fibers

Article Commingled Yarn Spinning for /Glass Fiber Composites

Niclas Wiegand and Edith Mäder *

Leibniz-Institut für Polymerforschung Dresden e.V., Dresden D-01069, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-351-465-8305

Academic Editor: Stephen C. Bondy Received: 25 April 2017; Accepted: 17 July 2017; Published: 20 July 2017

Abstract: Online commingled yarns were spun with three different polymeric matrices, namely (PP), (PA) and (PLA) and glass fibers. Tailored sizings were applied for the three matrices and the resulting mechanical performance of unidirectional composites was evaluated and compared. Significant improvements in the fiber/matrix bonding were achieved by employed sizing chemistry in order to achieve multifunctional interphases. The pure silane coupling agents provide the best performance for all matrices investigated. However, an additional film former has to be added in order to achieve fiber processing. Film formers compatible to the matrices investigated were adapted. The consolidation behavior during isothermal molding was investigated for polypropylene matrix. Different fiber volume contents could be realized and the resulting mechanical properties were tested.

Keywords: commingled yarns; Polypropylene; Polyamide; Polylactic Acid; sizing; glass fibers; thermoplastic composites; multifunctional interphase

1. Introduction Thermoplastic polymer matrices enable to manufacture composites at very short cycle times which makes them feasible for high volume productions. In contrast to thermosetting composites, they exhibit better impact resistance, higher toughness, recyclability and in the case of polylactic acid (PLA), even biodegradability. However, fiber impregnation is more difficult due to the high melt viscosity of the thermoplastic matrix. Different impregnation techniques have been proposed and performed. One of the most promising routines is provided by online commingled yarns which are based on the principle of homogeneous distribution of continuous matrix filaments and reinforcement glass filaments (GFs) during melt spinning [1]. This technique is more advantageous compared with air jet texturing because fibers were not damaged during commingling. The homogeneous fiber/matrix distribution of online commingled yarns especially leads to short impregnation paths and low void contents reflected by the high mechanical performance of the thermoplastic composites. Besides our previous and main focus on polypropylene (PP)-glass fiber composite development and characterization [1–5] as well as highlighting the importance of the role of film formers [2] for both properties of fibers, processing and performance of composites, industrial demands for technical and higher temperature stability are increasing for structural components to be designed for mechanical engineering. One objective of our work is to enhance the thermal stability by introducing polyamide (PA) which also leads to higher strength and stiffness of continuous fiber reinforced thermoplastics. Therefore, the innovative approach of online commingled yarns consisting of PA and GF was carried out by using our unique and multi-tasking pilot plant. On the other hand, biodegradable polymers like PLA have often been used in special non–load-bearing biomedical applications like tissue engineering or targeted release of active

Fibers 2017, 5, 26; doi:10.3390/fib5030026 www.mdpi.com/journal/fibers Fibers 2017, 5, 26 2 of 15 ingredients. In order to use PLA in orthopedics for bone fixation, it can be spun as online hybrid yarn with silica-based bioglass filaments to achieve the required high tensile strength and stiffness [6]. For this purpose, it is necessary to improve the adhesion strength between PLA and glass fiber to reach the properties of bone and to tailor the degradation behavior upon healing. An appropriately modified sizing can be used in order to reach these encountering challenges and being biocompatible. Interphases in fiber-reinforced composites are vital regions for the stress transfer between fibers and matrices [7–10]. Implementing a percolated carbon nanotube (CNT) network into composite interphases allows the detection of mechanical stress–strain behavior by monitoring electrical response [3–5,11–17] and contributes to additional effects besides silane coupling agents and polymeric film formers. The change in electrical resistance gives a quantitative parameter for the deformation as well as the early stage damage formation in highly strained regions of the interphase [5], as shown in our previous work with online commingled yarns. The objective of this work is to investigate the opportunities of the pilot plant to manufacture online commingled yarns with different polymeric matrices. As a comparison, PP, PLA and PA66 were spun together with GF. Besides the polymer matrix material aspects, the sizings, especially the influence of the different components within the sizing, will be discussed in terms of mechanical properties of GF composites. Online commingled yarns with different fiber volume contents will be spun and the effects on the mechanical properties of the composites are investigated. Another important topic is to investigate the consolidation parameters during isothermal in order to reduce the void content, thus increasing mechanical performance. Here we will perform tensile and compression shear tests in order to determine the interphase effects due to sizings, compatible to different thermoplastic matrices.

2. Experimental

2.1. Processing of Online Commingled Yarns and Interphase Modification The processing of online commingled yarns differs significantly from other commingling techniques. Glass and polymer filaments are simultaneously spun and commingled while passing the sizing applicator (Figure1). This approach combines different advantages towards traditional commingling techniques, such as air texturing: the filament distribution homogeneity is reasonably high since commingling is done at a state where both matrix and glass fiber yarns do not possess pronounced fiber integrity. The online commingled yarn integrity is performed simultaneously with applying a sizing on the sizing applicator roll. The mechanical load on the yarn during commingling is negligibly low as compared to air jet texturing. Neither glass fibers are broken nor polymer fibers are stretched which results in high yarn strength and avoids thermal shrinkage during consolidation. Depending on the matrix polymer, different sizings were applied during the fiber spinning, as shown in Table1. All sizings consisted of a silane coupling agent and at least one polymeric film former. In order to investigate GF/matrix adhesion strength, silane and polymeric film former were systematically changed. In this study, two different types of GFs were used during commingled yarn spinning at the Leibniz-Institut für Polymerforschung (IPF). For the preparation of structural composites, E-glass fibers with an average diameter of 17 µm were continuously spun and sized with aqueous solutions. As matrix material for the commingled yarn spinning, PP and PA6.6 (HG455 FB, Borealis and Ultramid A27, BASF, Ludwigshafen, Germany) were used. For the preparation of the biomedical composites, silica-based bioglass fibers, having the same diameter as the E-glass fibers, were online commingled with PLA fibers ( 2002D, NatureWorks LLC, Minnetonka, MN, USA). In-situ commingled yarns consisting of 204 glass and 102 polymer filaments, respectively, were always spun. Fibers 2017, 5, 26 3 of 15 Fibers 2017, 5, 26 3 of 15

Figure 1. Principle of online hybrid yarn spinning.spinning.

Table 1. 1.Sizing Sizing formulations formulations applied applied during during the hybridthe hybrid yarn yarnprocessing processing and glass and filament glass filament (GF) volume (GF) fractionsvolume fractions of manufactured of manufactured composites. composites.

