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. E~ - xposure bite ant . Weatherometer Evaluations of Synthetic Polymers W. LINCOLN HAWKINS The environmental stability of synthetic polymers is important in applying them as protective coatings or sealants for historic stone buildings and mon- uments. These materials are attractive as protestants because of their ability to form highly impervious films or to penetrate into ceramic materials and so function as sealants. Care must be exercised in selection of the synthetic polymer having the highest level of stability under conditions to be encoun- tered, while at the same time not losing other important properties. The addition of stabilizers can do much to extend the useful life of synthetic polymers, and test procedures have been developed to predict the stability of these materials during outdoor exposure. Synthetic polymers are often used as coatings to protect buildings and other structures from the damaging effects of weathering. The use of paints, many of which have a polymeric component, to protect wooden structures dates back to the time of the pharaohs. Acrylic finishes for wood or metal are more modem applications, as is impregnation of stone and concrete with synthetic polymers, which is often employed as a technique for sealing out moisture. Despite these applications, however, the vulnerability of polymer coatings to various elements of the environment is often overlooked. W. Lincoln Hawkins is Director of Research, Plastics Institute of America, Hoboken, New jersey. 322
Exposure Site and Weatherometer Evaluations of Polymers 323 Almost all synthetic polymers are organic in composition. The only important exceptions are the siloxanes, and although these polymers are basically compounds of silicon and oxygen, they do contain carbon groupings in their structures. Most organic compounds, and polymers in particular, react readily with oxygen finder a variety of conditions. In the case of synthetic polymers, oxidation usually results in rapid degradation, and properties important to protective film applications are adversely affected. Moisture also degrades certain polymers by breaking molecular chains into smaller fragments. Ibe stability of synthetic polymers in the face of outdoor weathering varies widely. Degradation depends, in the first instance, on the pres- ence of chemical groups in the polymer molecule that absorb ultra- violet radiation in the frequency range to which the polymer will be exposed. Polyethylene, which is related to paraffin wax, should be transparent to ultraviolet radiation. However, this polymer has poor stability against weathering, apparently the result of oxygen-containing groups present as imperfections in the polymer molecules. Polymers with ring structurespolystyrene is an important example absorb strongly in the ultraviolet range and yellow rapidly dunng outdoor exposure. Another familiar polymer, polyvinyl chloride!, absorbs ul- traviolet radiation as a result of unsaturated groups in the polymer molecules and discolors extensively when exposed out-of-doors. Poly~methy! methacrylate), marketed under the trade names of Lu- cite and Plexiglas, has very good resistance to ultraviolet radiation. As a result, it is used as a glazing material. Although polytetrafluoro- ethylene has very good resistance to weathering, it is difficult to fab- ricate into protective films. The siloxanes are among the most stable polymers in outdoor applications and also function well as moisture repellents. FACTORS IN WEATHERING It is important to understand the complex factors that are responsible for loss of stability and eventual failure of synthetic polymers during weathering. A variety of reactive chemical agents are present in the outdoor environment, and many of these contribute to degradation. Oxygen, the most pervasive of these reactants, attacks all polymers, although stability to oxygen varies considerably for individual poly- mers. In the form of ozone, oxygen may be an important catalyst for oxidative degradation, although this has not been clearly established. Moisture is the second most damaging reactant in the atmosphere. Loss of stability by hydrolysis is very damaging to nylons, polyesters,
324 CONSERVATION OF HISTORIC STONE BUILDINGS and similar polymers made by condensation reactions. Polyethylene, polystyrene, and the polyacrylates (Lucite or Plexiglas) are resistant to attack by moisture. Catalysts, which are present in the atmosphere at all geographic locations and particularly in industrial areas, can rapidly promote loss of stability. These catalysts include oxides of sulfur and nitrogen, ozone in abnormal concentrations, and organic peroxides from automobile exhausts. These catalysts may accelerate oxidation Inch, when acidic or basic, can cause the rapid hydrolysis of such polymers as nylons and polyesters. Thus, these polymers will degrade rapidly in contact with the "acid rain" that may occur in heavily industrialized areas. When such polymers are used as impregnants for stone structures, acidic for basic) leachates from the stone or from concrete may cause them to degrade rapidly. Adverse weathering may appear to be slow, but the rate increases rapidly as conditions are intensified. Heat from solar radiation is a primary energy source that promotes the loss of stability during out- door exposure. Ultraviolet