Preparation of PP-g-(AA-MAH) Fibers using Suspension Grafting and Melt-blown Spinning and its Adsorption for Aniline in Wastewater

This paper uses polypropylene (PP) as the matrix and acrylic acid (AA) and maleic anhydride (MAH) as functional monomers to prepare PP-g-(AA-MAH) fibers by suspension grafting and melt-blown spinning technology that are easy to industrially scale-up. The fibers can be used to adsorb aniline from wastewaters. Results showed that the grafting ratio reached the maximum of 12.47%. The corresponding optimal conditions were grafting time of 3h, AA : MAH = 0.75, total monomer content of 55%, benzoyl peroxide 1.4%, xylene concentration of 6 mL/g PP, and deionized water content of 8 mL/g PP. Owing to its good fluidity and thermal stability, the product of suspension grafting can be used for melt-blown spinning. Infrared spectroscopic and nuclear magnetic resonance spectroscopic analyses indicated that AA and MAH were successfully grafted onto PP fibers. After grafting, the hydrophilicity of PP-g-(AA-MAH) fiber increased. Therefore, it had higher adsorptivity for aniline and the adsorption capacity could reach 42.2 mg/g at 45 min. Moreover, the PP-g-(AA-MAH) fibers showed good regeneration performance.


Introduction
Water discharged from industrial activities, if left untreated or is not treated up to proper standard, inevitably causes water pollution, which is one of the major environmental issues of the day [1,2]. Aniline is one of the common organic compounds, used as an intermediate in chemical industry, and hence its market demand has risen sharply [3][4][5]. This provides more channels for aniline to enter water bodies, thus causing environmental pollution. Data [6][7][8][9] shows that in China itself, annually, nearly 30,000 tons of aniline enters the water bodies, through various pollution routes. In the United States, on an average about 600,000 tons of aniline-containing wastewater is discharged into the environment annually, whereas in Europe, this figure is about 1.4 times that of the United States. Since aniline does not readily undergo degradation, it accumulates in the environment. Moreover, being carcinogenic, teratogenic, mutagenic and highly toxic substance, aniline has a severely high risk of causing potential damage to the environment and organisms [10][11][12]. Therefore, aniline has been included in the list of PP resin especially for melt-blown process was provided by Shanghai Expert New Material Co. Ltd. Other chemical reagents, such as AA, MAH, xylene, benzoyl peroxide (BPO), acetone, absolute ethanol, sodium hydroxide, hydrochloric acid, etc. were all analytically pure and commercially available.

Preparation of PP-g-(AA -MAH) fibers
A certain amount of PP resin and xylene were charged into a magnetically-stirred reactor and the temperature was raised to 140 °C under stirring. Then, a certain amount of deionized water was rapidly pumped at normal temperature into the reactor to cool down the temperature of the reaction mixture. After this, the temperature of the reactor was maintained at 90 °C and then the mixture was allowed to swell for 30 minutes. Then, the monomers AA and MAH and BPO initiator were added to the reaction kettle through the high-pressure feed port and graft polymerization was carried out for a certain period of time under strong stirring. The obtained products were washed with deionized water and acetone to remove unreacted monomers and homopolymers. The washed product was dried and then passed through a melt-blown spinning machine [38] to obtain PP-g-(AA-MAH) fibers.

Calculation of grafting ratio
PP-g-(AA-MAH) fibers (0.5 g) were placed in a 250 mL round-bottom flask. Xylene (100 mL) and sodium hydroxide-ethanol solution (0.05 mol/L, 30 mL) were added in succession. The mixture was allowed to boil at 120 °C for 30 min. After the solution was cooled to room temperature, 3-5 drops of thymol blue indicator was added and then the mixture was titrated with hydrochloric acid-isopropanol solution (0.05 mol/L) until the blue color just disappeared. The ungrafted PP fibers were used as the blank sample for comparison. The grafting ratio was calculated as follows: where (%) denotes the grafting ratio; (L) and (L) represent the volumes of HCl-isopropanol solution consumed to titrate PP fibers and PP-g-(AA-MAH) fibers, respectively; (mol/L) denotes the concentration of HCl-isopropanol solution; denotes the mass of the fiber sample (g); 98.06 g/mol and 72.06 g/are the molar masses of MAH and AA, respectively.

