Extrusion Coating Blog

Beth Replies to comment to 'Understanding and Troubleshooting Your Cast Feed Throat'

Thank you for the question, David.  My feeling is that any temperature measurement is better than none.  I have seen surface mounted temperature measurement in many plants.  The advantage to this type of temperature measurement is that you can put it anywhere on the feed cylinder.  You can move it around to check different locations also.  Sometimes smaller extruders do not have a separate section from the barrel.  Instead there are two clamped on halves with limited cooling.  In this case, I have found that the cooling is very poor to non-existent in the top half.  The main disadvantage to a surface mounted unit is that you are getting the temperature through the metal and not the actual water temperature.  As long as you realize it won’t be as accurate, you should be fine.

 

Beth

Understanding and Troubleshooting Your Cast Feed Throat

Contributed by:  Beth M. Foederer, PE - Optex Process Solutions, LLC

As a feed screw designer, I often find that everyone wants to know what the metering depth or mixer design is for a particular screw.  I rarely get asked, what is the depth or pitch for the feed section?  The feed section is a very important part of screw design.  If a material does not feed well into the screw, you could get poor output, surging and quality problems.  The feed rate of the screw is examined as closely as the metering and mixing when designing a screw. 

The temperature of the cast feed throat can also influence the feed rate of a polymer.  The principle of the flighted screw is to force the polymer along the screw by getting it to stick to the wall and slide on the screw root and letting the flight push the material forward.

Adding temperature readout to the cast feed throat can provide an early warning of processing problems or a troubleshooting aid.  Cast feed throats often have problems in that minerals and deposits fill the small water channels in the cylinder casting over time (especially if using hard water).  Once this occurs, water flow is restricted.  As the contamination builds over time, the cast feed throat does not cool as it should.  A hot inner surface can lead to resin feeding problems and if it becomes too hot, a bridge or melt block. 

Most cast feed throats have a flow meter on the water line.  Many times, these flow meters either do not function correctly or become difficult to see with dirt and dust build up.  Even if the flow is turning in the meter, it may not be carrying heat away due to contamination in the channels. 

The temperature measurement can be a simple thermometer that is mounted in to the throat (as shown in the photo) or it can be as complicated as thermocouple readout to a panel that alarms the operator when the temperature is too high. 

 

Suppose you have a feed section that is not cooling properly?  How can you correct the problem?  If the section is not completely blocked, you can have the section acid flushed.  If the blockage is at an entrance or exit, the holes can be drilled out if you are very careful.  It is best to just use a small drill to see if you can break a hole into the channel and then use an acid flush.  If the acid flush does not work, the feed cylinder may be so fouled that you will need to replace it.

As previously mentioned, a hot feed cylinder can cause a bridge or melt block.  If you get one of these melt blocks, you can use HDPE sticks to break it up.  These sticks can be round or square, about 1” in width, height or diameter.  They are usually 2-4” long.  Feed these one at a time into the extruder throat.  The sticks will push the melt block further into the screw where it can break up and melt.  Use of these sticks can help eliminate the need to pull the screw.

When running low melting point adhesive polymers, it is good to also cool the root of the screw.    

Take a look at the following feed throat photos.  The barrel is part of the throat and contains the opening.  The cooled section of the throat clamps onto the barrel and is constructed in two halves.  You will notice that the bottom half is not cooled at all and the top half has minimal cooling.  The design is not good for low melting point or COF polymers due to the poor cooling ability.  Replacement of the barrel and clamp on throat with a separate barrel and cast feed throat is recommended.

 Taking care and monitoring the temperature of the feed cylinder will prevent unnecessary processing problems in the future.

Avoiding Coextrusion melt disturbance in extrusion lamination

When extrusion coating with a coextrusion, it is important to insure that there are no melt disturbances in the extrudate.  This can be insured by making the outer coextruded layers lower viscosity than the inner of core layer in a two or three layer coextrusion. 

How do I know this to be true?  Well let’s look at what is happening in the flow of a polymer melt.  There is a phenomenon called encapsulation which happens in the flow of two melts in a pipe which demonstrates the viscosity behavior or misbehavior if we don’t set about things right. 

