Assessment of the adhesive properties of the bacterial polysaccharide FucoPol

Accepted Manuscript Title: Assessment of the adhesive properties of the bacterial polysaccharide FucoPol Author: Diana Ara´ujo Vitor D. Alves Joana Campos Isabel Coelhoso Chantal Sevrin Christian Grandfils Filomena Freitas Maria A.M. Reis PII: DOI: Reference:

S0141-8130(16)30822-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.07.035 BIOMAC 6309

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

19-4-2016 1-7-2016 11-7-2016

Please cite this article as: Diana Ara´ujo, Vitor D.Alves, Joana Campos, Isabel Coelhoso, Chantal Sevrin, Christian Grandfils, Filomena Freitas, Maria A.M.Reis, Assessment of the adhesive properties of the bacterial polysaccharide FucoPol, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.07.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Assessment of the adhesive properties of the bacterial polysaccharide FucoPol

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Diana Araújo1, Vitor D. Alves2, Joana Campos1, Isabel Coelhoso3, Chantal Sevrin4, Christian

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Grandfils4, Filomena Freitas1,* , Maria A. M. Reis1

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Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal

UCIBIO-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,

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Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisbon, Portugal

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Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal

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Belgium

LEAF – Linking Landscape, Environment, Agriculture and Food, Instituto Superior de

LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,

Interfacultary Research Centre of Biomaterials (CEIB), University of Liège, B-4000, Liège,

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UCIBIO-REQUIMTE, Departamento de Química

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Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa

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Campus da Caparica

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2825-516 Caparica, Portugal

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Tel.: +351 212948300

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[email protected]

Corresponding Author:

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Abstract

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To address the industry’s interest in finding novel biobased glues, the adhesive properties of

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the bacterial polysaccharide FucoPol were evaluated through shear bond strength tests. A

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FucoPol solution was used to bond different materials, namely, wood, glass, cardboard and

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cellulose acetate film. The shear strength was compared to that of the same adherends bonded

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with commercial synthetic glues. Wood-wood joints bonded with FucoPol formulation

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withstood 742.2±9.8 kPa shear strength without detachment. FucoPol adhesive capacity for

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cardboard was comparable to that of the tested commercial glues (425±8.9 kPa), yielding

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similar shear strength values (416.0±12.9 kPa), while improved performance was shown for

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glass (115.1±26.2 kPa) and cellulose acetate film (153.7±11.3 kPa) comparing to the

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commercial glues (67.7-97.5 kPa and 79.4-92.7 kPa, respectively). This study demonstrates

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the adhesive properties of FucoPol, opening up the opportunity of using this bacterial

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polysaccharide for the development of new natural water-based glues, suitable to bond

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different materials.

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Keywords: FucoPol; Polysaccharide; Rheology; Shear bond strength; Natural adhesive

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1. Introduction

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Growing concern about the environmental pollution caused by the manufacture and

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use of petrochemical-based adhesives, together with increasing demand of adhesives and

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limited availability of resources, have driven the industry to consider alternative sources for

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the development of new and ecofriendly adhesives [1-3]. Over the last decades, a wide range

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of natural polymers have been proposed as sources for the development of biobased

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adhesives [4]. However, to be able to compete with petroleum-based polymers that have high

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bonding performance, as well as heat and water resistance, biobased polymers must have

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identical performance and/or introduce new valuable properties [5]. The use of biobased

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polymers is currently limited by their low bond strength, low durability, low water resistance,

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and high viscosity [2,5,6]. The major advantages of using biopolymers for the development

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of novel adhesives is that they are generally non-toxic, biocompatible, biodegradable and

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produced from renewable resources [7,8]. Still, the production costs are also an important

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issue to be considered.

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Different biopolymers, such as proteins, polyhydroxyalkanoates and polysaccharides

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have been proposed as sources of novel biobased adhesives and their bonding capacity for

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different materials were studied. Proteins from milk, blood and soybean were used as

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adhesives for thousands of years before being replaced by synthetic adhesives [6]. Nowadays,

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soy protein-based adhesives are used in the wood and paperboard industries [3,9]. A frog

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protein-based glue was tested to bond wood specimens, reaching high shear strength values

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(~1.7 MPa) [10]. Recently, a medium chain length PHA (mcl-PHA) was demonstrated to

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have adhesion properties to wood and glass, though low shear strength values were achieved

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(67 and 65 kPa, respectively) [11]. Polysaccharides, on the other hand, have been the subject

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of intensive research and different natural sources have been considered, including animals

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(e.g. chitosan), plants (e.g. starch, guar gum, konjac glucomannan) and microorganisms (e.g.

