Interplay Between Viscoelastic and Chemical Tunings in Fatty-Acid

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Interplay Between Viscoelastic and Chemical Tunings in Fatty-AcidBased Polyester Adhesives: Engineering Biomass toward Functionalized Step-Growth Polymers and Soft Networks Richard Vendamme,* Katrien Olaerts, Monica Gomes, Marc Degens, Takayuki Shigematsu, and Walter Eevers Nitto Denko Corporation’s European Research Committee (ERC), Nitto Europe NV, 22 Eikelaarstraat, 3600 Genk, Belgium S Supporting Information *

ABSTRACT: This Article describes the synthesis and characterization of renewable self-adhesive coatings with tunable viscoelastic properties and equipped with well-defined amounts of carboxylic acid “sticker” groups with adhesion promoting characteristics. Hydroxyl-ended polyesters with various architectures (linear, branched) were synthesized by melt polycondensation of dimerized fatty acids and fatty diols and then cured with maleic anhydride-modified triglycerides (such as maleinized soybean oil) in the presence of the amidine catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene. The curing reaction of alcoholysis has the dual effect of chain extending/cross-linking the base polymers via creation of polymeric half-esters linkages while introducing carboxylic acid functions within the gel structure. We demonstrated how the adhesion properties can be finely tuned from molecular design and formulation of the network precursors and how the rheology and functionality of the coatings influence the adhesive bond formation and development. These renewable polyester adhesives proved to be suitable materials for pressure-sensitive adhesives applications with respect to adhesion strength, viscoelasticity, and functionality. In addition, the environmental benefits of such materials are briefly discussed.

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INTRODUCTION Among the different classes of adhesives, pressure-sensitive adhesives (PSAs) are one of the most widespread types. Typically, PSAs must be able to form a bond upon a short contact with a substrate, that is, to establish molecular contact without any chemical reaction (even on a rough surface).1,2 Nowadays, self-adhesive tapes and labels are ubiquitous in everyday life and can be found virtually everywhere (electronic industry, medical products, automotive parts, etc.). PSAs are viscoelastic materials combining a liquid-like dissipative character necessary to form molecular contact under a light pressure and a solid-like character to resist macroscopic stress during the debonding phase.1 This combination of properties can be achieved with sparsely cross-linked polymer networks with a low glass-transition temperature. Fine-tuning of the network properties and of the gel content is used to control the adhesive performances. Besides this viscoelastic tuning, incorporation of functional groups into PSA is known to improve various properties such © 2012 American Chemical Society

as the bond strength and the cohesion of the adhesive join. In particular, carboxylic acid groups can improve the wetting onto the adherent surface and accelerate the rate of bond establishment (tack) via the formation of hydrogen bonding, noncovalent interactions, or both.3 Intra- and intermolecular hydrogen bonding interactions between carboxylic acid-functional polymer chains also induce remarkable modifications to the bulk properties of the adhesive (cohesion, viscoelasticity, etc.) and therefore influence its adhesive properties. The effect of carboxylic acid groups in acrylic-based adhesives obtained by copolymerization of acrylic esters with acrylic acid is welldocumented.4,5 Today the world is faced with major environmental challenges such as global warming, the increasing emissions of green house gases, and the recognized depletion of the earth Received: April 3, 2012 Revised: May 3, 2012 Published: May 21, 2012 1933

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Table 1. Linear and Branched Biobased Polyesters Prepared from a Dimer Fatty Acid (Pripol 1009), a Dimer Fatty Diol (Pripol 2033), and a 25% Dimer Acid/75% Trimer Acid Blend (Pripol 1040) polyester

feed composition (in g) Pripol 1009/1040/2033

COOH mol %a

P1 P2 P3 P4 P5 P6

133.94/-/191.06 140.45/-/184.55 146.96/-/178.04 150.20/-/174.80 153.45/-/171.55 156.70/-/168.30

0.400 0.420 0.440 0.450 0.460 0.470

P7 P8 P9 P10 P11

132.84/7.60/184.55 116.64/23.81/184.55 100.90/39.55/184.55 85.52/54.94/184.54 70.44/70.02/184.55

0.420 0.420 0.420 0.420 0.420

branching (μmol/g)b Linear Polyesters 12.15 12.15 12.14 12.14 12.14 12.14 Branched Polyesters 33.13 77.84 121.29 163.75 205.34

Mw (g/mol)

Mn (g/mol)

PDI

AV (mgKOH/ g)

9073 10872 15246 17298 19361 24699

4282 4658 6773 7594 6049 6380

2.12 2.33 2.25 2.28 3.20 3.87

0.01 0.01 0.02 0.03 0.14 0.07

40.52 33.04 25.58 19.46 15.33 11.36

(40.51) (32.45) (24.41) (20.36) (16.33) (12.29)

12040 17564 23089 29314 63846

4870 6391 6228 5759 8162

2.47 2.75 3.71 5.09 7.82

0.02 0.04 0.01 0.01 0.01

33.63 34.00 35.00 36.39 37.58

(32.67) (33.12) (33.56) (33.98) (34.41)

OHV (mgKOH/ g)c

a [COOH]/([COOH]+[OH]) molar ratio in the monomer mixture. bCalculated molar percentage of trifunctional monomers (COOH and OH functional) per gram of polyester. cAs measured by titration (and as calculated from the formulating ratio assuming a full conversion).

remarkable dual effects of modifying bulk viscoelastic properties of the adhesive while introducing carboxylic acid functions in the gel. This Article describes how the adhesive properties of such materials can be adjusted by fine-tuning the structure of the adhesive’s precursors, the formulation ratio, or the curing and aging conditions.