Glass Fiber Sizing Sizing Formulation Matrix PolymerMatrix Glass Fiber Volume Fraction [%] Sizing Sizing Formulation Volume 3-Aminopropyl-triethoxysilane, maleic anhydride Polymer grafted PP film former (varying surfactants types, MA-g-PP 40.1–61.2Fraction [%] surfactant contents and molecule weights) 3-Aminopropyl-triethoxysilane, maleic anhydride grafted PP film 3-Aminopropyl-triethoxysilane MA-g-PP 50 Maleicformer anhydride (varying grafted surfactants PP film former types, reference surfactant contents MA-g-PP and MA-g-PP 50 40.1–61.2 Deionizedmolecule water weights) MA-g-PP 50 3-Aminopropyl-triethoxysilane, maleic anhydride 3-Aminopropyl-triethoxysilane PPMA-g-PP 50 50 grafted PP film former Maleic anhydride grafted PP film former reference MA-g-PP 50 3-Aminopropyl-triethoxysilane PP 50 Deionized water MA-g-PP 50 Maleic anhydride grafted PP film former reference PP 50 3-Aminopropyl-triethoxysilane, maleic anhydride grafted PP film Deionized water PPPP 50 50 3-Aminopropyl-triethoxysilane,former V1 PA6.6 50.4 film former 3-Aminopropyl-triethoxysilane PP 50 3-Ureidopropyltrimethoxysilane, polyurethane V2 PA6.6 45.6 Maleic anhydridefilm former grafted PP film former reference PP 50 3-Aminopropyl-triethoxysilane,Deionized water PP 50 V3 PA6.6 45.6 V1 3-Aminopropylpolyurethane/polyacrylate-triethoxysilane, film former polyurethane film former PA6.6 50.4 3-Ureidopropyltrimethoxysilane, V4 PA6.6 46.0 V2 3-Ureidopropyltrimethoxysilane,polyurethane/polyacrylate film former polyurethane film former PA6.6 45.6 3-Aminopropyl-triethoxysilane, polyurethane/ V5 3-Aminopropyl-triethoxysilane, polyurethane/polyacrylatePA6.6 film 43.7 V3 film former PA6.6 45.6 3-Ureidopropyltrimethoxysilane, polyurethane/epoxyformer V6 PA6.6 44.7 film former 3-Ureidopropyltrimethoxysilane, polyurethane/polyacrylate film V4V7 3-Aminopropyl-triethoxysilane PA6.6PA6.6 45.9 46.0 former S0 bioresorbable finish [3] PLA 37.5 V5 32-Aminoethyl-3-aminopropyl-trimethoxysilane,-Aminopropyl-triethoxysilane, polyurethane/epoxy epoxy film former PA6.6 43.7 S1 PLA 38.5 film former V6 3-Ureidopropyltrimethoxysilane, polyurethane/epoxy film former PA6.6 44.7 3-Ureidopropyltrimethoxysilane, polyurethane S2 PLA 37.6 V7 3film-Aminopropyl former -triethoxysilane PA6.6 45.9 2-Aminoethyl-3-aminopropyl-trimethoxysilane, S0S3 bioresorbable finish [3] PLAPLA 37.9 37.5 chitosan film former 2-Aminoethyl-3-aminopropyl-trimethoxysilane, epoxy film S1 PLA 38.5 2.2. Composite former S2 3-Ureidopropyltrimethoxysilane, polyurethane film former PLA 37.6 Unidirectional thermoplastic/glass fiber composites were produced using dry filament winding 2-Aminoethyl-3-aminopropyl-trimethoxysilane, chitosan film (IWTS3 Wickeltechnik, Erlangen, Germany) of the commingled yarnsPLA followed by isothermal37.9 former

Fibers 2017, 5, 26 4 of 15 compression molding. In detail, commingled yarns were wound onto a mandrel until it was covered to the desired specimen thickness (1, 2 and 4 mm, depending on mechanical testing standards). After filament winding, the mandrels were transferred to a personal computer (PC)-controlled platen press (KV207, Rucks, Germany) for the consolidation process. Each mandrel was heated up from room temperature to isothermal temperature at a constant rate of 10 K/min and a pressure of 0.43 MPa. After reaching the temperature, pressure was maintained along the defined consolidation time followed by a pressure increase up to the consolidation pressure and kept constant for 2 min. Afterwards, the mandrels were cooled down to room temperature at 50 K/min while maintaining the consolidation pressure. As a standard program, the following parameters shown in the Table2 were used.

Table 2. Parameters for composite manufacturing.

Isothermal Temperature Consolidation Time Consolidation Pressure Matrix Polymer [◦C] [min] [MPa] PP 225 21 2.25 PA6.6 295 20 2.25 PLA 170 20 1.5

Composites with GF volume fractions between 38 and 61% (cf. Table1) were produced. Polished cross sections of the GF laminates were used to determine the void content via optical microscopy (VHX2000, Keyence, Japan). In order to highlight voids within the polished cross sections, a blue dye was applied during specimen preparation. For a better understanding of the heat transfer during the consolidation process, 5 thermocouples were embedded along the thickness of the composite. Specimens for mechanical testing were cut out of the unidirectional plates using a rotating diamond saw.

2.3. Design of Experiments and Statistical Analysis For the systematic investigation on the process variables that influence the mechanical properties, designed experiments were used for PP/GF commingled yarns. In order to study the interaction between the sizing content and the fiber volume content, a full factorial design was chosen. The sizing content was varied on six levels from 0 to 15 wt % whereas 3 levels from 41 to 60% fiber volume content were investigated. Based on these variables, composites were fabricated and the resulting mechanical properties where studied. For the consolidation investigation, a Box-Behnken design resulted in 15 experiments according to three levels, and three factors was chosen. Besides the stepwise variation of the moulding pressure (0.75; 1.5 and 2.25 MPa), time (7, 14 and 21 min) and temperature (195, 210 and 225 ◦C) were changed and the resulting mechanical properties were used in order to determine consolidation quality. The statistical analysis of the experimental results was performed by analyzing the factorial design with statistics software (Statgraphics, Centurion, Miami, FL, USA). By creating regression equations, a model fit was obtained relating the results of the mechanical testing to main and secondary effects, respectively.

2.4. Physical Characterization For mechanical testing, a universal test machine (Allround Line, Zwick Roell, Uim, Germany) with a contact strain extensometer (multiXtens, Zwick Roell, Uim, Germany) was utilized. Compression shear tests (CST) [5], tensile tests (ISO 527-5), transverse tensile tests (ISO 527-4), and 4 point bending tests (DIN ISO 14125) were performed. Both pyrolysis at 700 ◦C in air and thermogravimetic analysis (Q500, TA Instruments, New Castle, DE, USA) were applied in order to determine the sizing content of the coated yarns. The crystallization and melting of the different matrix polymers was studied using differential scanning calorimetry (Q2000, TA Instruments, New Castle, DE, USA). The viscosity of the matrix and sizing polymers were Fibers 2017, 5, 26 5 of 15 determined using a rational rheometer (ARES G2, TA Instruments, New Castle, DE, USA). Prior to the investigations, all polymers were dried at 40 ◦C in a vacuum oven.

