Impacts of Goat Horn/Ammonium Polyphosphate on Mechanical and Thermal Properties of Epoxy Composites

Abstract

ABSTRACT

In this paper, the ammonium polyphosphate (APP) is mixed with epoxy to fabricate APP/epoxy goat horn composites. The composites were obtained by casting method with different weight ratios of APP of 0%, 10%, 15%, and 20% for improving the mechanical and thermal properties. This is the first paper to discuss about both mechanical and thermal properties of APP/epoxy goat horn composites. The mechanical properties were evaluated by tensile test and wear test. For the goat horn composites with a content of 20 wt% of APP, the tensile strength was 30 MPa, shows that adding APP led to composites having a better tensile strength. From limiting oxygen index test results, it is clear that goat horn composite is fire retardant, and that the fire‐retardant value improves with the addition of APP up to a weight ratio of 15% APP. The thermogravimetric analysis (TGA) reveals that the degradation of composites in relation to temperature. From the TGA analysis, the bio composites without APP gives the better results comparing with incorporation of APP in composites up to 40% of weight loss. At the same time, the 50% of weight loss occur at the degradation temperature of around 380°C for 20 wt% of APP composites. It shows the addition of APP improves the thermal stability of the composites. The ignition time from the horizontal flammability test of the composites of 0, 10, 15, and 20 wt% APP were 20, 22, 24, and 25 (in s) respectively. It shows that the addition of filler material into the composites, the ignition time also increases. The water absorption percentage was peak at 15% of APP and lower at 10% of APP. All the samples were saturated after 80 h. The lower percentage of water absorption in composites can be attributed to the improved interfacial bonding between the fibers and matrix. From scanning electron microscopy (SEM) analysis the morphological structure and dispersion of ammonium polyphosphate and goat horn particles in epoxy bio composites were studied.

Introduction

In recent, the plastics industry has been actively searching for environmentally friendly materials to use in their products due to growing global social awareness of the negative environmental effects of plastics [1]. The bio composites are now a days being preferred more because of their bio degradable, less environmentally polluting, economically cheaper compared with synthetic composites and have less density which results in good tensile strengths. Natural fiber is a kind of renewable resource and the latest in reinforcing and supplements for the polymers [2].

The interlocking ability of the KPLF (King Pineapple Leaf Fiber) and epoxy matrix is influenced by the epoxy matrix's superior capacity to wet the KPLF fiber surface [3]. Timoho fiber's (TF) high cellulose content has made it a viable reinforcement option for composites. The cellulose extraction process proved to be efficacious and is highly recommended as a sustainable cellulose bio composite material [4]. For improving the mechanical strength and thermal stability, the various natural fibers and filler materials were reinforced with epoxy resin. Glass fiber in polymer composites is currently substituted with natural fibers as fillers [5]. The alkali treated natural fibers have better properties compare with non-treated fibers. After alkalization, the cellulose content increased while the hemi-cellulose, lignin, and wax contents decreased in fiber, according to the results of the chemical analysis [6].

Walikukun fiber is one of the many natural fibers that are utilized as an alternative for synthetic fiber. For polymer composites, 6% potassium hydroxide (KOH) concentration is ideal since it improves tensile strength, thermal stability, fiber characteristics, and crystallization index, all of which are useful for reinforcement [7]. Because of its benefits, thermosetting polymers make it easier to fabricate the constituent mixtures of nature fiber reinforced composites [8]. Additionally, it was discovered that epoxy resin could have a more stronger surface interaction compared with unsaturated polyester. Furthermore, compared with thermoplastic polymers, thermosetting composites can be processed at significantly lower processing pressures [9].

When natural fiber was added to neat polymers, the heat release was decreased and the flame-retardant performances were improved in comparison to pure matrices. Adding fire retardant at a small loading level could further reduce the bio composite's flammability behavior at the expense of a small reduction in mechanical properties [10–12]. When the fiber volume contents were increased at the ideal loading of 20%, the flammability and tensile properties of Oil Palm Empty Fruit Bunches (OPEFB) fiber reinforced fire-retardant epoxy composites were decreased, with the values being 11.47 mm/min and 4.29 kPa, respectively. The SEM morphological test revealed that the surface ruptured had more flaws, which reduced the composites' tensile strength [13]. After 10 wt% iron powder filler was added, the fire combustion speed rate dropped from 31.49 to 20.25 mm/min, resulting in a very significant gain in thermal stability. At the same time the composite's tensile strength dropped, because of the filler was not evenly distributed [14].

