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Thermodynamics of phenolformaldehyde polycondensation
THERMODYNAMICS
OF
PHENOLFORMALDEHYDE
POLYCONDENSATION* M. I . SILINO a n d I . V. ADOROVA Scientific Research Institute of Physics
(Received 6 April 1970) IZCFOR~-~TrON concerning the thermodynamics of preparing polymers is essential in evaluating the extent of polymerization a n d the reversibility of this process, the yield and molecular weight distribution of the polymer and for heat calculation related to the apparatus, etc. There are no detailed thermodynamic data in the literature concerning phenofformaldehyde polyeondensation. There are only a few papers [1-7] which deal with the thermoehemistry of these processes. Polycondensation of phenol with formaldehyde involves three mahl types of reaction: hydroxymethylation of phenols in the ortho- or para-positions; reaction of the methylol group with the hydrogen atom of the aromatic nucleus with the separation of water and formation of a methylene bridge between the phenol rings and the formation of a methylene ether bond b y the reaction of two methy]ol groups. For the initial stages of polycondensation these reactions can be described as: OH
OH
(r) I
CH2OH OH
I
OH
+
OH
~
OH
CH
+H,O
(II)
CH2OH OH
OH + ~OH
OH -+
OH
"
+H~O
(III)
CH2OH
Thcrmodynanfic constants of phenol, formaldchyde and water arc given in the literature (Table 1). Corresponding values for methylolphenol, dihydroxydiphenylmethane a n d dihydroxydibenzyl ether were derived by the authors using approximate methods of calculation. Several methods were used to obtain more reliable results. Wc denote by AHcomb * Vysokomol. soyed. A13: No. 9, 2129-2138. 1971.
2392
Thermodynamics of phenolformaldehyde polyeondeneation
2393
t h e h e a t of c o m b u s t i o n o f t h e m a t e r i a l in t h e l i q u i d s t a t e , zIH 0 is t h e s t a n d a r d h e a t o f f o r m a t i o n o f t h e m a t e r i a l a t 25 °, S ° is t h e s t a n d a r d e n t r o p y o f t h e m a t e r i a l a t 25 °, A Z ° is t h e s t a n d a r d i s o b a r p o t e n t i a l o f t h e f o r m a t i o n o f t h e m a t e r i a l , K is t h e e q u i l i b r i u m c o n s t a n t o f t h e r e a c t i o n s , J H ° is t h e v a r i a t i o n o f t h e s t a n d a r d h e a t o f f o r m a t i o n as a r e s u l t o f t h e rea c t i o n , J Z ° is t h e v a r i a t i o n of t h e s t a n d a r d i s o b a r p o t e n t i a l as a r e s u l t o f t h e r e a c t i o n . I n d i c e s , g, 1 a n d s d e n o t e t h e gaseous, liquid a n d solid s t a t e s o f t h e m a t e r i a l .
-AZ' ',kcal/mole
4z;
120
f
80 ~0 I I I i I I
f I I
I
I
6
I
FiG. 1
I
I
t
8 IOC
I
~
I
8 0I I0
I
-,4 Hi,
~I
I
kca//mole
F~o. 2
FI('. 1. Dep(mdence o f t h e i s o b a r p o t e n t i a l o f n - a l k a n e s on t h e n u m b e r o f c a r b o n a t o m s iu t h e i r molecules. Ft(~. 2. R e l a t i o n b e t w e e n JZ~I a n d JH~I for v a r i o u s b e n z e n e d e r i v a t i v e s : 1 - - p - h y d r o x y benzoic acid; 2 - - b e n z o i c acid; 3 - - h y d r o q u i n o n e ; d - - p h e n o l ; 5 - - b c n z y l alcohol; 6 - - p - x y l e n e ; 7-- t o l u e n e . W e give a b r i e f d e s c r i p t i o n o f m e t h o d s o f c a l c u l a t i o n used for d c t o r m i n i u g J / ~ l a n d AZ ° o f m c t h y h ) l p h e n o l , d i h y d r o x y d i p h e n y h n c t h a n c a n d d i h y d r o x y d i b e n z y l e t h e r . l. U s i n g t h e m e t h o d p r o p o s e d p r e v i o u s l y [10, 13], values o f A/~g a n d S~g a n d t h e n AZ ° o f t h e c o m p o u n d s were c a l c u l a t e d , p - X y l e n c was selected as t h e initial c o m p o u n d for ('alculating t h e r m o d y n a m i c c o n s t a n t s o f m e t h y l o l p h e n o l a n d d i h y d r o x y d i p h e n y l m e t h a n e al~d d i e t h y l e t h e r was used for d i h y d r o x y d i b e n z y ] e t h e r . Values of AHg a n d S~s o f t h e initial m a t e r i a l s were t a k e n f r o m a f o r m e r s t u d y [9], a n d t h e c o r r e c t i o n s f r o m a n o t h e r p a p e r [10J. ( ' o r r c c t i o n o f t h e v a l u e J/~g in r e s p e c t o f t h e s u b s t i t u t i o n o f t h e CI-Ia g r o u p w i t h p h e n o l h y d r o x y l is a n e x c e p t i o n , t h e v a l u e o f w h i c h ( - - 3 2 . 7 k c a l / m o l e ) is in p o o r a g r e e m e n t w i t h e , ) n v c n t i o n a l values for p h e n o l s [9]. A n a n a l y s i s of t h e s e results i n d i c a t e s t h a t t h e a v e r a g e v a l u e o f t h i s c o r r e c t i o n is close to - - 3 5 k c a l / m o l c . W c u s e d t h i s v a l u e in t h e c a l c u l a t i o n s . F,~r t h e t r a n s i t i o n f r o m AZ°~ to A Z [ a l l o w a n c e was m a d e for t h e f a c t t h a t in t h e case o f m ( , l o - a n d d i - s u b s t i t u t e d b e n z e n e d e r i v a v i v e s t h e difference (JZ°s--AZ [ ) (1.5-3.5 k c a l / m o l e ) [it] c a n a p p r o x i m a t e l y be r e g a r d e d a~q c o n s t a n t a n d e q u a l t o 2.5 k c a l / m o l c on a v e r a g e . 2. U s i n g t h e R e m o l o Ciola [14] m e t h o d t h e v a r i a t i o n o f J Z wae c a l c u l a t e d for t h e p r o c e s s itl w h i c h t h e r e a c t i n g s u b s t a n c e s are in t h e gaseous s t a t e . As t h e difference (AZ ° - - A Z 0) i~ slight a n d a p p r o x i m a t e l y c o n s t a n t a n d t h e c o r r e s p o n d i n g v a l u e for w a t e r is also low (2-1 k c a l / m o l e ) , it m a y be a s s u m e d t h a t JZ~ o f r e a c t i o n s (1)-(3) will be close t o 5Z~. 3. F o r a n a p p r o x i m a t e e s t i m a t i o n o f AZ ] P a r k s a n d I-Iaffman [12] p r o p o s e d correcl i~)ns w h i c h allow for t h e v a r i a t i o n o f t h e i s o b a r p o t e n t i a l of h y d r o c a r b o n s o n a d d i n g f u n c t i o n a l z r o u p s t.o t h e molecule. As t h e s e c o r r e c t i o n s were b a s e d on o b s o l e t e results, we d e c i d e d to
2394
M . I . SrLxNO and I. V. A_voRovA
define t h e i r values m o r e a c c u r a t e l y using m o d e r n i n f o r m a t i o n concerning J Z I o f aliphatic a n d a r o m a t i c h y d r o c a r b o n s , alcohols, phenols a n d ethers [8, 9, 15]. Values o f JZ~ o f m e t h a n e , e t h a n e , p r o p a n e a n d n - b u t a n e were d e r i v e d b y e x t r a p o l a t i o n of t h e linear d e p e n d e n c e o f JZ~° o f C6-C10 alkanes [15] on t h e n u m b e r of c a r b o n a t o m s in t h e molecules (Fig. 1). T h e following corrections (kcal/mole) were o b t a i n e d : s u b s t i t u t i o n o f t h e H a t o m in the C - - H b o n d b y a C6H~CH~ g r o u p - - --36, s u b s t i t u t i o n of the H a t o m in t h e C - - H b o n d by a C6H5 g r o u p - - -- 35, i n t r o d u c t i o n o f a n Olql group in t h e benzene r i n g - - -- 41, substitution of t h e ]-I a t o m b y an O H g r o u p to f o r m p r i m a r y a l c o h o l - - --35.5, a d d i t i o n of an - - 0 - - group b e t w e e n two c a r b o n a t o m s to f o r m an e t h e r - - --22. F r o m these results values of JZ~ of c o m p o u n d s we are interested in were d e t e r m i n e d by the equations:
CH,0H
--7.5
--48-5; OH
//
//\
CH,0H
CH,
--49.2
--13.7
OH
CH,
27.3 OH
OH
J
a/ 66-2
--15.8;
//
--19.7
21.3
--13"7
//\
0-0
--~SHI--CH, -~ --6"0
&. &
64"0
27.3
0
v--CH,0CH,-~ 42"0
~
OH
42"5
--27"5
OH
H,0CI~, ---40-0; - - 3 9 . 5 ; - - 4 0 . 7
OH
OH
0H
t
6 0d CH, 27"3
CHs --13"7
22-3
0"3
I n this l a y o u t t h e n u m b e r s u n d e r the structural formulae d e n o t e J Z I ; information for t h e initial c o m p o u n d s were t a k e n from t h e literature [9]. 4. I n t h e p r e v i o u s calculation it was assumed t h a t J Z ° of t h e a r o m a t i c c o m p o u n d on a d d i n g a phenol h y d r o x y l group in its molecule changes to a c o n s t a n t value. I t follows f r o m this a s s u m p t i o n t h a t t h e v a r i a t i o n of the isobar p o t e n t i a l in reactions (I) and (II) will be a b o u t t h e s a m e as for reactions o f f o r m a t i o n of benzyl alcohol and d i p h e n y l m e t h a n e , r e s p e c t i v e l y (reactions (Ia) a n d (IIa))
Thermodynaxnies of phenolformaldehyde polycondensation
2395
CTIzOH
~
+CH,O~ ~
%/
(Ia)
~CH2__ ~
(IIa)
CH~OH Thermodynamic characteristics of reactions (Ia) and (IIa) were calculated from the information previously obtained [9]. 5. The fol]owing linear relation is often observed with many similar compounds dZ ° =A + BztH °
