was stirred mechanically. Then, NaOCl solution was added at a ratio [
The typical process of the synthesis of LPCA-WPU elastomers was as follows. 0.10 g LPCAs and 10 mL distilled water were added into a 100 mL beaker, and then 2 mol L−1 NaOH solution was added to make LPCAs just dissolved. The solution of LPCAs was diluted to 75 mL with distilled water to be ready for the synthesis of LPCA-WPU elastomers.
10.00 g PPG-1000 was added into a 500 mL four-neck round-bottom flask and nitrogen gas was purged continuously. Under mechanical stirring, 4.36 g TDI was added into the flask at room temperature. After that, the temperature of the water bath was raised to 70°C and the mixture was stirred for 2 h. Next, the temperature of the water bath was reduced to 60°C. 0.94 g DMPA, 50 mL acetone, and 2 drops of dibutyltin dilaurate were added successively. The reaction was carried out for 2 h. During the reaction, acetone would be added additionally to lower the viscosity of the mixture. When the reaction finished, the temperature of the water bath was reduced to 40°C and 0.70 g triethylamine was fed into the flask. The mixture was stirred for 10 min. And then, the solution of LPCAs was added and the stirring was sped up properly and kept for 1 h. Later, the water emulsion was poured into a mold and dried at 60°C for 48 h to get the elastomer. Then the elastomer was put into a sample bag and was kept at room temperature for 1 week to be ready for characterization.
The LPCA loading in LPCA-WPU elastomers is expressed as the weight ratio of LPCAs vs PPG-1000,
The content of carboxyl was determined by titration analysis [
The molecular weight of lignin was measured by gel permeation chromatography (GPC) using Agilent PL-GPC 220. The temperature of the column was 70°C. The type of the column was PLgel.
The water resistance test was carried out in room temperature. A sample with an area of approximately 1cm2 was cut from each elastomer. Later, the samples were placed into distilled water for 12 h. Afterwards, the samples were taken out and were wiped up by filter paper, and then the final weight of the samples were measured. The percentage of the weight change was figured out according to (
The thermogravimetric analysis was performed by a thermal analyzer (NETZSCH STA 449C) under a nitrogen atmosphere from 40 to 800°C at a heating rate of 10°C min−1.
The tensile performance was characterized by a microcomputer-controlled electronic universal testing machine (Jinan Liangong Testing Technology Co. Ltd. CMT-20) with a tensile rate of 100 mm min−1 according to the China National Standard GB/T 1040-2006. The temperature of the room for the tensile test was kept as 23°C and the relative humidity was kept as 50% according to the China National Standard GB/T 2918-1998. The elastomers were cut into strips with dumbbell-like shapes, of which the width of the central part was 4 mm. Both ends of the strips were clamped tightly in the tensile test to just let the central part of them stretched. For each elastomer, at least 5 strips were used to calculate the average value and the standard deviation.
The content of carboxyl, number-average molecular weight (
The titration analysis and GPC characterization results of acid-precipitated lignin and LPCAs.
Acid-precipitated lignin | LPCAs | |
---|---|---|
9.02% | 11.10% | |
2.91 × 104 | 1.23 × 104 | |
3.48 × 104 | 1.25 × 104 |
After lignin was oxidized by NaOCl solution, the content of carboxyl increased and the molecular weight decreased, indicating that LPCAs were prepared successfully.
Figure
The photos of LPCA-WPU elastomers immersed in water after 12 h.
The percentage of the weight change of LPCA-WPU elastomers with different loadings of LPCAs.
LPCA loading | |
---|---|
0 wt% | 167.8% |
1.0 wt% | 102.2% |
2.5 wt% | 53.9% |
5.0 wt% | −22.9% |
7.5 wt% | −42.3% |
Figure
TG and DTG curves of LPCA-WPU elastomers with different loadings of LPCAs.
Weight percentage of the residue (
LPCA loading | |||
---|---|---|---|
0 wt% | 1.132% | 308.06 | 401.14 |
1.0 wt% | 3.782% | 266.41 | 402.54 |
2.5 wt% | 3.206% | 268.47 | 402.20 |
5.0 wt% | 4.439% | 253.75 | 398.90 |
Figure
Illustration of interactions between LPCAs and WPU elastomers.
Figure
Effect of LPCA loading on tensile stress at 100% strain (
Tensile stress at 100% strain (
LPCA loading | |||
---|---|---|---|
0 wt% | 2.96 ± 0.12 | 5.16 ± 0.07 | 22.2 ± 1.6 |
1.0 wt% | 5.02 ± 0.32 | 7.49 ± 0.49 | 65.5 ± 4.8 |
1.5 wt% | 5.09 ± 0.27 | 7.61 ± 0.58 | 61.1 ± 5.8 |
2.5 wt% | 6.97 ± 0.37 | 9.83 ± 0.49 | 107.1 ± 6.1 |
5.0 wt% | 5.26 ± 0.36 | 7.78 ± 0.35 | 88.5 ± 5.8 |
7.5 wt% | 3.95 ± 0.30 | 6.27 ± 0.30 | 51.8 ± 7.0 |
Figure
Effect of LPCA loading on tensile strength (
Tensile strength (
LPCA loading | ||
---|---|---|
0 wt% | 13.48 ± 1.73 | 736 ± 108 |
1.0 wt% | 14.93 ± 1.12 | 702 ± 84 |
1.5 wt% | 14.25 ± 1.12 | 673 ± 85 |
2.5 wt% | 18.60 ± 1.37 | 707 ± 31 |
5.0 wt% | 13.96 ± 0.65 | 650 ± 46 |
7.5 wt% | 12.83 ± 1.79 | 679 ± 99 |
Lignin was oxidized to prepare LPCAs, which was later introduced into WPU elastomers and exhibited modification effects on many aspects of properties. Water resistance test suggested that LPCAs increased the water solubility of WPU elastomers. DTG curves of LPCA-WPU elastomers showed that the weight loss peak of the soft segments of WPU moved to the lower temperature after LPCAs were added. LPCAs made the soft segments decompose more easily, indicating that LPCAs enlarged the interspace of the soft segment regions. The relatively high content of carboxyl helped LPCAs interact with the soft segments. LPCAs hindered the movements of the soft segments, resulting in an increase of the mechanical properties of WPU elastomers. Comparing with the blank WPU elastomer, when the LPCA loading was 2.5 wt% (based on PPG-1000),
All relevant data are within the paper.
The authors declare that there are no conflicts of interest.
This work was partly supported by the National Natural Science Foundation of China (no. 51502108), the Foundation of Jilin Province Development and Reform Commission, China (no. 2014N145), the Science and Technology Innovation “Double Ten Project” (no. 55SS06), and the Changchun Science and Technology Bureau (no. 15SS06).