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Technical Note (National Research Council of Canada. Division of Building Research), 1965-05-01
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Mechanism of Weathering of Paints
Yamasaki, R. S.
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DIVISION OF BUILDING RESEARCH
'IfE
C
lHI N
J[
CAlL
NOTE
ND.
438
PREPARED BY R. S. Yamasaki
PREPARED FOR General Distribution
CHECKED BY EVG APPROVED BY NBH
DATE May 1965
SUBJECT
MECHANISM OF WEATHERING OF PAINTS
Deliberations at the first meeting of the Associate Committee on Paint Research, held in November 1964, led to a request for a literature survey on the degradation of paints on exposure to weathering. This information was believed to be essential towards setting up reliable
performance tests for the evaluation of paints. Therefore, this Technical Note was prepared summarizing the current knowledge and understanding of the mechanism of weathering of paints primarily used to protect wood.
Before proceeding, perhaps it might be useful to dwell briefly on the subject of paint performance. In considering the possibility of setting up tests to predict paint performance the question comes to mind, "What is meant by a good paint performance?'· According to Ta1en (1), "A g:ood performance means that the paint coating, shortly after application and the formation of the film fulfills the requirements specified, and that in course of time, the film changes so little in its properties, that it will continue to fulfill these requirements for a long time. II
There is no single method for the prediction of paint performance. A sound prediction will involve knowledge of:
(a) The task the paint system is expected to accomplish.
(b)- The factors influencing the paint performance, derived from the exposure environment and the substrate.
(c) The type of degradation to be expected, with different types of film formation, with more detailed information relative to the type of film former and the type of pigment in the paint being examined.
These data being given, an indication of the perfor:mance to be expected :may be obtained fro:m quantitative assess:ment of aging pheno:mena,
i.e. fro:m rate s of change in particular propertie s with ti:me. Such aging pheno:mena can be considered syste:matically as follows:
(i) Surface pheno:mena: los s of glos s, develop:ment of chalking and effects of erosion or abrasion.
(ii) Internal fil:m changes, leading to changes in :mechanical properties and thence to cracking, checking, etc.
(iii) Substrate/fil:m interactions leading to los s of adhe sion, bliste ring, flaking, etc.
In order to understand the proble:m of deterioration of paints, let us first consider what happens to paint ·syste:ms when they are exposed to the ravages of natural weathering.
EFFECTS OF NATURAL WEATHERING
Nor:mal deterioration of oil house paint according to Browne (2) proceeds as follows: soiling, flatting or loss of gloss, chalking, fissure for:mation, disintegration and advanced break-up. Abnor:mal paint behaviour. resulting in pre:mature failure, :may be caused by unsuitable spacing of paint coats or repainting, often leading to intercoat peeling and :moisture blistering. Also, unsuitable coating thickness :may bring about rapid erosion if the fil:m is too thin and cracking if it is unduly thick. Inco:mpatibility in properties of different layers of paint such as in swelling capacity on exposure to water :may lead to develop:ment of stress in the syste:m and cause cracking and
scaling. Finally, blistering :may be caused by the develop:ment of pressure in a gas or a liquid at the fil:m/ fil:m or fil:m/ substrate interface.
Alkyd paints, in general, are :more durable than oil paints but nevertheless they do undergo phases of nor:mal deterioration si:milar to that of the oil paint (3,4). They :may fail pre:maturely by excessive erosion when for:mulated as paints pig:mented with only titaniu:m dioxide and extender (5). Alsq they :may fail as recoat because of the poorer wetting characteristic of the alkyd vehicle and the differential e:mbrittle:ment at the repaint interface, re suIting fro:m a reaction of the alkyd vehicle with the zinc pig:ments of the original fil:m.
