Effect of matrix composition on properties of polyamide 66/polyamide 6I-6T composites with high content of continuous glass fiber for optimizing surface performance

Jiaxiang Xie

doi:10.1515/epoly-2022-8111

Article Open Access

Effect of matrix composition on properties of polyamide 66/polyamide 6I-6T composites with high content of continuous glass fiber for optimizing surface performance

Jiaxiang Xie

Published/Copyright: March 10, 2023

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From the journal e-Polymers Volume 23 Issue 1

Abstract

To optimize the surface performance of the composites of polyamide 66 and continuous glass fiber (50/50, weight ratio), poly(hexamethylene isoftalamide-co-terephthalamide) (PA6I-6T) was introduced. The composites were prepared by a twin-screw extruder, and the effect of PA6I-6T on the thermal and mechanical properties, as well as surface appearance was investigated. The results showed that the crystallization temperature and the melting point of the composites decreased with increasing the amount of PA6I-6T. The Jeziorny method proved that the presence of PA6I-6T did not change the nucleation mode and the crystal growth of PA66 in the hybrid matrix, and the Mo method revealed that the crystallization rate of PA66 decreased first and then increased with the increase of the PA6I-6T content. The reduction in the “floating fiber” on the surface of the composites was attributed to the decrease of the crystallization rate of PA66 in the presence of PA6I-6T. It was shown that the composites with smooth surface had a large melt flow index. Moreover, the composites with high content of PA6I-6T had low heat deformation temperatures. The mechanical properties of the composites did not change significantly with the increase of the PA6I-6T content. Scanning electron microscope (SEM) images revealed that glass fiber was evenly distributed in the matrix and strongly oriented parallel to the injection direction.

Keywords: PA6I-6T; PA66; glass fiber; high glass fiber; glass fiber reinforcement

1 Introduction

With the improvement of the industrial development level and product quality requirements in recent years, the research involving high-performance general engineering plastics has been put out and has become the key research direction and development frontier of modified plastics (1,2,3). Among them, glass fiber (GF)-reinforced thermoplastic polyamide (PA) composites have been used in many fields such as mechanical industry, automotive parts, precision instruments, electronics, and electrical appliances. In particular, the modified PA composite with high GF content (GF mass fraction not less than 50%) has high mechanical properties (especially rigidity), excellent dimensional stability, long-term heat resistance, chemical resistance, and oil resistance, and shows a broad application prospect in the field of automobile lightweight (4,5,6). It can replace some metal materials (such as aluminum alloy materials) to be used in structural parts such as automobile parts and mechanical parts, which can effectively reduce product costs and weight to achieve the purpose of energy saving and consumption reduction. Therefore, it has attracted the attention of many researchers in the field.

Relevant research showed that the melt viscosity of the PA matrix increased with the increase of the GF content. In turn, GF was more difficult to uniformly disperse in the matrix (7,8,9). The resulting poor fluidity and nonuniformity in the composites caused floating fiber on the product surface and reduced the surface quality. The fiber floating makes the surface relatively rough, reduces the smoothness of the surface, and in some cases reduces the performance of the composite (7,8,9). In fact, GF exposure was a common problem in GF-reinforced nylon composites during the production process (10,11), and especially severe at high GF content (12,13). Therefore, optimizing the surface performance of PA/GF composites with high GF content was one of the concerns of various modified plastics enterprises.

GF-reinforced polyamide 66 (PA66/GF) composites are attracting more interest than others in the PA/GF composites due to advantages such as high strength (14), high-heat resistance (15), self-lubrication (16), and high toughness (17). PA66-based composites with high GF content have a wide range of applications in the aerospace, automotive, and electrotechnical industries. However, the research on PA66 composites with high GF content mainly focused on mechanical properties, toughening modification, flame retardant modification, etc., and there were few reports on improving surface properties (18,19,20,21,22,23).

