Abstract
Waterborne polyurethane (WDPU) comprising polycaprolactone diol and hexamethylene diisocyanate was prepared by using tartaric acid (TA) as an ionomer. WDPU was further modified with polymethyl methacrylate (PMMA). Hybrid WDPU-PMMA coatings were formed by using unsaturate end capping agents such as 2-hydroxyethylmethacrylate. WDPU-PMMA blend coatings were formed by using chain extenders such as ethylene glycol, ethylene diamine and mixed with PMMA. Thermal behavior and structural characterizations were investigated by means of thermogravimetric analysis and Fourier transformm infrared (FT-IR) spectroscopy, respectively. Chemical and solvent resistance was checked against various chemicals and solvents. Bio-stability was evaluated in CoCl2/H2O2 solution. Gloss, film flexibility, tackiness, abrasion resistance and adhesive prosperities of WDPU-PMMA coatings were also checked by applying these coatings on a leather sheet.
1 Introduction
Polyurethane (PU) is a hetero chain polymer with a ur-ethane group (carbamate group) and may also contain other functional groups, such as biuret, urea, ester, isocyanurate, allophanate, ionic groups and carbodiimide, structurally similar to aliphatic polyamides (1). Incompatibility of hard segments (polar, crystalline phase) and soft segments (nonpolar, amorphous phase) of PU produces a multiphase micro structure that is responsible for the rubber like properties through the physical cross-linked structure by strong hydrogen bonding (2). A recent trend in the development of PU is the production of sustainable and biodegradable products due to the exceptional physical properties of PU such as hydrolytic stability, low in vitro protein adsorption and platelet adhesion (3). PU can be applied as molding, varnishes, adhesive, finishing coatings and paint (4).
Nowadays coating industries have developed new synthetic techniques such as waterborne powder coatings and radiation curable PUs due to environmental factors (5). Waterborne polyurethane (WDPU) has appreciably replaced solvent based coatings in order to reduce volatile organic compounds (VOCs) (6). Aqueous PUs were commonly used as adhesives/coatings for various substrates like textile fabrics, leather, plastics, wood, glass fibers and metals (7). WDPU has special application as leather finishing to improve its beauty, lifetime and quality. Water dispersible PU was synthesized by using external emulsifier or reaction of Na+H- with the NH group of PU (8). But in the present study emulsifier free water dispersible PU was synthesized by using tartaric acid (TA) as an ionomer, embedded in PU chain. TA is an environmental friendly, cheap and renewable material (9). Waterborne PU coatings have some drawbacks such as low solvent/chemical resistance, low pot life, slow drying time, high hydrophilicity and low thermal stability (10). These drawbacks can be improved by copolymerization of PU with acrylate polymers. In a continuation of our previous work, PU dispersion was blended with less expensive acrylic latexes to upgrade the film properties of coatings (7, 11). The main object of this research was to synthesize PU-acrylate blend/hybrid coatings of superior properties such as high glossiness, fast drying time, room temperature cure characteristics, outstanding flexibility, excellent abrasion resistance, transparency, nonflammable and resistance to various chemicals/solvents. Thermal, mechanical, chemical, surface and optical properties of PU-acrylate hybrid coatings and PU-PMMA blend coatings were compared in order to achieve a better understanding of the structure properties relationships of PU-acrylate coatings. PU-acrylate coatings with required properties will be synthesized by controlling the following factors such as the structure of the polycaprolactone diol; type of diisocyanate; type of the chain extender; molar ratio NCO/OH; soft segment concentration and acrylate monomer (1, 12). WDPUs offer an attractive approach toward the reduction of VOCs and the superior properties of new WDPUs have great potential applications (13).
2 Materials and methods
2.1 Materials
Hexamethylene diisocyanate (HDI, ≥99.0%), polycaprolactone diol (PCL, Mw=1250 g/mol), ethylene glycol (MEG, ≥98.5%), ethylene diamine (EDA, >99%) methyl methacrylate (MMA, >98%), 2-hydroxyethylmethacrylate (2-HEMA, 98.5%), acrylamide (C3H5NO, 97%) triethyl amine (TEA, >99%), tartaric acid (TA, 98.5%), potassium persulfate (KPS, >98%) acetone (C3H6O, >99%), 1,4-benzoquinone (C6H4O2, >95%), deionized water, hydroxyethyl cellulose (HEC, >98%), sodium stearoyl lactylate (SSL, 34%), and dibutyl tin dilaurate (DBTDL, >97%) were purchased from Merck Chemical Company (Lahore, Pakistan) and were used as received without further purification.
2.2 Synthesis of polyurethane-acrylate coatings
2.2.1 Synthesis of WDPU-PMMA hybrid coatings by acetone process
Polymerization was conducted in a 0.5 dm3, four-neck, round bottom flask equipped with a mechanical stirrer, oil-bath, temperature controller, condenser, dropping funnel and nitrogen inlet. HDI (50.4 g, 300 mmol), PCL (125 g, 100 mmol), TA (15 g, 100 mmol) and acrylamide (7.1 g, 100 mmol) were dissolved separately in dry acetone to form 50% by weight solution each. The amount of the PCL and TA solutions were calculated in the round bottom flask along with two drops of DBTDL catalyst (25–100 ppm in acetone). Under vigorous stirring, HDI solution was added by dropping funnel over 25–30 min at room temperature and then reflux for 3 h to prepare an NCO terminated polyurethane prepolymer (pre-PU) (14). Unsaturated end capping pre-PU was formed by the reaction of 5% w/w acrylamide (2-HEMA in the case of PUAC-13) as an end capping agent with pre-PU for 2 h in the presence of few drops of 1,4-benzoquinone as an inhibitor and neutralized with TEA at room temperature to form carboxylate ammonium salt of pre-PU. Unsaturated end capping pre-PU was slowly poured into a resin kettle charged with 220 g of deionized water at room temperature with strong agitation (15). Hydroxyethyl cellulose (1.2 g) as an protecting colloid and sodium stearoyl lactylate as an emulsifier (2 g) were added for the emulsification of monomer and the polymer. The emulsified MMA monomer (60 g) was fed into the reaction mixture drop wise with agitation along with the potassium persulfate (K2S2O8) catalyst (0.5 g in 10 g water) through a dropping funnel at 68–75°C (16). The addition of the MMA monomer was completed in 2 h and then the temperature was increased up to 95°C for 20 min. The resulting milky bluish hybrid WDPU-PMMA emulsion was referred to as PUAC-14 shown in Scheme 1. Similarly a PUAC-13 hybrid coating was synthesized using 2-HEMA as an unsaturated end capping reagent.

