Manufacturing Process

Pre-expansion Expanded polystyrene is supplied as plastic (Polystyrene) beads in which an expanding (or blowing) agent, usually pentane, has been dissolved. In the presence of steam the thermoplastic polystyrene softens and the increasing vapour pressure of the expanding agent causes the beads to expand up to 50 times their original volume. During this stage the degree of expansion is controlled to achieve the desired density. Expanded polystyrene does not contain any ozone depleting substance and none is used in it’s manufacture - Ozone Depleting Potential is zero (ODP=0).


From the pre-expander the beads are gently transported to large hoppers for ageing. The time of ageing is set to cool and stabilise the beads and allow for infusion of air to replace the expanding agent in the cells.


After conditioning, the beads are charged into a closed mould where they are further expanded and fused together by steam heating.


The freshly moulded blocks of Isolite® are passed through temperature controlled ovens to remove moisture and the final traces of the expanding agent, and to provide blocks of constant dimensional stability and dryness.

Environment Aspects

Isolite® EPS it contains no hydro fluorocarbon (HFC) or hydro chlorofluorocarbon (HCFC) blowing agent that might cause depletion of ozone in the upper atmosphere. Ozone Depleting Potential is zero (ODP=0).

Manufactured to a Standard

Isolite® EPS is manufactured to AS 1366 Part 3-1992, Rigid Cellular Plastic Sheets for Thermal Insulation, Rigid Cellular Polystyrene, in six classes. The standard designates a colour to identify each of the six classes:

Class L:


Class M:


Class SL:


Class H:


Class S:


Class VH: 


The standard specifies the minimum physical property limits for each of the six classes and methods for determination of compliance.

Quality control

To meet with the compliance requirements of the standard, the quality control system monitors and controls each stage of the manufacturing process and assures that Isolite® conforms to AS 1366 Part 3 - 1992 within 95% confidence limits by on site testing of density and key physical properties.

Properties of Isolite®

The physical properties are primarily determined by the moulded density for well made oven cured EPS.

However, these properties will be affected by raw material and manufacturing variations, and for this reason AS1366 Part 3-1992 specifies the classes in terms of performance properties rather than density.

The standard lists Nominal Density for each class, but these densities should be regarded as a guide only as the physical properties shown in Table 1 may be achieved by EPS of other densities.

Mechanical Properties

The density dependency of the main physical properties of Isolite® can be seen in (Fig.1 to 4): Compressive strength, Cross Breaking strength (flexural strength) Tensile strength and Shear strength. Compressive Creep It is common to report only the compressive stress at 10% deformation but the latter is often taken from complete stress-strain curves as shown in (Fig.5). Although it appears to deform elastically over a range of comprehensive loads, Isolite® that has been stressed will, with the release of all stress, retain some permanent deformation.

(Fig.1 to 5) can be useful for short-term loads where some deformation is acceptable. For long term loads showing compressive creep under constant loads versus time, should be used.


It should be noted that compressive Strength in AS 1366 Part 3 1992 is a performance characteristic at 10% deformation and should not be taken as a universal design loading recommendation.

Floatation Properties

The density of Isolite® is low compared with water, with a nominal density range from 10 to 25 kg/m3 compared with water at 1000 kg/m3. The water buoyancy per cubic metre of Isolite® is determined by subtracting its kg/m3 density from 1000. The result is the weight in kilograms which a cubic metre of Isolite® can support when fully submerged in water.

Thermal Properties

The low thermal conductivity (K value) of Isolite® characterises its exceptional insulating properties as shown below.

EPS has a remarkably high R-value compared with most other insulating material used in similar applications as shown below.

Isolite® EPS gains its thermal resistance from the stabilised air trapped within its cellular structure; it contains no hydro fluorocarbon(HFC) or hydro chlorofluorocarbon (HCFC) blowing agent that might cause depletion of ozone in the upper atmosphere.

Design Thermal Properties

As AS 1366 Part 3 - 1992 is a minimum conformance standard, the thermal resistances quoted will be achieved, as a minimum, in 97.5% of cases in a statistical sample, when tested at a mean sample temperature of 23ºC.Thermal resistance varies with mean insulation temperature, where mean insulation temperature is the average of the temperature on either side of the insulation.

For design purposes the average thermal resistance is a better guide than the minimum thermal resistance. A full listing of design thermal conductivity values for each class of EPS at differing mean temperatures is shown on Table 7.

Insert Pic : Isolite_Table7

Low Temperature Operation

Isolite® does not become brittle at sub-zero temperatures. The testing of specimens at -75ºC for 48 hours demonstrates almost no loss of impact resistance compared with specimens tested at +23ºC.

Isolite® is able to withstand temperature cycling and thereby assure long-term performance without the loss of structural integrity of physical properties; core specimens taken from 20 year old freezer rooms show no deterioration. Unlike some other insulating materials, the K value of Isolite® decreases at lower average mean temperatures.

High Temperature Operation

The effect of elevated temperatures on the mechanical properties is an accelerating decline in the values shown in Fig.1 to 5 until at approximately 85ºC the so-called zero strength is reached.


Isolite® should not be continuously exposed to temperatures in excess of 80ºC as expansion and blistering may occur.

Effect of Moisture on K Value

The dimensional stability and mechanical properties of Isolite® are not affected by water, but because absorbed water will increase the K value, as with all insulating materials, care should be taken in designing insulated structures to take account of water and water vapour that may be present.

