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ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 2023, VOL. 9, NO. 1, 2205796 https://doi.org/10.1080/20550340.2023.2205796 Effects of reactive and non-reactive tackifying agents on mechanical neat resin and composite performance for preforming processes and Liquid Resin Infusion (LRI) techniques Florian Helber, Stefan Carosella and Peter Middendorf Institute of Aircraft Design (IFB), University of Stuttgart, Stuttgart, Germany ABSTRACT ARTICLE HISTORY Received 2 February 2023 Preforming processes can be used for automated manufacturing of fiber reinforced polymers. Accepted 18 April 2023 Different technologies are used for processing of dry textile fabrics into 2D or 3D preforms. Due to missing tack of dry fabrics, auxiliary binder systems are used for fixation of the fabrics KEYWORDS onto a substrate material and in order to achieve sufficient adhesion between layers. In this Dry fiber placement; binder study, seven reactive and non-reactive tackifying agents have been dissolved in neat resin systems; carbon fiber samples of three epoxy resin systems, showing different degrees of solubility and a variation reinforced polymer; on neat resin tensile properties (Dr ¼ 20%) as well as a reduction on thermal properties AVG preforming; thermoset; (up to DT ¼18 C). Subsequently, fiber reinforced polymers were manufactured using g mechanical performance; Liquid Resin Infusion techniques in order to characterize the influence of binder systems on glass transition temperature; water water absorption (c ¼ 1.38 wt%) and Interlaminar Shear Strength (ILSS). It was shown s,max absorption that ILSS properties are negatively affected by non-reactive tackifying agents (up to 27%). GRAPHICAL ABSTRACT 1. Introduction become a popular manufacturing alternative to the cost-intensive autoclave procedure, in order to meet Fiber reinforced polymers (FRP) are manufactured by the growing demand for high performance composite embedding man-made (e.g. glass fibers; carbon fibers, components in various industries [2, 3]. Within the etc.) or natural reinforcement fibers (e.g. flax fibers, majority of these processes, low-viscosity, thermoset- hemp fibers, etc.) in a polymer matrix system. In order ting matrices are utilized to impregnate dry fiber pre- to accommodate for the applied loads, fibers are forms. In both manual and automated composite aligned in pre-defined fiber orientations and quantity. preforming processes several layers of dry reinforce- The use of continuous fiber reinforced composites and ment fabrics are stacked, according to a predefined their superior weight-specific mechanical properties stacking sequence. In order to achieve the optimal allows for the engineering of tailored materials, while mechanical performance in the final composite part, keeping them lightweight at the same time. As a result fiber orientations of the reinforcement fabrics have to of these potentials, fiber-reinforced polymer composites be precisely met and maintained. Automated manufac- are used in the aerospace, automotive, transport, and construction sectors [1]. Liquid Resin Infusion (LRI) turing technologies respectively reduce the risk of fiber and Out-of-Autoclave (OoA) processes in general have misplacement and manual rework [4, 5]and a variety CONTACT Florian Helber florian.helber@ifb.uni-stuttgart.de Institute of Aircraft Design (IFB), University of Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany. 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 F. HELBER ET AL. of different automated fiber deposition technologies are preforming step is finalized and ready for subsequent available, in order to produce load-path oriented and processing and resin impregnation. One preforming approach, which is recently near-net shape preforms [6]. Especially, the Dry Fiber acquiring increasing relevance is the Dry Fiber Placement (DFP) technology has recently gained Placement process, as a subcategory of the increasing interest throughout the various industrial Automated Fiber Placement (AFP) process [15]. sectors, due to its high material properties, flexibility This is due to the fact of minimized scrap rates and and the possibility of producing composite components the possibility to produce load-path oriented fiber with low fiber undulations and reduced fiber areal orientations within a preform. Most commonly, weights (FAW) [7]. robotic AFP systems are used for deposition of spread or slit fiber materials. Generally, it can be 1.1. Preforming processes – state of the art distinguished between the layup of pre-impregnated tape materials (AFP), thermoplastic tapes (TAFP) In the context of composite lightweight engineering, [16] and the deposition of spread, non-impregnated one can distinguish between prepreg and preforming but bindered semi-finished products (DFP) [17]. manufacturing methodologies. While the automated Based on the presence of matrix material in prepreg manufacturing of composite components with unidir- and thermoplastic semi-finished products during ectional (UD) pre-impregnated semi-finished prod- AFP and TAFP, tape fixation is realized due to the ucts will lead to homogenous and superior part adhesive behavior of the matrix material after ther- quality, due to reduced undulation and a high fiber mal heat input [18]. Dry fiber materials on the other volume fraction, the automated processing of UD hand do not possess an inherent tack and auxiliary prepreg materials is also linked to high capital invest- tackifying agents are required for fiber fixation [19]. ment, such as autoclave and automated processing Due to the growing interest in DFP processes and technologies, as well as refrigerated storage capacities. its industrial application the availability of bindered However, for the majority of industries, in which tow materials with different tackifying agents has composite materials are relevant, processing of dry