Improving the Hydrolysis Resistance of Poly(lactic acid) - American

Feb 23, 2015 - finishing on PLA fiber inhibiting hydrolytic degradation to a great extent ... stability of finished and nonfinished PLA fibers was per...
0 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/IECR

Improving the Hydrolysis Resistance of Poly(lactic acid) Fiber by Hydrophobic Finishing Mingbo Ma† and Wenlong Zhou*,†,‡ †

College of Materials and Texitles, ‡Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Ministry of Education), Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China ABSTRACT: Poly(lactic acid) (PLA) fiber has a tendency to undergo hydrolysis under wet conditions, which has restricted the application of PLA fiber. In this research, we report a new idea for improving the resistance of PLA fiber to hydrolytic degradation with controlled impairment of the fiber properties. The conventional pad-dry-cure method was used for hydrophobic finishing on PLA fiber inhibiting hydrolytic degradation to a great extent under the wet conditions studied. The evaluation of the stability of finished and nonfinished PLA fibers was performed by examining the weight loss, surface morphology, molecular weight, tensile properties, and thermal properties of the samples after being treated with pH 7.4 buffers at 50 °C. Hydrophobic finishing of PLA fiber could be a simple and effective method to endow PLA fiber with good resistance toward hydrolysis, although the elevated curing temperature would cause slight damage to the fiber. filament yarn loses about 20% of its strength and 25% of its modulus when washed 10 times at pH 10, 55 °C.17 A decrease of the MW of the polymer of about 50% has been observed when the fabrics were stored at 40 °C and 80% humidity for 262 days, and for those finished with softeners the degradation was more serious due to the improvement in hydrophilicity of the fabrics.18 Improving the resistance of PLA fiber to hydrolysis is therefore necessary. Some approaches, such as stereocomplexation via polyblending or block copolymerization19−21 and cross-linking through radiation modification,22 have been shown to be feasible to improve the hydrolysis resistance of PLA polymer. However, very limited efforts have been made on PLA fiber. Reddy et al.23 reported that this goal coupled with improvement in the dyeability can be achieved by spinning PLA into polyblend fibers with polypropylene (PP). The obtained 80/20, 50/50, and 20/80 PLA/PP fiber has more strength retention of 52%, 66%, and 79%, respectively, compared to the total strength loss of the pure PLA fiber after being in water at neutral pH and 50 °C for 20 days. Blend spinning of PLA certainly provides a new way to deal with the problem, but it causes a remarkable reduction in mechanical properties of the blend fibers due to the phase separation of the component polymers. Furthermore, it is imaginable that the polyblend PLA fibers show a performance that is characteristically different from the pure PLA fiber. Hydrolysis of PLA polymer is affected by water uptake, the degradation rate increases with the decreasing of the hydrophobicity of the polymer chain.20−22,24 Similarly, hydrolytic degradation of PLA matrices, we assume, is affected by the accessibility of water. In this study, therefore, we have investigated the possibility of improving the hydrolysis resistance of PLA fiber by imparting PLA fiber with a

1. INTRODUCTION Poly(lactic acid) (PLA) fiber, a biodegradable aliphatic polyester derived from renewable natural resources (such as cornstarch), has drawn considerable attention due to its environmental benefits and favorable comprehensive properties. It has a wide spectrum of applications in apparel, homeware, hygiene, industrial, medical, and other products.1,2 Fabrics produced from PLA fiber have excellent hand, moisture management, and easy-care properties, as well as good flammability properties, and thus are favored for applications in apparel and considered to be a promising alternative to common synthetic fibers.3,4 Textile materials are required to possess good stability during the processing and usage stages. However, as an apparel material which must undergo wet processing and frequent laundering, PLA fiber unfortunately is not chemically stable enough in wet conditions. The PLA polymer is inherently susceptible to degrade in wet conditions, even to trace water in the environment, which can induce simple hydrolysis of ester bonds leading to the decrease of the degree of polymerization.5,6 The hydrolytic degradation of the polymer can be accelerated by ultraviolet radiation7 and increased ambient humidity,7,8 temperature, and pH,5,8−10 particularly the aqueous high temperature and/or alkaline conditions under which fabrics usually suffer in downstream processing. This inherent drawback of the PLA polymer has considerably affected the processing and the use of PLA fabrics, as they will cause a reduction in the molecular weight (MW) of the polymer and thus the tensile properties of the fabrics. Substantial research has suggested that degradation of PLA fiber occurs during the routine wet processing, such as scouring, bleaching, and dyeing.11−15 For example, more than 22% reduction in MW of the polymer and 35% reduction in yarn strength occurs when PLA fabric is dyed at 110 °C, pH 5, for 90 min,11,14 and the situation is exacerbated by an increase in dyeing pH, temperature, and time.16 Furthermore, PLA fabrics have also been reported to have experienced substantial degradation during home laundering and even storage. The © 2015 American Chemical Society

