Abstract

Composites of urea-formaldehyde/clay were synthesized and analyzed in the present study. The analyses of XRD, FTIR, DSC, and TGA were performed to investigate the variation of urea-formaldehyde (UF)/clay composites' structural and thermal properties with different clay loading levels. It was discovered that the clay loading level influences the glass transition temperature of UF resin. Thermo Gravimetric Analysis (TGA) has shown that the degree of clay loading and heating rate affects the composites' thermal stability. Furthermore, the curing reactions' activation energy was investigated using Kissinger's and Ozawa's methods, confirming the presence of interactions between the polymer chain and clay particles.

1. Introduction

Clay is defined as a small crystal of alumino silicates of different ratios, having a substitution of iron and magnesium by alkali and alkaline earthy elements ¹. Most of the clay is found as a natural rocks or soils that include many minerals. Generally, clay is a fine material separated from other materials by composition structure and size. The dispersion amount of clay in clay/polymer composites is developed along with many better properties. These composites have improved properties compared to the plane polymer matrix. PCN (polymer clay composites) is generally prepared by using clay of nano or micro range. Only a small quantity of clay dispersed into the matrix can make an excellent increment in conventional polymers' properties. High clay loading level reduced the polymer's strength and toughness ^{2, 3, 4, 5, 6}. In this way, the addition of clay into the polymers comes with a value-added material. In the coating materials, the presence of organoclay alleviate the rheological properties of the coating material. This formatted coating material would control the pigment settling and sink on surfaces ⁷. Many studies have been done on clay's ability to boost the properties of the material ^{8, 9, 10}. Thermal properties of grease with clay are also studied. It is concluded that the addition of organoclay improved the thermal properties ^{11, 12}. Moreover, nano clay also has an application in cosmetics and ink due to its color retention ^{7, 13}. Petroleum industries also use organoclay to remove the hydrocarbon from refineries ¹³. Again for pharmaceuticals and pesticide industries, clay is used to remove toxic chemicals. Due to clay's absorption properties, it is also used to remove heavy metals, polychlorinated biphenyl, and organic materials ¹⁴, for removal of radionuclide's from water ^{15, 16} and application as a therapeutic agent drug vehicle ¹⁷. The innovator of nanocomposite science is the Toyota company research group. This company's research group successfully prepared Nylon6 nanocomposites with excellent properties ¹⁸. For the enhancement of these properties, clay is proposed as a filler. Under heat operations, the order of thermal expansion is as follows:

Polymer > metal > ceramic

Therefore to decrease the thermal expansion and increase the thermal stability clay (ceramics) is used as filler in the polymer matrix. This study includes the preparation of urea-formaldehyde/ball clay composites. The objective of this study is the formation and characterization of urea-formaldehyde/clay composites by using clay with different loading levels. The clay used in this study is locally available ball clay in Bikaner, district of Rajasthan.

2. Experiment Procedure

Ball clay sample was collected from industry located in the Bikaner district's kolayat region (western part of Rajasthan). Urea is taken in the form of balls and liquid formaldehyde from Green chemistry laboratory, Dungar College, Bikaner.

Urea was dissolved in formaldehyde (HCHO) and heated at 60C for about 2-3 hours with continuous stirring. The mixture was then cast on a flat rectangular Petri dish and left to dry at room temperature to prepare standard urea-formaldehyde to achieve constant weight. For the formation of clay/UF composite (CUF), the same methodology has been followed by mixing ball clay in the mixture. After drying, composites were cut into appropriate dimensions for different tests. For this study, the particle size of used clay is 2 µm. Different clay contents ( 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30wt%) of ball clay were used to produce clay-modified composites. The labeling of composites is shown in Table 1.