Synthesis of WDPU-PMMA hybrid coatings (PUAC-14 and PUAC-13).
2.2.2 Synthesis of WDPU-PMMA blend coatings by acetone process
PUAC-11 and PUAC-12 blend coatings were formed in a similar way by using ethylene glycol and ethylene diamine as chain extenders. WDPU-PMMA blend coatings (PUAC-11 and PUAC-12) were simply formed by the free radical polymerization of methyl methacrylate in PU dispersion as shown in Scheme 2. These blends have no chemical linkage with each other but physical interactions such as hydrogen bonding were operating within them. WDPU-PMMA hybrid coatings (PUAC-13 and PUAC-14) have chemical linkage between PU and polymethyl methacrylate (PMMA) along with physical interactions. They were formed by intoducing an unsaturated end capping group at the endings of PU molecule which are sandwiched between PMMA repeating units by free radical mechanism. The chemical compositions of WDPU-PMMA coatings are given in Table 1.

Synthesis of WDPU-PMMA blend coatings (PUAC-11 and PUAC-12).
Chemical composition of WDPU-PMMA coatings.
Properties | WDPU-PMMA coatings | |||
---|---|---|---|---|
PUAC-11 | PUAC-12 | PUAC-13 | PUAC-14 | |
Diisocyanate | HDI | HDI | HDI | HDI |
Polyol | PCL | PCL | PCL | PCL |
Chain extenders | Ethylene glycol | Ethylene diamine | – | – |
Unsaturate end capping agents | – | – | 2-HEMA | Acrylamide |
Ionomer | TA | TA | TA | TA |
Acrylic monomer | MMA | MMA | MMA | MMA |
Molar ratio (HDI:PCL:TA) | 4:1:1 | 4:1:1 | 4:1:1 | 4:1:1 |
Neutralizing amine | TEA | TEA | TEA | TEA |
NCO/OH | 1.33 | 1.33 | 1.33 | 1.33 |
Polymer backbone | Aliphatic | Aliphatic | Aliphatic | Aliphatic |
2.3 Characterization
For different characterizations, WDPU-PMMA films were formed by casting onto glass Petri dishes and allowing them to dry for 24 h at room temperature. Fourier transform-infrared (FT-IR) spectroscopy experiments were conducted on a Thermo Scientific Nicolet 6700 FTIR Spectrometer over the wavelength range of 4000–400 cm-1. The thermal decomposition profiles of coatings were thermogravimetrically analyzed using a Toledo (TGA/SDTA 851E) TGA instrument. Adhesion of films to the leather substrate was determined by employing a cross-hatch adhesion test according to ASTM D-3359. Knoop hardness was measured by using a QV-1000AAT Micro Hardness Tester with a diamond pyramid indenter. Flexibility of the coating was measured at an angle of 180° using a Conical Mandrel (ASTM F 23) (Japan). Drying time of the WDPU-PMMA films was determined according to the relevant polish standard. A 20°/60° dual angle Navo Lite (BGD 510) Gloss meter was used to measure glossiness of the coating. Abrasion resistance was determined by a Taber-5130 Abraser using CS-17 abrasive wheels in accordance with ASTM D4060. The study of chemical resistance of WDPU-PMMA film was considered according to the ASTM D 543-67 method. Biodegradability (oxidative degradation) of WDPU-PMMA films was determined by exposing the film of WDPU-PMMA into H2O2/CoCl2 solution, for a period of 24 days. The percentage swell in different solvents was determined for specimens after 2 weeks of immersion at room temperature.
3 Results and discussion
The main object of this research was to synthesize WDPU-PMMA blend/hybrid coatings of superior properties such as high glossiness, fast drying time, room temperature cure characteristics, outstanding flexibility, excellent abrasion resistance, transparency, nonflammable and resistance to various chemicals/solvents. Modifications in the PU coatings were made by using TA as an internal emulsifier and methylmethacrylate as a acrylic monomer. The chemical composition of WDPU-PMMA coatings are given in Table 1. WDPU-PMMA coatings have been classified into two groups, the WDPU-PMMA blend system (PUAC-11 and PUAC-12) and the polyurethane-acrylate hybrid system (PUAC-13 and PUAC-14). In WDPU-PMMA hybrid coatings, there is a permanent covalent linkage between PU and PMMA but no such bond in case of case of WDPU-PMMA blend coatings. The FT-IR spectroscopy, thermogravimetric analysis, adhesion test, hardness test, flexibility test, abration resistance test, gloss test, solvent/chemical resistance tests and biostability test were performed for investigation and comparasion of morphology, thermal properties, mechanical properties, glossiness, solvant/chemicals resistance and biodegradebility of WDPU-PMMA hybrid coatings with blend coatings.
