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PHYSICAL PROPERTIES OF SILICONE ELASTOMERS. REVIEW ARTICLE

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Kati. World Journal of Pharmaceutical Research
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PHYSICAL PROPERTIES OF SILICONE ELASTOMERS. REVIEW
ARTICLE
Firas Abd Kati (Lecturer)*
Department of Optical Techniques, College of Health and Medical Technology, Baghdad,
Middle Technical University, Baghdad, Iraq.
INTRODUCTION
The materials that are used for fabrication of extra-oral prosthesis
should have desirable physical properties. These properties involve
hardness, wettability, non-water absorption, colour stability, high
tensile, elongation percent and tear strengths (Aziz et al., 2003;
Thomas, 2006).
PHYSICAL PROPERTIES
1. Hardness
The hardness property expresses the flexibility of silicone material.
This characteristic is an essential because it is very important to have a
material with similar hardness to the missing facial defect (Aziz et al.,
2003).
Overall, the hardness of silicone elastomers can be measured by durometer shore A scale
ranging from zero to 100, where the grade (zero) indicates the softness while the grade (100)
means the hardest. The silicones which are used for facial and body prostheses range from 5
to 20 shore A. Table 1demonstrates the shore A scale for different types of silicone (Thomas,
2006).
World Journal of Pharmaceutical Research
SJIF Impact Factor 8.074
Volume 8, Issue 6, 50-57. Review Article ISSN 2277 7105
Article Received on
04 March 2019,
Revised on 24 March 2019,
Accepted on 15 April 2019,
DOI: 10.20959/wjpr20196-14837
*Corresponding Author
Prof. Firas Abd Kati
(Lecturer)
Department of Optical
Techniques, College of
Health and Medical
Technology, Baghdad,
Middle Technical
University, Baghdad, Iraq.
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51
Material
Durometer Shore A
MDX4-4210 Silicone Elastomer (Factor II)
27
Silicone MED 40072 ( Rhodia Silicones Inc.)
25
A-2186 Silicone Elastomer (Factor II)
30
A-588 Variable Durometer (Factor II)
Variable 12,20, 29
VST-50 Versital (Factor II)
30
Techsil 3455ST ( Techsil Ltd)
40
Elastosil RTV 625 ( Wacker Chemic Gmbh)
25
Silbione RTV 4408 ( Rhodia Silicones Inc)
8
Silopren LSR 2020 TP 3364(G.F. Bayer)
22
MED 4940( Nusil)
48
To measure the hardness of silicone samples, the specimens are placed on a flat surface and
the pressor foot of the durometer scale is applied on the samples with enough pressure in so
that contact between the samples and the pressor foot occurs. When the contact occurs, the
scale will determine the hardness of the specimen (Wolfaardt, et al., 1975). Figure 1
demonstrates Application of durometer to specimens for hardness testing (Wolfaardt, et al.,
1985).
Figure 1: Application of durometer to specimens for hardness testing (Wolfaardt, et al.,
1985).
2. Tensile strength
The tensile strength of silicone elastomer is defined as the ability of a material to resist the
stretch until it tears. Therefore, the material which is used for construction of extra-oral
prostheses should have a high tensile strength (Aziz et al., 2003).
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In addition, the elongation percent of the samples at break is recorded where means the
ability of material to be flexible before fracture. Thus, it is significant to have material with
high percentage of elongation especially when peeling the prosthesis from the tissue. The
tensile strength of material is measured in pounds per square inch (psi). To measure the
tensile strength, the sample is placed in tensile machine where one end of the sample is
clamped in one grip and the other end is clamped in other grip. Samples are then exposed to
the load (such as 1 kilo Newton) until its breaks. For each sample, the tensile stress and the
elongation percentage are calculated by computer software using the formula below (Aziz et
al., 2003).
Stress = Load / initial cross-sectional area
Percentage strain % = Extension / Original length x 100
According to these formulas, the tensile strength of a material depends upon the load which is
applied on the sample and cross sectional area. As well as the elongation percentage depends
upon extension of the sample after exposing to the load and sample’s length. Table 2
demonstrates the tensile strength and elongation of different types of silicone elastomers
(Thomas, 2006).
Table 2: Tensile strength and elongation percent of different types of silicone elastomers
(Thomas, 2006).
Material
Elongation
percent
MDX4-4210 Silicone Elastomer (Factor II)
500
Silicone MED 40072 ( Rhodia Silicones Inc.)
