Secondary Rheology Modifier
Hydroxypropyl Starch Ether — HPS
MICHEM HPS 305 is a hydroxypropyl starch ether designed to refine the fresh-state rheology of dry-mix mortar. It is used where the formulation needs stronger anti-sag behavior, more controlled consistency and smoother application without relying on a large increase in primary cellulose-ether viscosity.
Formulation Strategy
HPS is a rheology fine-tuning tool, not a replacement for cellulose ether
Hydroxypropyl starch ether is often added at a much lower level than the main cellulose ether, yet its effect on application behavior can be immediately visible.
That creates a common misunderstanding. Because HPS can make a mortar feel more stable, it is sometimes treated as another thickener that can simply replace HPMC or HEMC. That is not the most useful way to formulate with it.
HPMC and HEMC normally carry the broader responsibility for water retention, consistency and the usable working window of a dry-mix mortar. HPS is better used to reshape the rheology inside that window. It can help a wall-applied material resist movement at rest while remaining workable when force is applied with a trowel, pump or mixing tool.
MICHEM’s HPS 305 technical information positions the grade around thickening, water retention, improved workability, reduced sagging, extended open time and smoother application. It can also be used together with cellulose ether when a formulation needs more precise rheology control or a better balance between performance and additive cost.
The formulation question is therefore not:
“Can HPS replace HPMC?”
It is:
“Can HPS solve the remaining rheology problem more efficiently than increasing HPMC or HEMC further?”
That distinction is the foundation of correct HPS selection.
Selective Structure at Rest
Providing resting stability and sag prevention while maintaining low drag during troweling.
Physics & Suspension Mechanics
Why a small amount of starch ether can change mortar behavior so strongly
Fresh mortar is not simply a viscous liquid. It is a concentrated suspension of cement or gypsum, fillers, sand, polymers and dissolved additives.
When the material is moving under shear, it needs to spread, pump or trowel efficiently. When movement stops, it often needs enough internal structure to remain stable. A tile adhesive should not continue sliding after the tile is placed. A render should not slump down the wall. A skim coat should hold its shape rather than slowly flowing after the applicator moves on.
Hydroxypropyl starch ether can influence this balance by modifying interactions within the particle-water-polymer system.
Industry research on starch derivatives in cementitious materials shows that modified starch can increase yield stress and change low-shear rheology. This is important because yield stress is closely connected to whether a mortar begins to move under its own weight. A formulation can therefore become more resistant to sag without requiring the same proportional increase in viscosity at every shear condition.
This is why “thickening” is an incomplete description of HPS.
The practical value is often selective structure: stronger stability when the mortar is resting, but acceptable mobility when the installer applies force.
Rheological Levers
Yield Stress Explains More Than Bulk Viscosity
Distinguishing stress-threshold activation from bulk solution thickening and shear recovery.
Yield stress explains more than bulk viscosity
Consider two wall mortars with the same apparent viscosity measured under one laboratory condition.
One remains in place after application. The other slowly sags.
The difference can come from yield behavior and structural recovery rather than the headline viscosity number.
Yield stress is the stress that must be exceeded before a structured material begins to flow. Raising that threshold can improve resistance to gravity-driven movement. For vertical mortar, this may translate into better anti-sag performance. In tile adhesive, it can contribute to reduced tile slip after placement.
Hydroxypropyl starch ethers are useful because they can influence this part of the rheology profile strongly.
For the formulator, the implication is important: if the mortar already has adequate water retention and basic viscosity but still moves too much at rest, increasing the main cellulose ether may be an inefficient response. A starch ether may address the failure more directly.
The choice still has to be confirmed experimentally because binder type, filler grading, cellulose ether, RDP, water level and other admixtures all affect yield behavior.
HPS and thixotropy should be considered together
Thixotropy describes structural breakdown under shear and rebuilding after shear is reduced. In practice, a good vertical mortar should move under the trowel but recover enough structure after placement to resist sag or tile slip.
HPS can help refine this balance, but the target is not maximum thixotropy. Excessive or overly rapid recovery can make the mortar sticky, heavy or difficult to adjust.
