Synthesis and Thermal Characterization of Polyurethanes Obtained from Cottonseed and Corn Oil-Based Polyols

The use of vegetable oils to replace fossil feedstock has become an area of opportunity and a priority for study in the fi eld of polymer science. Vegetable oils are considered as renewable resources with high potential, low cost and full availability. The aim of this study is the synthesis of biobased polyols from cottonseed oil (Gossypium barbadanse) and corn oil (Zea mays) as feedstock. Their synthesis was successfully performed, as can be concluded from the determination of their hydroxyl index as well as the structural and thermal characterization carried out in this work. Polyurethanes from biobased polyols were synthesized with two different isocyanates (HDI and MDI) and variable polyol/isocyanate ratio and they were further characterized by infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy, concluding that cottonseed and corn oils could be considered as valid alternatives for the synthesis of biobased polymeric materials.


INTRODUCTION
In an age of increasing environmental problems, such as global warming and waste disposal, the change from fossil feedstock to renewable resources can considerably contribute to the sustainable development [1].The differences in the resource consumption are so high that those products based on vegetable oils (VOs) could clearly be considered an environmentally benign alternative to petrochemical products [2].In the chemical industry, VOs are used as ingredients or components in many manufactured products, such as surfactants (soaps), lubricants, plasticizers, cosmetic products, monomers (e.g., dimer acids and polyols) and agrochemicals.In addition, they have been used for decades in paint formulations, as fl ooring materials, coatings and resin applications [3].VOs are part of a major family of chemical compounds known as fats or lipids, mainly conformed by triesters of glycerol and fatty acids, commonly known as triglycerides.Their composition and properties give them high versatility to permit their participation in the preparation of a sort of different poly meric materials [4].However, VOs should be functionalized to increase their ability to form chemical bonds with other monomers and to serve as building blocks for polymers [5].Triglycerides of vegetable oils, whose basic chemical structure is shown in Figure 1, are highly functionalized molecules and consequently they have been used in the synthesis of crosslinked polymers by two main approaches.The fi rst one takes advantage of the naturally occurring functional groups present in triglycerides, such as the internal double bonds, hydroxyl groups and oxirane rings, which can be polymerized by different methods.The second strategy is based on chemical modifi cations taking place prior to polymerization [6].Some of the VOs already studied and with industrial applications include soybean oil, palm oil, castor oil, linseed and saffl ower oils [1,7].
The composition of cottonseed oil is typical of the group of unsaturated fatty acids and it is conformed mainly of oleic acid (33.1%) and linoleic acid (39.6%), as reported by Valdes et al. [8], while corn oil is rich in oleic acid (25.8%), linoleic acid (58.9%) and linolenic Scrivener First acid (1.1%) (Figure 2) [8,9].Even when these oils contain a certain amount of saturated fatty acids, the fatty acid profi les of both VOs provide them the potential as raw materials for the replacement of petroleumbased building blocks and monomers.In addition, corn and cotton crops are a highly social culture in Mexico, generating direct and indirect labor, as they are able to produce a high number of original products and by-products from their manufacturing, such as original cotton and seed in the case of the cotton industry.Around 35% of cotton (fi ber form) is used for the fabrics and spun manufacturing, while 54% (seed form) is used for new sowings and edible oils extraction for further use as feed and other industrial applications.The rest of the initial mass of cotton (byproducts) is used for the livestock industry [10].
The aim of this study is the use of cottonseed and corn oils as feedstock in the synthesis of bio-polyols and biobased polymeric materials such as polyurethanes.These materials were synthesized and further characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM).

Preparation of Polyols
The synthesis of polyols was carried out in a glass reactor where 100 g of cottonseed or corn oil and glacial acetic acid (35 g) were mixed by mechanical stirring and heated at 85 °C.Then, 30% (v/v) hydrogen per oxide (110 g) and 0.5 mL of concentrated sulfuric acid were added dropwise to the reaction chamber and the whole mixture was reacted for 4 h with peracetic acid formed in situ.
Prior to analysis, all polyols were washed with a saturated sodium bicarbonate solution until neutral pH [11].These products were dried in a vacuum oven at 60 °C for 24 h and then characterized by FTIR spectroscopy.The hydroxyl numbers in all formulations were determined by following the general method of analysis MGA 0491 [12].The hydroxyl number is defi ned as the number of milligrams of potassium hydroxide needed to neutralize the acetic acid produced by the hydrolysis of one gram of acetylated fat. Figure 3 shows a general schematic representation of the hydrolysis reaction that the unsaturated moieties of fatty acids suffer during the synthesis of polyols from VO.

Synthesis of Polyurethanes
Polyurethanes (PUs) were obtained by reaction of the above-synthesized polyols with MDI and HDI.Biobased polyols and HDI or MDI (diisocyanate index 1.2, 1.4 and 1.6) were separately heated at 50 °C.Warm polyols and the corresponding isocyanate were vigorously mixed for 10 minutes and they were cast on aluminum trays and allowed to stand for 15 min.Then, trays were placed in a vacuum oven at 80 °C for 24 h to complete curing.Materials were fi nally removed from the trays and the analysis of their structure, thermal and morphological properties was carried out.

