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he November 1996 column

dealt with the damaging ef-

fects of compressive and ten-

sile stresses resulting from the ex-

pansion and contraction of paint

films as they respond to changes in

humidity. This month, we will con-

sider in more detail the effect of

small molecules as penetrants intothe dry film. While the film’s re-

sponse to such penetration parallels

its response to humidity, the magni-

tude of the response may be signifi-

cantly greater, and the consequences

may be more catastrophic.

This article will review the action

of penetrants through films, the in-

fluence of free volume and glass

transition temperature (Tg), penetra-

tion-related primer failures, and the

measurement of solvent resistanceof coatings.

Free Volume and Penetrant

The January 1996 column intro-

duced the concept of free volume.

In discussing the volumetric expan-

sion of a film caused by the entry

and accumulation of penetrants into

the film, we revisit free volume.

It is largely through free volume

holes that small penetrants enter in-

tact films, working their way down- wards into the film through adjacent

free volume sites in successively

deeper molecular layers. These sites

or holes become available as the

polymeric structure of the film vi-

brates with the kinetic energy of the

molecule. (These vibrations are of a

very high order , 1012 times each

second.) As the molecule or pene-

trant moves from one molecular

ingress and the accumulation of

even more penetrant.

In the case of thermoplastic sys-

tems, active solvent penetrants pro-

gressively dissolve the film. In cross-

linked systems and in thermoplastics where the penetrant is not an active

solvent, the extent of penetrant accu-

mulation and therefore swelling will

depend on the solubility of the pen-

etrant in the coating and the rate of

its diffusion within the film. These

are affected by the molecular struc-

ture of the polymer and penetrant,

the size and shape of the penetrant,

the affinity of the penetrant for

groups within the polymer, the

cross-link density of the film, and therelationship T-Tg. In less amorphous

thermoplastics, film structure is very

important, as is the molecular weight

of the polymer, the preponderance

of crystalline areas, the degree of

secondary valency bonding between

chains, and again T-Tg. These factors

will largely dictate the rate of entry

and diffusion of the penetrant.

layer to the next deeper layer, the

vacant site in the original layer is

taken up by a new molecule of pen-

etrant that enters the film at the lo-

cation being vacated.

The insertion of a highly polarmolecule, like water, into the poly-

meric continuum may provide a site

to which other water molecules are

attracted. This site will feature a hy-

drogen-bonded accumulation of

water molecules.

In other cases, functional groups

on the polymer structure of the

binder will facilitate the ingress and

accumulation of penetrants, for ex-

ample, hydroxyl groups on epoxies

and amide groups on urethanes. Theaccumulation of penetrant molecules

at such hospitable sites within the

polymeric matrix may eventually pry

apart the chains of the polymer. The

penetrants overcome some of the

secondary associations that, espe-

cially in thermoplastics, hold the film

together and thus reduce the Tg of

the film by plasticization. The weak-

ening of the film allows increased

MAY 1997 / 69

T

continued

TROUBLE with PAINT

Effects of Solvents onCoating Films

Fig. 1 - Solvent attackon a cross-linkedepoxy filmFigures courtesyof the author

by Clive H. Hare,Coating System Design Inc.

Copyright ©1997, Technology Publishing Company

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When the take -up of penetran t

is sufficient to induce severe changes

in non-soluble films, severe compres-

sive stresses build up within the

solvated film and cause mechanical

failure. Initially, this failure is centered

at the interface of the film and itssubstrate and is first manifested as

blistering from localized de-adhesion.

In some cases, the stress-induced de-

adhesion may be facilitated by an

accumulation of penetrant at this in-

terface, which displaces the coating

film. With the more aggressive sol-

vents, this effect is often secondary

to film distention, which precedes

any interfacial accumulation. In the

more severe cases, the solvated film

may expand so much that it delami-nates entirely, wrinkling into furrows

(Fig. 1). Delamination may be accom-

panied by concomitant vertical cohe-

sive failure, cracking, or breaking of

the film, as the film is strained be-

yond its elongation at break. (The

mechanical properties of the film, in-

cluding modulus, yield values, tensile

strength, and elongation properties,

constantly change during solvent at-

tack. The film becomes softer and

weaker, if more flexible, than before

solvent impact.) This type of failure is

typical of the action of paint re-

movers. (See pp. 73-74.)

