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					Tensile Testing
Introduction This short article reviews the meaning of the tensile strength test, its relevance and shortcomings when considering contact lens polymers. The origins of the test will be briefly discussed and the difficulties associated with the sample preparation when trying to test contact lenses. Background Tensile testing is used for providing information on the strength of a wide range of materials, from a number of industrial applications. Perhaps, not surprisingly, the equipment used to carry out the test is called a tensometer. Early examples go back to the mid-1800s, when attempts were being made to quality control the materials evolving from the Industrial Revolution. Different industries developed tests and associated equipment suitable for their particular applications. Hence, the paint industry developed gloss and surface scratch tests, the building industry developed impact strength tests whilst the textile and paper industries developed tensometers for the strength testing of fibres, yarns, cloths and papers. The arrival of rubbers and, later in the 1930s, polymers, saw the development of the tensile test into these industries. Strength on stretching to break, coupled with the degree of elasticity, were clearly mechanical properties relevant to such materials. Early tensometers relied on simple motors to move clamping jaws apart at predetermined rates, with weights to provide the load and pendulum systems to record the actual load to break. Modern instruments now use electronic load cells to record the stress being applied to the sample, with chart recorders and, more recently, computers, to provide the stress versus strain data, required for data analysis. Tensile Information Essentially, the results of a tensile test give a plot of the load (force) required to cause gradual elongation of the test sample, up to ultimate break. The information is normally presented on a graph of stress (force per unit cross sectional area of the sample) versus strain (the ratio of the actual elongation and the original length, often quoted as a percentage). The total area under the stress/strain line (or curve) is the well known Young’s Modulus. The units of Young’s Modulus are those of stress - since the units of strain are dimensionless - and are therefore those of a force divided by an area (g.cm-2, kg.cm-2 etc). Depending upon the origin of the particular test and the vogue at the time, the units used have been prone to change. When comparing results of different materials, it is important to be sure that the units used are the same, otherwise

All products are produced to BS EN ISO 9001:2000 and EN ISO 13485:2003 Audited by the EU Notified Body SGS UK Ltd

convert if necessary. Since the days when every school child measured Young’s Modulus by hanging weights on the end of a steel wire, the units used as a matter of convention have changed three times. Currently, the scientific fraternity favour the MKS System (metre, kilogram, second) and so the units of force are called Newtons and hence Newtons per square metre (i.e. Pascals or MegaPascals). The most frequently quoted number, for comparing materials, is the load at break (or tensile strength). Also useful is the elongation at break and the initial modulus - a measure of the force required to cause the initial deformation or stretch. Hence, as an example, the stress/strain curves in figure 1 are clearly different and illustrate why the tensile strength alone does not give the full information we require for these two materials. Hence, although the tensile strengths are more-or-less the same, the materials are clearly different in behaviour. Material A rises quickly to its maximum load at break with little or no extension (high initial modulus) whilst material B creeps relatively slowly to its load at break with a resultant high extension (low initial modulus). Hence, material A is relatively stiff or brittle whilst B is softer and rubbery. Typically, A could be a brittle polymer (such as a rigid gas permeable type) and B could be a strong elastomer (or hydrogel type). Contact Lenses/Sample Size The use of the tensile strength test for a contact lens is clearly not all that relevant, in view of the basic origins of the tensile test. Contact lenses are not pulled, as such in use, but rather flexed, rolled, pinched, scratched or torn from the edge. No specific strength tests have been developed for contact lens materials. Out of all the ISO Standards on contact lenses and materials, none appertain to strength in any way. It would seem the tensile strength test has been used and quoted for contact lens materials, more by default than for any other real scientific rationale. The equipment is well developed and available, and so contact lenses were tested on them as they were there. Normally, tensile tests are carried out on long/thin samples relevant to the application. For polymer films, a dumb-bell shape would be cut from the sheet (or film) and the wider ends clamped in the jaws of the tensometer. Clearly, very often, when the only sample may be a contact lens itself, this presents difficulties. It is possible to cut thin strips from the centre of a hydrogel lens and it is possible to devise clamps for the whole rigid gas permeable lens. This is not ideal and great care and dexterity is needed by the operator. It is very easy to generate meaningless results when handling contact lenses. A good approach is to prepare polymer membranes under the same polymerisation conditions as the contact lens button (or rod) and then good tensile samples can be prepared from the membrane (or film). However, this

All products are produced to BS EN ISO 9001:2000 and EN ISO 13485:2003 Audited by the EU Notified Body SGS UK Ltd

approach has not always been adopted and some quoted tensile strength results may not stand scrutiny. Meaning of Results With all tensile testing, the conditions of the test and the size of the sample can dramatically affect the stress/strain curve. Hence, different industries have standardised on sample sizes, speed of extension, clamping systems etc, and these are available as ISO standards or ASTMs. There are no such standards for contact lenses and normal sample sizes for other polymer applications are frequently not possible. Consequently, results need to be interpreted with caution and it makes comparison of different sets of data very difficult. In general (and certainly within Vista) we do not regard the tensile test as a routine QC test. The natural variations of the test, coupled with the operator dependence, render it inappropriate for routine testing. The same thing can be said, incidentally, for oxygen permeability measurements.

Stress

Fig 1

Fig 2

Stress

Strain

Strain

All products are produced to BS EN ISO 9001:2000 and EN ISO 13485:2003 Audited by the EU Notified Body SGS UK Ltd


				
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