Novel route of synthesis for cellulose fiber- based hybrid polyurethaneF H Ikhwan, S Ilmiati, H Kurnia Adi, R Arumsari, M Chalid
Department of Metallurgy and Materials Engineering, Universitas Indonesia, UI Depok New Campus,
Depok 16424, West Java – Indonesia
E-mail : chalid@metal.ui.ac.id
Abstract. Polyurethanes, obtained by the reaction of a diisocyanate compound with bifunctional or
multifunctional reagent such as diols or polyols, have been studied intensively and well developed.
The wide range modifier such as chemical structures and molecular weight to build polyurethanes led
to designs of materials that may easily meet the functional product demand and to the extraordinary
spreading of these materials in market. Properties of the obtained polymer are related to the chemical
structure of polyurethane backbone. A number polyurethanes prepared from biomass-based
monomers have been reported. Cellulose fiber, as a biomass material is containing abundant hydroxyl,
promising material as chain extender for building hybrid polyurethanes. In previous researches,
cellulose fiber was used as filler in synthesis of polyurethane composites. This paper reported a novel
route of hybrid polyurethane synthesis, which a cellulose fiber was used as chain extender. The
experiment performed by reacting 4,4’-Methylenebis (cyclohexyl isocyanate) (HMDI) andpolyethylene glycol with variation of molecular weight to obtained pre-polyurethane, continued byadding micro fiber cellulose (MFC) with variation of type and composition in the mixture. The
experiment was evaluated by NMR, FTIR, SEM and STA measurement. NMR and FTIR confirmed
the reaction of the hybrid polyurethane. STA showed hybrid polyurethane has good thermal stability.
SEM showed good distribution and dispersion of sorghum-based MFC.

