Many preconceptions revolve around the use of densified LDPE, the result of careless production and uses with expectations too high

The LDPE waste that comes from plastic waste from separate waste collection should be a selection of plastic films, single product, to be sent for recycling.

In reality, many times, these waste streams can contain different materials, in the form of other plastics and pollutants, such as labels , paper and other fractions.

The lack of a real reference market in the sale of densified LDPE leads the recycling industry to prefer granulation of the material trying, in the extrusion phase, to reduce these foreign bodies in order to best qualify the raw material.

In this case, a priori, we give up paying more attention to the selection and desification phase of the LDPE waste.

The result, often, is a granule that remains at the low end of the market, which can be used for molding non-metallic articles. aesthetic, such as vases and tubs for the fruit and vegetable sector, but hardly lends itself to the production of films with thin thicknesses or the production of tubes.

At this point, sometimes, one wonders whether it is not better to qualify the densified material for the injection molding sector, rather than spending time, money and energy to granulate LDPE.

To follow this path it is necessary to better qualify the densified, so that its use in presses does not make you regret the injection process with a filtered granule.

But let's see what densified LDPE is

The term “densified”, in relation to LDPE, refers to the polymer that has been compacted, in the context of mechanical recycling.

The production of LDPE densified from post-consumer waste is an integral part of the recycling process of this material. The production process can be divided into these phases:

- LDPE waste is acquired from designated collection points, which deal with waste from separate collection,

- Once it arrives at a recycling plant, LDPE waste is separated from other materials. This separation can be done manually or through machines such as air separators.

- The LDPE waste is then washed to remove impurities such as food residue, soil or other contamination. This ensures that the final product is of good quality.

- After cleaning, the LDPE is shredded into small pieces or flakes. This facilitates the densification process.

- There are several techniques to densify LDPE:

- By agglomeration: the ground LDPE is exposed to heat and agitation. This causes the partial melting of the pieces, which agglomerate forming larger lumps.

- By compaction: The process involves the use of compacting machines that press the material into blocks or agglomerates.

It is important to underline that the quality of the LDPE densified largely depends on the purity of the starting material and the effectiveness of the cleaning and separation processes.

Therefore, special attention is given to these steps to ensure that the densified product is of good quality and free from significant contamination.

How to create a high-performance compound with densified LDPE

LDPE (Low Density Polyethylene) is often used in combination with other plastic resins, to exploit the complementary characteristics of the different polymers and obtain products with specific properties.

However, the decision to blend post-consumer LDPE with other polymers depends on various factors, including the desired properties of the final product, the compatibility of the polymers themselves, and the presence of compatibilizers.

Let's see some combinations:

- HDPE (High Density Polyethylene): LDPE and HDPE are often compatible with each other and can be mixed to obtain products with intermediate properties between the two. For example, a blend of LDPE and HDPE might offer a combination of flexibility and strength.

- EVA (Ethylene Vinyl Acetate): The addition of EVA to LDPE can improve the toughness and elasticity of the product the final. EVA is also used to improve the UV resistance and flexibility of LDPE.

- PP (Polypropylene): Although polypropylene and polyethylene are not intrinsically compatible, they can be mixed in the presence of specific compatibilizers. This blend can be used in specific applications where you want to combine the properties of both polymers.

- LLDPE (Linear Low Density Polyethylene): LDPE and LLDPE can be mixed to adjust the mechanical properties and the workability of the final product.

Care should however be taken because not all plastics are compatible with each other, and mixing incompatible polymers can lead to products with unwanted or inadequate properties.

Furthermore, the presence of contaminants or additives in post-consumer materials can influence the compatibility and properties of the mixed product.

What are the melting temperatures ideal for producing finished products in LDPE 

LDPE (Low Density Polyethylene) has a branched structure, which means that it does not have the same arrangement regular and ordered molecular chains like other polyethylenes, for example HDPE (High Density Polyethylene).

