Water absorption of (RTV) silicone denture soft lining material

Journal of Dentistry Vol. 24, Nos 1-2, pp. 105-108, 1996

0300-5712(95)00032-1

Copyright0 1996Elsevier Science Ltd. All rights reserved Printed in Great Britain. 0300-5712/96 $15.00+ 0.00

Water absorption lining material

of (RTV) silicone denture soft

M. G. J. Waters*, R. G. Jagger? and R. W. Winter+ *Department of Basic Dental Science, i-Department of Prosthetic Dentistry, Cardiff Dental School and $-School of Chemistry and Applied Chemistry, University of Wales College of Card@ UK

ABSTRACT Objectiw: The reason for high water absorption by room temperature vulcanizing (RTV) silicone

denture soft lining materials has not been demonstrated previously. An experimental room-temperature vulcanizing (RTV) silicone soft lining material which had been shown to have good mechanical properties had high water absorption at equilibrium whilst losing only small amounts of soluble material after desorption. The volume change in the material was also high. A variety of formulations of the experimental material was devised in order to determine the cause of the high water sorption values. Methods: Water sorption values were determined using standard experimental techniques. Sections of the polymerized materials were examined by scanning electron microscopy for microporosity. Results: Neither removal of residual cross-linker nor changing the method of polymerization reduced the water absorption. Scanning electron microscopy did not demonstrate a porous structure of the polymerized material. A correlation was seen between filler content and water absorption. A formulation without filler showed a greatly reduced water absorption and volume change. Conclusion: It was concluded that the filler was directly responsible for the water absorption of the RTV material. KEY WORDS: Denture soft liner, Water absorption J. Dent. 1996; 24: 105-l 08 (Received

4 May 1993; reviewed

22 June 1993; accepted 14 July 1994)

INTRODUCTION Water absorption by a denture soft lining material is undesirable because of the resultant dimensional change which may lead to stress at the liner/denture base interface and reduce bond strength’. Thus it is desirable that a soft lining material absorbs the same small amount of water as the denture base, although water diffusing to the bond site causing hydrolysis of the bond will also reduce strength2. Preliminary investigations have demonstrated that an experimental room-temperature vulcanizing (RTV) silicone soft lining material has favourable mechanical properties but high water absorption3. Similar high water absorption values for RTV silicone rubbers have

been noted previously2,4, although the reason for the high values is not known. Braden and Wright4 theorized that the type of filler

and the way that it is bonded to the polymer could be responsible for the high water absorption seen in the RTV silicone materials, and the heat-cured silicone materials could have better bonding to the filler. They

Correspondence should be addressed to: Mr R. G. Jagger, Department of Prosthetic Dentistry, University of Wales College Medicine, Dental School, Heath Park, Cardiff CF4 4XY, UK.

of

also suggested that heat-cured silicone materials may exhibit greater cross-linking, and this, coupled with the application of pressure, produces a denser material. As a result no micropockets of water would exist within the material. A further factor associated with the present experimental material is that it is cross-linked using a mixture of silanes, and many silanes readily undergo hydrolysis with alcohol being a by-product5. In general, alcohols are water-immiscible and may act as humactants as a result of hydrogen bonding. Thus any residual crosslinking agent may contribute to water absorption. To investigate causes of water uptake in the experimental material: Post-cures were applied to a series of specimens in order to force out any retained cross-linker. The experimental silicone was cured using heat and an alternative cross-linker, to establish the role of the original RTV cross-linking agent in water uptake. Formulations with different filler quantities were tested to establish the influence of the filler content on water uptake. Investigation of the material for microporosity using scanning electron microscopy was undertaken.

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J. Dent. 1996; 24: Nos 1-2

To predict clinical behaviour, both the amount of water absorbed and the amount of soluble material lost must be measured4. Thus in the present study the water absorption and solubility of the experimental RTV formulations were determined. The associated dimensional changes were also measured.

