Development of Flexible Polyurethane Nanostructured Biocomposite Foams Derived from Palm Olein-Based Polyol

This study examined the effect of organoclay montmorillonite (OMMT) on the mechanical properties and morphology of flexible polyurethane/OMMT nanocomposite (PU/OMMT) foams prepared from petroleumand palm olein-based polyols. Palm-based PU foams exhibited inferior mechanical strength as compared to neat petroleum PU foams. However, addition of OMMT significantly improved the foams strength of flexible polyurethane/OMMT nanocomposite foams prepared from palm olein-based polyol (PU bionanocomposite foam). The morphology analysed by scanning electron microscopy (SEM) showed that the cell size of the foam decreased with increasing OMMT content. PU bionanocomposite foam with 5wt% of OMMT had the most improved tensile (63%) and tear (48%) strengths compared to its neat counterpart. Transmission electron microscopy (TEM) revealed the exfoliated structure of the respective foam. It was concluded that OMMT improved mechanical properties and morphology of PU foams.


Introduction
Polyurethanes (PUs) are recognized as the most versatile polymers.They consist of soft and hard segments.Factors that influence the properties and application suitability of the polyurethane include segmental flexibility, chain entanglement, interchain forces, and cross-linking [1].Generally, polyurethanes are widely used in coatings, adhesives, foams, elastomers, and composites [2].In making PUs, polyol is one of the main raw materials.Almost all polyurethanes are derived from petroleum-based raw materials.However, issues surrounding petrochemical derived feedstock including unpredictable petroleum price, sustainability, stability of production, environment impact, and waste disposal have led to studies on renewable raw materials.Over the years, polyols have been successfully developed from natural resources.Vegetable oils such as soybean, castor, palm, and canola oils have been reported to be the potential sources for natural polyols [3][4][5][6][7].Polyols synthesised from cashew nut shell were also reported in the literature [8].Nonetheless, the properties of vegetable oil derived PU are generally inferior in comparison to the petroleum derived counterparts due to the position of hydroxyl groups that are pendent in the aliphatic backbone of the triglycerides structure.In contrast, the petroleum-based polyols are telechelic polymers [9].This contributes to lower physical properties of PU foam made from the former [9][10][11][12].
In general, to cope with the limitations such as low stiffness and low strength of polymers, particularly PUs derived from vegetable oils, inorganic fillers such as talc, glass, Al 2 O 3 , CaCO 3 , and SiO 2 were used to enhance the mechanical properties of polymer composites [13].Three main attributes of the fillers as reinforcement agents that impact the development of mechanical properties are chemistry, size, and shape [14].The mechanism of the reinforcement is based on the higher resistance of rigid filler materials against straining due to their higher module.When a rigid filler is added to the soft polymer matrix, it will carry major portion of applied load to 2 Advances in Materials Science and Engineering the polymer matrix under stress conditions, if the interfacial interactions between filler and matrix are adequate [15,16].
It has been shown that dramatic improvements in mechanical properties can be achieved by incorporation of a few weight percentages (wt%) of inorganic clay minerals consisting of layered silicates in polymer matrices [17][18][19][20][21].A commonly used layered silicate, montmorillonite (MMT), is dioctahedral clay of smectite group that have a thickness of ∼1 nm and lateral dimensions of ∼30 nm to several microns or larger.The large aspect ratios of layered silicates dominate the interaction with polymers, resulting in enhanced mechanical properties of particulate-polymer nanocomposites.
Nanocomposites can be defined as composites having more than one solid phase with a dimension in the range of 1-20 nm [1].Intercalation of polymer chains between individual platelets of layered silicates introduced into the polymer is the key to the polymer nanocomposite technology.Therefore, it is crucial to completely disperse the silicate layers in the polymer matrix for the development of remarkable polymer nanocomposites.This is accomplished by the surface modification of montmorillonite (MMT) with organophilic groups.Since MMT is hydrophilic and lacks affinity with hydrophobic organic polymers, modification of MMT is needed in order to give partially hydrophobic character.OMMT is produced by exchange of metal cations in MMT with organic ammonium salts.The affinity of PU to the surface of the clay and the organic surfactant of the OMMT is essential to promote favourable interaction between these two materials [22].
