Influence of Resins on the Heat Bodying of Oils R. C. SHUEY Bakelite Corporation, Bloomfield, N. J.
erts less than normal dilution effect) at all concentrations, are proportionately superior for cooking with the slower oils which normally receive hightemperature treatment. In general, the greater this acceleration, the quicker the drying produced. This class produces varnishes of the highest durability. 3. Resins which exert normal or more than normal dilution retardation of polymerization should not be cooked with the oils but added after the heat treatment is effected. Resins seldom, if ever, form true solutions in the oils, and colloidal effects often overshadow the more hidden chemical effects which are important in the improvement of serviceability-the primary purpose of heat treatment.
The course of change during the heat bodying of oils is markedly influenced by the presence of resins during the heating. The resins exert specific effects other than that of simple dilution, and these effects often are manifested in both application and service of the resultant varnishes. A method is proposed for classifying resins by comparison of their influences on the bodying of tung oil, and the properties common to the major groups are compared: 1. Heat-hardenable resins may so exceed the oil in polymerization rate that utilizable consistency may be passed before oil component of a varnish has attained any appreciable degree of polymerization. 2. Nonheat-hardenable resins, if of the class which accelerates polymerization of tung oil (ex-
T
HE heat treatment of oils as a method of modifying their properties is subject to control by a change in both temperature and concentration. The other materials present often appear to exert effects quite different from those of simple dilution, and these effects may be of great value in obtaining properties unattainable by bodying the oil alone, even if the same materials are dissolved in it after bodying. The presence and state of dispersion of resins, driers, etc., in an oil or varnish have as great an influence on the properties of the 61ms produced as have carbon and other substances on the properties of steels. A particular heat treatment suitable for the best processing of the oil without admixture generally is unsuited for the best processing of the oil when mixed with other substances. The differences obtained may manifest themselves both in the working properties and in the service and endurance of the finished product. The literature is replete with studies of the bodying of oils and the variations in properties so produced, but is comparatively deficient in data quantitatively classifying resins as to their value in the further modification of these effects.
such as change of particle size and gelation or solidification. A change from liquid to solid produced by a reduction in temperature cannot be strictly considered a form of polymerization. Also under the heat in the kettle it may be possible to produce virtual solidification without the production of appreciable change in chemical properties detectable by such tests as iodine number determination (27, 28) and specific refraction (18). The solution or dispersion of a resin in an oil, in the absence of other variables, produces changes in all the physical constants of the oils. It also often complicates determination of the chemical constants. In few cases is the value found to be that of the mathematical mean of the components. A systematic study of changes, therefore, must generally be based upon comparison with simpler or more definitely known cases supposed to have at least some measure of parallelism.
Effects on Gelation Among the most apparent changes taking place during heat treatment is an increase in body or viscosity which may be continued to virtual solidification, especially with the more reactive oils. The addition of resins generally changes the rate of this “bodying”, and since different resins have various degrees of effect, it was thought desirable to establish some sort of a zero point from which to evaluate the specific effects of the various resins. The primary effect of addition might be considered to be simple dilution; therefore the addition of a simple diluent and a study of its effect on the course of the heat treatment would produce an idealized curve to serve as the zero point from which to evaluate changes. Tung oil, the most common quickly polymerizable oil, was studied, and soybean oil was chosen to serve as the “inert” diluent. Work by Bolton and Williams (4, 6) indicated that in comparison to tung oil soybean oil polymerized SO slowly
Heat Treatment An accurate evaluation of the effects of heat treatment on oil mixtures is complicated by the changes and additions of other materials intervening between the final heat treatment and the formation of the film. Furthermore, many of the changes are of the irreversible type. A correlation of cause and effect is therefore more often inferential than direct. The progress of the heat treatment of oils, often referred to as polymerization, may be followed by periodic observation of both the chemical and physical changes. Strictly speaking, polymerization is a specific term for a particular chemical change, and there is a question as to whether the term should be allowed to include the idea of physical changes of state, 921
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INDUSTRIAL AND ENGINEERING CHEMISTRY
that the amount of insoluble material produced on gelation of a mixture of the two oils is practically proportional to the tung oil content. The Browne gelation method (7), with slight modification (S4), was used to establish the curve of gelation of tung oil by running tests at various temperatures from 392" to 662" F. (200' to 350' (3.). Because of the chance of interference due to oxidation, parallel tests were also made in closed and sealed tubes. Both curves are shown in Figure 1. It is apparent that a t the low temperatures, where the time was long, oxidation produced acceleration, whereas at the higher temperatures oxidation appeared to produce slight retardation.
OF TEMPERATURE ON THE GELATIOX FIGURE1 . EFFECT TI~ZIE OF TUXG OIL
The increase in gelation rate with increasing temperature is such that, roughly, the time is halved for each increase of 25' F., or quartered for each 50" F. increase. For linseed oil there is a halving of the time for each 37.5" F. temperature increase. These values apply only through that portion of the curve which is approximately a straight line. By repeating these determinations on tung oil diluted with 1 0 , 2 0 , 3 0 , 4 0 , and 50 per cent of soybean oil, equivalent curves were prepared which can serve as the framework of the gelation rates in the presence of the designated percentages of inert diluent (Figure 2). At any given temperature, the change in gelation rate due to dilution can be calculated by a comparison of these values. .kt 392" F. the formula for this rate change was calculated to be approximately: T mix = T oil (l/c)'*' where T mix = time required t o gel mixture T oil = time required t o gel tun oil c = concentration of tung oif in mixture At higher temperatures the retarding effect was greater. With increasing dilution the temperature required to prevent gelation is lowered as shown by the upper portion of the lines becoming horizontal at lower temperatures. Subjecting mixtures of tung oil and various resins to equivalent gelation tests gives values for the change in gelation rate due to these resins (IS). Any marked departure from these values produced by an equivalent weight of resin under the same conditions may be considered as due to some factor other than simple dilution, and it is possible by comparing such departures to evaluate to a certain extent the different effects of various resins. For the sake of brevity, only the values obtained at 482' F. (250' C.) will be discussed (Figure 3). Percentage of diluent (resin or other oil) is plotted vertically and gelation time horizontally. Considering the gelation rate
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of the oil used as unity and dividing the gelation rate of the mixture by that value was the device used for making all determinations comparable. This derived value is termed the "gelation rate factor". This factor multiplied by 30 (an average gelation time for tung oil a t 482" F.) gives comparable gelation times in minutes. The formula referred to above as representing the effect of the dilution of tung oil with soybean oil, or inert dilution, a t 392 " F. is shown as curve 1; the observed values for the same mixtures a t 482" F. are shown as curve 2. The increase in r e tarding effect with increasing diluent content a t the higher temperature is apparent. It is evident that increased temperature within certain ranges produces an effect similar to increased diluent content. Linseed oil as a diluent is shown in curve 3. Since it has an appreciable content of linolenic glyceride, which is more easily polymerized than the linolein and olein of soybean oil, linseed oil would be expected also to polymerize to a certain extent and thus give the effect of acceleration in comparison to the inert diluent. Curve 4 represents chlorinated diphenyl as the diluent. This resin is supposed to have no effect on the polymerization of tung oil and therefore might be considered an inert diluting resin. This resin appears to produce more relative accelerstion than linseed oil. Such an apparent increase may be due to various causes. For example, an increase in melting point and viscosity causes the test sample to be stiffer, with a consequent consideration that the mixture is gelled a t an earlier stage in the actual polymerization of the oil. I n other words, high melting or very viscous diluents appear to produce the effect of premature gelation before polymerization has progressed to the same stage which is adjudged as "gelled" with a liquid diluent. This generalization is supported by a comparison of determinations on various lots of resins of different melting points at different temperatures. At lower temperatures this melting point effect is invariably accentuated.
