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Study of the performance of polyurethane coatings derived from cottonseed oil polyol.

Abstract Cottonseed oil (CSO) was converted into a polymerizable polyol by in-situ epoxidation and hydroxylation in the presence of water as nucleophile and sulfuric acid as catalyst. This hydroxylated CSO (HCSO) was polymerized with isophorone diisocyanate at different NCO-to-OH ratios to furnish polyurethane coatings of different composition. The mechanical properties of the resulting moisture-cured films were studied. The adhesion, abrasion resistance, impact strength, flexibility, water vapor permeability, and chemical and/or solvent and corrosion resistance of the coatings were investigated.

Keywords Cottonseed oil, Hydroxylation, Crosslink density, Mechanical properties


Natural oils, a major class of renewable resources, have, after partial or total chemical modification, been widely used as substitutes for petroleum feedstocks. Polyurethanes (PU) are usually synthesized from petroleum polyols; for several reasons, including increasing petroleum prices, depletion of fossil fuel, and other environmental concerns, much effort has been devoted to developing bio-polyols based on renewable resources, especially vegetable oil. Several methods have been used to attach hydroxyl and other functional group(s) to the triglyceride backbone of oils. (1-8) Epoxidation of vegetable oil is a viable means of constructing intermediates for synthesis of macromolecules. Epoxidation then ring-opening of the epoxide with nucleophile(s), e.g. hydrolysis or alcoholysis in the presence of an acid catalyst, is a popular method for introduction of hydroxyl groups to vegetable oil. Several studies have revealed that hydroxylation of plant oils, by opening epoxy rings, is a crucial step in the formation of polyols. (7-21) Many investigators have developed a variety of polyols from different vegetable oils, for example castor, linseed, cottonseed, soyabean, nahar, olive, tung, sunflower, palm, corn, and canola oil; these polyols have been successfully used for preparation of PU. (2-6-22 28)

Cottonseed oil (CSO) has recently been used as starting material in the synthesis of intermediate macromolecules and for subsequent development of polymers for coating applications. (29-33) Because of the availability of CSO in India and its ability to provide the desired functionality, for example double bonds and ester linkages, it may be a potential, vital, and inexpensive raw material for production of plant oilbased resin for coating applications. In CSO the ratio of unsaturated (65-75%) to saturated (26-35%) fatty acids is 2:1; of its unsaturated fatty acids 18-24% are monounsaturated and 42-52% are polyunsaturated. (13-34-36) The main objective of this work was to develop hydroxylated CSO polyol (HCSO) and isophorone diisocyanate (IPDI) formulations for PU coatings. The effects of NCO-to-OH ratio, i.e., hard segment content (HSC), on the mechanical and physicochemical properties of the resulting PU were studied and are discussed.



Cottonseed oil (CSO; Jayajothi Industries, Hyderabad, India), isophorone diisocyanate (IPDI), dibutyl tindilaurate (DBTDL) (Sigma-Aldrich), hydrogen peroxide (30% w/v), 98% sulfuric acid, sodium sulfate, sodium bicarbonate, hydrochloric acid, sodium chloride, sodium hydroxide, xylene, glacial acetic acid, toluene, acetone (s.d. Fine Chemicals, Mumbai, India), 85% formic acid (Finar Chemicals, Hyderabad, India), commercial cellosolve acetate (CA), and other reagents were used without further purification.


Fourier-transform infrared (FTIR) spectra of the CSO, epoxidized cottonseed oil (ECSO), HCSO, and fully cured PU films were recorded by use of a PerkinElmer Spectrum 100 (USA). Neat liquid samples were analyzed as KBr discs and films were analyzed by use of a universal attenuated total reflectance (UATR) polarization accessory. Each sample was scanned eight times within the range 450-4000 [cm.sup.-1] with resolution of 4 [cm.sup.-1], [sup.1]H NMR spectra of ECSO and HCSO were recorded, at ambient temperature, by use of a Bruker 500 MHz spectrometer; CD[Cl.sub.3] was used as solvent and tetramethylsilane (TMS) as standard.

