Thermogravimetric analysis of Calotropis procera fibers and their influence on the thermal conductivity and flammability studies of polymer composites

K Yoganandam; B NagarajaGanesh; P Ganeshan; K Raja

doi:10.1088/2053-1591/ab3bbe

1. Introduction

Environmental friendly materials gain attention of the materials community people on account of their increasing environmental regulations throughout the world. Natural fibers are considered as the interesting and important source for the material science people as they are becoming substitute for the synthetic fibers on account of the favorable properties such as sustainability, abundant availability and biodegradability to name a few [1]. Fibers are small thread like tiny strands that are used to reinforce the polymers. Relatively reasonable strength, less power consumption and damage to the tooling and processing equipment, non-toxicity and minimal health hazards makes natural fibers as efficient polymeric reinforcements [2]. Fibers can be extracted from the plants, animals and also from the agricultural leftovers and residues. Most of the natural fibers reinforced studies are based on fibers obtained from plants although increasing interest is also found on fibers from other sources also [3]. The prime ingredient of the plant fibers are cellulose, hemicellulose, lignin, pectin, wax and other micro-constituents. Cellulosic fibers are derived from various parts of the plants namely stem (jute, kenaf, hemp, flax and ramie) fruit (coir, luffa, kapok), leaf (pine apple, abaca), grass (rice, corn, wheat and maize) and also from the woods [4–8]. These fibers are used as reinforcements for polymer matrices as these fibers impart strength to the matrices on account of their stiffness and modulus. The mechanical behavior of kenaf fibers reinforced polypropylene under different fiber loadings is studied and reported by Akhtar et al [9] that the mechanical properties increase with increase in fiber loadings. Effect of fiber content on mechanical properties of lignocellulosic fruit fibers reinforced hybrid composite show that 30 weight % is the optimum composition for enhanced mechanical properties of the composites [10]. Water absorption is a major problem with natural fibers and water absorption studies on the natural fiber composites are conducted and reported by Collins [11]. Henequen fiber based composite materials are fabricated and studied by Herrera-Franco and have concluded that mechanical properties such that the tensile strength, flexural strength are maximum when the fiber volume content 20% [12]. In a study by Dabade et al using palm and hemp fibers reinforced polyester composites it is found that 55 weight % of the fibers exhibit maximum tensile strength [13]. Investigation on the crystallinity, fiber modification, thermal stability, weathering resistance and durability of jute/plastic composites is done by Al-Oqla and Sapuan in addition to determine the suitability of these composites to produce eco-design composites to the automotive industry [14]. Paul et al studied the thermal conductivity and thermal diffusivity of the propylene / banana fibers reinforced composites and have found that the thermal conductivity decrease with fiber loading [15]. A comparative study on the fire fighting behavior of glass and flax fiber reinforced composites is made and reported by Chai et al (2012 )which proves that composite samples reinforced using flax fibers are easily affected by fire and they release high heat and gets easily combusted with sudden deformation in their structure unlike the glass fiber composites [16]. In another similar study using Sansevieria fiber composites, huge volume of carbondioxide and smoke are evolved from the composites during combustion as against the pure polyester composites [17]. Though many studies related to mechanical, structural and electrical studies are found, comparatively few works are conducted and reported on the fire retardant studies of natural fiber based composites. As a result of increasing environmental regulations, the natural fiber based materials are finding their place in many applications like automobile, packaging, structural, electronics, textile, pharmaceutical and marine. Plant fibers naturally contain cellulose, hemicellulose and lignin as major constituents and they are easily susceptible to fire and can decompose quickly when exposed to such kind of environments. Studies on flammability characteristics are found scanty and a material's response to fire needs to be known to avoid unprecedented failures due to fire. Hence, thermal and flammability studies on natural fibers and their composites can open up new avenues for exploring further applications in this regard.

