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Effects of poly(vinyl chloride) morphological properties on the rheology/aging of plastisols and on the thermal/leaching properties of films formulated using non-conventional plasticizers Sofia Marceneiro, Rafael Alves, Irene Lobo, Isabel Dias, Elizabete Pinho, Ana M.A. Dias, Maria G. Rasteiro, and Herminio C. C. de Sousa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03097 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Industrial & Engineering Chemistry Research

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

Effects of poly(vinyl chloride) morphological properties on the rheology/aging of plastisols and on the thermal/leaching properties of films formulated using nonconventional plasticizers Sofia Marceneiro1,2, Rafael Alves1, Irene Lobo2, Isabel Dias2, Elizabete de Pinho2, Ana M.A. Dias1*, M. Graça Rasteiro1*, Hermínio C. de Sousa1* 1

CIEPQPF, Chemical Engineering Department, FCTUC, University of Coimbra, Rua Sílvio Lima, Pólo II – Pinhal de Marrocos, 3030-790 Coimbra, Portugal 2 TMG Automotive, S. Cosme do Vale, Apartado 14, 4761-912 Vila Nova de Famalicão, Portugal

Abstract The efficiency of poly(vinyl chloride) PVC plasticization depends predominantly on the strength of PVC-plasticizer interactions, which ultimately depends on the intrinsic physicochemical properties of the plasticizer, but also of the polymer. The aim of this work was to study the influence of the morphological properties of different PVC grades (two emulsion (with different K values) and one micro-suspension grades) and of non-conventional greener plasticizers on the rheological/aging properties of PVCbased plastisols and on the thermal and plasticizer leaching properties of films obtained from those plastisols. Commercially available castor oil based CITROFOL® AHII and GRINDSTED® SOFT-N-SAFE were employed as plasticizers and the phosphoniumbased ionic liquid (trihexyl(tetradecyl)phosphonium bistriflamide ([P6,6,6,14][Tf2N]) as a co-plasticizer. Plastisols formulated with the conventional plasticizer di-isodecyl phthalate (DIDP) were also prepared for comparison. Obtained results showed that the highest shear stress and plastisol aging were observed for plastisols formulated using the lower molecular weight emulsion PVC grade (E70-PVC) and the conventional plasticizer DIDP. The E70-PVC systems formulated using DIDP and citrate-based plasticizer presented a pseudo-plastic behavior while all the other systems presented a Newtonian profile. Lower mixing enthalpies were also calculated for PVC/DIDP systems, indicating more favorable interactions for PVC/phthalate systems over nonconventional plasticizers. The incorporation of [P6,6,6,14][Tf2N] as a co-plasticizer significantly decreased the enthalpy of mixing of all the prepared plastisols, showing that its presence in the formulations may favor PVC/plasticizer interactions. Moreover PVC films obtained from plastisols formulated using this ionic liquid presented higher long-term thermal stability due to its negligible vapor pressure that avoids loss during usage. Keywords: emulsion poly(vinyl chloride), micro-suspension poly(vinyl chloride), nonconventional plasticizers, trihexyl(tetradecyl)phosphonium bistriflamide, plastisol rheology, PVC film properties Corresponding authors: E-mail address: [email protected] (A.M.A. Dias); Phone: +351-239-798758; Fax: +351-239-798703 E-mail address: [email protected] (H.C. de Sousa); Phone: +351-239-798749; Fax: +351-239-798703 E-mail address: [email protected] (M.G. Rasteiro); Phone: +351-239-798725; Fax: +351-239-798703

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1. INTRODUCTION

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The automotive industry employs large quantities of synthetic polymers to manufacture

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flexible products based on elastomers or artificial leather.1 The global awareness for

53

material’s sustainability, as well as the high-cost and limited resources of natural leather

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has stimulated the development of novel synthetic composite materials for automotive

55

interiors. These materials are currently used to produce car door trims, steering wheel

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covers, gear boot, seat upholstery, instrument panel and sun visors, by using transfer,

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calendering, extrusion and/or rotary screen coating manufacturing processes.2

