Wood Adhesives Based on Natural Resources: A Critical Review: Part IV. Special Topics

Various naturally-based chemicals can be used directly as wood adhesives or are precursors for the synthesis of adhesive resins. Liquefaction and pyrolysis of wood yield various smaller chemicals derived from the different wood components, which then are used in the preparation of adhesives by replacing mainly phenol as raw material. The possible replacement of formaldehyde in aminoplastic and phenolic resins would solve the question of the subsequent formaldehyde emission.<br/> The multiple unsaturations of the triglycerides in vegetable oils enable polymerization for the direct synthesis of thermosets, as well as bases for polyfunctionalization and crosslinking.<br/> Natural polymers, such as poly(lactic acid)s (PLAs), natural rubber, or poly(hyhydroxyalkanoate)s (PHAs) are thermoplastics and can be used for various special applications in wood bonding, in case they can also be crosslinked. For other thermoplastic wood adhesives, such as PUR or PA, chemicals based on natural resources can at least replace a part or even all synthetic raw materials (monomers); these monomers derive from targeted decomposition of the wood material in biorefineries.<br/> Cellulose nanofibrils (CNFs) can be used as as sole adhesives or as components of adhesives. Hydrogen bonding has a key function in binder applications related to adhesion between cellulose nanoparticles and other materials. CNFs are able to establish strong bonding between wood particles/fibres through flexible and strong films by a simple drying process.<br/> Cashew nut shell liquid (CNSL) is a by-product of the cashew nut processing with cardanol (CD) as main component. CD-formaldehyde resins show improved flexibility compared to phenol-formaldehyde (PF) resins; CD can replace up to 40% of the phenol.

Cold Atmospheric Pressure Plasma Technology for Modifying Polymers to Enhance Adhesion: A Critical Review

This review summarizes the results of cold atmospheric pressure plasma technology application in polymers surface treatment. Attention is given to results of changes in the hydrophilic property of polymer surfaces by incorporation of polar functional groups when exposed to atmospheric pressure plasma, depending on the time of treatment, applied voltage, gas flow rate, and distance from the surface. We have successfully developed a plasma device that is able to generate cold atmospheric pressure argon plasma of low temperature (20 – 26) ° C downstream using a high-voltage power source which can be widely used in materials processing. Therefore, a cost-effective system of generating a plasma jet at atmospheric pressure with potential applications has been developed. Cold atmospheric pressure plasma jet (CAPPJ) has shown a lot of applications in recent years such as in materials processing, surface modification, and biomedical materials processing. CAPPJ has been generated by a high voltage (0-20 kV) and high frequency (20-30 kHz) power supply.<br/> The discharge has been characterized by optical and electrical methods. In order to characterize cold atmospheric pressure argon plasma jet, its electron density, electron temperature, rotational temperature, and vibration temperature have been determined using the power balance method, intensity ratio method, Stark broadening method, and Boltzmann plot method, respectively. The improvement in hydrophilicity of the cold plasma-treated polymer samples was characterized by contact angle measurements, surface free energy analysis, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Contact angle analysis showed that the discharge was effective in improving the wettability of polymers after the treatment. Furthermore, atmospheric plasma can be effectively used to remove surface contamination and to chemically modify different polymer surfaces. The chemical changes, especially oxidation and cross-linking, enhance the surface properties of the polymers.

Mussel-Inspired Underwater Adhesives- from Adhesion Mechanisms to Engineering Applications: A Critical Review

Recent progress in the adhesion mechanism of mussels has led to great excitement in the field of adhesive materials. Although great progress has been made in the mussel adhesion mechanism and underwater adhesives, there are still many unknowns and challenges in this area. Thus, it is highly important to review the recent progress in mechanisms of mussel adhesion and mussel-inspired adhesives and predict trends for the future. In this review, we (1) summarize the research progress in fundamental interaction mechanisms in natural mussels; (2) discuss the application of the mussel interaction mechanism in the biomimetic mussel adhesive materials, from permanent/high-strength adhesives to temporary/smart adhesives; (3) briefly state the potential applications of the mussel-inspired adhesives in multiple fields, such as engineering applications, smart robotics and biomedicine; (4) summarize the future perspectives and unsolved challenges of mussel adhesion mechanisms and mussel-inspired adhesive materials. We envision that this review will provide an insightful perspective in understanding the mussel adhesion mechanism and directions to further explore, and promote the development of novel biomimetic mussel adhesive materials.

