The first developed cellulose-based adhesive was trimethylsilylcellulose TMSC. It is a very well-known cellulose derivative introduced by Schuyten et al. The TMSC was synthesized through the reaction of cellulose with different organo-chlorosilanes in the presence of pyridine. Only signals for the substituent 0. Later some improvements in the TMSC synthesis led to products soluble in some organic solvents, such as chloroform, 1,1,1-trichloroethane and o -xylene [ 43 ].
Reproduced from [ 45 , 46 ] with permission from Wiley and Copyright Clearance Center, Generally, the synthesis of TMSC comprises several steps involving cellulose dissolution in a non-volatile solvent, such as N,N-dimethylacetamide with LiCl, derivatization in homogenous phase, and phase separation of the obtained TMSC. The product can be finally dissolved in a common suitable organic solvent tetrahydrofuran or toluene [ 44 ].
The obtained silylated cellulose is highly hydrolysable in the presence of water or another hydroxylated compound. Additionally, polymer crosslinking can be achieved as a result of either some secondary reactions of the trimethylchlorosilane itself, or due to the presence of some impurities in the chlorosilane, such as higher chlorinated silanes.
The latter are known to be considerably more reactive than trimethylchlorosilane [ 43 ]. The main challenge is to control the crosslinking process, enabling the reaction of the crosslinker chlorosilane with the OH groups of the substrate.
Another interesting route to obtain cellulose-based compounds with potential application as AS was reported by Stiubianu et al. The reaction proceeded at room temperature, which is quite beneficial for industrial proposes. Nevertheless, an important limitation of this procedure is related to the catalyst, which should be maintained away from cellulose acetate or other material containing OH groups due to it enhanced reactivity after being mixed with the other reagents [ 47 ].
Cellulose silylation and the subsequent regeneration using acid vapor. Reproduced from [ 47 ] with permission from Springer and Copyright Clearance Center, In a related work by Klemm et al. Multilayered supramolecular silylated cellulose structures were formed after applying a Langmuir—Blodgett technique.
These ultrathin films may be also suitable for AS purposes. Robles et al. The process is suggested to occur in four steps, but it basically involves a chemical grafting on the hydroxyl groups of cellulose chains Figure 7. However, some other works have demonstrated the possibility of curing of ATS at room temperature [ 50 ].
The use of ATS as a curing reagent for cellulosic raw materials appears as an interesting approach for bio-based AS, due to the possibility of chemical crosslinking between ATS and biopolymers contained in the AS or surfaces containing OH groups. Adapted from [ 49 ]. Farnaz Eslah et al. The acetylation of the cellulose nanocrystals reduces the crystallinity of cellulose. The SF reaction with acetic anhydride converts the amine and hydroxyl groups into amides and esters, respectively.
Primary and secondary amines in the ASF-based adhesive formulation disappeared, which may suggest the reduction of the amine content and formation of amides. These bio-based adhesives were found to fulfil the requirements for interior plywood, according to the American National Standards.
Khanjanzadeh et al. Reproduced from [ 53 ] with permission from Elsevier and Copyright Clearance Center, The modified CNC were incorporated in urea-formaldehyde adhesives and their performance investigated.
It was observed that the incorporation of modified-CNC improves the mechanical and other physical properties of the medium density fiberboard panels, while the formaldehyde emission significantly decreases. Draman et al. Cellulose was chemically modified with epoxy to display adhesion properties.
Different ratios of toluenesulfonic acid TSA -doped PPy with epoxypropyl cellulose were tested and their electrical and conductivity properties evaluated as summarized in Table 1. Thermal conductivity and electrical conductivity of the nanocomposites synthesized [ 54 ].
The authors have also shown a decrease in electrical conductivity in the cellulose-based adhesives, since cellulose works as an insulator. This non-metal material causes interface resistance, which is believed to also be the blockage for the thermal conductivity [ 56 ]. The thermal conductivity displayed suggests that this cellulose-based material can potentially be used in small electronic devices.
