Miura et al.

Int Chin J Dent 2010; 10: 57-62.

Effect of surface treatment agents on bond strength ofauto-polymerized acrylic resin to poly(ethylene terephthalate)
Eiji Miura, DDS, PhD,a Yasuko Takayama, DMD, PhD,a Yukihiro Mizuno,b Minoru
Abe, DDS, PhD,a Yoshiyuki Kawai, DDS, PhD,a and Chikahiro Ohkubo, DMD, PhDaab

Department of Removable Prosthodontics, and Dental Technician Training Institute,Tsurumi University School of Dental Medicine, Yokohama, Japan
Purpose: This study evaluated the adhesive strength between a thermoplastic poly(ethylene terephthalate) (PET)
sheet and an auto-polymerized acrylic resin (Unifast III).
Materials and Methods: The shear bond strength of auto-polymerized acrylic resin to PET with or without
surface treatment was measured. Four surface treatment agents, Unifast III Liquid, Resin Primer, Adhesive, andGP Solvent, were used.
Results: The shear bond strengths were 28.8 MPa for GP Solvent, 26.5 MPa for Resin Primer, 24.6 MPa forAdhesive, 24.4 MPa for the untreated control, and 23.4 for Unifast III Liquid. Significant differences werefound between the control and GP Solvent, between Unifast III Liquid and GP Solvent, and between Adhesive
and GP Solvent (p<0.05). SEM observation showed that the surfaces of the thermoplastic sheets had dissolved
due to the application of Unifast III Liquid, Adhesive, and Resin Primer.
Conclusion: Specimens treated with GP Solvent, Resin Primer, and Adhesive exhibited higher bond strength
value compared to the control specimens. GP Solvent had the highest adhesive strength. SEM observation
indicated that the surface changes of the PET treated with Resin Primer were the most remarkable.
(Int Chin J Dent 2010; 10: 57-62.)
Key Words: auto-polymerized acrylic resin, poly(ethylene terephthalate), shear bond strength, surface treatment
agent, thermoplastic sheet

Introduction
Dental processing of thermoplastic resins is broadly classified into two fabrication methods: injection molding
with thermoplastic resin pellets and fabrication under pressure or suction of the heated sheet-type thermoplasticresin. The use of thermoplastic sheets has recently increased1-7 for splints for temporomandibular disorders,nightguards, obturator prostheses, splint-type temporary dentures, treatment or diagnostic dentures, and oral
appliances for obstructive sleep apnea (OSA).4,5

The fabrication procedure of prosthetic devices using

thermoplastic sheets is very simple and can be accomplished in a short time. Four types of sheets, namely,
polycarbonate,1 poly(ethylene terephthalate) (PET),4 cellulose acetate butyrate,7 and ethylene vinyl acetate(EVA)2,5 are available for clinical use. One of these kinds of thermoplastic sheets can be selected for eachsituation, depending on the case and purpose as well as the hardness needed.
In prosthodontic practice, the combination of thermoplastic resin sheets and auto-polymerized acrylic resin isnecessary in some situations. For instance, splint-type dentures or OSA appliances are easy to produce using
these materials, resulting in an effective treatment.3,4,7 To use these devices clinically over a long period of time,
the bonding of thermoplastic resin sheets and auto-polymerized acrylic resin becomes an important factor. Thusfar, such bonding has not been totally successful since debonding of the acrylic resin from the thermoplasticsheets has frequently occurred during short-term clinical observations. One reason for the debonding may be
due to the stress concentration at the bonding interface since thermoplastic resin is very flexible. The methods
for increasing the adhesive strength between a denture base acrylic resin and auto-polymerized acrylic resin
include roughening the bonding surface of an acrylic resin using a carbide bur,8 abrading with alumina particles,9and using surface treating agents.10-13 Thus far, the influence of the surface treatment agents on the bonding
between the thermoplastic resin sheets and the auto-polymerized acrylic resin has not been investigated.

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The purpose of this study was to evaluate the influence of surface treatment agents on the bonding strengths
between thermoplastic sheets and auto-polymerized acrylic resin. Using PET as the thermoplastic resin, theshear bond strengths of auto-polymerized acrylic resin treated with one of four types of surface treatment agentswere measured.

Materials and Methods
Specimen fabrication
The materials used in this study and their chemical compositions are presented in Table 1. Figure 1 shows adiagram of the specimen fabrication process based on the laboratory situation. The thermoforming of PET (3mm thick) on the plasterboard was performed using a pressurized thermoforming device (Mini-star S, Morita,Tokyo, Japan) (Heating temperature: 220°C for 80 s; pressurized power: 0.3 MPa). The thermoformed PET was
cut into 10×10 mm pieces using a supersonic wave cutter (Labo Sonic Cutter NE87 Nakanishi, Kanuma, Japan)
and then fixed to an auto-polymerized acrylic resin block (30 mm diameter, 20 mm high; Unifast III) withcyanoacrylate adhesive (Aron Alpha; Toagosei, Tokyo, Japan). The surface of the PET was cleaned using asteam cleaner for 5 s.
Table 1.

