Potential of Organosolv Lignin Based Materials in Pressure Sensitive

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Potential of Organosolv Lignin Based Materials in Pressure Sensitive Adhesive Applications Gopakumar Sivasankarapillai, Elahe Eslami, and Marie-Pierre G. Laborie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b01670 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Sustainable Chemistry & Engineering

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Potential of Organosolv Lignin Based Materials in Pressure Sensitive Adhesive

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Applications

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Gopakumar Sivasankarapillai1,2, Elahe Eslami1, Marie-Pierre Laborie1,2

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1

Freiburg Material Research Center, Stefan-Meier-Strasse 21, D-79104, Albert-Ludwig- University

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of Freiburg, Freiburg, Germany

6

2

7

6, D-79085, Albert-Ludwig- University of Freiburg, Freiburg, Germany

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E-mail: [email protected]

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ABSTRACT

Chair of Forest Biomaterials, Faculty of Environment and Natural Resources, Werthmannstrasse

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Blending lignin with other polymers has long been utilized as a potent approach to explore novel

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applications for lignin. Yet, combinations of organosolv lignin (OSL) with superplasticizer have been

12

hardly explored as a potential to design novel material properties. In this study, a commercial

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superplasticizer, a polycarboxylate polyether (PCE) is blended with a beech-based organosolv lignin

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and their miscibility and adhesive properties are studied over the entire compositional range. All

15

blend compositions exhibit a single glass transition temperature (Tg) that follows the Kwei model of

16

miscible polymer blends, while a thermal stabilization effect is evidenced in blends containing up to

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50% lignin. Fourier transform infrared spectroscopy (FTIR) investigations reveal specific

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interactions between functional groups in PCE and lignin OH groups as molecular basis for

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miscibility. The rheological properties, tackiness and adhesive peel strength suggest that these

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blends have potential as base formulation for pressure sensitive adhesive applications, albeit

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needing some fine-tuning.

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KEYWORDS: Organosolv Lignin, Polycarboxylate Polyether, Pressure Sensitive Adhesive, Blend

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Miscibility, Thermal & Rheological Properties

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INTRODUCTION 1 ACS Paragon Plus Environment

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As a by-product of the pulp and paper industry and the second most abundant biopolymer on

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earth, lignin has long been considered a valuable biomacromolecule for material development. In

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particular, lignin-based polymer blends have long garnered attention from scientists as testified by the

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number and the timeframe of reviews on this topic. 1–7

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Recognizing lignin potential in the late 80’s, Glasser and his group pioneered blending approaches

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of lignin and its derivatives with a wide range of polymers including biodegradable polymers such

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as cellulose acetate butyrate (CAB), hydroxypropyl cellulose (HPC), Polyhydroxybutyrate (PHB),

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Polycaprolactone (PCL), lignin esters, starch, and petroleum-based polymers such as PVC. 8 With

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the application as carbon fibers in view, Kadla and coworkers later studied the miscibility between

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hardwood Kraft lignin and synthetic polymers 9-12 particularly investigating blend properties and

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intermolecular interactions in hardwood kraft lignin and synthetic polymers such poly(ethylene

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oxide) (PEO), poly(vinyl alcohol) (PVA), poly(ethylene terephthalate) (PET) and poly(propylene)

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(PP). Also prevalent in the literature since the late 80s is the utilization of lignin in adhesive

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applications13 and particularly in structural adhesives. Here lignin can be used as phenol substitute

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in wood adhesives while lowering their carbon footprint and cost.14

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More recently biomass-derived polymers including industrial lignin have also been considered for

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pressure sensitive adhesive (PSA) applications.15-16 Pressure sensitive adhesive (PSAs) are soft

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viscoelastic solids that adhere onto the substrate on application of a slight pressure in a very short

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time18. PSA require three important properties: all PSAs require some degree of stickiness or tack;

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removable PSA require a controlled peel force and adhesive failure for debonding while permanent

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PSAs must exhibit minimal creep.17 To meet these requirements, PSA are formulated with multiple

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components, with at least one component operating well above its Tg. For example, water-based

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PSAs consist of dispersions of a high Tg component and a low Tg component, whose composition

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determines the critical properties of the PSA i.e. the tack, peel strength and shear strength.

