POLYMER MODIFIED BITUMEN

Asif Nadeem

M.Sc. Chemical Engineer

INTRODUCTION

In Pakistan, the broad temperature swings cause roadways to expand and contract with fluctuating temperatures, a condition that ultimately reached to severe cracking, rutting and potholes.

Polymer modified bitumen provides the solution to these problems. It is offering superior storage stability, easier lay down, longer life and reduced surface maintenance for any blacktop- roads, airport runways, parking lots etc.

DESCRIPTION

Ethylene terpolymer reacts with bitumen asphalt to form a stable compound, which provides maximum protection against cold-weather cracking and resists rutting problems in summer. In test, soft bitumen modified, performed better under high temperature conditions than a non-modified hard bitumen.

Modified bitumen allows lower temperature applications and also act as a lubricant allowing the aggregate to be mixed, coated and tightly compacted to form a smooth dense surface.

During heated transportation and storage, asphalt binder made with a non-reactive modifier can separate into components with varying softening points; in contrast, a reacted modifier creates a stable compound with more consistent performance.

One of the full-scale applications of asphalt modified underscores its easy handling benefits. It allows lower temperature paving allowing the compactor to follow the spreader for easy compacting. Less equipment ?sticking? means faster cleanup. Modified bitumen would not separate in the tank during equipment shutdowns.

After cooling modified bitumen acts as the glue to hold the aggregate together in a solid matrix. In this finished state bitumen is considered viscoelastic. It has both elastic and viscous characteristics.

Modified bitumen provides both an increase in viscosity and an increase in elasticity. This means that a road lay with modified bitumen under heavy axle load can ?bounce back? after deformation and non-modified bitumen based road can?t.

This kind of response to a load can be related conceptually to an automobile?s shock absorbing system. These systems contain a spring and a liquid filled cylinder. The spring is elastic and returns the car to an original position after hitting a bump. The viscous liquid within the cylinder dampens (reduce the strength) the force of the spring and its reaction to the bump. Any force exerted on the car causes a parallel reaction in both the spring and the cylinder.

In hot mix bitumen, the spring represents the immediate elastic viscous reaction of the bitumen, particularly in warmer temperatures. Most of the response is elastic or viscoelastic (recoverable with time) while some of the response is plastic and non-recoverable.

HANDLING OF BITUMEN

As compared to other modified asphalt it is very easy to handle. Other modified asphalt can separate since the modifier used is just blended with asphalt while this one reacts and forms a permanent bond.

Other modified asphalt will be hard to pump as it will have a higher viscosity but it will behave as neat asphalt.

Ordinary modified asphalt tends to separate from the aggregates if the job is shutdown or postponed. (By rain, mechanical problems, etc.) On the other hand it will never because of reactive ethylene terpolymer, which forms a permanent bond with the asphaltenes in asphalt.

It can be compacted with either rubber or steel compactors while other modifiers needs steel compaction rollers.

ROAD PERFORMANCE

Other modified asphalts will rut sooner, since either it much poorer elasticity or stiffness. Other modified asphalts will fatigue crack sooner, it will last 5 ? 10 times longer in fatigue performance. Other modified asphalts tend to cold crack sooner. (They have poor cold temperature performance for equivalent hot temperature performance.)

CHEMICAL COMPOSITION

Ethylene terpolymer is especially designed for Asphalt modification and has three chemical components:

Ethylene backbone: it ensures that polymer blends into asphalt easily and improves its viscosity.

Butyl Acrylate: it improves asphalt elastic properties due to which it gives better performance on the road.

Glycdyl Methacrylate: it reacts with asphalt forming a permanent chemical bond and provides storage stability and can be reheated as many times.

CHARACTERISTIC FEATURES

Following are the some important and major features of polymer-modified bitumen.

THERMAL RESISTANCE

Modified bitumen has lower susceptibility towards temperature variations under heavy axle load. It will not be heated due to friction of heavy loaded vehicle?s tire and as a result, its temperature will not rise.

