Mechanical and thermal characterization of condom industry waste reinforced natural rubber composites - Circular economy approach

P Bashpa; K Bijudas; P Dileep; Mehar Singh; Silviya Elanthikkal; Tania Francis

doi:10.4491/eer.2023.098

Environ Eng Res > Volume 29(2); 2024 > Article

Bashpa, Bijudas, Dileep, Singh, Elanthikkal, and Francis: Mechanical and thermal characterization of condom industry waste reinforced natural rubber composites - Circular economy approach

Research

Environmental Engineering Research 2024; 29(2): 230098.

Published online: June 8, 2023

DOI: https://doi.org/10.4491/eer.2023.098

Mechanical and thermal characterization of condom industry waste reinforced natural rubber composites - Circular economy approach

P Bashpa^1,², K Bijudas², P Dileep³, Mehar Singh⁴, Silviya Elanthikkal⁵, Tania Francis^1,^†

¹Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Kozhikode, Kerala, India - 673008

²Department of Chemistry, N. S. S. College, Ottapalam, Palakkad, Kerala, India - 679103

³J.J. Murphy Research Centre, Rubber Park India (P) Limited, Valayanchirangara, Ernakulam, Kerala, India 683556

⁴Department of Chemistry, Panjab University, Chandigarh, India 160014

⁵Deanship of Scientific Research, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, Dammam, Saudi Arabia

^†Corresponding author: E-mail: francistania76@gmail.com, Tel: +91 98957 26068, Fax:

Received February 23, 2023 Revised April 14, 2023 Accepted June 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Condom industry waste mainly consists of light magnesium carbonate (LMC), which is extensively used as a finishing powder during the final stages of condom production. This LMC waste (LW) is usually disposed of as a landfill, causing an increase in the hardness of water. LW generated in HLL Lifecare Limited, India, was procured, characterized by analytical techniques, and reutilized in natural rubber (NR) as a reinforcing filler. The prepared composites were subjected to rheological, mechanical, thermal and sorption characterizations. In comparison to NR neat, the composite with 3 phr (parts per hundred of rubber) LW showed a rise in tensile strength, tear strength, and modulus at 300% elongation by 22%, 20%, and 28%, respectively. A remarkable decrease in abrasion loss is also evidenced. The activation energy (E_a) for degradation, calculated by the Coats-Redfern (CR) method, showed 10 kJmol⁻¹ increase for composite with 3 phr LW, proving its better thermal stability. It also exhibited higher solvent uptake resistance, as established by sorption experiments. The superior properties of this composite have been attributed to the uniform LW distribution and ameliorated NR-LW interaction. Hence, the prepared composites find considerable potential in manufacturing industrial NR components accomplishing a circular economy.

Keywords: Circular economy, Condom industry waste, Light magnesium carbonate, Mechanical properties, Natural rubber composite, Thermal characterization

Graphical Abstract

Keywords: Circular economy, Condom industry waste, Light magnesium carbonate, Mechanical properties, Natural rubber composite, Thermal characterization

1. Introduction

Today, the usage of male condoms is one of the most prominent and widespread practices of birth control [1]. The principal constituent in condoms is latex, which is vulcanized to increase the strength and resilience of the rubber. Fatty acids as binding agents and surfactants for latex stabilization are added during production. Zinc oxide is another constituent that protects rubber from fungi attacks and UV light. Some antioxidants and accelerators are also incorporated to prevent oxidative degradation and harmful nitrosamine formation. A silicone-based lubricant is added to retain slippery properties which is better than water-based lubricants. Magnesium carbonate and calcium carbonate powders are extensively used to reduce the stickiness of condoms [2]. Nowadays, pharmaceutical-grade LMC is used to reduce the stickiness of condoms by various manufacturers. The condom market in India is projected to attain $180 million by 2022 due to higher consumer awareness about sexually transmitted infections (STIs) and human immunodeficiency virus (HIV) through the efforts of the Government of India. Prominent players dominating the condom market in India include HLL Lifecare Limited, Reckitt Benckiser (India) Ltd, Mankind Pharma, TTK Protective Devices Limited, Cupid Limited and Raymond Limited.