This m a y be used to find the value o f ~ Z ° of the compound from the well-known value of AH ° [16]. This method was used to determine ziZ°l for methylolpheno] (Fig. 2). Standard heats of formation of methylolphenol, dihydroxydiphenylmethane and dihydroxydibenzyl ether were determined from the combustion heats of these compounds, calculated from the Karrash formula [10, 17]. R e s u l t s o f c a l c u l a t i o n s are s h o w n in T a b l e 2. (Values o f SZ~l d e t e r m i n e d b y m e t h o d s 2 a n d 4 w e r e c o n v e r t e d t o zfZ~l o f t h e materia]s). TABLE 1. THERMODYNAMIC C O N S T A N T S
OF
MATERIALS
OF PKENOLFORMALDEHYDE
Material
AH o,
State
TAXING
PART
IN
THE
FORMATION"
POLYMERS
kcal/mole
ztZ °, kcal/mole
-- 39.46
-- 12-45
--36.2
--12"3"
[91 [9, 10]
--35"9
--31.0
[8, 9]
References
i
Phenol Formaldehyde
Formaldehyde Hexamethylene tetraminc Water Ammonia
Solid i i Liquid : Unhydrolysed in dilute aqueous solution Hydrolysed in an aqueous solution Solid Liquid Gaseous
[8l
--lll-1 28-8 --68.3 --ll.0
102-7 --56"7 3.9
[]1] [9] [9J
* The variation of JZ ° on melting phenol was calculated by tile authors using a me/hod formery described [1'-']. A p p r o x i m a t e t h e r m o d y n a m i c c h a r a c t e r i s t i c s o f r e a c t i o n s ( I ) - ( I I I ) are g i v e n in T a b l e 3. R e s u l t s s h o w n in T a b l e s 2 a n d 3 refer t o p a r a - s u b s t i t u t e d p h e n o l d e r i v a t i v e s . C o r r e s p o n d i n g o r t h o - s u b s t i t u t e d c o m p o u n d s c a n be f u r t h e r s t a b i l i z e d b y t h e f o r m a t i o n o f a n i n t r a m o l e c u l a r h y d r o g e n b o n d . T h e r e is d e t a i l e d e x p e r i m e n t a l i n f o r m a t i o n [18-24] t o p r o v e t h e e x i s t e n c e o f this b o n d . A c c o r d i n g t o t h e in-
M. I. S~INO a n d I. V. ADOI~OVA
2396 TABLE
2. THERMODYlCAMIC
CHARACTERISTICS
PHE I~]'OLFORMALDEHY'DS AZ~, k c a l / m o l e
Average value
methoq o f c a l c u l a t i o n
Material 1
2
3
4
5
Methylolphenol
46-9
48.4
48.5 49-2
49.5
49.0
Dihydroxydiphenylmethane
12"6
17o8
15.8 19.7 39.5 40.0 40.7
17.2 --
Dihydroxydibenzyl ether
35"6
--
OF INITIAL
PRODUCTS
OF
POLYCONDEI~,TS A T I O N
48"6
-- J Hcomb
-~/i7,
kcal/mole
kcal/molo
846'6
85"0
16"6
1563.0
69.5
39.0
1686.7
108-2
formation formerly described [25] for the formation of an intramolecular hydrogen bond of the 0 - - H . . . 0 type 5Z ° is (1-3) kcal/mole. Thus, values of A~ for ortho-substituted phenols examined will be lower by 1-3-kcal/mole t h a n the values given in Table 2 for the corresponding para-isomers. Table 3 indicates that the thermodynamic characteristics of reactions I - I I I differ considerably. The equilibrium constant for the hydroxymethylation of TABLE
3. THERMODYNAMIC
C H A R A C T E R I S T I C S OF ."~-IN R E A C T I O N S OF P H E N O L F O R I M t A L D E H Y D E POLYCONDENSATION
Reae
°
tion, :No. I II III IV
V
VI
VII
Reaction
Formation of methylolphenols Formation of a methylene bond b e t w e e n t h e p h e n o l rings Formation ofadimethylene ether b o n d b e t w e e n t h e p h e n o l rings Transition of the dimethylene ether bond to the methylene bond with the separation of formaldehyde P h e n o l y s i s o f c o m p o u n d s containing a dimethylene ether bond Formation of a methylene bond by the reaction of phenols with hexamethylene tetramine Recombination of compounds containing methylene bonds
az~, kcal/mole
keal/mole
-- 12-9 --
16"6
--
Equilibrium constant 25 °
100 °
--5'3
8 × 10 *
10 ~
12-4
100
3 × 106
8 X 10 -2
9 X 10 -*
2 × 10 e
5 × 106
--6"5
1"5
2.8
--8'6
--10.1
--13"9
2 × 10 ~°
5 × 10 8
--11,8
5 × 10 8
2 × 10 7
--9"2
Thermodynamics of phenolformaldehyde polyeondensation
2397
phenol is fairly high, for t h e f o r m a t i o n of a m e t h y l e n e b o n d b e t w e e n t h e phenol rings this v a l u e is higher b y 4 orders of m a g n i t u d e : t h e s e r e a c t i o n s are practicaUy irreversible. A t t h e s a m e t i m e t h e e q u i l i b r i u m described b y r e a c t i o n ] I 1 is c o n s i d e r a b l y displaced in t h e direction of h y d r o l y s i s of t h e d i m e t h y l e n e ether bond. F r o m a t h e r m o d y n a m i c p o i n t of view the f o r m a t i o n of a m e t h y l e n e bridge is m u c h m o r e f a v o u r a b l e in this ease t h a n t h a t of a d i m e t h y l e n e e t h e r bridge, t h e c o r r e s p o n d i n g equilibrium c o n s t a n t s differing b y 8-9 orders of m a g n i t u d e . Experimental information in the literature is ba,~ically in agreement with results of calculations. The separation of CH,.O from nmthylol derivatives of phenols is only noticeable in the case of some phenolic alcohols and takes plat(, to an insignificant extent [26-29]. o-Methylolphenols are particularly resistant in this aspect which can, of course, be due t(~ the formation of an intrarnolecular hydrogen bond. There is no evidenee m ~he literature indicating the possibility of hydrolytie decomposition of the methylene bond t,) fi~rm mcthylol derivatives. According to our results obtained using paper chromatography [30], prolonged heating of an aqueous dioxane solution of 4,4'-dihydroxydiphenylmctham' in neutral, acid or alkaline media does not result in the appearance; in the mixture of phenolic alcohols or isomeric dihydroxydiphenylmethanes. Informavion concerning the inst~ability q*f dihydroxydibenzyl ethers in an aqueous medium is contained in separate papers [31-33]. During self-condensation of p-methylolphcnol in a dilute aqueous alkaline solution ~,, traces of 4,4'-dihydroxydibenzyl ether could be detected [33]. 2,2'-Dihydroxydibenzyl ether. stabilized by an intramolecular hydrogen bond [18, 21, 29], is more stable. The low value of the equilibrium constant of reaction I I helps to explain why dihydroxydibenzyl ether~ are synthesized from methylolphenols in an anhydrous medium, normally a phenol alcohol melt [29, 32, 33.]. R e s u l t s o f T a b l e 2 help to d e t e r m i n e the t h e r m o d y n a m i c characteristics n o t only of r e a c t i o n s I - I I I , b u t also of s o m e o t h e r r e a c t i o n s which can h a v e a signific a n t role in t h e s y n t h e s i s a n d processing of p h e n o l f o r m a l d e h y d e resins. T h e separ a t i o n of f o r m a l d e h y d e f r o m d i h y d r o x y d i b e n z y l ethers is first of all of interest OH
OH
OH
OH
(fv)
ACH~OCH2 ~/
~CH2--~/
T a b l e 3 shows t h a t t h e e q u i l i b r i u m in this reaction is p r a c t i c a l l y c o m p l e t e l y displaced t o t h e right. I n fact, it is k n o w n [20] t h a t a t high t e m p e r a t u r e s f o r m a l d e h y d e is i r r e v e r s i b l y s e p a r a t e d f r o m ethers to f o r m a m e t h y l e n e bond. P h e n o l y s i s of d i h y d r o x y d i b e n z y l ethers a p p e a r s to be f a v o u r a b l e from a t h e r m o d y n a m i c p o i n t of v i e w OH
OH
OH
OH
C~H20H
OH
OH
~---CH ~'--~
2398
M.I. S~o
and I. V. ADOROVX
T h e possibility of these reactions was e m p h a s i z e d in a n earlier s t u d y [34], where it was f o u n d b y p a p e r c h r o m a t o g r a p h y t h a t h e a t i n g 2 , 2 ' - d i h y d r o x y d i b e n z y l , e t h e r w i t h phenol p r o d u c e s saligenin, 2,2'- a n d 2 , 4 ' - d i h y d r o x y d i p h e n y l m e t h a n e s , c o m p o u n d s w i t h t h r e e a n d four nuclei w i t h m e t h y l e n e bridges. R e a c t i o n s o f t y p e I - V m a y t a k e place n o t o n l y w i t h t h e initial p r o d u c t s o f p h e n o l f o r m a l d e h y d e p o l y c o n d e n s a t i o n ( P F P C ) , b u t also w i t h c o m p o u n d s of this series o f higher m o l e c u l a r weight. T h e t h e r m o d y n a m i c characteristics o f t h e r e a c t i o n s should n o t m a r k e d l y change. This conclusion follows f r o m t h e a s s u m p t i o n t h a t as in t h e case o f phenol, for a n y p r o d u c t of P F P C t h e s u b s t i t u t i o n of t h e H a t o m of t h e ring b y CH2OH, C6H4(OH)CH~ a n d C6H4(OH)CH20CH~ g r o u p s alters t h e v a l u e of ztZ~l of t h e initial c o m p o u n d b y a v a l u e w h i c h is app r o x i m a t e l y c o n s t a n t for e a c h of t h e s u b s t i t u t i n g g r o u p s * . I t follows f r o m T a b l e 2 t h a t this change will be - - 3 6 . 3 for t h e C H ~ O H group, - - 4 . 3 for C6tt,(OH)CH~ a n d - - 2 6 . 