Method for preparation of stable bitumen polymer compositions

The present invention provides a method for preparing an asphalt and thermoplastic elastomer composition. The process comprises heating an asphalt cut in a stirred tank to a temperature sufficient to allow the stirring of the asphalt in the tank. A thermoplastic elastomer or rubber is added to the asphalt while continuing to stir the asphalt. The mixture is stirred at a speed and for a period of time sufficient to increase the distribution of the elastomer into the asphalt. The stirring speed is reduced and the temperature is increased to add an oil dispersion of crosslinking agents to the tank. Stirring is continued for a period of time sufficient to improve the distribution of the crosslinking agent dispersion in the asphalt.

BACKGROUND OF THE INVENTION
 This invention is directed to bitumen compositions, which are prepared from
 bitumen, polymers such as copolymers of styrene and a conjugated-diene,
 and defined amounts of crosslinking agents such as sulfur. The bitumen
 compositions described herein are useful in industrial applications, such
 as in hot mix asphalts useful in preparing aggregates for road paving.
 The use of bitumen (asphalt) compositions in preparing aggregate
 compositions (bitumen+rock) useful as road paving material is complicated
 by at least three factors, each of which imposes a serious impediment to
 providing an acceptable product. First, the bitumen compositions must meet
 certain performance criteria or specifications in order to be considered
 useful for road paving. For example, to ensure acceptable performance,
 state and federal agencies issue specifications for various bitumen
 applications including specifications for use as road pavement. Current
 Federal Highway Administration specifications designate a bitumen
 (asphalt) product, for example, AC-20R as meeting defined parameters
 relating to properties such as viscosity, toughness, tenacity and
 ductility (see Table 1). Each of these parameters define a critical
 feature of the bitumen composition, and compositions failing to meet one
 or more of these parameters will render that composition unacceptable for
 use as road pavement material.
 Conventional bitumen compositions frequently cannot meet all of the
 requirements of a particular specification simultaneously and, if these
 specifications are not met, damage to the resulting road can occur,
 including permanent deformation, thermally induced cracking and flexural
 fatigue. This damage greatly reduces the effective life of paved roads.
 In this regard, it has long been recognized that the properties of
 conventional bitumen compositions can be modified by the addition of other
 substances, such as polymers. A wide variety of polymers have been used as
 additives in bitumen compositions. For example, copolymers derived from
 styrene and conjugated dienes, such as butadiene or isoprene, are
 particularly useful, since these copolymers have good solubility in
 bitumen compositions and the resulting modified-bitumen compositions have
 good rheological properties.
 It is also known that the stability of polymer-bitumen compositions can be
 increased by the addition of crosslinking agents such as sulfur,
 frequently in the form of elemental sulfur. It is believed that the sulfur
 chemically couples the polymer and the bitumen through sulfide and/or
 polysulfide bonds. The addition of extraneous sulfur is required to
 produce the improved stability, even though bitumens naturally contain
 varying amounts of native sulfur.
 Thus, U.S. Pat. No. 4,145,322, issued Mar. 20, 1979 to Maldonado et al.,
 discloses a process for preparing a bitumen-polymer composition consisting
 of mixing a bitumen, at 266.degree.-446.RTM. F. (130.degree.-230.degree.
 C.), with 2 to 20% by weight of a block copolymer, having an average
 molecular weight between 30,000 and 300,000, with the theoretical formula
 S.sub.x -B.sub.y, in which S corresponds to styrene structure groups and B
 corresponds to conjugated diene structure groups, and x and y are
 integers. The resulting mixture is stirred for at least two hours, and
 then 0.1 to 3% by weight of sulfur relative to the bitumen is added and
 the mixture agitated for at least 20 minutes. The preferred quantity of
 added sulfur cited in this patent is 0.1 to 1.5% by weight with respect to
 the bitumen. The resulting bitumen-polymer composition is used for
 road-coating, industrial coating, or other industrial applications.
 Similarly, U.S. Pat. No. 4,130,516, issued Dec. 19, 1978 to Gagle et al.,
 discloses an asphalt (bitumen) polymer composition obtained by
 hot-blending asphalt with 3 to 7% by weight of elemental sulfur and 0.5 to
 1.5% by weight of a natural or synthetic rubber, preferably a linear,
 random butadiene/styrene copolymer. U.S. Pat. No. 3,803,066, issued Apr.
 9, 1974 to Petrossi, also discloses a process for preparing a
 rubber-modified bitumen by blending rubber, either natural or synthetic,
 such as styrene/butadiene rubber, with bitumen at 293.degree.-365.degree.
 F. (145.degree.-185.degree. C.), in an amount up to 10% by weight based on
 the bitumen, then adjusting the temperature to 257.degree.-320.degree. F.
 (125.degree.-160.degree. C.), and intimately blending into the mix an
 amount to sulfur such that the weight ratio of sulfur to rubber is between
 0.3 and 0.9. A catalytic quantity of a free-radical
 vulcanization-accelerator is then added to effect vulcanization. This
 patent recites the critical nature of the sulfur to rubber ratio, and
 teaches that weight ratios of sulfur to rubber of less than 0.3 gives
 modified bitumen of inferior quality.
 Although polymer-modified bitumen compositions are known, these previously
 described compositions are not necessarily useful for road paving
 applications. For example, mixing NorthWest paving asphalt having an
 initial viscosity of 682 poise at 140.degree. F. (60.degree. C.) with 3.6
 weight percent Kraton.RTM.-4141, a commercially available
 styrene-butadiene tri-block copolymer which contains 29 weight percent
 plasticizer oil, and 0.25% sulfur gives a modified-asphalt composition
 with a viscosity of 15,000 poise at 140.degree. F. (60.degree. C.). This
 viscosity, however, greatly exceeds the acceptable viscosity range set by
 the widely-used AC-20R specification for paving asphalt. This
 specification, issued by the Federal Highway Administration, requires
 bitumen compositions to have a viscosity in the range of 1600-2400 poise
 at 140.degree. F. (60.degree. C.). Thus, the modified bitumen compositions
 produced by the procedures of U.S. Pat. No. 4,145,322 using
 Kraton.RTM.-4141 would be unacceptable for use in road paving under the
 AC-20R specification.
 The second factor complicating the use of bitumen compositions concerns the
 viscosity stability of such compositions under storage conditions. In this
 regard, bitumen compositions are frequently stored for up to 7 days or
 more before being used and, in some cases, the viscosity of the
 composition can increase so much that the bitumen composition is unusable
 for its intended purpose. On the other hand, a storage stable bitumen
 composition would provide for only minimal viscosity increases and,
 accordingly, after storage it can still be employed for its intended
 purpose.
 The third factor complicating the use of bitumen compositions concerns the
 use of volatile solvents in such compositions. Specifically, while such
 solvents have been heretofore proposed as a means to fluidize
 bitumen-polymer compositions containing relatively small amounts of sulfur
 which compositions are designed as coatings (Maldonado et al., U.S. Pat.
 No. 4,242,246), environmental concerns restrict the use of volatile
 solvents in such compositions. Moreover, the use of large amounts of
 volatile solvents in bitumen compositions may lower the viscosity of the
 resulting composition so that it no longer meets viscosity specifications
 designated for road paving applications. In addition to the volatile
 components, reduction of other emissions during asphalt applications
 becomes a target. For example, it is desirable to reduce the amount of
 sulfur compounds that are emitted during asphalt applications.
 Asphaltic concrete, typically including asphalt and aggregate, asphalt
 compositions for resurfacing asphaltic concrete, and similar asphalt
 compositions must exhibit a certain number of specific mechanical
 properties to enable their use in various fields of application,
 especially when the asphalts are used as binders for superficial coats
 (road surfacing), as asphalt emulsions, or in industrial applications.
 (The term "asphalt" is used herein interchangeably with "bitumen."
 Asphaltic concrete is asphalt used as a binder with appropriate aggregate
 added, typically for use in roadways.) The use of asphalt or asphalt
 emulsion binders either in maintenance facings as a surface coat or as a
 very thin bituminous mix, or as a thicker structural layer of bituminous
 mix in asphaltic concrete, is enhanced if these binders possess the
 requisite properties such as desirable levels of elasticity and
 plasticity.
 Previously, various polymers have been added to asphalts to improve
 physical and mechanical performance properties. Polymer-modified asphalts
 are routinely used in the road construction/maintenance and roofing
 industries. Conventional asphalts often do not retain sufficient
 elasticity in use and, also, exhibit a plasticity range which is too
 narrow for use in many modern applications such as road construction. It
 is known that the characteristics of road asphalts and the like can be
 greatly improved by incorporating into them an elastomeric-type polymer
 which may be one such as butyl, polybutadiene, polyisoprene or
 polyisobutene rubber, ethylene/vinyl acetate copolymer, polyacrylate,
 polymethacrylate, polychloroprene, polynorbornene,
 ethylene/propylene/diene (EPDM) terpolymer and advantageously a random or
 block copolymer of styrene and a conjugated diene. The modified asphalts
 thus obtained commonly are referred to variously as bitumen/polymer
 binders or asphalt/polymer mixes. Modified asphalts and asphalt emulsions
 typically are produced utilizing styrene/butadiene based polymers, and
 typically have raised softening point, increased viscoelasticity, enhanced
 force under strain, enhanced strain recovery, and improved low temperature
 strain characteristics.
 The bituminous binders, even of the bitumen/polymer type, which are
 employed at the present time in road applications often do not have the
 optimum characteristics at low enough polymer concentrations to
 consistently meet the increasing structural and workability requirements
 imposed on roadway structures and their construction. In order to achieve
 a given level of modified asphalt performance, various polymers are added
 at some prescribed concentration.
 Current practice is to add the desired level of a single polymer, sometimes
 along with a reactant which promotes cross-linking of the polymer
 molecules until the desired asphalt properties are met. This reactant
 typically is sulfur in a form suitable for reacting. Such current
 processes are discussed in various patents such as U.S. Pat. Nos.
 4,145,322 (Maldonado); 5,371,121 (Bellamy); and 5,382,612 (Chaverot), all
 of which are hereby incorporated by reference.