Tg

and Penetrant Action

Because free volume availability and

molecular mobility within the coat-

ing film are greatly reduced at tem-

peratures below the Tg of the film, it

follows that penetration is signifi-

cantly reduced as T-Tg becomes

negative. There is some free volume

(about 2.5 percent of the volume of

the film) below the Tg. Some re-

searchers1,2 have concluded that

minimization of diffusion is found at

some secondary threshold below theTg, T∞, at which there is no configu-

rational entropy.

If impermeability to solvent is the

paramount design criterion, the

binder selection should ensure that

film Tg is above the service tempera-

ture (i.e., T-Tg is negative). It should

also remain above the service tem-

perature. The plasticization of a film

by the absorption of a penetrant that

depresses the Tg of the film below

the service temperature may initiatesignificantly increased absorption

and system failure. This degree of

plasticization may occur in immer-

sion service, where the temperature

of the liquid is temporarily increased

above the Tg. Without post curing,

in fact, some researchers such as

Wicks believe that maintaining the

Tg well above T can be difficult with

films cured at ambient temperature.

Cross-linking of a thermosetting

polymer becomes 2 or 3 times slow-er as the Tg rises during cure above

the ambient temperature. Essentially,

air drying thermosets never achieve

Tg levels much above the tempera-

ture at which the film is formed.

This, Wicks believes, is a factor in

the superior corrosion resistance of a

baked system compared to air dry-

ing offsets.3

TROUBLE with PAINT

70 / Journal of Protective Coatings & Linings

continued

Fig. 2 - Wrinkling of oil-based primerrecoated with a high performancethermoplastic coating (lacquer)



Copyright ©1997, Technology Publishing CompanyMAY 1997 / 73

TROUBLE with PAINT

The ingress of penetrants into a

coating film decreases Tg in both

thermosets and thermoplastics. Not

only does decreased Tg reduce the

system’s resistance to the penetra-

tion of additional molecules of the

same penetrant, but also the in-

creased free volume will facilitate

the penetration of other types of

small molecules. The corrosion resis-

tance of a highly solvated film will

be poorer than one that is free

of solvents.

Residual hydrophilic solvents such

as cellosolve acetate in high-build

vinyls have compromised the perfor-

mance of such systems in water

service, for example. Similarly, the

depression of Tg from the ingressof moisture into the film also de-

creases the film’s resistance to inva-

sion by oxygen.

In extreme cases, high Tg ther-

mosets of high cross-link density

(e.g., trimerized cyanate esters and

thermosetting polyimides) will take

up little water and solvent and will

change little dimensionally, even

in immersion service. In films of

highly crystalline polymers, such as

those based on the fluoropolymers,

acrylonitrile, or polyolefins, the

polymer will exhibit little take-up

of even the most powerful pene-

trants, and consequent swelling will

again be minor. Systems having

lower cross-link density, such as

epoxy polyamides and those lower

density systems rich in polar groups

(alkyds), will, conversely, exhibit

considerable volumetric expansion

in the presence of water and sol-

vents. Fortunately, cer tain amine-cured epoxies, polyesters, and

vinyl esters give adequate imperme-

ability to oxygen, solvents, and,

relatively speaking, to water, after

ambient cure.

Lifting Failure with

Solvent-Sensitive Primers

In recoating one film with another,

the absorption of carrier solvent

from the recoat systems into the al-

ready applied films of the primer,

undercoat, or old coating is to some

degree inevitable, as is the conse-

quent deformation of the existing

film. Solvent absorption may or may

not cause problems. Lacquers and

latex system films may partially or

completely redissolve in the solvent

systems of the newly applied finish,

even allowing some interfacial asso-

ciations between molecules of one

coat and another. These films rarely

give problems. The resultant solvent

weld or disappearing interface be-tween the 2 layers will normally

guarantee adhesion.

