Nanolignin as Nucleating Agents Promoting Crystallization of Isotactic Polypropylene

Chen, Jing; Yang, Tao; Zhu, Yongchen; Luo, Zhenyang; Ma, Xiaofeng

doi:https://doi.org/10.1155/2023/7454857

Polymer Crystallization

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 7454857 | https://doi.org/10.1155/2023/7454857

Nanolignin as Nucleating Agents Promoting Crystallization of Isotactic Polypropylene

Jing Chen,¹Tao Yang,²Yongchen Zhu,¹Zhenyang Luo,^1,3and Xiaofeng Ma^1,3

Academic Editor: Dario Cavallo

Received09 Aug 2022

Revised07 Mar 2023

Accepted08 Mar 2023

Published23 Mar 2023

Abstract

Lignin is the second most abundant pollution-free biomass material. However, most of lignin is discarded as waste in river or burned as fuel, which results in serious environmental pollution problems and low utilization efficiency of lignin at present. Thus, high-value utilization of lignin has become a hot research field. Herein, nanolignin (nano-Lig) is prepared successfully by the self-assembly method, and then nano-Lig and isotactic polypropylene (iPP) are mixed to prepare a series of nano-Lig/iPP composites by the solution blending method. Nano-Lig not only enhance thermostability of iPP but also improve crystallization properties of iPP. When 1.0 wt% nano-Lig was added in iPP, the crystallinity of iPP increased by 6.39% compared to iPP. Nano-Lig can increase the crystallization rate of iPP through investigation of kinetics of nonisothermal crystallization, suggesting nano-Lig can be used as a nucleating agent for iPP.

1. Introduction

Isotactic polypropylene (iPP) is one of the most used, large volume, low price polymorphic semicrystalline thermoplastic polymers [1, 2]. It has several unique properties such as high toughness, elasticity, ease of processability, excellent water, and chemical resistance. Therefore, it can be found in various applications, such as the automobile industry, food packaging, and the manufacture of toys and furniture [3–6].

The fabrication of iPP products involves heating and cooling during processing. It melts and crystallizes upon heating and cooling, respectively. The ultimate properties of iPP products are mainly controlled by the polymer structure, which can be affected by these phenomena of melts and crystallizes during processing. iPP crystallizes into four forms, namely, monoclinic α, trigonal β, orthorhombic γ, and a mesomorphic form, under different thermal and mechanical conditions and the usage of nucleating agents (NAs) [7–12]. Different kinds of nucleating agents can affect not only the crystallization rate of iPP in the forming process but also the crystalline structure.

NAs can be divided into natural NAs, synthetic NAs, and inorganic NAs. The natural NAs mainly include rosin, sorbitol, and sorbitol derivatives such as dibenzylidene sorbitol and 3, 4-dimethyldibenzylidene sorbitol [13–24]. The synthetic NAs include polycyclic aromatic hydrocarbons, organic acids/salts, amides, [25] benzoic acid salts, [26] hydrazide compounds, [27] and various organic pigments. Inorganic NAs have various salts oxides, such as CaCO₃ [28, 29], talc [30, 31], and Al₂O₃ [32]. Three kinds of NAs can increase the rate of PP crystallization, and the temperature at which the maximum rate of crystallization occurs upon cooling from the molten state. However, the disadvantage of synthetic NAs is that the technology of their production is not environmentally friendly, which usually include several stages of organic syntheses using specialty compounds, as well as toxic solvents. Talc and other inorganic fillers are also suitable NAs; they are inexpensive and serve as reinforcing agents. However, high concentrations of these materials are needed.

