REMOVABLE PROSTHODONTICS 5XCTION LIKI-OR!,

LOUIS BLA-ITERFEIN

S. HOWARD I’AYSL.

CEORC;L .4 ZARB

Coloring acrylic denture base resins

Sheldon Winkler, D.D.S.,* and Harold M. Vernon** State University of New York at Buffalo, School of Dentistry, Buffalo, N. k

A little more than 100 years ago the art of vulcanizing rubber was discovered. One early use of hard rubber was for denture bases. Until the intro- duction of methyl methacrylate (acrylic resin) to dentistry, many other materials were tried, but vulcanite remained the only satisfactory denture base material.

The natural color of vulcanite is a dark olive brown. Coloring it meant the addition of a consider- able amount of inert material, which decreased its strength and durability. It is also opaque: so that only surface color is visible. On the other hand, since acrylic resin in its pure state is colorless, its esthetic possibilities are unlimited.

Artificial teeth in the early days were opaque and unnatural looking. Wisely, the dental profession interested itself first in function and patient comfort and tried to design the denture base so that it showed very little.

THE PROBLEM

It is impossible to actually duplicate the color effect of the gingivae, but an illusion can be created which suggests and simulates it. To do this, it is necessary to borrow from both the impressionist and the camoufleur. The impressionist does not mix color on the palette to obtain his gradations. He stipples pure colors on the canvas in the form of dabs or dots in such a way that the eye fuses them to his desired effect when his paintings are examined at viewing distances. The camoufleur confuses the eye by color

This article was originally prepared by the authors a little over a decade ago. It has been slightly revised and is being submitted

at this time on the anniversary of the death of Harold M. Vernon to once again bring before the profession the name of the man who perhaps more than anyone else was responsible for the introduction of methyl methacrylate (acrylic resin) to

dentistry. *Associate Professor, Removable Prosthodontic Department.

**Deceased; formerly Research Director, Vernon-Benshoff Co.

and line so as to cause the camouflaged object to merge with the surroundings and be difficult to detect.

No tissue in the human body is of uniform color. All are aggregates of a number of hues, and most have varying shadings of the same hue.

The complaint that an upper and lower complete denture do not match in color has often been heard. Viewed outside of the mouth they probably do not, but in the mouth and backed by oral tissue they do match. This simply means that reproducing surface color and the color effect of a translucent body are different problems. The color effect of any denture base must be evaluated in its ultimate environment. the wearer’s mouth.

EARLY ATTEMPTS AT A SOLUTION

When acrylic resins were first used in dentistry. there was no background of experience or adequate research information available to the manufacturers. They were satisfied to produce it colorless with good optical properties. The first 100 or so clinical dentures made with resin supplied by the Vernon- Benshoff Company* were colorless.

Early color experiments with ester-soluble dyes were not encouraging. The available dyes were either degraded or destroyed by the catalysts or the heat or turned out to be inhibitors. The mixtures to which they were added simply failed to polymerize.

Water-soluble dyes were also tried. Results with them were not satisfactory. especially in regard to color stability: dentures colored with these dyes tended to bleach to a lighter shade in the mouth. Most of them also were transparent, and a good deal of opacifier had to be used to attain the proper opacity. The resulting effect was often an unaccept- able pale pink.

*Albany. N. \

4 IULY 1976 VOLUME 10 NUMBER 1 0022-3913/78/0140-ooo(100.40/0C 1978 The C. V. Morhy Co

COLORING ACRYLIC RESINS

Fig. 1. Pigment has attached to the external surface of polymer spheres. (Magnification x 450. Courtesy of the L. D. Caulk Co., Milford, Del.)

Fig. 2. Polymer particles are internallv pigmented .., throughout their structure. (Magnification x 4.50. Courtr- sy of the L. D. Caulk Co., Milford, Del J

INORGANIC PIGMENTS

The earliest successful coloring of denture base resins was accomplished with inorganic pigments. Although the production formulas of different shades of commercial products have remained a trade secret, it can be assumed that most manufac- turers are still making extensive use of them.

The production procedure is simple. The dry, finely ground pigments are mechanically blended with the polymer particles for a specified amount of time. Commonly used pigments include cadmium sulfide, cadmium sulfa-selenide, iron oxide, zinc chromate, and mercuric sulfide. Besides the coloring agents, opacifiers are also used. Titanium dioxide is employed most frequently. When viewed under the microscope, the small pigment particles appear to be concentrated on the exterior surface of the polymer spheres (Fig. 1). The amount of pigment used is very small in proportion to the powder, and because it is inert it does not enter into the polymerization reaction.

smaller particles yielded a more homogeneous color- ation.

When ball milling is used to apply pigments to polymer particles, the resin is subject to considerable variation in resultant color because of variations either in the ball milling or in the resin itself. The continued difficulty in maintaining constant color led to experimentation with organic pigments, which are more effective coloring agents. Using them can reduce the total amount of pigment added to the resin and simplify the manufacturing procedure.