3. Result and Discussion

UF resin is formed when a condensation reaction of urea and formaldehyde takes place. This resin is chemically identified as poly (methylene ether hydroxymethyl ureas). Clay which contains SiO₂ particles has a SiOH group known as silanol group, which may react with macromolecular end groups and mainly with hydroxyl group ¹⁹. During urea and formaldehyde reaction, Dimethylol Urea is produced in which two hydroxyl groups are readily available to interact with silanol groups of SiO₂ ²⁰. The FTIR spectra of the UF resin is shown in Figure1. The complexity of the polymer structure results in multiple and broad peaks, as shown in this spectra. The broad peak related to the hydrogen-bonded OH and NH group is present around 3300–3350 cm⁻¹. The hydrogen bond is formed between the OH group of monomers (water and formaldehyde) and reactive functional groups CH₂OH, NH₂, and NH ²¹. It is mentioned that the peak at 3440 cm⁻¹ is the characteristic peak of the free NH₂ group, while the peak at 3340 cm⁻¹ is the characteristic peak of bonded NH group ²⁰. In the pure UF resin spectrum, the peak is found at 3341 cm⁻¹ which suggests that the bonded NH group is present in the resin. A weak absorption band appears in the pure UF resin spectrum of 3040-2900 cm⁻¹, attributed to the symmetric CH stretching mode of CH₂, CH₂OH, and NCH₂ ²². The peak at 1699 cm⁻¹ is attributed to the stretching of carbonyl (C=O) (amide I) in the CONH₂ group ²³. A weak band at 1471 cm⁻¹ and 1346 cm⁻¹ may be ascribed to CH bending mode (CH₂ methylene bridges) in CH₂/CH₂OH/NCH₂N ^{22, 23}. A peak at 1245 cm⁻¹ is due to CN stretching ²⁴. The other observed peak at 1138 cm⁻¹ and 1035 cm⁻¹ are assigned to CO aliphatic ether and methylene bridge (NCH₂N), respectively ²⁵. The weak and small peak at 770 cm⁻¹ is for NH bending and wagging vibration of amide I and amide II, respectively ²⁶.

The FTIR spectra of clay is shown in Figure 2. The band observed at the 3690 cm⁻¹ can be attributed to the stretching vibration of AlOH, and the weak band at 3426 cm⁻¹ may be assigned to hydroxyl stretching vibrations ²⁷. The small peak at 2990-2900 cm⁻¹ is attributed to CH₂ asymmetric stretching vibration ²⁴. The peak at 1626 cm⁻¹ is related to H-OH bending vibrations of water molecules absorbed on clay ²⁷. A peak at 1041 cm⁻¹ is attributed to the stretching of Si-O-Si bonds ²⁸.

The FTIR spectra of the CUF composites is shown in Figure 3. All the peaks are shown in the Table 2 for UF resin and CUF composite. The peaks around 3690 cm⁻¹ of clay, related to OH stretching vibrations caused by layered silicates, disappeared, which can be due to resin existence inside the layer ²⁴. The peaks in the range of 3343-3358 cm⁻¹ are assigned to stretching of NH bonds formation. The peaks for all the composites in the range of 2963-2874 cm⁻¹ are attributed to within phase stretching of CH₂ hydrocarbon, which shows the dispersion of clay platelets that have intercalated the polymer chains ²⁹. Peaks in the range of 1640-1675 cm⁻¹ belong to the NH-C=O group. Peaks in the range of 1537-1593 cm⁻¹ are assigned to NH stretching ²⁴. Peaks observed at 1340-1350 cm⁻¹ are the deformation vibration peaks of methylene indicates a further polycondensation reaction in UF resin ²⁸. Peaks observed in the range of 1236-1253 cm⁻¹ for CUF composites are due to CN stretching. In the range of 790-742 cm⁻¹, obtained peaks are due to Si-O-Si deformation ²⁴. In the range of 1150-1130 cm⁻¹ a medium absorption band is obtained which is due to the asymmetric stretching vibration of NCH₂N group, the vibration of (C-O-C), vibration of (Si-O-C) and vibration of asymmetric Si-O-Si of silicone ²².

X Ray Diffraction technique is useful for confirming the degree of dispersion of clay in polymer matrices ¹⁰. By using the Bragg equation and XRD test results, the distance between the layers of clay can be calculated. The wavelength of radiation (λ) used was 1.541874 A⁰. The interlayer space can be calculated using Bragg's equation which is

where n is the integer number for wavelength (in this case n=1), λ is the wavelength of the X ray, d is the platelet interlayer spacing in A⁰, θ is the X ray maximum diffraction angle.

