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Stephen C. BayneStephen C. BayneDepartment of Operative DentistryDepartment of Operative DentistrySchool of DentistrySchool of DentistryUniversity of North CarolinaUniversity of North CarolinaChapel Hill, NC 27599Chapel Hill, NC 27599--74507450

RPDRPDACRYLIC MATERIALSACRYLIC MATERIALS

Now that we have examined the cast metal frameworks which are part of RPDs, let’s consider the acrylic portions.

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Acrylic Portions of Acrylic Portions of RPDsRPDs1.1. RPD Acrylic portions = RPD Acrylic portions =

denture teethdenture teeth (DT) and (DT) and denture basedenture base (DB)(DB)

KeyEvents

2.2. Potential materials problems:Potential materials problems:a.a. DT wearDT wearb.b. DT bonding to DBDT bonding to DBc.c. PMMA porosityPMMA porosityd.d. DB distortionDB distortione.e. DB stainingDB stainingf.f. DB bonding to alloy frameworkDB bonding to alloy frameworkg.g. AcrylicAcrylic--metal leakage metal leakage

RPDs typically involve acrylic resin teeth bonded to acrylic saddles made from denture base material. There are a variety of problems that may arise [CLICK] as one can see by reference to the cross-section through the posterior section of an assembly [CLICK]. Starting at the top, denture tooth wear can occur, reducing its intraoral function. Denture teeth must be effectively bonded to the denture base – and this often is a problem. During the processing of denture base acrylic, porosity may arise from poor laboratory fabrication that leads to a range of problems. The denture base may become distorted by changing stress distributions on the assembly. Extrinsic acrylic discoloration often arises from staining of surface irregularities. Poor stress transfer from the acrylic to the metal mesh occurs if there is only mechanical bonding between the two materials. Finally, denture base staining often arises from microleakage along the interface of the metal mesh under the acrylic.

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STRAIN (STRAIN (εε, % elongation), % elongation)

STR

ESS

(ST

RES

S ( σσ

, MPa

), M

Pa)

CoCo--CrCrRPDRPDAlloyAlloy

AuAuRPDRPDAlloyAlloy

Modulus for RPD ComponentsModulus for RPD Components

DentalDentalPorcelainPorcelain HumanHuman

SkinSkin

AcrylicAcrylicDentureDentureTeethTeeth

DentureDentureBaseBaseAcrylicAcrylic

The materials involved have a wide range of moduli. [CLICK] Co-Cr RPD alloy (or even Au RPD alloy) has a much high modulus [CLICK] than acrylic denture teeth, acrylic denture base, or soft tissues that will ultimately accept much of the stress. Dental porcelain teeth are rarely used – but have an extremely high stiffness. [CLICK] Ideally there should not be a major difference in modulus as stress is transferred from one material to another. Otherwise, the stress tends to build up and cause failures at the boundary. To partly compensate for this problem, it is crucial to try to chemically bond the metal surfaces to the denture base acrylic if possible.

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DENTURE TEETH (DT)DENTURE TEETH (DT)A.A. Classification:Classification:

1.1. Porcelain (high fusing ceramic)Porcelain (high fusing ceramic)2.2. PMMAPMMA (acrylic) (acrylic) –– lightly crosslinked with TEGDMAlightly crosslinked with TEGDMA3.3. IPNIPN (Interpenetrating Network) (Interpenetrating Network) –– 22--phase polymerphase polymer4.4. IsositIsosit (composite; crosslinked acrylic) (composite; crosslinked acrylic) –– heavily crosslinkedheavily crosslinked5.5. (Experimental [fiber(Experimental [fiber--reinforced])reinforced])

PorcelainTeeth

Acrylic ResinTeeth

Denture teeth are the same types and variety as used for a complete denture. Teeth may be porcelain, PMMA, IPN, or Isosit. PMMA and IPN teeth are the most common. PMMA teeth are not as strong but are easily bonded to the denture base material. Other teeth are designed to be more wear resistant but are difficult to bond to the denture base. Porcelain teeth are slightly better in esthetics but generally not desirable for a range of other reasons.

