E. R. MUELLER and CLARA D. SMITH Battelle Memorial Institute, Columbus, Ohio
Effect of Driers on Linseed Oil and Varnish Films Composition a n d Physical Characteristics of A i r - D r i e d Films
Infrared absorption studies, chemical and weight gain data, oxygen functional group analyses show that. . . Varnish films dried with drier gain nearly twice a s much oxygen a s those dried without @ linseed oil films dried without drier gain slightly more oxygen than those dried with
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INVESTIGATION was started in this laboratory about 6 years ago to determine the effect of various drier combinations on the air-drying and film properties of tall oil varnishes (3, 6). In subsequent work ( 5 ) ,infrared absorption spectra were obtained on some of these films to help explain chemically the physical changes observed. The changes during air drying at the wave lengths characteristic of OH and/or OOH groups and C=O structure were qualitatively in agreement with anticipated changes based on drying oil data in the literature. The quantitative results were unexpected. Stronger OH and/or OOH and carbonyl absorption intensities were observed in the varnishes set with driers present than in the varnishes set without driers. These results focused attention on the lack of experimental data and consistent definitions behind the prevailing concept that films dried with drier contain less oxygen than films dried without drier. Experiments were planned to:
1. Evaluate the significance of infrared data in terms of per cent oxygen by comparing simultaneous elemental analyses, weight gain data, and infrared absorption spectra on drying films. 2. Compare analytical results for various “set” conditions such as cotton-
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free time and tack-free time as well as for “fully-dried” films (arbitrarily chosen as films 3 to 4 months old). 3. Compare these analytical results for varnishes with similar analytical data for linseed oil
tribute relatively more to per cent oxygen than to infrared absorption intensities. Principal experimental observations for pentaerythritol tall oil varnish are:
The varnish used was a pentaerythritol ester (PE) of a 4470 rosin-acid tail oil (Unitol S, Union Bag and Paper Corp.). This varnish is essentially equivalent to a 121/2-gallon soybean oil ester-gum varnish. I n agreement with the earlier infrared results, there was more oxygen in films of this varnish when it was dried with driers present than when it was dried without driers. Values obtained were 25% oxygen for the varnish with drier and 17YGfor the varnish without drier. These data were obtained on films 4 months old. The oxygen percentages correlate well with the infrared absorption spectra obtained a t this time. At tack-free time, which occurred during a period of rapid chemical changes, the oxygen contents of the films were only a few tenths of a per cent apart. The elemental analyses a t this point in air drying did not correspond as well quantitatively with the infrared absorption spectra as they did further along in the air-drying process. This is reasonable when it i s considered that hydroperoxides and peracids con-
with drier than in the film without drier. 2. A higher level of C 4 builds up and remains in the film with drier than without. 3. Total per cent oxygen and the amount of C=O and 0-H in a dried film after 3 to 4 months are still higher with drier than without drier. 4. Films become tack free at nearly the same per cent increase in total oxygen with and without drier. Values obtained were 6.3Y0 oxygen increase with drier and 6.87, oxygen increase without drier. 5. An immobile film, as determined by cotton-free, set-to-touch, and tackfree tests, is obtained during a period of rapid chemical changes. 6. Indications are that the major proportion of oxygen entering the film remains as C=O and 0-H in the airdried film. After 2 years, the absorption at 2.9 to 3.0 microns and 5.8 to 5.9 microns is still strong. This observation does not rule out the possibility that some C-0-C cross linking may be present. However, this does mean that a large proportion of the oxygen is incorporated into the final film in forms that are not cross-linking structures.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1. A higher level of OH and/or OOH builds u p and remains in the film
There are several differences in the changes between the pentaerythritoltall oil varnishes and linseed oil : 1. By 2 months, the 2.9-micron OH and/or OOH absorption band was the same intensity in linseed oil films whether driers were used or not. However, as in the varnish films, the carbonyl intensity remained stronger with drier than without through the last test made at 7 months. 2. For linseed oil, the total per cent increase in oxygen became higher without drier before it leveled off at about 10 days than it did with drier before it leveled off at about 20 hours. 3. The linseed oil film without drier contained a higher per cent oxygen at set-to-touch time than the linseed oil with drier. Values obtained are lO.8yOgain in oxygen without drier and 4.3y0 gain in oxygen with drier. 4. The linseed oil films lost weight rapidly after becoming set-to-touch and returned to a point of only 1 to 3% net weight increase. As high as 1470 net weight gain was observed at the peak.
