1
ZINC PHOSPHATE AND ZINC POLYCARBOXYLATE CEMENTS
Seminar By
Dr. S. HIMAGIRI Postgraduate Student
DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS SRI RAMACHANDRA DENTAL COLLEGE AND HOSPITAL CHENNAI
2
CONTENTS
ZINC PHOSPHATE Introduction
1
Classification
1
Composition
2
Chemistry of setting reaction
3
Mixing procedure
9
Factors influencing working time and setting time
12
Properties
12
ZINC POLY CARBOXYLATE Introduction
13
Composition
14
Setting reaction
15
Factors influencing working time and setting time
17
Properties
19
3 INTRODUCTION Among all the dental cements zinc phosphate cement has been used for the longest period (since 1879). It has low film thickness and sufficient working time to allow complete seating of restoration. Initially on mixing it has low pH, which can irritate the pulp (2.15pH) and gives rise to transient pain. It is the oldest cement of all those used in dentistry. It also known the names “crown and bridge” cement and zinc oxy phosphate cement. It produces the thinnest film thickness of the available cements yet provide is height compressive strength. PRINCIPAL USES Luting agents for restorations and orthodontic appliances. Secondary
uses
intermediate
restorations,
thermal
insulating bases and root canal restorations. APPLICATIONS 1.
Luting of restorations
2.
High strength bases
3.
Temporary restorations
4.
Luting of orthodontic bands and brackets
CLASSIFICATION ADA specification No.8. Designates them as 1.
Type I
–
Fine grained for luting Film thickness should be 25 µ or less
2.
Type II
-
Medium grain for luting and filling Film thickness should not be more than 40µ
4 MODE OF SUPPLY Available as 1.
Powder and liquid system.
2.
Capsules of pre-proportioned powder and liquid variety of shades also available like yellow, gray, golden-brown, pink and white.
COMMERCIAL NAMES 1.
Confit
2.
Harvard
3.
Zinc cement improved
4.
Modern tenacin
COMPOSITION Powder Zinc oxide
- 90.2%
Magnesium oxide - 8.2%
- Principle constituent - Aids in Sintering
Other Oxides Bismuth trioxide, Calcium oxide, Barium oxide – 0.2% Improves smoothness of mix. Silica – 1.4%
- Filler
Liquid Phosphoric Acid
- 38.2%
- Reacts with zinc oxide
Water
- 36%
- Controls rate of reaction
-
5 CALCINATION PROCESS The
ingredients
of
the
powder
are
heated
together
at
temperature ranging from 1000°C to 1300°C for approx 4 to 8 hours or longer depending on the temperature. This calcination results in a fused or sintered mass this mass is ground and pulverized to a fine powder, which is sieved to recover selective particle sizes. LIQUID Aluminum and zinc is add to liquid, partially it will neutralized the H3PO4 and temper the reactivity of the liquid and is desired as buffering. WATER Presence
of
additional
water
shortens
the
setting
time.
Insufficient amount of water increases setting time CHEMISTRY OF SETTING REACTION When excess of zinc phosphate cement powder is brought in to contact with the liquid wetting occurs and a chemical reaction is initiated.
The acid liquid resulting in an “exothermic reaction”
dissolves the surface of the alkaline powder.
The set zinc phosphate
cement is essentially a hydrated amorphous network of zinc phosphate that surrounds incompletely dissolved particles of zinc oxide. The amorphous phase is extremely porus. The alumina of the liquid is essential to cement formation without its presence a non-cohesive crystalline structure matrix of tertiary zinc phosphate, Zn3 (PO4)2 4H20 (HOPEITE) would be formed. The alumina of the liquid complexes with phosphoric acid to form zinc alumino phosphate gel.
