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MODIFIED STARCHES FOR ENHANCED PAPER QUALITY
Ashok Kumar Mishra
Starch application in papermaking dates back to the invention of paper itself when starch was applied to the paper for obtaining a stronger smoother writing surface about two millennia ago.
For mineral filler containing grades of paper, starch is the highest volume raw material after water, fibres, and the fillers used for manufacturing paper worldwide. Starch contributes to papermaking in many ways because it provides functional properties to the paper and works as a process aid as well. Paper mills use starches from various raw material sources such as regular corn, waxy maize, tapioca, potato and wheat, mostly based on the availability and economics in a given region. It is estimated that worldwide paper starch consumption consists of 67% corn, 15% potato, 8% tapioca, and 3% waxy maize.
These starches are normally purchased in dry powder form and cooked onsite in paper mills prior to application. Starch manufacturers sell native and several types of modified starches to the paper industry. Paper mills simply cook the modified starches obtained from the starch manufacturer before application or modify the native starches onsite prior to cooking and application. Since starch manufacturers continuously invest in technology and resources to remain competitive, the modified starches obtained from starch manufacturers in general provide better value than native starches to the paper manufacturers due to consistent starch quality coupled with the starch supplier's application expertise.
Starch utilization rate in papermaking depends on the type of paper, other raw materials used, papermaking technology, desired end properties of paper, and paper machine productivity needs. For example, tissue grades of paper utilize low amounts of starch (ed: to increase dry strength) or no starch whereas fine writing and printing papers could use up to 10% starch by paper weight. A higher amount of starch is used with paper grades containing higher mineral fillers to maintain strength and printing properties. Traditionally, starch was used as a dry strength and surface improvement aid, but in alkaline papermaking, starch is a critical part of the wet-end sizing and with micro-particle retention systems, starch is an integral part of the wet-end retention system. Surface starch also works as a binder, water holding agent, and priming coating for the surface sizing chemicals and other functional additives.
It is estimated that about 5 million tons of starch are currently used by the world paper industry, that is about 1.5% starch by weight including all grades of paper and paperboard. The use of modified starches is more common in the United States where modified starches are not only used to achieve higher productivity, but also higher paper quality. For example in 2004, of all the corn-based starch used by the paper industry in the United States, 76% was modified, according to data obtained from the Corn Refiners Association. Corn-based starches are estimated to account for more than 95% of all starch utilization by the paper industry in the United States. Modified cornstarch consumption grew by 14% in 2004 over 2003 whereas native cornstarch consumption declined by 10.5% for the same time period. Including starches based on other raw materials, the share of modified paper starches in the United States would be even higher than 76%. In fact of all modified cornstarch shipped by the members of the Corn Refiners Association, 66% went into paper manufacturing. Thus, papermaking is the largest modified starch application of all applications in the United States.
In contrast, the Asian paper industry uses less than 60% modified starch of the total starch consumption. In Africa, the percentage of modified starch of the total starch use is only about 35%, which is far lower than Asia. As the environmental enforcement becomes stricter and the quality of paper rises in Asia and Africa, the trend will be to use more modified starch to control waste discharge, enhance paper quality, and increase productivity. As Asian and African economies and standards of living continue to grow, paper production is expected to rise correspondingly. Modified starch opportunities in Asia are expected to grow at a faster rate than paper production growth due to improvement in paper quality and utilization of higher than usual amounts of recycled fibres, agricultural fibres, and mineral fillers.
Contributions of Starches to Papermaking
As mentioned before, starches were traditionally used for imparting dry strength and enhancing the surface integrity for improved writing and printing when paper was primarily made of fibres. Over time, the requirements have changed as papermaking has evolved into a complex process utilizing nontraditional sources of fibre including recycled fibres and agricultural fibres, mineral fillers, and a myriad of chemicals for improved papermaking process and functional paper properties. Now starch is used in the wet-end with the pulp to enhance dry strength, including ply bond and stiffness of paper as well as to improve fines and chemical retention, drainage, internal sizing, formation, printability, and to lower refining energy, biochemical oxygen demand, and overall paper manufacturing costs. Surface starches improve surface as well as internal strength and printability. A pictorial display of starch contributions to papermaking is presented below in Figure 1:
Figure 1. Illustration of starch contributions to papermaking. The primary use of starch has been for dry strength
For example, when used properly, modified wet-end starches allow higher utilization of low cost mineral fillers to replace expensive fibres and reduce refining energy needs by providing additional strength to the paper. Reduction in refining allows additional drainage on wire leading to energy savings in pressing and drying operations. Wet-end starch also works as a protective colloid for reactive sizes in alkaline papermaking. Wet-end starch anchors and distributes the reactive sizes to the papermaking fibres, thus enhances the cleanliness of the papermaking system and improves productivity. One practical example of wet-end starch enhancing the productivity was when additional wet-end starch was able to eliminate wet press picking and hence, allowing the paper machine to run for a longer period of time without breaks. In another case, use of modified wet-end starch in place of synthetic polymer as an emulsifier for the reactive alkaline size eliminated the deposits on the paper machine elements, and thus increased productivity. Yet another paper mill was able to reduce the overall starch usage and reduce streaks on the paper machine when it used modified surface starch in place of onsite converted native starch. Use of modified starch at this mill not only reduced the paper rejects, it also simplified the starch preparation system allowing the paper mill to focus on other important matters.
