1. Introduction
Natural rubber (NR) is an essential raw material in several products such as medical devices, gloves, aircraft tires and other engineering products [
1]. Because of its distinctive properties, e.g., resilience, elasticity, abrasion and impact resistance, efficient heat dispersion and malleability at cold temperatures, NR is barely able to be substituted by synthetic rubber in imperative applications [
2]. Therefore, despite the availability of synthetic rubber made from petroleum, NR yet shares 40% of the world’s total rubber demand [
2,
3]. NR is mainly produced from the rubber tree Hevea brasiliensis, with a global annual supply of more than 12 million tonnes [
3,
4]. Southeast Asia is the main production region, providing more than 90% of the annual supply mentioned above; the main production countries are Thailand, Malaysia and Indonesia [
1].
Thailand was the world-leading NR producer in 2015, providing approximately 4.5 million tonnes and exporting about 86% of its NR production [
3] in four different forms: compound rubber, concentrated latex, block rubber and ribbed smoked sheet. More than 60% of the NR is processed into block rubber or rubber sheets to reduce the moisture content before export using two major conventional processes: direct solar drying and hot air drying [
5]. Direct solar drying is employed by mostly smallholder NR producers in the open air, which takes several days and is environmentally dependent [
6], especially in Southeast Asia where the ambient humidity is relatively high and with frequent rainfall. As a result, the quality of the direct solar dried NR sheets is compromised by fungus, which grows easily on rubber sheets due to the uncontrollable drying conditions, high relative humidity and warm temperature [
5,
7].
Energy used for the hot air dryers in the NR industry, especially for block rubber drying, is mainly supplied from non-renewable resources, e.g., diesel and liquid petroleum gas (LPG), resulting in considerable amounts of greenhouse gas emissions [
8]. For ribbed smoked sheets, biomass, e.g., wood, is the main energy source for the drying process [
9,
10]. As the TSR block rubber shares the highest fraction in the NR market, its drying process, which consumes fossil fuel as a primary energy source, is responsible for the principle greenhouse gas emissions in the NR industry, with about 306 kg CO
2-eq/ton product, compared to 143 and 20 kg CO
2-eq/ton product for concentrated latex and ribbed smoked sheet, respectively, in Thailand [
8]. With about 42% of block rubber production in Thailand in 2015, approximately 1.89 million tonnes, around 0.58 million tonnes of CO
2-eq per year can possibly be released into the atmosphere during pre-processing before being internationally exported or domestically used. For illustration, the CO
2-eq emission in Thailand for block rubber pre-processing in 2015 was almost 30% of the annual CO
2-eq emission of the entire country of Lao PDR in 2014. To follow the sustainable development plan from the UN, the drying process for NR should be independent from fossil fuels and more renewable resources, such as solar energy, should be increasingly utilized.
Despite the free solar energy resource used for drying rubber sheets by smallholder NR producers, the uncontrollable quality of the direct solar-dried rubber sheets is not preferred in the NR market. In preference, the technically specified rubber (TSR) grades as the block rubber form, e.g., TSR10 and TSR20, shares around 70% of the world NR market [
7]. Plasticity Retention Index (PRI), Wallace initial plasticity (P
0), and volatile matter (VM) content are the three main parameters used to indicate NR grades for TSR block rubber. The main volatile matter in NR is moisture and minimal volatile organic compounds (VOCs). With standard grades used to specify the TSR, a hot air drier is widely used in industrial-scale NR production as temperature and humidity are controllable in a drying chamber leading to higher quality NR that meets the standard and shortens the drying time.
Although the NR drying process releases significant amounts of greenhouse gas annually, there are currently very limited pieces of research that propose new sustainable ways for the NR drying process. Sonthikun et al. [
10] used a solar-biomass hybrid dryer to dry NR sheets where solar radiation passed through a transparent roof to heat the air and rubber sheets during the daytime and a biomass furnace was used to maintain the temperature of the drying chamber when solar irradiance was insufficient. The hybrid dryer significantly reduced the drying time from several days when using a conventional approach to only 48 h, with a moisture content of less than 0.34% and decreased biomass consumption during the drying process. Mei, Yong-Zhou, Guang, and Xiao-Ping [
11] compared NR granules dried using a 115 °C hot air and a microwave. The results revealed that microwave drying took only 13.47 min, compared to 210 min using hot air drying, to reduce the moisture content to 0.8%, which is the maximum amount of moisture content using the Chinese national standard. Moreover, the initial plasticity (P
0) and plasticity retention index (PRI) of the microwave-dried NR are higher than that of NR dried using hot air; therefore, the microwave was preferable for NR drying in this research.