NR-based composites find a broad scope of industrial applications in various fields, such as in the production of footwear, tires, sports goods, hoses, glues, belts, gaskets etc. Vulcanization is the most crucial process in the rubber industry, whereby mechanical properties can be considerably enhanced [3]. Curing agents like accelerator, cross-linking agent, activator etc., are added to NR during vulcanization. However, NR composites with curing ingredients alone cannot meet the requirements of an industry today. To overcome this defect, certain insoluble foreign materials, called fillers, which considerably improve various properties, are added to the neat rubber [4]. Enhanced properties of rubber-filler composites depend upon the filler’s specific surface area, shape, particle size, and aspect ratio [5]. Carbon black and silica are commonly used as fillers for reinforcing NR in earlier days [6,7]. But both these fillers have the disadvantage of forming aggregates in the rubber matrix [8,9]. Above-mentioned defect is resolved by introducing nano and hybrid fillers with a large specific surface area, improving rubber composite’s mechanical characteristics by increasing the rubber-filler interaction [10–12]. Rubber-filler compatibility is a major criteria for homogeneous filler dispersion on the matrix, which yields better performance. But most of the fillers lack this advantage mainly due to the difference in polarity between filler and rubber [13]. The rubber industry currently encounters the above-mentioned issues while producing high-performance NR-filler composites.

Globally, researchers now focus on the reutilization of industrial by-products and waste materials in diverse applications and as fillers in rubber to achieve the ultimate aims of an industry, cost reduction and pollution prevention in a circular economy manner [14–16]. HLL Lifecare Limited, a Government of India enterprise, has eight condom manufacturing units all over the country and its industrial unit at Peroorkada, in Thiruvananthapuram, Kerala, was established in 1969. This unit has undertaken continuous modernization over the years, with an annual production volume of 1947 million condoms in number. LMC and silica were utilized as a finishing powder during the final stages of condom production. Around 75–85 tons of LW has generated annually as a waste in this plant, and no attempt has been reported regarding its utilization. This waste is usually discarded as a landfill, causing the hardness of potable water. Water pollution due to the unscientific disposal of condom industry waste propelled us to think about its reutilization and to carry out this work. Even though few reports are available on using magnesium carbonate as a reinforcing filler [17,18], such investigations on the usage of LW in NR as a reinforcing filler are not available in the literature to the best of our information. Hence, we report the preparation of LW-reinforced NR composites by employing standard methods. The developed systems have been characterized using rheological, mechanical, thermal and sorption studies. Young’s modulus values obtained from rheological studies were compared with those obtained from theoretical models proposed by Einstein and Guth. The E_a values for the prepared composite’s thermal degradation was studied using the CR method. The scope of the present work is the reutilization of industrial waste as reinforcing fillers in natural and synthetic rubbers to produce high-performance industrial components. Apart from the industrial application, this method highlights an efficient route for solid waste management from a circular economy perspective.

2. Materials and Methods

2.1. Materials

LW was collected from HLL Lifecare Limited, Thiruvananthapuram, India and is used as such. Natural Rubber (ISNR 5), having a plasticity retention index of ≈ 60, was acquired from Rubber Research Institute of India, Kottayam, India. The additional ingredients for compounding, namely, zinc oxide (ZnO), tetrame-thylthiuramdisulfide (TMTD), 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ), stearic acid, N-cyclohexyl-2-benzothiazolesulfenamide (CBS) and sulfur (S) were purchased from Thermo Fisher Scientific India Pvt. Ltd. and Merck Millipore India Pvt. Ltd. and utilized as such.

2.2. Compounding

NR-LW composites were compounded using a laboratory two-roll mixing mill of 6 ″×12 ″ size (ASTM D3184). ZnO and stearic acid were added after the mastication of NR, and to this mixture, weighed quantity of LW was added, followed by other ingredients, namely, TMTD, CBS, sulfur, and TMQ. The resultant mixture was subjected to homogeneous mixing, and the mixes were passed through a 3 mm tight nip gap for 3–5 times to get identical sheets. These NR-LW composite sheets were allowed to maturation by keeping them at room temperature before molding. The developed mixes were of the formulation as given in Table 1 and named as NR-LW (1) to NR-LW (5). The mixes were vulcanized in standard molds at 150 °C using an electric hydraulic press (12 ″ × 12 ″) at a pressure of 14.71 MPa.