MATERIALS

AND METHODS

The constituents of the experimental soft lining material are shown in Table 1. All the silicone material used was from the same batch. Vulcanization of the elastomer took place at room temperature by a polycondensation reaction with the elimination of alcohol. The experimental formulations that were tested contained: 1. 1.3% ww catalyst, 2.3% ww cross-linker and 29% ww filler. 2. 1.3% ww catalyst, 2.3% ww cross-linker and 39% ww filler. 3. 5% ww catalyst, 5% ww cross-linker and 0% ww filler. The catalyst and cross-liner were added dropwise to the elastomer, and mixing was carried out with a spatula on a Perspex block. In an additional formulation dichlorobenzoyl peroxide activated by heat was used to cross-link the material. This formulation contained: 4. 2% ww dichlorobenzoyl peroxide and 29% ww filler. Specimens 45 mm in diameter and 1 mm thick were prepared by packing dough into moulds which were formed by investing Perspex blanks of appropriate size in 50: 50 stone plaster mixture using a conventional dental flasking technique. Specimens of formulations 1, 2 and 3 were cured for 24 h at room temperature. Formulation 4 specimens were cured for 4 h at 120°C in a dry heat oven. Five specimens of each formulation were prepared. On completion of the cure the silicone rubber discs were removed, dried to a constant weight in a desiccator to an accuracy of 0.0001 g, and placed in distilled water at 37°C. The discs were removed at intervals, excess water was blotted and they were reweighed as before. This procedure was continued until equilibrium was reached. The materials were then desorbed in a desiccator at 37°C weighing at recorded time intervals again until equilibrium was achieved. Tab/e 1. Constituents rubber Polymer Filler Catalyst Cross-linker

of experimental

polydimethylsiloxane

Hyroxy end-blocked polydimethylsiloxane Pyrogenic hydrophobic silica filler Dicarboxylate tin Mixture of alkoxy silanes

RTV

The percentage absorption and solubility were determined as follows: 1.

% Absorption = w2-w3 w1

2.

% Solubility = w1-w3 w1

x100 x100

(1) (2)

where Wl = initial weight, W2 = weight after absorption or desorption, W3 = final weight after desiccation. Ideally further absorption and desorption cycles should be performed until all the soluble material is extracted and the sorption values are constant4. However equilibrium uptake is not reached for some 2 years and thus the figures are based on the first absorption and desorption cycles. Dimensional change during absorption was recorded by measuring the diameter and thickness of the specimens in millimetres using a digital micrometer to an accuracy of 0.001 mm (Micro 2000, Moore and Wright, Sheffield, UK). Volume was calculated using the formula s-R2 x t,

(3)

where R = radius, t = thickness. Ten additional specimens of formulation 1 were constructed using the method described earlier. Five specimens were each subjected to post-cures: A. Additional 2 h at 100°C in a dry heat oven. B. Additional 4 h vacuum oven 60°C at 207 kPa. These specimens were then tested as before for absorption and desorption data. Finally five specimens of formulation 1 were placed in the vacuum chamber of an Ion Etching machine (Ion Technology, Teddington, Middlesex, UK). The surfaces of the materials were subjected to continual ion bombardment for 2 h and then removed. The etched surface of the material was studied using scanning electron microscopy, (SEM; IS1 Super 3A, ISI, UK), at a magnification of X 500, and photomicrographs were taken.

RESULTS As water was taken up, all specimenswere seen to turn from a translucent white to a milky-white colour. On drying the original colour was restored. Figures I and 2 depict the percentage water uptake and desorption values respectively of the various formulations. The results are based on the mean percentage value of the five specimens of each formulation. Table II shows the percentage absorption and percentage solubility of the formulations at equilibrium together with the percentage volume change at equilibrium uptake.

Waters et al.: Water absorption of an RTV silicone soft lining material

160

107

*

‘ii1 1

/Formulation

1 Formulation 2 Formulation 3 Formulation 4 Post cure A Post cure B

+ s II x o

I

200

0

600

400

800

Time (days) Fig. 7. Percentage water uptake of formulations A and B as a function of time.

Fig. 3. Photograph of the ion-etched surface of formulation 1 using SEM at x500 magnification showing no evidence of a microporous structure. Scale bar=0.03 mm.

l-4 and post-cures

“-

The original experimental soft lining material (formulation 1) has excellent mechanical properties but would be unsuitable for clinical use because of its high water sorption values and large associated dimensional change (Table 11). Although reasons for the high water uptake associated with previously investigated RTV denture lining materials have been proposed, no reports have appeared substantiating these suggestions. The post-cures applied to formulation 1 were devised to force out any excesscross-linker in the matrix which could undergo hydrolysis and form alcohol. As can be seen, neither post-cure improved the water absorption figures (Table 11). Thus either all the cross-linker is used up in the reaction or any retained is not causing water absorption. The structure of the cured experimental formulation 1 was examined by SEM. Specimens were prepared for SEM by ion-etching. If a material is prepared for microscopy by cleaving with a sharp knife, there is a possibility of a smear layer being produced on the surface. Ion-etching avoids this technical problem. All samples were free of air pockets that can be caused by mixing and there was no evidence of any microporous structure that would retain substantial quantities of water (Fig. 3). As mentioned previously 4, it is suggested that heatcured silicone materials may have greater bonding to

-10

-60

-70

0

* I 2

I

I

I

4

6

8

I 12

10

I

I

14

16

Time (days) Fig. 2. Percentage water loss of formulations and B as a function of time.

l-4

and post-cures A

Figure 3 shows the etched surface of a representative specimen of formulation 1 at a magnification of X500.

