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Sulfonation of low-density polyethylene and its impact on polymer properties.

INTRODUCTION

Imparting certain functional properties is of considerable advantage in polymers and it is a normal industrial practice for the surface treatment of polyethylene films for this purpose [1-3], Several methods have been proposed for imparting functionality to polymer surfaces. The changes in functional moieties within structure of a solid organic polymer can influence its dielectric properties, degradability, hydrophilicity, permeation, adhesion, dye absorption, thromboresistance, and cellular attachment, which are important factors for engineering and biomedical applications [4]. Though considerable work has been carried out on films surface modification, we have modified the process by swelling low-density polyethylene pellets using solvents for in-depth functionalization. According to Billmeyer [5] there are two stages involved in dissolution process of polymers, the polymer swelling is the first step and the dissolution is the second step. When the polymer is added to an appropriate solvent or a combination of solvents, according to polarity of substrate, attraction and dispersion forces begin acting between the solvents and polymer substrate. If the polymer-polymer attraction forces are lower than the polymer-solvent interactions, the solvent molecules penetrate into chain segments of polymer increasing volume and swelling out into a gel like structure. The swelling of LDPE plays an important role in chain disentanglement. When polymer comes in contact with a thermodynamically compatible solvent, the solvent diffuses into the polymer and starts chain disentanglement which takes some time to complete. In the presence of appropriate mixing of polymer and solvents, a gel like swollen layer is formed after a required time [6]. To the best of our knowledge there is no reported process using a combination of polar and nonpolar solvents to homogenously sulfonate LDPE, especially for application in technical textiles.

As this process was particularly devised for making technical textiles, which are textile products manufactured for nonaesthetic purposes, and where their function is the primary criterion. This new generation of textiles uses innovative materials, production methods, and materials technologies for imparting functional properties and not for their appearance or aesthetics. Terms such as industrial textiles, functional textiles, engineered textiles and high-tech textiles are also used in various contexts instead of technical textiles [7]. Polyethylene in particular has attractive properties for various shaped textile articles including fibres, filaments, yarns, ribbons, tapes, and fabric structures. This study investigates an easily accessible, cost-effective polymeric material with improved properties, which dissipates static charge and renders the material more susceptible to common dyestuff, while retaining the essential mechanical properties of low-density polyethylene for its intended use in technical textiles [8]. Polyolefins films have been treated with mineral acids under different reaction conditions in order to achieve a rough film surface and to introduce polar groups [9-11]. Common polyolefin are used in the packaging industry and most of the research work reported in the literature is related to only chemical surface treatment of films [12]. Furthermore, very little work has been carried out for specific application of functionalized polyethylene in nonconventional areas like technical textiles. Sulfonation is also one of the most common methods employed for surface modification of polyethylene [13]. Sulfonation includes treatment with gaseous S[O.sub.3] [10, 14], hot concentrated sulfuric acid [15], and fuming sulfuric acid [13, 16, 17]. Polyethylene has attracted special interest as a functionalized polymer because it is chemically inert, ecologically clean and abundantly available [18]. Among many varieties of polyethylene, low-density polyethylene (LDPE) has more branching, thus higher number of tertiary carbons are attached to the labile protons which are more amenable to chemical modification. Hence low-density polyethylene is more readily attacked by acids converting it to a functionalized derivative. In addition to its functionality, LDPE offers high tensile strength and modulus making it a good candidate as technical textile material. As stated earlier in comparison to just surface treatment of polyethylene i.e. change in its surface chemical composition and morphology [19]; we have pursued acid treatment methodology that allows functionalization of polymer's inner layers. Rasmussen et al. while listing the shortcomings for the definition of "polymer surface" pointed out that the impact of solvents would swell polyethylene, allowing bulk sulfonation reaction [4]. Thus, in-depth functionalization of low-density polyethylene was achieved utilizing a volumetric ratio of 4:1 for cyclohexane and ethanol as non-polar and polar swelling agents, respectively. Presence of volatile reagents enabled the maintenance of the batch under isothermal conditions producing a vapor-liquid equilibrium. Polyethylene was made miscible using a mixture of cyclohexane and ethanol with continuous agitation and heating. This was done for the intended fabrication of cost effective and durable composite fibres for its application in technical textiles [20], The advancing contact angle of water on the sulfonated LDPE correlates well with the concentration and hydrophilicity of the functional groups at the interface. The influence of increased surface hydrophilicity on the contact angle appears to reach saturation relatively quickly and may be lost due to extended exposure of the polymer surface to various environments. However, the in-depth sulfonation of LDPE reported here will result in imparting a deeper and more durable hydrophilic and adhesive character to the polymer and will provided prolonged functionally. Furthermore, the increase in the polymer porosity due to the in-depth sulfonation will also be helpful in the use of the modified LDPE in many functional textile and biomedical applications.

