Simultaneously improving the tensile strength and modulus of aramid fiber by enhancing amorphous phase in supercritical carbon dioxide

Abstract

The aramid fiber made from poly(p-phenyleneterephthalamide) PPTA has high strength and high modulus due to a perfect orientation of rigid macromolecular chains. Many attempts to improve the mechanical performance of the aramid fiber were proposed during the last decade; however, simultaneously improving the strength and modulus of the aramid fiber is still a great challenge as most methods can only improve the modulus at the expense of reducing the tensile strength. The mechanism of broken aramid fibers indicates that the weak point of the aramid fiber is at amorphous phase. Therefore, if the amorphous phase can be enhanced, the mechanical performance of PPTA fibers would be improved. Here a novel method is reported to enhance the amorphous phase of PPTA fibers by treating the aramid fiber in supercritical carbon dioxide (ScCO₂) with or without hexamethylene diisocyanate (HDI). In this new process, the ScCO₂, with or without the crosslinker HDI in ScCO₂, is diffused into the amorphous phase of PPTA fibers and the amorphous phase is enhanced by re-arrangement of molecular segments or HDI reacting with the groups (–NH₂, –COOH, and –NHCO–) of PPTA fibers. This is done without breaking the crystalline structure and orientation by carefully controlling the reaction temperature, pressure and time. The results showed that the tensile strength and modulus had simultaneously increased. Molecular weight could be increased under appropriate treatment, while the crystalline structures of PPTA fibers were almost unchanged. Dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) showed that the glass-transition temperature had increased. All the results implied the chain crosslinking mainly took place in the amorphous phase of the PPTA fibers and the performance improvement was attributed to the amorphous enhancement.

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1. Introduction

Poly(p-phenylene terephthalamide) (PPTA) fibers are organic fibers that can be used as reinforcement for advanced composites. These composites have great potential application in fields of aeronautical and astronautical, military, industrial and other advanced domains due to their excellent high tensile strength, modulus, great thermal stability and good solvent resistivity.¹ The exceptional strength of the fibers is a direct result of the chemical structural characteristics of PPTA, including its stiff, extended chain conformation and nearly perfect uniaxial orientation.² In PPTA, the presence of amide groups at regular intervals along the linear macromolecular backbone also facilitates extensive hydrogen bonding in a lateral direction between adjacent chains. This, in turn, leads to efficient chain packing and high crystallinity.¹

Aramid fibers are composed of fibrils or microfibers in their microstructure. The bonding between these fibrils is rather weak, which enables the fibers to be easily damaged, due to external abrasion as well as their low compression strength. This is important in composites. Weak inter-fibril bonds also limit the degree of improvement in aramid/matrix interfacial bond strength because the fibrils on the surface of the fibers can be peeled away before the fiber/matrix interface fails.^3,4 Much attention has been paid to improving the performance of fibers made from the rigid-rod polymers by increasing the transverse interactions or by increasing the crystallinity at the polymer chain level. For example, heat treatment is a modification process used previously on polyethylene terephthalate fibers by Peszkin et al.⁵ The high modulus fibers of Kevlar 49 were produced by heat treatment of Kevlar fiber under tension by DuPont, and after the treatment the Young's modulus was improved, while the tensile strength was to some extent reduced, as the heat treatment causes the products to degrade.^4,6–9 The compressive strength is important in composites; many researchers have studied the crosslinking at the molecular level and incorporation of bulky groups into the backbone of the polymer to disrupt its nematic packing order.^8,10–13 To improve inter-fibrillar bond strength, Mathur and Netravali et al.¹⁴ reported that both the tensile and compressive strengths of fibers increased because plasma treatments could potentially improve the inter-fibrillar adhesion or cohesion and thus improve the fibers' tensile strength, as well as the IFSS.

ScCO₂ has frequently been employed as an extraction medium due to its combination of low viscosity and negligible surface tension.¹⁵ Zhao et al.¹⁶ designed a new pretreatment process in which the organometallic complex was impregnated into Kevlar® fabrics/fibers using ScCO₂ to make electroconductive composite fabrics/fibers. Lesser et al. investigated the use of supercritical carbon dioxide (ScCO₂) to directly wash the residual phosphoric acid out of the poly-p-phenylene-benzobisoxazole fibers.¹⁷

To improve the properties of PPTA, we have developed a new pretreatment process to enhance the amorphous phase of its fibers by treating the aramid fiber in ScCO₂ with or without HDI as a crosslinker. In this new process, the ScCO₂, with and without crosslinker HDI, is diffused into the amorphous phase of PPTA fibers, and the amorphous phase is enhanced by re-arranging the molecular segment or by HDI reacting with the groups (–NH₂, –COOH and –NHCO–) of PPTA fibers.

