No abstract

Some OF THE most important rheological properties of asphalt depend on chemical constitution. Systmatically reblending chemical type components shows the influence of composition on penetration, softening point, penetration index, viscosity, viscosity-temperature slope, shear susceptibility, and complex stress-strain modulus.A study of the constitution of asphalt in terms of distributions of molecular sizes and definite chemical classes of compounds led to the development of a group of separation procedures (6). These procedures were applied to a representative group of paving asphalts, and the individual fractions have been characterized with regard to molecular weight, hydrocarbon type composition, elemental composition, viscosity, viscosity-temperature slope, and durability(3), The Venezuelan asphalt referred to in that study is the basis for the work reported here.

Experimental latices of synthetic cis‐1,4‐polyisoprene and films and cellular objects produced from them are described. The latices of the synthetic polymers with molecular weights of about 2,000,000 are stabilized by anionic surfactants and have weight median particle diameters of about 0.9 μ and log normal particle size distribution. Tensile properties and flow characteristics of unvulcanized latex films could be regulated by introduction of controlled, small, amounts of crosslinking into the latex. Procedures used for crosslink density determination on cis‐polyisoprene latex at various stages of use are described. Vulcanized latex films having maximum tensile strengths (5500–6000 p.s.i. and 800–950% elongation) were obtained when the final crosslink density was in the range 1.0–1.6 × 10−4 moles of effective chains/cc. Use of the synthetic cis‐polyisoprene latex in production of foam is described. Gelation with sodium silicofluoride at room temperature occurs within 2–10 minutes. Production of a controlled level of crosslinking in the latex prior to frothing and gelling was found to improve the uniformity of cell structure of foams and to increase their resistance to compression. Excessive crosslinking in the latex during maturation was shown to cause a loss of tensile strength in cured products presumably because the coalescence of latex particles during gelation is adversely affected. The properties of synthetic cis‐1,4‐polyisoprene latices and the physical properties of films and foams derived from them are shown in general to be very similar to those of natural rubber latex and its products.

A sliding plate microviscometer for determining the viscosity of asphalt in absolute units is described which utilizes the same constant temperature water baths normally found in control and materials testing laboratories. The instrument is sufficiently accurate and simple to permit bringing the measurement of viscosity in fundamental units into the control laboratory and eventually into asphalt specifications. The sliding plate microviscometer is suitable for routine measurement of viscosity-temperature relationships and viscosity-shear rate dependence. It is compact, rugged, and simple to operate and has a working range of 100 to 100 billion poises. This range permits overlapping with a capillary viscometer in the low viscosity region and allows measurement of the viscosity of the hardest paving grade asphalt at 32 F. Methods for expressing the viscosity-temperature relationship of asphalt are reviewed. Two equations which give straight-line relationships between asphalt viscosity and temperature are discussed. Data obtained with the microviscometer are presented to illustrate these relationships.

No abstract