Synthesis and properties of foams from a blend of vegetable oil based polyhydroxyurethane and epoxy resin

Abstract

This article aims to highlight the synthesis of foams from a blend of hydroxyurethane of castor oil and epoxy resin. An epoxidized castor oil of 4% oxirane oxygen was first converted to cyclic carbonate of castor oil at 120°C, 1 atm CO₂ pressure and then it was reacted with three different aliphatic diamines to yield amine terminated Polyhydroxyurethane (PHU). Foams were prepared in a metal mould from the blend of PHU, epoxy resin, epoxy hardener and polymethylhydrogensiloxane blowing agent which releases hydrogen gas upon reaction with amine. FTIR and ¹H NMR of cyclic carbonate of castor oil and PHU of castor oil were done to confirm their chemical structures. Optical microscopy and scanning electron microscopy of foams was done to assess their cellular morphology along with DSC and TGA to evaluate their thermal properties. Both flexible and rigid type of foams were synthesised in this study. Resilience of flexible foams was inspected using a ball rebound test and compression-recovery test while thermal insulation property was checked by measuring thermal conductivity, thermal diffusivity and R-values of rigid foams from heat transfer study using a heat transfer apparatus.

Keywords

Polyhydroxyurethane cyclic carbonate of castor oil epoxy resin blowing agent

Introduction

Polymer industry was boosted in the 20th century not just with the advent of world war but also with the catapulting emergence of technology. In 1937, it was Otto Bayer et al.¹ who first synthesised and introduced to the world, polyurethane, from polyols and isocyanates in an attempt to replace natural rubber. Since then, polyurethane have been commercially synthesised by a polyaddition reaction between di/poly-isocyanates and diols/polyols.² Polyurethane exhibits a wide range of mechanical properties such as flexibility, rigidity or elasticity, good damping properties and also resistance to weathering, abrasion and impact.^3–5 Owing to these peculiar characteristics PU applications cover foams (67%) and elastomers (6%),² adhesives and sealants (6%),^6,7 coatings (3%)⁸ and others (18%).⁹ Production of polyurethane is done by various isocyanates such as methylene diphenyl diisocyanate (MDI),¹⁰ toluene diisocyanate (TDI),¹¹ hexamethylene diisocyanate (HDI),^12,13 isophorone diisocyanate^14–16 and methylene bis-(4-cyclohexyl-isocyanate).¹⁷ However, it is a disappointing fact that most of these isocyanates contribute to certain acute and chronic ailments which is often overlooked especially when it comes to their commercial use in synthesis of polyurethane. Kapp¹⁸ proposed toxicological studies on various isocyanates and reported that under mild or prolonged exposure to these chemicals a person is likely to acquire symptoms or ailment from any of the following: asthma, dermatitis, eczema, skin and respiratory sensitization, pulmonary oedema, coughing and respiratory distress, immune sensitization, infertility, foeticide, eye irritation and more. Moreover, down the decades researchers have assessed the adverse effects of TDI and MDI on human health^19–21 and concluded that their prolonged exposure may become the concern of causing tumours in human organs and eventually cancer. Therefore, the European Council in the Commission Regulation for Chemicals deemed these diisocyanates to be Carcinogenic, Mutagenic and Reprotoxic. In the latest edition, COMMISSION REGULATION (EU) 2020/1149 of 3 August 2020,²² some serious restrictions were imposed on the use of diisocyanates in terms of concentration to be used, training and safety protocols to be followed and marketing with adequate information.

In order to overcome the toxicity related issue in conventional isocyanate-based polyurethane, a new pathway has emerged that completely discard the use of isocyanates designated as Non-isocyanate Polyurethane (NIPU). This pathway has been practised a lot recently as it utilizes a simple polyaddition reaction between a 5-membered cyclic carbonate and an amine to yield polyurethane with a secondary hydroxyl group in the main chain, hence also called as polyhydroxyurethane.^23–36 Although amines are readily available substances, synthesis of cyclic carbonates correspond to key aspect of a methodical approach toward PHU. So far in literature synthesis of cyclic carbonates have been carried out from 1,2-diols,^37,38 olefins,³⁹ halohydrins,⁴⁰ propargylic alcohols,^41,42 urea^43–45 and linear carbonates.^46–48 However, the most cheap, convenient and preferred route to obtain cyclic carbonates has been attributed to the chemical fixation of CO₂ into epoxides.^{33–36,49–55}

