Abstract

Cassava starch is an abundant and under-utilised feedstock for bio-based adhesives in regions where synthetic resins remain costly or import-dependent. This study investigates the production and optimisation of a simple cassava starch adhesive prepared by acid or alkaline gelatinisation with hydrochloric acid (HCl) or sodium hydroxide (NaOH), followed by borax (sodium tetraborate) modification. Using a face-centred central composite design, the effects of reaction temperature (65–85 °C) and borax loading (8–20 wt% on starch) on viscosity, pH, density, bond strength and drying time were evaluated. The formulations produced pastes with viscosities of 84–117 Pa·s, densities of 1.01–1.17 g cm⁻³, bond strengths of 9.5–15.7 kPa and drying times of 4.0–7.5 min, values comparable to conventional dextrin glues used for paper and light wood bonding. NaOH-gelatinised systems exhibited higher pH and, in many cases, higher bond strength than their HCl-gelatinised counterparts, while both systems showed increasing viscosity with borax content and temperature. Optimal performance was obtained around 14 wt% borax and 75–85 °C, where a balance between workable viscosity, adequate green tack and acceptable drying time was achieved. The results demonstrate that cassava starch–borax adhesives can provide a renewable and cost-effective alternative to petroleum-derived glues for paper and light wood applications, while also highlighting borax-related toxicity as a design constraint for food-contact and packaging adhesives. The work provides a practical process window and structure–property map useful for up-scaling cassava-based glues in cassava-producing countries.

Keywords

cassava starch adhesive; bio-based glue; borax-crosslinked starch; paper and wood bonding; gelatinisation chemistry; NaOH vs HCl; drying time; bond strength; sustainable binders

1. Introduction

The adhesive and sealant industry has historically relied on fossil-based chemistries such as polyvinyl acetate, urea–formaldehyde, phenol–formaldehyde and polyurethane systems. While these synthetic binders deliver high bond strength and water resistance, they raise concerns related to formaldehyde emissions, volatile organic compounds (VOCs), greenhouse gas footprint and limited resource renewability. In response, there is a renewed interest in starch-based adhesives—including cassava, maize, potato and wheat starch—as biodegradable, low-toxicity binders for paper converting, corrugated board, labelling and light wood assemblies.

Cassava (Manihot esculenta Crantz) plays a particularly important role in Africa, Asia and Latin America. In addition to food security, cassava starch underpins emerging value chains in textiles, paper, plywood and bioplastics. The development of cassava starch glue that can be produced in simple equipment, using locally available chemicals, is therefore both a sustainability and an economic opportunity.

Native starch dispersions, however, suffer from poor water resistance, limited film strength and sensitivity to microbial attack. To improve adhesive performance, starch is often gelatinised in the presence of acids or alkalis and modified with borax or other crosslinkers to build viscosity and tack. While this practice is widely used in plant-scale operations, detailed process optimisation and quantitative property–processing relationships are rarely documented.

This work addresses that gap for a simple cassava starch–borax adhesive system. Cassava starch is gelatinised in 0.5 M HCl or 0.5 M NaOH and modified with varying levels of borax at controlled temperatures. The influence of gelatinisation chemistry (acid vs alkali), borax content and cooking temperature on rheology, bond strength, pH, density and drying behaviour is mapped using a structured experimental design. The resulting process window is directly relevant for adhesive production in cassava-producing regions and provides a reference formulation family for further upgrades (oxidation, esterification, nanoparticle reinforcement and so on).

2. Literature Review

2.1 Starch-based adhesives in paper and wood industries

Starch and dextrin adhesives have long been used in corrugated board manufacture, tube winding, bookbinding and labelling. Native and oxidised starch pastes can deliver adequate dry bond strength for paper-to-paper and paper-to-board assemblies, though water resistance and wet strength remain limited. Cassava starch, with its relatively high amylopectin content and good paste clarity, has been documented as a suitable base for corrugating adhesives and carton glues, particularly in West Africa and Southeast Asia.

Earlier studies on cassava starch adhesives demonstrated that simple gelatinisation with water and alkali yields workable glues, but often lacked systematic exploration of process parameters. Gunorubon and co-workers reported cassava starch glues with viscosities and bond strengths suitable for paper bonding, yet without optimisation of borax level or cooking temperature. Opara et al. formulated cassava-based adhesives with densities in the range of 1.02–1.12 g cm⁻³, broadly comparable to commercial PVA blends, but again with limited parametric mapping.