The optimum therefore depends on the application and must be tested in the full formulation.
Additive Synergy
HPS 305 should work with HPMC or HEMC, not compete with them
MICHEM HPS 305 can be combined with cellulose ether. This is one of the most useful formulation strategies for dry-mix mortar because the two additive families can be assigned different jobs.
A practical division of responsibility is:
| Formulation function | HPMC / HEMC | HPS 305 |
|---|---|---|
| Primary water retention | Main role | Supporting effect |
| Base consistency | Main role | Fine adjustment |
| Open working window | Main role | Can contribute |
| Anti-sag / anti-slip | Contributes | Strong fine-tuning role |
| Low-shear structure | Contributes | Important adjustment lever |
| Application feel | Strong influence | Can refine feel |
| Cost optimization of rheology package | Depends on grade and dosage | Can help avoid overbuilding cellulose-ether viscosity |
This table is a formulation framework, not a claim that one ingredient always controls only one property. The effects overlap. The useful concept is that HPMC or HEMC establishes the broad operating window, while HPS can help shape the internal rheology of that window.
Decision Framework
Formulation Decision Guide
Systematic criteria for determining whether to adjust primary cellulose ethers or introduce HPS 305.
When should you increase HPMC or HEMC instead of adding HPS?
If the main problem is rapid water loss, short wetting time or insufficient water retention, start by reviewing the primary cellulose ether.
HPS should not be used to disguise an underperforming cellulose-ether system.
For example, if a tile adhesive skins too quickly on an absorbent substrate, the root cause may involve cellulose ether type, dosage, cement, environmental conditions or water demand. Adding HPS only because the adhesive also slips may improve vertical stability without solving the water-management problem.
You should also review HPMC or HEMC when the complete mortar lacks enough general body across the application cycle rather than only at rest.
Typical signals include:
- water retention is below the formulation target
- the mortar loses workability too quickly
- the base consistency is too low under both rest and shear
- a change in cellulose-ether grade has created a broad rheology deficit
- open time or substrate wetting is the main failure rather than sag
In these situations, fix the primary cellulose-ether platform first.
When should you add HPS instead of increasing HPMC or HEMC?
HPS becomes especially interesting when the main cellulose ether is already doing its job but one specific rheology problem remains.
For example:
- the mortar has acceptable water retention but still sags
- a tile adhesive is workable but tile slip is too high
- increasing HPMC improves sag but makes troweling too heavy
- the formulation needs stronger rest structure without a large rise in application resistance
- the producer wants to refine workability and anti-sag through a secondary rheology modifier rather than move to a much higher-viscosity cellulose ether
- the formulation needs a more economical balance between the primary and secondary rheology package
This is where HPS 305 should be evaluated.
The right trial is not “with HPS versus without HPS” at only one dosage. The better approach is to compare different HPMC/HEMC + HPS combinations and identify which package gives the widest acceptable process window.
Application Domains
Targeted Application Strategies
Optimizing formulation balance, slip control, trowel drag, and pumping efficiency across dry-mix sectors.
Tile adhesive: use HPS to separate anti-slip from excessive viscosity
Tile adhesive is one of the clearest examples of why starch ether can be useful.
The adhesive must be easy to comb with a notched trowel. After the tile is placed, it must provide enough stability to limit downward movement. At the same time, the installer may need to adjust the tile before the adhesive develops too much structure.
A simple way to reduce slip is to increase cellulose-ether viscosity. But if this also makes the adhesive harder to comb or reduces wetting, the formulation has solved one problem by creating another.
HPS offers a different route.
By strengthening low-shear structure and yield behavior, it can help the adhesive remain stable at rest while allowing the formulator to keep the primary cellulose-ether system within a workable range.
During qualification, test tile slip together with troweling and open time. A formulation that produces almost no slip but becomes difficult to spread is not commercially optimized.
Wall putty and skim coat: control body without making the product heavy
Wall putty and skim coat need enough internal structure to remain on the substrate and provide a smooth finishing window.
