Effects of sulfonated polyol on the properties of the resultant aqueous polyurethane dispersions

Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 176–185

Effects of sulfonated polyol on the properties of the resultant aqueous polyurethane dispersions Hsun-Tsing Lee a,∗ , Sheng-Yen Wu b , Ru-Jong Jeng b a

b

Department of Chemical Engineering, Vanung University, Chung-Li 320, Taiwan, ROC Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Received 19 June 2005; received in revised form 21 September 2005; accepted 21 October 2005 Available online 28 December 2005

Abstract A polyol containing long sulfonated side chain, designated as PESS, was used to prepare aqueous polyurethane (PU) dispersions through the prepolymer mixing process in this study. The effects of this sulfonated polyol on the properties of the resultant waterborne polyurethanes (WPUs) were studied. Structures and properties of the WPUs were examined by FTIR, GPC, particle sizer, viscometer, TGA, DMA, DSC, tensile tester and surface resistance meter. When the content of total hydrophilic groups (i.e. sodium sulfonate group of PESS and quaternary ammonium carboxylate group of neutralized dimethylol propionic acid, DMPA) remains unchanged, the WPU dispersion with higher PESS content possesses higher average particle diameter and film decomposition temperature due to its higher average molecular weight (Mw). The WPU with higher PESS content possesses higher dispersion viscosity and film ionic conductivity due to the higher dissociation degree or electrostatic interaction of sodium sulfonate groups than quaternary ammonium carboxylate groups. PESS enhances the antistatic property of the WPU films, which can be used in electronic, wrapping, and coating industries. Moreover, the WPU film with higher PESS content exhibits lower modulus but higher elongation at break due to the lower hard segment content. The glass transition temperature of these amorphous films is at about −45 ◦ C, irrespective of the PESS/DMPA ratio and hydrophilic group content (HGC). © 2005 Elsevier B.V. All rights reserved. Keywords: Dispersion; Hydrophilic group; Polyurethane; Sulfonate group; Waterborne

1. Introduction Polyurethanes (PUs) are functional polymers that their properties can be tailor-made by simply adjusting the compositions. Because of this, the PUs are of great interest for applications in coatings, adhesives, medical devices, binders, sealants, and textiles industries [1–5]. Conventional polyurethane products such as coatings and adhesives contain significant amount of organic solvents and some also contain free isocyanate monomers [6]. These solvent-based polyurethanes have been gradually replaced by the waterborne polyurethanes (WPUs), due to the increasing concern about environmental pollution and health and safety risks. Conventional polyurethane is insoluble or undispersible in aqueous media. For polyurethane to be dispersible in water, ionic or hydrophilic groups were incorporated to the polymer chains. The WPUs can be prepared from these PU ionomers or ∗

Corresponding author. Tel.: +886 3 4515811x537; fax: +886 3 4514814. E-mail address: [email protected] (H.-T. Lee).

0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.034

hydrophilic PUs. Many processes have been developed to prepare WPUs. The earliest process was the acetone process [7,8]. First, a solution of polyurethane ionomer is prepared in a lot of hydrophilic solvent, e.g. acetone. The solution is subsequently mixed with water, and then the solvent is removed by distillation. An aqueous dispersion of the polyurethane ionomer is obtained. A different prepolymer mixing process to prepare WPUs [7–9] was used in this study. This process is intended to avoid the use of a lot amount of organic solvent. In this process, NCO-terminated polyurethane prepolymers containing ionic groups are dispersed in water. The chain extension reaction proceeds subsequently by adding primary diamines to the dispersion instantly to react with NCO groups. An aqueous dispersion of the polyurethane ionomer is thus obtained. During the chain extension reaction, the NCO groups react primarily with diamines [8–10] instead of water due to two reasons. The first is that the reaction rate of NCO group with NH2 group is about 1000 times faster than that of NCO group with water [11]. The second is that the NCO groups of the prepolymer droplets were surrounded and protected by the hydrophilic ionic moieties in the outer part of the droplets [8–10].