Lignin is the second most abundant biopolymer on the earth next to cellulose and is obtained as a major industrial waste material from pulp and paper industries [33]. Whereas 98% of the lignin is burned or discharged into rivers, thereby causing serious environmental pollution and grievous waste of renewable resources [34]. It has attracted much interest in scientific and industrial field because of its unique properties, such as environmentally friendly, sustainable, biodegradable, thermal stability, and low cost, which can contribute to the reduction of the environmental problem [35–41]. Thus, lignin has been applied to many fields, such as binder, carbon materials, fire retardant, and biomedical application [42–45]. In the past decades, nano-Lignin (nano-Lig) had been developed to design high-value materials. Nano-Lig has many merits, such as more uniform size, shape, and a high surface area to volume ratio, which helps to reinforce and strengthen polymer matrices and improve polymer thermal stability [46–48]. In addition, nano-Lig also shows superior dispersion and miscibility in the hydrophobic matrices [46]. The study of Nano-Lig mainly focus on lignin-based hydrogels [47] and enhancement mechanical properties of polyvinyl alcohol and polylactic acid [48]. However, to the best of our knowledge, there is still no report on using nano-Lig as NA for iPP, even though there was report on lignin could act as NA for iPP [49]. It is expected that nano-Lig can act as an effective NA for iPP; in other words, a smaller amount of nano-Lig can promote crystallization of iPP.

Herein, we report that nano-Lignin (nano-Lig) can serve as a NA for iPP. Nano-Lig is prepared by a simple self-assembly method using tetrahydrofuran as good solvent and H₂O as poor solvent. First, nano-Lig/iPP composites are prepared by the solution blending method. Second, the effect of nano-Lig on the melting properties, crystallization structure, and thermal stability of iPP are investigated. Finally, nonisothermal kinetics of crystallization of iPP and nano-Lig/iPP composite are demonstrated.

2. Experimental Section

2.1. Materials

iPP was purchased from Sinopec Yangzi Petrochemical Co., Ltd., China. Enzyme-hydrolyzed lignin was purchased from Shandong Longli Biotechnology Co., Ltd., China. Tetrahydrofuran (THF, AR, 99%) and xylene (AR, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd., China. Deionized water was obtained from Milli-Q.

2.2. Preparation of Nano-Lig

Nano-Lig aqueous solution was prepared by a simple self-assemble method according to a previous report [50]. The preparation process of nano-Lig was as follows (Scheme 1): enzyme-hydrolyzed lignin of 0.50 g was dissolved in 50 ml THF at room temperature. Then, deionized water of 200 mL was slowly dropped to the above lignin/THF solution under stirring. After dropping, the final solution was stirred for 4 h until the suspension was formed. Finally, the suspension was transferred into a dialysis bag (: 14000) and dialyzed in deionized water. The water was changed every 6 h and repeated for 5 times to obtain nano-Lig aqueous solution. Nano-Lig powder was obtained after the vacuum freeze drying process of nano-Lig aqueous solution.

2.3. Preparation of Nano-Lig/iPP Composites

A series of nano-Lig/iPP composites (0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt% nano-Lig) were prepared by the solution blend method. Here, we just show the preparation process of 1 wt% nano-Lig/iPP composite. 4.95 g iPP was dissolved in xylene at 130°C to obtain iPP/xylene solution. Then, nano-Lig of 0.05 g was added into iPP/xylene solution. After stirring and heating the solution at 80°C for 4 h, then the white nano-Lig/iPP composite was obtained after drying the solid product at 75°C under vacuum for 12 h.

2.4. Dynamic Light Scattering (DLS) Analysis

The diameter of nano-Lig in H₂O was determined by DLS analyzer (BIC-90plus PALS, Marven, England). About 1 mL nano-Lig aqueous solution was balanced for 5 min at room temperature before measurement, and the measurement result was the average of the 10 collected data.

2.5. Fourier Transform Attenuated Total Reflectance Infrared Spectroscopy (FTIR-ATR)

The surface of the samples was scanned with FTIR-ATR. Each sample was scanned 32 times in the wavenumber range from 4000 cm⁻¹ to 400 cm⁻¹.