E:muls.ion paint syste:ms when applied properly :may be just as
durable as, or better than alkyd paints. Then they usually fail by erosion (6). In the interi:m they :may undergo soiling, chalking, checking and cracking (4). Latex paints :may fail pre:maturely because of poor fil:m for:mation when
•
They may also fail by grain cracking on bare wood (8) and severe loss of adhesion to oil paints when wet (9), and as a repaint on oil paint because of poor adhesion to the chalky surface (5). When top-coated with a less permeable oil or alkyd paint, severe blistering and loss of adhe sion can result (9).Where a good pigmented house paint is expected to last about 5 years, a good clear coating such as a phenolic varnish may last only 2 years or less. Urethanes, epoxies and alkyd resin varnishes give even poorer durability as shown by studies at the Division of Building Research. Clear coatings may undergo soiling, loss of gloss, erosion and checking (3), and eventually fail by cracking, peeling and delamination (lO). It is believed that degradation of the wood at the wood-coating interface affects the durability of the coating as results at D. B. R. have shown. Dr. Desai of the Department of Forestry, Ottawa, is currently studying the degradation of cellulose and has prepared a review and bibliography on the photochemical degradation of cellulose and cellulosic materials (II).
WEATHERING AGENTS
Factors which are mainly responsible for destructive weathering action on paints are water, light, oxygen and temperature. They may act at different levels, e. g. variation in intensity of solar radiation, or may act at different rates, e.g. shock effects due to rapid thermal change or the
sorption of water. Agents may act independently or together in various combinations and in different sequences, e.g. oxidation after stress effect differs from oxidation before stress. It is not surprising,:lherefore, that because of these variations, inconsistencies in results are often obtained.
Weather records
U2L
as shown in Table I, for four differe nt N. R. C. exposure sites in Canada illustrate the variability of the weather with respect to location and time of the year.The weathering agents bring about deterioration in chemical, physical and mechanical properties of the paints, causing eventual failure of the protective property of the coating.
EFFECTS OF WEATHERING AGENTS
To elucidate the mechanism of degradation of paints, the effects of the destructive action of the individual environmental agents on paints have been studied. In this connection it has been found that the nature of the deterioration of the fibn depends on the type of film formation so it is appropriate to discuss degradation under such a classification.
•
(1) Chemical processes - chemical reaction between components as for linseed oil paints. Degradation of the film isintimately connected with its formation.
(2) Physical processes - by evaporation of the solvent or non-solvent as for emulsion paints, or solidification of the film by cooling. Changes due to degradation occur
subsequent to film formation.
(3) Combined physical and chemical processes - chemical reaction between components accompanied by solvent evaporation as for oil-alkyd paints and oleoresinous varnishe s. Degradation is connected with its formation.
OIL PAINT
Chemical Change s
The film formation and degradation occur simultaneously and are intertwined. For linseed oil the film formation proceeds as follows: when linseed oil is exposed to air it undergoes oxidation, taking up 10 to 12 per cent of oxygen, isomerization from a non-conjugated to a conjugated unsatu-ration, and formation of hydroperoxides at carbons adjacent to the double
carbon to carbon bonds. These hydroperoxides then combine with the unreacted ester to form dimers and polymers with perhaps peroxide and ether linkages and solidify to an insoluble and infusible film of three -dimensional network. These reactions probably continue throughout the life of the film but at decreasing rates.
With respect to deterioration, Elm (13) has summarized our
knowledge and understanding up to 1949. He concluded that scission reactions, along the bridge linkages such as ether and peroxide linkages formed during film formation, and to a lesser extent, of the end groups such as hydroperoxide, hydroxyl and keto groups, were responsible for the fragmentation and breakdown of the film. Crecelius et al. (14, 15) have since studied the chemical effects of UV and oxygen on thick (200 11) and thin (20 ｾＩ dried linseed oil films by infrared ｳｰ･｣ｴｾｯｳ｣ｯｰｹＮ They found that the films degraded into carbon dioxide, carbon monoxide, and low molecular weight aldehydes and ketones. The rate of formation of carbonyl and OR and/ or OOH:groups for the thick film differed from that of the thin film. Methyl alcohol was indentified in the gaseous phase of the decomposition of the thick film. Miller (16), Browne (2) and Cowan (17) have written more recent reviews. Also, studies have been conducted and are still continuing at D.B.R. on the chemical kinetics of photo-oxidative
•
degrarilation of both dried linseed oil and of one of its components,trilinolein, by infrared spectrophotometry. Free films and films attached to magnesium fluoride substrate because it transmits infrared have been
studied up to 2, 000 hr of exposure.
