Response of Properties and Chemicals to Preparation Parameters in the Oxidation Processing of Refined Montan Wax.

Refined montan wax (RMW) is a lignite-based chemical product with wide application and high added value. However, research on its processing and performance is very limited. Currently, four parameters in the key preparation procedure for the oxidation bleaching of RMW, including the concentration of two oxidants (H2SO4 (P1) and CrO3 (P2)), oxidation time (P3), and the mass ratio of CrO3 used in two oxidation steps (P4), were systematically evaluated in regard to their impact on the properties and chemistry of RMW. The results showed that the four tested parameters visibly affected RMW, and each parameter had a different impact on the properties of RMW by range analysis, of which P1 showed a greater influence on its acid value; P2 influenced its friability, specific surface area, and aperture; P3 affected its color, initial melting point, and saponification value; and P4 had a higher impact on its final melting point, melting range, and hardness. Gas chromatography with flame ionization detection-mass spectrometry analysis revealed that the compounds found in RMW samples (RMWs) under different oxidation conditions differed significantly, with major differences in the content and amount of these components. Among the compounds in RMWs, 16 different compounds (variable importance of projection > 1) were found by the orthogonal projections to latent structures discriminant analysis method, nine of which have a strong relationship to the different performances of RMWs. This work provided a basis for the development of performance-oriented preparation processing technology for RMW.

Introduction

Montan wax is a natural wax extracted from lignite that demonstrates good performance and wide application. Compared to synthetic or semisynthetic waxes, montan wax contains natural long-chain fatty acids, polyhydroxy fatty acid esters, and a small amount of free fatty acids, granting it a higher melting point and better stability.[1−6] Therefore, it has been considered as a potential substitute for palm wax. Currently, montan wax products include crude montan wax (CMW), deresined montan wax (DMW), and refined montan wax (RMW), which differ not only in appearance but also in performance and application. In its industrial production, CMW is directly extracted from lignite using organic solvents by continuous extraction under atmospheric pressure, which contains resin with a content of 15–30 wt %. The color of CMW is almost black and is usually used as a substitute or supplement for expensive natural animal or vegetable waxes as well as for the production of carbon paper, leather shoe polish, floor wax, glazing wax, and metal polishing agents.[6−9] DMW is prepared from CMW through a deresination process, in which the resin content is controllable (4–15 wt %) by continuous extraction at low temperatures. The color of CMW is close to dark brown and is usually used as a raw material to prepare RMW or applied in areas that require a higher performance compared to CMW.[10] RMW is prepared from DMW by oxidative bleaching, through which the color of RMW becomes close to yellow or even white and the resin content is reduced to trace amounts. Compared to CMW and DMW, RMW has noticeable advantages in appearance for use in high-value products such as lipsticks, hair gels, creams, and precision casting.[11−15]

However, for this wide applicability, the requirements of the appearance and performance of MW are quite different. For example, the RMW used in lipsticks requires a harder wax than in hair gels, while a higher melting point is needed in precision casting than for creams.[15−18] Precisely because of these personalized needs, the precise preparation of these materials has become an important focus of the material chemical industry. In our previous investigation, the influence of the deresination procedure on the performance of DMW compared to CMW and the impact of residual montan resin (MR) as the raw material in DMWs on the appearance and performance of RMW had been systematically researched. The results showed that the deresination time and temperature were two key factors affecting the CMW deresination rate.[19] As the residual MR content in DMW increased, its friability, acid value, and saponification increased but the melting point of DMW decreased. Furthermore, as the residual MR content in DMW increased, the RMW color deepened and its friability and saponification increased; however, the melting point, acidity, and hardness decreased.[8,10,19] This indicated that processing and the surrounding parameters had a strong influence on the appearance and performance of the products.

