Cleaner Potential for Natural Rubber Drying Process Using Microwave Technology Powered by Solar Energy

Abstract

To reduce carbon dioxide emissions from traditional drying methods, this research investigated the use of microwave technology for drying Standard Thai Rubber (STR) in Thailand. Commercial microwave ovens were modified and integrated with the microwave emitting power control system to maintain the appropriate temperature levels to evaporate the moisture from rubber. Throughout the drying process, the temperature of the rubber was measured both internally and outside. The results revealed that STR5L and STR20 could be dried satisfactorily and met the requirements for standard Thai rubber properties by utilizing 500 W for 140 and 120 min, respectively. By keeping the temperatures less than 150 °C, rubbers’ molecular structure is not destroyed from internal heat stress. Although utilizing less power for a longer period of time is possible, more energy was used, which is unfavorable. Compared to traditional hot air drying technologies, which take approximately 4–6 h for the drying process, microwave technology potentially reduces the drying time by half or more. If solar energy is used to supply electrical energy, 300,000 tons of Carbon dioxide can potentially be eliminated annually in the STR drying industry in Thailand by promoting approximately 1115 MW of Photovoltaic technology installations with the solar resources in southern Thailand.

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 CO₂-eq/ton product, compared to 143 and 20 kg CO₂-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 CO₂-eq per year can possibly be released into the atmosphere during pre-processing before being internationally exported or domestically used. For illustration, the CO₂-eq emission in Thailand for block rubber pre-processing in 2015 was almost 30% of the annual CO₂-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 (P₀), 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 (P₀) 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.

2. Methodology

This study was performed based on an experimental approach to determine appropriate solutions for drying STR rubber by using microwave technology. Firstly, the block rubber types which have the highest share in Thailand were chosen, then appropriate sample sizes were studied, which corresponded to the microwave penetration depth in the NR. After that, a microwave oven was selected with adequate power for the intended sample sizes, then the control unit was developed to enable a smoothly adjustable power-output function. Finally, the method for gathering crucial parameters for analyzing the quality of the microwave-dried rubber was introduced along with the analytical method of cleaner production in the rubber drying industry. The detail of each step is presented as follows.

2.1. NR Sample Preparation

In Thailand, block rubber is technically specified and named following the Standard Thai Rubber (STR) grades, which are STR XL, STR 5L, STR 5, STR 5CV, STR 10, STR 10CV, STR 20 and STR 20CV. Because STR 20 is the most produced standard, this paper therefore used the STR 20 for the microwave drying experiments and STR 5L was also used because of its local availability and to ensure that microwave drying is capable for a wide range of NR standards. The STR standard properties for three block rubber types are presented in

Table 1. The properties’ standard for STR 5L, STR 10 and STR20 block rubber.

Properties’ Standard	Type of Block Rubber
	STR 5L	STR10	STR20
Maximum dirt content (%wt)	0.04	0.08	0.16
Maximum ash content (%wt)	0.40	0.60	0.80
Maximum Nitrogen content (%wt)	0.60	0.60	0.60
Maximum volatile matter content (%wt)	0.80	0.80	0.80
Minimum Wallace initial plasticity (P0)	35	30	30
Minimum Plasticity Retention Index (PRI)	60	50	40

as a reference for the properties’ tests of the STR samples in the drying experiment.