There are three types of nanocomposites structures, which depend on the OMMT opening degree after integration with polymer matrix.The composites are classified as exfoliated or delaminated when silicate layers are fully dispersed in the matrix.This type of composite yields the greatest improvement in properties because maximum reinforcement is reached.Most of the composites reported in the literature are intercalated.Intercalated composites are categorised when the layers are partially open.Composites with closed layers (tactoid) are classified as immiscible [23].
The effect of MMT on the palm oil-based rigid PUFs was studied by Chuayjuljit et al. [35].Rigid PUFs were prepared with incorporation of 1, 3, and 5 wt% MMT in the formulation.Foam with incorporation of 5 wt% MMT showed the highest compressive strength of 172 kPa as compared to the neat foam of 117 kPa.In another study, integration of modified diaminopropane montmorillonite (DAP-MMT) into palm olein-based polyol improved the compressive strength of the rigid PU nanocomposite foams.It was reported that DAP-MMT was capable of reducing the cell size of the rigid PU nanocomposite foams without altering the chemical structure.Rigid PU nanocomposite foams exhibited exfoliated structure due to uniformly dispersed DAP-MMT within PU matrix.It was suggested that the formation of urea linkages between -NH 2 groups of DAP-MMT and -NCO groups of diisocyanates could enhance the interfacial adhesion between filler and the matrix [36].In a study carried out by Piszczyk et al. [37], modified MMT enhanced the compressive stress at 20% strain from 100 to 174 kPa of the rigid PUF.It was recommended that the presence of hydroxyl group of OMMT facilitates the dispersion of the nanofillers in the polyol mixture, hence resulting in improved compressive stress.In a similar way, comparative study of the properties of rigid PU/OMMT nanocomposite foams prepared using organoclay as blowing agent was conducted by Xu et al. [38].The resultant foams demonstrated uniform and finer cell structures as compared to the rigid PUFs prepared from unmodified clay.Incorporation of up to 8 phr organoclay in the formulation revealed that rigid PUF with 2 phr organoclay resulted in the improvements of 110 and 152%, in the tensile and compressive strengths, respectively.The study also highlighted that the highest carbonyl hydrogenbonding index (2.17) was achieved at 2 phr of organoclay.The index decreased (0.96) when incorporation of organoclay was more than 4 phr.The results from the study proved that finer cell structure of rigid PU/OMMT nanocomposite foams can be accomplished using organoclay as blowing agent.In addition, more hydrogen bonding between PU and organoclay contributes to the improvement of the strengths.
A great number of literatures addressed the improvement of mechanical and thermal performance of rigid polyurethane/organoclay nanocomposite foams [35][36][37][38][39][40][41].These properties include heat and flame resistance, mechanical strength, gas barrier resistance, thermal stability, and ionic conductivity.However, studies on the effect of OMMT on palm oilbased flexible PUF are quite scarce.
The objectives of the study were to prepare flexible PU nanocomposite foams using palm olein-based polyol, Pioneer E-135 (US 7,932, 409) [42], and petroleum-based polyol with OMMT as nanoclay.The effects of the Pioneer E-135 as a drop in replacement for petroleum-based polyol in the formulation and the effects of OMMT on mechanical properties and morphology of flexible PU foams prepared from petroleum-and palm olein-based polyol and the respective PU/OMMT nanocomposite foams were investigated.The flexible PU foams produced could have a high potential to be used for mattresses or car seat.purchased from Bayer (Malaysia).Pioneer E-135 was prepared by Malaysian Palm Oil Board (MPOB).In all formulations studied, water was used as the blowing agent.Catalysts, Dabco 33LV and Niax A-1, were purchased from Kimia Cergas (Malaysia) and dibutyltin dilaurate (DBTDL) was obtained from GoldShmidt (Malaysia).Surfactant, Tegostab B 4113, was purchased from Evonik (Malaysia).Lumulse POE 26 as a cell opener was obtained from Lambert Technologies (Malaysia).All materials were used as received.

