Tinctorial Response of Recycled PET Fibers to Chemical Modifications

Article pubs.acs.org/IECR

Tinctorial Response of Recycled PET Fibers to Chemical Modifications during Saponification and Aminolysis Reactions Vasilica Popescu,*,† Augustin Muresan,† Ovidiu Constandache,† Gabriela Lisa,‡ Emil Ioan Muresan,‡ Corneliu Munteanu,§ and Ion Sandu∥,⊥ †

Faculty of Textiles, Leather Engineering and Industrial Management, ‡Faculty of Chemical Engineering and Environment Management, and §Faculty of Mechanical Engineering, “Gheorghe Asachi” Technical University, Iasi, 700050, Romania ∥ ARHEOINVEST Interdisciplinary Platform, “Alexandru Ioan Cuza” University, 11 Street Carol I, G Building, Iasi, 700506, Romania ⊥ Romanian Inventors Forum, 3 Street Sf. P.Movila, L11, III/3, Iasi, 700089, Romania S Supporting Information *

ABSTRACT: In this article we show that poly(ethylene terephthalate) (PET) fibers obtained from the recycling process of PET bottles can be chemically modified and used to create materials destined to become clothing articles. The modifications of the characteristics of PET fibers through saponification and aminolysis reactions have been studied. The work represents a comparative study of the behavior of PET virgin (of synthesis) fibers versus recycled PET fibers (from PET bottles). Comparisons have been made between the modifications which appear in physical and chemical structures, thermal stability, mechanical properties, and dyeability under the action of some reactions with NaOH, with ethylene diamine, or with their mixture at 20 °C. The FTIR, SEM, EDAX, XRD, DSC, and TGA analyses made evident the differences/similarities between the two types of studied polyester fibers. The recycled PET fibers are much less crystalline, more stable at high temperature, and easier to dye with nonspecific (anionic and cationic) dyes. After the treatment with AgNO3, the virgin/recycled PET fibers, chemically modified (which have NH2 groups), have antimicrobial activities due to their affinity for Ag ions. The silver presence on the treated samples can be seen even after 10 cycles of repeated home laundering. The only inconvenience of the treatments meant to modify the PET chemical structure is tenacity diminution, manifested in different ways, depending on the reagent type, concentration, and duration. However, the recycled PET fibers can be used to create materials destined to become clothing articles (by themselves or blended with synthetic/natural fibers) which can be more easily and more economically dyed in a single bath and a single stage.

1. INTRODUCTION In the textile industry, polyester fibers are the most utilized synthetic fibers, due to their remarkable properties, such as good breaking resistance, thermal resistance, excellent wash and wear ability, antiwrinkle character, and processability.1 Their replacement by recycled poly(ethylene terephthalate) (PET) fibers is growing, as these are cheaper and have the same chemical structure as the virgin PET fibers.1 These reasons make recycled PET fibers usable for the realization of carpets, tapestry, toys, sleeping bags, or clothes; for instance, a 1 m2 carpet can be realized with PET fibers obtained as the result of recycling 60 PET bottles of 2 L, a sleeping bag is obtained from PET fibers produced by recycling 35 bottles, and the necessary fibers for certain clothes are obtained from recycling five PET bottles (for a T-shirt) or 20 bottles (for a sweater).2 Therefore, by processing used PET bottles, at the end of a mechanical processing or of a chemical recycling, one obtains recycled PET fibers.3−6 In the case of mechanical recycling, the polymer from the new product (the fiber) has the same properties as the original polymer, with its physical structure being yet modified.3,4 Chemical recycling implies reactions of hydrolysis,7,8 aminolysis,9−11 and glycolysis12−15 to which the used PET bottles are subjected; depending on the working conditions (temperature (usually greater than 130 °C), reagent concentration, and © 2014 American Chemical Society

catalyst presence), total and/or partial PET depolymerizations occur, when monomers and/or oligomers are obtained. These are subjected to a separation and purification process, and the purified monomers are then used in a new polymerization process16 which results in poly(ethylene terephthalate). Irrespective of the origin of the poly(ethylene terephthalate) (synthesis or recycling), the shortcoming of poor reactivity and implicitly dyeability still remains;1,17 it can only be dyed with disperse dyes at temperatures over 120 °C, or at 100 °C in the presence of accelerators. This involves dyeing in two baths when PET fibers are mixed with other natural and synthetic fibers to form yarns/garments. The tinctorial capacity can be improved by creating functional groups as the result of certain treatments for chemical structure modification.17,18 There are in the literature numerous pieces of information about the methods to obtain recycled fibers,19,20 but extremely few of these concern the manner of producing the chemical modification of recycled fibers and of their tinctorial answer.17 In this paper we have studied the behavior of two types of polyester fibers: recycled (from PET bottles) and virgin Received: Revised: Accepted: Published: 16652

July 22, 2014 October 1, 2014 October 2, 2014 October 2, 2014 dx.doi.org/10.1021/ie5028974 | Ind. Eng. Chem. Res. 2014, 53, 16652−16663

Industrial & Engineering Chemistry Research

Article

Table 1. Experiment Planning and Sample Codification sample code expt no.

virgin PET

recycled PET

0 1 2 3 4 5 6 7 8 9 10

0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10V

0R 1R 2R 3R 4R 5R 6R 7R 8R 9R 10R

treatment conditions (at 20 °C, 24 h)

expt type control samples saponification aminolysis

saponification + aminolysis (simultaneous)

saponification + aminolysis (individually, 24 h)

− 50 50 50 50 50 50 50 50 50 50

mL mL mL mL mL mL mL mL mL mL

of of of of of of of of of of

2 M NaOH 0.5 M ED 1 M ED 2 M ED 2 M NaOH + 50 mL of 0.5 M ED 2 M NaOH + 50 mL of 1 M ED 2 M NaOH + 50 mL of 2 M ED 2 M NaOH, t = 24 h; +50 mL of 0.5 M ED, t = 24 h 2 M NaOH, t = 24 h; +50 mL of 1 M ED, t = 24 h 2 M NaOH, t = 24 h; +50 mL of 2 M ED, t = 24 h

