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Topographical changes in polyester after chemical,physical and enzymatic hydrolysisDeepti Guptaa, Harshita Chaudharyab & Charu Guptab

a Department of Textile Technology, Indian Institute of Technology, New Delhi, Indiab Department of Fabric and Apparel Science, Institute of Home Economics, University ofDelhi, New Delhi, IndiaPublished online: 03 Jul 2014.

To cite this article: Deepti Gupta, Harshita Chaudhary & Charu Gupta (2014): Topographical changes in polyester afterchemical, physical and enzymatic hydrolysis, The Journal of The Textile Institute, DOI: 10.1080/00405000.2014.934046

To link to this article: http://dx.doi.org/10.1080/00405000.2014.934046

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Topographical changes in polyester after chemical, physical and enzymatic hydrolysis

Deepti Guptaa*, Harshita Chaudharya,b and Charu Guptab

aDepartment of Textile Technology, Indian Institute of Technology, New Delhi, India; bDepartment of Fabric and Apparel Science,Institute of Home Economics, University of Delhi, New Delhi, India

(Received 1 April 2014; accepted 9 June 2014)

Polyester was treated with alkali, VUV excimer, and a hydrolytic enzyme. The effect of each treatment on thephysicochemical properties of polyester (PET) was studied and compared by qualitative and quantitative methods.Scanning electron microscopy and atomic force microscopy analysis show that alkali treatment causes severe fiberdegradation with formation of pits on the fabric surface. In contrast, excimer treatment results in uniform pitting atnanometer level, which is restricted to the surface. In case of enzyme-treated samples, non-uniform roughness wasobserved due to deposition of residual protein formed due to enzyme hydrolysis. K/S value and moisture regain value ofexcimer-treated samples were found to be higher than alkali- or enzyme-treated PET. Out of the three methods, excimertreatment appears to be the best method for PET modification with maximum number of polar groups being created onhydrolysis with marginal loss in strength and weight.

Keywords: polyester; surface functionalization; topographical analysis; enzyme treatment

Introduction

Synthetic fibers form an important part of the textileindustry. They hold a 54% market share in the fibermarket. Of this market share, polyethylene terephthalate(PET) polyester alone accounts for almost 50% of allfiber materials in 2008 (Guebitz & Paulo, 2007).Polyester is a synthetic fiber composed of terephthalicacid (TA) and ethylene glycol. PET fibers haveoutstanding chemical, physical, and mechanicalproperties but suffer from drawbacks due to the intrinsichydrophobic and inert nature of the constituent polymer.Low moisture regain and poor wettability limit theirusage for a variety of applications such as in sports andactive wear, protective clothing, medical textiles,undergarments and in numerous technical applications(Zimmermann & Billing, 2011). In recent years, the fieldof fiber modification has gained momentum forimparting special properties to textile materials.

Treatment can be carried out to create reactivefunctional groups on the fabric surface which in turn canform bonds with desired molecules, thus improvingwettability or other desirable properties. It also bringsabout physical changes in polymer, resulting innanostructuring of surface and thus improving itsadhesion. An effective technique of modification shouldretain the mechanical strength of the fiber core whileimparting surface functionality to fabric (Goddard &Hotchkiss, 2007). Various chemical, biochemical andphysical modification methods have been employed forPET modification. Each of these techniques varies in

terms of their action mechanism and the effect producedon the fabric surface.

Alkaline hydrolysis of PET using sodium hydroxideis the most conventional method for rendering PEThydrophilic mainly because of its simplicity andimplementability on a manufacturing scale. It has beenwidely investigated and applied at industrial level as atool to enhance their processing and end useperformance such as better dyeability, finishingproperties, and moisture management (Dave, Kumar, &Srivastava, 1987). However, this method is high energydemanding, chemical consuming and is considered quiteaggressive towards PET textiles.