Thermogravimetric study demonstrated that the addition of wool fibers to the epoxy matrix as a toughening agent can increase the material's durability at elevated temperatures. By using a natural waste, wool fiber-epoxy composites have been shown to be a promising insulating material, as seen by the 30% decrease in coefficient of thermal conductivity when compared with neat epoxy, a finding further supported by simulation [15]. According to thermogravimetric analysis (TGA), the thermal stability of defatted horn fiber (HF) and polypropylene (PP) composites increases as the amount of fiber increases. Good compatibility between HF particles and PP is shown by the SEM micrograph. The composite with 15 wt% of HF particles performs best among the HF/PP composites [16].

But in a polymeric formulation, increasing the concentration of APP is frequently linked to decreased mechanical performances and unavoidably higher material costs; hence, it is normally preferable to have the least effective fire-retardant loading possible, which might be as low as 20 wt% [17]. In the Underwriters Laboratories 94 (UL-94) test, the addition of fire retardants to PALF-reinforced polymer composites reduced the rate of burning, increasing fire resistance. In the horizontal burning test, ammonium polyphosphate (APP) 30% showed excellent fire resistance, burning 10.83 mm/min [18].

There are not many published reports on the use of APP to improve the flammability, thermal stability, and mechanical properties of natural fiber-based composites, and no prior research has looked into how APPs affect the fire resistance of goat horn fiber reinforced epoxy-based composites [19, 20]. Most of the studies were taken for physical and mechanical properties of horn fiber reinforced polymer composites [21, 22]. Improved fire safety was demonstrated by EP/BADM-X, and the limiting oxygen index (LOI) raised to 28.2% [23]. Given its LOI value of 23.5% and inability to self-extinguish in the UL-94 test (no rating), the neat EP appears to have a highly flammable nature. The fire-retardant properties of EP/G@SiO2@FeHP nanocomposites are markedly enhanced upon the addition of G@SiO2@FeHP [24]. The 1.50 wt% phosphorous transesterification vitrimer (TPN1.50) exhibits desirable fire retardancy, with a vertical burning (UL-94) V-0 classification and a LOI of 35.2% [25].

The purpose of this work was to investigate the effects of APP/goat horn particles mixed with epoxy on the mechanical properties, flammability and thermal stability of the resulting composite. This was achieved by performing a number of characterizations in order to determine the optimal constituent loading composition.

Experimental Work

Materials

The materials used for fabricating the APP/epoxy goat horn composites are LY556 Epoxy with Araldite Hardener (HY951) and fire-retardant APP were purchased from Haritha agencies, Trichy. The horn of Ovis aries species of goat used as a natural fiber reinforcement were purchased from a local butcher shop at Trichy. NaOH solution and distilled water is also used for the purpose Alkali treatment of fibers.

Methodology

The goat horn of the O. aries species and the horn was first cleaned with tap water, then it was immersed in sodium hydroxide (NaOH) solution for a time period of 24 h in order to remove impurities and then it was immersed in distilled water for 1 h, later it was removed from the solution and allowed to dry in the open atmosphere for 3 days. After drying in the atmosphere, the horn was then dried in an oven at a temperature of 60°C for 2 h, after removal from the oven, it was machined to small chips with the help of a vertical milling machine. The collected chips were then turned to a fine powder with the help of a mixer grinder. Then the powder was allowed to dry again in atmospheric temperature in order to completely remove the moisture content present in it. The mold was prepared using an aluminum plate, the plate was milled with the help of a computer numerical control as per American Society for Testing and Materials (ASTM) standards of test like tensile, water absorption, wear test, and so forth. The fabrication process was carried out using LY556 epoxy resin as matrix material and Araldite hardener HY951 are used in the ratio of 3:1. The preparation of goat horn/APP epoxy composite is presented in Figure 1a,b.