Received: Revised: Accepted: Published: 2599

December 10, 2014 February 6, 2015 February 23, 2015 February 23, 2015 DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) Water contact angle and surface fluoride content of finished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution. For the nonfinished fiber before and after hydrolysis, the water contact angles were maintained at 75°−79° and the surface fluorides were not detected. (b) Photographic images of a water droplet on PLA fabric with and without the hydrophobic finishing.

hydrophobic surface through finishing. A commercially available perfluorochemical finishing agent and the conventional pad-dry-cure method have been used. The effects of finishing on the original fiber properties and the resistance of the finished fiber to hydrolysis have been investigated.

instead of PLA fiber because of the test difficulties of the static water contact angle of fiber surface. The finished and nonfinished PLA fabrics were deposited in the buffer solutions which were used in the hydrolysis procedure below, in order to monitor the hydrophobicity changes of the fiber surface. The measurements were made at each time interval after the samples were rinsed 3 times in distilled water and dried at 50 °C. Water contact angle tests were performed under standard atmospheric conditions (20 °C, 65% RH) using an automatic Krüss K100MK2 surface tensiometer. A drop (4 μL) of ultrapure water was dripped on the fabric surface of each sample using a microsyringe and photographed with a black and white CCD camera. The contact angle was determined at different locations for at least six times and the mean value was reported. The fluorine content of the samples was determined by using a Hitachi S-4800 field emission scanning electron microscope and an Oxford-INCA energy dispersive X-ray analyzer (SEM-EDX) operated at an accelerating voltage of 5 kV with a magnification of 1000. The SEM images of all the fiber samples after being coated with Au were collected at an accelerating voltage of 1 kV and with a magnification of 1000. 2.4. Degradation Procedure. Hydrolytic degradation of the finished and nonfinished PLA fiber was performed with 0.1 M potassium phosphate buffers of pH 7.4 at 50 °C, respectively. About 5 g of each sample was incubated in 300 mL of the buffers; samples were drawn at regular time intervals, then rinsed and dried at 50 °C. During the treatment, the buffers were replaced with fresh ones at 1-week intervals to keep the pH constant at predetermined levels. 2.5. Sample Weight Measurements. The weight loss of the samples was evaluated by measuring the dry weight of the samples before and after hydrolysis. Measurements were carried out after drying the samples in a closed weighing bottle in vacuo at 60 °C for 5 h. All reported data points resulted from duplicated measurements performed in two specimens. 2.6. Molecular Weight Measurements. The molecular weight of the samples was determined by gel permeation chromatography (GPC) analysis. Measurements were performed on a Verotech PL-GPC 50 Plus system equipped with a PL-RI detector and two Polar Gel-M organic (300 mm × 7.5 mm) columns from Varian. Tetrahydrofuran (40 °C) was used as eluent and was delivered at a flow rate of 1.0 mL/min. All samples were dissolved in tetrahydrofuran and the injection volume was 20 μL. The Mn of the fiber samples was calculated

2. EXPERIMENTAL SECTION 2.1. Materials. The PLA fiber used in this study was a staple fiber (length 46 mm; diameter 20 μm; thickness 6.7 dtex) with a glass transition temperature of 60−65 °C and a melting temperature of 160−175 °C. The number-average molecular weight (Mn) and polydispersity index (Mw/Mn) of the initial PLA polymer were 1.26 × 105 and 1.53, respectively. The fiber as well as a commercial PLA plain-woven fabric with a basic weight of 160 g/m2 was supplied by Jiangyin Gao Xin Chemical Fiber Co., Ltd. (Jiangshu, China). The hydrophobic finishing agent (Repellan XPF), which mainly comprises a copolymer carrying a perfluoroalkylethyl acrylate segment, an n-alkyl acrylate copolymers segment, and a polymethylolmelamine cross-linking group, was purchased from Pulcra Chemicals (Shanghai, China). The finish could form a hydrophobic thin film on the surface of the textile fiber through self-cross-linking and the esterification of the hydroxyl on the methylolmelamine moiety with the carbonyl end-group of PLA polymer. Citric acid was analytical-reagent grade and was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). 2.2. Finishing Procedure. The hydrophobic finishing conditions used below are the optimized conditions. Many trials of finishing were conducted by varying the concentration of finishing agent, curing temperature, and curing time. The optimized conditions were decided based on the resultant hydrophobic effects and fiber damages. In the optimized condition, first the fiber was immersed in the aqueous solutions of 50 g/L Repellan XPF and 0.5 g/L citric acid for 10 min followed by padding (3 dip 3 nip) to reach an approximate pick-up rate of 70% and then dried at 90 °C for 10 min. Finally, the fiber was cured at the temperature of 150 °C for 3 min to obtain hydrophobic surface. 2.3. Characterization of Finishing Effects. The effects of hydrophobic finishing of the PLA fiber were evaluated by determining the water contact angle and the surface fluorine content of PLA fabrics which was finished at the same condition for the fiber. In this study, PLA fabric was used 2600

