A Comparison of the Advantages and Disadvantages of Roll Coating versus Die Coating of Hot Melt Adhesives. by Bob Dages

Transcription

1 A Comparison of the Advantages and Disadvantages of Roll Coating versus Die Coating of Hot Melt Adhesives by Bob Dages...~... Thank you for the opportunity to discuss the relative merits of the two coating application methods that are named in the title of the presentation. As noted, my name is Bob Dages, and I work for an OEM of coating and laminating equipment, Kroenert Corporation. In my work, I have noticed a recent increased interest in hot melt coating applications, This surge in interest can be attributed to a few factors: First, improvements in the chemistry of UV cross-linked hot melts have expande d the range of applications for this product. Second, producers are looking for methods of incrementally expanding capacity or serving niche markets without accepting the large commitments of capital that are typically associated with solution adhesives. A third driving force is the manufacturers' desire for the elimination of thermal inputs and solvent handling considerations from their operations for health, safety and environmental reasons. As an OEM, we are often asked to recommend the best alternative for a given application or to provide information for the buyer to make an informed decision. Consequently, we often / provide the prospective buyer with alternative proposals. From this perspective, we have compiled the information that is presented today. My co-presenter, Jeff Seckora of EDI, will highlight the areas of performance and specifications for the die portion of the comparison. I will provide the roll coating considerations. First, we will look at the basics of the systems to insure that we are using similar vocabulary and providing the overview knowledge of the systems that is required for discussion, 177

2 A coating system is in line layout concept, illustrated in drawing 1.0 located at the end of the text. This particular system shows the arrangement of a die applicator. Please note that the only difference with the roll coater is in the coating area only. All other line components are identical. -:. The components of flexible lip die are described as follows and are indicated in drawing 2.0. The die coater consists of a body that is divided into top and bottom halves, with removable lip inserts that can be manipulated to provide optimum cross profiling by means of a flexible lip adjuster mechanism. The removable lips can provide flexibility for differing rheologies and to correct for wear in the lips. The internal cavity of the die includes a coat hanger-style manifold that distributes the fluid evenly behind the lips of the die. The die halves can be offset by means of an adjustment to improve the coating appearance. The die is heated entirely, by means of either liquid or electrical heating mechanisms. The die is mounted opposing a rubber roll that is cored for heating. This roll provides a solid backing against which the coating is applied and the grinding tolerances of this roll are critical to maintaining good coat weight control. Polymer-covered rolls are used to eliminate the thermal expansion inconsistencies that would be present in the steel roll and to absorb the high pressures formed at the point of application. 178

3 Ultracoat Production Slot Die Coating Head Body materials: stainless steel or Flex lip slot / steel with plating adjustment '!U I/ i il S :_ :,, Flexible lip--~,~.eoova,o lip inserts ~ I t lli.-: Coathang ---, :~"~" Liquid or electri temperature d control -.,~" 9" (230mm) ~,.~ entrance i Drawing 2.0 The die system includes a support carriage that is used to precisely position the die for application of the coating, retract the die for cleaning or inspection and it isolates the die from vibrations that may influence the performance of the die. The attack angle of the die to the substrate can also be varied through the carriage. Both the roll coater and the die coater require substantial equipment behind the application area in order to melt and transfer the coating to the applicators. The die coating must be supplied in a pulse-free manner through a positive displacement pump or by the extrusion.?. method which is becoming more common for hot melt adhesives. The roll coater builds a reservoir of coating above the application point and can withstand some variation in the supply rate. The roll coating equipment can be observed by exploring the different methods available for coating application. A detailed look at the roll coater reveals the following application ). methods can be used within the concept of "roll coating". 179

