Preparation and Properties of Poly(ethylene terephthalate)(PET) Coated by Chitosan (CS) Based on Sol-gel Method

YUAN Kai( )， HU Zuming ()， *， DUAN Guangyu()， YU Junrong ()， ， WANG Yan ( )， ， ZHU Jing ( )，

1 College of Material Science and Engineering， Donghua University， Shanghai 201620， China2 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials， Donghua University， Shanghai 201620， China

Abstract: Sucrose ester (SE) was fixed on surface of poly(ethylene terephthalate)(PET) fibers to improve surface activity. Chitosan (CS) was used to graft onto pretreated PET fibers by sol-gel method. The transformations of surface chemical structure， microcosmic morphology and thermodynamic property were investigated by Fourier transform infrared spectroscopy(FTIR)， X-ray photoelectron spectroscopy (XPS)， scanning electron microscope (SEM)， X-ray diffraction technique (XRD)， and thermo gravimetric analysis (TGA)， respectively. The wettability and antistatic property of PET fiber were significantly improved after modification by SE and CS.

Key words: poly(ethylene terephthalate)(PET)； sucrose ester(SE)； chitosan(CS)； surface modification; sol-gel method

Introduction

Poly (ethylene terephthalate) (PET)， processing high strength， good wear resistance， excellent dimensional stability and chemical stability， has been widely used in various fields. However， it possesses hydrophobic and non-antistatic due to high crystallinity and lack of polar groups. Surface modification methods are useful ways to improve hydrophilicity and enhance antistatic property of PET material. Conventional approaches include plasma treatment[1-3]， UV irradiation[4-6]， and grafting polymerization[7-8]. Recently， researches on surface modification of PET with polyethylene glycol (PEG) were reported that the adhesive coating of PEG showed promising effects on improving biocompatibility and reducing bacterial adhesion[9-10]. This paper describes an new method to make PET surface more active. Under high temperature and pressure， the chains of PET can generate numerous gaps， which provide a large number of sites for other molecules to be anchored. When recovered to room temperature， the activity of chain segment deceased， leading to the fixation of molecules.

Chitosan (CS) is the deacetylated derivative of chitin which widely exists in cuticles of crustaceans， insects and mollusks. Owing to the favorable biodegradability， biocompatibility， antimicrobial activity and non-toxicity， CS has been extensively applied in many fields such as biotechnology， textile， waste water treatment and food processing[11-13]. In recent years， the applications of CS in textile are attracting more attention， and quantities of researches had focused on the functionalization of textile products[14]. CS possesses amino groups and can be dissolved in various acid solutions， and the optimal choice is acetic acid[15]. As for the final product， washing in alkaline solution to remove the residual acid is a kind of common approach[16]. However， it is worth noting that the common method may destroy CS coating irreversibly. To avoid the mentioned defect， there is an alternative method to prepare CS coating. As is known to all that CO2can be dissolved into water to form carbonic acid， and the acidity of carbonic acid is stronger than acetic acid， which is an ideal acid solution to dissolve the pretreated CS to obtain CS coating and avoid the mentioned defect[17-18].

In this paper， the sucrose ester (SE) surfactant was fixed onto surface of PET fiber to strengthen the surface activity firstly. Then， CS coating adhered to surface of modified PET fiber via sol-gel method. The structure and related properties of prepared samples were studied in detail.

1 Experimental

1.1 Materials

Polyester fibers (30 mm， 1.2 dtex) were kindly provided by Sheng Hong Group(China). CS (90% deacetylation) was obtained from Sigma-Aldrich(St. Louis， MO， USA). Sucrose eater was purchased from GT Food Chemical Co.， Ltd. (China). Malic acid was obtained from Sinopharm Chemical Reagent Co.， Ltd (China). Carbonic acid gas (CO2gas) was gotten from CG gas industry Co.， Ltd. (China). Other reagents used in this paper were analytical grade.