The study of the degradation of drying oils which is essentially oxidative is made difficult by the fact that reactions involved in film
formation, non-film formation and deterioration all take place simultaneously, probably throughout the life of the film. Even as the oil is air-drying,
the hydroperoxides initiate chain reactions to split molecules of drying oil and of the polymer into a succession of smaller oxygenated fragments. Free radicals are formed and become carriers of degradation reactions. Oxygen acts like a free diradical and attacks the organic molecules causing cleavage to occur at the heart of the molecule resulting in loss of film strength, or at the ends resulting in evolution of the volatiles. Meanwhile, film-forming cross-linking reaction has also proceeded, beneficial at first but detrimental when it has gone beyond the optimum causing loss of elasticity.
The UV radiation accelerates the deterioration of oil films. The shorter components attack the surface of the film causing erosion and
production of volatile carbon compounds. The longer components penetrate deeper into the bulk of the film promoting evolution of hydrogen as water, formation of carbonyl groups and cross -links causing further shrinkage and embrittlement of the coating. To obtain more detailed information, studies on the relative effects of radiation bands in UV on paints are being initiated at the British Paint Research Station (18).
Water alone, but even more in conjunction with UV, also accelerates deterioration by stimulating production of volatile and water-soluble
decomposition products.
Higher temperature in general is expected to accelerate the degradation reaction depending on the energy of activation of the reaction, and may even change the nature of the reaction.
Pigments materially alter the course of deterioration of oils by changing either the physical or chemical phases of the oxidation reactions. Some pigments such as zinc oxide and rutile titanium dioxide absorb UV and thus reduce its depth of penetration and retard deterioration. Other pigments such as anatase titanium dioxide participate chemically in the deterioration reaction by acting as catalyst in the oxidation of film by atmospheric oxygen (19). Ultraviolet absorbers are used as screeners for
clear coatings but are not as efficient as pigments (19).
Driers produce both deleterious and beneficial, effects in the deterioration but these effects are relatively small (20).
Physical and Mechanical Changes
Gloss. - According to Browne (2), loss of gloss is due to loss of volatile products and leaching of soluble products which proceed most rapidly at the exposed surface, destroy the superficial layer of oil vehicle and lay bare the granular bed of pigments. In addition, contraction in volume from deterioration of deeper parts of the coating and distortion from repeated swelling and shrinking contribute to the roughening of the surface or
flatting. Continuation of the flatting eventually de stroys the binding mediUm of the pigment causing loosening of the pigment or chalking.
Fissure (2). - Young coatings are able to withstand the internal stresses caused by shrinkage in volume and by swelling in water followed by redrying because they can undergo a high degree of both elastic and plastic deformation. On aging, such distensibility diminishes at a rate determined by the progress of deterioration under the condition of exposure. Loss by volatilization and leaching of low molecular decompostition products that have plasticizing action, and increased cross -linking of the vehicle gel embrittle the coating. The internal stresses from changes in volume become even greater until the cohesion of the coating is exceeded. Fissures then occur in the coating to relieve the stresses, first as checking then later as cracking.
Peeling (2). -When checking penetrates to the bottom of the coating, water gains direct access to the wood underneath. Adhesion between coating and wood is greatly weakened by water. Moreover, the internal stresses
within the coating reach a maximum at the ｷｑｯ､ＭＧｾｯ｡ｴｩｮｧ interface. Peeling eventually results.
Abnormal Paint Behaviour (2). _The unsuitable spacing of paint coats may result in failure due to intercoat peeling or by moisture blistering. The relatively high content of water-soluble substances and greater swelling capacity of young coatings may well be responsible.
If the coating is too thick, intercoat stresses may disrupt the coat1ng,
swelling. The junction between coatings of markedly dissimilar swelling capacity becomes the seat of very high stresses when the coating absorbs water and dries out again, causing cracking.
Blistering; ';'i ,Aecording to Werthan (21,22), moisture blistering is the most troublesome and the most publicized paint defect. The susceptibility of a paint film to moisture vapour blistering depends on four factors:
(1) The permeability of the film to moisture vapour.
(2) The swelling of the film on saturation with moisture.
