Effect of NCO index on the physico-mechanical properties of Poly(urethane-urea) foams using amine based polyol and non-amine based polyol

Abstract

Poly (urethane-urea) foam (PUU Foam) from an amine-based polyether polyol (APP) is different from Sucrose-Sorbitol based polyether polyol (SSPP). In this study, rigid PUU Foams produced from an amine-based polyol, (Amine value: 100 mg KOH/g, Hydroxyl value of 408 mg KOH/g) was compared with Sucrose-Sorbitol based polyether polyol, (Hydroxyl value:380-420 mg KOH/g, Amine value: 0 mg KOH/g) using varying isocyanate ratio (Isocyanate index). Effect of varying Isocyanate index on foaming parameters, physical, chemical, morphological as well as thermomechanical properties were evaluated in both kinds of foams. A higher NCO index resulted in increased crosslinking, which improved the compressive strength of PUU foams through the formation of isocyanurates, thus enhancing their thermal stability. Fourier-transform infrared (FTIR) spectroscopy was utilized to analyze the chemical properties of the foams. Cell size through scanning electron microscopy (SEM) and closed cell content and dimensional stability of the foams were also carried out to assess the morphological structures of the foams. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed to examine the thermal properties of the foam. The thermal conductivity of the foams, prepared with various isocyanate indexes, ranges from 25 to 45 mW/m K, especially for APP based foams with NCO index of 175 resulted in low thermal conductivity of 25 mW/m K (at RT) and 14 mW/m K (at LN2), indicating their potential suitability for cryogenic insulation.

Keywords

Poly(urethane-urea) foam amine-based polyol polyether polyol NCO index cryo insulation

Introduction

Polyurethane (PU) ranks among the most adaptable polymer materials and finds extensive use in various industries, including thermal rigid insulation foam, flexible foams, coatings, adhesives, and elastomers, due to their excellent porosity, low density, and high strength.^1–4 The suitability of its application is determined by its properties, which are greatly affected by the specific polyol and isocyanate utilized during its manufacturing process.^5,6

Generally, polyurethane (PU) materials are obtained by polyaddition reaction of polyols and polyisocyanates. Polyols are organic compounds characterized by having several hydroxyl (OH) groups. The reaction between the OH groups of the polyol and the isocyanate (NCO) groups leads to a polymerization process, resulting in a polymer with monomer units connected by urethane linkages, known as PUs. Depending on the desired application, the types of polyols, isocyanates, and catalysts can be modified to create foams with varying properties, such as high-temperature resistance, flame retardancy, acoustic insulation, and high resilience.^7,8

Rigid Polyurethane foams are highly cross-linked polymers with closed-cell structure, are typically produced using polyfunctional isocyanates like p-MDI, which contribute to crosslinking and thermoset behaviour. In the processing of PUU foams, a key attribute is the presence of an closed cell structure and lower thermal conductivity which makes the rigid foams a good thermal insulation material. Closed cells reduce the movement of gas, which reduces convective heat transfer and therefore improves insulation.⁹ The formation of cell structures in the PUU foams is influenced by two primary chemical reactions: the gelling process and the blowing process. The gelling process occurs when isocyanate reacts with polyol to create urethane bonds, while the blowing process generates urea bonds through the reaction of isocyanate with water, producing carbon dioxide gas. The outcomes of these reactions can also be significantly influenced by various essential components, such as polyols (either polyether/ester or amine-based), isocyanates (TDI or MDI), chain extenders, silicon surfactants, blowing agents (either physical or chemical), and catalysts (both amine and tin-based), which are included in the foam formulations.^10–13

Stoichiometric analyses of various NCO/OH molar ratios are crucial for assessing the properties of PUU foams. The NCO index (NCO equivalents/OH equivalents × 100) is commonly utilized to express the NCO/OH molar ratio in the synthesis of PUU foams. Umar Adli Amranet al. investigated the effect of the NCO index on the physical and chemical characteristics of EFB polyol-based polyurethane foams (EFBPUFs).¹⁴ Kim et al. (2008) found that different NCO indexes affected the foaming properties of PU Foams.¹⁵ Hejna et al. (2017) reported that increases in the NCO index had affected the crosslink density of PU Foam.¹⁶ A change in NCO index value affects the PU foaming process itself and the mechanical strength of the foams. To reduce the flexibility of the foams, the NCO index can be increased, i.e., the content of rigid segments is increased due to the formation of additional urea bonds.¹⁷ Mateusz Barczewski and his team examined how varying the amount of water and the NCO index affects the cellular structures and physical-mechanical attributes, including the thermal insulation capabilities and dimensional stability of open-cell polyurethane foams.¹⁸ Roger G. Dingcong. Jr. and his research group explored the autocatalytic properties of a highly functional biopolyol (p-CDEA) obtained from coconut oil (CO) and diethanolamine (DEA), providing strong evidence for the improved sustainability of p-CDEA-based polyurethane-acrylate hybrid foam in comparison to petroleum-based polyurethane.¹⁹ So far, the influence of the NCO index on the properties of rigid polyurethane-urea foams produced from an anime based polyol has not been described.

