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Mechanical properties of polyurethane(PU)–starch biocomposites

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Polyurethane - starch (PU-Starch) biocomposite was prepared by incorporating starch into PU polymer matrix by casting method. Mechanical and morphological properties of the PU-starch biocomposites were characterized using universal testing machine (UTM) and scanning electron microscope (SEM). Mechanical studies showed that the mechanical stability of the PU-starch biocomposite increased compared to that of PU. The mechanical tests include tensile strength, flexural strength, modulus and impact strength. The fractured surfaces morphology of the PU-starch biocomposites were studied under SEM.
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1
Mechanical Studies on Polyurethane (PU)-Starch Nanocomposites
Tayser Sumer Gaaz1, 3*, M.N.M.Ansari1, 2, Robert A. Shanks2, Ing Khong2
1Centre for Advance Materials, College of Engineering, University Tenaga Nasional, Malaysia.
2School of Applied Sciences, RMIT University, Box 2476 GPO, Melbourne, VIC 3001, Australia.
3Department of Agriculture Machinery Equipment Engineering Techniques, Technical College
Al-musaib, Foundation of Technical Education, Ministry of Higher Education, Iraq.
Corresponding Author’s Email*: tay_alimy@yahoo.com
ABSTRACT
Polyurethane - Starch biocomposite (PU-Starch) was prepared by incorporating starch into PU polymer
matrix by casting method. Mechanical and morphological properties of the PU-starch biocomposites were
characterized. Mechanical studies showed that the mechanical stability of the PU-starch biocomposite
increased compared to that of PU. The mechanical tests include tensile strength, flexural strength, impact
strength and scanning electron microscopy. The fractured surfaces of the PU-starch biocomposites were
studied under scanning electron microscope and showed no presence of any micron-sized Starch bundles.
Keywords: Starch, Nanocomposite, Nanotube, Mechanical properties, Morphology, Interface, Dispersion
1. INTRODUCTION
Starch is a biodegradable polymeric material that is, renewable and available worldwide at low cost
making it attractive as a substitute for petroleum based thermoplastics [1]. Polymer blends of this type are
currently receiving increased attention because of the biodegradability of the Starch component [2].
Starch is a natural renewable polysaccharide obtained from a great variety of crops and is a promising raw
material for production of biodegradable products [3]. TPS applications as bioplastics are especially
under investigation. As with synthetic polymers, the mechanical properties of TPS depend upon the
crystallinity of the constituting polymers [4]. Natural alternatives include polysaccharides such as
cellulose and Starch, which are abundant, biodegradable, and of lower cost than protein and synthetic
additives [5]. One paramount condition for producing polymer blends that take advantage of the Nanosize
of reinforcements is to obtain a sufficient dispersion in the polymer [6]. During the past 40 years, there
have been numerous studies and excellent publications on the preparation and structural, thermal,
mechanical and morphological characterization of TPU systems by various research groups [7].
The potential impact of PUs continues to be strong and promising in many emerging fields, such as
biomaterials, tissue engineering, optoelectronics, shape-memory materials, conducting polymers,
molecular recognition and smart surfaces [8]. The ability of PUR scaffolds to promote new tissue
formation in excisional wounds in rat skin, achieved through local delivery of platelet derived growth
factor (PDGF), suggested a potential utility of biodegradable PUs both as a supportive scaffold and as a
protein delivery system for tissue restoration [9]. PUs are widely used for the production of flexible
foams, automotive paint, insulating materials, adhesives and commercial moulded components [10].
Thermo-sensitive PUs are defined as functional materials with the ability to sense and respond to external
thermo-stimuli in a predetermined temperature range [11]. PUs are one of the important classes of
polymeric materials that have varied applications such as biomedical, construction, textile and automotive
[12]. PUs are a broad group of elastomers that contain one common element, a urethane linkage [13] and
these segments typically phase separate[14], while providing biocompatibility and biodegradation due to
the biobased feedstock. Biocompatability is required in the current polyurethanes for preparation of
thermoplastic Starch blends. Structure, tensile and impact properties were characterised.
2
2. MATERIALS AND METHODS
2.1 Materials
A polyurethane system with polyol (Part A) and isocyanate (Part B) was processed under room
temperature (20-25 °C). The Starch was an unmodified maize Starch (Gelose 80).
