Exterior exposure study of stains and clear finishes

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Canadian Paint and Finishing, 48, 2, pp. 13-16, 1974-02

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Exterior exposure study of stains and clear finishes

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NATIONAL RESEARCH COUNCIL OF CANADA

CONSEIL NATIONAL DE RECHERCHES DU CANADA

EXTERIOR EXPOSURE STUDY OF STAINS AND CLEAR FINISHES

by

H.

E.

_<;

ASHTON

REPRINTED FROM

583-

<i3

CANADIAN PAINT AND FINISHING

VOL. 48, NO.

2,

FEBRUARY 1974

RESEARCH PAPER NO. 599

OF THE

DIVISION OF BUILDING RESEARCH

OTTAWA

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ETUDE DE L'EXPOSITION DES TEINTURES E T DES

COUCHES DE FINITION CLAIRES AUX

INTEMPERIES

SOMMAIRE

L'auteur examine la composition de matQiaux montks sur des pan-

neaux de ckdre rouge et exposks aux intempkries pendant prks de cinq

ans relativement

B

_{des essais d'exposition antkrieurs. On fait le lien}

entre les rksultats de l'exposition et la composition des mathiaux. Une

couche supplkmentaire de vernis amkliore le comportement du vernis

p-phknylphknolique conventionnel. L'effet d'un absorbant d'ultra-

violet sur la durabilitk des couches de finition claires est variable. Les

changements du pigment dans la formule de teinture du "U.S. Forest

Products Laboratory" influencent le comportement de la teinture.

(4)

Exterior exposure study

of

-

7

.\

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;.!

-, *

COVER FEATURE

-.

7

stains and clear finishes

by H . E. ASHTON,

Research Officer, National Research Cosrncil of Canada

The Building Materials Section of the Division of Building Research has received many inquiries regard- ing the use of clear finishes and stains on exterior wood. Advice to users has been based on the results of exposure tests initiated in 1955 and 1960 and, to a limited extent, on accelerated weathering studies. The results of these exposure tests and the outcome of some refinishing studies have been reported previously ( 1 ) .

Because so few clear finishes have performed adequately, additional exposure tests were carried out to discover whether durability could be enhanced. Most attention was devoted to the use of ultraviolet absorbers and the number of coats required for improved performance.

Accelerated weathering studies Ilad shown that UV absorbers gave improved durability, provided they were added to coatings in quantities at least 10 times greater than the 0.5 percent used in the 1960 studies. The effect of pigmentation on the

U.S. Forest Products Laboratory

(U.S. FPL) stain formula ( 2 ) was also investigated because of inquiries about changing the original dark color. This present report de- scribes the materials tested and the

results of additional exposure

studies.

As before, coatings were exposed at the Ottawa exposure site, although some workers have ques- tioned whether there is sufficient sunlight at Ottawa to cause much degradation of clear coatings. It should be noted that Ottawa's latitude is the same as that of Venice, Italy and Portland, Oregon. Al- though latitude is not the only variable, the test results should be ap- plicable to most of Europe as well

as to Canada and the Northern United States.

Materials and methods

The phenolic varnish and alkyd resin solution that had the best durability in their classes in the 1960 series of tests were selected for the present study. A new batch of varnish was cooked as before and a new lot of alkyd resin obtained from the manufacturer. Urethane coatings had previously performed rather poorly but a few were included to determine whether UV absorbers would upgrade their performance. The most successful acrylics in an accelerated weathering study were added to the exterior program. The U.S. Forest Products Laboratory stain formula was made in the same color as the 1-GP-145 alkyd, so that

direct comparison could be made. The level and type of pigmentation in the U.S. FPL stain was also varied to see whether light colors would perform satisfactorily. The general composition of the coatings

is given in Table

I.

The materials were applied in the laboratory to Western red cedar panels. For increased confidence in the results, four panels were coated with each material, except for the stains where two additional white pine panels were included. Panels coated with phenolic and alkyd finishes were exposed on the test fence on 15 July, 1967, and the re- maining materials on 20 September, 1967.

Although it is known that a longer life is obtained if clear coatings are maintained at yearly inter-

(5)

vals, the panels were allowed to weather without touch-up. They were removed from the test fence in May, 1972, after almost five years of exposure. Any clear finish still in good condition after four summers can be considered to have very good durability, since most fail after two summers.

