REINFORCED PLASTICS SYMPOSIUM
UNSATURATED POLYESTERS EARL E. PARKER N e w light is cast on th effects of side reactions .f unsaturated acid or acid anbdride and glycol
constituents on the end properties .f unsaturated polrester resins
E. Parker is Senior Research Associate at the Research and Deuelopmazt Iabmatmies of Pittsburgh Plate GIass Co.'s Coatings and Resins Division. He is widely experienced and published in several areas of the polyestersjeld. AUTHOR Earl
he simplest commercial polyester resins contain an T unsaturated polyester, a monomer, and an inhibitor system. Before these are converted into the end product, a free-radical catalyst and any of the following agents may be added: fillers, reinforcements, pigments, dyes, thixotropic additives, flame-retardant additives, lubricants, promoters, accelerators. This paper discusses the chemistry of unsaturated polyester as it affects the properties of the final product. These properties are affected not only by the chemical composition of a raw material charge but also by side reactions that occur during the polyesterification reaction. The most important reactions other than polyesterification are as follows: -Isomerization of maleate to fumarate -Addition of glycol to maleate and fumarate double bonds -Oxidative destruction of double bonds -Lou of glycol
I
4
i'2
imm Figwe 1. Possible fumarate content as predicted by the product of &grm of double-bond retention, and the &gree of iSanm'mtian is shown as a function of temperature f m two pred'ction metho&
Figure 2. Chic# of glycol is i y i w c d by the pcxwal strength of the end pmfuct polyestg. For dtxerent unroturakd add concentrations, difereni glycolsprode difereni paural strengths
80
70
50
50
UI 30 20
uwmm NIOWErmm
IO
F i p e 3. The effect of glycol choice on modulus i n p e w e is shown with the limiting thresholds of unsaturated acid concentration, below which the modulus drops off com'hably exceptf m butrmadiol-?,3 V O L 5 8 NO. 4
A P R I L 1 9 6 6 53
ItIt:.-: --
y
Y
110
riw
QO
,.b..(
I M l H t YUtAlE fKIwAlATE
~..""'AlUlAWtDlOl
100 90
...
ai
-0MliIEW4l€?"U4li 1.3 WlUlE fUlHAU11
70 60 62 S 40 30 20 UNSANUI~D 100 you t i m
80
I"
Figwe 5. Hcut distwtion point is consisteatly higher f w polyestns based on the propylene glycol even at low unratwatc con-
Figurs 4. Diethylm glycol promdcs for higher 1m.b strength in poly&z~s except for low unrnhrrated acid concentrations
Figwe 6. Water absorption is lowcst for butanediol-l,3 and highest for diethylene glycol
cdmtrntiar Maleak Iromeriralion
Internal fumarate double bonds are more active in copolymerization reactions than maleate double bonds. Because maleic anhydride has usually been considerably cheaper than fumaric acid, it is indeed fortunate that conventional polyesterification techniques cause the isomerization of maleate to fumarate.
-
0
I1
0
II
II
H-C-GOR-
II
-P
-RO-C-GH
II
H - G M R -
II
0
0 Maleate
Fumarate
TABLE 1.
1,4-Cyclohexanedimethanol Diethylene glycol
Z,Z-Dimethyl-l,3-propanediol (neopentyl glycol) Ethylene glycol 1,Z-Propylene glycol
Early workers studied the isomerization of maleate to fumarate by the use of infrared analysis (3),by measurements of the heat distortion temperature of cured polyester resins, and by polarographic analysis of hydrolysis products of polyesters (8). Unfortunately, none of these methods is capable of giving unequivocal results. Recently Curtis and others ( I ) studied this problem by the use of nuclear magnetic resonance. Their results are eiven in Table I which shows that the Dercentaee of fumarate increases considerably as the glycol becomes more stearically hindered. Isomerization is also increased by the presence of an aromatic dibasic acid. Curtis also showed that the rate of isomerization increased with the increasing esterification temperatures, but that changing the esterification temperatures did not affect the total amount of isomerization that occurred over the range of temperatures studied.