increased. In previous studies it was shown that pre- fibers and fabrics with subsequent resin impregnation form and composite properties can be affected by is of increasing interest, due to various benefits, such fiber placement processes [2, 20]. Furthermore, as higher design freedom and material flexibility Grisin et al. [21] have described the manufacturing (a.o.). According to the literature, a preform is of fixedTow [22] and bindered materials [23]. In defined as a non-impregnated fiber structure with a general, it can be stated that binder technology and given orientation and load-path oriented fiber align- its application in sequential preforming processes is ment results in a near-net-shape geometry [8]. It can critical for high-performance composite components be stated that methodologies for preform manufactur- and was therefore subject of numerous studies. ing are divided into direct and sequential processes [8]. Direct preforming processes generally create 1.2. Impact of binder systems on composite three-dimensional geometries and integral structures performance [9] and the utilization of auxiliary binder systems is not necessarily required. Among others, radial braid- Various authors have studied the effects of tackifying ing [10], 3D weaving, Tailored Fiber Placement (TFP) agents on the impregnation behavior of preforms as [11, 12] are some of the most common direct pre- well as the resin rheology and mechanical perform- forming methods for continuous FRPs. During ance of fiber reinforced plastic components, mainly sequential preforming, layers of semi-finished prod- driven by matrix-dominant properties. Ringwald [24] ucts are initially cut to size and subsequently stacked, and Shih et al. [25] state that the activation process of according to a predefined ply book and the fixation tackifying agents, in particular the effect of activation or joining of the layers can be realized using embroi- temperature, consolidation pressure and duration, dery or binder technology [13]. When using the can affect the preform permeability. Shih et al. [25] embroidery technology, the stacked layers are formed state that inadequate binder activation (e.g. insuffi- into a near-net-shaped geometry and are fixed cient activation temperature or duration) or excessive together with a sewing yarn [14]. When using tackify- amount of binder material [26] will lead to accumula- ing agents for layer fixation, an auxiliary material is tion of binder material in the interface region placed in between two adjacent textile layers. The between two adjacent reinforcement layers, leading to stack is then heated upon the activation temperature a blockage of resin flow channels, hence affecting the of the auxiliary binder system and consolidated into corresponding mechanical behavior of the composite shape (e.g. hot pressing). After adequate consolida- laminate. On the other hand, Dickert et al. [26] report tion and removal from the draping process, the that the presence of tackifying agents can cause a ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 3 spacing effect within a preform, resulting in enhanced to achieve adequate composite performance. The main permeability by the creation of macroscopic flow aim of this study is to investigate further thermal and channels. However, this effect is strongly dependent mechanical effects of a variety of different binder sys- on the binder particle size [26, 27]. Additional studies tems used in different preforming processes. The majority of past studies have focused on the direct on the solubility behavior of different powder binders comparison of two binder materials with similar phys- and their effect on rheological effects have been ical appearance (e.g. particulate tackifying agents). studied by Schmidt et al. [28] and Brody [29], show- Novel tackifying agents, such as SAERfix EP or ing different degrees of binder solubility. The tensile strength of composite components is dominated by EPIKOTE MGS PR685, have not been addressed in recent scientific studies so far. However, due to the lack the tensile properties of the reinforcing fibers. For of thermal binder activation processes for SAERfix EP that reason, studies on tensile composite properties in and EPIKOTE MGS PR685, reduced application and regards to the effects of binder system are rare and it handling complexity as well as resin compatibility, has been shown that binder systems have little effect those two tackifying agents are of high practical rele- on composite tensile properties [28, 30, 31]. vance and were thus considered in this study. Considering compression properties, various authors Furthermore, the commonly used binder system have come to different conclusions. Schmidt et al. AIRTAC 2E was also not analyzed within previous [28] have shown that the compression modulus in studies. Therefore, this study includes the analysis of fiber direction is increased by the presence of binder seven reactive and non-reactive binder types with dif- systems and compression strength is increased with ferent binder formats, including three powder binders, elevated binder fractions. Tanoglu and Seyhan [32] one textile veil, one adhesive film, one hot-melt-binder on the other hand point out that with elevated binder and one spray adhesive, as it can be seen in Figure 1, fractions ( 3 wt%) compressive behavior is reduced, including novel tackifying agents. Next to their physical which is supported by the findings of Geßler et al. appearance, the tackifying agents can be distinguished [30]. Schulz et al. [33] have investigated the bending by means of their chemical composition, activation properties of composite laminates with a variety of temperature and their field of application. For that rea- different tackifying agents (Binder Areal Weight son, binder solubility was analyzed at distinct curing (BAW): 4–10 wt%.). It was shown that bending temperatures, using three different epoxy resins. The