Thermal stability of natural fiber reinforced biodegradable composites

Polymer composites reinforced with natural fibers are being increasingly developed by researcher and scientist in the recent field of material science due to their various applications in aerospace, marine and industries. The hydrophilic natural fibers are incompatible with the hydrophobic polymer matrices this leads to less interfacial bonding between fibers and matrix. In this paper, fibers were collected from desert plant prosopis juliflora and NaOH treatment was done to increase interfacial bonding of fiber-Matrix. Prosopis Juliflora fiber reinforced phenol formaldehyde composites were prepared with different fiber loading up to 20wt% and then characterized by thermo gravimetric analysis. This paper describes thermal properties composites materials by Thermo gravimetric analysis TGA and Differential scanning calorimetric DSC analysis of composite materials with different heating rates and hence establishes a connection between temperature and physical properties of substances. This study highlights the potential of alkali treatment in improving the thermal stability of the composites. This paper concludes that by, increasing the fiber weight percentage (fiber loading) in PF resin does increase the thermal stability of the resulting composite. The mass residue of untreated fiber reinforced PF composites with fiber loading 15% wt. UTFRPFC 15was 35%, while treated fiber reinforced PF composites with fiber loading 15% wt. ATFRPFC 15 had a mass residue of 75% at a temperature of 400°C. This clearly shows that alkali treatment significantly enhances the thermal stability of the composites. Alkali pre-treatment activates the fibers’ surface and helps increase the fiber’s mechanical strength.


Introduction
In the recent years, natural fiber reinforced composites NFRCs, has gained considerable attention in the field of material science due to the sustainable and bio-degradable nature.Thermal properties of NFRCs are compared to the synthetic fibers composites but due to completely biodegradable and improved physical and mechanical nature NFRCs have various applications in the field of engineering and material science.The mechanical properties like tensile strength, lightweight and biodegradable natural fiber are found good as compared to synthetic fiber.Furthermore, natural fiber composites also have other limitations including incompatibility between the hydrophilic natural fibers and hydrophobic polymers.Among the numerous factors that influence the performance of the polymer composites reinforced with natural fibers is the resulting chemical modification.Thus proper surface treatment and efficient combination of natural fiber and polymer is needed to get reinforced composites.To overcome the hydrophobic nature various surface treatments are applied.The nature, size, orientation and aspect ratio of fiber as well as the matrix properties decide the mechanical and physical properties of the composites.The basic components of NFRCs are biopolymers and natural fibers, these fibers includes cellulose based fibers, manmade cellulose, lignin based fibers, proteins based fibers or a mix of these fibers.These composite materials are used for the product ranging from automotive applications to aircraft components.They show excellent mechanical properties with high damping capacity which make them attractive for the different industrial use.NFRCs are been used in damping, electrical and thermal insulation, aerospace industry, automotive industry, civil engineering applications and construction materials.The use of natural fibers offers environmental and economic advantages as compared to synthetic fibers composites.There is now vast scope to use natural fibers reinforced composites in industry, which have potential to replace the synthetic polymers and metels.[1,2].

Extraction of fibers
Prosopis Juliflora is a desert plant it can easily thrive and survive in very harsh desert environments and commonly found in the western Rajasthan [3].Initially, Stems were collected from plant prosopis juliflora.After that stems were washed from normal water and dipped into normal water for 10 days.The stem becomes softer, and its outer greenish layer is removed.After it all stems were taken out from water and washed with water until all the surface impurities are removed.Now, fiber extraction was done from the each stem Alkali treatment of PJ fiber was done to remove surface hemi-cellulose lignin and impurities.Sodium hydroxide (NaOH,) with 6 wt% concentration was used to promote better interaction between the Prosopis juliflora fiber and polymer matrix.Fibers were soaked in NaOH solution for 24 h at room temperature.Neutralization of the fibers was done with immersion of fiber into running tap water.Then fibers were dried in an oven at 40˚C temperature for 24 h and chopped in desired length for further using to fabricate composite materials.NaOH treatment gives high mechanical strength to the fiber and creates roughness on surface of fiber to increase the bonding between fiber and matrix [2].

Synthesis of Natural fiber reinforced Composites
Phenol formaldehyde resin is a Thermo-set plastic having properties including high strength, high resistance to chemical and solvents, good thermal stability and low in flammability.For the preparation of standard Phenol formaldehyde resin (PF), phenol crystal were dissolved in formaldehyde (HCHO) in the presence of alkali medium (NaOH), and the mixture was heated at 60˚C for 2 hours with continuously stirring.The mixture was then poured into a flat rectangular petri dish and allowed to dry at normal temp erature for 3-4 days.After that sample was kept in vacuum oven at 80C till constant weight was achieved.After drying composites were cut into required dimension for further test.

