Method for corona treating thermosets

This development is a method for accelerating thermoset resin cure and enhancing adhesions of coatings to thermoset articles comprising the steps of providing a thermoset article having at least one surface and corona treating the thermoset article to increase the surface energy thereof. The corona treating allows for painting in line with the pultrusion process for producing thermoset articles.

TECHNICAL FIELD 
This invention relates to a method for corona treating a thermoset article 
to increase the polarity of the surface thereof More particularly, 
continuously advancing elongate members are so treated to accelerate paint 
cure and enhance paint adhesion. 
BACKGROUND OF THE INVENTION 
Applying a coating, such as paint, to all or part of an elongate member, 
such as an FRP pultruded lineal used to fabricate windows, continues to 
require improvement. When the elongate member is pultruded, advantages 
exist in coating contemporaneously or in-line with the pultrusion process. 
Painting in-line, however requires coordinating a multitude of variables. 
The process must coordinate painting steps with pultrusion steps. Painting 
in-line in particular requires addressing paint adhesion to the 
continuously advancing article. This is especially true where the article 
comprises a thermoset resin. 
DISCLOSURE OF INVETION 
We now have developed a method for accelerating cure and enhancing adhesion 
of coatings to thermoset articles comprising the steps of providing a 
thermoset article having at least one surface and corona treating the 
thermoset article to increase the polarity of the surface thereof. The 
corona treating oxidizes the surface of the thermoset article. The treated 
surface is thereby more polar and has a higher surface energy level. The 
corona treating also forms mechanical sites on the surface which further 
aids adhesion of the coatings thereto. Preferably, the articles is an 
advancing elongate member and the corona treating is applied continuously. 
Our method preferably includes the step of coating the surface of the 
article after the corona treating. Typically, the thermoset article is a 
polyester and the coating is a two part acrylic modified urethane which 
can be solvent diluted.

BEST MODE OF CARRYING OUT THE INVENTION 
FIG. 1 illustrates a double-hung window 10 including a frame 12 and upper 
and lower window sashes 14 and 16 constructed of lineal structural 
members. Each of frame 12 and sashes 14 and 16 has straight top, bottom 
and opposite side members. Each sash 14 and 16 is shown with an insulating 
glass unit 18, although removable double glazing may be used instead. 
FIG. 2 shows shaped fibrous glass structural member 20. Core 22 for a 
structural member 20 is a glass fiber board including glass wool 
impregnated with about 20% or less, suitably 14% by weight of a phenolic 
resin binder such as phenol-urea-formaldehyde and molded and cured to a 
density of less than 20 pounds per cubic foot, suitably 6 to 8 pounds per 
cubic foot, and to an appropriate thickness. The board is appropriately 
grooved at opposite ends and slip into core 22 of appropriate rectangular 
cross-section. A casing encases core 22 and comprises mats 26 and 28 and 
rovings 30 impregnated with resin 32. The casing provides a cover around 
core 22 having a high quality, void-free surface finish that is 
reinforced. Generally, mat 28 is a polyester veil, mat 26 is a continuous 
glass strand mat and resin 32 is a polyester resin. Mat 28 may be a 
conductive veil capable of being grounded. 
Structural member 20 may be made by any continuous process such as by 
pultrusion. A preferred method and apparatus for producing the continuous 
elongate member is that U.S. Pat. No. 4,681,722 discloses. 
The thermoset polymers solidify or set irreversibly when heated. Thermoset 
usually means a cross-linking reaction of the molecular constituents 
induced by heat or radiation. In many cases, one needs to add "curing" 
agents such as organic peroxides or (in the case of rubber) sulfur. For 
example, linear polyethylene cross-links to a thermosetting material 
either by radiation or by chemical reaction. Phenolics, alkyds, amino 
resins, polyesters, epoxides and silicones are thermosetting; but the term 
also applies to materials where additive-induced cross-linking is 
possible, e.g. natural rubber. 
Use of corona treatment enhances adhesion of protective and decorative 
surface treatments on thermoset parts, achieves higher cure levels and 
eliminates the need for primers or abrasives in conjunction with thermoset 
parts coating. Corona treatment increases adhesion of coatings/tie layer 
adhesives to thermosets without need for primers or abrasives. The 
treating also increases the cure level of thermoset parts. 
Prior to investigations of corona treatment strategies, thermoset lineals 
had been surface prepped with a commercial power blaster at a cost 
penalty. Corona treatment is a methodology whereby atmospheric ionized 
gases are directed onto a surface. The chemical species on the surface are 
oxidized by reaction with the generated ion rich corona plume and some 
mechanical surface alteration also takes place. The oxidized surface 
species are polar and thus the surface now has higher surface energy. This 
allows materials such as paints to adhere to this treated surface through 
strong Van der Waals' attraction to the polar species and some mechanical 
sites formed in the process. Use of corona treatment allows parts to be 
painted or coated without abrasive treatment or priming and at very low 
surface treatment cost. 
We investigated corona treatment on a thermoset system in conjunction with 
liquid paints, powder paints, moisture-cure hot melt adhesives and 
thermoplastic coatings. We saw improved adhesion for all systems; the need 
for alternative mechanical surface prep was eliminated. Since our surface 
is a filled free radical initiated polyester resin, we also documented a 
increase in resin cure level achieved by corona treatment. This stems from 
interaction of the resin with free radicals (ions) from the corona plume 
which essentially increase the concentration of free radicals which force 
further polymer cure. The free radical or ions at the surface propagate 
through standard mechanisms into the resin. Corona treatment of thermosets 
is enabling technology for in-line painting processes. It is applicable to 
all pultrusion processes requiring protective or decorative surface 
finishes. 
The paint we prefer is a two part acrylic modified urethane which can be 
solvent diluted. 
Conventional paints and stains which also can be used for coating include 
the following: phenolic, urethane, epoxy, acrylic cationic latex, acrylic 
anionic latex, water-reducible polyester, thermoplastic and latexes. We 
also can use powder coating techniques, as well as transparent or 
translucent stains. 
EXAMPLE 
Corona treatments have been found effective using a wide range of 
equipment. Most work has been done using a corona field generated with 60 
Hz and 30 K electrode volts, but is equally effective with fields 
generated over a range of frequencies and voltages, such as, 2 M Hz and 
20K-250 K volts, Hz to 30 K Hz and 30 K volts. 
As an example, a paint adhesion run was made on a thermoset polyester resin 
lineal, using a 60 Hz, 30 K volt corona unit. The corona plume was applied 
at two different locations having lineal temperatures of 265.degree. F. 
and 145.degree. with a lineal speed of 5 FPM. Adhesion was measured 
separately for each temperature and line location. The control for both 
temperatures was 0 to 1 or no paint adhesion. After corona treatment, 
adhesion was 4 to 5. Paint adhesion was measured using the industry 
standard cross hatch method and a two part acrylic modified urethane paint 
system. No adhesion has a value of zero and complete adhesion has a value 
of 5. Acceptable adhesion is 3 or higher. 
A second example looked at the effect of corona treatment on accelerating 
resin cure. Under this condition the corona plume was applied to the 
window lineal under the conditions above and discharge end of the forming 
die. Without the corona unit, the control gave a DMA7 cure index of 13.8. 
The index was 5.3, with the corona on. The acceptable range is a maximum 
of 12 and a preferred range of 6 to 7 or less.