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The hydrophilic group is of primary importance to the synthesis of WPU. The dimethylol propionic acid (DMPA) containing pendant carboxyl group is one of the most commonly used hydrophilic compounds, which can be incorporated into the PU polymer chain due to the reactivity of its two OH groups. The DMPA based aqueous PU dispersions can be electrostatically stabilized with carboxyl groups, which are neutralized by tertiary amines [12–16]. The compositional effects and process variables as raw material species, NCO/OH molar ratio, DMPA content and neutralization degree on the properties of DMPA based WPU had been extensively investigated [5,8,14,17–19]. Moreover, further crosslinking reaction of WPU with respective aziridine [20], carbodiimide [21], glycidyl compounds [22], amino-formaldehyde and melamine-formaldehyde resins [23] were also studied. To enhance the WPU performance, another selected polymer was incorporated to PU to form a multi-phase structure in the dispersion through various techniques as blending, seeded emulsion polymerization or interpenetration network formation [24–28]. The DMPA based WPU had also been investigated for potential polymer electrolyte usage through neutralizing DMPA with LiOH [29] or through adding LiClO4 [30] or CF3 SO3 Li [31] salts. In addition to DMPA, compounds containing pendant sulfonate group can also be used to prepare anionic PU solution or dispersion by several methods. A series of sulfonated PU ionomers were prepared by reacting urethane NH group of different classical PUs with NaH and 1,3-propane sultone in organic solvents [32–41]. The morphology, physical and conductive properties of the films cast from these ionomer solutions were studied [32–35]. The interactions between the solvent and polymer, and the ionic aggregate structure of the ionomer solutions were also investigated [36–39], The above PU ionomers could be used as macromolecular electrolytes in water during the electropolymerization of pyrrole to form conductive polypyrrole/PU composites [40,41]. Moreover, various sulfonated PU ionomers based on different sulfonated diols were directly prepared in organic solvents. The structure, and physical and conductive properties of the ionomer films were studied [42–45]. Other sulfonated PU dispersions based on sulfonated diols were prepared by a modified acetone process. The obtained PU dispersions could be used as dispersants for TiO2 pigment in water-soluble acrylic paints [46]. The dispersible PU could also be end-capped by silanol to become a self-crosslinkable dispersion [47]. In addition, some PU dispersions based on both DMPA and sulfonated diamines were prepared by a modified acetone process, and investigated for potential usage as polymer electrolytes [15,29,31,48,49]. From the above elucidation, the sulfonated PUs reported in literature were prepared in organic solvents or by a modified acetone process, whereby plenty of organic solvents needs to be removed. The removal of volatile solvents would increase the number of preparation steps and degree of inconvenience to obtain the final products. In this study, the prepolymer mixing process was used to avoid the use of large amount of organic solvents. The as-synthesized aqueous PU dispersions can be used directly and nonhazardously. The PU ionomers prepared from different synthesis processes would exhibit different character-

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istics and properties. In addition, the pendant sulfonate groups used in literature were directly incorporated to the PU backbone or linked by short chains such as propylene, phenylene, etc. The sulfonated diol used in this study possesses a long side chain with 23–24 repeat units of ethylene oxide and/or propylene oxide, and end-capped with a sulfonate group, designated as PESS. The PU ionomers with the sulfonate groups at the end of long flexible side chains would exhibit different dispersion and film properties as compared to those reported in literature. Moreover, both DMPA and sulfonated diamine were used together to prepare PU ionomers as polymer electrolytes by a modified acetone process in literature. DMPA was used in the prepolymer synthesis step, whereas sulfonated diamine was used in the chain extension step. The sulfonated diamine was used in a way not intended to replace the use of DMPA. In this study, both DMPA, and PESS with long sulfonated side chain were used to prepare aqueous polyurethane dispersions by the prepolymer mixing process. PESS was used to replace different amount of DMPA to obtain a series of WPU dispersions. The effects of PESS on the dispersion and film properties of the resulting WPU were investigated. The methodology used in this study is different from those in literature. The sodium sulfonate group of PESS is a salt of a strong acid and a strong base, and is freely dissociated in water. The carboxylic acid group of DMPA is neutralized by tertiary amine to form quaternary ammonium carboxylate during the preparation of WPU. The quaternary ammonium carboxylate is a salt of a weak acid and a weak base. When this salt is dissociated in water, some of the resulting quaternary ammonium cations and carboxylic anions react with water to form undissociated quaternary ammonium hydroxide and carboxylic acid, respectively. Thus, both the dissociation degree and efficiency for stabilizing WPU of DMPA are lower than those of PESS. The essence of free dissociation and strong electrostatic interaction of sodium sulfonate groups of PESS would strongly affect the properties of the resulting WPU dispersions and films. The above essence of PESS would also impart the antistatic property to the WPU films. The antistatic PESS based WPU can be used as electronic parts in electronic industry or as antistatic wrappings and coatings [50,51]. Furthermore, the molecular weight of PESS (1400 g/mol) is much higher than that of DMPA (134 g/mol). The molecular weight of a diol or polyol is expected to exhibit significant influence on the properties of the resulting polyurethane. Thus, we make use of PESS to replace certain amount of DMPA and investigate the effects of PESS on the properties of the resulting WPU. An understanding of this system would provide valuable information for applications of WPU in coatings, adhesives, binders, antistatic wares and other industries. 2. Experimental 2.1. Materials Isophorone diisocyanate (IPDI), DMPA and dibutyltin dilaurate (T-12) were of synthetic grade obtained from Lancaster. 1-Methyl-2-pyrrolidone (NMP) of synthetic grade was obtained