Characterization methods
X-ray diffractometer (X'TRA) was used to characterize the crystal structures of sample before and after quenching. The fluidity and thermal stability of the suspension grafting product were studied by melt indexer (RL-Z1B1) and thermal analyzer (Pyris 1 DSC), respectively. The functional groups of the modified fibers were analyzed using Fourier transform infrared spectrometer (Nexus 670) and nuclear magnetic resonance spectrometer (DRX500).

Hydrophilicity of PP-g-(AA-MAH) fiber
The Washburn equation [39] can determine the contact angle of a dynamic liquid by measuring the speed at which the liquid penetrates into the filled powder (Eqn. 2). However, the penetration height of wetting liquid is difficult to determine. As the penetration height changes, the air in the capillary tube gets compressed and the air pressure increases [40]. Therefore, Eqn. 2 can be revised by the relationship between osmotic pressure and height and Eqn. 3 is obtained. Let Kɵ can be obtained from the linear relationship between (ΔP) 2 and t. In this study, the water contact angle of degreased cotton with good hydrophilicity was set to 0, that is, when ɵ= 0, then the equation becomes K0 = LV   . Since the bulk densities of all the test samples was almost the same and deionized water was used as the wetting liquid, the β, η, and ϒLV values of the test samples were approximately the same. Due to these similarities, Eqn. 4 can be obtained for calculating the water contact angle on the surface of tested fiber. The schematic diagram of the self-made device is shown in Figure 1.
where h denotes the penetration height of the wetting liquid in time t; Reff denotes the effective capillary radius; ϒLV denotes the surface tension of the wetting liquid; θ denotes the water contact angle; η denotes the viscosity of the wetting liquid; t denotes the penetration time; ΔP denotes the change in osmotic pressure, and β denotes the parameters related to the stacking mode of the stacked bed. A certain amount of PP-g-(AA-MAH) fibers was taken in a conical flask containing aniline solution to achieve a concentration of 300 mg/L, as that of simulated wastewater. After adsorption at 25 °C under shaking, the fibers were taken out. After allowing to stand for 5 minutes, the aniline concentration was determined and the adsorption capacity was calculated from equation.
where Q (mg/g) denotes the adsorption capacity; C0 (mg/L) and C1 (mg/L) denote the concentrations of aniline in the solution before and after adsorption; V (L) denotes the volume of the solution; and m (g) is the mass of fiber as absorbent.
The adsorption-saturated PP-g-(AA-MAH) fiber was desorbed by shaking in 0.5 mol/L HCl solution for a certain period of time. The concentrations of aniline in the solution were measured at different times and the desorption ratios were calculated from Eqn. 6.
where D (%) denotes the desorption ratio; C denotes the concentration of aniline in the eluent (mg/L); Q is the adsorption capacity of the fiber (mg/g); V is the volume of the eluent (L), and m denotes mass of fiber (g).

