The tensile strength of a composite resin reinforced with carbon fibers

Barry M. Kilfoil, D.D.S.,+ Richard A. Hesby, D.D.S., M.S.D.,** and George B. Pelleu, Jr., Ph.D.*** National Naval Dental Center, Bethesda, Md.

F ixed prosthodontics is the art and science of restoring damaged teeth with cast metal or porcelain restorations and of replacing missing teeth with fixed, or cemented, prostheses.’ Although they are among the finest ser- vices rendered, fixed prostheses and cast restorations may not be the restorations of choice because of the prognosis for the abutment teeth, pulp size, or the patient’s health, availability, or finances. Advances in the area of acid-etched bonded fixed partial dentures, however, have made it possible to place fixed restora- tions where they were previously contraindicated.*

In selected patients, pontics attached with composite resin to acid-etched enamel on abutment teeth can be thought of not as a substitute for cast restorations but as an alternative to mucostatically retained removable partial dentures, space maintainers, or bulky splints. Although acid-etched bonded fixed partial dentures are conservative and economical, limitations in the strength of the composite resin have restricted the use of these restorations.

Studies in the field of synthetic resins have shown a qualitative improvement in the physical properties of expoxy and acrylic resins reinforced with linear carbon fibers. Schreibe+’ evaluated denture base resins rein- forced with carbon fibers and found that the transverse and impact strengths were increased by 50%. Manly and Bowman5 demonstrated that the addition of carbon fibers to standard denture base resins tripled their tensile strength. Bowman et a1.6v7 investigated the

The opinions or assertions contained herein are the private ones of the authors and arc not to be construed as official or as rcfleeting the views of the Department of the Navy.

Supported through funds provided by the Bureau of Medicine and Surgery under Navy Medical Research and Development Com- mand Research Work Unit No. M0095-.003-3014.

Semifinalist, American College of Prosthodontists Annual Research Award Competition, Monterey, Calif.

‘Lieutenant Commander (DC) USN; Dental Department, U.S.S. Saratoga (CV-60); formerly resident in prosthodontiq NNDC.

**Captain (DC) USN; Chairman, Prosthodontics Department. ***Chairman, Research Department.

40

C 6+

Fig. 1. Top and side views of test sample: composite bar (shaded urea) in clear acrylic resin block. Dimen- sions in millimeters.

biocompatibility of carbon fibers and found no evidence of long-term toxicity or carcinogenicity when these fibers were implanted in mice for periods of 2 years.

If the addition of carbon fibers to composite resin increases the resin’s resistance to fracture, some of the shortcomings associated with acid-etched bonded fixed partial dentures might be eliminated. At this time, however, there are no published reports evaluating the use of carbon fiber-reinforced composite resin with these restorations.

The purpose of this study was to compare the flexure strength of unmodified and carbon fiber-reinforced composite resins mechanically interlocked with simu- lated acrylic resin denture teeth.

MATERIAL AND METHODS

Nuva-Fil P.A. (L. D. Caulk Co., Milford, Del.) composite resin was selected for use in this study because of its unlimited working time. Clear acrylic resin blocks were used rather than actual denture teeth because the blocks could be more precisely indexed into

JULY 1983 VOLUME 50 NUMBER 1

Table I. Carbon fiber content of 0.175 gm Table II. Maximum load at fracture and composite resin samples flexure strength of composite resin samples

Group Fiber content

(gm) % of fiber by weight Carbon fiber

content (gm)

No. of samples

Maximum load at

fracture (kg) (mean + SD)

Flexure strength (kg/cm*) I (control) 0 0

II 0.001 0.57 III 0.005 2.8 IV 0.035 20

a flexible mold, and their transparency would allow maximum penetration of ultraviolet light into the depth of the mold as well as visual inspection of the composite resin for voids or cracks. Dimensions of the blocks were those given by Wheeler’ for an average permanent central incisor.

Fifty acrylic resin blocks were made from heat-cured resin (Palatex, Moyco Industries Inc., Philadelphia, Pa.) and trued to dimensional accuracy. A transverse groove 3.5 mm X 1 mm was machined in each block with a l&inch carbide cutter. An aluminum jig was used in the milling process to ensure uniformity. In a separate procedure, the transverse groove was undercut with a No. 37 inverted cone carbide bur modified to cut only on its circumference, 1 mm from the tip. The edge of the groove served as a guide for the modified bur.