The XRD spectrum patterns of the ball clay is shown in Figure 4. XRD spectrum of ball clay revealed broad elevated background showing the significant proportion of crystalline constituents, with major mineral phases identified as SiO₂ with prominent peaks, is about 64 to 66% of the sample and the remaining 34 to 36% is composed of amorphous material. The sharp and narrow peaks show the crystalline structure of clay, while a broad peaks are the sign of an amorphous structure.

The peaks as observed in the diffraction pattern of ball clay and corresponding d-space (the interlayer spacing of neat ball clay) are shown in Table 3.

XRD spectrum of CUF composites is shown in Figure 5.

The intercalation depth is determined by calculating the inter-gallery spacing, d₀₀₁, which is present between the polymer and the layered silicates. After mixing this clay with UF resin, the (001) peak is still present, but it shifted to a lower 2θ value corresponds to which d-spacing are given in Table 4. So there was a small but discernible shift downward in diffraction peak angle for all wt% of clay. Therefore clay mixed with UF indicating a small degree of derangement (intercalation) of the crystal structure. This decrease in 2θ value indicates an increase, although relatively small, in the distance between the clay layers.

The calculated d-spacing of clay in the UF resin increased from 7.14A⁰ to approximately 7.73A⁰. It appears as the intercalation of the UF resin into the clay (meaning a small increase in clay interlayer spacing) to some degree between the clay layers. Incomplete dispersion of clay in UF resin is evidence of some crystalline (partially) clay existing in the resin mix. For UF resin, when mixed with clay, a slightly higher d-spacing in composites could have resulted in monomers of UF resin being smaller and therefore is likely to enter interstitial spaces within the clay particles more easily.

From the data revealed in Table 4, It is evident that intercalation is considerably promoted for different loading levels of clay as positive relative intercalation (RI) values are observed. This is an indication of increment in the clay interlayer space ³⁰. From the data reported in Table 4, it is concluded that composite CUF 20 B has shown the maximum RI (%) value which means intercalated to a greater extent at 20 wt% loading level of clay.

The crystallinity of clay and CUF composites can be determined by using XRD pattern. To calculate the crystallinity following formula is used.

Crystallinity (%) = Area of crystalline peaks * 100 / Area of all peaks (Crystalline + amorphous)

From XRD patterns, less sharp and less intense crystalline peaks are observed, which approaches the composite towards amorphous nature. Due to this, the crystallinity of the composites decreases compare to clay. Crystallinity in % is given in Table 5. Destruction of the crystallization in the composites is the main reason for the cross linking for building the 3D network structure. In this way, this cross linked network restricts the promotion of crystalline domains, which leads to amorphous structure ³¹. The crystallinity and strength of the final product is related. It is stated that the low bonding strength of the three-dimensional network is the result of high crystallinity ³².

Moreover, It is also reported that UF resins' adhesion strength is affected negatively by the formation of crystalline structures within UF resins ³³. The main reason for the poor bonding strength is that the molecules that participated in the crystalline domain's development did not join in cross linked structures and were connected by weak intermolecular bonds ³². Besides, the lower value of crystallinity could be due to the interaction of particles of clay in UF resins, in which hydrogen-bonded molecules restricted the growth of crystalline domains.

Thermo Gravimetric Analysis (TGA) is an analytical technique used to verify a material's thermal stability by observing the mass loss on heating of the sample. This study tries to explain the dispersion of clay in resin. As the temperature increases, corresponding weight loss also occurs due to the release of moisture or gases from the material's decomposition. In the end, only nonvolatile and thermally stable residue remains. The TGA study is done for all CUF composites up to 400°C at a different heating rate (5°C, 10°C, 15°C, 20°C, 25°C). Figure 6 - Figure 12 show the shift in temperature for mass loss values as a function of the heating rate. From the presented TGA curves, it can be seen that the first mass loss step almost occurs between 45°C and 100°C for all samples, which corresponds to a loss of water from the samples. At temperatures, 100°C to 255°C, the broken polymer chains disassembled the network, and cyclic structures in the polymer chain occur. This degradation step results in broad polymer disintegration. The release of formaldehyde from dimethylene ether groups begins with the degradation of cured resins, and the maximum degradation occurred when the stable methylene ether linkages were destroyed ²⁰. Mass loss occurs from 255°C to 310°C due to dehydration of silane group present in amorphous silica particles. Further above this temperature degradation is due to chemically bound water loss from the SiO₂ particles.

