Oil-Soluble Phenolic Resins Influence of Substituents on Properties V. H. TURKINGTOE AND IVEY ALLEN, JR. Bakelite Corporation, Bloomfield, K. J.
T
HE oil-soluble phenol-formaldehyde resins, which have steadily increased in importance during recent years as raw materials for oleoresinous coatings, are generally based upon substituted phenols in combination with formaldehyde. The term “substituted phenol” embraces literally hundreds of known or possible compounds varying widely in their chemical and physical properties, but generally capable of yielding resins when combined with formaldehyde. Naturally the resins also vary over an extremely wide range in properties. This has often led to confusion and sometimes to erroneous conclusions as to the properties that may be considered characteristic of the entire class of products. It may, therefore, be helpful to examine their chemical structures in relation to the specific properties that determine their value in protective and decorative coatings. Although only a few of these compounds are in actual commercial use, many others have been examined by research workers EO that sufficient basis exists for drawing several general conclusions as to effects of the various substituent groups and their positions in the phenol nucleus. Unsubstituted phenol (COHbOH) may be considered as the parent substance from which the substituted phenols are derived by addition of alkyl, aryl, carboxyl, hydroxyl, halogen, or other groups or various combinations of them. The unsubstituted phenol has been u-idely utilized in production of oil-soluble resins in combination with rosin or other acidic natural resins, but its resinous formaldehyde condensation products are generally too insoluble in drying oils to permit successful use without such modification. The following discussion, therefore, is limited to consideration of those resins which are soluble in drying oils without any added solubilizing or modifying agents. Probably the most important characteristic of a resin intended for use in an oleoresinous varnish is its solubility or dispersibility in the available drying oils. If the resin cannot be dispersed in oil by relatively simple heat treatment, it is generally classed as insoluble and its usefulness is limited in scope, even though it may be possible by special processing methods or by the use of added solubilizing materials to obtain homogeneous “solutions” in oil. It mill be understood, then, that the term “oil solubility” as used in the varnish industry and in this discussion does not necessarily mean true molecular solution but refers only to the ability of a resin to form visually homogeneous dispersions with drying oils which remain stable after cooling to room temperature. Resins which do not meet this requirement have, fcr present purposes, been omitted from further evaluation in coating materials. Table I presents a partial list of forty-four phenolic compounds, together with the properties of resins resulting from their condensation with formaldehyde under the conditions described and, in cases where the resins are oil soluble, their behavior as clear varnish coatings. All such varnishes are
on the basis of 100 parts of resin t o 200 parts of tung oil. Because of the wide divergence in behavior of the resins with oil, no attempt n-as made to standardize such factors as cooking time and temperature, thinners, and metallic drier concentration. Each varnish, hovever, was brought within the limits of practical varnish-making procedure to contain 50-60 per cent solids a t a satisfactory brushing consistency. Drying and color behavior were observed by brushing the varnishes onto white porcelain panels and allowing them to dry a t room temperature (25” (2.). A careful perusal of Table I leads to the following general conclusions. Oil Solubility Vhile this property depends somewhat upon the size and the nature of the substituent group, its position in the phenol nucleus is the most important factor. Among the alkyl-
There are hundreds of known or possible phenolic compounds capable of reacting with formaldehyde to form resins; many of them are soluble in fatty oils. As these phenolic compounds vary wTidely in properties, so also do the resins and the oleoresinous varnishes made from them. A representative list of phenolic compounds, substituted in various positions with alkyl, aryl, carboxyl, hydroxyl, or methoxyl groups, is tabulated, together with a comparison of the properties of their formaldehyde condensation products. Particular emphasis is placed upon their relative effects when combined with drying oils for use in oleoresinous coating materials. Such important properties as oil solubility, oil reactivity, color stability, durability, drying rate, and resistance to moisture and alkali are shown to be closely related to the structure and position of the substituent groups in the phenolic material used in producing the resin. 966
August, 1941
962
INDUSTRIAL AND ENGINEERING CHEMISTRY OF RESINS AND VARNISHESFROM PHENOLIC COMPOUNDS AND FORMALDBHYDE TABLE I. PROPERTIES
Phenolic Compound
Resin Reaction Conditions Mol. ratio, CH20: Time, Temp., O C. Catalyst phenol hr.
