Journal of Dentistry, 6, No. 4, 1978, pp. 299-304.
Printed
in Great Britain
The effect of a cross-linking agent on the abrasion resistance and impact strength of an acrylic resin denture base material Alan
Harrison,
BDS, PhD, FDS RCS”
Robin Huggett, CGIA, LBIST, ARSH Robert C. Jagger, BDS, MScD Department of Restorative Dentistry, Dental School, Cardiff
ABSTRACT Many acrylic resin teeth and denture base materials contain cross-linking agents in varying percentages, and claims of superior physical properties as a result of these agents are common. In this investigation the abrasion resistance and impact strength of specimens of a denture base material containing a cross-linking agent (ethylene glycol dimethacrylate) in concentrations up to 100 per cent of monomer volume were determined to see if any relationship existed between abrasive wear, impact strength and the percentage of the cross-linking agent in the monomer. The effect of the cross-linking agent on the abrasion resistance of a denture base material was found to be of no significance, and it was shown that there was no correlation between impact strength and concentration of the cross-linking agent. These results are in general agreement with those of other workers.
INTRODUCTION Low abrasion resistance may make dentures Abrasion action
such
of a denture
difficult
resistance as rubbing,
base material
to clean and unaesthetic
may be defined scraping
as the ability
or erosion
which
is undesirable
since loss of surface
detail
and lead to loss of fit (Fig. I). of a material tends
to withstand
progressively
mechanical
to remove
material
(ASTM Spec., D. 1242-521‘). The measurement of abrasion resistance is very complex and may be affected by many factors, such as the mechanical properties of the material (Steijn, 1967; Lancaster, 1968). Ratner (1967) stated that, despite the differences in the properties of plastics, their wear by abrasive paper is abrasive in nature, i.e. it is connected with micro-cutting. It should, therefore, depend on the work to failure which may be determined by measuring the impact strength. He showed a relationship between the abrasion resistance of plastics during wear by abrasive paper and the impact strength of a notched specimen. Lancaster (1969) reported that, in view of the high localized strain rates involved during the abrasive wear process, the relevant property is most likely to be the notched impact strength. It has been shown previously (Atsuta et al., 1969, 1971) that the addition of the crosslinking agent 2, 2_di (4methacryloxyphenyl) propane increases the abrasion resistance of polymethyl methacrylate. However, the curing procedures were not those used for denture base production. In the present investigation the abrasion resistance and impact strength of specimens of a denture base material which contained a cross-linking agent in concentrations from
its surface
*Present address: Department of Restorative Dentistry, School of Dentistry, Leeds.
Journal of Dentistry,
300
Fig. 1. Loss of surface detail of denture to wear.
Vol. ~/NO. 4
bases due
of up to 100 per cent of monomer volume were determined to see if any relationship existed between abrasive wear, impact strength and the percentage of the cross-linking agent in the monomer.
MATERIALS
AND METHOD
Kallodent 333 clear polymethyl methacrylate denture base powder and methyl methacrylate monomer liquid were the constituents of the denture base. The cross-linking agent used was ethylene glycol dimethacrylate which was added to the monomer in concentrations ranging from 0 to 100 per cent of monomer volume. The powder to liquid ratio was 35 : 1 by volume. Moulds for the abrasion resistance specimens were prepared by drilling holes 45 mm in diameter into gypsum. Moulds for the impact specimens were prepared by investing pattern blocks of Perspex 50 x 50 x 7 mm into gypsum. Both preparation procedures followed the conventional dental flasking techniques. After mixing and packing, the polymer was cured by heating for 14 hours at 70 ‘C in a thermostatically controlled waterbath. The flasks were then bench cooled. After deflasking, the specimens were stored in water at room temperature for 30 days before testing. Abrasive wear tests were made using a previously described machine (Harrison and Lewis, 1975), which is designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances and contact times encountered in the human masticatory cycle. Ten specimens of each cross-linking percentage were mounted on the pins of the abrasion machine. The combined length of pin and specimen was measured using a bench micrometer calibrated to O-2 pm, the mean value of 5 determinations being recorded. Each group of specimens was then abraded on the machine against 600-grit silicon carbide paper under water. After 1 hour the paper was changed and the machine run for another hour with an identical pressure on each specimen of 0.25 N/mm*. The mean length for each pin and specimen was again determined and the loss calculated. All the groups were tested under identical conditions, the mean wear for the group being found from the wear on each of the 10 pins within the group.
