-Unsaturated scale production. Eager and watchful eyes are beamed on this development, and there are almost as many opinions regarding its probable course as there are eyes. Production methods certainly need improvement. It is generally agreed that there is a place for reinforced polyesters in the automotive field. It remains to be seen where it will be-whether it is best suited for prototype and sports models only or whether larger scale production will be economically realized. If in this development top consideration is given to the unique properties of reinforced polyesters (strength weight ratio, nondenting properties, noncorrosiveness, and ability to be formed a t low working pressures), the right answer will certainly be found quickly.
Polyesters-
Again, all these parts in these three fields lean heavily on characteristic A to make them practical as this enables production of large shapes at lowpressures* I n any consideration of applications, numerous flat sheet materials made by standard press methods must be included; these are finding wide utility in electrical and mechanical products. A180 growing rapidly are the alkyd molding resins and various reinforced “premix” compounds. RECEIVED for review December 7, 1953.
.kCCEPTED
February 3 , 1984.
Physical Properties of Polyester Resins EARL E. PARKER AND E. W. MOFFETT Pittsburgh Plate Glass Co., 235 E. Pittsburgh Ave., Milwaukee 1, Wis.
T h e ratio of unsaturated to saturated dibasic acid was varied i n 10 mole % steps in four series of unsaturated polyesters. One of these was mixed with 20, 30, 40, and 50% styrene, while the others were mixed with 30% styrene. Flexural strength, modulus in flexure, tensile strength, modulus in tension, elongation, water absorption, and heat distortion points were determined on castings prepared from each mixture. The results are discussed.
C
OMMERCIAL interest in the chemistry of linear polyesters was initiated by the work of Carothers. The subsequent development of one branch of this field resulted in the present unsaturated polyester resin industry. Commercial polyester resins usually contain three essential components: polyester, monomer, and inhibitor. Styrene is the most common commercially used monomer and was used exclusively in this work. An inhibitor is necessary in a polyester-styrene resin in order to provide storage life of the product before it is catalyzed and working life of the liquid resin after it is catalyzed. Hydroquinone was used for this purpose in the resins used in this work. The polyester usually constitutes 50 to 85% of a commercial polyester resin. These polyesters are prepared by esterifying one or more dibasic acids with a glycol. One of the dibasic acids must be ethylenically unsaturated and is commonly introduced as maleic anhydride or fumaric acid. A variety of ethylenically saturated acids is used, but phthalic anhydride is the most common. Ethylene, diethylene, and propylene glycols are the most frequently used glycols. The following equation shows the reaction between maleic acid and ethylene glycol to produce the simplest unsaturated polyester, ethylene maleate HOOC-CH=CH-COOH
+ HOCHzCHzOH
H( OCHZ-CHsOOC-CH=CH-CO)x
+
OH
Polyesters of this type are conveniently prepared with a molecular weight of approximately 800 to 2000 representing some 6 to 15 repeating units. Commercial polyester resins are usually sold in the form of sirups, These are converted to the solid form by dissolving a peroxide-type catalyst in the resin and heating the solution for a short time. Combinations exist that will cure in a few hours a t room temperature while others cure in as little a8 ’/z minute a t 110’ C. The chemical reaction that takes place involves a reaction between the double bonds in the unsaturated polyester August 1954
and the vinyl group in the monomer resulting in the polyester becoming cross-linked in a number of places. There is a fairly strong tendency for an alternating-type copolymer to form, but a large excess of styrene can be consumed without the formation of any appreciable amount of polystyrene. This indicates that several monomer units may be consumed in cross-linking two polyester molecules. This paper deals primarily with the relationship between the physical properties of the cured resin and the chemical composition of the uncured material. Three types of variation were studied: 1. The degree of unsaturation of the polyester 2. The ratio of polyester to styrene in the resin 3. The chemical components of the polyester Four series of polyesters were used to study these variables 1. 2. 3. 4.
