MXPA98003913A - Polymer for modification of asfa cement - Google Patents

Abstract

This invention describes a process for synthesizing a styrene-butadiene polymer, which is particularly useful for modifying asphalt to improve strength ductility, elastic recovery, hardness and toughness, by a process comprising the steps of: (1) continuously loading monomer from 1,3-butadiene, a lithium organ compound, a polar modifier and an organic solvent to a first polymerization zone, (2) allow the 1,3-butadiene monomer to polymerize in the first polymerization zone to a conversation of when minus about 90 percent to produce a living polymer solution that is comprised of the organic solvent and living polybutadiene chains having a number average molecular weight that is within the range of about 20,000 to about 60,000, (3) continuously removing the solution of living polymer from the first reaction zone, (4) continuously loading styrene monomer ene, divinylbenzene and the living polymer solution which is being withdrawn from the first polymerization zone into a second polymerization zone, (5) allowing the styrene monomer and the divinylbenzene monomer to polymerize in the second polymerization zone to produce a solution of styrene-butadiene polymer having a number-average molecular weight that is in the range of about 30,000 to about 85,000, and (6) continuously withdrawing the styrene-butadiene polymer solution from the second polymerization zone.

Description

POLYMER FOR MODIFICATION OF ASPHALT CEMENT Background of the Invention The importance of roads and highways has been appreciated since the time of the Roman Empire. In about 300 BC, the first section of the Via Appia was built, which extends from Rome to Capua. Some more than 93,000 kilometers of road finally built in the Roman Empire were built with heavy stone. However, not much progress was made in the field of road construction from the era of the Roman Empire to the development of motor vehicles. such as cars and trucks. For centuries, stone blocks, blocks of wood, vitrified brick and natural asphalt (tar) have been used to pave roads and highways. However, at the beginning of the automobile era, most of the rural road surface consisted of broken stone or gravel. These roads were often rough. dusty and clearly unsuitable for modern cars and truck traffic. At present, the United States has the most extensive road system in the world of approximately 3,700,000 kilometers of paved road. Napoleon noted the importance of road systems and built such a system in France that still has the second largest system of pavement roads in the world covering approximately 926,500 kilometers. Germany. Japan, Great Britain and India and Australia also currently have paved road systems that extend well over 185,300 kilometers. In addition to these public roads there are countless paved roads and parking lots throughout the world. At present, roads, highways, highways, and parking lots are frequently paved with asphalt concrete. Can the pavement be made with asphaltic concrete that is dust-free, smooth? that offer the resistance required for the modern traffic of cars and heavy trucks. Asphalt concrete is usually made by mixing aggregate (sand and gravel or crushed stone) with the appropriate amount of asphalt cement at an elevated temperature. The hot asphalt concrete is then placed by a layer-forming machine or paver on the surface being paved and tamped down completely before the asphalt concrete mix cools. Asphalt concrete is usually applied at a thickness that varies from about 25 to about 100 millimeters. The asphalt concrete pavements can be made to be very smooth, which offers surprising frictional resistance for the vehicles that operate on them. This asphalt concrete pavement can be repaired simply by adding additional hot asphalt concrete to holes or other types of defects that develop on the surface. Asphalt concrete pavements can also be easily improved by adding additional layers of hot asphalt concrete to old surfaces that need repair. Even though asphalt concrete offers numerous benefits as a paving material, its use is not free of problems. A major problem encountered with asphalt concrete pavements is the loss of the adhesive bond between the aggregate surface and the asphalt cement. This breakage of the adhesive bond between the asphalt cement and the aggregate surface is known as "detached". The release of asphalt binder from aggregate surfaces results in shorter pavement life and many millions of dollars of maintenance work on American roads each year. The reduction of this tendency to detachment is of great interest when it comes to improving road conditions and reducing these maintenance costs. Over the years, various methods have been developed to reduce the tendencies to detachment. For example, amines and lime are known to act as anti-peel former and are frequently applied to the surface of the aggregate before mixing it with the asphalt cement when making the asphalt concrete. United States Patent 5, 219,901 discloses a technique for reducing stripping tendencies involving coating the aggregate with a thin, continuous film of a water-insoluble, high molecular weight organic polymer, such as an acrylic polymer of a styrene-acrylic polymer. U.S. Patent 5,262,240 discloses a technique for providing aggregate with a high level of water release resistance, which comprises: (1) mixing the aggregate with latex to form a latex / aggregate blend that is comprised of about 0.005 per weight percent to about 0.5 weight percent dry polymer; (2) heating the latex / aggregate mixture to a temperature that is within the range of about 66a to about 232aC; (3) maintaining the latex / aggregate mixture at the elevated temperature for a time which is sufficient to reduce the moisture content of the latex / aggregate mixture to below about 0.7 weight percent and to allow the polymer in the Latex is reticulated on the surface of the aggregate to produce the coated aggregate. At high service temperatures, such as those experienced on hot summer days, asphalt concrete may experience furrowing and shifting. On the other hand, at low service temperatures, such as those experienced during cold winter nights, asphalt concrete may also experience low temperature cracking. To combat these problems, it is known in the art to modify the asphaltic cements with rubbery polymers, such as styrene-butadiene rubber (SBR). In some approaches, the SBR is added to the asphalt as a dry rubber while in others it is added as a latex. These modification techniques can greatly improve the resistance to furrowing, displacement and cracking at low temperature. However, the rubbery polymers used in such applications have a tendency to separate the phase of the hot asphalt cements due to the low compatibility. A solution to the problem of low compatibility is offered by the technique described in U.S. Patent 5,002,987. U.S. Patent 5,002,987 relates to a modified asphalt cement containing from about 90 to about 99 parts by dry weight of an asphalt cement and from about 1 to about 10 parts by dry weight of a rubbery latex having a weight molecular weight by weight of less than 250,000 and a Mooney viscosity of less than 50. The latex is a random polymer comprising from about 60 to 100 weight percent of at least one conjugated diolefin containing from 4 to 6 carbon atoms and from about 0 to 40 weight percent styrene. This latex polymer is highly compatible with asphalt and provides good ductility resulting in good resistance to cracking at low temperature. However, the use of the rubbery polymers described in U.S. Patent 5,002,987 in asphaltic cements provide little improvement in elastic recovery or firmness. In this way, its use results in compromised characteristics of displacement and furrowing. Consequently, is there a current need for a modifier that is compatible with asphalt cement and that improves the strength of the asphalt concrete made? - with the same for displacement, furrowing and cracking at low temperature. The US Patent 5,534,568 discloses an asphalt concrete which is comprised of (A) from about 90 weight percent to about 99 weight percent of an aggregate and (B) from about 1 weight percent to about 10 weight percent of a modified asphalt cement which is comprised of (1) from about 90 weight percent aa about 99 percent by weight of asphalt and (2) from about 1 weight percent to about 10 percent by weight of a rubbery polymer which is comprised of repeat units which are derived from (a) about 64 weight percent to about 84.9 weight percent of a conjugated diolefin monomer, (b) about 15 weight percent to about 33 weight percent of a vinyl aromatic monomer and (c) from about 0.1 weight percent to about 3 weight percent isobutoxymethyl acrylamide. U.S. Patent 4,145,322 discloses a process for making a tar-polymer composition consisting of contacting each other, at a temperature between 130 ° C and 230 ° C, 80 to 98 weight percent of a tar exhibiting a value of penetration between 30 and 220, and 2 to 20 weight percent of a block copolymer with an average molecular weight between 30,000 and 330,000 having the theoretical formula Sx-by wherein S corresponds to groups styrene structure, B corresponds to the groups of conjugated diene structure and x and y are integers, stirring the obtained mixture for at least two hours, then adding 0.1 to 3 weight percent of elemental sulfur with respect to the tar and maintaining the mixture obtained in this way under stirring during at least 20 minutes Batch polymerization techniques are commonly used when synthesizing block copolymers are used when modifying asphalt in order to achieve the desired performance characteristics. However, it would be highly desirable from a cost point of view to be able to synthesize said polymers using continuous polymerization techniques. It would also be highly desirable to increase the strength ductility, elastic recovery, firmness and toughness of asphalt which is modified with said polymers.