The purpose of the study is to assess the first-time comparison of PUU foams prepared from an amine-based polyol (APP) vs. a non-amine polyether polyol based on sucrose –sorbitol (SSPP) at various NCO index ranging from 100 to 200 in order to develop a suitable foam formation. We have also examined the influence of NCO index on the physical and mechanical characteristics of the PUU foams, dimensional stability of the PUU foams and thermal insulation property especially for cryo insulation applications. Conductivity through the polyurethane-urea itself is also an important feature, influenced by the appropriate wall thickness and closed cell structure, which is reflected in the apparent density. From the available reported literature knowledge, there have been no studies reported related to the use of an amine-based polyol in the preparation of Polyurethane-urea foam for cryo insulation.

Materials and methods

Materials

Empeyol R444 (APP with an amine value of 100 mg KOH/g, hydroxyl value of 408 mg KOH/g and viscosity of 9600 cP at 30°C) was procured from M/s. Manali Petrochemicals, Chennai and SUSOR 400N (SSPP with hydroxyl value of 380-420 mg KOH/g and viscosity of 4800 cP at 30°C, was procured from M/s Expanded Polymers, Mumbai), Polymeric methylene diphenyl diisocyanate (p-MDI) used in this study, is not a pure diisocyanate but a mixture with average functionality >2 (commonly around 2.7 with ≈32 wt.% NCO content, was supplied by M/s Surabhi Industries, Pune), HFO (Opteon 1100, physical foam expansion agent obtained from Chemours, density: 1.36-1.39 g/cc at 25°C), DBTDL (tin content: 18-18.6%,Make; Alfae aesar), Kosmos 75 (potassium octoate content: 70 ± 5%,Make:EVONIC), DABCO TMR 4 (nitrogen content: 5-7%,Make Air products), Tegostab B-8408(viscosity: 500-600 cP at 30°C,Make:EVONIC) were used in this study. Deionized Water was used as a chemical blowing agent, which, in reaction with isocyanate, causes the formation of carbon dioxide.

Preparation of rigid Poly(urethane-urea) foam

Rigid PUU foams were prepared using two distinct polyols through a one-shot foaming process. The formulations of the PUU Foams varied with different NCO indexes (i.e., 100, 150, 175, and 200). Open molds made from paper boxes measuring 15 cm × 15 cm × 10 cm (length × width × height) were utilized for the PUU Foams. First, a pre-mix of polyol, surfactants, blowing agent, and catalyst was prepared in a 500 mL disposable plastic cup according to the composition listed in Table 1 and Table 2, then mixed with a mechanical stirrer (1/8 HP, 2000 rpm) for 5 min. Following cooling to room temperature, a predetermined quantity of p-MDI (based on the NCO index) was incorporated into the mixture and stirred at 4000 rpm for 3 to 4 s. The mixture was promptly poured into a paper mould, allowing the foam to expand at ambient conditions (25°C, RH = 37%). Cream time, rise time and tack free time were measured and the resulting PUU foams were labelled and allowed to cure for 24 h before characterization.

Table 1.

Poly(urethane-urea) foam formulation based on APP.

Foam formulation	Components	Quantity, g
Foam formulation	Components	NCO-100	NCO-150	NCO-175	NCO-200
Polyol, g	Empeyol R444	100	100	100	100
Catalyst, g	DBTDL	0.37	0.37	0.37	0.37
	Dabco TMR 4	2.92	2.92	2.92	2.92
	Kosmos 75	0.49	0.49	0.49	0.49
Surfactant, g	Tegostab B 8408	5.84	5.84	5.84	5.84
Blowing agent, g	Water	2.43	2.43	2.43	2.43
Blowing agent, g	HFO	28.28	28.28	28.28	28.28
Isocyanate, g	MDI	94.25	141.4	165	188.57

Table 2.

