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Polyurethane (PU) composites were prepared using a palm oil-based polyol, ... calorimetry (DSC), dynamic mechanical analyses (DMA) and Fourier transform ...
EFFECT OF COMPACTABILIZATION OF POLYMER ON THE PROPERTIES OF POLYURETHANE-PALM FIBER COMPOSITES Yaakob, Z.1, Min Min A.1∗, Mohd Hilmi M.2, Khairul Zaman Hj M.D2 and Kamarudin.S.K.K1 1

Department of Chemical & Process Engineering, Faculty of Engineering, National University Malaysia , 43600 Bangi, Selangor D.E, Malaysia 2 Radiation Processing Technology Division, Malaysia Nuclear Agency (Nuclear Malaysia), Bangi, 43000 Kajang, Malaysia

ABSTRACT Polyurethane (PU) composites were prepared using a palm oil-based polyol, polycaprolactone (PCL) and Oil Palm Trunk (OPT) fiber. Glass transitions and degree of phase separation were derived from differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA) and Fourier transform infrared (FTIR) measurements. The analyses showed that between PU/PCL and OPT at higher Tg values. The different composition of polyurethane becomes more flexible at the higher polyol content in the polyurethane composite. FTIR spectra do not show the changes in presence of OPT fibers. Effect of PCL adding shows in the thermal degradation of polyurethane composite and influence the basic mechanism of biobased PU/OPT composite. The morphology of fractured surfaces of the composites, examined by a scanning electron microscope showed an improvement in the adhesion between the fiber and matrix was achieved. Compared to the characteristic features of neat PU material, more definite plastic deformation was observed in the fracture morphology of the PU composites. Keywords: Polyurethane; Polycaprolactone; polyol; OPT ∗

Corresponding author. Tel.: +603 89216420; fax: +603 8921 6148, E-mail address: [email protected] or [email protected]

INTRODUCTION In recent years, the development of biocomposites from biodegradable polymers to natural fibers has attracted great interests in the composite science, because they could allow complete degradation in soil or by composting process /1-16/. The biodegradable polymers, in particular polyurethanes, (PU) are an interesting family of materials. PU has a large range of applications, which can be attributed to the fact that it can be obtained as linear and flexible polymer or as rigid and highly cross-linked PU /4, 5/. Also, the versatile physical properties of the PU are attributed to their micro phase-separated structure arising from the thermodynamical incompatibility between the hard and soft segments /6/. Composite materials formed by natural fibers and polymeric matrices constitute a current area of interest in composites research. A great development in this field has been noticed, mainly driven by the automotive industries. Polyurethane resins are attractive due to their structural versatility (as elastomer, thermoplastic, thermosetting, rigid and flexible forms), as well as the fact that they can be derived from either petroleum or vegetable oils. At present, they (PU resins) are still particularly more compatible to vegetable fibers than other resins, due to the possible reactions between the hydroxyl groups of vegetable fibers and the isocyanate groups of PU /17/. Poly (e-caprolactone) (PCL) is a biodegradable, semicrystalline, aliphatic polyester polyol. It is hydrophobic and insoluble in water but degradable through the hydrolytic attack of the ester bonds. The degradation rate of PCL is very slow in most cases. The preparation of polymers from renewable sources such as vegetable oil based materials is currently receiving increasing attention because of the economic and environmental concerns /18-22/. Vegetable oils are triglycerides of various fatty acids. In order to use these compounds as starting materials for polyurethane synthesis, it is necessary to functionalize them to form polyols. The use of oleochemicals such as from soya oil or castor oils as plasticizer, and building blocks for polymers such as polyesters or polyamides are becoming more demanding or popular due to their renewability and more environmentally friendly /23/. The utilization of lignocellulosic materials in making composites has received considerable attention in recent time particularly, in the thermoplastic composites /24-27/. Utilization of neat vegetable oil-based polymer consider has quite low properties in mechanical properties although researchers are trying to modify

