Stickies are adhesive substances that can cause a number of problems in the machine cycle. Solving the problem of their formation requires the evaluation of a series of parameters and the choice of how and where to intervene. Understanding this scientifically means identifying the correct method of action.
Whether they result from the use of certain fibrous raw materials or are caused by chemical reactions within the machine cycle, stickies have always been a problem that paper mills struggle with. These sticky substances can create a number of problems in paper production. From sheet breakage with consequent interruption of the production cycle and loss of time and money, to problems with machine clothing that loses its permeability, to a drop in efficiency in the dryer. In addition to the difficulties in the production process, there is also the damage that stickies can cause to the finished product, such as stains, pinholes, incorrect porosity etc. It is therefore necessary to understand how they form and why, and how to remedy them. This was explained by Luca Paccagnella, paper specialist at Acquaflex who spoke about it at the Aticelca 2021 Congress.
It has been said that the use of certain raw materials, in particular when it comes to fibres derived from recycled paper, waste paper or leaflets, can influence the formation of stickies, as can the use of various chemical auxiliaries which, by reacting with each other, cause the formation of these substances. The increasingly widespread use of special surface treatment products, for example, is a further contributory factor.
These substances, explains the expert, can have adhesive properties even at room temperature or acquire adhesive characteristics when temperatures rise; in the paper production cycle this happens in particular in the dryer or press area. This is where action is needed.
Acquaflex has been producing and marketing products and speciality chemicals for the paper industry for many years, but the approach it presents here to solving the stickies problem is first and foremost a matter of method.
An approach, explains Paccagnella, which is useful as a general rule for dealing with any problem. It starts by defining the problem or how the mill perceives the problem, then moves on to analysing the stickies to understand what defects they cause on the paper or what deposits they create. «We do a thorough analysis of the process water, detailed as it relates specifically to this type of problem; we set up an industrial-type test together with the mill, defining exactly what the target is; we monitor the test with numbers, so that we can make the assessment quantitative, and thus evaluate whether or not we have achieved the solution to the issue that the mill had raised».
The first step is to define what Paccagnella refers to as «the boundaries conditions of the problem» and then determine «the parameters that identify it». Firstly, the point at which the stickies occur is located, then in which sections of the machine the problem occurs and what defects the stickies create on the paper. «We specify the number of breaks, black spots, pin holes, etc. and then we quantify exactly what the problem is with the paper mill. Together we define the minimum target we want to achieve, trying as much as possible to base it on objective and measurable parameters so that we can quantify it as detailed as possible. In this way we can carry out a statistical analysis».
The procedure for analysing Acquaflex stickies and process water starts with a traditional – i.e. quantitative – chemical analysis of both the organic and inorganic fractions. «We can thus determine the composition of the organic fraction and the percentages of the various elements present. We use infrared spectroscopy techniques, in particular FT-ATR, to identify the composition of the stickies by comparing it with the database that we at Acquaflex have built up over more than 15 years of experience with this type of problem». This enormous collection of information allows the company to identify the nature of the adhesive substance and its origin.
As far as process data analysis is concerned, continues Paccagnella, «we carry out detailed chemical and physical characterisation of the water, so we check pH, conductivity, determination of various hardnesses, temperature, redox potential and other special measurements. In addition to these, we make specific measurements of the paper mill water, in particular we analyse the charge requirements and the Z potential, which are parameters that can influence the creation of deposits».
Analysis and the fluorescence technique
In order to carry out this phase of the analysis at the paper mill, Acquaflex has developed a mixture for use with the fluorescence technique. The latter is a method of investigation using a specific dye that reacts with the stickies, developing a fluorescent effect. The Milan-based company’s technicians have designed a special product that guarantees high reproducibility of the data being observed and, above all, is easy to use, using techniques and instruments that can be used on site.
«In water, the dye binds to the stickies, making them identifiable precisely through spectroscopic techniques» explains Paccagnella. «The interesting thing is that these adhesive substances made fluorescent can also be easily measured with field instruments and therefore directly at the paper mill». This makes the work much easier, as it is no longer necessary to take samples from the cycle and then bring them to the laboratory for analysis. Everything is greatly speeded up and simplified.
