Preliminary Chemical Compatibility Testing of an Alternate CPVC Fire Sprinkler System Antifreeze
Dr. Duane Priddy, Plastic Expert Group
There is a need in CPVC fire sprinkler systems for an improved antifreeze that is chemically compatible with CPVC and also is non-flammable, even better, a fire retardant material. Currently the only antifreeze NFPA 13 approved for use in CPVC fire sprinkler piping is pure (USP or CP) glycerin mixed with water at no higher than 48% v/v. There have been failures of CPVC fire sprinkler systems due to the use of impure glycerin.
Client believes they have a fire retardant liquid material that will work as an antifreeze in CPVC fire sprinkler systems. They are seeking to determine if the new material is chemically compatible with CPVC. In order to make an assessment, we performed testing to compare the effects of the new material on CPVC pipe samples relative to 44% v/v (=50% by weight) glycerin/water solution as a control. In order to accelerate the kinetics of chemical interaction with the CPVC pipe test samples, the test specimens were placed under various levels of mechanical stress and immersed in the antifreeze solutions.
The results of the preliminary testing indicate that new developmental antifreeze is at least as chemically compatible with CPVC as 44% v/v glycerin in water.
CPVC is incompatible with most hydrocarbon chemicals. Exceptions include those chemicals located at the extremes of the chemical polarity spectrum. On the non-polar end of the spectrum there is mineral oil and at the other end there is glycerin. Most carbon-hydrogen containing chemicals in between the polarity extreme of glycerin and mineral oil are not compatible with CPVC. Exposure of CPVC pipes and fittings to incompatible hydrocarbon chemicals results in failure called environmental stress cracking or ESC. ESC failure takes place because CPVC under stress will absorb most hydrocarbons allowing the polymer chains to become plasticized and mobile so they disentangle. Since polymer chain entanglement is one of the key factors that control the mechanical strength of plastics, chain disentanglement allows the normally ductile material to turn brittle.1
ESC is the most common cause of the failure of CPVC fire sprinkler piping. ESC is caused by the combination of two factors: stress and the presence of an incompatible chemical. Remove or eliminate either stress or an incompatible chemical from the system and ESC will not take place. Unfortunately, fire sprinkler pipes are under hoop stress from the water pressure inside the pipes so some stress is already present. Therefore the only thing required for ESC failure is the presence of an incompatible chemical.
There have been many failures of CPVC fire sprinkler system piping caused by the use of incompatible antifreeze. NFPA requires the use of glycerin based antifreeze at concentrations >48% v/v. Propylene glycol should not be used as an antifreeze because it is not compatible with CPVC at concentrations >25%.
Need for Alternate Antifreeze
Only high purity (USP or CP grade) glycerin is allowed for use in CPVC fire sprinkler piping. A low cost source of glycerin is from Soy. Soy based glycerin is generally contaminated with significant amounts of fatty acid esters which are highly incompatible with CPVC.
The complete 10 page article is available upon request.