Gopal as a Sustainable Alternative: Chemical, Rheological, and Mechanical Insights

Abstract

The availability of petroleum asphalt, derived from non-renewable natural sources, is steadily declining in tandem with dwindling petroleum reserves. To mitigate the reliance on petroleum, alternative renewable natural sources are being explored for use as both modifiers and replacements for petroleum asphalt, particularly as binders in asphalt mixtures. The development of bio-asphalt represents a significant innovation aimed at reducing or even eliminating the dependence on petroleum as a source of asphalt. This paper examines the chemical, rheological, and mechanical properties of Gopal (Gondorukem Asphalt), a bio-asphalt derived from Gondorukem (gum rosin) and CPO (Crude Palm Oil). Two types of Gopal, Gopal-GEM130 and Gopal-GEG90, were analyzed using FTIR (Fourier Transform Infra-Red) and EDX (Energy Dispersive X-ray) tests, with Pen 60 petroleum asphalt serving as a control for comparison. The results indicate that the chemical groups of Gopal-GEG90 and Gopal-GEM130 share 86% similarity with those of Pen 60 petroleum asphalt. Compared to Pen 60, Gopal-GEM130 is less toxic and less alkaline, while Gopal-GEG90 is also less toxic but more alkaline. Rheologically, Gopal-GEG90 and Gopal-GEM130 fall within the same classification as Pen 60, based on the Pen 60 classification grade of asphalt. Gopal-GEG90 exhibits slightly better stripping resistance and lower aging resistance than Pen 60, whereas Gopal-GEM130 demonstrates significantly better stripping resistance but lower aging resistance. Performance-wise, both Gopal variants belong to the same performance grade (PG64S) as Pen 60 petroleum asphalt. However, Gopal-GEG90 has slightly better rutting resistance compared to Pen 60 but lower than Gopal-GEM130, and it ages faster with lower fatigue resistance. Conversely, Gopal-GEM130 has superior rutting resistance but lower fatigue resistance and ages faster than Pen 60 petroleum asphalt.

1. Background

Declining petroleum resources necessitate the exploration of sustainable alternatives for asphalt. A multitude of studies has investigated bio-oils sourced from crops, particularly highlighting soy and rapeseed [1,2]. Despite positive results from these bio-oils, the potential rosin ester and crude palm oil (CPO), especially from gum rosin, as a bio-asphalt, remains underexplored. The development of biobased innovations, including bio-asphalt, is influenced by the urgency of circular economy implementation, which can hinder progress [3].

The binder utilized in Hot Mix Asphalt (HMA) is critical for ensuring the performance and longevity of the pavement. As the availability of petroleum reserves diminishes, obtaining cost-effective and high-quality asphalt becomes increasingly challenging, thereby fostering a heightened interest in bio-asphalts. These bio-asphalts, which can function as modifiers, extenders, or rejuvenators for aged asphalt, provide both functional advantages and environmental benefits [4].

Numerous investigations have analyzed bio-asphalt and its constituent materials pertinent to bio-asphalt manufacturing [5]. Nonetheless, these bio-asphalts are predominantly employed as modifiers and have yet to entirely supplant petroleum-derived asphalt. The collective effort led by [6] has presented an inventive bio-asphalt formulated with Gondorukem, showcasing an impressive capacity to wholly replace the conventional petroleum asphalt.

Prior investigations suggest that resins and esters significantly influence bitumen properties under aging conditions [7,8] explored gum rosin esterification for industrial stability, though not specifically for asphalt. Ref. [9] highlighted the potential of bio-based binders, particularly vegetable resins, for pavement construction, underscoring the importance of alternative materials.

Gopal, or Gondorukem Asphalt, is a bio-asphalt variant using Gondorukem (gum rosin) as the main component. Gondorukem undergoes transformation into rosin esters (GEG-90 and GEM-130) and is combined with CPO to meet the 2018 Bina Marga specifications [10]. This study rigorously evaluates the chemical, rheological, and mechanical properties of Gopal-GEG90 and Gopal-GEM130 compared to Pen 60 petroleum asphalt, assessing their viability as sustainable road construction alternatives.

