The Effect of ChoppedPoly(methyl methacrylate) Fibers

on Some Properties of AcrylicResin Denture Base Material

Daryll C. Jagger, BDS, MSc, FDS RCS'Alan Harrison, TD, BDS, PhD, FDS Res'"

Purpose; The fracture of acrylic resin dentures remains an unresolved prohleni. Over theyears, various approaches to strengthening acrylic resin have been suggested, includingmodifying or reinforcing the resin. The aim of this study was to investigate the effect ofchopped pofylmetliyl methacrylate] (PMMA) fibers on some properties of acrylic resindenture base materidi. Materials and Methods: PMMA in the form of fibers 0,75 mm indiameter and 5 mm in length was added to acrylic resin denture base material in variouspercentages to form a composite material. The influence on doughing and manipulationtimes and transverse strength was examined. The results were subjected to statisticalanalysis using a one-way analysis of variance and, where appropriate, the Scheffé test.Results: The results showed that the doughing time was decreased by the addition offibers, with the manipulation and setting times showing inconsistent changes. There wasa significant difference between the materials in terms of the transverse strength. Whenthe amount of PMMA fibers in the acrylic resin was increased, there was a decrease inthe modulus of rupture and a decrease in (he modulus of elasticity. The differences wereshown to be statistically significant in some groups. Conclusion: The doughing time wasdecreased by the addition of fibers, while the manipulation and setting times showedinconsistent changes. The incorporation of chopped, randomly oriented PMMA fibersinto acrylic resin had no advantage over the unmodified polymer in terms of strength andcannot be recommended as a reinforcing agent for acrylic resin denture base material,IntJ Prosthodont 1999:12:542-546.

The material most commonly used in the construc-tion of dentures is polylmethyl methacrylate)

(PMMA), and although few would dispute that satis-factory esthetics can beachieved with this material, interms of mechanical properties it is still far from ideal.The fracture of acrylic resin dentures remains an un-resolved prohlem that may result from impact failure,for example dropping the denture accidentally, orfiom fatigue failure caused by repeated flexing undermasticatory forces. Several studies have investigatedthe prevalence and types of denture fractures,''^

'Lecturer, Restorative Dentistry, Department of Oral and DentalScience, Bristol Dental School and Hospital, Bristol, UK.••Professor and Head, Department of Orai and Dentai Science,Bristol Denial School and Hospital, Bristol, UK.

Reprint requests: Dr D. C. /agger, Department of Oral and DentaiScience, Division of Restorative Dentistry fProsthodontics), BristolDental School and Hospitai, Lower Maudlin Street, Bristol BSi 2LY,United Kingdom, e-mail: D.C.lagger@bris.ac.uk

Over the years, various approaches to strengtheningacrylic resin have been suggested, including modify-ing or reinforcing the resin. Strengthening has been ap-proached through chemical modification to producegraft copolymers of rubber methacrylate, referred to ashigh-impact resins,^ or by incorporating various metalforms" and several types of fibers to provide rein-forcement of fiacture-prone areas. Several types offibers in varying forms (chopped, woven, and linear),random or oriented, have been added to acrylic den-ture resins to improve their physical and mechanicalproperties. Reinforcement has been achieved throughthe inclusion of polyaramid fibers,^^ sapphirewhiskers,'' and carbon,"'-' In recent years, the inclusionof glass fibers'""'- and oriented or woven ultrahigh-modulus polyethylene fibers'^"'^ has produced en-couraging results; however, the extra laboratorysteps'^''" coupled with the additional time requiredhave limited their use. The incorporation of high-mod-ulus polyethylene beads did not have a significant ef-fect on the mechanical properties of the

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laagerflHarrison Efíecl of PMMA Fihers on Acryiic Resin Denture Base Mate

Many attempts to strengthen acrylic resin have failedbecause areas of stress concentration occur around em-bedded materiais, and the overall effect is to weakenrather than strengthen the denture base. Various sug-gestions have been made to improve the interface be-tween the denture base material and the reinforcingmaterial, such as sandblasting,'^ silanization,''' theuse of metal-adhesive resins,^" and plasma treatment,^'but failure at the interface remains a problem.