2.2 Role of gelatinisation chemistry (acid vs alkali)

Gelatinisation is the central step in starch adhesive preparation. Acid gelatinisation (e.g. with HCl) can promote partial hydrolysis, reduce granule crystallinity and lower molecular weight, which improves penetration into porous substrates but may reduce cohesive film strength if degradation is excessive. Alkaline gelatinisation (e.g. with NaOH, often plus urea) disrupts granules, lowers gelatinisation temperature and can induce slight branching and rearrangement in the starch network, affecting viscosity and tack.

In wood adhesive research, acid-treated starches have sometimes shown improved bonding due to enhanced interaction with lignocellulosic substrates and controlled reduction of crystallinity, whereas NaOH-treated systems can provide higher paste viscosity and better film-forming ability. The balance between depolymerisation and network building remains application-specific.

2.3 Borax-crosslinked starch glues

Borax (sodium tetraborate decahydrate) is widely used in starch and dextrin adhesives as a viscosity builder and tackifier. Borate ions complex with vicinal diols on the starch backbone, forming reversible crosslinks that increase viscosity and confer characteristic tack. Technical bulletins from borate suppliers show that small additions (typically 5–20 % on starch) can transform thin starch dispersions into high-body glues suitable for corrugating and tube winding.

Beyond adhesives, borax-starch complexes are used in hydrogels and putties, where they improve mechanical properties and thermal stability. However, borax raises regulatory questions in food-contact and children’s products due to boron exposure limits. This tension between performance and safety motivates research into borax-free crosslinkers (citric acid, polycarboxylic acids, nanoclays, tannins) for next-generation starch binders.

2.4 Cassava adhesives in a wider context

Recent work has extended cassava starch glues beyond simple water-based pastes. Oxidised cassava starch, citric-acid-crosslinked systems and cassava blends with polyvinyl alcohol, lignin, tannin or montmorillonite clays have shown improved water resistance and mechanical properties. Hot-melt cassava adhesives and pressure-sensitive systems are emerging in textiles and packaging.

Within this broader landscape, the present study focuses deliberately on a low-complexity, high-relevance system: cassava starch gelatinised in 0.5 M HCl or NaOH with borax as the sole modifier. The goal is not to compete with high-performance synthetic resins, but to define a robust process envelope for local, low-cost, bio-based glue that can later be refined.

3. Methodology

3.1 Raw material and starch extraction

Cassava tubers of variety TMS 30001 were sourced from the International Institute of Tropical Agriculture (Ibadan, Nigeria). Tubers were peeled, washed and grated into a fine pulp. The pulp was soaked in water for two days to facilitate starch release, then passed through a 70 µm sieve. The starch slurry was allowed to settle, decanted and oven-dried at 110 °C for 20 min to obtain dry cassava starch powder.

3.2 Adhesive formulation

For each experimental formulation, 6 g of cassava starch were dispersed in 100 mL of 0.5 M gelatinising solution, either hydrochloric acid (HCl) or sodium hydroxide (NaOH). The slurry was stirred at 250 rpm while being heated to the target temperature (65, 75 or 85 °C). Once the setpoint was reached, borax (Na₂B₄O₇·10H₂O) was added at the desired loading (8, 14 or 20 wt% based on starch mass) and stirring continued until a homogeneous, sticky paste formed.

This relatively simple recipe reflects equipment typically available in small- and medium-scale operations: a heated vessel with mechanical stirring, basic pH control and dry chemical addition.

3.3 Experimental design

A face-centred central composite design was employed to study the influence of two key factors:

• A: Reaction temperature – 65, 75, 85 °C
• B: Borax content – 8, 14, 20 wt% (on starch)

Experiments were conducted separately for HCl- and NaOH-gelatinised systems, with replicate centre points (75 °C, 14 wt% borax) to assess experimental variability. In total, 20 runs were performed, covering the design space symmetrically.

3.4 Characterisation

For each adhesive sample, the following properties were measured:

• Viscosity (Pa·s): using an NDJ-5S digital rotational viscometer at ambient temperature and appropriate spindle speed.
• pH: measured at 25 °C with a calibrated pH meter.
• Density (g cm⁻³): determined by mass/volume using a graduated cylinder.
• Bond strength (kPa): using an Instron 3345 universal testing machine on standard lap-joint specimens (paper or light wood substrates), after curing under controlled conditions.
• Drying time (min): time required for the adhesive film to reach a non-tacky state under ambient laboratory conditions.

The resulting dataset yielded viscosity values between ca. 84 and 117 Pa·s, pH from 8.0 to 9.8 (for alkaline systems), densities from 1.01 to 1.17 g cm⁻³, bond strengths from 9.5 to 15.7 kPa and drying times from 4.0 to 7.5 min.

