CA2354750C - Polyethylene modified asphalt - Google Patents

Abstract

The use of styrene butadiene block elastomer and sulfur enables the preparation of stabilized polyethylene modified asphalt cement compositions. These compositions have excellent low temperature flexibility and excellent resistance to rutting-type deformation at high service temperature and are comparatively inexpensive to manufacture. The compositions are particularly suitable for use in the paving of roads which are subjected to high traffic loads.

Description

Polyethylene Modified Asphalt FIELD OF THE INVENTION

This invention relates to polymer modified asphalt cement.
BACKGROUND OF THE INVENTION

Asphalt cement is widely used as a paving material. Different grades of asphalt are employed for different environments. One characteristic of asphalt which is used to distinguish between different grades is the "penetration value", or pen value. In general terms, the pen value indicates the depth to which a specific, weighted instrument will penetrate into asphalt cement. Thus, a soft asphalt cement will have a higher pen value that a hard asphalt cement (indicating that the weighted instrument penetrated further).
It is desirable to use a paving asphalt cement which is deformation resistant at high service temperatures (to reduce "rutting" of pavements subjected to heavy traffic loads) but which is soft/flexible at cold service temperatures (to reduce the cracking of pavement at low service temperatures). Conventional asphalt cements generally do not display this most desirable type of temperature/ property characteristics.

However, it is known to improve the temperature susceptibility of asphalt cements with polymers.

For example, it is known to modify asphalt cement with a combination of styrene-butadiene-styrene ("SBS") polymer and sulfur, as disclosed in U.S. patent 4,145,322 (Maldonado). The use of SBS
PSC4MCase9100/SP-CAN.045 - 2 -produces excellent polymer modified asphalt, but SBS is comparatively expensive. Accordingly, an invention leading to the production of a less expensive polymer modified asphalt cement (PMA) would represent a useful addition to the art.

Polyethylene is one polymer which is a seemingly good candidate for the production of cost effective PMA in that polyethylene is inexpensive and is largely chemically saturated. However, the use of polyethylene to produce PMA has been hampered by an incompatibility between the polyethylene and the asphalt, leading to gross phase separation. We previously invented a solution to this problem by treating the polyethylene with a source of oxygen (such as ozone) - so as to produce an 0-modified polyethylene which is more compatible witil asphalt. This invention is disclosed in U.S. patent 5,302,638. However, this invention still involves the expense associated with the additional process of treating the polyethylene with the oxygen or ozone.

We have now discovered a cost effective PMA composition which contains polyethylene, sulfur and a styrene-butadiene polymer. The preparation of such modified asphalt cement is comparatively inexpensive, convenient and produces a PMA with enhanced properties and good phase stability.

SUMMARY OF THE INVENTION
The present invention provides:

PSCrm/Case9100/SP-CAN.045 - 3 -A polymer modified asphalt composition comprising asphalt cement, block styrene-butadiene rubber (SB), polyethylene (PE), and sulfur (S), wherein the composition is characterized by containing:

(a) from 1 to 4 weight % block styrene-butadiene elastomer;
(b) from 1 to 4 weight % polyethylene;

(c) from 25 ppm to 2 weight % sulfur; and (d) asphalt cement.

The compositions of this invention demonstrate enhanced resistance to rutting/deformation at high service temperatures (as illustrated by viscosity at 60 C), while still maintaining good flexibility at low service temperatures (as illustrated by penetration values at 0 C).
Moreover, the present compositions have good resistance to phase separation - which is a significant advantage in comparison to simple asphalt cement/polyethylene compositions.

Whilst not wishing to be bound by any particular theory, it is believed that:

(a) both of the polyethylene and the SB contribute to the enhanced physical properties of the present composition; and (b) the SB functions as a "compatibilizer" for the polyethylene and asphalt cement (i.e. the SB copolymer is believed to act as a type of emulsifier for the polyethylene, thus preventing the polyethylene from separating out of the composition).

PSCrjm/Case9100/SP-CAN.045 - 4 -DETAILED DESCRIPTION

The compositions of the present invention utilize asphalt cement (which is also known as "bitumen"). The asphalt cement may be any type of asphalt cement which is conventionally employed in road paving.
Suitable types of asphalt cement include natural asphalt and asphalt cement derived from petroleum distillation processes. A preferred source of asphalt cement is the residue obtained from the vacuum distillation of heavy crude oil.

