Document x5DmkQLknjMppz9p8Qa9YZqGQ

474 G . H. HUTCHINSON July TRANSACTIONS AND COMMUNICATIONS Some Recent Advances in the Chemistry and I Technology of Drying Oils* By G. H. Hu t c h in s o n In t r o d u c t io n Much has been added to our knowledge of the chemistry of drying oils over the past twelve to fifteen years. It is the object of this paper to survey some of the important advances, and against this fundamental background to discuss a number of technical developments in processed drying oils. Rather than present a general review a limited number of topics will be discussed in an attempt to show how much industry has come to depend on fundamental research for a better understanding of its technical oil media, and to stimulate ideas for new developments in drying oils. Reference will also be made to some work carried out in the author's laboratory on the heat treatment of drying oils. Th e Po l y e s t e r Co n c e p t o f Dr y in g Oil Po l y me r s In the earlier studies on drying-oil polymerisation a good deal of attention was paid to studies of addition reactions and the role of the unsaturation of drying oils. The knowledge that drying-oil polymers could be formed by condensation as well as by addition reactions came as a result of studies on alkyd resins and other polymers. With the recognition that drying-oil polymers are polyesters, new knowledge is being gained of the behaviour of drying oils on thermal and oxidative treatment and during the complex processes of film formation. It will be useful to dwell a little longer on this aspect of polymer chemistry. The functionality requirements of acids and alcohols, which in condensation reactions lead to the building up of polymers, are that reaction between a dihvdric alcohol and a dimeric acid leads to the formation of a linear polymer, while reaction between a trihydric alcohol and a dimeric or trimeric acid leads to the formation of a three-dimensional polymer. It can be seen that a stand oil formed by heat treatment of unsaturated drying-oil glycerides may be regarded as a polyester formed by reaction of glycerol with a mixture of mono-, di- and tribasic acid units. Stand oils are not in fact manufactured in this way, but the final product is the same, whether it is made by esterifying the mixed polymeric acids with the alcohol, or by thermal treatment of the drying-oil glycerides to give the same degree of polymerisation. When the so-called modified drying oils are synthesised the reactivity of the drying oil molecules is generally increased, e.g,, by replacement of the alcohol or fatty acids with polyols or polybasic acids of higher functionality. The polyester concept applies not only to stand oils, blown oils and modified drying oils, but also to the formation of dried films. In the manufacture of a stand oil the ester polymer is built up to a stage short of the gel point. When this product is incorporated in a paint and the latter is applied in film form, the polyesterification is carried to the gel stage by the linking together of further monomeric (monobasic) acid units. This linkage is brought about following oxidation of the reactive centres *Read before the Hull Section on 7 October. 1957. 1958 SOME ADVANCES IN THE CHEMISTRY AN in the fatty-acid chains. On drying, the f proportion of polymer and a high proper material. Hardening and ageing leads to a h Co n s t it u t iv e As p e c t s o f C A great deal of attention has been givei years. Development of new analytical te-. crystallisation of fatty acids and glycerides fi analysis, has made it possible to obtain m constituent fatty acids of the technical dryi surface-coating industry. Attention may be The Chemical Constitution of Natural Fats1, results up to 1955. In 1943 Hilditch suggested that, in seim for the surface-coatings industry, more attc component fatty acids and glycerides in the shortage of drying oils in the years follow] studies of this kind of vital importance to im out in this country, on the Continent, in Ind aspects of paint oils: perhaps the greatest made by Hilditch and the Liverpool School The fatty-acid composition chart (Table of a number of drying and semi-drying oils. TABLE I Co mp o n e n t Fa t t y Ac id s o f Dr y iv ~~~\ ~~T ~~ i i Iodine Oil ! Value Perilla Conophor .. Linseed Candlenut Rubber seed.. Soya Bean .. 1 Niger seed . . Sunflower seed Safflower seed , Tobacco seed ; 205 200 ISO 164 138 133 136 136 133 142 Satu rated Acids 7 11 10 .13 21 19 15 10 7 5 Oleic Linok Acid Acit: ?0 5 12 12 20 20 10 ' 49 20 38 l 22 50 15 66 22 68 26 67 27 66 Len x (Lin -- Len) x I0~2. T(Lm -f 2L.cn) w ere Lin == % Linoleic acid. Len (l Linoleniv N41534 July J958 SOME ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 475 UNICATIONS ) Chemistry and <> Oils* ' ? % hemistry of drying oils over his paper to survey some of ntal background to discuss .1 drying oils. Rather than ics will be discussed in an j :o depend on fundamental oil media, and to stimulate e will also be made to some eat treatment of drying oils. g Oil Po l y me r s good deal of attention f the unsaturation of __ could be formed by te as a result of studies on on that drying-oil polymers he behaviour of drying oils : complex processes of film on this aspect of polymer \\ tols. which in condensation e that reaction between a mation of a linear polymer, neric or trimeric acid leads \ ::t can be seen that a stand /ing-oil glycerides may be 8?S. ol with a mixture of mono-, manufactured in this way, I Ae by esterifying the mixed reatment of the drying-oil ation. When the so-called %% the drying oil molecules is l Icohol or fatty acids with i le polyester concept applies j rving oils, but also to the j l stand oil the ester polymer j 1. his product is incorporated j polyesterification is carried j m Jonomeric (monobasic) acid tion of the reactive centres) in the fatty-acid chains. On drying, the film contains only a relatively small proportion of polymer and a high proportion of unchanged but still reactive material. Hardening and ageing leads to a highly cross-linked polymer structure. Co n s t it u t iv e As p e c t s o f Dr y in g -Oil Ch e mis t r y A great deal of attention has been given to studies of this nature in recent years. Development of new analytical techniques, such as low-temperature crystallisation of fatty acids and glycerides from solvents and spectrophotometric analysis, has made it possible to obtain more accurate information about the constituent fatty acids of the technical drying and semi-drying oils used in the surface-coating industry. Attention may be drawn to Hilditch's latest edition of The Chemical Constitution of Natural Fats1, which includes most of the relevant results up to 1955. In 1943 Hilditch suggested that, in searching for additional sources of oils for the surface-coatings industry, more attention should be given to studies of component fatty acids and glycerides in the oils under consideration. The acute shortage of drying oils in the years following the end of World War II made studies of this kind of vital importance to industry. Much work has been carried out in this country, on the Continent, in India, and in the U.S. A. on constitutive aspects of paint oils; perhaps the greatest contribution to this field has been made by Hilditch and the Liverpool School. The fatty-acid composition chart (Table 1) shows the fatty-acid composition of a number of drying and semi-drying oils. Although these are typical analyses, TABLE 1 Co mp o n e n t Fa t t y Ac id s o f Dr y in g a n d Se mi-Dr y in g Oil s Iodine , Satu Oil Value ' rated Acids Peril la . . : 205 7 Conophor 200 11 Linseed 180 10 Candlenut . . 