Stable emulsions (solid content: 38%) of emulsifier-/N-methylpyrrolidone (NMP)-free crosslinkable waterborne polyurethane-acrylic hybrids with a fixed acrylic monomer content (20 wt.%) and different molar ratios (mole%) of dimethylol propionic acid (DMPA)/crosslinkable pentaerythritol triacrylate (PETA)(17/23, 22/17, 27/11, 32/5) were successfully prepared. This study examined the effect of mole% of DMPA/PETA on the stability and viscosity of hybrid emulsions, the tensile properties/dynamic mechanical thermal properties of hybrid film samples and the adhesive strengths of formulated adhesives for footwear at both dry and wet states. The tensile strength/modulus, storage modulus and Tgs increased with increasing PETA content. The adhesive strength at dry state increased with increasing DMPA content up to 27 mole%, and then decreased a little. However, the adhesive strength at wet state decreased with increasing DMPA content. The optimum DMPA/PETA contents were found to be 27/11 mole% to achieve high performance adhesive properties.
Generally, adhesives used for sole attachment are either solvent-based adhesives or water-based adhesives. Many studies have already been done in solvent based polyurethane adhesives (1–4). However, solvent-based adhesives are gradually being changed with water-based adhesives. It is chiefly because the VOCs evaporate into the atmosphere so there can be hazardous to the health of workers if proper ventilation is not performed. These VOCs are regulated in many countries, including the U.S. EPA.
Waterborne polyurethane (WBPU) dispersions are being widely used in coatings and adhesives (5–11). Most WBPUs contain ionic groups in their molecular structure, and show excellent mechanical properties due to the presence of inter-chain Columbic forces and hydrogen bonding (9). The hydrophilic pendant carboxylic salt groups of dimethylol propionic acid (DMPA) in the WBPUs will act as anionic centers and internal emulsifiers.
Generally, dried WBPU films are water sensitive because of the presence of hydrophilic ionic groups. Accordingly, the ionic content should be kept to a minimum for the formation of water-resistant WBPUs. We found that it was very difficult to obtain stable WBPU dispersions with low content of ionic moieties (<10 mole%, 2 wt.%) (11). Thus, it is very important to adjust the water resistance and dispersion stability via the subtle control of the hydrophilic-hydrophobic balance through the use of the hydrophobic component and enough ionic moieties for WBPUs (7–9, 11–14).
Kwak et al. (7) investigated the effect of dimethylol propionic acid (DMPA) content on adhesive strength of polyester-based WBPU. They reported that the adhesive strength increased with increasing DMPA content. Nakamae et al. (3, 4) found that the adhesive strength on aluminum plates at both dry and wet states increased with increasing hard segment content and carboxyl group content. Sanchez-Adsuar et al. (1) reported that the adhesive strength increased with increasing hard/soft segment ratio. Yang et al. (12) studied the effect of different types of neutralizing agents on adhesive strength of polycaprolactone-based WBPU. Crosslinker is used to increase the adhesive force of WBPU Kwon and Kim (13). However, few studies are available on WBPU adhesive in open scientific literature.
The crosslinking of water-based materials is also very important to improve water resistance as well as mechanical properties. Park et al. (15) studied the effect of vinyl triethoxysilane (VTES, self-crosslinking agent) on the properties of WBPU/self-cross-linked fluorinated acrylic copolymer hybrids for high-performance water-repellent coating materials. They reported that the optimum VTES/BA content was found to be about 9/6 wt.% to obtain high-performance water-repellent coating materials.
Generally, WBPUs are environment-friendly materials with good adhesion, excellent elasticity/flexibility and abrasion resistance. However, WBPUs have the shortcomings of high cost, low pH stability and limited outdoor durability. On the other hand, acrylic polymers have excellent water and weather resistance, proper mechanical properties, good pigmentability and low cost, however, they exhibit poor elasticity and abrasion resistance. Consequently, formulators have sought ways of combining the advantages of WBPU and acrylic polymer (16–20). The polyurethane/acrylic hybrid emulsion was developed to exploit the potential cost reduction and good water resistance afforded by the acrylic polymers and maintain a greater share of the advantageous WBPU properties.
