An efficient and environment friendly bio-based polyols through liquefaction: Liquefaction temperature and catalyst concentration optimization and utilized for rigid polyurethane foams

Abstract

Aiming towards the liquefaction of paddy straw was accumulation as well as providing a technically viable route leading to preservation of the natural resources and environment, the paddy straw was chemically liquefied. Paddy straw were liquefied into bio-based polyol in the presence of castor oil and blend of castor and karanja oil as depolymerizing agent and P-Toluene sulfonic acid as catalyst. Liquefied product was characterized by chemical as well as analytical techniques. The agricultural waste base paddy straw was eventually converted into polymeric precursor (polyol) monomer with nearly 80 to 95% yield by employing 2% catalyst concentration and at optimized temperature of 180°C. Synthesized polyol can be utilized further in formulating high quality rigid polyurethane foams. The foams were characterized in terms of their physical, mechanical, thermal and morphological properties. All foams exhibit good compressive strengths and thermal stability. Thermal conductivity of foams varied between 0.012 and 0.023 Kcal/mh°C, with the lowest being of foam from liquefied (LP), making it suitable for utilization as an insulation material.

Keywords

Lignocellulosic agricultural residue liquefaction non-edible oil renewable resource rigid PU foam

Introduction

Looking to present scenario, concerns over the depletion of petroleum resources as well as utilization of agricultural waste, extensive research has been concentrated on developing bio-based polyols (biopolyols) and PU products from renewable sources. In some previous studies, polymers obtained from different renewable resources have been reported.^1,2 Rice has always been valuable crop in South Asia of the world play an important role in the economic and social lives of the people of these zones.³ Among these enormous amounts of agricultural residues, only some off portion of the waste is used as animal feedstuff or domestic fuel. Nevertheless, a huge quantity of the remaining straw and stems is not used as industrial raw materials and is burnt in the fields or on the road. The pollution therefore is a serious problem by burning agricultural residues in this area. The use of agricultural residues in polymeric precursor, bio-based composite, pulping and papermaking has, therefore, many advantages including reducing the need for disposal and environmental deterioration through pollution, fires and pests.⁴

In the exploration of additional fields of applications of this abundant residual waste, one possible solution is its conversion to materials with high additional value. By this wisdom, among the plentiful methods studied in relation to green chemistry, the liquefaction of natural polymeric substrate has been an attractive field. In this process the biomasses directly transformed into liquid polyols for the preparation of ecologically approachable polymeric products.^5
-7 Liquefaction of lignocellulosic biomass embraces a complex set of reactions taking place on the polymeric components of the material, like esterification or etherification of free hydroxyl groups in cellulose or lignin as well as reactions that cause degradation of the biomass constituents.^8,9

Wood from different species^10
-12 and several agricultural crops residues^13,14 have been converted to polyols through liquefaction process. Nevertheless, as far as we know, the liquefaction of paddy straw has never been subjected to study.

The purpose of this frame work was to found the feasibility of chemically modify rice residue by means of liquefaction processes and the characterization of the subsequent polyols and in that way to discover a valuable industrial application for this plentiful biomass resource. The liquefaction of paddy straw residue was carried out using castor oil and blend of castor oil and karanja oil as liquefied solvents, respectively. And also optimized the process parameters like catalyst and without catalyst, temperature, and time towards yield concern. The viscous polyols were characterized by FT-IR spectroscopy. Elemental analysis and GPC was studied in terms of molecular weight for its different application. The hydroxyl numbers, acid numbers and viscosities were determined for the further preparation of polyurethanes, were also determined for the resulted polyols.

Material and methods

Paddy straw residue collected from local farm was used as raw material, Moisture content of rice straw was found to be 11.08%. Powders the collected residue (0.90 mm to 0.50 mm) were used in the experiment. Non-traditional oil use as liquefaction agent (castor oil, Karanja oil).