164 13 Rubber seed.. Soya Goan . . Niger seed 138 133 136 21 19 15 Sunflower seed Safflower seed 136 133 10 7 Tobacco seed 142 5 Oleic i Linoleic Tinolenic Total Acid Acid ' Acid Drying Acids Drying Power Hilditch Greavt Index* Index 20 5 68 ' 73 50 141 12 12 65 77 50 142 20 20 50 70 3 s 120 10 49 no 77 n 105 20 38 21 59 22 50 9 59 ! 15 66 4 70 12 80 s 68 3 74 22 68 26 67 27 66 0 68 0 67 2 68 0 68 0 67 1 70 I 476 G. H. HUTCHINSON July it is important to bear in mind that considerable variation in fatty-acid composition can occur in any botanical species, due to factors such as climatic conditions during seed development. In the last two columns of the chart are figures calculated from the so-called quick-drying index which has been used as an estimate of drying power of an oil from a consideration of its component linoleic and linolenic acids. The index derived by Hilditch2, Len x (Lin ~ Len) x 10~2. does not assign drying power to oils containing solely linoleic acid in their constituent acids. That by Greaves3, (Lin + 2 Len), is of more general value, since by its use values can be assigned to the linoleic-rich oils, such as tobacco seed, safflower, sunflower, poppyseed, etc. However, the need for indices of this type is passing, since with a new knowledge of the glycer'-de composition of drying oils, the drying properties of an oil may be directly assessed so far as glyceride composition is concerned from the proportions of linolenic and linoleic acid in its component acids6. Non-conjugated drying oils can be divided into two classes, linolenic-rich and linoleic-rich. As a result of the extensive studies of the Liverpool school2'*'5'6'7'8, the following facts emerge: The technical value of non-conjugated drying j oils depends on their contents of linolenic and linoleic acid, and on the wayj in which these acids are combined in the mixed glycerides of the oils. A good | drying oil must contain at least two polyethenoid acid groups in practically i all its glyceride molecules. For a quick-drying oil the presence of a definite j proportion of linolenic acid is desirable. The total content of polyethenoid j acids is an approximate measure of drying power and should not be less than i about 65-70 per cent2. To be equivalent in drying power to a good linseed i oil a linolenic-rich oil should contain at least 60 per cent di- and tri-linolenic s glycerides and at least 80 per cent di- and tri-polyethenoid glycerides (including I 25 per cent tripolyethenoid). All glycerides should contain one linolenic acid j group. For replacement of linseed oil a linolenic-rich drying oil should contain j m its mixed fatty acids not less than 70 per cent (linolenic-flinoleic) drying! fatty acids, of which linolenic acid should form at least 50 per cent of the | total fatty acids6. The suitability of a linoleic-rich oil so far as its use in paints j is concerned is that it should contain not less than 67 per cent linoleic acid j in its total fatty acids; it must consist of glycerides, 90 per cent of which should j contain two or three linoleic groups in each triglyceride molecule, ?.e., it must! not contain more than 10 per cent mono-linoleic glycerides5. All linoleic-rich I oils which contain the requisite 67 per cent or more linoleic acid in their total} acids are identical materials and completely interchangeable for technical! purposes irrespective of agricultural origin (always provided, of course, that| unduly large amounts of anti-oxidant or natural plant pigment do not interfere8). This last statement is of considerable importance, since it emphasises the! importance of recognising the chemical status of natural drying oils rather j than botanical origin. One of the important findings of Hilditch's work was that natural drying oils! conformed very closely with the rule of even distribution. Barker and Hilditch6! examined the component glycerides of a number of linoleic-rich oils (Niger! seed, safflower seed and six sunflower seed oils of differing linoleic acid content).! Their results show that at 70 per cent linoleic acid content the proportions! of di-linoleo glycerides reaches a maximum (about 90 per cent). No trilinoleinl 1958 SOME ADVANCES IN THE C Is present until linoleic acid 1 dyceride structure of these oil W Dutton and Cannon9 claim countercurrent distribution m distribution pattern. They is linseed oil, a glyceride not per Scholfield and Hicks10 have composition of soya-bean oik is proposed. Further studies o of distribution in glyceride oi unsaturation (where both line Segregation The upgrading of drying attention in recent years. The and the liquid propane (Sole. operated commercially, the > on fish oils and sunflower-seec carried out in packed towers, by Gloyer11. Examples cited value 180 into an extract (7: (25 per cent) of iodine value into fractions of iodine value I The Solexol process as oj Le Riche and Stubbs12 with and fish (pilchard) oils. Befl be emphasised that any segr* and, if the non-drying low-ioc over the original oil, the wh( drying fraction. Jordan et a of a solvent-fractionating ph furfural process used for so\ used for up-grading linolenn Up-grading of linoleic oils s probably necessitate some p! Of the fatty acids in the glycc limit to what can be achieve have discussed the propane 4(K70 per cent linoleic acid, linoleic acid is set for the dryii those oils containing less than fraction containing 67 per ce seed oil of 56.3 per cent linolei ofl examined in the author'' Claims that the oil was an exc 25 Per cent of tung oil m component fatty-acid an that of a low-iodine-value lins <V> <V ^o. Ipr.' TCHINSON ^ 1 1958 SOME ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 477 : considerable variation in fatty-aci< species, due to factors such as ciimati i the last two columns of the chart ari ck-drying index which has been used from a consideration of its componeifi -rived by Hilditch2, Len x (Lin + Len] `.o oils containing solely linoleic acid i; ~s3, (Lin -f- 2 Len), is of more general- signed to the linoleic-rich oils, such jppyseed, etc. However, the need ': ith a new knowledge of the glyceride properties of an oil may be directlyit i is concerned from the proportions of lent acids6. Non-conjugated drying oi] -rich and linoleic-rich. ies of the Liverpool school2*4*5'6,7* nical value of non-conjugated dryinj and linoleic acid, and on the wa] j Ked glycerides of the oils. A gootL o.vLchenoid acid groups in practically .