Harrison et al.: Properties
Table 1. Abrasion
of denture
loss at each percentage
Percentage crosslinking agent
0
301
of cross-linking
Mean abrasion loss Imml
Coefficient of variation 1%)
O-0962 o-1310 0.1224 0.1059 00915 0.0758
of the six readings: mean 1.0730,
o-
agent
Standard deviation
1.0420 1.1134 1 a0648 1.0713 1.1105 1 a0465
20 40 60 80 100 Summery
base material
20
range 0.0714,
40
00
9.2 11.8 11-6 9.9 8.2 7.3
s.d. 0.0317.
80
100
PERCENTAGECRCS9LUWltKiAGENT
Fig. 2. Abrasive wear loss of denture base material specimens containing O-100 per cent ethylene glycol dimethacrylate.
The impact specimens, measuring 45 x 6.35 x 6.35 mm, were cut blanks, 10 specimens of each cross-linking percentage being produced. using hand-operated apparatus,* and the impact resistance tests were Hounsfield Plastics Impact Testing Machine, following the recommended Tensometer Ltd in their publication No. 124/59.
from the processed Notches were cut conducted using a method detailed by
RESULTS Abrasion resistance The mean abrasion loss, standard deviation and coefficient of variation of the 10 specimens at each percentage of cross-linking agent are shown in Table I. A summary of the means obtained for the different percentages of cross-linking agent is also presented. An analysis of variance was performed on all the results and it was found that there was no significant difference between the groups of each cross-linking percentage (P > O-05). The results showing the plot of wear against cross-linking percentage are shown in Fig. 2. *Tensometer
Limited, Croydon, Surrey.
Journal of Dentistry,
302
Tab/e II.
Impact
resistance: energy absorbed at each percentage
Percentage crosslinking agent 0 20 40 60 80 100 Summary
of cross-linking
Vol. ~/NO. 4
agent
Mean impact energy (N. M.J
Standard deviation
Coefficient of variation (%I
0.04665 0.04560 0.04795 0.04536 0.04198 O-04282
0.0074 o-0051 0.0049 0.0063 0.0055 0.0065
15.9 11.1 10.2 13.9 13.2 15.2
of the six readings: mean 0.0450,
range 0.00597,
s.d. 0.0023.
MEANtSD= I
I 60 -
. OF
20
40
PERCENTAGE
60
60
100
CROSSLINKINGAGENT
Fig. 3. Impact energy absorbed of denture material specimens containing O-100 per ethylene glycol dimethacrylate.
base cent
Impact resistance In Table II the mean impact energy absorbed, standard deviation and coefficient of variation of the 10 specimens at each percentage of cross-linking agent are shown. A summary of the means is also presented. The results showing the plot of impact energy against cross-linking percentage are shown in Fig. 3. An analysis of variance of the results showed that there was no significant difference (P > 0.05) between the groups, except for those containing 40 and 80 per cent cross-linking agent which were significantly different from each other (P< O-05).
