Agentes de colagem do papel: controlo de qualidade, métodos de ensaio e normas do setor

1. Introduction: The Critical Role of Quality Testing in Sizing Performance

In the papermaking industry, sizing agents are among the most consequential chemical additives used in the manufacturing process. The effectiveness of a sizing agent directly determines the final paper’s resistance to water, ink, and other liquids—properties that are essential for everything from packaging board to printing papers. Yet the performance of a sizing agent is not simply a matter of what is written on the product specification sheet. It must be verified, measured, and monitored through rigorous testing at every stage: from raw material acceptance and production quality control to final product verification and in-process monitoring at the paper mill.

Testing of sizing agents is not merely a production step—it is a critical quality assurance process that ensures product consistency, optimizes usage efficiency, and prevents costly production failures【15†L0-L2】. Through scientifically rigorous testing methods, manufacturers can effectively enhance the performance of sizing agents and ensure product quality stability【15†L1-L2】.

This article provides a comprehensive technical overview of sizing agent testing methods, covering the full spectrum of quality parameters, international and national standards, analytical techniques, and specialized considerations for AKD and other modern sizing agents. For papermakers, this knowledge enables informed purchasing decisions, optimized application strategies, and consistent product quality. For chemical manufacturers, it provides a framework for quality management and regulatory compliance.

LY Chem Technology Co., Ltd. , with over two decades of experience in paper chemicals manufacturing, maintains comprehensive quality control systems across all our product lines—including our LY-AKD301 and LY-AKD302 sizing agents, LY-CRS and LY-ARS rosin sizes, and LY-BB201 surface sizing agent. Our products are manufactured under ISO 9001 quality management and are subject to rigorous testing to ensure consistent performance and regulatory compliance.

2. The Scope of Sizing Agent Testing: A Comprehensive Framework

2.1 Testing Coverage

The testing of sizing agents encompasses a broad range of parameters that span the entire product lifecycle—from raw materials and intermediates to finished products and application performance evaluation【11†L0-L1】. Testing applies to all major categories of sizing agents【11†L1-L2】:

• Acidic sizing agents (rosin-based)
• Neutral sizing agents (AKD, ASA)
• Agentes de colagem sintéticos
• Surface sizing agents (starch, PVA, styrene-acrylate emulsions)
• Reactive sizing agents
• Emulsion-type sizing agents
• Powdered sizing agents
• Bio-based sizing agents
• Specialty functional sizing agents
• Composite sizing agents

2.2 The Full Spectrum of Testing Parameters

Comprehensive sizing agent testing covers an extensive array of quality indicators and performance metrics【11†L0-L1】【14†L0-L1】:

Physical and Chemical Properties:

• Solid content
• Viscosity
• pH value
• Surface tension
• Particle size distribution
• Zeta potential (for emulsion stability)

Performance Characteristics:

• Film-forming properties
• Water resistance and hydrophobic performance
• Mechanical strength enhancement
• Sizing degree
• Sizing agent distribution analysis

Stability and Durability:

• Storage stability assessment
• Thermal stability testing
• Hydrolysis resistance (for AKD)
• Aging resistance

Application Performance:

• Compatibility with other wet-end chemicals
• Retention and drainage efficiency
• Impact on paper physical properties
• Application process optimization

Safety and Environmental:

• Toxicity and safety testing
• Environmental impact assessment
• Residue analysis (e.g., solvent residues)

Economic Considerations:

• Cost-benefit analysis
• Consumption rate optimization

3. Core Quality Parameters and Their Significance

3.1 Solid Content

Solid content is the most fundamental quality parameter for liquid sizing agents. It represents the percentage of active sizing material in the emulsion or dispersion and directly determines the effective dosage required to achieve target sizing performance.

Test Method: Solid content is typically determined by gravimetric analysis—a sample is weighed, dried to constant weight at a specified temperature (usually 105°C), and reweighed. The solid content is calculated as the ratio of dry residue to initial sample weight.

Test Standard: In China, solid content determination for sizing agents follows GB/T 2793-2022【14†L0-L1】.

Significance: Accurate solid content measurement is essential for:

• Calculating correct dosage rates
• Ensuring batch-to-batch consistency
• Verifying product concentration claims
• Optimizing application economics

3.2 Viscosity

Viscosity is a critical rheological property that affects the handling, pumping, and application behavior of sizing agents. For surface sizing agents in particular, viscosity directly influences film formation and penetration into the paper surface.