3.1 FT-IR spectroscopy
FT-IR spectroscopy experiments were conducted on a Thermo Scientific Nicolet 6700 FTIR Spectrometer over the wavelength range of 4000–400 cm-1. FT-IR studies were carried out by focusing on three principle regions, -CH stretching (2700–2950 cm-1), -NH stretching (3500–3300 cm-1), and the C=O stretching (1620–1740 cm-1) (17). PU are capable of forming several kinds of hydrogen bonds due to the presence of a donor N-H group and a C=O acceptor group in the urethane linkage. These bands have been widely used to characterize the hydrogen bonding state, and were used in correlation with the phase separation in the coating. It is well known that in hydrogen-bonded urethanes, N-H and C=O bands appear at lower wave numbers than the bands that appear in urethanes free from hydrogen bonding. The band at 1720–1740 cm-1 (amide I, C=O stretching), 1538–1560 cm-1 (amide II, bending N-H and stretching C-N), 1230–1250 cm-1 (amide III, C-N stretching and N-H bending) and 1140–1120 cm-1 (C-O-C asymmetric stretching) confirm the formation of the urethane group as shown in Figure 1 (18). As hydrogen bonding alters the distribution of electrons, hydrogen bonded groups absorb at lower frequency than the non-bonded groups. Here we used FT-IR assignments reported in the literature for the analysis of our data. However, it should be kept in mind that the differences in structural factors (difference in chemistry), and environmental factors (difference in the degree of order) could give rise to minor deviations in the observed absorption frequencies (19). Hydrogen bonded urea N-H groups in an ordered environment absorb from 3340 to 3320 cm-1, while those from an disordered environment absorb above 3340 cm-1 in the case of PUAC-12. The adjectives disordered and ordered refer, respectively, to the regions of sample that were primarily amorphous, and the regions that have some degree of regularity. Similarly, urethane C=O in the ordered region absorb at around 1722 cm-1, whereas C=O absorption at about 1737 cm-1 corresponds to the disordered state as in the case of PUAC-11. Similar assignments for carbonyl region of polyurea were ordered C=O absorption at about 1628 cm-1 and disordered C=O absorption at about 1689 cm-1 as in the case of PUAC-12. Thus, absorption wave numbers serve as an indicator of the strength as well as order of hydrogen bonding. The FTIR spectrums of all the WDPU-PMMA coatings are shown in Figure 1. For PUAC-11 the absorption bands around 3328 cm-1 (urethane N-H stretching), 1737 cm-1 (free urethane C=O), and 1722 cm-1 were assigned to the urethane linkage. The linkage at 1140 cm-1 (C-O-C stretching) showed the formation of the urethane linkage. Similarly the structure of the WDPU-PMMA samples as PUAC-12, PUAC-13 and PUAC-14, were also verified by FTIR spectroscopy. The absorption bands around 3320 cm-1 (urea N-H stretching) and 1628 cm-1 (H-bonded urea C=O) were assigned to the urea linkage for PUAC-12. The peak at 3332 cm-1 (urethane N-H stretching), 1720 cm-1 (H-bonded urethane C=O), 1258 cm-1 (sym. CH2 bending) and 1128 cm-1 (C-O-C stretching) showed the formation of the urethane linkage in the case of PUAC-13. In the case of PUAC-12, the appearance of a single N-H band at 3328 cm-1 suggested that most of its N-H groups were hydrogen bonded. Moreover the absence of a band at 2270 cm-1 confirms that unreacted NCO groups were not present in the final product (20). In the case of PUAC-13 and PUAC-14, the absence of a characteristic band of unsaturation (C=C bond) at 1644 cm-1 conformed the successful grafting of acrylate monomers onto the reactive site of PU dispersion. The N-H groups free from hydrogen bonding have a stretching vibration at 3450 cm-1 (21). In contrast, the groups involved in hydrogen bonding have much lower frequencies, ranging from 3300 cm-1 to as low as 3200 cm-1. The exact position depends on the strength of the hydrogen bonding formed. The peak at 3300–3450 cm-1 belonging to urethane -NH groups was broader. Therefore, the peaks for free (3440 cm-1) and hydrogen bonded N-H groups (3330 cm-1) were overlapped. The absorbance in the range from 2790 to 2980 cm-1 was attributed to the CH2 vibration. The peaks at 1128 cm-1 arise from the stretching of C-O-C of the urethane linkage. The broad peaks at 1732 cm-1 mean -C=O stretching at urethane linkage and ester linkage. A urethane linkage thus has one donor and one acceptor, while a urea linkage has two donors and one acceptor. These groups primarily hydrogen bond with each other forming hard segment domains. In spite of their structural similarity, differences in electron delocalization in the urea and urethane linkages lead to differences in the frequency at which their carbonyl groups absorb in the IR region (17). The stretching mode of urea carbonyl appears at a lower frequency (free carbonyl=1691 cm-1) compared to the urethane carbonyl (free carbonyl=1730 cm-1) due to greater electron delocalization in the π-bonds in the former. In general for WDPU-PMMA coatings, the interpretation of carbonyl region in the FTIR spectra was complicated because of closely located carbonyl absorption modes in urea and urethane linkages and also because of the presence of both urea and urethane groups in the same polymer (18). In comparison, the N-H stretching modes were not much influenced by the delocalization of electrons making interpretation less difficult. For this reason we have primarily used N-H absorption to understand the hard segment hydrogen bonding.

FT-IR spectra of WDPU-PMMA coatings.
