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Article Green Polyurethanes from Renewable and Biobased White Dextrins

Jakob Konieczny 1,2 and Katja Loos 1,2,*

1 Macromolecular Chemistry and New Polymeric Materials, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands; [email protected] 2 Dutch Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands * Correspondence: [email protected]; Tel.: +31-50-3636867

 Received: 11 September 2018; Accepted: 31 January 2019; Published: 3 February 2019 

Abstract: Polyurethanes (PUs) are an important class of polymers due to their low density and thermal conductivity combined with their interesting mechanical properties—they are extensively used as thermal and sound insulators, as well as structural and comfort materials. Despite the broad range of applications, the production of PUs is still highly petroleum-dependent. The use of carbohydrates in PU synthesis has not yet been studied extensively, even though, as multihydroxyl compounds, they can easily serve as crosslinkers in PU synthesis. Partially or potentially biobased di-, tri- or poly-isocyanates can further be used to increase the renewable content of PUs. In our research, PU films could be easily produced using two bio-based isocyanates—ethyl ester L-lysine diisocyanate (LLDI] and ethyl ester L-lysine triisocyanate (LLTI)—, one commercial —isophorone diisocyanate (IPDI), and a bio-based white dextrin (AVEDEX W80) as a crosslinker. The thermal and mechanical properties are evaluated and compared as well as the stability against solvents.

Keywords: starch; carbohydrates; polyurethanes; film formation; renewable resources

1. Introduction Polyurethanes (PUs) are versatile polymers, created from the reaction between a and a polyisocyanate, that are used in a large range of applications, such as automotive, furniture, construction, footwear, etc. The PU industry is strongly dependent on fossil-based and polyisocyanates. Developing novel sustainable polyols from valuable biobased building blocks is a first step toward strong and durable development [1–3]. In recent years, the preparation of PUs from renewable sources such as -based materials [4], lignin [5], limonene [6], and even coffee grounds [7] has been receiving increasing attention because of economic and environmental concerns [8–13]. The use of carbohydrates in PU synthesis has not yet been studied extensively, even though, as multihydroxyl compounds [14–19], they can easily serve as crosslinkers in PU synthesis. Furthermore, they can impart mechanical strength, biodegradability, and biocompatibility to the produced PUs. Carbohydrates are reported to be embedded in PU networks by using them as composites/fillers and by covalent linkage with isocyanate to form crosslinked networks [20]. Solanki et al., for instance, synthesized -based PUs crosslinked with starch and reported excellent mechanical properties of the produced materials [21]. Carbohydrates, such as cellulose [22–26] or starch [27] nanocrystals, have been used as useful fillers in PUs. We have recently reported PU synthesis and film formation capacity using an asymmetric cyclic aliphatic diisocyanate and acetylated and pristine partially hydrolyzed amylopectin/white dextrin (AVEDEX W80) as a crosslinker [28,29]. The next logical step towards a greener PU formulation is then, of course, the use of partially or potentially biobased di-, tri- or poly-isocyanates. While synthetic pathways to biobased isocyanates still

Polymers 2019, 11, 256; doi:10.3390/polym11020256 www.mdpi.com/journal/polymers Polymers 2019, 11, 256 2 of 10 require as a petroleum-based reagent, several commercial isocyanates with high renewable content exist, such as isocyanates based on fatty acids or amino acids. L-lysine-based diisocyanates have frequently been used for biomedical PUs in the last several years as drug delivery systems, hydrogels, implant materials, etc. [30–36]. In some very inspiring recent research, the L-lysine based ethyl ester L-lysine diisocyanate was compared to petrol-based hexamethylene diisocyanate, and isophorone diisocyanate in terms of reactivity and final properties of the PU [37–41] and even fully renewable poly(ester urethane )s were produced [42]. More studies on enzymatic breakdown/biodegradability of (biobased) PUs are conducted in recent years proving the importance of environmentally friendly PU applications [9,43–47]. In our previous work, we found that pristine AVEDEX W80 resulted in the best PU films [29,48] (i.e., with no cracks, inhomogeneity, or swelling) and we, therefore, continue with this system and two bio-based isocyanates—ethyl ester L-lysine diisocyanate (LLDI) and ethyl ester L-lysine triisocyanate (LLTI)—and one commercial isocyanate—isophorone diisocyanate (IPDI). The optimal reaction conditions in terms of casting time and temperature, isocyanate used, amount of solvent, and excess of isocyanate were also determined.