FIGURE2.
EFFECTOF TEMPERATURE AND DILUTION ON THE GELATION T I N EOF TUNGOIL
Among the older resins commonly used with tung oil are rosin and limed rosin shown in curves 5 and 6. Both of these retard gelation markedly more than the previous diluents, SO may be considered as producing actual retarding of the effect of heat on the tung oil. Ester gum (acid number 6) shown in curve 7 produces more rapid bodying than any of the previous substances; Congo resin, curve 8, is still more rapid. It has been shown (IO)that acid oils are definitely slower in bodying than the neutral glycerides. We have observed that rosin varnishes are definitely slower than ester gum varnishes. Now we find an acid material (Congo resin) which produces faster bodying than
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
923
of many other types of "100 per cent phenolics". Curve 12 represents a polybasic acid-phenol resin which changes the bodying rate of the oil but little, irrespective of resin concentration. In fact, its tung oil varnishes can be cooked on the schedule of straight tung oil a t any resin concentration up to 50 per cent and a t any temperature from 450" F. up to the highest point a t which the operator mould have sufficient time t o handle tung oil. However, it does not impart so great durability to the very shorboil varnishes and is inferior to the previous three in gasproofing. The phenyl-substituted phenol resin shown in curve 13 starts toward the left (marked acceleration) and maintains this acceleration almost up to the 50 per cent concentration point. This is the most effective of all in the production of gasproofing, alkali resistance, and improvement of durability with increase in resin concentration. The last two curves (14 and 15) represent GELATIOW R A T E FAClOR.1 2 two of the more active substituted phenolic resins of the heat-reactive type. The bend in FIGURE3. EFFECTOF OILS ASD RESINSON THE GEL-LTION TIMEOF TUNQ these curves is in the opposite direction to all the OIL AT 482" F. others. Since these resins polymerize with heat. anv effect thev mav have on the oil is the neutral rosin ester. Evidently another factor is operating. hidden by the polymekzation of the resin reducing the apparent cooking time. At the higher resin concentrations this The degree of dispersion of the resin appears to have an influence, the more coarsely dispersed materials producing less polymerization of the resin has reduced the solidification dilution effect per unit of weight than the ones which are more time to less than the time required to warm the 1-gram sample nearly in solution. Reasoning of this sort is subject to the used to the temperature of the bath in which it is immersed. criticism that molecular weight determinations on colloidal Because of the limited time available for heat treatment of materials are unreliable (I@, and the apparent molecular the oil, gasproofing is difficult and the indications are that the oil is essentially unchanged in this short time. The greatest weight is dependent largely upon the conditions of determination and may bear little relation to the dispersion in another utility of these heatrhardenable resins is in hardening the solvent or dispersant (2). softer resins such as ester gum and limed rosin. When diAmong the straight phenol-aldehyde resins we find a luted with such soft or retarding resins, this type of resin can greater spread in activity than in all other examined classes be held in the kettle sufficiently long and a t sufficiently high combined. An aliphatic substituted phenolic type (curve 9) temperatures to produce gasproofing, and consequently alkali shows an apparent retarding effect a t 50 per cent resin (12.5 resistance and durability improvement in proportion to the gallon varnish), in which it is exceeded only by rosin and limed amount of phenolic resin used. rosin. At 25 per cent resin (37-gallon varnish), however, it Generalizations are always dangerous; yet we may dram has a markedly faster rate than ester gum. some fairly safe conclusions from this array of apparently disSince this resin is not appreciably polymerizable by heat, cordant observations: the acceleration in long-oil varnishes cannot be ascribed to 1. Acceleration of gelation rate produced by a resin which itresin polymerization being mistaken for oil polymerization. self is hardened by heat is not necessarily connected with an inThe fact that even at 450" F. (232' C.) its long-oil varnishes crease in the rate of polymerization of the oil with which it is are easily gasproofed, and thus produce varnishes of excellent cooked. durability and high waterproofness and alkali resistance, 2. Acceleration of the oil gelation rate by a resin which is not of the heat-hardenable type appears to be connected with accelerindicates that normal oil polymerization or something akin to ation of oil polymerization or a reaction similar t o oil polymeriit has taken place. zation in effect. This seems a logical conclusion from the Two other aliphatic substituted phenolic types (curves 10 increased gasproofingand durability obtained in these cases. Furand 11) show gelation rates of the order of ester gum. The ther evidence will be presented later. 3. The resins which show comparatively little increase in resins with the highest melting points (curves 9 and 11) show curvature with increase in resin concentration produce better the more rapid rate. The one with the lowest melting point drying varnishes when cooked a t high temperatures than do the (curve 10) shows slower cooking rates a t all concentrations. resins which produce marked curvatures (Figure 3, curves 8, 11, They are all exceptionally good a t gasproofing tung oil. If 12, and, under limited conditions, 13). The proof lies in comparison of similar curves a t higher temperatures. cooked with tung oil a t still higher temperatures, the resin of 4. The change in gelation rate producible by an inert diluent is curve 11 maintains its rapid bodying rate while 10 and 9 lag ractically never produced by the varnish resins in common use. markedly. The order of the drying rates of the varnishes is Soybean oil and chlorinated diphenyl are the only observed cases the same as the order of the cooking rates a t high temperaof dilution producing a change in rate exponentially roportional t o the reciprocal of the concentration of the tung oif) Each of tures even though the drying tests are made on varnishes the commonlyused resins, therefore, appears t o exert some specific cooked a t 482" F. or below; resin 11 is the most rapid and 9 effect on the course of the bodying of the oil. For differentiation the least rapid. There appears to be some connection beof these effects, confirmation must be looked for in a study of tween proximity to the unity time (30 minutes) a t 20 to 30 per the other properties of the varnishes. cent resin and quick drying. These three resins impart remarkable exterior durability to tung oil even in short-oil In considering resins as diluents, if consideration is given to varnishes, a fact not true of the resins previously discussed nor the three factors-direction of curvature, degree of curvature,