Propylene maleate phthalate Propylene fumarate phthalate Propylene maleate adipate Diethylene fumarate phthalate
The ratio of ethylenically unsaturated to saturated acid was ’ steps, so that there were 10 numbers of each varied in 10 mole % series. An attempt was made to prepare all of the polyesters so that they had approximately the same viscosity and, therefore, roughly equal molecular weights. Each member of the propylene maleate phthalate series was mixed with 20, 80, 40, and 507, styrene in order to obtain the effect of varying the quantity of styrene as well as the degree of unsaturation of the polyester. EXPERIMENTAL
The polyesters were prepared in 5-liter three-necked flasks equipped with a thermometer, an inlet tube for inert gas, and a device for azeotropic removal of water. I n most cases 20 moles of acid (or anhydride) and 22 moles of glycol were refluxed with xylene as a solvent until the acid number was reduced to approxi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1615
mately 50. At this point a vigorous stream of inert gas was introduced t o remove the xylene and t o increase the molecular weight of the polyester until a 60% solution in ethyl Cellosolve had a viscoslty of H-J on the Gardner-Holdt scale. This was done in an attempt t o obtain polyesters of roughly equal molecular weights. A11 polyesters were inhibited with 0.02% by weight of hydroquinone based on the weight of the charge exclusive of solvent.
able in most cases, no serious attempt was made to obtain the highest possible values from a given combination. EFFECT OF VARYING AMOUNTS OF STYRENE AND DEGREE OF UIVSATUR4TION ON PHYSIC4L PROPERTIES OF RESINS
Exothenn. The term exotherm, as used here, is defined am the highest temperature that is reached when a standard size test tube containing a standard volume of catalyzed resin is placed in a constant temperature water bath a t 180' F. Figure 1 shows the type of time versus temperature curve that is obtained, The exotherms of the various propylene maleate phthalate resins are shown in Figure 3. At high levels of maleic unsaturation, the exotherm is higher as the styrene content increases. As the mole per cent of maleic is decreased, however, the resins containing 50% of styrene are diluted by the excess of styrene and their exotherm becomes lower than those containing 40% styrene and 30% styrene. Although it might be expected that the highest exotherm would be obtained with equimolar quantities of styrene and maleic unsaturation, in practice the highest values are obtained when more than the theoretical amount of styrene is present. For example, the theoretically equivalent amount of styrene a t 50 mole 7,maleic anhydride in this series ia roughly 207,, but the exotherm for the 30%, 407,, and 50% styrene resins are all higher than that of the resin containing 20% styrene. This is another bit of evidence that more than 1 mole of styrene may be consumed in reacting with 1 mole of maleic unsaturation. Flexural Strength. Castings from resins containing high or low amounts of maleic anhydride are quite brittle and their flexural strengths (Figure 2) are relatively 101~. Higher values for flexural strength are obtained between these extremes. The flexural strength is higher as the quantity of styrene decreases except in the resins containing low unsaturation. Here the situation is reversed and the flexural strength is higher as the quantity of styrene increases. hIodulus in flexure (Figure 2 ) behaves in a manner similar to flexural strength in that higher values are obtained on resins containing intermediate amounts of unsaturation. The values also tend to increase as the quantity of styrene decreases, but here the results were not as clear cut aa the flexural strength data. Tensile Strength. The highest tensile (Figure 2) values were obtained on resins containing the most styrene. However, fairly high values were exhibited over a broader range as the quantity of styrene was decreased. High tensile values are obtained when one component is in large excess. At 1 0 0 ~ omaleic the polyester unsaturation is in the largest excess in the case of the resin with 207, styrene, and it has the highest tensile strength.
