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Diminishing Discoloration in Methacrylate Accelerator Systems
Diminishing discoloration in methacrylate accelerator systems
R. L. Bowen, DDS H. Argentar, BS, W ashington, D.C.
A method was devised to accentuate the f o r m ation o f discoloration products th a t may f o r m during the free radical polym erization o f m ethacrylates at r o o m tem perature. By this method, various accelerators and stabilizers were compared on an equim olar basis. The selection o f the m aterials was based on lite ra ture concerning current theories o f free radical polym erization and the absorption o f visible lig h t by organic compounds. The use o f the ste rica lly hindered phenol, butylated hydroxytoluene, did not produce discoloration, whereas the use o f hydroquinone o r the m onomethyl ether o f hydroquinone had a darkening effect. A compound (N ,N -d im e th yl-3 , 5 -d im e th ylan iline) was synthesized w hich was an effective accelerator and w h ich produced less discolora tion than N ,N -d im e th y l-p -to lu id in e .
To investigate factors that lead to undesirable color formation in direct-filling resins,1 in coldcuring polymer composites,2 and in surface-active comonomers or coupling agents capable of im proving the bonding between polymers and hard 9 18
tooth tissues,3 a comparison was made of various systems of chemical compounds used in the poly merization of methacrylate monomers at room temperature. The tendency of these compounds to form discolored products is reported here. Discoloration can occur during the hardening process as well as during prolonged periods of use. A distinction between these or among the relative roles played by absorption of light4’5 or thermal energy,6 by oxidation,5 8 or by mechanical6 or other5 factors, in the production of discoloration will not be made. The sorption of extraneous col ored materials will not be considered; only color formation resulting from interaction between in gredients of the specified formulations under the test conditions is described. Table 1 ■ M a te ria ls used fo r c o lo r-fo rm a tio n tests. Grade
M a te ria l Benzene Ethanol Benzoyl peroxide N ,N -d im e th y l-p to lu id in e N ,N -d im e th y l-m to lu id in e N ^ - d im e th y l- B ^ d im e th y la n ilin e H ydroquinone M o no m e th yl eth e r o f hydroquinone B utylated hydroxyanisole B utylated hydro xytolu en e Dodecyl m ercaptan M e th a crylic acid
Source
C e rtifie d reagent USP Technical
Fisher S cientific P ublicker Ind., Inc. Fisher S cientific
Reagent
Eastman Kodak Co.
Reagent
Eastman Kodak Co.
U ndeterm ined Technical
Synthesized Eastman Kodak Co.
Technical
Eastman Kodak Co.
Food grade
Eastman Kodak Co.
Food grade Reagent Glacial (containing 0 .1 0 percent HQ as inh ib itor)
Eastman Kodak Co. Borden Chem ical Co. Rohm & Haas Co.
Table 2 ■ Experim ental liq u id fo rm u la tio n s * tested fo r color s ta b ility. Formulation
W e ig h t (% }
BIS-G M A (Epoxylite CF no. 8760 o r S typol 46-4005) M e thyl m ethacrylate
Function
Source
49-60
Cross-linking dim ethacrylate
49-38
Com onom er to reduce viscosity o f BIS-GMA U ltra vio le t abso rb er to im prove c o lo r stability C oupling a ge nt to bond resin to silica Polym erization a c c e le ra to r and to im prove c o lo r stab ility Amine polym erization a c ce le ra to r To im prove c o lo r stab ility S tabilizer to increase working time Fluorescent agent
1.0
UV a b so rb e r 9 12-hydroxy4-m ethoxybenzo phenone) A-174 (7 -m ethacryloxypropyltrim ethoxysilane D odecyl mercaptan
0.75
N , N -d im e th yl-p -to lu id in e *
0.50
M e tha crylic acid Tenox BHT (2, 6-di-ter/-butyl-p-cresol)
0.32 0.20
Tinopal PCR
0.02
1.0
The E poxylite C o rp . o r Freeman Chem ical C o rp . Rohm & Haas Co. American Cyanamid Co. Union C arb id e C o rp . Borden Chemical C o . Eastman Kodak Co. Rohm & Hass Co. Eastman Kodak Co. G e ig y Chemical C orp.
*Four form ulations w e re com pared. They differed in the amine a ccele ra tor. In place o f 0.5 percent N ,N -dim eth yl-p -toluidin e , the o th e r three form ulations contained equim olar concentrations o f N ,N -dim eth yl-m -tolu id in e, N ,N -dim eth yl-3 , 5-dim ethylaniline, and half as much N ,N -dim ethyl-3, 5-dim ethylaniline, respectively.
M aterials and methods N,N-dimethyl-3,5-dimethylaniline (D M D A ) [also known as N,N-dimethyl-sym-m-xylidine (D M SX ), or N,N,3,5-tetramethylaniline] was synthesized for this investigation by the method of Noelting and T rautm an:9 boiling point — 221°C .; n29D = 1.5393; yield: 10 percent based on 3,5-dimeth ylaniline. The compound was then made available through a commercial custom synthesis,* and it has subsequently become commercially available, f Most of the compounds that were used are listed in Table 1. They were used as received. The reactions were carried out in a solvent mix ture of 13.7 weight percent ethanol in benzene, in which hydroquinone and all the other ingredients were soluble. This solvent system resembled comonomer systems of interest210 11 in many re spects, except that it did not contain the reactive methacrylate esters. Benzoyl peroxide (B P) was added (1.00 percent by weight to this benzenealcohol solvent. In addition, equimolar (with re spect to B P) concentrations of an amine accel erator and other compounds that might be used in resin systems were added. The accelerator reacts with the BP, producing free radicals. The interac tion of these free radicals in this solvent yielded colored products in higher concentration than were found in methyl methacrylate monomer, making comparisons more sensitive than would be possible in resins during polymerization. Small, clear-glass vials with screw caps were used as containers. They were coded according to contents, and ranked according to color by subjective, double-blind evaluation. In this study, the shift in absorption to longer wavelengths (bathochromic shift) was accompa nied by an increase in the intensity of absorption.