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pullulan, xanthan, levan) [1,5,6].

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Some polysaccharides possess adhesive properties that are mainly due to the presence

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of hydroxyl groups in their structure. However, these groups also have a negative impact on

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the adhesive’s resistance to water and compromise their integrity under high humidity and

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immersion in aqueous media [1,2]. For this reason, polysaccharide-based adhesives may have

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lower adhesive strength than synthetic products. Nonetheless, such natural adhesives are

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suitable for use in many types of adhesives, including pressure sensitive adhesives, denture

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and medical adhesives, pharmaceutical tablet binders, among others [1,5,12]. Furthermore,

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the adhesive properties of polysaccharides can be improved by chemical modification,

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blending with synthetic adhesives or formulation with other additives [1,2].

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In this study, the adhesive properties of FucoPol, a bacterial polysaccharide

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synthesized by the bacterium Enterobacter A47 using glycerol as carbon source [13-15],

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were assessed for the first time. The polymer formulation was characterized in terms of its

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rheological properties and contact angle at the surface of each tested material. Single lap

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shear strength tests were performed to assess the FucoPol formulation bonding capacity using

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as adherends different materials, namely, balsa wood, glass, cellulose acetate and cardboard.

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The performance of FucoPol as adhesive was compared with commercial synthetic glues

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under the same conditions.

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2. Materials and Methods

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2.1. Physical-chemical characterization of FucoPol

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FucoPol was produced by cultivation of the bacterium Enterobacter A47 in glycerol,

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as described by Alves et al. [13], and extracted from the cultivation broth as described by

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Ferreira et al. [16]. The polymer was analyzed in terms of sugar monomers and acyl groups

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composition, as well as total inorganic and protein contents. For determination of the sugar

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composition, dried samples of FucoPol (~5 mg) were dissolved in deionized water (5 mL)

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and hydrolyzed with trifluoroacetic acid (TFA) (0.1 mL TFA 99%), at 120 ºC, for 2 hours.

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The hydrolysate was used for the identification and quantification of the constituent

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monosaccharides by liquid chromatography (HPLC) using a Carbopac PA10 column

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(Dionex), equipped with an amperometric detector, as described by Freitas et al. [17]. The

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analysis was performed at 30ºC with sodium hydroxide (NaOH 4 mM) as eluent, at a flow

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rate of 0.9 mL/min. The acid hydrolysates were also used for the identification and

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quantification of acyl groups. The analysis was performed by HPLC with and Aminex HPX-

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87H 300×7.8mm (Biorad), coupled to an infrared (IR) detector, using sulphuric acid (H2SO4

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0.01 N) as eluent, at a flow rate of 0.6 mL/min and a temperature of 30 ºC.

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The total inorganic content of the samples was determined by subjecting them to

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pyrolysis at a temperature of 550 ºC, for 24 hours. For the determination of the protein

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content, 5.5 mL samples of aqueous FucoPol solutions (4.5 g/L) were mixed with 1 mL 20%

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NaOH and placed at 100 ºC, for 5 min. After cooling on ice, 170 µL of CuSO4.5H2O (25%

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v/v) were mixed. The samples were centrifuged (3500×g, 5 min) and the optical density was

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measured at 560 nm. Albumin (Sigma-Aldrich) solutions (0.5-3.0 g/L) were used as protein

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standards.

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Number and weight average molecular weights (Mn and Mw, respectively), as well as

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the polydispersity index (Mn/Mw), were obtained by size exclusion chromatography coupled

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with multi-angle light scattering (SEC-MALS), as described by Freitas et al. [17]. Briefly,

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FucoPol solutions (0.2 g/dL) were dissolved in 0.1 M Tris-HCl, NaCl (0.2 M), pH 8.1 buffer,

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which was also the SEC mobile phase. The SEC columns (PL aquagel-OH mixed 8 μm, 30 x

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7.5 mm) were equilibrated for 24 h before running the analysis at a flow rate of 0.7 ml/min at

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room temperature. In order to follow the purity and molecular mass distribution of the

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polysaccharide signals from MALLS were recorded in parallel and treated with Astra (V

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4.73.04). A dn/dc of 0.190 mL/g was assumed to calculate the Mw of the FucoPol.