2.2. NR Sample Size

The sample size is determined by its correspondence with the microwave penetration depth (PD) into the NR, as calculated from Equation (1), where PD is the depth to which the microwave can penetrate the medium until its energy decreases to 37%, to ensure that the deepest point of the samples are able to receive microwave energy, which influences the even heat and mass transfer of moisture inside the samples to the ambient conditions. In Equation (1), λ is the microwave wavelength calculated from λ = c/f, where c is the speed of microwaves in free space (3 × 10⁸ m/s) and f is the microwave frequency (2.45 GHz). ε is the complex dielectric constant calculated from Equation (2), where ${\mathsf{\epsilon }}_{\mathrm{g}}$ is the absolute dielectric constant (8.85 × 10⁻¹² F/m), ${\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }$ is the relative dielectric constant and ${\mathsf{\epsilon }}_{\mathrm{r}}^{″}$ is the dielectric loss factor. To easily specify the microwave energy absorbability of a matter, a loss tangent (δ) calculated from Equation (3) is employed. With a high loss tangent, the high energy absorbability of a medium matter that the microwave penetrates through is expected, which means the PD is low. Therefore, knowing the relative dielectric constant (${\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }$) and the dielectric loss factor (${\mathsf{\epsilon }}_{\mathrm{r}}^{″}$) of NR and water is beneficial to set the appropriate size of the samples, as the pre-process NR mainly consists of NR and moisture. The ${\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }$ and ${\mathsf{\epsilon }}_{\mathrm{r}}^{″}$ are gathered from [13] as presented in

Table 2. The dielectric properties of water and NR [13].

Matter	Relative Dielectric Constant $\left({\mathsf{\epsilon }}_{\mathbf{r}}^{\prime }\right)$	Dielectric Loss Factor $\left({\mathsf{\epsilon }}_{{r}}^{″}\right)$	Loss Tangent (δ)
Water (distilled @ 25 °C)	78.0	12.00	0.1538
NR (uncured)	2.1	0.02–0.05	0.0095–0.0238

.

Following Equation (1) with the dielectric properties of water and NR in Table 2, we found that the penetration depth of the microwave with a 2.45 GHz frequency can penetrate through a 14.3 mm and 564.8–1412 mm depth of water and NR, respectively. Therefore, the microwave oven dimensions used for rubber drying should not exceed 1410 mm, otherwise another magnetron may be installed at the opposite side of the existing one to double the oven size for the NR drying process. In this paper, the microwave oven dimensions are 200 × 300 × 250 mm, less than the threshold of 1412 mm to ensure that the microwave can penetrate thoroughly into the NR sample.

$$\mathrm{PD}=\frac{\mathsf{\lambda }\sqrt{{\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }}}{2\mathsf{\pi }{\mathsf{\epsilon }}_{\mathrm{r}}^{″}}$$

(1)

$$\mathsf{\epsilon }={\mathsf{\epsilon }}_{\mathrm{g}}\left({\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }-\mathrm{j}{\mathsf{\epsilon }}_{\mathrm{r}}^{″}\right)$$

(2)

$$\mathsf{\delta }=\frac{{\mathsf{\epsilon }}_{\mathrm{r}}^{″}}{{\mathsf{\epsilon }}_{\mathrm{r}}^{\prime }}$$

(3)

2.3. Microwave Oven Modification

There are six main components in microwave drying technology, namely a power supply, a magnetron, a waveguide, a heating chamber, a control unit, and a front door. The power supply usually consists of a transformer, to convert a regular AC supply (220 V_ac, 50 Hz) into two outputs, a low voltage high current (3 V, 10 A) output for electron cloud generation at the magnetron’s cathode when the filament is at high temperature [14], and a high voltage low current (4 kV, 0.5 A) output where the positive side is connected with a capacitor for electron charging, and the negative side is connected with the voltage multiplier part to double the voltage at the magnetron cathode for microwave generation with 50 Hz production frequency to avoid the magnetron overheating. The magnetron’s anode is at ground voltage and is relatively very high compared to the negative voltage at the cathode (−4 kV to −8 kV), which is separated by a vacuum cavity with a permanent magnetic field. The high voltage anode attracts the electron from the cathode to the cavity full of the magnetic field, which induces the electron to radiate the microwave (2.45 GHz for the magnetron used in this study) resonating in the cavity, then it is guided into the waveguide, made by copper or aluminium, to the heating chamber. The heating chamber is sealed with a Faraday’s cage to maintain the microwave inside the chamber while being able to release moisture to the ambient environment.

