Toward Eco-Friendly Rubber: Utilizing Paper Waste-Derived Calcium Carbonate to Replace Carbon Black in Natural Rubber Composites

Abstract

The growing concerns for the environmental impact of resource depletion and carbon emissions has led to the current study of using novel, sustainable materials in natural rubber compounds. The principal goal of this study was to reduce the usage of the non-renewable filler carbon black (CB). For this purpose, two waste-derived calcium carbonates were introduced in natural rubber compounds as a partial replacement for CB. To enhance their performance, the compounds were modified using alpha-lipoic acid and a titanate as in situ coupling agents. The effect of these renewable fillers and coupling agents on the in-rubber properties was analyzed using various characterization methods. Remarkably, by replacing 10 phr of carbon black with a calcium carbonate filler and introducing the alpha-lipoic acid coupling agent, a compound was obtained with performance levels similar to the CB-filled reference compound. These findings contribute valuable insights into the replacement of carbon black with renewable calcium carbonate fillers.

1. Introduction

In the wake of the Paris Agreement and the global push for sustainability, there has been an increase in focus on developing sustainable materials and decreasing carbon emissions worldwide [1]. In order to obtain a net-zero emissions economy, diverse sectors are seeking for sustainable solutions. The rubber industry is one of the sectors with a major impact in reducing greenhouse gas emissions. Enhancing the efficiency of rubber products and using novel, sustainable materials are key factors in achieving this goal.

One of the main constituents in a rubber compound is the filler, which improves rubber compound properties such as tensile strength, abrasion resistance, tear resistance, and modulus. Carbon black (CB) is a widely used filler due to its superior in-rubber performance. However, it is derived from non-renewable sources, and its production is associated with substantial environmental impacts, including greenhouse gas emissions and resource depletion [2].

As the production and use of carbon black heavily depends on the depletion of petroleum products, interest has grown in deriving particulate fillers from alternative resources [3]. These innovative fillers do not only contribute to reducing the environmental impact but also present opportunities for enhanced performance and cost-effectiveness in rubber formulations. Alternative fillers are identified based on their renewability. A non-renewable substance is a substance that is used up more quickly than it can replenish itself over time. The supply of these substances is limited. Renewable substances are replenished over time, so the supply can be seen as infinite. Mineral fillers such as calcium carbonate, talc, or clay are considered non-renewable, even if their supply is not likely to run out in the near future. On the other hand, particulate (mineral or synthetic) fillers obtained from waste industries will be classified as renewable, even if the strict definition of this term would not include these fillers, as the use of these fillers significantly increase the sustainability of the rubber industry [3,4,5].

A variety of renewable fillers have been investigated as eco-friendly fillers in rubber compounds throughout the years, some improving in-rubber properties more than others. Organic, biodegradable materials such as chitin, chitosan, and starch show some reinforcing behavior but are difficult to integrate into the rubber matrix due to their incompatibility with the polymer, often necessitating modification of the filler or polymer matrix [4]. Other waste-derived rubber fillers include industrial dust [6,7,8], wood flour [9,10], (silane-modified) tea waste [11,12], fly ash [13,14,15], and more. These fillers typically consist of large particles and have limited compatibility with rubber matrices, restricting their effectiveness in rubber reinforcement.

One of the most-used fillers in the rubber industry is calcium carbonate (CaCO₃). In the last few years, renewable calcium carbonate has become an attractive alternative to develop more sustainable rubber products. Renewable calcium carbonates derived from fishbone and eggshells have been integrated into NR compounds by Mogy et al. (2020). The powdered materials were compared to commercial calcium carbonate (CC) compounds. The study notes the similar performance and dispersion of the eggshell-filled compared to the CC-filled compounds [16]. In another study by Bhagavatheswaran (2019), eggshell particles were functionalized with a terpolymer to improve compatibility between the filler and the Acrylonitrile Butadiene Rubber (NBR). Enhanced interaction was attributed to the peptide group on the eggshell surface, which are not present on the surface of pure calcium carbonate, thus presenting new opportunities for filler integration. Roy (2019) used malleated NR (MNR) as a compatibilizer for eggshell-derived nano CaCO₃ and NR. The interaction between the amine group on the filler surface and the MNR was the reason behind the increased interfacial adhesion of the filler and the polymer [17].