limited petroleum reserves. The growing awareness of society toward environmental issues combined with the recent governments regulations and incentives toward the reduction of carbon dioxide emissions is pushing the chemical industry to develop greener products and to find alternative growth strategies based on sustainable economical models.6 In this context, the use of raw materials derived from renewable feedstock seems a particularly relevant option.7 Biomass, which is created when solar energy and atmospheric carbon dioxide are stored as organic molecules through photosynthesis, emerges as one of the carbon sources of the future because it is abundant, renewable, and versatile. However, finding sustainable and efficient ways to transform biofeedstocks into highly functional materials and coatings is still very challenging.8−10 The design of functional biobased materials able to compete with their highly optimized petrochemical counterparts can be seen as the foundation for the successful development of the biorefinery concept.9 On the basis of the above statements, it is no surprise that the design of PSAs derived from renewable resources is currently attracting a lot of attention in several academic and industrial laboratories throughout the world.11−14 Fatty acid derivatives are an attractive resource for the development of biobased adhesives because of their intrinsically low glasstransition temperature and the ever-increasing range of chemical and biorefining operations available for plant oils.15,16 In the arena of paints and coatings, alkyd resins (i.e., polyester resins incorporating petrol-based carboxylic acids, anhydrides functional monomers, or both in combination with polyols and unsaturated “drying” fatty acids) are still the focus of intensive research, despite their long history.17 However, the alkyd technology does not allow the design of PSA with complex, and often contradictory, physicochemical properties. We report herein the design of self-adhesive coatings from plant oil renewable resources with tunable viscoelastic properties and equipped with carboxylic acid groups playing the role of adhesion promoters. Our design starts with the synthesis of biobased polyols from bulk polycondensation of dimer fatty acid, dimer fatty diol, and eventually a trifunctional branching agent. These polyols are then reacted with an anhydridemodified plant oil cross-linking agent, in the presence of a suitable catalyst. The alcoholysis curing reaction has the

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EXPERIMENTAL SECTION

Materials. The high-purity, fully hydrogenated dimer fatty acid and dimer fatty diol were obtained from Croda (Gouda, The Netherlands) under the trade name Pripol 1009 and Pripol 2033, respectively. A mixture containing 25% of dimerized fatty acids and 75% of trimerized fatty acids was also obtained from Croda (Pripol 1040). Titanium tetrabutoxide [Ti(O n Bu) 4 ], 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), soybean oil, and dodecanol were purchased from Aldrich. All chemicals were used as received, without further purification. The anhydride-functionalized triglycerides X1 were obtained from Vandeputte Oleochemicals (Mouscron, Belgium) and used as received. Polycondensation. Polymer P1 was synthesized according to the following protocol. Pripol 1009 (100 g, 176 mmol) and Pripol 2033 (142.65 g, 265 mmol) were weighted into a 1 L round-bottomed glass flange reactor. The reactor was fitted with a dean-stark apparatus mounted with an Allihn condenser to collect the condensation product and with a digital overhead stirrer. During the first part of the synthesis, the setup was continuously flushed with nitrogen to prevent oxidation and facilitate transport of the polycondensation byproduct (water). While stirring, the mixture was heated to 180 °C for 4 h using an electromantle connected to a temperature controller. The temperature was then increased to 200 °C, and vacuum processing (2mbar) was applied for 30 min. [Ti(OnBu)4] (0.02 mol % relative to the carboxylic acid functions) dissolved in xylene was added to the melt, and the reaction was continued under vacuum for 3 h at 220 °C. Finally, the polymer was cooled to 140 °C and discharged from the reactor. Other polymers were prepared according to similar procedures. Feed compositions and properties of polymers P1 to P12 are given in Table 1. Spectroscopic data of the hydroxy-ended fatty polyesters can be summarized as follows: FTIR (cm−1): 3464 (OH); 2925, 2854, 1462 (C−H); 1739 (CO);1246 (C−O). 1H NMR (CDCl3): δ 4.1 (t, 2H, COOCH2), 2.3 (m, 2 H, α-CH2 adjacent to ester group), 1.6 (m, 7 H, β-CH2 adjacent to ester group), 1.3 (m, 45H, −CH2−), 0.9 (m, 10 H, −CH3). Synthesis of Maleinized Triglycerides. The maleinized soybean oil X2 was synthesized following the experimental protocol of Wool et al.18 In short, soybean oil (250 g, 0.287 mol) was placed in a flask under N2 fitted with a condenser, magnetic stirrer, and thermometer. Maleic anhydride (23.5 g, 0.24 mol) was added, and the mixture was 1934

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heated to 180 °C. A second portion of maleic anhydride (23.5 g) was added after 1 h, and the temperature was increased to 195−200 °C. A slight overpressure in the reaction vessel prevented the partial sublimation of maleic anhydride. The last fraction of anhydride (23.5 g, 0.24 mol) was added, and the temperature was increased to 230 °C. After 3 h at 230 °C, the clear solution was cooled and discharged from the reactor. FTIR (cm−1): 3007, 725 (C−H); 2926, 2856, 1464 (C−H); 1862, 1783 (OCOCO); 1744 (C O); 1224, 919 (OOC-O−COO). 1H NMR (CDCl3): δ 5.4−5.2 (m, 7 H, HCC, α-CH, CH glycerol unit), 4.3 (dd, J = 11.7, 4.3 Hz, 2 H, −CH2− from glycerol unit), 4.1 (dd, J = 11.7, 5.9 Hz, 2 H, −CH2− from glycerol unit), 2.8 (m, 4 H, β-CH2 adjacent to double bond), 2.3 (m, 7 H, α-CH2 adjacent to ester group), 1.9 (m, 7 H, α-CH2 adjacent double bond), 1.6 (m, 6 H, β-CH2 adjacent to ester group), 1.3 (m, 61 H, −CH2−), 0.9 (m, 9 H, −CH3). Analytical Measurements. The average molecular weights of the fatty acid based polymers were determined by gel permeation chromatography (GPC) instrument (Waters model pump 515 and Waters 2414 refractive index detector) with styragel columns relative to polystyrene (PS) standards using tetrahydrofuran (THF) as eluent. Fourier transform infrared (FTIR) spectroscopy analysis of the polyester adhesives was performed using a Thermo Scientific Nicolet 8700 spectrometer. The acid and hydroxyl functionalities of the polymers were monitored via standard titration methods as described in the Supporting Information. Adhesives Formulation, Coating and Curing. Predetermined amounts of polyol and DBU were weighted with a microbalance. Toluene was then added to decrease the viscosity of the mixture and facilitate the coating step. (The final solid content of the formulations was fixed at 80 wt %.) Once this initial solution was well mixed, the required amount of maleinized oil was further added and homogenized. The adhesive formulations are always designated by the codes Pα-β%Xδ, where Pα refers to the base polymer (Table 1), β corresponds to the wt % of maleinized oil in the formulation (relative to the weight of base polymer Pα), and Xδ refers to the anhydride compound (Table 2).