7 k c a l / m o l e for C6I-I4(OH)CI-I~OCI~2. T h e s e results help to derive t h e a p p r o x i m a t e v a l u e of ztZ~l for a n y o f t h e m a i n p r o d u c t s of P F P C , which m a y b e used t o e s t i m a t e t h e t h e r m o d y n a m i c possibility o f various reactions w i t h these compounds. B e a r i n g in m i n d t h e results o b t a i n e d , let us e x a m i n e t h e processes which t a k e place d u r i n g t h e synthesis a n d t r e a t m e n t o f p h e n o l f o r m a l d e h y d e p o l y m e r s . Polycondensation o f phenols w i t h f o r m a l d e h y d e results in t h e f o r m a t i o n o f p o l y m e r s , in t h e molecules of which t h e phenol rings m a y be joined b y m e t h y l e n e or d i m e t h y l e n e e t h e r bridges. The effect of the dimethylene ether bonds on PFPC has often been exaggerated in the literature. For example, Bilmeier [35] when dealing with the formation mechanism of phenolformaldehyde resins writes that "the reaction of ether formation is the most important reaction of condensation". According to thermodynamic considerations this reaction cannot be of great significance under normal conditions of PFPC. In fact when preparing phenol resins formaldehyde is normally used in 37~/o aqueous solution, and consequently the reaction takes place in the presence of a considerable amount of water. Under these conditions the formation of dimethylene ether bonds cannot be of considerable importance owing to the low value of K for reaction I I I . Vansheidt's results are in agreement with this [36]; according to these results conventional novolak phenolformaldehyde resins do not contain a noticeable number of ether bonds. When examining novolak resins by NMR [37] no ether bonds were found. The use of this method for the study of resol type oligomcrs [38, 39] indicates that only resins prepared in a medium which is close to neutral contain a small amount of dimethylene ether bonds. This can be understood if we bear in mind that the neutral medium mainly promotes reaction on the ortho-positions of the ring [37]; consequently, conditions are created for the formation of o,o'-dimethylene ether bonds stabilized by intramolecular hydrogen bonds. A reduction in water content in the system for example by the replacement of formalin by paraform may also promote an increase in the proportion of ether bridges. These bonds may form in some quantity during the stage of drying phenol re,sins. * This assumption is similar to the one on which the evaluation of ~Z~ is based using a third method of calculation. On the other hand, it is the equivalent to the conventional assumption normally used in physical chemistry concerning the same equilibrium constant in all stages of polycondensation.
Thermodynamics of phenolformaldehyde polycondensation
2399
Reactions ]E and II are of main importance in PFPC. As these reactions are practically irreversible, the composition of the products obtained will be chiefly determined by the ratio between the rates of the various directions of the process. This m a y be borne out in particular by the fact that the isomeric composition of products of P F P C depends greatly on the p H of the medium and the t y p e of catalyst used [37], i.e. on factors which influence the kinetics, not the thermodynamics of the reactions. As the equilibrium constant of reaction II is high, the dilution of the reaction mixture with water need not affect either the degree of completion of the reaction and polymer yield, or the molecular weight. The molecular weight of phenolformaldehyde novolak polymers, which is maximum in respect of thermodynamic considerations, can be estimated using the well known dependence P-----~/K/W, where P is the degree of polycondensation, W is the concentration of the low molecular weight compound, in this case water, K is the equilibrium constant of the separate stage of polycondensation. A combination of reactions I and II should be regarded as this stage in the case examined. The equilibrium constant of this stage will be equal to the product of the equilibrium constants of these reactions. Assuming that W is 20 mole/1, we find that even if polycondensation is carried out in an aqueous solution at 100 ° the preparation of a polymer with a molecular weight of the order of 10e is possible from a thermodynamic point of view. Consequently it is not thermodynamic factors which lead to the low molecular weight of phenolformaldehyde polymers. Values of 5H ° for reactions I and II (Table 3) are of interest in the thermochemistry of PFPC. The heat of formation of a methylene bond is much greater than that of hydroxymethylation of phenol. A similar conclusion was drawn in earlier studies [4, 7]. For transition from 5 H ° to the heat effects observed the heats of solution of the reagents in the reaction mixture should be taken into account. These values determined previously [4] for materials in the solid state were 2.7 for phenol; 3.8 for methylolphenol and 4.6 kcal/mole for dihydroxydiphenylmethane (solution took place with heat absorption). Using the wellknown formula [17] - - A H m e l t ~ (9-11) Tmelt the heat of fusion of phenol alcohols and dihydroxydiphenylmethanes can be estimated (3.4 and 3.6 kcal/mole respectively). Using these results and data of Table 1 we find from heats of fusion of phenol and dehydration of formaldehyde that the heat of reaction I should be 5-0 and of reaction II 15.Skcal/mole. The experimental values [4] are 4.1 and 16-9 kcal/mole respectively. The combined heat of reactions I and II is 20.Skcal/ /mole according to our results and the experimental values derived by various authors [3-5] vary between 19.7 and 23kcal/mole. According to earlier results when preparing novolak resins [1] 17.5 kcal/mole heat is liberated per mole phenol used; this value was found to be 14-1 kcal/mole [2]. This variation of results m a y be explained if we consider that the heat per mole of phenol depends on the molecular weight of the resin formed. For a
2400
M . I . SrL1NG a n d I. V. ADOROVA
polymer with n phenol rings this value will be n--1
Q=(Q,+Q~) ....
,
n
where Q1 and Q2 are the heat effects of reactions I and II. For Q ~ + Q ~ = 2 0 . S k c a l / /mole and n----3-6 the Q "value will vary between 13.7 and 17.1 kcal/mole, which corresponds to results of earlier studies [1, 2]. The resin obtained in this study [2] apparently had a lower molecular weight. Hardening. The transition of phenolformaldehyde resol resin to resite, an infusible and insoluble product, involves reactions of types II and III. Compared with polycondensation, the role of formation of dimethylene ether bonds somewhat increases. From a thermodynamic aspect this is favoured by the absence of a large amount of water from the system. Furthermore, if hardening takes place in a neutral medium the kinetic characteristics of the process also evidently favour reaction III. The breakdown of ether bridges with separation of formaldehyde remains thermodynamically favourable. These reactions take place at higher hardening temperatures [20]. To convert phenolformaldehyde novolak resin to the infusible and insoluble state, hexamethylene tetramine is normally used as "crosslinking" agent. Hardening in this ease takes place by the formation of methylene and nitrogen-containing bridges between the phenol rings [20]. Using results shown in Tables 1 and 3, we calculated the thermodynamic characteristics of the formation of a methylene bond by the reaction of novolak resin with hexamethylene tetramine. This process can be depicted as: OH
OH
OH
0:H (Vl)
+
+ ~ C6H,IN,--*
+
The results of calculation shown in Table 3 point to the practically complete irreversibility of this reaction. Differential-thermal analysis is now being increasingly used to study hardening of phenolformaldehyde polymers [40]. When interpreting results obtained by this method the thermodynamic characteristics of the reactions studied should be borne in mind. In view of this, previous results [40] are open to doubt; according to these results the heat of formation of a dimethylene bridge is 20.2 kcal/ /mole in hardening phenol resins. As shown by Table 3, this value is much nearer to the heat of formation of the methylene bond. During hardening of phenolformaldehyde polymers recombination and decomposition m a y take place in addition to reactions resulting in the formation of new bonds between the phenol rings. We will now examine these processes, which increase in importance with higher temperatures of hardening.