3.2 Thermogravimetric analysis
The thermal decomposition profiles of samples were thermogravimetrically analyzed using a Toledo (TGA/SDTA 851E) TGA instrument. A 10 mg sample was loaded in a platinum pan at a scanning rate of 10°C/min and stability of the sample was studied from 25°C to 600°C in the nitrogen atmosphere. TGA analysis was used to analyze the decomposition behavior of WDPU-PMMA coatings. TGA curves of WDPU-PMMA coatings were shown in Figure 2. WDPU-PMMA coatings were subjected to thermogravimetric analysis. WDPU-PMMA coatings showed two stage degradation patterns which indicate two phase morphology. Thermal degradation of WDPU-PMMA was a complex process, it is general rule that the more easily formed PU is less stable, i.e. more easily dissociated (15). PU was thermally degraded through three basic mechanisms: urethane bond dissociation into basic components, i.e. alcohol and isocyanate; breaking of the urethane with the formation of primary amine, carbon dioxide, and olefin; and finally splitting the urethane bond into secondary amine and carbon dioxide (16).

TGA of WDPU-PMMA coatings.
The thermal index is the criterion of thermal stability, it can be inferred that acrylate modified WDPU-PMMA hybrid PUAC-13 and PUAC-14 have higher stability than blends. This behavior can be attributed to the presence of stronger interactions between the acrylate groups with PU backbone due to grafting. Percentage weight loss in each stage indicates that the first stage degradation was due to the PU phase and the second stage was caused by a soft segment and the acrylate phase. For example, percentage weight loss in the first stage degradation of WDPU-PMMA was found to be 64.3% and in the second stage it was 26%. The degradation temperature and intermolecular forces such as hydrogen bonding and dipole-dipole interactions (which are very important in PU) decided bond stability (22).
Onset temperature of weight loss in the first stage remains very close to each other varying in a range of only 275–315°C for all samples. Initial thermal decomposition occurs due to a hard segment, composed of urethane linkages. When an urea linkage was present, the thermal stability was higher (as in case of PUAC-12), probably because of the higher hydrogen-bonding capacity of this group when compare to urethane. A major objective of this research was to investigate the degradation and combustion process of PU and to determine whether one could reduce the risk of combustion by synthesizing PU, which produces significant char during degradation, thereby disrupting the combustion cycle (23). Thermal stability is more dependent on the stability of the urethane bond, which is the weakest linkage in the PU structure. It was proposed that the thermal degradation of PU was primarily a depolycondensation process, which starts at about 250°C. It was in the hard segment, composed of urethane linkages that the initial thermal decomposition occurs (19).
3.3 Coating properties
Coating properties were checked by applying it on leather sheets. The leather sheets were degreased in alkali solution and subsequently swabbed with xylene to remove any type of oily material or contaminant. As soon as the sheets were dry, coatings were applied on them without any delay. The coated panels were examined for drying time, adhesion test, flexibility test, knoop hardness, solvent resistance, biodegradability and chemical resistance by standard methods.
3.3.1 Gloss
A 20°/60° dual angle Navo Lite (BGD 510) Gloss meter was used to measure glossiness of the coating. It measures the reflective appearance of the surface by shining a light to at a set angle and measuring the amount of reflection at the same and opposite angle. Gloss values were quoted in gloss unite (GU). A high gloss coating can measure 100 GU. The glossiness of WDPU-PMMA coatings were based on a combination of factors, including low weight percent of hard segment, low crystallinity or good matches in the refractive index of PU and acrylate, i.e. phase mixed regions. All the samples had formed high glossy film on leather sheets. Gloss values of all the samples were varied from 91 to 96.5 GU and no abnormal change was found (Table 2). This improvement can be attributed to excellent structural compatibility of components, i.e. between PU and PMMA (19).
Coating properties of WDPU-PMMA coatings.
Property | WDPU-PMMA coatings | ||||||
---|---|---|---|---|---|---|---|
PUAC-11 | PUAC-12 | PUAC-13 | PUAC-14 | ||||
60° Specular gloss | 91 | 93.5 | 96.5 | 95.9 | |||
Adhesion | Good | Good | Good | Good | |||
Appearance of emulsion | Milky | Milky | Milky bluish | Milky bluish | |||
Flexibility test | Pass | Pass | Pass | Pass | |||
Knoop hardness (HK 0.5) | 265 | 274 | 280 | 291 | |||
pH | 8 | 8.7 | 7.5 | 8.5 | |||
Abrasion resistance (mg per 100 cycles) | 0.09 | 0.07 | 0.06 | 0.05 | |||
Solid contents (%) | 44.10 | 41.78 | 47.25 | 45.8 | |||
Surface dry time (h) | 0.3 | 0.25 | 0.34 | 0.31 | |||
Tack-free dry time (h) | 0.6 | 0.5 | 0.8 | 0.7 |
3.3.2 Drying time (h)
Drying time of the WDPU-PMMA films was determined according to the relevant polish standard. The films were checked for surface dryness and tack-free dry stages at regular intervals of time. While moving a finger on the film without applying any pressure – if an impression of fingerprint was not observed on the film, it was said to be surface dry. If the thumb was pressed on the film and twisted while applying some pressure and yet no thumb impression or detachment of film was observed then it was said to be tack free dry. Time to achieve a tack free coating was noted (24). From the results as shown in the Table 2, it was suggested that these films gave good surface dryness and good tack-free dry properties. Am important property for any room temperature cured coating was the time it takes to become tack-free (25). It might be expected that drying time would be extended in waterborne coatings because of the relatively slow evaporation of water from the film due to strong interactions but the drying time of WDPU-PMMA was found to be less than pure PU. Acrylate resin decreased the drying time of WDPU which was more pronounced in WDPU-PMMA blend coatings rather than hybrid coatings. Table 2 shows that the hybrid films have higher drying time because solvents molecules become entrapped in the cross-linking network of the hybrid polymer and it is difficult to escape.
