Trisphenols as Renewable Antioxidants for

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Ferulic acid-based bis/trisphenols as renewable antioxidants for polypropylene and poly(butylene succinate) Armando F Reano, Sandra Domenek, Miguel Pernes, Johnny Beaugrand, and Florent Allais ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01429 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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ACS Sustainable Chemistry & Engineering

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Ferulic acid-based bis/trisphenols as renewable

2

antioxidants for polypropylene and poly(butylene

3

succinate)

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Armando F. Reano,a,b,c Sandra Domenek,*b Miguel Pernes,c Johnny Beaugrand,c Florent

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Allaisa,d

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a

7

Terres, 51110 Pomacle, France

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b

9

Massy, France

AgroParisTech, Chaire Agro-Biotechnologies Industrielles (ABI), CEBB 3 rue des Rouges

UMR Ingénierie Procédés Aliments, AgroParisTech, INRA, Université de Saclay, F-91300

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c

11

d

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78850 Thiverval-Grignon, France

UMR FARE, INRA, Université de Reims Champagne-Ardenne, 51100, Reims, France UMR GMPA, AgroParisTech, INRA, Université de Saclay, Avenue Lucien Brétignières F-

13 14

Corresponding author: Sandra Domenek, 1 rue des Olympiades, F-91300 Massy, France

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Email: [email protected]

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Tel. +33 (0)169935068

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ABSTRACT 1 ACS Paragon Plus Environment


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Green chemistry principles recommend the use of renewable feedstocks and biocatalysis to

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decrease environmental impact of chemicals manufacturing. In this scope, three ferulic acid

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based bisphenols and one trisphenol were synthesized using enzymatic catalysis. Their

4

antioxidant activity at polymer processing and service temperature was investigated in

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polypropylene (PP) and polybutylene succinate (PBS), and benchmarked against the

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commercial antioxidant Irganox 1010®. The analysis of the Oxygen Induction Time (OIT) of

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the polymer degradation at high temperatures showed that Irganox 1010® was more efficient

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to protect PP than the ferulic acid based bis/trisphenols, while, in the case of PBS, the

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biobased antioxidants, and in particular tris-O-dihydroferuloyl glycerol (GTF), were more

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efficient. FT-IR analysis of neat and formulated PP with different antioxidants stored for two

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years at room temperature showed no degradation. Aging studies of PBS at room temperature

12

in dry atmosphere showed that all antioxidants had an equal stabilizing effect on the

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molecular weight averages of the polymer. In conclusion, ferulic acid based antioxidants can

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be efficient primary antioxidants for the thermo-oxidative stabilization of polymers.

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Keywords: antioxidant, oxidative degradation, chemical aging, phenol, polymer, Hildebrand

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solubility parameters, biobased

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INTRODUCTION

2 ACS Paragon Plus Environment

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Polymer industry representing a production of more than 300 Mtonne of plastics per year,1

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research in this field is logically important and developed, especially for commodity polymers

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(e.g., PP, PE, PS, PVC).2-6 Polypropylene (PP) is a commodity polymer widely used in

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applications, such as packaging or automobile parts.1 Poly(butylene succinate) (PBS) is a

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partially biobased and biodegradable polymer,7-8 which could be an alternative to some

6

applications of PP, such as food trays or composites.7, 9 During processing and service life,

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polymers are subjected to oxidative degradation due to light and temperature. Usually a

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mixture of primary and secondary antioxidants is employed to stabilize polymers against

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oxidative degradation. Primary antioxidants act as radical scavengers and are generally

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hindered phenols and secondary amines. Secondary antioxidant act as peroxides decomposers

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and are usually phosphites and thioesters.10

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The thermo-oxidative degradation of PP is an autocatalytic reaction widely described.

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External factors like light or temperature can create alkyl radicals on the polymer backbone,

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which in the presence of oxygen are rapidly converted into peroxides. The bond scission of

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the peroxides produces peroxy radicals, which propagate the degradation reaction. The

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coupling of two radicals leads to termination.11-13 Phenolic antioxidants prevent the initiation

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reaction by scavenging peroxy radicals due to the abstraction of the phenolic hydrogen that

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then leads to a radical of low reactivity.11-13 The oxidative degradation of PBS is scarcely

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investigated, major part of PBS degradation studies concern its biodegradability.7-8 According

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to Rizzarelli and Carroccio14. and Kim et al.15, PBS can undergo three different degradation

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mechanisms depending on the storage conditions. Like all polyesters, PBS is sensible to

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hydrolysis when in contact with liquid water or water vapor. In the absence of water, heat, UV

23

light, or mechanical stress can produce primary radicals on the polymer backbone leading to

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the formation of peroxyl radicals in the presence of oxygen caused by radical-radical coupling



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of an oxygen molecule on a carbon atom centered free radical. This process can be interrupted

2

by primary antioxidants.

