Coloured bitumen concrete or mastic asphalt are frequently used to create coloured surfaces for streets, squares, etc. In particular, red surfacing materials are used for cycle paths, pavements and parking areas. Inorganic pigments such as iron oxide, chrome oxide, titanium dioxide and mixed phase pigments (e.g. cobalt blue) are especially suitable for colouring bituminous mixes, the base products for bitumen concrete and mastic asphalt. These pigments are both lightfast and weatherstable. Organic pigments do not usually have the necessary heat and weather stability. They are also more expensive so are generally not considered for this application. The binder used, i.e. the bitumen, also plays a role in the manufacture of coloured bituminous mixes. If only dark road bitumen or the somewhat lighter "bitume pigmentable" (widely used in France) are available, only iron oxide red and, perhaps, chrome oxide green are suitable as pigments. Brilliant shades or white can only be produced using pale (colourless) binders. Most bitumen manufacturers (Shell, Total, BP, Esso, etc.) market such products. Used in sufficiently high concentrations, iron oxide red pigments, e.g. Bayferrox Red, with high tinting strength are able to mask the dark colour of normal bitumen and yield satisfactory red shades. For this reason, iron oxide red is the most important pigment for this application. Midshade iron oxide reds, equivalent to Bayferrox 130, are the most suitable pigments for colouring normal bitumen. If a pale bitumen is available, there is a wider range of pigments to choose from. Iron oxide yellow, brown and black, as well as Chrome Oxide Green pigments can also be used. In certain applications, the use of iron oxide yellow, brown and black is restricted because of their poorer heat stability. At Page 1 of 7
temperatures above 180 C, iron oxide yellow and black convert to iron oxide red. The same applies to iron oxide brown pigments which are usually mixtures of iron oxide black, yellow and red. This fact must be borne in mind when manufacturing bituminous mixes, especially mastic asphalt because the mixing temperature is usually above 180 C. Suitable pigments for red mixes are Bayferrox red pigments. Heatstable zinc ferrite products, e.g. Colortherm 3950, can be used for yellow mixes and Bayferrox 645 T and 303 T (mixed manganeseiron oxides) for brown and grey mixes. The heat stability of the other pigments, e.g. iron oxide red, titanium dioxide, chrome oxide green and mixed phase pigments, is high enough for this application. As already stated, titanium dioxide pigments can be used for shade reduction in bituminous mixes and to produce white surfacing materials. These pigments are available as rutile or anatase modifications. It is normally recommended that the rutile modification is used because anatase pigments act as catalysts in the photodegradation of the binder. Poor results are obtained by lightening dark bitumen with titanium dioxide and then shading it with colour pigments. Trials have shown that the lightening effect of the titanium dioxide reduces the intensity of the shade from the very start. In other words, the white pigment reduces the tinting strength of the colour pigment. From all that has been stated so far, it can be concluded that iron oxide red is by far the most important pigment in the colouring of bituminous mixes. If pale binders are available, other shades ranging from yellow to blue can also be produced. Page 2 of 7
The key inorganic pigments manufactured by LANXESS Deutschland GmbH for this application are summarised in Table 1 and Fig. 1. Table 1: Colouring of bituminous mixes Bitumen Type of Recommended Recommended shade pigments grades pigment concentration (%) Normal Red Bayferrox 130 3 4 bitumen Yellow Brown Green Special pale Red Bayferrox 130 bitumen Yellow Colortherm 3950 Brown Bayferrox 645 T Green Chrome Oxide Green GN Fig. 1 Page 3 of 7
How much pigment is required? When using dark bitumen, the addition of about 3 to 4 % of a synthetic iron oxide red pigment with high tinting strength, e.g. Bayferrox 130, is usually sufficient to ensure virtual saturation. The amount given is calculated on the total mix. Increasing the amount of pigment added has little effect on the colour intensity. This statement applies only to synthetic iron oxide pigments with high tinting strength. If the iron oxide red used has a lower tinting strength, the saturation point is shifted to higher pigment concentrations. In the case of other pigments, e.g. chrome oxide green, which are less suitable for masking the natural colour of dark bitumen, a pigment concentration of 4 % is by no means sufficient to yield the required saturation. Shades other than red are therefore almost impossible to produce using the normal (black) types of bitumen. The only way of improving the shade is to add aggregates whose natural colour approximates that of the pigmented bitumen. The pale types of bitumen mentioned earlier have been used for some time in a number of countries. The disadvantages of these products are their higher price and, in some cases, poorer weather stability. However, their use makes it easier to produce certain shades. For example, very attractive yellows and greens are yielded by adding a pigment concentration of between 1 and 2 % pigment, calculated on the total mix. Page 4 of 7
What metering options are available? Two types of plants are used in the manufacture of bituminous mixes. These are: 1. batch mixing plants (all products, Fig. 2) 2. continuous mixing plants (mainly bitumen concrete, Fig. 3) Aggregate Extender Mixer Mix Fig: 2: Batch mixing plant Page 5 of 7
Extender Mix Mixer Aggregate Fig. 3: Continuous mixing plant Given the large amount of iron oxide red used, the question naturally arises as to the easiest way of adding the pigment to the mix. In batch mixing plants, the pigment can be and is added manually. Bayferrox Red is supplied in 20kg and 25kg polyethylene sacks for this application. The pigment can be added directly to the mix via a chute or a trop. The sacks do not have to be opened because the polyethylene dissolves completely at 130 140 C. The advantage of this method is that no dust is formed. There is no effect on the mixing time, i.e. 60 to 90 seconds are long enough. To prevent the formation of agglomerates (pigment lumps), the pigment should be added before the bitumen and premixed with the aggregates for 10 to 15 seconds. Manual addition of the pigments is usually out of the question in continuous mixing plants. The plants are not normally designed with large enough filling openings. Moreover, these plants have a capacity of around 400 tonnes per hour, making manual addition very difficult. It is therefore more usual to equip such a plant with a pigment metering unit. Page 6 of 7
To summarise, it can be said that coloured bitumen concrete and mastic asphalt can be produced in conventional mixing plants, provided the points covered here are borne in mind. Modern pigments permit safe manufacture under difficult conditions such as those often found in continuous mixing plants. NB: This information and our technical advice whether verbal, in writing or by way of trials are given in good faith but without warranty, and this also applies where proprietary rights of third parties are involved. Our advice does not release you from the obligation to check its validity and to test our products as to their suitability for the intended processes and uses. The application, use and processing of our products and the products manufactured by you on the basis of our technical advice are beyond our control and, therefore, entirely your own responsibility. Our products are sold in accordance with our General Conditions of Sale and Delivery. LANXESS Deutschland GmbH Business Unit Inorganic Pigments D51368 Leverkusen Page 7 of 7