TISSUE SOFTNESS

Introduction.

Tissue products, at least facial tissue and paper towel, were developed to replace cloth items, which were soft and absorbent. Not surprisingly, once a necessary level of absorbency has been reached, further consumer acceptance of the tissue products was strongly influenced by their softness. In fact, softness has become the most important determiner of consumer preference and purchase intention. Curiously, there have been few technical articles dealing with this all-important property; most have been restricted to a narrow area of topics. In this article I aim at a wider range. I will show that softness is quite different from the other tissue properties. Not only is it subjective, but it is a compound property, made up of components like surface smoothness, flexibility, and cushioniness. I will discuss the two main methods for its determination: instrumental measurements of the individual components, and subjective evaluation using standards. I will also suggest a way to establish an "absolute softness scale". I will describe the most important relationships between softness and other variables. Finally, I will indicate several ways in which softness may be improved at reasonable cost.

What is softness?

Few people realize that the most important tissue property, softness (also called handfeel by some writers), is quite different than the other properties, tensile strength, bulk, and basis weight (BW), for instance. The latter are objective,

well-defined, and have easy-to-understand units. Tensile strength, for instance, is defined as force per linear area, its units are gm/cm, or oz/in; BW is weight per unit area, its units being gm/cm², or lbs/3000ft². A product that that is twice as strong requires twice as great a force to break it. In contrast, softness is subjective and not well defined. In fact it is not a single property but a combined one, standing for a subjective combination of several more basic properties. None of the authorities on softness agree on exactly what these properties--I call them the components of softness--are, and how they combine to yield a single measure of softness. Most agree, however, that the main components are surface smoothness, flexibility, and cushioniness. In my work I have found that these three components are sufficient to define softness. It is difficult even to decide what units to use for softness or what it would mean to say that a product is twice as soft as another. The most important tissue property is shrouded in mystery.

What is required for a product to be soft?

For a tissue product to be soft, it is necessary that its density be low. In fact, as a first approximation, the lower the paper density the higher its softness. This can best be seen by contemplating wrapping paper, for instance. Wrapping paper is not soft at all, because it is extremely dense. If the paper was slowly

de-densified, by reducing wet pressing and refining, and by adding a debonder, its softness would increase (and its strength decrease). This low-density product would be cushiony and maybe have good flexibility; that is to say two of its three components would be soft. But its surface may not be smooth enough. To have high softness the paper has to have both, low density and high surface smoothness. In fact, in facial and toilet tissue, consumers value surface smoothness above the other two components.

Measurement and scaling.

My preference lies with measuring the above-mentioned individual components separately, using suitable measuring devices which are readily available. The benefit of this approach is that it employs reproducible physical measurements to characterize the individual softness components. However, these methods take time, and gain their legitimacy by using statistical correlation methods to subjective softness evaluation. As an alternative, I would like to discuss the use of a single, subjective, number to represent softness.

At first glance, trying to represent such a compound property as softness by a single number seems doomed to fail. Yet, it works: most consumers are able to integrate these components into a single, highly reproducible, documentable impression of softness. The rest of this article will be based the use of such a single number. This somewhat idealized method of determining softness requires the use of standards, set to different levels of softness, against which products of unknown softness are evaluated by an expert panel. Most companies use such methods. The standards themselves are generally set by a process whose origins are often more historical than scientific. Standards, however, can be manufactured by the following much more logical process. The standards are made on a single paper machine, keeping all properties except tensile strength constant. Varying tensile strength allows the production of various levels of softness. The density of these samples is next established, and the numerical values of the standards can be set in such a manner that they are inversely proportional to their density. This, proportional scale, allows the establishment of an "absolute scale", which would be a completely valid concept if softness were only density dependent which it isn't. Still, this, density-based softness is at least based on firmer grounds, in that softness values vary inversely with tensile strength and thus, for weakly bonded tissue products, bond strength. The numbering of the standards is arbitrary, generally 20-100; a product with a softness value of 80 is approximately half as bonded as one of 40. It is this inverse relation that allows one to state that a product has 20% higher softness than another.

Surprisingly, this type of softness correlates extremely well with consumer surveys, in which participants rank softness on a 10 point scale. Moreover, Softness correlates highly with purchase intention.

How to change softness? Softness relationships.

The first thing one notices when trying to increase softness is that there is no softness button on the paper machine. One can increase strength by increasing refining or adding dry strength resin, one can increase BW by increasing stock flow, but how does one increase softness? This is merely to introduce the idea that, while tensile strength and BW are independent variables, softness is a dependent variable. It can only change when we change one of the many independent variables that softness depends on.

Strength.

Softness-strength relationship is one of the most basic relationship in tissue papers. It is the exact parallel with the famous pressure-volume relationship for gases. When all other independent variables (BW, furnish, formation, type of paper machine, and some others) are kept constant, softness varies inversely with strength. The exact functionality of the relationship has never been established, but I have had excellent results when I used the following semi-empirical relationship: softness = [a/(tensile strength)] + b,

where "a" and "b' are constants. Such a relationship is shown on Figure 1.

The graph points out the bitter reality of tissue making: both properties are essential, and a trade-off between them has to be made.