Methods.
The foam was prepared by mixing palm oleinbased polyol (Pioneer E-135), commercial petroleum-based polyols, amine and tin catalyst, silicone surfactant, and water together in a plastic cup.The mixture was stirred under high shear rate with a mechanical stirrer at 2500 rpm for one minute.Then, an appropriate amount of pMDI which was calculated based on the isocyanate index was poured into the mixture.Stirring was continued and stopped just before the cream time.The mixture was then quickly poured into a plastic container (20 × 20 × 10 cm).The foam was allowed to rise and cured at 80 ∘ C in the oven for 10 minutes.The demoulded foams were hand crushed to open the cell windows.Mechanical and morphology analysis were conducted after aging the foams at 25 ∘ C for a minimum of 7 days.Pioneer E-135 was synthesised from 100% RBD palm olein.Schematic diagram of the synthesis and flowchart of the production are given in Figures 1 and 2, respectively.The properties of Pioneer E-135 were provided by MPOB as shown in Table 1.In this study, four sets of PU foams were prepared.The first set of foams were prepared from 100% petroleum-based polyol and followed by PU foams made from 10%, 20%, and 30% palm olein-based polyol as a drop in replacement of the petroleum-based polyol.All foams prepared were incorporated with 3, 5 and 7 wt% OMMT.Formulations of PU foams prepared are shown in Table 2. Foams densities were in the range of 45 to 48 kg/m 3 .The designations of the PU foams are tabulated in Table 3.

Fourier Transform Infrared (FTIR).
Identification of functional groups of neat petroleum-and palm-based PU  foams and the nanocomposite foams was conducted using Perkin Elmer, Spectrum 100 FT-IR Spectrometer (Llantrisant, UK).The samples were scanned between 4000 and 650 cm −1 wavenumbers.

Tensile Properties.
The test was conducted according to the ASTM D3574 (Test E).Foams were cut into flat sheets of 12.5 ± 1.5 mm thickness and stamped to dumb-bell shape as described in ASTM D 412.The test was carried out using Hounsfield S-Series Machine (Surrey, UK).The specimens were placed in the grips of the testing machine and pulled at a speed of 500 ± 50 mm/min.The tensile strength of the foam was obtained using the average value from three samples.

Tear Resistance.
Tear resistance of the foams was determined using Hounsfield S-Series Machine (Surrey, UK) according to the ASTM D3574 (Test F).The specimens were clamped at the jaws of the testing machine and pulled across at the speed of 500 ± 50 mm/min.
2.6.Resilience.Foam resilience was measured according to ASTM D3574 (Test H).This test is principally a ball rebound test in which a steel ball is dropped from a prescribed height onto the sample and the percentage of recovered height is recorded.The specimen size was 100 mm × 100 mm × 50 mm.
Average value of three specimens from different locations of a sample was recorded.

Scanning Electron Microscopy (SEM).
Morphology of the foam such as cell size was observed using Zeiss, Leo 1450 VP Scanning Electron Microscopy (Oberkochen, Germany).A thin piece of foam was carefully sliced with a sharp blade and stuck to aluminium stubs.The samples were then sputtercoated with a total of 15 nm of Au/Pd and observed under the microscope employing an accelerating voltage of 10 kV and a probe current of 6 × 10 −11 amps.

Transmission Electron Microscopy (TEM).
Morphology of the PU/OMMT foam with 5 wt% OMMT was also studied using CM 12 Philips Transmission Electron Microscopy (Eindhoven, Netherlands).The 70 nm sectioned ribbons were placed on 400-mesh copper grids for imaging using TEM.The samples were imaged at high magnification of 28 000X with accelerating voltage of 100 kV.

Preparation of Polyurethane Bionanocomposite Foams.
PU bionanocomposite foams were prepared by replacing petroleum-based polyol with 10 (PUF10), 20 (PUF20), and 30% (PUF30) Pioneer E-135 with incorporation of 3, 5, and 7 wt% OMMT in the formulation.Figures 3 and 4 show the PUF10 and PUF20 with incorporation of 3, 5, and 7 wt% OMMT, respectively.PUF10 exhibited uniform cell structures.In the case of PUF20 and PUF30, coarse cell structures were clearly evident.Therefore, the respective foams were not evaluated further due to the defects of the foams.Another defect, shrinkage phenomenon, was reported by Pawlik and Prociak [12] when more than 15% palm oleinbased polyol was incorporated in the formulation.It was found that significant changes in the foam formulation are required in order to eliminate undesirable effects such as shrinkage, coarse cell structures, and collapse.The optimisation of the foam formulation, including quantities of catalysts and surfactant to be added, has to be studied.