The microwave oven’s control unit was modified in this study to the microwave power output desired to heat the chamber to study the preferred drying conditions for NR drying. For typical microwave ovens, the output power is controlled by outputting a maximum fixed microwave power with different periods of time. For example, when 300 W power is desired, the control unit may output a maximum power of 1000 W for 3 s then the power is shut off for 7 s, resulting in a 300 W average power output into the heating chamber. This typical control strategy is simple, but it may lead to overheating of the NR in some parts and an uneven heat and mass transfer inside the block rubber. Therefore, this research introduced a new control strategy to control the input voltage of the transformer to obtain the desired microwave output power with the phase control strategy.

A simplified phase control circuit was considered, containing a capacitor (C1), a resistor (R1), a variable resistor (VR1), a DIAC and a TRIAC, as shown in Figure 1a. The line voltage (220 V, 50 Hz) was supplied from the utility passing through the protection circuit to the capacitor C1 and the TRIAC (BTA26-600BWRG). The TRIAC is used as an electronic switch [15] to block the input voltage for a period of time (time delay) where the specific time delay is obtained by adjusting the variable resistor VR1, resulting in a variable time charging and discharging the capacitor C1. At different levels of resistance, the capacitor takes a specific time to charge itself to a specific voltage level (Triger level in Figure 1b). This triggers the DIAC to let the current go to the TRIAC and opens the input voltage to the transformer (the voltage output waveform in Figure 1b). With different values of VR1, the time delay can be adjusted as shown in Figure 1c; therefore, by corresponding to the resistance value of VR1, different levels of duty cycle of the transformer voltage input were achieved, leading to variable microwave power outputs as presented in

Table 3. The relationship between the step of the variable resistor, duty cycle of the transformer input voltage, and the microwave power output.

Power Control Step (Variable Level of Resistance)	Duty Cycle (%)	Power Output (W)
1	50.4	90
2	55.2	135
3	60.0	230
4	60.8	390
5	61.6	405
6	62.4	415
7	63.2	435
8	64.0	595

.

2.4. Experimental Procedure

1.

The STR 20 and STR 5L samples were supplied from the rubber estate organization (REO) of Thailand, Nakhon Si Thammarat branch. The REO gathered coagulated NR from local producers and its own plantation then washed out some dirt and ash to meet the standards of each STR grade before creping, cutting, and granulating to wet random sized granules that attach together.

2.

The uncured NR supplied from the REO was then separated into small portion samples with an initial weight of approximately 300 g each as shown in Figure 2. As seen in Figure 2, the raw NR samples have a porous structure. It is difficult to control the initial porosity; therefore, the initial porosity of the NR samples was not controlled in the experiments.

3.

STR20 and STR5L samples were dried with the modified microwave ovens with variable microwave power, from 50 W to 700 W, to observe the temperature response of the samples using temperature sensors (thermocouple type K) in three layers: surface layer (SL), centre layer (CL), and a middle layer (ML) between the surface and the centre as shown in Figure 3. The temperature at each layer was measured every 20 min by opening the oven and inserting 3 thermocouples to the mentioned layers, then closing the oven until all temperature readings reached their maximum (steady state). During the temperature measuring process, the microwave power was temporarily turned off so as to not excite the thermocouples and raise their temperature. It took no longer than one minute for the temperatures to reach their steady states, then the drying process was continued.

3. Analytical Calculations on the Cleaner Potential of the NR Drying Process

3.1. Potential Input Power from Photovoltaic Technology

Considering the global horizontal irradiation (GHI) in Thailand of around 1700–1950 kWh/m², as shown in Figure 4, compared to high GHI areas of roughly 2200–2500 kWh/m² in Africa, the Arabian Peninsula, and Australia [17], Thailand has the potential to increase its solar share by converting solar energy into electricity to supply power to microwave technology via photovoltaic technology. In Thailand, rubber trees are planted mostly in the south; almost 60% of all rubber products are produced in the south, followed by the east and northeast regions where solar energy is sufficiently available.

