BACKGROUND OF THE INVENTION Asphalt is a common component in roofing products, such as shingles, roofing felts, and impregnated fiberglass mats. Asphalt has several desirable properties which make it suitable for use in roofing applications. Asphalt is inherently waterproof, inexpensive, weatherable, exhibits stronger adhesive strength than cohesive strength, and passes through a phase transition from a solid to a liquid at relatively low temperatures. Although the decrease in viscosity as a function of temperature facilitates the processing of asphalt and asphaltic compounds, the thermal sensitivity of these compounds also limits the performance of asphalt after processing.

For example, modified asphalt which is pumpable at processing temperatures of 250° will exhibit dimensional instability in service as a roofing material.

Many attempts have been made to find an additive that will alter and enhance the mechanical properties of asphalt after processing, and particularly when used in roofing products. Adding small amounts of sulfur to asphalt is known to polymerize asphalt from a soft, flexible material which will liquefy with heat into a hard, stiff compound which will not liquefy. Sulfur polymerization of asphalt is an exothermic and self perpetuating reaction which makes processing impractical. The reaction also results in a highly crosslinked, stiff and hard end product that is of little value as a roofing material.

Styrene butadiene rubbers have also been added to asphalt to enhance the thermal stability of the asphalt. While the softening point of the asphalt rubber compound does increase somewhat, the compound remains soft and is highly susceptible to shear stress under thermal load and mechanical deformation. These asphalt compounds modified with styrene butadiene rubber have also met with limited success in roofing products due to the decrease in mechanical properties as the temperature increases.

Polyethylene has also been added to asphalt to increase the thermal stability and creep resistance of the asphalt. Past attempts to modify asphalt with polyethylene have failed due to either incompatibility and gelling during processing or phase separation, immediately after mixing. Prior processing techniques have met with limited success by breaking the extended polymer chain into small independent blocks of carbon chains. The small, broken carbon chains are then held in suspension in the asphalt with minimal contribution to the properties of the mixture. Other processing techniques formulate a modified asphalt with linear low density polyethylene in

which the phase separation is sufficiently retarded to allow processing. The linear low density polyethylene polymer is a small molecule with minimal branching and exhibits similar properties to the broken carbon chains. Other polyethylene modified asphalts require a special asphalt composition with a number of additives that allow the polyethylene to remain in suspension in the asphalt.

Accordingly, a need exists for a modified asphalt compound with improved mechanical properties (e. g. puncture resistance, impact resistance, tear resistance, tensile strength), increased thermal stability, and self-sealing characteristics while maintaining ductility and flexibility. A modified asphalt compound is needed that will extend the service life of asphalt roofing products. A need also exists for a method for preparing a polyethylene modified asphalt in which the polyethylene is maintained in suspension in the asphalt without excessive breakdown of the polymer chains and without phase separation.

SUMMARY OF THE INVENTION The present invention features a method for preparing modified asphalt from a source of polyethylene material and an asphalt composition. The method comprises the steps of: providing pre- processed polyethylene material; combining the pre-processed polyethylene material and the asphalt composition to form a polyethylene asphalt blend; and high shear mixing and heating the polyethylene asphalt blend, for transforming the pre-processed polyethylene material into a three dimensional reinforcing matrix within the asphalt composition.

One example of the method includes the step of pre-processing scrap polyethylene material by reducing the scrap polyethylene material to polyethylene chips. The step of pre-processing the scrap polyethylene material preferably includes grinding the polyethylene material in a grinder until the scrap polyethylene material is reduced to polyethylene chips having a gross diameter between about. lcm to . 6cm. The scrap polyethylene material can include recycled linear polyethylene material, such as recycled or scrap milk, soft drink and food containers. The polyethylene material is preferably a linear high density polyethylene material added to asphalt in an amount of at least about 2% by weight.

The step of combining the pre-pr ocessed polyethylene material with the asphalt composition is preferably performed in a high shear mixer in which the pre-processed polyethylene material is further reduced in combination with the asphalt composition to form the polyethylene asphalt blend.

According to the preferred method, the step of mixing and heating the polyethylene asphalt blend is also performed in the high shear mixer. The polyethylene modified asphalt blend is preferably heated to a temperature in the range of about 240 ° to 350° F.

The method optionally includes mixing at least about 2% by weight of pre-processed styrene butadiene rubber with the asphalt composition to form a styrene butadiene rubber asphalt blend. The styrene butadiene rubber asphalt blend is then mixed with the polyethylene asphalt blend to form the modified asphalt blend. The mixing of the styrene butadiene rubber and the asphalt and the mixing of the polyethylene and the asphalt is preferably done separately in one or

more high shear mixers. The mixing of the styrene butadiene rubber asphalt blend and the polyethylene asphalt blend is preferably done in a low shear mixer.