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B.B. Attachment mechanisms of DT to DB:Attachment mechanisms of DT to DB:

1.1. Gross mechanical retentionGross mechanical retention((diatoricdiatoric holes, undercuts)holes, undercuts)

2.2. Micromechanical retentionMicromechanical retention(bur roughening, grinding, sandblasting)(bur roughening, grinding, sandblasting)

3.3. PseudoPseudo--chemical bondingchemical bonding(DB monomer penetration into DT)(DB monomer penetration into DT)

a.a. Minimizes interfacial leakage Minimizes interfacial leakage and staining (hygienic and esthetic problem)and staining (hygienic and esthetic problem)

b.b. Facilitates stressFacilitates stress--transfer transfer preventing cracks or crazes near interface with basepreventing cracks or crazes near interface with base

4.4. THERE IS NO CHEMICAL BONDINGTHERE IS NO CHEMICAL BONDING

DENTURE TEETH (DT)DENTURE TEETH (DT)DT/DB Attachment MechanismsDT/DB Attachment Mechanisms

Denture tooth (DT) attachment to denture base (DB) material is usually a combination of mechanical and pseudo-chemical bonding. Gross mechanical retention occurs with undercuts, including holes (diatorics), for retention. However, this is not very good. Generally, the teeth are roughened to increase micromechanical retention as well. With PMMA teeth, it is possible for the MMA in the denture base material to be sorbed into the surface and permit chain growth from the denture base into the surface of the teeth, creating pseudo-chemical bonding (or chain entanglement). This is very strong and the ideal situation. Wear-resistant teeth (e.g., IPN or Isosit) that are made from composites have much more difficulty in developing pseudo-chemical bonding with the denture base. [CLICK] There is no true chemical bonding procedure that is possible.

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C.C. Denture Tooth Wear:Denture Tooth Wear:

1.1. Denture tooth Denture tooth vsvs natural tooth wearing surfaces natural tooth wearing surfaces –– minimize wear minimize wear a.a. [Porcelain] x [Porcelain][Porcelain] x [Porcelain]b.b. [Gold] x [enamel, gold, or other restorative materials (not [Gold] x [enamel, gold, or other restorative materials (not porcporc)])]c.c. [Acrylic] x [enamel, acrylic][Acrylic] x [enamel, acrylic]

DENTURE TEETH (DT)DENTURE TEETH (DT)Wear PropertiesWear Properties

Denture teeth must be matched in hardness to their counterparts to insure that there is no unwanted wear occurring. Porcelain teeth should be matched with porcelain teeth. Porcelain teeth will wear natural teeth. Acrylic teeth work well against acrylic or natural teeth.

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DENTURE BASES (DB)DENTURE BASES (DB)A.A. Classification:Classification:

1.1. HighHigh--Impact Acrylic (PMMA = Impact Acrylic (PMMA = LucitoneLucitone 199; Lightly 199; Lightly XlinkedXlinked) ) 2.2. Experimental (polyethylene fiberExperimental (polyethylene fiber--reinforced)reinforced)3.3. Other (polystyrene, Other (polystyrene, nylonnnylonn, , polcarbonatepolcarbonate, epoxy, SS), epoxy, SS)

B.B. Attachment mechanisms of DB to RPD framework:Attachment mechanisms of DB to RPD framework:

1.1. Gross mechanical retentionGross mechanical retention (interpenetration of framework holes)(interpenetration of framework holes)

2.2. Micromechanical retentionMicromechanical retention (roughened surface (roughened surface –– SB)SB)a.a. Single phase alloys cannot be effectively etchedSingle phase alloys cannot be effectively etchedb.b. Sandblasting with Al2O3 is bestSandblasting with Al2O3 is best

3.3. NO PSEUDOCHEMICAL BONDINGNO PSEUDOCHEMICAL BONDING

4.4. Chemical bonding via couplingChemical bonding via coupling (4(4--META, Silane, META, Silane, RocatecRocatec))a.a. Achieve about 15Achieve about 15--20 MPa bond strengths20 MPa bond strengthsb.b. Discourages leakage and aids stress distributionDiscourages leakage and aids stress distribution

Denture base materials are almost always made from PMMA but have been fabricated from other polymers or reinforced materials. The denture base cannot be pseudo-chemically bonded to the metal framework but can take advantage of gross mechanical retention (i.e., holes in the metal mesh), micromechanical retention (i.e., sandblasted metal surfaces), and chemical bonding (using coupling agents and/or silicate treatments of the metal surface to fasciliate chemical bonding).