SUMMARY OF THE INVENTION This invention describes a technique for synthesizing, through a continuous polymerization process, a styrene-butadiene polymer that is highly suitable for modifying asphalt. In fact, the asphalt which is modified with the styrene-butadiene polymer of this invention exhibits improved strength ductility, elastic recovery, firmness and tenacity. This invention more specifically describes a process for synthesizing a styrene-butadiene polymer that is particularly useful for modifying asphalt by a continuous polymerization process, the process comprising the steps of: (1) continuously charging 1,3-butadiene monomer, a compound of organolithium, a polar modifier and an organic solvent to a first polymerization zone, (2) allow the 1,3-butadiene monomer to be polymerized in the first polymerization zone at a conversion of at least about 90 percent to producing a solution of living polymer which is comprised of the organic solvent and chains living polybutadiene having an average molecular weight of which is within the range of about 20,000 to about 60,000, (3) continuously withdrawing the solution of living polymer from the first reaction zone, (4) continuously charge styrene monomer, divinilben and the living polymer solution being withdrawn from the first polymerization zone into a second polymerization zone, (5) allowing the styrene monomer and the divinylbenzene monomer to polymerize in the second polymerization zone to produce a solution of styrene-butadiene polymer having a number-average molecular weight that is within the range of about 30, 000 to about 85,000, and (6) continuously withdrawing the styrene-butadiene polymer solution from the second polymerization zone. The present invention further discloses an asphalt concrete that is comprised of (A) from about 90 weight percent to about 99 weight percent of an aggregate and (B) from about 1 weight percent to about 10 weight percent. by weight of a modified asphalt cement that is comprised of (i) from about 90 weight percent to about 99 weight percent asphalt; (ü) from about 1 percent by weight to about 10 percent by weight of a styrene-butadiene polymer made by a process that is comprised of the steps of: (1) continuously charging 1,3-butadiene monomer, a lithium organ compound, a polar modifier and an organic solvent to a first polymerization zone, (2) allow the 1,3-butadiene monomer to be polymerized in the first polymerization zone at a conversion of at least about 90 per cent. percent to produce a living polymer solution that is comprised of the organic solvent and living polybutadiene chains having a number average molecular weight that is within the range of about 20,000 to about 60,000, (3) continuously removing the polymer solution live from the first reaction zone, (4) continuously charge styrene monomer, divinylbenzene and the living polymer solution that is being removed from the to the first polymerization zone to a second polymerization zone, (5) allow the styrene monomer and the divinylbenzene monomer to be polymerized in the second polymerization zone to produce a styrene-butadiene polymer solution having an average molecular weight in number that is within the range of about 30,000 to about 85,000 and (6) continuously withdrawing the styrene-butadiene polymer solution from the second polymerization zone; and (iii) from about 0.1 weight percent to about 5 weight parts of sulfur per 100 parts by weight of the styrene-butadiene polymer. The present invention also discloses a styrene-butadiene polymer that is particularly useful for modifying asphalt to improve strength ductility, elastic recovery, firmness and toughness, the styrene-butadiene polymer being comprised of a portion of butadiene and a portion of styrene. , wherein the butadiene portion is comprised of repeating units that are derived from 1,3-butadiene, wherein the butadiene portion has a vinyl microstructure content that is within the range of about 35 percent a about 80 percent, wherein the butadiene portion has a number average molecular weight that is within the range of about 20,000 to about 60,000, wherein the styrene moiety branches to multiple arms at branching points that are derived of divinylbenzene, and wherein the styrene-butadiene polymer has a number-average molecular weight that It is within the range of around 30,000 to about 85,000.