In view of the disadvantages of poor appearance and low processing fluidity of PA66-based composites with high GF content, the composites were modified by introducing poly(hexamethylene isoftalamide-co-terephthalamide) (PA6I-6T) to obtain excellent properties and good appearance at 50% of GF mass fraction in the present study. It is generally believed that improving the fluidity of the matrix melt can reduce the “floating fibers” (12). Currently, introducing semi-aromatic polyamide (SAPA) into GF-reinforced PA-based composites to improve the fluidity of the matrix has become one of the methods to avoid GF exposure (14,16). As a typical SAPA, PA6I-6T has a relatively slow crystallization rate due to its large benzene ring structure (24). The steric hindrance of the benzene ring can delay the crystallization of PA66, which is conducive to the dispersion of GF in the PA66 matrix. In the study, the effect of the composition ratio of PA66 and PA6I-6T in the matrix on the surface of the composites was evaluated by an ultra-deep field microscope, and the crystallization behavior and the corresponding nonisothermal crystallization kinetics of PA66 in the matrix was characterized by using a differential scanning calorimeter (DSC). The dependence of the melt fluidity on the matrix composition in the composites was also revealed. The distribution and orientation of glass fiber in the matrix were observed by using scanning electron microscope (SEM). In addition, mechanical properties and heat deformation temperatures (HDTs) of the resulting composites were also studied.

2 Experiment

2.1 Materials

PA66 (EPR27) with a melt flow index (MFI) of 10.8 g per 10 min and a relative viscosity of 2.64–2.70 was purchased from Henan Shenma Nylon Chemical Co., Ltd, Henan, China. PA6I-6T (PA-TI1207) was purchased from Shandong Guangyin New Materials Co., Ltd, Shandong, China. GF, EDR17-2400-988A, modified by silane coupling agent, was purchased from China Jushi Co., Ltd, Zhejiang, China. Carbon black masterbatch and processing aids were provided by Henan Hairuixiang Technology Co., Ltd, Henan, China.

PA66 and PA6I-6T were dried in a circulating-air stove at 100°C for 12 h. The composites of PA66/PA6I-6T/GF were made in a twin-screw extruder (SHJ-36, Nanjing Ruiyajie, China) at temperatures ranging from 265°C to 275°C along the nine zones. The screw speed controlled at 730 rpm (screw of diameter 36 mm with L/D = 40), and the feed speed was 440 rpm. In addition to PA66, PA6I-6T, and GF, the composites also contained carbon black (CB) masterbatches and flow additives. The CB masterbatch contained 50% CB, and PA6 was used as the carrier. The test formulations of the composites are listed in Table 1. Extruded composite pellets were collected and stored at room temperature.

Table 1

Test formula of PA66/PA6I-6T/GF composites

Sample no.	PA66	PA6I-6T	GF	CB masterbatche	Flow additive
H₅₀G₅₀	50	0	50	2	1.4
H₄₅T₅G₅₀	45	5	50	2	1.4
H₄₀T₁₀G₅₀	40	10	50	2	1.4
H₃₀T₂₀G₅₀	30	20	50	2	1.4

2.2 DSC testing procedures

A DSC (DZ-DSC 300, Nanjing Dazhan, China) was used to investigate melting and crystallization behaviors of the composites. Each specimen of 20 ± 1 mg was sealed into an aluminum crucible, and the test was carried out in a nitrogen atmosphere. Temperature and heat flow scales were calibrated using high-purity indium. Both the warming and cooling rates were set at 20°C·min⁻¹. The specimen was first heated from 30°C to 320°C and held in the molten state for 5 min to eliminate thermal history. The specimen was subsequently cooled to 40°C. After 3 min at 40°C, the specimen was reheated to 320°C. The first warming, cooling, and the second warming scans were recorded.

Immediately after the crystallization experiments, DSC heating traces were recorded at a heating rate of 10°C·min⁻¹ to determine the degree of crystallinity. All heating traces were corrected by subtracting the baseline recorded with an empty pan in the sample and reference oven. The crystallinity weight fraction or crystallinity ratio was calculated from this relationship.

