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Materials Express 2158-5849/2015/5/377/013

Copyright © 2015 by American Scientific Publishers All rights reserved. Printed in the United States of America

doi:10.1166/mex.2015.1254

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Influence of NCO/OH and transesterified castor oil on the structure and properties of polyurethane: Synthesis and characterization Sonalee Das, Priyanka Pandey, Smita Mohanty∗ , and Sanjay Kumar Nayak Laboratory for Advanced Research in Polymeric Materials, Central Institute of Plastics Engineering and Technology, Bhubaneswar 751024, Odisha, India

ABSTRACT

Keywords: Castor Oil, Partially Biobased Isocyanate, Flory Rehner Equation, Phase Separated Surface Morphology, NCO/OH.

1. INTRODUCTION In recent years there has been growing demand for the development of biobased products from renewable agricultural based materials owing to depletion of fossil fuels, global warming and waste disposal. Thus, extensive studies and researches are being undertaken for the synthesis of biobased materials which can replace the conventional petrobased materials as reported.1 2 Amongst the various groups of polymers the most distinguished are polyurethane (PUs). The cause for its huge demand is its high strength resiliency, good resistance to abrasion and weather conditions.3 PUs find huge application in the field of composites and nanocomposites owing to ∗

Author to whom correspondence should be addressed. Email: [email protected]

their flexibility and abrasion resistance.4 Most notable natural nanofillers that have attracted huge attention for reinforcing agents in PU composites and nanocomposites are chitin5 and cellulose nanocrystals6 7 since these are derived from natural sources. It has been understood that the molecular weight and NCO/OH ratio are the detrimental factor for determining the structure and properties of PUs.8–10 Gurunathan et al.11 have reported the effect of varying NCO/OH molar ratio on the final properties of isocyanate terminated castor oil based PU prepolymer (COPUP). In this study it was revealed that the varying NCO/OH molar ratio had a profound effect on the physicochemical properties of COPUP. Similar research was also investigated by Huang et al.12 where a series of graft interpenetrating polymer network (graft-IPNs) was synthesized from castor oil and

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A series of polyurethane (MBPUs) with varying stoichiometric balance i.e., isocyanate (NCO): hydroxyl groups (OH) ratio (viz. 1:1, 1.2:1 and 1.4:1), was synthesized using transesterified castor oil and partially biobased IP: 5.10.31.211 On: Thu,stability, 10 Jan morphology 2019 12:45:08 isocyanate. The chemical properties, kinetics, thermal and mechanical properties of the Copyright: Scientificthe Publishers MBPUs were analyzed systematically. FTIR American results confirmed synthesis of MBPUs and the increase in Delivered Ingenta hydrogen bonding with the increase in NCO/OH molarby ratio. A three step degradation mechanism for MBPU films was observed through TGA analysis. The DSC study revealed that the glass transition temperature (Tg ) increases with increase in NCO/OH molar ratio. Broad scattering halos observed from WAXD pattern of MBPU films revealed its amorphous nature. The phase separated surface morphology of MBPUs was noticed in SEM and AFM images. Flory Rehner Equation was used for determining the molecular weight between crosslinks and degree of crosslinking. From the mechanical analysis, it was inferred that the NCO/OH molar ratio had a profound effect on the tensile properties and modulus.

Article

Materials Express

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

nitrolignin with 1,4 butanediol as chain extender with vary2. EXPERIMENTAL DETAILS ing NCO/OH molar ratio. It has been mentioned here that 2.1. Materials with increase in NCO/OH molar ratio the tensile properties Castor oil (CO) used in this study was procured from and crosslinking density increases whereas the elongation M/s.SD Fine chemicals (Kolkata, India). The CO was at break decreases. So, NCO/OH molar ratio can be condried at 90  C under vacuum and characterized for sidered as an effective route to dictate and formulate the its hydroxyl value (160 mg KOH · g−1  and acid value desired physicochemical properties of PUs. 1.96 (mg KOH · g−1  as per ASTM D standards D-1957 Generally the synthesis of PUs is usually carried out and D-1639 respectively before use. Partially aliphatic bio by a variety of methods utilizing di or polyfunctional based Isocyanate (Tolonate™ X FLO 100) was kindly propolyols and di or polyfunctional isocyanates derived from vided by M/s. Vencorex Chemical, France. Pentaerythripetroleum resources. Conventionally petroleum based syntol, Lead oxide (PbO) and 1, 4 Butanediol was procured thetic diisocyanates have been used for the synthesis of from M/s. Himedia, India. The catalyst dibutyltin dilauPUs.13–15 These diisocyanates are toxic and harmful to rate (DBTDL) was procured from M/s. Sigma Aldrich, the environment since they are derived from phosgene and Germany and was used as received. Acetone of analytirelease diamines, free diisocyanate, and hydrogen cyanide cal grade was procured from M/s Fischer Scientific, USA. on degradation.16 Due to the legislative restrictions an The isocyanate was characterized for its NCO content by alternative to these synthetic isocyanates based on renewASTM-D 2572. Table I presents the complete information able resources is being required.17 about the materials used. In current scenario almost all PUs are synthesized from polyols derived from vegetable resources with petrochem2.2. Preparation of Polyol ical based diisocyanates.18 19 Numerous efforts have been The synthesis of polyol was carried out by the transesundertaken to develop isocyanates from alternative routes terification process of castor oil with pentaerythritol as from vegetable oil in order to reduce the carbon footprint. described elsewhere.23 The synthesis was carried out in Till date, very few reports have described the synthesis and a three necked flask equipped with a stirrer, reflux conusage of diisocyanates based on vegetable oil.16 20 denser, thermometer and nitrogen inlet charged with a Nowadays, castor oil (CO) is extensively attracting weighed amount of CO and pentaerythritol as shown in IP: 5.10.31.211 On:due Thu, 10 Jan 2019 12:45:08 attention for the synthesis of biobased polyurethane, Table II Publishers in presence of litharge catalyst (PbO) at 0.05%. 21 Copyright: American Scientific It is low cost, abunto its inherent hydroxyl groups. Delivered byThe Ingenta reaction was carried out at 210  C for 3 h with condantly available renewable agricultural resource. Previous tinuous stirring. The derived polyols were dried at 70  C researches studies have shown that PUs obtained from CO under vacuum. The transesterified CO was represented as are typically water resistant, flexible due to the presence MCO. For the synthesis different molar ratio of pentaery21 22 However, as reported the of long fatty acid chain. thritol/CO (i.e., 0.1 and 0.2) was used to modify the CO properties of PUs obtained from CO includes low moduand represented as MCO1 and MCO2. The physicochemlus, low tear strength and sluggish rate of curing due to a ical properties like acid value, hydroxyl value and vislow hydroxyl number and structural irregularity. To concosities of polyol were determined by ASTM standards quer these limitations attention has been paid to the modiD1824, D1957 and D 7253 respectively. The results are fication of CO by transesterification reaction with alcohols summarized in Table II. The schematic diagram depictlike glycerol, pentaerythritol, trimethylol propane for the ing the reaction pathway for the transesterification of synthesis of PUs.21–23 castor oil with pentaerythritol is illustrated in Figure 1. Hence, in this study, we aimed at developing From Table II it is noticed that MCO2 showed the higher polyurethane using transesterified castor oil and aliphatic hydroxyl value and viscosity as compared with MCO1. partially biobased isocyanate. The chosen isocyanate is These observations revealed the higher degree of transesdeveloped from palm oil, extremely low viscous in nature terification occurred in MCO2. Hence MCO2 was used for due to the presence of intramolecular hydrogen bonding, the synthesis of PUs. possessing 25% renewable material according to ACDV’s (Association Chimie Du Végétal) evaluation with green carbon content of approx. 32% (14 C measurement as per ASTM-D6866) which makes it a promising material for the synthesis of solvent free PUs with reduced emission of VOCs. The properties of the synthesized PUs were characterized in terms of varying NCO/OH molar ratio. The purpose of this study was the successful synthesis of biobased PUs by optimizing the parameters and to investigate the effect of NCO/OH molar ratio. It is hoped that the relevant findings from this research will explore new ideas and expand the application of synthesized PUs. 378