4 5
1
POLYESTER RESIN FXOTPERM CL'9LE
T UE
Ih V I N U T E S
Figure 1. Polyester Resin Time versus Temperature Curve
The finished polyesters were partially cooled, blended with styrene, and cooled t o room temperature. Castings were prepared by catalyzins the resins with 1 % benzoyl peroxide, heating the mixture at 130 F. until gelation took place, and then curing 1 hour at 170" F. and one hour a t 250' F. This system worked well considering the wide variety of resins that was used. The exotherm test represents the highest temperature reached in a 15 X 125 mm. test tube '/z full of catalyzed resin when it is placed in a water bath a t 180' F. The temperatures were measured with a Brown Instrument Co. recording potentiometer (No. 153XllP-X-29) using an iron-constantan needle-type thermocouple, The other tests were run by standard ASTM procedures. DISCUSSION
The physical properties that were determined on the resins and castings are discussed separately. The values shown in Figures 2-5 were taken from the average results on five specimens from each of two experiments. While they are believed to be compar-
Figure 2.
Effect of Varying Amounts of Styrene and Degrees of Unsaturation on Tensile Strengbh, Flexural Strength, ModuIus in Tension, and Modulus in Flexure of Polyesters A 50% Styrene f 3 40% Styrene X 3Oyo Styrene 0 20% Styrene
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Val. 46, No. 8
-Unsaturated
The large excess of styrene present in the ca6e of the 50% styrene series results in high values a t 30 to 40 mole yomaleic anhydride. This peak occurs at 40 t o 50 mole % maleic anhydride in the 40% styrene series. Modulus In Tension. A low percentage of styrene favors a high modulus (Figure 2) except in the region where the unsaturation is quite low. In this region the large excess of styrene is beneficial, and the resins that are highest in styrene have higher moduli than the others. Elongation. Elongation (Figure 3) and tensile strength are closely related. The highest values were obtained a t low maleic concentrations with the 50% styrene series. At high maleic concentrations the resin containing the lowest amounts of styrene gave the highest values. Heat Distortion Point. This value is a measure of crosslinking density and high values are obtained with polyesters that have high unsaturation. Some of the values shown in Figure 3 could be increased substantially, if the castings were postcured a t a higher temperature. The data show that low values are obtained with 20y0styrene, and that the 40% and 50% styrene containing resins tend to give higher values than the resins with 30% styrene.
mu
I s>
I I I I I VOL'; i.&*Tu&TEE~cl?
8,
Figure 4.
I
Polyesters-
Water Absorption. The lowest water absorption (Figure 3) was found in the region of 20 to 30 mole % maleic anhydride. This appears to be due primarily to the beneficial effect of phthalic anhydride, a t least to the point where the cross-linking density of the resin becomes very low. The effect of the amount of styrene is not great, but the lowest values were obtained with the lowest amount of styrene in the resins that contained over 40 mole % maleic anhydride in the polyester. When the polyesters contained less than 40 mole % maleic anhydride, the resins containing high amounts of styrene tended to provide slightly lower values. EFFECT OF CHEMICAL COMPONENTS ON PHYSICAL PROPERTIES OF POLYESTER
In this series the styrene concentration was held constant a t
30%, and the effect of varying the individual components of the polyester was studied. Thus, there is a comparison between fumaric acid and maleic anhydride, phthalic anhydride and adipic acid, and propylene glycol and diethylene glycol. Exotherm. The propylene maleate adipate series (Figure 5 ) shows a lower exotherm than the others, and the propylene
I
' Effect of Chemical Components of Polyesters on Tensile Strength, Flexural Strength, Modulus in Tension, and Modulus in Flexure Io
A Diethylene fumarate phthalate I I Propylene maleate adipate
X Propylene fumarate phthalate 0 Propylene maleate phthalate
August 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1617
Figure 5.