(3) Change in the cohesive and adhesive strength of the film on absorption of moisture.
(4) Pressure exerted by moisture vapour behind the paint film.
The conventional linseed oil house paint is susceptible to moisture blistering because the film has low moisture permeability and upon saturation with water, the hydroxyl or hydroperoxyl groups attract the water molecules to such an extent as to cause swelling. Further, the polar groups involved in paintf substrate bonding are hydrated resulting in lowering of adhesion. The vapour pressure developed behind the film then provides sufficient force to cause blistering.
At the present time there are two schools of thought on whether pressure is necessary to cause blistering: Bullett and Rudram (23) have argued for the osmotic pressure theory. However, Brunt (24) considers that blisters are not osmotically formed. He explains that blisters are formed by volume increase by swelling by water and loss of adhesion.
For current information on blistering, consult review articles by Holbrow (25,26).
EMULSION PAINT
Chemical Changes
Vehicles commonly employed for emulsion paints are vinyl
acetate polymer, acrylic resin or polymer, and s:ty'renefutadiene copolymer.
Grassie (27) classifies polymer degradation reactions into:
(i) Chain-scission reactions - depolymerizations.
(ii) Non chain-scission reactions.
(b) Reactions induced by chemical agencies (oxidation, hydrolysis, etc.).
(i) Chain-scission reactions.
(ii) Non chain-scission reactions.
In general, degradation reactions induced by physical agencies tend to proceed by initial attack at a few specific points in the molecule followed by a train of reaction along the chains from these points, where mostly labile structural abnormalities prevail (I).
Thermal degradation of acrylic, styrene, and butadiene polymers proceeds by chain-scission-depolymerization reactions forming corresponding monomers as degradation products. Vinyl acetate polymer degrades by
non chain-scission mechanism forming acetic acid and olefin as products.
Photochemical ultraviolet degradation of acrylic and vinyl acetate polymers proceeds predominantly by chain-scission reactions. Pigments and UV absorbers decrease the rate (28).
In chemically-induced reactions, structural abnormalities play a relatively minor role and the nature of the attack is random. The reaction consists mainly of an attack of the reagent at some structural characteristic of the monomer mits, each of which, in any sample of the material, is
equally vulnerable.
Oxidative degradation of styrene -butadiene copolymer because of olefinic unsaturation proceeds by formation and subsequent decomposition of hydroperoxides accompanied by chain-scission or cross-linking reactions. In oxidative degradation the attack is largely at random so that new groupings which appear in the early stages are much more difficult to identify, because they are attached to the large fragments into which the original molecule is known. On the other hand, the products of extensive degradation, although they are volatile and more readily' identified, are relatively useless as an aid to the classification of the early stages of the reaction.
Acrylic and vinyl acetate polymers undergo some photo-oxidative degradation by virtue of residual monomers, with olefinic unsaturation, and
end-groups not blocked by steric hindrance (29). Since styrene -butadiene copolymer possesses olefinic unsaturation as opposed to both acrylic and vinyl acetate polymers, it undergoes more vigorous photo-oxidative degradation (29). Oxygen attacks the molecule at the butadiene double bond. Carbonyl is formed and cross -linking results. In addition, scission and volatilization take place so the film is completely degraded.
Physical and Mechanical Changes
Rosa and Elm (30) found that "non-oxidizing" acrylic resin
and polyvinyl acetate underwent erosion due to oxidative deterioration when exposed to UV.
Marshall (29) in his studies on the effect of light of different wavelengths (200ml-l - infrared) in the degradation of clear coatings found that:
(a) Acrylic resin - Short UV (200-365 ml-l) and visible-infrared (500 ｭｬＭｬｾ and up) radiations produced films with the highest early tensile strength at break because of cross -linking which rapidly deteriorated on account of scission and volatilization. The UV -visible (200-600 ml-l) radiation initially produced film of low tensile strength (scission) which gradually increased on further exposure (cross-linking). Elongation at break at all exposures decreased due to scission, volatilization, oxidation and cross-linking.
Here, low to high elongation means hard to flexible film. Low to high tensile strength means brittle to tough film.
