Tính lưu biến của chất làm đặc tổng hợp trong in lụa dệt may: Hiện tượng loãng do cắt, khả năng phục hồi và độ sắc nét của bản in

Synthetic thickener selection for textile screen printing should be based on rheology, not on one viscosity number. A printing paste is almost stationary in the color kitchen, experiences moderate shear during pumping, much higher shear under the squeegee and through the screen, then returns to low shear after it reaches the fabric. The thickener therefore needs enough low-shear body to control spreading, enough shear thinning to pass through the screen, and enough structural recovery to rebuild the pattern after transfer. If recovery is too slow, the print can bleed or lose definition; if recovery is too fast or elasticity is too high, poor leveling, stringing or mesh marks can appear. This guide explains how to evaluate synthetic-thickener rheology for textile flat and rotary screen printing and connect laboratory measurements with actual print definition.

What Rheology Should a Synthetic Thickener Provide?

A useful synthetic thickener for textile screen printing should generally provide three linked behaviors:

High Enough Structure at Low Shear → Lower Apparent Viscosity Under Printing Shear → Controlled Structural Recovery After Transfer

These three behaviors help the paste:

• Remain stable in the tank
• Move through pumps and pipes
• Pass through the screen under squeegee force
• Deposit evenly on the fabric
• Recover enough structure to limit spreading
• Maintain edge definition until drying begins

The best grade is therefore not the synthetic thickener with the highest measured viscosity.

It is the grade with the most useful flow-and-recovery profile for the actual screen, machine, fabric and pigment-binder formula.

Why One Viscosity Number Is Not Enough

A Brookfield or rotational-viscometer value normally represents one specific measurement condition.

For example:

30,000 mPa·s at one spindle, one RPM and one temperature

does not tell the mill how the paste behaves at much higher shear under the squeegee.

Textile-printing research shows that thickener rheology affects:

• Screenability
• Paste flow through screen openings
• Penetration into fabric
• Độ sắc nét khi in
• Hiệu suất màu
• Levelness

Two pastes with the same low-shear viscosity can therefore give different printing results if they differ in:

• Shear-thinning rate
• Recovery speed
• Độ đàn hồi
• Yield stress
• Thixotropy

Same viscosity does not mean same rheology.

The Screen-Printing Shear History Map

The printing paste experiences several mechanical environments during one production cycle.

Process Stage	Relative Shear Condition	Rheology Requirement
Tank / Rest	Low	Body, stability, anti-settling
Pumping / Circulation	Low to medium	Easy flow without permanent breakdown
Squeegee Movement	Medium to high	Strong shear thinning
Screen Opening	Cao	Low enough apparent viscosity for transfer
Fabric Deposit	Rapidly falling	Controlled recovery
Before Drying	Low	Enough structure to limit spreading

Published screen-printing rheology work describes this same principle: paste is relatively high in viscosity at rest, becomes easier to flow under squeegee and screen shear, then rebuilds viscosity after transfer.

This is why rheology must be evaluated as a curve and a recovery process rather than one point.

1. Low-Shear Body: Holding the Paste Before and After Printing

At low shear, the printing paste needs enough structure to remain stable and resist uncontrolled flow.

Low-shear body helps control:

• Pigment distribution
• Phân phối bìa hồ sơ
• Spreading after printing
• Penetration into the fabric
• Fine-line definition

If low-shear structure is too weak, possible symptoms include:

• Bleeding
• Haloing
• Loss of fine detail
• Excess penetration
• Uneven solid areas

But excessive low-shear viscosity can create:

• Difficult pumping
• Poor screen filling
• High squeegee pressure requirement

The target is sufficient structure—not maximum structure.

2. Shear Thinning: Helping Paste Pass the Screen

Most useful screen-printing pastes are pseudoplastic or shear-thinning.

As shear rate increases, apparent viscosity decreases.

This behavior allows the paste to:

• Move more easily under the squeegee
• Pass through screen openings
• Transfer to the textile with lower mechanical resistance

A strongly shear-thinning paste can have high body at rest and still flow efficiently during printing.

This combination is one reason synthetic polyacrylate thickeners are useful in selected textile-printing systems.