STRENGHT

It has greater resistance to deformation of its assembled shape in working environments on the roads as compared to ordinary bitumen, which under heavy loads of vehicles and severe conditions of weather deformed and loose its shape.

BINDING FORCE

It has better binding of aggregate and bitumen than ordinary bitumen. Its greater binding characteristic prevents it in heavy rainy seasons to dissolve in rainy water.

CRACK PROOF

Its internal molecular structure is such that it prevents cracks in structure under all conditions of temperature and load.

RUT PROOF

Due to its greater strength it has more resistance to rutting phenomena. It will deshape a road in such a way that deep passages formed when heavy loaded vehicles continuously passed over the same way.

LONG LASTING

Polymer modified bitumen increases overall life of the motorways and high ways by reducing the maintenance cost.

ECONOMIC VIABILITY

From economical point of view polymer modified bitumen also favorable.

Some important aspects are as follows:

Roads, which are constructed polymer modified bitumen, have lower maintenance cost than the ordinary roads. It does not allow them to deform by any means such as cracking or rutting, which are usually major factor for the destruction of any road. Its greater binding force prevents roads under severe conditions of temperatures.

During constructional works its wastage is very less because of its improved ways of handling and usage that the conventional ways.

It has a very low percentage addition of 1.5 % ---- 2.5 % as compared to 5 % ----- 15 % of conventional modifiers.

MIXING PROCEDURE WITH CATALYIST

1A lab mix study needs to be performed to determine optimum reactive ethylene terpolymer and acid levels prior to producing commercial pounds of product. (Typically reactive ethylene terpolymer levels range from 1.5% to 2.5%, and catalyst levels are less then 0.25%.) Do not increase polymer or acid levels above laboratory levels without first running a lab test to see if PMB (Polymer Modified Bitumen) will gel at the increased levels of polymer and/or acid.

2.The very first PMB production run should be a small-scale production. (~50 tons). This small run will be used to check the accuracy of the lab scale versus actual production scale.

3.Inspect the mix tank. Flush and empty if necessary before adding the PMB base stock.

4.Check that the PMB base stock meets all cold weather bid specifications. Remember that reactive ethylene terpolymer modifies the hot weather properties. The base stock must be formulated to meet the cold weather properties.

5.Begin adding the PMB base stock to the tank. Heat the base stock up to 330°F (385°F if catalyst will not be used.)

6.Sample the PMB base asphalt in the blend tank. Measure the viscosity at 140°F (60°C). Check other asphalt properties for baseline values.

7.Turn agitator and recirculation pump on. Check the top of the mix tank to assure tank is being vigorously agitated. (Should see a vortex or rough churning on the surface of the asphalt.) Do not proceed if tank is not properly agitated.

8.Begin adding the reactive ethylene terpolymer in reactor at a rate of 30-lb/min.. Check the top of the tank periodically for polymer buildup (iceberg like lumps). If polymer buildup is encountered, shut down polymer addition and allow polymer to mix into the asphalt. After polymer is mixed in, startup polymer addition at a rate of 5 lb/min less than previous rate. Repeat this step until no polymer buildup is detected after one hour of continuous polymer addition.

9.If no polymer buildup is encountered at 30 lb/min for one hour, increase polymer addition rate by 10 lb/min, wait one hour, if no polymer buildup, increase rate by another 10 lb/min. Repeat this step until either polymer build up occurs, or maximum polymer feeder rates are encountered. (Note: It is rare to be able to exceed 100 lb/min without having polymer buildup.) If a polymer buildup is detected, reduce feed rates back to the previous level that did not produce polymer buildup. If the buildup continues, discontinue polymer addition until the buildup is gone.

10.Continue reactive ethylene terpolymer until the desired amount has been added.

11.After the optimum polymer addition rate is determined, check this rate over the next few production runs to ensure there is no buildup on top of the tank.

12.Continue maintaining tank temperature at 330°F(385°F if no acid is being used.)

