Environmental Engineering Reference
In-Depth Information
solution ensures that there is no kinetic limitation of the reaction of the
hydroxylized metallic surface (Me-OH) with the silanol groups (Si-OH)
forming hydrogen bonds which can be dehydrated during the curing pro-
cess to form the i nal Me-O-Si layer (Reaction 2.1) [34].
Si-OH
(solution)
+Me-OH
(metal surface)
↔
Si-O-Me
(interface)
+ H
2
O
(2.1)
Excess SiOH groups which are adsorbed on the metals but not interact-
ing with Me-OH groups condense among themselves and form a siloxane
(Si-O-Si) surface i lm (Reaction 2.2).
Si-OH
(solution)
+Si-OH
(solution)
↔
Si-O-Si
(silane i lm)
+H
2
O
(2.2)
At er the hydrolysis step, the silanol groups can be involved in conden-
sation reactions, resulting in polymerization and/or precipitation. Silanol
groups are important to form a covalent bond at the metal/i lm interface
and also for crosslinking in the bulk of the silane layer. In the solution, the
hydrolysed molecules can react to form dimers, trimers and bigger spe-
cies through condensation [16]. h e kinetics and equilibrium of hydrolysis
and condensation of silanes in solution are inl uenced by the nature of the
organofunctional groups, the pH of the solution, the concentration of the
silane itself, the temperature, and the aging of the solution [29, 35,36].
In the case of a bis-sulfur silane (PSS) solution [30], the total hydrolysis
reaction equilibrium is (Reaction 2.3):
(OR)
3
Si(CH
2
)
3
S
4
(CH
2
)
3
Si(OR)
3
+ H
2
O
↔
(OH)
3
Si(CH
2
)
3
S
4
(CH
2
)
3
Si(OH)
3
+3ROH
(2.3)
where, S - sulfur atom (4 represents average number of the S atoms con-
tained by each bis-sulfur silane molecule); OR - hydrolyzable alkoxy groups
or OC
2
H
5
in the case of the bis-sulfur silane; SiOH - silanol. During the
silane surface treatment the curing/drying step is much more important
than the immersion step compared to the conventional chromating pro-
cess where the i lm thickness is proportional to the contact time. Due to
the fact that hydrolysis and condensation reactions are catalyzed by acids
or bases, the silane aqueous solution stability is strongly dependent on the
pH [37, 38]. h us, in acidic or alkaline medium, the rates of both hydroly-
sis and condensation reactions are high, while at or near neutral pH they
are slow. If the reaction is catalyzed by HO
-
, condensation is favored with
rapid formation of a gel; in the case of a H
+
catalyzed reaction a high rate of
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