Synthesis of a novel glycerol based B₃-type monomer and its application in hyperbranched polyester urethane–urea coatings

Abstract

Glycerol (GLY) based hyperbranched polyester polyols (HBP) were synthesized via the reaction of GLY and succinic anhydride (SA) as a B₃-type monomer with GLY as the core moiety without using any solvent. Acid-terminated B₃ monomer and HBP were characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infra-red (FTIR) spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The degree of branching present in HBP was determined by NMR spectroscopy. The HBP was further reacted with different ratios of isophorone diisocyanate (IPDI) to obtain an isocyanate-terminated polyurethane pre-polymer, which was cured under atmospheric moisture to obtain the desired coating films. The thermo-mechanical, viscoelastic and contact angle properties of these films were also evaluated. The glass transition temperatures (T_g) of the cross-linked networks were found to be in the range of 95–125 °C, and the water contact angles were in the range of 78–82°. The T_g and hydrophobic character of the coating films were found to increase with increasing NCO : OH ratio. This study provides an effective and promising way to prepare glycerol based HBPs for obtaining high-performance coatings.

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1. Introduction

Over the recent years, sustainability is becoming increasingly important for the chemical industry; thus, polymers from renewable resources are attracting a lot of interest, especially due to the high price and limited availability of crude oil. Our research group has explored the direct utilization of glycerol to produce HBPs.^1–3 The use of glycerol for making polymers would be an efficient application for the coating industry because glycerol is the principal co-product in the transesterification process by which biodiesel is produced.⁴ Recently, HBPs have received increased attention because of their low viscosity and highly branched architecture, which can be produced at a low cost for large scale applications with a wide range of physical and chemical properties;^5,6 moreover, they can be prepared from commercially available raw materials. Therefore, HBPs have attracted considerable attention in the research communities and industries involved in developing coatings, additives, sealants, and lubricants.^7,8 Polyurethanes (PUs) are a class of polymer with unique and versatile chemistry, which have wide range of applications.⁹ Most importantly, hyperbranched polyurethanes (HBPUs) have been used in the coating industry due to their low melting viscosity, good solubility in different solvents, high density of terminal functional groups and their high glass transition temperature.¹⁰

The most common synthesis method of hyperbranched (HB) polymers in single monomer pattern is the intermolecular polymerization of AB₂ strategies^11,12 where A and B represent different functional group monomers that are intermolecularly polymerized to build HB structures. However, AB₂ monomers are expensive and have to be synthesized prior to the polymerization step.¹³ An alternative method to build HBP was proposed by Flory,¹⁴ in which he described the critical gel point for AB₂ HBP produced from AB₂ monomers. Apart from AB₂ polymerization, another route, A₂ + B₃, which involves double monomer methodology (DMM) followed by the development of HBP through this process.^15–18

Because there are many limitations such as the commercial unavailability of monomers, gelation, solubility and uncontrollable molecular weight, a new synthetic method was established, namely, couple monomer methodology (CMM). This methodology was developed by Yan and Gao^19–21 and involves strategies such as A₂ + BB₂*, A₂ + CB₂, AB + CD_n, A* + B_n and AA*B₂. The A₃ + B₃ type approach adopted in the synthesis of HBP is considerably less explored, which gives complete symmetry to the monomers and provides a better chance of the maximum formation of HBP without gelation. In the present study, we wish to establish the potential for using glycerol as a renewable resource to replace petroleum based monomers (high price monomers). Towards this achievement, an acid terminated monomer (B₃) was synthesized by the nucleophilic ring-opening addition reaction of GLY and SA, which was further reacted with GLY (A₃) monomer in a solvent-free, one-pot reaction to form HBP. Herein, we describe the synthesis as well as the thermal and mechanical properties, contact angles, and morphological properties of PU films based on HBP (Scheme 1). To develop a high-performance moisture curable coating composition, we successfully studied the effect of the NCO/OH ratio on HBP and its effect on the thermo-mechanical properties in moisture curable systems. The synthesized HBP was further reacted with isophorone diisocyanate (IPDI) at different NCO/OH equivalent ratios to obtain NCO-terminated pre-polymers. The physical and thermal properties were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical thermal analysis (DMTA), and XRD measurements. Our research efforts are focused on the development of a novel synthetic method for HBP, which can be a potential candidate for replacing or partially replacing petroleum-based PU coatings.

