Abstract
In this research work, the recycling of waste tea dust particles was done by preparing the injection moulded polypropylene (PP) composites using different (10–40 wt.%) loadings of tea dust particles as a filler. The tea dust particles were treated with maleic anhydride (MA) and its esterification effect was studied on physico-mechanical (morphology, mechanical, thermal and swelling) properties of tea dust:polypropylene (TD:PP) composites. Surface modification of tea dust particles was observed due to the MA treatment i.e. the esterification effect. The surface morphology of the composites showed better interfacial bonding between the maleated tea dust particles and PP than the untreated particles. The mechanical properties of the maleated TD:PP composites were found to be improved (200–300% higher tensile strength, 250% higher tensile modulus, 43% higher flexural strength and 10% higher hardness) as compared to untreated TD:PP composites. Crystallinity and thermal properties of the maleated TD:PP composites showed increasing trend with increase in filler content. Water absorption tests showed that maleated TD:PP composites absorb less water than that of untreated one. The increment in all these properties is due to the greater compatibility in tea dust particles and PP by MA treatment. Thus, tea dust particles can be suggested for preparation of composite materials.
1 Introduction
Stiffness, hardness and dimensional stability of plastics can be improved by incorporating the lignocellulosic fillers which offer advantages such as low density, low production costs, biodegradability and renewability [1], [2]. But, the very poor compatibility between the polymer and natural filler was observed [3]. Reinforcement of natural fibre depends upon the nature and crystallinity of cellulosic material [4]. The degree of improvement in compatibility depends on the origin, size, shape and fraction of filler, as well as interaction between polymer and filler after surface modification [5]. Anhydrides [e.g. maleic anhydride (MA), phthalic anhydride and succinic anhydride] treated filler showed improvement in tensile and flexural strengths, flexural and young’s moduli as well as swelling resistance of the polymer composites (e.g. treated wheat straw, sorghum straw and can bagasse fibres in novolac resin) as compared to that of untreated filler composites [6]. The Young’s modulus of maleated fibre composites was observed to be increasing with increase in fibre content as compared to untreated fibre composites (e.g. MA treated banana, hemp and sisal fibres based novolac resin composites). The impact strength and Shore-D hardness were also reported higher in maleated fibre composites than that of untreated one [7].
Two main factors inhibit the mechanical properties of natural fibre based composites and these factors also limit their large scale production. One factor is the incompatibility between the hydrophilic filler and the hydrophobic polymer [8], while high water absorption tendency is another factor of natural filler composites which ultimately lowers the strength. However, higher water ingestion can enhance the important characteristic such as biodegradability of the polymer which is in demand for various applications [3]. Numerous scientists [9], [10], [11] have found the incompatibility between moisture and fibres, and reported that high moisture containing filler based composites exhibit voids or porosity in their structures [12]. The hydrophilic characteristic of natural fillers adversely affects adhesion to the hydrophobic polymer and as a result dimensional changes may occur in the composites. A surface modified filler is required to reduce the hydrophilic character and to improve their adhesion properties [7], [13], [14], [15], [16], [17], [18]. The impact strength and tensile modulus of PP composites were found to be improving due to self-hybridization [19]. Comparison of different sizing of fibres and the use of a coupling agent showed a significant impact on properties of injection moulded fibre-PP composites [20]. The MA grafted fibres and agro waste based polymer or resin based composites showed an improvement in mechanical properties [21], [22], [23], [24] as compared to non-modified fibres based polymer or resin composites [25]. PP can be reinforced with wood flour to increase stiffness and impact resistance. Structure and adhesion can be controlled by the addition of MA treated fillers to polymers [26].
