fibers Article
Commingled Yarn Spinning for Thermoplastic/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 polypropylene (PP), polyamide (PA) and polylactic acid (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 thermoplastics 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
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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 compression molding 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 (Figure 1). 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 Table 1. 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 (Ingeo 2002D, NatureWorks LLC, Minnetonka, MN, USA). In-situ commingled yarns consisting of 204 glass and 102 polymer filaments, respectively, were always spun.
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Figure 1. Principle of online hybrid yarn spinning. Table applied during the hybrid yarn yarn processing and glass (GF) volume Table1.1.Sizing Sizingformulations formulations applied during the hybrid processing andfilament glass filament (GF) fractions of manufactured composites. volume fractions of manufactured composites. Sizing
Sizing
Sizing Formulation
Matrix Polymer
Sizing Formulation
3-Aminopropyl-triethoxysilane, maleic anhydride grafted PP film former (varying surfactants types, surfactant contents and molecule weights)
MA-g-PP
Glass Fiber Glass Fiber Volume Fraction [%] Matrix Volume Polymer 40.1–61.2 Fraction [%]
3-Aminopropyl-triethoxysilane, maleic anhydride grafted PP film 3-Aminopropyl-triethoxysilane
MA-g-PP
former (varying surfactants types, surfactant Maleic anhydride grafted PP film former reference Deionizedmolecule water
contents and MA-g-PP
weights)
MA-g-PP
3-Aminopropyl-triethoxysilane, maleic anhydride 3-Aminopropyl-triethoxysilane grafted PP film former
PP
Maleic anhydride grafted PP film former reference
PP
Maleic anhydride grafted PP film former reference 3-Aminopropyl-triethoxysilane PP Deionized water
3-Aminopropyl-triethoxysilane, maleic anhydride grafted PP film Deionized water PP V1
3-Aminopropyl-triethoxysilane, former polyurethane film former
V2
3-Ureidopropyltrimethoxysilane, polyurethane Maleic anhydride grafted PP film film former
V3
V1 V4 V2 V5
V3
V6 V7 V4 S0
50
MA-g-PP
PA6.6
polyurethane/polyacrylate film former
3-Aminopropyl-triethoxysilane, polyurethane film former 3-Ureidopropyltrimethoxysilane, 3-Ureidopropyltrimethoxysilane, polyurethane/polyacrylate film formerpolyurethane
film PA6.6 former
3-Aminopropyl-triethoxysilane, polyurethane/epoxy 3-Aminopropyl-triethoxysilane, polyurethane/polyacrylate PA6.6 film film former
former 3-Ureidopropyltrimethoxysilane, polyurethane/epoxy film former
MA-g-PP MA-g-PP MA-g-PP PP
3-Aminopropyl-triethoxysilane
PA6.6
former bioresorbable finish [3]
PP PP PP
44.7
PA6.6
PA6.6 PLA
2-Aminoethyl-3-aminopropyl-trimethoxysilane, epoxy film
2.2. Composite Manufacturing S2
former
3-Ureidopropyltrimethoxysilane, polyurethane film former
45.9
46.0
37.5
PA6.6
S1
50.4
45.6
film former 3-Aminopropyl-triethoxysilane
PLA
45.6
50 50 50
43.7
3-Ureidopropyltrimethoxysilane, polyurethane/epoxy film former 3-Ureidopropyltrimethoxysilane, polyurethane [3]
45.6
PA6.6
V6
2-Aminoethyl-3-aminopropyl-trimethoxysilane, bioresorbable finish chitosan film former
50
45.6
PA6.6
V7 S0S3
50
PA6.6
3-Aminopropyl-triethoxysilane, polyurethane/epoxy film former 2-Aminoethyl-3-aminopropyl-trimethoxysilane, epoxy PLA PLA
50
50 50 50
46.0
V5 S1 S2
50
PA6.6
PLA
film former
50
50.4
PA6.6
3-Ureidopropyltrimethoxysilane, polyurethane/polyacrylate film
40.1–61.2
50
PA6.6
3-Aminopropyl-triethoxysilane PA6.6 former reference Deionized water 3-Aminopropyl-triethoxysilane,
50
38.5
43.7 44.7
37.6 37.9
45.9 37.5
PLA
38.5
PLA
37.6
Unidirectional thermoplastic/glass fiber composites were produced using dry filament winding 2-Aminoethyl-3-aminopropyl-trimethoxysilane, chitosan film (IWT followed by isothermal S3 Wickeltechnik, Erlangen, Germany) of the commingled yarns PLA 37.9 former
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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 Table 2 were used. Table 2. Parameters for composite manufacturing. Matrix Polymer
Isothermal Temperature [◦ C]
Consolidation Time [min]
Consolidation Pressure [MPa]
PP PA6.6 PLA
225 295 170
21 20 20
2.25 2.25 1.5
Composites with GF volume fractions between 38 and 61% (cf. Table 1) 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
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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. Figure 2 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.
