The effect of cross-linking on sorption properties of a denture-base material R.G. Jagger1. R. Huggett2
1*Department of Prosthetic Dentistry University of Wales College of Medicine The Dental School Heath Park Cardiff CF4 4XY South Wales, UK 2Department of Prosthetic Dentistry University of Bristol Dental School Lower Maudlin Street Bristol BS2 1LY, UK Received November 21, 1989 Accepted May 1, 1990 Dent Mater 6:276-278, October, 1990
Abstract-Cross-linking of the polymer of the interstitial matrix of a heat-cured resin denture base is said to provide craze resistancefor the denture base. The mechanism of crazing can be related to both water sorption and solvent attack. The present study shows that the addition of ethylene glycol dimethacrylate in concentrations of 0-100% of monomer volume has little effect on water sorption properties but is an efficient method of providing solvent resistance.
ured poly(methyl methacrylate) (PMMA) d e n t u r e - b a s e resin consists of the remains of polymer particles of the original powder embedded in an interstitial matrix of newly formed polymer. Ethylene glycol dimethacrylate (EGDM) is a cross-linking agent commonly added to acrylic denturebase monomer liquid in order to produce a cross-linked gel network in the resultant interstitial matrix (Hill, 1981). Although studies have shown that cross-linking of denture-base materials with EGDM has little effect on mechanical properties of the denture base (Wolff, 1962; Jagger and Huggett, 1975; Harrison et al., 1978), cross-linking is said to provide resistance to crazing (Phillips, 1982). The development of crazing in acrylic denture-base resins has been related to both water sorption and solvent action (Bevan and Earnshaw, 1969). It was, therefore, considered of interest for the relationship between the concentration of the cross-linking agent EGDM and the water sorption and solvent resistance of a heat-cured PMMA denture-base resin to be examined.
C
MATERIALS AND METHODS
Test specimens were produced in moulds made by investment of pattern blocks of Perspex (38 × 38 × 1.6 mm) in gypsum in the conventional dental-flasking technique. Kallodent 333 clear poly(methylmethacrylate) 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% by volume. Powder-toliquid ratio was 31/2:1 by volume, and the polymer was cured by being heated for seven h at 70°C followed by one h at 100°C in a thermostati-
cally controlled w a t e r bath. The flasks were then bench-cooled. After being processed, the specimens were hand-finished by progressively finer g r a d e s of silicon c a r b i d e p a p e r (grades 280-600). The size of the finished specimen was 38 mm × 38 mm × 1.3 mm _+ 0.03 mm. Water 8orptian. - T h e specimens were dried at 37°C in a desiccator containing phosphorus pentoxide until a constant weight was obtained. They were then immersed in distilled water at 37°C and weighed every 15 min for the first hour, every hour for four h, and then every day until equilibrium was attained. The specimens were weighed one min after removal from water, having been dried until free from visible surface moisture. The diffusion coefficient is an index of rate of water uptake, and this was calculated as suggested by Braden (1964). Toluene $orptian. - T h e specimens were weighed and then immersed in toluene in separate glass containers which had airtight metallic tops. They were re-weighed once every week for four weeks. The readings were taken 30 s after removal from the toluene, the specimens having been shaken dry for 10 s. RESULTS
Diffusion coefficients (D) and the equilibrium water uptake expressed as percentages of original specimen dry weight are shown in Table 1. The mean, range, standard deviation, and coefficient of variation of water uptake and D are also given. The addition of cross-linking agent in concentrations of 0-100% of monomer volume has no effect on diffusion coefficient. Percentage water uptake increased to 60% of crosslinking agent concentration and then decreased, but the range was extremely small.
