By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jun 12 2024A recent review article accepted for publication in Heliyon systematically examined several factors that influence the bonding strength of bamboo-based composites, which are gaining prominence as a green building material.
Study: Enhancing Bonding Strength in Bamboo-Based Composites. Image Credit: youcanpick/Shutterstock.com
Background
Historically, wood and bamboo have been key construction materials worldwide. However, bamboo suffers from inconsistent mechanical qualities and variable geometry and structure; its irregular tube shape and diameter complicate joint and connection formation. To this end, engineered bamboo products such as bamboo lumber, laminated bamboo, bamboo scrimber, and bamboo-timber composites have emerged as robust alternatives.
The durability and quality of these products largely depend on the strength of the interfacial bonds between the bamboo fibers. The efficacy of these bonds is influenced by several key factors, including the depth of adhesive penetration, the rate of adhesive curing, and the conditions under which the materials are clamped during the manufacturing process. These factors collectively determine the structural integrity and longevity of the finished products, making them viable and sustainable alternatives to traditional materials.
This study comprehensively discussed the variables influencing the bonding properties of bamboo products, highlighting the optimal parameters and procedures essential for enhancing the quality and reliability of bamboo-based composites.
Gluing Factors
The bonding strength and overall quality of bamboo-based constructions are heavily influenced by the choice and application of adhesives, given their extensive use in bamboo composites. Factors such as the type of adhesive, its solid content, rate of spread, application method, and any modifications to the adhesive formula must be examined closely to ensure optimal bonding.
Achieving the best performance from bamboo-based composites requires a harmonious interplay between the bamboo's inherent properties, the characteristics of the adhesive, and the specifics of the bonding process. Adhesives like Melamine Urea Formaldehyde (MUF) and Phenol Resorcinol Formaldehyde (PRF) are identified as having the most suitable properties for bamboo products intended for both exterior and interior structural applications.
PRF adhesives, in particular, are noted for their effectiveness when the resin content ranges from 10 % to 18 %. Selecting the optimal adhesive within these parameters is crucial for establishing strong interfacial bonds that support the structural integrity and durability of bamboo products, making them reliable for various construction applications.
Bamboo Components
The basic structural and physical properties of adherent bamboo affect the bonding quality of the composites. Bamboo, a biomass material, has an uneven microscopic structure, and its culm mainly consists of parenchyma cells and fiber cells. The bonding strength is significantly influenced by the species of bamboo, the area where it is grown, and its age under outdoor conditions. Furthermore, characteristics such as porosity, density, the nature of the cell walls, moisture content, extracellular spaces, the presence of nodes, and the type and size of elements are critical determinants of product performance.
The chemical components found within bamboo culms can adversely affect interfacial bonds, thereby weakening the properties of the product and limiting its potential for structural applications. Nevertheless, implementing stringent material quality control and employing processes to eliminate these harmful constituents can significantly improve the bonding properties of bamboo.
Specifically, among various bamboo species, Phyllostachys heterocycla and Guadua angustifolia kunt are noted for their superior interfacial bonding capabilities. However, the strength of these bonds can gradually degrade over time, especially under conditions of prolonged and sustained loading.
Elevated Temperatures
The thermal performance of adhesives plays a critical role in determining failure models and mechanical properties, such as the bonding strength of bamboo products. This influence can vary, being either beneficial or detrimental, depending on the temperature. Consequently, temperature resistance is a crucial consideration in bamboo-based construction.
MUF adhesives demonstrate superior connection strength at lower temperatures, comparable to that of solid wood. However, their chemical structure begins to degrade when exposed to temperatures exceeding 150 °C. In contrast, PRF adhesives maintain robust interfacial bonding even at high temperatures, making them better suited for heat-stressed applications.
Manufacturing Methods
Engineered bamboo products are crafted using various methods to transform irregular, circular canes into regular construction materials with straight edges. The mechanical properties of these products, such as bonding strength, are influenced by numerous factors including the arrangement of layers, type of treatment, clamping pressure, duration under pressure, pressing type, temperatures, surface characteristics, and loading direction.
The bleaching treatment is favored over caramelizing for bamboo products as it enhances surface properties crucial for bonding. This improvement is largely due to the altered chemical content of lignin during bleaching, which increases the surface energy.
Moreover, surfaces with higher roughness tend to exhibit stronger bonding strength compared to smoother ones. Any surface treatment should precede adhesive application, as post-treatment can weaken the glue line. Proper surface preparation before manufacturing is essential for achieving optimal product performance.
Challenges and Future Prospects
Despite its potential, several challenges still hinder the widespread adoption of bamboo composites in construction. Achieving high bonding strength in bamboo composites at a low cost remains challenging, necessitating innovative approaches in material science to develop cost-effective and environmentally friendly adhesives.
Currently, there are no universally accepted standards and codes for bamboo products, complicating the assessment, comparison, and certification of bamboo-based materials. Establishing standardized testing methods and protocols can reduce variability in bamboo quality and ensure consistent long-term performance of composite materials.
Environmental factors like corrosion and insect infestation pose additional challenges to engineered bamboo construction, requiring innovative manufacturing techniques to scale production while maintaining quality and durability. Additionally, the variability in bamboo properties across different geographic locations adds complexity to standardization and application, emphasizing the importance of understanding and categorizing these variations.
In conclusion, this comprehensive review outlines the key variables that affect the bonding properties of bamboo products, contributing to the development of high-quality, reliable applications in bamboo-based construction.
Journal Reference
Sewar, Y., Amran, M., Avudaiappan, S., Gamil, Y., & Rashid, R. S. M. (2024). Bonding strength performance of bamboo-based composite materials: An in-depth insight for sustainable construction applications. Heliyon, e32155. https://doi.org/10.1016/j.heliyon.2024.e32155, https://www.sciencedirect.com/science/article/pii/S2405844024081866
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