3

In the aim of proposing biobased antioxidants, the use of natural hindered phenolic

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compounds exhibiting antioxidant activity has received large academic interest. Examples are

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α-tocopherol,16-17 lignin,18-20 carvacrol,21 thymol,22 or curcumin23-24. The main drawback of

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the use of extracted natural phenols is their generally limited temperature stability, which

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causes their degradation during polymer melt processing. Furthermore, natural phenols have

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often limited solubility in polymers such as PP19-20 and their efficiency lacks behind

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petrochemical additives.25

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A way to adapt natural antioxidants to their use in polymers is their derivatization to increase

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their activity and thermal stability using biocatalysis to be coherent with green chemistry.26-28

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In a previous paper, we reported the enzyme-catalyzed synthesis of polyphenols from ferulic

13

acid,27 a p-hydroxycinnamic acid that can be extracted from agricultural by-products such as

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wheat and rice bran and beetroot pulp.29 Following a lipase-mediated biocatalytic procedure,

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bis/trisphenols could be obtained in high yield (>90%) and having a thermal stability

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compatible with polymer extrusion processes. The analysis of structure - antiradical activity

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relationships in solution showed that bis/trisphenols based on saturated ferulic acid with rather

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flexible linkers between the phenol rings (e.g., 1,4-butanediol, glycerol) had the best

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performance29. In order to investigate their performance in use conditions, we studied their

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capacity to stabilize PP and PBS. Their performance was compared to the one of Irganox

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1010® (tetrakis(methylene-(3,5-di-t-butyl-p-hydroxy-cinnamate))methane), a widely used

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commercial antioxidant for polyolefins.30-32 Formulations of PP and PBS with different

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bis/trisphenols charges were prepared. Their thermo-oxidative stability was recorded at high

24

temperatures using the measurement of the Oxygen Induction Time (OIT) to mimic


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degradation at polymer processing temperatures33-34. Furthermore, the long-term stability of

2

formulated PP and PBS was followed for at maximum two years at room temperature in the

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aim to analyze the performance of the antioxidants at polymer service conditions.

4 5

EXPERIMENTAL SECTION

6

Materials

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Poly(butylene-co-succinate) (PBi 003) was supplied by NaturePlast (France) and used as

8

received. Polypropylene (Solvay Eltex P HV001PF) was kindly donated by Solvay Eltex

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(France). All reagents and Irganox 1010® were purchased from Aldrich Chemical Co.

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(France) and used as received. Solvents were purchased from Thermo Fisher Scientific

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(France). Recombinant Candida antarctica lipase B immobilized on resin (ref. L477-10G,

12

expressed from Aspergillus niger, activity ≥ 5000 propyl laurate units per g) was purchased

13

from Aldrich Chemical Co.

14

Synthesis of biobased bisphenols

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The synthesis of the bisphenols is based on the procedure published by Pion et al.27 and

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shown in scheme 1.

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Synthesis of ethyldihydroferulate (2). Ferulic acid (1) (25 g, 0.13 mol, 1 eq) was dissolved

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in ethanol (900 mL, 250 g L-1) in presence of a few drops of concentrated hydrochloric acid

19

and heated at reflux for 1 day. The reaction mixture was then cooled to room temperature

20

(r.t.), evaporated under vacuum and solubilized in ethyl acetate (900 mL). Then it was washed

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three times with NaHCO3 (500 mL) to remove unreacted ferulic acid. The reaction mixture

22

was subsequently put under argon before adding Pd/C 10% w/w. The mixture was stirred 18 h

23

at r.t. under H2 flow and the reaction was monitored by NMR until complete disappearance of

24

the typical signal of the double bond. The solution was filtered on Celite/silica 5:5 and the



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1

solvent removed under vacuum. The final product (2) slowly crystallized yielding a white

2

powder (24 g, 85 %, mp 4.2. °C).