6 MIXING PROCEDURE INITIAL STAGE By the initial incorporation of small portions of the powder into the liquid minimal heat is liberated and easily dissipated the heat of the reaction is most effectively dissipated when the cement is mixed. 1. Over a large area 2. Over a cooled glass slab 3. With a relatively long narrow bladed stainless steel spatula. During the neutralization of the liquid by the powder the temperature of the mixing site is inversely proportional to the time consumed in accomplishing the mix. If a large volume of powder is carried to the liquid all at once results in increase in temperature at reaction site and speeds the reaction and hinds control over consistency. MIDDLE STAGE – II During this stage larger amount can be incorporated to further saturate the liquid with newly forming complex phosphate’s, the quantity of unreacted acid is less at this time because of the prior neutralization gained from adding small increments of the initial powder the amount of heat liberated will be less and can be dissipated adequately by the cooled glass slab. FINAL STAGE III Finally smaller increments of powder again are incorporated, so that the desired consistency is detained.
7 The phosphoric acid attacks the outside of the powder particle forming a crystalline matrix of phosphate compounds, and traps the un-dissolved powder as the cement sets. Three critical factors must be considerer when working with zinc phosphate cement: 1. Solubility 2. Strength 3. Setting time All above factors are under control of dentist 1. Zinc phosphates are extremely water-soluble when used as a luting agents, it is important to keep the area of cement not to expose oral fluids or saliva during setting, if contact occurs with saliva or oral fluids surface of the cement will become dull, soft, easily dissolved.
This can results in open margin, secondary
caries ď ś Strength It is adversely effects by improper mixing of the cement. If mixes too rapidly, the crystalline formation will be disturbed. ď ś Control of setting time It
is
crucial
in
developing
an
optimal
luting
consistency for zinc phosphate cement factor governing the setting time is a)
Mixing time
b)
Temperature
c)
Water content
d)
Powder liquid ratio
e)
Speculation of the powder in to the Liquid
f)
Concentration of H3PO4
8 Even slight changes in concentration of H 3PO4 can result in drastic changes in setting time. neutralized
diluted
H3PO4,
The liquid consist of partially
which,
when
exposed
to
humid
atmosphere, will absorb water: when expose to dry air it will loose water. Water content of liquid H3PO4 effects the setting time the reason is weak H3PO4 in the presence of water will convert to less stable from of ortho phosphoric acid, which is easily dissociated, leads to decrease setting time because increased dissociation so cap should always replaced immediately after dispensing. The liquid contain should be discarded when 80% of the constant have been used because of possible absorption of water from repeated opening of the bottle. Loss of water
-
prolonged setting time
Addition of water
-
shorted the setting time
CHEMISTRY OF REACTION The acid liquid resulting in exothermic reaction dissolves when powder is brought in contact with the liquid the surface of alkaline powder. When the powder is mixed with the liquid the H 3PO4 is attacks the surface the particles and releases zinc ions in to the liquid. The aluminum, which already forms a complex with the H 3PO4 react with zinc, results in zinc alumino phosphate gel on the surface of the remaining portion of the particles.
Set cement contain amorphous
matrix of un-reacted zinc oxide, covered with cohesive amorphous matrix of zinc alumino phosphate.
If excessive moisture is present
this zinc phosphate react with moisture to form zinc hopeite [Zn3(PO4)2.4H20]
9 WORKING AND SETTING TIME Working time is the time measured from the start of mixing during which the viscosity (consistency) of the mix in low enough to flow readily under pressure to form a thin film. Setting time means that the matrix formation has reached a point were external physical disturbance will not cause permanent dimensional changes. It can be measured with a 4.5N (1 pound) Gilmore needle: A reasonable setting time for zinc phosphate cement is between 5 and 9 minutes (ADA NO 8) MIXING TIME Exceed the mixing time 60-90 sec results in weakening of the cement mass by the breaking down of the matrix because it tends to form and bind the un-dissolved powder particle together. FACTORS INFLUENCING WORKING TIME AND SETTING TIME POWDER, LIQUID RATIO Working time and setting time can be increase by reducing the powder, liquid ratio if not it effects on physical properties and compressive strength of cement. RATE OF POWDER INCORPORATION Introduction of small quantity of powder in to the liquid for the first few increments increases working time and setting time by reducing the amount of heat generated and permits more powder to be incorporated in to the mix there fore it is the recommended procedure for zinc phosphate cement.