All blocks were inspected for uniformity of dimen- sions with a Boley gauge and machinist’s square, and weighed on an analytic balance (Type 24N, William Ainsworth & Sons, Denver, Colo.). The completed blocks were stored in water at 100” F.

Two representative blocks were selected. A wax bar was placed in the transverse groove of each block, completely filling it and extending 3.5 mm laterally on both sides. Each bar and block unit was attached, wax side down, to a glass slab and boxed with sheet wax to form a mold space 1% inches in diameter and % inch thick. This mold space was filled with RTV silicone pattern-making material (Dow-Corning Corp., Mid- land, Mich.) and vacuum treated to remove air. Each rubber mold was allowed to cure for 48 hours and then was separated from its wax border.

After separation, the wax bars were removed from the acrylic resin blocks and the blocks were removed from the rubber molds. The resulting molds indexed the acrylic resin blocks and provided a precise space for forming the composite resin in the construction of test samples. One of the molds was selected for all sample construction and the other was set aside.

Five test samples (Fig. 1) were constructed with unmodified composite resin to refine the technique and determine the mean amount of composite resin

0 (control) 13 5.50 2 1.65 28.3 0.001 10 4.99 + 0.90 25.7 0.005 10 3.16 + 0.54 16.2’ 0.035 8 2.33 + 0.73 12.0,

*Significantly different from control @ < .Ol, Student’s t-test); the samples containing 0.001 gm of carbon fiber did not differ signifi- cantly from the control samples (p > .05). Statistical analyses were made with the use of maximum load at fracture values.

required for each sample (0.175 gm). Then four groups of samples were constructed (Table I). For the first group, which served as the control, the bar portions of the acrylic resin blocks were filled with unmodified composite resin. The bar portions of the blocks in each of the remaining groups were filled with a different concentration of carbon fiber-reinforced composite resin (sized carbon fibers, Gougeon Bras., Bay City, Mich.). The completed samples were stored in water at 100” F and then thermocycled alternately between 60” F and 130” F, 5 cycles/day for 6 days.

Flexural strength was determined on an Instron Universal testing machine (Instron Corp., Quincy, Mass.) with a crosshead speed of %oo inch/min. An aluminum test fixture (Fig. 2) supported the samples between points 12 mm apart. The load was applied midway between the points in a plane perpendicular to the sample with a hardened steel insert 2 mm in diameter. Each sample was tested to the point of fracture as determined by the maximum load on the recording graph of the testing machine. All samples were tested in a random manner on the same day. Statistical comparisons for significance were made between maximum load values by means of the Stu- dent’s t-test.

To calculate flexure strength the follo,wing formula was used9:

S=3WL

2 b dZ

where S = flexure strength L = distance between supports b = width of specimen W = mean maximal load at fracture

RESULTS

The maximum load at fracture and the flexure strength values for the composite resin with and

THE JOURNAL OF PROSTHETIC DENTISTRY 41

KILFOIL, HESBY, AND PELLEU

LOAD CELL

Fig. 2. Fixture used to support samples on Instron load cell during test.

without carbon fibers are shown in Table II. All but two samples fractured through the bulk of composite resin at or just inside the junction with the acrylic resin block. Flexure strength values at fracture ranged from 28.3 kg/cm2 for the control group to 12 kg/cm2 for the composite resin containing 0.035 gm of carbon fiber. The mean flexure strength of composite resin samples containing 0.005 gm and 0.035 gm of carbon fiber was significantly lower than that of the control group (p < .Ol).

DISCUSSION

The addition of carbon fibers increases the strength of polymethyl methacrylate resins used in denture basest0 Epoxy composite resins are also strengthened by the addition of carbon fibers.11~‘2 The results of our study, however, indicate that carbon fibers added to a restorative composite resin decrease rather than increase the flexural strength of the material.

Skinner and Phillips’ defined the term “composite material” as a three-dimensional combination of at least two chemically different materials with a distinct interface separating the components. Nuva-Fil P.A., the composite resin used in this study, is a two-phase system consisting of an organic resin and quartz particles bound at their interface with a silane coupling agent. The coupling agent chemically bonds the phases with a weak yet sufficient bond.