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Lipase-catalyzed transesterification. One equivalent of di- or triol (isosorbide, 1,3-

4

propanediol, 1,4-butanediol, glycerol) and 2 (1.5 eq. per ester function) were melted and

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magnetically stirred at 75 °C. The lipase CAL-B (10 % w/w relative to the total weight of

6

di/triol and 2) was added. The reaction mixture was kept under reduced pressure for 4 hours to

7

3 days depending on di/triol used (exact conditions can be found in Reano et al.24). The

8

reaction mixture was then dissolved in acetone (150 mL) and filtered to remove CAL-B

9

beads. The solvent was evaporated under vacuum and the crude product was purified by flash

10

chromatography on silica gel eluted with cyclohexane:ethyl acetate 70:30 until affording

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unreacted 2, then 50:50 to elute the final product.

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The following abbreviation code will be applied to name bis- and trisphenols: XYF, where X

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= I (isosorbide), P (1,3-propanediol), B (1,4-butanediol), G (glycerol), Y = D (disubstituted)

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or T (trisubstituted), and F (ferulic acid). For example, BDF corresponds to the bisphenol

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deriving from ferulic acid and 1,4-butanediol.

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Fabrication of formulated polypropylene films

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A mixture of 6 g of PP and additive (mass depending on the percentage w/w of additive

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targeted) was prepared by melt processing. PP is typically compounded with additives at a

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concentration around 2% w/w.32 In the aim of working on comparable concentrations and

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explore concentration effects, the concentration range of formulation was chosen from 2 to

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10% w/w. A co-rotating twin screw micro-extruder (Thermo Haak minilab) was used in cycle

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extrusion (screw diameter 5/14 mm conical; screw length 109.5 mm). The mixture was

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introduced through the feeder and extruded at 180 °C during 1 minute at 60 rpm. Formulated

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strands were cooled at room temperature and pelletized manually. Films (thickness about 100


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µm) were prepared by hot molding at 5 bar at 200 °C for 1 min using an Atlas Manual

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Hydraulic press supplied by Specac.

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Fabrication of formulated polybutylene succinate films

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The casting method was used for the preparation of PBS films. 2 g of PBS were dissolved in

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10 mL of CHCl3, and then bis/trisphenol (quantity depending on the targeted percentage) was

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added to the solution. The quantities of antioxidants used were chosen starting from

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concentrations used in literature (for example 0.2% w/w in PBS)15. In order to analyze the

8

impact of the incorporation yield and to override the impact of additives already present in the

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polymer matrix, additives were used from 0.5 to 2% w/w. The polymer solution was cast on a

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flat surface using an adjustable film applicator to form a wet film of uniform height. The

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solvent was evaporated at r.t. under the fume hood for 24 h and films were subsequently dried

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under vacuum for 72 hours. After solvent evaporation, films with a thickness about 100 µm

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were obtained.

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Calculation of solubility parameters

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Solubility parameters (molar volumes and molar interaction constants of the polymer matrix

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and antioxidant) were calculated with the van Krevelen and Hoftyzer atomic group

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contribution method. Tables can be found in van Krevelen et al.’s work.35 The Hansen

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solubility parameters (HSPs) of each antioxidant were calculated with eqs. 1–3, where δd is

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the dispersion component of the solubility parameter (J1/2 cm-3/2), δp is the polar component of

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the solubility parameter (J1/2 cm-3/2), δh is the hydrogen-bonding component of the solubility

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parameter (J1/2 cm-3/2), Fdi is the dispersion contribution of the molar attraction constant [(J1/2

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cm-3/2)/ mol-1], Fpi is the polar contribution of the molar attraction constant [(J1/2 cm-3/2)/mol-1],

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Ehi is the hydrogen-bonding- energy contribution of the molar attraction constant (J/mol), and

24

V is the molar volume contribution of the chemical group involved (cm3/mol).



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1

δd = ∑

2

δp =

3

δh =

4

Constants used in equation 1-3 are presented in Table 1. From HSPs equations we can obtain

5

Hildebrand solubility parameter (δ or HiSP), with the simplified eq. (4).

6

δ = δ ²d + δ ² p + δ ²h

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The lower the difference of HiSP between two compounds, the higher their solubility.