276 JAGGER & HUGGETT/SORPTION PROPERTIES OF A CROSS-LINKED DENTURE BASE
Table 2 shows the percentage uptake of toluene (by weight) of two separate groups of specimens that contained from 0 to 100% cross-linking agent concentration and that were cured on separate days. The crosslinked denture base absorbs less toluene than the non-cross-linked material, the amount of uptake being inversely proportional to the amount of cross-linking agent. Toluene sorption by the non-cross-linked material is high. Initially, sorption reduces rapidly, to a level of around 1 wt% at 60% cross-link concentration. Above this concentration, sorption values are independent of time and are below 1 wt%. The range of sorption values obtained indicates that the cross-linking network increases to approximately 50% of EGDM. DISCUSSION
Cross-linking of a heat-cured denture base has little effect on mechanical properties below the glass transition point. This s t u d y has shown that the addition of EGDM has little effect on water sorption [which agrees with the finding of Rose et al. (1955)], and that the rate of sorption/desorption is similarly unaffected. It has been shown here that crosslinking may be an efficient means of providing solvent resistance for a denture-base resin. The cross-linking a g e n t in the monomer liquid causes an insoluble cross-linked gel network to be formed during polymerization. Geometrical restrictions of polymer chains and gel formation prevent the cross-linking reaction from going to completion, and the interstitial matrix of the cured polymer contains unreacted methyl methacrylate and E G D M m o n o m e r s as well as p e n d e n t methacrylate groups of partially unreacted EGDM (Loshaek and Fox, 1953). F a c t o r s affecting the efficiency of the cross-linking reaction of the denture base are not well-investigated, but the present results indicate that, under the present conditions, the cross-linking network increased to approximately 50% of monomer volume of EGDM. Cross-linking of a denture-base resin is said to provide craze resistance (Phillips, 1982). A craze is a
TABLE 1 PERCENTAGEWATER UPTAKEAND DIFFUSIONCOEFFICIENTS Percentage Cross-linking 0 10 20 30 40 50 60 70 80 90 100 Analysis of the eleven readings
Mean Range S.D C of V
Percentage Water Uptake 2.56 2.66 2.74 2.72 2.75 2.77 2.79 2.74 2.71 2.74 2.64 2.71 0.23 0.06 2.33
Mean Range S.D. C of V
Diffusion Coefficient (10-Scm2sec-~) 1.46 1.44 1.65 1.45 1.65 1.67 1.44 1.48 1.72 1.65 1.24 1.53 0.48 0.15 0.59
TABLE 2 PERCENTAGEUPTAKE OF TOLUENE BY WEIGHT Percentage Cross-link Specimens Group 1 Agent 14 Days 21 Days 28 Days 0 243.2 257.1 288.4 10 19.2 26.1 36.2 20 6.1 7.1 10.4 40 0.1 0.1 0.9 60 1.3 1.3 1.3 80 0.4 0.2 0.3 100 0.2 0.1 0.0 *Specimens disintegrated when handled.
layer of polymer less than a few micrometers thick which has underg o n e p l a s t i c d e f o r m a t i o n in a direction approximately normal to the craze plane as a response to tension applied in this direction (Kambour, 1986). The plastic def o r m a t i o n p r o d u c e s an o p t i c a l change and can result in surface discoloration. Crazing is thus a result of applied stress. B e v a n and E a r n s h a w (1968) showed that both water-saturated and completely dry specimens do not readily craze when exposed to a solvent. They suggested that partial chTing of specin~ens results in a water g r a d i e n t which c a u s e s i n t e r n a l stresses in the material. Absorption of solvent into the partially dried resin on a molecular scale causes polymer separation in craze planes related to the internal stresses. Crosslinking of the interstitial matrix can provide sufficient solvent resistance to prevent solvent crazing. This study also supports the conclusion of Bevan and Earnshaw (1968)
Specimens Group II 1-4 Days 21 Days 28 Days 210.4 278.2 * 41.1 53.2 * 11.6 16.7 29.6 1.8 2.4 4.4 0.9 0.7 1.2 0.8 0.7 0.6 0.7 0.6 0.6
that resistance to solvent crazing associated with cross-linking is not due to alteration in water sorption properties. CONCLUSIONS The addition of the cross-linking agent in concentrations of 0-100% of monomer value has no effect on diffusion coefficient. Percentage water uptake increased to 60% of crosslinking agent and then decreased, but the range was extremely small. The range of toluene sorption values indicates that the cross-linking network increases to approximately 50% of EGDM concentration and that the degree of cross-linking to this limit is proportional to the amount of cross-linking agent that is added. REFERENCES BEVAN, E.M. and EARNSHAW,R. (1968): Part I - T h e Role of Water Sorption in the Solvent Crazing of Acrylic Resins, Aust Dent J 13: 265-273. BEvy, E.M. and EARNSHAW,R. (1969):
Dental Materials/October 1990 277
Part I I - T h e Role of Water Sorption in the Solvent Crazing of Acrylic Resins, Aust Den~ J 14: 190--196. BRADEN, M. (1964): The Absorption of Water by Acrylic Resins and Other Materials, J Prosthet Dent 14: 307-316. HARRISON, A.; HUGGETT, a.; and JAEGER, R.G. (1978): The Effect of a Cross-linking Agent on the Abrasion Resistance and Impact Strength of an Acrylic Resin Denture Base Material, J Dent 6: 299-304. HILL, R.G. (1981): The Cross-linking Agent Ethylene Glycol Dimethacrylate Content of the Currently Avail-
able Acrylic Denture Base Resins, J
and Glycol Dimethacrylate, J Polym
Den~ Res 60: 725-726.
