Sato et al.

Int Chin J Dent 2010; 10: 23-27.

Discoloration of auto-polymerized resin applied using a brush-ontechnique
Yohei Sato, DMD, PhD, Hikari Chiba, DMD, PhD, Tomoko Osada, DMD, PhD, Ema
Muraishi, DMD, Chikahiro Ohkubo, DMD, PhD, and Toshio Hosoi, DDS, PhD
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine,Yokohama, Japan
Purpose: The discoloration of auto-polymerized resin has frequently been observed after denture repair. This
study is an investigation of the discoloration of auto-polymerized resin applied using a brush-on technique.
Materials and Methods: Rectangular resin plates were fabricated so that five auto-polymerized resins (UnifastII, Unifast Trad, Provinice, Metafast, and Miky) could be packed in a mold (15x30x1 mm) using a brush-on
technique (resin brush, horsehair) in six increments for each mold. As controls, the five resins were poured in
the molds after the polymer and monomer were mixed at 0.5 mL/g for 10 s. After polymerization, half of theresin plates were covered with aluminum foil. All the plates were soaked in distilled water (37°C) and exposed
to xenon light for 24 hours according to ISO 7491. They were soaked in water again for 5 days. After removing
the aluminum foil, the discoloration was measured using a color difference meter (Shade eye system) three times
for each specimen. The color differences (∆E: n=5) were analyzed by ANOVA/Tukey’s test (α=0.05).
Results: For the brush-on technique, Provinice had the greatest color change of any of the resins tested. There
were significant differences between the brush-on technique and the controls for Unifast II and Miky.
Conclusion: The brush-on technique resulted in fewer color changes than the mixing technique on all resins
except for Provinice, as shown by the fact that many porosities were found in the control specimens.
(Int Chin J Dent 2010; 10: 23-27.)
Key Words: autopolymerized acrylic resin, brush-on technique, color change, discoloration

Introduction
A series of studies on auto-polymerized resins using a brush-on technique was conducted for chair-side
denture repair.1-4

The characteristics of auto-polymerized repair resins, namely, mechanical strength,4dimensional accuracy,2 fluidity,1 and handling efficiency,3 were compared, and points were suggested for clinicaland laboratory procedures. However, the problem of discoloration of auto-polymerized resins after denture
repair remains unresolved and has not been sufficiently investigated, although heat-polymerized resin andpour-type resin have been thoroughly examined.5-10 In particular, the discoloration of pink auto-polymerizedresin used for denture repair has not been studied in depth; however, there have been a few reports regardingivory resin.11-13
Several experimental methods have been used in research on the discoloration of resins. They include thefollowing: 1, soaking in stain solution;14 2, soaking in warm water (60˚C) for one month;15,16 3, exposure tosunlight and ultraviolet rays;17-21 4, soaking in warm water and synthetic saliva of 37˚C for a long time.22,23
Alternatively, ISO 7491, a dental material-determination system of the color stability of dental polymericmaterials, was enacted as a tone stability examination standard.
One reason for the discoloration would be the lower degree of polymerization of auto-polymerized thanheat-polymerized resins. However, few details are known about the discoloration of repair resins. The purposeof this study was to investigate the discoloration of commercial auto-polymerized acrylic resins using brush-on
and conventional mixing techniques.

Materials and Methods
The auto-polymerized resins used in this study are listed in Table 1.

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Table 1.

Int Chin J Dent 2010; 10: 23-27.
Auto-polymerized resins used in this study.

Brand name
Unifast II
Unifast Trad
Provinice
Metafast
Miky

Manufacturer
GC, Tokyo, JapanGC
Shofu, Kyoto, JapanSun Medical, Moriyama, Japan
Nissin, Kyoto, Japan

Fig. 1.

Shade#3 Pink
#3 Pink
U3#2 Pink#2

Lot number
Powder 0503172, Liquid 0504261
Powder 0504261, Liquid 0503221
Powder 020507, Liquid 030555
Powder 41101, Liquid 41103
Powder PEIL, Liquid ELG

Illustration of the specimen.

Specimen fabrication
As shown in Fig. 1, rectangular resin plates were fabricated so that five auto-polymerized resins could beapplied in a mold (15x30x1 mm) using a brush-on technique (resin brush, horsehair; Seiundo, Tokyo, Japan).
The resin slurry was applied six times for each mold. As controls, the five resins were poured into the molds
after the polymer and monomer were mixed at 0.5 mL/g for 10 s according to the manufacturer’srecommendation. After polymerization, half of the resin plates were covered with aluminum foil (Myfoil,Sumikei Aluminum-Foil Co., Ltd., Tokyo, Japan). All the plates were soaked in distilled water (37°C). Five
specimens were made for each resin; a total of 50 specimens were fabricated.
Discoloration test
All the specimens were exposed to xenon light using a xenotest apparatus (Suntest CPS, Heraeus Kulzer,Hanau, Germany) for 24 hours according to ISO 7491. They were then soaked in water again for 5 days. Afterthe aluminum foil was removed, the discoloration was measured using a color difference meter (Shade eyesystem, Shofu, Kyoto, Japan) three times for each specimen. The color differences of the five resins (∆L*, ∆a*,∆b*: n=5) were analyzed by ANOVA and Tukey’s multiple comparisons test at a significance level of α=0.05.
Between the brush-on and conventional mixing techniques, discoloration (∆E*: n=5) was analyzed by the t-test
at a significance level of α=0.05. After the discoloration test, each specimen was macroscopically observed toconfirm the presence of porosities and color changes.