EXPERIMENTAL

Sulfonation Theory and Mechanism

Generally sulfonation reactions are characteristically electrophilic substitution when sulfuric acid, oleum, chlorosulfonic acid, and free sulfur trioxide are used as sulfonation agents, but it also can be nucleophilic when sulfites are used. As illustrated in reaction scheme in Fig. 1, sulfonation of polyolefin commences by sulfonic group electrophilic substitution to a carbon atom and then dissociation of hydride ion to produce double bonds. Gordon and Main illustrated sulfonation, de-sulfonation as most significant chemical change that occurs during the reaction between concentrated sulfuric acid and polyolefin [15]. Reaction mechanism in Fig. 1 reaction proceeds with sulfonation of polyethylene with substitution of S[O.sub.3]H group on carbon atom, and then desulfonation due to abstraction of hydride ion from the polyethylene chain. This sulfonation and desulfonation mechanism leads to olefinic conjugation [15]. In their article titled as "Homogenous and heterogeneous sulfonation of polymers" Kucera and Janear pointed out that even though the dissociation energy value of carbon to hydrogen is lower i.e. 384 kJ/mol for aliphatic compounds and 428 kJ/mol for aromatic compounds, formation of double bond via sulfonation is comparatively difficult in aliphatic compounds. They also classified sulfonation reactions of polymers into two states namely homogenous and heterogeneous sulfonation. According to these researchers, usually sulfonation reactions are carried out as heterogeneous reaction and homogenous reactions and are customarily performed in hydrocarbon and chlorinated solvents [21]. In studying the reaction mechanism, when using fuming sulfuric acid Kaneko et al. suggests that double bonds react with S[O.sub.3] resulting in production of beta, gamma and delta sulfones, sulfonic acid and other sulfate groups [13].

Materials

Low-density polyethylene pellets (Petlin, Malaysia) with reported melt flow index of 5 and polydispersity index (PDI) of 9.2 were used in the sulfonation experiments. Analytical grade sulfuric acid 96% (Fischer Scientific Malaysia) was used to make 2.5, 5, 10, and 15 molar solutions with deionized water (Millipore Elix). Cyclohexane (Merck Germany) and Ethanol 95% (HmbG Chemicals) of analytical grade were used as nonpolar and polar reagents, respectively.

Experimental Setup

Low-density polyethylene pellets were swelled in a mixture of cyclohexane, ethanol and at 75[degrees]C in a three-neck reaction flask. Stirring was carried out using an overhead variable speed motor, with fluid seal attached to the agitator in order to trap vapors. A constant stirring speed 200 rpm was used. Reflux condenser with tap water as cooling medium and temperature control were attached to other two necks of the reaction flask as illustrated schematically in Fig. 2. The swollen polyethylene samples were subjected to sulfuric acid of varying concentrations from 2.5M to 15.0 M under continued stirring for a fixed period of one hour to obtain uniform sulfonation reaction. Heating was isothermally adjusted at 72[degrees]C, as cyclohexane and ethanol vapors were refluxed back into the reaction vessel. According to Beth et al. polymers do not dissolve instantaneously and the dissolution process is a controlled disentanglement of polymer chains at solvent interface [22]. In this manner enhanced effect of sulfuric acid in swollen polyethylene is achieved in comparison with polyethylene in solid form as illustrated in Fig. 3. This figure is adapted from Vrentas and Vrentas and represents a swellable polymer system before and after disentanglement due to solvent action [23].

