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Closed cellular rubber and plastics provide excellent service in a wide range of applications from simple air, dust and moisture seals, all the way to sophisticated thermal protection, high temperature engineered situations, automotive, aerospace and a broad range of applications in between.
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A thorough understanding of both the application and the required performance is necessary as well as a similar knowledge of the various types of cellular materials available and their property strengths and limitations. This document will assist, in general terms, in the matching of an application with a product based on some general assumptions of the properties of a specific material. In some cases, reference to industry accepted test methods will be used and expanded to paint a clearer picture of just what a specific property means.
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This implies just what it sounds like, how dense? or how heavy the material is? More specifically it means that a cube of material 12 inches’ square on all sides (1 cubic foot) will weigh a certain amount. What this means in a real world application is low density materials will be lighter and in most cases softer, (compression deflection, durometer). If pressure is applied to a specimen of a light density product of a particular size it will be compressed more than a similar higher density product. This is not 100% true in all cases, particularly in modified low density plastic/rubber blends. It is a design consideration in weight sensitive applications such as automobiles and aircraft.
This property is specific to the measured and perceived firmness or compressibility as mentioned in the density section. The values given indicate what pressure in pounds it would take to compress a 1″ x 1″ square of material to of its thickness (25%) the value is stated in PSI (pounds per square inch) and is an accepted method of stating comparative firmness.
This is a small scale indicator once again of a material’s firmness, its value is stated as an arbitrary value and can be equated to the firmness one might feel by pinching a material and perceiving the different firmness of products. Its major value is in its simplicity and may be used in applications where more exacting dynamic properties are not needed. In a general and broad ranges, it may correlate to compression deflection. Sometimes referred to as shore durometer, the durometer test instrument is a small spring loaded instrument which allows quick relative surface hardness measurements. Type 00 is used for very soft materials. There are other instruments used for measuring the firmness produced (eg: type D and A).
This is a property which is best described as one that indicates a material’s relative ability to recover from being compressed for an extended time. The lower values indicate a material with superior properties. This would be of concern in applications where a seal would be subjected to repeated opening and closing. Recovery would be necessary to maintain seal integrity? On the other hand, in an application where a seal would be mechanically held in position for an extended period or even the life of an assembly, a material with less of a recovery ability (higher compression set) would be acceptable. As a rule of thumb, closed cell materials of low density and low compression deflection will usually exhibit higher compression set.
This property is an indicator of the strength of a material. Imagine a strip of cellular material that is 1? thick and 1? wide and 2 feet long. You pull on each end until the strip breaks. The amount of force in pounds is how strong it is and the value is stated as PSI. It is generally performed on precisely prepared specimens and serves as an indicator of a products integrity to with stand handling, stretching etc. The higher the tensile? the stronger the product.
This property goes hand and hand with the tensile strength. It is an indicator of what extent a material can be stretched (elongated) without it breaking or tearing. This would be of great concern in stretching of seals over a large end of an assembly or an application which might see intermittent movement. The higher the value indicates the material may be stretched to that % of its original free dimension without failure.
This is a key property of closed cellular materials. This is the ability to resist absorption making these materials excellent as a sealant from moisture and water. The value stated is an indicator of how much water a specific sized specimen will absorb and it is stated as a percent (%) of the materials weight. The lower the value indicates the greater water penetration resistance.
Materials with absorption of 5% or less (materials 10 PCF or greater) and 10% (materials less than 10 PCF) are considered to be closed cell. Open cell sponges are materials with exploded cells absorbing much higher levels of water and are not suitable in certain sealing applications.
This proportion serves as a standard indicator of a materials ability to resist hydrocarbon based fluids such as gasoline, oils, alcohols and other solvent type fluids. It is difficult to indicate a materials resistance to all possible media, but this property value can be valuable when selecting from the general properties of various materials. The lower the absorption value reported is an indicator of petroleum resistance.
Linear shrinkage indicates the amount of contraction in cellular material as it naturally ages. It predicts how much oversize to the cut seal, assuring it will not contract or compromise the seal integrity. Equally important in some applications is the product must fall within the limits of a useable size. It is stated as a percent (%) of its original dimension. Lower values indicate better performance.
Ozone (O3) is a highly active form of oxygen and is present in the atmosphere naturally. It is responsible for natural degradation of elastomeric products and manifests itself as aging cracks seen in rubber products which have been outdoors or in extended service. This is a good indicator of how well the product will perform (lifetime) in an outdoor or otherwise high ozone atmosphere. The lower the rating the better the products weatherability is implied, 0(zero) indicating the degree of deterioration under controlled test conditions.
Note: This should not be confused with UV(ultraviolet) and IR (infrared exposure).
This property is how well or how poorly the material conducts electricity. A poor conductor might be considered as an insulator, the value is stated as? K value, an accepted method of stating a materials comparative thermal insulating value. The lower the K? value, the better the insulator (sometimes insulation values are stated as R? values).
The property of how springy? or bouncy? a material is called Resilience. The values reported correspond to how high a controlled object will bounce when dropped onto the cellular material. It is stated as a percent of the height from which it is dropped, and how well a product might recover or conform to irregular surfaces or bounce back from multiple compressions.
It is a general indicator that a product contains a high level of rubber instead of filler and will have a higher percentage of resilience. The higher the value? the more resilient the product.
These values help in selection of a product where the environment is known. For example, a freezer, oven door, or an aerospace application where temperature extremes are expected. These type requirements might require further investigation of end usage. The value listed is the lowest temperature at which the material will remain flexible and the highest temperature without affecting performance.
These are the accepted ratings for automotive (FMVSS), consumer goods (UL), and aerospace (FAR), and offer comparative ratings to assist in selection when flammability and listing with a regulatory agency is an engineering requirement. It is recommended it be fully investigated, even though the guide can assist in the choosing of the candidate product to explore suitability.
It is important to note that these ratings are based on small comparative lab tests and do not imply how a material will perform in actual fire conditions.