In addition to PET beverage bottles, HDPE recycled material for milk and juice bottle recycling is in high demand because this recycled material does not contain pigments, giving HDPE a stable performance.
Most milk, mineral water and juice bottles are now made from HDPE, although milk is usually packaged in cardboard boxes and glass bottles.
HDPE, meanwhile, is widely used in detergent and shampoo bottles.
HDPE bottle recycling process
Old milk bottles arrive at recycling plants after being separated from most PET, vinyl, or non-PE containers at material recovery plants.
In addition, there are some detergent bottles among these packages. Detergent bottles, although made of HDPE, are usually coloured and made of HDPE copolymer, and it is best to recycle them separately.
In most operations, recycled HDPE is usually olive green (or grass green), and the green colour of HDPE receipts obtained from milk bottles is due to the introduction of milk bottle caps (typically green and red) during the recycling process.
Bottle caps are also made of HDPE and are, therefore, difficult to separate by the float-down process. Cap removal is a very important procedure and is currently achieved through colour sorters and other equipment for efficient automatic cap separation.
Characteristics of recycled milk and juice bottles
The advantages of recycling HDPE for milk and juice bottles are:
- Easy identification of shapes and abundant sources.
- Stable MI and density of recycled products from different batches because the resin used for dairy is a strictly required blow moulding resin.
- The recycling of HDPE milk containers ends up with a clean, colourless product. Moreover, since the final recyclate does not undergo any thermal degradation during the recycling process, it has the same rheological properties as the original resin.
Characteristics of recycled dairy products
However, some drawbacks limit the widespread use of PCR for dairy-based HDPE, which include.
- The resin for dairy is a low MI material (i.e. 0.6g/10min) that can be blow moulded into milk bottles and juice bottles; due to its high molecular weight, the production cycle cannot be shortened during injection moulding.
- Resins for dairy are homopolymers with low environmental stress cracking resistance (ESCR) and, therefore, cannot be used for bottles for detergents or bleach.
- Colour fluctuations due to caps and shampoo bottles make them unsuitable for use in light-coloured applications.
- Experiments revealed that injection moulded products produced from blends containing recycled HDPE milk bottles exhibited greater shrinkage than products produced from pure HDPE injection moulding resin.
It can be seen that the low MI and poor processing properties of dairy products determine that it is not suitable for injection moulding.
Moreover, it is also unsuitable for the film due to its higher density and lack of toughness.
Its high rigidity can lead to cracks in the film.
It can also no longer be used for milk bottles because it has not passed the contact food permit. It also lacks toughness and rigidity, so it cannot be used directly in large blow moulded products (IPBM).
Therefore, on most occasions, recycled dairy resins must be modified in some way (e.g., blended with other polymers and additives) to compensate for the deficiencies of homopolymerized HDPE recyclates.