Sci 75: 3544-3550.
JAGGER, R.G. and HUGGETT, R. (1975): The Effect of Cross-linking on Indentation Resistance Creep and Recovery of an Acrylic Denture Base Material, J Dent 3: 15-18. KAMBOUR,R.P. (1986): Crazing. In: Encyclopedia of Polymer Science and Engineering, 2nd ed., New York: John Wiley & Sons, Inc., pp. 299-323. LOSHAEK,S. and Fox, T.G. (1953): Crosslinking Polymers. I. Factors Influencing the Efficiency of Cross-linking in Copolymers of Methyl methacrylate
PHILLIPS, R.W. (1982): Skinner's Science of Dental Materials, 8th ed., Philadelphia: W.B. Saunders, p. 201. ROSE, E.E.; LAL, J.; GREEN, R.; and CORNELL,J. (1955): Direct Resin Filling Materials: Coefficient of Thermal Expansion and Water Sorption of Polymethylmethacrylate, J Dent Res 34: 589-596. WOLFF,E.M. (1962): The Effect of Crosslinking Agents on Acrylic Resins, Aust Dent J 7: 439-444.
278 JAGGER & HUGGETT/SORPTION PROPERTIES OF A CROSS-LINKED DENTURE BASE
1*Department of Prosthetic Dentistry University of Wales College of Medicine The Dental School Heath Park Cardiff CF4 4XY South Wales, UK 2Department of Prosthetic Dentistry University of Bristol Dental School Lower Maudlin Street Bristol BS2 1LY, UK Received November 21, 1989 Accepted May 1, 1990 Dent Mater 6:276-278, October, 1990
Abstract-Cross-linking of the polymer of the interstitial matrix of a heat-cured resin denture base is said to provide craze resistancefor the denture base. The mechanism of crazing can be related to both water sorption and solvent attack. The present study shows that the addition of ethylene glycol dimethacrylate in concentrations of 0-100% of monomer volume has little effect on water sorption properties but is an efficient method of providing solvent resistance.
ured poly(methyl methacrylate) (PMMA) d e n t u r e - b a s e resin consists of the remains of polymer particles of the original powder embedded in an interstitial matrix of newly formed polymer. Ethylene glycol dimethacrylate (EGDM) is a cross-linking agent commonly added to acrylic denturebase monomer liquid in order to produce a cross-linked gel network in the resultant interstitial matrix (Hill, 1981). Although studies have shown that cross-linking of denture-base materials with EGDM has little effect on mechanical properties of the denture base (Wolff, 1962; Jagger and Huggett, 1975; Harrison et al., 1978), cross-linking is said to provide resistance to crazing (Phillips, 1982). The development of crazing in acrylic denture-base resins has been related to both water sorption and solvent action (Bevan and Earnshaw, 1969). It was, therefore, considered of interest for the relationship between the concentration of the cross-linking agent EGDM and the water sorption and solvent resistance of a heat-cured PMMA denture-base resin to be examined.
