Comparison of ISO and ASTM Standards in Determining The Flexural Strength of Denture Base Resin
Comparison of ISO and ASTM Standards in Determining The Flexural Strength of Denture Base Resin
Comparison of ISO and ASTM Standards in Determining The Flexural Strength of Denture Base Resin
Introduction
How to cite: Chander NG, Jayaraman V, Sriram V. Comparison of ISO and ASTM standards in This work is licensed under Creative
determining the flexural strength of denture base resin. Eur Oral Res 2019; 53(3): 137-40. Commons Attribution-NonCommercial
4.0 International License
138 Chander et al.
bending tests are commonly employed and few studies have stage. A polythene sheet was placed over the resin and
used outdated guidelines (13). The distribution of stress trial closures in the hydraulic press (Hydraulic Press P400,
varies between three and four point bending test. The four SIRIO Dental SRL) was done to ensure even flow of the resin
point bending causes stress distribution whereas three point throughout the mold space. This was repeated until no flash
leads to stress concentration. The difference in standards was observed. The flask was then tightened to 100 N using
influence the sample size, shape, analysis procedure and the hydraulic press machines and bench curing was done for 20
outcome (2,13,14). Consensus is required in following the min. The packed acrylic resin was processed by conventional
particular protocol for dental material analysis (3). short polymerization cycle, 70ºC for 90 min and boiled for
The objective of this study is to evaluate the quantitative 1 hour. Once the curing process was finished, the flasks
differences in the flexural strength in three and four point were bench cooled for 30 min. The samples were de-flasked
bending test of heat cure acrylic denture base resin. using a wooden mallet and plastic knife. The samples were
finished and polished. The test specimens were subjected to
Materials and Methods grinding with acrylic burs. All the irregularities on the edges
were adjusted using conventional acrylic burs by holding the
Sample preparation specimen in a low speed dental lathe and followed by fine
surface smoothness using 600 grid sand papers. Mechanical
The consent and approval for the study was obtained from polishing performed with pumice slurry and chalk powder
Institutional review board and ethical committee. The master in combination of water for 30 s. Group 1 (ISO samples)
die for the test samples were prepared in accordance to ISO sample was cut into three equal strips before testing
and ASTM regulation. The dimension of 65mm x 40mm measuring 64mm in length, 10mm in width and 3.3mm in
x 5mm die were fabricated for testing ISO 20795.1.2013 height. The strips were trimmed and polished, all the edges
samples and 127mm x 12.7mm x 3.2mm die were fabricated and faces were smoothened and flattened to required size.
for ASTM D790 (8,9). The master die was duplicated with The dimensions were verified using digital vernier calipers.
addition silicone impression material (Aquasil soft putty, The test specimens of ISO and ASTM were stored in water at
Dentsply, Germany, Batch no:3162). The duplicated index was a temperature of 37°C for 50 hours prior to flexural testing
used to prepare wax test samples. 30 wax patterns for each (14).
group of the above mentioned dimension were prepared for
both ISO and ASTM D790 standards. The dimensions of the Flexural strength
specimens were verified using a calibrated digital vernier
caliper. The wax samples were polymerized by conventional The flexural strengths of the specimens were determined
reverse flasking method. Type III gypsum dental stone (Gem using a three-point and four-point bending test device in
stone Mahindra traders Chennai) was used for investing the a universal testing machine INSTRON (Autograph universal
wax pattern in dental flask. A layer of separating media (cold testing machine, Shimadzu corp, Japan). The ISO specimen
mold seal) was applied between two investment segments. (65mm x 40mm x 5mm) were rested on two supports and
With the final set of investment stone in the flask, the flask are loaded by means of a loading nose midway between
is placed in dewaxing unit to eliminate wax. Any residual the supports on the Universal Testing Machine for flexural
wax was manually removed using the hand shower of the strength evaluation. Load was applied at the center of the
same machine. The cavity in the dental flask was used as specimen with a cross head speed of 1.50mm/min and a
matrices for the fabrication of heat polymerized acrylic resin span length of 40.00 mm. The maximum load before fracture
specimens. A thin layer of cold mold seal (DPI) was painted was measured. Flexural strength was calculated using
over the stone of both flask halves. Heat cure acrylic resin the equation (M=3WI/2bd2). The mean flexural strength
was mixed with monomer in ratio of 3:1 in a porcelain jar. of group was calculated, tabulated and the values were
Acrylic resin was packed into the mold space in dough statically analyzed (Table 1 and 2)(15).
The flexural strength of the ASTM specimen was evaluated The mean flexural strength of Group ISO is 60.49 MPa
by four point bend test. The test specimens (127mm x and Group ASTM is 61.44 MPa. The results matched
12.7mm x 3.2mm) were rested on the cylindrical support arm the manufacturer and ideal values of flexural strength
of the universal testing machine. In order to avoid excessive of denture base materials. The test found no statistical
indentation, or failure due to stress concentration directly differences between the two methods. But quantitatively
under the loading noses, the radii of the loading noses and ASTM is slightly higher than ISO. Flexural properties in both
supports was standardized to 5.0±0.1 mm. The maximum protocols may vary with in accordance specimen depth,
load before fracture was measured. Flexural strength was temperature, atmospheric condition and rate of strain. The
calculated using the equation S=3PL/4bd2. The mean quantitative variability in this study can be due to the stress
flexural strength of group was calculated, tabulated and the distribution. In 4-point bending test the axial stress are
values were statically analyzed (Table 1 and 2) (3). uniformly distributed between the loading points compared
to 3- point bending test where the maximum axial stress is
Results located immediately under the loading points (9).