P
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and location of the line with respect to the tung oil under inert dilution-it is possible to form certain conclusions as to probable behavior of other oils as well as tung oil when bodied in the presence of these resins. Let us turn for a moment to the gelation rate of linseed oil, on which dilution data are much more fragmentary; it has been observed that the resins which show but little increase in curvature with increased resin concentration (generalization 3 above) are most suitable for use with linseed oil, either alone or in admixture with tung oil. They produce better drying and durability, the ones farthest to the left of the formula line (acceleration) showing the greatest durability, and the ones to the right (retardation) showing the least (Figure 3). Conversely, the resins showing the greatest curvature are the least suited for use with linseed and the slower oils (curves 5,6,7,9,and 10). There is a possibility that this is connected with the observation that they produce poorer drying in tung oil varnishes cooked a t the higher temperatures. The question may be raised that, since varnish cooking is seldom carried to the gel point, gelation rate evaluations are beside the point. However, the Worstall test and the Browne test, both gelation rate evaluations, have been the accepted methods for the purpose throughout the history of the modern use of tung oil. I n comparison let us examine the other most apparent changes during bodying.
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minations in the kettle with any of the mechanical types of viscometers, such as the De Vilbiss or Brookfield, or even with a pipet kept immersed to ensure uniform temperature, and simply withdrawn during the timing of the efflux. All three of these instruments have been used in collecting the data for the following discussion. If tung oil is brought to a given temperature a t the rate of 10' F. per minute, calculation from the data of Figure 1 shows that the warm-up period is equivalent to 4-5 minutes a t that temperature. If the oil is held a t that temperature and the viscosity determined periodically, a curve similar to that of Figure 4 will be obtained; that is, the change in viscosity is approximately hyperbolic.
Viscosity Increase during Heat Treatment Both the string test and the finger test on a cold drop of the cook are modified viscosity tests. It is true that they are not subject to accurate numerical expression, but they are amenable to remarkably accurate duplication by a skilled operator desiring to reproduce a known cook accurately.
FIGURE4. EFFECTOF HEATINQ TIMEAT 450' F. ON THE VISCOSITY OF TUNG OIL
Viscosity determined on a chilled sample of an undiluted varnish by a Gardner-Holdt tube, pipet, or other instrument is not necessarily proportional to the viscosity which will be found on a sample of the same material after dilution to a given degree with a given solvent. Nor are viscosities in different solvents always related directly to one another (3). The type and degree of dispersion may change during dilution or storage. Viscosity determinations made during the cooking process on the material of the cook a t the temperature of the cook do, however, give a measure of the physical state of the cook a t that time. The change occurring during cooling or solution can thus be eliminated by making viscosity deter-
[FIGURE 5 . EFFECTO F RESINSON THE VISCOSITY CH.4XGE OF TUNG OIL AT 450" F.
If, however, the points shown in Figure 4 are plotted as time us. log viscosity, they fall exactly on the lines shown as curve 0 of Figure 5. Figure 4 was derived from the lines of Figure 5 merely to illustrate why plotting arithmetically, as is generally done, fails to show the breaks in the curves as drawn in Figure 5. Curve 0 of Figure 5 is derived from the average of a number of determinations a t various times on different oils by all three instruments. The only correction applied was to consider a viscosity of 200 centipoises a t 450"F. as complete gelation, arbitrarily scaling all determinations on a basis of complete gelation (200 centipoises) occurring in a corrected time of 66 minutes. This was done because the remainder of the curves were plotted from varnishes made from 66-minute oil. Before interpreting the observations on the varnishes of Figure 5, we should endeavor to find the significance of the breaks in the oil or reference curve. Viscosities in the warmup period were calculated to 450" F. from observations a t the lower temperatures and are of doubtful value on that account. The line of no change in viscosity ends at about 20 minutes after reaching 450",the time when the oil begins to become incompletely soluble in acetone on dilution of 1 volume to 20 with acetone. Insolubility in acetone increases progressively throughout the next stretch, reaching a practical maximum a t the break shown a t 25 centipoises. At this point dilution in the same manner, but using mineral spirits in place of acetone, begins to show a precipitate insoluble in mineral spirits. This progressively increases through the final stretch of the curve up to solidification. From the 200-centipoise point to solidification is a matter of a fraction of a minute. The 200-centipoise point, for all practical purposes, is identical with the end point of the Browne test. For purposes of identification, the point at which the first material insoluble in mineral spirits a p pears will be called the "mineral spirits point". The time required to reach this point is close to 80 per cent of the time required to accomplish gelation, and the time of arrival a t the acetone point is approximately 35 per cent of the gelation time.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