The viscoelastic behavior of PU films was analyzed by use of a dynamic mechanical thermal analyzer (DMTA; DMA-Q-800; TA Instruments, USA) in tensile mode at a frequency of 1 Hz and a heating rate of 5[degrees]C [min.sup.-1]. The sample dimensions were 15 mm x 10 mm x 0.15 mm and the temperature range was -100 to 150[degrees]C. The tensile strength and percentage elongation of free PU films were determined by use of a Shimadzu (Japan) model AGS-10k NG universal testing machine (UTM) at room temperature. The instrument was connected to an autograph controller/measurement unit. Test specimens were in the form of dumbbells, in accordance with ASTM D638-10. The gauge length was 50 mm and the crosshead speed was 10 mm [min.sup.-1]. The results reported are averages from three experiments. Adhesion to a metal surface (cast iron) was tested by use of a type-V self-aligning adhesion tester (automatic) on the metal surface, in accordance with ASTM D4541-09. The stress applied by the detaching assembly (U) was in MPa. ASTM D4060-10 was followed to measure the abrasion resistance of all fully cured PU coatings by use of a Taber (USA) model 5131 instrument; results are expressed as wear index (I) (mg/1000 cycles). A load of 500 g on each arm and abrader wheel H-10 was used, in accordance with the standard.

Impact-resistance testing of PU coatings was performed on steel panels (24-gauge) by use of an aggregate impact tester (Sheen Instruments, UK) in accordance with the ASTM D2794-93 standard. The test panel was placed on an aperture at the base of the instrument and a weight of 10.5 lb (including indenter) was released from a height of 70 cm. Testing was performed on both sides (direct and reverse of coated surface). Flexural testing was performed in accordance with the ASTM D522-93a standard by use of a 180[degrees] conical mandrel tester (Sheen Instruments); the mandrel was 200 mm long with a diameter of 3 mm at one end and a diameter of 38 mm at the other end. Sample size was 150 mm x 100 mm x 0.8 mm. Results, as pass/fail, are averages from three experiments.

Free PU films were subjected to exposure to a variety of chemical media to study their behavior, viz. change of weight, overall appearance, loss of gloss, solubility, among others. The immersion test was performed in accordance with ASTM D543-06 for 7 days. The water vapor transmission (WVT) test was measured in accordance with ASTM D1653-03 (wet-cup method) to study the rate of moisture permeation through the free PU film. During the test, cups were kept under very low relative humidity (~0) at room temperature in a desiccator. The weights of the cups, initially and subsequently for the next 10 days, were recorded every 24 h. WVT (g [m.sup.-2] [day.sup.-1]) was calculated by use of the equation:

WVT = (G)/(T x A)

where G is the weight loss (g), T the elapsed time (days), and A the exposed area of the specimen ([m.sup.2]).

The salt spray (fog) test was performed in a salt-spray chamber in accordance with ASTM B117-11, to study the corrosion resistance of the coating when applied to mild steel panels. Coated panels with cross cuts were exposed to a controlled corrosive environment (5% NaCl solution at room temperature) for 300 h, and were examined periodically after 100 h for blistering associated with corrosion (ASTM D714-56), loss of adhesion, and rusting (ASTM D610-43), among others, in the scribed and unscribed areas.

Epoxidation of cottonseed oil

CSO (250 g) and a mixture of 26.62 g formic acid (double bond-to-HCOOH ratio 1:0.5) and 1.90 g [H.sub.2]S[O.sub.4] catalyst (catalyst loading 3 wt% of HCOOH + [H.sub.2][O.sub.2]) were placed in a three-necked, round-bottomed flask and 40.14 g 30% aqueous [H.sub.2][O.sub.2] (double bond-to-[H.sub.2][O.sub.2] ratio 1:1.2) was added dropwise to the reaction mixture (maintained at ~10[degrees]C) at a flow rate of 4.5 ml [min.sup.-1]. The temperature was increased to 60[degrees]C after complete addition of the [H.sub.2][O.sub.2] and the reaction continued until the oxirane oxygen content reached a maximum (5 h). Samples were withdrawn at regular intervals during the course of the reaction, dissolved in ethyl acetate, then washed successively with distilled water and NaHC[O.sub.3] solution and dried over anhydrous [Na.sub.2]S[O.sub.4]. The solvent was then removed and the product analyzed for oxirane oxygen content (ASTM D1652).


Hydroxylation of epoxidized cottonseed oil

After completion of the epoxidation reaction the reaction temperature was increased to 90[degrees]C and maintained at this temperature until the oxirane value was minimum (15 h). The ring-opening reaction was monitored by determining the oxirane oxygen value at different times (Fig. SI, Supporting Information); the reaction was arrested when the oxirane value reached a constant. The reaction mixture was worked up as described above and the final product was analyzed for its hydroxyl value (ASTM E222-10). In-situ epoxidation followed by hydroxylation is shown in Scheme 1 and the properties of the HCSO are reported in Table 1. The polyol had a hydroxyl value of 167 mg KOH [g.sup.-1].