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with the functional groups of the film former, which then form entanglements with the matrix 6 of 15 polymer, or directly reacts with the functional groups of the matrix polymer.
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Figure 2. Influence of the matrix grafting (polypropylene (PP) un-grafted, left, MA-g-PP, right) and the Figure 2. Influence(1ofwtthe grafting un-grafted, left, MA-g-PP, right) and sizing components % matrix silane and 10 wt(polypropylene % MA-g-PP film(PP) former) on the mechanical performance the sizing components (1 wt % silane and 10 wt % MA-g-PP film former) on the mechanical of unidirectional GF/PP composites having a fiber volume content of 50%. 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 plasticizers. Figure 3A 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 plasticizer 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. Figure 3B 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 Figure 3B, 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 Figure 4A, 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.
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Figure 3. Influence Influence of of molecular weight MA-g-PP film former (A)(A) and thethe components within Figure 3. of molecular weight of of a MA-g-PP film former (A) and the components within aa a Figure 3. Influence molecular weight a MA-g-PP film former and components within film former (B). film former (B). film former (B).
Figure 4. Influence Influence of of thethe surfactant charge (A)(A) and thethe surfactant content onon thethe glass fiber surface Figure 4. of the surfactant charge (A) and the surfactant content on the glass fiber surface Figure 4. Influence surfactant charge and surfactant content glass fiber surface (B) on the mechanical properties of GF/PP composites. (B)(B) onon thethe mechanical properties of GF/PP mechanical properties of GF/PP composites.
In order toto investigate thethe influence ofof the surfactant content (Figure 4B) independent ofof thethe MAorder investigate influence independent MAIn In order to investigate the influence of the the surfactant surfactantcontent content(Figure (Figure4B) 4B) independent of the g-PP, mixtures of silane and surfactant were applied to the glass fiber surface. A very low content of g-PP, mixtures ofof silane MA-g-PP, mixtures silaneand andsurfactant surfactantwere wereapplied appliedtotothe theglass glassfiber fibersurface. surface.AAvery verylow lowcontent content of 0.75 wt % surfactant already reduces the strength by almost one third, compared with the reference surfactant already already reduces reducesthe thestrength strengthby byalmost almostone onethird, third,compared compared with the reference of 0.75 0.75 wt wt % % surfactant with the reference without surfactant. This highlights that surfactants cannot be beneficial for strength but have to bebe without surfactant. This highlights that surfactants cannot beneficial strength but have without surfactant. This highlights that surfactants cannot bebe beneficial forfor strength but have to to be applied with care in order to achieve stable film former emulsions. applied with care in order to achieve stable film former emulsions. applied with care in order to achieve stable film former emulsions. 3.2. Influence of of Fiber Sizing Content onon thethe Mechanical Performance 3.2. Influence Fiber Sizing Content Mechanical Performance 3.2. Influence of Fiber Sizing Content on the Mechanical Performance InIn thethe previous chapter, thethe fundamental influence ofof thethe sizing chemistry onon thethe strength ofof previous chapter, fundamental influence sizing chemistry strength In the previous chapter, the fundamental influence of the sizing chemistry on the strength of aa a unidirectional composite was discussed. Within thethe following paragraph, thethe influence ofof thethe amount unidirectional composite was discussed. Within following paragraph, influence amount unidirectional composite was discussed. Within the following paragraph, the influence of the amount of sizing which is applied during glass fiber spinning will be further examined. Thus, the sizing sizing which is applied during fiber spinning be examined. further examined. sizing of of sizing which is applied during glass glass fiber spinning will bewill further Thus, theThus, sizingthe system system always consists ofof thethe same APS silane 1plus wt % plus a MA-g-PP film former varying system always same APS 1 wt plus a MA-g-PP film former varying from always consists ofconsists the same APS silane at 1silane wt at %at a% MA-g-PP film former varying from 0from up to0 0 up to 15 wt % of the sizing. By increasing the weight percent of the film former, the solid content ofof of the By sizing. By increasing the weight of the film former, thecontent solid content 15up wtto % 15 of wt the%sizing. increasing the weight percentpercent of the film former, the solid of the the sizing is