Poly(urethane-urea) foam formulation based on SSPP.

Foam formulation	Components	Quantity, g
Foam formulation	Components	NCO-100	NCO-150	NCO-175	NCO-200
Polyol, g	SUSOR 400N	100	100	100	100
Catalyst, g	DBTDL	0.37	0.37	0.37	0.37
	Dabco TMR 4	2.92	2.92	2.92	2.92
	Kosmos 75	0.49	0.49	0.49	0.49
Surfactant, g	Tegostab B 8408	5.84	5.84	5.84	5.84
Blowing agent, g	Water	2.43	2.43	2.43	2.43
Blowing agent, g	HFO	28.28	28.28	28.28	28.28
Isocyanate, g	MDI	129	193.53	225.78	258.04

The quantity of curing agent used for the fixed quantity of polyol and water in the foaming process was determined using the following Equations:

Equivalent weight of p - MDI = \frac{42}{(% NCO content of p - MDI)}

Equivalent weight of Polyol = \frac{56100}{Hydroxyl value of the Polyol}

{\frac{Wt of Polyol}{Eq . wt . of Polyol} + \frac{2}{18} \times m_{water}} \times NCO index = \frac{Wt of p - MDI}{Eq . Wt . of p - MDI} \times 100

Where, m_water = mass of water (g)

Foaming properties

According to the standard, cream time, free rise time, and tack-free time of the PUU foams were measured and determined. The definitions of the terms related to the foaming reaction are as follows: (a) Cream time, often referred to as initiation time, is the duration from when all reactants are combined until tiny bubbles begin to form. (b) Gel time (pull time) indicates the time at which long strings of tacky material can be pulled away from the surface of the foam when the surface is touched by the edge of the tongs. (c) Free-rise time denotes the period during which the PUU ceases to expand. (d) Tack-free time indicates the moment when the surface of the PUU can be touched without any adhesion.

Characterization of poly(urethane-urea) hybrid foams

The thermal conductivity of each PUU foam samples were evaluated using the Hotwire method in an air atmosphere at 1 atm pressure, following the guidelines of ASTM C 1113 (sample dimensions 140 × 80 × 20 mm). The densities of the foam samples were assessed in accordance with ASTM D1622-14. The PUU foam specimens were sliced into 50 mm × 50 mm × 50 mm cubes (length × width × height) and measured with a calliper.

Next, the specimens were weighed on an analytical balance. The apparent density of the PUU foams was determined using the following equation:

D = \frac{m_{s}}{V}

Where, D = density of specimen (kg/m³), m_s = mass of specimen (kg) and V = volume of specimen (m³).

The compressive strengths of the foam samples were assessed using a Universal Testing Machine, specifically a 50 kN device equipped with compression plates, in accordance with ASTM D1621. The thermal transitions of the formulated foams were analyzed via differential scanning calorimetry (DSC) under an argon atmosphere, reaching temperatures up to 200°C with a heating rate of 10°C per minute, (sample weight between 5 and 10 mg). Thermo-gravimetric analysis (TGA) was performed in a nitrogen atmosphere up to 600°C at a heating rate of 10°C per minute (sample weight of 5-10 mg). The surface morphological characteristics of the foams were examined using scanning electron microscopy (SEM) with an EPSEM-CARL ZEISS EVO-5 at a magnification of 60X. FT-IR analysis of the PUUFs with varying NCO indexes was carried out using a Nicolet is 50 ATR FT-IR spectrometer. Each sample was scanned 64 times at a resolution of 2 cm^-1 across the wave number range of 4000-700 cm^-1. Closed cell content of the foam was determined using gas displacement air pycnometer, Micromeritics ACCUPYC II 1340 as per the ASTM standard D6226-21.

Results and discussion

FT-IR analysis

The impact of varying NCO indexes on the chemical composition of PUU foams was examined through the evaluation of their functional groups using FT-IR spectroscopy. The primary reaction involved in PU is the interaction between the hydroxyl groups of a polyol and the isocyanate groups of diisocyanates (p-MDI) to create urethane linkages. In addition to urethane as the repeating units, a PU also comprises urea, ether, ester, and aromatic substances within its structure.²⁰

IR peaks at 1698-1706 indicates the urethane carbonyl group, 1590-1597 indicates urea carbonyl group, 1408-1412 indicates isocyanurate linkages. FTIR curve is given in Figure 1 and peak datas are given in Table 3.