To alter the filler–filler and polymer–filler interaction, calcium carbonates are often coated with low-molecular-weight organic compounds like stearic acid, other stearates, or fatty acid materials [18]. These coatings help to wet the filler surface and improve the filler incorporation and dispersion within the rubber matrix [19]. The calcium carbonate fillers are mostly pre-modified (ex situ) with these coatings, which mainly reduce the filler–filler and polymer–filler friction. In situ modification makes use of coupling agents, whose chemical composition should allow coupling to both the filler and the polymer.

In this work, two post-consumer waste-derived calcium carbonate fillers obtained from the paper industry were used to partially replace carbon black in natural rubber compounds. In order to investigate the effect of these fillers on the compound performance, the fillers were first characterized on their Brunauer–Emmett–Teller (BET) surface area and on their thermal stability and decomposition behavior using a thermogravimetric analysis (TGA). Afterwards, natural rubber compounds were prepared with a filler system consisting of both carbon black and the waste-derived calcium carbonate. Alpha-lipoic acid and titanate were used as in situ coupling agents for the compounds.

2. Materials and Methods

2.1. Materials

The rubber compounds presented in this study were prepared using Natural Rubber TSR10 grade (WEBER & SCHAER GmbH & Co., KG, Hamburg, Germany).

TSR stands for “Technically Specified Rubber”, which is a standardized grading system for natural rubber. TSR10 has relatively low dirt contamination and ash content and is used in applications where good mechanical properties are required. The fillers used in this study were carbon black (CB) and two different types of calcium carbonate (CC). Carbon Black N550 (Birla Carbon, Hannover, Germany) is used as the reference filler. It is a semi-reinforcing filler with a BET surface area of 42 m²g⁻¹. Two types of calcium carbonate (CaCO₃) were supplied by the company Alucha, which reclaims calcium carbonate from paper mill sludge (PMS). The particle size distribution for both calcium carbonate grades is as follows: 50% of all the particles (d50) have a size below 1 μm, while 98% (d98) have a size below 5 μm. The PMS used to produce these CC types was sourced from a waste stream generated during the production of new paper products from recycled paper. This recovery process [20] converts the organic part of the PMS into bio-oils, while the inorganic fraction, consisting of mainly calcium carbonate, is separated and recovered. For the preparation of the rubber compounds, Zinc oxide (ZnO) and stearic acid were used as activators (Millipore Sigma, Hamburg, Germany); sulfur, N-tert-butyl-benzothiazole sulfonamide (TBBS), and TetraBenzylThiuram Disulphide (TBzTD) (Caldic B.V., Rotterdam, The Netherlands) as curatives; and treated distillate aromatic extracted (TDAE) (Hansen & Rosenthal, Hamburg, Germany) as oil. 6PPD (6-Phenyl-1,3-dihydro-2H-benzimidazole-2-thione) was used prevent degradation caused by ozone and heat. Paraffinic wax was employed to prevent degradation caused by oxygen.