increased, which tends to enhance the viscosity of the sizing. Consequently, the fiber the sizing is increased, whichtotends to enhance the viscosity of the sizing. Consequently, the up fiber sizing is increased, which tends enhance the viscosity of the sizing. Consequently, the fiber picks picks up more sizing when it passes the sizing applicator roll yielding to an increased sizing content picks up more sizing when it passes the sizing applicator roll yielding to an increased sizing content more sizing when it passes the sizing applicator roll yielding to an increased sizing content and thus thus a thicker average layer onBesides thethe fiber. thethe sizing thethe fiber and thus a thicker average sizing layer on fiber. Besides sizing content, fibervolume volume aand thicker average sizing layersizing on the fiber. the Besides sizing content, the content, fiber volume content was content was systematically changed from 42 until 62 vol %. The design of experiments was used forfor content was changed systematically changed 42 The untildesign 62 volof%.experiments The design was of experiments was used systematically from 42 until 62from vol %. used for planning and planning and analyzing thethe strength and stiffness results ofof the composite testing. planning and analyzing strength and stiffness results the composite testing. analyzing the strength and stiffness results of the composite testing. As the Pareto chart in Figure 5 shows, the film former has a significant influence on the strength. Pareto chart Figure 5 shows, the film former has a significant influence the strength. AsAs thethe Pareto chart in in Figure 5 shows, the film former has a significant influence onon the strength. It tends to decrease the transverse tensile strength. The fiber volume content is not significantly tends decrease transverse tensile strength.The Thefiber fibervolume volumecontent contentis is not significantly It It tends to to decrease thethe transverse tensile strength. not significantly changed from a statistical point ofof view. The coupling effect between both tends toto bebe right onon thethe changed from a statistical point view. The coupling effect between both tends right edge of significance, indicated by the blue line in the chart. Quite surprisingly, it is not the fiber edge of significance, indicated by the blue line in the chart. Quite surprisingly, it is not the fiber
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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 2017, 5, 26 indicated by the blue line in the chart. Quite surprisingly, it is not the fiber 8volume of 15 of Fibers significance, content which dominates the strength, the sizing content. principle strengthindue volume content which dominates the but strength, but the sizingThe content. The reduction principle in reduction volume content which dominates the strength, but the sizing content. The principle reduction in tostrength the increased sizing level can be explained by the chemical nature of the film former. In due to the increased sizing level can be explained by the chemical nature of the film contrast former. to strength due to the increased sizing level can be explained by the chemical nature of the film former. the matrix PP, the film former consists of a significantly lower molecular weight PP. To a large In contrast to the matrix PP, the film former consists of a significantly lower molecular weight PP.extent, To In contrast to the matrix PP, the film former consists of a significantly lower molecular weight PP. To the properties, e.g., strength and stiffness, areand influenced molecular As a bulk a physical large extent, the physical properties, e.g., strength stiffness,by arethe influenced byweight. the molecular a large extent, the physical properties, e.g., strength and stiffness, are influenced by the molecular weight. the As a bulkformer material, the film polymer very brittle and hasofa about low strength about 7 material, polymer isformer very brittle andis a low strength 7 MPaof and a strain weight. As afilm bulk material, the film former polymer ishas very brittle and has a low strength of about 7 MPa and a strain to failure of only 4%. In contrast, the matrix polymer shows intensive yielding and to MPa failure of only 4%. In contrast, the matrix polymer shows intensive yielding and strength of about and a strain to failure of only 4%. In contrast, the matrix polymer shows intensive yielding and of about 25increased MPa. Based onconcentrations the increased strain concentrations forcontents higher fiber volume 25strength MPa. Based on the strain higher fiber volume [20], one would strength of about 25 MPa. Based on the increased for strain concentrations for higher fiber volume contents [20],aone would rather assume a waste reduction in strength for increased fiber levels. rather assume waste reduction in strength for increased fiber levels. contents [20], one would rather assume a waste reduction in strength for increased fiber levels.