Detailed Description of the Invention The rubbery polymer that is used to modify the asphalt cement in accordance with this invention is made by a continuous solution polymerization technique. In the first step of the process used, 1, 3-butadiene monomer, an organolithium initiator, a polar modifier and an organic solvent are continuously charged to a first polymerization zone. The first polymerization zone will typically be a polymerization reactor or some other type of reaction vessel. The organic solvent can be one or more aromatic, paraffinic or cycloparaffinic compounds. These organic solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquid under the conditions of polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal bexane, benzene, toluene, xylene, ethylbenzene and the like, alone or as a mixture. It is often desirable to use a mixture of different isomers of hexane as the organic solvent. Such a mixture of hexane isomers is often referred to simply as "hexanes". In the solution polymerizations of this invention, there will usually be about 5 to about 35 weight percent of monomers and polymer in the polymerization medium in the first polymerization zone and in the second polymerization zone. Said constant state polymerization media, of course, are comprised of the organic solvent, the monomer, the polymer, the polar modifier, the organolithium initiator and, optionally, gel inhibiting agent. In most cases, it will be preferred that the polymerization medium contain from 10 to 30 weight percent of monomers and polymer. It is generally more preferred that the polymerization medium contain 20 to 25 weight percent of monomers and polymer. A polar modifier is added to the first polymerization zone in an amount that is sufficient to produce a living polybutadiene chain having a vinyl content that is within the range of about 35 percent to about 80 percent. The living polybutadiene chain will preferably have a vinyl content that is within the range of about 40 percent to about 60 percent and more preferably, will have a vinyl content that is within the range of about 45. percent to about 55. The segment or block in the styrene-butadiene polymer of this invention that is derived from butadiene will naturally have the same vinyl content as it was in the living polybutadiene chain. The ethers and tertiary amines that act as Lewis bases are representative examples of polar modifiers that can be used. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, NNN ', N'-tetramethylethylenediamine (TMEDA), N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and the like. Dipiperidinoethane, dipyrrolidinoethane, diethylene glycol dimethyl ether, TMEDA and tetrahydrofuran are representative of highly preferred modifiers. U.S. Patent 4,022,959 describes the use of ether and tertiary amines as polar modifiers in greater detail. It is generally desirable to include a gel inhibiting agent in the polymerization medium. In this way, it will normally be desirable to load a gel inhibiting agent into the first polymerization zone. In most cases, 1, 2-butadiene will be charged to the first polymerization zone as a gel inhibiting agent. Polymerization is initiated by adding an organolithium compound to the polymerization medium. The organolithium compound that can be used includes organomonolithium compounds and multifunctional organo lithium compounds. The multifunctional organo lithium compounds will typically be organodilithium compounds or organotrilithium compounds. Some representative examples of suitable multifunctional organolithium compounds include, 1,4-dilithiobutane, 1, 10-dilithiodecane, 1, 20-dilithioeicosane, 1, 4-dilithiobenzene, 1,4-dili-ionephthelene, 9, 10-dilithioanthracene, 1,2-dilithio-1,2-di-yl-ethane, 1,3,5-tritylthiopentane, 1,5, 15-trilithioeicosane, 1,3,5-tritylcyclohexane, 1,3,5, 8-tetralithiodecane, 1,5,10, 20-tetralithioeicosane, 1, 2, 4,6-tetralithiocyclohexane, 4,4'-dithiobiphenyl and the like. The organolithium compounds that can be used are usually organomonolithium compounds. Preferred organolithium compounds are alkyl lithium compounds which are represented by the formula R-Li, wherein R represents a hydrocarbyl radical containing from 1 to about 20 carbon atoms. Generally, these monofunctional organolithium compounds will contain from 1 to about 10 carbon atoms. Some representative examples of organolithium compounds that can be used include methyl lithium, ethyl lithium, isopropyl lithium, n-butyl lithium, sec-butyl lithium, n-octyl lithium, tert-octyl lithium, n-decyl lithium, phenyl lithium, 1- Naphthyl lithium, 4-butylphenyl lithium, p-tolyl lithium, 4-phenylbutyl lithium, cyclohexyl lithium, 4-butylcyclohexyl lithium and 4-cyclohexylbutyl lithium. As a general rule, in all anionic polymerizations, the molecular weight (Mooney viscosity) of the polymer produced is inversely proportional to the amount of the initiator used. As a general rule, from about 0.01 to about 1 phm (parts per hundred parts of monomer by weight) of the organolithium compound will be employed. In most cases, it will be preferred to use from about 0.015 to about 0.4 phm of the organolithium compound being more preferred to use from about 0.13 phm to 0.25 phm of the organolithium compound. In any case, the amount of organolithium initiator employed will be adjusted to produce living polybutadiene chains having a number average molecular weight that is within the range of about 20,000 to about 60,000. It is usually preferred to use an amount of organolithium initiator which will result in the living polybutadiene chains having a weight average molecular weight that is within the range of about 30,000 to about 50,000. It is generally preferred to use an amount of lithium organ initiator that will result in the living polybutadiene chains having a weight average molecular weight that is within the range of about 35,000 to about 45,000. This will typically correspond to a dilute solution viscosity (DSV) that is within the range of about 0.5 to about 0.7. The polymerization temperature used in the first polymerization zone and the second polymerization zone will typically be within the range of about -10aC to about 150SC. It is usually preferred that the polymerization medium be maintained at a temperature that is within the range of about 40SC to about 110SC through the polymerization. Typically it is more preferred that the polymerization temperature be within the range of about 602C to about 95SC. The pressure used will normally be sufficient to maintain a substantially liquid phase under the conditions of the polymerization reaction. The residence time in the first polymerization zone will be sufficient for there to be a substantially complete polymerization of 1,3-butadiene monomer towards polymer. The conversion of monomer to polymer in the first polymerization zone will typically be at least about 90 percent. It is usually preferred that the monomer conversion in the first polymerization zone be at least about 95 percent with conversions of at least about 97 percent being more preferred. The residence time in the first polymerization zone will typically be from the scale of about 0.1 hours to about 2 hours. A residence time from about 0.2 hours to about 1 hour is normally preferred and a residence time of from about 0.3 hours to about 0.7 hours is usually most preferred. The living polybutadiene solution made in the first polymerization zone is continuously removed from the first polymerization zone and fed into the second polymerization zone. The styrene and divinylbenzene (DVB) will also be continuously fed to the second polymerization zone. The amount of loaded styrene will typically be from about 15 weight percent to about 35 weight percent of the total monomer charge. In this way, the styrene-butadiene polymer being synthesized will typically contain from about 65 weight percent to about 85 weight percent of repeating units that are derived from 1,3-butadiene and from about 15 percent by weight. weight to about 35 weight percent of repeating units that are derived from styrene. It is usually preferred to load from about 20 weight percent to about 30 weight percent styrene, based on the total monomer charge. It is usually more preferred to load from about 24 weight percent to about 28 weight percent styrene, based on the total monomer charge. In this manner, the styrene-butadiene polymer being synthesized will preferably contain from about 70 weight percent to about 80 weight percent of repeating units that are derived from 1,3-butadiene and from about 20 weight percent to about 30 weight percent of repeating units that are derived from styrene. It is more preferred that the styrene-butadiene polymer being synthesized contains from about 72 weight percent to about 76 weight percent of repeating units which are derived from 1,3-butadiene and from about 24 weight percent. weight percent to about 28 weight percent of repeating units that are derived from styrene.

