liquid paraffin

 

An environmentally responsible polyester dyeing technology using liquid paraffin

liquid paraffinAbstract:Two major challenges for sustainability of the textile industry are water consumption and hazardous industrial effluents. A solvent dyeing technology for polyethylene terephthalate fabrics was developed to facilitate the reduction of chemical consumption and to eliminate the use of water. Liquid paraffin was selected as the optimum dyeing medium from a collection of 110 organic solvents using a combination of Hansen solubility parameters and environmental, health, and safety profiles. The solvent dyeing method developed in the present work offers three advantages over traditional aqueous dyeing: high quality dyed goods without using water and auxiliaries, efficient reduction of surface oligomers without additional reduction cleaning and easy reuse of spent dye liquors. As a result, negative environmental impacts can be minimized compared to conventional aqueous process. Substantial savings in chemicals (115 kg per metric ton of dyed polyester) and water (70 m 3 per metric ton of dyed polyester) can be achieved. The result of a 7-cycle reuse sequence demonstrates excellent color consistency of dyed fabrics.

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By implementing the principle of reducing waste, reusing, and recycling resources, the solvent dyeing process developed in this study effectively reduces the consumption of chemicals and eliminates the use of water. Compared with traditional aqueous dyeing method, the solvent dyeing is a promising alter-native technology for the protection of scarce freshwater resource and for sustainability of the textile industry.
1.    Introduction
Two biggest sustainability challenges for textile industry arewater consumption and hazardous industrial effluents (Alkaya andDemirer, 2014). For example, conventional dyeing process forpolyethylene terephthalate (PET, polyester) utilizes a large amountof water and chemicals (DeSimone, 2002; Ferrero and Periolatto,2012; Hicks and Dietmar, 2007). The discharged wastewater contains various kinds of hard-to-destroy surfactants and unused dyes(Hicks and Dietmar, 2007). Dyeing one kilogram of fiber mayrequire almost 100 L of water (DeSimone, 2002). Given the highproduction volume of PET (48 million metric tons in 2014)(Association, 2015), dyeing process not only leaves large waterfootprint on the planet but also suffers from the increasing costs ofwastewater treatment (Chico et al., 2013; Hoque and Clarke, 2013;Jiang et al., 2010). Therefore, it is highly desirable to develop awater-free and effluent-free dyeing process in order to conserve thescarcewater resources, reduce the negative environmental impactsand improve the economic sustainability.
Considerable efforts have been made tofind new dyeing mediato replace the role of water in dyeing of PET in past decades. Themost activefields are supercritical carbon dioxide (ScCO2) dyeingtechnology (Banchero, 2013; Hou et al., 2010; Long et al., 2014) andsolvent dyeing technology (Gebert, 1971; Kim et al., 2006). Each hasits strengths and weaknesses.
ScCO2 dyeing is a waterless dyeing process that exploits the unique properties of super critical fluids. The advantages of ScCO2 dyeing have been described in several excellent reviews (Bach et al., 2002; Hou et al., 2010; Lewis, 1999). This new dyeing technique appears to be ecologically attractive for the textile industry. How-ever, one disadvantage of ScCO2 dyeing technology is the high pressure requirement in the dyeing process. Process conditions of up to 300 bars are unusual for the dyeing mills (Bach et al., 2002).
The high cost of dyeing machines is a clear obstacle to the commercial adoption of the technology. Another disadvantage of ScCO2 dyeing is the migration of oligomers from within the PET to the surface of textile and to the interior surface of the dyeing equipment (Bach et al., 1996; Hou et al., 2004; Montero et al., 2003). The deposition of oligomers can affect the final quality of the dyed products and cause substantial operational problems in dyeing machineries. This phenomenon is considerably more pronounced in ScCO2 than in water (Hou et al., 2004). However, no method currently available can effectively eliminate the influence of oligomers on dyeing process (Yang and Li, 2000).


