The effect of colorants on injection molded plastic parts is complex. Though rarely at the top of the list of considerations in project planning, it should be addressed at the material selection stage. The process requires several important steps. Coloring selection will depend on factors including the actual plastic involved and the final application, which may impose demands on durability, environmental factors and cost.
Relevant plastic properties include melt flow rate, melt or processing temperature, transparency, structure (crystalline or amorphous), and the reinforcements and fillers being used.
Colorants can be dyes or pigments. Pigments do not dissolve into the plastic, whereas dyes go into solution in the plastic and are easily mixable. They are used to color transparent plastics and where powerful tinting is needed. Dyes give brilliant color, yet do not obscure surface imperfections as well as pigments do. Dyes also tend not to perform well in outdoor applications.
Pigments are partially soluble (organic) or insoluble (inorganic). Organic pigments are more effective than inorganic ones for tinting capacity and coloring strength, creating brighter and glossier colors at lower percentages (typically 0.2-0.5% in contrast with around 2%). They can also be used to color transparent plastics. Inorganic pigments are less likely to migrate than dyes and organic pigments, so they are used to color plastics that require a high level of opacity. Inorganic pigments also have superior heat resistance and weathering properties compared to both dyes and organic pigments. However, some inorganics are perceived as health hazards and, as a result, industry is shifting towards organic pigments.
Effects of colorants on polymer properties
Adding colorants to plastic can affect properties of the plastic part so it is important to consider the compatibility between the chemistry of the polymer and the chemistry of the colorant. The effect of colorants on the properties of the polymer also depends on the amount of colorant.
Some of the compounds in colorants can break down the chemistry of the polymer and weaken original properties, such as impact resistance. The high heat used in the injection molding process can also influence the degree to which the colorant affects the polymer. The colorant itself must also be capable of surviving the high processing temperature. An individual colorant may affect one polymer differently than another. The interaction between polymer and colorant can also be affected by other additives. For example, a polycarbonate otherwise unaffected by a colorant may have a different reaction to it when a flame retardant is added.
There are also several different methods of coloring plastics, including masterbatch, compounding, surface coating and dry blending. The method used can influence the mechanical properties of the plastic. For example, in the masterbatch method, pellets of natural color would be blended with a masterbatch of pellets with a high pigment content. Because most polymers do not tend to mix well with other polymers, care must be taken to ensure material compatibility.
Different colorants can affect a polymer in different ways. Transparent colors can be made with dyes which may affect the polymer structure less than the pigments used in opaque colors. Pigments are made of particles, and the size of those particles can impact the performance of the material. Thus, moulding the same part in different colors can result in differing part dimensions, which can be important if very tight tolerances are critical.
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Warping
The degree of crystallinity and the speed of crystallization determine the final properties of a plastic part. Organic pigments are known to influence these parameters during the cooling phase of plastic processing, particularly in HDPE injection moulding.
Transparency
Usually, transparency is improved by reducing pigment particle size as much as possible. This is achieved by surrounding the particles as soon as they are formed with a coating, which prevents the growth of crystals. The most common products used for this coating are rosin or rosin derivatives. The dispersion process can also influence transparency, as it involves breaking up agglomerates of particles to individual primary particles.
References
- https://www.aimplas.net/blog/colouring-of-plastics-how-to-choose-a-colourant/
- https://www.steinwall.com/wp-content/uploads/2016/05/Colour-in-Plastic-Parts-Part-3.pdf
- https://polymer-additives.specialchem.com/selection-guide/pigments-for-plastics
- https://www.manufacturingtomorrow.com/article/2018/05/how-colourants-affect-plastic-characteristics/11518/
- Harris, Ronald M. Colouring Technology for Plastics. New York: Plastics Design Library, 1999.
- Kanu, Rex C., Thomas H. Spotts, Michael Chesebrough. “The Effects of Some Organic and Inorganic Pigments on the Tensile and Impact Properties of Injection-moulded Polypropylene.” International Journal of Modern Engineering Fall 2001.
http://www.ijme.us/issues/fall2001/articles/polypropylene.htm - Herbst, W., Klaus Hunger. Industrial Organic Pigments. Weinheim: VCH, 1997.
- Zeno W. Wicks, Jr., Frank N. Jones, S. Peter Pappas. Organic Coatings: Science and Technology. 2nd ed. New York: Wiley, 1999.
- Kim, Dae-Jin, Kwan-Ho Seo, Ki-Heon Hong, Sang-Youl Kim. “Effects of dispersing agents on dispersity and mechanical properties of carbon black/PET.” Polymer Engineering and Science 39.3 (1999): 1548-2634.
- Bung, Hans-Henning. “Metallic Looking Plastics with New Silver and Coloured Aluminum Pigments.” ANTEC 1998.
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Dear Sir:
Thanks for publishing such an informative column about color masterbatches.
However, if we talk about the general use of masterbaches in PP or HDPE’s commodity products made by injection and blow molding processes.
Please, guide us what is the maximum ratio of masterbtches can be used and if the maximum limit is breached what will be effect on the properties of plastic products?
Thanks
color masterbatches are an effective way for coloring plastic products.
Indeed. Something which you could develop into your own more detailed answer.
The maximum masterbatch loading is dependent upon the carrier polymer, the type of colourants and possibly regulatory constraints. Polymer specific masterbatches can often be used at very high addition rates as the carrier polymer is mostly the same as the polymer to be coloured, avoiding compatibility issues.
As the author mentioned, organic (soluble) dyes tend to impact mechanical properties less than (solid) pigments; unfortunately dyes must be avoided in polyolefins as they will migrate which leaves you with pigmented masterbatches for PE and PP.
The main problem with overloading the masterbatch (apart from cost) will be reduced impact and flexural properties. With this in mind a rule of thumb would be 2% addition for universal type masterbatches and 5% addition for polymer specific types without needing to worry too much. Beyond that, mechanical testing would be prudent.
Thank you for bringing some light to this topic.
We recently (and sadly) noticed the potential of a high impact on material properties by using coloring pigments in polymers.
A grey masterbatch (with the same matrix as the colored polymer) dropped the impact resistance more than you would expect.
Hello.
There is a wide use of Diffusive additives for lighting applications.
Should those additives be treated similar as colorants?
I am interested to learn how an Iron blue (C.I. Pigment Blue 27) can be recommended for the coloration of Plastics. The thermal stability is in my opinion far too low. (ca, 160C). It is suitable for Printing inks and seed coloration but I question it’s suitability here.
Cobalt Blues – Yes: C.I. Pigment Blue 28 and Blue 36 would be my recommendations.
Thank you for sharing that experience. Most valuable.
In response to Henning and the grey masterbatch affecting impact properties, this can happen with highly loaded TiO2. There are alternatives and fillers you can use when formulating highly loaded white and grey masterbatch to avoid this.