Results
Table 2 shows three color change factors, namely, lightness (∆L*, chromatic changes), red-green color change(∆a*), blue-yellow color change (∆b*), and discoloration (∆E*) of five resins. The discoloration (∆E*) of eachresin using both the brush-on technique and the mixing technique is shown in Fig. 2.
All of the resins demonstrated color changes toward green and yellow. The lightness (∆L) of Provinice wassignificantly greater than that of all the resins tested using the brush-on technique (p<0.05). In contrast, thered-green color change (∆a) of Provinice was the smallest. Miky had the greatest blue-yellow color change (∆b)
of all the resins tested except for Trad using the mixing technique. Of the brush-on technique, Provinice had the24Sato et al.

Int Chin J Dent 2010; 10: 23-27.

greatest color change (∆E) of all the resins tested (p<0.05). There were significant differences in color changes(∆E) between the brush-on technique and the mixing technique for Unifast II and Miky (p<0.05).
Table 2.

Chromatic and color changes of the auto-polymerized resins.

Mixing technique

Brush-on technique

Unifast II

Unifast Trad

Provinice

MetafastMiky∆L*

5.37 (6.89)

0.44 (1.19)

4.20 (1.85)

1.03 (1.11)

2.11 (0.77)∆a*-1.14 (0.99)a-1.18 (3.57)a-7.82 (0.81)-0.31 (2.19)a-1.318 (1.12)a∆b*

0.83 (1.06)a

1.82 (2.47)ab

0.31 (0.73)a

1.21 (1.34)a4.28 (0.93)b∆L*

1.24 (0.76)a-0.81 (1.9)a4.25 (1.68)

0.21 (0.91)a0.73 (1.18)a∆a*-1.18 (0.73)a-0.92 (2.7)a-8.92 (0.88)-0.10 (1.99)a-0.86 (0.58)a∆b*

1.13 (0.77)a0.95 (1.2)a0.04 (0.85)a0.54 (1.02)a

2.91 (0.42)

Means and standard deviations in parentheses. Identical letters in same rows are not significantly different (p>0.05).

Fig. 2.

Fig. 3.

Discoloration (∆E*).

Specimens after being exposed to xenon light. Upper, mixing technique; Lower, brush-on technique

Macroscopic observations of each resin surface are shown in Fig. 3. A significant amount of porosity wasobserved in the Unifast II specimens using the mixing technique. Although Provinice had severe discoloration,
there was little discoloration in Metafast. When Miky was prepared with the mixing technique, many small
porosities and a color change toward yellow were observed.

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Int Chin J Dent 2010; 10: 23-27.

Discussion
Ivory auto-polymerized resin has been used for provisional, short-term restoration.

However, pink

auto-polymerized resin with a gingival color has been utilized for long-term denture repair and attachment set-up.
When such resin is applied in the vicinity of the anterior tooth, the discoloration may significantly affect theesthetics. Therefore, not only the mechanical strength of auto-polymerized resin but also its color stability has to
be investigated.
Generally, the discoloration of the resin is determined by two factors, i.e., the decrease of ∆a* from the
red-to-green direction and the rapid increase of ∆b* from the blue-to-yellow direction. In previous studies, thecolor difference was determined by visual inspection. Eye perception may be clinically sufficient because
discoloration can be determined by the eye, even if it is without a high degree of precision. In this study,
macroscopic observations and measurements with apparatuses were performed, and both results were similar.
The mixing technique showed a greater ∆b* value than that of the brush-on technique of all resins tested
except for Unifast II. As a reducing agent tert-amine is contained in most auto-polymerized resins. The
tert-amine would contribute to the change to yellow. However, barbituric-acid derivatives, used for Unifast II asa polymerization initiator, have demonstrated excellent tone stability; therefore, Unifast II had a smaller ∆b*.
The brush-on technique yielded a smaller color difference than the mixing technique on all resins except for
Provinice. Particularly, there were significant differences between the brush-on technique and the mixing
technique for Unifast II and Miky (p<0.05). The reason is that the monomer/polymer ratio for the brush-on
technique (0.31 to 0.38 mL/g) was less than that for the mixing technique (0.5 mL/g). The greater L/P ratio may
contribute to lower mechanical strengths and less color stability. Significant porosity was observed in theUnifast II and Miky specimens using the mixing technique. Similarly, these specimens showed greater ∆L*(brightness) in the mixing technique. Osada et al.4 examined the bending strength of the same resins used in thisstudy. The bending strength of Miky was significantly lower using the brush-on technique (p<0.05). Many
porosities in Miky affected the bending strength and discoloration. In contrast, the greater ∆L* (brightness) ofthe Provinice would be caused by the bleaching. It was assumed that the kind of colorant in Provinice could beeasily discolored by the xenon light.
The brush-on technique resulted in fewer color changes than the mixing technique on all resins except for
Provinice, as shown by the fact that many porosities were found in the control specimens. Provinice had thelowest resistance to light, and Miky showed the greatest color change to yellow among all the resins tested.

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Correspondence to:
Dr. Yohei Sato
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: yohee.japan@gmail.com
Accepted March 24, 2010.

Copyright ©2010 by the International Chinese Journal of Dentistry.

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