(b) Polyvinyl acetate copolymer (vinyl acetatevinyl chloride) -All radiations raised both the tensile strength and elongation but not sufficiently to overcome over-all decrease on normal aging due to scission and volatilization. In other words, radiations were beneficial in that they produced a tougher, more flexible film.
(c) Styrene-butadiene copolymer - Tensile strength decreased with exposure to all radiations. The UV -visible and visible-infrared radiations initially produced films of lower tensile strength than that of short UV.
Elongation of film irradiated with short UV increased to a maximum after three weeks and then decreased. Magnitude of elongation was high, about 1500 per cent. The UV -visible
radiation decreased elongation. Visible -infrared radiation had no net effect. This seems to demonstrate the action of heat in augmenting UV and accelerating scission and
volatilization and degradation.
Thus, in general, short wavelengths increased toughness (cross -linking), and long wavelengths increased brittleness (scission, volatilization and cross -linking).
Jaffe and Fickenscher (8) have found that grain cracking of polyvinyl acetate on new or unpainted wood occurred because the coating
was unable to undergo dimensional change s demanded by the substrate. Films with excellent elongation and moderate tenSile strength for sufficient
defcmrrahility did not crack.
When emulsion paints are top-eoated with less permeable oil or alkyd paints, severe blistering and loss of adhesion can result. Phillips (31) has proposed a mechanism for such failure. Now organic films can behave as permeability valves in that water vapour passes more quickly in one direction through a composite membrane than in the reverse direction. At high humidity the top coat, undercoat and the primer in succession slowly sorb water. When the system is subse quent1y subjected to drying, as the drying out process takes place at a more rapid rate than the sorption process, and the system is less permeable in the reverse direction, water is trapped within the primer. The system is rendered soft by the water and also loses adhesion with the result that the pressure of the water attempting to pas s through the system ｢ｨｳＮｴ･Ｇｲ［ｳｾ
it.
OIL·.., ALKYD PAINT
Chemical Change s
The oil-alkyd polymer undergoes air-drying by cross-linking of the fatty acid parts of the chain as for oils:but, of course, at lower concentration (1).
Fitzgerald (32) has studied the photo-oxidative degradation of oil-alkyd (50 per cent oil length-glycery1 phthalate) films. He has found that the oxidative deterioration begins in the oil part of the molecule with the same degradative mechanism as for the oil film and then in the presence of radiation spreads to the glycery1 and phthalate structures, which are ultimately converted into volatile products. Slight excess of :the scission
process over cross-linking occurs at an early stage in the exposure but soon the rate of cross-linking increases rapidly and it becomes the predominant feature.
Physical and Mechanical Changes
Loss of gloss and chalking of a pigmented film are due to loss of binder as volatile products. "Volatilization is caused chiefly by the shorter UV radiation (32). Zinc oxide and rutile titanium dioxide pigments offer some protection, that of the former being greater (30,33).
The shorter UV radiation was most effective in decreasing strength (29). The longer UV radiation, because of its greater penetration into the bulk of the film, caused shrinkage and embrittlement and loss of elongation of the film (29, 32). Higher temperature also contributed to a decrease in elongation (29) and stiffening of the film, as did humidity and water by leaching out solubles (34). Combination of these factors brings about rapid embrittlement leading to cracking if the extension of the substrate results in deformation of the film.
Oil-alkyds (22) are less susceptible to water blistering than those based on oil only. This is because the alkyd resins contribute to the film, polymers formed by condensation rather than by oxidation, so the specific adhesion of the vehicle to the wood surface is very likely due to attraction between polar groups other than hydroxyl and hydroperoxyl, and is therefore not nearly so markedly affected by moisture. In addition, such paints
(owing to the lower concentration of hydratable groups in the dry binder) do not absorb as much water as dry linseed oil film and hence swell considerably less.
PHENOLIC VARNISH
*
Chemical Change sPhenolic varnishes undergo film formation by oxidation and polymerization.
In oxidative degradation, as for oil-alkyd films, the oil molecule' is first attacked by oxygen. Peroxides, polyketones and free radicals are formed. The free radicals then attack the aromatic ring structure and phenolic
':< Para-phenyl phenol formaldehyde and China-wood oil and castor oil, or linseed oil and tung oil, or alkyd modified and linseed oil (29).
hydroxyl, producing cross -linking which is three -dimensional in the case of the first two formulations. Chain-scission reaction also occurs.