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from Ferak. Triethyl amine and ethylene diamine were obtained from Tedia. PESS with an average molecular weight of 1400 was obtained from Po Chiun Co., Taiwan. The chemical structure of PESS is shown as follow:

DMPA per 100 g of solid moiety of the WPU. A PU film can be obtained by drying the dispersion at 60 ◦ C for several days and then under dynamic vacuum at 60 ◦ C for 24 h. 2.3. Characterization

Poly(hexylene adipate) diol with an average molecular weight of 1900 was obtained from Yong Shun Chemical Co., Taiwan, and is designated as PHAO. The PESS and PHAO polyols were dried and degassed at 100 ◦ C under dynamic vacuum at 3–5 mmHg for 4 h. 2.2. Preparation of aqueous PU dispersions Aqueous PU dispersions were prepared by the prepolymer mixing process as shown in Fig. 1. The formulations used were shown in Table 1. The PHAO diol, PESS diol, DMPA, and NMP were added to a 1000 mL four-necked glass flask equipped with a heating mantel, a mechanical stirrer, a thermometer, a reflux condenser, and a nitrogen inlet. A few drops of T-12 catalyst were added to the flask and the reaction mixture was heated to 75 ◦ C. Then, IPDI was slowly added to the flask to maintain the reaction temperature at 75 ◦ C. The reaction proceeded until the amount of residual isocyanate groups reached a theoretical end-point, as calculated when all hydroxyl groups had reacted with isocyanate groups. The NCO content of the prepolymer was measured by the dibutylamine back titration method [52]. An NCO-terminated prepolymer was therefore obtained. The polyurethane anionomer was obtained by cooling down the prepolymer to 50 ◦ C and adding triethylamine to neutralize the COOH groups of DMPA. The polyurethane anionomer was then dispersed in water and the polymer chain was extended by reacting with ethylene diamine. The presence of small amount of NMP would reduce the viscosity of the polyurethane anionomer and facilitate the dispersing of anionomer in water. The obtained aqueous polyurethane dispersion had a solid content of 25 wt%. The hydrophilic group content (HGC) is defined as the miniequivalents of PESS and