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2.2. Adhesive formulation preparation

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A 3 wt% FucoPol solution was prepared by dissolving the polymer in deionized

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water, under vigorous stirring, at room temperature, during 12 hours. The solution thus

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obtained was heated at a temperature of 50-60 ºC, for 6 hours, to evaporate part of the solvent

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and concentrate the solution. A FucoPol solution with a concentration of about 7.6 wt% was

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obtained and used as the adhesive formulation in the subsequent tests. Sodium azide (10

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ppm) was added to prevent microbial growth and the formulation was stored at 4 ºC.

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2.3. Apparent viscosity and viscoelastic properties

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The apparent viscosity and viscoelastic properties of the formulation prepared as

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described above were studied using in a controlled stress rheometer (HAAKE MARSIII,

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Thermo Scientific) equipped with a plate-plate serrated geometry (diameter 20 mm), with a

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gap of 1 mm. The samples were equilibrated at 20 ºC, for 5 min, after which the flow curves

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were obtained using a steady-state flow ramp in the range of shear rate from 10-3 to 1000 s-1.

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The viscoelastic properties were evaluated by carrying out stress sweeps, at constant

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frequency (1 Hz) for a stress range from 0.01 to 5000 Pa; and frequency sweeps at a constant

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tension within the linear viscoelastic region, for a frequency range from 0.01 to 100 Hz.

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2.4. Shear bond strength

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The adhesion strength of the FucoPol formulation was determined by single lap shear

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strength according to the method used by Kim and Netravali [9] with some modifications.

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Briefly, samples of balsa wood (Leroy Merlin Store, Portugal), glass (Deltalab S.L.),

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cellulose acetate film (Staedtler, lumocolor, photocopy film) and corrugated cardboard

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(Leroy Merlin Store, Portugal), cut in 26 × 76 mm strips, were used as adherends. The

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thickness of the wood, glass, cellulose acetate and cardboard samples were 30, 11, 0.1 and 50

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mm, respectively.

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For determination of the shear bond strength, wood-wood, glass-glass, cellulose

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acetate-cellulose acetate and cardboard-cardboard joints were prepared by homogenously

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applying 0.15±0.003 g of the FucoPol formulation in a superficial area of 5.0 cm2, at the end

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of the strips. The strips were overlapped, hand-pressured for 10 s and kept pressed using two

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commercial binder clips in the bonded area at room temperature for 24 hours. For each

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adherend, three replicates were prepared and tested.

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The shear strength measurements were carried out using a TA-XT plus texture

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analyzer (Stable Micro Systems, Surrey, England), as described by Cruz et al. [11]. Briefly,

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the adherends were attached on tensile grips and an axial force at a crosshead speed of 0.5

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mm/s was applied until the joints were totally separated or the maximum capacity of the

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equipment was reached. UHU Universal Glue (UHU GmbH & Co. KG, Buhl, Germany) and

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UHU stick (UHU GmbH & Co. KG, Buhl, Germany) were used as reference commercial

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adhesives.

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2.5. Contact angle

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A KSV contact angle meter CAM 100 was used to measure the contact angle between

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the FucoPol formulation and the different adherends. The formulation drops were delivered

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onto the adherends with a 1 mL syringe (Injekt®-F Tuberculin, B Braun), using a needle (0.6

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× 25 mm, BD Microlance™ 3). Images were taken immediately after applying the solution

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drop and after 4 seconds. Five drops were performed for each adherend and the contact

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angles determined are average of the five measurements.

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3. Results

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3.1. FucoPol characterization

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FucoPol used as bioadhesive was characterized in terms of chemical composition and

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molecular mass distribution. The glycosyl composition analysis of the polymer revealed that

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it was a heteropolysaccharide composed of neutral sugars (fucose, galactose and glucose) and

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the uronic acid, glucuronic acid, as previously reported [14,17]. The heating procedure used

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to concentrate the FucoPol solution and obtain the adhesive formulation did not cause any

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change in the polymer’s sugar monomer composition. Similarly, there were no significant

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changes on the acyl groups content that accounted for 23-25 wt% of the polymer’s dry

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weight.