Chemical and elemental composition of non-traditional oils, rice straw and liquefied product

Chemical and elemental composition data of this straw are given in Table 1. And Table 2 illustrates the fatty acid profile of castor and karanja oil.

Table 1.

Elemental analysis of tobacco stalk rice straw and liquefied product.

Material	% N (RSD)	% C (RSD)	% H (RSD)
Rice straw	0.52	41.97	7.00
Liquefied Product	0.63	51.86 (2.0)	12.14 (1.0)

^* RSD = Relative standard deviation

Table 2.

The fatty acid profile of karanja and castor oil.

Fatty Acid	Percentage
Karanja Oil
Palmitic Acid	09.85
Stearic Acid	06.13
Oleic Acid	56.96
Linoleic Acid	16.94
Linolenic Acid	02.87
Unknown Fatty Acids	07.25
Castor Oil
Palmitic Acid	02.39
Stearic Acid	03.83
Oleic Acid	07.41
Linoleic Acid	09.11
Linolenic Acid	0.65
Ricinoleic Acid	76.41
Unknown Fatty Acids	0.20

Liquefaction

Air dry paddy straw (10 g) and 100 to 120 g non-traditional oil were premixed. Then the mixture was transferred into a 250 ml, three necked flask equipped with a mechanical agitation system and a spherical reflux condenser. The flask was heated in an oil bath at 130–190 ± 1°C. After a pre-setting time, the flask was cooled to room temperature immersing in cold water bath of about 5°C to quench the liquefaction reaction, the products were diluted by dioxin and then filter it by glass filter and separate out the residue and filtrate. Collect the product for chemical and analytical characterization. The % conversion (w/w) of feed was calculated according to the following equation.

% C o n v e r s i o n (l) = \frac{(w_{0} - w)}{W_{0}} \times 100

where W₀ = Weight of the initial dry rice straw. W = Mass of the residue obtained after the liquefaction.

FT-IR analysis

Infrared spectra of paddy straw liquefied were examined by using Fourier Transform Infrared Spectrophotometer (FTIR-3000 E). All measurements were carried out using the KBr disc technique.

Hydroxyl number of liquefied (LP)

The hydroxyl numbers of liquefied was determined as follows: a mixture of 1 g LP and 25 ml of a phthalation reagent was heated for 20 min at 110°C. This was followed by addition of 50 ml of 100% 1, 4-dioxane and 25 ml of distilled water and the mixture was titrated with a 1 M sodium hydroxide solution to the equivalence point using a pH metre. The phthalation reagent consisted of a mixture of 150 g phthalic anhydride, 24.2 g imidazole and 1000 g dioxane. The hydroxyl number in mg KOH/g of sample was calculated by the following equation:

H y d r o x y l n u m b e r (m g \frac{K O H}{g}) = \frac{(c - B) N \times 56.1}{w} + A c i d n u m b e r

where A is the volume of the sodium hydroxide solution required for titration of LP sample (ml); B is the volume of blank solution (ml); N is the normality of the sodium hydroxide solution and W is the weight of LP (g).

Acid number of LP

A mixture of 1 g LP sample, 10 ml 100% 1,4-dioxane and 20 ml water was titrated with 1 M KOH solution to the equivalence point. The acid number in mg KOH/g of sample was calculated by the following equation:

H y d r o x y l n u m b e r (m g \frac{K O H}{g}) = \frac{(c - B) N \times 56.1}{w}

where V is the titration volume of sodium hydroxide solution at equivalence point (ml); N is the normality of Potassium hydroxide solution and W is the weight of LP.

Rheological actions of LP1 and LP2 polyols

The rheological performance of the two polyols (LP1 and LP2) was examined by flow-curve analysis. Figure 1 represents the relation of viscosity (η) and shear stress (τ) against shear rate (γ). Viscosity of the LP1 polyol found to be around 51,421 mPa.s, which is higher than that of LP2 polyol (around 21,880 mPa.s).

Figure 1.