liquid paraffinAnother non-aqueous dyeing method is solvent dyeing. Organic solvents have many advantages over water, including requiring no auxiliary chemicals and eliminating the problem of effluent control (Ritter, 1969). The most researched solvents were chlorinated hydrocarbon solvents, especially perchloroethylene (Gebert, 1971; Love, 1978). However, very few of solvent dyeing researches have resulted in direct applications. One problem of perchloroethylene is that the disperse dyes have a higher solubility in perchloroethylene than in water. The higher solubility results in a lower dye uptake which cannot satisfy the production requirements (Love, 1978).
Another problem of perchloroethylene is the environmental, health and safety concerns. Perchloroethylene is one of the substances listed as hazardous air pollutants under the federal Clean Air Act in USA. As with most chlorinated solvents, acute exposure to perchloroethylene primarily affects the central nervous system and causes skin and eye irritation. Chronic exposure to perchloroethylene can adversely affect the neurological system, liver and kidneys.
The objective of this work is to develop a new solvent dyeing method for polyester to facilitate the reduction of chemical consumptions and to eliminate the use of water. Although ScCO2 dyeing had been the focus of waterless dyeing in recent years, solvent dyeing technology was chosen in the present study for the following three reasons. First, solvent dyeing technology does not require expensive high pressure equipment. Aqueous dyeing ma-chines could be adapted for solvent dyeing with minor modifications. Second, solvent dyeing provides a potential way to remove the surface oligomers. Surface oligomers primarily consist of hydrophobic cyclic trimers (Yang and Li, 2000). A lipophilic solvent is expected to have a considerable dissolving ability for those hydrophobic oligomers. Third, solvent dyeing method for cotton had been recently established in our laboratory (Chen et al., 2015).
Developing a solvent dyeing method for polyester would open up a new route for one-bath dyeing of polyester/cotton blends, which would be of great interest to the textile industry. In contrast, ScCO2 dyeing of cotton is still considered a great challenge.
For solvent dyeing technology, the selection of a solvent as the optimum dyeing medium and successful implementation of dye-bath reuse are key to minimizing the negative environmental impacts. Previous solvent dyeing studies indicated that solvent assessment should address the issues related to dyeing performance as well as the potential environmental, health and safety (EHS) impacts. Dye uptake is a very important dyeing parameter. It is an indication of colorfastness and directly relates to dyeing cost and effluent control. Therefore, one of the key criteria of solvent selection is to maximize the dye uptake. In the present paper, C. I. Disperse Blue 56 was selected for study because of its popularity.
Potential solvents were selected using a screening protocol combining Hansen solubility parameters and EHS profiles. The computational predictions were further validated through experimental measurements.
Dyebath reuse is a viable means for reducing the discharge of chemicals. The feasibility of dyebath reuse was examined using a 7-cycle reuse sequence. The content of surface oligomers on PET fabrics was measured to examine the oligomer removal using sol-vent dyeing and the effect of oligomer buildup on the reuse of dyebath. Finally, the environmental impacts of new solvent dyeing procedure were assessed in terms of water and chemical consumption.
2.    Experimental section
This section is organized as follows: first, the solvent selection protocol and dyeing procedures are described. The characterizations of the dyes and fabrics are then provided in detail. Finally, the water and chemical consumption analysis of dyeing process is presented.
2.1.    Materials
