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Effect of surface preparations on the repair strength of denture baseresin
Hiroshi Shimizu, DDS, PhD,a Kenneth S. Kurtz, DDS,b Masaharu Yoshinaga, DDS,a
Yutaka Takahashi, DDS, PhD,a and Tetsuya Habu, DDS, PhDaaDivision of Removable Prosthodontics, Department of Oral Rehabilitation,
Fukuoka Dental College, Fukuoka, Japan, and bMontefiore Medical Center/Albert
Einstein College of Medicine, Bronx, NY, USA
Purpose: The objective of this study was to evaluate the effect of surface preparations on the repair bondstrength of an acrylic denture base resin material.
Materials and Methods: Rectangular specimens (n = 80) were prepared with a heat processed denturebase resin (Lucitone 199) and divided into five groups. Four sets of 16 specimens were cut in half, and theremaining one set was not cut and served as the reference group. The surface preparations for the cut
specimens were; 1) without preparation, 2) application of dichloromethane, 3) air-abrasion with 50 µm
alumina, and 4) alumina air-abrasion followed by application of dichloromethane. These four groups were
repaired with an autopolymerizing acrylic repair resin (Repair Material). The specimens were immersed in
37˚C water for either one-day or 30 days. Transverse strengths were determined with a three-point bending
test.
Results: Repair bond strengths in MPa of the denture base material after one-day and 30 days were; 73.0
and 52.0 for group 1, 74.8 and 57.3 for group 2, 76.7 and 54.7 for group 3, and 76.3 and 64.9 for group 4,
whereas transverse strengths of uncut group were 85.5 and 85.0. Of the four surface preparations assessed,
group 4 showed the greatest 30-day bond strength, although reduction in bond strength after aging was
remarkable and the value was inferior to that of uncut group (p<0.05).
Conclusion: Within the limitation of the current in vitro study, air-abrasion with 50 µm alumina followedby the application of dichloromethane was the most effective surface preparation for the denture base resin.
(Int Chin J Dent 2002; 2: 126-133.)
Clinical Significance: Alumina air-abrasion combined with application of dichloromethane facilitates bondstrength to a heat processed denture base resin material when the fracture site is repaired with an
autopolymerizing acrylic repair resin.
Key words: acrylic resin, alumina, denture base, dichloromethane, repair.

INTRODUCTION
Acrylic resin currently is the most dominant denture base material utilized in clinical prosthodontic

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practice.1 Acrylic denture base fracture is an infrequent, yet sometimes unavoidable complication of
denture care.

Repair of this fracture must be easily performed and have adequate repair strength.

Autopolymerizing acrylic repair resin is often utilized as a repair material because of its easy handling andrapid polymerization properties. However, high bond strength between heat processed denture base resin
and autopolymerizing acrylic repair resin is not always predictable.2,3 To gain optimum strength for repairs,
it is essential that a good bond exists between these two types of acrylic resin.
The fundamental difference between the heat processed denture base resin and autopolymerizing acrylic
resin is the method by which benzoyl peroxide is decomposed to yield free radicals. Generally, theconversion achieved using chemically autopolymerizing acrylic repair resin is not as complete as that
achieved using heat activated systems. Chemically activated resin exhibits physical properties that are
somewhat inferior to those of heat processed denture base resin4 and displays lower transverse strength.1,5
To improve bond strength, several researchers have reported preparations to improve adhesion between
broken denture base segments or for attachment of denture base resin to denture teeth. These include; 1)application of a solution of equal volumetric parts of dichloromethane (methylene chloride) andautopolymerizing (cold-curing) monomer,6 2) application of chloroform,7 3) application of a 1:1 mixture ofdichloromethane and heat-polymerization monomer,8 4) application of autopolymerizing monomer,9 5)application of bonding agent of visible light-cured resin and heat-polymerization monomer,10 and 6)
application of dichloromethane.11 Limited information, however, is available about combined effect of
mechanical and chemical retentions for the denture base repair. The purpose of the present study was to
examine the transverse strengths of repaired denture base resin specimens, in which the repair surfaces had
been modified using three surface preparations. Another purpose of this study was to investigate thedurability of the bond of the repaired specimens after water storage.

