Nondestructive testing of defects in adhesive joints

2. Experimental Details
Polypropylene was obtained from Reliance Industries Ltd, India (REPOL H200F, melt flow index 20
g/10min). MWNT were obtained from DMSRDE, Kanpur. Various compositions of PP/MWNT
composites were prepared by melt-mixing in which MWNT were varied from 0.5-3 wt% in a conical
twin-screw microcompounder (Micro 5, DSM Research, Netherlands) at optimized melt mixing
condition of 260 0 C with a rotational speed of 200 rpm for 15 min. Melt compounded PP/MWNT
samples were dried at 80 0 C under vacuum over night. They were melt-spun using a small scale
spinning machine manufactured by the Bradford University Research Ltd. UK, using a single hole
spinneret of 1 mm diameter. The fiber spinning was carried out at 200 0 C with ram speed of 1.4
mm/min. The take up speed was kept at 20 m/min for all compositions. The melt spun fibers were
further drawn at draw ratio of 8 by varying the post drawing parameters (temperature and drawing
speed). Details of variation of post drawing parameters provided in Table 1 which depicts 12
exiperments were performed for each MWNT concentration of 0.5, 1 and 3 wt% respectively. The
mechanical properties of drawn fiber were tested on a Favimat fiber tester (Textechno) with gauge
length of 20 mm and at a rate of 20 mm/min for 50 filaments.
3. Result and Discussion
3.1 Mechanical properties
If the composite fibers are to be obtained of high tenacity, high modulus of elasticity and low
elongation at break; nanotube orientation distribution along the fiber axis should be higher. In this
context, it is also reported that CNT/polymer interface influences significantly the mechanical
properties of the composite fibers [10].
Figure 1 exhibits the distribution of modulus, tenacity and elongation at break of composite fibers at
various post drawing parameters. It is observed that variation in post drawing parameters lead to the
dramatic changes in mechanical properties of composite fibers.
Tenacity (which is a measure of fineness of fiber) and modulus increases with decreasing drawing
speed and corresponding increase in temperature (Figure 1a-b). High modulus values are achieved for
those combinations of post drawing parameters which exhibits less elongation. This may be due to
the addition of MWNT which prevent the polymer chain from slipping past each other thereby
improving modulus and reducing the elongation. In case of pure PP, failure is found to occur due to
PP chains sliding past each other according to Smith Irvine model [11].
It is observed that at lowest drawing speed i.e 8 m/min modulus values are found to be higher than
composite fiber drawn at higher drawing speed for all the combination of temperature and various
weight fractions of MWNT. It is the orientation distribution of MWNT [10] at a definite post drawing
parameter which contributes to the superior mechanical properties of composite fiber. The highest
modulus and tenacity value are observed at post drawing parameters of 100 0 C, 8 m/min for 1 wt % of
MWNT and 120 0 C, 8 m/min for 0.5 wt % of MWNT (Figure 1a-b).
3.2 Discussion
It is interesting to note that with an increase in temperature modulus increases at low drawing speed.
At 80 0 C, drawing of fiber much above the glass transition temperature, the interaction between chains
and interaction in between chain and nanotubes restrict the achievable orientation of MWNT.