Thermodynamics of phenolformaldehyde polyeondensation
2401
Recombination and decomposition. During heat t r e a t m e n t of phenol formaldehyde polymers, particularly if this process takes place with acid or basic catalysts, recombination, or as it is normally termed, interchain transfer (reaction VII) may take place.
OH --
---H-~ / / ' ~ - -
CH2
-
-
--H--+
(VII)
We have examined the assumption t h a t the substitution of an H atom of the phenol ring by a C~H~(OH)CH2 group alters A Z ° of the initial compound by a constant value. Assuming the foregoing the following expression can be derived for A Z ° of a novolak molecule with a number of phenol units (n-I-l): A Z ° = - - 12.3-~n (--4.3) I t is obvious t h a t for interchain transfer, when n-+-m=r-ks, the isobar potential varies about zero. (A similar conclusion can also be drawn in relation to 5H°). Consequently the equilibrium constants of these reactions should be nearly 1.* Recombination in the phenol resin series can be experimentally confirmed [41, 42]. I t was found in the former s t udy t h a t in industrial novolak resins prepared with non-volatile acids, during distillation of the phenol the viscosity increases up to the point of gel formation. The same effect was also observed when phenol had been eliminated. It has been shown [42] t h a t heating individual dihydroxydiphenylmethanes with acid results in the formation of products of higher molecular weight, as well as isomerization of these compounds. I t is noteworthy t h a t isomerization of dihydroxydiphenylmethanes mainly results in the formation of a 2,2'-isomer. This once more points to the noticeable effect of the intramolecular hydrogen bond on the thermodynamic stability of the products of PFPC. Phenolysis of novolak resins from a t herm odynam i c aspect is a particular case of reaction (VII), when n = 0. The equilibrium constant of phenolysis for any value * .It is obvious that these conclusions can apply to interchain transfer of any polymer. In the general case the isobar potential for the polymer molecule may be described as follows: AZ ° =-a-'Fnb, where a is the isobar potential of the initial monomer, b is the isobar potential variation as a result of chain extension by one unit, and (n~-1) is the number of units in the poly~ner molecule.
2402
IV[. I. StLINOand I. V. KUOROV&
of m should be close to unity. I t is natural t h a t during reaction of phenol with polymers of the novolak type the molecular weight of the resins decreases [41-44], i.e. decomposition takes place. I f as a result of phenolysis the polymer is in a state close to equilibrium, the final molecular weight will be determined by the ratio of the overall amount of phenol in the system to the overall amount of formaldehyde. In this case the molecular weight m a y be calculated by well-known formulae which characterize its dependence on the monomer ratio (see e.g. [45]). There is no significant difference from a thermodynamic point of view as to which individual CH2 bond is ruptured by phenolysis, the one connecting the phenol rings in the linear macromolecule or t h a t which functions as "crosslink". In the light of this the tendency of hardened phenolformaldehyde polymers to decompose during prolonged heating with various phenols to form soluble and fusible products [46], becomes understandable. Finally, we note t h a t the thermodynamic characteristics of P F P C obtained in this study may be used for the approximate description of the polycondensation of formaldehyde with other phenols, such as alkylphenols, resorcinol, etc, CONCLUSIONS
(1) The thermodynamic characteristics of the following main reactions of phenolformaldehyde polycondensation were determined by approximate methods of calculation: formation of methylolphenols; formation of a methylene and dimethylene ether bond between phenol rings; transition of the dimethylene ether bond to a methylene bond; phenolysis of compounds containing a dimethylene ether bond; formation of a methylene bond by the reaction of phenols with hexamethylene tetramine; recombination of compounds containing methylene bonds. (2) The results obtained were used for the study of processes taking place during synthesis and processing of phenolformaldehyde polymers. Translated by E. S~.M~RS REFERENCES
1. K. P. M:ISHCHENKO and A. V. KUPFER, Prom-st' organich, khimii 7: 84, 1940 2. L. M. PESIN and Ch. G. ]gHASANOV, Prom-st' organich, khimii 7: 314, 1940 3. E. MANEGOLD and W. PETZOLD, Kolloid-Z. 94: 284, 1941 4. T. JONES, J. Soc. Chem. Industr. 65: 264, 1946 5. E. IMOTO and T. KIMURA, J. Chem. Soc. Japan, Industr. Chem. Soc. 53: 9, 1950 6. G. B. RAVICH and A. A. FROLOVA, Dokl. AN SSSR 90: 391, 1953 7. O. VLK, Plaste und Kautsehuk 4: 127, 1957 8. Selected Values of Chemical Thermodynamic Properties, p. 1 and 2, Washington, 19651966 9. M. Kh. KARAPET'YANTSand M. L. KARAPET'YANTS, Osnovnye termodinamicheskio konstanty neorganicheskikh i organicheskikh veshchestv (Main Thermodynamic Constants of Inorganic and Organic Materials). Izd. "Khimiya", 1968 10. Spravoehnik khimika (Chemist's Handbook). Vol. 1, Goskhimizdat, 1962 11. S. CHANG and E. WESTRUM, J. Phys. Chem. 64: 1547, 1960
Thermodynamics of phenolformaldehyde polycondensation
2403
12. G. PAP,KS and G. K H A F ~ , Svobodnyc energii organicheskikh soycdinenii (Free Energy in Organic Compounds). ONTI, 1936 13. O. HOUGEN, K. WATSON and R. RAGATZ, Chemical Proce~ Principles, P. II, New York, 1959 14. RENOLO CIOLA, Industr. and Engng. Chem. 49: 1789, 1957 15. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds, Amerieau Petroleum Institute, 1953 16. M. Yds. KARAPET'YANTS, Metody sravnitel'nogo rascheta fiziko-khimiehoskikh svoist (Methods of Comparative Calculation of Physical and Chemical Behaviour). Izd. "Nauka", 1965 17. M. Kh. KA~APET'YANTS, Khimicheskaya termodinamika (Chemical Therm(niynamies). Goskhimizdat, 1949 18. K. HULTZSCH, Chemie der Phenolharze, 1950 19. R. RICItARDS and H. TOMPSON, J. Chem. See., 1260, 1947 20. R. MARTIN, The Chemistry of Phenolic Resins, New York, 1956 21. K. HULTZSCH, Ktmstoffe 53: 166, 1963 22. R. MITRA and S. Ctt.ATTEP, JEE, I n d i a n J. Chem. 2: 85, 1964 23. A. ZAVITSAS, J. Chem. Data 12: 94, 1967 24. J. KOLINSKY, M. VASTA, R. CH:ROMECFAKand M. BONDANRCKY, Collect. Czechoslov. Chem. Communic. 31: 2714, 1966 25. D. PIMENTEL and O. MAC-CLELLAN, Vodorodnaya svyaz' (Hydrogen Bond). Izd. "Mir", 1964 26. J. J. DEJONG ~ald J. DE JONGE, Rocueil tray. chim. 72: 497, 1953 27. G. FREEMAN and C. LEWIS, J. Azner. Chem. See. 76: 2080, 1954 28. G. SCHIEMANN and E. HAI~TMANN, Makromolek. Chem. 63: 162, 1963 29. E. ZIEGLF_~ and K. LERCHER, Ber. 74: 841, 1941 30. I. V. A.DOROVA, V. Ya. KOVNER and M. I. SILING, Plast. massy, No. 1, 60, 1968 31. A. ZINKE and E. ZIEGLER, Ber. 74: 541, 1941 32. J. REESE, Kmmtstoffe 45: 137, 1955 33. L. YEDDANAPALLI and D. FRANCIS, Makromolek. Chem. 55: 74, 1962 34. D. FRASER, R. HALL and J. RAUM, J. Appl. Chem. 7: 676, 1957 35. F. BIL'MEIER, Vvedenie v khimiyu i tekhnologiyu polimerov (Introduction to the Chemistry and Technology of Polymers). Izd. inostr, lit., 1953 36. A. A. VANSHEIDT, Prom-st organich, khimii 3: 385, 1937 37. M. I. SHJNG, O. S. M A ~ A , O. A. MOCHALOVA, V. P. PSHENITSYNA and I. Ya. SLONIM, Vysokomol. soyed. A l l : 1943, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2215, 1969) 38. J. WOODBREY, H. HIGGINBOTTOM and H. CULBERTSON, J. Polymer Sci. A3: 1079, 1965 :~9. W. BROCK_MANN, H. B R O C ~ N and H. BUDZIKIEWICZ, Kautschuk und Gummi 21: 679, 1968 40. Z. KATOVIC, J. Appl. Polymer Sei. 11: 85, 94, 1967 41. H. HOYT, H. KEUCHEL and R. DEAN, P a i n t and Varnish Product 48: 33, 1958 42. Kh. I(JIORIUTI, Koge kagaku dzassi 63: 1651, 1960 43. Kh. KHORI-UTI, Koge kagaku dzassi 66: 139, 1963; 14ZhKhim, l l C I 2 8 , 1964 44. Kh. KHORIUTI, Koge kagaku dzassi 66: 145, 1963; RZhKhim., 19C137, 1964 45. V. V. KORSILa,K and S. V. VINOGRADOVA,,Ravnovesnaya polikondensatsiya (Equilibrium Polyeondensation). Izd. "Nauka", 1968 46. K. ELLIS, K h i m i y a sinteticheskikh stool (Chemistry of Synthetic Resins). ONTI, Vol. 1, issue l, 1938
OF
PHENOLFORMALDEHYDE
POLYCONDENSATION* M. I . SILINO a n d I . V. ADOROVA Scientific Research Institute of Physics
(Received 6 April 1970) IZCFOR~-~TrON concerning the thermodynamics of preparing polymers is essential in evaluating the extent of polymerization a n d the reversibility of this process, the yield and molecular weight distribution of the polymer and for heat calculation related to the apparatus, etc. There are no detailed thermodynamic data in the literature concerning phenofformaldehyde polyeondensation. There are only a few papers [1-7] which deal with the thermoehemistry of these processes. Polycondensation of phenol with formaldehyde involves three mahl types of reaction: hydroxymethylation of phenols in the ortho- or para-positions; reaction of the methylol group with the hydrogen atom of the aromatic nucleus with the separation of water and formation of a methylene bridge between the phenol rings and the formation of a methylene ether bond b y the reaction of two methy]ol groups. For the initial stages of polycondensation these reactions can be described as: OH
OH
(r) I
CH2OH OH
I
OH
+
OH
~
OH
CH
+H,O
(II)
CH2OH OH
OH + ~OH
OH -+
OH
"
+H~O
(III)
CH2OH
Thcrmodynanfic constants of phenol, formaldchyde and water arc given in the literature (Table 1). Corresponding values for methylolphenol, dihydroxydiphenylmethane a n d dihydroxydibenzyl ether were derived by the authors using approximate methods of calculation. Several methods were used to obtain more reliable results. Wc denote by AHcomb * Vysokomol. soyed. A13: No. 9, 2129-2138. 1971.
