Water sorption of plasticized denture acrylic lining materials

Water sorption of plasticized denture acrylic lining materials S. Kalachandra D.T. Turner* Department of Operative Dentistry Dental Research Center University of North Carolina Chapel Hill, North Carolina 27599-7455 Received May 13, 1988 Accepted October 31, 1988 *deceased This investigation was supported by USPHS Research Grant No. DE06201 and General Research Support No. 0533. Dent Mater 5:161-164, May, 1989

Abstract-The objective was to characterize water sorption of plasticized denture acrylic lining materials, with determination both of uptake and diffusion coefficient (D). Feasibility was investigated first for simple binary mixtures of poly(methyl methacrylate), PMMA, and various phthalate plasticizers prepared by radiation polymerization of methyl methacrylate/plasticizer solutions. These were shown to be tractable when samples were saturated with water and then characterized in desorl~tion measurements. Plasticizers were found to decrease the uptake of water. This decrease was attributed to the hydrophobic nature of the plasticizers and also to their ability to fill microvoids in PMMA which, in their absence, would be available to water. Values of D increased monotonically with increasing volume fractions of plasticizer. It appeared that values of D increased more rapidly in cases where large proportions of plasticizer resulted in samples which were above their glass transition temperature, Tg, at the temperature of testing water sorption. Desorption measurements were extended satisfactorily to a model system made from poly(ethyl methacrylate) powder, methyl methacrylate, and dibutyl phthalate, which was polymerized with a conventional redox initiator. Water desorption results were compared with various proprietary materials which were also characterized with respect to Tg and, cursorily, to composition.

p

lasticized soft lining materials were used extensively in prosthetic dentistry (Braden, 1964; Braden and Clarke, 1972), and their clinical application is well-documented. They are important in providing comfortable dentures but have only a limited service life, typically about six months. Analysis of longterm data on w a t e r sorption by acrylic lining materials is complicated by gradual loss of plasticizer (Braden and Wright, 1983; Wright, 1981). These materials undergo two processes when immersed in water: Plasticizers and other soluble materials are leached into the water, and water is taken up by the polymer. These two factors affect the dimensional stability and compliance of the materials. Earlier studies failed to characterize the water sorption of these materials completely, either because they have failed to separate the two processes (Travaglini et al., 1960; Craig and Gibbons, 1961; Eick et al., 1962) or because the length of study chosen was insufficient to represent clinical use (Storer, 1962; Bates and Smith, 1965; Suchatlampong, 1975). As a result of diffusion processes occurring in these materials, their composition changes during use, and hence their physical and mechanical properties change with time in the patient's mouth. These diffusion processes include the loss of ethanol and the sorption of water (Wright, 1982). Proprietary acrylic materials are somewhat numerous and are generally thought to be acrylic polymers and copolymer powders along with a liquid containing an acrylic monomer and plasticizer. The role of plasticizer is to lower the glass transition temperature of the polymer to a value below mouth temperature so that the softness of the material is reduced to a satisfactory level. The objective of the present in-

vestigation was to make detailed analyses of the short-term desorption data in which loss of plasticizer may be neglected and to find whether detailed studies can be made to provide estimates of both uptake and diffusion of water in these materials. MATERIALS AND METHODS Following manufacturers' instructions, plane sheets of 6.45 cm 2 (0.1 cm thick) were made from Truliner (Harry J. Bosworth Co., Skokie, IL), Kooliner (Coe Laboratories, Inc., Chicago, IL), and Neotone (Neology Products Co., Posen, IL). Sheets were also made from self-curing model systems containing poly(ethyl methacrylate), methyl methacrylate, and dibutyl phthalate. Products were analyzed with respect to glass transition temperature by differential scanning calorimetry (DuPont 990 Thermal Analyzer). Gas chromatographic analyses were performed by a Hewlett-Packard 5750 instrument with a 3.66 mm × 3.18 mm stainless steel column packed with 20% SF-96 on 60-80 mesh chromosorb W. A ramping program was run at 8°C per minute, from 50-150°C, by use of a flame ionization detector. Tetradecane was used as an internal standard. Mixtures were made of methyl methacrylate (MMA) (Aldrich, WI, USA), poly(ethyl m e t h a c r y l a t e ) (Aldrich), with dibutyl phthalate (DBP) (Aldrich), diethyl phthalate (Aldrich), and butyl phthalyl butyl glycolate (BPBG) (Morflex Chemical Company, Inc.). Model mixtures of methyl methacrylate and plasticizer in various proportions were polymerized by exposure to a 137Cs -y-ray source (dose rate = 0.8 Mrad hr ~1) at an ambient temperature of 35°C in a nitrogen atmosphere. The samples of Neotone, Kooliner, and Truliner were prepared according to manufacturers' instructions. Also,

