Polyamide 6/carbon fibre composite: An investigation of carbon fibre modifying pathways for improving mechanical properties

Abstract

Carbon fibre-reinforced polyamide 6 (PA6) composites exhibit exceptional mechanical properties, making them a highly attractive material option for various industries. Their ability to combine the advantages of carbon fibres with the versatility of PA6 provides opportunities for lightweight, high-performance applications. This work presented the fabrication of a PA6/carbon fibre composition with carbon fibre was modified by two different chemicals: nitric acid and diglycidil ether bisphenol A (DGEBA). The tensile strength at break, flexural strength, Rockwell hardness and Izod impact were measured to compare the mechanical properties of composites based on the PA6 matrix with two different modified carbon fibres. The morphology of PA6-based composite with different modified carbon fibres was observed by scanning electron microscopy. The results showed that PA6 is more compatible with acid-modified carbon fibre than DGEBA-modified carbon fibre and unmodified carbon fibre. In comparison to composite materials containing unmodified carbon fibre and DGEBA-modified carbon fibre, composite containing acid-modified carbon fibre increase tensile strength (16–38% and 7–15%, respectively), flexural strength (18–66% and 51–70%, respectively), Izod impact (23–65% and 32–78%, respectively) and Rockwell hardness (50–400% and equivalence, respectively).

Keywords

plastic composite carbon fibre surface modification polyamide 6 plastic enhanced mechanical properties

Introduction

One of the most widely used materials in a variety of fields of life, science, and technology is carbon fibre. High strength (2–7 GPa), high-stress modulus (200–900 GPa), low density (1.75–2.2 g/cm₃), low thermal expansion and high conductivity and heat transmission (800 W/m/K) are only a few of the excellent properties of carbon fibre.^1–6 Carbon fibres can be used directly to manufacture materials, such carbon fibre textiles, or indirectly to improve the qualities of materials by adding them to polymer composites.^7,8 Carbon fibres are incredibly thin strands made primarily of carbon atoms, usually less than one-tenth the diameter of a human hair.⁹ These fibres possess several remarkable characteristics that make them highly desirable for use in composites. Firstly, carbon fibre has an exceptionally high strength-to-weight ratio, making it stronger than steel but significantly lighter.¹⁰ This property enables the development of lightweight structures that can withstand significant loads, crucial for applications such as aerospace, automotive, and sports equipment.^11,12 Furthermore, carbon fibre exhibits remarkable stiffness, providing exceptional rigidity and resistance to deformation.¹³ This stiffness contributes to improved structural integrity, precision, and stability in composite components. In addition to its mechanical properties, carbon fibre also possesses excellent resistance to corrosion, chemicals, and fatigue, ensuring the durability and longevity of composite structures.^14,15

However, carbon fibres have a chemically inert surface, which can make it challenging for them to form strong bonds with the matrix material.¹⁶ A weak interfacial bond can result in reduced mechanical properties and compromised structural integrity. Furthermore, carbon fibres are susceptible to degradation when exposed to harsh environmental conditions or during processing.¹⁷ Thus, modifying carbon fibres for composites is essential to improve interfacial adhesion, enhance compatibility with the matrix material, control the fibre-matrix interface, prevent fibre degradation, and optimise specific properties.^18,19 Many approaches have been effectively employed for the modification of carbon including, but not limited to, surface treatment (oxidation, plasma treatment, or chemical etching), sizing agents (coatings to enhance their compatibility with specific matrix materials), surface modification with nanoparticles, chemical functionalization (chemical reactions or grafting techniques, or functional groups such as carboxylic acids, amines, or epoxides), and hybridisation with other fibres.^20–23 These modifications aim to improve the interfacial bonding, adhesion, and compatibility between the carbon fibres and the matrix material in composite structures. By enhancing the compatibility, the composite's mechanical properties, such as strength, stiffness and impact resistance, can be significantly improved, leading to more reliable and high-performance composite materials.