2.2 Preparation of Nanocomposites
A 250 mL beaker equipped with mechanical stirrer and thermometer was used as a reactor to prepare the
PU containing polyol (Part A) of 100 pph added with isocyanate (Part B) of pph. Then the PU mix was
added with the required quantity of filler (Starch) and stirred mechanically to disperse the Starch
uniformly into the prepolyurethane mix. The soft segment-forming reaction was carried out at 40 °C for
4.0 h, followed by a hard-segment-forming reaction at 60 °C to 70 °C for 2.0 h. A neutralization reaction
was carried out at 40 °C for 20-30 min.
Table 1. Abbreviations for PU-Starch Nanocomposites combined with isocyanate.
Samples
PU
(g)
Starch
(g)
Isocyanate
(g)
PU
21.87
0
13.90
PU-0.5% Starch
21.80
0.109
13.95
PU-1.0% Starch
21.74
0.217
13.91
PU-1.5% Starch
21.67
0.325
13.87
PU-2.0% Starch
21.61
0.432
13.83
2.3 Characterization
The composites were prepared by a casting method intoa rectangular shape of 150 x10 x3 mm. Tensile
tests was carried out with using Instron universal test instrument (Model No. 8801, UK) at 27 °C,
according to ASTM D638, with a crosshead speed of 10 mm/min. Flexural testing used a three-point bend
test carried out with a 100KN universal test instrument according to specification stated by Instron 8801
(UK). Specimens were tested according to ASTM D790. The Charpy impact strength of PU and PU-
Starch composites were determined according to ASTM D5942 using a pendulum impact instrument
(MT3016, UK). The Charpy impact tests were performed on both un-notched and single-notched
specimens at room temperature.
3. RESULTS AND DISCUSSION
3.1 Tensile strength
Table 2 presents the results of tensile stress, Young's modulus, maximum load and elongation at break for
six specimens tested for each composite. The results show that the modified PU-Starch matrix laminates
exhibited enhanced mechanical properties. From Figure 1 we observe that the 1.5 %·w/w PU-Starch
composite had the highest tensile strength of 9.62 MPa (from 0 to 1.5 %·w/w Starch loading) showed
increased tensile strength, and we observe that there is a fall in the curve at 2.0 %·w/w PU-Starch with a
lower tensile strength of 9.45 MPa (from 1.5 to 2.0 %·w/w Starch loading). At 1.5 %·w/w Starch loading,
the composites showed better interaction between the matrix and the reinforcing Starch surface leading to
an improved bond strength, resulting in the highest tensile strength. Whereas, at higher loading, for
example above 1.5 %·w/w, the tensile strength decreases may be due to agglomeration of Starch particles
resulting in poor interaction of the matrix and Starch surface.
Table 2. Tensile strength of the PU-Starch Nanocomposites.
Specimen
Tensile Stress
(MPa)
Young's Modulus
(MPa)
Max. Load
(N)
PU
8.19
1069
246
PU-0.5 % Starch
8.51
1185
255
PU-1.0 % Starch
9.35
1388
280
PU-1.5 % Starch
9.62
1311
289
PU-2.0 % Starch
9.45
1262
284
3
From Figure 2 we observe that the 1.0 %·w/w PU-Starch composite has the highest Young's modulus of
1388 MPa, showing an increasing Young's modulus from 0 to 1.0 %·w/w of starch loading. PU-Starch
with 1.5 %·w/w was similar in modulus, while we observe a lower Young's modulus of 1262 MPa for 1.5
to 2.0 %·w/w Starch with the smallest modulus. The Nano-sized Starch fillers were expected to be
bonded to the PU molecular chain so that the mobility of the molecular chains was arrested.
Figure 1. Tensile strength of the PU-Starch
Nanocomposite.
Figure 2. Tensile strength of the PU-Starch
Nanocomposite.
From Figure 3 we observe that the maximum load increased at 1.5 %·w/w PU-Starch at a maximum load
value of 289 N (from 0 to 1.5 %·w/w Starch loading) showed an increased maximum load meaning that
the composition of PU-Starch had a greater stiffness. The observed slope at 2.0 %·w/w PU-Starch had a
lower maximum load of 284 N (from 1.5 to 2.0 %·w/w Starch loading) showed decreased maximum load
and the largest slope, meaning the modulus, followed by 2.0 %·w/w PU-Starch at a load of 283.52 N.