2. Materials and methods

2.1 Materials

PPTA fibers were supplied by Hebei Silicon Valley Chemical Co., Ltd. China. The yarn is made of 1000 filaments with the diameter of 12.7 μm and the specific linear density of 1111 dtex. Carbon dioxide (CO₂) (purity: +99.99%) was purchased from Shanghai Chenggong Gases Co., Ltd., China and used as received. Hexa-methylenediisocyanate (HDI) (purity: 99%) was purchased from J&K Chemical Ltd (Shanghai, China) and used without further purification. Acetone (chemically pure grade) was brought from the Shanghai Ling Feng Chemical Reagent Co., Ltd. (China).

2.2 Treated aramid fiber in ScCO₂

PPTA fibers were washed in the Soxhlet extractor with acetone at 75 °C for 20 h and dried in an air oven at 110 °C for 3 h to thoroughly remove acetone prior to preceding the ScCO₂ pretreatment. Details of the experimental procedure and the apparatus employed are given elsewhere.¹⁶ The reactions were carried out in high-pressure stainless steel vessels. All parts of the vessels were washed in clean acetone, acid bathed (2.0 M aqueous sulfuric acid), then thoroughly rinsed in clean water, and finally dried at 120 °C. The ScCO₂ impregnation process was carried out in a 50 cm³ cartridge. Samples were rolled, tied on a stainless steel formwork. The concentration of HDI in ScCO₂ was 10 wt%. When the vessel reached the set temperature, the cartridge was introduced. Then, CO₂ gas was supplied via a high-pressure syringe pump and maintained at a preset pressure. Subsequently, the system was kept stable during a certain period of time to allow the HDI to dissolve into the ScCO₂ and to react with the aramid fibers. Finally, decompression was slowly conducted. The impregnation processes were carried out under static conditions.¹⁸

2.3 Characterization

2.3.1 Intrinsic viscosity [η]. The intrinsic viscosity of the fibers was determined at 30 °C by a solution containing 0.5 g of polymer per 100 mL of concentrated sulfuric acid (H₂SO₄) using an Ubbelohde viscometer.

2.3.2 Tensile experiment. Tensile tests were carried out by using a tensile strength tester 0–100 CN (XQ-1A) with a head speed of 10 mm min⁻¹ at a gauge length of 10 mm. The average values were calculated from at least 20 samples.

2.3.3 Fourier transform infrared (FTIR). A spectrophotometer (Nicolet 8700) was used to obtain the spectra of untreated and treated fibers at room temperature ranging from 4000 to 400 cm⁻¹.

2.3.4 X-ray diffraction (XRD). The crystallites of the fibers were tested using a Rigaku D/Max-2550 PC X-ray diffractometer (Rigaku Co., Japan) with CuKα radiation as the source.

2.3.5 Thermomechanical analysis (TMA). TMA experiments were performed on a Thermal Mechanical Analyzer (TMA) (TA Instruments, Model 400EM) in nitrogen with a constant applied load of 20 mN at a heating rate of 5 °C min⁻¹ by tension mode from 30 to 400 °C.

2.3.6 Dynamic mechanical analysis (DMA). DMA measurements for PPTA fibers were conducted using a dynamic mechanical analyzer (DMA Q800, TA Instruments, USA). The DMA test was performed in the fiber fixture mode and the testing temperature ranged from 30 to 450 °C at a heating rate of 10 °C min⁻¹ and a frequency of 1 Hz.