Formula
Phenol H
o
a
Alkali Acid
3.0:l 1.O:l
40 5
26 100
~
~
Type H. H.
P. F.
Resin Propertiesa Melting goint, C. Color Liquid 86
Light resistance
Varnish Propertiesb Gelation Drying Color Oil time, time, stanun. sol. hr. bility
W. W.
Good Poor
Insol. Insol.
Yellow Red tint
V. poor V. poor
Sol. Sol.
L.
V. poor Fair
Insol. Insol.
L. L.
Fair Good Poor
L.
ALKYL-SUBSTITUTED AND RELATED COYPOUNDS
o-Cresol
Acid Alkali
1.O:l 3.0:l
10 24
m-Cresol
Acid Alkali
1.O:l 4.0:l
J/2
Acid Alkali
1.O:l 4.0:l
None Alkali
p-Cresol
HOCI>CHa
100 25
P. F. 81. H. H.
80 Liquid
V. poor V. poor
51
*.
24+ 24f
Sol. Sol.
24
5 6
Fair Good
Partly sol. Insol.
20
10
Poor
..
..
...
48
100 25
P. F. H.H.
108 Liquid
48
3
100 25
H . H.
P.F.
104 Crystals
1.O:l
5
100
P. F.
89
Yellow
2.0: 1
12
25
H. H.
101-103, crystals
Yellow
L. Fair yellow Yellow
801.
27
8
Poor
Sol.
23
8
Fair
Red tint
CHa Xylenol (3 5dimethyl: phenol)
H
O
c
j dHs CHa
Xylenol (3 4dimethyiphenol) Xylenol (2 5dimethyiphenol)
l,OC_I)CH*
..
Acid
1.O:l
1
100
P. F.
104
Alkali
2.0:1
40
25
H.H.
100-102, crystals
Acid
1.O:l
15
100
P. F.
123
Yellow
Fair
Sol.
30
7
Poor
Acid
1.O:l
5
100
P. F.
72
Yellow
Fair
Sol.
27
7
Fair
..
CH5
,
O
b I
CH5 Xylenol (2,4dimethylphenol)
CH5 H0t)CHa CaHs 1.O:l
3
100
P. F.
81
L.
Poor
Sol.
30
7
Poor
Acid
1.O:l
3
100
P. F.
86
L.
Fair
Sol.
27
6
Fair
Alkali
2.0:l
24
25
H. H.
Crystals
Sol.
..
6
Good
Add
o-Ethyl
H
p-Ethyl
HOC>C~H'
O
b
yellow
yellow L. Good yellow
(Continued o n page 068)
and aryl-substituted compounds, practically all those substituted in the ortho position yield resins readily soluble in oil with either alkaline or acid condensing agents. It is not surprising, then, that the first oil-soluble phenolic resin to be recorded in the literature was obtained from o-cresol'. Compounds substituted in the para position usually yield oil-soluble resins, while those in the meta position tend strongly toward insoluble products. The explanation lies in the fact that the three most reactive positions in the phenol ring structure are the para and the two ortho positions. When any one of these positions is occupied by a substituent group, the tendency to form cross-linked or "three-dimensional" insoluble polymers with formaldehyde is diminished. Meta-substituted phenols, such as m-cresol or m-xylenol, have all three reactive positions unoccupied and thus behave much like unsubstituted phenol unless the substituent groups are large enough to cause steric hindrance. Alkaline catalysts also favor formation of the oil-insoluble structure on continued heating, generally producing insoluble resins with meta-substituted phenols and often with parasubstituted phenols unless the reaction is stopped a t an unfinished intermediate stage. These latter products are of considerable commercial value, because of the fact that in 1 Aylsworth,
