Abstract
The relationship between the types of pulp, the tissue making technologies, and shelf price of bath tissue was evaluated for the North American market. Twenty-four market tissue samples (representing approximately 80% of the current market offering) were sourced and analyzed along with their nationwide price information. Pulp composition, drying technologies, market share, sustainability advertising, and tissue properties were evaluated. Tissue properties, including softness, ball burst strength, water absorbency, density, tensile strength, and tensile modulus were measured. Among all the drying technologies, creped through-air dry (CTAD) and creped through-air dry belt (CTADB) seemed to improve tissue softness most. The UCTAD maximized tissue bulk by drying the tissue web solely using a through-air (TAD) cylinder. Tissue samples with freeness between 575 to 650 mL seemed to have their properties improved more significantly through advanced drying technologies. It was found that the retail prices of these bath tissues were directly related to softness, bulkiness, water absorbency, and basis weight. A mathematical model was conducted to predict the retail price of bath tissue (based on product performance and attributes). This paper also identified the effect of “sustainability” on the retail price.
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Effect of Pulp Properties, Drying Technology, and Sustainability on Bath Tissue Performance and Shelf Price
Yuhan Wang,a Franklin Zambrano,a Richard Venditti,a Sudipta Dasmohapatra,b Tiago De Assis,a Lee Reisinger,c Joel Pawlak,a,* and Ronalds Gonzalez a,*
The relationship between the types of pulp, the tissue making technologies, and shelf price of bath tissue was evaluated for the North American market. Twenty-four market tissue samples (representing approximately 80% of the current market offering) were sourced and analyzed along with their nationwide price information. Pulp composition, drying technologies, market share, sustainability advertising, and tissue properties were evaluated. Tissue properties, including softness, ball burst strength, water absorbency, density, tensile strength, and tensile modulus were measured. Among all the drying technologies, creped through-air dry (CTAD) and creped through-air dry belt (CTADB) seemed to improve tissue softness most. The UCTAD maximized tissue bulk by drying the tissue web solely using a through-air (TAD) cylinder. Tissue samples with freeness between 575 to 650 mL seemed to have their properties improved more significantly through advanced drying technologies. It was found that the retail prices of these bath tissues were directly related to softness, bulkiness, water absorbency, and basis weight. A mathematical model was conducted to predict the retail price of bath tissue (based on product performance and attributes). This paper also identified the effect of “sustainability” on the retail price.
Keywords: Bath tissue; Pulp composition; Drying technology; Tissue softness analyzer (TSA); Softness; Shelf price; Market share; Sustainability
Contact information: a: Tissue Pack Innovation Lab, Department of Forest Biomaterials, North Carolina State University, P. O. Box 8005, Raleigh, NC 27695-8005, USA; b: Department of Statistical Science, Duke University P. O. Box 90251, Durham, NC 2770, USA; c: ReiTech Incorporated, 26 The Point, Coronado, CA, 92118, USA;*Corresponding authors: joel_pawlak@ncsu.edu; rwgonzal@ncsu.edu
INTRODUCTION
The hygiene tissue industry has a global $100 billion and ca. 38 million tons market per year, growing at 3% compound annual growth rate over the past five years (Fastmarkets RISI 2017). Previous studies in the authors’ research group have quantified the effect of fiber, technology, and sustainability on the retail price for kitchen towel tissue in North America (Assis et al. 2018). In this paper, the authors quantified the effects of products’ performance and attributes on the retail price for bath tissue.
In general terms, the performance of tissue products is related to the composition of furnish, chemical additives, and manufacturing technology (Gigac and Fišerová 2008). Virgin and recycled fibers are the two major components of hygiene tissue products (FisherSolve International 2017). Softwood and hardwood virgin pulps have been utilized in a certain ratio to make tissue products soft and strong (FisherSolve International 2017; Assis et al. 2018). Softwood furnish with long fiber length forms a strong and flexible web for bath tissue (Assis et al. 2018). Hardwood fibers with short fiber length produce more free fiber ends on the surface, which ultimately gives a velvet feeling on the surface (Wang et al. 2019). Usage of recycled fibers typically decreases the bulkiness, softness, and water absorbency of bath tissue. Multiple rounds of fibrillation of recycled fibers during the refining process results in excess bonding, producing a denser and stiffer paper web (Banavath et al. 2011). Depending on the market segment of the tissue product, the content of recycled fiber varies.
