Iron oxide pigments after titanium dioxide are the most important pigments which are used in a variety of applications. These consist of colouring construction materials, paints and wood protection systems.
Natural Iron Oxides
These were the first type of iron oxide colourants used and date back to pre-historic times and are found in cave paintings. The iron oxide content of these is dependent on the type of mineral deposit. Natural red iron oxide occurs in the mineral hematite. It has the composition of α-Fe2O3. Yellow iron oxide, natural goethite is the main source and has the composition of α-FeOOH.
Synthetic Iron Oxides
These have the same composition of natural iron oxides but contain higher and more consistent levels of iron oxide contents. The particle sizes of red iron oxide typically range from 0.1 µm to 1.0 µm, have colours/appearance ranging from yellow-red to violet and are easily dispersible. Yellow iron oxide ones tend to give dilatant rheology whilst the red ones tend to increase in viscosity over time and separate. This is down to their high density, around 5 g/cm3.
Iron oxide pigments also have excellent hiding power depending on their particle size, lightfast, and long-term outdoor stability, and for iron oxide has excellent heat stability. However, the yellow iron oxides tend to have poor heat stability and can dehydrate at temperatures less than 200°C, but this can be increased by the incorporation of aluminium in the pigment.
Transparent Iron Oxides
As mentioned above the hiding power of iron oxides depend on their particle size. Transparent iron oxides can be prepared which have very small particle sizes. This results in pigments with high surface areas and high oil absorption figures. Since the primary particles are very small, high interfacial forces occur between the primary particles. As a consequence of these forces, aggregates are formed which are difficult to wet and disperse. Hence the need to bead mill to achieve the transparent properties of these pigments. When they are dispersed they do not scatter light and are fully transparent. These types of pigments can be used as colourants in metallic effect coatings and also in transparent paints.
Technical Work
In this work, we initially looked at standard Pigment Red 101 and future work will be undertaken on Pigment Yellow 42 and both transparent forms.
We used a micronized version of Pigment Red 101 - Bayferrox 110M (ex-Lanxess). This is a yellowish-red iron oxide.
We made a series of simple binder-free aqueous dispersions based on the formulations below:
Pigment Red 101 | 60 |
Dispersant (100% active) | 6.9 |
Biocide | 0.1 |
Defoamer – Dfoam AX1 | 0.01 |
Water | 32.99 |
Total | 100 |
The ingredients were all added apart from the pigment and mixed on a high-speed stirrer for 5 minutes. The pigment was then added and mixed for a further 5 minutes before being transferred to a Silverson homogenizer for mixing at 4000rpm for 15 minutes.
The following products were tested as use as a dispersant:
Product | Comment |
100% active - BioLoop | |
80% active - BioLoop | |
75% active - BioLoop | |
100% active - BioLoop | |
80% active - BioLoop | |
75% active - BioLoop | |
40% active - BioLoop | |
80% active - Synthetic | |
80% active - BioLoop | |
40% active - Polyacrylate | |
70% active - Wetting agent |
*Product is 100% biobased.
The BIO products are based on soybean derivatives and the RPS products are rapeseed oil based.
Lansperse LT87 is around 40% biobased and Lansperse BA6 is around 85% biobased.
Viscosity and Stability of Dispersions
In iron oxide dispersants, one of the most important parameters is the ability to produce a dispersion that is stable over time without settling. This is mainly due to the high density of around 5g/cm3.
Since the viscosity of these formulations was very low in viscosity, the viscosity was measured in seconds using a DIN4 flow cup. Viscosities were measured over some time and also noted was the appearance of the dispersions i.e. was any settlement noted.
Time (days) | Lansperse BIO691 | Lansperse BIO801 | Lansperse BIO868 | Lansperse RPS11 | Lansperse RPS25 | Lansperse RPS43 | Lansperse LT87 |
| Seconds | Seconds | Seconds | Seconds | Seconds | Seconds | Seconds |
2 | 26 | 21 | 28 | 22 | 32 | 67 | 31 |
9 | 20 | 21 | 29 | 21 | 31 | 145 | 33 |
29 | 17 | 18 | 29 | 19 | 30 | 149 | 47 |
36 | 18 | 20 | 30 | 22 | 31 | 158 | 49 |
None of the above showed any settlement/separation after the viscosity tests were completed.