-drying oil the presence of a definite le. The total content of polyethenoid ing power and should not be less than nt in drying power to a good linseed t least 60 per cent di- and tri-linolenic d tri-polyethenoid glycerides (including' ides should contain one linolenic acid' linolenic-rich drying oil should contain 70 per cent (linolenic+linoleic) drying mid form at least 50 per cent of the loleic-rich oil so far as its use in paintf lot less than 67 per cent linoleic acid glycerides, 90 per cent of which should each triglyceride molecule, /.<?., it must 10-linoleic glycerides5. All linoleic-ricK :ent or more linoleic acid in their tot$ ipletely interchangeable for technics igin (always provided, of course, th; ^ latural plant pigment do not interfere8^ importance, since it emphasises tliSj 1 status of natural drving oils rathdT itch's work was that natural drying oifej even distribution. Barker and Hilditch;! a number of linoleic-rich oils (Nigslj d oils of differing linoleic acid content| linoleic acid content the proportion^ ^nim (about 90 per cent). No trilinoletf' is present until linoleic acid forms about 60 per cent of the total acids. The glyceride structure of these oils approximates to the even-distribution type. Dutton and Cannon9 claim, from studies of glyceride constitution by the countercurrent distribution method, that linseed oil conforms to the random distribution pattern. They isolated about 18 per cent of trilinolenin from linseed oil, a glyceride not permitted under the strict even-distribution pattern. Scholfield and Hicks10 have used a similar technique to study the glyceride composition of soya-bean oil, and here again the random-distribution pattern is proposed. Further studies of this type may throw more light on the problem of distribution in glyceride oils which contain polyethenoid acids of differing unsaturation (where both linolenic and linoleic acids are present in quantity). Segregation The upgrading of drying oils by solvent segregation has received some attention in recent years. The furfural process of The Pittsburgh Plate Glass Co. and the liquid propane (Solexol) process of the M.W. Kellogg Co. are both operated commercially, the former mainly on soya-bean oil and the latter on fish oils and sunflower-seed oil. Both are countercurrent extraction processes carried out in packed towers. The furfural process has been discussed in detail by Gloyer11. Examples cited included the segregation of linseed oil of iodine value 180 into an extract (75 per cent) of iodine value 196 and a raffinate (25 per cent) of iodine value 132, and of sova-bean oil of iodine value 138 into fractions of iodine value 152 (60 per cent) and iodine value 118 (40 per cent). The Solexol process as operated in South Africa has been described by Le Riche and Stubbs12 with reference to the up-grading of sunflower seed and lish (pilchard) oils. Before enlarging on this particular subject it must be emphasised that any segregation process results in at least two fractions, and, if the non-drying low-iodine-value fraction does not command a premium over the original oil, the whole of the segregation cost must be borne by the drying fraction. Jordan et a/.13 have discussed the case for the installation of a solvent-fractionating plant in the U.K. and consider that, although the iurfurai process used for soya-bean oil fractionation in the U.S.A. could be used for up-grading linolenic oils, such as rubber seed and candlenut oil, up-grading of linoleic oils such as tobacco, sunflower and safflower would probably necessitate some plant modifications. The natural even distribution } the fatty acids in the glycerides of drying or semi-drying oils sets a definite limit to what can be achieved by solvent segregation. Le Riche and Stubbs have discussed the propane segregation of sunflower-seed oils varying from 40-70 per cent linoleic acid, and have shown that, if a target of 67 per cent linoleic acid is set for the drying segregate, only very small yields are obtained for those oils containing less than 60 per cent linoleic acid, e.gonly 15 per cent of a fraction containing 67 per cent linoleic acid can be obtained from a sunflower seed oil of 56.3 per cent linoleic acid content. The parcels of segregated sova-bean ff examined in the author's laboratory some years ago did not live up to claims that the oil was an excellent substitute for linseed oil. Only by extension ^Jth 25 per cent of tung oil did it approach linseed oil in drying properties, ine component fatty-acid analysis for the segregated oil is very different from of a low-iodine-value linseed oil (only 15 per cent linolenic acid as compared J 478 G. H. HUTCHINSON July with about 55 per cent in linseed oil14;. It was perhaps unfortunate that thd segregated soya oils should have been described as a replacement for linseed oil In the past, arguments for and against the installation of solvent-segregatio plant in the U.K. have been influenced by economic factors--mainly pric' and availability of drying oils, considered together with the high capital cos of the plant itself. There is, however, a technical aspect of the subject, a study of which may bring about a renewed interest in segregation techniques in the not too distant future. Now that the importance of the chemical rather than the botanical status of natural glyceride oils is becoming recognised, an > with the new knowledge on the constitution of drying oils, the industry should be better equipped to tackle problems relating to the choice of oil component for a particular film-forming medium. In choosing the oil component of paint medium, whether it is to be modified as, for example, by alkyd formation or processed by oxidation or thermal treatment, it should be known wha is the ideal combination of glyceride molecules containing no, one, two, Oh three reactive (polyethenoid) acids which will contribute to a film with good physical properties, without at the same time leaving appreciable amounts^ of polyunsaturated centres in the film which can only undergo oxidative