modulus and strength were reduced by 10–15 wt% by selection of the amine cured epoxy resins was based on all of the binder systems tested. The interlaminar their specific curing cycles and respective glass transi- shear strength (ILSS) is a quality criterion, which tion temperatures (T ). Therefore, the impact on the describes the adhesion of adjacent layers within a glass transition temperature was studied using composite part. The ILSS test acc. DIN EN ISO 14130 Differential Scanning Calorimetry (DSC). Additionally, is a measurement of the resistance of composite lami- water absorption and its impact on the ILSS behavior nates under shear forces parallel to the layers of the was studied and compared to non-conditioned speci- laminate and thus the adherent interface. Since tacki- mens. Finally, tensile testing of neat resin binder sam- fying agents are placed exactly in between two adja- ples was carried out. cent layers, it can be assumed that the binder system has a major effect on these properties. Shih and Lee [25] have analyzed the impact of the PT500 binder on 2. Materials and methods ILSS properties (resin PR500). For common binder 2.1. Materials fractions (5–10 wt%), ILSS values show no significant difference to the reference specimens, which is sup- 2.1.1. Fabrics ported by the findings of Girdauskaite [34], while HEXForce G0827 BB1040 [38] carbon fabric was Hillermeier and Seferis [35] state that the use of purchased from HEXCEL Composites SASU PT500 led to a reduction of mechanical properties (Dagneux, France). It is a unidirectional plain weave (BAW: 5.7 wt%; resin RTM6). Further studies have fabric with a fiber areal weight of 160 g/m , contain- been carried out by Brody and Gilespie [29], Tanoglu ing 3k TORAYCA FT300B carbon fibers (198 tex, [36] as well as Bulat et al. [37]. All authors conclude density: 1.76 g/cm ) in warp direction (97 wt.%) and that the application of auxiliary tackifying agents will EC5 5.5 2 glass fibers in weft direction (3 wt.%). lead to reduced mechanical properties. 2.1.2. Tackifying agents Table 1 lists seven different binder systems, which 1.3. Scope were considered for further assessment. The selec- The various studies concerning the different effects of tion of tackifying agents was based on the criteria tackifying agents reveal that the utilization of auxiliary listed in Section 1.3 and shall therefore cover a wide binder systems need to be carefully addressed in order range of application. Reactive tackifying agents are 4 F. HELBER ET AL. Figure 1. Tackifying agents; (a) Powder binder, e.g. E5390; (b) Textile veil, e.g. PA1541; (c) Adhesive film, e.g. SAER; (d) Hot- melt binder, e.g. PR685; (e) Spray adhesive, e.g. AIR2E. Table 1. Description of tackifying agents characterized within this study. Name Abbreviation Supplier Type Appearance Chem. Composition Activation Temperature [ C] EPIKOTE 05311 E5311 HEXION Reactive Powder Bisphenol A 97–107 EPIKOTE 05390 E5390 HEXION Reactive Powder Bisphenol A 75–105 Araldite LT3366 LT3366 HUNTSMAN Reactive Powder Bisphenol A 160–200 Spunfab PA1541 PA1541 Spunfab Non-Reactive Veil Copolyamide 87–100 SAERfix EP SAER SAERTEX Reactive Veil / Bisphenol A RT Transfer Film EPIKOTE MGS PR685 PR685 HEXION Reactive Hot-Melt Bisphenol A 120 / RT AIRTAC 2E AIR2E AIRTECH Non-Reactive Spray Rubber based RT based on Bisphenol A and can therefore participate Table 2. Curing cycles applied for different epoxy resins. in the cross-linking of the selected epoxy resins due Resin Post-curing Tg [ C] to their similar chemical composition. Non-reactive RIMR135/RIMH137 8 h 70C90 tackifying agents are composed from a different Biresin C120/120-6 12 h 120 C 115 Resoltech 1500/1504 3 h 50 Cþ 3 h 100 Cþ 3 h 150 C 141 polymeric material (e.g. a thermoplastic copolya- Values obtained from corresponding TDS. mide) and will therefore not participate in the cross-linking process. RIMH137 from HEXION GmbH (Esslingen, EPIKOTE 05311 [39], EPIKOTE 05390 [40] and Germany) [47], Biresin CR120 with the hardener Araldite LT3366 [41] are supplied as white powder Biresin CH120-6 from SIKA AG (Bad Urach, with varying grain size distributions (E5311: 90– Germany) [48] and Resoltech 1500 with the hard- 125 mm; E5390: 50–90 mm; LT3366: 160–200 mm) ener 1504 from Resoltech Advanced Technology [22]. Spunfab PA1541 [42] was purchased as a copo- Resins (Rousset, France) [49] are considered for fur- lyamide veil from Spunfab Ltd. (Cuyahoga Falls, 2 ther characterization. Those three low-viscosity, USA) with a binder areal weight of 13 g/m . The self- amine curing epoxy resins are suited for LRI proc- adhesive veil SAERfix EP [43] was purchased from esses. Mixing ratios are stated in the respective tech- SAERTEX GmbH (Saerbeck, Germany). SAERfix EP nical data sheets (TDS) and are identical for all is compatible with epoxy resins due to chemical resins with 100:30 ± 2 wt.% (resin: hardener). Curing cross-linking in the course of curing. HEXION was accomplished within 24 h at room temperature, EPIKOTE Resin MGS PR685 [44] is a high-viscous followed by resin-specific post-cure cycles according resin component, based on Bisphenol A. At room to the respective TDS (Table 2). temperature PR685 shows a high viscosity ( 30 000 mPas). For application, the binder has to be heated up in a pneumatic hot-melt glue gun BUHNEN 2.2. Manufacturing of neat resin specimen HB700 K spray from Buhnen € GmbH (Bremen, In order to analyze the solubility of tackifying agents Germany) [45] and is applied at elevated tempera- within different epoxy resins, neat resin (NR) sam- tures above 100 C (2000–3200 mPas). AIRTAC 2E ples were produced with a binder fraction of 5 wt%. [46] was purchased from AIRTECH Europe Sarl Neat resin samples of 10 g were manufactured and (Haneboesch, Luxembourg) and is a rubber-based binder fractions were subsequently added and spray adhesive for temporary fixation of consumables manually stirred for 3 min at room temperature. A and fabrics. AIRTAC 2E is commercially available in pipette is used for extraction of a resin drop, which aerosol cans and is tacky at room temperature (RT) is applied onto a slide for subsequent microscope once the solvents have evaporated. analysis. The aforementioned procedure is not applicable for all binder types, since AIRTAC 2E is 2.1.3. Resin systems EPIKOTE Resin MGS RIMR135 and the corre- supplied in an aerosol can and needs to be sprayed sponding hardener EPIKURE Curing Agent MGS into the mixing bucket beforehand. The epoxy ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 5 sample is poured in afterwards. The samples are were applied manually with a targeted binder areal degassed for 5 min at 50 mbar absolute pressure and weight of 5 wt.