2. Gondorukem

Gondorukem, also known as gum rosin or colophony, is derived from the resin of pine trees through a distillation process that removes volatile turpentine components. The resulting solid resin ranges in color from light to dark, depending on its origin and processing method [11]. While Gondorukem is insoluble in water, it dissolves readily in organic solvents like alcohol, acetone, and chloroform, making it valuable across various industrial sectors. Gondorukem consists of approximately 90% acidic components, mainly rosin acids like abietic and pimaric acids, and 10% neutral compounds [8]. These rosin acids are tricyclic diterpenes with reactive double bonds, which render Gondorukem susceptible to oxidation and property changes, particularly under heat exposure [12,13].

The abietic acid content, typically 30–40%, is crucial for its quality, influencing crystallization and solidification rates [8]. Higher concentrations of abietic acid accelerate the crystallization process, causing the rosin to solidify more quickly. To enhance the stability and performance of Gondorukem, chemical processes such as hydrogenation, isomerization, and esterification are utilized to reduce the abietic acid content [14]. Among these, esterification is particularly effective as it improves the thermal and chemical resistance of the resin, making it more suitable for advanced applications like bio-asphalt production. Esterification transforms Gondorukem into rosin ester, a substance with a higher softening point and improved durability [14]. While Gondorukem’s modification through esterification significantly enhances its industrial applicability, the broader context of pine resin utilization highlights its potential in various sectors. Pine forests, a source of Gondorukem, also contribute to carbon sequestration and offer a sustainable alternative to petroleum-derived products, aligning with green chemistry principles.

The properties of rosin ester can be further modified by using alcohols such as glycerol and pentaerythritol, with higher molecular weight esters providing greater flexibility and thermal resistance [15]. Additional additives like malic acid and ethylene glycol are often introduced to fine-tune their properties [11]. Esterified Gondorukem finds wide applications in industries such as perfumery, textiles, and polymers. In this study, it is employed in the formulation of Gopal, a bio-asphalt binder. The enhanced stability of esterified Gondorukem makes it a promising alternative to petroleum-based asphalt for flexible pavement applications, providing both environmental benefits and durability.

3. Crude Palm Oil

Crude Palm Oil (CPO) is the primary product obtained from the processing of palm oil fruit, specifically through the extraction or pressing of the fruit’s flesh (mesocarp). Unlike palm kernel oil (PKO), which is derived from the kernel of the palm fruit and lacks beta carotene, CPO is characterized by its reddish hue, owing to its high beta-carotene content [16]. CPO finds wide applications across various industries, including food, cosmetics, chemicals, and animal feed, owing to its versatile chemical composition. In recent years, its potential as a material for bio-asphalt production has also gained attention, such as [10] utilizing CPO in the production of Gopal (Gondorukem Asphalt), a bio-asphalt binder made from gum rosin and CPO.

The quality of the CPO used in bio-asphalt production plays a critical role in determining the final product’s rheological properties, as outlined by [10]. Two main factors govern the quality of CPO: its color and acidity level, the latter being measured in terms of Free Fatty Acid (FFA) content [17]. While the color of CPO has minimal impact on its functionality in asphalt production, the FFA content is a significant determinant of the resulting performance. High FFA content in CPO can negatively impact the stability and flexibility of bio-asphalt, making it less effective for road construction applications [18]. Ref. [19] concluded that only CPO with an FFA content exceeding 30% is suitable for suboptimal rheological behavior.

This insight underscores the importance of selecting the right CPO for bio-asphalt production, as variations in FFA content can directly affect the flexibility, stability, and overall performance of the final product. The focus on maintaining low FFA levels is essential for ensuring the oil’s stability and effectiveness in various applications, including asphalt. The use of high-FFA CPO in the production of Gopal allows for the development of a bio-asphalt that not only meets the necessary specifications for road construction but also contributes to sustainability by utilizing renewable resources.

4. Methodology

The Gopal preparation involved melting rosin esters, GEG90 or GEM130, at 100 °C and 140 °C, respectively, for 15 min. Crude Palm Oil (CPO) with over 30% FFA was subsequently integrated into the mixture. The weight ratios of rosin ester to CPO were established at 82.5:17.5 for GEG90 and 83.5:16.5 for GEM130. During heating at a constant temperature, the continuous stirring of the mixtures at 5000 rpm for 10–15 min ensured homogeneity. The products, devoid of solid residues, were labeled Gopal-GEG90 and Gopal-GEM130.