1.2 Alkaline treatment of polyester

Alkaline treatment was carried out in 2% NaOH-ethylene glycol/water(1/1， v/v) solution at 80 ℃ for 1 h using a liquor-to-goods ratio of 30∶1. After being incubated for a given time， polyester was taken out and washed with diluted acetic acid (2%) to neutralize residual sodium hydroxide. After sequentially rinsed in water， polyester was dried in electric heat oven at 80 ℃ to obtain the surface-hydrolyzed PET fiber.

1.3 Preparation of SE-PET

SE-PET fiber was prepared by high temperature and pressure method as described previously[19]. Briefly， SE-PET fibers were prepared as follows: 2.0 g of hydrolyzed PET fibers were immersed in 60 mL of sucrose ester aqueous solution (2 g/L) and then kept in infrared dyeing machine(Roaches Pyrotec 2000， England) at 130 ℃ for 1 h. Finally， samples were subsequently washed with water and dried at 70 ℃ for 1 h and named as SE-PET.

1.4 Coated by Chitosan-CO2

Chitosan-CO2solution was prepared by reported method[18]. Firstly， 1.5 g CS powder was dispersed in 100 mL of 0.5 mol/L acetic acid. Then 0.5 mol/L sodium hydroxide solution was added with stirring to neutralize the solution until the pH reached 7.5. Secondly， precipitate of translucent CS gels was collected by centrifugation and washed with distilled water repeatedly. The obtained gels were dispersed in distilled water and formed homogeneous colloidal solution with stirring. Thirdly， pure CO2gas was dissolved into colloidal solution until the solution became transparent. Finally， quantificational malic acid as cross-linking agent was dissolved into prepared chitosan-CO2solution before impregnating pretreated PET fiber. Two hours later， the processed PET fiber was taken out and dried in fume hood at 25 ℃ for 24 h. The sample was marked as CS-SE-PET， and the whole procedures were depicted in scheme 1.

Scheme1Procedures for preparing CS-SE-PET fibers： (i) Alkali treatment on surface of PET; (ii) surface was modified by sucrose ester via high-temperature and high-pressure treatment; (iii) CS was grafted onto surface of modified PET

1.5 Characterization

Fourier transform infrared spectroscopy (FTIR) was recorded on Nicolet 6700 FTIR spectrophotometer(America). X-ray photoelectron spectroscopy (XPS)(Thermo Scientific Escalab 250Xi, America) was used to analyze surface of samples. The surface morphology was observed with Field Emission Scanning Electron Microscope (Hitachi SU8010， Japan). To enhance the electrical conductivity， PET fiber was sputtered with gold under vacuum before observation.

The degree of hydrolysis (DH) of PET fiber was calculated according to the previously reported method[20].DHwas calculated using Eq. (1):

DH/%=(W0-W1)/W0× 100，

(1)

whereDHis degree of hydrolysis (%)；W0andW1are weights of PET before and after hydrolysis， respectively.

The thermal property test was performed by thermo-gravimetric analyzer (TGA； Q50 TA Instruments, America) with a heating rate of 10 ℃·min-1from 30 ℃ to 700 ℃ under N2condition The fixation rate of SE on surface of PET material was calculated by Eq. (2):

Rf/%=(Wt-Wi)/Wi×100，

(2)

whereRfis fixation rate (%) of SE，Wiis initial weight of sample before SE treatment， andWtis final weight of sample after treated by SE.

Wettability of modified PET fiber was measured 3 times by contact angle measuring analyzer(Data-physics Instruments GmbH， German) at room temperature.

Half decay time was measured to determine the antistatic property via YG401 electrostatic induction analyzer(NGT Instrument Co.， Ltd., China). Samples were placed under given condition(65% relative humidity (RH)， 20 ℃)for 12 h before testing.

The moisture regain of PET fiber was calculated by Eq. (3).

R/% =(W1-W0)/W0×100,

(3)

whereRis moisture regain (%)，W1is initial weight of sample after drying in 80 ℃ for 24 h， andW0is the final weight of sample after placing for 24 h under 65% RH.