Polyamide 6 (PA6), also known as nylon 6, is a versatile engineering thermoplastic with excellent mechanical strength, toughness, and chemical resistance.²⁴ It exhibits good heat resistance, dimensional stability, and low friction properties. When combined with carbon fibres, PA6 achieves enhanced mechanical and thermal properties.²⁵ Carbon fibres, as previously mentioned, are exceptionally strong, lightweight and rigid. By incorporating carbon fibres into the PA6 matrix, the resulting composite benefits from the high strength-to-weight ratio, increased stiffness, and improved dimensional stability offered by carbon fibres.²⁶ PA6/carbon fibre composites find applications in various industries such as automotive sector, aerospace, and sports equipment.^27,28 For example, Botelho's group fabricated the PA6/carbon fibre composite without any modification of carbon fibre and investigated the mechanical properties of the resultant composite.²⁵

Specific research on PA6 and carbon fibre-based composite materials in the world has been abundant in the last two decades.^29–33 Studies have highlighted the advantages of using carbon fibre as reinforced material, such as tensile strength, flexural strength or impact resistance, as well as the way in which carbon fibre surfaces are modified to increase the adhesion and dispersion of carbon fibres in PA6.^34,35

Nevertheless, existing researches primarily concentrate on the utilisation of long carbon fibres for enhancing PA6-based-composite material manufactured by molding. There is limited discussion regarding the implementation of micrometer-sized carbon fibres, despite their significance in the production of 3D printing materials. These small-sized reinforcements are crucial in preventing blockages during the Fused Deposition Modeling (FDM) 3D-printing process, which aligns with contemporary manufacturing techniques widely adopted worldwide. Three-dimensional printing materials on PA6 substrate reinforced by micromet carbon fibre are now commercially available such as Markforged's Onyx fibre, Airwolf's 3D carbonite nylon, 3Dxtech's Carbon X fibre, Matterhacker's Nylon X fibre, etc.

Herein, the carbon fibre with a length of less than 300 micrometers are modified using different chemicals, which are nitric acid and diglycidil ether bisphenol A (DGEBA). The modified carbon fibres are employed as additives for fabrication of PA6-based composites. The mechanical properties of the PA6/carbon fibre composite are studied to select the appropriate modifying pathway for the carbon fibre. Through chemical reactions with acidic solutions, this treatment removes surface contaminants and amorphous carbon, creating a roughened surface morphology on carbon fibres to promote mechanical interlocking with the PA6 matrix and enhance adhesion via exposure of functional groups for chemical interactions with the PA6 matrix. A DGEBA is anticipated to establish covalent linkages with carbon fibre surfaces via its reactive epoxy groups, particularly interacting with functional groups like amide groups, thereby enhancing interfacial adhesion and facilitating load transfer between the fibre and polymer matrix through chemical bonding mechanisms.

Experimental section

Materials

Basf PA6 resin (code B33L) with a melting point of 220°C and a density of 1.12 g/cm³ was purchased from HuuLoc Plastic Company Limited. Carbon fibre with a diameter of 5 micrometers, a density of 1.75 g/cm³, carbon content of greater than 99.9%, and fibre length of less than 300 micrometers, was obtained from Toray Industries, Inc. Acetone (AR) and nitric acid HNO₃ (AR) were purchased from Xilong chemical company (China). Diglycidil ether bisphenol A was bought from Merck (Germany). All chemicals were used as received without further purification.

Modification of carbon fibre

Carbon fibres were rinsed with acetone to remove any contaminants on the carbon fibre's surface, and then dried at 80°C for 4 h.

Modification of carbon fibre with acid

Following acetone cleaning, 150 g of carbon fibre was weighed and added to the 300 mL of concentrated nitric acid (68%). The mixture was constantly agitated for 8 h at 80°C. The modified carbon fibre was then thoroughly washed utilising distilled water. Dried in vacuum oven for 4 h at 80°C to obtained acid-modified carbon fibre.