The crystallinity (X _m) was determined from Eq. 1:

(1) X m = Δ H m H m 0 · w PA 66 × 100 %

where Δ H m 0 = 195.5 ± 5 J·g⁻¹ for 100% crystalline PA66 in the α-crystal form (23) and w PLA is mass% of PA66 in the specimen.

2.3 Nonisothermal crystallization analysis

To observe the nonisothermal crystallization characteristics, the aforementioned DSC technology was used. The specimen was heated to 300°C at a warming rate of 20°C·min⁻¹ and held for 5 min in a nitrogen atmosphere. Subsequently, the specimen was cooled down to room temperature at cooling rates of −5°C·min⁻¹, −10°C·min⁻¹, −15°C·min⁻¹, and −20°C·min⁻¹, and reheated to 300°C at 20°C·min⁻¹. In the nonisothermal crystallization process, the crystallization temperature and crystallization time were transformed by Eq. 2:

(2) t = ( T o − T ) φ

where t is the crystallization time, T _o is the initiate crystallization temperature, T is the temperature at time t, and φ is the cooling rate (°C·min⁻¹).

2.4 Test and characterization

The composite pellets were dried in an oven at 100°C for 4 h before further testing.

The dried pellets were made into standard strips by an injection molding machine at 265–270°C and injection time of 26 s. The strips were left for at least 24 h before testing. The surface morphology of the strips was investigated by using an ultra-deep field microscope (VHX-6000, Keyence). The operation was performed at room temperature in atmospheric air. The mechanical testing was conducted by a universal testing machine (WDW-2010, Chengde Juyuan, Hebei, China). Following the GB/T 1040-2006 and the GB/T 9341-2008 standards, the tensile and the flexural tests were performed at a constant speed of 5 and 2 mm·min⁻¹, respectively. Following the GB/T 1843-2008 standard, a notched impact test was performed using an Izod impact strength machine (XJU-5.5D, Chengde Juyuan).

The MFI was obtained by using a melt index tester (XNR-400C, Chengde Juyuan Co., Ltd). Following the GB/T 3682-2000 standard, the tests were carried out at 270°C under the specified load of 2.16 kg.

Vicat softening point of the composites was obtained by using a heat deformation temperature tester (ZWK1302-2, Meister). Specimens with length, width, and thickness 80 mm × 10 mm × 4 mm were tested at a warming speed of 2°C·min⁻¹ with a load of 1.8 MPa.

The water absorption was determined by using a moisture meter (QL-720C, Xiamen Meade, China) at room temperature in atmospheric air (Eq. 3):

(3) A = ( C − C o ) / C o × 100 %

where A is the absorption rate, C ₀ is the initial sample weight, and C is the weight after water absorption.

Thermogravimetric analysis (TGA) was performed by using a TGA/DSC simultaneous thermal analyzer (STA409PC, Netzsch, China) at a warming speed of 10°C·min⁻¹ in the temperature interval of 25–600°C in a nitrogen atmosphere.

The half-time of crystallization, t _1/2, is defined as the time required to reach 50% of the final crystallinity.

3 Results and discussion

3.1 Melting and crystallization behaviors of PA66/PA6I-6T/GF composites

The DSC scans of PA66/PA6I-6T/GF composites are shown in Figure 1, and the results derived from the first warming scan are listed in Table 2.

Figure 1

DSC curves of the first warming (a), the cooling (b), and the second warming (c) scans of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

Table 2

DSC results derived from the first warming scan for H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀

	T _m (°C)	∆H _m (J·g⁻¹)	X _m (%)
H₅₀G₅₀	263	11.6	11.9
H₄₅T₅G₅₀	265	14.9	16.9
H₄₀T₁₀G₅₀	261	15.3	19.6
H₃₀T₂₀G₅₀	257	14.9	25.4