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research Table 1. Effects of Hydrophobic Finishing on the Properties of PLA Fiber breaking Tg

Tc

Tm,L

Tm,H

deg of crystallinity

Mn × 105

tenacity

elongation

Young’s modulus

samples

°C

°C

°C

°C

%

g/mol

cN/dtex

%

cN/dtex

control finished

63 63

116 124

165 164

169 169

42.8% 37.2%

1.26 1.23

2.66 ± 0.31 2.47 ± 0.45

27.3 ± 6.3 25.2 ± 7.5

61.5 ± 6.6 59.8 ± 9.5

by reference to a universal calibration curve relative to polystyrene standard. 2.7. Tensile Property Measurements. Tensile properties of the fiber samples were tested according to the Testing Method for Breaking Strength and Breaking Elongation of Synthetic Staple Fibers in China (GB/T 14337-1993) using a XQ-1 laboratorial fiber tensile tester. The breaking tenacity, elongation and Young’s modulus were measured at a gauge length of 20 mm and a strain speed of 20 mm/min, and every reported datum was the average of 50 measurements. 2.8. Thermal Analysis. Thermal analysis was performed using a TA differential scanning calorimeter (DSC Q2000 V24.10 Build 122) at a heating rate of 10 °C/min with a temperature range from ambient to 200 °C and the second scans of the samples are reported.

retain a relatively durable hydrophobic surface in buffer solutions for hydrolysis resistance evaluation. Fiber Structure and Properties. Table 1 shows the effect of finishing on the thermal properties, crystallinity, and mechanical properties of PLA fiber. The finishing did not result in apparent change in thermal properties as the almost constant glass transition temperature (Tg) and double melting points (Tm,L and Tm,H) of PLA fiber before and after finishing are observed. As expected, after finishing, the cold crystallization temperature (Tc) of PLA fiber increased by 8 °C as an evidence of interpenetration of the finishing agent in the fiber which might form cross-linking between the PLA and finishing agent molecules after curing. This also indicates that the finish could go into the fiber and form a hydrophobic inner surface, which of course benefits the resistance of the finished fibers to hydrolysis. The cross-linking leads to a restricted segmental mobility of the macromolecular chains and thereby makes the crystallization of PLA fiber more difficult. Furthermore, microstructural changes of PLA fiber also occurred after the finishing. The X-ray diffraction (XRD) measurement suggested that the crystallinity of PLA fiber declined significantly after finishing, from the initial 42.8% to 37.2%, in contrast to the fact that the curing of nonfinished PLA fiber under the same condition resulted in a slight increase of crystallinity (because of cold crystallization, the XRD patterns are not shown). This disparity can be interpreted in terms of the disturbing effect of the chain cross-linking in the amorphous−crystalline boundary. It suppressed the molecular motion for crystallization, and therefore introduced interfacial shear stress, and in turn resulted in a partial disorder of the crystal lattice and crystal defects during the curing-induced chain rearrangement and crystal reorganization process.22,27 Partial phase transition of the stable perfect crystals to disordered imperfect crystals was also observed in the DSC curves of finished and nonfinished samples, and detailed information is discussed in the following section. The finishing also caused a slight decrease (2.3%) in molecular weight which is due to the high-temperature hydrolytic degradation of amorphous chains during the curing process. Such degradation is well investigated by Tsuji in the temperature range of 120−190 °C.28 As a result of these microstructural damages, loss in tensile properties of finished PLA fiber occurred. After the finishing, the breaking tenacity, breaking elongation, and Young’s modulus of PLA fiber decreased by 7.1%, 13.5%, and 2.8%, respectively. 3.2. Resistance to Hydrolytic Degradation. Weight Loss. Weight loss of PLA fiber during the hydrolysis process is caused by the formation of water-soluble monomer and oligomer lactides and their release into the surrounding aqueous media. Figure 2 shows the weight loss of the finished and nonfinished samples as a function of hydrolysis time. The weight loss of nonfinished fiber accelerated as the hydrolysis proceeded, indicating that accelerated hydrolytic degradation of the fiber occurred. This is due to the exponentially produced carboxylic acid end-groups, which autocatalyzes the hydrolysis of PLA chains via a chain-end scission mechanism.24,29 After 8

