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Thermoformed Paperboard Coatings

Thermoformed Paperboard/Corrugated Coatings

One of the big advantages of thermoformed paperboard and thermoformed corrugated plates and trays is that they can be engineered to each specific end use.  Much of the variety of end uses is controlled by the coatings applied to the interior and exterior of the material.  Unlike plastic and/or aluminum foil containers, the inexpensive base material of paper which makes up at least 90% of the total basis weight of the material can remain the same for many, many applications while the coating which is usually less than 10% by weight can be customized to the application.    

While the coating on the inside of the plate or tray defines the end use characteristics for the thermoformed tray, the exterior of the tray should not be neglected.  The exterior coating serves two purposes.  The first purpose is to enhance the formability of the material by reducing the coefficient of friction between the material and the forming die.  This is critical to being able to thermoform certain materials such as materials with a high recycled content due to their lack of tensile strength.  This cannot be emphasized enough.  Many materials will simply not be able to thermoform without the aid of an exterior slip agent coating.  The second purpose of the exterior coating is to protect the outside of the tray from inadvertent exposure to grease and/or moisture during the food packing and/or distribution process.   

Coatings for the interior of plates and trays have been developed specifically for dual ovenable, microwave only, frozen foods, agricultural packaging for fruits and vegetables, bakery products, take away foods, serving trays, deli trays, paper plates, and many, many other applications.   These coatings fall into three basic categories:  Extrusion Coating, Laminated Coatings, and Press Applied Coatings.  The following is discussion of a few of the different types of coatings available within each of these categories.  Please understand that this is not an exhaustive list as new coatings are being developed each year.

1.)    Extrusion Coatings

a.    PET – Polyethylene Teraphthalate (Polyester) – When extrusion coated, this material finishes in an amorphous state (non-crystalline), which means that it has good sealing properties, but is more susceptible to sticking at in the forming dies at lower temperatures than crystallized PET.  It has a melting temperature of approximately 185 degrees F, but will begin to stick to tooling at a much lower temperature (approximately 125 F) due to the high pressure and dwell time.   If for some reason a male die overheats due to the press being closed and subsequently the PET coated material sticks to the die, do not try to peal the tray off the die.  Simply close the die under heat and pressure for about a minute.  The PET will crystallize and immediately release from the male die with no sign of sticking.   PET is suitable for use in microwave and conventional ovens up to about 400 degrees for 30 minutes as long as there is a good food load in the container to absorb much of the heat.  Because it is amorphous, it has a tendency for foods to stick to it such as bakery products and other foods with high sugar content.  For good adhesion to the paperboard and to maintain a solid coating with no pin holes, this coating is generally applied at approximately 25 pounds per ream.  In the end use of the tray (cooking process), the coating can begin to crystallize and shrink.  As it shrinks, it has a tendency to pull on the paperboard, thereby warping and bowing the tray.

b.    PBT – Polybutylene Teraphthalate – This material is a derivative of PET and is very difficult to extrude.  It is also very difficult to run on forming machines as it has a tendency to stick to the male dies and blister very easily.  Despite applying a heavy coat (approximately 25 pounds per ream) it does not adhere well to the paperboard and therefore blisters easily.  However, foods have less of a problem releasing from this coating, and it is more dimensionally stable through the cooking process than standard PET.  For these reasons, PBT is used for bakery products and food products requiring some higher degree of release than can be provided by PET.  PBT is suitable for use in microwave and conventional ovens up to about 400 degrees for 30 minutes as long as there is a good food load in the container to absorb much of the heat.

c.    PMP – Polymethylpentene – Also known by its trade name, TPX, this material is very well suited for thermoforming paperboard and/or thermoforming corrugated materials.  It has a high melt temperature so it does not stick to the forming dies in the thermoforming press, and it is dimensionally stable throughout the end use cycle of the tray.  It has high release characteristics which make it ideal for bakery items and foods with high sugar content.  The one drawback of this material is that butter fat can permeate it and soak through to the base paperboard.