Figure response surface surfaceplot plot(right) (right)for forthe thetransverse transverse tensile strength Figure5.5.Pareto Pareto chart chart (left) (left) and and response tensile strength Figure 5. Pareto chart (left) and response surface plot (right) for the transverse tensile strength depending on the fiber volume content and the film former content. depending on the fiber volume content and the film former content. depending on the fiber volume content and the film former content.
contrasttotothe the strength results, results, Figure 66 shows content hashas a major InIn showsthat thatthe thefiber fibervolume volume content a major Incontrast contrast to thestrength strength results, Figure Figure 6 shows that the fiber volume content has a major influence on the Young’s modulus perpendicular to the direction of fibers. Increasing the fiber influence to the the direction direction ofoffibers. fibers.Increasing Increasing fiber influenceononthe theYoung’s Young’smodulus modulus perpendicular perpendicular to thethe fiber volume content results in increased Young’s modulus, which corresponds to the well-established volume content results in increased Young’s modulus, which corresponds to the well-established volume content results in increased Young’s modulus, which corresponds to the well-established micromechanical models [21]. On the other hand, the film former has a detrimental influence on the micromechanicalmodels models[21]. [21].On On the the other hand, influence on on thethe micromechanical hand, the thefilm filmformer formerhas hasa adetrimental detrimental influence modulus. Throughout all of the investigated film former levels, the modulus is significantly reduced. modulus.Throughout Throughoutall allof ofthe the investigated investigated film is significantly reduced. modulus. filmformer formerlevels, levels,the themodulus modulus is significantly reduced. Although the Young’s modulus of the bulk film former and the matrix polymer are only marginally Although theYoung’s Young’smodulus modulusof ofthe the bulk bulk film former areare only marginally Although the formerand andthe thematrix matrixpolymer polymer only marginally different, within the fiber interphase the low molecular weight film former clearly reduces the overall different, within the fiber interphase the low molecular weight film former clearly reduces thethe overall different, within the fiber interphase the low molecular weight film former clearly reduces overall stiffness of the composite. stiffness thecomposite. composite. stiffness ofof the
Figure6.6.Pareto Pareto chart(left) (left) and response response surface plot modulus depending on on Figure plot(right) (right)for forthe theYoung’s Young’s modulus depending Figure 6. Paretochart chart (left)and and response surface surface plot (right) for the Young’s modulus depending on the fiber volume content and the film former content. thethe fiber volume fiber volumecontent contentand andthe thefilm filmformer former content. content.
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3.3. Influence of Time, Temperature and Pressure During Consolidation on the Mechanical Performance The of continuous fiber-reinforced thermoplastics has been widely addressed in Fibersconsolidation 2017, 5, 26 9 of 15 the literature [22–31]. It is well accepted that the impregnation during the consolidation consists of a 3.3. Influence of Time, Temperature and Pressure During on the Mechanicalmolten Performance macroscale and microscale impregnation. During the Consolidation macroscale impregnation, matrix flows between fiber bundles, whereas during the microscale impregnation, fibers are infiltrated The consolidation of continuous fiber-reinforced thermoplasticsthe hassingle been widely addressed in by the molten matrix,[22–31]. whichItisisroughly considered Darcy’s law.during the consolidation consists of a the literature well accepted that theby impregnation macroscale During macroscale molten matrix flows The aim ofand thismicroscale chapter impregnation. is to investigate thetheinfluence ofimpregnation, the processing parameters on the between fiber bundles, whereas during the microscale impregnation, the single fibers are infiltrated transverse tensile strength using a Design of Experiments (DOE) approach. For a better understanding the molten matrix, which isand roughly considered by Darcy’s law. cross sections for each specimen of thebyimpregnation behavior the void content, polished 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 Figure 7 were demonstrates that, during the isothermal consolidation trials, thedistribution time and the specimen prepared. In order to monitor the heat transfer and the temperature temperature have a significant impact on the mechanical properties. This behavior correlates well during the consolidation, each plate was equipped with thermocouples along the thickness. with previous composite consolidation studies [28]. Besides these linear factors, a quadratic term Figure 7 demonstrates that, during the isothermal consolidation trials, the time and the have a significant impact temperature on the mechanical This behavior well in of thetemperature time and an interaction between and properties. time are responsible forcorrelates the reduction with previous composite consolidation studies Besides linear factors,mentioned a quadratic ones term of strength. The influence of the pressure is much[28]. lower thanthese all previously which time and to anthe interaction betweenfiber temperature and time responsible for the reduction in and can betheattributed homogeneous distribution levelare in the online commingled yarns strength. short The influence of the is much lower than all previously mentioned ones which can consequently distances for pressure the matrix to flow. 