Properties of Perspex® Cast Acrylic PROPERTIES OF PERSPEX® CAST ACRYLIC CONTENTS 1. INTRODUCTION. ........................................................................................................... 2 1.1 Limitations of Data ............................................................................................ 2 1.2 The ‘Perspex® Cast Acrylic Range .................................................................. 2 2. GENERAL PROPERTIES ............................................................................................. 3 2.1 Clarity………………………………………………………………………… ........... 3 2.2 Weathering resistance ...................................................................................... 3 2.3 Thermoplastic Behaviour ................................................................................. 3 2.4 Water Absorption .............................................................................................. 3 2.5 Abrasion Resistance ........................................................................................ 3 2.6 Relative Density ............................................................................................... 3 2.7 Flammability ..................................................................................................... 5 2.8 Permeability to Gases ...................................................................................... 6 3 MECHANICAL PROPERTIES ......................................................................................... 6 3.1 Tensile Modulus and Tensile Strength ............................................................. 6 3.1.1 Tensile Modulus ........................................................................................... 7 3.1.2 Tensile Strength ............................................................................................ 7 3.2 Flexural Modulus .............................................................................................. 7 3.3 Dynamic Flexural Modulus ............................................................................... 8 3.4 Flexural Strength ............................................................................................ 11 3.5 Compressive Modulus .................................................................................... 11 3.6 Shear Strength .............................................................................................. 11 3.7 Poisson’s Ratio ....................................................................................... 11 3.8 Creep ....................................................................................................... 12 3.9 Fatigue ..................................................................................................... 12 3.10 Crazing ................................................................................................. 13 3.11 Impact Strength ..................................................................................... 14 3.12 Hardness ............................................................................................... 16 4. THERMAL PROPERTIES ............................................................................................ 17 4.1 Vicat Softening Point ...................................................................................... 17 4.2 Temperature of Deflection underload ............................................................ 17 4.3 Demoulding Temperature ............................................................................... 17 4.4 Coefficient of Thermal Expansion ................................................................. 17 5. OPTICAL PROPERTIES ............................................................................................. 19 5.1 Clear Perspex® Cast Acrylic ......................................................................... 19 5.2 Strain optical Coefficient ............................................................................... 19 5.3 Transport coloured Perspex® Cast Acrylic ................................................... 19 5.4 Translucent and Opaque coloured Perspex® Cast Acrylic ........................... 21 5.5 Whiite Opals .................................................................................................. 21 6. ELECTRICAL PROPERTIES ....................................................................................... 24 6.1 Permittivity and Loss Tangent ...................................................................... 24 6.2 Surface Resistivity ....................................................................................... 24 6.3 Electrical Strength ....................................................................................... 24 6.4 Volume Resistivity ........................................................................................ 24 7. CHEMICAL RESISTANCE .......................................................................................... 28 www.perspex.co.za PAGE 1 1. INTRODUCTION Before considering the properties of Perspex® Cast Acrylic in detail, it is necessary to examine certain overriding factors which affect the interpretation of the results of tests applied to thermoplastic materials. One of the features distinguishing thermoplastics is the effect of temperature on their mechanical properties. Broadly speaking, when thermoplastics are heated, they become mechanically weaker and softer; conversely, when cooled, they become harder, stiffer and less tough. This behaviour permits easy processing, which partly accounts for their widespread use in industry, but at the same time it introduces complications into the discussion of their mechanical properties, which must always be interpreted with reference to temperature. A further characteristic of thermoplastics is the marked dependence on their mechanical properties on the rate at which they are stressed or strained. Again, this factor must be taken into account when interpreting test data for design purposes. Although these two factors apply to all thermoplastics, the precise relationship will depend upon the particular material. 