AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS*

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g ). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.

     

The Relationship of Glass Transition Temperature to Adhesive Strength

The effects of glass transition temperature (T_g) on mechanical properties have been further demonstrated by the observation of adirect relationship between the T_g of an epoxy adhesive and its lap shear bond strength to metal at elevated test temperatures. An additive (coupling agent) which lowers the T_g from a point near or above the test temperature to below it causes a subsequent decrease in the strength of the system and generally increased cohesive failure. Therefore effects on the T_g of the adhesive are more important than on interfacial properties. The end result is that differential thermal analysis (DTA) can be utilized as a effective screening method for adhesives and additives, and can be a good indicator of maximum use temperature. From these data T_g can also be used to estimate adhesive strength at a given use temperature.     

Transient Thermal Deformations of the Interphase in Polymer Composites

Heterodyne micro-interferometry was utilized to measure out-of-plane transient displacements in the interphase due to thermal cycling. In-situ measurements were made on single carbon fiber/epoxy samples with interphases of varying glass transition temperature. Interphase properties were tailored such that one set of samples had fibers which were coated with a low T_g resin, another set had a higher T_g coating, and in the third set the fibers were uncoated. The interferometric data demonstrated that interphase T_g has a significant effect on the rate and magnitude of the thermal deformations at the fiber/matrix interface. The presence of a low T_g interphase caused an increase in the magnitude of the thermal displacements due to a local softening of the matrix and increase in coefficient of thermal expansion. In addition, the rate at which the displacements increase was also higher due to the reduction in T_g. Samples with untreated fibers (no tailored interphase) behaved as if a low T_g interphase had formed. Experimental displacement profiles were also compared with finite element predictions to assess the behavior of the tailored interphases.     

Drying Characteristics of Malaysian Paddy: Kinetics & Grain Cracking Quality

Drying kinetics of Malaysian paddy dried in a laboratory scale batch rapid bin dryer was studied. Quality of paddy after drying at temperatures of 60 and 80°C, bed heights of 2 and 4 cm, and at different drying stages was presented in terms of cracking percentage. Moisture content of paddy reduced exponentially with time and became stable at equilibrium moisture content. Characteristic drying rate curves exhibited mainly falling rate period, with an induction period in deep beds, but without the constant rate period. A custom-made light box was built to visually analyze the cracking in paddy kernel. The extent of breakage is directly related to the crack percentage. The percentage of cracks increased with drying and tempering temperatures and was higher after 2 weeks of storage before milling. Bed heights did not significantly affect the head rice yield as much as tempering between two drying stages. Temperatures above 80°C caused severe cracking in paddy kernel and produced miserable yield of head rice after milling, which could be slightly improved by tempering. The use of a proper miller is also important in ensuring good head yield results and whiteness quality.     

Drying Characteristics of Malaysian Paddy: Kinetics & Grain Cracking Quality

Abstract

Drying kinetics of Malaysian paddy dried in a laboratory scale batch rapid bin dryer was studied. Quality of paddy after drying at temperatures of 60 and 80°C, bed heights of 2 and 4 cm, and at different drying stages was presented in terms of cracking percentage. Moisture content of paddy reduced exponentially with time and became stable at equilibrium moisture content. Characteristic drying rate curves exhibited mainly falling rate period, with an induction period in deep beds, but without the constant rate period. A custom-made light box was built to visually analyze the cracking in paddy kernel. The extent of breakage is directly related to the crack percentage. The percentage of cracks increased with drying and tempering temperatures and was higher after 2 weeks of storage before milling. Bed heights did not significantly affect the head rice yield as much as tempering between two drying stages. Temperatures above 80°C caused severe cracking in paddy kernel and produced miserable yield of head rice after milling, which could be slightly improved by tempering. The use of a proper miller is also important in ensuring good head yield results and whiteness quality.     

