Extrusion Coating Blog

An Active Packaging Lamination

Scavenging Hexanal from a package

During the storage of foods in a packaging lamination, the diffusion of oxygen into the product can oxidize oils in the foods generating hexanal, a decomposition product from the oxidation of oils and fats in foods.  As hexanal is an earlier indicator of oil rancidity, a packaging material which could absorb hexanal would aid in maintaining the freshness of products after the onset of rancidity.  Such a packaging material has been discovered and was patented in 1994.  US Patent 5,284,892 “Aldehyde scavenging compositions and methods relating thereto” describes a lamination which contains an aldehyde scavenging molecule.  In this particular case the scavenging material is a high molecular weight polyalkylene imine ("PAI"), and in particular the primer polyethylene immune (PEI).   In this system the imine reacts with the aldehyde forming a condensation product and water. 

The imine can be placed in the package several ways, by forming a layer of the imine between other layers as if used as a primer in a lamination or it can be mixed into a polyolefin polymer which is then formed into a film or a layer in a film.  The patent gives various ranges of PEI concentrations and aldehyde absorption to show the effectiveness of the approach.   This composition can be used to produce what is known as an active package, i.e. a packaging material which is interacting with the packaged product.

The approach works in part due to the volatility of the hexanal and the reactivity of the imine and hexanal.  However, at some point after the onset of the rancidity the further oxidation of the remaining fat molecule which gave off the hexanal will begin to become noticeable and the product will appear to the consumer to be rancid.

Why Not Replace a Perfectly Good Screw?

Contributed by:  Beth Foederer, PE – Optex Process Solutions, LLC

 

If you have an extruder that is more than 10 years old chances are that there are new materials that can enhance your products.  The physical and rheological properties of these materials may be different than the original materials targeted for the current extruder.   As you run these materials, you may see higher melt temperatures, higher motor amps, edge instability or various other indicators that these materials process differently.  The choice of materials is always determined by product requirements, not processability.  These process differences may have an impact on your run speed and productivity.  You will need to evaluate how and where the best place to run these materials is.

The first step is to look at the product mix to be run on the extruder.  What are the materials?  What is the output range of each material?  What is the target melt temperature?  Where do you run today and is this the optimum condition to run these materials?  For example, your extruder may be designed to run LDPE.  You now run low melt index mLLDPE/PE blends and see high temperature.  On the other end of the spectrum, you may have an extruder designed for low temperature, shear sensitive resins such as an acid copolymer that needs to run an extrusion coating grade of LDPE.  The extruder won’t likely get to 600F for the LDPE.  If these are minor components of your product mix, this may be a perfectly acceptable situation.  If the trend is away from your original design, it may be time to evaluate modifications to your system.

The easiest fix may be to move products around on your existing lines.  For example, you could place all the low shear requirements on one line and all the high temperature materials on another.  The differences for each extruder should be evaluated and quantified on one spreadsheet to better compare the options.  You should examine temperature uniformity, output potential, mixing quality and limitations caused by process stability issues.  One way to evaluate this is by process simulation.  Process models can be developed that accurately reflect current system performance.  These models can be easily used to evaluate changes such as running alternate conditions, alternate material or design modifications.  An example of such simulation software is Compluplast VEL (Virtual Extrusion Laboratory).  This software can be used for the complete extrusion modeling such as feed screws, feed pipes, Coextrusion adaptors, flat dies, round dies, cooling and haul off.

Depending on the available extruders and options, you may need to consider a design optimization.  After going through the above exercise, you may be better off than before but still may have a number of extruders that can benefit from a feed screw redesign.  You have identified an extruder that is not meeting your performance expectations.  You need to go back to the list of materials and priorities that need to be run on this extruder.  The benefit of having an existing extruder is that the existing design can be modeled and evaluated for the priorities that have been established for this extruder.  It may be possible to optimize performance by making minor modifications to the existing screw.  The demands of production may also dictate that the machine cannot be down waiting the 2-3 weeks needed for modification.  In that case, we would recommend a new screw with the old screw to later be sent to be modified as a spare.  If the new priority mix is significantly different that the original target, a complete new design may be required for best performance.  It is not unusual to see 10-15% output improvement, better melt quality, reduced down time, reduced changeover and improved stability from the new design.   Investment can be readily justified by these improvements.