10 SPATULATION TIME Operators who
prolong the spatulation time are effectively
destroying the matrix that was forming. TEMPERATURE OF MIXING SLAB If we use cool glass slab can increase working time and setting time. MANIPULATION The proper amount of powder should be slowly incorporated in to the liquid on a cool glass slab (approximately 21째C) to attain the desired consistency of cement. MIXING SLAB Most chemical reactions are accelerated by the presence of heat because of an increase in molecular actively of the reactants. When we mix powder, liquid result in liberation of heat in the immediate environment of the reaction, this heat must be dissipated readily or other wise the reaction will proceed too fast towards completion, so properly cool glass slab with sufficient thick enough.
It should be
sufficient thick. Glass slab should not influence by environment. The mixing slab temperature should be low enough to be effective in cooling the cement mass but not be below the dew point unless the frozen slab technique is used [a temperature of 18째 to 24째C is indicated when room humidity permits].
11 POWDER LIQUID RATIO The amount of powder that can be incorporate into a given quantity of liquid greatly determines the properties of the mixed mass of cement as much powder as possible should be used to obtain a particular consistency. CARE OF THE LIQUID Liquid contains of the partially neutralized diluted H3PO4 when this liquid is exposed to a humid atmosphere, it will absorb water, and where as exposure to dry air tends to result in loss of water. While using some dispensing bottles of liquid the cap is off for a sufficient period of time to significantly affect the water content of the liquid. There fore keeping the bottle tightly closed when not using is good practice.
The polyethylene squeeze bottles do not require
removal of dropper and therefore eliminated the tendency for gain or loss of water from the liquid. The setting time and mixing time is quite noticeably affected by water addition or subtraction from the cement liquid. The addition of water causes a more rapid reaction, with the powder resulting in a shorter setting time. A loss of water lengthens the setting time. MIXING PROCEDURE FROZEN SLAB METHOD In this method glass slab is cooled in a refrigerator at 6째C or a freezer at -10째C.
No attempt is made to prevent moisture from
condensing on the slab when it is brought to room conditions. Cement in mixes on the cold slab by adding the powder until the
12 correct consistency is reached.
The amount of powder incorporated
with the frozen slab method is 50% to 75% more than with normal procedure the compressive and tensile strengths of cements prepared by the frozen slab method are not significantly different from those prepared normal mixes. ADVANTAGES Substantial increase in the working time Shorter setting time This method has been advocated for cementation of bridges with multiple pins Cementations of orthodontic bands PHYSICAL PROPERTIES STRENGTH Initial powder liquid composition, the powder and liquid ratio and the manner of mixing and handling of cement influence strength during its placement. SOLUBILITY AND DISINTEGRATION When tested according to ADA it showed relatively low solubility - 0.06-wt percentages. The solubility is grater is dilute organic acids like acetic, lactic citric acid. EFFECT OF POWDER AND LIQUID A thicker mix shows less solubility and disintegrated than thicker mix.
13 WATER CONTENT Any change in water content accompanied by increases solubility. THERMAL PROPERTIES Primary use of zinc phosphate is an insulting base beneath the metallic restoration. These are good thermal insulators and may be effective in reducing galvanic effect. The premature contact of incompletely sets cement with water results in the dissolution and leaching of that surface, for this reason the use of the term hydraulic to describe any of he zinc phosphate cement is improper because they do not harden or set with desirable physical properties when sub merger in water. OPTICAL PROPERTIES Set cement is opaque COMPRESSIVE STRENGTH Zinc – phosphate cements are stronger than zinc – oxide evening but not as strong as silico phosphate cements. The set cement gains 75% maximum strength in the first hour, maximum strength is attained in the first day (1500 psi) The strength of cement is sufficient when used as a base or luting agent, however when it is exposed to the oral environment, e.g. temporary restorations its brittleness and low strength causes it to fracture and disintegrate.
The prolonged contact with the oral
fluids or water gradually decreases its strength; this may be due to the slow dissolution of the cement.
14 FACTOR AFFECTING COMPRESSIVE STRENGTH ARE Powder and liquid ratio: the more the powder added in to the liquid, the greater is the strength. Water content: a change in the water content of the liquid, either loss or gain, reduces the strength. Tensile strength the cement is weaker in tension, thus making it brittle. 5.5mpa (800psi) MODULUS OF ELASTICITY It is comparatively high this makes it stiff and resistant to elastic deformation even when it is employed as a luting agent for restorations that are subjected to high masticatory stresses. 13.5 Gpa (1.96 X106psi). COPPER CEMENT Silver slats or copper oxide are some times added to the powder of zinc phosphate to increase their antibacterial properties. APPLICATION 1. Temporary fillings in children 2. Intermediate restorations CLASSIFICATION Classified according to the percentage of copper oxide that used as a replacement of zinc oxide. COMPOSITION Copper oxide, zinc oxide. When cuprous oxide is used the color of the cement is red, when
cupric oxide is used the color of the
cement is black. BIOLOGICAL PROPERTIES Poor biological properties because pH of the cement is 5.3
15 They are bactericidal or bacteriostatic. ZINC POLYCARBOXYLATE ♦ Developed by Dennis Smith 1968, in an effort to produce cement that could adhere to tooth structure ♦ They adhere via chelation to dental surfaces are also reduce pulpal problems associated with low pH of the traditional cements ♦ Zinc poly carboxylate was the first cement system that develops an adhesive bond to the tooth structure these cements are first to have adhesive bond to the hard tooth structure. ♦ In liquid the Phosphoric acid
is substituted by an acid functional
polymer firming the matrix ♦ As they are less acidic hence they do not irritate pulp ♦
The strength is almost same as zinc phosphate cement
♦
They are used for 1. Cementing cast metal restorations 2. Directly cementing orthodontic brackets on the tooth 3. They can also be used for temporary filling 4. Cavity base, cavity liners
COMMERCIAL NAMES Poly f (DENTSPLY)
HARVARD CC
DURELON
CAROXYLON
CARBCO (VOCO)
CERMCO, ”P.C.A”
SELFAST
16 DISPENSING Generally
supplies as a Type A
powder liquid
Type B
water settable powder
COMPOSITION Powder consist of Zinc oxide – 90 to 95 %
Magnesium oxide (or) stannic oxide 5 to 10% which have been sintered and ground to reduce the reactivity of the zinc oxide and adjust Working time
Bismuth oxide, alumina is also added in small proportions. Stannous fluoride in very small quantity (It act as a flux) 1. To modify setting time 2. To improve manipulative properties
The fluoride is released in negligible amount, hardly 15 to 20 % of the mount released from glass ionomer and silico phosphate cements. The water settable cements contain 15% to 18% poly acrylic acid coated on the powder particles
LIQUID Consist of aqueous solution of poly-acrylic acid or co-polymer of acrylic acid used c unsaturated carboxylic acid such as itaconic acid about 32 % 42 %.
17
The cement may be called as polyalkenoic cement, as these cement are based on solutions of polymers. The molecular weight of poly-acrylic acid is 28,000 to 50,000 the viscosity of the liquid can be controlled by 1. Varying the molecular weight of the polymer 2. By adjusting pH by adding sodium hydroxide
To stabilize the liquid, to increase the setting time and working time it conic or tartaric acids are added Ammonium sulphate is added as activator Isopropyl
alcohol
to
help
in
chain
transfer
during
polymerization
Liquid should be kept in stoppered bottle, as loss of water the liquid will lower its pH and slow down the setting.