FTIR Spectroscopy
The FTIR tests were collected in transmission mode using a Bruker IFS 66/S infrared spectrometer (Bruker Analitik, Germany) equipped with an attenuated total refl ectance (ATR) device.The FTIR spectra were recorded in the 4000-500 cm −1 region, using 128 scans and 4 cm -1 of spectral resolution.A background spectrum was performed before each measurement to avoid the effect of humidity and carbon dioxide presence in the air by spectra subtraction.

Thermogravimetric Analysis (TGA)
The raw cottonseed and corn oils, as well as the biobased polyols and all the polyurethanes synthesized in this study, were analyzed by using a TGA/SDTA 851e Mettler Toledo thermal analyzer (Schwarzenbach, Switzerland).Samples were weighed in the range of 5-7 mg and were heated from 30 °C to 700 °C at 10 °C min -1 under nitrogen atmosphere (fl ow rate 50 mL min -1 ) in order to ensure the complete degradation of samples.The initial degradation temperature (T 5 ) was calculated from the TG curve as the temperature where 5 wt% of the initial mass was lost, while the temperature at the maximum degradation rate (T max ) was determined as the maximum peak from the fi rst derivative of TG curves (DTG curve).

Differential Scanning Calorimetry (DSC)
The DSC thermograms for all samples were recorded with a TA Instruments Q2000 calorimeter (New Castle, DE, USA) under nitrogen atmosphere (50 mL min -1 ).Samples (5-10 mg) were sealed in aluminum pans and heated from −90 °C to 150 °C at 10 °C min -1 , followed by quenching to −90 °C and further heating to 150 °C at 10 °C min -1 .Glass transition temperatures (T g ) were determined from the second heating scan as the infl ection point of the region where a shift in the signal baseline was detected.

Scanning Electron Microscopy (SEM)
Morphological analyses of the surface of PUs were performed with a scanning electron microscope JEOL model JSM-840 (Jeol USA Inc., Peabody, MA, USA), operated at 10 kV.Samples were previously coated with gold to turn them into conductive materials with a metallizer (Au)/Evaporator© Balzers, model SCD 004 (Oerlikon Balzers, Liechtenstein).

RESULTS AND DISCUSSION
The FTIR spectra of polyols and cottonseed and corn oils are shown in Figure 4.The spectra of raw vegetable oils showed the characteristic peak at 1740 cm -1 due to the stretching of the C=O bond, while the C=C-H stretching vibration band was observed at 3005 cm -1 .Both bands are considered as the fi ngerprint of the carboxylic acid and the straight unsaturated chain of the fatty acids, respectively [9,12].After the hydrolysis reaction, which transforms oils in polyols, a reduction in the intensity of the stretching C=C-H signals as well as the appearance of broad signals at about 3500 cm -1 , corresponding to the stretching of O-H groups, were observed.These variations in the FTIR spectra were indicative of the modifi cation of C=C-H bonds of the VO and therefore they represent the confi rmation of the successful formation of polyols.The hydroxyl number (mg KOH g -1 ), which is the most frequent way of expressing the concentration of hydroxyl groups in polyols [11,13], was also determined for both polyols, which showed high hydroxyl numbers: 216 and 165 mg KOH g -1 for corn and cottonseed oil-based polyols, respectively.These values are similar to those reported for polyols based on soybean oil, palm oil, castor oil, linseed oil and saffl ower oil, which are the vegetable oils more commonly used for the production of biobased polyols [1,7,11,13].
Once confi rmed, the successful formation of polyols and polyurethanes were synthesized with diisocyanate indexes 1.2, 1.4 and 1.6 (with both HDI and MDI). Figure 5 shows the infrared spectra of polyurethanes obtained from polyols of cottonseed and corn oils with MDI and HDI 1.2.It can be observed that the 3380-3400 cm -1 band corresponding to the stretching signal of the C-OH group in the polyols disappeared, while the appearance of the N-H urethane stretching signals at 3300 cm -1 is a clear indication of the successful reaction between biobased polyols and isocyanates.Combined stretching signals were obtained for urethane and carbonyl groups -NH-CO-O and CO-O at 1730 cm -1 and for N-H and C-N at 1540 cm -1 [14,15].The isocyanate group (-NCO) has a characteristic stretching signal at 2270 cm -1 , which could be indicative of isocyanate residues in polyurethanes and can be used to determine if the amount of isocyanate added is enough to complete the reaction with polyols [13,14].Only in the case of PUs synthesized with MDI residual isocyanate was observed, which could be attributed to the more rigid structure of MDI compared to that of HDI.This lack of fl exibility could restrict the interaction with OH moieties available in polyols, leaving some isocyanate groups unreacted [16].
Thermal stability of PUs was studied by TGA under nitrogen atmosphere.Table 1 shows the initial decomposition temperature (T 5 ) and the temperature at the maximum degradation rate (T max ) determined for polyurethanes and their corresponding corn and cottonseed-based polyols.It should be noted that urethanes are relatively unstable in their thermal behavior.Some authors indicated that the urethane bond decomposition in biobased polyurethanes starts at about 150-220 °C by training a primary amine or olefi n, resulting in the formation of secondary amines and carbon dioxide, followed by the degradation of the polyol backbone at about 400 °C [14,17].All the polyurethanes synthesized in this work showed high thermal stability, since T 5 values were about 315 °C and T max values, associated with the polyols structure, were between 420 and 460 °C (Table 1).Figure 6 shows the thermal behavior of PU synthesized with polyols from cottonseed and corn oils using MDI as crosslinker.As can be seen, both PUs show a similar thermal behavior.The high thermal stability of polyurethanes obtained from corn and cottonseed polyols could be related with the ester linkages in these polyols, as already discussed and observed by FTIR.The high thermal stability of PUs is an advantage when considering their processing window.
The T g values of both monomers were determined from the second heating scan in DSC analysis as −45.9 °C and −60.5 °C for corn and cottonseed polyols, respectively (Table 2).Only one T g value was observed for each polyurethane, indicating the success in their synthesis, without the presence of excess polyol.The T g values for PUs obtained from corn polyol and both isocyanates (HDI and MDI) were between 10 °C and 48 °C, respectively; while those obtained for PUs based on cottonseed polyol and both isocyanates were between 13 °C and 43 °C.As can be observed, all T g values for those PUs synthesized with HDI were below room temperature, while those prepared with MDI were above room temperature.These results indicate that PUs prepared with MDI have a more rigid structure than those prepared with HDI due to the rigidity of phenyl groups in MDI compared with the fl exibility of CH 2 groups in HDI [16].Furthermore, a slight effect of the polyol/isocyanate ratio was observed on these T g values.Polymer networks based on triglycerides contain dangling chains arising from the polyol.The increase in the glass transition temperature of polyurethanes with respect to those of the polyols is related to the nature of the monomer, and therefore, with the number of available hydroxyl groups in the polyols, the position of the hydroxyl group in the fatty acid chain (middle or end of the chain) and the isocyanate structure [5].Table 2 Glass transition temperatures (T g ) determined from the second heating scan in DSC analysis (10 °C min -1 ) of polyurethanes obtained from corn and cottonseed polyols.

