Journal of Oral Rehabililalion, 1992, Volume 19, pages 151-I6t)

Bond strength and rupture properties of some soft denture liners D. SINOBAD, W.M, MURPHY* R. HUGGETT'l' S. B R O O K S ' " Klinika za proleliku, Beograd, Yugoslavia, ^Department of Prosthetic Detitistry, University of Wales College of Medicine, Denial School, Cardiff, and fDeparlmenI of Frosllietic Dentistry and Dental Care of the Elderly, University of Bristol, Dental School, Bristol, U. K.

Summary The bond strength and rupture properties of three soft acrylic liners (Coe-Soft, Coe Super-Soft and Vertex Soft) and two silieone liners (Molloplast-B and Flexibase) wer-e tested immediately after processing and following itntnersion in water at 37°C for 7 and 90 days. The bond test gave variable results, as some materials partly peeled and then stretched until ruptur'e occurred. After saturatioti the acrylic liners exhibited an increase in tear resistance, whilst silieone materials deteriorated, in the case of Flexibase to approximately 25% of its original value. Vertex Soft was the exception, as tear resistance decreased as water sorption increased. Scanning electron microscopy revealed indefinite demarcation between soft acrylic resins and hard denture base, which was not altered after saturation in water. On the other hand, the interface between denture base and silieone liners was clearly visible before and after saturation. The results indicated that denture soft liners had variable water sorption values depending on their basic structure, and some properties changed after immersion in water, a finding that is of relevance to prosthodontie practice. Introduction The clinical benefits of resilient denture liners have been recognized in prosthodontie practice for many years. They act as stress absorbers, enabling uniform distribution of pressure on denture-bearing tissues, and reduce diseomfort for patients with sharp or severely r'esorbed alveolar ridges and sensitive rnucosa. Despite efforts to develop a clinically acceptable material, current soft liners do not fulfil all clinical requirements, and numerous shortcomings have been reported (Travaglini, Gibbons & Craig, 1960; Eiek, Craig & Peyton, 1962; Stor'er, 1962a, b; Bates & Smith, 1965; Gonzalez & Laney, 1966; Wilson & Tornlin, 1969; Suchatlampong, Davies & Von Fratinhofer, 1976; Wright, 1980, 1981, 1984; Amin, Fletcher & Ritchie, 1981; Graham, Jones & Sutow, 1989). Soft liners can be divided into two main types: plasticized acrylics and silieone elastomers. Both types are available in autopolymerizing and heat-curing forms, differing in the percentage of plasticizers, crosslinking agents, catalysts and fillers. The aim of this paper is to deseribe the bond strength and rupture properties of five soft denture liners that are commonly used in prosthodontie practice. Coi't'cspotidenee; Mr W.M. Murphy, Department of Prosthetie Dentistry, University of Wales College of Mcdicitic, Dental School, Heath Park, Cardilf CF4 4XY, U.K. 151


D. Sinobad et al.

Materials and methods The materials investigated were three plastieized acrylic resins and two silieone elastomers (Table 1). Coe-Soft is manufactured as a powder containing poly ethyl/methyl rnethacrylate and a liquid mixture of aromtitie ester plasticizer and ethyl alcohol penetrant. This material is more accurately described as a tissue conditioner. Coe Super-Soft is heat cured, and consists of a powder containing poly ethyl-methyl rnethacrylate, benzoyl peroxide activator and possible other eopolymers, and a liquid eontaining an alkyl rnethacrylate monomer and an aromatic ester plasticizer. Vertex Soft is deseribed by the manufacturer as a heat-curing acrylic resin, and is therefore probably similar to Coe Super-Soft, differing primarily in filler content. Flexibase is an autopolymerizing silieone liner, and is manufactured as a powder containing polydimethyl siloxane, an organic silicate cross-linking agent and a filler, and a liquid eontaining an organo-tin catalyst. Molloplast-B is a heat-curing singlepaste system containing a hydroxy-terminated polydimethyl siloxane which is crosslinked with acryl-oxyalkyi silane after initiation by benzoyl peroxide. The filler is pr'obably some form of silica. Peel test

The peel test was designed to measure the force necessary to peel the lining material from a rigid denture base material under a controlled rate of tension (Wright, 1982). The rigid denture base material used was Biocryl, a Type I Class I denture base polymer*. Specimens of soft lining and Biocryl of thickness 3mm, width 25mm and length 75 mm were constructed with a 25-mtn length of soft lining bonded to the Biocryl plate, while the remaining 50-inm length was unattached. The unattached soft lining tab was bent backwards and subjeeted to tensile forces under a constant crosshead speed of 20mmmin^' in an Instron testing machine (Fig. I). The force and nature of failure for each specimen was recorded. As auto-polymerizing silieone materials do not adhere naturally to polymerized denture base material, an adhesive was applied to the processed Biocryl specimen before attaching Flexibase. In the ease of the hcat-euring acrylic liner, both the Biocryl plate and the liner were inserted in the dough state in a denture Hask and processed together according to the manufacturer's instructions. This processing technique was attempted with Molloplast-B, but specimen thickness could not be controlled due to the high viscosity of the elastomer paste. Consequently, the acrylic base was processed fir'st, coated with an adhesive, and the liner was then heat-cured in the usual