DISCUSSION Cross-linking of high polymers such as polymethyl methacrylate might be expected to increase such properties as modulus of elasticity, surface hardness, resistance to moisture and resistance to solvent swelling (Mark, 1942). Manufacturers commonly include cross-linking agents in varying percentages in many acrylic resin teeth and denture base materials in order to modify the properties. The present study has shown that the addition of a cross-linking agent in a
Harrison et al.: Properties
of denture
base material
303
wide range of concentrations has no significant effect on the properties of abrasion resistance and impact strength of a denture base material. However, the cross-linking reaction is complex, and because polymerization temperatures are below the Tg of polymethyl methacrylate, the geometrical limitations of the polymer network prevent 100 per cent efficiency of cross-linking (Loeshak and Fox, 1953). The unreacted cross-linking agent in the form of a residual monomer or pendant chains will then act as a plasticizer. Polymethyl methacrylate denture base materials consist of two phases. Granules of the original polymer powder are embedded in an interstitial matrix of newly formed polymer. The addition of a cross-linking agent produces cross-linking in the relatively small interstitial phase only. Causton (1972), investigating the viscoelastic properties of polymethyl methacrylate denture base materials, found that addition of a cross-linking agent decrea&d the fracture surface energy but increased the inherent flaw size. The net mechanical effect was decreased tensile strength. These results were in agreement with previous work on single phase, bulk-polymerized polymethyl methacrylate (Berry, 1963). Although the addition of a cross-linking agent to a polymethyl methacrylate denture base material produces resistance to solvent swelling proportional to the amount of crosslinking agent that is added (Jagger, 1975), the cross-linking has little effect on any mechanical properties previously investigated besides tensile strength and resistance to repeated blows (Wollff, 1962; Jagger, 1975; Jagger and Huggett, 1975). It has been shown that up to 20 per cent of monomer concentration of cross-linking agent has little effect on tensile strength, but that above this concentration, the tensile strength is reduced (Wollff, 1962; Causton, 1972; Jagger, 1975). Of particular relevance to the present results is the finding of Cornell et al. (1960) that cross-linking of a denture base material with ethylene glycol up to a concentration of 20 per cent of monomer volume increased the resistance to fracture by repeated blows. The relationship of their ‘falling-ball’ test to impact strength is uncertain, since failure of the material is progressive over a series of blows. Similarly, the clinical significance of their results is not known. The test, however, illustrated a change in mechanical properties related to the addition of the cross-linking agent. The present study has shown that the properties of both abrasion resistance and impact strength are uninfluenced by the cross-linking agent ethylene glycol dimethacrylate in a denture base material produced by a conventional dental processing technique. REFERENCES
Atsuta M., Mirasawa T. and Masuhara E. (1969) Hard methacrylic polymers. (1) On the physical properties of the highly cross-linked poly (methyl methacrylate). J. Jap. Sot. Dent. Appar. Mater. 10, 52-56.
Atsuta M., Nakabayashi N. and Masuhara E. (1971) Hard methacrylic Copolymers of methyl methacrylate and 2, 2-di (4-methacryloxyphenyl)
polymers. (II) propane. J.
Biomed. Mater. Res. 5, 183-195.
Berry J. P. (1963) Fracture processes in polymeric materials. (IV) Dependence of the fracture surface energy on temperature and molecular structure. J. Polymer Sci. 1, 9931003.
Causton B. E. (1972) The physical and chemical properties of some polymeric dental materials. PhD Thesis, University of London. Cornell J. A., Tucker J. L. and Powers C. M. (1960) Physical properties of denture-base materials. J. Prosthef. Dent. 10, 5 16-524.
Journal of Dentistry, Vol. ~/NO. 4
304
Harrison A. and Lewis T. T. (1975) The development of an abrasion testing machine for dental materials. J. Biomed. Mater. Rex 9, 341-353. Jagger R. G. (1975) The effect of the curing cycle on polymethyl methacrylate denture base - resins. MScD Thesis, University of Wales. Jagger R. G. and Huggett R. (1975) The effect of cross-linking on indentation resistance, creep and recovery of an acrylic resin denture base material. J. Dent. 3, 1 S- 18. Lancaster J. K. (1968) The wear of polymers at elevated temperatures and its dependence on mechanical properties. Royal Aircraft Establishment. Technical report 68045. Lancaster J. K. (1969) Abrasive wear of polymers. Wear 14,223-239. Loeshak S. and Fox T. G. (1953) Cross-linked polymers. I. Factors influencing the efficiency of cross-linking in copolymers of methyl methacrylate and glycol dimethacrylates. J. Am. Chem. Sot. 75,3544-3550.
Mark H. (1942) Intermolecular Eng. Chem.
Ratner S. B. (1967) Comparison Abrasion of Rubber. London, Steijn R. P. (1967) Friction and Wollff E. M. (1962) The effect 439-444.
forces and mechanical
behaviour
of high polymers.
Indust.