Fourier Transform Infrared (FTIR).
In the synthesis of polyurethane (PU), there are a number of reactions that happen concurrently due to reactive isocyanate group, which reacts with molecules that have "active hydrogen" such as polyol (hydroxyl group), water, and amine [43], as illustrated in Figure 5.The most important reaction is between isocyanate and hydroxyl group of polyol (Reaction 1).This reaction leads to production of urethane group, which forms the majority of functional groups found in PU products.
Water is used as a source of blowing agent in the production of PU foams, where it reacts with isocyanates to form unstable carbamic acid (Reaction 2).The unstable carbamic acid decomposes further to form amine compound and gaseous carbon dioxide.This reaction is a very convenient source of a gas, which is necessary to generate the cellular structure of polyurethane foams.The amine, which comes from diethanolamine (chain extender) or decomposed unstable carbamic acid, reacts with an isocyanate group and generates symmetrical disubstituted urea (Reaction 3).In this study, the reaction (Reaction 1) was catalysed by an organotin compound known as dibutyltin dilaurate (DBTDL).At high temperature, the reaction between isocyanate and urethane group leads to formation of an allophanate (Reaction 4) while the reaction between urea group and isocyanates leads to formation of biuret linkage (Reaction 5) [44].Some of the reactions discussed above can be monitored via FTIR through their functionalities.
FTIR spectra of neat petroleum-and palm-based PU foams prepared with 3, 5, and 7 wt% OMMT are illustrated in Figures 6 and 7, respectively.The characteristic of FTIR spectra for PU nanocomposite and PU bionanocomposite foams was almost unchanged when compared to the neat petroleum-and palm-based PU foams.This could indicate that chemical structures of the neat petroleum-and palmbased PU foams were not affected by incorporation of OMMT [35,45].Broad stretching of hydrogen-bonded urethane, N-H, was observed at 3405 cm −1 .The band at 2995-2860 cm −1 was attributed to the C-H stretching vibration.There was no stretching vibration band at 2270 cm −1 which is a characteristic peak of isocyanate (-N=C=O) group, indicating that all of the isocyanate groups reacted during polymerization.Wavenumbers at 1731-1718 cm −1 and at 1685-1706 cm −1 are assignable to the stretching of Advances in Materials Science and Engineering  hydrogen-bonded carbonyl groups which leads to ordered and disordered conformation, respectively.These hydrogenbonded carbonyl groups can be observed at lower wavenumbers to non-H-bonded (free) carbonyls which appears at 1731-1733 cm −1 [45].Combined motion of H-N-C=O in amide II was observed at 1510 cm −1 [46].46.7 kPa and 42.0 kPa to 62.2 kPa and 68.4 kPa, respectively (Figure 8).The tear strength increased 13% (petroleum) and 48% (palm-based) from 147.5 N/m and 137.0 N/m to 167.3 N/m and 202.5 N/m, respectively (Figure 9).However, further incorporation of OMMT (7 wt%) reduced its strength due to the agglomeration of the OMMT within the PU matrix.According to Chan et al. [47] a large amount of nanoclay added in the system may agglomerate or cluster of nanoclay will be formed.It was observed that OMMT had a more significant effect on mechanical properties of PU bionanocomposite foams compared to PU nanocomposite foams.The strength of the PU bionanocomposite foams was higher than the PU nanocomposite foams, regardless of the amount of OMMT added, although the palm-based PU foams had a lower strength as compared to the neat petroleum PU foam.This phenomenon was supported with smaller cell size of PU bionanocomposite foams compared to the cell size of PU nanocomposite foams as shown by SEM images.It is well known that improved strength of the nanocomposite foams can be achieved by smaller and uniform cell sizes.According to Wilkinson et al. [48] strong H-bond formation between the edge hydroxyl groups of  the silicate lamellae (mainly silanol, Si-OH, aluminol, Al-OH) with urethane groups was to improve the strength of the nanocomposite A possible interaction mechanism of hydrogen bonding between the and OMMT is shown in Figure 10.In addition, the intercalant quaternary ammonium salts of the OMMT act as the "bridge" connecting the MMT layers and polymer chains [29].Moreover the OMMT can interlock the polymer chains and eventually form strong barriers once it is subjected to a loading [47].