Prior to this process, Gondorukem underwent glycerol esterification at 250 °C for 6 h, maintaining glycerol to resin ratio of 1:3, and malleic esterification at 150 °C for 1 h with a ratio of 1:1, yielding GEG90 (Glycerol Esterified Gondorukem, with a softening point of 90 °C) and GEM130 (Melic Esterified Gondorukem, with a softening point of 130 °C).

The chemical characterization of Gopal-GEG90 and Gopal-GEM130 involved FTIR spectroscopy for functional group identification and EDX analysis for elemental composition. FTIR was conducted at a spectral resolution of 4 cm⁻¹ and EDX analysis achieved a detection limit of 0.1% for major elements, with comparisons drawn to Pen 60 petroleum asphalt as a control.

Rheological properties such as penetration and softening point viscosity were assessed through standard methods, and mechanical properties via Dynamic Shear Rheometer (DSR) and Multiple Stress Creep Recovery (MSCR) tests [19]. DSR tests spanned various temperatures with stress measurements noted to an accuracy of ±0.01 kPa, while MSCR tests evaluated rutting resistance and elastic recovery under varying stress conditions, adhering to [5] standards.

Calibration of all instruments preceded testing to ensure measurement accuracy and precision. Each test was repeated at least three times for result validation, with standard errors (±SE) incorporated into data tables to reflect measurement variability.

For comparative purposes, identical testing protocols were applied to Pen 60 petroleum asphalt, establishing a control baseline for the performance evaluation of variants.

5. Results and Discussion

5.1. Chemical Analysis

The FTIR (Fourier Transform Infra-Red) analysis revealed notable differences between Gopal-GEG90, Gopal-GEM130, and Pen 60 petroleum asphalt, as shown in Figure 1. Both Gopal variants exhibit additional peaks that are not present in Pen 60, primarily due to the esterification process. These peaks, especially the C=O stretch at 1227 cm⁻¹, indicate the presence of ester groups in Gopal, which are absent in petroleum-based asphalt. The presence of esters in Gopal enhances its adhesive properties and improves stripping resistance, making it more effective in maintaining aggregate-binder adhesion under moisture exposure.

In

Table 1. Wavelength and Transmittance Peak of Gopal and Oil Asphalt Pen 60.

No	Wavelength (cm−1)
	Gopal-GEG90 ± SD	Gopal-GEM130 ± SD	Oil Asphalt Pen 60 ± SD
1	721.12	720.60	720.94
2	921.48	808.39	808.39
3	946.20	946.20	946.20
4	1089.97	1089.97	−
5	1135.59	1135.59	−
6	1227.21	−	−
7	1384.66	1375.34	1375.34
8	1457.02	1456.61	1455.61
9	1579.95	1579.95	1579.95
10	1727.45	1727.45	1727.45
11	1779.26	1782.26	1782.26
12	2850.47	2849.11	2849.11
13	2920.29	2917.49	2917.49
14	3405.29	−	−

, the wavelength positions of the major peaks observed in the FTIR spectra are listed. These peaks correspond to specific chemical bonds that contribute to the mechanical and rheological properties of the binders.

The data in Table 1 show that Gopal-GEG90 and Gopal-GEM130 share many peaks with Pen 60, indicating a significant chemical similarity (approximately 86%). However, the unique C=O stretch peak at 1227 cm⁻¹ in Gopal variants reflects the esterification process, which leads to enhanced moisture resistance and improved stripping resistance, as discussed earlier.

In addition, Gopal variants exhibit additional peaks in the 1000–1300 cm⁻¹ range related to CO stretches, which are absent in Pen 60. These peaks suggest a higher concentration of light oil fractions in Gopal, contributing to its superior flexibility.

EDX (Energy Dispersive X-ray) analysis, summarized in

Table 2. RDX Test Results of Gopal and Oil Asphalt Pen 60 Chemical Elements.

Chemical Elements	Asphalt Types
	Gopal-GEG90	Gopal-GEM130	Oil Asphalt Pen 60
C	74.68	71.33	61.52
O	18.73	25.37	11.45
Na	1.09	−	0.40
Mg	0.99	0.46	2.85
Al	1.19	1.20	0.39
Si	1.06	1.64	0.56
S	−	−	11.48
Ca	0.88	−	10.91
Ti	0.56	−	0.45

, provides further insights into the elemental composition of Gopal variants compared to Pen 60.