independent of strain). Frequency sweep measurements were performed from 0.01 to 100 Hz. Creep tests were obtained in a similar geometry (⌀20 mm, gap 0.6 mm). A shear stress of 1000 Pa was instantaneously applied to the gel, and the resultant strain was monitored as a function of time. After 10 min of creep, the shear stress was removed while the strain was still monitored for 10 min. Tensile Testing. Tensile tests were conducted on a Zwick Z005 testing machine with a speed of 300 mm/min. The initial distance between grips was 10 mm, and the initial cross-section of the cured adhesive sheets was 10 × 0.8 mm. Three tests per specimens have been analyzed. Adhesion Force. The adhesive strength has been investigated with a 180° peel test. First, a 20 ± 1 μm thick layer of biobased glue (dry thickness) was laminated and cured on a 50 μm thick layer, as described above. Then, 2 cm wide, 10 cm long stripes of adhesive tapes were cut and placed on a table (adhesive face on top). Half of the tape was covered with a second 50 μm-thick PET film. This assembly was turned upside down, and the remaining free adhesive zone was applied on the reference surface. A 2 kg cylinder was rolled twice on the tape, and a dwell time of 15 min was observed. Peel tests were conducted on a Zwick Z005 testing machine at a constant peeling speed of 300 mm/ min. We defined the initial adhesion as the peel force measured after a dwell time of 15 min from application of the tape onto the substrate. Failure mode of the adhesives was investigated under an optical microscope (Olympus TH3) equipped with a digital camera.

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RESULTS AND DISCUSSION Synthesis of the Fatty Acid-Based Prepolymers. Polyesters with varying molecular weight and branching degrees have been prepared via bulk polycondensation of dimer fatty acids, dimer fatty diols, and eventually trimer fatty acids. Figure 1 shows the structure of a dimer acid (Figure 1B) and trimer acid (Figure 1D) that can be obtained by a Diels− Alder reaction of linoleic acid (figure 1A). Because the reaction is a step-growth polymerization, a statistical distribution of chain lengths is obtained, with average molecular weight and chain end groups controlled by monomer stoichiometry. Because the target was hydroxyl-functional polymers, a welldefined excess of the diol was used. As shown on Table 1, precise control of the stoichiometry is essential for controlling polymer molecular weight. For linear polyesters (polymers P1 to P6), molecular weight increases as the ratio of diol to diacid monomers nears the stoichiometric equivalence. Polymers P7 to P11 were prepared with an almost equal stoichiometric imbalance of diol to diacid, but the addition of an increasing amount of trimer acid leads to polymers with a wide range of branching degrees. Increasing the concentration of branching agent is associated with an increase in Mw and a widening of the polydispersity index, in accordance with Carothers theory of polycondensation.19 The extent of polymerization and conversion of acid groups were monitored via GPC and end-groups titration (Table 1). The acid values of all polymers were negligible, confirming that the polymerization proceeds to a point at which the acid reactant is completely used up and all of the chain ends possess the same functional hydroxy group in excess. Theoretical OH values have been calculated, assuming a full conversion of the acid groups into esters and taking into account the mass loss resulting from the removal of the reaction byproduct (water). The very good correlation between experimental and calculated hydroxyl values confirms that a full conversion is reached under these reaction conditions and that there is no preferential loss of one of the monomers. Synthesis of the Anhydride-Functionalized Triglyceride. Thermal reaction of maleic anhydride with the unsaturated

Table 2. Characterization of the Maleic Anhydride-Modified Natural Oil Triglycerides item

base oil

AV (mgKOH/g)

ANHV (mgKOH/g)a

anhydride per triglycerideb

X1 X2

linseed soybean

143.85 224.40

71.93 117.20

1.35 (±0.1) 2.50 (±0.1)

a Anhyride value (ANHV) = avid value (AV)/2. bAverage number of maleic anhydride groups per triglyceride.

Coating and curing of the resin were made as follows. A 40 μm adhesive layer (wet thickness) was roll-coated manually with an approximate speed of 5 cm/s on a 50 μm thick polyethylene terephtalate (PET) base film. This bilayer stack was cured in a thermal oven (Heraeus LUT 6050) at 130 °C during 20 min. Dry thickness of adhesive layers was 20 ± 1 μm. After curing, the free adhesive surfaces were covered with an antiadhesive siliconized paper to prevent contamination of the adhesive prior to characterization. Gel Content. Around 0.2 g of cured adhesive was packed into a sealed pocket made of a porous, folded PTFE membrane (Temish membrane from Nitto Denko). After 2 weeks of immersion in a large excess of toluene, the dialysis bag is removed and dried. The gel content (gel %) of the biobased adhesives is given by the equation gel % = 100·(wadh − wsol) ÷ wadh, where wadh is the initial weight of sample (before dialysis), (wadh − wsol) is the weight of sample after extraction, which refers to the gel (cross-linked) fraction, and wsol is the lost weight during extraction. The gel content was determined as the average of two measurements. Rheology. Rheological profile was monitored at 25 °C with an AR 1000 rheometer (TA Instruments) in parallel plate geometry (20 mm diameter and 0.6 mm gap) in dynamic mode with a strain of 0.01 (within the viscoelastic regime of the adhesives, that is, with G′ and G″ 1935

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Figure 1. Molecular structures of (A) 9,11-linoleic acid (the precursor of the dimer acid), (B) the vegetable oil dimer fatty acid Pripol 1009, (C) the dimer diol Pripol 2033, (D) the trimer acid Pripol 1040, (E) titanium(IV) tetrabutoxide, (F) an hypothetical triglyceride that have been modified with maleic anhydride via an ene-type reaction (i.e., the two upper maleate groups) and via a Diels−Alder mechanism (lower maleate group), and (G) 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).