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. Figure 8 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 expected by the results the DOE, the voidof content decreases with increasing temperature of the composite. Figure of 8 reports the evolution the voids during different consolidation conditions. and holding time, whereas the pressure only has a minor influence illustrated for the 14 minand 195 ◦ C As expected by the results of the DOE, the void content decreases with increasing temperature sample (Pressure and 2.25 The temperature hasinfluence a major illustrated impact onfor thethe consolidation holding time,0.75 whereas the MPa). pressure only has a minor 14 min 195 quality, °C ◦ C consolidation sample (Pressure and 2.25 MPa).temperature The temperature has a major impact on the as expected. From 1800.75 onwards, the void content canconsolidation significantly be quality, as expected. 180dotted °C consolidation temperature onwards, void content can reduced, highlighted by theFrom orange line within the temperature graphs the in Figure 8. The influence significantly be reduced, highlighted by the orange dotted line within the temperature graphs in of the time on the mechanical performance can be clearly espaliered by a close look on the temperature Figure 8. The influence of the time on the mechanical performance can be clearly espaliered by a close distribution through the thickness along consolidation time. As soon as the heatable platen 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 defined temperature whereas the lowest plies level, are almost 100 colder.plies Theare temperature the press have alreadylevel, reached the defined temperature whereas theKlowest almost difference and bottom becomes less with proceeding time.less After 21proceeding min (green line) it 100 K between colder. Thetop temperature difference between top and bottom becomes with time. is almost Since the temperature is introduced via the top plie, void via content is not Afterin 21equilibrium. min (green line) it is almost in equilibrium. Since the temperature is introduced the top plie, void content is not homogeneously distributed through the thickness. Only the bottom areas homogeneously distributed through the thickness. Only the bottom areas which have not reached ◦ C along the which have notabove reached temperature levels above 180 °Ctend alongtothe holding time tend to have voids. temperature levels 180 holding time have voids.
FigureFigure 7. Pareto chart (left) and (right)for forthe thetransverse transverse tensile strength 7. Pareto chart (left) andresponse responsesurface surface plot plot (right) tensile strength depending on the processing parameters, time and temperature, for a constant pressure of 1.5 MPa. depending on the processing parameters, time and temperature, for a constant pressure of 1.5 MPa.
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Figure 8. (Left) Temperature distribution through thickness isothermal ◦ C °C Figure 8. (Left) Temperature distribution through thethe thickness forfor 195,195, 210210 andand 225225 isothermal temperature at the beginning of the isothermal holding (black), after 7 min (red), after 14 min (green) temperature at the beginning of the isothermal holding (black), after 7 min (red), after 14 min (green) and after 21 min (blue); (right) Corresponding polished cross section to the temperature and time and after 21 min (blue); (right) Corresponding polished cross section to the temperature and time with with voids highlighted in blue (for a pressure constant of pressure of 0.75 MPa despite top orange one). The voids highlighted in blue (for a constant 0.75 MPa despite the middlethe topmiddle one). The orange dotted line highlights the temperature from where on void free composites are generated. dotted line highlights the temperature from where on void free composites 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 Figure 9. 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, Figure 8 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 Figure 7. 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 macroand microscale appears to be void–free, it is still to be questioned what happens on the nanoscale
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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 on the mechanical performance. Fibers 2017, 5, 26 11 of 15
Figure Figure9.9.(Left) (Left)Viscosity-temperature Viscosity-temperatureplot plotfor formatrix matrixand andfilm filmformer formerPP; PP;(right) (right)aacloser closerlook lookon onthe the consolidation consolidationbehavior behaviorin incommingled commingledyarn yarncomposites: composites:Plastic Plasticdeformation deformationofofPP PPfilaments, filaments,sintering sintering and andintrabundle intrabundlevoid voidreduction reductionfollowed followedby bypolymer polymerflow flowinto intothe theglass glassfiber fiber bundles. bundles.