If I take two polymer streams and coextrude them with the lower viscosity material in the center of the flow (figure 1) this is what happens.  First the center flow moves to the wall and then it begins to coat the wall until it completely surrounds the higher viscosity polymer.  When this happens the pressure drop decreases and it takes less energy to pump the tow polymers thru the pipe.  This is why the put water in oil when they pump it as the water coats the wall of the pipe and lowers the horse power requirements for pumping the water.  This is why ethanol cannot be delivered like oil, because the water dissolves in the ethanol and the pumps are undersized for pumping with out water lubrication

Figure 1

Now we can look at why it happens.  Figure 2 shows the viscosity as a function of shear rate for a HDPE resin.  In Figure 2 we see that the viscosity drops as shear rate increases, i.e. the polymer shear thins.

Figure 2

 In Figure 3 we have the velocity profile for the flow of a HDPE polymer which is perfectly homogeneous in temperature and composition. 

Figure 3

Now the slope of the velocity profile is the shear rate and at the center of the flow the shear rate is zero, while at the wall the slope is highest and the shear rate at the wall is highest.  Understanding that the viscosity is a maximum, when the shear rate is zero, we see clearly that for a single polymer flow the viscosity at the wall is lower than that away from the wall because the polymer is shear thinning. 

So when we coextrude and layer polymers on purpose we should take care to put the low viscosity polymer on the outside or we will get an encapsulation flow and drive a flow inversion which will give a melt disturbance of the third kind.

Book Review: Polymer Surface Modification and Characterization

From time to time all of us will have a film or substrate which is not wetting or giving acceptable adhesion when printed or laminated.  In these cases the need arises to analyze the film surface to see what is different.  The question is what is the best way to determine the surface characteristics which are controlling the problem?   The book “Polymer Surface Modification and Characterization” written by Chi-Ming Chan was published by Hanser in 1994 and is an excellent source of information on polymer surface chemistry and will help you in answering this question. 

Chapter 1 begins with an overview description of surface analysis and handling techniques which are available and should be helpful in determining how to proceed in determining what is on your substrate surface.  The chapter finishes up with a description of the many ways to actively modify a substrate surface.  From this background the next few chapters cover in great detail the surface measuring techniques.  Chapter 2 is on Contact Angle Measurement followed by Chapter 3 on x-ray Photoelectron Spectroscopy or XPS.  Both of which are techniques I have discussed using in the various treatment posts.  This is followed by Chapter 4 on Secondary Ion Mass Spectrometry or SIMS.  These are techniques which are available from commercial laboratories and which will be suggested for use if you are in need of determining what your substrates surface is or how it may have changed.  Becoming better acquainted with them will only help in understanding what the lab is measuring for you.

The last three chapters are about specific polymer and surface modification methods which are quite common.  Chapter 5 is on Surface Grafting, based on the use of, light, ionizing radiation and pretreated surfaces.  This is followed by Chapter 6 on Plasma Modification which is more and more available today as a surface modification method in metallizing chambers and on films lines at atmospheric plasma treaters.  Then the book winds up with Chapter 7 Corona and Flame Treatment.

I found the book to be well written and well documented with figures and literature references.  I have been using it myself in my work and as an aid in explaining and better understanding my own findings in surface chemistry studies of substrate surfaces.  

Polymer Surface Modification and Characterization, Chi-Ming Chan, Hanser/Gardner Publications, Cincinnati, Ohio, (1994) ISBN 3-446-15870-7 New York,

Slip transfer in laminations-

When a laminated film or any film, is wound into a roll, the transfer of slip from one surface of the lamination to the other surface will occur.  This transfer of slip (or other migratory additives such as antistat or silicone oil additives) can have an impact on COF.  Figure 1 shows the impact of an additive layer on the inside surface of a lamination film on the COF of the other laminate surface. 

In this example a metallized film is fabricated with erucamide in the sealant layer alone.  In this case the sealant layer is approximately 1.5 microns in thickness for an 18 micron film.  Typical sealant thicknesses range from 1 to 1.5 microns depending on the film end use.

