Nondestructive testing of defects in adhesive joints

Excess PVA solution was drained off and the membranes were dried at ambient temperature. The
membranes were subsequently heated in oven at a temperature of 125±2 ºC for 30 minutes.
Characterization of Membranes:
FT-IR: The IR spectrum of PSF ultrafiltration and cross-linked PVA TFC membranes (of size 6 cm × 3
cm) were recorded in the middle infrared region using a Perkin Elmer 400 FT-IR spectrometer.
Contact angle (θ): Contact angle of resulting PVA TFC membranes was studied by Wilhelmy plate
method using Dynamic Contact Angle Tensiometer (DCAT 21, Data Physics).
Scanning electron microscopy (SEM): Phase morphology (Cross-sectional and topography) of the
membranes were studied using Leo microscope at 15 kV accelerating voltage.
Membranes performance testing: The salt rejection characteristics of membranes were studied in RO test
cell by passing aqueous feed solutions of mono and bivalent salts separately through the membranes at
different pressure (150-350 psi). Permeate flux was studied and solute rejection was calculated with the
help of equation 1. The concentrations of inorganic salts in feed and permeate were determined by
inductively coupled plasma spectrometry (ICP), chemical analysis and conductivity measurements.
⎡ Concentrationof
solutein
permeate⎤
Soluterejection(
R)
(%) = ⎢1
−
⎥×
100
( 1)
⎣ Concentrationof
solutein
feed ⎦
Molecular weight cut-off (MWCO): A mixed solution containing glucose, sucrose, raffinose and glycerol
each of 500 ppm was passed through the membranes. The percent organic solute in feed and permeate
was analyzed by High performance liquid chromatography (HPLC) and Total organic carbon (TOC)
analysis. The solute rejection was calculated by using the formula 1. The solute rejection above 85% was
taken as MWCO of membranes.
Result and Discussion
The base PSF substrate was prepared from 17% PSF solution and PVA layer was coated with 1% PVA
solution containing MA as curative in the concentration range from 2-50% (w/w with respect to PVA).
The IR spectrum of PSF base substrate membrane is shown in figure 1(graph I). The existence of PVA at
the surface of PSF TFC membranes is indicated by the peak at 3,200-3,400 cm -1 (Figure1, graph II),
which is the characteristics of OH of PVA. The peak at 1,335 cm -1 is due to the ester linkages formed by
the cross-linking of PVA with MA.
Contact angle reflects the hydrophilicity of any surface. PVA coating provides the hydrophilicity to
hydrophobic PSF membranes. The hydrophilic nature of PVA TFC membranes are reflected by their
average lower contact angle (81º) value compare to that of hydrophobic PSF membranes (89.5 º).
Cross sectional morphology of PSF membranes show a wide open pore structure (Figure 2A). However
coating of PVA partially masks the polysulphone layer and overall reduction in the average pore size is
observed (Figure 2B).
Effect of different process parameters on membranes performance:
Effect of variation of curative dose on membrane performance: To see the effect of MA concentration on
stability of rejection layer, the flux (Figure 3) and rejection (Figure 4) profiles of the membranes (varying
in the cross-link density of the rejection layer) were tested in continuous mode for a period of 24 hours
using 500 ppm MgSO4 solution. The membranes were cured at a temperature of 125±2 ºC for duration of
30 minutes. It is observed from Figure 3 that lowers the curative dose higher the membrane flux.
However all the membranes show almost consistent flux over the studied period irrespective of the crosslinker
dose. It is also observed (Figure 4) that in the rejection profiles there is a slight fall with time for
membranes prepared with lower curative dose of 2 and 5% MA. However for MA dose >5% the rejection
remains constant with time. Hence 5-10% concentration of MA may be considered as optimum for curing
of such membranes.
Effect of cure time on membrane performance: Membranes were prepared from 17% PSF and 1% PVA-1
with MA concentration of 10% (w/w) cured by varying the cure time from 10-40 minutes at a temperature
2