If shear thinning is too weak:

• Screen passage can be poor.
• Pressure may need to increase.
• Transfer can become incomplete.

If shear thinning is too strong:

• Paste can become excessively fluid under the squeegee.
• Penetration can become too deep.
• Recovery demand becomes more important.

3. Structural Recovery: Rebuilding After the Screen

When the paste exits the screen, the high shear condition disappears.

The thickener should rebuild enough structure to stabilize the deposit.

This is structural recovery.

Recovery helps control:

• Pattern edge sharpness
• Dot or fine-line stability
• Surface spreading
• Paste leveling
• Độ thâm nhập

Polyacrylate-based printing systems can show rapid recovery after shear, but the exact rate depends on the grade and formula.

A useful synthetic thickener should therefore be evaluated for both:

Viscosity Loss Under Shear

và:

Viscosity / Structure Recovery After Shear

What Happens When Recovery Is Too Slow?

Slow recovery leaves the paste too fluid after transfer.

Possible results include:

• Bleeding
• Edge spreading
• Loss of fine lines
• Excess fabric penetration
• Lower surface color concentration

In this case, simply increasing low-shear viscosity may not be the best correction.

A grade with faster recovery can sometimes improve definition without making pumping or screen transfer excessively difficult.

What Happens When Recovery Is Too Fast?

Very fast recovery can also create problems.

If the paste rebuilds structure immediately after passing the mesh, it may have insufficient time to level.

Possible symptoms include:

• Mesh marks
• Uneven solid areas
• Surface texture
• Poor leveling

General screen-printing research shows that recovery must be balanced: too slow allows spreading, while excessively rapid recovery can preserve mesh structure in the deposited layer.

The objective is therefore a controlled recovery window.

4. Elasticity and Stringiness

Synthetic printing pastes are often viscoelastic.

This means they show both liquid-like and elastic behavior.

Some elasticity helps the paste rebuild after shear.

Too much elasticity can create:

• Stringing
• Filament dragging
• Poor screen release
• Tail defects
• Uneven transfer

Polyacrylate thickener studies have specifically evaluated filament dragging and thixotropy because these properties can influence print quality.

A visually “thick” paste can still be unsuitable if it stretches into strings during screen separation.

5. Yield Stress and Rest Stability

Some synthetic-thickener systems develop an apparent yield stress.

Below this stress, the paste resists flow.

Above it, the paste begins to move more readily.

A useful yield-stress region can support:

• Rest stability
• Anti-settling behavior
• Pattern control after transfer

But excessive yield stress can make pumping and screen transfer difficult.

Yield stress should therefore be interpreted together with shear-thinning and recovery behavior.

6. Thixotropy and Time-Dependent Rheology

Thixotropy means the paste structure changes with both shear and time.

A thixotropic paste can become thinner during sustained shear and gradually rebuild when shear stops.

This matters in production because:

• High-speed mixing can reduce viscosity temporarily.
• Pumping can continue to shear the paste.
• Rotary-screen circulation exposes paste to repeated shear.
• Rest periods allow recovery.

Therefore, viscosity should be measured after a defined shear history and a defined recovery time.

Otherwise, the same paste can produce different laboratory readings depending on when the sample was taken.

Why Synthetic Thickener Architecture Matters

“Synthetic thickener” includes several polymer architectures.

Examples can include:

• Conventional polyacrylate systems
• Alkali-swellable acrylic emulsions
• Hydrophobically modified associative acrylic thickeners
• Compound synthetic systems

These products can differ in:

• Độ nhớt ở điều kiện cắt thấp
• High-shear response
• Phục hồi
• Độ đàn hồi
• Khả năng dung nạp chất điện giải
• Khả năng tương thích của chất kết dính

Therefore, rheology should be measured on the actual commercial grade rather than inferred from the generic word “acrylic.”

Pigment and Binder Can Change the Rheology

Water-only rheology is not the final printing rheology.

Pigment dispersion and binder can change:

• pH
• Electrolyte level
• Cân bằng chất hoạt động bề mặt
• Chất rắn
• Polymer interaction

This can alter shear thinning, recovery and elasticity even when the final Brookfield viscosity looks acceptable.