From CMW extraction to RMW preparation, there are two key steps in the overall process: deresination and oxidation.[8,20,21] Generally, the content of wax in lignite is 3–8 wt %, and the resin content in CMW is 15–30 wt %. The purpose of deresination was to reduce the resin content in CMW, while oxidation bleached DMW and improved its performance.[10,22] While oxidation processing is the most important and complex process for the preparation of RMW, how the preparation parameters affect the properties and chemistry of RMW remains unknown. Thus, the four main parameters in the oxidation bleaching of RMW, including the concentration of the two oxidants (H2SO4 (P1) and CrO3 (P2)), the oxidation time (P3), and the mass ratio of CrO3 used in two oxidation steps (P4), were evaluated systematically for their respective impact on the properties and chemistry of RMW in order to provide a basis for the development of performance-oriented preparation processing technology for RMW.

Materials and Methods

Materials

Deresined montan wax (DMW) was provided by Yunnan Shangcheng Biotechnology Co., Ltd. (Yuxi, China), in which the residual resin content was 10 ± 1.2 wt %, as determined by the MW analysis method prescribed by the national standard.[10,23] All chemical reagents used in this work were of analytical grade and purchased from Tianjin Fengchuan Chemical Reagent Scientific Co., Ltd. (Tianjin, China). The oxidation bleaching process of RMW preparation from DMW involved four procedures (Figure ): stage-one oxidation, stage-two oxidation, pickling, and water washing. The material ratio of DMW with 10 wt % residual MR/CrO3/sulfuric acid used in this reaction was 1:1.2:5.

Figure 1

Flow chart of oxidation bleaching of RMW.

First, 10 g of DMW was added to a 250 mL three-neck flask with a stirring device and was mixed with 50 g of sulfuric acid solution with a given concentration (P1). The reactant was heated and stirred until the sample melted. Then, a certain amount of chromium trioxide oxidant aqueous solution (P2) was slowly added to the sample through a dropping funnel at the temperature range of 105–110 °C. After reacting for several hours (P3), the sample and waste liquid were stratified, and the waste liquid in the lower layer was decanted after cooling. The procedure for stage-two oxidation was similar to that of stage-one oxidation with the exception of the mass ratio of chromium trioxide used in the two oxidation steps (P4). Notably, the total mass of chromium trioxide used in stage-two oxidation was 12 g. In the third stage, the sample was washed twice using 40% sulfuric acid solution for 1 h. Finally, the sample was washed with 50 mL of pure water four times. The washing liquid was discarded, and the product was obtained, dried in an oven at 50 °C, melted at 105 °C, cooled, and molded to obtain RMW.

In the above-mentioned process, four parameters, including the concentration of the two oxidants (H2SO4 (P1) and CrO3 (P2)), the oxidation time (P3), and the mass ratio of CrO3 used in the two oxidation steps (P4), were adjusted. A series of RMW products were prepared under different conditions with different parameters, which are specified in Table . In addition, the product yields of the RMW samples (RMWs) under different conditions are listed in Table S1 in the Supporting Information.

Table 1

The Parameters in Oxidation Bleaching of RMWs and Their Change Levels

variable parameters	level 1 (L1)	level 2 (L2)	level 3 (L3)	level 4 (L4)	invariant parameters
P1: concentration of H2SO4 in each stage of oxidation	30%	40%	50%		50% CrO3 aqueous solution; 3 h; 7:3
P2: concentration of CrO3 aqueous solution in each stage of oxidation	30%	40%	50%	60%	50% H2SO4; 3 h; 7:3
P3: oxidation time in each stage of oxidation	1 h	2 h	3 h		50% H2SO4; 50% CrO3 aqueous solution; 7:3
P4: the mass ratio of CrO3 in two stages of oxidation	6:4	7:3	8:2		40% H2SO4; 50% CrO3 aqueous solution; 3 h

Performance Analysis of RMWs

Appearance

The colors of the RMWs prepared under different conditions were observed in natural light by three staff members working in the QC department of Yunnan Shangcheng Biotechnology Co., Ltd. (Yuxi, China). To quantitatively describe the differences in appearance, the color of each RMW was scored using a color comparison ruler according to staff evaluation (Figure ).