%, which is considered as industrial subsequent curing of binder/epoxy resin samples at standard providing sufficient tack and processability at elevated temperatures was carried out in a convec- minimal BAW. Powder binders (E5390, E5311, tion oven UF260plus from Memmert GmbH LT3366) were manually applied using a sieve container (Buchenbach, € Germany) [50]. Different samples are and visually checked for homogeneous distribution cured at varying temperatures. The curing tempera- quality, while SAER and PA1541 were applied with the ture was increased from room temperature to given binder areal weight and binder distribution. 120 Cin 20 C increments. AIR2E and PR685 were manually applied using the For mechanical characterization of neat resin spe- given aerosol cans (AIR2E) and a hot-melt glue gun cimen and the effect of tackifying agents, a binder BUHNEN HB700 K spray. The distribution quality fraction of 2 wt.% was set. Due to agglomeration was checked visually. Preform stacks were subse- effects for PA1541 at a binder fraction of 5 wt%, the quently activated in the heating cabinet UF260 plus at binder ratio was reduced to 2 wt% in order to ensure elevated temperatures (ET) for 20 min (E5311 and comparability within the NR test series. Mixtures PA1541: 110 C; E5390: 100 C; LT3366: 200 C) using with an overall amount of 100 g were prepared and a caul plate for consolidation (100 N). For AIR2E, corresponding fractions of resin, hardener and binder SAER and PR685 no activation at ET was necessary were measured using the digital scale TGD 50-3C and respective preform consolidation was carried out [51] from KERN&SOHN GmbH (Balingen- at RT. Frommer, Germany). For mixing and homogeneous dispersion of the binder resin mixture, the centrifugal 2.4. Manufacturing of CFRP specimen via LRI mixer Mazerustar KK-250S [52] from Kurabo Industries Ltd. (Osaka, Japan) was used. For a homo- Vacuum Assisted Resin Infusion (VARI) was chosen for resin impregnation of the preform materials and geneous dispersion of binder particles and yarns, the the VARI setup can be seen in Figure 2. During binder was initially dissolved in the resin component VARI processing, the resin pot is generally left at using pre-set program 4 [52], which is specified for mixing and degassing. After subsequent addition of atmospheric pressure (approx. 1 atm), while the resin outlet is connected to the vacuum vent. Once the hardener, the neat resin binder mixture was again the resin inlet is opened, the resulting pressure gra- centrifuged (program 4). The superimposed revolu- dient within the VARI setup draws the resin tion and rotation of the mixing containers allows sim- ultaneous blending and degassing. Therefore, further through the preform. degassing was not required. Resoltech 1500 and the corresponding hardener Mechanical characterization of neat resin samples 1504 from Resoltech Advanced Technology Resins (Rousset, France) were used for the manufacturing acc. to DIN EN ISO 527-2 [53] requires specific dog of the CFRP specimens and were manually mixed bone specimen with an overall length of 170 mm and a thickness of 4.0 ± 0.2 mm (Specimen Type 1 A). Silicone according to the respective mixing ratio. The resin- casting molds were manufactured according to the hardener mixture was degassed for 5 min at 11 mbar absolute pressure prior to RT infusion. dimension stated in DIN EN ISO 527-2 using the casting Ambient conditions were recorded and documented silicon KDSV-25 from R&G Faserverbundwerkstoffe GmbH (Waldenbuch, Germany) [54]. The neat resin at all times (25.2 C; 42.5% rH; 964 hPa). After suc- binder mixtures were carefully poured into the mold cessful infusion of the CFRP specimens (330 mm x and cured at room temperature. Visual inspection did 240 mm x 2 mm), curing was carried out at room temperature conditions for 24 h. Subsequent post- not indicate pore formation. After RT curing, the speci- mens were taken from the silicon mold and post-cured curing was executed according to the corresponding according the corresponding TDS using the heating cab- TDS at 3 h at 50 C, 3 h at 100 C and 3 h at 150 C. inet UF260plus. Grinding of the specimens to the Different samples for mechanical testing were cut required thickness was carried out using the rotary on a water-cooled rotary saw (DIN EN ISO 14130 sander DP-U3 with a granulation of 125. [55]). Conditioning (DIN EN ISO 175 [56]) and fiber volume determination were carried out accord- ing to DIN EN 2564 [57](Table 3). 2.3. Preform manufacturing Assyt Bullmer Premiumcut ST from Bullmer GmbH 2.5. Microscopic analysis (Mehrstetten, Germany) was used for cutting of the G0827 fabric. In order to achieve a specimen thickness For microscopic analysis of specimen cross-sections of 2 mm for testing of interlaminar shear strength, 12 and neat resin samples, they were first embedded in layers of carbon fabric are required. Tackifying agents Epofix Resin from Struers GmbH (Willich, 6 F. HELBER ET AL. Figure 2. Manufacturing of CFRP specimen via LRI (a) Schematic VARI set-up (b) Experimental VARI-setup. Table 3. Description of CFRP specimen. linking. The three epoxy resins RIM 135/RIMH137, Binder Fiber volume Biresin CR120/CH120-6 and Resoltech 1500/1504 Specimen Thickness [mm] fraction [wt%] fraction [%] were used for DSC analysis. According to Section CF-NR 1.93 – 59.22 2.2 different binder materials with a binder fraction CF-E5311 2.13 4.5 54.52 CF-E5390 2.02 4.5 56.06 of 5 wt% have been added and the specimens have CF-LT3366 2.29 4.5 51.52 been cured in a convection oven Memmert UF260 CF-PA1541 2.59 7.4 42.36 CF-SAER 1.99 4.8 57.62 Plus with the corresponding cure cycle. Six DSC CF-PR685 2.08 16.1 57.22 samples for every binder resin mixture and the neat CF-AIR2E 2.01 3.7 57.36 1 resin have been prepared with a sample weight of Due to manual application, BAW of 5 wt% was not achieved. Average STD: 0.02 mm. 12 mgs. Differential Scanning Calorimetry analysis has been carried out using the DSC2920 from TA Germany). Grinding and polishing was carried out Instruments (NewCastle, USA). In order to delete on the rotary sanding machine Tegramin30 [58] the thermal background, two DSC cycles from room (Struers GmbH, Willich, Germany). Water-based temperature to 180 C at a heating rate of 10 C per wet grinding was done with the grinding disc MD- minute are executed with a cooling cycle at 20 C Gekko and varying SiC films (120; 220; 320), while per minute in between. For the determination of the polishing was done with diamond suspension T , the second DSC cycle was analyzed using the DiaPro Allegro 9 lm and DiaPro Dac 3 lm (Struers half height analysis algorithm. GmbH, Willich, Germany) on different polishing discs MD Largo and MD Dac. Final surface finish 2.7. Water absorption test was done with a Si-Suspension OPS Non Dry and a rotary polishing cloth MD Chem from Struers In order to analyze the effect of binder systems on GmbH (Willich, Germany). Inbetween the different water absorption and the potential degradation of grinding and polishing steps, the samples were mechanical properties, FRP specimens were condi- cleaned in a water-based ultrasonic bath Emmi 20 tioned in distilled water according to DIN EN ISO HC [59] from EMAG Technologies (Morfelden- 175. Prior to water storage, the specimens were con- Walldorf, Germany). Micrographs were obtained ditioned at 50 C for 24 h, cooled within an exsicca- using the Metallux 3 stereo metallurgical microscope tor for 3 h and the initial mass m was documented from Ernst Leitz Wetzlar GmbH (Wetzlar, with a digital scale TGD 50-3C from KERN&SOHN Germany). GmbH (Balingen-Frommer, Germany). Every 24 h the specimens were extracted from the water bath and the corresponding mass m was documented 2.6. Differential scanning calorimetry (DSC) until no further water absorption was evident (satur- In this study, the effects of different tackifying ation). For analysis of the water absorption, three agents on thermal properties were analyzed via specimens per FRP batch were measured and the Differential Scanning Calorimetry. With the help of average value was determined. calorimetry, the heat quantity is analyzed that is necessary for physical or chemical transformation of 2.8. Mechanical testing a material and can be used for determination of the glass transition temperature T . The temperature The tensile properties of the neat resin binder sam- range in which the transition occurs is called glass ples were evaluated according to DIN EN ISO 527-2 transition area and starts at the Onset Point using the tensile testing machine Hegewald & (T ). T and T are both dependent on the Peschke Inspekt table 20-1 and the corresponding Onset g Onset chemical structure as well as the degree of cross- software LabMaster from Hegewald & Peschke ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 7 Meß- und Pruftechnik € GmbH (Nossen, Germany) be detected. When the curing temperature is was used for data documentation and visualization increased, the remaining particles decrease and at with a measuring resolution of 50 Hz. Each speci- 120 C the binder is completely dissolved within the men type was tested at least six times. For strain epoxy resins. LT3366 does not dissolve at room measurement a video extensometer (VE) with a tele- temperature, as it can be seen in Figure 5a. Particle centric objective (focal length 334 mm) was used edges can be seen in every epoxy resin mixture. At (see Figure 3). Testing speed was set to 1 mm/min elevated temperatures (60 C) first changes in according to DIN EN ISO 527-2. appearance are obvious. Less particles can be Interlaminar Shear Strength testing was carried detected and edges are rounded. At 120 C this out for composite samples according to DIN EN observation is even more evident, since only few ISO 14130 using the Hegewald & Peschke particles remain in the resins. Furthermore, a halo Universalprufmaschine € Inspekt table 20-1 around the particles is visible. It is assumed that the (Hegewald & Peschke Meß- und Pruftechnik € halo results from dissolved binder. The self-adhesive GmbH, Nossen, Germany) testing machine with a veil SAER is compatible with epoxy resins and sup- set traverse speed of 1 mm/min. A minimum of 6 posed to participate in the chemical cross-linking samples were tested for dry and conditioned speci- during the curing process. At elevated temperatures, mens (see Figure 4). For determination of the inter- these binder fragments completely dissolve in the laminar shear strength s, the first maximum F resins, so that no residue is detectable. PR685 is a max,1 of the load curve is used within Eq. (1), with a given hot melt binder, consisting of a high viscous resin specimen width b and thickness h according to DIN component only and is supposed to participate in EN ISO 14130. the chemical cross-linking. Micrographs at RT and elevated temperatures reveal that the PR685 shows 3 F s ¼ (1) good solubility in all epoxy resins (Figure 5b). The 4 bh fact that even at RT no binder residue is apparent confirms that the PR685 participates in the chemical crosslinking. 