Majority of polyurethane applications are covered by flexible foams for mattresses and cushions, semi-rigid and rigid foams for thermal insulation. Production of polyurethane foams is done by making use of physical blowing agents like hydrocarbons (HC’s), hydrofluorocarbons (HFC’s), hydrochlorofluorocarbons (HCFC’s), and chemical blowing agents such as dialkyl carbonates, azo compounds and peroxides.⁵⁶ Chemical blowing agents decompose to produce gas that blows the polymer matrix whereas physical blowing agents expand quickly at foaming temperature as a result of vaporization. Unfortunately, during NIPU synthesis no such blowing agent is produced in any step or reaction. Therefore, requirement here is of a chemical species which is non-toxic, stable and which will produce enough gas to blow the polymer matrix to an appreciable volume. Polymethylhydrogensiloxane has been used as a blowing agent in literature for synthesis of foams especially in the system where amines are primary reactants.^57–62 It undergoes a reaction with amines to yield hydrogen gas.

Researchers worldwide have reported the synthesis of non-isocyanate polyurethane foams from cyclic carbonates and amines as starting materials. Cornille et al.^61,62 synthesized NIPU foam using cyclic carbonates, diamines and polymethylhydrogensiloxane as blowing agent. While Figovsky et al.⁶³ for the first time patented the composition of a sprayable NIPU foam, Lauth et al.⁶⁴ patented NIPU foams from a carbonate blend, hexamethylene diamine and HFC’s as a blowing agent. Also, Grignard et al.⁶⁵ reported NIPU foams of low density by utilizing supercritical CO₂ as a foaming medium. Glucose-based NIPU foams from linear carbonates^66,67 along with sorbitol and lysine based NIPU foams⁶⁸ have also been reported recently in literature.

The biggest limitation of NIPU technology is the low reactivity between cyclic carbonate and amine which imparts inferior properties to the polymer such as low molecular weight and poor performance. In order to circumvent these challenges hybrid NIPU materials have been introduced constituting polyhydroxyurethane, epoxy and/or acrylic as major components.^54,69–71 Cornille et al.⁷² prepared epoxy-urethane polymers of high molecular weight (1,200–3,000 g mol⁻¹) from oligo polyhydroxyurethane. In this article, cyclic carbonate of castor oil and aliphatic diamines have been used to prepare PHU based foams. Due to limitation of low reactivity and poor conversion reaction of CCO, synthesis of foams from PHU of castor oil alone was far from possible. Therefore, commercial epoxy/hardener system was used and it was blended with PHU as a supporting medium.⁷³ Amine terminated PHU of castor oil was prepared from reaction between cyclic-carbonate of castor oil (CCO) and various diamines such as ethylene diamine, hexamethylene diamine and diethylene triamine. The blend of ingredients was blown in a metal mould using polymethylhydrogensiloxane as a chemical blowing agent to obtain foams of different formulations.

Experimental

Materials

Epoxidized castor oil (ECO) (4% oxirane oxygen, MW∼980 g mol⁻¹) was purchased from Jayant Agro-Organics Ltd. Mumbai, India. Ethylene diamine (EDA), diethylene triamine (DETA), tetrabutylammonium bromide (TBAB) (catalyst for carbonation of oil) and sodium sulphate (used for washing the carbonated oil) were purchased from Amrut Chemicals Mumbai, India. Hexamethylene diamine (HMDA) was purchased from Thermo Fischer Ltd. India. Commercial epoxy resin (EEW∼190 g eq⁻¹) and its hardener was obtained from Bhor Bond Ltd. India. 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) (catalyst for synthesis of PHU of castor oil) was purchased from Avra Chemicals Ltd. Bangalore, India. Polymethylhydrogensiloxane (PMHSO) (MW∼1900 g mol⁻¹) blowing agent was purchased from Alfa Aesar Ltd. India. Deionized water was used for product washing purposes. All chemicals were used as such without any further purification.