2. Materials and Methods Already degraded amylopectin–white dextrin (AVEDEX W80) was obtained from AVEBE (Veendam, The Netherlands) and used without further purification. Iodine was provided by Boom and purified by sublimation twice. Isophorone diisocyanate (IPDI) was provided by Merck (Darmstadt, Germany) and ethyl ester L-lysine diisocyanate (LLDI) and ethyl ester L-lysine triisocyanate (LLTI) were provided by Shanghai Infine Chemicals Co. Ltd. (Shanghai, China). Acetic anhydride ((CH3CO)2O) (≥99.0%) and propionic anhydride ((CH3CH2CO)2O) (≥96.0%) were provided by Fluka (Bucharest, Romania), sodium thiosulfate (NaS2O3)(≥98.0%) and DMSO (99%) were provided by Sigma-Aldrich (St. Louis, MO, USA) and ethanol (≥99%) was purchased from Merck (Darmstadt, Germany) and used without further purification. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) measurements were characterized by a Bruker IFS88 FT-IR spectrometer (Leiderdorp, The Netherlands). For each sample, 128 scans were performed. Thermal transitions were measured by differential scanning calorimetry (DSC) on a TA-Instruments Q1000 (Hertfordshire, UK). The heating and cooling rates were 10 ◦C/min. To remove the remaining water and solvents in the polymer, the tested sample was first heated to 100 ◦C at 10 ◦C/min, maintained at this temperature for 5 min, and then cooled to room temperature before the standard DSC measurement. Each sample was run for 4 cycles, and the Tg was calculated from the second. Specimens were made from PU samples and were tested on a tensile tester to study their mechanical properties, such as stress–strain behavior and Young’s modulus. The samples were kept between two Teflon plates to ensure a smooth and straight sample form, especially samples that had the tendency to break easily. Mechanical properties were tested on an Instron Tensile Tester (Boechout, Belgium) with a pulling speed of 10 mm/min. The distance between the clamps was 25 mm with a pressure of 7 bar.

2.1. Crosslinking of Acylated Polyols with Di- and Tri-Iisocyanates Point two gram polyol AVEDEX W80 was dissolved in 7.5 mL of solvent (DMSO). A 1.05-fold excess of a di- and/or tri-isocyanate (i.e., a 1.05-fold excess of –NCO groups to –OH groups) was added, the mixture was stirred for 1 to 2 min, and the resulting homogenous mixture was poured onto a petri dish. The petri dish was placed on a heating block at a defined temperature (120 ◦C), covered with filter paper and the heating block was sealed to provide a solvent atmosphere. After a defined period of time, the petri dish was removed from the heating block. Polymers 20192018,, 1110,, 256x FOR PEER REVIEW 3 of 10

2.2. Resistance against Organic Solvents 2.2. Resistance against Organic Solvents To investigate the resistance of the obtained polyurethane films against organic solvents, samplesTo investigate were weighed, the resistance after drying of the to obtained constant polyurethane weight, before films and against after organic immersing solvents, them samples in an wereorganic weighed, solvent. after drying to constant weight, before and after immersing them in an organic solvent.

3. Results The synthesis of polyurethane filmsfilms by the crosslinkingcrosslinking of carbohydrates as polyol components with isocyanates—seeisocyanates–see Scheme Scheme 11–was—was successfully successfully co conducted.nducted. The The selection of isocyanatesisocyanates availableavailable for PU PU synthesis synthesis with with polysaccharide polysaccharide polyols polyols is al ismost almost unrestricted. unrestricted. As aromatic As aromatic isocyanates isocyanates might mightintroduce introduce a color into a color the final into material, the final we material, did not we consider did not them consider here. Two them bio-based here. Two isocyanates— bio-based isocyanates—ethylethyl ester L-lysine ester diisocyanateL-lysine diisocyanate (LLDI) and (LLDI) ethyl and ester ethyl L-lysine ester L -lysinetriisocyanate triisocyanate (LLTI)—and (LLTI)—and one onecommercial commercial isocyanate—isophorone isocyanate—isophorone diisocyanate diisocyanate (IPDI)—were (IPDI)—were used. used.

