Drying is the necessary criteria for quality processing of plastics, particularly with the increased usage of thermoplastics / engineering polymers / resins.
Engineering polymers / resins absorb moisture during manufacturing, transportation and when in storage prior to processing. It can be safely assumed that all batches of resins potentially contain surplus/excessive moisture, regardless of the circumstances.
Thus, the moisture must be removed from the resins prior to processing. Excessive moisture in the resin results in severe material inconsistency during processing. Apart from resulting in sub-standard product, both in terms of appearance and property, moisture trapped inside the resin can cause silver streaking or splay. More disastrous can be damages, which are not visible.
Plastics resins which are affected by moisture, can be classified as:
– Non-Hygroscopic
– Hygroscopic
Non-Hygroscopic resins collect moisture only on the surface of the pallet. This surface moisture can be dried by exposing the resin to a continual blow of the hot air Hygroscopic resins, collect moisture inside the core of the pallet and can be best dried by dehumidifying dryers.
Hygroscopic resins like PA, ABS, PET, PC, PS etc., which have a high affinity for moisture, need to be dried thoroughly and properly to ensure quality finish of final product. Improper and inadequate drying can result in loss of structural, impact strength and tensile strength, cosmetic defects and many other moisture related defects and deficiencies.
In this paper we will elaborate, the importance of proper drying of engineering plastic resins, to maximize quality of moulded parts and minimize possibilities of degradation. We will also be discussing various variables of drying and equipment / methods available for effective drying of engineering plastic.
One of the most important and universal advantages of engineering plastics is their virtual freedom from attack by ambient moisture or from surface attacks compared to the rusting of steel or the atmospheric corrosion of copper and aluminium.
However, plastics in the resin state may be hygroscopic (moisture penetrates the pellets) and absorb moisture from the atmosphere during storage or before being processed, which adversely affects the final quality of the moulded part.
Even non-hygroscopic plastics (moisture accumulates on the surface) are susceptible to surface moisture contamination that should be removed before processing.
Common problems of many resins like – Nylon, ABS, Polythermide, Polysulphone, Polycarbonate due to improper drying are –
Surface Appearance
Moisture bubbles trapped in the resin can ruin a part’s surface appearance by causing:
Property Performance
Moisture can cause a loss in material properties. Trapped moisture bubbles can cause voids in the interior of the part. Moisture can also cause a loss in molecular weight due to hydrolysis. (A chemical process of decomposition that occurs with the addition of water).
Loss of Physical properties may result in –
For resins such as polycarbonate and polyester where impact strength is a critical property, dry resin is a must.
Hence, proper drying of many plastic resins is the first critical step towards obtaining optimum performance of molded parts and minimizing possibilities of degradation. Without exception, the longer and hotter a given forming operation is, the drier the pellets must be.
Resin’s Moisture Content
The amount of moisture a resin will absorb depends upon several key factors. Moisture content depends on the type of resin, for each resin has its own absorption characteristics. Some polymers have a greater affinity to water than others and therefore will absorb moisture more readily. Not only does every resin family have its own moisture absorption characteristics, but each resin grade does as well. The weather conditions or the moisture content of the air as well as the temperature of the ambient air and the pellets also affect the resin’s moisture content.
Drying Variables
Just as in moisture absorption, the variables that will affect moisture removal or drying are: the nature of the resin; the dryness of the air; the temperature of the air and the pellets; and the time that the resin is exposed to the air.
Proper Drying
Drying reverses the absorption process, removing moisture from the pellet to a very low level moisture content that is suitable for processing. Proper drying improves processing consistency by removing moisture from the pellet prior to processing and preheating the material to predetermined temperature. The drying specifications outline three criteria; how ‘dry’ the air must be: how high the air temperature must be; and how long the resin must be dried.