Acetone, n-hexane, phenol, sodium hydrosulfite, acetic acid (HAc), sodium acetate (NaAc) and sodium hydroxide were analytical grade. Chlorobenzene and liquid paraffin were chemically pure grade. These chemicals were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Pure C.I. Disperse Blue 56 was purchased from Longsheng Co. (Zhejiang, China). Disperse agent NNO (condensates of b-naphthalene sulphonic acid and formaldehyde) was purchased from Jingci Chemical Co. (Qingdao, China). All chemicals were used as received.
The fabrics used for the dyeing studies were woven fabrics from 100% PET yarns obtained from Taoyuan Silk Factory (Jiangsu, China). The linear density of PET yarns was 150denier. The density of PET fabrics was 80 g/m 2. PET fabrics were scoured in a solution containing 2.0 g/L of anionic surfactants at 80  C for 20 min. The multifiber adjacent fabrics used for colorfastness assessment were purchased from Testfabrics Inc. The chemical structures of Disperse Blue 56 and PET are shown in Fig. 1.
2.2.    Solvent selection protocol
The solvent selection protocol consisted of four steps. First, an initial pool of commonly used solvents was built. Additional sol-vents were added based on literature (Kim et al., 2005; Kim and Son, 2005). Final size of collection was 110. Second, solvents were ranked based on predicted dye uptake. Hansen solubility parameters were used to rank the dye uptake for each solvent (Hansen, 2007). The predictions were experimentally validated by measuring color yields in selected solvents. Third, top ranked sol-vents from dye uptake screening were further filtered using EHS profiles. The key parameters for environmental impact evaluations were acute toxicity on fish, daphnia or algae, log octanol/water partition coefficient and biodegradability. The key parameters for health evaluation were acute toxicity, occupational exposure limits, and skin/eye irritation. The key parameters for safety evaluation were boiling point, flash point and autoignition temperature.
Finally, the best solvent was tested for possible adverse effects on dye and polyester under typical dyeing condition.
2.3.    Dyeing procedures
For the solvent dyeing, PET fabrics weighing 5.0 g were dyed in different solvent mediums with 5.0% on the weight of fabric (o.w.f.) of Disperse Blue 56 and afixed liquor-to-goods ratio of 50:1. Dyeing started at ambient temperature. Temperature was raised to the dyeing temperature at a rate of 2C/min and held at the dyeing temperature for 4 h. After dyeing, the fabrics were washed using a mixture of n-hexane and acetone for 10 min at 40C.
For the conventional aqueous dyeing, PET fabricsweredyed with a dye concentration of 5.0%o.w.f. at 130  C for 60 min. The liquor-to-goods ratio was 15:1. The pH was 5.0 adjusted by HAc/NaAc. The concentration of disperse agent NNO was 2.0 g/L. The dyeings were performed with a Rapid H-12SF laboratory dyeing machine. After dyeing, the samples were reductively cleaned in a bath containing 2 g/L sodium hydrosulphite and 2 g/L sodium hydroxide for 10 min at 80 C, then rinsed in water, and dried at room temperature.
For the adsorption isotherm study, PET fabrics weighing 0.3 g were dyed in the selected solvent with different concentrations of Disperse Blue 56 and afixed liquor-to-goods ratio of 300:1. Dyeing was performed at 130 C for 4 h. The data were evaluated for compliance with Nernst, Langmuir, Freundlich and BrunauereEmmetteTeller (BET) isotherm models.
For the reuse of the spent dye liquors, dyeing was carried out at 130  C for 4 h with 5.0% o.w.f. of the dye and a liquor-to-goods ratio of 15:1. At the completion of the dyeing cycle, a sample from the dyebath was analyzed for residual dye concentration. The dyebath was reconstituted to the concentration required for the following dyeing. A second batch of fabrics was placed in the dyebath for a second dyeing cycle. The sequence was repeated for six times.K/S values and color difference (DE) were used to assess the color reproducibility of dyed fabrics.
 



