Sustainable Epoxy Composites with UV Resistance Based on New Kraft Lignin Coatings

Citation: Seoane-Rivero, R.; Ares-Elejoste, P.; Gondra, K.; Amini, S.; de Hoyos, P.-L.; Gonzalez-Alriols, M. Sustainable Epoxy Composites with UV Resistance Based on New Kraft Lignin Coatings. Molecules 2024, 29, 3697. https://doi.org/10.3390/ molecules29153697 Academic Editor: Jin Li Received: 12 July 2024 Revised: 1 August 2024 Accepted: 2 August 2024 Published: 5 August 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). molecules Article Sustainable Epoxy Composites with UV Resistance Based on New Kraft Lignin Coatings Rubén Seoane-Rivero 1,* , Patricia Ares-Elejoste 1, Koldo Gondra 1, Sara Amini 2, Pedro-Luis de Hoyos 2 and Maria Gonzalez-Alriols 2 1GAIKER Technology Centre, Basque Research and Technology Alliance (BRTA), Parque Tecnológico de Bizkaia, Edificio 202, 48170 Zamudio, Spain 2Biorefinery Processes Research Group, Chemical & Environmental Engineering Department, Faculty of Engineering, Gipuzkoa, University of the Basque Country UPV/EHU, Plaza Europa 1, 20018 Donostia-San Sebastián, Spain *Correspondence: [email protected] Abstract: Currently, the composite industry is focusing on more environmentally friendly resources in order to generate a new range of biobased materials. In this manuscript, we present a new work using lignocellulosic wastes from the paper industry to incorporate into biobased epoxy systems. The manufactured materials were composed of kraft lignin, glass fiber, and a sustainable epoxy system, obtaining a 40% biobased content. Using a vacuum infusion process, we fabricated the composites and analyzed their mechanical and UV resistance properties. The findings reveal a significant correlation between the lignin content and flexural modulus and strength, showing an increase of 69% in the flexural modulus and 134% in the flexural strength with the presence of 5% of lignin content. Moreover, it is necessary to highlight that the presence of synthesized lignin inhibits the UV degradation of the biobased epoxy coating. We propose that the use of lignocellulosic-based wastes could improve the mechanical properties and generate UV resistance in the composite materials. Keywords: sustainable; composite; epoxy; biobased; UV resistance; kraft lignin 1. Introduction The need to develop materials in terms of strength, stability, lightness, and cost has been driven by industry and has generated rapid growth and strong demand from customers in different sectors, such as the automotive [ 1 , 2 ], wind [ 3 , 4 ], and train sectors [ 5 ]. Recently, there has also been an increased focus on the sustainability variable and that is why, nowadays, more and more researchers are working in biobased materials [ 6 – 8 ]. In addition to more sustainable resin systems, the use of renewable additives in composite materials has been considered in recent years, in order to improve properties and increase the biobased content. Composites are materials made from two or more constituent materials with significantly different physical or chemical properties. When combined, these constituents produce a material with characteristics different from the individual components [ 9 ]. The importance of composites in materials science lies in their ability to offer enhanced performance properties, such as higher strength, lighter weight, improved durability, and resistance to environmental degradation. These enhanced properties make composites critical in various applications, including the aerospace, automotive, and construction industries and for sporting goods [10]. Epoxy composites, a subset of this family of materials, are particularly significant due to their excellent mechanical properties and strong adhesive capabilities. They are formed by an epoxy resin matrix, which is usually reinforced with different types of additives [ 11 ]. The resulting material generally exhibits high strength-to-weight ratios, excellent thermal stability, and superior resistance to chemicals and moisture [ 12 ]. Throughout this work, we have worked with biobased gelcoat and epoxy resins. Although their price in comparison Molecules 2024,29, 3697. https://doi.org/10.3390/molecules29153697 https://www.mdpi.com/journal/molecules Molecules 2024,29, 3697 2 of 12 to other polymer systems are considerably higher, it is important to note their novelty and the renewable raw materials used for their synthesis. For example, the biobased gelcoat is formulated with 51% biobased carbons, reducing their carbon footprint without compromising performance. Moreover, it should be noted that they show very good properties compared to other