Abrasion and Polishing

8/3/2019 Abrasion and Polishing

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ABRASION AND POLISHING

Introduction:

The finishing and polishing of restorative dental materials are

important steps in the fabrication of clinically successful restorations.

The techniques employed for these procedures are meant not only for

the removal of excess material but also to smoothen the rough surfaces.

The finishing of dental restoration prior to their placement in the

oral cavity has therefore three benefits viz.

A) To promote oral hygiene - by resisting the accumulation of food

debris and pathogenic bacteria.

B) Enhance oral function - a well polished surface aids in gliding

movement of the feed over occlusal surfaces and between

embrasure spaces (smooth restoration contacts minimise wear

rates on opposing and adjacent teeth. e.g. materials like ceramics

which contain phases harder than the teeth enamel and dentine).

C) To improve aesthetics.

The materials employed for finishing and polishing of dental

restorations are generally termed as Abrasives. Thus an understanding

of the characteristics/features with their properties of these materials and


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the process of abrasion would aid in 'improved clinical usage of these

materials and techniques.

Individual Considerations:

I) Abrasion:

Craig has defined abrasion as:

"A process of wear whereby a hard rough surface (like a sand

paper disk) or hard irregular shaped practicles (like those in an abrasive

slurry) plough grooves in a softer material and cause materials from

these grooves to be removed from the surface".

It has been stated by Skinner's that abrasive could be a two

body or a "three body process."

To understand this more specifically the terms viz. Wear,

Abrasive Wear and Erosive Wear need more specific understanding

I) Wear - is a material removal process th3t can occur whenever

surfaces slide against each other. In dentistry the outermost particle

or the surface material of an abrading instrument is referred to as

ABRASIVE. The material finished is termed as a SUBSTRATE.

II) Abrasive Wear


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This could be of twotypes

a) Two body wear - occurs when abrasive particles are firmly

bonded to the surface of the abrasive instrument and no other

abrasive particles are used. E.g. a diamond bur abrading a teeth.

b) Three body wear - When the abrasive particles are free to

translate and rotate between two surfaces . E.g. dental

prophylaxis pastes (between tooth and rubber cup).

III) Erosive Wear

This is caused by articles impacting a substrate surface, carried

by either a stream of air or stream of liquid. Erosive wear should be

differentiated from chemical erosion more commonly known as acid

etching which involves acids and alkalis instead particles to remove the

substrate material.

Chemical erosion is employed to prepare teeth surfaces for

bonding purposes and not for finishing of materials.

Desirable Characteristics Of An Abrasive-

1. It should be irregular in shape so that it presents a sharp edge.


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[-Jagged particles are more effective. Round sand particles and cubicle

particles of sand paper are poor abrasives].

2. It should be harder than the work it abrades.

If it cannot indent the surface to be abraded then it cannot cut it

and therefore wears out.

3. Abrasive should posses a high impact strength / body strength.

Abrasive point should always fracture rather than dull out so that

always a sharp point or edge is available. The cuts also helps in

shredding debris accumulated from the work e.g. grinding wheel

against a metal.

4. Abrasive should possess attrition resistance so that it does not wear.

Design Of Abrasive Instruments

The abrasives employed could be one of the following three types, viz.:

A) Abrasive Grits

B) Bonded Abrasives

C) Coated Abrasive Disks And Strips

A) Abrasive Grits

Abrasive grits are derived from (abrasive) materials that have

been crushed an passed through series of mesh screens to obtain


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different particle size ranges. The grits are classified as - COARSE,

MEDIUM COARSE, MEDIUM FINE and SUPER FINE according to

the particle size ranges.

B) Bonded Abrasives

These consists of abrasive particles incorporated through a binder

to for grinding tools.

The abrasive particles bonded by four general methods viz.:

i) Sintering

ii) Vitreous Bonding [Glass/ Ceramic]

iii) Resinoid Bonding [Usually perenolic resin]

iv) Rubber Bonding [usually silicon rubber]

i). Sintering: Sintered abrasives are the strongest variety since the

abrasive particles are fused together.

ii). Vitreous Bonded: are mixed with a glassy / ceramic matrix

material, cold pressed to the instrument shape and fired to fuse

the binder.

iii). Resin Bonded: are cold/hot pressed and then heated to cure the

resin.


