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hydroxyl groups with TBDMS-Cl, and the resulting material could be incorporated into low-density polyethylene (LDPE) blends forming a hydrophobic polymer matrix. 75 Lignin is a versatile sca ff oldfordi ff erentmodi cations depending on the desired application. For the production of epoxy resins, epoxi- dation with epichlorohydrin is a common technique. This approach has also been combined with CO 2 xation resulting in cyclic carbonates being incorporated in the lignin. 76 2.4. Analysis techniques Techniques to assess lignins and lignocellulosic biomass have long been a topic of great interest, both for quantitative and qualitative characterization. Such techniques are also critical to probe and assess chemical modi cations. A summary of common methods is given in Table 1. Di ff erent techniques are o en combined to provide a better overall picture. For example, chemical modi cation of lignin may be probed in terms of molecular weight, i.e. , by using size- exclusion chromatography, and abundance of functional groups, as determined by FTIR or 2D NMR analysis. The tech- nique of choice can depend on factors such as the target groups of interest, but also on availability and cost. The polydisperse nature of technical lignin can sometimes make accurate measurements di ffi cult. This is manifested, for example, in the incomplete ionization of phenolic moieties during titration or UV spectrophotometry, as the con guration and side chains of phenolic moieties induce varying degrees of resonance stabilization.
steam explosion, CO 2 explosion, ammonia ber expansion (AFEX) and liquid hot water (LHW) pretreatments. 46 Ionic liquids have also been successfully used for lignin isolation. Five cations with good solubilizing abilities were identi ed: the imidazolium, pyridinium, ammonium and phosphonium cations, while the two large and non-coordinating anions [BF 4 ] − and [PF 6 ] − were found to disrupt dissolution of the lignin. 46 The chosen extractive method will not only a ff ect the characteristics of the resulting lignin, but also the amount that is extracted. Several methods have been developed for the deligni cationof sugarcane bagasse, e.g. , milling, alkaline or ionic liquid extraction, where yields of 17 – 32% were obtained depending on the method of choice. 71 2.3. Chemical modi cation Chemical modi cation of technical lignins is well explored and include a huge variety of techniques (see Fig. 6 for illustrative examples). Technical lignins have been modi ed by a myriad of techniques, such as esteri cation, phenolation and ether- i cation. 6 Urethanization with isocyanates has been explored towards polyurethan production, 72 and allylation of phenolic OH groups enabled Claisen rearrangement into the ortho -allyl regioisomer which is of interest for its thermoplastic proper- ties. 73 The solubility and charge density of technical lignins can be a ff ected by sulfomethylation or sulfonation, 17,74 and meth- ylation of the phenolic OH groups have led to lignin with an increased resistance to self-polymerization 17 The thermal stability of lignins has also been improved by silylating the
Fig. 6 Examples of chemical modi fi cations of technical lignin.
12534 | RSCAdv. , 2023, 13 , 12529 – 12553
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