Normally, from about 0.1 to about 1.5 moles of divinylbenzene will be charged to the second reaction zone per mole of charged organolithium compound to the first reaction zone. It is usually preferred to charge from about 0.2 to about 0.8 moles of divinylbenzene per mole of the organolithium initiator. It is usually more preferred to load from about 0.3 to about 0.7 moles of divinylbenzene per mole of the organolithium initiator. The residence time in the second polymerization zone will be sufficient for there to be a substantially complete polymerization of styrene and divinylbenzene monomer towards the polymer. The conversion of monomer to polymer in the second polymerization zone will typically be at least about 90 percent. It is usually preferred that the monomer conversion in the second polymerization zone be at least about 95 percent with versions of at least about 97 percent being more preferred. The residence time in the first polymerization zone will typically be in the range of about 0.2 hours to about 4 hours. A residence time of from about 0.4 hours to about 3 hours is usually preferred and a residence time of from about 0.7 hours to about 1.5 hours is usually the most preferred. The styrene-butadiene polymer typically made will have a number average molecular weight that is within the range of about 30,000 to about 85,000. The styrene-butadiene polymer made preferably will have a number average molecular weight that is within the range of about 40,000 to about 75,000 and more preferably will have a number average molecular weight that is within the range of about 50,000 to approximately 65,000. A solution of the styrene-butadiene polymer is continuously removed from the second polymerization zone. The made styrene-butadiene polymer can then be recovered from the organic solvent by conventional techniques; such as, separation by steam, followed by decanting, filtration, centrifugation and the like. It is often desirable to precipitate the polymer from the organic solvent by the addition of lower alcohols containing from 1 to about 4 carbon atoms to the polymer solution. Lower alcohols suitable for precipitation of the rubbery polymer from the polymer cement include methanol, ethanol, isopropyl alcohol, n-propyl alcohol and t-butyl alcohol. The use of lower alcohols to precipitate the polymer from the polymer cement also "kills" the living polymer chains by inactivating the lithium end groups. After the polymer is recovered from the organic solvent, steam scrubbing can be employed to reduce the level of volatile organic compounds in the styrene-butadiene polymer. The styrene-butadiene polymer can then be employed in making the modified asphalt compositions of this invention. The asphalt cement can be modified with the styrene-butadiene polymer of this invention by simply mixing the styrene-butadiene polymer in hot asphalt. The styrene-butadiene polymer will typically be mixed to the asphalt at a temperature that is within the range of about 130aC to about 230SC. The styrene-butadiene polymer of this invention can be added to the asphalt in an amount that is within the range of about 1 part by weight to about 10 parts by weight. Preferably, from about 1 part by weight to about 6 parts by weight of the styrene-butadiene polymer is added with amounts within the scale from about 2 parts by weight to about parts by weight, with particular preference being given. To achieve a good dispersion of the styrene-butadiene polymer through the asphalt, this mixing will normally take at least about 2 hours. After the styrene-butadiene polymer has been well dispersed through the asphalt, the sulfur element is added to the polymer / asphalt mixture. Normally from about 0.1 to about 5 parts by weight of sulfur are added per 100 parts by weight of the styrene-butadiene polymer. In most cases, it is preferred to use from about 1 to about 4 parts by weight of sulfur per 100 parts by weight of the styrene-butadiene polymer. Typically it is more preferred to use from about 2 to about 3 parts by weight of sulfur per 100 parts by weight of the styrene-butadiene polymer. After the styrene-butadiene polymer and the sulfur have been completely mixed with the asphalt cement, the modified asphalt cement should be stored at elevated temperatures to prevent solidification before use. Virtually any type of asphalt can be employed in making the asphalt cement compositions of this invention. Asphalt is defined by ASTM as a dark brown to black cementitious material in which the predominant constituents are tars that occur in nature or are obtained in the processing of petroleum. Asphalts typically contain very high molecular weight hydrocarbons called asphaltenes. These are essentially soluble in carbon disulfide and chlorinated aromatic hydrocarbons. Tar is a generic term defined by ASTM as a class of black or dark colored (solid, semi-solid or viscous) cementitious substances, natural or manufactured, composed mainly of high molecular weight hydrocarbons, of which asphalts, pitches, bitumens natural, asfaltites are typical. ASTM also classifies asphalts or bituminous materials as solids, semi-solids or liquids that use a penetration test for consistency or viscosity. In this classification, solid materials are those that have a penetration at 25SC under a load of 100 grams applied for 5 seconds, no more than 10 decimililiters (1 millimeter) and a penetration at 25SC under a load of 50 grams applied for 1 second of no more than 35 millimeters. Semisolid and liquid asphalts predominate in commercial practice today. Asphalts are usually specified in varying degrees for the same industry, differing in hardness and viscosity. Specifications of asphalt paving cements usually including five grades that differ at any viscosity level at 60SC or penetration level. The sensitivity of the viscosity to temperatures is usually controlled in the asphalt cement by its viscosity limits at a higher temperature such as 135SC and a penetration limit or viscosity at a lower temperature such as 25SC. For asphalt cements, the newest designation of viscosity grade is the midpoint of the viscosity scale. The asphalt materials that can be used in the present invention are those typically used for road paving and repair and maintenance purposes. Petroleum asphalts are the most common source of asphaltic cements. Petroleum asphalts are produced from petroleum refining and are predominantly used in paving and roofing applications. Petroleum asphalts, compared to native asphalts, are organic with only trace amounts of inorganic materials. Some representative examples of asphalt cements that can be used in the present invention have an ASTM grade of AC-2.5, AC-5, AC-10, AC-20 and AC-40. Preferred asphalt cements include AC-5, AC-10 and AC-20. In addition to the styrene-butadiene polymer, sulfur and asphalt cement, the modified asphalt cement of the present invention may contain other conventional additives. Examples of conventional additives include anti-detachment compounds, fibers, release agents and fillers. Some specific examples of additives that may be employed include kaolin clay, calcium carbonate, bentonite clay, sand dust and cellulose fibers. After the asphalt cement has been modified, it can be mixed with aggregate to make asphalt concrete using conventional equipment and procedures used to make asphalt concrete. As a general rule, from about 1 weight percent to about 10 weight percent of the modified asphalt cement and from about 90 weight percent to about 99 weight percent aggregate will be included in the asphalt concrete. It is more typical for asphaltic concrete to contain from about 3 weight percent to about 8 weight percent of the modified asphalt cement and from about 92 weight percent to about 97 weight percent of the aggregate. It is usually preferred that the asphalt concrete contain from about 4 weight percent to about 7 weight percent of the modified asphalt cement and from about 93 weight percent to about 96 weight percent of the aggregate. The aggregate is mixed with the asphalt to reach an essentially homogeneous asphalt concrete. The aggregate is mixed with the asphalt cement using conventional techniques and conventional equipment. For example, the aggregate can be mixed with the asphalt to produce asphalt concrete on a continuous basis in a conventional mixer. The conventional aggregate can be used in the practice of this invention. The aggregate is essentially a mixture containing rocks, stones, crushed stone, gravel and / or sand. The aggregate will typically have a wide distribution of particle sizes that varies from dust to the size of a golf ball. The best particle size distribution varies from application to application. In many cases, it will be advantageous to coat the aggregate with latex in accordance with the teachings of U.S. Patent 5,262,240 to improve the peel strength by water. The asphalt concrete made using the modified asphalt cement of this invention can then be used to pave roads, roads. exit ramps, streets, highways, parking lots, airport runways or airport runways that use conventional procedures. However, pavements made using the asphaltic concrete of this invention offer resistance to loosening, displacement and cracking at low temperature. Additionally, they can be applied without encountering processing difficulties due to the latex used for the modification that is incompatible with the asphalt. This invention is illustrated by the following examples which are merely for the purpose of illustration and are not to be construed as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, all parts and percentages are given by weight.