Fdi (1)

∑V

i

∑F² ∑V

pi

(2)

i

∑E ∑V

hi

(3)

i

(4)

8 9

Analytical methods

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Morphological analysis of polymer films. Confocal laser scanning microscopy was

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performed on a Leica type TCS SP2 with an objective 63.0 x 1.40 OIL. Excitation was at 488

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nm and emission was recorded at 493-650 nm, zoom 1.9, image size 125x125 µm.

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Quantification of antioxidant concentration of polymer films. Compounded PP films were

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dissolved in a minimum (approximately 20 g/L) of warm xylene (150 °C). A known amount

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of ferulic acid was added into the xylene solution and used as internal standard. After few

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minutes at reflux, the solution was added dropwise in cold methanol (200 mL), allowing PP to

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slowly precipitate. The precipitate was then filtered. The methanol supernatant was analyzed

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by HPLC using a Thermo Scientific Ultimate 3000 apparatus, fitted with an autosampler and

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a Diod Array Detector (280 nm). A column βmax Neutral from Thermo Scientific (150 x 4.6

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mm, 5µm) inside a column oven at 40 °C was used. Eluent flow was 1 mL/min. The analysis


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method consisted in an isocratic step for 2 minutes at 5% of ACN, a subsequent ramp to 60 %

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of ACN for 3 min, and second isocratic step for 9 min. The conditions were returned to the

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initial solvent composition within 3 min and held for further 3 min. External calibration with

4

bis/trisphenols was used for quantification. Analyses were carried out in duplicate.

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PBS films were dissolved in CHCl3 (about 3 g/L) and analyzed by Gel Permeation

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Chromatography (GPC). GPC analysis was performed on an Agilent 1260 Infinity system

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equipped with a UV detector (280 nm). Two PLgel MIXED-D (300 x 7.5 mm, 5 µm) columns

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were purchased from Agilent Technology and used at 40 °C in CHCl3 (stabilized with

9

ethanol). The elution method consisted in an isocratic step at 1 mL/min flow of CHCl3 for 20

10

min. The calibration was done with polystyrene standards (PS calibration kit). The

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antioxidants were quantified using the corresponding GPC peak recorded by the UV-visible

12

detector at 280 nm with the help of an external calibration curve. All analyses were carried

13

out in duplicate.

14

Thermal properties of polymer films. Differential Scanning Calorimetry (DSC) analyses

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were recorded by a Q20 (TA Instruments) calibrated with Indium and Zinc standards. TZero

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pans were used. Sample weight was approximately 5 mg. The temperature program consisted

17

in a heat/cool/heat cycle at 10 °C/min under N2 flow (50 ml/min) from -50 to 200 °C. Glass

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transition temperature (Tg), melting temperature (Tm), and crystallinity degree (χ) were

19

determined during the second heating cycle. For the calculation of χ the melting enthalpy of

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207 J/g of PP36 and of 110.3 J/g of PBS37 were used.

21

Thermal degradation of polymer films. Thermo-Gravimetric Analyses (TGA) were

22

recorded by a Q500 (TA Instruments) calibrated in weight and temperature with standard

23

weights and the Curie point of Ni. For the analysis of the thermal stability of additives and

24

polymers, the heating ramp was 10 °C/min from r.t. to 500 °C under N2.



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1

The Oxygen Induction Time (OIT) analyses were carried out on the same TGA apparatus in

2

ramp mode, following the ASTM standard33. The ramp mode was developped for the sake of

3

easier comparison because it allowed using the same experimental method for all samples,

4

while the isothermal method required the change of the degradation temperature due to the

5

different thermal stability of PP and PBS. The heating rate was 5 °C/min from r.t. to 500 °C

6

under oxygen. OIT represents the time between the atmosphere switch (at the start of the

7

experiment) and the onset of mass loss. All analyses were carried out in triplicate.

8

Aging tests of polymer films. PBS sample films were placed in an oven at 40 °C under air at

9

2% Relative Humidity (RH). Aging was monitored by GPC (control of Mn, Mw, and Ð) and

10

DSC (control of Tg, Tm, and χ) in duplicate after 11 and 24 weeks with the help of the

11

analytical protocols described above.

12

PP films were stored in the dark at r.t. under air for two years. At appropriate time intervals

13

samples were monitored by FT-IR. An Agilent Cary630 FTIR spectrometer was used to

14

observe the carbonyl bands of PP oxidation.