2392
Thermodynamics of phenolformaldehyde polyeondeneation
2393
t h e h e a t of c o m b u s t i o n o f t h e m a t e r i a l in t h e l i q u i d s t a t e , zIH 0 is t h e s t a n d a r d h e a t o f f o r m a t i o n o f t h e m a t e r i a l a t 25 °, S ° is t h e s t a n d a r d e n t r o p y o f t h e m a t e r i a l a t 25 °, A Z ° is t h e s t a n d a r d i s o b a r p o t e n t i a l o f t h e f o r m a t i o n o f t h e m a t e r i a l , K is t h e e q u i l i b r i u m c o n s t a n t o f t h e r e a c t i o n s , J H ° is t h e v a r i a t i o n o f t h e s t a n d a r d h e a t o f f o r m a t i o n as a r e s u l t o f t h e rea c t i o n , J Z ° is t h e v a r i a t i o n of t h e s t a n d a r d i s o b a r p o t e n t i a l as a r e s u l t o f t h e r e a c t i o n . I n d i c e s , g, 1 a n d s d e n o t e t h e gaseous, liquid a n d solid s t a t e s o f t h e m a t e r i a l .
-AZ' ',kcal/mole
4z;
120
f
80 ~0 I I I i I I
f I I
I
I
6
I
FiG. 1
I
I
t
8 IOC
I
~
I
8 0I I0
I
-,4 Hi,
~I
I
kca//mole
F~o. 2
FI('. 1. Dep(mdence o f t h e i s o b a r p o t e n t i a l o f n - a l k a n e s on t h e n u m b e r o f c a r b o n a t o m s iu t h e i r molecules. Ft(~. 2. R e l a t i o n b e t w e e n JZ~I a n d JH~I for v a r i o u s b e n z e n e d e r i v a t i v e s : 1 - - p - h y d r o x y benzoic acid; 2 - - b e n z o i c acid; 3 - - h y d r o q u i n o n e ; d - - p h e n o l ; 5 - - b c n z y l alcohol; 6 - - p - x y l e n e ; 7-- t o l u e n e . W e give a b r i e f d e s c r i p t i o n o f m e t h o d s o f c a l c u l a t i o n used for d c t o r m i n i u g J / ~ l a n d AZ ° o f m c t h y h ) l p h e n o l , d i h y d r o x y d i p h e n y h n c t h a n c a n d d i h y d r o x y d i b e n z y l e t h e r . l. U s i n g t h e m e t h o d p r o p o s e d p r e v i o u s l y [10, 13], values o f A/~g a n d S~g a n d t h e n AZ ° o f t h e c o m p o u n d s were c a l c u l a t e d , p - X y l e n c was selected as t h e initial c o m p o u n d for ('alculating t h e r m o d y n a m i c c o n s t a n t s o f m e t h y l o l p h e n o l a n d d i h y d r o x y d i p h e n y l m e t h a n e al~d d i e t h y l e t h e r was used for d i h y d r o x y d i b e n z y ] e t h e r . Values of AHg a n d S~s o f t h e initial m a t e r i a l s were t a k e n f r o m a f o r m e r s t u d y [9], a n d t h e c o r r e c t i o n s f r o m a n o t h e r p a p e r [10J. ( ' o r r c c t i o n o f t h e v a l u e J/~g in r e s p e c t o f t h e s u b s t i t u t i o n o f t h e CI-Ia g r o u p w i t h p h e n o l h y d r o x y l is a n e x c e p t i o n , t h e v a l u e o f w h i c h ( - - 3 2 . 7 k c a l / m o l e ) is in p o o r a g r e e m e n t w i t h e , ) n v c n t i o n a l values for p h e n o l s [9]. A n a n a l y s i s of t h e s e results i n d i c a t e s t h a t t h e a v e r a g e v a l u e o f t h i s c o r r e c t i o n is close to - - 3 5 k c a l / m o l c . W c u s e d t h i s v a l u e in t h e c a l c u l a t i o n s . F,~r t h e t r a n s i t i o n f r o m AZ°~ to A Z [ a l l o w a n c e was m a d e for t h e f a c t t h a t in t h e case o f m ( , l o - a n d d i - s u b s t i t u t e d b e n z e n e d e r i v a v i v e s t h e difference (JZ°s--AZ [ ) (1.5-3.5 k c a l / m o l e ) [it] c a n a p p r o x i m a t e l y be r e g a r d e d a~q c o n s t a n t a n d e q u a l t o 2.5 k c a l / m o l c on a v e r a g e . 2. U s i n g t h e R e m o l o Ciola [14] m e t h o d t h e v a r i a t i o n o f J Z wae c a l c u l a t e d for t h e p r o c e s s itl w h i c h t h e r e a c t i n g s u b s t a n c e s are in t h e gaseous s t a t e . As t h e difference (AZ ° - - A Z 0) i~ slight a n d a p p r o x i m a t e l y c o n s t a n t a n d t h e c o r r e s p o n d i n g v a l u e for w a t e r is also low (2-1 k c a l / m o l e ) , it m a y be a s s u m e d t h a t JZ~ o f r e a c t i o n s (1)-(3) will be close t o 5Z~. 3. F o r a n a p p r o x i m a t e e s t i m a t i o n o f AZ ] P a r k s a n d I-Iaffman [12] p r o p o s e d correcl i~)ns w h i c h allow for t h e v a r i a t i o n o f t h e i s o b a r p o t e n t i a l of h y d r o c a r b o n s o n a d d i n g f u n c t i o n a l z r o u p s t.o t h e molecule. As t h e s e c o r r e c t i o n s were b a s e d on o b s o l e t e results, we d e c i d e d to
2394
M . I . SrLxNO and I. V. A_voRovA
define t h e i r values m o r e a c c u r a t e l y using m o d e r n i n f o r m a t i o n concerning J Z I o f aliphatic a n d a r o m a t i c h y d r o c a r b o n s , alcohols, phenols a n d ethers [8, 9, 15]. Values o f JZ~ o f m e t h a n e , e t h a n e , p r o p a n e a n d n - b u t a n e were d e r i v e d b y e x t r a p o l a t i o n of t h e linear d e p e n d e n c e o f JZ~° o f C6-C10 alkanes [15] on t h e n u m b e r of c a r b o n a t o m s in t h e molecules (Fig. 1). T h e following corrections (kcal/mole) were o b t a i n e d : s u b s t i t u t i o n o f t h e H a t o m in the C - - H b o n d b y a C6H~CH~ g r o u p - - --36, s u b s t i t u t i o n of the H a t o m in t h e C - - H b o n d by a C6H5 g r o u p - - -- 35, i n t r o d u c t i o n o f a n Olql group in t h e benzene r i n g - - -- 41, substitution of t h e ]-I a t o m b y an O H g r o u p to f o r m p r i m a r y a l c o h o l - - --35.5, a d d i t i o n of an - - 0 - - group b e t w e e n two c a r b o n a t o m s to f o r m an e t h e r - - --22. F r o m these results values of JZ~ of c o m p o u n d s we are interested in were d e t e r m i n e d by the equations:
CH,0H
--7.5
--48-5; OH
//
//\
CH,0H
CH,
--49.2
--13.7
OH
CH,
27.3 OH
OH
J
a/ 66-2
--15.8;
//
--19.7
21.3
--13"7
//\
0-0
--~SHI--CH, -~ --6"0
&. &
64"0
27.3
0
v--CH,0CH,-~ 42"0
~
OH
42"5
--27"5
OH
H,0CI~, ---40-0; - - 3 9 . 5 ; - - 4 0 . 7
OH
OH
0H
t
6 0d CH, 27"3
CHs --13"7
22-3
0"3
I n this l a y o u t t h e n u m b e r s u n d e r the structural formulae d e n o t e J Z I ; information for t h e initial c o m p o u n d s were t a k e n from t h e literature [9]. 4. I n t h e p r e v i o u s calculation it was assumed t h a t J Z ° of t h e a r o m a t i c c o m p o u n d on a d d i n g a phenol h y d r o x y l group in its molecule changes to a c o n s t a n t value. I t follows f r o m this a s s u m p t i o n t h a t t h e v a r i a t i o n of the isobar p o t e n t i a l in reactions (I) and (II) will be a b o u t t h e s a m e as for reactions o f f o r m a t i o n of benzyl alcohol and d i p h e n y l m e t h a n e , r e s p e c t i v e l y (reactions (Ia) a n d (IIa))
Thermodynaxnies of phenolformaldehyde polycondensation
2395
CTIzOH
~
+CH,O~ ~
%/
(Ia)
~CH2__ ~
(IIa)
CH~OH Thermodynamic characteristics of reactions (Ia) and (IIa) were calculated from the information previously obtained [9]. 5. The fol]owing linear relation is often observed with many similar compounds dZ ° =A + BztH °