theVolume void Content contentincannot be the only reason for an increased mechanical 3.4. Nevertheless, Effects of the Fiber GF/PP Composites on the Mechanical Properties performance throughout the consolidation trials. Especially composites which do not show any sign The(e.g., fiber volume content online yarns was varied differences by the spinning pump settingstensile during of voids above 195 °C/14ofmin or hybrid 210 °C/7 min) display in the transverse the polymer yarn spinning process within the region of about 40 to 60 vol %. In order to manufacture strength, as presented in the response surface plot in Figure 7. Since the heating and cooling rates specimens for mechanical testing, unidirectionalthese composites made onattributed filament winding were not changed throughout all experiments, changeswere cannot be based directly to any ◦ C using a heatable platen press at a of hybrid yarns followed by isothermal consolidation at 225 difference in the melting or crystallization state during consolidation. Although the consolidation on pressure ofand 1.5 MPa for 21 min throughout the variation thetofiber volume content. fiber/matrix the macromicroscale appears to be void–free, it is of still be questioned what The happens on the distribution homogeneity was very high and was not affected by the fiber volume variation. 10 nanoscale near the glass fiber surface. The important influence of the glass fiber interphaseFigure on the shows exemplarily polished cross sections of unidirectional GF/PP composites containing 42 vol mechanical performance has been already addressed in the two previous chapters. An increase in% and 60 vol % glass fibers, respectively. Figure 11 reveals that it is possible modulus temperature always goes along with increased molecule mobility,to tailor whichYoung’s facilitates the and to optimize the fiber dominated mechanical strength properties of the unidirectional composites interpenetration of the matrix into the sizing network and vice versa. Furthermore, the probabilityin longitudinal and transverse fiberreact direction the fiber volume content. Both Young’s that each functional group can with with a counterpart is also increased. It isthe likely that moduli all thesein ◦ ◦ 0 andtend 90 fiber direction increased linearly tensile and flexural loading as a function effects to have a positive impact on thedetermined mechanical after performance. of the fiber volume content (Figure 11a–c). Simultaneously, the strain to failure values decreased. ThisEffects is also to the nearlyContent linear elastic behavior up to which is also displayed by the 3.4. of due the Fiber Volume in GF/PP Composites onthe the failure Mechanical Properties stress–strain curves (Figure 12). In transverse fiber direction, the tensile strength and compression The fiber volume of with online hybrid yarns varied by thewhich spinning pump settings shear strength slightlycontent decrease enhancing fiber was volume content, is due to the stress during the polymer yarn spinning process within the region of about 40 to 60 vol %. In order to concentration and limited matrix yielding within the interphase region due to decreased matrix paths manufacture specimens for mechanical testing, unidirectional composites were made based on between the single reinforcement fibers (Figure 11b,d). The Charpy impact toughness is also slightly filament winding hybrid yarns followedcontent by isothermal consolidation at 225 °C using 11d). a heatable deteriorated with of increasing fiber volume due to limited matrix yielding (Figure 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
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Figure 10. Polished cross sections of unidirectional unidirectionalGF/PP GF/PPcomposites composites containing vol % (left) (left) and Figure10. 10.Polished Polishedcross cross sections sections of containing 42 42 volvol % (left) andand Figure unidirectional GF/PP composites containing 42 % 60 vol % (right) glass fibers. The glass fibers are highlighted in blue color. 60 vol % (right) glass fibers. The glass fibers are highlighted in blue color. 60 vol % (right) glass fibers. The fibers are highlighted in blue color.