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In this work we hypothesize that blends of high Tg lignin with a very low Tg polymer might well

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perform as a PSA, opening new applications for lignin. We choose to investigate the suitability of

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aqueous blends of organosolv lignin, a high Tg macromonomer, with a superplastizer as the low

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Tg component, for PSA application. We select organosolv lignin because in contrast to industrial

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lignin, organosolv lignin exhibits higher purity (no sulfur) and homogeneity, lower molecular weight

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and glass transition temperature.18, 19 We further select as the low Tg component a

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polycarboxylate polyether (PCE), a commercial superplasticizer commonly used for admixtures in

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concrete. Besides its very low Tg, its functional groups suggest that it is able to interact with lignin

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(Scheme 1). In concrete, lignin has for example been repeatedly used for the modification of the

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aliphatic superplasticizer, 20-22 Intermolecular interactions and miscibility among the components

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are in fact critical to the PSA performance as it influences its viscoelastic properties. 23-25

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In this paper we first report on the miscibility and intermolecular interactions in the organosolv

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lignin/PCE pair. We then examine the rheological behavior, tack, shear and peel strength of blends

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of varying composition, to assess their suitability for PSA applications.

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EXPERIMENTAL SECTION

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Raw Materials:

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Polycarboxylate Polyether (PCE) in aqueous solution/sodium salt (ETHACRYL.HF) was received

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from Arkema (France) and used without further modification or purification (Scheme 1).

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Commercial crepe paper latex-impregnated tape, (Würth), was used as the substrate for testing

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adhesive properties. Beech Organosolv lignin (OSL) was received from Fraunhofer CBP,

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Germany.

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Raw Materials Characterization:

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The molecular weight of the PCE was analyzed on a GPC SECurity 1200 system (PSS-Polymer

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Standards Service, USA) (Table 1). Polystyrene narrow standards were used for establishing the 3 ACS Paragon Plus Environment


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calibration curve. For lignin, the total hydroxyl group content was determined with 31P NMR

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(Table1) following published procedure.26

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76 77

Scheme 1: Hypothetical partial structure of beech wood organosolv lignin 27 (above) and commercial

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PCE (bottom).

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Table 1. Molecular weight, glass transition temperature and 31PNMR data of raw materials 31P

NMR (mmol/g OSL

Raw

Tg Mw(g/mol)

Polydispersity

Aliphatic

pH

Aromatic (oC)

material

OSL(Beech)

Solid

4300*

3.9*

46535

1.69

OH

OH

2.43

2.39

content

114

-

-60

3.7*

-

PCE(in _

_

40*

aq.solution) 80

*Data received from the manufacturer

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In brief, an exact amount of 25–30 mg of the OSL samples was diluted in 400 µL CDCl3/pyridine

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(1:1.6) ,150 µL of a solution of chromium(III) acetylacetonate (3.6 mg/mL) as relaxation agent and 4 ACS Paragon Plus Environment

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cyclohexanol (4.0 mg/mL) as an internal standard in CDCl3/pyridine (1:1.6) were added and the

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solution was stirred for 5 min. 2-Chloro-4,4,5,5-tetramethyl-1,2,3-dioxaphospholane (TMDP, 70 µL)

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was added and the solution was transferred into an NMR tube for analyzing in a Bruker 300 MHz

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spectrometer with 128 scans and a delay time of 15 sec. The chemical shifts relative to the

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reaction product of TMDP with water at 132.2 ppm are assigned to the functional groups at δ =

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150.0–145.5 (aliphatic-OH), 145.5–144.7 (cyclohexanol), 144.7–136.6 (phenolic-OH), 136.6–133.6

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(carboxylic acids) ppm.26 OSL and the PCE display very distinct Tg and the PCE appears as a

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rather low molecular weight polymer.

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Blends Preparation

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OSL and the PCE aqueous solution were mixed thoroughly in a beaker at 100 0C for 15 min with

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constant stirring on a hot plate in PCE:OSL w/w blend compositions of 100:0, 80:20, 70:30, 60:40,

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50:50, 40:60, 20:80, 0:100 (5g solid content batch). Then all the samples were dried in a vacuum

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oven for 1h at 1500C. The moisture content of the dried blends was assessed with

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thermogravimetric analysis and did not exceed 3%. The dried blends were subjected to various

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analyses to determine miscibility, nature of molecular interactions, rheological behavior and

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adhesive performance of PSA.