According to world atlas report, India is second largest tea producer in the world after China, and yearly tea production is approximately ~900,000 tonnes which is ~30% of total production in the world. Average ~25 kg of dry tea is manufactured from 100 kg green tea leaves and approximately 20 kg tea is packed for the actual use in the market and other 5 kg of dry tea material is wasted. Such a great production and consumption with large quantities of tea wastes (From the Cafeteria, tea stalls and households) are usually discarded into the environment without any treatment. Recycling of waste tea dust can reduce their utilization related environmental issues. Untreated tea dust (UTD) is light and cheap material and can be obtained from natural resources, large amount from Cafeteria, tea stalls, tea industries and households. Also, the tea leaves are rich in poly phenols (tannins) [27], [28]. On the basis of this property and presence of lignocellulose, the recycling of waste TD particles is focused to use them as organic filler up to a certain loading level in commodity polymers. This research work is focused on the utilization of TD particles for cheap products especially in building and construction sectors [29], [30], [31]. In this study, the TD particles were treated with MA to make them compatible and dispersive in PP as well as to enhance properties of composites. So, our main goal is to explore the use of TD particles for the preparation of the tea dust-polypropylene (TD:PP) composites and to study the physico-mechanical (morphological, mechanical and thermal) properties of maleated tea dust-polypropylene (MTD:PP) composites.
2 Materials and methods
2.1 Materials
Polypropylene (PP grade ICORENE®4014, Icorene Polymers, France) was used. This commercial PP had melt flow index of 1.5 gm/min at 230°C/2.16 kg and density of 0.90 gm/cm3. Acetone, methanol, toluene, acetic anhydride (AA), MA were purchased from S.D. Fine Chemicals Ltd. (Mumbai, India).
2.2 MA treatment on the surface of tea dust particles
Waste TD particles were collected from the tea stalls and used as filler because they contain cellulosic materials. All TD particles were initially washed with water and then by acetone to remove the traces of water. These pure TD particles were dried and ground in a mortar pestle to obtain particle size of 35 mesh. These particles were noted as UTD particles. A 50 gm of clean UTD particles were esterified using 2 ml of MA (a commercial compatibilizer) in 100 ml toluene (solvent). This mixture was allowed to reflux for 60 min. The resulting MA treated TD particles were dried in oven at temperature of 40°C. These particles were noted as MTD particles. The surface modification of TD particles allows to penetrate and deposit into luminous of the cell wall of filler, which minimizes the penetration of possible extend of moisture.
2.3 Preparation of composites
The TD particles (both UTD and MTD) were taken in the ratios of 10:90, 20:80, 30:70 and 40:60 (wt/wt) and reinforced with PP matrix to prepare TD:PP composites by an injection moulding machine. The TD particles (both UTD and MTD) and PP were precisely weighed in the required ratios before mixing in Micro compounder (HAAKETM Minilab II, Thermo scientific, Karlsruhe, Germany). Thereafter, mixed material of all TD:PP composites was then fed at 30°C into an injection molding machine by keeping the temperatures at 150, 170 and 180°C of the feed, compression and metering zones, respectively. The mixing and the molding process were carried out at 150–180°C for the duration of 15 min by applying a compression pressure of 5 MPa. Direct contact between the PP and metal platens during the heating and pressing process was carried using greasy paper for proper separation. All TD:PP (UTD and MTD:PP) composites specimens were cut from the pressed sheet in rectangular size having dimension of (64×12.5×2.1) cm.
2.4 Analysis of maleated tea dust particles
2.4.1 Fourier transform infrared (FTIR) spectroscopy
Particles of the UTD and MTD were finely grinded and dispersed in a KBr powder for analysis. FTIR spectrophotometer (8400, Shimadzu, Tokyo, Japan) was used to obtain the IR spectra which comprehend the bond formation due to the treatment as well as to judge the resistance of the polymer composites to water. For both TD particles, total 45 scans were taken and recorded in the wavelength range of 4000–400 cm−1 with resolution of 4 cm−1 in the transmittance mode.