Scheme 1 Schematic representation of the synthesis of B₃ and HBP.

2. Experimental

2.1. Materials

Glycerol (GLY) was procured from Qualigens (India). Isophorone diisocyanate (IPDI, 98%) and succinic anhydride (SA, 99%) were supplied by Alfa Aesar (MA, USA). Tin(II) chloride (SnCl₂, ≥99.99%) and dibutyltin dilaurate (DBTDL, 95%) were obtained from Aldrich (Milwaukee, WI). All other chemicals were of analytical grade and were used without further purification.

2.2. Synthesis of acid terminated hyperbranched polyester (B₃)

The acid terminated hyperbranched polyester was synthesized in a four-necked reaction flask by charging GLY and SA in a mole ratio of 1 : 3. The reaction was maintained at 120 °C in the presence of 1 wt% of SnCl₂ as a catalyst. It was monitored using FTIR spectroscopy by collecting product samples during the reaction time at regular intervals and stopped when the anhydride peak at 1850 cm⁻¹ disappeared completely. The acid value (A.V) of the B₃-type monomer was determined according to the ASTM standard D 1639-89. A schematic representation of the synthesis of B₃ is shown in Scheme 1.

B₃: ¹H NMR (CDCl₃, δ, ppm): 2.5–2.6 (CH̲₂–COOH and CH̲₂–COO–), 4.1–4.3 (–CH̲₂–OCO–), 5.2 (–CH̲–O–CO–); ¹³C NMR (CDCl₃, δ, ppm): 28.1 (–C̲H₂–COOH & –C̲H₂–COO–), 61.7 (–C̲H₂–OCO–), 68.5 (–C̲H–O–CO–), 172.75 (–C̲OO–), 174.17 (–C̲OOH); FTIR (KBr, cm⁻¹): 3423 (s, OH str, –COOH), 1732 (s, –C=O str), 1162 (s, O=CO–C str), 1069 and 1026 (m, O=C–OH str).

2.3. Synthesis of hydroxyl terminated hyperbranched polyester (HBP)

Hydroxyl terminated hyperbranched polyester was synthesized by charging B₃ and A₃ (GLY) in a mole ratio of 1 : 3 and reaction was continued at about 150–160 °C. The eliminated water was collected using a Dean–Stark apparatus. The reaction was monitored periodically by checking the acid value through a simple titration method and stopped when the acid value was below five. The synthesized hydroxyl terminated polyester was named as HBP. The hydroxyl value of the synthesized HBP was determined according to the ASTM standard D 4274-94. A schematic representation of the synthesis of HBP is shown in Scheme 1. The product was further characterized by FTIR and ¹H and ¹³C NMR spectroscopy.

HBP: ¹H NMR (CDCl₃, δ, ppm): 2.6 (–CH̲₂–COO–), 4.03 and 4.23 (–CH̲₂–OCO–), 5.23 (–CH̲–O–CO–), 3.39 and 3.54 (–CH̲₂–OH), 3.69 and 3.92 (–CH̲–OH). ¹³C NMR (CDCl₃, δ, ppm): 28.1 (–C̲H₂–COO–), 62.08 (–C̲H2–OCO–), 69.09 (–C̲H–O–CO–), 171.34–172.03 (–C̲OO–), 62.88 and 59.75 (–C̲H₂–OH), 66.16 and 69.43 (–C̲H–OH). FTIR (KBr, cm⁻¹): 3425 (s, –OH str), 2931 and 2895 (m, –CH₂ str), 1728 (s, C=O str), 1029 (s, –C–OH str).

