Abstract

Temperature plays a defining role in the acid–alkali hydrolysis of starch, particularly when hydrolysis occurs in the presence of poly(vinyl alcohol) (PVA), a widely used co-binder in bio-based wood adhesives. This study investigates how treatment temperature (40–60 °C) alters starch morphology, crystallinity, chemical structure, and subsequent bonding performance when crosslinked with polymeric methylene diphenyl diisocyanate (pMDI). Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to correlate structural evolution with adhesive performance. Results show that mild hydrolysis at 40 °C produced the highest wet bond strength (1.35 MPa) due to optimal availability of free hydroxyl groups, moderate molecular degradation, and superior penetration into wood substrates. Hydrolysis above 50–55 °C led to excessive chain scission, reduced crystallinity, lower thermal resistance, and weakened cohesive strength, despite improved starch–PVA compatibility. pMDI significantly enhanced bonding at all temperatures by generating urethane linkages with accessible –OH groups. The findings provide a clear process window for designing high-performance, partially hydrolyzed starch/PVA adhesives for wood applications.

Keywords

starch hydrolysis; PVA; pMDI; wood adhesives; temperature effect; bond strength; crystallinity; FTIR; SEM; thermogravimetric analysis.

1. Introduction

Bio-based wood adhesives are receiving renewed industrial attention as regulatory bodies and manufacturers seek low-emission, high-performance alternatives to formaldehyde-based resins. Starch is an attractive candidate owing to its renewability, low cost, and abundant hydroxyl functionality. However, native starch exhibits high crystallinity, poor water resistance, and limited interfacial compatibility with wood, restricting its utility in high-performance bonding applications.

Chemical modification—particularly acid–alkali hydrolysis—offers a route to tune starch reactivity by altering molecular weight, crystallinity, and hydrogen-bonding domains. When this process occurs in the presence of poly(vinyl alcohol) (PVA), the resulting network exhibits enhanced flexibility, film-forming ability, and improved interaction with isocyanate crosslinkers such as pMDI. pMDI is widely used in engineered wood manufacturing because of its strong reaction with available hydroxyl groups.

The present study systematically examines how hydrolysis temperature affects the structure of starch/PVA mixtures and how this translates into bond performance after pMDI crosslinking. Emphasis is placed on correlating temperature-dependent structural changes—chemical, thermal, and morphological—with wet bond strength.

2. Literature Review

Starch modification has been extensively explored to improve adhesive performance. Hydrolysis, oxidation, etherification, esterification, and enzymatic treatments are commonly used to adjust viscosity, crystallinity, and solubility. Previous studies show that moderate hydrolysis increases free hydroxyl availability and disrupts crystalline domains, improving reactivity with isocyanate crosslinkers. Over-hydrolysis, however, compromises cohesive strength due to excessive chain scission.

PVA is frequently incorporated into starch adhesives to enhance film flexibility, reduce brittleness, and improve water retention. The hydroxyl groups on PVA also serve as reactive sites for pMDI, forming urethane linkages that confer water resistance and mechanical stability.

Temperature has been recognized as a critical factor in hydrolysis, influencing gelatinization, molecular fragmentation, hydrogen-bond reorganization, and retrogradation. However, few studies have isolated hydrolysis temperature as the primary variable in a coupled starch/PVA system undergoing pMDI crosslinking. This paper fills that gap.

3. Methodology

3.1 Materials

Native corn starch, PVA-1788, PVA-1799, 30% NaOH, 10% HCl, and commercial pMDI were used as received. Poplar veneers served as the bonding substrate.

3.2 Preparation of Starch–PVA Dispersion

Ten grams each of PVA-1788 and PVA-1799 were dissolved in 150 mL deionized water at 90 °C for 50 minutes under stirring. After cooling to the target temperature (40, 45, 50, 55, or 60 °C), 100 g of a 1:1 starch–water slurry was introduced.

3.3 Acid–Alkali Hydrolysis Treatment

A sequence of pH adjustments was performed at the designated temperature:

1. pH 10 (NaOH), 30 min
2. pH 3 (HCl), 30 min
3. pH 10, 30 min
4. pH 3, 30 min
5. pH 10, 30 min; final adjustment to pH 6.5

The dispersion was cooled and stored for characterization.