The fiber used in this study is extracted from Calotropis procera plant that belongs to the flowering species in the dogbane family called Asclepidaceae, and a native of North Africa. Indo-China and South Asia and cultivated for producing fibers to make textiles. This Calotropis procera is a drought resistant plant that grows along the road sides and also in areas where cultivation is hardly seen due to absence of rain fall. This plant is cultivated in South America and about 0.5 ton per hectare is produced for production of fibers which is identified to surpass the usage of cotton for producing fabrics as Chinese and Korean scientists are using these plants as a source for making textiles [18, 19]. Recent studies on these plants have proven that the plant could emerge as a cash crop for the farmers since the parts of the plant such as leaves, seeds, fibers of this plant are used for antioxidant and antibacterial activities, producing high quality pulp, as a renewable energy stock and as a potential polymeric reinforcement for manufacturing composites [20–23]. Chemical assay shows that the fibers of this plant possess cellulose, hemicellulose, lignin, ash, moisture content and wax as 54.5%, 12.3%, 14.8%, 7.6%, 9.3% and 1.5% respectively and are considered as prospective reinforcements. The usage of natural fibers in plastics are constantly growing and to extend their deployment to a wide variety of applications, the present work is intended to assess the thermal stability of these fibers and to study the thermal conductivity, flammability studies on Calotropis procera reinforced composites to ascertain whether they can be used as thermal shields or can be used as fight resistant materials.

2. Materials and methods

2.1. Materials

Initially, a twig is cut from the Calotropis procera plant of about 2 years old and it is soaked in normal water after removing the outer layer. Then, Calotropis procera fibers are extracted mechanically from the twig after which they are washed with water to remove any dirt and unwanted impurities present in them. The fibers are then dried under sunlight to remove moisture content present in the surface of the fibers. A small portion of these fibers are powdered for Thermogravimetric analysis and the remaining fibers are chopped and used for the fabrication of composites. Epoxy resin commonly called as bisphenol A diglycidyl ether is used as matrix along with hardener NN'-bis 2-aminoethylethane-1, 2-diamin. Epoxy resin(PER 526) and the hardener (PHG- 110) used in this study are specially formulated chemicals for hand layup applications are used as supplied by Pliogrip Resins and Chemicals Pvt. Ltd, Thane, Maharashtra State, India.

2.2. Methods

2.2.1. Thermogravimetric analysis

The most commonly employed method used to determine the thermal stability of the fiber is Thermogravimetric analysis where, a mass loss of a weighed quantity of the powdered fiber is studied as a function of time. Shimadzu Thermal Analyzer is used to perform the thermo-gravimetric analysis of the powdered Calotropis procera fiber by heating it from room temperature to 800 °C in a nitrogen atmosphere with a constant heating rate of 20 °C min⁻¹.

2.2.2. Sample fabrication

Composite samples are fabricated using handlayup technique where the hardener and the matrix are mixed in the ratio 1:10 as specified by the supplier. The dimension of the fabricated laminates is 200 mm × 200 mm × 3 mm. Curing time of about 24 h is allowed at room temperature after which the samples are post cured at 105 °C for 4 h.

2.2.3. Thermal conductivity test

The transverse thermal conductivity of the composite samples are measured using Unitherm model 2022 guarded heat flow meter in accordance with ASTM E1530 standards [24]. Five test samples of size 50 mm in diameter and 10 mm in thickness are prepared and are placed between two polished surfaces and the pressure of 0.07 MPa is applied on the top portion of the stack and the thermal conductivity of the samples are obtained. This test method is suitable for materials having thermal conductivity approximately from 0.1 to 30 W m⁻¹K⁻¹ over the approximate temperature range from 150 to 600 K [24].

2.2.4. Underwriters laboratories test (UL-94)

UL-94 vertical burning tests are conducted in accordance with ASTM D3801 standards using a controlled flame that ignites the free end of a vertically mounted sample supported by a stand as shown in figure 1, for two 10 s intervals separated by the time it takes for flaming combustion to cease after the first application. Cotton wick is placed at the bottom of the sample so that flaming drips, if any, can burn the cotton wick. Five specimens are tested to measure the ability of the material to either self-extinguish or to spread the flame by classifying them as one of the following types namely V-0, V-1, V-2 or no rating (NR) and the average of them is reported [25].