DetHal Mtlteriols/.VIa!/1989 161

TABLE 1 GLASS TRANSITION TEMPERATURE (Tg) OF SOME POLY ALKYL METHACRYLATES

1.0

0.8

8

Substituent Methyl Ethyl Propyl Butyl (normal) Butyl (iso)

0.6-

~E 0.4-

0.2-

I

I 400

I

I 800

I

I 1200

I

I 1600

I

I

2O0O

tl/2/2! (cm-' sec 1/2)

Fig. 1. Kinetics of sorpUon [o] and desorption (e) of PMMA with 15% DBP at 37°C.

similar sheets were made from selfcuring model systems containing poly (ethyl methacrylate) (7 gin), methyl methacrylate (4 gms), and dibutyl phthalate (DBP) 15% (wt) of the total sample. The cylindrical polymerized products were cut under a stream of water with a high-speed diamond saw to provide samples of the following dimensions: diameter = 1.4 cm and thickness = 0.1 cm. With up to 25% plasticizer, the products were generally transparent and appeared to be homogeneous. Samples were immersed in distilled water, usually at 50°C, until equilibrated with water; they were dried at room temperature over anhydrous calcium chloride, and weighed periodically. The water uptake was determined relative to the dry weight of the sample (Turner, 1982; Turner and Abell, 1987). Values of T~ were estimated by differential scanning calorimetry, as described previously (Kalachandra and Turner, 1987a & b).

uptake by desorption are in general slightly g r e a t e r than values estimated by sorption. For this reason, uptake of water was estimated, more reliably, from desorption data. The rate of desorption and sorption was analyzed by reference to conventional solutions by Fick's laws of diffusion (Eqs. 1 & 2) for plane sheet geometry (Crank, 1957): M t

8 ....

1

M~ - 1 - ~-~ ~--o (2n + 1)e e x p [ - ( 2n + 1)"-rreDt]

j M, _ 2

i~

~/~

(2)

where Mt and M® are the masses of water sorbed or desorbed at times t and =, respectively, and 2e is the thickness of the specimen. Data both in sorption and desorption conformed approximately to Eq. (2) (Fig. 1). Observation of a higher diffusion rate of desorption, com-

Tg (°C) 105 65 35 20 53

pared with sorption, is similar to previous findings on PMMA alone and interpreted as due to a dependence of diffusion coefficient on water content (Barrie, 1968). More reliance is placed on values of D calculated from desorption data at 37°C, for which duplicate runs agreed within a few percent. Nevertheless, most measurements were made at 50°C so that the time of testing would be reduced. The glass transition temperatures of the alkyl methacrylates are summ a r i z e d in Table 1. Poly(butyl methacrylate) should require a relatively small amount of plasticizer to soften it, since its T~ is in the vicinity of room t e m p e r a t u r e . Poly (methyl methacrylate), on the other hand, should require a relatively large quantity of plasticizer, because its T~ has to be lowered from 105°C to mouth temperature. The acrylic soft liners may vary from hard-elastic to soft-plastic types of material. This variation indicates significant differences in the compositions of these materials. Compositions and glass transition temperatures of some acrylic lining materials and model system are shown in Table 2. The water uptake decreased markedly, with increasing plasticizer content up to about 15% (Fig. 2). At higher plasticizer

RESULTS

Samples i m m e r s e d in w a t e r increased in weight and reached values which are stationary in the sense that apparently constant values were attained over a period of several days. Such stationary values are used in the present work, but it should be noted that on prolonged immersion a slow decrement in weight was detected, presumably due to leaching out of plasticizer. This effect may also be a factor in accounting for the observation that estimates of water

162

TABLE 2 COMPOSITION AND Tg OF SOME ACRYLIC LINING MATERIALS

Dibutyl Phthalate Powder(PEMA), Monomer, (DBP) in the Material wt (gm) wt (gm)* Monomer (wt%) Tg (°C) Neotone (Neology) 7 4 35 6 Model System 7 4 32 35 Kooliner (Coe) 7 4 43 Truliner (Bosworth) 7 4 8 67 Proprietary samples were prepared according to manufacturers' instructions. The model system included a redox initiator and was prepared similarly. *The predominant component was methyl methacrylate, except in the case of Kooliner, which contained isobutyl methacrylate.