15 16

RESULTS AND DISCUSSION

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Solubility of bis/triphenols in PP and PBS

18

The compounding of PP with the different additives was done with the help of a micro-

19

extruder equipment due to the small available quantities of materials, although it is known

20

that its processes do not have the same mixing efficiency as larger scale extruders. During

21

processing difficulties due to the aspect and solubility of ferulic acid-based bis/trisphenols

22

were met. Indeed, PDF, GTF, IDF are viscous oils, leading to sticking in the feeder and

23

condensation on the extrusion equipment. Moreover, formation of a chalky layer was noticed

24

in PP formulated with BDF. These qualitative observations of the samples led to the


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conclusion that the bis/trisphenols were apparently merely soluble in the PP matrix and

2

additives were lost during the process. The actual incorporation concentration of the different

3

additives in the polymer matrices was quantified and the results are shown in the Table 2. The

4

actual incorporated concentrations for all additives lacked far behind the target value. Irganox

5

1010® (crystalline powder, melting point 115 °C) was included at maximum at a third of the

6

target value because of the low incorporation efficiency of the micro-extrusion process. The

7

recovered quantity of Irganox 1010® (Table 2) was low compared to the maximum solubility

8

of this antioxidant in PP, which was measured by Frank and Frenzel38 (10.1 wt% of Irganox

9

1010® in atactic PP at 169 °C). The Table 2 shows furthermore that the inclusion yield

10

seemed not to be influenced by the chemical structure of the biobased antioxidant. For better

11

understanding the morphology of the produced materials, confocal microscopy was carried

12

out, taking advantage of the natural fluorescence of bis/trisphenols. Phase separation of

13

included antioxidants and PP at room temperature is expected, because generally the

14

antioxidant solubility decreases with decreasing temperature.38-39 Irganox 1010® was

15

described as moderately soluble in PP, and a maximum solubility below 1 % w/w was

16

evaluated39. This quantity is lower than the incorporated concentration of Irganox 1010®

17

(Table 2). Therefore, whatever the additive, their incorporation resulted in the formation of

18

large aggregates in PP (Table 3, column PP). Better dispersion is expected upon processing

19

with a more efficient mixing extruder, i.e. with a one hundred grams size one.

20

In the case of PBS, a different incorporation method could be chosen. Indeed, PBS is co-

21

soluble with the bis/trisphenols in common solvents, such as chloroform. The casting process

22

can thus be easily carried out, which allows for intensive mixing of both compounds in the

23

solution. The inclusion of all additives in the PBS matrix is almost quantitative (Table 2, PBS

24

column). Morphological analysis of the cast films showed homogeneous distribution of the



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1

additives in PBS. The Table 3 (column PBS) shows that only a few larges aggregates can be

2

observed. Interestingly, the formulations of PBS with Irganox® 1010 showed a fluorescent

3

picture very comparable to formulated PP. Some large aggregates were clearly observed

4

whatever its target concentration in PBS. We conclude that phase separation of Irganox 1010®

5

in PBS occurred most probably because the solubility limit at room temperature was

6

exceeded.

7

The solubility of additives in the polymer is a very important parameter for successful

8

homogenous distribution in the matrix. The semi-empiric Hansen and Hildebrand solubility

9

parameters are widely used in the field of polymer for assessing solubility of additives. They

10

were calculated for all additives in the aim quantifying, in a first approach, differences of

11

solubility among the bis/trisphenols included in one polymer matrix and solubility differences

12

of given molecule in PP and PBS. The Table 4 shows a significant difference of δ between

13

PP35 and PBS,40 which is most likely due to the differences in the contribution of the polarity

14

and hydrogen bonding contributions. Although δp and δh values of PBS could not be found in

15

literature, polyesters with similar structure show the importance of these contributions. To

16

give some examples, the solubility parameters of poly(hydroxybutyrate-co-valerate)

17

(PHBV)41 and polylactide (PLA)42 are displayed in the Table 4. The ferulic acid-based

18

bis/trisphenols had higher contributions in the dispersive, polar and hydrogen bonding factors

19

leading to higher δ compared to Irganox 1010®. Among the bis/trisphenols, IDF had the

20

highest polarity linked to the polarity of the isosorbide group, followed by GTF, which

21

includes a third phenol group. Solubility of one component in another is described by the

22

difference of the solubility parameters. Clearly, as observed in Table 4 (column δ), the

23

biobased additives are more soluble in PBS than in PP. This is coherent with the higher


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1

inclusion percentage (Table 2) and the confocal microscopy pictures (Table 3), which show

2

more homogenous distribution of the additives in PBS.

