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 DOI: 10.1177/0892705709356335

published online 16 December 2009Journal of Thermoplastic Composite MaterialsM. Khairul Zaman

Z. Yaakob, A. Min Min, M. N. Satheesh Kumar, S. S. K. Kamarudin, M. M. Hilmi and H.Fiber Dust

Oleic Acid-based Polyurethane and its Biocomposites with Oil Palm Trunk  

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Oleic Acid-based Polyurethaneand its Biocomposites with Oil Palm

Trunk Fiber Dust

Z. YAAKOB,* A. MIN MIN, M.N. SATHEESH KUMAR

AND S.S.K. KAMARUDIN

Faculty of Chemical & Process Engineering, Faculty of EngineeringNational University of Malaysia, 43600 Bangi, Selangor D.E, Malaysia

M.M. HILMI AND H.M. KHAIRUL ZAMAN

Radiation Processing Technology Division, Malaysia Institute forNuclear Technology (MINT), Bangi, 43000 Kajang, Malaysia

ABSTRACT: The main objectives of the present research are (i) to obtain amonoglyceride from oleic acid through a direct esterification process, (ii) utilizationof the obtained monoglycerides to prepare polyurethane (PU) sheet, and (iii) thepreparation of a series of biocomposites using prepared PU and different amounts ofoil palm trunk (OPT) fiber dust. The tensile strength of OPT fiber dust incorporatedPU biocomposites is found to be increased with the increase in the OPT fiber dustcontent up to 20% in the composites. The tensile strengths of PU and itsbiocomposites with 20% OPT fiber dust were 0.6 and 2.15MPa, respectively. Thethermal properties of PU and its biocomposites having varied amounts of OPT fiberdust of 0–40% have been investigated using dynamic mechanical analyzer (DMA)and thermogravimetric analyzer (TGA). TGA studies revealed that PU and itsbiocomposites have undergone two- and three-step degradation, respectively. FromDMA studies, a shift in the tan � value was noticed for the biocomposites containing30 and 40% OPT fiber dust. The morphology of the tensile fractured compositespecimens was analyzed using scanning electron microscope.

KEY WORDS: Palm, OPT fiber, PU, composites, DMA, TGA, SEM.

INTRODUCTION

THE INCREASED GREEN house gas emission and the anticipated scarcityof fossil fuel reserves in the near future directed the development of

Journal of THERMOPLASTIC COMPOSITE MATERIALS, Vol. 00—Month 2009 1

0892-7057/09/00 0001–12 $10.00/0 DOI: 10.1177/0892705709356335� The Author(s), 2009. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

*Author to whom correspondence should be addressed. E-mail: zahirayaakob@gmail.com

doi:10.1177/0892705709356335Journal of Thermoplastic Composite Materials OnlineFirst, published on December 16, 2009 as

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alternative renewable resource-based material technology [1]. The fabrica-tion of biocomposites using renewable resources offers many advantages [2]such as low cost, lightweight, biodegradability, and no release of toxicbyproducts when burnt.

Oil palm trunk (OPT) fiber is one of the agricultural byproducts of oil palmbiomass found in the field on replanting and pruning. The un-utilized OPTbyproduct may pose serious environmental threats [3]. The reported alphacellulose content in OPT byproduct was 41%, which is lower than that ofwood (45–50%) [4]. The OPT lumber has found an application in theproduction of blockboard [5] and furniture [6]. The extensive studies onthe fabrication of materials using OPT lumber through pilot plant andsmall-scale production have lead to the development of a technology that canbe commercialized [7]. The processing of OPT lumber during themanufacturing of blockboard or furniture generates a lot of dust. Theutilization of such OPT fiber dust or filler to generate value-added materialsmay reduce the cost of the furniture.

Oleic acid is a monounsaturate fatty acid found naturally in many plantsources and animal products [8]. It is a monoglyceride also known as mono-acylglycerol consisting of one fatty acid chain covalently bonded to a glycerolmolecule [9] through an ester linkage (Scheme 1). Numerous research papersare available in the literature on the preparation of monoglycerides fromvegetable oils [10–13]. The preparation of monoglyceride involves eitherglycerolysis of oils and fats at high temperature or direct esterification of fattyacids and glycerol with or without catalysts [14]. The reported yield ofmonoglycerides was typically in the range of 50–70%, but the moleculardistillation process appears to provide more than 90% yield [15]. Themonoglycerides separated from the fatty acid source consists of two reactivehydroxyl groups (Scheme 1), which can react with diisocyanates to formpolyurethane (PU) [12].

PUs are a class of polymeric materials with a wide spectrum of propertiesthat make these materials of great use to the daily life of mankind. Forexample, PUs are used in furniture, clothing, automotives, carpets, and many

H2C

H2C

HC

O C R

O

OH

OH

Scheme 1. Structure of a typical monoglyceride.