Example 1 In this experiment, a styrene-butadiene polymer was synthesized using the continuous polymerization process of this invention. A two reactor system consisting of a first reactor of 3,785 liters and a second reactor of 7.57 liters was used with the polymerization temperature being maintained at approximately 88SC. In the procedure used, 1, 3-butadiene was charged to the first reactor at a rate of 115.2 grams per minute, n-butyl lithium was charged to the first reactor as a 0.25 M solution at a rate of 1.6 grams per minute and TMEDA was loaded onto the first reaction as a 0.25 M solution at a rate of 1.04 grams per minute. The 1,3-butadiene monomer was charged to the reactor as a 16 weight percent premix solution in hexane. The residence time in the first reactor was 0.51 hours. The solution of the living polybutadiene made in the first reactor was charged to the second reactor. A solution of styrene premix in hexane and divinylbenzene was also charged to the second reactor. The styrene was charged to the second reactor at a rate of 27.9 grams per minute and the divinylbenzene was charged to the second reactor as a 0.5 M solution at a rate of 0.80 grams per minute. The styrene monomer was charged to the second reactor as a 22 weight percent solution in hexane. The residence time in the second reactor was 0.82 hours. The styrene-butadiene polymer that is discharged from the second reactor was mixed with a solution containing 5 weight percent of an antioxidant and 5 weight percent of isopropyl alcohol that was added at a rate of 4.91 grams per minute. . The styrene-butadiene polymer made in this experiment had a Mooney viscosity ML-4 at 100 ° C of 25 and a glass transition temperature of about -63 ° C. The styrene-butadiene polymer was also determined to have a number average molecular weight of about 55,000 and a weight average molecular weight of about 338,000.