Liquid that
appears cloudy after water loss should be discarded SETTING REACTION When we mix the powder and liquid ↓ Surface of the powder particles are dissolved by the acid present in the liquid ↓ Results in release of zinc, magnesium, strontium ions ↓
18 The ions are bind to the
polymer chain via the carboxylic groups ↓
And ions also react with carboxyl groups of adjacent poly acid (CHAIN) ↓ In this way cross – linked salt is formed as the cement sets ↓ The set cement is a zinc poly acrylate ionic gel amorphous matrix in which un-reacted zinc oxide particles are suspended
The fresh mobile cement paste wets and adheres to the enamel apatite surface by hydrogen bonds provided by free carboxylic acid groups. As the cement reaction precedes most of these hydrogen bonds are replaced by metal ions to give metal ion bridges that provide adhesion of the cement to the enamel.
Zinc poly carboxylate cement work developed by Dennis smith in 1968 in an effort to produce cement that there is some evidence that the cement may also bond to the collagen of the dentin. Colleges contain some branches chains that terminates in 1.carboxylate groups and 2. Other that terminates in amino groups The carboxylate group can link to the cement mass by metal ion bridging, while the amino group link to the cement mass by hydrogen bridges. During setting the cement heat is given out since the reaction
19 is exothermic. The bond strength of this cement to enamel is 3.45 Mpa to 13.1 Mpa and to dentin is 2.07 of Mpa. SETTING TIME DEPENDS ON (A)
POWDER This is generally prepared commercially by heating the ingredients to above 1000째C, causing granulation, of sintering process subsequently the solid is ground to very fine powder. The rate of reaction of powder with the liquid depends on 1. The temperature, to which it is heated, A higher temperature reduces the viscosity 2. The particle size similarly it reacts more quickly since a greater surface area is exposed to the liquid.
(B)
MANIPULATIVE VARIABLE
Five factors will accelerate the setting reaction 1. High Powder liquid ratio 2. Fast rate of addition of powder is liquid 3. Presence of moisture 4. Higher temperature
(C)
TARTARIC
REACTION
ACID
ADDED
TO CONTROL
THE
SETTING
20
COMPARISON OF ZINC PHOSPHATE AND POLY CARBOXYLATE CEMENT Zinc S.No
Zinc Poly
PROPERTIES
1
Phosphate Adhesive bond with the calcified Absent
Carboxylate Present
2
tooth structure Viscosity of the mix with same Low
High
3 4
powder liquid ratio Working time Longer Shorter To increase working time, powder All can be Only liquid liquid and glass slab, spatula can cooled
cannot
5 6 7 8 9 10
be cooled Compressive strength Tensile strength Modulus of elasticity Solubility in water pH of liquid On mixing powder
cooled More More Less Very low 1.5 Quickly
11 12 13
increases Size of acid molecule Small Pulpal irritation More Film thickness, viscosity of liquid, Less dimensional stability
liquid
Less Less More Low 1.8 pH Slowly
Large Very less More
be
21 WORKING TIME AND SETTING TIME Working time of poly carboxylate cement is half than zinc phosphate cements as with zinc phosphate cement lowering the temperature] increases working time. But with zinc poly carboxylate cement only the powder should be cooled as cooling of glass slab pr liquid makes the liquid thicker, which makes mixing difficult. Setting time is between 7 to 9 minutes.
PROPERTIES Compressive strength Is inferior to zinc phosphate cement Compressive strength is around 7250 psi studies shown that this compressive strength is satisfactory to good fit. After 24 hours compressive strength of poly acrylate cements for luting is 8300- 14400psi, which is much lower, then zinc phosphate cement, which is a value of 14000 – 19000 psi. However tensile strength is about 40% higher than zinc phosphate cement, modulus of elastically about 1/3 that of zinc phosphate cement. Strength poly acrylate cement appears to be less sensitive than zinc phosphate cement to small fluctuation in powder liquid ratio.