No external surfactants are present to contribute adversely to water sensitivity of WBPU-based materials. A previous study reported the optimum composition of a high-performance coating material with stable WBPU/acrylic hybrid latex formation without an external surfactant by in situ polymerization using a prepolymer mixing process (14, 17). In the previous study, the adhesive property of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylate emulsions with various acrylic monomer contents was investigated.
One disadvantage of both urethane-acrylic hybrid emulsions and WBPU emulsions is the inclusion of N-methylpyrrolidone (NMP) solvent, which is commonly a necessary processing solvent included at levels ranging from about 3 to 15%. Consequently, NMP-free types of waterborne urethane-acrylic hybrid emulsions are becoming increasingly popular choices as coatings and adhesives. However, most of the researches on NMP-free water-based adhesives have been made in industrial laboratories which were barely published in open literature.
A series of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic (C-WPUA) hybrid with different mole% of dimethylol propionic acid (DMPA, 13–32 mole%)/crosslinkable pentaerythritol triacrylate (PETA, 23–5 mole%) were prepared from acrylate terminated urethane prepolymers [H12MDI/PTAd/DMPA/PETA/DBTDL/TEA/acrylic monomers (BA/MMA/GMA)] were prepared in this study. This study examined the effect of DMPA/PETA mole percentages (17/23, 22/17, 27/11, 3/5) on the stability and viscosity of the C-WPUA hybrid emulsions, water swelling%, tensile properties/dynamic mechanical thermal properties of C-WPUA hybrid film samples and adhesive strengths of formulated adhesives (C-WPUA hybrid emulsions/thickener/hardener) between upper (synthetic leather) and sole (EVA rubber) at both dry and wet states.
Poly(tetramethylene adipate glycol) (PTAd Mn= 2000 g/mol; DAEWON, Korea) was dried at 90°C under 1–2 mm Hg for 3 h before use. 4,4′-dicyclohexymethane diisocyanate (H12MDI, Aldrich Chemical, Milwaukee, WI, USA), triethylamine (TEA, Aldrich Chemical, Milwaukee, WI, USA), acetone (Aldrich Chemical, Milwaukee, WI, USA) were used after dehydration with 4 Å molecular sieves for 1 day. Dimethylol propionic acid (DMPA, Aldrich Chemical, Milwaukee, WI, USA) was dried in a vacuum oven for 5 h at 100°C. Dibutyltin dilaurate (DBTDL, Aldrich Chemical, Milwaukee, WI, USA), distillated deionized water, pentaerythritol triacrylate (PETA, Aldrich Chemical, Milwaukee, WI, USA), butyl acrylate (BA, Aldrich Chemical, Milwaukee, WI, USA), methyl methacrylate (MMA, Aldrich Chemical, Milwaukee, WI, USA), glycidyl methacrylate (GMA, Aldrich Chemical, Milwaukee, WI, USA), ammonium persulfate (APS, Aldrich Chemical, Milwaukee, WI, USA) were used without further purification. Ethylene vinyl acetate (EVA, Haksan, Korea), synthetic leather (Haksan, Korea), thickener (UH420, Adeka Korea Corporation, Korea), hardener (ARF40, Henkel Technologies, Korea), UV primer (P-7-2, Henkel Technologies, Korea) and primer (W-104, Henkel Technologies, Korea) were used as received.