Preparation of crosslinked PU coatings

Crosslinked PU was prepared by reaction of HCSO with IPDI monomer. The calculated amounts of HCSO and catalyst (DBTDL, 0.2 wt% of HCSO and IPDI) were dissolved in CA (Table 2), then the calculated amount of IPDI monomer was added such that the NCO-to-OH ratio ranged between 1.4 and 2. The resulting formulations are denoted PU-141, PU-161, PU-181, and PU-201 respectively. For example, PU-141 denotes a formulation in which the NCO-to-OH ratio is 1.4. The solids content was fixed at 85%. The solution was stirred vigorously at room temperature and degassed by applying a mild vacuum. The films were cast on tin foil, supported on a glass plate, by use of manual square applicator; curing was performed at 70[degrees]C for 10 h and post curing at 100[degrees]C for 1 h, as described elsewhere. (2) Excess unreacted NCO present in the films was left to moisture cure at room temperature and under laboratory humidity conditions (40-60%) for 15 days. The supported films on tin foil were extracted after amalgamation and cleaned. For evaluation of mechanical and interfacial properties, different PU formulations were cast on metal panels of a specific size in accordance with the test specifications.

Results and discussion

Spectral analysis of cotton seed oil derivatives: epoxidized oil and polyol

Figure 1 shows FTIR spectra of CSO, ECSO, and HCSO. The absorption band at 1745 [cm.sup.-1] corresponds to -C=0 stretching vibrations of the ester group of triglyceride moieties, that at 1660 [cm.sup.-1] to C=C stretching. -CH stretching appears in the 2800 to 3000 [cm.sup.-1] range. When the oil is epoxidized, the peak corresponding to =CH stretching vibration at approximately 3000 [cm.sup.-1] disappears and a new peak at approximately 843 [cm.sup.-1] appears; this is ascribed to oxirane C-O-C stretching. When the epoxy ring is opened to yield the hydroxylated compound, the peak corresponding to oxirane C-O-C disappears and the absorption at approximately 1700 [cm.sup.-1] becomes broader (combined peaks corresponding to -OH bending and ester -C=0 stretching) and the -OH stretching vibration is observed as a broad peak at approximately 3427 [cm.sup.-1]. However, oxirane oxygen is to some extent lost because of hydroxyformate formation and crosslinking. All these data confirm the formation of the epoxy group followed by ring opening to give the hydroxylated product. Proton NMR analysis data is available in the Supporting Information (Fig. S2).


Properties of the PU films

All PU films were transparent with a glossy finish. Pendant fatty acid chains of triglycerides contribute to the glossiness of the coatings. (29)

Figure 2 shows the FTIR spectra of PU-141, PU161, PU-181, and PU-201 films. The absence of an N=C=0 peak at 2200 [cm.sup.-1] for all the PU films confirms that all the isocyanate groups reacted during the polymerization. Characteristic peaks at 1700-1750 [cm.sup.-1] (-C=0 stretch of amide-I), 15-30 1540 [cm.sup.-1] (amide-II stretch consisting of a mixture of peaks [v.sub.N-H]. [v.sub.C-N] > and [v.sub.C-C]), 1230-1240 [cm.sup.-1] (amide-III stretch consisting of [v.sub.C-N], and 770 [cm.sup.-1] (amide-IV CO-NH out-of-plane vibration) are indicative of urethane formation. The absorption band at ~1600-1650 [cm.sup.-1] which corresponds to -C=0 of urethane groups, increases in intensity as HSC increases because of the higher concentration of biscarbamate linkages. The peak corresponding to the ester group of the triglyceride molecule observed at approximately 1718 [cm.sup.-1] has shifted from 1740 [cm.sup.-1] (observed in HCSO, Fig. 1); this is because of the effect of H-bonding with the -NH group of the urethane moiety. These results confirm the presence of H-bonding between -NH and -C=0 groups which increases with increasing HSC. (22,37-39)