4 The first coating method shown is capable of direct roll coating (Fig. 1.1). In this method, the coating is transferred directly from the applicator roll to the web with the direction of rotation of the transfer roll in the same direction as the web travel.,i :. ::.,:. j..". ;~:... D jffer-.ential SUPPORT ROLL Figure 1.1 The roll train consists of an applicator roll, a metering roll, a backing roll and a support roll. Characteristics of each roll are described in Table 1.1. In narrow machines, the support roll can be omitted from the machine as the deflection in the backing roll is minimal. The second common method is the reverse roll coating technique indicated in Figure 1.2. This method provides for the coating to be applied to the web from an applicator roll that is rotating in opposition to the web travel. This can provide some surface improvement in the coming. RE VERSE ROLL SUPPOtPT ROLL Figure 1.2 Each of the roll positions, generic constructions and functions, is summarized in Table 1.1 and Figure

5 Roll No. Roll Description Drive System Speed Ratio % Roll Temp, 1 Adjustable 2 Adjustable to #1 3 Fixed Position 4 Adjustable Rubber Backing ' Roll Steel Back-Up Roll Steel Applicator Roll Steel Metering Roll Fine Variator Gear Box. Usually Driven from Chill Roll Not Required Regenerative DC Motor Worm Gear and AC Motor 95-1o0% See A and B below Stationary - has drive to advance for cleaning.purp. Chilling ~ Recommend ed Up to 460o F" Up to 4600 F,'.~. < Table 1.1 A. Applicator roll speed can be varied up to 10 times of the backing roll speed. B. Applicator roll runs same speed or 1% faster than backing roll. 4 3 tj. Figure 1.3 WEB The various application parameters of each method will now be explored. It should be noted that each parameter described is complementary to the other parameters and creates an operating window for the technique described. For example, a system coating 15 lbs. per ream of 10,000 cps material at 1,300 fpm may sacrifice some speed for additional coat weight or a 181

6 higher viscosity. The die is capable of delivering a coating thickness range of 0.75 mil up to 10 mils. This translates into 11 lbs./ream, up to 140 lbs. per ream. Viscosity ranges for the use of die coating can vary from low-end applications of hot melts at 500 cps up to 100,000 cps. The speed of 1,800 fpm has been achieved for the die performance on hot melts. Typical speeds are in the 300 to 1,000 fpm range. The roll coater techniques are substantially different in the delivery of coat weight to the substrate. The direct method can be used to coat as low as 4 lbs. per ream, up to 60 lbs. per ream. The reverse roll method is capable of coating higher coat weights, but suffers at the low end of the coat weightrange. It can apply from 15 lbs. per ream up to 310 lbs. per ream. The roll coater is capable of delivering the hot melts and resins from 2,000 cps up to 250,000 cps. The lower end is determined by the ability of the coating material to be retained within the edge dams and the coating nip. It has been observed that higher viscosity coatings have been delivered at very reduced speeds. We are aware of an application of 500, 000 cps materials at 20 fpm. The roll coating techniques have been successfully operated at speeds of 1,200 fpm. Often, the limiting factor is the ability of the operation to practically melt and supply the hot melt at high application rates- some combination of width, speed and coat weight. Also, a match of the hot melt application capacity with downstream processes often leads the operation into the purchase of a process that is run at less than the maximum operating speed. For example, a 48-inch wide hot melt PSA coater and laminator can supply several narrow width printers. Roll coating can achieve a machine direction stripe pattern by an indirect roll transfer capability. This method is indicated in Figure 1.4 and uses deckling wipes that are positioned and sized according to the desired stripe. The coating is wiped from the intermediate transfer roller and transferred to the applicator in the direct reflection of the desired stripe pattern. The 182

