Rigid polyurethane (PU) foam is used as a thermal insulating and supporting material in domestic refrigerator/freezers and it is produced by reaction injection molding (RIM) process. There is a need to improve the thermal property of rigid PU foam but this is still a challenging problem. Accordingly, this work investigates the RIM process parameters to evaluate their effects on rigid PU foam's structure and hence property. It has been found that mold temperature is a key parameter whereas curing time has negligible effect on structure of PU foam. Cell size, strut thickness, and foam density have been found very critical in controlling the thermal and mechanical properties. Upper and lower values of 30 to 32 kg/m 3 density are critical to observe contribution of radiation and solid conductivity separately. Finally, PU foam with 160 mm average cell size, 16 mm strut thickness, below 10% open cell content, and 30 to 32 kg/m 3 density allow obtaining better thermal insulation without significant reducing in the compressive strength. The presented work provides a better understanding of processing-structure-property relationship to gain knowledge on producing highquality rigid PU foams with improved properties.
In this study, bone china body was reformulated by completely replacing Cornish stone with nepheline syenite and quartz. Effect of controlled milling/mixing on the technological properties and microstructural evolution was also studied. Specimens prepared both from reformulated and controlled milled/mixed bodies were sintered between 1200 and 1250°C with 25°C increment. Sintering and technological properties of reformulated bodies were not being adversely affected but conversely, the measured flexural strength values (55 MPa) were half of the value that was published for bone china (100 MPa). Microstructural investigations showed that enlarged pore formation was the reason for strength reduction. However, improvement in particle packing by controlled milling/mixing eliminated enlarged pore formation and in response, flexural strength values increased to conventionally quoted levels. Detailed microstructural investigations revealed that the reason behind enlarged pore formation was heterogeneous distributions of body components, especially CaO and quartz grains. It was suggested that variations of CaO and SiO 2 to form improper ratio between them would affect the viscosity of glassy phase and crystallization, which would prevent gases in pores to dissolve away. The obtainment of homogenous distribution of body components by controlled milling/mixing has a strong influence on the evolution of microstructure and improvement of technological properties.
The effect of adding 0.1, 0.3, 0.6, 1.0 wt.% halloysite clay, which calcined at 600, 800, 1000 and 1200°C, to the standard wall tile slurry was studied. Wet milled halloysite was sieved with different sizes to examine the halloysite grain size effect. The granulated samples were pressed by applying a pressure of 30 MPa using a hydraulic press and made ready for sintering at 1150°C for 30 minutes. Detailed technological, mechanical and microstructural characterization studies were done on sintered samples. Compared to standard wall tile, almost all the halloysite added samples displayed similar technological properties but higher green and fired strength values. The produced sample with adding 0.6% halloysite (at 600°C calcined) by weight showed a higher firing strength with 28.75 MPa value than the minimum requirement (17.75 MPa). Observation of needlelike microstructure in the porcelain body led to establishing a simple relationship between the interlocking effects of thin needlelike mullite grains and increasing strength values. This study has shown that the addition of calcined halloysite may be an alternative to produce a thinner section wall tile product.
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