Page 20 - Handbook of Adhesion Promoters

2.3 Diffusion and entanglement 13

2.3 DIFFUSION AND ENTANGLEMENT

There are two important stages in
this mechanism, if polymers are
involved, namely diffusion and
entanglement formation. Diffu-
sion process permits chain of one
material (e.g., adhesive) to pene-
trate the other material (e.g.,
adherent). In most cases, interdif-
fusion occurs due to the diffusion
Figure 2.10. Steps involved in fusion bonding of thermoplastic products of both materials (adher-
surfaces. [Adapted, by permission, from Asseko, ACA; Cosson, B;
Lafranche, E; Schmidt, F; Le Maoult, Y, Composites, B97, 53-61, ent and adhesive) to cross the
2016.] boundary between them (called
interface) and to form an interme-
diate layer which has a composi-
tion different than any of the two
components and it is called inter-
phase. Figure 2.10 illustrates
these two steps leading to the for-
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mation of interface. From the
illustration, it becomes pertinent
that temperature is one of the most
Figure 2.11. Welded joint interphase. [Adapted, by permission, important variables of the pro-
from Asseko, ACA; Cosson, B; Lafranche, E; Schmidt, F; cess. It is also quite obvious that
14
Le Maoult, Y, Composites, B97, 53-61, 2016.]
thermal welding processes are the
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primary examples of this type of the mechanisms of adhesion. Figure 2.11 shows the
result of welding of polycarbonate containing glass fiber (top) and polycarbonate contain-
14
ing carbon black (bottom). Both glass fiber and carbon black are found in the inter-
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phase. Macromolecular structures on each side of the welding interphase penetrated to
14
establish an entangled network.
The effect of temperature can be clearly seen from Fick’s law of diffusion:
E

⁄
x ∝ exp   ----------- t 12 [2.1]
2RT 
where:
x average interpenetration depth
E diffusion activation energy
R gas constant
T temperature
t time of contact
Considering that Fick’s law of diffusion has limitations in the description of polymer
chain movements, reptation theory was later developed and adopted to the description of
interdiffusion of amorphous polymers in welding processes. The distance of interpenetra-
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tion of chain through interface is given by the following equation: