Neither treatment is a universal winner. In the most directly relevant pineapple-leaf-fiber (PALF)/epoxy study, silane produced the larger interfacial-shear-strength gain in a single-fiber fragmentation test, while the combined alkali–silane sequence performed best in a droplet test. The result is therefore conditional on fiber, chemistry, process severity and test method.
What the direct evidence says
Zaman and Khan (2023) compared untreated PALF with sodium-hydroxide (NaOH) treatment, 3-aminopropyltriethoxysilane (3-APS) treatment and a sequential alkali–silane treatment in an epoxy system. Their single-fiber fragmentation analysis reported a 6% apparent interfacial shear strength (IFSS) improvement for NaOH-treated PALF and a 47% improvement for silane-treated PALF, both relative to untreated fiber.
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The same paper used a droplet test and reported a different ranking: the combined alkali-plus-silane treatment had better interfacial stress strength in that test, whereas the silane-only fiber had the highest result in the fragmentation analysis. The authors summarize the distinction as follows: “The silane-treated fibers exhibited the highest interfacial stress strength, whereas the PALF treated with alkali and silane had better interfacial stress strength, according to the droplet test.”
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute| Treatment | Direct PALF/epoxy finding | Test and qualification |
|---|---|---|
| Untreated PALF | Reference condition | Used as the comparison baseline in Zaman and Khan (2023) |
| NaOH alkali | 6% IFSS increase | Single-fiber fragmentation result versus untreated PALF |
| 3-APS silane | 47% IFSS increase | Single-fiber fragmentation result versus untreated PALF |
| Alkali followed by silane | Best direction reported among the compared treatments | Droplet-test result; the source does not provide a percentage in the supplied findings |
These figures describe one PALF preparation, one epoxy formulation and the procedures used in that experiment. They should not be converted into expected gains for flax, hemp, jute, ramie, sisal or another fiber without a matched study.
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Why alkali and silane change the interface differently
Alkali treatment: cleaning and roughening the surface
Natural fibers contain cellulose together with non-cellulosic constituents such as hemicellulose, lignin, waxes and surface impurities. An alkaline bath is commonly used to remove part of this non-cellulosic material. The exposed cellulose-rich surface can become rougher, increasing mechanical interlocking with cured epoxy. Cleaning can also remove weak boundary layers that otherwise interfere with wetting.
The benefit has a limit. Excessive concentration, temperature or exposure can attack the cellulose structure, reduce fibril strength and create defects. Alkali therefore changes both sides of the interface equation: it may improve contact with resin while simultaneously weakening the load-bearing fiber. Washing and drying also matter because residual alkali, absorbed water or incomplete drying can alter the subsequent epoxy cure and the measured bond.
Silane treatment: a chemical bridge
Silane coupling agents are intended to improve compatibility between a hydrophilic lignocellulosic surface and a comparatively less polar epoxy matrix. In the PALF study, the silane was 3-aminopropyltriethoxysilane (3-APS). The treatment is generally understood as creating a more chemically compatible interphase: hydrolyzed silane species can interact with hydroxyl-rich fiber surfaces, while the organofunctional end is selected to interact with the matrix or its curing chemistry.
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Why a sequential process can outperform either single treatment
Alkali can expose a cleaner, higher-area surface; silane can then modify that surface for better chemical compatibility. The sequence is attractive when the manufacturing line can accommodate two baths, thorough washing and a controlled drying step. It also introduces more opportunities for variability, contamination and fiber damage. The combined treatment should therefore be treated as a separate process, not as proof that every alkali or silane recipe will deliver the same result.
How to read the two PALF interface tests
Single-fiber fragmentation
In a fragmentation test, a single embedded fiber is loaded through the surrounding matrix until the fiber breaks repeatedly. The spacing and stress state around fragments are used to estimate an apparent IFSS. This is a focused probe of local fiber–matrix load transfer. The 6% NaOH and 47% 3-APS values belong specifically to this analysis.