Flow performance of LP1 and LP2.

Preparation of foam

The rigid polyurethane foam was synthesized by following the procedure reported in the literature.¹⁵ The foams were labelled as PUF 1(prepared using LRS-CO bio-based polyol in the presence of castor oil), PUF 2 (prepared using LRS-CKO blend of castor and karanja oil based polyol). Recipes for the formulations are given in Table 3. The PU foams were prepared by adding PTDI to the polyol mixture, which consisted of a LRS-CO and LRS-CKO polyol, distilled water, catalyst and Tegostab (surfactant), with stirring. At the creamy stage, the mixture was decant into a coffee cup and left undisturbed to rise freely. The full rise time (the time from mixing to full expansion of foaming) was recorded. Later, the foam was removed from the mould and allowed to post cure for 2 days at room temperature before cutting it into test specimens. The conventional foam sample used for the comparison was prepared by the same conditions using petroleum based polyol. Samples were cut after 1 week as per the test requirement and then properties of foams were measured. Cream time, rising time, and gel time was also determined. Figure 2 is the image of prepared polyol and foam sample.

Table 3.

Foam formulations.

Sr. No	Sample code	Foam composition (g)
Sr. No	Sample code	Polyol	Surfactant	Catalyst(DMCHA)	TDI	Water
1	LRS-CO	50	2.0	1.0	80	1.0
2	LRS-CKO	50	2.0	1.0	80	1.0

Figure 2.

Prepared polyol and foam image.

Thermogravimetric analysis (TGA)

TGA was carried out using Mettler Toledo thermogravimetric analyzer (TGA) model (TGA/STDA 851 e). The weight loss of samples during temperature ranging from 50°C to 610°C was measured under nitrogen flow 10 ml min⁻¹ with a heating rate of 10°C min⁻ ¹.

Thermal conductivity

Thermal conductivity of prepared foam was measured by divided base method. In this method a thin sample plate is sandwiched between two metal roads. The cross-section of sample is the same as the shape of metal roads. Heat is provided from one end of the block. Temperature are measured at given point and its allow us to determine the thermal conductivity.

Scanning electron microscope (SEM)

A study of morphological properties by SEM images of the samples were obtained in a Theophylline Microspheres (FEG-SEM) operating at 5 kV. The samples were placed on a plastic support.

Result and discussion

Elemental analysis

The elemental analysis proves that the derived product shows, high carbon and hydrogen percentages were found in rice straw. From that it is confirm the availability of hydroxyl groups in this substance and nitrogen was not detected. The analysed data of element values listed in Table 1 are in agreement with those reported for other kinds of raw materials that could be used as precursors for polyol preparation.¹⁶

Process parameter

In order to establish technically and commercially viable process for utilizing the agricultural residues, the process parameters like temperature, catalyst concentration as well as time were altered and further the influences of the process parameters on the yield of the product was studied. Table 4 shows the study of correlation of catalyst concentration, temperature, time as well as without catalyst for the yield concerns and liquefied reagent used like castor oil and blend of castor and karanja oil (30:70). The oil was varied for castor to blend of castor and karanja oil, from these two we get good result in the blend of castor and karanja oil. The concentration of catalyst was varied from 0 to 2% with step wise increase of 0.5% for each trial. The effect of temperature was studies eventually between 130°C and 180°C. The reaction time for the process was diversely between 120 min to 180 min. The best result was obtained when the reaction proceeded with catalyst concentration of 2% at temperature 180°C for 180 min, which increased the yield of the liquefied product up to 94%, which was dark brown in colour and liquid polyol. Even without catalyst we also get comparable results of yield in same process conditions. In order to establish technically and commercially viable process for utilizing the residual rice straw, the process parameter like liquefied reagent (castor oil and blend of castor with karanja oil) catalyst, temperature (180°C) as well as time (1 hr to 3 hr). According to study it is indicating that in the presence of catalyst (PTSA), the appropriate high yield conversion in to liquefied product compared to without catalyst. The study also indicate that the yield of liquefied product is get high using of blend of castor oil through karanja oil(LP2), (30:70) then castor only (LP1) used as liquefied reagent. So form it is conformed that not only castor oil used for liquefaction, but the other oil also use if the required modification done.

