J. Dent. 1992;
240
A denture base resin with absorption
20: 240-244
low water
M. J. Barsby Department
of Prosthetic
Dentistry,
The London
Hospital
Medical
College, UK
ABSTRACT The aim ofthis study was to investigate poly(isobuty1 methacrylate) as a potential denture base material. It was found to have very low water absorption but poor mechanical properties and a low glass transition temperature. The latter features render the material itself unsuitable for use as a denture base material, but indicated that isobutyl methacrylate may be a useful co-monomer for reducing water absorption. KEY WORDS:
Denture
base materials,
J. Dent. 1992; 1992)
20: 240-244
(Received
Physical
properties
2 July 1991;
reviewed
18 September
Correspondence should be addressed to: Dr M. J. Barsby, Department Hospital Medical College, Turner Street, London El 2AD, UK.
1991;
of Prosthetic
accepted
Dentistry,
20 January
The London
INTRODUCTION The absorption of water by denture base resins is a phenomenon of considerable importance since it is accompanied by dimensional changes (Souder and Paffenbarger, 1942; Woelfel et al., 1961). A further undesirable effect of absorbed water in a denture base resin is to reduce the tensile strength of the material (Smith, 1961; Barsby, 1981). Fracture mechanics conceives of two important parameters that constitute ‘strength’. One is the energy per unit area created on fracture; the other is the size of natural flaws in the structure of the material. Clearly the larger the flaw size, the weaker the material. Berry (1965) has shown that water reduces the intrinsic flaw size. Work by Shen et al. (1983) shows that water absorbed from either the vapour or liquid phase reduces the fatigue life of poly(methy1 methacrylate) by a factor of four. Hence the effect of water absorption is to shorten the useful life of the material. For conventional dental resins based on poly(methy1 methacrylate) the equilibrium value for water absorption is about 2 per cent by weight at 37°C and is linear with temperature up to 72°C (Braden, 1964). Water solubility data (Bandrup and Immergut, 1975) indicates that the higher methacrylates should have lowerwater absorption. Other methacrylates are used in dentistry. Some used for fabricating temporary crowns and materials @ 1992 Butterworth-Heinemann 0300-57 12/92/040240-05
Ltd
bridges comprise poly(ethy1 methacrylate) polymer powder with either n-butyl or isobutyl methacrylate monomer (Schwarz and Braden, 1973; Bradenet al., 1976). The advantage of these systems over those based on conventional poly(methy1 methacrylate) is very much better soft tissue and pulpal tolerance. This is in general accord with the work of Mir et al. (1973), who have made a very detailed study of the pharmacological properties of methacrylate systems, showing in general that toxic tissue effects decrease with increasing length of the alkyl sidechains. Unfortunately, the glass transition temperature also decreases with the length of the alkyl side-chain and the lower transition temperatures of the poly(ethy1 methacrylate)/butyl methacrylate systems referred to above have been reported by Clarke and Braden (1982). Poly(isobuty1 methacrylate) seemed a priori of some interest, because its reported dilatometric glass transition temperature is higher than both ethyl and n-butyl polymers, it is less irritant than methyl methacrylate and there are therefore reasons for expecting lower water absorption. It has not previously been studied as a polymer powder/monomer system as used in dentistry and other biomedical applications. This study examines the water absorption characteristics and some mechanical and viscoelastic properties.
Barsby: Low water absorption
resin
241
Tab/e 1. Poly(isobutyl methacrylate): effect of ethylene glycol dimethacrylate (EGDM) and methacrylic acid (MAA) on mechanical properties (standard deviations in parentheses; n = 5)
Composition
Tensile strength (MPa m-‘)
of monomer
100% lsobutyl methacrylate (IBM) IBM with 3% EGDM IBM with 6% EGDM IBM with 9% EGDM IBM with 12% EGDM IBM with 1% MAA IBM with 2.5% MAA IBM with 5% MAA IBM with 10% MAA
38.91 29.02 32.34 26.06 22.48 33.43 37.55 61.24(f 38.86
Conventional poly(methyl methacrylate) denture base material
73.80
MATERIALS
AND
(k (i (f (+ (+ (k (+ (i
2.77) 2.67) 2.92) 4.16) 1.94) 3.76) 1.83) 7.19) 3.16)
METHODS
Poly(isobuty1 methacrylate) polymer (Cole Polymers Ltd. Croydon, UK) was used in combination with isobutyl methacrylate monomer of 98 per cent purity (0876K KochLight Laboratories Ltd. Colnbrook. UK). Manipulative properties were investigated by using a the most range of powder : liquid ratios to determine suitable proportions to produce a workable ‘dough’. It was found possible to use a higher powder : liquid ratio than that commonly employed with conventional poly(methy1 methacrylate)-based materials. A powder : liquid ratio of 3 g : 1 ml was chosen and specimens were processed in the form of rods and flat sheets using a dry-heat curing cycle of 7 h at 70°C followed by 3 h at 100”. Processed specimens were subsequently machined to forms suitable for: 1. Bending tests to determine Young’s modulus and flexural strength. Samples were cut from a flat parallel-sided plate of the material approximately 1-2 mm in thickness. The edges
240
A denture base resin with absorption
20: 240-244
low water
M. J. Barsby Department
of Prosthetic
Dentistry,
The London
Hospital
Medical
College, UK
ABSTRACT The aim ofthis study was to investigate poly(isobuty1 methacrylate) as a potential denture base material. It was found to have very low water absorption but poor mechanical properties and a low glass transition temperature. The latter features render the material itself unsuitable for use as a denture base material, but indicated that isobutyl methacrylate may be a useful co-monomer for reducing water absorption. KEY WORDS:
Denture
base materials,
J. Dent. 1992; 1992)
20: 240-244
(Received
Physical
properties
2 July 1991;
reviewed
18 September
Correspondence should be addressed to: Dr M. J. Barsby, Department Hospital Medical College, Turner Street, London El 2AD, UK.
