The preparation of modified polyamide clay nanocomposite/recycled maleic anhydride polyamide 6 and blending with low density polyethylene for film blowing application

Abstract

Generally, polyamide cannot be used as film blowing material because of its unsuitable properties. In this study, polyamide 6 clay nanocomposite (cPA) and styrene maleic anhydride copolymer (SMA) were mixed in various ratios for the preparation of modified polyamide 6 clay nanocomposite S_xcPA_y resins by reactive extrusion. The S₁cPA₁₄ resin was blended with recycled maleic anhydride polyamide (rPA) to form the (S₁cPA₁₄)_x rPA_y resins. Finally, they were mixed with LDPE in 1:9 ratio to afford (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins, respectively, followed by film blowing and the analyses of the physicochemical properties of resins. The FTIR spectrum illustrated that the C=O symmetric and asymmetric absorption fingerprint peaks in the anhydride (-OC-O-CO-) group of SMA disappeared and the new characteristic absorption peak of-CO-N-CO- of imides was observed. The anhydride functional group of SMA underwent reactive extrusion with the terminal amino group of cPA to generate the imides structure. The thermal properties showed that the glass transition temperature and crystallinity of S_xcPA_y and (S₁cPA₁₄)_x rPA_y resins increased with increasing SMA and S₁cPA₁₄ contents. The Tg (85.4.0°C) of (S₁cPA₁₄)₁₂ rPA₁ resin were enhanced significantly, with 30°C higher than cPA. In terms of tensile mechanical properties, S₁cPA₁₄ test pieces demonstrated the highest Young’s modulus and tensile strength. After mixing with LDPE, the tensile mechanical properties of (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins and films were both higher than that of LDPE. ((S₁cPA₁₄)₁₂ rPA₁)₁LDPE₉ film shown the best tensile properties and barrier performance compared with other films due to the optimal rPA content could assisted SMA as a better compatibilizer to improve the dispersion and compatibility of cPA in HDPE. It was worth noting that (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins were formed by film blowing at the processing temperature of 140°C followed by successful preparation of the film.

Keywords

Reactive extrusion polyamides low-density polyethylene film blowing

Introduction

Polyamide (PA) is one of the most widely engineering plastics due to its outstanding properties, such as excellent chemical resistance, barrier performance, high tensile strength, and flexibility.¹ But the price of PA is high, and cannot be used as film blowing material. Low-density polyethylene (LDPE) is one of the most commonly plastics because of good processability and low cost, and it is a good for blowing film.² By mixing PA and LDPE, the advantages of the two polymers can be combined to obtain a resin with blowing film, low cost, and better barrier property.

The development of polymer blends usually successfully enhanced the physical properties of single polymer material and it is a simple and quick method. The technical development of polymer blends is a market trend to serve as an economical and effective method for developing new resins.^3,4 The polymer blending methods can be categorized as: 1. miscible blends, such as polyphenylene oxide/polystyrene and poly(vinyl chloride)/selected polyesters. The properties of such blends are usually additive. Moreover, mutual interactions occur between the polymers in the blends with the same glass transition temperature (Tg).^5–8 2. Immiscible blends: The mutual compatibility of such blends is extremely low, requiring the addition of another functional polymer such as surfactant or a compatibilizer. The addition of a compatibilizer can reduce the surface tension between incompatible polymer blends^1–11 It can prevent the coagulation of disperse phases and improve their stability in the matrix along with the fine distribution characteristics.^12–14