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C.C. DB Fabrication:DB Fabrication:

1.1. Design considerations: avoid thin sections of acrylic near alloyDesign considerations: avoid thin sections of acrylic near alloy

2.2. Curing considerations:Curing considerations:a.a. [MMA [MMA →→ PMMA] = chain reaction, fast, exothermicPMMA] = chain reaction, fast, exothermicb.b. Chain Rx = ACT (heat/BPO), INIT, PROP, TERMChain Rx = ACT (heat/BPO), INIT, PROP, TERM

3.3. Composition:Composition:a.a. Liquid Liquid = MMA, EGDM, HQ (Inhibitor)= MMA, EGDM, HQ (Inhibitor)b.b. Powder Powder = PMMA / PS / BMA / EGDMA, BPO, = PMMA / PS / BMA / EGDMA, BPO,

colorants, fibers (mimic arteries)colorants, fibers (mimic arteries)

DENTURE BASES (DB)DENTURE BASES (DB)Composition of MaterialsComposition of Materials

MMA is a liquid and similar to water making it unusable in that consistency. Denture base material is produced in the laboratory from a mixture of MMA and PMMA that generates a dough-like consistency which is easy to place and mold.

MMA is polymerized to PMMA. In the process, the MMA matrix that forms becomes pseudo-chemically bonded to the PMMA beads in the mixture so that there appears to be one continuous system of PMMA.

As shown above, a variety of modifiers are added to the mixture. The liquid normally includes ethylene glycol dimethacrylate to produce some crosslinking and limit the amount of water absorption of the denture base. HQ is an inhibitor. The powder includes polystyrene (PS), butyl methacrylate (BMA), and ethylene glycol dimethacrylate (EGDM) along with colorants and chopped nylon red fibers to mimic blood vessels. BPO is the initiator. An SEM picture of some of the powder is shown in the lower right hand corner before being mixed with MMA.

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MMALiquid

PMMAPowder

Old and newpolymer chainsintertwined at themolecular level.

PMMAPowderParticle MMA

Liquid

Acrylic dough

Heat and pressure

4.4. Manipulation of acrylic dough:Manipulation of acrylic dough:

a.a. Mixing of P/L: Dissolution of monomer into surface of powderMixing of P/L: Dissolution of monomer into surface of powderb.b. Thermal decomposition of BPO on heating; PolymerizationThermal decomposition of BPO on heating; Polymerizationc.c. New polymer becomes pseudoNew polymer becomes pseudo--chemically bonded to old polymer.chemically bonded to old polymer.

A schematic summary of the pseudo-chemical bonding between the MMA and PMMA is shown above. [CLICK] The MMA partially dissolves the surfaces of the PMMA particles and penetrates into the particles – as shown on the left. [CLICK] In the laboratory heat and pressure are applied to run the polymerization reaction. Newly growing PMMA polymer chains cross over into the PMMA particles and become entangled with pre-existing chains to generate pseudo-chemical bonding – as shown in the center. [CLICK] At the end the material is entirely PMMA.

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5.5. FlaskingFlasking options with acrylic dough:options with acrylic dough:a.a. TrialTrial--packing, trimming, repacking packing, trimming, repacking b.b. PackingPacking--onlyonlyc.c. Poured resin (Poured resin (e.g.e.g., , LucitoneLucitone FasFas--PorPor))

6.6. Heat and pressure control: avoids porosity; maximizes conversionHeat and pressure control: avoids porosity; maximizes conversiona.a. MMA: MMA: TbpTbp = 100= 100°°C (P= 1 C (P= 1 atmatm); 140); 140°°C (P= 2 C (P= 2 atmatm))b.b. Reaction is thermally activated and generates heat as wellReaction is thermally activated and generates heat as wellc.c. Reaction conversion is about 98 to 99.5%Reaction conversion is about 98 to 99.5%

7.7. Flask processing conditions:Flask processing conditions:a. a. 4040--30 Cycle = cure at 7130 Cycle = cure at 71--7272°°C for 40 min + 100C for 40 min + 100°°C for 30 minC for 30 minb.b. Fast Cycle = cure at 71Fast Cycle = cure at 71--7272°°C for 30C for 30--90 min + 10090 min + 100°°C for 30 min. C for 30 min. c. c. Slow CycleSlow Cycle = cure at 71= cure at 71--7272°°C for 10 hrs. C for 10 hrs.