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more applications in the form of foams, elastomers, coatings, adhesives,sealants, and composites. The monoglycerides present in various vegetableoils have been separated and utilized by many researchers for the preparationof PU [12,16]. Tanaka et al. [17] reported the preparation and characteriza-tion of PU foams using the monoglycerides obtained from palm oil. The bio-based PU were synthesized by Bakare et al. [18] using the rubber seed oil(RSO) monoglyceride together with xylene and hexamethylene diisocyanate.Okieimen and Bakare [19] separated the monoglycerides from RSO andutilized it for the fabrication of PU and its biocomposites with sisal, jute, andbanana fiber. Davaran and Entezami [20] prepared biodegradable PUs fortherapeutic application by reacting 1,6-hexamethylenediisocyanate andethylene glycol with ibuprofen monoglyceride as a chain extender.

As oleic acid can be obtained from many renewable resources and hasthe ability to provide a monoglyceride with two reactive hydroxyl groups,the utilization of such a renewable resource-based monoglyceride for thepreparation of PU is expected to minimize the dependence on petroleum-based raw materials. The present research investigation employed the directesterification method to obtain monoglycerides from oleic acid. Theprepared monoglycerides have been reacted with methylene diphenylisocyanate (MDI) to obtain PU sheet. A series of biocomposites havebeen prepared by the incorporation of different amounts of OPT fiber dustcontaining 0–40% of the PU pre-polymer. The mechanical and thermalperformance of the PU sheet and their OPT fiber dust biocomposites havebeen investigated.

EXPERIMENTAL

Materials

Materials used were oleic acid obtained from palm oil (70%, Acid ChemMalaysia), dodecyl benzene sulfonic acid (DBSA) from Fluka, glycerol(Sigma), molecular sieve 3A (Sigma), 1-4 butanediol (Fluka) and 4-4methylene diphenyl isocyanate (MDI, Merck). Oil plam trunk (OPT) fiberdust (a mixture of parenchyma and vascular bundles) with an average particlesize of 500 mm was kindly supplied by Malaysian Palm Oil Board (MPOB).The OPT fiber dust was oven dried at 1058C for 5 h prior to use.

Synthesis of Monoglycerides from Oleic Acid

The synthesis of monoglycerides from oleic acid was carried out as per theprocedure described elsewhere [21]. The typical reaction was carried out in afour-necked 250mL round-bottomed flask equipped with heating mantle,

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mechanical stirrer, thermometer, an inert gas inlet tube, and condenser.Oleic acid (0.2mol), glycerol (0.4mol), DBSA (0.025mol), and molecularsieve (grade 3A) were placed in the reaction flask and heated to 120–1308Cfor 3 h. After 3 h, the reaction flask was allowed to stand overnight at roomtemperature. The monoglycerides were removed from the excess glycerol byadding sodium chloride solution and ethanol. The final product was dividedinto two layers in the flask. The excess glycerol at the bottom layer wasremoved and the upper monoglyceride layer was distilled using vacuumevaporator.

Preparation of PU Prepolymer

The preparation of PU prepolymer was carried out by reactingmonoglyceride with MDI at an NCO to OH ratio of 1 : 1.2. In a typicalreaction, the synthesis of PU prepolymer was carried out in a four-neckedjacketed glass reactor equipped with a heating mantle, mechanical stirrer,thermometer, and an inert gas inlet tube. First, the monoglyceride waspoured into the reactor under nitrogen atmosphere and heated to 708C.MDI was then added into the reaction flask when the temperature reached708C followed by 2–3 drops of dibutyl tin diluarate. The reaction wasstopped after 3.5 h, degassed, and stored in a tightly covered Teflon mold.The prepolymer was allowed to cure at room temperature for about 10 h andat 708C for another 24 h.

Preparation of Oil Palm Trunk Fibre-Polyurethane (OPTPU) Biocomposites

The melt blending technique was adopted to prepare OPTPU biocompo-site granules having different amounts of OPT fiber dust. Melt blending wascarried out at 808C for 10min in a blender attached to a Haake Rheometer(Polydrive, Germany). The OPTPU biocomposite granules having differentamounts of OPT fiber dust were compression molded at a temperature of808C and pressure of 150 kg/cm2 for 15min. After 15min, the biocompositeswere cooled between two plates in a cold press maintained at 258C.The OPTPU biocomposites having 1mm thickness were postcured for about10 h at 708C.