and improve them to achieve good properties based on vegetable oil based polymer. Oil palm trunk (OPT) fiber is one of the available and natural fibers in Malaysia. OPT is a biomass of oil palm industry. It consists of 84% cellulose and 20% lignin /28/. The aim of this work is to develop a new biomass composite material from a polyurethanes derived from palm oil based oleic acid, with oil palm trunk (OPT) fibers and with addition of polymer that is PCL. This OPT is a by-product of palm oil industry, in which the palm trunk consists of about 84% holocellulose and 20% lignin. A comparative study on the variation of thermal properties of PU/PCL/OPT composites will be performed with the aim of finding useful utilization of the OPT in automotive applications. 2.1 Materials Materials used were palm oil-oleic acid (70% Acid Chem Malaysia), 4Dodecylbenzene sulphonic acid (DDBSA, Fluka), glycerol (Sigma), molecular sieve 3A (Sigma), 1,4-butanediol (1,4-BDO, Fluka), dibutyltin dilaurate (DBTDL, Merck) and 4,4-diphenylmethane diisocyanate (MDI, Merck). Polycarpolactone (PCL) with number average molecular weight of 50,000 was provided from Solvay Company, UK, and oil plam trunk fiber (OPTF) was obtained from Malaysian Palm Oil Board, Biomass (MPOB). The proximate chemical composition of OPTF is given in Table 1. No coupling agent and no fiber treatment were used in this study. Table 1 Chemical composition of OPTF Chemical composition Ash Lignin Holocellulose Alfa-Cellulose Hemi cellulose Alcohol/benzene soluble Hot water soluble Alkali 1% NaOH soluble

Weight (%) 01.5 14.6 67.8 40.5 33.5 01.0 08.9 23.6

2.1.1 Synthesis of palm oil-oleic acid based polyol/diol Palm oil-oleic acid (0.2 mol), glycerol (0.4 mol), DBSA (0.025 mol) and 3A molecular sieve were added into the 4-neck glass reactor equipped with a glass stirrer, thermometer, heating mantle, and nitrogen gas inlet system. The temperature of the reactants was raised up to 120 oC and controlled from 120130 oC for 3 hours. Thereafter, the reactants in the flask were allowed to stand overnight at room temperature. Excess glycerol was removed by adding sodium chloride solution and ethanol. The final products are separated into two layers in the flask. The bottom layer of excess glycerol was removed, and the upper layer of the diol/polyol was distilled under vacuum evaporator at 60-70 oC. The yield of palm oil-diol was 60% and the molecular weight was determined by using GPC and the hydroxyl number of polyol was measured by using ASTM D-4272 method C. 2.1.2 Synthesis of Prepolymer Palm oil- polyol and MDI (NCO/ OH ratio at 1.0), NCO-terminated urethane prepolymers were synthesized by the reaction between a difunctional polyol and an excess of diisocyanate. The syntheses were carried out in a 4-neck glass reactor equipped with a glass stirrer, thermometer, heating mantle, and nitrogen gas inlet system. First, polyols were poured into the reactor under a nitrogen atmosphere and heated up to 70oC. When the temperature in reactor reached at 70oC, MDI was added to the polyol. The system was kept at 70oC and mechanically stirred at 250rpm.The exothermicity of reaction was controlled by cooling. The reaction of urethane linkage formation started immediately after the MDI addition. During synthesis the changes of absorbance peaks belonging to characteristic functional groups of polyol, MDI and urethane were monitored by FTIRATR KBr. After the NCO terminated, the reaction was stopped by cooling and the prepolymer stored in a sealed glass bottle under nitrogen. 2.1.3 Processing of Polyurethane Elastomers The prepolymer was heated at 90-100oC and a specified amount of the prepolymer was weighted into a 250 mL plastic cup. The chain extender (1,4BDO), which was preheated at 100oC was added to the prepolymer under vigorous mixing for 5 mins. Then it was cast into the Teflon mold and

pressed at 70oC for 15 mins. Lastly, it was placed in a vacuum oven at 7080oC for 18 hrs. 2.1.4. Preparation of Composite Composites with PCL contents of OPT fiber loading (Table 2) were prepared by melt blending at 80°C for 10 min in a blender attached to a Haake Rheometer ( Polydrive, Germany) until torque reached equilibrium. The 1mm thick sheets were compression molded by hot pressing the granules between hydraulic press at 80°C. A pressure of 150 kg.cm2 was applied for 15 min to press the compound between the plates. The sheets were then immediately cooled off between two plates of cold press at 25°C. For post curing, samples were placed in an oven at 70°C for 3 hrs and kept in the humidity chamber for a week for further testing. 30% OPT fiber loading as an optimum loading in terms of thermal and physical properties of PU/OPT composite /29/. Table 2 Composition of PU/OPTF/PCL composite Composite 0 10 20 30 40

PU (wt %) 100 90 80 70 60

PCL (wt %) 0 10 20 30 40

OPTF (wt %) 0 30 30 30 30

2.2 Characterization Thermal stability of biocomposites PU/OPT samples are analyses having been carried out by Differential Scanning Calorimetry (DSC) using a Mettler DSC30, equipped with low temperature probe, ranging from -60°C to +200°C. The heating rate was 10°C/min. All the temperature scans were performed under nitrogen atmosphere (flux: 20ml/min). The TGA thermograms were obtained using Perkin Elmer Pyris 1 TGA thermal analyzer at a heating rate of 10 oC min-1 over the temperature range from 40 to 700oC. The weight of the sample used was about 4-5 mg in all the cases. DMA was performed for individual specimens using a DMA7 (Perkin Elmer

Corp., High Wycombe, UK) with a parallel plate disc. The sample temperature range of -100 oC up to +150 oC at heating rate of 5 oC/min and the frequency of 1 Hz in the parallel disc configuration. The value of tan δ and the storage modulus were recorded for each sample. DATR-FTIR spectra were recorded using the Perkin-Elmer Spectrum, one spectrometer with golden gate ATR (Attenuation Transform Infrared) attachment with diamond crystal. The transmittance measurements were carried out in the range of 4000cm-1 – 500cm-1. In order to study the morphological changes, the composites of polyurethane were fractured and examined using SEM/EDX QUANTA 400 scanning electron microscope. Analytical gel permeation chromatography (GPC) was performed with a Perkin-Elmer LC- chromatograph equipped with refractive index detector LC- RI to determine the changes in molecular weight distribution of polyol (diol/triol) and polyurethane. The viscosities of polyol and polyurethane measured using a Brookfield Viscometer Model RV-DVII Pro were in the range of 100-1000cP at 25 oC. Density measurements were carried out according to ASTM D1505-72 using the Electronic Densimeter Model MD200S by means of liquid mode. The hydroxyl number of polyol was measured according to ASTM D 4274 method C. Tensile tests were carried out using a universal mechanical testing machine Instron, Model 4301, with a load of 11kN at 23°C and 50% humidity using a crosshead speed of 5 mm/min in accordance to ASTM D1822-L. The test specimens were cut in dumbbell shape according to the ASTM method. At least five specimens were tested for each set of samples being the mean values reported. 3. Results and Discussion 3.1 Mechanical properties Figure 1 shows the effect of PCL on the tensile strength (Ts) and modulus of PU/PCL. A gradual increase in Ts with the addition of PCL for PU/OPT was observed in Fig.1. The mechanical properties of a composite material depend primarily on the strength and the chemical stability of the matrix and the effectiveness of the bonding between matrix and fibers in transferring stress across the interface. The mechanical properties of fiber composites depend on the fiber volume fraction /30-32/. Comparison of the polyurethane composites based on fiber loading studied showed that the highest loading of

fiber content at 40% showed the highest Ts compared with pure PU. The same trend at adding PCL at PU/OPT composites. The surface adhesion between the fibers and the polymer matrix plays an important role in the transmission of stress to the matrix which contributes to the better performances of the composites. For structural applications, good interfacial adhesion between fibers and matrix is required to achieve high strength and good dimensional stability of the composites. The interfacial adhesion between fibers and matrix is a key factor that determines the interlaminar shear strength of the fiber-polymer matrix composites /33-35/. The composites of vegetable oil based polyol did not show any significant changes in the interlaminar shear strength values. This could be attributed to the very good compatibility between palm oil based PU/PCL with OPT fiber.

3

90 80

2.5

Ts (MPa)

2

60 50

1.5 40 1

30

Modulus (MPa)

70

20 0.5 10 0

0 Pure PU

10%PCL

20%PCL

30%PCL

40%PCL

Fig. 1: Tensile strength and modulus of PU/PCL/OPT composites In the reaction with aromatic diisocyanates, it gives a rigid, highly crosslinked PU network, and the complete polyester chains are part of the network. Palm oil-oleic acid based polyol had internal hydroxyl groups positioned in the middle of the 18 carbon fatty acid chain. Thus when crosslinking was completed, a portion of the chains was not included in the network and was left dangling, increasing the free volume in the polymer network and acting as a plasticizer. This is an undesirable characteristics of the resin intended to be used in composites. To increase the rigidity of the matrix resin, a crosslinker such as low molecular weight alcohol (e.g. 1,2diethylene glycol instead of I,4-butanediol ) or other compounds such as

methylene-bis-ortho-chloroaniline (MOCA) should be added. Crosslinkers increased the crosslinking density and contributed to the better mechanical properties /36/. 3.2 Synthesis and characterization of the polyol and polyurethane Palm oil is a naturally occurring vegetable oil having a mixture of triglycerides of various fatty acids. One way of producing ester polyol is from vegetable (palm) oil oleic acid. Thus from functionality point of view, oleic acid is roughly 70% difunctional and 30% trifuctional. According to the measurements, data from polyol synthesized from palm oil oleic acid are shown in Table (3). Table 3. Properties of oleic acid and poly Oleic acid