[A slow cycle is better with larger amounts of material.][A slow cycle is better with larger amounts of material.][Generally, slow cures result in better dimensional accuracy.][Generally, slow cures result in better dimensional accuracy.]

DENTURE BASES (DB)DENTURE BASES (DB)Flask ProcessingFlask Processing

To insure that the acrylic dough (e.g., mixture of PMMA and MMA) is well-adapted to the teeth and framework, the entire prosthesis is processed in a flask using plaster to define the remaining mold space for the acrylic resin. The mold is trial packed with acrylic dough, trimmed, repacked and then processed. There are alternative techniques but we will focus on the standard one.

What happens next depends on the properties of the methyl methacrylate monomer (MMA) which is very volatile and boils at 100°C at one atmosphere of pressure. Since the polymerization reaction is exothermic, excess heat trapped in the flask can easily raise the temperature above 100°C and cause localized boiling of the monomer which is bad. Since the polymerization reaction involves formation of a linear polymer, there are no steric problem of monomer addition to the growing chain. Almost complete reaction conversion is achieved.

There are wide range of curing cycles for denture base materials – as referenced on the chart above. A slow cycle works the best for controlling the reaction conditions and insuring that the denture base contains no defects. The conditions of temperature, time, and pressure will be discussed in detail next.

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00 2020 4040 6060 8080TIME (minutes)TIME (minutes)

MMA MMA bpbp

2020

4040

6060

8080

100100

120120

140140

TEM

PER

ATU

RE

(TE

MPE

RA

TUR

E ( °°

C)C)

BPO thermalBPO thermaldecompositiondecompositionrapid at 60rapid at 60--7070°°CC

waterwateracrylicacrylic

•• No pressure control.No pressure control.•• No temperature control.No temperature control.•• Time control.Time control.•• General acrylic porosity.General acrylic porosity.

00 2020 4040 6060 8080TIME (minutes)TIME (minutes)

MMA MMA bpbp

2020

4040

6060

8080

100100

120120

140140

TEM

PER

ATU

RE

(TE

MPE

RA

TUR

E ( °°

C)C)

•• No pressure control.No pressure control.•• Temperature control.Temperature control.•• Time control.Time control.•• Local acrylic porosity.Local acrylic porosity.

TEM

PER

ATU

RE

(TE

MPE

RA

TUR

E ( °°

C)C)

00 2020 4040 6060 8080TIME (minutes)TIME (minutes)

MMA MMA bpbp

2020

4040

6060

8080

100100

120120

140140

•• Pressure control.Pressure control.•• Temperature control.Temperature control.•• Time control.Time control.•• NO POROSITYNO POROSITY

Processing ConditionsProcessing Conditions

The three charts above, describe differing conditions of temperature, time, pressure control for denture base processing. The vertical axis is temperature in °C. The brass metal processing flask (containing the acrylic dough, framework, teeth, plaster) is placed in a tank of heated water. A dashed line indicates the water temperature in the bath surrounding the flask. Remember that MMA boils at one atmosphere at 100°C. The horizontal axis is time in minutes.

[CLICK] In the first situation, the acrylic dough temperature rises at about the same rate as the water bath until at ~65°C, when thermal decomposition of the benzoyl peroxide initiator starts the exothermic polymerization reaction, releasing heat, and raising the internal temperature quickly to about ~135°C. This causes local boiling of some of the monomer and produces porosity throughout the denture base, particularly near the center of the acrylic mass close to the framework. This is a bad situation.

[CLICK] The second situation includes some temperature control. To partially control the effects of the exotherm, the water bath can be heated to only ~65°C so that only the exothermic heat is responsible for further temperature rises of the acrylic dough. The temperature rise stays below ~100°C but some sections may become hotter and include porosity. After the initial exotherm has happened the temperature is raised in the water bath to push the reaction to near completion.