2.4. Measurements
2.4.1. Solubility measurement
 To measure the solubility of oligomers in liquid paraffin at room temperature, 10 mg of the oligomers was placed in 10 mL of the solvent at 25 C. The solution was centrifuged and filtered to remove undissolved solids. The concentration of the oligomers in filtered solution was determined spectrophotometrically. The solubility was then calculated.
To measure the solubility of oligomers in liquid paraffin at 130 C, 10 mg of the oligomers was placed in 10 mL of the solvent at 130 C for 120 h. The sample from supernatant was quickly diluted using cold dioxane. The concentration of oligomers was then determined spectrophotometrically. The solubility was then calculated.
2.4.2. Dye uptake
The dye remaining in the fabric was stripped using a hot extractor. The extractor was a mixture of chlorobenzene and phenol with a mass ratio of 1:1. A fixed quantity of acetone was added after the fabric was dissolved. The concentration of dye on fabric was determined by a Shimadzu UV-1800 spectrophotometer.
2.4.3. Color yield
Depth of shade was assessed in terms of the color yield (K/S) values. TheK/Svalue was determined by a Datacolor 650 spectrophotometer. The reflectance at the wavelength of maximum ab-sorption (lmax¼630 nm) was used to calculate the color yield of dyed fabrics.
2.4.4. Colorfastness
Colorfastness to laundering was determined using a Darong SW-12 washing colorfastness tester according to American Association of Textile Chemists and Colorists (AATCC) test method 61e2006 (2A): Colorfastness to Laundering, Home and Commercial: Accelerated (AATCC, 2006). Dry and wet colorfastness to crocking were examined using an Atlas AATCC Mar CM-5 Tester according to AATCC test method 8-2005: Colorfastness to Crocking: AATCC Crock meter Method (AATCC, 2007).
2.4.5. Oligomer analysis
For oligomer analysis, all the fabrics were treated using the same dyeing procedure without dye. The surface oligomers were extracted using a Soxhlet apparatus with tetrachloroethylene (Yang and Li, 2000). The solvent was then removed. The extracts were dissolved in dioxane and quantified by a Shimadzu UV-1800 spectrophotometer.
2.4.6. Characterization of textile
The X-ray diffraction (XRD) measurements were performed on a Rigaku D/max 2550 PC X-ray diffractometer with a Cu X-ray tube operated at 40 KV and 200 mA. The scan was done in the 2qrange from 10 to 60with a step of 0.02.
A Nicolet Nexus-670 Fourier transform infrared spectroscopy (FTIR) spectrometer was used to obtain all the infrared spectra via the attenuated total reflection method. Data were collected with 10 scans over a resolution of 0.48 cm 1.
Thermogravimetric analysis (TGA) was measured using a Netzsch TG-209-F1 apparatus from 30 to 600C under nitrogen atmosphere with a flow rate of 40 mL/min. The heating rate was 10C/min. The weight of the samples was 4.5±0.5 mg.
Tensile testing was carried out on a H10K-S tester in accordance with the method detailed in American Society for Testing Material (ASTM) D 5035-06: Breaking Force and Elongation of Textile Fabrics: Strip Method (ASTM, 2006).
2.4.7. Characterization of dye
To investigate any possible shade change of disperse dyes in solvent at high dyeing temperature, UVevis analysis of Disperse Blue 56 in liquid paraffin was performed using a simulated dyeing process without fabrics. The dyebath was heated at 160C for 4 h without the polyester and then scanned from 400 nm to 800 nm using a Shimadzu UV-1800 spectrophotometer.
2.5. Water and chemical consumption analysis of dyeing process
The water and chemical consumption analysis was based on the dyeing process for 1 metric ton of PET fabrics. For aqueous dyeing, 5.0% o.w.f. of disperse dyes was assumed. The liquor-to-goods ratios for dyeing and reduction cleaning were set to 10:1 and 20:1, respectively. The auxiliary chemicals consisted of pH buffer; disperse agents, sodium hydroxide and sodium hydrosulfite. For solvent dyeing, 5.0% o.w.f. of disperse dyes was assumed. The liquor-to-goods ratios for dyeing and rinsing were set to 15:1 and 20:1, respectively. No auxiliary chemicals were used. The detailed analysis is provided in the supplementary data.
3. Results and discussion
To develop a new solvent dyeing technology, potential solventswere selected using a screening protocol combining Hansen solubility parameters, EHS profiles, and experimental measurements.
The effects of selected solvent on polyester and dye were characterized. The solvent dyeing parameters were optimized and theperformance of dyed fabrics was reported. The surface oligomerremoval using solvent dyeing was then examined. The dyebathreusewas implemented to eliminate the use of water and to reducethe consumption of chemicals.
Finally, environmental friendliness of the new solvent dyeingtechnology was quantitatively evaluated in terms of water andchemical consumption analysis.
3.1. Solvent selection
The first step of solvent selection was to collect the commonly used organic solvents. The complete list of the 110 solvents is provided in supplementary data (Table S1). These candidates covered a diverse set of solvents, including alcohols, esters, ketones, ethers, hydrocarbons, chlorinated solvents, aromatics, and dipolar aprotics. Table S1also lists the Hansen solubility parameters of each solvent.
In the second step, solvents were ranked based on the values of absolute difference of solubility parameters between solvent and dye (jDsj). There is a generally accepted relationship between the values of jDsjand the dye uptake on fiber (Kim et al., 2005; Kim and Son, 2005; Urbanik, 1983). A large value of jDsj indicates a poorer solubility of the dyes in the solvent, altering the partition coefficient in favor of the dye in the fiber. Therefore, the dye uptake in each solvent could be predicted using the values of jDsj.
To experimentally validate the predictions on dye uptake, the K/ S values of dyed PET fabrics were measured in selected solvents.
Table 1shows the solubility parameters of nine different solvents and the corresponding K/Svalues of dyed PET fabrics. As shown in Table 1, the K/Svalues generally increase with the increasing values of jDsj. It should be noted that the K/S values were only an approximation for the dye uptake. Still, the data in Table 1 supported the idea of using the value of jDsj to predict the dye uptake.
Consequently, top 20 solvents with the highest values of jDsj were passed to the next step of solvent selection.
The third step of solvent selection involved EHS evaluations. Table S2shows the environmental, health and safety profiles for each solvent. Most of solvents were eliminated due to the flammability potential. Only four solvents passed the safety evaluation:
octadecane, glycerol, liquid paraffin and formamide. Formamide and octadecane were further removed from the candidate list on the basis of health evaluation results. Therefore, only 2 out of 110 solvents passed the tests of solvent selection protocol.
In the subsequent dyeing experiments, glycerol was removed from the candidate list since the viscosity of glycerol (152) was substantially larger than that of liquid paraffin (<2.5). High viscosity might result in levelness and mass transfer problems.
Therefore, on the basis of dyeing performance and EHS profiles, the liquid paraffin was chosen for the following research.
 