3. Results and Discussion

3.1. Fundamental Aspects of Tailored Interphases in Commingled Yarns In contrast to thermosetting resins, thermoplastic filaments do not undergo chemical reactions during consolidation. Nevertheless, thermoplastics can be divided into polar polymers containing functional groups (e.g., PA6.6) and non-polar polymers. The non-polar PP is a typical example for a nonreactive polymer, but even the reactivity of PP can be increased by maleic anhydride grafting at compounding temperatures [7]. In order to further enhance adhesion between the glass fiber and matrix, coupling agents are applied via a sizing approach during the glass fiber spinning process. In general, sizings are mixtures of silane coupling agents, polymeric film formers, antistatic agents and lubricants [8]. Only silane coupling agents are necessary to promote adhesion between the glass fiber surface and the matrix. In detail, the silanol groups can form either hydrogen or covalent bonds to the glass fiber and the organofunctional groups can react with the matrix polymer during processing. Other ingredients are added to the sizing in order to achieve additional properties such as fiber protection against abrasion, strand integrity, fiber wet-out, lubrication and anti-static which are necessary for handling and processing. Figure2 illustrates the principle influence of PP grafting and sizing components on the transverse tensile strength of the composites. The laminate strength is up to 3 times higher for the MA-g-PP versus the PP matrix. This seems reasonable since the chemical reactivity is increased by the functional maleic anhydride groups at the end of the polymer chain. Nevertheless, the sizing does play an important role on the strength properties in both matrix systems. Only a mixture of silane and MA-g-PP film former (sizing formulation consists of 1 wt % silane and 10 wt % MA-g-PP film former) results in reasonable strength properties around 7.5 MPa, whereas pure film former and pure silane provide strength levels below 2.5 MPa. The performance of each sizing can be explained by the chemical nature. The mixture of silane and film former can form a covalent bond to the glass fiber surface and to the functional group of the film former via the silane. The film former then cocrystallizes via physical entanglements with the molecular chains of the PP matrix. Thus, stress transfer from the matrix into the fiber is possible. The pure silane sizing can only form bonds with the glass fiber surface. Due to the absence of functional groups within the molecular chains of the matrix, the organo functional group of the silane is missing its counterpart, resulting in poor stress transfer. Vice versa, a pure film former sizing can form entanglements with the matrix but does not form bonds to the glass fiber surface, resulting in poor transverse strength as well. Quite interestingly, glass fibers sized only with deionized (DI) water show increased strain to failure levels of up to 10% at low strength levels, although no bonds are formed between the glass fiber and the matrix. One explanation may be that only the matrix Poisson contraction is hindered due to the glass fibers. The same sizings perform differently in a MA-g-PP matrix. Pure film former and deionized water result in low strength levels, as expected. In contrast to the previous results, the silane sizing shows the highest strength and strain to failure results. This can be explained by the covalent bonds which are formed between the glass fiber surface and the grafted matrix molecular chains via the silane. The sizing containing a mixture of MA-g-PP film former and silane shows a lower strength and strain to failure in comparison to the pure silane sizing. It is likely that the silane will form covalent bonds with the glass fiber surface, whereas the organo functional complex can either react with the functional groups of the film former, which then form entanglements with the matrix polymer, or directly reacts with the functional groups of the matrix polymer. Fibers 2017, 5, 26 6 of 15

with the functional groups of the film former, which then form entanglements with the matrix Fibers 2017, 5, 26 6 of 15 polymer, or directly reacts with the functional groups of the matrix polymer.

Figure 2. Influence of the matrix grafting (polypropylene (PP) un-grafted, left, MA-g-PP, right) and the sizingFigure components 2. Influence (1 of wt the % matrix silaneand grafting 10 wt (polypropylene % MA-g-PP film (PP former)) un-grafted, on the mechanicalleft, MA-g-PP, performance right) and ofthe unidirectional sizing components GF/PP composites(1 wt % silane having and a fiber 10 volume wt % MA content-g-PP of film 50%. former) on the mechanical performance of unidirectional GF/PP composites having a fiber volume content of 50%. Although the pure silane sizing shows the best mechanical results, it cannot be used in real Although the pure silane sizing shows the best mechanical results, it cannot be used in real processing, as previously mentioned. Therefore, the purpose of the following investigations was processing, as previously mentioned. Therefore, the purpose of the following investigations was to to determine how the performance of a film former/silane mixture is influenced by its ingredients. determine how the performance of a film former/silane mixture is influenced by its ingredients. The The commercially available film formers do not only consist of a MA-g-PP, but also contain surfactants commercially available film formers do not only consist of a MA-g-PP, but also contain surfactants and . Figure3A shows the influence of the molecular weight on the transverse tensile and plasticizers. Figure 3A shows the influence of the molecular weight on the transverse tensile strength. The molecular weight affects the amount of entanglements which each chain forms with strength. The molecular weight affects the amount of entanglements which each chain forms with other chains. Higher weights result in more entanglements, thus achieving higher tensile strength other chains. Higher weights result in more entanglements, thus achieving higher tensile strength properties [18]. The lowest molecular weight film former shows the lowest strength. The strength properties [18]. The lowest molecular weight film former shows the lowest strength. The strength can can be increased by increasing the molecular weight but remains unaffected as soon as a reasonable be increased by increasing the molecular weight but remains unaffected as soon as a reasonable molecular weight is applied. Besides the molecular weight, the surfactant and the can molecular weight is applied. Besides the molecular weight, the surfactant and the plasticizer can have have an influence on the performance of a composite. Figure3B shows the influence of an additional an influence on the performance of a composite. Figure 3B shows the influence of an additional surfactant and plasticizer within a film former in comparison to a reference one. The purpose of the surfactant and plasticizer within a film former in comparison to a reference one. The purpose of the surfactant is to stabilize the MA-g-PP within the film former emulsion, whereas the plasticizer facilitates surfactant is to stabilize the MA-g-PP within the film former emulsion, whereas the plasticizer the emulsification process [19]. From a manufacturing point of view, it is therefore desirable to apply facilitates the emulsification process [19]. From a manufacturing point of view, it is therefore these substances in excess. Nevertheless, both substances are likely to reduce the strength properties of desirable to apply these substances in excess. Nevertheless, both substances are likely to reduce the the composites. As it is shown in Figure3B, a 10 wt % increase of surfactant content (red) in comparison strength properties of the composites. As it is shown in Figure 3B, a 10 wt % increase of surfactant to the reference film former yields no significant change in the strength properties. On the other hand, content (red) in comparison to the reference film former yields no significant change in the strength the same increase of plasticizer content results in a significant reduction of the strength. properties. On the other hand, the same increase of plasticizer content results in a significant As highlighted in Figure4A, besides the surfactant level, it is also the surfactant charge which reduction of the strength. affects the transverse tensile strength. From a processing point of view, it is desirable to generate As highlighted in Figure 4A, besides the surfactant level, it is also the surfactant charge which stable sizings with desired charge. It was possible for all surfactant charges to produce glass fibers affects the transverse tensile strength. From a processing point of view, it is desirable to generate in quantities necessary for composite manufacture. Only the nonionic surfactant tends to reduce the stable sizings with desired charge. It was possible for all surfactant charges to produce glass fibers in strength properties significantly, whereas cationic, anionic and nonionic/anionic mixtures result in quantities necessary for composite manufacture. Only the nonionic surfactant tends to reduce the nearly the same strength. strength properties significantly, whereas cationic, anionic and nonionic/anionic mixtures result in nearly the same strength. Fibers 2017, 5, 26 7 of 15 FibersFibers 2017 2017, 5,, 265, 26 7 of7 of15 15

FigureFigure 3.3. Influence3.Influence Influence ofof of molecularmolecular molecular weightweight weight ofof of aa MA-g-PPMAa MA-g--PPg-PP filmfilm film formerformer former ((AA )()A andand) and thethe the componentscomponents components withinwithin within aa a filmfilmfilm formerformer former ((BB ).()B. ).