Wood Adhesives Based on Natural Resources: A Critical Review Part I. Protein-Based Adhesives

This series of critical reviews on Wood Adhesives Based on Natural Resources (in total four parts) describes the experience and actual status of wood adhesives based on natural resources and gives an outlook into the future of these materials. Desite boundless results and papers in the development, purely naturally based wood adhesives are in industrial use only in negligible amounts; therefore this review series also reports on combinations of naturally based adhesives with synthetic components, such as modifiers or crosslinkers.<br/> Part I of this series concentrates on general topics and questions related to wood adhesives based on natural resources, such as systematic overview on the various types of naturally based wood adhesives, including cases where the adhesive is not applied separately but is used in situ, originating from the various components of the wood material. As a first product group, proteins from plants and animal sources and their use as wood adhesives will be described in this Part I.

Wood Adhesives Based on Natural Resources: A Critical Review Part II. Carbohydrate-Based Adhesives

This Part II of the series of critical reviews on Wood Adhesives Based on Natural Resources concentrates on the various carbohydrates and their decomposition products, which might be used as wood adhesives. This includes the use of native carbohydrates as such as adhesives, and also with chemical modification of carbohydrates by natural and synthetic components. Crosslinking in order to improve moisture and water resistance is achievable by natural and synthetic chemicals. Most promising options are realized by decomposition of various carbohydrates to small, well-defined monomers, which then can undergo various reactions, yielding again polymers in order to create a bondline. Suitable monomers are various furan moieties, such as 5-hydroxymethylfurfural. Activation of cellulose and hemicellulose at the wood or fibre surface by chemicals, such as citric acid, enables bonding effects without addition of an external adhesive. So far, however, carbohydrates are used only in very small amounts as industrially applied wood adhesives, despite tremendous R&D effort made and a huge number of scientific papers and reports published.

Bioadhesives in Biomedical Applications: A Critical Review

The necessity for a long time contact between the drug and mucus layer/epithelial cell or a combination of the two requires a bioadhesive. A bioadhesive is known to intensify contact between the two and help in controlled release of drugs. The conventionally used bioadhesives are known to have poor adhesion strength and can have toxic side effects. This review focuses on the various types of polymers and their composites for use as bioadhesives which can overcome the previously mentioned issues. These include some naturally occurring bioadhesives such as collagen, chitosan, albumin, dextran and some synthetic bioadhesives like gelatin, poly(ethylene glycol), poly(acrylic acid), poly(lactic-co-glycolic acid) based bioadhesives. The clinical trials prove the effectiveness of these bioadhesives and they are found to be more efficient than the commercial glues and hence possess great potential for use in the biomedical industry.

Study and Characterization of Sisal Fiber/Zein Resin Interface

The interfacial shear strength (IFSS) between natural sisal fiber and zein protein resin was explored using the microbond test. Commercially available zein protein was processed into resins and their IFSS with sisal fiber was measured. Effects of sorbitol plasticizer content and microfibrillated cellulose (MFC) reinforcement loading on the IFSS with the resin were studied. Scanning electron microscopy (SEM) was used to characterize the fracture surfaces before and after the microbond test. Energy dispersive X-ray spectroscopy (EDX) was utilized to map the residual resin on the sisal fiber surface after the microbond test. The results showed that sisal fiber/ zein IFSS decreased with sorbitol content. At 20 wt% sorbitol content 53% decrease in IFSS was observed. IFSS increased with MFC loading from 1.32 MPa (control) to 2.40 MPa for resin containing 15 wt% MFC. Physical entanglements between sisal fibers and MFC are believed to be responsible for this enhancement in the IFSS.