34, 1343-1348.
of the abrasion of rubbers and plastics. In: James D. I. (ed.) Maclaren. wear of plastics. Metals Eng. Q. 1,9-2 1. of cross-linking agents on acrylic resins. Aust. Dent. J. 7,
Printed
in Great Britain
The effect of a cross-linking agent on the abrasion resistance and impact strength of an acrylic resin denture base material Alan
Harrison,
BDS, PhD, FDS RCS”
Robin Huggett, CGIA, LBIST, ARSH Robert C. Jagger, BDS, MScD Department of Restorative Dentistry, Dental School, Cardiff
ABSTRACT Many acrylic resin teeth and denture base materials contain cross-linking agents in varying percentages, and claims of superior physical properties as a result of these agents are common. In this investigation the abrasion resistance and impact strength of specimens of a denture base material containing a cross-linking agent (ethylene glycol dimethacrylate) in concentrations up to 100 per cent of monomer volume were determined to see if any relationship existed between abrasive wear, impact strength and the percentage of the cross-linking agent in the monomer. The effect of the cross-linking agent on the abrasion resistance of a denture base material was found to be of no significance, and it was shown that there was no correlation between impact strength and concentration of the cross-linking agent. These results are in general agreement with those of other workers.
INTRODUCTION Low abrasion resistance may make dentures Abrasion action
such
of a denture
difficult
resistance as rubbing,
base material
to clean and unaesthetic
may be defined scraping
as the ability
or erosion
which
is undesirable
since loss of surface
detail
and lead to loss of fit (Fig. I). of a material tends
to withstand
progressively
mechanical
to remove
material
(ASTM Spec., D. 1242-521‘). The measurement of abrasion resistance is very complex and may be affected by many factors, such as the mechanical properties of the material (Steijn, 1967; Lancaster, 1968). Ratner (1967) stated that, despite the differences in the properties of plastics, their wear by abrasive paper is abrasive in nature, i.e. it is connected with micro-cutting. It should, therefore, depend on the work to failure which may be determined by measuring the impact strength. He showed a relationship between the abrasion resistance of plastics during wear by abrasive paper and the impact strength of a notched specimen. Lancaster (1969) reported that, in view of the high localized strain rates involved during the abrasive wear process, the relevant property is most likely to be the notched impact strength. It has been shown previously (Atsuta et al., 1969, 1971) that the addition of the crosslinking agent 2, 2_di (4methacryloxyphenyl) propane increases the abrasion resistance of polymethyl methacrylate. However, the curing procedures were not those used for denture base production. In the present investigation the abrasion resistance and impact strength of specimens of a denture base material which contained a cross-linking agent in concentrations from
its surface
*Present address: Department of Restorative Dentistry, School of Dentistry, Leeds.
Journal of Dentistry,
300
Fig. 1. Loss of surface detail of denture to wear.
Vol. ~/NO. 4
bases due
of up to 100 per cent of monomer volume were determined to see if any relationship existed between abrasive wear, impact strength and the percentage of the cross-linking agent in the monomer.
MATERIALS
AND METHOD
Kallodent 333 clear polymethyl methacrylate denture base powder and methyl methacrylate monomer liquid were the constituents of the denture base. The cross-linking agent used was ethylene glycol dimethacrylate which was added to the monomer in concentrations ranging from 0 to 100 per cent of monomer volume. The powder to liquid ratio was 35 : 1 by volume. Moulds for the abrasion resistance specimens were prepared by drilling holes 45 mm in diameter into gypsum. Moulds for the impact specimens were prepared by investing pattern blocks of Perspex 50 x 50 x 7 mm into gypsum. Both preparation procedures followed the conventional dental flasking techniques. After mixing and packing, the polymer was cured by heating for 14 hours at 70 ‘C in a thermostatically controlled waterbath. The flasks were then bench cooled. After deflasking, the specimens were stored in water at room temperature for 30 days before testing. Abrasive wear tests were made using a previously described machine (Harrison and Lewis, 1975), which is designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances and contact times encountered in the human masticatory cycle. Ten specimens of each cross-linking percentage were mounted on the pins of the abrasion machine. The combined length of pin and specimen was measured using a bench micrometer calibrated to O-2 pm, the mean value of 5 determinations being recorded. Each group of specimens was then abraded on the machine against 600-grit silicon carbide paper under water. After 1 hour the paper was changed and the machine run for another hour with an identical pressure on each specimen of 0.25 N/mm*. The mean length for each pin and specimen was again determined and the loss calculated. All the groups were tested under identical conditions, the mean wear for the group being found from the wear on each of the 10 pins within the group.
