Preparation and characterization of tung oil-based flame retardant polyols☆

Wei Zhou ,Puyou Jia ,Yonghong Zhou ,Meng Zhang ,*

1 Institute of Chemical Industry of Forestry Products,CAF,Nanjing 210042,China

2 National Engineering Lab for Biomass Chemical Utilization,Nanjing 210042,China

3 Key Lab of Forest Chemical Engineering,SFA,Nanjing 210042,China

4 Key Lab of Biomass Energy and Material,Nanjing 210042,China

Keywords:Tung oil Transesterification Epoxidation Flame retardant polyols

A B S T R A C T Three kinds of tung oil-based structural flame retardants polyols(TOFPs)were prepared by new methods in this paper.First,tung oil was used to produce monoglyceride and diglyceride by transesterification with glycerol by sodium methoxide.The products after transesterification were epoxidized by peracetic acid which was in-situ generated from acetic acid and hydrogen peroxide in the presence of sulfuric acid catalyst.And then,TOFPs were prepared from epoxidized alcoholysis tung oil(EGTO)with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO),diethyl phosphate(DEP)and diethanolamine(DEA)by ring-opening reactions,respectively.GPC was used to evaluate the conversion rate,at optimum reaction conditions,selectivity for monoglyceride in transesterification.The influence of different parameters such as temperature,mole ratio or mass ratio on the conversion rate of transesterification and epoxidation were investigated.The molecular structures of TOFPs were characterized by FTIR and 1HNMR.Finally,tung oil-based polyurethane foams(TOPUFs)were prepared by a one-shot process using TOFPs with polyisocyanate.The LOI values of TOPUFs whose content of DOPO-EGTO,DEP-EGTO and DEA-EGTO were 100 wt%can reach to 26.2%,25.1%,and 24.4%,respectively.

1.Introduction

Polyurethane foams(PUFs)are common materials used in construction industry,which can be used as polymeric concrete components,insulating materials,sealants etc.[1].The fluctuations of crude oil price affect the production of PUFs prepared from petroleum-based polyols with isocyanate.At the same time,the massive consumption of PUFs arouses people's concern about the environmental pollution[2].People's awareness of environmental deterioration and dependency on depleting fossil feedstock attract researchers'interests on finding renewable energy to replace the petroleum.Recently,much attention has been paid to the biomass-based polyols,which are claimed to be more environmentally friendly than petroleum-based polyols[3–5].Vegetable oilis one kind of biomass resources with low cost,biodegradability,universal availability,environmental acceptance,renewability and non-toxic nature[6],it is triglyceride with a structure of an ester formed from glycerol and three fatty acids[7].Vegetable oil will be the most promising option as feedstock for green polyols to produce polyurethane materials.Several kinds of vegetable oils,including soybean oil,castor oil,palm oil and olive oil,have been utilized as starting materials for polyurethane materials[8,9].Non-edible woody plant oils such as tung oil,which is a typical natural drying oil obtained from the seeds of tung tree,are being used for the production of PUFs[10].PUF has the property of low density,large specific surface area and high opening rate.It can produce melt dripping and large amounts of toxic smoke which contains carbon monoxide,hydrogen cyanide when was burn in the air.What's more,the porous structure of PUF leads to flameless smoldering[11].The high porous and combustible nature of polyurethane foams further facilitates the spread of flame rate[12].Many researchers have engaged in improving the flame retardant properties of PUFs.The most representative flame retardants[13]are mineral flame retardants,halogen-based flame retardants,phosphorusbased flame retardants,nitrogen-based flame retardants and siliconbased flame retardants.Traditionally,the halogen-based flame retardants for PUFs such as pentabromodiphenyl ether and chloroethyl phosphate may endow PUF excellent flame retardation[14].When burning,such retardants release a lot of toxic gases that are harmful to the environment and human health[12,15,16].Thus,the environmentally-friendly flame retardants with good flame retardancy are urgently needed.Many reports[17–20]have suggested that phosphorus,nitrogen and silicon-based flame retardants exhibit good flame retardant efficiency because they provide excellent fire protection with less smoke and lower toxicity.Phosphorus-based flame retardants like phosphonates [21],phosphoramidates[22]and DOPO based flame retardants[23]have been shown to be quite efficient in flame retardation of PUFs.Nitrogen-based flame retardants can provide an inert diluent in the flame[24,25].Silicon-based flame retardants include silicones and silica.They can degrade,forming thermally stable silica,which have a tendency to migrate to the char surface serving as a protection layer to prevent further degradation of char at high temperature in the flame.

