Renewable malic acid-based plasticizers for both PVC and PLA polymers

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Abstract

Acetylated malic acid alkyl esters (AcMAE-Cn) with different alkyl chain lengths, as alternative plasticizers for both poly(vinyl chloride) (PVC) and poly(lactic acid) (PLA), were synthesized by the esterification of renewable malic acid with alcohols and subsequent acetylation. The molecular structure of the synthesized plasticizers was confirmed using 1H NMR and FT-IR spectroscopy. PVC or PLA blends containing 50 phr of the plasticizers were evaluated by dynamic mechanical analysis (DMA), tensile testing, and migration testing to evaluate the plasticization efficiency. The plasticized PVC films presented good plasticizing performance, as demonstrated by the results of tensile and migration testing and the Tg values (as determined by DMA). Among the plasticizers, AcMAE-C6 and AcMAE-C8 presented enhanced plasticizing performance compared with that of the commercial dioctyl adipate (DOA) and di(2-ethylhexyl) phthalate (DEHP) plasticizers. Additionally, the plasticizing properties of the plasticized PLA films depend significantly on the structure of the plasticizers. It was found that plasticizers (AcMAE-C4 and AcMAE-C6) bearing short alkyl groups (n-butyl and n-hexyl, respectively) may act as excellent plasticizers. In summary, the AcMAE-C6 plasticizer showed potential as an alternative plasticizer for both PVC and PLA polymers.

Introduction

Poly(vinyl chloride) (PVC) is one of the most widely used polymers in industrial fields such as the manufacturing of packaging, toys, biomedical products, and building materials [1], but its use is generally limited due to the brittle nature of the polymer. Thus, it is often compounded with plasticizers to enhance its flexibility and toughness for various applications. Some of the most common plasticizers that are currently being utilized for PVC are aromatics and aliphatic esters, typically di(2-ethylhexyl) phthalate (DEHP) and dioctyl adipate (DOA). There have been concerns over the use of DEHP, as it negatively impacts human health and the environment [2]. More specifically, DEHP is a known endocrine disruptor in animals and suspected to also affect humans, in which it could potentially damage not only reproductive function but also immune function, behavior and memory [3]. Many alternative plasticizers have been developed with the discontinuation or restriction of DEHP use in European nations and elsewhere [4], [5], [6]. These alternatives should have similar or superior mechanical properties to phthalates in order to function as suitable replacements. Ideally, alternative plasticizers should also be biodegradable and have less propensity than phthalates to leach into the surroundings in order to minimize their negative environmental and health effects. There have already been many studies performed on biobased plasticizers for PVC, such as succinate [7], maleate [8], fumarate [9], vegetable derivatives [10], [11], [12], and lactic acid [13], in hopes of dethroning phthalates as a popular plasticizer. However, the environmentally friendly plasticizers developed so far may have problems competing with phthalates in terms of cost as well as plasticization efficiency.

Poly(lactic acid) (PLA) is a popular biodegradable aliphatic polyester derived from natural resources such as corn starch, tapioca root chips or starch, or sugar cane. PLA has a broad range of applications in packaging, textiles, agriculture, and medicine [14], [15], [16]. PLA is a popular alternative to conventional petroleum-based polymers due to the low cost of substrates, lower temperature requirements, and low energy consumption for chemical synthesis [17]. However, because PLA is a brittle and stiff polyester (Tg = 60 °C, elastic modulus of 3000 MPa, maximum tensile strength of 50 MPa, and elongation at break of 15%), it is blended with plasticizers and/or modifiers to confer the mechanical properties of higher flexibility, higher toughness, and/or higher impact resistance [14], [15], [16]. Several compounds have been investigated as modifiers of PLA, such as lactide oligomers [18], citrate esters [19], polyethers [20], tartaric acid [21], starch [22], and various glycerol-based modifiers [23], [24].

The issue presented here is that these polymers in their pure form exhibit poor mechanical performance. However, the solution to this problem has previously been to add plasticizers that may be harmful to the environment and to humans or do not confer sufficiently desirable mechanical qualities to the polymer. Therefore, development and testing are needed to find an ideal plasticizer that will solve these major issues. To successfully plasticize polymers, the plasticizer molecule typically contains a portion that is of comparable polarity to the polymer chains and at the same time a portion that disrupts the intermolecular forces between polymer chains to create free volume and/or to reduce the intermolecular friction between the polymer chains. The first portion is necessary for successful mixing and compatibility of the plasticizer with the polymer, and the second portion is what enhances qualities such as flexibility and ductility as the forces between the polymer chains are disrupted, allowing freer movement of the molecules [25]. In PVC and PLA, the polarity of the polymers is similar to that of esters, which is why ester-based plasticizers are quite commonly used with these polymers [26]. A sufficiently long carbon alkyl chain attached to the central ester structure disrupt the intermolecular forces between the polymer chains, as the alkyl chain is nonpolar. Accordingly, malic acid is a good candidate and was chosen as the raw material for PVC and PLA plasticization, as it possesses the ester groups required for compatibility and additional reactions can be performed to add alkyl chains to the molecule.

