Determining Hydrolytic Degradation Thresholds in Virgin Resin Feedstocks
Hydrolytic degradation thresholds require drying virgin polyesters below 0.02 percent and polyamides below 0.10 percent water before melt processing.
Relative Humidity
Meaning
Ratios of partial pressure of water vapor to the equilibrium vapor pressure determine the moisture load in the air. Atmospheric relative humidity impacts the state of hygroscopic polymers during storage and processing. When the air is moist, resins like nylon 66 quickly absorb water from the surroundings, necessitating longer drying times before moulding.
Monitoring this variable in the warehouse helps predict how much moisture a batch of resin will gain before it reaches the hopper.
Polymers Equilibrium
Resins eventually reach a state where they neither gain nor lose water to the surrounding air. The level of relative humidity determines the final moisture percentage the material will hold at this equilibrium point. For many engineering plastics, this point is well above the maximum allowable moisture for injection moulding.
Understanding this relationship allows processors to calculate the required drying capacity for different seasonal conditions.
Ambient Window
Conditions in the moulding shop can narrow the time available to process material after it leaves the dryer. High relative humidity causes dry resin to pick up moisture as it sits in the machine hopper or a container. If the material absorbs too much water before it enters the screw, the resulting parts may suffer from silver streaks or reduced molecular weight.
Shop floor management often involves sealing hoppers and using desiccant dryers to combat these environmental effects.
Long-term Effect
Exposure to varying air moisture levels can change the physical properties of some resins. Consistent relative humidity in a climate controlled warehouse prevents the wide fluctuations in moisture content that make drying cycles difficult to manage. Unprotected bags of resin stored in a damp area will require significantly more energy to dry than those kept in a dry environment.
Managing the storage atmosphere is a cost effective way to improve the stability of the production process.
Karl Fischer Determination of Trace Water in Polyethylene Terephthalate
Polyethylene terephthalate trace moisture determination relies on Karl Fischer coulometric oven extraction at 170 to 180 degrees Celsius to prevent chain scission.