3. RESULTS AND DISCUSSION 3.1. Finishing on Properties of PLA Fiber. Hydrophobic Effects and Durability. High temperature curing is necessary for textile chemical finishing. When fluoro-based finishes are applied, high temperature curing of the treated substrates results in cross-linking of the finished molecules and external orientation of the perfluoroalkyl segments which improves the durability and repellency effects.25 A curing temperature of 130−180 °C (usually for 1−3 min) is commonly used for textile finishing,26 but high curing temperatures should be avoided because of a relatively low melting point of PLA fiber (160−170 °C) and the accelerated hydrolytic degradation induced by high temperature.1 A curing temperature of 150 °C for 3 min was used in this study after the resultant hydrophobic effects and fiber damages were comprehensively taken into account. The effects of finishing on the surface hydrophobicity of a PLA fiber are shown in Figure 1. With a surface fluoride content (SFC) of 3.25%, the PLA fabric obtained a remarkably improved hydrophobic surface (Figure 1b), of which the water contact angle (WCA) increased to 133° from 79° of the control. To evaluate the availability of the proposed method, a durable hydrophobic fiber surface is required. To investigate the durability of the hydrophobic surface, an evaluation experiment was performed by treating the finished PLA samples in a buffer solution at pH 7.4 and 50 °C on the fabric instead of dispersed fiber, since it is difficult to measure the static WCA of fiber bundles accurately. The buffer solution media for the durability evaluation was identical to that for the hydrolysis trial in the following experiment, aiming to reflect the changes of surface hydrophobicity during the hydrolysis process. As shown in the figure, the WCA of the finished fabric decreased by 6° accompanied by a significant decrease (1.12%) in SFC after the first week of treatment. It was probably due to the removal of the uncross-linked finish in the buffer solution. Although both the WCA and the SFC of finished fabric decreased slowly with increasing treatment time, an appreciable hydrophobicity was still maintained (i.e., WCA = 120°, SFC = 1.85% after 8 weeks). It suggests that the finished fiber could 2601

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research

Figure 2. Weight loss of finished and nonfinished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution.

weeks in the buffer solution, the nonfinished sample had lost 15.8% of its original mass, compared to the finished sample that had lost 2.0%. The mass of the finished sample also decreased (1.1%) significantly in the first week but did not proceed as progressively as the nonfinished samples did. It should be ascribed to the removal of the uncross-linked finish from the fiber surface to the buffer solution, which is in accordance with the reduction in SFC as discussed above. This suggests that a large extent of hydrolytic degradation might not occur. Morphology. Figure 3 represents the SEM images of finished and nonfinished samples before and after hydrolysis. The images, which are typical of those obtained from the samples studied, showed remarkable differences in original surface morphology and degradation extent. Compared to the smooth surface of the nonfinished sample, the finished surface was shown to have a coarse deposit layer which acted as a liquid−water barrier layer. By increasing hydrolysis time, progressive degradation of the fiber surface was revealed for the nonfinished samples. The deepening pores and crack-like grooves are undoubtedly due to the inhomogeneous hydrolysis of the fiber surface. No significant morphological changes were observed for the finished samples, indicating that no significant surface degradation had occurred. Molecular Weight. The hydrolysis of PLA molecules occurred as directly evidenced by the decrease in the molecular weight (MW). Figure 4 shows that the MW changes of finished and nonfinished samples treated in pH 7.4 buffers were in good agreement with the weight loss and fiber damage. It can be seen that the finished sample had a 3.5% lower initial MW than the nonfinished sample, the MW loss undoubtedly resulted from the degradation during the curing process because PLA has been shown to be susceptible to hydrolysis under a combination of elevated temperature and moisture conditions.28,30 The MW of the nonfinished samples decreased gradually during the first 3 weeks of hydrolysis and then accelerated to give a loss of 89.8% after 8 weeks. While the decrease for finished samples was considerably slower, the final MW loss was only 19.3%. The hydrolytic degradation rate constant k (g/mol day−1) for the hydrolysis period of 0−3 and 3−8 weeks was calculated using the following equation:5,31,32