3 4

Thermal properties of PP and PBS compounds

5

The impact of bis/trisphenols on the thermal properties was analyzed and the results are given

6

in the Table 5. Biobased bis/trisphenols did not influence the thermal properties of both

7

polymers. Moreover, the absence of a change in the Tg verified the complete evaporation of

8

solvent from the cast PBS samples, as no plasticizing of the samples was observed. The fact

9

that all samples of one polymer show similar thermal properties and degree of crystallization

10

rules out matrix effects induced by different degrees of crystallinity in the analysis of the

11

thermo-oxidative stability.

12 13

Thermal degradation of PBS and PP films under nitrogen

14

The thermal stability of the bis/trisphenols under nitrogen was described by Reano et al.29 The

15

temperature at maximum mass loss rate (Tmax) was 350, 340, 339, and 344 °C for IDF, PDF,

16

BDF, and GTF, respectively. These values are comparable to that of Irganox 1010® (355 °C).

17

The thermal stability of the different polymer samples under nitrogen was measured by TGA

18

(Figure 1). The numerical values are given in the Table 6 as a function of the molar

19

concentration in phenol groups (µmolOH/g) in order to take into account the different number

20

of phenols present in the different molecules (2, 3 or 4).

21

The PP matrix (Figure 1a) shows thermal stability up to almost 430 °C (Td5%), which is in

22

agreement with the literature.22, 43-44 The temperature of the maximum degradation rate (Tmax)

23

was not affected by the incorporation of the additives. All antioxidants seemed to slightly

24

decrease the onset of the degradation of PP at all loads, though. The Td5% was significantly



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

lower than the blank, which can be attributed the thermal degradation of the antioxidant itself.

2

The quantity of included additive (Table 6) had no effect on the degradation temperatures

3

under inert atmosphere. Moreover, the onset of the thermal degradation was higher than the

4

micro-extrusion temperature (180 °C), which led us to the conclusion that additive volatility

5

cannot account for the poor inclusion yield.

6

Thermal degradation data of formulated PBS are also given in Table 6 (column PBS) and

7

figure 1b. PBS showed a thermal stability up to 310 °C (Td5%). This result is in agreement

8

with the literature.45 Its temperature of maximum degradation rate was not affected by the

9

presence of the antioxidants. Irganox 1010® seemed to have no influence on the thermal

10

stability of PBS. Biobased bis/trisphenols caused an increase in the thermal stability, namely

11

in the Td5% value. PBS degradation under N2 atmosphere proceeds preferentially by β-

12

hydrogen-transfer bond scission leading to the production of succinic anhydride14, which

13

should not be impacted by the bis/trisphenols. This mechanism together with the observations

14

that i) the PBS degradation onsets before the degradation of the bis/trisphenols, and ii) the

15

Td5% value was correlated with the thermal stability of the used molecule led to the

16

conclusion that the overall thermal stability increase of the material could be attributed to the

17

stability of the additives themselves.

18

Thermal degradation of PP and PBS films under oxygen

19

The degradation of the polymer samples under oxygen was analyzed. The Table 6 shows the

20

evolution of OIT of PP and PBS films as a function of the number of moles of phenolic

21

groups added in one gram of polymer (column µmolOH/g).

22

In the case of PP, Irganox1010® showed clearly the highest antioxidant efficiency, i.e. the

23

highest OIT at the lowest concentration. Interestingly the OIT at the higher Irganox 1010®

24

concentrations decreased, which evidences the necessity of optimization of additive


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Page 15 of 37

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1

concentrations in polymer matrices. All biobased additives had a stabilizing effect on PP, and

2

BDF and GTF seemed to be the most efficient. However they reached lower maximum OIT

3

and only at higher concentration compared to Irganox 1010®.

4

On the contrary, the biobased antioxidants were more efficient in PBS compared to Irganox

5

1010®. The measured OIT at any concentration was higher than the ones at similar

6

concentration of Irganox®. A small concentration effect was detected. The OIT of the lowest

7

concentration of phenol groups in the material (approximately 20 µmolOH/g) was always

8

significantly lower compared to the OIT obtained at concentrations higher than 40 µmolOH/g.