Asphalt cement is conventionally described by a number of ASTM
tests. Preferred asphalt cements for the present use have a penetration value (or "pen" value) of from 5 to 300 dmm (as measured by ASTM D-5 at 25 C).
As used herein, the term block styrene-butadiene ("SB") elastomer or block SB rubber refers to a family of "blocky" elastomeric copolymers which are prepared by the polymerization of a conjugated diene monomer selected from butadiene, isoprene and 2,3 - dimethyl - 1,3 butadiene and a vinyl aromatic monomer which is styrene or a polymerizable derivative thereof (such as alpha-methyl styrene). Such polymers are sometimes also referred to by those skilled in the art as "block SBS rubber" or "SBS
rubber". The word "block" is well known to those skilled in the art and is used to distinguish between "random" elastomers (in which the monomer is incorporated "at random" into the elastomer chain) from the present block elastomers (in which the monomer is incorporated in repeat or PSC/JmlCase9100/5F-cAN.045 - 5 -blocky units). Thus the words "block" or "blocky" refer to the requirement that the present copolymers contain "blocks" of repeating monomer units (for example, "blocks" of styrene and "blocks" of butadiene).

The preparation of block SB elastomers is well known and widely reported and may, for example, be undertaken by the initial solution polymerization of styrene in the presence of a butyl lithium catalyst, followed by the subsequent addition and polymerization of butadiene.
Suitable examples of block SB elastomers are sold by the Shell Oil Company (Houston) under the trademark "KRATON" and by the Fina group of companies under the trademark "FINAPRENE".

Many of the block SB elastomers sold by the Shell Oil Company are hydrogenated so as to produce an essentially saturated product.
These saturated polymers have improved aging resistance in comparison to non-hydrogenated elastomers. However, for use in the present invention, the block SB elastomer must be partially unsaturated.
Preferred block SB elastomers are non-hydrogenated, though a partially hydrogenated block SB elastomer may also be suitably employed.

The present compositions also contain polyethylene, which is a widely available thermoplastic polymer. The polyethylene may be a copolymer (i.e. a polymer containing a major portion of units derived from ethylene monomer and a minor portion of units derived from another alpha olefin monomer - such as butene or hexene) or a homopolymer. Any of PSCfjrtUCas99100/SP-CAN.045 - 6 -the commercially available polyethylene materials may be employed. The use of post-consumer or recycled polyethylene is also contemplated.

The compositions also require the use of sulfur. The use of conventional sulfur is satisfactory. "Special preparations" of sulfur, such as-those which may be used in certain rubber compounds are not required and constitute an unnecessary expense for use in the preparation of these compositions.

The relative amounts of asphalt cement, block SB elastomer, polyethylene and sulfur used in the present invention may vary considerably, provided that the amounts fall within the following limits:

(a) 1 to 4 weight % block SB elastomer;
(b) 1 to 4 weight % polyethylene;
(c) from 25 parts per million to 2 weight % sulfur; and (d) the balance, to 100 weight %, asphalt cement.

The use of more than 4 weight % block SB elastomer provides good properties but is generally too expensive for commercial use. The preferred amount of block SB elastomer is from 2 to 3 weight %.

The combined use of polyethylene and block SB elastomer provides good properties at lower cost than the use of block SB elastomer (due to the comparatively high price of block SB elastomer). The preferred amount of polyethylene is from 2 to 3 weight %.

It will be appreciated that the acceptable range of sulfur usage is quite large. The minimum figure represents the minimum amount required PSqmlCase910U/SP-CAN.045 - 7 -to produce compatibilization of the polyethylene (as evidenced by a lack of gross phase separation). The maximum figure represents a guideline to indicate where the beneficial effect attributed to the addition of incremental sulfur is significantly diminished. Larger amounts of sulfur may be employed but are economically wasteful. The preferred amount of sulfur is from 25 to 500 parts per million.

The preparation of these compositions will be comparatively simple for the skilled artisan. Conventional asphalt mixing equipment will provide good results. As will be appreciated by those skilled in the art of preparing polymer modified asphalts, the mixing temperature should be between about 140 C and 230 C (to maintain satisfactory viscosity) and the mixing time for commercial preparations should be at least 45 minutes, preferably 2-3 hours.

Further details of the invention are illustrated by the following examples.

EXAMPLES
Procedures For all experiments, polyethylene and SB block elastomer were added (in the amounts shown in table:~ 1 and 2) to vigorously stirred asphalt cement at a temperature of about 180 C. The compositions were agitated for 45 minutes. Sulfur was then added (in the amounts shown in tables 1 and 2) and mixing was continued for a further 45 minutes.
Samples from the resulting mixed compositions were poured into a brass PSCfjrtJCase9100/SP-CAN.045 - 8 -cylinder (having a diameter of 2 cm and a height of 25 cm) and aged for 5 days at 160 C. At the end of the aging period, the material was separated into top and bottom half sections and the softening points of the two halves were measured according to ASTM procedure D-6.