Scheme 1.1.Reaction Reaction scheme scheme of theof crosslinkingthe crosslinking of carbohydrates of carbohydrates as polyol as components polyol components with isocyanates. with isocyanates. PUs derived from the diisocyanates IPDI and LLDI were too hard and brittle to test after drying. LLTI-basedPUs derived PUs behaved from the morediisocyanates like a rubber IPDI thanand LLDI a ; were aftertoo hard drying and the brittle material to test became after drying. hard, butLLTI-based it was still PUs flexible. behaved PUs more from like IPDI a rubber always than resulted a plastic; in a hardafter anddrying brittle the materialmaterial (afterbecame drying), hard, but theyit was are still the flexible. only ones PUs that from are transparentIPDI always due resulted to the yellowin a hard color and of brittle the biobased material isocyanates. (after drying), PUs frombut they LLDI are also the became only ones brittle that after are drying,transparent but thedue material to the yellow was soft color and of easy the tobiobased scratch. isocyanates. PUs fromFT-IR LLDI spectra also of became all films brittle show after successful drying, PU bu synthesis;t the material Figure was1 shows soft and the easy FT-IR to spectrum scratch. of an IPDI-basedFT-IR spectra PU. The of peaks all films of theshow amide successful I and amide PU sy IInthesis; bands Figure from the 1 shows newly the formed FT-IR polyurethane spectrum of linkagesan IPDI-based are visible PU. The at 1550 peaks cm of− 1theand amide 1650 I cmand−1 amrespectively.ide II bands All from materials the newly are formed not fully polyurethane reacted, i.e., notlinkages all of are the OH-functionsvisible at 1550 of cm the−1 polyoland 1650 formed cm−1 urethanerespectively. linkages All materials with the di- are and not tri-isocyanates, fully reacted, i.e., not evennot all while of the using OH-functions a 50% excess of ofthe isocyanate. polyol formed It seems urethane to be impossible linkages with to convert the di- all and free tri-isocyanates, OH groups in pristinenot even AVEDEX while using W80 a as 50% crosslinking excess of becomes isocyanate. too high.It seems In our to be previous impossible work, to we convert also observed all free thatOH PUgroups films in with pristine an excessively AVEDEX highW80 crosslinkingas crosslinking become becomes brittle too so high. that a In full our conversion previous iswork, not desirable we also fromobserved the start.that PU films with an excessively high crosslinking become brittle so that a full conversion is not desirable from the start.

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absorbance [a.u.]

3500 3000 1500 1000 wavenumber [cm-1]

Figure 1. Fourier transform infrared (FT-IR) spectrum of isophorone diisocyanate (IPDI) based polyurethanesFigure 1. Fourier (PU) (75% transform IPDI/25% infrared ethyl ester(FT-IR)L-lysine spectrum triisocyanate of isophorone (LLTI)). diisocyanate (IPDI) based polyurethanes (PU) (75% IPDI/25% ethyl ester L-lysine triisocyanate (LLTI)). To improve the properties of synthesized PUs, we decided to perform experiments with mixtures of isocyanatesTo improve from thisthe series.properties Two seriesof synthesized of experiments PUs, with we differentdecided ratiosto perform of two isocyanatesexperiments were with carriedmixtures out, of with isocyanates the diisocyanate from this always series. the Two main series component of experiment of the isocyanates with different and up ratios to 50% of (by two volume)isocyanates LLTI were as a “co”-isocyanate. carried out, with Combinations the diisocyanate of the always two diisocyanates the main component were also of prepared, the isocyanate with differentand up ratiosto 50% of (by these volume) two isocyanates. LLTI as a “co”-isocyanate. Combinations of the two diisocyanates were alsoMechanical prepared, with properties different of theratios produced of these two films isocyanates. were tested on an Instron Tensile Tester with a pullingMechanical speed of 10 properties mm/min andof the are produced summarized films in were Table tested1—the on stress–strain an Instron curves Tensile are Tester shown with in a Figurespulling2– 4speed. of 10 mm/min and are summarized in Table 1—the stress–strain curves are shown in Figures 2–4. Table 1. Average Young’s modulus, maximum stress and maximum strain of PUs with different isocyanateTable 1. ratios.Average Young’s modulus, maximum stress and maximum strain of PUs with different isocyanate ratios. % LLTI % IPDI Young’s Modulus (MPa) Stress at Max Load (MPa) % Strain at Max Load % LLTI % IPDI Young’s Modulus (MPa) Stress at Max Load (MPa) % Strain at Max Load 8 92 1126 9 2 8 1292 881126 15369 332 2 12 2588 751536 155433 442 3 25 2975 711554 142744 463 5 29 71 1427 46 5 % LLDI % IPDI % LLDI % IPDI 42 4258 581600 160044 443 3 50 5050 501501 150122 222 2 58 5842 421585 158541 413 3 % LLDI% LLDI% LLTI % LLTI 34 66 22 2 60 34 66 22 2 60 50 50 195 6 92 50 50 195 6 92 66 6634 3418 182 2123 123