[IMAGE OMITTED. SEE PDF]

The first sample was prepared to take a considerable amount of epoxy and goat horn powder, the mixture was mixed with the help of a magnetic stirrer for 1 h. Then the Hardener was poured and then carefully poured in the prepared mold and was allowed to solidify for the next 24 h at room temperature. After 24 h the specimen was carefully removed from the mold. similarly, the second sample was fabricated but in this sample 10% of APP filler was added. in the same manner the third and fourth samples were fabricated with 15% and 20% of APP respectively. All the samples were collected and segregated as per proportion shown in Table 1.

TABLE 1 Composites with content composition.

Specimen no.	Specimen composition
Specimen 1	70% epoxy + 30% goat horn powder + 0% APP
Specimen 2	60% epoxy + 30% goat horn powder + 10% APP
Specimen 3	55% epoxy + 30% goat horn powder + 15% APP
Specimen 4	50% epoxy + 30% goat horn powder + 20% APP

Tensile testing was done using a tensile strength tester and a load cell of 10 kN using ASTM D638 in order to find out the tensile characteristics of the composite samples. With measurements of 165 mm × 19 mm × 3 mm, three specimens were evaluated for each case. For the wear test up to 400 grade emery paper was polished; before to testing, the samples were dried and cleaned with acetone. These samples' wear characteristics were examined using a pin-on-disc machine. Limiting oxygen index is the lowest oxygen concentration required to either sustain a material's flame burning for 3 min or to consume a sample's length of 5 cm. The United States has standardized it (ASTM D 2863). Using a burner to ignite the sample's top, the sample is positioned vertically in a regulated environment. Thermogravimetric analysis was used to investigate the glass transition temperature and thermal stability of composite materials. A sample of 10 mg of composite was placed in an aluminum crucible and heated to 550°C at a heating rate of 10 K/min while under nitrogen environment. Underwriters Laboratories UL-94 HB Horizontal flammability tests were conducted in compliance with DIN EN 60695-11-10, using specimen dimensions of 127 mm × 12.7 mm × 3 mm. For the burning rate ≤ 40 mm/min and test specimen thickness 3–13 mm, the flammability rating HB will be identified. Water absorption behavior of goat horn particle reinforced epoxy composites was investigated by water absorption experiments conducted in compliance with ASTM570 standards. Over the course of 120 h, the composite samples (25 mm × 25 mm × 3 mm) were immersed in deionized water at room temperature. Over various time periods, the percentage of weight gain was calculated. A Tescan Oxford scanning electron microscope was used to examine the morphology of the composites' tensile fracture surfaces. To make the samples conductive, a thin layer of gold needs to be vacuum-coated on them.

Result and Discussions

Mechanical and Thermal Characteristics of Composites

This chapter presents the mechanical properties and thermal properties of the goat horn with epoxy resin and the addition of APP to create reinforced polymer composites prepared for this present investigation. Details of the processing of these composites and tests conducted on them have been described in the previous chapter. The results of various characterization tests are reported here. This includes evaluation of tensile strength, limited oxygen test, horizontal flammability, thermo-gravimetric analysis, scanning electron microscopy and water absorption test has been studied and discussed. On considering the thermal properties includes evaluation of thermo-gravimetric analysis and in terms of the weight loss function and the results have been studied and discussed. The interpretation of the results and the comparison among various composite samples are also presented.

Mechanical Characteristics of Goat Horn

The test results for tensile strength are shown in Table 2. It is found that with increase in the composition of APP the tensile strength of the composite also increases linearly.

TABLE 2 Tensile strength of goat horn/ammonium polyphosphate epoxy composite.

Weight percentage of APP	Tensile strength (MPa)
0%	22.1
10%	25.6
15%	27.4
20%	30.3

All the samples have good ultimate tensile strength values above 20 MPa. The specimen 4 has the highest tensile strength with the addition of 20% of APP and the tensile value is around 31 MPa. It can be inferred that with addition of APP the tensile strength has almost increased by 30%–50% depending upon the composition of the filler material. On the whole it can be concluded that sample 4 has a good tensile strength compared with the other weight ratios. Figure 2 shows the effects of APP on tensile strength of goat horn/APP epoxy composite.

[IMAGE OMITTED. SEE PDF]

Wear Test

Wear test is used to identify the lifetime of a material. The wear test off all the samples were taken along with the frictional analysis and the results are represented in Figure 3. The co-efficient of friction of goat horn/APP epoxy composite is presented in Table 3 shows the values in reduction from 1.5 to 0.5.