[CLICK] In the third situation, pressure control is included as well. To completely control the effects of the exotherm, the water bath and flask are put under 2 atmospheres of pressure. That increases the boiling point for monomer to 140°C. Now, the water bath temperature can be raised quickly or slowly – without any worries that exothermic heat will induce monomer boiling. Examples of processing schedules are shown in the last figure.

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DB ProblemsDB Problems8.8. Analysis of POROSITY problems:Analysis of POROSITY problems:

a.a. Internal porosityInternal porosity: P/L heterogeneity and air incorporation : P/L heterogeneity and air incorporation (spherical pores)(spherical pores)

b.b. Internal porosityInternal porosity: localized MMA boiling : localized MMA boiling (common in thicker portions)(common in thicker portions)

c.c. External porosityExternal porosity: insufficient pressure or dough : insufficient pressure or dough (surface blisters and pores)(surface blisters and pores)

9.9. DIMENSIONAL CHANGES on processing:DIMENSIONAL CHANGES on processing:

a.a. Expansion on heating flask; Expansion on heating flask; b.b. Expansion on polymerization exotherm; Expansion on polymerization exotherm; c.c. Contraction on polymerization (21vol.%); Contraction on polymerization (21vol.%); d.d. Contraction on cooling to room temperature; Contraction on cooling to room temperature; e.e. Expansion on swelling in water; Expansion on swelling in water; f.f. Expansion on thermal change to 32Expansion on thermal change to 32°°C.C.

10.10. CRACKS and CRAZES:CRACKS and CRAZES:

a.a. Created by thermal and mechanical cyclingCreated by thermal and mechanical cycling Reversible Irreversible

CRAZE CRACK

Porosity is the major problem for denture base properties but other things may occur as well.

Internal porosity may also arise from mixing problems that create local heterogeneity or trap air. If the dough is not well packed into the mold, then insufficient pressure occurs during processing and surface blisters or pores may occur.

There are distortions of the acrylic during processing due to a wide range of temperature changes. These are itemized above. Generally it is assumed that the functioning temperature of the denture is above room temperature but below body temperature and in the range of ~32°C.

During use, the denture base and framework thermally and mechanically cycle. On the surface of the denture base there is a strong tendency to lose and gain water at the same time. This may produce crazes and cracks that will affect the esthetics and pick up stain.

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D.D. DB reprocessing:DB reprocessing:

1.1. Hard and soft tissue changes every 5Hard and soft tissue changes every 5--8 years 8 years require modifying denture base:require modifying denture base:a.a. Relining Relining ≡≡ resurfacing of the tissue surfaceresurfacing of the tissue surfaceb.b. Rebasing Rebasing ≡≡ replacement of entire denture basereplacement of entire denture base

2.2. SoftSoft--liners generally not practicalliners generally not practical

Maintenance of Acrylic PortionsMaintenance of Acrylic Portions

E.E. DB hygiene:DB hygiene:

1.1. Clean with toothbrush and warm soapClean with toothbrush and warm soap--andand--waterwater2.2. Avoid oxidizing or Avoid oxidizing or ClCl--containing materialscontaining materials3.3. Diligently clean both the top and tissueDiligently clean both the top and tissue--borne surfacesborne surfaces

Acrylic portions of complete dentures or RPDs may require repair or replacement for a variety of reasons. Generally hard and soft tissue changes under the denture base lead to poor adaptation and require adjustments. [CLICK] Relining produces a new palatal surface that is properly adapted. [CLICK] Rebasing is the complete replacement of the denture base. With complete dentures, soft liners are often used to manage conditions of changing soft tissues or irritation problems. However, these are too complicated to be used with RPDs since there is little space between the framework and palatal surface to add soft liners into the design. If the acrylic thickness is too small then it tends to fracture.

[CLICK] Patients often are confused about what they can utilize to clean an RPD since there are so many different materials involved. [CLICK] The best solution is to use a soft brush with soap and warm water. Avoid using anything that will attack the casting alloy or acrylic.

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Thank you.