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Preparation of renewable polyol as raw material for polyurethane (PU) resin is become more important nowadays. In this research, polyol was prepared from epoxidized soybean oil (ESO) and applied as PU coatings on wood. Additional nucleophilic attack method with glycerol and ethylene glycol (EG) was performed to provide hydroxyls functionality in fatty acid backbones. By reacting the modified polyol with Isophorone diisocyanate (IPDI) adduct, where different ratios of NCO/OH were used, PU of various compositions had been obtained. The samples of polyols and PU obtained were characterized by Fourier Transform Infrared (FTIR) spectroscopy. Types of polyols prepared with various ratio of NCO/OH in each type of modification had been studied for coating properties (hardness, tensile and adhesion strength onto wood substrate). The result showed that the hardness and tensile strength of PU dependent on the ratio of NCO/OH. Polyol from glycerol provide higher tensile strength and adhesion on merantin wood substrate.
Keywords - Polyols; Polyurethane; Isophorone diisocyanate; soybean oils; coatings
In recent years, polymeric materials prepared from natural and renewable resources have attracted the attention of many researchers due to their potential to substitute petrochemical derivatives . Currently, interest in cheap biodegradable polymeric materials has recently encouraged the development of such materials from readily available, renewable, inexpensive natural sources such as starch, polysaccharide and edible oils .
Oils and fats of vegetables and animal origin make up the greatest proportion of the current consumption of renewable material in chemical industry. They offer a large number of possibilities in chemistry that can be rarely met by petrochemical . Vegetable oils contain unsaturated side on fatty acids bond, which have potential in polymerization of biopolymers [4-5]. Numerous fatty acids are now available in purity and that make itself more attractive for synthesis and raw material for industrial use especially in oil-base polymer.
In this research, polyol from ESO were synthesized for two components (2K) PU and applied as coating resin. Hydroxylation of ESO with ethylene glycol (EG) and glycerine has been done to provide the primary hydroxyl onto fatty acid backbones in triglyceride. On the other hand, the usage of glycerine in this reaction provides primary and secondary hydroxyl on the fatty acid backbones. It is an effort to have more hydroxyl functionality but at difference reactivity to the isocyanate.
Epoxidize soybean oil (ESO) supplied by Cognis and used without additional purification. Glycerine, ethylene glycol (EG), hydrogen bromide, phosphoric acid, glacial acetic acid, potassium phthalate, butyl alcohol, sodium sulphate anhydrous, N-methylpirrolydon (NMP),dibutyltin dilaurate (DBTL) (Sigma Aldrich, Germany), acetic anhydride, diethyl ether, ethanolic potassium hydroxide (R&M Marketing Essex, UK) analytical grade was used. Isophorone diisocyanate (IPDI) prepolymer (Desmodur 4470) from Bayer, Germany was used as received.
2.2 Synthesis of Triglyceride Polyol
ESO was modified with glycerine and EG to form two type of polyols. All Polyols were synthesized through hydroxylation process at Polymer Laboratory, UiTM Shah Alam.
Hydroxylation was done with excess of two moles EG or glycerines to the epoxies functionality. Phosphoric acid was used as catalyst (0.05% w/w).
500-ml multi-neck reaction flask equipped with a mechanical stirrer, a thermometer, a water cooled condenser and a dropping funnel, EG or glycerine, phosphoric acid in a specified proportion, was charged. The mixture was brought to the reflux condition by heating at a slow rate under mechanical stirring. The temperature of the reaction mixture was in 120 ÂÂ°C. ESO was dissolved in a portion of reaction solvent and added drop wise into the flask over a period of 45 minutes. After addition of the ESO solution was complete, heating was continued and the reaction sample was periodically tested for the acid value and the percentage of the oxirane contents. Reaction was stopped after 6 hours or when the oxirane contents was found to be.
The yields from the reaction were dissolved in diethyl ether and washed using deionise water for 4 times in separation funnel until the pH was 7 to remove excess glycerine or EG. Oxirane ring test was done according to the AOCS Cd.9-57, reapproved 1997 to measure the decreasing of epoxies functionality. The reaction was ended when the percentage of oxirane rings is nearly to zero. Hydroxyl value test was done according to AOCS Cd. 4-40 to measure the percentage of hydroxylation from the epoxidize oil.
Samples were dried using sodium sulphate anhydrous and it was filtered out after 2 hours. Diethyl ether was evaporated from the samples using rotational evaporator at 55 ÂÂ°C in vacum condition. Samples were mixed with IPDI at specified proportion and 0.05% (w/w) DBTL used as catalyst.
Viscosities of polyols were measured using Brookfield viscometer. Molecular weights were obtained from Gel Permeation Chromatography (GPC). FTIR was used to characterize the reaction between isocyanate and polyols during formation of polyurethane coatings.
Synthesized resin was applied as coating on meranti wood as depicts in figure 1 with different ration of IPDI and Polyol. NMP was used as film formation solvent and DBTL is a catalyst.
Figure 1: Preparation of PU coatings
2.3 Physical Testing
Physical properties of PU films were tested on their hardness (Pencil Hardness Test, ASTM D3363), Adhesion (Defelsco, Pull off Adhesion test, ASTM D4541) and Tensile test (ASTM D882). Gel content was done by soxhlet extraction using tolune as solvent.