Synthesis of cyclic-carbonate of castor oil

Epoxidized castor oil and TBAB catalyst (0.035 mol per mole of epoxy group³⁵) was charged into a glass reactor and heated to dissolve the catalyst. The carbonation reaction of oil was carried out by passing CO₂ gas for 70 h at 120°C and 1 atm. For every 5 h, oil samples were withdrawn and checked for their reduction in oxirane value by HBr in acetic acid titration method as per ASTM D 1652. The reaction was stopped after a constant value was obtained as shown in Figure 1. The product obtained was washed twice with DI water and sodium sulphate solution to remove the catalyst. The reaction pathway for this step is shown in Scheme 1. The yield of carbonation reaction of epoxidized castor oil was estimated from equation (1).

% Yield = \frac{{OV}_{2} - {OV}_{1}}{{OV}_{1}} x 100

(1)

where OV₁ = oxirane value of oil at the start of carbonation step, OV₂ = oxirane value of oil at the end of carbonation step.

Figure 1.

Reduction in oxirane value of oil with reaction time.

Scheme 1.

5-membered cyclic-carbonate from epoxidized castor oil.

The lowest oxirane value was 1.3 at the end of carbonation step (Figure 1), hence from equation (1), % yield = (4–1.3/4) × 100 = 67.5%.

Carbonate equivalent weight (CEW) of CCO was determined by integrating the carbonate peak in ¹H NMR spectra by using trimethylsilane standard.

AHEW of EDA, HMDA and DETA were calculated from equation (2).

AHEW = \frac{Molecular weight of amine}{number of active hydrogen}

(2)

CEW and AHEW were used in setting the molar ratio for synthesis of PHU of castor oil.

Synthesis of amine terminated PHU of castor oil

CCO and DBU catalyst (0.5 mol% w.r.t CCO) were charged into a round bottom flask equipped with a stirrer, reflux condenser and temperature controller. After a few minutes of stirring amine was added and the reaction was proceeded at 80°C under nitrogen atmosphere. The molar ratio of CCO and amine was kept at 1:1 calculated from CEW and AHEW respectively. Amine value of the product was determined according to ASTM D 2074 and reaction was continued until a constant amine value of the product was obtained (Table 1). In the end a dark brown viscous product was obtained. The product obtained by reaction between CCO and ethylene diamine was named as PHU-EDA. Similarly, PHU synthesized from other amines, i.e. hexamethylene diamine and diethylene triamine were named as PHU-HMDA and PHU-DETA respectively. The reaction pathway for this step is shown in Scheme 2.

Table 1.

Reduction in amine value of PHU with reaction time.

PHU of castor oil	Amine value (mg KOH/g) with reaction time
PHU of castor oil	0 h	2 h	4 h	6 h	8 h
PHU-EDA	1,760	1,238	779	342	152
PHU-HMDA	932	603	320	135	108
PHU-DETA	1,582	1,008	657	361	129

Scheme 2.

Amine terminated PHU from cyclic carbonate of castor oil.

Synthesis of foams

All foams were synthesized in a metal mould according to the following procedure: PHU and epoxy resin was taken in the mould and stirred mechanically at 2,000 r/min for 1 min. Then epoxy hardener was added to this mixture and stirred at 1,500 r/min for 2 min till a homogeneous mixture was obtained. Finally, polymethylhydrogensiloxane was added and stirred at 3,000 r/min for 1 min. PMHSO reacts with amine to yield hydrogen gas as shown in Scheme 3. According to technical datasheet, 1 g of PMHSO react with amine to release approximately 350 mL of hydrogen gas therefore a fixed amount of PMHSO was used in all formulations in order to avoid overblowing and rupture of foams. When mixture started to turn creamy stirring was stopped and the mould was kept undisturbed on a flat surface and placed inside the oven where foaming and curing took place at 90°C for 1 h. Foams obtained in this study were named according to the amine used. Table 2 shows the formulation of all foams synthesized in this work.

Table 2.

Formulation of foams (quantities in parts by weight).