Materials used in this study

Materials

CodeComposition

Lot number

Manufacturer

071016

Sheu-Dental GmbH,
Iserlohn, Germany

Powder: poly(methyl methacrylate)Liquid: methyl methacrylate (MMA)

P: 0711012
L: 0712121

GC Corp., Tokyo, Japan

Thermoplastic poly (ethylene terephthalate) sheet
DuranPETpoly(ethylene terephthalate)Auto-polymerized acrylic resin
Unifast III
Surface treatment agent
Unifast III Liquid

MMA

0712121

GC Corp.

Resin PrimerRP

dichloromethane, MMA

0712111

GC Corp.

Adhesive

AH

ethyl acetate

782

Tokuyama Dental Corp., Tokyo, JapanGP Solvent

GPn-hexane, d-limonene

UB5

Nippon Shika Yakuhin Co. Ltd.,
Shimonoseki, Japan

Fig. 1. Schematic illustrating the study design
Four surface treatment agents, Unifast III Liquid, Resin Primer (RP), Adhesive (AH), and GP Solvent (GP),were used in this study. They were applied only once on the PET surface using a new writing brush (GC Corp.,Tokyo, Japan). The agents on the PET surface volatilized immediately and did not remain. Specimens without
surface treatment were also prepared as a control. After the surface treatment, a piece of masking tape with a58

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Int Chin J Dent 2010; 10: 57-62.

3-mm diameter hole was adhered to the treated PET surface to indicate a prescribed area. A stainless steel tube
(4 mm inner diameter, 4 mm high) was placed on the masking tape, and auto-polymerized acrylic resin (UnifastIII) was applied within the tube using the brush-on technique. The resins were polymerized at room temperature(25°C) and ambient atmospheric pressure, and the polymerized specimens were stored in distilled water for 48hours. Ten specimens for each agent (total of 50 specimens) were fabricated. Only one dental technician with
30 years’ experience made the specimens since operator dexterity has been found to significantly influence thespecimen quality.
Shear bond strength and Knoop hardness
The specimens were mounted on a universal testing machine (Instron 5565; Instron Corp., Canton, MA, USA),and shear testing was performed at a 0.5 mm/minute crosshead speed. The shear bond strengths (MPa) werecalculated by dividing the peak load at fracture by the surface area.
In addition, to examine the influence of the surface treatment agents on the PET surface, the Knoop hardnessof the treated surfaces was measured (MVK-E hardness tester, Akashi, Tokyo, Japan) with an applied load of 10g for 30 s. The Knoop hardness of the control and the specimen without thermoforming were similarly
measured. The hardness was measured at five points on the surface of each specimen, and the mean hardness
values were calculated.
The data collected on the shear bond strengths and Knoop hardness were statistically analyzed using a
one-way analysis of variance (ANOVA) and Bonferroni multiple comparisons test at a significance level of
α=0.05. SPSS for Windows Ver.12.0 (SPSS Japan Inc., Tokyo, Japan) was used for both statistical analyses.
Observation of treated surfaces and fractured surfaces
The treated surfaces were observed using a scanning electron microscope (SEM; JSM-5600LV, JEOL,Akishima, Japan).

After shear testing, the fractured surfaces of the PET were examined using astereomicroscope (SZH-10 Olympus, Tokyo, Japan) and SEM.

Results
The results of the shear bond strength and hardness testing are shown in Table 2. The mean shear bondstrength of the agents ranged from 23.4 MPa to 28.8 MPa. GP had the highest strength (28.8 MPa) among theproducts tested, followed by RP and AH. There were significant differences between GP and the control,between GP and Unifast III Liquid, and between GP and AH (p<0.05).
Table 2.

Shear bond testing and Knoop hardness testing results

Surface treatment agent

Shear bond strength (MPa)

Knoop hardness number (KHN)

MeanCategory

SD

Mean

CategorySD

Without thermoforming

NANANA

10.70

a, b, c0.19

Control

24.4

a

1.8

10.23

d, e

0.24

Unifast III Liquid

23.4

b

4.3

5.23

a, d, f

1.77

RP

26.5

2.2

≤3.00

AH

24.6

2.8

7.39

b, e, g

0.44

GP
28.8
a, b, c2.0
9.99
NA, Not available; Identical letters indicate significant difference (p<0.05).

c, f, g0.15cKnoop hardness testing results of the treated surfaces on the PET sheet are also summarized in Table 2. There
was no significant difference between the conditions before and after thermoforming. After applying the surface