Table 4.

Study of process parameters.

Rice Straw (g)	Castor oil (ml)	Castor/Karanja oil (ml)	% Catalyst PTSA	Temperature (°C)	Reaction time (min)	Liquefied product (g)	% Yield
20	130	–	–	130	120	–	–
					150	5.15	25.75
					180	7.70	38.50
20	130	–	–	150	120	1.40	7.00
					150	6.05	30.25
					180	8.10	40.50
20	130	–	–	180	120	5.70	28.50
					150	9.15	45.75
					180	13.77	68.85
20	–	30:70	–	130	120	–	–
					150	5.80	29.00
					180	8.50	42.50
20	–	30:70	–	150	120	2.40	12.00
					150	7.20	36.00
					180	8.70	43.50
20	–	30:70	–	180	120	6.15	30.75
					150	10.05	50.25
					180	14.01	70.05
20	130	–	0.5	130	120	2.35	11.75
					150	7.50	37.50
					180	9.70	48.50
20	130	–	1.0	150	120	4.30	21.50
					150	8.90	44.50
					180	10.09	50.45
20	130	–	2.0	180	120	7.10	35.50
					150	13.40	67.00
					180	7.10	85.50
20	–	30:70	0.5	130	120	2.80	14.00
					150	8.50	42.50
					180	10.70	53.50
20	–	30:70	1.0	150	120	6.20	31.00
					150	9.70	48.50
					180	10.00	50.00
20	–	30:70	2.0	180	120	8.60	43.00
					150	14.60	73.00
					180	18.81	94.05

Characterization of liquefied products (LP)

The physiochemical properties of the LP are display in Table 5. It was found that the acid value castor oil based as well as blend of castor oil by means of karanja oil based was in the range of 1.5 to 2.5 mg KOH/g and hydroxyl value are between 250 to 500 mg KOH/g which clearly indicates the LP used as polymeric precursor. The liquefied products investigated by (gel permeation chromatography) GPC for the molecular weight of two LP based on castor (LP1) and blend of castor with karanja oil (LP2). Figure 3 shows the chromatogram of castor oil based LP; it demonstrates sharp peak at the retention time of 8.784 min corresponding to no average molecular weight of 1490 g mol⁻¹ with molecular weight 1917 g mol⁻¹ with a polydispersity ratio of 1.286 which indicates the product is near to mono-dispersed in nature, while the chromatogram of LP2 (Figure 4) portray a single narrow peak at retention time 8.775 min and molecular weight of 2059.5 g mol⁻¹ which conform the molecular weight of LP monomer. The Mw/Mn ratio of the LP2 fraction obtained was 1.3 which designates mono dispersion of fraction 2 as well as suitable for different polymeric application.

Table 5.

Physiochemical properties of fractions (LP1 and LP2).

Sample	I_OH (mg KOH/g)	Viscosity (Pa s at 25°C)	Acid Value (mg KOH/g)	Molecular weight	Polydispersity
LP1	370	1.20	3.58	1917	1.286
LP2	295	2.15	2.14	2059.5	1.26

Figure 3.

GPC of liquefied product (LP) 1.

Figure 4.

GPC of liquefied product (LP) 2.