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Figure 2. Stress–strain behavior of PUs with different IPDI/LLTI ratios.

Gradual replacement of the hard diisocyanate with the soft triisocyanate resulted in a softer, less rigid and breakable material. At 29% LLTI, the strain at maximum load was twice as large as at 12% LLTI and samples were much more flexible. The curve progression was straight at 8% LLTI, but at 29% LLTI the curve already looked much more like a stress–strain curve from a rubber, with a plateau at its maximum andFigureFigure much 2. 2. Stress–strainhigherStress–strain strain behavior behavior than of at PUs of 8% PUs wi LLTIth with different (see different Figure IPDI/LLTI IPDI/LLTI 2). ratios. ratios.

Gradual replacement of the hard diisocyanate with the soft triisocyanate resulted in a softer, less rigid and breakable material. At 29% LLTI, the strain at maximum load was twice as large as at 12% LLTI and samples were much more flexible. The curve progression was straight at 8% LLTI, but at 29% LLTI the curve already looked much more like a stress–strain curve from a rubber, with a plateau at its maximum and much higher strain than at 8% LLTI (see Figure 2).

FigureFigure 3. 3. Stress–strainStress–strain behavior behavior of PUs of PUs with with different different IPDI/ IPDI/ ethyl ethylester esterL-lysine L-lysine diisocyanate diisocyanate (LLDI) (LLDI) ratios. ratios.

Figure 3. Stress–strain behavior of PUs with different IPDI/ ethyl ester L-lysine diisocyanate (LLDI) ratios.

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FigureFigure 4. 4.Stress–strain Stress–strain behavior behavior of of PUs PUs with with different different LLDI/LLTI LLDI/LLTI ratios. ratios.

Gradual replacement of the hard diisocyanate with the soft triisocyanate resulted in a softer, less Shifting the ratio of two diisocyanates slightly from a 1:1 ratio toward either side did not change rigidthe andmechanical breakable properties material. of At the 29% material. LLTI, the As strainseen in at Table maximum 1, neither load the was average twice as modulus, large as at nor 12% the LLTIstress and or samples the strain were at much max moreload flexible.change Thesignificantly. curve progression Only the was curve straight progression at 8% LLTI, becomes but at 29%more LLTIrubbery, the curve like alreadyin the sample looked with much 58% more LLDI. like a stress–strain curve from a rubber, with a plateau at its maximumGradually and much replacing higher LLDI strain with than LLTI at 8% during LLTI (seesolvent Figure casting2). resulted in a much harder material comparedShifting to the PUs ratio consisting of two diisocyanates only of LLDI. slightly At approximately from a 1:1 ratio 30% toward LLTI, either the material side did has not the change most thedesirable mechanical properties, properties as the of surface the material. is hard, As but seen the in whole Table sample1, neither is still the averageflexible enough modulus, to bend, nor the and stressthe strain or the before strain atbreaking max load is changeover 100%. significantly. The LLDI/LLT Only theI PU curve is by progression far the one becomes with the more greatest rubbery, strain likeat inmaximum the sample load with (Table 58% 1). LLDI. GraduallyIncreasing replacing the percentage LLDI with of LLTI LLTI changes during solvent the PU castingproperties resulted even infurther, a much with harder the materialflexibility comparedalmost lost to PUsand the consisting stress the only sample of LLDI. can Atwithst approximatelyand before breaking 30% LLTI, very the materiallow compared has the with most the desirableother combinations properties, asof isocyanates. the surface isThe hard, maximum but the strain whole is sample also grea istly still reduced flexible with enough an increase to bend, of andLLTI the above strain 30%. before breaking is over 100%. The LLDI/LLTI PU is by far the one with the greatest strain atThe maximum thermal properties load (Table of1). the produced PUs were assessed by differential scanning calorimetry. Increasing the percentage of LLTI changes the PU properties even further, with the flexibility The highest Tg was observed for the IPDI-based PU (Tg = 157 °C). To investigate the dependence of almost lost and the stress the sample can withstand before breaking very low compared with the other Tg on the ratio of isocyanates, samples with different ratios were tested; this data is presented in combinationsFigure 5. In the of isocyanates.case of the IPDI-b The maximumased PUs. the strain effect is alsois clearly greatly visi reducedble; lowering with anthe increase IPDI ratio of lowers LLTI above 30%. the Tg. In the case of the LLDI/LLTI-based PUs, change of the LLDI/LLTI ratio does not result in The thermal properties of the produced PUs were assessed by differential scanning calorimetry. g g significant change in T ; increasing the percentage of LLDI results◦ in T being slightly shifted towards Thelower highest temperatures. Tg was observed At this for point, the IPDI-basedwe cannot expl PU (Taing =the 157 oneC). outlier To investigate at around the 80% dependence LLDI content. of Tg on the ratio of isocyanates, samples with different ratios were tested; this data is presented in Figure5. In the case of the IPDI-based PUs. the effect is clearly visible; lowering the IPDI ratio lowers the Tg. In the case of the LLDI/LLTI-based PUs, change of the LLDI/LLTI ratio does not result in significant change in Tg; increasing the percentage of LLDI results in Tg being slightly shifted towards lower temperatures. At this point, we cannot explain the one outlier at around 80% LLDI content.