Sample Preparation

The resultant sulfonated polymer was washed several times with tap water and deionised water then dried in a vacuum oven. The polyethylene samples were named SAE 2.5M, SAE 5.0M, SAE 10.0M, and SAE 15.0M in accordance with the molarity of sulfuric acid used for the sulfonation of low-density polyethylene. The functionalized samples were then grinded and allowed to age at the ambient conditions for 30 days before further analysis. All the grinded samples are shown in Fig. 4. The sample with circular mark is LDPE treated with 15.0M solution indicating the colour change.

Determination of Density and Pore-Volume

Density and pore-volume of the untreated LDPE and treated polymer was measured using gas pycnometer (Accupyc 1340) obtained from Micromeritics. This is a fast and fully automated instrument which provides high speed and high precision volume measurements and density calculations. The chamber size of 10 [cm.sup.3] was used in accordance with the ASTM standard 6226 and eight cycles per samples were repeated for accurate results.

Viscosity and Molecular Weight Determination

Ten milligrams of LDPE was fed into a 100 ml Erlenmeyer flask and 45 ml of tri-chlorobenzene (TCB) was added to the flask. The flask was heated to 150[degrees]C [+ or -] 2 and stirred for one hour. The flask was examined for any undissolved polymer. The clean viscometer was placed into constant temperature oil bath at 135[degrees]C, allowed to equilibrate and flow of the solvent time was measured thrice and an average value was then obtained. The solvent was removed from the viscometer which was vacuum-washed and dried with air. The hot polymer solution was fed into the viscometer and allowed to equilibrate at 135[degrees]C and the flow time of the solution was measured thrice and an average value was recorded. The same procedure was applied in order to determine the intrinsic viscosity of LDPE with xylene as the solvent.

Elemental Analysis

Simultaneous determination of the treated polyethylene for carbons, hydrogen, nitrogen, sulfur, and oxygen was carried out using Elementar Germany's, Vario MACRO Cube CNHOS elemental analyser. Using quantitative high temperature decomposition, solid or liquid substances are changed into gaseous combinations. The gas mixture is cleaned, separated into its components and through efficient detectors determined with a precision and accuracy.

Fourier Transform Infra-Red spectroscopy

Attenuated total reflectance (ATR) Fourier Transform InfraRed spectra of the treated polymer powder were obtained using Perkin Elmer Spectrum 100 apparatus.

Contact Angle Measurements

Treated polyethylene powder was heated to prepare films and contact angles for double distilled water droplets were taken at least on eight different positions along the film. Angles were measured at 25[degrees]C using a goniometer 14[degrees] horizontal beam comparator. (G-23, serial no 91314, KRUSS, GmbH, Hamburg Germany)

Thermo Gravimetric Analysis (TGA)

TGA measurements were carried out using thermo gravimetric analyser (TA Instruments, TGA Q500), under nitrogen using flow rate of 20 [cm.sup.3]/min, with samples placed in platinum pans.

Differential Scanning Calorimetery (DSC)

Thermal analysis of the treated polymer samples were performed using DSC model Q10, TA Instruments. The samples were accurately weighed in the range of [+ or -] 2 to 3mg and then heated from 35[degrees]C to 150[degrees]C at a rate of 10[degrees]C/min (runl). They were then cooled to 30[degrees]C using a scan rate of 10[degrees]C/min (run 2) and finally the samples were subjected to the last heating cycle from 35[degrees]C to 150[degrees]C of 10[degrees]C/min (run 3).

Scanning Electron Microscopy (SEM)

Treated polymer samples were air dried and then made into films for observation under scanning electron microscope (EVO 50, Brand ZEISS). Films were fixed to a metal base using double-sided tape, and then sputter coated with platinum to make specimen conductive prior to SEM analysis.