Compared to chemical hydrolysis, enzymatichydrolysis is performed under mild conditions and hasthe advantage of maintaining mechanical stability of thefabric. In the past few years, significant progress hasbeen made in the use of enzymes for modification ofsynthetic polymers, including polyester. Enzymes thathydrolyze ester bonds of polyester include cutinases,lipases, and esterases (O’Neill, Araujo, Casal, Guebitz,& Paulo, 2007). Enzymes because of their large size arenot able to penetrate inside the fiber and act only on thefiber surface. Treatment of PET with several lipases hasbeen shown to improve wettability, dyeability, andabsorbent properties of PET fabrics, while strengthproperties were retained (Fischer Colbrie et al., 2004).Studies have been conducted on comparing thetechniques of alkali and enzymatic hydrolysis based onthe release of the degradation products TA and mono

*Corresponding author. Email: deepti@textile.iitd.ernet.in

© 2014 The Textile Institute

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(2-hydroxyethyl) terephthalate by HPLC (Brueckner,Eberl, Eberl, Rabe, & Guebitz, 2008). The amount ofdegradation products released by alkali hydrolysis wasfound to be higher than the products released by enzymetreatment.

Physical hydrolysis methods based on plasmatechniques (Dong & Bell, 1999; Morent et al., 2008) orUV radiation present certain advantages over the others(Table 1). Compared to others, they are both eco-friendlyas they can be carried out under atmospheric pressure, asa dry application media (Borcia, Anderson, & Brown,2006; Zaman, Liu, Xiao, Chibante, & Ni, 2013). Thisresults in lowering the consumption of chemicals, water,as well as energy employed. These processes are knownto modify only the outermost atomic layers of a materialsurface without interfering with the bulk properties(Periyasamy, Gupta, & Gulrajani, 2007a). UV excimerlamps are more recent and hold the promise forlarge-area industrial applications for surface modificationof textiles. UV excimer irradiation uses the dielectricbarrier discharge principle and emits intensemonochromatic light in the UV region of the spectrum(Periyasamy, Gupta, & Gulrajani, 2007b). The excimerlamps combine the simple handling with thephotochemical properties of a monochromatic lightsource such as an excimer laser (Kerkeni, Behary,Perwuelz, & Gupta, 2012). Because of their uniqueproperties and simplicity of construction, large emissionarea with low cost, and availability in multiplewavelengths, excimer sources are an attractive alternativeto other techniques of surface modification (Gupta,Siddhan, & Banerjee, 2007). Researchers have used UVradiation to alter the PET surface to create a morehydrophilic surface with improved adhesion with nilamount of weight loss (Zhu & Kelley, 2004). Surfacegrafting on excimer-modified PET has also beenattempted by researchers in order to improve itsproperties (Kerkeni, Gupta, Perwuelz, & Behary, 2010).

This study was conducted with the aim of comparingthe techniques of alkaline hydrolysis, excimer treatment,and enzymatic hydrolysis of polyester. The effect of eachtreatment on the physicochemical properties of polyesterhas been studied by qualitative and quantitative methods.

Materials and methods

Hundred percent polyester fabric having plain weave ofdensity 90 g per square meter weight with 55 ends/cm;33 picks/cm was used for the study. Polyester fabricsamples were scoured to remove any impurities andtemporary spin finishes that might be present on thefabric. Scouring was done with 5% Lissapol N(non-ionic wetting agent; 99% purity, procured fromImperial chemicals industry, UK) at 100°C for 1 h(Gupta et al., 2007). Deionized water was used for all

experiments. The fabric samples were rinsed, dried, andconditioned for 24 h at 65 ± 2% relative humidity and 20± 2°C before further treatment.

Techniques of PET modification

Alkali treatment

Laboratory grade sodium hydroxide (Merck, USA), aceticacid (Lobachemie, India) was used for alkali hydrolysis.Deionized water was used in all the experiments.

PET was treated with 40 g/L of sodium hydroxide,for 45 min at 80°C. The material to liquor ratio wastaken as 1:50. Following the alkaline treatment, thesamples were rinsed repeatedly with deionized water,neutralized with a solution of 1% aqueous acetic acidsolution, and washed in distilled water until the rinsewater was neutral to litmus paper. The samples wereair-dried and after drying in an oven at 80°C, werecooled, conditioned at standard temperature andhumidity in a dessicator, and weighed.

