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LABORATORY WAXES Stephen C. Bayne University of Michigan School of Dentistry Ann Arbor, Michigan sbayne@umich.edu Use of WAXES in dentistry are almost exclusively confined to dental laboratory settings.

OVERVIEW OF DENTAL WAXES 1. Definition of dental wax = thermoplastic molding material that is solid at room temperature. 2. General Composition of Waxes: a. BASE Wax: (1) Hydrocarbon [eg, PARAFFIN] or ester types; (2) High or low MW b. MODIFIER Waxes: (1) Hydrocarbon or ester types; (2) High or low MW Technically, 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:

BASE AND MODIFIER WAXES Name: Origin: Composition: Melting (C) Density (20C) PARAFFIN Mineral Hydrocarbon mixture 50-57 0.90 CERESIN Complex hydrocarbons 61-78 0.91-0.92 BEESWAX Animal Ester mixture 62-65 0.95-0.96 CANDELILLA Plant C21 hydrocarbons 68-70 0.95-0.99 CARNAUBA Hydrocarbon, Ester, Fatty Acid 82-86 0.99-0.999 GUM DAMMAR Aromatic resin ca 120 1.040-1.120 ROSIN Aromatic resin acid 100-150 1.08 PARAFFIN 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

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.

INLAY WAX 1. Overview: a. Objective: Pattern material to accurately represent desired 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 temperatures 2. Inlay Wax Composition: a. 60% Paraffin Wax = BASE Wax b. 25% Carnuba Wax = MODIFIER Wax c. 10% Ceresin = MODIFIER Wax d. 5% Beeswax = MODIFIER Wax e. <1% Colorants = COLORANT Let’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%).

Physical Properties – Melting Range INLAY WAX Physical Properties – Melting Range COMPOSITION (%) TEMPERATURE (C) Paraffin Carnuba 40 30 20 60 50 70 80 90 25 75 100 LIQUID LIQUID + SOLID SOLID Melting 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

Physical Properties – Thermal Expansion of Components INLAY WAX Physical Properties – Thermal Expansion of Components 25 30 40 45 35 50 1.2 1.0 0.8 0.6 0.4 0.2 TEMPERATURE (C) EXPANSION (%) KERR HARD WAX Paraffin Beeswax 250 ppm/C Carnuba During 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 (30-600 ppm/°C). [CLICK] A typical value for an inlay wax is 250-300 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

Mechanical Properties and Chemical Properties INLAY WAX Mechanical Properties and Chemical Properties Mechanical Properties: Flow < 1% Ductility = moderate Residual Stress = none Chemical Properties: Homogeneity = good Contact Angle = low Oxidation = complete There 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.

Thanks It 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.