3. Results and discussions

3.1 Simultaneously improving tensile strength and modulus of aramid fiber

The results of mechanical properties testing with temperature, pressure, time for as-received and HDI treated PPTA fibers in ScCO₂ with or without HDI are summarized in Tables 1–3. Compared with that of as-received fibers, the mechanical properties (the average tensile strength and the Young's modulus) of the treated fibers with or without HDI in ScCO₂ tend to increase with the increase of the heating temperature as used. However, the mechanical properties of the fibers treated without HDI were improved less than that of fibers treated with HDI. For the fiber treated in pure ScCO₂, the improvement of mechanical properties was due to the compact microstructure and the reduced defects aided by the plasticizing ability of ScCO₂. When the fibers were treated in the ScCO₂ at the hydrostatic pressure, structural reorganization has occurred as the chain–chain interaction decreased, while the segment and chain mobility increased.

Table 1 The mechanical properties of aramid fibers treated at various temperatures in ScCO₂ (pressure 13 MPa, treatment time 40 min)

Temperature (°C)	Tensile strength (cN per dtex)	Young's modulus (cN per dtex)	Tensile strain (%)
As received	21.7	480.7	2.4
Treated in ScCO2 without HDI
40	22.0	490.4	2.3
60	22.3	510.6	2.6
90	22.6	541.9	2.5
120	22.1	522.3	2.4
Treated in ScCO2 with HDI
40	22.8	515.4	2.6
60	23.1	531.6	2.7
90	23.6	550.6	2.8
120	22.5	464.3	2.5

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Table 2 The mechanical properties of aramid fibers treated at various pressures in ScCO₂ (treatment time 40 min, temperature 90 °C)

Pressure (MPa)	Tensile strength (cN per dtex)	Young's modulus (cN per dtex)	Tensile strain (%)
As received	21.7	480.7	2.4
Treated in ScCO2 without HDI
10	21.9	498.2	2.6
13	22.6	541.9	2.5
15	22.1	502.3	2.4
18	22.4	513.5	2.3
Treated in ScCO2 with HDI
10	22.2	515.9	2.6
13	23.6	550.6	2.8
15	23.2	520.7	2.5
18	22.7	476.8	2.7

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When the HDI was added, most of the properties were improved more than in those treated under the same condition without HDI, as shown in Tables 1–3. The reason was that except for the effect mentioned above that was introduced by the ScCO₂, the functional group NCO– reacted with the group having the H-bonding and formed crosslinking in the fibril, for example, the NH₂–, –NH–CO– and COOH. However if the temperature gets too high, it will cause the decomposition of the fibers and decrease the tensile strength; the same results can result from high pressure or longer treatment time as well. When PPTA fibers were treated for 90 min, as shown in Table 3, the tensile strength and modulus were improved; apparently this might have resulted from increase in the crosslinking reaction, but actually the fibers treated at this condition were too stiff to be of further use. Compared with different treatment conditions, seen from the Tables 1–3, treatment time plays a more important role than treatment pressure and temperature. The fibers exhibit good properties under conditions of pressure of 13 MPa at 90 °C and a treatment time of 40 min: the tensile strength increased by 8.7% and the modulus increased by 14.5%, likely due to an improved interfibril bond strength.

Table 3 The mechanical properties of aramid fibers treated at various times in ScCO₂ (treatment pressure 13 MPa, temperature 90 °C)

Time (min)	Tensile strength (cN per dtex)	Young's modulus (cN per dtex)	Tensile strain (%)
As received	21.7	480.7	2.4
Treated in ScCO2 without HDI
40	22.6	541.9	2.6
90	22.4	520.2	2.5
120	22.3	510.1	2.4
180	20.2	494.3	2.8
Treated in ScCO2 with HDI
40	23.6	550.6	2.8
90	25.4	570.8	2.4
120	23.1	530.4	2.6
180	21.0	462.8	3.0

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3.2 Structure characterization

To investigate the mechanism of the effect of the ScCO₂ and HDI on the fibers, the surface chemical structure of fibers ((fibers treated in ScCO₂/90 °C/40 min/13 Mpa HDI), fibers treated in ScCO₂/90 °C/40 min/13 MPa, as received) were characterized. As can be seen from Fig. 1, these FTIR spectra are very similar. The spectrum of fibers treated with HDI has an extra peak located at 2940 and 2852 cm⁻¹, which is attributed to stretching vibrations of CH₂ groups involved in the HDI. The amide II band is a mixed mode with contributions from the N–H in-plane bending, the C–N stretching, and the C–C stretching vibrations.¹⁹ After treatment with HDI in ScCO₂, the peak intensity at 1514 cm⁻¹ is stronger than the others, which are assigned to –CO–NHR. The results indicated that new groups were formed after treated with HDI. Possibly, –NCO groups in HDI reacted with –NH₂, –NHCO– and –COOH groups in PPTA fibers to form a certain degree of grafting and cross-linking.