The electrical energy output from PV technology was validated against a 0.99 MW solar rooftop project on a factory in the studied area where the roof faces south with a 10° incline angle from the horizontal plane. Global Horizontal Irradiance data at latitude 8.5° and longitude 99.5° was collected for 4 months via a C-Class pyranometer through the rainy season in the region to ensure that even during low incoming solar irradiation times, there will be adequate input electricity for the drying process. Ambient temperature and wind speed were also collected for the calculation of the electrical power output. When considering energy management using solar energy for the microwave drying process of rubber, it is favorable that on rainy days, there will be significantly less rubber to be dried as farmers cannot harvest latex from rubber plants at this time.

3.2. Electrical Power Production from PV Technology

Rubber drying factories were constructed several years before solar PV technology was affordable; hence, most factories’ rooftops were not designed with optimum rooftop angles to maximize the annual electrical energy yield. Therefore, this study used real electrical power production data from an actual factory with a 0.99 MW PV installation located at latitude 8.4°, longitude 99.7°, and tilted at an angle of 10° facing south for 75% of the installation area and tilted at an angle of 10° facing north for the rest. The factory is not a rubber drying factory, but this paper used the production data for validation purposes. The solar panels used at the studied site is shown in Figure 5 and the specification of the installed panels is shown in

Table 4. The X21-350-BLK solar panel specification.

Parameters	Values
Nominal Power	350 W
Panel Efficiency	21.5%
Rated Voltage (Vmpp)	57.3 V
Rated Current (Impp)	6.11 A
Open-Circuit Voltage (VOC)	68.2 V
Short-Circuit Current (ISC)	6.5 A
Power Temperature Coefficient	−0.29%/°C
Voltage Temperature Coefficient	−167.4 mV/°C
Current Temperature Coefficient	2.9 mA/°C

. A pyranometer was installed on the rooftop of the rubber drying factory to measure horizontal irradiance, which can be used to calculate the irradiance at any tilted angle by using the mathematical models from [19].

3.3. Preheat Potential from the Installed PV for the Rubber Drying Process

Solar energy can be converted to not only electrical energy but also thermal energy, which is not preferred if electrical energy is the focus as thermal energy causes a PV cell’s temperature to rise, which decreases the PV cell’s efficiency. However, if both electrical and thermal energies are desired, the Photovoltaic-Thermal (PV/T) collector can be used to obtain much higher total efficiency (electrical efficiency and thermal efficiency combined), which leads to a higher solar fraction for the overall energy consumption part. For the rubber drying process, thermal energy is required to heat up moist rubber until its desired moisture content is reached. Conventionally, the source of thermal energy is mostly derived from a natural gas combustion chamber via air convection. Fresh air with ambient temperature is fed into the combustion chamber, then flows into the rubber drying chamber with a controlled temperature ranging from 110–130 °C for rubber quality control purposes [20]. With the conventional drying process mentioned above, PV/T technology can be used to preheat the fresh air before being fed into the microwave drying chamber to circulate the air for quality control purpose.

According to information from the rubber drying industries, the moist air discharge rate is approximately 4.2 m³/s per ton of initial rubber weight to maintain the desired temperature and pressure inside the drying chamber, while the relative humidity is reduced to allow more moisture to diffuse out from the wet rubber. With the aforementioned air discharge rate, energy reduction (preheat energy) can be calculated from Equation (4) where $\rho$ is the ambient air density (kg/m³), $\dot{Q}$ is the volume flow rate (m³/s), C_p is the ambient air specific heat capacity (kJ/kg‧K), $T_{preheated}\left(t\right)$ is the time-dependent air output temperature from the PV/T preheat process, and $T_{amb}\left(t\right)$ is the time-dependent ambient air before being fed into the PV/T collector.

$$E_{preheat}=\underset{t_0}{\overset{t_1}{{\displaystyle \int }}}\rho \dot{Q}{C}_p\left(T_{preheated}\left(t\right)-T_{amb}\left(t\right)\right) dt$$