While Table 2 shows that certain amounts of water are absorbed by EPS under various conditions, Table 3 demonstrates that the loss of R-values in EPS as a result of this moisture absorption is minimal. Overseas research [Ref(i)] has revealed that the decay in thermal resistance caused by moisture is considerably less for EPS than for either extruded polystyrene foam or cellular glass. (see Fig.11)

As with other building materials care should always be taken to keep Isolite® dry before and during installation.

Water Vapour Transmission Properties

Although Isolite® has a low water vapour transmission rate it is not considered a vapour barrier. This breatheability characteristic reduces any tendency towards the formation of vapour dams. As Fig.12 shows, of all the material used for insulation purposes, EPS is one of the most resistant to the adverse affects of moisture.

In applications where the high humidity and high temperature differentials are likely a vapour barrier should be installed. Normally the vapour barrier should be installed on the warm side of the structure with the insulation as near as possible to the cold side.

Acoustic Properties

Because Isolite® has a closed cell structure, it offers only a limited absorption of airborne sound. Structure borne sounds, transmitted through such structures as walls and pipes, may be effectively isolated by the use of floating floor systems. For this type of sound insulation, Isolite® with the required dynamic stiffness can be obtained by compressing the sheets by 50 to 60 percent and then allowing them to recover to 80 to 90 per cent of their original thickness.

Chemical Properties

Isolite® is resistant to virtually all aqueous media including dilute acid and alkalis. In addition, it is resistant to water-miscible alcohols such as methanol, ethanol and i-propanol, and also to silicon oils.

Isolite® has limited resistance to paraffin oil, vegetable oils, diesel fuel and vaseline. These substances may attack the surface of Isolite® after long term contact. Isolite® is not resistant to hydrocarbons, chlorinated hydrocarbons, ketones and esters.

Paint containing thinners and solutions of synthetic adhesives naturally fall in to the same category, and this should be taken into account in any painting or bonding operation.

Anhydrous acids such as glacial acetic acid or fuming sulphuric acid destroy Isolite®.

Prolonged exposure to UV light causes yellowing and embrittlement of Isolite®, which should therefore be protected from direct outdoor exposure.

Resistance to specific reagents is given in the RMAX technical data sheet Isolite® Chemical Resistance and detailed in the table below.

Download: Isolite Chemical Resistance.pdf

Resistance to Fungi and Bacteria

Fungus attack has not been observed on Isolite®, and it does not support bacterial growth. Surface spoilage (in the form of spilt soft drink, sugar, etc) can however supply the nutrient for fungal or bacterial growth.

Resistance to Ants, Termites, Rodents and Marine Borers

Since is has no food value, Isolite® does not attract ants, termites, or rodents, however, it is not a barrier to them. Ants, termites and rodents will chew through Isolite® to reach food or establish a comfortable home.

Marine borers can attack EPS, as they do wood and Isolite® should be protected by an anti-fouling paint over a suitable primer.

Electrical Properties

The electrical characteristics of Isolite® (See Table 5) and air are similar. This applies to arc resistance, as well as other electrical properties. The EPS melts about the path of an arc as soon as the arc penetrates it. Dielectric loss of Isolite® is quite low.

Flammability Properties

Expanded polystyrene products are combustible and should not be exposed to open flame or other ignition sources. As with all other organic material, insulation products must be considered combustible and to constitute a fire hazard if improperly used or installed.

The material contains a flame retardant additive to inhibit accidental ignition from small fire sources. Table 6 shows test results for Isolite® and other common building materials to provide a guide as to how these products compare.

Coefficient of Linear Thermal Expansion

The coefficient of linear thermal expansion for Isolite® is 6.3 x 10-5 m/m deg K. It is constant across the range of densities used in Isolite® products.


The heat of combustion of solid polystyrene polymer is 40, 472kJ/kg; combustion products are carbon dioxide, water, soot (carbon), and to a lesser extent carbon monoxide.

A CSIRO report [Ref (ii)] comments that the toxicity of gases associated with the burning of EPS is no greater than that associated with timber.

Extensive research programs have been conducted overseas [Ref (iii)] to determine if thermal decomposition products of EPS present toxicity hazard. The test results have revealed that the toxicity of the decomposition products appears to be no greater than for wood and decidedly less than other conventional building products.

Thermal conductivity design values – W/m K

a) Determine mean temperature of insulation in °C

To = Temperature on outside surface of insulation

Ti = Temperature on inside surface of insulation

T Mean = To + Ti 2

b) Select the class of EPS from AS 1366 Part 3 – 1992

c) Look up relevant K value in the table for the mean temperature in °C Thermal conductivity quoted in W/mK

The information contained in this brochure is presented as a guide to users of EPS, and while to the best of RMAX's knowledge it is correct and reliable, no responsibility can be taken by the company for the applications in which Isolite® EPS may be selected or the way in which it is used.


(i) Wayne Tobiasson and John Ricard, US Army Cold Regions Research and Engineering Laboratory, 'Moisture gain and its thermal consequences for common roof insulations'.

(ii) P.R. Nicholl and K.G. Martin, 'Toxicity considerations of combustion products from cellular plastics'.

(iii) H.Th Hofmann and H. Oettel, 'Comparative toxicity of thermal decomposition products'.