Can Eastman plasticizers be used in applications involving contact with food either directly or indirectly?
Selected Eastman plasticizers can be used under current FDA food additive regulations with limitations and restrictions on type of plasticizer, applications, concentrations, and type of contact.
Does Eastman offer a kosher-certified plasticizer for use in materials that have direct or indirect food contact?
Eastman DOA plasticizer, kosher, is offered to meet this need. A certifying agency has confirmed that the material is kosher. To achieve this certification, the manufacturer must supply a complete, detailed list of every ingredient in the product, including preservatives, release agents, stabilizers, or other inert ingredients. In addition, every step in the manufacturing process, every cleansing agent used on the equipment, and all other products produced on the same premises require close investigation and supervision. The manufacturer agrees to make no changes of ingredients or suppliers without prior written consent of the agency. The actual on-site inspector (mashgiach) will verify that the company is complying with the contract.
What are the common sources of contamination in pellet handling systems?
A single speck of metal in a half-million pounds of plastic pellets (.002 parts per billion) is enough to interrupt a typical conversion process once per week. The stringent demands for cleanliness which this places on the production, transportation, and handling of pellets cannot be overemphasized. An ongoing process of education and diligence involving management, production and maintenance personnel as well as provision of proper shelter, working conditions, procedures, and inspections are the only way to meet these demands. The pellet producer and the converter are partners in this process.
Source of contamination
Possible sources or occasions for contamination entering the system include:
- Any hose connection opened to atmosphere
- A truck or rail loading or unloading station
- Hose or tubing couplings, especially those with metal-to-metal contact
- Defective or missing filters where air enters either a vacuum or pressure convey system
- Hose changes
- Pipe changes
- Any operation disconnecting part of a handling or drying system or even opening an access door
- Any moving parts within the system, such as injection dies
A typical pellet storage silo discharges in funnel flow, which means that pellets from the top of the storage bed flow down a channel in the center of the silo and all other pellets are static until the pellet level drops to their position. The pellets near the bottom of the silo, therefore, remain undisturbed indefinitely unless the silo is emptied completely. Any contamination in the lower portion of the silo will remain in the silo, being released a little at a time when the level reaches a low point or when the static portion of the pellet bed is disturbed by some vibration or the impact of new material entering the silo. Since pellets are flowing away from the walls and down a channel at the center of the silo, there is no movement or scrubbing action along the walls. So even if the silo is emptied, contamination in contact with the wall is likely to stick to the wall for later discharge.
Many problems and much expense may be avoided by meticulous cleaning of all piping, silos, and equipment prior to being put into service.
How do I maximize capacity of my pellet handling system?
Vacuum or pressure pneumatic conveying systems are generally limited in capacity by the pressure or vacuum they can develop for conveying. Since pressure drop is proportional to the length of the conveying pipe and the square of the conveying velocity, it is important to keep piping runs short and velocities as low as possible. Bends or obstructions add pressure drop or equivalent length to the system. Air leaks reduce the pressure or vacuum which the system can develop. The maximum safe operating pressure must be determined by the individual system or equipment supplier.
Following are some points to consider in maximizing your system capacity:
- Leaks may occur at joints or couplings which do not have proper gaskets or are not properly tightened, worn or damaged hoses or piping, or defective relief valves.
- Obstructions include bent or damaged pipe, kinked hose, and foreign objects in the system.
- Bends are equivalent in pressure drop to many feet of straight pipe and should be kept to a minimum.
- Hoses have higher pressure drop than smooth pipe and are subject to leaks as well kinks and the tendency to follow circuitous paths. They should be avoided if possible, and when used, they should be cut to a length and supported in a way which provides the most direct path. Hoses should be rigid as possible to prevent kinking or sagging and as smooth as possible inside for minimum resistance to flow.
- Storage vessels and conversion equipment should be located for minimum conveying distance. Piping should be routed by the most direct path with a minimum number of bends. Conveying capacity is inversely proportional to conveying distance.
- For dilute phase conveying systems, pressure drop is proportional to the square of the air velocity so conveying capacity will usually be maximum at the lowest stable conveying velocity for the particular material being conveyed.
How do I minimize fines and streamers in my pellet handling system?