The EDX results show that both Gopal-GEG90 and Gopal-GEM130 have carbon (C) and oxygen (O) as their primary elements, similar to Pen 60. However, Gopal variants lack sulfur (S), which is present in Pen 60, making Gopal less toxic and more environmentally friendly. Moreover, Gopal-GEM130 is less alkaline than Gopal-GEG90 due to the absence of sodium (Na), which could lead to better stability in certain conditions.

5.2. Rheological Analysis

The rheological properties of Gopal variants were assessed using the Dynamic Shear Rheometer (DSR) and Multiple Stress Creep Recovery (MSCR) tests, with Pen 60 petroleum asphalt serving as the control. As shown in

Table 3. Rheological properties of Gopal made from Gondorukem with CPO diluent.

No	Type of Tests	Test Results			Specification
		Gopal-GEG90	Gopal-GEM130	Oil Asphalt Pen 60	Oil Asphalt Pen 60	PG 70	PG 76
1	Penetration at 25 °C, 100 g, 5 s (dmm)	60	63	60.1	60–70	Reported	Reported
2	Kinematic Viscosity at 135 °C (cSt)	330	2045	445	≥300	≤3000	≤3000
3	Softening Point (°C)	49	49	48.3	≥48	Reported	Reported
4	Ductility at 25 °C, 5 cm/s (cm)	140	>140	>140	≥100	-	-
5	Flash Point (COC) (°C)	242	237	233	≥232	≥230	≥230
6	Solubility in CHCl (%)	99.3	99.9	99.7	≥99	≥99	≥99
7	Specific Gravity	1.070	1.065	1.093	≥1.0	-	-
8	Paraffin Content (%)	0.033	0	0	≤ 2	-	-
9	Storage Stability (Softening Point Difference) (°C)	-	-	-	-	-	≤2.2
Residue Testing after TFOT at Temperature 163 °C, 5 h
10	Loss on Heating (%)	0.8	0.7	0.005	≤0.8	≤0.8	≤0.8
11	Penetration at 25 °C, 100 g, 5 s (% initial)	64	73	72	≥54	≥54	≥54
12	Ductility at 25 °C, 5 cm/s (cm)	140	>140	>140	≥50	≥50	≥25

, both Gopal-GEG90 and Gopal-GEM130 meet the requirements outlined in the General Specifications for Bina Marga Rev-2 of 2017 [20] for use as asphalt binders [21].

The results in Table 3 show that Gopal-GEG90 has a lower viscosity (330 cSt) than Pen 60, indicating better workability at lower temperatures. On the other hand, Gopal-GEM130 has a significantly higher viscosity (2045 cSt), which may improve its performance in high-temperature applications but could present challenges during handling. Both Gopal variants exhibit slightly better softening points than Pen 60, suggesting better resistance to deformation under heat.

5.3. Stripping Resistance

The stripping resistance of an asphalt binder is crucial for its ability to maintain adhesion to aggregate surfaces in the presence of moisture [22]. In this study, a boiling test was conducted to simulate the moisture conditions pavements experience over time by immersing the aggregate-binder samples in water at elevated temperatures. As shown in Figure 2, Gopal-GEM130 demonstrated superior stripping resistance compared to both Gopal-GEG90 and Pen 60 petroleum asphalt. This higher performance is attributed to the elevated ester content in Gopal-GEM130, which enhances its adhesive properties, even under moisture exposure.

Gopal-GEG90, although it performed slightly worse than Gopal-GEM130, still exhibited better stripping resistance than Pen 60 petroleum asphalt. The lower ester content in Gopal-GEG90 compared to Gopal-GEM130 explains its slightly reduced water resistance. However, the presence of ester groups still provides Gopal-GEG90 with a significant advantage over Pen 60. In contrast, Pen 60 petroleum asphalt displayed the lowest stripping resistance, which is a characteristic of petroleum-based binders due to their lack of ester bonds. This deficiency leads to poor adhesion in moist environments, causing early pavement failures such as stripping, raveling, and potholing.