For pigment-printing grades, measure rheology in the complete:

Pigment + Binder + Thickener + Fixer + Auxiliary

system.

Flat Screen: What Rheology Usually Matters Most?

Flat-screen printing exposes the paste to intermittent squeegee strokes and dwell periods.

Important properties include:

• Low-shear hold during pauses
• Flow during the squeegee stroke
• Recovery after transfer
• Resistance to screen drying
• Repeatable behavior after restart

A paste that recovers too slowly may spread during the dwell period.

A paste that becomes too structured during pauses may require increased pressure on restart.

Flat-screen qualification should therefore include pause/restart testing.

Rotary Screen: What Rheology Usually Matters Most?

Rotary-screen printing is continuous and places stronger emphasis on repeated shear and long-run stability.

Important properties include:

• Khả năng bơm
• Continuous-shear viscosity
• Recovery after screen transfer
• Foam control
• Mid-run stability
• End-run stability

A paste that prints well for the first few meters but progressively thins during circulation has not passed rotary production qualification.

Screen Mesh and Open Area

Finer screen openings increase sensitivity to:

• High-shear viscosity
• Gel particles
• Elastic stringing
• Filtration quality

A more open screen can increase paste deposit and penetration.

Do đó:

Fine Mesh ≠ Automatically Lower Viscosity

và:

Open Mesh ≠ Automatically Higher Viscosity

The complete rheological profile must be matched to screen geometry.

Squeegee Pressure, Speed and Angle

Squeegee settings change both mechanical force and shear history.

Higher pressure or faster movement can increase the shear applied to the paste and change:

• Apparent viscosity during transfer
• Lớp lắng đọng dạng bột nhão
• Độ thâm nhập
• Pattern definition

If a paste requires unusually high squeegee pressure to transfer, the problem may be insufficient high-shear flow rather than inadequate low-shear viscosity.

Machine settings and rheology should therefore be diagnosed together.

Penetration vs. Surface Color

Rheology controls where the paste goes after it reaches the fabric.

Too little penetration can produce:

• Surface-heavy print
• Uneven coverage on some fabrics

Too much penetration can produce:

• Lower surface color concentration
• Reduced edge sharpness
• Higher reverse-side show-through

The optimum penetration depends on:

• Fabric weight
• Construction
• Absorbency
• Print design
• Pigment / binder system

The thickener should therefore control both screen transfer and post-transfer penetration.

How Rheology Controls Print Definition

Print definition is the combined result of:

Screen Transfer + Paste Deposit + Penetration + Recovery + Drying

Good definition generally requires:

• Enough shear thinning for clean transfer
• Enough recovery to limit spreading
• Low enough elasticity to avoid stringing
• Enough leveling to avoid mesh marks

This explains why “the thickest paste” is not automatically the sharpest paste.

Print definition is a balance between flow and structure.

How to Use Brookfield Viscosity Correctly

Brookfield viscosity remains useful for incoming QC and production control.

But the method must be fixed.

Bản ghi:

• Nồng độ
• Nhiệt độ
• Spindle
• RPM
• Thời gian đọc
• Sample age

Use Brookfield viscosity to confirm batch consistency.

Do not use one Brookfield number as the entire rheological specification.

Build a Multi-Speed Viscosity Profile

If a rheometer is not available, a multi-speed rotational-viscometer test can provide a practical shear-response profile.

Measure the same paste at several appropriate speeds within the instrument’s useful torque range.

Sau đó, hãy so sánh:

Low RPM Viscosity → Medium RPM Viscosity → Higher RPM Viscosity

A stronger decrease with increasing speed indicates stronger shear-thinning behavior.

Do not combine readings from unsuitable spindle/torque conditions into one curve.

The method must remain repeatable.

Run a Simple Shear-Recovery Test

A practical laboratory test can compare:

1. Initial viscosity after standard rest.
2. Viscosity immediately after a controlled high-shear mixing step.
3. Viscosity after a short recovery period.
4. Viscosity after a longer recovery period.

Bản ghi:

Recovery (%) = Recovered Viscosity ÷ Initial Viscosity × 100

The objective is not necessarily 100% immediate recovery.

