WHAT IS KRAFT PAPER?

Kraft paper was named after the German word kraft, meaning "strength," which refers to a specific type of pulping process that ensures strong paper. Developed in 1879 by Carl F. Dahl, a patent was issued five years later, and the first kraft paper-making company opened its doors in Sweden in 1890. Combined with the invention of the recovery boiler in the early 1930s, kraft paper production entered a new, more efficient and environmentally-friendly stage.

With the recovery boiler, kraft paper manufacturers now reach a nearly closed-loop production cycle, meaning they recover and reuse almost all of the water and major chemicals used in the kraft pulping process. By the 1940s, kraft paper producers surpassed traditional sulfite process manufacturers, making them the dominant manufacturing type for wood pulp in the world.

Kraft paper is durable, a machine-made paper that's made mostly from wood pulp. Unlike other papers, kraft can use all types of wood and non-wood for manufacture, including resinous pine, bamboo, and agri-residues.

Kraft paper products are known for their thickness and strength and, consequently, are coarse, have a high tear resistance and contain highly-oriented fibers. There are benefits to packaging with kraft paper. Since it is lightweight, kraft wrapping paper keeps shipping costs down. It also offers better protection than many other packing materials, thanks to its coarse texture, durability, and flexibility. Kraft papers used in multiple ways across the globe because of these including:

• Paper sacks for industrial materials such as cement and chemicals
• Flour and food bags at the grocery stores
• Multiwall sacks
• Envelopes
• Paper bags
• Corrugated layered cardboard

When unbleached, you can find it most often as a wrapper or packaging material. Kraft paper makers can make products that are watermarked, striped, or calendared and kraft paper can be printed on as well.

Brown kraft paper is normally made from unbleached kraft pulp. A white kraft paper, by contrast, is normally made from bleached kraft pulp.

Since bleaching weakens paper, kraft papermaking companies make their strongest products in the natural brown color.

LET'S TAKE A LOOK AT HOW KRAFT PAPER IS MADE

RAW MATERIALS FOR MAKING KRAFT PAPER:

Fibers

Softwood and hardwood can be used for making kraft paper. Kraft paper can also use non-wood agri-residues and waste paper in the manufacturing process.

Chemicals

Pulp cooking chemicals are also called "white liquor." This mixture has active ingredients, like NaOH and Na2S, that are used for cooking the wood chips in the digester for pulp production. While there isn't a hard and fast rule on the ratio of chemicals, in general, 25 to 35% sulfidity is maintained. Any lower, especially below 15%, affects the reaction rate and pulp quality of the resulting products.

KRAFT PULPING PROCESS

The kraft process, sometimes called "Kraft Pulping" or the "Sulfate Process," is the process by which wood is turned into wood pulp, the main component of the paper. Wood chips are treated with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S) in large pressure vessels called digesters. These large devices can hold 10-20 tons of pulp and come in two processing types: batch and continuous. This chemical mixture breaks down the bonds that link lignin, hemicellulose, and cellulose. One of the major benefits of kraft pulping is that roughly 90% of the pulping chemicals can be recovered and reused in an economical manner. This makes the process more sustainable and affordable at the same time.

In general, "kraft pulp" and "sulfate pulp" are synonymous but many countries around the world deem the two terms different based on the pulp manufacturing process. In these countries, the paper is also sold using different names to mark the subtle differences in production, and thus the end products.

TYPES OF KRAFT PAPER

Here are a few types of kraft paper and how they might meet your business needs.

A. UNBLEACHED KRAFT PAPERS

Virgin Natural Kraft Paper

Virgin natural kraft paper is clean, durable and affordable. This type of paper is most often used for heavy-duty applications where high tear resistance and superior bursting strength are critical. It can be printed on as well, making it perfect for branding, protective layering and other instances where it needs to be strong and bear certain markings or logos. Virgin kraft can contain up to 5% recycled materials.

Virgin natural kraft paper can be used for wrapping, book covers, paint masking, carrier sheets, stationery products, floor protection liners, and pallet interleaving, to name a few applications. Layers of the paper are used to make corrugated boxes.

Recycled Kraft Paper

Recycled kraft paper is not as strong as natural virgin kraft, but it is a very eco-friendly option and costs less than its non-recycled counterpart. Compared to natural Kraft, recycled Kraft has a lower tear and bursting strength and breaks down more easily when exposed to friction or moisture. It is more economical and offers many beneficial qualities.

This type of paper is used for bundling and stuffing, including as newspaper bottom wrap, garment underlay paper, textile wrappers, interleave, internal carton packaging, void filler or dunnage, dust covers, floor protection liners, carrier sheets, paint masking, and box/tray liners.

Black Kraft Paper

With its deep color, black kraft is one of the most popular papers. You've seen seen black kraft paper used as backing for photo frames, and it's a very popular choice for a variety of craft applications.