Figure 2

Color appearance of RMWs prepared from DMWs under different parameters in oxidation bleaching and the color comparison ruler used for quantitative description of the color.

Physical and Chemical Properties

The basic physical properties of RMWs under different conditions were determined in this work. The melting point was measured using a WRS-3 melting point instrument (Shanghai, China), the friability was measured with an FT-2000 friability tester (Tianjin, China), and the microhardness was determined using an HXS-1000 Vickers microhardness tester (Shanghai, China). The determination methods of the melting point, melting ranges, microhardness, and friability had been detailed in our previous publication.[10] To determine the specific surface area and aperture, 0.5 g of RMW was weighed and treated with nitrogen at 65 °C for 20 min, and its specific surface area and aperture were measured using a Mike ASAP2020 specific surface area and porosity analyzer (Shanghai, China).

The chemical properties of RMWs under different conditions, including acid and saponification values, were determined according to the GB/T 2559-2005 method of lignite wax. The procedure was similar to the one mentioned in our previous publication.[10,24]

Composition Analysis by Gas Chromatography–Mass Spectrometry (GC–MS)

GC–MS analysis was carried out on a GC7890B/MS5977A instrument (Agilent, Palo Alto, USA) equipped with an HP-5 capillary column (30 m × 0.32 mm × 0.25 μm, Agilent). An ether solution of diazomethane used was prepared by our laboratory according to the literature.[25] Bis(trimethylsilyl)trifluoroacetamide (BSTFA + TMCS) was purchased from Tokyo Chemical Industry (Tokyo, Japan). First, 4 mg of analytical-grade hexatriacontane (GC > 99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) was weighed and dissolved in 10 mL of toluene to prepare a 0.4 mg/mL hexatriacontane solution as the internal standard solution. Then, 5 mg of powdered RMW was placed in a 5 mL derivatization vial, in which 2 mL of a toluene solvent and 500 μL of the internal standard solution were added. Next, the cap was covered, and the vial was heated to 60 °C to dissolve the RMW. The derivatization step was carried out according to the literature.[10]

The GC-FID analysis conditions were as follows: the injection volume was 2 μL, the injection port pressure and temperature were 17.81 psi and 300 °C, respectively, the injection mode was splitless, and the chromatographic column had a constant flow. The carrier gas was high-purity N2 with a flow rate of 3 mL/min, the fuel gas was high-purity H2 with a flow rate of 45 mL/min, and the supporting gas was high-purity air with a flow rate of 300 mL/min. The detector temperature was set as 300 °C, and the temperature program was as follows: the initial temperature was 160 °C and maintained for 3 min, increased to 235 °C at a rate of 4 °C/min, and finally increased to 300 °C at a rate of 3 °C/min and maintained for 6 min. For MS analysis, the transmission line temperature of MS was 300 °C, and the quadrupole temperature was 150 °C. The electron energy was 70 eV, with the ion source temperature was set at 230 °C. The acquisition mode was set to full scan, and all isolated compounds were identified and analyzed by the NIST 08 database. The relative content of each component in the sample was calculated according to the following formula:where A1 is the peak area of the composition to be tested, A2 is the peak area of the internal standard, and C is the internal standard concentration.

Statistical Analysis

The Excel 2019 software was used to process the experimental data of each group, and the single-factor analyses of ANOVA, LSD, and the Duncan method were performed using the SPSS statistical analysis software (p < 0.05) to analyze the significance of the difference. In addition, the influence or relationship of the oxidation parameters to the properties or chemistry of the sample was analyzed by range analysis, principal component analysis (PCA), and orthogonal projections to latent structures discriminant analysis (OPLS-DA) using the Origin 2019 software.