resins, such as polyester resins. Traditionally, the additives incorporated into the epoxy matrix are derived from petrochemical sources, which are non-renewable and contribute to carbon emissions. Consequently, the introduction of biobased products into epoxy composites is essential to address the growing need for sustainable materials in response to environmental concerns [ 13 ]. In fact, by incorporating biobased materials, such as plant-derived oils, starches, or lignocellulosic components, the environmental impact can be significantly reduced. Moreover, these biobased materials offer several advantages, namely, a reduction in the carbon footprint, enhanced biodegradability, and resources renewability. Lignocellulosic molecules, particularly lignin, play a crucial role in the development of sustainable epoxy composites. Lignin is a natural polymer found in the cell walls of plants and is a major byproduct of the paper and bioethanol industries [ 14 ]. Furthermore, its integration into epoxy composites offers several benefits, regarding the production process and the material properties. On the one hand, lignin is one of the most abundant organic polymers on Earth ensuring a readily available and sustainable raw material source, and it is relatively inexpensive, potentially reducing the overall cost of the composite production [ 15 ]. On the other hand, this natural polymer can enhance the mechanical strength and rigidity of epoxy composite materials. Furthermore, its aromatic structure contributes to better thermal stability of this type of composites, making them suitable for high-temperature applications [ 16 ]. Accordingly, in recent years, the incorporation of lignin into epoxy matrix-derived composites has attracted a great deal of attention. Thus, the synthesis of epoxy resins by reacting epichlorohydrin with lignin has been widely reported in the literature [ 17 , 18 ]. In this case, the lignin is an environmentally friendly source of hydroxyl groups, which can replace the use of other synthetic polymers such as bis phenol A (BPA). However, a common issue in the substitution of BPA with lignin can be its high polydispersity index, high molecular weight, and low solubility in some organic solvents and water [ 19 ]. To overcome this challenge, some studies have focused on the utilization of low-molecular weight lignin or lignin-derived oligomers [ 20 , 21 ]. In addition to that, another approach employed is the pretreatment of the lignin, i.e., its chemical modification. In this regard, various lignin modification reactions are implemented, e.g., phenolation [ 22 ], demethylation [23], esterification [24], glycidation [25], and amination [26]. Concerning the properties of the resulting epoxy–lignin composites, once the previously mentioned hurdles are solved, it can be confirmed in general that the incorporation of lignin results in epoxy composites with enhanced mechanical and thermal properties. In a recent study by Xue et al. 2021, the elaboration of epoxy composites with different degrees of substitution of BPA with lignin was presented, and they showed the mentioned improved tendency regarding the mechanical performance of these materials. Thus, they found that the composites with 100% of lignin were the ones showing the highest values for tensile strength [ 27 ]. Another work by Zhen et al. 2021 proved that the introduction of lignin not only achieved higher values for flexural strength and storage modulus but also promoted a certain degree of flame retardancy in the resins [ 28 ]. In addition to yielding some level of fireproofing, lignin added into the epoxy composites produced an increment in the thermal properties as well. Thereby, it has recently been reported by X. Wang et al. [ 29 ] that an increase in the amount of lignin in different epoxy resin formulations leads to a higher percentage of residue after thermogravimetric analysis. Likewise, they showed that the formulation with the higher amount of lignin was the one with the highest value of T50%. In this work, a biobased epoxy gelcoat was combined with lignin in order to fabricate new sustainable composites with UV resistance capabilities. We synthesized new kraft lignin from the Zicuñaga pulp and paper industry (Gipuzkoa, Spain), which was integrated into the coating of the composite. Glass fiber was used as reinforcement and Molecules 2024,29, 3697 3 of 12 epoxy resin with 38% of biobased content was used in order to manufacture the composite by an infusion process. 