Extrusion Coating: A Process Manual

Contents

PREFACE

1 THE EXTRUSION COATING PROCESS

2 EXTRUDER AND SCREW DESIGN

3 DIE DESIGN

4 STRETCHING FLOWS AND NECK-IN

5 ADHESION

6 COEXTRUSION

7 ADHESION PROMOTION METHODS

8 POLYOLEFINS FOR EXTRUSION COATING

9 BARRIER POLYMERS

10 SUBSTRATES AND FILMS

11 EXTRUSION COATED PRODUCTS

12 OPTIMIZING PROPERTIES FOR FOOD PACKAGING

13 HEAT SEALING WITH PE RESINS

PREFACE

Extrusion coating was one of the first continuous processes developed, to combine in situ, an extruded thermoplastic to a flexible substrate. The first commercial plants appeared in the USA during the late 1940’s. Over the years this technology has grown into one of the most sophisticated plastic converting processes in use today. The latest machines incorporate several extruders with two or more coating stations with coextrusion and laminating facilities. Commercial machines with line speeds in excess of 300 mpm and die widths of three metres have been constructed. These plants include supervisory’ computer management that can access the process recipes for immediate implementation with minimal down time.

In its genesis, extrusion coating was used to apply a moisture proof thin polyethylene layer onto paper. The earliest applications were paper bags and multiwall sacks. The process then took a significant step forward in the liquid packaging carton market by replacing paraffin wax coatings with polyethylene. Over the years this was extended to produce a variety of composite structures to satisfy the needs of markets in flexible packaging, photographic reproduction, health care, cosmetic, construction and industrial applications. The substrates can vary from heavy boards, light papers, aluminium foil, plastic films and woven materials. Although the bulk of production is centred on low density polyethylene other polymers such as, polypropylene, polyester, ethylene vinyl alcohol resins and a wide range of ethylene copolymers and blends are used in specialized applications.

This comprehensive study of extrusion coating technology combines experimental studies with computer simulations to provide insight and assistance in process optimization and problem solving for engineers, technologists and production personnel. The extensive use of experimental data combined with computer modelling using powerful software provides a unique approach to learning and understanding this technology. Melt flows in screws and dies and stretching and cooling in air gaps and on chill rolls are analyzed. Τ slot and coat hanger dies are compared with different polymers. This approach is invaluable in the design of critical items of extrusion coating equipment.

The control of process parameters to maximize productivity and adhesion of the polymer to the substrate are evaluated in depth. The effects on adhesion of polyethylene oxidation, comonomer and stress relaxation are discussed thoroughly using experimental and numerical methods. The use of copolymers and surface treatment methods to improve adhesion, product resistance and productivity are comprehensively studied.

Melt velocity, draw rates, shear stress and thickness distributions and their effects on interlayer instabilities in coextrusion coating dies with various polymers are analyzed. Heat transfer between layers in feedblock and dual manifold dies are predicted. Computer calculations arc used to predict the cooling efficiency of chill rolls in relation to their diameter, temperature and mass flow rates. Finally, reviews of key polymers, copolymers and substrates that are commonly available to the extrusion coater are discussed and will prove invaluable to the reader searching for innovation. The final chapters cover in detail aspects of heat sealing and organoleptic properties in food packaging and provide an invaluable source of data for product designers.

In this study much use is made of computer generated modelling to help in demonstrating and evaluating polymer flows in various parts of the processing equipment. This PC software is designed to simulate non-Newtonian flows in a variety of flow fields including screws, dies and coextrusion processes. The program solves the equations of conservation of mass momentum and energy for polymer melts through process equipment. The finite element method (FEM) is used, which is a rigorous solution of the differentia] equations. The Galerkin formulation is used to obtain solutions expressed as polynomials within small triangular elements and finally the individual solutions are assembled together to provide the velocity, pressure and temperature throughout the whole field of flow. From the velocity field the streamlines, local shear and stretch rates, stresses and particle residence time are calculated.