Test Methods for Fibre/Matrix Adhesion in Cellulose Fibre-Reinforced Thermoplastic Composite Materials: A Critical Review

Due to the increasing discussion about sustainable and CO2-reduced materials, the demand for cellulose-based fibres as a reinforcing component in thermoplastic composites has increased considerably. Knowledge about the possibilities of modifying fibres for improved adhesion to the plastic matrix is essential in this context. The fibre/matrix adhesion in cellulose fibre-reinforced polymers is of considerable importance for the design of composite materials. Unfortunately, there are no standards for many essential methods to determine fibre/matrix adhesion. In this review article, various methods for measuring the interfacial shear strength between fibres and matrix, as an indirect characterisation of adhesion, are presented. Additionally, a brief overview of different methods for surface modification of cellulose fibres to improve the adhesion to a thermoplastic matrix is given. This review focuses on the fact that the parameters for the production of test specimens as well as the test method itself can vary considerably from study to study. Because of this, the comparison of data from different publications is not always possible. Therefore, in this article, the main influencing factors and differences in the methods are presented and discussed. Based on a systematic review and a clear description and discussion of the methods, the reader is given a broad basis for a better understanding of characteristic values for fibre/matrix adhesion.

Experimental Study of Cold Atmospheric Pressure Plasma Jet and Its Application in the Surface Modification of Polypropylene

The cold plasma technology is gaining popularity as one of the most effective tools for a wide range of applications. Cold atmospheric pressure plasma jet (CAPPJ) has attracted considerable attention in recent times for materials processing such as surface modification and biomedical applications. The cold atmospheric pressure plasma jet sustained in pure argon has been used here to modify the surface properties of polypropylene. CAPPJ has been generated by a high voltage power supply 5 kV at an operating frequency of 20 kHz. This paper reports the diagnostics of CAPPJ in argon environment by electrical and optical methods and its application in the surface modification of polypropylene (PP). The surface properties of the untreated and plasma-treated PP samples were characterized by contact angle measurements, surface free energy determination, scanning electron microscopy and Fourier transform infrared spectroscopy analysis. Most of the previous work has used RF power supply which is more expensive compared to the power supply used in the present study. The plasma jet is designed with locally available materials and can be used for continuous treatment for long time. We have successfully developed a plasma device that is able to generate a non-equilibrium atmospheric pressure argon plasma jet of low temperature. Therefore, a cost-effective system of generating a plasma jet at atmospheric pressure with potential applications in materials processing and biomedical research has been developed.

A Direct Method for the Assessment of Cohesive Zone Models for Thin Adhesive Layers Loaded in Mode I, Mode II, and Mixed-Mode I/II

In the context of increasing the strength-to-mass ratio of lightweight structures, the adhesively bonded joining technology appears to be an attractive solution. Nevertheless, the adhesive bonding method is important when the structural integrity of joints has to be ensured. In the literature, the cohesive zone models (CZMs) are shown to be able to predict both the static and fatigue strengths of adhesively bonded joints. The strength prediction is dependent on material laws and associated material parameters, characterizing the bondline behaviour mainly under pure mode I, mode II and mixed-mode I/II. The characterization methods are thus crucial. This paper aims at assessing the capabilities to identify the parameters of a particular CZM for both the inverse method, based on the energy balance associated with the path independent J-integral, and of a direct method described in this present work. The particular CZM has a classical shape based on the definition of a bilinear law for each of both pure modes, associated with pure mode interaction energy laws for initiation and propagation under mixed-mode I/II. The methodology used in this paper is based on a numerical test campaign only, involving the macro-element (ME) technique. A new approach for the fast formulation and implementation of ME modelling of two bonded beams is described.