As it is said before that TGA gives information about weight loss at a particular temperature. Figure 6 shows the TGA curve for the UF resin and Figures 7-12 show TGA curves for CUF B composites at different heating rates, said above. The temperature for mass loss of 5% for all the composites is shown in Table 6. The observation for thermal stability of the samples can be taken from the TGA curve. Figure 6 shows that for pure UF resin, 5% mass loss occurs at 82°C, while in all composites, it appears at more than 82°C, as shown in Figure 7 – Figure 12. It is concluded here that the thermal stability of composites improved significantly. Similar results are observed at all heating rates. Since the applied heating rate controls the reaction time, which is available at a particular temperature, kinetics degradation directly depends on the heating rate. Generally, it is considered that if the heating rate is lower, the time available for the reaction will be longer. A general trend in the TGA curve is followed that sample degrades at a lower temperature at a lower heating rate, while it decomposes at a higher temperature at a higher heating rate. In this way, thermal stability strongly depends on the heating rate ³⁴. It is confirmed from Table 6 that as the heating rate increases, the temperature for a particular mass loss (5% mass loss) also increases.

At 300°C, From the TG curve, it can be observed that the pure UF produced 51% mass residue, while the composites produced mass residue above 51%, as shown in Table 7. This indicates that the mass residue at a particular temperature is higher for CUF composites. Therefore it is proved that the sample composition CUF B composites are correct as they had been formulated. Among all the composites, CUF 20 B exhibited better thermal stability than pure UF. The better interaction that exists between the components in the composite is the reason behind this stability. Due to the formation of the hydrogen bond, the interaction between clay and UF has been greatly enhanced, and therefore strong adhesion between clay and UF exists.

Further, it is concluded from Table 7 that as the wt% of clay increases up to 20 wt%, the mass residue is higher for all heating rates. The mass residue percentage is high for all CUF composites compared to UF resin, indicating that clay-modified UF composites provide more thermal stability than pure UF resin. This confirms the dispersion between the clay and UF resin.

TGA shows an increase in decomposition temperature in CUF composites which indicates the formation of clay exfoliation or intercalation. This exfoliated or intercalated clay disrupts the polymer decomposition by inhibiting oxygen's movement into the material ^{35, 36}. Hence, it is concluded that any composite's thermal stability is strongly influenced by the morphology and by the loading level of clay particles.

The TGA study indicates that the intercalation or exfoliation of clay in resin shows thermal stability to be better than pure resin. The modified clay has shown better thermal stability, probably due to these reasons.

a. The hydrophilic clay contains SiO₂, which is thermally stable for temperatures up to 500^oC, resulting in the enhanced residual weight with 20% of clay. The presence of a higher % of inorganic substance like Sio₂, Fe in clay plays an important role in the thermal stability of CUF composites.

b. When mixed with UF resin, the clay would alleviate UF resin's hydrolysis, which is responsible for this thermal stability.

c. Further, clay has a large surface area; therefore, intercalated or exfoliated clay in composites shows better thermal stability. This is because of the formation of carbonaceous silicate char on the surface during burning. These char layers insulate the underlying material and slow down the escape of volatile products generated during decomposition. So overall, studies show that adding clay will enhance thermal stability.

The cross linking of polymer molecules in the curing process is exothermic, creating a positive peak in the DSC curve that is usually obtained just after the glass transition. It is known as the crystallization process. The crystallization process occurs above the glass transition temperature. Below the glass transition, molecular mobility is restricted, so there is no crystallization process, and above the glass transition temperature, small crystals are formed at relatively low temperatures. This phenomenon is known as cold crystallization. In the DSC curve, the exothermic peak temperature is considered as crystallization temperature. Usually, activation energy is composed of the activation energy of nucleation E_n and growth energy E_g because nucleation and growth are responsible for the crystallization phenomenon of glasses. Generally, both are combined into activation energy to represent the whole crystallization process. In this study, two theoretical models Ozawa's and Kissinger's equations are used to study crystallization kinetics. Activation energy has been obtained using Kissinger's and Ozawa's model, and their results have been compared in this part.