degradation. Attention may be drawn to a paper by Lundberg15 who ha,, stressed the need for research on the influence of glyceride composition o? film-forming properties. Once information on this aspect becomes available,* the whole question of solvent fractionation may well have to be considered in a new light. Linseed, Tobacco Seed and Soya Bean Oils It is appropriate to discuss some constitutive aspects of linseed, tobacco* seed, and soya-bean oils, since the first named is the principal drying oil, an the last two are in considerable use, particularly for alkyd-resin manufactur where their non-yellowing characteristics can be used to advantage. With bot semi-drying oils it is important to' realise that there can be considerable variations between various parcels of oil. Table II gives some idea of th extreme variation. TABLE II Va r ia b il it y o f Se mi-Dr y in g Oil s i Content of | Tobacco seed | Soya Bean (%) i (%) Saturated acids .. .. j 10-15 ; 12-14 Oleic .. .. .. .. | 15-30 ' 22-34 Linoleic.. .. .. .. |1 55-75 50-66 Linolenic trace ! 2-9 Iodine value 135-147 1 103-152 1 It will be appreciated that in alkyd-resin manufacture there are fact, influencing drying and film properties other than chemical constitution* the modifying oil, e.g., oil length, polyalcohol used, degree of condensati July nate that the >r linseed oil. it-segregation mainly price i capital cost Dject, a study niques in the 1 rather than ognised, and iustry should ii component iponent of a yd formation, known what one, two, or m with good ible amounts rgo oxidative -g15 who has mposition on ties available. )e considered . ^58 SOME ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 479 and polymerisation, and whether some modification such as styrenation or / xfialeinisation is used. Nevertheless it should be appreciated that variations ih fatty-acid composition of semi-drying oils may lead to variations in drying &nd yellowing of the film. In the formation of long-oil alkyds it is important to consider whether the oil component has the necessary 65-70 per cent drying fatty acids in its glycerides. In some notes on non-yellowing oils, Greaves16 has stated that a linoleic acid content of 70 per cent or over should be guaranteed, as well as some greater constancy in composition, if there is to be an increased and sustained interest in tobacco-seed oil as a drying oil. Even linseed oil, the principal drying oil, is subject to considerable variation as between different parcels. Agricultural factors such as climate and environ ment in seed development are responsible for the differences. Painter17 has stated that adverse climatic conditions, high temperature and insufficient moisture while the seed ripens, are factors sometimes responsible for the production of low-iodine-value linseed oil. The B.S. Specification for linseed oil does not include a clause describing linseed oil in terms of fatty acid composition. In addition to such identification tests as saponification value, refractive index, unsaponifiables, etc., it states that the iodine value shall not be less than 175. There is not much the oil processor can do about variations between parcels of oil and economic factors do not enable him to supply, on request, oils to a close specification with regard to iodine value. )obaccooil, and manufacture *e. With both considerable ` idea of the Since the importance of recognising the chemical rather than the botanical status of technical oils has been discussed, it would seem appropriate to consider the chemical constitution of various linseed oils with reference to some recently published work. Pouchon and Massoni18 have examined a large number of linseed oils from different sources. Their results indicate that the same quality of a seed cultivated in the same country can yield oils of different composition depending on climatic conditions. These workers collected data from 300 different linseed oils, and plotted the content of linolenic, linoleic, oleic and saturated acids against the iodine value. One can obtain an idea of the composition of linseed oil of a known iodine value from these curves, which are a fair average, but to get the true composition of an oil it is necessary to carry out the analysis in the usual way. Fig. 1 shows the relation between composition and iodine value, based on the data of Pouchon and Massoni. It indicates the following trends for oils of iodine value 175-200: An almost linear increase of linolenic acid from 48-61 per cent with increase in iodine value, an almost constant content of linoleic acid (ca. 14 per cent) with increasing I-V., a slow decrease of oleic acid content from 25 to 20 per cent, and a decrease of saturated acids from 11 to 6 per cent. : are factors nstitution of condensation Recently Fauve19 carried out an investigation in which he compared linseed ds of low and high iodine value, in stand-oil and alkyd-resin manufacture, and in the drying and yellowing of white paints based on these oils. He concluded 480 G. H. HUTCHINSON Jul| Fig . 1.Va r ia t io n o f Co mpo s it io n o f Lin s eed Oil s w it h Io d in e Va l u e [By courtesy of Peintures, Pigments, Verms] that high-iodine-value oils of high linolenic acid content polymerised mor| rapidly in stand oil manufacture than oils of low iodine value (and hencf lower linolenic acid content). Long-oil glycerol alkyds based on low-iodine-valu| oils bodied slower and were slower drying than those based on high-iodinej value linseed oils. In rather shorter oil alkyds, high-iodine-value oils gav| rise to a more serious risk of gelation. Comparing linseed oils refined and bleacheoj to the same degree, the high-iodine-value oils yellowed more than oils of lowej unsaturation. (In this work Fauve took particular care to compare oils fron which impurities, such as phosphatides and anti-oxidants, had been removed' by careful refining and which had been stored out of contact with air an4' light prior to their use.) He concluded that the yellowing, drying and reactivit| of an oil was closely related to the iodine value, itself a function of densitj and refractive index. Fauve emphasised the importance of accuracy of measurement in determiniiij the physical and chemical constants of linseed oil. In relating iodine valiF to the refractive index and density of an oil, he pointed out that measuremen must be made on properly refined pure oils. (Oils stored in contact with ail which have undergone oxidation, will have an increased refractive indpj 1958 soMi (R.I.), lov to several Fame's o Hence. 