Foam	PHU: epoxy	Epoxy: hardener	PMHSO^a	Cream time (s)	Foam time (s)
PHU-EDA-F1	4:1	100:45	0.05	53	126
PHU-EDA-F2	3:2	100:45	0.05	52	133
PHU-EDA-F3	2:3	100:45	0.05	55	130
PHU-EDA-F4	1.5:3.5	100:45	0.05	54	128
PHU-HMDA-F1	4:1	100:45	0.05	51	126
PHU-HMDA-F2	3:2	100:45	0.05	53	122
PHU-HMDA-F3	2:3	100:45	0.05	55	125
PHU-HMDA-F4	1.5:3.5	100:45	0.05	58	124
PHU-DETA-F1	4:1	100:45	0.05	57	118
PHU-DETA-F2	3:2	100:45	0.05	59	119
PHU-DETA-F3	2:3	100:45	0.05	63	120
PHU-DETA-F4	1.5:3.5	100:45	0.05	68	121

^aWeight percentage of PMHSO with respect to amine.

Scheme 3.

Reaction of PMHSO with amine to yield hydrogen gas.

Swell index and gel content

Swell index of foam samples was calculated as follows: Foams were immersed in 30 mL THF for 24 h. Weight of the samples measured before (w₁) and after (w₂) immersion was noted and swell index (%) was obtained from equation 3,

Swell index (%) = \frac{w_{2} - w_{1}}{w_{1}} × 100

(3)

Gel content of foams was determined as follows: Foam samples were dried in oven at 60°C for 24 h. Weight of samples taken before (m₁) and after (m₂) drying was used to calculate gel content (%) from equation 4,

Gel content (%) = \frac{m_{2}}{m_{1}} x 100

(4)

Density of foams

Density of foams was obtained by measuring dimensions of foams with the help of vernier calliper. Volume was obtained from dimensions and with known mass density was calculated from equation 5,

Density = \frac{m}{V}

(5)

Nuclear magnetic resonance spectroscopy

Chemical structures of cyclic carbonate of castor oil and PHU of castor oil was studied under ¹H NMR spectroscopy using a SA-Varian 400 MHz NMR spectrometer equipped with a OneNMR pulse-field gradient probe at room temperature. CDCL₃ was used as solvent and trimethylsilane (TMS) as external reference.

Fourier transform infrared spectroscopy

Chemical structures of products were determined by Infrared Spectroscopy on Bruker instrument in the ATR (attenuated total reflectance) mode from 700 cm⁻¹ to 4,000 cm⁻¹ wavelength range with 100% transmittance.

Cellular morphology of foams

Internal cellular structure, cell size and cell compactness of the foams was studied by optical microscopy and scanning electron microscopy at various magnifications. Olympus BX41 polarized optical microscope (POM) and Hitachi N-3400 scanning electron microscope (SEM) were used.

Thermogravimetric analysis

TGA study was done to determine thermal stability of foams by using a Perkin Elmer Pyris 1 thermogravimetric analyser. Heating was done in the range of 30–600°C with 20 mL/min nitrogen purge at a heating rate of 10°C min⁻¹.

Differential scanning calorimetry

Thermal properties of foams were studied by employing TA Q100 DSC analyzer to determine T_g where heating and cooling cycles were recorded between −40 and 100°C under nitrogen atmosphere at heating and cooling rate of 5°C min⁻¹. Temperature versus heat flow was plotted and T_g of the foams were determined accordingly.

Thermal conductivity

Thermal conductivity of rigid foams synthesized from DETA was determined by Lee’s Disc experiment. The experimental setup consisted of circular brass metal disc, brass steam chamber, steam source and thermometers. Foam sample having 115 mm diameter and 2–3 mm thickness was kept between steam chamber at the top and metal disc at the bottom where steam chamber was equipped with inlet and outlet for steam. Two thermometers were inserted through holes, one in steam chamber and one in metal disc. Steam was then passed through the chamber and sample until the temperature indicated by thermometers became steady. Then steam chamber was removed and the bottom metal disc with sample kept over it was heated to 10°C above the noted steady temperature. Reduction in temperature with time was observed till the temperature of the bottom disc fell 10°C below the steady state temperature. Lastly time-temperature graph was plotted (Figure 10) and thermal conductivity of foam was determined by equation 6,

λ = \frac{m s (\frac{dT}{dt}) x}{A (θ_{1} - θ_{2})}

(6)

Where λ is thermal conductivity of foam, m is mass, s is specific heat of metal disc, $dT / dt$ is the slope of time-temperature graph, x is thickness of sample, A is area of sample, Ɵ₁ is steady state temperature of steam chamber and Ɵ₂ is steady state temperature of bottom metal disc.