 

Using an Adjustable Depth Thermocouple for Improved Process Evaluation

Contributed by:            Beth Foederer, PE  Optex Process Solutions, LLC

 

Typically the OEM supplies a thermocouple for melt temperature readout at or after the discharge end of the extruder.  The most common type is fixed depth, flush mount.  This type is designed for the thermocouple to be in contact with the polymer melt flow to measure and feedback the melt temperature PV (present value).  Additional melt probes less commonly used include adjustable depth thermocouple probes and infrared probes.  These also are designed to contact the melt to measure and feedback the PV of the melt. 

 

All of the melt temperature probes must be mounted in a port in either the extruder barrel or the polymer flow piping.  This port can be located at any position in the system that provides a relatively smooth and straight internal flow surface.  The port must be drilled through the steel component to the polymer flow channel so the probe can be mounted to be in contact with the molten polymer.

 

A flush mount probe is positioned such that the tip of the thermocouple is at the edge of the melt stream.  While this is an ideal position to keep the melt flow undisturbed, it may not be a representative measurement of the average melt temperature.  Figure 1 shows a typical flush mounted melt probe.  Since polymers are poor conductors of heat, if the steel wall temperature is different than the bulk polymer temperature, a flush mount thermocouple will always feedback a PV close to the actual steel temperature (the control thermocouple setpoint).  This type of thermocouple will be limited in how helpful it will be to troubleshoot or improve the process since it seldom gives feedback that is representative of the average bulk melt temperature.

 

 

FIGURE 1

 

A fixed depth thermocouple (Figure 1) is positioned with the tip into the melt stream.  It does create some melt disturbance but provides polymer temperature feedback more representative of the average melt temperature.  The depth into the melt stream is commonly from 3mm to 13mm (0.12-0.5”).  If the probe depth is longer it is more prone to being damaged or bent due to the high polymer viscosity during lower temperature shut down, start up or purging.  While this type of probe is more representative than the flush mount, it is still just a one spot measurement of temperature.

 

The location of these melt temperature readouts may be in a poor position, such as an elbow or turn.  The velocity profile through the elbow as well as heating and control issues may significantly influence the measured temperature.  The best location for the melt thermocouple is after all interacting devices such as the screw, valve, screen changer, static mixer and elbows and before final forming.  It is best in a straight feed pipe section just prior to entering a feed block or die.  Due to the polymer viscosity dependence on temperature; understanding the temperature variation before forming will allow control adjustments that may significantly improve the product quality and consistency.

 

The adjustable depth thermocouple significantly improves the range and accuracy of the data that can be gathered from the melt stream.  The adjustable depth thermocouple is a much better tool for troubleshooting melt temperature problems as the tip of the thermocouple can traverse from the edge of the melt stream to the center and in most cases, beyond center.  During normal operation, the tip can be left slightly in the melt stream or flush with the melt stream.  For process analysis, troubleshooting or optimization; the thermocouple can be put into the melt stream by turning the thumbwheel (see photo, below).  The temperature can be continuously recorded as the tip is moved to different positions across the melt stream.  This data can now be analyzed to understand the position dependent temperature variation across the melt stream.  The distribution of this variation can provide us insight into the system performance to guide us in adjusting the control set points to improve melt temperature uniformity.  For example; a peak temperature at the wall (above the zone set point) may indicate too high a temperature setting at the discharge end of the barrel.

 

By using an exposed junction (fast response) thermocouple and the appropriate high speed data acquisition device, this thermocouple can also be used to evaluate time dependent temperature variation as well.  The thermocouple can be adjusted to a specific depth and left over a longer period of time.  The variation recorded over time by the data acquisition system will indicate time dependent temperature variation.  This is often indicative of poor mixing, cycling heaters, or feeding instabilities.  Again, the insight gained by the data collected can guide our adjustment of the operating conditions to improve uniformity.   

 

One concern when using the adjustable thermocouple is that the user needs to remember to back the tip near to the wall when finished.  If the tip is too far into the melt stream, the probe can bend when the machine is shut down or purged.  It is difficult to remove this tool after it is bent.

 

Evaluating Extruder Performance to insure product design

Within the last two weeks I was working on a line start up and took the time to evaluate the performance of the extruders which were installed on the line.  There were several different extruders for the line and several had the same screw design for the same resin.  We had been running and all seemed OK, a little too much head pressure variation but we could make product and would be able to work on that as needed (if there was a detectable MD gauge variation).