In the present work, the chemicals alpha-lipoic acid (Sigma Aldrich, Zwijndrecht, The Netherlands) and the titanate Ken-React^®CAPOW^®L^®12/H (Kenrich Petro-chemicals Inc., Bayonne, NJ USA) were reviewed as in situ coupling agents for the calcium carbonate-filled compounds. Alpha-lipoic acid is a small organic compound that is found inside the human body, where it carries out important metabolic and antioxidant functions. It has also been used as a dietary supplement in multivitamin and anti-aging formulas [18]. Its chemical structure is shown in Figure 1 (left). It is theorized to react to the calcium carbonate via the carboxylic acid group on the molecule in a similar manner as stearic acid [21]. In a typical scenario, the deprotonated carboxylic group (-COO⁻) reacts with the calcium cations (Ca²⁺) through ionic bonding. Furthermore, this group might covalently react with surface hydroxyl (-OH) groups through a condensation reaction. The disulfidic moiety on the molecule is particularly attractive as the S-S bonds are strong but can undergo cleavage upon heating [21], photo-irradiation [22], or mechanical stress [23,24]. Heat-induced disulfide exchange reactions can be expected during mixing and curing, increasing interaction between itself and other sulfur-containing substances in the compounds as well as with double bonds of natural rubber during vulcanization [25].

2.2. Compounding and Mixing

Rubber compounds were prepared in an internal mixer Brabender Plasticorder 350S, Duisburg, Germany) with a fill factor of 0.7, initial temperature of 70 °C, and rotor speed of 70 rpm. For the present study, the samples were prepared according to the formulation shown in

Table 1. The formulation of the rubber compounds.

Ingredient	CB	A/B-10	A/B-LA10	A/B-Ti10	A/B-20	A/B-LA20	A/B-Ti20
NR—TSR10	100	100	100	100	100	100	100
TDAE 1	10	10	10	10	10	10	10
Zinc oxide	4	4	4	4	4	4	4
Stearic acid	2	2	2	2	2	2	2
6PPD 2	2	2	2	2	2	2	2
Paraffin wax	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Sulfur	1.5	1.5	1.5	1.5	1.5	1.5	1.5
TBBS 3	1	1	1	1	1	1	1
TBzTD 4	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Carbon Black N550	60	50	50	50	40	40	40
Calcium Carbonate A/B	-	10	10	10	20	20	20
Alpha-Lipoic Acid	-	-	0.7	-	-	1.4	-
Titanate	-	-	-	0.5	-	-	0.5

¹: Treated Distilled Aromatic Extract; ²: N-(1,3-dimethylbutyl)-N’-Phenyl-p-PhenyleneDiamine; ³: N- tert-butyl-benzothiazole sulfonamide; ⁴: TetraBenzylThiuram Disulphide.

and mixing procedure in

Table 2. Mixing procedure.

Time (min)	Action
	Step 1 pre-heating 70 °C—70 rpm
0.00	Addition of rubber
1.00	Addition of 1/2 filler, processing oil
2.30	Addition of 1/2 filler, anti-degradants, and activators
4.00	Ram up, sweep for 15 s
6.00	Stop mixing
	Step 2 pre-heating 50 °C—50 rpm
0.00	Addition elastomer batch stage 1
1.00	Addition curing system
3.00	Stop mixing

.

Alpha-lipoic acid and the titanate were incorporated into the compound formulation in the first stage of mixing procedure. As advised by the manufacturer, 0.5 phr of the titanate was introduced to all compounds. The amount of added alpha-lipoic acid was 0.7 phr for the A10 and B10 compounds and 1.4 phr for A20 and B20. The amount of alpha-lipoic acid was adjusted to the amount of filler A or B in an equimolar relation following Michelin’s green tire recipe for silica–silane compounds, irrespective of the filler surface area [26].

2.3. Filler Characterization

2.3.1. Brunauer–Emmett–Teller (BET) Surface Area Measurement

The surface area of the fillers was determined using the BET method. By analyzing the adsorption isotherms of nitrogen, the BET method provided accurate surface area calculations, enabling assessment of the fillers’ surface characteristics.

2.3.2. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was performed using a TGA 550 from TA Instruments, New Castle, DE, USA. The TGA measurements involved subjecting samples to a controlled temperature ramp in order to determine their thermal stability and decomposition behavior. TGA was conducted under air atmosphere. A temperature ramp of 20 °C/min⁻¹ was used with an end temperature of 800 °C.