The panels were moved to the laboratory so that thorough exam- inations and comparative assess-

3 C O A T S 8 9 3 l t -

-

6 1 0 D U R A B I L I T Y R A T I N G 5 P: :

2

z

..

m z $ 2 1 2

Figure 1. Effect o f number and type o f coats

2 C O A T S 8 9 3 1 C O A T 9 1 4 7- l C O A T 8 9 3 2 C O A T S 9 1 4 9 1 3 C O A T S 9 1 4 4 C O A T S 8 9 3

-

2 C O A T S 8 9 3 2 C O A T S 9 1 4

ments could more easily be made. The panels were rated on the usual 0 to 10 scale, where 10 is perfect condition and 0 is complete failure. This scale is not linear as one or two isolated failure spots lower the rating to nine, while larger failures would be required to decrease the rating from six to five. For clear finishes, a rating of five indicates fairly numerous small breaks with streaks of whitening, showing loss of adhesion. Fairly ex- tensive preparation would be needed before the panels could be recoated. Two observers first rated the panels independently, and then com- pared their ratings to resolve dif- ferences. The correlation coefficient

3

1 m

8 9 3 P H E N O L I C

9 1 4 A L K Y D

D U R A B I L I T Y R A T I N G

Figure 2 . E f f e c t o f UV absorber in phenolic and alkyd (three coats)

TABLE

I

COMPOSITION OF TEST MATERIALS

(a) Clear Finishes

Formula Oil % Oil on Per Cent

No. Resin Type TYW Sol~ds Solids 893 p Phenylphenolic Tung 66.7 5 0

914 Alkyd Soya 56 5 0

850 Polyisocyanate Modified 48.1 50.1 Castor

1042 Polyisocyanate Castor 63.3 49.8 1043 Urethane Prepolymer None 0 49.8 1005 Acrylic Lacquer None 0 35

-

Acrylic Emulsion None 0 4 5

(b) Stains

Sample Formula Resin Per Cent Per Cent

No. No. Type Oil Type Sol~ds Pigment Notes 25

-

None Linseed 75.5 0 Clear vehicle of

U.S. FPL Formula 29 848 None Linseed 76.9 4.6 U.S. FPL Formula 3 0

-

None Linseed 77.0 7.6 White pigment 26

-

None Linseed 76.9 5.0 Ferrite yellow

and red 27

-

None Linseed 76.2 2.5 Reduced ferrite

yellow and red 28 870 Alkyd Soya 35.7 5.4 1GP-145

Linseed

between the independent ratings of the clear finishes was 0.927, showing excellent agreement between the observers. For the stains the coefficient was lower at 0.81, but this was still sigruficant at the 99.9 percent confidence level. The lower coefficient resulted because one ob- server tended to rate the stains above four while the other gave lower ratings.

The final combined ratings of the two observers are given in Tables I1 and 111. The average ratings shown are based on both the median and the mean. The difference between them was usually small, but the latter frequently resulted in fig- ures with two decimal places, an unrealistic situation when visual ratings are being used.

Results

The first objective of the study was to determine whether additional coats of phenolic varnish, or a combination of alkyd and varnish films, would improve durability. The results after four years and ten months of exposure are shown in Figure 1. Comparison of samples 1 and 2 shows that an additional coat of phenolic did improve performance markedly. The combination of the two types of clear finish did not perform as well as expected from the accelerated weathering test. Per- formance was poor because the alkyd tended to delaminate from the varnish film, in some cases trapping dirt, and this did not happen in the accelerated test.

It is also evident from the results that the durability of the combination is chiefly due to the phenolic varnish. Two coats of alkyd and one of varnish had a lower rating than three coats of varnish; two coats of alkyd and two of varnish rated lower than four coats of varnish. Anomalously, two coats of varnish and one of alkyd rated higher than three coats of varnish. Three coats of alkyd alone had the lowest rating in this group. The durability of three coats of phenolic was corn- parable to that in the previous series when NRP 893 had a rating of four after three and one half

years of weathering ( 1 )

.

The effectiveness of ultraviolet absorbers in improving the performance of phenolic and alkyd clear Finishes is illustrated in Figure 2.

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The absorber was more effective in the alkyd than in the phenolic, prob- ably because alkyds are transparent to lower wavelengths of UV light. The paraphenylphenolic resin used in these varnishes tends to act as an UV absorber because of the two benzene rings coupled together in the resin molecule. Hence, there is a smaller possibility that the add- itive will improve the varnish performance. This is shown by the fact that an extra coat of varnish was considerably more effective than 7.5 percent UV absorber in three coats

of varnish. The UV _{absorber im-}

proved the performance of the al-

kyd to a certain level and additional absorber seemed to be without effect.

