Investigation of vibration,flexural,tribological and flame retardant properties on rubber crumb-reinforced polymer composites

Abstract

This research aims to fabricate polymer matrix composites with improved mechanical, flame retardant, and damping properties by incorporating waste rubber crumbs as the reinforcement. The damping characteristics of the polymer matrix composites reinforced with different weight percentages of rubber crumb such as 0, 2.5%, 5%, and 7.5% were analyzed using a free vibration test. The experimental results confirmed that the addition of rubber crumb significantly improved the damping characteristics of the material. In addition, the rubber crumb-reinforced composites showed improved mechanical and fire-retardant properties. The flexural strength improved exceptionally with the increase in rubber reinforcements up to 7.5%, whereas the tensile strength increased up to 2.5% weight and reduced with the further increase in rubber. Enhanced flame-retardant properties were witnessed in the polymer composites reinforced with rubber crumb than that of the plain Epoxy - Glass fibre composites. Thermogravimetric analysis (TGA) were also used to ascertain the thermal properties. The scanning electron microscope was employed to analyze the surface and fractography of the polymeric composites. The overall results showed the contemporary effect of rubber crumbs on polymer composites.

Keywords

Polymer composites rubber crumb damping flame retardant strength wear

Introduction

Polymer composites (PMCs) are being used in a variety of applications such as construction, transport (aerospace, automobiles, trains, ships), and electronics applications,¹ where vibration and fire risk are the major issues causing environmental pollution and health hazard. Composites with high damping capacity are favorable in automobiles in which noise, vibration, and harshness (NVH) were the critical factors affecting vehicle stability. Polymers with different types of reinforcement such as long fibres, short fibres, and nanofillers were used to improve the composites’ damping and flame-retardant characteristics. Faster damping of vibrations supported by glass fiber-reinforced composites reduces the shock transmitted to the neighboring components in vehicles.

For effective vibration absorption, high-damping reinforcements should be incorporated into the PMCs for increased durability, service life, and noise pollution.² High-damping polymer composites can be attained passively with a proper selection of reinforcements. Several approaches were practiced to enhance the damping capability of GFRP composites, among them interphase engineering plays a major role in the fiber/matrix interphase.³ The vibration energy can be absorbed effectively when the fiber slips from the matrix. This could be advantageous in many applications and can be improved by coating the viscoelastic polymers and nanofillers over the fiber.

In recent times, nanoparticles are commonly used as the potential reinforcement for increasing the vibration damping, mechanical and thermal properties. Energy dissipation may be effectively facilitated by the frictional stick-slip processes between neighboring carbon nanotube (CNT-CNT) interactions.⁴ Arulmurugan et al., used Nanoclay for improving the vibrational characteristics and concluded that the increase in nano clay increases the natural frequency and damping factor.⁵ The effect of graphene nanoplatelets on the modified epoxy composite’s vibration and damping characteristics was reported by Rafiee et al.⁶ The static and dynamic mechanical behavior of the polymer composites can be influenced by modifying the carbon fiber by heat treatment, coating, and GSD (Graphitic Structures by Design) technique. The GSD-coated carbon fiber-reinforced epoxy polymer composites demonstrated improved loss modulus within the frequency range of 1–60 Hz.⁷ The graphene nanoplatelets improved the damping factor and strength considerably with a small wt. % of graphene.

The effect of CNTs on vibration damping, flame retardancy, and mechanical properties has been reported by many researchers. The damping ratio of the GFRP-CNT hybrid nanocomposites increased linearly with the CNT content due to the slippage of CNTs from the fibre–matrix interface.⁸ The properties of the CNTs such as nanoscale, low density, and higher surface area escalate the friction between the matrix and nanoparticle resulting in higher damping properties of the composite material.⁹ Difficulty in handling the nanoparticles is the major problem, also recent research testifies to the potential release of hazardous nanoparticles when subjected to mechanical and thermal loading.^10,11 So, a high-damping reinforcement material with environmental concerns is the need in the current scenario.

E-Waste material using plastics, waste carbon fibre, and worn-out rubber/tyres, tyre tubes are being dumped in landfills and cause serious health hazards. Utilizing these wastes as the potential reinforcement material in PMC is the most common disposal method. Also, we can improve the damping, wear, and fire-retardant properties of the composites by employing these cheap and environmentally friendly waste materials. Over one billion tyres are produced each year in the world, and an equivalent quantity is permanently removed from vehicles and ends up as waste. Such wastes are disposed of improperly in landfills, which causes severe environmental issues. Also, there is a risk of fire and harmful fumes from the piled-up tyres. So, there is a need for proper handling, disposal, and reusable technique for rubber/tyre wastes. The addition of scrap tyres and their effect on the polymer composite’s mechanical properties,¹² and water absorption¹³ were reported. The waste rubber had been used as the reinforcement in the concrete structure and its effect on the mechanical properties^14,15 was reported elsewhere.