3.3.3 Knoop hardness
Knoop hardness was measured by using a QV-1000AAT Micro Hardness Tester with a diamond pyramid indenter. The loading force was in the range of 10–1000 g force (micro-hardness range). The Knoop hardness test is applied for testing soft material and thin coatings the Knoop number (HK) is calculated by the formula (26)
F is applied load in kg and L is long diagonal of the impression in mm. It can be observed from Table 2 that all samples exhibit better hardness properties than conventional PU coatings due to strong interactions and cross-linkages. This improvement can be attributed to the excellent structural compatibility of components in the WDPU-PMMA blend coatings which form a cross-linked polymer (27). WDPU-PMMA coatings had greater hardness than simple PU. The Knoop hardness of WDPU-PMMA coatings lies in the range of 265–291 gf.
3.3.4 Abrasion resistance
Abrasion resistance was determined by a Taber-5130 Abraser using CS-17 abrasive wheels in accordance with ASTM D4060. For every 100 cycles the loss of weight of the film was determined in mg (28). Abrasion resistances of overall WDPU-PMMA coatings were very good. Hybrid WDPU-PMMA coatings (PUAC-13 and PUAC-14) had higher abrasion resistance than WDPU-PMMA blend coatings (PUAC-11 and PUAC-12). Abrasion resistance of WDPU-PMMA coatings was found between 0.05 and 0.09 mg loss/100 cycles and remained the same for all samples as shown in Tabel 2.
3.3.5 Flexibility
The flexibility of the coating was measured at an angle of 180° using a Conical Mandrel (ASTM F 23) (Japan). Flexibility test was carried out using mandrels which had specific rod diameters. A test panel was inserted between the hinges and rod in such a way that the coated side was kept outside to the direction of bending. The hinge was closed at a single stretch without jerking in about a second causing the test panel to bend through an angle of 180°. The panel was examined for presence of cracks or loss of adhesion without removing the panel from the mandrill. WDPU-PMMA blend and hybrid coatings yield a hard film (Knoop hardness). However, with the exception of PUAC-12, all the films possess a significant flexibility (Table 2). Samples were passed without any crack or break on film during the flexibility test (28). In PUAC-12, flexibility was found to be less because of greater inter-linking by the presence of hydrogen bonding due to the urea group.
3.3.6 Adhesion (%)
Adhesion of films to a leather substrate was determined by employing the cross-hatch adhesion test according to ASTM D-3359. A crosshatch adhesion test was carried out after 24 h of coating application. By using a sharp edged knife, 10 parallel lines 1 mm apart from each other were drawn on the film. Another set of such lines at right angles of 90° to previous lines was superimposed to give a pattern of squares consisting of 100 squares with each square having a 1 mm side length. Self-adhesive tape was stuck over the square pattern in such a way that no air was present between the tape and film. Intimate contact between the tape and film was assured by pressing the tape over the length with fingers. The tape was kept in contact for 10 s and then the tape was rapidly pulled off in a single stroke at an angle of 120° approximately. The test was rated passed if not more than 5% of squares were removed. All the four samples showed 100% adhesion on leather sheets (Table 2). The reason for good adhesion strength of WDPU-PMMA coating may be due to formation of stable intermolecular hydrogen bonding (28).
3.3.7 Chemical resistance
The study of chemical resistance of the WDPU-PMMA film was considered according to the ASTM D 543-67 method. The samples were suspended in the different reagents for 2 weeks and change in appearance and weight was tested. All WDPU-PMMA coatings showed excellent acid, alkali, and solvent resistance as shown in Figure 3. No significant changes in the physical appearance of WDPU-PMMA coatings were seen except in H2SO4 and CHCl3. Specimens became brittle after drying. It was also found that alkali resistance of samples was better than acid resistance. The degradation of PUs was induced by chemical environments such as acid, base and oxidizing agents (13). The hydrolytic degradation was induced not only by water but also by acids and bases. The oxidizing agent helps degradation of PUs by the oxidation reaction. From the results obtained (Table 3), it has been found that both PU and acrylate resins were stable towards acids, bases and oxidants. But in the case of WDPU-PMMA blend there was a very small percentage of degradation (≈0.5–1.75). In the case of oxidant the percentage of weight loss ranges from 0.20 to 0.3 mg. The hydrolytic degradation of the soft segment PUs in acid medium may be due to increase of hydronium ions. The base induced hydrolytic degradation was due to the abstraction of hydrogen by the hydroxyl group of the base. The hydrolytic attack on acrylate based PU was centered mainly on the urethane group (29).

Chemical resistances of WDPU-PMMA coatings.
Chemical resistance of WDPU-PMMA coatings.
Chemicals | WDPU-PMMA coatings (percentage weight loss) | |||
---|---|---|---|---|
PUAC-11 | PUAC-12 | PUAC-13 | PUAC-14 | |
CCl4 | 1.37 | 2.81 | 2.73 | 1.58 |
CH3COOH (25%) | 1.19 | 1.60 | 2.67 | 2.54 |
CHCl3 | 0.85 | 1.04 | 2.01 | 2.05 |
H2SO4. (25%) | 0.89 | 1.70 | 2.31 | 2.33 |
HCl (25%) | 0.57 | 1.44 | 1.34 | 0.49 |
HNO3 (25%) | 1.44 | 1.52 | 2.33 | 1.49 |
Methylethyl ketone | 2.21 | 2.68 | 1.41 | 1.21 |
NaCl (10%) | 0.71 | 2.12 | 2.27 | 2.11 |
NaOH (10%) | 0.44 | 1.42 | 2.46 | 1.21 |
NH4OH (10%) | 0.06 | 0.07 | 0.08 | 0.09 |
Toluene | 0.60 | 0.33 | 2.12 | 2.47 |
Water | 1.05 | 1.61 | 1.78 | 1.34 |
The hydrolysis at the urethane linkage leads to amine termination. The hydrolysis at allophanate linkage leads to the formation of urea-linkage. Hydrolytic degradation was more effective for polyol chains, and less with aliphatic diisocyanate (19). It was thought that hydrophobicity and hard segment formation seem to resist the hydrolytic stability of PUs. In general, the hydrolytic stability of PUs can be related to that of their polyol component, with the stability of the polycaprolactone segments being greater than the other polyester. In addition it has been found that the longer the hydrocarbon chain of the glycol portion, the more resistant the PU is to hydrolysis (1). The different media under study has the same influence on the hydrolytic degradation. The PUs matrix was less susceptible to ionic permeation and hence the sodium ions and chloride ions have less effect on the degradation. Hence all the synthesized PUs were found to possess very good hydrolytic stability (Table 3).