90 °C, time 30 min) and then rinsed in quantity with deionized water at 40 °C until pH 7 was reached for the wastewater that resulted from washing. Finally, several rinsings with deionized water at 20 °C were performed. The samples were then dried at room temperature. We specify that the residual floats (from each treatment and from the respective washing and rinsing) have been collected for treatment before passing them to wastewaters; in the situations presented in this paper, the ethylene diamine residues (from residual floats) were removed by means of the photooxidation treatment using hydrogen peroxide as oxidant. 2.3. Methods of Analysis. 2.3.1. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Analysis. FTIR analysis was carried out on a multiple internal reflectance accessory (SPECAC, SUA) with an ATR KRS-5 crystal of thallium bromide iodide, having 25 reflections and the investigation angle of 45°. This accessory device was attached to the spectrophotometer FTIR IRAffinity-1 Shimadzu (Japan), the spectra registration being realized with 250 scans in the 1800−600 cm−1 range. After the registration, the absorption spectra have been electronically superposed (using the Panorama soft from LabCognition Co.). 2.3.2. SEM and EDAX Analyses. A QUANTA 200 3DDUAL BEAM electron microscope was used, which is a combination of two systems (SEM and focused ion beam (FIB)), by whose means, by sending an electron beam on the treated samples, three-dimensional images could be obtained, with a magnification of 100000×. Moreover, by use of energy-dispersive X-ray analysis (EDAX), elemental analyses were possible for the identification of the surface characteristics and a high resolution chemical analysis. 2.3.3. XRD Analysis. An X’PERT PRO MRD X-ray generator (produced by PANalytical Holland) with a copper target was used for collection of intensity data. Using the indications from the literature,21 the following identical experimental conditions were maintained for all the samples tested in this paper: tube voltage = 35 kV, tube current = 20 mA, vacuum pressure = 1/2 mbar, slit width = 80 μm, counterslit width = 250 μm, wavelength of Cu Kα radiation λ = 1.5418 Å, countersample distance (A) = 20 cm, and capillary diameter (d) = 1 mm. The room temperature was maintained at 22.5 ± 0.5 °C. Monochromatic Cu Kα (λ = 1.54 Å) radiation was obtained using a 10 μm thick nickel filter to irradiate polyester fibers packed in Mark capillary tube of 1 mm diameter. The results were interpreted by the X’PERT PRO MRD software, and in the end the material diffractogram was obtained.

(synthesis). Comparisons have been made among the modifications appearing under the action of NaOH, of a diamine, or of a NaOH and diamine mixture, in the physical and chemical structures, the thermal stability, and dyeability. The analyses by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) revealed the differences/similarities between the two types of studied polyester fibers. The recycled PET fibers are much less crystalline, more stable at high temperature, and easier to dye with nonspecific (anionic and cationic) dyes, and they have a higher affinity to Ag ions, which gives them antimicrobial capacity. In this paper we shown that polyester fibers obtained from the recycling process of PET bottles can be chemically modified and used to create materials destined to become clothing articles.

2. EXPERIMENTAL SECTION 2.1. Materials. Virgin polyester fibers (3.37 dtex) and recycled PET fibers of fineness 3.38 dtex acquired from Green Fibers Buzau S.A., pure ethylene diamine (ED) (from Merck Co.), NaOH (Merck Co), Eosin Y (C.I. Acid Red 87 from Merck), Methylene Blue (C.I. Basic Blue 9 from Fluka AG), and disperse dye Foron Briliantrot S-GL (C.I. Disperse Red 121 from Clariant Co.) were utilized. The chemical structures of the dyes are presented in Figure S1 in the Supporting Information. Recycled PET fibers were obtained at the end of a mechanical recycling, and their main properties (as compared to those of virgin PET fibers) are presented in Table S1 in the Supporting Information. 2.2. Working Procedure. The reactivity of virgin and recycled fibers was improved by saponification and aminolysis operations. Saponification (at 20 °C, 24 h) consisted in treating samples of 2 g of fibers with a solution of 2 M NaOH, and aminolysis treatment was with 50 mL of ED of 0.5−2 M concentration. At the same time, mixed saponification (50 mL of 2 M NaOH) followed by aminolysis performed immediately or after 24 h (with 50 mL of 0.5−2 M ED) has been tested. When the saponification and aminolysis phases occur separately, the samples are treated with 1 mL of concentrated HCl and 150 mL of distilled water for 15 min at 20 °C, immediately after saponification, in order to convert the COONa groups into COOH. The experimental protocol is presented in Table 1. After each performed treatment, the samples were washed with a solution of 1 g/L nonionic surfactant (temperature of 16653