**Figure 1.** Schematic representation of vertical burning test.
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2.2.5. Rate of burning test

Rate of burning test is done on the samples as per ASTM D635 standards. This test is conducted to compare the rate of burning of different materials, or as a measure of deterioration or change in the burning characteristics of the samples prior to or during use [26]. The rate of burning depends on factors as density, anisotropic nature of the material, addition of pigments, fillers and the thickness of the specimen. Tests are conducted on five samples and the average value is reported.

2.2.6. Mechanical characterization

In order to determine the usage of these Calotropis procera fibers reinforced composites for load bearing applications mechanical characterization studies namely flexural and tensile testing are conducted in accordance with ASTM D790 and ASTM D638 standards respectively [23, 27]. Tests are conducted at a crosshead speed of 3 mm min⁻¹ using an Instron universal testing machine. The tensile test samples size is 160 mm × 12.5 mm × 3 mm while flexural test samples size is 60 mm × 12.5 mm × 3 mm. Tests are conducted on five sets of samples to check consistency.

3. Results and discussion

This section deals with the thermogravimetric analysis on the Calotropis procera fibers, thermal conductivity and flammability studies conducted on the polymer composites reinforced using Calotropis procera fibers.

3.1. Thermo-gravimetric analysis

The TGA/DTG profile of Calotropis procera fiber shows that mass loss has taken place in four stages as seen in figure 2. It is quite natural to see that the thermal degradation of the natural fibers to take place in three or four stages [27, 28]. The mass loss in the fibers taken in order of increasing temperature are due to loss of evaporation of moisture and absorbed water content, degradation of hemicellulose, degradation of cellulose and finally the degradation of lignin. These stages range between 100 °C and 600 °C and depend upon the constituents in the fibers and their relative percentage present in them. In the present study, the first stage mass loss occurs between 37 and 157 °C which is due to the release of moisture content present in the fiber. About 10% mass loss takes place at this stage which is similar to other natural fibers. The second degradation takes place in the temperature range 156 to 309 °C representing hemicellulose degradation followed by the degradation of cellulose in the temperature ranging from 309 to 396 °C. This is similar to the degradation of natural cellulosic fibers such as Typha angustifolia, Cocos nucifera , Calotropis gigantea, fish tail palm, Luffa cylindrica, Sterculia urens and Senna auriculata [24, 27–32]. Though, lignin degradation initiates about 200 °C, complete degradation occurs in the temperature range from 396 to 518 °C due to the strong aromatic bonds present in them. At this stage, the colour of the fibers turns dark. The corresponding mass losses in these stages are 7.45%, 47.09%, 22.11% and 3.89%. In the first stage the degradation rate is slow and it is maximum when the temperature is 90 °C. The temperatures at which the maximum degradation occurs are at 290 and 376 °C that correspond to the thermal degradation of the biopolymers present in the fibers. Beyond 600 °C, the mass loss is slow and is due to carbonization. This study shows that these fibers are capable of withstanding a temperature of about 200 °C similar to other natural fibers [27, 28, 30, 32].