Start with clean pellets
Work with suppliers to ensure that pellets are clean prior to entering your plant. If there is a significant level of streamers in a shipment, they will usually be obvious on top of the load. Streamers tend to stay suspended during loading and settle on top of the load when the turbulence generated by loading subsides. Screens placed at the discharge of silos may catch streamers that are present in incoming product or that were produced in the unloading conveying systems; however, these screens would be expected to clog frequently, thereby becoming a maintenance issue themselves.
Keep conveying velocities low
Fines and streamer generation increases exponentially with increasing conveying velocity, making this the single most important factor to consider. Typical dilute phase conveying pickup velocities for plastic pellets should be 3600 fpm for pressure systems and 4000 fpm for vacuum systems. Velocities as low as 500 fpm may be appropriate in dense phase conveying systems where pellets move through the pipe in intermittent surges. While considerable expense is involved in converting dilute phase systems to dense phase systems, the reduction of fines and streamers may justify the expense.
Consider treated pipe
Appropriate pipe treatment will prevent long streamers from forming by eliminating the smooth surfaces where impinging plastic pellets tend to form a film on the inner surface of the pipe. In treated pipe, only the high points of the pipe surface can be coated, so when the film peels off, it peels off in short lengths rather than the long streamers characteristic of smooth pipe. While this may not reduce the total weight of streamers produced, the short streamers do not present the handling problems of the longer variety. Spiral grooving is the preferred treatment, but other surfaces (e.g., shot peened and dimpled) are available.
Eliminate piping bends
Since pellets are forced against the pipe wall by centrifugal force in bends, fines and streamers are formed at a much higher rate in bends than in straight pipe. Therefore, reducing the number of bends may be expected to significantly reduce fines and streamers; and treatment of the interior of bends as described, even without treating the rest of the piping, will have a significant impact on streamer formation.
Reduce gas temperatures
Since streamer formation increases with temperature, high convey gas temperatures as well as warm ambient conditions and even piping warmed by direct sunlight are a concern in conveying systems. As the blower compresses gas to provide energy for conveying the pellets, the gas temperature increases about 13°F for each additional pound of pressure developed. Depending on the size of the blower and the differential pressure, temperatures can rise more than 200°F between the inlet and discharge. Under extreme conditions of high pressure and low pellet flow, the glass transition temperature of the pellet may even be exceeded, causing a dramatic increase in streamer formation. The lowest stable conveying velocity which produces an acceptable conveying rate will also be the velocity which minimizes pressure and temperature at that rate.
Monitor truck unloading systems
Since truck unloading is usually controlled by the truck driver and the objective is to unload as quickly as possible, pressures, temperatures and velocities in those systems are variable and often quite high. Actually, unloading rates may be increased somewhat by reducing the truck blower speed and operating the blower at the same pressure. This reduces maximum velocity by reducing air flow and further reduces average velocity by heavily loading the convey stream. Individual convey routes as well as the particular blower and piping arrangement on each truck must be analyzed in order to determine optimum settings.
Clean pellets prior to processing
If pellets are conveyed in a high velocity, dilute phase system, fines and streamers will always be generated. The only way to eliminate these from your feed material other than switching to low velocity, dense phase conveying systems is to clean the pellet stream following the final conveying step. Elutriators and dedusters are both effective for cleaning pellets; however, both require a sizeable capital investment.
Filter dryer gas streams
Gas streams in dryer beds can carry over dust particles that are too fine for a cyclone to remove. These fines can reduce desiccant life and blind gas diffuser plates. It is very important that dryer gas loops have filters for removing these fines upstream of the desiccant. It is also important that the filters are properly maintained and that new filters are purchased when old filters become worn or deformed.
Larger convey pipe sizes tend to produce less streamers per pound conveyed due to the fact that pipe surface area per pound conveyed is less. Similarly, higher rates conveyed at the same velocity in the same pipe (higher product-to-air ratio) will produce a lower percentage of fines and streamers.
What extruder and processing conditions should be used to blow mold Eastar™ 6763 copolyester?
The same extruder design is suggested for extrusion blow molding Eastar™ 6763 copolyester as is used for PVC resins. Eastar 6763 is a relatively viscous polymer and requires a low-work screw to prevent melt temperature override. Best results are obtained when using a low-shear barrier screw designed for this material.