radiation also contributes to the degradation of most synthetic polymers. The protective ozone layer in the upper atmosphere screens out ultraviolet radiation of frequencies below about 2900 A. However, most polymers are degraded by ultraviolet radiation between 3000 and 3500 A, and considerable energy within this frequency range reaches most areas of the earth's surface. Thus, oxygen, moisture, many of the catalysts in the atmosphere, and solar heat and radiation constitute a very damaging combination of reactants. The adverse effects of weathering are the principal lim- itation on the use of polymers out-of-doors. ANTIWEATHERING STABILIZERS Considerable research has been directed toward developing stabilizers to offset adverse weathering effects. The chemical reactions responsible for photodegradation have been studied extensively, and as a result very effective stabilizers are now available. By prudent selection of stabilizers, the useful life of polymers can be extended considerably. Thermal oxidation of polyethylene, polypropylene, and similar poly- mers occurs by a free-radical-initiated chain reaction. Once reaction is initiated at some site within the material, degradation continues by autocatalysis, in which a chain reaction results in further attack at hundreds of additional sites. Understanding of this mechanism has led to development of two types of thermal stabilizers or ~antioxidants.
Exposure Site and Weatherometer Evaluations of Polymers 325 The first of these inhibits free-radical initiation, and the second inter- rupts the chain reaction so that a few steps, rather than hundreds, result from an initiation reaction. For instance, unprotected polyeth- ylene is completely degraded in about two years at temperatures slightly above ambient. When properly stabilized, however, this polymer resists degradation for 40-50 years at similar temperatures in accelerated tests. The life of polyethylene would be even greater under natural outdoor conditions because temperatures are usually much Tower at night. Many of the newer stabilizers can be used in clear formulations; they do not impart color to the polymers. Effective protection can be realized with only a tenth of a percent of added stabilizer. Protection against ultraviolet radiation is provided by pigments, which screen out the radiation, or by ultraviolet absorbers, which absorb the damaging radiation and then dissipate the absorbed energy in a manner not hallnful to the polymer. Absorbed energy may dissipate through fluorescence, emission of visible light, or by chemical reactions of the stabilizer. Carbon black is by far the most effective of all available light screens. However, its use in protective films is limited to those applications in which black would be an acceptable finish. Ultraviolet absorbers, on the other hand, afford good protection for clear or light- colored films. Although not as effective as carbon black, several of the newer ultraviolet absorbers can extend the outdoor exposure life of some polymers by an order of magnitude. When stability is lost and failure occurs during out-of-doors expo- sure, the extent to which thermal energy may have contributed to photooxidation must be considered. It is difficult to distinguish be- tween concurrent reactions and the effects resulting from each. For this reason, combinations of an ultraviolet absorber and a thermal antioxidant are used when maximum protection against weathering is required. These combinations often result in a synergistic effect, giving better protection than would be anticipated from the effective- ness of the individual components. ACCELERATED TESTS Accelerated tests for stability against weathering fall into two cate- gories, outdoor and indoor tests. Outdoor exposure is the most reliable test, but the time required to measure the stability of well-protected polymers is too long to be practical. Acceleration by a factor of about two can be realized through location of exposure sites in areas of intense solar radiation. In the United States, locations in the Southwest (the Sun Belt) and in Florida are preferred. The United Kingdom uses
326 CONSERVATION OF HISTORIC STONE BUILDINGS exposure sites in South Africa and similar areas. However, site location alone does not give sufficient acceleration. Conventional exposure sites employ an angle of 45° facing south for test samples. This fixed angle does not give the maximum exposure but has been adopted as a stan- dard in order to make possible a reasonable level of cross-checking between different locations. A higher degree of acceleration is obtained by rotating test samples so as to follow the sun's path across the sky. These solar-tracking devices con reduce the test time by perhaps an additional factor of two. The most advanced of the solar-tracking de- vices uses a group of polished metal mirrors to accumulate solar ra- diation and concentrate the energy on test samples. The combined effects of solar tracking and accumulation of solar radiation have been 1 0,000 1 ,000 - 1 00 10 ....... 1 1 1 2700 2900 3100 3300 3500 WAVELENGTH IN ANGSTROMS (a) Sunlight (b) Xenon Arc (c) Carbon Arc (d) Mercury Lamp (e) Fluorescent Lamp .~ it. : ,, ! ~1 i /1 i i / \,,-~(~__' ibJ _-~' .(c) I, 1 1 `_1 :. ~ 11 _ I (e) I '__~ ~ 1 3700 3900 4100 FIGURE 1 Comparative spectra of ultraviolet light sources. SOURCE: John Wiley ~ Sons.