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Thermal and Spectroscopic Characterization of PCE:OSL Blends

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Differential scanning calorimetry (DSC): about 8-10 mg of sample was placed in a pin-holed and

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sealed 30 μl Aluminum pan (3 replicates) and analyzed on a DSC8500 Pyris 1 (Perkin Elmer,

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USA) at 100C/min under nitrogen atmosphere. The PCE sample was heated from 30 to 100°C at

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20°C/min in the first scan, then the sample was cooled to -90 at 20°C/min to minimize the enthalpy

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relaxation in the second heating scan. After two minutes of holding time, the sample was reheated

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to 150°C at 10°C /min in order to determine the glass transition temperature (Tg). The starting (-90



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to -30°C) and the end temperatures (100°C to 250°C) were changed depending on the blend

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composition.

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Thermogravimetric analysis (TGA): about 5 mg of samples placed in thermogravimetric analyzer,

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Pyris 1 (Perkin Elmer, USA) and heated to 9000C at 10 0C /min under nitrogen stream. The TGA

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was used to evaluate the water content and the thermal stability of the PCE:OSL blends by virtue

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of the thermal decomposition temperature, and the temperatures at 5% and 10% mass loss,

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respectively.

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Fourier transform infrared spectroscopy (FTIR): spectra were collected on a FTIR spectrometer 65

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(Perkin Elmer, USA) operating in the attenuated total reflection (ATR) mode (ZnSe crystal). Each

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spectrum was taken as an average of 32 scans at a resolution of 4 cm-1. For the 30wt% OSL

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content blend, the theoretical spectrum was mathematically constructed according to the rule of

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mixture and compared with the experimental spectrum in order to delineate specific interactions in

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the blends.

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Rheological Properties of PCE:OSL Blends

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Rheological measurements were carried out on a HAAKE Mars-II rheometer with Modular

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Advanced Rheometer System. The oscillatory experiments of the samples with 20 mm diameter

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and 1.0-0.5 mm thickness were performed on the parallel-plate fixture. The angular frequency

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ranged from 0.01 to 100 Hz in the linear viscoelastic zone (5%) at Tg+700C temperatures.

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Adhesive Performance of PCE:OSL Blends

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The blends were evaluated for performance in relation to a possible use as a PSA. Namely, tack,

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peel adhesion and shear strength were determined for 0-40wt% OSL compositions.

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The tack values were determined with the rolling ball tack method as the distance traveled by the

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ball on an adhesive layer before stopping in accordance with ASTM D 3121 standard.28 In 6 ACS Paragon Plus Environment

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general, rolling ball travel distances are divided to three zones: i) 0 to 100 mm is good tack zone; ii)

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100 to 200 mm is medium tack zone and iii) 200 to 300 mm is low tack zone.29 The 180o peel test

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was performed in accordance with the Pressure Sensitive Tape Council standard for 180o peel test

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(PSTC 101, test A).30 The universal tensile tester, Inspect mini Hegewald & Perschke was used for

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running 180o peel test. A 24 ± 0.5mm wide and 300mm long strip of the PCE:OSL coated film was

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prepared. Tests were performed on 120 micrometer (μm) thick films with a 20 μm coating of

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adhesive. The thickness of adhesive was measured by “Mitutoyo” Thickness Gages (Accuracy: ± 2

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μm). After preparation of the films, they were adhered on the test panel. The test panel is a 50 by

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125mm and 1.1mm thick stainless-steel panel in accordance with specification of ASTM A 666.31

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The stainless-steel panel was clamped into the bottom jaw of the adhesion tester, the tape doubled

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back at an angle of 180o and clamped into the upper jaw. Tensile testing machine was operated at

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5.0±0.2 mm/s, at room temperature (23±-2OC and 50±5% relative humidity). The motion of

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movable jaw causes the mechanically peeling of the film. The average force per unit width

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obtained during peeling is considered as the adhesion value. Five different specimens were tested

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for each composition.