Results and Discussion

Color Response to the Preparation Parameters in the Oxidation Processing of RMW

Through oxidation bleaching, the colors of the RMWs had significantly improved compared to those of DMW. However, because the parameters in the oxidation process were different, the colors of the appearance-improved RMWs also differed. The shade of the color for each RMW was quantified using numbers from low (light) to high (dark) (Figure and Table S2 in the Supporting Information). Range analysis indicated that P3 had the greatest influence on the color followed by P2, P4, and P1 (Figure and Table S3 in the Supporting Information). In other words, the color of the RMW was influenced by a comprehensive effect induced by each parameter in the oxidation processing of the RMW. However, the impact of each parameter was different.

Figure 3

Changes in physical and chemical properties of RMWs prepared from DMWs under different parameters in oxidation bleaching. (a) Initial melting point (n = 3); (b) final melting point (n = 3); (c) melting range (n = 3); (d) Vickers hardness under a 50 gf test force (n = 3); (e) friability expressed by the weight loss ratio (n = 3); (f) BET surface area (n = 1); (g) aperture (n = 1); (h) acid value (n = 3); (i) saponification value (n = 3). Different small letters in each figure indicate the significant differences among different RMW samples (p < 0.05).

It is well-known that differences in appearance reflect the internal differences of the products, including their properties and compositions. These results revealed that the four parameters tested in the oxidation process have obvious and various effects on the color of the RMW. A prior investigation showed that the residual MR content in DMW had a positive correlation with its resulting color, as the color of the RMW was found to deepen as the residual MR content in DMW increased.[10,26] In practical production, the average residual MR content in DMW is usually ∼10 wt % by industrial deresination technology.[10,23] Thus, the color requirement of RMW could be achieved by adjusting these four parameters in the oxidation process of RMW and optimizing them based on their influence on the color.

Effect of Preparation Parameters on the Properties of the RMWs after Oxidation Processing

Five main physical properties, including the melting point, microhardness, friability, Brunauer–Emmett–Teller (BET) surface area, and aperture, as well as two chemical properties, including acid and saponification values, were detected by their corresponding methods.[27−29] As a result, these seven properties differed in various degrees for RMWs prepared under different conditions, indicating that the four parameters considered in oxidation processing affected the performance of the RMWs (Figure ). The influence of each parameter on the properties of the oxidation processing of RMW was analyzed by range analysis and ordered as follows: for the color, P3 > P2 > P4 > P1; for the initial melting point and the saponification value, P3 > P1 > P2 > P4; for the final melting point, P4 > P1 > P2 > P3; for the melting range, P4 > P2 > P3 > P1; for the hardness, P4 > P3 > P1 > P2; for the friability and aperture P2 > P3 > P4 > P1; for the specific surface area, P2 > P3 > P1 > P4; and for the acidity, P1 > P4 > P2 > P3 (Figure ).

Figure 4

Influence of each parameter in the oxidation process by range analysis. P1: the concentration of H2SO4; P2: the concentration of CrO3; P3: oxidation time; P4: the mass ratio of CrO3 used in each oxidation stage.

Performance is the basis of material application, which can be determined and adjusted by the processing parameters. For widely used chemical products, the different applications have different requirements for RMWs. For instance, RMW added to car wax requires a higher melting point than when added to floor wax, while the required hardness of RMW added to leather wax is higher than for glazing wax.[30,31] On the other hand, the acidity and the saponification value are two indexes that characterize the chemical properties of RMWs.[10,25] Our previous investigation showed that they strongly affected the physical properties of RMWs.[10] In addition, the response of each property to each parameter in the oxidation processing of RMW showed obvious differences. Based on the influence of each parameter on the properties of RMW, the performance of RMW can be improved or optimized to meet the personalized requirements for different applications.