2.3. Synthesis of Modified Bio Based Polyurethane (MBPUs) The polyurethane was synthesized by one step bulk polymerization process using transesterified castor oil (MCO) and partially biobased isocyanate. All reactions were carried out in a silicone oil bath with three necked round bottom flask equipped with a magnetic stirrer, fitted with dry nitrogen inlet, thermometer. A weighed amount of moisture free MCO2 was placed in RB flask (flushed with nitrogen) and charged with 1,4 BDO. Then Tolonate™ Mater. Express, Vol. 5, 2015

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Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

Table I. Chemicals used for the synthesis of modified biobased polyurethane (MBPUs). Role

Chemicals

Structure

Density (g/cm3 )

Fnc

Isocyb Cont. (%)

Partially biobased diisocyanate

Tolonate™ X FLO 100

1041

2–2.1

12

Natural oil

Castor oil

961

2.7

–

Polyol

Pentaerythritol

140

4

–

Catalyst

DBTDL

1050

–

–

Catalyst Chain extender

Lead oxide 1,4 Butanediol

953 101

– 2

– –

PbO OH(CH2 4 OH

On: Thu, 10 Jan 2019 12:45:08 X FLO 100 was added drop wiseIP:in5.10.31.211 order to maintain USA with omnic software for data collection and analysis Copyright: NCO/OH ratio 1, 1.2, 1.4 in the presence of American catalyst Scientific equippedPublishers in the range of 4000–400 cm−1 with attenuated Delivered by Ingenta DBTDL (0.05 weight% with respect to polyol). The temtotal reflectance (ATR) attachment. The kinetics of the perature of the silicon oil bath was maintained at 60–75  C reaction between MCO and Tolonate™ X FLO 100 for for a specified period of time. During the reaction, aceanalyzing the isocyanate (NCO) conversion of the MBPU1 tone was added to the mixture for decreasing the visfilm in the presence of catalyst was also determined by cosity and to facilitate proper mixing. The mixture was FTIR-ATR technique. degassed under vacuum and poured into teflon moulds, Wide angle X-ray Diffractometer (M/s. Shimadzu and allowed for curing at 90  C for 12 h. The films were XRD-700L, X-Pert MPD, Japan) equipped with graphite named as MBPUs since they were synthesized from modmonochromatic and source of Cu K radiation operated ified castor oil and the number indicates the NCO/OH at 40 kV and 30 mA was used to characterize the MBPU molar ratio. Clear transparent films with 2 mm thickness films. The scanning 2 angle ranged from 2–70 with a were obtained. Details of the composition and reaction scan speed of 5 /min. parameters are summarized in Table III. The schematic Scanning electron microscopy (SEM) was used to diagram representing the reaction pathway for the syntheobserve the cross sectional morphology of MBPU films. sis of MBPUs from transesterified castor oil and Tolonate™ The samples were cryofractured in liquid nitrogen and X FLO100 is illustrated in Figure 2. were mounted on the SEM stubs by carbon sticker, sputter coated with gold and the palladium mixture to 2.4. Characterization make the surface conductive. After coating, the samples FTIR spectroscopy analysis of the MBPU films and MCOs were inserted into SEM (M/s. EVO MA 15, Carl Zeiss, were recorded using M/s. Nicolet 6700, Thermo Scientific, Germany) instrumental chamber and the morphological Table II.

Value of hydroxyl content, acid number and viscosity of castor oil based polyols.

Sample code

Molar ratio pentaerythritol/castor oil

Castor oil (gm)

Pentaerythritol (gm)

Hydroxyl value (mg KOH/g)

Acid number (mg KOH/g)

Viscosity (cps) 25  C

CO MCO1 MCO2

0 01 02

40 40 40

0 06 116

160 190 234

196 168 140

930 980 992

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Notes: b stands for isocyanate content; c stands for functionality.

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Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

fewer crosslinks per unit weight of polymer whereas high vc implies high crosslink content.25 Mc =

V Vp1/3 − Vp /2

ln1 − Vp  + Vp + Vp2 vc = p /Mc ve =

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Fig. 1. Reaction pathway for the transesterification of castor oil with pentaerythritol.