dx.doi.org/10.1021/ie5028974 | Ind. Eng. Chem. Res. 2014, 53, 16652−16663


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TINIUS OLSENH5K-T type STM-466 dynamometer respecting the SR EN ISO 5079-2000 standard. 2.3.7. Treatment with AgNO3: Conductibility of Residual Solution. Taking into account that the recycled PET fibers originate from the transformation of PET bottles from various sources, antimicrobial treatment was a must. Testing was made indirectly, by means of an AgNO3 solution. Namely, the conductibility of the solution remained after AgNO3 treatment was measured: the smaller the conductibility, the more silver ions were attached on the PET fiber, giving it antimicrobial properties. The tests were made on samples of 0.1 g of fibers of virgin PET and recycled PET, and 85 mL solution of 5 g/L AgNO3 for each sample. The solution was intensely stirred, and then the sample was introduced to it and was kept in the dark (in a thermostat) for 0.1−120 h. Conductibility was measured after 0.1, 1, 2, 3, 24, 48, 72, 96, and 120 h with the apparatus WTW Cond 3210 SET1/TetraCon 325. 2.3.8. Durability. The durability effect of treatment with AgNO3 was determined with the home laundering test. According to the standard test SR EN ISO 105-CO6:1999, samples were subjected to 10 repeated home laundering cycles. The tests were carried out with a Mathies Polycolor 2002 machine from Bezema Co., followed by rinsing with distilled water at 40 °C and then drying at room temperature. The durability of treatment with AgNO3 has been assessed by measuring the Ag content remaining on the washed fibers using the EDAX option of the electronic microscope. 2.3.9. Dyeability and Color Measurement. 2.3.9.1. Dyeability. The samples of virgin PET and recycled PET were dyed with two dyestuff classes: specific (disperse) and nonspecific (anionic and cationic) dyes for polyester fibers. Dyeing with disperse dye was performed on a Mathis Polycolor 2002 machine at 125 °C for 60 min with 4% Foron Brilliantrot S-GL (C.I. Disperse Red 121) and 1 mL/L of 30% CH3COOH. The samples were then subjected to soaping at 60 °C with 2 g/L nonionic surfactant for 15 min, rinsing with warm and cold water, and drying at room temperature. Dyeing with nonspecif ic dyes was carried out to prove that the applied treatments result in fiber chemical modifications. The modifications generated by saponification (COO− groups) are revealed by a higher color intensity at dyeing with a cationic dye. The presence of the NH2 groups (resulting from PET aminolysis) makes possible dyeing with anionic dyes. The presence of two new types of groups (COO− and NH2) will determine a more intense dyeing with the class of dyes specific to the prevailing group. Dyeing with the cationic dye Methylene Blue (C.I. Blue Basic 9) was performed with 4% dye and 1% acetic acid at 125 °C for 60 min. After dyeing, the samples were soaped with 2 g/L nonionic surfactant at 60 °C for 15 min, then rinsed with warm and cold water, and dried at room temperature. Dyeings with the anionic dye Eosin Y (C.I. Acid Red 87)) were carried out in two stages: maintain the PET fibers in an acid solution in order to promote the protonation of NH2 groups, and then the sample dyeing operation itself. Protonation was done with 1% acetic acid for 15 min at 20 °C. Dyeing was performed with 4% dye for 60 min at 100 and 125 °C, respectively. After having been dyed, the samples were soaped with 2 g/L nonionic surfactant at 60 °C for 15 min, rinsed with warm and cold water, and dried at room temperature. 2.3.9.2. Color Measurements. Color measurements were performed for colors obtained after dyeing the PET samples

Bragg angles 2θ were determined from diffractograms, while the d-spacing of the crystalline part of PET and t (crystallite size) were calculated according21 to eq 1:

d=

nλ 2 sin θ

(1)

from Bragg’s law: nλ = 2d sin θ

where d is the Bragg spacing (Å), n is the order of reflection (n = 1), λ is the radiation wavelength (λ = 1.542 Å), and 2θ is the diffraction angle (deg). According to the literature,21 the crystallite size was calculated using eq 2:

t=

0.9λ Δω cos θ

(2)

where t is the crystallite size (Å), λ is the radiation wavelength (λ = 1.542 Å), Δω is the peak width at half-maximum (rad), and 2θ is the diffraction angle (deg). Initially, the elements for the calculation of Δω expressed in degrees were determined from diffractograms, and then Δω values were converted to radians (radians = degrees·π/180). 2.3.4. Calorimetry Analysis. The DSC curves have been recorded with a Mettler Toledo DSC1 apparatus in inert atmosphere (nitrogen) at a flow rate of 150 mL/min, with a heating rate of 10 °C/min. Three cycles have been performed: one of heating from room temperature to 300 °C to obtain the melting patterns, another of cooling (300−25 °C) to obtain crystallization data, and then reheating (25−300)°C for remelting. Significant parameters of the samples were calculated: melting temperature (Tm), enthalpy of melting (ΔHm), crystallization temperature (Tc), enthalpy of crystallization (ΔHc), and glass transition temperature (Tg). The mass of the analyzed samples was around 2.5 mg. Crucibles are made of aluminum and have a capacity of 40 μL (Al crucibles, 40 μL with pin). An empty pan was used as reference. The operational parameters were maintained constant for all the tested samples, in order to obtain comparable data. DSC curves were used for calculate the crystallinity. Crystallinity (%) was calculated with eq 3, according to the literature:22 Cr =

ΔHfusion · 100 ΔH100%

(3)

where ΔHfusion is the enthalpy of fusion and is given by the area under the melting endothermic peak (J/g) and ΔH100% is the enthalpy of fusion for a 100% crystalline sample (for PET, ΔH100% = −140 J/g). 2.3.5. Thermogravimetric Analysis. Thermal analysis (TGA) was carried out with a Mettler Toledo TGA-SDTA 851e derivatograph, in nitrogen atmosphere, with a flow rate of 20 mL/min with a heating rate of 10 °C/min between 25 and 800 °C. The sample mass was around 2.5 mg. Aluminum oxide crucibles with 70 μL capacity were used (ME-24123, aluminum oxide crucibles, 70 μL). Operational parameters were maintained constant for all the tested samples, in order to obtain comparable data. 2.3.6. Mechanical Properties. The mechanical properties (tenacity and elongation) of the fibers were determined on a 16654



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Figure 1. Comparison of FTIR spectra of the control samples: 0V (virgin PET) and 0R (recycled PET).

of the original polymer, as they have the same chemical structure, but their physical structure was modified. This could only lead to increasing the capacity of diffusion of disperse dyes without allowing the use of nonspecific dyes or changing the class of dyes used in dyeing; therefore, the dyeability is not changing. Yet, in order to significantly improve the tinctorial capacity, the chemical structure of the PET fibers (virgin and recycled) has been modified by means of NaOH and ED (as in the four treatment variants presented in Table 1), as follows: 1. Saponification reactions23 occur when polyester gets functional groups, according to eq 4.

that were chemically modified (through saponification and aminolysis) and for the colors obtained by dyeing the control samples (namely, “0V” for the series of virgin samples and “0R” for the series of recycled samples; we specify that both dyeing operations and final treatments (after dyeing) applied to the control samples were carried out under the same conditions as in the case of chemically modified virgin/recycled PET samples). For color measurements, a Spectraflash SF 300 type (DataColor) was used, while Micromach2000 software was used to determine the chromatic parameters using the CIELAB (L*a*b*) system. The colors were viewed under the CIE D65 daylight illuminant/10° observer. Using the option Color Difference, the values of L*, a*, and b* of the tested/dyed samples were compared with those of the control samples (untreated but dyed). We have thus obtained the differences ΔL* (lightness difference), Δa* (difference on the red/green axis), and Δb* (difference on the yellow/blue axis). Information offered by these differences is the following: if ΔL* < 0, then the treated sample has a smaller lightness (i.e., it is more intensely dyed) than the control sample; if Δa* > 0, then the tested sample is redder than the control sample; if Δb* > 0, then the tested sample is yellower than the control sample.