The useful recovery profile is the one that gives good screen transfer and print definition.

What a Rheometer Can Add

A rheometer can provide deeper information such as:

• Flow curve
• Yield stress
• Thixotropic loop
• Storage modulus (G’)
• Loss modulus (G”)
• Quá trình phục hồi sau khi chịu lực cắt có kiểm soát

These measurements can help distinguish two pastes that look identical in a single-viscosity test.

However, rheometer data should still be connected to actual screen printing.

The most sophisticated rheology curve has little production value if it is not correlated with transfer, penetration and definition.

Define an Approved Rheology Window

Do not approve one perfect laboratory value.

Define a working window that includes:

• Low-shear viscosity range
• High-shear response
• Recovery behavior
• Holding-time stability
• Screen-transfer result
• Print-definition result

The working window should remain acceptable across:

• Normal temperature variation
• Expected pigment loading
• Approved binder level
• Expected production holding time

This makes the specification useful for actual manufacturing rather than one ideal lab condition.

Phê duyệt thử nghiệm sản xuất

Laboratory rheology is only the screening stage.

During a controlled production trial, record:

• Thickener grade and batch
• Liều lượng
• Complete paste formula
• Start viscosity
• Độ nhớt giữa chu kỳ
• Độ nhớt khi chảy qua khe hở
• Tốc độ máy
• Cài đặt màn hình / lưỡi gạt
• Pattern definition
• Độ thâm nhập
• Beginning-to-end shade
• Finished hand and fastness

A candidate should pass a production window, not one short print swatch.

Bảng khắc phục sự cố

Vấn đề được ghi nhận	First Rheology Check	Đừng vội kết luận
Paste spreads after printing	Low-shear body and recovery	Only the Brookfield viscosity is too low
Paste is difficult to push through screen	High-shear flow / shear thinning	More squeegee pressure is the only fix
Fine lines blur	Recovery, deposit, penetration	Higher viscosity always improves sharpness
Mesh marks remain visible	Recovery may be too fast / leveling too low	Fast recovery is always better
Stringing during screen release	Elasticity / filament dragging	Viscosity alone explains the defect
Rotary run thins over time	Thixotropy, continuous shear, temperature	Initial lab viscosity proves long-run stability
Same viscosity, different print definition	Full shear/recovery profile	The two grades are equivalent
More thickener improves body but hand worsens	Polymer solids and paste deposit	Maximum viscosity is the target

Tổng chi phí trong quá trình sử dụng

Rheology affects cost through:

• Liều lượng chất làm đặc
• Pumping energy
• Squeegee pressure
• Tốc độ máy
• Vệ sinh màn hình
• Sửa lại
• Vải dệt tay

Một mô hình hữu ích là:

Total Cost in Use = Thickener + Preparation + Machine Efficiency + Downtime + Curing / Finishing + Rework + Quality Loss

A higher-priced synthetic thickener can be cheaper if it gives better shear response and allows lower dosage or higher machine efficiency.

A cheaper product can be more expensive if it causes poor screen transfer, excessive corrections or unstable long runs.

Bạn nên cung cấp những thông tin nào cho nhà cung cấp chất làm đặc?

For useful rheology matching, provide:

• Current thickener name, sample or TDS
• Liều lượng hiện tại
• Độ nhớt và phương pháp thử nghiệm toàn diện
• Dung dịch phân tán sắc tố
• Hệ thống chất kết dính / chất cố định
• Final paste pH
• Rây phẳng hoặc rây quay
• Screen mesh / engraving
• Squeegee settings
• Vải
• Tốc độ máy
• Thời gian giữ
• Main defect: transfer, spreading, stringing, penetration, definition or long-run drift

FSX Chemical có thể sử dụng thông tin này thông qua Mẫu và việc kết hợp to compare current and candidate thickener rheology under controlled printing conditions.

Đánh giá Các ứng dụng của chất làm đặc trong in ấn dệt may for process-based route selection.

How Should a Mill Evaluate Synthetic-Thickener Rheology?