Figure 2. Proposed reaction mechanism for the organo-catalytic alcoholysis between fatty acid-based polyols and maleinized oils in the presence of the superbase DBU.

fatty acids of soybean oil was carried out at 230 °C and monitored by FT-IR (Figure S1 of the Supporting Information). The carbonyl zone of virgin natural oil is characterized by a single carboxyl adsorption band at 1745 cm−1 (ester CO, stretch). After reaction with maleic anhydride, two new carbonyl adsorption bands appear at 1864 (anhydride CO, antisymmetric stretch) and 1783 cm−1 (anhydride CO, symmetric stretch), together with the typical C−O stretching band of cyclic anhydrides at 916 cm−1. A hypothetical structure of maleinized soybean oil is shown on Figure 1F. The two maleinized triglycerides used in this study were further characterized by titration. Maleinized oils were first dissolved in a mixture of pyridine and water and hydrolyzed at 100 °C for 15 min, and the acid values were measured as described in the Experimental Section. Formulation Procedure. The fatty-acid-based polyol was cured with the anhydride-functional plant oils via an alcoholysis reaction (i.e., addition esterification), which can be catalyzed by tertiary amines. DBU is an amidine compound with a stronger basicity than standard tertiary amines such as triethylamine or 2-ethyl imidazole. The catalytic efficiency of DBU in the production of biodesel (alcoholysis reaction of natural triglycerides with low-molecular-weight alcohols such as methanol) has been reported.20 Besides its strong basicity, DBU was also chosen here because it is a liquid compound at room temperature and thus can be easily mixed with the adhesive precursors.

It was observed that DBU reacts rapidly with cyclic anhydride groups to form a reddish product. As a result, the formulation sequence Polyol → maleinized oil → DBU always resulted in the formation of small solid particles that cannot be easily extracted from the mixture. Interestingly, Webster et al. reported very recently a similar observation during the preparation of high biobased content epoxy-anhydride thermosets prepared from epoxidized sucrose esters and catalyzed by DBU.21 Therefore, the mixing sequence Polyol → DBU → (solvent) → maleinized oil, which always results in homogeneous solutions, was adopted. Upon adding the maleinized cross-linker to the polymer/DBU mixture, the viscosity of the mixture slightly increased and the formulation became faintly orange. To facilitate the coating of the formulations on the base film, we always added a small amount of solvent (toluene) to decrease the viscosity of the mixture. (The solid base of all formulations was fixed at 80% throughout this work.) Curing Reaction Mechanism and Network Development. Alcoholysis is a nucleophilic reaction that preferentially takes place via the formation of the semiproduct A* (Figure 2) in the presence of a tertiary amine such as DBU due to the increase in the partial charge on the carbon neighboring the nitrogen atom.22 Reaction of DBU with the maleinized oils undoubtedly takes place in our system. Evidence of this is the faintly orange coloration appearing when the two substances 1936

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are mixed. Neither maleinized plant oils nor DBU absorb in the visible part of the spectrum. Representative FT-IR spectra of the formulation P5-50%X2 before and after curing are shown on Figure 3. In the uncured

Figure 3. Overlaid FTIR spectra of P5-50%X2 adhesive before curing, after curing at 130 °C for 20 min, and after aging.

Figure 4. (A) Schematic representation of maleinized soybean oil (mixture of ditopic and tritopic anhydride-functional molecules). (B− G) Possible macromolecular clusters resulting from the alcoholysis of polyols and maleinized plant oils. The function of the clusters in a cross-linked gel is also indicated: end-chains (B,C), chain extensions (D,E), or branching/cross-linking points (F,G). The hollow (○) and filled (●) circles represent ester and carboxyl functions, respectively. Black arrowheads indicate continuation of the cross-linked gel matrix.

formulation, the carbonyl region is composed of the four peaks 1864, 1783 (both corresponding to the anhydride groups), and 1747 cm−1 (related to the ester bonds of soybean oil and of the polyester-polyol). After curing at 130 °C for 20 min, a sharp decrease in the anhydride adsorption bands at 1864, 1783, and 916 cm−1 is observed, denoting that the curing reaction effectively takes place under these experimental conditions. At the same time, a shoulder appears on the right side of the carbonyl band of the ester peak. This band (1710 cm−1) corresponds to the carbonyl group of the carboxylic acid ester, supporting the cross-linking reaction between one hydroxyl end group and the anhydride moieties to form a polymeric halfester linkage and a free carboxylic acid group. Because the formulation P5-50%X2 has been formulated with an excess of anhydride relative to hydroxyl functions, anhydride bands are still clearly visible after 20 min of curing. Increasing the curing time to 2 h did not modify the intensity the residual anhydride bands, suggesting that after 20 min at 130 °C all hydroxyl functions have been consumed and prevent the curing from proceeding further. It was also observed that the residual anhydride groups tend to hydrolyze upon aging, and after 2 months at room temperature the anhydride bands could not be detected anymore (Figure 3). From a mechanistic point of view, FT-IR suggests that alcoholysis is the main curing mechanism. However, other possibilities may also be involved such as acidolysis, transesterification, and esterification resulting from the condensation of hydroxyl and carboxylic acid functionalities. The drying temperature (130 °C) was mainly chosen to get fast evaporation of the solvent. However, high temperatures are not necessary to get gelation. Curing also takes placed at room temperature. For instance, formulations that were left at room temperature for a few days were cured to the same extent as formulations cured at 130 °C for 1 h. This finding opens interesting opportunities for low-temperature adhesive systems, requiring less energy than conventional high-temperature curing processes. Figure 4 schematically illustrates this tandem functionalization-curing approach. Cross-linker X2 has an average of 2.5 anhydride groups per triglyceride and can be conceptually represented as a mixture of tritopic and ditopic molecules