[IMAGE OMITTED. SEE PDF]

TABLE 3 Co-efficient of friction of goat horn/ammonium polyphosphate epoxy composite.

Weight percentage of APP	Co-efficient of friction
0%	1.5
10%	1.3
15%	1.2
20%	0.5

A lower friction coefficient results in less resistance to a relatively moving surface, which lowers the wear rate. The 20% APP in horn/epoxy composite Sample gives lesser wear rate compare with 0% APP sample.

From the Figure 3, the wear rate corresponding to the data points are plotted and shows the reduction on wear rate with addition of APP in composites. More content of APP in the composites reduces the wear rate because of APP intermolecular bonding with epoxy. The friction force graph shows the very less amount of friction forces was generation and maintained gradual decreases in wear rate after addition of APP in composites [26].

Thermal Characteristics of the Composites

Limiting Oxidation Index

The test results for LOI are shown in Figure 4. It is found that at various ratios of APP the LOI is gradually increasing from sample 1–4. Sample 3 and sample 4 have the highest oxidation therefore it is a good fire retardant compared with other samples. Generally, all the samples have value of oxygen percentage above 21, it can be inferred that materials with LOI value above 21% are naturally classified as self-extinguishing or fire retardant. From the results obtained it can be stated that goat horn composite is a fire-retardant material and with addition of APP the LOI increases up to the weight ratio of 15% of APP [27]. Furthermore, addition of APP was incorporated into the epoxy composites, the same dispersion was occurring because of APP agglomeration in epoxy composites clearly viewed in SEM image shown in Figure 9, that shows the unchanged value of LOI for both 15% and 20% APP composites.

[IMAGE OMITTED. SEE PDF]

Effect of APP Weight (%) on TGA Analysis

In this specimen, the APP is mixed with the composite of (0%, 10%, 15%, and 20%) and so the result shows the sharp curve of the weight reduction of the composite at high temperature and it is also clear that the weight decreases rapidly due to the changes made in addition of the fire retardant which is the APP. The TGA is presented in Table 4.

TABLE 4 Thermogravimetric analysis.

Sample	5% weight loss at temperature (°C)	Main decomposition began at temperature (°C)	Tmax (°C)	50% weight loss at temperature (°C)	Residue weight loss at 550°C (%)
0% APP	130	300	380	392	5.79
10% APP	180	290	365	376	16.63
15% APP	188	285	340	350	19.76
20% APP	195	305	360	369	18.02

Figure 5 shows the effects of APP on thermal stability of goat horn/APP epoxy composite. This TGA analysis measures the amount of weight change of a material, as a function of increasing temperature and isothermally as a function of time, in an atmosphere of nitrogen. The temperature range of each sample was identified and graphed. It is observed that there is an increase in temperature for the addition of APP TGA [28].

[IMAGE OMITTED. SEE PDF]

The main degradation stage of the 0% APP sample took place at temperatures higher than 300°C (92 wt%). At the main decomposition, the highest temperature (T_max) was 380°C (84 wt%). About 5.79% of the 0% APP was still present at 550°C. Decomposition started with the 10% APP sample at 365°C with just 90 wt%, and the T_max was 376°C with 78 wt%. 15% APP samples had a maximum temperature of 350 and a decomposition temperature of 340. For the 20% APP samples, the maximum temperature was 369 and the decomposition was 360. However, the residues for the 10%, 15%, and 20% APP samples at 550°C were approximately 16%, 19%, and 18%, respectively. Due to improved APP agglomeration in epoxy composites, it is determined that adding 20% APP improves thermal stability when compared with other compositions.

Differential Thermal Analysis (DTA)

Technique in which the difference in temperature between the sample and a reference material is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. In this testing the APP is mixed with composite of with various percentage (0%, 10%, 15%, and 20%) with epoxy and goat horn. The result shows that the composite with 20% is maximum. Figure 6 shows the Effects of APP on DTA of goat horn/APP epoxy composite.