2.6. X-Ray Diffraction (XRD) Analysis

The iPP and nano-Lig/iPP composites were crystallized at 120°C for 2 h, and then those samples were measured by X-ray diffractometer (D/Max 2500, Japan Science Co., Ltd.) to determine the crystal structure of iPP. The current of the instrument was 200 mA, the acceleration voltage of the instrument was 40 kV, λ = 0.154 nm, the scanning step was 0.02°, the range was 10°∼25°, and the scanning step frequency was 2°/min.

2.7. Differential Scanning Calorimeter (DSC) Analysis

The crystallization and melting behavior of iPP and nano-Lig/iPP composites were studied by DSC (NETZSCHTG 214). The test atmosphere was N₂ and the sample was weighed about 3 mg. The testing process is as follows: Heating up to 200°C at a heating rate of 30.0°C min⁻¹, eliminating thermal history at 200°C for 1 min, cooling to 30°C at a cooling speed of 20°C min⁻¹ at 30°C for 1 min, and then heating up (second heating) to 200°C at a heating rate of 20.0°C min⁻¹. The enthalpy of crystallization during the cooling process and the melting enthalpy during the second heating process were recorded. The crystallization peak temperature and melting temperature of the sample were obtained from the crystallization and melting curve, respectively. According to the assumption of the two-phase model, when the polymer is melted, only the crystal part of the polymer changes; so, the melting heat is essentially the heat needed to destroy the crystal structure. Thus, the higher the crystallinity is, the larger the melting heat is. In other words, the melting heat of the polymer is proportional to its crystallinity. Using this principle, the crystallinity X_c of the polymer can be calculated by equation (1):where and (207 J·g⁻¹) [51] are the heats of fusion of the experimental sample and the perfect (defect-free) iPP crystal, respectively. The is the weight fraction of nano-Lig in the composite.

Nonisothermal kinetic test: the sample was heated to 200°C at a heating rate of 30.0°C min⁻¹. After the heat history was eliminated at 200°C for 1 min, it was cooled to 50°C at the cooling rates of 2.5°C min⁻¹, 5.0°C min⁻¹, 10.0°C min⁻¹, and 20.0°C min⁻¹, respectively. The crystallization curve and crystallization peak temperature at each cooling rate were recorded.

2.8. Thermogravimetric (TG) Analysis

Under N₂ atmosphere, the sample of 3 mg was weighed, the temperature increased from room temperature to 600°C at a heating rate of 20.0°C min⁻¹, and the mass change of the sample during the test was recorded.

3. Results and Discussion

3.1. Characterization of Nano-Lig and Nano-Lig/iPP Composites

Figure 1(a) shows the nano-Lig particle size distribution in H₂O at room temperature. The average particle size (hydrodynamic radius) of nano-Lig in H₂O was 153.2 nm, the particle size distribution (PDI) was 0.200, and the PDI was relatively narrow. Such size of nano-Lig is comparable with that of the previous report [50]. This result indicated that nano-Lig was prepared successfully. The FTIR spectra of iPP, nano-Lig, and nano-Lig/iPP composites are shown in Figure 1(b). The peaks from 2837 cm⁻¹ to 2953 cm⁻¹ and the peaks from 1375 cm⁻¹ to 1454 cm⁻¹ were assigned to stretching vibration and bending vibration of methylene of iPP, respectively. The wide peak from 1500 cm⁻¹ to 1800 cm⁻¹ was the stretching vibration of the aromatic ring skeleton of nano-Lig. A new wide peak from 1500 cm⁻¹ to 1800 cm⁻¹ was shown in all nano-Lig/iPP composites, which could be assigned to the characteristic peaks of the aromatic ring skeleton of nano-Lig. The results demonstrated that nano-Lig/iPP composites were prepared successfully.