3.2. Effect of liquid paraffin on polyester and dye
Under high temperature dyeing condition, the physicochemical properties of PET and disperse dyes might be altered by a solvent.
To further investigate the feasibility of liquid paraffin as non-aqueous dyeing medium, the effect of solvent on PET fabrics was studied using a simulated dyeing process without dyes. The effect of liquid paraffin on Disperse Blue 56 was studied using a simulated dyeing process without fabrics. The results are shown in Fig. 2.
XRD measurements were performed on solvent treated and untreated specimens (Fig. 2a). The crystallinity of the fiber was calculated from the ratios of the areas under the specific regions of crystalline and amorphous diffraction scattering curves (Table S3).
The crystallinity of PET was 44.96±2.36% for untreated PET. The crystallinity of PET was 54.12±2.81% for liquid paraffin treated PET.
The increased crystallinity of PET was caused by the recrystallization of incomplete parts of crystals at high temperature in liquid paraffin. Such phenomenon was also observed when the water was used as dyeing medium, where the crystallinity of PET is increased to 51.72±2.43% (Table S3). A nova analysis indicated that the difference in the crystallinities between liquid paraffin and water treated PET samples was insignificant (P-value: 0.41).
FTIR studies were carried out to examine any possible alteration of existing groups as a consequence of solvent dyeing (Fig. 2b). The spectrum of liquid paraffin treated sample was essentially identical to that of untreated samples. There was no drastic change in the chemical structure of PET after liquid paraffin treatment.
TGA was used to examine the thermal stability of solvent treated polyester (Fig. 2c). The thermogravimetric curves were virtually indistinguishable for liquid paraffin treated and untreated polyester before the thermal decomposition occurred around 400C.
Therefore, solvent treatment did not deteriorate the thermal stability of polyester in the temperature range for textile applications. To investigate any possible degradation or shade change of disperse dye in liquid paraffin, UVeVis analysis was performed (Fig. 2d). The absorption of Disperse Blue 56 in the visible region was unchanged after liquid paraffin treatment. Therefore, the dyeing condition in liquid paraffin had a negligible effect on the shade of Disperse Blue 56.
Overall, available results indicated that the liquid paraffin did not have any adverse impact on the PET or disperse dyes. These results are not surprising since disperse dyes have been designed for high temperature dyeing condition and liquid paraffin is a chemically inert solvent. Together with high color yield and favorable EHS score, multiple lines of evidences suggested that the liquid paraffin was a promising candidate for new non-aqueous dyeing medium.