The first warming scans of the composites in the initial state are shown in Figure 1a. The melting peaks of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀ were at 263°C, 265°C, 261°C, and 257°C, respectively. The crystallization ability of PA6I-6T was very poor, while PA66 was a crystalline polymer. Therefore, the melting process during the warming was reasonably attributed to the melting process of the PA66 crystalline phase in the composites. For the series of H₅₀G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀, the peak temperature decreased with the increase of the PA6I-6T content. The anomalous increase in the peak temperature of H₄₅T₅G₅₀ was attributed to the fact that a small amount of PA6I-6T favored the perfection of the crystal structure of PA66. The X _m values of the composites calculated according to Eq. 1 are listed in Table 2. As shown earlier, the values increased with the increase of the PA6I-6T content.

The cooling scans of the composites without thermal histories are shown in Figure 1b. As seen, one crystallization process attributed to PA66 occurred. For H₅₀G₅₀ and H₄₅T₅G₅₀, the crystallization peaks were at 231°C and 230°C, respectively, with a difference of 1°C. For H₄₀T₁₀G₅₀ and H₃₀T₂₀G₅₀, the crystallization peaks were at 219°C and 198°C with a decrease of 12°C and 33°C compared with H₅₀G₅₀, respectively. The delay of PA66 crystallization in the presence of PA6I-6T made GF to have more time to disperse in the matrix, which was conducive to the uniform dispersion of GF. In addition, the resulting PA66 crystals formed at lower temperatures had small grain size and imperfect structure, and thus, the melting temperature was low during the second warming (as shown in Figure 1c).

3.2 Nonisothermal crystallization kinetics of the PA66/PA6I-6T/GF composites

3.2.1 DSC nonisothermal scans

Given that PA66 was a crystalline matrix, its crystallization had an impact on the properties and appearance of the composite. It was necessary to investigate the crystallization kinetics of PA66/PA6I-6T/GF composites. Here, nonisothermal crystallization methods were used. Figure 2 shows the DSC curves of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀ at rates of −5, −10, −15, and −20°C·min⁻¹. It was seen that there was only one crystallization process for each cooling curve, which was attributed to the PA66 crystallization process, not to the PA6I-6T crystallization process.

Figure 2

DSC curves of the cooling scans at rates of −5, −10, −15, and −20°C·min⁻¹ for H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d), respectively.

Figure 3 presents the plots of X _t versus t at the cooling rates of −5, −10, −15, and −20°C·min⁻¹ for the composites. It was seen that all these curves had the same sigmoidal shape, which was due to the slowing down of the crystallization rate caused by the collision and extrusion of spherical crystal boundaries with each other in the late stage of the crystallization of PA66. As shown in Figure 3, the cooling rate had a significant effect on their crystallization rates for all the composites, and the crystallization rate increased significantly as the cooling rate increased. The characteristic parameters obtained from Figures 2 and 3 are listed in Table 3. As listed in Table 3, t _1/2 became smaller as the cooling rate increased, indicating that the crystallization rate increased, and the completion time decreased with increasing the cooling rate.

Figure 3

Plots of X _t versus t at the cooling rates of −5, −10, −15, and −20°C·min⁻¹ for H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d), respectively.

Table 3

DSC results derived from the scans at the cooling rates of −5, −10, −15, and −20°C·min⁻¹ for H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d), respectively

	Φ (°C·min⁻¹)	T _c (°C)	∆H _c (J·g⁻¹)	X _c (%)	t _1/2 (min)
H₅₀G₅₀	−5	235	13.0	14.4	1.33
	−10	231	13.6	15.0	0.91
	−15	230	13.2	14.6	0.55
	−20	229	13.1	14.5	0.38
H₄₅T₅G₅₀	−5	231	12.8	15.9	1.12
	−10	227	14.0	17.3	0.84
	−15	225	12.3	15.2	0.52
	−20	224	11.6	14.4	0.43
H₄₀T₁₀G₅₀	−5	227	13.1	18.4	1.27
	−10	221	13.6	19.1	0.71
	−15	219	12.5	17.7	0.51
	−20	217	11.7	16.5	0.36
H₃₀T₂₀G₅₀	−5	212	10.1	19.6	1.94
	−10	201	9.44	18.3	0.84
	−15	197	8.45	16.4	0.56
	−20	193	7.89	15.3	0.48