DISCUSSION This investigation has determined water-sorption values of a variety of formulations of an experimental denture soft lining material using standard experimenal techniques4a6. Table Il. Mean percentage Formulation

Formulation Formulation Formulation Formulation Post-cure A Post-cure B

absorption,

Time in distilled water at 3PC fm th) 1 2 3 4

22 23 20 23 22 19

solubility and volume change at equilibrium % Absorption

% Solubility

%Volume change

R

SD.

R

SD.

x

SD.

22.71 39.70 4.28 156.23 38.58 29.55

1.25 2.31 0.16 6.21 2.21 1.72

0.99 2.09 10.79 4.96 2.14 0.59

0.02 0.07 0.63 0.14 0.08 0.01

25.76 42.19 -1.2 82.12 45.29 27.65

1.63 2.25 0.01 6.67 2.22 1.38

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J. Dent. 1996; 24: Nos l-2

the filler and greater cross-linking and that these factors could be partially responsible for heat-curing materials having lower water absorption than room-temperature curing silicones. Formulation 4 uses 2,4-dichlorobenzoyl peroxide and the application of heat to cross-link the elastomer base. The polymerization of formulation 4 is dependent on free radicals arising from the peroxide on their decomposition, abstracting hydrogen atoms from methyl groups on the polydimethylsiloxane. Ethylene links can thus be formed between the siloxane chains7. This is a polyaddition reaction without the liberation of alcohol. This heatcured formulation, however, shows exceptionally high water absorption. It was noted, however, that porosity was present in these samples, centred around unreacted pieces of 2,4-dichlorobenzoyl peroxide powder. This would contribute significantly to the absorption values. Increasing the filler content from 29% to 39% in formulation 2 brought a 16.99% increase in water absorption, clearly indicating a connection between< filler content and water absorption (Table II). The unfilled formulation 3 was cured with 5% catalyst and cross-linker. Preliminary investigation had shown that this was the minimum amount needed to produce a non-tacky vulcanite. The lack of filler polymer bonds which produce the rigidity and strength in the rubber necessitates the need for such catalyst and cross-linker contents. The 3% weight loss of formulation 4 (Fig. I> is likely to be due to the high amount of catalyst that has been used in this formulation, which would be extracted by water. The alcohol is given off in the reaction, which has been shown not to be retained within the matrix, will also be extracted by the distilled water4. These factors account for the high solubility figure of 10.79% seen in Table II. Also, although there is a net weight loss, there is a 4.28% water absorption figure at equilibrium because of the desorption desiccation removing any water absorbed into the material. The percentage volume change of the various formulations was monitored throughout the study. Since soft lining materials will always be in an aqueous environ-

ment while in use, the information that has been derived from the initial absorption cycle, as in this study, is the most relevant4. The results have confirmed that lining materials with high water absorption are dimensionally unstable. The unfilled formulation 4 has a volume reduction at equilibrium probably caused by excesscatalyst leaching out into the distilled water. This study has demonstrated that the pyrogenic silica filler was the major cause of high water absorption seen in the experimental RTV silicone denture lining material. A correlation was seen between the amount of filler and degree of water absorption. When filler was removed completely from the polymeric matrix, water absorption was minimal. The filler was selected because of its hydrophobicity. During the condensation polymerization the hydrophobocity would seem to be lost. Further work is being undertaken to identify a surface-modified filler which may overcome this problem

Acknowledgements This study was supported by the Welsh Scheme for the Development of Health and Social Research. References 1. Bates JF, Smith DC. Evaluation of indirect resilient liners for dentures: laboratory and clinical tests. J Am Dent Assoc 1965; 70: 344-353. 2. Wright PS. Soft lining materials: their status and prospects. JDent 1976; 6: 247-256. 3. Waters MGJ, Jagger KG. Properties of an experimental silicone soft lining material. f Dent Res 1994; 73: 807 (abstr). 4. Braden M, Wright PS. Water absorption and water solubility of soft lining materials for acrylic dentures. J Dent Res 1983; 62: 764-768. 5. Union Carbide Silanes: Method of application/Technical data sheet 1. Union Carbide, 1992. 6. Stafford GD, Braden M. Water absorption of some denture base polymers. J Dent Res 1968: 47: 241. 7. No11W. Chemistry and Technology of Silicones. London: Academic Press. 1068; 494-516.