The output check was begun by having all of the extruders (this is a coex line) running at specific purging rates to set a base level of output.  The melt was cut from the die lip and the stop watch begun.  A timed sample of six minutes was taken.  During the six minutes data from the drive power, barrel settings, melt temperature, head pressure and variation, and down stream pressure and variation from each extruder were taken down for future reference.  As a special check each extruder rpm was checked to make sure the read outs were accurate (you don’t want to know why I check that EVERY time)

At this point one of the extruders was increased in speed to about 25 rpm.  The melt temperature and head pressure were monitored to determine when steady state was reached.  At this point the extruder drive, melt temperature, pressure and pressure variability were recorded.  Once this was complete, the melt was cut from the die and a timed sample taken and weighed.  Screw speed was increased and from three to five rpm settings were tested for each extruder.  The output was calculated by subtracting the purge weight from each weighed sample and plots of output vs. rpm, melt temperature vs. rpm, motor power vs. rpm and melt temperature vs. output were prepared.

This simple straightforward work yielded huge dividends in this case, because the line was set up with specific output values for the screw to calculate the relative layer thicknesses of the extrudate.  The experimental conformation found that none of the input specific output values were correct.  once the experimental values were put into the control program the proper layer thicknesses were obtained and the product was alright.  Another interesting finding was that the screws which were the same did not perform the same, each was different.

The moral of the story is always check the performance of an extruder during the start up of a line to make sure of what you are actually making instead of what someone else says you should be making.

Barrier laminations

In many existing laminations, inexpensive films are combined to produce improved laminations for many purposes. These films have been developed and used for many years and have excellent machining properties. But today there are ever increasing demands for improved barrier films, either for oxygen or perhaps flavor and aroma. Replacing one of the laminate films with a new substrate is full of surprises and often times the suprises are not so much fun, especially if you have to develop new formulations for slip and blocking resistance and performance at the end user.

A good way to avoid many of these problems without disturbing the existing surface properties, while adding the desired barrier property is to coextrusion laminate. Earlier I discussed the use of adhesive polymers for enhanced adhesion to films and foils while keeping cost in line by using LDPE as the core of the coextrusion. To add the desired barrier will now require the addition of a barrier polymer into the lamination system.

Typically this would result in a five layer extrudate for the lamination adhesive. If we assume we want the same over all thickness of the lamination adhesive we wind up with a structure like:

LDPE/tie layer/EVOH/tie layer/LDPE

In this case we are using EVOH as the barrier layer but have to use a tie layer to have it adhere to the LDPE layers used for adhesion to the laminating films. Of course nylon could be used if you don’t need all of the oxygen barrier that the EVOH affords. Nylon would be good for toughness as well as chemical barrier and a moderate oxygen barrier improvement over a simple LDPE laminating resin

Improving adhesion with coextrusion

In my last post I discussed the impact on adhesion of resin selection and the processing temperature of LDPE.  But as line speeds are increased it is more difficult to have the LDPE properly surface modified in the air gap by oxidation alone.  In these cases productivity will be limited as you can expect that the adhesion will drop with increasing line speed, all other factors held constant.   This is because the oxidation time is reduced with increasing line speed.

A way around this limitation is to use a resin which is already functionalized for adhesion, and typically this means partially oxidized.  This is readily achieved by the use of a copolymer of ethylene and an oxygen containing comonomer.  While several resins of this type are available, they are more expensive than simple LDPE.  Resins such as EVA, EMA, and EAA, etc. all are candidates for modifying LDPE extrusion resins for laminations.  This also adds a lot of flexibility as the copolymer can be chosen to optimize adhesion with specific materials offering overall better results than with the single oxidized LDPE of the typical single extrusion adhesive.   Optimum adhesion for foil and various films are obtained with different resins. 

Blending of the copolymers with LDPE is possible if you have only a single extrusion system, but this will require larger percentages of copolymer (10% to 50%) to achieve the best results.  Some testing of blend concentration is necessary to obtain the desired adhesion level to a particular substrate.  A more cost effective approach is to coextrude the copolymer with a LDPE core.  This also allows optimization of adhesion of different substrates as it is possible to have a three layer melt curtain with different copolymers on each side.  A two extruder system can give a two or three layer coextrusion as well.

Coextrusion is more cost effective because it allows lower overall percentages of the copolymer to be used relative to blending, perhaps 5% a side.  Because the cost of resin is always variable this permits the lowest cost as it minimizes the use of the expensive copolymer while locating it at the surface of the melt curtain where it is needed.  In blends a lot of the material will be ineffective for surface bonding as it is dispersed in the center of the adhesive melt curtain.