2.2 Preparation of emulsifier-/NMP-free crosslinkable wateborne polyurethane-acrylic (C-WPUA) hyrid emulsions
Emulsifier-/NMP-free crosslinkable wateborne polyurethane-acrylic (C-WPUA) hybrid were synthesized using a prepolymer mixing process (Scheme 1). This process was divided into three steps . The first step is the formation of vinyl-terminated urethane prepolymer by reacting PETA (5–23 mole%) with NCO-terminated urethane prepolymer prepared from H12MDI (44–48 mole%)/PTAd (Mn=2000, 14–15 mole%, soft-segment content: 60 wt.%)/DMPA (17–32 mole%): PTAd and DMPA were placed in a four-neck, round-bottom flask equipped with a thermometer, mechanical stirrer, condenser with a drying tube, an inlet for dry nitrogen and a heat jacket, and was degassed in a vacuum at 90°C for 1 h. The mixture was allowed to cool to 50°C with moderating stirring (175–200 rpm). H12MDI was then dropped slowly into the flask, and the reaction mixture was allowed to react at 85°C until the theoretical NCO content was reached. The change in NCO value during the reaction was determined using the standard dibutylamine backtitration method (ASTM D 1638). The reaction mixture of NCO-terminated urethane prepolymer was cooled to 45°C, and acetone (10 wt.% based on urethane prepolymer weight) was added to the NCO-terminated prepolymer mixture to adjust the viscosity of the solution. Then, PETA was added dropwise. To obtain a vinyl-terminated urethane prepolymer, the capping reaction of NCO-terminated urethane prepolymer with PETA was continued until the NCO-content reached zero, as evidenced by the disappearance of the IR NCO peak . The second step is the neutralization of vinyl-terminated urethane prepolymer using tertiary amine TEA and the formation of mixtures of neutralized vinyl-terminated urethane prepolymer and acrylic monomers (BA/MMA/GMA) to allow copolymerization between vinyl-terminated urethane prepolymer and the acrylic monomers: The acrylic monomer mixture (BA/MMA/GMA: 20 wt.%) was then added to the vinyl-terminated prepolymer mixture to adjust the viscosity of the solution. TEA was added to the reaction mixture to neutralize the carboxyl group of the vinyl-terminated prepolymer. After 30 min neutralization, the reaction mixture was cooled to 40°C, and distilled water was added to the mixture with vigorous stirring (1000–1300 rpm) . The third step involves the dispersion of vinyl-terminated urethane prepolymer/acrylic monomers in water and the copolymerization of various vinyl groups by adding a water-soluble radical initiator (APS): A water/radical initiator (APS: 2 wt.% based on the acrylate-content) was added to the emulsion, and radical polymerization of acrylate groups (vinyl group) was performed by slowly heating the mixture to 65°C until the vinyl group peak in the IR spectra had disappeared. The emulsifier-/NMP-free crosslinkable wateborne polyurethane-acrylic (C-WPUA) hybrid (38 wt.% solid content) was obtained by the evaporation of acetone. Table 1 lists the sample designation, composition, viscosity, average particle size and stability of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic (C-WPUA) hybrid emulsions.
|Sample designation||Composition of C-WPU (molar ratio)||PTAd (wt.%)||DMPA/PETA (mole%)||Acrylic monomer (AC, wt.%)||C-WPU/AC (wt.%)||Mean particle size (nm)||Viscosity (cP/25°C)||Stability|
2.3 Preparation of C-WPUA hybrid films
C-WPUA hybrid films were prepared by pouring the dispersion into a Teflon disc and dried under ambient conditions for 24 h. The films were then peeling off from the Teflon disc. The films were vacuum dried at 45°C for 24 h under 20 mm Hg to remove the moisture and unreacted monomer. The vacuum dried films were stored in a vacuum desiccator at room temperature.
2.4 Formulation of adhesives for footwear
Footwear adhesive materials were formulated from C-WPUA hybrid emulsions, a thickener and a hardener. An appropriate amount of C-WPUA hybrid emulsion was mixed with the thickener (UH420, 1.5 wt.% based on C-WPUA) and the hardener (ARF40, 5.0 wt.% based on C-WPUA) to obtain a homogeneous mixture at room temperature.
