Effect of rapid materials

curing

procedures

on polymer

Lawrence Gettleman, D.M.D., M.S.D.,* Dan Nathanson, Richard 1. Myerson, MS.*** Harvard School of Dental Medicine, Boston, Mass.

impbtt

D.M.D.,**

and

D II o ymethyl

methacrylate (PMMA) has been utilized for many years as a versatile biocompatible material for fabrication into various forms of porous and nonporous dental implants. This material benefits from adequate mechanical strength when well cured and is capable of rapid processing which is advantageous to the dentist and patient \\heIl ustbd in th(, tooth-replica implant tectlnique.‘.‘8 Kapid curing shortells the time between creation of the socket and final implantation of the replica. The purpose of this study was to examine various curing conditions in order to minimize processing time and maxirniye physical properties of four heat-cured PMMA-based polymers. Rapid and complete curing of these materials is beneficial to rnany other procedures in clinical dentistry. MATERIALS

AND

METHOD

The polyrners utilized are listed in Table I. Hue-Lent (resin a) is a proprieta? heat-cured resin used in prosthetic dentistry, consisting of (I ) hornopolymethyl methacrylate beads (Fig. I ( part a) and (2) a liquid of 81 per cent methyl methacrylate monomer (MMA) cross-linked with 16 per cent ethylene glycol dirnethacrylate (EGDMA!. and it is inhibitrd by hydroquinone. ‘I‘tle reaction is initiated bvith residual benzoyl peroxide in the polymer. Polymer h was the material used by Hodosh and co-authors for implant work.‘. “. Ii Polymers c and d are small and large PMMA beads also made from Hue-Len (Fig. I. 6 and ( ) which are similar to those used b> Read

before

Supported the National ‘Assistant **Instructor ***Lecturer tThe

74

the .4merican

Assuciation

for Dental

by United States Public Health Institute of Dental Research. Professor

of Prosthetic

in Operative in Prosthetic

I.. D. Caulk

Company,

Dentistry.

Dentistry. Dentistry. Milford,

Del.

Service

Research, Research

General Grant

Session. No. R23

DE

3690

from

Effect of rapid

Volume Number

37 1

Table

I. Dense and porous implant

PMMA with foaming

c. Porous PMMA d. Porous PMMA

with small beads with large beads

size reported

I Gm. PMMA, DNPT, Chopped bone, I Gm. I Gm.

agents

as mean

(see text),

The

anorganic

§N-tributyl

Table

II. Curing

chips,

Allied

conditions

Liquid

(27 f 10 Mm beads) 0.65 Gm.* 0.15 Gm.t Kiel 0.20 Gm.$ (I 10 f 28 pm beads) (383 f 78 pm beads)

Company, Opex

bone

phosphate,

75

MMA, 0.46 ml.* MMA, 0.40 ml.* NTP, 0.06 ml.§

MMA, MMA,

0. I5 ml.* 0.15 ml.*

+ 1 S.D. L. D. Caulk

~Dinitrosopentamethylenetetramine, $Chopped

implants

Powder

a. Clear nonporous b. Porous PMMA and bone chips

*Hue-Lon

on polymer

polymers

Types of polymers

Bead

curing

Kiel

bone,

Chemical,

Unilab,

New

for polymer

Milford,

Del.

100, National York,

Polychemicals,

Inc.,

New

Wilmington,

York,

Mass.

N. Y.

N. Y.

implant

materials

Pressurepotf Air oven* 45 min. at 220” C.

*Blue

(B.P.

M oven,

tAcri-Dense

model

SV-57A,

Blue

M Electric

curing

unit,

Coe

No. 777, Wilmot-Castle Radarange,

11Megacycles

-

Table

Ill.

Glycerin (B.P. = 291” C.) 25 p.s.i. for 45 min.; 100” and 135°C.

pneumatic

$Speed-Clave §Amana

20% NaCl = 137” C.) 25 p.s.i. for 45 T3i;:;dW and

per

Ultimate

Air oven at 220” C. for 45 min. 40.3 MPa (3.4*) or 5,850 p.s.i.

model

MR-1,

Steam auloclave$

Company,

Laboratories,

Company, Amana

Blue Inc.,

Rochester,

Refrigeration

Microwave

25 p.s.i.; 15 and 30 min. for resins a and 6 and 22 min. for resins c andd; 121’C. Island,

Chicago,

111. 111.