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The lap joint shear strength test was carried using tensile testing machine (Inspeck mini Hegwald

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& Perschke) to determine the shear strength of PCE:OSL blends coated on glass strips according

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to ASTM D1002. Specimen size for lap shear specimen was 25.4 mm wide, 76 mm long with an

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overlap of 12.7 mm. The blends were coated in thin layers (25 g/ m2 and 20±5 μm) and

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conditioned at around 23OC with 50± 5% relative humidity before the test. The testing distance was

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50 mm. The loading applied was 80 to 100 kg/ cm2 of the shear area per min, and pulled at

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1.3mm/min. until rupture occurs.

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Finally, Shear resistance was investigated by examining the adhesive strength of the PCE:OSL

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blends under a constant load according to PSTC-107 testing procedure.32 A 20mg of PCE:OSL

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blend was coated on a PET silicon tape and the coated strip was placed on a horizontally mounted 7 ACS Paragon Plus Environment


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stainless steel plate with a contact area of 25mm x 25mm. A 100g weight was attached to the free

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end of the PET strip with a metal clamp. The experiment was started by recording the time taken to

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pull the adhered tape away from the stainless steel plate. The experiment was repeated with 0-40

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wt% of OSL samples.

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RESULTS AND DISCUSSION

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Thermal Properties and Blend Miscibility

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Miscible blends on a 10-20 nm ranges display one single glass transition temperature (Tg),

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whereas immiscible blends exhibit the two glass transition temperatures of the parent polymers.33

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Figure 1 shows the DSC curves from the second run of the PCE:OSL blends prepared by hot

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mixing. An endothermic step is observed for all the samples and provides a signature of their Tg

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(Figure 1). For vacuum-dried OSL and PCE, the Tg appears at 1140C and -610C, respectively.

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The PCE:OSL blends also exhibit a single Tg, which varies between the values for the parent

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polymers, depending on the blend composition, suggesting complete miscibility.34 Various models

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have been proposed for predicting the composition dependence of Tg in miscible polymer blends,

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shedding further light on the possible nature of the interactions. In particular, the Fox equation,

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Gordon–Taylor (GT) and Kwei equations are commonly used for miscible polymer blends.35-37

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(1)

Fox,

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(2)

GT, (Tg) =

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(3)

𝐾𝑤𝑒𝑖, ( 𝑇𝑔 ) =

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Here, Tg is the glass transition temperature of the blend, Tg1, Tg2, w1, w2 are the glass transition

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temperatures and weight fractions of the parent polymers, respectively. In the Gordon-Taylor and

1 w1 w2 = + Tg Tg1 Tg 2 w1 Tg1 + kw2 Tg2 w1 + kw2 𝑤1 𝑇𝑔1 + 𝑘𝑤2 𝑇𝑔2 + 𝑞𝑤1 𝑤2 𝑤1 + 𝑘𝑤2


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Kwei model, k and q are adjustable parameters reflecting intermolecular interactions. Normally the

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q parameter corresponds to the strength of hydrogen bonding.38, 39

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b) 80

Tg (0C)

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


Expt Fox GT Kwei

40 0 -40 -80 0

177

20

40

60

80

100

OSL wt%

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Figure 1: DSC curves (left), and glass transition temperatures of PCE:OSL blends Vs OSL wt%

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with experimental data points and theoretical predictions from the Fox, Gordon-Taylor and Kwei

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models (right).

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In Figure 1b, all three models globally describe the general trend in Tg with composition with the

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Fox model resulting in the lowest goodness of fit (R = 0.96). The Kwei model provides the best fit

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(R=0.99) to the experimental Tgs of blends with k =1.52±0.5 and q = -0.01, followed by the GT

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model with kGT value of 2.35 (R= 0.98). The fact that the GT and Kwei model provides the best fit

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to the data compared to the Fox equation9,37 suggests specific interactions between the parent

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polymers. The value of k in the Kwei model indicate that OSL contribution to the final TgKwei is

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proportionally smaller than its mass fraction.37 As a result, the curve obtained by the TgKwei shows a

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negative shift (U-shaped) from the simple weight-averaged curve. The parameter q portrays the

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balance between the breaking of self-associations and the forming of inter-associations between

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the two parent polymers.39, 40 The q value close to zero implies that there is a good balance

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between the breaking of intra-molecular geometry of OSL and the formation of intermolecular

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interactions between PCE and OSL. Kadla et al investigated the supramacromolecular lignin 9 ACS Paragon Plus Environment


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complexes break up through specific intermolecular interactions between polyethylene oxide

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(PEO) and individual Kraft lignin components and showed that increasing PEO incorporation

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disrupts the supramacromolecular structure of lignin.12 Likewise, the value of q in the present

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PCE:OSL blend suggests that the dissociation of lignin supramolecular structure nullified the effect

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on Tg due to the formation of supramacromolecular structure of PCE:OSL blends through H-

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bonds.

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Nature of intermolecular Interactions based on Fourier transform infrared (FTIR)

200

spectroscopy.

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FTIR analysis might help shed light on the existence and nature of interactions in miscible polymer

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blends.12,39,40 FTIR spectra of the PCE:OSL blends show significant and systematic changes upon

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addition of OSL to PCE, in particular in the OH, C=O stretching region and in the C-O-C region

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(Figure 2). With increasing OSL content, the hydroxyl band of OSL and PCE merge towards one

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another, as might be expected from a simple mixture effect (Figure 2b). In addition, a shoulder

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band appears around 3290 cm-1 in the experimental spectrum. This new band is not present in the

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theoretical spectrum obtained by mathematical superposition of the parent spectra in the 30wt%

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OSL blend (Figure S3). It thus suggests the appearance of additional hydrogen bonds in the

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blends. The same trend is observed in the C-O-C region of PCE at 1096 cm-1 shifts to the lower

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region, reaching for example 1089 cm-1 in blends with 30wt% OSL (Figure 2a), even farther away

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from the lignin C-O-C vibration at ca. 1118 cm-1. Again, this differs from the theoretical spectrum

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(Figure S3) where the band center at 1094 cm-1. Finally, the carbonyl vibrations of OSL (1732 cm-

213

1

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simply obeying the law of mixture. The appearance of new OH stretching vibrations and the shifts

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in C-O-C band center which do not simply obey a rule of mixture reveal the presence of specific

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intermolecular interactions. Namely, the polyether graft chains of the PCE could be involved in H-

217

bonds with the lignin OH groups. The systematic trend in band center shifts for C-O-C vibrations

) and PCE (1725 cm-1) also shift in the blends, albeit in proportion to the component ratios, thus


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(Figure 3) upon OSL addition confirms that specific interactions involving these functional groups

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are taking place. All the samples show changes in ether and in carbonyl regions with broadened

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bands at lower wavenumber (1729 cm-1, Figure S4) probably due to the presence of different

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carbonyl groups in different environments from both blend components.

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To assess whether esterification did take place in the blends, a set of blends were prepared at

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various temperatures and further characterized by FTIR (Figure S5). Blending at higher

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temperatures, viz. at 170 OC and 190 OC revealed a new shoulder around 1759 cm-1, which is

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characteristic of ester carbonyls.41 In the normal preparation conditions of the blends, this shoulder is not

226

detected, suggesting that esterification does not take place . Instead, the observed band changes

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confirm the hypothesis that H-bonding takes place between the hydroxyl groups of OSL and the

228

carboxylic and ether functionalities of the PCE.

229 230

Figure 2: FTIR spectra of PCE:OSL blends and its components: PCE:OSL with 30wt%OSL),

231

70wt% OSL, PCE and OSL, FTIR spectra of (a) C-O-C region (b), OH stretching region.



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Figure 3: The relationship of absorbance of OSL and PCE based ether groups (band center

234

position) against wt% of OSL (left) and hypothetical architecture of supramolecular aggregates of

235

PCE:OSL blend (right)

236

Altogether the thermal analysis and vibrational spectroscopy provide strong evidence for miscibility

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across the entire compositional range of PCE and OSL arising from specific H-bonding between

238

the lignin OH groups and the PCE component. As lignin /PEO miscibility is clearly established it is

239

no surprise that PCE, which comprises PEO branches exhibit also strong miscibility with lignin.12

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Rheological Properties

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Rotational rheological properties of the PCE:OSL were investigated using a parallel plate set up for

242

PCE:OSL blends with up to 50wt% OSL content. For all tested PCE:OSL blends, the storage

243

modulus (G’) is lower than the loss modulus (G”) in the entire range of frequencies (ω) tested

244

(Figure 4), which is indicative of a viscous behavior.42 This reveals that there is no chemical

