Soil Burial Tests: Trends in Material Behavior After Eight Years of Soil Exposure

It is difficult to pick out trends from the d a t a discussed in the preceding papers but there are some trends t h a t m a y aid design engineers in developing longer lasting structures for buried plant. T h e effects are categorized for discussion as follows: (i) M o i s t u r e / p r o p e r t y («) Biological (Hi) Site and depth of burial These effects also are shown in Table I. 1 T h e numbers in this table, however, are not exclusive (i.e., some materials may be affected in more t h a n one way) and therefore are not additive. II. MOISTURE/PROPERTY CHANGE A minimum of one-third of the materials in any one class in Table I (except the adhesives and thermoplastics) has shown a significant property change (arbitrarily chosen as ^ 25 percent) due to chemical or moisture influences. For instance, the effects of soil burial on the insulation resistance of certain elastomers and thermoplastics and thermosetting plastics is shown in Table I I . The plastic materials t h a t have markedly changed are the cellulosics, polyamide type 6, melamine glass, styrene polyester glass, alkyd glass, the silica-filled styrenebutadiene copolymer, and all the elastomers except the natural rubber jacket. With the exception of the cellulosic esters, the polyamides which can undergo hydrolysis, and possibly the styrene materials, the dc resistance of thermoplastic compounds has not been affected by soil burial. In contrast to this, it is very apparent t h a t all the thermosetting materials, phenolics, diallyl phthalates, melamine, styrene polyester glass, alkyd and styrene-butadiene glass, Which can absorb considerable