Tensile Measurements

Mechanical properties of the treated LDPE specimens were determined according to ASTM D638-05 standard test methods, using SHIMADZU (model AG-1) Universal Testing Machine with load cell of 5 kN, crosshead speed of 5 mm/min and gauge length of 30mm. Tests were performed until tensile failure occurred.

RESULTS AND DISCUSSION

Density and Pore Volume

The density values reported were measured using the gas pycnometer and are an average of eight readings. The densities of the untreated and sulfonated samples are presented in Table 1. The table also indicates the amounts of porosity increase with increase in sulfuric acid concentration. It was observed that increasing sulfonation molarity over low-density polyethylene caused an increase in the mass of the polymer. This is due to the increase in the number of sulfonic groups attached to the polymer chain. As described in Table 1, the measured density results compare well with those reported by Arribas Rueda for the sulfonation of low-density polyethylene films [24] and Fonseca et al. [25] for surface etching of LDPE by sulfuric acid. The density increased from initial value of 0.9238 to 0.9707 showing an overall increase of five percent in the density value, which is more than previous reported value of 4% [25]. A significant increase in the porosity of the sulfonated LDPE film was also observed. However, paradoxically, the increase in mass was also accompanied by an increase in the porosity value as measured automatically by the pycnometer. The increase in sulfuric acid concentration not only raises the amount of functional group moieties but physically renders treated LDPE more porous. Nevertheless the sulfonation reaction impacts overall density of LDPE in positive direction and the relative porosity increase is accompanied by the development of greater number of pores per unit volume of the sulfonated LDPE samples.

LDPE Viscosity and Molecular Weight

The intrinsic viscosity results of LDPE using trichlorobenzene and xylene were 1.506 dL/gm and 2.439 dL/gm, respectively. Once intrinsic viscosity of LDPE was measured, the Mark-Houwink relationship [[??]] = K [M.sup.[alpha]] was used to determine both the viscosity average ([M.sub.v]) and weight average molecular weights ([M.sub.w]). Strazielle and Benoit [26] values were used to calculated weight average molecular weight with a focus on viscometric behaviour LDPE in TCB, where k = 7 X [10.sup.-4] and [alpha] = 0.67 resulting in weight average molecular weight of 94,226 g/g.mol. Similarly viscosity average molecular weight was calculated using Trementozzi [27) values valid for xylene with k = 17.6 X [10.sup.-3] and [alpha] = 0.83 resulting in a value of 38,051 g/g mol.

Elemental Analysis

The elemental concentrations were measured using Vario MACRO Cube elemental analyser, for detection of carbon, hydrogen, oxygen and sulfur contents. The percentages of carbon and hydrogen decreased with increase of sulfuric acid concentration compared with the untreated low-density polyethylene. Higher concentration of acid increases the amount of sulfur and oxygen signifying an overall increase in the density of the samples. Table 2 shows a decrease in the content of both carbon (14%) and hydrogen (6.5%) with respective increase of 1.35 and 12% of sulfur and oxygen. Table 3 and Fig. 5 represent the elements in important ratios. These results show that as the sulfuric acid molarity was increased, a decrease in carbon to hydrogen ratio and increase in sulfur and oxygen ratio to carbon was obtained. These results indicate proportional abstraction of hydrogen and subsequent formation of double bonds signifying proportional increase of sulfonation rate with sulfuric acid concentration [13].

