High quality NBSK for premium tissue

By Paul Bicho, Ph.D., manager, optimisation and innovation, Canfor Pulp Innovation

The makers of premium bathroom and facial tissue strive to maximise softness and bulk while maintaining tensile strength to ensure runnability and a high converting efficiency, all while meeting a final quality target. Bulk and softness start with a good furnish, which is followed by the processes and chemicals used on the tissue paper machine, as well as in converting.

Premium tissue requires the use of high quality furnish. Incorporating quality softwood kraft pulps into a fibre furnish improves tissue strength and other quality parameters. The challenge facing tissue makers is maximizing softness and bulk while maintaining the strength specifications of the product all while controlling costs.

Tissue makers have many tools at their disposal to ensure that strength targets are met while not compromising softness and bulk. Foremost is the judicious selection of a softwood kraft pulp. Softwood pulps with high initial strength and good fibre development during refining can help tissue makers reach their strength targets with either lower softwood pulp levels, less refining or both. In this way tissue makers can minimize the negative impacts that refining has on softness and bulk.

During refining, pulps are subjected to mechanical forces that alter their morphology in a manner that will enhance certain properties, typically tensile strength. This is achieved by fibre straightening during the early stages of refining. As refining progresses, pulp fibres are delaminated and surface defibrillation occurs. The former tends to contribute to fibre collapse while the latter increases the surface area of a fibre which improves fibre to fibre bonding. The demarcation of these three phases is not clear and there is overlap. At some point the gain in strength due to refining levels off, typically occurring at freenesses (CSF) that are too low for most tissue applications.

If done properly, a little softwood kraft pulp refining can go a long way. However, tissue makers are well aware that excessive refining detrimentally affects other properties that are important for tissue, specifically bulk decreases, CSF drops (SR increases) and softness that is adversely affected. Furthermore, if refining of softwood fibres is not done properly, fibre cutting can occur which can undo the benefits that refining is meant to generate. For these reasons, tissue makers that focus on premium quality tissue grades pay close attention to the softwood kraft pulp that they use and how they refine it. What softwood kraft pulp attributes are desired by tissue makers? Specifically, thin-walled (fine), long fibred pulps with low fines content that have high initial tensile strengths and whose beneficial characteristics are readily developed with minimal refining. Northern bleached softwood kraft pulps, like those produced by Canfor Pulp, are highly prised for these attributes by the makers of quality tissue.

Figure 1: Effect of refining (expressed as freeness or °SR) on tensile strength (breaking length in km) for 33 northern softwood kraft pulps that are commonly used in premium tissue furnishes. The coarseness of these fibres range from 11 to 17 mg/100m. The two lower coarseness Canfor pulps are highlighted in red.

Many factors contribute to the overall quality of softwood kraft pulps. Shorter growing seasons but longer days during growth result in trees with lower proportions of coarser late-wood fibres. Species plays an important role in this with some species naturally producing finer fibres.

These include the spruces and some pines (such lodgepole pine). It is important to emphasize that some pine species (southern pines, radiata pines and some northern pine species) produce very coarse fibres so tissue makers need to be wary of kraft pulps containing these species.Longer fibres are naturally produced by the tree during the mature wood phase of growth – typically after 60 years or more of growth. Trees that are harvested with a high proportion of shorter fibred juvenile wood will have finer yet significantly shorter fibres. Shorter fibred pulps are weaker in terms of tensile strength and will require either higher proportions in the full furnish or more refining to achieve the desired strength.

Northern bleached softwood kraft pulps from the Canadian interior are intrinsically finer and longer than many of the species found in many parts of the world that have faster growing plantations where volume is valued over fibre quality. In the central interior of British Columbia, Canfor Pulp is fortunate to have three softwood species (white spruce, lodgepole pine, subalpine fir) that are known to be inherently fine fibred.

Furthermore, the rapid onset of cold winters results in slow growing trees with very high proportions of fine fibred earlywood and little or no coarser fibred latewood. In addition, the age of the trees (well over 100 years at time of harvest) ensures a high proportion of long fibred mature wood. Forest utilization practices ensure that the inner part of the log is directed towards lumber while the outer part – which is all long fibred mature wood – is directed towards kraft pulping.

In this way, a good fibre for tissue making is made even better. The result is that Canfor pulps are inherently strong and require significantly less refining to develop strength than most other softwood kraft pulps (Fig 1). Tissue makers can take advantage of this by either refining these pulps less or reducing their content in a tissue furnish – or some optimal combination of both. In either case, the result is a soft, bulky sheet that meets the strength expectations of a premium tissue. Other benefits are that higher strengths are reached at higher CSF (lower °SR) which can improve drying efficiency.

Tissue makers appreciate the importance of furnish optimisation in production of high quality premium tissue products. Blends of hardwood and softwood kraft pulps are typically used to enhance the softness, bulk and strength demanded for the particular finished tissue product.It is well known that separate refining of each pulp allows for optimization of each component of the furnish, to obtain a higher quality tissue product.

In many mills, the wide range of products with different softwood contents makes optimal refining of this fibre challenging.In practice this is often overcome by co-refining the hardwood and softwood pulps. Machine, or touch-up, refiners can also be used.

Recent studies by Canfor, at the Voith Pilot Tissue Machine, have confirmed that co-refining hardwood and softwood improves strength while bulk and softness are lowered. By refining only the softwood, higher bulk and softness are obtained but with the possible need to increase the softwood content to maintain strength. The final decision rests with the tissue maker, depending on the processing equipment they have at their disposal and the requirements of their specific situation.

It is important that tissue makers realize that not all softwood kraft pulps have refining characteristics that are suitable for premium bathroom or facial tissue products. In these grades, softness and  bulk are highly valued and excessive refining can rapidly deteriorate these properties. Selecting a superior strength softwood kraft pulp gives tissue makers options to optimize refining and softwood to hardwood pulp proportions to provide the quality required at a reasonable cost. We, at Canfor Pulp Innovation, are well versed in the analysis of both pulp and tissue products, and refiner optimisation for the tissue industry. We can provide the technical support needed to maximise value recovery from our superior quality bleached pulps.

This article was written for Tissue World magazine by Paul Bicho, manager, optimisation and innovation, Canfor Pulp Innovation.

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