Treatment using VUV excimer lamp

A Xenon Excimer UV lamp (XERADEX 20W/L40/120/SBSX46/KF50) emitting light at 172 nm, power supply(DBD 110 V/230 V 50 Hz/60 Hz) supplied by MessrsRadium Lampenwerk Wipperfurth of Germany was usedto irradiate PET fabric. The lamp emits almostmonochromatic light in the VUV region with anirradiation power of 50 mW/cm2.

Scoured PET samples were irradiated for 15 min onboth the sides under excimer lamp keeping a distance of5 mm at atmospheric conditions.

Modification using Lipase from Candida antarctica

Lipase Cal A (NovoCor AD L) from Candida antarcticaproduced by submerged formation of a geneticallymodified Aspergillusoryzae microorganism, supplied bySigma-Aldrich, USA, was used without furtherpurification. The activity of enzyme was determined tobe 6KLU/G. Trishydroxymethyl amino methane (Merck,USA) abbreviated as Tris was used as a buffer. To keepionic properties of treatment solution constant, Trisbuffer solution is used as the basis for all applications.The pH was adjusted using 1M hydrochloric acid(Merck, USA).

PET fabrics were treated with 200% lipase Cal A onweight of fabric, at 50°C for 90 min, pH 4.2 in Tris buffersolution, using a material and liquor ratio of 1:30. Afterthe enzyme treatment, bath temperature was raised to 90°Cfor 10 min to inactivate the enzyme. After inactivation, thesamples were soaked in ethanol for 5 min to removesorbed enzyme if any. This was followed by soaping with5 g/L Lissapol N at 40°C with a liquor ratio of 1:50.

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After soaping, the samples were rinsed thoroughlywith hot and cold water, dried and conditioned at 21°Cand 60% relative humidity for 24 h before furthercharacterization. All treatments were carried out in ashaking water bath (Julabo SW22, Germany) at 150 rpm.

Characterization of modified PET

Topographical analysis

Scanning electron microscopy. Scanning electronmicroscopy (SEM) is commonly used for examining thesurface morphology and structures of textile surfaces.The effect of modification on the morphology of thefiber was examined qualitatively under the scanningelectron microscope EVO 50 (Zeiss, Germany) at 10 Kmagnification.

Atomic force microscopy. Investigation using a Nanonics(Israel) was carried out for atomic force microscopy(AFM) imaging in the tapping mode. Budget Sensor(Nanonics, Israel) tapping mode tips, 50 μm in length,made of a monolithic silicon probe with aluminum reflexcoating, at a resonance frequency of 300 kHz, were used.The tapping mode was preferred to the AFM⁄ lateralforce microscopy (contact mode AFM), as the tappingmode overcomes problems associated with friction,adhesion, and electrostatic forces, which may distortimage data (Kerkeni et al., 2012). Tapping modeimaging was carried out in ambient air. The fiber surfaceroughness parameter (Ra) was calculated directly fromAFM signals using the computer software.

Evaluation of fabric characteristics

Weight loss: Weight loss was determined by measuringthe difference in weight of the untreated and modifiedsamples. The weight loss (WL) is expressed as relativeWL according to the equation:

Weight loss %ð Þ ¼ W1�W2ð ÞW1

� 100; (1)

where W1 and W2 denote the weight of samples beforeand after treatments.

Tensile strength: The tensile strength of fabricsamples was measured according to ASTM D5034 TestMethod (grab tensile method) on PARAMOUNT digi

Strength tester for Textiles at a constant sample length of10 cm and speed of the traverse of the lower jaw at30 cm/min.

