ReviewSilane coupling agents used for natural fiber/polymer composites: A review
Section snippets
Introduction and background
Natural fiber reinforced polymer composites (NFPCs), as an important branch in the field of composite materials, have been studied for decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Natural fibers have different origins such as wood, pulp, cotton, bark, nut shells, bagasse, corncobs, bamboo, cereal straw, and vegetable (e.g., flax, jute, hemp, sisal, and ramie) [10], [11], [12], [13]. These fibers are mainly made of cellulose, hemicelluloses, lignin and pectins, with a small
Silane structures
To effectively couple the natural fibers and polymer matrices, the silane molecule should have bifunctional groups which may respectively react with the two phases thereby forming a bridge in between them. Silane coupling agents have a generic chemical structure R(4−n)Si(R′X)n (n = 1,2) where R is alkoxy, X represents an organofunctionality, and R′ is an alkyl bridge (or alkyl spacer) connecting the silicon atom and the organofunctionality. In the past decades, various silane structures have been
Fiber surface coating and cell wall modification
In the present literature, there are several different means reported to apply silanes to natural fibers. These can be divided into fiber surface treatment and cell wall modification. Spraying is a relatively easy way to treat the fiber surface with a silane solution. The silanes are dissolved into certain organic solvents or solvent/water mixtures and the prepared solution is directly sprayed onto the fibers. If the solution is water-free, the sprayed silanes can be partly hydrolyzed via the
Inter-phase compatibility
The interaction mode between the silane-treated fiber and the polymer matrix is a crucial factor for the mechanical properties of the resulting NFPCs. Physical blending of the silane-treated fibers and the thermoplastic matrices enhances their mutual adherence via inter-molecular entanglement, or acid–base interactions (ABI) [43]. The interfacial shear strength (ISS) between jute fibers and PP, determined by a microdroplet micromechanical test, was improved by treating jute fibers with a 0.5%
Outdoor performance
The moisture sorption of NFPCs measured according to standard ASTM D-1037 [137] generally indicates very low moisture contents (in a range of ca. 2%), although this depends upon the fiber contents. This has been attributed to the effect of the encapsulation of fibers by polymers [138]; however, with the NFPCs in service there often appears deleterious effects such as surface colour fading and erosion, warpage, mold growth, fungal decay, and strength loss after a long-term exposure to an
Summary
Most established silanes used for natural fiber/polymer composites are trialkoxysilanes bearing a non-reactive alkyl or reactive organofunctionality. Silane is hydrolyzed forming reactive silanols and is then adsorbed and condensed on the fiber surface (sol–gel process) at a specific pH and temperature. The hydrogen bonds formed between the adsorbed silanols and hydroxyl groups of natural fibers may be further converted into covalent bonds by heating the treated fibers at a high temperature,
Acknowledgement
The author Dr. Yanjun Xie would like to thank the German Academic Exchange Service (DAAD) for a research grant support.
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