More About Paperboard

This blog is a follow-up to the “6 Steps to Making Paperboard” blog.  We will briefly discuss the types of wood used to make virgin paperboard, the most important characteristics of paperboard, and the properties of the various grades of paperboard.

 

Wood

Virgin paperboard can be made from two types of wood – softwood from pine trees and hardwood from deciduous trees (lose their leaves every year). Each has different characteristics that lend themselves to different applications.  The primary difference is that softwood has longer fibers than hardwood does.  Those long fibers lend themselves to better tear resistance and better stiffness since the longer fibers have more surface area to bond together.

 

 

However, hardwood’s short fibers lend themselves to a smoother surface because, if on occasion fibers do not lay completely flat on one another, they stick up slightly toward the surface of the sheet.  Short fibers stick up less than long fibers do, resulting in a smoother sheet.  All coated paperboard is smooth because of the coating applied to the top of the sheet, but paperboard from short fibers is smoother.

 

The eight most important characteristics of paperboard:

1. Machine direction (MD)– the direction in which the fibers naturally line up in the paperboard making process, effects stiffness and tear strength.

2. Cross direction (CD)– perpendicular to the machine direction, also impacting stiffness and tear strength.

3. Smoothness– how smooth the sheet is, affecting its printability.

4. Tear strength– long fibers “grab” each other better than short fibers. For example, beverage can cartons need strong handles that won’t tear easily.  Tear strength is greater in the cross direction (across the fibers) than in the machine direction (parallel to the fibers).

5. Stiffness– long fibers provide more stiffness, again because of the greater surface area. Stiffness is important in gluing cartons and in high-speed filling because those operations stress the cartons.  Stiffness is greater in the cross direction (across the fibers) than in the machine direction (parallel to the fibers).

6. Basis weight– the weight of 1,000 square feet of paperboard.

7. Caliper– the thickness of paperboard in thousandths of an inch. Typically paperboard for folding cartons ranges from .0014” to .026”.

8. Brightness– how white and bright the sheet is.

 

Six Steps to Making Paperboard

Step 1 – Pulping

Pulping is the process of reducing wood or old paper and paperboard down to the fiber level.  Think of fibers as small splinters.

For virgin paperboard (paperboard that is made from wood), pulping is accomplished by “chipping” logs down to wood chips and then pressure cooking those chips in a solution that breaks down the wood’s natural glue (called lignin).

Once the chips are reduced to the fiber level they become pulp.

The pulp is washed to remove impurities and, depending on the type of paperboard being made (white or natural brown), it may be bleached.  For environmental reasons the bleach is made from oxygen and hydrogen peroxide, not chlorine.

For recycled paperboard the source of fibers is old paper, corrugated boxes and paperboard.  The old materials are mixed with water in a pulper (a machine that looks like a very large blender).

The blades in the pulper beat on the material until it is reduced to fibers. The recycled pulp gets washed to remove impurities like staples, wires, tape and paperclips.  The larger impurities are removed mechanically from the solution that resembles oatmeal.  To clean the smaller ones out, the solution passes through a series of screens.  At this point, when fibers have been cleaned, they are called pulp.

 

 

Step 2 – Forming

It can be helpful to think of paperboard manufacturing as reducing the wood or recycled material down to the fiber level, then reassembling those fibers again.

Regardless of whether the pulp solution is made of recycled or virgin fibers it is pumped into the headbox at the front of the machine.  The headbox is a container the width of the machine that collects the pulp solution and then pumps it out evenly through a long orifice onto a moving piece of fabric, similar to a conveyor belt, traveling at 1,000 feet per minute or more.  The amount of pulp solution that is pumped out of the headbox determines the paperboard’s weight and is one of the determinants of the paperboard’s caliper or thickness.

The pulp solution is between 98 – 99.5% water as it leaves the headbox.  In essence, the paperboard machine’s primary purpose is to remove water from the pulp solution as the fibers travel down the machine.  Water falls through the fabric as the solution travels down the fabric conveyor. More water can be removed by vacuum boxes located underneath the fabric, so that by the end of the forming section the material is down to 75% – 90% water and is able to support itself.  It is now referred to as a web.

 

Step 3 – Pressing

The web now leaves the fabric and transfers to the press section of the machine. The press section is a series of top and bottom cylinders the width of the machine.  As the web travels through this section more water is squeezed out so that by the end of the section it is down to approximately 65% water.

 

Step 4 – Drying

The drying section is the longest part of a machine that can easily be 100 yards or more from beginning to end.  It is a series of many (over 100) steam heated cylinders over and under which the web passes.  Each cylinder removes a little more moisture so that by the end of the drying section the web’s moisture content is down to only 6% or 7%.

 

Step 5 – Coating

A clay coating (essentially latex house paint) is applied to the web to smooth out the surface, provide whiteness, and to ensure that ink stays on the surface of the sheet during printing.  A coating roller the width of the machine dips into a vat of coating and then contacts and transfers that coating to the top side of the web.  A smooth rod also the width of the machine wipes away the excess coating and leaves what had been a somewhat rough paperboard surface, smooth.

After a quick pass through an infrared dryer another roller applies a second coat of the coating.  The second coat provides more whiteness.  This time instead of a rod wiping away the excess, an air knife blows it off.

 

Step 6 –Finishing

The last steps are to wind the web onto a large reel and then slit the reel down to the desired roll widths.  Some machines are more than 200 inches wide, so the reel can weigh in at 25 tons and yield several rolls almost 7 feet tall.

 

For more information…

https://www.youtube.com/watch?v=QDNJRfgGNsA

https://www.youtube.com/watch?v=E4C3X26dxbM