Figure 5(a) and (b) shows the FT-IR spectrum of LP1 and LP2. The absorption of 3397.9 cm⁻¹ and 3394.22 cm⁻¹ indicated the presence of hydroxyl (–OH) alcoholic group. A band at 2959.38 cm⁻¹ indicates C–H stretching, a narrow sharp band of C=O was observed at 1731.25 cm⁻¹ and 1725.38 cm⁻¹, C–O–C was observed at 1056.26 cm⁻¹ and 1055.17 cm⁻¹ and bands between 675 and 900 cm⁻¹ demonstrated relation with aromatic ring while, –OH was identified at 1129.35 cm⁻¹ and C–H aromatic bonds at 1402–1502 cm⁻¹. The C–O ester asymmetric vibration was noted at 1230.16 cm⁻¹ and C–O ester symmetric vibrations at 1055.17 cm⁻¹ were also detected. The FT-IR spectrum indicated the functional groups as contained by polymeric precursor monomer.

Figure 5.

(a) FT-IR of liquefied product (LP) 1. (b) FT-IR of liquefied product (LP) 2.

Characteristic data of viscous polyols

The acquired products were evaluated to determine their appropriateness as replacements of low cost synthetic precursors in formulations of environment friendly polymeric products. In that sense, the hydroxyl number [IOH], acid value, colour and viscosity are the most relevant factors as far as the preparation of polyurethane is concerned. Table 5 shows the characteristic data of two samples with suitable yield during the liquefaction by using castor oil and blend of castor in karanja oil.

A result compared with published value in the literature for the other raw materials used for liquefaction technique revealed that the IOH obtained in this method (370 mg KOH/g) was comparable with those reported for polyol prepared from several biomass residues using castor oil and blend of castor/karanja oil similar experimental conditions [reaction temperature: 165°C, IOH: 200–400 mg KOH/g.¹⁷

The liquefied product existing IOH value of 370 (mg KOH/g) that was very nearly in the matching range with those achieved in the liquefaction of different hardwoods and soft woods studied by Kunaver et al. [varied from 200 to 400 mg KOH/g],¹⁸ Japanese wood species investigated by Kurimoto et al. [varied from 275 to 329 mg KOH/g].¹⁹ On the basis of the above results, polyols obtained by liquefaction processes acquire the potential to be used as precursors for the preparation of polyurethanes, due to their suitable IOH and viscosities values (IOH between 300 and 400; viscosity below 300 Pa s).²⁰

Characterization of rigid polyurethane foam samples

Processing Parameters and Density the foaming process parameters of cup foam for polyols are listed in Table 6, with the cream times, full cup times, string times, tack-free times and end of rise times of the PUF-1, and PUF-2. It can be seen that the foaming rate of PUF 1 is slightly higher due to viscosity of polyol is lower than LP2, and wire illustrate time of the polyester-based foam systems were approximately the same, indicating that their reactivates were almost equivalent.

Table 6.

Foaming processing parameters of foam for polyols.

Sample Code	Cream Time (s)	Full Cup Time (s)	String Time (s)	Tack-Free Time (s)	End of Rise Time (s)
PUF-1	36	41	50	63	92
PUF-2	32	39	46	58	84

Density of prepared foam was measured according to ASTM D1662. The specimen was cut into dimension of 30 × 30 × 30 mm. The density of three specimens was measured and averaged. The density of foams is greatly affected by the blowing agent. Water is a low cost and readily available blowing agent used for preparing PUFs. Increasing the amount of water in the formulation of 1 to 10 parts per hundred parts of polyol (pphp), the density of foam decreased from 210 to 132 kg/m³ as shown in Figure 6(a). The observed diminish in density of the foam with expanding the measure of blowing agent is straightforwardly identified with the cell size of the prepared foams. As the water content was expanded the synthetic response among water and diisocyanate produces more measure of carbon dioxide, which is trailed by more air pocket arrangement. These air pockets coagulate bringing about greater cell size because of which mass per unit volume diminishes. The cell size of the foam increased with increase in amount of water, reducing the area of surface boundary around the bubbles ultimately reducing the compression bearing capacity of the foam.

Figure 6.

(a) Density of the foams (pphp parts per hundred parts of polyol). (b) Compressive strength of the foams (pphp parts per hundred).