Examining Figure 1 shows a clear drop in COF of the laminate for erucamide concentrations of 3000 ppm and above with little extra benefit for concentrations above 3000 ppm.  Also, below 2000 ppm there is little effect on outside laminate COF.  This is because below the 2000 ppm in the sealant layer alone, the concentration of slip is not enough to saturate the film and there is not enough slip available to cover the film surface for transfer to the opposite face of the laminate.

 

 

Figure 1

 

 

Laminating Resins - HDPE

Continuing the earlier discussion of polyethylene resins and their structure, branched for LDPE, linear for LLDPE and HDPE, I want to discuss their use as laminating resiss.  15 or more years ago, one of my coworkers was looking at the use of HDPE for laminations to make them stiffer.  I believe his purpose was to allow down gauging of the lamination while maintaining stiffness for specific packaging machines.  As I recall, the bond strength was not as good as with the LDPE and stiffness gain may not have been exactly where he wanted it.  I believe the bond strength may have been lower due to the stiffness increase as we are not really measuring bond strength but peel strength and a stiffer sheet could force a crack along the HDPE/laminate interface.  Not 100% sure the exact cause but that is a possibility.  The more compliant LDPE would permit some more energy absorption in the peel giving a higher value for the peel strength. 

This earlier work was with a single layer lamination; if the use of a three layer lamination had been used with say a LDPE/HDPE/LDPE coextrusion for the lamination adhesive, then bonding may have been increased along with the stiffness.  Of course the use of the HDPE would also improve moisture barrier relative to the LDPE it would displace. 

Today there are many new PE based resins and thermoplastic elastomers (TPE) with very low density and increased elasticity that if combined with other cores by coextrusion could give a wide range of potential benefits to the lamination.

PE and wax are "cousins"?

I have received a comment from Bill about the similarity of the chemical nature of wax and Polyethylene.  This is correct but today polyethylene is such a rich word, I thought I would narrow it down a little.

 

Chemically, wax is a linear combination of ethylene (H₂C=CH₂) and contains from 30 to 100 carbon atoms .  Solid waxes are obtained by dewaxing light petroleum products and are a mixture of various chain length molecules.  It is able to crystallize and melts around 48 to 66 C according to the Britannica (http://www.britannica.com/EBchecked/topic/442604/paraffin-wax)

 

Polyethylene is composed of much longer chains of ethylene and to be accurate only linear polyethylene (HDPE) has the same molecular structure as the shorter chain wax.  It is the extreme chain length of liner polyethylene which gives it its excellent physical properties.  The molecules are so long that they physically entangle together to give the physical properties which we admire.  In the case of ultrahigh molecular weight polyethylene UHMWPE the chains are so long that there are very few chain ends.  This gives the polymer excellent wear properties as the sliding friction is low (remember waxing sliding boards with your wax paper sandwich wrap?)and the lack of chain ends helps prevent surface peeling giving the UHMWPE the wear resistance.

 

A good analogy to use to see the effect of chain length would be cooked spaghetti.  Imagine a plate of spaghetti which is gut into short 1 inch lengths.  This is not possible to twirl onto a fork to eat.  But long spaghetti is able to be wound onto a fork.  But if the length of the spaghetti is say 12 to 18 inches long then when you twirl the fork it picks up the whole plate because the individual “chain” are physically wrapped around each other preventing the disentanglement needed to capture just a bite sized portion on the fork.

 

In comparison, the old technology LDPE was made by a high pressure and temperature free radical polymerization.  This produces branched molecules which have long chain lengths as well and good molecular weights.  But the branches limit the crystallization of the molecules and lower the crystallinity of the bulk solid polymer and therefore the density, moisture barrier , mechanical properties, stiffness etc.  It is not possible to produce linear polyethylene with this process and it was the invention of the Ziegler-Natta catalyst systems which are responsible for many of the linear polymers we use today such as polypropylene.