400
A-2186 Silicone Elastomer (Factor II)
600
A-588 Variable Durometer (Factor II)
325-700
VST-50 Versital (Factor II)
480
MED 4940( Nusil)
452
3. High tear resistance
The material that used in the construction of extra-oral prostheses should have high resistance
to tearing. The thin edge of extra-oral prosthesis (ear, eye, and nose) which is retained by
medical adhesive is subjected to tearing due to the insertion and removal of prosthesis by the
patient. Hence, the tearing of the edge causes the deterioration of the prosthesis over time
(Aziz et al., 2003).
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The tear of silicone material is measured in pounds per square inch (ppi) or pounds per linear
inch (pli). Table 3 shows the tear strength for different types of silicone materials (Thomas,
2006).
Table 3: Tear strength for different types of silicone materials (Thomas, 2006).
Material
Tear
strength(PPi)
MDX4-4210 Silicone Elastomer (Factor II)
90
Silicone MED 40072 ( Rhodia Silicones Inc.)
80
A-2186 Silicone Elastomer (Factor II)
90
A-588 Variable Durometer (Factor II)
45
VST-50 Versital (Factor II)
112
MED 4940( Nusil)
252
4. Water absorption
Another physical property is water absorption, where the absorbed water may affect the
physical properties and colour stability of silicone elastomer. The silicone consists of two
main components polydimethylsiloxane chain and silica filler. The interaction between these
components affects the physical properties of silicone. Hence, the material that used for
extra-oral prosthesis should be hydrophobic nature (Waters et al., 1996; Aziz et al., 2003).
5. Wettability
It is important that the material used for fabrication of maxillofacial prosthesis should have
high wettability, where the material that is easily wetted will form a superior lubrication
between the tissues and reduce the abrasion between the tissue and prosthesis and patient
discomfort (Polyzois et al., 1991; Waters et al., 1996, Aziz et al., 2003).
6. Viscosity
The material that used in the fabrication of extra-oral prosthesis should have a low viscosity
to allow the material to enter into all parts of the mould easily and as the same time high to
permit the settling of pigments inside the mould (Lewis and Castleberry, 1980). It is
measured in centipoises (cps) or milli-pascals (mpas). Table 4 shows the viscosity of different
types of silicone materials (Thomas, 2006).
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Table 4: Viscosity of different types of silicone materials (Thomas, 2006).
Material
Viscosity(cps)
MDX4-4210 Silicone Elastomer (Factor II)
70,000
Silicone MED 40072 (Rhodia Silicones Inc.)
10,000
A-2186 Silicone Elastomer (Factor II)
90,000
A-588 Variable Durometer (Factor II)
85,000
VST-50 Versital (Factor II)
12,000
Techsil 3455ST ( Techsil Ltd)
45,000
Elastosil RTV 625 ( Wacker Chemic Gmbh)
45,000
Silbione RTV 4408 ( Rhodia Silicones Inc)
1,900
Silopren LSR 2020 TP 3364(G.F. Bayer)
200
MED 4940( Nusil)
Translucent paste
7. Colour stability
The material that is used for construction of facial prosthesis should be able to accept and
retain intrinsic and extrinsic pigments. Tints must be soluble, and pigments and fibers must
be dispersible. This property is measured by the solubility parameter. The ideal value for a
prosthetic material is from 9 to 11 cals½ (Lewis and Castleberry, 1980).
Discolouration of silicone elastomer is one of the major factors which lead to remake of
extra-oral prosthesis. This problem is caused because of colour instability of intrinsic and
extrinsic pigments due to environmental factors (sun light, air pollution and body oil
accumulation), ultraviolet light and cleaning agents (Polyzois, 1999).
Several studies were undertaken to assess the colour stability of different silicone elastomers
after exposing to outdoor and artificial weathering. For example, Lemon et al., 1995 assessed
the colour stability of silicone elastomer in two conditions (natural and artificial weathering).
The samples were measured before and after exposing to artificial and natural weathering
using a spectrophotometer. They reported that artificial aging caused a change in the colour
of silicone more than outdoor weathering.
Another study by Polyzois (1999) assessed the colour stability of three silicone elastomers:
Ideal (Orthomax, Bradford, UK); Silskin 2000 (De Puy Healthcare, Leeds, UK), Elastosil
M3500 (Wacker-Chemie GmbH, Munchen, Germany). He exposed the specimens to the
sunlight and then he measured the colour stability using colourmeter. He found that the
colour stability of Silskin 2000 was lower than the other two elastomers. He concluded that
the type of silicone elastomer and time exposure to weathering were the main factors that
affect the colour stability.
