KENNETH S. RQSAI AND SAMUEL XI. WEISBERG National Dairy Research Laboratories, Inc., Oakdale E . I . , >V.Y .
S
OME of the more pertinent properties to be considered in
the formulation of a latex paint are: viscosity stability, freeze-thaw stability, brushing and leveling, and good pigment dispersion. While all the components in a latex paint system have some effect, the stabilizer, although only a minor constituent quantitatively, appears to play a major role in helping to control these properties. To be a good stabilizing agent, a substance must not only have protective colloidal properties but must also be compatible with a variety of compounds found in a typical latex formulation. According to Melsheimer and Hoback ( 4 ) , it is not accidental that casein and soybean proteins are almoet universally used, A number of starches and soluble gums could meet the bodying requirement, but they do not provide the unique combination of dispersing and stabilizing, exhibited by casein. This paper discusses the relation of protein stabilizers t o viscosity stability and freeze-thaw stability and the effect of modification of a protein on these properties. VISCOSITY STABILITY
S'iscosity stability is a function of a number of variables, which include the mechanical stability of the latex (a), and the chemical and physical forces between the numerous components in a latex paint. One of the major reactions that can occur between components is that between polyvalent cations and the protein. The polyvalent ions may come from the use of an extended pigment or they may occur as an impurity. Regardless of the source, the reaction can disrupt the stability of the paint unless it is controlled. hlelsheimer and Hoback ( 4 )have shown that premature precipitation of the pigment by reaction with the protein can greatly detract from its color value The latex itself can be easily destabilized by polyvalent cations ( 5 ) . Sormal proteins react very readily with polyvalent cations and this causes a large increase in viscosity and, in some instances, precipitation of the protein. If the reaction occurs in a fully formulated paint, there will be a continual uncontrolled increme in viscosity of the paint and eventually the paint will become unusable. As a practical matter, calcium ions occur most often in these systems and so attention was focused on calcium. A study was therefore undertaken to determine whether modification of the protein could increase its cation compatibility. Calcium in the form of calcium sulfate was chosen as a typical cation, which could originate from both pigments and hard water. The sulfate was chosen over the carbonate because of its greater solubility, permitting a greater number of calcium ions in solution, and thus making a more severe test. Alkali dispersions of various modified proteins were prepared and tested in the following manner. One sample was held as a control while to the other was added 10% calcium sulfate hemihydrate. The viscosity of the samples was measured on a comparative basis against samples from an unmodified alkali-protein dispersion by means of a Brookfield viscometer. Calcium ions in all probability react with the carboxyl groups of the protein, causing bridges between protein particles and thus increasing the viscopity of the dispersion. The following methods of modification were therefore used in order to increase calcium ion compatibility: controlled fragmen-
tation, blocking of functional groups, heat treatment, and addition of reducing agents. COXTROLLED FRAGMENTATION. Controlled fragmentation can be accomplished by an enzymatic hydrolysis. Because the particle size after fragmentation is greatly reduced, it may be possible to hydrolyze t o such a degree that no great increase in viscosity is noted even when calcium bridging takes place. Hydrolysis also causes an increase in the number of carboxyl groups and amino groups. By increasing the number of carboxyls per particle, it may be possible to have the calcium bridging take place within particles rather than between them. This would make the protein act in a calcium-compatible manner. The increased number of amino groups could increase the sequestration power of the protein, thereby preventing calcium bridging, Casein a t a 23% concentration was solubilized by borax a t a p H of 7.7. This dispersion was hydrolyzed with trypsin. Samples were removed every 15 minutes for 1.5 hours. The enzyme was inactivated by heat treatment, the samples were brought to 25' C., and 10% calcium sulfate hemihydrate was added to half of each sample. The other half of the sample was used as the control. Viscosities were taken on all samples. The results are recorded in Table I, after all samples were held a t 26" C. for 48 hours.