Effect of Chemical Components of Polyesters on Elongation, Water Absorption, Heat Distortion, and Exotherin h Diethylene fumarate phthalate Propylene maleate adipate X Propylene fumarate phthalate 0 Propylene maleate phthalate
maleate phthalate and propylene EuInarale phthalate series are fairly close together. However, the diethylene fumarate phthalate series shows higher values than might be expected. Although, on the basis of a higher molecular weight per unsaturated acid unit, this series should liberate somewhat less heat than the propylene glycol resins, the heat is liberated more rapidly and this results in approximately the same temperatures for the three series. The reason for this is not Tyell understood, hut one practical result i s that such resins cure very rapidly. Flexural Strength. The propylene maleat'e series has a some\That higher flexural strength (Figure 4) tlian the corresponding fumarate series a t most levels of unsaturation. The diethylene fumarate phthalate series is higher than the others a t high levels of unsaturation and lower than the propylene glycol resins at' the lower levels of unsaturation. The propylene maleate atiipate series tends to be flexible as the unsaturat,ion is reduced and, thus, has flexural strengths that are much lower than the others. The propylene fumarate phthalate series has a higher modulus in flexure (Figure 4 ) a t high levels of unsaturat'ion than the maleate series. At low levels it is more flexible and has a lower modulus. The diethylene fumarate phthalate and the propylene maleate adipate series show the plast'icixing effect of the diethylene glycol and the adipic acid and, thus, have much lower moduli. Tensile Strength. The propylene maleate phthalate series has a higher tensile strength (Figure 4) than the corresponding fumarate resins except a t very Ion- levels of unsaturation. This is quite similar to the situation that exists with flexural strength. The diethylene fumarate phthalat,e resins have considerably higher tensile strengths than the propylene fumarate phthalates except at low levels of unsaturation. Adipic acid again shom up poorly as compared with pht'halic acid. Modulus In Tension. Although the propylene maleate phthalate resins have a higher tensile strength, the propylene fumarate phthalates have higher moduli in tension (Figure 4) in all cases. The diethylene fumarate phthalates have relatively low moduli in spite of their high tensile strengths. The propylene nialeat,e adipates have both low tensile strengths and low moduli. Elongation. All four series have low elongation (Figure 6) at high levels of unsaturation. This is a general characteristic of
1618
polyester resins as well as most other highly cross-linked polymers. At loiv levels of unsaturation the adipic acid series shows substantial elongation, and the diethylene fumarate phtlialatc series shows even higher values. Heat Distortion. The propylene fumarate phthalate d e s lias slightly higher heat, didortion points (Figure 5 ) than tlic propylene maleate series a t all levels of unsaturation. The propylene maleate adipate series again is lower than the corresponding phthalate series. The diethylene fumarate phthalate series is also substant,ially lower than the propylene fumarate phthalate seiies. Water Absorption. The propylene maleat,e phthalate series has a slight' advantagc over the propylene fumarate phthalates in having a slightly lon-er water absorption (Figure 5 ) . The beneficial effect of phthalic is shown by the lowel water absorptions a t lorn degrees of unsaturation. The diethylene fumarate phthalates absorb slightly more water than the propylene funiamte phthalates, and this effect becomes more pronounced as the unsaturation becomes very low. The propylene maleate adipate series absorbs more water t>hanthe other series, and this effect becomes decidedly more pronounccd as the quantity of adipic wid increases. CONCLUSIONS
The highest flexural strength was obt'ained nith propylene maleate (40%) phthalate (60%) with 20% styrene; the highest tensile strength was obtained IT-ith the diethylene fumarate phthalate series a t 30(1, st,grene; the highest heat distortion points were obtained m-ith resins coiitaining 100 mole yo unsaturated acid and the 1on;est nater absorpt'ion values m r e obtaiiml at 20 mole $c unsaturated acids. There are many important properties of polyester resins ot,her than t,hose enumemted here that are best obtained with still other compositions. Thus, a situation exists in which no one polyester resin or eren a small group of polyester resins can possibly hope to fill adequately all of the commercial needs. The selection of the best compositions for each application remains an important assignment for the chenikt in this field. RECEIYCDfor review October 9. 1G.53.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCJCPTEDklaicii 2 5 , l Q 5 4 ,
Vol. 46, No. 8
