Nondestructive testing of defects in adhesive joints

Experimental
Polyamide6 (PA6 with zero shear viscosity = 180 Pa s at 260 o C) was obtained from GSFC,
Gujarat, India (Gujlon M28RC, relative viscosity 2.8, Mv is 38642 in 85% formic acid).
Polypropylene (H200MA) was obtained from Reliance Industries Ltd with melt flow index (MFI)
of 23. Acrylonitrile-butadiene-styrene (ABS) copolymer (Absolac-120, with composition as:
acrylonitrile 24 wt %, styrene 59.5 wt % and rubber content 16.5 wt %) was obtained from Bayer
India Ltd. Styrene-maleic anhydride copolymer (SMA) with 8% MA content (Dylark 232) was
supplied by Nova Chemicals, USA. The ternary blends of PA6/PP/ABS were prepared by melt
mixing in a conical twin-screw microcompounder (Micro 5, DSM Research, Netherlands) at
260 o C with a rotational speed of 150 rpm for 15 min.
Morphological investigation was carried out by scanning electron microscopy (SEM, Hitachi
S3400N).
Results and discussion
Depending on the composition two different types of morphologies have been observed
in PA6/PP/ABS ternary blends viz. core-shell and co-continuous morphology for the
compositions having 80/10/10 and 40/30/30 PA6/PP/ABS respectively.
The phase morphology of melt-mixed 80/10/10 PA6/PP/ABS blends has been investigated
through SEM and can be found in Figure1. Core-shell type morphology has been observed in
cryofractured etched surface of the extruded strands of the blends. By selective extraction of PP
(by hot xylene) and ABS (by THF) it is revealed from the SEM micrographs that ABS is found to
encapsulate the PP phase i.e. PP phase forming the core and ABS phase becomes the shell.
In case of 80/10/10 PA6/PP/ABS ternary blends system PA6 forms matrix and PP along with
ABS form dispersed phases or more specifically core-shell type of morphology is observed from
the SEM micrographs (Figure 1).The formation of this kind of morphology can better be
understood from the spreading co-efficients of the respective polymer pairs [9]. The spreading
coefficient λPP/ABS of PP phase over ABS phase can be described as:
λ PP/ABS = σ PA6/ABS – σ PA6/PP – σ ABS/PP -------------------------------------------- (2)
where σ ij is the interfacial tension between i and j components.
If λ PP/ABS is >0 then PP phase becomes shell and ABS phase core. Similarly, if λ ABS/PP (Eq. 3) is
>0 then ABS forms shell and PP core.
λ ABS/PP = σ PA6/PP – σ PA6/ABS – σ PP/ABS -------------------------------------------- (3)
If both λ PP/ABS and λ ABS/PP are negative, the PP and the ABS phases will disperse separately in
the PA6 matrix.
To determine the spreading co-efficient value, Interfacial tension (σ ij) data were calculated for
the three polymer-polymer interfaces present in the blends, i.e. PA6/ABS, PA6/PP and PPABS
using the harmonic mean equation [8]:
σ12 = σ1 + σ 2 - 4 σ d 1 σ d 2 / σ d 1 + σ d 2 - 4σ p 1 σ p 2 / σ p 1 + σ p 2 -------------------(4)
where σ ij is the interfacial tension between the components i and j. σ i is the surface tension of
component i, σ d i is the dispersive fraction of the surface tension of component i and σ p i is the
polar fraction of the surface tension of component i.