Control of Granule Application
For both economic as well as esthetic reasons minimizing the quantity of applied mineral surfacing while maximizing coverage is an important consideration during the manufacturing process of bitumen (asphalt) roofing. It is usually more economical to achieve proper shingle weight with coating bitumen (asphalt) than it is with colored roofing granules, and a sheet with a minimum amount of granules necessary for good coverage results in a smoother and better appearing sheet. Also, since all of the granules have adequate bitumen contact, granules adhere to the base better.
Primary factors affecting the quality of roofing granules adhered to a bitumen coated sheet are: the viscosity of the bitumen at the time of granule application; and various roofing machine operating parameters, such as tension, rate of granule feed, velocity of feed, and drum diameter. In addition to those considerations, the specific gravity, shape, and gradation of the granules themselves affect the surfacing weight.
Viscosity of the Coating Bitumen
Proper coating viscosity is important in the application of roofing granules to bitumen coated sheet. If the viscosity is too high, making the surface harder than it should be the granules will tend to remain on the surface of the coating, rather than partially sinking in. Adhesion is likely to be poor, and the amount of granule surfacing will be too low for good coverage. If, on the other hand, the viscosity is too low, the granules will sink in too deeply and interstitial bitumen squeeze-up will pick up excess granules. The primary means of controlling viscosity in a given bitumen coating is through temperature control. For most modified bitumen, temperatures in the range of 180°C to 190°C, and for oxidized asphalts in the range of 190°C to 220°C at the coater are adequate to obtain good adhesion while maintaining relatively high viscosity. These temperature ranges cannot be considered universal, however. Because of differing viscosity characteristic, some asphalt formulations must be hotter to provide good adhesion. A simple test will determine the minimum asphalt temperature necessary for “wetting” the granules. Simply pick out several well-embedded granules from a finished sheet. If the granules pull away some of the coating, the temperature is high enough. However, if the granules do not pull a bit of coating loose and are only stained, the temperature should be raised slightly. Continue checking the finished product until the granules show evidence of having beet “wet.”
The next most important means of controlling coating viscosity is the incorporation of a mineral stabilizer in the coating.
Roofing Machine Adjustment and Modification
Granule surfacing weights can be influenced significantly by the manner in which a roofing machine is operated. (Generally, a machine is operated with as large a margin for error as possible). Often, through changes in adjustment and minor modifications, the amount of granules used in a sheet can be reduced and still achieve optimum coverage.
While roofing machines vary in their adjustability, all of them provide some means of controlling sheet tension. Proper adjustment of tension in certain machine sections can reduce granule usage. The critical point is in the section immediately following granule application. Within 300 to 900 mm of the point where granules are applied, the sheet passes around a drum, variously known as a “slate,” “mica,” “sanding,” “wrap-around,” or “turnover” drum. At this point in the process, the surface of the sheet has an excess of granules on it and high tension on the sheet as it passes around this drum will cause more of the excess granules to be squeezed into contact with the asphalt and remain on the sheet. The tension at this point should be only high enough to pull the sheet through the coater.
For minimum tension around this drum, a pull-through type of coater with low drag is more desirable than a dip coater.
Rate of Granule Feed
In order to save granules, an easy machine adjustment is to reduce the amount of granules fed from the hoppers.
Machine operators will normally apply a large excess of granules to the sheet to make sure that a “stick-up” does not occur on the slate drum, causing an expensive and time consuming shutdown. The application of a large excess of granules to the sheet invariably results in higher granule surfacing weights. Experience has generally shown that a greatly reduced application rate can be used without seriously increasing the danger of stick-ups on the slate drum. This particular problem is more often caused by plugging in the hopper or the feed spout, which completely cuts off the granule supply.
Velocity of Granule Feed
In addition to the quality of granules applied, the method by which the granules are applied also has an effect on the surfacing weight. At the point of granule application, the bitumen coating is fairly fluid. If the granules strike the soft surface at a high velocity, many will be practically buried. Since some granules have one long dimension or one thin dimension, those landing on an end or edge will tend to remain in an upright position, and not lie flat, if they strike the soft surface at high velocity. Obviously, more granules will be required in order to cover the surface. Any technique which reduces the velocity with which the granules strike the sheet will permit more granules to tip over to a flat position, thus reducing the quality of granules necessary for coverage.
Dropping the granules from a point as close to the sheet as possible will reduce the velocity of impact. This technique will have the additional benefit of preventing spot segregation, caused when the falling granule layer separates into streams when dropped from too great a height.
Even more important than the height from which the granules are dropped is the fact thaton some roofing machines the applicator rolls apply the granules in the direction opposite to sheet travel. When operating at high volume production speeds, the granule feed roll imparts horizontal velocity to the granules. Often the granules’ path of fall may deviate as much as 45° from vertical. This velocity, when added to the rapid movement of the sheet in the opposite direction, can combine to give the granules a high impact force. Under these conditions, many of the granules will be driven into the coating, and nearly buried.
By installing a properly designed, curved baffle plate under the hopper, the granules can be diverted into the direction of sheet travel to reduce the relative velocity to little or nothing. This method lays the granules on the surface of the coating gently, and most of the granules will tend to lie flat, thus reducing the amount of granules necessary for good coverage.
In addition to reducing the quality of granules needed for good coverage, the curved baffle also sharpens the dividing line between colored granules and headlap granules. Because the dividing line is sharper at lower relative velocity, it is possible to adjust the hopper dividers following the slate drum in such a manner that fewer colored granules are down-graded to headlap. Depending upon sheet design, approximately 4% to 6% fever colored granules will be used, by saving 6 mm on each side of the colored granule lanes.
Even when all of the previous adjustments have been made, and the roofing machine is
carefully “tuned,” there remains one point at which further surfacing weight reductions can usually be made. Some excess granules generally remain on the face of the sheet as it leaves the slate drum. Many are loosely held between other granules simply because the granule surfaces are somewhat irregular. If the drum which follows the slate drum, used to turn the sheet granule-side up, is smaller in diameter, the sheet will be flexed, permitting these trapped granules to fall off or return to the feed hopper.
All of the suggestions outlines here are presently being used within the asphalt roofing industry and have proven successful in actual operation. To derive the most benefit from these adjustments and modifications, it is necessary to impress upon machine operators that these changes will be successful, but only if used properly and conscientiously.
Please Contact Us to learn more about Granule Surfacing.