Why is kraft pulp stronger than soda pulp?

The kraft process produces stronger pulp than the soda process because of the specific chemical composition of its cooking liquor.12

While the soda process relies on caustic soda alone, the kraft process incorporates sodium sulfide alongside caustic soda. This chemical modification, introduced by Carl F. Dahl in 1884, results in a more effective pulping environment that preserves the integrity of the wood fibers, leading to a significantly stronger and more durable end product.12

Because of this superior strength, many mills that originally used the soda process were converted to the kraft method. The term "kraft" itself is derived from the German and Swedish word for "strong," reflecting this primary advantage over earlier alkaline pulping techniques.21