Abstract
Plasticized cellulose acetate (CA) is one of the most applied bio-based polymers due to its structural properties and easy processing. Plasticizers are added to CA to increase workability, prevent degradation under processing conditions and ensure thermo-mechanical properties suitable for the intended final application. Moreover, inexpensive and non-toxic solvents enable its processing into fibers, films, and solid blocks. However, when incorporated in the polymer matrix, plasticizers are prone to migration. CA products can suffer embrittlement, cracking, warping, or discoloration during their life cycle, affecting the material's integrity and durability. The design of new plasticizers compatible with the polymer at high concentrations, tailored to be effective in lowering the glass transition temperature, and with a low tendency to migration could considerably reduce material degradation over time. This review offers a perspective on the current plasticizers and comprehensively depicts the plasticization mechanisms in CA for internal and external plasticization. Understanding the plasticization mechanisms paves the way to identify a rationale for designing new plasticizers for this polymer. Cellulose acetate (CA), a cellulose ester derived from the partial acetylation of cellulose, has numerous applications in a wide range of industrial fields due to its transparency, high gloss, colorability, warm and pleasant feel, and suitability for direct and prolonged contact with human skin. The main applications of CA include eyewear frames [1,2], tool handles, cigarette filters [3-6], safety glasses and shields, protective films for LCD polarizing panels [7-10], costume jewelry, buttons, sequins [11] and decorations. Other applications of CA include drug delivery systems [12] [13] [14] [15] [16] [17] , membranes [18] [19] [20] [21] [22] [23] [24] , battery separators [25, 26] , biodegradable films [27], foaming applications [28] , and photographic films [29] [30] [31] . CA has also been proposed for sensors of metal ions [32] , gases [33] , humidity [34, 35] , formaldehyde [36] , and ascorbic acid [37] . CA features a strong dipolar interaction network, which gives it a high glass transition temperature (Tg) and a narrow window between flow temperature (Tf) and decomposition temperature (Td) [38, 39] . To overcome this issue, CA has traditionally been blended with low molecular weight organic compounds that act as plasticizers to broaden the processing window and increase its workability [40] . This method is generally termed external plasticization. [41] CA thermoplastic processing commonly uses 15-35 wt% plasticizers. [11] Other cellulose esters with longer chains, such as cellulose propionate (CP), cellulose butyrate (CB), or longer substituents, are more flexible and require lower quantities of plasticizer to reach the same flowability at a given temperature due to an internal plasticization effect [11, 42] . Copolymerization with a small amount of a second monomer or grafting substituents with high side chain flexibility is called internal plasticization [43] . However, external plasticization is the more generalized approach. As the other relevant cellulose ester, CA is processed using most of the plastic production methods, such as injection molding [44] , compression molding [45] , blow molding [46] , extrusion [47] , film casting [48] and fiber spinning [49] . CA is subjected to solvent dissolution in acetone in the last two industrial processes. In the case of fiber production, the dope solution is dry spun to produce textile filaments or cigarette filters. It is worth recalling that the traditional solvent block process is still employed for obtaining thick patterned and multicolored sheets for applications in the eyewear and fashion industry (Fig. 1 ) [11] . Although industrial processes are optimized by incorporating a certain amount of plasticizers, plasticized CA products are generally subject to plasticizer migration and volatilization to such an extent that CA becomes prone to cracking, warping, discoloration, and powdering as it ages [50, 51] . Moreover, plasticizers that migrate out of the material could harm human health and contaminate the environment [27, 52] . In this regard, understanding the mechanisms of CA plasticization is crucial to find room for improvement in tailoring cellulose acetate properties with a well-designed plasticizer molecule. Even though numerous works in the literature focus on plasticizer design for polymers [53] [54] [55] [56] [57] [58] [59] , there are no recent studies on the development of plasticizers for CA. In one recent work, six plasticizer candidates with slight structural variations were designed to evaluate the effects of the inclusion of oligolactide segments, in combination with variations in end groups (hydroxyl or levulinate ester) and structural differences in the alcohol cores on the thermal, mechanical, and migration patterns of plasticized poly(lactic acid) (PLA). The plasticizer candidates were designed from potentially renewable chemicals using a flexible (1,4-butanediol) or rigid (isosorbide) core and combined with three different types of flanking groups [53] . The same authors [60] further studied eugenol, a natural aromatic compound: bio-based resources such as levulinic and valeric acids were combined to produce dual-functioning plasticizers, with different plasticizing and antibacterial performances, depending on phenol-, carboxyl-, and alcohol group content. In this scenario, fundamental parameters such as solubility, compatibility, polarity, molecular weight, and functional groups of plasticizers play a crucial role in determining the success of plasticized systems while mitigating the drawbacks of the external plasticization [61] . Internal plasticization of pure cellulose or CA is an interesting approach to prevent plasticizer migration, but lower plasticizing efficiency and technological limitations hinder its industrial scaling-up [62, 63] . However, the substituent structure seems to lower glass transition temperature, and acylating reagents consisting of bulky terminal moieties spaced from CA chains by a linear chain efficiently separate macromolecular chains without generating detrimental stiffening interactions [64, 65] . This review provides an overview of CA plasticization approaches and sets general principles and guidelines for selecting a suitable plasticizer or designing a new molecule. After an introduction to the general properties of cellulose and its derivatives, mainly CA, and the main plasticization theories, we thoroughly present the modification strategies of internal plasticization and the more common external plasticizers used for CA. Through a more detailed analysis of effective plasticizer molecules, we then investigate molecular structures for a possible future design of new molecules to be used in CA, focusing on some critical aspects such as substituent moieties (ester groups, hydroxyl groups, alkyl chains, aromatic rings), as well as molecular weight. We finally provide the reader with guidelines for proper CA plasticizer design. Fig. 1 . Solvent block process: CA flakes are mixed with the plasticizer and the solvent(s) to produce a transparent paste. The paste is colored by adding pigment and pressed in a calender to produce CA sheets. These colored sheets are then cut into cubes, randomly reassembled in a mold, and hot pressed to generate the wet block. The wet block with the desired pattern is then cut into sheets. Finally, the sheets are dried, flat-pressed, and trimmed for subsequent processing.
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