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A. LEE SMITH, L, H. BROWN, L. J. TYLER, and
M. J. HUNTER
Dow Corning Corp., Midland, Mich.
Thermal Stability of Resins Infrared Spectroscopic Study of Isomeric Phthalic Alkyds and Silicone Infrared analysis, applied to thermal stability in resins for the first time, is a promising indicator of their practical value
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R E C E N T L Y , A NUMBER of 'heat-stable resins and varnishes have been developed for applications such as electrical ingulation. One of the techniques for studying these materials is infrared spectroscopy, which has proved valuable for following the progress of chemical changes in polymers (7, 2 ) . In the infrared technique, a thin film of resin is deposited on a rock salt plate and then heated in an oven. The spectrum of the film is recorded intermittently during the heating period, and in this way a dynamic picture is obtained of chemical changes in the functional groups. The approach is not new; it was used in 1951 to study thermal degradation of polystyrene ( Z ) , poly(viny1 formal) (3), and cellophane (4). However, no published degradation studies on silicone-containing resins have been found, nor has this method been used to compare thermal stability of different resin types. Although infrared data indicate only chemical changes in structure, they usualky correlate well on a relative basis with physical properties such as flexibility, craze life, or dielectric strength which actually determine the practical value of the material. Since the films studied are considerably thinner than those encountered in normal usage, the heat treatment constitutes a severe test, and any numerical data obtained by infrared methods are probably conservative.
Experimental Details Resins discussed here include isomeric phthalic alkyds, their silicone modificatons, and a straight silicone. They were prepared in a conventional manner ( 5 ) in these laboratories. The silicone alkyds are copolymers (Table I).
Table Resin
A B C
D
E
F G @
I.
Compositions Studied
of
Resins
Phthalic,
Oil,
Silicone,
Tvt. %
%
%
0 0 0 29.9 29.9 37.0 0
0 0 0 25.0 25.0 24.9 100"
60.2 64.3 64.3 29.7 29.7 22.8 0
(ortho) (iso) (tere) (ortho) (iso)
(tere)
Dow Corning 994.
Specimens were prepared by putting a few drops of the resin solution on a polished rock salt plate, which was then "wiped" on a smooth flat surface to give a reasonably uniform film. Films showing suitable absorbances were usually obtained in two or three attempts, A 1hour cure a t 175' C. was used to remove the solvent and harden the films which were estimated a t 0.005 mm. in thickness. The sample, mounted on the rock salt plate, was then subjected to consecutive heating, cooling, and scan-
ning cycles. A special sample holder ensured consistent placement of specimens in the spectrometer and thus minimized errors from nonuniformity of films. Samples were heated in forced draft ovens a t 200°, 250°, and 300' C. To prevent the salt plates from cracking by thermal shock during the heating and cooling cycles the plates were nested in glass wool in individual metal pill boxes. Infrared spectra were run on a Baird double-beam spectrometer with a sodium chloride prism. T o obtain per cent of the group remaining after heat treatment, absorbances of the bands were measured in the usual manner and compared to those measured for the cured but unheated resin. Wave lengths of the bands used are: ortho- and terephthalates, 7.9 microns; isophthalate, 8.1 ; carbonyl, 5.8; SiCsHs, 14.3; SiCH3, 7.9; and siloxane, 9.3-9.7.
Results I n thermal degradation reactions, several effects are important. One is volatility, where low polymers are lost because of their vapor pressure. This effect usually occurs during initial heating of a resin. Another is bond rearrangement which may cause formation of volatile fragments. This is especially true of orthophthalic alkyds in which much of the loss is caused by formation and volatilization of phthalic anhydride. Also, atoms or small groups of atoms may VOL. 49, NO. 11
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H O U R S HEATED Figure 1.
Progress of thermal degradation of phthalic alkyds at 200" C.
oxidize to form products which may be volatile or remain with the resin. Most resins are subject to all of these effects during heating, and their relative importance depends on, among other
things, history of the sample, nature of the substratum, time of exposure, thickness of the film, and nature of the surrounding atmosphere. Figure 1 shows the progress of thermal
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Figure 2.
1904
Infrared spectra of a silicone-modified terephthalic alkyd at 200"
INDUSTRIAL AND ENGINEERING CHEMISTRY
C.