Old, well-cured oxidized systems,

baked systems, and other thermosets

continued

Paint Removers

A n extreme example of penetrant

attack on coating films is seen in the

action of paint removers (Fig. a).

Such removers are based on designed

solvent mixtures primarily made up

of small molecules such as methylene

chloride and methanol. These mole-

cules readily insert themselves into

the free volume spaces within the

polymeric matrix. Strong electronega-

tive groups (chlorine, hydroxyls)

allow for good secondary valency

bonding of the solvent with the poly-

mer, which encourages absorption.Evaporation of the solvent (a process

that opposes absorption) is reduced

by trapping the solvent against the

film using wax additives. The addi-

tives float to the top of the paint re-

mover surface. There, they precipitate

and form a barrier above the solvent.

Excessive free volume expan-

sion, resulting from the absorption of

sensitive substrate. The characteristic

wrinkling or puckering of the film be-

large quantities of solvent by the

coating, produces interfacial failure of

the paint system with a less solvent-

Fig. a - Mechanism of paint remover activity

PAINT REMOVERS continued on page 74




are also rarely affected by solvent at-

tack in recoating. However, their

cross-link density is usually high

enough to resist excessive solvent

absorption and consequent film de-

formation. In fact, it is often more of

a problem to secure good adhesion

to these films because of the ab-

sence of solvent attack. A recoat

window related to the degree of

primer cure is often utilized when

recoating epoxies and urethanes.

Entirely more vulnerable to attack

by a wide range of solvents are ther-

mosetting systems of very low cross-

link density, such as recently ap-

plied oxidizing films (long oil

alkyds). These films are slow to de-

velop appreciable cross-link density and, therefore, solvent resistance.

Vi ny lat ed al kyd copo lymers

(styrene and vinyl toluenated

alkyds) are particularly vulnerable in

this respect. Here, a rapid full cure is

sacrificed for fast initial dry.4 The

films are sensitive to solvent during

a long intermediate cure period be-

tween the initial drying (lacquer

mechanism) and full cure (oxidative

mechanism). The large amounts of

vinylation reduce the available un-

saturation, so that oxidation takes far

longer than with the equivalent oil-

modified alkyds. Other systems

showing fast touch-dry/slow

through-dry properties, such as very

high-solids medium and long oil

alkyds, are also very susceptible to

this type of attack. Here, aromatic

solvents and oxygenated solvents

will rapidly penetrate the still incom-

pletely cured primer, producing se-

vere swelling of the film and, as aconsequence, a wrinkling and lifting

from the subfilm (Fig. 2).

Less attack is likely using finish

coats based on aliphatic hydrocar-

bons such as mineral spirits and

VM&P naphtha. While a slightly more

aggressive solvent than mineral spir-

its, VM&P naphtha evaporates very

rapidly, which diminishes contact

time. Made up of larger, more un-

wieldy branch chain molecules in-

stead of the small, compact structures

that typify the aromatics, these sol-

vents are similarly far less dangerous

to use over sensitive, newly applied

films because they are less easily in-

serted into the polymeric matrix.

Factory-primed with alkyds and oil

paints, structural steel members,

light miscellaneous metal pieces, or

other equipment often exhibit lifting

failure in demanding environments

where surfaces are to be refinished

with epoxies, urethanes, and otherhigh performance topcoats. During

application of the finish coat or soon

after, the primer wrinkles and lifts

from the metal as if it had been

TROUBLE with PAINT


trays the large increase in surface

area (as compared to the area of thesubstrate) that accompanies the volu-

metric increase of the film. The film

may remain locally adherent at dis-

crete points where stress has been

dissipated locally by the loss of adhe-

sion of the film in adjacent areas. Lo-

calized adhesion leads to locally un-

restricted film distortion and reduced

stress on the remaining adjacent ad-

hesive anchorages. However, lateral

propagation of adhesion loss at the

remaining anchorages usually results

in eventual total delamination. In thiscase, the film puckers up.