The present invention also features a polyethylene modified asphalt made according to the process set forth above and materials coated with the polyethylene modified asphalt.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein: Fig. 1 is a schematic diagram of a system for producing the modified asphalt compound according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The modified asphalt compound according to the present invention, in general, includes polyethylene combined with asphalt. The modified asphalt compound optionally includes styrene butadiene rubber combined with the asphalt and polyethylene. The asphalt, polyethylene, and optional styrene butadiene rubber are mixed and heated, as will be described in greater detail below, such that the polyethylene and styrene butadiene rubber blend into the asphalt and act as a reinforcing matrix without excessive breakdown of the polymer chains in the polyethylene.

One use for the modified asphalt compound is in the manufacture of roofing materials including, but not limited to, asphalt impregnated fiberglass mats, asphalt roofing shingles, asphaltic roof

coating and mastics, and exposed roofing membranes. The modified asphalt compound, made according to the present invention, enhances the performance and durability of such roofing products by imparting increased tensile strength, tear resistance, impact resistance, weatherability, thermal stability with self-sealing properties that are typically counter to the mechanical properties, cold temperature flexibility and durability. The present invention also contemplates use of the modified asphalt made according to the present invention with other substrates or materials.

The polyethylene used in the present invention includes high or low density polyethylene material. The preferred polyethylene material is a high or low density polyethylene that does not dissolve or break down when mixed with the asphalt, thereby enhancing performance when the unbroken polymer chains form a reinforcing matrix structure within the asphalt. A preferred range for the specific gravity of the polyethylene is between about 0.9 to 1.0 (g/cm3). The polyethylene material is preferably pre-processed until reduced to having a size and shape that will allow the polyethylene to blend with the asphalt and form a reinforcing matrix. In one example, the pre- processed polyethylene includes polyethylene chips having a gross diameter between about. lcm and. 6 cm.

One source for the high-density polyethylene used in the present invention includes recycled polyethylene material, such as recycled milk bottles, soft drink containers or other scrap polyethylene material. The scrap polyethylene material is pre-processed using a grinder to form the polyethylene chips, as described in greater detail below. Alternatively, the polyethylene material is obtained in a pre-

processed form for use in the system and method of the present invention. The present invention also contemplates other sources for the polyethylene.

The styrene butadiene rubber optionally used in the present invention preferably includes a pre-processed SBS rubber material, such as the crumbed SBS commercially available from Shell or Fena.

Styrene butadiene rubber imparts low temperature flexibility, tack and minor resistance to flow at temperatures between about 100°F and 180°F. Butadiene is a soft, gummy rubber. Styrene is a stiff, rigid polymer. The low molecular weight asphaltenes dissolve the styrene portion of the polymer and the resulting"gaps"in the molecules are filled with asphalt. The remaining butadiene portion of the polymer forms a soft, pliable, highly elastic matrix which enhances flexibility at the expense of thermal stability. The styrene, which ranges between about 10% to 40% by polymer weight improves performance, i. e., thermal stability increases and cold temperature flexibility and tack decrease as styrene content increases. When the SBS rubber is used, the combination of the SBS rubber with the polyethylene has a synergistic effect in that the SBS rubber and the polyethylene form a three dimensional reinforcing matrix.

The modified asphalt blend preferably includes at least about 2% by weight high-density or at least about 3 1-% low density polyethylene and most preferably about 2%-5% by weight. For use with fiberglass mats or other similar roofing materials, about 2%-3% by weight high- density polyethylene is used. The optional styrene butadiene rubber is preferably added in an amount of at least 2% by weight of styrene butadiene and most preferably about 3%-10% by weight depending on the

desired properties of the modified asphalt compound. The present invention contemplates higher percentages of polyethylene and styrene butadiene for other applications, such as extrusion applications and for molding shingles.

The asphalt used with the present invention includes any type of oxidized or unoxidized new or recycled asphalt. In one preferred example, the asphalt of the present invention is obtained from recycled asphalt shingles, such as organic or fiberglass-based roof shingles. One example of a system and method of recycling asphalt shingles is disclosed in greater detail in U. S. Patent No. 5,848,755 and in U. S. Patent Application Serial No. 09/104,085, both assigned to the assignee of the present invention and incorporated herein by reference. In this example, only about one-half as much SBS rubber or polyethylene is needed, for example, about 1-3% by weight. A synergistic effect occurs between the polymers in the asphalt composition and the organic wood fibers or fiberglass fibers in the shingles. The asphalt can also include other additives known to those skilled in the art, although the use of the polyethylene and styrene butadiene rubber avoids having to use additives with the asphalt composition.