Dry Air
The air used to dry the pellets must have a dew point of at least – 40° F/ °C Only if the air is this dry, can a pellet reach the point of equilibrium to maintain a moisture level >0.05%. The drying temperatures and times specified assume that the air in the dryer will have a dew point of at least – 40° F/° C or lower.
There are three methods of resin drying;
Manual Drying
Manual drying is an outdated, labour intensive process, with high probability of spoilage of pallets because of improper control of temperature.
Hot Air or Hopper Drying
The hopper (Hot air) drying, on the other hand, allows for continuous drying in the hopper and is preferable for long runs but only for non-hygroscopic resins i.e where moisture is only on the surface of pallets.
Dehumidifying Drying
For hygroscopic resins i.e. where moisture is inside the core of the pallet, dehumidifying drying is the only solution.
Higher temperatures provide a lower RH, and hence the rate of drying is faster. However, with heating, the dewpoint and moisture content remain unchanged. As a result, there is no positive removal of moisture from the environment. The moisture equilibrium content of the plastic pellet can be limited. Thus, the degree of dryness will also be limited. This is particularly true for all hygroscopic plastic resins. Hot air drying is recommended for non-hygroscopic resins where the moisture is present on the surface.
As mentioned earlier, pellets come to equilibrium with the surrounding environment. Since the environment cannot always be controlled during shipment and storage, the initial moisture content before drying will vary. The drying time should be adjusted upward as the initial moisture content increases. E.g a rule of thumb for ABS is if the moisture content is increased by a factor of two, the drying time should be doubled as will.
Basically, smaller pellets dry faster than larger pellets because the average diffusion path (heat flow inward and moisture outward) is shorter.
The drying rate is greatly affected by differences in dewpoint. Lower dew points allow pellets to approach a lower equilibrium state of dryness. The extra degree of dryness is essential for producing quality parts. Furthermore, the lower the dewpoint, the less time it takes to reach a particular final moisture content.
A desiccant bed is composed of desiccant enclosed by two solid sides and tow sides with perforated openings, which allow the passage of air. The desiccant is a porous material, which by nature is extremely hygroscopic. Since desiccants have a natural affinity for water, an air sample exposed to a desiccant will give up its moisture to the desiccant. At the same time, the desiccant is unchanged both in size and shape after taking the moisture out of the air.
When the desiccant has completed its drying job, it is removed from the process air stream and separately regenerated with heat. The presence of heat will cause the desiccant to give up the moisture (through evaporation) that it had previously adsorbed. Once the accumulated moisture has driven from the desiccant, the bed is ready again for re-adsorption.
Air Velocity Across the Beds
There is a basic relationship between air velocity across the desiccant bed and desiccant adsorption capacity. The greater the velocity the less capability the desiccant has to adsorb moisture. This is similar to cooling coils where the face area is sized to achieve the proper velocity across the coil. Accordingly, the proper designing of CFM to desiccant bed face area is critical and should be optimised to achieve proper balance between desiccant adsorption capability and size of the bed (i.e. size of the bed is limited by space requirements).
Depth of Desiccant Beds
There is an increasing rate of adsorption capability as the desiccant depth is increased up to a point, after which diminishing returns come into play. The desiccant bed depth should be deep enough to reach this critical point and also have some additional depth to provide a safety margin for desiccant contamination.
Again, relating this to cooling, up to a point each additional row of coils provide further cooling, after which, no work will be done.
Type of Desiccant Used
The adsorption capability of the desiccant bed is dependent upon the type and size of desiccant used. The types of desiccant used for air-drying include silica gel, activated alumina and molecular sieve. Molecular sieve is used for plastic dryer applications due to its superior adsorption capability at low dewpoints and high entering temperatures. The sieve size of the molecular sieve is also important. Like plastic pellets, the larger the size the greater is the average diffusion path of heat and moisture. Thus the larger the desiccant size, the harder it is to reactivate the desiccant and drive out the moisture that has been adsorbed.
Accordingly, smaller desiccant sieve sizes allow for easier reactivation and greater adsorption.