Modification of carbon fibre with DGEBA

Carbon fibre was treated with 10% DGEBA in acetone using the same mass ratio as acid (150 g carbon fibre in 300 ml of 10% DGEBA solution, stirred continuously for 8 h). Then, dried in vacuum oven at 80°C for 4 h to obtained DGEBA-modified carbon fibre.

A high-precision balance (HR-200 from A&D Company Limited, with accurate of 0.1 mg) was utilised to determine the mass of carbon fibres before and after modification.

Fabrication of PA6/carbon fibre composite

PA6 material is dried in oven for 8 h at 110°C to remove moisture prior to use. Toray carbon fibre with a length of less than 300 micrometers that has not been modified, modified by acid nitric, modified by DGEBA was employed to fabricate the composites.

Composites were prepared by mixing PA6 and carbon fibres (according to mass composition ratios of 95/5, 90/10, 85/15, 80/20, 75/25, 70/30), the sample's weight was accurately calculated to assure fill factor of 0.7 in the mixing chamber (Haake internal mixer-Rheomix 610, with chamber volume of 120 ml, max. torque of 160 Nm). Typically, 65 g of the mixture was introduced to the mixing chamber and processed at temperature of 240°C, the screw speed of 50 rpm, and the mixing time of 5 min. Then the molten sample was removed and transferred to hydraulic press TOKYOSEIKY (Japan), where it was pressed into a flat sheet at 220°C and 20 MPa. The samples were cooled and stored in a desiccator at room temperature.

The sample used to measure physical and mechanical properties were prepared in the dogbone shapes (the compression moulded material was used as stating material for preparing the dogbone shapes) by injection molding and prototyping using Haake Mini Jet equipment with the following measurement modes: extrusion temperature of 265°C, mold temperature of 160°C and pressure of 750 bar.

Characterisations and mechanical properties analysis

Fourier transform infrared spectroscopy on the TENSOR II device (Brucker) was employed to study the functional groups on the surface of the modified carbon fibres. A small quantity of carbon fibres was ground and combined with potassium bromide (KBr) to create pellets for Fourier-transform infrared spectroscopy (FTIR) analysis using the mode: resolution: 4/cm, sample scan: 16 with the deuterated L-alanine doped triglycene sulphate (DLAGTS) detector.

Rheological parameters (torque values) are determined directly on the Haake internal mixer with measurement modes: temperature of 240°C, screw speed of 50 rpm, and mixing time of 5 min. The physical and mechanical properties of the material, including tensile strength and flexural strength, were measured using the Zwick Z2.5 (Germany) instrument (with the test speed 5 mm/min) in accordance with the ASTM D638 standard and ASTM D790, respectively. Samples made for tensile testing comply to type 4 dimensions of ASTM D638, while samples made for bending testing follow dimensions LxWxH: 4.5 × 0.4 × 0.12 inch defined by ASTM D790. All samples utilise HAAKE's Mini Jet equipment with molds of suitable dimensions for production.

The hardness of the material was measured using an instrument called the HR-320MS Mitutoyo (Rockwell hardness testing machine) in accordance with ISO 2039 with samples measured in mode HRT15: iron balls with a diameter of 1/16 inch, head pressure F₀= 29.42 N (3 kgf), and rear pressure F₁= 147.1 N (15 kgf). The Izod impact strength of the material was determined using Testresources instruments in accordance with ASTM D256. The surface structure morphology of materials was observed using field emission electron microscope (FE-SEM) on HITACHI S-4800 with measuring mode of 5 kV in vacuum environment. On the basis of the ASTM D790 flexural strength measurement, a LxWxH: 4.5 × 0.4 × 0.12 inch sample is prepared for measuring the cross-sectional surface. The sample is subsequently chilled with liquid nitrogen and shattered with a plier; the fractured surface is designated as the SEM measuring surface.

Results and discussion

The FTIR spectroscopy was employed to study chemical nature of the modified carbon fibres. Figure 1 presents FTIR spectra of unmodified, acid-modified, and DGEBA-modified carbon fibres.