For the varnishes, a relatively small weight decrease from 14 to 12% was observed for the sample with drier. The no-drier film had not yet reached a weight increase peak when last measured a t 7 months’ age.
Infrared Absorption Spectra The infrared absorption spectra of a variety of oil and varnish films have been examined before, during, and after air drying. It appears that the qualitative spectral changes discussed in the following paragraphs can be considered representative of air-oxidation effects on drying-oil spectra. Vehicles studied extensively are linseed oil ; unmodified pentaerythritol ester of tall oil; and maleic-modified, soybean oil-extended
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pentaerythritol ester of tall oil. Each of these was investigated both with and without drier; the varnishes were studied a t three different drier combinations. Experimental. Spectra were determined on a double-beam Perkin-Elmer Model 21 spectrophotometer. Films from 0.4- to 0.7-mil thick were spread or sprayed on rock salt plates. Thickness was determined by comparing band intensities with the same sample placed in a calibrated fixed-thickness cell. It was necessary to place the linseed oil films in a horizontal position to avoid thickness changes from film flow of undried samples. Raw linseed oil was used instead of alkali-refined varnish linseed oil because of experimental difficulties encountered with the latter. Puddling or other kinds of uneven film formation occurred throughout many efforts to use varnish linseed oil without drier addition. A drier combination of o.03y0 cobalt, 0.06% manganese, 0.5% lead, and 0.5% calcium in the form of salts of naphthenic acids was used. This choice was based on the fast-dry performance of this combination reported earlier (5). Results. Spectra run on a linseed oil film 0.4-mil thick during air drying are shown in Figure 1. The broad band at 6.3 to 6.4 microns is caused by the drier salts. There is some contribution to C-H absorption bands by the drier salts, but this cannot be distinguished in admixture. The other bands in the first spectrum, which was made before pronounced oxidation started, are characteristic of fatty esters. The major spectral changes observed during drying are summarized below. Some of these changes have been discussed in other articles ( 2 ) and are presented here only for immediate reference purposes.
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From 2.85 to 2.95 microns, a broad absorption band with its maximum a t 2.90 microns appears. This band arises stretching vibrations and, from 0-H therefore, can be caused by either 0-H or OOH groups. A sharp absorption maximum a t 3.3 microns decreases. This band is caused by C-H when the carbon is unsaturated. Absorption by incoming carbon)-l groups is apparent adjacent to the ester carbonyl on the long wave length side of it. Acids, aldehydes, ketones, and esters absorb throughout this wave length interval. Quantitative measurements show that a strong maximum occurs a t 5.85 microns. Interference in this region from absorption by ester groups in the original oil films makes this band less readily apparent than the 2.9-micron 0-H band. A band which appears at 6.15 microns is probably caused by C=C groups and possibly by conjugation. Since C=C vibrations are sensitive to type and symmetry of substitution, the appearance of this band does not necessarily mean that additional double bonds are being formed. I t could be accounted for as well by a transformation of one olefin group into another. The generally increasing intensity observed from 7 to 10 microns is much more difficult to define than the changes previously discussed. In general, it is attributable to C-0 linkages. The band which becomes increasingly more pronounced a t 10.3 microns is attributed to the disubstituted ethylene trans-o1efin
H (R-
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Support is given to this common inter-
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W a v e Length, Microns
Figure 1.
Infrared absorption spectra of linseed oil films with drier at consecutive stages of autoxidation VOL. 49, NO. 2
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Figure 2.