-
ISOMERIZATION TO MALEATE TO FUMARATE
I
X X
II
X
X
2,2,4-Trimethyl-1,3-pentanediol
X
X
X
X
X
X
X X
X
X X
Phthalic anhydride
x x x x x x x x x x x x x x x x x x x x x x x x x
Per cent famarate
95 96 71 72 65 94 84 85 82 52
Maleic anhydride Isophthalic anhydride
54
INDUSTRIAL A N D ENGINEERING CHEMISTRY
93 75 . 5 0 50 33
10.0
a
aI
Figure 7. Choice of saturated acid has great beming on the pcxwal shength of the end product matwatedpolyeskr matmil
Figure 8. Modulus inj4emre of maturated polyesters based on phthalic anhydride is conriitdly higher than for o h saturated acid bases
Glycol Addition
Felici and co-workers (2) have studied the disappearance of unsaturation from polyesterification reactions using polarographic methods of analysis. This work showed that about 15% of the unsaturation was destroyed by addition of glycol to the double bond in the case of a formulation containing maleic anhydride and about 10% of the unsaturation was destroyed in a formulation containing fumaric acid. It was also shown that the rate of destruction was considerably slower in the case of fumaric acid. A further loss of double bonds occurred when any oxygen was present.
0
H H O
I1 I I II
ROH+HO-C-CbC-C-OH+ O H H H-C-C-OH
0
I1 I I I1 I I OR H
Turunen (7)has shown in a study of model compounds that this loss of unsaturation follows a reaction mechanism that appears to be a free-radial polymerization with molecular oxygen as the initiator. The reaction products identified by Turunen were mainly dimers, and it was found that the reaction could be retarded by the addition of polymerization inhibitors. Because both the isomerization of maleate to fumarate and the destruction of unsaturation are favored by higher temperatures, it follows that there should be an optimum temperature at which the highest amount of fumarate would be obtained. Studies to determine an optimal temperature have been conducted on a polyester consisting of 1 mole of maleic anhydride, 1 mole of phthalic anhydride, and 2 moles of diethylene glycol. The degree of double-bond retention was measured both by
Figure 9. Concmbotion of mduroted wid in the end product is on essential factor in selecting saturated Ocid for polyesterr
nuclear magnetic resonance (NMR) and by a bromine number determination. The degree of isomerization was measured by NMR. The product of these two values gives a percentage of the possible fumarate content (Figure 1). Although the fumarate content obtained by NMR methods is in considerable disagreement with that obtained by the bromine number method, both experiments showed that the optimum temperature for this particular polyester is about 210' C. At lower temperatures too little of the maleate is isomerized to fumarate, and at higher temperatures too much of the fumarate is destroyed by side reactions. There has been considerable discussion in the literature as to whether polyesters prepared from maleic anhydride and fumaric acid are the same or different. I n 1954, Parker and Moffett (6) compared the physical properties of propylene maleate phthalate and propylene fumarate phthalate and found that in all instances the physical properties studied were slightly different. On the other hand, a number of workers studied the chemical composition of the polyesters and failed to find a detectable difference in composition. On the basis of this, Hayes, Read, and Vaughn (4) concluded that the differences observed by Parker and Moffett (6) were due to a different arrangement of the molecules along the chain. However, more recent work indicates that the chemical composition of such polyesters is actually different. Not only is the isomerization of maleic anhydride to fumaric acid seldom 100% but, because of the greater reactivity of the cis double bond, polyesters prepared from maleic anhydride have a substantially higher degree of branching and a lower degree of unsaturation. Thus, it now seems clear that the differences in physical properties found in the two types of polyester resins are due, at least in part, to actual differences in the chemical composition of the polyesters themselves. VOL 5 8
NO. 4
APRIL 1 9 6 6
55
i w m BO
70 60 50 40 30 MlTUUllD ACID MOLE m N I
m io
Figure 70. FWhaItc anhydride p i o d e s fm the highest hat distwtion point in unsaturated polyesters 41 all unsaturated acid conc&aliotu
ioom BO
70 M 50 40 30 UYIINUlED ACID YOU M E m
Figure 77. Both diglycolic and d ) i c m'ds fm rather poor water absorption properlies in unsahuatedpolyester malm'a&
@or&
Olycol Loss by Dislill@ion
The loas of glycols by distillation from the polyesterification reaction can largely be avoided by the proper design of the overhead equipment. HGever, a number of chemical side reactions must be considered by the resin formulator. Propylene glycol decomposes in the presence of acid to a mixture of cyclic ethers. This material may appear as a low boiling oily layer in the water distilled from the reaction. Butanediol-1,4 decomposes into tetrahydm furan in the presence of acid. 2,2,4-Trimethyl pentanediol-1,3 undergoes an acid-catalyzed decomposition into a complex mixture of low boiling products. Reactions such as these, along with the addition of glycols to the double bonds of maleic anhydride and fumaric acid, account for the fact that polyesters are normaUy formu-
-
m io
Figwe 72. At high bwls of unsaturated m2 cowenIration, diethylene maleate shows higher pexurul shengths than two otherpolyesters
lated with a small excess of glycol over the stoichiometrically required amount.