 Figure 1

Furnish.

When all other variables kept constant, softness depends on furnish quality. The higher the furnish quality, the higher the softness. Furnish quality is notoriously difficult to define but generally the higher quality fibers cost more. Longer, more slender cellulosic fibers yield softer products. I have had good success with estimating the softness of a furnish by combining morphological properties, like fiber length, coarseness, and wall thickness. In this paper I want to merely indicate the importance of furnish quality.

In my experience most manufacturers do not use optimum furnish for a given furnish cost. By using the correct relationships between fiber properties and softness, using the weight average blending relationship for the different fibers, one can determine the optimum cost-softness relationship for a given paper machine. Once that relationship is known, the relationship between consumer preference and softness can be determined. From this knowledge product quality can be set to yield optimum profit. However, these topics are beyond the aim of this article.

Manufacturing technology.

Major manufacturing technologies affect softness considerably. It is well known that at the same BW, strength, and furnish composition, through air drying (TAD) technology, as practiced by Procter and Gamble in the US, makes products with superior softness levels when compared with those made by conventional wet pressed technologies. The reason for this superiority is that TAD process does not compact the paper, producing a low density product. In contrast, wet pressing compresses the sheet, producing higher densities.

All advanced paper machines come with stratification capability. The effect of stratification on softness, however, is not often understood. Stratification per se does not improve softness; if the same furnish is used alone or stratified, the resulting softness will be the same. Where stratification is most effective is to achieve superior softness with inferior furnish, by placing the low quality fibers in the middle of the sheet and keeping the superior one on the surface, where it comes in contact with the fingers. I always think of stratification as a cost-saving operation. For paper machines which do not have a stratified headbox, one can calculate the economic trade-off between the one-time cost of installation and continued interest and the continued cost of a more expensive furnish. I have often found that it may make more economic sense not to install a stratified headbox on an older paper machine.

Basis Weight.

Basis Weight has a strange effect on softness. At a constant level of bonding, say at a fixed level of refining, a small increase in BW has beneficial effect especially if the product BW is low to start with. If however BW is increased beyond a certain level, the tissue paper becomes stiff and softness decreases. The exact form of this relationship has never been determined. For economic reasons most manufacturers produced paper at the lowest possible level of BW, which may not be the most prudent course of action, because softness will suffer. A sure way to increase softness via BW change is to run the product at a constant tensile strength but increase BW. This strategy produces a product with lower level of bonding, hence lower density, and softness increases. This is one way for the manufacturer using conventional, wet press, technology to produce a product with softness levels equal to those obtained with TAD technology (at lower BW). Many manufacturers contemplate upgrading their technology to TAD, however the cost of the initial investment is many million dollars. It may well be possible that keeping their technology but running at a higher BW is more economically viable.

Calendering.

Calendering is one of the best ways I know to effect small improvements in product softness at low extra cost. Calendering, especially steel-to--steel calendering, imparts extra surface smoothness to the tissue product, hence it improves overall softness. The relationship is logarithmic, the softness benefit decreases with increasing level of calendering. If carried beyond a certain point the sheet can be so compressed as to decrease softness. Also, the higher the initial softness, the lower the benefit calendering can provide. The effect of calendering is shown on Figure 2.

Figure 2.

Formation.

Formation plays a major effect in producing softness. However, the mechanism is often not well understood. When formation is bad, fibers are not bonded effectively, and it is necessary to use excessive refining or chemical strength additives to achieve desired product strength. The resulting sheet becomes too dense, and softness suffers. When formation is optimum, extra strengthening is not required, and softness improves. Formation, unfortunately, is one of those variables that always gets worse, seemingly by itself. One of the available formation testers may be used to monitor it. There other methods I favor; these however require handsheet-making facilities.

Forming fabrics and press felts.

There have been new developments in recent years in this field. So-called patterned fabrics and felts have been developed which can increase the bulk of the sheet considerably. This increase is beneficial by itself, but can be used to improve softness by converting the bulk gain into softness gain. The above-mentioned calendering step can be used for this purpose. The combined effect of pattern fabrics and calendering can be considerable.

Other variables.

Manufacturers spend much time trying to improve softness by "tweaking" other variables, wet end and creping. There are constant trials with debonding and softening agents. Their primary effect is to reduce tensile strength and increase softness by travelling along the strength-softness curve shown in Figure 1. At constant tensile strength (which is the only way to compare them) I have not found any softness increase using these chemicals, although such improvements are often talked about, mostly by suppliers, and in several patents by Procter and Gamble claim such improvement.

Similarly, at constant strength, I have found little improvement by altering creping arrangement if they work properly to start with.

Ultimate in softness.

For the highest level of softness, one would require the highest quality furnish on a stratified TAD machine at optimum formation and at optimum level calendering.

No manufacturer does this. In fact, superior manufacturing technology enables the companies to save on furnish cost and still produce softness levels superior to those obtainable with conventional technology. As was pointed out in this article, the softness of products produced by conventional technology can be improved by judicious furnish optimization, improved formation, and through the use of patterned fabrics, and calendering. These steps can close the gap between TAD and conventional wet press technology.