MATERIALS AND METHODS
A heat processed denture base resin (Lucitone 199, Dentsply International Inc., York, PA, USA) and an
autopolymerizing acrylic repair resin (Repair Material, Dentsply International Inc.) were selected for the
study (Table 1). A total of 80 rectangular specimens of the heat processed denture base resin, 3.0 x 10.5 x
68.5 mm in dimension, were prepared according to the manufacturer’s instructions. The poly(methyl
methacrylate) (PMMA) powder (21 g) and the liquid (10 mL) were mixed for 15 s. The dough time toreach packing plasticity was 9 minutes. Procedures for mixing and packing the resin in stone molds
followed conventional laboratory procedure for denture processing, as powder/liquid ratio can contribute todistortion.12 After processing, the specimens were abraded under running water with silicone carbide paper
to 600 grit, to a final dimension of 2.5 x 10.0 x 68.0 mm, after which the specimens were stored in 37˚C
distilled water for 30 days. The intact 16 specimens were used as references, and the remaining 64
specimens were cut in half with a band saw under running water guided by a standardized positional jig.
The resulting rectangular shape would not contribute mechanical advantage to the repair site,13 and waseasiest to duplicate. Cut surfaces were made parallel to each other and perpendicular to the long axis of the127SHIMIZU ET AL

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specimens by abrading under water with 100 grit silicone carbide paper to simulate roughening of therepair surface of the denture with laboratory burs.
Table 1. Materials used.
Materials

Manufacturer

Batch number

Dentsply International Inc., York, PA, USA

Powder 991129, Liquid 9911014

Dentsply International Inc.

Powder 000118, Liquid 000404

Denture base resin
Lucitone 199
Denture repair resin
Repair Material

The abraded surfaces were divided into four groups; 1) untreated, 2) application of dichloromethane withpreparation duration of 5 s,14 3) 50 µm alumina air-abrasion at a right angle to the surface from 5 mmdistance for 10 s at an emission pressure of 0.48 MPa using a grit blaster (Microblaster, Comco Inc.,
Burbank, CA, USA), and 4) 50 µm alumina air-abrasion followed by the application of dichloromethanewith preparation duration of 5 s.

All specimens were air-blasted to remove volatile component ofdichloromethane or excess alumina particles on the test surfaces.
The parallel halves of the specimens were placed in open-ended stone molds of the same dimensions as
the original intact specimens for the transverse strength test. The open-ended stone molds were used so thatthe ends to be repaired could be moved apart to leave a 3 mm rectangular butt joint gap. The repair
surfaces were wetted with the MMA monomer liquid of the repair material before autopolymerizing acrylic
repair resin was applied to the joint space. The duration of MMA wetting for the repair was 180 s.9 Thisgap was then filled with the autopolymerizing acrylic repair resin. The mixing ratio (powder to liquid) was
2:1 (w/w) and the free-flowing mix was poured into the joint space to allow for a slight excess to ensure acomplete joint. After the polymerization process was completed at 23˚C to mimic the most basic clinicalcircumstances, the repaired specimens were then trimmed to their original dimensions with 600 grit silicone
carbide paper, eliminating the unpolymerizing surface layer.15 Half of the specimens were kept in distilledwater at 37˚C for one-day before testing was performed. The remainder of the specimens was kept indistilled water at 37˚C for 30 days to evaluate the influence of aging in water on bond strength. This would
simulate immediate and aged clinical repair circumstances.
A three-point bending test was performed with the universal testing machine (TCM-200, Minebea Co.
Ltd., Tokyo, Japan). The distance of 60 mm between the supports and a crosshead speed of 10 mm/minute
was used during loading. The load at fracture was converted to transverse strength (S) by the formula S =3PL/2bd2, where P is the breaking load, L is the length between the jig supports (60 mm), b is the width of
the specimen and d is the thickness of the specimen. All tests were performed under uniform atmospheric128

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conditions of 23˚C and 50% relative humidity. The data were analyzed statistically using two-way analysis
of variance (ANOVA).

The variables were surface preparation and the period of water immersion.

Newman-Keuls post-hoc comparison was applied when appropriate (95% confidence level).
To determine the morphologic changes to the surface after preparations, a few extra specimens wereprepared according to the original protocol. They were sputtered with gold, and observed with a scanningelectron microscope (SEM, JSM-T 330, JEOL Inc., Tokyo, Japan) operated at 5 kV. For all specimens, theinterface where failure occurred was observed through an optical microscope (92052, Nikon, Tokyo, Japan)
at x30 magnification.