A Fourier transform infrared spectrophotometer (FTIR; Perkin-Elmer Model Paragon 500) was used to identify the chemical structure of polyurethane. The dispersion sample was coated as a thin liquid film on thallium-bromide/thalliumiodide crystal surface and dried for examination. The thalliumbromide/thallium-iodide crystal is water insensitive. A gel permeation chromatograph (Analytical Scientific Instruments Model 500) with a reflection index (RI) detector (Schambeck RI2000) and two columns of Jordi gel DVB mixed ˚ bed in series at 30 ◦ C was used to measure the bed and 10,000 A molecular weight distribution relative to polystyrene standards. The calibration curve was obtained by eight standards in the molecular weight range 3420–2.57 × 106 . The carrier solvent was tetrahydrofuran at a flow rate of 1 ml/min. A particle sizer (Brookhaven Model BI-90 plus) was used to measure the average particle diameters and their distributions of the dispersions. The PU dispersion was diluted with deionized water to 0.12 wt% before the measurement. A viscometer (Brookfield Model DV III) equipped with a thermostat was used to measure the viscosities of the aqueous PU dispersions at 25 ◦ C. A thermogravimetric analyzer (TGA; Seiko Model TG/DTA220) was used to measure weight losses of WPU films in the temperature range 50–500 ◦ C with a heating rate of 10 ◦ C/min and under a nitrogen stream. Prior to the temperature scan, each specimen was subject to a heating scan from room temperature to 100 ◦ C in the TGA furnace to remove any adsorbed moisture. A dynamic mechanical analyzer (DMA; Thermal Analyzer Model Q800) was used to measure flexural, loss moduli (E , E ), and tan δ of the PU films in the temperature range −140 to 50 ◦ C with a heating rate of 2 ◦ C/min and frequency of 1 Hz. The size of the specimens was about 20 mm long, 5 mm wide, and 0.3 mm thick. After mounting a specimen in the sample chamber, the specimen length subject to cyclic flexural motion was about 15 mm. A differential scanning calorimeter analyzer (DSC; Thermal Analyzer Model 2010) was used to examine thermograms of

Table 1 Recipe of waterborne polyurethane dispersionsa Hydrophilic group content (HGC, meq/100 g solid)b

Weight of raw materials (g) IPDI

PHAO

PESS

DMPA

NMPc

TEAd

H2 Oe

EDA

20 25 30 35

25.0 25.0 25.0 25.0

74.1–74.9 65.5–70.8 57.5–64.4 50.3–58.8

1.4–16.4 1.7–19.6 1.9–22.5 2.1–25.2

1.6–2.7 1.9–3.2 2.2–3.6 2.4–4.0

26.8–29.3 25.2–28.0 23.7–26.8 22.5–25.7

1.2–2.1 1.4–2.4 1.6–2.8 1.8–3.0

294.5–322.0 276.8–307.8 261.1–294.8 247.3–282.9

3.0 3.0 3.0 3.0

a b c d e

At NCO/OH molar ratio = 1.8 and PESS/DMPA molar ratio: 1/20, 1/15, 1/10, 1/5, 1/2, 1. HGC = [miniequivalents of (PESS + DMPA)]/[grams of (IPDI + PHAO + PESS +DMPA + EDA)] × 100. NMP content is about 6.17 wt% of the dispersion. Moles of TEA equal to those of DMPA. Appropriate amount of water is added to form a dispersion of solid content 25 wt%.

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Fig. 1. Schematic representation of the WPU preparation.

WPU films in the temperature range −140 to 200 ◦ C with a heating rate of 10 ◦ C/min and under a nitrogen stream. Uniaxial stress–strain testing was performed using a Shimadzu Model EZ Test machine at room temperature with a crosshead speed of 50 mm/min. Dumbbell-shaped samples were stamped out of cast films with a standard ASTM D1708 die. The water resistance of the films (40 mm × 20 mm × 0.46 mm in size) was measured by immersing preweighed films in deionized water for 24 h at room temperature. After the residual water was wiped from the film surface using filter paper, the weight of the swollen film was measured immediately [47,53]. The water resistance or water uptake (WU) was expressed as the weight percentage of water in the swollen film. WU (%) =

Ws − Wd × 100% Wd

where Ws is the weight of the swollen film and Wd , the weight of the dry film. A surface resistance meter (Monroe Electronics Model 272A) was used to measure the surface and volume electrical resis-

tances of WPU films at room temperature. The size of the specimens was about 68 mm in diameter and 0.3 mm in thickness. 3. Results and discussion 3.1. Structure of the WPU IR spectra of the WPU films with PESS/DMPA molar ratios of 1/20 and 1 at a HGC of 30 meq/100 g are shown in Fig. 2. PHAO is a polyester diol, whereas PESS is a polyether diol. These two polyols and DMPA diol were used for synthesizing the WPU, so that the WPU consists of basic functional groups as urethane, ester and ether. The characteristic absorption peaks are: 2949 and 2867 cm−1 which are due to alkane CH stretching vibration, 1732 cm−1 due to C O stretching of urethane and ester groups, 1557 cm−1 due to CHN vibration of associated secondary urethane groups, 1462 cm−1 due to CH2 and CH3 deformation vibration, 1385 and 1365 cm−1 due to C N stretching, 1242 cm−1 due to ester C O C asymmetric

