Foam stability and thermo-mechanical properties of micro/nano filler loaded castor oil based flexible polyurethane foam

Use of fillers in polymers is to improve thermo-mechanical properties of the resulting material. Fillers are also used in polymeric foam as cell openers. Flexible polyurethane (PU) foams undergo major loss in structural stability when synthetic polyol is replaced with castor oil in the formulation as an alternate polyol. This study probes the effect of various micro and nano-fillers on PU foams prepared using blend polyol containing castor oil and synthetic polyol at a ratio of 1:1. Physical and cellular properties such as foam height, cell diameter, strut thickness and cell number density were evaluated to probe the structural stability of the foam. All foams prepared were found stable while it was found that the densities of the PU foams synthesized were greater than that of the conventional PU foams. Addition of fillers found to enhance thermal and mechanical properties of the foam. Moreover, all foam samples were found to observe thermal stability over and above 258 °C. Minimum glass transition temperature was recorded for 15% HG samples (i.e., −35.5 °C). Highest tensile strength was observed for 15% Si samples whereas, highest elongation was observed for 10% NC.


Introduction
Polyurethanes (PU) are most important part of the broad ranging and supremely diversified family of polymers and plastics capable of adapting today's challenges and providing sustainable solutions [1].The mechanical strength of these materials depends on the complexity of the cell walls (the shape of the cells, wall width, and size distribution) and the internal properties of the polymer [2].Generally, polyols used in the industrial synthesis of flexible polyurethane foams are derived from crude oil.In recent times, research is focused towards investigating the incorporation of environment friendly (i.e., vegetable derived) constituents for the synthesis of polyurethane foam [3,4].Various vegetable oils and plant derived polyols such as soy oil [5][6][7][8][9][10], rapeseed oil [11,12], palm oil [13][14][15][16], castor oil [17][18][19][20][21][22][23][24][25][26][27], olive oil [28], and lignocellulose [29,30] are used in the synthesis of flexible polyurethane foams as an alternate polyol which is blended with synthetic polyol.Most of the vegetable oils and plant derived polyol requires modification by chemical reaction for introducing hydroxyl group in their parent chain [31].On the other hand, castor oil can be used in the synthesis of the polyurethane foam without any further modification, as it has inherent hydroxyl group in its parent chain [18].It has been observed that the use of castor oil in flexible polyurethane foam causes structural instability leading to a partial/limited replacement of synthetic polyol [18].Therefore, to improve the structural, mechanical and thermal properties of the flexible PU foams, micro/nano fillers are used.
A brief literature showing improvement in properties of flexible PU foams follows.Bryskiewicz et al has shown that fillers prepared using walnut shells and hazelnut shells represents the improvement in thermal stability and mechanical properties of the foam [32].Sung et al has reported the incorporation of various inorganic fillers in PU foam and found the increase in tensile strength of the flexible PU foam and decrease in compression strength and elongation.In addition, foam collapsing phenomenon has also been observed [33].Various other works on the effect of fillers on mechanical properties of flexible polyurethane foam were also reported [34].Fan et al, has reported the effect of microsphere and nanofiller in the soybean oil (15%) based polyurethane foam.The compressive strength, hardness and the bulk density of the resultant foams increased by using microsphere (>7%) and nano-silica as filler, however, a decrease in the flexibility of the foams were observed [35].In a similar study, addition of micro silica in PU foams decreased the mechanical properties [36].Fillers are also used as a cell opener in the flexible polyurethane foams which can control cell growth and stability of the rising foam [37].Nanofillers were used to enhance the stability of PU foams loaded with microfillers for enhanced mechanical and thermal properties [38].Javni et al, have reported the influence of micro and nanofiller in the flexible polyurethane foam.They have found that the incorporation of micro filler marginally improves the properties of the foam, whereas nanofillers with hydrophilic surface drastically enhance the mechanical properties [39].Reza et al, have found the reduction in cell size with the incorporation of nanoclay.Tensile properties of the foams were found to increase with the addition of clay, whereas compressive properties deteriorate [40].
From the summary of literature it has been understood that the use of fillers (micro/nano) in the polyurethane foam can improve structural strength of the expanding foam which can be used to overcome the stability issues in the vegetable oil based formulation.In this work, flexible polyurethane foams were synthesized using castor oil and synthetic polyol in 1:1 ratio loaded with various micro and nanofillers (at different weight percentages).Effects of these fillers on the stability of these foams were studied by analyzing physical, cellular, mechanical and thermal properties of the resultant foams.

