Kraft paper is made primarily from wood pulp, specifically the cellulose fibers extracted from softwood trees like pine, spruce, hemlock, and fir. What sets it apart from other papers is the chemical process used to break down the wood, which preserves longer, stronger fibers and gives kraft paper its characteristic toughness and brown color.
The Wood Behind the Paper
Softwood trees are the backbone of kraft paper production. Southern pines account for more than half of the softwoods used in pulping in the United States, with spruce, balsam fir, and hemlock making up much of the rest. These species are favored because their fibers are naturally long, which translates directly into a stronger finished product.
Hardwoods play a smaller role, representing roughly 14 percent of the wood used for pulping. Species like aspen, cottonwood, sweetgum, red maple, and elm are sometimes blended in. Hardwood fibers are shorter, so they produce a smoother sheet but sacrifice some of the tear resistance that makes kraft paper useful for packaging and bags.
How Wood Becomes Kraft Pulp
The word “kraft” comes from the German word for strength, and the process earns the name. Wood chips are cooked at high temperature and pressure in a chemical solution called white liquor, a mix of sodium hydroxide and sodium sulfide dissolved in water. These two chemicals work together to dissolve lignin, the natural glue that holds wood fibers together, while leaving the cellulose fibers largely intact.
This is the critical difference between kraft pulp and mechanically ground wood pulp. Mechanical processes physically shred the wood, breaking fibers into shorter pieces and leaving most of the lignin behind. The kraft process chemically separates fibers at full length, which is why kraft paper can absorb a lot of force before tearing. Research on kraft paper has consistently shown that tensile strength and fracture toughness increase with fiber length, and tearing strength is the property most affected when fibers are shortened.
The trade-off is color. Residual lignin left in the pulp after cooking gives unbleached kraft paper its familiar brown tone. Bleaching can remove this color, but many kraft paper products skip that step entirely because the remaining lignin may actually help the paper last longer. Studies on cellulose pulp have found that lignin’s natural compounds can act as a shield against degradation, capturing damaging molecules that would otherwise break down cellulose chains over time.
Why Kraft Paper Is So Strong
Three factors combine to give kraft paper its durability. First, the long softwood fibers create more overlap and contact points between fibers in the finished sheet. More contact means more bonding area, and more bonding area means the sheet holds together under stress. Second, the chemical cooking process preserves the structural integrity of individual fibers rather than grinding them down. Third, the cellulose chains themselves remain long. Pulp scientists measure this using a value called the degree of polymerization, essentially how many sugar units are linked in each cellulose chain. Higher values mean longer chains and stronger paper. In kraft pulping, this value can range from around 1,000 to over 3,000 depending on how aggressively the wood is cooked.
Cooking intensity involves a balancing act. More aggressive cooking removes more lignin and produces a lighter-colored pulp, but it also shortens the cellulose chains and reduces tear resistance. Less aggressive cooking leaves more lignin in the pulp, preserving fiber strength but producing a darker, rougher sheet. Manufacturers adjust this balance based on what the paper will be used for.
Additives That Improve Performance
Raw kraft pulp produces a strong, porous sheet, but many applications demand extra properties like water resistance or dimensional stability. To achieve this, manufacturers mix additives into the pulp before forming the paper sheet.
Wet-strength agents are among the most common additives. Untreated paper loses most of its strength when wet because water disrupts the hydrogen bonds between fibers. Wet-strength agents create crosslinks between fibers that hold up even when saturated. The most widely used synthetic option is a polymer called PAE, but manufacturers also use formaldehyde-based resins and various acrylamide-based polymers. On the natural side, modified starch and chitosan (derived from shellfish shells) can serve the same purpose. Starch treated with organic acids becomes more water-resistant, making it a practical bio-based alternative.
Sizing agents are another common addition. These reduce how quickly the paper absorbs liquids, which matters for applications like food packaging or shipping containers that might encounter moisture. The specific combination of additives varies widely depending on whether the kraft paper is destined for grocery bags, corrugated boxes, industrial wrapping, or envelopes.
Unbleached vs. Bleached Kraft
The brown kraft paper you see in shopping bags and shipping boxes is unbleached. It retains a small percentage of lignin, typically in the low single digits, which gives it that natural tan-to-brown color. This version is the strongest form of kraft paper because the fibers haven’t been subjected to additional chemical processing beyond the initial cook.
Bleached kraft paper goes through extra steps to remove residual lignin, producing a white or near-white sheet. This makes it suitable for printing, food-grade packaging, and products where appearance matters. Bleaching does reduce fiber strength somewhat, but bleached kraft is still considerably stronger than paper made from mechanical pulp. White kraft paper is what you typically find in premium gift bags, medical packaging, and high-quality envelopes.
What Makes It Different From Regular Paper
Standard office paper and newsprint often rely on shorter hardwood fibers or mechanically processed pulp, sometimes blended with fillers like clay or calcium carbonate to improve smoothness and printing quality. These additions make the surface better for ink but weaker under physical stress. Kraft paper goes in the opposite direction: it prioritizes fiber length and bonding strength over surface finish. That’s why it feels rougher to the touch but can support heavy loads without ripping.
Kraft paper is also one of the most recyclable paper products. Because it starts with strong, long fibers, those fibers can survive multiple rounds of recycling before becoming too short to form a usable sheet. Recycled kraft pulp is commonly blended back into new kraft paper or used in corrugated cardboard, extending the life of the original wood fibers through several product cycles.