2.3. Characterization of LW and NR-LW Composites

Rheological characterization of NR-LW composites was done in 100 cpm frequency range and 0.5° angle using Rubber Process Analyzer (RPA 2000, Alpha Technologies, USA) according to ASTM D5289. The Eq. (1) determines the cure rate index (CRI), where t₉₀ denotes the cure time, and t₂ is the scorch time.

(1)

C R I = \frac{100}{t_{90} - t_{2}}

The tensile strength and resistance to tear of the NR-LW composites were examined using Instron universal testing machine (UTM) having 5 kN load capacity (ASTM D412 and ASTM D624, respectively) at a speed of 500 mm/min crosshead to assess the stress and elongation at break, Young’s modulus, and the modulus at 300% elongation at room temperature. The hardness and abrasion resistance of NR-LW composites were determined by a Durometer (shore A type, ASTM D2240) and Bareiss DIN abrader, Germany, as per ASTM D5963. A densimeter was employed to measure the specific gravity of samples as per ASTM D297. The compression set was determined by a compression set apparatus with a 9.5 mm spacer thickness (ASTM D395). Dynesco Goodrich flexometer (USA) was employed to measure the heat build-up of the samples (ASTM D623).

Structural characterization of LW and NR-LW composites was carried out using various analytical methods. The NR-LW interactions and structural elucidation of LW were studied using Shimadzu IRAffinity-1S Fourier-transform infrared spectrometer (FTIR) ranging from 4000 cm⁻¹ to 500 cm⁻¹. The structural characteristics of LW were investigated by employing PANalytical X’Pert Pro X-ray diffractometer (XRD). The average size of LW particles was evaluated using Horiba Particamini-LA-350 dynamic light scattering (DLS) analyzer. Field emission scanning electron microscopy (FESEM) analysis (MIRA3 XMU, Czech Republic) and high-resolution tunnelling electron microscopy-energy dispersive X-ray (HRTEMEDS) analysis (Hitachi H-9500, Japan) was employed for the morphological studies of fractured NR-LW composites in liquid nitrogen and elemental composition analysis. The area of the surface and volume of the pore of LW was measured by Brunauer–Emmett–Teller (BET) equation using a BET analyzer (MicrotracBEL, Japan). The decomposition features of the composites were studied on a Hitachi STA 7200 thermogravimetric analyzer (TGA) by heating the samples at 10°C/min rate between 40°C to 600°C, under the atmosphere of nitrogen at 100 cc/min purge rate. The temperatures, namely, onset (T_i), maximum value observed in the derivative thermogram (T_max), and the residue found at 600°C, were recorded.

The CR method is used to determine the E_a for the thermal decomposition of NR and NR-LW composites as given in the following Eq. (2). [19].

(2)

ln ⌊ \frac{- ln (1 - α)}{T^{2}} ⌋ = ln (\frac{A R}{β E_{a}}) - \frac{E_{a}}{R T}

where A is the Arrhenius frequency parameter, T is the heating temperature, R is the universal gas constant, and β is the heating rate. The term α can be calculated by the given Eq. (3).

(3)

α = (\frac{C_{0} - C}{C_{0} - C_{f}})

C_0, C, and C_f are the masses of composites at the initial, preferred and final temperatures. E_a is determined from the plot of

⌊ \frac{- ln (1 - α)}{T^{2}} ⌋

against 1/T.

2.4. Swelling Behaviour of NR-LW Composites

The swelling behaviour of NR-LW composites (ASTM D471) was carried out to determine the cross-link density and swelling index in toluene, and the latter is calculated by using the following Eq. (4).

(4)

S w e l l i n g i n d e x = (\frac{W_{s} - W_{i}}{W_{i}}) \times 100

where Ws is swollen, and Wi is the sample’s initial weight. The cross-link density of the samples was assessed by employing the Flory-Rehner equation [Eq. (5)] as given below [20].