2. Results The KL sample’s characterization by a GPC analysis indicated that the lignin presented a molecular weight (Mw) of 4936 g/mol and a polydispersity value (Mw/Mn) of 4102, which means that the lignin presented a medium molecular weight and polydispersity value, suitable for being used in a wide variety of formulations [30]. The characterization of the KL samples in terms of the hydroxyl groups’ content, conducted with a 31P NMR spectroscopy, presented the results included in Table 1. The concentrations of the OH groups in aliphatic, C5-substituted, guaiacyl, p-hydroxyphenyl, and carboxylic acid forms were measured (mmol OH/g lignin units). The syringyl/guaiacyl (S/G) monomers’ ratio was also calculated by dividing the OH groups of C5 substitutes (considered as the syringyl OH groups as a result of signal overlapping) into guayacyl OH groups. This ratio provides information about the number of β -O-4 bonds present in the lignin network. The formation of a monomeric unit requires the cleavage of two β -O-4 bonds, one on each side of the aromatic moiety. This means that, during the lignin biosynthesis, G units tend to form C–C bonds, resulting in a decrease in the formation of the ether bonds that generate monomers. Table 1. Content of different hydroxyl groups in KL samples (mmol/g lignin) obtained after RNM analysis. Aliphatic OH C5-Substituted OH Guaiacyl OH p-Hydroxyphenyl OH Carboxylic Acid OH S/G Ratio 2.04 4.63 0.28 1.56 0.28 16.38 The obtained results are in good concordance with the biomass source (hardwood), due to the low content of p-hydroxyphenyl units, which appear mainly in coniferous wood types and herbaceous species. The value of the S/G ratio, much bigger than one, also corresponded to a hardwood species [ 31 ]. The results of the elemental analysis of the KL sample and SGi128 + KL (5% wt.) coating sample are included in Table 2. Table 2. Results of the elemental analysis. Sample % C % H % N % S % O (by Difference) KL (5% wt.) 61.63 5.63 0.13 6.60 26.01 KL + SGi128 41.11 6.16 14.15 2.56 36.02 The results of the elemental composition reveal a high nitrogen percentage provided by the coating formulation. The FTIR spectra of the KL (5% wt.) sample, SGi128 coating sample, and KL (5% wt.) + SGi128 coating sample are presented in Figure 1. The main bands identified in the fingerprint region are classified in Table 3. The bands related to the G and S structures are coherent with the raw material type (hardwood), and the spectrum of the lignin plus gelcoat shows a good integration of lignin in the matrix. DSC tests were carried out to characterize the thermal transitions of the polymeric system and the effect of the lignin in the gelcoat. The obtained results can be found in Table 4. In the analyzed systems, in the first run from an ambient temperature to 280 ◦ C, an exothermic peak with a maximum exothermic temperature was obtained, due to the curing reaction of both epoxy systems. Molecules 2024,29, 3697 4 of 12 Molecules2024,29,xFORPEERREVIEW4of12    Figure1.InfraredspectraobtainedbyFTIRanalysis. ThemainbandsidentifiedinthefingerprintregionareclassifiedinTable3. Table3.IdentificationofthebandsinthefingerprintregionfromthekraftligninFTIR[32,33]. Wavenumber(cm−1)BandAssignment 3450StretchingofthealiphaticO–Hbond 2934,2831C–Hstretchinmethylandmethylenegroups 1600AromaticskeletonvibrationsplusC–Ostretching 1510AromaticvibrationsofGunits 1574AromaticskeletonvibrationsplusC–Ostretching 1458C–Hdeformations(asymmetricin–CH3and–CH2–) 1423AromaticskeletonvibrationscombinedwithC–Hinplanedeformations 1383AliphaticC–HstretchinginCH3andphenolicOH 1363AliphaticC–HstretchinginCH3andphenolicOH 1299GringplusC+Ostretching 1245C–C , C–O , andC=Ostretching 1183TypicalforHGSligninsamples;C–Oinestergroups 1099AromaticC–HdeformationofSunits 1038AromaticC–HinplanedeformationplusC–OdeformationinprimaryalcoholsplusC–Hstretching(unconjugated) 911C–Houtofplane(aromaticrings) 827C–Houtofplaneinpositions2,5,and6(Gunits) ThebandsrelatedtotheGandSstructuresarecoherentwiththerawmaterialtype (hardwood),andthespectrumoftheligninplusgelcoatshowsagoodintegrationoflignin inthematrix. DSCtestswerecarriedouttocharacterizethethermaltransitionsofthepolymeric systemandtheeffectofthelignininthegelcoat.Theobtainedresultscanbefoundin Table4.Intheanalyzedsystems,inthefirstrunfromanambienttemperatureto280°C, anexothermicpeakwithamaximumexothermictemperaturewasobtained,duetothe curingreactionofbothepoxysystems. Figure 1. Infrared spectra obtained by FTIR analysis. Table 3. Identification of the bands in the fingerprint region from the kraft lignin FTIR [32,33]. Wavenumber (cm−1) Band Assignment 3450 Stretching of the aliphatic O–H bond 2934, 2831 C–H stretch in methyl and methylene groups 1600 Aromatic skeleton vibrations plus C–O stretching 1510 