Foam synthesis
The polyol blend was prepared by adding equal amount of polyether polyol (6 g) and castor oil (6 g) in a paper cup with a volumetric capacity of 600 ml.Then, different fillers (10 wt% and 15 wt% with respect to the polyol blend) were added to the paper cup and sonicated using a probe sonicator for 5 min for homogeneous dispersion of particles in the polyol blend.Next, 0.32 g of catalyst blend (catalyst to surfactant ratio of 3:1) and distilled water (0.54 g) was added to the cup and stirred with a mechanical stirrer at 4000 rpm till cream color appears.Following that, the isocyanate oligomer (9.32 g equivalent to isocyanate index 100) was added into the polyol blend and stirred for 10 s at 4000 rpm.The blend was allowed to expand freely to obtain polyurethane foam and was kept for curing at room temperature for 24 h.

Characterization
Sol fraction was measured by taking 3 small cubes (2.5 cm × 2.5 cm × 2.5 cm) from each foam samples.Further, the samples were immersed in a beaker filled with dimethylformamide (DMF) and kept undisturbed for 72 h.Next, the samples were removed and placed in a hot air oven at 80 °C for 48 h to ensure complete removal of solvent.Finally, the sol fraction of PU foams with different fillers and varying concentration is obtained by calculating the ratio of weight loss by the sample before and after drying to that of the actual weight of the sample.Thermogravimetric analysis of the samples was done using Q50 (Make and model).Approximately 3 g of sample were placed in the alumina pan and temperature was gradually increased from room temperature (26 °C) to 700 °C at a heating rate of 10 °C min −1 .Glass transition temperature of the foams were measured using Q200 (TA Instruments).Approximately 3 mg of sample were placed in the alumina pan and temperature was gradually decreased from room temperature (26 °C) to −80 °C at a cooling rate of 5 °C min −1 .Tensile strength of the foams were measure using Universal Testing Machine (UTM TFT-50KN-C).Samples were prepared as per ASTM D-638 and a rate of strain was fixed at 5 mm min −1 .FTIR of the foam samples were obtained using IRAAffinity-1 (Shimadzu).Scanning electron micrographs were obtained using EVO 18 Research: Zeiss.The samples were coated with gold using a sputter unit before imaging.

Results and discussion
3.1.Physical properties, cellular morphology and foam stability Bulk density of foam indicates the applicability of the foam for a suitable application.PU foams loaded with fillers show high bulk densities as compared to conventional PU foam. Figure 1(a) shows that silica containing 15% concentration acquires the highest density compared to other polyurethane foams.Silica with 10% concentration also comparatively has a higher density value but slightly lesser in comparison with hollow glass.Density reveals the quality and durability of the PU foams.Higher density depicts that the formulated PU will be heavier due to increased amount of fillers.However, a general trend is found in which the 10% fillers inculcated in PU foams acquire lower bulk density.This is due the fact that the fillers induced in PU foams with higher concentration acquire high bulk density value.
From the conducted sol fraction experiment (figure 1(b)), we can conclude that PU foams containing a percentage concentration of 15% GB as filler has the least sol fraction which indicates very low loosely bounded molecules [18].The reaction between diisocyanate and polyol blend occurs extensively (∼97%) leaving a mere  percentage of unreacted molecules.HG loaded samples have the highest sol fraction (∼32%) because of the hollow structures of the microspheres not allowing the trapped polyol to interact with isocyanate.The polyol trapped in the hollow spheres is during the sonication process.PU containing 10% concentration of fillers leaves a higher amount of unreacted diisocyanate compared to the ones in case of 15%.
From the observation of FTIR data (figure 2), the data shows the type of functional group present in the region ranging from 2200 to 2400 cm −1 which is known for isocyanate stretch [38].It contains different types of functional groups like N=C=O, C≡N.The peak information is useful in estimating the amount of unreacted isocyanate present in the formulated PU foams by comparing the area under the curve.The areas under of the curves were evaluated by deconvoluting the peaks for all the samples.For 10% filler samples, areas under the curve are 0.010, 0.013, 0.011 and 0.017 for GB, NC, SI and HG respectively, whereas 15% filler samples show an area of 0.009, 0.012, 0.010 and 0.015 for GB, NC, SI and HG respectively.These results are in agreement with the sol fraction values of the samples.
The visual verification of the foam stability can be confirmed by measuring the foam rise height (see table 1 and inset image of figure 3).The cellular morphology of polyurethane foam with different fillers and varying concentration are depicted using scanning electron microscope shown in figure 3. The open cell structure of PU foams is observed indicating flexible nature of the foams.Moreover, the mean cell diameter (MCD), cell number density (CND) and mean strut thickness (MST) at various points in the image is obtained and an average of each is attained and is compared with different fillers incorporated in the polyurethane foam samples.PU foams induced with 10% fillers acquire a greater open cell structure compared to the rest due to presence of low concentration of fillers.Therefore, these foams containing 10% fillers acquire a more flexible characteristic than those in the case of 15% containing rigid properties.A clear indication of more number of cells has been observed upon increasing the concentration of particles in the formulation (see CND values in table 1).Mean strut thickness of all the samples are approximately found between 35 to 75 μm.
Figure 4 shows the FTIR curve of PU foams samples in the region ranging from 1550 to 1800 cm −1 .This region is known as the carbonyl stretching region.It consists of various groups in the polyurethane band.The major observation made with regards to the hydrogen bonding between polyurea-polyurea domains at around 1675 cm −1 wavenumber.Presence of H-bonded urea peak represents interaction between urea-urea hard domains.This indicates the possible agglomeration of hard domains leading to structural instability.Absence of H-bonded urea peak indicates good dispersion of hard domain in the urethane soft matrix, giving structural stability to the expanding polymer film during foaming [27].Moreover, a series of multiple absorption peaks were also noted and labeled in the figure 4.