The mechanism of suspension grafting polymerization of two monomers
The schematic diagram for the mechanism of suspension grafting polymerization of AA and MAH monomers [33] is shown in Figure 2. In the suspension grafting reaction system, under the action of shear force provided by high-speed stirring, each PP resin particle swelled up and was surrounded by xylene. It dispersed into an independent "reaction bed" in deionized water as the dispersant. The grafting polymerization in each "reaction bed" mainly occurred in the amorphous regions of PP resin. In this study, PP resin was quenched before the graft polymerization, in order to enlarge the amorphous region and PP transformed from a resin to a particle with surface micropores. The swelling effect could not only expand the amorphous region further but also enabled the BPO initiator and AA and MAH monomers to diffuse through the surface and into the pores of PP particles more uniformly. Especially, under the action of xylene, they could migrate to the internal micropores of PP more conveniently.
Generally, the initiator first decomposes to form primary free radicals. The primary free radicals can generate PP macromolecular free radicals denoted as α, by capturing α-H from the PP molecular chains (Reaction 1). The macromolecular free radicals, α, react with monomers to form the target product, ε (Reaction 3). However, side reactions like homo-polymerization of monomers (Reaction 4) and PP degradation (Reaction 2), also occur during this process. In the presence of AA, AA first attacks the PP macromolecular radicals α during the grafting polymerization, since AA is an electron provider and has higher reactivity. It can rapidly react with the PP macromolecular radicals α. Moreover, it inhibits side reactions like degradation to form stable PP-AA macromolecular free radicals. The macromolecular free radicals further react with AA or MAH, which is the main reaction pathway. At the same time, due to low reaction temperature, it becomes difficult for the degradation products to react with the monomer (Reaction 5). The PP used in this research is a special raw material for melt-blown process. It has high crystallinity, but the suspension grafting modification is mainly carried out in the amorphous regions of PP. Therefore, the PP melt can be quenched to reduce its crystallinity, which makes the grafting polymerization easier. It can be seen from the X-ray diffraction patterns of PP before and after the quenching treatment in Fig. 3 that the dispersion peak intensity (shaded area in the figure) of the amorphous region of the quenched PP was significantly greater than that of the original PP. Since the quenching process greatly shortens the crystallization time of PP melt, some of the PP macromolecular chains have no time to move, transform from the disordered coils and then grow into crystals, which results in lower crystallinity.

Effects of different parameters on grafting ratio in suspension grafting polymerization
The main factors affecting grafting in suspension grafting polymerization include the grafting time, the AA/MAH ratio, the total amount of monomers, and the dosages of BPO, xylene, and deionized water. Therefore, effects of the six factors on the grafting ratio were all investigated and the results are shown in Figure 4. It can be seen from Figure 4A that the grafting ratio increased rapidly with increase in grafting time and it reached a stable state after 3 hours. The half-life period of the initiator BPO at 90 °C is 1 h [33]. In the initial stages of the reaction, BPO decomposed into a large number of primary free radicals, which rapidly initiated the grafting reaction. In addition, the monomer concentration was also high at this time, due to which the grafting ratio increased rapidly. After 3h, BPO was basically decomposed and the monomer concentration was also lower, exhibiting a more stable grafting ratio.
From Figure 4B it was evident that when the mass fraction of AA was 0, the grafting ratio was only 1.38%. Subsequently, as the AA/MAH ratio increased, the grafting ratio increased at first and then decreased. With increase in the amount of monomer AA added, the concentration of macromolecular free radicals with AA chains increased. This increased the probability of free radicals contacting with PP. In addition, the monomer MAH could not undergo self-polymerization, due to its steric hindrance. When AA was added, it could form a copolymer with MAH and then graft with the PP segment [41], due to which the grafting ratio increased. However, as the amount of AA added continued to increase, the probability of capturing primary free radicals increased, thereby forming more number of AA-containing macromolecular free radicals. The probability of termination due to recombination between these free radicals increased, which resulted in lower grafting ratio. In addition, more the number of AA molecules, more was the probability of AA initiation, which then reduced the probability of formation of macromolecular free radicals of PP, thereby affecting the grafting ratio.
It can be seen from Figure 4C that as the total amount of monomers AA and MAH increased, the grafting ratio increased at first and then decreased. Higher monomer concentration increased the effective collisions between the monomers and PP macromolecular radicals, thereby increasing the grafting ratio. However, grafting polymerization mainly occurred in the amorphous regions of PP. Although the amorphous regions of PP expanded after quenching, the reaction space was still limited. Too high monomer concentration led to more homopolymerization as the side reaction. Additionally, the mass transfer of monomer and initiator were also affected, so the grafting ratio decreased.
With increase in the amount of initiator BPO, the grafting ratio first increased and then decreased. When the amount of added BPO accounted for 1.4% of PP, the grafting ratio reached the maximum value of 12.47% ( Figure 4D). On heating, BPO first decomposed into benzoyl radicals. Due to steric hindrance, benzoyl radicals were further decomposed into benzene radicals with stronger capability for H atom. Benzene radicals flowed and diffused slowly in the PP particles within limited space, depriving the α-H from the PP chains to form PP macromolecular radicals and initiating monomer polymerization. Therefore, by appropriately increasing the amount of BPO more number of benzene radicals could be generated. This increased the number of monomers participating in the grafting reaction and increased the grafting ratio. However, excessively high concentration of benzene radicals could cause explosive polymerization and lead to an increase in homopolymerization side reactions. This would again affect the progress of the grafting reaction and also result in degradation of PP [42].
It can be seen from Figure 4E that with the increase in amount of xylene, the grafting ratio increased rapidly and then decreased slightly. With xylene concentration of 6 mL/g PP, the grafting ratio reached the maximum value of 12.47%. The monomers used in this study possessed polar groups, and hence their solubility parameters were quite different from that of PP, which implied lower swelling capability of PP. The addition of certain amount of xylene helped to increase the swelling degree of PP and provided greater space for the grafting reaction. Meanwhile, the addition of xylene also kept the concentration of water-soluble monomers at the interface to a low level. This effectively reduced homo-polymerization as side reaction and increased the grafting ratio. However, too much of xylene readily formed a film covering the surface of PP, which hindered the grafting reaction between monomer and PP and affected the grafting ratio. Figure 4F shows that as the amount of deionized water as dispersant increased, the grafting ratio increased rapidly and then decreased. As the amount of deionized water reached 8 mL/g PP, the grafting ratio reached the maximum value. If the amount of deionized water added was too less, the dispersion of PP into multiple independent "reaction beds" became difficult, since PP particles agglomerated in the reactor to form agglomerates, resulting in a low grafting ratio. With too much of deionized water, although the PP particles could form numerous independent "reaction beds", the concentration of monomer dissolved in the water decreased. Hence, the monomer concentration gradient between the dispersion medium and the interfacial reaction layer became smaller. As a result, it was difficulty for the monomer molecules to enter the interfacial layer and participate in the grafting polymerization reaction, due to which the grafting ratio decreased.

Effect of suspension grafting on the performance of melt-blown spinning product
Melt-blown spinning has strict requirements of fluidity and thermal stability of raw materials. PP resin itself has greater fluidity, but after its modification with polar groups, the interactions between molecular chains increase and the fluidity is affected. In addition, in the melt-blown spinning process, the raw materials are melted at high temperature and extruded through the screw. Hence, only better thermal stability can ensure that no decomposition of the raw materials occurs during the process. Therefore, the melt flow-rate (MFR) and thermogravimetric (TG) of PP and PP-g-(AA-MAH) were investigated to determine whether it can be spun into fibers. The results are presented in Figure 5. It can be seen from Figure 5A that MFR (570 g/10 min) of the product PP-g-(AA-MAH) of suspension grafting was lower than that of the PP raw material (1296 g/10 min), implying decrease in fluidity. However, the MFR was still higher than that required for the melt-blown process of PP melt (400 g/10 min). The decrease in MFR could be attributed mainly to the strengthening of hydrogen bonds between the grafted polar chains [33,38]. Compared with the TG curves of unmodified PP in Figure 5B, the product of suspension grafting PP-g-(AA-MAH) had better thermal stability. PP-g-(AA-MAH) showed no significant weight loss before 250 °C, and hence the melt-blown spinning at maximum temperature of 220 °C basically did not cause decomposition.

