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An investigation into the transverse and impact strength

of `high strength' denture base acrylic resins

D. C . J AGGER * , R . G. J AGGER , S. M. ALLEN & A . H A R R I S O N * *Department of Oral and DentalScience, Division of Restorative Dentistry (Prosthodontics), Bristol Dental School and Hospital, Bristol and Department of Adult Dental Health,

Dental School, University of Wales, Cardiff, UK

SUMMARYSUMMARY A range of materials, often marketed as

high strength resins is available. These materials are

often expensive options to conventional heat-cured

acrylic resin. The aim of this study was to investigate

transverse and impact strength of ve `highstrength' acrylic resin denture base materials. A

conventional heat-cured acrylic resin was used as a

control. Specimens were prepared as specied in the

International Standard Organization (ISO 1567:

1988) and British standards for the Testing of

Denture Base Resins (BS 2487: 1989) and the British

Standard Specication for Orthodontic resins (BS

6747: 1987) for transverse bend and impact testing.

The impact strength was measured using a Zwick

pendulum impact tester and the transverse bend

strength measured using a Lloyds Instruments

testing machine. The results showed that Metrocryl

Hi, Luctitone 199 and N.D.S. Hi all had an impact

strength which was signicantly higher than thecontrol. For the modulus of rupture, there was a

signicant difference between Sledgehammer and

the other groups. There was no signicant difference

between the other groups and the control. For the

modulus of elasticity, Sledgehammer produced the

highest value followed by the control. The remain-

ing four materials had a modulus of elasticity less

than the control.

Introduction

The material most commonly used in the construction

of dentures is poly (methyl methacrylate), however

this material it is not without limitations, particularly

in terms of exural and impact strength. Attempts to

improve the mechanical properties of poly (methyl

methacrylate) have taken the researcher through

many avenues and the reinforcement of denture base

materials has been reviewed (Jagger et al., 1999).

Alternative materials to poly (methyl methacrylate)have been introduced only later to be withdrawn

(Mutlu et al., 1989). Over the years various types of

bres or beads, such as carbon (Bowman & Manley,

1984; Ekstrand et al., 1987), polyethylene (Clarke

et al., 1992; Ladizesky et al., 1994), glass (Solnit,

1991; Vallittu, 1996), aramid (Grave et al., 1985;

Berrong et al., 1990) and poly (methyl methacrylate)

(Jagger & Harrison, 1999; Jagger et al., 2000) have

been added to acrylic resin in an attempt to improve

its mechanical properties. Metal inserts in the form of

wires, meshes and plates have been incorporated into

dentures in an attempt to reinforce areas potentially

vulnerable to fracture (Vallittu & Lassila, 1992;

Polyzois, 1995).

The chemical modication of acrylic resin through

the incorporation of rubber in the form of butadiene

styrene has been successful in terms of improving the

impact strength (Rodford, 1990; Rodford & Braden,

1992). However, the incorporation of rubber has notbeen entirely successful in that it can have detrimental

effects on the modulus of elasticity and hence the

rigidity of the denture base. A wide range of materials,

often marketed as high strength resins is available.

These materials are often expensive options to conven-

tional heat-cured acrylic resin.

The aim of this study was to investigate transverse

and impact strength of a selection of `high strength'

2002 Blackwell Science Ltd 263

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acrylic resin denture base materials. A conventional

heat-cured acrylic resin was used as a control.

Materials and method

The materials used in the investigation are presented in

Table 1. A series of polymerized blanks was produced

by mixing the appropriate amount of polymer with

monomer according to the manufacturers' instructions.

The mix was allowed to reach the dough stage prior

to loading into a gypsum mould in a dental ask.

Moulds were prepared by investing master Perspex*

blanks in gypsum. Following a trial closure, the

plate specimens were cured in a thermostatically

controlled water bath for 7 h at 70 C and 3 h at

100 C. The asks were allowed to bench cool before

opening. The cured plates were carefully removed from

the mould, the excess ash was removed and the

specimens nished using a Kemet polishing machine

with wet, self-adhesive. waterproof silicon carbide

paper discs of 203 mm diameter, 320 and 600 A grit

size. Each plate was of sufcient size to be cut into four

specimens using a band saw. The specimens were then

returned to the polishing machine and carefully

nished to the dimensions of 64 10 25 mm for

the transverse bend tests and 50 6 4 mm for the

impact tests, as specied in the International Standard

Organization (ISO 1567: 1988) and British standards

for the Testing of Denture Base Resins (BS 2487: 1989)

and the British Standard Specication for Orthodontic

Resins (BS 6747: 1987). A tolerance of 0 03 mm was

accepted. Ten specimens were prepared for each

percentage group and stored in a water bath at

37 2 C until fully saturated. The impact specimens

were taken from the water bath and stored in air for

1 h prior to testing.