Polyimide Adhesives and Aluminum

This study examines the effect of thioether sulfur in the polyimide backbone, polyimide T_g, and adherend surface pre-treatment on aluminum bond strengths as determined with both peel and wedge specimens. Surface pretreatment and T_g had more of an effect on peel strength than the presence of sulfur in the polyimide backbone. NaOH etching and comparatively low T_g polyimides combined to produce the highest peel strengths. Together, these factors combined the removal of surface oxide from the adherend with a flexible polyimide which could better relieve stress during testing. Little difference was observed between the peel strengths of sulfur and non-sulfur containing polyimides, and no oxidation of sulfur was observed in the peel samples. NaOH etching also caused both wedge and peel specimens to fail more within the polyimide than in the oxide layer of the adherend. Thus, the NaOH etch appeared to increase interfacial adhesion between the aluminum and the polyimide. The low T_g polyimides performed better than the high T_g polyimides in the wedge test, with the polyimide derived from 4,4'-bis(3,4-dicarboxyphenoxy) diphenyl sulfide dianhydride and 4,4'-diaminodiphenyl ether (BDSDA/ODA) performing the best. This observation could be due to a metal-sulfur interaction since oxidized sulfur was surprisingly observed on the failed surfaces of these bonds regardless of the environment or surface pretreatment. A metal component in the aluminum alloy rather the aluminum is believed to promote the sulfur oxidation.     

STUDY ON ROUGH RICE FISSURING DURING INTERMITTENT DRYING

The Assuring of rough rice during intermittent drying was studied through experimental and numerical methods. The moisture distribution inside the rice kernel during drying and tempering was obtained by diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying.

Discontinuing the drying process with tempering can decrease the hydro stresses in the rice kernel. Decreased unit drying time or increased intermittent ratio caused decrease of the stresses in the rice kernel during intermittent drying. Less fissured rice was also observed by discontinuing the drying process in the experiments. Higher intermittent ratio or lower unit drying time caused lower percentage of fissured rice in the experiments.     

Viscoelastic Behavior, Thermodynamic Compatibility, and Phase Equilibria in Block Copolymer-Based Pressure-Sensitive Adhesives

The viscoelastic behavior, thermodynamic compatibility, and phase equilibria in block copolymer-based pressure-sensitive adhesives were investigated. The block copolymers investigated were: (1) polystyrene-block-polybutadiene-block-polystyrene (SBS) copolymer (KRATON® D-1102, Shell Development Company) and (2) polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer (KRATON® D-1107, Shell Development Company). The tackifying resins investigated were: (1) WINGTACK® 86 (Goodyear Tire & Rubber Company) and (2) PICCOTAC® 95BHT (Hercules Inc.). Samples of various compositions were prepared by a solution-casting method with toluene as solvent. Measurements of dynamic storage modulus (G'), dynamic loss modulus (G'), and loss tangent (tan δ) were taken, using a Rheometrics Mechanical Spectrometer. It was found that: (1) both WINGTACK 86 and PICCOTAC 95BHT were equally effective in decreasing the plateau modulus (G^O_N), and increasing the glass transition temperature (T_g) of the polyisoprene midblock of KRATON 1107; and (2) WINGTACK 86 was very effective in decreasing the G^O_N and increasing the T_g of the polybutadiene midblock of KRATON 1102, whereas PICCOTAC 95BHT was not. The observed difference between WINGTACK 86 and PICCOTAC 95BHT in decreasing the G^O_N and increasing the T_g of the polybutadiene midblock of KRATON 1102, whereas PICCOTAC 95BHT was not. The observed difference between WINGTACK 86 and PICCOTAC 95BHT in decreasing the G^O_N and increasing the T_g of the polybutadiene midblock of KRATON 1102 (perhaps to SBS block copolymers in general) is explained by the values of the interaction parameter for WINGTACK 86 and KRATON 1102, and for PICCOTAC 95BHT and KRATON 1102. The interaction parameter was determined, using the piezoelectric quartz sorption method. Phase diagrams were constructed for the four block copolymer/tackifying resin systems investigated, using information obtained from both dynamic viscoelastic measurements and optical microscopy. It was found that when mixed with KRATON 1102, PICCOTAC 95BHT formed separate domains whereas WINGTACK 86 did not over the range of concentrations and temperatures investigated. This confirms the evidence obtained from two other independent experimental techniques, namely, dynamic viscoelastic measurements and the piezo-electric sorption method. We have concluded from the present study that PICCOTAC 95BHT is not as an effective tackifying resin as WINGTACK 86, when each is mixed with KRATON 1102. It is pointed out further that information on the order-disorder transition temperature T_r, which was determined from a rheological technique proposed by us, is valuable in determining optimal processing conditions for block copolymer-based pressure-sensitive adhesives.     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g. Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