1 2Mc

(1) (2) (3)

Where V and p is the density of the polymer respectively, Vp is the volume fraction of the respective polymer films and, is the polymer solvent interaction parameter.

micrographs were monitored at an accelerated voltage The polymer solvent interaction parameter can be calculated by using the following equation,8 of 5 KV. Surface morphology of MBPU 1.4 film was recorded Vs p − s 2 by using AFM (M/s. Park scientific instrument, XE-100, (4) 12 = 034 + RT USA) in contact mode and a commercial probe was used Where, p and s are the solubility parameters of the polyat room temperature and moderate pressure. mer and solvent respectively in (cal/cm3 1/2 . TGA of MBPUs was carried out using TGA Q 50, M/s.  is the lattice constant whose value is 0.34, TA Instrument, and USA as per ASTM E 1868 with a heatVs is the molar volume of solvent (cm3 /mol), ing rate of 10  C/min from room temperature to 800  C R is the gas constant, 1.983 (cal/molK) and under nitrogen atmosphere. Corresponding onset and final T is the temperature, K. degradation temperature (Tdonset , Tdend  char residue and (T98% , T50% ) was determined respectively. IP: 5.10.31.211 On: Thu, 10 The Jan 2019 12:45:08 test was carried out with sample dimensions Publishers12 × 15 × 1 mm (length × width × DSC of MBPUs was carried out Copyright: using DSC,American Q 20, Scientific of approximately Delivered Ingenta The weighed samples were immersed into M/s TA instruments, USA. About 5–10 mg of sample was bythickness). taken and heated at a rate of 10  C/min from −70  C to toluene for 72 h until the swollen equilibrium was 100  C to determine the glass transition temperature (Tg ) obtained. The surface of the specimen was then dried with under N2 atmosphere. blotting paper and weighed again.26 The volume fraction of the polymer (Vp ) in the swollen polymer network was The contact angle measurement was performed using calculated using the equation below,8 M/S. Phoenix SEO instrument using water as a probe material at room temperature. For each sample, 5 data Mp /p (5) Vp= were collected and the average contact angle has been Ms /s  + Mp /p  reported. The swelling characterstics, molecular weight between Where Ms = mass of solvent in the swollen polymer sample at equillibirium (in grams) crosslinks (Mc ), crosslink densities (ve ) and degree of MP = mass of the dry weight of polymer at the initial crosslinking (vc ) of the MBPU films were determined 8 24 state (in grams) using Flory Rehner equation. Mc is defined as the = density of solvent (g/cm3   average molecular weight of chains between two crosslink s p = density of the respective polymer film (g/cm3  sites and degree of crosslinking (vc ) determines the extent of crosslink per unit weight polymer. Low vc refers to The tensile properties of the MBPU films were determined in accordance with ASTM D882-91. The analysis was performed at room temperature carried with a test speed of Table III. Details of the composition and reaction parameters. 10 mm/min. An average of five tests was taken for calculating the tensile properties of each MBPU film. Hard a The density of each MBPU film was determined as per segment Tolonate , ASTM 792 by sinker method using a reference liquid. Sample Polyol TM X FLO100 1,4 BDO content code MBPU1 MBPU1.2 MBPU1.4

NCO/OH

(ml)

(ml)

(ml)

(wt%)

1:1 1.2:1 1.4:1

30 30 30

25.21 31.26 43.76

10 10 10

53 58 64

Note: a Tolonate™ X FLO 100: partially biobased isocyanate.

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3. RESULTS AND DISCUSSION 3.1. FTIR Analysis of MCO FTIR spectrum of original castor oil and polyols derived after the transesterification reaction is shown in Figure 3. Mater. Express, Vol. 5, 2015

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

Materials Express

Fig. 2. Reaction pathway for the synthesis of (MBPU) from transesterified castor oil and tolonate™ X FLO100.

Fig. 3. FTIR spectra of (a) Castor oil (b) Polyol (MCO1) (c) Polyol (MCO2).

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Fig. 4. FTIR spectra of (a) Tolonate™ X FLO100 (b) MBPU 1.4 (c) MBPU 1.2 (d) MBPU 1.

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In all the cases (i.e., CO, MCO1 and MCO2) the char1050 cm−1 ( CHbend ) and 1161 cm−1 (C–O–C) remained −1 unaltered even after the transesterification of castor oil. acteristic peak at 3400 cm , corresponding to hydroxyl group was noticed. An increment in the band intensity at 3400 cm−1 of transesterified CO (i.e., MCO1 and 3.2. FTIR Analysis of MBPU Films MCO2) revealing the increase in the hydroxyl value of The FTIR spectra of MBPUs (i.e., MBPU1, MBPU1.2 the castor oil obtained after transesterification reactions.23 and MBPU1.4) with varying NCO/OH ratios as shown IP: band 5.10.31.211 Thu, 10 2019 12:45:08a characteristic band of urethane The highest intensity of absorption at 3400 On: cm−1 in Jan Figure 4 showed American Scientific Publishers −1 was noticed for MCO2 indicating Copyright: that a substantial stretching at 3336 cm which might be due to hydrogen Delivered by Ingenta increase in the hydroxyl value after modification with bonded N–H group in disordered form.27 28 The absence respect to the increase in pentaerythritol content. This of peak at 3440 cm−1 in the spectra of MBPUs indiwas in accordance with the calculated hydroxyl valcates that no free –NH group are present.28 The comues (Table II). The other characteristic absorption bands bination of NH out of plane bending and CN stretching was observed in the region 1532 cm−1 . In case of all the observed at 2923–2853 cm−1 (–CH2 7 , 1742 cm−1 (esters), MBPUs (i.e., MBPU1, MBPU1.2, and MBPU1.4) absence of NCO stretching, vibration band at 2260 cm−1 (Fig. 2)

