Asphalt is a common material utilized for the preparation of paving and roofing materials and also for coatings such as pipe coatings and tank liners. While the material is suitable in many respects, it inherently is deficient in some physical properties which it would be highly desirable to improve. Efforts have been made in this direction by addition of certain conjugated diene rubbers, ethylene containing plastics like EVA and polyethylene, neoprene, resins, frillers and other materials for the modification of one or more of the physical properties of the asphalt. Each of these added materials modifies the asphalt in one respect or another but certain deficiencies can be noted in all modifiers proposed. For example, some of them have excellent weather resistance, sealing and bonding properties but are often deficient with respect to warm tack, modulus, hardness and other physical properties; and some of them improve only the high temperature performance- of asphalt, some only improve the low temperature performance of asphalt, while some lack thermal stability or mixing stability with asphalt.
Since the late 1960s, diene polymer rubbers such as styrene-butadiene rubber and styrene-rubber block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers have been used to dramatically improve the thermal and mechanical properties of asphalts. Practical application of the rubber addition approach requires that the blended product retain improved properties and homogeneity during transportation, storage and processing. Long term performance of elastomer-modified asphalts also depends on the ability of the blend to maintain thermal and chemical stability.
To be suitable for paving materials, the asphalt polymer mixtures should meet the following requirements:
(a) The polymer must be mixable in asphalt and stay mixed during subsequent processing--compatibility. In a modified asphalt composition, compatibility is important. The polar asphaltene fraction of the asphalt is generally incompatible with the polymer and phase separates over time. This phase separation leads to a serious deterioration in physical properties.
(b) The mixture must have the right rheological (flow) properties to prevent rutting which is the permanent deformation of a road caused by repetitive traffic loads. Viscosity is important but elasticity is the most important aspect since the material must be able to recover rather than just resist deformation. This characteristic is most important in warm climates.
(c) The mixture must have good low temperature properties, i.e. resistance to cracking. As a road cools, stresses develop because it cannot shrink uniformly and eventually this will cause cracking. Traffic-caused stresses also contribute. The polymer will lower the temperature at which cracking will occur. This characteristic is more important in cold climates.
(d) Temperature susceptibility of a polymer modified asphalt is a major consideration. Ideally, one would want a binder (asphalt and polymer) which would be "hard" and elastic at elevated temperatures to resist permanent deformation.
To be suitable for synthetic roofing materials, the asphalt polymer mixtures should meet the following requirements:
(a) sufficient resistance to flow at high temperatures, PA1 (b) sufficient flexibility at low temperatures, PA1 (c) workability according to the conventional methods used in the roofing technique, PA1 (d) adequate hot storage stability, PA1 (e) adequate hardness to prevent deformation during walking on the roof, and PA1 (f) if it is to be used as an adhesive, sufficient adhesion.
British Patent 1,584,504 made bituminous emulsions which were made with an aqueous solution of a polyamine. The emulsion optionally contained a functionalized polymer but the bitumen was modified by functionalizing it with carboxylic acid anhydride groups. The functionalization of asphalt is a difficult and impractical step and is not used commercially.
At the present time, unfunctionalized polymers are being used in paving and roofing applications. Unfunctionalized polymers have certain disadvantages which can cause problems when used in applications such as these. Such disadvantages include undesirably low adhesion to polar materials such as some asphalts, fillers, aggregates, substrates, reinforcing mats, and the like.