C
MATERIALS AND METHODS
Test specimens were produced in moulds made by investment of pattern blocks of Perspex (38 × 38 × 1.6 mm) in gypsum in the conventional dental-flasking technique. Kallodent 333 clear poly(methylmethacrylate) 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% by volume. Powder-toliquid ratio was 31/2:1 by volume, and the polymer was cured by being heated for seven h at 70°C followed by one h at 100°C in a thermostati-
cally controlled w a t e r bath. The flasks were then bench-cooled. After being processed, the specimens were hand-finished by progressively finer g r a d e s of silicon c a r b i d e p a p e r (grades 280-600). The size of the finished specimen was 38 mm × 38 mm × 1.3 mm _+ 0.03 mm. Water 8orptian. - T h e specimens were dried at 37°C in a desiccator containing phosphorus pentoxide until a constant weight was obtained. They were then immersed in distilled water at 37°C and weighed every 15 min for the first hour, every hour for four h, and then every day until equilibrium was attained. The specimens were weighed one min after removal from water, having been dried until free from visible surface moisture. The diffusion coefficient is an index of rate of water uptake, and this was calculated as suggested by Braden (1964). Toluene $orptian. - T h e specimens were weighed and then immersed in toluene in separate glass containers which had airtight metallic tops. They were re-weighed once every week for four weeks. The readings were taken 30 s after removal from the toluene, the specimens having been shaken dry for 10 s. RESULTS
Diffusion coefficients (D) and the equilibrium water uptake expressed as percentages of original specimen dry weight are shown in Table 1. The mean, range, standard deviation, and coefficient of variation of water uptake and D are also given. The addition of cross-linking agent in concentrations of 0-100% of monomer volume has no effect on diffusion coefficient. Percentage water uptake increased to 60% of crosslinking agent concentration and then decreased, but the range was extremely small.
276 JAGGER & HUGGETT/SORPTION PROPERTIES OF A CROSS-LINKED DENTURE BASE
Table 2 shows the percentage uptake of toluene (by weight) of two separate groups of specimens that contained from 0 to 100% cross-linking agent concentration and that were cured on separate days. The crosslinked denture base absorbs less toluene than the non-cross-linked material, the amount of uptake being inversely proportional to the amount of cross-linking agent. Toluene sorption by the non-cross-linked material is high. Initially, sorption reduces rapidly, to a level of around 1 wt% at 60% cross-link concentration. Above this concentration, sorption values are independent of time and are below 1 wt%. The range of sorption values obtained indicates that the cross-linking network increases to approximately 50% of EGDM. DISCUSSION
Cross-linking of a heat-cured denture base has little effect on mechanical properties below the glass transition point. This s t u d y has shown that the addition of EGDM has little effect on water sorption [which agrees with the finding of Rose et al. (1955)], and that the rate of sorption/desorption is similarly unaffected. It has been shown here that crosslinking may be an efficient means of providing solvent resistance for a denture-base resin. The cross-linking a g e n t in the monomer liquid causes an insoluble cross-linked gel network to be formed during polymerization. Geometrical restrictions of polymer chains and gel formation prevent the cross-linking reaction from going to completion, and the interstitial matrix of the cured polymer contains unreacted methyl methacrylate and E G D M m o n o m e r s as well as p e n d e n t methacrylate groups of partially unreacted EGDM (Loshaek and Fox, 1953). F a c t o r s affecting the efficiency of the cross-linking reaction of the denture base are not well-investigated, but the present results indicate that, under the present conditions, the cross-linking network increased to approximately 50% of monomer volume of EGDM. Cross-linking of a denture-base resin is said to provide craze resistance (Phillips, 1982). A craze is a
TABLE 1 PERCENTAGEWATER UPTAKEAND DIFFUSIONCOEFFICIENTS Percentage Cross-linking 0 10 20 30 40 50 60 70 80 90 100 Analysis of the eleven readings
Mean Range S.D C of V
Percentage Water Uptake 2.56 2.66 2.74 2.72 2.75 2.77 2.79 2.74 2.71 2.74 2.64 2.71 0.23 0.06 2.33
Mean Range S.D. C of V
Diffusion Coefficient (10-Scm2sec-~) 1.46 1.44 1.65 1.45 1.65 1.67 1.44 1.48 1.72 1.65 1.24 1.53 0.48 0.15 0.59
TABLE 2 PERCENTAGEUPTAKE OF TOLUENE BY WEIGHT Percentage Cross-link Specimens Group 1 Agent 14 Days 21 Days 28 Days 0 243.2 257.1 288.4 10 19.2 26.1 36.2 20 6.1 7.1 10.4 40 0.1 0.1 0.9 60 1.3 1.3 1.3 80 0.4 0.2 0.3 100 0.2 0.1 0.0 *Specimens disintegrated when handled.