This branched structure makes LDPE more flexible but also less dense and with a lower melting point than HDPE.

The melting temperature of LDPE generally varies between 105°C to 115°C (220°F to 240°F).

However, when it comes to transforming LDPE through techniques such as extrusion or injection molding, temperatures can vary depending on the specific needs of the application and the presence of any additives.

Here are some general guidelines for processing LDPE

- Extrusion: 150°C to 220°C (300°F to 430°F).

- Injection molding: 140°C to 250°C (285°F to 480°F).

These temperatures are general guidelines only and may vary based on LDPE waste, machine conditions and other factors.

What physical characteristics does the addition of a densified LDPE product in a compound with PP bring?

Mixing LDPE (Low Density Polyethylene) and PP (Polypropylene) is a common practice in some applications, especially when you want to take advantage of the complementary properties of both polymers.

The addition of a densified LDPE in a compound with PP can influence the physical characteristics of the blend in various ways:

Compatibility

First, it is essential to note that LDPE and PP are not inherently compatible. This means that without the use of compatibilizers or modification of casting conditions, the two resins tend to separate into distinct phases, potentially leading to inferior or inadequate mechanical properties in the final product.

Elasticity and Flexibility

LDPE is generally more flexible and ductile than PP. The addition of LDPE can therefore increase the flexibility and toughness of the blend, while reducing stiffness.

Melting Point

Since LDPE has a lower melting point than PP, mixing the two can lead to a decrease in the overall melting point of the blend, depending on the proportions used.

Transparency

LDPE is typically more opaque than PP. Its addition can therefore reduce the transparency and brilliance of the blend, making it more opaque or milky.

Chemical Resistance

LDPE and PP are both resistant to many chemicals, but their combination may have a slightly different chemical resistance profile than pure polymers.

Transformation

The workability of the mix can change with the addition of an LDPE densifier. For example, viscosity during extrusion or injection molding may change, affecting ideal processability conditions

What blemishes can be created in the production of LDPE products by using a melting temperature that is too high

The use of an excessively high melting temperature when processing LDPE (Low Density Polyethylene) can lead to various blemishes and problems of quality in finished products. We can recall some of the potential problems:

- LDPE can degrade when exposed to too high temperatures. This degradation can cause changes in the mechanical properties of the material and produce gases and/or volatile compounds that can form bubbles or voids in the finished product.

- Thermal degradation can also lead to discoloration of the polymer. An overheated LDPE can take on a yellowish or brown color.

- Thermal degradation can produce compounds with unpleasant odors. This can be particularly problematic for applications where the presence of odor is an important factor, such as in the case of food packaging.

- Excessively high temperatures can cause uneven cooling during part formation, leading to incorrect deformations or shrinkage.

- The use of too high temperatures can cause the formation of streaks or superficial stains on the product, especially if there are impurities or additives in the material.

- Thermal degradation can adversely affect the mechanical, thermal and chemical properties of LDPE. This could result in products with reduced strength, toughness or durability.

- At excessively high temperatures, LDPE may become too fluid, making it difficult to form precise details or maintain desired tolerances .

Problems with aluminum labels in densified LDPE

It often happens that, despite the decantation and centrifuge washing of the LDPE plastic waste, in the densified product there is still the presence of flexible aluminum parts.

We must keep in mind the difference between the impurities made up of rigid aluminum fractions and those made up of leaf aluminum.

If in the first case the rigidity of the metallic impurity cannot be tolerated, due to a series of negative problems that these can cause to the injection systems, which are the result of poor selection and washing, the presence of aluminum leaf parts do not create technical problems.

These parts are made up of packaging labels which can contaminate the films but, being soft, do not cause damage to the systems or final products.

There remains an aesthetic aspect that must be considered, but with a view to creating non-aesthetic products, the bright dot which recalls the presence of aluminum foil must be considered “part of the game”.

This acceptance of the impurity of aluminum foil can bring significant price advantages on the final product and a considerable availability of raw material on the market.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.