1991;
of Prosthetic
accepted
Dentistry,
20 January
The London
INTRODUCTION The absorption of water by denture base resins is a phenomenon of considerable importance since it is accompanied by dimensional changes (Souder and Paffenbarger, 1942; Woelfel et al., 1961). A further undesirable effect of absorbed water in a denture base resin is to reduce the tensile strength of the material (Smith, 1961; Barsby, 1981). Fracture mechanics conceives of two important parameters that constitute ‘strength’. One is the energy per unit area created on fracture; the other is the size of natural flaws in the structure of the material. Clearly the larger the flaw size, the weaker the material. Berry (1965) has shown that water reduces the intrinsic flaw size. Work by Shen et al. (1983) shows that water absorbed from either the vapour or liquid phase reduces the fatigue life of poly(methy1 methacrylate) by a factor of four. Hence the effect of water absorption is to shorten the useful life of the material. For conventional dental resins based on poly(methy1 methacrylate) the equilibrium value for water absorption is about 2 per cent by weight at 37°C and is linear with temperature up to 72°C (Braden, 1964). Water solubility data (Bandrup and Immergut, 1975) indicates that the higher methacrylates should have lowerwater absorption. Other methacrylates are used in dentistry. Some used for fabricating temporary crowns and materials @ 1992 Butterworth-Heinemann 0300-57 12/92/040240-05
Ltd
bridges comprise poly(ethy1 methacrylate) polymer powder with either n-butyl or isobutyl methacrylate monomer (Schwarz and Braden, 1973; Bradenet al., 1976). The advantage of these systems over those based on conventional poly(methy1 methacrylate) is very much better soft tissue and pulpal tolerance. This is in general accord with the work of Mir et al. (1973), who have made a very detailed study of the pharmacological properties of methacrylate systems, showing in general that toxic tissue effects decrease with increasing length of the alkyl sidechains. Unfortunately, the glass transition temperature also decreases with the length of the alkyl side-chain and the lower transition temperatures of the poly(ethy1 methacrylate)/butyl methacrylate systems referred to above have been reported by Clarke and Braden (1982). Poly(isobuty1 methacrylate) seemed a priori of some interest, because its reported dilatometric glass transition temperature is higher than both ethyl and n-butyl polymers, it is less irritant than methyl methacrylate and there are therefore reasons for expecting lower water absorption. It has not previously been studied as a polymer powder/monomer system as used in dentistry and other biomedical applications. This study examines the water absorption characteristics and some mechanical and viscoelastic properties.
Barsby: Low water absorption
resin
241
Tab/e 1. Poly(isobutyl methacrylate): effect of ethylene glycol dimethacrylate (EGDM) and methacrylic acid (MAA) on mechanical properties (standard deviations in parentheses; n = 5)
Composition
Tensile strength (MPa m-‘)
of monomer
100% lsobutyl methacrylate (IBM) IBM with 3% EGDM IBM with 6% EGDM IBM with 9% EGDM IBM with 12% EGDM IBM with 1% MAA IBM with 2.5% MAA IBM with 5% MAA IBM with 10% MAA
38.91 29.02 32.34 26.06 22.48 33.43 37.55 61.24(f 38.86
Conventional poly(methyl methacrylate) denture base material
73.80
MATERIALS
AND
(k (i (f (+ (+ (k (+ (i
2.77) 2.67) 2.92) 4.16) 1.94) 3.76) 1.83) 7.19) 3.16)
METHODS
Poly(isobuty1 methacrylate) polymer (Cole Polymers Ltd. Croydon, UK) was used in combination with isobutyl methacrylate monomer of 98 per cent purity (0876K KochLight Laboratories Ltd. Colnbrook. UK). Manipulative properties were investigated by using a the most range of powder : liquid ratios to determine suitable proportions to produce a workable ‘dough’. It was found possible to use a higher powder : liquid ratio than that commonly employed with conventional poly(methy1 methacrylate)-based materials. A powder : liquid ratio of 3 g : 1 ml was chosen and specimens were processed in the form of rods and flat sheets using a dry-heat curing cycle of 7 h at 70°C followed by 3 h at 100”. Processed specimens were subsequently machined to forms suitable for: 1. Bending tests to determine Young’s modulus and flexural strength. Samples were cut from a flat parallel-sided plate of the material approximately 1-2 mm in thickness. The edges