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Poly(vinyl chloride) (PVC) is one of the most frequently used synthetic resins to

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produce artificial leather for automotive interiors due to its low cost and tunable

60

capacity through appropriate formulation.1-5 Soft and resilient PVC based artificial

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leather materials are obtained by plasticization of the resin using considerable amounts

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of plasticizers (> 60 % wt/wt) which provide a liquid continuous phase to disperse the

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PVC particles, originating plastisols.5-9 In this process PVC particles are first swollen by

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the plasticizer and the final flexible product is obtained by gelation during heating of the

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swelled particles.2,8,10 PVC resins commonly used to formulate plastisols are obtained

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by emulsion (E-PVC) and micro-suspension (MS-PVC) polymerization. Although MS-

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PVC resins present lower price/performance ratio than E-PVC grades, they can be used

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as secondary resins and are often applied in low-fogging automotive applications.2,3

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There is a large number of commercially available PVC resins used for industrial

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formulations, with different molecular weight, particle size/distribution, porosity,

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morphology and crystallinity.2,5,11 They can be classified by their K-value or viscosity

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number, which is an indicator of their molecular weight and degree of polymerization.

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K-values are determinant to control PVC processing since it affects the plastisol

74

stability during storage and gelation.8 For automotive interior materials, the K-values of


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PVC resins range between 64 < K-value < 80 (corresponding to medium to high

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molecular weights).5-11 The size, structure and distribution of the PVC particles in

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plastisols, as well as their tendency to deagglomerate and progressively swell (by the

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incorporation of the plasticizer), are important variables that influence the rheological

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properties of the plastisols12-15 and consequently define formulation compositions, and

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processing conditions, that will lead to the envisaged properties of the final product.15-19

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The efficient large scale processing of PVC plastisols depends not only on the

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properties of the polymer particles but also on the type and concentration of the

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plasticizers used, which also have a direct influence on the rheological properties of the

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plastisols, and on the thermomechanical properties of the processed polymer.20 One of

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the main requisites of an efficient plasticizer is that it should be compatible with the

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polymer, which occurs when both the plasticizer and the polymer have similar

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intermolecular forces. The strength of the plasticizer-polymer and plasticizer-plasticizer

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interactions depends on the plasticizer properties including its polarity, molecular

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weight, molecular volume and conformation.2-8 Those interactions will affect the

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properties of the plastisol formulation (size, structure, distribution profile and

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agglomeration of the swelled PVC particles) and consequently the properties of the final

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material, as discussed above, and as previously reported in the literature.6-9,21

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Ultimately, they have impact over the long-term thermomechanical stability of the final

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products and on the emission/migration of the plasticizers (and of other additives) from

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those materials.28

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So far, most commonly used plasticizers include both low and high molecular weight

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phthalates (ester side-chain lengths with less or more than 6 carbons (or Mw > 334.45

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g.mol-1), respectively) which are highly compatible with PVC.8,9 Phthalate ester

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plasticizers derived from isomeric C10 alcohols are the most frequently used in



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automotive interiors mainly due to their relatively low volatility when compared to

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other conventionally used low molecular weight phthalates (e.g. bis(2-ethylhexyl)

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phthalate (DEHP), dioctyl phthalate (DOP), diisononyl phthalate (DINP)). However,

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and since they are not covalently bonded to the polymer network, and are volatile (even

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if to a lower extent than others as just mentioned),24,25 plasticizer migration/leaching

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from the polymer matrix is likely to occur during PVC processing and/or usage.26 In

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recent years, and due to more restrictive legislation that limits the incorporation of these

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compounds in plastic-based materials, the automotive industry is exploring the use of

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new greener, non-volatile and non-toxic phthalate-free plasticizers to overcome

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sustainability, performance, environmental and toxicity issues.10,16-18,20-23,27 In this

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context, natural-based plasticizers (such as citrates, epoxidized derivatives of natural

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oils, bio-derived succinate esters, etc) are being proposed as promising friendlier

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alternatives to phthalates in PVC plasticization.10,18,19,28,29 Nevertheless, and to date, the