Dynamic mechanical thermal analysis

The thermal properties of polyurethane depend on such factors as secondary forces, crosslink density (ve), molecular weight and its distribution, phase separation, and HSC (22,23,38,40-42) Figure 3 shows the storage modulus (E') of the PU films. A drop in E' in the range -50 to 70[degrees]C was observed for all PU except PU-181, for which the temperature range was -50 to 100[degrees]C. E' was observed to increase with increasing NCO-to-OH ratio and crosslink density (3,24) except for PU-181, for which E' was less at low temperature and higher in the high temperature range than for other PU. The E' value is affected not only by crosslinking density and HSC but also by less hindrance of dangling chains in domains, which increases segmental mobility and acts as a plasticizer. (24,37) It was observed that the glass transition temperature ([T.sub.g]) of all the PU increased with increasing NCO-to-OH ratio (Fig. 4). Shifting of the tan [delta] peak to higher temperature can be explained by increasing crosslink density as H-bonding increases; this reduces the segmental mobility of polymer chains and dispersion of energy throughout the polymer decreases, hence [T.sub.g] increases. (3,6,15,24,37,43) The single tan [delta] peak indicates the absence of phase separations between the soft and hard domains; in fact, more crosslinking promotes phase mixing. (2,3,44)

The crosslinking density has a crucial effect on thermal, mechanical, and physicochemical properties. The [v.sub.e] and molecular weight between crosslinks ([M.sub.c]) of cured specimens was deduced from the E' value in the rubbery plateau region in accordance with rubber elasticity theory (24,45):

E! = 3[v.sub.e]RT

Mc = [rho]/[v.sub.e]

where E' is the storage modulus at ([T.sub.g] + 30)[degrees]C (MPa), R the universal gas constant (8.314 J [mol.sup.-1] [K.sup.-1]), T the absolute temperature (K), and [rho] the density of the polymer (g [cm.sup.-3]).

The [v.sub.e] values (Table 3) of the PU increase with increasing excess NCO because as the NCO-to-OH ratio increases, more crosslinking and network formation occurs. Molecular weight between crosslinks ([M.sub.c]) was found to increase with decreasing NCO-to-OH ratio, which indicates that the flexible chains have increased inter-molecular distance and enhanced soft segmental motion. (6,44) In addition, secondary interactions restrict segmental motion. (22)

Mechanical properties

Mechanical properties of all the PU coatings are listed in Table 4. The mechanical performance of the PU coatings were strongly affected by the number of urethane and/or urea groups, virtual or physical crosslinking, intra and intermolecular interactions, and phase mixing or separation. It was observed that tensile strength ([sigma]) and modulus (E) increased whereas % elongation (% El) at break decreased as the NCO-to-OH ratio increased. This is because of increasing H-bonding, dipolar interaction, crosslinking density, and urea groups with increasing NCO-to-OH ratio; these reinforce the PU matrix and make it more compact and stiff. (3,22,23,38,39,44) These findings are attributed to the fact that crosslink density increases with HSC. The higher average [M.sub.c] value and decreasing secondary interactions at low NCO-to-OH ratios reduce the hindrance effect and enhance molecular motion in the PU. The triglyceride chain has a plasticizing effect, thereby increasing elongation. (25) Nevertheless, %El for PU-181 was found to be unexpectedly high despite the high HSC. This may be because of better phase mixing between hard segment (HS) and soft segment (SS). These findings suggest that the PU-181 is the most appropriate formulation for good mechanical properties.

Adhesion is caused by chemical bonding, physical bonding, and/or mechanical interlocking at the interface, and several other factors, for example type of resin, crosslinking density, and such interfacial properties as the nature of surface. (4,46) The pull-off adhesion test was performed to determine the adhesive strength of the coatings on a metal substrate; the results are reported in Table 4. Pull-off strength (U) of PU coatings was found to increase up to NCO-to-OH ratios of 1.8, but thereafter decreased (PU-201). Increasing crosslink density and concentration of NH groups of urethane, urea, or allophanate linkages led to enhanced H-bonding both with the substrate and with the oxygen atom of ester groups of the polymer itself. (5,22,26,29,47) However, adhesive strength of PU-201 was found to be low. The highly crosslinked network structure of PU-201 creates internal stresses leading to a decrease in adhesive strength. (47,48) These findings suggest that PU-181 is the most appropriate formulation for applications in which adhesive strength is a prime factor.