(although other species should be present in the product, such as mono- or tetra-functionalized triglycerides). Depending on the initial anhydride/hydroxyl ratio and the extent of curing reaction, a wide variety of macromolecular entities will be created. For instance, a ditopic maleinized oil reacting with two linear polyols will have a chain-extending effect and introduce two acid groups at the chain extending site (Figure 4E). A tritopic anhydride compound reacting with three linear polyols will have a branching effect while introducing three carboxyl groups at the branching point (Figure 4F). A tritopic anhydride compound reacting with only two linear polyols will have a chain-extending effect, introduce two carboxyl groups, and leave one residual anhydride moiety unreacted (Figure 4D). Considering that this residual anhydride group could be further hydrolyzed, the chain-extending unit Figure 4D could even bear a total of four carboxyl units. It is possible to decouple the chain-extending/branching effects from the formation of carboxylic acid groups by using branched polyols. For instance, a ditopic maleinized oil reacting with one linear polyol and one branched polyol will create a branching point (Figure 4G) while introducing only two acid groups. Above a critical conversion threshold (and if the stoichiometry allows it), some of these macromolecular clusters will percolate to form a macroscopic polymeric gel.23 Because of the randomness of the chain-extending, branching, and crosslinking processes, it was anticipated that the obtained networks would be relatively loosely defined with structural defects (dangling chains, loops, etc.).24 Far from being a drawback, this macromolecular diversity is very useful for the design of adhesives because good PSAs should possess a number of relaxation modes with widely different characteristic times and strengths. It was also expected that the gel and sol fractions of the cured adhesives could be manipulated by tuning the stoichiometry of the networks precursors. 1937

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Influence of Stoichiometry on the Gel Structure. Gelation of step-growth macromolecular networks depends on several parameters such as the stoichiometry of the adhesive’s precursors (i.e., the hydroxyl/anhydride molar ratio), the functionalities of the prepolymer and cross-linker, as well as the conversion and conditions of the curing reaction. The level of conversion pgel at which gelation should occur in the system can be estimated from eq 1 pgel 2 = [r ·(f − 1) ·(g − 1)]−1

limit the extent of curing achievable by the networks. Finally, side reactions of the cross-linking process may also be envisaged. For instance, anhydride ring-opening with moisture (from the oven) could reduce the effective functionality of the cross-linker and thus modify the overall stoichiometry of the system. Control over the sol/gel content is a key parameter for the design of functional PSA because it is related to the ability of the material to dissipate energy through deformation. It was found that the gel contents of the adhesives reported herein could be manipulated from the molecular weights of the polymers, the functionality of the anhydride cross-linker, as well as the formulation ratio. Higher gel contents (above 80%) were obtained for adhesives formulated from the branched polyols such as P10 or P11. For a comparison purpose, multipurpose commercial PSAs typically contain about 50−70% of insoluble fraction (in good agreement with some of the formulations presented in Figure 5), and certain labels used as surface protection films need to have very low adhesion power and are formulated with much higher gel content (up to 90% of insoluble fraction). Although this Article mainly focuses on coatings prepared with the cross-linker X2, it is interesting to note that useful PSA can also be obtained with cross-linker X1, which possesses only 1.4 anhydride groups per triglyceride in average. Although adhesives formulated from linear polymers and X1 displayed rather low gel content ( 10 Hz), all samples display an elastic behavior (G′ > G″), and the G′ curves tend to superimpose, irrelevantly of X2 amount. In fact, the stress relaxation at high frequency is mainly controlled by short-range chain motions, which are expected to be independent of the molecular weight between cross-links. In the low (ω < 0.01 Hz) and intermediate (0.01 Hz < ω < 10 Hz) frequency zones, the rheological profile is strongly influenced by X2 amount. The network with the highest gel content (P5-30%X2) exhibits an elastic response (G′ > G″) over the whole frequency range. Deviation from this optimum formulation lowers the networks gel contents and cross-linking densities (Figure 5). The gels P5-25%X2 and P5-40%X2 deviate only slightly from the optimal stoichiometry and still demonstrate a characteristic elastic response comparable to P530%X2 (although with slightly lower G′ values at low frequencies). Further discrepancies toward the optimal cross-linking stoichiometry induce more profound modifications of the rheological profiles. For instance, networks P5-20%X2 and P550%X2 are characterized by an elastic behavior (G′ > G″) over the whole frequency range but also by G″ values only slightly

lower than G′ (G′ ≈ G″) at low frequencies. Experimentally, it was found that these samples are stickier to the touch (tacky). In dynamic mechanical analysis, softer gels usually demonstrate a stronger frequency dependence than harder ones; therefore, the slope of G′ as a function of oscillation frequency is a good indication of the cross-linking degree of an adhesive. In Figure 7A, the slope of G′ = f(ω) is higher for the networks P5-20%X2 and P5-50%X2 than for the network P5-30%X2, indicating that these two networks display a stronger liquid-like character at low frequency. The loosely cross-linked networks P5-17.5%X2 and P5-60%X2 display crossovers between G′ and G″ at 2 and 0.8 Hz, respectively, and are therefore characterized by viscous behaviors at low frequency (G″ > G′). Finally, the linear frequency dependence of G′ observed with the formulation P570%X2 is typical for linear (or branched) flexible polymers without cross-linking. Because none of the P5-X2 specimens show G′ plateau values at low frequencies, their structures should be considered as soft networks. The use of a Cole−Cole plot can help to clarify further the viscoelastic properties of the P5-X2 adhesives, as shown in Figure 7B. In this graph, the loss modulus G″ is plotted over the storage modulus G′ as a function of the oscillation frequency. When the material is located on the right side of the equimoduli line (G′ = G″), it has a higher degree of elastic behavior. Data on the left of the equi-moduli limit are characteristic of viscous materials. The gels with the higher cross-linking density such as P5-30%X2 always remain well within the elastic side of the graph. The loosely cross-linked gels (P5-17%X2 and P560%X2) present a crossover with the equi-moduli line, denoting their viscous character at low frequency. Figure 7B also highlights the peculiar rheological profiles of P5-20%X2 1939

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and P5-50%X2, with their G″ = f(G′) curves almost superimposing with the equi-moduli line (but still slightly remaining on the elastic side of the graph). Creep and Tensile Properties. In addition to the property of adhesiveness, useful pressure-sensitive adhesive must also possess an elastic cohesiveness and a resistance to flow under shear (known as resistance to creep).32,33 In Figure 8, a model

Figure 9. Representative tensile curves for the networks P5-17.5%X2, P5-20%X2, and P5-30%X2.