Radiation (200 ml-l -infrared) accelerates and intensifies the above degradation mechanism, short components promoting
cross-linking and long components cross-cross-linking with scission and volatilization.
When the phenolic varnish was modified with a linear alkyd, deterioration occurred more rapidly probably due to the lack of three-dimensional network.
Physical and Mechanical Changes
For phenolic varnish incorporating China-wood oil, all radiations (200 ml-l - infrared) impaired an increase in tensile strength.at break or
toughening, and produced a decrease in elongation at break or hardening. The UV -visible (200-600 ｭｾＩ component had the greatest weakening effect. The visible-infrared (500 ml-land above) radiation had the greatest effect on elongation, a sure sign of wide cross-linking and resultant brittleness.
For phenolic varnish containing linseed and tung oils, the tensile strength first increased and then decreased with aging, radiations accentuating the effects. The shortest and longest wavelengths had the greatest effect. The elongation decreased steadily, the visible -infrared portion having the greatest effect. This varnish had the lowest elongation of three types inve stigated which indicate s that aromatic cros s -linking and conjugation occurred very early. This may be partially due to the presence of tung oil which has a more powerful drying action than linseed oil.
Finally, for phenolic varnish modified with a linear alkyd, all radiations decreased the elongation and the tensile strength to bring about complete physical failure in eight weeks. Ultraviolet-visible component had the greatest effect on tensile strength while visible -infrared portion had the least. For elongation, visible -infrared radiation had the greatest influence and short UV (200 - 365 ml-l) had the least. No doubt the linearity of this
polymer renders it weaker than the three-dimensional para-phenyl phenol resin of the other two.
ARTIFICIAL WEATHERING
As the combined environmental a;gents and their interactions are the real agencie s in natural weathering, it will be seen that it is unwise to predict the weathering characteristic of paints from research where only one
or two of the factors are 'exerting an influence. Therefore, paint te chnologists have re sorted to artificial weathe ring te sts.
Bloch (35) defines artificial aging as "a controlled and reproducible laboratory process aimed at achievirig :poss'ibly within a short period the same deterioration of an item that is usually obtained
as the result of its long-term outdoor ej{posure in a particular environment'f. With the advent of more durable coatings, acceleration of deterioration has become of paramount importance.
Concerning the developmento£a,reliable accelerated weathering method, studies (30) have shown that for oil and oleoresinous paints the results are surprisingly good in view of the somewhat arbitrary nature of the test. For most paints, colour and gloss changes correlate well with those that occur outside; embrittlement of films also follows the same general pattern. Blistering by water has been over -emphasized while the more advanced forms of failure such as flaking and peeling are rarely
produced. The general conclusion is that for this class of paint the accelerated treatment is doing the job of chemical aging of the films reasonably
satisfactorily, but does not always produce the visible film failure which would occur on natural exposure.
Investigations (37) are currently under way in the United Kingdom to develop tests capable of simulating natural weathering, employing
various accelerated weathering cycles with a range of paints, both
oleoresinous and alkyd, and designed to show specific types of breakdown on outdoor exposure. Some encouraging results have been obtained.
CONCLUSION
Degradation of paint systems on exposure to weathering takes place at the surface of the film, inside the film and at the interface between the film and the substrate. Weathering agents degrade paint systems by bringing about detrimental chain-scis sion and cross -linking reactions in the film, causing erosion and deterioration in the mechanical compatibility with the substrate, and by undermining adhesion and providing force to de stroy film- substrate integrity.
