News

Choose wet-end starch when the main objective is stronger bonding within the sheet and the mill can retain the added starch effectively. Choose surface starch when the priority is surface strength, reduced picking or dusting, or control of the sheet’s response to printing and converting. Many paper grades benefit from both, but the balance depends on where strength is missing, how the furnish behaves, and what the application equipment can deliver.

For technical evaluators comparing modified starch for paper industry applications, the first decision is therefore the point of action. A starch that performs well in the stock system may be unsuitable for a size press, while a surface-sizing grade with excellent viscosity control may contribute little if added to a furnish that cannot retain it. Start with the sheet’s failure mode, then work backward to starch chemistry, preparation conditions, and application limits.

Identify the property that needs improvement

“Higher strength” is too broad a specification for selecting starch. Low internal bond, weak tensile performance, surface picking, and poor resistance to converting stresses can require different interventions. A sheet may have adequate bulk strength yet release fibers during printing. Another may have a strong surface but insufficient bonding through its thickness. Increasing starch consumption without distinguishing these conditions can add cost while leaving the limiting property largely unchanged.

Wet-end starch supports fiber bonding within the sheet when it is retained and distributed effectively. Surface application places starch at and below the surface, with penetration governed by the base sheet and application conditions. It can improve properties beyond the outermost layer, but the depth and uniformity of that contribution must be established for the grade concerned.

Also distinguish surface sizing from water resistance. Starch can strengthen and consolidate the surface, but starch alone should not be assumed to provide the required resistance to liquid water. Where water absorption is a specification, evaluate the complete sizing system, including internal or surface hydrophobic sizing agents and their compatibility with the starch formulation.

Before requesting samples, define the limiting property, the relevant test method, and the operating conditions under which the problem appears. For a packaging grade, that might involve strength after conditioning and converting. For a printing grade, surface integrity under representative printing conditions may be more revealing than a general strength result.

Wet-end selection depends on retention and charge compatibility

Cationic starch is a common starting point for wet-end strength development because its positive charge can promote adsorption onto negatively charged furnish components. Its value depends on how much reaches useful bonding sites. Fibers, fines, fillers, and dissolved or colloidal substances all influence that distribution, so the starch addition rate alone says little about effective utilization.

A furnish with substantial anionic demand can consume part of the starch’s charge before it contributes effectively to fiber bonding. Recycled furnish can introduce variability through dissolved contaminants, fines, and residual process chemicals. Conductivity and changes in water-system closure can further alter adsorption and chemical interactions. A starch selected under relatively clean laboratory conditions may therefore behave differently in circulating machine water.

Higher cationic substitution is not an automatic route to better performance. It changes adsorption behavior and interactions with the rest of the wet-end program. Depending on the system, it may improve utilization or create an unfavorable balance with retention aids, fixatives, and other charged additives. Compare grades in representative furnish and process water, keeping the addition sequence and mixing conditions relevant to the machine.

Amphoteric starch can be a candidate where its combination of charged groups suits the furnish chemistry. However, the label alone does not establish suitability for a particular pH, conductivity, or contaminant load. Ask suppliers to describe the grade’s charge characteristics and intended operating window, then verify the response under mill conditions.

Cooking quality deserves equal attention. Incomplete gelatinization, inconsistent solids, or deterioration during storage can undermine an otherwise suitable chemistry. The evaluation should cover the mill’s actual cooking method, dilution practice, holding temperature, and residence time. A laboratory preparation that the production system cannot reproduce provides a weak basis for selection.

During a trial, track strength alongside drainage, formation, retention, and white-water behavior. A gain in tensile strength may be commercially unattractive if it comes with poorer formation or a drying constraint. Likewise, additional starch that produces little incremental strength may indicate limited adsorption capacity, interference from other substances, or a different bottleneck in sheet development. Raising dosage should follow diagnosis.

Surface sizing requires a workable viscosity–solids balance

At the size press or film press, the starch must form a stable application solution, transfer consistently, and reach the appropriate depth in the sheet. Selection therefore centers on cooked viscosity, solids concentration, solution stability, film properties, and interaction with the base paper. Charge remains relevant, especially in mixed formulations, but wet-end selection criteria cannot simply be carried across.

Oxidized, enzyme-converted, and other viscosity-adjusted starches can offer useful routes to surface application. These descriptions identify broad modification approaches, not interchangeable performance grades. Molecular degradation, functional groups, botanical source, and preparation conditions can all influence the resulting solution and film.

Reducing viscosity can allow application at higher solids, potentially reducing the water introduced for a given dry starch pickup. That opportunity must be balanced against strength development and film integrity. Excessive molecular degradation can compromise performance, while a solution that is too viscous for the equipment can cause uneven application or restrict the achievable solids level.