9

No significant differences can be observed between the higher concentration levels

10

(approximately 40 and 80 µmolOH/g). Optimization of the concentration of each antioxidant

11

will be clearly necessary allowing for efficient protection of the total volume of the polymer

12

at minimum quantity.

13

GTF was the most efficient antioxidant structure yielding at all concentrations the highest

14

OIT. The obtained ranking of antioxidant activity (GTF > BDF = PDF > IDF) corresponded

15

to the antiradical activity in our previous work29, in which structure-antiradical activity

16

relationships of a library of bis/trisphenols was analyzed using a test in solution (DPPH). In

17

this work we proposed that the regeneration of the oxidized phenols proceeds through an

18

intra-molecular coupling, which explained the antiradical activity of biobased bis/trisphenols.

19

Flexible linkers between the phenol rings proved also to promote antiradical activity.

20

Therefore, we assume that the star-like structure of GTF was responsible for quenching more

21

free oxygen radicals compared to linear structures at equal amount of phenol groups. In the

22

polymer matrix, the mobility of the bisphenols is strongly limited compared to a solution,

23

which might decrease the regeneration ability. However, in areas of high additive



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

concentration, such as the aggregates shown in Table 3, this coupling might still be possible

2

and promoted by the branched structure of GTF.

3 4

Stabilization of polymer films by biobased antioxidants at service conditions

5

The aging of the formulated polymers at service conditions was studied with the help of aging

6

tests. PP was aged at room temperature and humidity for 2 years in the dark. The apparition of

7

the carbonyl signals, which are characteristic for oxidative degradation, was monitored by FT-

8

IR analysis. Neither virgin nor compounded polymer samples showed the apparition of

9

carbonyl groups over the detection threshold, as illustrated in Figure 2. Based on this

10

observation, no difference in the activity could be evidenced among the different antioxidants.

11

It can at least be concluded that the biobased antioxidants perform, within the limits of the

12

present experiment, equal to Irganox 1010®.

13

PBS was subjected to accelerated aging at 40 °C for under dry air to avoid hydrolysis and the

14

polymer degradation was monitored by GPC and DSC analyses. The DSC analyses (Table 7)

15

showed that the Tg of PBS was not impacted significantly during the aging experiment. The

16

crystallinity degree slightly increased during storage, which might be attributed to slow cold

17

crystallization of PBS stored at a temperature beyond its glass transition.

18

The Table 7 shows furthermore the evolution of chain length when stored at 40 °C at 2 % RH

19

and statistical groups of the samples using the Student test realized for each line. Most of the

20

ഥ௪ and ‫ܯ‬ ഥ௡ after 11 weeks of aging. All samples samples showed no significant change of ‫ܯ‬

21

presenting significant differences in molecular weight averages compared to the initial sample

22

corresponded to the lowest additive concentration. The not formulated PBS film showed no

23

significant differences compared to initial mass averages (line Φ in Table 7), but had largely

24

increased standard deviation. This can be attributed to increased structural heterogeneity.


Page 16 of 37

Page 17 of 37

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1

ഥ௪ or ‫ܯ‬ ഥ௡ or both. The relative After 24 weeks, all samples showed significant decrease in ‫ܯ‬

2

decrease was smaller than the one of the not formulated sample. Irganox 1010® and biobased

3

bis/trisphenols thus decreased the degradation of the macromolecules in the PBS films. The

4

concentration effect was similar to the one observed in the OIT experiment, namely the

5

protecting effect of the antioxidants was better at 40 µmolOH/g compared to the lower

6

concentrations, and at higher concentrations no significant differences were found. The

7

analysis of the GPC chromatograms showed that the aged blank sample presented a tailing of

8

the GPC peak towards low molecular masses, compatible with the higher dispersity value.

9

The stabilized aged samples showed a shift of the peak towards longer retention times without

10

ഥ௪ of neat PBS ഥ௡ and ‫ܯ‬ much broadening. The numerical analysis of the changes in ‫ܯ‬

11

ഥ௪ . ഥ௡ decreased to a higher extent compared to ‫ܯ‬ compared to the initial values showed that ‫ܯ‬

12

ഥ௡ of the formulated samples decreased equally or globally less than On the contrary, the ‫ܯ‬

13

ഥ௪ . This indicates that the degradation in the PBS films proceeded by random ‫ܯ‬

14

macromolecular chain scission involving backbone rupture and that the antioxidants acted on

15

that process. Irganox 1010® and GTF seemed to have the highest stabilization effect.