13.Once all the reactive ethylene terpolymer has been added, check the top of the tank to ensure all the polymer has been dissolved. Do not proceed with adding catalyst until the entire polymer is dissolved. It is very hard to disperse reactive ethylene terpolymer in asphalt after the acid has been added.

14.Sample tank and run the absolute viscosity at 140°F. The viscosity measurement will be used to monitor the rate of the reaction.

15.Begin injecting the phosphoric acid into the recirculation line of the tank. If 0.25% acid (by weight) is desired in the asphalt, inject 0.25% into the recirculation line. (i.e., if recirculation flow is 100,000 lb/hr, and 0.25% acid is desired, inject at 250 lb/hr.)

·Should re-circulation flow be lost at any point during the catalyst injection, immediately shut down the acid flow. Injecting acid into a no-flow asphalt line could cause the asphalt to gel, plugging the line. It is highly recommended that the acid pump be interlocked down on the loss of the recirculation pump.

16.Continue injecting acid until the desired amount has been injected. Make sure that the following is occurring both during the acid addition and until the reaction is complete:

Agitator and recirculation pump are running.

Tank is maintained at 330°F.

Viscosity of tank is checked every hour to track the reaction rate/time.

17.Once the viscosity has become stable for 24 hours, the reaction is complete. The PMB should be tested to ensure that it meets all the end use specifications. The PMB can now be pumped to a finished product storage tank, and held at 325°F. The storage tank should have a gentle mixer (10-15 HP side mixer) at the bottom of the storage tank. The PMB will not separate or settle out of the asphalt, because the reactive ethylene terpolymer is incorporated into the backbone of the asphalt molecule. If longer-term storage of the PMB is needed, the tank can be cooled, and reheated.

MIXING PROCEDURE WITHOUT CATALYST

1. A lab mix study needs to be performed to determine optimum reactive ethylene terpolymer level prior to producing commercial pounds of product. (Typical reactive ethylene terpolymer levels range from 1.5 to 2.5 %) Do not increase polymer above laboratory levels without first running a lab test to see if PMB (polymer modified asphalt) will gel at the increased levels of polymer.

2. The very first PMB production run should be a small-scale production. (~50 tons). This small run will be used to check the accuracy of the lab scale versus actual production scale.

3.Inspect the mix tank. Flush and empty if necessary before adding the PMB base stock.

4.Check that the PMB base stock meets all cold weather bid specifications. (Remember that reactive ethylene terpolymer modifies the hot weather properties. The base stock must be formulated to meet the cold weather properties.)

5. Begin adding the PMB base stock to the tank. Heat the base stock up to 385 °F.

6. Sample the PMB base asphalt in the blend tank. Measure the viscosity at 140°F (60°C). Check other asphalt properties for baseline values.

7.Turn agitator and recirculation pump on. Check the top of the mix tank to assure tank is being vigorously agitated. (You should be able to see a vortex or rough churning on the surface of the asphalt.) Do not proceed if tank is not properly agitated.

8. Begin adding the reactive ethylene terpolymer at a rate of 30 lb/min check the top of the tank periodically for polymer buildup (iceberg like lumps). If polymer buildup is encountered, shut down polymer addition and allow polymer to mix into the asphalt. After polymer is mixed in, restart polymer addition at a rate of 5 lb/min less than previous rate. Repeat this step until no polymer buildup is detected after one hour of continuous polymer addition.

9. If no polymer buildup is encountered at 30 lb/min for one hour, increase polymer addition rate by 10 lb/min and wait one hour. If there is no polymer buildup, increase the addition rate by another 10 lb/min. Repeat this step until polymer buildup occurs, or maximum polymer feeder rates are encountered. (Note: It is rare to be able to exceed 100 lb/min without having polymer buildup.) If a polymer buildup is detected, reduce feed rates back to the previous level that did not produce buildup. If the buildup continues, discontinue polymer addition until the buildup is gone.

10.Continue adding reactive ethylene terpolymer until the desired amount has been added.