Harrison et al.: Properties
Table 1. Abrasion
of denture
loss at each percentage
Percentage crosslinking agent
0
301
of cross-linking
Mean abrasion loss Imml
Coefficient of variation 1%)
O-0962 o-1310 0.1224 0.1059 00915 0.0758
of the six readings: mean 1.0730,
o-
agent
Standard deviation
1.0420 1.1134 1 a0648 1.0713 1.1105 1 a0465
20 40 60 80 100 Summery
base material
20
range 0.0714,
40
00
9.2 11.8 11-6 9.9 8.2 7.3
s.d. 0.0317.
80
100
PERCENTAGECRCS9LUWltKiAGENT
Fig. 2. Abrasive wear loss of denture base material specimens containing O-100 per cent ethylene glycol dimethacrylate.
The impact specimens, measuring 45 x 6.35 x 6.35 mm, were cut blanks, 10 specimens of each cross-linking percentage being produced. using hand-operated apparatus,* and the impact resistance tests were Hounsfield Plastics Impact Testing Machine, following the recommended Tensometer Ltd in their publication No. 124/59.
from the processed Notches were cut conducted using a method detailed by
RESULTS Abrasion resistance The mean abrasion loss, standard deviation and coefficient of variation of the 10 specimens at each percentage of cross-linking agent are shown in Table I. A summary of the means obtained for the different percentages of cross-linking agent is also presented. An analysis of variance was performed on all the results and it was found that there was no significant difference between the groups of each cross-linking percentage (P > O-05). The results showing the plot of wear against cross-linking percentage are shown in Fig. 2. *Tensometer
Limited, Croydon, Surrey.
Journal of Dentistry,
302
Tab/e II.
Impact
resistance: energy absorbed at each percentage
Percentage crosslinking agent 0 20 40 60 80 100 Summary
of cross-linking
Vol. ~/NO. 4
agent
Mean impact energy (N. M.J
Standard deviation
Coefficient of variation (%I
0.04665 0.04560 0.04795 0.04536 0.04198 O-04282
0.0074 o-0051 0.0049 0.0063 0.0055 0.0065
15.9 11.1 10.2 13.9 13.2 15.2
of the six readings: mean 0.0450,
range 0.00597,
s.d. 0.0023.
MEANtSD= I
I 60 -
. OF
20
40
PERCENTAGE
60
60
100
CROSSLINKINGAGENT
Fig. 3. Impact energy absorbed of denture material specimens containing O-100 per ethylene glycol dimethacrylate.
base cent
Impact resistance In Table II the mean impact energy absorbed, standard deviation and coefficient of variation of the 10 specimens at each percentage of cross-linking agent are shown. A summary of the means is also presented. The results showing the plot of impact energy against cross-linking percentage are shown in Fig. 3. An analysis of variance of the results showed that there was no significant difference (P > 0.05) between the groups, except for those containing 40 and 80 per cent cross-linking agent which were significantly different from each other (P< O-05).