Table 3.

Spectral shifts of urethane, urea and isocyanurate linkages at each NCO index.

NCO index	Urethane carbonyl		Urea carbonyl		Isocyanurate
NCO index	APP	SSPP	APP	SSPP	APP	SSPP
100	1701.00	1701.00	1597.00	1597.00	1411.16	1410.70
150	1699.00	1699.0	1595.83	1594.87	1410.70	1410.70
175	1701.39	1698.04	1592.00	1594.00	1410.19	1409.73
200	1705.73	1701.90	1591.95	1592.94	1410.19	1408.77

It was clear from the above table that there is no spectral shifts with change in polyol or change in NCO index.

Cellular morphology and cell size distribution of PUU foams

Cellular morphology was analysed through SEM, about 50 number of cells in each image were evaluated using image J software. Results of the analysis of all the PUU foams are shown in Figure 2.

It was observed that foams with APP based polyol showed larger cells and cell size decreased when the NCO index increased from 100 to 200. Rather than the viscosity aspects, the higher reactivity of APP based polyol compared to the SSPP based polyol might affected on the cell morphology differences.

Closed cell content and dimensional stability

The closed cell content is one of the main characteristics of thermal insulation materials, Higher the closed cell content, lower the thermal conductivity. The measured closed cell content of all the developed rigid PUU foam samples are shown in Table 4. It was the found that closed cell content increases with the NCO Index.²¹

Table 4.

Closed cell content % of PUU Foams at different NCO indexes.

Closed cell content, %	NCO index
Closed cell content, %	100	150	175	200
APP	38.86	63.16	96.94	97.94
SSPP	35.56	70.52	97.13	97.89

% Shrinkage of all the developed rigid PUU foam samples are shown in Table 5. All the developed foams except APP 100 and SSPP 100 were dimensional stable. At lower NCO index at 100, both APP based foam shown substantial shrinkage and SSPP based foam also exhibit some shrinkage. The shrinkage of the foam might be due to lower crosslink density at lower NCO index.²²

Table 5.

Shrinkage % of PUU foams at different NCO indexes.

NCO index	% Shrinkage, (APP)	% Shrinkage, (SSPP)
100	10	0.44
150	0.25	0.17
175	<0.05%	0.17
200	<0.05%	0.11

Effect of NCO index on the foaming properties of PUU foams

The foam reactive parameters of APP and SSPP were examined by assessing the cream, gel and rise times of the PUU reaction, as given in Table 6.

Table 6.

Foaming properties of PUU foams at different NCO indexes.

NCO index	APP based foam				SSPP based foam
NCO index	Cream time, s	Gel time, s	Rise time, s	Tack free time, s	Cream time, s	Gel time, s	Rise time, s	Tack free time, s
100	1	22	26	30	4	40	44	48
150	1	23	27	31	3	43	51	54
175	4	25	30	34	4	50	56	60
200	5	26	31	34	3	60	65	75

A notable difference in foaming parameters observed in APP in comparison to SSPP, a non-amine polyol. This increased reactive parameter of APP (reduced gel time and tack free time compared to SSPP) is attributed by the presence of intrinsic amine groups in APP. Its catalytic properties enhance the reaction between water and isocyanate, facilitating quicker bubble-size expansion.²³

Increasing tendency of tack free time with NCO index was reported earlier in Literatures.²⁴ The increasing tendency of tack free time is due to the increased production of low molecular weight polymers, including the unreacted monomers, as the stoichiometric imbalance between the reacting groups is increased. Because of their low free energy, the low molecular weight species are typically exposed to the free surfaces, causing adhesiveness and prolonging tack-free time.²⁴

Effect of NCO index on apparent density of PUU foams

The density is a significant physical characteristic of PUU foams that influences their mechanical properties. Figure 3 illustrates the impact of various NCO index on the apparent density of PUU foams. According to the findings, the apparent density of the PUU foams increased as the NCO index was increased from 100 to 200.