1.4626) X His ex[ 0.60658) > In detei and const found tha oils, the 1 The ab of Zelcnv the foliov The rc! test, on oi iodine val In this of invest i remains t< the chcmi part play' processes industry,. in the sur Many autoxidati important research 1 of the pa; to, and it advances a relatjvel of dimer than tetra ments, wl is now be take into oils are e varying I) monbmeri viscosity i of 120,00 I 200 ALUE 'ernis] merised more e (and hence v-iodine-value 1 high-iodinelue oils gave 1 and bleached i oils of lower )are oils from been removed with air and and reactivity ion of density in determines ? I iodine value; measurements; ntact with airfractive index - 1958 SOME ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 481 lower iodine value (l.V.), and perhaps increased viscosity.) He referred to several expressions relating I.V. with R.I., and l.V. with density of the oil. Fauve's own expression is: 1.V.=(/2i,--1.4626) x 104. Hence, an oil having an R.I. at 20C of 1.4815 will have an I.V. of (1.4815-- 1.4626) X 10,000=189.0. His expression relating density (p) at 20C with I.V. is: I.V.=(0.6733p-- 0.60658) Xl04. In determinations of the R.I. it is essential to have an accurate refractometer and constant temperature conditions. In the author's laboratories it has been found that for freshly refined and bleached linseed, tobacco-seed and soya-bean oils, the l.V. is related to the refractive index by the expression: I.V. = (wff -- 1.4570) / 0.00012 The above expression agrees quite closely with the experimental findings of Zeleny and co-workers20, who state that with linseed and soya-bean oils the following relationship between I.V. and R.I. at 25C holds: I.V, = (/fg -- 1.45765) / 0.0001164 The refractive index, if determined accurately, constitutes a useful sorting test on oil deliveries, since it can be carried out much more rapidly than the iodine value determination. In this paper attention has been drawn to some of the important results of investigations of the chemical constitution of drying oils. A good deal remains to be done, and perhaps one of the widest gaps in knowledge concerns the chemistry of the non-glyceridic components of technical glyceride oils. The part played by the natural pigments, trace metals, and anti-oxidants in the processes of autoxidation has attracted the attention of chemists in the edible oil industry, and is a field which should be investigated more fully by technologists in the surface-coatings industry. Th e r ma l Po l y me r is a t io n o f Dr y in g Oil s Many investigations of the mechanisms of thermal polymerisation and autoxidation have been carried out in recent years. In this country many important contributions have come from the Paint Research Station, and research has been active abroad, particularly in the U.S.A. At the beginning of the paper, the polyester concept of drying-oil polymerisation was referred to, and it is appropriate to refer to it again before discussing some recent advances in the knowledge of the chemistry of thermal polymerisation. Only a relatively short time ago it was thought that stand oils were made up largely of dimer and trimer glyceride polymers, and contained no higher polymer than tetramer. This was based on number-average molecular-weight measure ments, which for stand oils yield values of the order of 3000. More attention is now being given to weight-average molecular-weight determinations, which take into account the presence of large polymers. It is known that stand oils are extremely heterogeneous and contain a range of molecular species varying from very high to very low molecular weight polymers, as well as oionomeric unreacted glycerides. For example, a linseed stand oil with a viscosity as low as 20 poise at 25 C may contain fractions of molecular weight of 120,000 or more. Hoeve and Sutton21'22 have shown, with slow-bodying 482 G. H. HUTCHINSON Jull oils such as linseed oil, that, provided the various acids are randomly esterifid with the alcohol groups in the various glycerides, Flory's theory of polyl condensation reactions can be applied, enabling changes in molecular-weighf distribution to be followed as polymerisation proceeds. Also the relationship!] between the distribution and the viscosity can be studied. Studies of this kincp involve determinations of the amount of mono-, di- and tri-basic units the polymerised oils (determined by methanolysis of the polymerised oil followed by molecular distillation of the methyl esters so as to segregate thf monomeric, dimeric and trimeric acid units). Having obtained these data| the amounts of monomeric glycerides and various polymeric glyceride species! can be calculated using Stockmayer's equations23. In this way Hoeve22 hal| shown that a linseed stand oil polymerised for 6 hours at 300C contains; appreciable amounts of glyceride polymer with a molecular weight higher? than pentamer (/.<?., five interlinked glyceride units). There is no doubt thatf fundamental studies of this kind are of practical importance. They might! also be applied to changes in molecular-weight distribution during the dryingp of an oil film, in which case it might be possible to investigate drying and# mechanical properties as a function of molecular-weight distribution. Mechanisms of Thermal Polymerisation m The precise mechanisms by which addition polymerisation of unsaturatedjf fatty acid chains takes place has been the subject of considerable research! and discussion. With conjugated esters such as tung or oiticica oil it has beenj established that dimerisation takes place between a double bond on one chain| and a conjugated diene on another, uniting the chains by a six-membered|l cyclic group (Diels-Alder mechanism). Chemical evidence for the existence^ of a six-membered ring substituted in four neighbouring positions in the dimerl of methyl (3-elaeostearate has been obtained by Ciingman et al.24. On substitutiveg bromination followed by dehydrobromination and then oxidation, the dimerj yields prehnitic acid (1 : 2 : 3 :4-benzene tetracarboxylic acid). It has been| generally accepted that dimerisation of non-conjugated drying-oil esters| proceeds first by conjugation of the double bonds in a linoleate or linolenate radicle. Two such radicles can dimerise by a Diels-Alder reaction. AdditionJ can also occur between an isolated double bond on a non-conjugated radicle and the conjugated diene group in an isomerised linolenate or linoleate radicle| R -- CH = CH R -- CH = CH CH2 -- CH = CH -- R' -----* conjugation CH = CH -- CH, -- R'" R -- CH = CH -- CH CH -- CH., + R -- CH = CH -- R' R' CH R CH, -- R' 1958 SOME A Evidence workers25 us linseed and that it is the et air1 coul( the bromina of methyl li made by Ri dimerisation linoleate. Tl preceded by second-ordei by the react CH,[ClTh CH,[CH,]4 Abstracts first linolcat* Cl I JCl l], Transfer <. gives the fro CH:,(C11,], The radio limiting fori fCHjcHJ CHa[CHa; CHalCH*; If a rever reversal stej (IT) will pr< normal lino formation. The free-r of the beht anthraquinc be one whi< radicles. Aj catalysis of atom from radicle and  >fr July iterifie f poly.' -weighty ionship* us kind*' mits in} sed oil ;ate the* e data, species, 'e22 has ontains higher ibt that ' might drying ;ng and iturated esearch as been ie chain m bered Ad v a n c e s in t h e c h e mis t r y a n d . t e c h n o l o g y o f d r y in g o il s 483 Mnce fr this mechanism has been provided by Waterman and co|ers5 using refractive-index methods with polymerised drying oils (tung, te<jband poppyseed), but there has been no conclusive chemical evidence llijfs the sole operative mechanism. It is possibly significant that Clingman 'could obtain only very low yields of prehnitic acid when they applied ifShunation, dehydrobromination, and oxidation sequence to the dimers .