DSC thermograms of CUF B for the above said clay loading level at heating rates 5°, 10°, 15°, 20°, and 25°C/minute has shown in Figure 13-Figure 18.

The crystallization phenomenon has been analyzed in terms of crystallization temperature T_c and activation energy E_c. Using DSC thermograms, the value of the composites' crystallization temperature at different heating rates is given in Table 8.

On heating again in DSC, crystallization activation energy is the energy absorbed by the atom when it transitions between local potential minima. The energy barriers separate these minima. Among all the local minima, the most stable has the lowest internal energy in the glassy region. Therefore atoms with minimum activation energy have a high probability of making the transition to stable minima and can be stable.

It has been shown ³⁷ that three types of activation energies have to be considered in the crystallization process. These are known as the activation energy of nucleation E_n, the activation energy of growth E_g, the activation energy of whole crystallization process E_c. Again, some studies ³⁸ show that the activation energy for growth may be equal to the activation energy of the whole crystallization if it is evaluated using the sample's thermal analysis. Experimental data are analyzed based on Kissinger's and Ozawa's equation ³⁹ for non isothermal crystallization.

Kissinger’s model for activation energy

Figure 19 – Figure 24 show the plots of ln (β/T_c²) as a function of 1000/T_c for CUF B composites for different loading levels of clay at the heating rate 5, 10, 15, 20, 25°C/min, respectively. Slope of the straight line gives information about the activation energy in this region for variously said composites and are given in Table 9.

Ozawa’s model for activation energy

Figure 25 - Figure 30 show the plots of ln (β) as a function of 1000/T_c for CUF B composites for different loading levels of clay at the heating rate 5, 10, 15, 20, 25°C/min, respectively. The straight line of obtained curves gives information about the activation energy in this region for variously said composites and is given in Table 9.

It can be seen that for the kinetic analysis of composites, the calculation of the activation energies from Kissinger's and Ozawa's equation are in good agreement, as shown in Table 9. The little difference in the values of activation energies is due to approximations taken to derive these equations. The activation energy is due to the hydrogen bonds forming between the SiO₂ particles of clay and polymer chains of UF resin. The hydrogen bond altered inter phase around particles or aggregate. Hence mobility of the polymer chain, which is needed to cure the UF resin, is less in composites samples with higher activation energy. Therefore they need more energy for the cross linking process.

Further, it is also reported that clay particles create an obstacle that disrupts the matrix's continuity. Therefore, a problem is created for the reactive group of urea and formaldehyde to come close and interact. Thus in this way, the higher activation energy of the curing is an indication of the existence of UF resin/clay particle interactions. Similar results were obtained by other researchers in composites' research work ^{20, 40}. It is clear from Table 9 that composite sample CUF 20 B has the minimum activation energy, which shows that it needs less energy to cure UF resin ⁴¹. XRD results also support this result as the above said composite shows less crystallinity among all the composites shown in Table 5. The reason behind the less crystallinity is the earlier curing of UF resin by the interaction of clay with urea-formaldehyde (UF) resin. Hence, composite does not get enough time to make crystals.

4. Conclusion

1. Clay/urea formaldehyde composites were prepared by varying the clay loading level.

2. FTIR study confirmed the formation of CUF composites.

3. XRD results confirmed the increase in d­spacing of silicate layers, and positive RI values were observed indicating the intercalation of UF resin into clay layers. Composite CUF 20 B showed the maximum d­spacing. Poor crystallinity of CUF 20 B composite also favors a better cross linking network between clay and UF resin, in which hydrogen-bonded molecules restricted the growth of crystalline domains.

4. TGA study revealed that the mass residue percentage is high for all CUF composites compared to UF resin, indicating that clay clay-modified UF composites provide more thermal stability than pure UF resin. It is also found that thermal stability strongly depends on the heating rate and clay loading level.

5. The curing reaction's activation energy has been calculated from DSC thermograms, and it was reported that clay loading level affects the activation energy. Higher activation energy is an indication of the existence of clay/UF composites. Earlier curing and less crystallinity of CUF 20 B composite are observed as it showed minimum activation energy.

References