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Stickies Monitoring at a Newsprint and Packaging Mill

Jerome Andrew*1, Asheena Hanuman1 and Bruce Sitholé1, 2

1CSIR Natural Resources and the Environment – Forestry and Forest Products (FFP) Research Centre
2University of KwaZulu Natal – Department of Chemical Engineering

INTRODUCTION

One of the many challenges facing paper manufacturers both globally and in South Africa is the problem of stickies in recovered paper. Stickies refer to the tacky hydrophobic, pliable organic materials found in recycled paper systems (Putz, 2000; Hoekstra et al., 2001). Although there are no figures available in South Africa, it has been estimated that stickies-related problems are costing the United States paper industry several million US dollars per year (Friberg, 1997; Bajpai, 2006). This is mainly due to product quality problems such as holes, dark spots and reduced strength of the finished product (Putz, 2000; Sitholé & Filion, 2008). Runnability problems such as sheet breaks that arise due to deposits on the wire, felts, press rolls, drying cylinders and calenders also result in long periods of downtime for cleaning (Friberg, 1997).

In response to a questionnaire that was developed by the CSIR's Forestry and Forest Products (FFP) Research Centre and distributed within the recycling sector of South Africa, many paper recycling mills highlighted the measurement and removal of stickies as an area of concern for their operations. Stickies are derived mainly from pressure sensitive adhesives (PSA), hot melt adhesives (HMA), or a combination of both. Pressure sensitive adhesives are used in labels and tapes and appear tacky and flexible at room temperature. Hot melts are used in bindings and are usually solid at room temperature. Other main sources of stickies include printing inks and coating binders. Generally, stickies are classified as either primary or secondary (Sarja et al., 2004). Primary stickies remain solid throughout the papermaking process, and are further classified into macrostickies (>100μm in size) or microstickies (<100μm in size) (Doshi & Dyer, 2000). Secondary stickies originate from agglomeration of dissolved or colloidal substances as a result of abrupt physico-chemical changes in process conditions (e.g. temperature shock, pH shock, addition of cationic poly-electrolytes, etc.). Potential secondary stickies are the colloidal or dissolved substances that have not yet formed secondary stickies but have the potential to do so (Carré et al., 1998). A schematic description for the size classification of stickies is given in Figure 1. This size classification is important as it influences the strategies for removing each size class, and also influences the approach taken for minimising their effects on papermaking (Doshi & Dyer, 2000).

Figure 1
Figure 1. Size classification of stickies (Doshi et al., 2003).

The literature is replete with methods for the measurement of macrostickies and microstickies in recycled pulps. For some examples, see Doshi et al. (2003), Sitholé & Filion (2008) and Doshi (2009). In the case of macrostickies, there are three standardised methods available – TAPPI standard method T277, INGEDE (International Association of the Deinking Industry) method 4, and a method by the International Organisation for Standardisation (ISO) that is based on either visual inspection (ISO 15360-1:2000) or image analysis (ISO 15360-2:2001) of the screenate. Most South African mills utilising recovered fibre in papermaking are currently quantifying macrostickies using a screening method that is based, in part, on the TAPPI standard method T277. In this method, the sample is screened to separate/concentrate the macrostickies from the pulp, followed by heat-setting of the macrostickies between a black filter paper and coated paper, and finally the macrostickies are quantified by image analysis. Following the concentration step there are several divergent methods for analysing the macrostickies in the screenate, with most mills following the procedure described by Aquan-Yuen et al. (1999). This procedure involves lamination of the filter paper containing the screenate and subsequent quantification of macrostickies by image analysis. In the case of microstickies and potential secondary stickies, there are no standard methods available for measurement, and very few, if any, paper recycling mills in South Africa carry out these measurements on a regular basis. Some of the shortcomings of existing methods for measurement of macrostickies and microstickies that were highlighted by mill personnel during the survey included: (1) poor definition of stickies – depending on the size of the screening device macrostickies can be defined as larger than 75, 100 or 150µm; (2) long turn-around times; (3) the procedures are labour intensive and complicated; (4) in some instances there is need for sophisticated equipment and expertise; and (5) procedures often have poor separation of macrostickies and microstickies. Consequently, measurement of macrostickies and microstickies at some mills is not carried out on a regular basis. This results in lots of missing data, poor monitoring and incomplete understanding of the process with regards to stickies.

As a result of this, mill personnel have expressed their need for methods that are simple and quick and which can be easily implemented in a mill environment. In addition, the methods should not require significant capital investment. In response to this need, the CSIR developed methods that are based on existing methods in the literature but which have been modified to meet the requirements mentioned above. The objective of this study was to demonstrate the applicability of the new methods by undertaking stickies audits in a newsprint and a packaging mill to test the sensitivity of the methods to detect changes in the concentration of stickies at various stages in the process. The efficiency of the new methods was also compared with existing methods that are available in the literature for measurement of stickies.

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