Aromatic vibrations of G units 1574 Aromatic skeleton vibrations plus C–O stretching 1458 C–H deformations (asymmetric in –CH3and –CH2–) 1423 Aromatic skeleton vibrations combined with C–H in plane deformations 1383 Aliphatic C–H stretching in CH3and phenolic OH 1363 Aliphatic C–H stretching in CH3and phenolic OH 1299 G ring plus C+O stretching 1245 C–C, C–O, and C=O stretching 1183 Typical for HGS lignin samples; C–O in ester groups 1099 Aromatic C–H deformation of S units 1038 Aromatic C–H in plane deformation plus C–O deformation in primary alcohols plus C–H stretching (unconjugated) 911 C–H out of plane (aromatic rings) 827 C–H out of plane in positions 2, 5, and 6 (G units) Table 4. Thermal properties of different epoxy systems. Formulation Tm (◦C) ∆H (J/g) Infugreen 810 resin + SD8822 hardener 122 496 SGi128 coating + SD228 hardener 99 225 SGi128 coating + SD228 hardener + lignin (5% wt.) 97 210 All the polymer systems showed a single T max on the DSC measurements, as can be seen in Figure 2. It is important to note that these exothermic peaks were between approximately 97 ◦ C and 120 ◦ C. Once the lignin was introduced, the exothermic temperature decreased by around 20 ◦ C. Therefore, it can be considered that lignin has no effect on either the exothermic temperature or the curing process. This result is expected, because there are some studies that demonstrate that lignin has little impact on thermal properties [34,35]. Molecules 2024,29, 3697 5 of 12 Molecules2024,29,xFORPEERREVIEW5of12   AllthepolymersystemsshowedasingleTmaxontheDSCmeasurements,ascanbe seeninFigure2.Itisimportanttonotethattheseexothermicpeakswerebetweenapproximately97°Cand120°C.Oncetheligninwasintroduced,theexothermictemperature decreasedbyaround20°C.Therefore,itcanbeconsideredthatligninhasnoeffecton eithertheexothermictemperatureorthecuringprocess.Thisresultisexpected,because therearesomestudiesthatdemonstratethatligninhaslittleimpactonthermalproperties [34,35].  Figure2.Thermalpropertiesofpolymersystems. Table4.Thermalpropertiesofdifferentepoxysystems. FormulationTm(°C)ΔH(J/g) Infugreen810resin+SD8822hardener122496 SGi128coating+SD228hardener99225 SGi128coating+SD228hardener+lignin(5%wt.)97210 ThemechanicalpropertiesofthemanufacturedcompositesareshowninFigure3. Ascanbeseenintheaforementionedfigure,ligningivesbettermechanicalpropertiesto thecomposites’structures.Moreover,asthelignincontentincreases,theflexuralstrength andmodulusalsoimprovesignificantly.Theresultsconfirmthattheflexuralmodulus increasesaround31%withthepresenceof2.5%wt.lignincontentand69%with5%wt. Regardingtheflexuralstrength,thiseffectismorenoticeable,asitincreasesaround34% withthepresenceof2.5%wt.lignincontentand134%with5%wt.Thisfactshowsthat theintroductionofthisbiomassadditivehasasignificantimprovementontheflexural properties.Thiscanbeanindicatorofthereinforcementeffectoflignininthebiobased gelcoat.Notmuchinformationisfoundintheliteratureonthemechanicalpropertiesof thermosetcompositescontaininglignin,but,forexample,WuZheet.al.analyzedhow theadditionofligninfiberchangedthecompressioncharacteristicsofthematrixmaterial, resultingintheappearanceofastrengtheningstage[36]. Figure 2. Thermal properties of polymer systems. The mechanical properties of the manufactured composites are shown in Figure 3. As can be seen in the aforementioned figure, lignin gives better mechanical properties to the composites’ structures. Moreover, as the lignin content increases, the flexural strength and modulus also improve significantly. The results confirm that the flexural modulus increases around 31% with the presence of 2.5% wt. lignin content and 69% with 5% wt. Regarding the flexural strength, this effect is more noticeable, as it increases around 34% with the presence of 2.5% wt. lignin content and 134% with 5% wt. This fact shows that the introduction of this biomass additive has a significant improvement on the flexural properties. This can be an indicator of the reinforcement effect of lignin in the biobased gelcoat. Not much information is found in the literature on the mechanical properties of thermoset composites containing lignin, but, for example, Wu Zhe et. al. analyzed how the addition of lignin fiber changed the compression characteristics of the matrix material, resulting in the appearance of a strengthening stage [36]. Molecules2024,29,xFORPEERREVIEW6of12    Figure3.Mechanicalpropertiesofcomposites. RegardingtheUVeffectonthemechanicalproperties,itcanbesaidthatthesamples didnotsufferhighchangesafterdegradation,becausethisattackisfocusedonthegelcoat. Itwasobservedthatthesampleswithoutligninincreasedtheirmodulusandstrength, whereasthesampleswiththisbiofillerata5%wt.contentshowedhighervaluesofthese parametersafterUVirradiation,asobservedinpreviousstudies[37].Itisremarkablethat themechanicalpropertiesoftheKL(5%wt.)samplesreportedbettervaluesthantheones thatdidnotcontainligninanddidnotsuffertheUVattack.Withthegoalofcharacterizing theUVresistancecapabilityofthemanufacturedcompositesinthepresenceoflignincontent,thebrightnessandcolorwereanalyzed.Figure4showsthebrightnessvariationand colorchanges. Ascanbeseen,theeffectofUVattackandwatercondensationat50°Cismoresignificantinthecolorthaninthebrightnessproperties.Thismaybebecausethesamples weremattandwhiteincolor.  