This m a y be used to find the value o f ~ Z ° of the compound from the well-known value of AH ° [16]. This method was used to determine ziZ°l for methylolpheno] (Fig. 2). Standard heats of formation of methylolphenol, dihydroxydiphenylmethane and dihydroxydibenzyl ether were determined from the combustion heats of these compounds, calculated from the Karrash formula [10, 17]. R e s u l t s o f c a l c u l a t i o n s are s h o w n in T a b l e 2. (Values o f SZ~l d e t e r m i n e d b y m e t h o d s 2 a n d 4 w e r e c o n v e r t e d t o zfZ~l o f t h e materia]s). TABLE 1. THERMODYNAMIC C O N S T A N T S
OF
MATERIALS
OF PKENOLFORMALDEHYDE
Material
AH o,
State
TAXING
PART
IN
THE
FORMATION"
POLYMERS
kcal/mole
ztZ °, kcal/mole
-- 39.46
-- 12-45
--36.2
--12"3"
[91 [9, 10]
--35"9
--31.0
[8, 9]
References
i
Phenol Formaldehyde
Formaldehyde Hexamethylene tetraminc Water Ammonia
Solid i i Liquid : Unhydrolysed in dilute aqueous solution Hydrolysed in an aqueous solution Solid Liquid Gaseous
[8l
--lll-1 28-8 --68.3 --ll.0
102-7 --56"7 3.9
[]1] [9] [9J
* The variation of JZ ° on melting phenol was calculated by tile authors using a me/hod formery described [1'-']. A p p r o x i m a t e t h e r m o d y n a m i c c h a r a c t e r i s t i c s o f r e a c t i o n s ( I ) - ( I I I ) are g i v e n in T a b l e 3. R e s u l t s s h o w n in T a b l e s 2 a n d 3 refer t o p a r a - s u b s t i t u t e d p h e n o l d e r i v a t i v e s . C o r r e s p o n d i n g o r t h o - s u b s t i t u t e d c o m p o u n d s c a n be f u r t h e r s t a b i l i z e d b y t h e f o r m a t i o n o f a n i n t r a m o l e c u l a r h y d r o g e n b o n d . T h e r e is d e t a i l e d e x p e r i m e n t a l i n f o r m a t i o n [18-24] t o p r o v e t h e e x i s t e n c e o f this b o n d . A c c o r d i n g t o t h e in-
M. I. S~INO a n d I. V. ADOI~OVA
2396 TABLE
2. THERMODYlCAMIC
CHARACTERISTICS
PHE I~]'OLFORMALDEHY'DS AZ~, k c a l / m o l e
Average value
methoq o f c a l c u l a t i o n
Material 1
2
3
4
5
Methylolphenol
46-9
48.4
48.5 49-2
49.5
49.0
Dihydroxydiphenylmethane
12"6
17o8
15.8 19.7 39.5 40.0 40.7
17.2 --
Dihydroxydibenzyl ether
35"6
--
OF INITIAL
PRODUCTS
OF
POLYCONDEI~,TS A T I O N
48"6
-- J Hcomb
-~/i7,
kcal/mole
kcal/molo
846'6
85"0
16"6
1563.0
69.5
39.0
1686.7
108-2
formation formerly described [25] for the formation of an intramolecular hydrogen bond of the 0 - - H . . . 0 type 5Z ° is (1-3) kcal/mole. Thus, values of A~ for ortho-substituted phenols examined will be lower by 1-3-kcal/mole t h a n the values given in Table 2 for the corresponding para-isomers. Table 3 indicates that the thermodynamic characteristics of reactions I - I I I differ considerably. The equilibrium constant for the hydroxymethylation of TABLE
3. THERMODYNAMIC
C H A R A C T E R I S T I C S OF ."~-IN R E A C T I O N S OF P H E N O L F O R I M t A L D E H Y D E POLYCONDENSATION
Reae
°
tion, :No. I II III IV
V
VI
VII
Reaction
Formation of methylolphenols Formation of a methylene bond b e t w e e n t h e p h e n o l rings Formation ofadimethylene ether b o n d b e t w e e n t h e p h e n o l rings Transition of the dimethylene ether bond to the methylene bond with the separation of formaldehyde P h e n o l y s i s o f c o m p o u n d s containing a dimethylene ether bond Formation of a methylene bond by the reaction of phenols with hexamethylene tetramine Recombination of compounds containing methylene bonds
az~, kcal/mole
keal/mole
-- 12-9 --
16"6
--
Equilibrium constant 25 °
100 °
--5'3
8 × 10 *
10 ~
12-4
100
3 × 106
8 X 10 -2
9 X 10 -*
2 × 10 e
5 × 106
--6"5
1"5
2.8
--8'6
--10.1
--13"9
2 × 10 ~°
5 × 10 8
--11,8
5 × 10 8
2 × 10 7
--9"2
Thermodynamics of phenolformaldehyde polyeondensation
2397
phenol is fairly high, for t h e f o r m a t i o n of a m e t h y l e n e b o n d b e t w e e n t h e phenol rings this v a l u e is higher b y 4 orders of m a g n i t u d e : t h e s e r e a c t i o n s are practicaUy irreversible. A t t h e s a m e t i m e t h e e q u i l i b r i u m described b y r e a c t i o n ] I 1 is c o n s i d e r a b l y displaced in t h e direction of h y d r o l y s i s of t h e d i m e t h y l e n e ether bond. F r o m a t h e r m o d y n a m i c p o i n t of view the f o r m a t i o n of a m e t h y l e n e bridge is m u c h m o r e f a v o u r a b l e in this ease t h a n t h a t of a d i m e t h y l e n e e t h e r bridge, t h e c o r r e s p o n d i n g equilibrium c o n s t a n t s differing b y 8-9 orders of m a g n i t u d e . Experimental information in the literature is ba,~ically in agreement with results of calculations. The separation of CH,.O from nmthylol derivatives of phenols is only noticeable in the case of some phenolic alcohols and takes plat(, to an insignificant extent [26-29]. o-Methylolphenols are particularly resistant in this aspect which can, of course, be due t(~ the formation of an intrarnolecular hydrogen bond. There is no evidenee m ~he literature indicating the possibility of hydrolytie decomposition of the methylene bond t,) fi~rm mcthylol derivatives. According to our results obtained using paper chromatography [30], prolonged heating of an aqueous dioxane solution of 4,4'-dihydroxydiphenylmctham' in neutral, acid or alkaline media does not result in the appearance; in the mixture of phenolic alcohols or isomeric dihydroxydiphenylmethanes. Informavion concerning the inst~ability q*f dihydroxydibenzyl ethers in an aqueous medium is contained in separate papers [31-33]. During self-condensation of p-methylolphcnol in a dilute aqueous alkaline solution ~,, traces of 4,4'-dihydroxydibenzyl ether could be detected [33]. 2,2'-Dihydroxydibenzyl ether. stabilized by an intramolecular hydrogen bond [18, 21, 29], is more stable. The low value of the equilibrium constant of reaction I I helps to explain why dihydroxydibenzyl ether~ are synthesized from methylolphenols in an anhydrous medium, normally a phenol alcohol melt [29, 32, 33.]. R e s u l t s o f T a b l e 2 help to d e t e r m i n e the t h e r m o d y n a m i c characteristics n o t only of r e a c t i o n s I - I I I , b u t also of s o m e o t h e r r e a c t i o n s which can h a v e a signific a n t role in t h e s y n t h e s i s a n d processing of p h e n o l f o r m a l d e h y d e resins. T h e separ a t i o n of f o r m a l d e h y d e f r o m d i h y d r o x y d i b e n z y l ethers is first of all of interest OH
OH
OH
OH
(fv)
ACH~OCH2 ~/
~CH2--~/
T a b l e 3 shows t h a t t h e e q u i l i b r i u m in this reaction is p r a c t i c a l l y c o m p l e t e l y displaced t o t h e right. I n fact, it is k n o w n [20] t h a t a t high t e m p e r a t u r e s f o r m a l d e h y d e is i r r e v e r s i b l y s e p a r a t e d f r o m ethers to f o r m a m e t h y l e n e bond. P h e n o l y s i s of d i h y d r o x y d i b e n z y l ethers a p p e a r s to be f a v o u r a b l e from a t h e r m o d y n a m i c p o i n t of v i e w OH
OH
OH
OH
C~H20H
OH
OH
~---CH ~'--~
2398
M.I. S~o
and I. V. ADOROVX
