ELSEVIER 0142-9612(95)00330-4

~iomoteriak 17 (1996) 1627-1630

0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved

0142-9612/96/$X.00

Dynamic mechanical thermal analysis of denture soft lining materials

Mark Waters*, Robert Jagger**, Keith Williams* and Vjeko Jerolimov+ *Department of Basic Dental Science, **Department of Restorative Dentistry, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY, UK; ‘Department of Removable Prosthetics, Faculty of Stomatology, University of Zagreb, Zagreb, Croatia

The purpose of this study was to characterize the deformation properties of a range of long-term

denture soft lining materials using dynamic mechanical thermal analysis. Specimens were subjected

to sinusoidal shear deformation at a frequency of 1 Hz and a strain of 64 microns peak to peak over a

temperature range of 30-70°C. The technique simulated the type and rate of deformation soft lining

materials would experience clinically, thus producing more relevant results than previously used

methods. Shear modulus G’, dissipative modulus G” and mechanical loss tangent G”/G’ were obtained

for each material. With increasing temperature the moduli of the acrylic materials decreased, indicat-

ing an increased compliance and greater deformation. The silicone (Molloplast B) and alternative

formulation (Novus) properties were generally insensitive to temperature over the range chosen. The

mechanical loss tangent values of the acrylics fell continuously with temperature, suggesting that the

plasticization process lowered the Tg values below 3O”C, the lowest temperature of testing. 0 1996

Elsevier Science Limited

Keywords: Dental materials, denture liners, dynamic mechanical thermal analysis, deformation

properties

Received 16 July 1995; accepted 3 November 1995

Of the physical properties of soft lining materials, the resilience and the capacity of a material to absorb energy are of particular importance.

Methods previously used to determine deformation characteristics have been static compression set, torsional pendulum and mechanical impedance tests. Compression set measurements invoke measuring the rate of recovery of the material after the application of a given stress’. However, this is a static load application technique and does not predict in viva

behaviour. Braden and Clarke’ realized the importance of measuring deformation at rates appropriate to those experienced in viva. Hence they measured the response of the materials to cyclic (sinusoidal) forces using a torsional pendulum, previously described by Braden and Stafford3. However, the technique is insufficient to maintain a constant amplitude over a set temperature range. Mechanical impedance testing4 has limitations in precision of measurement owing to specimen size requirements.

DMTA has been used to evaluate denture-base polymers6, but investigation of resilient liners using this technique has not been reported.

The purpose of this investigation was to compare the moduli of selected resilient lining materials using DMTA.

THEORETICAL CONSIDERATIONS

When a sinusoidal stress is applied to a perfectly elastic solid the deformation (and hence the strain) occurs exactly in phase with the applied stress, hence the modulus is not time dependent. A completely viscous material will respond with the deformation lagging behind the applied stress (Figure I).

Many of the limitations of the above techniques are overcome by the use of the recently developed dynamic mechanical thermal analysis (DMTA). The technique involves the application of a sinusoidally oscillating stress to a material which enables measurement of the strain at a fixed frequency or range of frequencies over a specific temperature range and also isothermally as a function of time5.

When a sinusoidal stress is applied to a viscoelastic material it will behave neither as a perfectly elastic nor as a perfectly viscous body and the resultant strain will lag behind the stress by some angle(s) where 0 = 90” (Figure z). The magnitude of the loss angle is dependent upon the amount of internal motion occurring in the same frequency range as the imposed stress.

For a perfectly elastic material deforming in shear, it is possible to write:

y(f) = $

Correspondence to Mr M.G.J. Waters. Where y(t) is the shear strain, o(t) is the shear stress and

1627 Biomaterials 1996, Vol. 17 No. 16

1628 DMTA of denture soft liners: M. Waters et al.

Elastic t T Viscous

mawrial mawid

Figure 1 Schematic representation of the response of perfectly elastic and perfectly viscous materials to an applied stress.

G is the shear modulus and the time dependence of oscillation (0). The stress is given by:

a(t) = 60 cos wt (2)

and the strain as:

y(t) = 2 cos wt (3)

For a perfectly viscous rod the deformation behaviour is:

The strain rate, d/dt responds linearly to stress for a viscous body with a proportionately constant 7, the viscosity. The rate of strain is a maximum when the stress is a maximum and a minimum when the stress is a minimum. Integrating Equation 4 yields:

a S1ress t

b

Figure 2 a, Schematic representation of the response of a viscoelastic material to an applied sinusoidal stress. b, Schematic representation of the resolution of the in-and- out-of phase stress components.

The stress and strain are therefore 90” out of phase for a sample whose properties are defined by Equation 4.

For a viscoelastic material, the strain will lag behind the stress by an amount normally called the loss angle (6). It is normally convenient to separate the viscoelastic response into ‘in phase’ and ‘out phase’ components. The ‘in phase’ stress and strain result in elastically stored energy which is completely recoverable, while the ‘out of phase’ stress and strain result in the dissipation of energy. The ‘in phase’ and the ‘out phase’ shear moduli G’ and G” are:

G, = 4 = Amplitude of in phase stress component

‘i Y

G,,= c = Amplitude of out of phase stress component

*J ‘/

The shear compliances are similarly:

J’=; J” = $

and

” J” Y” ” tan&Cr=_=_=G (T’ J’ y’ G’

The storage modulus is the elastic response and corresponds to completely recoverable energy, whereas the loss modulus is the viscous response corresponding to energy lost through internal motion.