2 Results and Discussion

2.1 FTIR analysis

Infrared spectra of pristine PET， hydrolyzed PET， SE-PET and CS-SE-PET fibers are shown in Fig. 1. Figure 1(a) depicts the basic characteristics of pristine PET fiber， the absorption peak at 2 969 cm-1is C—H stretching vibration. The four absorption peaks at 1 600 cm-1， 1 578 cm-1， 1 505 cm-1and 1 470 cm-1are ascribed to stretching of benzene skeleton. The 1 343 cm-1absorption peak is attributed to wagging vibration of —CH2. The absorption peaks at 1 248 cm-1and 1 050 cm-1are assigned to asymmetric and symmetric stretching of C—O—C group of aromatic ester， respectively[21]. In fact， there is almost no obviously difference between pristine PET and hydrolyzed PET fiber， which indicates that hydrolysis scarcely affect the chemical structure of the fiber. However， some new absorption peaks emerge in infrared spectra after modifying by SE. The absorption peaks at 2 917 cm-1and 2 850 cm-1are revealing SE has been introduced onto PET fiber successfully. For CS-SE-PET， the 3 438 cm-1absorption peak is attributed to —NH2of CS， which demonstrates that CS has grafted onto SE-PET fiber. Figure 1(b) shows difference spectrum between SE-PET and hydrolyzed PET fiber， the clear absorption peak from 3 200 cm-1to 3 600 cm-1corresponds to stretching vibration of the hydroxyl SE， furtherly illustrating SE has coating onto PET fiber.

(a)

(b)

Fig.1 Changes in chemical structure of PET fibers: (a) FTIR spectra of pristine PET and modified PET fiber; (b) difference spectra obtained by subtracting transmittance values of hydrolyzed PET and SE-PET fiber

2.2 XPS characterization

Fig.2 XPS survey spectrum of PET fiber

Table 1 Experimental atomic compositions obtained from the areas of core level photoemission peaks corrected by sensitivity factors

2.3 XRD analysis

The XRD patterns of pristine PET， hydrolyzed PET， SE-PET and CS-SE-PET are displayed in Fig. 4. Characteristic peaks are observed of 2θvalues at 17.8°， 23.1° and 26.2°，which can be corresponded to [0 1 0]， [1 1 0] and [1 0 0] reflections， respectively. For four kinds of PET fibers， the XRD patterns show no apparent difference with related peaks intensity and position. This result indicates that the crystalline structures of hydrolyzed PET， SE-PET and CS-SE-PET fiber were similar to pristine PET fiber， demonstrating SE as well as CS did not affected the crystal structure of PET fiber.

(a)

(b)

(c)

Fig.4 XRD patterns of pristine PET， hydrolyzed PET， SE-PET and CS-SE-PET

2.4 Effects of DH on mechanical property and SE fixation rate of PET fiber

Figure 5 shows influence ofDHon fracture strength of Hydrolyzed PET fiber and SE fixation rate. It can be observed that fixation rate of SE(dotted line) on hydrolyzed PET surface increase rapidly at lowDH(less than 15%)， but decrease whenDHexceeds 15%. It can be explained that alkali treatment provided micro-pits and carboxyl groups， which were benefit for forming H-bonds and Van der Waals forces between PET and SE. However， excess hydrolysis reduced the volume and specific surface area of PET fiber， leading to SE fixation rate down. In addition， the fracture strength of PET fiber(black solid line) decreases with the increasing ofDH. It can be attributed to the fact thatDHdestroyed the surface structure of PET fiber， and the higherDH， the more reduction of fracture strength. Therefore， in order to balance the fracture strength and fixation rate， the value ofDHbetween 10% and 15% is admissible in this experiment.

Fig.5 Effects of DH on fracture strength and SE fixation rate

2.5 Microscopic characterization

The FTIR spectra have obviously showed the changes of PET fiber after modified by sucrose eater and CS. Figure 6 reveals the effects of alkali treatment， sucrose ester fixing and CS coating on micro-topography. SEM images of pristine PET fiber and hydrolyzed PET fiber are manifested in Figs. 6(a) and 6(b). The hydrolyzed PET fiber presents a relative rough surface compared with pristine PET fiber. Figures 6(c) and 6(d) display the morphologies of SE-PET and CS-SE-PET fiber. It is obviously to observe that SE-PET is rougher than hydrolyzed PET fiber， demonstrating SE has been successfully coated onto surface of hydrolyzed PET fiber. As for CS-SE-PET fiber， it seems smoother than SE-PET， it can be explained that polar groups of SE can strengthen the function of hydrogen bonds and chemical bonds between SE and CS. From SEM images of four kinds of PET fiber， it can be concluded that SE and CS can be grafted onto hydrolyzed PET and SE-PET effectively.

