Presentation on theme: "Copyright 2008, Dr. Stephen Bayne."— Presentation transcript:
1 Copyright 2008, Dr. Stephen Bayne. Unless otherwise noted, the content of this course material is licensed under a Creative Commons Attribution 3.0 License.Copyright 2008, Dr. Stephen Bayne.The following information is intended to inform and educate and is not a tool for self-diagnosis or a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional. You should speak to your physician or make an appointment to be seen if you have questions or concerns about this information or your medical condition. You assume all responsibility for use and potential liability associated with any use of the material.Material contains copyrighted content, used in accordance with U.S. law. Copyright holders of content included in this material should contact with any questions, corrections, or clarifications regarding the use of content. The Regents of the University of Michigan do not license the use of third party content posted to this site unless such a license is specifically granted in connection with particular content objects. Users of content are responsible for their compliance with applicable law. See for more information.Mention of specific products in this recording solely represents the opinion of the speaker and does not represent an endorsement by the University of Michigan.Viewer discretion advised: Material may contain medical images that may be disturbing to some viewers.
2 LABORATORY WAXES Stephen C. Bayne University of Michigan School of DentistryAnn Arbor, MichiganUse of WAXES in dentistry are almost exclusively confined to dental laboratory settings.
3 OVERVIEW OF DENTAL WAXES 1. Definition of dental wax = thermoplastic molding materialthat is solid at room temperature.2. General Composition of Waxes:a. BASE Wax:(1) Hydrocarbon [eg, PARAFFIN] or ester types;(2) High or low MWb. MODIFIER Waxes:(1) Hydrocarbon or ester types;(2) High or low MWTechnically, the definition of a wax is a “thermoplastic molding material that is sold at room temperature.” By implication, heating a wax will convert it to a liquid phase and make it much more easily moldable. Not all waxes require melting to be used in indirect dental fabrication procedures.[CLICK] Waxes are composed of 3 major components – a BASE wax (that is almost always paraffin), [CLICK] MODIFIER waxes (to contribute properties such as increased hardness, stickiness, or brittleness), and [CLICK] COLORANTS (which represent only about 1% of the composition in general). There are no fillers because, in almost all instances, waxes need to be pyrolyzed at some point. Materials being pyrolyzed are being burned to the point that they melt and/or decompose into water vapor and carbon dioxide.c. COLORANT:
4 BASE AND MODIFIER WAXES Name:Origin:Composition:Melting(C)Density(20C)PARAFFINMineralHydrocarbon mixture50-570.90CERESINComplex hydrocarbons61-78BEESWAXAnimalEster mixture62-65CANDELILLAPlantC21 hydrocarbons68-70CARNAUBAHydrocarbon, Ester, Fatty Acid82-86GUM DAMMARAromatic resinca 120ROSINAromatic resin acid1.08PARAFFIN is the major component of almost all dental waxes. Materials that we call waxes are usually medium molecular weight organic materials (linear or aromatic) that are derived from mineral, animal, or plant sources. Paraffin is a linear hydrocarbon that is relatively low molecular weight (a couple dozen carbon atoms long). It is not pure because it is so difficult to isolate a single molecular weight. It is collected during the fractionation of oil into its components. Therefore, it is reported as a mixture of hydrocarbons. Its molecular weight is just high enough that at room temperature it is solid (Tm = 50-57°C). In general, as the molecular weight of wax increases, the melting temperature increases, and the density increases. That is obvious from the table above.[CLICK] A variety of modifier waxes are routinely added to paraffin to control the final properties. Ceresin and carnuba tend to increase the hardness and water resistance of wax. Beeswax increases the stickiness. Rosin increases the brittleness. These are melted together carefully to make the final wax.Notice that all of these have relatively low melting ranges (and so does each mixture). Therefore, during use it is crucial not to overheat a wax during melting while it is being manipulated or else some of the base or modifier waxes can be decomposed. That would change the overall properties of the wax.INLAY WAX = Paraffin + Carnuba + Ceresin + Beeswax + Colorants
5 CLASSIFICATION OF DENTAL WAXES There are tremendous number of dental laboratory applications which involve waxes. Waxes are chosen over synthetic polymeric alternatives because they are much less expensive.[CLICK] In general, dental waxes can be divided into 3 major classes based on their general use – (1) PATTERN waxes, (2) IMPRESSION waxes, and (3) PROCESSING waxes. [CLICK] Within each class, they can be subdivided by dental application. For example, pattern waxes can be subdivided as (a) inlay waxes, (b) casting waxes, and (c) baseplate waxes. [CLICK] Within each application category, waxes can be further subdivided on the basis of a defined set of properties (generally classified by ADA standards committees and called “types” like Type 1 or Type 2).[CLICK] Waxes are supplied in geometric shapes that are convenient for their particular dental application (e.g., sticks, rods, sheets, ropes, …). [CLICK] The COLOR assigned to each form is intended to signify the general application. However, manufacturers do not consistently apply the color coding for anything but their own products. Therefore, inlays waxes could be green, or blue, or purple.
