AS/NZS 4766 - Compliance of raw materials

Section 5 and 6.1 of the standard focus on the raw material used in manufacturing tanks. They define requirements for material properties that play a vital role in the longevity of a tank. You can use the list below to navigate through the information on this page.

And, in case you are interested in still more details, ARMA have put together a booklet a few years ago which contains information on a variety of material properties and their influence on the properties of a tank as well as detailed information on how the different tests are carried out. You can get a copy through ARMA.

 

AS/NZS 4766 compliance requirements for raw materials

 

General Requirements

The material used must be a colour compound. Dry blends, produced by mixing natural polyethylene powder with dry pigment, cannot be used to produce AS/NZS4766 compliant tanks. Reground, recycled or reprocessed material is also not to be used.
 

Melt Flow Index (MFI)

The MFI of the material must not vary by more than 20% from the value nominated by the compound manufacturer. Limiting the MFI variance ensures that there are no significant variations in properties that are dependent on molecular weight like ESCR, long term strength and toughness of the material. Even though the standard lists this test as a type test only, this is a routine test and suppliers will carry out MFI tests on every batch they produce.
 

Thermal stability of the base resin

The thermal stability of the base resin is verified through an oven ageing test where the polymer is kept at 100C for 100 days. The MFI of the sample before and after ageing must not vary by more than 20%. This test is meant to ensure that the base resin contains sufficient antioxidants to protect the material from thermal degradation during both the compounding and rotomoulding processes. This is a type test and only needs to be carried out if there are changes to the antioxidant package.


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UV resistance

Polyethylene is attacked by UV light and in order to prevent degradation of the material, which leads to a loss of long term strength and ultimately failure of the tank, stabilisers are added. UV stabilisation of a material is verified by conducting tensile tests on unpigmented material and comparing its elongation at break before and after exposure to UV light in a weatherometer. Elongation at break is effected by the fine cracks in the sample surface that occur when exposure to UV light causes degradation of the material. For compliance with AS/NZS 4766, elongation at break of the material after 8,000 hrs in a weatherometer must be at least 50% of the tensile strength before exposure. This corresponds to a UV8 rating. This is a type test that only has to be carried out if there are changes to the UV stabiliser package.
 

Pigment/Additive dispersion

The standard requires all pigments and additives to be dispersed evenly throughout the polymer to ensure even distribution of all components. This is particularly important in materials that use non UV stabilised base resins, with the stabiliser introduced as part of the pigment masterbatch. In these materials, where there is no pigment there is likely to be no UV stabiliser. Too much pigment, on the other hand, can have a negative effect on material strength. AS/NZS4766 compliant powders must be inspected for good pigment dispersion at regular intervals. The maximum allowed pigment content is 2%, with the exception of carbon black when it is used as UV stabiliser. This requires a higher add rate. Every batch of powder produced must be inspected for proper pigment dispersion at regular intervals.
 

Light Penetration

If enough light penetrates through the walls of a tank, algae growth inside the tank can occur. Therefore, AS4766 defines a limit for the light transmittance through the materials used. Since the light is blocked out by the pigments contained in the material, opacity will vary from colour to colour (high opacity = low light transmission, low opacity = high light transmission). This is a type test that has to be repeated every five years for each colour masterbatch used.


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Stress crack resistance

Exposure to some substances can cause cracks to form in parts of a product that are under stress. This isn't necessarily limited to stresses occuring through load on the part but can also be caused by stresses introduced through part geometry or processing conditions. Stress cracking can lead to premature failure of a tank. A material's stress crack resistance (also referred to as environmental stress crack resistance or ESCR) is influenced by a number of factors including molecular weight, density and copolymer type and some grades will be more susceptible to it than others. AS/NZS4766 requires a minimum stress crack resistance of 500hrs in order for a material to be compliant. Please note that there are different test methods for this property and their results can't be directly compared to one another (AS/NZS refers to ASTM D1693, condition A, using the stress cracking agent at 100%). This is a type test that is carried out on the base resin.
 

Chemical resistance

If a tank is intended for the storage of chemicals, then it is obviously important that whatever is stored doesn't have a negative effect on the tank material. Polyethylenes generally show a very good resistance to a wide range of chemicals. However, there are some that will lead to a physical (softening, stress cracking) or chemical attack (degradation) and both affect the material's long term strength and can lead to premature failure of a tank. Chemical resistance is often assessed based on data available in databases or suppliers' chemical resistance tables. Typically, these are made up of generic test data obtained through short term (30-60 days) immersion in the chemical followed by a tensile test and aren't necessarily linked to a specific grade. While these tables can give a first indication as to whether PE is likely to be affected by a chemical, they often don't show up effects like stress cracking, softening or sometimes even degradation of the material. The decision to supply a tank for a specific chemical should never be based on this resistance data alone but also take into account other factors such as known effects of similar chemicals, experience in the field, expected service life of the tank and the consequences of a failure.


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Food and potable water contact

It goes without saying that materials used to make tanks that are intended for storing drinking water must be approved for potable water contact. The standard ensures this by making potable water testing to AS/NZS4020 a requirement for drinking water tanks. To meet the requirements of AS/NZS4020, a material must pass a number of tests including the growth of microorganisms in the water that's in contact with the material and even a taste test. In addition, base resin and pigments used must be suitable for food contact as per AS/NZS2070. Potable water testing must be carried out for every colour that compliance is claimed for and the test has to be repeated every five years. It is worth noting that both AS4020 and AS2070 permit the presence of heavy metals in materials used for food and potable water contact applications, albeit at low levels. However, most Australian powder suppliers have eliminated heavy metals from their tank materials as part of an ARMA initiative to make Australian rotomoulding heavy metals free.
 

Design - mechanical test data

Compliance with AS/NZS4766 requires the tank design to be based on finite element analysis (FEA) and for the relevant mechanical test data to be available. FEA is carried out using data from both short term (such as flexural or tensile testing) and long term tests (such as creep testing) on the base resin and usually the polymer manufacturer will make this data available to the design engineer on request. The mechanical data listed in technical data sheets is not detailed enough for use in designing a tank. Contact your Price Plastics representative if you need data for FEA - we'll organise for a data package to be supplied to your engineers.
 

Design - Hydrostatic Design Basis

The hydrostatic design basis (HDB) can be used to determine the wall thickness required for straight walled, cylindrical tanks. It is based on the material's hydrostatic long term strength, obtained through long term pressure tests on rotomoulded pipe.


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Design - derating

Most mechanical data is measured under standard laboratory conditions, most importantly at a temperature of 23C. However, in real life, the service termperature of a tank can vary significantly from this standard temperature. Stiffness and long terms strength of polyethylene are temperature dependent and there is already a noticeable loss in strength when going from 23C to 40C. This has to be taken into account when the expected service temperature of a tank is above 23C, for example by derating the material's HDB. Some polymer suppliers will also have FEA relevant data obtained at 40C which can be used to design tanks with an expected service temperature up to 40C.