layer of polymer less than a few micrometers thick which has underg o n e p l a s t i c d e f o r m a t i o n in a direction approximately normal to the craze plane as a response to tension applied in this direction (Kambour, 1986). The plastic def o r m a t i o n p r o d u c e s an o p t i c a l change and can result in surface discoloration. Crazing is thus a result of applied stress. B e v a n and E a r n s h a w (1968) showed that both water-saturated and completely dry specimens do not readily craze when exposed to a solvent. They suggested that partial chTing of specin~ens results in a water g r a d i e n t which c a u s e s i n t e r n a l stresses in the material. Absorption of solvent into the partially dried resin on a molecular scale causes polymer separation in craze planes related to the internal stresses. Crosslinking of the interstitial matrix can provide sufficient solvent resistance to prevent solvent crazing. This study also supports the conclusion of Bevan and Earnshaw (1968)
Specimens Group II 1-4 Days 21 Days 28 Days 210.4 278.2 * 41.1 53.2 * 11.6 16.7 29.6 1.8 2.4 4.4 0.9 0.7 1.2 0.8 0.7 0.6 0.7 0.6 0.6
that resistance to solvent crazing associated with cross-linking is not due to alteration in water sorption properties. CONCLUSIONS The addition of the cross-linking agent in concentrations of 0-100% of monomer value has no effect on diffusion coefficient. Percentage water uptake increased to 60% of crosslinking agent and then decreased, but the range was extremely small. The range of toluene sorption values indicates that the cross-linking network increases to approximately 50% of EGDM concentration and that the degree of cross-linking to this limit is proportional to the amount of cross-linking agent that is added. REFERENCES BEVAN, E.M. and EARNSHAW,R. (1968): Part I - T h e Role of Water Sorption in the Solvent Crazing of Acrylic Resins, Aust Dent J 13: 265-273. BEvy, E.M. and EARNSHAW,R. (1969):
Dental Materials/October 1990 277
Part I I - T h e Role of Water Sorption in the Solvent Crazing of Acrylic Resins, Aust Den~ J 14: 190--196. BRADEN, M. (1964): The Absorption of Water by Acrylic Resins and Other Materials, J Prosthet Dent 14: 307-316. HARRISON, A.; HUGGETT, a.; and JAEGER, R.G. (1978): The Effect of a Cross-linking Agent on the Abrasion Resistance and Impact Strength of an Acrylic Resin Denture Base Material, J Dent 6: 299-304. HILL, R.G. (1981): The Cross-linking Agent Ethylene Glycol Dimethacrylate Content of the Currently Avail-
able Acrylic Denture Base Resins, J
and Glycol Dimethacrylate, J Polym
Den~ Res 60: 725-726.
Sci 75: 3544-3550.
JAGGER, R.G. and HUGGETT, R. (1975): The Effect of Cross-linking on Indentation Resistance Creep and Recovery of an Acrylic Denture Base Material, J Dent 3: 15-18. KAMBOUR,R.P. (1986): Crazing. In: Encyclopedia of Polymer Science and Engineering, 2nd ed., New York: John Wiley & Sons, Inc., pp. 299-323. LOSHAEK,S. and Fox, T.G. (1953): Crosslinking Polymers. I. Factors Influencing the Efficiency of Cross-linking in Copolymers of Methyl methacrylate
PHILLIPS, R.W. (1982): Skinner's Science of Dental Materials, 8th ed., Philadelphia: W.B. Saunders, p. 201. ROSE, E.E.; LAL, J.; GREEN, R.; and CORNELL,J. (1955): Direct Resin Filling Materials: Coefficient of Thermal Expansion and Water Sorption of Polymethylmethacrylate, J Dent Res 34: 589-596. WOLFF,E.M. (1962): The Effect of Crosslinking Agents on Acrylic Resins, Aust Dent J 7: 439-444.
278 JAGGER & HUGGETT/SORPTION PROPERTIES OF A CROSS-LINKED DENTURE BASE