Coextrusion lamination has been used to significantly increase laminator line speeds with excellent adhesion levels being obtained with the proper copolymer selection.  Well worth considering for a new line or for upgrading an existing single layer line.

Extrusion Lamination variables

As many of us are aware, successful extrusion lamination requires a combination of things to occur all at once.  If I define success as forming a good sheet with flat profile and with acceptable bonds as fast as I can go, then the primary interactions are between the output rate, the melt temperature, the melt strength and the oxidation of the extrudate before it enters the nip.  This will require that several things work together to achieve this at all operating points.

I am going to begin a series of posts to define some of the interrelationships and the difficulties associated with achieving them and some of the equipment set ups that help.  After resin selection, a key factor for melt strength, perhaps the most important factor controlling the success of the extrusion lamination process is the screw performance characteristics. 

In general extrusion situations this would be the measured output and melt temperature as a function of screw rpm.  If the screw characterization work is done really well the output vs. melt temperature curve will be obtained at several of back (head or screen) pressure.  Usually the characterization is performed against a nonadjustable outlet nozzle such as a sheet or film die and therefore we are limited to output and melt temperature as a function of screw rpm with pressure as an uncontrolled variable.   If this is all that was available we would have a limiting situation in terms of process as in general we want to operate with a melt temperature of say 293 C to 332 C (560 F to 630 F) and for a simple extruder/die combination we would have a limited range of screw speeds over which to work.  This may not be practical depending on the product widths we need to run and line speed limitation of the laminator. 

So we need a means of increasing the melt temperature so we can run at the best temperatures at all outputs which fit with our product mix and drive or winder constraints.  This need is generally met with the addition of a back pressure valve between the extruder and the die.  This allows us to increase the discharge pressure on the screw which will increase the residence time in the screw (lowers the output) increasing the melt temperature.  Now of course we must have a screw which can deliver the required output at the required melt temperature.  This will require a balance between the screw technology (type of screw such as barrier, standard profile etc.) and the melt pumping characteristics of the screw design (what may be termed a “hard” or “soft” screw) measured by the specific output of the screw.

With a back pressure valve, in order to increase melt temperature you simply turn the valve towards the closed position.  But this decreases the flow rate so you have to increase the screw speed to discharge the same amount at the higher pressure.   In the mid screw speed range this works well enough but if the screw is “soft” you may have to increase the screw speed a lot and then you can run out of output due to the screw performance.

In my next post I will begin to show some screw characteristics as a function of back pressure and start to define the impact of the melt temperature on the lamination bond strength.  Also to look at characterizing an extruder so you can determine the output range you have available with the screw you have installed.  This of course is the first step in determining if you need to modify your extrusion process to improve perductivity.

Ordering the AIMCAL Metallizing Handbook

Haim Korkus asks where he can purchase the AIMCAL Metallizing Technical Reference.

The Technical Reference was published by AIMCAL, the Association of Industrial Metallizers, Coaters and laminators.  Their web site is:  www.aimcal.org.

Once at the web site you can go to the bottom of the home page to publications: then click on Reference Materials to open the link to the Metallizing Technical reference.  Then you can fill out the online form or call the office to order and arrange shipping.

Laminating and coating resins

  Extrusion lamination and coating is an old art / Science and is basically practiced by the use of low density polyethylene (LDPE) as an adhesive for bonding together two films or as a coating on the substrate.  The use of extrusion lamination and coating allows for the combination of dissimilar materials such as Foil/paper/films/scrim/nonwovens and other materials.  Bonding is often a key factor in determining success of the coating or laminating step.  Bonding is basically controlled by the ability of the LDPE extrusion to wet and adhere to the substrate.  These properties are generally controlled by the surface oxidation of the LDPE and its melt viscosity at the nip point.  Both of these factors are controlled by the melt temperature and the distance between the die lips and the laminating/coating nip.

As the hot melt passes from the die to the nip point, it is suspended in air.  While in the air it will oxidize at the melt surface and also begin to cool.  To increase oxidation we want a long residence time in the air gap, however, to insure good wetting we need a fluid surface and the longer the melt is in air the colder the melt surface will become.  If the melt surface cools too much it will not be able to flow and contact wet the substrate surface as well.  Die/nip gap also has other effects but these two are what we will focus on today.