d.    LDPE – Low Density Polyethylene – This material is usually extruded at a rate of approximately 15 pounds per ream.  Because of its low melt temperature, it can be very difficult to form on forming machines.  The coating can become sticky and cause the material to stick to the forming dies simply through transfer heat.  Therefore, when using LDPE, the thicker the substrate, the better in order to prevent transfer heat through the material to the male die and to prevent the heat from quickly transferring through the substrate to the LDPE.  Forming usually requires lower temperatures and higher dwell times to obtain good quality trays.  This material is suitable for microwave use only and even at that, for foods that are only to be re-warmed, not cooked.  Foods with high fat content or high sugars can become much hotter than the melting point of LDPE in the microwave oven.

e.    HDPE – High Density Polyethylene - This material is usually extruded at a rate of approximately 15 pounds per ream.  Because of its relatively low melt temperature, it can be very difficult to form on forming machines, although it is much better than LDPE.  Just like LDPE, the coating can become sticky and cause the material to stick to the forming dies simply through transfer heat.  Forming usually requires lower temperatures and higher dwell times to obtain good quality trays.  This material is suitable for microwave use only.  It will handle temperatures high enough to be compatible with most foods.

f.    PP – Polypropylene - This material is usually extruded at a rate of approximately 15 pounds per ream.  Polypropylene is much easier to form on the forming machine that either of the PEs.  Transfer heat is not as much of a problem for PP.  Although heat cannot be applied directly to the male forming die, transfer heat is not as much of an issue with this material as it is with PE.  Forming usually requires lower temperatures and higher dwell times than PET (although not to the extreme as with PE) to obtain good quality trays.  This material is suitable for microwave use only.  It will handle temperatures high enough to be compatible with most foods.

2) Laminated Coatings

a.    PET – Polyethylene Teraphthalate (Polyester) – PET can be applied to paperboard via adhesive laminating.  In this process, a roll of PET film (also known as Mylar) is unwound at the same rate as a roll of paperboard and is pressed onto the paperboard between rollers with a layer of glue that has been applied to either the film or the paperboard.  This particular film is usually made via a “casting” process and as a result is crystalline in nature.  Therefore it is possible for the male forming die to be heated to some degree (approximately 125-135 degrees F).  This provides for less dwell times and subsequently higher speeds.  It is imperative that the correct type and amount of adhesive is utilized in order to create a fiber tear bond between the PET and the substrate.  Otherwise, the process of heat, dwell and pressure combined with the moisture in the paperboard will cause the film to delaminate from the paperboard.  In addition, the type of adhesive may need to be modified if printing is to be applied either on the inside of the film or on the board before laminating.  It is nearly impossible to achieve fiber tear adhesion if clay coated board or oil base inks are on the surface of the board prior to laminating.  Laminated PET also provides better food release than amorphous PET.  Mylar is suitable for use in microwave and conventional ovens up to about 400 degrees for 30 minutes as long as there is a good food load in the container to absorb much of the heat.  Because it is already crystalline in nature, it is much more dimensionally stable through the end use process (cooking) than is extrusion coated PET.

b.    Susceptor Materials – Susceptor materials are Mylar (crystallized PET film) that has had a very thin layer of metal (typically aluminum – sometimes other metals such as stainless steel, titanium, etc.) applied to the inside surface.  This metal is generally applied via a vacuum metalization process whereby the thickness is measured in angstroms.  A typical means for measuring the thickness is by using an optical densitometer.  Hence, the thickness is sometimes referred to as optical density.  The purpose of the metalization is to heat the food load when subjected to microwave energy.  The microwaves excite the molecules of aluminum and cause them to rub together creating a tremendous amount of friction heat.  The surface of a susceptor material can reach instantaneous temperatures over 1000 degrees F.  Even at those high temperatures, not much heat is generated as a result of such a thin cross sectional area.  This heat then has a tendency to brown and crisp foods that come in contact with the susceptor in the microwave that would otherwise remain soggy.  A delicate balance in terms of metal thickness must be maintained for use as a susceptor heating pad.  Too thick and the metal will short out causing arcing in the oven and potential fires.  Too thin and susceptor will not be hot enough to produce enough heat for its intended purpose.  This material is applied to the paperboard in the same manner as described above under the PET laminate.  However, due to its different nature, the adhesive (glue) will be of a different type than is used with the PET.  It is imperative that the correct type and amount of adhesive is utilized in order to create a fiber tear bond between the LDPE and the substrate.  Otherwise, the process of heat, dwell and pressure combined with the moisture in the paperboard will cause the film to delaminate from the paperboard.  Due to the heat generated by the metalization, a very aggressive adhesive must be utilized.  No printing, clay, or any other contaminate can be applied between the film and paperboard.  The high temperature of the metalization can cause nasty substances to be given off by any materials present including the adhesive, paperboard, and PET itself.  Therefore, it is important to keep these substances to a minimal level by not introducing any other foreign materials such as ink or clay.  In most cases, the temperatures become so hot that the PET actually fails and cracks.  Because this material is crystalline in nature, it is possible to provide heat to the male forming die thereby decreasing dwell times and increasing speeds.   Also because of the crystallinity, it is much more dimensionally stable through the end use process (cooking) than is extrusion coated PET.