ATR-Fourier Transform-Infra-Red Spectroscopy

A comparison of the FT-1R transmittance spectra and FT-1R transmittance spectra with ATR method for sulfonic group analysis by Tada and Ito [16] showed that the ATR technique gives better results. ATR-FT-IR spectra shown in Fig. 6 indicate significant changes in the chemical composition of the acid treated low-density polyethylene and provide visual comparison of the various untreated and sulfonated LDPE samples. The appearance of bands and increase in their intensities are consistent with formation of sulfonic functional group moieties. Generally, the peaks intensify with the increase in sulfuric acid concentration. The FT-IR results correlate well with the elemental analysis data which indicate an increase in sulfur and oxygen percentages in the overall composition of the functionalized polymer with increasing acid concentration. When comparing the spectra of untreated low-density polyethylene and 2.5M treated sample, the results show that the peak at 2335 [cm.sup.-1] disappears and the dual peaks at 2848 [cm.sup.-1] and 2916 [cm.sup.-1] due to olefinic carbon to hydrogen bond stretching becoming sharper. The broad peak at around 3800 [cm.sup.-1], corresponding to hydroxyl groups pertaining to sulfonic acid, appeared only in the treated samples and became stronger with the increase in sulfuric acid concentration. The broad band spectral changes in the region of 1700 to 1600 [cm.sup.-1] due to the action of concentrated sulfuric acid on polyolefins have been attributed to olefinic unsaturation [15]. Thus this new peak at 1640 [cm.sup.-1] is attributed to carbon-carbon double bond (isolated), which intensify gradually as the sulfuric acid concentration is increased from 2.5 molar to 10.0 molar. Hao et al. attributed FTIR(ATR) absorption between 1250 and 840 [cm.sup.-1] to multiple sulfonic groups such as O=S=O, S=O, S[(=O).sub.2]OH, -S[O.sub.3]H, and S-O-C, etc. Last portion of Fig. 6 represents a general view of the changes in ATR spectra of LDPE after sulfonation attributed to sulfonic groups. This portion pertaining to SAE 5.0M is magnified in Fig. 7 for illustration of multiple peaks observed from 500 to 1400 [cm.sup.-1]. These peaks correspond to carbon-sulfur bond, symmetrical and asymmetrical stretch of sulfoxides and sulfonates in the treated samples [28].

Contact Angle Measurements

Generation of functional group moieties increases hydrophilicity of the originally hydrophobic polymers. Functionalized polyethylene surface establishes hydrogen bonding with the water droplet, consequently spreading of water droplet on the surface and thus lowering its contact angle [19], Treated LDPE samples were grinded and made into films by melting. The films were then tested for their contact angle values. The results presented in Table 4 show that the contact angle values decrease almost linearly with an increase in the concentration of sulfuric acid used in the sulfonation reaction. Significant changes in contact angle values for all samples after sulfonation were observed. The surfaces become more hydrophilic with increasing degree of sulfonation, i.e. the contact angle of water drops falls due to the improved wetting character of the polymer surface. Therefore, a considerable increase in the hydrophilicity of the LDPE film can be achieved via the sulfonation of the polymer. The contact angle values of the sulfonated LDPE compare well with those of the commercially used textile polymers such as polyethylene terephthalate (72.5 [theta][degrees]) and nylons (68.3 [theta][degrees]).

Thermogravimetric Analysis (TGA)

Thermal stability of LDPE is an important selection criterion in many applications of the polymer, especially when it is used a thermoplastic matrix material in composites. Therefore

TGA was performed on the sulfonated LDPE specimens in order to assess the effect of degree of sulfonation on the thermal stability of the polymer. The accurately weighed polymer samples

([+ or -]5 mg) were scanned in the temperature range from 25[degrees]C to 600[degrees] C at a heating rate of 10[degrees]C/min. Weight loss of treated samples is illustrated in Fig. 8. All sulfonated LDPE samples showed similar weight loss behaviour upon heating, however, the LDPE samples treated with 5.0 and 10.0M sulfuric acid solutions show somewhat lower weight loss than those treated with other concentrations. The samples treated at the lowest (2.5M) and the highest (15.0M) sulfuric acid concentrations showed higher weight loss with increasing temperature. A reasonable explanation for this observation is that there exists a useful sulfonation concentration for the thermally stable sulfonated LDPE.

