THE INVISIBLE ADVANTAGE:
Near-Infrared Light Revolutionises Polymer Identification

With the increasing requirements for polymer recovery and recycling, the use of NIR (Near Infra-Red) detector technology has been seen to allow the separation of different polymer materials to aid in their recycling.

In some cases, this has required a need to consider alternative pigment types to minimise the detrimental effect on NIR detection ability, e.g. carbon black being substituted with more suitable NIR transparent black pigments.

In the case of aluminium effect pigments, various studies have been conducted to determine the effect on NIR detectability of various polymers to give guidance on best selection and formulation.

SPECTRAL SIGNATURES:
How NIR ‘Reads’ the Identity of Every Plastic

From Metaflake’s experience, the following have been found to be significant contributing factors for NIR detection in polymers containing aluminium pigments:

1) Aluminium Pigment Particle Size:

a: Smaller sized aluminium pigments can reduce how efficiently the polymer can be detected by NIR.

b: Above 15um aluminium pigment size, the effect is relatively minimal at standard addition levels.

c: With aluminium pigments above 15um, the dosage can be in the “standard” level of 0.5%-1% without significantly
disrupting NIR.

2) Aluminium Pigment Additional Level:

a: The additional level of aluminium pigment can be a factor, especially when combined with the above small particle
size characteristic.

b: With aluminium pigments below 15um, the dosage should be below 0.5% (or even below 0.3%) to minimise the
negative effect on NIR detectability.

c: With aluminium pigments above 15um, the dosage can be in the “standard” level of 0.5%-1% without disrupting
NIR detectability unacceptably.

3) Aluminium Pigment Shape:

a: Silver dollar aluminium pigments (round flakes) give significantly lower negative effect on NIR detectability than
“cornflake” aluminium pigments.

b: This means that even <15um, a dosage for good opacity in the range of 0.3-0.5% with a silver dollar aluminium
pigment can give acceptable results in NIR detectability.

c: Above 15um, the effect on NIR detectability with silver dollar aluminium pigments is generally not found to be a
significant issue.

4) Substitute with Non-Aluminium Pigments:

a: The use of mica (synthetic or natural) or glass flakes has been shown to give less or no negative effect on NIR
detectability, even when used at higher additional levels.

b: The significantly lower opacity with this type generally requires a much higher additional level to achieve opacity
in the final article.

c: It is also difficult to achieve a true metallic effect with these alternative pigments types on their own.

d: As a result, they can sometimes be useful in combination with aluminium pigments (cornflake or silver dollar types)
where necessary.

e: These non-aluminium alternatives tend to be significantly more expensive rendering them less interesting for
colouring of plastics for general consumer packaging items.

IN SUMMARY:
For best results in NIR detectability of polymers:

1) Use a silver dollar aluminium pigment.

2) Use as large a particle size as is possible in line with desired visual effect.

3) Where a small cornflake (<15um) is required, try to keep additional level in the final article below 0.3% or switch
to a silver dollar or cornflake/silver dollar combination

4) In some cases, mixing mica or glass flakes into the formulation together with aluminium flakes can bring extra
opacity without reducing NIR detectability further.

All above information is offered for guidance only and Metaflake
is available to discuss further in more details where of interest.

PRECISION SORTING SOLVED:
Metaflake fine to medium particle sizes aluminium pigments for high
opacity metallic effects in NIR-Ready applications.