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180 80

160 70

140 60 120 50 100 40 [°C] [°C]

g 80 g T T 30 60

20 40

20 10

0 0 30 40 50 60 70 80 90 100 30 40 50 60 70 80 90 100 IPDI [%] LLDI [%]

Figure 5. Relationship between Tg and isocyanate ratio of IPDI-based (left) and LLDI-based (right) Figure 5. Relationship between Tg and isocyanate ratio of IPDI-based (left) and LLDI-based (right) PU films. PU films. To test the resistance of the PU films against organic solvents, different samples were chosen and To test the resistance of the PU films against organic solvents, different samples were chosen immersed in DMSO for 24 h. DMSO was chosen because it showed the most aggressive behavior and immersed in DMSO for 24 h. DMSO was chosen because it showed the most aggressive behavior toward the films. The difference in weight before and after immersion after drying until constant toward the films. The difference in weight before and after immersion after drying until constant weight was calculated; this data is presented in Table2. As observed by FT-IR measurements, the weight was calculated; this data is presented in Table 2. As observed by FT-IR measurements, the samples are not changed chemically during the solvent resistance test, indicating that the weight loss samples are not changed chemically during the solvent resistance test, indicating that the weight loss mainly results from washing away of unreacted polyol from the PU sample (as indicated by the lower mainly results from washing away of unreacted polyol from the PU sample (as indicated by the lower OH-peak and higher amide I and II peaks). Samples made with IPDI seem to be more resistant against OH-peak and higher amide I and II peaks). Samples made with IPDI seem to be more resistant against attacks from DMSO. The maximum weight loss in samples with IPDI is 7%, as compared to 12.5% for attacks from DMSO. The maximum weight loss in samples with IPDI is 7%, as compared to 12.5% for LLTI and almost 30% for LLDI. LLTI and almost 30% for LLDI. Table 2. Weight loss of different ethyl ester polyurethanes (PU) samples after immersion in DMSO forTable 1d. 2. Weight loss of different ethyl ester polyurethanes (PU) samples after immersion in DMSO for 1d. isocyanate IPDI LLDI LLDI LLTI LLDI/LLTI IPDI/LLTI LLDI/LLTI IPDI/LLTI LLDI/LLTI LLDI/LLTI LLDI/LLTI isocyanate IPDIratio LLDI100 100 LLDI 100 LLTI 100 LLDI/LLTI 50/50 66/33IPDI/LLTI 66/33LLDI/LLTI 75/25 IPDI/LLTI 71/29 LLDI/LLTI 92/8 LLDI/LLTI 79/21 LLDI/LLTI weight ratio 100 0.0582100 0.0750100 0.0713100 0.1223 0.0812 50/50 0.0578 66/33 0.1060 66/33 0.0474 75/25 0.0955 71/29 0.1033 0.1335 92/8 79/21 before (g) weight weight 0.0582 0.05410.0750 0.05300.0713 0.0650 0.1223 0.1350 0.0809 0.0812 0.0547 0.0578 0.0927 0.1060 0.0442 0.0474 0.0948 0.0955 0.0970 0.1233 0.1033 0.1335 before (g) after (g) difference weight −7.04% −29.33% −8.84% 10.38% −0.37% −5.36% −12.55% −6.75% −0.73% −6.10% −7.64% 0.0541% 0.0530 0.0650 0.1350 0.0809 0.0547 0.0927 0.0442 0.0948 0.0970 0.1233 after (g) difference −7.04% −29.33% −8.84% 10.38% −0.37% −5.36% −12.55% −6.75% −0.73% −6.10% −7.64% % 4. Conclusions Unmodified AVEDEX W80 was able to form homogenous films with the tested di- and 4. Conclusions tri-isocyanates, after optimization of several reaction conditions. With commercially used IPDI, the resultingUnmodified material AVEDEX is transparent, W80 was rigid, able and to tendsform toho breakmogenous quite films easily, with especially the tested after di- drying and intri- theisocyanates, vacuum oven. after optimization of several reaction conditions. With commercially used IPDI, the