Towel/facial wet end optimisation: utilising strength additives and functional promoters
A TW report, by Kemira’s Clayton Campbell, Chen Lu, Junhua Chen and Adrian Stuart
In the manufacturing of wet strength containing grades such as towel and facial tissue, effective wet end process management is vital in order to achieve the desired sheet quality attributes and optimum machine operating efficiency at the lowest chemical treatment spend.
This technical paper focuses on improving wet and dry strength performance efficiency by utilising novel functional promoters and dry strength resins. Case studies will be presented to illustrate how selecting the correct process chemical additives can provide significant beneficial impacts on both sheet quality and machine overall operating efficiency. Functional promoters have successfully demonstrated the ability to increase sheet dewatering rate and enhance the fixation of fines, fillers, and functional chemicals.
Unlike conventional retention/drainage aids, functional promoters show no negative impacts on sheet formation and bulk and are able to reduce the usage of strength additives by up to 35%. Furthermore, this paper reveals the synergistic benefits of combining two different strength additives, polyamidoamine epichlorohydrin and cationic glyoxalated polyacrylamide, to effectively achieve sheet strength targets.
In the manufacturing of towel and facial grades, wet strength additives are widely added to the pulp suspension to provide wet strength. The term “wet strength” refers to the strength of finished paper or paperboard after it has been rewetted by water. The term does not refer to the strength of paper web prior to pressing and drying. The term for this property is “wet web strength.”
One of the most common wet strength additives is polyamidoamine epichlorohydrin (PAE) resin which contains cationic azetidinium functional groups. During the papermaking process, azetidinium groups can interact with the anionic sites on fibres and fines (primarily carboxyl groups). In addition, PAE self-crosslinks to increase paper wet strength.
The performance efficiency of wet strength additives (WSPE) is defined as the wet tensile index divided by the amount of added wet strength additives. For example, if 10 kg/t of PAE is required to achieve a 10 N.m/g CD wet tensile index, then the WSPE is 1.0 (kN.m/g). The key is to achieve the highest WSPE number that allows the manufacturer to either reduce chemical usage or increase the wet/dry tensile ratio.
WSPE is highly dependent on the type of pulp furnish. Most bleached virgin pulps have low conductivity and low cationic demand. However, commercial PAE resins have a high cationic charge density, typically in the range between 2 and 3 meq/g. PAE converts the pulp suspension to cationic at high dosages, resulting in un-retained PAE resins and low WSPE. Recycled pulps, especially those from mixed office wastes, contain a relatively high percentage of fines, fillers, and other anionic materials. Due to their high surface area, those fines and fillers carry a great portion of wet strength additives. Un-retained fines and fillers decrease WSPE significantly. In contrast, increasing the fixation of fines and fillers enhances the incorporation of wet strength additives into the sheet, leading to an increase of WSPE. As demonstrated in Figure 1, white towel products produced from mixed office waste (MOW) recycled fibres would have a lower WSPE than brown towel products from old corrugated cardboard (OCC) recycle fibres. Many bleaching processes lower fibre surface charge density. This in turn reduces the number of covalent bonds between wet strength resins and fibres, leading to a reduction of paper wet strength. Furthermore, MOW typically contains higher ash content than OCC. Un-retained ash consumes added PAE resins and lowers WSPE.
Poor WSPE often causes paper machine runnability issues. Un-retained wet strength resins accumulate in the water system, leading to poor drainage, wire and felt filing, sheet breaks and holes, and higher defoamer usage. Furthermore, excessive un-retained wet strength resin deposits on the Yankee surface and harden creping coating, resulting in poor creping performance. Plugged felts also produce high moisture streaks, which in turn strip crepe coating.
This technical paper focuses on improving WSPE by utilising novel anionic functional promoters and cationic glyoxalated polyacrylamide dry strength additives. Case studies will be presented illustrating how selecting the correct process chemical can provide a significant beneficial impact to sheet quality and overall operating efficiency.
Various functional chemicals have been developed in the past to enhance PAE performance but with limited success. Carboxymethycellulose (CMC) is frequently applied with PAE to increase both wet and dry tensile strength. However, CMC is often delivered in the form of dry powder. Costly make-down units are needed in order to dissolve CMC in water before the application. Synthetic anionic solution polymers are used as an alternative to CMC if the customer requires an easy-to-handle product in liquid form. Nevertheless, both CMC and solution polymers offer very little improvement on the retention of fines and fillers. Conventional retention/drainage aids are effective to incorporate fines, fillers, and PAE resins into the sheet. However, those products are typically high molecular weight linear polymeric flocculants, which can cause a myriad of problems such as poor sheet formation, lower tensile strength, and lower sheet brightness. In addition, those flocculants are shipped in the form of either dry powder or water-in-oil emulsion. A post-dilution aging tank is typically required to provide an extended period to hydrate those flocculants properly to achieve optimum retention performance. A more cost-effective and easy-to-handle PAE promoter is still highly desired by many towel/facial producers.
Kemira developed a series of unique anionic functional promoters which increase the fixation of functional chemicals without the negative impacts caused by the conventional flocculants. Kemira’s anionic functional promoters have a three dimensional micro-network structure with an estimated particle size of 50 nm and a net anionic charge. During the papermaking process, anionic functional promoters interact with cationic PAE and enhance the incorporation of fines, fillers, and PAE into the sheet, resulting in a higher WSPE. In addition, anionic functional promoters produce compact micro-flocs owning to their three dimensional structure, leading to uniform sheet formation. Maintaining sheet uniformity is critical to produce high quality tissue products. Most tissue products have fairly low basis weights. A slight disruption of sheet uniformity decreases strength properties significantly. By using Kemira’s anionic functional promoters, towel/facial producers will be able to improve the fixation of PAE into the sheet while maintaining sheet formation.