J. W., U. S. Patent 1,111,287 (Sept. 22. 1914).
their intermediate stage they are soluble in oils and com patible with other resins, but during subsequent heating they continue to polymerize without separation from the added ingredients, produce rapid thickening or hardening, and greatly improve such varnish properties as drying, durability, and resistance to moisture and alkali. Acid catalysts generally yield resins of the permanently fusible type and are favored for the production of the very hard, high-melting resins. I n these the formaldehyde condensation reaction can be carried to completion and still retain satisfactory oil solubility. While these acid-catalyzed resins can be, and often are, used in combination with low-cost natural resins, they do not depend upon such additions for securing good solubility and are most useful in undiluted form in varnish formulations requiring the highest resistance to weathering, moisture, weak acids, and alkalies. It may also be noted that a high ratio of oxygen in substituent groups, regardless of their position, tends to diminish oil solubility. Thus, hydroxyl, methoxyl, or carboxyl groups generally yield psor solubility. The carboxyl materials, however, because of their ability to enter into esterification reactions with glycerol or other polyhydric alcohols, are useful as intermediates in the formation of highly complex polyesters or alkyd type products. When used in conjunction with rosin or other acidic natural resins or with
968
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 8
TABLE I. PROPERTIES OF RESINSAND VARKISHES FROM PHENOLIC COMPOCNDS AND FORMALDEHYDE (Contd.) Phenolic Compound
Formula
Resin Reaction Conditions Mol. ratio C H d : Time, T$rnz:, Catalyst phenol hr.
Type
point, C.
Color
Varnish Propertiesa Gelation Drying Color time, time, staOil min. hr. bilitv sol. ~
Resin Properties6
Melting Light resistance
ALKYL-SUBSTITUTED AND RELATEDCOMPOUSDS (COntd.) CH3
hid
1.O:l
12
100
Alkali
2.O:l
40
25
2.O:l
1.O:l
17 46
1.0:l
p-Isooctyl
Thymol
p-tert-Butyl H o O L - CC ' H,H ,
p-tert-Amyl
HO(r)(-CH*-CHs
CH8
P. F.
96
n. n.
Liquid
100 26
P. F. H. K.
88 Liquid
18
100
P. F.
1.1:1
13
100
1.0:1
3
1.0:l
Acid
Alkali Acid
L. Fair yellow W.W. Good
Sol.
34
5
Fair
Sol.
35
6
Good
Yellow Fair L. Good yellow
Sol. Sol.
26
6
Good
85
L. Fair yellow
Sol.
36
6
Fair
P. F.
91
S1. Fair yellow
Sol.
39
6
Fair
100
P. F.
92
Yellow
Poor
Sol.
39
18+
Poor
6
100
P. F.
97
Yellow
Poor
Sol.
36
184- Poor
1.O:l
ti
100
P. F.
77
Black
V. poor
Insol
..
Black
V. poor
CHa p-tert-Hexyl
Carvacrol
32
5'12 Fair
\
CH(CHaji
Guaiacol
I
.
...
OCHa
...
...
Eugenol
HOhCH-CH=CH%
Acid
1.O:l
8
100
Liquid
Isoeugenol
H O ~ C H = C H - C H S Acid
1.O:l
5
100
P . F.
70
Black
V. poor
Sol.
39
24
Poor
1.0:1
8 8 11
100 100 100
P. F.
85 75 88
Brown V. poor Yellow Good Yellow Fair
Sol. Sol. Sol.
32 45 31
10 8 6
Poor Good Fair
TrirnethylCaHz (CHdaOIl phenol Terpene phenols Mixed isomers Diisobutyl Mixed isomers
Acid Acid Acid
1.0:1 1.O:l
fatty acids, they yield oil-soluble products which combine the good properties of both phenolic and alkyd type resins within the same molecule, and therefore form more homogeneous products than can be obtained by simply mixing phenolic and alkyd type products.
Oil Reactivity Quite apart from their mere solubility in fatty oils, there is strong evidence that certain types of phenol-aldehyde resins react chemically with drying oils. This property, however, is not common to all phenolic resins, or a t least the ability to combine varies greatly among the numerous members of the class. Also, as might be expected, the chemical structure of the oil must be considered in any discussion of resin-oil reactivity. Those oils which contain a conjugated double bond system show the greatest evidence of combination, as measured by changes in specific refraction and in viscosity or gelation comparisons, though even those oils which contain only isolated double bonds show evidence of some chemical combination with the more active types of phenolic resins. Although the exact mechanism of such reactions cannot be stated definitely, the comparison in Figure 1 does indicate that t8hestructure of the substituent group exerts a powerful influence upon the relative reactivity of various