Example 2 In this experiment, asphalt was modified with the styrene-butadiene polymer made in Example 1 with a conventional styrene-butadiene block copolymer (for comparative purposes). The conventional styrene-butadiene block copolymer which was evaluated was Solprene® 1205 25/75 styrene / butadiene linear block copolymer: An asphalt AC-20 having an absolute viscosity of 2000 poises at 60aC was used in this experiment. In the process used, 15.5 grams of the styrene-butadiene polymer of this invention or 15.5 grams of the block copolymer Soprene (R) 1205 were slowly stirred into the asphalt for a period of about 45 minutes at a temperature of about 177SC-182SC . Then, the polymer / asphalt mixes were mixed for about 15 minutes in a high speed Ross mixer which was operated at a speed of 4200 rpm. Elemental sulfur was subsequently mixed into the polymer / asphalt mixture for a period of about 2 minutes and the mixture was then stirred slowly for a period of one hour at a temperature of 177SC-182SC. The physical properties of the modified asphalt cements made were then determined using conventional testing procedures. The strength and flexibility of asphalt binder cement at moderate or low temperatures was measured by force ductility, firmness and tenacity. These properties measure the resistance to deformation. Increasing strength and firmness provides greater resistance to surface abrasion and wear and provides better aggregate retention. Ductility was determined by ASTM D113. The ductility strength, elastic recovery, firmness and tenacity of two samples of modified asphalt are reported in Table 1. The styrene-butadiene polymer was also determined to have excellent compatibility with asphalt. This was determined using a separation test where the modified asphalt sample was placed in a tube having a diameter of 2.54 cms. and a length of 14 cms. and heat in a 163SC oven for 48 hours. The tube was maintained in an upright position through the heating passage. The tube containing the asphalt sample was then placed in a freezer at approximately -7 ° C for a minimum of 4 hours. Then the sample was separated from the freezer and cut into three equal length portions. The ring and ball softening point of the upper and lower portions of the sample was then determined by the method of ASTM D36. The compatibility is considered to be excellent in cases where the temperature difference between the softening points between the upper and lower samples is not greater than 2aC. In the case at hand, this temperature difference was only 0.4aC which indicates excellent compatibility.