7 applicator roll then transfers the hot melt to the web. Edge deckling uses the same wipes as the hopper box end seals and adjusts the deckling by moving the end seals on a frame mounted over the hopper box. H...,,,...,,!... ~-,,..;...,,".... L... /..~,.~. ~""-,...,... ~r "~, ~, "",,~:.. Figure 1.4 ',,._LJ 4... ~... : "... DtR:CT " ' E,,'~ DOLL -::- ============================ ~. D / FFE P, EN Ti A L S P EEO STRIPE COA TING L~A. h"f,/b ' E_ R : R O L -.:.i...'17 <. The die coater can accomplish stripe coating by one of four methods. The first possibility incorporates the use of an external rake - the pattern of the hot melt reflecting the geometry of the rake. The rake is positioned over die lips, as shown in Figure 1.5. The second method requires internal shimming that blinds the outlet of the die and provides for uncoated areas at the blinded lanes. The removable lips can also be machined for lane coating or, finally, an internal rake can be installed in the die before the lips. For deckling, the die uses internal shims to blind the end of the application area of the die. This can be motorized with the device shown in Figure 1.6. This device drives the shim to the edge point of the desired deckle width. Because of the geometry of the device, a minimum deckle reduction of 25% is suggested in order to minimize space problems in the line arrangement. 183

8 A. RAKE-STYLE DECKLING B. SHIM-STYLE DECKLING Figure ! a(c(rvlk PLAT(-.J / r~lgt(x e~:~l~g S(AL ~sh PLAT(---' -).~--O( ~L( rt/~ (~ INTERNAL DECKEL Figure 1.6 In any operation, it is desirable to maintain a high degree of cleanliness in order to prevent contamination in the product. Given the nature of the process of hot melt coating, it is very wise to give careful consideration to cleaning and process changes. Cleaning of the roll coater can be made easier by release coating all wetted surfaces. When discontinuing operation for a weekend, the rolls and hopper should be run as dry as possible, with the final residual captured onto a catch pan. The pan can be employed into the gap between the applicator roll and the backing roll when the backing roll is lowered and captures the material between the applicator and metering rolls that are opened when cleaning. The applicator roll and metering rolls are then started at a slow speed and at temperature against a 184

9 scraper, with the scrapings falling into the above-mentioned catch pan. An experienced operator can accomplish this clean up in 60 minutes. Consequent to this arrangement, any major clean up of the equipment at a cold temperature has to be accomplished by mechanical power washing or other abrasive means. The die cleaning involves splitting the die by removing the bolts that attach the two halves to one another. The top half is then removed away from the bottom half, and the internal of both halves can be scraped clean. The removable lips can also be cleaned off-line with minimal interruption of the process. Estimated breakdown of the die components, clean up and reassembly is approximately 3 hours. It should be noted that heat up of the die and peripheral equipment can take up to two hours to reach a soaked condition. < Changing the coating media chemistry is similar to a cleanup or can be painless - depending on the compatibility of the coatings. This approach will be true of either the die or the roll coater. Greater problems typically are encountered in the feed system (everything behind the applicator system) than at the point of application when changing from one coating to the other. The die coating thickness changes can be accomplished in by two methods. The first method accomplishes the weight change by the line speed relative to the flow rate of the coating from the die. Increasing the relative speed stretches the film andreduces the coating thickness. Alternately, the reduction of the web speed relative to a constant flow rate of coating will increase the film thickness. The same phenomena can be achieved by changing the flow rate of i.:.. the coating relative to a fixed web speed. A roll coater coat weight change employs the same principles. The gap between the metering roll and applicator roll controls the application amount onto the applicator roll. The applicator roll can then be varied in speed relative to the web speed to give coat weight changes. The lower limit of the coat weights of the roll coater is limited by the safe gap distance of

10 mm between the applicator and metering roll. The qualifier "safe" describes the need to maintain a suitable gap in order to sustain an even laydown of the coating. When running in the reverse roll mode, the speed differential between the applicator roll and the web is minimal with only a 1% overspeed as typical. This reduces the options of coat weight changes to only the gap changing method. Direct roll coating allows for a speed differential of the applicator roll running up to 10 times the web speed. This allows the direct method to be operated at much lower coat weights. Some polishing of the coating surface can occur due to the differential in speeds and this limits the manipulation of coat weight by speed differential. An advantage of this polishing effect can be improved surface characteristics of the coated hot melt..., Closed loop control can be accomplished by either speeding up or slowing down the delivery rate of the material to the die, as dictated by a coating thickness/weight change measurement device. The rate of application can also be automatically changed by speeding or slowing the line speed relative to the application rate. Both of these methods of closed loop weight control are subject to the limitations within a relevant range of line speed and coating weight or thickness. The closed loop control should only be used for small weight changes after the gross target is achieved. The other parameters of the line operation could become an unstable "slave" to the widely varying weight changes. Closed loop control of a roll coater is accomplished by varying the gap between the applicator and metering rolls or by changing the relative speeds between the applicator roll and the web speed, as dictated by a coating weight sensor. Again, caution should be exercised when controlling the application rate across a wide range of coat weights. Cross web coating thickness variations of a die can be minimized by use of the flex lip adjustment device. The lip gap is adjusted by varying a pressure application to the lip in various zones. The pressure is achieved by changing the temperature in a heating block that is mounted 186

11 above the lip. The thermal expansion and contraction of the blocks modulates the gap opening. This adjustment is used with a coating thickness scanning gauge. The roll coater uses a roll skewing device that varies cross machine the gap relationship between the applicator and metering rolls. The skewing is accomplished by tilting the metering roll out of level. This method cannot be zone controlled or coupled with a gauge and can give only an averaged improvement over the width of the material.. :-. i The capital cost differential of a die versus a roll coater are negligible. This is particularly true when measured against the cost of an entire process. The roll coater employs the same web handling strategy, cooling/curing system and must deliver the coating to the web through a heated medium. The die system employs a backing roll that should be cored for heating or L cooling. Gap adjustment is common to both systems. The roll coating system's greater expense can be attributed to the additional rolls, the drives associated with the rolls, a hopper box and slightly more complex heating system. The actual difference- as a percentage of total investment in the processing line- is approximately 4%, with the die being the slightly cheaper approach. Variations, such as an additional smoothing bar following the die or a complex deckling system, can offset the higher price of the roll coater. The operating costs follow much the same pattern with the major determinant being the production profile of a given user. Both systems utilize nearly the same amount of coating, and timing of the changeovers slightly favors the roll coaters. The amount of power consumption is :>, very similar, with the extra roll drives of the roll coater adding some small expense. Material consumption and yields would be significant only when the particular operation has characteristics in its product mix that suggest the operation should favor the use of one system over the other. These differences are summarized in the Table

12 i,-, : _. ; ROLL VS. DIE HOT MELT COAT NG ROLL COATING DIE COATING...~ COAT WEIGHT VISCOSITY WIDTHS GAP COATING FINISH SHORT RUNS COAT WT. TOLERANCE ON 20 GSM STRIPE COATING CAPITAL COST gsm 500 gsm possible 2, ,000 cps up to 2.1 m 0.4 mm equal - more easily cleaned +/- 5% gsm ,000 cps up to 3.5 m 20 micron equal - +/- 6% manual CD control +/- 3% automatic CD control yes yes 5% less SPEED 370 m/min. 550 m/min. OPERATING COST less for short runs same for long run narrow product max. applications Table 1.2 The following line schematics illustrate the typical arrangements of the coating methods. Note that on the an:angements, the coated surface is always exiting the application area, with the coating facing the operator for inspection purposes. Also, please note the systems are compatible for use with either cooling only and with the UV curing. The web should be oriented with the UV source above the coated web so tension losses do not result in the coating smearing the UV sources. 188

13 ,,,==.~..... o -,'! % ~,, -r~ " try.: i i =,, , 1 2 m I Drawing 1.0 (- -I -1 t..,17.:2 i 83LSr-.,~.7;-3- ~ Drawing 3. 0 Finally, designs are available where the roll coater and the die coater can be incorporated into a single process line when a broad range of product mix or product development needs dictate both methods should be considered. ACKNOWLEDGEMENT In closing, I would like to acknowledge the considerable input from the persons at MMK, the Germany parent company of Kroenert Corporation, and thank my co-presenter, Jeff Seckura of EDI, for providing the information about the extrusion dies. 189