As shown in Table 3, for each composite, T _c decreased with increasing the cooling rate. For the series of the composites in the study at the same cooling rate, T _c decreased with increasing the content of PA6I-6T. On the other hand, the introduction of PA6I-6T improved the crystallinity of PA66 in the composites. As shown in Table 3, the t _1/2 values of H₅₀G₅₀ and H₃₀T₂₀G₅₀ were higher than those of H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ at a given cooling rate. This suggested that 5 parts and 10 parts PA6I-6T accelerated the whole crystallization process regardless of the cooling rate, but 20 parts PA6I-6T did not.

As Ф increased from −5 to −20°C·min⁻¹, the crystallization peak gradually moved toward the low temperature. This was due to the fact that the rearrangement of polymer chains into the lattice was a relaxation process, which took some time to complete. If the crystallization rate did not catch up with the cooling rate, the crystallization peak on the DSC curve occurred at a lower temperature. Moreover, when the cooling rate increased, the crystallization peak continued to move toward the lower temperature. Compared with H₅₀G₅₀ without PA6I-6T, the crystallization temperatures of the other composites with PA6I-6T in the study were all lower as shown in Figure 2 and Table 3, suggesting that PA6I-6T was able to reduce the motility of PA66 molecular chains. Moreover, the crystallization temperature of H₃₀T₂₀G₅₀ was lowest among them, and its crystallization peaks moved toward the lowest temperature as the cooling rate increased, indicating that the movement of PA66 molecular chains in H₃₀T₂₀G₅₀ was more difficult than that in H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀.

The crystallization of H₅₀G₅₀ and H₃₀T₂₀G₅₀ was slower than that of H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ at the same cooling rate. The slower crystallization of H₅₀G₅₀ was due to strong forces between PA66 molecular chains (such as hydrogen bonding), but that of H₃₀T₂₀G₅₀ was due to the too strong pulling effect of PA6I-6T on the PA66 molecular chain movement. In other words, the controlled amount of PA6I-6T facilitated the PA66 molecular chain movement.

3.2.2 Jeziorny method

The Avrami model was widely used to describe isothermal crystallization kinetics, in which X _t was expressed in Eq. 4 (25):

(4) 1 − X t = exp ( − Z t t n )

where n is the Avrami crystallization index, which depends on the nucleation mechanism and growth size, t is the crystallization time, and Z _t is the crystallization rate constant, which depends on the nucleation and crystal growth.

Eq. 4 can be transformed into the following linear relationship:

(5) ln [ − ln ( 1 − X t ) ] = ln Z t + n ln t

According to Eq. 5, n and Z _t are obtained from the slope and intercept of the curve between ln[−ln(1 − X _t)] and ln t as shown in Figure 4. However, the Avrami model was not applicable to the nonisothermal crystallization because the temperature constant was variable, and nucleation and crystal growth were temperature dependent. Jeziorny proposed that Z _t was influenced by the cooling rate ( φ ) and verified by having a cooling rate term as shown in Eq. 6:

(6) ln ( Z c ) = ln ( Z t ) / φ

Figure 4

Avrami plots for H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d).

The values of n and Z _c are listed in Table 4.

Table 4

The parameters of nonisothermal cooling crystallization for H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀, respectively

	Φ (°C·min⁻¹)	n	Z _c
H₅₀G₅₀	−5	4.13	0.67
	−10	3.62	0.94
	−15	3.98	1.11
	−20	3.62	1.15
H₄₅T₅G₅₀	−5	3.48	0.80
	−10	3.58	0.98
	−15	4.20	1.13
	−20	4.53	1.16
H₄₀T₁₀G₅₀	−5	4.46	0.67
	−10	3.11	1.05
	−15	4.12	1.15
	−20	3.61	1.17
H₃₀T₂₀G₅₀	−5	4.29	0.49
	−10	4.43	1.03
	−15	4.34	1.15
	−20	4.25	1.17

The n value was related to the nucleation mode and the crystal growth dimension. As shown in Table 4, there was no significant change in the n value as PA6I-6T was increased to 20 parts in H₃₀T₂₀G₅₀, which meant that the introduction of PA6I-6T did not change the nucleation mode and the crystal growth of PA66.