Differential Scanning Calorimetery (DSC)

Understanding of the degree of crystallinity for a polymer is also important as crystallinity affects many properties such as storage modulus, permeability, density, and melting temperature. DSC technique was also used to measure the heat flow into and out of the LDPE samples as a function of both time and temperature. Polymer crystallinity was determined by quantifying the heat associated with melting (fusion) of the polymer. DSC melting curves of the acid-treated low-density polyethylene were obtained using a heating rate of 10[degrees]C/min under nitrogen blanket. As shown in Fig. 9, all the DSC curves show a sharp melting peak except 15.0M sulfuric acid treated LDPE sample which shows characteristics of a premelting plateau before the main melting peak. These DSC curves also indicate that the annealing procedure plays a major role in acid treatment procedure [28]. Enthalpy and crystallinity values are illustrated in the form of graph in Fig. 10. Crystallization behaviour of the untreated and sulfonated polyethylene samples were calculated using enthalpy change values of each curve automatically using following formula [X.sub.c] = {[DELTA][H.sub.f] /[DELTA][H.sub.f]*} x 100, where [DELTA][H.sub.f] is change in enthalpy and [DELTA][H.sub.f] * is enthalpy of fusion for 100% crystalline polyethylene sample having value of 293.1 J/g [29, 30].

The results presented in Table 5 compare the onset melting temperature, onset melting peak temperature, enthalpy, and crystallinity values of the untreated LDPE and sulfuric acid treated LDPE samples. These results show that in general acid treatment of LDPE has very little effect on the melting behaviour of the polymer except that for the specimen treated with 15.0M sulfuric acid (SAE15.0M), which shows a slight decrease in the melting temperature and some broadening of the melting peak, with a shoulder appearing just before melting. Though, the results presented in Table 6 and illustrated in Fig. 10 exhibited marked decrease in enthalpy of fusion and crystallinity percentage for 10.0M treated LDPE sample. Using DSC time for 50% weight loss along with the weight percentage degradation rate per degree Celsius for untreated and sulfonated LDPE samples under two ranges of 50 to 300[degrees]C and from 300 to 500[degrees]C were calculated and presented in Table 6. Nearly all sulfonated samples showed similar weight loss behaviour upon heating with the exception of SAE 10.0M which has [t.sub.1/2], of 37 min in comparison to 40 min half-life for other sulfonated samples. As for as the degradation rate of LDPE samples is concerned, the samples treated with 5.0 and 10.0M sulfuric acid solutions show somewhat lower weight loss in comparison to 2.5 and 15.0M molar treated samples during initial temperature range of 50 to 300[degrees]C. Contrariwise, samples treated with 2.5M and 15.0M sulfuric acid concentrations showed lower degradation rate for temperature range of 300 to 500[degrees]C. The degradation rate value ranges from the lowest value of 0.5577 to the highest of 0.6178 wt% per [degrees]C for temperature range of 300 to 500[degrees]C showing a similar trend between untreated and sulfonated LDPE. Conversely, a gradual increase in the degradation rate of LDPE until 5.0M treated sample and then the decrease in degradation rate for 10.0M treated sample was observed.

The decrease in enthalpy of fusion and crystallinity for LDPE treated 10.0M sulfuric acid percentage correlate with the [t.sub.1/2] of 37 min. In comparison to 10.0M treated sample slight increase in both enthalpy and crystallinity was observed for 15.0M sulfuric acid treated sample. The results suggest that acid treatment modifies the crystal structure of the polymer by incorporation of side chains due to sulfonation. These side chains mainly contain sulfonic groups and reduce the symmetric and crystalline portion of polymer. Thus, it may be concluded that sulfonation imparts additional amorphous character to treated polyethylene. The decrease in crystallinity and enthalpy is almost directly related to the concentration of the sulfuric acid up to the molarity of 10.0. Phillips et al. have demonstrated that the inclusion of sulfonic acid side groups in polyethylene cause a decrease in the percent crystallinity of polymer as a function of the acid concentration [31]. The slight increase in these values may be due to the formation of new type of crystals as indicated by the appearance of a shoulder in the DSC curve of this sample (SAE15.0M) or this phenomenon may be related to the movement of the domains formed by hydrogen bonding arising from the high concentration of the sulfonic acid groups.