Moisture regain: Moisture regain was measured inaccordance with ASTMD 629-99 by comparing the dryweight and moisture conditioning weight of the samples.Samples were dried in an oven at 105°C for 1 h andthen weighed after cooling in a desiccator for 30 min.Moisture conditioning was carried out at 20°C and 65%relative humidity for 24 h. The moisture regain wascalculated using the following equation:

Moisture regain ð%Þ ¼ Wm �Wd

Wd� 100; (2)

where Wm is the weight of fabric in a state of moistureequilibrium at 20°C and 65% relative humidity, and Wd

is the weight of fabric dried at 105°C for 90 min.

Dyeing with basic dyes

An indirect method suggested by Gupta et al. (2007),based on dyeing of polyester with cationic dyes wasused to estimate the extent of modification on the PETsurface. Negatively charged groups generated on thefabric surface form ionic bonds with positively chargeddye molecules, resulting in increase in dye uptake. Thedye uptake by the fabric was measured by K/S values. Itwas expected that higher is the amount of chemicalfunctionalization, higher would be the dye taken up bythe fiber.

Samples were treated with the basic dye, MethyleneBlue, at 0.5% on weight of fabric, 1:30 (material-to-liquor ratio) at 100°C, for 60 min at pH 5 (adjusted withsodium acetate and acetic acid buffer). Dyed sampleswere cold rinsed and soaped with 0.5 g/L Lissapol N at50°C for 10 min, followed by cold rinsing and drying.The depth of shade obtained on polyester wasdetermined in terms of K/S values calculated fromreflectance data according to the Kubelka–Munkequation:

K=S ¼ 1� Rð Þ22R

; (3)

where K is the absorption coefficient, S is the scatteringcoefficient and R is the reflectance of the dyed fabric atthe wavelength of maximum absorption of the dye at

Table 1. Comparison of techniques used for modification of PET.

Alkaline treatment VUV excimer lamp treatment Enzyme treatment

Cost Cheap Expensive Very expensiveChemical usage Very high Nil LowWater consumption Very high Nil HighEnvironmental impact Very high Nil Medium

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665 nm. Colorimetric measurements of the dyed sampleswere performed on Color-Eye 7000A computer colormatching system (Gretag Macbeth, USA).

Results

The effect of three different treatments onphysicochemical properties of polyester was studied byvarious qualitative and quantitative methods. Morphologyof the fiber was studied with SEM analysis, surfaceroughness using AFM, chemical functionalization bydyeing with basic dyes, and moisture-related property bymoisture regain.

Topographical analysis

SEM analysis

Morphological structure of untreated and modified PETby all the three techniques was studied by SEM analysisat 10 K magnification, Figure 1. Surface of untreatedpolyester fiber shown in Figure 1(a) is smooth anddistinct. In case of alkali-treated samples, Figure 1(b),

severe damage can be seen. The surface appears to beeroded and etched with distinct small pits at variousintervals. Pits or cavities of about 1.1 × 0.5 ± 0.16 μm insize can be seen on the fiber surface. Some depositswhich are absent in the untreated fiber can also be seen.These could be the oligomers formed during hydrolysisof polyester which areredeposited as clumps on the fibersurface.

Figure 1(c) shows that excimer treatment gives auniformly roughened surface on polyester. The pitting isuniform and appears to be restricted to the fiber surfaceunlike the observations in case of alkali-treated fibers.The fiber surface as well diameter remains essentiallyunchanged. On the other hand, the enzyme treatedsamples, seen in Figure 1(d), show severe damage. Thefiber diameter is seen to have become non-uniform dueto uneven treatment. A heavy powdery deposition isseen on the fiber surface. This effect could be due tosevere action of enzyme on fiber surface andredeposition of intermediate and final products ofenzymatic hydrolysis of polyester.

(d) Enzyme Treated(c) Excimer treated

(b) Alkali Treated(a) Control

Figure 1. SEM micrographs of PET. (a) Control, (b) Alkali treated, (c) Excimer treated, and (d) Enzyme treated.

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AFM analysis

Topographical analysis was carried out with AFMcomparing untreated and modified PET samples. TheAFM results of 2 × 2 μm2 area after surface modificationare shown in Figure 2. It can be seen that untreated PET,Figure 2(a) has a rough surface. This could be due to thedeposits observed on fiber surface in the SEM images.

Since the polyester sample used in the study was acommercial sample obtained from the market, theroughness is most likely caused by residual finishingmaterials which did not get removed during scouring.

AFM images of alkali-treated PET, shown inFigure 2(b), validate the presence of pits observed in theSEM images. Alkali treatment, as mentioned earlier has

Figure 2. 3D views of AFM images of polyester samples. (a) Control, (b) Alkali treated, (c) Excimer treated, and (d) Enzymetreated.

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a severe effect on polyester fiber. Large-size pits arecreated intermittently on the surface, while the rest ofthe surface is completely smoothened. Ridges are alsoobserved on the fabric surface while the rest of thesurface seems to be smoothened out. Surfacesmoothening can be attributed to the removal of surfaceimpurities observed in Figure 2(b). It is likely that thesurface layers of the fiber have been removed thusmaking it appear completely smooth except for thepresence of pits.

In case of excimer-treated samples, shown inFigure 2(c), the etching is more dense, uniform, andevenly distributed. Enzyme treated fabrics, Figure 2(d)have higher roughness and non-uniformity. Theseobservations corroborate the findings of SEM analysis.

More precise analysis of the etching effect ofvarious treatments on fiber surface can be done withthe help of histograms shown in Figure 3. Histogram,Figure 3(a), recorded for the untreated fiber sampleshows roughness values in the range of 200–300 nmwith maximum frequency being at around 250 nm. Ontreatment with alkali, the roughness becomes higherwith distribution ranging from 1.5 to 2 μm withmaximum roughness of about 1.8 μm, Figure 3(b). Itcan be seen from Figure 3(c) that the roughness causedby excimer treatment is in the same range as observedfor control PET i.e. between 150 and 300 nm.However, the effect seems to have been intensified. Incase of enzyme-treated PET, Figure 3(d), there is aslight increase in roughness ranging between 250 and350 nm. However, this observed roughness is not dueto surface etching as in case of other treatments but isdue to the redeposition of by-products of enzymehydrolysis.

Surface roughness values of modified PET samplesare given in Figure 4. The roughness values (Ra) aftermodification with of alkali were almost twice comparedto excimer (Ra = 26.48 ± 0.34 nm). Surface roughnessvalues of enzyme-treated samples (Ra = 18.08 ± 0.5 nm)were closer to the roughness values of untreated PET(Ra = 17.39 ± 0.38 nm).

Evaluation of fabric characteristics

Weight and tensile properties

Weight loss and tensile strength properties of PET werefound to reduce on treatment with alkali. The results aregiven in Table 2. In case of alkali-treated PET, weightloss was found to be around 6.25 ± 0.25% with about22% reduction in strength. However, excimer-treatedPET showed nil weight loss and no reduction instrength. On treatment with enzymes, weight loss ofaround 0.15 ± 0.15% was observed with no reduction instrength properties.

Moisture regain

Moisture regain is the tendency of fibers to pick up andgive off ambient atmospheric moisture until they reach astate of equilibrium moisture content at a giventemperature and humidity level. Untreated polyester hasa low moisture content of 0.60 ± 0.10%. After treatment,moisture regain values for all PET samples were seen toimprove. It can be seen from the results compiled inFigure 5, that maximum enhancement in moisture regainis observed in case of excimer-treated sample, followedby that of alkali-treated sample. Moisture regain ofenzyme-treated samples remains nearly unchanged at0.68 ± 0.10%.

Dyeing with basic dyes

Treated and untreated polyester was dyed with basic dyeMethylene Blue. Results are plotted in Figure 6. Ondyeing the control sample, only a slight coloring wasobserved which may be due to the presence of anionicimpurities on the fiber surface as observed in the SEMsince polyester per se does not have any charged groupson its surface. K/S values increase marginally aftertreatment with alkali and enzyme. Excimer-treated PETsamples showed maximum dye uptake, with K/S valuebeing 10 times higher than the other two samples at 7.083.