2. The aminolysis reaction24,25 (in the presence of ED) is according to eq 5:

3. The simultaneous mixed treatment of saponification and aminolysis results in the end in the formation of both amide terephthalate groups and free carboxylate groups (according to eq 6). In this case, the two bases (NaOH and ED) are more aggressive and cause a significant scission of the polymeric chain (as in the samples codified with 5V−7V and 5R−7R). These transformations are also confirmed by FTIR analysis through the appearance of peaks related to a primary amine and

3. RESULTS AND DISCUSSION 3.1. Chemical Mechanisms. The recycled PET samples have been obtained at the end of a mechanical recycling process; taking into account the indications from the literature,3,4 one can infer that they preserved the properties 16655



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Figure 2. Overlapping of FTIR spectra corresponding to recycled PET samples (same codes as in Table 1).

secondary amide is obtained). One can see that polyester has acquired polar groups; therefore it will have some affinity for water and/or dyeing solutions (for nonspecific dyes: anionic). 3.2. ATR-FTIR. Figure 1 shows the comparison of FTIR spectra of the samples 0V and 0R. Bands characteristic of the 0V sample (virgin polyester), according to Bio-Rad software and the literature,27−29 are presented in Table S2 in the Supporting Information. By comparing the FTIR spectra of 0V and 0R (Figure 1), one can see that through the mechanical recycling process one obtains PET fibers which have identical chemical structures with those of the virgin PET fibers obtained by synthesis.1−3 The only difference between the 0V and 0R spectra appears at the absorption bands characteristic of the COOH group: slight increases of the peaks characteristic of CO stretching (1709 cm−1) and CO stretching (from 1236, 1092, and 1016 cm−1) prove the existence of carboxyl end groups that appeared during recycling.27,29 In Figure 2 is shown the comparison of FTIR spectra of the recycled samples: control sample (0R) and treated samples (1R, 4R, 7R, and 10R). Chemical treatments performed have generated the following changes, highlighted by the FTIR spectra: The aminolysis reaction results in the attachment of an amine group on the polymer chain (sample 4R), a fact confirmed by the appearance of peaks from 1548 cm−1 (NH2 stretching, primary amines have two bands) and 1338−1089 cm−1 (C−N stretching). Simultaneous treatment with NaOH and ED (sample 7R) results both in COONa and OH groups (see reaction 6) and in

of COONa groups, as well as through the decrease of the peaks related to the length of C−C chains.

4. Individual mixed treatments of saponification (for 24 h), followed by acidification and then aminolysis (reaction time 24 h, in the cases of samples 8V−10V and 8R−10R) proceed according to eqs 7 and 8. The COONa groups that resulted during saponification (according to eq 4) are transformed into COOH groups through reaction with 1 mL of concentrated HCl and 150 mL of distilled water, for 15 min at 20 °C.26

During this treatment, there will be both COONa groups (which confirm the saponification) and amide groups (as result of the N-acylation reaction when an N-acylated amine or 16656



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Figure 3. Overlapping of FTIR spectra corresponding to virgin PET samples (same codes as in Table 1).

amide terephthalate groups. NaOH and ED used in the treatment determine an extended action on the polyester, which affects the length of the C−C chain, decreasing it the most (very small peaks within the range 2931−2838 cm−1). The presence of COONa and OH groups is confirmed by the height of peaks from 1709, 1236, 1092, and 1016 cm−1 (for CO stretching), 3311 cm−1 (OH stretching), and 1406 cm−1 (for OH band). Formation of the amide terephthalate group is confirmed by the peaks at 3311 cm−1 (NH stretching overlap with OH stretching), 1668 cm−1 (shoulder for CO stretching), and 1563 cm−1 (combination NH deformation and C−N stretching). Individual treatment with NaOH and ED (in two stages, according to reactions 7 and 8; sample 10R) has both saponification and aminolysis effects; the aminolysis effect prevails, which is also confirmed by the bigger peaks from the range 2931−2838 cm−1 and the extension of a less high peak at 3291 cm−1 characteristic of the COOH group. Virgin samples behave similarly to recycled samples (Figure 3), yet the effect of chemical substances is less visible on virgin samples, since these are less predisposed to chemical attacks, having a bigger crystallinity. 3.3. SEM and EDAX for PET Treated by Saponification and Aminolysis Reactions. SEM images highlight the modifications that appeared at the surface of PET fibers (Figure 4), namely pores, voids in the case of samples 1V and 1R, and some accumulations in the cases of 4V, 4R, 7V, 7R, 10V, and 10R. The EDAX analysis indicates the increase of percent attributable to O (expressed in weight percent) in all

Figure 4. SEM images for PET fibers (same codes as in Table 1). “MF” is the magnitude factor.

samples (virgin and recycled) treated with NaOH or NaOH + ED mixture, as compared to 0V and 0R, respectively. This fact 16657