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The polymer used in this study had total protein and inorganic salts contents of 5.0

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wt% and 9.5 wt%, respectively. The total protein content is within the values reported for

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FucoPol (5-15 wt%) extracted using dialysis or diafiltration [14,17,19]. The inorganic salts

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content was higher than the values reported in previous studies, wherein contents below 4

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wt% were detected [17,19].

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In terms of average molecular weight (Mw), the results have shown that FucoPol was

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a homogenous polymer with low polydispersity index (1.3) and has an average molecular

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weight of 1.8×106. These values are in accordance within the range reported for FucoPol

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(1.7×106-5.8×106) [14,17,19]. Envisaging its use as adhesive, the high molecular weight of

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FucoPol is an important characteristic since it allows for cohesive strength [5]. Many of the

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polysaccharides that have been reported to have adhesive properties, including xanthan gum,

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guar gum and levan, have Mw values of the same order of magnitude (2.6×106-7.0×106)

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[1,6,14,].

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3.2. Rheological properties of FucoPol adhesive formulation

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The apparent viscosity of the FucoPol formulation, an aqueous solution with a

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polymer concentration of 7.6 wt%, was measured for shear rates ranging from 10-3 to 1000 s-

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followed by a shear-thinning behavior for shear rate values above 0.1 s-1. This behavior is

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typical of polysaccharides in aqueous media [17] and indicates that the formulation was

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composed of entangled polysaccharide macromolecules [20,21].

. The flow curve (Fig. 1a) showed a well defined Newtonian plateau at low shear rates,

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The experimental data was fitted with the simplified Cross model, Eq. (1) [19]:

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=

(

̇ )

(1)

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where ηa is the apparent viscosity (Pa.s), η0 is the viscosity of the first Newtonian plateau

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(Pa.s), λ is the relaxation time (s) and m is a dimensionless constant. The experimental data

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was very well described by the equation. The estimated Cross model parameters are

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presented in Table 1. 9

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As shown in Table 1, the FucoPol formulation was a highly viscous solution. The

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Cross model parameters, such as the viscosity of the first Newtonian plateau (η0) and the

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relaxation time (λ), were considerably higher (η0 = 48.9 Pa.s and λ = 0.90 s) than the values

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achieved for a 1.2 wt.% solution (η0 = 2.5 Pa.s and λ = 0.55 s), as reported by Torres et al.

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[20]. This difference is due to the higher polymer concentration of the formulation, leading to

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higher number of entanglements between the polymer chains, increasing the viscosity and the

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relaxation time of the sample [20].

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The flow behavior of FucoPol formulation was similar to other natural

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polysaccharides, such as chitosan [22], GalactoPol [25,26] and guar gum [24], since all of

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these biopolymers were reported to exhibit a Newtonian plateau at low shear rates, followed

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by a decrease in viscosity (shear-thinning behavior) with the increase of the shear rate.

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Xanthan gum also exhibits a similar behavior [23], but only at low concentrations (below 0.5

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wt%). For higher concentrations, xanthan gum was reported to form a gel-like structure,

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characterized by a continuous increase of the viscosity when the shear stress is lowered (i.e.,

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a Newtonian plateau is not perceived at low shear rates) [27]. A completely different flow

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behavior is displayed by the bacterial polysaccharide levan, whose aqueous solutions were

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shown to have a Newtonian behavior for concentrations up to 30 wt% [21]. Levan’s aqueous

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solutions viscosity increased as the polymer concentration increased, but only for

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concentrations above around 30 wt% a weak shear-thinning behavior was perceived.

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The mechanical spectrum shown in Fig. 1b provides information about the

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viscoelastic properties of FucoPol formulation. It shows a loss modulus (G’’) slightly higher

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than the storage modulus (G’) both highly dependent on the frequency (Fig. 1b). This result

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demonstrates that the polymer solution has a higher viscous behavior than the elastic one. In

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addition, at high frequencies, the values of G’ and G’’ tend to the same value, where a cross-

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over is observed at an angular frequency of about 1.8 Hz. The same trend was shown for a 1

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wt.% FucoPol solution, in which the cross-over occurred at around 2.8 Hz [17]. This behavior

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is typical of liquid-like fluids, indicating the presence of a high number of entanglements,

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which is consistent with the higher viscosity observed for the FucoPol formulation (Fig. 1a).