A combination of these will be referred to as “color.” Color-stability tests were conducted, as de scribed previously,2 on 1-mm. thick, cast speci mens of the compositions indicated in Tables 2 and 3. Half of the flat surface of each disk was covered with opaque reflective aluminum foil. The other half of the surface was exposed to the light and ultraviolet emissions of a standard sunlamp for 24 hours. Scale atomic models were used to indicate the configurations of compounds that could be formed according to current theories on free radical reac tions.
Results The solutions discolored to varying extents. The results shown in Figures 1 and 2 are typical. In the benzene-alcohol solutions, amine accelerators
Table 3 ■ One o f the e xp e rim e n tal powder fo rm u la tions tested fo r color sta b ility.
Powder Spheroldalfused silica
C ataflex "2 0 2" sign beads (x-ray opaque barium -titani um glass) M in-U -Sil (ground quartz) Benzoyl peroxide
Estimated p article size
True volume
W e ig h t
W
<%)
<%>
Glass density (Gm/ml.)
5-37
48
38
44-53
22
0.5-5
30
C oupling agent used* 1% )
Source
2.2
1.0
33f
4.1
0.27
Processed by TAFA, Division o f Humphreys Corp. C ataphote C o rp .
28
2.65
2.5
1.0
Pennsylvania Glass Sand C orp. Fisher Scientific
*Each p o w d e r was tre ate d separately w ith the given w e ig ht percent o f 3(trim ethoxysilyl) p ro pyl m ethacrylate (Z-6030, D ow C orning C o rp .). fA b o u t 33 percent o f this p o w d e r is sufficient to yie ld composites w ith X-ray o p a c ity a b o u t equal to that o f hard to o th tissues.
Bowen— A rg e n ta r: D ISCO LO RATIO N IN M E TH AC R Y LA TE S ■ 9 1 9
DMPT MAA
Fig. 1 ■ C ontrol (C ) via l contains o benzene-alcohol solution o f the re action products o f 1.0 0 percent benzoyl peroxide w ith an equim olar co n ce ntra tio n o f N ,N -d im e th y l-p to lu id in e ; the o th e r vials contain th e reaction products resulting from a co m b in a tio n o f the ingredients o f the co n tro l w ith o th er equim olar com pounds as in d ica te d: HQ, h yd ro q u in o n e ; M A A , m e th a c ry lic a cid ; BHT, b u tyla te d h yd ro xyto lu ene; D D M , dodecyl m ercaptan; N 2, th e so lu tio n was flushed w ith n i trogen gas.
produced color in the decreasing order: N,Ndimethyl-p-toluidine (D M PT) > N,N-dimethylm-toluidine (D M M T) > N,N-dimethyl-3,5-dimethylaniline (D M D A ). For example, the control (C ) in Figure 1, containing DMPT, is darker than control (C ) in Figure 2, containing DMDA (D M SX ). These findings are in accord with a report12 that the colors resulting from the polymerization of methyl methacrylate with 2 percent benzoyl perox ide and 2 percent of each of the following amines were N,N-dimethylaniline (D M A ), amber; DMPT, medium yellow, and DMMT, pale yellow. Hg C,
/CHg
B, C.
BHT
bilizer) > butylated hydroxytoluene (BHT; 2, 6-di-ierf-butyl-p-cresol; 4-methyl-2,6-di-feri-butylphenol). It can be seen that the addition of the stabilizer BHT did not produce discoloration, whereas the addition of HQ had a darkening effect compared with the control, C. H
0 r ^
? h3
O ¿Hs
HQ
,CIfc
l
- U — cc — CH, I CKfe
BHA
DMA Hb C , n / CH3
Ife C
HME CHa
Ha C
DMMT
DMDA (DMSX)
When certain stabilizers were compared, in the presence of BP and DMPT, the decreasing order of color formation was: hydroquinone (FIQ) > > butylated hydroxyanisole (BHA; a mixture of 2and 3-teri-butyl-4-hydroxyanisole) — monometh yl ether of hyroquinone (H M E ) > control (no sta 920
® J A D A , Vol. 75, O ct. 1967
BHT
When the solutions were flushed with nitrogen or argon gas, there was less color formed than when the reactions occurred in the presence of air (Fig. 1). The formation of color was reduced in the pres ence of dodecyl mercaptan (D D M ). Depending on the concentration, DDM appears to enhance the accelerating action of certain tertiary amines in the polymerization of methacrylate with BP. DDM is also a free radical transfer agent.13
DMSX MAA
DDM
DDM +BHT
DDM + B HT +M A A Fig. 2 ■ C ontrol (C) contains a b enzene-alcohol solution o f the reaction products o f 1.0 0 percent benzoyl peroxide w ith an e q u i m olar co n ce ntra tio n of N ,N d im e th y l-s y m -m -x y lid in e ; the o th er vials co n ta in the reaction prod ucts resu ltin g from a co m b in a tio n o f the co n tro l ingredients together w ith e q u im o la r com pounds as in d i ca te d : M A A , m e th a crylic acid; BHT, b u tyla te d h ydroxytoluene; D D M : dodecyl m ercaptan.