Abrasion resistance (Table 4) of fully cured PU coatings are indicative of such physicomechanical properties as hardness, toughness, and tensile strength. These findings gives an indication of factors that could be modified when formulating tough coatings for such specific applications as floor coatings, depending upon wear index (I) value. The smaller the value of I, the harder and tougher will be the coating film. In general, the abrasion resistance of coatings can be improved by maximizing the degree of crosslinking. (4,22,38) In this study, the wear index of PU coatings was found to be almost the same up to PU-181 but PU-201 had the highest I value. The highly crosslinked network structure may be the reason for this behavior. It is noticeable that wear index values for the all PU films, from the lowest to the highest NCO content, are well within the limits of ASTM standards, suggesting that these materials are suitable for floor coatings. (4)

Impact resistance and flexural testing of PU coatings were performed to study the plasticizer effect of the triglyceride groups; results are reported as pass or fail in Table 4. Impact resistance is the ability to absorb the applied external energy and dissipate this to adjacent molecular networks. (23) All PU coatings passed (>70 cm) the impact resistance test direct and reverse to the coated site, except PU-201, which failed the impact to the direct site. This failure can be attributed to its compact structure, which restricts molecular movement and leads to brittleness, because of the very high crosslink density with high NCO content, as already explained. All CSO-modified PU coatings had adequate flexibility and can be bent on a conical mandrel of diameter 3-38 mm. It is believed that pendent chains of triglyceride groups improve the degree of freedom of molecular movement and molecular relaxation, and thus function as plasticizers that reduce polymer rigidity and improve polymer flexibility. (22,23,42) 2 This result clearly indicates that the long hydrocarbon chain of triglycerides, which is the backbone of the polyol, imparts sufficient flexibility, strength, and toughness to the entire network. Some specimens were also studied for cracking, film thickness, and contact with the metal surface across the tested area by use of the naked eye; no significant changes were observed for coatings which passed the test. It is concluded that NCO-to-OH ratio of 1.8 is the optimum for obtaining a tough and flexible coating.

Physicochem icaI properties

Free PU films were exposed to a variety of chemical media to study their behavior, viz. change of weight, overall appearance, loss of gloss, and solubility; results in the form of % weight gain (+) or % weight loss (--) are reported in Table SI. The results show that PU with NCO-to-OH ratios of 1.6 and 1.8 (PU-161 and PU-181) have good acid and alkali resistance, i.e., weight gain or weight loss is small. All PU films have very good resistance to water. In salt and organic solvent, weight loss decreased as HSC increased. Increasing crosslink density, secondary interactions with increasing NCO content, make the film compact and insoluble, resulting in a network stable against chemical media. The solubility of the matrix in organic solvent increased with decreasing NCO-to-OH ratio; this may be because of high proportion of SS from triglyceride exposed to media. It is also believed that the more crosslinked structure prevents penetration by media. (5-26-29-47)


The WVT was used to measure the rate of water permeation through uniform PU coating films. As is apparent from Fig. 5a, weight loss increases linearly with time for all coating systems. It is also observed that the slope of the line decreases with increasing HSC, which clearly indicates that the water loss and/or permeation through film decrease as the NCO-to-OH ratio increases. Figure 5b shows WVT curves, which represent average water permeation through the film per day. PU-181 has the lowest average WVT (13.6 g [m.sup.-2] [day.sup.-1]) and PU-141 has the highest average WVT (50.14 g [m.sup.-2] [day.sup.-1]). The reason could be high crosslinking and better phase mixing between HS and SS as a result of H-bonding, (38) higher crosslink density, less water permeation through the film, (29,49) and lower WVT value. The HS of highly crosslinked films will act as the impenetrable part whereas at low crosslink density with flexible SS, moisture can diffuse through. Unexpectedly, the WVT for PU-201 was found to be higher than that for PU-181. This finding helps us to choose a type of coating which can to be used in a moist environment and suggests that PU-181 is the most appropriate formulation.

The salt spray test was performed to estimate the corrosion resistance of PU coatings. Uniform corrosion was observed for all PU coatings (Fig. 6). Corrosion resistance was found to increase with decreasing HSC, because smaller corrosion-affected areas were observed with decreasing NCO-to-OH ratio. Similar results were observed by Ismail et al. (5) No sign of blistering, loss of gloss, or delamination or failure was observed for any coatings except PU-201. Failure of PU-201 could be because of low adhesive strength (Table 4) and higher WVT than PU-181 (Fig. 6)--water permeation can lead to more corrosion.



In this work, renewable and inexpensive CSO-based polyols with high potential were successfully synthesized by in-situ epoxidation then hydroxylation, with water as nucleophile and sulfuric acid as catalyst. Four PU coatings with different NCO-to-OH ratios were prepared from a cyclo aliphatic diisocyanate (IPDI) and hydroxylated CSO polyol (HCSO), and were investigated for their structure-property relationships and coating properties. Films were transparent with a glossy finish. The crosslink density of the polymers was found to increase with increasing NCO-to-OH ratio, owing to increase in free isocyanate content and a variety of secondary interactions. Dynamic mechanical study revealed that the glass transition temperature and storage modulus increased with increasing segment content. Tensile strength and elastic modulus improved with increasing HSC; maximum values were 16.59 and 28.67 MPa, respectively. Percentage elongation was highest (96%) when the NCO-to-OH ratio was 1.8. Adhesive strength, wear resistance, and impact resistance were found to increase up to an NCO-to-OH ratio of 1.8. All PU coatings had adequate flexibility, because they bent on conical mandrel of 3 mm diameter. All PU films had satisfactory resistance against acid, alkali, salt, and organic solvents, and excellent resistance against water. The lowest water vapor transmission was observed when the NCO-to-OH ratio was 1.8. High corrosion resistance was observed for PU-141, but satisfactory results were obtained for PU-161 and PU-181. In conclusion, PU-181 was the most appropriate formulation for high-performance coatings. Value-addition to cottonseed oil has been achieved to give an environmentally benign product.

DOI 10.1007/s11998-015-9741-9

P. Narute

Department of Surface Coating Technology, Laxminarayan Institute of Technology, Nagpur 440033, India

A. Palanisamy ([mail])

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India


Acknowledgments The authors thank the CSIR XII 5-year plan Project Intelcoat (CSC 0114), for financial support, and LIT, Nagpur, for providing facilities to carry out this work.


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Table 1: Properties of CSO and HCSO

Sample                                         Specific
code     Physical appearance and color         gravity     AV (d)

CSO      Liquid, clear, light golden yellow   0.9250 (b)    0.39
HCSO     Pale yellow Liquid, C = 2.7 (a)      1.0064 (c)    9.92
                                              0.9993 (b)

Sample                                   Viscosity @
code     IV (e)   % 00 (f)   HV (g)   30[degrees]C (cps)

CSO      99.88       --        --             50
HCSO       --       1.7      167.21          5450

(a) Lovibond scale

(b) At 30[degrees]C

(c) At 15[degrees]C

(d) Acid  value (mg KOH [g.sup.-1])

(e) Iodine value (g of iodine per 100 g sample)

(f) % Oxirane oxygen (g per 100 g sample)

(g) Hydroxyl value (mg KOH [g.sup.-1] sample)

Table 2: PU formulations

Sample code   NCO-to-OH ratio   HCSO (g)    IPDI (g)

PU-141              1.4            10         4.60
PU-161              1.6            10         5.30
PU-181              1.8            10         5.96
PU-201              2.0            10         6.62

DBTDL, 0.2 wt% of HCSO and IPDI; CA, to maintain 85%
solid content

Table 3: Thermomechanical properties of the PU

Sample    [rho] (g       [T.sub.g]       E' at ([T.sub.g]
code     [cm.sup.-3])   ([degrees]C)   + 30)[degrees]C (MPa)

PU-141       1.40            65                3.36
PU-161       1.16            75                4.63
PU-181       1.09            95                4.52
PU-201       1.28            88                6.75

Sample   [v.sub.e] at ([T.sub.g] + 30)    [M.sub.c] at ([T.sub.g] +
code     [degrees]C (kmol [mm.sup.-3])         30)[degrees]C
                                              (kg [mol.sup.-1])

PU-141         3.67 x [10.sup.-4]                   3809
PU-161         4.90 x [10.sup.-4]                   2363
PU-181         4.55 x [10.sup.-4]                   2407
PU-201         6.91 x [10.sup.-4]                   1852

Table 4: Mechanical properties of PU

Sample      Static mechanical properties    U (a) (MPa)
          [sigma] (MPa)   E (MPa)   % El

PU-141        6.44         6.81     94.54      4.21
PU-161        8.80        12.06     72.96      4.32
PU-181       15.40        15.95     96.56      4.41
PU-201       16.59        28.67     57.86      3.35

Sample    I (b) (mg/1000   Impact resistance    Flexural test
code         cycles)
                           Direct    Reverse

PU-141         24.2         Pass       Pass         Pass
PU-161         21.4         Pass       Pass         Pass
PU-181         20.4         Pass       Pass         Pass
PU-201         47.8         Fail       Pass         Pass

(a) Uniform pull-off strength in adhesion test

(b) Wear index in abrasion resistance
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Author:Narute, Prashant; Palanisamy, Aruna
Publication:Journal of Coatings Technology and Research
Geographic Code:9INDI
Date:Jan 1, 2016
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