DISCUSSION Cross-linking of high polymers such as polymethyl methacrylate might be expected to increase such properties as modulus of elasticity, surface hardness, resistance to moisture and resistance to solvent swelling (Mark, 1942). Manufacturers commonly include cross-linking agents in varying percentages in many acrylic resin teeth and denture base materials in order to modify the properties. The present study has shown that the addition of a cross-linking agent in a
Harrison et al.: Properties
of denture
base material
303
wide range of concentrations has no significant effect on the properties of abrasion resistance and impact strength of a denture base material. However, the cross-linking reaction is complex, and because polymerization temperatures are below the Tg of polymethyl methacrylate, the geometrical limitations of the polymer network prevent 100 per cent efficiency of cross-linking (Loeshak and Fox, 1953). The unreacted cross-linking agent in the form of a residual monomer or pendant chains will then act as a plasticizer. Polymethyl methacrylate denture base materials consist of two phases. Granules of the original polymer powder are embedded in an interstitial matrix of newly formed polymer. The addition of a cross-linking agent produces cross-linking in the relatively small interstitial phase only. Causton (1972), investigating the viscoelastic properties of polymethyl methacrylate denture base materials, found that addition of a cross-linking agent decrea&d the fracture surface energy but increased the inherent flaw size. The net mechanical effect was decreased tensile strength. These results were in agreement with previous work on single phase, bulk-polymerized polymethyl methacrylate (Berry, 1963). Although the addition of a cross-linking agent to a polymethyl methacrylate denture base material produces resistance to solvent swelling proportional to the amount of crosslinking agent that is added (Jagger, 1975), the cross-linking has little effect on any mechanical properties previously investigated besides tensile strength and resistance to repeated blows (Wollff, 1962; Jagger, 1975; Jagger and Huggett, 1975). It has been shown that up to 20 per cent of monomer concentration of cross-linking agent has little effect on tensile strength, but that above this concentration, the tensile strength is reduced (Wollff, 1962; Causton, 1972; Jagger, 1975). Of particular relevance to the present results is the finding of Cornell et al. (1960) that cross-linking of a denture base material with ethylene glycol up to a concentration of 20 per cent of monomer volume increased the resistance to fracture by repeated blows. The relationship of their ‘falling-ball’ test to impact strength is uncertain, since failure of the material is progressive over a series of blows. Similarly, the clinical significance of their results is not known. The test, however, illustrated a change in mechanical properties related to the addition of the cross-linking agent. The present study has shown that the properties of both abrasion resistance and impact strength are uninfluenced by the cross-linking agent ethylene glycol dimethacrylate in a denture base material produced by a conventional dental processing technique. REFERENCES
Atsuta M., Mirasawa T. and Masuhara E. (1969) Hard methacrylic polymers. (1) On the physical properties of the highly cross-linked poly (methyl methacrylate). J. Jap. Sot. Dent. Appar. Mater. 10, 52-56.
Atsuta M., Nakabayashi N. and Masuhara E. (1971) Hard methacrylic Copolymers of methyl methacrylate and 2, 2-di (4-methacryloxyphenyl)
polymers. (II) propane. J.
Biomed. Mater. Res. 5, 183-195.
Berry J. P. (1963) Fracture processes in polymeric materials. (IV) Dependence of the fracture surface energy on temperature and molecular structure. J. Polymer Sci. 1, 9931003.
Causton B. E. (1972) The physical and chemical properties of some polymeric dental materials. PhD Thesis, University of London. Cornell J. A., Tucker J. L. and Powers C. M. (1960) Physical properties of denture-base materials. J. Prosthef. Dent. 10, 5 16-524.
Journal of Dentistry, Vol. ~/NO. 4
304
Harrison A. and Lewis T. T. (1975) The development of an abrasion testing machine for dental materials. J. Biomed. Mater. Rex 9, 341-353. Jagger R. G. (1975) The effect of the curing cycle on polymethyl methacrylate denture base - resins. MScD Thesis, University of Wales. Jagger R. G. and Huggett R. (1975) The effect of cross-linking on indentation resistance, creep and recovery of an acrylic resin denture base material. J. Dent. 3, 1 S- 18. Lancaster J. K. (1968) The wear of polymers at elevated temperatures and its dependence on mechanical properties. Royal Aircraft Establishment. Technical report 68045. Lancaster J. K. (1969) Abrasive wear of polymers. Wear 14,223-239. Loeshak S. and Fox T. G. (1953) Cross-linked polymers. I. Factors influencing the efficiency of cross-linking in copolymers of methyl methacrylate and glycol dimethacrylates. J. Am. Chem. Sot. 75,3544-3550.
Mark H. (1942) Intermolecular Eng. Chem.
Ratner S. B. (1967) Comparison Abrasion of Rubber. London, Steijn R. P. (1967) Friction and Wollff E. M. (1962) The effect 439-444.
forces and mechanical
behaviour
of high polymers.
Indust.
34, 1343-1348.
of the abrasion of rubbers and plastics. In: James D. I. (ed.) Maclaren. wear of plastics. Metals Eng. Q. 1,9-2 1. of cross-linking agents on acrylic resins. Aust. Dent. J. 7,