Starch Application Methods
Starches are applied in the papermaking process at several steps by several different methods to achieve desired results. For example, the use of uncooked spray starch in the wet-end in between plies in multiply grades has been found to be effective in increasing ply bond strength whereas the mixing of cooked modified wet-end starches with the pulp improves strength, sizing, retention, drainage, formation, waste water quality, and productivity. Proper selection and application of modified starches ensures the benefits listed above. Starch addition rate, point of addition, and compatibility with other wet-end chemicals are also important points to cover for optimum performance of the selected wet -end starch. The application of starches in various papermaking steps is shown below in Figure 2:
Figure 2. Starch application points in papermaking
A major portion of the total starch is applied by the size presses on the surface of the paper. These size presses are commonly of conventional pond or metering type. Surface starches applied with size presses increase internal strength in addition to improving the surface integrity, printability, and surface strength of the paper. In some paperboard grades, surface starches are applied utilizing calenders to reduce fuzz and improve stiffness, printability, surface strength, and curling tendency. Some paper mills use starch solutions at calender and size press as pre-coats prior to the application of expensive coating chemicals to reduce coating costs. In coated grades, starch acts as binder and rheology modifier in the aqueous coating and reduces costs by replacing expensive synthetic chemicals. Starches also improve the environment by being a natural, renewable, and biodegradable polymer.
Advantages of Modified Over Native Starches
The advantages of modified starches over native starches are manifold. The most significant of all are the increase in productivity and improvement in quality. As the papermaking has become increasingly complex, paper quality has improved, and paper machines have become faster and wider, and the impact of downtime and off-grade paper has become very costly to the paper mills. Therefore, the best value provided by the modified starch over native starch is the reduction in downtime and improvement in paper quality. Additional benefits offered by the modified starches are:
One paper mill saw dramatic improvement in the white water and paper quality by replacing native starch in the wet-end by a cationic starch. This mill also reduced the starch usage with the cationic starch to achieve similar results. Another paper mill saw reduction in overall manufacturing costs, improvement in paper properties, and simplification of the starch preparation system when it used modified surface starch by replacing onsite converted native starch.
There are several types of starches available to the papermaker to choose from. These starches are based on various raw materials and have been modified by several different methods. Potato starch (ed: also called farina starch) used to be the dominant modified wet-end starch in the USA about a decade ago; however, this starch has been mostly replaced by modified cornstarches due to cost and availability reasons. (ed: The chemical difference between corn and farina starches lies in the relative ratio of amylopectin (branched chain) to amylose (non-branched polymer) present.) Now, the wet-end systems have been adapted to use the modified cornstarches. In Europe also modified cornstarches have penetrated the wet-end due to cost issues in spite of the region well known for its potato starches. In Asia and Africa, several paper mills are using tapioca based starches whereas Australia is using wheat-based starches. In most cases the wet-end starches are cationic or amphoteric starches whereas the surface starches are oxidized or hydroxyethylated. Some of the relatively newer wet-end starches include cross-linked, anionic, liquid or dry pre-gel, and highly charged wet-end starch etc. Other modified starches used for surface applications include hydroxypropylated, acetylated, acid modified, phosphate ester, and dextrins etc. The changing needs of papermaking have provided opportunity to starch manufactures to develop newer grades of modified starch to help improve paper quality and productivity. Starch application experts help the paper maker choose the right type of starch for a given application for optimum productivity, quality, and manufacturing cost.
Various types of starches have played a key role in paper production to improve paper quality and productivity and reduce overall cost for several centuries. Being a natural biodegradable product, starch is considered environmentally friendly compared with synthetic chemicals. Since starch is an abundant renewable product, it offers economic value to the papermaker unparalleled by another chemical. It still offers the best value when it comes to paper strength. With increasing use of alkaline papermaking, starch has become an essential part of the alkaline sizing program while starch is used as a polymer for micro-particle retention systems. Increased use of modified starch is on the horizon not only to improve quality and productivity, but also to meet or exceed the rising environmental standards. Some paper mills use the native starch only because of its lower price, but the overall cost of using native starch could be higher than modified starch due to lost productivity and lower paper quality. It is important for the paper mills to focus on their primary goal of producing paper and board at the highest quality at lowest cost. Since starch preparation systems in many paper mills are not considered to be of high priority, onsite converted native starches could in fact increase the over all cost of manufacturing due to: 1. Off quality starch wastage, 2. Use of labour, chemicals and energy, and 3. Off grade paper production. Starch suppliers with proper selection and application of the modified starches could help the papermakers meet or exceed their goals.
(paper reviewed by Bob Heimann)
Comments, questions, and additional information can be directed to the author at: firstname.lastname@example.org .
Ashok Kumar Mishra has over 27 years of experience of working in the paper and starch industries in various capacities. He has graduate degrees in paper science and management and lives in Naperville, IL USA.