Solubility and Disintigration Poly carboxylate cements or less soluble in water but solubility increases when exposed to organic acids, with a pH of 4.5 or less.
22 Film Thickness Clinically a film thickness of 25µm or less is observed It meets the requirements that has been establish for the cementation of precision fillings and castings. Although poly carboxylate mix prepared with correct powder liquid ratio appears to be very thick and viscous as compare with corresponding mixes.
Bond strength ♦ Optimum bonding however requires clean tooth surface. ♦ Sand blasting (or) electrolytic etching of the gold alloy surface is necessary to achieve optimum bonding. Failure usually occurs cohesively or at the cement metal interface.
Adhesive bonding can be attained by electroplating the casting with Sn (Tin) and there by obtain a bond through the reaction of carboxyl group in the cement all the Tin.
Dimensional Stability It shows linear contraction when setting at 37°c to the amount of contraction various from 1% for wet specimen at 1 day to 6% for a dry specimen at 14 days.
23 VISCOSITY Viscosity after couple of minutes is less than that Zinc phosphate cement even though viscosity of poly carboxylate was initially higher. The freshly mixed poly carboxylate has the property of being pseudo plastic and show shear thinning on mixing. Cement will react 80% of its final strength with in 1 hour.
MANIPULATION
Powder liquid ratio required to produce a cement of suitable consistency vary among brands. But generally 1.54g parts of power in 1g part of liquid by weight cement mixed with water has a powder liquid ratio of 5:1.
MIXING
Mixing should be done on thick glass slab and not on paper pads, because glass slab maintains coolness for longer time. The liquid should not be refrigerated and should dispended before mixing. Because
liquid
looses
water
in
the
dry
atmosphere
rapidly.
Refrigeration and loosing water both makes liquid more viscous, which produce weak cement.
24 DURING MIXING
Half of the powder should be incorporated at once and mixer quickly followed by rest, only powder should be refrigerated before mixing by which the working time can increase to 8 to 10 min required for multi unit bridges without reducing the strength.
Glass lab and spatula should be cooled to about 4째C but not below the due point.
Cement should be mixed within 30 to 60
seconds.
USES 1. Luting permanent restoration to tooth. 2. Providing bases. 3. Cementing bands and brackets directly to enamel in orthodontic treatment. 4. Root canal filling in endodontics.
BIOCOMPATIBILITY
Average mix of the poly carboxylate cement after 2 minutes of the mixing have pH of 3.45, which is the lowest acidity among the all cements. It is used for luting and bonding. The acidity very quickly declines and causes lesser irritation to the pulp.
25 The pH after 10 minutes of the mixing is 4.5. After 1 hour it is 5 to 10 and after 24 hours 6.00. It is minimally irritating to the pulp, because of large dimension of the poly acrylic acid molecules an their attachment to the main bulk of the cement mass.
The acid has low
diffusion mobility in to the underlying dentin due to its immediate complexing
with
the
dentinal
proteinacious components.
fluoride,
calcium,
and
some
The complex product prevents further
penetration of acid.
The effect of poly carboxylate cement at different effective depths. ♦ Whenever pulp dentin organ has 1mm effective depth (or) greater, a healthy reparative reaction can be expected. ♦ Whenever the material comes in contact with the pulp a destructive reaction with definitely be initiated. ♦ In vivo studies shows that poly acrylic causes pulpal response similar to that causes by zincoxide eugenol with a slight to moderate response after 3 days and only mild, chronic inflammation after 5 weeks. ♦ Reparative dentin formation is minimal with these cements and thus they are recommended only in cavities with intact dentin in the floors of the cavity preparation.
26 ACIDITY Zinc poly acrylate cements are slightly more acidic than zinc phosphate cement when first mixed, but the acid is only weekly dissociated, and penetration of the high molecular weight polymer molecules toward pulp tissue is minimal.
Histological reactions to
poly acrylate cements appears to be similar those of zincoxide eugenol cements. Although the production of reparative dentin under the poly acrylate cement is more evident.