16

In the aim of getting more insight in the protection mechanism the GPC-UV signals of the

17

antioxidants themselves were investigated. The Figure 3 shows the peak of Irganox 1010® and

18

BDF as an example. No changes are observed in the case of Irganox 1010® (Figure 3a). In the

19

case of BDF and the other ferulic acid-based bis/trisphenols a novel peak corresponding to a

20

population of higher molecular weight than the initial ferulic acid-based bis/trisphenol can be

21

observed (Figure 3b). This could be assigned to a possible dimerization of ferulic acid-based

22

bis/trisphenols as proposed by Reano et al.29 Other biobased antioxidants such as eugenol,

23

showed similar behavior in liquid media46. To account for this finding, we propose the

24

reaction mechanism displayed in the Scheme 2. Radical-radical biaryl coupling can be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

occurring with a di-oxidized ferulic acid-based bis/trisphenol. We assume that the concurrent

2

reaction involving the radical centered on the 1-position, which undergoes radical-radical

3

coupling with another radical on 1-position or a radical on 5-position is thermodymanically

4

disfavored because such a coupling cannot regenerate the aromaticity of the benzene ring

5

carrying the radical on 1-position (as there is no H that can be transferred). The formation of

6

the 5-O-4 dimers might be possible, but has not been observed in our previous work28.

7

Depending on the rigidity of the internal diol, this coupling could be intra-molecular or inter-

8

molecular. Irganox 1010® cannot react by this mechanism, because its carbons in ortho-

9

position of the phenol group are substituted.

10 11

CONCLUSION

12

The efficiency of four novel bis/trisphenols derived from ferulic acid to stabilize PP and PBS

13

against oxidative degradation was studied. The thermo-oxidative degradation at high

14

temperatures, which are relevant in extrusion processes, was investigated with the help of OIT

15

analysis. The oxidative degradation of the macromolecules at polymer service conditions was

16

studied with the help of standard aging tests at room temperature. The performance of the

17

biobased molecules was compared to Irganox 1010®, a widely used petrochemical

18

antioxidant. The performance of the different antioxidants ranked as follows: OIT values of

19

PP showed lower activity of bis/trisphenols compared to Irganox 1010® (Irganox

20

1010®>GTF>BDF=PDF>IDF). On the contrary, bis/trisphenols exhibited an activity slightly

21

higher than Irganox 1010® in PBS (GTF=PDF>BDF>IDF>Irganox 1010®). Room

22

temperature aging tests of PP during 2 years did not allow for discrimination between samples

23

as no degradation was observed. Accelerated aging tests of PBS during 6 months showed that

24

all antioxidants had a positive impact on the stability of molecular mass averages, but with no


Page 18 of 37

Page 19 of 37

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1

significant differences whatever their structure. The reaction mechanism of the ferulic acid

2

based antioxidants apparently includes radical scavenging and intermolecular dimerization,

3

favored by more flexible molecular structures. In conclusion, results showed that ferulic acid

4

based bis\trisphenols are an efficient alternative to petrochemical antioxidants and can be

5

successfully applied to stabilize oxidation sensitive polymers.

6 7

ACKNOWLEDGEMENTS

8

The authors acknowledge the financial support of the Region Champagne-Ardenne, the

9

Conseil Général de la Marne, and Reims Métropole. The authors thank Alain Lemaitre from

10

INRA for his technical assistance.

11 12 13

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30.

4



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1

Page 26 of 37

Tables and Figures

2 3

Table 1. Molar constant for the calculation of Hansen solubility parameters. Fd

V

Structural 1/2

(J

-3/2

Fp²

Eh

(J cm-3 mol-1)

(J/mol)

(cm

cm

group

Irganox 3

mol-1)

BDF PDF IDF GTF 1010®

mol-1)

-CH3

420

0

0

33.5

24

2

2

2

3

-CH2-

270

0

0

16.1

12

8

7

6

8

-CH-

80

0

0

-1

0

0

0

4

1

>C

Trisphenols as Renewable Antioxidants for

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