 However, cost of the polymer adds significantly to the overall cost of the
 resulting asphalt/polymer mix. Thus, cost factors weigh in the ability to
 meet the above criteria for various asphalt mixes. In addition, at
 increasing levels of polymer concentration, the working viscosity of the
 asphalt mix becomes excessively great and separation of the asphalt and
 polymer may occur.
 One result of the high viscosities experienced at increased polymer
 concentrations is that it makes emulsification of the asphalt difficult.
 As is known in the art and used herein, emulsification of asphalt refers
 to forming an emulsion of asphalt and water. Asphalt emulsions are
 desirable in many applications because the emulsion may be applied at
 lower temperatures than hot-mix asphalts because the water acts as a
 carrier for the asphalt particles.
 For example, hot-mix asphalts, mixes of asphalt, aggregate, and a single
 polymer, commonly are applied at a temperature of 350.degree. Fahrenheit
 (F.) to 450.degree. F. (177.degree. Centigrade (C.) to 232.degree. C.) to
 achieve the requisite plasticity for application. In comparison, an
 asphalt emulsion typically may be applied at 130.degree. F. to 170.degree.
 F. (54.degree. C. to 77.degree. C.) to achieve the same working
 characteristics. Emulsified asphalt products are generally used to reduce
 the release of environmentally-harmful volatile organic compounds normally
 associated with asphalts diluted with light carrier solvents such as
 diesel fuel, naphtha, and the like. Emulsification basically requires that
 the asphalt and any desired performance-enhancing additives be combined
 with an emulsifying agent in an emulsification mill along with about 20 to
 40 percent by weight of water. However, high polymer loading in an asphalt
 produces high viscosities and melting points, making emulsification of the
 polymer-asphalt composition difficult.
 The bitumen/polymer compositions are prepared in practice at polymer
 contents range from about 3% to 6% by weight of bitumen depending on the
 nature and the molecular weight of the polymer and the quality of the
 bitumen. Gelling of the bitumen/polymer composition, which is observed
 fairly frequently during the preparation of the said composition or while
 it is stored, occurs as soon as the polymer content of this composition
 exceeds the above-mentioned threshold. It is thus difficult, in practice,
 to produce non-gellable bitumen/polymer compositions with a high polymer
 content, which would act as bitumen/polymer concentrates, and are more
 economical to prepare and to transport than bitumen/polymer compositions
 with a lower polymer content, and which could be diluted at the time of
 use, by addition of bitumen, in order to obtain the corresponding
 bitumen/polymer binders with a lower polymer content which are usually
 used to make coatings.
 In view of the above, bitumen compositions, which simultaneously meet the
 performance criteria required for road paving, and which are substantially
 free of volatile solvent would be advantageous. Additionally, viscosity
 stable bitumen compositions would be particularly advantageous. Further, a
 method for efficiently introducing the polymer into the bitumen
 composition would be desirable. In preparing the composition, significant
 mixing is needed to insure the uniform addition of both the polymer and
 any crosslinking agents. The crosslinking agents are usually added as a
 dry powder and mixed with the asphalt compositions.
 TABLE 1
 Properties of Various Asphalt Grades
 AASHTO M-226
 TEST AC 2.5 AC 5 AC 10 AC 20 AC 30
 AC 40
 Viscosity @ 140.degree. F., poise 250 .+-. 50 500 .+-. 100 1000 .+-. 200
 2000 .+-. 400 3000 .+-. 600 4000 .+-. 800
 (AASHTO T-202)
 Viscosity @ 275.degree. F.; cSt, 125 175 250 300
 350 400
 minimum (AASHTO T-201)
 Pen. @ 77.degree. F.; minimum 220 140 80 60
 50 40
 AASHTO t-49)
 Flash Point, COC 325 350 425 450 450
 450
 Minimum .degree.F.
 Ductility After TFOT 100 100 75 50 40
 25
 (AASHTO T-179)
 @ 77.degree. F., 5 cm/min,
 minimum
 Viscosity After TFOT 1000 2000 4000 8000 12000
 16000
 (AASHTO T-179)
 @ 140.degree. F., poise
 minimum
 TEST AR1000 AR2000 AR4000 AR8000
 AR16000
 Viscosity @ 140.degree. F., poise 1000 .+-. 250 2000 .+-. 500 4000 .+-.
 1000 8000 .+-. 2000 16000 .+-. 4000
 (AASHTO T-202)
 Viscosity @ 275.degree. F., cSt, 140 200 275
 400 500
 minimum (AASHTO T-201)
 Pen. @ 77.degree. F., minimum 65 40 25 20
 20
 (AASHTO T-49)
 Percent of Original -- 40 45 50 52
 Pen. @ 77.degree. F., minimum
 Ductility @ 77.degree. F., 100 100 75 75
 75
 minimum, 5 cm/min
 As can be seen from the above, the art is replete with methods to improve
 the mixing of asphalt and polymer compositions. The needed elements for
 the commercial success of any such process include keeping the process as
 simple as possible, reducing the cost of the ingredients, and utilizing
 available asphalt cuts from a refinery without having to blend in more
 valuable fractions. In addition, the resulting asphalt composition must
 meet the above-mentioned governmental physical properties and
 environmental concerns. Thus, it is a target of the industry to reduce the
 cost of the polymers and crosslinking agents added to the asphalt without
 sacrificing any of the other elements.