Figure 3. SEM images of finished and nonfinished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution.

ln M n(t 2) = ln M n(t1) − kt

Figure 4. Change in the molecular weight of finished and nonfinished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution.

where Mn(t2) and Mn(t1) are Mn values at the hydrolysis times of t2 and t1, respectively. The k values for the finished and nonfinished fiber were 1.8 × 10−3 and 8.2 × 10−3 in the first 3 weeks, and 4.5 × 10−3 and 37.5 × 10−3 in the period of 3−8 2602

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research

good retention in mechanical properties which is favorable for practical applications, due to the limited degradation extent. Thermal Properties. Surface hydrophobic finishing of PLA fiber leading to significantly higher stability of the fiber structure to hydrolysis is confirmed by the DSC measurements. Figure 6 shows the DSC thermographs of the finished (Figure

weeks, respectively, indicating a much shower hydrolysis speed of the finished fiber. It also suggested that hydrolysis occurred among finished samples, although no significant evidence was observed on the fiber surface. This was probably due to the fact that hydrolysis of PLA proceeds heterogeneously and goes faster inside than at the surface,33−35 being induced by the moisture penetrated from the external solution. Mechanical Properties. One of the key properties of PLA fiber affected by hydrolysis is its mechanical properties, which reflects the density of tie chains remaining in the amorphous region.31,32 Figure 5 panels a−c illustrate the changes in

Figure 6. DSC heat graphs of (a) finished and (b) nonfinished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution.

6a) and the nonfinished (Figure 6b) fiber samples hydrolyzed for 1, 3, 5, and 8 weeks, respectively. A weak endothermic peak at 63 °C, and a broad exothermic peak from 100 to 130 °C as well as a double endothermic peak in the range of 156−170 °C are related to the glass transition, cold crystallization, and melting process of the PLA, respectively. It is revealed that the Tg did not change as a function of the hydrolysis time, irrespective of the finished and the nonfinished samples. While the Tc of the nonfinished samples decreased gradually to 110 °C from 115 °C after 8 weeks of hydrolysis. Crystallization of PLA becomes easier after hydrolysis due to the increase in the segmental mobility of the broken polymer chains.36,37 Such a decrease was not observed in the case of all the hydrolyzed finished samples. This was probably due to the very limited hydrolyzed degradation that occurred among the finished samples. Double melting behavior is a common phenomenon observed from the DSC thermograms of poly(L-lactic acid) products (including PLA fiber) annealed at a temperature between 105 and 120 °C due to the coformation of the perfect and imperfect small crystals.38−41 The peaks at the low temperature (Tm,L = 162 °C) and the high temperature (Tm,H = 168 °C) represent melting of the imperfect and perfect crystals, respectively. Owing to the increased amount of

Figure 5. (a) Breaking tenacity, (b) elongation-at-break, and (c) Young’s modulus of finished and nonfinished PLA fiber after treatment in buffer solution of pH 7.4 at 50 °C as a function of hydrolysis time.

breaking tenacity, breaking elongation, and Young’s modulus of finished and nonfinished PLA fiber with increasing hydrolysis time. It is apparent from the data that the loss in mechanical properties is much less significant for finished samples than the nonfinished. After hydrolysis for 8 weeks, the finished sample had lost 27.9%, 31.7%, and 17.4% of its original strength, elongation, and modulus, respectively, compared to the nonfinished sample which had completely lost its mechanical properties. This suggested that finished PLA fiber had a much 2603

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research imperfect crystals produced during the fiber curing, the peak height of Tm,L relative to Tm,H became higher. It is interesting to observe that it decreased with increasing hydrolysis time for the nonfinished samples (Figure 6b), indicating a significant loss in the relative amount of imperfect crystals. For the finished samples, however, such a decrease is much less significant. The hydrolysis of the imperfect crystals is probably related to their low thermal and chemical stability since they lack intermolecular interactions and have looser chain packing.42,43 To illustrate more clearly the differences in hydrolysis rates toward the two crystals of the finished and nonfinished samples, the enthalpy changes in melting which reflect the changes in the amount of corresponding crystals, were calculated after Gaussian peak fitting. Because the samples crystallized during the DSC heating, the final melting enthalpy of perfect crystals (ΔHm,H ′ ) does not represent the initial one of the starting perfect crystals (ΔHm,H). The new crystals are speculated to be perfect crystals due to the higher heating temperature (above 120 °C) during the DSC measurement that is beneficial for crystal growth in PLA fiber.43 Therefore, the melting enthalpy of perfect crystals in the starting sample may be evaluated by the following equation:

crystallinity frequently observed.9,34,43−45 In our case, this phenomenon was also observed although degradation of the imperfect crystals occurs. The crystallinity of nonfinished sample had increased significantly from 15.8% to 12.8% over the hydrolysis period, while that of the finished samples had remained almost unchanged. After 8 weeks, the increase in ΔHm,H was 3.44 J/g for the nonfinished sample as compared to almost no change for that of the finished sample. The good retention in ΔHm,L and ΔHm,H also indicated that degradation of amorphous and imperfect crystalline regions in the finished fiber was very limited as compared to the extensive degradation of those in the nonfinished fiber.

4. CONCLUSION This research demonstrated that effective improvement in hydrolysis resistance of PLA fiber can be achieved by imparting high hydrophobicity to the PLA fiber. The hydrophobic finished PLA fiber showed considerably higher hydrolysis resistance in comparison to the nonfinished fiber. The finished fiber had 72.1%, 68.3%, and 82.6% retention in tenacity, elongation, and modulus, respectively, compared to the nonfinished fiber that lost its strength completely when it was treated in pH 7.4 buffers for 8 weeks. Hydrophobic finishing of PLA fiber could be an easy and multioptional method to improve the resistance of PLA fiber to hydrolytic degradation and it provides an alternative to other methods, such as polyblend spinning. This method could alleviate the fiber damage caused by hydrolytic degradation and prolong the use life of PLA textiles. On the other hand, hydrophobic finished PLA fiber could also provide water-repellency to PLA fabrics, which are well suited for waterproof applications. Furthermore, controlled degradation of PLA products could be achieved by endowing them with suitable hydrophobicity. It should be noted that the hydrophobic finishing of PLA fiber resulted in the improvement of hydrolysis resistance, but such improvement was not limited to the perfluoro-based finish and the pad-dry-cure method used in this research. There are many previous studies carried out to endow PLA fiber with high hydrophobicity using novel methods and other chemicals.46,47 We believe that the hydrophobic PLA fibers that they obtained are also more resistant to hydrolysis than the untreated fiber as long as the finished fibers have a certain durability to the circumstances involved.

′ − ΔHC ΔHm,H = ΔHm,H

As shown in Figure 7, the enthalpy change of the imperfect crystals (ΔHm,L) is higher, and that of the perfect crystals

Figure 7. Changes in the melting enthalpy of imperfect (ΔHm,L) and perfect (ΔHm,H) crystals of finished and nonfinished PLA fiber as a function of hydrolysis time at 50 °C in pH 7.4 buffer solution.

■

(ΔHm,H) is lower, for the initial finished sample than that of the nonfinished sample confirming again the occurrence of partial phase transition. As the hydrolysis proceeded, the decrease in ΔHm,L was much more remarkable for the nonfinished samples than for the finished ones, especially after 3 weeks. The ΔHm,L of nonfinished samples had decreased by 1.57 J/g, compared to that of the finished samples that only had a decrease of 0.21 J/g during the 8 weeks’ hydrolysis. It is evident that both the imperfect crystals in the two samples underwent hydrolysis but those of the finished samples showed much better resistance to hydrolysis. The ΔHm,H of the nonfinished sample, however, displayed an increased trend as a function of hydrolysis time. This occurs because hydrolysis of the PLA proceeds much more easily from the structurally loose regions than from the ordered crystalline domains. The degradation of the former part of the polymer results in a larger percentage of the crystalline phase which would be verified by the increase in

AUTHOR INFORMATION

Corresponding Author

*Tel: +86-057186843016. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by the Program for Zhejiang Leading Team of Science and Technology Innovation (No. 2010R50038) and the National Natural Science Foundation of China (No. 51373156).

(1) Gupta, B.; Revagade, N.; Hilborn, J. Poly(lactic acid) fiber: An overview. Prog. Polym. Sci. 2007, 32, 455. (2) Lunt, J.; Shafer, A. L. Polylactic acid polymers from com. Applications in the textiles industry. J. Ind. Text. 2000, 29, 191.