(
i3 s
ÌJ-/7
1 3 .Ì -1 0
Among the various combinations tested in the presence of BP, the lightest color resulted from the combined use of DMDA, DDM, BHT, and methacrylic acid (M A A ), as seen in Figure 2. The role of M AA (whether as a result of its unsatu rated, acidic, or other properties) in reducing the color is not completely understood. Reinforced resin composites were prepared from materials such as those listed in Tables 2 and 3. In this group of formulations, the param eter was the amine accelerator used. The portions of the specimens not exposed to the sunlamp did not vary greatly in color; subtle differences appeared to correlate with those of the benzene-alcohol solutions wherein the amines were compared. The discoloration of the portions exposed to the sun lamp for 24 hours, however, showed a different ranking: DM M T (bright yellow) > DM PT > DMDA. Since, in these composite formulations, D M DA showed greater effectiveness, that is, a shorter hardening time, than did equimolar con centrations of DM PT, a formulation (Table 2, 3) was also prepared with half the molar concentra tion of DMDA. This reduction in the amine con centration resulted in a further reduction of discoloration; the color change of the portion ex posed to the sunlamp was only slightly perceptible. By increasing the ultraviolet absorber to 1.5 percent, by replacing the 0.02 percent of organic fluorescent agent ( “Ottalume 2115,” Ottawa fluorescent agent with 1.0 percent of an inorganic Chemical C o .), and by using 0.275 percent DM DA (D M SX ) for DMPT, a formulation other wise the same as that shown in Tables 2 and 3
I
3iJ l i
yielded composites that did not show perceptible color change after exposure to the sunlamp.
Discussion Further spectroscopic examination of the color formation is being made. However, an interpreta tion of the color formation (discoloration) in these experiments is as follows: Free radical combinations,5-14 15 disproportion ations,5 transfers or displacements,5’13,15 and other reactions such as oxidation5'8 yield, among other things, compounds with alternating single and double bonds. These compounds can absorb visi ble light, resulting in colored products. Light ab sorption increases with the num ber of alternating single and double bonds, including those in aro matic rings, and with electron-donor substituents (such as amino, alkoxy, alkyl, and other bathochromic or auxochromic groups) that are in these conjugated systems, as well as with the length and freedom from geometric distortion of the conju gated system.18-17 Steric or structural restrictions, which prevent alternating single and double bonds from assuming a flat, planar configuration, can reduce the amount of undesired light absorption. Reduction of coplanarity of double bonds, together with a minimizing of electron-donating groups that can become part of chromophoric systems, may shift the absorption frequencies out of the visible range,16'18 resulting in colorless products. Generally, results support this interpretation in
Bowen— A rg e n ta r: D ISCO LO RATIO N
IN M E T H A C R Y LA TE S ■ 921
that increased substitution and steric hindrance of the aromatic ring compounds coincided with less formation of color during the free radical reac tions. However, the introduction of groups that can impose steric restrictions may, in some in stances, tend to increase the color by donating electrons to the conjugated system18 and may ac count for the fact that HM E did not yield more color than BHA. Attention to only the steric and substituent ef fects, however, can be misleading if the nature of the free radical reactions involved is not ade quately understood. Current knowledge of free radical reactions is too incomplete for quantitative predictions regarding color formation, but cur rent theories can guide empirical experimenta tion.5'8’13"1*’19 The use of antioxidants to stabilize polymers20 21 entails complications in some cold-curing resin systems since the free radicals needed during poly merization may interact with the antioxidants during the hardening reactions. Ultraviolet absorb ers may be used to make the material less vulner able to light that contains ultraviolet radiation.4’20 Discoloration during prolonged use should be fur ther investigated. Analysis of the atomic and molecular structures involved may be helpful in avoiding or eliminating relatively unstable chemical bonds5 in the resulting polymer. Such bonds, on disruption by absorption of light,4’5 thermal,6 or mechanical energy,6 or by chemical reactions,5 8’20 may lead to discolored products through various free radical reactions, somewhat similar to those postulated for the test results presented here. Food-grade BHT (2,6-di-feri-butyl-p-cresol), which is a relatively innocuous compound22 that has almost no systemic toxicity,23 should be con sidered for use (as a stabilizer for methacrylate monomers) in place of HQ or HM E, both of which produce color and are moderately22 24 and slightly22 to moderately24 toxic, respectively. High concentrations of cross-linking monomers (for example, in Table 2) tend to make such materials gel too soon after the start of the mixing procedure when initiators are present in sufficient concentrations for maximum conversion of mono mer to polymer. Since the inhibitor (or retarder), BHT, did not produce colored products, BHT can be used in concentrations sufficiently high to give adequate working time (induction period) to such formulations. Additional study should determine if color for mation could be further reduced by a diminution 9 2 2 ■ JA D A , V o l. 7 5 , Oct. 1967
in the concentration of BP or by its replacement with a peroxide that, on decomposing, cannot produce phenyl radicals14’15 capable of coupling to form highly resonant systems.
Conclusion Although it is beyond the scope of the research discussed here to analyze the reaction products of these experiments or to prove that the formation of a planar conjugated system of double bonds, associated with auxochromic groups, is the cause of the discoloration occurring in the systems, it does seem possible that certain configurations of the molecules in a formulation could be used to reduce the intensity of the color that develops by spacial or steric restrictions on coplanary of the conjugated systems that may form. Further ex ploration of the possibility of the use of electronwithdrawing substituents on appropriate com pounds in such formulations should be made. The combined use of such electron-withdrawing sub stituents, steric hindrance to the formation of planar conjugated systems, and the exclusion of compounds or groups that are subject to air oxi dation (autoxidation) may result in further im provements in color stability.
T his investigation was supported in p a rt by USPHS research g ra n t D E -0 0 5 8 9 -0 7 , Study o f Adhesion M echa nisms in Dental M aterials, fro m th e N ational In s titu te o f Dental Research, N ational In s titu te s o f H ealth, Bethesda, M d., to th e A m erican Dental Association and is p a rt o f th e dental research program conducted a t th e N ational Bureau o f Standards, in cooperation w ith the A m erican D ental A ssociation; the A rm y D ental Corps; Dental Ser vices D ivision o f the USAF School o f Aerospace M edicine, and th e Veterans A d m in istra tio n . Doctor Bowen and M r. A rg e n ta r are research associates o f th e A m erican Dental Association Research Division, N a tio n a l Bureau o f Standards, W a sh in g to n , D.C. 2 0 2 3 4 . ^D is tilla tio n Products Industries, Eastman Kodak Co. tC e rta in com m ercial m aterials and e q u ip m e n t are id e n tifie d in th is paper to specify ad e qu a te ly th e experim ental procedure. In no instance does such id e n tific a tio n im p ly recom m endation or endorsem ent by the N a tio n a l Bureau o f Standards or th a t the m aterial o r e q u ip m e n t id e n tifie d is necessarily the best available fo r th e purpose. 1. Caul, H, J., and Schoonover, I. C. C olor s ta b ility o f d ire c t fillin g resins. JA D A 4 7 :4 4 8 O ct., 1953. 2. Bowen, R. L. Properties o f a silica -re in fo rce d p o ly m er fo r dental restorations. JA D A 6 6 :5 7 Jan., 1963.
3. Bowen, R. L. Adhesive bonding o f various m aterials to hard to o th tissues. I. J Dent Res 4 4 :6 9 0 Ju ly-A u g ., 1 9 6 5 ; II. J Dent Res 4 4 :8 9 5 Sept.-O ct., 1 9 6 5 ; III. J D ent Res 4 4 :9 0 3 Sept.-O ct., 1 9 6 5 ; IV. J D ent Res 4 4 :9 0 6 S ept.-O ct., 1965. 4. S chm itt, R. G., and H irt, R. C. Investigation o f the pro te ctive u ltra v io le t absorbers in a space environm ent. II. Photochemical studies. J Polymer Sei 6 1 :3 6 1 O ct., 1962. 5. W a llin g , C. Free radicals in solution. N e w Y o rk, John W ile y & Sons, Inc., 1957. 6 . Neim an, M . B. (editor), A g in g and sta b iliza tio n o f polymers. N ew Y o rk, C onsultants Bureau, Enterprises, Inc., 1965. 7. Lundberg, W . O. A u to xid a tio n and antioxidants, vol. 1. N ew Y o rk, John W ile y & Sons, Inc., 1961. 8 . Ingold, K. U. In h ib itio n o f the a u toxida tio n of organic substances in the liq u id phase. Chem Rev 61 :563 Dec., 1961. 9. N o e ltin g , E., and Trautm an, E. Condensationsproducte aus O rthoam eisensäureester m it D im e th ylm e ta to lu id in und D im e th y l-s -M e ta x y lid in , Berichte der Deutschen Chemischen Gesellschaft. BD. 1. 2 4 :5 6 1 Jan.June, 1891, (footnote p. 563). 10. Bowen, R. L. Dental fillin g m aterial com prising vin yl silane treated fused silica and a b inder consisting o f th e reaction p ro d u ct o f bisphenol and gylcid yl acrylate. U.S. Patent no. 3 ,0 66 ,1 1 2, Nov. 27, 1962. 11. Bowen, R. L. M ethod o f preparing a m onom er having phenoxy and m ethacrylate groups lin ke d by hy droxy glyceryl groups. U.S. Patent no. 3 ,1 7 9 ,6 2 3 , A p ril 2 0 , 1965. 12. Brauer, G. M .; Davenport, R. M ., and Hansen,
W . C. A ccelerating e ffe cts o f amines on polym erization o f m ethyl m ethacrylate. M od Plastics 3 4 :1 5 3 Nov., 1956. 13. Ham, G. E. C opolym erization. N ew Y o rk, In te r science Publishers, Inc., 1964. 14. Fuson, R. C. Reactions o f organic compounds. N ew Y o rk, John W ile y & Sons, Inc., 1962. 15. Bevington, J. C. Radical p o lym e riza tio n . N ew Y ork, Academ ic Press, Inc., 1961. 16. Brooker, L. G. S., and Van Lare, E. J. Color and co n stitutio n o f organic dyes. Encyclopedia o f Chemical Technology, ed. 2. 5 :7 6 3 N ew Y o rk, Interscience Pub lishers, Inc., 1964. 17. Gould, E. S. M echanism s and structu re in organic chem istry. N ew Y o rk, H o lt, R inehart & W in sto n , Inc., 1959. 18. Klevens, H. B., and Platt, J. R. Geometry and spectra o f substituted anilines. J A m e r Chem Soc 71 :1 71 4 M ay, 1949. 19. Ayrey, G. Uses o f isotopes in a d d itio n polym er ization. Chem Rev 6 3 :6 4 5 Dec., 1963. 20. Scott, G. A n tio xid a n ts, C hem istry and Industry, no. 7. Feb., 1963, p. 27 1. 21. Holdsw orth, J. D .; Scott, G., and W illia m s , D. Mechanisms o f a n tio xid a n t a ctio n : s u lp h u r-co n ta in in g an tioxidants. J Chem Soc (London), Dec., 1964, p. 4 6 9 2 . 22. Patty, F. A. (editor), In d ustria l hygiene and to x i cology, ed. 2, vol. 2, N ew Y o rk, John W ile y & Sons, Inc., 1962. 23. Stecher, P. G. (editor), T he M e rc k index o f chem icals and drugs, ed. 7. Rahway, N ew Jersey, M e rck & Co., Inc., 1960. 24. Sax, N. I. Dangerous properties o f industrial m a terials. N ew Y ork, Reinhold P ublishing Corp., 1957.
Now You Know Flag Day was o rig in a te d by Dr. Bernard J. C igrand, a te a che r-de n tist, who began to cam paign fo r a special day honoring the Stars and Stripes in 1885. President W ilson issued a proclam ation m aking June 14 Flag Day on M ay 30, 1916.
Philadelphia Evening Bulletin, June 14, 1967.
Bowen— A rg e n ta r: DISCO LO RATIO N
IN M ETH A C R Y LA TE S ■ 923
R. L. Bowen, DDS H. Argentar, BS, W ashington, D.C.
A method was devised to accentuate the f o r m ation o f discoloration products th a t may f o r m during the free radical polym erization o f m ethacrylates at r o o m tem perature. By this method, various accelerators and stabilizers were compared on an equim olar basis. The selection o f the m aterials was based on lite ra ture concerning current theories o f free radical polym erization and the absorption o f visible lig h t by organic compounds. The use o f the ste rica lly hindered phenol, butylated hydroxytoluene, did not produce discoloration, whereas the use o f hydroquinone o r the m onomethyl ether o f hydroquinone had a darkening effect. A compound (N ,N -d im e th yl-3 , 5 -d im e th ylan iline) was synthesized w hich was an effective accelerator and w h ich produced less discolora tion than N ,N -d im e th y l-p -to lu id in e .
To investigate factors that lead to undesirable color formation in direct-filling resins,1 in coldcuring polymer composites,2 and in surface-active comonomers or coupling agents capable of im proving the bonding between polymers and hard 9 18
tooth tissues,3 a comparison was made of various systems of chemical compounds used in the poly merization of methacrylate monomers at room temperature. The tendency of these compounds to form discolored products is reported here. Discoloration can occur during the hardening process as well as during prolonged periods of use. A distinction between these or among the relative roles played by absorption of light4’5 or thermal energy,6 by oxidation,5 8 or by mechanical6 or other5 factors, in the production of discoloration will not be made. The sorption of extraneous col ored materials will not be considered; only color formation resulting from interaction between in gredients of the specified formulations under the test conditions is described. Table 1 ■ M a te ria ls used fo r c o lo r-fo rm a tio n tests. Grade
M a te ria l Benzene Ethanol Benzoyl peroxide N ,N -d im e th y l-p to lu id in e N ,N -d im e th y l-m to lu id in e N ^ - d im e th y l- B ^ d im e th y la n ilin e H ydroquinone M o no m e th yl eth e r o f hydroquinone B utylated hydroxyanisole B utylated hydro xytolu en e Dodecyl m ercaptan M e th a crylic acid
Source
C e rtifie d reagent USP Technical
Fisher S cientific P ublicker Ind., Inc. Fisher S cientific
Reagent
Eastman Kodak Co.
Reagent
Eastman Kodak Co.
U ndeterm ined Technical
Synthesized Eastman Kodak Co.
Technical
Eastman Kodak Co.
Food grade
Eastman Kodak Co.
Food grade Reagent Glacial (containing 0 .1 0 percent HQ as inh ib itor)
Eastman Kodak Co. Borden Chem ical Co. Rohm & Haas Co.
Table 2 ■ Experim ental liq u id fo rm u la tio n s * tested fo r color s ta b ility. Formulation
W e ig h t (% }
BIS-G M A (Epoxylite CF no. 8760 o r S typol 46-4005) M e thyl m ethacrylate
Function
Source
49-60
Cross-linking dim ethacrylate
49-38
Com onom er to reduce viscosity o f BIS-GMA U ltra vio le t abso rb er to im prove c o lo r stability C oupling a ge nt to bond resin to silica Polym erization a c c e le ra to r and to im prove c o lo r stab ility Amine polym erization a c ce le ra to r To im prove c o lo r stab ility S tabilizer to increase working time Fluorescent agent
1.0
UV a b so rb e r 9 12-hydroxy4-m ethoxybenzo phenone) A-174 (7 -m ethacryloxypropyltrim ethoxysilane D odecyl mercaptan
0.75
N , N -d im e th yl-p -to lu id in e *
0.50
M e tha crylic acid Tenox BHT (2, 6-di-ter/-butyl-p-cresol)
0.32 0.20
Tinopal PCR
0.02
1.0
The E poxylite C o rp . o r Freeman Chem ical C o rp . Rohm & Haas Co. American Cyanamid Co. Union C arb id e C o rp . Borden Chemical C o . Eastman Kodak Co. Rohm & Hass Co. Eastman Kodak Co. G e ig y Chemical C orp.
*Four form ulations w e re com pared. They differed in the amine a ccele ra tor. In place o f 0.5 percent N ,N -dim eth yl-p -toluidin e , the o th e r three form ulations contained equim olar concentrations o f N ,N -dim eth yl-m -tolu id in e, N ,N -dim eth yl-3 , 5-dim ethylaniline, and half as much N ,N -dim ethyl-3, 5-dim ethylaniline, respectively.
M aterials and methods N,N-dimethyl-3,5-dimethylaniline (D M D A ) [also known as N,N-dimethyl-sym-m-xylidine (D M SX ), or N,N,3,5-tetramethylaniline] was synthesized for this investigation by the method of Noelting and T rautm an:9 boiling point — 221°C .; n29D = 1.5393; yield: 10 percent based on 3,5-dimeth ylaniline. The compound was then made available through a commercial custom synthesis,* and it has subsequently become commercially available, f Most of the compounds that were used are listed in Table 1. They were used as received. The reactions were carried out in a solvent mix ture of 13.7 weight percent ethanol in benzene, in which hydroquinone and all the other ingredients were soluble. This solvent system resembled comonomer systems of interest210 11 in many re spects, except that it did not contain the reactive methacrylate esters. Benzoyl peroxide (B P) was added (1.00 percent by weight to this benzenealcohol solvent. In addition, equimolar (with re spect to B P) concentrations of an amine accel erator and other compounds that might be used in resin systems were added. The accelerator reacts with the BP, producing free radicals. The interac tion of these free radicals in this solvent yielded colored products in higher concentration than were found in methyl methacrylate monomer, making comparisons more sensitive than would be possible in resins during polymerization. Small, clear-glass vials with screw caps were used as containers. They were coded according to contents, and ranked according to color by subjective, double-blind evaluation. In this study, the shift in absorption to longer wavelengths (bathochromic shift) was accompa nied by an increase in the intensity of absorption.
A combination of these will be referred to as “color.” Color-stability tests were conducted, as de scribed previously,2 on 1-mm. thick, cast speci mens of the compositions indicated in Tables 2 and 3. Half of the flat surface of each disk was covered with opaque reflective aluminum foil. The other half of the surface was exposed to the light and ultraviolet emissions of a standard sunlamp for 24 hours. Scale atomic models were used to indicate the configurations of compounds that could be formed according to current theories on free radical reac tions.
Results The solutions discolored to varying extents. The results shown in Figures 1 and 2 are typical. In the benzene-alcohol solutions, amine accelerators
Table 3 ■ One o f the e xp e rim e n tal powder fo rm u la tions tested fo r color sta b ility.
Powder Spheroldalfused silica
C ataflex "2 0 2" sign beads (x-ray opaque barium -titani um glass) M in-U -Sil (ground quartz) Benzoyl peroxide
Estimated p article size
True volume
W e ig h t
W
<%)
<%>
Glass density (Gm/ml.)
5-37
48
38
44-53
22
0.5-5
30
C oupling agent used* 1% )
Source
2.2
1.0
33f
4.1
0.27
Processed by TAFA, Division o f Humphreys Corp. C ataphote C o rp .
28
2.65
2.5
1.0
Pennsylvania Glass Sand C orp. Fisher Scientific
*Each p o w d e r was tre ate d separately w ith the given w e ig ht percent o f 3(trim ethoxysilyl) p ro pyl m ethacrylate (Z-6030, D ow C orning C o rp .). fA b o u t 33 percent o f this p o w d e r is sufficient to yie ld composites w ith X-ray o p a c ity a b o u t equal to that o f hard to o th tissues.
Bowen— A rg e n ta r: D ISCO LO RATIO N IN M E TH AC R Y LA TE S ■ 9 1 9
DMPT MAA
Fig. 1 ■ C ontrol (C ) via l contains o benzene-alcohol solution o f the re action products o f 1.0 0 percent benzoyl peroxide w ith an equim olar co n ce ntra tio n o f N ,N -d im e th y l-p to lu id in e ; the o th e r vials contain th e reaction products resulting from a co m b in a tio n o f the ingredients o f the co n tro l w ith o th er equim olar com pounds as in d ica te d: HQ, h yd ro q u in o n e ; M A A , m e th a c ry lic a cid ; BHT, b u tyla te d h yd ro xyto lu ene; D D M , dodecyl m ercaptan; N 2, th e so lu tio n was flushed w ith n i trogen gas.
produced color in the decreasing order: N,Ndimethyl-p-toluidine (D M PT) > N,N-dimethylm-toluidine (D M M T) > N,N-dimethyl-3,5-dimethylaniline (D M D A ). For example, the control (C ) in Figure 1, containing DMPT, is darker than control (C ) in Figure 2, containing DMDA (D M SX ). These findings are in accord with a report12 that the colors resulting from the polymerization of methyl methacrylate with 2 percent benzoyl perox ide and 2 percent of each of the following amines were N,N-dimethylaniline (D M A ), amber; DMPT, medium yellow, and DMMT, pale yellow. Hg C,
/CHg
B, C.
BHT
bilizer) > butylated hydroxytoluene (BHT; 2, 6-di-ierf-butyl-p-cresol; 4-methyl-2,6-di-feri-butylphenol). It can be seen that the addition of the stabilizer BHT did not produce discoloration, whereas the addition of HQ had a darkening effect compared with the control, C. H
0 r ^
? h3
O ¿Hs
HQ
,CIfc
l
- U — cc — CH, I CKfe
BHA
DMA Hb C , n / CH3
Ife C
HME CHa
Ha C
DMMT
DMDA (DMSX)
When certain stabilizers were compared, in the presence of BP and DMPT, the decreasing order of color formation was: hydroquinone (FIQ) > > butylated hydroxyanisole (BHA; a mixture of 2and 3-teri-butyl-4-hydroxyanisole) — monometh yl ether of hyroquinone (H M E ) > control (no sta 920
® J A D A , Vol. 75, O ct. 1967
BHT
When the solutions were flushed with nitrogen or argon gas, there was less color formed than when the reactions occurred in the presence of air (Fig. 1). The formation of color was reduced in the pres ence of dodecyl mercaptan (D D M ). Depending on the concentration, DDM appears to enhance the accelerating action of certain tertiary amines in the polymerization of methacrylate with BP. DDM is also a free radical transfer agent.13
DMSX MAA
DDM
DDM +BHT
DDM + B HT +M A A Fig. 2 ■ C ontrol (C) contains a b enzene-alcohol solution o f the reaction products o f 1.0 0 percent benzoyl peroxide w ith an e q u i m olar co n ce ntra tio n of N ,N d im e th y l-s y m -m -x y lid in e ; the o th er vials co n ta in the reaction prod ucts resu ltin g from a co m b in a tio n o f the co n tro l ingredients together w ith e q u im o la r com pounds as in d i ca te d : M A A , m e th a crylic acid; BHT, b u tyla te d h ydroxytoluene; D D M : dodecyl m ercaptan.
(
i3 s
ÌJ-/7
1 3 .Ì -1 0
Among the various combinations tested in the presence of BP, the lightest color resulted from the combined use of DMDA, DDM, BHT, and methacrylic acid (M A A ), as seen in Figure 2. The role of M AA (whether as a result of its unsatu rated, acidic, or other properties) in reducing the color is not completely understood. Reinforced resin composites were prepared from materials such as those listed in Tables 2 and 3. In this group of formulations, the param eter was the amine accelerator used. The portions of the specimens not exposed to the sunlamp did not vary greatly in color; subtle differences appeared to correlate with those of the benzene-alcohol solutions wherein the amines were compared. The discoloration of the portions exposed to the sun lamp for 24 hours, however, showed a different ranking: DM M T (bright yellow) > DM PT > DMDA. Since, in these composite formulations, D M DA showed greater effectiveness, that is, a shorter hardening time, than did equimolar con centrations of DM PT, a formulation (Table 2, 3) was also prepared with half the molar concentra tion of DMDA. This reduction in the amine con centration resulted in a further reduction of discoloration; the color change of the portion ex posed to the sunlamp was only slightly perceptible. By increasing the ultraviolet absorber to 1.5 percent, by replacing the 0.02 percent of organic fluorescent agent ( “Ottalume 2115,” Ottawa fluorescent agent with 1.0 percent of an inorganic Chemical C o .), and by using 0.275 percent DM DA (D M SX ) for DMPT, a formulation other wise the same as that shown in Tables 2 and 3
I
3iJ l i
yielded composites that did not show perceptible color change after exposure to the sunlamp.
Discussion Further spectroscopic examination of the color formation is being made. However, an interpreta tion of the color formation (discoloration) in these experiments is as follows: Free radical combinations,5-14 15 disproportion ations,5 transfers or displacements,5’13,15 and other reactions such as oxidation5'8 yield, among other things, compounds with alternating single and double bonds. These compounds can absorb visi ble light, resulting in colored products. Light ab sorption increases with the num ber of alternating single and double bonds, including those in aro matic rings, and with electron-donor substituents (such as amino, alkoxy, alkyl, and other bathochromic or auxochromic groups) that are in these conjugated systems, as well as with the length and freedom from geometric distortion of the conju gated system.18-17 Steric or structural restrictions, which prevent alternating single and double bonds from assuming a flat, planar configuration, can reduce the amount of undesired light absorption. Reduction of coplanarity of double bonds, together with a minimizing of electron-donating groups that can become part of chromophoric systems, may shift the absorption frequencies out of the visible range,16'18 resulting in colorless products. Generally, results support this interpretation in
Bowen— A rg e n ta r: D ISCO LO RATIO N
IN M E T H A C R Y LA TE S ■ 921
that increased substitution and steric hindrance of the aromatic ring compounds coincided with less formation of color during the free radical reac tions. However, the introduction of groups that can impose steric restrictions may, in some in stances, tend to increase the color by donating electrons to the conjugated system18 and may ac count for the fact that HM E did not yield more color than BHA. Attention to only the steric and substituent ef fects, however, can be misleading if the nature of the free radical reactions involved is not ade quately understood. Current knowledge of free radical reactions is too incomplete for quantitative predictions regarding color formation, but cur rent theories can guide empirical experimenta tion.5'8’13"1*’19 The use of antioxidants to stabilize polymers20 21 entails complications in some cold-curing resin systems since the free radicals needed during poly merization may interact with the antioxidants during the hardening reactions. Ultraviolet absorb ers may be used to make the material less vulner able to light that contains ultraviolet radiation.4’20 Discoloration during prolonged use should be fur ther investigated. Analysis of the atomic and molecular structures involved may be helpful in avoiding or eliminating relatively unstable chemical bonds5 in the resulting polymer. Such bonds, on disruption by absorption of light,4’5 thermal,6 or mechanical energy,6 or by chemical reactions,5 8’20 may lead to discolored products through various free radical reactions, somewhat similar to those postulated for the test results presented here. Food-grade BHT (2,6-di-feri-butyl-p-cresol), which is a relatively innocuous compound22 that has almost no systemic toxicity,23 should be con sidered for use (as a stabilizer for methacrylate monomers) in place of HQ or HM E, both of which produce color and are moderately22 24 and slightly22 to moderately24 toxic, respectively. High concentrations of cross-linking monomers (for example, in Table 2) tend to make such materials gel too soon after the start of the mixing procedure when initiators are present in sufficient concentrations for maximum conversion of mono mer to polymer. Since the inhibitor (or retarder), BHT, did not produce colored products, BHT can be used in concentrations sufficiently high to give adequate working time (induction period) to such formulations. Additional study should determine if color for mation could be further reduced by a diminution 9 2 2 ■ JA D A , V o l. 7 5 , Oct. 1967
in the concentration of BP or by its replacement with a peroxide that, on decomposing, cannot produce phenyl radicals14’15 capable of coupling to form highly resonant systems.
Conclusion Although it is beyond the scope of the research discussed here to analyze the reaction products of these experiments or to prove that the formation of a planar conjugated system of double bonds, associated with auxochromic groups, is the cause of the discoloration occurring in the systems, it does seem possible that certain configurations of the molecules in a formulation could be used to reduce the intensity of the color that develops by spacial or steric restrictions on coplanary of the conjugated systems that may form. Further ex ploration of the possibility of the use of electronwithdrawing substituents on appropriate com pounds in such formulations should be made. The combined use of such electron-withdrawing sub stituents, steric hindrance to the formation of planar conjugated systems, and the exclusion of compounds or groups that are subject to air oxi dation (autoxidation) may result in further im provements in color stability.