T h e possibility of these reactions was e m p h a s i z e d in a n earlier s t u d y [34], where it was f o u n d b y p a p e r c h r o m a t o g r a p h y t h a t h e a t i n g 2 , 2 ' - d i h y d r o x y d i b e n z y l , e t h e r w i t h phenol p r o d u c e s saligenin, 2,2'- a n d 2 , 4 ' - d i h y d r o x y d i p h e n y l m e t h a n e s , c o m p o u n d s w i t h t h r e e a n d four nuclei w i t h m e t h y l e n e bridges. R e a c t i o n s o f t y p e I - V m a y t a k e place n o t o n l y w i t h t h e initial p r o d u c t s o f p h e n o l f o r m a l d e h y d e p o l y c o n d e n s a t i o n ( P F P C ) , b u t also w i t h c o m p o u n d s of this series o f higher m o l e c u l a r weight. T h e t h e r m o d y n a m i c characteristics o f t h e r e a c t i o n s should n o t m a r k e d l y change. This conclusion follows f r o m t h e a s s u m p t i o n t h a t as in t h e case o f phenol, for a n y p r o d u c t of P F P C t h e s u b s t i t u t i o n of t h e H a t o m of t h e ring b y CH2OH, C6H4(OH)CH~ a n d C6H4(OH)CH20CH~ g r o u p s alters t h e v a l u e of ztZ~l of t h e initial c o m p o u n d b y a v a l u e w h i c h is app r o x i m a t e l y c o n s t a n t for e a c h of t h e s u b s t i t u t i n g g r o u p s * . I t follows f r o m T a b l e 2 t h a t this change will be - - 3 6 . 3 for t h e C H ~ O H group, - - 4 . 3 for C6tt,(OH)CH~ a n d - - 2 6 . 7 k c a l / m o l e for C6I-I4(OH)CI-I~OCI~2. T h e s e results help to derive t h e a p p r o x i m a t e v a l u e of ztZ~l for a n y o f t h e m a i n p r o d u c t s of P F P C , which m a y b e used t o e s t i m a t e t h e t h e r m o d y n a m i c possibility o f various reactions w i t h these compounds. B e a r i n g in m i n d t h e results o b t a i n e d , let us e x a m i n e t h e processes which t a k e place d u r i n g t h e synthesis a n d t r e a t m e n t o f p h e n o l f o r m a l d e h y d e p o l y m e r s . Polycondensation o f phenols w i t h f o r m a l d e h y d e results in t h e f o r m a t i o n o f p o l y m e r s , in t h e molecules of which t h e phenol rings m a y be joined b y m e t h y l e n e or d i m e t h y l e n e e t h e r bridges. The effect of the dimethylene ether bonds on PFPC has often been exaggerated in the literature. For example, Bilmeier [35] when dealing with the formation mechanism of phenolformaldehyde resins writes that "the reaction of ether formation is the most important reaction of condensation". According to thermodynamic considerations this reaction cannot be of great significance under normal conditions of PFPC. In fact when preparing phenol resins formaldehyde is normally used in 37~/o aqueous solution, and consequently the reaction takes place in the presence of a considerable amount of water. Under these conditions the formation of dimethylene ether bonds cannot be of considerable importance owing to the low value of K for reaction I I I . Vansheidt's results are in agreement with this [36]; according to these results conventional novolak phenolformaldehyde resins do not contain a noticeable number of ether bonds. When examining novolak resins by NMR [37] no ether bonds were found. The use of this method for the study of resol type oligomcrs [38, 39] indicates that only resins prepared in a medium which is close to neutral contain a small amount of dimethylene ether bonds. This can be understood if we bear in mind that the neutral medium mainly promotes reaction on the ortho-positions of the ring [37]; consequently, conditions are created for the formation of o,o'-dimethylene ether bonds stabilized by intramolecular hydrogen bonds. A reduction in water content in the system for example by the replacement of formalin by paraform may also promote an increase in the proportion of ether bridges. These bonds may form in some quantity during the stage of drying phenol re,sins. * This assumption is similar to the one on which the evaluation of ~Z~ is based using a third method of calculation. On the other hand, it is the equivalent to the conventional assumption normally used in physical chemistry concerning the same equilibrium constant in all stages of polycondensation.
Thermodynamics of phenolformaldehyde polycondensation
2399
Reactions ]E and II are of main importance in PFPC. As these reactions are practically irreversible, the composition of the products obtained will be chiefly determined by the ratio between the rates of the various directions of the process. This m a y be borne out in particular by the fact that the isomeric composition of products of P F P C depends greatly on the p H of the medium and the t y p e of catalyst used [37], i.e. on factors which influence the kinetics, not the thermodynamics of the reactions. As the equilibrium constant of reaction II is high, the dilution of the reaction mixture with water need not affect either the degree of completion of the reaction and polymer yield, or the molecular weight. The molecular weight of phenolformaldehyde novolak polymers, which is maximum in respect of thermodynamic considerations, can be estimated using the well known dependence P-----~/K/W, where P is the degree of polycondensation, W is the concentration of the low molecular weight compound, in this case water, K is the equilibrium constant of the separate stage of polycondensation. A combination of reactions I and II should be regarded as this stage in the case examined. The equilibrium constant of this stage will be equal to the product of the equilibrium constants of these reactions. Assuming that W is 20 mole/1, we find that even if polycondensation is carried out in an aqueous solution at 100 ° the preparation of a polymer with a molecular weight of the order of 10e is possible from a thermodynamic point of view. Consequently it is not thermodynamic factors which lead to the low molecular weight of phenolformaldehyde polymers. Values of 5H ° for reactions I and II (Table 3) are of interest in the thermochemistry of PFPC. The heat of formation of a methylene bond is much greater than that of hydroxymethylation of phenol. A similar conclusion was drawn in earlier studies [4, 7]. For transition from 5 H ° to the heat effects observed the heats of solution of the reagents in the reaction mixture should be taken into account. These values determined previously [4] for materials in the solid state were 2.7 for phenol; 3.8 for methylolphenol and 4.6 kcal/mole for dihydroxydiphenylmethane (solution took place with heat absorption). Using the wellknown formula [17] - - A H m e l t ~ (9-11) Tmelt the heat of fusion of phenol alcohols and dihydroxydiphenylmethanes can be estimated (3.4 and 3.6 kcal/mole respectively). Using these results and data of Table 1 we find from heats of fusion of phenol and dehydration of formaldehyde that the heat of reaction I should be 5-0 and of reaction II 15.Skcal/mole. The experimental values [4] are 4.1 and 16-9 kcal/mole respectively. The combined heat of reactions I and II is 20.Skcal/ /mole according to our results and the experimental values derived by various authors [3-5] vary between 19.7 and 23kcal/mole. According to earlier results when preparing novolak resins [1] 17.5 kcal/mole heat is liberated per mole phenol used; this value was found to be 14-1 kcal/mole [2]. This variation of results m a y be explained if we consider that the heat per mole of phenol depends on the molecular weight of the resin formed. For a
2400
M . I . SrL1NG a n d I. V. ADOROVA
polymer with n phenol rings this value will be n--1
Q=(Q,+Q~) ....