RESULTS
Two-way ANOVA and Newman-Keuls post-hoc comparison revealed that there were significant
differences in transverse strength due to the variables surface preparation and period of water immersion,
whereas there were no significant differences in their interaction (p<0.05). There were no significant
differences in transverse strength due to the surface preparations after one-day water immersion (73.0 to
76.7 MPa) whereas after 30 days water immersion, 50 µm alumina air-abrasion followed bydichloromethane application showed higher strength (64.9 MPa) than the other methods and the control(52.0 to 57.3 MPa). The transverse strengths of the original intact specimens of the Lucitone 199 material,
which was not included in the statistical analysis, were 85.5 MPa for one-day water immersion and 85.0
MPa for 30 days water immersion. Contrast evaluation revealed that there was no significant difference
between these strengths.

The average strengths, standard deviations, statistical categories, and thepercentages (%) of the transverse strengths to that of the original intact specimens are summarized in Table
2. Modes of failure of all specimens except for the original intact specimens are presented in Table 3.
Table 2. Transverse strengths of the repaired denture base material.
Surface preparation

1 day immersion

30 days immersion

Mean

SD

Category

Mean

SD

Untreated

73.0

2.9

a (85)

52.0

10.2 c (61)Dichloromethane

74.8

4.8

a (88)

57.3

8.6

c (67)

Alumina air-abrasion

76.7

1.5

a (90)

54.7

8.7

c (64)

Alumina air-abrasion + dichloromethane

76.3

3.8

a (89)

64.9

7.3

b (76)

Uncut intact specimen

85.5

0.9

- (100)85.0

3.5- (100)Transverse strength (MPa)

SD: standard deviation. Identical letters indicate that the values are not statistically different (p>0.05).
Numbers in parentheses show the percentage of the strength to that of the corresponding intact specimen.

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Fig. 1 depicts SEM views after surface preparations. SEM view of denture base resin surface by
abrading with 100 grit silicone carbide paper showed lots of well-regulated scratches in one way direction.
Table 3. Mode of bond failure.
Surface preparation

1 day immersion

30 days immersion

Untreated

CCCCCCCCM M AAAAAA

Dichloromethane

CCCCCCCAC C M AAAAA

Alumina air-abrasion

CCCCCCCAC C M AAAAA

Alumina air-abrasion + dichloromethane

CCCCCCCA

CCMMMMMA

A: Adhesive failure at the denture base resin-repair resin interface; C: Cohesive failure within the repair resin; M: Mixture ofcohesive failure and adhesive failure. Each letter code corresponds to a separate specimen.

SEM view of denture base resin treated with alumina air-abrasion exhibited rougher surface compared withthat by abrading with silicone carbide paper. SEM views of denture base resin treated with alumina
air-abrasion followed by dichloromethane and that with dichloromethane showed an almost homogenous
surface void of irregularities with pores and channels of less than 1 µm in diameter. One thing to be
noticed was that the surface treated with alumina followed by dichloromethane looked a little bit uneven.

a

b

c

d

Fig. 1. Scanning electron micrographs of the denture base material (a: ground; b: ground +dichloromethane; c: alumina-blasted; d: alumina-blasted + dichloromethane).

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Della Bona and van Noort16 described that the tensile strength test is more appropriate than shear bondstrength test for evaluating the adhesive capabilities of resin composite to ceramics, because that shear
bond strength test has as its inherent feature the measurement of the strength of the base material rather
than the strength of the adhesive interface. This fact can be applied to the combination of a denture baseresin and an autopolymerizing acrylic resin.17 The tensile strength test, however, needs exactly accurateload direction to the specimen and this factor, which is too difficult to control, considerably affects theresults.18 The validity of three-point bending test was found to have close correlation with the failure modeof specimens.18 Therefore the three-point bending test was performed for the current study.
Tables 2 revealed that a heat processed denture base resin had a higher transverse strength to an
autopolymerizing acrylic repair resin after one-day water immersion than that after 30 days water
immersion and none of the surface preparations significantly improved the transverse strength of the repair
specimens after one-day water immersion. A comparison in failure mode of all specimens after one-day
water immersion did not show much difference. Cohesive failure of the repair material indicates that asufficient bonding to the repair surface has been achieved.9 After 30 days water immersion, however, thetransverse strengths of all groups significantly decreased and the highest transverse strengths corresponded
to 50 µm alumina air-abrasion followed by dichloromethane application. The result demonstrates that thetransverse strength between a heat processed denture base resin and an autopolymerizing acrylic repair
resin have a tendency to decrease with the period of immersion in water. Surface preparation of 50 µm
alumina air-abrasion followed by dichloromethane reduces the decline of the transverse strength between
two types of resin.
Dichloromethane, is an organic and nonpolymerizable solvent, which swells the surface and permits adiffusion of the polymerizable material.6 The strength of the bond depends upon the degree of penetrationof the solvent and the strength of the interwoven polymer network formed thereafter.6 Dichloromethane
preparation can create surface pores and channels approximately 1 µm in diameter on a conventional
acrylic resin tooth, and these channels tend to interconnect frequently.11 This morphological change also
occurs when dichloromethane is applied to heat processed denture base resin (Fig. 1). Prepolymerizing
PMMA pearls present in the denture base resin should allow diffusion of the dichloromethane solvent.
Vallittu previously described diffusion of MMA solvent into the matrix and interpenetrating polymer
network of dental polymer.19 Since the prepolymerizing PMMA pearl are not crosslinked with the matrix,
application of solvent can create crosslinking.
Alumina air-abrasion is utilized to roughen the repair surface of the denture base (Fig. 1) to increase thearea of bond contributing to micromechanical retention and eliminating substances which adhere to thesurface of the repair region, mechanically facilitating application of solvent or MMA monomer. The SEM
view of denture base resin surface treated with 50 µm alumina air-abrasion followed by dichloromethane(Fig. 1) nearly looks like that of treated with dichloromethane.