Exposure Site and Weatherometer Evaluations of Polymers claimed to give an acceleration factor as high as 11 over exposure at a fixed angle in a temperate zone. These methods may, however, pro- vide an improper balance between thermal and radiation damage. It is difficult to maintain the temperature of test samples near what it would be under normal exposure conditions. And despite all the ad- vances made in outdoor test procedures, exposure time is lost at night. In an attempt to obtain 24-hour exposure, indoor tests have been developed that use an artificial ultraviolet source. Ultraviolet lamps and arcs have been used in indoor testing in devices known by the generic term "weatherometer." Weatherometers are further classified by the ultraviolet light source used. The comparative spectra of several common ultraviolet sources are compared with the solar spectrum in Figure 1. It is apparent from these data that the xenon arc corresponds most closely with the solar spectrum. Unless there is a close match 327 TABLE 1 Comparative Stability of Polymers During Outdoor Weathering (E excellent, G good, F fair) Polymer Ultraviolet Radiation Heat Moisture Polytetrafluoroethylene (Teflon) Urea-formaldehyde resin {Melamine) Polyidimethoxy siloxane) Poly~methyl methacrylate) {Lucite or Plexiglas) Epoxy resins Polyurethanes Ester type Ether type Polylethylene terephthalate) {Hytrell Polyethylene {Polythene or Alathon) Polycarbonates {Lexan) Nylons lZytel) Poly(vinyl chloride) {Exon, Geon, Tygon) - NOTE: These relative stabilities are for ~.nprotected polymers. Selected trade n~mes appear under the technical names and are in parentheses. Some of the polymers listed are generic names, and there may be significant differences between members in a family. E E E E G F F F F F E E E G G F F F F F E E E G G F F G E G F G
328 CONSERVATION OF HISTORIC STONE BUILDINGS between the solar spectrum and that of the artificial light source, it is not possible to make an accurate prediction of weatherability. In Table 1, polymers likely to be used as protective coatings are grouped according to their relative stabilities during outdoor exposure. The data shown are for unprotected polymers. Relative ratings could be altered somewhat for stabilized polymer compositions. CONCLUSION This paper is intended to alert potential users of synthetic polymers of the vulnerability of these potentially useful materials to degradation under ordinary outdoor exposure. Careful choice of polymers to be used is most important. Much more can be gained by incorporating an efficient stabilizer system into the base polymer. The final caution to be raised concerns a proper test procedure for a selected polymer ~ . . . ~ . composition, one that will give a reasonable prediction of the time to . failure of the protective coating. BIBLIO GRAPHY ,7 ~ ~ Grassie, N., ed. Chemistry of High Polymer Degradation Processes. Wiley-Interscience: New York 1956. Hawkins, W.L., ed. Polymer Stabilization. Wiley-Interscience: New York, 1972. Kamal, M.K., ed. Applied Polymer Symposium No. 4. Wiley-Interscience: New York, 1967. Lundberg, W.O., ed. Autoxidation and Antioxidants. Wiley-Interscience: New York, 1961. Scott, G., ed. Atmospheric Oxidation and Antioxidants. Elsevier: New York, 1965.