3.3.8 Bio-stability
Biodegradability (oxidative degradation) of WDPU-PMMA films was determined by exposing the film of WDPU-PMMA into H2O2/CoCl2 solution, which simulated the oxidative component in the living organism (in vivo) environment, for a period of 24 days. The 24-day experiment showed that minimum degradation was observed in case of WDPU-PMMA hybrid coatings. A brittle surface layer was formed in case of the ethylene glycol base WDPU-PMMA film. Ethylene diamine based WDPU-PMMA film was marked by numerous pits and dimples. The 24-day experiment showed that shallow pits were appeared infrequently on all surfaces. Some debris was apparent on some sections of the film. No cracks appeared on any of the surfaces. This study used an accelerated in vitro test to demonstrate that a WDPU-PMMA hybrid film was more stable toward oxidative degradation than a WDPU-PMMA physical blend. There was no weight loss, shape change or embrittlement due to environmental exposure in the bio-degradability test which indicates that these WDPU-PMMA coatings were non-biodegradable (23). Very small variations in the chemical structures of polymer could lead to large changes in their biodegradability. The biodegradability depends on the molecular weight, molecular form and crystallinity (24). It decreases with increase in molecular weight; while monomers, dimers and repeating units degrade easily. PU produced by the diisocyanate poly-addition process was the characteristic chain link of urethane bond. Growth of microorganisms could not be supported by PUs and so the biodegradation was also found incomplete. PU degradation proceeded in a selective manner, with amorphous regions. Also, PUs with long repeating units and hydrolytic groups would be less likely to pack into high crystalline regions as normal PU, and these polymers were accessible to biodegradation (26). Cross linking was considered to inhibit degradation, some microorganisms like papain were found to diffuse through the film and break the structural integrity by hydrolyzing the urethane and urea linkage producing a free amine and hydroxyl group (28). Bio stability (oxidative degradation) of WDPU-PMMA coatings showed that hybrid WDPU-PMMA coatings were more stable towards oxidative degradation than WDPU-PMMA blend coatings.
3.3.9 Solvent resistance
The degree of cross-linking was calculated from the maximum degree of swelling. Samples of film weighing approximately 0.1 g each were immersed in solvent and the degree of swelling was calculated. After the maximum degree of swelling was reached, the sample was weighed and dried to constant solids content. Percentage swelling (solvent contents) was calculated by the following equation.
G1 is the weight of sample before drying and G2 is weight of sample after drying.
From the data presented in Table 4, it can be seen that all samples showed excellent solvent resistance, especially hybrid coatings in the range of 2.2–9.2 g/100 g of sample. The scheme also indicates the high degree of crosslink aging as observed by its insolubility and low extent of swelling in solvents.
Solvent resistance of WDPU-PMMA coatings.
Solvents | WDPU-PMMA coatings (weight in g/100 g of the sample) | |||
---|---|---|---|---|
PUAC-11 | PUAC-12 | PUAC-13 | PUAC-14 | |
CCl4 | 9.0 | 7.3 | 3.9 | 4.2 |
CHCl3 | 9.2 | 7.9 | 5.1 | 4.5 |
Methylethyl ketone | 7.5 | 6.3 | 4.7 | 4.9 |
Toluene | 5.0 | 4.3 | 3.7 | 2.9 |
Water | 4.0 | 4.1 | 2.2 | 2.8 |
This factor makes the determination of molecular weight impossible especially for WDPU-PMMA hybrid coatings (30). Results of solvent absorption are shown in the Figure 4. The solvent absorption decreased with increase in molecular weight of WDPU-PMMA coating and degree of cross-linking (31). Higher cross-linking does not allow solvent to penetrate and in the present case of PUAC-13 and 14, less penetration was observed because of higher degree of cross-linking.

Solvent resistance of WDPU-PMMA coatings.
4 Conclusions
Micro-phase segmented WDPU-PMMA co-polymers were synthesized via conventional two step solution polymerization process. Excellent mechanical and coating properties in term of gloss, adhesion, flexibility were found for WDPU-PMMA coatings especially for hybrid coatings. All WDPU-PMMA coatings formed clear transparent, highly gloss film on leather sheets. WDPU-PMMA hybrid coatings were found to be much glossy, flexible, and stronger than WDPU-PMMA blend coatings. Very good solvent resistance (2.2–9.2 g/100 g of sample) of all coatings was found except in carbon tetrachloride solvent. WDPU-PMMA coatings were not biodegradable and very small percentage of degradation was observed (0.5–1.75%) in 20% CoCl2/0.1 m H2O2 solution. Percentage weight loss of WDPU-PMMA coatings in various chemicals were found between 0.06 to 2.81. Hybrid WDPU-PMMA coatings had higher drying time than WDPU-PMMA blend coatings. Knoop hardness was 265–291 gf and gloss value was 91–96.5 GU for all coatings. All these extreme property differences between WDPU-PMMA hybrid and WDPU-PMMA blend coatings were attributed to better hydrogen bonding in the former. This research provided valuable evidence about the importance of hydrogen bonding in WDPU-PMMA coatings properties.
