The average particle sizes of the prepared raw materials (expandable polystyrene) and the expanded polystyrene (EPS) beads are given in Table 2 together with the measured pentane % within the prepared samples.
Table 2
The expansion ratio and pentane levels of the prepared samples with the initial and after expansion average sizes
| Sample No. | Expandable PS Average Size (mm) | EPS Average Particle Size (mm) | Expansion Ratio (%) | Measured Pentane % |
| 1 | 1.01 | 3.12 | 3.09 | 5.96 |
| 2 | 0.91 | 2.63 | 2.89 | 6.18 |
| 3 | 0.94 | 2.8 | 2.98 | 6.06 |
| 4 | 0.87 | 3.27 | 3.75 | 5.98 |
| 5 | 0.98 | 2.63 | 2.69 | 6.23 |
The average particle sizes of the expandable PS beads are quite close due to the advantage of hybrid system that uses cutting in order to obtain them. When the trapped pentane percentage is compared with the expansion ratio (Fig. 1), it is seen that the highest pentane amount possess the lowest expanding ratio. This shows that not only pentane amount and initial average size but also the microstructure of the beads should be considered for expansion performance.
The cross sectional areas of the expanded beads are shown in Fig. 2. It is observed that the foaming conditions caused larger pores for pentane excalation which decreased the expanding ratio. This decrease might be also as a result of relatively higher pentane amount within the samples 2, 3 & 5. In order to avoid aggressive foaming that causes such inhomogeneties and lower expanding ratio performance, an optimization is necessary for the expansion parameters such as applied steam temperature, pressure and contact time of the steam. Because the main aim for this study was to work all samples in similar conditions, a further optimization was not done at this step and out of the scope of this study. The best expansion performance was obtained for sample no.4 whose homogenous closed cell microstructure after expansion can also be tracked in Fig. 2.
The densities of the expanded particles are given in Table 3 together with the initial raw material and carbon additive amounts. The samples no. 2, 3 and 5 have relatively higher densities than the samples that contain graphite powder whose amount was also lower than the carbon black additive. Therefore, the carbon based additives seem to have a strong influence on the sample density regardless of the expansion ratios given in Table 1 .The higher density with 26.6 g/cm3 was obtained for Sample 5 which is composed of both GPPS and exp. PS raw materials and also carbon amount. Hence, even though the sum of the raw materials is nearly 90.7%, the additive impact on the density is tracked better for this sample. Moreover, sample no.4, whose raw material amount is similar to the raw material amount of sample no.5, has a density of 25.3 g/cm3. As a rule of thumb, density has an inverse relation with expansion performance. Therefore, one would expect a lower density for sample no.4. However, expanded average size for this sample was the highest (3.27 mm) among other samples. This also shows that carbon black additive increased the weight resulting in denser expanded particles.
Table 3
Density of the obtained samples compared with initial raw material amounts
| Sample | GPPS (%) | Exp. PS (%) | CB (%) | Graphite (%) | Density (g/cm3) |
| 1 | 0 | 93.7 | 0 | 3 | 23.7 |
| 2 | 90.7 | 0 | 6 | 0 | 24.8 |
| 3 | 93.7 | 0 | 0 | 3 | 22.9 |
| 4 | 0 | 90.7 | 6 | 0 | 25.3 |
| 5 | 20 | 70.7 | 6 | 0 | 26.6 |
The glass transition temperatures extracted from differential scanning calorimeter (DSC) against raw material types are depicted in Fig. 3. For polystyrene, glass transition temperature is generally between 90–110 ºC. Obtained temperatures are within this range. This shows that all materials are suitable for further applications that are used in the industry. The highest glass transition temperature was recorded for sample no 1. Hence, the addition of graphite powder to expandable PS shows a synergistic effect while the addition of graphite powder to GPPS has a contrary effect. Hence, the lowest glass transition temperature was recorded for GPPS with graphite powder. On the other hand, carbon black added samples with either GPPS or exp. PS have similar glass transition temperature but an increase in glass transition temperature was observed for the sample (no.5) containing both exp. PS and GPPS.
Thermal conductivity measured at 25 and 40 ºC according to ISO 8301 standard is given in Fig. 4. Basically, it is desired to obtain lower thermal conductivity values for insulation boards since it is defined as the ability to conduct heat. Depending on the insulation material type and thickness, this value may vary. As the thickness of the insulation board increases, thermal conductivity is expected to decrease. Among commercial EPS insulation boards, this value varies between 0.029–0.037 W/mK [8, 10] depending on the additives, density and the thickness of the end material. In general, EPS insulation boards, which graphite or carbon black are added, have a lower thermal conductivity value in the range between 0.003–0.008 W/m.K than the insulation boards without graphite or carbon black content.
The obtained thermal conductivities are in good agreement with the commercial materials. Sample no.2, which graphite was added to GPPS raw material, has the lowest thermal conductivity value at 25 and 40 ºC. Similar thermal conductivity values were observed for the mixed sample which was blended with carbon black. However, when carbon black is added to only GPPS or exp. PS material, thermal conductivity values at 40 ºC are approximately the same. The highest thermal conductivity was recorded for the sample that has graphite and expandable polystyrene. The compatibility of graphite powder with GPPS is better than expandable polystyrene as raw material in terms of lower thermal conductivity from foam extrusion process. Hence, employing GPPS material would be more economical when the raw material prices for GPPS and EPS are considered [25].