It is not knou-n why there should be no appreciable change in viscosity during the first 20 minutes at 450’ F. Chemical change is certainly taking place, for nearly half of the low in iodine number occurs during this time. Whatever the substance being formed, it must have a viscosity at that temperature close to that of raw tung oil. The acetone-insoluble material has a definitely greater viscosity than tung oil. It a p pears to consist of dimer, a t least to a great extent. The mineral-spirits-insoluble portion appears also to be largely dimer (6, l 7 ) , a t least in so far as the Diels-Alder type of reaction is involved, but in a different state of dispersion or aggregation. The break a t the commencement of insolubility in mineral spirits coincides with the point of string from the spade as determined by the practical varnish maker. il’aturally the presence of a resin (which will be solid when cold) advances the string point because of its presence in the oil and thus allows the varnish maker to anticipate the reaching of this point before the cook progresses into the stage of “false body”, or what we may call “dispersed gel”. Viscosity determination at kettle temperature can thus be used for numerically recording the progress of a cook and estimating arrival at the desired end point. For the cooks containing 20 per cent resin (Figure 5 ) , curve 1 represents the viscosity changes when the resin is rosin, curve 2, limed rosin, curve 3, rosin ester or ester gum; and curve 4,a phenyl-substituted phenol resin. Comparison with the curves for these resins in Figure 3 shows that there is some relation between the gelation test and the progress of the viscosity change. The phenyl-substituted phenol resin, which produced marked acceleration in the gelation test (Figure 3), owes this acceleration more to advancement of the mineral spirits point in the viscosity curve than to an increase in the formation of what has been designated as dispersed gel. In fact, the dispersed-gel (mineral spirits point to gelation) portion of the curve has a more gentle slope than has tung oil alone, that of the phenyl-substituted phenol varnish being practically the same as that of rosin ester varnish a t this concentration. This leads to the conclusion that both these resins have equivalent holding power against the formation of dispersed gel and possibly to the deduction that the acceleration of the former is due to a speedier reaction in the zone between the acetone point and the mineral spirits point. The acetone point is essentially unchanged in the presence of 20 per cent of phenyl-substituted phenol resin, while the meager data a t hand indicate that i t has been delayed in the presence of the other resins. It is worthy of note that the phenyl-substituted phenol resin varnish became gasproof at the acetone point a t 450” F. whereas the other varnishes and tung oil had to be carried beyond the mineral spirits point a t this temperature to pass the same gas-checking test. The optimum of serviceability of the phenylphenol resin varnishes occurs a t the acetone point (22),while for best service the others must be cooked a t sufficiently higher temperatures to bring the gasproohg point a t or to the left of the mineral spirits point, This leads to the conclusion that the production of acetone-insoluble material in the phenyl-substituted phenol resin varnish is neither necessary nor desirable, but that the production of acetoneinsoluble material is required in the case of the other varnishes. Presumably, then, the acceleration of reaction has taken place before reaching the acetone point.
Chemical Polymer Formation Since no dependably comparable determinations of iodine numbers could be made on varnishes containing the resins, we are forced to rely on a comparison of viscosity values and iodine numbers of the oil only for any inferences as to possible
0 5 IO
20
30
TIME A T 450°F. ,CORRECTED 0 40 50 60 MIN.
OF IODINE FIGURE6 (above). COMPARISON NUMBER DECREASE AND VISCOSITY INCREASE OF TUNG OIL BODIEDAT 450“ F. OF VISCOSITY 7 (below). COMPARISON FIGURE AND ACETONE SOLUBILITY CHANGE I N TUKG OIL AKD ITS20 PER CENTPHEKYLPHENOL
RESINVARNISH
chemical significance in this difference in behavior. Such a comparison is shown in Figure 6, with the iodine number values inverted for the purpose of superimposing loss of iodine number and’gain in viscosity. A comparison of the progressive acetone and mineral spirits tests of tung oil and its 50gallon phenyl-substituted phenol resin varnish is given in Figure 7. Approximately half of the loss in iodine number occurs before arrival at the acetone point (no appreciable viscosity change), and almost 90 per cent of the iodine number loss has occurred upon arrival at the mineral spirits point (25 per cent of the viscosity change), leaving only 10 per cent of the iodine number change to take place during 75 per cent of the viscosity change shown. Virtual solidification took place during cooling of the last sample. If iodine number loss is considered evidence of primary valence polymerization, then half of this polymerization has taken place without appreciable change in viscosity or without
926
INDUSTRIAL ,4ND ENGINEERING CHEMISTRY
the formation of a substance insoluble in acetone; only 10 per cent of the chemical polymerization is accomplished during the 75 per cent increase in viscosity conveniently described as formation of dispersed gel. Steger and Van Loon (19) showed that loss in iodine value is not directly proportional to the loss of double bonds, and that there is no sudden change in chemical composition during gelling (80). Marcusson (IS) concluded that the solid and liquid polymer are chemically identical. Without resorting to explanations based on the above reasonings, we have the observation that, a t the point of only half the iodine number change in the oil, the phenol-substituted phenolic resin has effectively gasproofed the oil; this indicates that the component responsible for gas checking has disappeared. Such varnishes stopped a t this point show durability in service surpassing that otherwise producible only by continuing the cook until practically 90 per cent of the total iodine number change has been produced.