c.    Other Metalized PET (Mylar) – For decorative purposes, some companies may adhesively laminate vacuum metalized Mylar to the base substrate.  The thickness of this metalization is much heavier than that used for susceptor materials.  Therefore, it cannot be put into the microwave oven without risk of fire.  However, all other processing parameters are the same as with susceptor materials.

d.    LDPE – Low Density Polyethylene – This material is applied in the same manner as described above under the PET laminate.  However, due to its different nature, the adhesive (glue) will be of a different type than is used with the PET.  It is imperative that the correct type and amount of adhesive is utilized in order to create a fiber tear bond between the LDPE and the substrate.  Otherwise, the process of heat, dwell and pressure combined with the moisture in the paperboard will cause the film to delaminate from the paperboard.  In addition, the type of adhesive may need to be modified if printing is to be applied either on the inside of the film or on the board before laminating.  It is nearly impossible to achieve fiber tear adhesion if clay coated board or oil base inks are on the surface of the board prior to laminating.  Because of its low melt temperature, it can be very difficult to form on forming machines.  The coating can become sticky and cause the material to stick to the forming dies simply through transfer heat.  Therefore, when using LDPE, the thicker the substrate, the better in order to prevent transfer heat through the material to the male die and to prevent the heat from quickly transferring through the substrate to the LDPE.  Forming usually requires lower temperatures and higher dwell times to obtain good quality trays.  This material is suitable for microwave use only and even at that, for foods that are only to be re-warmed, not cooked.  Foods with high fat content or high sugars can become much hotter than the melting point of LDPE in the microwave oven.

e.    HDPE – High Density Polyethylene - This material is applied in the same manner as described above under the PET laminate.  However, due to its different nature, the adhesive (glue) will be of a different type than is used with the PET.  It is imperative that the correct type and amount of adhesive is utilized in order to create a fiber tear bond between the HDPE and the substrate.  Otherwise, the process of heat, dwell and pressure combined with the moisture in the paperboard will cause the film to delaminate from the paperboard.  In addition, the type of adhesive may need to be modified if printing is to be applied either on the inside of the film or on the board before laminating.  It is nearly impossible to achieve fiber tear adhesion if clay coated board or oil base inks are on the surface of the board prior to laminating.  Because of its relatively low melt temperature, it can be very difficult to form on forming machines, although it is much better than LDPE.  Just like LDPE, the coating can become sticky and cause the material to stick to the forming dies simply through transfer heat.  Forming usually requires lower temperatures and higher dwell times to obtain good quality trays.  This material is suitable for microwave use only.  It will handle temperatures high enough to be compatible with most foods.

f.    PP – Polypropylene - Polypropylene is available in several different forms as is PE.  BOPP and OPP (biaxially oriented polypropylene and oriented polypropylene) provide superior gloss.  PP materials are applied in the same manner as described above under the PET laminate.  However, due to its different nature, the adhesive (glue) will be of a different type than is used with the PET.  It is imperative that the correct type and amount of adhesive is utilized in order to create a fiber tear bond between the PP and the substrate.  Otherwise, the process of heat, dwell and pressure combined with the moisture in the paperboard will cause the film to delaminate from the paperboard.  In addition, the type of adhesive may need to be modified if printing is to be applied either on the inside of the film or on the board before laminating.  It is nearly impossible to achieve fiber tear adhesion if clay coated board or oil base inks are on the surface of the board prior to laminating.  Polypropylene is much easier to form on the forming machine that either of the PEs.  Transfer heat is not as much of a problem for PP.  Although heat cannot be applied directly to the male forming die, transfer heat is not as much of an issue with this material as it is with PE.  Forming usually requires lower temperatures and higher dwell times than PET (although not to the extreme as with PE) to obtain good quality trays.  This material is suitable for microwave use only.  It will handle temperatures high enough to be compatible with most foods.