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crystallite sizes (t). This is also confirmed by the higher accessibility of recycled samples to chemical substances (dyes used for dyeing including): they are dyed more intensely than dyed virgin samples. 3.5. DSC. The DSC curves were used to calculate the maximum values of the temperatures of melting (Tm), crystallization (Tc) and glass transition (Tg), as well as the areas of peaks corresponding to melting (ΔHm), remelting (ΔHrem), and crystallization (ΔHc). These areas were obtained after performing the normalized integration. They were then used to calculate crystallinity (Table 4). The curves related to the first heating and the second heating (Figure S3 in Supporting Information) have an endothermic peak situated in the range 220−270 °C; the area of this peak (calculated through a normalized integral) offers indications concerning the heat necessary for melting (ΔHm from Figure S3a in Supporting Information) and remelting (ΔHrem from Figure S3c in Supporting Information) of the PET. The curves related to cooling present an exothermic peak between 180 and 240 °C that offers indications concerning the heat released during the crystallization process, namely ΔHc (Figure S3b in Supporting Information). By comparing the values of the melting temperature, Tm, with values of the remelting temperature, Trem (from Table 4 and Figure S3a,c in Supporting Information), one can see that the Tm values are shifted to higher temperature than the Trem values. The enthalpy necessary for melting is larger in the first scan (Table 4); this fact might indicate a higher initial crystallinity percent than that calculated after the second heating. Among the tested samples, the sample 0V has the highest crystallinity (in agreement with XRD analysis). In Table 4 are also indicated the values of the glass transition temperature (Tg), which is one of the most important parameters that characterize the PET products of saponification or aminolysis. Tg depends on the heating rate, on the sample thermal history, and on any molecular parameter that can affect chain mobility.31,32 The data from Table 4 indicate that the values of Tg for recycled PET samples range between 76 and 78 °C, as indicated also in specialty literature.22,33−35 The glass transition temperature Tg is due to the PET molecular movement22 and is determined from the second heating scan. Being a second order thermal transition, Tg provides information regarding the softening of the amorphous portion of a polymer as the temperature is increased.36 The Tg values for the recycled samples, saponified and saponified + aminated, are smaller than those of the virgin sample 0V, which proves that these treatments determine physical modifications of PET. The smaller values of Tg also indicate the existence of polar groups which formed intermolecular hydrogen bonds with water;37 from this point of view, the order of the samples with increasing Tg is 1R, 7R, 4R, 8R, 0R, and 0V. The highest Tg corresponds to the sample 0V, which has the largest crystallinity, confirmed by XRD analysis; the treatment with NaOH, ED, or their mixture modifies the crystallite size (t) from PET samples, decreasing it (Table 3). 3.6. TGA. The TGA curves indicate mass loss due to increased temperature during heat treatment. Also, the TGA curves provide information about the thermal stability of the samples tested. PET degradation during pyrolysis takes place in one or two stages depending on how the PET structure is modified by saponification treatments or aminolysis. The TGA curves (Figure S4 in Supporting Information) and derivative thermogravimetric (DTG) curves (Figure S5 in

signifies the increase in the number of COONa and OH groups due to the saponification reaction.30 The presence of N both in virgin samples (4V, 7V, 10V) and in recycled samples (4R, 7R, 10R) indicates the existence of some NH2 groups formed due to the aminolysis reaction or to the combined action of saponification + aminolysis (according to the proposed mechanism). The samples 7V and 7R have the biggest C:O:N ratios (Table 2), which justifies the good color intensity after dyeing with anionic dyes. Table 2. Elemental Analysis of PET Samples sample type virgin PET

recycled PET

a

sample codea

C (%)

O (%)

N (%)

C:O:N

0V 1V 4V 7V 10V 0R 1R 4R 7R 10R

54.16 49.20 45.55 42.86 47.16 49.87 49.68 43.76 42.71 45.21

45.85 50.80 49.77 48.35 48.62 50.13 50.32 48.67 49.73 49.60

− − 4.68 8.79 4.22 − − 7.57 7.56 5.19

1:0.846:0 1:1.032:0 1:1.092:0.102 1:1.128:0.205 1:1.031:0.089 1:1.005 1:1.012 1:1.112:0.173 1:1.164:0.177 1:1.097:0.114

Sample code is the same as in Table 1.

3.4. XRD. Diffractograms of PET samples obtained after scanning with X-rays are presented in Figure S2 in the Supporting Information. Parameters specific to these diffractograms are comparatively presented in Table 3. The names of Table 3. Parameters Specific to XRD Analysis for PET Samples sample codea 0V/0R

1V/1R

4V/4R

7V/7R

10V/10R

a

peak no.

2θ (deg)

d = λ/(2 sin θ) (Å)

t = 0.9λ/Δω cos θ (Å)

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

17.79/17.71 22.66/22.62 25.88/25.81 17.64/17.74 22.67/22.67 25.88/25.80 17.65/17.57 22.59/22.51 25.73/25.66 17.57/17.49 22.59/22.40 25.66/25.51 17.56/17.34 22.59/22.44 25.66/25.36

4.98/5.008 3.92/3.93 3.44/3.45 5.03/5.01 3.93/3.94 3.44/3.45 5.03/5.04 3.94/3.95 3.46/3.47 5.04/5.07 3.94/3.97 3.47/3.49 5.05/5.11 3.94/3.96 3.47/3.51

53.65/53.29 53.00/53.82 70.94/70.02 53.28/53.12 51.65/51.65 67.42/62.74 46.78/46.64 51.48/45.04 62.02/59.52 46.77/46.63 51.48/44.69 62.73/62.71 47.32/47.31 51.48/48.10 62.73/61.16

Sample code is the same as in Table 1.

these parameters are as follows: 2θ is the diffraction angle, d is the Bragg spacing, and t is the crystallite size. Table 3 indicates that all the treated samples (from 1V to 10V and from 1R to 10R, respectively) have smaller crystallites (t) than their untreated control samples (0V and 0R, respectively). By comparing the virgin and recycled samples that were subjected to the same treatment, it was determined that all the recycled PET samples have smaller crystallinities than the virgin PET samples, because they have smaller 2θ angles, bigger d-spacings in crystalline zones, and smaller 16658



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Table 4. Main Characteristics Obtained from DSC Curves first heating

a

cooling

second heating

sample codea

Tm (°C)

ΔHm (J/g)

Crb (%)

Tc (°C)

ΔHc (J/g)

Tg (°C)

Trem (°C)

0V 0R 1R 4R 7R 8R

255.68 254.04 242.17 250.84 251.84 251.17

−50.86 −48.62 −36.70 −48.30 −40.35 −44.08

36.32 34.72 26.21 34.50 28.82 31.48

208.40 208.43 219.29 203.13 206.11 201.29

43.94 43.08 33.00 43.65 37.81 37.74

77.82 77.60 72.16 76.83 73.17 77.15

252.64 251.77 246.55 247.56 248.57 248.06

ΔH

rem

(J/g)

−37.54 −37.35 −30.98 −36.71 −32.75 −33.21

Crb (%) 26.81 26.67 22.12 26.22 23.39 23.72

Sample code is the same as in Table 1. bCr is the crystallinity of PET (calculated by eq 3).

Table 5. Main Parameters Taken from TGA and DTG Curves sample codea 0V 0R 1R 4R 7R 8R

stage

Tonsetb (°C)

Tpeakc (°C)

Tendsetd (°C)

We (%)

Rf (%)

I I I II I I I

387.91 392.77 389.88 602.96 388.15 398.22 388.41

427.55 432.11 430.85 654.8 431.26 438.81 427.63

456.25 462.51 458.97 692.06 457.34 451.53 462.69

96.68 93.62 76.27 8.75 91.05 93.81 87.62

3.32 6.38 14.98 8.95 6.19 12.38

a

Sample code is the same as in Table 1. bTonset is the temperature at which the degradation stage begins. cTpeak is the temperature at which the degradation rate is maximum. dTendset is the temperature at which the degradation stage is finished. eW is the mass loss (expressed in percents) during each stage. fR is the residue (the mass remaining after heating the samples) at 800 °C.

Figure 5. Mechanical properties (tenacity and elongation) of PET fibers.

has the smallest tenacity: the tenacity loss is 53.89% compared with the control sample, 0R. The order of decreasing of tenacity is as follows: 0V, 7V, 1V, 4V, and 10V and 0R, 7R, 1R, 4R, and 10R, respectively (Figure 5). The samples 7V and 7R have the highest tenacity values from all the treated samples, these being with 3.42 cN/tex smaller than the tenacity value of the control sample (0V) and, respectively, with 4.34 cN/tex smaller than the tenacity value of the control sample (0R). The explanation of the behavior of the samples 7V and 7R could be that, under simultaneous action of NaOH and ED, the PET fibers form both COONa and NH2 groups (end-group type, as in the proposed mechanism), and amide-type bridges, supplementary. These amide bridges can be formed as the result of the penetration of ED molecules through the voids/pores formed under NaOH action and of the reaction of the two NH2 groups (from ED) with some ester groups existent in the closest oligomers (neighbors); partial networks can appear. Therefore, these amide bridges are formed inside the PET fiber and the FTIR analysis (which is a surface analysis) does not detect the presence of those CH2 groups derived from ED involved in the bridges from partial networks. The idea of amide bridge formation was launched by Hoang and co-workers,25 who indicated that, under the action of NaOH and ED, oligomers with amide bridges are formed instead of ester bridges, when the molar ratio ED:PET is small. They studied the oligomers’ thermal behavior, but not the mechanical properties. The presence of the amide bridges could consolidate the structure of the PET fiber. This structure consolidation does not remain valid in the cases of 10V and 10R samples, because many voids created during the saponification stage fade or are closed until the moment when fibers get in contact with the ED molecules (the treatment is performed in two distinctive stages). Therefore, there is no possibility of ED penetration through these voids and formation of amide bridges in the PET fiber. Moreover, in these cases the 10R and 10V fibers are in contact

Supporting Information) were used to collect the following values, presented in Table 5: Tonset, Tpeak, Tendset, mass loss (W), and residue (R). In Table 5 one can see that the main degradation stage appeared in the temperature range 387− 462 °C. This stage appears for all the tested PET samples, and Tonset offers information concerning the thermal stability of each sample. The higher the Tonset, the higher is the thermal stability. From Table 5 it follows that all recycled PET samples are more stable than the virgin control sample (0V). Within the temperature range 387−462 °C the PET degradation occurs, resulting in mass losses of 96.68% for 0V and 93.62% for 0R. All recycled samples have mass losses (W) smaller than that of 0V. Stage II (602−692 °C) appears only in the sample 1R and is the result of degradation of oligomers formed during the saponification reaction. In Table 5 and Figure S4 in the Supporting Information, one can see that all recycled PET samples have larger residues than 0V (3.32%), namely varying between 6.19 and 14.98%. 3.7. Mechanical Properties. There are indications in the literature concerning the diminution of mechanical properties of the PET treated with NaOH23 or ED,24 but no indications concerning their behavior in the presence of the action of both bases exerted in a single phase or in two successive stages. In Figure 5 one can see that the virgin untreated/treated samples have higher tenacities than the recycled PET fibers. This could be explained by a bigger crystallinity as compared to the recycled samples (in agreement with the data from Tables 3 and 4 for the XRD and DSC analyses). In the series of virgin PET samples, the sample 10V has the smallest tenacity: the tenacity loss is 35.32% compared with the control sample, 0V. In the series of recycled PET samples, 10R 16659



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with the reagents for 48 h and 15 min, i.e., the longest duration of treatment (24 h with NaOH, 15 min with HCl, and 24 h with ED; see the mechanism). Theoretically, these samples (10V and 10R) have very many NH2 groups (covalently attached on the PET fiber), as well as many scissions of the macromolecular chain, what makes after the first washing shorter chains and so fewer groups of NH2 remain. In Figure 5 one can see that the recycled samples have larger elongations than the virgin samples, because they have a smaller crystallinity and a larger amorphous area (according to Table 4). 3.8. Results of Treatment with AgNO3. 3.8.1. Conductibility of Residual Solutions of AgNO3. The values obtained for conductibility were used to elaborate Figure 6. One can see a

Figure 7. SEM images for PET samples treated with AgNO3 solution (same codes as in Table 1). Magnitude factor = 2500.

