J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 3 (1984) 1 0 2 9 - - 1 0 3 0

Plasticization of acrylic resins used in dentistry

ANDREASG. ANDREOPOULOS Laboratory of Special Chemical Technology N. 7". U. Athens, Greece

GREGORY L. POLYZOIS* Department of Prosthodontics, Division of Removable Prosthodontics, Faculty of Dentistry, University of Athens, Greece

It is well known that the incorporation of a plasti- cizer in a polymer improves its impact strength. In the case of acrylic resins the use of phthalic acid diesters, such as dibutyl phthalate (DBP)or dioctyl phthalate (DOP) can be recommended but these additives tend to migrate contaminating the environment and modifying the resin proper t ies [1-31.

Nowadays plasticized (soft) acrylics are widely used in dentistry and especially in prosthodontics. Soft acrylics have been used in a variety of clinical situations. Their properties and defects have been well documented and their chemical structure elucidated [4-6] .

Briefly, soft acrylics comprise a polymer powder of poly(ethyl methacrylate) and a monomer liquid of any of a range of higher methacrylates (e.g. n-butyl, 2-ethoxyethyl methacrylate, lauryl metha- crylate) with a phthalate plasticizer. Whilst such materials have excellent adhesion to poly(methyl methacrylate), they have poor elastic properties and harden rapidly due to the leaching out of plasticizer. The basic problem is to formulate soft acrylics that do not contain leachable plasticizer. Therefore, it is useful to examine the behaviour of some internal plasticizers, i.e. those which can be attached to the macromolecule eliminating the possibility of leaching out.

Two series of specimens were prepared using a dental heat-cured acrylic resint and dibutyl phthalate (DBP) and diallyl phthalate (DAP) as external and internal plasticizers correspondingly.

Powder to liquid ratio was 3 : 1 by volume and the amount of plasticizer was added during the mixing. The processing was carried out using steel

TABLE I Results of impact strength tests, mean values (kp cm/cm)

Percentage plasticizer Impact strength

0 2.4 4.5 DBP 4.7

10 DBP 4.8 2 DAP 4.5 4 DAP 4.9

moulds in a heat platen press for 20 min at 100 ° C. Specimens for impact tests satisfied the require- ments of ASTM, D-256, Izod method, notched specimen [7].

The migration measurements were run using specimens which had the form of sheets about 1 mm thick. The environment for the migration study was 95% ethanol and the measurements were carried out gravimetrically at 30 ° C.

The results from the impact tests are shown in Table I. As these data indicate the impact strength is improved by both plasticizers. For DBP a limit- ing effect is observed for 4.5%. The amount of DAP needed for the same effect is much lower. The results from the migration measurements are shown in Table II. As the data indicated, for the case of DBP, there is considerable plasticizer migration, while in the case of DAP there is little or no migration. It is therefore evident that the incorporation of DBP in an acrylic resin cannot be recommended for the improvement of its impact strength. The leaching out of DBP con- taminates the surrounding area (e.g. the oral environment). On the other hand the plasticizer loss results in a decrease of the impact strength

*Present address: Depaztment of Prosthodontics, Faculty Athens, 608, Greece. tVertex RS, Dentimex, Zeist, Holland.

of Dentistry, University of Athens, 2 Thivon Street, Goudi,

0261-8028/84 $03.00 + .I 2 ©1984 Chapman and Hall Ltd. 1029

TABLE II Plasticizer migration in 10 days. Specimens immersed in 95% ethanol at 30 ° C

Percentage Weight Migration of plasticizer loss (%) plasticizer (%)

0 0.2 0 4.5 DBP 4 3.8

10 DBP 8.3 8.1 2 DAP 0.2 0 4 DAP 0.25 0.05

which tends to re turn to its initial value. The use

o f DAP seems to be an effect ive way to improve

the impact strength o f acrylic resins used in den-

t istry. This internal plasticizer having unsaturated

groups can be chemical ly a t tached to the acrylic

macromolecu les (at least in low concent ra t ions)

excluding the leaching ou t in the mouth .

References 1. E. KAMPOURIS, F. REGAS, S. ROKOTAS, S. POLY-

CHRONAKIS and M. PANTAZOGLOU, Polymer 16 (1975) 840.

2. E. KAMPOURIS, Polymer Eng. ScL 16 (1976) 59.

3. E. KAMPOURIS, Europ. Polym. J. II (1975) 705. 4. B. ELLIS, SUAD AL-NAKASH, D.J. LAMB, M.P.

McDONALD, jr. Dent. 7 (1979) 133. 5. M. BRADEN, J. Dent. Res. 49 (1970) 145. 6. P.S. WRIGHT, Jr. Dent. 4 (1976) 247. 7. Annual Book of ASTM Standards, Part 35, 1977,

pp. 94-112.

Received 13 April and accepted 16 May 1984

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