Solar energy is a promising source of clean energy for several applications, especially at locations located close to the equator with high annual solar irradiance. For example, a typical solar rooftop or solar farm projects for electricity production, or a specific application, such as a solar pump in an agricultural area without an irrigation system [
12]. However, those solar energy usage applications are mainly utilized for loads that consume electrical energy. If solar energy is preferred for other applications, which do not consume electrical energy as a primary source, it may not be economically applicable. For example, in a rubber drying industry, if conventional hot air drying is supplied by grid electricity through an air heater, the energy cost may be significantly higher than when LPG is used. Therefore, to promote the use of solar energy in the rubber drying industry, an innovative process and technology should be proposed.
It is evident that detailed studies of using microwave technology for NR drying is very limited. Moreover, the temperature response of each type of block rubber as a function of drying time has never been studied although this is crucial information to be able to control the power of the microwave at an appropriate level. Without the temperature responses studied in this research, that are associated with microwave power levels, there is no promising evidence to develop more efficient microwave technology for the rubber drying industry. Moreover, there is no study related to a solar PV system design that can supply the electrical energy for microwave technology currently. Without studying the required PV installation capacity, policymakers may not be able to release an effective subsidizing policy to promote the use of solar PV technology in the rubber drying industry and electricity from the grid might be used, which is not sustainable as grid electricity is still mainly produced from fossil fuel sources. Therefore, the study of how to control microwave power to maintain NR temperatures with optimum power usage in this article is novel.
The main objective of this research is to study the possibility of using a more sustainable way to dry NR, focusing on industrial block rubber that has the highest share in the world NR market. Solar energy will be the only energy source used for the drying process, which is converted into two different forms of energy: electricity, which is used to supply the magnetron for microwave drying, combined with thermal energy, which is used to heat the ambient air to the desired temperature level. Therefore, the air-type hybrid Photovoltaic/thermal (PVT) collector is recommended for future NR drying processes to convert solar energy to supply the NR drying industry. The study is expected to reveal the optimum operating microwave power required to successfully dry NR with the solar energy supply available in Thai weather conditions. The novel information revealed by this study suggests the total installation PV power that is required from PV technology to sufficiently supply clean energy for the microwave technology for the block rubber drying industry, which may lead to a sustainable NR industry in Thailand with minimal greenhouse gas emissions in the near future.
4. Results and Discussion
4.1. Temperature Responses of the Microwave-Dried STR Rubbers
The temperature responses of the STR5L block rubber under power-controlled microwave drying of 420, 500, 560 and 650 W with the drying time ranges from 0 to 160 min are presented in
Figure 6. The above test results show that during the first 20 min of the rubber drying process, the temperatures of the rubber increased rapidly both at the center (75–105 °C) and at the surface (75–95 °C) depending on supplied power. After 40 min of drying, when 420 W and 500 W of power were used, the temperature responses were almost identical both at the center and at the surface of the samples. When 560 W of power was used, the center of the rubber had a slightly higher temperature compared with the cases using lower power, while the surface temperature response was roughly the same compared with when 420 W and 500 W of power were used. However, after 60 min, the surface temperatures in the case of the 560 W samples were slightly higher than the other two cases using the lower power supply, although the center temperatures increased with a similar pattern. The temperature responses at both the center and surface of the samples approached specific saturated temperatures associated with a supplied power that was less than the 560 W cases. Surface temperatures were at the saturation values of 105, 107, 112 and 125 °C when 420, 500, 560, and 650 W of power was used, respectively. When 650 W of power was used, the center temperature raised to over 150 °C, which led to the degradation of the rubber and which will be mentioned later.
Considering the weight loss of the rubber samples, used as a drying-level indicator, it was found that during the first 60 min, the weight dropped rapidly due to the early evaporation of water inside the samples. After that, the weight continued to drop to points where the weight was almost constant. For the STR 5L, an average of approximately 24% reduction in the samples’ weight was achieved as shown in
Figure 7. As expected, it was found that when higher power was used, drying time can be reduced. When 420, 500, and 560 W of power were used, the drying time should be 160, 140, and 100 min to meet the standard. When using 650 W of power, the weight reduced to 24% within only 60 min, however, the rubber did not meet the standard due to degradation at the center caused from the extensively high temperature.