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Fig. 2. IR spectra of the WPU films with PESS/DMPA molar ratios of 1/20 and 1 at a HGC value of 30 meq/100 g.

stretching vibration, 1174 cm−1 due to coupled C N and C O stretching vibration, 1142 cm−1 due to ether C O C asymmetric stretching vibration, 1062 cm−1 due to ester and ether C O C symmetric stretching vibration [54]. The asymmetric and symmetric stretching vibrations of SO3 Na, supposedly located at 1195–1175 cm−1 and 1065–1050 cm−1 , respectively [54], are masked by the above absorption peaks. The asymmetric and symmetric stretching vibrations of COO− , supposedly located at 1610–1550 cm−1 and 1420–1335 cm−1 , respectively [54], are also masked by the above absorption peaks. These IR spectra confirm the formation of a polyurethane structure. Furthermore, no free NCO group of the WPU at 2250–2285 cm−1 was observed in these spectra. Thus, the NCO-terminated prepolymer was fully reacted with ethylene diamine during the chain extension step. The weight average molecular weights (Mws) of WPUs with different PESS/DMPA molar ratios and HGCs are shown in Fig. 3. The Mws are in the range 37,000–52,700 g/mol. The

Fig. 3. The weight average molecular weights of WPUs with different PESS/DMPA molar ratios and HGC values (the symbols are experimental data and curves are the results of curve-fitting in all figures).

number average molecular weights (Mns) are in the range 14,700–24,200 g/mol, which possess the same trend as Mws do. The average molecular weights are calculated using a calibration curve of polystyrene standards each with a known molecular weight. When the content of hydrophilic groups (i.e. SO3 Na group of PESS and COOH group of DMPA) remains unchanged, the WPU with higher PESS content, i.e. lower DMPA content, possesses higher molecular weight as shown in Fig. 3. The molecular weight of PESS is 1400, but that of DMPA is 134. The prepolymer chain should be longer as higher ratio of PESS was present at a fixed total amount of PESS and DMPA. When HGC value was larger, i.e. more PESS and DMPA were present, less PHAO diol would be used to maintain NCO/OH molar ratio at 1.8 for the preparation of WPU. Furthermore, the molecular weights of PESS and DMPA are lower than that of PHAO, i.e. 1900. Thus, the WPU with a larger HGC value possesses lower molecular weight at a fixed PESS/DMPA molar ratio as shown in Fig. 3. The constituent with higher molecular weight would impart a higher molecular weight to WPU, when the NCO/OH molar ratio and reaction conditions are unchanged. The properties of WPUs are strongly affected by their molecular weights, which will be discussed in the following sections. 3.2. Physical characteristics of the WPU dispersions The WPU dispersions exhibited unimodal particle size distributions. Fig. 4 shows the number average particle diameters of the WPU dispersions with different PESS/DMPA molar ratios and HGC values. The average particle diameters of the WPU dispersions with PESS/DMPA ratios of 1/20 and 1 are 123 and 303 nm, respectively, at a HGC value of 30 meq/100 g. When the HGC value remains unchanged, the WPU with higher PESS content possesses higher average particle diameter as shown in Fig. 4. The molecular weight of prepolymer increases with increasing PESS/DMPA ratio at a fixed total amount of PESS and DMPA as illustrated in the above section. The prepolymer with higher molecular weight is more viscous. Therefore, the

Fig. 4. Number average particle diameters of the WPU dispersions with different PESS/DMPA molar ratios and HGC values.

