Effect of Nylon Fiber Reinforcement on Mechanical Behavior of Expanded Perlite/Sodium Silicate Composites

In this research, expanded perlite/sodium silicate composites reinforced with nylon fibers were manufactured with varying percentages of nylon fibers without changing the quantity of expanded perlite and sodium silicate solution. The composites were made by compression molding and cured at a temperature of 120 degrees Celsius for 48 hours. The flexural and compression tests were conducted according to the ASTM standards in the universal testing machine. The maximum flexural and compressive properties are found for 1.62 percent nylon fiber reinforcement. The addition of a higher quantity of nylon fiber in the composites deteriorates the compressive and flexural properties. The energy absorption during flexural and compression tests is improved significantly due to nylon fiber reinforcement. The fiber-reinforced composites show the ability to carry a significant amount of load even after crack initiation.


Introduction
Expanded perlite particles are known for their excellent properties, including lightweight, high thermal insulation, high acoustic insulation, fire resistance, and environment-friendly.Many researchers working on developing expanded perlite-based composites for various applications [1] such as building wall and ceiling insulation for heat and acoustic resistance, replacement of traditional concretes, pipe insulation, fire resistant doors, rocket insulation, energy absorbing metal syntactic foam, etc. Expanded perlite has drawn the attention of researchers considerably in recent years.Ai et al. [2] studied the thermal insulation behavior of the polyurethane/expanded perlite composites and found that the compressive strength and heat insulation have improved with the increase of expanded perlite content in the composites.Vimmrova et al. [3] investigated the foamed gypsum with expanded perlite particles and found promising results in terms of compressive strength and thermal conductivity.Wang et al. [4] produced a building thermal insulation board by modifying expanded perlite particles with aerogel and sodium silicate solution as a precursor and their investigation shows that the incorporation of aerogel in the pores of expanded perlite decreases the thermal conductivity of the composites.Tian et al. [5] studied the effect of various manufacturing parameters on the physical and mechanical properties of the expanded perlite composites.Alam et al. [6] investigated expanded perlite-fumed silica composite for effective vacuum insulation panel core and reported that the inclusion of perlite would reduce the cost of the panel with good insulation properties.Expanded perlite-based composite panels were manufactured and their physical and mechanical characteristics were investigated by many researchers using various binder systems such as 1305 (2024) 012033 IOP Publishing doi:10.1088/1757-899X/1305/1/012033 2 potato starch [7], sodium silicate solution [8][9][10][11], epoxy [12], sodium silicate solution with corn starch [13], boric acid modified sodium silicate solution [14], recycled polystyrene [15], etc. Expanded perlite particles have also been studied for manufacturing lightweight concrete [16][17][18][19][20][21].The most of research work was carried out for physical, thermal, and mechanical properties and researchers have tried to improve those properties.However, a very limited study can be found in the literature where the effect of fiber reinforcement on expanded perlite composites was investigated for the improvement of mechanical properties.Recently, Yildizel [22] fabricated the glass fiber reinforced gypsum/expanded perlite/silica sand composites to investigate the mechanical performance and found encouraging results.Fiber reinforcement is proven to be effective for the improvement of the mechanical properties of expanded perlite-based composites.Therefore, in this work, short nylon fiber-reinforced expanded perlite/sodium silicate composites were manufactured and the effect of fiber reinforcement on the mechanical properties of the developed composite was investigated.The compression and flexural tests were carried out to analyze compressive strength, compressive modulus, flexural strength, flexural modulus, energy absorption in compression, and energy absorption in flexural loading.The failure mechanisms during compression and flexural loading were also investigated.

Material
Expanded perlite particles of commercial grade were obtained from Xinyang King Caster Company Ltd., Henan, China.Particles of 3-4 mm size were used in this study.The major constituents of expanded perlite are aluminum oxide and silicon dioxide.The bulk density of expanded perlite particles was measured to be 0.075 g/cm 3 .Sodium silicate solution was obtained from Silica solution, in Chittagong Bangladesh.It was used as a binder in the composite.The density of the sodium silicate solution was 1.381 g/cm 3 , the solid content percentage was 37-38 % and the silica to sodium oxide ratio was 3.20.The nylon fiber rope was collected from a local store and fibers were separated and cut into a length of 10 mm to use as reinforcement in the composite.