2.4. In-Rubber Tests

2.4.1. Mooney Viscosity

The viscosity of the compounds was measured at 100 °C using a Mooney viscometer 2000E (Alpha Technologies, Hudson, OH, USA) with a large rotor, following the ASTM D1646 standard. One measurement per compound was performed.

2.4.2. Cure Behavior

Cure behavior analysis was carried out using the RPA Elite Rubber Process Analyzer (TA Instruments). The measurements were performed at 160 °C by applying a deformation of 6.98% at a frequency of 1.667 Hz. The t90, that is, the time to reach 90% conversion, of each compound was used as molding time for vulcanizing the rubber samples in a hydraulic press.

2.4.3. Payne Effect

Payne effect measurements were conducted to evaluate filler–filler and polymer–filler interactions. The RPA Elite Rubber Process Analyzer (TA Instruments) was utilized for these measurements at a temperature of 60 °C. Samples were cured before performing the measurement. The storage modulus (G’) values were recorded during shear deformation, employing a strain sweep range of 0.84–100% at a frequency of 1 Hz.

2.4.4. Macro-Dispersion

The macro dispersion of the filler in the rubber compound was studied by means of a Dispergrader—Dispersion Tester Alpha View (Alpha Technologies, Hudson, OH, USA). The rubber samples were investigated by optical light microscopy with a 30° irradiation angle and at 100× magnification.

2.4.5. Tensile Behavior

The tensile strength of the cured samples was measured by a universal testing machine Zwick Z05 (Zwick, Ulm, Germany) operation with the crosshead speed of 500 mm/min according to ASTM D412 standard.

3. Results and Discussion

3.1. BET Analysis

The surface area of the fillers was determined using the BET method. By analyzing the adsorption isotherms of nitrogen, the BET method provided accurate surface area calculations, enabling assessment of the fillers’ surface characteristics. The results are shown in

Table 3. The BET surface area of the studied fillers.

Filler Name	Code	BET SA [m2g−1]
Carbon Black N550	CB	42
Calcium Carbonate A	CCA	11.0
Calcium Carbonate B	CCB	14.9

. Both calcium carbonates show a significantly lower surface area than carbon black, which can have a negative impact on the in-rubber properties of the compounds. Comparing A and B, it is shown that calcium carbonate B has a higher surface area than A.

3.2. Thermogravimetric Analysis (TGA)

The TGA graphs are presented in Figure 2. The carbon black’s TGA shows the start of the weight loss at approximately 500 °C; the weight loss observed up to 800 °C equals 0% because of a complete thermo-oxidative decomposition of CB particles in the oxidative atmosphere of synthetic air. The onset temperature of the decomposition is significantly higher for CB than for CCB. This is most likely caused by the much more regular chemical structure of CB in comparison to the cellulose-originated biochar present on the calcium carbonate surface. The calcium carbonates A and B exhibit decomposition above 600 °C, which is the breakdown of the inorganic CaCO₃ into CaO (solid) and CO₂ (gas), resulting in weight loss. However, weight loss is also observed above 350 °C. This weight loss is an indication of the decomposition of organic matter or other impurities. It is evident that the fillers are composed of more than only one constituent.

According to manufacturer data, filler A contains primarily calcium carbonate (up to 95%), but dolomite (2–5%), kaolinite (1%), and talc (1%) are also present in this filler. The slight decomposition for this material below 600 °C may be attributed to moisture evaporation and the kaolinite releasing its strongly bonded hydroxyl group as water above 450 °C [27,28]. The composition of filler B is the same as similar to filler A. The only difference is that B also contains biochar. Observing the thermal decomposition curve in Figure 2, a maximum calcium carbonate content of 91% is assumed in filler B. Due to its similar feedstock material, the rest of the filler may consist of components similar to filler A and carbon-based residues (<8%).