But, there was scanty evidence available on the influence of waste rubber (tube) crumb on the mechanical, vibration, and flame-retardant properties of PMCs. Therefore, an attempt has been made to relate the vibration properties with the waste rubber as the reinforcement in the polymer matrix composites. The rubber crumbs are reinforced in the polymer composites with different wt. %. The mechanical, vibration, wear, and flame retardancy properties are studied and compared for varying concentrations of reinforcements.

Methodology

Materials

A commercially available Unidirectional E-glass fiber is used for this study, which was purchased from Marktech Composites Pvt. Ltd Banglore, India. The epoxy matrix (Araldite LY 556) and the hardener (Aradur HY 951) were purchased from a commercial source of Huntsman India. The vehicle tube rubber crumb granule with an average size range of ∼1–5 mm (Figure 1) was obtained by shredding the used rubber tubes, which were purchased from a commercial retailer in Chennai, India.

Figure 1.

Shredded rubber crumb.

The shredded rubber crumbs were uniformly dispersed in epoxy resin through a mechanical stirrer for 45 min at 200 rpm speed. The epoxy/rubber crumb solution was ensured for uniform dispersion. The rubber crumbs were dispersed in three varying concentrations that is, 2.5 wt.%, 5 wt.%, and 7.5 wt.% in the epoxy-hardener mixture.

The E-glass fibre laminae were cut into a square shape with a dimension of 30 cm × 30 cm. The glass fiber ply was aligned in a 0° sequence of four layers as shown in Figure 2(a). According to the manufacturer’s recommendations, a 10:1 ratio of epoxy resin to hardener was used. Now the required amount of epoxy/rubber crumb/hardener mixture was poured over the glass fibre laminae, which was originally kept on the waxed die as shown in Figure 2(b). The composite specimen with and without the rubber crumbs was fabricated by repeating the above step. Four different composite plates with 0, 2.5%, 5%, and 7.5% wt. rubber crumbs were fabricated and the corresponding designation was reported in Table 1.

Figure 2.

Fabrication of rubber crumb reinforced composite. (a) Stacking sequence of glass fiber; (b) hand layup process.

Table 1.

Details of the epoxy, glass fibre-rubber crumb weight in the composites.

Sample name	Epoxy (g)	Glass fiber (g)	Rubber crumb (g)
E-G-R0	600	350	0
E-G-R2.5	600	350	23.75 (2.5%)
E-G-R5	600	350	47.5
E-G-R7.5	600	350	71.25

Composite characterization

Static test

The tensile, flexural, flame retardant, wear, and vibration specimens were cut from the fabricated square plate. The tensile tests were carried out on the rectangular specimen as per the ASTM D638 standard. Three-point bending tests were carried out on the different composite samples as per ASTM D790. The addition of reinforcements on the tensile strength and flexural tests were compared concerning the parent material. Both tests were carried out on the INSTRON universal testing machine with a crosshead speed of 1 mm/min. To avoid sampling error, three specimens were taken for each test. The flame retardancy tests (ASTM - D635) were also carried out to figure out the hindrance provided by the waste rubber against burning. The wear characteristics of the composites were analyzed using the rotating drum¹⁶ covered with abrasive. The specimen with 15.7 mm × 7 mm width × 3 mm thick and a drum covered with abrasives having 150 mm diameter and 500 mm length was used for the wear tests. The stationary composite sample was pressed against the rotating drum and the worn-out particles were collected against a specific rpm. The wear rate and abrasion loss were compared for different rubber reinforcement percentages. Also, thermal stability and functional groups of the rubber/polymeric composites were measured using the thermogravimetric analyzer and Fourier transform infrared spectroscopy. The samples for TGA and FTIR spectroscopy were obtained by grinding the different polymer composite samples against emery sheets. Individual emery sheets were employed for each specimen to avoid the residual mixing of other composites.

Free vibration characteristics

The vibrational characteristics of the neat GFRP composites were compared against the waste rubber-reinforced composite materials. The free vibration tests were carried out on different composite specimens, and the response of the beam was analyzed by using the accelerometer and data acquisition software. The vibrational characteristics of rubber crumb-modified composite beams can be analyzed using the following continuous beam model.

