Polymer Distribution of Varnish Resins H. E. ADAMS AND P. 0. POWERS,Armstrong Cork Company, laneaster, Penna. An approximate estimate of the distribution of polymers in varnish resins may he made by preeipiteting the resin from dilutesolutionwithm.arured amounts of a nonsolvent, while determining the decrease in light transmisaion t h u g h the medium. This titration is reasonably rapid and can be carried out in 20 minutes. This method bas h e n used with hydrocarbon resins, rosinmodified phenolic and malaic resins, and pure phenolic resins using toluene as the solvent and methanol or hexane as the nonsolvent. The titration curves ohtained in this maonen m a y be used to ulmpare resins of the same type. To estimate polymer distribution from these cum=, the eoncenaationof nonsolvent at whkh carefuuy separated fractions of a resin are pmeipitated must be determined.
tion. Thua the distribution of polymers in a resin may be estimated if the transmission during precipits,tion and the cancentration of nonsolvent required to precipitate a polymer are known. Even where the polymer-nonsolventooneentration relationship has not been established, the results of precipitation are valuable in comparing resins of the same class.
Apparatus and Method The titration was conducted in a square pint jar which had been painted b l x k except for two windows an o posite sidea about 2.5 x 7.5 om. (1 X 3 inches).' Light entmeithrough one window from an automobile head Lamp with a parabolic reflector, using B &volt storage battery as B iouree of-current. The m fleetor was covered except for a small window laced in front of the titration jar (Figure 1). The photronio o d beston No. 694)
T
HE distribution of polymers in commercial resins used in the manufacture of paints and varnishes has never been reported. Since these resins are considerably lower in molecular weight than the reens used BS plastios whose polymer distribution has in m m y caees been established, the determination might be expected to be rsther Bimpler. However, the polymers in commercial resins are often built up fmm eeveral daerent types of molecules. This results in B greater complexity in the types of polymem formed. Determination of the cloud point (#) of varnish resins in many instance giv, a nseful index of the solubility and hence the degree of polymerization of a resin. It has been found in some oases, however, that two resins of the same type and of the same cloud point &re not identical in all their properties. This di5erence is probably due to a di5erence in the distribution of the polymers in the two resins. It was desired to develop a method that would quickly establish such differences. Polymer distribution has often been effected by fractional precipitation of the resin from solution. The fractions are separated and their propertiea determined. This method is timeconsuming and while i t has been used in these laboratories, it is much too slow for mutine examination of samples of resins. Nephelometric methods have been used widely to measure the amount of a precipitate. McNally (1) has described a method for messurmg the trs&ion of light through a solution of B synthetic resin during precipitation hy a nonsolvent. A photronic cell was used to measure the transmission, which wried inversely with the m o u n t of resin precipitated. Elchub (3') hae shown that the m o u n t of nonsolvent required to precipitate a polymer is B memure of its degree of polymeriza-
FIQWB1. TITRATION Appmm '111
712
INDUSTRIAL AND ENGINEERING CHEMISTRY
t,iometer WBR wire-wound and had a spread of about 180'. A dial reading 0 to 100 was used to meawre the position of the null poirit. Resist.ances Rl to R' were carbon resistors. The values arc those actually used, hut are not critical. These resistors make it possiblc to titrate solutions of dnrk-colored resins, since chnnKeE in transmission at low intensities can be measured if a high resistance is used.
Vol. 15, No. 12
reading juat before precipitation started waa taken aa a measure of the initial t,ransmiasion, 1 0 . Small amounts of precipitant were added and the transmission, I, was determined after each addition. Decrease in transmission occurs rapidly at first and the titration is complete when further addition of precipitant does not further decrease the transmission.
It might be expected that transmission would increase by dilution, but generally such increase was not observed. The appar% tus and method wcre checked by precipitating inorganic salts and it was found the extinction, -log ]/IO, measured the amount of the precipitate at low concentration.