A practical sequence is:

Define Complete Paste → Measure Low-Shear Viscosity → Measure Higher-Shear Response → Apply Controlled Shear → Measure Recovery → Print → Evaluate Transfer / Penetration / Definition → Confirm Long-Run Stability

Các nguyên tắc chính là:

1. Same viscosity does not mean same rheology.
2. Useful screen-printing paste normally needs higher structure at low shear and easier flow at high shear.
3. Recovery should be fast enough to limit spreading but not so fast that leveling becomes poor.
4. Elasticity and thixotropy can affect screen release and long-run stability even when viscosity looks normal.
5. Flat and rotary screen machines expose the paste to different shear histories.
6. The final rheology specification must be validated by actual print definition and production stability.

Các câu hỏi thường gặp

1. What rheology is best for textile screen printing?

A useful paste normally has enough low-shear structure to control spreading, shear-thins under the squeegee and then recovers enough structure after transfer.

2. Is higher viscosity always better for print definition?

No. Excess low-shear viscosity or elasticity can reduce screen transfer, while poor recovery can cause spreading. The complete rheological profile matters.

3. What is shear thinning?

Shear thinning means apparent viscosity decreases as shear rate increases. This helps printing paste move through the screen under squeegee force.

4. What is structural recovery?

Structural recovery is the rebuilding of paste viscosity or internal structure after high shear is removed.

5. What happens if recovery is too slow?

The paste can remain too fluid after printing, causing bleeding, loss of fine detail or excessive penetration.

6. Can recovery be too fast?

Yes. Excessively rapid recovery can reduce leveling and contribute to mesh marks or uneven solid areas.

7. Why can two synthetic thickeners with the same Brookfield viscosity print differently?

They can have different shear-thinning, elasticity, yield stress, thixotropy and recovery behavior.

8. How can I measure shear thinning without a rheometer?

A controlled multi-speed rotational-viscometer test can provide a practical comparison, provided the spindle and torque range remain appropriate.

9. How can I test recovery in a normal laboratory?

Measure initial viscosity, apply a standardized high-shear mixing step, then measure immediately and again after defined recovery times.

10. Does rotary screen require lower-viscosity paste than flat screen?

Not as a universal rule. Rotary printing places more emphasis on continuous shear and long-run stability, while flat screen includes repeated strokes and dwell periods.

11. Should pigment and binder be included in rheology testing?

Yes. They can change pH, electrolyte level, solids and polymer interactions, so final rheology should be measured in the complete pigment paste.

12. What should I send FSX Chemical for rheology troubleshooting?

Send the current thickener or TDS, dosage, viscosity method, pigment, binder, pH, machine, screen, squeegee conditions and the observed transfer or print-definition problem.

Match Synthetic-Thickener Rheology to Your Screen Printing Process

If your synthetic thickener reaches the target viscosity but still gives spreading, poor transfer, stringing, excess penetration or unstable long runs, FSX Chemical can help separate low-shear viscosity from shear-thinning and recovery behavior.

Để nhận được bài đánh giá kỹ thuật hữu ích, vui lòng gửi:

• Mẫu chất làm đặc tổng hợp hiện tại của quý khách, TDS hoặc COA
• Liều lượng hiện tại
• Độ nhớt và phương pháp thử nghiệm toàn diện
• Dung dịch phân tán sắc tố
• Hệ thống chất kết dính / chất cố định
• Final paste pH
• Rây phẳng hoặc rây quay
• Screen mesh / engraving
• Squeegee settings
• Fabric and machine speed
• Thời gian giữ
• Current transfer, definition, penetration or stability problem

Bắt đầu bằng Mẫu và việc kết hợp for a controlled current-vs-candidate comparison.

Đánh giá Các ứng dụng của chất làm đặc trong in ấn dệt may for process-based selection.

Bạn cũng có thể Yêu cầu báo giá trực tiếp từ nhà máy after the suitable synthetic thickener and working rheology window are confirmed or Liên hệ với FSX Chemical để thảo luận về các vấn đề kỹ thuật📧 Email: Service@fsxchemical.com

The best synthetic thickener is not the one with the highest static viscosity. It is the one that flows easily under screen shear, recovers at the right rate after transfer and keeps the printed pattern stable until drying and curing are complete.