|thyl linoleate and linolenate. A most important contribution has been Ife by Rushman and Simpson26, who carried out a kinetic study of the fllliferisation and conjugation reactions in the heat polymerisation of methyl These workers have shown that polymerisation is not necessarily .lif&ided by a conjugation step. Both conjugation and dimerisation follow 'kf^ond-order kinetics, and the dimers and conjugated isomers are formed by the reaction of free radicles according to the following reaction scheme: ; CHdCH2]4 - CH = CH -- CH2 - CH - CH - [CH2]7 - COOCH3 (first chain) CH.ICHJ4 CH = CH -- CH2 -- CH = CH (second chain) [CH2]7 -- COOCH3 Abstraction of hydrogen atom from and active methylene group on the linoleate chain gives the free radicle, Kx: CBdCHsh -- CH = CH -- CH -- CH = CH -- [CH2]7 -- COOCH3 Transfer of a hydrogen atom to a double bond in the second linoleate chain gives the free radicle. R2: CH,[CH2]4 -- CH2 -- CH -- CH2 -- CH = CH -- [CH2]7 -- COOCH3 stetfrfve e dimer as been i esters lolcnate vddition radicle radicle. I The radicle Rx is a resonance hybrid which can be represented in the three limiting forms: 'CHa(CK,](1 -- CH = CH -- CH = CH -- CH -- [CH2]7 . COOCH3 (I) CHo(CH2]4 -- CH -- CH = CH -- CH = CH -- [CH2]7 . COOCH3 (II) CH8[CH2]4 -- CH = CH -- CH -- CH = CH -- [CH2]7 COOCH3 (III) w If a reversal of the hydrogen-transfer reaction occurs, the products of the reversal step will depend on which resonance structure is involved. (I) and (fl) will produce conjugated linoleate isomers, while (III) will regenerate a normal linoleate molecule. Combination of Rx and R2 radicles leads to dimer formation. The free-radicle hypothesis of Rushman and Simpson furnishes an explanation i.the behaviour of quinone-type catalysts in stand oil manufacture, e.g., jmthraquinone. The most suitable catalyst in the free-radicle reaction would H^ne cou^ offer a low energy path to the formation of Rx and R2 guides. Anthraquinone is easily reduced and it is postulated that, in the ^talysis of the free-radicle process, the first step is abstraction of a hydrogen *rom an active methylene group in a linoleate chain to produce an Rx -^aide and a semi-quinone-type free radicle. The latter acts as an intermediate $*75$ 484 G. H. HUTCHINSON carrier of the hydrogen atom and in the second step donates this hydroge atom to another normal linoleate chain to produce an R2 radicle. The eatalys forms a readily reversible oxidation reduction system. -- CH CH CH CH -- O CH = CH -- CH -- CH = CH -- tRx radicle) o- semi-quinone free radicle O CH = CH -- CH, -- CH = CH CH, CH CH2 -- CH = CH -- (R, radicle vw oII Rushman and Simpson have in fact shown that low concentration (0.5 per cent) of the quinone catalyse both dimerisation and conjugation Concentrations above 0.5 per cent do not appear to catalyse dimerisatiq but under these conditions the relationship between catalyst concentratin' and conjugation is almost linear. The free-radicle polymerisation of methyl linoleate does not precluv dimerisation between conjugated isomers, by, for example, a Diels-Ald^ addition mechanism, but indicates that the bulk of dimer is formed by diret dimerisation of the linoleate chains. Although the precise mechanisms of theriT polymerisation have not been established with certainty in the case of ncf conjugated drying oils, the present state of knowledge perhaps indicates tht both mechanisms (free radicle and Diels-Alder addition) are operative. radicle >1958 SOME ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 485 filter- and Intramolecular Polymerisation In the polymerisation of drying oils the main reaction is one of dimerisation between unsaturated fatty acid chains on separate triglyceride molecules (interpolymerisation). A relatively small amount of intrapolymerisation (between fatty acid chains on the same triglyceride molecule) takes place in the early stages of the heat polymerisation27. There is also some evidence that some intra-acyl cyclisation also takes place during the heat bodying of both conjugated and non-conjugated drying oils28'29*30. Interchange Reactions Radioactive isotopes have been used at the Paint Research Station31 to study interchange reactions such as occur in stand oil and varnish manufacture. Using esters of drying-oil acids it has been shown that all three types of reaction __acidolysis, alcoholysis, and trans-esterification--proceed at measurable rates at 300C in the absence ofcatalysts; trans-esterification is slower than alcoholysis and acidolysis, and at 200C no measurable trans-esterification occurs in the absence of catalysts, and at this temperature alcoholysis is faster than acidolysis. It is concluded that in most practical systems the rate of rearrangement for all types of reaction at temperatures around 300 will be sufficiently great to ensure a close approach to the equilibrium distribution. Studies of the Rate ofPolymerisation of Drying Oils In calculating the rate of bodying of drying oils it is useful to adopt the K value measurement, which is the slope of the curve of log (viscosity) against time of bodying. Cannegieter32 describes the straight line relationship between log (viscosity) and time, and calculates a polymerisation constant, K: K ------ jggfr-at constant temperature CH -- (R2 radicle) ow concentrations i and conjugation, alyse dimerisation, alyst concentration does not preclude tple, a Diels-A'defr is formed by di eel chanisms of thermal in the case of non* rhaps indicates that *e operative. where rnt2 is the viscosity (poise) at time tli2 (min.). Anderson and Porter33 have found that on plotting log (viscosity) against time for various drying oil blends bodied at constant temperature the curves obtained are in three sections (between 0.5-100 poise at 25C): a straight line of slope A^, a straight line of slope AT2, and a