Figure4.Color(left)andbrightness(right)variationoffabricatedcomposites. Moreover,itisimportanttonotethatthecolorvariationdecreaseswiththelignin content,soitcanbeconcludedthattheeffectofligninisconsiderableinUVresistance, decreasingtheΔE(color)bymorethan10betweenthereferenceandthecompositewith KL(5%wt.). AscanbeobservedinFigure5,thecolorchangesarevisibletothenakedeye,dueto theyellowingcausedbyUVradiation.Itshouldbetakenintoaccountthatthereisanotabledifferenceinthecolorbetweenthesamplewithoutlignincontentandthesample thatcontains5%wt.InordertocharacterizetheeffectofUVtreatmentonsurface Figure 3. Mechanical properties of composites. Regarding the UV effect on the mechanical properties, it can be said that the samples did not suffer high changes after degradation, because this attack is focused on the gelcoat. It was observed that the samples without lignin increased their modulus and strength, whereas the samples with this biofiller at a 5% wt. content showed higher values of these parameters after UV irradiation, as observed in previous studies [ 37 ]. It is remarkable that the mechanical properties of the KL (5% wt.) samples reported better values than the ones that did not contain lignin and did not suffer the UV attack. With the goal of Molecules 2024,29, 3697 6 of 12 characterizing the UV resistance capability of the manufactured composites in the presence of lignin content, the brightness and color were analyzed. Figure 4shows the brightness variation and color changes. Molecules2024,29,xFORPEERREVIEW6of12    Figure3.Mechanicalpropertiesofcomposites. RegardingtheUVeffectonthemechanicalproperties,itcanbesaidthatthesamples didnotsufferhighchangesafterdegradation,becausethisattackisfocusedonthegelcoat. Itwasobservedthatthesampleswithoutligninincreasedtheirmodulusandstrength, whereasthesampleswiththisbiofillerata5%wt.contentshowedhighervaluesofthese parametersafterUVirradiation,asobservedinpreviousstudies[37].Itisremarkablethat themechanicalpropertiesoftheKL(5%wt.)samplesreportedbettervaluesthantheones thatdidnotcontainligninanddidnotsuffertheUVattack.Withthegoalofcharacterizing theUVresistancecapabilityofthemanufacturedcompositesinthepresenceoflignincontent,thebrightnessandcolorwereanalyzed.Figure4showsthebrightnessvariationand colorchanges. Ascanbeseen,theeffectofUVattackandwatercondensationat50°Cismoresignificantinthecolorthaninthebrightnessproperties.Thismaybebecausethesamples weremattandwhiteincolor.  Figure4.Color(left)andbrightness(right)variationoffabricatedcomposites. Moreover,itisimportanttonotethatthecolorvariationdecreaseswiththelignin content,soitcanbeconcludedthattheeffectofligninisconsiderableinUVresistance, decreasingtheΔE(color)bymorethan10betweenthereferenceandthecompositewith KL(5%wt.). AscanbeobservedinFigure5,thecolorchangesarevisibletothenakedeye,dueto theyellowingcausedbyUVradiation.Itshouldbetakenintoaccountthatthereisanotabledifferenceinthecolorbetweenthesamplewithoutlignincontentandthesample thatcontains5%wt.InordertocharacterizetheeffectofUVtreatmentonsurface Figure 4. Color (left) and brightness (right) variation of fabricated composites. As can be seen, the effect of UV attack and water condensation at 50 ◦ C is more significant in the color than in the brightness properties. This may be because the samples were matt and white in color. Moreover, it is important to note that the color variation decreases with the lignin content, so it can be concluded that the effect of lignin is considerable in UV resistance, decreasing the ∆ E (color) by more than 10 between the reference and the composite with KL (5% wt.). As can be observed in Figure 5, the color changes are visible to the naked eye, due to the yellowing caused by UV radiation. It should be taken into account that there is a notable difference in the color between the sample without lignin content and the sample that contains 5% wt. In order to characterize the effect of UV treatment on surface roughness, a confocal microscopy was also used. The surface topography measurements were conducted before and after the UV attack. Molecules2024,29,xFORPEERREVIEW7of12   roughness,aconfocalmicroscopywasalsoused.Thesurfacetopographymeasurements wereconductedbeforeandaftertheUVattack.  