3.3.2.
Resilience.Neat petroleum PU foam had better resilience than palm-based PU foam.Higher hard segment content in palm olein-based polyol decreases the resilience of PUF as reported by Rojek and Prociak [49].Addition of OMMT had no apparent effect on the resilience of PU nanocomposite foams.However, resilience of the PU bionanocomposite foams increased 19-21% as compared to the palm-based PU foam as shown in Figure 11.The results indicated that the OMMT could help to improve resilience of the palm-based PU foam.This was due to the uniform dispersion of the OMMT within the PU matrix as the OMMT contains one long alkyl tail that leads to better dispersion of OMMT platelets in PU matrix [22].PU bionanocomposite foams with addition of OMMT had resilience of more than 40%.ASTM D3574 (Test H) describes that flexible PU foams are referred to as having high resilience if resilience is greater than about 40%, thus reflected to PU bionanocomposite foams.

Scanning Electron Microscopy (SEM).
Information on cell shape and domain size can be determined by morphology of the foam.Figure 12 illustrates the cellular structure of the neat petroleum PU and the nanocomposite foams produced with 3, 5, and 7 wt% of OMMT, while the structures of the palm-based PU and the bionanocomposite foams produced with 3, 5, and 7 wt% of OMMT are shown in Figure 13.The micrographs revealed that neat petroleum PU foams and palm-based PU foam have fewer cells and a larger cell size than the respective nanocomposite foams.As the content of OMMT was increased, the average cell size decreased for neat nanocomposite foams incorporated with 3, 5, and 7 wt% OMMT.Cell size of neat petroleum PU foam has the average values of Ferrets diameter of 870 m.It was reduced to 600, 523, and 450 m with incorporation of 3, 5, and 7 wt% OMMT, respectively, whereas average values of Ferrets diameter for palm-based PU foam were 371 m and those of PU bionanocomposite foams with incorporation of 3, 5, and 7 wt% OMMT were reduced to 290, 250, and 215 m, respectively.
OMMT can act as a nucleation agent and affects nucleation efficiency due to its particle size [50,51].It serves as nucleation site for cell formation and since a higher number of cells start to nucleate at the same time, there is less gas available for their growth and this leads to a decrease in the size of the cell [52,53].From the micrograph, PU bionanocomposite foams exhibited uniformity in size and shape compared to PU nanocomposite foams due to secondary hydroxyl groups in palm oil [54].

Transmission Electron Microscopy (TEM).
Nanocomposite foams with incorporation of 5 wt% OMMT for both petroleum-and palm-based showed the highest improvement in tensile and tear strength; thus the exfoliation of the silicate layers was confirmed using TEM.A TEM image (Figure 14) of the PU nanocomposite foam with 5 wt% OMMT demonstrates intercalated structures.The dark lines represent the individual layers of OMMT which are aligned in the same direction whereas darker lines (circled area) show stacked silicate layers due to clustering or agglomeration.PU bionanocomposite foam prepared using 5 wt% OMMT shows a higher degree of exfoliated structures as shown in Figure 15.It can be seen that OMMT layers have reached nanometer scale where the average thickness appears to be just a few nanometers and the average length is about 100 nm.Exfoliated structures of PU bionanocomposite foam with 5 wt% OMMT correlate well with significant improvement of its mechanical properties.

Conclusion
Incorporation of 5 wt% OMMT improved the mechanical properties and morphology of PU nano-and bionanocomposite foams as compared to their neat PU foams.PU bionanocomposite foams showed the most significant improvement.TEM images revealed homogenous dispersion of OMMT in polymer matrix as it exhibited exfoliated structure.Smaller cell sizes were observed for PU bionanocomposites foam with incorporation of 5 wt% OMMT and this in return improved 48%, and 21% of tensile and tear strength and resilience, respectively.Incorporation of more than 5 wt% OMMT however reduced the average performance of the PU nanocomposite foams.

Figure 2 :
Figure 2: Flowchart of the production of Pioneer E-135.

Figure 9 :
Figure 9: Tear resistance of PU nanocomposite foams and PU bionanocomposite foams with different amounts of OMMT.

Figure 10 :
Figure 10: A possible interaction mechanism of hydrogen bonding between the PU chain and OMMT.

Figure 11 :
Figure 11: Resilience of PU nanocomposite foams and nanocomposite foams with different amounts of OMMT.

Table 2 :
Formulation of petroleum-and palm-based PU foams.The concentrations of all ingredients are expressed in parts per hundred parts of polyol.bTheamount of isocyanates used relative to the theoretical equivalent amount.Isocyanate index is defined as the ratio of the equivalent amount of isocyanate used relative to the theoretical equivalent amount times 100. a

Table 3 :
Designations of the prepared PU foams.