Using 'Perspex' for Jacuzzi Manufacturing
'Perspex' is established as the most widely used material for the construction of spa and bath shells and has a long history of development to suit the needs of spa manufacturers. It has excellent resistance to degradation in varying climatic conditions, combined with suitable mechanical and physical properties. These advantages account for its reputation as a material second to none for durable and attractive spas.
PROPERTIES
Weathering resistance The resistance of' ‘Perspex' to outdoor exposure is outstanding and in this respect it is superior to other thermoplastics. After many years under tropical conditions the degree of colour change of both clear and coloured materials is very small. The best stability is developed if the 'Perspex' is shaped.
Thermoplastic Behaviour'Perspex' softens gradually as the temperature is increased above about 100C. At a temperature of 150-160C it is sufficiently rubber-like to be shaped easily. Because it is a true thermoplastic, it retains the property of softening on re-heating even after shaping. When the temperature of a shaping is raised above a particular level the material demoulds and will revert in time to its original form. Provided that the temperature does not rise above 80C the shaping will remain stable indefinitely. When 'Perspex' is first heated to its shaping temperature it will shrink approximately 2% in both length and breadth, this shrinkage being accompanied by an increase in thickness sufficient to maintain the total volume constant.
Water absorption'Perspex' has a low water absorption but although the equilibrium water content is small, its effect on dimensions may not be negligible and absorbed water may have a slight effect on mechanical properties, acting to some extent as a plasticiser. The water content of' ‘Perspex' as supplied is in the range 0.5-0.8% by weight.
Abrasion resistanceThe abrasion resistance of' ‘Perspex' is roughly comparable with that of aluminium but because the material is indented rather than removed, the resultant visual effect is rarely noticed in service.
Relative densityThe low relative density of' ‘Perspex', 1.19, enables large components to be made which are sufficiently strong to be self-supporting and yet light in weight.
FlammabilityAlthough 'Perspex' will burn, it is difficult to ignite except by an open flame playing on an edge. A typical rate of burning measured by BS 2782 method 508A using 6 mm clear 'Perspex' is 21 mm/min. In the surface spread of flame test specified in BS 476 Part 7 it receives a Class 3 grading, which includes hard woods such as mahogany. When tested to BS 476 Part 6 fire propagation tests it fails under both indices. Unlike wood and similar materials, burning 'Perspex' does not produce smoke, nor does it continue to smoulder after the fire has been extinguished.
CreepEngineers are familiar with the concept of creep in metals-i.e. a non-recoverable deformation which increases with time under static load. 'Perspex' shows a somewhat analogous behaviour but, in contrast to metals, the strain, which is sensitive to temperature and stress level, is wholly recoverable at a rate depending on temperature.
CrazingIf the surf ace tensile stress of a 'Perspex' component exceeds a critical value, the phenomenon known as crazing may occur. Crazing consists of multiple hair -line surface cracks which, being very narrow and relatively deep and sharp at the bottom, act as notches and may cause a serious drop in mechanical strength. In common with other properties, crazing is a time-dependent phenomenon, and so it may occur at lower stresses after longer periods of loading.
Impact Various arbitrary impact tests have been devised from time to time, but the strength amount of information they give is limited. The impact strength of a carefully prepared specimen determined by an arbitrary test does not necessarily give an accurate indication of the resistance to breakage of a fabricated component in service. In the design of' ‘Perspex' components, particularly those liable to be subjected to impact, sudden changes in cross-section should be avoided because they may lower the effective impact resistance of the component Hardness is another complicated property to express in a single figure. Numerous test methods have been devised and a simple guide for plastics is the pencil scratch method using a set of graphite pencils covering the hardness range from 6B to 9H. 'Perspex', which is one of the hardest of the thermoplastics, is scratched by a 9H pencil only.