Effect of Heat Treatment and Resins on Application Properties CONSISTENCY A N D SOLUBILITY.Oils cooked only to the acetone point require but little dilution to bring them to brushing consistency. If cooked to the mineral spirits point, oils will stand dilution to 45 or 50 per cent nonvolatile content to obtain brushing consistency. Most resins show very low viscosities when diluted to 50 per cent with appropriate solvents. KO general rule can apply to oil-resin mixtures under dilution, for the choice of end point, the type of solvents used, and the effects of driers and other added materials must all be taken into account. A rosin or limed rosin varnish bodied on the down heat may be carried to the mineral spirits point or slightly beyond to obtain the maximum amount of thinner addition. Raising the melting point of the rosin or limed rosin by fusing it with the heat-reactive type of substituted phenolic resin produces some additional body and thus allows for equivalent dilution without passing the mineral spirits point of the oil. Conversely, varnishes stopped a t the acetone point or slightly beyond may require only 35 per cent of volatile to bring them to brushing consistency and thus produce 140 per cent as much film at the same covering rate per gallon. I n this case two coats would be practically equivalent to three coats of the varnish cooked to the mineral spirits point and thus save the cost of one application. The choice of solvents, simply from the standpoint of economy, often centers around the use of the maximum amount of mineral spirits. If all components are infinitely soluble in mineral spirits, it may be the sole diluent, although brushing properties are generally improved by some turpentine or dipentene. Of these two, turpentine, the most active oxygen carrier, is preferred for varnishes requiring the maximum of oxidation for film formation; dipentene is indicated in varnishes which require the minimum of oxygen intake or are extremely sensitive to oxidation. If a component or reaction product is not soluble in mineral spirits, some strong active solvent of the aromatic class is indicated, or if higher nonvolatile is desired for a given cook and viscosity, increasing the percentage of strong solvents will generally produce a certain reduction in viscosity. SOLIDIFICATION AND DRYING. The so-called drying of an oil in the absence of a solvent is simply a change from the liquid to the solid phase. The change from liquid to solid may be adjudged to have taken place as soon as the percentage of solid phase has increased to the point where i t has absorbed all the liquid phase and become rigid enough to display solid film properties. As colloids age, they lose their liquid-holding properties; therefore, if the solid ages faster than the
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liquid phase disappears, we may observe sweat-back or aftertack. Sweat-back is the true syneretic squeezing out of excess liquid with the aging of the film if the film contains large quantities of liquids, such as oils high in stearin and olein, which do not solidify with age. Aftertack may be defined as slow solidification of a certain liquid portion which lags behind the main film-forming substance, but finally does solidify and become a part of the solid film. Since resins are solids a t room temperatures, they are just as true film formers as are the polymerized portions of the oils. Normally the addition of resins thus increases the percentage of solid phase. Low-melting resins or resins liquefiable by a relatively small percentage of liquid are generally of comparatively low value as film formers because of their tendency to accentuate the aftertack and sweat-back in the film as it ages, especially under warm or humid conditions. High-melting resins, especially those with great liquid absorption properties, are therefore preferable. Oils held for any appreciable time above the temperature of maximum gelation rate, as shown in Figures 1 and 2, have their nonsolidifiable portion increased so that they become slow drying with a tendency toward tackiness. This maximum rate temperature is apparently lower with tung oil and higher with perilla and oiticica than with most of the other oils. For best drying, therefore, the indicated temperatures should not be exceeded. The practice of bodying the slower oils in the absence of resins while adding the resins on the down heat is an attempt to avoid this slow drying. As mentioned in the discussion of Figure 3, resins differ greatly in the effect produced by increased concentration. In general, increased temperatures produce effects similar to increased concentrations. The polybasic acid-phenol resin (curve 12, Figure 3) should therefore produce exceptional drying of the slower oils at all concentrations, even when high temperatures are used. This is borne out in practice. The mechanism of solidification has not been worked out completely. It is generally conceded, however, that both polymerization and oxidation reactions are involved and that their relative rateavary greatly with temperature as well as other conditions. Solidification by baking is largely polymerization. At room temperatures oxidation assumes greater importance, particularly in compositions that require a considerable degree of oxidation to activate the h a 1 stages of polymerization, However, some oxidation products of oils are liquids and overoxidation (sometimes called “maloxidation”) acts as a retardant. Since the oxidation products are less durable and less resistant to chemical attack than the polymerization products, oxidation should be held a t a minimum by the use of the lowest adequate amount of driers. Rapid oxidation, especially as a stimulant to polymerization in accordance with the chain reaction theory of Stephens (21) and others, produces quick drying, but if oxidation does not cease when the film is dry, premature aging by oxidation and ultimate destruction of the film will ensue.
Effect of Heat Treatment and Resins on Service Behavior CHALKINGus. CHECKING. No satisfactory correlation of adhesion and durability has been worked out, perhaps more because of other conilicting factors than because of inability to measure adhesion forces accurately. For example, shrinkage takes place (increase in density in addition to oxidationvolatilization losses) during the aging of oil films, and if the film becomes too rigid, cracking or checking must ensue. If tensile strength exceeds adhesion strength, cracks may be few. On the other hand, for a given shrinkage the pull exerted where a crack does form will be proportionately greater; and since
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I N DUSTRIAL AND ENGINEERING CHEMISTRY
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These figures clearly indicate the lower tensile strength of the surface of most films is a little harder than the interior, linseed oil compared to tung oil and the softening effect of slight slippage tends to result at the interface with the subrosin, even though hardened with lime, when compared to the strate producing alligatoring, or the coating curls away at the other resins. It also shows the enormous tensile strength of cracks and produces peeling. If the tensile strength of the the phenylphenol resin-tung oil varnish and the high tensile film is low, a multiplicity of cracks will form; and if the strength combined with high elongation of equivalent varcracks are numerous enough, they may be so fine as to evade nishes a t half linseed and half tung oil. It is unfortunate that a detection with the unassisted eye and produce virtual failure straight linseed oil film could not be made commensurate in of the whole film before they are apparent. If the multiaging time with the above values, but 7.5 hours at 260" F. tudinous cracks are only superficial and the body of the film (127" C.) were necessary to obtain a film which could be retains its integrity, this effect is often called "chalking", and handled and measured. At this age the values of varnish no particular harm has been done to the protective value of the films were as follows (25): film. Superficial chalking, then, would appear preferable to checking, and checking preferable to peeling. Composition, % Elongation, Tensile Strength, Most oleoresinous films have much more adhesive strength Bodied linseed oil Phenylphenol resin 70 Kg./Sq. Cm. than is required for good service. They are far superior in 12.6 ..5 36.8 100 17.3 56 95 adhesion to such films as straight nitrocellulose. 