Titration of Varnish Resins
FIGURB 2. PHOTRONIC CELLCIRCUIT
To establish the range of prccipitation and conditions for titration, a varicty of varnish resins was dissolved in toluene (0.5 pcr cent solution) and precipitated by the addition of methanol. Figure 3 shows that an cstimate of polymer distribution is possible for several types of varnish resins. H1, H2, and H3are coumaroneindone resins of successively decreasing molecular weight. A malcic rosin resin ( M R l ) and a roiin-modified phenolic (PRl) resin were also titrated, and the results are shown in Figure 3. These resins are typical of the less soluble varnish resins. The
Bo. 1.6 volts
Ro. IOO? o h m R I . 05; ohrlla
Plintrmio cell
C.
20
Q. Microauiueter
Rs. 6108 oh1118 R I . 9489 ohrris R,. 27,500 o h m Rg. 97.000 o h w
The solution of the resin and the nonsolvent, were brought to
25" C. .To conduct a tit,rat,ion,80 cc. of the solution of the renin, usual1 in toluene at 0.03 Iwr vent solids, were added to the jar
and t i e nonrolvent was added with stirring by Hn air-driven sgitntur. The rate of stirring must he Puffic.ient,lyprwt t u ensure complete rnisirig, but must not cwse the introduc-tion of air buhl)les into t,he solution. A nonsolvent, usually methanol, WDB added uni.il it was apparent that proripitation was nbliut to occur. The microsmmc*ter wm bnlltnwd. using the witch Petting giving the largest reding on the potentiometer scale. Small amounts of methanol were added and the circuit was again balanced. The
I-
I
0
IO
w
3
& X
FIGURE 4. EFFECT O F
30 I-
L?
g 20
s X
IO
2 50
40
4
6 P OLY M E R
10
8
60
TITRATION OF
50 X
NON
SOLVENT
75
40
50 Y NON SOLVENT
60
ANALYTICAL EDITION
December 15, 1943
results show the wide range of composition of commercial resins. Polystyrcne (PS) is also included; it is much higher in molecular weight than the other resins. The decrease in transmission is due to the resin settling out of solution; this can be overcome by using a more dilute solution. Titrations are probably more significant if run a t lower concentration. Figure 4 shows the results of titrating the same resin a t three different concentrations. It will be noted that higher concentration of nohsolvent may be required to start precipitation at low concentration. Estimates of molecular size from precipitability determinations may therefore depend on concentration of the resin, which should be constant for such determinations.
Polymer Distribution The titration curves are believed to be a sufficient basis for comparison of commercial varnish resins. However, to corrclate a titration with polymer distribution, a coumarone-indene resin was fractionated by partial solution, resulting in seven fractiona, and the molecular weight of the fractions was determined by depression of the melting point of pure benzene. The titration curves of these fractions are shown in Figure 5. The first two more soluble fractions did not precipitate on titration. The other fractions gave varying amounts of precipitate, only the last fraction being entifely precipitated, showing presence of lower polymers in these fractions. Since the results of the titration and molecular weight determinations indicated that the decamer was precipitated a t 37 per cent nonsolvent and the tetramer at 73 per cent nonsolvent, the &huh (3) equation where y*
X and
--
y* =
A
713
was used to estimate the range in which each polymer is thrown down. From the above values =
0.13
2.42 +x
The range for each polymer by this relationship is shown in Figure 6. The amount in each fraction is estimated by the differential of -log Ill0 and the values for the fractions are summed in Figure 7. A similar estimate of the distribution of the resin was made from the titration of the original resin and, as might be expected, shows a somewhat narrower distribution. It is apparent that the lower polymers are not estimated by this method. The results also suggest a mutual solubility effect of various polymers, resulting in incomplete separation. I n many cases the high polymers determine the character of the resin, and an estimate of the range and amount of these polymers afford significantinformation on the composition of a resin. To determine if titration would measure small differences in the composition of a resin, mixtures of two hydrocarbon resins of somewhat similar properties were examin'ed. It will be noted (Figure 8 ) that the curves indicate the presence of larger amounts of higher polymers as the content of the less soluble resinas in-
I
FIGURE 12.
+ B/X
concentration of nonsolvent degree of polymerization
A and B = constants 50
70
60 X N O N SOLVENT
40
50
70
60
X NON SOLVENT X NON SOLVENT
0.6