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full replacement of conventional high-molecular weight phthalate-based plasticizers by

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these bio-derived alternatives has not been accomplished due to their higher

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cost/performance ratios.19,29,30

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Ionic liquids (ILs) belong to a family of compounds that have been also proposed as

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alternative plasticizers/stabilizers for PVC, mainly because of their high thermal

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stability,

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physicochemical properties such as hydrophilicity, viscosity, miscibility, toxicity and

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volatility can be tailored by selecting the proper combination of cations and anions.31-35

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This feature can potentiate PVC-plasticizer compatibility and miscibility, avoiding their

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migration/volatilization from the plasticized polymer network.19,31-33,36 Previously

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reported data showed that phosphonium-based ILs (PhILs) were able to successfully

low

flammability

and

negligible

volatility.19,31-35

Moreover,

their


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plasticize PVC and improve its long-term thermal stability,19,33,36 mainly when

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bis(trifluoromethanesulfonyl)amide (or bistriflamide) is used as counter-anion.19,35,37-39

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The aim of this work was to study the influence of the morphological properties of

127

emulsion and micro-suspension poly(vinyl chloride) (PVC) and of non-conventional

128

greener plasticizers on the rheological/aging properties of PVC-based plastisols and on

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the thermal and plasticizer leaching properties of films obtained from those plastisols.

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For that purpose two emulsion (with different K values) and one micro-suspension PVC

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grades were formulated with two commercially available castor oil based plasticizers

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(CITROFOL® AHII and GRINDSTED® SOFT-N-SAFE) and a phosphonium-based

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ionic liquid (trihexyl(tetradecyl)phosphonium bistriflamide, [P6,6,6,14][Tf2N]) which was

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used as a co-plasticizer. The results were compared with those obtained when using the

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conventional plasticizer di-isodecyl phthalate (DIDP). This study is a continuation of

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our previous work19 which now also addresses the effect of the morphology of PVC

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properties on PVC/plasticizer and/or PVC/(plasticizer+PhIL) interactions and

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consequently on the properties of the plastisols and of the PVC films obtained from

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those plastisols.

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2. EXPERIMENTAL SECTION

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2.1. Materials

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Two emulsion PVC powders, with different K values (VICIR E 1970 P (K value = 70)

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produced by Cires, S.A., Portugal and Vinnolit® P 75 SK (K value = 75) produced by

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Vinnolit GmbH & Co) and one micro-suspension PVC powder (Vinnolit® P 70 HT

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produced by Vinnolit GmbH & Co, K value = 70) were kindly provided by TMG

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Automotive. In this work, the PVC samples obtained by emulsion polymerization were

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identified as E70-PVC and E74-PVC (according to their K values of 70 and 74,



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respectively) and the sample obtained by micro-suspension polymerization was

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identified as MS-PVC (with K value of 70). Di-isodecyl phthalate (Jayflex® DIDP) was

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supplied by ExxonMobil Chemical Europe, Belgium. Acetyltri-2-ethylhexyl citrate

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(CITROFOL® AHII, hereafter abbreviated as ATEC) was supplied by Jungbunzlauer

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Inc, Switzerland. Acetylated monoglycerides of fully hydrogenated castor oil

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(GRINDSTED® SOFT-N-SAFE, hereafter abbreviated as COMGHA) was supplied by

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Danisco

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trihexyl(tetradecyl)phosphonium

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(commercially available as Cyphos IL 109) was supplied by Cytec Industries, Canada,

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with purity > 98 %. The chemical structures and some physicochemical properties of

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the plasticizers used in this work are presented in Table 1. Other chemicals such as n-

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heptane and n-hexane (purities > 99 %) were supplied by José M. Vaz Pereira SA,

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Portugal while acetone (purity > 99 %) and Linseed Oil (purity > 98 %) were supplied

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by Sigma-Aldrich, Portugal. All chemicals were used as received without further

163

purification.

S/A,

Denmark.