Specifically, a fluorescence microscope is used in the field, which “also allows us to take photographs with which we can carry out an initial qualitative investigation of the stickies. We can then identify the size and – very importantly – the shape of the stickies, thus the degree of coagulation of the stickies and their structure. From here, quantitative measurements are made with a portable fluorometer which provides, as mentioned, reproducible values, but above all very quickly. In this way, a characterisation is made in what we call macro and microstickies». A definition which, Paccagnella points out, is not exactly correct, but which serves to distinguish between the stickies that must or must not be taken into consideration during analysis. Specifically, he explains, microstickies are defined as those between 0 and 30 µm, macrostickies as those between 30 and 150 µm.
Designing treatment in the paper mill
The collection of analytical data is then used to design the most appropriate treatment for the specific situation.
«First of all, the collection of analytical data allows us to design the treatment to be proposed to the paper mill quite easily. However, the key moment of the design» as pointed out by the Acquaflex expert, «is the choice of the anti-stickies technology to be used». The definition “technology” is not made at random; Paccagnella explains that it is not just a matter of deciding which products to use, but also which application points should be chosen and which application methods should be used. Pagnanella adds that «these technologies of ours can either replace or complement other traditional technologies and materials or products commonly used in these cases, such as talcum powder or aluminium salts. The range of technologies we have at our disposal is very wide indeed.
The decisive factor, it is said, is the point of application. «They can be applied in the wet end, in forming cloths, in dryer cloths and in dryer cylinders. A different technique will be chosen for each section of the paper machine with its own particular challenges».
He explains that during the start-up phase of the trial all the same analytical checks made during the treatment study phase are monitored and these parameters allow a quick check of how the system is performing. He points out that «it doesn’t guarantee the result, but it allows us to understand whether the system we are applying is working in the way we intended. This allows us, very quickly – within a couple of days – to bring the industrial test up to optimum conditions».
The case study
The method was applied to a real case at a paper mill in northern Europe. «The customer complained of numerous breaks in the dryer, despite the fact that he had special cleaning treatments for the cylinders and wires» says Paccagnella. The problem occurred at the cylinders that first came into contact with the cover side of the paper – the PM was a cover and back machine. «The chemical and physical analysis of the water showed us that the problem was on the cover side of the paper and on the cylinders we could see a lot of stickies, mainly due to vinyl glues».
The first step was to choose where to carry out the analysis. «We placed our sampling points in the machine wires, underlays and inflow boxes, and detailed the stickies».
«The analysis showed a significant presence of macrostickies in the cover line and the hypothesis developed was that these had been the cause of the problems in the dryer. We therefore turned to a treatment that we carried out directly on the cover line. The treatment involved the use of products based on natural solvents in emulsion form, capable of reducing the adhesion of the stickies and with a dispersing action». The product, as Paccagnella explains, had a dual purpose, «it was used to disperse the macrostickies and, at the same time, to prevent them from re-adhering, but above all it was intended to remove their adhesiveness».
During the first phase of the trial, the technicians recorded a significant reduction in the number of macrostickies in the cover line. At this point, the choice was made to proceed with a small modification from the application point of view, «basically, the dosing point was moved and we were able to reduce the macrostickies to practically zero».
This change in dosing point resulted in a slight increase in microstickies, but this was a positive effect, explains Paccagnella, because it meant that the chosen application was working; «the aim was to reduce macrostickies and possibly convert microstickies to non-clotting. The interesting fact is that the average macrostickies really did reduce dramatically».
The chosen treatment reduced the number of breaks that the mill was experiencing on a daily basis; specifically, recalls Paccagnella, «it saw a reduction in paper sheet breaks of almost 60% day by day».
Innovation is in the method
Ultimately Acquaflex has created a system in which the innovation lies not so much in the type of products used – although some are very special – but in how the mill’s problems are managed. «We are able to deal with the problem very quickly and analyse the progress of the trials» says Paccagnella. «The detailed study of the machine, combined with the experience we have gained over the years, promises us to find tailor-made solutions for each individual paper machine with a remarkable speed of treatment set-up». He concludes that «this means a guarantee that the method with which we are applying the proposed technology is definitely the best».