The tangent of this loss angle, tan 6, is dimensionless and is equal to the ratio of energy lost (dissipated as heat) to energy stored per cycle:

tan6 = Loss modulus G”

Storage modulus = ??

MATERIALS AND METHODS

The materials investigated are listed in Table I. Samples were prepared by packing dough into 50%/ 50% stone/plaster moulds that had been prepared by investing acrylic blanks 75 x 25 x 3mm in a conventional dental flashing technique. Mixing, packing and cureing were carried out according to manufacturers’ instructions. The cured strips of material were then cut to specimen dimensions of 10 mm diameter and 3 mm thick, using an appropriate cutting instrument.

Five pairs of specimens were produced for each material and were stored in air at 37°C. They were then tested in a dynamic mechanical thermal analyser (DMTA) apparatus (Polymer Laboratories Ltd., Loughborough, UK) model MKII, linked to a Compaq 386 IBM compatible computer. The specimens were subjected to sinusoidal shear deformation at a frequency of 1 Hz and a strain of 64 microns peak to peak (x4) over a temperature range of 30°C to 70°C. Parameters measured were G’, storage modulus (MPa); G”, loss modulus (MPa) and tan 6, loss tangents, with readings at 30”C, 37”C, 50°C and 70°C.

Biomaterials 1996, Vol. 17 No. 16

DMTA of denture soft liners: M. Waters et al.

Table 1 Type and manufacturer of materials used

1629

Material Type Manufacturer

Coe Super Soft Vetrex Soft Malloplast B Flexibase RTV Flexor Novus

Plasticized acrylic Plasticized acrylic Silicone elastomer Silicone elastomer Copolymer resilient liner Polyphosphazene fluoroelastomer

Coe Laboratories Inc., Chicago, Illinois, USA Dentimex EIV., Zeist, Holland Regneri G-MBH & KG, Karlsruhe, Germany Flexico Developments Ltd., London, UK Weil-Dental G-MBH, Rosbach, Germany Hyginic Corporation, Akron, Ohio, USA

Table 2 Shear parameters measured at 37°C

Material N = 5 G’ (MPa) G” (MPa) Tan 6 (G”/G’)

Mean SD cv Mean SD cv Mean SD cv

Coe Super Soft 0.311 0.055 17.6 0.331 0.079 23.9 1.078 0.126 11.7 Vertex Soft 0.654 0.104 15.9 0.536 0.088 16.5 0.826 0.044 5.4 Molloplast B 0.439 0.094 21.4 0.029 0.008 28.9 0.066 0.015 22.7 Flexibase 0.509 0.117 20.0 0.030 0.014 17.4 0.158 0.012 7.4 Flexor 0.766 0.115 15.1 0.070 0.008 11.7 0.092 0.010 11.1 Novus 0.355 0.058 16.5 0.041 0.003 7.7 0.117 0.014 12.3

tan cs = 1.078. 0 = 47”.

Means, standard deviations and coefficients of variation were calculated for all parameters. Means were tested by Student’s t-test. Results were tested by one-way ANOVA and multiple comparisons of means, together with p-value calculations.

RESULTS

Table 2 gives the mean, standard deviation, and coefficient of variance at 37°C for G’, G”, and tan 6.

The shear storage modulus G’ plotted against temperature is shown in Figure 3. The acrylic materials Vertex Soft and Coe Super Soft are particularly temperature dependent in this temperature range, while the remaining materials indicate G’ values which are virtually insensitive to temperature. At 37”C, Flexor exhibits the highest modulus value and would be expected to develop the highest energy absorbing capacity, with Coe Super Soft the poorest in this respect. Vertex Soft is within the max-min range of moduli at 37°C but falls below the non-acrylic materials above

1 .o r+

n Coe super-soft

+ Vertex SOfL

* Mollophs~-R

0 Flcxihase

x Flexor

0 Novus

25 30 35 40 45 SO 55 60 65 70

“C

Figure 3 Shear storage modulus measured over tempera- Figure 4 Dissipative modulus measured over temperature ture range 30-70°C. range 30-70°C.

50”C, while Coe Super Soft exhibits the lowest G’ value above 35°C. Since all materials where cycled over a strain range of 64 microns this figure would also be expected to indicate the resilience of the respective materials, i.e., Flexor the best and Coe Super Soft the poorest at 37°C. A plot of the dissipative modulus G” against temperature is shown in Figure 4. The non- acrylic materials show a virtually zero modulus change which is indicative of highly elastic and resilient materials in the temperature and strain amplitude range chosen. Both acrylic G” values are particularly temperature dependent and since they fall continuously, suggest a Tg value below 30°C.

The mechanical loss tangent plotted against temperature (Figure 5) merely confirms the elastic behaviour of the non-acrylic materials.