2.5 Process of adhesion between upper (synthetic leather) and sole (EVA) of footwear
The steps typically required to bond the upper (synthetic leather) to the EVA sole are as follows: UV primer (P-7-2) was coated onto EVA sole, and then dried at 60°C for 2 min followed by UV-cured. Mixture of primer (W-104)/hardener (ARF40: 5.0 wt.% based on W-104) was coated onto both the UV primer treated EVA sole and upper leather, and then dried at 60°C for 5 min. Formulated adhesive is brushed onto the sole surface by hand, and then allowed to dry at 60°C for 5 min. The two surfaces (leather and sole surfaces) were brought into contact, and pressed two times using roller, and then dried at room temperature for 30 min and 24 h.
The mean particle size of the C-WPUA hybrid emulsions was measured at 25°C using a LS 13,320 laser diffraction particle size analyzer (Beckman Coulter, USA). The viscosity of the C-WPUA hybrid emulsions was measured at 25°C using a Brook field LVDVII+Digital viscometer (Brookfield, USA). The measurements were performed at 100 rpm using a spindle RV-3. The chemical components of the pristine C-WPUA hybrid samples were confirmed by a NICOLET iS5 Fourier transform infrared spectrometer (Thermo scientific, USA). The FT-IR spectra of the sample were recorded in the range of 4000–650 cm-1 at a resolution of 4 cm-1 and 32 scans. A constant compression load was applied to the samples. The IR samples (NCO-terminated urethane prepolymer, acrylate-terminated urethane prepolymer and C-WPUA hybrid emulsions) were prepared by spreading the coated viscose emulsions on the KRS-5 disc 25/4 mm and drying them using hair drier. The dynamic mechanical properties of C-WPUA hybrid film samples were examined by dynamic mechanical analysis (TA Instrument, DMA Q800, USA) at 1 Hz and a heating rate of 10°C/min over a temperature range from -100 to 150°C. The tensile properties were measured at room temperature with a 5582 system universal testing machine (Instron, USA) according to the ASTM D 638 specifications. A cross-head speed of 100 mm/min was used throughout these investigations to determine the ultimate tensile strength and modulus and the elongation at break for all samples. The values quoted are the average of three measurements. To measure the swelling in water, the films were immersed in water for 48 h at 25°C. The water swelling of the films was calculated using the following equation:
where W0 is the weight of the dried film and W is the weight of the film at equilibrium swelling.
The adhesion strengths of dry-samples (width/length: 2/10 cm) dried for 30 min and 24 h at room temperature and wet-samples (24 h dried samples soaked in water for 24 and 48 h at room temperature) were measured using a LT2100C universal testing machine (LABTRON CO., Korea) operated at a crosshead speed of 150 mm/min according to T-peel test. The values quoted are the average of five measurements.
3 Results and discussion
3.1 Preparation and shelf-stability of C-WPUA hybrid emulsions
Stable emulsions (solid content: 38 wt.%) of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic hybrid with different mole% of dimethylol propionic acid (DMPA, 17–32 mole%)/crosslinkable pentaerythritol triacrylate (PETA, 23–5 mole%) and a fixed soft segment content (60 wt.%) were successfully prepared in this study. They were also shelf-stable for a period of at least 4 months. However, the as-polymerized C-WPUA13/28 emulsion containing 13/28 mole% of DMPA/PETA was found to be unstable, indicating that around 13 mole% of self-emulsifying DMPA could not stabilized the external emulsifier-/NMP-free crosslinakble waterborne polyurethane-acrylate (C-WPUA13/28) emulsion. The radical co-/homo-polymerization of vinyl groups in acrylic monomers (BA/MMA/GMA)-absorbed acrylate-terminated urethane prepolymer mixture took place simultaneously. Therefore, the chance of interpenetrating polymer network formation for polyurethane-acrylates/polyacrylates is pretty excellent.