N. Y. Company,

Amana,

Iowa.

second.

tensile strength

of clear, nonporous

Pressure pot at 25 p.s.i. for 45 min. 20% NaCi 47.2 or 50.4 or

100” c. MPa(l.S*) 45.0 6,840 p.s.i. or 135 o c. MPa (0.7;) 52.8 7,310 p.s.i. or

Glycerin MPa (9.5*) 6,520 p.s.i. MPa (3.3;) 7,660 p.s.i.

PMMA

(resin a)

r

Steam autos at 121” C. and 25 p.s.i. 15 min. 38.2 MPa (2.9*) or 5,540 p.s.i. 30 min. 63.0 MPa (1.72*) or 9,130 p.s.i.

Microwave 43.3 or 57.1 or 59.4 or 34.3 or

n = 5. Powder/liquid *Standard

oven3

4, 5, 6, and 7 min.; 2,450 MHz!. 32 kilojoules per min. (30 BTU per min.)

= 1 Cm. per 0.46 ml. deviation in MPa.

oven

4 min. MPa (4.9*) 6,280 p.s.i. 5 min. MPa (10.8*) 8,280 p.s.i. 6 min. MPa (3.6*) 8,610 p.s.i. 7 min. MPa (8.3*) 4,980 p.s.i.

76

Gettlernan,

Fig.

1, a-c.

Fig. 2. Pneumatic which compress Fig. 3. A clamped the chamber. The waves.

Nathanson,

Scanning

and Myerson

electron

press designed polymers c and

micrographs

beads

of the polymer

beads

specially

prepared

of resins

a, G, and

(100 psi.) pressure to machined flask for tensile bars.

flask is positioned in a microwave oven. Radiation top and bottom of the flask are removed to permit

were

Dent.

January.

to deliver 0.7 MPa d into a five-specimen

Healey and Taylor,7 Taylor and mixed with a minimal amount of material.* All materials were packed in a doughy stage was reached in 5 *The

I. 1’1osthrt.

1977

d.

plungers

enters from the top of penetration of the micro-

Smith,8 and Weinstein and associates.!’ They were monomer to arrive at a porous, “SinteEd” implant water-saturated to 10 minutes. by The

L. D.

split plaster of Paris molds after Polymers c and d were condensed Caulk

Company,

Milford,

Del.

Volume Number

37 1

Effect

Fig. 4. (A) Split grips disassembled positioned in testing machine. Table

IV. Ultimate

tensile strength

20% NaCl

2 I .2 MPa (2.6*) or 3,080 psi.; 27% pores;

n =

19.7 MPa

(2.8*)

or 2,920 p.s.i.; 18% pores; 8=48Mmt

(25*)

tensile

of porous

1

curing

bar

PMMA

Glycerin 14.6 MPa(l.7*) or 2,120 p.s.i.; 14% pores; 0 = 41 flrnt

(4.1

with

on polymer

and

12.5 mm.).

foaming

Sleam autoclave at 121’ C. and 25 p.s.i. 15 min. 20.7 MPa (3.5*) or 3,020 p.s.i.; 2 1% pores; 0= 46Nmt 30 min. 22.4 MPa (3.3*) or 3,250 p.s.i.; 17% pores; Q, = 54umt

77

inplants

(E)

agents

Specimen

(resin b)

Microwave oven for 4 min. 21.4 MPa(2.0*) or 3,100 p.s.i.; 23% pores; 0 = 57flmt

5.

Powder/liquid *Standard f0

rapid

Pressure pot at 135” C. and 25p.s.i.for45 min.

Air oven at 220” C. for45 min.