Based on Meirovich’s model for the cantilever beam, the natural frequency can be calculated using equation (1).^17,18

f = \frac{1}{2 π} {(\frac{1.875}{L})}^{2} \sqrt{\frac{E I}{ρ A}}

(1)

ξ = \frac{(ω_{\max} - ω_{\min})}{2 ω_{n}}

(2)

Where the length of the sample is L, the Stiffness of the sample is E, the unit cross-sectional area is A, the geometrical moment of inertia is I and the density of the sample is ρ. The dimension of the proposed cantilever beam used in this experimental work is 125 mm × 20 mm × 3 mm. The damping responses were also determined using equation (2) based on the half-band width technique.

Results and discussion

FTIR studies

The FTIR is an important tool for analyzing the functional groups of the shredded rubber crumbs with epoxy and glass fiber. The critical peaks of the rubber crumbs based on absorbance are illustrated in Figure 3. The absorbance peaks of 3200 and 3650 cm⁻¹ signify the O-H stretching vibrations of hydroxyl groups of epoxy which affect the strength of the composites.¹⁹ Asymmetric and symmetric stretching of C-H bonds and CH₃ bonds were connected to the noteworthy peaks that were obtained in the range of 2800 cm⁻¹ and 2900 cm⁻¹. The absorbance peaks of 1050 cm⁻¹ and 1200 cm⁻¹ were related to the sulphonic group, which ensures the presence of rubber in the composites. The CH₂ bonds can be noticed at the absorbance peaks of 1300 and 1450 cm⁻¹. Similarly, the stretching of C-H bonds is obtained between the 810 and 910 cm⁻¹ which is a double bond of Carbon and Carbon. The carbon and sulfur groups were identified in between the absorbance peaks of 620 and 740 cm⁻¹.

Figure 3.

FTIR responses of epoxy – glass fiber and rubber crumbs with (a) 0%, (b) 2.5%, (c) 5%, and (d) 7.5%.

TGA studies

The thermal stability of the epoxy–glass fiber and rubber crumbs is investigated using TGA in an N₂ atmosphere. Thermal stability is defined as the ability of the composite material to resist the deterioration of material properties due to temperature. The TGA responses of the rubber/polymeric composites are illustrated in Figure 4. The cross-linking of the rubber and polymer plays a key in the thermal stability of rubber-reinforced GFRP composites. The illustration shows the dual regions of the thermal degradation where the first region is held between 230°C and 330°C. In this region, the aliphatic and weak bonds get broken. The strong bonds were broken in the second region of thermal degradation, which is associated with temperatures between 510°C and 590°C.

Figure 4.

TGA curves of the epoxy – glass fiber and rubber crumb with wt. % of 0, 2.5, 5, and 7.5%.

The mass of the polymeric composites is lost at a faster rate and the real decomposition originated after the temperature reaches 300°C. The disintegration of volatile wastes from the polymeric chains is the main reason for the degradation. The mean loss in mass of the polymeric composites below 300°C is due to the moisture release. In contrast, polymer decomposition is associated with the loss in mass after 300°C. The complete volatile wastes are chased away from the rubber/polymeric composites and finally ensure the residual char. This study on rubber/polymeric composites is important from an engineering perspective because it validates the use of composites in high-temperature situations. Thermal degradation of polymer composites was improved by the addition of rubber crumbs.

Tensile strength

The tensile properties of the composites with and without rubber crumbs are shown in Figure 5. The ultimate tensile strength of the 2.5% rubber crumb composites was maximum (325 MPa) while comparing the other wt. % composites. The adhesion rate of a smaller percentage of untreated rubber was found to be good and significantly captivates the applied tensile stress. The interface of the reinforcement and glass fibre with the matrix plays a vital role in the composite material. An appropriate amount of reinforcement will result in better mechanical properties by having sufficient bonding between the matrix. Accordingly, the composites with 2.5% wt. rubber showed a peak in tensile strength. The tensile strength of the composite samples with higher wt.% of rubber such as 5% and 7.5% showed poor strength than the other two composite materials. This is because of the decreased wettability and void formation while tensile loading. The applied tensile load causes the debonding of reinforcement which causes the voids. These voids act as the stress concentration site and led to the failure of composites. The elongation also decreased linearly with the wt.% of rubber addition due to the void formation. The tensile modulus of the composites increased linearly from 8.1 GPa to 8.8 GPa by adding the reinforcements. The chemical treatment of rubber crumb may result in better interface bonding with the matrix so that the strength can be further improved in the composites with high rubber wt. %.²⁰

Figure 5.

Tensile strength and tensile modulus versus rubber weight (%).