Starch and cellulose are two very similar polysaccharides but with different configurations of the linkages between the anhydroglucose units. This conformational difference not only makes starch less crystalline than cellulose but also more easily solubilized. Biodegradable composites with high robustness and elastic properties based on corn starch and PDMS have been developed by Ceseracciu et al.
As can be observed in Figure 9 , the degree of transparency of the obtained films change significantly with the amount of starch but overall the materials are quite mechanically robust.
It was also demonstrated that the naturally adsorbed moisture on the starch surface enables the auto-catalytic rapid hydrolysis of the polyorganosiloxane forming Si—O—Si networks. Additionally, corn starch granules have also excellent compatibility with the addition-cure polysiloxane chemistry.
Regardless of the starch concentrations used, all the developed bio-elastomers have hydrophobic surfaces with low friction coefficient and much less water uptake capacity than thermoplastic starch. The bio-elastomers are biocompatible and estimated to biodegrade rapidly even in an aquatic environment, thus avoiding one of the main drawbacks of standard silicones [ 57 ].
Sugih et al. Then hexamethyldisilazane HMDS was added to the gelatinized mixture to initiate the silylation reaction. The purpose of silylation was to make starch more hydrophobic by partial substitution of the OH groups by trimethylsilyl groups. After 2—4 h reaction time, toluene was added to solubilize the precipitated and partially silylated starch. The reaction scheme is shown in Figure Schematic illustration of the reactions involved in the silylation of starch with hexamethyldisilazane HMDS.
Adapted from [ 58 ], with permission from Elsevier and Copyright Clearance Center, Note that the partially silylated starch is more hydrophobic than pristine starch and thus more soluble in organic solvents.
The synthesized product has been suggested as a potential compatibilizer in starch-polymer blends [ 58 , 59 ]. Wei et al. The use of long side hydrocarbon chain of HDS was expected to provide an efficient and simple way to improve the hydrophobicity of SNC [ 60 ]. The SNC were prepared by acid hydrolysis of waxy maize starch according to the method of Angellier et al [ 61 ].
Afterwards, the silane modified SNC was prepared by physical adsorption, chemical grafting, and condensation reactions between the hydroxyl and silanol groups, as represented in Figure Adapted from [ 60 ], with permission from Elsevier and Copyright Clearance Center, As can be observed in Figure 12 , the hydrophobicity and the hydrophobic stability of the modified SNC was found to increase with the HDS content.
The dispersion of modified SNC was significantly improved due to the introduction of the long chain hydrocarbon. Moreover, the hydrophobically modified SNC show great compatibility with the non-polar solvents.
It is an inexpensive renewable resource which possesses numerous attractive properties, such as high thermal stability, biodegradability, high carbon content, antioxidant activity, and favorable stiffness.
The use of lignin in different applications has been a topic of interest for many researchers [ 4 , 62 ]. Nowadays, most of the aromatic feed chemicals originate from non-renewable fossil sources. It is believed that lignin could be a very appealing future source of natural polyphenols to compete and eventually substitute petroleum-based precursors. Despite its great potential to be used in the preparation of novel materials, lignin finds limited applications.
The majority of what remains is simply disposed or burnt as a low-grade fuel. Stiubianu et al. The developed elastomers have low water sorption due to their hydrophobic features and low dielectric constant. These properties were enhanced with the lignin content added to the formulation.
Despite the presence of lignin, which could facilitate the conduction of electrical currents, all the samples were observed to present low conductivity, thus behaving as very interesting insulators. Feldman et al. The results obtained suggest that lignin acts as a reinforcing agent adding rigidity to the polymeric matrix. This was further confirmed by the increase in toughness and shore of the blended sealants. The curing mode of PU was also observed to change with the addition of lignin.
Moreover, the initial setting time was reduced with the addition of lignin, but the rate of curing was found to remain constant. This suggests that the matrix is hardened in direct proportion to the amount of lignin present in the blend.