 

3.3. Dyeing of PET in liquid paraffin
The effect of dyeing parameters was investigated to obtain theoptimal conditions for dye uptake. Unlike conventional aqueousdyeing, auxiliaries are not required when liquid paraffin is used asdyeing medium. The pH of dyebath is also not considered. Thedyeing temperature is the only parameter to be optimized.PET fabrics were dyed at 100, 110, 120, 130, 140, 150 and 160C.
The concentrations of dye onfiber ([D]f) at each dyeing temperaturewere measured. The results are presented inFig. 3. As shown inFig. 3, the [D]increased with increasing dyeing temperature. This is due to the enhanced chain mobility at elevated temperatureswhich created more free volume and facilitated the dye sorption (Yang and Huda, 2003). The [D]f reached saturation when temperature reached 140C. Although the dye sorption still increased when temperature went above 130C, 130C was selected as the dyeing temperature for further studies because it is the temperature used in the industry for polyester dyeing with disperses dyes.
Adsorption isotherm studies were performed to explore the thermodynamic origin of dyeing behavior. Fig. 4 shows the adsorption isotherms of Disperse Blue 56 on the polyester substrateat 130 C using liquid paraffin as the exhaustion medium. The adsorption data were tested against four common isotherms: Nernst, Langmuir, Freundlich and BET.Fig. 4 shows the BET adsorption isotherm, which was the best fit isotherm with a correlation coefficient of 0.995. Thefitted number of adsorbed layers is 1.024, indicating a monolayer adsorption. The standard affinity of disperse dye to PET was calculated to be 7.57 kJ/mol. The fitting results for other three types of isotherms are shown in the supplementary data (Figs. S1eS3).
 3.4. Fastness and mechanical properties of dyed polyester
 Colorfastness to washing and staining of polyester are listed in Table 2. The solvent dyed polyester had acceptable wash fastness for apparel applications. The gray scales for staining of fabrics dyed in solvent were comparable to those of aqueously dyed fabrics. Colorfastness to crocking of dyed polyester is shown in Table 3. The dry and wet crock fastness of fabrics dyed in liquid paraffin was the same as those of fabrics dyed in water.
The breaking strength and breaking elongation of polyester in warp direction are also presented in Table 3. The mechanical properties of polyester dyed in liquid paraffin were slightly worse than those of aqueously dyed polyester and undyed control.
However, since PET is a strong fiber, 1% loss of its breaking strength and 5% loss of breaking elongation should not be a problem for most applications.
The results of wash fastness, rub fastness and mechanical properties clearly demonstrated that liquid paraffin had potential as a new non-aqueous dyeing medium for polyester.
 