 

Other short chain branched polyethylenes are made by copolymerization of ethylene with other higher olefin molecules such as, propylene, butene, hexane and octane.  When introduced into the linear polymer molecule these comonomers give regular length side chains which also disrupt the crystallization of the molecule giving the lower densities characteristic of these linear low density polyethylenes (LLDPE).  Because they also have very long chain lengths the branching also affects the rheological properties of these materials.

 

Linear polyethylene with its higher stiffness and barrier is used sometimes as a laminating resin, but generally in combination with LDPE skins to give better interfacial adhesion.  Sometimes the stiffness is a problem but that will depend on the application.

Advantages of Extrusion coating and laminating

Early packaging materials were all made using adhesive laminations.  This was for the simple reason that at the time there were no available extrudable polymers as we know them.  The primary polymers available were either natural polymers or a few polymers such as PVC, PVdC, polystyrene etc made by free radical polymerizations, no polyolefins such as LDPE or PP and the wide variety we have now.

All the first packaging substrates such as paper, foil and cellophane were not meltable and were glued together to make laminations to improve performance.  A primary adhesive and coating material was wax (remember chewing gum wrappers?) and later some of the vinyl polymers.  As you can imagine there were real limitations to what could be produced.  Then as discussed in a previous posting the invention of LDPE helped to change that.

Polyethylene (PE) when considered as a chemical, is recognized as the same chemistry as that of wax, simple chains of carbon and hydrogen combined as methylene groups -(CH2)-.  It almost seems that it is too simple an idea to replace the wax with the new material to make products with similar, but improved properties.  The big difference between wax and PE is the size (length) of the chain of methylene groups.  Where wax has a chain length of perhaps 30 to 100, PE has a chain length of several hundred to a million.  This changes the fluid properties of the PE relative to the Wax quite a bit with wax being a low viscosity fluid when molten and PE is a thick highly viscous, elastic fluid when molten.  This change in viscosity forces the change in coating methods when transitioning from wax to PE.  PE coating requires the use of an extruder to melt and homogenize the PE and to generate the pressure necessary to have it flow and be coatable.  PE also can be used as a sealant due to its high viscosity and its ability to fuse together when melted and forced together so now a layer of PE could replace another coating as well if desired.

In the next posting I will begin to discuss how the use of PE as an extrusion coating lead to a variety of new products and then how the use of PE as a laminating adhesive can lead to lower overall product costs.

The Start Of Extrusion Coating And Lamination

The use of extrusion coating and lamination as we know it started approximately 60 years ago.  Film to film and film to foil laminations existed prior to that made use of laminating adhesives and the use of extrusion based adhesives required the commercial availability of polyethylene.  As I understand it, Polyethylene was invented sometime in the mid to late 1930’s and was a strategic material during WWII and not commercially available until after the war.   With the commercial availability of LDPE came the early inventive work to make use of it in packaging. 

In 1948, H. P. Smith developed the Loxol process which coated paper with PE using an extruder fed calendaring process to melt and coat the paper substrate.  This process resulted in product with a LDPE coating but the coating had more pinholes in it than the current extrusion coating process employed today.  Much of the early extrusion coating work appears to have been conducted by DuPont working with Hartig, Eagan, St. Regis Paper, International Paper and Sealright located in what used to be the Empire State and is currently known as New York.  The work took place in Upstate NY due to the focus on producing LDPE coated paper and the heavy production of paper which used to be done in Upstate NY.  This is the reason some of the manufactures of converting equipment, such as Black Clawson are located in NY State. 

St Regis Paper appears to be the first company to an extrusion based process with a slot die which is what is still practiced today.  By 1951 Sealright was coating thin paper using an 8 inch diameter extruder with a 90 inch wide slot die.  By 1956 Sealright, DuPont and Frank Eagan were experimentally coating paper board with PE for milk cartons.  This process was developed to compete with paper board which was solvent coated with vinyl ultimately replacing the vinyl coated paper board with PE coated paper board for milk cartons.  The PE coated paperboard appears to be one of, or the first major use of LDPE and propelled it to the first billon pound polymer.

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.