2.4. Synthesis of polyurethane–urea coating films

The coating films were prepared using the formulations summarized in Table 1. The synthesized HBP was further reacted with IPDI in the presence of 0.01 wt% of DBTL as catalyst at different NCO : OH equivalent ratios (1.2 : 1, 1.4 : 1, 1.6 : 1 and 2 : 1) at around 70–80 °C to obtain different –NCO terminated pre-polymers. The –NCO terminated hyperbranched PU pre-polymers (HBPPU) prepared above were used further for the preparation of the final coatings. The –NCO terminated pre-polymer films were casted on a tin foil supported on a glass plate using a manual driven square applicator. The films were placed in the open atmosphere under laboratory humidity conditions for a period of 15 days to obtain the moisture cured HBPPU–urea films. They were removed from the glass plate and the free films of the coatings were obtained using an amalgamation technique. The disappearance of the –NCO peak at 2270 cm⁻¹ in the FTIR spectrum was considered as a measure of complete cure. The different formulations and mole ratios are shown in Table 1 and a schematic representation of the formation of poly(urethane–urea) is shown in Scheme 2.

Table 1 Different formulations and mole ratios

Sample code	NCO : OH (IPDI)
HBPPU-2	2 : 1
HBPPU-1.6	1.6 : 1
HBPPU-1.4	1.4 : 1
HBPPU-1.2	1.2 : 1

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Scheme 2 Schematic representation for the formation of poly(urethane–urea) from HBP.

3. Characterization

Characterization of the polymers was carried out using gel permeation chromatography (GPC: C-R4A Chrotopac; Shimadzu, Kyoto, Japan). The samples were dissolved in THF used at the ratio of 0.1 g/10 mL, and the experiments were carried out at a flow rate of 1.0 mL min⁻¹ using THF as the mobile phase. Columns were calibrated using polystyrene standards obtained from Aldrich. The structures of the polymers and coating films were characterized using Fourier transform infrared (FTIR) spectroscopy on a Thermo Nicolet Nexus 670 spectrometer. Each sample was scanned 128 times with a resolution of 4 cm⁻¹ and averaged to obtain a spectrum. All the spectra were scanned within the range of 400–4000 cm⁻¹. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded in CDCl₃ solution using a Varian-Inova-500 MHz spectrometer. Chemical shifts (δ) are given in ppm with tetramethyl silane as a standard. Thermal analysis was carried out on a dynamic mechanical thermal analysis (DMTA) IV instrument (Rheometric Scientific, USA) in tensile mode at a frequency of 1 Hz with a heating rate of 3 °C min⁻¹ by scanning the films from 27 °C to 200 °C. Storage modulus (E′) and tan δ as a function of temperature at a constant frequency were observed. Chemical resistance of the films was studied using ASTM D543-67 (1972) method. Thermogravimetric analysis (TGA) measurements were performed on a TA Q500 (TA Instruments, Inc.) at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere. The weight of the samples was varied from 5 to 10 mg. Differential scanning calorimetry (DSC) analysis was recorded on a Mettler Toledo DSC 821^e, Switzerland. The samples were heated from −70 °C to 200 °C at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere at a flow rate of 30 mL min⁻¹. The viscosity of the polymers was determined using a Haake rotational viscometer 2.1 system M5/SV2 (HAAKE, Germany). Contact angle was measured on a G10 (KRUSS) instrument using the sessile drop method. Powder X-ray diffraction patterns (XRD) of the coating films were recorded at different diffraction angles (2θ) using a Siemens/D-5000 diffractometer with CuKα radiation (λ = 1.5406 Å).

4. Results and discussion

4.1. Characterization of B₃ and HBP

Fourier transform infrared (FTIR) spectroscopy was used to characterize the chemical structure of the polymers. The progress of the reaction between GLY and SA in the presence of SnCl₂ as catalyst at 120 °C was monitored by FTIR spectroscopy and the resulting final spectrum is shown in Fig. 1. In the spectrum, we can observe the disappearance of the anhydride peak between 1779–1850 cm⁻¹ and the appearance of a strong ester carbonyl peak at 1723 cm⁻¹, which confirms the completion of the reaction. The presence of the ester group was reconfirmed by the appearance of a peak at 1280 cm⁻¹ due to the –(CO)–O–C– stretching mode, and the appearance of the C–O acid and C–O–C ester stretching peaks at 1727 and 1280 cm⁻¹, respectively, indicates the completion of the reaction between GLY and SA. The FTIR spectra of the synthesized B₃ and HBP are shown in Fig. 1. In HBP spectrum, a broad peak at 3425 cm⁻¹ and a sharp peak at 1754 cm⁻¹ were attributed to the –OH stretching absorbance and the –C=O group of the ester linkages, respectively.²² The broad peak of –OH stretching band at about 3425 cm⁻¹ was due to the combined effect of the differently associated hydroxyl groups, i.e., intra/inter molecular hydrogen bonding between the different –OH groups or between the –OH and –C=O groups. The structures of B₃ and HBP were further supported by ¹H and ¹³C NMR spectroscopy. The ¹H and ¹³C NMR spectra of both B₃ and HBP are shown in Fig. 2 and 3, respectively. Dendritic and hyperbranched units exhibited different chemical shift values based on their environment in both ¹H and ¹³C NMR, and the values are given in the Table 2. All these values are consistent with the previously reported studies as well.^23,24 To correlate the units of HBP and describe the structure of HBP quantitatively, Frechet and co-workers were first to propose an equation for determining the degree of branching (DB), which is displayed as eqn (1).