 SUMMARY OF THE INVENTION
 In accordance with one embodiment of the present invention, the bitumen is
 heated up to the desired temperature in a stirred tank. The temperature in
 the tank is sufficient to allow the stirring of the asphalt and is usually
 320.degree. F. or more. A suitable rubber or thermoplastic elastomer is
 then added and mixing is continued for a period of time. Preferred
 thermoplastic elastomers are styrene butadiene copolymers having a styrene
 content of fifty percent (50%) or less. A composition of crosslinking
 agents comprising zinc-mercaptobenzothiazole and sulfur is introduced to
 the mixture in the tank. The concentration of the crosslinking agents in
 the overall composition in the tank should be at least 0.02 percent by
 weight of zinc-mercaptobenzothiazole and 0.06 percent by weight of sulfur.
 Elemental sulfur is preferred but sulfur donating compounds are also
 utilized. Zinc oxide may also be added as part of the crosslinking
 composition. The crosslinking agents are added in various forms such as
 dry components, in an oil dispersion, or as a water emulsion. The emulsion
 or dispersion preferably has a crosslinking chemicals content of about
 fifty percent or more and are stable during shipping and storage.
 Preferably, the dispersion is an oil dispersion comprising about fifty
 percent (50%) active ingredients. In a preferred embodiment, the
 dispersion comprises an oil dispersion wherein the oil has a flash point
 above 450.degree. F. and is liquid at room temperature. The crosslinking
 agents utilized in one embodiment comprised ZMBT (zinc
 2-mercaptobenzothiazole): ZnO (zinc oxide): S (sulfur) in a 1:1:8 weight
 ratio.

DESCRIPTION OF THE INVENTION
 As used herein, the term "bitumen" (sometimes referred to as "asphalt")
 refers to all types of bitumens, including those that occur in nature and
 those obtained in petroleum processing. The choice of bitumen will depend
 essentially on the particular application intended for the resulting
 bitumen composition. Preferred bitumens have an initial viscosity at
 140.degree. F. (60.degree. C.) of 600 to 3000 poise depending on the grade
 of asphalt desired. The initial penetration range (ASTM D5) of the base
 bitumen at 77.degree. F. (25.degree. C.) is 50 to 320 dmm, preferably 75
 to 150 dmm, when the intended use of the copolymer-bitumen composition is
 road paving. Bitumens which do not contain any copolymer, sulfur, etc.,
 are sometimes referred to herein as a "base bitumen."
 As used herein, the term "volatile solvent" refers to a hydrocarbon solvent
 which has a distillation point or range which is equal to or less than
 350.degree. C. Such solvents are known to vaporize to some extent under
 ambient conditions and, accordingly, pose environmental concerns relating
 to hydrocarbon emissions.
 The term "substantially free of volatile solvent" means that the complete
 (final) bitumen composition contains less than about 3.5 weight percent of
 volatile solvent. Preferably, the bitumen composition contains less than
 about 2 weight percent of volatile solvent and more preferably, less than
 about 1 weight percent of volatile solvent.
 "Elastomeric Polymers" are natural or synthetic rubbers and include butyl,
 polybutadiene, polyisoprene or polyisobutene rubber, ethylene/vinyl
 acetate copolymer, polyacrylate, polymethacrylate, polychloroprene,
 polynorbornene, ethylene/propylene/diene (EPDM) terpolymer and
 advantageously a random or block copolymer of styrene and a conjugated
 dienes. It is preferred to use styrene/conjugated diene block copolymers,
 linear, radial, or multi-branched. Styrene/butadiene and styrene/isoprene
 copolymers having an average molecular weight of between 30,000 and
 300,000 have been found to be particularly useful in the present
 invention.
 "Conjugated-dienes" refer to alkene compounds having 2 or more sites of
 unsaturation wherein a second site of unsaturation is conjugated to a
 first site of unsaturation, i.e., the first carbon atom of the second site
 of unsaturation is gamma (at carbon atom 3) relative to the first carbon
 atom of the first site of unsaturation. Conjugated dienes include, by way
 of example, butadiene, isoprene, 1,3-pentadiene, and the like.
 "Block copolymers of styrene and conjugated-dienes" refer to copolymers of
 styrene and conjugated-dienes having a linear or radial, tri-block
 structure consisting of styrene-conjugated diene-styrene block units which
 copolymers are represented by the formula:
EQU S.sub.x --D.sub.y --S.sub.z
 where D is a conjugated-diene, S is styrene, and x, y and z are integers
 such that the number average molecular weight of the copolymer is from
 about 30,000 to about 300,000. These copolymers are well known to those
 skilled in the art and are either commercially available or can be
 prepared from methods known per se in the art. Preferably, such tri-block
 copolymers are derived from styrene and a conjugated-diene, wherein the
 conjugated-diene is butadiene or isoprene. Such copolymers preferably
 contain 15 to 50 percent by weight copolymer units derived from styrene,
 preferably 25 to 35 percent derived from styrene, more preferably 28 to 31
 percent derived from styrene, the remainder being derived from the
 conjugated diene. These copolymers preferably have a number average
 molecular weight range between 50,000 and 200,000, more preferably between
 100,000 and 180,000. The copolymer can employ a minimal amount of
 hydrocarbon solvent in order to facilitate handling. Examples of suitable
 solvents include plasticizer solvent which is a non-volatile aromatic oil.
