Theory and Method
Chemical attack on geosynthetics follows different routes depending on the polymer. Polyester carries ester linkages along its backbone, and hydroxide ions in alkaline solution split those linkages through hydrolysis. Each break shortens a molecular chain, molecular weight falls, and tensile strength follows it down. That vulnerability explains why polyester reinforcement placed against concrete or lime-stabilized fill gets scrutinized so closely, since cementitious pore water sits at pH 12 to 13. Polypropylene and polyethylene have no hydrolyzable groups on the chain and resist both acid and alkali well, but their stabilizer packages and any mineral filler can still be leached or attacked, so neither polymer gets a free pass.
Screening measures the outcome rather than the mechanism. Specimens are immersed in defined acidic and alkaline solutions held at elevated temperature for a set period, then removed, rinsed, conditioned, and pulled in tension. Dividing the result by the strength of an unexposed control from the same material gives retained strength as a percentage, which is the number a specification calls for. Running acid and alkaline arms on specimens cut from one roll matters, because material variability between rolls would otherwise show up as a difference between the two chemistries.
Temperature does the accelerating. Hydrolysis and most other degradation reactions roughly double in rate for each 10 °C increase, so exposure at 60 °C compresses years of ambient attack into days of laboratory time. That relationship is also why control tolerance carries weight: a chamber drifting several degrees above setpoint accelerates the reaction beyond what the method intends and reports a product as worse than it is, while running cool flatters it. The AA holds ±1 °C at the 60 °C setpoint, and its two tanks reach temperature independently so one arm is not waiting on the other.