3.)    Press Applied Coatings – Paper Plates: 

The coating applied to paper plates serves multi-purposes.  It serves to hold out grease, moisture, etc. during the end use as well as provide a gloss to the graphics to make them more appealing.  However, just as importantly is the release that the coating provides to the forming process.  Inadequate levels or improper coating will cause the clay and/or inks to pick off from the paperboard and transfer to the male forming die.  In worst cases, the plate will stick to the forming die and cause jam ups.

a.    Nitrocellulose - Prior to the late 1980s, the most popular coating used in the U.S. for paper plates was Nitrocellulose.  Nitrocellulose provides excellent gloss, good barrier and good heat resistance for the forming process.  However, due to the high VOCs associated with using nitrocellulose and the physical danger of fire and explosion in handling it, very few companies are willing to use it today - especially with the advent of much safer and nearly as good aqueous coatings.

b.    Michelman PPCX20 – This coating is a water based acrylic and was one of the first successful coatings to substitute for Nitrocellulose.  It can be applied via the Flexo or Gravure process.  Although it supplies adequate barrier for paper plates, it is not a vapor barrier as it allows the paperboard to “breath” thereby eliminating problems caused by trapped moisture.

c.    Michelman PPCX40 – This coating is also a water based acrylic.  It is a generation newer than the PPCX20 and provides superior gloss, holdout, and release to the forming process as compared to PPCX20.

d.    Michelman PPCX70 – Also a water based acrylic, this coating has higher gloss than PPCX20 or PPCX40.  It also is recoatable and can be used over mylar.  However, it requires higher coat weights and is more expensive than other alternative coatings.

e.    Michelman PK910 – This acrylic coating is a low cost, mainstream coating for paper plates.  It does not have as high of gloss as the other Michelman coatings, but it also contains less ammonia and foams less than the others.

f.    Other paper plate coatings are available from Roymal, Premier, Sun, and Cork as well as others.

5.) Press Applied Coatings – Inside of Ovenable Trays: 

For many years coating companies and ovenable tray producers have attempted to replace the extruded or laminated coatings with press applied, water based coatings for ecological reasons.  Unfortunately, to date no coating has been developed which can withstand the rigors of moistening, scoring, cutting and forming.  Many, many, many companies claim to be able to achieve this, but we have never tested a product that proves to be satisfactory from the processing through end use.

Application Information

All of the coatings for paper plates and the outside of ovenable trays are in the range of id="mce_marker".00 per pound ($2.20 per kg), give or take a few cents.  To achieve a degree of barrier, approximately 1 to 1.5 pounds (4.8 – 7.3 kgs) of wet coating needs to be applied per 1000 square feet (1000 square meters) of material.  This can be accomplished by using a 180 line screen with a cell volume of 10-12 cbm (cubic billion microns).  For slip and mold release purposes only (back of ovenable paperboard trays), approximately 0.5 – 1.0 pounds (2.4 – 4.8 kgs) of wet coating needs to be applied per 1000 square feet (1000 square meters) of material.  This can be accomplished by using a 220 – 230 line screen anilox with a cell volume of 5 – 7 cbm (cubic billion microns).

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Paper Plate and Ovenable Tray Coating Supplier Information

Michelman Inc.

9080 Shell Road

Cincinnati, OH 45236-1299 USA

Tel: 513-793-7766

Fax: 513-793-2504

Michelman International & Co. SNC

Zoning Industriel B-6790
    Augange, Belgium
    Tel: 32-63-38-9609
    Fax: 32-63-38-9692   

Michelman Asia-Pacific & Co.

    1, Tuas Avenue 13
    Singapore  638924
    Tel: 65-862-5055
    Fax: 65-862-5066Premier Ink Systems


P.O. Box 670
Harrison, OH  45030
PH: (513)367-2300
FX: (513)367-5346