Adhesion Performances. Adhesion forces of the P5-β%X2 adhesives coated on a PET film have been investigated by a 180° peel test on BA Steel and polypropylene (PP) after a dwell time of 15 min. The peel test determines the force necessary to tear a 20 mm-wide strip of tape from a solid substrate at a constant speed, and is expressed herein in cN/20 mm. Other authors have performed the same type of tests with different tape widths, for instance, 10 mm in refs 11 and 12 or 25 mm in ref 13, and report adhesion values accordingly normalized (i.e., N/10 mm and N/25 mm, respectively). All adhesion results reported in this contribution can be critically compared with the literature values by normalizing the reported peel forces with the specific geometries employed (for instance, 500 cN/20 mm = 2.5 N/cm = 6.25 N/25 mm). The influence of the X2 ratio on the initial peel force is reported in Figure 10A,B for BA steel and PP, respectively. In addition, manual 180° peel tests have been performed under an optical microscope to record the failure mode of each formulation (Figure 11). At low cross-linking level (P5-17% X2), the adhesive behaves like a viscous liquid and does not possess enough cohesiveness to sustain the mechanical stress of the peeling process, and therefore it fails cohesively. Increasing the X2 amount to 20% leads to a considerable increase in adhesion force of more than 1000 cN/20 mm (Figure 10). The failure is still slightly cohesive on steel (very few sticky patches can be observed on the steel substrate after peel) but becomes interfacial on PP. It is generally agreed that the transition from cohesive to interfacial failure modes is associated with the transition from liquid-like viscous to rubber-like elastic behavior.1 As we previously observed in Figure 7B, the gel P5-20%X2 shows indeed a very good balance between elastic (G′) and viscous (G″) properties. The adhesive bond is difficult to break near this transition from cohesive to interfacial failure because the adhesive exhibits considerable fibrillation and a large deformation is required to cause debonding. Although P520%X2 has a soft response during the slow bonding process onto the substrate so that the polymer chain mobility and wettability become high enough to maximize rapidly the contact, this adhesive is still not enough resistant to external stress. Further increasing X2 ratio to 22.5% results in an adhesive that fails in an interfacial mode with a peel strength of 400 cN/ 20 mm on steel. Interestingly, small interfacial fibrils are observed (Figure 11A), and typical holes resulting from the

Figure 8. Creep test (strain as a function of time) for P5-β%X2 adhesives.

creep test has been done for the system P5-X2 with various cross-linking levels. The extent of strain after applying a stress for 10 min is the lowest for the system P5-30%X4, which confirms again the solid-like character of this network. The strain percentage of recovery after creep is related to the density of elastically active chain of the network (G′) whether the lost percentage is related to the energy dissipated (in the form of heat) by the system during the creep phase. The systems P5-20%X2 and P5-50%X2 show again similar profiles during both the creep and recovery phases. An interesting difference is observed between the two loosely networks P517%X2 and P5-60%X2. Although P5-17%X2 displays a high percentage of recovery after creep typical of viscoelastic polymers, P5-60%X2 shows a more viscous character with a low recovery. Figure 8 also confirms that sample P5-70%X2 is not cross-linked at all (very high creep without any recovery). Although the small-strain rheological profile describes the time-dependent relaxation of the networks, it does not provide any information on their large-strain (nonlinear) mechanical behavior occurring during the debonding phase.34 Therefore, tensile experiments are useful to provide this information. From Figure 9, the specimen P5-30%X2 shows the higher strain hardening effect of all samples. P5-20%X2 also strain-hardens to a lower extent, whereas P5-17.5%X2 behaves as a viscous gel. Strain hardening at high level of strain is a useful property in PSA and is generally related to the requirement of the adhesive layer to fail without leaving a sticky residue on the surfaces.35 The above structural and mechanical analyses demonstrate that the P5-β%X2 networks are viscoelastic materials exhibiting both viscous and elastic responses to stress. Interestingly, all P5-β%X2 gels display G′ values inferior to 105Pa at 1 Hz and therefore satisfy with the Dahlquist’s criterion of stickiness.36,37 The rheological study confirms that these fatty acid-based elastomers are good candidates for pressure-sensitive adhesives and suggests that these different rheological behaviors would induce different adhesion and cohesion strengths. 1940

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Figure 10. (A,B) Initial adhesion forces (i.e., adhesion after 15 min dwell) for the system P5-β%X2 on steel (A) and on PP (B). (C,D) Initial adhesion forces for the system Pα-25%X2 (with α = 1 to 6) on Steel (C) and on PP (D). (E,F) Initial adhesion forces for the system Pα-30%X2 (with α = 7 to 11) on Steel (E) and on PP (F). Polymers P7, P8, P9, P10, and P11 have similar OH values but increasing branching degrees. (See Table 1 for details.) The stars (★) indicate a cohesive failure.

50%X2 shows a rather high adhesion level (500 cN/20 mm) while preserving an interfacial failure mode. At 60%X2, a new failure mode transition is observed, this time from interfacial to cohesive. Again, this transition is characterized by very large macroscopic fibrils (Figure 11C) and by a high peel force of 1300 cN/20 mm. Finally, the sample P5-70%X2 fails cohesively on steel with a peel force of 800 cN/20 mm. Coupling versus Decoupling between Viscoelastic and Chemical Tunings: Implications on the Adhesion Force. Figure 10A demonstrates how the adhesion of Pα-β% Xδ adhesives can be manipulated by changing the cross-linker ratio β while keeping α and δ unchanged. In that case, the viscoelastic and chemical tunings are intrinsically coupled (cf. Figure 5 and 6). In a similar manner, varying the hydroxyl values of the base-polymer (Pα) while keeping the cross-linker ratio constant (β%) also creates a tandem modulation of the mechanical and functional properties of the adhesives because the anhydride-hydroxyl ratio of the network precursors has been altered. For instance, Figure 10C displays the adhesion forces of various Pα-25%X2 adhesives prepared from different linear polymers. We have investigated how tuning of the gel content could be decoupled from the formation of carboxylic acid groups. Figure 10E,F displays the adhesion strength for the adhesives Pα-30% X2 on steel and on PP, respectively. In this case, the base polymers had similar OH values, the resulting coatings were prepared from stoichiometric amounts of anhydride and hydroxyl functions (1:1 molar ratio), and we anticipated that all the adhesives would present very similar acid values. This assumption was supported by FTIR observations (Figure S2 of the Supporting Information), as the relative intensity of the carbonyl band from the carboxylic acid of the adhesives P8-30% X2, P9-30%X2, and P11-30%X2 remains very similar. However, because of the very different branching degrees of the base polymers employed (P7 to P11), the dynamic mechanical properties and the gel content of the resulting coatings were very different. The adhesive P7-30%X2 has a pronounced viscous character and fails cohesively on both steel and PP (P7