Table

IV

_{illustrates the addition}

of W absorber to the combined

alkyd-phenolic system. Addition to only one coat of a three-coat system had no effect, but when two coats of a three- or four-coat sys- tp,m contained UV absorber the per- formance was improved markedly. Whether the absorber was added to the phenolic or to the alkyd did not change the result. When the absorber was present in all four coats the performance was excellent, with one individual panel rating nine after almost five years of exposure. When urethanes were first intro- duced, it was hoped that they would solve the problem of clear finishes for exterior wood. Unfortunately, this has not been the case, as is shown in this and the previous study.

In Figure 3 it may be seen that two urethanes without UV absorber were slightly poorer than the alkyd and the other slightly better. Addi- tion of five percent absorber did not improve durability sufficiently to warrant the extra cost. There was no essential difference between the different types of urethanes exposed. Clear aliphatic urethanes, without UV absorber, should not be expected to be an improvement over the aromatic urethanes tested here, since they transmit more W radia- tion than do the aromatics.

Two acrylic finishes were also exposed. In an accelerated test they had performed the best of several acrylics, although not so well as the standard phenolic. The results for the lacquer are shown in Figure 3. It is evident that the UV absorber

markedly improved the performance of the acrylic lacquer to the point where it was the second best clear f i s h . Addition of absorber to the acrylic emulsion was not attempted because most absorbers are solvent soluble.

The effect of pigment in enhanc- ing exterior durability of stains may

be readily observed in Table 111.

The unpigmented vehicle failed completely in four years and eight months; for the samples containing the normal quantity of pigment the lowest average rating was 2.75. With the sample in which the amount of pigment was reduced by 50 percent, the rating also decreased to about half. Similarly, two coats of U.S. F P L stain were considerably more durable than one coat, con- firming the findings of the 1960 test. White stain was inferior to iron oxide containing stains in the U.S. FPL vehicle. On cedar, two coats

of the original pigmentation were more durable than the stain containing yellow and red iron oxide, but on pine the reverse occurred. This may be due to the difference in wood, since only two pine panels

I I I I 8 5 0 U R E T H A N E

I

I I I I I

I

0 2 4 6 8 1 0 D U R A B I L I T Y R A T I N G Figure 3 . E f f e c t o f UV absorber i n urethane and acrylic

I 8 1 9 2 0 2 1 2 2 2 3

TABLE II

DURABILITY RATING OF CLEAR FINISHES

%

uv

Sample Formula Description No. of Ab- Average

No. No. Coats sorber Rating

1 893 Phenolic Varnish 3 0 2 2 893 Phenolic Varnish 4 0 5.5 3 893 Phenolic Varnish 3 5 2.5 4 893 Phenolic Varnish 3 7.5 3.5 893 Phenolic Varnish

:

8 )

_2.75

:

1 z;:

:rl:olic Varnish

1 ]

2.5

{

:k:

irl:olic Varnish _I _1.5 2 0 1

8

{

iii

F$iolic Varnish 1 0 1

2 5 J 5.5 9 914 Alkyd 3 0 0.5 10 914 Alkyd 3 5 4.75 11 914 Alkyd 3 7.5 4.5 J 893 Phenolic Varnish l2

1

914 Alkyd

f

8 )

2.75 l3

i

893 Phenolic Varnish 914 Alkyd 2 2 5 1 0 1 6.25 l4

{

893 Phenolic Varnish 914 Alkyd

f

1 )

6.25 l5

i

893 Phenolic Varnish 914 Alkyd

f

)

7.75

16 850 Urethane Prepolymer-Modified Castor3 0 1.25

17 850 Urethane Prepolymer-Modified Castor3 5 2.75

18 1042 Urethane Prepolymer-Castor Oil 3 0 0

19 1042 Urethane Prepolymer-Castor Oil 3 5 2.5

20 1043 Urethane Polymer-Catalyst 3 0 0.25

2 1 1043 Urethane Polymer-Catalyst 3 5 2.5

22 1005 Acrylic Lacquer 4 0 1.25

23 1005 Acrylic Lacquer 4 5 7.5

2 4 Acrylic Emulsion-Rhoplex WN-8084 0 0

@

-

Registered Trade Mark

0 % 1 0 4 2 U R E T H A N E 5 % I 1 0 9 6 1 0 4 3 U R E T H A N E 5 % I 00/ol 1 0 0 5 A C R Y L I C 5 % _I

(7)

of each sample were exposed. In

most cases the performance of stains was better on cedar than on pine.