The exemplary system 10, Fig. 1, for preparing the modified asphalt compound, according to the present invention, includes hoppers 14,15 for receiving the polyethylene material 12 and the styrene butadiene rubber material 13, respectively. Where scrap polyethylene material 12 is used, the system 10 further includes a polyethylene grinder 11 for pre-processing the scrap polyethylene material 12 into polyethylene chips 18. Alternatively, if the polyethylene material 12

is already pre-processed, the polyethylene grinder 11 can be eliminated. The system 10 also includes a blower conveyer 16 or other similar device for conveying or otherwise transporting the polyethylene chips 18 to a grinder or high shear mixer 24 or preferably to a hopper 19 that feeds the chips 18 to the high shear mixer 24, for example, with a screw feeder (not shown).

The system 10 also includes a blower conveyer 17 or other device for conveying the styrene butadiene rubber crumbs 13 from the hopper 15 to the high shear mixer 24 or hopper 19 connected to the high shear mixer 24. A source 28 of asphalt 26 is coupled to the high shear mixer 24 to supply the asphalt 26 to be mixed with the polyethylene chips 18 and/or the styrene butadiene rubber crumbs 13 into a hot liquid blend. The high shear mixer 24 preconditions the asphalt 26 and mixes the polyethylene chips 18 and/or the styrene butadiene rubber crumbs 13 into the asphalt 26 to form a modified asphalt blend 30 having a substantially homogenous mixture of polymers and asphalt.

The high shear mixer 24 operates at about 3,600 RPM and masticates the polyethylene in chips 18 with a particle size between about 0.0001 inch and 0.05 inch. The modified asphalt blend is preferably mixed under high shear stress until blend is homogeneous. Timing of mixing will vary with formulation, raw materials and processing temperature.

According to a preferred embodiment, the polyethylene chips 18 and the rubber crumbs 13 are separately mixed with the asphalt 26 to form a polyethylene asphalt blend and a styrene butadiene rubber asphalt blend respectively. The blends can be mixed sequentially in the high shear mixer 24 or alternatively, could be mixed simultaneously in separate high shear mixers. The polyethylene

asphalt blend and the styrene butadiene rubber asphalt blend are then combined in the low shear mixer 32.

The modified asphalt blend 30 is also preferably heated during the mixing to a temperature between about 240 ° to 350° F. The combination of high shear mixing and heat prevent phase separation of the polyethylene from the asphalt without dissolving or breaking down the polymer. The blend of asphalt, polyethylene, and styrene butadiene is thereby transformed into a modified asphalt compound in which the polyethylene and styrene butadiene polymers form a three dimensional reinforcing matrix within the asphalt.

The modified asphalt blend (s) 30 is (are) then transferred to a mixing tank 32 where recycled asphalt may be added and the asphalt blend (s) 30 is (are) mixed under low shear stress so that the polyethylene and styrene butadiene polymers are maintained in the three dimensional matrix within the asphalt. The mixing intensity in the mixing tank 32 is sufficient to disperse the polyethylene and styrene butadiene polymers without breaking the polymer chains and interfering with matrix formation. The mixing tank 32 is preferably heated in generally the same manner and temperature range as the high shear mixer 24.

The hot modified asph alt blend 30 is then pumped or otherwise transported by way of a supply line 36 to a coater 38 or other equipment, which applies the hot modified asphalt blend 30 at a precise thickness to a substrate including, but not limited to, a fiberglass mat, scrim, paper or polymer film. The coating of the fiberglass mat employs conventional coating technology, well known in

the roofing industry. Any excess modified asphalt blend is returned to the mixing tank 32 by way of a return line 40.

Accordingly, the method of the present invention allows asphalt to effectively be modified with polyethylene and styrene butadiene such that a substrate coated with the modified asphalt compound according to the present invention imparts waterproofing, high mechanical strength, durability, thermal stability, weatherability and flexibility to the substrate. The polymer matrix provides mechanical strength and dimensional stability to the asphalt, thereby eliminating cracks, splits and cratering when coating the hot asphalt onto the substrate and making the coated substrate resistant to weathering and cracking.

Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention which is not to be limited except by the claims which follow.

What is claimed is: