Gilsonite Application Summary: Paints, Coatings and StainsIn the paint applications, Gilsonite is usually used in combination with bitumen (asphalt). In most cases, if Gilsonite is used alone, the final paint will be very hard and brittle after drying. If normal straight-run bitumen is used alone, the finished paint is too soft and tacky . Therefore, a combination of Gilsonite and bitumen is used to achieve the desired hardness (penetration) and drying time of the finished paint. In addition to imparting hardness to the paint, Gilsonite is also increasing the paint's: (1) adhesion; (2) gloss; (3) chemical resistance; (4) water resistance; and (5) body.
For hardness, Gilsonite has a zero (0) penetration (at 25°C; 100 gm, 5 sec.) compared to the 60-70 pen, 80-100 pen or softer bitumens commonly available from petroleum companies or asphalt manufacturers. Approximately 90% of all bitumen is used for road construction and these hardness grades are acceptable for that purpose. However, they are too soft by themselves for the manufacture of paint and surface coatings which require hardness values around 5 penetration for acceptable paint drying. Paint films composed of bitumen alone are tacky if the penetration is more than 10, and are brittle if less than 4.
Air-blowing soft, road paving bitumens down to 5 pen is a common practice but this is an advanced aging process which damages the bitumen. Air-blowing upsets the colloidal balance of the bitumen which can lead to films of oil or wax exuding from the bitumen. If these films exude to the surface of the paint, the result is a loss of gloss. If these films migrate to the interface of the substrate, the result is a loss of adhesion which causes the paint to peel. Other disadvantages of using air-blown bitumen in paint formulations include gelling with certain solvents and large viscosity increases during the mixing of the paints or during their storage.
Therefore, rather than air - blowing a soft bitumen down to 5 penetration, that same soft bitumen may be transformed into a hard bitumen by modifying it with Gilsonite, avoiding the disadvantages of oil/wax migration and gelation. For example, adding about 35 to 50% Gilsonite to an 80 - 100 pen etration bitumen will transform it to 5 - 10 penetration, suitable for paint making.
The preferred formula, (C), shows that a combination of 25 parts each of 80 - 100 bitumen and Gilsonite in 50 parts mineral spirits yields a paint with the same penetration as using air - blown bitumen. It dries in 10 - 15 minutes and has greatly improved gloss and viscosity stability. Alternate formula (D) uses some air - blown bitumen and less Gilsonite, to reduce raw materials costs, giving a higher softening point but gloss and viscosity stability are reduced.
The adhesion of a bitumen - based paint may be increased with Gilsonite addition because of Gilsonite's relatively high nitrogen content, about 3.0% for Gils onite versus 0.3% for bitumen. The addition of nitrogen to the paint formula results in a high percentage of polar compounds which improve the adhesion of the paint to the substrate. Because Gilsonite is hydrophobic and nearly chemically inert, both the wa ter resistance and chemical resistance of paints are increased with Gilsonite addition.
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