It can be obvious that the acid-modified carbon fibre is of similar FTIR spectrum to the unmodified carbon fibre. This might be due to the low modifying degree of the acid for the carbon fibre. On the surface of carbon fibre modified by DGEBA, FTIR infrared spectroscopy reveals peaks at positions 2965/cm, 2925/cm and 2871/cm, which are typical for the C-H bonding of -CH₂ and -CH₃; 1603/cm, 1506/cm, for the C=C bond in the aromatic ring; and 1229/cm, 1026/cm and 824/cm, for the C-O bond in epoxy group.

Effect of modified carbon fibre on melting viscosity of the composites

Some properties of the composite materials such as crosslinking, reaction between phase, melt viscosity and breaking of polymer chain could be determined through torque measurements.³⁶ Figure 2 shows the torque-time curves of PA6-based composite materials reinforced with pristine carbon fibre, acid-modified carbon fibre and DGEBA-modified carbon fibre. According to Figure 2(a) to (c), adding unaltered and modified carbon fibre to composites enhances their melt viscosity proportionally to the filler content, as illustrated by the rise in equilibrium torque with increasing carbon fibre content. This might be due to the introduction of carbon fibre increasing the internal friction between components in the composite, leading to an increase in the kinematic viscosity of the material; as a result, increasing the viscosity of the composite.^37,38 In the torque-time curves of all PA-based composites reinforce with either pristine or modified carbon fibre, the torque values gradually increased throughout the first minute, reaching a maximum at 4 min, and then practically remained constant, indicating that the carbon fibres evenly distributed in the PA6 matrix. Figure 2(d) reveale the compation in the torque-time curves of the composites reinforced with 25%w/w of pristence, acid-modified, and DGEBA-modified carbon fibres. It can be clearly seen that the melt viscosity of the composite reinforced with DGEBA-modified carbon fibre is greater to that of pristine and acid-modified carbon fibres. This might be due to the well interaction between DGEBA-modified carbon fibre with the PA6 polymer to form homogeneous matrix of the composite. The uniform distribution of the carbon fibre in the PA6 matrix demonstrates that the carbon fibre modified with the DGEBA is suitable for the modification of carbon fibre used as additive for the reinforcement of the PA composite.

Two steps are involved in the heating and dispersing of carbon fibre for the composite containing DGEBA-modified carbon fibre: first, the torque rises and then falls suddenly; this is how carbon fibre is mixed with the plastic; and second, the torque rises and then falls gradually. Following thermal cure, epoxy is now distributed differently within the basic resin.

Effect of modified carbon fibre on the melting temperature

The melting temperature of the PA6-based composites with and without reinforcement of the carbon fibres was studied using differential scanning calorimetry (DSC) diagram as shown in Figure 3. The sample without addition of the carbon fibre has a melting point of 226 °C, exhibiting characteristic peak for the melting of the α phasein the crystalline structure of the PA6.^39–41 When 25%w/w of pristine and acid-modified carbon fibres were added into the composite, negligible change in the melting temperatures of the resultant composite compared to the neat PA6 sample are observed with 224 °C for pristine carbon fibre and 225 °C for acid-modified carbon fibre. In the PA6 + acid-modified carbon fibre samples, a broadening of the melting peak was observed at around 190°C, indicating the melting of γ crystal phase.³⁹ However, the γ phase is less durable than the α phase. Intrudingly, the melting point of the PA6 composite reduces significantly upon introduction of the DGEBA-modified carbon fibre to 213 °C. This might be due to the well integration of the DGEBA-modified carbon fibre in the PA6 matrix, which is consistent with previous published works.^42,43 The introduction of epoxy modification decreases the melting temperature of the composite by establishing crosslinking bonds with the amide group of PA6. This process leads to the formation of an amorphous structure while reducing the crystalline structure of PA6. When epoxy and amide groups react and form crosslinking bonds, they create a network structure within the composite material. This network structure can disrupt the regular packing of polymer chains in PA6, leading to an increase in the amorphous regions of the polymer. The amorphous regions of a polymer typically have a lower melting point than the crystalline regions. By increasing the amorphous content, the overall melting behaviour of the PA6 component in the composite may be influenced, potentially resulting in a lower apparent melting point. These findings indicate that the carbon fibres modified with DGEBA affect the degree of crystallinity, and the specific value is influenced by the rate at which the material cools down.