3.0 Result & Discussion
Synthesis of Polyol
The preparation of polyol from ESO and glycerine or EG are essentially a nucleophilic substitution reaction involving the nucleophilic attack of hydroxyl on an electrophilic protonation of oxirane oxygen by an acid catalyst. The nucleophilic attack with acid catalyst in epoxidize fatty acid is described in schematic figure as below:
Figure 2: Addition alcohol component to epoxidize fatty acid .
Figure 3: The reducing of Epoxy groups during hydroxylation process
Figure 4: Molecular weight of Polyol
According to the result in table 1, most of the epoxy functionality in ESO reacted during hydroxylation process. The reducing of epoxy in soybean fatty acid was monitored by percentage of oxygen content (%OOC) (Figure 3) and hydroxyl value at the end of the reaction. The opening reaction of epoxy rings with hydroxyl shows the proportionate trends with hydroxyl value. The hydroxyl value increased at the end product due to the increasement of hydroxyl from glycerin and EG. These phenomenon is shown during hydroxylation process in figure 3. The high hydroxyl value justifies the fact that excessive oxirane groups are not lost in oligomerization. It is important to note that under the conditions of all the reactions, no appreciable glyceryl ester hydrolysis has occurred, as indicated by fairly narrow polydispersity index (Mw/Mn) for all the samples.
However, from the yield of reaction, 89.2% (Polyol-EG) and 83.7% (Polyol-Glycerin) were converted to the OH group from the epoxy group. More than 10% of epoxy was suspected to form an ether linkage and form the dimers when glyceryn was used in the reaction. The present of dimers can be shown in figure 4. Higher molecular weight and interaction of hydroxyls in polyol increased the viscosity properties and it could be seen in Polyol-Glycerine.
Phosphoric acid was used as a reactant as well as a catalyst. Phosphoric acid, a polybasic acid has three dissociation constants associated with each of its protons, giving rise to anions having significantly varying nucleophilicity. The used of phosphoric acid in the reaction produced a dark yield of polyol and it is disadvantage to the polyol for further formulation with isocyanate.
Table 1: Properties of modified Polyol
Percentage of oxirane rings (%OOC)
Hydroxyl Number (mgKOH/g)
Acid Number (mgKOH/g)
Average weight of Molecular Weight (Mw)
Number of Average Molecular Weight (Mn)
3.2 Effects of different Polyols at 40% IPDI (w/w)
FTIR spectrum in figure 3 depicts the reaction between isocyanate and hydroxyl group in IPDI and Polyol during the formation of PU at 40% IPDI content. The reducing peak at 2260 cm-1 (isocyanate) and 3200 cm-1 (hydroxyl) during the curing process at 60Â°C shows the reaction was occurred to form urethane linkage. After 12 hours, all the isocyanate had been reacted and remaining hydroxyl (according to theoretical calculation) is still in PU system.
Pencil Hardness, pull of adhesion and tensile strength were tested on coatings film. The result depicts that the hardness, tensile and adhesion strength are higher in Polyol-Glycerine than Polyol ââ‚¬"EG at the same percentage of IPDI. Higher functionality of hydroxyls in Polyol Glycerines provides random crosslink density and could attribute to the higher hardness and tensile strength. The adhesion of two pack PU (isocyanate and polyols) on wood substrate provide the possibility of chemical bonding between isocyanate and hydroxyl in wood fibers. On the other hand, the remaining hydroxyls in Polyol-Glycerine will promote the interaction between hydroxyl in wood and resin. Appropriate viscosity below than 2000CPs could provide an adsorption and mechanical interlocking between wood and resin.
Figure 5: FTIR spectrum of PU base on Polyol-EG (40% IPDI)
Table 2: Physical properties of coatings
Type of Polyol
Pull off Test Adhesion (MPa)
Tensile Strength (MPa)
Effects of different percentage of IPDI to the
hydroxyl functionality on coatings properties
Various ratios of IPDI was used to the polyol to investigate the properties of polyurethane properties. The amount of IPDI in polyurethane system could contribute higher density of crosslink when higher isocyanate is reacted to the hydroxyl in polyol. Table 3 illustrates the effect of IPDI in PU at various composition.
Table 3: Physical properties of coatings film with different percentage of IPDI to the hydroxyl functionality.
Type of Polyol
Pull off Adhesion (MPa)
Tensile Strength (MPa)
Gel Content (%)
Polyol- Glycerine (20%)
Polyol- Glycerine (40%)
Polyol- Glycerine (70%)
Polyol- Glycerine (90%)
From the result, two type of samples which is tacky or cured sample were produced when adding IPDI to the polyol. Tacky sample is a low or inediquate of crosslink density due to the low isocyanate content. Density of crosslink was measured using gel content test and the result depicts the increasing trend when higher percentage of IPDI was used.
Consequently, when the composition of IPDI was increased, the tensile and hardness of PU film also increased. The same trend showed in pull off adhesion. In pull off adhesion, adhesion of sample to the substrate and the tensile strenght of the sample is the main factor to the value of adhesion.
Hydroxylation process using EG and glycerines have high potential as renewable coatings resin due to the higher hardness, adhesion and tensile strength to the wood substrate. Higher functionality of hydroxyl in Polyol-Glycerine provide more active side for isocyanate and more polar resin to the polar substrate.