As for the crystallization rate, Z _c of each composite almost always increased significantly as the cooling rate increased as shown in Table 4. This indicated that the total crystallization rate of each composite increased with the cooling rate.

3.2.3 Mo method

By developing the Avrami model, the Ozawa model was used to describe the nonisothermal crystallization assuming that the nonisothermal crystallization process was a small step in isothermal crystallization. The fractional crystallinity ( X t ) at time t is shown in Eq. 7 (26):

(7) X t = [ 1 − exp ( − K o m / φ ) ]

The linear form of Eq. 7 is more commonly used in Eq. 8:

(8) ln [ − ln ( 1 − X t ) ] = ln ( K o ) − m ln ( φ )

where m and K _o are obtained from the slope and interception on the y-axis of the graph from the relationship between ln [ − ln ( 1 − X t ) ] and ln ( φ ) , respectively.

The Mo method combined the Avrami equation with the Ozawa equation to describe the nonisothermal crystallization process. The Mo equation is as follows (26):

(9) ln φ = ln F ( T ) − α ln t

where F ( T ) is the Mo-modified crystallization rate parameter, and α is the ratio of n in the Avrami equation to the exponent m in the Ozawa equation related to crystallization dimension. Obviously, plotting ln φ versus ln t at a given value of relative crystallinity yielded a series of straight lines, as shown in Figure 5. Then, F ( T ) and α are obtained from the intercept and slope of the straight lines. Table 5 lists the parameters obtained from the plots of ln φ versus ln t for all samples.

$Figure 5 Plots of lg φ {\rm{lg}}\hspace{.5em}\varphi versus lg t {\rm{lg}}\hspace{.5em}t for the cold crystallization of H50G50 (a), H45T5G50 (b), H40T10G50 (c), and H30T20G50 (d).$

Figure 5

Plots of lg φ versus lg t for the cold crystallization of H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d).

Table 5

Parameters obtained from the plots of ln φ versus ln t for H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀

	X _t (%)	F(T)	ɑ	r ²
H₅₀G₅₀	20	5.76	1.04	0.94548
	30	6.37	1.05	0.94108
	50	7.63	1.07	0.93534
	60	8.34	1.09	0.93506
	80	10.42	1.10	0.91774
H₄₅T₅G₅₀	20	4.48	1.36	0.93364
	30	5.35	1.32	0.91536
	50	6.58	1.33	0.93857
	60	7.39	1.31	0.92790
	80	9.60	1.27	0.90373
H₄₀T₁₀G₅₀	20	5.07	1.08	0.99135
	30	5.62	1.09	0.99228
	50	6.67	1.12	0.99192
	60	7.24	1.13	0.99194
	80	8.95	1.15	0.99051
H₃₀T₂₀G₅₀	20	7.14	0.87	0.99585
	30	7.79	0.88	0.99589
	50	8.94	0.91	0.99468
	60	9.55	0.92	0.99553
	80	11.05	0.92	0.99491

From Figure 5, it was seen that the curves were essentially linear, indicating that the Mo model was suitable for analyzing the nonisothermal crystallization kinetics of the studied composites. As shown in Table 5, F(T) increased with X _t increasing for each composite, indicating that a faster cooling rate was required to achieve a higher X _t value per unit time. At the same X _t, the F(T) decreased and then increased with the addition of PA6I-6T from H₅₀G₅₀ to H₃₀T₂₀G₅₀, indicating a lower crystallization rate of PA66 in H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀.