0 = 54wmt

and polymer

of

=

=

1 Gm.

per 0.463

ml.

deviation. Mean

pore

diameter.

into the flasks under 0.7 megapascals (MPa) (100 p.s.i.) pressure in a small pneumatic press (Fig. 2). All flasks were pressed or clamped shut during curing. The polymers were cured by the methods described in Table II. The air-oven method was used by Hodosh and associates? and our laboratory for clinical implants. It was found in preliminary work that the temperature within the polymer in the wet plaster flasks under these conditions plateaued at 100” C. at 24 minutes and remained there until 68 minutes. Therefore, a 45 minute period was

78

Gettleman,

Nathanson,

and Myer.ron

Fig. 5. Photomicrograph of a cross section of the porous polymer material agents and bone chips. Specimen is stained with Sudan black and phloxine pore size for this material is an average of both large and small pores.

containing foaming B in ethanol. Mean

chosen as suitable for curing. ‘[‘he immersion media used in the pressure-pot technique wcrc chosen on the basis of their ability to incrcasr the rate of heat transfer and, in cases such as nonflasked curing. to reduce crazin,g and the surface inhibition of polymerization.“‘~ ‘I The steam autoclave and micro\vave oven (Fig. 3) were utilized to provide rapid heating of the polymer. Microwave heating has recently been tried as a sterilizing method and has proved unsuccessful.” The metal backs of the plaster of Paris flasks were removed for curing in pressure pot, autoclave. and microwave oven to allow penetration of either immersion media or strarri or nlicrowavc radiation. Use of metal flasks and clamps within the microwave chamber is permissible if good electrical continuity is maintained between the parts. To prevent overheating of the magnetron tube, a beaker of water may be placed in the oven when small objects are to be cured. The water absorbs some microwa\rc: energy. and slightly more time will be required for an equivalent cure. Tensile bars, measuring 4.1 mm. in diameter by 12.5 mm. of reduced area. \\‘crc made of each material and cured by each method. Specimens were recovered fJTJJJ1 the plaster of Paris molds and stored in distilled water at 37O C. lIetermination of ultimate tensile strength was done on 3.~1 Instron machine* (Fig. 4. A) at 0.2 inch per minute (5.08 mm. per minute\ in grips designed to apply a uniaxial load (Fig. 4, R). Cylinders of 6 by 6 mm. were made by similar methods, potted in polyester resin, and sectioned with a wafering saw. They were metallographically ground using graded SIC papers and polished with 0.05 ~JTJ levigated alumina. Microscopic porp volume and mean pore size were determined by linear counting rnethods after staining with a solution of Sudan black and phloxine R in alcohol. *Instron

model

TM-L,

Instron

Corp.,

Canton,

Mass.

VOlUrnf Number

Table

37 1

Effect

V. Ultimate

tensile strength Pressure

20% NaCl Air oven at 220” C. for 45 min.

at 25p.s.i. and 135’ C. for 45 min.

of rapid

of porous

PMMA

VI. Ultimate

tensile strength

with

on polymer

implants

110 pm beads

79

(resin c)

got G, G-i at i5p.s.i. and 135” C. for45 min.

Steam autoclave at 25p.s.i. and 121’ C. for 22 min.

9.20 MPa (1.7’) 8.69 MPa (O.?9*) 6.4OMPa(l.S*) or 1,260‘p.s.i.;’ or 1,330‘p.s.i:; or 930 p:s.i.; ’ 3 1% pores; 32% pores; 40% pores; 0 = 7Ormt 0= 55pmt 0 = 79pmt n = 5. Powder/liquid = 1 Gm. Per 0.15 ml. *Standard deviation in MPa. t0 = Mean pore diameter. Table

curing

of porous

PMMA

9.11 MPa (2.6*) or 1,320‘p.s.i:; 40% pores; 0 = 79pmt

with

I Microwave oven for 6 min.

8.36 MPa 12.4*) or 1,210‘p.s.i:; 39% pores; 0 = 79pmt

383 pm beads

(resin

d)

Air oven at 220” C. for 45 min.

8 .26 MPa (0.8*)

10.0 MPa (l.O*) 6.57 MPa (1.2*) or 1,200 p.s.i.; or 1,460 p.s.i.; or 953 p.s.i.; 35% pores; 30% pores; 34% pores; 0 = 182pmt 0 = 161 pm? 0 = 169pmt n = 5. Powder/liquid = 1 Gm. per 0.15 ml. *Standard deviation in MPa. t0 = Mean pore diameter.