The author believes the information discussed to be accurate and reliable. No liability is assumed or warrant) given for the application of the data in this document. The data is based on technical information which is available to the general public and to the best knowledge of the author it contains no confidential data received from a third party. The user of this publication agrees to protect the author’s copyright by making all efforts to prevent third parties from obtaining access to the information.

1 THE EXTRUSION COATING PROCESS

In extrusion coating a thin film of very hot molten polymer is pressed into a substrate passing through a cooled nip roll assembly as shown in Fig. 1.1. Coating thickness will normally range from 10-75 g/m². For fabric and textile coating, thickness may be as high as 0.5 mm.

Fig. 1.1 POLYMER MELT FEEDING INTO THE NIP ROLL ASSEMBLY

The section of the equipment that combines the polymer melt with the substrate(s) is sometimes referred to as the laminator. Usually the term coater is more descriptive of the function to avoid confusion with the adhesive lamination process. The polymer melt at uniform temperature flows from the lips of the slot die and is drawn down into the nip formed by the chill roll and the rubber sleeved pressure roll. The substrate is simultaneously fed into the same nip where it combines with the molten polymer under pressure. A compression force is generated in the nip, which forces the melt to penetrate the substrate. The cooled weh is then stripped from the chill roll as it passes over a stripper roll and is pulled by the haul-off and winding equipment at constant tension. The positioning of the extruder relative to the chill roll is critical. The extruder should be mounted on a chassis, which allows movement in three directions. The melt entry angle and height (air gap) of the die relative to the nip is critical. The extruder should be equipped with a screenpack unit to filter any contamination from the melt.

In extrusion laminating two substrates fed from rolls are combined with the polymer melt acting as the adhesive between the two materials. Fig. 1.2 shows the extrusion lamination process. This technique is widely used to laminate aluminium foil to paper with low density polyethylene (LDPE).

The coating section is the heart of the machine where the melt has to spread and adhere to the substrate(s). In the nip the melt must be fluid enough to flow and penetrate into the moving surface. The polymer side in contact with the chill roll will freeze immediately as a temperature gradient rapidly develops across its interface. The bond must be formed virtually instantaneously. Optimum nip pressure is required to maximize adhesion between the melt and the substrate. A pneumatic pressure of 9-18 kg/linear cm is a useful starting point. Excessive pressure will not improve adhesion and may cause

unnecessary stress on the roll bearings. The nip force will not influence the thickness of the coating, which is determined by the output of the extruder relative to the rotational velocity of the chill roll. A water cooled (15-20 °C) steel back-up roll removes excess heat from the surface of the rubber pressure roll as shown in Figs. 1.1 and 1.2, This back-up roll should be of double shell construction to maximize cooling efficiency.

Fig. 1.2 EXTRUSION LAMINATION

Fig. 1.3 shows a schematic cross section of a typical extrusion coating machine. The substrate, at constant tension, arrives into the coating nip via a series of idler rolls. A steam heated roll is a useful option to improve adhesion by preheating/drying the paper substrate before it arrives in the nip. The surface of the substrate will usually by either corona or flame treated to improve adhesion. The corona discharge electrodes should be positioned as close as possible to the melt stream exiting the die.

Fig, 1.3    SCHEMATIC CROSS SECTION THROUGH A COATING LINE

The melt enters into the nip almost tangential to the chill roll. An overlap of 3-5 mm on the pressure roll prior to entry in the nip is recommended. The air gap or distance between the die and nip will have a profound influence on adhesion. A useful starring distance is 150-200 mm (6-8 inches) of air gap. However, further adjustments will be necessary as experience with different polymers and operating

conditions are developed. The effect of time in the air gap on oxidation and its influence on adhesion is very critical and is comprehensively discussed in Chapter 5.

The coated product is stripped off the chill roll by a set of driven nip rolls, which are synchronized with the drives of the chill roll and winder to accurately control web tension. The practical recommended arrangement is to position the stripper roll (Fig. 1.1) so that the web comes off the chill roll along a vertical line from its horizontal axis. Increasing the overlap beyond this point does not seem to improve chill roll release. As will be shown later the chill roll temperature, its diameter, line speed and surface finish are crucial in determining the operational effectiveness of the coater.