 However, when the hydrocarbon solvent is a volatile solvent (as defined
 above), care should be taken to ensure that the amount of solvent
 contained in the final bitumen composition is less than about 3.5 weight
 percent.
 The term "sulfur" refers to elemental sulfur in any of its physical forms
 or any sulfur donating compound. Sulfur donating compounds are well known
 in the art and include various organic compositions or compounds that
 generate sulfur under the mixing or preparation conditions of the present
 invention. Preferably, the elemental sulfur is in powder form known as
 flowers of sulfur.
 The term "desired Rheological Properties" refers to bitumen compositions
 having a viscosity at 140.degree. F. (60.degree. C.) of from 1600 to 4000
 poise before aging; a toughness of at least 110 inch-pound (127
 cm-kilograms) before aging; a tenacity of at least 75 inch-pound (86.6
 cm-kilograms) before aging; and a ductility of at least 25 cm at
 39.2.degree. F. (4.degree. C.) at 5 cm/min. pull rate after aging. Each of
 these desired Rheological Properties are necessary parameters in meeting
 the AC-20(R) specifications for bitumen compositions suitable for use as
 road pavement material (See Table 1).
 Viscosity measurements are made by using ASTM test method D2171. Ductility
 measurements are made by using ASTM test method DI13. Toughness and
 tenacity measurements are made by a Benson Method of Toughness and
 Tenacity, run at 20 inches/minute (50.8 cm/minute) pull rate with a 1/8
 inch (2.22 cm) diameter ball.
 By "storage stable viscosity" it is meant that the bitumen composition
 shows no evidence of skinning, settlement, gelation, or graininess and
 that the viscosity of the composition does not increase by a factor of
 four or more during storage at 325.degree.+-0.5.degree. F.
 (163.degree.+-2.8.degree. C.) for seven days. Preferably the viscosity
 does not increase by a factor of two or more during storage at 325.degree.
 F. (163.degree. C.) for seven days. More preferably the viscosity
 increases less than 50% during seven days of storage at 325.degree. F.
 (163.degree. C.). A substantial increase in the viscosity of the bitumen
 composition during storage is not desirable due to the resulting
 difficulties in handling the composition and in meeting product
 specifications at the time of sale and use.
 The term "aggregate" refers to rock and similar material added to the
 bitumen composition to provide an aggregate composition suitable for
 paving roads. Typically, the aggregate employed is rock indigenous to the
 area where the bitumen composition is produced. Suitable aggregate
 includes granite, basalt, limestone, and the like.
 As used herein, the term "asphalt cement" refers to any of a variety of
 substantially unblown or unoxidized solid or semi-solid materials at room
 temperature which gradually liquify when heated. Its predominant
 constituents are bitumens, which may be naturally occurring or obtained as
 the residue of refining processing. The asphalt cements are generally
 characterized by a penetration (PEN, measured in tenths of a millimeter,
 dmm) of less than 400 at 25.degree. C., and a typical penetration is
 between 40 and 300 (ASTM Standard, Method D-5). The viscosity of asphalt
 cement at 60.degree. C. is more than about 65 poise. Asphalt cements are
 often defined in terms specified by the American Association of State
 Highway Transportation Officials (AASHTO) AR viscosity system. Two sets of
 typical specifications are shown in Table 1.
 The asphalt terms used herein are well known to those skilled in the art.
 For an explanation of these terms, reference is made to the booklet
 SUPERPAVE Series No. 1 (SP-1), 1997 printing, published by the Asphalt
 Institute (Research Park Drive, P.O. Box 14052, Lexington, Ky 40512-4052),
 which is hereby incorporated by reference in its entirety. For example,
 Chapter 2 provides an explanation of the test equipment, terms, and
 purposes. Rolling Thin Film Oven (RTFO) and Pressure Aging Vessel (PAV)
 are used to simulate binder aging (hardening) characteristics. Dynamic
 Shear Rheometers (DSR) are used to measure binder properties at high and
 intermediate temperatures. This is used to predict permanent deformation
 or rutting and fatigue cracking. Bending Beam Rheometers (BBR) are used to
 measure binder properties at low temperatures. These values predict
 thermal or low temperature cracking. The procedures for these experiments
 are also described in the above-referenced SUPERPAVE booklet.
 Asphalt grading is given in accordance with accepted standards in the
 industry as discussed in the above-referenced Asphalt Institute booklet.
 For example, pages 62-65 of the booklet include a table entitled
 Performance Graded Asphalt Binder Specifications. The asphalt compositions
 are given performance grades, for example, PG 64-22. The first number, 64,
 represents the average 7-day maximum pavement design temperature in
 .degree. C. The second number, -22, represents the minimum pavement design
 temperature in .degree. C. Other requirements of each grade are shown in
 the table. For example, the maximum value for the PAV-DSR test (.degree.
 C.) for PG 64-22 is 25.degree. C.