Diol polyol

Hydroxyl value (mg KOH/g)

-

93

ASTM D-4272

Acid value (mg KOH/g)

22.3

9.3

AOCS

Viscosity at 25 oC (cP)

45

540

Viscometer

pH value

4

3.5

pH paper

Density

0.96

0.976

Densimeter

Molecular weight (g/mol)

873

1208

GPC

Functionality (f)

-

2

-

3.3 FTIR Spectra The FTIR was utilized in the verification of the completion of resin and composite curing reaction in cured composites. The FTIR spectra, shown in Fig. 2 for the pure polyurethane and PU/PCL/OPT composites, indicated that all isocyanate groups reacted and the resin was completely cured. Slight changes of the absorbance were noticed in the spectra between pure PU and PU/PCL/OPT, especially at 1700 cm-1 (-C=O vibration) due to the addition of PCL.

Fig. 2: The FTIR spectra of pure PU and PU/PCL/OPT composites

3.4 TGA TGA curves of the pure PU and PU/PCL/OPT composite are shown in Fig.3(a). All the decompositions started at approximately 100°C. The shapes of the weight loss curves of PU/PCL/OPT composites were almost similar in the temperature range of 100–250°C and showed significant differences in the range of 350–450°C temperature range. Weight loss was very gradual until 250°C, where a rapid drop followed and ended at approximately 490°C for pure PU. Two decomposition stages can be seen in all compositions and pure PU at around 200°C with a residue of 20%. The second stage of PU decompositions presented a principal degradation at around 400°C, but with 15% weight loss at 250°C without residue. Typical two-step decomposition curves can be seen in all compositions of PU/PCL/OPT composites. 40% PCL decomposition shifts to a higher temperature side (400°C) with increased PCL content. Mass residue at 300-400°C increased linearly with increasing PCL content. As the previous studies are based on TG-FTIR /37-38/, the first step of PU decomposition is attributed to the decomposition of isocyanate, and the second step of the thermal decomposition is attributed to the total decomposition of organic compounds.

120 Pure PU PCL10%

Weight (%)

100

PCL 20% PCL 30%

80

PCL 40%

60 40 20

(a)

0 50

150

250

350

450

Temperature (°C)

-9

550 Pure Pu

-8

10% PCL 20% PCL

Weight percentage derivative (%/min)

-7

30% PCL

-6

40% PCL

-5 -4 -3 -2 -1

(b)

0 50

150

250

350 o Temperature ( C)

450

550

650

Fig. 3: The TGA curves (a) PU/PCL/OPT composite and (b) their derivative curves It is well known that the first stage of degradation is dominated by urethane bond decomposition and that the amount of residue is correlated with the amount of unreacted isocyanate in polymers /39/. Javni et al. /40/ have found that although the early stage of degradation is dominated by urethane bond decomposition, the polyol component may contribute to the weight loss at higher conversions, causing an increase in energy activation. This suggests that the first step in the pure PU loss is likely due to a higher conversion of –NCO groups to urethane bonds and all of the composition of PU/PCL/OPT composites, as well. Basically, the shape of the curve showed some changes with PCL adding (Fig. 3b). The first downturn is observed above 300°C. The residual weight is

correlated to the amount of PCL added. Table 4 shows the degradation temperature of stage by stage. Derivative TGA curves corresponding to neat polyurethane reveals four maximum peaks: 300, 350, 400 and 490°C, suggesting at least four main degradation processes (Fig. 3b). The almost similar peaks are observed in the composites with 10, 20 and 30 wt% of PCL while two peaks are seen for the two other composites (40wt % and pure PU). They showed the same patterns (Fig 3b) and the more PCL adding the more decomposition temperature shifts to a lower values and lesser residue due to the biodegradable PCL polymer. Decomposition of urethane bonds starts at around 150°C /41-44/. The polyol component contributes to the degradation at higher temperature /45/. From these results, PCL did influence the basic mechanisms of the biobased PU thermal degradation and improved the thermal stability of the composites. Table 4 Degradation temperature of pure PU and PU/PCL/OPT composites Sample Pure PU PU/30%OPT/10%PCL PU/30%OPT/20%PCL PU/30%OPT/30%PCL PU/30%OPT/40%PCL

