Chemistry of Synthetic Varnish Resins IVEYALLEN,JR., V. E. MEHARG,AND JOHNH. SCHMIDT, Bakelite Corporation, Bloomfield, N. J.
The chemistry of the nine outstanding types of synthetic resins used in the paint and varnish industry today is reviewed.
A
CENTURY ago there were no synthetic resins, and throughout the following fifty years chemists were content to make no attempt to improve on Nature’s handiwork. Some fifteen years after the Columbian Exposition, Baekeland announced the result of his researches on phenol-formaldehyde resins. This marked the beginning of the synthetic resin industry. Varnish manufacturers remained quite conservative, however, and, while Albert and Berend, shortly after Baekeland’s discoveries, introduced a modified, oil-soluble, phenol-formaldehyde resin, it was not until “the development of nitrocellulose lacquers gave new impetus to research in synthetic resins” (12) that rapid and definite progress was made. Today the outstanding types of synthetic varnish resins are : 1. Ester gum 2. Cumar-indene resins 3. Phenolic resins a. 100 Der cent b. Redhced 4. Polghgdric-polybasic resins a. -Air-drying b. Baking and of lesser importance: 5. Vinyl and styrene resins
6. Amide-formaldehyde resins
others on the constitution of glucose and cellobiose laid the foundation for our knowledge of the structure of cellulose. The work of Harries was fundamental to a n understanding of the rubber molecule, and our present theories as to the structure of the protein fibers, silk, and hair, originated in the monumental works of Emil Fischer on the amino acids and the polypeptides. Willstatter’s work on the structure of chlorophyll may yet lead us to an understanding of how the plant can synthesize (from carbon dioxide and water) cellulose, rubber, and natural resins. Fortunately fundamental organic research has not been entirely neglected by investigators in the field of synthetic resins, and, while knowledge regarding resin structure is still meager, definite progress has been made.
ESTERGUMS The glyceryl esters of rosin were first introduced into ilmerica in 1893, but it was twenty years later before they received wide application. Since they are simple esters of the natural resin acids, it is obvious that they possess a similar structure. Following a suggestion of Wallach’s that the structure of the terpenes could be related to that of naphthalene which in turn “might be considered as having been built up by the condensation of isoprene units,” Ruzicka ($1) has regarded the problem of the structure of abietic acid as one of terpene chemistry. Without going into the mass of experimental evidence which Ruzicka has advanced, suffice it to say that a t present the s t r u c t u r e of a b i e t i c acid is given a3 follows :
7. Chlorinated-diphenyl resins 8. Petroleum-hydrocarbon resins 9. Rubber-derivative resins Chemically the synthetic resins are usually grouped under the all-inclusive and much abused term “polymers.” I n fact, as early as 1901 K r B n s t e i n (16) a d vanced the hypothesis that “resins owe their formation to polymerization.” Since that time chemists have been too easily satisfied to say that resins are polymers or that they contain resinophoric g r o u p s. The time-honored methods of organic chemistry for determining the constitution of c o m p o u n ds-namely, Abbau or degradation and ilufbau or synthesis-have been somewhat n e g l e c t e d for the more enticing methods of physical and colloid chemistry. These o l d e r m e t h o d s have achieved, h o w e v e r , quite n o t a b l e s u c c e s s in elucidating the structure of THE ART OF VARNISHING AND LACQUERING other high polymers. Thus Reproduoed from “Aikin’s Dictionary of Chemistry” ( ublished in Lont h e w o r k of H a w o r t h , don in 1807) for the “Century of Progress” Program of &e Paint and VarIrvine, Freudenburg, and nish Division, AMERICAN CHEMICAL SOCIETY, Chicago. 1933. 663
H3C\ /COOH
Ruzicka and his colleagues have extended their investigation to copal and kauri, and here a g a i n we find that, while their structure has not been definitely determined, the structure of copal bears a close relationship to trimethylnaphthalene and pimanthrene. The same investigators have shown that an ether-soluble a m o r p h o u s acid in kauri is structurally related to methylethylnaphthalene and retene, while the cryst a l l i n e acid is related to pimanthrene. It is of passing interest to note that nature may possibly synthesize from the one hydrocarbon, isoprene, both
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kauri and copal, and rubber. Between those two extremes certainly lies the perfect varnish resin which some chemist may yet discover.