Kemira’s functional promoter series can also provide effective control and management over sticky contaminants and machine deposits. The furnish ash content can play a significant role in machine and converting operating efficiency and sheet quality. If not removed through the washing and cleaning process, ash can accumulate up to levels of 50% or higher in the headbox, resulting in a high ash content in the sheet varying from 2 to 5%. Ash particles also act as nuclei onto which the hydrophobic micro-stickies particles tend to adsorb, increasing in size and eventually forming sticky agglomerates. Kemira’s functional promoters can effectively prevent ash accumulation, and fix ash and stickies to the sheet and remove them from the process.
Additionally, Kemira’s anionic functional promoters offer a simple “pump and go” application solution over CMC and conventional retention/drainage aids. Those anionic functional promoters are shipped in liquid form and mix easily with dilution water by using an in-line static mixer. There is no need for either an expensive makedown unit or an extra post-dilution aging tank.
Cationic glyoxalated polyacrylamide Glyoxalated polyacrylamide
(GPAM) is widely applied in a variety of paper grades to increase paper dry strength. GPAM is typically added in the pulp suspension before paper sheet formation. Upon drying the treated paper sheet, GPAM is believed to form covalent bonds with cellulose to increase paper dry strength. In addition, GPAM increases temporary wet strength since the covalent bond between GPAM and cellulose is reversible in water.
Recently, Kemira introduced its latest generation of GPAM product Fennobond dry strength series which delivers superior strength performance over the conventional GPAM products. This report demonstrates that Fennobond unique dry strength technology provides synergistic impact in combination with PAE resins during the production of wet strength paper grades. PAE resins are often applied to increase both paper wet strength and dry strength.
However, many recycled pulp furnishes are rather challenging for PAE resins alone to provide enough dry strength to meet the requirements. Quite often, when a high dosage of PAE resin is applied, the wet tensile test result is well within the production specification while the dry tensile test result is still below the target.
Further increasing PAE dosage will cause machine runnability issues. In this situation, Fennobond technology can be applied in combination with PAE to provide necessary dry strength enhancement. Fennobond technology also increases sheet de-watering rate and sheet wet strength, leading to a reduction of PAE usage.
In the manufacturing of towel and facial grades, PAE resins are added to pulp furnishes to provide wet strength. Pulp furnishes often contain high contents of fines and fillers, which reduce PAE wet strength performance efficiency (WSPE) significantly. Kemira developed novel approaches to increase WSPE through the uses of unique anionic functional promoters and high efficiency cationic glyoxalated polyacrylamide. With Kemira’s anionic functional promoters, towel/facial producers will be able to improve sheet wet strength properties by increasing the fixation of PAE into the sheets while maintaining sheet formation. GPAM can be applied in situations where paper dry strength properties are below the target. GPAM also increases paper wet strength properties and reduces PAE usage.
Kemira’s Clayton Campbell is a senior manager, global business development, Chen Lu is a senior research scientist, R&D, Junhua Chen is a research scientist and Adrian Stuart is a field application specialist.
Case Study #1 Anionic Functional Promoter
A tissue mill in Southern China produces high wet tensile facial and towel grades. The mill typically uses 100% virgin bleached Kraft containing about 30% of softwood, 65% of hardwood, and 5% of broke. The mill experiences high wet strength resin usage (12.5~14.0 kg/ton). A charge titration study discovered that both headbox filtrate and white water were cationic. This indicates that wet strength resin was not fixed into the sheet, and WSPE was relatively low. This could also be the potential cause of low first pass retention, slow drainage, high defoamer usage, and poor creping performance.
Kemira recommended an anionic functional promoter (Fennopol technology) trial for this customer. The objective was to increase WSPE, machine first pass retention, and fibre usage efficiency. The anionic functional promoter was applied to the machine at the inlet of fan pump post wet strength resin addition point. The dosage was between 0 to 1 kg/ton.
Figure 2 shows the trial result of Fennopol anionic functional promoter and Table 1 summarizes trial benefits. During the trial, the dosage of wet strength resin was reduced by 35%. In the same time, wet tensile index increased about 4%, thus improving the overall MD WSPE by 61%. The turbidity of white water decreased about 48%. The headbox and white water soluble charge declined significantly and was close to iso-electrical point. Detailed benefits are summarized in Table 1. The anionic functional promoter provided about 90RMB/t of savings on wet strength resin usage alone. The return of investment (ROI) achieved 180%. Table 1.
Case study #2 Cationic glyoxalated polyacrylamide
A tissue mill in Northern US produces brown towel with a wet crepe tissue machine. The pulp furnish was 100% OCC. 4.2 kg /ton of PAE resins were added to increase sheet wet strength. The mill experienced both low wet and dry tensile strength, resulting in “tabbing” issues. “Tabbing” refers to situations that towel products are torn apart when being pulled out from the dispenser by consumers in order to dry their hands. Kemira proposed a trial of the new Fennobond GPAM product in combination with PAE resin to resolve this issue.
As shown in Table 2, the addition of 1.8 kg/ton GPAM increased MD dry tensile index by 26%. CD wet tensile index was increased by 17% and the PAE dosage was reduced by 0.8 kg/ton which was equal to 19% of the original dosage, thus producing an overall improvement in the CD WSPE by 44% . Additionally, the trial achieved a reel speed increase by 11%.