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iv). Rubber Bonded: made in a manner which is similar to resin

bonded.

As far as bonded abrasives are concerned the type of bonding

employed affects greatly the grinding behaviour of the tool on the

substrate. Bonded abrasives that tend to disintegrate rapidly on the

substrate are weak and result in increased abrasive costs. Bonded

abrasives that disintegrate very slowly tend to clog with the grinding

debris and result in loss of abrasive efficiency. Thus an ideal binder

would be one which would hold the abrasive to the instrument

sufficiently long as to cut, grind or polish the substrate and yet release

the abrasive particle before its efficiency is lost or before heat build-up

causes thermal damage to the substrate.

The type of binder is also intimately related to the life of the tool

in use. In most cases the binder is impregnated throughout which an

abrasive so that as an abrasive particle is wrenched during use another

abrasive particle takes its place as the binder wears e.g. for diamond

rotary instrument electroplated with a nickel base matrix is used as a

binder.

The heat generated during abrasive activity is dependent on the

efficiency of the abrasives. Cooling however maybe required for e.g. in


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abrading polymeric materials, excessive heat should be avoided as it can

cause stress relief and warpage (E.C.Combe).

It is important that a bonded abrasive must always be TRUED

and DRESSED before use.

Truing - procedure where bonded abrasive is first run against a harder

abrasive block until it rotates in the handpieces without eccentricity or

run out when placed on the substrate. Therefore TRUING is done for

proper shaping of the bonded abrasive.

Dressing - Like Truing is done for proper shape of the abrasive and also

for two other purposes.

1) Dressing reduces width to correct working size and shape

2) Users to remove clogged debris (ABRASIVE BLIN I KING]

from the abrasive instrument to restore working efficiency.

C) Coated Abrasive Disks And Strips

These abrasives are supplied as disks and finishing strips. They

are fabricated by securing abrasive particles to a flexible backing

material [heavy weight paper or Mylar].


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The disks are available in different diameters with thin and very

thin backings. Moisture - resistant backings are advantageous as the

abrasive stiffness is reduced by water degradation.

Abrasive Action

The mode of action of the abrasives is similar to that of the dental

burs that is it is merely a cutting action. Each fine abrasive particle thus

presents as a sharp edge which cuts through the surface similar to a

pointed chisel. During this cutting process the shaving thus formed is

powdered usually clogs the abrasive which thus makes periodic cleaning

ofthe abrasive necessary.

Abrasive Action On Metals

On abrading metallic restorations, the metallic grain structure

usually gets disoriented. The more abrasion the greater is the associated

disorientation. Strain hardening of the abraded metal usually

accompanies the grain disorientation. (Strain hardening is a process

wherein - the metal becomes stronger, harder and less ductile - when

greater stresses are induced at the grain boundaries to provide slipping

of the lattice network].

In the abrading of metals the crystalline structure of the surface is

disturbed, sometimes to a depth. of 10 m, However this surface affect


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varies with different metals. e.g. inductile metals [such as gold] less of

the surface metal may be removed by the abrasive than in a brittle metal.

Abrasive Action On Resins

The surface disturbance produced by abrasion on resins however

leads to the creation of surface stresses that may cause distortion if the

abrasion is too rigorous. The heat - generated during this abrasion

partially relieves the stresses.

If this heat generated is excessive it may even relieve stresses

created during processing which leads to warpage of the resin along

with melting of the surface of the resin. The heat generated is directly

proportional to the efficiency of the abrasives.

Factors Affecting Rate

Rate of abrasion of given material by a given abrasive is

determined primarily by three factors.

1) Size of the abrasive particle - larger the size - greater

the abrasion.

2) Pressure of the work against the abrasive


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When work is pressed against the abrasive scratches are deeper

and abrasion is more rapid - so greater chances of abrasive to fracture.

3) Speed at which the abrasive particle travels across the

work.

Greater the speed greater would be the rate of abrasion.

Speed employed is two types. viz.

- Rotational speed

- Linear speed - speed with which particles pass over the work.

- Linear speed required is 5000 ft/min.