Gopal-GEM130, with its superior resistance to moisture-induced stripping, is an excellent option for regions with high rainfall or humid climates. Its ability to maintain strong adhesion to aggregates under wet conditions can significantly extend pavement lifespan, reduce maintenance costs, and improve overall road performance in moisture-prone areas. By retaining better aggregate–binder bonding than both Gopal-GEG90 and Pen 60, Gopal-GEM130 proves to be a more durable and sustainable alternative for road construction.

5.4. Mechanical Analysis

The mechanical properties of the asphalt binders were assessed using Dynamic Shear Rheometer (DSR) testing, focusing on rutting and fatigue resistance. The rutting factor (G/sin δ)* measures the binder’s ability to resist permanent deformation under heavy traffic loads and high temperatures.

Table 4. Rutting Factors of Gopal and Pen 60 Oil Asphalt.

Parameter		G*/sin δ (kPa) Test Samples
		Original Binder			After RTFOT
		Gopal		Oil Asphalt Pen 60	Gopal		Oil Asphalt Pen 60
		GEG90	GEM130		GEG90	GEM130
Temp (°C)	64	1099	2114	1.379	2499	3279	2267
	70	0.486 (Fail)	1024	0.642 (Fail)	1020 (Fail)	1436 (Fail)	1038 (Fail)
	76	−	0.494 (Fail)	−	−	−	−
P.G		−	70	64	−	64	64
Specification		≥1000 kPa			≥2200 kPa

shows that Gopal-GEM130 consistently exhibited higher rutting resistance compared to Gopal-GEG90 and Pen 60 petroleum asphalt, particularly at 64 °C and 70 °C. This indicates that Gopal-GEM130 is more suitable for high-temperature environments where deformation risks are higher. In contrast, Gopal-GEG90 met the required rutting factor at 64 °C but failed at 70 °C, signaling lower performance at elevated temperatures.

The fatigue factor (G*sin δ), which evaluates resistance to cracking, was also analyzed, and the results are presented in

Table 5. Fatigue Gopal Factors and Pen 60 Oil Asphalt.

Parameter		G*Sin δ (kPa) Test Samples After PAV
		Gopal		Oil Asphalt Pen 60
		GEG90	GEM130
Temperature (°C)	31	9397 (Fail)	1887	1783
	28	−	3294	2663
	25	−	6223 (Fail)	3790
	22	−	-	5156 (Fair)
Specification		≤5000 kPa

. Gopal-GEG90 exhibited significantly higher fatigue factors, indicating a greater susceptibility to cracking over time, especially at 31 °C. Gopal-GEM130, on the other hand, showed better fatigue resistance, with values closer to those of Pen 60 petroleum asphalt, suggesting that Gopal-GEM130 can perform more reliably under repeated loading conditions. Despite this, Pen 60 petroleum asphalt remains the most reliable binder for fatigue resistance, particularly in colder regions where fatigue cracking is more of a concern.

5.5. Deformation Resistance

The Multiple Stress Creep Recovery (MSCR) test was conducted to evaluate the binders’ ability to recover from deformation after repeated stress.

Table 6. MSCR Gopal Test Results and Pen 60 Oil Asphalt.

Parameter		After RTFOT Test Samples
		Gopal		Oil Asphalt Pen 60	Gopal		Oil Asphalt Pen 60
		GEG90	GEM130		GEG90	GEM130
Temp (°C)	64	4235	3211	3.94	3.4	9.8	10.2
Performance Grade (PG)		64S	64S	64S	Pass	Pass	Pass
Traffic Levels	Standard (S)	Max. 4.5 kPa−1			Max. 75%
	Heavy (H)	Max. 2.0 kPa−1			Max. 75%
	Very Heavy (V)	Max. 1.0 kPa−1			Max. 75%
	Extremely Heavy (E)	Max. 0.5 kPa−1			Max. 75%

presents the Jnr3.2 values, with lower values indicating better resistance to permanent deformation. The results show that Gopal-GEM130 exhibited the best deformation resistance, outperforming both Gopal-GEG90 and Pen 60. This makes Gopal-GEM130 a suitable candidate for pavements exposed to heavy traffic or high-stress environments, as it can effectively recover from deformation. Although Gopal-GEG90 performed better than Pen 60, it still lagged behind Gopal-GEM130, emphasizing the superior mechanical properties of the latter.