Thermal diffusivity is related to thermal conductivity and it was calculated using equation (7).

α = \frac{λ}{ρ Cp}

(7)

Compression and recovery study

The mechanical compressions of the foam samples were done on a UTM with compression holders using 20 mm × 12 mm × 10 mm foam slabs. A maximum force of 40 N with application time of 200 s and a maximum of 80% of strain was applied to the sample. Time of recovery of foam sample after removal of load was measured in seconds.

Resilience by ball rebound test

Resilience of foam was measured by the ball-rebound test according to a standard method. In this test, a steel ball weighing 16 g and 10 mm diameter was dropped from 30 cm height on the foam slab of 70 mm × 30 mm × 15 mm and the rebound height was then measured in percentage compared to the height of drop as shown in equation (8). The resilience value of regular Polyurethane (PU) foams is between 25 and 40%.

Percentage rebound of ball (resilience value) = \frac{h_{2}}{h_{1}} x 100

(8)

where h₂ is height of rise of ball after rebound and h₁ is height from which ball is dropped.

Determination of R-value

R-value also known as thermal resistance relates to insulation characteristic of foam. For closed cell polyurethane foams, it is around 0.97–1.4 m²KW⁻¹. R-value of the foam was determined by heat transfer study on a heat transfer apparatus. The foam slabs of 120 mm × 90 mm × 18 mm dimensions incorporated between aluminum panels (Figure 13) were inserted in the heat transfer apparatus (Figure 2) between two sections which measured the temperatures T_A and T_B respectively. T_A was gradually raised from room temperature and finally maintained at constant temperature of 40°C while changes in T_B with respect to time were noted. R-value was determined at steady state conditions by employing equations (9–11).

Q = - λ A \frac{dT}{dx}

(9)

Φ q = \frac{Q}{A}

(10)

R - value = \frac{ΔT}{Φq}

(11)

Figure 2.

Heat transfer apparatus for determining R-value.

Results and discussion

Spectroscopy by NMR and FTIR

Figure 3a shows ¹H NMR spectra of cyclic-carbonate of castor oil in which a peak at 4.5 ppm shows carbonate functionality obtained from carbonation of castor oil. Similarly Figure 3b shows NMR spectra of PHU of castor oil. In this spectra, disappearance of carbonate peak at 4.5 ppm and appearance of new peaks at 2.8 and 3.2 ppm corresponded to formation of urethane linkages. Also, peak at 4.6 ppm showed formation of hydroxyl group in the PHU of castor oil. Carbonate equivalent weight of CCO was determined from integration of peak at 4.5 ppm in ¹H NMR titration using TMS standard, and it was found to be 340.

Figure 3.

¹H NMR spectra of (a) cyclic carbonate of castor oil and (b) PHU of castor oil.

Figure 4 shows FTIR spectra of ECO, CCO, Castor oil PHU and PHU foam. It was observed that epoxy peak at 1240 cm⁻¹ of ECO disappeared and carbonyl peak at 1790 cm⁻¹ of CCO appeared after carbonation reaction of the oil. Also, during synthesis of castor oil based PHU, carbonyl peak at 1790 cm⁻¹ disappeared while CN stretch at 1070 cm⁻¹, CO band at 1690 cm⁻¹ and NH stretch at 3328 cm⁻¹ could be seen which concluded the presence of urethane functionality. The appearance of SiH elongation at 2164 cm⁻¹ showed the use of PMHSO blowing agent in the synthesis of PHU foam. CH₂ symmetric and asymmetric stretching vibrations of linear chain of carbon atoms at 2925 cm⁻¹ and 2857 cm⁻¹ along with C=O at 1728 cm⁻¹ of ester linkage could be seen in all spectra thereby confirming the use of castor oil in the synthesis of foams.

Figure 4.

FTIR Spectra of ECO, CCO, PHU of castor oil and PHU foam.