Fig. 1 FTIR spectrum of PPTA fibers before and after treatment in ScCO₂ (with HDI).

The viscosities of the fibers in concentrated sulfuric acid were measured to evaluate the molecular weight of the fibers. The results are summarized in the Table 4. The viscosity of the fibers treated in ScCO₂ was higher than that of untreated fibers, which might be caused by the extraction effect of ScCO₂ that removed small molecules. When treated with HDI in ScCO₂, the viscosity was the highest and was significantly improved compared with the others. This is due to some fibers consisting of polymer chains with slipping for terminal segment,²⁰ some end groups of molecular chains can react with the NCO groups of HDI, the number of chains will be reduced and the length of the molecular chains would increase, so the molecular weight of the polymer will increase.

Table 4 The viscosities of PPTA in the fibers

Sample	η (dl g^−1)
As received	6.96
ScCO2 without HDI	7.02
ScCO2 treated with HDI	7.23

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To gain insight into the influence of ScCO₂ and the HDI on the crystalline structure of PPTA fibers, XRD studies were performed on specimens treated in ScCO₂. Fig. 2 shows that there is little difference of the XRD results among these samples. Two strong equatorial diffraction peaks can be due to the 110 and 200 planes at 20.7° and 23.1°, respectively. As can be seen from Table 5, the crystallinity increased slightly, indicating the recrystallization of molecules on the surface of the lamellae.²¹ The peak's 200 plane intensity was enhanced and the crystallites were improved after the fibers were treated in ScCO₂ without HDI; their microstructures in the amorphous region were recognized and the chain compacted and defects reduced as the result of the plasticizing ability of ScCO₂.¹⁵ Panar et al.²² postulated that surface fibrils are uniform, axially oriented, whereas fibrils in the fiber core are imperfectly packed and ordered. This suggests that the core may be more readily affected. As shown in Table 5, the treatment conditions show no difference with respect to the crystallite sizes; moreover, these results imply that the perfection of crystallites increased. However, for the samples treated with HDI, there was little effect on the intensity of the diffraction peak, indicating that the crosslinking and the molecular weight improvement might happen in the amorphous regions of the fibers.

Fig. 2 The wide-angle X-ray diffraction of the PPTA fibers.

Table 5 Apparent sizes of crystallites

Sample	Crystallinity (%)	(110) (Å)	(200) (Å)	d(110) (Å)	d(200) (Å)
as-received	64.08	52.56	49.42	4.30	3.87
ScCO2 without HDI	66.92	54.37	48.00	4.23	3.80
ScCO2 treated with HDI	63.03	54.96	50.97	4.23	3.80

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The glass transition temperature (T_g) can generally be measured by differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), and dynamic mechanical analysis (DMA); however, the DSC has a sensitivity that is too low to characterize the difference among our samples whose main chains are difficult to move over a wide range due to the rigidity of the molecular chains. As TMA and DMA can clearly show the T_g, we use TMA and DMA measurements to determine the T_g of PPTA fibers. There are only a few studies published on the TMA and DMA study on para-aramid fibers.^23–25 TMA measures the dimensional change of a sample as a function of temperature, when it is being compressed or stretched. The TMA curves are shown in the Fig. 3 for the PPTA fibers. The inflection point of thermal expansion curve is defined as TMA T_g. The TMA curves showed a change of slope at about 100 °C due to dehydration of the fibers, and an inflection near 250 °C for PPTA fibers, which was attributed to the glass transition. The T_g of PPTA fibers treated in ScCO₂ with HDI (at about 275 °C) is higher than that of the fibers not treated and treated in ScCO₂ without HDI (250 °C), implying some crosslinking reactions occurred in the amorphous region of the fibers, making movement of molecular chains more difficult.

Fig. 3 TMA curves of the PPTA fibers.