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Similar behavior was reported for xanthan gum [23] but, due to higher viscosity of the

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polymer’s solution at low concentrations, the cross-over occurred at lower frequencies (~0.02

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Hz). Nevertheless, at concentrations similar to FucoPol formulation (e.g. 7 wt.%), xanthan

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gum has a different behavior: both moduli are only weak functions of frequency and G′ is

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higher than G′′ over the whole frequency range, which is indicative of a gel-like structure

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[27].

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The bonding capacity of the adhesives can be correlated with their bulk rheological

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properties [28,29]. In fact, the viscoelastic properties are frequently used in adhesive studies

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to evaluate the adhesion properties [29]. A good adhesive is characterized by a high ratio of

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G’ at high frequency to G’ at low frequency, concomitant with a G’’ higher at higher

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frequencies [29]. This behavior was observed for FucoPol formulation, which showed a high

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frequency dependency for both moduli that increased for higher frequencies (Fig. 1b).

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The viscosity of the adhesive has also a significant impact on its application

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performance. The adhesive should be able to flow over the surface of the adherend to

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guaranty a proper wetting and some degree of penetration into the material [5]. Highly

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viscous adhesives may be difficult to spread evenly throughout the adherend surface, thus

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compromising the bonding performance. Although the 0 viscosity of the FucoPol

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formulation (43 Pa.s) is higher than that of some commercial synthetic glues such as

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polyvinyl acetate adhesive (12-15 Pa.s) [30] and certain polyurethanes (www.axson-

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technologies.com) (0.25 – 5.7 Pa.s) [31], it is within the values (0.07-280 Pa.s) reported for

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other biopolymer solutions that were tested as adhesive formulations, including a soybean

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meal-based adhesive, 26 Pa.s [2]; xanthan gum, 43 Pa.s; guar gum, 280 Pa.s [5]; chitosan, 35-

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91 Pa.s [32]; and levan, ~0.06 Pa.s [21].

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3.3. Adhesive properties of FucoPol

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The adhesive properties of FucoPol formulation were evaluated by submitting the

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bonded specimens to shear stress tests, since the shear strength is a commonly used index to

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evaluate the quality of bonded materials [30]. Single lap joints was the design chosen to

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evaluate the shear bond strength of the formulation since they are simple to prepare and

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adequate to subject the adhesive to a force at its strongest direction [33]. As mentioned

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above, for determination of the shear bond strength, four types of materials were tested: balsa

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wood (a natural organic material), corrugated cardboard (a processed natural material),

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cellulose acetate film (a semi-synthetic material) and glass (an inorganic material). In order to

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compare FucoPol with commercial synthetic glues, specimens of the same materials were

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glued using UHU universal glue and UHU stick, under identical bonding conditions.

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The highest adhesivity was found for the wood joints glued with FucoPol formulation

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and with the commercial glues, since the maximum strength of the available equipment

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(742.2 ± 9.8 kPa) was not enough to detach the bonded specimens. In fact, for some of the

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replicates, there was a wood failure without detachment at the area bonded with the FucoPol

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formulation (Fig. 2). A similar behavior was observed for the commercial UHU Universal

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glue. This is mostly because the rough wood surface allows the penetration of the adhesives

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into the wood matrix, providing a higher adhesive-wood interaction [9] and resulting in a

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stronger bond. This high interaction was confirmed by measuring the contact angle between

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the FucoPol formulation and the wood surface (Table 2). Immediately after application of the

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adhesive, a contact angle of 120.65±7.65º was measured, but after 4 seconds it was reduced

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to 70.87±7.03º. Considering the rough porous structure of the wood surface, this high

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decrease was mostly due to the adhesive penetration into the wood matrix.

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The adhesives used for wood bonding applications should have shear strength values

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higher than the strength required to cause failure of the wood materials, which depend on the

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type of wood, as well as on the joint design and the adhesive loading [1]. Typical values

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range between 1000 and 50000 kPa, for petrochemical-based adhesives, such as

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polyvinylacetate, polyurethane and phenol-formaldehyde [1,6,30]. Most polysaccharide-

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based adhesives have lower adhesive strength and their integrity is often compromised by

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rehydration. Nevertheless, they can still be suitable for many wood applications [1]. For

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example, chitosan was described as one of the most interesting polysaccharide for wood

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bonding [32,35]. According to Patel et al. [32], 4 and 6% (w/v) chitosan formulations

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exhibited shear bond strength values of 4200 and 6100 kPa, respectively, thus demonstrating

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the polymer’s good adhesive properties.