The physical compatibility of fiber and matrix is animportant factor in the success of fiber-reinforced com-posites. The concept of self-reinforcement of poly-mers has been discussed,--"-"' although to date not inthe dental literature. PMMA is a thermoplastic mater-ial and it is possible to impart molecular orientation intothe polymer by thermomechanical processing. Thehighly oriented molecules impart improved strengthcharacteristics over the randomly oriented and entan-gled polymer chains. The potential advantage of self-reinforced composite materials is that they shouldhave improved mechanical properties over the amor-phous random polymer. This study investigated theself-reinforcement of PMMA denture base resin.

The aim of this study was to investigate the eftectof chopped PMMA fibers on some properties ofacrylic resin denture base material. The properties in-vestigated were the doughing, manipulation, and set-ting times ofthe material, and its flexural strength. Inthis study, the fibers were incorporated into the acrylicresin polymer powder so that conventional curingand packing procedures could be used to avoid prob-lems relating to fiber orientation.

Materials and Methods

The materials used in this study are listed in Table 1.The ü.75-mm-diameter PMMA fibers were suppliedin precut 5-mm lengths. The short fiber length rep-resented a convenient size for manipulation and in-clusion in the acrylic resin dough. The percentagesof fibers used were 0, 5, 15, 20, and 25% by weight.

Transverse Bend Test

A series of polymerized blanks was produced bymixing 24 g of polymer, which included the appro-priate percentage of fibers, with 10 mL of methylmethacrylate monomer. The mixture was left to standin a plastic mixing vessel. The mixing procedurelasted 60 seconds each time and the mix was al-lowed to reach the dough stage prior to loading intoa gypsum mold in a dental flask. Molds were preparedby investing master perspex blanks in SYPSLrn-

Following a trial closure, the blanks were cured ina thermostatical ly control led water bath for 7 hours at

Table 1 Materials Used in the Study

Material Manufacturer Description

Trevalon Polymer

Trevalon Mcncmer

Dentsply

Dentsply

0.75-mm PMíií A fibersin 5-mm lengths

Heat-cured PMMA Ipowder (93%) *

BenzoyI peroxide(0.4%)

tVlMA monomerEthylenegl/colDimetfiacrylate (6%)Hydroquinone (0.006%)Highly drawn PMMA j .

tibers ,;

PMMA = poly(methyl msthacrylate)

7O''Cand 3 hours at lOO^C. The flasks were allowedto bench cool before opening. The cured plates werecarefully removed from the mold, the excess flashwas removed, and the specimens were finished usinga Kemet polishing machine with wet, self-adhesive,waterproof silicon carbide paper discs of 203-mm di-ameter and 320- and 600-A grit size. Each plate wasof sufficient size to be cut into four specimens usinga band saw. The specimens were then returned to thepolishing machine and carefully finished to dimen-sions of 64 mm X 10 mm x 2.5 mm as specified bythe International Standards Organization (ISO) andBritish Standards Institution (BSI) specification for thetesting of denture base resins.-^'^^ Ten specimensfrom each group were prepared and stored in a waterbath at 37 ±2°C for 7 days.

The transverse bend test was carried out using aLloyd's Instrument Material Testing Machine, modelL2000R using the 3-point method. The test rig is de-scribed in BSI specification BS 2487,-^ The test rigconsisted of a loading wedge and a pair of support-ing wedges placed 50 mm apart, which represents theaverage intermolar distance of a denture. Each testspecimen was centered on the testing rig so that theloading wedge, set to move at a speed of 5 mm/min,engaged the center of the upper surface of the spec-imen until the specimen broke. The specimens weretested in a water bath at 37 ± f C . Moduli of ruptureand elasticity were recorded.

Packing Plasticity Test

For the evaluation of the flow behavior of the pow-der/liquid mixtures, a packing plasticity test was per-formed using samples from each powder mix. Eachsample was made of 24 g of polymer (which includesthe appropriate percentage of fibers) with 10 mL ofmonomer. Three samples from each powder mix weretested and the doughing, manipulation, and settingtimes for each group were calculated. The packingplasticity was performed using a needle penetrometer.