The

phosphonium

ionic

bis(trifluoromethylsulfonyl)imide,

liquid

(PhIL),

[P6,6,6,14][Tf2N]

164 165

2.2. Characterization of the PVC powder

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The three different PVC powders studied in the present work were characterized for

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their density, pore size distribution, particle size distribution, surface charge and

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microstructure. The real density of the particles was measured by helium pycnometry

169

(Accupyc 1330 Micromeritics, Micromeritics Instrument, USA). The pore size

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distribution, total pore area and bulk density of the powders were measured by mercury

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porosimetry (Autopore IV Micromeritics, Micromeritics Instrument, USA) after

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purging the samples for 5 minutes at 50 µm Hg to remove adsorbed water and other

173

impurities. The surface area and the pore volume of PVC powders were determined by


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nitrogen adsorption (Micromeritics, model ASAP 2000, 20Q-34001-01) using the

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Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method,

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respectively. The particle size distribution of each powder was measured by Laser

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Diffraction Spectroscopy (LDS) (Mastersizer 2000, Malvern Instruments, UK) and their

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average surface charge (zeta potential) was measured by electrophoretic light scattering

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(Zetasizer Nano ZS, Malvern Instruments, UK) with the solid particles dispersed at 1 %

180

(wt/wt) in milli-Q water. The microstructure of the PVC powders was analyzed by

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Scanning Electron Microscopy (SEM) (Jeol JSM-5310, Japan) on gold-coated samples

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with an operating voltage of 10 kV.

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2.3. Preparation of plastisol samples and corresponding gelled films

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Plastisols were prepared at room temperature by mechanical stirring (Janke & Kunkel

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GmbH & Co, IKA-Werk, RW 24 basic, Germany) of all components (150 parts of total

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plasticizer amount per 100 parts of polymer) at 500 rpm for 15 minutes. The total

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plasticizer amount considers the use of plasticizers (DIDP, ATEC or COMGHA) alone

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or their mixtures with PhIL (at 15 % (wt/wt)). Previous works reported that at this

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composition PhIL is miscible and homogenously dispersed through the PVC matrix33,40.

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The composition of the prepared plastisols is given in Table S1 as Supporting

192

Information. After stirring, the homogeneous mixture was vacuumed for 1 h at 1 mbar

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to eliminate entrapped air and stored in a desiccator containing silica gel at room

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temperature (~ 20 °C) during the entire aging period (up to 14 days) to avoid moisture

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sorption. Each sample was coded as PVC grade/plasticizer type or as PVC

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grade/plasticizer type/PhIL, depending if the plastisol was formulated without or with

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[P6,6,6,14][Tf2N], respectively. PVC films with a thickness of 1 mm were obtained for

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each formulation, at t = 0 and t = 14 days, by spreading the plastisol over a pre-heated



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special support paper (release paper) and gelation in a ventilated oven at 210 ºC for 1

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minute.

201 202

2.4. Characterization of the plastisol formulations

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2.4.1. Rheological measurements

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Rheological measurements were performed at constant temperature (~ 23 ºC) using a

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controlled stress rheometer (Model RS1, Haake, Germany) with a cylindrical sensor

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system Z34 DIN connected to a thermo controller recirculation bath (Haake Phoenix II,

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Germany), working in controlled stress mode and at shear rates ranging between 0.5 and

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30 s-1. Flow measurements were performed for different aging times (0, 1, 2, 7 and 14

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days) using 50 mL of each plastisol and the results are presented as an average of at

210

least three measurements for each sample.

211 212

2.4.2. Particle size distribution measurements

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The particle size distribution (PSD) of the PVC particles in the plastisols was measured

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by Laser Diffraction spectroscopy (LDS) using a Mastersizer 2000 (Malvern

215

Instruments, UK) for the different aging times referred previously. Particle size

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measurements were carried out without ultrasounds using plastisol samples dispersed in

217

n-heptane (at 1 % wt/v), following previously reported procedures.15 The results are

218

presented as an average of at least three measurements for each sample.

219 220

2.5. Characterization of the PVC films (obtained from the plastisol formulations)

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2.5.1. Gelation degree

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The acetone immersion test (AT) was performed on gelled films following the ASTM

223

D2152 standard.41 The gelled PVC samples (2 cm × 3 cm) were completely immersed


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in acetone for 45 minutes and examined for fractures/fragmentation, which occur in

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case of inefficient gelation.