We hypothesize that the addition of carbon fibers to the Nuva-Fil P.A. system can weaken it in one of three ways. First, the carbon fibers can act as simple

inclusion bodies and create stress lines that promote early fracture. Second, if the composite resin and the carbon fibers have different coefficients of thermal expansion, any bond formed as the resin undergoes polymerization may be broken. The third and perhaps most likely reason is that the surface treatment of the carbon fibers used by the manufacturer to enhance bond strength with epoxy and polyester resins in the fluid state may be incompatible with the much more viscous Nuva-Fil P.A. and may not allow the surface of the carbon fibers to be wetted by the composite resin.

Plueddemannt3 stated that adhesion between organic resins and unmodified carbon fibers is good but that even if the adhesion is perfect, a weak composite resin could result. This weakness may be due to cohesive failure in a weak boundary layer just below the surface of the carbon. Composites of oxidized carbon and nonpolar resins probably will require silane coupling agents at the interface to allow stress relaxation while retaining mechanical properties after exposure to moisture.

Perhaps treating the surfaces of the carbon fibers with a silane coupling agent or using a composite resin that would better wet the surfaces of the fibers will increase the strength of dental composite resins.

SUMMARY

A study was undertaken to compare the flexure strength of unmodified and carbon fiber-reinforced composite resins. If the addition of carbon fibers to a

42 JULY 1983 VOLUME 50 NUMBER 1

TENSILE STRENGTH OF COMPOSITE RESIN

dental composite resin increases the resin’s resistance to fracture, some of the shortcomings associated with acid-etched bonded fixed partial dentures can he over- come. The findings of the study, however, showed that the addition of carbon fibers to a dental composite resin actually decreased the flexure strength.

REFERENCES

1.

2.

3.

4.

5.

6.

Shillingburg, H., Hobo, S., and Whitsett, L.: Fundamentals of Fixed Prosthodontics. Chicago? 1978, Quintessence Publishing co., p 9. Ibsen, R., and Neville, K.: Adhesive Restorative Dentistry. Philadelphia, 1974, W. B. Saunders Co., p 139. Schreiber, C. K.: Polymethylmethacrylate reinforced with carbon fibres. Br Dent J 130~29, 1971. Schreiber, C. K.: The clinical application of carbon fibre/

polymer denture bases. Br Dent J 137~21, 1974. Manly, T. R., and Bowman, A. J.: Denture bases reinforced with carbon fibres. Br Dent J 146~25, 1979. Bowman, A. J., Cook, M., and Jennings, E. H.: Technique for

studying tissue reaction to embedded carbon fibers. J Dent Res 52~982, 1973.

7. Bowman, A. J., Cook, M., Jennings, E. H., and Rannie, I.: An interim report on long-term toxicity studies on carbon fibre inplants. J Dent Res 53~1080, 1974.

8. Wheeler, R. C.: A Textbook of Dental Anatomy and Physiol- ogy, ed 4. Philadelphia, 1968, W. B. Saunders Co., p 126.

9. Skinner, E. W., and Phillips, R. W.: The Science of Dental Materials, ed 6. Philadelphia, 1968, W. B. Saunders Co., p 38.

10. Wylegela, R. T.: Reinforcing denture base material with carbon libres. Dent Tech 26~97, 1973.

Il. McCreight, L. R.: Overview of fiber composites. J Dent Res 46~1167, 1967.

12. McFarland, S. R.: Fiber reinforced composites for orthodon- tics, prosthetics and mobility aids. J Biomed Sci Instr 11:151, 1975.

13. Plueddemann, E.: Interfaces in Polymer Matrix Composites. New York, 1974, Academic Press, Inc., vol 6, pp 206-207.

Reprint requests to: DR. GEORGE B. PELLEU, JR.

NATIONAL NAVAL DENTAL CENTER

BETHESDA, MD 20814

I Bound volumes available to subscribers

Bound volumes of the JOURNALOF F%x3THETIC DJWTLWRY are available to subscribers (only) for the 1933 issues from the publisher at a cost of $39.00 ($49.00 international) for Vol. 49 (January-June) and Vol. 50 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact Mr. Deans Lynch at The C. V. Mosby Co., 11830 Westline Industrial Drive, St. Louis, MO 33146, USA.

Subscriptions must be in force to qualii. Bound volumes are not available in place of a regular JOURNAL subscription.

THE JOURNAL OF PROSTHETIC DENTISTRY 43