A floor had three coats of polyurethane applied, a sealer and two finish coats, and in one location the third layer delaminated.
The probable causes of the delamination were:
- Inactivated surface on coat 2
- Improper mixing coat 2 or 3
Contamination creates a barrier film between adjacent coating surfaces, preventing intimate contact. Reduced adhesion and ultimately delamination are the consequences. Potential sources of contamination could include:
- Aerosol sprays - Most will cause reduced adhesion, but not necessarily rejection of the coating. The reduced adhesion may not manifest as an actual failure until some time (months) later when film stresses occur in use;
- Dissimilar products - e.g. rollers previously used for sealer application being used for finish coat applications. Dissimilar materials result in a "stress fault line" if they occur in between coatings of the same material. Stresses at the interface between coats can result in adhesion failure;
- Surface active additives - e.g. Anti-silicone drops. These function by forming a film on surfaces. These films contribute to adhesion reduction, even at recommended dosage levels. This reduction is generally insignificant unless overdosing has occurred, or mixing in has not occurred. These additives are usually acrylic or polysiloxane (silicone family) in nature and
- Extraction of migratory species from substrate. e.g. oils and chemical treatments in the timber can be extracted by the solvent in the sealer to sit on the surface of the sealer. Application of a coat of polyurethane can then re-dissolve these extractables so that they will then sit on the surface of that polyurethane layer. Surface tension characteristics of the polyurethane finish/sealer interface mean that a rejection condition is rarely evident after the first finish coat. The second finish coat will then possibly show reduced adhesion as a result of being applied onto the extractable species sitting on top of the first finish coat.
2. lnactivated surface on coat 2 (finish coat 1).
Polyurethane cured surfaces are highly inert and will not adhere to other coats unless they are surface activated.
- Initial activation - applying a second coat at too early a state of cure will "re-dissolve" the first coat giving a condition called "frying". Too late a state of cure and no external activation will take place, resulting in reduced adhesion;
- Chemical activation – Recoat fluids provide a two fold action via micro-roughening of the surface and chemical activation of the polyurethane molecules in the cured film and
- Mechanical activation – Again provides a two fold action of surface roughening to allow for mechanical anchorage of the applied film, and also removal of surface oxidised material. Maximum disturbance of the cured film is desirable for maximal subsequent film anchorage or adhesion.
3. Improper formulation of coat 2 or 3 (finish coat 1 or 2).
- Improper mixing – When using a two component product, an improper mixing ratio due to inaccurate measurement or poor mixing in the application bucket can result in reduced adhesion of the coating. e.g. excess hardener component B will give a higher static adhesion, but the reduced film extensibility will increase interfacial stress and can contribute to delamination;
- Catalyst settling - All polyurethanes employ catalysts to assist cure. These materials are typically organometals of high specific gravity. This will result in them tending to settle with time. It is good practice to shake all cans before use. High catalyst concentrations will provide for reduced adhesions in a dynamic (surface film movement) situation and
- Added catalyst – The observed practice of catalyst dosing the first finish coat to enable all coats to be applied in the one day in inadvisable. Excess catalyst levels can reduce adhesion retention with ageing.
RECTIFICATION OF DELAMINATED FLOORS
Two approaches are possible:
- The "Band Aid" – The lowest cost rectification technique is to utilise a heavy screen back to remove the flaking material and apply another finish coat with a special additive. This may give a reasonable result. However, delamination may possibly occur on previously non-delaminated areas as evidenced by marks on the edges of previous delaminations and
- The correct remedy – Unfortunately, if one area of a floor has delaminated, other areas are likely to delaminate in the future. Sanding back to bare timber and repeating the complete coating process is the only guaranteed remedy.
Reference: Toby Technical bulletin TB003