2604

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605

Article

Industrial & Engineering Chemistry Research (3) Gruber, P.; O’Brien, M. Polylactides: NatureWorks PLA in Biopolymers. Polyesters III: Applications and Commercial Products; Wiley-VCH: Weinheim, Germany, 2002. (4) Farrington, D. W.; Lunt, J.; Davies, S.; Blackburn, R. S. Polylactic acid fibres. In Biodegradable and Sustainable Fibres.; Blackburn, R. S., Ed.; Woodhead Publishing Limited: Cambridge, UK, 2005. (5) Auras, R. A.; Lim, L. T.; Selke, S. E.; Tsuji, H. Poly(lactic acid): synthesis, structures, properties, processing, and applications. Part IV: Degradation and Environmental Issues; John Wiley & Sons: NJ, USA, 2011. (6) Hakkarainen, M.; Albertsson, A. C.; Karlsson, S. Weight losses and molecular weight changes correlated with the evolution of hydroxyacids in simulated in vivo degradation of homo-and copolymers of PLA and PGA. Polym. Degrad. Stab. 1996, 52, 283. (7) Copinet, A.; Bertrand, C.; Govindin, S. Effects of ultraviolet light (315 nm), temperature and relative humidity on the degradation of polylactic acid plastic films. Chemosphere 2004, 55, 763. (8) Ho, K. L. G.; Pometto, A. L.; Hinz, P. N. Effects of temperature and relative humidity on polylactic acid plastic degradation. J. Environ. Polym. Degr. 1999, 7, 83. (9) Cam, D.; Hyon, S.; Ikada, Y. Degradation of high molecular weight poly (L-lactide) in alkaline medium. Biomaterials 1995, 16, 833. (10) Yang, Y.; Huda, S. Dyeing conditions and their effects on mechanical properties of polylactide fabric. Am. Assoc. Textile Chem. Color. Rev. 2003, 3, 56. (11) Phillips, D.; Suesat, J.; Wilding, M. Influence of different preparation and dyeing processes on the physical strength of the Ingeo fibre component in an Ingeo fibre/cotton blend. Part 1; Scouring followed by dyeing with disperse and reactive dyes. Color Technol. 2004, 120, 35. (12) Phillips, D.; Suesat, J.; Wilding, M. Influence of different preparation and dyeing processes on the physical strength of the Ingeo fibre component in an Ingeo fibre/cotton blend. Part 2; Bleaching followed by dyeing with disperse and reactive dyes. Color Technol. 2004, 120, 41. (13) Avinc, O. Maximizing the wash fastness of dyed poly(lactic acid) fabrics by adjusting the amount of air during conventional reduction clearing. Text. Res. J. 2011, 81, 1158. (14) Avinc, O. Clearing of dyed poly(lactic acid) fabrics under acidic and alkaline conditions. Text. Res. J. 2011, 81, 1049. (15) Lunt, J.; Bone, J. Properties and dyeability of fibers and fabrics produced from polylactide (PLA) polymers. Am. Assoc. Textile Chem. Color. Rev. 2001, 1, 20. (16) Karst, D.; Hain, M.; Yang, Y. Mechanical properties of polylactide after repeated cleanings. J. Appl. Polym. Sci. 2008, 108, 2150. (17) Karst, D.; Hain, M.; Yang, Y. Care of PLA Textiles. Res. J. Text. Apparel. 2009, 13, 69. (18) Avinc, O.; Wilding, M.; Phillips, D. Investigation of the influence of different commercial softeners on the stability of poly(lactic acid) fabrics during storage. Polym. Degrad. Stab. 2010, 95, 214. (19) Karst, D.; Yang, Y. Molecular modeling study of the resistance of PLA to hydrolysis based on the blending of PLLA and PDLA. Polymer 2006, 47, 4845. (20) Karst, D.; Yang, Y. Effect of arrangement of L-lactide and Dlactide in poly [(L-lactide)-co-(D-lactide)] on its resistance to hydrolysis studied by molecular modeling. Macromol. Chem. Phys. 2008, 209, 168. (21) Andersson, S. R.; Hakkarainen, M.; Inkinen, S. Polylactide stereocomplexation leads to higher hydrolytic stability but more acidic hydrolysis product pattern. Biomacromolecules 2010, 11, 1067. (22) Quynh, T. M.; Mitomo, H.; Nagasawa, N. Properties of crosslinked polylactides (PLLA & PDLA) by radiation and its biodegradability. Eur. Polym. J. 2007, 43, 1779. (23) Reddy, N.; Nama, D.; Yang, Y. Polylactic acid/polypropylene polyblend fibers for better resistance to degradation. Polym. Degrad. Stab. 2008, 93, 233.