FigureFigure 4.4. Influence4.Influence Influence ofof of thethe the surfactantsurfactant surfactant chargecharge charge ((AA ())A andand) and thethe the surfactantsurfactant surfactant contentcontent content onon on thethe the glassglass glass fiberfiber fiber surfacesurface surface ((BB)()B onon) on thethe the mechanicalmechanical mechanical propertiesproperties properties ofof GF/PPofGF/PP GF/PP composites composites. composites. .

In order to investigate the influence of the surfactant content (Figure 4B) independent of the MA- InIn order order to to investigate investigate the the influenceinfluence of the the surfactant surfactant content content (Fig (Figureure 4B)4B) independent independent of ofthe the MA- g-PP, mixtures of silane and surfactant were applied to the glass fiber surface. A very low content of MA-g-PP,g-PP, mixtures mixtures of of silane silane and and surfactant surfactant were were applied applied to to the the glass glass fiber fiber surface. surface. A A very very low low content content of 0.75 wt % surfactant already reduces the strength by almost one third, compared with the reference of0.75 0.75 wt % surfactant already reducesreduces thethe strengthstrength by by almost almost one one third, third compared, compared with with the the reference reference without surfactant. This highlights that surfactants cannot be beneficial for strength but have to be withoutwithout surfactant. surfactant. This This highlights highlights that that surfactants surfactants cannot cannot be be beneficial beneficial for for strength strength but but have have to to be be applied with care in order to achieve stable film former emulsions. appliedapplied with with care care in in order order to to achieve achieve stable stable film film former former emulsions. emulsions.

3.2.3.23.2. InfluenceInfluence. Influence of of Fiber Fiber Sizing Sizing Content Content on on thethe the MechanicalMechanical Mechanical PerformancePerformance Performance InInIn thethe the previousprevious previous chapter,chapter chapter, thethe, the fundamentalfundamental fundamental influenceinfluence influence ofof of thethe the sizingsizing sizing chemistrychemistry chemistry onon o then the strengthstrength strength of of aa a unidirectionalunidirectional compositecomposite composite waswas was discussed.discussed. discussed. Within Within thethe the following following paragraph,paragraph paragraph, thethe, the influenceinfluence influence ofof of the the amount amount of ofsizing sizing sizing which which which is is applied is applied applied during during during glass glass glass fiber fiber spinning fiber spinning spinning will be will will further be be further examined.further examined. examined. Thus, the Thus, Thus, sizing the the system sizing sizing alwayssystemsystem consistsalways always consists of consists the same of ofthe the APS same same silane APS APSat silane 1silane wt at % at1 plus wt1 wt % a % MA-g-PPplus plus a MAa MA film-g-PPg- formerPP film film former varying former varying varying from 0from up from to 0 0 15upup wtto to15 % 15 ofwt wt the% %of sizing. ofthe the sizing. Bysizing. increasing By By increasing increasing the weight the the weight weight percent percent percent of the of filmofthe the film former, film former former the, solidthe, the solid contentsolid content content of the of of sizingthethe sizing sizing is increased, is isincreased increased which, which, tendswhich tends to tends enhance to toenhance enhance the viscosity the the viscosity viscosity of the sizing.of ofthe the Consequently,sizing. sizing. Consequently, Consequently, the fiber th picks the fibere upfiber morepickspicks sizingup up more more when sizing sizing it passes when when theit passesit sizingpasses the applicator the sizing sizing applicator rollapplicator yielding roll roll toyielding anyielding increased to toan an increased sizing increased content sizing sizing and content content thus aand thickerand thus thus average a thickera thicker sizing average average layer sizing on sizing the layer fiber. layer on Besides on the the fiber. fiber. the Besidessizing Besides content, the the sizing sizing the content,fiber content, volume the the fiber content fiber volume volume was systematicallycontentcontent was was systematically changedsystematically from changed 42changed until from 62 from vol 42 %.42 unt Theuntil 62il design 62 vol vol % of .% The experiments. The design design of was ofexperiments experiments used for planningwas was used used and for for analyzingplanningplanning and the and strengthanalyzing analyzing and the the stiffness strength strength results and and stiffness ofstiffness the composite results results of ofthe testing. the composite composite testing. testing. AsAs the the ParetoPareto Pareto chart chart in in FigureFigure Figure5 5 shows, shows,5 shows, thethe the filmfilm film formerformer former hashas has aa significant significanta significant influenceinfluence influence onon on thethe the strength.strength. strength. ItIt It tends tends tends to to to decrease decrease decrease the the the transverse transverse transverse tensile tensile tensile strength. strength. strength. The The fiberfiber fiber volume volume content content is is not not significantly significantly significantly changedchanged from from a statisticala statistical point point of ofview. view. The The coupling coupling effect effect between between both both tends tends to tobe be right right on on the the edgeedge of of significance significance, indicated, indicated by by the the blue blue line line in in the the chart. chart. Quite Quite surprisingly, surprisingly, it it is is not not the the fiber fiber Fibers 2017, 5, 26 8 of 15 changed from a statistical point of view. The coupling effect between both tends to be right on the edge Fibers 2017, 5, 26 8 of 15 ofFibers significance, 2017, 5, 26 indicated by the blue line in the chart. Quite surprisingly, it is not the fiber8 volume of 15 contentvolume which content dominates which dominates the strength, the but strength the sizing, but thecontent. sizing The content. principle The reduction principle inreduction strength in due volume content which dominates the strength, but the sizing content. The principle reduction in tostrength the increased due to sizing the increased level can sizing be explained level can be by explained the chemical by the nature chemical of the nature film former.of the film In contrastformer. to strength due to the increased sizing level can be explained by the chemical nature of the film former. theIn matrix contrast PP, to thethe filmmatrix former PP, the consists film former of a significantly consists of a significantly lower molecular lower weight molecular PP. Toweight a large PP. extent, To In contrast to the matrix PP, the film former consists of a significantly lower molecular weight PP. To thea physicallarge extent properties,, the physical e.g., strengthproperties and, e.g., stiffness, strength are and influenced stiffness, by are the influenced molecular by weight. the molecular As a bulk a large extent, the physical properties, e.g., strength and stiffness, are influenced by the molecular weight. As a bulk material, the film former polymer is very brittle and has a low strength of about 7 material,weight. theAs a film bulk former material polymer, the film is former very brittle polymer and is has very a lowbrittle strength and has of a aboutlow strength 7 MPa of and about a strain 7 MPa and a strain to failure of only 4%. In contrast, the matrix polymer shows intensive yielding and toMPa failure and of a onlystrain 4%. to failure In contrast, of only the 4% matrix. In contrast, polymer the shows matrix intensive polymer shows yielding intensive and strength yielding of and about strength of about 25 MPa. Based on the increased strain concentrations for higher fiber volume 25strength MPa. Based of about on the 25 increased MPa. Based strain on concentrations the increased for strain higher concentrations fiber volume for contents higher fiber[20], one volume would contents [20], one would rather assume a waste reduction in strength for increased fiber levels. rathercontents assume [20], a one waste would reduction rather assume in strength a waste for increasedreduction in fiber strength levels. for increased fiber levels.