Corn PU materials T g (°C)
Cottonseed PU materials T g (°C) The SEM micrographs of the polyurethane surfaces are shown in Figure 7.The PUs synthesized with cottonseed polyol and HDI 1.4 (a) and MDI 1.4 (b) showed irregular surfaces, but with a small quantity of pores (bubbles) in the latter case, which can be attributed to the effect of the vacuum during the curing process, since the micrographs were taken from the surface of the PU fi lms.In the case of PUs synthesized with corn polyol and HDI at 1.4 (c), irregular surfaces were observed, but no signals of the formation of pores during processing were appreciable.However, for the PU obtained with corn polyol and MDI 1.4 (d) a homogeneous and smooth surface, with no irregularities or pores, was observed.These results suggest that, in general, the polyurethanes obtained from biobased polyols are compact and nonporous materials.This apparent homogeneity could be attributed to the networks formed between isocyanates and hydroxyl groups of polyols, with no noticeable differences between all the formulations.

CONCLUSIONS
The synthesis of biobased polyols obtained from cottonseed and corn vegetable oils was successfully performed and these polyols were characterized by their main structural, thermal and morphological properties.The FTIR spectra showed the typical structure reported for polyols obtained from vegetable oils, with high hydroxyl numbers.Polyurethanes synthesized by using both types of biobased polyols and HDI and MDI showed homogeneous and nonporous morphologies with high thermal stability.The effect of diisocyanate was observed only on the T g values, indicating that PUs synthesized with MDI are harder than those synthetized with HDI.Therefore, cottonseed and corn oils could be considered as valid alternatives for the synthesis of biobased polyols with high functionalities for further use in the production of environmentally friendly polyurethanes.

Figure 1
Figure 1 Chemical structure of triglycerides of vegetable oils.

Figure 2
Figure 2 Chemical structure of unsaturated fatty acids of vegetable oils.

Figure 3
Figure 3 General modifi cation scheme of the unsaturated bonds of vegetable oils in the synthesis of polyols.

Figure 4 Figure 5
Figure 4 FTIR spectroscopy analysis of cottonseed and corn oils and their corresponding synthesized polyols.

Figure 6
Figure 6 Thermogravimetric analysis of PU obtained from cottonseed and corn polyols with MDI under nitrogen atmosphere.

Table 1
Thermal degradation parameters (T 5 and T max ) of polyurethanes obtained from corn and cottonseed polyols, determined under nitrogen atmosphere by TGA.