T his investigation was supported in p a rt by USPHS research g ra n t D E -0 0 5 8 9 -0 7 , Study o f Adhesion M echa nisms in Dental M aterials, fro m th e N ational In s titu te o f Dental Research, N ational In s titu te s o f H ealth, Bethesda, M d., to th e A m erican Dental Association and is p a rt o f th e dental research program conducted a t th e N ational Bureau o f Standards, in cooperation w ith the A m erican D ental A ssociation; the A rm y D ental Corps; Dental Ser vices D ivision o f the USAF School o f Aerospace M edicine, and th e Veterans A d m in istra tio n . Doctor Bowen and M r. A rg e n ta r are research associates o f th e A m erican Dental Association Research Division, N a tio n a l Bureau o f Standards, W a sh in g to n , D.C. 2 0 2 3 4 . ^D is tilla tio n Products Industries, Eastman Kodak Co. tC e rta in com m ercial m aterials and e q u ip m e n t are id e n tifie d in th is paper to specify ad e qu a te ly th e experim ental procedure. In no instance does such id e n tific a tio n im p ly recom m endation or endorsem ent by the N a tio n a l Bureau o f Standards or th a t the m aterial o r e q u ip m e n t id e n tifie d is necessarily the best available fo r th e purpose. 1. Caul, H, J., and Schoonover, I. C. C olor s ta b ility o f d ire c t fillin g resins. JA D A 4 7 :4 4 8 O ct., 1953. 2. Bowen, R. L. Properties o f a silica -re in fo rce d p o ly m er fo r dental restorations. JA D A 6 6 :5 7 Jan., 1963.
3. Bowen, R. L. Adhesive bonding o f various m aterials to hard to o th tissues. I. J Dent Res 4 4 :6 9 0 Ju ly-A u g ., 1 9 6 5 ; II. J Dent Res 4 4 :8 9 5 Sept.-O ct., 1 9 6 5 ; III. J D ent Res 4 4 :9 0 3 Sept.-O ct., 1 9 6 5 ; IV. J D ent Res 4 4 :9 0 6 S ept.-O ct., 1965. 4. S chm itt, R. G., and H irt, R. C. Investigation o f the pro te ctive u ltra v io le t absorbers in a space environm ent. II. Photochemical studies. J Polymer Sei 6 1 :3 6 1 O ct., 1962. 5. W a llin g , C. Free radicals in solution. N e w Y o rk, John W ile y & Sons, Inc., 1957. 6 . Neim an, M . B. (editor), A g in g and sta b iliza tio n o f polymers. N ew Y o rk, C onsultants Bureau, Enterprises, Inc., 1965. 7. Lundberg, W . O. A u to xid a tio n and antioxidants, vol. 1. N ew Y o rk, John W ile y & Sons, Inc., 1961. 8 . Ingold, K. U. In h ib itio n o f the a u toxida tio n of organic substances in the liq u id phase. Chem Rev 61 :563 Dec., 1961. 9. N o e ltin g , E., and Trautm an, E. Condensationsproducte aus O rthoam eisensäureester m it D im e th ylm e ta to lu id in und D im e th y l-s -M e ta x y lid in , Berichte der Deutschen Chemischen Gesellschaft. BD. 1. 2 4 :5 6 1 Jan.June, 1891, (footnote p. 563). 10. Bowen, R. L. Dental fillin g m aterial com prising vin yl silane treated fused silica and a b inder consisting o f th e reaction p ro d u ct o f bisphenol and gylcid yl acrylate. U.S. Patent no. 3 ,0 66 ,1 1 2, Nov. 27, 1962. 11. Bowen, R. L. M ethod o f preparing a m onom er having phenoxy and m ethacrylate groups lin ke d by hy droxy glyceryl groups. U.S. Patent no. 3 ,1 7 9 ,6 2 3 , A p ril 2 0 , 1965. 12. Brauer, G. M .; Davenport, R. M ., and Hansen,
W . C. A ccelerating e ffe cts o f amines on polym erization o f m ethyl m ethacrylate. M od Plastics 3 4 :1 5 3 Nov., 1956. 13. Ham, G. E. C opolym erization. N ew Y o rk, In te r science Publishers, Inc., 1964. 14. Fuson, R. C. Reactions o f organic compounds. N ew Y o rk, John W ile y & Sons, Inc., 1962. 15. Bevington, J. C. Radical p o lym e riza tio n . N ew Y ork, Academ ic Press, Inc., 1961. 16. Brooker, L. G. S., and Van Lare, E. J. Color and co n stitutio n o f organic dyes. Encyclopedia o f Chemical Technology, ed. 2. 5 :7 6 3 N ew Y o rk, Interscience Pub lishers, Inc., 1964. 17. Gould, E. S. M echanism s and structu re in organic chem istry. N ew Y o rk, H o lt, R inehart & W in sto n , Inc., 1959. 18. Klevens, H. B., and Platt, J. R. Geometry and spectra o f substituted anilines. J A m e r Chem Soc 71 :1 71 4 M ay, 1949. 19. Ayrey, G. Uses o f isotopes in a d d itio n polym er ization. Chem Rev 6 3 :6 4 5 Dec., 1963. 20. Scott, G. A n tio xid a n ts, C hem istry and Industry, no. 7. Feb., 1963, p. 27 1. 21. Holdsw orth, J. D .; Scott, G., and W illia m s , D. Mechanisms o f a n tio xid a n t a ctio n : s u lp h u r-co n ta in in g an tioxidants. J Chem Soc (London), Dec., 1964, p. 4 6 9 2 . 22. Patty, F. A. (editor), In d ustria l hygiene and to x i cology, ed. 2, vol. 2, N ew Y o rk, John W ile y & Sons, Inc., 1962. 23. Stecher, P. G. (editor), T he M e rc k index o f chem icals and drugs, ed. 7. Rahway, N ew Jersey, M e rck & Co., Inc., 1960. 24. Sax, N. I. Dangerous properties o f industrial m a terials. N ew Y ork, Reinhold P ublishing Corp., 1957.
Now You Know Flag Day was o rig in a te d by Dr. Bernard J. C igrand, a te a che r-de n tist, who began to cam paign fo r a special day honoring the Stars and Stripes in 1885. President W ilson issued a proclam ation m aking June 14 Flag Day on M ay 30, 1916.
Philadelphia Evening Bulletin, June 14, 1967.
Bowen— A rg e n ta r: DISCO LO RATIO N
IN M ETH A C R Y LA TE S ■ 923