 One of the methods commonly utilized in the industry to standardize the
 measure or degree of compatibility of the rubber with the asphalt is
 referred to as the compatibility test. The test comprises the mixing of
 the rubber and asphalt with all the applicable additives, such as the
 crosslinking agents. The mixture is placed in tubes, usually made of
 aluminum or similar material, referred to as cigar tubes or toothpaste
 tubes. These tubes are about one inch in diameter and about fifty
 centimeters deep. The mixture is placed in an oven heated to a temperature
 of about 162.degree. C. (320.degree. F.). This temperature is
 representative of the most commonly used asphalt storage temperature.
 After the required period of time, most commonly twenty-four (24) hours,
 the tubes are transferred from the oven to a freezer and cooled down to
 solidify. The tubes are kept in the vertical position. After cooling down,
 the tubes are cut in one thirds, three equal sections. The softening point
 of the top one third is compared to the softening point of the bottom
 section. This test gives an indication of the separation or compatibility
 of the rubber within the asphalt. The rubber would have the tendency to
 separate to the top. The lower the difference in softening point between
 the top and bottom sections, the more compatible are the rubber and
 asphalt. In today's environment, most states require a difference of
 4.degree. F. (2.degree. C.) or less to consider the asphalt/rubber
 composition as compatible. Few standards allow a higher difference. The
 twenty-four hour test is used as a common comparison point.
 In accordance with one embodiment of the present invention, an asphalt
 composition is prepared by adding the asphalt or bitumen to a mixing tank
 that has stirring means. The asphalt is added and stirred at elevated
 temperatures. Stirring temperatures depend on the viscosity of the asphalt
 and can range up to 500.degree. F. Asphalt products from refinery
 operations are well known in the art. For example, asphalts typically used
 for this process are obtained from deep vacuum distillation of crude oil
 to obtain a bottom product of the desired viscosity or from a solvent
 deasphalting process that yields a demetalized oil, a resin fraction and
 an asphaltene fraction. Some refinery units do not have a resin fraction.
 These materials or other compatible oils of greater than 450.degree. F.
 flash point may be blended to obtain the desired viscosity asphalt.
 Rubbers, elastomeric polymers, or thermoplastic elastomers suitable for
 this application are well known in the art as described above. For
 example, Finaprene products available from Fina Oil and Chemical Company
 are suitable for the applications of the present invention. This example
 is not limiting for the technology which can be applied to any similar
 elastomeric product particularly those produced from styrene and
 butadiene.
 Various crosslinking agents for asphalt applications were tested as shown
 in the tables below. In a preferred embodiment, elemental sulfur and
 organic zinc compounds are used. These crosslinking agents are normally
 sold in powder or flake form.
 TABLE 2
 Number 932-163 950-38 950-39
 % Asphalt 100 95.69 95.6
 % Rubber 3.99 3.98
 % ZMBT 0.2 0.1025
 % ZnO 0.2
 % Sulfur 0.12 0.12
 24 hour compat, top, F. 156 173.4
 24 hour compat, difference, F. 1.9 10.4
 Binder DSR 65.9 81.4
 RTFO DSR 66.4 77.8
 PAV DSR 22.2 18.8
 BBR m Value -14.68 -18.35
 BBR s value -13.89 -18.34
 PG Grade PG64-22 PG76-28
 The data in Table 2 shows the values for a control sample without any
 rubber addition and samples where elemental sulfur was used in addition to
 organic zinc and zinc oxide. The organic zinc is zinc
 2-mercaptobenzothiazole (CAS Reg. No.: 155-04-4), hereinafter referred to
 as ZMBT. The rubber utilized is Finaprene 401, available from Fina Oil and
 Chemical Company in Dallas, Tex. Finaprene 401 is a styrene-butadiene
 block copolymer having a butadiene/styrene ratio of 78/22. Two procedures
 were utilized for testing the present invention in two separate
 refineries. The procedures vary in temperature and only depend on the
 capabilities or nuances of the refinery. The results are comparable
 regardless of the procedure. The stirring time can also vary depending on
 the asphalt and how easily compatibility is obtained.
 In one procedure, the asphalt was added to the tank and stirred at
 340.degree. F. at 2500 rpm for 45 minutes. The shear rate was subsequently
 reduced to 500 rpm and the temperature increased to 350.degree. F. The
 crosslinking agents were added and stirring was continued for one hour.
 Samples were taken and placed in an oven for 24 hours to conduct the
 compatibility test. SHRP tests may also be run.
 In the second procedure, the rubber was sheared into the asphalt at
 350.degree. F. at 2500 rpm for 45 minutes. The mixture was placed on the
 low shear mixer and the temperature was adjusted to the desired level. The
 crosslinking agent was added and stirring was continued for one hour while
 raising the temperature to 380.degree. F. Samples were taken for testing.
 Unless otherwise indicated, the first procedure is utilized in reporting
 the data.
