Nina Deng and Frederic Parent, FPInnovations

 

Advanced techniques and tools developed by FPInnovations to characterize tissue web uniformity are providing new opportunities for tissue mills to improve their tissue making and converting efficiency.

 

Challenges in tissue converting

 

For many tissue mills, converting efficiency is relatively low (40-70%) so the low tissue converting efficiency is often a bottleneck in tissue production. It represents a large potential improvement area for overall tissue manufacturing efficiency. Tissue converting efficiency is affected by three main factors including 1) poor operation and set-up time, 2) equipment failure caused by poor maintenance and 3) tissue web non-uniformity which leads to rejects and web breaks. Tissue properties can vary in both machine direction (MD) and cross-machine direction (CD). Over the years, FPInnovations has developed unique measurement tools to map the non-uniformity of printing and writing paper to relate it to web performance and runnability in pressrooms. These tools have been applied to characterize tissue non-uniformity and relate it to tissue converting performance and efficiency.

 

Roll Testing Facility (RTF)

 

FPInnovations developed its Roll Testing Facility (RTF) 15 years ago to quantify CD non-uniformity of paper rolls (Figure 1) and it has been used to diagnose and solve many web performance issues such as web breaks, bagginess, wrinkles and other roll structure defects.  An important step in its evolution came in 2015 when a rebuild of the equipment allowed the RTF to test low basis weight paper rolls such as tissue (all grades). The timing was good as the needs in tissue testing, for example in the uniformity as well as problems seen in embossing such as wrinkles, were increasing. In addition, all the knowledge FPInnovations developed from paper roll testing can be applied to tissue rolls.

 

For example, we applied RTF to test tissue rolls from a mill which experienced some “bagginess” problems. The good rolls, without any bagginess, were also tested for comparison. As can be seen in Figure 2, the good roll has a rather uniform tension across CD while the baggy roll has a significant variation of tension across CD. This explains why the mill has experienced some bagginess problems leading to wrinkling and web breaks. The mill took action on the tissue machine to correct the tension variation and the bagginess then disappeared.

 

Figure 1: FPInnovations’ Roll Testing Facility (RTF)

 

Figure 2: CD Tension Variation of Tissue: Baggy roll vs. good roll

 

The m-factor 

 

Tissue has very low basis weight so it is more sensitive to strength variations. To address this, we have applied a concept called “strength uniformity” which was developed for predicting web breaks of lightweight paper in the pressroom to quantify the strength uniformity of tissue in MD. Analyzing strength uniformity (m-factor) is a useful tool in troubleshooting web breaks in tissue converting. The low strength uniformity entails that there is a greater number of weak areas in the tissue; when the weak areas meet tension variations on the converting line, the risk for web breaks greatly increases. An m-factor analysis consists in measuring the variations of MD tensile for a certain tissue length, and then calculates the Weibull modulus (m-factor) of the tensile distribution using PapTune software which was developed at FPInnovations (Figure 3). The higher the m-factor, the higher the strength uniformity and the lower the risk of having web breaks.

 

Figure 3: Definition and Quantification of m-factor

 

We carried out benchmarking of strength uniformity of tissue samples produced on different tissue machines. As shown in Figure 4, there is a wide range of strength uniformity (m-factor from 10 to 18) among different machines. For the machines producing tissue with low strength uniformity factors, we worked with the mill staff to identify the potential causes and made recommendations to improve it.

The factors that could affect tissue strength uniformity include the variations in basis weight, caliper, fiber orientation, formation, non-uniform creeping…., etc. The contributions of these factors are being investigated under FPInnovations’ tissue research program.

 

Figure 4: How m-factor is measured and calculated

 

Stretch variation

 

Stretch is another important parameter as the tissue can have about 20 times higher stretch than traditional P&W paper grades. Stretch is created by creping and significant variations can be created by non-uniform creping. Tissue stretch is the key in many converting issues so FPInnovations has focused on achieving a more uniform stretch profile, finding the root causes of stretch variation that can lead to performance issues.

 

Below is an example of non-uniform stretch caused by poor creping due to the deterioration of Yankee surface. We measured the stretch profile of tissue samples collected along CD. As seen in Figure 6, there is a significantly lower stretch in one location. This was attributed to the local deterioration of the Yankee surface, as measured through surface profile.

 

Figure 6: CD stretch profile of the tissue samples before and after Yankee re-surfacing

 

After re-surfacing the Yankee, we measured the stretch profile again. As seen in Figure 6, the stretch profile was improved especially in the poor creping area. This indicates that Yankee surface quality has a significant effect on creping, thus tissue stretch variation. It is important for the tissue mills to monitor the Yankee surface and re-surface it if needed to ensure a uniform stretch profile.

 

Summary

 

Tissue web uniformity is critical for tissue making and converting efficiency as it can cause many performance issues including bagginess and web breaks. FPInnovations has developed unique measurement tools such as RTF, strength uniformity (m-factor) analysis, etc. to map out the tissue non-uniformity in both CD and MD. These tools have been used by tissue producers to identify the causes of tissue non-uniformity and minimize them through improving tissue making operations.