Test Method: Viscosity is measured using rotational viscometers (Brookfield-type) under standardized conditions of temperature and shear rate.

Test Standard: Viscosity testing typically follows ISO 2555:2018 (Plastics — Resins in the liquid state or as emulsions or dispersions — Determination of apparent viscosity using a single cylinder type rotational viscometer method)【14†L0-L1】.

Significance: Viscosity monitoring ensures:

• Consistent application performance
• Proper pumpability and flow characteristics
• Optimal film formation for surface sizing
• Batch-to-batch uniformity

3.3 pH Value

The pH of a sizing agent affects its stability, reactivity, and compatibility with other wet-end chemicals. For AKD emulsions, pH is particularly critical—hydrolysis is strongly influenced by pH, with hydrolysis rates minimized at pH 4.5–5.0.

Significance: pH monitoring helps:

• Predict and control hydrolysis rates (for AKD)
• Ensure compatibility with papermaking system chemistry
• Maintain emulsion stability
• Verify product quality and consistency

3.4 Surface Tension

Surface tension is an important indicator of the wetting and spreading characteristics of sizing agents, particularly for surface sizing applications where uniform coverage is essential.

Significance: Surface tension measurement helps:

• Predict penetration behavior into paper surfaces
• Optimize size press application parameters
• Ensure uniform film formation

3.5 Particle Size and Zeta Potential (For Emulsion Sizing Agents)

For emulsion-type sizing agents such as AKD and rosin dispersions, particle size distribution and zeta potential are critical stability indicators. Smaller, more uniform particles with higher zeta potential (surface charge) generally exhibit better stability and more efficient retention on cellulose fibers.

Analytical Equipment: Particle size and zeta potential are measured using nanoparticle size and zeta potential analyzers【14†L1-L2】.

Significance: These parameters:

• Predict emulsion storage stability
• Indicate resistance to hydrolysis and coalescence
• Correlate with retention efficiency on fibers
• Guide formulation optimization

4. Sizing Performance Testing: Evaluating the End Result

While testing the sizing agent itself is essential, the ultimate measure of quality is the performance of the sized paper. Several standardized test methods have been developed to evaluate sizing effectiveness.

4.1 The Cobb Test (Water Absorptiveness)

The Cobb test is the most widely used method for determining the water absorptiveness of sized paper and board. It measures the mass of water absorbed by a specified area of paper under standardized conditions over a defined time period—typically 60 seconds or 120 seconds.

Test Standards:

• ISO 535:2014 — Paper and board — Determination of water absorptiveness — Cobb method
• GB/T 1540-2002 — Paper and board — Determination of water absorption — Cobb method
• TAPPI T 441 — Water absorptiveness of sized (non-bibulous) paper, paperboard, and corrugated fiberboard (Cobb test)

Test Principle: A paper sample is weighed, placed in a Cobb testing apparatus (a metal ring clamped to the paper surface), and exposed to water for a specified time. The water is then removed, the paper is blotted, and the sample is reweighed. The Cobb value is calculated as the mass of water absorbed per square meter (g/m²).

Application: The Cobb test is applicable to sized paper, paperboard, and corrugated fiberboard. It is not recommended for paper with grammage below 50 g/m², embossed paper, or unsized paper.

Significance for Papermakers:

• Lower Cobb values indicate better water resistance
• Provides a quantitative measure of sizing effectiveness
• Enables comparison between different sizing agents and dosage levels
• Essential quality control parameter for packaging papers and liquid packaging board

4.2 The Hercules Size Test (HST)

The Hercules Size Test measures the time required for a dye solution to penetrate through paper sufficiently to reduce reflectance to a specified value, typically 80%. This method is particularly sensitive to the degree of sizing and is widely used in North America.

Significance: The HST provides a more sensitive measure of sizing effectiveness than the Cobb test, particularly for highly sized papers.

4.3 The Ink Float Test

The ink float test is a qualitative method in which a paper sample is floated on a standard ink solution, and the time for ink penetration to occur is observed.

Significance: This simple test provides a quick indication of sizing effectiveness, though it is less quantitative than Cobb or HST methods.

4.4 The Klemm Test

The Klemm test measures the absorptiveness height of paper over a specified testing time, typically 10 minutes. It is independent of basis weight and provides an alternative measure of sizing performance.

4.5 The Stöckigt Method

The Stöckigt method (also known as the sizing degree test) is widely used in Asia for determining the sizing degree of paper. It involves measuring the time required for a test solution to penetrate through the paper sheet.

4.6 Special Considerations for AKD Sizing Testing

AKD presents unique testing challenges due to its curing behavior. Unlike rosin-based sizing agents that develop sizing immediately upon drying, AKD requires a curing period for full sizing development—typically 12–24 hours at room temperature.

Accelerated Curing for Testing Purposes: To enable timely quality control, AKD-sized paper samples are typically subjected to accelerated curing: the paper sample is placed in a 105°C oven for 10 minutes, then cooled before testing【7†L0-L1】. This accelerated curing procedure allows the sizing degree to be evaluated without waiting for the full room-temperature curing period.

Fast-Cure AKD Formulations: Recent innovations have produced fast-cure type AKD formulations with higher cationicity that show better sizing degree than conventional AKD even at low addition levels and without extended curing treatment. When sufficient fixing agent is added, sizing degree is improved for both conventional and fast-cure AKDs.

4.7 Contact Angle Measurement

Contact angle measurement is an advanced technique for evaluating the hydrophobicity of sized paper surfaces【11†L0-L1】. A droplet of water is placed on the paper surface, and the angle between the droplet and the surface is measured using a video contact angle measurement system【14†L1-L2】.

Significance:

• Higher contact angles indicate greater hydrophobicity
• Provides direct measurement of surface energy modification by sizing
• Enables evaluation of sizing uniformity across the sheet
• Useful for research and development applications

4.8 Strength Testing

Sizing agents contribute not only to water resistance but also to paper strength. Testing of mechanical properties is therefore an essential component of sizing performance evaluation【14†L1-L2】:

• Ring crush strength — Critical for corrugated board and packaging applications
• Burst strength — Measures resistance to rupture under pressure
• Tensile strength — Indicates fiber bonding effectiveness
• Folding endurance — Important for papers subjected to repeated folding

5. Analytical Chemistry Techniques for Sizing Agents

Beyond physical and performance testing, chemical analysis is essential for characterizing sizing agent composition, identifying impurities, and ensuring regulatory compliance.

5.1 Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) is widely used to identify chemical functional groups and verify the chemical composition of sizing agents【11†L0-L1】. It can detect the characteristic lactone ring structure of AKD and confirm the absence of unwanted contaminants.

5.2 Gas Chromatography-Mass Spectrometry (GC-MS)

GC-MS is used for the analysis of alkyl ketene dimers (AKDs), which are used as sizing agents in paper manufacture. The method is based on the hydrolytic extraction and quantification of long-chain ketones, the identities of which are confirmed by combined gas chromatography-mass spectrometry.

Significance: GC-MS enables:

• Quantification of AKD and its hydrolysis products (dialkyl ketones)
• Detection of solvent residues (e.g., toluene)
• Verification of product purity
• Compliance with food-contact regulations

5.3 Pyrolysis-Gas Chromatography

Pyrolysis-gas chromatography is used for the determination of styrene copolymer sizing agents in paper. Styrene monomer derived from the styrene sequences in the copolymer is used as the key product in the pyrograms of paper samples.

5.4 Scanning Electron Microscopy (SEM)

SEM is used to examine the distribution of sizing agents on fiber surfaces and to identify surface defects such as waxy spots or uneven coverage【11†L0-L1】.

5.5 Thermogravimetric Analysis (TGA)

TGA measures the thermal stability of sizing agents and can be used to determine decomposition temperatures and composition【11†L0-L1】.

5.6 Accelerated Aging Tests

Accelerated aging tests evaluate the permanence of sizing by subjecting paper samples to elevated temperatures in an oven and measuring changes in whiteness and physical properties. This is particularly important for evaluating the long-term stability of AKD-sized papers, which can suffer from sizing reversion over time.

6. Quality Assurance and Regulatory Compliance

6.1 ISO 9001 Quality Management

LY Chem Technology Co., Ltd. operates under ISO 9001 quality management systems, ensuring that all products are manufactured to consistent quality standards. This includes:

• Documented quality control procedures
• Calibrated testing equipment
• Trained quality control personnel
• Traceable raw material and finished product testing
• Continuous improvement processes

6.2 International Standards Compliance

Our testing protocols align with major international and national standards:

Parâmetro	Standard	Description
Water absorptiveness	ISO 535:2014	Cobb method for paper and board
Water absorption	GB/T 1540-2002	Chinese national standard, Cobb method
Water absorptiveness	TAPPI T 441	Cobb test for sized paper and board
Solid content	GB/T 2793-2022	Determination of non-volatile matter content【14†L0-L1】
Viscosity	ISO 2555:2018	Rotational viscometer method【14†L0-L1】

