sion over PVDF finishes have also been demonstrated,
which do not involve significant VOCs. The most effective solution, in fact, may be the simplest: a thorough
cleaning of the surface, followed by scuff sanding. This
procedure seems to be very effective in generating a
contamination-free PVDF surface that can be readily
adhered to by PVDF-acrylic hybrid dispersion formulations; in fact, excellent adhesion and water blister resistance was obtained by even formulation C1, directly
applied over the fresh commercial PVDF substrate S1,
when this substrate preparation strategy was followed.
For situations involving the touch up or repair of relatively new PVDF finishes, scuff sanding the substrate is
very highly recommended.
Another low-VOC strategy which seems to be effective
in many cases is to use a waterborne primer based on a
low Tg acrylic emulsion polymer. These kinds of primers
can dramatically improve the water blistering and adhesive properties of PVDF-Acrylic Hybrid A coatings.
The above approaches to enhance adhesion generally can be used in combination as well. Finally, as was
observed in this study, attention must be focused on
the formulation details because the excessive use of hydrophilic surfactants and rheology modifiers can greatly
influence the water sensitivity of a coating, especially
with latex-based systems. Such ingredients should be
used judiciously and in limited amounts.
Acrylic Tg Effect
Coating C4, prepared from the lower Tg PVDF-Acrylic Hybrid B, showed much better adhesion and
high water-blister resistance on top of the S3 (aged
Lab-PVDF) substrate, compared to C1 with PVDF-Acrylic Hybrid A. Since the lower Tg latex particles in
the Hybrid B gave lower modulus coatings, as demonstrated by mechanical testing, we hypothesize that the
adhesion improvement is due at least in part13 to improved contact between the topcoat and the aged PVDF
finish, resulting from the lower coating modulus. An
additional factor contributing to the improved wet adhesion may be the much lower level of coalescent used
in Coating C4 (about one third the amount used in C1;
see Table 2), which could help reduce the total residual
hydrophilic content of C4 relative to the other coatings
tested (a lower acrylic Tg would also tend to promote a
lower residual hydrophilic content, by enhancing the
diffusion rate).
Whatever the exact reason for the improvement, the
good adhesion performance of Coating C4, based on
the PVDF-Acrylic Hybrid B, is especially noteworthy as
this formulation is below the 50 g/L VOC limit that is
in effect for restoration coatings in the South Coast Air
Quality Management District (SCAQMD) region.
Influence of Substrate
The wetting properties and roughness of surfaces play
a dominant role in adhesion between coatings and surfaces. 13 In this study, the adhesion of PVDF-acrylic hybrid
coatings on a new commercial PVDF finish was poor
( Table 3), unless some mechanical means like scuff sanding was used to prepare the surface. In a series of publications, Hinder et al. 7 have shown that the low molecular
weight wetting and flow agents which are often used
in the primer and topcoat layers of commercial PVDF
finishes, such as siliconized additives, can migrate to the
topcoat surface. Such species would reduce the surface
energy, potentially reducing the wetting of the surface by
any subsequent overcoat and reducing the adhesive energy, and could also impede the generation of mechanical linkages across the interface due to polymer reptation.
The high advancing water contact angle of Substrate S1,
as described in the section on surface characterization,
strongly suggests the presence of low energy species on
the commercial finish S1, and explains why careful cleaning and mechanical treatments are required to achieve
good adhesion in such cases, even when waterborne
PVDF-acrylic hybrid coatings are applied to them.
On the other hand, when a PVDF finish is made
in the absence of such additives, as was the case for
lab prepared Substrate S2, there seems to be sufficient
wetting and adhesive force that a PVDF-acrylic hybrid
dispersion topcoat can achieve excellent adhesion in
both dry and wet conditions, even without scuff sanding and other mechanical treatments. The wet adhesive
properties are largely lost, however, when the same
substrate has been weathered for an extended period of
time in the outdoors (Substrate S3), despite an increase
in the surface roughness which might normally be expected to improve the adhesion (see Figure 2). In this
case, it seems likely that photooxidized materials on
the surface, which are hydrophilic enough to lead to a
reduction in the water contact angle, provide pockets at
the interface where water and hydrophilic components
can migrate and reside. This leads to a build up in the
osmotic pressure which causes the observed blistering
and wet adhesion failure. In the case of aged PVDF finishes, however, it appears that these impediments can
be rather easily overcome, either through careful surface
preparation, the use of primers, formulation (as demonstrated with PVDF-Acrylic Hybrid Dispersion A), or by
the optimization of the PVDF-Acrylic Hybrid Dispersion
itself (Coating C4, based on Hybrid Dispersion B).
CONCLUDING REMARKS
In this study, we have discussed approaches to im-
prove the adhesion of restoration coatings based on
newly developed, water-based PVDF-acrylic hybrid