RESULTS

AND

7.12MPa(l.O*) or 1,030 p.s.i.; 39% pores; 0= 139pmt

6.4; MPa(l.7*) or 935 p.s.i.; 35% pores; 0 = 134pmt

DISCUSSION

The ultimate tensile strength of resin a (clear, nonporous PMMA) under various curing conditions is presented in Table III. Data are presented in megapascals as well as pounds per square inch and are the result of five determinations. It is apparent that curing this material under pressure produces properties superior to those obtained by curing in open air alone. There is little difference between curing in salt or in glycerin solution at the two temperatures investigated, but much greater strengths were generated in the polymer cured in a steam autoclave for 30 minutes at 25 p.s.i. Curing under pressure would be expected to reduce the formation of bubbles or other defects in the polymer. Processing in the microwave oven revealed strength properties which improved dramatically with small increases in time. Maximal properties were established at 6 minutes, but it was noted that the thick ends of the specimens began to develop bubbles. These bubbles appeared throughout the reduced sections in the 7 minute specimens, accounting for the reduced strength properties observed and in contrast to the dense, nonporous microstructures with shorter curing times. It is apparent that full polymerization of PMMA in a microwave oven is highly dependent on the quantity of heat delivered into the flask, with limitations, after

80

Gettleman,

Nathanson,

and Myerson

Fig. 6. Photomicrograph

of a cross section of the large-bead material.

which excessive microwave heating causes degradation of the polymer. ‘I-his is in contrast to other methods of curing which are controlled by the constant temperature of boiling water in the bath or in the wet plastc,r. It is known that absorption of microwave energy is proportional to the dielectric loss factor, the amount and configuration of material in the chamber, and the absence of obstructions to microwave penetration.‘, Considerable local heat conduction and temperature control in this system are accounted for by the presence of dipolaf molecules of water in the plaster of Paris flask. I,ack of water results in rapid overheating and actual burning of the resin in 1~s tllan 4 minutes. Therefore, the process has to be controlled and carefully tailored to the particular object’s dimensions in order to avoid over- or undercuring. But. the energy and time-saving advantages are self-evident. Similar results \yere obtained bv Nishii” for curing of complete dentures. The strength and porosity of resin h (porous PMMA containing foaming agents and anorganic bone chips) are presented in Table IV. This material has roughly one half to one third of the tensile strength of nonporous PMMA. Clear-cut differences in strength of this material are not apparent between the different curing processes. except w+lcn using the glycerin Inediurn. But. processing in the autocla\.e for 30 minutes, or in the microwavr oven for 4 minutes, produced properties ecluivalent to those obtained by curing in an air oven or a pressure pot at 45 minutes. Internal volunletric. porosity of this rnatrrial was some\vhat reduc,cd by pressure curing, but had little effect on thr mean pore diameter. Although pores ranged from very small to very large, the mean pore dianleter was uniform between curing methods (Fig. 5). The increment of in-flask pressure brought about by these curing methods may bca sulall. considering the pressure gelzratecl by clamping the flasks. Similar properties for resin c (porous PMMA material made from small beads] are shown in Table V. As with the aforementioned material, processing with ,glyc’erin resulted in considerably reduced strength. A similar situation existed when using resin d (large beads) (’ I-able VI:, except that the rapid curing methods produce

Volume Number

EJect

37 I

Fig. 7. Photomicrograph

of rapid

cwing

on polymer

implants

81

of a cross section of the small-bead material.