The typical basis weight of bath tissue in the North American market is approximately 40 g/m2, with products ranging from 15 g/m2 to 50 g/m2. Within bath tissue properties, softness is one of the most important (Hollmark and Ampulski 2004; Wang et al. 2019). Unlike the manufacturing of printing or writing paper that involves wet-pressing before drying, tissue making procedures are designed to minimize or avoid pressing, thus preserving the bulk of the tissue sheet (Kullander et al. 2012). There are four major types of drying process technologies being used in bath tissue manufacturing, namely light drying-crepe (LDC), creped through-air dry (CTAD), creped through-air dry belt (CTADB), and uncreped through-air dry (UCTAD).
The LDC uses a combination of gravity, vacuum, press nip, and a Yankee dryer to dry the wet tissue web (Kullander et al. 2012). The press nip is a roller that is set against the Yankee cylinder, which applies sufficient pressure to the paper web to remove approximately 20% of water (Kullander et al. 2012). The tissue web then enters the Yankee cylinder and is dried by heat. A creping blade is used at the end of the drying process to gently scrape on the surface of the tissue web (Kullander et al. 2012). The creping process expands the tissue web on the Z-direction, creates folds and physical ridges on the tissue web to increase the bulkiness, stretchability, and creates free fiber ends on the surface to increase the velvet feeling (Padley 2012). Such increase of bulkiness and velvet feeling is a trade-off from the loss of tensile strength.
To further increase softness and bulkiness, a thru-air drying (TAD) process was later introduced to the bath tissue industry. Instead of using light pressure to remove water, TAD uses an air cylinder for water removal. After the wet web is transferred to the TAD cylinder, a constant pressure drop and heated air (100 to 250 °C) are applied to the wet web through the honeycomb surface structure (Valmet 2014). The most common TAD dry machine is the CTAD that combines the TAD cylinder with the Yankee Dryer.
Kimberly-Clark invented the UCTAD by removing the Yankee dryer and using only the TAD cylinder to dry the web (Wendt et al. 1998). This process results in higher productivity because the creping speed no longer limits the production rate.
Instead of using TAD fabric in drying (CTAD), CTADB uses a woven fabric belt cast with urethane as the carrier for the tissue web (Smurkoski et al. 1992). The molding can provide both uncompressed pillows and compressed lines (Assis et al. 2018). The CTADB is capable of providing bath tissue with high strength and softness, but the belt itself is expensive and less durable compared to a conventional fabric (Assis et al. 2018).
Much of the work related to drying technologies has focused on attempting to improve the softness of tissue sheets. Softness has historically been difficult to quantify. Panel tests are often used, in which human perception determines the softness values. To attempt to quantify softness in terms of physical behavior, a “softness analyzer” was developed. Giselher Grüner (Grüner 2016) invented the tissue softness analyzer (TSA). The instrument measures the physical attributes correlated with the softness of a paper by spinning a lamellar structure type fan on the surface of the sample. A previous study showed that TSA provides a relatively accurate quantifiable value of softness (Wang et al. 2019).
In this work, the authors conducted a value analysis between the shelf prices of bath tissue and their key physical properties (e.g., basis weight, strength, softness, and density). Based on the results, it is possible to explore the value created by different manufacturing technologies and physical properties. Additionally, the effect of sustainability (products marketed as sustainable) on product price was discussed.
EXPERIMENTAL
Materials
Bath tissue samples
Table 1. Summary of Commercial Samples’ Properties
*Drying method: LDC, UCTAD, CTAD, or CTADB; A specialist in tissue paper manufacturing reviewed each sample to determine the technology used (ReiTech 2018).**Type of brand: National brands are brands owned by manufacturers, and private label are brands are owned by wholesalers or retailers (Assis et al. 2018), while sustainable labels are the brands that claim to use sustainable materials and/or processes
A total of 24 bath tissue samples were sourced across the US, comprising approximately 80% of the total consumer market for bath tissue. The samples were purchased in stores across the US. The shelf price for each sample, excluding any price discount, was recorded. Because the size of the packages (e.g., number of rolls, number of sheets per roll, and total area) influences product prices, careful attention was paid to select packages with the approximate same total area (tissue area). Table 1 provides a description of each sample, including the drying method and type of brand. Machine technology was evaluated by tissue machine specialists (ReiTech 2018).