Difference spectrum for air-dried linseed oil and undried linseed oil films Air-dried film in sample beam; undried film in reference beam
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Figure 3.
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Infrared absorption spectra of linseed oil film without drier a t consecutive stages of autoxidation
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Figure 4.
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Infrared absorption spectra of film of tall oil pentaerythritol ester with drier at consecutive stages of autoxidation
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Figure 5. Infrared absorption spectra identifying rosin ester bands which decrease during air oxidation of tall oil pentaerythritol ester
pretation by the concurrent decrease observed in cis-olefin absorption. This &-olefin band is the broad 13.5- to 14.5-micron band which in the first spectrum is so strong that it nearly masks the 13.88-micron terminal chain band. These spectral changes are shown in somewhat clearer fashion by a difference spectrum, which results from placing a n oxidized film in the sample beam and an unoxidized film in the reference beam of an infrared spectrometer. Only the differences in absorption of the two samples is then recorded. This kind of difference spectrum for linseed oil is presented in Figure 2. The functional groups which show up as much stronger in the oxidized film are the 2.9-micron OOH and OH band, the 5.85-micron C=O band, and C-0 broad absorption from 7 to 11 microns. The 6.15-micron and 10.3-micron bands which were mentioned before as probably specific olefin types are stronger in the oxidized sample. The olefin bands strongest in the unoxidized sample are at 3.3 microns and a t 13.5 to 14.5 microns (cis-olefin) . There are interesting spectral differ-
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Figure 6. Rate of structural group change during air drying of film of tall oil pentaerythritol ester with 0.03% cobalt, 0.06% manganese, 0.5y0lead, 0.5% calcium VOL. 49, NO. 2
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Figure 7. Intensity change in infrared absorption bands during air oxidation of films of tall oil pentaerythritol ester
Figure 8. Per cent oxygen increase at consecutive times during air drying of films of tall oil pentaerythritol ester
ences which show up, the significance of which has not been fully explored. The broad C=O band is interrupted a t 5.75 microns by strong reference beam absorption. This means that either some of the original ester groups are destroyed or some molecular changes occur close enough to the ester group to alter its frequency. The types of structural changes which would be most generally recognized as apt to alter the ester C=O frequency are alpha or beta unsaturation or alpha or beta oxygen substitution (7). This spectral change would appear to be unexpected enough in terms of usual drying oil theories to warrant some additional investigation. Qualitatively, much the same thing is observed to happen to the infrared spectrum of linseed oil when it is air
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dried without drier as when it is air dried with drier. Comparison of spectra obtained during the drying of a nodrier film can be made from Figure 3. As was shown in Figure 1 on linseed oil with drier, the 0-H and carbonyl absorption and 10.3-micron absorption increase during air drying. The broad 13.5- to 14.5-micron cis-olefin absorption
Table I.
