ENZYMES: using Mother Nature’s tools to control man-made stickies
By Pulp & Paper Canada
Stickies control methods fall into two main categories, mechanical and chemical. Mechanical methods, screens, cleaners, DAF's and washing stages can all remove stickies. Each equipment type is effecti...
By Pulp & Paper Canada
Stickies control methods fall into two main categories, mechanical and chemical. Mechanical methods, screens, cleaners, DAF’s and washing stages can all remove stickies. Each equipment type is effective in removing stickies in a given size range. In most cases stickies removal has to be balanced against yield loss. With all of the mechanical methods only a certain percentage of the stickies will be removed. This is true even with the best equipment and process conditions.
Chemical control programs take two main approaches. There are stock additions, trying to tie up the stickies and to stop them from depositing and “point of problem” type applications. Traditional chemical programs have been effective but do not really attack the stickie itself.
Buckman took a new approach to stickies control by looking at enzymes that would break down and change the nature of the stickie. Esterase-type enzymes were found that actually break down the stickies into smaller, less tacky particles. This new product line is called Optimyze.
Stickies come as part and parcel of recycled fibre and are a challenge in all grades of paper. Runnability and quality problems caused by stickies can be hard to control. Sometimes, certain types or sources of recycled fibre have to be avoided, as the problems caused by their stickie content are impossible to live with. The global increase in the use of recycled fibre and the subsequent reduction in quality means these problems will not be going away. There is no reason to believe that they will not become increasing difficult.
Stickies are tacky, hydrophobic, pliable organic materials found in recycled paper systems. They have a broad range of melting points and different degrees of tackiness depending upon their composition. Stickies found in recycled fibre can be adhesives, SBR, rubber, vinyl acrylates, polyisoprene, polybutadiene and hot melts, to name a few. (This, of course, is not a complete list.)
The control of stickies involves mechanical and chemical approaches. Mechanical methods use screens, cleaners, dissolved air floatation (DAF) and washing stages to remove the stickies from the stock. Depending on the nature of the stickie, its size, melting point, conformability, etc., they may be removed in one or more of the various types of equipment. This variability means that while some stickies will be removed in one step of the process there is not one piece of equipment, or even a combination that will remove all the stickies, all of the time.
The mechanical methods can be made more efficient by manipulating process parameters such as temperature and pH. Process conditions can be set so that the stickies are in an optimum form to be removed by cleaners, screens, etc. However, once again the variability of the stickies means that not all of the stickies will be in the optimum form or that there even exists a single set of conditions that will allow the mechanical removal of all stickies.
Even with the best equipment and conditions, mechanical methods will not remove all of the stickies. This is where chemical methods come in to the picture. Chemical control methods have mainly involved dispersants, polymers and/or absorbents. The various chemicals used have two main modes of action. The first method is to tie-up or passivate the stickie. These are stock applications. The second mode of action is “point of problem” type applications. These passivate a surface or clean off the stickies after they have deposited on equipment.
A new direction
What was required was a family of products that would break down and change the nature of the stickies. Buckman started to look at enzymes as a way to attack them. Enzymes have been used in a variety of industries, including pulp and paper, for many years. Enzymes are mother nature’s catalysts that drive the chemical reactions that are in all living things.
Enzymes have the following properties:
* They are effective in very small amounts — a few enzyme molecules will catalyze thousands of reactions per second.
* They are unchanged and are not consumed in the reaction.
* They reduce the activation energy of a reaction and therefore increase the speed of reaction.
* They are very specific to a reaction.
* They have a specific pH and temperature range that they are active in.
The fact that enzymes are specific to a certain reaction allows enzymatic products to be tailored to specific needs. For example, a cellulase enzyme will break down cellulose but will not break down starch. This means that the enzyme used to control stickies will not affect fibre or other papermaking additives. The trick was to find enzymes that would break down stickies.
Enzymes act as a catalysis for specific chemical reactions. The first step was to determine which chemical bonds were predominant in stickies. A study of stickies revealed that many have a large number of ester type chemical bonds that link the basic building blocks of the stickie. Therefore, what was required to breakdown the stickies was an esterase. A number of esterase type-enzyme mixtures were studied to find one that had the ability to break down a high percentage of stickies. Not only will the enzyme break the ester bonds in the stickie and reduce their size but also as the stickie compound is broken down into smaller components, there will be a greatly reduced chance that the stickies can reagglomerate further down the process.
Unique family of stickies control products
The Optimyze product line is the result of this work. This family of products is unique patented esterase enzymes. The family started with three products, one each for MOW (mixed office waste), ONP/OMG (old newspaper, old magazine) and OCC (old corrugated containers).
The enzymes have been selected for their ability to break down a variety of stickies into smaller components. Figure 1 is photomicrographs that show PVA (a major stickie source) before and after treatment with the enzyme. The reduction in size of the stickie particles is dramatic. Another important effect on the stickies is the enzyme modification of the surface of the stickie. The photomicrographs in Figure 2 show a PVA type stickie before and after treatment with the enzyme. A distinct change in the nature of the surface of the stickie can be seen. This change results in a less tacky stickie. A less tacky stickies results in less deposition.
As discussed before, enzymes have specific pH and temperature ranges in which they are effective. To ensure the most effective use of these products, the following conditions are required:
* The pH must be greater than 6.5 and less than 10 in the process where treatment occurs;
* The temperature must be between 25C (80F) and 70C (160F);
* There must be no or minimal oxidizing chemistry residual in the process where treatment occurs; and
* There must be 45 minutes contact time.
The application point for these products is determined by the nature of the stickies problem and the parameters and design of the process. A complete system survey is done to quantify the number of stickies and their size distribution. For macro-stickies, a measurement method has been developed that uses a screen, a heated press and then a scanner to measure the amount and size distribution of the macro-stickies. A Pulmac Masterscreen set up to screen stickies is used to collect stickies from a pulp sample. The stickies are collected on the reject pad. The pad is dried and then the stickies are transferred to filter paper using a heated press. You can also use a transparency and a laminator. The stickies on the filter paper or transparency are then measured using a flatbed scanner and scanning software.
Typical results from the macro-stickies test are shown on Figure 3.
The graph details how the size distribution changes with the application of the enzyme. The number of larger stickies on the right of the graph has been reduced to zero as they are broken down. The total number will drop as the stickie size is reduced to below the detection limit of the scanning software. Also, as the stickies are reduced in size, they will pass through the screen.
Smaller, less tacky stickies that do not reaglommerate lead to a reduction in stickies deposition. Figure 4 shows a doctor blade that had a large build-up of stickies. Figure 5 is the same doctor blade after application of the enzyme. The enzyme treatment has eliminated this build-up with subsequent improvements in machine runnability.
The family grows
The esterase products have proven their effectiveness in providing major improvements in the control of troublesome stickies. All of the early work involved applying the product into the stock. A recent development was the investigation of “point of problem” type applications. Applying the enzyme directly onto paper machine clothing to reduce stickie deposition was investigated and found to be effective. The enzyme has been applied on forming fabric and press felts with positive results. The application is through a shower bar, full coverage is important. The mechanism involved is the enzyme’s detackifing effect on the stickies. Making the stickies less “sticky” allows the cleaning showers to more effectively remove them from the fabrics.
A green family
Enzymes have the important benefit in that they can be considered a “green’ product. They are natural occurring compounds with little adverse impact on the environment. In 2004, the Optimyze family of products was awarded a US Environmental Protection Agency Presidential Green Chemistry Challenge Award.
The use of recycled fibre continues to grow. As the demand increases, the quality of the recycled fibre will decrease. The result: more stickies. More stickies, more stickie headaches. Mechanical and traditional chemical control approaches have worked but they are not the complete answer. Enzymes are a new approach in the battle against stickies and have proven effective.
In-stock applications have been shown to be effective and now paper machine clothing applications are being proven. Work continues in increasing the understanding of the mechanism of these products and also to expand the applications available. The Optimyze product line from Buckman is successfully helping to minimize or eliminate the problems caused by stickies in many mills around the world.
David R. Jones is an Industry Specialist at Buckman Laboratories of Canada Ltd.