Stress fissuring and process duration during rough rice convective drying affected by continuous and stepwise changes in air temperature

During rough rice drying, gradients of moisture content and glass transition temperature cause thermal and mechanical stresses inside the kernel. These stresses eventuate to kernel fissuring during the milling process. In this study, convective drying of Hashemi (long grain) rough rice was applied to investigate the effect of continuous and stepwise changes in air temperature on stress cracking index and process duration. Toward this objective, the concepts of glass transition and analysis of moisture contents distributions within the rice kernel were determined through a numerical modeling of mass transfer. For stepwise temperature change, the drying experiments were conducted at temperatures above the glass transition temperature. Results indicated that the stress cracking index under stepwise temperature change conditions (i.e., within the rubbery state) was reduced compared to the continuous mode probably due to a drip in the moisture content gradients created inside the kernels during the drying process. Moreover, the drying duration significantly was shortened when the kernel was dried within the rubbery state due to faster diffusion moisture within the kernel.     

STUDY ON ROUGH RICE FISSURING DURING INTERMITTENT DRYING

Abstract

The Assuring of rough rice during intermittent drying was studied through experimental and numerical methods. The moisture distribution inside the rice kernel during drying and tempering was obtained by diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying.

Discontinuing the drying process with tempering can decrease the hydro stresses in the rice kernel. Decreased unit drying time or increased intermittent ratio caused decrease of the stresses in the rice kernel during intermittent drying. Less fissured rice was also observed by discontinuing the drying process in the experiments. Higher intermittent ratio or lower unit drying time caused lower percentage of fissured rice in the experiments.     

Comparative Study of Effects of Drying Methods and Storage Conditions on Aroma and Quality Attributes of Thai Jasmine Rice

This article describes the effects of various drying treatments and storage conditions on the main quality attributes of Thai jasmine rice; i.e., aroma, milling, pasting, and cooking properties. The experimental results show that drying treatments using various high temperatures (115-150°C) combined with 30 min tempering time between the pass or ambient air drying affect the composition of volatile compounds as well as the commercially accepted quality characteristics of Thai fragrant rice. High-temperature drying followed by ambient air drying can retain most of the quality attributes of Thai jasmine rice than multi-pass high-temperature drying with a tempering period between passes.     

Comparative Study of Effects of Drying Methods and Storage Conditions on Aroma and Quality Attributes of Thai Jasmine Rice

This article describes the effects of various drying treatments and storage conditions on the main quality attributes of Thai jasmine rice; i.e., aroma, milling, pasting, and cooking properties. The experimental results show that drying treatments using various high temperatures (115–150°C) combined with 30 min tempering time between the pass or ambient air drying affect the composition of volatile compounds as well as the commercially accepted quality characteristics of Thai fragrant rice. High-temperature drying followed by ambient air drying can retain most of the quality attributes of Thai jasmine rice than multi-pass high-temperature drying with a tempering period between passes.     

Evolution of mechanical properties of parboiled brown rice kernels during impinging stream drying

Information on mechanical properties of parboiled brown rice kernels upon impinging stream drying, which is important for effective control of kernel fissure and head rice yield, is reported. Experiments were performed at the drying temperatures of 130, 150, and 170°C; inlet air velocity of 20?m/s; impinging distance of 5?cm and paddy feed rate of 40?kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle, the paddy was tempered for a period of either 0 (without tempering) or 30?min. The moisture evaporation rate was noted to be very high during the first two drying cycles and rapidly dropped in the later drying cycles. When tempering was included after a particular drying cycle, the drying rate in a subsequent cycle was higher than without tempering. At the kernel moisture contents immediately after drying of 25.3–47.5% (d.b.), the drying temperature and existence of tempering did not affect the mechanical properties although microcracks were formed in the kernels. However, both factors played a more important role on the mechanical properties when the kernels were evaluated at 16% (d.b.). The head rice yield correlated well with the tensile strength of the kernels.     