Solvents used in paint removers

are readily imbibed into even highly

cross-linked films, such as epoxies

and polyurethanes, as well as into

aged oxidizing films. These solvents

will dissolve lacquers and latex.

Methylene chloride is, of course,

the target of regulation. (Toxicity con-

These groups may attack vulnerable

sites on the polymer, causing hydrol-

ysis and thereby further facilitating

solvent ingress and deformation of the film. This type of remover is

rarely necessary with alkyds or even

polyamide-cured epoxies. Carboxylic

acids may, however, be valuable in

removing more highly cross-linked

epoxies. Paint removers based on so-

lutions of sodium and potassium hy-

droxide in gelled solvents may be

used to remove urethanes and oil

systems. Small amounts of water and

surfactants may be introduced to fa-

cilitate alkaline hydrolysis of the ester

or amide groups within the film. Thesame principle without the solvent is

used in poultice-type removers popu-

lar for the field removal of lead-based

paint systems. These highly alkaline

materials are trapped against the film

with water-impermeable paper or

plastic sheet, and the hydrolysis of

the polymer proceeds over a more

extended period of time. ❍

cerns have always limited the use of

methanol in these compositions to

about 4 percent.) Finding non-toxic re-

placement solvents of equal effective-ness poses some difficulty. Methylene

chloride and methanol are small,

streamlined, highly polar materials that

are low in molecular weight and effec-

tively penetrate and expand the free

volume of even the most cross-linked

systems. Some small, compact mole-

cules, such as the cyclic lactam, n-

methyl-2-pyrrolidone, have similar

properties but are slower to evaporate

than methylene chloride and

methanol. Delamination, therefore,

may take much longer than is the case with conventional removers.

Other small, active molecules

have also been used in paint re-

movers. These include the lower car-

boxylic acids (formic acid and acetic

acid) as well as substituted aromatics

such as phenol and cresylic acid.

These materials also have benefits be-

cause of their functional groups.

continued

PAINT REMOVERS

continued from page 73




MAY 1997 / 77

TROUBLE with PAINT

treated with paint remover. In some

systems (e.g., the vinylated alkyds

noted above), there is a viable recoat

window both before and after the pe-

riod of solvent sensitivity wherein

these systems may be recoated safely.

Shortly after application, the lacquer-

type drying predominates without oxi-

dation, and the film may be recoated

without mishap. In the very long run,

the film may become more fully oxi-

dized, and here again resistance to sol-

vent attack improves enough to allow

good recoatability. In the long interme-

diate period, however, the conse-

quences of recoating with strong sol-

vent topcoats may be lifting of the

primer. Rapid recoating of such insuffi-

ciently oxidized films with chemically curing systems may actually lock in a

partially uncured state within a primer

of this type. Thereafter, this type of de-

ficient system will be unusually sensi-

tive to lateral cohesive failure under

applied stress.

Still available is a class of universal

lift-resistant primers that dry rapidly

and may be satisfactorily coated

with lacquers and other strong sol-

vent-containing finish coats. They

are generally inhibitive primers for-

mulated from special phenolic and

rosin-modified short to medium oil

alkyds. Similar alkyds are sometimes

externally modified with phenolic

dispersion resins, giving still further

improvements in recoatability.

The use of solvents in a recoat film

to obtain improved adhesion of the

recoat to the primer (or undercoats)

involves much the same considera-

tions as are noted here, although in

the former case, solvent attack is con-

trolled and used productively.

Solubility and Attack of Solvents

Solvency and the judicious applica-

tion of solubility parameters5,6 are

the keys to predicting solvent sensi-

tivity of amorphous thermoplastics.