TABLEI. RELATIVEVISCOSITYOF ENZYXATICALLY DIGESTED CASEIS
(28% protein concn. with a n d without, addition of CaSO4,1/aHzO. All viscosities are comparative, are recorded in Brookfield units a t 2 5 O C. and 10 r.p.m. using No. 4 spindle) Brookfield Viscosity Digestion Time, Digested protein Digested protein Nin. (no CaSOd) 10% CaSOcl/&O 0 20 loo+ 15 10,6 18.3 4 5 30 6.5 46 2.5 6.0 2.5 60 5 0 75 2.4 2.6 90 2.4 2.4 105 2.3 2.3
+
The same procedure was repeated with alpha-soy bean protein. It may be noted from Table I that increasing the hydrolysis time decreases the viscosity of the protein but increases the calcium compatibility. After 90 minutes of digestion, no increase in viscosity is noted on addition of 10% calcium sulfate hemihydrate. OF FUNCTIONAL GROUPS. I n order to determine BLOCKIXG whether the amino groups play a part in the calcium compatibility of a protein, the amino groups of casein were blocked by the addition of formaldehyde. Formaldehyde is known to react with amino groups of proteins.
X 20% alkaline dispersion of casein was prepared. To a portion of this %-asadded 0.47, of formaldehyde (40%) based on the weight of casein. Both portions of the dispersion (with and without formaldehyde) were adjusted to pH 8.0 and divided into two samples each. One sample from each group served 8% the control, while 10% calcium sulfate hemihydrate was added to the other. Viscosities of all samples were taken after 48 hours a t room temperature. The viscosities were run a t 50" C., and are recorded in Table 11. The formaldehyde-treated sample shows a much greater viscosity increase on the addition of calcium ions than does the un-
174
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1954
COMPATIBILITY OF CASEINSMODIFIED BY TABLE 11. CALCIUM BLOCKIXG A FUNCTIONAL GROUP ( 4 s measured b y change in viscosity on the addition of CaSOc'/zHzO. All viscosities are comparative, recorded in Brookfield units, taken a t 10 r.p.m. using latge paddle, after 48 hours a t equilibrium temperature) Viscosity Temp. a t Time Control % Protein of Viscosity, (protein Protein Reagent Used in Sample c. alone) (10% CaSO4)
+
Propylene oxide None Formaldehyde None
20 20 20 20
25 25 50 50
21.1 8.3 5.1 1.2
21.0 69.1
loo+
10.8
treated casein sample. This would indicate that the amino groups play a sequestering role in the calcium compatibility of a protein. If the original theory is COrreCt, that calcium tivity is due to bridges between carboxyl groups, it should be POSsible to increase the compatibility by blocking this group. A reaction was carried out a t room temperature between a 20% casein dispersion and propylene oxide. This reaction can be properly controlled to block carboxyl groups selectively (3). After reaction, the product was adjusted to pH 8.0 and tested for calcium compatibility. The results of this test can also be found in Table 11. Table I1 indicates that no increase in viscosity occurred with this derivative on the addition of calcium eightfold viscosity rise occurred with the untreated casein control. This would indicate that the carboxyl groups of casein can act in conjunction with calcium ions to form bridges. Blocking this group will produce a calcium-compatible protein. HEAT TREATMENT. By varying the time, temperature, and pH of protein dispersions, it should be possible to achieve a controlled fragmentation of the protein, which could also possibly achieve calcium compatibility. A fourth variable was addedchoice of cation to achieve dispersion of the protein. Casein dispersions (20%) were prepared by using potassium and sodium hydroxides. ~h~~~ dispersions m,ere adjusted to p ~ ' 7s , 8, and 9. Samples mere heabtreated a t 1500 and 1 7 5 O F, for various lengths of time. At pH's 7 and 8 regardless of the temperature or time of heating, no increase in calcium compatibility was noted. The results for pH 9 are tabulated in Table 111.
775
heating was increased. At 175" F. a slight increase in compatibility was noted after 30 minutes. These results indicate ammonium ions to be superior to potassium ions in attaining a calcium-compatible casein. However, the increased compatibility achieved by heat and ammonium ions is rather poor in comparison to enzymatic hydrolysis and propylene oxide treatment. To determine whether the difference in compatibility apparently due to ion types can also be noted in a different type of heating, the following experiment was carried out. Commercially prepared spray-dried potassium and ammonium caseinates were tested for calcium compatibility by preparing 20% dispersions, adjusting to pH 8.0, and adding 10% calcium sulfate hemihydrate to a portion of each caseinate. The viscosities of all samples are recorded in Table IV. The viscosity data in Table IV indicate that the spray-dried samples have a lower viscosity than those not spray-dried; the munonium spray-dried sample was even lower than the potassium. The results also indicate ammonium ions to be superior in achieving calcium compatibility by means of heat treatment. Excellent calcium compatibility is exhibited by the spray-dried ammonium caseinate. ON CALCIUM TABLE IV. EFFECTOF HEATTREATMENT
COMPATIBILITY
(As measured b y change in viscosity on addition of CaSOd.I/2HxO. All viscosities are comparative, recorded in Brookfield units, taken a t 10 r.p.rn. using large paddle) Temp. a t Time Viscosity % Protein of Viscosity, Control Protein (protein 10% Cas04 Reaaent Used in Sample alone)
+
c.
