The coloring of a recycled plastic polymer, especially if its input is post-consumer waste, is subject to many factors that influence the final result

It is not enough to choose a masterbach of the desired color and follow the technical sheets, which may indicate the percentage to be added to the polymer melt, to expect the desired color.

Especially if the polymer we are about to create comes from post-consumer plastic waste, the desired color requires various considerations upstream, well before turning on the extruder, even from the plastic waste that we still have to select.

Risky? Exaggerated? No, as each plastic element that is selected will bring with it its history, in terms of quality, performance, smell and also influence on the final color.

Making the color of a recycled polymer brighter and more uniform can be a real challenge, due to the impurities and degradation that can occur during the life cycle of the polymer. However, there are several strategies that can be used to improve color brilliance:

Thorough cleaning

Thorough cleaning of the recycled polymer, understood as good selection, good washing and good filtration, can remove a good part of impurities or residues that negatively affect the appearance of the polymer.

Compatibilisers

Using compatibilizers can improve the mixing of different polymers or additives, leading to better uniformity and gloss.

Optical additives

Optical brighteners (OBAs) can be used to make recycled polymers whiter or brighter. They work by absorbing ultraviolet light and releasing it as blue visible light, thus offsetting unwanted yellowish hues.

Nucleating agents

They are additives that can influence the crystallization process of semi-crystalline polymers, such as polypropylene. Controlled crystallization can lead to improved optical properties and improved color brilliance.

Dispersion improving additives

These additives help in the uniform dispersion of pigments and other additives in the polymer, ensuring uniform color.

High quality dyes

Using high-quality pigments and dyes, specific to recycled polymers, can produce more vivid and brighter colors in the recycled polymer.

Extrusion process

Optimizing extrusion conditions, such as temperature and speed, can improve the brilliance of the finished polymer, avoiding thermal stress on the material with negative repercussions on the quality of the surfaces.

Finishing techniques

After processing, finishing techniques such as polishing or coating can be used to improve the gloss of the final product.

UV stabilizers

Exposure to UV rays can cause color degradation over time. Using UV stabilizers can help protect the color from fading and keep it bright.

Oxidation reduction

Oxidation can affect the brilliance of the color. Using antioxidants can help protect the polymer from oxidation during processing.

Blending

Mixing the recycled polymer with a small amount of virgin polymer can, under certain conditions, improve color brilliance.

It is important to underline that the strategy or combination of multiple strategies to be adopted will depend on the specific needs and conditions of the recycled polymer in question. You may need to experiment with different options to get the results you want.

How caco3 (calcium carbonate) affects colors in recycled polymers

Calcium carbonate (CaCO₃) has a significant effect on the colors of recycled polymers when used as a filler. Let's see how it can influence the aesthetic appearance of polymers:

Opacity

CaCO₃ has a whitish nature and can increase the opacity of the polymer. This means that when added to a transparent or semi-transparent polymer, it can reduce its transparency. Furthermore, an increase in the percentage quantities used may cause changes in the base color towards irregular and opaque shades.

Brightness

The addition of CaCO₃ can increase the brightness of a polymer, not to be confused with gloss, due to its white nature. If the recycled polymer has a dark or gray color due to previous impurities or additives, adding CaCO₃ can make it slightly lighter.

Interactions with other additives

If there are other additives or colorants in the recycled polymer, calcium carbonate can interact with them. Which could influence the final appearance of the polymer in terms of color and opacity.

Diffusion of light

CaCO₃ has the ability to scatter light, so this behavior can affect the visual appearance of the polymer, making it less shiny or less transparent.

Importantly, the effect of CaCO₃ on the color and appearance of a recycled polymer can vary depending on the size and distribution of the particles of this mineral filler, as well as the amount of filler added and the properties of the base polymer.

How talc affects color quality in recycled polymers

Talc, a magnesium silicate mineral, is commonly used as a filler in plastic compounds. In the context of recycled polymers, talc can affect color quality in various ways:

Opacity

Like CaCO₃, talc can also increase the opacity of the polymer. This means that adding talc to a transparent or semi-transparent polymer can reduce its transparency.

Color shades

Due to its white-grey nature, the addition of talc can affect the color tone of the recycled polymer, potentially making it paler or dulling bright colors.

Color uniformity

Talc can help provide a more uniform appearance to the polymer, especially if the recycled material initially has a non-uniform color due to impurities or previous additives.