“Apparent” is important: the calculated value depends on assumptions about fiber diameter, fragment length, stress transfer and the specimen geometry. A larger IFSS indicates more effective transfer under that test configuration, but it is not the same as a laminate tensile strength, impact strength or moisture-durability measurement.
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Droplet testing
A droplet test embeds a short fiber section in a small resin droplet and evaluates the force or stress needed to debond it. Differences in droplet size, fiber geometry, loading rate and failure mode can change the ranking. In Zaman and Khan’s work, the combined alkali–silane treatment was favored by this method even though silane alone led the fragmentation result.
Practical implication of the disagreement
The two outcomes are not contradictory evidence that one test is wrong. They show that treatment performance is coupled to the interface test and its failure mechanics. When comparing papers, put the test name beside every number and do not pool fragmentation, droplet, pull-out, laminate tensile and flexural results into one league table.
What later alkali-treated PALF data can—and cannot—show
Anand (2026) examined an alkali-treated PALF/epoxy composite across fiber loadings and identified 15 wt% PALF as the reported optimum. The alkali protocol used 5% NaOH. That formulation reached 78 MPa tensile strength, compared with 45 MPa for neat epoxy, and reported a 3.5 GPa tensile modulus and 3.7 GPa flexural modulus.
| Reported value | What it describes | What it does not establish |
|---|---|---|
| 5% NaOH | Alkali concentration used in Anand’s PALF preparation | It is not a universally optimal concentration for other fibers or epoxies |
| 78 MPa tensile strength | 15 wt% alkali-treated PALF/epoxy at the reported optimum | It is not a head-to-head alkali-versus-silane result |
| 45 MPa tensile strength | Neat epoxy comparator in Anand (2026) | It does not isolate the contribution of NaOH from fiber reinforcement, loading or processing |
| 3.5 GPa tensile modulus | Reported optimum alkali-treated PALF/epoxy system | It is not a silane benchmark |
| 3.7 GPa flexural modulus | Reported optimum alkali-treated PALF/epoxy system | It cannot be used to rank interface treatments measured by IFSS |
The 78-versus-45 MPa comparison demonstrates that this reinforced composite exceeded the neat-resin control under the study’s conditions. It does not show that NaOH is more effective than 3-APS, because no matched silane composite is the comparator.
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Fiber damage and the cost of over-treatment
Improved bonding is useful only if enough fiber strength remains to carry the applied load. Sun et al. (2022), working with ramie and cellulose acetate rather than epoxy, found improved interface adhesion after both alkali and silane treatments, while single-fiber tensile properties declined. In that particular setup, alkali caused greater damage than silane.
The matrix difference matters: a cellulose-acetate result is not a direct PALF/epoxy prediction. It is nevertheless a warning about process severity. A rougher surface or stronger chemical coupling can coexist with a weaker fiber. The 2026 Springer Nature review likewise identifies excessive alkali treatment as a potential source of structural damage and notes that inconsistent reporting of treatment parameters makes cross-study comparisons difficult.
- Measure fiber tensile properties after treatment, not only composite strength.
- Inspect for fibrillation, pitting, discoloration or broken filaments when microscopy is available.
- Record mass loss and moisture content before impregnation.
- Do not assume that a longer or stronger alkali exposure is automatically better.
Process specifications that must be reported
Readers cannot reproduce or fairly compare a treatment from its chemical name alone. For NaOH, state the concentration, solution pH, bath temperature, exposure duration, fiber-to-solution ratio if controlled, rinsing or neutralization method, and final drying conditions. Anand’s 2026 PALF study used 5% NaOH, but that single value should be treated as a study-specific protocol rather than a default recipe.
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For silane, identify the exact agent—in the direct PALF comparison it was 3-APS—and report solution concentration, pH, hydrolysis or aging time, application method, drying temperature and time, and whether excess coating was removed. The retrieved PALF findings do not establish a universal 3-APS concentration or pH. Omitting those details makes a nominally identical “silane treatment” impossible to evaluate.