Volume 12, Number 6, 1999 543 Tlie International |ou

Efiecl of PMMA Fibers on Acryiic Resin DeiiLure B.ise Material lagger/iHarrison

E£

elra

tio

S.

•

13-

12-

11 -

10-

9-

a-

7-

6-

5-

4 -

3 .

2-

1 •

0 -

\\ \\ \V V\ \\\ w\\\

V. ^ V '

N

1 1 —10 20

- . -- » --*—- T —

- * -

130

Time (min)

Controi5%15%20%25%

*

^ ; ; : : : : : ^ !

S •T40

~T 150 60

Ffg 1 Effect o¡ the addition otPMMA fibers (Q.7S-mm diarrie-ter, 5-mm length) on the (low be-havior ot Trevaion i"--ryli<: reSindenture base rnaieii-si

I i I

1

JControl 6%group group

iDougining lime

15% 20% 25%group group group

JManipulation lime | |SeHing time

Fig 2 Etfect of fhe addition ofPMMA fibers (0.75-mm diame-fer, 5-mm iength) on fhe dough-ing, manipulation, and seffingtimes of Trevaion acrylic resindenture base material.

which has been shown to be an effective method ofmeasuringdoughing and manipulation times.^''Usingthis technique, fhe point at which the penetration de-creases sharply (penetration < 10 mm) indicates the be-ginning of the dough stage ¡ie, doughing time). At apenetration of < 3 mm the material is not workable andthe setting time is therefore defined as the time frommixing untii the material becomes rubbery and un-

workable (at the completion of tbe manipulation time).The manipulation time is the period between dough-ing (penetration < 10 mm) and setting time (penetra-tion < 3 mm) in which it is possible to manipulate andpack the dough. The results presented in Figs 1 and 2and Tables 2 and 3 were subjected to statistical analy-sis using a one-way analysis of variance (ANOVA)and, where appropriate, the Scheffé test.

Tlie Infemalionai Jouniai ol Prostiiodonlics 544 Tiel2, NumberE.

I agger/Harrison Etfen of PMMA Fibers on Acrylic Resin Denture 8ase Material

Results

The results showed that there was a significant dif-ference between the materials in terms of the trans-verse strength. Increasing the amount of PMMA fibersin the acrylic resin decreased the modulus of rupturefrom 74.0 MPa (control] to 57.1 (25%) (Table 2) anddecreased tbe modulus of elasticity from 1,831 MPa(control) to 1,617 MPa 125%] (Table 3). The differ-ences were shown to be statistically significant. Theresults showed that the manipulation and settingtimes (Fig 2] gradually increased as the percentage offibers in the acrylic resin powder increased, with theexception of the addition of 25% fibers.

Discussion

Transverse strength is a measure ofthe stiffness andresistance to fracture. The ISO 1567-= and BSI2487^''for denture base resins have specified transverse de-formation limits of from 1 to 2.5 mm for a force of 15to 35 N and from 2 to 5 mm for a force of 15 to 50N. The mean breaking force of acrylic resin shouldnot be less than 55 N. In this respect the control ma-terial and the moditied specimens containing 5%fibers satisfied the standard.

A one-way ANOVA demonstrated a significant dif-ference in the modulus of rupture between the con-trol specimen and tbe modified specimens. The dif-ferences, however, were small, with the controlspecimen demonstrating the highest value. The ad-dition of a low percentage of fibers |5%l producedonly a 2% decrease. However, with an increase in thepercentage of fibers (25%], tbere was a 23% reduc-tion in the modulus of rupture,