1. Introduction
Cellulose, the main structural component of plant that confers strength and stability to theplant cell wall, is widely used in commercial material due to its properties such as biodegradability.
Cellulose that provided from fibers has many hydroxyl functional groups which makes it potential as achain extender in polyurethanes polymerization, but there has not been any study that proved it. In last
decade, some efforts have converted to several forms like, MFC, microcrystalline (MMC) with several
end use application [1]. The aim of this study, is to determine the availability of Sorghum bicolor plant
cellulose as a chain extender in polyurethane polymerization.
Sorghum bicolor is a plant with a high level of photosynthesis. It takes only 3 – 5 months forthe sweet sorghum to mature, and the species can be planted in very dry areas. The top grain, leaves
and stalks can be used for forage, and sugars can be extracted from stalks for use in syrup andfermenting ethanol [2]. According to a study conducted by Bakeer and his colleagues in 2013,
sorghum containing 32.30% of fiber with high crystallinity [3]. Sorghum bicolor and sorghum bagasse
are only different in name, but the same sweet sorghum plant [4]. According to those study, sorghum
bicolor worthy to be utilized as chain extender because its fiber contain providing many hydroxyl
groups.
Polyurethane (PU) chemistry has been studied intensively and is well developed. Typically,polyurethanes are obtained by the reaction of a diisocyanate with bifunctional or multifunctionalreagent such as diols or polyols. Material design is achieved by varying the chemical structures andmolecular weights of both the polyol and the diisocyanate compounds as the polymer precursors. Anumber of polyurethanes prepared from biomass-based monomers have been reported. So far, thesepolymers were synthesized by using several stages, such as pre-polymerization followed by chain
extension, and have relatively low molecular weights. Also the direct polymerization of relatively highmolecular weight monomers is a promising approach to obtain high molecular weight polyurethanes[5]. A study in 2013 explains that a lignin based polyurethane thermoplastics have a high tensile
strength shown by its young’s modulus. The study shows that by utilizing lignin’s rigid cell properties,
one can synthesize a high tensile strength polyurethane [6]. This study proves that sorghum bicolor
can be a chain extender in this hybrid polyurethane (HPU) synthesis and serves a novelty route ofhybrid polyurethane synthesis. The hybrid polyurethanes are obtained by pre-polymerization reactionof polyethylene glycol as the soft segment with the molecular weight of 4000 and 6000 and highmolecular diisocyanate, 4,4′-Methylenebis (cyclohexyl isocyanate) (HMDI) as the hard segment,followed by copolymerization reaction which cellulose as a chain extender.
Over the past few years, several materials used by the automotive industries have been using
PU – incorporated materials. The mentioned materials are especially referred to the materials needed
for body kits, tires, and interiors. However, PU – incorporated materials usually do not have the ability
to degrade naturally.
2. Materials and Methods
2.1 Materials
Polyurethanes were synthesized from mixtures of Polyethylene glycol (PEG) Mw:4000g/mole and 6000 g/mole obtained from Sigma – Aldrich as the soft segment with the composition
0.0025 moles (10g) and 4,4′-Methylenebis (Cyclohexyl isocyanate) (HMDI) obtained from Sigma –Aldrich as the hard segment with the composition 0.01 moles (2.45 mL), Dibutyltin dilaurate (DD)obtained from Sigma - Aldrich as the polymerization catalyst with the composition 2 drops (0.1 mL),
dimethyl formamide (DMF) as the solvent, Sorghum bicolor plant cellulose obtained from Chandra
Asih as the chain extender with the composition ranging from 0.5– 2.0 g, and cellulose obtained fromSigma – Aldrich as the chain extender with the composition ranging from 0.5 – 2.0g.
2.2 Cellulose Preparation
Sorghum was crushed by using a crushing machine and sieved to get uniform size (50 mesh).
50g of fiber were soaked with 0.25 M of NaOH solution overnight, then washed 4 times with distilledwater until the pH of fiber became 7. After that, the fiber trough bleaching stage, in this stage, fiber
was soaked by using 5% NaClO solution for 4 h and then it was washed with distilled water until thepH became 7. Next stage was acid hydrolysis stage with 25% wt H2SO4 solution for 2 h and washedwith distilled water. In all stages mentioned, fiber was immersed and stirred using hot plate magneticstirrer. In the last step, cellulose obtained was dried by oven, at 60oC for 6 h.
2.3 Synthesis of Hybrid Polyurethane
The process started by preparation of schlenk tubes filled with DMF solvent and PEG wasfluxed using vacuum suction machine and nitrogen gas three times. All of the flasks were then dipped
into the oil bath and heated. The temperature is then raised to 75°C and stirring for 1h. After 1 h,
HMDI were injected into the flasks in a nitrogen atmosphere. After that, 2 drops of Dibutyltin
Dilaurate (DD) catalyst were added into each flask. Then stirring were conducted for 2 h at a constanttemperature at75°C [7].
After the solution became as white as milk, each of the schlenk tubes were filled by 0.5g chain
extender (see each composition in Table 1 with the temperature raised to 80oC and stirring for 1h. The
process was then terminated using 20 mL of water and was left intact for 24 h until a phase separation
occurred, and the hybrid polyurethane will precipitate. After that, the precipitate will be separatedusing a filtration cloth and dried under room temperature for 3 days and will be continued withcharacterization afterwards.
2.4 Characterization
1H-Nuclear Magnetic Resonance (1H-NMR) and 13C-NMR was used to analyze the structureof hybrid polyurethanes.1H-NMR was used to identify the functional groups within the polyurethanes
and to measure the amount of phenolic hydroxyl functional groups in the polyurethanes.
Characterization procedure was conducted at DTMM FTUI using NMR Jeol 500MHz. Measurement
condition are 23⁰C for the temperature and 6 Pa for the pressure (vacuum).
Fourier Transform – Infra Red (FTIR) to analyze and confirm the NMR data about thefunctional groups formed in the hybrid polyurethanes. Specifically, FTIR was used to identify thefunctional groups in the lignin. Characterization procedure was conducted at DTMM FTUI using
FTIR PerkinElmer UATR Two Spectrum Two. The instrument has a limitation of range of wavelengthfrom 450 – 4500 λ.
Simultaneous Thermal Analysis (STA) consisted of Thermogravimetric Analysis (TGA) andDifferential Scanning Chromatography (DSC) to analyze physical changes such as Tg and Tm(thermal) and thermal stability. Characterization procedure was conducted at DTMM FTUI using
STA PerkinElmer STA6000. The instrument has a limitation of rate of 5 min /oC and max temperatureof 1000oC.
3.

