Direct Flame Method at Weyco Mill

Direct-Flame Method 
Improves Board Surface Properties 
at Weyco Mill

Longview, Wash.,bleached paperboard mill uses direct-flame system for both pre- and posttreatment for better quality, safer operation
By Clive Granville
reprinted from PULP & PAPER 1991

It is well-known that surface properties of substrates such as bleached paperboard must be modified before they can be coated, laminated, or printed on. Weyerhaeuser Co.’s Pulp & Paperboard Div. in Longview, Wash., has long employed various standard methods to accomplish this task.

The mill produces laminated bleached paperboard for sale to converters in the Pacific Northwest, South America, and the Pacific Rim. The end products are milk and juice cartons, and the company is one of the world’s top suppliers.

However, when the mill’s surface treatment methods did not perform optimally, extruder superintendent Jim Loren and a group of experts from the mill began to investigate modernizing the current method or switching to a new system.

Weyco’s Surface Treatment System. Paperboard surface treatment occurs at two points–prior to application of the laminate to assure proper chemical bond and after application to permit printing on the carton exterior. For many years, Longview’s pretreater has been a direct-flame system supplied by Flynn Burner, New Rochelle, N.Y. The conventional Flynn system, consisting of a gas burner, control cabinet, and piping apparatus, applies flame directly to both sides of the substrate as it moves through the web press at 900 fpm.

Applied consistently and uniformly across the 10-ft web, the flame burns off fibers and, by oxidating the surface, increases its polarity so that a chemical bond is established between the substrate and the film. Immediately after the film is applied in an extrusion process, the web moves through a chill roll that reduces the temperature from over 600°F to room temperature.

Applied consistently and uniformly across the 10-ft web, the flame burns off fibers and, by oxidating the surface, increases its polarity so that a chemical bond is established between the substrate and the film. Immediately after the film is applied in an extrusion process, the web moves through a chill roll that reduces the temperature from over 600°F to room temperature.

Next the web moves into a posttreatment stage of the exterior only, preparing it to accept the printing inks applies by the converter. Finally, the web passes through a sophisticated defect detection system and on to rewind.

A host of variables impacts the quality and cost factors of the product. The film, primarily low-density polyethylene, is the most expensive component of the operation, and the mill constantly battles waste and, of course, rejection by the customer, often due to leakage. Some customers are so demanding that they may reject an entire shipment after detecting holes in one roll.

Although the basic chemical reaction of posttreatment is the same as during pretreatment, the direct-flame method had until recently been employed only at pretreatment. The generally accepted method for post-treatment had been via the corona discharge technique. Using this method, the desired surface alteration is achieved electrically, with a voltage buildup in an electrode ionizing the air in a gap between the electrode and the film.

Analyzing Alternate Treatment Methods. Post-treatment at Longview was in the corona discharge mode after installation of the two extrusion lines because it seemed the best way to cope with the conditions affecting application of polyolefins to paper and other substrates. Still, Loren said various factors were always troubling. In fact, he said that corona treating in extrusion coating is consistently one of the major topics at industry gatherings, “the one thing that everybody is not satisfied with.”

So in 1987, as the system on one of Longview’s two machines began to show normal signs of wear, Loren analyzed the system’s performance and investigated what the marketplace offered. Three options were considered by the mill:

• Replace the covered-roll corona discharge treater with an updated version.
• Switch to a bare-roll corona, which was in use on the other machine.
• Go to direct-flame treatment at both ends of the line.

Even to consider the last option was risky, according to conventional knowledge at the time. Direct flame was fine to prepare the web for the film, but to use it as a film treatment was not normally considered a viable option.

Loren instructed a group of experts within the company to scrutinize all options, forgetting preconceived notions about what was and wasn’t done, in favor of taking an objective look at where improvements could be made.

An extruder is a costly system that is replaced only after much consideration, but, as with many systems, constant evaluation of upgrades will help keep a company competitive. In fact, it was a major upgrade to the production line–the addition of the laser defect detector–that prompted the study of the corona discharge system. The new station, an Intec 5000, was to be installed just after posttreatment to perform 100% inspection of the web. It required considerable space.