3.2 Mean particle size and viscosity of the C-WPUA hybrid emulsions
In our earlier works [15, 20], we found that increased hydrophilic component DMPA content enhanced the shelf stability of WBPU and resulted in smaller particle and higher viscosity. The particle size distributions of C-WPUA hybrid emulsions are shown in Figure 1. The mean particle size and viscosity versus DMPA content are shown in Figure 2. As DMPA content increased, the mean particle size of C-WPUA (126–83 nm) was significantly decreased, however, the viscosity (27–43 cP) of the emulsion increased a little. Generally, smaller particles lead to larger hydrodynamic volumes and, therefore, induce higher viscosities. It is generally known that the mean particle size is not directly related to the physical properties of waterborne polyurethane (WBPU) cast films. However, the control of the mean particle size is important with respect to the particular application of WBPU dispersion. For example, relatively larger particles are preferred in surface coatings for rapid drying, and smaller ones are desirable when the deep penetration of the dispersion into a substrate is essential. In any case, it was found that there was no problem with using C-WPUA emulsions prepared here as adhesives for footwear.
3.3 Identification of the chemical structure of C-WPUA hybrid films
Figure 3 shows FT-IR spectra of (A) NCO-terminated urethane prepolymer, (B) acrylate-terminated urethane prepolymer and (c) C-WPUA hybrid emulsion. The NCO terminated sample has the NCO peak at 2270 cm-1. The characteristic bands at 1082–1085 cm-1, 1713–1720/1744–1730 cm-1, 2855–2955 cm-1 and 3300–3500 cm-1 confirm the ether (C-O-C) of the ester group, carbonyl groups of urethane/acrylate, methylene/methyl group and amide group in C-WPUA, respectively. The carbonyl (C=O) groups of the urethane and acrylate in C-WPUA sample were identified by the characteristic peaks at 1713–1720 cm-1 (1713 cm-1) and 1744–1730 cm-1 (1731 cm-1), respectively. Acrylate vinyl group (C=C) peak at 1610 cm-1 was appeared in acrylate-terminated urethane prepolymer sample. However, no peak of acrylate vinyl group at 1610 cm-1 in isocyanate terminated urethane prepolymer as well as C-WPUA samples was observed, indicating the complete reaction of all acrylate vinyl groups in C-WPUA samples.
3.4 DMA results, mechanical properties and water swelling of C-WPUA hybrid films
Figure 4 shows the storage modulus and tan δ curves as function of temperature for C-WPUA hybrid film samples. The DMA results are shown in Table 2. The storage modulus of the film sample in the temperature range of -100 to 150°C was mostly decreased with increasing DMPA content. As the DMPA content increased, the Tgs shifted from -46.1°C to -51.3°C and the Tgh shifted from 87.9°C to 37.3°C. The shifts of Tgs and Tgh suggest that the soft/hard segments of the polyurethane are partially miscible with the acrylic monomer components in C-WPUA materials. The partial miscibility might be due to the intimate molecular mixing through the formation of acrylic monomer-absorbed acrylate-urethane prepolymer as well as copolymerization/crosslinking reaction between urethane-acrylate containing tri-functional group and acrylic monomer.
|Sample designation||DMA results||Mechanical properties|
|Tgs (°C)||Tgh (°C)||Tensile strength (MPa)||Elongation at break (%)||Modulus at 5% strain (MPa)|
Figure 5 shows the stress-strain curves of C-WPUA hybrid film samples. The tensile strength/modulus and elongation at break of C-WPUA hybrid film samples are shown in Table 2. As the DMPA content in DMPA/PETA increased, the tensile strength/modulus decreased, however, the elongation at break increased. The higher tensile strength/modulus and lower elongation at break might be attributable to the higher cross-linking density with increasing PETA content (decreasing DMPA content).
Figure 6 shows the water swelling of the C-WPUA hybrid film samples. The water swelling% of C-WPUA hybrid film samples increased from 13.27 to 15.48% with increasing DMPA content from 17 to 32 mole%. This should be due to the increase of hydrophilic ionic DMPA units/urethane groups. The water swelling% of C-WPUA materials is directly related with the adhesives strength at wet state as described below.