It was hypothesized that the incorporation of lignin may contribute to an increase in the degree of crosslinking of the PU sealant [ 64 ]. Another interesting study was reported by Scarica et al. The material was synthesized via the functionalization of softwood lignin with succinic anhydride SAn Figure 13 [ 65 ]. Schematic representation of the functionalization reaction between lignin and succinic anhydride SAn to produce succinylated lignin. Process of curing the succinylated lignin in a substrate.
This work introduces a straightforward strategy to develop new high-lignin-content PE thermosetting systems that may be suitable for bio-derived coatings and adhesive alternatives [ 66 ].
In another interesting report, lignin-based anticorrosive coatings have been developed after lignin fractionalization LF and silanization LF-S [ 67 ]. Schematic representation of the silanization of lignin. The authors report four different methods to obtain lignin-based coatings. The first two methods comprise solution-based deposition of LF and LF-S, followed by appropriate thermal treatment. The third coating was obtained from a formulation containing LF-S and acetic acid, which worked as a co-crosslinking agent.
In the last formulation, the cross-linking agent tetraethyl orthosilicate TEOS was also incorporated in order to evaluate its ability to act as organic-inorganic hybrid crosslinker in the lignin-based coating system.
Schematic representation of the covalent bonding between lignin fractionalization and silanization LF-S and metal substrate promoted by the presence of acetic acid and heat. The latter method, where TEOS was used as a cross-linker, was found to be the most promising due to the more rigid structure originated from the presence of the inorganic phase that partially hinders macromolecular movement in the cross-linker system [ 67 ].
Vegetable oils present versatile chemical options for different applications since they host reactive sites, such as ester groups, double bonds, and allylic hydrogens [ 68 , 69 , 70 ]. On the other hand, the use of vegetable oils as a raw material has many advantages, such as availability, price stability, sustainability, physico-chemical properties, and ecological reasons e.
Rapeseed is the most important oilseed in Europe and largely used in the production of biodiesel [ 71 ]. From a chemical point of view, the long aliphatic chains present in a typical vegetable oil can be used for the synthesis of for example, new silanes and polysiloxanes. These novel systems with hydrophobic features could find applications as coatings for the protection of different surfaces, such as wood, metal, or even concrete [ 69 ].
Several plants that produce unsaturated oils are claimed to be important in a healthy human diet. Their reactive unsaturated bonds also make them suitable candidates for other applications. For instance, these unsaturated oils have been used as coatings, such as the case of catalyzed oxidative curing of linseed oil finishes [ 72 ].
Another pursued application is the conversion of the unsaturated carbon bonds into polyols, which can be further transformed into PU [ 73 ].
As mentioned before, PUs have been widely used as adhesives since they only require mild curing temperature conditions, using the existing moisture of wood.
Moreover, PU is more capable of bonding wood matrixes with higher moisture contents than are phenolic and amino-based resins and adhesives. Fatty oils for non-PU routes are also being investigated for making adhesives and coatings [ 74 ].
For example, vegetable oil alternatives to silanes have been developed by attaching the siloxane groups to the carboxylic groups of fatty acids from rapeseed oil [ 69 ]. Due to the problems related to glycerine removal formed during vegetable oil saponification, the process was preceded by the reaction of oil transesterification leading to methyl ester of fatty acids.
The purified methyl esters were subjected to a posterior saponification and then the resultant sodium salts of fatty acids were subjected to nucleophilic substitution with 3-chloropropyltrimethoxysilane in the presence of potassium iodide as the catalyst Figure Scheme reaction of the synthesis of rapeseed-based silane. In fact, the obtained silane following the procedure described in Figure 17 has been employed in the creation of a coating capable to protect wood against water.
As introduced before, due to water exposition hydrolysis and condensation reactions may occur, leading to the formation of Si—OH groups. Schematic illustration of the covalent bonds formed between the silane groups of the vegetable oil-based coating and OH groups of wood. The coating is found to be attached to the surface of the modified element following the essential principle of the adhesive formulations. Furthermore, the aliphatic chain bonded to the silicon atom makes the surface of wood substrate more hydrophobic in comparison to the unmodified sample, thus forming a real protection against moisture-rich environments.