(5)

C r o s s l i n k d e n s i t y = \frac{1}{2 M_{C}}

where M_c represents the molar mass of the samples between successive cross-links, which is computed with the given Eq. (6).

(6)

M_{C} = - \frac{1}{2 V_{2}} ρ_{r} V_{s} V_{r}^{\frac{1}{3}} / [ln (1 - V_{r}) + V_{r} + χ V_{r}^{2}]

where V_s be the molar volume of toluene (106.2 cm³/mol), ρ_r be the density of NR (940 kg/m³), V_r be the volume fraction of NR samples at equilibrium swelling, and χ be the interaction parameter between NR and toluene which is 0.3787 from the literature available [21]. V_r can be calculated using the Ellis and Welding equation [Eq. (7)] [22].

(7)

V_{r} = [(d - f w) ρ_{r}^{- 1}] / (d - f w) ρ_{r}^{- 1} + A_{s} ρ_{s}^{- 1}

where d and w represent the test sample’s deswelling weight and initial weight, f is the fraction volume of the filler, A_s is the weight of toluene absorbed, and ρ_s be the solvent density (for toluene, 0.867kg/m³).

3. Results and Discussion

3.1. Characterization of LW Powder

3.1.1. Phase analysis, structural interpretation, BET measurements and thermal stability analysis of LW

The structural characteristics and phase analysis of LW were carried out using powder XRD (PXRD) measurements, as shown in Fig. 1a. The sharp peaks obtained reveal the presence of crystalline MgCO₃ with traces of CaCO₃. Peaks related to MgCO₃ were observed at 2θ = 32.68°, 35.82°, 43.21°, and 53.82°, corresponding to the hkl planes (104), (006), (113), and (116), respectively [23].

FTIR of LW provided vital information about the structure and conformation of the material. In Fig. 1b, a strong doublet band can be spotted at 1481 and 1427 cm⁻¹, corresponding to the C-O stretching (asymmetric) vibrations and an intense band at 870 cm⁻¹ denoting the CO₃²⁻ asymmetric deformation. The weak band observed at 1026 cm⁻¹ belongs to the carbonate group’s C-O deformation (symmetric) vibration. The prominent peaks at 802 and 702 cm⁻¹ are characteristic bands of the calcite form of CaCO₃ [24].

BET isotherm analysis of LW was conducted at 80°C for 16 hours, and the nitrogen adsorption-desorption isotherms are demonstrated in Fig. 1c. As per the IUPAC classification, the obtained isotherms can be classified as type IV [25]. Type IV isotherms are predominantly detected in adsorbents of mesoporous nature with 6–10 nm pore size. In pore condensation, the gas condenses as a liquid-like phase at a pressure (p) lesser than the bulk liquid’s saturation pressure (p_o). Accordingly, final saturation plateaus of varying lengths are a distinctive aspect of type IV isotherms, and capillary condensation is accompanied by hysteresis. The nanosphere constituted by the void spaces between nanoparticles creates these pores [26]. The LW pore size distributions were assessed by the Barret-Joyner-Halenda (BJH) plot [inset of Fig. 1c]. The total pore volume and mean pore diameter are 1.6976 × 10⁻² cm³g⁻¹ and 9.61 nm, respectively. The adsorption capacity of the compound containing MgCO₃ and CaCO₃ rises with equilibrium pressure due to the condensation of N₂ molecules in mesoporous adsorbents at high pressures. The obtained surface area indicates less porosity of the material due to the applied pre-treatment and is found to be 7.0660 m²g⁻¹. The BET-specific surface area is a significant parameter for adsorption studies and is found to be low, which may be due to poor porosity results from the LW distribution. Therefore, this material can be used to modify the properties of other materials according to the required outcomes.

TGA of LW is given in Fig. 1d and shows three different decomposition stages of LW. The first stage of decomposition is the removal of water of crystallization which extends up to 236°C. The second stage decomposition of MgCO₃ into MgO and CO₂ occurred between 410 – 500°C, accounting for more than 60–65 % of overall weight loss [27]. In the third stage, 30–35% of CaCO₃ decomposes to CaO and CO₂ within 600 to 800°C [28].