FigureFigure 5. 5. ParetoPareto chart chart ( (leftleft) and response surface surface plot plot (right (right) ) for for the the transverse transverse tensile tensile strength strength Figure 5. Pareto chart (left) and response surface plot (right) for the transverse tensile strength dependingdepending on on the the fiber fiber volume volume contentcontent and the film film former former content. content. depending on the fiber volume content and the film former content.

In contrast to the strength results, Figure 6 shows that the fiber volume content has a major InIn contrast contrast to to the the strength strength results, results, Figure 66 showsshows that the fiber fiber volume volume content content has has a a major major influence on the Young’s modulus perpendicular to the direction of fibers. Increasing the fiber influenceinfluence on on the the Young’s Young’s modulus modulus perpendicular perpendicular to the the direction direction of of fibers. fibers. Increasing Increasing the the fiber fiber volume content results in increased Young’s modulus, which corresponds to the well-established volumevolume content content results results inin increasedincreased Young’sYoung’s modulus,modulus, which which corresponds corresponds to to the the well well-established-established micromechanical models [21]. On the other hand, the film former has a detrimental influence on the micromechanicalmicromechanical models models [ 21[21].]. OnOn thethe otherother hand hand,, the the film film former former has has a adetrimental detrimental influence influence on onthe the modulus. Throughout all of the investigated film former levels, the modulus is significantly reduced. modulus.modulus. Throughout Throughout all all of of thethe investigatedinvestigated film film for formermer levels levels,, the the modulus modulus is significantly is significantly reduced. reduced. Although the Young’s modulus of the bulk film former and the matrix polymer are only marginally AlthoughAlthough the the Young’s Young’s modulus modulus ofof thethe bulkbulk filmfilm former and and the the matrix matrix polymer polymer are are only only marginally marginally different, within the fiber interphase the low molecular weight film former clearly reduces the overall different,different within, within the the fiber fiber interphase interphase thethe low molecular weight weight film film former former clearly clearly reduces reduces the the overall overall stiffness of the composite. stiffnessstiffness of of the the composite. composite.

Figure 6. Pareto chart (left) and response surface plot (right) for the Young’s modulus depending on FigureFigure 6. 6.Pareto Pareto chart chart ( left(left)) and and responseresponse surface plot plot ( (rightright) )for for the the Young Young’s’s modulus modulus depending depending on on the fiber volume content and the film former content. thethe fiber fiber volume volume content content and and the the filmfilm formerformer content.

Fibers 2017, 5, 26 9 of 15

3.3. Influence of Time, Temperature and Pressure During Consolidation on the Mechanical Performance

TheFibers consolidation 2017, 5, 26 of continuous fiber-reinforced thermoplastics has been widely addressed9 of 15 in the literature [22–31]. It is well accepted that the impregnation during the consolidation consists of a macroscale3.3. Influence and microscale of Time, Temperature impregnation. and Pressure During During the Consolidation macroscale impregnation,on the Mechanical molten Performance matrix flows between fiberThe consolidation bundles, whereas of continuous during the fiber microscale-reinforced impregnation, thermoplastics thehas singlebeen widely fibers areaddressed infiltrated in by the moltenthe literature matrix, [22 which–31]. It is is roughly well accepted considered that the by impregnation Darcy’s law. during the consolidation consists of a Themacroscale aim of and this microscale chapter im ispregnation. to investigate During the the influence macroscale of impregnation the processing, molten parameters matrix flows on the transversebetween tensile fiber strengthbundles, usingwhereas a Design during ofthe Experiments microscale impregnation (DOE) approach., the single For fibers a better are understanding infiltrated of theby impregnation the molten matrix behavior, which is and roughly the voidconsidered content, by Da polishedrcy’s law. cross sections for each specimen The aim of this chapter is to investigate the influence of the processing parameters on the were prepared. In order to monitor the heat transfer and the temperature distribution during the transverse tensile strength using a Design of Experiments (DOE) approach. For a better consolidation, each plate was equipped with thermocouples along the thickness. understanding of the impregnation behavior and the void content, polished cross sections for each Figurespecimen7 demonstrates were prepared. that, In order during to monitor the isothermal the heat transfer consolidation and the temperature trials, the distribution time and the temperatureduring the have consolidation a significant, each impact plate was on equipped the mechanical with thermocouples properties. along This the behavior thickness. correlates well with previousFigure composite 7 demonstrate consolidations that, during studies the [isothermal28]. Besides consolidation these linear trials factors,, the timea quadratic and the term of thetemperature time and anhave interaction a significant between impact temperatureon the mechanical and properties. time are responsible This behavior for correlates the reduction well in strength.with previous The influence composite of the consolidation pressure isstudies much [28 lower]. Besides than these all previouslylinear factors, mentioned a quadratic onesterm of which can bethe attributed time and to an the interaction homogeneous between fiber temperature distribution and time level are in theresponsible online commingled for the reduction yarns in and consequentlystrength. shortThe influence distances of the for pressure the matrix is much to flow. lower than all previously mentioned ones which can be attributed to the homogeneous fiber distribution level in the online commingled yarns and According to the literature [25], the void content tends to determine the mechanical performance consequently short distances for the matrix to flow. of the composite. Figure8 reports the evolution of the voids during different consolidation conditions. According to the literature [25], the void content tends to determine the mechanical performance As expectedof the composite. by the results Figure of8 reports the DOE, the evolution the void of content the voids decreases during different with increasing consolidation temperature conditions. and ◦ holdingAs expect time,ed whereas by the results the pressure of the DOE, only the has void a minorcontent influencedecreases with illustrated increasing for temperature the 14 min and 195 C sampleholding (Pressure time 0.75, whereas and 2.25 the MPa). pressure The only temperature has a minor has influence a major illustrated impact on for the the consolidation 14 min 195 quality,°C as expected.sample (PressureFrom 180 0.75◦C consolidation and 2.25 MPa). temperature The temperature onwards, has a majorthe void impact content on the can consolidation significantly be reduced,quality, highlighted as expected. by the From orange 180 dotted °C consolidation line within the temperature temperature onwards graphs, the in Figure void 8 content. The influence can of thesignificantly time on the be mechanical reduced, highlighted performance by can the be orange clearly dotted espaliered line within by a closethe temperature look on the graphs temperature in distributionFigure 8. through The influence the thickness of the time alongon the consolidationmechanical performance time. As can soon be clearly as the espaliered heatable by platen a close press look on the temperature distribution through the thickness along consolidation time. As soon as the reaches its pre-defined temperature level (0 min time, black line), the temperature distribution along heatable platen press reaches its pre-defined temperature level (0 min time, black line), the the 48 plies of the composite is somehow different. The top plies near the press have already reached temperature distribution along the 48 plies of the composite is somehow different. The top plies near the definedthe press temperature have already level, reached whereas the defined the lowest temperature plies level are almost, whereas 100 the K lowest colder. plies The are temperature almost difference100 K betweencolder. The top temperature and bottom difference becomes between less withtop and proceeding bottom becomes time. less After with 21 proceeding min (green time. line) it is almostAfter in 21 equilibrium.min (green line) Since it is almost the temperature in equilibrium. is introduced Since the temperature via the top is introduced plie, void via content the top is not homogeneouslyplie, void content distributed is not homogeneously through the thickness. distributed Only through the the bottom thickness. areas Only which the have bottom not areas reached temperaturewhich have levels not abovereached 180 temperature◦C along thelevels holding above 180 time °C tend along to the have holding voids. time tend to have voids.

