Linear thermal expansion coefficient (LTEC) was measured for compression molding samples of polypropylene (PP)/clay composites with clay loading of 0 to 7 wt%. Composites were prepared by internal batch mixer and specimens were prepared by compression molding. These processing methods are not anticipated to have a preference for orientation; therefore effect of anisotropy was minimal. The LTEC was measured along three different faces of the compression molding sheets, parallel to compression direction S1 and perpendicular to compression directions S2 and S3. The LTEC for neat PP measured by current research,
Polypropylene (PP) is a versatile polyolefin that finds a strong demand in some of the advanced applications such as automotive industry. What makes PP a good candidate for such advanced applications is the excellent chemical and mechanical properties due to its superior crystallinity nature. PP is a highly crystalline polyolefin with relatively higher melting and softening temperature compared to other commodity vinyl polymers such as polyethylene (PE) and polystyrene (PS). Some of the interior parts of an automotive may be made of PP. Recently, glass fiber reinforced PP was introduced to be used in the under-the-hood parts in automotive. Despite this versatility in applications for the PP, this polymer suffers from dimensional instability due to high linear thermal expansion coefficient (LTEC). Neat PP has different values of LTEC depending on the orientation of crystalline chain domain [
In our study we tried to relate the processing nature to the extent of thermal expansion of PP/clay composites. In other words we intended to minimize the influence of orientation on the magnitude of LTEC by changing the process type. The reader may realize that most of the studies conducted on thermal expansion coefficient of either neat PP or PP modified by inorganic fillers and fibers have used samples taken from injection molding specimens where the orientation effect is very high. Here we used compression molding as a means to prepare our samples for the LTEC tests in order to have less oriented chain segments. We would like to investigate the anisotropy nature of LTEC for unoriented PP/clay composites.
Polymer matrix used in this study was a homopolymer PP (MH418), a courtesy of PETKİM Petrochemical, Turkey. Polypropylene grafted with malice anhydride PP-g-MA (Fusabond M613-05) was used as a coupling agent for PP/clay composite systems. The clay used in this study was as received Turkish Montmorillonite with no surface modification or treatment.
The PP/clay was prepared by a Haake batch mixer (Polylab) at 190°C and 80 rpm. After 2 min of melting the PP, PP-g-MA was added to the PP and let melt mix for another 2 min. After that, clay in the amounts ranging from 1 to 7 wt.% was added into the mixer and continued to mix for 7 min. The ratio of clay to PP-g-MA was 1 : 3. The blended samples were collected and left for cooling. After cooling, the blends were pressed into 100 mm × 100 mm samples having a thickness of 2 mm using a hot press (Carver- Hydrolic Press 25 ton) at 190°C.
XRD was used to determine the characteristic peaks of the prepared composites. Scanning electron microscope (FEI, model NNL 200) was used to investigate the fracture surfaces of the composites. Thermogravimetric (PerkinElmer TGA-7) was used to determine the exact loading of the clay within the PP matrix and to assess the thermal stability of the composites.
Small parallelogram pieces with the dimension of 2 mm × 3 mm × 3 mm were cut of the compression molding sheets as shown in Figure
Schematic of TMA specimens showing the three surfaces where LTEC was measured (S1 parallel to compression direction and S2 and S3 perpendicular to compression direction).
Figure
XRD spectra of PP/clay composites at various clay contents.
Comparison of XRD peak intensity at
Comparison of XRD peak intensity at
TGA curves for the prepared PP/clay composites.
Molded samples at various clay content after natural cooling.
SEM micrographs for the fractured surfaces of the compression molding specimens at various clay loadings (wt%). (a) 0; (b) 1; (c) 3; (d) 5; (e) 7.
The increase in specimen length relative to its original length, that is,
LTEC values at the three designated surfaces; S1, S2, and S3 for the PP/clay composites at various clay contents.
Clay wt% | Surface | LTEC ×10−4 |
( |
|
---|---|---|---|---|
0 | S1 | 1 | 25 | 0.9995 |
0 | S2 | 1 | 29 | 0.9949 |
0 | S3 | 1 | 33 | 0.9992 |
1 | S1 | 2 | 40 | 0.9963 |
1 | S2 | 1 | 27 | 0.9996 |
1 | S3 | 1 | 27 | 0.9998 |
3 | S1 | 4 | 70 | 0.9973 |
3 | S2 | 1 | 13 | 0.9933 |
3 | S3 | 1 | 14 | 0.9982 |
5 | S1 | 1 | 32 | 0.9981 |
5 | S2 | 1 | 10 | 0.9996 |
5 | S3 | 1 | 16 | 0.9997 |
7 | S1 | 2 | 31 | 0.9983 |
7 | S2 | 2 | 25 | 0.9393 |
7 | S3 | 0.8 | 16 | 0.9968 |
Change in specimen’s length upon heating by TMA. (a) S1; (b) S2; (c) S3.
LTEC for the PP/clay composites at various clay contents.
Dimensional specimen’s change at 0°C.
The LTEC of the less oriented neat PP prepared by compression molding was seen not to be affected by direction of test whether it was parallel or perpendicular to compression direction. The LTEC of the PP/clay composites was seen to remain unchanged at moderately high clay loading of 5 wt% regardless of the direction of the surface at which the LTEC was measured. Addition of clay to neat PP was seen to minimize its shrinkage or contraction behavior.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank King AbdulAziz City for Science and Technology (KSCT) in Saudi Arabia and Izmir Institute of Technology (IZIT) in Turkey for providing required support to carry out this research. Dr. Alsewailem thanks his employer, KACST, for granting him a postdoctoral assistantship during his visit to IZIT.