(24) De Jong, S. J.; Arias, E. R.; Rijkers, D. T. S. New insights into the hydrolytic degradation of poly(lactic acid): Participation of the alcohol terminus. Polymer 2001, 42, 2795. (25) Rao, N. S.; Baker, B. E. Textile Finishes and Fluorosurfactants; Springer: New York, 1994. (26) Yang, C. Q. Infrared spectroscopy studies of the effects of the catalyst on the ester cross-linking of cellulose by poly(carboxylic acids). J. Appl. Polym. Sci. 1993, 50, 2047. (27) Bowden, P. B.; Young, R. J. Deformation mechanisms in crystalline polymers. J. Mater. Sci. 1974, 9, 2034. (28) Tsuji, H.; Saeki, T.; Tsukegi, T. Comparative study on hydrolytic degradation and monomer recovery of poly (L-lactic acid) in the solid and in the melt. Polym. Degrad. Stab. 2008, 93, 1956. (29) Madhavan, N. K.; Nair, N. R.; John, R. P. An overview of the recent developments in polylactide (PLA) research. Bioresour. Technol. 2010, 101, 8493. (30) Carrasco, F.; Pagès, P.; Gámez-Pérez, J. Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties. Polym. Degrad. Stab. 2010, 95, 116. (31) Tsuji, H.; Del Carpio, C. A. In vitro hydrolysis of blends from enantiomeric poly(lactide)s. Part 3. Homocrystallized and amorphous blend films. Biomacromolecules 2003, 4, 7. (32) Tsuji, H. In vitro hydrolysis of blends from enantiomeric poly(lactide)s Part 1. Well-stereo-complexed blend and non-blended films. Polymer 2000, 41, 3621. (33) Pistner, H.; Bendi, D. R.; Mühling, J. Poly(L-lactide): a longterm degradation study in vivo: Part III. Analytical characterization. Biomaterials 1993, 14, 291. (34) Tsuji, H.; Ikada, Y. Properties and morphology of poly(Llactide). II. Hydrolysis in alkaline solution. J. Polym. Sci., Polym. Chem. 1998, 36, 59. (35) Vert, M.; Mauduit, J.; Li, S. Biodegradation of PLA/GA polymers: Increasing complexity. Biomaterials 1994, 15, 1209. (36) Vyavahare, O.; Ng, D.; Hsu, S. L. Analysis of structural rearrangements of poly(lactic acid) in the presence of water. J. Phys. Chem. B 2014, 118, 4185. (37) Nasef, M. M.; Saidi, H.; Dahlan, K. Z. M. Electron beam irradiation effects on ethylene−tetrafluoroethylene copolymer films. Radiat. Phys. Chem. 2003, 68, 875. (38) Magoń, A.; Pyda, M. Study of crystalline and amorphous phases of biodegradable poly(lactic acid) by advanced thermal analysis. Polymer 2009, 50, 3967. (39) Furuhashi, Y.; Kimura, Y.; Yoshie, N.; Yamane, H. Higher-order structures and mechanical properties of stereocomplex-type poly (lactic acid) melt spun fibers. Polymer 2006, 47, 5965. (40) Zhang, J.; Tashiro, K.; Tsuji, H. Disorder-to-order phase transition and multiple melting behavior of poly(L-lactide) investigated by simultaneous measurements of WAXD and DSC. Macromolecules 2008, 41, 1352. (41) Nofar, M.; Zhu, W.; Park, C. B.; Randall, J. Crystallization kinetics of linear and long-chain-branched polylactide. Ind. Eng. Chem. Res. 2011, 50, 13789. (42) Zhang, J.; Duan, Y.; Sato, H. Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy. Macromolecules 2005, 38, 8012. (43) Pan, P.; Kai, W.; Zhu, B. Polymorphous crystallization and multiple melting behavior of poly(L-lactide): molecular weight dependence. Macromolecules 2007, 40, 6898. (44) Fischer, E. W.; Sterzel, H. J.; Wegner, G. Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reactions. Kolloid Z. Z. Polym. 1973, 251, 980. (45) Iwata, T.; Doi, Y. Morphology and enzymatic degradation of poly(L-lactic acid) single crystals. Macromolecules 1998, 31, 2461. (46) Khoddami, A.; Avinc, O.; Mallakpour, S. A novel durable hydrophobic surface coating of poly(lactic acid) fabric by pulsed plasma polymerization. Prog. Org. Coat. 2010, 67, 311. (47) Bae, G. Y.; Jang, J.; Jeong, Y. G. Superhydrophobic PLA fabrics prepared by UV photo-grafting of hydrophobic silica particles possessing vinyl groups. J. Colloid Interface Sci. 2010, 344, 584. 2605

DOI: 10.1021/ie504814x Ind. Eng. Chem. Res. 2015, 54, 2599−2605