Figure 11. Failure mode of P5-22.5%X2 (A,D,G), P5-30%X2 (B,E,H), and P5-60%X2 (C,F,I) from BA steel surfaces. The first row (pictures A−C) shows side-views of the peeling test. The second (D−F) and third (G−I) rows represent top-view images of the adhesive and substrate surfaces after peeling, respectively. The scale on panel I is valid for all micrographs. c.Fib: cohesive fibrils; i.Fib: interfacial fibrils; Cavi.: cavitation patterns; d.Wet: “dewetting-like” patterns. The star (★) indicates cohesive failure.

cavitation process associated with the debonding can be clearly observed on the adhesive surface after peel (Figure 11D). The adhesives cross-linked at or near the optimal stoichiometric conditions (P5-30%X2 and P5-40%X2) display an interfacial failure on both substrates and are associated with the lowest adhesion levels (∼200 cN/20 mm). In fact, P5-30% X2 is the most elastic sample, and its adhesion hysteresis is not large enough to cause the formation of fibrils. After a dwell time of 15 min, P5-30%X2 is characterized by a “soft” debonding process without the formation of cavitation holes but instead with the appearance of dewetting-like patterns (Figure 11E). Above the optimal cross-linking conditions, a further increase in X2 concentration results in an opposite scenario. Sample P51941

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Figure 12. (A,B) Initial adhesion and adhesion after aging (2 days at room temperature) for the adhesives P5-22.5%X2 and P5-40%X2, respectively. PC: polycarbonate; PS: polystyrene; ABS: acrylonitrile butadiene styrene; PMMA: poly(methyl methacrylate); PVC: poly(vinyl chloride); PP: polypropylene. The star (★) indicates a cohesive failure.

branching degree = 33.13 μmol/g). When the branching degree of the polymers is increased to 77 μmol/g (polymer P8), the adhesive fails in an interfacial mode and with peel strength of 200 cN/20 mm. Further increase in the branching degrees of the base polymer enhances the overall cross-linking density of the adhesives, which is translated by a gradual decrease in the peel strength. Ultimately, P11-30%X2 (P11 branching degree = 205 μmol/g) has a very low tack and an adhesion force below 50 cN/20 mm on steel. Molecular Development of the Adhesive Bond. An interesting feature of these biobased adhesive is the possibility to tune the functionality of carboxylic acid groups in the polyester networks. Such functional groups are known to improve the strength of adhesive joins, and to support this idea, investigations of the adhesion strength of samples P5-22.5%X2 and P5-40%X2 were performed on several substrates with different polarity and for different aging times. These two networks had rather similar gel contents (Figure 5) as well as rheological behaviors (Figure 7) but differ significantly in functionality (Figure 6). After a dwell period of 2 days at room temperature, P5-22.5% X2 shows a significant increase in its adhesion force on all substrates (Figure 12A) (notably a 228% adhesion increase on steel). Although P5-40%X2 initially displays slightly lower adhesion levels than P5-22.5%X2, its adhesion force dramatically increase on all substrates after 2 days (up to 725% increase on steel, Figure 12B). It is instructive to note that on a polar substrate such as steel the most polar adhesive (P5-40% X2) shows the higher adhesion level associated with a cohesive failure (denoting that the work of adhesion has overcome the cohesive energy of the network). On nonpolar substrates such as PP, the adhesive with the lowest carboxylic acid functionality (P5-22.5%X2) shows a better adhesion. From the above evidence, it is assumed that the reorganization of the molecular structure near the interface, and especially the progressive formation of hydrogen bonds or acid−base interactions between the carbonyl functions of the adhesive and the substrates, is responsible for the peculiar adhesive behaviors of P5-40%X2. This statement is illustrated on Figure S5 of the Supporting Information, where the peel curves and the corresponding work of adhesion of P5-40%X2 on ABS are shown. The adhesive bond development represented by the work of adhesion increases as the dwell time increases. Establishment and development of the adhesive bond can be summarized in the following scenario: for short contact times, the peel forces can be nicely correlated with the rheological behavior of the networks, and the only interfacial forces active

should be weak interactions such as van der Waals forces. For longer contact times, the overall adhesion mechanism becomes rather complex and is the result of a delicate balance between the viscoelastic character of the adhesive and the increased interfacial energy resulting from macromolecular rearrangements and the development of a 2D network of hydrogen bonds near the adhesive/substrate interface. The time dependence of the adhesion presented in Figure 12 and Figure S5 of the Supporting Information is reminiscent of the research conducted by Lee and Gong et al. using model carboxylated poly(butadiene) and can be interpreted based on a surface restructuring model of the PSA layer in contact with the solid substrate.38−41 In this model, the adhesion energy ψ between the polymer and the solid substrate can be expressed by eq 2, where ϕX is the mole fraction of carboxyl sticker groups X in the polymer, ϕY is the surface coverage of receptor groups Y on the substrate, and χ is the strength of the X−Y interactions (acid−base type and/or H-bonding type).38−41 ψ ≈ χ ·ϕX ·ϕY

(2)