82.7 20 54 80 Although nitrocellulose alone displays poor adhesion on many surfaces, the addition of plasticizers (liquids which disI n the first series drier quantities commonly used for tung oil tend the iilm and reduce its tensile strength) and resins varnishes were used; in the second series larger quantities of (which have good adhesive properties) has successfully overdriers more suitable for linseed were used. come this objection. If peeling or curling is to be guarded After aging 100 hours a t 260" F. plus 160 hours at 302" F. against, addition of a distending liquid (preferably a film (150' C,), the values had changed to: former and not a liquid plasticizer) should reduce the tensile strength to a value commensurate with the adhesion strength. Elongation, Tensile Strength, Composition % Bodied linseed oil Phlnylphenol resin. 70 Kg./Sq. Cm. Similar results have been obtained by the cooking of linseed 290 3.35 100 .. oil into tung oil varnishes, which otherwise would have had 276 7.4 5 95 7.8 320. 20 SO tensile strength many times that required and therefore far in excess of the more than adequate adhesive strength (22). Thus the same elongation or flexibility relation was mainThe addition of the more flexible oils for this purpose may be tained with age, and numerical expression is given to the complicated by a slowing of the drying rate of the varnish unfrequently observed high flexibility of these varnishes in pracless a resin is used which will produce quick drying even when tice, even when compared to the oil itself. these oils are used in comparatively high percentages. OXIDATION.As previously mentioned, oxidation, after Films composed of resins which are themselves resistant to film formation is complete, tends to become film destruction oxidation do not shrink excessively, so that addition of such (11,16). This is evidenced by increase in acid number in the resins to oils in the making of varnishes should proportionfilm and subsequent loss of weight through the formation of ately reduce both checking and peeling if the oils are properly volatile products. Some resins tend to increase the oxygen processed. The phenyl-substituted resins, and to a lesser intake of oil films, others may retard it. As far as observed, extent some of the other phenolic resins, have this property; all resins appear to retard the beginning of the loss-in-weight therefore greater durability can be imparted to their short-oil stage, possibly because dilution of the oil with resin gives a varnishes than can be obtained in their long-oil varnishes, measure of physical interference with the ingress of oxygen which are in turn more durable than the oils or than other and egress of the oxidation products. varnishes made from these oils (8, 15). Most resins, espeFilms of bodied oils and varnishes with identical heat treatcially those of the retarding and low gasproofing class, reduce ment containing identical driers and no volatiles, were spread the durability of their short-oil varnishes markedly below a t the same weight per unit area. (Weighed quantities in that of their longer oil varnishes. TOUGHNESS, TENSILE STRENGTH, ELASTICITY, ELONGATION,small tin cans spread completely over sides and bottom by rotation until set.) Previous experiments had shown that FLEXIBILITY. These properties can hardly be treated sepalonger cooking would give lower oxygen absorption values, but rately because of their interrelation. Toughness and tensile we desired to keep the viscosity of all varnishes low enough for strength are generally associated, as are elasticity, elongation, spreading without the use of volatiles. The changes in weight and flexibility, although very viscous liquids may display with time were recorded for a period of nine months. Exelongation without elasticity. posure was under indirect sunlight in a room free from laboraThe oils themselves when dried to films tend to have high tory fumes. Figure 8 shows the weight changes observed elongation and comparatively low tensile strengths. In 27with tung oil cooked 30 minutes a t 400" F. (204" C.) and with gallon varnishes Nelson (14) showed the following values: linseed oil cooked 30 minutes a t 540" F. (282" C.), Composition Elongation Tensile Strength Tung oil alone reached a peak a t 13 per cent increase in % Ko./eq. cm. weight in about 800 hours or 1 month, dropping back to Ester gum-tung oil 100 62 about 12 per cent in the following 8 months. The inclusion of Limed rosin-tung oil 42 52 Limed rosin-linseed oil 88 28 30 per cent ester gum during the bodying gave essentially an Kauri gum-linseed oil 80 30 identical percentage change (calculated on the oil component) during the first 2 weeks but continued to increase to a maxiThe values can be compared approximately to 33-gallon mum of 15 per cent at the end of 9 months. Thirty per cent phenylphenol resin varnishes (24) of the same age (5 hours a t Congo resin, under similar conditions, also departed from the 100" C. or 212" F.): oil line in about 2 weeks but rose more slowly to 13.5 per cent Phenylphenol Resin with: Elongation Tensile Strength a t the end of the experiment. % Ko./sq. em. Thirty per cent of phenylphenol resin stimulated weight Tung oil 81 212 increase during the first day but retarded it thereafter to hold 75% tung-25% linseed oil 82 158 50y0 t ~ n g - 5 0 7linseed ~ oil 113 98 the maximum down to 8.5 per cent in 2 months, with no later 25% t ~ n g - 7 5 7linseed ~ oil 102 63 change. Repeated experiments verified this first increase (a Linseed oil 91 10
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VOL. 32, NO. 7
by 9 months. Thirty per cent phenylphenol resin, on the other hand, held back the weight increase to practically 7.5 per cent in 2 weeks, with no change thereafter. This probably has a direct bearing on the greater flexibility retention indicated by the elongation values. SOLID-LIQUIDPHASE CHANGES. Long et al. (11, 18) studied the gradual reduction in liquid phase during the aging of linseed oil films and concluded that loss of absorbed liquid phase is the direct cause of loss of flexibility and production of embrittlement with age. This is believed to be applicable to the other oils as well. The addition of a hard resin normally reduces the proportional amount of liquid phase and consequently lowers the flexibility. The Kauri reduction test is based on this effect. However, the inclusion of Kauri gum in the heat treatment does not produce the same embrittlement as the same amount of kauri added as a cold cut solution. The colloidal dispersion or phase distribution appears to be influenced favorably by the heat treatment. Zanzibar resin is reputed to stand above all others of the natural resins in this production of favorable effect by heat treatment. However, time appears essential in the development of this property; and in the days when Zanzibar resin and linseed oil were used for marine varnishes, long continued cooking of the oil with the resin was accepted practice. Choice and percentage of resin and heat treatment can thus be utilized in modifying the properties of oil films. For rubbing varnishes, hard resins added a t the end of the cook tend to produce rigidity and the comparative brittleness required for easy sanding. To increase the toughness of the varnish, resin should be present during a greater percentage of the heat treatment; this generally modifies the phase distribution favorably and reduces the brittleness. For the maximum of toughness and flexibility, the presence of a resin of high oil or liquid-phase absorption throughout the heat treatment, is accepted practice. If the resin retards oxidation, as indicated by the weight increase curves, this should tend OB TUNG OIL VARNISH FIGURE 8. WEIGHTCHANGESDURING DRYING to prolong the flexible life of the film. ComFILMS(above) AND LINSEEDOIL VARNISHFILMS (below) binations of oils and resins displaying sweatback (low liquid phase retention) would make for the poorest flexibility retention with age. For marine and other services requiring the maximum of plausible explanation of quick drying) as well as ultimate toughness, those resins should be chosen which display the lower intake under all cooking conditions and at all concentragreatest freedom from sweat-back, perhaps the surest easy tions of this resin. The lowering of the weight increase per criterion of high liquid-phase holding ability. unit of oil was found to be proportional to the amount of resin CHEMICALAND WEATHERRESISTANCE. Many relations used, reaching the low value of only about 2 per cent total between physical properties of films and their ability to stand weight increase at 50 per cent resin, or a 12.5-gallon varnish. weathering are apparent. The relation between chemical Confirmation of this observed resistance to the effects of properties and weathering are often less apparent, for the oxidation was also found in the determination of the acid films are almost never soluble in the chemical substances to number of varnish films exposed to the weather; the acid which they are exposed. number of the phenylphenol varnish remained essentially unMoisture absorption of the film, allowing the ipgress of changed in 9 months, whereas the other varnishes showed ingases and chemicals through their solubility in water, might creases of several hundred per cent. be considered the most important link between physical prop Linseed oil alone reached a peak of 10.5 per cent in about erties and chemical effects; the exclusion of water practically 12 days, dropping back almost to 9 per cent a t the end of 9 excludes all commonly occurring deleterious substances not months. Thirty per cent ester gum gave a more rapid rise to themselves soluble in the film. over 14 per cent in about a month with slight indication of loss