(a) (a)
(b)
(c) (c)
(d)
(b)
(d)
Figure11. 11.Young’s Young’smodulus, modulus, tensile tensile strength, strength and strain to failure Figure strength,bending bendingstiffness, stiffness,bending bending strength and strain to failure ofGF/PP GF/PP composites depending on (a): 0°0tensile test; (b):and 90°strain tensile ◦ tensile ◦ tensile Figure 11. composites Young’s modulus, tensile strength, bending stiffness, bending strength totest; failure of depending onthe thefiber fibervolume volumecontent. content. (a): test; (b): 90 test; (c): 0° bending test; (d): Charpy impact test and compression shear test. ◦ of GF/PP composites depending on the fiber volume content. (a): 0° tensile test; (b): 90° tensile test; (c): 0 bending test; (d): Charpy impact test and compression shear 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. Effects of Sizings in composites GF/PLA andcontaining GF/PA6.6 aComposites on Their Mechanical For the GF/PLA constant silane content of 1 wtPerformance % 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 wellfilm Forcontent the GF/PLA composites containing a constant silane content of former 1 wt % (S1) and constant former of 10 wt % in the sizing formulations, the epoxy film based sizing, as the chitosan-based sizing (S3), showed a significant increase of interphase strength being reflected former content of 10 wt % in the sizing formulations, the epoxy film former (S1) based sizing, as well asinwell as themechanical chitosan-based sizing (S3), showedtransverse a significant increase of such interphase strength improved properties of the composites to fiber direction, as transverse as the chitosan-based sizing (S3), showed a significant increase of interphase strength being reflected being reflected improved mechanical properties of 12a). the composites transverse to fiber direction, tensile strengthinand compression shear strength (Figure The especially biodegradable chitosanin improved mechanical properties of the composites transverse to fiber direction, such as transverse such as sizing transverse tensile strength compression (Figure 12a). The especially based S3 will be selected for and further componentshear designstrength and application. The stress–straintensile strengthchitosan-based and compression shear strength (Figurefor 12a). The component especially biodegradable chitosanbiodegradable befibers selected design finish and application. curves of GF/PLA compositessizing based S3 onwill sized S0 withfurther bioresorbable commercial and S2 based sizing S3 will be selected for further component design and application. The stress–strainThe stress–strain-curves GF/PLA composites based on sizedmaximum fibers S0 with bioresorbable commercial with polyurethane filmof former indicated significantly lower values for stress and strain, curves of GF/PLA composites based on sized fibers S0 with bioresorbable commercial finish S2 finish and S2 with film indicatedinterphases. significantly lower maximum valuescurves forand stress which reveal poorpolyurethane stress transfer of former the composites The average stress–strain with polyurethane film former indicated significantly lower maximum values for stress and strain, and strain, reveal stress transferstrength of the composites interphases. The average stress–strain (Figure 13)which confirm the poor different adhesion leading to different mechanical performance of which reveal poor stress the transfer of the composites interphases. average stress–strain curves GF/PLA composites. curves (Figure 13) confirm different adhesion strength leading to The different mechanical performance (Figure 13) confirm the different adhesion strength leading to different mechanical performance of of GF/PLA composites. GF/PLA composites.
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For the GF/PA6.6 composites, the variations in terms of transverse tensile strength and Fibers 2017, 5, 26 shear strength (Figure 12b) are even more pronounced, beginning from 3 MPa 13 (V5) of 15 and compression For the GF/PA6.6 composites, the variations in terms of transverse tensile strength and ending at 73 MPa (V7). These findings are also reflected by the stress–strain curves (not shown here), For shear the GF/PA6.6 composites, the variations in pronounced, terms of transverse tensile strength and compression strength (Figure 12b) are even more beginning from 3 MPa (V5) and which indicated a very early failure for poor interphase strength when polyurethane/polyacrylate compression shear strength (Figure 12b) are even more pronounced, beginning from 3 MPa (V5) and ending 73 MPa (V7). findings areboth alsostrength reflected bystrain the stress–strain curves (not shown here), filmatformer (V3) wasThese applied, whereas and increased significantly with improved ending at 73 MPa (V7). These findings are also reflected by the stress–strain curves (not shown here), which indicated a very early failure for poor interphase strength when polyurethane/polyacrylate interfacial interaction. Unexpectedly, GF with 1 wt % 3-Aminopropyl-triethoxysilane and without which indicated a very early failure for poor interphase strength when polyurethane/polyacrylate film film former (V3) was strength and strain increased significantly former (V7) applied, revealedwhereas the bestboth transverse tensile strength. It indicates that allwith filmimproved formers film former (V3) was applied, whereas both strength and strain increased significantly with improved (constant 10 wt % in sizing formulations) used in the sizing formulations decreased tensile strength interfacial interaction. Unexpectedly, GF with 1 wt % 3-Aminopropyl-triethoxysilane and without interfacial interaction. Unexpectedly, GF with 1 wt % 3-Aminopropyl-triethoxysilane and without film to a(V7) certain extent andbest didthe notbest contribute to improved toughness. Further research is necessary to former revealed the transverse tensile strength. It indicates that all film (constant film former (V7) revealed transverse tensile strength. It indicates that allformers film formers reveal the mechanisms influenced by local nanoscale interphase properties. 10 wt(constant % in sizing formulations) used in the used sizing decreased tensile strength to a certain 10 wt % in sizing formulations) in formulations the sizing formulations decreased tensile strength
extent did not contribute to contribute improvedtotoughness. Further research is necessary to reveal to and a certain extent and did not improved toughness. Further research is necessary to the reveal theinfluenced mechanisms by localinterphase nanoscale interphase properties. mechanisms byinfluenced local nanoscale properties.