226 227

2.5.2. Thermal stability

228

The short-term thermal stability of the PVC films obtained immediately after preparing

229

the plastisol (t = 0 days) and after aging (t = 14 days) was analyzed using a

230

Thermogravimetric Analyzer (TA Instruments, Q500, USA). Experiments were carried

231

out between 25 ºC up to 600 ºC, at 10 ºC.min-1 under dry nitrogen atmosphere (at 100

232

mL.min-1). The long-term thermal stability of the films was measured by placing

233

rectangular samples (∼ 500 mg, 1 mm × 1 cm2) in an oven at 120 ºC and following their

234

weight loss until constant weight was observed.

235 236

2.5.3. Leaching of the plasticizers from the PVC films in different solvents

237

The amount of plasticizer(s) leached from the PVC films was measured gravimetrically

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by immersing rectangular samples (∼ 250 mg, 1 mm × 1 cm2) in 3 ml of different

239

solvents: milli-Q water, n-hexane and linseed oil. Experiments were carried out at 25 ºC

240

without stirring. Samples were periodically weighed, after removing the excess of

241

solvent from the surface with a filter paper, and until constant weight was achieved. The

242

plasticizer(s) leached amount was calculated using the following equation:

243

Plasticizer leached amount (%) =

244

where W0 is the weight of the initial dried sample and W1 is the final weight of the dried

245

sample after leaching. Measurements were performed in triplicate for each sample.

W1 − W0 ×100 W0

(1)

246 247

2.5.4. Wetting properties of the PVC films



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The wetting properties of the PVC films surfaces were evaluated by static water contact

249

angle measurements performed at room temperature (20-23 ºC) using the sessile-drop

250

(10 µL of milli-Q water) method (OCA 20, Dataphysics Instruments, Germany). The

251

results are presented as an average of three measurements that were performed on

252

different points on each sample. The water sorption capacity of the PVC films was

253

determined gravimetrically at room temperature (20-23 ºC) by monitoring the weight

254

changes of previously dried samples (1 mm × 1 cm2), at fixed time periods, after

255

immersion in milli-Q water. The water sorption capacity (WSC) of the films was

256

calculated using the following equation:

257

WSC (%) =

258

where Wd and Ws are the weights of the dried and water-swelled films (at time t),

259

respectively. These values were calculated neglecting the amount of plasticizer that is

260

leached from the films.

Ws − Wd ×100 Wd

(2)

261 262

3. RESULTS AND DISCUSSION

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3.1. Characterization of the PVC powders

264

The properties measured to characterize the E-PVC and MS-PVC grades studied in this

265

work are summarized in Figures 1, 2 and S1 and in Table 2. The real density of the PVC

266

powders, measured by helium picnometry, is within the range known for PVC industrial

267

grades (0.97-1.45 g.cm-3)2. This property depends on the particle/grain size distribution

268

of the powders, on their compactation degree, on their morphology (shape, size and

269

porosity) and also on the chlorine content of the PVC backbone2 (since chlorine ions

270

hamper inter-chain attraction). The bulk density of the PVC powders, measured by

271

mercury porosimetry, follows an opposite trend to that observed for the real density, and

272

increases in the order E70-PVC < MS-PVC < E74-PVC. 10 ACS Paragon Plus Environment

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The results obtained by mercury porosimetry (Figure 1) and BET isotherm analyses

274

(Figure S1) show that, in general terms, all the powders present a multimodal profile

275

indicating the presence of intra-particle (0.001 to 0.1 µm), inter-particle (0.1 to 10 µm)

276

and inter/intra-grain (10 to 1000 µm) pores. This multimodal profile is more

277

pronounced for E-PVC samples when compared to MS-PVC (which presents an intense

278

peak around 1µm characteristic of inter-particle pores). These results are in agreement