3.2. Thermal conductivity

The thermal conductivity studies conducted on the composite samples according to the ASTM E 1530 standards are shown in figure 3. The thermal conductivity of the composites reinforced with 10, 20, 30 and 40 weight% fiber loading of Calotropis procera fibers at 328 K is 0.146, 0.141, 0.138 and 0.137 Wm⁻¹K⁻¹ respectively. The thermal conductivity of plain epoxy matrix is found to be 0.210 Wm⁻¹K⁻¹. It can be noticed that the increase in fiber loading has caused decrease in thermal conductivity values. In a study involving the hybridization of sisal and banana fibers as reinforcements in polyester matrix, the thermal conductivity of the composites are observed and reported by Maries Idicula to fall between 0.153 and 0.140 Wm⁻¹K⁻¹ with 20 to 40 volume % of fiber loading [33]. In a similar study with hybrid composites prepared with cotton and sisal, the thermal conductivity values vary between 0.231 and 0.250 Wm⁻¹K⁻¹. Ramie and cotton fibers reinforced hybrid polyester composites show thermal conductivity values in the range 0.100–0.237 Wm⁻¹K⁻¹ for 0.64 volume % of jute fibers and for the ramie fibers based hybrid cotton composites using polyester matrix, thermal conductivity values are 0.190–0.220 Wm⁻¹K⁻¹ with 0.77 volume fraction of ramie fibers [34]. The thermal conductivity values obtained in the present study agrees well with the values found in literature. This is because of the fact that the natural fibers are bad conductors of heat and electricity and is mainly attributed to the geometry of the fibers. Plant fibers contain a primary cell wall and three secondary cell walls with a central opening in the fiber cells called as lumen. Lumen is a small cavity through which the plant inducts water, air and other nutrients into it. The decrease in thermal conductivity of the composites on increasing the fiber content is influenced by the lumen diameter of the fibers as air enters and fills the cavity [35]. Air is an excellent thermal insulating medium possessing a thermal conductivity of 0.026 Wm⁻¹K⁻¹ at 298 K. When the lumen area is more, more air enters and fills the lumen making it good thermal insulator. So on increasing the fiber content, there is more space for air to enter, thus it makes the composites as good insulators [36]. This study shows that the Calotropis procera fibers reinforced epoxy composites are good thermal insulators. The average thermal conductivity of Typha angustfolia composite is reported as 0.137 Wm⁻¹K⁻¹ [24], while, the thermal conductivities of the fish tail palm fiber reinforced polyester composites are reported to lie in the range between 0.163 and 0.176 W m⁻¹K⁻¹ [29] and for bamboo fiber reinforced polyester composites the values vary between 0.185 and 0.211 Wm⁻¹K⁻¹ [37] agreeing with the present research. Based on the above results, it can be concluded that Calotropis procera composites are good thermal insulators and can be used in the manufacture of the interior parts of automobiles, electronic items, building construction, and sports goods that do not need thermal conductivity properties.

A comparison graphic showing the thermal conductivities of composites reinforced using natural fibers [29, 33, 34, 36] is given in figure 4. It is obvious that the thermal conductivity of Calotropis procera fiber composites is quite comparable with the other natural fiber reinforced composites and similar to that of sisal and banana fibers reinforced hybrid composites. Typha angustfolia composites possess similar thermal conductivity as Calotropis procera fibers reinforced composites [24].

3.3. Underwriters laboratories test

The following observations are noticed during the UL-94 test for flammability for the epoxy matrix composite samples reinforced with Calotropis procera fibers. When the samples are ignited by applying a flame for 10 s, the samples burnt completely. During burning, the unburnt part of the samples gradually gets heated and as a result of this preheating the composite samples burn easily by spreading the flames. Hence the necessity for applying second flame did not arise as per the testing method. Also, during burning, no drips are noticed and eventually the cotton wick placed under the sample does not burn. This shows that the samples are not fire retardant and are easily affected by fire. The results obtained from the experiments conducted show that there are no UL ratings for these samples. Hence these composites are classified as NC-No classification and are shown in table 1. The flammability characteristics of polypropylene / Kenaf nanocomposites and Phenolic composites reinforced with Kenaf fibers and pine apple leaf fibers are classified as NC corroborating the present study [37, 38].Generally, composite samples prepared with natural fibers support burning, since a layer produced during burning enhances the flame and spreads to the un-burnt material. Hence, phosphorous based materials such as Ammonium polyphosphate, Glycerol phosphate, halogen free materials, nanohydroxyapatite fillers and ammonium sulfamate [39–41] can also be used while preparing the composite samples to augment fire fighting characteristics of the composites so that they can be used for high temperature applications that are susceptible to fire hazards.

Table 1. Thermal conductivity, flammability studies of Calotropis procera polymer composites.