(a)
(b)
(a) performance for GF/polylactic acid (PLA) composites (b) with different sizings S0 Figure 12. Mechanical to S3 (a)12. and GF/PA6.6performance compositesfor with different sizings V1 tocomposites V7 (b). with different sizings S0 Figure Mechanical GF/polylactic acid (PLA) Figure 12. Mechanical performance for GF/polylactic acid (PLA) composites with different sizings 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
Stress [MPa] Stress [MPa]
4035 3530 3025
S0
2520
S0 S1 S1 S2 S2 S3 S3
2015 15
10
10 5
5 0
0
-5
-5
0,0 0,0
0,5 0,5
Strain [%]
1,0 1,0
Strain [%]
Figure 13. Average stress-strain curves for GF/PLA composites with different sizings S0 to S3. Figure 13. Average stress-strain curves for GF/PLA composites with different sizings S0 to S3. Figure 13. Average stress-strain curves for GF/PLA composites with different sizings S0 to S3.
4. Conclusions
4. 4. Conclusions Conclusions
Glass fiber commingled yarns with different matrix polymers and sizing chemistry were Glass fiber commingled commingled yarns yarnswith withdifferent differentmatrix matrix polymers sizing chemistry Glass fiber polymers andand sizing chemistry werewere investigated in terms of mechanical performance and consolidation behavior upon mechanical loading. investigated in terms terms of of mechanical mechanicalperformance performanceand and consolidation behavior upon mechanical investigated in consolidation behavior upon mechanical Based on the results,on thethe influence of theinfluence sizing, inofparticular the variation of the sizing components and loading. results,the theinfluence sizing, particular variation of sizing the sizing loading. Based on the results, of thethe sizing, in in particular the the variation of the weight contents in polypropylene/glass fiber composites had a significant influence on the mechanical components and fiber composites hadhad a significant influence components and weight weightcontents contentsininpolypropylene/glass polypropylene/glass fiber composites a significant influence performance of the later composites. The influence ofThe the sizing was also visible throughout the on the the mechanical mechanical performance ofofthe later composites. influence of the sizing was was also also visible on performance the later composites. The influence of the sizing visible 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.
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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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Mäder, E.; Rausch, J.; Schmidt, N. Commingled yarns-Processing aspects and tailored surfaces of polypropylene/glass composites. Compos. Part A 2008, 39, 612–623. [CrossRef] Mäder, E.; Moos, E.; Karger-Kocsis, J. Role of film formers in glass fibre reinforced polypropylene—New insights and relation to mechanical properties. Compos. Part A 2001, 32, 631–639. [CrossRef] Rausch, J.; Mäder, E. Health monitoring in continuous glass fibre reinforced thermoplastics: Manufacturing and application of interphase sensors based on carbon nanotubes. Compos. Sci. Technol. 2010, 70, 1589–1596. [CrossRef] Rausch, J.; Mäder, E. Health monitoring in continuous glass fibre reinforced thermoplastics: Tailored sensitivity and cyclic loading of CNT-based interphase sensors. Compos. Sci. Technol. 2010, 70, 2023–2030. [CrossRef] Rausch, J.; Mäder, E. Carbon nanotube coated glass fibres for interphase health monitoring in textile composites. Mater. Technol. 2011, 26, 153–158. [CrossRef] Lehtonen, T.; Tuominen, J.; Rausch, J.; Mäder, E. Biodegradable continuous fibre reinforced composites based on hybrid yarns. In Proceedings of the ICCM 18, Jeju, South Korea, 21–26 August 2011. Russell, K. Free radical graft polymerization and copolymerization at higher temperatures. Prog. Polym. Sci. 2002, 27, 1007–1038. [CrossRef] Plueddemann, E.P. Silane Coupling Agents; Springer Science + Business Media: New York, NY, USA, 1991; pp. 29–45. Kim, J.-K.; Mai, Y.-W. High Strength, High Fracture Toughness Fibre Composites with Interface Control—A Review. Compos. Sci. Technol. 1991, 41, 333–378. [CrossRef] Pukanszky, B. Interfaces and Interphases in Multicomponent Materials: Past, Present, Future. Eur. Polym. J. 2005, 41, 645–662. [CrossRef]
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