third portion in curved form (of continuously increasing slope). These workers have plotted the overall values of K (/>., between 0.5-100 poise) for a number of polymerised drying and semi-drying oil blends against various functions of the component fatty acid analysis. They have shown that the function: 2.0 (per cent linolenic)-r 1.6 (per cent linoleic)+0.6 (per cent oleic), gives a figure which predicts the bodying rate (up to 100 poise) better than does the iodine value. In the author's laboratory curves similar to those of Anderson and Porter have been obtained in the relationship log (viscosity) (stoke) against time of polymerisation, following heat-bodying studies with drying and semi-drying oils. In a recent investigation35 of the behaviour of various drying and semi-drying oils and mixtures of these oils (linseed, tobacco-seed, soya-bean) in the manufacture of long-oil pentaerythritol alkyds, the overall polymerisation constants have been calculated in the preparation of low-viscosity (1.0 stoke at 25CC) stand oils which have been used for the preparation of the alkyds. It has been found 486 H. HUTCHINSON Jull that there is good correlation between the overall K value and the following function (F) of the fatty acid analysis34 : F = (4.5 Le + 2.0 Lo)2 / 1000 where Le -- % linolenic acid, and Lo = % linoleic acid Rates of polymerisation have been determined at two reaction temperatures^ 282C and 240C, and in both cases an almost linear relationship is obtained35^' between the K value and the function (4.5 Le~-2.0 Lo)2/1000. An interesting1! feature of these experiments is the slow but measurable rate of polymerisation! of the drying oils at 240C (alkyd-making temperature). An estimate of thej bodying rate of a non-conjugated drying oil can be obtained from a consideration^ of the component fatty acid composition. Effects of Oxygen in the Heat Polymerisation of Drying Oils In comparing the properties of open- and closed-pot stand oils, it is necessary' to take into account the manufacturing conditions, particularly for the open-pof stand oils. Access of air (oxygen) has the effect of accelerating the rate of , bodying to an extent depending on a number of factors, such as mass of oil/f area of surface exposed to air, and degree of agitation of the oil. An investigation! has been carried out in the author's laboratory in an attempt to determine* the effects of oxygen on the heat polymerisation of linseed oil refined to varying degrees. In the apparatus adopted for the experiments, a constant quantity' of gas was passed over the surface of the oil during heat treatment, the oxygeh content being varied for each experiment by diluting the gas with carbon dioxide.^ Although this work is still proceeding, the following relationship appears to hold:| K _ *;(', + where: K = bodying rate of drying oil processed under gas stream with! oxygen of partial pressure p atmospheres, K' = bodying rate of the same drying oil processed out of contact! with air (/.<?., where />=0), A = area of surface exposed to gas stream (cm.2), a -- a constant, the value of which depends on the shape of thel vessel and whether stirring is used, M = mass of oil (grams), and p -- density of oil (g./cm.3). For a cylindrical vessel the expression becomes: K 1+ where: L -- depth of oil in vessel in cm. This follows since --M -- volume andj volume L ' 1958 sc Cons of dryii at the c to the | types c Thr occu-r the ir uneh; react! IU grout Ini Pn Te Dea radi on i lino lind smperatures, is obtained3* a interesting lymerisation mate of the onsideration is necessary he open-pot the rate of nass of oil, ivestigation determine i to varying it quantity the oxygen on dioxide, irs to hold: with of contact pe of the lumear.d )jV*E;ADVANCES IN THE CHEMISTRY AND TECHNOLOGY OF DRYING OILS 487 Au t o x id a t io n o f Dr y in g Oil s qnsiderable advances have been made in our knowledge of the autoxidation |drying oils. Not many years ago classical theory held that oxidation occurred af the double bonds in both non-conjugated and conjugated drying oils, leading , to the production of cyclic peroxides, which could further react to form various ^types of oxygen-containing rings between chains: -- CH = CH -- + Oo '1 -- CH -- CM -- CH - CH O -O i J. CH -- CH :o o -- CH --- CH -- The work of Farmer and his associates36 first demonstrated that oxidation occurs at an active methylene group with the formation of a hydroperoxide, the initial unsaturation of the fatty acid chain (lmoleate, linolenate) remaining unchanged in the primary oxidation stage. The oxidation is a free-radicle chain reaction and takes place according to the following scheme: RH " linoleate or linolenate chain (H is a hydrogen of the active methylene group). Initiation: RH -> R + H (promoted by U.V. light or metal catalysis and traces of peroxides) Propagation: R + 02 -> R02 R02 -f RH ROOH -- R (considerable conjugated hy droperoxide is formed as a result of resonance). Termination: 2 R02 R -----O -- R R + R -> R -- R 09 R02 + R->R -- O -- O -- R Decomposition of hydroperoxides (thermal): ROOH heat 100CC ------------ ----------- > RO or action of driers radicle attack on unchanged linoleate and linolenate. RO + RH --> ROH -f R OH + RH R HoO OH + OH 490 G. H. HUTCHINSON Jll of hydroperoxides, but some cyclic peroxides are formed by 1 :2 or 1 m addition. -- CH = CH -- CH = CH -- +02 -- > -- CH -- CH -- CH = CH -- O- -o or CH -- CH = CH -- CH !I o--------------------------------o Most of the polymer formed from the oxidation of conjugated esters appeal to be carbon-carbon linked, the high molecular weight of the polymer portion suggesting that a chain mechanism is operative. The mechanisn by which conjugated species polymerise in the autoxidation of drying oil requires further study, and presents one of the gaps in our knowledge of tK mechanism of film formation. It is certain, however, that an oil film durin drying is an extremely dynamic system involving the reactions of free radicle in the building up of a macromolecular structure. Catalysis of Autoxidation It is generally understood that the action of those driers that exist in tvf valency states, e.g., cobalt and manganese, is to decompose the hydroperoxide which are formed in the primary stage of autoxidation, at temperatures nornm associated with drying. In the decomposition of hydroperoxides, these metal are oxidised from the lower to the higher valency state. Co** + ROOH -> Co*** + RO OH or RO + OH Co- RO Co** + RO Co- OH Co* OH The RO and OH radicles then attack unchanged unsaturated fatty aci chains at the reactive methylene groups as already described. The oxidation of cobalt from the divalent to the trivalent state by dryb) oil peroxides is used as a preliminary sorting test in the author's laboratoj to assess the extent of oxidation of drying oils during storage. Some tin? ago it was found37 that alkali-refined linseed oil of high peroxide value, wh* used for alkyd-resin manufacture, gave rather dark alkyds which also pop merised at an abnormal rate during their manufacture. It has now been fotf that, by shaking up equal volumes of a refined and bleached linseed oil a solution of cobalt naphthenate (0.001 g. Co per ml.) in White Spirit, ti extent of oxidation in terms of peroxide oxygen can be assessed approximate by the development of colour from violet to green. A colorimetric meth is used which relates Lovibond colour in red, yellow and blue units with t peroxide value in mg.