~mstof composition An Proparties The physical properties of polyester resins are affected to a great extent by the chemical composition of the polyester. This relationship is shown in a series of graphs in which cornpition is plotted against five different physical properties. In all cases, a mixture of 70 parts of polyester and 30 parts of styrene was used. Castings were prepared between glass plates and were gelled at 140' F. and then heated for 1 hour at 170' and 1hourat250" F. The catalyst was 1% benzoylperoxide. Glycol Comparison. In Figures 2 through 6, propylene glycol, diethylene glycol, and butanediol-1,3 are compared with each other in glycol maleate phthal-
DIflUI1INE YAlEAll M10KfNDAlE
3.-- -0 DlElHlltHE Y A l t A l t F H l U A U l l
A......-.A
DIETHILINE NYARAlE VHTULLP'
13
Figure 73. D i ~ t ~ l mmleate c chlornulnte Figure 74. Brittlems of chlorendic m'd poduces a rmtarkobly p a t modulus in p c ~ c polyester resins showad up in t m ' l c tests of th curve a( reuealed in recmt labmatmy tcsb three polyesters under laboralmy inwsfigation 56
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
Figure 75. Markedly higher heat distortion poi& of chlorcndic m'dpolycsters 4/50 showed Up in laboratmy studies of the three resitu
gure 16. Rdariualy w water absorption by
lmmdic acid polyes& rim is evident from the its of the three rerim
ate systems. The flexural strengths of the diethylene glycol series are higher than the others at high levels of unsaturation. On the other hand, butanediol-1,3 is better than the other two glycols at low levels of unsaturation. The flexural moduli show that the propylene glycol series is stiffer than the butanediol-1,3 series and the butanediol-1,3 series is stiffer than the diethylene glycol series. The highest tensile strengths are shown by the diethylene glycol series followed by the butanediol-i,3 series and then the propylene series. The heat distortion points also follow the expected pattern. The water absorption of the diethylene glycol series is poorer than the propylene glycol series, while the water absorption of the butanediol-1,3 series is lower than either of the other two. Saturated Acid Comparison. In the next series of graphs (Figures 7 through l l ) , phthalic acid, adipic acid, and diglycolic acid are compared in the same manner. As might be expected, the flexural strengths
follow the order of phthalic > diglycolic > adipic. The flexural moduli and the tensile strengths follow the same pattern. The heat distortion point curve for diglycolic acid is well helow that of adipic acid which might not be expected. Both diglycolic acid and the adipic acid lead to rather poor water absorption results, as shown in Figure 11. Unsaturated Polyester Comparison. In the next series (Figures 12 through 16), diethylene maleate chlorendate is compared with diethylene maleate phthalate and diethylene fumarate phthalate. As might be expected, the curve for diethylene maleate phthalate shows a higher flexural strength than the curve for diethylene fumarate phthalate at high levels of unsaturation. Because diethylene glycol cause rather low isomerization of maleate to fumarate, much of the unsaturation of the diethylene maleate phthalate series is not available for cross-linking and, consequently, these resins are less brittle than the more highly crosslinked diethylene fumarate phthalate series. The diethylene maleate chlorendate series shows substantially higher flexural strengths at low levels of unsaturation than the diethylene maleate phthalate or the diethylene fumarate phthalate series. The higher fumarate content of the diethylene fumarate phthalate series shows up in the modulus in flexure curves. The diethylene maleate chlorendate series produces a remarkably flat modulus in flexure curve. Because chlorendic acid produces rather brittle polyester resins, it is not surprising that the tensile strength of this series is substantially lower than that of the diethylene maleate phthalate and diethylene fumarate phthalate series. The incomplete isomerization of maleate to fumarate is again demonstrated in the tensile strength curves. The water absorption curves show that chlorendic acid is considerably more hydrophobic than phthalic. The heat distortion point curves for this set of resins are very interesting.
M
15
!-lo
I/
3
Figure 17. With 30% styraw monomer conrcnt, thepexwal strength of the three rerim stdied proved to be about
the same
C'
Figure 18. With 30% styrcw monomer content, the isophthalatc polyester proved to have a considnably h i g h modulus inflexwe
Figure 19. With 30% styrene monom# contenf, the efecl of salurated acid choice on t m ' l c sfrengthof andproduct polyester is minimal .VOL 5 8
NO. 4
APRIL 1966
57
s1.
u
Figure 20. With 3% slyrcne monomer mnrcnt, the w& abswption is uniformly slightly Iowm Uumfor thc o r b resins
,...!...oh&
&E
I,,&(mnaE
WUlt
Figure 21.