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
Since the vegetable oils have a certain measure of moistureabsorbing and -transmitting ability, one of the functions of resin is to control water absorption. The United States Forest Products Laboratory (16)found the percentage efficiency of three coats of the following films in resisting moisture penetration to be: Linseed oil Spar varnish (supposedly tung oil, resin unnamed) Rubbing varnish (supposedly tung oil, resin unnamed)
17%
60 89
The low value of the oil and the increase in protection produced by the resin are indicated. Because of its physical characteristics a high resin rubbing varnish would be the least durable of the three under exposure, but this early work indicated the high moisture-excluding value of resins compared to oils. The problem then becomes one of obtaining the maximum water resistance without loss of physical properties requisite for maximum durability-in other words, the use of the maximum amount of resin consistent with the greatest freedom from checking and peeling. For marine service the resins which are the best preventives of sweat-back are therefore again indicated. Since the vegetable oils are saponifiable oils, they form soaps or salts with basic substances, even in the polymerized state. Many of these soaps are water soluble. On the other hand, dilute acids tend to cause the splitting by hydrolysis of the glycerides to fatty acid and glycerol. It is here that freedom from water absorption has great value in excluding the destructive chemicals. The use of an alkali resistance test in evaluating chemical resistance of varnish films is based much more on measurement of resistance to moisture than on measurement of the saponifiability of the oils. There is no question as to the saponifiability of the oils; i t is simply a question of whether the alkali has an opportunity to reach the oils to get in its work. Many chemists have probably failed to see the significance of this distinction. Chemical effects are often accelerated by exposure to sunlight, but unless the resin itself is deleteriously affected by sunlight, the reactions on the oil, most of them hydrolytic, will still be retarded by the exclusion of moisture. Superficial chalking, as distinguished from checking and the deeper forms of film failure, thus appears to be related to the moisture-excluding value of the film,for with the practical effect of water confined to the exterior, a film of proper colloidal construction should have the weathering effects limited t o the surface of the fYm only. I n some cases the chalking thus produced may be removed by the elements, but in most cases where high gloss is desired, occasional washing or wiping is necessary to remove accumulated grime, and the chalking is generally removed with the grime.
Choice of Heat Treatment and Resins for Best Utilization of Newer Oils This discussion has been based principally on tung oil and linseed oil, because they are most commonly used and more data are available. However, much of the discussion is equally applicable to oiticica, dehydrated castor, fish oil concentrates, soybean, and many others. The approximate composition of these oils is known (Q),and proper blending to obtain equivalent colloidal effects offers no problem, especially if resin effects as well as oil properties are given full consideration. The heat treatment required by the different oils is dependent principally on their composition. It is essential in any case that each oil be polymerized in the kettle to its optimum point; otherwise the solid-liquid phase relation will be out of balance during both application and service.
929
Resins which assist oil polymerization (essentially vertical type, Figure 3) are preferably added at the commencement of the cook. There is often a definite advantage in bodying the slower oils in the presence of such resins, adding the faster oils later so that both will profit by the beneficial effects of the resins and reach the optimum point simultaneously. Oiticica oil contains approximately the same amount of polymerizable components as tung oil, but approximately 12 per cent of fats of the stearic acid class as compared to 4 per cent in tung oil. The saturated or nearly saturated fats, which may be present as mixed glycerides ( I ) , may or may not be crystallizable in the raw oil. As the unsaturates polymerize, however, their ability to hold the saturated fats appears to decrease and the portions which were originally effective as plasticizers become some of the worst offenders in producing the syneretic effects evidenced as sweat-back and brittleness. Oiticica oil thus requires plasticization to reduce or delay embrittlement with age. If plasticized with the slower oils, the percentage of saturated fats is seldom reduced, so that the tendency toward aftertack is increased without much permanent reduction in aging effects. Experience has shown that plasticization is best accomplished by the use of resins of high liquid-phase retention, such as the phenylphenol type. These resins, if properly cooked into the oil, not only preserve the flexibility of oiticica films but permit the use of as much as 50 per cent of the slower oils without significant reduction of drying time or appreciable loss of required film characteristics. Dehydrated castor oil is practically free from saturated acids, but lacks conjugated acids of the quickly polymerizable type and therefore requires acceleration. Dehydrated castor oil and oiticica together resemble an equivalent mixture of linseed and tung in many properties. Rosin and ester gum tend to produce too soft a film with such a combination. Much better drying is accomplished by the use of the resins which give essentially vertical lines in Figure 3, aftertack being greatly reduced if not eliminated. Recently fractionation of fish oil has been accomplished commercially; either bodied or unbodied oils are produced which contain only the unsaturated portions. The natural percentage of saturates in fish oil may be of the order of 25 or 30 per cent. Cold pressing may be used to reduce it to about 16 per cent. These new products, however, consist essentially of the higher molecular weight unsaturated portions separated from the nondrying and semidrying components by fractional distillation or fractional precipitation. Therefore they produce hard films because of lack of plasticizing fractions. Although clupanodonic glyceride is more highly unsaturated than eleostearic glyceride, the lack of conjugated bonds prevents these oils from drying as rapidly as tung oil. Mixed with tung oil they can be made to dry faster and harder than tung oil. These oils are not subject to sweat-back, as are fish oils, but may display some aftertack because of the low degree of conjugation and lack of chain reaction stimulation during drying. If the high-curvature resins are used, they should be added after bodying is accomplished; but best results are obtained by the vertical- and minimum-curvature types, which will give better drying and flexibility retention on aging. This discussion has been limited to the treatment of oils by heat bodying as influenced by the presence of resin; many types of finishes containing oil have been omitted which are not directly related to the routine processing of oils by polymerization.
