Calendering is King

Article from PLASTICS ENGINEERING January ’87 pp. 29-32

Calendering is still king for high-volume PVC sheet

Though calenders are expensive, calendering remains the Preferred method of producing large amounts of polyvinyl chloride sheet at high rate. Few new calenders are expected to be built in coming years, and most calenders now in use are relatively old. But they are continually being upgraded with new technology.

Stephen J. Gust
American Hoechst Corporation
Ottowa, Illinois
 

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The calendering process came into being in the 19th century with E.M. Chaffee’s patent for a multiple-roll device to make rubber sheet. Of course, the process, still used to manufacture

golf ball windings and elastic bands for clothing had to be tried on thermoplastics. PVC sheet was first successfully calendered in Germany in the 1930s. By the end of that decade, calendering was being used there for both rigid and plasticized compounds (the tough differentiation between the two types is that plasticized compounds contain at least 20-percent plasticizer). German Processors greatly expanded their production of rigid PVC sheet during the 1940s and 1956s, while U.S. companies of that era concentrated on plasticized PVC, moving slowly into calendering rigid PVC.

Today, calendering is practiced worldwide. with rigid PVC production approaching a billion pounds annually. Roughly 95 percent of all calendered production is PVC. But over the last 15 years, the total number of PVC calenders in operation in the United States has remained nearly constant. Only about 25 of the 150 PVC calenders now operating are used for rigid production. A few of the 150 are new, but many are the same ones that have been operating for all of the 15 years. Despite the age of current calender lines, major productivity increases and product quality changes have been made, for calendering technology has been forced by competition both at home and abroad to strive for better rates and quality.

 Figure 1.) Calenders, originally designed for use in the rubber industry, were adapted for use with thermoplastics such as PVC. "Globs" of fluxed resin are squeezed into thin, flat sheets at very high rates.
 

Why use a calender?

A calender forms molten Plastic into a homogeneous flat sheet. During the process, usually one and sometimes both surfaces are given a textured finish. Just to make the sheet flat and bring it down to the proper thickness, calenders have four or more rolls, each over two feet in diameter and over four feet in length (Fig.1). With a calender's massive size and weight comes a massive investment, which may lead manufacturers to consider any of a number of alternative processing meehods, Indeed, why use a calender at all if other methods are available?

Calendered sheet is usually made between 2 and 50 thousandths of an inch thick. Sheet may also be produced on an extruder followed by a polishing roll stack, or on a calenderette, which, like a polishing roll stack, has two or three methods (see the Table) indicates that calendering is the only option where high rates are required and less-than-perfect clarity is permissible.
 

The calendering process

At the beginning of the calendering process, the major raw materials in a formula are automatically weighed and added to a blender. Smaller amounts of materials, usually pigments and stabilizers, are masterbatched (combined with some of the larger additives for greater accuracy) ahead of time so they too can be automatically weighed and added.

Next, the blend is fed to a fluxing machine, a kneader for example, which "melts" it and drops it onto a holding mill from which a continuous strip is cut off to feed the calender's first bank. The bank is continuously formed into a rough sheet by the first nip (roll clearance). At the second nip, another lesser bank is formed and the sheet is thinned and widened. The sheet is taken off the last calender roll by a series of small stripper rolls and guided through a set of embossing rolls, and then over and under a series of cooling rolls, before finally being wound on a tube or cut into sheets for shipment. As simple as the sequence of operations may seem, each phase is complex and has evolved to its present state over many years. (See Fig. 2 for the entire process.)
 

Figure 2.) Most current calenders have been in use for over 15 years. Many that originally batch-processed rubber were adapted to make textured or laminated PVC sheet continuously and automatically from beginning to end.

The formulas

Because of the complexity of controlling a calender, most formulas are closely related to and even specifically designed for the type of calender and auxiliary equipment in the calender line. Ingredients are usually kept secret but tvpicallv fall into the following categories: PVC, impact modifiers, stabilizers, process aids, lubricants, and plasticizers.

PVC resin for rigid compounds, made either by the suspension (emulsion or colloid) or mass (bulk polymerization) processes, is best in the 1.7-to-2.0 relative-viscosity range. Homopolymer grades are used alone and in combination with 2 to 10 percent acetate copolymer.

Impact modifiers, the mini shock absorbers used to improve PVC's poor impact resistance. come from a large selection of ABS, MBS, CPE, or acrylic polvmers chosen on the basis of their impact efficiency, clarity, weatherability, and stress-whitening, as well as their processing characteristics.

Stabilizers are needed. When heated, PVC naturally tends to degrade, first by yellowing, then by turning dark brown and losing its physical properties. Stabilizers-added to a formula between 0.1 and 5 percent-retard degradation by tying up hydrochloric acid (HCL) generated by the heat of processing.

Many types of stabilizers are used. Metallic salts, mixed metal salts, organotins, and tin mercaptides are the major categories. The amount and type- and therefore the efficiency- of a stabilizer chosen for use in a particular calender line must be tempered by FDA and EPA requirements on the use of heavy metals. At the same time, processability must be balanced against cost. Even reactive problems with the stabilizer in processing or calendering must be accounted for. In spite of the fact that PVC is one of the least stable polymers, it enjoys one of the highest production volumes of any thermoplastic because of effective stabilization and the use of flow modifiers.

Process aids assist stabilization and increase the melt strength of the sheet during calendering and post processes such as thermoforming. Added to PVC formulas in small amounts, process aids help fluxing and reduce process temperatures, thereby decreasing the amount of expensive stabilizer needed. Acrylic process aids are used most because of their strength and versatility.

 

Figure 3.) In four- and five-roll designs, both top and bottom feed, the most  popular for PVC is the four-roll inverted L.

 

Lubricants are added to reduce the tendency of PVC to stick to the hot metal of mill and calender rolls. Lubricants also save on stabilizer use because they reduce frictional heat buildup between particles during fluxing and between molecules during calendering.

Lubricants are classified as either external or internal, depending on where they are most effective. Stearic acid, organic and inorganic stearates, and soaps of many kinds are mostly classified as internal lubricants. Paraffin and polyethylene waxes are almost entirely external, but montan waxes combine both internal and external properties. As little as 0. 1 to 0.5 percent lubricant is normal. Thicker sheet requires less lubrication because it generates less frictional heat on the calender.

Plasticizers make PVC more flexible but therebv lower melt viscosity, moduli, and transition temperatures. When plasticizers are used in amounts less than 5 percent. they are generally considered lubricants. If they are used in quantities greater than 20 percent, the formula is classified as flexible, that is, plasticized PVC. In either case, the effect on calenderability is nearly the same as lubricants. Two common plasticizer groups are phthalates such as diocryl phthalate and epoxides such as epoxidized soybean oil.
 

Blending

Blending the raw materials is one of the most underrated aspects of calendering technology. The reason, perhaps, is at U.S. manufacturers first concentrated on flexible compounds rather than rigid ones. Blending flexible compounds is done differently, usually in large batch blenders, first heated and then cooled for better absorption and storage of masterbatched plasticizer and pigment. So, when U.S. manufacturers finally under took compounding of rigid PVC formulas, there was a tendency not to spend capital dollars on the high-intensity mixers used extensively in Europe. Because of their high speed, not only do they blend thoroughly and quickly but fuse ingredients in place preventing separation. U.S. compounders had been relying on ribbon blenders, which are much less efficient. Banburys, from the Farrel Corp, were used to flux batch by batch and essential mixing was accomplished in the fluxing step. But with the newer continuous fluxing machines, little effective mixing is done after a compound leaves a blender.
 

Processing

Fluxing

Ingredients leave a blender as a freeflowing powder at room temperature, sometimes with regrind blended in. Fed to batch or continuous machines, the blends are formed into homogeneous melt streams under high pressure and temperatures around 150° C.
 

Heating and delivery

The output from a fluxing machine is seldom totally adequate as a direct feed to a calendar. Either a two-roll mill or an extruder is used to form strands, chunks, ribbons or other acceptable forms. A mill or an extruder, in series with the fluxing machine, does double duty in partially degassing the melt and serving as a reservoir to help prevent running calender rolls together-an expensive error in both repair and downtime if stock momentarily runs out. At this stage of the process, melt is maintained and delivered to the calender at around 140° to 160° C.
 

Calender rolls

The most common calender roll configurations, shown in Fig. 3, all squeeze a plastic melt into a flat sheet. The inverted L is a favorite for PVC.

In calendering, PVC tends follow the hotter and faster roll. The progression is, therefore, to have each roll hotter and faster than the previous one in the stack. Heated either by steam or hot oil, roll temperatures range from 150° C to 200° C.

Fluxed material delivered to the first calender nip is regulated to form a 6 inch-diameter rolling bank. The sheet passing the first nip forms another bank about 2 inches in diameter between the second and third rolls and so on until, at the final nip. the desired thickness is obtained from the smallest bank possible to minimize stress in the sheet.

Forces created by melt in the nip are sufficient to cause the calendar rolls to deflect affecting sheet profile adversely. And roll-separating forces are a function of roll speed, the type of roll surface, sheet thickness, and a compound's rheology. Making the rolls of forged steel that has twice the elastic modulus of cast iron helps minimize deflection but at a significant cost increase. Even with forged steel, further correction of roll contour is necessary to make flat sheet. There are three available means of adding extra correction: crowning, roll bending, and roll crossing. All can be used at once, if necessary.

Normally only the last roll is corrected by crowning: machining a bow of several thousandths of an inch along the length of the roll compensates for some of the bending.

Roll deflection from compound to compound may not be constant, and bending the last roll, bowing it in either a positive or negative direction in additional compensation, is accomplished by applying force outboard or inboard of the main bearings (bottom of Fig. 4).

Figure 4.) Roll crossing and roll blending are two methods that are used for correcting sheet profile.
 

Rotating the axis, skewing one roll with respect to another (top of Fig. 4). is called roll crossing. It increases a sheet's edge thickness while keeping the thickness at the center of the sheet constant.
 

Sheet takeoff and post-processing

Stripping. Thickness of sheet coming from the last nip is purposely oversize. This is to make up for as much as a 2/3 gage reduction that occurs when the sheet is stretched-from 30 to 150 percent-as it is stripped from the last calender roll by a series of at least two small rolls. Therefore, the design and operation of these stripping rolls affects shrinkage and flatness of the sheet.

Embossing and laminating. To create a leather-like grain or other surface, the sheet coming from the stripper rolls is passed between a textured steel roll nipped by a rubber roll that forces the PVC into the grain.

To laminate another sheet or a fabric to the calendared sheet, it may be nipped against the sheet on the last calender roll or a laminating station may be installed in place of the embossing rolls.

Cooling. Passing the sheet alternately over and under a series of cooling rolls brings it to room temperature. The sheet is then trimmed to width and wound on a tube or cut to length and stacked as sheets. Edge trim. taken at this stage, is recycled.
 

Improving the system

Because of the extremely high cost of building new calender lines, it is doubtful that there will be a large number of new lines forthcoming without more high-volume products. Immediate efforts will be directed toward increasing quality and consistency coming from present calendars through better blending, improving fluxing and delivery methods, and updating calender and takeoff process controls.

The more consistent the input, the more smoothly the process runs and the more consistent and reliable is the output. Closed-loop weighing and blending is now becoming the norm and with improved blending available, present fluxing systems become adequate. But providing a more uniform feed to the calender remains a challenge.

Stronger materials and better designs could result in newer calenders with smaller rolls and more precise bearings. In the main, however, existing calenders will be overhauled and provided with better drives and bearings.

Closed-loop thickness control is currently practiced but is still being refined as improving sensor technology for feedback control becomes available. On-line defects sensed by lasers, for example, will enable 100 percent inspection as quality requirements will, in general, continue to tighten.

With those improvements already in place and others constantly on the way to meet growing competition, calendering will remain the most efficient way to make high-volume PVC products.

 

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