3.3 Thermal performance

Figure 6 presents the HDTs of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀. Compared with that of H₅₀G₅₀, there was a small increase in HDT of H₄₅T₅G₅₀ with 5 parts PA6I-6T, and a small decrease in HDT of H₄₀T₁₀G₅₀ with 10 parts PA6I-6T. But for that of H₃₀T₂₀G₅₀ with 20 parts PA6I-6T, there was a significant decrease, nearly 58°C. HDT was used to characterize the superiority or inferiority of heat resistance of a polymer or polymer material, and it was obvious that a small amount of PA6I-6T was beneficial to the improvement of heat deformation ability of PA66/GF composites, but an excessive amount of PA6I-6T was not. HDT mainly depended on the structural characteristics of the material such as molecular alignment, residual stress, crystal structure, crystallinity, and filler orientation. The low heat resistance of H₃₀T₂₀G₅₀ was attributed to the low crystallinity of PA66 in the composite.

Figure 6

HDTs of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

Figure 7 presents the MFIs of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀. It was obvious that PA6I-6T was very beneficial to the melt flow capacity of the studied composites. In particular, the MFI value of H₄₅T₅G₅₀ was nearly twice that of H₅₀G₅₀, reaching 42.0 g·min⁻¹. The MFI of H₄₀T₁₀G₅₀ increased by 41% compared with H₅₀G₅₀, and that of H₃₀T₂₀G₅₀ increased by 34%. However, the MFI value decreased with the increase of PA6I-6T from H₄₅T₅G₅₀ to H₃₀T₂₀G₅₀, indicating the blocking of PA66 chain movement by PA6I-6T.

Figure 7

MFIs of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

Figure 8 presents the thermogravimetric curves of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀. As shown in Figure 8, the thermogravimetric curves of H₅₀G₅₀ and H₄₅T₅G₅₀ almost overlapped, while the thermogravimetric curves of H₄₀T₁₀G₅₀ and H₃₀T₂₀G₅₀ shifted from 441°C in H₅₀G₅₀ to 452°C in H₄₀T₁₀G₅₀ and 464°C in H₃₀T₂₀G₅₀, respectively, at 70% mass. This suggested that the presence of PA6I-6T facilitated the thermal stability of the composites.

Figure 8

Thermogravimetric curves of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

3.4 Tensile properties analysis

Figure 9a–c presents the tensile strength, flexural strength, and modulus of the investigated composites in the study. Figure 10 presents the impact strength. As shown, H₄₀T₁₀G₅₀ showed the higher mechanical performance. Despite containing 20 parts of PA6I-6T, the mechanical properties of H₃₀T₂₀G₅₀ were not the highest.

Figure 9

Tensile strength (a), flexural strength (b), and flexural modulus (c) of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

Figure 10

Impact strength of H₅₀G₅₀, H₄₅T₅G₅₀, H₄₀T₁₀G₅₀, and H₃₀T₂₀G₅₀.

3.5 Surface appearance

Figure 11 presents the ultra-deep field microscope photos of the composites, suggesting that the “floating fiber” of H₅₀G₅₀ and H₃₀T₂₀G₅₀ was more obvious than that of H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀.

Figure 11

Ultra-deep field microscope photos of H₅₀G₅₀ (a), H₄₅T₅G₅₀ (b), H₄₀T₁₀G₅₀ (c), and H₃₀T₂₀G₅₀ (d), respectively.

GF exposure was a common phenomenon in high-content GF-reinforced nylon composites. The factors that caused floating fiber were multiple. The composites in the study differed from each other only in the amount of PA6I-6T. Therefore, the difference in surface appearance was attributed to the difference in the amount of PA6I-6T in the composites. For composites of thermoplastic polymers reinforced by GF, a “fountain” effect was formed when the matrix melt was injected into the mold cavity, and GF flew from the inside to the outside, thus making contact with the surface of the cavity. When the temperature of the mold surface was low, GF was frozen instantly. If the matrix melt covered GF in time before freezing, GF was prevented from being exposed. Therefore, improving the fluidity of the matrix melt and delaying its crystallization were beneficial to eliminating floating fibers. H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ had high MFIs (as shown in Figure 7), so that their surfaces were smoother than that of H₅₀G₅₀ and H₃₀T₂₀G₅₀.