Figure5.(a)Reference;(b)referenceafterUVtreatment;(c)KL(2.5%wt.)sampleafterUVtreatment;and(d)KL(5%wt.)sampleafterUVtreatment. ThesamplesthatdidnotundergotheUVattackshowedasimilarroughness,with Savaluesofaround1.6µm,ascanbeseeninFigure6.Incontrast,aftersufferingwater condensationat50°CandUVradiation,thesamplesincreasedtheirroughnessconsiderably;inthecaseofthereference,theroughnessincreasedaround171%,whilethesamples thatcontainedligninshowedapercentagelower:157%fortheKL(2.5%wt.)sampleand 132%inthecaseoftheKL(5%wt.)sample,ascanbeseeninTable5.Thephotodegradationeffectwashigherinthesampleswithlesslignincontent.  Figure6.(a)Reference;(b)referenceafterUVtreatment;(c)KL(2.5%wt.)sample;(d)KL(2.5%wt.) sampleafterUVtreatment;(e)KL(5%wt.)sample;and(f)KL(5%wt.)sampleafterUVtreatment. Figure 5. (a) Reference; (b) reference after UV treatment; (c) KL (2.5% wt.) sample after UV treatment; and (d) KL (5% wt.) sample after UV treatment. The samples that did not undergo the UV attack showed a similar roughness, with Sa values of around 1.6 µ m, as can be seen in Figure 6. In contrast, after suffering water condensation at 50 ◦C and UV radiation, the samples increased their roughness considerably; in the case of the reference, the roughness increased around 171%, while the samples that contained lignin showed a percentage lower: 157% for the KL (2.5% wt.) sample and 132% in the case of the KL (5% wt.) sample, as can be seen in Table 5. The photodegradation effect was higher in the samples with less lignin content. Molecules 2024,29, 3697 7 of 12 Molecules2024,29,xFORPEERREVIEW7of12   roughness,aconfocalmicroscopywasalsoused.Thesurfacetopographymeasurements wereconductedbeforeandaftertheUVattack.  Figure5.(a)Reference;(b)referenceafterUVtreatment;(c)KL(2.5%wt.)sampleafterUVtreatment;and(d)KL(5%wt.)sampleafterUVtreatment. ThesamplesthatdidnotundergotheUVattackshowedasimilarroughness,with Savaluesofaround1.6µm,ascanbeseeninFigure6.Incontrast,aftersufferingwater condensationat50°CandUVradiation,thesamplesincreasedtheirroughnessconsiderably;inthecaseofthereference,theroughnessincreasedaround171%,whilethesamples thatcontainedligninshowedapercentagelower:157%fortheKL(2.5%wt.)sampleand 132%inthecaseoftheKL(5%wt.)sample,ascanbeseeninTable5.Thephotodegradationeffectwashigherinthesampleswithlesslignincontent.  Figure6.(a)Reference;(b)referenceafterUVtreatment;(c)KL(2.5%wt.)sample;(d)KL(2.5%wt.) sampleafterUVtreatment;(e)KL(5%wt.)sample;and(f)KL(5%wt.)sampleafterUVtreatment. Figure 6. (a) Reference; (b) reference after UV treatment; (c) KL (2.5% wt.) sample; (d) KL (2.5% wt.) sample after UV treatment; (e) KL (5% wt.) sample; and (f) KL (5% wt.) sample after UV treatment. Table 5. Surface roughness values of reference, KL (2.5% wt.) sample, and KL (5% wt.) sample before and after the UV radiation. Sa (µm) Reference KL (2.5% wt.) KL (5% wt.) Initial 1.6 ±0.1 1.8 ±0.2 1.4 ±0.1 500 h UV 4.4 ±0.3 4.6 ±0.3 3.4 ±0.2 3. Materials and Methods 3.1. Epoxy Resin System A commercial epoxy resin, InfuGreen 810, with 38% of biobased content (a carbon of plant origin) was used. The hardener that was used was SD 822, supplied by Sicomin Epoxy Systems (France). This system shows a very low viscosity at room temperature, and it can be mixed with different hardeners. The most relevant physicochemical properties of the above-mentioned system are shown in Table 6. Table 6. Physicochemical properties of the epoxy resin used. Property Units Value Viscosity at 20 ◦C mPas 215.0 Density at 20 ◦Cg/cm31.2 Pot life at 20 ◦C 500 g. h 4.7 Molecules 2024,29, 3697 8 of 12 The applied coating was also supplied by Sicomn Epoxy Systems and was formulated with 51% of the biobased gelcoat called SGi 128 combined with the SD 228 hardener. This is a halogen-free and fire-resistant epoxy coating with low levels of smoke density and gas toxicity. The reinforcement used was a quadriaxial glass fiber with a grammage of 450 g/m2. 3.2. Kraft Lignin Kraft lignin (KL) was precipitated from black liquors provided by the Zicuñaga pulp and paper industry (Gipuzkoa, Spain). This industry processes eucalyptus wood. Kraft lignin is considered a medium–high-molecular weight lignin with values ranging from 200 to 200,000 g/mol, high carbohydrate contents due to condensation reactions during the kraft process, and 1–3% sulfur contents [ 38 ]. KL is precipitated by the addition of acid, protonating the phenolic compounds present in lignin, lowering their hydrofiliation, and precipitating as a result. In this case, precipitation was carried out by acidifying the liquor to a pH of 4 using glacial acetic acid. Once the lignin precipitation was completed, the solution was left to stand for 24 h. Then, a vacuum filtration was performed, and the lignin sample recovered in the filtrate, which was washed with distilled water until it was neutralized. Finally, the recovered lignin was dried for 2 h at 28 ◦C and stored. 