3. Results and discussion The non-reactive, thermoplastic binder veil PA1541 3.1. Binder solubility is not expected to dissolve in the neat epoxy resin sam- Micrographs for every resin/binder combination ples. When looking at the micrographs (Figure 6a)the were analyzed at different curing temperatures. assumption is verified. No dissolution can be observed Representative micrographs can be seen in Figures 5 at room temperature or at elevated temperatures. and 6. Due to comparable solubility behavior, Especially at RT and 60 C a clear outline of the veil LT3366 was chosen as characteristic illustration for structure is evident. The micrograph at 120 Cshows a powder binder solubility (Figure 5a). The same slight difference in appearance. applies to the solubilities for SAER and PR685, for Similar to PA1541, the spray adhesive AIR2E is which PR685 was chosen as characteristic illustra- expected not to dissolve in the neat resin samples tion and can be seen in Figure 5b. E5311 does not due to its rubber-based composition. As it can be dissolve at room temperature, as a high quantity of seen in the micrographs (Figure 6b) at RT and ET, binder particles can be detected in the micrographs. the AIR2E does not dissolve or change its appear- With increasing temperature, the number of visible ance in any of the three epoxy resins. particles is significantly reduced and particle edges For the assessment of binder solubility, a distinc- rounded. At elevated temperature at 120 C nearly tion in four different degrees of solubility is selected, no residue is apparent. Best solubility can be seen in based on morphological changes in the binder resi- the Resoltech resin. At 60 C only few particles are due. Solubility implies that no or only few particles left and vanish at 120 C. E5390 shows similar or binder residue are visible in the micrographs at behavior. At room temperature binder particles can room temperature, as it can be seen for SAER and Figure 3. Tensile testing of neat resin samples acc. DIN EN ISO 527-2 (a) Test set-up (b) Detailed view of valid test specimen. 8 F. HELBER ET AL. Figure 4. ILSS testing acc. DIN EN ISO 14130 (a) Test set-up before load application (b) Detailed cross-sectional view. Figure 5. Reactive binder solubility analysis (a) LT3366 (b) PR685. Figure 6. Non-reactive binder solubility analysis (a) PA1541 (b) AIR2E. PR685 (Figure 5b). Solubility for PR685 was study reveals that PR685 is also well soluble within expected within the RIM135 resin. Both, tackifying other amine curing epoxy resins, such as CR120 and agent as well as epoxy resin, are provided from RES1500. Partial solubility states that after heat HEXION GmbH. The TDS of PR685 states that the application/curing at elevated temperatures binder binder consists of a high-viscous resin component particles disappear or the quantity of particles is sig- only and is especially suitable for dissolution in nificantly reduced, as it can be seen for E5311 and HEXION’s LRI matrix systems [44]. However, this E5390. Binder materials can be classified as critically ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 9 soluble, when particle edges are clearly rounded and E5390, LT3366, SAER, PR685 and PA1541 have a the remaining quantity of particles in the micro- minor effect on the T with a DT of ±2 C. The g g graph is reduced at elevated temperature, as it can rubber-based spray adhesive AIRTAC 2E reveals a be seen for LT3366. Insolubility states that even at DT of 9.41 C resulting in a T of 131.42 C. g g elevated temperatures binder residue are still clearly Differential Scanning Calorimetry analysis shows detectable in the neat resin sample, e.g. PA1541 and that especially the non-reactive tackifying agent AIR2E (Figure 6). The thermoplastic threads of AIR2E significantly affects the thermal properties of PA1541 at elevated temperatures (e.g. 120 C) all neat resin epoxy samples that have been charac- appear wider and not as sharp as in the graphs with terized. An average reduction of DT of 12.04 C lower temperature. This can be explained by the was measured, which might be linked to the non- melting temperature of PA1541, which lies between soluble rubber particles (see Figure 6b). 87 and 100 C. These morphological changes can therefore be explained by melting of the veil and 3.3. Water absorption not by dissolving in the resins. Conditioning in distilled water according to DIN EN ISO 175 of the CFRP specimens covered a 3.2. Thermoanalytical characterization period of 49 days. Within the first 10 days, a signifi- It is to be expected that the neat binder materials cant increase of the water content can be noted, and their solubility behavior will have an impact on which flattens afterwards and reaches a saturated the thermal properties and the glass transition tem- state after approximately another 30–40 days. As it perature T of the epoxy resins. Table 2 lists the can be seen in Figure 8, the conditioned specimens glass transition temperature of the resin systems with the tackifying agent PR685 has the highest used. Benchmark measurement series of six refer- saturated water absorption c (1.38 wt%), while espe- ence samples have been analyzed via DSC for every cially the powder binders E5311 (0.84 wt%), E5390 resin/binder mixture and were used for further com- (0.86 wt%) and LT3366 (0.79 wt%) show a reduced parison. The binder/resin samples are compared water content, when compared to the neat resin against the corresponding neat resin sample to sample (1.01 wt%). Table 5 lists relevant parameters, make a statement about the effects of different tacki- such as saturated water absorption cs, diffusion fying agents on the glass transition temperature coefficient D (acc. Eq. (2)) and time until saturation (Table 4). TM1, which were obtained according to Fick’s Looking at the effect of the tackifying agents on Law. the thermal behavior of the RIM135 resin it can be stated that all binder systems show only little influ- D ¼ (2) ence on the T behavior with the exception of p t AIR2E which reduces the T to 77.34 C(DT ¼ g g 9.06 C). For the epoxy resin SIKA Biresin The analysis of the water absorption test reveals CR120/CH 120-6, higher deviations can be that particulate tackifying agents exhibit reduced observed. The T was reduced by approx. 