These polymers remain soluble in

the classes of solvents that character-

ize the solubility of the dry resin

type. Even high molecular weight

systems, including thermoplastic

powder coatings, organosols, plasti-

sols, and latex paints, will soften, if

not dissolve, in these solvents.

The actual dissolution of ther-

mosetting ambient-cured epoxies,

urethanes, baked amino, and phenyl

formaldehyde cross-linked films is

precluded by the primary bonded

network of the film structure. How-

ever, solvents may penetrate and

swell systems of this type, especially

those of lower cross-link density. In

the latter case, the more open, cross-

linked structure allows the easier in-sertion of penetrants. Penetration is

also partially dependent upon solu-

bility characteristics, certainly on the

affinity of the penetrant for the

structural make-up of the film. As

continued




noted above, cross-linked systems

rich in alcohol groups, for example,

will be vulnerable to alcohols and

other polar solvents, while non-

polar coatings will be more resistant

to attack from this type of solvent.

Testing for Solvent Sensitivity

Solvent sensitivity in thermosetting

systems is more likely, therefore, to

be an indicator of the extent of cure.The rub test with an appropriate sol-

vent (usually methyl ethyl ketone, or

MEK) is a standard qualitative test for

the assessment of cure. In this test, an

MEK-soaked cotton, cloth, or felt tip

of an engineered marker is reciprocal-

ly rubbed across the film, and the

number of rubs required for film re-

moval, loss of gloss, or other change

is noted. Systems showing no effect

after 200 double rubs are considered

“cured,” although this is hardly a

measure of the extent of cross-link-

ing. In addition, the test is not much

good for differentiating between 2 or

more systems that are satisfactorily

cross-linked. In systems where a rea-

sonable degree of cross-linking is ex-

pected and the film is removed in 20

to 50 rubs, the result would indicate

some problem in the cure condition

(cure time and temperature or mixing

ratio). Immediate solubility of the

films supposedly laid down from two-

pack mixes such as urethanes and

epoxies is usually evidence that one

or another of the components is miss-

ing. Solvent rub techniques are often

used in the field as part of the failure

analysis process. The general degree

of cure may be confirmed by infra-

red spectrophotometry.

Solvent immersion testing may bea better measure of relative solvent

resistance in highly cross-linked sys-

tems using either the watch glass test

or preferably some kind of covered

immersion test. Coatings are com-

pared by their timed response to the

solvent, which progresses from soft-

ening or blistering to dissolution or

delamination. Watch glass tests using

volatile materials can be particularly

frustrating because in spite of the

cover, solvent evaporates rapidly. In

addition, the test requires continual

monitoring with many applications.

The effect of repeated solvent ab-

sorption and desorption is probably

fatiguing to the film but thwarts even

a rough qualitative assessment of the

relative effect of different solvents on

the same film or the response of

multiple films to the same solvent.

A more quantitative assessment of

solvent sensitivity may be obtained by

weighing small coupons of totally

coated metal or glass before, during,

and after solvent immersion, and plot-

ting the weight change data against

time (Fig. 3). In this way, both the

amount of absorbed material and the

rate of absorption may be assessed,

as well as the extent of dissolution (or

solid removal) after the test is termi-

nated. Differences in the weight of

the fully dried films before and after

immersion may show the loss of ex-

tractables. Some error may occur be-

cause of weighing time, but this usu-

ally takes less than 30 seconds with a

modern analytical balance.

Matching the temperature of the

test with the service application isalso important in testing for solvent

sensitivity. Hot solvent is far more de-

structive than cold solvent, and hot

films (above their Tg) are more sensi-

tive than cold films. Solvent contact at

high temperature may yield quite dif-

ferent results from low temperature

testing on the same film. In applica-

tions where solvent contact is not en-

closed, the mitigating effects of evap-

oration will also be magnified in the

case of hot solvent.

Recoatability

Solvent sensitivity is often used to

advantage in the design of coating

systems, where it may be used to

improve the intercoat adhesion be-

tween successive films.