It was found that the adhesive strength and the failure mode (cohesive or interfacial) depend on the optimal values of sticker and receptor groups. The polymer−solid interface restructuring model of Gong et al. and Lee et al. has been further extended by Wool and Bunker with a percolation model of entanglements to determine the optimal sticker group concentration ϕ*X (which was found to be ∼1 mol %).42 Remarkable agreement was observed between the experimentally observed and the calculated ϕ*X values. These insightful surface-restructuring models mentioned above38−42 may be used to design rationally and further optimize the adhesion performance of the fatty acid-based polyester adhesives described in the present contribution. Biobased Content and Sustainability Assessment. The emerging biobased manufacturing industry is producing large numbers of products that contain mixtures of both biobased materials and petroleum-derived materials. Therefore, the biobased content is becoming an increasingly important technological and economical tool. The biobased content (BIO%) is defined as the percentage of carbon in a product that is derived from biobased materials, relative to the total amount of carbon (bio plus fossil) in the product (i.e., BIO% = [Cbio]/ [Cbio + Cpetro]), and is conveniently determined by radiocarbon analysis.43 The dimer acids are derived from renewable oleic and linoleic acids (themselves extracted from rapeseed, sunflower, or pine tree oils) and are therefore 100% biobased. DBU is a fully petrochemical-based compound. On the other end, the 1942

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properties. Most importantly, the adhesive properties can be controlled and tuned by careful selection of the networks precursors. The establishment of a 2D network of hydrogen bonds at the adhesive−substrate interface significantly enhances the work of adhesion upon aging. Besides these good technical performances, the adhesive ingredients are widely available from biomass and rather cheap, creating a cost-effective valuechain among agriculture, biorefining, and the adhesive industry. The outlook for the synthesis of renewable functionalized polyesters seems to be promising due to an ever-increasing demand for versatile materials with lower environmental footprint, better ecocompatibility, and potential biodegradability.47 Applications of renewable polyester-based adhesives are not restricted to the industrial arena but can be extended to a variety of biomedical applications requiring a careful control over polymers elasticity, stickiness, biofunctionality, and bioresorption. In fact, functionalized polyesters are now attracting attention for several applications in medicine ranging from surgical sealants and glues, pharmacological patches, to wound healing devices.48−51

maleinized triglycerides are hybrid compounds containing a biobased core and a petrochemical fraction (maleic anhydride). The biobased content of the polyol-maleinized oils−DBU networks can be estimated from the elemental composition of the individual compounds on a weight basis, the nature of the carbon atoms (bio vs fossil) within each constituent, and the weight fraction of each compound in the formulation. For instance, DBU (C9H16N2) is 71% carbon-based on a weight basis ([12 × 9]/[12 × 9 + 14 × 2 + 1 × 16] = 0.71), 100% of DBU’s carbon is derived from petrochemicals, and DBU represents 1 wt % of the total adhesive weight. When only carbon atoms are considered in the system by weight, the theoretical biobased content of our adhesives can be expressed by eq 3, where x is the weight fraction of maleinized oil (relative to the polymer weight), y is the average number of maleic anhydride groups per triglycerides, and z is the wt % of DBU in the formulation (relative to the total adhesive weight): 82.4575 + BIO% = 0.824575 + x·

(

(

68460.99·x 88457.39 + 9902.48·y

684.6099 + 48.0428·y 88457.39 + 9902.48·y

)

■

) + 0.00710647·(1 + )·z x 100

(3)

ASSOCIATED CONTENT

S Supporting Information *

Equation 3 reveals that the biobased content of our renewable adhesives is very high (between 90 and 98% depending on the compositions). In the long term, the use of a biomass-derived maleic anhydride (for instance, obtained by dehydration of renewable succinic acid)44 may further increase the biobased content to 99%+. Preliminary life-cycle analysis (LCA) investigations conducted on our fatty acid-based adhesives tapes reveals that these biobased PSAs have a significantly lower impact on the environment than conventional acrylic tapes or even natural rubber-based PSAs. Typical advantages of the plant oil-derived PSA are: lower energy consumption, lower global warming potential, lower carbon footprint (up to four times lower than acrylics), and lower depletion of nonrenewable resources (crude oil, hard coal, natural gas, etc.). The environmental footprint of biobased adhesive tapes is further improved by laminating thin layers of the biobased glue onto Bioplastic films such as PLA, thus creating a fully renewable tape construction. Another advantage of the technology described herein lies with the end-of life of the adhesive tapes and especially with their potential biodegradability. In fact, our plant-derived adhesive gels are fully built upon ester linkages (Figure 4), which are known to degrade hydrolytically under various environments such as industrial composting.45,46 Conventional acrylics or polyolefins-based PSA do not degrade and contribute to the increasing pollution of natural and marine environments. Biodegradability of our fatty acid-based adhesives is currently investigated and will be reported in due time.

Experimental protocols for the end-group titrations (acid value and hydroxyl value) and for the synthesis of cross-linker X2; overlaid FT-IR spectra of pure soybean oil, and of the crosslinkers X1 and X2; overlaid FT-IR spectra of adhesives P8-30% X2, P9-30%X2, P10-30%X2; experimental acid values for the model systems dodecanol-X1 and dodecanol-X2 plotted as a function of the anhydride molar ratio and extrapolation for P5X2 cross-linked adhesives; and optical micrograph of a tape made of a 20 μm-thick layer of P5-30%X2 biobased adhesive coated on a PET film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +32-(0)89-36 04 95. Fax +32-(0)89-36 22 42. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). We are indebted to Croda for providing us with the dimerized fatty acid and especially to Angela Smits, Johan Harmsen, and Hans Ridderikhoff for the interesting meetings. We acknowledge Satoshi Tanigawa, Tetsuo Inoue, Hitoshi Takahira, Satomi Yoshie, Shigeki Ishiguro, Hiroki Senda, and Tetsuya Iwai (Nitto Denko Corporation, Japan) for insightful discussions on adhesive design. The management of Nitto Denko is acknowledged for authorizing the publication of this work.

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CONCLUSIONS Several hydroxy-functional polyesters based on renewable resources were synthesized via bulk polycondensation of diacids and diols. Adhesive gels were formulated from these polyols in combination with maleinized triglycerides. This synthetic strategy takes full advantage of the Diels−Alder reaction (for the dimerization of unsaturated fatty acids and the thermal grafting of maleic anhydride on the plant oil) and of esterification processes (condensation esterification for the polyol synthesis and addition esterification during the curing step). Cured coatings demonstrate very promising adhesion

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