 For the experiments shown in Tables 3-5, an asphalt product was used. The
 asphalt is an AC-30 product prepared from asphaltenes, and outside flux
 oils. It is the starting material for all the other blends. This is a
 particularly difficult cut to make rubber compatible. This asphalt has
 about a 30.degree. F. separation on the compatibility test with Finaprene
 401 (FP401). For this set of experiments, the asphalt was heated to
 340.degree. F. A high shear mixer was used at 2500 rpm before adding 4% of
 FP401. After 45 minutes, the mixture was placed on low shear at 250 rpm
 and the temperature was adjusted to the desired level. The crosslinking
 agent was added either in powder form or as an emulsion as indicated in
 the tables. The mixture was stirred for 30 minutes after the addition of
 the crosslinking agent. Samples were taken for compatibility tests and
 other tests such as DSR. The mixture was stirred for another 30 minutes
 and additional samples were taken for testing. With some samples, heating
 was continued for 4 hours with samples taken every hour.
 TABLE 3
 Number 11 14 control 15 16 17
 % Asphalt 100 100 100 100 100 100
 % Rubber 4 4 0 4 4 4
 Temperature 380 350 350 380 320
 (F.)
 Speed low 750 750 750 750 750
 shear mixer
 (RPM)
 Time on low 5 5 1 1 1
 shear (hours)
 % ZMBT 0.01 0.01 0 0.01 0.01 0.01
 % ZnO 0.008 0.008 0 0.008 0.008 0.008
 % Sulfur 0.06 0.06 0 0.06 0.06 0.06
 Binder DSR 67.9 82.1 82.9 82.4
 RTFO DSR 69.4 78.7 78.8 79.6
 PAV DSR 27.1 24.6 24 24.3
 BBR m Value -13.63 -10.97 -10 -9.49
 BBR s value -14.58 -16.09 -16.02 -16.19
 PG Grade
 Compatibility, 24
 1 hour
 Compatibility, 7.7 27.8
 2 hour
 Compatibility, 2 22.3
 3 hour
 Compatibility, 0.9 13 21
 4 hour
 Compatibility, 5.8 28.9
 5 hour
 Compatibility, 1.5 8.3 5 5.9
 24 hour
 Compatibility, v 1.8
 48 hour
 TABLE 4
 Number 23 24 45
 % Asphalt 100 100 100
 % Rubber 4 4 4
 Temperature (F.) 350 350 350
 Speed low shear mixer (rpm) 750 750 750
 Time on low shear (hours) 1 1 1
 % ZMBT 0.01 0.013 0.017
 % ZnO 0.008 0.01 0.013
 % Sulfur 0.06 0.075 0.1
 Aquamix including water 0.16 0.2 0.25
 Binder DSR 80 80.5 82.7
 RTFO DSR 79.8 79.3 78.9
 PAV DSR 23.5 24.5 23.3
 BBR m Value -11.14 -10.73 -14.67
 BBR s value -16.57 -16.73 -18.24
 Compatibility, 24 hour 6.9 3.8 0.8
 TABLE 5
 Number 21 22 9 20 8 10 12 13
 18 19
 % Asphalt 100 100 100 100 100 100 100 100
 100 100
 % Rubber 4 4 4 4 4 4 4 4
 4 4
 Temperature 380 350 350 365 350 380 380 350
 350 380
 (F.)
 Speed low 750 750 750 750 750 750 750 750
 750 750
 shear mixer (rpm)
 Time on low 3 3 5 1 5 5 5 5
 1 1
 shear (hours)
 % ZMBT 0.01 0.01 0.03 0.03 0.05 0.05 0.05 0.05
 0.05 0.05
 % ZnO 0.008 0.008 0.012 0.012 0.004 0.004 0 0
 0 0.004
 % Sulfur 0.06 0.06 0.12 0.12 0.3 0.1 0.3 0.1
 0.1 0.1
 Binder DSR 81.8 81.9
 RTFO DSR 78.7 79
 PAV DSR 23.4 24.9
 BBR m Value -11.27 -11.28
 BBR s value -16.64 -16.73
 PG Grade
 Compatibility, 10.7 Gelled 11.5 Gelled 11.5
 1 hour
 Compatibility, 3 1.5 10.4
 2 hour
 Compatibility, 2.7 1 4.5
 3 hour
 Compatibility, 1 0.4 2.4
 4 hour
 Compatibility, 2.3 0.7
 5 hour
 Compatibility, 1.9 4.2 3
 3 0
 24 hour
 Compatibility,
 48 hour
 As indicated from the data above, one must utilize at least 0.06% elemental
 sulfur to render the asphalt and rubber composition compatible. The
 crosslinking reaction is dependent on the temperature of the asphalt (see
 runs 11, 14 and 17). Comparing blend 11 versus blend 16 and blend 14
 versus blend 15, shows that prolonged stirring after the addition of the
 crosslinking agent improves the compatibility of the rubber and asphalt.
 Blends 9, 20,8,10 and 12 show that one can obtain gels at high sulfur
 concentrations (0.3% relative to asphalt) and zinc (ZMBT+ZnO=0.05%)
 levels. Blends 16, 17, 20, 18 and 19 indicate that SHRP properties are
 affected by the amount of ZMBT and ZnO added to the asphalt. The upper
 grades and the s-value were not affected by the addition of the
 crosslinking agent. It the concentration of ZMBT and ZnO (ZMBT+ZnO) is
 lower than 0.03%, the bottom grade (m-value) of the asphalt increases
 (worsens) by 2.degree. C. Conversely, if the total zinc concentration is
 equal to or greater than 0.03%, the m-value is decreased (improved) by
 2.degree. C. Table 5 shows the results of adding the crosslinking agents
 as an emulsion. The emulsion used a ratio of ZMBT/ZnO/S of
 0.01/0.008/0.06. The emulsion was about 50% water.
