Silicone ear impression materials have two primary properties that hearing aid and hearing conservation professionals must take into account for each patient, depending on the texture and elasticity of the outer third, cartilaginous portion of the ear canal. These two oft-conflated impression material properties are the viscosity (“thickness”) of the impression material before it cures, and the durometer (“hardness”) after it cures …And even most supply houses & earmold labs conflate these two properties as well, hence this article to explain the difference, and why it is important.
There are actually several other properties of silicone impression materials, such as lubrication, shrinkage, and thixotropy; but these shouldn’t really concern the clinician, as the manufacturers all take these into account to produce good materials that easily release from the ear, and easily flow through the mixing tip when gun-type injectors are used.¹
• Viscosity is a measure of a fluid’s resistance to flow, describing the internal friction of it while moving. For example, imagine a styrofoam cup with a hole in the bottom. If we then pour honey into the cup we will find that the cup drains very slowly. That is because honey’s viscosity is large compared to other liquids’ viscosities. If we fill the same cup with water, the cup will drain much more quickly.
The SI physical unit of viscosity is the Poise, which is in pascal-seconds (Pa•s), which is equivalent to N•s/m² or kg/(m•s); and in the lab it is measured with a Brookfield viscometer, which measures the resistance of a spinning disc or drum — See images at right →
• Durometer (or more accurately, Shore durometer) is a measures of the “hardness” of a given material; and is defined as a material’s resistance to indentation. The “durometer” of the material is measured on various Shore scales: The two most common ones, using slightly different measurement systems, are the ASTM D2240 type A and type D scales. The A scale is for softer plastics, including the “silicone” (vinyl polysiloxane) used for ear impressions and earmolds, while the C and (mostly) D scales are used for harder, more rigid plastics.
• Impression material types There are two basic types of materials used to make ear impressions: The obsolete methacrylate powder-and-liquid, and RTV Silicone (room temperature vulcanizing vinyl polysiloxane). Silicone comes in two basic varieties: Two-part alkoxy condensation cure, and and two-part additive cure, with most of the material used today being of the additive cure variety. The way to tell the difference is that condensation cure materials use a small amount of hardener from a tube, while additive cure materials use an equal “A+B” mix of materials. All mixing gun materials use additive cure silicone.
• Shrinkage and deformation: All impression materials shrink after curing to some degree; however while silicone shrinkage is for all intents and purposes nil, the same cannot be said for methacrylate, as it shrinks 5-10%, making it difficult for the earmold lab to guess how much wax to add to get a comfortable fit. In addition, powder-and-liquid impressions must be trimmed, sprayed with clear coating, and glued to the bottom of the impression box before shipping to the lab, as they are subject to deformation if they toss about in the box. Worse, in warm weather, the canal portion can droop, causing the impressions to be unusable.
The Hearing Blog recommends against using methacrylate “powder-and-liquid” impression materials for any patient ear impression: If your audiologist pulls out the pill bottle of powder and the vial of liquid, get up and walk out: If s/he is cutting this corner, how many other corners are also being cut?
NOTE: This article is short a couple of photos; however we needed to publish this for the AudiologyOnline class on IIC’s on 6 August 2013
1) Thixotropy is a third property of silicone impression materials that manufacturers take into account in their formulations; and this goes to why not all silicone materials lend themselves to injector gun mixing tips. Thixotropic liquids become less viscous over time when shaken, agitated, or otherwise subject to shear stresses. This is an important property for materials that are injected through a mixing tip, as without this temporary “loosening up” it would be very difficult to pull the trigger on the gun.
Another example of a thixotropic liquid is nail lacquer: While in the bottle for weeks on end, the tendency is for the pigments — Especially the white titanium dioxide used for “cream” shades — to settle out, looking ugly in the bottle. However, if the polish were that thick while applying it to the nail, it would result in poor flow, leading to an uneven surface with brush marks. This is solved by compounding the nitrocellulose base to have thixotropic properties, where once you get the stainless steel ball inside the bottle moving, it literally “loosens up” the liquid, allowing for a smooth flow while applying. Good nail lacquer lines generally use two viscosities: A thinner base for transparent to translucent colors, and a thicker base for opaque, creme and metallic shades, preferably using two agitator balls.
2) The Rockwell hardness test scale is similar to the Durometer scale, except it is used on metals. It is defined in the ASTM E18 specifications.
- Shore® (Durometer) Hardness Testing of Plastics
- Durometer comparison chart
- ASTM D2240 – 05(2010): Standard Test Method for Rubber Property—Durometer Hardness
- ASTM E18 : Standard methods for Rockwell hardness and Rockwell superficial hardness of metallic materials