Discussion

Topographical analysis of PET shows that untreated PETin itself has some impurities which impart an inherentroughness and hydrophilicity to the fiber surface. Thisfinishing chemical, most likely a softener, could not beremoved during the scouring treatment imparted to thesample before the study.

On treatment with alkali, the outermost layer of fiberis leached out. Due to the severity of the treatment, thefiber undergoes a high strength and weight lossindicating that the treatment is not restricted to thesurface but affects the bulk properties of the fiber. Alkalitreatment also results in peeling of the fiber surfaceresulting in generation of pits. High weight loss andstrength loss can also be explained by the formation ofthese pits. Moreover, the residue of hydrolytic activitygets deposited on the fiber and appears as deposits onthe surface, but does not appear to affect the surfaceroughness. Because of removal of impurities, the surfaceof the fiber becomes smoother but micron-level pitsresult in surface damage.

Excimer-treated samples show uniform pitting whichis restricted to a few nanometers depth indicating theabsorption of UV photons mainly on the surface.Moisture regain values of excimer-treated PET werefound to be maximum as compared to alkali and

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Figure 3. Histograms of PET samples. (a) Control, (b) Alkali treated, (c) Excimer treated, and (d) Enzyme treated.

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enzyme. This indicates that maximum surfacefunctionalization in terms of generation of polar groupsis occurring in this case. These groups lead to improvedhydrogen-bonding capacity with water molecules,resulting in better moisture properties. That thesegroups are anionic in nature is further validated bydyeing studies where maximum color was taken up byexcimer-treated PET. Although alkali hydrolysis wascomparatively more intense with considerable reductionin weight and strength loss, it can be seen that a higherweight and strength loss does not translate into highersurface functionalization. The reverse is true for excimertreatment.

Enzyme treatment proved to be the least desirableand least effective as the fiber diameter was renderednon-uniform and surface roughness was caused byadsorption of hydrolyzed by-products on the surface. Incontrast with the alkali- and enzyme-treated PET,excimer treatment appears to be the most effective andsuitable means of PET surface modification. The surfacewas modified uniformly with negligible reduction inweight and tensile strength of the fiber.

Conclusion

Out of the three techniques used for modification ofPET, alkali appears to have the most severe degradationeffect on the fiber morphology. Along with the weightloss, the core properties of PET-like tensile strength gotaffected, with nominal increase in functional groups.Enzyme also appears to degrade the fiber in terms ofmaking fiber non-uniform as indicated by diameter ofthe fiber. However, no change in fiber roughness wasobserved. Enzymes had no effect on fiber-core propertieswith insignificant increase in polar groups. Out of thethree methods, excimer treatment appears to be the bestwith maximum number of polar groups being created onhydrolysis with minimal strength and weight loss.Roughness imparted by VUV excimer lamp is dense,uniform, and concentrated on the surface. Apart frombeing effective, VUV excimer is also an eco-friendly,dry, chemical-free, and cost-effective technology.

AcknowledgementWe would like to thank Department of Biotechnology, Ministryof Textiles for providing us the financial support to carry outthe analytical tests. We would also like to acknowledge theinputs provided by Prof. M.N. Gupta, Department ofChemistry, Indian Institute of Technology, Delhi for enzymatictreatment.

Figure 4. Surface roughness of PET samples (AT: Alkalitreated, ExT: Excimer treated, EzT: Enzyme treated).

Table 2. Reduction in fabric weight and tensile strength ofPET.

Weight loss (%) Loss in strength (%)

Control 0 0Alkali treated 6.25 ± 0.25 22Excimer treated 0 0Enzyme treated 0.15 ± 0.15 0

Figure 5. Moisture regain values of PET samples (AT: Alkalitreated, ExT: Excimer treated, EzT: Enzyme treated).

Figure 6. Color values of PET samples (AT: Alkali treated,ExT: Excimer treated, EzT: Enzyme treated).

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