245

network developing in the blends. Likewise, no crossover of G″ and G′ can be observed at room

246

temperature over the entire frequency range (1-100 rad/ ω) also supporting that gel formation does

247

not occur in all blend compositions at room temperature. Finally, G’ and G” are found to increase 12 ACS Paragon Plus Environment

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with increasing OSL content. Yang and Chang suggested that the viscoelastic properties of a

249

polymeric system as obtained by rheology provide insight as to its potential performance as a

250

pressure sensitive adhesive.43 At a frequency of 1 Hz, viz., in the bonding region the polymeric

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systems should satisfy the Dahlquist criterion, which states that G’ had to be less than about 0.1

252

MPa before any adhesive tack was observed. 44. This criterion predetermines whether the material

253

has the required tackiness in the bonding region. This is clearly the case for all PCE:OSL

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formulations tested including the neat PCE. In turn, the 180 O peel strength of a PSA can be

255

deduced by observing viscoelastic properties at a frequency of circa 435 rad/s. To accede to this

256

region, master curves were created for the blend systems by virtue of Time Temperature

257

Superposition (TTS) using Tref =30 OC and isothermal data acquired at 10 OC intervals between -20

258

O

259

the entire frequency and temperature ranges used and yielded smooth master curves and shift

260

factor curves (Figure 4, right). However, when examining the master curve of a single blend, for

261

example the 40wt% OSL compositions over a broad range of temperature, a crossover frequency

262

for G’ and G’’ clearly reveals the existence of a gel point close to the debonding frequency. In other

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words, with increasing temperature the blends are able to form a physically crosslinked network

264

adopting a solid-like behavior.

C to 70 OC. The TTS and the William Landel Ferry equation appeared valid for all samples over



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Figure 4. Dynamic modulus (G’ &G’’) against the angular frequency of PCE:OSL blends with

266

different OSLwt% at Tg+700C (left), and master curve(right) for the 40wt% OSL blend showing a

267

gel point at ω = 103 rad/s. (Dahlquist line G’< 3. 105 Pa)

268

The ratio of the loss tangent to the storage modulus (tan δ /G’) provides information on the ability

269

of the adhesive to release energy at the debonding frequency.45 For example a value of 5 MPa-1

270

and 10 MPa-1 is recommended for stainless steel and polyethylene, respectively. For the neat

271

PCE, the tan δ/G' value is ca. 2.7 MPa-1 [Figure 5]. With initial addition of OSL this value increases

272

to ca. 4 MPa-1 and 3 MPa-1 for 10 wt% and 20wt% OSL respectively, and then decreases again

273

upon further addition. In the case of these two compositions, blending with lignin increases energy

274

dissipation mechanisms, possibly as a result of the supramolecular blend assembly by multiple

275

hydrogen bonds.46,47 While small amounts of lignin and thus the occurrence of a PCE:OSL

276

supramolecular assembly thus contribute to the energy release ability of the system, it remains

277

insufficient to provide adequate dissipation of energy at the debonding frequency. In other words,

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the blends maintain a strong elastic character such that debonding on steel surfaces may occur by

279

interfacial cracks. Further tuning of the system is therefore required to improve energy dissipation

280

capacity.

281 14 ACS Paragon Plus Environment

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282 283

Figure 5: The ratio tan δ /G’ as a function of frequency and the tan δ /G' for PCE:OSL blends with different amounts of OSL measured @ 23±2 OC and 1 Hz (inset figure).

284

The viscoelastic data can be further utilized to determine the usable window of a PSA.18,48 In this

285

analysis materials falling into quadrant 1 are typically not suited for PSA applications. Quadrant 2

286

relates to PSA which necessitate high shear strength. Quadrant 3 applies to PSA with low peel

287

strengths and therefore that are easily removable. Quadrant 4 is the region of low G‘ and G ‘‘ and

288

stands for quick-stick PSA. In Figure 6, it is apparent that the neat PCE does not fit in the general

289

PSA window. In contrast, adding OSL to PCE shifts the viscoelastic window closer to the general

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PSA. In particular the 20 and 30 wt% OSL content blends center on Quadrant 3, revealing possible

291

application as removable PSA. The 40 wt% OSL blend centers around Quadrant 2, suggesting

292

potential application as high cohesive strength PSAs. Regardless of the special attributes of the

293

lignin blend as PSA, it is clear that blend composition is a useful tool to tune the specialty of the

294

PSA from a removable PSA to a high shear PSA.