T1 (°C) 300 300 300 300 300

T2 (°C) 350 350 350 350 349

T3 (°C) 400 390 399 399 389

T4 (°C) 490 489 489 489 485

3.5 Thermal properties The results of the investigation of dynamic mechanical thermal behaviour of the composites in Fig. 4 illustrated the variations of glass transition at low and high temperatures. The thermal properties of pure PU and PU/PCL/OPT composites were also studied by DSC, and their results are shown in Table 5. Polyurethane chains are built of soft and hard segments. The soft segments are derived from a macrodiol, usually polyether or polyester polyol, whereas the hard segments are formed by the diisocyanate with urethane linkages or low molecular chain extenders. The soft and hard segments arrange themselves in so called soft and hard domains, the resulting phase segregated polymers will have two respective glass transition temperature (Tg): one (at

lower temperature, Tgs) belonging to soft segments an the other (at higher temperature, Tgh) belonging to hard segments /46/. 0.9

Tan δ

0.8

Pure PU 10% PCL

0.7

20% PCL

0.6

30% PCL

0.5

40% PCL

0.4 0.3 0.2 0.1 0 -100

-50

0 Temperature (°C)

50

100

Fig. 4: DMA curves for tan δ of pure PU and PU/PCL/OPT composites Table 5 Glass transition temperature of pure PU and PU/PCL/OPT composites Sample Pure PU PU/30%OPT/10%PCL PU/30%OPT/20%PCL PU/30%OPT/30%PCL PU/30%OPT/40%PCL

Tgs (°C) -33 -20 -23 -13 -

Tgh (°C) 50 50 52 52 55

Tgh (°C) 83 91 95 99

The glass transition temperature (Tg) of PU/PCL/OPT composites were increased as the PCL composition increased from 0 to 40 %. It seems that in this case, the restricting effect of the PCLon the polyurethane molecules and the hydrogen bonding between the urethane groups of the PU and PCL molecules contributed to this behaviour. The higher Tg value showed strong crosslinking between PU, PCL and OPT. Since adding PCL, there have two high Tg temperature. At higher temperatures, the Tg values increase with increasing content of PCL although around 50°C temperature have all composition due to the PCL melting temperature. Also Fig. 4 shows that the value of tan δ as a function of temperature. The results at low temperature (Tgs) as soft segment and the higher temperature (Tgh) as hard segment were almost similar to the ones showed by DSC measurements. At low

temperature (Tg) it was shown that by increasing a PCL content, the tan δ peak associated with glass transition shifts to a higher temperature /46/. The tan δ spectra strongly demonstrated an elastomeric behavior of the composite curing with lower Tg values.

A

B

C

D

E

Fig. 5: SEM Micrographs (A) pure PU, (B) 10% PCL, (C) 20% PCL, (D) 30% PCL, (E) 40% PCL The improvement in the interfacial adhesion between fibers and polyurethane matrix can be clearly seen from SEM micrographs of the tensile

fracture surface, as shown in Figure 5. With a weak interfacial bond, the fracture is more likely to lead to interfacial debonding and extensive fiber pullout, as shown in Figure with OPT fibers. However, if the bonding is strong as in the cases, the failure occurs with fiber breakage at the fracture point, as shown in Figure 5. The fracture surface morphology of the PU/PCL/OPT after tensile tests were shown in Fig 5 (B-E). Compared to the characteristic features of neat PU material, more definite plastic deformation was observed in the fracture morphology of the PU composites. The reason for this phenomenon is speculated to be the strong reinforcing effect of the fibers in the polymer, as well as higher amorphous potions of the PU inside the composites, which allowed increased deformation beyond the elastic behaviour.

4. CONCLUSION Polyester diol/polyol was prepared using palm oil-oleic acid after hydrolysis reaction with glycerol, followed by alcoholysis with triethanolamine. PU was prepared by the isocyanation of the palm oil-polyol with diisocyanate compounds such as MDI together with a catalyst and with or without a solvent. The improvement in the interfacial adhesion between fibers and polyurethane matrix can be clearly seen from SEM micrographs of the tensile fracture surface. Dynamic mechanical analysis showed that the effect of adding PCL to the glass transition temperatures was higher due to the content of OPT. For PCL, Tg value slightly increased by increasing PCL content in PU/OPT composite. The shifting of tan δ peaks to the lower temperatures can be attributed to the increase in the elasticity of the systems with OPT fibers. These findings should have important implications for designing and manufacturing automotive materials and further research could be recommended.

ACKNOWLEDGEMENTS The authors acknowledge University Kebangsaan Malaysia (UKM) (grant number UKM-OUP-BTT-25/2007) for the financial support, the staff of Malaysian Oil Board (MPOB) and Malaysian Nuclear Agency (Nuclear Malaysia) for their help.

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