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What you need to know about starch in papermaking.

Starch application in papermaking dates back to the invention of paper itself 2000 years ago, when starch was applied to paper for a stronger, smoother writing surface. For mineral filler containing papers, starch is the highest volume raw material after water, fiber, and fillers.

Starch contributes to papermaking by providing functional properties and serving as a process aid. Paper mills use starches from various sources, such as regular corn, waxy maize, tapioca, potato, and wheat. Usage depends on availability and economics in a given region. Worldwide paper starch consumption consists of 67% corn, 15% potato, 8% tapioca, and 3% waxy maize, according to market estimates.

Starch manufacturers sell "native" and several types of modified starches to the paper industry. Mills normally purchase starch in dry powder form and cook it onsite prior to application. Modified starches in general offer better value than native starches, providing more consistent starch quality coupled with the starch supplier's application expertise.

STARCH UTILIZATION

Starch utilization in papermaking depends on the type of paper, other raw materials used, papermaking technology, desired end properties, and paper machine productivity needs. For example, tissue grades typically use low amounts of starch or no starch while fine printing and writing papers can use up to 10% starch by paper weight. A higher amount of starch is used in paper grades containing higher mineral fillers to maintain strength and printing properties.

Traditionally, starch has been used as a dry strength and surface improvement aid. But in alkaline papermaking, starch is a critical part of wet-end sizing. Starch is an integral part of microparticle retention systems. Surface starch also works as a binder, water holding agent, and carrier for surface sizing chemicals and other functional additives.

The global paper industry uses about 5 million tons/yr of starch. That amounts to about 1.5% starch by weight across all grades of paper and paperboard. Modified starch usage is more common in the United States, where modified starches not only increase productivity, but also 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 the Corn Refiners Association.

Corn-based starches account for more than 95% of all starch utilization by the paper industry in the United States. Modified corn starch consumption grew by 14% during 2004 from 2003, while native corn starch consumption dropped 10.5% in the same period. Including starches based on other raw materials, the share of modified paper starches in the United States is even higher than 76%. Of all modified corn starch shipped by the members of Corn Refiners Association, 66% went into paper manufacturing.

By contrast, less than 60% of the starch used by the Asian paper industry is modified starch. As environmental enforcement becomes stricter and the quality of paper rises in Asia, the trend will be to use more modified starch to control waste discharge, enhance paper quality, and increase productivity. As Asian 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 due to improvement in paper quality and utilization of higher than usual amounts of recycled fibers, agricultural fibers, and mineral fillers.

CONTRIBUTIONS OF STARCHES TO PAPERMAKING

As mentioned before, starches were traditionally used for imparting dry strength and enhancing surface integrity when paper was made primarily from fiber. Over time, requirements have changed as papermaking has evolved into a complex process using nontraditional raw materials, including recycled fiber, agricultural fiber, mineral fillers, and myriad chemicals designed to improve the papermaking process and functional paper properties.

Today, starch is used in wet end applications with pulp to enhance dry strength, including the ply bond and stiffness of paper, and to improve fines and chemical retention, drainage, internal sizing, formation and printability. It is also used 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 in Figure 1. For example, when used properly, modified wet end starches allow higher utilization of low cost mineral fillers to replace expensive fibers and reduce refining energy needs by providing additional paper strength. Reduction in refining allows additional drainage on the 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 fibers, thus enhancing the cleanliness of the papermaking system and improving productivity.

There have been many examples of increased productivity. In one case, a mill using additional wet end starch eliminated wet press picking, allowing its paper machines to run for longer periods 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 deposits on paper machine elements, increasing productivity. Yet another paper mill was able to lower 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 paper rejects, it also simplified the starch preparation system.

[FIGURE 1 OMITTED]

STARCH APPLICATION METHODS

Starches are applied in the papermaking process at several stages by 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 increase ply bond strength. Mixing cooked, modified wet-end starches with the pulp improves strength, sizing, retention, drainage, formation, wastewater 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 critical to optimum performance of the selected wet-end starch. The application of starches during various papermaking steps is shown in Figure 2.

A major portion of starch is applied by pond or metering size presses on the surface of the paper. Surface starches applied with size presses increase internal strength and improve surface integrity, printability, and surface strength. In some paperboard grades, surface starches are applied with calenders to reduce fuzz and improve stiffness, printability, surface strength and curling tendency.

To reduce coating costs, some paper mills use starch solutions at the calender and size press as pre-coats prior to the application of expensive coating chemicals. In coated grades, starch acts as binder and rheology modifier in the aqueous coating and reduces costs by replacing expensive synthetic chemicals. Since they are natural, renewable, and biodegradable, starches are environmentally friendly.