DISCUSSION

A necessary requirement of soft lining materials is their ability to elastically recover following deformation

0.8

n

. +

-\

. Coe super-soft

+ Vertex soft

* Molloplrst-B

0 Flexibasc

+\ X Flexor

“25 30 35 40 45 SO 55 60 65 70

“C

Biomaterials 1996, Vol. 17 No. 16

1630 DMTA of denture soft liners: M. Waters et al.

I .2 r l y_ n Coe super-soft + Vertex soft

* Molloplast-R

m 6 -) 0.4-

0.2 - *--

0 I I r I I I r 25 30 35 40 45 50 55 60 65 70

OC

Figure 5 Mechanical loss tangent measured over tempera- ture range 30-70°C.

during mastication over reasonably long time periods. This elastic recovery response is normally quoted as ‘resilience’, i.e. the material’s ability to consistently and immediately return to its original shape following deformation. Unfortunately, polymeric materials tend to exhibit time-dependent behaviour during and following deformation’s ‘3 7. Thus it would be expected that materials deforming at temperatures near and above their Tg would demonstrate time-dependent properties which may include viscoelastic behaviour. The greater the degree of elastic response elicited during a deformation event, the greater the chance of success as a soft lining material. Another important property requirement is the elastic modulus, since the lower this value the greater the energy absorbed at a given load during the deforming cycle. This behaviour will help in allowing the point applied loads to be spread over a larger area of the material with lower loads transferred to the underlying tissue. These important parameters can be measured in a number of ways as already outlined’y277. However, an important requirement for time-dependent materials is to simulate masticatory deformation by cycling the materials through a given stress-strain regime at a frequency close to that of chewing. Past methods have generally failed to simulate masticatory deformation with periods of cycling as long as 5 mini.

In this work, ‘elastic’ properties have been measured using a dedicated DMTA testing machine which allows measurement of both the elastic and viscous response of materials at specific testing rates over a series of temperatures. The storage modulus measures the perfectly elastic response of the materials. Thus if resilience is the prime requirement of a soft lining material, then at 37°C Flexor would be expected to show ideal properties, with Novus the poorest of the non-acrylic materials at 37°C. Since the materials were deformed to a specific strain, this behaviour does not represent the energy-absorbing capacities of these soft liners for a given load. If the capacity to absorb energy is a more realistic requirement particularly in terms of patient comfort, then the ranking of materials shown may not represent the in vivo requirement.

Evidently, the behaviour of the acrylic materials is highly temperature sensitive, showing good elastic response at 37°C in line with the other candidate

materials but falling away rapidly with temperature. Since both acrylics demonstrate an element of viscous behaviour as evidenced by the dissipative modulus (G”) (Figure 4) and tan 6 (Figure 5) plots, this viscous proportion to the total deformation decreases with temperature. The acrylic materials therefore demonstrate improved elastic cycling response with increasing temperature.

The clinical consequences of these temperature- dependent changes are difficult to interpret from one type of test. Certainly, an increased proportion of elastic response is desirable compared with time-dependent recovery events (i.e., viscoelastic and viscous). However, at higher temperatures, the modulus of the acrylics may be too low to sustain a given loading in thicknesses normally applicable to soft lining materials. Thus the deforming force may simply move the material allowing direct loading onto the underlying tissues.

The DMTA apparatus did prove to be a rapid and convenient method for determination of the deformation properties of soft lining materials under clinically relevant loading and cycling. Further work will look at methods to determine the exact time/ deformation event which is necessary to further classify viscoelastic and viscous materials at or near body temperature.

CONCLUSIONS

(11

(2)

(31

(4)

(5)

The acrylic materials were less resilient than the other materials. The acrylic materials were temperature sensitive with all values decreasing with increasing temperature, whilst the other materials indicated moduli which were relatively constant over the temperature range. Flexor was the stiffest material at 37”C, i.e. highest modulus. Molloplast was the most resilient material. Dynamic mechanical thermal analysis (DMTA) proved to be a rapid, reliable and convenient method for the determination of viscoelastic properties of resilient liners.

REFERENCES

Wilson HJ, Tomlin HR. Soft lining materials: Some relevant properties and their determination. 1 Pro&et Dent1969;21:244-250.

Braden M, Clarke RL. Visco-elastic properties of soft lining materials. I Dent Res 1972; 51: 1525-1528. Braden M, Stafford GD. Viscoelastic properties of some denture base materials. 1 Dent Res 1968; 47: 519-523. Clarke RL, Braden M. Determination of visco-elastic properties of dental polymers by mechanical impedance measurements. JDent Res 1982; 61: 1245-1249. Wetton RE, Marsh RDL, Wan-De-Velde TG. Theory and application of dynamic mechanical thermal analysis. Thermochim Acta 1991; 175: 1-11. Clarke RL. Dynamic mechanical thermal analysis of dental polymers. 1. Heat-cured poly(methy1 methacrylate)-based materials. Biomaterials 1989; 10: 494-498. Wright PS. Soft lining materials: their status and prospects. J Dent 1976; 4: 247-256.

Biomaterials 1996. Vol. 17 No. 16