Specimen Manufacturing
Sodium silicate solution was taken in a plastic container.For the control sample with no nylon fiber, perlite was added to the sodium silicate solution and mixed uniformly.For the nylon fiber reinforced samples, first nylon fiber was added to the sodium silicate solution and mixed properly then expanded perlite was added to the mixture.After mixing, the mixture was poured into the mold and compacted to a thickness of 18 mm.After compacting, samples were taken out and inserted into an electric oven at 120℃ and a 4 kg weight was placed on top of the sample to restrict distortion.After 24 hours of drying the support weight was taken out and the sample was kept in the oven for another 24 hours.The final specimen thickness was found to be 18 mm because of the expansion.The amount of perlite and sodium silicate solution was 60.75 g and 243 g respectively for all composites.Only the mass of fiber was varied i.e. 0 g, 5 g, and 7.5 g.For each fiber content, three samples of size 160×160×18 mm 3 were made.The sizing of the samples was done with a hax-saw to perform mechanical testing.

Mechanical Testing
The compression test was done according to the ASTM standard C365/C365M-11a in a universal testing machine (Shimadzu AGX 300 kN).The sample size for this test was chosen to be 25 mm × 25 mm ×18 mm.The test speed was 0.5 mm/min.Six specimens were tested for each fiber content.The flexural test was done according to the ASTM standard C393 in the universal testing machine (Shimadzu AGX 300kN) at a speed of 5 mm/min.It was a three-point bending test and the sample size was chosen to be 150 mm ×25 mm ×18 mm.