ADVANTAGES OF MODIFIED OVER NATIVE STARCHES

The advantages of modified starches over native starches are manifold and include increased productivity and improved quality. As paper machines have become faster and wider, the impact of downtime and off-grade paper has become very costly. Therefore, the best value provided by modified starch over native starch is the reduction in downtime and improvement in paper quality. Additional benefits include.

* Improvement in wastewater discharge quality with charged starches

* Elimination of chemicals and equipment for on-site conversion of native starches

* Reduction in labor costs due to the simplicity of cooking and using modified starches

* Improvement in cooked starch stability and hence minimum starch rejection

* Attainment of consistent quality starch leading to reduced process variation

* Enhancement in overall operation due to vendors' service and application of best practices

One paper mill saw dramatic improvement in white water and paper quality by replacing native starch in the wet end with a cationic starch. This mill also reduced starch usage with 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 instead of onsite converted native starch.

[FIGURE 2 OMITTED]

STARCH TYPES

There are several types of starches based on raw materials and modification methods. Potato starch used to be the dominant modified wet end starch in the United States a decade ago; however, this starch has been mostly replaced in the wet end by modified corn starches due to cost and availability. In Europe, too, modified corn starches have penetrated the wet end due to cost issues in spite of the region being 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, wet end starches are cationic or amphoteric, whereas surface starches are oxidized or hydroxyethyalted. Some of the relatively newer wet end starches include cross-linked, anionic, liquid or dry pre-gel and highly charged wet end starch. Other modified starches used for surface applications include hydroxypropylated, acetylated, acid modified, phosphate ester and dextrin.

The changing needs of papermaking have provided the opportunity for starch manufacturers to develop newer grades of starch to help improve paper quality and productivity. Starch applications experts help the papermaker choose the right type of starch for a given application for optimum productivity, quality and manufacturing cost.

THE MOVE TO MODIFIED

For several centuries, various types of starches have played key roles in improving paper quality, increasing productivity, and reducing overall cost. Being a natural biodegradable product, starch is considered environmentally friendly compared to synthetic chemicals. Since starch is an abundant, renewable product, it offers economic value to papermakers unparalleled by other chemicals. It still offers the best value when it comes to paper strength.

With increased use of alkaline papermaking, starch has become an essential part of alkaline sizing programs while starch used as a polymer has become essential for microparticle retention systems. Increased use of modified starch is on the horizon not only to improve quality and productivity, but also to meet or exceed rising environmental standards.

Some paper mills use native starch 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. Paper mills must focus on their primary goal of producing paper and board at the highest quality and at the lowest cost. Since starch preparation systems in many paper mills are not considered to be a high priority, onsite converted native starches could in fact increase the overall cost of manufacturing due to:

* Off quality starch wastage

* Use of labor, chemicals and energy and

* Off grade paper production.

Starch suppliers that offer a proper selection and application of modified starches can help papermakers meet or exceed their goals.

ABOUT THE AUTHOR

Ashok Kumar Mishra is director, Asia/Africa division, Corn Products International Inc., Westchester, Illinois, USA. Comments, questions, and additional information can be directed to the author by email at: ashok.mishra@cornproducts.com. He has over 27 years of experience working in the paper and starch industries in various capacities. He has graduate degrees in paper science and management and lives in Naperville, Illinois.

[ILLUSTRATION OMITTED]

ASHOK KUMAR MISHRA, CORN PRODUCTS INTERNATIONAL INC.

WHAT YOU WILL LEARN

* How starch contributes to the papermaking process

* Global trends in starch usage

* Starch application methods

* The benefits of using modified starches

ADDITIONAL RESOURCES

* Starch and Starch Products in Surface Sizing and Paper Coating, H.W. Maurer, ed. ISBN: 1930657560. This monograph discusses the properties and uses of starches, specifically the application of starch and starch products for surface sizing and coating of paper and paperboard. 2001. 170 pages, soft cover. This text is available from TAPPI Press. For more information, go to www.tappi.org and enter the following Product Code in the search field. 0101R297. Or, call 800 332-8686. Member Price: $102.00 Non-Member Price: $156.00
COPYRIGHT 2005 Paper Industry Management Association
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Article Details
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Title Annotation:BACK TO BASICS
Author:Mishra, Ashok Kumar
Publication:Solutions - for People, Processes and Paper
Date:Aug 1, 2005
Words:1951
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