Epoxy matrix weight %	Fiber weight %	Thermal conductivity W/mK	UL-94 code	Rate of burning mm/min
90	10	0.146	NC	20.24
80	20	0.141	NC	24.12
70	30	0.138	NC	26.38
60	40	0.137	NC	27.66

3.4. Rate of burning

The rate of burning of the epoxy composites reinforced with varying weight % of Calotropis procera fibers are shown in figure 5. The rate of burning of the Calotropis procera fibers reinforced composites increase with increase in fiber loadings. This is because of the fact that natural fibers are easily vulnerable to fire and when the weight % of the fibers are increased in the composites, the chance for ignition too increases causing the rate of burning to increase [37]. Moreover when the fiber loading is increased, there may be chances for the fibers to abstain from proper wetting which exposes the cellulosic fibers to the free atmosphere making them to catch fire easily. The rate of burning of the epoxy composites reinforced with 10, 20, 30 and 40 weight % of the Calotropis procera fibers is 20.24, 24.12, 26.38 and 27.66 mm min⁻¹ respectively. This is in good agreement with the previous researches [36–39, 42]. In case of hybrid composites containing varying weight % of sisal/coir fibers, the rate of burning of the composites are reported as 33, 41, 48, 63 mm min⁻¹ for 10, 20, 30 and 40 weight % of the fibers which is far high than the present study [43] and is due to the poor adhesion and loose bonding between the fibers and resin increase rapid propagation of flame. In a study, on Kenaf fiber reinforced composites it is reported that the rate of burning of the composites is 13.6 mm min⁻¹ and the rate of burning of pineapple leaf fibers reinforced composites is 15.71 mm min⁻¹ [44]. It is reported that the rate of burning of 25 weight % filled portakarb ethylene-propylene copolymer is 27.6 mm min⁻¹ and 30 mm min⁻¹ for 34.8 weight % filled portakarb ethylene-propylene copolymer [45]. These values are quite comparable with the results obtained in this study.

3.5. Mechanical characterization

The tensile tests conducted on the composite samples show an increase in tensile modulus with increase in fiber loading exhibiting a maximum tensile modulus of 896 MPa for 30 weight % of fiber loading after which a decline in value is noticed as reported by various researchers [7, 9, 10, 23]. The tensile modulus of 10, 20 and 40 weight % fibers reinforced Calotropis procera composite samples is 732, 829, 884 MPa respectively. This is similar to the values obtained for chicken feather and madar fiber composites [3, 46].

Similarly the flexural modulus of the composites for the fiber loading taking in increasing order of fiber loading is 2758, 3165, 3426 and 4289 MPa respectively. These values agree with the flexural strength values of rice straw/chicken feather, Cocos nucifera and madar fibers reinforced polymer composites [7, 27, 46].

The tensile modulus and flexural modulus of the composites are exemplified in figures 6 and 7. These appreciable mechanical properties along with their thermal behavior can make Calotropis procera fibers reinforced composites suitable for low temperature thermal insulating products which are susceptible to light loads.

**Figure 6.** Tensile modulus of *Calotropis procera* fibers reinforced composites.
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**Figure 7.** Flexural modulus of *Calotropis procera* fibers reinforced composites.
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4. Conclusion

This study thus shows that the Calotropis procera fibers are thermally stable till 200 °C and they can be used as polymeric reinforcement. Thermal conductivity of the composites decreases with increase in fiber loading and is influenced by the fiber lumen diameter. These composites can be used for making automobile interiors, electronic items and sports goods. Flammability tests conducted on the composites show that these composites are easily flammable with NC classification and addition of fire retardants during the sample preparation can improve the fire fighting capabilities of the samples so that they can be used for applications that are vulnerable to fire hazards. Inclusion of fibers in the matrix hastens the rate of burning of the composites. Thus, it can be concluded that Calotropis procera fibers can be good candidates for low temperature thermal insulation purposes but are poor fire fighters.

Acknowledgments

The authors are thankful to Mr A Balasubramanian, retired Tamil Nadu Government Forest Engineering Division Officer, for his kind support in fiber extraction and sample preparation.

Conflicts of interest

None declared

Thermogravimetric analysis of Calotropis procera fibers and their influence on the thermal conductivity and flammability studies of polymer composites

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Abstract

1. Introduction