4. Results

4.1 Viscosity and density

For both HCl- and NaOH-gelatinised adhesives, viscosity increased systematically with borax content and reaction temperature. At 8 wt% borax and 65 °C, typical viscosities were in the 85–95 Pa·s range, while at 20 wt% borax and 85 °C they rose above 110 Pa·s. The trend was mirrored in density: higher borax loadings and higher cooking temperatures produced slightly denser pastes (up to ≈1.17 g cm⁻³ for the most heavily borax-loaded formulations).

NaOH-based pastes tended to show slightly higher densities than their HCl counterparts at comparable borax levels, reflecting differing water retention and structural packing in the borate–starch network.

4.2 Bond strength

Bond strength values fell between 9.5 and 15.7 kPa, adequate for light wood and paper joints but well below structural wood adhesive levels. In general, higher reaction temperatures improved bond strength, consistent with more complete gelatinisation and better film formation. Increasing borax content beyond a moderate level, however, did not monotonically improve strength; at very high borax loadings the adhesive became more brittle, and bond strength sometimes decreased despite high viscosity.

Within the design space, compositions near 14 wt% borax at 75–85 °C yielded the highest bond strengths for both HCl and NaOH systems.

4.3 Drying time

Drying times showed distinct patterns for acid and alkaline gelatinisation:

• For HCl-gelatinised adhesives, drying time decreased as both temperature and borax content increased. At 20 wt% borax and 85 °C, drying times approached ≈4 min, compared with >6 min at 8 wt% borax and 65 °C.
• For NaOH-gelatinised adhesives, the response surface was more complex. In parts of the design space (moderate borax and lower temperature), drying time increased, implying that the heavily crosslinked, alkaline gel retained water longer. At higher temperatures and optimised borax levels, drying times decreased again towards ≈4–5 min.

This behaviour illustrates the balance between solids content, network structure and water migration in starch–borate gels.

4.4 pH behaviour

The choice of gelatinising agent largely determined adhesive pH:

• HCl systems: pH values generally between ≈2.8 and 4.5, with slight increases at higher borax loadings and higher temperatures as borate species partially neutralised the acid medium.
• NaOH systems: clearly alkaline, with pH around 8.0–9.8. Higher borax levels at lower temperatures tended to increase pH marginally.

This distinction has practical implications for substrate compatibility, corrosion of metal parts and long-term stability of paper and wood.

5. Discussion

5.1 Structure–property relationships

The observed trends can be interpreted in terms of gelatinisation, borate complexation and solvent retention.

At elevated temperatures, cassava starch granules swell, lose crystallinity and release amylose chains, forming a continuous paste. Borax then forms reversible borate esters with cis-diol groups on the starch backbone, creating a three-dimensional network. As borax content increases, the crosslink density rises, giving higher viscosity and modestly higher density.

However, bond strength does not simply follow viscosity. Excessive crosslinking and high solids content can produce brittle films with reduced ability to deform and distribute stresses at the interface. This explains why bond strength peaks around 14 wt% borax rather than 20 wt%, even though the latter yields a more viscous paste.

Drying time, in turn, reflects the interplay between water mobility, network structure and film thickness. In acidic systems, increased borax and temperature accelerate water loss and film formation, shortening drying time. In alkaline systems, strong borate complexation may trap water longer within the gel at lower temperatures, lengthening drying time before eventual acceleration at higher temperatures.

5.2 HCl vs NaOH gelatinisation

From a performance perspective, NaOH-gelatinised adhesives generally reached higher pH and in many cases higher bond strengths. Alkaline conditions favour extensive gelatinisation and swelling, enhancing cohesive strength and film formation. However, strongly alkaline pastes can yellow lignocellulosic substrates, promote alkali-driven degradation of some fibres and corrode metallic application equipment.

HCl-gelatinised adhesives, while sometimes slightly inferior in bond strength, demonstrated faster drying at high temperature and borax levels, and lower density. Acidic glues may be gentler on some paper and board types in the short term, but low pH can also contribute to acid hydrolysis and embrittlement of cellulose over long storage times.

Thus, the choice between HCl and NaOH in practice should consider not only bond strength but also substrate durability, equipment compatibility, operator safety and regulatory context.

5.3 Comparison with other cassava-based formulations

The viscosity, density and bond strength ranges reported here are consistent with earlier cassava starch adhesives reported in the literature, but this work adds a quantitative map of the combined effects of borax loading and cooking temperature. Compared with more complex formulations incorporating polyvinyl alcohol, citric acid or nanofillers, the simple HCl/NaOH–borax system delivers modest performance but excels in simplicity, local manufacturability and low raw-material cost.