A weak formulation can sag, pull or feel watery. An overbuilt formulation can feel heavy, sticky or difficult to polish and finish.
HPS 305 can be screened when the cellulose ether already provides acceptable water retention but the product needs more controlled body after placement.
The formulator should compare:
- smoothness under the trowel
- sag after application
- edge definition
- wet tack
- drag
- water demand
- finishing behavior over time
The best result is not the sample that feels thickest in the bucket. It is the sample that remains stable on the wall and still works efficiently under the applicator’s tool.
Render and plaster: think about application method
Hand-applied render needs body and spreadability; machine-applied material must also remain pumpable. A rheology package that works by hand can create excessive pumping resistance, so HPS trials should reproduce the real application method whenever possible.
Gypsum-based mortar: include the set-control system
Gypsum rheology changes as hydration proceeds, so HPS should not be approved from an immediate consistency test alone. Record performance through the real working period and evaluate the starch ether together with cellulose ether and the retarder package.
Self-leveling compounds require particular caution
Self-leveling compounds need enough stability to prevent segregation but low enough yield stress to flow under gravity. Because HPS can increase low-shear structure, excessive addition can reduce leveling. Evaluate flow, segregation, air and surface closure, and never transfer a wall-mortar dosage directly into a self-leveling formula.
Metrology & Application Testing
Do not judge HPS by a single viscosity measurement
The effect of starch ether may not be represented well by one conventional viscosity number.
A solution viscosity test can be useful for material characterization, but a dry-mix mortar is a concentrated multiphase system. What matters to the applicator is how the full formulation behaves at low shear, during shear and after shear.
A strong HPS qualification program therefore focuses on application properties.
For vertical mortar, useful measurements can include:
If sophisticated rheometry is available, yield stress, plastic viscosity and thixotropic loop or structural recovery can help explain why two formulations feel different. If it is not available, well-controlled application tests can still give valuable comparative data.
HPS interacts with the whole admixture package
Starch ether does not work in isolation. HPMC or HEMC controls the main water-retention and rheology platform; RDP can change fresh workability as well as hardened properties; PCE can reduce yield stress while starch derivatives may increase structural resistance; defoamers and set-control additives can also change the apparent rheology over time.
Keep these ingredients fixed during the first HPS comparison. Once the starch-ether response is understood, optimize the package as a system and record performance at several time points rather than only immediately after mixing.
Evaluate: Cellulose Ether • RDP • PCE Superplasticizer • Defoamer • Retarder / Accelerator
Troubleshooting
Common failure patterns after adding HPS
Diagnose and resolve common fresh-state imbalances when formulating with starch ethers.
Sag improves, but troweling becomes heavy
The total rheology package is likely too strong. Reduce HPS, review cellulose-ether viscosity or examine whether filler grading is already creating excessive yield structure.
Tile slip remains high
Do not assume more HPS is automatically required. Check water level, tile weight, cellulose ether, open time, trowel geometry and particle packing. Anti-slip is a system property.
The mortar becomes sticky
The HPS and cellulose ether combination may be overbuilding structure or changing application feel. Compare a lower HPS level before changing the water demand.
Flowable mortar loses leveling
Yield stress may have increased beyond the allowable window. In self-leveling systems, this is a strong signal to reduce the starch-ether contribution or redesign the rheology package.
Laboratory performance is good but plant batches vary
HPS is used as a fine-adjustment additive, so dosing precision and dry-mix uniformity matter. Check weighing accuracy, premixing, powder distribution and raw-material moisture.
Validation Framework
A practical HPS 305 qualification protocol
Start with a stable production formulation and one defined failure.
If the issue is sag, measure sag. If the issue is tile slip, measure tile slip. If the problem is application feel, define what “better” means before starting the test.
Keep the binder, filler system, water, cellulose ether, RDP and other additives fixed for the first trial. Run the reference formulation and the HPS 305 candidate under the same mixing procedure.
