By Suhas Kulkarni
Most resins, especially those from condensation polymers absorb moisture. Nylons, acetals and polyesters are examples of such resins. The presence of moisture during the melt processing of these resins can cause the polymer to degrade resulting in the loss of properties. Other effects are surface defects such as splay and voids in the end product. Processing difficulties such as gassing and foaming of the melt as in the case of nylons is encountered. Therefore, drying the resin before it is processed is important. Since the water molecules form a chemical bond with the polymer, depending on the type of base polymer, the resin is subjected to a specified drying time and temperature. The resin manufacturer supplies this data. Typical drying times are three to four hours and typical drying temperatures are between 120 deg F to 240 deg F.
Additives are often added to plastic to enhance their properties for specific applications and/or to reduce their cost. In volume, fillers such as glass and minerals are the most widely used additives. Other additives include plasticizers, lubricants, flame retardants, heat stabilizers, colorants, blowing agents and biocides which are added to the polymers in small percentages. These are usually low molecular weight compounds also called oligomers.
Studies on PBT and Nylons:
Working with PBT and Nylon resins, we found that if the suggested drying times were exceeded, there was a danger of producing non-quality parts. Drying the PBT resin over 8 to 10 hours produced brittle parts and loss in surface appearance. Parts that looked black with high gloss now looked a dull grey. Drying Nylons over about 12 hours continually increased the viscosity of the plastic causing the molding process to have to be adjusted constantly.
In case of PBT, TGA (Thermogravimetric Analysis) studies have shown that when the resin is subjected to excessive drying times, the low molecular additives either degraded or were carried out of the resin. Rheometric studies showed a marked drop in viscosity. The loss of a heat stabilizer probably caused the degradation resulting in lower melt viscosity. Naturally this led to the dull looking, brittle parts.
In case of the Nylon, rheometric studies showed there was a clear increase in the viscosity with increasing drying times. Water in the nylon acts as a viscosity reducer and regulator. As the water is pulled out from the system, the viscosity increases.
On the shop floor the most efficient way to prevent over drying is in the following ways:
- Size the hopper dryer such that the residence time of the plastic is equal to the drying time of the plastic. In some cases this is difficult because of the hopper sizes being larger than required. In these instances, retrofit the dryers with a level sensor and adjust the level to load in only the required amount of material.
- If the molding is not ready to be started after the material is dry, turn down the dryer temperature to about 80-100 deg F, but keep the dryer running. Supplying the hopper with low temperature dry air will keep the moisture out and will not have detrimental effects on the resin.
- If the machine is going to be down for more than 4 hours after the drying has taken place, shut the hopper dryer off as soon as possible.
Cautions and Recommendations:
- Drying in some resins such as PBT has a cumulative effect. The drying process removes the low molecular weight additives for good,s and so these are not added back if the drying is stopped and the resin returned to the shelf for the next run.
- However in case of nylons, the moisture can be absorbed back and the viscosity can be re-regulated.
- Since each resin and its additives are unique, customized experimentation would best suggest the type of control you would require for your individual process.
February 2, 2007