- Linear speed is related to rotational speed as follows:

V = dn where

V = Linear speed

d = diameter of abrasive

n = revolutions per minute.

Factors Influencing The Efficiency Of Abrasives

These factors are as follows:

i). The hardness of the abrasive particle of diamond is hardest,

punice, garnet etc., are relatively mild].

ii). The shape of the abrasive particle


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Particles with sharp edges are more effective.

iii). Particle size of abrasive

Longer particle size will cut deeper grooves.

vv) Mechanical Properties of the abrasive

If the material breaks - it should form a new cutting edge.

Therefore bareness can be an advantage.

v) Rate of movement of abrasive particles

Slower abrasion - deeper scratches.

vi) Pressure of applied to the abrasive

Too much pressure can fracture the abrasive instrument and

increase heat of friction that has evolved.

vii) Properties of material that is being abraded

A brittle material can be abraded rapidly whereas a malleable and

ductile material [e.g. pure gold] will flow instead of being removed by

the abrasive.

Types of abrasives

According to Craig: The abrasives used can be classified /

grouped as and


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A) Finishing Abrasives

B) Polishing Abrasives

C) Cleaning Abrasives

A) Finishing Abrasives: These are hard, coarse abrasives used

initially to develop desired contours and removal of gross

irregularities.

B) Polishing Abrasives : These have a smaller particle size and are

less hard than abrasives used for finishing. They are used for

smoothening surfaces that are typically roughened by finishing

abrasives.

C) Cleaning Abrasives : These are soft abrasives with small particle

size and are intended to remove softer materials that adhere to

enamel or a restoration.

These dental abrasives could be employed In the form of three

basic designs as mentioned previously.

Secondly Skinners has grouped the abrasives employed in

dentistry as follows:

A) Natural Abrasives B) Manufactured Abrasives

1) Arkansas stone - are generally preferred because of

their more predictable physical

properties


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2) Chalk - Silicon carbide

3) Corundum - Aluminium oxide

4) Diamond - Silicon carbide

5) Emery - Aluminium oxide

6) Garnet - Synthetic Diamond

7) Pumice - Rouge

8) Quar - Tin oxide

9) Sand

10) Tripoli

11) Zirconium silicate

12) Cuttle

13) Kieselguhr

Natural Abrasives:

1) Arkansas stone

Semitransclucent, light gray, siliceous sedimentary rock, mended

in Arkansas

- contains micro crystalline quartz.

- small pieces of attached to metal shanks and trued

to various shapes for fine grinding of tooth enamel and

metal always.

2) Chalk

- mineral form of calcite


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- contains calcium carbonate

- used as a mild abrasive paste to polish teethenamel, gold fill, amalgam and plastic materials.

3) Corundum

- is largely replaced by a - Aluminium oxide due to

its superior physical properties.

- However corundum is primarily used for grinding

metal always and is available as a bonded abrasive.

4) Diamond

- transparent, colourless mineral composed of

carbon called super abrasive - because of ability to abrade any

other known substance.

- Hardest substance known.

- used on ceramic and resin based composite

materials.

- supplied as - bonded abrasive rotary instrument.

- flexible metal backed abrasive strips.

- Diamond polishing pastes.


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5) Emery - supplied predominantly as wanted abrasive disks. used for

finishing metal alloys or plastic materials.

6) Garnet - the term garnet includes several minerals which possess

similar physical properties like Silicates of Al, Co, Fe, Mg and Mn.

- garnet is dark red, extremely hard and when

fractured 'during abrasive activity forms sharp chisel shaped

plates - therefore making Garnet an effective abrasive.

- garnet is available on coated disks and Arbor box.

- used in grinding metal alloys and plastic materials

7) Pumice - produced from volcanic activity

- flour of pumice is an extremely fine grinded

volcanic rock derivative from Italy

- used in polishing teeth enamel, gold foil, dental

amalgam and acrylic resins.

8) Quartz - the particles are pulverised to form sharp angular particles

which are useful in making coated disks.

- used to finish metal alloys and may be used to grind dental enamel.

9) Sand - mixture of small mineral particles predominantly silica

- particles have rounded to angular shape


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- used (1) to remove refractory in vestment material from

base metal castings.