3.3. Mooney Viscosity

The Mooney viscosity (ML) is the measure of the compound’s resistance to flow. The Mooney viscosity values of the studied samples are shown in Figure 3. Introducing both coupling agents decreases compounds’ resistance to flow, indicating that the agents coat the surface of the fillers and induce a decrease in the polymer–filler friction. The increased viscosity for filler B is due to its higher surface area compared to filler A. Increasing the amount of both fillers to 20 phr results in a decrease in viscosity due to the lower total filler surface area. However, the initial addition of 10 phr of the calcium carbonate fillers increased the Mooney viscosity. This may be explained by the hydrodynamic effect caused by significantly bigger particles of CC in comparison to CB. It is hypothesized that when the filler content is relatively low (10 phr), the hydrodynamic effect plays a more significant role in the Mooney viscosity, while at higher content (20 phr), the effect of the higher surface area is more crucial for the overall viscosity of the compounds.

3.4. Cure Behavior

The curing curves of the studied compounds are shown in Figure 4. As can be observed, both calcium carbonates without a coupling agent (A10, A20, B10, and B20) have a detrimental effect on the maximum cure torque (M_H). The M_H gives an indication of the physical properties of the rubber compound and is related to its crosslink density and internal interactions present, such as the filler–filler and polymer–filler interactions. The decrease in the maximum torque for the calcium carbonate compounds can be attributed to their lower surface area and consequently lower filler–filler and filler–rubber interactions within the compounds. In Figure 4, it is shown that the coupling agents have a significant influence on the curing curves. The introduction of both coupling agents results in higher maximum cure torque values. This could indicate that these compounds present an increased crosslink density, filler–filler and/or polymer–filler interactions, than can lead to improved mechanical performance. The effect of the coupling agents on the maximum torque is greater for the B-filled compounds, which can be attributed to the higher surface area of calcium carbonate B compared to A. The addition of the coupling agent alpha-lipoic acid showed a higher impact on the curing behavior than the titanate. Compounds A10 and B10 show slightly higher M_H values than the reference compound with carbon black.

3.5. Payne Effect

The Payne effect gives an indication of the filler–filler network strength within a compound. This parameter is usually correlated with the micro-dispersion of the filler. It is calculated as the difference between the storage modulus at low strain and at high strain. The Payne effect curves are displayed in Figure 5. It is visible that the filler–filler and polymer–filler interactions decrease when introducing the lower surface area calcium carbonates A and B, and the decrease is higher when increasing the calcium carbonate amount (A20 and B20). The addition of alpha-lipoic acid leads to a significant increase in the Payne effect of all compounds. Heat-induced disulfide exchange reactions during mixing or curing may be a cause of network formation between the fillers, increasing the filler–filler interaction [23]. As well as physically or chemically bonding to the calcium carbonate surface, the polar carboxylic group on alpha-lipoic acid might be reactive towards carbon black’s polar surface groups through van der Waals forces or hydrogen bonding, increasing the filler–filler interaction even more. These results are in agreement with the ones obtained in the vulcanization test. The introduction of the titanate coupling agent has a much lower impact on the Payne effect. For calcium carbonate A, the values obtained with the compounds in which titanate are used are similar to the ones without a coupling agent. For calcium carbonate B, the filler–filler interactions slightly increase with the addition of this coupling agent but in a lower extent than for the alpha-lipoic acid.

The modulus values at high strains are used as indicators for the polymer–filler interaction and are shown in

Table 4. The Payne effect and modulus at high strains of the studied compounds.

	ΔG’(kPa)	G’100%(kPa)
CB	1700	455
A10	1154	442
A10-LA	1430	470
A10-Ti	1070	462
A20	766	416
A20-LA	1020	486
A20-Ti	867	458
B10	1114	457
B10-LA	1350	544
B10-Ti	1255	530
B20	671	414
B20-LA	944	503
B20-Ti	880	492

. Greater values are observed after introducing both coupling agents, indicating enhanced polymer–filler compatibility. The disulfidic group on the lipoic acid molecule is expected to be a cause of this behavior, as it can interact with sulfur and carbon–carbon double bonds during vulcanization [23]. The long hydrocarbon chains on the titanate might entangle with the polymer chains and bond by van der Waals forces, increasing this indicator for polymer–filler interaction.

