An Ode to Speargun Rubber
Below are a few things to consider when selecting your own Speargun Rubber:
Elasticity 101:
Elasticity is the property of an object or material which causes it to be restored to its original shape after distortion. It is said to be more elastic if it restores itself more precisely to its original configuration. A speargun rubber band is easy to stretch, and snaps back to approximately its original length when released. However, scientifically speaking, it is not as “elastic” as a piece of piano wire for example. The piano wire is harder to stretch, but would be said to be more elastic than the rubber band because of the precision of its return to its original length. (A piano string can be struck hundreds of time without stretching enough to go noticeably out of tune).
Hookes Law:
A spring is a classic example of an elastic object. When stretched, it exerts a restoring force which tends to bring it back to its original length. It is this restoring force that causes a speargun to shoot out its spear. This restoring force is generally proportional to the amount of stretch, as described by Hooke's Law. Hooke's Law states that one of the properties of elasticity is that it takes about twice as much force to stretch a spring twice as far, such that there is a linear dependence of displacement upon the stretching force. (This is also the reason why fishing scales work).
Elastic Properties:
Tensile Strength reflects how much force can be applied to the rubber before it breaks. This is measured in various units and is expressed as a force per unit area. In a standard dumbbell/strength type test, a piece of known cross sectional area is stretched until it breaks. The force required to do so is recorded and expressed as force per unit area. This is slightly different to Tear Strength, which is measured as the force required to tear a standard test piece of rubber. In the stand tear test, pieces are designed to produce weak points where a tear is initiated.
Elongation refers to how far a rubber can be stretched before it snaps. Elongation is defined as the length at breaking point expressed as a percentage of its original length at rest. Therefore, if a rubber reaches twice its length before breaking its elongation is 100%.
Modulus is how much force is required to stretch the rubber. It is measured as the force per unit area required to extend a rubber to a stated percentage of its original length e.g. 100%, 200% or 300% (modulus in rubber is non-linear). It is often written as M100 = 3.0MPa (i.e. modulus at 100% = 3.0MPa). It is also an indication of how much force it will apply to the spear after release.
Permanent Set is the measure that reflects how efficiently a piece of rubber returns back to its original length after it has been stretched. A standard test piece of rubber with a known length is stretched by a stated percentage for a period of time and is then released. After recovery the length is measured and the change in length, the “unrecovered length,” is expressed as a percentage of the original length.
Aging. Heat ageing is widely used as a method of evaluating long term ageing properties. Hardness and dumbbell type tensile test pieces are placed in an air circulating oven for a specific period of time at a given temperature (for example 14 days at 60ºC). The properties of the rubber are then tested and compared with the properties before ageing. The percentage retained for each property is recorded to give the rubber an aging score.