Thermo-mechanical properties of polyurethane foams 4.1. Thermogravimetric analysis
The thermal degradation of PU foams with addition of various fillers and constant concentration is approximately equal to each other (see figure 5 and table 2).But among the fillers used, 10% Silica and 15% Nano clay achieves the highest temperature of 275.9 °C and 290.5 °C respectively.After the point, the bond between di-isocyanide and water will break leading to the formation of carbamic acid intermediate.The degradation of PU foam takes place till the temperature reaches 404.7 °C and 383.1 °C respectively.The rate of degradation remains constant and rate of mass of PU foams decreases, which reaches a temperature of 568.7 °C and 530.7 °C respectively where the maximum degradation of PU foams takes place.However, all the foams synthesized show a minimum degradation temperature of ∼250 °C, which indicates the thermal stability of the foams under high temperature application.

Differential scanning calorimetry
Figure 6 provides the differential scanning calorimetry curves of the resultant PU foams.Each curve acquired a transition temperature, which represents the glass transition temperature, at which the polyurethane foams starts to crystallize (i.e.turn from rubbery to glassy state).Table 3 shows the glass transition temperatures of the   different foams.It provides a clear picture of various trends between different concentration and fillers.Higher transition temperature was observed for 15% GB and 10% HG samples i.e., > −50 °C.This enables the synthesized foams to be used at lower temperatures without any change in its flexibility.Lower values of glass transition temperature was observed for 15% HG and 10% NC (i.e., < −40 °C).

Mechanical properties
Tensile strength and the elongation of PU foams indicate their ability to sustain tensile stress and stretch ability before they break/fail.The dispersion state of both micro filler and nanofiller within the polymer matrix is the major reason for the enhancement of mechanical properties.Tensile property is majorly related to the surface interaction of the polymer with filler upto a certain %loading, whereas, the distribution of filler majorly affects the elongation properties till the particles are not highly agglomerated in the polymer matrix [41].Figure 7 shows the tensile stress-strain plot for the synthesized PU foam samples.Highest tensile strength was observed for samples loaded with 15% Si as filler (see table 4).With constant displacement rate, the foam experiences  maximum stress at the center of the PU foam.It provided the maximum gauge value which can be concluded that the foam reaches breakage or rupture point slower compared to the other foams with different and varying fillers and concentration respectively.The PU foam containing with 10% nano clay filler acquire the highest elongation which indicates that these types of foams are very flexible.Reduction in tensile strength of 15% NC samples is possibly because of the particle agglomeration at this concentration leading to stress concentrated zones.These zones are the reason of failure of material at a lower loading [42].

Conclusion
Various micro and nano fillers were added in order to improve the stability of the castor oil based flexible polyurethane foam.Foam stability is measured using visual observation and cellular morphology of the samples.All the foams showed excellent stability upon loading and no cellular morphological defects were observed.Thermal and mechanical properties were measured to observe the effect of various fillers.As the concentration of various fillers is added to the foams the thermal and mechanical properties are enhanced.All the foam samples were found to observe the thermal stability over and above 258 °C.Minimum glass transition temperature was recorded for 15% HG samples (i.e., −35.5 °C).Highest tensile strength was observed for 15% Si samples whereas, highest elongation was observed for 10% NC.Finally, these varying properties of different castor oil based polyurethane foam composite samples can be used for the specific application according to the property requirement such as in cushioning, upholstery, automotive seating and packaging.

Figure 7 .
Figure 7. Stress-strain plot for various PU foam samples.

Table 1 .
Physical dimension and cellular morphological parameters.

Table 2 .
Degradation stages of the PU foam samples during TGA analysis.

Table 4 .
Tensile strength and elongation values of various PU foam samples.

Table 3 .
Glass transition temperature of various PU foams.