Elsevier

European Polymer Journal

Volume 142, 5 January 2021, 110154
European Polymer Journal

An efficient cold-resistant strategy: Synthesis and application of green cold-resistant bio-based plasticizer for poly(vinyl chloride)

https://doi.org/10.1016/j.eurpolymj.2020.110154Get rights and content

Highlights

  • There is no report about cold-resistant bio-based plasticizers, and this is the first one.

  • Strong acid cation exchange resin is not only a catalyst, but also a ring opener for the following reaction.

  • The PVC plasticized by these cold-resistant bio-based plasticizers basically does not migrate in various extreme environments.

  • The PVC plasticized by these plasticizers exhibits outstanding cold resistance.

Abstract

Nowadays, the most commonly used cold-resistant plasticizer, bis(2-ethylhexyl)adipate (DOA), not only has the poor migration resistance so that it can only be used as auxiliary plasticizer, but also causes huge pollution to the environment, especially water. Here, using renewable and green oleic acid as raw material, we had synthesized a series of oleate plasticizers. By means of capability tests, we detected that the Ti (initial degradation temperature) of PVC films plasticized with oleate plasticizers was 40 °C higher than that of DOA/PVC and 20 °C higher than that of DOTP/PVC. More importantly, in addition to illustrious migration resistance, they could maintain remarkable flexibility in cold environments as well. The low-temperature efficiency value of these oleate plasticizers exceeded 70 °C, which was much higher than DOTP and DOA. These results demonstrated that they possessed the potential to replace toxic DOP and poorly stable DOA, which provided an effective and green approach to solve the harm of plastic products to human health and the threat to the environment. In addition, these plasticizers broadened the application of PVC in cold environments, which was a boon for the application of PVC industry in the cold-resistant field, and contributed to sustainability of the hackneyed and oft-maligned PVC industry.

Graphical abstract

The cold-resistant bio-based plasticizer is synthesized by oleic acid, and the PVC plasticized by these plasticizers possesses remarkable migration resistance and cold resistance.

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Introduction

Ever since its inception in 1926, poly(vinyl chloride) (PVC) has always benefited modern civilization in a countless aspects [1]. As one of the most important engineering plastics, PVC has been widely used in various fields with its excellent performance and low price [2]. Recently, the market demand around the world for PVC materials has continued to grow, from 38.3 million tons in 2013 to 41.3 million tons in 2016 [3]. So far, as the most paramount additive in flexible PVC products, the global annual output of plasticizers surpasses 6.4 million tons [4]. Despite multifunctionality and inexpensiveness, the safety of flexible PVC products have been accused, that di-2-ethylhexyl phthalate (DOP), the most commonly commercial plasticizer in PVC, is also an endocrine disruptor and carcinogen, and easily migrates out of PVC [5], [6], [7], [8]. Over the past decades, numerous regulations have been carried out in many countries including the European Union and the United States, where the use of DOP have been rigorously restricted in various fields, such as medical devices, food packaging, children's toys and so on [9], [10]. As a consequence, phthalates accounted for 88% of global plasticizer consumption in 2005, and are expected to drop rapidly to 60% in 2022 [11]. In addition, phthalate plasticizers are all derived from petroleum. It is estimated that the crude oil and natural gas in the world are just enough for us to use for 50 years [12]. In this context, there is a perfervid interest in sustainable bio-based green plasticizer alternatives with excellent performance in academia and industry.

The winter temperature in more than half of the countries worldwide is below 0 °C, followed by the increasingly stringent requirements for plastic products, the application prospect of flexible PVC products with cold resistance is really prominent. However, due to the poor migration resistance of cold-resistant plasticizer DOA, it can only be used in PVC as an auxiliary plasticizer. Therefore, it is usually used in conjunction with DOP and DOA, the proportion of DOA does not exceed 20 wt%, to impart cold resistance to PVC. On the other hand, DOA is easier to migrate out from PVC products than DOP. Subsequently, DOA is particularly harmful to water and will cause serious effects on water organisms [13]. Not only that, in the study of oral DOA in mice, all mice developed adenomas and liver cancers, which may be a potential threat to human health as well [14]. At present, there is no relevant report on green sustainable cold-resistant plasticizers. Therefore, the green bio-based plasticizers with excellent migration resistance and cold resistance are provided with broad application prospects.