Results and Discussion
The reaction can be seen in Figure 1. The reaction was initiated by pre-polymerization

process of polyethylene glycol with a diisocyanate (HMDI). The isocyanate groups are reactive
towards active hydrogen groups such as amine (-NH), hydroxyl (-OH), water, carboxylic acid (COOH), urea, and amide. Isocyanate will attack hydroxyl group forming urethane group. In the pre –PU forming stage, urethane groups are formed by reacting HMDI and PEG with DD (Dibutyltindilaurate) as a catalyst and DMF (dimethyl formamide) as a solvent in nitrogen atmosphere. The
reaction followed by copolymerization, which cellulose as chain extender to obtain polyurethanes.
In this stage, the isocyanate group from HMDI reacted with the hydroxyl group from PEG
forming a pre – polymer which is an interspersed polyurethane copolymer. The pre – polymer in thisstage still has isocyanate (N=C=O) group which will be eliminated after the addition of cellulose in thesecond step. The reaction of pre – polymer and cellulose occurs in the second step. Isocyanate groupleft on the end of the pre – polymer’s chain attacked hydroxyl group in the cellulose forming aurethane chain. The reaction shows two groups of hydroxyl; aliphatic hydroxyl, and phenolic hydroxyl.
The reaction rate for isocyanate and aliphatic hydroxyl is higher than the reaction rate for isocyanate
and phenolic hydroxyl. The result of this process is the hybrid PU.

Figure 1. Purposed Synthesis Route of HPU

Figure 2 shows the FTIR spectra of each hybrid polyurethanes synthesized from PEG 4000 (a)and PEG 6000 (b) with addition of cellulose for each the macrodiol at the amount of 1.5 g for the topgraph, 1 g for the middle graph, and 0.5 g for the bottom graph. The structure of the polymer wasconfirmed by FTIR spectrum. The presence of the amine bonds (NH3) appears at 1280 – 1342 cm-1,CO Acyl bond at 1242 cm-1, and secondary amine at 2887 cm-1. Secondary amine indicating thatisocyanate bonds (N=C=O) opened and made urethane group bond with hydrogen. Urethane groupbond in shows that hybrid polyurethane was obtained. According to the figure, fiber addition has
influence on amount of polyurethane obtained, it is signed from high molecule vibration in 1242 cm-1,2887 cm-1 from polyurethane obtained and 1104 and 3361cm-1 from cellulose.

Figure 2. Top figure: FTIR Spectra of hybrid polyurethane (HPU) P4M05, P4M1, P4M15; Bottom figure: FTIR
Spectra of HPU P6M05, P6M1, P6M15

Figure 3. Top Figure:1H-NMR Spectra of P4M1; Bottom Figure: 1H-NMR Spectra of P4S1

Characterization of the polymers was performed by 1H- 13C-NMR, and FTIR. Arepresentative 1H and13C-NMR spectrum for as one of the novel hybrid polyurethanes is shown inFigure 3. The urethane C=O group and amide groups appear in all of the samples, the former appears
at about 0.865 until 1.075 ppm in 1H-NMR and in 162.737 ppm in 13C-NMR spectra. Meanwhile thechemical shift of the amide groups appear at about 8.2229 in spectrum for the NH.

Figure 4. Top Figure:1H-NMR Spectra of P6M1; Bottom Figure: 1H-NMR Spectra of P6S1

Based from the overall FTIR spectra from Figure 3 and 4, sample P4M1, P4S1, P6M1, P6S1
have a relatively similar absorbent bond. The structure of the hybrid polyurethane is also confirmed by
FTIR spectrum, the presence of the amine bonds appears at 1342 and 1280 cm-1, while the urethanegroup and the amide group appear at 1703 cm-1 and 1553 cm-1, respectively. According to the FTIR
spectra, compared with P4M1 (Hybrid Polyurethane based from PEG – 4000 and 1 gr of cellulose),
P4S1 (Hybrid Polyurethane based from PEG – 4000 and 1 gr of sorghum) has higher peak of OH thatappears at 3400 cm-1.
From the Figure 3 and 4, it was proven that peak increases at the addition of sorghum as achain extender (P4S1 and P6S1) because there were more formed products (main and side product).
The main product is the hybrid polyurethane while the side product is the N-O2 stretch in 1392 cm-1
which only occurred in the sample with the sorghum addition.