Experimental Design
A flow chart of the experiment design is shown in Fig. 1. The commercial products were disintegrated and reprocessed to make handsheets. Properties before and after reprocessing were measured and compared.
Fig. 1. Flow chart of experiment design
Physical properties
All bath tissue were conditioned in a room maintained at 50% relative humidity (RH) and 23 °C according to TAPPI T402 sp-08 (2013). The basis weight, defined as the mass of paper per unit surface area, was measured according to TAPPI T410 om-08 (2013). The caliper of the samples was measured using a 2 kPa caliper according to TAPPI T410 om-08 (2013). Density was calculated based on the result of basis weight and thickness.
Water absorbency capacity, defined as the mass of water absorbed per unit of sample mass, was measured according to ISO 12625-8 (2010).
Ball burst strength, defined as the maximum penetration force that a sample can withstand when a ball applies a perpendicular force, was measured in dry conditions (bursting force) according to ISO 12625-9 (2005). Tensile strength, defined as the maximum tensile force per unit of width that a sample can withstand before breaking in a tensile tester, was measured under dry conditions according to ISO 12625-4 (2005). The tensile strength, tensile modulus and tensile energy adsorption were measured for both directions (paper machine direction (MD) and cross direction (CD)).
Softness score was measured using a TSA (EMTEC Electronic GmbH, Leipzig, Germany). A 10 cm × 10 cm bath tissue sample was placed and clamped on the holder of the TSA. A fan with six lamellas moved vertically downward to touch the surface of the sample webs. The instrument used a TPII algorithm that correlated with a softness panel test to calculate a handfeel coefficient (HF), the results ranged between 0 (least soft) and 100 (softest). TPII was found to be the algorithm that correlated the best with the panel result (Wang et al. 2019).
Methods
Fiber quality analysis
Fiber length and width of each sample were measured by the HiRes fiber quality analyzer (FQA) from OpTest Equipment Inc. (LDA96; Hawkesbury, Canada). Samples were diluted to approximately 1 to 5 mg/L and disintegrated at 15,000 revolutions using a standard pulp disintegrator (Testing Machines Inc., New Castle, DE, USA). Fiber width values ranged from 7 to 60 µm. The fiber length and width distributions were obtained.
Tissue reprocessing
To study the change in the product’s properties as a function of the drying technologies, it is necessary to create a control dataset that somehow isolates the effects of the drying process. To isolate the finishing and drying process effect, 24 g of each bath tissue sample was diluted and disintegrated with 2000 mL of deionized (DI) water using a standard pulp disintegrator (Testing Machines Inc., New Castle, DE, USA). The furnish was further diluted to 0.3% consistency. A total of 20 handsheets were made for each data point according to TAPPI T205 7.2, 7.3 (2006) with the following variations. After forming the handsheet, the sheet was couched onto two dry blotter papers using the couching process. A dry blotter paper was placed on the top of the handsheet. The sandwiched handsheet was dried by a heated roller dryer (Formax 12” Drum Dryer; Adirondack Machine Corp., Hudson Falls, NY, USA) at 115 °C and 20 rpm for 5 min.
Multiple linear regression
A multiple linear regression was performed using software R (RStudio, RStudio, 3.0, Boston, MA, USA) to evaluate physical properties (basis weight, caliper, ball burst strength, density, water absorbency, HF, CD tensile strength, MD tensile strength, CD tensile modulus, and MD tensile modulus), which are correlated to retailing price.