H.-T. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 176–185

Fig. 5. Viscosities of the WPU dispersions with different PESS/DMPA molar ratios and HGC values at 25 ◦ C.

more viscous prepolymer containing higher PESS content is more difficult to be dispersed in water during the dispersion step and subsequently results in a dispersion with larger particle size. When PESS/DMPA molar ratio remains unchanged, the WPU with a larger HGC value possesses smaller average particle diameters (Fig. 4). This phenomenon is due to the lower molecular weight of WPU and a larger quantity of ionic groups available for stabilizing WPU particles at a larger HGC value. In other words, the WPU particles with lower molecular weight and higher ionic density for stabilization will possess smaller diameter. Fig. 5 shows the viscosities of the WPU dispersions with different PESS/DMPA molar ratios and HGC values at 25 ◦ C. The viscosities of the WPU dispersions with PESS/DMPA ratios of 1/20 and 1 are 7 and 18 cps, respectively, at a HGC value of 30 meq/100 g. In the dispersion, PESS contains SO3 − Na+ groups, whereas the neutralized DMPA contains COO− NH+ (C2 H5 )3 groups. The SO3 − Na+ group being a salt of a strong acid and a strong base, is freely dissociated in water. On the other hand, the COO− NH+ (C2 H5 )3 group being a salt of a weak acid and a weak base, is not freely dissociated in water. Therefore, the electrostatic interaction of PESS is stronger than that of DMPA. Thus, at a fixed HGC value, the viscosity of WPU dispersion increases with increasing PESS content. When PESS/DMPA molar ratio remains unchanged, the WPU dispersion with larger HGC value possesses smaller average particle diameter as described earlier (Fig. 4). A WPU dispersion with a smaller average particle diameter at a fixed solid content will have larger surface area and steric hindrance. Furthermore, a WPU dispersion containing more ionic groups possesses stronger electrostatic interactions between particles. Thus, the WPU dispersion with a larger HGC value possesses higher viscosity.

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Fig. 6. TGA curves of WPU films with different PESS/DMPA molar ratios at a HGC value of 30 meq/100 g.

of 30 meq/100 g. The WPU films at other HGC values exhibit the same thermal decomposition behavior as the samples with a HGC value of 30 meq/100 g. There is hardly any weight loss before 200 ◦ C for all these compounds. The weight loss of WPU is more pronounced with decreasing PESS/DMPA ratio when heated above 250 ◦ C. The WPUs are almost completely decomposed above 450 ◦ C. Generally, the temperature at which weight loss reaches to 5% is assigned to be the decomposition temperature (Td). The Tds for WPUs with different PESS/DMPA molar ratios and HGC values are shown in Fig. 7. The WPU with higher PESS content at a fixed HGC value has a higher Td. Moreover, the WPU with lower content of hydrophilic groups at a fixed PESS/DMPA ratio also possesses a higher Td. The rising of Td in these two cases is due to the higher molecular weight of WPU as discussed earlier (Fig. 3). DMA results of the WPU films with different PESS/DMPA molar ratios at a HGC value of 30 meq/100 g, and those of the WPU films with different HGC values at a PESS/DMPA ratios of 1 are shown in Figs. 8 and 9, respectively. The E curves of all these films have one peak centered at about −45 ◦ C

3.3. Thermal and mechanical properties Fig. 6 shows the results of TGA measurements for the WPU films with different PESS/DMPA molar ratios at a HGC value

Fig. 7. The decomposition temperatures (Tds) of WPU films with different PESS/DMPA molar ratios and HGC values.

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Fig. 8. DMA results of the WPU films with different PESS/DMPA molar ratios at a HGC value of 30 meq/100 g.

Fig. 9. DMA results of the WPU films with different HGC values at a PESS/DMPA molar ratio of 1.

and one shoulder located at about −89 ◦ C, irrespective of the PESS/DMPA ratio and HGC value. The peak at −45 ◦ C is the glass transition temperature (Tg) of the WPU film. This is because E drops by more than 2 orders during the transition. The shoulder of E curve appeared at about −89 ◦ C is the ␤ transition temperature attributed to the side chain thermal motion. The Tg is also determined by DSC for comparison. DSC results of the WPU films with different PESS/DMPA molar ratios and HGC values are shown in Fig. 10. The Tg (about −52 ◦ C) determined by DSC is close to that obtained by DMA measurement, irrespective of the PESS/DMPA ratio and HGC value. In some curves, a tiny endothermic peak located at about 0 ◦ C was due to the existence of moisture. The DSC results show no crystallinity in these WPU films. The magnitude of E of these WPU films in the range −140 to −60 ◦ C is nearly irrespective of the PESS/DMPA ratio as shown in Fig. 8(a). This is because these films are in glassy state. At temperature higher than Tg, for example, 25 ◦ C, the E -value decreases with increasing PESS/DMPA ratio (Fig. 11). PESS is a high molecular weight diol possesses many ether linkages and contributes to the soft segment domains of the WPU film. Whereas, DMPA is a low molecular weight diol and con-