DB = (no. of dendritic units) + (no. of terminal units)/total no. of units

DB = (D + T)/(D + T + L) (1)

where D is the total number of dendritic units, T is the total number of terminal units, and L is the total number of linear units. D units are tri-substituted GLY units, while two different L units as di-esterified GLY monomers, either in a 1,2-(L_1,2) or a 1,3-connection (L_1,3) are to be expected. As far as the T units are concerned, they can be of three different types, that is, mono-substituted GLY, either with two primary (T_1,3) or one primary and one secondary OH-group (T_1,2), with carboxylic acid end groups (TA).¹³ However, NMR spectroscopy was used to determine how many GLY units reacted and at which of the primary or secondary alcohol positions the diacids are located. Therefore, it characterized the types of branching patterns and measured their relative occurrence. This information can be seen in Fig. 4 and data is summarized in Table 2 as the degree of branching ratio (DB%). The DB values for HBP, calculated according to eqn (1), from ¹H and ¹³C NMR spectroscopy are 60.6% and 66.0%, respectively.

Fig. 1 FTIR spectra of B₃ and HBP.

Fig. 2 ¹H NMR spectra of B₃ and HBP.

Fig. 3 ¹³C NMR spectra of B₃ and HBP.

Table 2 ¹H and ¹³C NMR resonance assignments for different glycerol branching patterns

	Dendritic unit (D)	Linear unit (L)		Terminal unit (T)		Degree of branching (DB)%
		L1,2	L1,3	T1,2	T1,3
Ha	4.23	4.21	4.04	4.03	3.53	60.6
Hb	5.23	4.99	3.92	3.69	4.77
Hc	4.23	3.54	4.04	3.39	3.52
Ca	62.08	62.49	65.16	65.93	59.75	66.0
Cb	69.09	72.35	66.16	69.43	75.96
Cc	62.08	59.45	65.16	62.88	59.75
Cx=O	171.66	171.91	171.91	172.03	—
Cy=O	171.41	171.34	—	—	NA
Cz=O	171.66	—	171.91	—	—

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Fig. 4 Degree of branching explained by ¹H and ¹³C NMR spectroscopy.