ORGANIC PIGMENTS

As the unaided eye cannot detect the pigment particles, what is seen in the finished denture is the characteristic pink color only. The denture, being translucent, transmits the natural color of the oral

In the late 1950s the organic pigments began to appear on the market. The first organic pigments tested were insoluble in monomer, water, and alco- hol and were very potent. They gave some trouble in regard to stability under ultraviolet light \vhen used in low saturations and also were affected by the heat of polymerization.

-Most of the organic pigments were transparent. or nearly so, and all were inclined to cause the same problems as the water-soluble dyes. The resultant color was either a too-pale pink or 3 too-intense red.

tissue. Coloration by inorganic pigments does not affect

the physical properties of the resin; it affects only color and transparency. The mechanical properties of clear acrylic resin are the same as the properties of the powder pigmented to produce the pink shade.

INTERNALLY COLORED POLYMERS

Internally colored polymers are produced b) adding the pigment (either organic or inotganic) to the charge in the reactor kettle during production of the polymer. The resultant polymer particle is pigmented throughout its structure (Fig. 2~.

Although it would appear that internal coloring would be the answer to many problems, the tenden- cy of the polymerizing monomer is to reject inor-

Further research by the manufacturers with varia- tions of polymer particle size showed that larger particles would produce a mottled appearance, while

THE JOURNAL OF PROSTHETIC DENTISTRY

WINKLER AND VERNON

Fig. 3. End view of a rayon fiber showing lack of contact with the denture base resin. (Magnification X 1000.)

Fig. 4. Side view of a cracking rayon fiber showing a sliver.

ganic pigments, throwing them to the outside of the particle.

CHARACTERIZING FIBERS

Almost all denture base manufacturers produce polymers with fibers for increased characterization.

Many manufacturers at first used acetate rayon. While the rayon fibers were completely insoluble in monomer, they were capable of absorbing a consid- erable quantity of it. As a result the fibers changed in

length and diameter. The absorbed monomer was also found to leave the fibers slowly.

Rayon fibers also absorbed water after the denture was placed in service, and a considerable change took place in the fibers after being in the mouth a short time (Fig. 3). The fibers and the acrylic resin in which they are embedded have greatly different physical properties, and therefore the fibers are essentially foreign bodies within the denture base.

Cross-section swelling of acetate rayon fibers in

Fig. 5. Strain cracks in denture base resin occur dround the end of a rayon fiber.

Fig. 6. End view of an acrylic fiber showing excellent contact with denture base resin. (Magnification x 1000.)

water is 15%30%, while viscose rayon swells 45%

82s.’ Rayon fibers protruding from the surface of the denture will be affected by the moisture of the oral cavity. Fibers that are broken off leave voids

that can easily fill with oral fluids and other discol- oring matter (Fig. 4). While this is true only of fibers at or near the surface, it is also true that only the

fibers at or near the surface have any effect on the characterization of the denture. It was found that within a short time a great deal of this characteriza- tion was lost and the capillary effect was replaced by voids or a dirty brown discoloration. Minute strain cracks were also noted around the fibers (Fig. 5). Acrylic resin fibers, on the other hand, showed a negligible amount of swelling and were compatible with the denture base resin (Fig. 6). IKylon fibers

6 JULY 1976 VOLUME 40 NUMBER I

COLORING ACRYLIC RESINS

have also been successfully used to simulate the

minute blood vessels of the oral mucosa.

CHARACTERIZATION OF DENTURE BASES FOR PEOPLE OF COLOR

Pigmentation of the oral tissues is a frequent

clinical observation in many people of all races and nationalities.’ IC is evident in the gingivae, mucous

membranes, hard and soft palates, tongue, and

tissues of the floor of the mouth. Dental manufacturers have responded with

special shades for people with heavily pigmented

oral tissues. as the use of conventional pink denture base resins for these people can be distracting and

conspicuous. especially for those who have a high lip line.

Researchers have also described various tech- niques for the characterization of denture bases for

people of color. !-‘I

CONCLUSION

.4crylic denture base resins can be pigmented and characterized. Research has enabled dentists to

provide denture bases that are esthetically pleasing

to patients who have pigmented ginpi\.ae. ‘I‘hc

coloring of denture base resins has improved

throughout the years to complement the develop- ment of natural-looking artificial treth. ‘l‘oclav it is consistently possible to make virtualI!- ulldctectablt

complete dentures.

REFERENCES

Unpublished data. Vernon-Benshon‘ (:ompar!\-

Dummetr. C. 0 Oral pigmentation ,J I’WICI~~III 31::W.

1’W.

(Lhoudhary. S. (1.. Craig, J. F.. and Suls. I: ,J <:haracteri~- ing the denture base for non-Caucasian parirnt~ ,J PKOS.IIIEI

Drs1 33:i3. 1975. Gerhard. R.. and Sawyer. N.. Dentures to harmonize with heavily pigmented tissues. J Am Dent Assoc 73:~. 1960.

Quinliun, J I‘.: Characterization of drntuw bases. Dent

(Iin Sorth Am 19:321. 19X

\Vinklrr. S.. Morris. H. F., Ortman. H K.. Staab. R. M., and Baitz. H. C:.: CAaracterization of denture baw fur peopk of

color. J Am Dent :\ssor 81:1 $49, I970

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