4.5 C for saturated water absorption c . The punctual applica- E5311 and PA1541 and with E5390 and PR685 T g tion pattern of powder binders results in a non- reduction is about 8.88 C and 11.87 C respectively. closed binder surface, therefore the diffusion is Highest deviation can be observed from AIR2E with reduced, when compared to other bindered speci- a DT of 17.63 C leading to an absolute T of g g mens. On the other hand, the thermoplastic binder 103.90 C. The majority of the reactive and non- veil PA1541 exhibits a continuous web of thermo- reactive binder materials reveal no significant impact plastic yarns, as it can be seen in Figure 6a, which on the thermal properties of the Resoltech 1500 causes enhanced water diffusion within the inter- resin. As illustrated in Table 4 and Figure 7 E5311, layer area. The enhanced saturated water absorption of PR685 might be caused by the enhanced binder Table 4. DSC analysis of neat resin samples. fraction and should be analyzed in future studies. RIM135 CR120 Resoltech1500 The rubber-based spray adhesive AIR2E does not 1 1 1 Specimen T [ C] STD T [ C] STD T [ C] STD g g g participate in the chemical cross-linking of epoxy DSC-NR 86.40 1.89 121.53 0.98 140.84 0.47 resins. Therefore, binder residue will remain within DSC-E5311 84.61 0.59 117.01 2.32 136.81 1.18 DSC-E5390 83.54 0.57 109.65 0.75 139.22 0.60 the interlayer region of the FRP specimens. When DSC-LT3366 83.88 0.66 120.41 0.50 139.11 0.60 compared to particulate tackifying agents, the mois- DSC-PA1541 86.07 0.75 117.15 1.76 142.21 0.99 DSC-SAER 85.86 0.30 124.21 1.57 140.52 1.25 ture uptake of AIR2E specimens is enhanced and DSC-PR685 86.30 0.71 112.65 1.56 141.46 0.70 might be caused by the dense surface coverage of DSC-AIR2E 77.34 0.68 103.90 5.08 131.42 0.43 Average Values were obtained from 6 DSC measurements. the AIR2E binder application (see Figure 1e). 10 F. HELBER ET AL. Figure 7. DSC analysis neat resin samples and effect of tackifying agents. Figure 8. Conditioning acc. DIN EN ISO 175 of 8 different CFRP specimens. Table 5. Water absorption test analysis acc. Fick’s Law. show no significant impact. When addressing the Specimen cs [wt%] D [mm /s] TM1 [d] elongation at break in Figure 10, similar observa- WAT-NR 1.01 2.93 10 39 tions can be found. Especially powder binder sys- WAT-E5311 0.84 2.76 10 28 tems seem to affect the fracture strain for all neat WAT-E5390 0.86 3.91 10 27 WAT-LT3366 0.79 4.26 10 28 resin samples the most. In general, it can be stated WAT-PA1541 1.07 5.21 10 30 that binder systems seem to shift the fracture behav- WAT-SAER 0.96 5.86 10 27 WAT-PR685 1.38 2.93 10 41 ior to a more brittle fracture behavior. Similar find- WAT-AIR2E 0.95 3.61 10 28 ings were observed by Schmidt et al. [28]. The brittle behavior can be explained due to non-dis- 3.4. Neat resin characterization solved binder residue within the neat-resin samples. These inhomogeneities will lead to local mechanical Figure 9 and Table 6 reveal that the tensile strength of neat resin samples is significantly affected by the property gradients and a reduced elongation at presence of auxiliary binder systems. The highest break. The E-Modulus is the property which is least impact can be observed, when powder binders are affected by tackifying agents. The E-modulus is cap- dispersed to the neat resin mixture. Especially E5390 tured from the gradient between 0.05 and 0.25% leads to highest reductions of the tensile strength elongation. In this area, barely any effects are for CR120 (52.29%) and RES1500 (41.82%), detected, as illustrated in Figure 11. while LT3366 leads to significant reduction for RIM135 (45.48%). When assessing the tensile 3.5. Composite characterization strength of RIM135 it can be noted that the tackify- ing agents AIR2E, PA1541, PR685 and SAER barely The assessment of interlaminar shear strength for composite laminates was carried out according to affect the mechanical properties. For CR120, similar results can be observed for AIR2E and PR685, while DIN EN ISO 14130. ILSS values were measured in for RES1500 the tackifying agents SAER and AIR2E both, dry and conditioned state and are compared ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 11 Figure 9. Ultimate tensile strength acc. DIN EN ISO 527. Table 6. Analysis of neat resin tensile properties acc. DIN high-performance components, adequate dosing EN ISO 527. technology is a decisive factor in order not to com- Tensile strength Fracture strain Tensile modulus promise the mechanical composite properties, which 1 1 1 Specimen r [MPa] STD e [%] STD E [MPa] STD max max is supported by further studies [60]. RIM135-NR 65.70 0.25 8.05 0.47 2724.56 32.75 As illustrated in Table 7, the presence of tackify- RIM135-5311 51.39 1.26 2.24 0.10 2757.70 64.40 ing agents leads to a reduction of ILSS properties RIM135-5390 43.43 4.11 1.75 0.21 2825.76 115.86 RIM135-LT3366 35.82 1.82 1.38 0.08 2746.70 74.84 for all binder systems which have been assessed. RIM135-PA1541 61.71 2.12 3.43 0.40 2794.60 92.10 Powder binder systems and SAER seem to have a RIM135-SAER 61.10 0.58 3.75 0.16 2729.70 81.48 RIM135-PR685 67.20 0.51 6.03 0.29 2884.56 53.55 minimal effect on interlaminar shear strength. RIM135-AIR2E 66.52 0.23 6.23 0.65 2813.34 104.13 PA1541, PR685 and AIR2E significantly affect the CR120-NR 63.01 4.30 3.91 0.65 2614.70 114.98 CR120-5311 50.83 1.03 2.37 0.07 2749.52 54.27 ILSS properties, while PR685 leads to highest reduc- CR120-5390 30.06 10.65 1.23 0.48 2728.40 120.69 tions. It needs to be stated that for PA1541 the CR120-LT3366 42.90 2.24 1.86 0.15 2788.68 133.93 CR120-PA1541 59.55 0.98 2.86 0.09 2802.06 47.12 highest absolute loads