Morphology Studies Using SEM

Scanning electron microscopy (SEM) is the most widely used tool for surface analysis and morphological changes [32]. The films were prepared from the untreated and sulfonated LDPE samples. Topography and morphological changes were observed by scanning electron microscopy of the acid treated and untreated LDPE film samples as illustrated in Fig. 11. These SEM micrographs show very significant morphological differences between the surfaces of the three film samples. The untreated LDPE film sample shows uniform surface morphology, whereas the films prepared from the two sulfonated materials show very rough surface morphologies. In both cases, for the sulfonated samples, surface cracks were observed and there were considerable differences between the surface characteristics of the films prepared from the sulfonated LDPE using 5.0M and 15.0M sulfuric acid concentrations. The 15.0M treated sample showed much rougher surface than the 5.0M sample. The film prepared using the high concentration acid LDPE exhibited a considerable amount of pitting and surface fracture features.

Mechanical Properties

Tensile strength (TS) and elongation at break (EB) of low-density polyethylene samples after sulfonation with increasing acid concentration were measured as mechanical performance indicators for polymeric material [1-3, 7], For each sample type at least five specimens were tested in accordance to ASTM D-638-V specifications. Average tensile strength and elongation at break values were calculated and are shown as bi-axis graph in Fig. 12. The results indicated almost linear decrease in tensile strength between the values of 8 to 6 MPa as the strength of sulfuric acid was increased. However, a slight increase in the elongation at break was observed for the samples prepared from LDPE treated with 15.0M sulfuric acid with respect to 10.0M treated sample. This behaviour corresponds with the increase in the crystallinity as illustrated in Fig. 10. There are marked differences of 22.6% in the elongation at break values and 0.2 MPa in tensile strength values between the untreated and 2.5M acid treated low-density polyethylene. Overall, a decrease in both the tensile strength and elongation at break values were observed for the treated samples and this decreased is proportional to the increase in sulfuric acid concentration used to treat the LDPE samples. This decrease in the mechanical properties of the sulfonated LDPE correlates with the changes in the crystallinity and enthalpy values obtained for these specimens.

CONCLUSIONS

Commonly sulfonation of polyolefin films is carried out as a surface treatment in order to increase adhesion and dye receptivity for packaging applications. In this study, we have performed liquid phase in-depth sulfonation of low-density polyethylene in pellet forms. This homogenous sulfonation reaction was successfully achieved by swelling low-density polyethylene in cyclohexane and ethanol as non-polar and polar reagents at ambient temperature under continuous agitation. Sulfuric acid treated LDPE became progressively brownish in colour as a result of overall sulfonation treatment with increasing acid concentration between 2.5M and 15M. This colour change indicates that an in-depth sulfonation of LPDE was obtained and the reaction was not just limited to the surface of the treated LDPE samples. These samples were grinded in to the powder form in order to ascertain in-depth sulfonation and to study their hydrophilic and mechanical properties. ATR-FT-1R spectra of the treated samples indicated oxidation, which was also accompanied by sulfonation reaction and formation of double bonds. More than 30[degrees] decrease in the contact angle values exhibits enhanced hydrophilic character of the sulfonated LDPE samples. Scanning electron microscopy also provides evidence of surface roughening and better physical interaction. A notable decrease in the crystallinity and tensile properties of the sulfonated LDPE samples were obtained. Overall, the results indicate the formation of a modified material that is cost-effective and suitable for many technical textiles applications.

ACKNOWLEDGMENTS

The author thanks Prof. Ani Idris of Universiti Technologi Malaysia for her support in contact angle measurements.

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Mohib R. Kazimi, (1,2) Tahir Shah, (3) Saidatul Shima Binti Jamari, (1) Iqbal Ahmed, (4) Che Ku Mohammad Faizal (1)

(1) Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Malaysia

(2) Department of Applied Chemistry and Chemical Technology, University of Karachi, Pakistan

(3) Centre of Materials Research and Innovation, University of Bolton, UK

(4) Department of Chemical Engineering, Universiti Technologi Petronas, Malaysia

Correspondence to: Che Ku Mohammad Faizal; e-mail: mfaizal@ump.edu.my

Contract grant sponsor: Universiti Malaysia PAHANG; contract grant number: PGRS #100352.