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Several microbial polysaccharides have also been evaluated as adhesives for wood.

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Pullulan, xanthan gum and levan were reported to have high adhesivity for different types of

307

wood materials (e.g. pine, maple, particle board), with shear strength values above 2000 kPa

308

[1,5,36]. On the other hand, the polysaccharide adhesive viscous exopolysaccharide (PAVE)

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synthesized by Alteromonas colwelliana strains displayed a shear strength value of 545 kPa

310

[37], which is lower than the value obtained in this study for FucoPol formulation. Given

311

these results, it can be concluded that FucoPol formulation has potential to be used as an

312

adhesive for wood (Fig. 3), since it withstands a shear strength of at least 742.2±9.8 kPa.

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Fig. 4 shows the maximum shear bond strength values obtained for cardboard, glass

314

and plastic strips bonded with UHU universal glue, UHU stick and FucoPol formulation. As

315

can be observed, shear bond strength was strongly dependent on the selected material. The

316

shear bond strength values for cardboard specimens were clearly the highest among the three

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materials (Fig. 4). However, for the specimens bonded with the commercial glues, structural

318

failure of the cardboard strips was observed for shear strength values around 425.4±8.9 kPa,

319

which means that the adhesives are stronger than the adherend. On the other hand, the

320

maximum shear bond strength achieved with FucoPol formulation without failure of the test

321

material (416.0±12.9 kPa), was close to the shear strength required to cause cardboard

322

rupture, indicating that this formulation can have applicability to glue this type of material.

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The cohesive failures observed for this adherend confirms the high interaction established

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between the adhesive and the cardboard. Cardboard has a porous structure that allows the

325

penetration of the adhesive throughout its matrix. The strong FucoPol-cardboard interaction

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was also supported by the contact angle measurements (Table 2). Similarly to that observed

327

in the wood surfaces, the FucoPol formulation was able to penetrate value as shown by the

328

high decrease of the contact angle from an initial value of 117.40±6.02º to 83.31±4.03º within

329

4 seconds.

330

The main adhesives used in the card and paperboard industry are thermoplastic and

331

thermosetting resins, whose disposal after use is difficult and may have environmental

332

impact. Since paper and cardboard are widely used as packaging materials, there is also

333

concern about the migration of harmful adhesive components to the products [3]. In view of

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this, there is an increasing demand for the development of environmentally friendly adhesives

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from natural sources. Hence, the good adhesivity of FucoPol formulation for cardboard

336

demonstrates its promising potential for this area of application.

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Lower maximum shear bond strength values were obtained for the glass (115.1±26.2

338

kPa) and cellulose acetate (153.7±11.3 kPa) specimens bonded with the FucoPol formulation

339

(Fig. 4), which may be explained by the non-porous nature of the materials’ surfaces that

340

limits the polymer-material adhesion [38]. Actually, the ability of the adhesive to make a

341

good contact with the adherend can be affected not only by adhesive penetration into the

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342

material, but also by adhesive spreading over the surface [34]. In fact, comparing with wood

343

and cardboard materials, lower contact angles were observed for glass and cellulose acetate

344

film, both initially (81.19±6.49 and 106.43±4.26, respectively) and after 4 seconds

345

(45.74±4.28 and 65.12±5.55, respectively). These results are related to the spreading of the

346

FucoPol formulation over the surfaces of these materials.

347

Interestingly, FucoPol formulation performed better with both materials than the

348

tested commercial adhesives. In fact, the maximum shear bond strength obtained with

349

FucoPol formulation for glass (115.1±26.2 kPa) was 41.18 and 15.29% higher than the

350

values obtained for glass bonded with UHU universal glue (67.7 ± 4.2 kPa) and UHU stick

351

(97.5±3.9 kPa), respectively. FucoPol formulation performed even better than either of the

352

commercial glues for bonding the cellulose acetate films (Fig. 4). The shear strength of the

353

cellulose acetate films bonded with FucoPol adhesive formulation was improved by 48.34

354

and 39.69% in comparison with the specimens glued with UHU universal glue (79.4±14.9

355

kPa) and UHU stick (92.7 ± 6.9 kPa), respectively. In all tests, including the specimens

356

bonded with the FucoPol formulation and the commercial glues, there were cohesive failures

357

as shown by the presence of adhesive distributed on both faces of the failure, thus confirming

358

the strong contact that was established between the adhesive and both adherends.