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Materials Express

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

confirmed that, the isocyanate group have been reacted during polymerization. The disappearance of the broad hydroxyl group at 3400 cm−1 for all MBPUs showed that the OH group is completely used up. This indicates the utilization of OH group during polymerization. The band due to the carbonyl stretching vibration (amide I band) of the urethane is observed for all the MBPUs in the region 1716–1714 cm−1 . Additionally a band at 1742 cm−1 was observed for MBPU1. The bands observed in the region 1351 cm−1 for all the MBPUs were due to –CHbend in alkanes. The bands around 776 cm−1 for all the MBPUs were due to amide 1V, V and VI modes containing a significant contribution from the –NH out of plane deformation mode.13 All the above spectral data confirm the synthesis of MBPUs from transesterified castor oil and partially bio based isocyanate. The –C O band of polyurethane mainly conFig. 5. FTIR spectra of MBPU 1 at different reaction time intervals. sists of hydrogen bonded –C O in ordered crystalline domains at 1685–1706 cm−1 , hydrogen bonded absorbance (ANCO /ACH2  and the corresponding isocyanate –C O in disordered amorphous conformations at conversion (p). −1 1714–1718 cm and non –H-bonded (free) carbonyl The parameter (p) was calculated respectively by the −1 3 29 It is well known that, in groups at 1731–1745 cm . following equation, polyurethane the –NH group of urethane forms hydrogen bond with the urethane C O, urea C O, ester carbonyl (6) p = 1 − ANCO /ACH2 /ANCO /ACH2 0 and ether oxygen of soft segments. Previous studies have Thu, 10 Jan 2019 revealed that the with the increaseIP:in5.10.31.211 the NCO/OHOn: ratio is the integrated absorbance for the isoWherein ANCO12:45:08 American Publishers the intensity of the lower absorption Copyright: value increases due Scientific cyanate group, Delivered 10 by Ingenta to increase in the number of hydrogen bond formation. ACH2 is the integrated absorbance for the CH2 group and, Also the hydrogen bonded urethane NH and C O appears (ANCO /ACH2 0 is the relative absorbance extrapolated for at lower wavenumber as compared to the free urethane time zero. carbonyls.30 The results obtained from the FTIR specFrom the above study it was observed that with time tra for all the MBPUs also correlates to the finding menthe intensity of isocyanate peak decreased and the comtioned above, being more prominent for MBPU 1.4. This plete conversion of isocyanate to polyurethane occurred might be due to the formation of more number of H-bonds after 12 h of reaction. The polyurethane film samples were between the NH of urethane linkage with the C O carobtained in a solid state of thickness 1–2 mm without any bonyl group. air bubbles. 3.3. Polymerization Kinetics Since, this paper aims at synthesizing biobased polyurethane from a newly developed aliphatic partially bio based isocyanate so it was essential to set up the reaction parameters by reporting isocyanate conversion in terms of curing time. The kinetics of polyurethane formation were studied using FTIR in ATR mode.31 The absorption band corresponding to the isocyanate group of Tolonate™ X FLO 100 was observed at 2260 cm−1 , as shown in Figure 5 and the decay in the intensity of this peak was used to analyze the isocyanate conversion during the polymerization process. It is calculated in terms of ratio of the absorbance of –NCO group and CH2 stretch. The absorption band corresponding to –CH2 stretch was observed at 2854 cm−1 . Figure 5 shows the FTIR spectra of MBPU 1 at different reaction time interval and Figure 6 shows the corresponding evolution of the relative intensity of 382

Fig. 6. (a) Decay in the relative absorbance (ANCO /ACH2 ) and corresponding (b) Isocyanate conversion.

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Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

Materials Express

Fig. 7. SEM micrographs of (a) MBPU 1(b) MBPU 1.2 (c) MBPU 1.4.

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3.4. SEM Analysis dark regions signifies the presence of microphase morphology. From the figure a topographical heterogeneity was The fractured surface morphologies of MBPU films with IP: 5.10.31.211 On: Thu, 10 Jan 2019 12:45:08 observed Publishers which might be due to the ordering tendency in different NCO/OH ratios are shown Copyright: in Figures 7(a)–(c). American Scientific synthesized PU. The similar results have been reported A phase separated structure is observed with Delivered aggrega- bythe Ingenta earlier by Oprea et al.32 It is also noticed that the hard tion of hard domains which becomes more prominent segments appear in the form of agglomerates, grouped in for MBPU1.4. Yong He et al.28 reported that the pricompact areas. The result obtained from AFM analysis mary reason for such phenomena is the strong intermoleccorroborates the finding of SEM analysis. ular interaction between the urethane units which are capable of forming interurethane hydrogen bonds. Earlier reports by Dutta et al.10 have mentioned that the ten3.6. TGA Analysis dency of phase separation increases with the increase in TGA was used to evaluate the thermal stability of the NCO/OH ratio. The phase separation may be attributed MBPU films with different NCO/OH molar ratios. to N–H· · · O C (urethane) hydrogen bonding which was Figures 9(a)–(b) and Table IV summarizes the characterisconsistent with the observation investigated by FTIR. It tic degradation temperature of the MBPU films. Corcuera can be seen from Figure 7(c) that the continous phase et al.33 reported that the thermal stability of PU is related represents the soft segment whereas the hard segment of to the structure, chemical composition and molar ratio of MBPU tends to form aggregates clustering into separate the hard segment and soft segment. The mechanism to domains and thereby getting trapped into the domains of explain the degradation process of polyurethane is illussoft segments thus exhibiting a two phase structure. From trated below26 34 in Figure 10. the analysis, it is distinct that with the increase in the The DTGA curve of all the MBPU samples derived NCO/OH ratio, MBPU 1.4 shows a well defined phase showed a three step degradation curve. The same obserseparated structure. vation was reported by earlier literature.35 The first stage of degradation is due to urethane bond decomposition34 3.5. AFM Analysis which takes place through the dissociation of PU into isocyanate and alcohol, the formation of primary amines To further confirm the presence of phase separated morand terminal olefinic groups on the polyester chains, and phology AFM study of MBPU 1.4 was investigated in through the formation of secondary amines and CO2 36 contact mode with a scanning area of 5 × 5 m. The The second stage of degradation corresponds to the phase image of MBPU 1.4 is shown in Figure 8. Usually decomposition of soft segments such as alkyl chain, dissothe darker and brighter region represents the soft domains ciation of the ester bond through chain scission, dehydro(polyol) and the hard domains (isocyanate and chain genation and depolycondensation of alkyl groups present extender)32 respectively. The existence of both bright and

Materials Express (a)

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

(c)

(b)

Fig. 8. (a) Phase image (b) 3D image and (c) Topographic image of MBPU 1.4.