The base surface treated withdichloromethane is almost flat and smooth, whereas that treated with 50 µm alumina air-abrasion followed

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by dichloromethane is a little bit uneven.

This suggests that the preparation with 50 µm alumina

air-abrasion followed by dichloromethane has a synergistic effect of the morphologic changes by means ofdichloromethane and increasing of the surface area due to air-abrasion. However, the independent effect ofdichloromethane and 50 µm alumina air-abrasion in improving the bond between a heat processed denturebase resin and an autopolymerizing acrylic repair resin is not completely clear from the results of thecurrent study. Remnants of the dichloromethane solvent may have a deleterious effect on bond strength.
This study concludes that the surface preparation of 50 µm alumina air-abrasion followed by theapplication of dichloromethane yields the highest bond strength after 30 days immersion in water among
the three preparations in this study. This finding reveals a synergistic effect of dichloromethane and 50 µm
alumina air-abrasion.

This is explained in part by understanding the eliminative action of alumina

air-abrasion which prepares the surface of the denture base resin for a roughened structure to apply
dichloromethane. Furthermore, an analysis of failure mode demonstrated only one adhesive failure in thisexperimental group.

CONCLUSIONS
Alumina air-abrasion followed by application of dichloromethane facilitates bond strength to a heatprocessed denture base resin material when the fracture site is repaired with an autopolymerizing acrylic
repair resin. Although reduction after aging is remarkable, this protocol may be considered for clinical use
on repair sites of fractured acrylic resin denture bases.

ACKNOWLEDGMENT
The authors would like to thank Dentsply International for their sponsorship of the denture base resin andrepair resin used in this experiment.

REFERENCES
1. Anusavice KJ. Phillips’ science of dental materials. Tenth ed. Philadelphia: W.B. Saunders Co.; 1996. p. 237-72.
2. Beyli MS, von Fraunhofer JA. Repair of fractured acrylic resin. J Prosthet Dent 1980; 44: 497-503.
3. Berge M. Bending strength of intact and repaired denture base resins. Acta Odontol Scand 1983; 41: 187-91.
4. Bates JF, Stafford GD, Huggett R, Handley RW. Current status of pour type denture base resins. J Dent 1977; 5:
177-89.
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strength of repaired heat-polymerizing resin. J Prosthet Dent 1994; 72: 639-43.

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10. Lewinstein I, Zeltser C, Mayer CM, Tal Y. Transverse bond strength of repaired acrylic resin strips and temperature riseof dentures relined with VLC reline resin. J Prosthet Dent 1995; 74: 392-9.
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14. Shimizu H, Naka S, Inoue H, et al. Surface preparation agents for denture base resin Part 2: Effect of preparation
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Reprint request to:
Dr. Hiroshi Shimizu
Division of Removable Prosthodontics, Department of Oral Rehabilitation, Fukuoka Dental College
2-15-1, Tamura Sawara-ku, Fukuoka 814-0193, Japan
FAX: +81-92-801-0513
E-mail: simizuh1@college.fdcnet.ac.jp
Received on April 30, 2002. Revised on October 20, 2002. Accepted on October 25, 2002.
Copyright ©2002 by the Editorial Council of the International Chinese Journal of Dentistry.

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