 
3.5. Reduction of surface oligomers
Liquid paraffin is a strongly lipophilic solvent. It was hypothesized that hydrophobic oligomers deposited on the textile surface could be efficiently removed using solvent dyeing. Experiments were carried out to validate this hypothesis. Table 4lists the surface oligomer content on solvent dyed fabrics. Reduction cleaning is the most widely used method among the existing approaches. The results from reduction cleaning are also presented for comparison.
The surface oligomer content increased from 0.21% to 0.55% after aqueous dyeing. Increased oligomer content was due to the migration of oligomer in PET to the surface at high temperature.
After reduction cleaning at 80C, the surface content was reduced to 0.31%. In contrast, the surface content decreased remarkably to 0.02% after solvent dyeing. It is generally accepted that the surface oligomer content should be below 0.1% to avoid problems in sub-sequent processing (Yang and Li, 2000). Therefore, solvent dyeing method is superior to reduction cleaning in terms of oligomer removal.
Although surface oligomer content could be reduced using either reduction cleaning or solvent dyeing method, two approaches operated via distinct mechanisms. Reduction cleaning is a chemical way to remove surface oligomers. Oligomer removal is based on alkaline hydrolysis of the ester bonds (Yang and Li, 2000).
Hydrolysis products contain carboxylate groups, which lead to a greater dissolution in water. However, alkaline treatment would also hydrolyze the PET fibers, resulting in undesired effects, such as weight, strength and elongation loss (Yang and Li, 2000).
Solvent dyeing is a physical way to remove surface oligomers.
The oligomer removal is based on the fact that hydrophobic oligomers have a relatively high solubility in lipophilic solvents. The solubility of oligomers in liquid paraffin was determined to be 6470 mg/L at 130C. In contrast, the solubility of oligomers in water was merely 2 mg/L at 130 C(Dugal et al., 1973). The greater solubility of oligomers in liquid paraffin accounted for a 90% reduction in surface oligomers. In comparison, the same amount of reduction would require an alkaline treatment at 130e140C for 30e60 min (Yang and Li, 2000). In addition, unlike the alkaline treatment, there were no chemical reactions involved in solvent dyeing. Neither polyester nor oligomers were hydrolyzed. Therefore, the advantage of solvent dyeing is the high efficiency of surface oligomer reduction with minimal damage to the textile.
Another advantage of solvent dyeing is that unhydrolyzed oligomers could be recycled through precipitation and reused for polymer synthesis. For example, Burch et al. (Burch et al., 2000) reported that high molecular weight polyesters could be obtained from rapid polymerization of a series of cyclic oligomers. PET fibers contained approximately 1e3% oligomers by weight (Goodman and Nesbitt, 1960). Given the large production volume of PET fibers (48 million metric tons in 2014 worldwide) (Association, 2015), the recovery of oligomers is certainly a worthy endeavor.

 
 

3.6. Reuse of the spent dye liquors
To explore the feasibility of the reuse of dye liquor, repeated dyeing with replenished spent dye liquors was carried out for six times.Table 5lists theK/Svalues and color differenceDE of fabrics dyed in each recycling. As shown, the fabrics dyed with spent dye liquors in each recycling exhibited good shade consistency. TheK/S values were consistently high (around 18.5). The color differences were all smaller than 1.0. Thus the color differences were within acceptable limit for commercial applications.
Therewas a concern that the buildup of extractable oligomers in recycled bath might cause dullness in dyeing and precipitation on fabrics (Chakraborty and Sharma, 2001). The effect of oligomer buildup in dyebath on both dyeing behavior and surface oligomer content had been examined to address this concern.
 The good color reproducibility from Table 5suggested that the oligomer buildup did not interfere with the dyeing process up to the 7th batch. The oligomers would remain in solution without precipitation as long as the temperature of dyebath was kept hot enough. Fig. 5 illustrates the surface oligomer concentration during the reuse experiments. The variation of surface oligomer contents was small (less than 0.02%) from batch to batch. The maximum content was 0.04%. The data indicated that oligomer buildup in dyebath had a negligible effect on the surface oligomer reduction.
Given such low surface concentrations, the fabrics dyed in the reused bath were not expected to experience oligomer problems in the subsequent processing.
These results demonstrated that reuse of the spent dye liquor were feasible for the solvent dyeing procedure developed in this work. The data in Table 5suggested that good color reproducibility could be attained in this reuse system. The oligomer buildup in the recycled bath did not seem to have adverse effects on either dyeing behavior or surface oligomer reduction.
 