P. F. P. F.
substituted phenol resins. Structural formulas are given and the gelation times at 250" C. of mixtures of 200 parts tung oil with 100 parts of resin made from each phenol. AU three resins were prepared by reacting the phenol with an equimolecular ratio of formaldehyde and an acid catalyst, and all were of approximately the same melting point (85100' (2.). These three compounds are of approximately the same molecular weight and have the same number of carbon atoms (6) in the substituent group, but their resins differ markedly in behavior nTith tung oil. The greater activity of the phenyl as compared to the cyclohexyl or terthexyl groups suggests that the greater unsaturation of the phenyl group may be largely responsible for this difference by providing a larger number of active points for possible combination with the oil. Oil-resin reactivity of a different type is indicated in the case of the heat-hardenable phenolic resins made with excess formaldehyde and alkaline catalysts. As noted above, when the reaction is checked at an intermediate stage, these resins have the useful property of being readily soluble in oils and then, upon continued heating, of being further polymerized to yield high-viscosity resin-oil complexes. The elimination of water and consequent active foaming during this heating has often led observers to the conclusion that extensive chemical reaction must be occurring between the resin and
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1941
969
.
TABLE I. PROPERTIES OF RESINS AND VARNISHES FROM PHENOLIC COMPOUNDS AND FORMALDEHYDE (Contd.) Resin Reaction Conditions Mol. ratio C H d : Time, Temp., Catalyst phenol hr. C. ~
Phenolio Compound
Formula
Varnish Propertiesb
Resin Properties"
Type
pgint, C.
Color
Liq;ht reoistance
Celp-
Oil sol.
tion Drying time, time, min. hr.
Color stability
DI- A N D TRIHYDRIC PHDNOLE
OH None
0.73:l
Resorcinol
None
0.73:l
Hydroquinone
None
0.73:l
Pyrogallol
None
0.67:l
=/a
Monomethylresorcinol
Acid
1.O:l
14
Catechol
H O t )
2
8/r
1
..
*.
...
Insol.
.. ..
.. ..
... ...
V. poor
Insol.
..
..
...
Yellow
Poor
Insol.
..
..
...
Yellow
Poor Poor
Sol. Sol.
28 37
5 8
V. poor Poor
100
P.F.
96
V. dark V. poor
Insol.
100
H.H.
112
Black
V. poor
Insol.
100
H.H.
138
Black
V. poor
100
H.H.
159,
Black
25
P. F.
119
ARYL-SUQSTITUTDD PHH~NOLS
o-Phenyl
na-Phenyl
dOb
OH
bo
P.F.
Acid Alkali
2.0:l 1O.O:l
2 42
150 30
H. H.
110 90
L.
Acid Alkali
0.7:1 4 1O.O:l '14
100 35
H. H.
P.F.
100 Crystals
Yellow Poor L. Poor yellow
Sol. Insol.
24
..
4
..
Poor
P.F.
110
L. Fair yellow
Sol.
24
3
Fair
H.H.
Crystals
L. Good yellow
801.
ie
4
Good
yellow
...
Acid
1.5:1
3
130
Alkali
3.0:1
24
85
o-Benzyl
Acid
1.1:l
21
100
P. F.
73
L. Poor yellow
Sol.
40
24
Poor
p-Benzyl
Acid
2.3:l
8
140
P. F.
95
Red tint
Fair
Sol.
25
4
Fair
Acid
1.0:1
3
130
P.F.
100
L. Fair yellow
Sol.
45
5
Fair
p-Phenyl
Hoc>cT> Hn HZ
p-Cyclohexyl
(Contkusd on page 870)
oil. It is possible to visualize the phenol alcohols, known to be present in this type of resin, as reacting with active hydrogen atoms of the drying oil with elimination of water and consequent foaming. On long continued heating a t high temperatures, it seems probable that such a reaction does take place t o some extent. However, the foaming that occurs at temperatures below about 230' C. is traceable principally to
p-Phenylphenol* p-Cyclohexylphenol p-tert-Hexylphenol Mol. wt., 170 Mol. wt., 175 Mol. wt., 178 Gel time, 24 rmn. Gel time, 45 min. Gel time, 35 min.