TABLE 1 Conventional Reinforced-Butadiene Copolymer Estimator Polymer Force Ductility, 800% 1.24 kg 0.78 gr Force Ductility, 1000% 1.29 kg 0.77 gr Elastic Recovery @ 10eC 62.5% 65.0% Firming @ 25aC 2,181 kg / m 1716 kg / m Tenacity @ 25aC 1,627 kg / m 1072 kg / m As can be seen from Table 1, the asphalt that was modified with the styrene-butadiene polymer of this invention exhibited better ductility strength at 800 percent elongation, mere strength ductility at 1000 percent elongation, comparable elastic recovery , better firmness and better tenacity than the asphalt that was modified with the conventional styrene-butadiene block copolymer. While certain embodiments and representative details have been shown for the purpose of illustrating the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention.

Claims (10)

CLAIMS:

1. - A process for synthesizing a styrene-butadiene polymer that is particularly useful for modifying asphalt by a continuous polymerization process, the process being characterized by the steps of: (1) continuously charging 1,3-butadiene monomer, a compound of lithium organ, a polar modifier and an organic solvent to a first polymerization zone, (2) allow the 1,3-butadiene monomer to be polymerized in the first polymerization zone at a conversion of at least about 90 percent to produce a living polymer solution that is comprised of the organic solvent and living polybutadiene chains having a number average molecular weight that is within the range of about 20,000 to about 60,000, (3) continuously removing the living polymer solution from of the first reaction zone, (4) charge continuously styrene monomer, divinylbenzene and the living polymer solution which e is being withdrawn from the first polymerization zone into a second polymerization zone, (5) allowing the styrene monomer and the divinylbenzene monomer to polymerize in the second polymerization zone to produce a styrene-butadiene polymer solution which it has a number average molecular weight that is within the range of about 30,000 to about 85,000 and (6) continuously withdrawing the styrene-butadiene polymer solution from the second polymerization zone.

2. The styrene-butadiene polymer made by the process characterized in claim 1.

3. An asphalt concrete that is characterized in that it is comprised of (A) from about 90 weight percent to about 99 weight percent of an aggregate, and (B) from about 1 weight percent to about 10 weight percent of a modified asphalt cement that is comprised of (1) from about 90 weight percent to about 99 weight percent of asphalt, (2) from about 1 weight percent to about 10 weight percent of the styrene-butadiene polymer specified in claim 2, and (3) from about 0.1 weight percent to about 5 percent by weight of sulfur, based on the weight of the styrene-butadiene polymer.