Flexural properties

The effect of the rubber crumb on the flexural properties of the glass fibre composites were shown in Figure 6. Unlike the tensile properties, the flexural graph showed an increase in flexural strength with the increase in rubber wt. %. The rubber crumb at the fiber/matrix interface plays a crucial role in absorbing the flexural load and preventing it from crack formation and propagation. In three point bending test, the load is applied normally resulting in the generation of tensile and compressive stress on the bottom and top sides of the composites. The rubber crumbs will bear the compressive and tensile stress generated by the bending load. Due to this, the composite with 7.5 wt. % rubber crumb showed higher flexural stress than any other composite. The flexural strength were increased from 540 MPa to 760 MPa by the addition of rubber crump. Also, the increase in flexural strain offered by the rubber crumb reinforcement in the composite can be seen in Figure 6.

Figure 6.

Flexural stress-strain curve for different composites reinforced with varying concentrations of rubber crumb.

Vibration studies of the polymer composites

The testing of the vibration responses of the composite structures is highly important for industrial applications. Moreover, the resonance condition of the engineering structures when it is in use can be avoided based on the modal analysis of the beams. The vibrational characteristics of all the composite cantilever beams have been carried out as per the ASTM standard of E756-05. Also, the natural frequency of the samples can be specifically influenced by the moisture content present in the composite beams. Hence this factor has been considered and prepared the samples appropriately. The dynamic characteristics of the composite samples were carried out with the one-end free and one-end fixed boundary conditions.

The vibration test and frequency responses (FR) of the epoxy glass fiber composite samples have been recorded as shown in Figure 7. Using the FR diagrams, the natural frequencies of the composite samples were determined and tabulated in Table 2. The addition of rubber crumbs increased the order of natural frequencies due to the stiffness of the composites. The fundamental natural frequency of 7.5% rubber composites was found to be 41.2 Hz, which is the highest among other samples. Most of the vibration loadings are normal to the specimen, and the load absorbance is similar to the flexural loading. This is the main reason for obtaining the highest frequency with the addition of rubber crumbs.

Figure 7.

Experimental setup for vibration test on the samples of epoxy - glass fiber - rubber crumb composites.

Table 2.

Natural frequencies of the rubber composite materials.

Sample name	Natural frequency (hertz)
Sample name	Mode I	Mode II	Mode III
ER-0	36.7	293.5	391.3
ER-2.5	41.2	302.9	396.7
ER-5	42.4	324.1	398.5
ER-7.5	43.1	321.5	409.6

The natural frequency of the samples E-G-R2.5 and E-G-R5 was found to be 41.2 Hz and 42.4 respectively, which are slightly higher than neat composite. Further, the value of the natural frequency increased for the combination of 7.5 wt% rubber crumb composite (Figure 8(a)).

Figure 8.

Frequency responses of the epoxy-glass fiber with rubber crumb in various wt%. (a) First mode; (b) second mode.

Through the vibrational analysis, it was observed that the higher-order frequencies are negligible compared to the first natural frequency. Hence, the first three natural frequencies were considered in the experimental results of epoxy –glass fiber – rubber crumb composite beams. The fundamental frequency of the sample E-G-R5 is increased by 10% compared to E-G-R0. For instance, the frequency value of the sample E-G-R7.5 is observed and there is an increment of 13.88% due to the addition of rubber crumb. None of all other higher-order modes of vibration is significantly influenced by the composite beams. It is observed that the natural frequency is directly proportional to the stiffness of the cantilever beam as a material property.²¹

The effects of rubber crumbs on the damping ratios of the epoxy and glass fiber composite beams are illustrated in Figure 9. Generally, the damping ratio is directly proportional to the natural frequency²¹ and can be confirmed through the trends of the damping ratio [Figure 8]. The damping ratios for the composite beam of E-G-R5 are decreased by 3.01% than E-G-R0. Though the damping ratios of the composite beams E-G-R5 are dropped by 19.59% and the damping ratio of the beam E-G-R7.5 is raised to 23.11% respectively by adding the rubber crumb particles.

Figure 9.

Damping ratios of the rubber crumbs modified epoxy-glass fiber in various wt% for mode I.