NHlOH
4- spray-
K ~ + H spraydry KOH
20
25
2.1
20 20 20
25
10.0 5.3 9.0
25
25
2.4
loo+ 16.2
loo+
ADDITIONOF REDUCING AGENTS. The addition of a reducing agent could convert 8-8 linkages to SH groups. This could POSRibly decrease the size of the protein particle to a point where no large increase in viscosity takes place on t'he addition of calcium ions. Although the sequestering action of the SH group is admittedly weak, it could also play a minor role.
Two very strong reducing agents, both specific for S-S bonds, were used: cysteine and sodium thioglycollate. These were added to a 23% protein dispersion (casein), allowed to react with the protein for 24 hours a t room temperature, and adjusted to p H 8.0 before any calcium ions were added to the system. minutes of heating. The viscosity data for sodium thioglycollate are recorded in With potassium ions a t 150" F. increased compatibility was Table V. The cysteine data are exactly the same. There is a noted after 15 minutes but it gradually decreased as the time of definite viscosity drop in the sample that was treated with the thioglycollate. Absolute compatibility was achieved by this modification of the casein. O F HEATTRE.4TMENT AT pH 9 ON CALCIUM TABLE 111. EFFECT COMPATIBILITY OF SOLUBILIZED CASEIN Because of the odor of mercaptans, it was (811 viscosities are comparative, recorded in Brookfield units, taken a t 10 r.p.m. using large decided to try a reducing agent, paddle, after samples were a t 25" C. for 2 hours) sodium bisulfite. This was added to a 23% casein Viscosity Temp. V i s c o s ~ dispersion and the entire mixture wap heated for Temp. Time of . of, Control Protein of. 30 minutes, a t 150" F. The react,ion product Heating, Heating, (protein + 10% Heatmg, (protein Min. F. alone) Cas04 F. alone) was cooled to 25" C. and adjusted to p H 8.2 and a calcium compatibility test was performed.
At pH 9 using ammonium ions, increasing the time of heating up to 60 minutes a t 150" and 175" F. increases the compatibility. A slight decrease in compatibility was noted after 90
o
15 30 60 90 0 15
30 60 90
150 150 150 150 150 150 150
150 150 150
CASEINSOLUBILIZED BY P\"4OH 10.0 loo+ 175 6.8 32 175 6.0 27 175 6.0 25 175 6.0 30 175
10.0 4.0 4.0 4.1
loo+
CASEINSOLUBILIZED BY KOH 9.0 loo+ 175 7.0 40 175 7.0 50 175 6.0 60 175 7.0 80 175
9.0 3.5 3.0
loo+ loo+
3'3 3.3
'loo+ O0+
4.2
30 28 18 24
48
Table V lists the results obtained on the sodium bisulfite-treated casein, and indicates that an increase in viscosity rather than a viscosity drop resulted under these conditions. However, a modified protein with good compatibility was achieved. (It is believed that sodium bisulfite is not acting as a calcium precipitant. A clear protein solution modified with sodium bisulfite gave no indication of precipitation on the addition of calcium ions.)