Considering the other types of block rubber, STR20, the same levels of microwave power including 420, 500, 560 and 650 W were used to investigate temperature responses and drying time ranges from 0 to 160 min. The temperature responses are illustrated in
Figure 8 and the relationship between the drying time and percent weight loss is presented in
Figure 9. The test results for STR20 also show similar temperature trends compared with the STR 5L; however, the temperature responses of STR20 were slightly higher than the STR5L at the same power levels. For example, the surface and center temperatures of the STR20 samples were higher than that of STR5L for approximately 5 °C. The temperature responses of the STR20 samples at the surface approached the specific saturated temperatures associated with supplied power at less than the 560 W cases. Surface temperatures were at their saturation values of 102, 105 and 110 °C when 420, 500 and 560 W of power were used, respectively. There is no report on the saturation temperature of the samples when 650 W of power was used as the sample test does not meet the STR20 standard due to rubber melting at 80 min drying time and with the center temperature raising to over 155 °C. Unlike the STR5L samples, the temperatures at the center of the STR20 samples do not converge at the drying time of 160 min when the moisture content approached a specific value, which can be seen from a percent weight loss of approximately 32% in
Figure 9.
4.2. STR Rubbers’ Properties from the Microwave Drying Process
The STR5L and STR20 samples under different drying times and microwave power levels were tested against their standards. Volatile matter was tested to indicate the moisture content in the block rubber after the drying process. For STR5L and STR20, the volatile matter should be lower than 0.8% by weight; however, the preferred value after the drying process should be lower than 0.5% by weight to allow for hygroscopicity during storage and transportation times. Different parts of the samples were tested for volatile matter and the results are presented in
Table 5. Note that the STR20 contains several types of raw materials compared with the STR5L including cup lump, rubber bark, coagulated rubber, etc., which made the raw materials of STR20 contain high levels of gel resulting from their partially crosslinking prior to the drying process leading to a higher moisture content in the STR20 at the initial drying time than the STR5L, as illustrated in
Figure 7 compared to
Figure 9. However, by using a drying time of more than 140 min at every experiment with a power less than 640 W, the volatile contents from all samples of both STR5L and STR20 meet the standard as presented in
Table 6.
Table 5 also shows that using 420 W of power for 160 min for the STR 5L drying of tires is not enough to dry the samples because the averaged volatile matter content is still higher than the specified value of 0.5 %wt, especially at the surface and middle layers of the rubber, which corresponded to the physiological characteristics of cloudy white granules scattered throughout the sample at the surface and middle layers. This characteristic reflects that there is still high residual moisture in the outer thin layer of the rubber lump. Therefore, when using 420 W of power, 180 min drying time should be performed to thoroughly dry the STR 5L. For STR20, at 420 W power, 160 min drying time is adequate to obtain the 0.5% averaged volatile content although the volatile content at the sample surface is a bit higher than the preferred value but still meets the standard.
When 500 W of power were used, the volatile content in both the STR 5L and STR20 samples tended to decrease faster in all areas, but especially at the center of the samples. This reflected how the microwave power can penetrate through to the deep part of the rubber lump, and then the heated moisture in the rubber is transferred to the outside layer, which results in the outside layer drying more slowly. Therefore, if white spots appear scattered over about 10% of the surface area, this small amount of residual moisture can be removed by further drying at low microwave power for 20 min more. With 500 W microwave power, the STR5L samples need 160 min drying time, while the STR20 samples required only 100 min to meet the standard.
When 560 W of microwave power were used, it was found that only 100 min would make the volatile matter content reach the preferred value of 0.5 %wt for both STR5L and STR20. However, the volatile content at the surface layer was still higher than the standard when it was quite low at the center. We suggest stopping the drying process at this point and letting the moisture diffuse throughout the sample to conserve energy, or a short-term (10 min) low-power annealing can be performed to completely remove the surface moisture. The high-power microwave at 640 W was also tested; however, it was found that the short drying time of 80 min could cause the center of the samples to turn brown while the volatile content was still high, which results in the enzymes in the rubber molecules being oxidized. This reflects how using microwaves at such power levels to dry blocked rubber is inappropriate, as it may lead to deterioration corresponding with high temperatures of 140–150 °C.
Subsequently, the Initial Flexibility (P0), Flexibility Index (PRI) and Mooney Viscosity (ML 1 + 4 @ 100 °C) were tested for samples which met the volatile content standard to indicate the plasticity property of rubber and classify the quality of blocked rubber (Rubber is of good quality if it has high flow resistance capability). It was found that when microwave power of 420, 500 and 560 W were used for the STR5L samples, the P0 values ranged from 39–46 units, which is higher than the STR 5L standard (higher than 35 units for STR5L). When 640 W of power was used, some parts of the samples that turned a brown colour gave a P0 of only 16–31 units, which fails the STR5L standard. This data indicates that the rubber molecules in this area underwent oxidation due to a high temperature rise from the high microwave power. For the STR20 samples, at 420 W for 140–160 min and 500 W for 120 min, the dried samples had a P0 value in the range of 32–35 units, which meets the STR20 standard of a P0 of not less than 30 units. However, when longer time was used at 500 W of power (140–160 min), the P0 was in the range of 15–35 units and when 640 W of power was used, the P0 was even less than 15 units, which recommends that those drying conditions should not be performed.
The PRI value was also tested, and the results indicated that the microwave-dried rubber samples at all conditions met the STR5L and STR20 standards as shown in
Table 6, which ensures that all samples satisfy the plasticity property. For the Mooney viscosity, the results from the STR5L samples ranged between 79–82 units when 420, 500 and 560 W of power were used, and the Mooney viscosity was 76 units when 640 W of power was used. For the STR20 samples, the Mooney viscosity ranged from 66–77 units when 420, 500 and 560 W of power were used, which confirms that the properties of the STR 5L and STR20 samples under the microwave-drying process met their standards.
The samples were also tested for other properties as shown in
Table 6 from the testing laboratory of the Rubber Plantation Organization in Nakhon Si Thammarat Province, which is a laboratory licensed by the Department of Agriculture for Thai block rubber standards. It was found that the samples met all the requirements of the STR 5L and STR20 block rubber standards.
Interestingly, note that the STR5L samples dried with the microwave had a Lovibond Scale of 2, which was much better than the standard (standard value is no more than 6 Lovibond Scale); that is, the microwave-dried STR5L rubber has a very light color, even lower than the higher-grade block rubber of STR-XL of no more than 4 Lovibond Scale. This is likely due to the shorter time the rubber is heated compared with the normal drying process using hot air. Therefore, the microwave drying process of block rubber can enhance the quality class of block rubber, which adds value to the blocked rubber as well.
In addition, the microwave-dried samples were examined for their chemical structure by using the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) technique and compared to commercial STR 5L (in
Figure 10) and STR20 (in
Figure 11) block rubbers.
Figure 10 shows that the spectra of the samples (both surface and center parts) were not different compared to the commercial STR 5L using the hot air drying method. This indicates that they have the same chemical structure, no significant changes in the chemical structure were observed in the NR molecules (in the wavenumber range 3100–500 cm
−1). For the STR20 in
Figure 11, the spectra of the samples (both surface and center parts) were also not different compared to the commercial STR 20 using the hot air drying method, with the wavenumber range from 2000–500 cm
−1.
4.3. Cleaner Drying Potential by Using Solar Energy
4.3.1. Electrical Energy Usage for Drying One Kilogram of STR5L and STR20 Using Microwave Technology
A power meter was installed to measure the power used for drying the STR5L and STR20 samples. With successful cases for drying the STR5L and STR20 samples when 500 W of power for 140 min was used for the STR5L samples, and 500 W of power for 120 min was used for the STR20 samples, the total energy used for drying each type of block rubber could be calculated. For the STR5L, 1.08 kWh of energy was required to dry 1 kg of rubber while only 0.81 kWh of energy was consumed for successfully drying the STR20 sample of 1 kg. In Thailand, if electricity costs of 0.07 USD/kWh was approximated, the cost of drying 1 kg of STR5L and STR20 are 0.0756 USD and 0.0567 USD, respectively. Compared to conventional hot air drying, 4–6 h is required with roughly 30 kg of LPG to dry 500 kg rubber. With the cost of 0.715 USD/kg of LPG in Thailand, drying 1 kg of STR5L and STR20 costs approximately 0.0429 USD, which is cheaper than drying using microwave power. However, if the electricity is from solar panels installed on the rooftop of the drying factory, the cost of the electricity may be reduced as the solar energy resource is free and carbon credits maybe claimed from clean production processes.
4.3.2. Potential to Reduce CO2 Emissions from the STR5L and STR20 Rubber Drying Process in Thailand
Considering that LPG contains approximately 82 %wt of carbon and complete combustion is assumed, 1 kg of LPG contains 0.82 kg of carbon, may be completely combusted with 2.187 kg of O
2 and emits CO
2 for approximately 3 kg. Therefore, if conventional hot air drying was performed for 1 kg of raw natural block rubber, 0.06 kg of LPG is required and 0.18 kg of CO
2 is emitted. In 2020, approximately 1,676,073 metric tonnes of STR type rubber were produced in Thailand [
25], which potentially emits roughly 300,000 metric tonnes of CO
2 annually. If microwave technology with a solar energy supply is used, 300,000 tons of CO
2 per year can be eliminated in the STR rubber drying industry in Thailand.