K A L A C H A N D R A & TURNER~WATER S O R P T I O N OF A C R Y L I C L I N I N G M A T E R I A L S

c o n t e n t s , d e c r e a s e s w e r e less marked. The influence of plasticizer content on D is shown in Fig. 3. Data for the influence of temperature on values of D for PMMA with i5% DBP were plotted according to the Arrhenius equation (Eq. 3) up to 50°C (Fig. 4). The activation energy for water diffusion is 40.0kJ mole ' Data for influence of plasticizer content on T~ were plotted (Fig. 5): D = Do exp( - E/RT)

3.0-

o-e

oDEP eDBP ABPBG

2.0 O

".~~ O ~ o

O~

Q.

(3) 1.0-

DISCUSSION

The influence of plasticizer on the uptake of water may be analyzed into two regions (Fig. 2). Above about 15%, the influence is small and consistent with the replacement of polymer by the more hydrophobic plasticizer. Diethyl phthalate (DEP) was judged to be less effective as a microvoid filler than dioctyl phthalate (DOP) (Kalachandra and Turner, 1987b). This interpretation was based on the evidence that PMMA without plasticizer takes up about 2 wt% water but swells only by 1 vol%. It was suggested that one-half of the total water is accommodated in microvoids. It was further suggested that this plasticizer viz. DEP can fill microvoids and thereby exclude uptake of water. This explanation is now extended to include DBP and BPBG, which are judged to be more effective than DEP as microvoid fillers. Perhaps DBP and BPBG are less soluble in PMMA and their partition into microvoids more favored. It is seen that BPBG appears to behave more or less as does DBP (Fig. 2). Several factors might be anticipated to complicate the influence of plasticizer content on the diffusion of water through PMMA. First, differences in microvoid filling might affect r e s u l t s up to a b o u t 10%. Secondly, antiplasticization might affect results at higher contents. In this latter respect, it has been reported that the tensile strength of PMMA at 26°C exhibits minimum and maximum values with 7% and 26% dibutyl phthalate, respectively (Olayemi and Oniyangi, 1981). Despite such anticipations, there is a relatively simple monotonic increase in the diffusion coefficient with increasing proportion of plasticizer content (Fig. 3). In the absence of

0

0

I

1

l

I

I

1

r

1

5

10

15

20

25

30

35

40

Content of plasticizer (vol%)

Fig. 2. Influence of plasticizers on uptake of water at 50°C. 24.0-

eDBP /',BPBG g.-

o

20.0-

x

/

;

¢n

/

/

/

2

// /

16.o-

// /

/

e-

-~ (D O "O

c

12.0-

8.0-

0

E3

4.0-

/ @ J J

15

25

Content of plasticizer (vol%)

Fig. 3. Influence of plasticizer content on diffusion coefficient, in desorption at 50°C.

any theoretical guidance, a leastsquares straight line was drawn through diffusion values for all plasticized compositions judged to be in the glassy state by reference to values of T~, i.e., with about 15% plasticizer (Fig. 5). B a s e d on this procedure, it appears that the value of D increases more rapidly above T~. Additional evidence of a related change in mechanism was obtained in experiments on temperature de-

pendence (Fig. 4). These suggest an enhanced rate of diffusion at temperatures above 50-60°C, which is near the T~ value for this composition, i.e., 61°C. The activation energy for diffusion in the glassy state is 40.0 k J/mole. In summary, there are several factors which might be expected to influence the way in which a plasticizer affects water transport in a glassy polymer. One factor which

Dental Materials~Malt 1989 163

1.7-

1.5-

DBP and BPBG seem to exert almost similar effects on the water sorption of PMMA. Water sorption of complex plasticized acrylic resins, including proprietary materials, can be characterized from desorption measurements.

o

1.30

+

1.1-

oO

0.9-

P

0.7-

0.5

z9

31o

3'.2

3:3

14

IO=/T (K-')

F/O. 4. Influence of temperature on diffusion coefficient, in desorpUon, DBP, 15%; PMMA 85%

ACKNOWLEDGMENTS We thank Mr. Devendra Singh for the preparation of samples. We also thank Dr. Mark W. Tubergen for the analysis of samples by G.C. REFERENCES

120-

v

100-

~ 0.........