Analyses

The density and viscosity of oleic acid and its polyol were measured usingBrookfiled viscometer (Model RV-DVII) and electronic densimeter (ModelMD-200S), respectively. The hydroxyl number of polyol was determinedaccording to ASTM D 4274 method C. The acid values of oleic acid and

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polyol were determined per the procedure described in ASTM D 2689-73.The molecular weights of oleic acid and its polyol were determined using gelpermeation chromatography (Perkin Elmer LC-chromatograph) equippedwith a refractive index (RI) detector.

The tensile strength of neat PU and OPTPU biocomposites was measuredat 238C and 50%RHusing 4302model Hounsfield universal testing machine.The DMA measurements were carried out on DMA 7 (Perkin Elmer)dynamic mechanical analyzer using parallel plate disc. The selected heatingrate, temperature range, and the frequency for DMA are 58C/min, �100 to1508C, and 1Hz, respectively. The storage modulus and tan � were recordedas a function of temperature. The thermal stability of neat PU and OPTPUbiocomposites was investigated using Perkin Elmer Pyris 1 thermogravi-metric analyzer (TGA) under N2 atmosphere up to 7008C. A heating rate of208C/min was used. About 20mg of specimen was loaded for eachmeasurement. The morphological behavior of the tensile fractured specimenswas studied using QUANTA 400 scanning electron microscope (SEM).

RESULTS AND DISCUSSION

The direct esterification reaction between fatty acids such as oleic acid andglycerol is known to provide monoglycerides with two reactive hydroxylgroups (Scheme 1). The physicochemical properties of oleic acid and itsmonoglyceride are given in Table 1. The reduction in acid value, increasedviscosity, density, and molecular weight of monoglyceride as compared tothe neat oleic acid could be attributed to the esterification reaction betweenoleic acid (fatty acid) and glycerol. The calculated hydroxyl (–OH)functionality of monoglyceride from the hydroxyl value and molecularweight was 2.

The tensile strength of PU and biocomposites as a function of OPT fiberdust content is shown in Figure 1. The observed tensile strength of OPTPU

Table 1. Physicochemical properties of oleic acidand its monoglyceride.

Properties Oleic acid Monoglyceride

Hydroxyl value (mg KOH/g) – 93Acid value (mg KOH/g) 22.3 9.3Viscosity at 258C (cP) 45 540pH value 4 3.5Density (g/cm3) 0.96 0.976Molecular weight (g/mol) 873 1208Functionality (f) – 2

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biocomposites is higher than that of PU sheet. The tensile strength of theOPTPU biocomposites increased with increase in the fiber content up to20% and declined thereafter. The observed trend in the present work may beattributed to the fact that the OPT fiber dust used may have occupied thespongy PU matrix, creating some reinforcing effect and this possibly mayhave increased the tensile strength. The reduction in tensile strength above20% OPT loading suggests that the composite formulation may be suitablefor applications subjected to tensile load at this low filler loading [22].

DMA is a useful technique to measure the modulus (stiffness) andviscoelastic properties of composite materials. Dynamic storage modulus (E0)is an important property to assess the load bearing capability of a compositematerial. The temperature dependence of E0 of PU and OPTPU biocompo-sites having different amounts of OPT content is shown in Figure 2.In general, the biocomposites showed higher E0 than PU. This may be due tothe presence of rigid cellulose in OPT fiber dust [4], which supports the stresstransfer from the PU in composite. The measuredE0 value at 308C is shown inFigure 3. At this temperature, the biocomposites showed higher E0 than PU.The E0 value was found to increase with an increase in the fiber content up to20%, and a sharp decline can be noticed for the biocomposites containing 30and 40wt% OPT fiber dust.

Figure 4 shows the tan � versus temperature plots for PU andbiocomposites having different amounts of OPT fiber dust. From the

0

0.5

1

1.5

2

2.5T

ensi

le s

tren

gth

(MP

a)

SamplesNeat PU A B C D

Figure 1. Tensile strength of PU and its biocomposites containing (A) 10% OPT, (B) 20%OPT, (C) 30% OPT, and (D) 40% OPT.

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DMAmeasurements, the glass transition temperatures (Tg) were determinedfrom the peak of tan � (ratio of loss modulus to storage modulus) curves.The presence of only one Tg (60� 28C) confirms the single-phase system forPU. A similar behavior was noticed by Dwanisa et al. [23] for soy oil-basedPUs. The observed Tg of biocomposites containing 10 and 20% OPT fiberdust was almost the same as that of PU. But an increase in Tg was noticedfor the biocomposites containing 30 and 40% OPT fiber dust. The observed

0.00E+00

1.00E+07

2.00E+07

3.00E+07

4.00E+07

5.00E+07

6.00E+07

10 25 40 55 70 85 100 115 130 145Temperature (°C)

Sto

rage

mod

ulus

(E

′)

Neat PU

10% OPT

20% OPT

30% OPT

40% OPT

Figure 2. The temperature dependence of storage modulus of PU and its biocompositescontaining different amounts of OPT.