11. After the optimum polymer addition rate is determined, check this rate over the next few production runs to ensure there is no buildup on top of the tank.

12. Maintain tank temperature at 385°F.

13.Once all the reactive ethylene terpolymer has been added, check the top of the tank to ensure all the polymer has been dissolved.

14.Sample tank and run the absolute viscosity at 140°F. The viscosity measurement will be used to monitor the rate of the reaction. Continue monitoring the viscosity every few hours.

15. When the viscosity has remained stable for 24 hrs, the reaction is complete. The PMB should be tested to ensure that it meets all the end-use specifications.

16. The PMB can now be pumped to a finished product storage tank and held at 325°F. The storage tank should have a gentle mixer (10-1 to 15-hp side mixer) at the bottom of the storage tank. The PMB will not separate or settle out of the asphalt, because the reactive ethylene terpolymer is incorporated into the backbone of the asphalt molecule. If longer-term storage of the PMB is needed, the tank can be cooled and reheated.

COMPARISION OF MIXING PROCEDURES

Polymer consumption is 1.35 % by using the catalyst where as     without catalyst polymer consumption 1.5 %.

· Phosphoric acid 0.25 % is used as catalyst. Therefore process cost increased when using catalyst, which is not considered without catalyst process.

· Major advantage of catalyst is the reduction of reaction time. Process time without catalyst is 24hrs while using catalyst it is left only 1hr.

· Reaction temperature required for the process decreases by the catalyst. Reaction temperature reduces 50o from 380oC to 330oC with usage of catalyst.

· Capital investment for the plant reduces due to small size of plant with catalyst use, where as high capital investment is required to built a bigger plant for the same capacity.

· Process becomes difficult when catalyst is used because handling of highly pure phosphoric acid require high safety precautions and measures, also catalyst storage is another factor. Without catalyst all these difficulties are not to be considered. 

· Steam consumption for the process reduces due to catalyst use because the required reaction temperature reduces.

SPECIFICATIONS OF EQUIPMENT AT PMB PLANT

Acid Pump:

Bison gear and Engineering Corporation

GE Chemteck metering pumps system

Pulsaseeder

Series 400

Serial #

Max pressure 300 psi (21 kg/cm2)

Max Output @ 60hrz

17gal/hr (64.25 lit/ hr)

Reactors R1 and R2:

No.of Agitator/reactor 1

No. of Impeller/reactor 2

Type of Impeller: Flat blade turbine

No. of blade/impeller: 4

Diameter of impeller: 4Ft

RMP 700-100rpm

Motor Power; 15-20HP(for each agitator)

Motor type; TEFC (Advised by Safety Deptt.)

SAFETY AND HANDLING CONSIDERATIONS FOR

PHOSPHORIC ACID

Phosphoric acid is hydroscopic, i.e. it will absorb moisture from air. During storage and handling of acid, care should be taken to avoid contact with water and air.

Dilution of phosphoric acid with water is highly exothermic. If acid needs to be diluted for disposal, etc., dilution must be carried out slowly and with mixing.

Dilution in water causes acid to be more aggressive on skin contact.

Dilution in water causes acid to more aggressively attack mild steels.

Phosphoric acid will corrode reactive metals (mild steel, aluminum). The reaction rate increases drastically with water. Hydrogen gas released in the reaction, possibly lead to explosive hydrogen ? air mixture.; 316 stainless steel or equivalent is recommended for all construction seeing pure phosphoric acid.

Once acid is fully mixed into the polymer-modified asphalt, no special precautions need to be taken due to the acid addition.

Phosphoric Acid Formula:

H (PO3H)n OH

EQUIPMENT FOR ACID ADDITION

It is recommend that all equipment coming into contact with the pure phosphoric acid be constructed of 316 stain less steel, or comparable construction. After acid is incorporated into the asphalt, no special materials of construction are necessary.>/span>

Two concepts for acid storage and addition have been used:

1.                             Lower-cost option: (55-gallon drums only)

This method is recommended for making test batches of reactive ethylene terpoymer with the poly phosphoric acid technology.