tributes to the hard segment domains of the WPU film. It is anticipated that the hard segments improve the mechanical property such as storage modulus of the WPU. Therefore, E increase with increasing DMPA content or with decreasing PESS/DMPA ratio. Moreover, the WPU film with a higher HGC value at a fixed PESS/DMPA ratio possesses a higher E (Fig. 11). This phenomenon is also due to the increase of DMPA content. The hard segment contents of WPU films with different PESS/DMPA molar ratios and HGC values are shown in Table 2. Stress–strain curves for the WPU films with different PESS/DMPA molar ratios at a HGC value of 30 meq/100 g are shown in Fig. 12. Tensile properties are summarized in Table 3. Fig. 12 and Table 3 show that the WPU film with higher PESS content or lower hard segment content behaves more like a rubber, exhibiting a low tensile strength and a high elongation at break. The tensile strength increases with increasing hard segment content or decreasing PESS content, which is in agreement with the dynamic mechanical E data at 25 ◦ C (Fig. 11). Table 3 also shows the water resistances or water uptakes (WU) of these films. The WPU film with higher PESS content exhibits higher water uptake or less water resistance due to the more hydrophilic nature of sodium sulfonate group of PESS than that of carboxylic acid group of DMPA.

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Table 2 The hard segment contents (HSC) of WPU films with different hydrophilic group contents (HGCs) and PESS/DMPA molar ratios PESS/DMPA

1/20 1/15 1/10 1/5 1/2 1

HSC (wt%) HGC = 20 meq/100 g

HGC = 25 meq/100 g

HGC = 30 meq/100 g

HGC = 35 meq/100 g

27.9 27.8 27.6 27.0 26.0 24.6

30.1 30.0 29.7 29.0 27.7 26.0

32.3 32.1 31.8 31.0 29.5 27.4

34.6 34.3 33.9 33.0 31.2 28.7

3.4. Antistatic property The surface and volume resistances of the WPU films with different PESS/DMPA molar ratios and HGC values are shown in Figs. 13 and 14, respectively. The ionic conductivity of the WPU film is mainly contributed by both sodium sulfonate group (SO3 − Na+ ) of PESS and ammonium carboxylate group (COO− NH+ R3 ) of neutralized DMPA. As a result, the surface and volume resistances of the film decrease

with increasing HGC value at a fixed PESS/DMPA ratio. As stated quantitatively, the surface resistances of WPU films with HGC values of 20 and 35 meq/100 g are 7.8 × 1010 and 1.0 × 1010 /cm2 (corresponding volume resistances are 2.3 × 1010 and 5.6 × 109  cm), respectively, at a PESS/DMPA ratio of 1. Moreover, the surface and volume electrical resistances of the film decrease with increasing PESS/DMPA ratio at a fixed HGC value. This is because the dissociation

Fig. 11. E -values of the WPU films with different PESS/DMPA molar ratios and HGC values.

Fig. 10. DSC thermograms of the WPU films with different PESS/DMPA molar ratios and HGC values.

Fig. 12. Stress–strain curves for the WPU films with different PESS/DMPA molar ratios at a HGC value of 30 meq/100 g.