The T_g also can be obtained by dynamic mechanical analysis (DMA T_g). The changes in loss modulus and loss factor as functions of temperature are shown in Fig. 4. The dynamic mechanical loss tangent (tan δ), also called the dissipation factor, is the ratio of the loss modulus to the storage modulus; it is the most important parameter for investigation of the properties of materials. As the loss modulus peak temperature is very close to the T_g of the polymeric system, and in this paper we focus on discussing the glass transition temperature, here in Fig. 4 we chose only the curve tan δ and loss modulus to characterize T_g. The T_g of 205 °C has been reported for Twaron2000 (also a PPTA fiber) by Guo et al. using DMA testing.²⁵ The loss modulus of the fibers treated in ScCO₂ without HDI leveled off at 250 °C; this phenomenon was not observed in the untreated fibers, which may have been a result of crosslinking in the chain. When not adding HDI, the T_g was reduced slightly; because of the plasticizing effect of the supercritical carbon dioxide on fibers and the β relaxation in the crystalline region, the mobility of the amorphous phase was enhanced by relaxation.²² When HDI was added, the T_g increased slightly; this may be due to some crosslinking interactions between molecular chains, so that the intermolecular forces increased and the flexibility of the chain segment was reduced, leading to the decreased activity of the chains. The competition between the plasticizing effect of supercritical carbon dioxide on fibers and the crosslinking of HDI makes T_g change slightly; the curve peak appears at 250 °C, which was in good agreement with the TMA results. As the XRD shows the crystallinity did not changed, the cross-linking reaction happened only in the amorphous phase.

Fig. 4 Dynamic loss tangent (tan δ) and the loss modulus (E′′) as a function of temperature for PPTA fibers (a) as-received PPTA fibers, (b) ScCO₂ treated fibers without HDI, and (c) ScCO₂ treated fibers with HDI.

3.3 Mechanism of enhancement of properties

The property improvement of aramid fiber is attributed to the enhancement of the amorphous phase by rearrangement of the PPTA macromolecular segment or by HDI reacting with the PPTA macromolecular groups (–NH₂, –COOH, and –NHCO–) of PPTA fibers.

Chemical interactions could possibly occur at four positions, as shown in Fig. 5;²⁶ according to the PPTA molecular structure and the results shown in FTIR, the reaction may be taken place in the position 3. Possible chain interactions are as shown in Schemes 1–6. Schemes 1–6 show possible interactions for groups –NH–CO– in PPTA molecules; end groups –NH₂, COOH with groups –NCO in HDI can react with groups –NCO in HDI and form new molecular chains, and the chains will become longer as shown in Fig. 6(a). Owing to the ScCO₂ plasticizing effect, the newly formed chains will be moved and rearranged regularly. Because most of the end groups of a macromolecular chain exist in the amorphous regions, these reactions may be occurring in the amorphous region, as shown in Fig. 6(b). These mechanisms are considered to be responsible for the chemical composition changes that we observed. The tensile strength and modulus increased when treated in ScCO₂ with HDI, which may have been due to the presence of newly formed chemical bonds, increasing the intermolecular forces and their microstructures in amorphous region recognized; and the chain compacted and defects were reduced.¹⁵ Thus after treatment in ScCO₂ with or not with HDI, fibers show better performance than untreated fibers.

Fig. 5 Four potential reaction sites of PPTA.

Scheme 1 –NH₂ reacting with NCO–.

Scheme 2 –NH₂–CO–NH reacting with NCO–.

Scheme 3 –CO–NH reacting with NCO–.

Scheme 4 –COOH reacting with NCO–.

Scheme 5 –NHCO–/–COOH reacting with NCO–.

Scheme 6 NH₂–/–COOH reacting with NCO–.

Fig. 6 The mechanisms of the new chemical reaction in ScCO₂: (a) some reaction of the active groups, (b) reaction in the amorphous region.

4. Conclusions

The tensile strength and modulus of PPTA fibers can be simultaneously improved by treating solid aramid fibers in ScCO₂ with or without HDI. The structure analysis by FTIR, XRD, TMA, DMA and molecular weight measurements imply that the improvement of mechanical performance was due to the enhancement of the amorphous phase among fibrils in the aramid fiber by segment movement of the chain with ScCO₂ as plasticizer without HDI, and by the cross-linking reaction of HDI with active hydrogen group in CONH, NH₂ or COOH in amorphous phase among fibril of PPTA fiber with HDI as crosslinking agent.

Acknowledgements

This work was supported by National Program on Key Basic Research Project (973 Program) (2011CB606101).

Notes and references

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