The dispersions based on Lansperse BA6, Lansperse SPA, Lansperse DS200W and Lanwet JH1 thickened up either on initial mixing or after the first viscosity measurement.
The viscosity results above for 36 days show the following trend:
Lansperse BIO691, Lansperse BIO801, Lansperse RPS11, Lansperse BIO868, Lansperse RPS25, Lansperse LT87, Lansperse RPS43.
The last two dispersions can be eliminated due to their viscosity increase over time.
The above viscosity results appear to show that the 100% biobased dispersants are extremely good at giving a stable, low-viscosity dispersion.
Colour and Loss Analysis
The dispersions were added at a 5% level to a water-based high gloss medium base paint (Dulux Trade) and then mixed using a Thinky Intertronics Compounds Mixer. This machine automatically mixes homogeneously by planetary centrifugal motion and defoams the dispersion as well.
The prepared tinted white paint was then applied to the LENETA card using a TQC Automatic applicator at 5mm/second and 50-micron thickness.
Gloss Measurements
Gloss | Lansperse BIO691 | Lansperse BIO801 | Lansperse BIO868 | Lansperse RPS11 | Lansperse RPS25 | Lansperse LT87 |
20° | 80.85 | 82.45 | 78.05 | 79.83 | 76.58 | 78.35 |
60° | 91.55 | 90.85 | 89.95 | 92.75 | 91.25 | 90.03 |
85° | 96.13 | 96.75 | 97.93 | 99.63 | 99.13 | 97.3 |
Usually, for gloss measurements, gloss is first measured at 60°. If the reading is between 10 – 70, use the 60° reading. If it is < 10 then use the 85° reading, but if > 70 then use the 20° reading.
The above readings show that the 20° reading should be used and the results show:
Lansperse BIO801 > Lansperse BIO691 > Lansperse RPS11 > Lansperse LT87 > Lansperse BIO868 > Lansperse RPS25
The difference in gloss reading at 20° is around 7% between the highest reading and the lowest.
Colour Measurement
The coated samples were measured on a Konica Minolta CM-5 spectrophotometer and the L*a*b results are shown below:
Value | Lansperse BIO691 | Lansperse BIO801 | Lansperse BIO868 | Lansperse RPS11 | Lansperse RPS25 | Lansperse LT87 |
L | 59.31 | 60.45 | 57.48 | 61.04 | 60.73 | 60.29 |
a | 23.15 | 25.72 | 27.07 | 25.62 | 25.42 | 25.42 |
b | 17.10 | 18.53 | 19.79 | 18.48 | 18.39 | 18.59 |
As the pigment used is a yellowish-red iron oxide, then the +a and +b values are important.
The difference in the L*a*b values between two samples is typically used and the following equation can be utilised.
If we take Lansperse LT87 as the standard then the differences are shown below:
Sample | ΔE*ab |
3.09 | |
0 | |
3.36 | |
0.76 | |
0.54 |
Compared to Lansperse LT87 as standard the colour analysis also shows:
Lansperse BIO691 – Darker, less red and less yellow
Lansperse BIO801 – Lighter, redder and slightly less yellow
Lansperse BIO868 – Darker, redder and more yellow
Lansperse RPS11 – Lighter, very slightly less red, slightly less yellow
Lansperse RPS25 – Lighter, less red and less yellow
Conclusion
This programme of work has highlighted that the biobased range of products are very suitable as dispersing agents for Pigment Red 101 red iron oxides which as mentioned in the introduction, is one of the most used pigments in the world.
The initial results show that three products from our BioLoop range should be looked at are:
Lansperse BIO691, Lansperse BIO801 and Lansperse RPS11. The first two are from our soybean-based BioLoops and the last one is from our newly launched rapeseed variants.
All these are 100% renewable and are an important addition to the Lankem range in developing greener, sustainable products that are required by the industry in moving forward.