 Blends 24 and 25 further confirm the need for at least 0.03% total zinc to
 decrease the m-value by 2.degree. C.
 The above results show that the crosslinking reaction is temperature
 dependent. The minimum practical temperature is 320.degree. F. If mixing
 is adequate, one can reduce the time for crosslinking by increasing the
 temperature. The effect between 320.degree. F. and 340.degree. F. is
 dramatic. It is believed that this effect is because cyclic sulfur opens
 to linear sulfur at the higher temperature. The minimum sulfur
 concentration is 0.06% based on the asphalt.
 TABLE 6
 Block 1-2 2-2 3-2 4-2 1-3 2-3 3-3 4-3
 ZMBT % 0.005 0.005 0.02 0.02 0.02 0.02 0.005
 0.005
 ZnO % 0.005 0.02 0.02 0.005 0.02 0.005 0.005 0.02
 S % 0.06 0.06 0.06 0.06 0.12 0.12 0.12 0.12
 Temp, F. 320 320 320 320 320 320 320 320
 Sample Number 933-114 933-122 933-131 933-138 933-115 993-123 933-132
 933-139
 Compatibility 18.5 gelled 14.8 14.5 gelled 14.3 gelled
 gelled
 Binder DSR 82.1 84.4 81.4 81
 RTFO DSR 77.9 78 78.1 78.1
 PAV DSR 27.4 23.2 24.7
 BBR m -12.94 -12 -13.23
 BBR S -13.7 -14.25 -14.68
 Comments *1
 *1 gelled very slowly
 TABLE 7
 Block 1-1 2-1 3-1 4-1 1-4 2-4 3-4 4-4
 ZMBT % 0.013 0.013 0.013 0.013 0.02 0.02 0.005
 0.005
 ZnO % 0.013 0.013 0.013 0.013 0.005 0.02 0.02
 0.005
 S % 0.09 0.09 0.09 0.09 0.06 0.06 0.06 0.06
 Temp, F. 335 335 335 335 350 350 350 350
 Sample Number 933-113 933-118 933-130 933-137 933-116 933-124 933-133
 933-142
 Compatibility 0.8 1.3 14.5/7.8 15.9 16.1 12.6 15.8 8.4
 Binder DSR 82.7 82.2 82.4 84.4 83.9 81.7 83.1 83.5
 RTFO DSR 79.2 78.5 79.2 77.9 79.5 79 77.5 78.1
 PAV DSR 25.4 23.3 23.4 25.5 26.4 26.3 25
 BBR m -14.76 -13.7 -13.78 -9.94 -10.31 -10.73 -10.95
 BBR S -14.68 -15 -14.68 -14.21 -13.24 -13.05 -12.43
 Comments
 TABLE 8
 Block 1-5 2-5 2-5a 3-5 4-5
 ZMBT % 0.005 0.005 0.005 0.02 0.02
 ZnO % 0.02 0.005 0.005 0.005 0.02
 S % 0.012 0.012 0.012 0.012 0.012
 Temp, F. 350 350 350 350 350
 Sample 933-117 933-125 933-129 933-136 933-143
 Number
 Compatibility 1.7 3.7 1.5 2.3 1
 Binder DSR 82.9 83.2 82.9 82.8
 RTFO DSR 78.5 78.4 77.8 77.4
 PAV DSR 23.6 24.7 23 22.7
 BBR m -14.48 -14.03 -14 -14.73
 BBR S -15.12 -14.91 -14.77 -15.02
 Comments *2 *3 *4
 *2 thin hard crust
 *3 small thin hard crust 1/8 of surface
 *4 small thin hard crust 1/6 of surface
 TABLE 9
 Block 5-1 5-2 5-3 5-4 99-38 99-38
 ZMBT % 0.013 0.005 0.02 0.005 0.005 0.02
 ZnO % 0.013 0.02 0.02 0.005 0.02 0.02
 S % 0.09 0.12 0.12 0.06 0.12 0.12
 Temp, F. 335 380 380 380 350 350
 Sample 933-151 933-156 933-157 933-160 933-158 933-159
 Number
 Compatibility 14.9 2.4 1.2 11.4 *5 *5
 Binder DSR 81.29 90.7 83.1 81.6
 RTFO DSR 79.19 7823.1 79.7 73.9
 PAV DSR 22.6 -15.32 23.2 23.9
 BBR m -13.5 -15.15 -15.17 23.9
 BBR S -15.24 -14.38 -12.93
 Comments -13.85
 *5 gelled while aging
 *6 gelled while aging
 The data shown in Tables 6-9 give the results of various experiments
 varying the crosslinking compounds and the temperature for crosslinking.
 While certain representative embodiments and details have been shown for
 the purpose of illustrating the subject invention, it will be apparent to
 those skilled in the art that various changes and modifications can be
 made therein without departing from the scope of the subject invention.