3.7. Evaluation of environmental impacts
Using solvent dyeing with dyebath reuse, chemical consumption could be reduced and the waste effluents could be eliminated. The quantification of the environmental impacts is presented in Table 6.
The calculation details are provided in the supplementary data.
Compared to aqueous dyeing process, solvent dyeing process demonstrated a substantial reduction in the release of chemicals to environment (115 kg per metric ton of dyed PET fabrics). The majority of reduction came from auxiliary chemicals since the solvent dyeing did not require auxiliary chemicals as in aqueous dyeing. The contribution from dyes was small due to very high exhaustion value of disperse dyes in aqueous bath. By design, the solvent dyeing is a closed loop process. No fresh water was used and no wastewater was discharged. As a result, a sizable amount of water usage (70 m per metric ton of dyed PET fabrics) was saved. In addition, the oligomers in PET fibers could be conveniently collected and reused.
It is estimated that about 13.5 kg of oligomers per metric ton of dyed PET fabrics could be reclaimed. In contrast, hydrolyzed oligomers are thrown away as waste in conventional dyeing.
Traditionally, dyeing of PET is a resource intensive process with difficult wastewater treatment problems. The analysis of chemical and water consumptions indicated that solvent dyeing, as a water-free and effluent-free method, is a valuable approach to improve environmental performance and economic sustainability.
 

4. Conclusions
A solvent dyeing method for polyester was developed to facilitate the reduction of chemical consumption and to eliminate the use of water. The liquid paraffin was selected as the optimum dyeing medium from a collection of 110 organic solvents using a solvent assessment protocol considering both dye uptake and EHS characteristics for each solvent. Compared to the previous solvent dyeing approaches, using liquid paraffin as dyeing medium resulted in a higher dye uptake and a better EHS profile than chlorinated solvents. Compared to ScCO2dyeing, the solvent dyeing method described here did not require expensive high pressure equipment.
Compared to conventional aqueous process, solvent dyeing did not consume water. The color yields, fastness and mechanical proper-ties of solvent dyed fabrics were found to be comparable to those of aqueously dyed controls. The content of surface oligomers was reduced to 0.02% using solvent dyeing, much lower than that observed after reduction cleaning in aqueous dyeing. The result of a 7-cycle reuse sequence demonstrated excellent color consistency of dyed fabrics. With liquid paraffin as the dyeing medium and multi-cycle reuse of the dyebath, a water-free and effluent-free dyeing process for polyester was established.
Textile production, and especially dyeing, has major impacts on water supplies and quality. The new solvent dyeing method developed in this study facilitates the protection of scarce freshwater re-sources. By implementing the principles of reduction, reuse, and recycle, negative environmental impacts can be minimized. In addition, with recently established solvent dyeing method for cotton, it opens up a new route for one-bath dyeing of PET/cotton blends.
The solvent dyeing method using liquid paraffin as the dyeing medium is a promising technology for a sustainable textile industry.
Acknowledgments
This work was supported by the Chinese National High Technology Research and Development Program 863 Project (2013AA06A307) and USDA (NIFA Hatch Act Multi-State Project S-1054 (NEB 37-037)).
Appendix A. Supplementary data
Supplementary data related to this article can be found athttp://
dx.doi.org/10.1016/j.jclepro.2015.08.114.
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Liquid Paraffin

Specification

Method

Unit

Result

Density

ASTM D1298

Kg/m3

835

Distillation(IBP-FBP)

ASTM D1160

°C

245-530

Viscosity @ 40°C

ASTM D445

c.st

17.3

Pour Point

ASTM D97

°C

0

Flash Point

ASTM D92

°C

195

Color

ASTM D1500

-

2.5

Sulfur Content

ASTM D4294

WT%

<0.02