(2) coated on paper disks for grinding of metal

alloys and plastic materials

10) Tripoli - derived from a light weight, siliceous sedimentary rock

- could be weight, gray, pink, red or yellow

- gray and red most frequently used.

- used for polishing metal alloys and some plastic

materials.

11) Zirconium Silicate - Off white mineral, ground to various Sizes

used to make coated disks and strips.

- also as a component of dental prophylaxis pastes.

12) Cuttle - made from the pulvedsed internal shell of a Mediterranean

marine mollusc, this abrasive is obtained as a white calcareous

powder.

- the related synonyms for this abrasive are cuttlefish and

cuttle bone

- it is obtained as a coated abrasive and useful for delicate

operations like polishing of metal margins and dental

amalgam-restus.


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13) Kieselguhr - this abrasive is obtained from the siliceous remains of

minute aquatic plants known as diatoms.

- it is an excellent mild abrasive

- the coarser variety of this is known as DIATOMACEOUS

EARTH which is used as filler in many dental materials.

- appropriate precautions must be taken while handling this

abrasive as the risk for respiratory silicosis due to long term

exposure of this material is significant.

Manufactured Abrasives

Silicon Carbide:

- This is the first of the synthetic abrasive to be developed

- Two types were manufactured viz. i) green form ii) blue black

form

- Both having similar physical properties

- However the green variety is preferred because substrates are

more visible against the green colour.

- The cutting efficiency of the silicon - carbide abrasives is higher

as the particles are sharp and break to form new sharp particles


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- supplied as an abrasive in coated disks and vitreous and rubber

bonded instruments the silicon carbide is used in cutting metal

alloys, ceramics and plastic materials.

Aluminium Oxide - This is second synthetic abrasive to be

manufactured.

- This form of alumina is much harder than its natural counterpart

[CORUNDUM] because of its purity.

- The forms usually prepared are

i) White stones - made of sintered aluminium oxide - are used

for contouring of enamel and finishing metal and ceramic

materials.

ii) Variations of the aluminium oxide include those which

chronic compound additions - these, obtained in pink and

ruby colours are obtained as vitreous bonded non-

contaminating mounted stones - used for preparation of

metal ceramic alloys to receive porcelain.

[important to note that remnants of these abrasives should not

interfere with porcelain bonding to the metal studies Therefore show

that carbides are better for this purpose]


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Synthetic Diamond

- The synthetic/manufactured form of diamond is produced at five

times the level of the natural diamond abrasive

- This synthetic diamond is used for manufacture of diamond saws,

wheels and burs and also diamond locks employed for truing of

bonded abrasives.

- Synthetic diamond abrasives are used primarily on teeth

structure, ceramic materials and resin based components

Rouge

- principle component is iron oxide blended with various binders -

used to polish noble metal alloys

- may be impregnated on paper/fabric - CROCUS CLOTH

Tin Oxide

- is composed of very fine abrasive particles.

- may be employed In an abrasive paste form along with water,

alcohol or glycerine.

- used as a polishing agent for teeth and metallic restorations.


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Polishing

Polishing is the production of a smooth mirror like surface

without use of any external form.

A number of reasons are elucidated for the importance of

polishing dental restoration and teeth these are

i) The smooth polished restorative and teeth [enamel] surfaces are

less receptive to bacterial colonisation and dental plaque

formation.

ii) The polished layer on metallic restoration aids in prevention of

tarnish and corrosion.

iii) Finally, from a clinical view point unpolished restoration with

rough surfaces are uncomfortable for the patient.

The polishing procedure is one which is eventually initiated once

the abrasive mechanism eliminates or obliterates most of the finer

scratches leaving a smooth finish. This smooth layer or smooth finish

on the surface of the restoration is referred to by skinners as a polish.

However, it is difficult to draw or markout an exact line of

differentiation between an abrasive and a polishing agent. For E.g.

given agent having a large particle size and that which produces


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deep/large scratches on the restoration surface could be termed as an

abrasive. On the other hand a similar agent with a fine particle size

which produces a smooth finish could be termed. as a polishing agent.

The polishing materials also differ in the amount of material they

remove from t he surface.

They remove molecule by molecule and thus provide a smooth

surface. In the process fine scratches and irregularities are filled in by

powdered, particulate being removed from the surface.