Results and Discussion
The compressive strength and modulus of the composites are given for various fiber contents in Fig. 1.It is seen that the compressive strength and modulus both have increased with the addition of 1.62% nylon fiber by 12.38% and 119.18% respectively compared with the composite without fiber.However, the compressive strength and modulus have decreased significantly with the further addition of nylon fiber to the composite.The flexural strength and modulus of the composites are plotted in Fig. 2 for various fiber contents.It is observed that the flexural strength and modulus of the composite have increased by 34.15% and 12.46% respectively for a nylon fiber content of 1.62% compared with the composite with 0% nylon fiber content.Further addition of fiber has decreased the flexural strength and modulus significantly.The sodium silicate binder content was fixed in the composite and mixed with the fibers and the perlite particles.When there is no fiber in the composite all sodium silicate binder contributes to the bonding between the perlite particles as well as to the coating on the surface of the particles.But when the fiber is added to the mixture the sodium silicate binder adheres to both the fibers and the particles.A portion of the sodium silicate binder is distributed on the surface of the fibers and the other portion is distributed on the surface of the particles during mixing.On the other hand, the perlite particles are very fragile and it is expected that the failure would take place with the fracture of the particles.The decrease in the compressive and flexural properties due to the addition of 2.41% nylon fiber is due to the lack of sodium silicate binder on the surface of the particles.The more fiber in the composite causes the less amount of the sodium silicate binder on the surface of the particles to enhance the fracture resistance of the particles.Nonetheless, the fiber content of 1.62% may be a threshold for the compressive and flexural properties of the composites studied in this work.In summary, it can be said that the addition of nylon fiber in the perlite/sodium silicate composites up to a certain threshold improves the compressive and flexural properties of the composite for a fixed ratio of perlite and sodium silicate binder.Further research may be conducted to see the effect of binder content i.e. perlite/binder ratio to get more insight.Energy absorption was calculated from the area under the force-displacement curve up to a deflection of 6 mm for the compression test and 17 mm for the flexural test.The energy absorption is given for various percentages of nylon fiber in Fig. 3.It is observed that the composite with 1.62% nylon fiber content showed the maximum compressive and flexural energy absorption.Both the compressive and flexural energy absorption decreased with the further addition of nylon fiber.The increase in compressive energy absorption for 1.62% fiber content is found to be 38.64%compared to the sample of 0% fiber content.A tremendous increase of 426.82% in energy absorption is noticed during flexural loading for the composite with 1.62% nylon fiber content compared to the sample of 0% fiber content.It is interesting to see that the flexural energy absorption of the sample with 2.41% nylon fiber content is 392.16% higher than the sample with 0% fiber content although its compressive strength, compressive modulus, flexural strength, and flexural modulus are lower than the sample with 0% fiber content.The energy absorption is highly related to the nature of the stress-strain curves which is again associated with the failure mechanism which is discussed in the following section.
It is important to study the failure behavior of a composite along with the nature of stress-strain curves to investigate the reasons for failure and the ways for improvement.Typical stress-strain curves are given for composites with various fiber contents in Fig. 4 and photographs taken at various strains are given in Fig. 5.It is seen from Fig. 4 that the stress increased linearly up to a peak where the failure takes place for all samples.For the composite with 0% fiber content, after the peak, the stress decreases gradually up to a strain of 0.3 before the plateau starts and no densification is seen up to a strain of 0.55.On the other hand, for the nylon fiber-reinforced samples, after the peak, a sudden drop in stress is seen before a long plateau region followed by densification.The long plateau region at higher plateau stress for the fiberreinforced composites is the reason for the high energy absorption of the 1.62% fiber-reinforced composites.However, for 2.41% nylon fiber reinforced samples, the energy absorption is lower than the 0% fiber reinforced samples (See Fig. 3(a)) even though there is a long plateau region in the stress-strain curve because of the lower compressive strength and plateau stress.Looking at the photographs of failure at various strains in Fig. 5 for both fiber reinforced and unreinforced samples it is clear that the failure started at the loading side of the specimen by crushing the particles and propagated towards the fixed compression platen.Although the failure mechanism is the same for both reinforced and unreinforced samples, the photographs show that there is less disintegration of particles and cracking of particles in the fiber-reinforced samples.So, it is clear that the nylon fiber reinforcement improved the compressive failure behavior of the perlite/sodium silicate composites.Typical flexural stress versus strain curves of composites with and without nylon fiber reinforcement is shown in Fig. 6 and the photographs of the failed specimen under flexural loading are given in Fig. 7.All samples show a linear increase in flexural stress until a peak.The stress after the peak in the unreinforced perlite/sodium silicate composite dropped rapidly to zero.The nylon fiber-reinforced composites also showed a sudden drop in stress after the peak but did not become zero rather slightly lower stress and the fiber reinforced composites were able to carry loads for a very large deflection.The plateau stress after the peak in the fiber-reinforced sample is due to the fiber pullout effect.In the plateau region, the fibers were being pulled out of the composites as shown in Fig. 7(b).The crack propagation in the unreinforced samples was rapid and failure takes place at a very short deflection whereas the crack propagation in the nylon fiber-reinforced samples are delayed by the fiber reinforcement and the complete failure took place at a very large deflection.The higher energy absorption of 2.41% nylon fiber reinforced composite compared to unreinforced composite (See Fig. 4(b)) is due to the plateau region in the flexural stressstrain curve resulting from the fiber pullout effect.So, the nylon fiber reinforcement in the perlite/sodium silicate composite improved the load-carrying and energy absorption capacity of the composite significantly even after failure.

Conclusion
Expanded perlite/sodium silicate composites were prepared with and without nylon fiber reinforcement and mechanical properties with failure mechanisms were investigated in this work.The major findings are summarized as follows -• The nylon fiber reinforcement in the expanded perlite/sodium silicate composite has improved the compression, flexural, and energy absorption behavior of the composites.addition of a higher percentage of nylon fiber without increasing the binder quantity deteriorates the compression and flexural properties and further research may be conducted in this regard.
• The compressive and flexural failure behavior of the composites was significantly affected by the fiber reinforcement.The crack propagation in the composites during both compression and flexural loading was delayed because of the nylon fiber reinforcement.

Figure 1 .Figure 2 .
Figure 1.Compressive strength and modulus for various fiber contents (Standard deviations are given as error bars)

Figure 3 .
Figure 3. (a) Compressive and (b) flexural energy absorption for various fiber contents (Standard deviations are given as error bars)

Figure 4 .Figure 5 .
Figure 4. Comparison between stress-strain curves for various quantities of fiber during compression.

Figure 6 .
Figure 6.Typical flexural stress-strain curves for various fiber contents