DOI 10.1002/pen.23802

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Density and porosity of unmodified and sulfonated low-density
polyethylene samples.

Sample name   Density (gm/[cm.sup.3])   Porosity increase
                                         ([cm.sup.3]/gm)

LDPE          0.9238 [+ or -] 0.0023           N/A
SAE2.5M       0.9287 [+ or -] 0.0008         0.00570
SAE5.0M       0.9355 [+ or -] 0.0006         0.00780
SAE10.0M      0.9372 [+ or -] 0.0004         0.00200
SAE15.0M      0.9704 [+ or -] 0.0004         0.03650

TABLE 2. Elemental percentage of unmodified and sulfonated low-density
polyethylene samples.

Sample       Carbon      Hydrogen      Sulfur       Oxygen
  name     percentage   percentage   percentage   percentage

  LDPE       85.60        14.30         0.01         0.01
SAE 2.5      84.06        14.24         0.23         1.45
SAE 5.0      75.11        13.51         0.36        10.90
SAE 10.0     73.82        13.71         0.72        11.05
SAE 15.0     73.28        13.37         1.35        11.98

TABLE 3. Carbon to hydrogen ratio, sulfur to carbon ratio and oxygen
to carbon ratio of sulfonated low-density polyethylene using sulfuric
acid of varying concentration.

Sample name    C/H      S/C      O/C

SAE 2.5       5.9036   0.0028   0.0172
SAE 5.0       5.5616   0.0048   0.1465
SAE 10.0      5.4809   0.0098   0.1588
SAE 15.0      5.3843   0.0184   0.1635

TABLE 4. Effect of sulfuric acid concentration on contact angle and
Cos [theta] values of low-density polyethylene.

Sample name   [theta][degrees]   Cos [theta]
                  (water)

LDPE          99.0 [+ or -] 2       -0.15
SAE2.5M       80.1 [+ or -] 3       0.17
SAE5.0M       71.2 [+ or -] 2       0.30
SAE10.0M      70.9 [+ or -] 3       0.32
SAE15.0M      64.7 [+ or -] 3       0.42

TABLE 5. DSC data of untreated and sulfonated low-density
polyethylene.

                Melt           Melt
               onset           peak
Sample          temp           temp       Enthalpy   Crystallinity
name        ([degrees]C)   ([degrees]C)    (J/g)          (%)

LDPE           98.25          104.53       129.30        44.02
SAE 2.5M       98.63          103.94       103.00        35.15
SAE 5.0M       98.10          103.76       96.08         32.79
SAE IO.OM      98.80          104.10       74.25         25.25
SAE I5.0M      97.53          104.60       82.96         27.99

TABLE 6. 50% weight loss time and degradation rates of untreated and
sulfonated low-density polyethylene at temperature range of
50[degrees]C to 300[degrees]C and from 300[degrees]C to
500[degrees]C.

             [t.sub.1/2] half        Percentage weight
Sample      time for 50% weight   loss per/[degrees]C for
name            loss (min)          50 to 300[degrees]C

LDPE               40.0                   0.00866
SAE 2.5M           40.5                   0.03362
SAE 5.0M           41.0                   0.01622
SAE 10.0M          37.0                   0.01818
SAE 15.0M          40.0                   0.03896

               Percentage weight
Sample      loss per /[degrees]C for
name          300 to 500[degrees]C

LDPE                 0.5847
SAE 2.5M             0.5844
SAE 5.0M             0.6178
SAE 10.0M            0.6077
SAE 15.0M            0.5577
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Author:Kazimi, Mohib R.; Shah, Tahir; Jamari, Saidatul Shima Binti; Ahmed, Iqbal; Faizal, Che Ku Mohammad
Publication:Polymer Engineering and Science
Article Type:Report
Date:Nov 1, 2014
Words:5859
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