6.3 Food-Contact Compliance Testing

For sizing agents intended for food-contact applications, additional testing is required to demonstrate compliance with FDA, BfR, and EU regulations. This includes:

• Quantification of AKD and dialkyl ketone content (FDA limit: 0.4% combined total in paper)
• Migration testing into food simulants (BfR SML: 5 mg/kg)
• Solvent residue analysis (toluene and other volatile organic compounds)
• Heavy metal content verification (RoHS compliance)

LY Chem’s commitment: Our food-grade products, including LY-AKD301 and LY-AKD302, are manufactured using solvent-free processes and are subject to comprehensive testing to ensure compliance with FDA, SGS, and RoHS requirements.

7. Practical Recommendations for Papermakers

7.1 Receiving Quality Control

Upon receiving sizing agent shipments, papermakers should conduct or request the following checks:

1. Verify product identity — Confirm product name, batch number, and specification
2. Check physical appearance — Note any signs of separation, sedimentation, or unusual color
3. Measure key parameters — Verify solid content, pH, and viscosity against specification
4. Document temperature — Record product temperature upon arrival (critical for AKD)
5. Retain samples — Keep retained samples for future reference and dispute resolution

7.2 In-Process Monitoring

During paper machine operation, regular monitoring of sizing performance is essential:

1. Monitor Cobb values — Regular Cobb testing of finished paper to detect any drift in sizing performance
2. Track consumption rates — Compare actual consumption against theoretical requirements
3. Observe paper properties — Monitor for signs of sizing issues (poor water resistance, waxy spots, surface defects)
4. Adjust as needed — Fine-tune dosage rates based on test results

7.3 Supplier Qualification

When evaluating sizing agent suppliers, papermakers should consider:

1. Quality system certification — ISO 9001 or equivalent
2. Testing capabilities — In-house laboratory with appropriate equipment and trained staff
3. Regulatory compliance — FDA, BfR, RoHS certifications for food-contact products
4. Batch-to-batch consistency — Historical quality data and statistical process control
5. Technical support — Availability of application expertise and troubleshooting assistance

8. Conclusion: Testing as the Foundation of Quality

The testing of paper sizing agents is far more than a routine quality control procedure—it is the foundation upon which consistent paper quality, production efficiency, and regulatory compliance are built. From the raw material receiving dock to the finished paper testing laboratory, rigorous testing ensures that sizing agents deliver their intended performance, batch after batch.

Key takeaways for papermakers and chemical buyers:

• Comprehensive testing matters — Solid content, viscosity, pH, particle size, and zeta potential all provide essential information about sizing agent quality and stability
• Standardized methods enable comparison — ISO, TAPPI, and GB standards provide a common framework for quality assessment
• Performance testing validates results — Cobb testing, contact angle measurement, and strength testing confirm that the sizing agent delivers the expected paper properties
• AKD requires special attention — Accelerated curing (105°C, 10 minutes) is necessary for timely quality control of AKD-sized papers
• Regulatory compliance demands rigorous testing — Food-contact applications require migration testing, residue analysis, and certification

LY Chem Technology Co., Ltd. is committed to quality at every stage of the product lifecycle. Our comprehensive testing program, ISO 9001 quality management system, and regulatory certifications ensure that our sizing products—LY-AKD301, LY-AKD302, LY-CRS, LY-ARS, and LY-BB201—deliver consistent, reliable performance for papermakers worldwide.

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For more information about LY Chem’s sizing solutions, testing capabilities, and quality assurance programs, please contact our technical sales team.

Referências

1. Baidu Baike – Sizing Agent Testing (施胶剂检测)
2. GB/T 2793-2022 – Determination of Non-Volatile Matter Content
3. ISO 2555:2018 – Determination of Apparent Viscosity
4. ISO 535:2014 – Paper and board – Determination of water absorptiveness – Cobb method
5. GB/T 1540-2002 – Paper and board – Determination of water absorption – Cobb method
6. TAPPI T 441 – Water absorptiveness of sized paper, paperboard, and corrugated fiberboard (Cobb test)
7. AKD Sizing and Curing Behavior – KoreaScience
8. Analytical Methods for AKD Sizing Agents – NSTL / GC-MS
9. Hercules Size Test – ScienceDirect
10. Klemm Test for Sizing Evaluation – J-STAGE
11. Stöckigt Method for Sizing Degree – CNKI