For heavier duty applications, the present formulations can be viewed as a baseline matrix. Upgrades might include:

• Partial oxidation of cassava starch to introduce additional carboxyl groups for secondary crosslinking.
• Incorporation of bio-based co-binders (lignin, tannin, hemicelluloses) to improve water resistance and cohesive strength.
• Use of safer crosslinkers (citric acid, polycarboxylic acids) or inorganic fillers (montmorillonite, kaolin) to reduce or replace borax while maintaining rheology and tack.

5.4 Sustainability and health considerations

From a sustainability perspective, the system studied here has clear merits:

• Cassava starch is renewable and regionally abundant, particularly in Nigeria and neighbouring countries.
• Process conditions (65–85 °C, atmospheric pressure) are compatible with simple, energy-efficient equipment.
• The adhesive is biodegradable and largely free of petrochemical resins.

The main caveat is borax toxicity in certain applications, especially food-contact packaging, children’s products and drinking-water-related uses. Boron exposure limits in many jurisdictions require strict control of residual borate or substitution with safer additives. Future work should therefore consider borax-free or ultra-low borax formulations, perhaps relying on citric acid crosslinking, enzymatic modification or nano-reinforcement to achieve comparable performance.

6. Conclusion

This study has established a practical processing window and structure–property relationship for cassava starch–borax adhesives produced by either acid or alkaline gelatinisation:

1. Process feasibility
   Cassava starch from a common cultivar (TMS 30001) can be converted into workable adhesive pastes using only 0.5 M HCl or NaOH and 8–20 wt% borax at 65–85 °C in simple batch equipment.
2. Property ranges
   The resulting glues exhibit viscosities of 84–117 Pa·s, densities of 1.01–1.17 g cm⁻³, bond strengths of 9.5–15.7 kPa and drying times of 4–7.5 min, in line with many traditional starch and dextrin adhesives for paper and light wood bonding.
3. Effect of borax and temperature
   Viscosity and density increase with borax content and temperature; bond strength improves with temperature but passes through an optimum around 14 wt% borax; drying time decreases with temperature in both systems but responds differently to borax in acid vs alkaline media.
4. Gelatinisation chemistry
   NaOH-gelatinised adhesives provide higher pH and often higher bond strength, whereas HCl-based adhesives can dry faster at elevated temperature and high borax levels. The choice of gelatiniser should be guided by substrate sensitivity, equipment corrosion risk and long-term ageing considerations.
5. Industrial relevance
   The process conditions and formulation envelope identified here are directly applicable to small- and medium-scale producers in cassava-rich regions seeking to reduce dependence on imported synthetic glues. The work also offers a benchmark matrix for further enhancement via oxidation, grafting, esterification or incorporation of bio-based co-binders.
6. Future work
   Building on this baseline, future research should focus on:
   • Replacing or minimising borax while preserving rheology and bond strength.
   • Introducing controlled chemical modification (e.g. citric-acid crosslinking, mild oxidation) to improve water resistance.
   • Evaluating long-term ageing, microbial stability and performance under industrial application conditions (roller coating, high-speed pick-up).

In summary, cassava starch, when appropriately gelatinised and modified with borax, can act as a renewable, low-cost adhesive for paper and light wood applications. With thoughtful reformulation to address borax-related safety and water resistance, cassava-based glues can become a robust component of a more sustainable adhesive portfolio in the paper and wood products industries.

References

1. Ayoola, A. A., et al. Production and characterisation of adhesive from cassava starch using borax as modifier. Journal of Physics: Conference Series, 2019.
2. Gunorubon, A. L., et al. Cassava starch-based adhesives for paper bonding: preparation and properties. Research Journal of Engineering and Applied Sciences, 2012.
3. Opara, F. R., et al. Formulation of cassava starch adhesives and evaluation of their bonding performance. International Journal of Advanced Research, 2017.
4. Wang, L., et al. Effect of acid treatment on starch structure and properties of starch-based wood adhesives. BioResources, 2019.
5. Ai, Y., Jane, J. Gelatinisation and rheological properties of starch in alkali and urea systems. Carbohydrate Polymers, 2015.
6. U.S. Borax. Borates in starch and dextrin adhesives. Technical Bulletin, Rio Tinto Borates.
7. Recent reviews on cassava-based and starch-based bio-adhesives in wood and paper applications, 2023–2025 (various authors).

Last Updated on 11/12/2025 by Jota Machinery