The polished layer is therefore thought of as one made up of

minute crystals, thus known as the Micro Crystalline Layer or Beilby

Layer (named after a scientist who first noted such a surface layer after

polishing) which is easily kept clean and has high corrosion resistance.

Surface structure not more than 0.005 mm is removed during

polishing. It is therefore observed that the process of Finishing,

Cutting/Abrading and Polishing have not been differentiated well in

dentistry.

The term finish - would actually relate to the final surface

achieved/applied to a material or the character of the surface of the

material.


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If this explanation for the term finish is accepted then others viz.

cutting/abrading, grinding and polishing could be considered as a series

of steps encompassed within the process finishing.

However minor differences do exist in the cutting, grinding and

polishing procedures.

i) A cutting operation - as cited by Skinners "refer to the use of

bladed instrument or any instrument in a blade like fashion". The

substrate of the cutting operation may be divided into large

separate pieces or may sustain deep notches and grooves by the

cutting operation.

ii) A grinding operation removes small particles of a substrate

through the action of bonded orcoated abrasive Instruments. The

grinding Instruments may contain randomly arranged abrasive

particles e.g. a diamond coated rotary instrument.

It is important to note here the cutting a nd grinding are both

Unidirectional in their course of action.

iii) The Polishing Operation is one of the most refined finishing

process. It produces scratches so fine that they are visible only

when greatly magnified. The ideally polished surface is one


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which would be automatically smooth with no surface

imperfections.

Polishing is considered to be Multi Directional in its course of

action.

Examples of polishing: Instruments are

1) Rubber abrasive Points

- Fine particle disks and steps

- Fine particle polishing pastes - applied with soft felt

points, muslin wheels, prophy cups or buffing wheels.

iv) Electrolytic Polishing:

- is a reverse of electroplating

- alloy to be polished is made the anode of the electrolytic cell.

- as current passes ionic material from anode is dissolved leaving a

bright surface.

- excellent method for Co-Cr alloys, used in denture bases


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Polishing Procedure (for resins)

The steps involved are

i) Smoothen the work with a coarse abrasive to produce large

scratches.

ii) Remove large scratches with a finer abrasive but difference in

fineness should not be too great as it causes streaking in the final

surface. Keep changing direction of the abrasion so that new

scratches appears at right angles to the coarser scratches, to

ensure uniform abrasion.

iii) Continue using fine abrasive until scratches are no longer

visible to the eye then accomplish

- Primary polishing with punice flour with a canvas buff wheel

[mainly for resins].

- Then the work is cleaned to remove all abrasive particles and

then punice and water paste of a muddy consistency is applied

and buffing done till a bright and well polished surface is

obtained. Glycerine is preferred instead of H20 because water

could evaporate, glycerine maintains consistency.


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- Clean the work with soap water and change to a lannel (rotate at

high speed) buff wheel. Polishing cake with grease is held

against buff wheel to impregnate with the agent.

- Apply light pressure to avoid excessive heat generation.

Care should be taken to avoid use of abrasive tool or slurry in a

dry condition.

- Which would reduce efficiency of abrasive ard also increased

heat.

Speed employed for Polishing -

- is more than that for abrasion.

- linear speed of range 7500 - 1 0000 ft/min.

Composites :

- The smoothest surface on a freshly inserted composite can be

obtained by allowing polymerisation to occur against an inserted

Mylar matrix.

- Use of green / carbide stones / 12 - blade carbide burs is also

accepted for removal of excess near enamel margins of macro

filled resins.


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This is followed by use of

- Aluminium oxide disks - for accessible areas finishing

- White source stones of suitable shapes - inaccessible areas.

- Fine and micro fine diamonds - finishing of micro filled resins.

Hazards Encountered In Finishing And Polishing Procedures

Aerosol Hazard:

Aerosols, the dispersion of solid and liquid particles in air are

generated whenever finishing and polishing operations are performed.

The dental aerosols contain teeth structure, dental materials,

micro organisms.

These aerosols are potential sources of infections and chronic

diseases of eyes and lungs.

Silicosis/grinder's disease is a major aerosol hazard in dentistry

because of a number of silica based materials are used in processing and

finishing dental restorations.

