Page 72 - Handbook of Adhesion Promoters

4.2 Surface treatment 65

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(40 W). The water content greater than 90 ppm resulted in a rapid adhesion failure of
deposited films at the substrate-polymer interface during outdoor weathering, indicating
ageing reactions within the plasma polymer films and at the bulk of polymer-coating inter-
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face.
Microwave plasma surface treatment was applied to polyamide-11 and polyamide-
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12. Ammonia and nitrogen/oxygen mixtures were used. The wettability and adhesive
properties were greatly improved after plasma treatment, especially for the nitrogen/oxy-
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gen mixture.
4.2.5 FLAME
A poor adhesion of PP compounds can be improved by flame treatment, resulting in a
good paint adhesion to the PP parts which is important requirement in automotive applica-
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tions. Flame treatment can be used to oxidize the PP surface (~5-10 nm). For most PP
compounds an oxygen concentration of 10-12 at% (measured by XPS) is required for
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acceptable adhesion and only 4-5 at% for PE is adequate. The oxidation mechanism of
flame treatment can be described by initiating reactions (hydrogen abstraction, usually
from tertiary carbon; formation of hydroperoxides), propagation reactions (decomposition
of peroxides resulting in chains scission, crosslinking, and autocatalytic initiation of new
reaction sides) and termination reactions
resulting in the formation of end-products
(ketones can tautomerize forming enols
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active in hydrogen bonding). The air pro-
pane ratio of 22-26, flame distance of 10-15
cm, and conveyor belt speed of about 20 m/
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min gave the best results. A too high heat
input during the flame treatment (tempera-
o
ture above 80 C) or holding time at ele-
vated temperatures resulted in the
disappearance of the introduced O concen-
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tration and a loss of paint adhesion.
The silane having epoxy functional
groups significantly increased the adhesion
strength of acrylic lacquers with carboxyl
functionality to the flame-treated PP sur-
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face. The following conditions of flame
treatment were used: air/propane ratio: 24,
flame distance: 6-9 cm, flame speed: 4-
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8 m/min.
Flame treatment of polymeric materi-
als improves surface free energy and conse-
quently the wettability and adhesion of
Figure 4.12. Energy dispersive spectroscopy line analy- coatings, metallizations, varnishes and
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sis of cross-section of treated sample (5 kV, 25,000 glues. A mathematical model of surface
count 65,000×). % conc. of each element is shown (C, free energy and wettability was proposed. 31
Pt, Au, O). [Adapted, by permission, from Mazzola, L;
Bemporad, E; Carassiti, F, Appl. Surf. Sci., 257, 6, 2148- Also, focused ion beam and transmission
58, 2011.] electron microscopy were used to deter-