3.1.2. Morphology, particle size and elemental analysis

Fig. 1 e–g shows the FESEM, HR-TEM and particle size analysis of LW. FESEM results predict the agglomerated nature of the magnesium and calcium carbonate composite, as shown in Fig. 1e. Applying ultrasonication before HRTEM analysis contributed to the segregation of agglomerated nanoclusters. From the Fig. 1f, the presence of rhombohedral nanoparticles related to carbonates of Ca and Mg can be analyzed. The average size of rhombohedral nanoparticles is in the range of 100–200 nm as per the DLS analysis. The particles of 400–500 nm in diameter can be possible because of the scalenohedral MgCO₃ and CaCO₃. The elemental composition, as given in Fig. 1g, mainly consists of magnesium (24.52%), calcium (1.44%), carbon (15.18%) and oxygen (54.35%).

3.2. Characterization of NR-LW Composites

3.2.1. Surface morphology of NR and NR-LW composites

The tensile ruptured surface morphology of NR and its LW composites was examined by FESEM and is given in Fig. 2 a–d. The FESEM image of NR (Fig. 2a) generalizes the smooth morphology of the surface due to the similar chemical composition [29]. Fig. 2b proved the uniform distribution of LW particles on the surface of NR with 3 phr loading. The cross-sectional analysis and some areas of NR-LW have shown the epitaxial wavy structure due to the modifications in the chemical composition of the NR-LW, as shown in Fig. 2c. But on the 5 phr loading of LW, the filler particles tend to agglomerate, leading to uneven surface morphology, as shown in Fig. 2d [30].

3.2.2. Attenuated total reflection Fourier Transform infraredspectroscopy

The potential peaks obtained in the FTIR spectra of neat NR and its LW composites are given in Table 2. LW and NR-LW composites showed peaks corresponding to symmetric deformation vibration of C-O and CO₃²⁻ which are absent in NR neat.

3.2.3. TG analysis of NR and NR-LW composites

The thermal profile of NR neat and NR-LW composites at various filler loading is examined, and the onset decomposition temperature (T_i), highest thermal decomposition temperature (T_max), amount of residue at 600°C, and E_a for decomposition are provided in Table 3.

A one-step decomposition process is evidenced for NR neat, which starts at 326.7°C. NR-LW composites also showed similar decomposition behaviouras that of NR, and there are no significant changes in the values of onset decomposition temperature [31]. The CR method is employed to compute E_a for the decomposition of NR-LW composites. The E_a values increased up to 3 phr LW loading (≈10 kJmol⁻¹), indicating higher thermal stability of NR-LW (3) composite. But, on further loading, the thermal stability decreased due to poor dispersion and aggregation of LW in the NR matrix. The maximum degradation temperature showed a small hike for the composites compared to that of NR. As the filler content increases, the residue at 600°C showed a marginal increase as the filler requires a high temperature for decomposition [32].

3.3. Cure Characteristics

Table 4. explains the impact of LW loading on the cure features of NR-LW composites.

All the composites showed a decrease in cure time, and notably, NR-LW (3) composite showed a 12% decrease proving the co-curing activity of the filler. The cure rate index (CRI) indicates the extent of curing, and the CRI of NR-LW composites is greater than that of NR, confirming the ease of processing of composites. The scorch time of all the composites is almost similar, where the composite with 3 phr LW has the lowest value. The M_L and M_H values were increased on increasing filler loading since the filler reduces the mobility of the macromolecular chains in NR [33]. Maximum torque values of the composites showed a gradual increase indicating better NR cross-linking on the addition of LW and is further backed by the rise in the difference between the maximum and minimum torque (M_H-M_L) [34].

3.4. Mechanical Properties

The mechanical property values of NR-LW composites are summarized in Table 5.