279

with the SEM micrographs (inner images in Figure 1) showing that E-PVC powders

280

(E70-PVC and E74-PVC) present similar morphology and are essentially composed by

281

primary particles clustered into irregular shaped loose and larger grains and/or

282

agglomerates while the MS-PVC sample is composed by regular and non-agglomerated

283

primary particles. The higher inter-particle free volume of the E-PVC samples justifies

284

the higher BET surface area, total pore area and pore volume measured for these

285

samples, mainly for E70-PVC (13.2 m2.g-1, 38.8 m2.g-1 and 0.05 cm3.g-1, respectively)

286

as shown in Table 2. The particle size distribution of the PVC powders (Figure 2 and

287

Table 2) shows that their median diameter (d50) ranges between 2.9 and 20.3 µm,

288

following the sequence MS-PVC < E74-PVC < E70-PVC, whereas the MS-PVC

289

sample presents the highest d90/d10 ratio. Here again, the higher d50 and lower d90/d10

290

ratios measured for the E-PVC samples result from the fact that aggregates are being

291

measured instead of non-aggregated primary particles characteristic of the MS-PVC

292

sample.1,7,9,42

293

Finally, the results obtained from the zeta potential measurements showed that all the

294

PVC samples are stable, with low tendency to agglomerate, considering their negative

295

surface charge which is lower than -34 mV in all cases. The highest stability was

296

observed for the E70-PVC sample (zeta potential ~ -44 mV).

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3.2. Characterization of the plastisols

299

The flow test results obtained for the plastisols prepared with the three different PVC

300

grades (E70-PVC, MS-PVC and E74-PVC) and plasticizers (DIDP, COMGHA and

301

ATEC) are presented in Figure 3, for as prepared and aged plastisols (stored during 14

302

days). The system E70-PVC/ATEC has been previously studied in our group19, but it

303

was repeated in the present work to evaluate the consistency of the experimental

304

procedure applied to prepare and to characterize the plastisols, which was confirmed

305

considering that reproducible values and trends were obtained in both independent

306

works. In general terms, E70-PVC plastisols, formulated using DIDP and ATEC as

307

plasticizers, present a pseudo-plastic behavior while a more Newtonian profile was

308

observed in all the other formulations. Experimental data show that higher initial shear

309

stresses were obtained for E70-PVC plastisols, independently of the plasticizer, being

310

the highest value observed when using the phthalate-based plasticizer (DIDP) (∼ 45 Pa).

311

This higher initial yield stress indicates the presence of strong intermolecular PVC-

312

DIDP interactions15,43. As previously discussed in literature2-14, phthalate-based

313

plasticizers are highly compatible with PVC, which justifies its intensive use as an

314

efficient PVC plasticizer. The system E70-PVC/DIDP also presents the most

315

pronounced aging with maximum shear stresses increasing from 46 to 93 Pa, for as

316

prepared and aged plastisols, respectively. This system is also the only that presents a

317

thixotropic behavior, after aging and for the range of shear stresses applied, with

318

prominent hysteresis. This behavior was also previously observed for E70-PVC

319

additivated with diisononyl phthalate (DINP)19. However, this formulation presented

320

lower shear stresses, mainly for as prepared plastisols, indicating that an increase in the

321

plasticizer alkyl chain seems to promote PVC-plasticizer interactions. All the other


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studied systems are characterized by maximum shear stresses lower than ∼ 40 Pa, with

323

low or no aging effect.

324

An efficient plasticizer should be able to readily establish strong interactions with the

325

polymer. However, several parameters can compromise plasticizer/polymer miscibility

326

(e.g. length/ramification of plasticizer alkyl chains, presence of polar groups, plasticizer

327

molecular weight, etc). This plasticizer/polymer miscibility, which is an indirect

328

measure of the strength of the plasticizer/polymer intermolecular interactions, can be

329

inferred (with reservations19) by comparing the solubility parameters of both

330

components. According to the Gibbs free energy theory enhanced plasticizer solvency

331

towards a given polymer results in negative ∆G values as calculated from equation 1:

332

∆G =∆H-T∆S

(3)

333

where ∆H is the enthalpy of mixing (J.mol-1), ∆S is the entropy of mixing (J.mol-1.K-1)