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Variables that decide which treatment works
A treatment label is not a complete process description. For a meaningful comparison, report and match the following variables.
| Comparison axis | Why it changes the result |
|---|---|
| Fiber species and source | Cellulose, lignin, wax content, diameter and prior processing vary by species and harvest source. |
| Fiber preparation | Cleaning, cutting, drying and storage determine the starting surface and moisture level. |
| Alkali chemistry | NaOH concentration, solution pH, temperature and exposure time control cleaning versus cellulose damage. |
| Silane chemistry | The silane identity, solution pH, hydrolysis, concentration, deposition and drying determine the coupling layer. |
| Sequence | Silane-only, alkali-only and alkali-then-silane are different processes; order should not be inferred from the treatment name. |
| Post-treatment handling | Washing, neutralization, drying temperature and storage can leave different residues or moisture levels. |
| Epoxy and hardener | Polarity, cure chemistry, viscosity and cure schedule affect wetting and chemical interaction with the treated fiber. |
| Composite architecture | Fiber loading, orientation, length, dispersion and manufacturing route determine how interface quality appears in bulk properties. |
| Measured outcome | IFSS, tensile, flexural, impact, water absorption and durability answer different questions and are not interchangeable. |
A defensible way to choose a treatment
Choose alkali first when surface cleaning is the main problem
Alkali is a rational first experiment when microscopy or wetting observations indicate waxy contamination, non-cellulosic residue or poor mechanical keying. Use a severity study rather than a single arbitrary recipe: compare untreated fiber with at least two controlled alkali conditions, while measuring both interface performance and retained single-fiber strength.
Choose silane first when chemical compatibility is the main problem
Silane is a rational first experiment when the fiber is clean but the epoxy does not wet or bond effectively. The PALF fragmentation result gives a strong study-specific reason to test 3-APS: it produced the larger IFSS increase in that analysis. The formulation still needs optimization for the actual epoxy and hardener.
Test the sequence when two process stages are acceptable
Use alkali followed by silane as its own candidate when the additional bath, washing and drying steps are practical. The PALF droplet result supports testing this sequence, but it does not establish that the sequence will beat silane alone in every fiber or test.
Reject a treatment that damages the reinforcement
If IFSS rises while post-treatment fiber strength falls sharply, the process may fail in a structural composite. Set an acceptance rule that includes both interface and fiber metrics before selecting the recipe.
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Step-by-step comparison plan for a laboratory study
- Define the question. Decide whether the primary endpoint is IFSS, laminate tensile strength, flexural performance, moisture uptake or another property. Do not substitute one endpoint for another.
- Lock the materials. Use the same fiber source, geometry, epoxy, hardener, fiber loading, orientation and cure schedule for every treatment arm.
- Create matched treatment groups. Include untreated fiber, NaOH-treated fiber, 3-APS-treated fiber and, if feasible, alkali followed by 3-APS. Record chemical identity, concentration, pH, temperature and duration for every bath.
- Control post-treatment handling. Apply the same washing, drying, storage and moisture checks. A residue or moisture difference can masquerade as an interface effect.
- Measure fiber integrity. Record single-fiber tensile properties and inspect treated surfaces before embedding them in epoxy.
- Run the chosen interface test. If using fragmentation or droplets, document specimen geometry, fiber diameter, embedded length, loading rate and failure criterion.
- Validate in the intended composite. Confirm the interface ranking with the actual fiber loading and manufacturing route; an IFSS ranking alone does not predict every bulk property.
- Report uncertainty and failure modes. Give replicate counts, scatter, exclusions and whether failure occurred at the interface, within the fiber or in the epoxy.
Common interpretation mistakes
- Calling a neat-resin comparison a treatment comparison. Anand’s 78 MPa composite versus 45 MPa neat epoxy result does not rank NaOH against silane.
- Combining incompatible tests. The 47% fragmentation improvement for 3-APS cannot be compared numerically with the combined-treatment direction reported by a droplet test.
- Generalizing from PALF to every natural fiber. Surface chemistry, morphology and damage tolerance differ substantially among species.
- Reporting chemistry without severity. “Alkali-treated” or “silane-treated” is incomplete unless concentration, pH, duration, temperature and post-treatment handling are stated.