The stress-strain behavior of NR-LW composites, supports an increase in the elastic nature of NR on LW addition. This is attributed to ameliorated matrix-filler adhesion which instigates higher stress for the rupture of the composites [35]. The composite with 3 phr LW showed the highest tensile strength and increased by 22% to NR due to better dispersion and strong interfacial bonding of LW on the NR matrix. But on further filler addition, it started decreasing due to the LW aggregation on the NR matrix. The NR-LW interactions are collapsed above 3 phr LW loading, favoring LW-LW interactions which failed to support the effective transfer of stress applied to the composite [36]. Elongation at break exhibited a decreasing trend up to 3 phr LW followed by a slight increase proving the better adherence of LW on NR leading to the restricted motion of molecular chains in NR-LW (1) to NR-LW (3) composites. The modulus values at 300% elongation showed an upward trend (29% increase at 3 phr LW) and decreased on further filler addition due to LW agglomeration. Young’s modulus of the composites was computed from stress-strain plots which shows an upward trend till 3 phr LW loading and dwindling on further loading. The tear strength values of NR-LW composites are increased up to 3phr LW (20% increase) and found to decrease gradually on further addition of LW. At higher filler loading, the tendency for LW aggregation is more prominent, resulting in unequal filler distribution on the NR matrix lowering the mechanical properties [37]. Composite with 3 phr LW has a higher hardness value due to its ameliorated dispersion in the pores of NR. The abrasion loss (volume loss) of all composites is lesser than that of NR, which supports the better dispersion of LW up to 3 phr on NR [38]. The compression set values NR-LW composites are close to that of the NR matrix, proving the ameliorated matrix-filler interaction and uniform dispersal of LW on NR.

3.5. Theoretical Modelling of Young’s Modulus of NR-LW Composites

Factors like the concentration and distribution of the filler in the matrix and the filler-matrix adhesion affect the mechanical properties of prepared composites. Various theories were introduced to explain the above factors. Einstein and Guth models are commonly employed to predict Young’s modulus of particulate-filled composites and consider better dispersion of filler on matrix effecting proper adhesion [39,40]. Einstein proposed the following equation [Eq. (8)] to predict Young’s modulus of a rigid particle reinforced composite.

(8)

E_{c} = E_{m} (1 + 2.5 φ)

where φ be the filler’s volume fraction, E is Young’s modulus, and the subscripts c and m are the composites and the matrix, respectively. Einstein’s equation holds good only at low-volume filler fractions and assumes perfect matrix-filler adhesion and uniform distribution of filler particles. An extension of Einstein’s theory to explain rubber reinforcement was put forward by Guth, and the following Eq. (9) gives the increase in modulus due to a rigid spherical filler:

(9)

E_{c} = E_{m} (1 + 2.5 φ + 14.1 φ^{2})

Fig. 3 compares both experimental Young’s modulus values with that of Einstein and Guth models prediction. The Young’s modulus values calculated by both Einstein and Guth models showed an appreciable correlation with the experimental values of NR-LW composites at lower filler loadings and merged with the Guth model value at 3 phr LW loading. This behavior implies the uniform distribution of it in NR and better NR-LW adhesion. At higher filler loading, the experimental Young’s modulus showed a negative deviation from the theoretically predicted values, probably due to the LW agglomeration on NR weakening the NR-LW interaction.

3.6. Swelling Behaviour of NR-LW Composites

Cross-link density is affected by the rubber-filler interactions and chemical cross-links in the vulcanized composites [41]. Solvent transport through the NR matrix is mainly governed by the polymer chain’s mobility, free volume in the composite and molecular size of the employed solvent [42,43]. As per the obtained swelling indexing values, the molecular transport of the solvent showed a regular decrease up to 3 phr LW; on further LW addition, a slender hike is observed. The swelling index values (%) obtained are 289±3, 286±2, 278±2, 275±1, 276±1, and 277±2 respectively for NR neat and NR-LW (1–5) composites. The extent of matrix-filler interaction is assessed by cross-link density and showed a maximum value for 3 phr LW and decreased on further LW addition [44,45]. The cross-link density values (10⁻⁴ molg⁻¹) for NR neat and NR-LW (1–5) composites are in the order 1.21±0.03, 1.22±0.05, 1.27±0.03, 1.29±0.05, 1.25±0.07, and 1.23±0.04. The uniformly distributed LW in NR restricts the movement of solvent molecules on NR up to 3 phr LW. Beyond 3 phr filler addition, LW particles start agglomeration, leading to free spaces in the matrix and facilitating solvent uptake. The enhanced mechanical properties showed by NR-LW (3) composite due to the better interaction of NR and LW are further supported by the assessment of cross-link density and swelling studies.