3.1 Thermal Behavior of Hybrid Polyurethane
The thermal characteristics on the hybrid PU material was done by utilizing Differential
Scanning Calorimetry (DSC). The result obtained from DSC was shown as heat flow vs. temperaturegraph (Figure 5 left). The graph doesn’t show any heat flow decreasing peak, however it shows theincrease in glass transition (Tg) and melting temperature (Tm). The increase in Tg and Tm was causedby the higher concentration of O-H in sorghum than in cellulose so more urethane chain was formed inthe sample. Figure 5 right shows that the differences of thermal characteristics of a material werecaused by the differences in utilizing PEG as a soft segment.

Figure 5. Left Figure: Sorghum and Cellulose Hybrid polyurethane P4M2, P4S2 DSC curve
comparison; Right Figure: PEG 4000 and PEG 6000 DSC curve comparison; Bottom Figure:
P4M0.5 and P4M1.5 DSC curve comparison.

The higher the molecular weight, the higher the melting temperature because the PEG – 6000 has
more polyol chain than PEG – 4000. This causes PEG – 6000 have more cross linking with sorghum
in forming a PU. Figure 5, Bottom Figure shows that the addition of cellulose as a cross linker inpolyurethane affects greatly the thermal characteristics of a material. The greater the amount ofcellulose added to a material, the higher its melting temperature will be because there are more crosslinking happened in the process in forming a PU.

SampleP4M2
P4S2
PEG 4000
PEG 6000
P4M0.5
P4M1.5

Table 2. DSC curve analysis results on hybrid polyurethane
Tg (oC)Tm (oC)
76.49
415.32110.60431.78111.22410.3063.70
435.0059.07
410.05104.20415.07

3.2 Thermal Stability AnalysisThermogravimetric Analysis was utilized to analyze thermal stability. The result of the test
was shown as a graph that shows the reducing of polyurethane mass against temperature increase as
shown in Figure 6.

Figure 6. Polyurethane TGA Curve

According to the curve, it can be seen that, HPU cellulose obtained from sorghum bicoloraddition (P6S1.5) has better thermal stability than HPU with pure cellulose addition (P6M1.5). It is
predicted caused by the chemical strong bonds which come from the hydroxyl. The –OH functionalgroup give a strong Van Der Walls intermolecular force on cellulose structure and provide thecellulose high mechanical and thermal stability. As this van der walls force increased from the –OH
functional groups which has been bonded to the PU, it may arrange any ordered chain structure or
strong attraction bonds with other functional groups which make it more difficult to break when thetemperature is continuously increased. In this case, cellulose was obtained from sorghum predicted has
more –OH functional than the pure cellulose, because cellulose from sorghum still containing
hemicelluloses and lignin.

This result was supported by Figure 7, where in the right of Figure 7

shows that there was bundles of cellulose obtained from sorghum in hybrid polyurethane sample, withdiameter of cellulose about 16µm. While from the left Figure 7 it can be seen that surface ofpolyurethane with pure cellulose (P6M1.5) has no bundles like the right one does. It Indicates that thetwo type of cellulose has different size and diameter, so when it was mixed and reacted, the purecellulose is able to be blended well with all of composition. While, cellulose form sorghum, with

larger diameter and tend to congregate, gave a stronger influence on the thermal stability. The bundle
was hard to break when the sample was heated with high temperature.

Figure 7. Left Figure: SEM Result of HPU with pure cellulose with 1000x magnifying; Right Figure:
SEM Result of HPU with sorghum bicolor with 1000x magnifying.

4. Conclusion
The three composition of the base materials (PEG, HMDI, and Sorghum bicolor plantcellulose) have been successfully synthesized as hybrid polyurethane proven by the NMR and FTIR
results in showing C=O, NH, amide, and C-O-H functional groups in all of the samples. Concentration
variation of sorghum bicolor plant cellulose as a chain extender influences the thermal properties ofthe hybrid polyurethane proven by the DSC and TGA results in showing the glass transition andmelting temperature comparison between 2 kinds of cellulose. Overall, the synthesis of hybrid
polyurethane by using cellulose as the chain extender can be proven in this study.

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