Table 1. Materials investigated Type



Plasticized aerylics

Coe-Soft Coe Super-Soft Vertex Soft

Coe Lab. Ine., Illinois, U.S.A. Coe Lab. ltic, Illinois, U.S.A. Dctitimex, Zeist, The Netherlatids

Silicotie elastomers

Molloplast-B Flexibase

Costtier and Co., Germany Flexico Devcloptnent Ltd, Lotidon, U.K.

* Galcnika, Belgrade, Yugoslavia.

Soft den tt ire liners


Fig. 1. Peel test in which unattached soft lining tab is subjected to a tensile foree in an Instron testing maehine.

manner. Nine specimens of each material were tested before and after immersion in distilled water at 37°C tor 7 and 90 days. Peel strength (Ps) was calculated aeeording to the following equation (Wright, 1982), where peeling angle was eonsidered to be 180°:


4- I kN

where F = applied force, w = width of specimen in the peeling area, and \ = extension ratio of liner, i.e. the ratio of stretched to unstretched length. Rupture properties A modifieation of ASTM D624-86, Standard Test Method for Rubber Property-Tear Resistance was employed (Craig & Gibbons, 1961). Specimens measuring 50 X 12 X 2mm were constructed with a 45° notch on one side, the width at the notch being 6mm (Fig. 2). Nine specimens were prepared for eaeh test material. The tear test was performed in an Instron testing machine using a constant cross-head speed of 20mm min^'. The foree of failure for each speeimen was recorded. The tests were performed before and after itntnersion in distilled water at 37°C for 7 atid 90 days. Tear resistance (Ts) was calculated using the following equations: dw


D. Sinobad et al.

Fig. 2. Tear test in which a specimen with a 45° notch is subjected to a tensile force in an Itistron testing maehine.

where F = force of failure, d = thickness of the specimen, and w = width of the specimen at the noteh. Microscopy of interface -. -. -. • . : :' ^ i, ' The interface between soft liner and hard Biocryl base was examined by scanning electron microscopy. Specimens were prepared by cutting a slice measuring 15 X 5 mm from each peel specimen. Seleeted surfaces were sputtered with gold in a Polaron E50(J0 sputter-coating unit*. It was not possible to examine Coe-Soft by this technique, whieh was excluded from this part of the investigation. Water sorption Weight changes following water immersion were recorded. Four speeimens of each soft material measuring 50 x 50 x 2 mm were processed according to the manufaeturer's instructions. Specimens were weighed, placed in a desiccator eontaining P2O5, dried in an oven at 37°C and reweighed until a constant dry weight was obtained. They were then immersed in distilled water at 37°C and weighed after 1, 7, 30 and 90 days. Results In the peel test, most soft materials stretched until rupture occurred, rather than by peeling from the hard denture base, i.e. there was cohesive rather than adhesive * Polaroti Ltd, Watford, Merts, U.K.

Soft denture liners


failur'e (Fig. 3). In cases wher'e the liner began to peel and then failed due to cohesive failur'e, the term adhesive/cohesive failure is used. The latter mode of failure was generally observed for Coe Super-Soft, Vertex Soft and Molloplast-B at all time intervals. Cohesive failure oecurred for Coe-Soft and Flexibase even after immersion for 90 days. Before saturation, peel strength values for Coe Super-Soft and Vertex Soft were considerably higher than those for other liners (Table 2). Following immer'sion in water for 7 days, all materials showed an increase in peel strength, but this was statistically signifieant only for Coe-Soft, Molloplast-B and Flexibase ( P < 0-001). After 90 days, Coe-Soft and Coe Super-Soft eontinued to show increased values (P{).{)5)). After saturation for 90 days all materials were affected. Both Coe-Soft and Coe Super-Soft exhibited an incr'case in tear resistance, particularly Coe Super-Soft. On the other hand, the tear resistance of Vertex Soft and the silieone materials decreased significantly, particularly Flexibase (P

Bond strength and rupture properties of some soft denture liners.

The bond strength and rupture properties of three soft acrylic liners (Coe-Soft, Coe Super-Soft and Vertex Soft) and two silicone liners (Molloplast-B...
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