The penetration value (ASTM D-5 at 25 C and 0 C, as indicated in table 1) and viscosity (at 60 C, using a bob-in-cup rheometer) of the samples were also determined.

Materials 1. The polyethylene was recycled polyethylene.

2. The asphalt cement was a commercial asphalt cement described as a 200/300 pen grade (i.e. having a penetration value, as measured by ASTM D-5 of between 200 and 300 at 25 C).

3. The styrene butadiene (SB) block elastomers were commercially available products, as follows:

SB-1 was Fina 1205 (from Fina Chemicals); and SB-2 was KRATONT"" D1101 (from Shell Chemical Company).

Results The results are illustrated in tables 1 and 2.

Comparative examples 1 and 2 illustrate the gross phase instability of the simple polyethylene/asphalt cement compositions (as evidenced by the data in table 1- note the large difference in the softening point of the PSCfjmlCase9100/SP-CAN.045 - 9 -top half of the samples - where the polyethylene presumably was concentrated - and the bottom half of the samples).

The compositions of inventive examples 3, 4 and 5 demonstrate good phase stability/homogeneity, as evidenced by the data in table 1 which show very small differences between the softening point of the top and bottom halves. Turning now to table 2, the compositions of inventive experiments 3, 4 and 5 all show very good flexibility at 0 C (as indicated by the penetration and softening point results) and excellent viscosity at high temperature (as indicated by the viscosity at 60 C results). This combination of low temperature and high temperature properties is indicative of an asphalt cement that will be resistant to cracking at low service temperatures and resistant to "rutting" or deformation at high service temperatures.

For comparison, the pen values, softening points and viscosities of an unmodified (or "as is") hard asphalt cement and an unmodified soft asphalt cement are given in table 2. These data show that the unmodified "soft" asphalt cement (i.e. the 200/300 pen grade, having a measured pen value of 27 (dmm at 25 C) is still flexible at 0 C (as indicated by the comparatively good pen value of 70 dmm) but is too mushy at 60 C as indicated by the comparatively low viscosity of 44 Pa.s). Thus, the data which describe this unmodified soft asphalt cement are indicative of a material that will be flexible at low temperatures but susceptible to undesirable rutting when subjected to load at high temperatures. In PSCfjm/Case9100/SP-CAN.045 - 10 -contrast, the data in table 2 which describe the unmodified hard asphalt cement (i.e. pen values of 159 at 39 dmm at 25 and 0 C, respectively, and a viscosity of 99 Pa.s at 60 C) are indicative of a brittle material at low temperatures and a material of questionable rutting resistance at higher temperatures. It is particularly noteworthy that the inventive io compositions of experiments 3, 4 and 5 have better properties at both low and high temperatures than the unmodified hard asphalt cement.
Comparative examples 6 and 8 illustrate the criticality of the sulfur.

In the absence of sulfur, the asphalt compositions which contain both PE
and block SB elastomer are prone to phase separation (see softening point data, table 1).

Comparative example 7 illustrates that sulfur alone will not compatibilize PE and asphalt cement (see softening point data, table 1).
Comparative examples 10, 11 and 12 illustrate known technology in which sulfur and block SB elastomer are used to produce a PMA having an excellent balance of properties. However, the use of block SB
elastomer in the production of PMA has been limited because of the very high cost of block SB. Thus, the replacement of expensive block SB with less expensive PE represents a useful addition to the art.

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Claims (4)

1. A polymer modified asphalt composition comprising asphalt cement, block styrene-butadiene elastomer, polyethylene and sulfur, wherein the composition is characterized by containing:

(a) from 1 to 4 weight % block styrene-butadiene elastomer;
(b) from 1 to 4 weight % polyethylene;

(c) from 25 ppm to 500 ppm sulfur; and (d) asphalt cement having a penetration value of from 200 to 300 as measured by ASTM D-5 at 25°C, such that the total weight of said (a), (b), (c) and (d) is 100 weight %.

2. The composition of claim 1 which contains from 2 to 3 weight %
block styrene-butadiene elastomer.

3. The composition of claim 1 which contains from 2 to 3 weight %
polyethylene.

4. The composition of claim 1 wherein said polyethylene is recycled polyethylene.