From Figure 4 we observe that the pure PU showed the highest elongation at break of 1.75 %. The
observed slope followed by 0.5 %·w/w PU-Starch 1.395 %·w/w, 1.05 %·w/w PU-Starch 1.095 %·w/w,
1.5 %·w/w PU-Starch 0.985 %·w/w and 2.05 %·w/w PU-Starch 0.735 %·w/w, showing the smallest
stiffness that decreased the elongation at break when the Starch loading increased for other compositions
of PU-Starch. Finally, the elongation at break at 2.0 %·w/w PU-Starch was observed to be 0.73 % almost
the largest modulus.
Figure 3. Tensile strength of the PU-Starch
Nanocomposite.
Figure 4. Tensile strength of the PU-Starch
Nanocomposite.
3.2 Flexural strength
Table 3 presents the results of flexural strength at yield, load at yield and extension at yield values of six
specimens tested for each composite. The results show that the modified PU-Starch matrix composites
exhibit higher mechanical properties. From Figure 5 we observed that the 0.5 %·w/w PU-Starch had the
highest flexural strength at yield of 126 MPa, showing increased flexural strength at yield for a Starch
loading from 0 to 0.5 %·w/w this means it almost higher modulus, and we observe a drop at 1.0 %·w/w
PU-Starch had a lower flexural strength at yield of 123 MPa, showing a decreased flexural strength at
yield at a Starch loading from 1.0 to 2.0 %·w/w giving almost the same modulus.
8.19 8.51 9.35 9.62 9.45
0
5
10
15
0 0.5 1 1.5 2
Tensile Stress
(MPa)
Starch loading (%)
1068.86
1185.31
1388.28
1310.69
1262.08
0
500
1000
1500
2000
0 0.5 1 1.5 2
Young's modulus
(MPa)
Starch loading (%)
245.69 255.13 280.41 288.55 283.52
0
100
200
300
400
0 0.5 1 1.5 2
Maximum load (N)
Starch loading (%)
1.75
1.39
1.09 0.98 0.73
0
0.5
1
1.5
2
0 0.5 1 1.5 2
Elongation at
break (%)
Starch loading (%)
4
Table 3. Flexural strength of the PU-Starch
Nanocomposites.
Specimen
Flexural
Stress at
Yield
(MPa)
Load at
Yield
(N)
Extension
at Yield
(mm)
PU
100
74
20
PU-0.5 % Starch
126
76
24
PU-1.0 % Starch
124
74
28
PU-1.5 % Starch
122
72
26
PU-2.0 % Starch
119
71
24
Figure 5. Flexural strength at yield of the PU-Starch
Nanocomposite.
From Figure 6 we observe that the 0.5 %·w/w PU-Starch withstood the highest load at yield of 75.63 N
from 0 to 0.5 %·w/w Starch loading we observe the slope of 1.0 %·w/w PU-Starch has a lower load at
yield of 73.94 N from 1.0 to 2.0 %·w/w Starch loading. From Figure 7 we observe that the 1.0 %·w/w
PU-Starch withstood the highest maximum deflection of 28.35 mm, showing an increased maximum
deflection of this curve from 0 to 1.0 %·w/w Starch loading, we observe the slope of this curve at
1.5 %·w/w PU-Starch had a lower maximum deflection value of 26.17 mm from 1.5 to 2.0 %·w/w Starch
loading.
Figure 6. Load at yield of the PU-Starch Nanocomposite.
Figure 7. Extension at yield of the PU-Starch
Nanocomposite.
3.3 Impact strength
Table 4 presents the results of impact stress values for single-notched specimens with six specimens
tested for each composite. The results show that the modified PU-Starch composite exhibits increased
mechanical properties. From Figure 8 we observe that at 1.0 %·w/w PU-Starch we get the highest impact
strength of 6.33 x 10-3 J/mm2, while the impact strength was decreased at other loading levels for
example at 1.5 %·w.w PU-Starch we obtained an impact strength of 5.24 x 10-3 J/mm2. This good
compatibility has probably improved the interfacial adhesion between the components at the Starch
loading of 1.0 %·w.w PU-Starch, the impact strength of the PU composites decreased as the Starch
loading increased from 0.1 to 2.0 %·w.w. This may be due to entanglement of the Starch particles causing
the Starch to exist as a micron sized filler where there is poor interfacial adhesion between the two
components.