(4)

3.4. System Overall Performance

To maximize solar energy for the rubber drying process, the preheat energy (thermal part) and electrical energy used in the microwave technology can be employed by using PV/T technology. However, the air output temperature from PV/T should not be controlled to be higher than an optimum point to avoid an extensive electrical efficiency drop. The electrical efficiency of a PV panel can be obtained from a Power Temperature Coefficient in a panel’s datasheet, as shown in Table 4 corresponding to a cell’s temperature. The PV cell’s temperature (T_cell) can be measured from installed PV panels or approximately calculated from Equation (5) [19] where T_amb is the ambient temperature, and G_NOCT is the irradiance at nominal operating cell temperature. With the known T_cell and wind speed (w), the PV power output can be calculated when the incident irradiance normal to the PV surface (G_obs) is determined.

$$T_{cell}=T_{amb}+\left(\frac{G_{obs}}{G_{NOCT}}\times \left(NOCT-20°\mathrm{C}\right)\times \left(\frac{9.5}{5.7+3.8·w}\right)\times \left(1-\frac{{\eta }_{STC}}{\tau \alpha }\right)\right)$$

(5)

To simplify thermal energy gain without decreasing the electrical power output, and to allow heat transfer between the PV/T and the air, the maximum air temperature output (T_{air_out}) from the PVT is set to be at T_cell—3 °C. With the thermal efficiency of an air-based PVT technology of more than 25% [21,22,23], this paper used a thermal efficiency of 20% and T_{air_out} to calculate the air mass flow rate out of the PVT. Therefore, with the known horizontal irradiance from the pyranometer, ambient temperature from temperature sensors, and the wind speed from an anemometer installed at the studied site, the required energy for the rubber drying process supplied entirely from solar energy can be achieved.

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. The volatile content (percent by weight) of the dried STR5L and STR20 samples at different parts.

Drying Conditions	At Surface Layer		At Middle Layer		At Center		Averaged Values
	STR5L	STR20	STR5L	STR20	STR5L	STR20	STR5L	STR20
420 W—120 min	0.7	1.2	0.5	0.7	2.0	0.6	1.1	0.8
420 W—140 min	0.5	0.7	0.5	0.5	1.2	0.6	0.7	0.6
420 W—160 min	0.7	0.6	0.6	0.5	0.5	0.5	0.6	0.5
500 W—120 min	0.9	0.5	0.7	0.4	0.7	0.5	0.7	0.5
500 W—140 min	0.8	0.5	0.6	0.5	0.6	0.4	0.7	0.5
500 W—160 min	0.5	0.5	0.4	0.4	0.4	0.4	0.4	0.4
560 W—100 min	0.8	0.8	0.3	0.3	0.4	0.4	0.5	0.5
560 W—140 min	0.2	0.5	0.1	0.4	0.2	0.4	0.2	0.4
640 W—80 min *	4.0	0.4	0.8	0.4	0.4	0.4	1.8	0.8

* The rubber at the center turned brown and started to melt.

. 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. Properties of microwave-dried tested samples against the specifications of the STR5L and STR20 block rubber standards.

Properties	Standard Values [24]		Tested Values
	STR 5L	STR20	STR 5L	STR20
Dirt content (%wt)	<0.04	<0.16	0.02	0.13
Ash content (%wt)	<0.4	<0.8	0.2	0.6
Nitrogen content (%wt)	<0.6	<0.6	0.3	0.5
Volatile matter (%wt)	<0.8/<0.5 *	<0.8/<0.5 *	0.4	0.5
Wallace initial plasticity (P0)	>35	>30	42	34
Plasticity Retention Index (PRI)	>60	>40	88	76
Lovibond Scale	<6	-	2	-

* Preferred value from manufacturers.

.

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 O₂ and emits CO₂ 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 CO₂ 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 CO₂ annually. If microwave technology with a solar energy supply is used, 300,000 tons of CO₂ 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.