Conclusions

T o obtain fairly long-term performance from clear finishes, it is necessary to use four applications of a coating containing at least five percent of an effective UV absorber. In this study, the best durability was obtained from a svstem of two coats of a tung oil-paraphenylphenolic varnish and two coats of a medium oil alkyd resin, both materials containing five percent of 2, 2'-di- hydroxy bmethoxy benzophenone.

Four coats of a UV-stabilized acrylic lacquer, or two coats of the

U.S. F P L stain formula. were al-

most as durable as the phenolic-

alkyd combination. UV absorbers have been shown to be quite specific

( 3 ) and the. performance of the

previously mentioned clear finishes might be improved or reduced if other types of absorber were used. An additional coat of phenolic varnish without absorber improved durability considerably over the performance of a standard threecoat system. Four coats of varnish containing absorber may have performed as well as the phenolic-alkyd system because without absorber the varnish was superior to the varnish- alkyd system. However, in the threecoat systems the alkyd was superior to the varnish when both contained UV absorber. The combination tended to fail by delamination of

the alkyd from the varnish in a

TABLE

Ill

DURABILITY OF STAINS AND STAIN VEHICLE

Average Rating

No. Description Cedar Pine

29 Original U.S. FPL Burnt Sienna

-

Raw Umber

Pigment

-

2 coats

. . .

7.5 3.75

...

1 coat . . . 3

26 Ferrite Yellow and Red Pigment, U.S. FPL type

. .

5.75 4.25 27 Reduced Ferrite Yellow and Red Pigment, U.S.

FPL type

. . .

2.5 2.25 28 Alkyd Stain, 1GP-145, Ferrite Yellow and Red

.

2.75 3

. . .

30 White Pigment, U.S. FPL type 4.25 2.5

. . .

25 Clear U.S. Forest Products Vehicle 0.25 . . .

TABLE IV

UV ABSORBER IN PHENOLIC-ALKYD

COMBINATIONS

Sample Code

No. No. Description No. of Coats Absorber % UV Average Rating

. . . 893 Phenolic 2 0 . . . 5 914 Alkyd 1 0 2.75

. . .

893 Phenolic 2 0 . . . 6 914 Alkyd 1 5 2.5 . . . 893 Phenolic 1 0 . . . 7 914 Alkyd 2 0 1.5

. . .

893 Phenolic 1 0 . . . 8 914 Alkyd 2 5 5.5 . . . 893 Phenolic 2 0 . . . 12 914 Alkyd 2 0 2.75

. . .

893 Phenolic 2 5 13 914 Alkyd . . . 2 0 6.25 893 Phenolic

. . .

2 0 14 914 Alkyd

. . .

2 5 6.25 893 Phenolic

. . .

2 5 15 914 Alkyd

. . .

2 5 7.75

manner unfavorable to appearance. The reason for the great improvement in the performance of the acrylic with UV absorber may be due to the transparency of acrylics to UV light. Because the resin itself is not degraded, the UV absorber only has to protect the underlying wood. -If this hypothesis is correct, the best approach to durable clears is to use a material highly transparent to UV, but with sufficient absorber to protect the wood sub- strate and enough flexibility to with- stand wood movements, caused by moisture changes.

The acrylic lacquer was formulat-

ed with n-butyl methacrylate,

which has the highest elongation of the acrylic resins made by the resin supplier. Where the vehicle itself absorbs UV to protect the wood, it eventually becomes degraded, as is the case with phenolic varnish.

Aromatic-type urethanes per-

formed about the same as alkyd without UV absorber and not so well as alkyd when both were stabilized against UV.

Changing the pigmentation of the U.S. F P L stain to white, or a lighter color, tends to reduce durability. Where such colors are desired more frequent maintenance will be required.

The alkyd stain was again less durable than the fortified oil stain, possibly because of the much lower solids content which results in a thinner film. The performance of the alkyd might be improved by adding the water repellents, paraffin wax and zinc stearate, used in the U.S. F P L formula and by increas- ing the solids content.

Acknowledgements

The author wishes to thank K . A . O'Doherty, who prepared the varnish and the test panels and who also made the first evaluation of the ex- posed panels.

References

I . Ashton, H . E., Clear Finishes for Exterior Wood: Field Exposzrre Tests. J . Paint Tech., 39, 507, p. 212, 1967.

2 . Forest Products Laboratory Na- tural Finish, U.S. Forest Products Laboratory, Report N o . 2096 (r-e- vised 1961).

3. Philadelphia S o c i e t y for Paint

Technology, Ultraviolet Light Ab- sorbers in clear C o a t i n g s for Wood, J . Paint Tech., 39, 515,