In our previous study, PA, EVOH, and HDPE were blended. As the PA and EVOH materials contain the polar amide functional group and 55–75% of polar ethanol functional group, respectively, they were incompatible with nonpolar HDPE. During the co-extrusion or laminar blow-molding processes, these PE/PA, PE/PVA, PE/EVOH, and PE/NYC blends were thermally immiscible and mechanically incompatible. The compatibilizer precursor (CP) resin is usually used to improve the surface property of PE to PA, PVA, and/or EVOH and to enhance the barrier property of PE/PA/CP blends.^15–18 The carboxylic acid group on CP and the amine group on PA underwent reactive extrusion to afford the CP-grafted-PA copolymers similar to the cross-linked CP/PA copolymers. Such CP/PA copolymers significantly enhanced the melting-shear viscosity of Modified PA. As a result, its viscosity was increased with increasing CP percentage.¹⁶ Furthermore, in related studies, Lim et al.¹⁹ blended polyamide 12 (PA12)/styrene ethylene/butylene styrene (SEBS) and PA12/maleic anhydride grafted SEBS (SEBS-g-MA) individually by the double screw method. The differential scanning calorimetry (DSC) results indicated that PA12/SEBS-g-MA showed improved thermal stability than PA12/SEBS while scanning electron microscope (SEM) revealed favorable phase morphology of PA12/SEBS-g-MA. Padwa et al.²⁰ used PE and maleic anhydride for grafting modification to give PE-g-MA as CP followed by blending of nylon6/PE-g-MA. The results showed that, in the compatibilization study, identical outcome on the impact property to those in the publications of grafting maleic anhydride onto the polyethylene phase^21–25 were achieved.

Upon the arrival of the era of high oil prices, the prices of petroleum cracking products have increased dramatically. On the other hand, the plastic degradation requires as many as a hundred years and are associated with numerous issues such as the impact on the environment. As a result, the government of each country adopts various promotion measures with rewards to encourage the industries and the public to recycle waste, with satisfactory outcomes achieved. Nowadays, waste recycling has created tremendous profits in each country around the world. In 2012, the total income of waste recycling in Taiwan reached NTD $7,030,000,000, 498,000 tons of waste recycled. From 1998 to 2012, the volume of garbage clearance per capita per day was reduced from 1.14 kg to 0.39 kg (Environmental Protection Administration, Executive Yuan, the R.O.C.). Therefore, the objectives of waste reduction and transformation of garbage to gold were achieved. In the United States, recycled plastics rapidly grow by an annual rate of 5.9%. It is forecasted to reach 3,400,000,000 pounds in 2016 (market report by the Freedonia Group). PET and HDPE plastics account for the majority of recycled plastics market. The research and development of recycled products has also become mature. The sports apparels of 9 football teams including Holland and Germany in the 2010 Football World Cup were made from textiles produced from recycled PET in Taiwan. However, there were few studies and reports on the related products of recycled nylon 6 plastics, which is deemed as important as PET plastics in terms of synthetic plastics. Recycled maleic anhydride PA (rPA) is one of polymer waste from plastic processing factory. However, there was no study reports in rPA, it might be a good filler for polymer technology. Polyamide 6 clay nanocomposite (cPA) is an excellent barrier packaging material, which has been reported in previous study.¹⁵ However, cPA is very expensive, and cannot be used as blown material. Therefore, there are three main purpose of the study, the first is to reduce the cPA price, because the more LDPE is added, the lower the cost. The second is to improve the film blowing processability of cPA. The third is that the recycling of rPA can reduce environmental hazards and reduce material costs. It was surprising that rPA could also be used as a compatibilizer in a suitable proportion. Therefore, the present study investigated the blending of polyamide 6 clay nanocomposite (cPA) and styrene maleic anhydride copolymer (SMA) in varied ratios to prepare the modified polyamide 6-clay nanocomposite (S_xcPA_y) resin by reactive extrusion. The S₁cPA₁₄ resin and rPA were blended in different ratios to afford the (S₁cPA₁₄)_x rPA_y resin. Finally, they were mixed with LDPE in 1:9 ratio to afford the (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins attributed to LDPE is cheap, and easy to blow film, but poor barrier properties. The ratio of PA to LDPE is a continuation of previous study,¹⁵ all resins followed by film blowing for molding. The FTIR spectroscopy, differential scanning calorimetry (DSC), tensile mechanical properties, morphological properties, and optical properties of resins were also investigated and discussed.