0.00E+00

5.00E+06

1.00E+07

1.50E+07

2.00E+07

2.50E+07

3.00E+07

3.50E+07

Stro

rag

e m

odu

lus

(E′)

Samples Neat PU 10% OPT 20% OPT 30% OPT 40% OPT

Figure 3. The storage modulus of PU and its biocomposites containing different amounts ofOPT at 308C.

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Tg of the composites containing 30 and 40% OPT was 80 and 858C,respectively. This may be due to the incorporation of stiff fibers, whichincreases the Tg by restricting the movement of polymer molecules [23].

The TGA curve and its derivative thermogram of PU and itsbiocomposites containing different weight percentage of OPT fiber dust isshown in Figure 5. The PU has showed two-step degradation and theOPTPU bicomposites have showed three-step degradation. A similarbehavior of degradation was reported in the literature for PU [24] andnatural fiber reinforced polymer composites [25]. The first stage degradationof PU at 284–3908C may be due to the breakage of urethane bonds and thesecond stage degradation at 391–5318C may be due to the decomposition ofaromatic moieties. The analyzed percentage weight loss as a function oftemperature from the TGA thermograms is given in Table 2. The OPTPUbiocomposites have exhibited relatively lower thermal stability compared toPU, which can be observed from the temperature at 50% weight loss (T50).

The interphase morphology of the OPTPU biocomposites was investi-gated as a function of PU and OPT content. The SEM microphotographs ofPU and its biocomposites having different amounts of OPT fiber dust areshown in Figure 6(a)–(e). The observed fiber pull outs from the SEM can beattributed to the deposition of less amount of matrix per unit area due to theincorporation of bigger size filler (500 mm), which has smaller surface. Badriet al. [26] observed a similar trend with empty fruit bunch fiber incorporatedPU composites. The SEM results revealed that the incorporation of smallersize OPT may enhance the fiber–matrix adhesion.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10 20 30 40 50 60 70 80 90 100 110 120 130 140

Temperature (°C)

tan

dPU

10%

20%

30%

40%

Figure 4. Tan � and temperature plots of PU and its biocomposites with different amountsof OPT.

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Table 2. Data obtained from TGA analysis of neatPU and OPTPU biocomposites having different amounts of OPT fiber.

OPT content(%) in OPTPU

Temperature at differentweight loss �2 (8C)a Residual

weight atbiocomposites T0 T10 T20 T50 5008C (%)

PU 200 21.5 251 293 38010 198 14.9 249 289 37820 173 23.9 233 286 36530 159 26.1 234 280 36040 100 26.8 201 276 353

aT0, temperature of onset decomposition; T10, temperature of 10% weight loss; T20, temperatureof 20% weight loss; T50, temperature of 50% weight loss; Tmax, temperature at whichdecomposition is completed.

100

80

60

40

20

0W

t%100 200 300 400 500 600 7000

Temperature (°C)

4

2

0

Derv. w

t (%/°C

)

100

80

60

40

20

0

Wt%

100 200 300 400 500 600 7000Temperature (°C)

4

2

0

Derv. w

t (%/°C

)

100908070605040302010

Wt%

100 200 300 400 500 600 7000Temperature (°C)

4

2

0

Derv. w

t (%/°C

)

100

80

60

40

20

Wt%

100 200 300 400 500 600 7000Temperature (°C)

4

2

0

Derv. w

t (%/°C

)

100

80

60

40

20

Wt%

100 200 300 400 500 600 7000Temperature (°C)

4

6

2

0

Derv. w

t (%/°C

)

(a)

(b) (c)

(d) (e)

Figure 5. TGA and derivative thermogram of (a) PU and OPTPU biocomposites containingdifferent weight percentages of OPT; (b) 10, (c) 20, (d) 30, and (e) 40%.

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CONCLUSIONS

The present research investigation revealed the possibility of preparing(i) PU sheets from oleic acid-based monoglycerides and (ii) biocompositesusing prepared PU and OPT fiber dust at different weight percentages.

Figure 6. SEM microphotographs of (a) PU and OPTPU biocomposites containing differentweight percentage of OPT, (b) 10, (c) 20, (d) 30, and (e) 40%.

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The reduction in acid value of the monoglyceride as compared to oleic acidconfirms the completion of the esterification process. The tensile strengthand storage modulus were found to be higher in OPTPU biocompositescompared to PU. The DMA results showed that the incorporation of 30 and40% OPT fiber dust enhances the Tg compared to PU sheet. The PU sheethas showed two-step degradation whereas the biocomposites have showedthree-step degradation. The SEM microphotographs revealed the fiber–matrix adhesion and suggested that the OPT fiber dust having less than500 mm may enhance the fiber–matrix adhesion.

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