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A similar study was undertaken by Eleni et al 2008 assessed the colour stability of silicone
elastomers (Episil , Dreve- Dentamid GmbH, Unna, Germany) after exposing to ultraviolet in
different time periods (8, 24, 48, 72, 96, 120, 144, 168 hours). The samples were measured
before and after exposing to ultraviolet light using spectrophotometer. They reported that the
change in the colour of silicone depended upon time exposure and type of the material.
8. Ideal properties
It is necessary that the material used for construction of an extra-oral prosthesis should be
easy to use and process, easy to colour, translucent, non-toxic, biocompatible with the tissue,
resistant to the light aging and long service life (Beumer et al., 1979; Thomas, 2006).
Lewis and Castleberry, 1980 listed the ideal values for different properties and desirable
physical and mechanical properties. (Table 5 & Table 6).
Table 5: Ideal values for different properties (Lewis and Castleberry, 1980).
Processing characteristics
Goal
Viscosity at ambient temperature
< 75,000 cps
Colour
Colorless
Solubility parameter
9 to 11 cal”
Pot life ( working time)
15 to 60 min
Curing temperature
< 100o c
Curing time
I to 2 hr
Table 6: Desirable physical and mechanical properties (Lewis and Castleberry, 1980).
Desirable performance
characteristics
Goal
Tear strength
30 to 100 ppi
Tensile strength
1000 to 2000 psi
Modulus at 100% elongation
50 to 250 psi
Elongation at break
400% to 800%
Glass transition temperature
< 0°c
Heat distortion temperature
> 120° c
Critical surface tension
30 to 45 dynes/cm
Coefficient of friction
0.4 to 0.6
Hardness
25 to 35 Shore A scale
Water absorption
None
Overall, none of materials what are used in construction of facial prostheses has ideal
properties. Table 7 indicates some advantages and disadvantages of silicone elastomers which
are used in the construction of extra-oral prostheses (Thomas, 2006).
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Table 7: Advantages and disadvantages of some silicone elastomers (Thomas, 2006).
Material
Advantages
disadvantages
MDX4-4210 Silicone
Elastomer (Factor II)
Room temperature vulcanizing or heat cure
at 95 for 1.5 hours
Less resistant to light aging
Silicone MED 40072
(Rhodia Silicones Inc.)
Room temperature vulcanizing or heat cure
at 95 for 1.5 hours
---------------
A-2186 Silicone
Elastomer (Factor II)
Room temperature vulcanizing or heat cure
at 95 for 1.5 hours
Firm silicone,
Mixed silicone stay nearly gel
inside the moulds.
A-588 Variable
Durometer (Factor II)
Room temperature vulcanizing or heat cure
at 95 for 1.5 hours
Mixed silicone stay nearly gel
inside the moulds.
VST-50 Versital (Factor
II)
Room temperature vulcanizing or heat cure
at 95 for 1.5 hours This material shows a
reasonable working time (2 hours).Easy to
use and manipulate.
This material requires de-
gassing because of air voids
MED 4940( Nusil)
This material shows a reasonable working
time (2 hours).
Provides very thin edges
Cure inhibition, this can be
prevented by using 5% of
platinum catalyst 50 solution
CONCLUSION
The materials that are used for fabrication of extra-oral prosthesis should have suitable
physical properties. These properties are high tear resistance, high tensile strength, high
modulus of elasticity, non water absorption, high wettability , colour stability and easy to use
and manipulate by maxillofacial prosthetist. It is very important to consider all these
properties when making a maxillofacial prosthesis.
REFERENCES
1. Aziz T, Waters M, Jagger R. Analysis of the properties of silicone rubber maxillofacial
prosthetic materials. Journal of Dentistry, 2003; 31(1): 67-74.
2. Beumer J, Curtis T, Firtell D. Maxillofacial rehabilitation: Prosthodontic and surgical
considerations. St Louis: The C.V. Mosby Company, 1979; 8: 324.
3. Eleni P, Katsavou I, Krokida M, Polyzois G. Color Stability of Facial Silicone Prosthetic
Elastomers after Artificial Weathering. Dental Research Journal, 2008; 5(2): 71-79.
4. Lemon JC, Chambers MS, Jacobsen ML, Powers JM. Color stability of facial prostheses.
Journal of Prosthetic Dentistry, 1995; 74(6): 613-618.