CUMAR-INDENE RESINS These resins obtained by the polymerization of indene and of cumarone :
Bakelite C
have been known for many years. The structure of pindene as represented by Whitby and Katz (26) is:
Ht
E(
cI>b
Hz6
'c-bHI H
H
? $H
Novolak 1
\C=CH
H
and the structure of p-cumarone is probably similar:
@m 0
0
Novolak 3
HzH
H H
Such a structure is in accord with the chemical character of these resins-namely, their great stability towards alkalies and a certain susceptibility to oxidation.
PHENOLIC RESINS
strating the presence of O
There has been much speculation and controversy concerning the structure of the phenol-formaldehyde resins since Baekeland (2) tentatively proposed "until we have something better" the following formula: HI Hn HI Ht HZ HZ ~-C-CEHI-CO-CEHI-CO-C~"~-CO-~EHI-CO-CEH~-~ I 0-CHz-0 __----_I
Raschig (60) from a study of the reaction in alkaline media concluded that the following formulas more nearly represented the reaction:
More recently Baekeland and Bender (S), who had definitely isolated dihydroxydiphenylmethane as one of the intermediate products, proposed the following reactions as representative of the formation of the resin:
CHzO
+
CsHdOH CHP< OCiHs
+ HYC=-C'
C6HiOE \OCsHs
Abandoning mere speculation, Megson and Drummond (17) have attempted, like Baekeland and Bender, to isolate the intermediate products. They have succeeded in demon-
+ Ha0
C OH
H2 O
O
H
dihydroxydiphenylmethane) in the phenol condensation, of
HsC H O O g L d i H in o-cresol condensation, and
-
of H O A E f i O H in the m-cresol condensaw tion. From p-cresol condensation a product has been isolated presumably having the constitution:
Edell (7) claims to have identified the presence of 2,3'-dihydroxydiphenylmethane in the phenol condensation although von Auwers (1) and Granger (8) maintain that the reactivity in the phenol molecule seems to be limited to positions ortho and para to the hydroxyl group. Practically at the same time Koebner (IS) announced the isolation of two compounds from a cresol-formaldehyde reaction to which he ascribed the structure:
Very shortly thereafter Pollak and Riesenfeld (19) assigned the following formula to the Novolaks,
This final product conceivably would polymerize thus : Ht
Blumfeldt (4) has taken exception to the formulas of Raschig and contends that the following more nearly express the facts:
(2,4'-
Ht
Ha
Ht
HI
HY
June, 1934
I N 1)
uST HI A L
A N 11
E N G I N E E H I NG
c ii E AI
I S T I( Y
065
and stated that they on no account believe they have a polymerization p r o d u c t as s u p p o s e d by Baekeland and Bender, since in that case the low values of the molecular weight would not :be accounted for. Granger (8)has r&xamined the results of earlier investigators of phenol alcohols and reports the isolation of a product having presumably the formula:
In a more recent and thorough study of the p-cresol-formaldehyde condensation, K o e b n e r (14)has synthesized compounds varying from t h o s e possessing a binuclear structure thus,
?It ;)CH?O11
to those of a pentanuclear structure,
+ I
I 1 \/
x
llcsil1
+
S Nnuolak
The general scheme for synthesis of these comlxiunds is illustrated by the following reaction:
As a result of these investigations Koebner concludes that the acid-catalyzed, permanently fusible cresol-formaldehyde resins (Novolaks) are mixtures of di-, tri-, tetra-, and polynuclear compounds. The hydroxyl groups of tlie polynuclear compounds are unchanged and are difficultly soiuble in dilute alkalies, owing t o the formation of an insoluble sodium salt. Their resinous character is due to a mixture of innumerable isomers and the variety of large, and in some cases very large, molecules. The great variety of reactions possible between phenol and formaldehyde either in neutral, acid, or alkaline solutions, and either with phenol or formaldehyde in excess are represented by Koebner by the following simple equations:
(alkslina reaation
~
pbeiiol alcohol: (11
Nuvdnk
+
-
OB ,qcildJl%
;,i
,,*
011 OH ~-c~~~/\c~i,ol.l : I iI l d J
i) \,,
(idkniier
Y Kcsol = Re& (Bakelite
+
Y ivrmalde1,yde
Hex& = Resit
+
~
'::
Resit
niumonin
t
~
+
Rcsol = Bakelite .kl water
water
(31
:41 (51
(01
Equation 2 represents the formatioil oi tlie simplest Xovolak; Equation 3 the formation of the simplest Kesol (Bakelite A). I n Equation 2 one molecule of phenol reacts with 0.5 rnolecule of formaldehyde. As the ratio approaches 1 to I, the chains which constitute the Novolak hecome longer, resulting in a resin of higher melting point, of lesser solubility in caustic or alcohol. and finallv of infusibility and insolubilitv as the I to 1 limit is reactled. This limit 1 to 1 is reached onlv in the case of 0- and Pcresol, whereas phenol and mcresbl may combine with more formaldehyde, up to a ratio 1 to 1.5, according to the following scheme:
It is to be understood, however, that there IS 110 regular arrangement of the chains in the final infusible form of a phenol resin but rather a tangled mixture of various molecules whose structurc is similar to that pictured above. Some recont work in these laboratories by one of the authors (Ivey Allen, Jr.) is oi interest in this connection. He has determined the nature of tbe products formed in the
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we might easily obtain xylenols a.nd rnesitol (sym-trimethylphenol) from a resin made with pure phenol and formaldehyde. I n fact in the case of a resin in which the ratio of phenol to formaldehj-de was 1 to 2.6, these compounds were actually obtained. These theories of structure lor tlie phenol-formaldohydc resins are in harmony with the present views as to the structure of high po1,ymers. 8 s pointed out by Meyer and Mark (28), the tendency towards interlockingchains andthree-dimensional structures is easily possible in the reaction of phenol and formaldehyde. If we substitute for phenol, 0- or p-cresol, in w ~ c ono h reactive position i s blocked by a methyl group, we do not obtain such highmelting insoluble p r o d u c t s but rather compo11n d s which belong t,o tlie chain polymers.
Iiesrm Tlio constitution of polyhydricpolybasic renins tias been the subject of an extensive investigation by C a r o t h e r s , K i e n l e , Honel, and others. The st,iidies of Ciirotliers ( 5 ) and his collaborators on the esterification of dibasic acids and the glycols have definitely established that the recurring unit in these polymerizations coiiid lie represented tlms: I'oLYtrYnlliC-PoLYB.4sl(:
liydrolpii of the finished reailis. In this work tire rcsin was heated in an antoclai-c a t 300" C:. for several hours with I0 or 15 per cent sodium 11 oxide in order to efkct the Irydrolysis. These conditions are extreme, hut there is no reason to beliem that any rearrangement wonld occur which wonld yield the tfpe of product.s obtained. In fact, wlien o-cresol was Iieakd mit.h alkali iinder even more extreme conditions, tire effect iuas iiegligible. I11 the case of the resins the -1: drolysis product a1wa.y~contained a greater proportion of higher boiling phenols than did t . 1 mat~erial ~ from which the resin was made. That is, if the resin was made from a material containing 40 per cent phenol, 30 per cent cresol, and 30 per cent xylenols, tliesc ratios in tlie hydrolysis products might he 10 of phenol, 40 of cresol, and 50 of xylenol. This shift toward the higher phenols was more noticeable the further advanced tlro resin was and the higher tlie ratio of formaldehyde to phenol. Quite often phenols were formed rrhich did not occnr in the original raw metorial. Also there was always a considerable amrrunt of h?-drogen formed. The mechanism of the renetion is not entireiy clear and is prob:ibly somewhat. in~olved. The net effects can he shown by an equation in the following manner:
or1
Oil
. ..
