The Future of Plasticizers: Biobased and Oligomeric

1  Introduction

Plasticizers play an important role in polymer processing. This is particularly true for PVC, which may require 40%–60% plasticizer by weight for processing and use [1]. PVC is a polar polymer and, in the absence of plasticization, is a rigid, brittle material [2]. Traditionally, phthalate esters, most prominently, di(2-ethylhexyl) phthalate [DEHP], have been used as plasticizers. DEHP is produced from two inexpensive precursors, phthalic acid and 2-ethylhexanol. Phthalic acid is obtained from the oxidation of o-xylene, the least useful of the mixed xylenes from the reforming of naphtha [3]. The alcohol, 2-ethylhexanol, is obtained from the hydrogenation of the aldol adduct from butraldehyde, which is a byproduct of the hydroformulation of propylene [3]. Further, DEHP is effective in affording plasticization and workability to a polymer matrix into which it is incorporated. Consequently, it has become the most widely used plasticizer for polymeric materials. Unfortunately, human exposure to DEHP and other phthalates can lead to the development of a wide variety of disease states, most arising from endocrine disruption, but including cardiovascular and respiratory disorders [4]. This has led to the prohibition of the use of phthalate plasticizers in certain key applications [5,6]. The negative properties of phthalate ester plasticizers have stimulated a massive interest in the development of new, effective, nonmigrating, nontoxic and cost-effective plasticizers.

2  Discussion

The response to the need for non-phthalate plasticizers that are effective, nonmigrating and nontoxic has focused on the development of plasticizing compounds derived from biomaterials or biobased materials [7–9]. Most often, these have been designed for use in PVC [10–12]. Because of wide availability, lack of toxicity and low cost, crop plant oils have been attractive precursors to new plasticizers [13,14]. Nonedible oils have also been used as plasticizer precursors [15]. A potential drawback to the use of oil-based plasticizers is the somewhat limited compatibility with many polymers. To overcome this, polar functionality has often been introduced into the structure [16,17]. A great variety of other biobased precursors have been used for the generation of plasticizers [18]. Prominently among these have been cardanol [19–21] (a byproduct of cashew nut processing), tartaric acid [22–25] (a byproduct of winemaking) and derivatives of starch, isosorbide [26,27] and the furanics [28–32]. Branching in plasticizer structure is effective for promoting free volume effects in a polymer matrix [33–35]. Branching is most prominent in hyperbranched oligomers, and several have been prepared to function as effective, nonmigrating plasticizers, primarily for PVC [36–38].

Glycerol is a nontoxic, trifunctional biomonomer readily available from biodiesel production and soap making [39]. It represents a valuable starting point for the synthesis of hyperbranched oligomeric plasticizers. Because of the difficulties associated with direct polyesterification of glycerol, materials have often been produced from glycidol [40–42]. Direct polyesterification of glycerol generally leads to a highly crosslinked gelled product. This is useful for the generation of coatings but hampers the generation of discrete, stable oligomers suitable for further transformations. Traditionally, empirical approaches, generally a reaction to low monomer conversion, have been utilized in an attempt to avoid gelation [43,44]. This poses two problems. The desired product must be isolated from a large amount of unchanged reactants. More importantly, the materials generated contain two kinds of end groups, hydroxyl and carboxyl, and will gel on storage. Fortunately, newly developed technology, the Martin-Smith approach for determining initial monomer ratios, permits reaction to high monomer conversion without gelation to produce structures of any desired molecular weight with a single kind of end group, either hydroxyl or carboxyl [45,46]. These materials may be produced in the presence of acidic or enzymatic catalysts or in the absence of catalyst using standard methodology, essentially at one-pot synthesis. The ability to tailor the molecular weight and to control end group identity provides great flexibility for the design of plasticizers for a broad range of polymers. For example, capping the hydroxyl end groups of glycerol/adipic acid hyperbranched oligomer of 1300 molecular weight as esters of varied structure affords excellent, nontoxic, nonmigrating plasticizers for PVC [47–49]. These oligomers prominently contain ester functionality, a high degree of branching, and a large number of end groups. All these features facilitate effective plasticization.

3  Conclusions

Plasticizers of the future need to be effective, available via a noncomplex process, relatively inexpensive, nonmigrating, and most importantly, non-toxic. Oligomeric materials of optimum molecular weight and structure offer the potential to avoid problems associated with plasticizer migration. Structural branching strongly facilitates plasticization effectiveness. Plasticizer toxicity may be controlled through the use of abundantly available, renewable, biobased precursors for their generation. Most prominently, hyperbranched oligomeric esters derived from the readily available, renewable, nontoxic biomonomers, glycerol and adipic acid, are excellent nonmigrating plasticizers. These materials may be produced in a simple process. Using recently developed technology, these oligomeric plasticizers may be produced in any desired molecular weight without gelation to afford materials of precise structure and a single kind of end group. This greatly facilitates the development of nontoxic, nonmigrating plasticizers for wide use.

To meet the demands of concerns about environmental contamination, sustainability, governmental regulation and human toxicity, plasticizers of the future will need to be biobased and oligomeric.

Acknowledgement: None.

Funding Statement: The author received no specific funding for this study.

Availability of Data and Materials: Not applicable.

Ethics Approval: Not applicable.

Conflicts of Interest: The author declares that they have no conflicts of interest to report regarding the present study.

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