295 296

Figure 6. Viscoelastic window of PCE:OSL blends with different OSL wt% @ 23±2OC.

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Adhesive Properties of PCE:OSL Blends



Page 16 of 27

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The rheological characterizations on the blends have suggested that the PCE:OSL blends might

299

well function as PSA. Creton defines 3 properties that PSAs require.18 “First, they require some

300

tackiness in order to form good bonds, second they need a controlled peel force as a function of

301

peel velocity and precise control of the residue left, finally they must exhibit minimum creep”. To

302

further assess this possibility, adhesive tackiness, 180O peel strength and shear strength were

303

tested for 0-40wt% OSL blend compositions. Turning first to the tackiness, it is apparent that

304

adding OSL up to ca. 20wt% improves the material tackiness, at which point further addition

305

slightly lowers tackiness (Figure 7). A tackiness level similar to that of a commercial PSA (coating

306

thickness 17± 2 μm, similar to that of the PCE:OSL blends at 20± 2μm) could be obtained by the

307

addition of up to 20wt% OSL. The 30wt% OSL content blend maintains acceptable tackiness in

308

contrast to the 40wt% OSL content blend, which completely loses its tackiness. Tackiness is

309

further evidenced by the ability of the material to form fibrils upon extension and detachment in the

310

rheometer (Figure 8, right).

311 312

Figure 7:Tack, peel and shear strength measured @ 23±2OC for the PCE:OSL blends


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313 314

Figure 8: Shear strength of various composition of PCE:OSL blends @ 23±2OC(left) and a

315

PCE:OSL blend in rheometer set up (right)

316

The 180O peel test reveals that the neat PCE has a low peel strength (0.05 N/cm). According to

317

Lee and Shin 49, excellently removable PSA exhibit a peel strength below ca 0.5 N/cm. However,

318

as evidenced by the viscoelastic analysis, the neat PCE does not fall into the usable window of a

319

PSA. Increasing OSL loading up to 30wt% increases peel strength more than 10 fold reaching

320

around 7 N/cm for the 30% OSL content blend and 2.2 N/cm for the 20wt% OSL content blend

321

(Figure 7). This positions the 20wt% OSL content blend in the category of the removable and

322

repositionable or semi-removable PSA such as scotch tape, while the 30wt% OSL content blend

323

pertains to the permanent PSA such as duct tape, in respect to their peel load.49 Therefore, these

324

two blends have peel strengths well in line with the performance of commercial removable and/or

325

repositionable PSAs. Again, lignin content is clearly revealed as a tool to engineer the peel

326

strength of the resulting adhesive system.

327

The shear test was done to measure the shear resistance of the PCE:OSL blends. The neat PCE

328

showed no strength at the testing condition as in fact, the PCE coated strip fell from the steel plate

329

as soon as the weight was hanged. Figure 8 displays the stress-strain curve for the shear test of

330

the different systems and confirms a low shear strength for all PCE:OSL blends with less than 17 ACS Paragon Plus Environment


Page 18 of 27

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30wt% OSL content. It also clearly reveals that increasing OSL content up to 40wt% delivers a

332

shear strength that is similar to that of a commercial PSA (Würth tape). With its glassy state and

333

its ability to form hydrogen-bonded networks with PCE, lignin contributes resistance to shear to the

334

system. The reason for the drastic increase in shear resistance at exactly 40% is however

335

unknown. Perhaps a change in the blend microstructure occurs, but this was not specifically

336

monitored. The shear resistance was also measured by examining the adhesive strength of the

337

PCE:OSL blends under a constant load according to PSTC-107 testing procedure.32 Adding OSL

338

to the PCE here again improved the shear resistance of the system, albeit it remained low.

339

PCE:OSL blends with 10, 20 and 30wt% of OSL could withstand the load for 1, 4 and 9 min

340

respectively. The test with 40wt% OSL content blend failed due the lack of tackiness. The obtained

341

time from 0-30wt% OSL blends remains low in comparison to other reports in the literature with

342

other biobased PSA systems and commercial PSAs but approaches the values for post-it

343

tapes.17,49,50.