The above results proved the significance of NR-LW (3) composite for manufacturing common industrial components like gaskets, seals, drive couplings, pad assemblies, mud flaps, automobile parts etc. As huge quantity of LW is generated in condom industry, its recycling and reutilization is a matter of importance. Hence, we report an efficient and economical method for its utilization. The current approach is highly applicable in the rubber industry as it considerably reduces the production cost and helps to prevent environmental pollution.

4. Conclusions

LW generated in the condom industry (HLL Lifecare Limited, India), which is a water pollutant causing hardness, is characterized and reutilized in NR as a reinforcing filler. The prepared NR-LW composite’s rheological, mechanical, thermal, and sorption features were studied.

The composite of NR-LW with 3 phr LW showed a substantial increase in tensile strength, tear strength, and modulus at 300% elongation.
The morphology of NR-LW composite’s surface was analyzed by FESEM and established the fine dispersion of LW up to 3 phr and formation of aggregates on further addition.
Studies on the thermal profile of NR-LW composites proved that adding LW up to 3 phr loading enhances NR’s thermal stability, as evidenced by the CR model. An increase of 10 kJmol⁻¹ in E_a value for NR-LW (3) composite confirms the enhanced thermal stability.
The swelling index and cross-link density of NR-LW composites were evaluated. The composite with 3 phr LW showed the lowest swelling index and highest cross-link density, proving the ameliorated matrix-filler interaction and uniform dispersal of filler in NR.

The formulated NR-LW (3) composite with enhanced mechanical, thermal, and solvent-resistant features can be utilized in common manufacturing applications where NR can be replaced with NR-LW composites reducing the production cost. The future scope of this work involves the reutilization of LW in other synthetic rubbers for the fabrication of high-performance, cost-effective products.

Acknowledgement

The authors thank KSCSTE-SRS and DBT-STAR for the instrumentation facility at the Department of Chemistry, St. Joseph’s College (Autonomous) Devagiri, Kozhikode. Acknowledgement goes to Bharat Ratna Prof. CNR Rao Research Centre, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore and CIL & UCIM, Panjab University, Chandigarh for characterization analyses. We especially thank S. Venugopal, Joint General Manager, HLL Lifecare Ltd., Thiruvananthapuram, for providing LMC for this work. We also thank J. J. Murphy Research Centre, Rubber Park India (P) Ltd., Valayanchirangara, for their help in the rheological and mechanical analyses. This research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Notes

Declaration of conflict of interest

The authors declare that they have no conflict of interest.

Author Contributions

P.B. (Ph.D. student): Methodology, resources, data acquisition, analysis, investigation, interpretation, and writing-original draft, K.B. (Professor): Data acquisition, investigation, resources, and writing-original draft, P.D. (Assistant Director): Analysis, and data acquisition, M.S. (Ph.D. student): Analysis, and data acquisition, S.E. (Dean): Data analysis and interpretation, T.F. (Supervisor): Conceptualization, supervision, reviewing, and editing.

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Fig. 1

(a) PXRD patterns of LW, (b) FTIR spectrum of LW, (c) BET-Adsorption-desorption isotherms (inset figure-BJH plot) of LW, (d) TGA of LW, (e) FESEM of LW, (f) HRTEM of LW, (g) Elemental composition of LW

Fig. 2

FESEM images of (a) NR, (b) & (c) NR-LW (3), (d) NR-LW (5)

Fig. 3

Correlation between experimental and theoretical values of Young’s modulus

Table 1

NR-LW Composite Formulation

Ingredients^* (phr)	NR Neat	NR-LW (1)	NR-LW (2)	NR-LW (3)	NR-LW (4)	NR-LW (5)
ISNR-5	100	100	100	100	100	100
LW	0	1	2	3	4	5