Malic acid is a ubiquitous substance present in all living things, and it is most commonly used as a food additive that gives foods a sour taste. Malic acid, known as a top promising renewable chemical, is a scalable and non-toxic material [27], [28]. Malic acid with two carboxylic acids and one hydroxyl group is a good candidate for PVC and PLA plasticization, as the two carboxylic acid groups can be esterified to form esters with carbon chains of a chosen length. Another ester group can be formed through the acetylation of the alcohol group, giving the compound a total of three ester groups. The third ester group may further enhance the compatibility of the plasticizer, as in the case of citrates, triacetin, and others. There have been few published papers on the use of malic acid derivatives as plasticizers [29]. This study investigated the synthesis of plasticizers with different structures from malic acid and characterized their plasticization efficiency as sustainable plasticizers that can be used both in PVC and PLA according to plasticizer structure through DMA and thermal, tensile and leaching property testing.

Section snippets

Materials

dl-Malic acid (>99%), 1-butanol (>99.4%), 1-hexanol (98%), 1-octanol (>99%), 2-ethyl-1-hexanol (99.0%), p-toluene sulfonic acid (p-TSA, 98%), bis(2-ehtylhexyl) phthalate (DEHP, 99.0%), and dioctyl adipate (DOA, 99.0%) were purchased from Sigma-Aldrich (S. Korea), and isotridecanol (99.0%) was purchased from Sasol. Extra pure grade toluene, tetrahydrofuran, and chloroform were purchased from Samchun Chemicals (S. Korea) and used without purification. PVC with a degree of polymerization of 1000

Synthesis of plasticizers

As shown in Scheme 1, malic acid alkyl esters (MAE-Cn, n = 4, 6, 8, i8, i13, and that means the number of carbons in the ester chain) were synthesized via the acid-catalyzed esterification of malic acid with excess alcohol in the presence of p-TSA. During the esterification, the reaction progress can be tracked easily by the measurement of the amount of water formed in the Dean–Stark trap and removed via azeotropic distillation with toluene and by the conversion that were calculated from GC

Conclusions

Acetylated malic acid alkyl esters, as alternative plasticizers for the both plasticization of PVC and PLA polymers derived from renewable natural malic acid, were synthesized by the esterification of malic acid and subsequent acetylation in an isolated yield of 95% or more. The molecular structure of the synthesized plasticizers was confirmed using EIMS, 1H NMR, and FT-IR spectroscopy. A total of 50 phr of the synthesized plasticizers were blended with PVC and PLA, and the compatibility and

Conflict of interests

The authors declare no potential conflict of interests.

Acknowledgements

Y.-W. Kim and J. Shin thankfully acknowledges the financial support for this work provided by the Korea Research Institute of Chemical Technology (KRICT, SI2011-20) and the Technology Development Program to Solve Climate Change of the National Research Foundation (NRF) funded by the Ministry of Science and ICT of the Republic of Korea (NRF-2017M1A2A2049103).

References (39)

  • R.H. Waring et al.

    Mol. Cell. Endocrinol.

    (2005)
  • H.C. Erythropel et al.

    Chemosphere

    (2013)
  • H.C. Erythropel et al.

    Chemosphere

    (2015)
  • H.C. Erythropel et al.

    Polymer

    (2016)
  • A. Greco et al.

    Polym. Degrad. Stabil.

    (2010)
  • T. Liu et al.

    Polym. Test.

    (2017)
  • R.M. Rasal et al.

    Prog. Polym. Sci.

    (2010)
  • S. Farah et al.

    Adv. Drug Deliv. Rev.

    (2016)
  • S. Lee et al.

    Carbohydr. Polym.

    (2015)
  • J. Yu et al.

    Waste Manage.

    (2016)
  • N. Cao et al.

    Food Hydrocol.

    (2009)
  • N. Ljungberg et al.

    Polymer

    (2003)
  • P.H. Daniels

    J. Vinyl Addit. Technol.

    (2009)
  • J.A. Tickner et al.

    Am. J. Ind. Med.

    (2001)
  • O. Fenollar et al.

    J. Mater. Sci.

    (2009)
  • J.B. Fontelles et al.

    OJEU

    (2005)
  • Public Law 110-314 – August 14, Consumer Product Safety Improvement Act of 2008

    (2016)
  • B. Bouchareb et al.

    J. Appl. Polym. Sci.

    (2007)
  • B. Mehta et al.

    J. Appl. Polym. Sci.

    (2014)
  • Cited by (0)

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