One of the feasible ways to solve the above problems is to take advantage of natural resources as ingredient to develop renewable non-toxic plasticizers. Because of non-toxic, easily available, biodegradable properties and unique molecular structure, vegetable oil has attracted attention from researchers. Accordingly, there are endless reports on the use of soybean oil [15], [16], [17], tung oil [12], [18], castor oil [19], [20], [21], [22], [23], cardanol [24], [25], [26], waste cooking oil [27], [28], [29] and other vegetable oils as raw materials to prepare bio-based plasticizers. However, due to the cumbersome and complicated reaction process, harsh reaction conditions and certain defects in the performance of the plasticized PVC, these plasticizers do not have the potential for industrialization. Oleic acid derived from vegetable oils has a linear structure similar to aliphatic diesters (ADEs) which are common commercial cold-resistant plasticizers, and the double bond of aliphatic chains could be modified so that it is an ideal raw material for preparing cold-resistant plasticizers. In addition, oleic acid is a renewable biomass feedstock as well, which does not cause detriment to human health and the environment, and corresponds the green and sustainable requirements.

Actually, the cold resistance of a plasticizer depends on the freezing point, the lower the freezing point, the better the cold resistance. Generally speaking, under the condition of the same molecular weight, the freezing point of the linear structure is lower than that of the branched structure. Consequently, for designing cold-resistant plasticizers, the following factors are mainly considered: linear structure, suitable molecular weight and necessary polar groups in the structure. Based on the consideration of these factors, we synthesized a series of oleate plasticizers by the mild reaction conditions and simple reaction pathways. During the reaction process, the strong acid cation exchange resin, a green catalyst, could not only be used as a catalyst for the epoxidation reaction, but also be used in the next step of the ring-opening acetylation reaction. Hereafter, these oleic acid ester plasticizers were compared with commercial DOTP and DOA in terms of the mechanical properties, thermal stability, migration resistance and cold resistance of PVC samples. Accordingly, these researches provide alternatives to phthalates with unprecedented performance in a potential and green orientation to solve the safety conundrum of PVC in the field of cold resistance, as well as the challenge of sustainability antagonizing the PVC industry. Not only that, these cold-resistant plasticizers have potential applications in the fields of lubricants, rubber, polylactic acid (PLA), etc.

Section snippets

Materials

Oleic acid (90%, acid value = 201 mg/g, iodine value = 103 g I2 per 100 g), formic acid (88%), potassium hydroxide (>85%), acetic anhydride (>98.5%), sodium hydroxide (>96%), anhydrous alcohol (99.8%), tetrabutyl titanate (98%), 1,4-cyclohexanedimethanol (99%), hydrogen peroxide (H2O2, 50%), sodium bicarbonate (99%), strong acid cation exchange resin (>95%, granularity = 0.3–1.2 mm), 1,8-octanediol (98%), bis(2-ethylhexyl)adipate (DOA, 99%), dioctyl terephthalate (DOTP, 99%), ethyl acetate

FT-IR spectra of plasticizers

The FT-IR spectra of OA, CDD, E-CDD and A-CDD are shown in Fig. 1. The broad peaks at 2400–3400 cm−1 and 935 cm−1 were characteristic absorption peaks of the hydroxyl group on the carboxyl group. After reaction, these two peaks disappeared and the stretching vibration peak of C=O (1710 cm−1) shifted, indicating that OA and 1,4-cyclohexanedimethanol were esterified completely. Upon epoxidation, the double bond-derived absorption (3008 cm−1) disappeared completely and the epoxy bond (C–O–C, 840 cm

Conclusions

Currently, because of the poor migration resistance, the most commonly commercial cold-resistant plasticizer DOA, can only be used as an auxiliary plasticizer with no more than 20 wt% in PVC. More seriously, the precipitation of DOA in the PVC matrix is likely to cause great pollution to the environment, especially water. In this work, we have demonstrated a potentially general and green approach for synthesizing four types of cold-resistant plasticizers. This approach exploited strong acid

CRediT authorship contribution statement

Dekai Liu: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration. Yirui Shen: Writing - review & editing, Supervision, Software. Pingping Jiang: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Phyu Thin Wai: Data curation, Writing - review & editing. Zheming Zhang: Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge financial support to this work by the fundamental Research Funds for the Central Universities (JUSRP51507), MOE & SAFEA for the 111 Project (No. B13025), international exchange and cooperation projects (BX 2019018), and the International Joint Research Laboratory for Biomass Conversion Technology at Jiangnan University.

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