4.2. Molecular weight, solubility, viscosity and thermal properties of B₃ and HBP

The solubility results of HBP are compiled in Table 1S in the ESI.† The solubility behavior of the polymer prepared in this study was determined in excess solvents at room temperature for 24 h. The molecular weight and molecular weight distribution of the B₃ and HBP polymers were monitored using GPC technique. From the GPC technique, it was found that the average molecular weight of HBP (M_w 1363) was slightly higher in comparison to the molecular weight of B₃ (M_w 1181), and because the hydroxyl groups were incorporated into B₃, an increase of the M_W was expected. The viscosity is an important fluid property to study the rheological behavior of the polymers. Fig. 5 illustrates the viscosity (η) variation as a function of shear rate (γ) for the four different temperatures studied (30, 40, 50, and 60 °C). The HBP shows Newtonian shear characteristics with the variation of temperature ranging from 30 to 60 °C, i.e. the viscosity is dependent on the shear rate. As can be seen in Fig. 5, HBP exhibited the same viscosity pattern over the temperature range studied, which was a non-linear decrease in viscosity with increasing temperature. The viscosities of HBP at different temperatures with a shear rate of 23.3 s⁻¹ were as follows: 30 °C, 0.843 Pa S; 40 °C, 0.497 Pa S; 50 °C, 0.224 Pa S; and 60 °C, 0.122 Pa S. This temperature effect on the viscosity of the samples has been attributed to the decreased intermolecular interactions due to great thermal molecular movement. The thermal transitions of B₃ and HBP like the glass transition temperature (−30.8 °C and −32.2 °C for B₃ and HBP, respectively) were determined using DSC and are shown in Fig. 1S in the ESI.† To study the influence of the chemical structure on the glass transition temperature of a polymer, the phenomenon should be examined in these polymers. Generally, it is said that the T_g is directly proportional to the average molecular weight of the polymer. In our case, the results are consistent with this statement. The structural variation in the B₃ backbone was due to the introduction of the hydroxyl groups with the addition of GLY, and it significantly affected the glass transition temperature. In addition, the acid value and hydroxyl value of B₃ and HBP were measured using the aforementioned ASTM standards and were almost consistent with the theoretical values (B₃: theoretical acid value – 429, experimental acid value – 420; for HBP: theoretical hydroxyl value – 547 and experimental hydroxyl value −535). The theoretical values for B₃ and HBP were calculated from their expected structures and also according to the mole ratio of their reactants. TGA was used to measure a variety of polymeric phenomenon such as weight changes, sorption of gases, desorption of contaminants, and degradation. The thermal degradation study of B₃ and HBP were carried out in a N₂ environment at a heating rate of 10 °C min⁻¹. In the TGA thermograms for B₃ and HBP, two-step decomposition profiles were observed, as shown in Fig. 6. The initial degradation of the B₃ may be attributed to the loss of carboxylic groups, whereas in HBP, it was due to the degradation of the peripheral glycerol moiety. It was noticed from the TGA profile that the thermal stability of B₃ was higher than that of HBP. The main decomposition of the samples takes place in the second stage of degradation, i.e., above 370 °C. The values of T_ON (initial decomposition temperature for the degradation step), T_end (final decomposition temperature for the degradation step) and % weight remaining at 200 °C, 300 °C, 400 °C and 500 °C are summarized in Table 3.

Fig. 5 Rheology curves for HBP.

Fig. 6 TGA curves for B₃ and HBP.

Table 3 Thermal analysis data for B₃ and HBP

Sample	Tid (°C)	Tmax (°C)	Tdf (°C)	Weight percentage at
				200 °C	300 °C	400 °C	500 °C
B3	192.2	374.6	416.2	79.68	51.4	10.93	7.64
HBP	238.4	380.2	421.8	80.89	59.92	13.53	8.98

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4.3. FTIR analysis of PU films

The FTIR spectra of the different HBPPU films are shown in Fig. 2S in the ESI.† The isocyanate terminated HBPPU pre-polymer was characterized by FTIR spectroscopy, which shows that the characteristic absorption band at 2270 cm⁻¹ due to the existence of the isocyanate groups completely disappeared after the moisture curing. Moreover, the characteristic bands of urethane –N–H stretching at 3500 cm⁻¹, a combination of urethane carbonyl –NH–CO–O and esteric carbonyl –CO–O at 1742 cm⁻¹, and a combination of –N–H out-of-plane bending and –C–N stretching at 1532 cm⁻¹ were present in the spectra. In addition, the disappearance of the absorption bands at 3400 and 1412 cm⁻¹ confirmed the absence of free hydroxyl groups in the HBPPU pre-polymer films.

4.4. DMTA analysis

DMTA measures the deformation of a material in response to oscillating forces and this technique is used to detect the viscoelastic behavior of polymeric materials and yields quantitative results for the tensile storage modulus E′ and the corresponding loss modulus E′′. The loss factor tan δ can then be expressed as the quotient of loss and storage, E′′/E′. E′ and E′′ characterize the elastic and viscous components of a material under deformation; E′ is a measure of the mechanical energy stored under load. tan δ compares the amounts of dissipated and stored energy. The T_g values for the coating films are obtained from the peaks in the tan δ curves. The E′ values in the rubbery region at T > T_g are taken to calculate the crosslink density (υ_e) using eqn (2).^25,26

υ_e = E′/3RT (2)

where R is the universal gas constant and T is the temperature in K (T > T_g).