Most extrusion blow molding machines are equipped with a copper coil for cooling the barrel. Circulating air through the coil generally provides adequate cooling, although oil is the most commonly used medium. Oil supplied at a temperature of approximately 120°C/250°F is suggested for cooling. Barrel cooling can be employed on the front zones, if necessary, to prevent melt temperature override. The melt temperature should be in the 215°-225°C/415°-440°F range. Temperatures on the low side of the range provide better melt strength, while temperatures on the high side provide a better surface finish.
The two rear zone temperatures can be raised, if necessary, to relieve excessive motor load. In fact, it is sometimes desirable to use a reverse temperature profile (rear zones hotter than front zones). Cooling on the front zones can be used to lower the melt temperature if needed. Zone cooling should not be used if the melt temperature (as indicted by the melt strength) is satisfactory without it.
It should be kept in mind that high-compression screws designed for high-density polyethylene are generally not suitable for Eastar 6763, since excessive melt temperature and poor melt strength usually result.
What are the drying recommendations for Eastar™ 6763 copolyester?
Drying Eastar™ copolyester properly is critical to the success of the final part. Make sure that the resin is appropriately dried before starting.
A summary of dryer requirements follows:
- Type of dryer—Automatic desiccant
- Air dryness—Dew point of -30°C (-20°F)
- Air temperature—Maximum of 65°C (150°F) at the hopper inlet
- Circulation—0.062 m³/min per kg/h (1.0 cfm of air per lb/h) of resin to be dried and not less than 2.8 m³/min (100 cfm) per unit
It is important that the dryer have the correct design requirements for the plastic resin to be processed. Eastar 6763 should be dried at a maximum temperature of 65°C (150°F). Care should be taken to obtain a dryer that can be accurately controlled to deliver air at a temperature of 65°C (150°F), as some dryers tend to cycle uncontrollably hotter. Higher temperatures will cause the pellets to stick in the hopper. An aftercooler should be used to help control the temperature of the air leaving the desiccant bed. An inlet temperature gauge is a useful addition to the drying hopper.
A desiccant dryer is required. This type dryer provides drying air having a dew point of -30°C (-20°F) or lower. A dew point monitor, an option available on modern dryers, is worth the small additional cost.
The drying hopper should be large enough to allow 4 hours of drying time for the pellets during steady-state extruder operation. A good drying hopper has a screen and cone system in the bottom to ensure uniform airflow through the pellet bed. It is also designed to provide an even flow of pellets from top to bottom (plug flow) and has an air-lock loading system. Insulated hoppers should be used to maintain the pellet bed at the inlet air temperature.
Dryers are specified according to their capacity to deliver a given volume of air in a given amount of time. A good dryer design criterion allows 0.062 m³/min per kg/h (1.0 cfm per lb/h) of material to be extruded. For example, if 45.4 kg/h (100 lb/h) of material is to be extruded, a minimum blower capacity of 2.8 m³/min (100 cfm) is used. A dryer with a blower capacity of 2.8 m³/min (100 cfm) should be considered the smallest size for any use, since smaller blowers may not have sufficient airflow to prevent excessive heat losses.
What is the expected mold shrinkage of polyester?
Molecular weight does not have a significant effect on mold shrinkage for Eastman PET-based containers. Shrinkage is mainly thermal (related to the coefficient of linear thermal expansion of thermoplastic polyesters), so sheet temperature and mold temperature control the shrinkage. The CLTE of Eastman PET 9921 is 65 x E-6 in/in/°F, so changing the delta between sheet and mold temperature is not effective to change the shrinkage. There is a further cooling segment from the mold temperature down to room temperature, but this is a small effect.
Mold shrinkage is a property of the polymer, Eastman PET 9921 shrinks 0.2% to 0.3%. CLTE accounts for 0.16% of the shrinkage of Eastman PET 9921. Mold shrinkage can be affected by any mechanical advantage in the design of the part, such as ribs or turned flange, which can restrain the shrinkage.
As you go up in CHDM content, the coefficient of linear thermal expansion is higher. The CLTE for polyesters is not significantly different for temperature ranges below the glass transition temperature.
Orientation of the polyester sheet can affect the apparent shrinkage.
The typical mold shrinkage for Eastman PET 9921 = 0.002 to 0.003 inch per inch.