Compare viscosity values only when the measurement conditions are comparable. A supplier’s number has limited meaning without solids concentration, temperature, preparation method, and measurement procedure. Measurements taken after cooking and after a representative holding period are useful because viscosity drift, gel formation, or instability may emerge before the solution reaches the applicator.

The application system changes the selection window. A pond size press exposes the sheet to a different transfer and wetting process from a metered film press. Machine speed, metering arrangement, nip conditions, and solution temperature affect application behavior. A grade that works on one installation may require a different solids level or viscosity profile on another.

Base-sheet absorbency is equally influential. An open, absorbent sheet can draw starch inward, contributing to strength while leaving less at the surface. Greater holdout can favor surface coverage but may not produce the desired penetration. Internal sizing, porosity, moisture, and formation should therefore be recorded when comparing starches. Otherwise, a change in the base sheet can be mistaken for a difference between products.

Surface-sizing trials should measure actual dry pickup and its uniformity across the sheet. Equal bath concentration does not guarantee equal application. If one candidate achieves a higher strength result simply because more starch transfers, the mill needs to know whether that increased consumption, drying demand, and operating behavior remain acceptable.

Decide how much work each application should do

Wet-end and surface starch can complement each other, but they are not freely interchangeable. Moving starch to the surface may improve application efficiency for a surface-limited problem, yet it may leave an internal bonding weakness unresolved. Increasing wet-end starch may improve sheet strength without delivering the surface integrity required for printing or coating.

A useful starting allocation follows the location of the deficiency:

• For inadequate internal bonding, investigate wet-end starch retention and bonding response alongside furnish preparation and refining.
• For picking, dusting, or weak surface integrity, prioritize surface application, pickup uniformity, and penetration control.
• When both internal and surface properties are limiting, evaluate the combined program and identify the incremental contribution of each addition point.
• When water absorption is the main concern, assess the hydrophobic sizing system and its interaction with starch rather than treating starch dosage as the sole control.

These are starting hypotheses for trials. Some surface treatments contribute substantially to overall strength, and wet-end improvements can influence surface behavior. The decision should rest on the measured response of the specific furnish, sheet structure, and machine.

For a combined program, change one major variable at a time before testing a broader adjustment. Establish the response to wet-end dosage, then assess surface pickup while holding the base sheet as consistent as practical. This helps distinguish a useful combination from two additions compensating for an unresolved process issue.

Build the comparison around mill conditions and cost per saleable tonne

A practical evaluation can proceed through laboratory screening, a preparation-system check, and a controlled machine trial. Each stage should answer a different question: whether the chemistry has potential, whether the mill can prepare and handle it consistently, and whether it delivers the required sheet properties without unacceptable operating penalties.

For wet-end screening, use representative furnish and process water, with realistic pH, conductivity, chemical sequence, and mixing conditions. Compare candidates at a common dry dosage, then examine dosage response. Include retention and drainage observations so that strength results can be interpreted in context. Handsheets help screen candidates, but cannot fully reproduce machine shear, formation, and water recirculation.

For surface screening, apply candidates to comparable base sheets and record both dry pickup and preparation conditions. Assess the property linked to the intended use, together with relevant effects on porosity, stiffness, appearance, or subsequent coating. Laboratory application helps narrow the choice; transfer stability and behavior at production speed still require machine evaluation.

Agree on acceptance criteria before the machine trial. Record enough of the operating context to distinguish starch performance from furnish or process variation. A stronger sheet produced with different moisture, refining, or basis weight is not a clean comparison. Allow for the system’s response time, particularly where circulating water can delay stabilization after a wet-end change.

Commercial comparisons should then use total cost per saleable tonne. Include dry starch consumption, preparation requirements, drying load, production constraints, and quality losses where these can be measured. A lower purchase price can be offset by poor retention or higher pickup requirements. Conversely, a higher-priced grade needs a measurable benefit under the mill’s conditions to justify its premium.

The supplier discussion becomes more productive once this operating window is defined. Request grade-specific information on charge characteristics where relevant, viscosity under stated conditions, cooking requirements, solids and moisture specifications, and storage behavior. For paper intended for food-contact applications, request documentation appropriate to the exact grade, intended use, and destination market; a general product-family statement is insufficient.

The strongest selection is the grade that repeatedly meets the limiting paper property within the mill’s preparation, application, and drying capacity. If additional starch stops producing a useful response, revisit retention, penetration, and the underlying sheet structure before approving a higher consumption target. That is the point at which a wet-end versus surface-sizing comparison becomes a defensible operating decision.