2.4.2 Determination of hydroxyl value
Hydroxyl value (HV) is defined as milligram of KOH required to neutralise the amount of acid produced as a result of acetylation (of hydroxyl groups) of polymer or sample. It is also denoted by Equation (1) as shown in Appendix. HV is measured in mg of KOH per gm of polymer or sample. Hydroxyl number is useful in determining hydroxyl equivalent weight of polymer or sample [32]. Approximately, 1.5 gm of TD particles and 5 ml of acylating agent (AA: pyridine in the ratio of 1:7) were taken and refluxed for 240 min in the round bottom flask attached with the water condenser. The resulting mixture was allowed to cool in reflux assembly and then 10 ml of distilled water was added through the top of condenser. Again the contents were refluxed for 40 min for reacting excess of AA and flask containing mixture was allowed to cool under tap water at room temperature. Thereafter the condenser was rinsed with n-butanol to collect all the acid stuck to the wall of the condenser and washed in to the flask. TD particles in the contents were titrated against the standard alcoholic KOH using phenolphthalein as indicator. Volume of KOH consumed was recorded. For determination of HV, the acid value is also necessary. Similarly, acid value of TD particles was also determined by titrating TD particles against the standard alcoholic KOH using phenolphthalein as indicator. Volume of KOH consumed was recorded. Acid value is given by Equation (2) as shown in Appendix.
2.5 Characterization of polymer composites
2.5.1 Scanning electron microscopy
Morphological studies of TD:PP composites were carried by scanning electron microscopy (SEM) microscope (S4800, Type II, Hitachi, Tokyo, Japan) at an operation voltage of 10 keV and pressure of 1.33 gm/cm2. The TD:PP composites were gold coated to make them conductive and mounted on specimen tube prior to view on SEM.
2.5.2 X-ray diffraction analysis
X-ray diffraction (XRD) spectra were observed in X-ray diffractometer (D8, Brukers, Coventry, Germany) with CuKα1 radiation (λ=1.5404 Å) within the 2θ range of 20–80° operated at a voltage of 40 keV and with a current of 40 mA. A dwell time of 2 s per step was used. The crystalline phases and their relative contents in materials can be quantitatively obtained by location and number of diffraction peak and relative intensity of XRD pattern. Crystallinity values of each sample were reported during the XRD analysis.
2.5.3 Mechanical testing
Tensile tests were carried out as per ASTM D638 on UTM machine (INSTRON 5582, Buckinghamshire, UK) at the speed of 0.4 cm/min, with a span length of 0.4 cm and a load of 500 kg. Flexural tests were carried out per ASTM D790 on a UTM machine (Dipak Polyplast Private Limited 484, Ahmedabad, India). Impact strength was determined per ASTM D256 using Izod Impact Tester (Polyplast Equipment, Falling weight 293, Mumbai, India) with a notched specimen. Hardness tests were also performed on the Shore-D hardness tester (ASTM D2240). Total five test samples were used to observe the results and mean value of the results were reported.
2.5.4 Thermal testing
Vicat softening temperature (VST) was recorded on a Vicat softening tester (Shant Engineering, Mumbai, India), following the standards set forth in ASTM D1525, with a standard load of 1 kg and a heating rate of 2±0.2°C/min [33]. Here also, mean value of the total five test samples was reported.
The differential scanning calorimetry (DSC) was carried out on a DSC 60 (Shimadzu, Tokyo, Japan) for the thermal study of UTD:PP and MTD:PP composites. DSC measurements were carried out over the temperature range of 35–300°C at heating rate of 10°C/min. Nitrogen was used as purge gas and gas flow rate was kept 50 l per min. Approximately, 5–7 mg of composite sample was taken and sealed in aluminium pan prior to DSC measurement. Peak temperature and change in enthalpy (ΔH) were obtained from the maxima and area of the melting peak, respectively.