Record the fresh-state properties immediately and again during the working period. Depending on the application, include water demand, consistency, slip or sag, troweling resistance, wet density, air, open time and setting.
Next, build a controlled dosage series.
After identifying the useful HPS range, run a second series in which the cellulose-ether level or grade is adjusted. This is the stage where HPS can reveal its economic value: the best formulation may not be the highest-performing single-additive sample but the cellulose ether + HPS combination that meets all specifications at the most stable total formulation cost.
Finally, confirm the preferred package with normal raw-material variation and production-scale mixing.
TDS Source of Truth & Supplier Control
Use the TDS for supplier control and the mortar for application approval.
MICHEM’s supplied TDS identifies the product as HPS 305 and positions it as a hydroxypropyl starch ether for rheology modification. The approved TDS should remain the source of truth for grade-specific specification, packaging, storage and other commercial fields; missing fields should not be filled with generic market values.
Commercial & Quality Alignment
Match the purchase order, TDS, COA and sample label to the same HPS 305 identity, then approve performance in the actual mortar.
Frequently Asked Questions
HPS Technical & Commercial FAQ
Clear technical guidance for product selection, formulation trials and commercial qualification.
What is hydroxypropyl starch ether used for in dry-mix mortar?
It is used as a rheology modifier to refine consistency, workability and low-shear stability. In vertical applications, it is particularly useful when sag or slip needs to be reduced without simply increasing the viscosity of the entire formulation.
What is MICHEM HPS 305?
HPS 305 is MICHEM’s supplied hydroxypropyl starch ether grade. The available TDS positions it around thickening, water retention, improved workability, reduced sagging, extended open time and smoother application.
Can HPS 305 replace HPMC or HEMC?
It should not be treated as a one-for-one replacement. HPMC and HEMC normally provide the primary water-retention and rheology platform, while HPS is better used to refine anti-sag, low-shear structure and application feel.
When should I add HPS instead of increasing HPMC?
Consider HPS when water retention and basic consistency are already acceptable but the mortar still sags, tiles slip, or increasing HPMC makes application too heavy.
Does HPS only increase viscosity?
No. The more useful way to understand starch ether is through its influence on yield stress, low-shear structure and structural recovery. These effects help explain why a small addition can strongly change sag resistance.
Can HPS be used with HEMC?
Yes. MICHEM’s supplied information supports using starch ether together with cellulose ether. The combination should be optimized as one rheology package.
Can HPS affect open time?
MICHEM’s HPS 305 information includes extended open time among its functional benefits. The final open time still depends on cellulose ether, binder, substrate absorption, temperature, water demand and the rest of the formulation.
Is more HPS always better for anti-sag?
No. Excessive structure can make troweling heavy, increase stickiness or reduce flow. The objective is the lowest level that gives the required rest stability without damaging application.
Can HPS be used in self-leveling compounds?
It requires careful testing because self-leveling products need low enough yield stress to flow under gravity. Any starch-ether addition should be qualified for flow, segregation and surface closure rather than transferred from a vertical-mortar formula.
What information should I provide for an HPS 305 trial?
Provide the application, binder system, current HPMC or HEMC grade and dosage, water demand, RDP and other key additives, the failure you want to correct, and the application method. For a vertical mortar, include current sag or tile-slip performance if available.
Formulation Principle
Use HPS to reshape the rheology—not to overbuild it
The strongest reason to use hydroxypropyl starch ether is precision.
HPMC and HEMC establish the broad water-retention and consistency platform of many dry-mix mortars. HPS 305 gives the formulator another lever when that platform needs stronger rest stability, reduced sag, lower tile slip or a more controlled application feel.
The right formulation does not contain the maximum possible HPS.
It contains enough HPS to solve the specific rheology failure while preserving spreadability, open time, pumping or flow and the required hardened performance.
Build the cellulose ether first. Use HPS to fine-tune the structure. Then validate the combination in the finished mortar.
Technical & Sample Support
Our application laboratory helps you optimize the synergy between HPMC/HEMC and HPS 305 to eliminate tile slip, prevent render sag, and balance overall additive costs.