References
1. Lee SH, Cheon JM, Jeong BY, Kim H-D, Chun JH. Synthesis and properties of waterborne polyurethane acrylate adhesive. Adhesion and Interface 2015;16:156–61.10.17702/jai.2015.16.4.156Search in Google Scholar
2. Chandra S, Sharma S, Ali H, Mane JV, Naik YP, Chavan VM, Manjunath BS, Patel RJ. Effect of γ irradiation on the impact response of rigid polyurethane foam. J Polym Eng. 2016.10.1515/polyeng-2015-0360Search in Google Scholar
3. Abdolmaleki M, Tavakoli T, Jazani OM, Saeb MR. Blend membranes based on polyurethane and polyethylene glycol: exploring the impact of molecular weight and concentration of the second phase on gas permeation enhancement. J Polym Eng. 2016;36:513–9.10.1515/polyeng-2015-0254Search in Google Scholar
4. Jain A, Betancur M, Patel GD, Valmikinathan CM, Mukhatyar VJ, Vakharia A, Pai SB, Brahma B, MacDonald TJ, Bellamkonda RV. Guiding intracortical brain tumour cells to an extracortical cytotoxic hydrogel using aligned polymeric nanofibres. Nat Mater. 2014;13:308–16.10.1038/nmat3878Search in Google Scholar PubMed
5. Alishiri M, Shojaei A, Abdekhodaie MJ. Biodegradable polyurethane acrylate/HEMA-grafted nanodiamond composites with bone regenerative potential applications: structure, mechanical properties and biocompatibility. RSC Adv. 2016;6:8743–55.10.1039/C5RA19669HSearch in Google Scholar
6. Park YG, Lee YH, Rahman MM, Park CC, Do Kim H. Preparation and properties of waterborne polyurethane/self-cross-linkable fluorinated acrylic copolymer hybrid emulsions using a solvent/emulsifier-free method. Colloid Polym Sci. 2015;293:1369–82.10.1007/s00396-015-3504-0Search in Google Scholar
7. Saeed A, Shabir G. Synthesis of thermally stable high gloss water dispersible polyurethane/polyacrylate resins. Prog Org Coat. 2013;76:1135–43.10.1016/j.porgcoat.2013.03.009Search in Google Scholar
8. Huang J, Liang J, Zhao W. High-performance UV-cure polyurethane acrylate for UV transparent insulation inks. Material Science and Engineering: Proceedings of the 3rd Annual 2015 International Conference on Material Science and Engineering (ICMSE2015, Guangzhou, Guangdong, China, 15–17 May 2015); 2016: CRC Press. 117 p.10.1201/b21118-26Search in Google Scholar
9. Yuan C, Wang J, Cui M, Peng Y. Aqueous PUA emulsion prepared by dispersing polyurethane prepolymer in polyacrylate emulsion. J Appl Polym Sci. 2016;133:1–9.10.1002/app.43203Search in Google Scholar
10. Bi Y, Li Z, Wang N, Zhang L. Preparation and characterization of UV/thermal dual-curable polyurethane acrylate adhesive for inertial confinement fusion experiment. Int J Adhes Adhes. 2016;66:9–14.10.1016/j.ijadhadh.2015.11.009Search in Google Scholar
11. Seok S, Shin S, Lee TJ, Jeong JM, Yang M, Kim do H, Park JY, Lee SJ, Choi BG, Lee KG. Multifunctional polyurethane sponge for polymerase chain reaction enhancement. ACS Appl Mater Inter. 2015;7:4699–705.10.1021/am508101mSearch in Google Scholar PubMed
12. Cai L, Li Z. Preparation of fluoroalkylsilyl polymethacrylates and their waterproof application on cotton fabrics. Fiber Polym. 2015;16:2094–105.10.1007/s12221-015-5505-5Search in Google Scholar
13. Li K, Shen Y, Fei G, Wang H, Li J. Preparation and properties of castor oil/pentaerythritol triacrylate-based UV curable waterborne polyurethane acrylate. Prog Org Coat. 2015;78:146–54.10.1016/j.porgcoat.2014.09.012Search in Google Scholar
14. Sun J, Fang H, Wang H, Yang S, Xiao S, Ding Y. Waterborne epoxy-modified polyurethane-acrylate dispersions with nano-sized core-shell structure particles: synthesis, characterization, and their coating film properties. J Polym Eng. 2016.10.1515/polyeng-2016-0003Search in Google Scholar
15. Strankowski M, Piszczyk Ł, Kosmela P, Korzeniewski P. Morphology and the physical and thermal properties of thermoplastic polyurethane reinforced with thermally reduced graphene oxide. Pol J Chem Technol. 2015;17:88–94.10.1515/pjct-2015-0073Search in Google Scholar
16. Oprea S, Oprea V. Biodegradation of crosslinked polyurethane acrylates/guar gum composites under natural soil burial conditions. e-Polymers. 2016;16:277–86.10.1515/epoly-2016-0038Search in Google Scholar
17. Zhang C, Hu J, Li X, Wu Y, Han J. Hydrogen-bonding interactions in hard segments of shape memory polyurethane: toluene diisocyanates and 1, 6-hexamethylene diisocyanate. A theoretical and comparative study. J Phys Chem A. 2014;118:12241–55.10.1021/jp508817vSearch in Google Scholar PubMed
18. Zhang C, Hu J, Wu Y. Theoretical studies on hydrogen-bonding interactions in hard segments of shape memory polyurethane-III: isophorone diisocyanate. J Mol Struct. 2014;1072:13–9.10.1016/j.molstruc.2014.04.012Search in Google Scholar
19. Taheri N, Sayyahi S. Effect of clay loading on the structural and mechanical properties of organoclay/HDI-based thermoplastic polyurethane nanocomposites. e-Polymers. 2016;16:65–73.10.1515/epoly-2015-0130Search in Google Scholar