(a)

(b)

3.2. Thermal Stability of Nano-Lig/iPP Composites

Figure 2 shows the TG curves of iPP, nano-Lig, and nano-Lig/iPP. The TG results of iPP, nano-Lig, and nano-Lig/iPP are shown in Table 1. For nano-Lig, a mass loss occurred at around 100°C, possibly because of water evaporation in nano-Lig. The T_d (the temperature at which the initial mass of the material has been lost) of nano-Lig was 210.0°C. For iPP, the T_d and T_i (the temperature at which 5 wt% of the initial mass of the material has been lost) of iPP were 354.8°C and 400.0°C, respectively. For nano-Lig/iPP composites, the T_d and T_i of nano-Lig/iPP composites were larger than those of iPP compared with iPP; furthermore, the T_d and T_i of nano-Lig/iPP composites increased with the increase of the nano-Lig content in nano-Lig/iPP composites, suggesting all nano-Lig/iPP composites had higher thermostability compared to iPP. The T_i of nano-Lig/iPP composite increased by 34.4°C when the nano-Lig content was 2 wt% in nano-Lig/iPP composites. While a previous report showed that T_i of lignin/iPP composite prepared by melt mixing lignin (not nano-Lignin) and iPP increased by 33°C when the lignin content was 5 wt% in lignin/iPP composites [49]. Also, another previous report showed that T_i of lignin/iPP composite decreased by 7°C under N₂ atmosphere and increased by 23°C when the lignin content was 5 wt% in lignin/iPP composites [52]. This result demonstrated that nano-Lig had a more effective promotion in the thermostability of iPP than lignin. These results indicated that nano-Lig could improve the thermostability of iPP.

3.3. Thermal Property of iPP and Nano-Lig/iPP Composites

Figures 3(a) and 3(b) show the DSC heating and cooling curves of iPP and nano-Lig/iPP composites, respectively. The DSC results data are listed in Table 2. The melting temperature (T_m) of the nano-Lig/iPP composites were comparable with that of iPP, while the melting enthalpy (∆H) of the nano-Lig/iPP composites were higher than that of iPP. Thus, the crystallinity (X_c) of all nano-Lig/iPP composites were higher that of iPP. Compared with iPP, the highest X_c of nano-Lig/iPP composites increased by 6.39% when the nano-Lig content was 1 wt% in the nano-Lig/iPP composite. The crystallization temperature (T_c) of the nano-Lig/iPP composites shifted to a higher temperature compared to iPP. The results suggested that nano-Lig could serve as a nucleating agent for iPP because 1 wt% nano-Lig/iPP composite showed the highest X_c; thus, the characterization of 1 wt% nano-Lig/iPP composite are shown in the following parts.

(a)

(b)

3.3.1. Nonisothermal Crystallization Behavior of iPP and Nano-Lig/iPP Composites

Figure 4(a) shows the DSC crystallization curve of iPP at cooling rates of 2.5°C min⁻¹, 5.0°C min⁻¹, 10.0°C min⁻¹, and 20.0°C min⁻¹. The relative crystallinity (X_t) varying with temperature can be expressed by equation (2):where is the maximum enthalpy value reached at the end of the nonisothermal crystallization process and ∆H_T is the enthalpy evolved as a function of the crystallization temperature (T). and represent the crystallization start and completion temperatures, respectively. Both and ∆H_T can be acquired through the software of a standard DSC. Using equation (2), the relationship between X_t and crystallization temperature for iPP at different cooling rates can be obtained and is shown in Figure 4(b).

(a)

(b)

(c)

(d)

(e)

(f)

The relationship between T and the crystallization time (t) is shown in equation (3), where R is the cooling rate.

