Elastomer-modified bituminous compositions

Details Elastomer-modified bituminous compositions Elastomer-modified bituminous compositions

1999-12-02

2002-08-06

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S059000, C524S069000, C524S070000, C524S071000

Reexamination Certificate

active

06429241

ABSTRACT:

FIELD OF INVENTION
The present invention is concerned with the preparation of bituminous compositions with high performance characteristics, particularly compositions containing bitumen and high molecular weight elastomer(s). The bituminous compositions provided herein have numerous commercial uses, including road paving, roofing, asphalt emulsions and crack fillers.
BACKGROUND TO THE INVENTION
Bituminous compositions for various applications are known to contain elastomers, such as natural rubber (NR), styrene-butadiene rubber latex (SBR), Neoprene latex and styrene-butadiene block copolymers (SB). The SB copolymers having a conjugated diene structure are most commonly used to provide improved properties in specific applications. These elastomers improve the mechanical properties of the bitumen and hence their incorporation is desirable.
In general, low molecular weight (MW) elastomers, those having a MW of less than about 70,000, are readily dispersed in bitumen to form a compatible bitumen composition. In this specification, the term ‘molecular weight’ as applied to elastomeric polymers refers to their weight average molecular weight. However, low MW elastomers do not offer significantly improved properties when compared with those resulting from the use of high molecular weight elastomers as bitumen modifiers, unless the low MW rubber chains are expanded and strengthened using a cross-linking agent, such as elemental sulfur. Even when so cross-linked, the low MW elastomeric bituminous compositions are still inferior to those prepared from high molecular weight elastomers, those having a MW greater than MW about 200,000. Unfortunately, compositions prepared from the high MW elastomers, in most cases, give rise to compatibility problems for the elastomer in the bitumen, resulting in phase separation when the bituminous compositions are stored.
The prior art has disclosed that the compatability as well as the properties of high MW elastomer-modified compositions can be improved by in-situ vulcanization using sulfur as a cross-linking agent. However, the processes described in. the prior art result in severe limitations on the proportions of sulfur and/or elastomer which can be reacted in the bitumen before the composition forms an un-processible gel. As noted above, a vulcanization reagent, such as elemental sulfur in combination with elastomers, is frequently employed to enhance the properties or to improve the compatibility of the elastomer in modified bitumen.
Specific examples of such prior art include U.S. Pat. No. 4,154,710, U.S. Pat. No. 4,145,322, U.S. Pat. No. 4,242,246 and U.S. Pat. No. 5,371,121, EP 2023, BE 877 112, AU 59,875, WO 90/02776, GB 460854, GB 1 548 541 and GB 1 330 425. In each of these prior art patent publications, elemental sulfur was used to improve the performance and/or the compatibility of the elastomer-bitumen compositions. In other prior art patent publications, other types of reagents, such as sulfur-donors and peroxides, are disclosed. However, these additives are so expensive that they have not, in general, been used commercially.
In general, superior performance enhancement of bituminous compositions at a fixed loading of elastomer can be achieved by using higher molecular weight elastomers. However with these higher MW elastomers, poor storage stability and relatively high viscosity can result in difficult material handling (processing, pumping, polymer separation and reblending), even at high temperatures. A number of examples from the prior art teach that successful stabilization and property enhancement can be achieved in bitumen/styrene-butadiene/sulfur blends using relatively low molecular weight elastomers, normally well below about 150,000. Some examples, including those used in U.S. Pat. No. 4,154,710, have even been based on the use of very low molecular weight liquid rubber, to initiate the sulfur reaction, thereby forming a high molecular weight elastomeric network in-situ through vulcanization, avoiding both storage and processing problems. However, these compositions show sub-optimal performance, high cost or both.
Some other examples from the prior art (EPA 0174795) have used high MW elastomers (over about 150,000) which have reacted with sulfur to provide blends with good mechanical properties. However, such prior art examples have all suggested that sulfur-initiated reaction or vulcanization of a high molecular weight elastomer be carried out in bitumen/oil blends (that is, the bitumen is fluxed with a hydrocarbon oil). The reaction product may then be further diluted with a light fluxant oil to reduce the viscosity to prevent the formation by gellation of a highly viscous product. A similar prior art (BE 87712) discloses that the bitumen may be chemically modified by a mother solution in which a high molecular weight elastomer (SB di-block copolymer) and sulfur have been dissolved and pre-reacted into a light petroleum fluxant.
In a recent patent (U.S. Pat. No. 5,371,121), a process was disclosed for producing a stable bituminous composition without using light fluxant, and in which a high molecular weight (over about 150,000) tri-block thermoplastic elastomer was reacted with a defined amount of sulfur from 0.015% to 0.075% by wt. However, this disclosure is relevant to only one specific grade of bituminous asphalt cement (AC20R) and at relatively low sulfur loadings. At only slightly higher sulfur loadings of 0.1 and 0.25%, as disclosed in Examples 15 and 16 of the patent, gel formation resulted in viscosities exceeding the standard for AC20R. The process of the present invention overcomes these difficulties.
Most recent prior art (WO 97/45488) disclosed a new bitumen-polymer blend that utilizes a combination of two types of elastomers (SB di-block copolymer with 100,000 MW and SBS tri-block copolymer with 300,000 MW) at effective amounts and ratios, along with defined amounts of sulfur which is to be added and reacted into the asphalt to enhance performance. The remainder of the prior art refers only to the use of a single elastomer to be reacted in the system. However, all examples given in WO 97/45488 are limited to sulfur loading which do not exceed 4% by wt, based on the total amount of elastomers, otherwise the viscosity is too high to permit workability of the asphalt-polymer mix.
In principle, under reactive process conditions, especially those using a cross-linking agent, such as sulfur, increasing the quantity of high MW elastomer-incorporated into bitumen should require an equivalent relative increase in the quantity of sulfur, in order to achieve better stability of the dispersed elastomer, and acceptable mechanical properties of the product which is processable in varied industrial applications. However, to achieve this result represents a challenging combination of requirements. In practice, as disclosed above in the prior art on the practice of reactive processing of bituminous compositions, the skilled artisan tends to proceed with considerable caution when utilizing high molecular weight elastomers which are to be chemically vulcanized into bitumen alone or in combination with oil or fluxing agents. Usually, even at a fixed level of sulfur, as the concentration of the elastomer is increased, the viscosity of the asphalt mixture increases rapidly due to gellation and phase separation of the asphalt and elastomer may occur because of excessive inter-polymer cross-linking. Therefore, it is generally considered difficult to incorporate high MW elastomer into bitumen at higher loadings, especially for the situation in which the proportion of sulfur is also increased proportional to the increased load of elastomer. In addition, the limitation on the load level of elastomer which can be vulcanized into asphalt has restricted the application of the reactive process in important industrial applications, such as roofing, crack filler and so on.
The present invention is directed towards solving the above described problems whereby a sulfur reactive process is proposed for producing unsaturated elastomer/bitumen co

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