20. Cheon J-M, Lee S-G, Chun J-H, Lee D-J, Lee Y-H, Kim H-D. Preparation and properties of emulsifier-/NMP-free crosslinkable waterborne polyurethane-acrylic hybrid emulsions for footwear adhesives (II)–effect of dimethylol propionic acid (DMPA)/pentaerylthritol triacrylate (PETA) content. e-Polymers. 2016;16:189–97.10.1515/epoly-2016-0005Search in Google Scholar
21. Chen C, Yang Z, Qiu F, Cao T, Cao G, Guan Y, Yang D. Synthesis of azo polyurethane-urea and investigation of its thermo-optic properties. Z Phys Chem. 2016;230:211–29.10.1515/zpch-2015-0581Search in Google Scholar
22. Wosek J. Fabrication of composite polyurethane/hydroxyapatite scaffolds using solvent-casting salt leaching technique. Adv Mater Sci. 2015;15:14–20.10.1515/adms-2015-0003Search in Google Scholar
23. Chen G, Guan X, Xu R, Tian J, He M, Shen W, Yang J. Synthesis and characterization of UV-curable castor oil-based polyfunctional polyurethane acrylate via photo-click chemistry and isocyanate polyurethane reaction. Prog Org Coat. 2016;93:11–6.10.1016/j.porgcoat.2015.12.015Search in Google Scholar
24. Rokicki G, Parzuchowski PG, Mazurek M. Non‐isocyanate polyurethanes: synthesis, properties, and applications. Polym Adv Technol. 2015;26:707–61.10.1002/pat.3522Search in Google Scholar
25. Griffini G, Passoni V, Suriano R, Levi M, Turri S. Polyurethane coatings based on chemically unmodified fractionated lignin. ACS Sustain Chem Eng. 2015;3:1145–54.10.1021/acssuschemeng.5b00073Search in Google Scholar
26. Jeon JH, Park YG, Lee YH, Lee DJ, Kim HD. Preparation and properties of UV‐curable fluorinated polyurethane acrylates containing crosslinkable vinyl methacrylate for antifouling coatings. J Appl Polym Sci. 2015;132:1–10.10.1002/app.42168Search in Google Scholar
27. Sun P, Wang J, Yao X, Peng Y, Tu X, Du P, Zheng Z, Wang X. Facile preparation of mussel-inspired polyurethane hydrogel and its rapid curing behavior. ACS Appl Mater Inter. 2014;6:12495–504.10.1021/am502106eSearch in Google Scholar PubMed
28. Suh D, Tak H, Choi SJ, Kim TI. Permeability-and surface-energy-tunable polyurethane acrylate molds for capillary force lithography. ACS Appl Mater Inter. 2015;7:23824–30.10.1021/acsami.5b06975Search in Google Scholar PubMed
29. Sultan M, Islam A, Gull N, Bhatti HN, Safa Y. Structural variation in soft segment of waterborne polyurethane acrylate nanoemulsions. J Appl Polym Sci. 2015;132:1244–51.10.1002/app.41706Search in Google Scholar
30. Yu Y, Liao B, Li G, Jiang S, Sun F. Synthesis and properties of photosensitive silicone-containing polyurethane acrylate for leather finishing agent. Ind Eng Chem Res. 2014;53:564–71.10.1021/ie403534fSearch in Google Scholar
31. Cho H, Kim SM, Kang YS, Kim J, Jang S, Kim M, Park H, Bang JW, Seo S, Suh K-Y, Sung Y-E, Choi M. Multiplex lithography for multilevel multiscale architectures and its application to polymer electrolyte membrane fuel cell. Nat Commun. 2015;6.10.1038/ncomms9484Search in Google Scholar PubMed PubMed Central
©2016 Walter de Gruyter GmbH, Berlin/Boston
This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Articles in the same Issue
- Frontmatter
- In this Issue
- Full length articles
- Synthesis and characterization of polyHIPE composites containing halloysite nanotubes
- Influence of N-vynilcarbazole on the photopolymerization process and properties of epoxy-acrylate interpenetrating polymer networks
- Investigation on the application properties of epoxy resin and glass fiber in RTV mold rubber
- Modification of pristine nanoclay and its application in wood-plastic composite
- FT-IR spectroscopic and thermal study of waterborne polyurethane-acrylate leather coatings using tartaric acid as an ionomer
- The influence of bioactive additives on polylactide accelerated degradation
- Fabrication and characterization of brominated matrimid® 5218 membranes for CO2/CH4 separation: application of response surface methodology (RSM)
- Hybrid nanocomposites based on poly aryl ether ketone, boron carbide and multi walled carbon nanotubes: evaluation of tensile, dynamic mechanical and thermal degradation properties
Articles in the same Issue
- Frontmatter
- In this Issue
- Full length articles
- Synthesis and characterization of polyHIPE composites containing halloysite nanotubes
- Influence of N-vynilcarbazole on the photopolymerization process and properties of epoxy-acrylate interpenetrating polymer networks
- Investigation on the application properties of epoxy resin and glass fiber in RTV mold rubber
- Modification of pristine nanoclay and its application in wood-plastic composite
- FT-IR spectroscopic and thermal study of waterborne polyurethane-acrylate leather coatings using tartaric acid as an ionomer
- The influence of bioactive additives on polylactide accelerated degradation
- Fabrication and characterization of brominated matrimid® 5218 membranes for CO2/CH4 separation: application of response surface methodology (RSM)
- Hybrid nanocomposites based on poly aryl ether ketone, boron carbide and multi walled carbon nanotubes: evaluation of tensile, dynamic mechanical and thermal degradation properties