Nondestructive testing of defects in adhesive joints

Experimental
Materials
Natural crumb rubber (ISNR-5): Weight average molecular weight = 8,20,000; Intrinsic viscosity
in benzene at 30 0 C= 4.45 dL/g; Wallace Plasticity, P0=39 was supplied by Rubber Research
Institute of India, Kottayam, Kerala.Toluene diisocyanate (TDI) (was 80/20 mixture of 2,4-and
2,6- isomers) was supplied by Aldrich, Germany and was used as received. Dibutyl tin dilaurate
(DBTDL) supplied by Fluka, Switzerland, was used as catalyst without further purification.
Hydrogen peroxide (30% w/v) supplied by E. Merck, India was used without further purification.
Toluene (reagent grade) obtained from E. Merck; India was used as solvent without further
purification. Methanol (reagent grade) supplied by E. Merck, India was used without further
purification. Chloroform (laboratory reagent grade) was dried with anhydrous calcium oxide and then
distilled before use. It was supplied by E. Merck, India.
Polyethylene oxide (PEO) of molecular weight ( M n ) 1000 and 4000 (PEO-1000 and PEO-4000
respectively) supplied by Aldrich, Germany were used as received. NaCl (Merck, India), KCl
(Merck, India), MgSO
−
4 (Merck, India), BaCl2 (Merck, India), CaCl2 (Merck, India) were used as
received. Double de-ionized water was used for the preparation of metal salt solutions.
Synthesis
Preparation of hydroxyl terminated liquid natural rubber (HTNR)
HTNR was prepared by the less vigorous but efficient method developed by Ravindran [26] et al
using H2O2 and solar radiation.
Natural crumb rubber was masticated for 30 minutes at 40 0 C, 500gm of it was dissolved in 10L
toluene and the solution was charged into a flat bottomed flask made up of borosilicate glass and
500 ml of aqueous hydrogen peroxide (30%w/v) was added and thoroughly mixed with rubber
solution. The mixture was then homogenized to a certain extent by the addition of 750 ml of
methanol. The whole assembly with the material for irradiation was placed in sunlight under
stirring. After an exposure of 60 hours a layer of water separated at the bottom with some white
deposits. This was separated and the upper layer comprising liquid natural rubber in toluene was
distilled to remove the toluene to recover the liquid natural rubber (HTNR). It was purified by
precipitation by methanol twice from toluene solution and was dried in vacuum oven.
Synthesis of NR/PEO block copolymer
The second part of the experiment comprises the synthesis of NR/PEO block copolymer as given
below [27].
HTNR (24gm) was dissolved in 68ml of chloroform (35%w/v) to get a solution. The whole
solution was taken in a reaction vessel, dibutyl tin dilaurate catalyst (6% by weight of HTNR)
was added and the solution brought to reflux with stirring. The stoichiometric amount of TDI
(1:2.76:1 of HTNR) was added in drops, followed by the drop wise addition of 6gm of PEO-
1000/24gm of PEO-4000 in chloroform (35%w/v) within a period of 1.5 hour. The reaction was
allowed to continue for another 2 hours. Excess chloroform was then distilled off and the viscous
polymer solution was cast in trays treated with silicone releasing agent. The sheet was removed
from the tray after 24 hours, kept in a vacuum oven at 60 0 C to remove traces of solvent and then
cured at 70 0 C for 24 hours, followed by one week ageing at room temperature in a moisture free
atmosphere.