Compressive Strength: The compressive strength of the prepared PUFs samples was measured according to ASTM D 1521 under ambient conditions with Universal Testing Machine and strength decreased from 2.20 to 0.40 MPa as the water content in the formulation increased from 1 to 10 pphp as show in Figure 6(b). They got compressive strength of the readied PUFs tests was similarly acceptable when contrasted with past work.²¹ The explanation for the lessening in compressive strength again decreases with the cell size of the got foam. The cell size of the foam increase with the amount of water reduces the area of surface boundary around the bubbles and ultimately reduces the compression bearing capacity of the foam.

Thermo gravimetric Analysis: Thermal stability of the of the prepared PUFs sample (water 2 pphp) having great mechanical property was done to evaluate its high temperature characteristics. Figure 7 (a) and (b) displays the TGA profiles of both prepared foams. As shown in curve both foam sample were thermally stable up to 300°C, The degradation process was complex and depends on several factors such as polyurethane linkages, unreacted isocyanate, and other products formed due to the reaction of isocyanate with other substances of the formulation.²² The obtained curve shows that the decomposition of foam started at around 170°C due to the moisture. Pyrolysis of PU foam under nitrogen condition begins at 200°C and intensified at 280°C. The main decomposition range of PUFs took place between 300°C and 400°C. For bio-based foams weight loss is negligible after 600°C. For both prepared foams there is not much difference between their thermal stability.

Figure 7.

(a) TGA graph of the castor oil based polyurethane foam (PUF-1). (b) TGA graph of the blend of castor and karanja oil polyurethane foam (PUF-2).

Morphological Characteristics: The structure of the both prepared foam was relatively homogeneous with thin walled and well-oriented shape as shown in Figure 8(a), (b). Cells of approximately 0.2 mm diameter and uniformly distributed were obtained. During the study, the sample PUF-2 (B) having more compact structure and more uniform and spherical-shaped pores than sample PUF-1(A), which resulted in higher compressive strength, which shows the importance of morphology of the foam on its mechanical strength.

Figure 8.

SEM of the prepared foam (a) Sample PUF - 1, (b) Sample PUF - 2.

Conclusion

Paddy straw (residue) was successfully liquefied by castor oil and blend of castor with karanja oil, providing the most promising solution regarding solid waste or agricultural residual waste management. From the numbers of experimental set for study of influences of catalyst concentration, temperature, reaction time and without catalyst on the yield of the end product, the liquefaction process in presence of 2% of catalyst at 180°C in 180 min in blend of castor through karanja oil as liquefying reagent providing highest conversion and yield of LP2 that has proven to be an economically viable route. The orchestrated polyols were additionally planned for making PU foams having characteristics and physical properties like the showcased foams. The physical properties were significantly influenced by using blowing agent and the properties of the prepared foam were found to have an inverse proportional relationship. Moreover, the other properties such as the thermal stability, thermal conductivity, and the morphological state foremost to mechanical as well as thermal applicability were found to be in accordance to the advantage values. By this way, the optimized procedure has demonstrated its track in acquiring the polymeric previous circumstances from bio-waste for a broadly applicable to polymeric framework like foam.

Footnotes

Acknowledgment

Dr Chiragkumar M Patel would like to thank CSIR-IICT, Hyderabad, India, for granting research fellowship and allowing conducting experiments.

Author contributions

All authors contributed to the design of the work, interpreting results, and manuscript writing. All authors have given approval to the final version of the manuscript.

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

Chiragkumar M Patel

Nikhil R Dhore

References

Lligadas

Ronda

Galia

, et al. Synthesis and characterization of polyurethanes from epoxidized methyl oleate based polyether polyols as renewable resources. J Polym Sci Part A: Polym Chem 2006; 44: 634–645.

Gandini

Belgacem

. Recent contributions to the preparation of polymers derived from renewable resources. J Polym Environ 2002; 10: 105–114.