Density, swell index and gel content of foams

Figure 5 shows all the foams made in this study. Table 3 shows density, swell index and gel content of the foams. The highest density reported was 235 kg m⁻³ of PHU-DETA-F4 and lowest density was 155 kg m⁻³ of PHU-EDA-F1. The density values of each type of foam went on increasing with the decrease in quantity of castor oil based PHU in the formulation.⁷⁴ The foams made from EDA and HMDA were flexible due to long chain aliphatic ethylene and hexamethylene structures. Perhaps their density values were in the range 155–215 kg m⁻³. However, density values of foams made from DETA were comparatively higher 188–235 kg m⁻³. This is due to the cross-linking nature of DETA that imparts toughness and rigidity to the foam. Rigid foams also had low swell index values between 45 and 92% while the flexible foams had higher swell index between 162 and 325%. Furthermore, gel content of the foams made from DETA was highest at 99% compared to flexible foam at 91%. Gel content typifies the extent of cross-linking between the molecular chains⁷⁵ and it can be evidently said that for foams made from DETA it must be highest due to is cross linking nature. Other factors apart from structure of amine such as curing time and foaming time does play a significant role in fluctuation of gel content values.

Figure 5.

Polyhyrdoxyurethane foams.

Table 3.

Physical properties of foams

Foam	Density kg/m³	Swell index %	Gel content %
PHU-EDA-F1	155	245	91
PHU-EDA-F2	169	217	92
PHU-EDA-F3	185	185	94
PHU-EDA-F4	208	162	96
PHU-HMDA-F1	162	325	97
PHU-HMDA-F2	175	280	96
PHU-HMDA-F3	189	195	96
PHU-HMDA-F4	215	171	95
PHU-DETA-F1	188	92	99
PHU-DETA-F2	190	80	99
PHU-DETA-F3	208	54	99
PHU-DETA-F4	235	45	98

Cellular morphology of foams

Figure 6 shows the cell structure of foams observed under optical microscope and Figure 7 shows cell structure as seen under scanning electron microscope. Foams made from linear aliphatic diamines, EDA and HMDA, were flexible and had cells (Figure 6(a) and (b)) of 142 μm diameter compared to cells of foam from DETA (Figure 6c) which were 115 μm in diameter.

Figure 6.

Optical microscopic images of foams. (a) PHU-EDA-F2, (b) PHU-HMDA-F1, (c) PHU-DETA-F2.

Figure 7.

SEM images of PHU foam at varying magnifications. (a) 20X, (b) 30X, (c) 100X, (d) 200X.

At the instance of foaming, flexible foams exhibited significant expansion which encouraged their cellular growth and therefore most of the cells were abruptly larger than their neighbouring cells (Figure 6(a) and (b)). Whereas in the case of rigid foam the cross-linking reaction due to DETA was predominant and since the foams became rigid while curing, its cell growth was restricted and cells were compacted (Figure 6c). More importantly, the foams made in this study had close cell morphology (Figure 7(a) and (b)) with cell size ranging from 115 to 142 μm in diameter. The cellular morphology of polyurethane foams is dependent on a number of factors that assist in forming the proper cell structure such as type of raw material, temperature of foaming, amount of blowing agent, curing temperature and time.

TGA analysis of foams

Thermal properties of foams are studied under TGA to determine number of stages of degradation, temperature of onset of degradation and corresponding weight loss and maximum degradation temperature. Table 4 shows degradation temperatures and Figure 8 shows TG and DTG curves of three PHU foams made in this study. The foams underwent three steps of degradation. Initially, the first step of degradation commenced at around 80–90°C which can be due to loss of moisture and presence of blowing agent in the foam, as is the case in regular PU foam with the only difference being the temperature for PU is higher for this step. This can be attributed to lower thermal stability of PHU foams as opposed to PU foams. The second degradation step, similar to PU, is the destruction of weakest bonds which are nothing but the soft segments such as polyols of polyester/polyether and aliphatic chains of fatty oil which take place at 270–280°C. In this study, this step took place around 178–210°C which corresponded to weakening of linear aliphatic structure of castor oil followed by its scission prior to degradation. Lastly, third step of degradation was intense and maximum degradation at around 385–395°C could be seen in this step for all the foams. Generally, in this step hard segments of PU such as isocyanurates and diphenylmethane structures of foam disrupt at 300–350°C.⁷⁶

Table 4.

Degradation temperatures of foams.