1.1 LIMITATIONS OF DATA In any instance the exact method of test, and factors such as the size and shape of the specimen, can affect the result, and therefore, throughout this handbook, details of the test methods which have been employed are given. The data are restricted to the properties of Perspex® Cast Acrylic acrylic materials as supplied, but it must be remembered that when thermoplastics are heat-formed, their molecules inevitably undergo some reorientation and this change in structure can result in changes in properties. The designer may sometimes have to take into account the possibility of a slight variation from nominally quoted thickness. Details of available sizes and dimensional tolerance are published separately. 1.2 THE PERSPEX® CAST ACRYLIC RANGE ‘Perspex’ is produced in a variety of sizes, thickness and smooth or textured surface finishes. It is manufactured as clear sheet and in extensive range of opals, transparent and opaque colours, metallic, bi-signs and fluorescent colours. Apart from the standard range of clear and coloured sheet, special grades of Perspex® Cast Acrylic are available to provide a spectrum of desirable properties to meet the requirements of particular specialist applications. These grades include: • Cross-linked grades for sanitary ware such as baths, shower trays and vanity basins. • A special grade for kitchen sinks, ‘Karran’™, is available. • A grade suitable for applications where maximum light transmission in the UV band in required, such as in sun beds. • Easy shaping grades for higher definition mouldings. • A UV absorbing grade particularly suitable for use in conjunction with mercury vapour street lamps. • A grade combining maximum protection from UV radiation with excellent transparency, invaluable in ensuring the minimum risk of degradation to art treasures or permanent display in museums and art galleries. 2. GENERAL PROPERTIES 2.1 CLARITY One of the outstanding properties of Perspex® Cast Acrylic materials in clear form is their very high light transmission, combining high surface finish with complete absence of colour. These characteristics have made it possible to produce a very wide colour range, including highly efficient opals, transparent and opaque colours. The incorporation of dyestuffs and pigments normally has no effect on the mechanical, machining and shaping properties of www.perspex.co.za PAGE 2 Perspex® Cast Acrylic, but any anomalies of behaviour arising from pigmentation are noted in the text. 2.2 WEATHERING RESISTANCE The resistance of Perspex® Cast Acrylic to outdoor exposure is also outstanding, and in this respect it is markedly superior to other thermoplastics. After many years exposure, even under tropical conditions, the degree of colour change of both clear and coloured materials in extremely small. 2.3 THERMOPLASTIC BEHAVIOUR Perspex® Cast Acrylic does not have a sharp melting point but softens gradually as the temperature is increased. At a temperature of 145 to 165 ºC it is sufficiently rubberlike to be shaped easily and simply (see Thermal properties section). Because it is a true thermoplastic, Perspex® Cast Acrylic retains the property of softening on heating. This is true even after shaping. When the temperature of a shaping is raised above a certain critical level, the residual stress in the material is sufficient to cause demoulding and the material exhibits such a degree of ‘plastic memory’ as to cause it to revert in time to its original form. Provided however, that the temperature does not rise above 80oC the shaping will remain stable indefinitely. When Perspex® Cast Acrylic is first heated to its shaping temperature, it will shrink approximately 2% in both length and breadth, this shrinkage being accompanied by and increase in thickness sufficient to maintain the total volume approximately constant. 2.4 WATER ABSORPTION Perspex® Cast Acrylic has low water absorption, as shown in Figure 1. Although the equilibrium water content is small, its effect on dimensions may not be negligible, as shown in Figure 2 and absorbed water may have a slight effect on mechanical properties, acting to some extent as a plasticizer. The rate of absorption is slow and Figure 3 shows the behaviour of samples stored at 20oC under conditions of 60%, 80% and 100% relative humidity, starting at two different water contents. The water content of Perspex® Cast Acrylic, as supplied is in the range 0, 5 – 0, 8% by weight. 2.5 ABRASION RESISTANCE The abrasion resistance of Perspex® Cast Acrylic is roughly comparable with that of aluminium, but because the material is indented rather than removed, the resultant optical effect is rarely noticeable in service. For example, street lighting lanterns, after many years’ operations in heavily industrialized districts showed no deterioration in efficiency, although during this period they have inevitably been subjected to abrasion by wind-borne dust and repeated cleaning. 2.6 RELATIVE DENSITY The low relative density of Perspex® Cast Acrylic, 1,19, enables large components to be made which are sufficiently strong to be self-supporting and yet light in weight. www.perspex.co.za PAGE 3 Figure 1 Water Absorption: Saturation values for 2mm sheet Figure 2 Effect of water absorption on dimensions: 6mm sheet at 20 ºC. Initial water content 0, 17% www.perspex.co.za PAGE 4 Figure 3 Amount of water absorbed. 