Chemical Response to Preparation Parameters in the Oxidation Processing of RMWs

The chemical compositions of RMWs obtained under different oxidation refining conditions were analyzed by gas chromatography–mass spectrometry (GC–MS), and the relative content of each identified chemical component in RMW was determined by GC. In total, 33 chemical substances (matching degree > 80%) were identified from these RMWs, of which 10 were shared components (Figure a and Table S4 in the Supporting Information). Based on the composition and content determination results, obvious differences were observed in each RMW product under different conditions (Figure b). PCA analysis on the composition and content of different RMWs indicated a strong positive correlation shown among RMWs prepared under all three levels of conditions of P4 variation, a strong negative correlation among RMWs prepared under three of four levels of conditions of P2 variation, an even stronger negative correlation shown among RMWs prepared under the three levels of conditions of P3 variation, and a negative correlation between RMWs prepared under the two levels of conditions of P1 variation with the other level, suggesting that the influence of each parameter in the oxidation processing of RMWs on the composition and content of each was different (Figure c). OPLS-DA analysis showed 16 chemicals with significantly different compositions (variable importance of projection (VIP) > 1) in different RMWs among all detected chemicals (Figure d), which were probably the main cause of differences in appearance and performance.

Figure 5

Chemical analysis of RMWs prepared from DMWs under different parameters in oxidation bleaching. (a) GC chromatogram; (b) heatmap visualization of 33 chemicals in different RMWs (representation of composition by numbers in Table S4 in the Supporting Information). Rows: chemicals; columns: RMWs; the color key revealed chemical quantity values, white: lowest, deep red: highest. (c) PCA analysis on the composition and content of different RMWs; (d) OPLS-DA analysis on significantly different compositions.

The difference in the appearance and performance of the product is caused by the different chemicals in each product, and variation in processing parameters directly leads to changes in the chemical compositions. Through instrumental analysis, determining the composition of products is often more efficient, rapid, accurate, and sensitive than performance detection. Thus, research on the chemical response to preparation parameters in the oxidation processing of RMW would provide more direct evidence regarding the appearance and performance response to preparation parameters. On the other hand, the chemical compositions of raw materials will also impact the appearance and performance of the product.[10] In our previous publication, the experimental sample (DMW with about 10% residual montan resin) used in this study was determined.[32] A combined analysis of those data and our present chemical results indicated that most of the compositions detected in DMW were also detected in RMW, but the number of compositions detected in the DMW sample was fewer than in RMW. It was found that the oxidants primarily oxidized the components in montan resin to some degree and that different oxidant conditions led to differences in trace residual montan resin in RMWs.

Correlation Analysis of the Composition and Performance of RMWs under Different Preparation Parameters in Oxidation Processing

To investigate the relationship between chemical compositions and the performance of RMW, PLS regression analysis of all 13 RMWs prepared under different parameters was carried out, and the results were visualized by a correlation plot (Figure ). It could be seen that the compounds numbered C13, C15, C19, and C23 had strong positive correlations with the color, friability, and specific surface area; C5, C12, C27, and C31 had strong positive correlations with the initial melting point, hardness, aperture, and saponification value; C2, C25, and C30 had strong positive correlations with the final melting point; and C3, C9, and C22 had strong positive correlations with the melting range and acid value. Notably, among these above-mentioned related compositions, nine of them had significantly different compositions (VIP > 1), indicating the correlation between the difference in performance and the difference in chemical composition and providing some basis for the study of performance metrics based on the investigation of chemical composition.

Figure 6

Correlation between performance and chemicals of RMWs.

Conclusions

The four parameters considered in the oxidation bleaching of RMW, i.e., the concentration of two oxidants (H2SO4 (P1) and CrO3 (P2)), the oxidation time (P3), and the mass ratio of CrO3 used in two oxidation steps (P4), had significant impacts on the color, properties, and chemistry of RMW. It was determined that P1, P2, P3, and P4 each had a respectively greater influence on the acid value; the friability, specific surface area, and aperture; the color, initial melting point, and saponification value; and the final melting point, melting range, and hardness of RMW. Furthermore, the 33 chemicals detected by GC-FID/MS analysis in the RMWs under different oxidative conditions differed significantly, in which 10 were shared components and 16 were distinct components. Among these chemicals in RMWs, nine distinct components had a strong relationship to the performance of RMW, as determined by PLS analysis.