The excess polymer at the edges (overhang) is cut by the slitter system positioned passed the chill roll were web tension is controlled and held constant. The slitting can be by razor, score or shear cutting. The straight edged web is taken up by a turret or surface winder where rolls of predetermined size are collected. Commercial machines incorporate automatic splicing, which allows uninterrupted roll changes at both ends of the process. A typical commercial process will run in the range of 150-250 mpm line speed. Machines capable of much higher line speeds have been constructed.

Extruder sizes will vary from 90 mm (3½) to 150 mm (6) diameter depending on the required output relative to the die width and line speed specified. Extruders with polyethylene outputs of 1000 kg/hi. are in operation. Larger machines are offered.

Die widths will range from 1.2 to 3.0 m for most paper and board coating applications. Dies of 4.0 m width for fabric/textile coating have been built. Coextrusion, usually based on combining feed block designs, is widely used to include thin adhesive or barrier layers in the structure. These satellite extruders will vary in size from 60-90 mm diameter and must be optimally sized to minimize residence time and avoid melt stagnation with ver;’ thin layers of polymers of relatively low heat stability. For some applications dual manifold designs where the melts combine near or just outside the die exit are preferred.

Drool catchers must be provided to protect the machine from the melt curtain as the extruder and die are driven to and from the coater.

1.1 CHILL ROLL COOLING SECTION

The coater in its most basic form is made up of a rotating steel chill roll, a rubber sleeved pressure roll, which form the nip, plus a stripper roll to feed the coated web to the haul-off downstream equipment (see Fig. 1.1.)

The hot polymer melt is drawn onto the substrate by passing it through the nip where a pneumatic load is applied. The nip pressure loading should be precision controlled to ensure repeatable impressions across the chill roll.

The melt flow rate at the entry to the nip determines the coating thickness as the melt spreads across the die width. In the nip the cold roll exerts a compression force, which squeezes the melt into the surface of the substrate, usually paper, which is porous. The melt will bond to the surface by a combination of mechanical penetration and various chemical bonding forces. The complex subject of adhesion with different substrates and polymers is discussed in detail later in this study.

A schematic of a typical chill roll assembly is shown in Fig. 1.1.1. The efficiency of this section of the machine is a key factor in determining both quality and productivity. The chill roll is essentially a heat

exchange unit that cools the hot polymer to ambient temperature and enables the transfer of the coated web to the haul-off equipment with a minimum of pulling (stripping) force from the chill roll.

Good design and maintenance are essential to hold a uniform temperature over the total surface of the chill roll. Double walled spiral rolls arc utilized and designed to keep the temperature variation across the face to ±1-2 °C. A heavy inner shell supports a thin outer shell with spiralled flights for maximum heat transfer efficiency. The chill roll should be designed for rapid change over to enable the use of different surface finishes. As shown below rotary unions through which the cooling water flows should be designed for quick disconnects and the roll change should be made within half an hour.

Fig. 1.1.1 CHILL ROLL SET-UP

Chilled water at 15-20 °C is pumped through the roll. The surface temperature should be higher than the environmental dew point of water to avoid condensation. The most efficient chill rolls use a very thin outer steel shell with a special assembly procedure to prevent the shell collapsing. High conductivity steel must always be used in :he construction. The water flows through the rotary unions connected to the two ends. Clean soft water must be used to keep the inner channels clean. The water capacity on a commercial line should be around 1500 1/min. at a flow rate of 3-5 m/s. Periodic flushing, cleaning and de-scaling is recommended to maintain the system at maximum heat transfer efficiency.

Refrigerated water is necessary on high speed lines. If the chill roll temperature is too high the coated polymer will stick to its surface and may tear from the substrate as it is pulled away. In extreme cases this will result in a web break. However, heated chill rolls are sometimes used in specialized applications to maximize adhesion with specific materials. Finally, too low a temperature may decrease adhesion due to excessively rapid quenching of the melt.