The mechanism of stress evaluation can display a minor
The mean flexural strength of was 60.492 MPa and 61.470 variation in the strength value. Both the protocols are
MPa for ISO and ASTM specimens. The standard deviation reliable testing methods. The test sensitivity is less in 4 point
of 0.803 and 1.370 was observed in both ISO and ASTM compared to 3 point bending test. This makes the 4 point test
specimen. The results had 95% confidence interval, 0.310, more ideal for composite and brittle materials. Literatures
and 0.529 for both the groups. The distribution was equal have determined 10% variations between the ASTM and
and parametric t test was done to analyze the results. The older ISO protocols (9) . ISO has adapted and modified to
results were statistically significant with P value ≤ 0.001. the needs of the situations and for dental materials it is more
ideal in terms of sample fabrication to mechanical testing (8).
Discussion The study evaluated the conventional heat cure specimens
without any modifications to the compositions. Further
The flexural strength of heat cure acrylic resin was studies are required to determine the influence of testing
evaluated in according to ISO 20795.1.2013 and ASTM D 6272 protocol with changes in composition, reinforcement,
standards (3). These test methods are generally applicable composite materials of PMMA and the influence in the
to rigid and semi rigid material. The flexural properties testing protocols.
determined by these methods are mostly used for quality
control and research (8,9). The study was done to determine Conclusion
the choice and use of appropriate protocol between 3 and 4
point testing protocol. A comparison of the results from three-point and four-
ASTM is a national organization that is a part of ISO point bend tests of denture-base polymers showed no
organizations. ISO is an international organization that significant statistically and clinical differences in the flexural
has representations from all countries including ASTM. strength. However, flexural strength values were higher in
ISO establishes documents and updates the standards of four-point bending than in three-point bending.
testing materials with global consensus from the experts
of the associated national organizations. The products thus Türkçe Öz: Protez kaide reçinelerinin bükülme dayanımının belirlen-
established are safe, quality and reliable. ISO standards are mesinde ISO ve ASTM standartlarının karşılaştırılması. Amaç: Isı ile
polimerize olan polimetilmetakrilat (PMMA) protez kaide reçinelerinin
better valid since it developed and updated to the needs bükülme dayanımının belirlenmesinde kullanılan metodoloji standart-
with the opinion of internationally established experts. The larında farklılıklar mevcuttur. ASTM ve farklı ISO standartları, materyalin
initial protocols of ISO had variations in testing procedures. bükülme dayanımını belirlemek için literatürde uyarlanan protokolle-
Over the years constant modifications and changes have rdir. Etkili, kabul edilebilir ve standartlar arası farklılıkları belirlemeye
been made to the needs. Constant efforts have been made ihtiyaç duyulmaktadır. Gereç ve Yöntem: Bu çalışmanın amacı, protez
to match the testing protocols between the organizations to kaide reçinelerinin bükülme dayanımının değerlendirmek için ideal
standardın belirlenmesidir. Amaç, ısı ile polimerize olan PMMA protez
reduce the duplications of the tests and serve the community kaide reçinelerinin bükülme dayanımlarını ölçmek için kullanılan ASTM
better. The standards for the day to determine flexural D790 ve ISO 20795.1.2013 arasındaki farkları karşılaştırmaktır. 30 adet
strength is ISO. Though directions have been issued towards ısı ile polimerize olan protez kaide örneği ISO 20795.1.2013 ve ASTM
for universal adaptation of latest ISO standards still many D790 standartlarına uygun olarak üretilmiştir. Örnekler rutin olarak
literatures employ ANSI or outdated ISO protocols (8,9). kullanılan protocol ile bitirilip muhafaza edilmiştir. Bükülme dayanımı,
The difference between four and three point bending test 1.50 mm / dak yaklaşma hızında ve 40.00 mm bir açıklık uzunluğunda
üniversal test makinesi kullanılarak belirlenmiştir. Ortalama bükülme
exist in specimen size, shape, and thickness, load nose radius, dayanımı değerleri MPa olarak elde edilmiş, tabloya aktarılmış ve stu-
bending momentum, maximum allowable strain and axial dent t testi ile istatistiksel olarak analiz edilmiştir. Bulgular: ISO ve ASTM
stress (3). The test specimen was 65mm x 40mm x 5mm for tarafından bulunan ısı ile polimerize olan PMMA'nın ortalama bükülme
three-point testing and measurement of 127mm x 12.7mm dayanım değerleri, 60.492 MPa ve 61.470 MPa arasında değişmiştir.
x 3.2mm for four point bending test testing. In dentistry, the İki yöntem arasında anlamlı fark bulunmamıştır. Sonuçlar istatistiksel
samples for ISO testing were easy to fabricate in regular dental olarak P ≤ 0.05 anlamlı bulunmamıştır. Sonuç: Isı ile polimerize olan
PMMA protez kaide reçinelerinin bükülme dayanım değerlerinde ISO
flasks compared to larger specimens of four point bending
20795.1.2013 ve ASTM 790 protokolleri arasındaki sayısal farklılıklar
test. The ASTM samples required larger flasks to fabricate and mevcuttur. Fakat bu farklılıklar, istatistiksel ve klinik olarak anlamlı
polymerize PMMA specimen. The variations in samples sizes değildir. Anahtar kelimeler: ISO; ASTM; protez kaide reçinesi; polimetil-
and protocol do not differentiate the results significantly. metakrilat.
140 Chander et al.