In this work,tung oil-based structural flame-retardant polyols with novel structure are prepared by transesterification of tung oil with glycerol;then the epoxidation of the product of glycerolysis of tung oil(GTO)takes place;after that,the ring opened by compounds with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO),diethylphosphate(DEP),and diethanolamine(DEA),respectively.The effects of different parameters such as temperature,mole ratio or mass ratio of transesterification and epoxidation were studied,and the structure properties of the flame-retardant polyols were also investigated.Tung oil-based polyurethane foams(TOPUFs)have been prepared by a one-shot process with tung oil-based flame retardant polyols.The main flame retardant properties of TOPUFs were investigated by limiting oxygen index(LOI).

2.Experimental

2.1.Materials

Tung oil(TO)and polyether polyols(PPG4110,sucrose and diethylene glycol as initiator,hydroxyl value:403(mg KOH)·g-1,viscosity at 25 °C:3.6 Pa·s)were obtained from Jiangsu Qianglin Bioenergy MaterialCo.,Ltd.Glycerol,sulfuric acid,ethyl acetate and DEA were supplied by Nanjing Chemical Reagent Co.,Ltd.Hydrogen peroxide(30 wt%)was purchased from Shanghai Linfeng Chemical Reagent Co.,Ltd.Sodium methoxide,acetic acid,triethyl phosphate(TEP)and DOPO were provided by Aladdin Chemical Co.,Ltd.Catalyst employed is:dibutyltin dilaurate kindly supplied by Aladdin Chemical Co.,Ltd.;surface active agent:polysiloxane-polyether copolymer(AK8804)purchased from Jiangsu Maysta Chemical Co.,Ltd.;blowing agent:blend of 1,1,1,3,3-penta fluorobutane(HFC-365mfc)and water.DEP was produced by laboratory and Polyaryl polymethylene isocyanate(PAPI,30.0 wt%–32.0 wt%NCO)was obtained from Yantai Wanhua Polyurethane Co.,Ltd.

2.2.Transesterification of TO

TO(87.2 g,0.1 mol)and glycerol(55.2 g,0.6 mol)were added into a three-necked 250 ml round bottom flask which is equipped with a mechanical stirrer,thermometer and re flux condenser.The reaction temperature ranges from 180 °C to 230 °C,and the molar ratio between glycerol and TO was from 2 to 10.Sodium methoxide as catalyst in a 0.75%by the mass of the total reactants,the mixture was stirred in a heating mantle for 3 h.Successively,the reaction mixture was slowly cooled down to room temperature,the mixture was poured into the separating funnel,and it will hung out for 12 h to separate due to gravity difference[26].The upper layer contains predominately the monoglyceride and diglyceride,the lower layer contains the glycerol.The mixture of upper layer was collected in the 500 ml beaker;1 mol·L-1citric acid solution was added and stirred immediately to neutralize the catalyst.Then the ethyl acetate was added to extract the mixture,the organic layer was under reduced pressure to remove the ethyl acetate until a constant weight of the brown liquid(GTO)was produced.Fig.1 shows the transesterification of TO.

2.3.Epoxidation of GTO

Epoxidation of GTO is shown in Fig.2.GTO and acetic acid were charged into a 500 ml flask at 50°C under vigorous stirring.Then,sulfuric acid(5.4 wt%)as catalyst was used;the final concentration of sulfuric acid in mass is 0.5%.Hydrogen peroxide was added slowly using syringe per0.5 h.The reaction was continued at50°C for another 5 h.The effects of the different mass ratios of acetic acid/GTO and H2O2/GTO on epoxy group formation were studied.In the end,150 ml of ethyl acetate and 100 ml of distilled water were added;the liquid separates into 2 layers.The organic layer was neutralized,and then dried over anhydrous magnesium sulfate and filtered.Finally,epoxidized GTO(EGTO)was obtained after vacuum submitted to rotary evaporation at 38°C for 2 h and dried in a 70°C drying oven over night.