A one-way ANOVA showed a significant differencein the modulus of elasticity between the groups. Thedifference between tbe highest and lowest valuesrecorded was 12%. The control material exhibited thehighest modulus of elasticity, in this study, the fiberswere incorporated into the acrylic resin polymerpowder so that conventional curing and packing pro-cedures could be used to avoid problems relating tofiber orientation. Previous studies on the self-rein-forcement of PMMA-- used fibers that were alignedapproximately in the tensile stress direction (contin-uous fiber unidirectional composites], rather thanthe discontinuous, randomly orientated fibers used inthis study. The previous study reported an increase inihe tensile strength, tensile modulus, and tensile strainto failure for self-reinforced composite PMMA com-pared with conventional PMMA. In unidirectionalform the fibers were assumed to carry the bulk of thetransmitted load, but it is likely that for the discon-tinuous fibers the load is shared between the fiber and

Table 2 Modulus of Rupture

No. ot Mean transversespecimens bend strength ¡MPa) SD

TT + 5% FT * Z0% FT + 15%FT + 25%F

109

107

10

74.072,762.960,157.1

5.953.677.0Q :i.

10.397.45

"One-way ANOVA indicated thai there was a significant difterence betweenthegroLpslPîOOOOOl).Vertical bars indicate values llial were not significantly differeni accord-ing to tlie Sctieffé tesl,T = Trevalon acrylic resin demure base maleriak F = PMMA fitters (0.75-mm diameter, 5-rnm lengtti); SD = standard deviation.

Table 3 Modulus of Elasticity

Material'

TT + 5% FT + 30% FT + 15% FT + 25% F

No.otpecimens

109

107

10

Mean elasticmodulus (MPa)

18311801173717011617

SD

8610839 -,82

1378

'One-way AhJOVA indicated that there was a significant difterence bBtwaenlhiegroup£(P=0.0001).Vertical bars indícale i/alues ttiat were not significantly different accord-ing to the Sclieffé lest,T = Trevalon acrylic resin denture base material; F = PMMA tibers (0.75-mm diameter, 5-mm length]; SD = standard deviation.

the matrix. For the continuous fibers, the increasedtoughness was attributed to a number of deformationmechanisms operating in the samples: increased duc-tility in the self-reinforced composite PMMA fibersand at tbe fiber interface, diversion of tbe crack outofthe original crack plane by fiber splitting, and fibermatrix interface failure. Because of differences infiber orientation, comparison with the results ofGilbert et al-^ is inappropriate.

The rhéologie properties ofthe PMMA dough willaffect its ability fo flow and can influence the accu-racy/quality of the molded denture. Mutlu et a l "stressed the importance of measuring the flow prop-erties during dough formation and the need for aknowledge ofthe correct doughing and manipulationtimes, because it is important that the dough is packedduring the manipulation stage. The results ofthe pack-ing plasticity test in Figs 1 and 2 show that the dough-ing time was decreased by the addition of the fibers,although not significantly (control 7 min, 5% fibers 5min, 15% fibers 5 min, 20% fibers 6 min, and 25%fibers 3,5 min¡. There were similarities between all 5flow curves (Fig 11. There were significant differencesin the setting times with the addition of the fibers tothe polymer powder, although the changes were in-consistent. The manipulation times ranged from 14.5

r 6, 199S 5 4 5 The Intcrnatianai lournal of Prostliudoniics

Effect of PMMA Eibers on Acrylin Resin Denlure Base Material I agger/Harrison

min (5% fibers) to 25,5 min (25% fibers). There weresignificant differences in the manipulation times, al-though the differences were inconsistent. In practice,the manipulation time is the most relevant since it re-flects the time available to the dental technician topack the resin in the mold(s). An extended manipu-lation time is therefore essential because it allowsseveral cases to be packed from a single mix. The ma-nipulation time can be extended by refrigerating themix; this technique is used in some laboratories.

Conclusions

• The incorporation of chopped, randomly orientedpoly(methyl methacrylate) fibers to acrylic resinhad no advantage over the unmodified polymerin terms of strength, and it cannot be recom-mended as a reinforcing agent for acrylic resindenture base material in its present form,

• The doughing time was decreased relative to thecontrol by the addition of fibers, while the ma-nipulation and setting times showed inconsistentchanges.

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The intemaiional Journai ol Prosthûdûnlics 546 Volume 12, Number 6,