Experimental

Materials and sample preparation

The Polyamide 6 Clay Nanocomposite (cPA), compatibilizer precursor (SMA) and LDPE used in this study were obtained from Nanopolymer Composites Corporation (Tainan, Taiwan), wherein cPA resins with a trade name of NF3020 and SMA is a Styrene Maleic Anhydride copolymer. The modified cPA (S_xcPA_y) resin was prepared by reactive extrusion of the melt blending of SMA and cPA in various ratios. The SMA, recycled maleic anhydride polyamide 6 (rPA) and the low-density polyethylene resins (LDPE NA207-66) used in this study were obtained from Chembridge International Corporation (Taipei, Taiwan), Liang Haw Technology Corporation (Taipei, Taiwan) and USI Corporation (Taipei, Taiwan), respectively.

The SMA compatibilizer, cPA resins and rPA were prepared by reaction extrusion, the dried SMA, cPA and rPA resins at varying ratios using a Nanjing Jiant SHJ-20 twin-screw extruder. Before melt blending, SMA, cPA and rPA resins were dried in a vacuum oven at 80°C for 16 hrs. cPA resins were mixed with SMA and S₁cPA₁₄ resins were mixed with rPA by dry blending in varying ratios separately followed by twin-screw extruder operating at 240°C in the feeding zone and 230°C toward the extrusion die with a screw speed of 200 rpm. The compositions of the S_xcPA_y and (S₁cPA₁₄)_xrPA_y resins prepared in this study are summarized in Table 1.

Table 1.

Compositions of the S_xcPA_y and (S₁cPA₁₄)_xrPA_y resins.

Resins	S₁cPA₄	S₁cPA₁₄	S₁cPA₂₀	S₁cPA₂₄	(S₁cPA₁₄)₄ rPA₁	(S₁cPA₁₄)₈ rPA₁	(S₁cPA₁₄)₁₂ rPA₁
SMA(wt%)	20.0	6.670	4.760	4.00	5.336	5.928	6.156
cPA(wt%)	80.0	93.33	95.24	96.0	74.66	82.96	86.14
rPA(wt%)	—	—	—	—	20.00	11.11	7.700

The S_xcPA_y and (S₁cPA₁₄)_xrPA_y pellets prepared from the twin-screw extruder was dried at 80°C for 16 h, and then dry blended with LDPE at a 10:90 weight ratios. The mixed (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ pellets were then blown films in a Jonh Huah TPH-550 extrusion blown film molding machine using an extrusion temperature of 140°C and a screw speed of 50 rpm, respectively.

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR)

ATR-FTIR measurements of cPA, S_xcPA_y, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ specimens were recorded on a Jasco-460 ATR-FTIR spectrophotometer at 25°C and 20% relative humidity, wherein 32 scans with a spectral resolution of 4 cm⁻¹ were collected during each spectroscopic measurement. The cast films used in this study were prepared sufficiently thin enough to obey the Beer–Lambert law.

Thermal properties

The thermal properties of cPA, S_xcPA_y, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins were determined at 25°C and 20% relative humidity using a Maia 200-F3 differential scanning calorimetry (DSC) machine. All scans were carried out at a heating rate of 10°C/min and under flowing nitrogen of a flow rate of 25 mL/min. The instrument was calibrated using pure indium. Samples weighing about 7 mg were placed in standard aluminum sample pans for each DSC experiment.

Mechanical properties

Tensile strength tests of cPA, S_xcPA_y, S₁cPA₁₄)_x rPA_y, (S₁cPA₁₄)_x rPA_y)₁LDPE₉ and (S_xcPA_y)₁LDPE₉ sheets and films were carried out in an HT-9501 Universal Testing Machine Indicator, according to ASTM-D 638 at a cross head speed of 100 mm/min. The results reported are the average of 10 tests.

Morphology properties

In order to observe the deformation structures of cPA and S₁cPA₁₄ resins in the cPA₁LDPE₉ and (S₁cPA₁₄) ₁LDPE₉ sheets, respectively, these sheets were first sectioned by a scalpel, and then etched with formic acid. The etched samples were then gold-coated and examined by a scanning electron micrograph (SEM, JSM-6335F JEOL, Tachigawa, Japan).