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Table 3 Stress–strain properties and water resistance of WPU films with different PES S/DMPA molar ratios PESS/DMPA

1/20 1/15 1/10 1/5 1/2 1

HSC (wt%)

32.3 32.1 31.8 31.0 29.5 27.4

Tensile strength (MPa)

Elongation at break (%)

WU (%)

4.11 3.70 3.33 2.90 1.82 0.48

436 467 541 760 1032 1420

7 7 11 15 18 19

of 108 –1011 /cm2 would exhibit sufficient antistatic properties [50,51]. Therefore, one would conclude that the PESS is capable of enhancing the antistatic property of WPU films. 4. Conclusions

Fig. 13. The surface electrical resistances of the WPU films with different PESS/DMPA molar ratios and HGC values.

degree or conductance of sodium sulfonate group of PESS is larger than that of ammonium carboxylate group of neutralized DMPA. As expressed quantitatively, the surface resistances of WPU films with PESS/DMPA ratios of 1/20 and 1 are 5.4 × 1011 and 1.0 × 1010 /cm2 (corresponding volume resistances are 3.8 × 1011 and 5.6 × 109  cm), respectively, at a HGC value of 35 meq/100 g. The film with a surface resistance

Fig. 14. The volume electrical resistances of the WPU films with different PESS/DMPA molar ratios and HGC values.

PESS containing long sulfonated side chain was used to prepare aqueous PU dispersions through the prepolymer mixing process. The SO3 − Na+ group of PESS and COO− NH+ R3 group of neutralized DMPA were the two hydrophilic units used to stabilize our WPU dispersions. When the HGC value remains constant, the WPU dispersion with a higher PESS content possesses a higher average particle diameter and film decomposition temperature due to its higher average molecular weight. The viscosity of the WPU dispersion increases with increasing PESS content, due to the higher dissociation degree or stronger electrostatic interaction of SO3 − Na+ groups when compared to COO− NH+ R3 groups. The WPU film with higher PESS content exhibits lower modulus but higher elongation at break due to the lower hard segment content. These WPU films show no crystallinity and their glass transition temperatures appear at about −45 ◦ C, irrespective of the PESS/DMPA ratio and HGC value. The PESS based WPU films, which has a surface resistances of 1.0 × 1010 /cm2 for certain composition in our system, can be used as electronic parts in electronic equipments or as antistatic wrappings and coatings. Acknowledgement We wish to thank the National Science Council of ROC for financial aid through the project NSC 91-2622-E238-002-CC3. References [1] H.B. Park, Y.M. Lee, J. Membr. Sci. 197 (2002) 283. [2] G.A. Abraham, A.A.A. de Queiroz, J.S. Rom´an, Biomaterials 22 (2001) 1971. [3] A. Gugliuzza, G. Clarizia, G. Golemme, E. Drioli, Eur. Polym. J. 38 (2002) 235. [4] Y.I. Tien, K.H. Wei, Polymer 42 (2001) 3213. [5] K. Mequanint, R. Sanderson, Polymer 44 (2003) 2631. [6] M. Modesti, A. Lorenzetti, Eur. Polym. J. 37 (2001) 949. [7] G. Oertel, Polyurethane Handbook, second ed., Carl Hanser Verlag, Munich, 1994, p. 31. [8] J.Y. Jang, Y.K. Jhon, I.W. Cheong, J.H. Kim, Colloid Surf. A: Physicochem. Eng. Aspects 196 (2002) 135. [9] Y.K. Jhon, I.W. Cheong, J.H. Kim, Colloid Surf. A: Physicochem. Eng. Aspects 179 (2001) 71. [10] H.T. Lee, Y.T. Hwang, N.S. Chang, C.C.T. Huang, H.C. Li, Waterborne High-Solids and Powder Coatings Symposium, New Orleans, 1995, p. 224. [11] J.H. Saunders, K.C. Frisch, Polyurethanes: Chemistry and Technology, Interscience Publishers, New York, 1962, p. 173. [12] G.N. Chen, K.N. Chen, J. Appl. Polym. Sci. 71 (1999) 903. [13] I.H. Kim, J.S. Shin, I.W. Cheong, J.I. Kim, J.H. Kim, Colloid Surf. A: Physicochem. Eng. Aspects 207 (2002) 169. [14] M.C. Delpech, F.M.B. Coutinho, Polym. Test. 19 (2000) 939. [15] T.C. Wen, H.S. Tseng, Z.B. Lu, Solid State Ionics 134 (2000) 291. [16] H.L. Wang, A. Gopalan, T.C. Wen, Mater. Chem. Phys. 82 (2003) 793. [17] R.S. Dearth, H. Mertes, P.J. Jacobs, Prog. Org. Coat. 29 (1996) 73.

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