2.4.Synthesis of DEP

45.5 g TEP and 28.2 g phosphoric anhydride were charged into a 250 ml flask.The reaction was run for 1.5 h under vigorous stirring at 50°C to obtain the intermediates of polymeric phosphoric acid.DEP was prepared by the intermediates of polymeric phosphoric acid which blended with 16.8 g anhydrous ethanol for 16 h at 110°C.The liquid and solid products were separated by filtration when the vessel was cooled to room temperature.DEP was obtained by complete removal of the anhydrous ethanol using a rotary evaporator under vacuum.The yield of DEP was about 80%.

2.5.Preparation of tung oil-based flame retardant polyols(TOFPs)

EGTO(epoxy value=1.19%)was used to synthesize the tung oil based flame retardant polyols.Polyols were identified as DOPO-EGTO,DEP-EGTO and DEA-EGTO,respectively.

Polyol of DOPO-EGTO was prepared by mixing DOPO with EGTO in a flask under magnetic stirring at 160°C.The mass ratio of EGTO to DOPO was10 g/40 g.The reaction proceeded for5 h,the reactant was cooled to room temperature,then it was dissolved in ethyl acetate for 2 days to precipitate out the excess DOPO;the solid DOPOwas separated by filtration.The DOPO-EGTO which was dark brown,viscous liquid,was obtained by evaporation of ethyl acetate.The idealized reaction scheme for the synthesis of DOPO-EGTO is shown in Fig.3.

The synthesis of DEP-EGTO was carried out as follows:the first step was to obtain the reactant solution by using 30 g EGTO with 40 g toluene.40 g toluene,20 g DEP and 0.1 g triphenylphosphine were dripped in reactant solution during 30 min.The reaction was under 75°C for 4 h,after that the reactant solution was neutralized using 1 mol·L-1NaOH,water was separated by extraction and the toluene was removed by reduced pressure distillation.The idealized reaction scheme for the synthesis of DEP-EGTO is shown in Fig.4.

Fig.1.Transesterification of TO.

Fig.2.Epoxidation of GTO.

Fig.3.Preparation of polyol from EGTO ring-opened by DOPO.

Fig.4.Preparation of polyol from EGTO ring-opened by DEP.

The synthesis of DEA-EGTO was carried out as follows:A 250 ml flask was charged with 20 g EGTO and 40 g DEA,then 50 g isopropyl alcohol and 4 g HBF4were added.The reaction was run for 28 h under vigorous stirring at 70°C,after that isopropyl alcohol evaporates from the reaction solutions and the excessive DEA was removed by saturated sodium sulfate solution washing.The DEA-EGTO was obtained after water was removed by reduced pressure distillation.The idealized reaction scheme for the synthesis of DEA-EGTO is shown in Fig.5.

Fig.5.Preparation of polyol from EGTO ring-opened by DEA.

2.6.Preparation of tung oil-based polyurethane foams(TOPUFs)

A series of TOPUFs were prepared by the one-pot and free-rise method.Table 1 shows the formulations of TOPUFs.The TOFP was premixed at room temperature with catalyst,surfactant,blowing agent,water and PPG4110.After the mixture was mechanically stirred at 1500 r·min-1for 60 s,it was added PAPI and then stirred at 1800 r·min-1for another 15 s.The mixture was casted to an open steel box(25 cm×25 cm×5 cm)for foaming.The resulting TOPUF was placed in an oven for 24 h at 80°C to complete the polymerization reaction prior to characterization.

2.7.Characterization

2.7.1.Analytical methods

The final products were analyzed for iodine value,epoxy value and hydroxyl value.The iodine value of product was determined using Wijs method according to the method(ISO 3961-2009).The epoxy value was obtained by the direct method with the hydrochloric acid solution in acetone referring to Chinese National Standard GB/T 1677.The hydroxyl value was determined by the method(ISO 14900-2001).The acid number was determined by the method(ASTM D 7253-2006).Viscosity was measured at 25°C using Brook field DVS+viscometer.

Table 1Formulations of tung oil-based PUFs

2.7.2.Gel permeation chromatography(GPC)

Products were measured by using a Gel permeation chromatography(GPC)made by Waters,USA at 30 °C( flow rate:1 ml·min-1,column:mixed PL gel 300 mm × 718 mm,25 μm)using THF as solvent.