solid state, which remains in residue form.39 The diisocyanate formed by the thermal decomposition may get dimerized to produce cabodiimide linkages which can react with urethane groups to form crosslinked structure. In case of Td(onset) which exhibits the start of thermal degradation. It is noticed that, with the increase in NCO/OH ratio, the degradation temperature Td(onset) increases. This is attributed to the fact that with the increase in the NCO/OH greater than 1 the polyurethane formed will be isocyanate terminated.40 Hence, the free NCO groups will react duringJan the 2019 crosslinking IP: 5.10.31.211 On: Thu, 10 12:45:08process resulting in the formation of three dimensional Copyright: American Scientific Publishers allophanate and biuret linkages and Delivered bythereby Ingentaincreasing the number of urethane groups.40 The urethane linkages formed increases with the increase in NCO/OH ratio and they decompose considerably at higher temperature.10 The results observed are in good agreement with FTIR. Similar increasing trend was noticed in case of Td(end) which exhibits the end degradation temperture. This indicates the highest thermal stability of MBPU 1.4 revealing that with the increase in the NCO/OH molar ratio the crosslinking density increases as a result the polymer backbone chain undergoes closer packing, making the material harder and thus increasing the thermostability.10 Similar trend was also reflected in the T50% and T98% decomposition of MBPUs where the decomposition temperature increases with the increase in NCO/OH molar ratio. Thermal stability of the polymeric materials is related to the char residue.41 Greater the char residue percentage, indicates higher thermal stability. In all the cases of MBPUs the char residue was obtained at 500  C,

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in polyol.37 Furthermore the third stage of decomposition which was observed at higher temperature is associated with other segments of the remaining structure,36 decomposition of soft segments33 or might be due to C–C bond cleavage.38 This final stage may be attributed to the thermal decomposition of carbodiimide linkages, which leads to the production of CO2 and char in the

Table IV. TG and DTGA data of the MBPU films with varying NCO/OH molar ratios (T98% , T50% , Tinitial% , Tdmax and char residue). Sample code

Fig. 9. TGA and DTGA thermograms of (a) MBPU1 (b) MBPU 1.2 (c) MBPU 1.4.

384

MBPU1 MBPU1.2 MBPU1.4

Td(onset)  C

Td(end)  C

T50%  C

T98%  C

Char residue at 500  C wt%

207 213 234

535 540 610

336 333 339

510 518 605

4.32 6.00 8.41

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Table V.

Materials Express

DSC data of MBPU film with varying NCO/OH molar ratio.

Sample

Tg1 ( C)

MBPU1 MBPU1.2 MBPU1.4

−343 −288 −212

Tg2 ( C) 619 696 726

Fig. 11.

DSC Curve of (a) MBPU 1.4 (b) MBPU1.2 (c) MBPU 1.

Mater. Express, Vol. 5, 2015

Fig. 12. WAXD (c) MBPU 1.4.

patterns

of

(a)

MBPU

1

(b)

MBPU

1.2

385

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respectively. Thus it is revealed that Tg1 increases with increase in NCO/OH molar ratio which might be due to the fact that the H-bonding as well as the cross linking density increases with the increase in NCO/OH molar ratio.36 As a consequence, the mobility of soft segments is arrested with the increase in hard segment ratio. This result is in good agreement with that obtained by FTIR Fig. 10. General mechanistic pathway for polyurethane degradation. studies explained in earlier section. Similar findings were also observed recently by Gurunathan et al.14 Moreover the second peak (Tg2 ) for MBPU1.4, MBPU wherein the highest percentage of char was noticed in 1.2 and MBPU 1 appears at 72.6, 69.6 and 61.9  C MBPU 1.4. This revealed the highest thermal stability of which increases with increase in NCO/OH molar ratio. MBPU 1.4, confirming the relatively high degree of physThis might be attributed to the fact that intermolecular H35 ical crosslinking. Therefore, it was concluded that with bonding amongst the hard–hard segments increases with increase in the NCO/OH ratio the thermal stability can be increase in hard segment. Reasonably these hard segment improved. may act as physical crosslinker thereby decreasing the segmental mobility and enhancing the phase separation.35 3.7. DSC Analysis Thus the above findings further confirms the phase sepaThe DSC curves of MBPUs with varying NCO/OH molar rated morpholgy as observed in SEM and AFM. The above On: Thu, 10 Jan 2019 12:45:08 ratio is depicted in Figure 11 and IP: the 5.10.31.211 results are summaresults were also reported by Bao et al.35 Copyright: American Scientific Publishers rized in Table V. As observed from Figure 11 that two Delivered by Ingenta well defined Tg s are present for all the MBPUs (1:1, 1.2:1 3.8. WAXD Analysis and 1.4:1). Amongst the two Tg s one appears at negaFigure 12 shows the X-ray diffraction patterns of MBPU tive temperature (Tg1  representing the soft segment and films with different NCO/OH molar ratios. In all the cases the other at positive temperature (Tg2 ) representing the 14 broad amorphous peaks near 2 = 20 was noticed. The The Tg1 of MBPU 1.4, MBPU 1.2 and hard segments. WAXD curves of all the MBPU samples exhibited amorMBPU 1 were observed at −21.2, −28.8 and −34.3  C phous nature at room temp, although some amount of crystalline phase must be present due to the hard segments.

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Influence of NCO/OH and transesterified castor oil on the structure and properties

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Bao et al. reported35 that the small amount of crystalline phase is concealed by the presence of a long aliphatic hydrocarbon chain of MCO and the hydrogen bonding. The above results thus indicate that some ordered arrangement of hard segment exist but the presence of H-bonding within the –CONH group as evident from FTIR and DSC hinders the orientation of the molecule and thus lowers the crystallinity.42 Another responsible factor is the long aliphatic hydrocarbon chain which remains as a pendant group to the main chain backbone which can increase the flexibility and hinder the close packing thereby lowering the crystallinity. Thus the above observation are in agreement with DSC results indicating that these MBPU films are not organized in crystalline form rather arranged in amorphous domains. The intensities of the diffraction peak increases with the increase in the NCO/OH ratio which can be ascribed to the finite contribution of the hard segment.

Das et al.

