Page 174 - Handbook of Adhesion Promoters

8.22 Polyurethane 167

8.22 POLYURETHANE

Many commercial adhesion promoters can be used in polyurethanes, including Chartwell,
Chartsil, Dow Corning, Dynasylan, Geniosil, Isoexter, Priplast, and Silquest. There are too
many products to be listed by their grade names here but a full account of these additives
can be found in Databook of Adhesion Promoters which also contains additional informa-
tion on the use of these additives.
The isocyanate molecules adhere strongly to the metal substrates, which points to a
1
chemical adhesion mechanism. Urea-like species form on all metal surfaces from the
1
reaction of MDI with adsorbed water molecules. The concentration of urea groups is
greater on Al and Cu than on Au because more H O molecules are adsorbed on the Al and
2
1
1
Cu surfaces. Urea groups do not contribute to adhesion.
Vinyltrimethoxysilane pretreated aluminum alloy had good adhesion performance in
dry, wet states after coating with polyurethane at pH < isoelectric point which is related to
2
the silanol group orientation toward aluminum surface. Water sensitivity of unreacted
vinyltrimethoxysilane molecules leads to the poor protective performance of specimens
2
treated at pH > isoelectric point. The higher the vinyltrimethoxysilane concentration at
2
pH=5, the greater the adhesion strength. Silane pretreatment at pH=9 resulted in the low-
2
est adhesion strength at dry, wet and recovered states. The higher the vinyltrimethoxysi-
2
lane concentration at pH=9, the worse the adhesion strength.
A γ-mercaptopropyltrimethoxysilane was found to be good adhesion promoter for a
waterborne polyurethane paint coatings on galvanized steel. 3
An aqueous primer solution comprised water, an aminotitanate compound, and an
epoxysilane compound having one epoxy group and two alkoxy groups increased adhe-
4
sion of polyurethane sealants to various substrates.
REFERENCES
1 Nies, C; Fug, F; Otto, C; Possart, W, Int. J. Adh. Adh., 52, 19-25, 2014.
2 Zand, BN; Mahdavian, M, Surf. Coat. Technol., 203, 12, 1677-81, 2009.
3 Seré, PR; Deyá, C; Elsner, CI; Di Sarli, AR, Procedia Mater. Sci., 8, 1-10, 2015.
4 Yao, H; Rosenberg, S; Blank, N, US6511752, Sika Corporation, Jan. 28, 2003.