2.5.5 Physical testing
Physical tests for water absorption of TD:PP composites were performed as per ASTM D570 standards. The samples (20×10×2) mm were taken to measure the water absorption for duration of 0–96 h. First, the initial weights of TD:PP composites samples were measured and then immersed in a water bath at room temperature. The temperature of water was maintained at 25°C and loss of water due to evaporation was compensated by continuous addition of water. The specimens were removed from water at regular interval of time, wiped with filter paper to remove water from the surface and weighed. The specimens were re-immersed in water to permit the continuation of the water absorption until saturation established. A graphical nature of water absorption of TD:PP composites was prepared against time. Water absorption (% swelling) was calculated, at different interval of time (0, 24, 72 and 96 h), using Equation (3) as shown in Appendix. In equation (3), W0 is weight of sample before water absorption and Wt is weight of sample after water absorption at different time interval of 0, 24, 72 and 96 h.
A specific gravity measurement was carried out using an analytical balance equipped with a stationary support for an immersion of vessel, corrosion resistant for suspending the specimen and sinker for light specimen. The specific gravity of the test specimens was calculated using Equation (4) as shown in Appendix. In Equation (4), “a” is weight of the specimen in air, “b” is weight of specimen (with sinker and wire) in water, “w” is weight of totally immersed sinker and partially immersed wire.
3 Results and discussion
3.1 Evaluation of tea dust particles treatment by MA
Figure 1 shows schematic view indicating surface chemistry mechanism or surface modification of TD particles using MA treatment. As shown in reaction mechanism, when single molecule of MA is reacted with two MTD particles, long chain of MTD (2TD-C4H2O4) is formed and water is obtained as by-product. Thus, polar hydroxyl (OH) group attached to TD particles become nonpolar. Decrement in hydrophilicity of MTD particles as compared to UTD particles was observed from the HV. HV were reported as 1125 and 923 mg of KOH/gm for UTD and MTD particles, respectively. This attributes to the esterification of OH groups. Prediction of hydrophilicity based on OH content has already been done [18], [24], [28], [33], [34]. The MA treatment is helpful to improve the dispersion of TD particles in polymer composite. After MA treatment the surface energy of the TD particles is increased to a level much closer to the surface energy of the polymer matrix. Thus, a better wettability and a higher interfacial adhesion can be obtained. The PP chain permits segmental crystallization and cohesive coupling between the modified filler and the PP matrix. The MA treatment reduced a polar component and surface energy of the TD particles. Thus, the wettability of TD particles with PP could be improved by the action of MA treatment due to the reduction of polar component and surface energy of the TD particles.
Surface modification of TD particles using MA.
Figure 2 shows higher transmittance for the MTD particles and low transmittance in UTD particles, due to the formation of hydrophobic groups at the interface by using MA. FTIR spectrum of MA treated TD particles revealed the formation of new ester groups (1750 cm−1) or showing (C=O) stretching between hydroxyl groups of TD particles. Peaks were observed at 1535 cm−1 for MTD particles (Figure 2), which is due to (C=C) stretching. These vibrations are absent in UTD particles, which indicates the proper functionalization or surface modification of TD particles using MA treatment.
FTIR spectra of untreated and treated TD particles.