Fig. 14. Influence of varying NCO/OH molar ratio on Mc and vc of MBPU films prepared from transesterified castor oil.

crosslinks) and vc (degree of crosslinking). An increase in ve value was noticed by the increase in NCO/OH molar ratio, which was in accordance with the earlier findings reported by Nadeem et al.44 Further, it was noticed that 3.9. Swelling Test and Mc Determination vc (degree of crosslinking) of the MBPUs increases with Swelling experiments were carried out to determine the the increase in the NCO/OH molar ratio. This observation solubility parameter ( ) of the MBPU films. The polymer can be explained on the basis that a three dimensional netfilm samples were immersed in several solvents with soluwork was formed by the reaction between the OH groups bility parameter ranging from 7.0 to 13.5 (cal/cm3 1/2 and in MCO and –NCO groups in biobased isocyanate resultwere swollen to equilibrium at room temperature.38 ing in an increase in vc 45 Moreover, it can be seen from The solubility parameter of the IP: respective MBPU film Table that 12:45:08 the ve is high for MBPU 1.4 as compared 5.10.31.211 On: Thu, 10 JanVI 2019 was obtained by plotting a graph between the American swelling Scientific Copyright: Publishers with MBPU 1.2 and 1 respectively. This might be due Delivered bytoIngenta coefficient of films with respect to the solubility parameter the presence of more NCO groups that participate in ( ) of the solvents and their corresponding Gaussian fits43 crosslinking reactions.9 The hard domains, thus behave is depicted in Figure 13. The maximum on each curve is as effective cross-link site in the polymer matrix.44 The taken as the polymer solubility parameter (p . Hence the reduction in Mc can be explained on the basis that, it is value of p was assumed to be 9.52 (cal/cm3 1/2 since each inversely propotional to ve 2446 Thus the results obtained MBPU film demonstrated the maximum swelling coeffiwere in good agreement with the FTIR and SEM obsercient value in tetrahydrofuran (THF) as compared with vations. The swelling data of MBPU films with different other solvents. NCO/OH molar ratios are illustrated in Table VI. Figure 14 presents the effect of varying NCO/OH molar ratio on the parameters Mc (molecular weight between 3.10. Mechanical Properties of MBPU Films As shown in Figure 15 and Table VII, the tensile strength and modulus of MBPU films increases and elongation at break decreases with the increase in NCO/OH molar ratio. The reason can be ascribed to the fact that with the increase in the NCO/OH molar ratio the hard segment content in the polymer increases. The same observation was reported by Velayutham et al.47 where it has been mentioned that the hard segment act as a Table VI. Swelling data of MBPU films with different NCO/OH molar ratios. Sample code

Fig. 13. Gaussian fit for the swelling coefficient versus solubility parameter of (a) MBPU 1.4 (b) MBPU 1.2 (c) MBPU 1.

386

MBPU1 MBPU1.2 MBPU1.4

Density (g/cc)

Volume fraction (Vp )

Mc (g/mol)

ve × 10−3 (mol/cm−3 )

vc × 10−3 (mol/cm−3 )

102 103 104

0028 0031 0037

6232 5613 4191

08 089 119

163 183 248

Mater. Express, Vol. 5, 2015

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

Fig. 15. Mechanical properties of MBPU films as a function of varying NCO/OH molar ratios.

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Fig. 16. Contact angle images of (a) MBPU 1 (b) MBPU 1.2 (c) MBPU 1.4.

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crosslinker thus inhibiting the motion of the soft segThis is mainly due to the fact that with the increment in the ments. Hence, as the percentage of hard segment conNCO/OH ratio the hard segment content in the polymer tent increases the intermolecular attraction between the increases, generating more urethane groups which results hard segment increases.37 This remarkable increase in in effective intermolecular hydrogen bonding and physical the tensile strength with the increase in NCO/OH molar crosslinks.10 ratio can also be accounted for the hydrogen bonding formed between the NH and C=O of the urethane which 3.11. Contact Angle Studies of MBPU Films restricts the rotation of polymer segments. Therefore IP: 5.10.31.211 On:the Thu, 10 2019angle 12:45:08 TheJan contact of MBPU films with different NCO/OH elongation at break decreases while Copyright: the tensile American strength Scientific Publishers ratio is represented in Figure 16. It can be observed that Delivered by Ingenta has increased35 37 The appearance of higher crosslinkthe contact angle had slightly increased with the increase ing density was another indication47 37 for the observed in the NCO/OH ratio. higher tensile strength, which was in agreement with the This might be due to the effective hydrogen bonding swelling test results. Moreover, the phase segregation with formed as observed through FTIR analysis with increase the increase in NCO/OH as revealed by SEM images may in the hard segment, which act as physical crosslinks10 also result in improved tensile strength ratio.37 . Thus the restricting the wetting of the film surface. The results results obtained are in good agreement with the observed obtained could also be correlated to the observation conphenomena of FTIR, SEM and swelling characteristics. cluded from swelling experiments. The observed increase The observed increase in the tensile strength of MBPU in ve with the increase in the NCO/OH ratio (Table V) 1.4 can be explained on the basis of FTIR studies. Analysis also testifies the observed decrease in contact angle of the stretching vibration of the C O (Fig. 4), showed measurement.48 With a higher crosslinking density the that MBPU1, displayed splitting of –C O absorbance water absorption decreases which results in higher contact peak from free (1742 cm−1 ) and disordered hydrogen angle.48 bonds (1716 cm−1 ). However for MBPU1.2 and MBPU1.4 a single peak was observed at 1715–1714 cm−1 indicating 4. CONCLUSION hydrogen bond formation in amorphous domains.33 It was also noticed that the intensity of the lower absorption peak The present paper reports synthesis of modified biobased increases with increase in NCO/OH ratio which further polyurethane based on polyol and partially biobased supports the formation of more number of H-bonds.10 44 isocyanate Tolonate™ X FLO 100. A systematic investigation of the synthesized product revealed the dependability of various physicochemical properties on the NCO/OH Table VII. Tensile properties of MBPU films with varying NCO/OH ratio. The FTIR spectra revealed the characterstic absorpmolar ratio. tion bands of polyurethane confirming the formation of Sample Tensile Elongation at Young’s modulus biobased polyurethane. The IR study also showed that with code strength (MPa) break (%) (MPa) the increase in the NCO/OH molar ratio the intermolecular MBPU1 11.14 ± 0.65 209.8 ± 0.81 18.8 ± 0.08 hydrogen bonding in amorphous domain increases. DSC MBPU1.2 13.96 ± 0.45 190.1 ± 0.57 21.8 ± 0.4 result reveal that with the increase in NCO/OH molar ratio MBPU1.4 17.46 ± 0.18 140 ± 0.46 24.7 ± 0.12 the Tg increases although there is lowering of crystallinity.