To observe the effect of NCO : OH ratio on the dynamic mechanical properties, the E′ and tan δ temperature curves for the representative coating films are shown in Fig. 7 and 8 respectively, while the data is reported in Table 4. The T_g values for HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 are 124.9, 103.2, 97.76 and 95.56 °C, respectively. The E′ values for HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 coating films are 2.003 × 10⁹, 1.362 × 10⁹, 1.003 × 10⁹ and 1.479 × 10⁸, respectively, at 40 °C. Usually, hard materials have a modulus of 10⁹–10¹⁰ Pa and rubbery materials have a modulus of 10⁶ Pa.²⁷ This indicates the good mechanical integrity of the coatings. The data suggests that the material stiffness, T_g, and crosslinked density of the HBPPU coatings increase with an increase in the NCO/OH ratio. This may be due to the formation of more urethane/urea segments in the matrix with an increasing NCO/OH ratio, which restricts the chain mobility through hydrogen bonding.^28,29

Fig. 7 E′ vs. temperature plot for PU films.

Fig. 8 tan δ vs. temperature plot for PU films.

Table 4 DMTA data for PU films

Sample code	Tg (°C, DSC)	Tg (°C)	E′ at 40 °C [Pa]	tan δ	E′ at Tg + 5 °C [Pa]	υe (Tg + 5 °C) (mole cm^−3)
HBPPU-2	117.65	124.9	2.003 × 10^9	0.7	5.131 × 10^7	5.172 × 10^−3
HBPPU-1.6	92.08	103.2	1.362 × 10^9	0.86	3.639 × 10^7	3.88 × 10^−3
HBPPU-1.4	91.14	97.76	1.003 × 10^9	0.87	2.755 × 10^7	2.98 × 10^−3
HBPPU-1.2	85.32	95.56	1.479 × 10^8	0.66	3.816 × 10^6	4.153 × 10^−4

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4.5. Chemical resistance

The percentage weight loss of the PU films were calculated in sulphuric acid, hydrochloric acid, sodium hydroxide, sodium chloride, toluene, chloroform, methyl ethyl ketone (MEK) and distilled water. The PU films were suspended in various chemical environments for 7 days and tested for changes in weight and the corresponding data is reported in Table 5.

Table 5 Chemical resistance (in percentage weight loss) of the PU films

Chemical	HBPPU-2	HBPPU-1.6	HBPPU-1.4	HBPPU-1.2
25% H2SO4	1.660	1.665	1.749	1.875
25% HCl	1.421	1.477	1.652	1.639
10% NaOH	0.512	0.564	0.719	0.655
10% NaCl	0.575	0.650	0.865	1.094
MEK	0.781	0.833	1.230	1.554
CHCl3	2.658	2.718	2.701	2.662
Toluene	1.145	1.476	1.619	2.052
Water	1.734	1.660	0.860	0.653

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4.6. TGA analysis

The thermal stability of the HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 coating films was evaluated from the TGA data derived from thermogravimetric analysis under a N₂ environment. The TGA curves for the coating films are shown in Fig. 9. The values of T_ON (initial decomposition temperature for the degradation step), T_max (maximum decomposition temperature for the degradation step), T_end (final decomposition temperature for the degradation step) and % weight loss temperature at 20%, 50% and 90% are summarized in Table 6. All the coating films show a three step degradation profile. The data in Table 6 shows that the thermal stability of the coating films was increased with increasing NCO : OH ratio. For instance, the maximum degradation temperature and 50% weight loss temperature for the coating films HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 were 311.5, 309.2, 308.9, 307.2 °C and 316.8, 308, 307.6, 305.1 °C, respectively. This trend indicates the formation of more cross-linked and hydrogen-bonded structures at higher NCO/OH ratios. More crosslinking and hydrogen bonding brings the polymer backbones closer and thus reduces the molecular mobility and also increases the thermal stability. This behavior supports the increase in T_g for the samples with an increase in NCO/OH ratio in DMTA analysis.^30,31

Fig. 9 TGA curves for PU films.