116
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
TABLE V. EFFECT OB ADDITION OF REDUCING AGENTSON CALCIUM COMPATIBILITY (115 measured b y change in viscosity on addition of CaSOc'/zII*O. Ali viscosities are comparative recorded in Brookfield units, taken a t 10 r.p.m. using large paddle,'after samples were a t 2h0 C. for 48 houru) Viscosity Temp. a t Time Control yo Protein of Viscosity, (protein Protein Reagent Used in Sample c. alone) 10% CaSOa
+
I n almost every case of protein modification, a substantial viscosity decrease resulted. This would indicate that an additional thickening agent would be necessary in a paint formulation where these proteins were used. The protein mould be used only for its stabilizing properties. This practice is followed by many formulators today, in that the final viscosity of the paint is adjusted by addition of a thickening agent in order to compensate for differences in thickening power exhibited by various batches of protein. It s-ould, therefore, be advantageous to use a modified protein whose viscosity contribution to a paint formulation was predictable. This would give the formulator the exact amount of thickener he needed in each paint batch. Although some of the modified proteiiis exhibiting good compatibility have a greatly decreased viscosity, it is not merely a low viscosity which accounts for the compatibility. This is clearly shown in Table 111, where samples of essentially the same viscosity do not have the same compatibility.
LATEXPAINT FORMULAS (GRAhiS)
Standard Latex Formula (Used with CaSOa-Extended Pigments) Titanox (RCHT-X) 150 0 Igepal CTA (emulsifier) 4.5 20% Alkali-dispersedprotein 16.5 Water 52.0 Dow latex 762-K (48% Bolids) 111.0 1.0 Dowicide B, preservative D-C Antifoam A 0 5 Alkyd Modified Latex Formula (Used with CaSOa-Extended Piernontss) Titanox (RCHT-X) 326,O Titanox (RA-50) ' 130.0 2.0 preservative 2.0 Alkyd resin VM-1944 96.5 Latex (43% solids) 244.0 10% KOH 14.8 A d a i r e t (wetting agent) 29.4 Protein (alkali-dispersed) 21.0 Water 183,O 24% Pb 2.0 0.8 6% Co Defoamer 1.0 Straight Latex Formula Prepared with Lorite-Extended Pigments Ti-Pure 60 00 Lorite (80% CaCOa) 40.00 Water 66,OO Protein 3.38 Water 19.12 Dow latex 762 W (48% solids) 117.00 Coned. XHa 0.50 Defoamer 2.50 0.76 preservative 0.75 Water 1.50 Method of Preparation of Paint. All paints were prepared in a pebble mill. The pigment, pigment extender, water, protein, and emulsifier (only in the firat formula) are loaded into the mill and milled for 30 minutes. The remaining ingredients, latex, defoamer, and preservative (50% solution), are added. The entire mixture is then milled an additional 30 minutes.
gz?::$1
~~~~~~
PROTEIN COMPATIBILITY IN PAIhTS
STRAIGHTLATEX WITH CaLcruzr SULFATE-EXTENDED PIGThe next step was to determine rrhether the increased calcium ion compatibility achieved by modification of the protein carries over into a formulated paint. For this purpose, a calcium-extended pigment (Titanox) was used t o ensure a maximum amount of calcium ions. The straight latex formula (used with calcium sulfate-extended pigments) is by no means intended to be a paint maker's formulation, but was merely used to gather data on viscosity stability. The viscosity data recorded are on a comparative basis taken nith a Brookfield viscometer a t 25' C. and 10 r.p.m. All paints were prepared in a pebble mill. Only the initial viscosities and the viscosity after an accelerated storage period of 40 hours at 140' F. are recorded. It is felt that this accelerated test is strenuous enough to evaluate protein derivatives under the most trying conditions. The results tabulated in Table VI indicate that the calciumcompatibility test with protein alone is a good indication in most cases of how the protein will react in a paint. I n all instances, except with the propylene oxide-modified casein, the results show good correlation. The propyl ester incorporated in a paint shows a decrease in viscosity on accelerated storage. The possibility that hydrolysis of the ester caused the viscosity drop, was given substance upon noting a drop in p H of the paint. The paints with sodium bisulfite-treated protein show stability, but only fair pigment dispersion. The paints with the potassium hydroxide-modified casein shorn some increase in viscosity upon storage, The paints made with enzyme-treated proteins gave absolute stability and excellent pigment dispersion. ALKYD-MODIFIED LATEXPAIKT WITH CALCIUM SULFATEEXTENDED PIGIIESTS.T o determine whether a difference in formulation would change the viscosity stability of a calcium-compatible protein, the two enzyme-modified proteins were tested in an alkyd-modified latex paint. An extended pigment (Titanox) was also used in this formulation. The results are tabulated in Table VII. These modified proteins had excellent stability after MENTS.