3.5 Adhesive strength of formulated adhesives for footwear
Figure 7 and Table 3 show the adhesive strength of formulated adhesives (C-WPUA hybrid emulsions/thickener/hardner) for footwear (leather/sole) at (A) dry state and (B) wet state. The adhesive strength of formulated adhesives at dry state (peel strength of footwear dried at 30 min and 24 h at room temperature after adhesion) increased with increasing DMPA content up to 27 mole%, and then decreased a little. The adhesive strength at wet state [peel strength after soaking the dry-state (24 h) samples in water for 24 and 48 h] decreased with increasing DMPA content. This should be attributed to the decrease of crosslinkable PETA moieties. It was found that the formulated adhesives with C-WPUA17/23, C-WPUA22/17 and C-WPUA27/11 at both dry state (24 h) and wet state (48 h) were passed the footwear adhesion requirements (peel strength at dry state >27 N/cm, and peel strength at wet state >25 N/cm). The high adhesive strengths (see Table 3) were fairly acceptable for potential footwear applications. The optimum DMPA/PETA contents were found to be about 27/11 mole% to achieve high performance adhesive properties.
|Sample designation||Adhesive strength (N/cm)|
|Drying time||Soaking time|
|30 min||24 h||24 h||48 h|
To obtain high performance footwear adhesives, a series of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic (C-WPUA) hybrid emulsions with different mole percentages of dimethylol propionic acid (DMPA, 13–32 mole%)/crosslinkable pentaerythritol triacrylate (PETA, 28–5 mole%) were prepared from crosslinkable acrylate-terminated urethane prepolymers [4,4′-dicyclohexymethane diisocyanate (H12MDI)/poly(tetramethylene adipate glycol) (PTAd, Mn=2000 g/mol)/DMPA/PETA/dibutyltindilaurate (DBTDL)/triethylamine (TEA)/acrylic monomers [butyl acrylate (BA)/methyl methacrylate (MMA)/glycidyl methacrylate (GMA)]. The footwear adhesives were formulated from emulsifier-/NMP-free C-WBPUA hybrid emulsions, thickener and hardener. Emulsions of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic (C-WPUA17/23, C-WPUA22/17, C-WPUA27/11, C-WPUA32/5) hybrid were found to be stable after 4 months. However, as-polymerized C-WPUA13/28 emulsion containing 13/28 mole% of DMPA/PETA was found to be unstable, indicating that around 13 mole% of self-emulsifying DMPA could not stabilized the external emulsifier-/NMP-free crosslinakble waterborne polyurethane-acrylic (C-WPUA13/28) hybrid emulsion. indicating that this value was the below the limit value of self-emulsifying ability. This study focused on the effect of mole percentages of DMPA/PETA on the shelf stability and viscosity of emulsions, tensile properties/dynamic mechanical thermal properties of hybrid film samples and adhesive strengths of formulated adhesives between upper (synthetic leather) and sole (EVA) in both dry and wet states. The average particle size of C-WPUA hybrid emulsions increased with increasing DMPA content in DMAP/PETA, but the viscosity increased. The tensile strength/modulus and storage modulus of the hybrid film samples decreased with increasing DMPA content. As the DMPA content increased, the hard segment Tg (Tgh) decreased from 87.9 to 37.3°C and the soft segment Tg (Tgs) decreased from -46.1 to -51.3°C. The adhesive strength of formulated adhesives for footwear (leather/sole) at dry state (peel strength of footwear dried at 30 min and 24 h at room temperature after adhesion) increased with increasing DMPA content up to 27 mole%, and then decreased. The adhesive strength at wet state (peel strength after soaking the dry-state (24 h) samples in water for 24 and 48 h) decreased with increasing DMPA content. The formulated adhesives with C-WPUA17/23, C-WPUA22/17 and C-WPUA27/11 at both dry state (24 h) and wet state (48 h) were passed the footwear adhesion criteria (peel strength at dry state >27 N/cm, and peel strength at wet state >25 N/cm). From these results, it was found that the optimum DMPA/PETA contents were about 27/11 mole% to achieve high performance adhesive properties.
This research was supported by the Ministry of Trade, Industry and Energy Republic of Korea, Republic of Korea (10047652).
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