Increasing isocyanate index raised the amount of isocyanate in the polymer thereby increasing the total mass of polymer network. On the other hand, the foam volume was dictated by the amount of carbon dioxide generated by the reaction of water and isocyanate. This reaction was independent of the isocyanate index since isocyanate was not limiting when reacting with water. Therefore, the foam volume did not change with the polyol percentage which was also observed in the experiment. With more isocyanate in the foam formulation (increasing isocyanate index), foam density increased.²⁵

A higher NCO index promotes the formation of an isocyanurate ring, which can enhance the crosslink density of PUU foams. The rise in crosslink density of PUU Foams may, in turn, lead to an increase in the density of PUU foams.^26,27

However, understanding the cellular structure morphology of the PUU Foams is crucial for grasping the increase in apparent density. In a cellular polymeric structure like PUU Foams, morphological analysis could provide further insight into its apparent density.²⁸ Lower cell size or increased cell wall can result in increased density. This trend was clearly observed when NCO index varied from 100 to 200 as shown in Figure 4.

Mechanical properties of PUU foams

The compressive characteristics of a cellular polymer arise from intricate processes involving void spaces and a network of solid plates or struts that create the cell walls (cell membranes) and edges. Essentially, the mechanical properties of the majority of polyurethane foams (PUFs) are strongly linked to their densities and the types of cells (whether closed or open). The effects of different NCO indexes on the compressive strength and Tensile strength of PUU Foams are shown in Figure 5(a), and (b), respectively.

Increasing the isocyanate index increased the compressive strength of foams. This was due to more the isocyanate used in the foam formulation, more cross-links were formed from the reaction of isocyanate and the hydroxyl groups in the polyols. A higher NCO index promotes the formation of an isocyanurate ring, which can enhance the crosslink density of PUUFs. In addition, foam density increased with the isocyanate index. Both increases in cross linking and density contributed to higher compressive strength.²⁹

Tensile strength of APP based foam is higher than that of SSPP based foam except NCO index 200. It was observed that value lies between 170 and 260 kPa for APP based foam and 140 to 230 kPa for SSPP based foam, respectively.

Thermal analysis of PUU foams

TGA-DTG

Polyurethane or Polyurethane-urea (PU) experiences a complicated process of thermal degradation that includes various partial degradation reactions. The thermal degradation mechanism of PU or PUU consists of random-chain and chain-end scissions, along with cross-linking. Typically, thermal degradation occurs in two stages in a nitrogen environment or three stages in an oxidizing environment.³⁰ In the initial stage, urethane or urea hard segments begin to degrade. Subsequently, the degradation of polyols or soft segments (including ether and ester linkages) occurs in the second and third stages.²³

Thermogravimetric (TGA) and derivative thermogravimetric (DTG) curves of all PUU foam derived from APP (R444) and SSPP (SUSOR) at various NCO indexes are shown in Figures 6 and 7, respectively. The DTG curves reveal two distinct peaks that could offer additional insights into the thermal degradation of the PUUFs, which were not clearly seen in TGA. From the DTG curve, a minor peak at 60- 100°C in all the foams reflect the emission of volatile substances, including gases and water. The larger peak observed between 200 and 380°C corresponds to the degradation of urethane, urea, and isocyanurate linkages (hard segments), which occur at 200, 250, and 350°C, respectively. In addition, the subsequent peak around 536°C signifies the degradation of soft segments of urea and urethane linkages, respectively.^20,31–33

It is observed that with respect to the NCO index, variation of thermal stability is minimal or it is not showing any exact trend. However, the stability of APP based foam is slightly low in all the ratios than the SSPP foam.

The structure of isocyanate and alcohol used influences the temperature of initial decomposition of the urethane bond. The highest degradation temperatures are generally observed for urethanes formed from an alkyl isocyanate and an alkyl alcohol. As the reactivity in the following series increases, the decomposition temperature of the urethane bond decreases: alkyl isocyanate-alkyl alcohol (∼250°C)> aryl isocyanate-alkyl alcohol (∼200°C)> alkyl isocyanate-aryl alcohol(∼180°C)> aryl isocyanate-aryl alcohol (∼120°C).¹⁹ The higher stability of SSPP based foam compared to APP based foam observed from Figures 6 and 7 is in line with the above theory.

Table 7.

Thermal stability of APP & SSPP based foams.

NCO index	APP based PUU foam				SSPP based PUU foam
NCO index	100	150	175	200	100	150	150	150
T_onset (°C)	275	274	275	278	319	322	319	319
T_max1 (°C)	302	279	380	331	419	394	380	446
T_max2 (°C)	548	510	564	543	608	586	577	605

The key parameters of TGA and DTG curves are shown in Table 7.