Val. 46, No. 4
j;
room temperature storage for 13 months and behaved well under accelerated storage a t 140' F. for 40 hours. I n addition, the leveling and brushability of these paints remained good. STRAIGHTLATEXPAINTWITH LORITEEXTENDED PIGXENTS. Since the main assumption mas that viscosity stability can be correlated with polyvalent ions in the paint, whether as impurities or otherwise, the straight latex formula with Lorite-extended pigments was used. Lorite is 80% calcium carbonate. The
TABLEVI. VISCOSITYSTABILITYOF PAIXTSPREPARED WITH STRAIGHT LATEXFORXULA USIXGCALCIUN SULFATE-EXTENDED PIGMENTS (Ail viscosities are comparative and were recorded in Brookfield units a t 2 Z C C. and 10 r.p.m. using large paddle) Modified Casein Employed as Stabilizer Propylene oxidemodified casein NaHSOa-modified casein KOH-modified casein Enzyme-modified casein (Shef-Tex)a Enzyme-modified alpha-soybean protien Unmodified casein Q
Initial
Viscosity Pigment Dispersion After accel. staafter Accel. Stability test, 140' F., bility Test, 140° F., 40 hr. 40 HI.
9.0
3.7
Good
2.3 3.4
2.3 9.0
Fair Fair
0.3
0.3
Excellent
0.3 Excellent Paint completely gelled Trade-mark of Sheffield Chemical Co., Inc., Yorwich, Pi. Y 0.3
3.5
-.
April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE VII.
VISCOSITY STABILITY OF PAINTSPREPARED FROM ALKYD-MODIFIED LATEXWITH EXTENDED PIGMENTS (CALCIUM SULFATE) Viscosity after Storage, K.U. Initial 3 months 6 months 9 months R o o r TEMPERATURBI STORAOE
T y p e of Protein
Shef-Tex (enzyme-modified casein) 67 66 66 66 Enzymatically modified soybean protein 67 67 69 69 ACCELERATED STORAOE, 140' F., 40 HOURS Accel. Leveling Initial Storage Color Initial 6 months Shof-Tex (Enzyme-modified casein) 67 67 Good Good Good Enzymatically modified soybean protein 67 67 Poor Good Good
13 months
66 69 Brushing Initial 6 months Good Good Good Goo&
777 protein and the unmodified protein. In most paints prepared with unmodified proteins, a sequestrant is usually added, such as potassium polyphosphate. These data indicate that with proper protein modification, no such additive may be needed for its sequestering properties. FREEZE-THAW STABILITY
Freeze-thaw stability is of great concern to both the latex manufacturer and the paint formulator. Much research has been done to achieve freeze-thaw-stable latices and latex paints. Although this property depends on many factors, it can be controlled to a substantial extent by the stabilizer. The amount of unmodified protein necessary to make a paint freeze-thawstable varies with the latex and the protein ( 7 ) . Because it depends to a great extent on the protective colloidal action of the stabilizer, it was deemed possible that a modified casein could impart better stability. The protective colloidal action of a protein varies markedly with its modification. Paints made using the straight latex formula with Loriteextended pigments were therefore compared in Table X. Cycles of 24 hours a t 0' C. followed by 24 hours at room temperature before taking viscosity data were used. Table X shows that only the paint prepared with the enzymemodified casein was able to withstand four freeze-thaw cycles without a viscosity change. The sodium bisulfite-treated casein withstood one cycle without change. All the other oaints were changed by viscosity increases, pigment settling, or coagulation.