FIGURE1
the moisture and free formaldehyde present in the resin itself plus the moisture resulting from the normal completion of the phenol-formaldehyde condensation. A rapid rise in viscosity is not necessarily due to any extensive intermolecular reaction between resin and oil but may be merely the result of the completion of polymerization of the resin itself. Support of this view is found in the fact that the amount of water eliminated by heating resin and oil separately a t 230" C. checks closely with that found by heating resin and oil together. Here again it would be misleading to state that this is true for all phenolic resins of the heat-hardenable type, as some resins may be both heat hardening or heat reactive and a t the same time be oil reactive or capable of entering the unsaturated bonds of the oil. I n general, the degree to which either type of reactivity predominates depends largely upon the chemical structure of the substituent groups of the phenol or phenols used in making the resin.
Color Stability Table I also shows that practically all ortho compounds yield resins of poor color stability, while the para compounds are generally much better in this respect, and meta compounds
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
970
Vol. 33, No. 8
TABLEI. PROPERTIES OF RESINSAND VARNISHES FROM PHENOLIC COMPOCNDS AND FORMALDEHYDE (Contd.) Phenolic Compound
Formula
Resin Reaction Conditions Mol. ratio CHnd: Time, Temp., hr. C. Catalyst phenol
~
Type
Resin Piopertieaa Melting point, C. Color
L. Fair yellow Red Fdir
Sol.
..
Insol.
..
Dark T‘. poor brown
Sol.
Red
Fair
Sol.
29
...
1,.
Fair
Insol.
..
..
P.F.
129
L. Fair yellow
Sol.
..
ti
Fair
P. F.
126
L. yellow
Fair
Sol.
..
6
Fair
co
Acid
1.O:l
10
100
P. F.
78
Alkali
5.O:l
72
40
H. H.
...
Acid
2.1:1
1
130
P. F.
104
p Phenoxy
H O C > O C >
Acid
2.3:1
65
100
P. F.
57
Dihydroxydiphenylmethane
H O C I I > g 2 a O H
Acid
1 0:l
6
100
H. H.
Acid
1.O:l
10
100
Acid
1.0:1
10
100
p-Hydroxybenzophenone
H o o g c >
or-Naphthol
Hac o Benzyl-ocresol
Light resistance
Varnish Propertiesb Gelation Drying Color time, time, staOil hr. bility sol. min.
yellow
8
..
Fair
...
24+
V. poor
10
Fair
...
OH
CHs g-Benzyl-ocresol
CARBOXYLIC PHENOLS
S licylic acid
Acid
1.3:l
e
100
P. F.
126
Colorless
Good
p-Hydroxybenzoic acid
Acid
1.7:l
10
100
11. 1%.
101
Colorless
Good
a-Cresotinic acid
Acid
1.4:1
12
100
P. F.
White Colorcxsstals less
Poor
m-Cresotinio acid
Acid
1.Q:l
2
135
P. F.
105
L. Fair yellow
p-Cresotinic acid
Acid
1.4:l
2
140
P. F.
100
L. Good yellow
esterified.
L.
a
-
T pe. H. H . = heat hardening, P. F. = permanently fusible, SI. %ghi; ,V. = very.
-
slight; melting points by ball and ring method: color:
b All varnish properties determined on basis of 100 parts resin t o 200 parts tung oil; gelation tests a t 280‘
“print-free” stage.
are in between. The use of alkaline catalysts yields resins of better color stability than acid catalysts. The di- and trihydric phenols form resins of very poor color stability, regardless of the position of the hydroxyl groups, as might be expected from their pronounced affinity for oxygen and the known highly colored nature of their oxidation products. Alkali Resistance Although the resistance to alkalies of a dried varnish film depends upon many other factors, such as the time and temperature of cooking, the concentration and type of metallic driers, and the atmospheric conditions during drying, it is generally found that the para-substituted phenols yield better alkali resistance than ortho or meta compounds. Also, other conditions being equivalent, the aryl-substituted compounds are more resistant than other types, the phenyl group appearing especially outstanding in this respect. As might be expected there is fair correlation between the alkali resistance of varnish films and the water solubility of the alkali metal salts of the various phenols.