Thermal conductivity

Impact of the NCO index from 100 to 200 on the thermal conductivity values PUU foams at room temperature as well as LN2 conditions are illustrated in Figures 8 and 9 respectively. Regardless of the isocyanate index employed, the thermal conductivity coefficients were noted to have comparable values.

The PUU foam produced from APP with an NCO index of 175 exhibits a thermal conductivity of 25 mW/m.K at room temperature and 14 mW/m.K under LN2 conditions. Table 8 shows the specifications of a rigid foam for cryo insulation applications.

Table 8.

Aerospace cryo insulation requirement properties for polyurethane foams.

Property	Foam specifications for cryogenic insulation	PUUF/APP/NCO175
Thermal conductivity, mW/m.K
@ RT	35 (max)	25
@ LN2	25 (max)	14
Compressive strength, ksc	1.6 (min)	2.3
Density, kg/m³	30-35	33

The thermal conductive properties of the PUU foams obtained from both polyols comply with the ASTM requirements established for type 1 rigid structural sandwich panel cores. Moreover, the PUU foam prepared from APP with an NCO index of 175 is suitable for cryogenic insulation in launch vehicles.

Conclusion

In this research, rigid PUU Foams were prepared with varying NCO indexes using two distinct types of polyols ie. amine based polyol which is autocatalytic and Sucrose-Sorbitol based polyol which is non autocatalytic. The impact of different NCO indexes on the foaming, physical, and chemical characteristics of the PUU Foams was examined. Each PUU Foams presented a specific set of properties corresponding to different NCO indexes. The foaming characteristics influenced the density and cellular structure of the PUU Foams, which subsequently affected their compressive attributes. The peak compressive and tensile strengths of the PUU Foams were achieved at NCO indexes of 175 and 200. The presence of carbonyl groups identified through FT-IR analysis verified the establishment of urethane and urea linkages in all PUU Foams. Cell morphology, closed cell content and dimensional stability of all the PUU foams were also characterized. TGA analysis indicated a minor reduction in the thermal stability of APP based foam compared to the SSPP based foam. However faster kinetic parameters shown by APP based foam make it attractive for spray foaming applications. Based on the obtained properties and the cryoinsulation requirement outlined in Table 8, it can be concluded that the polyurethane-urea derived from an amine-based polyol APP at NCO index 175, is suitable for use in structural and cryogenic insulation, especially for launch vehicle applications.

Footnotes

ORCID iDs

Nallaperumal AM

Nikhil PS

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

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

References

De Souza

Kahol

Gupta

. Introduction to polyurethane chemistry. ACS Symp Ser 2021; 1380: 1–24.

Soon-Ki

KWON

Kil-Yeong Choi

SKC

. Synthesis and properties of polyurethanes and polyureas containing 2,5-Thiophenylene linkage. Brevity 2014; 25: 198–213.

Lee

Zeng

Cao

, et al. Polymer nanocomposite foams. Compos Sci Technol 2005; 65(15-16): 2344–2363.

Yanping

. The development of polyurethane. Mater Sci Mater Rev 2018; 1(1): 1–8.

Gopalakrishnan

Linda Fernando

. Influence of polyols on properties of bio-based polyurethanes. Bull Mater Sci 2012; 35(2): 243–251.

Fang

, et al. Synthesis and application of a novel bio-based polyol for preparation of polyurethane foams. New J Chem 2014; 38(8): 3874–3878.

Dhoke

Ojha

Chaudhary

, et al. Influence of carbon nanotubes on the properties of biopolyol based polyurethane foams. Cell Polym 2021; 40: 73–86.

Jonjaroen

Ummartyotin

Chittapun

. Algal cellulose as a reinforcement in rigid polyurethane foam. Algal Res 2020; 51: 102057.

Miedzinska

czionka

Kairyte

, et al. Rowanberry/vermiculite modified rigid polyurethane foams. J Cell Plast 2024; 60(2): 117–133.

10.

Gunashekar

Pillai

Church

, et al. Liquid flow in polyurethane foams for filtration applications: a study on their characterization and permeability estimation. J Porous Mater 2015; 22: 749–759.

11.