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Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

Further this is in good agreement with the WAXD results which reveals the fact that all the samples are amorphous in nature. All the MBPUs exhibited three steps of degradation as observed from the DTGA/TGA. The TGA analysis revealed the relatively higher thermal stability in the samples with higher NCO/OH ratio. This characteristic behavior can be due to the more hydrogen bond being formed between C O of the carbonyl group and –NH in urethane in case of MBPU1.4. The SEM and AFM images displayed a phase separated structure for MBPUs with higher NCO/OH value (i.e., MBPU1.4). The findings of this study demonstrated that with the increase in the NCO/OH molar ratio the tensile strength increases, while percentage elongation decreases. Finally from Flory Rehner equation, we concluded that with the increase in NCO/OH value the average molecular weight between crosslink decreases whereas the crosslinking density increases owing to a high degree of physical crosslinking and formation of 3D network. Moreover, the contact angle measurement showed that the value of  increases with increase in NCO/OH molar ratio. Hence, from the analysis, it is concluded that Tolonate™ X FLO 100 can be utilized to synthesize biobased polyurethane with desirable properties.

10. S. Dutta and N. Karak; Effect of the NCO/OH ratio on the properties of mesua ferrea L. seed oil-modified polyurethane resins; Polym Int. 55, 49 (2006). 11. T. Gurunathan, S. Mohanty, and S. K. Nayak; Isocyanate terminated castor oil based polyurethane prepolymer: Synthesis and characterization; Prog. Org. Coat. 80, 39 (2015). 12. J. Huang and L. Zhang; Effect of NCO/OH molar ratio on structure and properties of graft-interpenetrating polymer networks from polyurethane and nitrolignin; Polymer. 43, 2287 (2002). 13. T. Gurunathan, R. K. C. Rao, R. Narayan, and K. V. S. N. Raju; Synthesis, characterization and corrosion evaluation on new cationomeric polyurethane water dispersions and their polyaniline composites; Prog.Org.Coat. 76, 639 (2013). 14. M. Ossama Abo-Elenien, M. A. Elsaeed, and A. M. El-Sockary; Synthesis of new polyurethane coating based on rosin for corrosion protection of petroleum industries equipment; Int. Journal of Engineering Research and Applications 4, 148 (2014). 15. H. Murakami, R. Nishiide, S. Ohira, and A. Ogata; Synthesis of MDI and PCL-diol-based polyurethanes containing [2] and [3] rotaxanes and their properties; Polymer. 55, 6239 (2014). 16. L. Hojabari, X. Kong, and S. S. Narine; Fatty acid–derived diisocyanate and biobased polyurethane produced from vegetable oil: Synthesis, polymerization and characterization; Biomacromolecules 10, 884 (2009). 17. A. Pegoretti; Trends in composite materials: The challenge of singlepolymer composites; Express Polym Lett. 1, 710 (2007). 18. M. Heinen, A. E. Gerbase, and C. L. Petzhold; Vegetable oil-based rigid polyurethanes and phosphorylated flame retardants derived from epoxidized soybean oil; Polym. Degrad Stabil. 108, 76 (2014). 19. M. Zieleniewska, M. Auguscik, A. Prociak, P. Rojek, and Acknowledgment: The authors IP: acknowledge the On: finanJ. Ryszkowska; Polyurethane-urea substrates from rapeseed oil5.10.31.211 Thu, 10 Jan 2019 12:45:08 cial support of Department of Science and Technology, based Publishers polyol for bone tissue cultures intended for application in Copyright: American Scientific tissue engineering; Polym. Degrad Stabil. 108, 241 (2014). Ministry of Chemicals and Fertilisers, Govt Delivered of India by Ingenta 20. L. Hojabari, X. Kong, and S. S. Narine; Novel long chain unsaturated through Sustainable Green Materials project. diisocyanate from fatty acid: Synthesis, characterization and application in bio-based polyurethane; J. Polym. Sci., Part A: Polym.Chem. 48, 3302 (2010). References and Notes 21. M. Valero, J. Pulido, A. Ramirez, and Z. Zhendong; Simulta1. C. K. Williams and M. A. Hillmyer; Polymers from renewable neous interpenetrating polymer networks of polyurethane from resources: A perspective for a special issue of polymer reviews; pentaerythritol-modified castor oil and polystyrene: StructurePolym Rev. 48, 1 (2008). property relationships; J. Am .Oil .Chem. Soc. 86, 383 (2009). 2. A. Gandini; Polymers from renewable resources; A challenge for 22. A. Kaushik and S. Paramjit; Synthesis and characterization of castor the future of macromolecular materials; Macromolecules. 41, 9491 oil/trimethylol propane polyol as raw materials for polyurethanes (2007). using time-of-flight mass spectroscopy; Int. J. Polym. Anal. Charact. 3. K. Pietrzak, M. Kirpluks, U. Cabulis, and J. Ryszkowska; Effect of 10, 373 (2005). the addition of tall oil-based polyols on the thermal and mechanical 23. M. F. Valero and A. Gonzalez; Polyurethane adhesive system from properties of urea urethane elastomers; Polym Degrad Stabil. 108, 1 castor oil modified by a transesterification reaction; J. Elastom.Plast. (2014). 44, 433 (2012). 4. V. Sharma and P. P. Kundu; Condensation polymers from natural 24. M. F. Vallat, N. Bessaha, J. Schultz, J. Maucourt, and C. Combette; oils; Prog. Polym. Sci. 33, 1199 (2008). Adhesion behaviour of polyurethane-based materials; J. Appl. Polym. 5. N. Lin, S. Wei, T. Xia, F. Hu, J. Huang, and A. Dufresne; Sci. 76, 665 (2000). Green bionanocomposites from high-elasticity “soft” polyurethane 25. H. D. Rozman and G. S. Tay; The effects of NCO/OH ratio and high-crystallinity “rigid” chitin nanocrystals with controlled suron propylene oxide-modified oil palm empty fruit bunch-based face acetylation; RSC Adv. 4, 49098 (2014). polyurethane composites; J. Appl. Polym. Sci. 110, 3647 (2008). 6. S. Lin, J. Huang, P. R. Chang, S. Wei, Y. Xu, and Q. Zhang; Structure 26. M. A. Semsarzadeh and A. H. Navarchian; Effects of NCO/OH and mechanical properties of new biomass-based nanocomposite: ratio and catalyst concentration on structure, thermal stability and Castor oil-based polyurethane reinforced with acetylated cellulose crosslink density of poly (urethane-isocyanurate); J. Appl. Polym. nanocrystal; Carbohydrate Polymers. 95, 91 (2013). Sci. 90, 963 (2003). 7. N. Lin, J. Huang, P. R. Chang, J. Feng, and J. Yu; Surface acetylation 27. J. Vega-Baudrit, M. Sibaja-Ballester, P. Vazquez, R. Torregrosaof cellulose nanocrystal and its reinforcing function in poly(lactic Macia, and J. M. Martin-Martinez; Properties of thermoplastic acid); Carbohydrate Polymers. 83, 1834 (2011). polyurethane adhesives containing nanosilicas with different specific 8. A. Sirkecioglu, H. B. Mutlu, C. Citak, A. Koc, and F. S. Geune; surface area and silanol content; Int. J. Adhes. Adhes. 27, 469 (2007). Physical and surface properties of polyurethane hydrogels in relation 28. Y. He, D. Xie, and X. Zhang; The structure, microphase-separated with their chemical structure; Polym. Eng. Sci. 54, 1182 (2013). morphology, and property of polyurethanes and polyureas; J. Mat. 9. S. Sarkar and B. Adhikari; Synthesis and characterization of lignin– HTPB copolyurethane; Eur. Polym. J. 37, 1391 (2001). Sci. 49, 7339 (2014).