CONVECTION HEAT TRANSEER COEFFICIENTS AT CONVECTIVE DRYING OF POROUS MATERIALS*

Our measurements proved that the convective heat transfer coefficient (h) has a larger value h_wet at the constant drying rate period and after that it falls down to a minimum one: h_dry in the equilibrium dried state (see Fig.1). Our measurements showed also that the heat of vaporisation in the last phase of the falling drying rate period is far greater than it was in the constant drying rate period (see Fig.10).

Our first measurements were made on a gypsum plate. Afterwards we carried out measurement research with fine glass powder and cement-perlite plate and determined h_wet and h_dry heat transfer coefficients as function of Reynolds number. All of these measurements confirmed our establishment that h_wet is far greater than h_dry.     

Simulation of Recirculating Circular Grain Dryer with Tempering Stage

The diiffusion model describing internal diiffusion of moisture within a grain kernel during drying and tempering stages was incorporated in the cross-flow drying model to simulate the recirculating circular grain dryer with drying and tempering stages. Experiments were conducted on an experimental prototype recirculating circular grain dryer for wheat and rough rice drying. The simulated grain temperature and moisture content were compared with the experimental data of drying wheat and rough rice, the maximum deviation of the outlet grain temperature was 5°C and the maximum deviation ofthe final grain moisture content was 0.3% w.b. The simulating program for recirculating circular grain dryer was used for analyzing the effects of structure parameters and hot air parameters on the dryer performance. Recommendations for design of the recirculating circular grain dryers are drawn from the experiments and simulation.     

The Moisture Distribution of High Moisture Content Rough Rice During Harvesting, Storage, and Drying

The kernel moisture content (MC) distribution of five varieties of double season rice was tested during the process of harvesting, drying, and tempering. Significant kernel MC variance was found to exist within the panicle. Binodal kernel MC distribution frequency was found at harvest. The MC distribution of five varieties during harvesting and drying showed similar trend of uniformity. To investigate the mechanism of moisture transport among rice kernels under low temperature, the kernel MC distribution of rice planted in Northeastern China was also investigated after stacked for over one month under the temperature of -20°C. The moisture exchange among rice kernels was found greatly influenced by environmental factors.     

The Moisture Distribution of High Moisture Content Rough Rice During Harvesting,Storage, and Drying

Abstract

The kernel moisture content (MC) distribution of five varieties of double season rice was tested during the process of harvesting, drying, and tempering. Significant kernel MC variance was found to exist within the panicle. Binodal kernel MC distribution frequency was found at harvest. The MC distribution of five varieties during harvesting and drying showed similar trend of uniformity. To investigate the mechanism of moisture transport among rice kernels under low temperature, the kernel MC distribution of rice planted in Northeastern China was also investigated after stacked for over one month under the temperature of ?20°C. The moisture exchange among rice kernels was found greatly influenced by environmental factors.     

EFFECT OF TEMPERING ON SUBSEQUENT DRYING OF PADDY USING FLUIDISATION TECHNIQUE

Drying and tempering models for paddy drying by a fluidised bed technique have been developed to describe the moisture movement inside a single paddy kernel. The grain shape was considered as a finite cylinder. The internal diffusion is an important contribution to control the drying rate of paddy. The dependence of effective diffusion coefficient on drying temperature can be adequately explained based on Arrhenius form. The parameters of this equation were evaluated in the range of temperatures between 110°C and 170°C by using the regression analysis with 189 experimental drying data. As compared with no tempering, the faster drying rate can be obtained by tempering treatment between drying stages. The effect of degrees of tempering on determining the moisture reduction in the second stage has also been explored. According to the simulation results, a prediction equation of the required tempering time for the tempering index of 0.95 has been established in which the drying air temperature, initial moisture content and drying time are taken into account. The tempering time for 35 min is recommended for the continuous fluidised bed dryers being operated in rice mills.