There are however, two considerations that limit the degree to which sieve size can be reduced. The first is that the desiccant size must be larger enough to be able to be contained by the perforated bed screen; a smaller sieve size will induce a greater pressure drop.
Typical Operating Principle of a Dehumidifying Dryer (resistance) through the desiccant bed. Therefore, the size of the desiccant is limited by the static pressure capability of the system blower.
Direction of Process and Reactivation Air Flows
Counter-current airflow is such that the process and reactivation airflows are moving in opposite directions. Co-current airflow design occurs when both airflows are moving in the same direction.
We have found that counter-current airflow provides a significantly greater degree of reactivation and adsorption that does co-current. The reason for this lies in the fact that the desiccant on the process entering side of the bed adsorbs the greatest percentage of moisture.
When the bed is reactivated, instead of driving the bulk of the moisture through the entire length of the bed (as would be in the case of co-current air flow) it is much easier to drive the moisture out in the opposite direction (counter-current) from which it had come, since this path provides the shortest length.
Reactivation Temperature
The reactivation temperature of molecular sieve is substantially higher than other types of desiccants. Molecular sieve should not be reactivated with lower than 300°F. Typically (assuming cooling of the bed after reactivation), the higher the reactivation temperature the greater is the amount of moisture that is released from the desiccant and the greater it its future adsorption capability.
Again, the optimisation must be reached in which an appropriate balance between bed depth, KW requirements and dew points is reached.
Cycle Time
Cycle time is the amount of time one bed is in process and another bed is in reactivation.
Cycle time is important because it is critical that the reactivation bed is substantially regenerated before going into process (breakthrough of moisture has occurred). It is also important for the process bed to adsorb an optimal amount of moisture under the existing conditions. Once the desiccant has adsorbed the optimal amount of moisture, additional time in process results in marginal diminishing returns. Accordingly, it the reactivation time is too long, energy (KW) is not put to efficient use.
Therefore, the time cycle must be of proper length to optimise both moisture adsorption in process, and moisture removal in reactivation for the particular bed depth involved. Typically, larger bed depths have longer cycle times (2-4 hours) than smaller bed depths (17 minutes to 1 hour).
Purge Cycle (Bed Cooling After Reactivation)
After reactivation the desiccant itself is rather hot (400-500°F). If the desiccant bed is placed into process directly after it has been reactivated, the desiccant will not begin adsorbing moisture immediately. Since the desiccant is still hot, it will be wanting to continue to release than adsorb moisture.
In order to avoid this problem, a cooling stage should be employed after the reactivation stage. But before the process stage. An efficient cooling stage should be designed to that the desiccant bed is cooled below the process entering air temperature (usually 100° – 180°F). Efficiency would also imply that the bed not be cooled with ambient or moisture laden air which would only serve to load the desiccant with moisture before it even goes into the process stage. A closed-loop cooling system is the most preferable.
Return Air Temperature from Hopper
There is an inverse relationship between the return air temperature and desiccant moisture removal capacity. The higher the return air temperature from the hopper the lower is the adsorption capability of the desiccant and the dewpoint output from the dryer. As was shown above, the presence of heat will induce the desiccant to give up moisture rather than take it on.
This is the reason why in many applications in which materials require drying at high temperature (i.e. PET) an after-cooler is used to reduce the return air temperature.
Closed Loop Process System
In order to obtain low dewpoint performance (i.e. -40°F dp) it is absolutely necessary for the drying system to be totally closed-looped. This requires no leakage in air hoses and drying hoppers. Further, it necessitates the use of two separate blowers for process and reactivation air streams. If only one main blower is used for both process and reactivation, ambient (moisture-laden) air must be introduced into the system in order to make up the deficiency resulting from the reactivation outlet air being exhausted. Ambient make up air then serves only to significantly increase the dewpoints of both the process entering (return) air and the process (supply) air.