W. W.
~
-
water-white;
C.; drying time is approximate time to reach
Durability The ability to impart greatly improved resistance to outdoor weathering is perhaps the property most responsible for the R idespread use of phenolic resins in oleoresinous coatings. Durability depends upon a combination of many factors not easy to correlate and the influence of any one component, such as the resin, may be obscured by such other variables as the choice of cooking procedures, oils, driers, pigments, drying conditions, etc. A remarkable property of the better grades of phenolic resins has been their ability to exert a pronounced and easily recognizable influence upon durability even when present in relatively small percentages of the total film composition and when treated over a wide range of formulation and processing conditions. Many types of coatings in general use today contain less than 10 per cent of phenol-aldehyde resin and still provide sqrprising improvement in durability, drying rate, and moisture resistance, although the most durable coatings should contain 25-50 per cent of the total film weight.
August, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Among the most important basic causes of failure on exposure t o weather are the gradual changes in film properties resulting from oxidation, moisture absorption, and the catalytic effects of sunlight. These eventually produce volume changes and embrittlement sufficient to disrupt film continuity by such familiar types of failure as cracking, checking, chalking, or peeling. Oxidation and the gradual loss of volatile oxidation products are generally admitted to be the primary cause of film shrinkage which results in cracks or checks as soon as the strains so produced exceed the elastic limit of the coating.. -, while the alternate absorDtion and evaporation of moisture with varying weather conditions tend to open up or enlarge microscopic pores in the film and thus expose more and more area to oxidation. The suppression of either oxidation or moisture absorption, or preferably both, for long periods might therefore be expected to enhance durability. Most phenols are known t o be oxidation inhibitors, and to a lesser degree their formaldehyde condensation products partake of this characteristic. Here again the nature of the substituent group exerts great effect. Such materials as anaphthol, guaiacol, hydroquinone, and catechol are outstanding examples of phenolic antioxidants. For practical reasons, because some oxidation must be permitted in order to bring about drying, it is necessary to choose phenolic compounds which have a milder antioxidant effect. The resins from such compounds as p-phenylphenol, p-tert-butylphenol, p-cresol, and numerous other similar phenols have little or no antioxidant effect during drying while oxygen absorption is rapid, but later tend to check the long, slow, continuous oxidation and so prolong the useful life of the film. Water absorption of practically all phenol-formaldehyde resins is so low that there appears to be but little significant difference among various members of the group. However, when they are cooked with drying oils, especially those oils which normally have poor moisture resistance, considerable differences begin to appear. Those resins which show most evidence of chemical combination with oils, designated above as oil reactive, also have proved most effective for developing improved moisture resistance in varnishes containing high percentages of linseed, soybean, or dehydrated castor oils.
971
Drying The influence of substituent groups on the drying rates of tung oil varnishes is indicated in Table I. These varnishes are not all comparable as to drier content (the slower drying ones all contain additional driers) so that the differences are actually greater than those shown. With the one exception of o-phenylphenol, all ortho compounds are much slower drying than the corresponding para compounds. There is no significant difference directly attributable to the size of substituent groups, but it is clear that the phenyl group has more effect than any of the straikht or branched-chain alkyl groups. When linseed, soybean, or other slow-drying oils are substituted for a major part or all of the tung oil, the differences between resins become more marked. For instance, most ortho compounds with straight linseed oil actually prevent drying almost entirely, and many para compounds which cause rapid drying with tung oil fail to produce satisfactory drying with linseed. Since linseed requires more extensive oxidation than tung oil to reach the “dry” condition, it appears reasonable that this difference should be connected with the antioxidant effect of the phenolic resin. However, polymerization also plays an important role in the solidification of linseed varnish films, and i t is found in actual practice that those phenolic resins which show a marked tendency to accelerate polymerization of the oil during cooking are also the most effective in promoting drying; final solidification of the film is apparently accomplished by a continuation of this: polymerization process with the aid of only a moderate degree of oxidation. It is obvious that the oil length of varnishes and the relative melting points of resins as obtained by varying the technique of resin manufacturing processes have great influence upon drying properties as well as upon the other properties discussed above, particularly in the case of very short oil varnishes. It is suggested, therefore, that these conclusions regarding the influence of substituent groups be regarded as general in nature, perhaps useful as a guide to the choice of materials for meeting specific coating requirements, but still needing the usual exercise of skill and judgment by varnish technologists to secure the maximum results of which the various types of resins are capable.
, Courleay, School of Mineral Industries Qallery,^ThaPsnnlyloanio State CoZlegr