FigureFigure 7. Pareto 7. Pareto chart chart (left (left) and) and response response surface surface plot ( (rightright) ) for for the the transverse transverse tensile tensile strength strength dependingdepending on the on processingthe processing parameters, parameters time, time and andtemperature, temperature, for for a aconstant constant pressure pressure of 1.5 of 1.5MPa MPa.. Fibers 2017, 5, 26 10 of 15 Fibers 2017, 5, 26 10 of 15

FigureFigure 8. (8.Left (Left) Temperature) Temperature distribution distribution through through the the thickness thickness for for 195, 195, 210 210 and and 225 225◦C °C isothermal isothermal temperaturetemperature at theat the beginning beginning of theof the isothermal isothermal holding holding (black), (black), after after 7 min 7 min (red), (red), after after 14 14 min min (green) (green) andand after after 21 min21 mi (blue);n (blue) (right; (r)ight Corresponding) Corresponding polished polished cross sectioncross section to the temperatureto the temperature and time and with time voidswith highlighted voids highlighted in blue (for in blue a constant (for a pressureconstant ofpressure 0.75 MPa of 0.75 despite MPa the despite middle the top middle one). Thetop orangeone). The dottedorange line dotted highlights line highlights the temperature the temperature from where from on voidwhere free on composites void free composites are generated. are generated.

It is still open for discussion what really happens in the temperature region until 180 °C. The It is still open for discussion what really happens in the temperature region until 180 ◦C. The glass glass transition temperature of the PP matrix is somewhat in the region of −5 °C. Melting of the transition temperature of the PP matrix is somewhat in the region of −5 ◦C. Melting of the crystalline crystalline areas occurs between 155 and 170 °C. As a consequence, the viscosity of the PP drops areas occurs between 155 and 170 ◦C. As a consequence, the viscosity of the PP drops within the within the melting area by almost 3 decades, as reported in Figure 9. The drop in viscosity goes along melting area by almost 3 decades, as reported in Figure9. The drop in viscosity goes along with with different distinctive events which can be observed in a polished cross section of a specimen different distinctive events which can be observed in a polished cross section of a specimen (195 ◦C, (195 °C, 7 min, 0.75 MPa, Figure 8 right) containing perfect consolidated areas on the top and almost 7 min, 0.75 MPa, Figure8 right) containing perfect consolidated areas on the top and almost pristine pristine commingled yarn areas with no sign of consolidation on the bottom. At temperatures below commingled yarn areas with no sign of consolidation on the bottom. At temperatures below 165 ◦C, the 165 °C, the circular-shaped polymer filaments deform plastically changing their shape, but voids with circular-shaped polymer filaments deform plastically changing their shape, but voids with the polymer the polymer bundles are still visible. Between 165 and 180 °C, the polymer filaments sinter, bundles are still visible. Between 165 and 180 ◦C, the polymer filaments sinter, intrabundle voids are intrabundle voids are removed and macroscale impregnated areas become visible. Above 180 °C, the removed and macroscale impregnated areas become visible. Above 180 ◦C, the glass fibers are wetted glass fibers are wetted out by matrix which flows from the resin-rich pools into the glass fiber bundles. out by matrix which flows from the resin-rich pools into the glass fiber bundles. These findings go These findings go along with more recent literature results which try to explain commingled yarn along with more recent literature results which try to explain commingled yarn consolidation via a consolidation via a sintering approach [32] rather than by Darcy’s law. sintering approach [32] rather than by Darcy’s law. Nevertheless, the void content cannot be the only reason for an increased mechanical performance throughout the consolidation trials. Especially composites which do not show any sign of voids (e.g., above 195 ◦C/14 min or 210 ◦C/7 min) display differences in the transverse tensile strength, as presented in the response surface plot in Figure7. Since the heating and cooling rates were not changed throughout all experiments, these changes cannot be directly attributed to any difference in the melting or crystallization state during consolidation. Although the consolidation on the macro- and microscale appears to be void–free, it is still to be questioned what happens on the nanoscale Fibers 2017, 5, 26 11 of 15 near the glass fiber surface. The important influence of the glass fiber interphase on the mechanical performance has been already addressed in the two previous chapters. An increase in temperature always goes along with increased molecule mobility, which facilitates the interpenetration of the matrix into the sizing network and vice versa. Furthermore, the probability that each functional group can react with a counterpart is also increased. It is likely that all these effects tend to have a positive impact Fiberson the 2017 mechanical, 5, 26 performance. 11 of 15

FigureFigure 9. 9. (Left(Left) )Viscosity Viscosity-temperature-temperature plot plot for for matrix matrix and and film film former former PP PP;; (r (rightight) )a a closer closer look look on on the the consolidationconsolidation behavior behavior in in commingled commingled yarn yarn composites: composites: Plastic deformation deformation of of PP PP filaments, filaments, sintering sintering andand intrabundle intrabundle void void reduction reduction followed followed by by polymer polymer flow flow into into the the glass glass fiber fiber bundles. bundles.

