4.4.3 Selection of Cover Material

 

Assuming that the modeling shows that a case can be made for some degree of significant land coverage, the next step is to determine the type material to be used for surface albedo enhancement. The cover should be inexpensive to produce, install and maintain, highly reflective, puncture and tear resistant, stable for years in an outdoor environment and readily available today or within a few years.

 

Spray-on coatings have been considered in several proposals for use in crop production as agricultural mulches and on a larger scale to combat global warming (127, 128). In this case, the coating is prepared as a liquid emulsion and sprayed onto the surface of the ground. However, the coating used, whether plastic or natural material based, would require large quantities of water. The tendency for spray-on coatings is also for much of the material to soak into the soil, greatly increasing the quantity required to achieve the objective. These problems rule out the direct use of spray-on coatings. However, spray-on coatings may still have a role to play in maintenance of the surface cover and even in its manufacture.

 

Aluminum foil or paper-based coverings could be used to reflect significant amounts of sunlight, but they lack the structural integrity required and could not be kept clean easily if at all. Although plant fiber is quite inexpensive, most of the cost of paper is related to manufacturing, so even use of crop waste or recycled paper would not offer an economic advantage.

 

The best choice for a surface cover material is plastic film. Linear low density polyethylene plastic resin, manufactured from petroleum, is extruded and made into grocery bags, construction film, agricultural film and agricultural mulch on a vast scale, with several billion pounds produced in the U.S. alone each year (129).

 

The film itself is transparent. Additives are included during the extrusion process to modify its reflectivity and stability. Carbon black is added to make the plastic black, while titanium dioxide is used to give it a white color, causing it to reflect 70-80% of the visible light (130, 131). The titanium dioxide also absorbs the UV radiation, reducing, but not preventing the deterioration this radiation eventually causes. Antioxidants, UV inhibitors such as hindered amines and thermal stabilizers are also added to increase its longevity in the sun. Up to 6-8% UV absorbers can be added without affecting the physical properties of the plastic. Certain weatherable grades of titanium dioxide can also be surface coated on the plastic, reducing extrusion costs.

 

Black plastic mulch and construction film have been used for years to control weed growth in vegetable growing and to protect equipment and supplies from the weather or as inexpensive fencing. White plastic has more recently been used for similar purposes (132-134). White plastic mulch is used where cooler soil conditions are desired. It reflects sunlight, increasing the light levels reaching the leaves of growing plants and stimulating their growth, while at the same time, keeping the soil around the plants cooler. The latter is most useful in growing of summer crops that are not as heat tolerant, while the former improves yields in areas with short growing seasons, like northern states.