RESULTS AND DISCUSSIONS
Physical Properties Comparison
As shown in Fig. 2, the basis weight of the samples ranged from 17.4 to 49.7 g/m2. The basis weight of TAD-dried tissues had a minimum basis weight of 30 g/m2, whereas the basis weight of tissue dried by LDC can be as low as 15 g/m2. The caliper ranged from 88 µm to 678 µm. The samples dried with LDC had much lower caliper than the TAD-dried samples. The density ranged from 0.073 g/cm3 to 0.167 g/cm3. Bath tissues dried by TAD had an average density of approximately 0.085 g/cm3, whereas the bath tissues dried by LDC had an average density of 0.13 g/cm3. Through–air-drying methods increased the bulkiness of the paper, which resulted in higher compressibility and higher bulk softness (Wang et al. 2019). The LDC used a press nip to remove the excess water from the bath tissue and densified the sheet, which ultimately decreased the HF value of the sheet. The HF ranged from 68 to 98, where the TAD-dried tissues had much more consistent and higher softness than the LDC. The results from Fig. 2 indicate that processes involving TAD produced products that were softer, bulkier, and had higher water absorbency than products dried with LDC. The ball burst strength (BBS) ranged from 1.1 N to 3.8 N. There was no obvious relationship between the BBS and drying technologies. The majority of samples had BBS of approximately 2.5 N, which implied that the minimal requirement strength for a quality bath tissue is approximately 2.5 N. The variation of the BBS may be a consequence of fiber selections, fiber refining, basis weight, and the finish process (Kullander et al. 2012). Tissue dried with different technologies had a similar cross direction tensile strength. On average, the tensile strength on the MD was one fold higher than the CD tensile strength for LDC samples. The LDC-dried samples had a broad spread of MD tensile strength, which was due to the variation of the fiber resources, as explained in a later section of this work. The CD and MD tensile modulus for TAD-dried tissues were lower than the ones dried by LDC, which implied a higher stretchability of TAD tissues.
Fig. 2. Boxplots of physical properties comparison between LDC and TAD-dried tissues
Though the TAD drying process does not involve much pressing, the average strength of TAD tissues was still competitive to LDC products, even though TAD tissues had a lower modulus and thus higher flexibility than LDC tissues. This was probably due to the higher average basis weight of TAD tissue, and more importantly the usage of forming fabrics during the forming process of TAD products.
The TAD drying process does not use much pressing; therefore, the wet web needs to have an excellent formation uniformity to retain the necessary strength for the stretching on the machine direction as the web moves forward (Valmet 2014). A high mesh molding fabric is typically used in the forming section to ensure good fiber support (Patel and Herman 2007).
The materials used for weaving the forming fabric are commonly extruded polyethylene or nylon. The strand size ranges from 0.1 mm to 0.45 mm (Patel and Herman 2007). Potential strength agent, such as cationic starch or polyDADMAC, might be added to the different brands of TAD tissue as well (Miller et al. 2011).
As shown in Fig. 3, HF of the tissue increased with freeness; this means softer tissue samples. Higher freeness implied that there were more virgin fibers with lower surface fibrillation, fewer small fibers, and fines in the pulp (Hubbe 2007). Tissues manufactured by the TAD process typically use pulps with higher freeness. For tissue prepared using similar freeness pulps (620 to 650 mL), tissues manufactured with TAD processes were softer than LDC.
For the technology comparison of CTADB, UCTAD, CTAD, and CTADB, the CTAD products exhibited higher softness than the UCTAD, which showed the significance of creping to increase softness. The freeness had no linear correlations with BBS, cross-direction tensile strength (CDTS), machine direction tensile strength (MDTS), cross-direction tensile energy adsorption, nor machine-direction tensile energy adsorption. This was probably due to the complex and diverse manufacturing processes of the different samples.
The highest freeness correlation found corresponded to the CD tensile modulus. Lower freeness can be often attributed to the increased specific surface area of the fiber or more wet fiber flexibility. Both of these factors can lead to denser sheets with more bonding and higher tensile modulus. This also corresponded with the high CD and MD modulus values of LDC-dried tissues.
The creping process significantly decreased the MD modulus by creating the wavy structure on the tissue surface. Sample H that was dried with LDC had a competitive HF to other advanced drying technologies. This was due to a 3-ply structure used in sample H. The tissue became bulkier as more plies were added, which resulted in better softness. In contrast, a 3-ply tissue typically has a relatively high basis weight, which will result in higher cost on fibers.
As shown in Fig. 4, neither BBS, CDTS, nor MDTS had strong correlations with HF (P values were reported in Table S1). Though tissue softness is typically achieved at the cost of strength (Lavash 1985), this relationship may be inverted by the advanced manufacturing processes. The CD tensile modulus had a weak linear relationship to HF, which implies that sheet flexibility could be treated as one of the attributes to softness.