Use of a drum pump to pump directly out of the 55-gallon drum and into the storage tank's recirculation line.

If phosphoric acid with a purity above 105% is to be used, it is important to heat the drums, pump, and acid transfer line to lower the viscosity to manageable levels.

(In the case of 115%, acid temperatures should be at least 60°C [140°F].) Heating of drums of acid can take several days, since the highly viscous fluid will not mix upon heating.

Drums should be either lined, or made of 316 SS. It is highly recommended that the pump internals, and acid injection line be made of 316 SS or equivalent. To prevent the acid from absorbing moisture from the air, drums should not be left open to the atmosphere. If long drum pumping times are anticipated, the drums should be blanketed with nitrogen or dry instrument air to prevent moisture accumulation by the hydroscopic poly phosphoric acid.

A method to accurately measure catalyst flow is needed. The injection of too much catalyst can lead to the asphalt "gelling". One low-cost way to measure flow is to place the drum on a commercial scale. Take readings of the weight loss every 10minutes, and adjust flow to compensate.

Pump sizing should allow for 0.1% to 0.5% acid (weight) loadings of the asphalt in the recirculation line. (Acid pump should be sized for 0.1 to 0.5% weight flow of the recirculation pump.)

2.                             Higher-cost, more permanent investments: (typically bulk delivery)

Acid should be received by tank truck or rail car, and pumped into a 316 stainless steel tank or equivalent. The tank should be inerted with nitrogen or dry instrument air to avoid moisture pickup from outside air.

A positive displacement pump should be used to pump the acid from the tank. Dual lobe and other gear pumps have been successfully used to pump the phosphoric acid. These pumps offer good turndown ratios, and predictable pumping characteristics over the wide range of viscosity encountered with phosphoric acid. Make sure the pump suction is designed to allow for sufficient head to properly prime the pump.

If a phosphoric acid above 105% strengthis used, the tank should have a heater capable of holding the tank at least 60°C (140°F).

Care needs to be taken to ensure that no water is added to the tank from the heating system. Also, the transfer lines and pump should be traced to maintain at least 60°C (140°F).

The acid is very viscous at temperatures below 60°C (140°F). The tank will need to be heated for an extended period of time (days), or the tank will need to be agitated prior to pumping the acid to ensure the entire tank is above 60°C (140°F). (The highly viscous nature of phosphoric acid tends not transfer heat well, leaving cold "pockets" of acid in the tank.) It is not uncommon to lose acid flow during a campaign, due to a "slug" of cold acid starving the pumping system.

Pump sizing should allow for 0.1% to 0.5% acid (weight) loadings of the asphalt in the recirculation line. (Acid pump should be sized for 0.1 to 0.5% weight flow of the recirculation pump.)

CHEMICAL REACTION WITH ASPHALT

Reactive ethylene terpolymer is a random terpolymer comprising ethylene, normal butylacrylate and glycidyl methacrylate (GMA). The molecular weight and co-monomer levels can be varied during polymer manufacture. It is the GMA portion of the molecule that appears to be responsible for the reaction observed when reactive ethylene terpolymer is mixed and heated with asphalt.

EGA copolymers chemically react with asphalt to form a polymer-linked-asphalt system with improved performance properties. The epoxide ring in the glycidal structure is believed to undergo an addition reaction with various functional groups in a typical asphaltene molecule. The asphaltenes, which can have carboxylic acid functionality, open the epoxy ring and form an aromatic ester. This type of reaction is consistent with the I.R. and D.S.C. trace of the extracted reaction product. The D.S.C. also indicates that the reaction is 40 to 50 percent asphaltene like material.

Polymers with higher levels of GMA have been and evaluated in asphalt. These polymers appear to allow the use of fewer polymers to give the same response in high temperature SHRP properties.

Proposed Mechanism for Reaction of EGA Polymer with Asphaltene Molecule Containing Carboxylic Acid Group