Liu

. Preparation and characterization of polyurethane foam/activated carbon composite adsorbents. J Porous Mater 2012; 19: 567–572.

12.

Joanna

P-S

Bogusław

Joanna

. Application of waste products from agricultural-food industry for production of rigid polyurethane-polyisocyanurate foams. J Porous Mater 2011; 18: 631–638.

13.

Defonseka

. Practical guide to flexible polyurethane foams. Smithers Rapra Technology Ltd., (Shrewsbury) 2013; 1: 41.

14.

Amran

Salleh

Zakaria

, et al. Production of rigid polyurethane foams using polyol from liquefied oil palm biomass: variation of isocyanate indexes. Polymers 2021; 13: 3072.

15.

Kim

Lim

. Effect of isocyanate index on the properties of rigid polyurethane foams blown by HFC 365mfc. Macromol Res 2008; 16: 467–472.

16.

Hejna

Haponiuk

Piszczyk

, et al. Performance properties of rigid polyurethane-polyisocyanurate/brewers’ spent grain foamed composites as function of isocyanate index. e-Polymers 2017; 17: 427–437.

17.

Prociak

Malewska

Bąk

. Influence of isocyanate index on selected properties of flexible polyurethane foams modified with various bio-components. j renew mater 2016; 4: 78–85.

18.

Kami´nska

Barczewski

Kura´nska

, et al. The effect of a chemical foaming agent and the isocyanate index on the properties of open-cell polyurethane foams. Materials 2022; 15: 6087.

19.

Dingcong

Jr Malaluan

Alguno

, et al. A novel reaction mechanism for the synthesis of coconut oil-derived biopolyol for rigid poly(urethane-urea) hybrid foam application. RSC Adv 2023; 13: 1985–1994.

20.

Chattopadhyay

Webster

. Thermal stability and flame retardancy of polyurethanes. Prog Polym Sci 2009; 34: 1068–1133.

21.

Kairyte

Vejelis

. Evaluation of forming mixture composition impact on properties of water blown rigid polyurethane (PUR) foam from rapeseed oil polyol. Ind Crops Prod 2015; 66: 210–215.

22.

Ruamcharoen

Phetphaisit

Chanlert

, et al. Sago starch and esterified sago starch as eco-friendly fillers for rigid polyurethane foams. J Cell Plast 2024; 60(1): 23–40.

23.

Leng

Cai

, et al. A study on coconut fatty acid diethanolamide-based polyurethane foams. RSC Adv 2022; 12(21): 13548–13556.

24.

Ivdre

Abolins

Sevastyanova

, et al. Rigid polyurethane foams with various isocyanate indices based on polyols from rapeseed oil and waste PET. Polymers 2020; 12: 738.

25.

Fan

Ali

Suppes

, et al., Physical properties of soy-phosphate polyol-based rigid polyurethane foams, Int J Polym Sci 2012; 2012: 907049.

26.

Hejna

Haponiuk

Piszczyk

, et al. Performance properties of rigid polyurethane-polyisocyanurate/brewers’ spent grain foamed composites as function of isocyanate index. e-Polymers 2017; 17: 427–437.

27.

Petrović

Zhang

Zlatanić

, et al. Effect of OH/NCO molar ratio on properties of soy-based polyurethane networks. J Polym Environ 2002; 10: 5–12.

28.

Zhao

Zhong

Tekeei

, et al. Modeling impact of catalyst loading on polyurethane foam polymerization. Appl. Catal. A 2014; 469: 229–238.

29.

Amran

Zakaria

Chia

, et al. Polyols and rigid polyurethane foams derived from liquefied lignocellulosic and cellulosic biomass. Cellulose 2019; 26: 3231–3246.

30.

Jiao

Xiao

Wang

, et al. Thermal degradation characteristics of rigid polyurethane foam and the volatile products analysis with TG-FTIR-MS. Polym Degrad Stab 2013; 98: 2687–2696.

31.

Cao

, et al. A thermal self-healing polyurethane thermoset based on phenolic urethane. Polym J 2017; 49: 775–781.

32.

Yuan

, et al. Synthesis of bio-based polyurethane foams with liquefied wheat straw: process optimization. Biomass Bioenerg 2018; 111: 134–140.

33.

Das

Neeharika

TSVR

Sailu

, et al. Kinetics of amidation of free fatty acids in jatropha oil as a prerequisite for biodiesel production. Fuel 2017; 196: 169–177.