By and large the extrusion of the LDPE should be from 293 C to 332 C (560 F to 630 F) and the best temperature will depend on the application and the line speed, polyweight etc.  Higher polyweight (layer or coating thickness) can use lower melt temperatures because of the thermal mass of the coating layer while thinner coatings need higher temps to minimize the cooling rate.  For laminations the coating thickness will range form ½ to 1 mil in thickness while for sealant coatings 1 to 2 mils would be desired.   Thicker sealant layers are possible but will depend on the substrate thermal stability.  Many times the use of 3 mil sealants is met by adhesive lamination of a blown film to the substrate but could also be accomplished by a ½ to 1 mil lamination layer to an other sealant film.   Generally speaking the thicker the lamination layer and the thinner the film the less expensive the product would be to manufacture.

Adhesion can also be helped by blending or coextruding the laminating or coating layer placing a more polar resin in the blend or at the skin of the coextruded adhesive layer.  In this case more expensive copolymers of PE can offset speed limitations caused by lower extents of surface oxidation.  The ability to go really fast relies on coextrusion of the coating or laminate layer.

Next time I will focus on how to control the melt temperature of the LDPE and what resins to coextrude and blend to improve adhesion

Using Pressure Transducers for Process Evaluation – Standard versus Fast Response

Contributed by: Beth Foederer, Optex Process Solutions, LLC

 

The melt pressure transducer provides additional data for understanding extrusion system performance.  Since the system pressure is dependent on the polymer viscosity, flow rate, and resistance of the flow channels there should be a minimum of one (1) pressure transducer before each significant restriction in the flow channel.  Many extruders are supplied with only one (1) pressure transducer located at the head of the extruder.  This is woefully inadequate for fully understanding and regulating most extruder processes.

 

Most standard pressure transducers have the following specifics:

                        Accuracy: +/- 0.5% or +/-1% (Full Scale)

                        Repeatability: +/- 0.2%

                        Sampling rate: 10Hz

Note that in some cases the readout instrumentation will only display at 5Hz.  Although 10 samples per second seems fast, keep in mind that an extruder screw turning at 200 RPM makes more than 3 complete revolutions in a second.  If there is instability in the extruder, the sample rate may not be fast enough to catch the actual peaks and valleys of the pressure variation.

 

When an extruder supplier supplies only one pressure transducer it is typically mounted at the discharge (head) of the extruder.  This is necessary for safety as it will be the highest pressure point throughout the downstream equipment if there are no downstream melt pumps.  Frequently this transducer is positioned in the barrel and may be over the rotating screw flights.  If the transducer is located in this position the readout will reflect the changing pressure between the pushing and trailing edge of the rotating flight.  For a highly viscous polymer this can be a significant variation, possibly as much as ±5% of the average pressure.  Even if the transducer is located beyond the screw flight there will be a residual pulse from this screw rotation until the polymer stream transitions into fully developed pressure flow in the downstream equipment.

 

What does this mean for extrusion control and operation?  Say your average processing pressure is 2000 psi with a 10,000 psi full scale pressure transducer mounted at the head of the extruder and a ±2.5% flight pulse.  The variation sensed from the screw rotation will be ± 50 psi.  If the sensor accuracy is 0.5% full scale this is also ± 50 psi.  Since it is unlikely that the sample rate matches the screw rotational speed the flight pulse will appear to be a random variation.  If there is now a processing problem in order to consistently recognize the variation, it must be greater than ± 100 psi or 5%.  Recognize that extruder output directly correlates to pressure as:

ΔQ ₌  1  ΔP

 Q      n   P

where Q is output rate, n is the power law index, and P is the melt pressure.  For a linear polyethylene with a power law index of 0.5 that means we could have as much as a 10% output variation before we could reliably analyze the root cause using the installed pressure instrumentation.

 

Fast response pressure transducers have the following typical specifications:

                        Accuracy: +/- 0.25%

                        Repeatability: +/- 0.1%

                        Sampling rate: 100Hz

The accuracy and repeatability are twice as good as standard transducers but more importantly the sampling rate is 10 times more samples per second than the standard mode.  Even at 200 rpm, the sample rate is fast enough to see any surging or other phenomenon in the system.

 

Remember that the accuracy and repeatability numbers are based on the full range of the sensor.  If your typical operating range is 800 – 1200 psi but the extruder is instrumented with a 10,000 psi transducer you will never see the variation to allow you to understand and improve the process.  If the 10,000 psi transducer is required at the head of the extruder due to safety interlocks and downstream restrictions between the extruder and the die you may want to add a 2000 psi sensor closer to the forming die. 

 

Keep in mind that the limitations of each tool listed above must be considered when used for data collection and process analysis.  The pressure transducer is a valuable tool when properly used for process improvement.