,
n
where Q1 and Q2 are the heat effects of reactions I and II. For Q ~ + Q ~ = 2 0 . S k c a l / /mole and n----3-6 the Q "value will vary between 13.7 and 17.1 kcal/mole, which corresponds to results of earlier studies [1, 2]. The resin obtained in this study [2] apparently had a lower molecular weight. Hardening. The transition of phenolformaldehyde resol resin to resite, an infusible and insoluble product, involves reactions of types II and III. Compared with polycondensation, the role of formation of dimethylene ether bonds somewhat increases. From a thermodynamic aspect this is favoured by the absence of a large amount of water from the system. Furthermore, if hardening takes place in a neutral medium the kinetic characteristics of the process also evidently favour reaction III. The breakdown of ether bridges with separation of formaldehyde remains thermodynamically favourable. These reactions take place at higher hardening temperatures [20]. To convert phenolformaldehyde novolak resin to the infusible and insoluble state, hexamethylene tetramine is normally used as "crosslinking" agent. Hardening in this ease takes place by the formation of methylene and nitrogen-containing bridges between the phenol rings [20]. Using results shown in Tables 1 and 3, we calculated the thermodynamic characteristics of the formation of a methylene bond by the reaction of novolak resin with hexamethylene tetramine. This process can be depicted as: OH
OH
OH
0:H (Vl)
+
+ ~ C6H,IN,--*
+
The results of calculation shown in Table 3 point to the practically complete irreversibility of this reaction. Differential-thermal analysis is now being increasingly used to study hardening of phenolformaldehyde polymers [40]. When interpreting results obtained by this method the thermodynamic characteristics of the reactions studied should be borne in mind. In view of this, previous results [40] are open to doubt; according to these results the heat of formation of a dimethylene bridge is 20.2 kcal/ /mole in hardening phenol resins. As shown by Table 3, this value is much nearer to the heat of formation of the methylene bond. During hardening of phenolformaldehyde polymers recombination and decomposition m a y take place in addition to reactions resulting in the formation of new bonds between the phenol rings. We will now examine these processes, which increase in importance with higher temperatures of hardening.
Thermodynamics of phenolformaldehyde polyeondensation
2401
Recombination and decomposition. During heat t r e a t m e n t of phenol formaldehyde polymers, particularly if this process takes place with acid or basic catalysts, recombination, or as it is normally termed, interchain transfer (reaction VII) may take place.
OH --
---H-~ / / ' ~ - -
CH2
-
-
--H--+
(VII)
We have examined the assumption t h a t the substitution of an H atom of the phenol ring by a C~H~(OH)CH2 group alters A Z ° of the initial compound by a constant value. Assuming the foregoing the following expression can be derived for A Z ° of a novolak molecule with a number of phenol units (n-I-l): A Z ° = - - 12.3-~n (--4.3) I t is obvious t h a t for interchain transfer, when n-+-m=r-ks, the isobar potential varies about zero. (A similar conclusion can also be drawn in relation to 5H°). Consequently the equilibrium constants of these reactions should be nearly 1.* Recombination in the phenol resin series can be experimentally confirmed [41, 42]. I t was found in the former s t udy t h a t in industrial novolak resins prepared with non-volatile acids, during distillation of the phenol the viscosity increases up to the point of gel formation. The same effect was also observed when phenol had been eliminated. It has been shown [42] t h a t heating individual dihydroxydiphenylmethanes with acid results in the formation of products of higher molecular weight, as well as isomerization of these compounds. I t is noteworthy t h a t isomerization of dihydroxydiphenylmethanes mainly results in the formation of a 2,2'-isomer. This once more points to the noticeable effect of the intramolecular hydrogen bond on the thermodynamic stability of the products of PFPC. Phenolysis of novolak resins from a t herm odynam i c aspect is a particular case of reaction (VII), when n = 0. The equilibrium constant of phenolysis for any value * .It is obvious that these conclusions can apply to interchain transfer of any polymer. In the general case the isobar potential for the polymer molecule may be described as follows: AZ ° =-a-'Fnb, where a is the isobar potential of the initial monomer, b is the isobar potential variation as a result of chain extension by one unit, and (n~-1) is the number of units in the poly~ner molecule.
2402
IV[. I. StLINOand I. V. KUOROV&
of m should be close to unity. I t is natural t h a t during reaction of phenol with polymers of the novolak type the molecular weight of the resins decreases [41-44], i.e. decomposition takes place. I f as a result of phenolysis the polymer is in a state close to equilibrium, the final molecular weight will be determined by the ratio of the overall amount of phenol in the system to the overall amount of formaldehyde. In this case the molecular weight m a y be calculated by well-known formulae which characterize its dependence on the monomer ratio (see e.g. [45]). There is no significant difference from a thermodynamic point of view as to which individual CH2 bond is ruptured by phenolysis, the one connecting the phenol rings in the linear macromolecule or t h a t which functions as "crosslink". In the light of this the tendency of hardened phenolformaldehyde polymers to decompose during prolonged heating with various phenols to form soluble and fusible products [46], becomes understandable. Finally, we note t h a t the thermodynamic characteristics of P F P C obtained in this study may be used for the approximate description of the polycondensation of formaldehyde with other phenols, such as alkylphenols, resorcinol, etc, CONCLUSIONS
(1) The thermodynamic characteristics of the following main reactions of phenolformaldehyde polycondensation were determined by approximate methods of calculation: formation of methylolphenols; formation of a methylene and dimethylene ether bond between phenol rings; transition of the dimethylene ether bond to a methylene bond; phenolysis of compounds containing a dimethylene ether bond; formation of a methylene bond by the reaction of phenols with hexamethylene tetramine; recombination of compounds containing methylene bonds. (2) The results obtained were used for the study of processes taking place during synthesis and processing of phenolformaldehyde polymers. Translated by E. S~.M~RS REFERENCES
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Thermodynamics of phenolformaldehyde polycondensation
2403
12. G. PAP,KS and G. K H A F ~ , Svobodnyc energii organicheskikh soycdinenii (Free Energy in Organic Compounds). ONTI, 1936 13. O. HOUGEN, K. WATSON and R. RAGATZ, Chemical Proce~ Principles, P. II, New York, 1959 14. RENOLO CIOLA, Industr. and Engng. Chem. 49: 1789, 1957 15. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds, Amerieau Petroleum Institute, 1953 16. M. Yds. KARAPET'YANTS, Metody sravnitel'nogo rascheta fiziko-khimiehoskikh svoist (Methods of Comparative Calculation of Physical and Chemical Behaviour). Izd. "Nauka", 1965 17. M. Kh. KA~APET'YANTS, Khimicheskaya termodinamika (Chemical Therm(niynamies). Goskhimizdat, 1949 18. K. HULTZSCH, Chemie der Phenolharze, 1950 19. R. RICItARDS and H. TOMPSON, J. Chem. See., 1260, 1947 20. R. MARTIN, The Chemistry of Phenolic Resins, New York, 1956 21. K. HULTZSCH, Ktmstoffe 53: 166, 1963 22. R. MITRA and S. Ctt.ATTEP, JEE, I n d i a n J. Chem. 2: 85, 1964 23. A. ZAVITSAS, J. Chem. Data 12: 94, 1967 24. J. KOLINSKY, M. VASTA, R. CH:ROMECFAKand M. BONDANRCKY, Collect. Czechoslov. Chem. Communic. 31: 2714, 1966 25. D. PIMENTEL and O. MAC-CLELLAN, Vodorodnaya svyaz' (Hydrogen Bond). Izd. "Mir", 1964 26. J. J. DEJONG ~ald J. DE JONGE, Rocueil tray. chim. 72: 497, 1953 27. G. FREEMAN and C. LEWIS, J. Azner. Chem. See. 76: 2080, 1954 28. G. SCHIEMANN and E. HAI~TMANN, Makromolek. Chem. 63: 162, 1963 29. E. ZIEGLF_~ and K. LERCHER, Ber. 74: 841, 1941 30. I. V. A.DOROVA, V. Ya. KOVNER and M. I. SILING, Plast. massy, No. 1, 60, 1968 31. A. ZINKE and E. ZIEGLER, Ber. 74: 541, 1941 32. J. REESE, Kmmtstoffe 45: 137, 1955 33. L. YEDDANAPALLI and D. FRANCIS, Makromolek. Chem. 55: 74, 1962 34. D. FRASER, R. HALL and J. RAUM, J. Appl. Chem. 7: 676, 1957 35. F. BIL'MEIER, Vvedenie v khimiyu i tekhnologiyu polimerov (Introduction to the Chemistry and Technology of Polymers). Izd. inostr, lit., 1953 36. A. A. VANSHEIDT, Prom-st organich, khimii 3: 385, 1937 37. M. I. SHJNG, O. S. M A ~ A , O. A. MOCHALOVA, V. P. PSHENITSYNA and I. Ya. SLONIM, Vysokomol. soyed. A l l : 1943, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2215, 1969) 38. J. WOODBREY, H. HIGGINBOTTOM and H. CULBERTSON, J. Polymer Sci. A3: 1079, 1965 :~9. W. BROCK_MANN, H. B R O C ~ N and H. BUDZIKIEWICZ, Kautschuk und Gummi 21: 679, 1968 40. Z. KATOVIC, J. Appl. Polymer Sei. 11: 85, 94, 1967 41. H. HOYT, H. KEUCHEL and R. DEAN, P a i n t and Varnish Product 48: 33, 1958 42. Kh. I(JIORIUTI, Koge kagaku dzassi 63: 1651, 1960 43. Kh. KHORI-UTI, Koge kagaku dzassi 66: 139, 1963; 14ZhKhim, l l C I 2 8 , 1964 44. Kh. KHORIUTI, Koge kagaku dzassi 66: 145, 1963; RZhKhim., 19C137, 1964 45. V. V. KORSILa,K and S. V. VINOGRADOVA,,Ravnovesnaya polikondensatsiya (Equilibrium Polyeondensation). Izd. "Nauka", 1968 46. K. ELLIS, K h i m i y a sinteticheskikh stool (Chemistry of Synthetic Resins). ONTI, Vol. 1, issue l, 1938