With this imperative added to the aging of the corona discharge unit, Loren and the group of experts had ample reason to review experience and examine the latest offerings of vendors.

The first option–to install a bare-roll corona–was almost immediately rejected, even though it was said to be superior to the covered roll. “We had a bare roll on the other machine and were never too happy with it,” Loren said. “We had lots of quality problems.” The bare-roll corona system requires too much energy and blows too many pinholes and burnholes ( In the mill’s terminology, a pinhole occurs whenever there is a disruption of the surface. A wet spot is the usual result. A burnhole acts like a shotgun pellet, making a hole through the substrate and through the film on both sides. This eventually produces a leaky carton.).

The group then examined the covered-roll corona method but found several quality, health, and safety problems. “First, it’s a real bear to get uniform treatment across the sheet. One side is always much lower, no matter what you do to level it.”

A second quality factor concerns control. Most of the time, Longview’s corona treater operates at maximum output of voltage and amperage, so if the treatment “goes away” it is difficult to determine what has happened or how to cope with it.

The experience with the Flynn direct-flame system showed that it delivered a more uniform surface treatment, with a flame quality that produced a more consistent result. Since it is usually operated at about 75% of capacity, adjustment up or down to change treatment is at the touch of a dial. And the direct-flame method doesn’t produce pinholes or burnholes. 

Even those factors might not have been enough to cause the mill to consider using direct-flame treatment of laminated webs if not for two major safety and health factors. Loren pointed out that a corona treater “can be a tremendous hazard because it uses 10,000 volts.” In fact, one of the mill employees had been injured due to the treater.

The corona treater also produces ozone. Such by-products of industrial processes are currently of great concern. One other factor also was considered–with corona treatment, the machine couldn’t handle aluminum foil, obviously a major drawback. Such problems would not occur with direct-flame treatment.

Taking The Direct-Flame Approach. All the quality and safety factors caused the mill team to take a more serious look at the direct-flame approach, especially since Flynn Burner had been working to advance the technology. As noted earlier, the basic chemistry is the same at both ends of the production line. Whether in treating the surface of the substrate before application of film or after, to permit inking, the primary concern is maintaining uniform flame quality.

Although the many variables impacting this method are highly technical, a number of basic points can be made for the nontechnician. Assuring proper surface treatment requires definition of the key parameters and then control of those parameters as precisely as possible. The key parameters follow:

• Air-gas ratio
• Burner and web geometry
• Mixture flow rate
• Film chemistry
• Ambient conditions.

In a system focused on moving giant rolls of paper through a web press at 900 fpm, keeping track of such potentially volatile elements requires more than just human analysis. Flynn Burner concentrated much of its efforts on the questions of control.

For example, to deal with the critical air-gas ratio, it is vital to analyze and control what is known as plasma, an electrically neutral gas created when flame reaches high temperatures. After a number of years of experience and research, the company decided that the most practical method of controlling this ratio is through the flame quality.

Flynn developed the Plasma Analyzer, a microprocessor-driven unit that simulates flame quality at the surface of the web as it passes through the flame station. Taking a continuous small sample of the air-gas mixture, the analyzer produces a controlled flame in a closed chamber. The flame plasma generates an electrical signal, which is processed to produce the plasm value and a readout of key data.

Refinement of such techniques proved to Loren’s team that Flynn Burner’s method could handle the mill’s needs. Therefore, in June 1988, Weyerhaeuser began using a method of posttreatment that no other mill have ever used. A posttreater, encompassing a specially designed, burner, went on line within a week (Figure 1). After six months and some redesign, the bugs of this prototype system were worked out to Weyerhaeuser’s satisfaction. Loren summed up the system installation with a word of caution: “We’re happy and plan to put it on the other machine. However, other mills must evaluate it in their own situation.” 

Figure 1: Basic elements of a typical direct-flame surface treater. The Weyerhaeuser burner is positioned somewhat differently, but the basic concept is the same