JOURNAL OF MATERIALS SCIENCE 25 (1990) 3688-3692 Structural characteristics of peat bitumen and peat/petroleum bitumen blends, and consideration of their potential use as road binder materials J. J. LEAHY, J. A. G. DRAKE, C. BIRKINSHAW National Institute for Higher Education, Limerick, Ireland I. L. JAMIESON National Research Council, Strategic Highway Research Program, 818 Connecticut Avenue NW, Washington 2006, USA Bitumen derived from peat was blended with petroleum bitumen and subjected to laboratory evaluation for use as road binder material. Standard empirical procedures used included pene- tration, softening point and Fraass brittle point determinations. Rheological behaviour has been assessed at law shear rate using a sliding plate microviscometer and at high shear rate using a cone and plate instrument. Thermal analysis techniques including calorimetry and dielectric thermal analysis have been used to obtain fundamental structural information. Pro- perties have also been measured after accelerated ageing. Peat bitumen is shown to differ significantly from petroleum bitumen in having a relatively high (30%) crystal content; how- ever, as crystal melting commences below 20 ~C, the initial effect obtained in blending with petroleum bitumen is that of a plasticizer, reducing blend viscosity and increasing penetration. Crystallinity in the blends is lower than expected and this is ascribed to diffusional control of the crystallization process. Peat bitumens show a marked propensity to harden on heating, presumably due to condensation reactions increasing molecular weight, and this more than compensates for the plasticizing effect. The results suggest that age hardening will limit tech- nically useful blends to a maximum of 17%wt/wt peat bitumen. 1. Introduction The use of bitumens derived from peat may offer economic advantage to those regions possessing sub- stantial deposits of this raw material. Bituminous sub- stances can be obtained from milled peat by solvent extraction; the chemical nature, and in particular the polarity, of the extraction solvent determining the type and quantity of species extracted. Only limited information is available regarding the detailed chemical structure of peat bitumen, but compared to petroleum bitumen, materials derived from peat contain signifi- cantly higher proportions of aliphatic acids, C20 to C30 alcohols, and their esters [1], The potential of bituminous materials from peat has long been recognized in Eastern Europe [2, 3], but exploitation has been restricted in the West due to the ready availability of petroleum bitumens: Initial work [4] on extraction and characterization of bitumens from native Irish peat has suggested that detailed evaluation of these materials as constituents of road binder materials is justified. The work described here is an examination of the physical characteristics of peat bitumen, and blends with petroleum bitumen, considered potentially useful for road binder applica- tions. In addition to standard empirical procedures commonly used in classifying these materials, thermal 3688 analysis has been used to provide more fundamental structural information. 2. Bitumen extraction and blend preparation Peat bitumen examined in this study was obtained by extraction with SBP11 (Shell Chemicals). This solvent is known preferentially to remove aliphatic species, but is more environmentally and economically accept- able than solvents of higher aromatic content and greater extractive power. The product obtained was of a soft consistency at room temperature and therefore not suitable for direct use as a road binder material. Initial work suggested that technically useful blends should be based on a relatively hard petroleum bitumen; a standard 50 penetration grade material was considered appropriate. Blends containing 10%, 17% and 25% by weight of peat bitumen were prepared by mechanical mixing at 120~ C. Microscopic exami- nation of the blends showed them to be homogeneous. 3. Characterization Empirical characterization procedures adopted were substantially those traditionally used with petroleum- derived materials. These included penetration, soften- ing point and Fraass brittle point determinations, with 0022-2461/90 $03.00 + .12 9 1990 Chapman and Hall Ltd.