2.7.3.Fourier transform infrared spectroscopy(FTIR)

Fourier transform infrared spectroscopy(FTIR)of samples was investigated on a Nicolet iS10 FTIR(Nicolet Instrument Crop.,USA)Fourier transformed infrared spectrophotometer.The spectra were acquired in the range of 4000–500 cm-1at a resolution of 4 cm-1.

2.7.4.1HNMR

1HNMR spectra of samples were conducted on an AV-300 NMR spectrometer(Bruker Instrument Crop.,Germany)at a frequency of 400 MHz with CDCl3as solvent and tetramethylsilane(TMS)as an internal standard.

2.7.5.Limiting oxygen index(LOI)

LOI test was carried out according to the GB/T2406–1993 to determine the relative flammability of foam.Test specimen dimensions used were 100 mm×10 mm×10 mm(length×width×thickness).

3.Results and Discussion

3.1.Properties of TO,GTO and Tung oil-based flame retardant polyols

Iodine value,hydroxyl number,acid number and viscosity of these samples are summarized in Table 2.

The iodine value of TO and GTO indicated that there were lots of double bonds in the TO and GTO.The iodine value of DOPO-EGTO,DEP-EGTO and DEA-EGTO were decreased compared with GTO mirrored that some double bonds via epoxidized followed by ring open reaction.However,the remaining iodine value of the tung oil-based flame retardant polyols showed that double bonds were notepoxidized completely.The OHnumbers of the flame retardant polyols are higher than GTO and TO,this was explained by the fact that the epoxy group have been ring opened by the phosphorus-nitrogen containing compounds.It was theoretically possible for the flame retardant polyols to have lower hydroxyl number than GTO and TO.The acid numbers of flame retardant polyols indicated the presence of small amounts of fatty acid in the mixture.The viscosity of the flame retardant polyols is higher than GTO,because the molecular weight of the flame retardant polyols was increased significantly by the coupling of EGTO and DOPO,DEP,DEA,respectively.

3.2.GPC analysis

The resulted GTOhad a content of95.00%,the contents of monoglyceride and diglyceride were 70.73%and 24.27%,respectively.In the curve of GTO,the molecular weights of 390 and 910 represented monoglyceride and diglyceride,while there was only a single peak of 1256 in tung oil spectra.This result indicated that the alcoholysis reaction of tung oil and glycerol had occurred and the majority of the TO has converted to monoglyceride and diglyceride.In the present work,the monoglyceride and diglyceride from TO were further converted to EGTO by the epoxidation(Fig.6).

Fig.6.GPC spectra of TO and GTO.Glycerol/TO=8:1(mol/mol),reaction time=3 h,reaction temperature=190°C.

3.3.Transesterification reaction of TO

The effect of temperature and the mass ratio between TO and glycerol were investigated by GPC.The effect of temperature on the conversion rate is shown in Fig.7.

Transesterification reaction can increase reactive hydroxyl content in the structure of monomer.Monoglyceride was preferred in that itcontains two hydroxyl groups in one molecule.As can be seen in Fig.7,the optimal temperature of transesterification reaction was 190°C according to the conversion rate of monoglyceride and the total conversion rate of glyceride of TO.When the temperature was higher than 190°C,the conversion rate of monoglyceride and the total conversion rate decreased.This is explained by the fact that saponification was prone to occur,products reacted with the catalyst to form the saponified products[27].TO is unsaturated fatty acid esters,the double bonds in TO are continuous conjugate,when the temperature was higher than 190°C,polymerization of tung oil can be attributed to reaction via conjugated double bonds[28].

Table 2Properties of TO,GTO and tung oil-based flame retardant polyols

Fig.7.Effect of temperature on the conversion rate.Glycerol/TO=6:1(mol/mol),reaction time=3 h.

Fig.8 presents the effect of the molar ratio of glycerol/TO on the conversion rate.When the molar ratio of glycerol/TO was 8,the conversion rate of monoglyceride and the total conversion rate obtained the maximum values,which were 70.73%,95.00%,respectively.Increasing of the molar ratio of glycerol/TO,the total conversion rate and the conversion rate of monoglyceride firstly increased and then decreased whereas the conversion rate of diglyceride decreased continuously.Results were attributed to the excessive glycerol which was used to favor the equilibrium of this reversible reaction towards the formation of the monoglyceride.However,the higher ratio of glycerol/TO has an impediment to the separation of glycerol and fatty acid glyceride and make the reaction to the direction[29].Therefore,the optimal molar ratio of glycerol/TO was 8.