Effect of modified carbon fibre on mechanical properties of the composite

The increase in tensile strength of composite materials reinforced with carbon fibre is proportional to the content of carbon fibre. For composite reinforced by unmodified carbon fibre, when the fibre content is 5%, the tensile strength of composite decreases, however, as the carbon fibre content continues to increase, the tensile strength of the composites tarts increasing compared to the samples without addition of carbon fibre (Figure 4(a) and (b) and Supplemental Figures S1 to S3). This can be explained by the degradation of PA6 at the high melting temperature to form composite materials, which is consistent with previous works on the deterioration of material properties due to heat.^44,45 However, with a sufficient amount of carbon fibre (more than 5%) tensile strength of composite materials enhance even degradation of PA6 during the process. PA6 is susceptible to oxidation at elevated temperatures, especially in the presence of oxygen. Oxidation reactions can lead to the formation of carbonyl groups (such as ketones and aldehydes) along the polymer chain, which can further facilitate chain scission and degradation. Thermal degradation can cause PA6 to change colour, usually yellowing, due to the formation of chromophore groups during the degradation process. The change in colour of the PA6 was observed during these experiments.

Composites reinforced with acid-modified carbon fibres reveal higher tensile values than those reinforced with DGEBA-modified carbon fibres and unmodified carbon fibres, with a maximal tensile value of 80.3 MPa at the carbon fibre content of 25% by mass. Nevertheless, the composite using acid-modified carbon fibre exhibits a lower tensile modulus compared to the composite incorporating DGEBA-modified carbon fibre. This is explained by DGEBA alteration, which results in a stiffer surface and cross-linked structure, producing in a composite with a greater modulus.

The flexural strength of the material reinforced with acid-modified carbon fibre has a higher value than those containing DGEBA-modified and unmodified carbon fibre; the value of flexural strength is proportional to the carbon fibre contents (Figure 4(c) and (d) and Supplemental Figures S4 to S6). It reaches a maximum of 129.7 MPa when the acid-modified carbon fibre content reaches 25% by mass. The material properties including tensile strength and flexural strength begin to decrease when the carbon fibre content exceeds 25%. According to some studies^46,47 when the carbon fibre content is too high, clusters of fibre are formed due to electrostatic bonds leading to the uneven distribution of the carbon the PA6 matrix; as a results, the mechanical properties of obtained composite materials decrease. For materials containing DGEBA-modified carbon fibre, the tensile strength is higher than tensile strength of materials reinforced unmodified carbon fibre (Figure 4(a)), however, the flexural strength is lower for the same carbon fibre content (Figure 4(b)).

Effect of modified carbon fibre on the hardness and impact strength of the PA6-based composite material

The hardness of the PA-based composite materials were measured using Rockwell hardness measurement (HRT) as show in Figure 5(a). It can be obvious that the hardness of the composite materials enhances along with the increase of the carbon fibre contents. The composites reinforced with the modified carbon fibre reveal significant improvement in the hardness in comparison to that of unmodified carbon fibre. The hardness values of the composite reinforced with the acid-modified carbon fibre are similar to that of those reinforced with the DGEBA-modified carbon fibre. The highest hardness value of the PA6-based composite is approximately 55.7 HRT with the acid-modified carbon fibre content of 25% by weight. Further increase of the carbon content witness the decrease in the hardness values. This might be due to the aggregation of the carbon fibre in the PA6 matrix at the high content of the carbon fibre.