Fox

Fujita

Ngidang

Peluso

, et al. Policies, political-economy, and swidden in Southeast Asia. Human Ecology 2009; 37(3): 305–322.

Gheewala

Bonnet

Silalertruksa

. Environmental sustainability assessment of sugarcane bioenergy. In: O’Hara

Mundree

(eds) Sugarcane-Based Biofuels and Bioproducts. Hoboken, NJ: John Wiley & Sons, 2016, pp. 363–378.

Choi

Mathews

. Two-step acid hydrolysis process kinetics in the saccharification of low-grade biomass: 1. experimental studies on the formation and degradation of sugars. Bioresource Technol 1996; 58(2): 101–106.

Kurimoto

Doi

Tamura

. Species effects on wood-liquefaction in polyhydric alcohols. Holzforschung 1999; 53(6): 617–622.

Maldas

Shiraishi

. Liquefaction of biomass in the presence of phenol and H2O using alkalis and salts as the catalyst. Biomass Bioenergy 1997; 12: 273–279.

Krzan

Kunaver

. Microwave heating in wood liquefaction. J Appl Polym Sci 2006; 101(2): 1051–1056.

Shiraishi

. Liquefaction of wood without a catalyst, 1: time course of wood liquefaction with phenols and effects of wood/phenol ratios. J Japan Wood Res Soc (Japan) 1993; 39(4): 446–452.

10.

Yamada

Ono

. Rapid liquefaction of lignocellulosic waste by using ethylene carbonate. Biores Technol 1999; 70(1): 61–67.

11.

Alma

Altay Basturk

Digrak

. New polyurethane-type rigid foams from liquified wood powders. J Mater Sci Lett 2003; 22(17): 1225–1228.

12.

Luo

. Polyols and polyurethanes from the liquefaction of lignocellulosic biomass. ChemSusChem 2014; 7(1): 66–72.

13.

Shiraishi

Onodera

Ohtani

, et al. Dissolution of etherified or esterified wood into polyhydric alcohols or bisphenol A and their application in preparing wooden polymeric materials. J Japan Wood Res Soc (Japan) 1985; 31(35): 418–420.

14.

Wang

Chen

. A novel method of utilizing the biomass resource: rapid liquefaction of wheat straw and preparation of biodegradable polyurethane foam (PUF). J Chin Inst Chem Eng 2007; 38(2): 95–102.

15.

Jin

Ruan

Cheng

, et al. Liquefaction of lignin by polyethyleneglycol and glycerol. Bioresour Technol 2011; 102(3): 3581–3583.

16.

Carriço

Fraga

Pasa

VMD

. Production and characterization of polyurethane foams from a simple mixture of castor oil, crude glycerol and untreated lignin as bio-based polyols. Euro Polym J 2016; 85: 53–61.

17.

Serrano

Alriols

Briones

, et al. Oxypropylation of rapeseed cake residue generated in the biodiesel production process. Ind Eng Chem Res 2010; 49(4): 1526–1529.

18.

Kunaver

Medved

Čuk

, et al. Application of liquefied wood as a new particle board adhesive system. Bioresour Technol 2010; 101(4): 1361–1368.

19.

Kurimoto

Takeda

Doi

, et al. Network structures and thermal properties of polyurethane films prepared from liquefied wood. Bioresour Technol 2001; 77(1): 33–40.

20.

Cateto

Barreiro

Rodrigues

, et al. Optimization study of lignin oxypropylation in view of the preparation of polyurethane rigid foams. Ind Eng Chem Res 2009; 48(5): 2583–2589.

21.

Zhang

Pan

Zhang

, et al. Study of the mechanical, thermal properties and flame retardancy of rigid polyurethane foams prepared from modified castor-oil-based polyols. Ind Crops Prod 2014; 59: 135–143.

22.

Zhao

Yan

Feng

. Polyurethane foams derived from liquefied mountain pine beetle-infested barks. J Appl Polym Sci 2012; 123: 2849–2858.