6 INLAY WAX1. Overview:a. Objective: Pattern material to accurately representdesired mold space for inlays, onlays, and crowns.b. Requirements for Inlay Waxes:(1) Good adaptation to dies(2) Thermal stability at low temperatures(3) Complete pyrolysis at high temperatures2. Inlay Wax Composition:a. 60% Paraffin Wax = BASE Waxb. 25% Carnuba Wax = MODIFIER Waxc. 10% Ceresin = MODIFIER Waxd. 5% Beeswax = MODIFIER Waxe. <1% Colorants = COLORANTLet’s use INLAY WAX as an example for careful examination of the composition, structure, and properties of a dental wax. An inlay wax is used to make patterns for inlays, onlays, and crowns. It requires good adaptation to dies to pick up the proper size of the restorations and quality of the margins. It should have good thermal stability at low temperatures during manipulation procedures when it is being melted and cooled several times. Since it will be invested and ultimately burned out of the set investment material, it must be capable of complete and clean pyrolysis.[CLICK] A typical composition of an inlay wax is shown above. It is mostly paraffin (60%) with carnuba (25%) and ceresin (10%) added to increase the hardness. A small amount of beeswax (5%) is added so that it will stick to a die. The rest of the composition is colorant (<1%).
7 Physical Properties – Melting Range INLAY WAXPhysical Properties – Melting RangeCOMPOSITION (%)TEMPERATURE (C)ParaffinCarnuba40302060507080902575100LIQUIDLIQUID + SOLIDSOLIDMelting Onset (Solidus)Melting Completion (Liquidus)First, consider the melting properties of a simple binary mixture of paraffin and carnuba [CLICK] as representing the more complex mixture that was just described. One can actually develop a phase diagram for a wax (as shown above).The liquidus line (representing the temperature at which complete melting has occurred) [CLICK] increases quickly from 62°C as carnuba is added to the composition. The solidus line (below which the composition is entirely solid) [CLICK] is not affected much by the carnuba additions. [CLICK] Importantly, the solid+liqud range in between is quite broad (almost 40°C). To develop wax flow, the temperature only needs to be heated to a point within the solid+liquid range or up to the point of the liquidus line but not much higher. Excessive heating would cause decomposition.In most dental laboratories, wax is heated in a wax pot [CLICK] that maintains a constant but low temperature with the wax just barely melted. The alternative is to heat a wax instrument and dip it into the wax to melt it and pick up some material. However, this approach is very prone to overheating and decomposing the wax.Image and graph source: Steve Bayne, University of Michigan, 2008
8 Physical Properties – Thermal Expansion of Components INLAY WAXPhysical Properties – Thermal Expansion of Components2530404535501.21.00.80.60.40.2TEMPERATURE (C)EXPANSION (%)KERRHARDWAXParaffinBeeswax250ppm/CCarnubaDuring heating and cooling wax expands or contracts at very high rates. Compared to ceramics (1-15 ppm/°C) and metals (10-30 ppm/°C), polymers (and waxes) have very high coefficients of thermal expansion (and contraction) and over a broad range ( ppm/°C). [CLICK] A typical value for an inlay wax is ppm/°C. The rate of thermal expansion is the same as the slope of the line on the ‘expansion versus temperature’ graph shown above. A steep line has a high rate.[CLICK] Paraffin has the highest coefficient of thermal expansion. Addition of modifier waxes such as beeswax [CLICK] and carnuba wax [CLICK] decrease the rate for the overall wax (see the for KERR HARD WAX). Control of the coefficient of thermal expansion helps to decrease the susceptibility of the wax to distortion on cooling.Graph source: Steve Bayne, University of Michigan, 2008
9 Mechanical Properties and Chemical Properties INLAY WAXMechanical Properties and Chemical PropertiesMechanical Properties:Flow < 1%Ductility = moderateResidual Stress = noneChemical Properties:Homogeneity = goodContact Angle = lowOxidation = completeThere are a limited number of mechanical and chemical properties that are routinely monitored to insure that waxes have the same general qualities from manufacturer-to-manufacturer. These are listed above.Wax should be dimensionally stable once it has solidified. Therefore, the flow should be less than 1%. Wax should be capable of some plastic deformation (ductility) so that it will deform rather than fracturing. This also allows it to be carved or burnished. Some thermal stresses are developed whenever wax additions are cooled. The exterior surface tends to cool first. The molten interior slowly solidifies and contracts. This encourages distortion or flow. Hopefully the stress is relaxed and not important distortions result.[CLICK] Chemically, waxes should have the same properties throughout the solid (i.e., good homogeneity). When a wax is melted, it should WET the surface of the material to which it is being added (e.g., die) and spread easily onto it (i.e.,low contact angle). Finally, when it is pyrolyzed, the process of oxidation should completely transform it into water vapor and carbon dioxide so that no residue is left.
10 ThanksIt is critically important to recognize the limitations or potential problems of waxes and make sure these do not create problems along the way in laboratory fabrication of restorations.Thank you.