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treatment agents, the hardness significantly decreased compared to the control, except for GP. In particular, RP
became so soft that it exceeded the measurement range.
Each treated surface observed by SEM is shown in Fig. 2. The surfaces of Unifast III Liquid, RP, and AH had
dissolved after treatment. RP looked especially rough, and there were micro scratches on GP. The images seen
with the microscope and the SEM of the fractured surfaces after testing are found in Fig. 3. All specimens
exhibited mixed failures interfacial failure between PET and the auto-polymerized acrylic resin and cohesive
failure within PET. However, in the SEM images, the control and Unifast III Liquid looked like threads, while
RP, AH, and GP had flakes on the stratum.

Fig. 2. SEM images of treatment surfaces

Fig. 3. Fracture surfaces of PET side: left, microscope image (×40); right, SEM image (×200)

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Discussion
The adhesive strength between acrylic resin and PET greatly affects the durability of a prosthetic device. The
bonding surface of the denture base acrylic resin is usually roughened using a carbide bur or by abrading withalumina particles to enhance the adhesive strength to the auto-polymerized resin. In addition, it is beneficial totreat the surface with agents such as an organic solvent or primer.9-13 Methyl methacrylate (Unifast III Liquid),
dichloromethane (RP), and ethyl acetate (AH) are frequently used as surface treatment agents. Since theeffectiveness of dichloromethane was reported,10,11,13 the present study examined these four agents including
monomer and dichloromethane as surface treatments. GP, which is widely used for softening or dissolving thegutta-percha in endodontic therapy,14,15 showed the highest shear bond strength (28.8 MPa). The d-limonene(GP) has been used for many years and is considered to be a reliable and safe chemical. Although RP produced
high strengths, dichloromethane is specified as a dangerous chemical. In contrast, ethyl acetate exhibited alower bonding strength than that of RP and GP. Many studies reported that the ethyl acetate had little effect on
the adhesion to acrylic resin.11,12 It is reported that it is necessary to apply the monomer for a relatively longtime (180 s) to increase the adhesive strength of the denture base acrylic resin.10,16 In this study, Unifast III
Liquid did not apply to RET surface such a long time. However, the Knoop hardness deteriorated remarkably.
The shear bond strengths between the auto-polymerized acrylic resin and denture base acrylic resin werereported to be 9-17 MPa by Sarac et al.,16 20.2 MPa by Shimizu et al.,17 and approximately 35 MPa by Minami
et al.9 Takahashi et al. demonstrated that the shear bond strength between auto-polymerized acrylic resin andinjection-type PET had a constant value of 27-28 MPa.18 Moreover, the shear bond strengths between theauto-polymerized acrylic resin and other injection-type thermoplastic resin, nylon and polycarbonate wereapproximately 5 MPa and 15-25 MPa, respectively.18,19 PET has higher bonding strengths to auto-polymerizedacrylic resin compared to the other thermoplastic resins. In the present study, the shear bond strength of theauto-polymerized acrylic resin to the thermoplastic resins was about 23-28 MPa, similar to the value found by
Takahashi.18 Because PET is more flexible than acrylic resin, it was easy to bend under occlusal force, and it
seems that the bonded resin peels off easily. Therefore, the effect of the surface treatment agents was important.
Many small pores are commonly observed on the denture base acrylic resin surfaces treated bydichloromethane or ethyl acetate,10,11,16,17,20,21 However, there was no porosity on the surfaces treated with RPand AH; only swollen and dissolved surfaces were seen in this study. The adhesive strength of RP and GP
appeared to have increased because of the dissolubility, which caused the cracks and undercuts on the surface ofPET. However, the Knoop hardness of GP was similar to that of the control, Unifast III Liquid was 5.23, and RP
could not be measured. From this result, the influence of each surface treatment agent seemed to be different.
The PET surface was more severely dissolved by RP as evidenced by the Knoop hardness results. Because thePET surface has changed greatly after only one application using the brush, it is surmised that the PET surface
might have deteriorated because of the long treatment time with RP.
The results of the present study showed that the use of surface treatment agents tended to increase theadhesive strength of the auto-polymerized acrylic resin to the thermoplastic resin. However, further evaluationof the adhesive strengths after bending fatigue testing based on simulated clinical situations is necessary due tothe great difference between the flexure of thermoplastic sheets and acrylic resins.

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Correspondence to:
Dr. Eiji Miura
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine
2-1-3, Tsurumi, Tsurumi-ku, Yokohama 230-8501, Japan
Fax: +81-45-573-9599 E-mail: miura-e@tsurumi-u.ac.jp
Accepted November 29, 2010.

Copyright ©2010 by the International Chinese Journal of Dentistry.

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