Foam	1st degradation		2nd degradation		3rd degradation
Foam	% deg	T_max (°C)	% deg	T_max (°C)	% deg	T_max (°C)
PHU-EDA-F2	4.76	76.76	8.99	217.61	78.05	395.53
PHU-HMDA-F1	4.34	84.46	12.39	219.79	74.23	392.12
PHU=DETA-F3	5.84	87.67	8.72	178.36	70.12	385.65

Figure 8.

TG (solid lines) and DTG (dot lines) curves of foams.

In this study epoxy resin was blended in castor oil based PHU. For epoxy polymers, the maximum temperature of degradation is between at 410–500°C.⁷⁷ While degradation for a general PU may last to a maximum of 350°C or higher, for foams made in this study, the maximum degradation temperature was about 395°C due to presence of epoxy resin in the foam formulation which obviously had an influence in increasing the temperature of degradation. From Table 4, it can be seen that at maximum degradation temperature flexible foams made from EDA and HMDA degraded more (78%, 74% at 395°C, 392°C) than rigid foam from DETA (70% at 385°C) which means that rigid foams had higher thermal stability than flexible foams which is a universal trend in PU foams. This can open avenues for their use as potential insulation materials for applications where higher thermal resistance is a prerequisite.

DSC of foams

Figure 9 shows the Tg values of foams made from various amines. The highest T_g reported was 35.3°C for rigid foam made from DETA while it was lower for HMDA and EDA at 31.5°C and 28.6°C respectively. It is pertinent that T_g increases as molecular weight of any of the component in the formulation increases. Also, foam having cross-linking component (DETA) in the formulation will have higher T_g as opposed to linear aliphatic component (EDA, HMDA).⁷⁸

Figure 9.

Heat flow vs temperature curves of foams.

Moreover, foams contained epoxy resin in the formulation which caused an increase in T_g of the foams.⁵⁸ Therefore, unlike conventional flexible PU foams where T_g can go as low as −20°C, T_g values of PHU foams made in this study were between 28–36°C.

Thermal conductivity of foams

Rigid PU foams have insulative property due to their low thermal conductivity values between 48 and 50 mWm⁻¹K⁻¹.⁷⁹ Thermal conductivity of PU foam is dependent on cellular morphology, density and sometimes gas entrapped in pores of the foam which is released from blowing agent. Total heat transmission through closed cell foam is the sum of heat conducted through gas contained in the cells, heat conducted through the solid polymer of the cells and heat radiated across the cell walls.⁵⁶ Table 5 shows thermal conductivity and thermal diffusivity values of rigid foams made from PHU-DETA which were calculated from equations (9–11).

Figure 10.

Temperature vs time graph for thermal conductivity measurement.

Table 5.

Thermal conductivity, thermal diffusivity and R-value of foams.

Rigid foams	Thermal conductivity, λ mWm⁻¹K⁻¹	Thermal diffusivity α mm²s⁻¹	R-value, m²KW⁻¹
PHU-DETA-F1	129.67	0.28	0.14
PHU-DETA-F2	123.86	0.27	0.15
PHU-DETA-F3	112.03	0.24	0.16
PHU-DETA-F4	104.32	0.23	0.17

All foams had thermal conductivity in the range 104–130 mWm⁻¹K⁻¹ and thermal diffusivity in the range 0.2–0.3 mm²s⁻¹. Foams made from DETA were rigid and densely compact in cellular structure. Cornille et al.⁶² synthesized NIPU foams and stated that density and thermal conductivity values of the foams were closely related to each other. As density of the foams increased, the pore became smaller in size and larger in numbers which led to decrease in radiation of heat through the cells of the foam. Also, he stated that low thermal diffusivity values of NIPU foams (0.2–0.3 mm²s⁻¹) justified that temperature flow within these materials is slow as opposed to PU foam (0.93 mm²s⁻¹).

Compression and recovery analysis

Recovery of flexible foams made from EDA and HMDA after mechanical compression under UTM was tested and results were obtained as shown in Figure 11. All the foams showed full recovery after complete removal of load which can be attributed to excellent flexibility of foams made from castor oil, linear aliphatic diamines and epoxy resin. Foams with high density such as PHU-EDA-F3, PHU-EDA-F4, PHU-HMDA-F3 and PHU-HMDA-F4 took longer time to recover than foams with low density. Moreover, decrease in quantity of EDA and HMDA based PHU and increase in epoxy resin in the formulation led to slight increase in toughness of foams thereby imparting prolonged recovery times after compression of foams. However, complete recovery in all foams was observed due to influence of epoxy resin.