344

Significant differences in tack, peel resistance and shear strength due to the presence of OSL

345

were detected. Interestingly, increasing the volume fraction of OSL to 30 wt% improves all the

346

critical PSA properties, except the tackiness which is only very moderately lowered. The

347

microstructural origin of the performance of the PCE:OSL is difficult to pinpoint with this study.

348

However, a few statements can be safely made based on prior literature.46,47,51-54 The

349

supramolecular assembly of lignin and PCE via hydrogen bonding likely provides some additional

350

energy dissipation mechanisms and tackiness as observed for the 10 and 20wt% OSL blends.

351

The ability to form physically crosslinked polymer networks with the glassy lignin moieties further

352

provides shear resistance to the assembly. The higher the physical crosslinking density, as is

353

expected with increasing lignin content, the higher the shear resistance. It is thus the

354

supramolecular assembly via hydrogen bonding itself that would be responsible for the

355

improvement in tackiness, peel strength and shear resistance as lignin is incorporated in moderate

356

amounts into the PCE. Above a critical lignin content of between 20 and 30 wt%, lignin plays 18 ACS Paragon Plus Environment

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mainly the role of a glassy filler which enhance shear resistance and peel strengths but the minor

358

supramolecular assembly of PCE:OSL systems no longer suffices to enhance energy dissipation

359

capability. This particularly clear for the 40 wt % OSL blend, which exhibits maximum shear and

360

peel strength and minimum tan/G’ and tackiness. In contrast, the PCE with its Tg below room

361

temperature functions as sticky branches.

362

Considering all the performances together, it stands out that PCE:OSL systems are possible base

363

formulations for PSA applications. Further fine tuning of the formulations is needed to completely

364

optimize the systems for PSA application, especially regarding the energy dissipation ability.

365

CONCLUSION

366

Organosolv lignin-based blends have been produced using a commercial PCE. A single Tg well

367

modeled by the Kwei analysis for the PCE:OSL blends alongside FTIR analyses evidenced

368

PCE:OSL miscibility and the existence of specific interactions between the OSL hydroxyl and the

369

proton accepting sites of PCE. Miscibility between the two parent polymers improved the thermal

370

stability and viscoelastic response of the blend in view of their possible utilization as pressure

371

sensitive adhesives. In particular, the blends up to 30wt% OSL content exhibited favorable tack,

372

peel and shear properties that could be tuned by the OSL content to fit specialty application from

373

removable/repositionable or semi removable PSA. The blends supramolecular assembly by

374

hydrogen bonding is proposed to be in part responsible for additional energy dissipation

375

mechanisms in the blends, while the glassy lignin and the rubbery PCE are mostly responsible for

376

shear strength and tackiness, respectively. Nonetheless additional fine tuning, possibly including

377

mild covalent crosslinking is needed to completely match the ideal performance of any category of

378

PSA.

379

ASSOCIATED CONTENT

380

Supporting Information 19 ACS Paragon Plus Environment


Page 20 of 27

381

Thermogravimetric analysis, Mathematically constructed and experimentally observed FTIR

382

spectra 30wt%OSL blend; FTIR spectra of carbonyl stretching region of blends, FTIR spectra of

383

selected PCE:OSL blends prepared at different temperatures.

384

AUTHOR INFORMATION

385

Corresponding author

386

* Phone: +49 (0)761 203 4759. Fax:

387

E-mail: [email protected]

388

Notes

389

Conflicts of Interest: The authors declare no conflict of interest

390

ACKNOWLEDGMENTS:

391

This work was partially funded by the European Union’s Seventh Framework Program, FP7-NMP,

392

and Project number 604215 (CARBOPREC). The authors wish to thank Arkema (France) and

393

Fraunhofer CBP, Germany for providing Polycarboxylate Polyether (ETHACRYL.HF) and

394

(Organosolv lignin) respectively. The authors would like to thank Elke Stibal for her technical

395

assistance.

+49 761 203 37 63

396 397

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Graphical Abstract: For Table of Contents Use Only

555 556 557

Brief Synopsis

558

The potential use of organosolv lignin in the base formulation of pressure sensitive adhesive was

559

assessed by its adhesive performances.

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Potential of Organosolv Lignin Based Materials in Pressure Sensitive

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