3.6. Macro-Dispersion

The macro-dispersion of the prepared rubber compounds was analyzed by optical microscopy. Compounds with 20 phr of calcium carbonates A and B with and without coupling agents were selected. The images obtained are shown in Figure 6. In the images, it can be observed that the samples without a coupling agent present a similar macro-dispersion. The addition of calcium carbonate does not seem to have an impact on the dispersion of the carbon black. Regarding the samples in which a coupling agent was added (titanate or alpha-lipoic acid), they show a slightly worse macro-dispersion. Some small clusters can be observed in these samples. This results were expected due to the higher filler–filler interactions observed in the Payne effect study for these compounds.

3.7. Tensile Test

The stress/strain curves of the studied compounds are shown in Figure 7. It can be observed that the addition of calcium carbonate A has a detrimental effect on the mechanical properties compared to the carbon black reference. However, the addition of 10 phr of CCA combined with the coupling agents has a significant impact on the mechanical performance. Both compounds (A10-LA and A10-Ti) show a similar behavior to the CB reference. This can be attributed to the increase in the filler–polymer interactions due to the addition of the coupling agents. These results are in accordance with the ones observed in the study of the Payne effect. Regarding calcium carbonate B, it can be observed that all samples present a similar performance to the reference compound. The addition of the coupling agents does not have a significant impact on the mechanical properties. The better performance of CCB compared to CCA can be attributed to its higher surface area and therefore higher polymer–filler interactions. Also, the presence of the biochar on the surface of CCB is playing a positive role. It is hypothesized that the chemical structure and morphology of the biochar is similar to carbon black, resulting in similar reinforcement mechanisms based on dispersive interactions with rubber molecules and the formation of bound rubber. The good mechanical performance of both calcium carbonates can also be related to a better dispersion of the filler in these compounds, as observed in the study of the Payne effect.

4. Conclusions

The growing environmental concerns associated with resource depletion and carbon emissions has led to the current study of using novel, sustainable materials in rubber compounds. Carbon black (CB) is a widely used filler due to its superior in-rubber performance. However, it is derived from non-renewable sources and its production is associated with substantial environmental impacts, including greenhouse gas emissions and resource depletion. For this purpose, renewable fillers (calcium carbonate) were introduced in rubber compounds as a partial replacement of carbon black. In this paper, rubber compounds of 5.5 and 11 wt% circular content were produced using minerals from a post-consumer waste stream. The main goal of this study was to reduce the usage of the filler carbon black in natural rubber compounds while maintaining similar in-rubber performance. Upon introduction into the natural rubber compounds, the calcium carbonates induced inferior compound properties compared to the CB-filled reference compound due to their lower surface area and differing surface chemistry.

To improve their in-rubber performance, the calcium carbonate/CB-filled compounds underwent modification with titanate and alpha-lipoic acid. Incorporating alpha-lipoic acid and the titanate coupling agent, the hybrid calcium carbonate/CB-filled compounds showed improved processing behavior and mechanical properties compared to the non-modified compounds. By replacing 10–20 phr (16.7–33 wt.%) of carbon black with the calcium carbonates A and B and introducing the alpha-lipoic acid coupling agent, two compounds were obtained with performance levels on par with the CB-filled reference compound. These results validate the possibility of carbon black to be replaced by renewable alternative fillers, and the findings contribute valuable insights into the replacement of carbon black with renewable calcium carbonate fillers. In future works, it is recommended to investigate the coupling mechanisms in more detail. Moreover, enhancement of the surface area of these renewable calcium carbonates will pave the way for sustainable high-performance rubber compounds.