Nondestructive testing of defects in adhesive joints

polypropylene exhibits β-chain scission reaction (degradation) with the addition of peroxide.
Hence the use of only peroxide is limited to the preparation of PP based TPVs. An alternative
approach is to use coagent together with peroxide curing system. Generally coagents are
multifunctional vinyl monomers which are highly reactive towards free radicals either by addition
reaction and/or by hydrogen abstraction. Chain scission also could be retarded by stabilizing the
PP macroradicals by addition reaction across the double bond in the vinyl monomer (coagent).
Hence addition of coagent in the PP/EOC blend increases the crosslinking efficiency in the EOC
phase and decreases the extent of degradation in the PP phase. Different coagents have different
reactivity and efficiency in terms of increasing the degree of crosslinking and decreasing the
extent of degradation. The main objective of the present investigation is to study the influence of
three structurally different coagents as a function of concentration on the dicumyl peroxide cured
PP/EOC TPVs in terms of mechanical and rheological characteristics.
Experimental
Materials - The general purpose polyolefin elastomer Exact 5371 (specific gravity,0.870 g/cc at
23 °C; co-monomer octene content 13 %; melt flow index,5.0 @190 °C/2.16 Kg), was
commercialized by Exxon Mobil Chemical company, USA. Polypropylene (Specific gravity,
0.9 g/cc at 23 °C; melt flow index, 3.0 @ 230 °C/2.16 Kg) was obtained from IPCL, India.
Dicumyl peroxide (DCP) (Perkadox-BC-40B-PD) having active peroxide content of 40 %;
temperature at which half life time (t1/2) is 1 hour at 138°C; specific gravity of 1.53 g/cm 3 at 23
°C) was used as the crosslinking agent obtained from Akzo Nobel Chemical Company, The
Netherlands. Three different types of coagents, Triallylcyanurate (TAC), Trimethylol propane
triacrylate (TMPTA), N,N’-m-phenylene dimaleimide (HVA-2) were used as boosters for DCPcured
TPVs, were obtained from Sartomer Company, USA.
Preparation of PP/EOC TPVs - The TPV compositions employed are shown in Table 1. The
experimental variables are the type and concentration of different coagents. All TPVs were mixed
by a batch process in a Haake Rheomix 600 OS internal mixer, having a mixing chamber volume
of 85 cm 3 with a rotor speed of 80 rpm at180°C. Immediately after mixing, passed once through a
cold two-roll mill to achieve a sheet of about 2 mm thickness. The sheet was cut and pressed in a
compression molding machine (Moore Press, Birmingham, UK) at 200°C, 4 min and 5 MPa
pressure. The sheet was then cooled down to room temperature under pressure. Different
coagents not only differ in molecular weight but they also have different relative functionality.
Hence in order to compare different coagents, concentration employed should be in terms of
milliequivalents.
Testing Procedure - Tensile tests were carried out according to ASTM D412-98 on dumb-bell
shaped specimens using a universal tensile testing machine Hounsfield H10KS at a constant
cross-head speed of 500 mm/min. Tear strength were carried out according to ASTM D-624-81
test method using un-nicked 90° angle test piece. Phase morphology of the cryo- fractured and
etched samples was investigated by a JEOL JSM 5800 Digital Scanning Electron Microscope
(SEM). Melt rheology of the blend components were studied in Rubber Process Analyzer (RPA
2000, USA). Each samples underwent the following test in sequence and in this order: frequency
sweep (FS), strain sweep (SS) followed by relaxation period of 5 mins, frequency sweep, and
strain amplitude sweep. Frequency sweep was logarithmically increased from 0.05 to 32 Hz at
6.95 % strain, which was selected to ensure that the dynamic moduli are measured in the linear
viscoelastic region. For the strain sweep, amplitude ranges from 1 – 1250 % at 180°C with a
constant frequency of 0.5 Hz. The sample relaxation was monitored by observing the decrease in
shear rate with time.
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