Fig.8.Effect of molar ratio of glycerol/TO on the conversion rate.Temperature=190°C,reaction time=3 h.

3.4.Epoxidation reaction of GTO

The effect of CH3COOH/GTO(w/w)ratio on the epoxy value is shown in Fig.9,the epoxy value of EGTO was 1.19%obtained when the reaction condition of the CH3COOH/GTO mass ratio was 0.5/1.The epoxy value was increased firstly and then decreased with increasing the mass ratio of CH3COOH/GTO from 0.125 to 0.75.Epoxidation is mostly associated with side reactions,such as hydroxylation,oxidation,oxygenation,and dimer formation[30].

Fig.9.Effect of CH3COOH/GTO mass ratio on the epoxy value.GTO/H2O2=1:1(w/w),temperature=50°C.

The influence of mass ratio between H2O2with GTO on the epoxy value is summarized in Fig.10.The epoxy value decreased when the mass ratio of H2O2/GTO is above 1.0.The highest epoxy value of EGTO was 1.19%when the mass ratio of H2O2/GTO was 1/1.The production of peracetic acid was insufficient as the amount of H2O2decreased,the ability of the reaction system decreased.Increasing the amount of H2O2promoted the process of epoxidation because the formation of peracetic acid was easier.However,side reaction which epoxy group ring opened by water will reduce the epoxy value[31].

3.5.FTIR analysis

Fig.11 shows the spectra of TO,GTO and EGTO.The peak at 1740 cm-1was corresponded to the estergroups from triglyceride molecules,the existence of peaks at 965 cm-1and 725 cm-1indicate both cis and trans conformations were presented in the fatty acids that from the triglyceride.In TO and GTO,there are small peaks at1640 cm-1and 3010 cm-1that were assigned to the absorption of double bonds in cis conformation and corresponded to unsaturations,respectively.There are rather high peaks at991 cm-1corresponding to the wag of the conjugated unsaturations of the elaeostearic chains,which is consistent with literature[32].The most obviously difference among the spectra of all is the increase of the band intensity at 3400 cm-1assigned to hydroxyl absorption.The characteristic peaks at 1050 cm-1and 1040 cm-1represent the C--OH of primary hydroxyl in GTO and EGTO,respectively.Meanwhile there is no absorption due to C--OH(1040 cm-1or 1050 cm-1)in the infrared spectrum of TO.This indicates that the alcoholysis reaction happened and the primary hydroxyl group was formed.Due to the fact that double bonds were formed to epoxy group,the spectrum of EGTO shows a characteristic signal at 849 cm-1indicating the presence of epoxy group;it is obvious that the group of C=C at 3010 cm-1disappeared.(See Fig.12.)

Fig.10.Effect of H2O2/GTO mass ratio on the epoxy value.CH3COOH/GTO=0.5:1(w/w),temperature=50°C.

Fig.11.FTIR spectra of TO,GTO and EGTO.

In the spectrum of DOPO-EGTO,a signal characteristic for vibration absorption peak of P--O--Ph was found at 1020 cm-1and 981 cm-1,the vibration absorption peak of P--Ph at 1430 cm-1gradually appearing strong in the FTIR spectra of DOPO-EGTO and the characteristic peak of epoxy group at 849 cm-1disappeared[33].In the spectrum of DEP-EGTO,the peaks at 1232 cm-1and 1050 cm-1were ascribed to the stretching vibration of P=O and P--O--C[34],respectively.Consequently,the bands related to the epoxide ring at849 cm-1disappeared.In the spectrum of DEA-EGTO,the peak at 849 cm-1corresponding to epoxy group disappeared,stretching vibration absorption at 1230 to 1050 cm-1was attributed to the C--N of fatty amine.

Fig.12.FTIR spectra of DOPO-EGTO,DEP-EGTO and DEA-EGTO.

It is concluded that tung oil-based flame retardant polyols such as DOPO-EGTO,DEP-EGTO and DEA-EGTO which contained the phosphate group or nitrogen group have been formed.