Colored Kraft Paper

Also, a favorite among crafters, colored kraft paper is seen in everything from elementary school supplies to scrapbooks, to bulletin boards. This is more often recognized as "crafter's construction" paper.

Printed Kraft Paper

Because it is so versatile, custom printed kraft paper is used in many industries for branded wrapping, packaging, and in-store displays. Delis use this paper for wrapping sandwiches in paper bearing their logo, while many fashion manufacturers ship their garments with printed kraft paper sleeves or sheets between individual articles.

Steel Interleaving Kraft Paper

Part of the stainless-steel milling process involved cold-rolling on interleaving kraft paper. This gives the steel its signature shine.

Absorbent Kraft Paper

Absorbent kraft paper is used in a wide variety of applications where exposure to moisture is present. You'll find this type of kraft paper in building construction and furniture. Absorbent kraft paper features high wet strength, high porosity, and can be used as filter media and laboratory filter paper. Other application includes special bags for plantation of saplings.

Spinning Kraft Paper

Some papers can be made flexible enough for use in weaving. Spun kraft paper can be used in wicker furniture, cloth making, rugs, and even yarn. When waxed, spun kraft paper can even be used in matches!

Electrotechnical Kraft Paper

With our love of electronics, we wouldn't get far without electrotechnical kraft paper. Designed to be non-conductive by unique cleaning process for removal of impurities and charged ions from the pulp, electrotechnical kraft is used for insulating cables and electrical components.

Super Calendared Kraft Paper / Release Base Paper

This dense paper is designed to hold up to silicone coatings where a label-release is also required. It can also be cut very precisely using a die.

Sack Kraft paper

A sack paper is a porous paper made in kraft paper plants from any combination of fibers. This paper features high elasticity and high tear resistance and is designed for packaging products that need strength and durability.

Sack kraft paper strikes the perfect balance between strength and porosity, making it ideal for valve sacks for commodities like cement and other powdered materials. To impart sack kraft with barrier properties, it is provided PE coating.

Foaming Kraft paper

Foaming kraft is designed to release smoothly and easily from the object it covers and is used in applications with polyester and polyether foams. MG foaming kraft paper is mainly used in the polyurethane industry for excellent performance in leading foam lines where the glossy surface of MG kraft paper offers excellent chemical resistance. The density of the foaming kraft paper is high and the porosity low which prevents penetration of the chemicals. The glazed side prevents too much chemical penetration and makes it easier to remove the paper from the dry foam.

B. BLEACHED KRAFT PAPERS

The bleaching process renders the paper weaker than unbleached kraft, and it also has a higher price tag. This type of paper is more common for wrapping packages that demand a higher "value look" than packages wrapped in dark, non-bleached, or recycled paper.

What are the Paper Grades for Bleached Kraft Paper?

Bleached kraft paper comes in four distinct grades. They are listed below.

Machine Glazed (MG)

Glazed paper has a high gloss on one side for added opacity, printability and runnability.

Smooth Finished (SF)

This provides a softer finish which is needed for print applications have a better print quality, convertibility, and printability.

Machine Finished (MF)

This is a highly refined type of kraft and it is most commonly seen in commercial applications, such as carrier bags, product wrapping, laminated products, and protective paper coverings.

Clay Coated Kraft (CCK)

A bleached sheet coated with clay coating to improve opacity and increase smoothness and printability. Clay coated kraft paper, because of its dimensional stability and lay-flat benefits, is used in the graphic art industry. CCK is frequently coated with silicone on one side and used as a release liner.

Here are some of bleached kraft paper applications:

Shopping/Retail Bags & Carrier Kraft Paper

Retail shopping bags offer one of the best strength-to-weight ratios available in paper packaging, and they can come in white, printed, machine glazed, grease resistant, or even ribbed. These bags are often PE coated for moisture protection.

Folding Boxes

Solid bleached kraft paper is used in clay-coated folding boxes for frozen foods, butter, ice cream, cosmetics and cartons for milk, juices and other liquid and oily foods. You can also find it used for plates, dishes, trays and cups.

Laminated Kraft Paper

Laminated kraft papers offer high strength and excellent consistency. This paper features high dimensional stability, high stiffness, and runnability.

Hygiene Release Liners & Release Base Papers

This type of kraft paper is used in feminine care and light adult incontinence markets. You'll also find it in baby products such as diapers.

Medical Grade Bleached Kraft Paper

Medical-grade bleached kraft paper must adhere to FDA standards for sterilization. That's because these papers are used in wound covers and medical adhesives.

Paper Sacks

Sack kraft paper features high elasticity and high tear resistance and is designed for packaging that need strength and durability. The several different types of kraft papers are used for sacks, including natural, wet-strength, bleached, and extensible.

Sack kraft paper is ideal for commodities like cement and other powdered materials. If you need an extra moisture barrier, sack paper can have a PE coating. Sack kraft paper can also be used for packing commodities, including food products such as rice, sugar and potatoes.

Kraft Paper Primer