Water vapor leakage test

The water barrier properties of the LDPE, cPA₁LDPE₉, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ films were evaluated by measuring the weight loss of water contained in the 200 ml conical flask with 100 g water. The film sample was cut into 10 × 10 cm², the thickness of the film was 0.05 mm, adhered to the mouth of the conical flask with vacuum glue, and put into the oven at 40°C for 14 days. For each experiment, at least 3 samples are taken to calculate the average value. The water vapor permeation rate was calculated using the following equations:

\frac{G r a m}{D a y} = \frac{\sum (W_{0} - W_{X})}{14}

where W₀ was the weight before putting into the oven (conical flask + 100 g water); W_x was the weight of (conical flask + water) on the x day after placing in the oven.

UV-VIS transmission properties

Ultraviolet-visible (UV-vis) spectra of the (S_xcPA_y)₁LDPE₉ and (S₁cPA₁₄)_x rPA_y)₁LDPE₉ films were recorded on a Hitachi U3900 spectrophotometer at room temperature with the scanning range of 200 to 800 nm.

CIE LAB color intensity measurement

The color intensities of (S_xcPA_y)₁LDPE₉ and (S₁cPA₁₄)_x rPA_y)₁LDPE₉ thin films were measured by X-rite Color SP60 spectrophotometer with the standard D65 light source at the angle of 8 degrees.

Results and discussion

ATR-FTIR spectroscopy

Typical Fourier transform infrared spectra (ATR-FTIR) of SMA, S_xcPA_y, and cPA samples were shown in Figure 1. The distinguished absorption bands of SMA sample centered at 1225, 1600, 1779/1860 and 3433/3630 cm⁻¹ are most likely corresponding to the motions of C-O-C stretching of the anhydrides, C=C stretching of the styrene, O=C-O-C=O symmetric and asymmetric stretching of the anhydrides and O-H stretching vibration present in SMA molecules, respectively. Similar to those found by Yeh et al.,¹⁴ the FT-IR spectra of cPA exhibit three distinguished absorption bands centered at 1542, 1642, and 3295 cm⁻¹, which are attributed to the motions of N-H bending vibration, C=O stretching vibration, and N-H stretching of PA molecules, respectively. In contrast, the FT-IR spectra of S_xcPA_y are very similar to that of the cPA specimen, wherein the three main absorption bands centered at 1542, 1636, and 3298 cm⁻¹ were also found in the spectra of S_xcPA_y. However, the 1779/1860 (O=C-O-C=O symmetric and asymmetric stretching of the anhydrides) absorption bands originally shown on the FT-IR spectra of SMA sample almost disappeared completely, and found the new imides group absorption bands centered at 1734 cm⁻¹ in the S_xcPA_y samples. Presumably, this disappearance is due to the for formation of cPA-grafted-SMA copolymers through the reaction of carboxylic acid groups of anhydrides of SMA with the terminal amine groups of cPA molecules during the reactive extrusion of S_xcPA_y resins.^26,27 Typical ATR-FTIR of rPA, S₁cPA₁₄ and (S₁cPA₁₄)_xrPA_y samples are shown in Figure 2. After blending, the ATR-FTIR of the (S₁cPA₁₄)_xrPA_y samples were similar to that of the S₁cPA₁₄ sample. The possible mechanisms of the reaction extrusion of the S_xcPA_y resins are suggested in the Figure 3.

Figure 1.

ATR-FTIR spectra of SMA, S_xcPA_y and cPA specimens determined at 35°C.

Figure 2.

ATR-FTIR Spectra of rPA, S₁cPA₁₄ and (S₁cPA₁₄)_xrPA_y samples determined at 35°C.

Figure 3.

The possible mechanisms of the reaction extrusion of the SxcPAy resins.

Typical ATR-FTIR of (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)₁₂ rPA₁)₁LDPE₉ samples are shown in Figure 4. The distinguished absorption bands of (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)₁₂ rPA₁)₁LDPE₉ samples centered at 1437, 1542/1642, 1726 and 3295 cm⁻¹ were most likely corresponding to the motions of C-H bending of the LDPE, NH/C=O stretching of the cPA, -OC-N-CO- stretching of imides group of the cPA-grafted-SMA copolymers and N-H stretching vibration present in (S_xcPA_y)₁LDPE₉ molecules, respectively.

Figure 4.

FTIR-ATR spectra of (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ samples determined at 35°C.