Protein-based AS have been explored particularly for biomedical applications, such as cardiac, vascular, oral, and reconstructive surgery [ 75 ]. These protein-based systems are typically used directly or in combination with a crosslinking agent that forms covalent bonds with the tissue surface [ 76 , 77 ].
Proteins can be directly extracted from human or other animal sources, offering many advantages over other common glues regarding biocompatibility and mechanical properties. Strausberg et al. Some mussels and barnacles can produce remarkable moisture-resistant adhesives. For instance, the blue mussel Mytilus edulis synthesizes a specific polyphenolic adhesive protein, which plays a key role in the attachment to surfaces.
The protein is located in a thread-like structure known as byssus, which also contains several other proteins including collagen. When secreted and applied, the byssal adhesive is highly cross-linked and cannot be readily analyzed.
The adherence of the engineered mussel adhesive protein in an aqueous environment to various surfaces including polystyrene, glass, hydrogel, and collagen has been tested. In each case, the activation to the quinone form of the protein was required for good surface adhesion. The nature of the chemical bond involved in cross-linking has not yet been determined. However, the authors have suggested that quinone residues could bond to the c-amino group of lysine through a Michael addition reaction.
Kumar et al. An initial protein acylation Figure 19 was performed to improve functional properties, such as solubility and surface hydrophobicity. Acylation of an amino group of soy protein.
Adapted from [ 79 ], with permission from Elsevier and Copyright Clearance Center, It has been shown that the silanization using 3- 2-aminoethyl -aminopropyltrimethoxy silane as a coupling agent enhances the interfacial adhesion between the soy matrix and, for instance, glass fiber to produce fiber-reinforced composites Figure Silanation of soy protein for improving interfacial adhesion in glass fiber reinforced composites.
Copolymers of soy protein isolate SPI have also been prepared by heating an alkaline dispersion of SPI with cationic epoxide or an acrylate monomer to originate modified protein materials. Create Alert Alert. Share This Paper. Background Citations. Citation Type. Has PDF. Publication Type. More Filters. Influence of nanofillers on the properties of siloxane elastomers. In this study, the influence of nanosilicon IV -oxide with hydrophobic and hydrophilic functionalized surfaces on the properties of siloxane elastomers was studied.
The elastomers were prepared … Expand. Highly Influenced. View 6 excerpts, cites background. View 2 excerpts, cites background. Two-dimensional coordination polymers containing permethylated motifs - promising candidates for 2D emerging materials. What makes silicon so special that is has an entire valley in California named after it?
Read on. In nature, silicon is no loner. It's usually found linked up with a pair of oxygen molecules as silicon dioxide, otherwise known as silica. Quartz, an abundant ingredient in sand, is made up of non-crystallized silica. Silicon is neither metal nor non-metal; it's a metalloid, an element that falls somewhere between the two. The category of metalloid is something of a gray area, with no firm definition of what fits the bill, but metalloids generally have properties of both metals and non-metals.
They look metallic, but conduct electricity only intermediately well. Silicon is a semiconductor, meaning that it does conduct electricity. Unlike a typical metal, however, silicon gets better at conducting electricity as the temperature increases metals get worse at conductivity at higher temperatures.
Berzelius heated silica with potassium to purify silicon, according to the Thomas Jefferson National Accelerator Facility , but today the refinement process heats carbon with silica in the form of sand to isolate the element. Silicon is a main ingredient in very low-tech creations, including bricks and ceramics.
But the high-tech stuff is where the element really makes its mark. As a semiconductor, silicon is used to make transistors, which amplify or switch electrical currents and are the backbone of electronics from radios to iPhones. Silicon is used in various ways in solar cells and computer chips, with one example being a metal-oxide-semiconductor field effect transistor, or MOSFET, the basic switch in many electronics. To make silicon into a transistor, the crystalline form of the element is adulterated with trace amounts of other elements, such as boron or phosphorous, according to Lawrence Livermore National Laboratory.
The trace elements bond with the silicon atoms, freeing up electrons to move throughout the material, according to the University of Virginia.
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