80-

E

60-

tO

.~

09

40-

t-

¢n u~

20-

(3 0

0

10

15

20

25

C o n t e n t of p l a s t i c i z e r (vol %)

Fig. 5. Influence of plasticizer (DBP) on glass transition temperalure Tg.

seems to be important is that the plasticizer molecules might increase transport by decreasing forces of attraction operating between macromolecular segments. Such an effect is consistent with the observed monotonic increase in the diffusion coefficient up to Tg. The more marked increase in the diffusion coefficient above Tg would be in agreement with transport through a greater free volume caused by increased "main chain" motions.

SUMMARY The uptake of water by PMMA is reduced by inclusion of dibutyl

164

KALACHANDRA

& TURNER~WATER

phthalate, butyl phthalyl butyl glycolate, or by diethyl phthalate in a way which has been interpreted mainly to involve the filling of microvoids, which in the absence of such additives can accommodate water. Dibutyl phthalate appears to reduce water uptake by PMMA more effectively than does diethyl phthalate. The diffusion coefficient of water in PMMA increases monotonically with increasing contents of dibutyl phthalate. The diffusion coefficient increases less markedly when the plasticized polymers are in the glassy state.

SORPTION

OF ACRYLIC

LINING

CONCLUSIONS These measurements show that detailed analyses can be made of water sorption by these complex systems. Such analyses should prove useful in improving our understanding of the failure of soft lining materials in service. In this respect, the desorption method may be used on soft lining materials recovered from patients after clinical failure.

MATERIALS

BARRIE, J.A. (1968): Water in Polymers. In: Diffusion in Polymers, J. Crank and G.S. Park, Eds., London and New York: Academic Press, pp. 259-313. BATES, J.F. and SMITH, D.C. (1965): Evaluation of Indirect Resilient Liners for Dentures: Laboratory and Clinical Tests, J A m Dent Assoc 70: 344-353. BRADEN,M. (1964): The Absorption of Water by Acrylic Resins and Other Materials, J Prosthet Dent 14: 207316. BRADEN, M. and CLARKE, R.L. (1972): Viscoelastic Properties of Soft Lining Materials, J Dent Res 51: 1525-1528. BRADEN, M. and WRmHT, P.S. (1983): Water Absorption and Water Solubility of Soft Lining Materials for Acrylic Dentures, J Dent Res 62: 764-768. CRAIG, R.G. and GIBBONS,P. (1961): Properties of Resilient Denture Liners, J A m Dent Assoc 63: 382-390. CRANK, J. (1957): The Mathematics of Diffusion. Oxford: Clarendon Press, Chapter IV. LICK, J.D.; CRAIG, R.G.; and PEYTON, FA. (1962): Properties of Resilient Denture Liners in Simulated Mouth Conditions, J Prosthet Dent 12: 1043-1052. KALACHANDRA,S. and TURNER, D.T. (1987a): Depression of the Glass Transition T e m p e r a t u r e of Poly(Methyl Methaerylate) by Plasticizers: Conformity With Free Volume Theory, J Polym Sei Polym Phys 25: 1971-1979. KALACHANDRA,S. and TURNER, D.T. (1987b): Water Sorption of Poly(methyl Methacrylate): Effects of Plasticizers, Polymer 28: 1749-1752. OLAYEMI,J.Y. and ONIYANGZ,N.A. (1981): Three Stage Interaction of Dimethyl Phthalate, Dibutyl Phthalate and Poly (Vinyl Acetate) with Poly(Methyl Methacrylate), J Appl Polym Sci 26: 4059-4067. STORER, R. (1962): Resilient Denture Base Materials: Part I: Introduction and Laboratory Evaluation, Br Dent J 113: 195-203. SUCHATLAMPONG, C. (1975): Selected Properties of Resilient Liner~s for Dentures, M Phil Thesis, University of London. TRAVAGLINI, E.A.; GIBBONS, P.; and CRAIG, R.G. (1960): Resilient Liners for Dentures, J Prosthet Dent 10: 664-672. TURNER, D.T. (1982): Poly(Methyl Methacrylate) Plus Water: Sorption Kinetics in Volumetric Changes, Polymer 23: 197-202. TURNER, D.T. and ABELL,A.K. (1987): Water Sorption of Poly(Methyl Methacrylate): Effects of Crosslinks, Polymer 28: 297. WRIGHT,P.S. (1981): Composition and Properties of Soft Lining Materials for Acrylic Dentures, J Dent 9: 210223. WRIGHT, P.S. (1982): Characterization of the Adhesion of Soft Lining Materials to Poly (Methyl Methacrylate), J Dent Res 61: 1002-1005.