ZnO 5.0, TMQ 1.0, CBS 0.6, stearic acid 2.0, TMTD 0.2 and sulfur 2.5

Table 2

Potential Peaks in the FTIR Spectra

Band position (cm⁻¹)	Assignment
2995	Stretching vibrations of CH in NR (symmetric and asymmetric)
2910	Stretching vibrations of CH₂ in NR (symmetric and asymmetric)
2840	Stretching vibrations of CH in NR (symmetric and asymmetric)
2352	Vibrational mode of C-H of stearic acid in NR
1667	C = C stretching in NR (cis-1,4-isoprene units) (42)
831	C-H wagging in NR (42)
1447	Asymmetric stretching vibrations of C-O
1085	Symmetric deformation vibration of C-O (absent in NR) (43)
676	Symmetric deformation vibration CO₃ (absent in NR) (43)

Table 3

Thermal Degradation Characteristics

Properties	NR Neat	NR-LW (1)	NR-LW (2)	NR-LW (3)	NR-LW (4)	NR-LW (5)
Onset decomposition temperature, T_i (°C)	326.7	326.9	328.8	330.2	330.5	327.1
Maximum decomposition temperature, T_max (°C)	372	373.5	374.6	375.8	375.2	374.1
Residue at 600 °C (%)	4.86	5.52	6.04	6.23	7.06	6.90
Activation Energy, Ea (kJmol⁻¹)	115.36	121.01	123.48	124.26	123.69	122.44

Table 4

Cure Features of NR-LW Composites

Cure characteristics	NR Neat	NR-LW (1)	NR-LW (2)	NR-LW (3)	NR-LW (4)	NR-LW (5)
Cure time, t₉₀ (min)	4.18	4.12	4.07	3.68	3.87	3.93
Scorch time (min)	2.15	2.37	2.24	1.95	1.99	2.27
Minimum torque, M_L (dNm)	6	6.5	7.8	6.3	7.6	6.6
Maximum torque M_H (dNm)	72.9	74.1	76	78.6	75.8	76.3
M_H-M_L (dNm)	66.9	67.6	68.2	72.3	68.2	69.7
Cure rate index (min⁻¹)	49.26	57.14	54.64	57.80	53.19	60.24

Table 5

Mechanical Property Values of NR-LW Composites

Properties	NR Neat	NR-LW (1)	NR-LW (2)	NR-LW (3)	NR-LW (4)	NR-LW (5)
Hardness (Shore A)	41 ± 0.5	40 ± 0.5	41 ± 1	43 ± 1	41 ± 1	41 ± 1.5
Specific gravity	0.965 ± 0.005	0.974 ± 0.005	0.978 ± 0.010	0.981± 0.012	0.981 ± 0.012	0.997 ± 0.015
DIN abrasion loss (cc)	0.272 ± 0.015	0.226 ± 0.022	0.228 ± 0.025	0.22± 0.018	0.23 ± 0.033	0.264 ± 0.035
Tensile strength (MPa)	21.93 ± 0.25	23.72 ± 0.34	24.91± 0.28	26.66 ± 0.46	26.21 ± 0.52	23.62 ± 0.55
Elongation at break (%)	697 ± 12	673 ± 18	669 ± 14	667 ± 15	687 ± 12	689 ± 16
Modulus at 300% elongation (MPa)	3.09 ± 0.23	3.53 ± 0.25	3.83 ± 0.34	3.97 ± 0.22	3.52 ± 0.43	3.48 ± 0.32
Young’s modulus (MPa)	0.899 ± 0.04	1.031 ± 0.03	1.138 ± 0.02	1.165 ± 0.02	1.049 ± 0.05	1.029 ± 0.02
Tear strength (N/mm)	41.76 ± 0.72	46.42 ± 0.65	47.42 ± 0.78	50.31 ± 0.84	48.01 ± 0.72	48.78 ± 0.68
Compression Set (%)	18.68 ± 0.22	17.39 ± 0.24	15.49 ± 0.28	18.23 ± 0.25	18.86 ± 0.22	19.57 ± 0.26
Heat build-up (°C)	1± 0.34	1 ± 0.22	2 ± 0.28	1± 0.36	2.5 ± 0.42	2.5 ± 0.34