334

and T is the absolute temperature (K). By considering that the entropy of solution of an

335

essentially amorphous polymer like PVC is positive, it can be assumed that the sign of

336

∆G in Eq. 1 depends on ∆H which can be calculated from the Hildebrand equation: ∆H= φ sφ p (δ s − δ p )

2

337

(4)

338

where φ s , φ p are the volume fraction of the solvent and polymer, respectively, and δ s ,

339

δ p are their respective solubility parameters. As can be seen in Table S1, the enthalpies

340

of mixing calculated for plastisols formulated with ATEC (2.9-3.9 J.mol-1) and

341

COMGHA (3.1-4.5 J.mol-1) are almost twice those calculated for DIDP (1.4-2.0 J.mol-

342

1

343

The miscibility of different phthalate- and natural-based plasticizers towards PVC was

344

recently estimated following the Hansen solubility parameter theory and using the

345

interaction radius concept.44 It was reported that the presence of hydroxyl groups in the

346

plasticizer structure increases the plasticizer/polymer interaction radius, leading to poor

), which in turn are similar to those previously calculated for PVC-DINP systems.19



Page 14 of 42

347

miscibility between the components, due to the high contribution of the hydrogen

348

bonding term. Therefore, the higher enthalpies of mixing calculated for the natural-

349

based plasticizers studied in this work may result from the higher amount of hydroxyl

350

groups present in their structure and when comparing with DIDP. Furthermore, these

351

results are in agreement with the rheological data measured for ATEC and COMGHA-

352

based plastisols, which presented lower shear stress values (between ∼ 8 and 45 Pa for

353

ATEC and between ∼ 16 and 39 Pa for COMGHA) as well as less pronounced pseudo-

354

plastic behavior and aging effect. All together, these results may indicate lower

355

miscibility of the natural-based plasticizers studied in this work towards PVC. This can

356

be due to the higher molecular weight and molecular volume of ATEC and COMGHA

357

when compared to DIDP (Table 1), since both properties may affect the diffusion of

358

these plasticizers through the PVC network. The incorporation of [P6,6,6,14][Tf2N] in

359

plastisol formulations decreases the enthalpies of mixing (by 24 % when mixed with

360

natural-based plasticizers – Table S1) indicating that it may favor PVC/plasticizer

361

interactions.

362

As can be seen in Figure 4, plastisols formulated with E70-PVC also present higher

363

limit viscosities than those formulated with MS-PVC and E74-PVC (more than 3×

364

higher). The effect of aging over the E70-PVC plastisols is also clearly observed in

365

Figure 4a, more pronouncedly when DIDP is used as plasticizer. The observed increase

366

in the viscosity of these plastisols over time results from the chemical interactions that

367

are established between the plasticizer and the E70-PVC particles. Moreover and as

368

previously discussed, E70-PVC particles are organized as looser and less compact

369

aggregates with higher free volume in-between the primary particles and consequently

370

higher inter-particle porosity, which enhance the solvation and the swelling of the PVC

371

particles by the plasticizer. This correlation between the morphological properties of the


Page 15 of 42 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


372

PVC grades (expressed in terms of particle surface area) and the relative viscosity of the

373

plastisols (normalized taking into account the intrinsic viscosity of pure plasticizers) is

374

represented in Figure 5.

375

The particle size distribution (PSD) of the plastisols shown in Figure 6 (numerical data

376

summarized in Table S2), show that the systems E70-PVC/DIDP also present the

377

highest variation in the (d90-d10)/d50 ratio during aging, in agreement with rheological

378

data. According to Figure 6, it is also evident that changes in d50 values are more

379

significant when comparing as prepared and 2 days aged plastisols and that the PSD

380

profiles of the plastisols are almost constant indicating complete solvation/swelling of

381

the PVC particles by the plasticizer(s) after 2 days of storage. The higher dispersion

382

observed for (d90-d10)/d50) values results from the different mechanisms involved in the

383

PVC particle swelling process and which include de-agglomeration of larger aggregates,

384

swelling and diffusion of the plasticizer diffusion through the polymeric matrix. When