3.3. Kraft Lignin Characterization The lignin samples were characterized by several techniques. A gel permeation chromatography/size exclusion chromatography (GPC) was used to determine the average molecular weight and size distribution of the lignins. A Jasco Inc. chromatograph with an LC-NettII/ACD interface, a CO-2065Plus oven column, and an RI-2031PlusIntelligent Refractive Index detector was used. In addition, the chromatograph was equipped with a guard column and two PolarGel-M columns (Varian Inc., Palo Alto, CA, USA) in series. DMF was used as the mobile phase at a flow rate of 0.7 mL/min and a temperature of 40 ◦ C. The instrument was calibrated with polystyrene standards in the molecular weight range of 250–70,000 g/mol. For the analysis, 25–50 mg of the sample was taken and dissolved in 5 mL of the mobile phase. One of the most important properties of lignin is the content of its hydroxyl compounds, as it acts on solubility and reactivity. Consequently, the quantification of the different OH groups of the molecules is of great importance. 31 P NMR is a quantitative determination by spectroscopy. Phosphorylation was carried out followed by phosphorus nuclear magnetic resonance ( 31 P NMR) spectroscopy in Bruker AVANCE II 400 equipment to analyze the content of hydroxyl groups in the lignin samples. An elemental analysis was performed using a Leco TruSpec microanalyzer at 1050 ◦ C. Both the carrier gas (3X pure helium) and test gas (4X extra-pure oxygen) were supplied by Nippon Gases. The calibration was performed using Leco Sulfamethanize (C = 51.78%; H = 5.07%; and S = 11.5%). The samples were analyzed in triplicate. To evaluate the main structural level characteristics and functional groups of lignin, the characterization was performed with Fourier transform infrared (FTIR) spectroscopy using a PerkinElmer Spectrum Two FT-IR instrument equipped with an attenuated total reflectance (ATR) accessory and an internally reflecting diamond glass lens. The samples were analyzed using 64 scans with a resolution of 4 cm −1 and a wave number range of 4000–400 cm−1. 3.4. Biobased Resin System and Gelcoat Formulation Characterization To determine the reactivity of the resin system before using it in the infusion process, a thermal analysis was performed. In the same manner, the influence of the lignin on the reactivity of the gelcoat system was studied in order to prevent it from inhibiting the curing at a concentration of 5% wt. The thermal transitions of the resin and gelcoat were analyzed by a differential scanning calorimetry (DSC) on a Mettler Toledo (DSC 822e) calorimeter equipped with a liquid Molecules 2024,29, 3697 9 of 12 nitrogen accessory. The samples were weighed on a METTLER TG 50 4-digit analytical balance in 40 µ L aluminum pans. These samples were weighed in the liquid state (approximately 10 mg), in order to obtain the maximum exothermic temperature peak, T max , and the heat of the reaction. With regard to the operating conditions, the samples were heated from 25 ◦ C to 280 ◦ C at a rate of 10 ◦ C · min −1 and under an inert nitrogen atmosphere. The data in Table 4were taken from the first scan. Once the formulation was cured, to observe the effect of lignin on the systems, especially at the level of the elemental composition and morphology, a scanning electron microscopy (SEM), using a Zeis EVO 50 microscope at 20 kV and an energy dispersive X-ray (EDX) (INCA, Oxford Instruments, Oxford, United Kingdom), was conducted. The cured formulations were coated by a sputter Leica EM SCD005, before the measurements were conducted with gold palladium. Further, a 3D optical profiler PL µ NEOX (Sensofar) was used for the surface topography measurements. 3.5. Composite Development and Characterization The biobased epoxy composites were developed using a vacuum infusion process. This process provides high-performance composites for different sectors such as the wind, aeronautic, automotive, and railway sectors. The vacuum infusion technique is as represented in Figure 7. Molecules2024,29,xFORPEERREVIEW10of12   system,theresinwasinjecteduntilthefiberwasproperlyimpregnatedandcured,accordingtothemanufacturer’scuringandpost-curingconditions.  