JEC COMPOSITES MAGAZINE - Issue #119 - March 2018 - 79

Ensuring the durability of adhesive-bonded assemblies

Fig. 5: IR monitoring of an adhesive's chemical changes [3]

In situ characterization of the joint
During the water-saturating process, the bulk specimens are
monitored gravimetrically until saturation. The gravimetric
curve serves to determine a coefficient for the diffusion of
water in the adhesive.
The shape of the curve (Figure 6) is used to determine the
diffusion kinetic law:
- The simplest is of the Fick's law type, with a single phase
and a plateau with a stable saturation rate (smore epoxy
adhesives follow this law below their glass transition temperature).
- A more frequent model is of the Langmuir (or Carter and
Kibler) type, with two phases:
- A phase characterized by water molecules that diffuse into
the polymer and bind to the polymer's polar areas,
- A phase characterized by free molecules with no direct interaction with the matrix (they occupy the free spaces).

These two types of diffusion can be modelled using two parameters (Fick's law: diffusion coefficient D and saturation
rate Ms) or four (Langmuir model: D, α, β and Ms).
The saturation rate Ms varies linearly as a function of the temperature and the percent of HR.
The diffusion coefficient D and the coefficients α and β vary
as a function of the temperature according to an Arrheniustype law.
It is possible to use curves for water ingress into the bulk adhesive (according to the Fick or Langmuir/Carter and Kibler
model) to calculate the diffusion of water in the adhesive
within a bonded assembly, whatever the ageing conditions
applied (one-, two-, or three-dimensional modelling is possible). These models are well-described in the Mario thesis [4]
(see Figure 7).

Fig. 7: Modelling the diffusion of water in an adhesive line [4]

Two types of specimens are used to model the water diffusion:
- Specimens of the Thick Adherend Shear Test (TAST)
type (NF EN 14869-2), Figure 8, to obtain the intrinsic
adhesive properties (shear modulus and elastic limit).
These tests are carried out at ambient temperature, at the mini
and maxi temperatures stipulated in the specifications, in the
saturated state, and in the dried state. TAST tests can also be
carried out on a specimen that was subject to a stress (below
the adhesive's reversibility limit) during the sorption and desorption phases.
The influence of a stress applied to the adhesive during the
sorption and desorption phases has been studied in various
theses, although inconclusively. In some cases, applying the
stress apparently does not change the sorption/desorption
kinetics; in others, it apparently increases the diffusion and
saturation rates.
Fig. 6: Sorption/desorption curves: Fick's law; Langmuir's (Carter's and Kibler's)
law

- Specimens of the tension/shear type (according to NF EN
1465) with substrates that are representative of the assemN°119 March 2018

/ jec composites magazine

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #119 - March 2018