have been recorded. The fact CR120-SAER 51.02 6.25 2.69 0.59 2523.52 103.64 that PA1541 leads to an increase in specimen thick- CR120-PR685 59.99 3.75 3.18 0.39 2741.94 34.09 ness, results in reduced shear stress values. CR120-AIR2E 45.99 2.37 2.02 0.15 2711.92 22.98 RES1500-NR 55.17 7.27 1.75 0.28 3517.54 39.78 Moreover, it needs to be mentioned that the overall RES1500-5311 40.84 3.91 1.21 0.14 3596.68 39.77 binder amount for PR685 is elevated (15 wt%), RES1500-5390 32.10 1.07 4.61 4.52 3431.78 133.48 RES1500-LT3366 35.12 1.32 1.04 0.06 3456.34 159.25 therefore it is reasonable to assume that the reduced RES1500-PA1541 43.37 3.05 1.25 0.09 3693.24 45.27 ILSS values are due to the elevated binder fraction. RES1500-SAER 51.76 2.40 1.67 0.10 3397.36 120.61 RES1500-PR685 39.24 3.62 1.16 0.13 3636.22 163.68 Figure 12 also shows that the ILSS properties for RES1500-AIR2E 57.34 4.28 1.82 0.18 3574.14 13.79 conditioned specimens are reduced for all binder Neat polymer samples without binder are marked in bold for reference. systems that have been analyzed. In general, ILSS Average Values were obtained from 5 measurements. values are reduced between 2 and 5 MPa, when con- in Figure 12. Conditioning was carried out in order ditioned in distilled water acc. DIN EN ISO 175. to assess the influence of water absorption on the In general, it can be stated that no correlation mechanical properties. The majority of load curves between the neat resin properties (Section 3.4) and show a significant drop in force, which indicates a the ILSS properties was identified. shear failure within the layers of the composite When the ILSS properties between reactive and laminate and therefore a reduction of ILSS proper- non-reactive binder materials are distinguished, a clear tendency can be observed. In average reactive ties. However, the load curves of PR685 and PA1541 did not show distinct drops in load, which binder agents lead to a reduction of ILSS values by leads to a higher standard deviation. Based on man- 6.59% (10.06% after conditioning), while non-react- ual application with a pneumatical hand device, pre- ive tackifying agents cause a reduction of ILSS by cise dosing of PR685 was not feasible. Due to the 24.17% (30.52% after conditioning). Similar findings have been observed by other authors [29, 35, 37, 61, elevated binder content in the PR685 samples (BAW 15 wt%), a direct comparison is not valid. 62]. The significant reduction of ILSS properties of However, this finding reveals that for industrial non-reactive tackifying (PA1541 and AIR2E) agents application of binder technology within structural can be explained due to the present binder residue 12 F. HELBER ET AL. Figure 10. Elongation at break acc. DIN EN ISO 527. Figure 11. Young’s modulus E of neat resin samples and effect of tackifying agents. Figure 12. Graphical ILSS assessment acc. DIN EN ISO 14130. within the interlayer region of the ILSS specimens. that enhance microcracking and furthermore pro- The continuous thermoplastic veil PA1541 and the motes moisture absorption, which further reduces areal application of AIR2E lead to inhomogeneities ILSS properties upon conditioning. ADVANCED MANUFACTURING: POLYMER & COMPOSITES SCIENCE 13 Table 7. ILSS assessment acc. DIN EN ISO 14130. binder fraction during preform processes has to be Dry specimen Conditioned specimen reduced to a minimum. Another possibility for 1 1 1 1 F s F s binder effect reduction is the appropriate selection max max Specimen [kN] STD [MPa] STD [kN] STD [MPa] STD of a tackifying agent according to the resin system ILSS-NR 1.61 0.02 62.29 0.70 1.51 0.03 58.24 0.97 being used. Furthermore, the selective in-line appli- ILSS-E5311 1.65 0.02 58.99 0.97 1.62 0.02 56.39 0.63 ILSS-E5390 1.56 0.02 58.95 0.70 1.52 0.03 56.58 0.73 cation of binder systems during preforming proc- ILSS-LT3366 1.69 0.02 58.84 0.82 1.74 0.04 56.71 0.89 esses could result in superior overall composite ILSS-PA1541 1.92 0.08 49.89 2.11 1.57 0.03 45.30 0.78 ILSS-SAER 1.51 0.01 57.44 0.56 1.48 0.02 55.84 0.69 performance. However, the resulting local property ILSS-PR685 0.84 0.07 30.50 2.48 0.70 0.11 26.64 4.41 gradients and their effects on overall composite per- ILSS-AIR2E 1.20 0.02 45.18 0.62 1.12 0.02 41.81 0.62 1 formance need to be studied in detail. Average Values were obtained from 8 ILSS measurements. 4. Conclusions Disclosure statement In the present study, different reactive and non-react- No potential conflict of interest was reported by the ive tackifying agents and their respective effect on authors. composite performance has been studied. Binder solu- bility in three different epoxy resins was studied via Funding micrograph assessment, indicating solubility for react- This research was funded by the European research pro- ive tackifying agents, especially at elevated tempera- gramme Clean Sky 2. The ecoTECH project has received tures. For PR85 and SAER solubility was also verified funding from the European Union’s Horizon 2020 Clean at room temperature. For the non-reactive tackifying Sky 2 Joint Undertaking under the AIR FRAME ITD agents PA1541 and AIR2E no solubility was evident at grant agreement 945521. both, room temperature and elevated temperature. Thermal properties have been analyzed via Differential Data availability statement Scanning Calorimetry, revealing that the non-reactive The data that support the findings of this study are avail- tackifying agent AIR2E reduces the glass transition able from the corresponding author, FH, upon reasonable temperature of all epoxy resins (RIM 135: 9 C; request. 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Advanced Manufacturing Polymer & Composites Science – Taylor & Francis
Published: Jan 1, 2
Keywords: Dry fiber placement; binder systems; carbon fiber reinforced polymer; preforming; thermoset; mechanical performance; glass transition temperature; water absorption
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