Figure 6. Conductibility of residual solutions from treatment with AgNO3 of recycled PET samples chemically premodified through saponification and aminolysis reactions (same codes as in Table 1).

decrease of conductibility with time, which proves the increase of the amount of Ag ions which were bound on amine groups of the polyester fiber. Figure 6 confirms the presence of Ag on polyester fibers, which gives them antimicrobial properties to a certain extent. 3.8.2. SEM and EDAX Results for PET Samples Treated with AgNO3 Solution. The presence of silver on chemically modified polyester samples is confirmed by the SEM images (Figure 7) and EDAX results. EDAX analysis led to a series of pieces of quantitative information concerning the Ag content in PET samples, after the treatment with AgNO3 solution. The samples chemically modified through aminolysis (4R and 4V) or saponification + aminolysis (7R, 10R, 7V, and 10V, respectively) retain Ag: when these samples treated with AgNO3 were intensely washed and rinsed only with water (so were not intensely washed with surfactant, as in a home laundering), the highest Ag quantity was retained by the NH2 groups from 4R (5.80%) and 4V (5.65%). The sample 10R has 5.09% Ag, and 10V has 5.04%. The sample 7R has 4.98% Ag, and 7V has 4.85%. Taking into account information from the literature38 which indicates that the polyester that contains Ag nanoparticles has antimicrobial capacities, we infer that 4R, 7R, and 10R and 4V, 7V, and 10V, respectively, have antimicrobial capacities. EDAX analysis offers both quantitative and qualitative information. The qualitative results are those “maps” or images which indicate the distribution of chemical elements in a sample, as in Tables S3 and S4 in the Supporting Information. Yet, these antimicrobial effects slightly diminish as the number of washing cycles increases. 3.8.3. Durability. We present in Figure 8 the silver content remaining on PET fibers without household washing (without home laundering but intensely washed and rinsed

Figure 8. Content of silver remaining on PET samples after 10 repeated laundering cycles.

only with water), after one cycle of home laundering, and after 10 repeated laundering cycles. One can see that, after the first home laundering, the biggest decreases of the silver content were recorded for the samples 4R and 4V, which means that the laundering removed the silver superficially bound on PET fibers. The difference between Ag content before and after the first home laundering is 0.15% (for 7V) and 0.18% (for 7R) and larger in the other cases (1.05% was maximum, for 4V). After another nine repeated launderings (i.e., after cycle number 10), these differences are higher, namely, 1.33% (7R) up to 2.32% (4V) in comparison with the samples without home launderings, and 0.77% (in 10V) up to 1.32% (4R) as compared to the samples subjected to a single home laundering. Therefore, at the end of the 10 laundering cycles there is enough silver to give the samples an antimicrobial effect. 3.9. Color Measurements. 3.9.1. Dyeing with Disperse Dye. By comparing 0V with 0R, one can see that 0R is lighter (L* = 46.76) than 0V (L* = 46.66) because it is dyed less intensely with disperse dyes. This fact can be explained by the presence of COOH end groups which appear in 0R, during the recycling process; these groups do not have a high affinity for this class of nonionic dyes. In Figure 9a one can see that all the virgin samples treated with NaOH, ED, or their mixture are dyed less intensely than 0V (they have ΔL* > 0). These treatments resulted in the appearance of COONa or NH2 16660



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Figure 10. Color differences (ΔL*, Δa*, and Δb*) between PET samples dyed with cationic dye (C.I. Basic Blue 9) and control samples (0V and 0R) (same codes as in Table 1).

Figure 9. Color differences (ΔL*, Δa*, and Δb*) between PET samples dyed with disperse dye (C.I. Disperse Red 121) and control samples (0V and 0R) (same codes as in Table 1).

structure) dyed with cationic dye, at 125 °C. For interpretation of Figure 10 we took into account the rule that the smaller than zero ΔL* is, the darker the sample is. The colorimetric characteristics of the virgin/recycled PET samples treated with NaOH, ED, or NaOH + ED and dyed with cationic dye resulted in larger or smaller values of ΔL*, according to the smaller or larger number of the newly formed anionic groups (COO−). Figure 10 confirms the fact that the samples 6R and 6V have smaller lightness; therefore, they were dyed the most intensely (6R has L* = 31.57, ΔL* = −39.24, and 6V has L* = 37.59, ΔL* = −33.84). They are followed by the samples 1R (L* = 33.98, ΔL* = −36.84) and 1V (L* = 44.25, ΔL* = −27.17), because these have COO− groups obtained through saponification. In addition, Figure 10 demonstrates that the samples 3V, 9V, 3R, and 9R are dyed very little with cationic dye because they have many NH2 groups that have no affinity for the cations of the dye molecules. 3.9.3. Dyeing with Anionic Dye. Dyeing with acid dye (Eosin Y, C.I. Acid Red 87) was performed at both 100 and 125 °C. 3.9.3.1. Dyeing with Acid Dye, Eosin Y (C.I. Acid Red 87), at 100 °C. Color measurements for tested recycled samples (which have NH2 groups) and dyed with C.I. Acid Red 87 at 100 °C indicate more intense dyeings in comparison with the virgin samples; for example, the color difference ΔL* for 7R is −3.993 while the virgin sample 7V has ΔL* = −2.784. The shade is also modified after performed treatments: the recycled samples are redder and yellower, while the virgin samples are redder with a slighter yellow shade, but bluer. 3.9.3.2. Dyeing with Acid Dye, Eosin Y (C.I. Acid Red 87), at 125 °C. Color measurements for recycled samples, dyed with Eosin Y (C.I. Acid Red 87) at 125 °C (Figure 11), indicate that only the fibers which acquired NH2 groups were dyed more intensely than their control samples, having ΔL* < 0. The