strength properities inferior to those produced by the air oven. It is apparent that pore volume and size are unrelated to curing method ; rather, they are probably dependent on the packing technique. The pore volume and average pore diameter for the large-bead material are approximately 35 per cent and 150 pm, respectively (Fig. 6)) and for the small-bead material, they are 35 per cent and 70 pm (Fig. 7). Tensile strength for these materials is one sixth to one eighth of that of the dense PMMA. CONCLUSIONS This investigation of standard curing methods for PMMA implants has demonstrated that alternate means of using the air oven may produce good strength properties along with a considerable degree of porosity, when desired, in a relatively short period of time. Curing of polymers in a pressure pot offers few advantages owing to the length of time required to produce, at best, equivalent strength for the porous materials. The method also poses dangers inherent in the use of superheated salt solution or hot glycerin. The autoclave is widely used in dental offices for sterilizing and will fully and consistently cure polymers within 30 minutes. The best properties for the PMMA resin were achieved with this method (61 MPa or 9,130 p.s.i., tensile strength). The microwave oven has become relatively inexpensive in recent years, and it offers time savings of up to 90 per cent which would be beneficial in implant dentistry or in dental laboratory procedures in general. The exact condition for curing particular polymers must be carefully determined to adjust the time of irradiation in order not to under- or overcure the polymer object. Curing polymers which contain intrinsic foaming agents under pressure conditions slightly reduced the total pore volume. But, pore volume and pore diameter in the large-bead polymers are determined predominantly by packing conditions, not curing conditions. Biologic tolerance to materials cured by these methods in primates is presently being evaluated.

82

Gettleman,

Nathanson,

and Myerson

Acknowledgment is made to Mr. Emery W. Dougherty of The L. D. Caulk who prepared the small- and large-bead polymer material and to Dr. C. H. Pameijer University who prepared the SEM micrographs.

Company of Boston

References 1.

Hodosh, M., Povar, M., and Shklar, G.: The Dental Polymer Implant Concept, J. PROSTHET. DENT. '22: 371-380, 1969. 2. Hodosh, M., Povar, M., and Shklar, G.: The Totally Self-Supporting Tooth Replica Polymer Implant, Oral Surg. 33: 1022-1030, 1972. 3. Ehrlich, J., and Azaz, B.: Immediate Implantation of Acrylic Resin Teeth Into Human Tooth Sockets, J. PROSTHET. DENT. 33: 205-209, 1975. 4. Ashman, A.: The Acrylic Resin Tooth Implant. II. A Continuing Report, J. PROSTHET. DENT. 29: 549-555, 1973. 5. Lam, R. V., and Peon, K. Y.: Acrylic Resin Root Implants: A Continuing Report, J. PROSTHET. DENT. 22: 657-662, 1969. 6. Gettleman, L., Nathanson, D., Myerson, L., and Hodosh, M.: Porous Heat Cured Poly (Methyl Methacrylate) for Dental Implants, J. Biomed. Mater. Res. Symp. 9: 243-249, 1975. 7. Healey, K. W., and Taylor, D. F.: Osseous Response to Porous Poly (Methacrylate) Implants in Mongrel Dogs, Int. Assoc. Dent. Res. Abst. No. 261, Feb., 1973. Methyl Methacrylate as an Implant Material, 8. Taylor, D. R., and Smith, F. B.: Porous J. Biomed. Mater. Res. 2: 467-479, 1972. L. G.: Implantable Artificial Teeth 9. Weinstein, A. M., Klawitter, J. J., and Peterson, Fabricated from Polymethylmethacrylate With Porous Roots, J. Biomed. Mater. Res. 6: 243-249, 1975. 10. Stern, M., and Myerson, R. L.: Technique for Fabricating Acrylic Veneer Facings in Fixed Restorations, J. PROSTHET. DENT. 27: 451-461, 1972. 11. Heynold, W. von: Ein Neues Polymerisationsverfahren fiir Kunststaff-Kronen, -Inlays und -Einpressungen, Prosthetische Zahnheilkunde 9: 69-72, 1968. 12. Hume, W. E., and Makinson, 0. F.: Microwave Radiation in Dental Sterilizing, J. Dent. Res. 54: 652, 1975. (Abst.) 13. Piischner, H.: Heating With Microwaves, New York, 1966, Springer-Verlag. 14, Nishii, M.: Studies on the Curing of Denture Base Resins With Microwave Irradiation: With Particular Reference to Heat-Curing Resins, J. Osaka Dent. Univ. 2: 23-40, 1968. HARVARD SCHOOL OF DENTAL 188 LONGWOOD AVE. BOSTON, MASS. 02115

MEDICINE