In such recoating, there are no de-

vices such as the wax used in paint

removers to trap the solvent systems

against the old coating and retard

their evaporation long enough so

that they may attack the existing

film. Under most circumstances, the

solvent system will evaporate from

the finish before it can absorb ap-

preciably into the paint film and

produce the free volume expansion

necessary to good intercoat adhesion,

or, in extreme cases, compressive

stresses sufficient to induce failure.

TROUBLE with PAINT


continued

Fig. 3 - Solvent resistance—weight change over time, before, during, and after immersion




MAY 1997 / 81

TROUBLE with PAINT

Contact time between solvent and the

exposed film will be increased with

high film thickness recoats or where

barrier-type pigmentations (aluminum

flake) are used in the recoat. In some

cases, such excessive contact time

may be long enough to cause solvent

attack on the primer, particularly

where high boiling (slow evaporat-

ing) solvents are used. Low molecular

weight solvents (small molecules)

such as acetone and toluene, which

are more aggressive than larger mole-

cules and which might easily pene-

trate the film, usually evaporate too

fast to do so. Conversely, larger mole-

cules such as methyl isoamyl ketone,

diisobutyl ketone, and high flash

naphtha may stay in contact with theexisting film long enough to facilitate

entry, but they are often too large to

readily penetrate the existing film.

Compact solvents with cyclic struc-

tures, such as cyclohexanone and n-

methyl-2-pyrrolidone, are small

enough to enter the film easily but

are relatively slow to evaporate.

These solvents may be more aggres-

sive in solvating the surface of the

existing film. They have been used

to soften hard, well cured thermosetsbefore coating. In lacquer-based sys-

tems made up of amorphous ther-

moplastic-based coatings, the poten-

tial dissolution of a lower coat by the

solvents of a subsequently applied

coat ensure a good bond with sol-

vent and solubilized binder intermin-

gling across the interface.

The swelling of thermosetting

primers by solvents in the finish may

also be used, especially where the

state of cure of the primer film remainsincomplete. As cure advances with

time (as Tg increases), the technique

becomes progressively more difficult

to apply. This fact is indicated in the

recoat window of the system pub-

lished by coating manufacturers. In

many of the better product informa-

tion sheets, this recoat window is ex-

pressed in terms of cure temperatures

involved. Recoat times become pro-

gressively shorter as cure temperatures

increase. It is, of course, possible that

in recoating primers with Tg values in

the 80 to 90 F (27 to 32 C) range, ap-

plications at 90 to 100 F (32 to 38 C)

may actually facilitate recoating be-

cause of T-Tg

effects.

Shrewd use of solvents in the re-

coat paint may also improve the re-

coatability of well aged gloss films.

Conclusion

Next month, we will review aesthet-

ic difficulties associated with solvent

sensitivity in coating films. JPCL

References

1. G. Adams and J.H. Gibbs, Journal

of Chem. Phys. (Vol. 43, Number 139, 1965).

2. E.N. Dalal, “On the Relationship

between T g and T,” Journal of

Polymer Science, Polymer Letters

Edition (Vol. 22, Number 547,

1984), 547-548.

3. Z.N. Wicks, “Free Volume and

the Coatings Formulator,” Jour-

nal of Coatings Technology (De-

cember 1986), 23.

4. H.R. Hamburg and W.H. Mor-

gans, Hess’ Paint Film Defects,

3rd Edition (London: Chapman

& Hall, 1979), p. 284.

5. C.H. Hare, Protective Coatings—

Fundamentals of Chemistry and

Composition (Pittsburgh, PA:

Technology Publishing Co.,

1994), p. 141.

6. H. Burrell, “Solubility Parameters

for Film Formers,” Official Digest

(now the Journal of Coat ings

Technology ) (Vol. 27, Number

369, 1955), 726.

Back issues containing the earlier

segments of the Trouble with

Paint series are available from

JPCL at 800/837-8303.

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