388

Mater. Express, Vol. 5, 2015

Influence of NCO/OH and transesterified castor oil on the structure and properties Das et al.

39. C. Dick and R. B. Dominguez; The flammability of urethanemodified polyisocyanurates and its relationship to thermal degradation chemistry; Polymer 42, 913 (2001). 40. S. Desai, I. M. Thakore, and B. D. Sarawade; Effect of polyols and diisocyanates on thermomechanical and morphological properties of polyurethanes; Eur. Polym. J. 36, 711 (2000). 41. R. E. Lyon; Pyrolysis kinetics of char forming polymers; Polym. Degrad. Stabil. 61, 201 (1998). 42. H. P. Bhunia, R. N. Jana, A. Basak, S. Lenka, and G. B. Nando; Synthesis of polyurethane from cashew nut shell liquid (CNSL), a renewable resource; J. Polym. Sci., Part A: Polym. Chem. 36, 391 (1998). 43. M. Carme, C. Ferrer, D. Babb, and A. J. Ryan; Characterisation of polyurethane networks based on vegetable derived polyol; Polymer 49, 3279 (2008). 44. N. Ahmad, M. B. Khan, M. Xiaoyan, and N. Ul-Haq; The influence of cross-linking/chain extension structures on mechanical properties of HTPB-based polyurethane elastomers; Arab. J. Sci. Eng. 39, 43 (2014). 45. G. S. Tay and H. D. Rozman; Chemical modification of oil palm empty fruit bunch: Determination of optimum condition and characterization; J. Appl. Polym. Sci. 106, 1697 (2007). 46. S. Gopalakrishnan and T. L. Fernando; Influence of polyols on properties of bio-based polyurethanes; Bull. Mater. Sci. 35, 243 (2012). 47. T. S. Velayutham, W. H. Abd Majid, A. B. Ahmad, Y. K. Gan, and S. N. Gan; The physical and mechanical properties of polyurethanes from oleic acid polyols; J. Appl. Polym. Sci. 112, 3554 (2009). 48. H. H. Wanga, J. Moua, Y. H. Nib, G. Q. Feia, C. L. Sic, and J. Zou; Phase behaviour, interaction and properties of acetic acid lignin containing polyurethane films coupled with aminopropyltriethoxy silane; Express Polym Letters 7, 443 (2013).

IP: 5.10.31.211 On: Thu, 10 Jan 2019 12:45:08 Copyright: American Scientific Publishers Delivered by Ingenta Received: 18 December 2014. Revised/Accepted: 1 May 2015.

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Article

29. J. L. Sormana and J. C. Meredith; High-throughput discovery of structure-mechanical property relationships for segmented poly (urethane-urea) s; Macromolecules 37, 2186 (2004). 30. M. M. Coleman, K. H. Lee, D. J. Skrovanek, and P. C. Painter; Hydrogen bonding in polymers; Infrared temperature studies of a simple polyurethane; Macromolecules 19, 2149 (1986). 31. B. J. Rashmi, D. Rusu, K. Prashantha, M. F. Lacrampe, and P. Krawczak; Development of bio-based thermoplastic polyurethane formulations using corn derived chain extender for reactive rotational moulding; Express Polym Letters 10, 852 (2013). 32. S. Oprea; Synthesis and properties of polyurethane elastomers with castor oil as crosslinker; J. Am. Oil Chem. Soc. 87, 313 (2010). 33. M. A. Corcuera, L. Rueda, B. Fernandez d’Arlas, A. Arbelaiz, C. Marieta, I. Mondragon, and A. Eceiza; Microstructure and properties of polyurethanes derived from castor oil; Polym. Degrad Stabil. 95, 2175 (2010). 34. E. Hablot, D. Zheng, M. Bouquey, and L. Avérous; Polyurethane based on castor oil: Kinetics, chemical, mechanical and thermal properties; Macromol. Mater. Eng. 293, 922 (2008). 35. L. H. Bao, Y. J. Lan, and S. F. Zhang; Effect of NCO/OH molar ratio on the structure and properties of aqueous polyurethane from modified castor oil; IPJ. 15, 737 (2006). 36. S. V. Levchik and E. D. Weill; Thermal decomposition, combustion and fire-retardancy of polyurethanes a review of the recent literature; Polym. Int. 53, 1585 (2004). 37. C. W. Chang and K. T. Lu; Natural castor oil based 2-package waterborne polyurethane wood coatings; Prog. Org. Coat. 75, 435 (2012). 38. X. Kong and S. S. Narine; Physical properties of polyurethane plastic sheets produced from polyols from canola oil; Biomacromolecules 8, 2203 (2007).

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