and the 3.32-micron :=C-H absorption decrease during air drying. These spectra were obtained on the no-drier linseed oil film both before and after a set-to-touch condition was reached. Set-to-touch time was 92 hours. Spectra were selected for this figure with the closest possible approach to the same proportionate times to the set-to-touch condition as was used for the film with drier (Figure 3 ) . The same qualitative oxygen structural group changes which were observed during the air drying of linseed oil also were observed during the drying of varnish films. Spectra of a pentaerythritol ester of 44YG rosin-acid tall oil before and after a set-to-touch condition was achieved by air drying are shown in Figure 4. The 2.9-micron and 5.85-micron changes parallel those in the linseed oil films. The varnish films show spectral changes during air drying that are not directly attributable to oxygen groups which have entered the fatty ester molecules. They appear: instead, to be a loss of individual rosin-ester structure. These bands, which gradu-. ally disappear as air drying progresses, are evident in the spectra in Figure 5 at 8.92, 9.07, 9.45, 9.65, 11.05: 11.33, and 12.18 microns. That these bands originate from rosin esters is demonstrated in Figure 5, where pentaerythritol ester of. rosin is compared with the starting varnish film. These spectra show promise for studying the role rosin oxidation may play in varnish-film drying. The difference spectrum included in Figure 5 shows that these rosin-ester bands are stronger in a freshly prepared film than in an air-dried film 3 days old. There is one band shown in Figures 4 and 5 which decreases during drying and is not attributable to rosin esters or to unsaturation. This band is at 13.88 microns and is caused by terminal chains of five or more methylene groups. Although the absolute change in absorption is not great, quantitative measurements show that the percentage change is large and parallels OH and C=O percentage changes. Figure 6 shows the per cent increase at 2.9 microns and 5.85 microns and the per cent decrease at 13.88 microns for a pentaerythritol ester of tall oil. This particular varnish film was tack free in 10 hours and considerable leveling off of all reactions had
Analysis of Linseed Oil Film
% Oxygen at Different Ages Film Scraped from glasc Analyzed on platinum
INDUSTRIAL AND ENGINEERING CHEMISTRY
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occurred by 7 days. The band intensities a t that time were arbitrarily chosen as the 100% change figure to provide a basis for calculation. This decrease in terminal unsubstituted carbon chains could be caused by chain scission or by substitution at or near the end of the chain. This terminal chain band is also observed to decrease during the drying of linseed oil films. However, the strong, broad cis-olefin band interferes considerably, especially during the earliest phase of the air-oxidation reactions. For linseed oil, it would be possible to follow the 13.88-micron band better if the &-olefins were isomerized to tramolefins before exposure of the film to air is started. In the series of linseed oil and varnish spectra in Figures I, 3, and 4, the spectra run closest to a specified state of film immobility are labeled set-to-touch and tack-free. I t is readily apparent that these physically measured characteristics do not correspond to a discontinuity in the chemical reactions taking place. They occur during a period of rapid changes which progress further after film immobility is achieved. These facts are shown clearly by quantitative data. In Figure 7 is shown measured absorbance a t 2.9 microns and a t 5.85 microns a t consecutive times during air drying of a varnish film of pentaerythritol ester of tall oil. The 2.9-micron OH and OOH band is a t 0.14 absorbance unit at tack-free time (12 hours), which is about half the 0.30 value at which it levels off a t about 70 hours. The carbonyl maximum at 5.85 microns measured 0.35 absorbance unit a t tack-free time of 12 hours, 0.48 at 70 hours when the OH leveled off, and continued to rise to an absorbance greater than 0.7 a t 250 hours. Because the carbonyl band is so intense, this oil film is too thick for accurate measurements of the increasing carbonyl concentration beyond this time. It is interesting that the carbonyl increase continues after the 2.9-micron OH and/or OOH band levels off. Chemical and Weight Gain Data
T o aid in interpretation of the infrared absorption spectra and to determine how well the spectral data correlate with the oxygen content of the film, percentages of carbon, hydrogen, ash, and oxygen (by difference) were obtained on linseed oil and varnish films. These data were obtained concurrently with the infrared data on films made from the same stock. Experimental. Oil and varnish samples for films for all of the determinations made, including infrared, were taken from the same batch after all prehandling including drier addition, was completed.
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Legend E 18 - o With drier combination of Mn , l3 16 - 00 503% Co,Co,00506% % Pb -~ b L 0 No drier 2 1 4 - I Tack-free tlme .c m
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Figure 10. OH and/or OOH absorbance at consecutive times during air drying of films of tall oil pentaerythritol ester
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Figure 11. Carbonyl absorbance at consecutive times during air drying of films of tall oil pentaerythritol ester VOL. 49, NO. 2
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Figure 12. Per cent oxygen increase a t consecutive times during air drying of linseed oil films
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Figure 13. Per cent weight gain at consecutive times during air drying of linseed oil films
o With drier combination of 0.03% Co, 0.06 % Mn, 0.5 Ca, 0.5 Pb
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