Table 6 Thermal analysis data for PU films

Sample code	Tid (°C)	Tmax (°C)	Tdf (°C)	Wt% loss temperature (°C)
				20%	50%	90%
HBBPU-1.2	257.8	307.2	417.2	273.0	305.1	368.6
HBBPU-1.4	262.0	308.9	429.0	274.4	307.6	371.5
HBBPU-1.6	267.2	309.2	436.8	275.8	308.0	381.5
HBBPU-2	272.9	311.5	438.5	281.8	316.8	384.5

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4.7. DSC analysis

The glass transition temperature (T_g) properties of the coating films were studied using DSC, and the results are displayed in Fig. 10 and reported in Table 4. The glass transition temperatures for the HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 coatings are 117.65, 92.08, 91.14 and 85.32 °C, respectively. Generally, the DMTA technique provides T_g values higher than DSC due to the dynamic nature of the test.³² The glass transition temperature increases with increasing NCO : OH ratio; thus, it appears reasonable to assume that the mobility of the organic polyurethane chains was greatly restricted by hydrogen bonding. The effect may be observed due to the high cross-linking density of the films upon increasing the cross-linker (IPDI), which restricts the segmental motion of the polymer chains and increases the glass transition temperature.

Fig. 10 DSC curves for PU films.

4.8. Contact angle

The hydrophobic nature of the HBPPU-2, HBPPU-1.6, HBPPU-1.4 and HBPPU-1.2 coating films was evaluated by measuring the contact angle (CA). The water CA was in the range of 82°–78°. The CA data of the coating films is as follows: HBPPU-2, 82.2°; HBPPU-1.6, 81.3°; HBPPU-1.4, 79.8° and HBPPU-1.2, 78.1°. Thus, the improvement in the hydrophobic capacity of the coating films upon increasing the NCO : OH ratio was confirmed. The results show that the higher CA was observed for the formulation of HBPPU-2 coating film among the other coating films.

4.9. XRD analysis

Fig. 11 shows the X-ray diffraction curve for the PU–urea coating films. The X-ray diffraction pattern of the PU–urea films depends on the NCO : OH ratios and alters the symmetry and regularity of the polymer matrix, and therefore increases its crystalline character. All the diffractograms contain a broad peak at around 18° (2θ angle), which suggests that the diffraction is mostly due to an amorphous polymer region. The higher broad peak was obtained for the HBPPU-1.2 sample, but the intensity of the broad peak around 18° gets decreased in the case of HBPPU-2. This might be due to the formation of more polar urea groups in the structure and the formation of more inter chain hydrogen bonding between the macromolecular chains, which can induce disorganization.³³

Fig. 11 XRD patterns for PU films.

5. Conclusions

The present study describes the synthesis of HBP based on a renewable resource to obtain a pre-polymer with terminal hydroxyl groups and to develop poly(urethane–urea) coatings. For this purpose, the synthetic strategy (B₃ + A₃) proposed here was convenient and effective for HBP synthesis to produce highly branched polyester polyol with the degree of branching almost equal to 66%, which was further reacted with different ratios of isophorone diisocyanate to obtain an isocyanate terminated polyurethane pre-polymer. The excess of isocyanate in the pre-polymers was cured under atmospheric moisture to obtain the polyurethane/urea coatings. B₃ and HBP were characterized by ¹H NMR, ¹³C NMR, and FTIR spectroscopy as well as thermal techniques. The coatings were studied for their thermal, surface, and viscoelastic properties using different techniques. The coating properties such as contact angle were also determined. The contact angle was directly dependent on the NCO : OH ratio. The hydrophobic character of the coating films was found to increase with increasing NCO/OH ratio. The DMTA result suggests that the T_g and crosslinking density of the different HBPPU coatings increase with an increase in the NCO/OH ratio. The overall results will broaden the scope of coating formulations to design renewable-based HBP polymers with thermally resistant and tough moisture curable coating composition with an appropriate choice of NCO/OH ratio.

Acknowledgements

Varaprasad Somisetti and Shaik Allauddin would like to thank the University Grants Commission (UGC) and Council of Scientific and Industrial Research (CSIR), New Delhi, India, for their research fellowship. The present study was carried out under the Intel Coat (CSC-0114) project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11712g

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