By considering the second mode of vibration, the damping ratios of the composite beam dropped by 19.79% for the E-G-R5, and the damping ratios for the composite beam E-G-R2.5 and E-G-R7.5 raised to 4.91% and 25.97% repectively. The natural frequencies and damping ratios for the third and fourth modes revealed no substantial enhancements. Conclusively, the addition of rubber crumbs into the polymer composite has increased the natural frequencies of the composite beam due to the enhancement of the adhesion between the reinforcement agents and polymer. Hence, the good dispersion of rubber crumbs into the polymer can be enhanced the stiffness of the composite structures.²²

Rate of burning of the polymer composites

The rate of the burning test was carried out on the rubber crumb-reinforced composite materials as per ASTM D-635. The specimens were marked with two lines 1” and 4” from one end and mounted with its long axis horizontal and it is short axis at a 45° angle. The flame is applied to the free end of the specimen with a one” high blue Bunsen burner for 30 s. The time taken for the flame front to travel between the two-gauge marks is measured and tabulated (Table 3) in mm per minute. The samples with 0 and 2.5% rubber exhibited improved resistance to flame travel and acts as self-extinguishing material. The unburnt rubber crumbs at the bottom side of the burner confirm the debonding of the rubber crumb from the matrix at a higher wt. % of rubber crumb composites. Due to this, the composites with 5% and 7.5% showed an increase in the rate of burning. The findings of the TG analysis cannot be inconsistent with the Rate of burning results because of the usage of powered samples in TGA, where there were no debonding.

Table 3.

Rate of burning of the polymer composites.

Sample name	Rate of burning (mm/min)
E-G-R0	The flame front does not reach the 25 mm reference mark
E-G-R 2.5	The flame front does not reach the 25 mm reference mark
E-G-R5	12.68
E-G-R7.5	41.0

Wear test

The wear characteristics of the composites were analyzed using the rotating drum covered with abrasive Figure 10. As shown in Figure 11, the applied load of 1 kg on the specimen was applied against the drum at 40 ± 1 rpm.

Figure 10.

Schematic diagram of pin-on-drum apparatus.

Figure 11.

Abrasion loss and wear rate for different composites.

The wear rate and the abrasion loss were reduced with the increase in rubber percentage. The abrasion loss was maximum in the parent material (0.247 g) and wear resistance increased with the rubber weight % in the composite material (Figure 11). This increase in wear resistance is due to the good interaction between the composite and reinforcement. The efficient bonding between the matrix and reinforcement makes the detachment process of rubber crumbs tough. Further, the detached rubber crumbs form a layer over the drum and save the sample from wear. The abrasion loss was increased with sample contact time against the drum. The wear rate reduced from ∼0.20 to ∼0.13% due to the addition of rubber crumbs in the composites. The change in wear resistance was insignificant between the 5% and 7.5% rubber-reinforced composites. The increase in wear resistance was useful for the increased product life.

Scanning electron microscope

The scanning electron micrographs of the sample have shown the interaction between the rubber sample and glass fibre for 2.5% rubber crumb composite (Figure 12(a) and (b)). The tensile strength was found to be good in this composite, because of the excellent bonding between the glass fibre and matrix. Also, the rubber crumbs in between the two mats act as a damper whenever the vibration occurs. Since the rubber crumb acts as the vibration absorber, the damping properties were also excellent. Figure 12(c) and (d) shows the fractured image of the 5% rubber crumb-reinforced composites after tensile loading. The rubber crumb inside the composite was shown and confirms the reinforcement (rubber) debonding and fibre failure in Figure 12(c). The failure of the matrix was also shown after the tensile loading (Figure 12(d)). The different failures such as debonding, glass fibre fracture and matrix failure confirm the need for chemical treatment of rubber crumb.

Figure 12.

(a), (b) SEM surface micrographs of 2.5 wt.% rubber crumb; (c), (d) SEM fractography of 5 wt.% rubber crumb.

Conclusion

This research work presented the effect of rubber crumbs on the polymer with glass fiber reinforcements on the mechanical, free vibrational, wear, and flame retardant properties. The following important conclusions were made from the experimental work.

• The tensile and flexural strength improved after reinforcing the rubber crumb in the GFRP composites. Anyhow, the addition of rubber crumb above the critical limit decreased the tensile strength. Whereas, the flexural strength keeps on increasing with the rubber wt. %.

• The addition of rubber crumb in the composite increased the hindrance to flammability. Also, the wear resistance improved with the addition of rubber crumb.

• Based on the free vibration studies, the addition of rubber crumbs into the polymer glass fiber composite increases the natural frequency of the composite structures. Clearly, the effect of rubber crumbs with the 2.5, 5, and 7.5 wt% shows a higher natural frequency than the glass fiber-reinforced polymer composite beams. Also, the damping responses were improved with the addition of rubber crumbs.

As a result, the addition of rubber crumbs into the glass fiber-reinforced polymer composite has improved the flexural, flame retardant, vibrational, and wear properties. Further studies on this work can be focused to identify a better composite structure for industrial and structural foundation applications.

Footnotes

Author contributions

NTP formulated and supervised the work. NTP and MS carried out the experimental work and wrote the manuscript followed by comments from all authors. Characterization work and related suggestions were given by PSK and SVK.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

N Thangapandian

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