Figure 11.

Compression and recovery of flexible foams. (a) PHU-EDA, (b) PHU-HMDA.

Fauzi et al.⁸⁰ explained that complete shape recovery in epoxy foams is seen after removal of load. This can be due to optimum interconnection between cells, porous cell wall and homogeneous cell size. All these factors played a significant role in the PHU foams made in this study.

Resilience by ball rebound test

Resilience of foam is the property by virtue of which it can regain its original shape after being compressed by an object. Resilience of foams is checked by ball rebound test. Figure 12 shows resilience values of PHU foams obtained by ball rebound test. Flexible PU foams have resilience values between 25 and 40%. Resilience values of all flexible PHU foams made in this study were low compared to reference PU foam. A similar trend was observed between compression-recovery and resilience values of PHU foams. The foam with quickest recovery, 14 s, viz. PHU-EDA-F1 showed highest resilience value 16, followed by PHU-HMDA-F1 with resilience value 15. Similarly, the foam that took longest time to recover, 38 s, was PHU-EDA-F4 with the lowest resilience value 3. All the foams showed resilience values between 3 and 16. Resilience and flexibility are interdependent properties by virtue of which PU foams exhibit high compressibility along with complete and irreversible recovery.

Figure 12.

Resilience of flexible foams.

Figure 13.

PHU-DETA rigid foams incorporated between aluminium panels.

R-value of rigid foams

Figure 14 shows temperature gradient with passage of time inside heat transfer apparatus. A maximum of 5°C difference between section A and section B of heat transfer apparatus was observed for all the foams at steady state conditions. This difference ( $Δ$ T) was used in equation 9 to calculate R-value of PHU-DETA foams. Table 5 shows R-values of PHU-DETA rigid foams obtained from the heat transfer study.

Figure 14.

Temperature gradient in heat transfer apparatus with time.

It was observed that R-values of the foams increased with the decrease in thermal conductivity of the foams and decrease in thermal diffusivity of the foams. Cornille et al.⁶² explained this phenomenon on the basis of density and cellular morphology. That is, with the increase in density cell size decreases. The cluster of small sized cells resists the diffusion of heat through the foam thereby decreasing the thermal conductivity of the foams. From equation (9), it is clear that decrease in thermal conductivity will increase R-value of the foam. Therefore PHU-DETA-F4 had highest R-value of 0.17 mm²KW⁻¹ followed by PHU-DETA-F3 having R-value of 0.16 mm²KW⁻¹, PHU-DETA-F2 with R-value of 0.15 mm²KW⁻¹ and PHU-DETA-F1 with R-value of 0.14 mm²KW⁻¹. R-value is an important factor in determining the potential insulative properties of the rigid PU foams for their applications such as in building for insulation from solar heat, refrigerators, and thermal storage devices.

Conclusion

PHU foams were synthesised in this study by employing cyclic carbonate of castor oil, different aliphatic diamines and epoxy resin. Foams showed closed cell morphology with homogeneous cells. Thermal stability of foams was quite good due to presence of epoxy resin in the blend but lower than PU due to low reactivity of CCO and amine. Foams made from EDA and HMDA were flexible with low resilience values however they showed complete recovery after compression due to excellent shape recovery characteristics of epoxy foam. Foams made from DETA were rigid and had slightly high thermal conductivity than PU foams and low thermal diffusivity and low R-value. However, a maximum of 5°C of temperature difference was observed in heat transfer experiment. Foams with high R-values can be potential materials for insulation applications. Isocyanate free polyurethane route has been researched for decades now and it seems this route will soon be adapted provided toxic isocyanates are completely eradicated from chemical industries.

Footnotes

Acknowledgements

The authors would like to thank Department of Polymer and Surface Engineering, Institute of Chemical Technology Mumbai, for providing instrumental facility for successful completion of this research.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Prakash A Mahanwar

Zeeshan R Ahmad

Prakash A Mahanwar

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