359

The adhesivity of chitosan towards glass surfaces was evaluated by Yamada et al.

360

[39], but the tested formulation did not show any adhesive properties for this type of material.

361

Only by supplementing the formulation with additives (dopamine and melB tyrosinase), a

362

shear bond strength of around 400 kPa was obtained. This value is higher than that obtained

363

for FucoPol, but in this study no additives were used. The shear strength values obtained for

364

FucoPol formulation for bonding glass specimens was within the values reported (49-182

365

kPa) for PAVE isolated from several A. colwelliana strains [37].

15

366

The good adhesive performance of the FucoPol formulation for bonding glass and

367

cellulose acetate materials comparing to the synthetic glues evidences that this

368

polysaccharide may be used for the development of new biobased glues that may replace the

369

synthetic products in some applications.

370 371

4. Conclusions

372 373

The adhesive properties of the natural polysaccharide FucoPol towards several

374

material surfaces were demonstrated. The FucoPol adhesive formulation performed similarly

375

or better than UHU Universal Glue and UHU stick, two synthetic commercial glues. Even

376

though the high hydrophilicity and low resistance of FucoPol to liquid water limits the use of

377

such a glue, there is a wide range of applications for a novel hydrophilic natural glue, such as,

378

for example, the production of corrugated board, labeling of glass bottles, book

379

manufacturing, tissue products, pressure sensitive tapes or medical adhesives.

380

16

381

Acknowledgements

382

This work was supported by the Unidade de Ciências Biomoleculares Aplicadas-UCIBIO

383

which is financed by national funds from FCT/MEC (UID/Multi/04378/2013 and

384

UID/AGR/04129/2013) and co-financed by the ERDF under the PT2020 Partnership

385

Agreement (POCI-01-0145-FEDER-007728). F. Freitas acknowledges FCT/MEC for

386

fellowship SFRH/BPD/72280/2010.

387 388 389 390 391 392 393

17

394

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501

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502 503

22

504

Figure captions

505 506

Figure 1. Apparent viscosity and viscoelastic properties of FucoPol adhesive formulation: (a)

507

flow curve; (

508

Mechanical spectrum storage [G’ ( )] and loss moduli [G’’ ( )].

) experimental data. The line represents the simplified Cross model. (b)

509 510

Figure 2: Wood-wood joints bonded with FucoPol formulation.

511 512

Figure 3: Schematic representation of the shear bond strength values reported for different

513

adhesives tested for bonding wood materials.

514 515

Figure 4: Shear bond strength for cardboard, glass and cellulose acetate strips bonded with

516

UHU universal glue, UHU stick and FucoPol formulation. The error bars represent the

517

standard error of the measurements. * - At least one of the replicas suffered structural failure.

518 519 520

Table 1. Estimated parameters of the Cross model for aqueous solutions of FucoPol and

521

other natural polysaccharides. Concentration (wt.%)

η0 (Pa.s)

Chitosan

2.50

10.8

n.a.

0.63

[22]

Xanthan gum

0.40

39.0

41.0

0.78

[23

Guar gum

0.76

3.18 ± 0.12

0.28 ± 0.01

0.26 ± 0.07

[24]

GalactoPol

0.49 0.85

0.25±0.003 1.61±0.022

0.02±0.001 0.09±0.005

0.79±0.020 0.74±0.015

[25] [26]

Biopolymer

522

Cross model λ (s)

m

References

n.a. – data not available

23

FucoPol

0.45 0.80 1.20 7.60

0.07 ± 0.002 0.44 ± 0.04 2.50 ± 0.07 48.9 ± 1.50

0.02 ± 0.002 0.10 ± 0.02 0.55 ± 0.05 0.90 ± 0.14

0.63 ± 0.03 0.68 ± 0.04 0.66 ± 0.02 0.67 ± 0.05

[20] [20] [20] This study

523 524

Table 2. Contact angles between the FucoPol solution and the different adherends (θ0 – initial

525

contact angle; θ4s – contact angle measured after 4 seconds). Material Balsa wood Corrugated cardboard Glass Cellulose acetate film

Contact angle Θ0 (°) 120.65 ± 7.65 117.40 ± 6.02 81.19 ± 6.49 106.43 ± 4.26

Θ4s (°) 70.87 ± 7.03 83.31 ± 4.03 45.74 ± 4.28 65.12 ± 5.55

526 527 528 529 530 531

24