The key parameters that influence the stripping temperature of LDPE at the separation point from the chill roll are:

•   roll temperature

•   thin outer shell

•   high thermal conductivity materials

•   chill roll diameter

•   line speed (rotational velocity)

The effectiveness of the chill roll cooling process was studied by computer modelling. The process conditions entered in the model in Fig. 1.1.1 are as follows.

•   Chill roll diameter: 450 mm.

•   Pressure roll diameter: 250 mm.

•   Chill roll temperature: 18 °C.

•   Pressure roll surface temperature: 40 °C.

•   Width of rolls (web): 1500 mm.

•   LDPF. temperature at nip: 315 °C.

•   Paper: 0.15 mm. Temp. 35 °C.

•   Mass flow rate (LDPE): 500 kg/hr.

•   Line speed: 200 mpm.

•   Coating weight: ~27 g/m².

•   Ambient air temperature: 20 °C.

The first step was to calculate the heat exchange that occurs at the nip were the hot polymer is first pressed against the paper and starts cooling rapidly as it contacts the chill roll set at 18 °C. Fig. 1.1.2 shows the cross cut were the temperature profile is plotted.

Fig. 1.1.2 CROSS CUT AT PAPER-LDPE INTERFACE

Fig. 1.1.3 shows the temperature profile across the LDPE-paper interface as it passes through the nip The LDPE temperature drops instantaneously from 315 to 235 °C. The substrate (paper) enters the nip at 35 °C: it is assumed that some heat is transferred from the corona/flame treater and hot die. The temperature gradient through this section is critical since it will determine the wetting and penetration of the melt into the substrate’s surface.

Fig. 1.1.3 TEMPERATURE PROFILE OF CROSS CUT AT NIP

Further downstream at the separation (stripping) point from the chill roll the interface temperature has dropped to ~21.5 °C as shown in Fig. 1.1.4.

Fig. 1.1.4 TEMPERATURE AT SEPARATION POINT FROM CHILL ROLL

The simulation illustrates the efficiency of this cooling process as 500 kg/hr. of LDPE is drawn at a line speed of 200 mpm by the machine. Over a distance of half a revolution of the 450 mm diameter roll the melt temperature drops from 315 to approximately 21 °C. The coating is now sufficiently cooled to be readily released from the roll by the haul-off equipment. The optimum release temperature will depend on the surface finish of the roll and the characteristics of the polymer. From Fig. 1.1.4 the coated web has a temperature ranging from ~21-23 °C at an ambient temperature of 20 °C. This virtual experiment demonstrates the very rapid heat exchange from the melt to the chill roll over this relatively short distance of travel.

Using the same process conditions and model the effect of varying the chill roll temperature was evaluated from 10 to 24 °C. The results are in Fig. 1.1.5. At 10 °C the release temperature drops to 14 °C and increases as shown with the increasing chill roll temperature. With the chill roll at 24 °C the interface is 27 °C. Smooth release is now unlikely. Deciding on the optimum temperature setting will be based on achieving excellent release and maximizing adhesion. Too rapid cooling may compromise the

ability of the hot melt to interact with the substrate and will reduce melt stress relaxation time. The result is weakened adhesion.

Fig. 1.1.5 COOLING AT 4 CHILL ROLL TEMPERATURES

In Fig. 1.1.6 the effectiveness of three chill roll diameters are compared. The LDPE at 315 °C was fed at 1000 kg/hr, and line speed was 400 mpm. The coating weight was 27 g/m² as in the previous simulations. Chill roll temperature was set at 18 °C in all cases. The cross cuts were taken at the separation point from the chill rolls.

Fig. 1.1.6 TEMPERATURE AT CROSS CUT FOR 3 CHILL ROLL SIZES

The interface temperatures show a difference of approximately 4 °C (21-25 °C) between the three chill rolls at the separation zone. These simulations are at extreme conditions operating at a line speed of 400 mpm. At these conditions the largest of the three chill rolls is preferred.

In Table 1.1.1, the 450 mm diameter roll is run at four temperature settings and two throughputs.