Fourier transform infrared analysis (FT-IR)
PP and its products of suspension grafting -PP-g-AA, PP-g-MAH, and PP-g-(AA-MAH) were spun by melt-blowing and processed into fibers. Their FTIR spectra (the total monomer contents were the same during the suspension grafting process) are shown in Figure 6. The fibers of modified PP-g-AA, PP-g-MAH, and PP-g-(AA-MAH) showed unique characteristic absorption peaks of PP, viz. symmetrical (2924 cm -1 ) and asymmetrical (2842 cm -1 ) C-H stretching vibrations of -CH3, the flexural vibrations of -CH2-at 1380 cm -1 , and absorption of C-H at 1450 cm -1 . Compared with that of PP fiber, the FTIR spectrum of PP-g-AA fiber showed peak for C=O stretching vibrations of the carboxyl group (1720 cm -1 ), indicating the successful grafting of AA onto the PP chain. Compared to the spectrum of PP fiber, PP-g-MAH showed a peak at 1780 cm -1 , which was attributed to the symmetric and asymmetric vibrations of C=O of acid anhydride. The peak at 1720 cm -1 was ascribed to the C=O of carboxylic acid, formed by the hydrolysis of anhydride. The FTIR spectra of PP-g-(AA-MAH), PP-g-AA, and PP-g-MAH fibers all showed peaks for stretching vibrations of C=O of the carboxyl group at 1720 cm -1 . The difference was that the FTIR spectrum of PP-g-(AA-MAH) had stronger intensity for C=O peak, indicating that its carboxyl content was higher than those of PP-g-AA and PP-g-MAH fibers. However, the FTIR spectrum of PP-g-(AA-MAH) fiber did not show peaks for symmetrical and asymmetrical vibrations of C=O of acid anhydride in the PP-g-MAH spectrum. This was due to the fact that MAH was grafted on to PP in the form of maleic acid [42,43]. FTIR results confirmed the successful grafting of AA and MAH onto PP fibers.

1 H-NMR analysis
The carboxyl group in PP-g-(AA-MAH) fiber was linked to -CH-. From the 1 H-NMR analysis (Figure 7), it was evident that compared with PP fiber, PP-g-(AA-MAH) fiber showed a chemical shift at around 2.3 ppm [43], indicating that the carboxyl groups were successfully grafted onto the surface of PP fibers.   Under the conditions of 25 °C, pH = 7, and initial aniline concentration of 300 mg/L, the adsorptivities of aniline on PP-g-(AA-MAH) fibers at different adsorption times were investigated. Thereafter, the regeneration performance of PP-g-(AA-MAH) fibers was investigated using the adsorption capacity and desorption ratio of regenerated fiber, as shown in Figure 9.  Figure 9A showed that PP-g-(AA-MAH) fibers had faster adsorptivity for aniline within the first 45 min, with adsorption capacity reaching up to 42.2 mg/g. Then, the adsorption capacity increased slowly until it reached saturation at 120 min. In aqueous solution, PP-g-(AA-MAH) fibers were deprotonated and negatively charged. Aniline was present in the form of C6H5NH3 + , and so the adsorption between the two components was mainly electrostatic in nature [24,25]. The faster rate of adsorption within the first 45 min was the result of the large number of active sites on the surface of PP-g-(AA -MAH) fibers. With prolongation of adsorption time, the number of active sites on the surface of PP-g-(AA-MAH) fibers gradually decreased along with decrease in aniline concentration. As a result, the driving force for adsorption obviously weakened and the adsorption capacity increased slowly until it reached saturated adsorption.
It can be seen from Figure 9B that after seven adsorption-desorption cycles, the desorption ratio of the PP-g-(AA-MAH) fiber still remained above 80% and the adsorption capacity for aniline could still reach 25.72 mg/g, indicating that PP-g-(AA-MAH) fiber had good regeneration-adsorption performance.

Conclusions
With PP as the matrix, AA and MAH as functional monomers, PP-g-(AA-MAH) fiber was prepared by suspension grafting and melt-blown spinning technology. The technology can be easily scaled-up to industrial level and the fibers can be used to adsorb aniline from wastewaters. The maximum grafting ratio was 12.47% under the following optimal suspension grafting conditions: grafting time of 3 h, AA : MAH = 0.75, total monomer content of 55%, BPO content of 1.4%, xylene content of 6 mL/g PP, and deionized water content of 8 mL/g PP. Melt flow-rate and thermal analysis indicated that the suspension grafting product had good fluidity and thermal stability, and could be spun by melt-blown process. FTIR and NMR analyses showed that AA and MAH were successfully grafted onto PP fibers. The hydrophilicity of PP-g-(AA-MAH) fiber was enhanced. Therefore, the adsorption for aniline from wastewaters was fast and the adsorption capacity could reach up to 42.2 mg/g at 45 min. Moreover, the fibers had good regeneration performance.