[IMAGE OMITTED. SEE PDF]

Horizontal Flammability Test

Figure 7 effects of APP weight (%) on horizontal flammability of goat horn-APP epoxy composite. From the results of horizontal flammability test it can be observed that all the specimens were treated using propane flame for a flame length of 38 mm and from the below graph it can be inferred that, all the specimens start to ignite only after 20 s of direct contact of high flame, generally materials which ignite after 20 s of direct contact of flame are called fire retardant materials and with the addition of filler material the ignition time also increases.

[IMAGE OMITTED. SEE PDF]

All the sample's burning rate was less than 45 mm/min indicates the flammability rating was HB. Materials with minimal flame resistance that are employed in less fire-critical applications might be classified under the HB classification. On the whole it can be concluded that sample 4 has good results with horizontal flammability and acts as the best fire-retardant material among the four.

Water Absorption Test

The results of the analysis shows that the addition of APP diminishes the water absorption tendency of the composite material (Figure 8). Whereas, the samples having 0% and 5% are partially absorbing water at a rate of 0.01 g for every 16 h. But, sample 3 having 10% APP gradually increasing at a rate of 0.01 g for every 32 h. Figure 8 makes it clear that all of the goat horn composite samples absorb water very quickly at initially, then progressively decrease their soaking rates until they approach the saturation point.

[IMAGE OMITTED. SEE PDF]

After reaching the saturation point, 15% APP composite absorbed the most water, followed by 0%, 20%, and 10% APP composites. Out of all the composites, the lowest percentage of water absorption for the goat horn composite has been found to be 3.20% for the 10% APP sample. The composite's maximum water absorption rate, measured at 15% APP, was 4.0%. The absorption was stopped after 80 h for 0%, 10%, and 20% APP composites but the 15% APP composites absorption was stopped around after 90 h only. It shows the 10% APP composites was the optimized composite proportion for the water-oriented applications like marine industry.

SEM Analysis on Different Weight Ratio of APP

SEM analysis was performed on all the samples to identify the distribution of goat horn powder and APP in all the prepared composites.

Figure 9a it is seen that are well dispersed, but APP are not well attached to the epoxy resin. It shows the clear image of the breaking of composites in a brittle form. So, they are not well immersed in it are identified with the help of SEM analysis. In Figure 9b it is shows good amount of bonding with goat horn with APP and epoxy resin. Then it clearly shows molecule are scattered properly. In Figure 9c it is seen that the epoxy resin molecule is spread properly. In Figure 9d it is seen that darker particles goat horn and APP they are well dispersed in the composite. It is also seen that the goat horn particles are well attached to the epoxy resin.

[IMAGE OMITTED. SEE PDF]

Conclusion

In this study it shows the tensile strength of the composite material decreases with the addition of APP and it is also clear that the wear rate of the composites gradually reduced with the addition of APP. The limited oxygen Index test of the composite material increases with addition of APP but it increases gradually at 15% of APP. From the thermal tests, we can conclude that the addition of APP has a great effect on the composite at elevated temperature and the composite with 0% of APP gives partially poor result compared with other samples. The flammability test shows the less flammability occurs with addition of APP in clear manner. This shows the composites have compact and better thermal stability. The composite with the addition of 10% APP gives the lowest percentage of water absorption for the goat horn composite has been found to be 3.20%. It shows that the material will be used in water-oriented applications such as marine, aerospace, and insulation for electronics.

Author Contributions

N. Parkunam: writing – original draft, methodology, investigation, conceptualization. P. Gopal: conceptualization, investigation, writing – original draft, methodology. G. Navaneethakrishnan: conceptualization, writing – original draft, visualization. R. Palanisamy: conceptualization, writing – original draft, visualization. Beena Stanislaus Arputharaj: writing – review and editing, formal analysis, project administration. C. Ahamed Saleel: writing – review and editing, resources, funding acquisition. Qasem M. Al-Mdallal: writing – review and editing, project administration, supervision.

Acknowledgments

The authors wish to express his sincere thanks to the honorable referees and the editor for their valuable comments and suggestions to improve the quality of the paper. Additionally, all authors would like to express their gratitude to both the United Arab Emirates University, Al Ain, UAE, for providing financial support with Grant No. 12R283 and the Deanship of Research and Graduate Studies at King Khalid University for funding this work through Large Research Project under grant number RGP 2/582/45.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data used to support the funding of this study are available from the corresponding author upon request.

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