Diffusion of light

Talc particles dispersed in the polymer matrix can scatter light, affecting the visual appearance of the polymer and potentially making it less shiny, as happens with calcium carbonate.

Interactions with other additives

If the recycled polymer contains other additives, colorants or stabilizers, talc can interact with these components, affecting the final appearance of the material.

Effect on workability

While not a direct effect on color, the presence of talc can alter the flow properties of the polymer during processing. This can have an impact on the surface finishes of the products and, consequently, on the perception of color and gloss.

To maximize the color quality in a recycled polymer with talc, it is important to control the quantity and size of the talc particles, sometimes it may be necessary to balance the use of talc with other additives or stabilizers. As always, the optimal formulation will depend on the specific needs of the application and the desired results.

How to intervene in the polymer recycling phases to increase the quality of the colored granule produced

The quality of the colored granule produced from recycled polymers can be influenced by various factors during the recycling phases. Here are some strategies and interventions that can be implemented to improve quality:

Selection and Separation

This is one of the most critical and most important phases is the selection of plastic waste. Careful separation of different types of plastic can reduce contamination and ensure that the recycled material is as pure as possible.

Thorough washing

After separation, the plastic should be washed thoroughly to remove residue, dirt, adhesive labels and other contaminants.

Degassing

During extrusion, it is essential to have an effective degassing system to remove moisture, odors and volatile substances that can compromise granule quality and coloration.

Optimization of the Extrusion Process

The temperature, speed and extrusion conditions should be optimized to avoid polymer degradation and ensure good color mixing.

Particle Size Control

The size and shape of the pigment or dye particles can influence the appearance of the granule. Good dispersion is essential to obtain uniform coloring.

Testing and Quality Control

After production, it is essential to test the granules to ensure they meet the desired specifications. This may include testing for coloration, strength and other relevant properties.

Correct Storage

Store the granules in optimal conditions (protected from light, in a dry environment) to prevent degradation or color changes before use.

With which tools can we evaluate the quality and RAL correspondence of a recycled polymer

To evaluate the quality and color match (e.g. with the RAL scale) of a recycled polymer, various tools and techniques can be used:

Spectrophotometers

These instruments measure the reflectance or transmittance of a material at different wavelengths, allowing precise quantification of color. They can be used to compare the color of a sample with a reference standard, such as a RAL colour.

Colorimeters

Similar to spectrophotometers, colorimeters are less complex and quantify color in terms of color coordinates such as Lab*, which can be compared to a reference value.

Microscopy

Under a microscope, the dispersion of the pigment or dye in the polymer can be examined, ensuring that there are no aggregates or separations that could affect the quality of the color.

RAL comparison tables

These are physical cards or sample sets that show RAL standardized shades. While they are not as precise as electronic tools, they can offer a quick visual reference for color matching.

Accelerated aging test

These tests expose the polymer to extreme conditions (such as intense UV light or heat) to evaluate how quickly the color will change over time.

Color management software

These programs can help translate and compare color measurements between different scales, such as RAL, Pantone, and others. They can also help predict how changes in formulation will affect color matching.

When using instruments such as spectrophotometers or colorimeters, it is essential to standardize the measurement conditions (e.g., measurement angle, type of illuminant, etc.) and regularly calibrate the instrument to ensure accurate and repeatable measurements.

Finally, while these tools can provide quantitative color match data, it is always helpful to have a visual assessment from experts as well, as human perception of color can vary based on several factors.

What is the difference between a dye for virgin polymers and one for recycled ones

The coloring of polymers, both virgin and recycled, can be influenced by various factors. While many colorants can be used for virgin polymers, there are some differences and specific considerations when it comes to coloring recycled polymers:

Recycled polymers may contain impurities or residues from previous use cycles. This can affect the dye's ability to disperse evenly and can alter the final appearance of the color.

Due to impurities or changes in the molecular structure of recycled polymers, some dyes, which work well with virgin polymers, may not be as effective with recycled polymers.

Because recycled polymers may have residual or unwanted colors, it may be necessary to use stronger or larger dyes to achieve the desired shade.

Furthermore, recycled polymers may have undergone thermal degradation in previous processing cycles. This means they may be more sensitive to heat during subsequent processing. Dyes chosen for these materials should have good thermal stability.

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