4.3.3. Required PV Panels Installation Area for STR Rubber Drying Industry in Thailand
To make sure that the required area for solar panel installation is obtained, the electrical energy used for drying the SRT20 rubber is considered as it consumes more energy to be dried to meet the standard. To dry a kg of STR20, 0.81 kWh of electrical energy is required, therefore, for the annual STR rubber production in 2020 in Thailand of 1,676,073 metric tonnes, electrical energy of 1,357,619 MW h is demanded. By using the electrical energy production data from the 0.99 MW PV installation site for one month, we can validate the electrical production for a year by using available irradiance, ambient temperature, and wind speed data from the closest weather station in the studied area.
Figure 12 shows that the calculated power output from the mathematical model in this paper exceptionally matched the measured power production from the studied installation site both on a sunny day with intermittent cloud on 20 June 2022 and a cloudy day on 15 June 2022. The calculated energy production on 20 June 2022 was 5.115 MW h, compared with the measured energy production of 5.135 MW h with an error of 0.39%. For the cloudy day on 15 June 2022, the calculated energy production was 2.642 MW h while the measured energy production was 2.681 MW h with an error of −1.45%. Therefore, it can be ensured that the mathematical model for calculating the electrical power production in Thailand is accurate; thus, this paper used the data of irradiance, ambient temperature, and wind speed available from June 2021 until May 2022 to calculate the energy production in the south of Thailand as presented in
Figure 13. According to the potential to produce 1217.7 MW h per year from the 0.99 MW PV installation site in the south of Thailand, with the required electricity of 1,357,619 MW h, there should be approximately 1115 MW of PV installation for drying STR rubber annually in Thailand. It should be noted that the efficiency of the PV system reduces linearly, and PV technology is expected to work for 25 years with its power output gradually decreasing to around 80% to 85%. Moreover, some panels may degrade more rapidly than expected due to the ambient conditions such as a location close to the sea or in high temperature and high humidity regions [
26]. Those factors should be considered if precise economic study is required.
5. Conclusions
This article explores the potential for utilizing microwave energy technology with electricity generated by solar photovoltaic technology to reduce CO2 emissions from conventional hot air drying using LPG in the Standard Thai Rubber (STR) drying industry. Commercial microwave ovens were modified and integrated using designed power control systems for microwave emitting to maintain the proper temperature levels of rubber bulk to evaporate moisture.
The modified microwave oven was used to dry the STR5L and STR20 block rubber with a sample size diameter of 100–120 mm (roughly 0.4 kg) for thorough microwave transmittance. The temperature response of microwave-dried samples were monitored at the surface and at the center to ensure that the drying process with controlled powers do not degrade the rubber standards.
In the case of the STR 5L samples, the drying results suggested that using a microwave power of 500 W for 140 min is the most suitable power and time because the samples passed the rubber testing standard for STR5L with the lowest possible energy usage. Using lower microwave power with a longer time resulted in higher energy consumption, which is not preferred, and using higher microwave power leads to rubber degradation. For STR20, the 500 W of power for 120 min is favorable with the lowest energy usage for drying. The power and time suggested above can maintain the temperature of both the inside and surface of the STR rubber samples at or below 150 °C, which maintains the rubber properties to those when using conventional hot air drying processes, when testing with the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) technique. The volatile matter contents of the STR5L and STR20 samples were not more than 0.5%, which is preferred. This article explores the potential for utilizing microwave energy technology with electricity generated by solar photovoltaic technology to reduce CO2 emissions in the Standard Thai Rubber (STR) drying industry.
With the drying conditions suggested from the results, if electrical energy is supplied by solar energy, which is a sustainable energy resource, 300,000 tons of CO2 per year can potentially be eliminated in the STR rubber drying industry in Thailand as LPG is no longer required. Therefore, moving from the conventional hot air drying process to a microwave drying process is recommended. Given that the 0.99 MW PV installation site in southern Thailand has the capacity to produce 1217.7 MW h yearly and that Thailand needs 1,357,619 MW h of power each year to dry STR rubber, there should be roughly 1115 MW of PV installation required to eliminate conventional drying process in the rubber drying industry in Thailand. However, a subsidization policy may be necessary to attract rubber drying factories to invest in the proposed microwave drying technology.