Tests with 20% slyrcne monomer
mnkd rmd with thc cure used, less than optimal
heat distortion results m e obtained
Diethylene fumarate phthalate has a substantially higher heat distortion point than diethylene maleate phthalate, even though the amount of styrene used is insufficient to give optimum results with respect to this property. The heat distortion point curve for diethylene maleate chlorendate is not only different from the other two curves in Figure 16, but is substantially different from all of the other resins in this entire group. This indicates that the double bond in chlorendic anhydride may be participating in the cross-linking reaction. The high flexural modulus at low degrees of maleate unsaturation might also be interpreted in thii way. E5eet of Saturated Acid. Figures 17 through 21 show the comparison between propylene maleate phthalate, propylene maleate &phthalate, and propylene maleate terephthalate. The flexural strengths for all three phthalic isomers are about the same at 30% styrene as shown in Figure 17, but at higher levels of styrene the isophthalic series has a clear advantage. The flexural modulus of the isophthalic acid series is highest at all levels of styrene from 20 to 50%. The tensile strengths of the isophthalic series are higher than the others and the phthalic series is the poorest in thii case.
Water absorptions of the isophthalics are uniformly slightly lower than those of the others. In the case of the heat distortion temperatures, the cure used in this work does not produce optimum values. However, postcuring only serves to increase the advantage of the isophthalic and the terephthalic series over the phthalic series. Flame Retardants. If all other factors could be equalized, it would be highly desirable that all polyester resins be nonburning. Although a tremendous amount of research effort has been expended in this area, fireretardant resins that have been marketed up to now have always been more expensive than their nonfire-retardant counterparts and, furthermore, they have usually been inferior in one or more of their properties. The mast flame-resistant unfilled cast resins contain tetrabromo58
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Figure 22,
rnmkcdgect of chlorcndic
mz
com8dration On tha pme rcsisroncc of unsafurotcdpolyester rain is shown
phthalic anhydride along with a small amount of a phosphorus compound. Because tetrabromophthalic anhydride has only recently become commercially available, most of the flame-resistant resins that have been sold in past years have contained chlorine compounds. The best of these is probably chlorendic acid. Figure 22 shows the effect of the chlorendic acid content on flame resistance for a diethylene maleate chlorendate series containing 25% styrene. The flame test used here was ASTM D-635-63, which is relatively mild. Normally, a clear casting must contain from 25 to 27% chlorine before it will p a s a fairly mild flame resistance test and be rated as self-extinguishing. More severe tests on castings require either more chlorine or synergistic quantities of a phosphorus compound to be classed as self-extinguishing. In filled compositions, antimony oxide has been widely used as a synergist. Formulations containing as little as 10% of bromine as tetrabromophthalic acid are probably more flame-resistant than resins containing 25% chlorine. All of the halogen-containing resins have poorer light stability than their nonhalogen-containing counterparts. The requirement of a minimum of 25% of chlorine in this type of resin severely limits the flexibility of formulation that is normally enjoyed with nonfireresistant resins. These restrictions, along with considerably higher cost, have up to now greatly restricted the utility of chlorine-containing, flame-resistant p l y ester resins. BIBLIOGRAPHY (1) Curb, L. C., Edward*, L. D., Smonda, R. M.,Trent, P. I., Van Bramu, P. T., END. END. Cmar. Paoouo~Rsaabacr DEVELOP. 9,218-21 (1964). (2) PeM, M.,Mmcphiai, C., &cotti, E., Sbrolli, w., C . m . Zd. (Mdd 43, 169-72 (1963). (3) Gordon, M., Gricvcrmn, B. M.,McMillao, I. D., J . Poly- Sa'. 18, 497-514 (1955). (4) Hayu, B. T., Rcad, W. J., Vaughn, L. H.,Chm. e d . ( L a d 4 1957. pp. 1162-70. (5) Parker, E. E.,Md. PlmtiwS6, 135 (1959). (6) Parker, E. E.,Mouctt, E. W.,I m . EM. C ~ E Y 46, . 1615-18 (1954). (7) Turunc.4 L. I., Iwo. Ewc. C ~ a uPaooucr . Rma~am Dava~a~. I, 40-5 (1962). (8) V-bS=mcr&yi, MararOd&a, K., Maby-BMi E., J . P d p n Sti. 5% 2 4 1 4 8 (1961).