Figure7.Schemeofthevacuuminfusiontechnique. Afterthecompositewasmanufactured,severaltests,suchasmechanicaltests,aUV attack,andcolorandbrightnesscharacterizations,werecarriedout. Themechanicalpropertieswerefocusedontheflexuraltests.ThesetestswereperformedinaccordancewithISO14125:1998/A1:2011[39].Asupportspacingof50mmand atestspeedof1mm/minwereused.Samplesofdimensions15×2.9mmwereprepared andloadedtofailure.Themeasurementswerecarriedoutwithaloadcellof5kN.For eachcomposite,threesamplesweretestedandtheaveragevaluewasreported. TheUVattackwasperformedaccordingtoUNEENISO4892-3:2016(MethodA)[40], whichrequiresatotaltimeof500h.Thesamplesweresubjectedtocyclesof8hofUV exposureat70°Cwith340nmUVAlamps,followedby4hofwatercondensationat50 °C. ThecolormeasurementswereperformedaccordingtoUNEENISO11664-1:2011 [41].Afterthemeasurement,thecolorimeterrecordedtheL,a,andbparameters.Toobtainthecolorvariationasexpressedintheresultssection,aseriesoftransformationsdetailedinthestandardswereused.Moreover,glossmeasurementswereperformedaccordingtotheUNE-ENISO2813:1999standard[42].Thismethodusesthreeanglesofincidenceofthelightbeam:20°,60°,and85°.Forsimplification,themeasurementsobtained withthe60°anglewerecompared.Anelementalanalysisofthecompositewasperformed usingtheequipmentandconditionsindicatedinthelignincharacterizationsection. 4.Conclusions Nowadays,itcanbesaidthatthereisinterestfromthecompositesindustryinobtainingnewsustainablealternativestodecreasetheenvironmentalimpactofcomposite materials.Thismanuscriptintroducestheconceptofusinglignocellulosic-basedwastes fromthepaperindustryand,ontheotherhand,theincorporationofbiobasedepoxysystems(resinandgelcoat),inordertogeneratenewsustainableepoxycomposites. Inthismanuscript,wepresentaworkusinganewkraftligninfromtheZicuñaga pulpandpaperindustry(Gipuzkoa,Spain),whichwasincorporatedintoabiobased epoxysystem.Firstly,throughaGPC,elementalanalysis,andRMNcharacterization,we ensuredthatthesynthesiswasingoodconcordancewiththebiomasssource.Secondly,it isimportanttonotethesignificantimprovementofthemechanicalpropertiesafterintroducingsynthetizedlignininthecoatingofthecomposite.Theseresultsshowthatthe presenceof5%oflignincontentincreasedtheflexuralmodulusby69%andtheflexural strengthby134%.TheUVresistancewasalsocharacterizedbycolorandbrightnesstests andaconfocalmicroscopy.Whilethebrightnesstestdidnotshowanydifferencebetween thereferenceandsampleswith2.5and5%oflignincontent,thecolorandroughnessparametersdemonstratedthatthepresenceoflignininhibitedtheUVdegradationofthe biobasedepoxycoating.Consideringthepresentedargumentsandresearchfindings,it canbeconcludedthatwedemonstratethattheuseoflignocellulosic-basedwastescould Figure 7. Scheme of the vacuum infusion technique. Firstly, the release agent was applied on the substrate (the molding plate), in order to facilitate the subsequent demolding of the composite. Then, a layer of gelcoat (SGi 128 combined with the SD228 hardener) and different percentages of lignin (2.5 and 5% wt.) were deposited on the glass substrate and, after 4 h, when the gelcoat was in a tacky state, five layers of fiberglass were stacked. Once stacked, a peel ply was placed to facilitate the demolding of the material after the process. A distribution mesh was then placed to help to achieve a correct and homogeneous impregnation of the fiber. Next, two tubes were placed in the system, one for the vacuum and the other for the resin injection, and then the system was sealed with a film (comprising the counter-mold). After assembling the system, the resin was injected until the fiber was properly impregnated and cured, according to the manufacturer’s curing and post-curing conditions. After the composite was manufactured, several tests, such as mechanical tests, a UV attack, and color and brightness characterizations, were carried out. The mechanical properties were focused on the flexural tests. These tests were performed in accordance with ISO 14125:1998/A1:2011 [ 39 ]. A support spacing of 50 mm and a test speed of 1 mm/min were used. Samples of dimensions 15 × 2.9 mm were prepared and loaded to failure. The measurements were carried out with a load cell of 5 kN. For each composite, three samples were tested and the average value was reported. The UV attack was performed according to UNE EN ISO 4892-3:2016 (Method A) [ 40 ], which requires a total time of 500 h. The samples were subjected to cycles of 8 h of UV exposure at 70 ◦ C with 340 nm UVA lamps, followed by 4 h of water condensation at 50 ◦ C. The color measurements were performed according to UNE EN ISO 11664-1:2011 [ 41 ]. After the measurement, the colorimeter recorded the L, a, and b parameters. To obtain the