Nondestructive testing of defects in adhesive joints

processing of different commercial thermoplastic materials are given in figure 1 [21]. It may be
observed that though many high-volume commodity thermoplastics can be processed at or below
about 250°C, all the high-performance engineering thermoplastics need temperatures between
250°C – 350°C for processing, whereas some specialty polymers need up to 400°C.
To compare the state of art in nanoclay modification, some of the thermal stability indicators
namely the temperature at initiation of thermal degradation and the temperatures at maximum rate
of degradation, derived from the earlier works [17,22] are also presented in figure 1. It can be
observed (Fig.1) that ammonium cation based modifiers, which are most commonly used for
modification of nanoclays, have the lowest thermal stability among all the classes of modified
presented above. While at about 250°C the phosphonium, cation based nanoclay just initiate to
degrade and the other higher temperature nanoclays do not show any degradation, the ammonium
based nanoclays reaches almost the point of maximum rate of degradation. According to the TGA
data reported by Calderon et al., a typical ammonium nanoclay, with grade name Cloisite 10A,
attains a weight-loss rate of 0.25 %/ °C. Taking the heating rate for the TGA experiment of 20°C
/min [17], it can be inferred that the nanoclay degrades at a rate of about 5% per minute. For a
more stable ammonium modified nanoclay, Closite 15A, the rate was about 1.5% per minute,
which rapidly increases to 3.4% at 300°C and reaching the maximum of about 5.5% at 325°C.
Further, it should be noted that the TGA experiments were run under nitrogen atmosphere
whereas thermoplastics are melt-processed in air and a typical thermoplastic itself may have
functional groups that could be reactive to the organic modifier at the processing temperature,
leading to rather severe degradation of the modifier groups on the nanoclays. This causes the loss
of the alkyl long chains from the modifiers, which increases the interlayer interaction, preventing
the exfoliation of nanoclays. In some cases, if the nanoclays are already exfoliated at lower
temperatures (e.g. master-batching at lower temperatures using other processing methods),
exposure to high temperatures in subsequent processing steps may cause collapse of the nanoclay
layers on lager size particles.
In contrast to the ammonium based nanoclays, the nanoclays modified with higher temperature
modifiers show negligible degradation up to 300°C. The phosphonium based nanoclay, which
found to be least stable among the nanoclays studied, reaches a degradation rate of 0.15% per
minute or 3% per minutes only above a temperature of 400°C, at the same experimental
conditions. Similar observations may be made from figure 1 for the imidazolium and pyridinium
based nanoclays too. Hence, development of organic modifiers based on these thermally stable
cations should help in translating the technology to the systems of engineering thermoplastics.
Since modified nanoclays has been successfully used in PA-6, which has processing temperature
range that extends above 250°C, it may be hoped that some of the nanoclay modifier might
survive the processing temperatures of specialty, high-temperature polymers such as Ultem*, PPS
or PEEK.
* - Trademark of Sabic Innovative Plastics
Conclusion:
Though the technology of nanocomposites based on modified nanoclays has been demonstrated
in a number of different polymers, it could reach the market only in cases of polymers having
comparatively lower processing temperatures. Even in the market it is not very clear how well
these new materials are performing. There may still be further need for the technology to achieve
the right balance between the overall advantages in terms of property improvements and the cost.
It may be a greater challenge to achieve this in commodity thermoplastics. On the other hand,
there are strong market needs for special properties, such as thermo-mechanical stability,
toughness, low flammability, chemical stability, high gas-barrier properties in polymeric
materials. Nanocomposites of commodity plastics may match some of the properties (e.g. room