Thermo Gravimetric Analysis (TGA)
Thermo gravimetric analysis is a technique which is used to measure the mass loss of any specimen with respect to the increased temperature.It shows the thermal stability and volatile components of any sample by measuring weight change of sample when it is heated at a constant rate.Natural fibers are composed of cellulose, hemi-cellulose, lignin, pectin and wax etc. thermal degradation in natural fibers mainly depends on its major component cellulose.Thermal degradation of the hemi-cellulose occurs before cellulose, but due to its less content in the fiber limits its effect.The thermal decomposition of lignin occurs in a broader range that initiates earlier but extends to higher temperatures than those of hemi-cellulose and cellulose degradation However; its effect is also limited by the smaller content in the fiber [4].
Thermo gravimetric analysis was conducted in LNMIIT Jaipur.The fibers with a weight of 15mg were placed in an alumina crucible and underwent the pyrolysis process with nitrogen (30mL/min).The temperature was set between 30 •C and 500•C with a heating rate of 10˚C/min.The TGA analysis was employed to measure the changes in mass loss of the fiber sample subjected to temperature changes in a controlled atmosphere condition.The TGA result of the alkali treated PJ fiber indicates that the major thermal degradation of the sample occurs between the temperatures ranges 100˚C to 380˚C.In the first mass step almost occurs between 40˚C -100˚C, which corresponds to the water content removal from the sample.In the broader range of Second degradation step lignin is removed it.In initial stage of the Third stage degradation is due to decomposition of the hemi-cellulose content and last stage is due to decomposition of cellulose [4,5].In fig. 1 in can be seen that after giving the alkali treatment to PJ fibers by removing the lignin content from the fiber it degrades faster as compared to untreated fiber and the % weight loss of treated fiber is less than the untreated fiber.In above fig. 2 nd stage mass loss (%) in untreated is 23.46% more than treated fiber.For UTFRPFC 15 and UTFRPFC 20 mass residue at 300˚C is 76.96% and 81.13% and for ATFRPFC 15 and ATFRPFC 20 it is found 53.53% and 67.48%.It is concluded from above figure that as wt% of fiber increases 15% to 20% the mass residue is higher which indicates that modified alkali treated PJ fiber gives more thermal stability to PF composites.Hence it is concluded that thermal stability of composites strongly influenced by the fiber morphology, fiber weight and the heating rates [8].

Conclusion and future scope
• TGA curve of PJ fibers concluded that alkali treatment modifies the surface of fiber which can improve the bonding between fiber and the matrix so alkali treated PJ fibers are more thermally stable as compare to untreated PJ fibers.• It can be seen that after giving the alkali treatment to PJ fibers by removing the lignin content from the fiber it degrades faster as compared to untreated fiber.• The results of the comparison between UTFRPFC 15 and ATFRPFC 15 at a heat rate of 20˚C/min indicate that alkali treatment increases the thermal stability of the composites.The mass residue of UTFRPFC 15 was 35%, while ATFRPFC 15 had a mass residue of 75% at a temperature of 400˚C.This clearly shows that alkali treatment significantly enhances the thermal stability of the composites.• It is concluded by comparison results of UTFRPFC 15 and UTFRPFC 20 composites that thermal stability of composites also increases as fiber wt% (fiber loading) increases in PF resin.• DSC thermo-grams suggest that the alkali treatment of the fiber modifies its surface and thus, it interacts more with the PF resin.This increases the glass transition temperature (Tg) and the crystallization temperature (Tc) of the composite.Furthermore, the result of the treatment was an increased thermal stability.

Figure 1 .
Figure 1.TGA thermo-gram of treated and untreated PJ fiber.

4 .
Differential Scanning Calorimetric (DSC) DSC Differential scanning calorimetric is a technique which measures heat flow through any sample with respect to increasing temperature when any sample is heated at a constant heating rate.A general DSC curve describes Glass transition, Cold Crystallization and Melting formation of any material.The abrupt change in baseline or first negative endothermic peak represents Glass transition temperature Tg and the next positive exothermic peak is due to crystallization process and corresponding temperature is called Crystallization temperature Tc.The DSC curve are plotted between heal flow (mW) and Temperature (˚C) at constant Heating rate [6].

Figure 2 (
Figure 2(a).TG, DTG and DSC curve Figure 2(b).TG, DTG and DSC curve of Of untreated PJ fiber treated PJ fiber

Figure 3 (Figure 4
Figure 3(a).TGA curve of untreated PJ fiber Figure 3(b).TGA curve of treated PJ fiber Reinforced PF composites Reinforced PF composites

Table 1 .
Thermal parameters of treated and untreated PJ fibers.
[4]rales, luis et al. also concluded by the TG Analysis of cotton fibers that The weight loss due to water and moisture content is between 37˚C and 150°C.After that weight loss of occurs due to decomposition of non-cellulosic and cellulosic contents[4].

Table 2 (
a). TG, DTG and DSC parameters of untreated and treated PJ fibers.

Table 2 (
b). Glass transition temperature and Crystallization temperature of PJ fibers.

Table 2 (
a) describes the stages of weight loss of untreated and treated PJ fiber.The glass Transition temp.(Tg) for untreated fiber is found at 73.1˚C whereas in treated fiber it is found at 82.8˚C.The crystallization temp.
(Tc) is found in untreated fiber at 144.2˚C and in treated fiber it is found at 373.2˚C.

Table 3 .
[7,8] transition temperature for composite UTFRPFC 15 and ATFRPFC 15.It is clear from the table 3 that the highest Tg values are obtained when the heating rate is increased.This indicates that the heating rate has a significant effect on the Tg values of ATFRPFC 15 composite.The highest Tg values are obtained at the highest heating rate of 20°C/min.This indicates that the Tg values of the composite are more sensitive to the heating rate.The highest Tg values are observed for ATFRPFC 15 composite.This indicates that the ATFRPFC 15 composite has a higher thermal stability than the other composites.Alkali treatment is an effective way to improve the properties of polyester fiber reinforced polyester (PF) composites.The surface modification of the fiber increases the interfacial bonding between the PF resin and the fiber, resulting in an increase in the glass transition temperature (Tg)[7,8].

Table 4 .
Crystallization temperature for composite UTFRPFC 15 and ATFRPFC 15.Table 4 describes crystallization temperature for composite UTFRPFC 15 and ATFRPFC 15 at heat rate 5˚C/min.to 20˚C/min.It is clear from above DSC curve that as heat rate increases crystallization temperature Tc also increases.ATFRPFC 15 Composite has more Tc values than UTFRPFC 15 at heat rates 5˚C/min and 15˚C/min.