Impact test

For the impact tests on each specimen, a v-notch was

cut to a depth of 08 mm leaving an effective depth

under the notch of 32 mm. This was carried out with a

Zwick notch cutter (Model 5000) and a notch broach

69D. The impact strength was measured using a Zwick

pendulum impact tester (Model 5102). Impact tests

were undertaken in air at 20 2 C.

Transverse bend test

The transverse bend test used the three-point loading

method (British Standard Specication For Denture

Base Polymers 2487: 1989) with a cross-head speed of

5 mm min1. A Lloyds Instruments testing machine

(Model L2000R) was used. Specimens were tested in a

water bath at 37 C to simulate oral conditions. Moduli

of rupture, moduli of elasticity and peak load were

recorded.

Results

The results of the impact tests are presented in Table 2and the results of the transverse bend tests are

presented in Tables 35. The results in Tables 35

were subjected to statistical analysis using a one-way

analysis of variance and where appropriate the Scheffe

test.

Table 1. Materials used in the investigation together with the

manufacturer

Material Manufacturer

Metrocryl High Metrodent, Hudderseld, UK

Luctitone 199 Dentsply, Weybridge, UK

Sledgehammer Bracon, Etchingham, UKEnigma Hi-base Schott lander, Letchworth, UK

N.D.S. Hi Mobildent, St Annes, UK

Trevalon (control) Dentsply, Weybridge, UK

Table 2. Impact fracture energies (kJ m2) of denture base acrylic

resins. Zwick notched Charpy test. Specimens tested in air at

20 2 C

Material

Number of

specimens

Mean

(kJ m2) s.d.

Metrocryl Hi 10 1145 2

34

Luctitone 199 9 1058 203

N.D.S. Hi 10 913 103

Enigma Hi-base 9 773 163

Sledgehammer 9 740 220

Trevalon 10 494 329

A one-way analysis of variance demonstrated that there was a

signicant difference between some groups P < 0001.

A Scheffe test was performed and the vertical tie bars in the

table indicate values, which are not signicantly different between

one another.

*ICI, Welwyn Garden City, UK.Kemet, Maidstone, Kent, UK.

Zwick Testing Machines Ltd, Leominster, Hereford, UK.Lloyds Instruments plc, Southampton, UK.

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Discussion

The fracture of acrylic resin dentures remains an

unresolved problem and failure is probably because ofa multiplicity of factors rather than the intrinsic

properties of the denture base material. Various

attempts have been made to overcome the problems

associated with the fracture of acrylic resin dentures.

One approach is the development of denture base

materials marketed as high strength in comparison with

conventional heat-cured acrylic resin. This study

compared the impact and transverse strengths of ve

`high strength' denture base acrylic resins with a

conventional heat-cured acrylic resin.

Impact test

In all the impacts tests, the specimens broke with a sharp

fracture. This type of fracture is typical of brittle fracture

behaviour characterized by a lack of distortion of the

broken parts. Impact energy values satised the require-

ments of the British Standard Specication for ortho-

dontic resins BS 6747: 1987 although it should be noted

that these are auto-polymerizing resins. The resultsdemonstrated that there were signicant differences

between some materials. There was no signicant

difference between Enigma Hi-base (773 kJ m2) and

Sledgehammer** (740 kJ m2) compared with the

control (494 kJ m2). The impact fracture energies

ranged from 1145 kJ m2 for the highest value recorded

for Metrodent Hi to 494 kJ m2 for the conventional

heat-cured acrylic resin Trevalon. Metrocryl Hi,

Luctitone 199 and N.D.S. Hi had an impact strength

which was signicantly higher than the control.

An improvement in the impact strength of the `highstrength' acrylic resins was expected. This result should

be reected in a reduction in the clinical failure of

dentures through impact fracture manufactured from

Table 5. Peak load (N). Transverse strength of acrylic resin

denture base materials. (Test cross-head speed 5 mm min1, tested

in water at 37 C)

Material

Number of

specimens

Peak load

(N) s.d.


67

Trevalon 10 5258 606

Luctitone 199 9 5233 300


N.D.S. Hi 6 5133 304

Enigma Hi base 10 5097 507


signicant difference between Sledgehammer and the other

groups P < 0001.

A Scheffe test was performed and the vertical tie bars in the table

indicate values which are not signicantly different between one

another.

Table 3. Modulus of rupture (MPa). Transverse strength of

acrylic resin denture base materials. (Test cross-head speed

5 mm min1, tested in water at 37 C)

Number of Modulus of rupture

Material specimens (mean Mpa) s.d.