resultingPUs derived material from is transparent, LLDI are flexible, rigid, and but tends rather to milky break thanquite transparent easily, especially and they after experience drying in the a coarsevacuum surface. oven. They are more flexible than the IPDI based PUs but also tend to crack when fully dry as provenPUs by tensilederived tests. from As LLDI the biobased are flexible, LLDI but and rather LLTI aremilky colored, than transparent the resulting and films they have experience a slightly a yellowcoarse color surface. (more They pronounced are more inflexible the PU than films the of IPDI LLTI). based PUs but also tend to crack when fully dry as provenAddition by of tensile the trifunctional tests. As the isocyanate biobased LLTI LLD toI eitherand LLTI IPDI- are or LLDI-basedcolored, the formulationsresulting films resulted have a inslightly a dry material, yellow color which (more has a pronou high scratchnced resistancein the PU butfilms is of still LLTI). flexible without breaking. Ratios of the isocyanatesAddition play of an importantthe trifunctional role in theisocyanate final properties LLTI to of either the material. IPDI- or LLDI-based formulations resultedIn general, in a dry the material, IPDI-based which PUs has shows a high higher scratch stability resistance in DMSO but thanis still the flexible biobased. without Additionally, breaking. theRatios biobased of the PUs isocyanates tend to swell play inan organic important solvents, role in which the final is not properties the case of for the IPDI-based material. PUs. Besides differencesIn general, in brittleness, the IPDI-based differences PUs inshows Tg can higher be observed stability in allDMSO four than PUs. the Combining biobased.two Additionally, of these isocyanatesthe biobased results PUs intend changes to swell in Ting ,organic especially solvents, while replacingwhich is not IPDI the with case LLDI, for IPDI-based resulting inPUs. a drop Besides in Tgdifferences. Gradually in replacing brittleness, LLDI differences with LLTI in does Tg can not be result observed in significant in all four Tg PUs.changes Combining in the material. two of these isocyanates results in changes in Tg, especially while replacing IPDI with LLDI, resulting in a drop in Tg. Gradually replacing LLDI with LLTI does not result in significant Tg changes in the material.

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The mechanical properties of these materials depend on the isocyanate used for the PU formulation. If IPDI is the major component in the isocyanate mixture, the mechanical behavior is that of a hard and brittle material. Addition of the trifunctional isocyanate results in a much more rubbery material, visible in the stress–strain diagram. The slope of the curve starts to decrease at some point, reaching a plateau, which then leads to “necking” in the tensile tests.

Author Contributions: J.K. designed and performed the experiments reported in this publication and wrote parts of the paper. K.L. provided guidance, idea and suggestion to the research, helped in analyzing some data, revising and writing this paper. Funding: This research forms part of the research programme of the Dutch Polymer Institute (DPI), project #673. Acknowledgments: This research forms part of the research programme of the Dutch Polymer Institute (DPI), project #673. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

PU polyurethane AVEDEX W80 amylopectin/white dextrin LLDI ethyl ester L-lysine diisocyanate LLTI ethyl ester L-lysine triisocyanate IDPI isophorone diisocyanate

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