solubility of calcium carbonate is negligible in comparison to calcium sulfate. It was, therefore, assumed that the free calcium ions occurring in such a paint system could be treated as though they were impurities. The modified proteins which showed good calcium compatibility were used as stabilizing agents in paints and were compared with a paint made with normal casein (cut with potassium hydroxide) and with a formulation containing no stabilizing agent a t all. The data recorded are only for the first 30 days of storage. These paints have continued in storage for a 10-month period. The same trends noticeable after 30 days are evident after 10 months. These viscosity values along with the accelerated storage test give a good indication of their stability. These paints were all prepared in a pebble mill. The p H of all paints was adjusted to 9.4. Tables VI11 and I X indicate the excellent stability of paints prepared from an enzyme-modified and spray-dried ammoniamodified casein. They also show that the sodium bisulfitetreated casein provides good stability in comparison to relatively poor stability of the spray-dried potassium hydroxide-modified
SUMMARY
TABLE VIII. VISCOSITY STABILITY O F P A I N T S PREPARED WITH A STRAIGHT LATEXFORMULA WITH EXTENDED PIGMENTS (LORITE) (All viscosities are recorded in centipoises and were taken with a Brookfield 25O C. using spindle 2) Brookfield Viscosity, Days Stabilizer Initial 1 5 7 10 15 Ammonia-modified casein 118 175 200 217 217 217 .. 875 1300 1375 KOH-modified casein 225 590 Enzyme-modified casein (ShefTex) 420 150 150 150 150 150 h'aHS03-modified casein 625 1000 1000 1000 1025 . 2500 3500 Unmodified casein 225 650 88 100 320 .. Control (no stabilizer)
.. ..
..
...
viscometer a t 20 217 4000
150 150 1625 1875 23250 35500
TABLEIx. VISCOSITY STABILlTY STRAIGHT LATEXFORI\fULA
WITH
O F PAINTS MADE WITH A LORITE-EXTENDED PIGMENTS
(All viscosity values are comparative a n d were recorded with a Brookfield viscometer a t 12 r.p.m. a t 25O C. using spindle 3. Test made after paints were 30 days old.) Viscosity After accelerated Nature of Stabilizer Initial storage, 140° F., 40 hours 0 8 1.8 Control (no stabilizer) Unmodified casein 22 3 Paint unusable, nonuniform gelation occurred 9 3 19.0 KON-treated casein 3.3 4 3 NaHSOs-treated casein Enzyme-treated casein 0.6 0 6 Ammonia-treated casein 0.7 0.7 TABLE
x. FREEZE-THAW STABILITY O F P U N T S AS MEASURED BY
VISCOSITY CHANGE
(Taken with a Brookfield viscometer a t 1 2 r.p.m. a t 25O C. using large paddle) Comparative Tiisro.4t,y after Number of Freeze-Thaw Cycles Stabilizer 0 1 2 3 4 Control (no stabilizer) Unmodified casein Enzyme-treated casein Spray-dried ammonium caseinate Spray-dried potassium caseinate NaHSOs-treated casein 0 Pigment settles.
0.5 6.5 3.5 2.5 5.1 20.6
0.5 0.5a 1.4" 4.6" 14.6 23.5 3.5 3.5 3:5 3:5 50 Not usable Completely coagulated 20 loof
30 217 3250
...
...
Divalent cation compatibility can be achieved for a protein by modifying the protein in accordance with the concepts outlined. This is an important attribute for the viscosity stability of a latex paint. Casein suitably modified with an enzyme, ammonia, or a reducing agent permits formulation of latex paints with unextended or calcium sulfate-extended pigments with good viscosity stability. A suitable enzyme-modified casein permits a latex paint formulation with good freeze-thaw stability. ACKNOWLEDGMENT
The authors wish to acknowledge the helpful assistance of Louis Caruso. The authors are greatly indebted to Dorothy Schroeder and Fred Steig of Titanium Pigments Corp. for their valuable assistance in evaluating the latex paint formulas. REFERENCES
(1) DiGioia, F. A,, and Nelson, R. E., IND. ENG.CHEM.,45, 745-8 (1953). (2) D0.w Chemical Co., Dow Coatings Technical Service Bulletin, Dow 762K Latex. (3) Fraenkel-Contrat, H., J . Biol. Chern., 154, 227 (1944). (4) Meisheimer, L., and Hobaok, W., IND.ENQ.CHEM.,45, 717-25 (1953). (5) Peterson, N. R., and Henson, W. A., OBc. Dig. Federation Paint S Varnish Production Clubs, 331, 543 (1952). (6) Saunders, B., J . Oil Colour Chemists' Assoc., 31, No. 333, 95 (1948). (7) Technical Committee, Northwestern Club, Paint, OiZ S Chern. Rev., 52-6 (Dee. 4, 1952). RECEIVEDfor review August 11, 1953. ACCEPTEDDecember 24, 1953. Presented before t h e Division of Paint, Plastics, a n d Printing I n k Chemistry at the 124th Meeting of t h e AMERICAN CHEMICAL SOCIETY,Chicago, Ill.