Sledgehammer 10 77

35 4

49Trevalon 10 6291 700

N.D.S. Hi 6 6174 319


Luctitone 199 9 6144 361



signicant difference between Sledgehammer and the other

groups P < 0001.


indicate values, which are not signicantly different between one

another.

Table 4. Modulus of elasticity (MPa). Transverse strength of

acrylic resin denture base materials. (Test cross-head speed

5 mm min1, tested in water at 37 C)

Material

Number of

specimens

Modulus of elasticity

(mean MPa) s.d.


Trevalon 10 17370 398

N.D.S. Hi 6 15283 921



3

Luctitone 199 9 1465 1003


signicant difference between some groups P < 0001.


indicate values which are not signicantly different between one

another.

Schottlander, Letchworth, UK.

**Bracon, Etchingham, UK.Metrodent, Hudderseld, UK.

Dentsply, Weybridge, UK.Mobildent, St Annes, UK.

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these materials. It is not possible to discuss the

mechanisms of reinforcement of the individual acrylic

resins as most manufacturers of denture base materials

are reluctant to reveal the exact constituents of the

products, or the mechanisms of reinforcement used.

The development of high impact strength denture base

acrylic resin and the mechanisms of reinforcement has

been discussed in more general terms (Rodford &

Braden, 1992). The mechanism of reinforcement

described was acrylate terminated butadiene styrene

copolymers (Macromers) of relatively low molecular

weight and narrow molecular range together with non-

acrylate terminated block copolymers.

The results of this study are in agreement with

Murphy et al. (1982) in an investigation of some

mechanical properties of three denture base materials,

a conventional heat-cured acrylic resin, a rubber

reinforced heat-cured acrylic resin and an injectionmoulded, rubber-reinforced acrylic resin. They reported

a considerable improvement in the impact strength for

the rubber-reinforced polymers. However, in contrast,

Johnston et al . (1981), in an investigation of the

exural fatigue of 10 commonly used denture base

acrylic resins which included a `high strength' acrylic

resin, reported that the `high impact' acrylic resins as a

category did not exhibit superior results compared with

other resins.

Transverse bend test

The transverse (exural strength) of a material is a

measure of stiffness and resistance to fracture. Flexural

strength tests were undertaken as these were consid-

ered relevant to the loading characteristics of a denture

base in a clinical situation. A number of studies (Regli &

Kydd, 1953; Swoop & Kydd, 1966) have established

that dentures in service undergo only small deforma-

tions and Ladizesky et al. (1993) reported that exural

modulus should be measured at similar small deforma-

tions. In addition when deections are small the

calculated exural modulus may be regarded asYoung's (elastic) modulus of the material.

The machine chosen for the tests, the Lloyd's

Instrument Materials Testing Machine is routinely used

and widely accepted. The tests were carried out

according to BS 2487: 1989 such that the results were

directly comparable with previous studies. The Inter-

national Standard Organization (ISO 1567) (1988) and

the British Standard specication 1989 (BS 2487) for

denture base resins have specied transverse deforma-

tion limits which are from 1 to 25 mm for a force of 15

35 N and 25 mm for a force of 1550 N. The average

breaking force of acrylic resin should not be less than

55 N. In this respect, only Sledgehammer satised the

requirement. There were no signicant differences

between the other materials tested. The lowest recorded

peak load was for Enigma (5097 N).

Stafford et al. (1980) compared the properties of a

range of high impact polymers with conventional

acrylic resin denture base materials. They demonstrated

that some unreinforced conventional acrylic resin

materials had a higher fatigue life value compared with

the reinforced acrylic resins but demonstrated a large

scatter. In the present study, for the modulus of

rupture, a one-way analysis of variance demonstrated

a signicant difference between Sledgehammer and the

other groups. There was no signicant differencebetween the other groups and the control. An increase

in the modulus of elasticity is associated with an

increase in the rigidity of the denture base material.

Sledgehammer produced the highest value of

1999 MPa, followed by Trevalon. The remaining four

materials had a modulus of elasticity less than the

conventional heat-cured control. The reduction in the

modulus of elasticity may be a reection of the type of

reinforcement, for example the incorporation of rubber.

ConclusionsMetrocryl Hi, Luctitone 199 and N.D.S. Hi all had an

impact strength which was signicantly higher than the

conventional heat-cured acrylic resin control material.

For the modulus of rupture, there was a signicant

difference between Sledgehammer and the other

groups. There was no signicant difference between

the other groups and the control. For the modulus of

elasticity, Sledgehammer produced the highest value

followed by the control. The remaining four materials

had a modulus of elasticity less than the control.

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E-mail: d.c.jagger@ bris.ac.uk

H I G H S T R E N G T H D E N T U R E B A S E A C R Y L I C R E S I N S 267


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