3.4.Nevertheless, Effects of the Fiber the Volume void Content content in cannot GF/PP Composites be the only on the reason Mechanical for an Properties increased mechanical performance throughout the consolidation trials. Especially composites which do not show any sign of voidsThe ( fibere.g., volumeabove 195 content °C/14 of min online or hybrid 210 °C yarns/7 min) was display varied difference by the spinnings in the pump transverse settings tensile during strength,the polymer as presented yarn spinning in the process response within surface the regionplot in of Figure about 7. 40 Since to 60 volthe %.heating In order and to cooling manufacture rates werespecimens not changed for mechanical throughout testing, all experiments unidirectional, these composites changes were cannot made be baseddirectly on attributed filament winding to any ◦ differenceof hybrid in yarns the melting followed or bycrystallization isothermal consolidationstate during consolidation. at 225 C using Although a heatable the consolidation platen press aton a thepressure macro of- and 1.5 MPamicroscale for 21 minappears throughout to be void the– variationfree, it is ofstill the to fiber be questioned volume content. what Thehappens fiber/matrix on the nanoscaledistribution near homogeneity the glass fiber was surface. very high The and important was not affected influence by of the the fiber glass volume fiber variation.interphase Figure on the 10 mechanicalshows exemplarily performance polished has been cross already sections addressed of unidirectional in the two GF/PP previous composites chapters. containing An increase 42 vol in % temperatureand 60 vol % glassalways fibers, goes respectively. along with Figure increased 11 reveals molecule that it is possible mobility to, tailorwhich Young’s facilitates modulus the interpenetrationand to optimize theof the fiber matrix dominated into the mechanical sizing network strength and properties vice versa. of the Furthermore, unidirectional the composites probability in thatlongitudinal each functional and transverse group can fiber react direction with witha counterpart the fiber volumeis also increased. content. Both It is the likely Young’s that moduliall these in ◦ ◦ effects0 and tend 90 fiberto have direction a positive increased impact linearly on the determinedmechanical afterperformance. tensile and flexural loading as a function of the fiber volume content (Figure 11a–c). Simultaneously, the strain to failure values decreased. 3.4This. Effects is also of duethe Fiber to the Volume nearly Content linear elasticin GF/PP behavior Composites up toon the the failureMechanical which Properties is also displayed by the stress–strain curves (Figure 12). In transverse fiber direction, the tensile strength and compression shearThe strength fiber volume slightly content decrease of with online enhancing hybrid yarns fiber was volume varied content, by the which spinning is due pump to the settings stress duringconcentration the polymer and limited yarn spinning matrix yielding process within within the the interphase region of region about due 40 to decreased60 vol %. matrixIn order paths to manufacturebetween the single specimens reinforcement for mechanical fibers (Figure testing 11, unidirectionalb,d). The Charpy composites impact toughness were made is also based slightly on filamentdeteriorated winding with of increasing hybrid yarns fiber volumefollowed content by isothermal due to limited consolidation matrix yieldingat 225 °C (Figure using 11a d).heatable platen press at a pressure of 1.5 MPa for 21 min throughout the variation of the fiber volume content. The fiber/matrix distribution homogeneity was very high and was not affected by the fiber volume variation. Figure 10 shows exemplarily polished cross sections of unidirectional GF/PP composites containing 42 vol % and 60 vol % glass fibers, respectively. Figure 11 reveals that it is possible to tailor Young’s modulus and to optimize the fiber dominated mechanical strength properties of the unidirectional composites in longitudinal and transverse fiber direction with the fiber volume content. Both the Young’s moduli in 0° and 90° fiber direction increased linearly determined after tensile and flexural loading as a function of the fiber volume content (Figure 11a–c). Simultaneously, the strain to failure values decreased. This is also due to the nearly linear elastic behavior up to the failure which is also displayed by the stress–strain curves (Figure 12). In transverse fiber direction, the tensile strength and compression shear strength slightly decrease with enhancing fiber volume content, which is due to the stress concentration and limited matrix yielding within the interphase region due to decreased matrix paths between the single reinforcement fibers (Figure 11b,d). The Charpy impact toughness is also slightly deteriorated with increasing fiber volume content due to limited matrix yielding (Figure 11d). Fibers 2017, 5, 26 12 of 15 Fibers 2017, 5, 26 12 of 15 Fibers 2017, 5, 26 12 of 15

FigureFigure 10.10 10. Polished. Polished crosscross sectionssections ofof unidirectional unidirectional GF/PP GF/PPGF/PP composites composites containing containing 42 42 vol vol % ( %left (left) and) andand 6060 vol vol % % ( rightright(right)) ) glassglass glass fibers.fibers.fibers. TheThe glassglass fibers fibersfibers are areare highlighted highlightedhighlighted in inin blue blueblue color. color.color.

(a) (b) (a) (b)

(c) (d) (c) (d) FigureFigure 11. 11.Young’s Young’s modulus,modulus, tensile strength, bending bending stiffness, stiffness, bending bending strength strength and and strain strain to failure to failure ofFigureof GF/PP GF/PP 11. compositesYoungcomposites’s modulus, depending tensile on on thestrength, the fiber fiber volume bending volume content content.stiffness,. (a (): abending ):0° 0tensile◦ tensile strength test test;; (b ):and ( b90°): strain 90tensile◦ tensile to test failure; test; (ofc(): cGF/PP): 0 ◦0°bending bending composites test;test; ((dd depending):): Charpy impact impact on the test testfiber and and volume compression compression content shear shear. (a): test 0° test.. tensile test; (b): 90° tensile test; (c): 0° bending test; (d): Charpy impact test and compression shear test. 3.5. Effects of Sizings in GF/PLA and GF/PA6.6 Composites on Their Mechanical Performance 3.5. Effects of Sizings in GF/PLA and GF/PA6.6 Composites on Their Mechanical Performance 3.5. EffectsFor theof Sizings GF/PLA in compositesGF/PLA and containing GF/PA6.6 aComposites constant silane on Their content Mechanical of 1 wt Performance % and constant film For the GF/PLA composites containing a constant silane content of 1 wt % and constant film former content of 10 wt % in the sizing formulations, the epoxy film former (S1) based sizing, as well formerFor content the GF/PLA of 10 composites wt % in the containing sizing formulations, a constant silane the epoxy content film of former1 wt % (S1)and basedconstant sizing, film formeras the contentchitosan of-based 10 wt sizing % in (S3)the ,sizing showed formulations a significant, theincrease epoxy of filminterphase former strength (S1) based being sizing reflected, as well asin well improved as the mechanical chitosan-based properties sizing of the (S3), composites showed transverse a significant to fiber increase direction, of such interphase as transverse strength as the chitosan-based sizing (S3), showed a significant increase of interphase strength being reflected beingtensile reflected strength in and improved compression mechanical shear strength properties (Figure of 1 the2a). composites The especially transverse biodegradable to fiber chitosan direction,- in improved mechanical properties of the composites transverse to fiber direction, such as transverse suchbased as sizing transverse S3 will tensile be selected strength for and further compression component shear design strength and application. (Figure 12Thea). stress The– especiallystrain- tensile strength and compression shear strength (Figure 12a). The especially biodegradable chitosan- biodegradablecurves of GF/PLA chitosan-based composites sizingbased S3on willsized be fibers selected S0 with for further bioresorbable component commercial design finish and application. and S2 based sizing S3 will be selected for further component design and application. The stress–strain- Thewith stress–strain-curves polyurethane film offormer GF/PLA indicated composites significantly basedon lower sized maximum fibers S0 withvalues bioresorbable for stress and commercial strain, finishcurveswhich and of reveal GF/PLA S2 with poor polyurethanecomposites stress transfer bas film ed of formeron the sized composites indicated fibers S0 interphases. significantly with bioresorbable The lower average maximum commercial stress– valuesstrain finish curves for and stress S2 andwith(Fig strain, polyurethaneure 13 which) confirm reveal film the former poordifferent stress indicated adhesion transfer significantly strength of the composites leading lower to maximumdifferent interphases. mechanical values The averagefor performance stress stress–strain and strain, of curveswhichGF/PLA (Figurereveal composites. poor13) confirm stress thetransfer different of the adhesion composites strength interphases. leading to The different average mechanical stress–strain performance curves of(Fig GF/PLAure 13) confirm composites. the different adhesion strength leading to different mechanical performance of GF/PLA composites. Fibers 2017, 5, 26 13 of 15 Fibers 2017, 5, 26 13 of 15 For the GF/PA6.6 composites, the variations in terms of transverse tensile strength and compressionFibers 2017, 5, 26 shear strength (Figure 12b) are even more pronounced, beginning from 3 MPa13 (V5)of 15 and endingFor the at GF/PA6.673 MPa (V7). composites, These findings the are variations also reflected in termsby the stress of transverse–strain curves tensile (not strengthshown here), and For the GF/PA6.6 composites, the variations in terms of transverse tensile strength and compressionwhich indicate sheard strength a very early (Figure failure 12b) for are poor even interphase more pronounced, strength when beginning polyurethane/polyacrylate from 3 MPa (V5) and compression shear strength (Figure 12b) are even more pronounced, beginning from 3 MPa (V5) and endingfilm at former 73 MPa (V3) (V7). was Theseapplied, findings whereas are both also strength reflected and by strain the stress–strain increased significantly curves (not with shown improved here), ending at 73 MPa (V7). These findings are also reflected by the stress–strain curves (not shown here), whichinterfacial indicated interaction. a very early Unexpectedly, failure for poorGF with interphase 1 wt % 3 strength-Aminopropyl when- polyurethane/polyacrylatetriethoxysilane and without which indicated a very early failure for poor interphase strength when polyurethane/polyacrylate filmfilm former former (V3) was(V7) applied, revealedwhereas the bestboth transverse strength tensile and strain strength increased. It indicates significantly that all with film improved formers film former (V3) was applied, whereas both strength and strain increased significantly with improved interfacial(constantinterfacial interaction. 10 interaction. wt % in Unexpectedly, sizing Unexpectedly, formulations) GF GF with with used 1 wt1 wtin % the% 3-Aminopropyl-triethoxysilane 3 sizing-Aminopropyl formulations-triethoxysilane decreased and tensile without strength film formertofilm a (V7) certain former revealed extent (V7) revealed theand best did the transversenot bestcontribute transverse tensile to improved strength.tensile strength toughness. It indicates. It indicates Further that all that research film allformers film is necessary formers (constant to 10 wtreveal(constant % in the sizing 10mechanisms wt formulations) % in sizing influenced formulations) used inby thelocal used sizing nanoscale in formulationsthe sizing interphase formulations decreased properties. decreased tensile strengthtensile strength to a certain extentto anda certain did notextent contribute and did not to contribute improved to toughness. improved toughness. Further researchFurther research is necessary is necessary to reveal to the mechanismsreveal the influenced mechanisms by influenced local nanoscale by localinterphase nanoscale interphase properties. properties.