degradation in the alkyd resins. A logarithmic time scale is used for convenience in plotting stability as a function of time. The orthophthalate alkyd has disappeared completely after 5 days a t 200' C. The iso- and terephthalate alkyds remain 40 to 50 times as long at the same temperature. New bands appearing in the terephthalate resin spectrum at 6.4 and 7.2 microns are believed to be due to sodium terephthalate which forms by reaction of the resin with the rock salt mount. Because of this salt formation, heat stability studies using alkali halide crystals as mounts may not give completely valid data a t long heating times, although experiments in which powdered salt is mixed with the resin before heating indicate that the salt has little effect on thermal life of the resin. Another approach, now being tested, is using reflection spectra from a metal surface coated with the resin. The pattern of loss i s similar for all alkyds, but the orthophthalate is lost more rapidly than the other two resins. Such a major difference in thermal stability of the isomeric phthalic alkyds suggests a need for re-evaluating formulations in the many applications where orthophthalic alkyds suffer from inferior thermal stability. Effect of heat on the same series of resins, now modified by adding linseed oil and silicone, is shown in Figures 2 and 3. Stability of the alkyd portions, which seem to act more or less independently of the remaining resin, is of the same magnitude as before. This study of silicone-modified alkyds indicates the mechanism by which alkyd silicone copolymers attain improved thermal stability. The effect is not so much stabilization of the alkyd as it is establishment of a silicone matrix which remains after the alkyd portion disappears under thermal stress. In this series, also, the iso- and terephthalic alkyd silicones are the most stable. The series of spectra shown in Figure 2 show the path of degradation for the modified terephthalic alkyd. The 3.4micron aliphatic bands have largely disappeared after 500 hours at 200" c.,indicating loss of the oil. After 5000 hours, the alkyd portion is gone but a small number of organosilicon groups remain. The spectrum of the residue after 10,000 hours of heating resembles that of hydrated silica. Reappearance of the 2.8micron hydroxyl band after 10,000 hours suggests hydration of the silica film by the surrounding atmosphere, even a t this elevated temperature. Figure 3 shows that similar effects occur in the orthoand isophthalic silicone modifications. The orthophthalic alkyd is stabilized to some extent by the silicone but it is still less heat-resistant than the other isomeric alkyds.
THERMAL STABILITY STUDIES
b Figure 3. Infrared spectra of siliconemodified alkyds at 200" C. D, ortho;
E, isophthalic
Figure 4. Progress of thermal degradation of a silicone-modified isophthalic alkyd at 200" C.
70 60 50 -
Iz w
40 -
0 30 -
a w
a 20-
E
IO -
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I
HEATED
b Figure 5. Loss of groups b y thermal degradation of a silicone-modified terephthalic alkyd
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Il.606 5766 1900 946 442 46
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Figure 6. Infrared spectra of a silicone resin (994) undergoing heat treatment a t 300' C.
lo-
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'
I
,
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HOURS H E A T E D Figure 7. Progress of thermal degradation of a silicone resin at 300' c.
T E M P E R A TuR E,'C Figure 8. Comparison of thermal stability for alkyd resins and a silicone resin A, ortho;
1 906
8, iro;
C, terephthalic;
G, silicone
Figure 4 shows the quantitative behavior of the silicone-isophthalate, and Figure 5 shows the silicone-terephthalate at 200' and 250' C. Perhaps the most striking feature of these curves is behavior of the siloxane (SiOSi) absorption. I t initially drops, as expected, but subsequently increases in intensity. The probable reason for this is that as organic groups attached to silicon are lost or oxidized, an oxidative cross linking takes place between nearby silicon atoms, thus actually increasing the amount of siloxane in the resin. The carbonyl band intensity is probably the sum of the ester carbonyl plus a small amount of nonvolatile oxidation products. That the carbonyl and isophthalate curves in Figure 4 parallel each other rather closely in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
dicates that the amount of such nonvolatile oxidation products formed after the initial heating is small. Figure 5 shows that the pattern of loss, and therefore the mechanism of degradation, is similar a t 200' and 250' C. In both the alkyd and silicone-modified alkyd resins, the degradation process involves largely a loss of small organic groups, probably by bond rearrangement and oxidative and hydrolytic attack. Behavior of a pure silicone resin is shown in Figures 6 and 7. The phenomenon of increasing siloxane absorption also appears in this resin. Possibly siloxane resulting from the oxidation process forms a protective skin over the resin which tends to protect it from further oxidation. Extreme heat stability of this resin is demonstrated by the fact that an appreciable amount remains after it i s heated for 10,000 hours (over 1 year) at 300° C. Its total life a t this temperature is estimated (by extrapolation) to exceed 105 hours. After 11,000 hours, the spectrum of the resin begins to resemble the silica residue left by the modified silicones after extended heating. The terephthalic alkyd, probably the most stable of organic resins, disappears completely after 40 hours a t 300' C. If it is arbitrarily assumed that useful life of the resin is finished when 20% of the original material remains, stability of various resins can be compared (Figure S ) , when 2070 retention life in hours is plotted against a function linear with respect to the reciprocal of the absolute temperature. For the terephthalic alkyd, the three points obtained fall on a straight line. For the other resins, parallel lines were drawn based on single points, for approximate comparison of stability. Relative stability may be compared in two ways-along the 200' C. isotherm, expected lives of the resins are for orthophthalic alkyd, 42 hours; isophthalic alkyd, 2600 hours; terephthalic alkyd, 4100 hours; and silicone (extrapolated), 900,000 hours. Conversely, a life of 1000 hours is expected at temperatures of 153' for orthophthalic alkyd, 215' for isophthalic alkyd, 223' for terephthalic alkyd, and 350' C. for silicone. Literature Cited
(1) Achhammer, B. G., Anal. Chem. 24, 1925 (1952). (2) Achhammer, B. G., Reiney, M. J., Reinhart, F. W., J. Research Natl. Bur. Standards 47. 116 11951) . (RP . 2235). (3) Beachell, H. C., Fotis, P., Huchs, J., J . Polymer Sci. 7, 353 (1951). (4) Kmetko, E. A., Phys. Rev. 82, 456 (1951). ( 5 ) Paint, Oil, and Chem. Rev. 119, No. 3 , 10 (1956). RECEIVED for review November 10, 1956 ACCEPTED May 8, 1957 Division of Paint, Plastics, and Printing Ink Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955. ~