Table 4. Impact strength of the PU-Starch
Nanocomposites for single-notched specimens.
Sample ID
Impact Strength (J/mm2 x10-3)
PU
4.87
PU-0.5 % Starch
5.39
PU-1.0 % Starch
6.33
PU-1.5 % Starch
5.24
PU-2.0 % Starch
5.19
Figure 8. Impact strength of the PU-Starch composites
for single-notched specimens.
99.82
126.04 123.57 121.54 119.45
0
50
100
150
0 0.5 1 1.5 2
Flexural stress at
yield (MPa)
Starch loading (%)
74.33
75.63
73.94
72.46 71.33
68
70
72
74
76
78
0 0.5 1 1.5 2
Load at yield (N)
Starch loading (%)
20.32 23.85 28.35 26.17 24.27
0
10
20
30
40
0 0.5 1 1.5 2
Deflection (mm)
Starch loading (%)
4.87 5.39 6.33
5.24 5.19
0
2
4
6
8
0 0.5 1 1.5 2
Impact srrength
( J/mm2 x10-3)
Starch loading (%)
5
3.4 Scanning Electron Microscopy (SEM)
Figure 10 shows the jagged surface of the composite with 0.1 %·w.w Starch. The morphology exhibits
parallel alignment of the polymer matrix. When the starch content reached 2 %·w.w (Figure 11), the
alignment became more irregular but can still be distinguished. In Figure 11, the fracture surface of the
composite with 2 %·w.w Starch contains bumps and hollows with many crystallites randomly distributed
inside the PU matrix. The dispersion of starch particles in the PU matrix was generally uniform, though
most are in the form of particle clusters of different sizes, shown in Figure 12. There are some isolated
particle clusters of relatively large size up to several microns, particularly for the Nanocomposite
containing 2 %·w.w Starch. Although the starch particle clusters were not in nano-scale, they consist of
numerous Nanosized particles, mainly nanotubes. The PU resin indeed can penetrate into the clusters,
wetting these nanoparticles, which are somewhat similar to intercalated MMT particles. The matrix resin
has fully penetrated into a large starch particle cluster and formed a Starch-rich region.
3.4.1 Fractured surface of tensile specimen
Figure 9 SEM image shows the microstructure for 0.5 %·w.w PU-Starch on a scale of 1 mm and is
uniformly infiltrated using 100X magnification. Figure 10 SEM image shows the microstructure for
1.5 %·w.w PU-Starch on a scale of 200 µm and is uniformly infiltrated using 500X magnification.
Figure 9. SEM image for 0.5 %·w.w PU-Starch 100 X
magnification.
Figure 10. SEM image for 1.5 %·w.w PU-Starch 500X
magnification.
3.4.2 Fractured surface of impact specimen
Figure 11 SEM image shows the microstructure for 1.0 %·w.w PU-Starch on a scale of 200 µm and is
uniformly infiltrated using 500X magnification. Figure 12 SEM image shows the microstructure for
2.0 %·w.w PU-Starch on a scale of 200 µm and is uniformly infiltrated using 500X magnification.
Figure 11. SEM image for 0.1 %·w.w PU-Starch 500X
magnification.
Figure 12. SEM image for 2.0 %·w.w PU-Starch 500X
magnification.
4. CONCLUSIONS
PU-Starch composites of high modulus and strength were prepared, however they exhibited improved
properties when combined with isocyanate Nanoparticles in the matrix. These composites with bimodal
filler size provide traditional composite properties with the advantages of Nanocomposites combined. The
micronsized fibres have long fibres that are inter-woven facilitating stress transfer from the matrix.
Within the PU matrix the structure has been modified by nanoisocyanate reinforcement resulting in
physical crosslinks. The matrix is thus less ductile than a thermoplastic, approaching a thermoset via the
PU matrix
Fracture
Fracture
Starch matrix
Starch matrix
PU matrix
Starch matrix
6
physical crosslinks, though with a continued ability to be thermoformed into various shapes. PU-
isocyanate Nanocomposites were found to enhance the properties of PU; however inclusion of typical
microfibers has provided composites with higher volume fraction of fillers than available from nanofillers
alone.