3.2 Mechanical properties of the polymer composites
The most of mechanical properties of the TD:PP composites have been found to be increasing [27], [28]. The tensile strength was found to be increasing with increasing filler content in MTD:PP composites (Figure 3A). Tensile strength of MTD:PP composites was 200–300% more than that of UTD:PP composites. However, the tensile moduli were recorded as found to be decreasing with increasing filler content in UTD:PP composites, while increasing order with increasing filler content in MTD:PP composites, respectively (Figure 3A). As compared to UTD one, an increment of 300% in tensile moduli of MTD:PP was observed due to the surface modification of TD particles by MA grafting. Particles mesh size may also affect the tensile modulus (i.e. there may be increment in tensile modulus of TD particles having mesh size between 70 and 230, it may be decrement for TD particles having mesh size <70) [35]. An elongation at break was reported to be decreasing with increase in filler contents in both UTD:PP and MTD:PP composites (Figure 3B). An average 50–60% reduction is clearly seen in elongation at break in TD:PP composites as compared to that of MTD:PP composites. Flexural strength of MTD:PP composites was observed to be increasing as compared to that of UTD:PP composites, which is illustrated in Figure 3B. MTD:PP composites exhibited 43% higher flexural strength than UTD one, which is due to the enhancement of the tension transference in the filler-polymer interface [36]. The maximum flexural strength was observed in 40 wt.% MTD:PP composites. Consequently, flexural strength was also observed to be higher due to high crystallinity of MTD:PP composites. It is observed that the impact strength decreases with increase in the TD content in both TD:PP composites as shown in Figure 3C. However, these values for MTD:PP composites were 110% higher than that of UTD:PP composites. Impact properties of the composites are directly related to hardness [12]. It is observed from Figure 3C that the hardness decreases marginally with increase in the TD content in UTD:PP composites, and this trend is reverse in MTD:PP composites. The maximum hardness was observed in 40 wt.% of MTD:PP composites and the value was 10% higher than 40 wt.% of UTD:PP composites. The crystallinity (Table 1) and the compatibility of MTD:PP composites were observed to be increasing with increasing the filler content and as a result hardness was observed to be increasing.
Mechanical properties of TD:PP composites (A) tensile strength and tensile modulus (B) elongation at break and flexural strength (C) impact strength and hardness of TD:PP composite.
Change in enthalpy, degradation temperature and crystallinity of TD:PP composites.
| Polymer composites (composition) | Thermal analysis | % Crystallinity obtained from XRD analysis | ||||
|---|---|---|---|---|---|---|
| Temperature (°C) | ΔH (kJ/kg) | |||||
| Onset | Endset | Peak | Degradation | |||
| PP | 155 | 170 | 165 | 150 | –4.06 | 76.6 |
| MTD:PP (10:90) | 154 | 166 | 161 | 376 | –1.80 | 81.9 |
| MTD:PP (20:80) | 153 | 166 | 161 | 375 | –3.02 | 85.6 |
| MTD:PP (30:70) | 152 | 166 | 161 | 403 | –3.99 | 86.1 |
| MTD:PP (40:60) | 153 | 168 | 162 | 410 | –4.60 | 88.0 |
| UTD:PP (40:60) | 153 | 169 | 163 | 468 | –1.60 | 57.6 |
3.3 Surface morphology of the polymer composites
FE-SEM micrographs (Figure 4A) show the surface morphology of virgin PP, MTD:PP and UTD:PP composites. MTD particles are shown by arrows in Figure 4B and it also shows smooth and uniform dispersion of MTD particles on the PP surface as compared to UTD particles as shown in Figure 4C. Increasing the contents of TD particles in PP composites resulted high interfacial adhesion, low quantity of empty spaces and low surface roughness. As a result, stiffness and mechanical strength increased of the TD:PP composites as aforementioned in the mechanical analysis [27], [28].
FE-SEM images (A) PP (B) MTD:PP and (C) UTD:PP composites.
3.4 Thermal properties of the polymer composites
Figure 5 shows that the VST decreases with increasing TD content from 10 to 40 wt.% in TD:PP composites. The maximum value of VST was observed in case of 40 wt.% MTD:PP composites, which was with 23°C higher than 40 wt.% UTD:PP due to the better compatibility of treated filler (MTD particles) with the polymer (PP matrix).
VST of TD:PP composites.