385

comparing the PSD results of the neat E-PVC powders (Table 2) with the corresponding

386

as prepared plastisols (Table S2), a general decrease in d50 values was observed. These

387

results indicate that the studied plasticizers are able to efficiently solvate, and

388

consequently disaggregate the agglomerated E-PVC primary particles. The PSD results

389

of MS-PVC based plastisols present the less pronounced aging effect, with almost

390

constant (d90-d10)/d50 and d90/d10 ratios and with d50 values that range between 3.7 and

391

13.8 µm (Figure 6, Table S2). Moreover, d50 values of neat MS-PVC and corresponding

392

as prepared plastisols are similar. These results are justified by the morphology of the

393

micro-suspension PVC sample, composed by less friable and non-agglomerated primary

394

particles as previously discussed, which are less sensitive to the plasticizer solvating

395

effects.



Page 16 of 42

396

The effect of the partial replacement of each plasticizer by the PhIL ([P6,6,6,14][Tf2N]) is

397

shown in Figures 3-6. In general terms, and for both E-PVC and MS-PVC grades, it can

398

be concluded that the incorporation of [P6,6,6,14][Tf2N] induced the formation of

399

Newtonian and less viscous plastisols, mainly for E70-PVC/DIDP and E70-PVC/ATEC

400

systems which can be advantageous for industrial PVC plastisols processing. Moreover,

401

plastisols formulated with [P6,6,6,14][Tf2N] also present less accentuated aging, as

402

confirmed by the low variation in (d90-d10)/d50 and d50 values during the storage period

403

(Table S2 and Figure 6). Similar trends were previously reported for related E70-

404

PVC/DINP/[P6,6,6,14][Tf2N] and E70-PVC/ATEC/[P6,6,6,14][Tf2N] systems.19

405 406

3.3. Gelation and thermal stability of PVC films obtained from different plastisols

407

The efficiency of each plasticizer (or plasticizer + PhIL mixtures) to solvate and swell

408

the PVC particles was indirectly accessed by the acetone immersion method. Although

409

this is a simple method, it is still currently used in the industry as a control procedure

410

for plasticization efficiency. According to this method, the efficient gelation of PVC

411

plastisols leads to the formation of tougher films that maintain their structure and that

412

do not dissolve when in contact with acetone. As can be seen in Table 3, most of the

413

plastisols were efficiently gelled, except those formulated with MS-PVC and E74-PVC,

414

both using ATEC as plasticizer, with and without PhIL obtained from as prepared or

415

aged plastisols, which were fragile and easily fractured. As prepared plastisols

416

formulated with E70-PVC/COMGHA and MS-PVC/DIDP also originated fragile films

417

however the corresponding aged plastisols (after 14 days) originated tough gelled films.

418

Similar behavior was observed for the plastisols formulated with the (plasticizer +

419

PhIL) mixtures indicating that the incorporation of the PhIL does not affect the gelation

420

of the PVC plastisols. Among the PVC grades studied in this work, E70-PVC is the


Page 17 of 42 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


421

only one that presents good gelation for all plasticizers, and/or plasticizer/PhIL

422

mixtures, even if after 14 days of storage in some cases. The same behavior was

423

previously observed for the same E70-PVC grade formulated with different PhILs

424

and/or plasticizers19, indicating that this PVC grade is highly compatible with different

425

plasticizers.

426

The short- and long-term thermal stability analysis of the PVC/plasticizer(s) systems

427

that presented good gelation is summarized in Table 3 and Figure S2. Experimental data

428

reported in Table 3 was also graphically represented in Figure S3 to facilitate

429

comparison among the different samples. The thermal stability parameters of the neat

430

PVC grades and pure additives (plasticizers and [P6,6,6,14][Tf2N]) were also measured

431

and are presented in Table 4. The results obtained for the pure compounds show that the

432

thermal stability increases according to the following sequences for the PVC grades:

433

MS-PVC < E70-PVC < E74-PVC and for the plasticizers: ATEC

Effects of Poly(vinyl chloride) Morphological ... - ACS Publications

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