6mm sheet at 20 ºC 2.7 FLAMABILITY Perspex® Cast Acrylic is a combustible material and naked flames should not be allowed to come into contact with it as ignition may occur (except in the case of flame polishing which is carried out under controlled conditions). If it does burn, its burning rate is similar to hard woods, but unlike wood and similar materials, burning Perspex® Cast Acrylic produces little or no smoke and does not continue to smoulder after the fire has been extinguished. “Perspex’™ have the following flammability ratings: FLAMMABILITY TEST METHOD UNITS RATING DIN 4102 - B2 UL 94 - HB BS 476 PT 7 CLASS 3 BS 2782 1970 METHOD 3mm = 28 mm / MIN 508 A 6mm = 22 www.perspex.co.za PAGE 5 2.8 PERMEABILITY TO GASES The values listed below refer to the rate of diffusion of gases at 50oC through thin films of polymethyl methacrylate. It should be noted the permeability values can vary with temperature and test method, and figures obtained on thin films may not be accurate for sheet of commercial thickness. PERMEABILITY: SI UNITS m3 (STP) GAS m/m2 sPa Carbon dioxide 5.04 x 10-16 Helium 14.1 x 10-16 Nitrogen 0.53 x 10-16 Oxygen 0.86 x 10-16 3. MECHANICAL PROPERTIES It has already been emphasized that the mechanical properties of Perspex® Cast Acrylic depend markedly on the temperature at which they are measured, the rate at which the Perspex® Cast Acrylic is stressed or strained, and to a lesser extent on the presence of absorbed water, which tends to act as a plasticizer, but this last effect can usually be ignored. When considering data on the mechanical properties of Perspex® Cast Acrylic, it is therefore necessary to supplement the information conventionally quoted for the strength of materials (i.e. Data obtained at one temperature and one straining rate) by families of curves obtained at different temperatures and rates. A detailed discussion of mechanical properties is given in Ref 1. Values for the individual mechanical properties determined under conditions of short term testing are listed in Table 1. Although useful for comparison, they should not be used for design purposes. Mechanical design data for Perspex® Cast Acrylic at 20oC are given in Table 2. Figures 4 and 13 indicate the effect of temperature on design stress and modulus. 3.1 TENSILE MODULUS AND TENSILE STRENGTH A test specimen is extended at a constant rate and the load and extension are measured simultaneously; an extension gauge is normally attached to the gauge length for measurement of the modulus. Table 1 www.perspex.co.za PAGE 6 Table 2 3.1.1 TENSILE MODULUS The stress / strain curve of Perspex® Cast Acrylic is not linear and is sensitive to both temperature and strain rate. For this reason, for most engineering design purposes, the idea of a secant modulus at a stated strain (1% secant modulus = stress at 1% elongation divided by 0,01) is more useful than that of a tangent modulus. The results quoted in Figure 5 cover a wide range of temperatures and straining rates. For purposes of comparison, the dynamic modulus / temperature curve has also been included in Figure 5. The dynamic modulus is measured by a vibrational method (described under ‘Dynamic flexural modulus’) at a strain rate of the order of 1000 times faster than that used for the determination of the 1% secant modulus. This comparison is fair, since, although the flexural mode involved compression and tension on alternate sides of vibrating specimen, it is shown in the section dealing with compression that that the modulus in compression at low strains (tangent at the origin) is the same as that in tension. Because time enters into the deformation / load characteristics of Perspex® Cast Acrylic, the long-term behaviour of a stressed structural member cannot finally be determined without knowledge of the creep characteristics of ‘Perspex’ ™. 3.1.2 TENSILE STRENGTH The definition of tensile strength adopted for the present purposes is the load at failure divided by the original cross-sectional area. Failure is said to have occurred either when the specimen under test breaks below the yield point (Figure 6a) or when the specimen yields www.perspex.co.za PAGE 7 (Figure 6b). Beyond the yield point, extension proceeds under decreasing load and the material is evidently structural useless. Perspex® Cast Acrylic exhibits failure of type A at temperatures below 25 ºC but above this temperature, type B failure becomes more frequent. Tensile strength is also dependent on strain rate and temperature. Figure 7 shows the effect of strain rate on the tensile strength and on the 1% secant modulus at 20 ºC, and Figure 8 shows the effect of temperature on tensile strength at a rate of 0,044% per second. Details of time to failure under static load are given in Figure 9. 3.2 FLEXURAL MODULUS Modulus of elasticity has been measured in flexure. The method is to load the centre of a supported beam and note the displacement after one minute. The strain in tension on the convex side of the beam should not exceed 0, 25%. Figure 10 indicates the change of modulus with temperature using this method. 3.3 DYNAMIC FLEXURAL MODULUS A dynamic method of measuring flexural modulus is now widely employed and is described in Ref.2. Briefly, from the dimensions and resonant frequency of a small vibrating cantilever specimen, the modulus can be calculated. The experiment also yields information relating to the resilience of the material which is quoted as ‘loss angle’ or ‘percentage losses’. Near the softening point of Perspex® Cast Acrylic, as it passes from the “glassy” to the “rubbery” state, the losses increase rapidly and pass through a peak. This can be seen in Figure 11 which also shows the dependence of modulus and losses on frequency (rate). Figure 5 Tensile modulus as a Function of Temperature: Comparison with Dynamic Flexural Modulus www.perspex.co.za PAGE 8 Figure 6 Stress / Strain Relationship Figure 7 Effect of strain rate upon tensile strength and 1% secant modulus at 20 ºC Figure 8 www.perspex.co.za PAGE 9