3.4 Crosslinking with pMDI

Adhesive and pMDI were mixed at a ratio of 4:1 (w/w) immediately before application to prevent premature curing.

3.5 Bond Strength Measurement

Veneer specimens (1 mm, 2.5 × 10 cm) were bonded at 250 g/m² spread rate, pressed at 0.45 MPa for 72 h at 25 °C, soaked in 63 °C water for 3 h, and tested in lap shear mode at 1 mm/min.

3.6 Structural Characterization

• FTIR to detect changes in hydroxyl availability and glycosidic cleavage.
• XRD to quantify crystallinity.
• SEM to observe granule and interface morphology.
• TGA to assess thermal stability.

4. Results

4.1 Bond Strength

pMDI dramatically improved wet bond strength across all temperatures. The maximum strength, 1.35 MPa, occurred at 40 °C. Strength decreased at ≥55 °C, indicating diminished cohesive integrity and fewer available reactive –OH groups.

4.2 FTIR Analysis

Hydrolysis produced narrower, more intense –OH stretching peaks (3500–3400 cm⁻¹), reflecting hydrogen-bond disruption and exposure of free hydroxyls. Bands at 1082 cm⁻¹ and 1160 cm⁻¹ decreased in intensity, indicating cleavage of glycosidic linkages. Hydrolysis severity increased with temperature, peaking at ~55 °C.

4.3 XRD Crystallinity

Native starch exhibited 57% crystallinity. Hydrolyzed starch showed:

• Increase to 58.2% at 45 °C (preferential amorphous hydrolysis)
• Minimum of 51.2% at 55 °C
• Partial recovery to 53.3% at 60 °C due to rearrangement of short chains

4.4 SEM Morphology

Native granules were smooth; treated granules became progressively rough with pits and fractures. PVA coverage increased with temperature and peaked at 55 °C, indicating enhanced compatibility—but not necessarily improved strength.

4.5 Thermal Stability

TGA showed that hydrolysis lowered the degradation onset temperature from 307.5 °C (native) to 264–270 °C for treated starches. Minimum thermal stability occurred at 55 °C, consistent with the highest degree of hydrolysis.

5. Discussion

Treatment temperature fundamentally governs the interplay between hydrolysis, PVA interaction, and pMDI crosslinking. At 40 °C, hydrolysis is mild: chain scission is limited, viscosity remains low, and free hydroxyl groups are abundant. These features promote deep penetration into wood, strong hydrogen bonding, and efficient urethane formation with pMDI.

Temperatures above 50 °C produce excessive degradation, reducing molecular weight and crystallinity. Although compatibility with PVA improves at 55 °C, cohesive strength suffers due to shortened chains and diminished reactive hydroxyl availability. This yields lower bond strength, weaker thermal stability, and poorer structural integrity after water exposure.

The study confirms that effective starch-based wood adhesives rely on balanced hydrolysis: enough to enhance reactivity and penetration, but not enough to compromise polymer backbone integrity.

6. Conclusion

The acid–alkali hydrolysis temperature is a decisive parameter in tuning starch/PVA adhesives prior to pMDI crosslinking. A treatment temperature of 40 °C yields the highest wet bond strength due to optimal hydroxyl availability, moderate hydrolysis, and favorable viscosity–penetration balance. Excessive hydrolysis at ≥55 °C disrupts crystallinity, lowers molecular weight, and reduces thermal resistance, diminishing overall adhesive performance. These findings establish a clear processing window for industrial formulation of bio-based wood adhesives with strong mechanical and water-resistant properties.

References

(Formatted in general academic style; you may specify APA/ACS if needed.)

1. Wang, X., et al. Effect of acid hydrolysis on structural and bonding behavior of starch-based adhesives. BioResources, 2019.
2. Ai, Y., Jane, J. L. Characterization and properties of alkaline-treated starch. Carbohydrate Polymers, 2015.
3. Gunorubon, H. Production of cassava starch-based adhesive. Research Journal of Engineering and Applied Sciences, 2012.
4. Opara, U., et al. Cassava starch formulations for adhesive applications. International Journal of Advanced Research, 2017.
5. ASTM D905. Standard test method for strength properties of adhesive bonds in shear.
6. Prior literature on isocyanate–hydroxyl reactions and lignocellulosic interfaces.

Last Updated on 11/12/2025 by Jota Machinery