Sheet material made from cotton canvas has traditionally been used in making protective covers for motor vehicles, boats, and machinery. Canvas covers are strong and relatively durable. However, canvas sheeting suffers from the disadvantages that it is heavy, permeable to water, and subject to rapid degradation when left exposed to sun or moisture.
Lighter weight plastic sheet material made from polymer films has also been used in making protective covers. Unfortunately, most plastic films suffer from the disadvantage that they are not breathable, and therefore, they trap moisture beneath their surface where the moisture can damage the covered item. Plastic films also frequently suffer from the disadvantages that they are easily ripped or torn, and they do not drape well over a covered item.
Protective covers have also been made from nonwoven webs melt extruded from a variety of thermoplastic polymer resins, which webs have been bonded to a reinforcing scrim material. U.S. Pat. No. 5,182,162 discloses protective covers made from a melt extruded nonwoven polyolefin sheet of very uniform thickness that has been thermal bonded to a cross-laminated thermoplastic net-like web. The cross-laminated thermoplastic net-like material of U.S. Pat. No. 5,182,162 adds strength and stability to the nonwoven sheet, but such products do not demonstrate the moisture vapor transmissibility, strength and durability required for many protective cover applications.
Nonwoven sheets made from thermal bonded flash-spun polyethylene webs have been used to make protective covers. Such materials are lightweight, they are moisture vapor permeable, and they do not rot with prolonged exposure to moisture. Nonwoven sheets made from flash-spun polyethylene plexifilamentary fiber webs are disclosed in U.S. Pat. No. 3,169,899. One such commercial sheet material that has been used in automobile storage cover applications is TYVEK.RTM. spunbonded olefin sheet material sold by E. I. du Pont de Nemours and Company of Wilmington, Del. ("DuPont"). TYVEK.RTM. is a registered trademark of DuPont. TYVEK.RTM. sheets, are made from flash-spun polyethylene that has been thermal bonded to form lightweight sheets that have outstanding mechanical properties while also exhibiting good tolerance to ordinary weather conditions. Specifically, TYVEK.RTM. sheet material has a high moisture vapor transmission rate, good liquid water permeability resistance, and good tear strength. In addition, because TYVEK.RTM. sheet material is made from high density polyethylene, it is readily recyclable. TYVEK.RTM. sheet material can also be readily printed with advertising or other information and it can also be laundered repeatedly. Finally, TYVEK.RTM. sheet material can be made with additives that make the sheets more resistant to degradation resulting from prolonged exposure to heat or UV radiation.
Although TYVEK.RTM. flash-spun polyethylene sheets have been found to exhibit an array of superb physical properties for protective cover applications, such sheet material is not sufficiently strong and durable for certain applications. For example, TYVEK.RTM. sheet material ripped when used in protective covers that were applied to boats during highway transit from a factory to a dealership. Also, in applications where covers made from TYVEK.RTM. flash-spun polyethylene sheet material were exposed to UV radiation from the sun for extended periods of time, the strength and toughness of the sheets has been found to degrade. Accordingly, there is a need for a lightweight, recyclable, liquid impermeable, moisture vapor permeable sheet material that exhibits a weight, permeability, and moisture vapor transmission rate similar to that of TYVEK.RTM. flash-spun polyethylene sheet material, yet also exhibits increased strength, durability, and significantly improved resistance to degradation from exposure to UV radiation.