Figure 6 and Table 1 show the thermal behavior of neat PP, UTD:PP and MTD:PP composites. It is observed from the results that there were no much variations in onset, endset and peak temperatures of the TD:PP composites. However, the change in enthalpy was reported maximum in case 40 wt.% MTD:PP composites. These results show that the change in heat flow behavior was found to be increasing endothermic trend with increasing the amount of the filler up to 40 wt.%. The heat flow behavior of 40 wt.% MTD:PP composites was higher than that of 40 wt.% of UTD:PP composites, which was also recorded in Silane treated wood polymer composites [37]. Crystallinity values of each sample were reported during the XRD analysis and reported in Table 1. From crystalline values, it can be said that the MA treatment of TD particles improved the crystalline behavior of MTD:PP composites. This improvement shows the mere presence of MTD particles in the PP matrix and it can increase the crystallinity. The reduction in crystallinity of UTD:PP composite is related to the transcrystalline region due to restrictions by UTD particles in the lateral direction.
DSC thermograms of PP, MTD:PP and UTD:PP composites.
3.5 Physical properties
Figure 7 shows the water absorption by UTD:PP and MTD:PP composites. Water absorption in MTD:PP composites increases with increase in filler contents, but it is less as compared to UTD:PP composites. It was also observed that the water absorption increases with respect to time. Surface modification of TD particles using MA improves the compatibility. The polymer-treated filler interaction attracts them nearer to each other and sometime it may be restricted because of the water penetration [38], [39]. Presence of esterified OH groups is recorded by measuring the HV of all MTD particles. Thus, MTD particles become non polar in nature and form the bond with PP. The detailed mechanism has already been discussed which indicates that the MA esterifies the free OH groups present in TD particles and as a result water absorption is restricted [18], [34]. In general, natural materials used as fillers have a strong affinity with water [28], [40], [41], mainly due to the OH groups present in TD particles. In UTD particles, low water absorption was observed because the absorbed water may cause the swelling, decay and consequently lower down the dimensional stability and long-term performance. It should be emphasized that the PP does not absorb any moisture, which clearly indicates that all water was absorbed by the TD particles present in the UTD:PP composites. Thus, the hydrophilic character of these TD particles is responsible for absorbing water in the UTD:PP composites [28]. Figure 8 shows the density of both UTD:PP and MTD:PP composites. It is clearly seen that the density increases with increase in the TD content from 10 to 40 wt.% in TD:PP composites. The maximum density was observed in the case of MTD:PP composites as compared to UTD:PP composites.
Swelling of TD:PP composites.
Density of TD:PP composites.
4 Conclusions
Recycling of waste TD particles is done by using them as fillers to prepare PP composites. Physico-mechanical (morphological, mechanical, thermal and swelling evaluations) properties were investigated to study the effect of surface treatment using compatibilizer on these properties of TD:PP composites. The surface modified TD:PP composites are important because (i) MA treatment for surface modification of TD particles was observed by measuring the HV (1125 and 923 mg of KOH/gm of UTD and MTD particles, respectively). (ii) IR peaks were observed due to the (C=O) stretching of α-β unsaturated ester and (C=C) stretching. (iii) MTD:PP composites had 200–300% higher tensile strength, 250% higher tensile modulus and 43% higher flexural strength. On the other hand, there was a significant decrement in impact strength for UTD:PP composites as compared to the MTD:PP composites. The Shore-D hardness was observed to be increasing by 10% due to the esterification, which indicates better compatibility of MTD particles with the PP. (iv) The VST and the crystallinity of the MTD:PP composites were observed to be increasing with increase in filler content and also due to the MA treatment of TD particles, while UTD: PP composites showed reverse trends. (v) A significant decrement in water absorption was observed in the case of MTD:PP composites as compared to UTD:PP composites. The composites with higher proportion of MTD particles were more hydrophobic. Thus, this study shows the solution for the utilization or recycling of waste TD particles to prepare cheap products and these TD:PP composites could be applied in infrastructure and construction materials, in decorative products like skirting boards, frames and ornamental pieces.
Acknowledgments
Authors are thankful to TEQIP-II grants funded by NPIU, MHRD, New Delhi, Government of India for providing financial assistance to carry out this research work.
Appendix
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