Gopal-GEM130’s high rutting and deformation resistance, combined with its superior stripping resistance, make it an ideal bio-asphalt binder for use in high-temperature, high-traffic, and moisture-prone regions. In contrast, Gopal-GEG90, while offering good moisture resistance, requires further improvement in mechanical properties such as fatigue and deformation recovery to match the performance of Gopal-GEM130 and Pen 60 petroleum asphalt. The ability of Gopal-GEM130 to recover from stress-induced deformation highlights its potential as a long-term, sustainable alternative to petroleum-based binders in demanding applications.

5.6. Cost Analysis

The economic assessment of Gopal bio-asphalt, derived from Gondorukem and crude palm oil, includes factors such as raw material costs, energy use, and production techniques. Gondorukem, sourced from pine resin, is readily available in Southeast Asia including Indonesia, offering a renewable resource and a potentially economical substitute for petroleum-based asphalt, though its price fluctuates due to supply chain stability and market demand.

CPO, a vital element of Gopal bio-asphalt, is a renewable resource with stable production in Indonesia and Malaysia, but refining processes necessary to meet bio-asphalt specifications significantly impact production costs.

Energy costs from Gondorukem esterification and bio-asphalt production processes, particularly oven heating, significantly affect total expenses, with heating potentially accounting for 40–60% of energy usage [23]. Despite the environmental benefits of bio-asphalt, including a reduced carbon footprint, its economic feasibility compared to traditional asphalt depends on balancing material costs with long-term performance advantages. A comprehensive analysis of life-cycle costs, including maintenance and durability of Gopal-based pavements, is essential for evaluating its broader economic viability.

6. Conclusions

6.1. Chemical Performance

The study shows that both Gopal-GEG90 and Gopal-GEM130 demonstrate an 86% chemical similarity to Pen 60 petroleum asphalt, highlighting their potential as bio-based alternatives. Gopal-GEM130 is less toxic and less alkaline, making it an environmentally friendly option with a lower potential impact than petroleum asphalt. Gopal-GEG90, although more alkaline, is also less toxic than Pen 60, which aligns with sustainability goals in road construction. The presence of ester groups in both Gopal variants contributes to their enhanced moisture resistance, an important factor for improving pavement durability in wet environments.

6.2. Mechanical Performance

In terms of mechanical properties, Gopal-GEG90 showed 10% better stripping resistance than Pen 60, a significant advantage in moisture-prone areas. However, its 12% lower aging resistance compared to Pen 60 highlights the need for further improvements to increase its longevity. Gopal-GEM130, on the other hand, exhibited superior rutting resistance, making it a better candidate for use in high-traffic roads or regions with extreme temperatures and heavy vehicle loads. Despite these advantages, both Gopal variants showed lower fatigue resistance and faster aging than Pen 60, which suggests the need for enhancements in these areas to improve long-term performance.

6.3. Practical Implications and Future Work

Continued investigation is necessary to validate the long-term sustainability of Gopal variants as alternatives to petroleum asphalt, despite their shared performance grade (PG64(S)) with Pen 60. Assessing of field durability of hot mix using Gopal is crucial for understanding their performance under varying environmental conditions, such as extreme temperatures, freeze–thaw cycles, and heavy traffic.

Gondorukem (gum rosin) and CPO, are the significant components of Gopal bio-asphalt, which is a renewable resource with relatively stable production rates in regions like Indonesia and Malaysia. The costs associated with Gondorukem and CPO, alongside the energy-intensive esterification and mixing processes, will substantially impact production scalability. The economic evaluation of Gopal as a bio-asphalt, sourced from Gondorukem and CPO, encompasses various factors such as raw material costs, energy use, and manufacturing methods. Gondorukem, derived from the pine tree, is abundantly available in Southeast Asia, presenting a potentially economical substitute for petroleum-based asphalt. Nonetheless, its pricing remains susceptible to fluctuations dictated by supply chain dynamics and market demand. Future research should include a comprehensive life-cycle cost analysis, balancing initial production costs against the long-term performance and sustainability benefits of renewable materials in road construction, especially as a binder for hot mix asphalt. Analyzing the resilience and microstructural changes in these bio-asphalts, as well as their cost-effectiveness, will enhance understanding of their industrial significance.