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:
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.
RAW MATERIALS FOR MAKING KRAFT PAPER:
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.
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.
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 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.
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.
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, paperboard, and its derivates are classified under H.S. Code heading 4810.3200, but in some countries, the same goods are being reported to be classified under heading 4810.9200.
With so many applications, it's not surprising to find high volumes of kraft paper in the waste streams of manufacturing companies, distribution centers, and other operations that consume large quantities of packaged commodities. For such companies, diverting kraft from the waste stream is a great way to increase recycling rates and reduce waste to landfill.
The Recycling Process:
In general, kraft paper is recycled similarly to other paper products. The recycling process can be split into three steps:
Step 1: Collection and Sorting
Step one of the process is determining viability for recycling of the kraft paper products or kraft paper rolls. Manufacturing companies will often use products with poly linings, wax coatings or glazes to increase wet strength and oil resistance. Unfortunately, these additional materials can make kraft paper recycling more costly or difficult. As such, much of the first step in recycling also means sorting out viable materials from materials that cannot be recycled.
You should always contact your recycling service for instructions on what can be recycled and how they want the products sorted.
Step 2: Baling and Shipping
After you have collected and properly sorted out your kraft paper, you will need to bale the products for more efficient transportation. Discussing expectations with your recycling service provider will help you determine how they want to have it bundled for pickup.
Step 3: Milling
The final step is out of your hands: the actual recycling of the kraft paper. The products will be sent to kraft paper factories where it will be pulped and used to create new paper products. In general, this process is broken into multiple stages, including:
Common Problems Associated with Recycling Kraft Paper Products
There are two major issues that make recycling kraft paper difficult: contamination and market rates for recycling.
Contamination is the presence of materials other than the material being recycled, like the poly linings and wax coatings mentioned above. Other contaminants include dirt, grease, food waste, tissues, and scrap metal.
Broadly, contamination falls under two classifications: outthrows and prohibitive materials. Outthrows are any contaminants that can be picked out during the recycling process, things that can be thrown off the recycled material to make it viable. Prohibitive materials, by contrast, cannot be physically separated and generally include chemical contaminants that require further processing to remove fully. Both kinds of contaminants can damage recycling equipment or lower the value or quality of the finished recycled products.
Kraft paper products are commonly used as packaging, which makes them susceptible to contaminants of both kinds, especially if they were used to package food products or powdery materials like cement.
Any residues on the kraft paper products can be difficult to remove and sometimes can be classified as a prohibitive contaminant.
Most recycling service providers will allow for a percentage of contaminants on the products they receive, but any contamination can make their processing costlier and less efficient. Kraft paper recycling is valuable because of the fiber quality. Since contamination can directly affect fiber strength and length, the fewer contaminants, the better.
Market rate difficulties are related to producers being willing to buy the finished recycled products for remanufacturing. In general, virgin fiber costs the same amount as recycled fiber so the product must be of equal quality. Fewer contaminants mean the products can be comparable in quality and price, making it more market viable. Because of the higher degree of contaminants in kraft paper specifically, the final recycled products are considered riskier and less desirable.
KRAFT PAPER REMAINS A GOOD INVESTMENT
Although some difficulties in recycling are apparent, these issues can be dealt with. The bottom line is, the quality, versatility, and value of the various types of kraft paper will help you and your business improve the look and feel of your products, reduce shipping costs, or enhance your branding efforts and improving your profit margins.
Compiled by Umesh G.
Written by Sonal P.