The impact strength of the composite reinforces with different carbon fibre modification was also investigated as shown in Figure 5(b). It can be clearly seen that the enhanced impact strength value is in direct proportion to the content of carbon fibre in the sample. The izod impact strength of composite materials decrease in the following order: material containing acid-modified carbon fibre, material containing DGEBA-modified carbon fibre, and unmodified carbon fibre. While the impact strength value of the composite reinforced with DGEBA-modified carbon fibre is similar to that of those reinforced with unmodified carbon fibre, the PA6-based composite reinforced with acid-modified carbon fibre exhibits a remarkable enhancement in the impact strength values. The impact strength of the composite increase along with the increase of the acid-modified carbon fibre's content and reach a maximal value of of 50.9 J/m with the carbon fibre's content of around 25% by weight (Figure 5(b)). The mechanical properties results indicate that the modification of carbon fibre with acid might be the suitable modifying pathway for the carbon fibre as an additive to improve the mechanical properties of the PA6-based composite.

Morphology of PA6-based composite reinforced with modified carbon fibre

The distribution of the carbon fibre with the content of 25% by weight in the PA6 matrix was observed by scanning electron microscopy as shown in Figure 6. The results show that the PA6-based resin does not adhere to unmodified carbon fibre (Figure 6(b)), whereas the PA resin adheres to the surface of acid-modified carbon fibre as it can be clearly seen from the Figure 6(d) that the PA6 resin is uniformly cover all the surface of carbon fibre. For composite material containing DGEBA-modified carbon fibre, phase separation occurs, a portion of epoxy cures during the process of mixing (the reason for the increase of torque (Figure 2(d)), resulting in fragmentation when the material is cut (Figure 6(f)). This is the reason why tensile strength, flexural strength, hardness and impact strength of the sample modified with DGEBA are lower than of the sample modified with acid. The well-integration of the acid-modified carbon fibre could further explain for the enhanced mechanical properties of the PA6-based composite when reinforcing with the acid-modified carbon fibre.

The energy dispersive spectrometry (EDS) mapping of the PA6 reinforced with acid-modified carbon fibre was obtained to investigate the element distribution of the composite. The result is exhibited in Figure 7. It can be clearly seen from Figure 7 that the O and N elements are well-distributed on the surface of the carbon fibre, indicating the good adhesion of the PA6 resin with the modified carbon fibre. This result further confirms that the modification of carbon fibre with acid could significantly enhance the bonding between the PA6 resin and carbon fibre.

Conclusions

In short, the carbon fibre has been successfully modified with acid, and employed as an additive to improve the mechanical properties of the PA6-based composite. The study on the acid-modified carbon fibres revealed that the well integration in PA6 matrix compared to DGEBA-modified carbon fibre and unmodified carbon fibre. Due to its better compatibility, the PA6-based composite reinforced with acid-modified carbon fibre exhibited significant improvement in mechanical properties than those of unmodified and DGEBA-modified carbon fibre. In comparison to composite materials containing unmodified carbon fibre and DGEBA-modified carbon fibre, composite containing acid-modified carbon fibre increase tensile strength (16–38% and 7–15%, respectively), flexural strength (18–66% and 51–70%, respectively), Izod impact (23–65% and 32–78%, respectively) and Rockwell hardness (50–400% and equivalence, respectively). With these enhanced mechanical properties, the carbon fibre modified with the acid could be the reasonable approach to improve the compatibility of the carbon fibre in the resin matrix.

Supplemental Material

sj-docx-1-prc-10.1177_14658011241253553 - Supplemental material for Polyamide 6/carbon fibre composite: An investigation of carbon fibre modifying pathways for improving mechanical properties

Supplemental material, sj-docx-1-prc-10.1177_14658011241253553 for Polyamide 6/carbon fibre composite: An investigation of carbon fibre modifying pathways for improving mechanical properties by Dung The Dinh, Ha Duc Ninh, Hung Tran Nguyen, Dat Huu Nguyen, Giang Vu Nguyen, Tung Huy Nguyen, Kien Trung Pham and Duong Duc La in Plastics, Rubber and Composites

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Hanoi Department of Science and Technology (grant number 01C-03/07-2021-03).

Supplemental material

Supplemental material for this article is available online.

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