These aerosols can remain airborne for more that 24 hours before

settling and are therefore c2oable of cross contaminating other zones of

the treatment facility.


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Aerosols produced during the finishing procedures can be

controlled following three ways controlled at the source through

- adequate infection control procedures

- water spray

- high volume suction

Personal protection -

- safety glasses

- disposable face masks

Adequate ventilation of the entire unit which c--ii'iciently

removes any residual particulates from the air.

Review of Literature

Rotary instrument finishing of micro filled and small particle composite

resins. JADA Aug 1987.

- This study suggests that rotary instrument for finishing

composites must be selected in accordance with the type of

composites used.

- Tungsten Carbide Burs at high speeds for trimming and finishing

micro filled composites are contraindicated because they disrupt


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the composite resin surface therefore for micro filled and small

particle resins diamond burs at slow spends are used.

- Carbide burs at high speeds on small particle composites

produces a surface free from the characteristic striations and

grooves produced by diamond burs.

Effect of three finishing systems on four aesthetic restorative materials.

Operative Dentistry - 1998.

Two varieties of composites (Hybrid and Micro flied) and two

types of GIC viz. Traditional/conventional GIC and a Resin modified

GIC were employed for this study.

- Impregnated disks and diamond and carbide burs were used.

- No difference was seen in the surface of any of the restorations.

When the mylar matrix were used.

- However the study concluded with the result that abrasive

impregnated disks and aluminium oxide disks provided smoother

finished surfaces on the GIC and Composite than did the

diamond and carbide finishing burs.

A quantitative study of finishing and polishing techniques for a

composite Journal of Prosthetic Dent. 1988.


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As we know, finishing and polishing of composite has always

been a problem because the resin matrix and inorganic fillers differ in

hardness and do not abrade uniforms!.

This study evaluated and compared six finishing and polishing

techniques to identify the most effective one for micro filled composite

restorations.

These six techniques were as follows:

TECHNIQUE I - Use of disks of medium, fine and superfine grits in

descending order.

TECHNIQUE II - Use of polishing points

TECHNIQUE III - Use of polishing paste with rubber cap

TECHNIQUE IV- Finishing [40m] and polishing [15 m] diamond burs

were used.

TECHNIQUE V - IV followed by polishing points

TECHNIQUE VI - IV followed by polishing pastes


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Conclusion Of The Study:

Technique I produced the smoothest polished surface

Technique V was second best

Finishing glass polyalkenoate cements [gic] M.J.Woodfords BDJ

(1988).

This article reports on the SEM surface examination of GIC after

furnishing procedures using rotary and hard. instrumentation.

Anhydrous water hardening type and an encapsulated type of gics

were employed.

Finishing procedures employed were

1. White stones and Vaseline in the slow hand piece at 5000

rpm.

2. Soft disks (A12O3 disks)and Vaseline in the, slow hand

piece at 500 rpm.

3. Tungsten carbide blank in air turbine Hand piece with a

water spray as coolant.


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Results :

Ideal surface is produced by the matrix (any finishing inevitably

disrupts this surface).

- Soflex discs produce a relatively smooth surface

- Tungsten Carbide disrupts surface of even mature GIC

- Hand instrument cause marginal breakdown.

Summary and Conclusion

Though a varied range of abrasive and polishing agents have

been described with relation to individual dental materials an ideal

abrasive or a polishing agent which would satisfy all polishing needs of

the dental materials, one would say is yet to be developed.

References:

1. Science of Dental Materials

10th Edition Kennith Anusavice.

9th Edition Ralph W. Philips.

2. Notes on dental materials 6th Edition, E.C. Coombe.

3. Notes on dental materials V.S. Rao.


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4. Dental materials Properties and Manipulation 4th Edition,

1987, Craig OBrien powers.

5. Restorative dental materials.

10th Edition. 1996, Craig.

6. Contemporary fixed prosthodontics. 2nd Edition, 1998, Stephen F.

Rosensteil.

7. The polishing of gold castings. JPD, 1959. 9, 668. Troxell R.R.

8. Feudametal of fixed prosthodontics. 3rd Edition. 1996, Herbert T.,

S Hillenberg.

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Abrasion and Polishing

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