ACKNOWLEDGEMENT
The authors acknowledge the Universiti Tenaga Nasional for providing the research laboratory facilities to
carry out this work. We thank the School of Applied Sciences, RMIT University, and Austalia for their
support.
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We have prepared starch-EAA complexes from different varieties of starch under a number of different reaction conditions. Water dispersions of starch at either 1 or 5% solids were combined with solutions of EAA in aqueous ammonia. Mixtures were air-dried, and the resulting composite films were then extracted with a solution of 1,1,2-trichloroethane, isopropanol, and toluene (15:15:70, by volume) to remove uncomplexed EAA. EAA content of composites after extraction was determined by FTIR. Because of its inability to form helical inclusion complexes with EAA, the microbial polysaccharide dextran was used to establish extraction conditions. Pretreatment of polysaccharide-EAA composites with methanol-water prior to extraction was essential for efficient removal of uncomplexed EAA. It was also necessary that methanol-water solutions be acidified to convert any residual ammonium carboxylate in the composite to carboxylic acid. The amount of EAA complexed by starch increased with an increase in the temperature used for gelatinization and also with the amylose content of the starch sample. Jet-cooked starch samples afforded the highest levels of complexation. Complex formation with cornstarch was not enhanced by the removal of trace amounts of lipid. Increased complexing of EAA was observed if composites were prepared at 1% as opposed to 5% solids. Under these conditions, complex formation with potato starch was about the same as that observed with cornstarch, indicating that phosphate substituents in potato starch do not influence complexing ability.
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Hydroxy-terminated polybutadiene/isophorone diisocyanate (HTPB/IPDI) polyurethane rubber which was aged in air at elevated temperatures has been studied by infrared microspectroscopy. Spectra were collected in transmission mode on microtomed samples. Analysis of sets of spectra taken across the sectioned material showed that most of the degradation occurred in the polybutadiene part of the polymer and that the urethane linkage was essentially unchanged. The trans isomer of the polybutadiene appears to be preferentially degraded compared with the vinyl isomer. The IR technique does not provide significant information about the cis isomer. The IR spectra indicated that likely degradation products included acids, esters, alcohols, and small amounts of other products containing a carbonyl functional group. Band area ratios, supported by a principal components analysis, were used to derive degradation profiles for the material. These profiles were steep-sided indicating an oxygen diffusion limited process.
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Novel segmented polyurethanes with hard segments based on a single diisocyanate molecule with no chain extenders were prepared by the stoichiometric reactions of poly(tetramethylene oxide)glycol (Mn=1000 g/mol) (PTMO-1000) and 1,4-phenylene diisocyanate (PPDI), trans-1,4-cyclohexyl diisocyanate (CHDI), bis(4-isocyanatocyclohexyl)methane (HMDI) and bis(4-isocyanatophenyl)methane (MDI). Time dependent microphase separation and morphology development in these polyurethanes were studied at room temperature using transmission FTIR spectroscopy. Solvent cast films on KBr discs were annealed at 100 °C for 15 s and microphase separation due to self organization of urethane hard segments was followed by FTIR spectroscopy, monitoring the change in the relative intensities of free and hydrogen-bonded carbonyl (CO) peaks. Depending on the structure of the diisocyanate used, while the intensity of free CO peaks around 1720–1730 cm−1 decreased, the intensity of H-bonded CO peaks around 1670–1690 cm−1, which were not present in the original samples, increased with time and reached saturation in periods ranging up to 5 days. Structure of the diisocyanate had a dramatic effect on the kinetics of the process and the amount of hard segment phase separation. While PPDI and CHDI based polyurethanes showed self-organization and formation of well ordered hard segments, interestingly no change in the carbonyl region or no phase separation was observed for MDI and HMDI based polyurethanes. Quantitative information regarding the relative amounts of non-hydrogen bonded, loosely hydrogen bonded and strongly hydrogen bonded and ordered urethane hard segments were obtained by the deconvolution of CO region and analysis of the relative absorbances in CO region.