4. A modified asphalt cement characterized in that it is comprised of (1) from about 90 weight percent to about 99 weight percent asphalt and (2) from about 1 weight percent to about 10 weight percent. weight percent of the styrene-butadiene polymer specified in claim 2.

5. A process for preparing a modified asphalt cement characterized by (1) mixing from about 1 to about 10 parts by weight of the styrene-butadiene polymer specified in claim 2 to from about 90 to about 99 parts by weight of asphalt at a temperature that is within the range of about 130a to about 230aC to produce a polymer-asphalt mixture, and (2) mixing about from 0.1 to about 3 parts by weight of sulfur to the polymer-asphalt mixture to produce the modified asphalt cement.

6. A styrene-butadiene polymer having excellent compatibility with asphalt and which is particularly useful for modifying asphalt to improve strength ductility, elastic recovery, hardness and toughness, the styrene-butadiene polymer being characterized in that it is comprised of a portion of butadiene and a portion of styrene, wherein the butadiene portion is comprised of repeating units derived from 1,3-butadiene, wherein the butadiene portion has a content of vinyl microstructure that is within the scale of about 35 by about 80 percent, wherein the butadiene portion has a number average molecular weight that is within the range of about 20, 000 to about 60,000, wherein the styrene moiety branches to multiple arms at branching points that are derived from divinylbenzene and wherein the styrene-butadiene polymer has a number average molecular weight that is within the range from around 30,000 to about 85,000.

7. An asphaltic concrete that is characterized in that it is comprised of (A) from about 90 weight percent to about 99 weight percent of an aggregate and (B) from about 1 weight percent to about 10 weight percent. weight percent of a modified asphalt cement that is comprised of (1) from about 90 weight percent to about 99 weight percent asphalt, (2) from about 1 weight percent to about 10 weight percent. weight of a styrene-butadiene polymer which is comprised of a butadiene portion and a styrene portion, wherein the butadiene portion is comprised of repeating units that are derived from 1,3-butylene, wherein the butadiene has a vinyl microstructure content that is within the range of about 35 percent to about 80 percent, wherein the butadiene portion has a number average molecular weight that is within the range of at a scale of about 20,000 to about 60,000, wherein the styrene moiety branches to multiple arms at branching points that are derived from divinylbenzene and wherein the styrene-butadiene polymer has a number average molecular weight that is within the range of from about 30,000 to about 85,000, and (3) from about 0.1 to about 5 parts by weight of sulfur per 100 parts by weight of the styrene-butadiene polymer.

8. A modified asphalt cement characterized in that it is comprised of (1) from about 90 weight percent to about 99 weight percent asphalt, (2) from about 1 weight percent to about 10 weight percent. weight percent of a styrene-butadiene polymer which is comprised of a portion of butadiene and a portion of styrene, wherein the butadiene portion is comprised of repeating units derived from 1,3-butadiene, wherein the The butadiene portion has a vinyl microstructure content that is within the range of about 35 percent to about 80 percent, wherein the butadiene portion has a number average molecular weight that is within the scale of about 20,000 to about 60,000, wherein the styrene moiety branches to multiple arms at branching points that are derived from divinylbenzene and wherein the styrene polymer o-butadiene has a number average molecular weight that is within the range of about 30,000 to about 85,000, and (3) about 0.1 to about 5 parts by weight of sulfur per 100 parts by weight of the styrene-polymer butadiene.

9. A styrene-butadiene polymer as specified in claim 6, characterized in that the butadiene portion is a linear polymer chain that is comprised of repeating units derived from 1,3-butadiene; characterized in that the butadiene portion has a vinyl microstructure content that is within the range of about 40 percent to about 60 percent; characterized in that the butadiene portion has a number average molecular weight that is within the range of about 30,000 to about 50,000; and characterized in that the styrene-butadiene polymer is comprised of from about 15 weight percent to about 35 weight percent styrene and from about 65 weight percent to about 85 weight percent butadiene.

10. A styrene-butadiene polymer as specified in claim 6, characterized in that the styrene-butadiene polymer has a number average molecular weight that is within the range of about 40,000 to about 75,000; characterized in that the styrene-butadiene polymer is comprised of from about 20 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 80 weight percent butadiene; characterized in that the butadiene portion has a vinyl microstructure content that is within the range of about 45 percent to about 55 percent; characterized in that the styrene-butadiene polymer has a number average molecular weight that is within the range of about 50,000 to about 65,000; and characterized in that the butadiene portion has a number average molecular weight that is within the range of about 35,000 to about 45,000.