CLAY NATURAL KRAFT (CNK®), Also known as Solid Unbleached Sulfate (SUS®), this is a superior strength paperboard grade produced from a furnish containing at least 80 percent virgin unbleached, natural wood pulp. WestRock CustomKote® is a coated natural kraft® (CNK®) paperboard that stands up to the most stressful environments throughout the supply chain. What is Paperboard? · Solid Bleached Sulfate (SBS) · Coated Unbleached Kraft Paperboard (CUK) · Uncoated Recycled Paperboard (URB) · Coated Recycled. Also known as CNK (Clay Natural Kraft) – SUS paperboard is not bleached, which gives the sheet a draft/brown appearance all the way through the sheet. Solid Unbleached Sulfate (SUS) is also known by some manufacturers as Coated Natural Kraft (CNK).