(a) (b) Figure 12. Mechanical(a) performance for GF/polylactic acid (PLA) composites(b) with different sizings S0 to S3 (a) and GF/PA6.6 composites with different sizings V1 to V7 (b). FigureFigure 12. Mechanical 12. Mechanical performance performance for for GF/polylactic GF/polylactic acid acid (PLA (PLA)) composites composites with with different different sizings sizings S0 S0 to S3 (a) and GF/PA6.6 composites with different sizings V1 to V7 (b). to S3 (a) and GF/PA6.6 composites with different sizings V1 to V7 (b).

45

4540

4035

3530

3025 S0 2520 S0 S1 20 15 S1 S2

Stress [MPa] Stress 1510 S2 S3

Stress [MPa] Stress 10 S3 5 5 0 0 -5 -5 0,0 0,5 1,0 0,0 0,5 Strain [%] 1,0 Strain [%]

Figure 13. Average stress-strain curves for GF/PLA composites with different sizings S0 to S3. Figure 133.. AverageAverage stress stress-strain-strain curves curves for for GF/PLA GF/PLA composites composites with with different different sizings sizings S0 to S0 S3 to. S3. 4. Conclusions 44.. ConclusionConclusions Glass fiber commingled yarns with different matrix polymers and sizing chemistry were GlassGlass fiber commingled commingled yarns yarns with with different different matrix matrix polymers polymers and and sizing sizing chemistry chemistry were were investigated in terms of mechanical performance and consolidation behavior upon mechanical loading. investigatedinvestigated in in terms terms of of mechanical mechanical performance performance and and consolidation consolidation behavior behavior upon upon mechanical mechanical Based on the results, the influence of the sizing, in particular the variation of the sizing components and loading.loading. Based on on the the results results, ,the the influence influence of of the the sizing, sizing, in inparticular particular the the variation variation of the of sizing the sizing weightcomponentscomponents contents inand polypropylene/glass weightweight contents contents in in polypropylene polypropylene fiber composites/glass/glass fiber had fiber acomposites significant composites had influence had a significant a significant on the influence mechanical influence performanceonon thethe mechanic of thealal later performanceperformance composites. of of the the Thelater laterinfluence composite composite ofs. Thes the. The influence sizing influence was of theof also thesizing visiblesizing was was throughoutalso alsovisible visible the variation of the fiber volume content and the matrix polymer, emphasizing that tailored sizing should be carefully chosen in order to achieve excellent performance in composites. Fibers 2017, 5, 26 14 of 15

For polypropylene matrices, only a mixture of silane and MA-g-PP film former results in reasonable strength properties. The mixture of silane and film former can build a covalent bond to the glass fiber surface and to the functional group of the film former via the silane. The film former can cocrystallize via physical entanglements with the chain of the PP-matrix. Due to the absence of functional groups within the polymer chains of the matrix, the organo functional group of the silane is missing its counterpart, resulting in poor adhesion strength. Vice versa, a pure film former sizing can form entanglements with the matrix but does not form bonds to the glass fiber surface. The pure silane sizing shows the highest strength and strain to failure results within the MA-g-PP matrix, because covalent bonds are formed between the glass fiber surface and the grafted matrix chains via the silane. The sizing containing a mixture of MA-g-PP film former and silane shows a somewhat lower strength and strain to failure compared to the pure silane sizing. The same behavior was detected for PA6.6 matrices and PLA matrices. Although the pure silane sizing shows the best mechanical results, it cannot be used in continuous real fiber processing because of poor strand integrity. Therefore, detailed investigation on film former chemistry compatible with the polymer matrix and the influence of additives is needed to reveal the interphase properties in nanoscale. Surprisingly, for PLA matrices, sizings consisting of silane and chitosan film formers show the best performance in composites. Consolidation was experimentally investigated in-depth and void-free composites could be achieved by choosing a suitable combination of temperature and time. The influence of the sizing had clearly been identified, therefore it will be reasonable to employ multifunctional effects such as a conductive CNT network additionally via a sizing onto the heavily strained interphase region of the composites. Taking advance of the electrical resistance change upon mechanical loading and ultimate failure, structural health monitoring can be achieved.

Acknowledgments: The authors express their gratitude towards the Deutsche Forschungsgemeinschaft (DFG) for financial support within the Collaborative Research Centre 639 (SFB) project A1. Author Contributions: Niclas Wiegand and Edith Mäder conceived and designed the experiments; Niclas Wiegand performed the experiments. Niclas Wiegand and Edith Mäder analyzed and discussed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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