By using equation (3), the relationship between X_t and T (Figure 4(b)) can be converted to the relationship between X_t and t for iPP (Figure 4(c)). Figure 4(d) shows the DSC crystallization curve of nano-Lig/iPP at cooling rates of 2.5°C min⁻¹, 5.0°C min⁻¹, 10.0°C min⁻¹, and 20.0°C min⁻¹. Figures 4(e) and 4(f) show the relationship between X_t and T and the relationship between X_t and t of nano-Lig/PP, respectively. When X_t = 50%, the half time of crystallization (t_1/2) can be obtained from Figures 4(c) and 4(f). The value of the inverse of t_1/2 reflects the relative crystallization rate of iPP and nano-Lig/iPP, i.e., the smaller t_1/2 is, the larger the inverse of t_1/2 is and the faster relative crystallization rate is. The crystallization kinetic parameters of iPP and nano-Lig/iPP are obtained from Figure 4 and shown in Table 3. It was found that T_c of iPP and nano-Lig/iPP decreased with the increase of the cooling rate due to the thermal hysteresis phenomenon of iPP. At the same cooling rate, T_c of nano-Lig/iPP was higher than that of iPP, and t_1/2 of nano-Lig/iPP was smaller than that of iPP, suggesting that nano-Lig could promote the crystallization of iPP. The result is consistent with our expectation that a smaller amount of nano-Lig (only 1 wt%) can accelerate the crystallization kinetics because it acts as a nucleating agent. However, a previous report showed that lignin could promote iPP crystallization when the lignin content was 5 wt% in lignin/iPP composites [49].

3.3.2. Analysis of Dynamic Parameters of Avrami

Using the abovementioned X_t and t data, the isothermal crystallization of iPP and nano-Lig/iPP can be described by the modified Avrami equation (4) [53, 54].where Z_t is the crystallization rate constant; R is the cooling rate; and n is the Avrami exponent. To obtain Z_t and n values, equations (4) can be converted to (5).

Figures 5(a) and 5(b) are the straight lines obtained by plotting lg[−ln(1 − X_t)] versus lg(Rt) of iPP and nano-Lig/iPP, respectively. Values of n and Z_t were determined from the slope and the intercept of the straight lines in Figure 5, respectively, and shown in Table 4.

(a)

(b)

Z_t Values of iPP and nano-Lig/iPP were comparable at the same cooling rate, at the cooling rate of 10.0°C min⁻¹ and 20.0°C min⁻¹, and the n values of both iPP and nano-Lig/iPP were larger than 3. At the cooling rate of 2.5°C min⁻¹ and 5.0°C min⁻¹, the n values of nano-Lig/iPP were larger than 3 and the n values of iPP were smaller than 3. The results indicated the three-dimensional growth of iPP crystalline units is induced by heterogeneous nucleation.

3.4. X-Ray Diffraction Analysis

Figure 6 shows the XRD spectra of iPP and 1 wt% nano-Lig/iPP composite. iPP showed typical α-crystal diffraction peaks at 2θ = 14.1°, 16.8°, 18.5°, 21.1°, and 21.8°, which was related to (110), (040), (130), (131), and (111) crystalline planes, respectively [55–57]. The characteristic peaks of 1 wt% nano-Lig/iPP composite were similar to those of iPP, indicating that the nano-Lig’s effects on the crystallization behavior of iPP were negligible, and the crystalline structure of iPP remained unchanged in the composite.

4. Conclusion

In this work, nano-Lignin (nano-Lig) was prepared successfully by the self-assembly method. Nano-Lig could improve thermostability and crystallization properties of iPP. The T_i of nano-Lig/iPP composite increased by 34.4°C when the nano-Lig content was 2 wt% in nano-Lig/iPP composites. The crystallinity of iPP increased by 6.39% when 1.0 wt% nano-Lig was added in iPP. The study of kinetics with nonisothermal crystallization found that nano-Lig can increase the crystallization rate of iPP.

Data Availability

The research data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Authors’ Contributions

Jing Chen conceptualized (lead), performed the methodology (lead) and writing original draft preparation (equal). Tao Yang performed data curation (equal), investigation (equal), and writing original draft preparation (equal). Yongchen Zhu performed data curation, investigation, and writing original draft preparation (equal). Zhenyang Luo supervised (lead) the study. Xiaofeng Ma performed review writing & editing (Lead) and supervised (lead) the study. The authors Jing Chen, Tao Yang, and Yongchen Zhu contributed equally.

Acknowledgments

This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Copyright © 2023 Jing Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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