Although much has been learned, there still remains among other problems, ｴｨ･ｵｮ､･ｲｳｴ｡ｮ､ｩｾｯｦ how the interaction and sequential effects of the elements bring about actual failure of the paint systems. The development of accelerated weathering methods capable of simulating natural weathering
TABLE I
WEATHER RECORDS FOR FOUR N. R. C. EXPOSURE SITES
JANUARY
1964-Temperature Precipitation Wetness Sunshine Total Radiation Location (Mean Max & Min) (OF) (In. ) Period (Hr) Period (Hr) (Langleys)
L Halifax
(marine - industrial) 33-20 5.34· 319 80
2. Ottawa
(rural high humidity) 27-14 3. 19 252 98 4,309
3. Saskatoon
(rural low humidity) 17-1.6 1. 14 632 117 3,348
4. Esquimalt, B. C. 46- 38 >:< 6.75
*
671 49*
2,139**
(marine) I JULY 1964 ... ｾ I 1. Halifax 70-55 6.08 381 J61 2. Ottawa 80-62 3.81 141 270 16,430 3. Saskatoon 81-58 0.82 494 340 17,267 4. Esquimalt 69-53*
1. 15*
246 245*
14,198** >;C Victoria >;C* Departure Bay, B. C.REFERENCES
1. Talen, H. W. The Requirements for the Prediction of Paint Performance. Journal of the Oil and Colour Chemists' Association, 46, 1963, p. 940-972. 2. Browne, F. L. Understanding the Mechanisms of Deterioration of
HousePaint. Forest Prod. J.,
i,
1959, p. 417-427.3. Bragdon, C. R., Editor. Film Formation, Film Properties and Film Deterioration. Interscience Publishers Inc .• NeiW York, 1958, p. 161.
" •..-- I
.
--
.4. ｓｴｾ Louis Society for Paint Technology. Evaluation of Vehicles in Exterior White House Paints. Official Digest, 35, t963, p." 1188-1210. 5. Whitford, P. Exterior Primers and Special Paint Systems for Wood.
Official Digest, 30, 1958, p. 707-728.
6. A Panel Discussion, Conventional Exterior Paints vs. Emulsion Exterior Paints. Official Digest, ｾＬ 1960, p. 135-148.
7. Fickenscher, J. H. PVAC Emulsion House Paints. Official Digest, ｾＬ
1961, p. 1630-1634.
8. Jaffe, H. L. and J. H. Fickenscher. Stress Strain Measurements of P. V. A. Films as Correlated with Natural Exterior Exposure on New Wood. Official Digest, 33, 1961, p. 331-343.
9. Vannoy, W. G. Exterior House Paints. Official Digest, 33, 1961, p. 1611-1615.
10. Estrada, N. Exposure Characteristics of Clear Finishes for Exterior Wood Surfaces. Forest Prod. J.,.!!., 1958, p. 66-72.
11. Desai, R. L. A Review of Existing Information and Bibliography of Photochemical Degradation of Cellulose and Cellulosic Material's. Progress Report No.1, Forest Products Research Branch, Dept. of Forestry, Ottawa, 1964.
12. Meteorological Branch, Department of Transport, Monthly Record, Meteorological Observations in Canada.
13. Elm, A. C. Deterioration of Dried Oil Films. Ind. Eng. Chern., 41, 1949, p. 319-324.
14. Crecelius, S. B., R. E. Kagarise and A. L. Alexander. Drying Oil Oxidation Mechanism, Film Formation, and Degradation. Ing. Eng. Chern., 47, 1955, p. 1643-1649.
Fi11m Degradation Under Ultraviolet Irradiation. NRL Report PB 121330, Naval Research Laboratory, Washington, D. C., 1956. 16. Miller, C.D. Degradation of Drying Oil Films, J. Am. Oil
Chemists' Soc. , ｾＬ 1959, p. 596-600.
17. Cowan, J. C. After Dry, What? A General Survey. Official:Digest, 34, 1962, p. 561-574.
18. Private ｴｯｭｭｵｮｩ｣｡ｴｩｯｮｾ .
19. Elm, A. C. Pigmentation and Its Effect on the Life of Paints. Official Digest, 34, 1962, p. 642-651.
20. Myers, R. R. Catalysis of Film Formation and Degradation. Official Digest, 34, 1962, p. 575-589.
21. Panel Representing The American Zinc Institute. A Current Look at Exterior House Paints. Official Digest,
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1963, p. 671-690.22. Elm, A. C. and S. Werthan. Mechanism of Paint Blistering and Special Exterior House Paint Primers. Official Digest, 29, 1957, p. 838-845.
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