groups (Table S5 in the Supporting Information) which have no affinity for disperse dyes. In addition, these groups could prevent the disperse dye penetration inside the PET fiber. In Figure 9b one can see that most of the recycled samples treated with NaOH, ED, or their mixture are dyed more intensely than 0R (with some exceptions: 1R, 4R, 7R, and 9R). This fact cannot be explained only on the basis of the modification generated by chemical treatment agents, but must be explained also on the basis of changing the physical structure: the growth of amorphous zones (confirmed by XRD analysis) permits both an easier penetration of dyes into the fiber and their diffusion. Samples 1R, 4R, 7R, and 9R are brighter than the control sample 0R (have ΔL* > 0), and the explanation is based on the following hypothesis: the more numerous the groups (COO− and NH2 groups) acquired by treatment are, the more hindrances they form in the way of disperse dye penetration and diffusion within the PET fiber. 3.9.2. Dyeing with Cationic Dye. Dyeing with cationic dye (C.I. Basic Blue 9) highlighted the COONa groups formed as the result of saponification treatment of virgin/recycled PET (Table S5 in the Supporting Information) In fact, the COO− groups predominate in the 1V, 5V, 6V, and 7V samples of virgin PET and in the 1R, 5R, 6R, and 7R samples of recycled PET. Even if in the samples 8V, 8R, 9V, 9R,10V, and 10R COOH groups appeared (as the result of saponifiction from the first stage), they react with ED and in this way only very few of them remain free (as one can see by comparing the heights of the peaks from 3291 cm−1 in the FTIR spectra, in the cases 10V with 7V and 10R with 7R, respectively). Therefore, they are not able to significantly influence the dyeability with cationic dyes, since they are not predominant. Figure 10 shows the values ΔL*, Δa*, and Δb* for a few PET samples (with or without COO − groups in their 16661



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stage) because both PAN and modified recycled PET can be intensely dyed with this dye class (the same as 1R or 7R); if the recycled PET fibers are blended with polyamide fibers, then the obtained yarns can be dyed at 125 °C (or at 100 °C) in a single bath and a single stage by using anionic/acid dyes; if the recycled PET fibers are blended with wool or natural silk for producing an article of clothing, then they can be dyed at 100 °C with anionic dyes. Therefore, an additional phase for dyeing them in the same shade is no longer necessary. In addition, the chemically modified PET fibers can be used as adsorbents for cations or anions contaminants from wastewater.

samples 1V and 1R were dyed worse than 0V and 0R, respectively; therefore, they had bigger values for lightness (ΔL* > 0). This can be explained by more electrostatic rejections exerted between the anionic groups of the saponified PET fibers and the anions of the dye molecules. The shade is also modified after performed treatments: the virgin samples are redder (7V and 10V) or greener (1V and 4V) with a blue shade (1V, 4V, and 7V) or yellow shade (10V) (Figure 11a), while the recycled samples are redder and bluer (with exception of 1V, which is greener and bluer) (Figure 11b).

4. CONCLUSIONS The FTIR, SEM, EDAX, XRD, DSC, and TGA analyses reveal the differences between the two types of studied polyester samples: virgin PET and recycled PET. The recycled PET samples are less crystalline and more dyeable with anionic and cationic/nonspecific dyes. The saponification treatments resulted in the enrichment of virgin/recycled PET samples in COO− groups, which gave them a higher dyeability with cationic dyes. Aminolysis has created new functional groups (NH2 type) in the treated samples, which resulted in a more intense dyeing with anionic dye. Dyeing with a disperse dye has confirmed that the treated PET fibers modify their chemical structure, being enriched in polar groups which have no affinity for these nonionic dyes. After the treatment with AgNO3, the virgin/ recycled PET samples, chemically modified (which includes NH2 groups), have antimicrobial activities, since they include Ag ions. EDAX analysis indicates a small decrease of the Ag content after 10 cycles of repeated laundering cycles; therefore, the PET samples preserve their antimicrobial effect. The mechanical properties are affected differently, depending on the reagent type used in treatment, the concentration, and the duration. Yet, the recycled PET fibers can be used to create materials destined to become clothing articles (by themselves or blended with synthetic/natural fibers) which can be more easily and more economically dyed in a single bath and a single stage.

Figure 11. Color differences (ΔL*, Δa*, and Δb*) between PET samples dyed (at 125 °C) with anionic dye (C.I. Acid Red 87) and control samples (0V and 0R) (same codes as in Table1).

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In conclusion, Figures 10 and 11 demonstrate the increase of the tinctorial capacity of the tested PET samples: the PET samples became dyeable even with nonspecific dyes (cationic and anionic dyes); the lightness difference (ΔL*) reflects the extent of chemical modification of PET through saponification and aminolysis, i.e., the appearence of some new functional groups (Table S5 in Supporting Information) which become dyeing centers in the tinctorial process. This explains why the cationic dye (C.I Basic Blue 9) dyes very intensely the samples 1R, 6R, 1V, and 6V, i.e., those samples which have COO− groups. Eosin Y dyes more intensely those samples which have NH2 groups, namely 4R, 7R, and 10R and 4V, 7V, and 10V. In the cases of 1V, 1R, 7V, and 7R samples, whose dyeability was improved and the mechanical properties were very little affected, one can state that they can be used in the textile industry to create articles of clothing. The other tested samples can be used in the textile industry only blended with other fibers, due to the diminution of their tenacity. Even if this study indicates certain degradations of the treated PET samples, it can still offer solutions for dyeing articles of clothing realized by blending these chemically modified PET fibers with other synthetic or natural fibers. For instance, if a T-shirt is made of yarns constituted by a blend of recycled PET fibers and polyacrylonitrile (PAN), then it can be dyed very well and uniformly only with cationic dyes (in a single bath in a single

ASSOCIATED CONTENT

S Supporting Information *

Section S1, materials; section S2, FTIR analysis; section S3, XRD analysis; section S4, DSC analysis; section S5, thermogravimetric analysis; section S6, qualitative elemental analysis for PET samples treated with AgNO3 solution; section S7, groups that influence the tinctorial capacity of chemically modified PET fiber. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +40 (0)726371108. Notes

The authors declare no competing financial interest.

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REFERENCES

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