3.6.1HNMR analysis

As can be seen from1HNMR spectrum of GTO in Fig.13 the peak of the terminal--C H3was assigned to 0.85,and the chemical shift of the--C H2of the long chain of eleostearic acid was seen at 1.06 to 1.30.The protons of--C H2--near to C=C appeared at 1.60,the peak at 2.30 was assigned to--CO--C H2--the peak at 3.60 and 3.90 were assigned to--C H2--O--and--CH--O--,respectively.The peak at 4.20 was attributed to CH2--O--while the peak at 2.10 and 2.17 corresponded to the protons of primary hydroxyl group and secondary hydroxyl group,respectively.Chemical shifts at 5.32 to 5.70 were witnessed,indicating the appearance of double bonds.As can be seen from Fig.14,some new signals corresponded to EGTO after epoxidation,the peak at 5.27 was attributed to the--CH=CH--,while the peaks at 3.47 to 3.70 were assigned to epoxy group[34],respectively.This information confirms that EGTO formed.(Fig.15.)

Fig.13.1HNMR spectrum of GTO.

Fig.14.1HNMR spectrum of EGTO.

Fig.15.1HNMR spectrum of DOPO-EGTO.

The formation of DOPO-EGTO in Fig.10 is evident from the peaks shown at 7.15 to 7.84 which were assigned to the proton of aromatic ring[35],except for the peaks at 7.26 which was attributed to CCl3D.New chemical shifts at 3.56 to 3.64 were assigned to--C H--OH via epoxy ring opening by DOPO and the peaks at 4.02 to 4.07 were attributed to P--C H--and,the peak at5.30 was correlated with C=C.The result indicates that the synthesis of DOPO-EGTO is successful.

A spectrum of DEP-EGTO is shown in Fig.16.The appearance of peak 1,corresponding to--C H2--O--P,the peak at 3.66 was assigned to CH--OH,and the proton of--CH--O--P--appeared at the 2.12,the peak at 5.30 was represented by the C=C.This information obtained from1HNMR confirms the formation of DEP-EGTO.

Fig.16.1HNMR spectrum of DEP-EGTO.

Fig.17 shows the1HNMR spectra of DEA-EGTO,the region 2.73 was associated with the--C HOH--,the peak at 3.64 was attributed to N--C H2--,the peak at 3.75 was attributed to--CH--N--,while peaks of the range from 5.26 to 5.39 were assigned to--CH=CH--.Compared to the1HNMR,it was found that DEA-EGTO formed.

Fig.17.1H-NMR spectrum of DEA-EGTO.

3.7.Limiting oxygen index(LOI)

The LOI values of the samples under investigation are shown in Table 3.It can be seen that by increasing the TOFP content causes an increase of LOI value,the LOI increased to 26.2%,25.1%and 24.4%for DOPO-EGTO,DEP-EGTO and DEA-EGTO,respectively.LOI date givesthe minimum concentration of oxygen,in its mixture with N2,which is sufficient to maintain the flame.It is noted that DOPO was more effective than DEP and DEA in enhancing the LOI,this was explained by the fact that thermal decomposition of DOPO groups leads to the production of phosphoric acid,which condenses rapidly to produce pyrophosphate structures and literate water[13].What's more,the structure of benzene ring is beneficial to flame retardancy of TOPUF[36].

Table 3Fire properties of different PUFs

4.Conclusions

Tung oil-based flame retardant polyols were successfully synthesized from tung oil via transesterification of tung oil by three step processes.It was found that under condition of fixed amount of catalyst,temperature and the molar ratio of glycerol/TO can affect the conversion rate of transesterification,the highest total conversion rate is 95.00%.In the study of epoxidation,it can be concluded that the ratios of CH3COOH/GTO and H2O2/GTO have effects on the epoxy value of EGTO,and the highest epoxy value is 1.19%.Tung oil-based flame retardant polyols such as DOPO-EGTO,DEP-EGTO and DEA-EGTO can be used as raw materials to produce the polyurethane foams,these tung oil-based polyurethane foams have good flame retardancy.The value of LOI of TOPUFs which contains 100 wt%DOPO-EGTO,100 wt%DEPEGTO and 100 wt%DEA-EGTO,respectively,can reach up to 26.2%,25.1%,and 24.4%.