Acknowledgment The author wishes to express his appreciation to A. J. Weith, V. H. Turkington, W. H. Butler, H. E. Riley, and other members of the research staff of the Bakelite Corporation for inspiration, encouragement, and many of the data used.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Literature Cited (1) Behr, 0.M.,IND.ENG.CHEM., 28,299-301 (1936). (2) Bender, H. L., J. A m . Chem. Soc., 61,1812-16 (1939). (3) Bender, H. L., Wakefield, H. F., and Hoffman, H. A., Chem. Rev., 15, 123 (1934). (4) Bolton, E. R., and Williams, K. A., Analyst, 51,335-8 (1926). (5) Zbid., 55,360-7 (1930). (6) Brod, J. S.,France, W. G., and Evans, W. L., IND.ENG.CHEM., 31, 114-18 (1939). (7) Browne, Frank, Chem. News, 14-15,July 12,1912; Analyst, 37, 410 (1912). (8) Came, C. L., Bur. Standards J . Research, 4,247 (1930). (9) Fawoett, P. H., Drugs, Oils & Paints, 53,322-6, 359-68, 400-2. 430-4 (1938); 54,13-14,49-54 (1939). (10) Jameson, P.E., Analyst, 45,329-30 (1920). (11) Long, J. S.,Rhenish, A. E., and Ball, G. L., Jr., IND. ENO. CHEM.,25, 1086-91 (1933). (12) Long, J. S.,Zimmerman, E. K., and Nevins, S. C., Ibid., 20,8069 (1928). (13) Marcusson, J., Z.deut. &u.FettInd., 43,162(1923).
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(14) Nelson, H. A.,Proc. A m . SOC.Testing Materials, 23,290-9 (1923). (15) New York Paint & Varnish Production Club-Federation Paper presented Nov. 11-12, 1932,Paint, Oil Chem. Rev., 94 NO.10,63-8 (1932). (16) Rhodes, F.H., and Ling, T. T., IND.ENG.CHEM.,17, 408-12 (1925). (17) Rhodes, F. H.,and Wela, C. J., Ibid., 19, 68-73 (1927). (18) Rinse, J., Rec. tmv. chim., 51,529-32 (1932). (19) Steger, A.,and Van Loon, J., Ibid., 51,648,996(1932). (20) Ibid., 53,769-78 (1934). (21) Stephens, H.N., IND.ENG.CHEM.,24,918(1932). (22) Turkington, V. H.,Moore, R. J., Butler, W. H., and Shuey, R. C., IND.ENG.CHEM.,27,1321 (1935). (23) Turkington, V. H., Shuey, R . C., and Butler, W. H.. Zbid... 22.. 1133 (1930). (24) Ibid., 23,791 (1931). Shuey. R. C., and Shechter.. L... Zbid.. 30.984 (25) Turkington, V. H., (1938). (26) U.S.Forests Products Lab., Tech. Note 181 (1933). (27) Wolff, H.,Farben-Ztg., 31,2235,2292-4 (1926). (28) Wolff, H., 2. angew. C h m . , 37,729 (1924).
Plastic Properties of Bituminous EFFECT OF OXIDATION Coking Coals R. E. BREWER, C. R. HOLMES, AND J. D. DAVIS Central Experiment Station, U. S. Bureau of Mines, Pittsburgh>Penna.
OR many years it was generally believed that the coking properties of bituminous coals were affected deleteriously by oxidation. More recently, however, i t has been established (7,9, IO) that some high-volatile A coals which develop a high degree of fluidity during carbonization show improved coking characteristics after oxidation has progressed to a certain stage peculiar to each coal. Controlled oxidation, therefore, has found economic applications in the reduction of the fluidity of such coals to give more desirable products in several low-temperature carbonization and coalprocessing methods and in coking plants. Furthermore, limited oxidation has been found to reduce the dangerous expansion properties of low- and medium-volatile coals. The tendency of different coals to oxidize a t ordinary temperatures has been measured in a number of different ways. Among these evaluation tests for determining the extent of oxidation a t any given stage, proximate and ultimate analyses, the agglutinating number of caking index, carbonization followed by shatter and tumbler tests on the resulting coke, temperature measurements in and composition of the gaseous products evolved from the stored-coal pile, and determination of plastic properties have found wide application. Since many coals often tend to oxidize slowly a t ordinary temperatures accelerated oxidation tests have been devised to produce samples for evaluation of the extent of oxidation by the above-mentioned tests.
F
Mechanism of Coal Oxidation The oxidation of coal is attributed to the addition of oxygen and therewith to the oxidation of certain coal constituents. All coals, even anthracites, combine with oxygen with the generation of heat. If this heat is not dissipated, spontaneous combustion may result. The “absorption” of oxygen by coal begins as soon as i t is broken out from the mine, and is most 1
The first paper in this series appeared in June, 1940, page 792’
rapid with freshly mined coal. The extent of oxidation of a given coal depends on a number of factors. These factors are summarized and discussed in earlier investigations (2, 6, 6). Davis and Reynolds (6) and Bone and Himus (8) give excellent reviews of both the older iron pyrites and of the present coal-oxygen complex theory of oxidation of coals. The latter theory attributes oxidation mainly to the presence in the coal substance of “unsaturated” compounds, which are oxidized more or less readily according to their constitution to form compounds of the humic acid type. During the process the oxygen is first “absorbed” by the coal substance, possibly in some activated form, then is more or less loosely incorporated with it in some way to form an unstable “oxygenated” compound or compounds, which with increased temperature decompose into carbon dioxide, carbon monoxide, and water. The net result, therefore, is a four-way distribution of the oxygen into oxygenated coal (residue), carbon dioxide, carbon monoxide, and water whose relative proportions depend on the conditions of oxidation. The known facts about the mechanism of oxidation of coals have a close parallel, a t least physically, with corresponding changes in plastic properties. The reductions in fluidity and in expansion of bituminous coking coals caused by preoxidation have been attributed to the destruction of any excess of the binding constituents over that required to give a strong semicoke. The same effect can be produced by the proper blending of coals of different types or by addition of inerts or, alternately, by preheating the coal to a temperature somewhat under 400” C. for a suitable time. Undoubtedly, the chemical changes are not the same, but the net physical effects appear to be similar.
Apparatus and Test Procedures The accelerated oxidation method developed in this laboratory (9) has been extended for tests at higher temperatures (8). In the latter test procedure the coal is oxidized for various selected