5. Lewis DH, Castleberry DJ. An assessment of recent advances in external maxillofacial
materials. Journal of Prosthetic Dentistry, 1980; 43(4): 426-432.
6. Polyzois G. Color stability of facial silicone prosthetic polymers after outdoor
weathering. Journal of Prosthetic Dentistry, 1999; 82: 447-500.
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7. Polyzois G, Winter R, Stafford G. Boundary lubrication and maxillofacial prosthetic
polydimethylsiloxanes. Biomaterials, 1991; 12(1): 79-82.
8. Thomas K. The art of clinical anaplastology. Published in 2006 by Thomas S. Gh., 1996;
10: 164-170.
9. Wolfaardt JF, Chandler HD, Smith BA. Mechanical properties of a new facial prosthetic
material. Journal of Prosthetic Dentistry, 1985; 53(2): 228-234.
10. Waters M, Jagger R, Winter R. Effect of surface modified fillers on the water-absorption
of a (RTV) silicone denture soft lining material. Journal of Dentistry, 1996; 24(4):
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ResearchGate has not been able to resolve any citations for this publication.
Article
The limited service of facial prostheses is the result of degradation of the elastomer and color instability. Deterioration may be caused by many factors, which include environmental exposure and changes in humidity. This investigation assessed the efficacy of an additive, intrinsic, broad-spectrum ultraviolet light absorber on the color stability of a pigmented facial elastomer. Samples were weathered artificially and outdoors at exposure levels of radiant energy of 150 to 450 kJ/m2. The samples changed color slightly but perceptibly. Artificial aging caused a greater change than outdoor aging. The ultraviolet light absorber UV-5411 did not protect the samples from color changes.
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Polydimethylsiloxanes exhibit hydrophobic behaviour and poor wettability. These features impose some problems with the clinical performance of the material when used for the construction of intraoral and extraoral facial prostheses. Surface active agents were added to modify the surface energy of the polydimethylsiloxane and contact angle measurements were carried out to observe wetting characteristics. Five siloxane-alkene oxide block copolymers, one organosilicon quaternary ammonium chloride and an epoxyalkyl silane ester were used as surfactants. The best surfactant in terms of permanency and contact angle lowering achieved a value of 24 degrees when added at 10% level compared with 57 degrees for the unmodified silicone. The fairly high viscosity, combined with a comb-like structure, together with the high hydroxyl number of 75 provided the correct balance of permanence and wetting characteristics.
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The tensile strength, tear resistance, and hardness of four silicone elastomers were compared. Results suggest that Cosmesil has improved mechanical properties over commonly used facial prosthetic silicone elastomers.
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The purpose of the present study was to modify the filler content of an experimental room-temperature vulcanizing (RTV) lining material that demonstrated high water sorption in order to produce a material with low sorption properties. Three new formulations were prepared, each containing different hydrophobic silane-treated silica fillers. Water sorption properties for specimens obtained from these formulations were determined using standard experimental techniques. All formulations demonstrated greatly reduced water absorption and low volume change. An experimental RTV poly(dimethylsiloxane) denture soft lining material having low water sorption properties has been produced.
Article
Maxillofacial prosthetic materials are used to replace facial parts lost through disease or trauma. Silicone rubbers are the materials of choice, however it is widely accepted that these materials do not possess ideal properties. The objective of this study was to assess the properties of a range of commercially available silicone rubber maxillofacial materials and make recommendations for improvements. Specimens of five commonly used maxillofacial materials were prepared in dental flasks according manufacturers instructions. Tear strength, tensile strength, percentage elongation, hardness, water absorption and water contact angles were determined for each material. The tear strength of Factor II, Cosmesil HC and Nusil were all comparable and significantly higher than Cosmesil St and Prestige (p<0.001). Nusil had a significantly higher tensile strength and elongation in comparison to the other materials (p<0.001) and Cosmesil St and Cosmesil HC were significantly harder (p<0.001). Factor II was significantly less wetted and Prestige and Cosmsesil St had a significantly higher water absorption in comparison to the other materials. None of the commercially available silicone rubber materials possessed ideal properties for use as a maxillofacial prosthetic material. Factor II, however, showed more favourable properties due to it's high tear strength, softness and ease of manipulation.
The art of clinical anaplastology
  • K Thomas
Thomas K. The art of clinical anaplastology. Published in 2006 by Thomas S. Gh., 1996; 10: 164-170.