- Optimizing only for adhesion. A stronger interface can transfer load into a fiber that has already been weakened by over-treatment.
- Assuming short-term interface data prove durability. The available evidence does not establish a matched, standardized long-term moisture comparison for alkali and silane in natural-fiber epoxy composites.
What adjacent studies add—and what they do not
Sahai et al. (2022) reported a sequential NaOH-plus-silane protocol and water-absorption testing in a sisal/polyester system. That work can inform experimental questions about sequence and moisture, but polyester is not epoxy and sisal is not PALF. Its values should not be presented as direct evidence for the title comparison.
The broader 2026 review supports the same caution: treatment conditions vary widely, and inconsistent reporting prevents reliable cross-study ranking. A useful literature table therefore keeps fiber, matrix, treatment recipe and test method in separate columns instead of placing every percentage in one universal comparison.
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Frequently asked questions
Which treatment improved PALF/epoxy adhesion more?
In Zaman and Khan’s single-fiber fragmentation analysis, 3-APS silane produced a reported 47% IFSS increase versus 6% for NaOH. In their droplet test, however, the sequential alkali–silane treatment ranked best. The answer depends on the test.
Does alkali treatment always damage natural fibers?
No. It can improve surface cleaning and bonding, but excessive severity can damage cellulose. Sun et al.’s ramie/cellulose-acetate experiment found reduced single-fiber tensile properties after both treatments, with greater damage from alkali in that setup. Fiber strength must be measured rather than assumed.
Is 5% NaOH the recommended concentration?
Five percent NaOH was used in Anand’s 2026 alkali-treated PALF/epoxy study. It is a reported protocol, not a universal recommendation. Concentration, temperature and time must be optimized for the specific fiber and epoxy system.
Can the 78 MPa PALF composite result be compared with the 47% silane gain?
No. The 78 MPa value compares a 15 wt% alkali-treated PALF/epoxy composite with 45 MPa neat epoxy. The 47% figure is an IFSS increase in a fragmentation test. They measure different outcomes and have different comparators.
Is the combined alkali–silane process worth the extra steps?
It is worth testing when two baths, washing and controlled drying are practical, because the PALF droplet test favored the sequence. The added stages increase process burden and variability, and the sources provide no general cost advantage. Validate the sequence against silane-only and alkali-only controls.
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What must be reported for a publishable comparison?
Identify the fiber and source, epoxy and hardener, treatment chemistry and severity, pH, temperature, duration, washing and drying, fiber loading and orientation, test geometry, loading rate, replicates, scatter and failure mode. Without these details, percentages cannot be transferred confidently between studies.
Bottom-line decision
For the directly studied PALF/epoxy interface, start with silane if your immediate objective is maximizing IFSS in a fragmentation-style screening test: 3-APS produced a reported 47% improvement versus 6% for NaOH. If your process can support two controlled stages, include alkali followed by silane because the combined treatment performed best in the study’s droplet test. Select the final process only after checking retained fiber strength and validating it in the exact epoxy composite and loading condition you intend to use.
Frequently Asked Questions
Which treatment improved PALF/epoxy adhesion more?
3-APS silane produced the larger IFSS gain in the fragmentation test, while alkali–silane ranked best in the droplet test.
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No. Excessive severity can damage cellulose; measure post-treatment fiber strength rather than assuming the interface gain is beneficial.
Is 5% NaOH universally recommended?
No. It was the concentration used in Anand’s 2026 PALF/epoxy study, not a universal recipe.
Can composite tensile strength be compared directly with IFSS?
No. They use different specimens, comparators and failure mechanisms.
Is the combined treatment worth extra processing?
It merits testing when two controlled stages are practical, but added washing and drying increase process burden and variability.
What details are required for a fair comparison?
Report fiber, matrix, chemical severity, pH, temperature, duration, washing, drying, architecture, test geometry, replicates and failure mode.
The Bottom Line
Silane led the PALF fragmentation result, while alkali–silane led the droplet result. Choose only after matching the test, controlling treatment severity and confirming that fiber strength is retained.
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