EFFECTS OF BITUMEN PIGMENT AND LIGHT

Hot Asphalt's and Macadam's Bitumen Based

The most common of coloured asphalt's and macadams are red and are produced by adding iron oxide pigments to the mixer box in the form of dry powder. This has been made cleaner by the use of pre-weighed bags using low melt polythene and also by pellatising the pigment to lessen the amount of coloured dust.

The drawbacks with adding powdered pigment to the mix are:-

1) The pigment tend to blow about and makes a mess of the plant
2) The operators get contaminated with pigment.
3) Variations in colour from batch to batch due to grinding effect of aggregate.
4) Makes the mix stodgy, requiring higher temperature to lay the product.
5) Decreases the shelf life of product causing more waste for contractor laying the finished product.
6) Problems with safety on some plants.

These problems can be overcome by the use of dispersion. The early dispersions gave problems with softening of the finished product due to their being formulated on processing oils, attempts at producing water based dispersion also gave rise to further problems, the water makes the finished product stodgy and difficult to work and the steam produced causes problems with the filters on the macadam plants.

Advantages of using dispersions.

1) Cleaner to use than dry pigment.
2) Gives stronger colour than dry pigment.
3) Cuts down on mixing time against dry pigment.
4) Can be pumped into the bitumen kettle or mixer box.
5) Finished macadam is easier to lay than material made with dry pigments.
6) Dosage can be controlled from the weighing console.
7) Can be added to the macadam in any proportion as it neither softens nor hardens the bitumen binder.
8) Has a long shelf life, if kept warm to assist pumping settlement will occur and some gentle aditation will be required to stop settlement of the pigment.
9) Contains only low levels of volatile matter.

Dispersions can be produced with a variety of binders depending on the end use and colour required.

Effects of pigments.

While there are a great many iron oxide pigments available and even more blends all of which are capable of being used in red macadams and sand carpet, but the basic pigments fall into two groups, natural and synthetic pigments.
Natural pigments are in general weaker in tint strength and as a result give browner results when used in bitumen products even when used at high levels and it can very often be a false economy to use natural pigments as opposed to synthetic pigment.
With natural pigments, even when micronised, there is a tendency for a greater range of particle sizes and shapes than with synthetic pigments. This can have an effect on the finished product making it more difficult to process and achieve good compaction.
Synthetic pigments in general are far stronger and brighter though in general more expensive but often better value for money if every thing is taken into consideration.
Particle shape, size and distribution are all important, they effect far more than just the colour. To give some idea of what this means in practice four pigment samples where compared. The method was to make up a dispersion of each pigment to the maximum concentration that could be dispersed in a processing oil by using modern techniques and suitable wetting agents and still giving a product that would be capable of being pumped into the asphalt plant, this would not on its own be a suitable product for producing macadam but it cuts out other variables. The pigments where labeled A, B, C & D. A being the most expensive and D being the cheapest. These samples where then made up into macadams using 5% dispersion.

Maximum pigment loading.

A 74% Pigment.

B 58% Pigment<

C 68% Pigment<

D 75% Pigment<

Macadam mix used.

While all the synthetic pigments are nearly the same colour at full strength they show considerable differences in the final macadam. If we take B being the standard at 5% to obtain the same finished colour the following proportions need to be used:-

A Requires 3.2%
B Requires 5% (taken as standard)
C Requires 4.1%
D Even at 10 % level cannot match B

The result of this in the finished costs makes A the cheapest followed by C, B & D to obtain the same finished colour.

Effects of bitumen on colour

The colour of bitumen varies depending on the source of the crude oil that is used as the feed- stock. The following are two samples made with 2 different sources of bitumen of the same hardness.

The hardness of the bitumen effects the colour of the finished macadam, the harder the bitumen the darker the finished product. The following shows the difference between 50, 100 & 200 pen bitumen from the same source when made up into macadam using the same amounts of pigment in each sample:-

The aggregate also effects the colour of the finished macadam. The use of reddish aggregate improves the colour provided that it does not contain a lot of dirty fines. The use of white, limestone types results in a pinkish finished product after some time.

Effects of sunlight.

Sunlight effects the bitumen in the finished macadam, it makes the colour of the bitumen fade resulting red macadam becoming brighter. The time taken for this to happen will depend a lot on the bitumen grade and source. There are other factors which will effect the colour. If the burner on the asphalt plant is incorrectly set or using a poor quality fuel, it will produce carbon in the soot which will effect the colour of the macadam., this will not fade with light and the pigmenting effect of carbon black is strong. Keeping the bitumen at elevated temperature for long periods can accelerated carbonization which will make the bitumen blacker.
The use of dispersion with a clear binder accelerates the fading of the blackness of the bitumen because it lowers the amount of bitumen in the finished product.

Other Colours Bitumen Based

By careful choice of bitumen and aggregate and relying on the fading effect of sunlight on the black effect of the bitumen in the macadam an interesting range of colours can be produced.

While the major colour used for surface coating is red the following shows the effect of Green, Blue, White, Yellow and Red at a level of 3% in 200 pen bitumen.

Colour when laid ..................... Colour after 2 months.

Copyright © 1996 David J Higham