-equiv. 02 per kg. oil. This test alone does not give* idea of the previous history of the refined oil. For example, the peroxide val may have decreased from a fairly high value following decomposition i Ad v a n c e s in t h e c h e mis t r y a n d t e c h n o l o g y o f d r y in g o il s 491 ,, In conjunction with the cobalt drier test, a test is made for iSKion of decomposition products in the oil--rather on the lines of the m,:/lheiscthfohravraencuindditeyr.gPonaelumoxbiod3a8tihoans devised during a test to thermal differentiate between treatment and those have not. It is a colorimetric test for aldehydes: 20 ml. of oil are treated 4^sively with 10 ml. of a 1 per cent solution of alcoholic phloroglucinol - CH ^ f' MtflO ini- of concentrated hydrochloric acid. The development of a pink colour |tt tlte aqueous layer denotes the presence of aldehydic decomposition products. I --O ; "'fable III illustrates the application of the cobalt-drier and phloroglucinol > appears >olymeric chanisms ying oils ge of the tests. Two samples (500 g. each) of alkali-refined bleached linseed oil were stored in open porcelain dishes for 16 months at room temperature. To one sample (B)was added 0.1 per cent di-/p/Y.-butyl hydroquinone before storage. Sample A was untreated. *y TABLE III Co l o u r Re a c t io n s j radicles f ' Cobalt Drier Solution in two :* >eroxides ^ normally r ;e metals * After six weeks' storage 11 A Green After three months' storage .. Deep Green, After eleven months' storage.. Deep Green Phloroglucinol Test B Pink Pale Yellow Yellow-brown AB itty acid y drying boratory >me time tie, when Iso polysn found i oil a^d oirit, the >ximately method with the t give . n ide valae sition of After six weeks' storage Cherry Red Pale Pink After eleven months Oil A had skinned over and had bleached considerably, whereas Oil B had not skinned and had substantially the same colour as the fresh oil. On heating Oil A to stand-oil making temperature, it darkened considerably (typical of highly oxidised oils); Oil B was bleached on heat treatment. Co n c l u s io n s ^ No attempt has been made to review past developments in the various fields of modified drying oils, e.g., styrenated oils, epoxidised oils, isomerised oils, maleinised oils, etc. Research has been active in these fields, as reference to the published literature will show. More recent developments such as the new unsaponifiable drying oils from fatty acids will no doubt receive considerable attention in the future, and there is likely to be increased interest in the use of drying oils as raw materials, both for the preparation of chemical inter mediates, and for new and impioved sui face-coating media The object of this paper has been to show how much industry has come to depend on fundamental research for a bettei understanding of drying oil media A great deal of progress has been made in the past twelve years or so, and with new knowledge on the chemistry of drying oils at our disposal, mdustiy should be better equipped to tackle the many unsolved problems in its technology. Re f e r e n c e s Hilditch, T. P., The Chemical Constitution of Natural Fats, 3rd edition (Londi Chapman & Hall, 1956). Hilditch, T. P., J.O.C.C.A., 1948, 31, 1. Greaves, J. H., Oil Col Trades J., 1948, 113, 949. Hilditch, T. P., J.O.C.C.A., 1949, 32, 5. Barker, C., and Hilditch, T. P., J.O.C.C.A., 1950, 33, 6. Hilditch, T. P., and Seavell, A. J., J.O.C.C.A., 1950, 33, 24. Crawford, R. V., and Hilditch, T. P., J. Sci. Food Agric., 1950, 1, 230. Bridges, R. E., Chakrabarty, M. M., and Hilditch, T. P., J.O.C.C.A., 1951, 34, 354. Dutton, H. J., and Cannon, J. A., J. Amer. Oil Chem. Soc., 1956, 33, 46. Scholfield, C. R., arid Hicks, M. A., J. Amer. Oil Chem. Soc., 1957, 34, 77. .1 Gloyer, S. W., Ind. Engg. Chem., 1948, 40, 228; Gloyer, S. W., and Georgian, C. 0* Ind. Engg. Chem., 1948, 40, 1162; Gloyer, S. W., J. Amer. Oil Chem. Soc., 1949, 26, 162,k Le Riche, F. J. H., and Stubbs, A. L., J.O.C.C.A., 1954, 37, 8. Jordan, L. A., et al. Oils for the Paint Industry, p. 100 (London, Res. Ass. Brit. Mfrs., 1951). . Hilditch, T. P., and Seavell, A. J., J.O.C.C.A., 1950, 33, 49. . Lundberg, W. O., J. Amer. Oil Chem. Soc., 1955, 32, 207. . Greaves, J. H., J.O.C.C.A., 1956, 39, 934. . Painter, E. P., and Nesbitt, L. L., Oil & Soap, 1943, 20, 208. . Pouchon, G. F., and Massoni, R., Peint. Pigm. Vern., 1956, 32, 217. . Fauve, M., Peint. Pigm. Vern., 1956, 32, 672. . Zeleny, L., and Coleman, D. A., U.S. Dept. Agr. Tech. Bull., 1937, No. 554. . Hoeve, C. A. J., and Sutton, D. A., J. Amer. Oil Chem. Soc., 1956, 33, 312. . Hoeve, C. A. J., J. Polymer Sci., 1956, 21, 1. . Stockmaver, W. H., J. Polymer Sci., 1951, 9, 69; Stockmayer, W. H., J. Polymer Sci$ 1953,11 ,'42<L . Clingman, A. L., Rivett, D. E. A., and Sutton, D. A., J.C.S., 1954, 1088. . Waterman, H. I., Cordia, J. P., and Pennekamp, B., Research (London), 1949, 2, 483. . Rushman, D. F., and Simpson, E. M. G., Trans. Farad. Soc., 1955, 51, 230, 237; Rep. Pro Appl. Chem., 1954, 39, 626. . Paschke, R. F., and Wheeler, D. H., J. Amer. Oil Chem. Soc., 1954, 31, 208. . Paschke, R. F., and Wheeler, D. H., J. Amer. Oil Chem. Soc., 1955, 32, 473. . Mehta, T. N., and Sharma, S. A., J. Amer. Oil Chem. Soc., 1956, 33, 38. i. Macdonald, J. A., J. Amer. Oil Chem. Soc., 1956, 33, 394. . Rushman, D. F., and Sherwood, A. F., P.R.S. Technical Paper 198, 1955; Rep. Prp Appl. Chem., 1956, 41, 439. . Cannegieter, D., Paint Oil & Chem. Review, 1947, 110, No. 4, 17. . Anderson, L. V., and Porter, J. V., Ind. Engg. Chem., 1949, 41, 741. . Powers, P. O., J. Amer. Oil Chem. Soc., 1950, 27, 468. . Hutchinson, G. H., and King, C. W. H. K., John M. Hamilton & Co. Ltd., Techrn' Information Bulletin, No. 1. . Farmer, E. H., et al, Trans. Farad. Soc., 1942, 38, 340, 348. r. King, C. W. H., and Hutchinson, G. H., J.O.C.C.A., 1955, 38, 673. * !. Palumbo, M., Ind. della Vernice, 1952, 6, 113. John M. Hamilton & Co. Ltd., Wincolmlee, Hull. [Received 23 October 1957