(a) (b)

2.6 Thermo gravimetric analysis

Thermal behaviors of pristine and modified PET fibers were investigated by thermogravimetric analysis(TGA). Figure 7 shows the TGA and differential thermal gravity (DTG) curves of pristine PET， hydrolyzed PET， SE-PET and CS-SE-PET fiber. It is easy to notice that all TGA and DGT curves manifest identical trend， indicating that SE and CS slightly affect thermal decomposition mechanism of PET matrix. From Fig.7(a)， there is an obvious weight loss in the range of 400-550 ℃， which can be attributed to the decomposition of PET chains. It revealed that all the PETs have similar thermal stabilities. The related thermal characteristic parameters are shown in Table 2. It revealed that four samples display similar values ofTiandTmax. This phenomenon could be explained that hydrolysis， SE and CS did not undermine the internal structure and crystallinity of PET matrix.

(a)

(b)

Table 2 TGA and DTG data of PET fibers

aInitial decomposition temperature；

bThe maximum decomposition temperature， corresponding to the temperature peak in the DTG curves

2.7 Moisture regain of modified PET fiber

It is well known that SE and CS contain lots of hydrophilic groups， which can absorb moisture from wet environment. Table 3 depicts moisture regain of kinds of PET fibers， 5-SE-PET means the concentration of SE is 5 g/L， 1.0-CS-SE-PET， 1.5-CS-SE-PET and 2.0-CS-SE-PET represent 1.0%， 1.5% and 2.0% of CS by weight， respectively. As can be seen that moisture regain of PET fiber increased gradually with increasing CS concentration. The average moisture regain of 2.0-CS-SE-PET reaches 1.24%， which is 4 times higher than that of original PET fiber. This can be attributed to the fact that the more CS grafted onto PET surface， the more moisture can be absorbed from outside circumstance. This phenomenon endows PET fiber with appropriate hydrophilic behavior. In addition， the improved hydrophilic property of PET fiber can make antistatic of PET fiber better.

Table 3 Moisture regain of different kinds of PET fiber

2.8 Washing loss rate of SE

The washing loss rate of SE after 5 times washing in cleaning solution can be seen in Fig. 8. With increasing concentration of SE， the washing loss rate increases. It is worth noting that the washing loss rate is less than 0.1% when SE concentration is below 50 g/L. This result indicates that SE can fasten to the surface of PET fiber effectively. Due to the abundant —OH of SE， modified PET kept ideal hydrophilic property and better antistatic behavior. However， when SE concentration surpasses 10 g/L， the washing loss rate increases obviously. It can be attributed to reason that the number of surface active groups of PET fiber is constant with same roughness. When SE concentration further increases， more —OH groups can impair the connection between the surface of PET fiber and SE molecules.

Fig.8 Washing loss rate of SE on PET fiber

2.9 Hydrophilic performance and antistatic property

Fig.9 Contact angle and half-life of PET fibers

3 Conclusions

In this study， SE-PET and CS-SE-PET were successfully prepared by high-temperature and high-pressure method as well as sol-gel method， respectively. The degree of hydrolysis had a significant effect on the mechanical property of PET fibers and the fixation ratio of SE on the surface and the results indicated that theDHvalue between 10% and 15% is optimal. The application of CO2for CS dissolution made the CS-SE-PET preparation process more convenient and environmentally friendly. The FTIR， XPS and SEM results confirmed the presence of SE and CS coated on PET fibers. Surface modification by sucrose ester and CS showed a faint effect on their thermal stability and crystal structure characterized by TGA and XRD. What’s more， the decrease of contact angle and half decay time indicated the improvement in wettability and antistatic property of CS-SE-PET fibers.