3.6 SEM observation

Figure 12 shows SEM micrographs of the fractured surface of impact test strips of H₄₀T₁₀G₅₀ as the typical composite in the present study. The SEM images revealed a fairly homogeneous dispersion of the GF in the PA66/PA6I-6T matrix in the case of the H₄₀T₁₀G₅₀ composite. Moreover, the GF was strongly oriented parallel to the injection direction close to the sample surface compared to the center of the sample where the GF appeared to be somewhat disoriented. The close view of the SEM image in Figure 12b reveals that pieces of the matrix remained strongly stuck on the GF located at the fractured surface of the strip. This good adhesion was attributed to the silane surface treatment of the GF.

$Figure 12 SEM images of the impact-fractured surfaces of H40T10G50 composites (a and b).$

Figure 12

SEM images of the impact-fractured surfaces of H₄₀T₁₀G₅₀ composites (a and b).

4 Conclusion

The presence of PA6I-6T in the PA66/PA6I-6T matrix not only reduced the crystallization temperature of PA66 but also reduced its melting temperature, leaving more time for GF to disperse in the matrix during processing. However, this did not mean that the more PA6I-6T, the less floating fiber. The surfaces of H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ were smoother than that of H₅₀G₅₀ and H₃₀T₂₀G₅₀, which should be attributed to the decrease of crystallization rate of PA66 in H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ rather than the decrease of the crystallization temperature of PA66. The Mo method proved that the crystallization rates of PA66 in H₄₅T₅G₅₀ and H₄₀T₁₀G₅₀ were slower than that in H₅₀G₅₀ and H₃₀T₂₀G₅₀ at the same crystallinity. In addition, the Jeziorny method proved that the introduction of PA6I-6T did not change the nucleation mode and the crystal growth of PA66. Further research showed that H₄₅T₅G₅₀ had the best melt fluidity, followed by H₄₀T₁₀G₅₀, and H₅₀G₅₀ was the worst. This showed that a small amount of PA6I-6T improved the melt fluidity of the matrix, and further increase of PA6I-6T reduced the melt fluidity.

The large drop in the temperature of the PA66 crystallization peak was caused by the apparent substitution and isolation effects of PA6I-6T on PA66 and became more and more pronounced with the increase of the PA6I-6T content. The PA6I-6T chains entered between the PA66 chains with the aid of hydrogen bonds and van der Waals forces, replacing the original PA66 interchain hydrogen bonds (substitution effect) by forming new hydrogen bonds with the PA66 macromolecule. However, the total hydrogen bond density in the matrix decreased. On the other hand, the PA6I-6T molecular chains separated the PA66 chains, and the interchain van der Waals force of the PA66 chains was weakened (isolation effect). These two effects reduced the ability of PA66 chains to arrange in an orderly manner, and the PA66 chains needed to be at a much lower temperature to enter the PA66 lattice.

In addition, PA6I-6T had little effect on the mechanical properties of the PA66/PA6I-6T/GF composites but facilitated the thermal stability of the composites. On the other hand, an excessive amount of PA6I-6T significantly reduced the HDT of the composites.

Funding information: This study was supported by the Innovation Fund of Luohe Medical College (Grant Nos. 2020-LYZZHXM010 and 2019-LYZZHYB02).
Author contributions: Jiaxiang Xie was solely responsible for the entire work.
Conflict of interest: The author states no conflict of interest.

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Received: 2022-12-27

Revised: 2023-02-03

Accepted: 2023-02-11

Published Online: 2023-03-10

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-8111

Keywords for this article

PA6I-6T; PA66; glass fiber; high glass fiber; glass fiber reinforcement

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