Thermal behavior

Figure 5 summarized the DSC thermograms of the second heating of SMA, cPA, S_xcPA_y, (S₁cPA₁₄)_x rPA_y and rPA resins, including the glass transition temperature (T_mid) graph of cPA, and (S₁cPA₁₄)₁₂ rPA₁ resins.

Figure 5.

DSC thermograms of the second heating scan for SMA, cPA, S_xcPA_y, (S₁cPA₁₄)_x rPA_y and rPA resins.

In addition, the crystallinity, Xc, was calculated as follows²⁸:

X_{C} = Δ H_{f} / Δ H_{f}^{0} ω

where △H_f is the apparent enthalpy of fusion (indicated in DSC thermograms as the melting enthalpy per gram of blends) corresponding to the component, ω is the total weight fraction of the cPA in the blend, and △H_f⁰ is the average enthalpy of fusion per gram of the component in its completely crystalline state (240 J/g for PA; α form=241J/g and γ form=239J/g).²⁹ PE completely crystalline state (289 J/g for LDPE).²⁷ The T_g, T_m, △T_m, and X_c of cPA, S_xcPA_y, and (S₁cPA₁₄)_x rPA_y samples were illustrated in Table 2. As shown in the results, the T_g, T_m, △T_m, and X_c of cPA are 55.4°C, 219.1°C, 15.0°C, and 18.1%, respectively. It was found that the T_g of S_xcPA_y blending sample increased with increasing SMA, with the T_g (72.24°C) of S₁cPA₄ (small graph in Figure 5) was 16.8°C higher than that of CPA. In the (S₁cPA₁₄)_x rPA_y resin, it was observed that the sample with added edge trim showed higher T_g and X_c than S₁cPA₁₄ sample. Among them, (S₁cPA₁₄)₁₂ rPA₁ demonstrated the highest T_g and X_c of 85.4°C and 21.7%, respectively (small graph in Figure 5 and Table 2).

Table 2.

Melting behavior of cPA, S_xcPA_y, (S₁cPA₁₄)_xrPA_y and rPA resins.

Resins	T_g (°C)	T_m (°C)	△T_m (°C)*	△H_f (J/g)	X_c (%)
cPA	55.4	219.1	15.0	43.5	18.1
S₁cPA₂₄	59.5	220.2	15.3	49.5	21.5
S₁cPA₂₀	58.8	219.6	15.4	49.9	21.8
S₁cPA₁₄	64.4	219.5	12.7	41.3	18.4
S₁cPA₄	72.2	216.7	13.5	40.9	21.3
(S₁cPA₁₄)₄rPA₁	68.3	219.4	16.0	45.4	19.9
(S₁cPA₁₄)₈rPA₁	65.3	218.9	15.8	43.6	19.3
(S₁cPA₁₄)₁₂rPA₁	85.4	220.2	15.5	49.0	21.7
rPA	—	221.7	14.1	44.6	15.0

*△T_m is the half-height width of the T_m peak.

Figure 6 and Table 3 illustrated the DSC thermograms and thermal properties of the second heating of LDPE, (S_xcPA_y)₁LDPE₉, ((S₁cPA₁₄)_x rPA_y)₁LDPE₉, and cPA resins. The results revealed that the T_m and X_c of LDPE were 109°C and 42.9%, respectively. In the blending sample of (S_xcPA_y)₁LDPE₉, the T_m values of LDPE samples were found to be little changed. On the other hand, the T_m and X_c of S_xcPA_y samples were found to decrease significantly, with S₁cPA₁₄ showing the most significant decreases by 13.6°C and 13.59%. Based on the aforementioned results, the modified S_xcPA_y resin, following being blended in the LDPE matrix, aided in the crystallization of LDPE. Yet, the crystal plates of its own decreased, with S₁cPA₁₄ dropping the most (4.51%, Table 3). In the ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ samples, there were little changes in the T_m values of LDPE samples, yet X_c values were found to decline. In addition, the crystal melting points of nylon in the ((S₁cPA₁₄)₄ rPA₁)₁LDPE₉ and ((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉ disappeared (Table 3).

Table 3.

Melting behavior of LDPE, (S_xcPA_y)₁LDPE_9, ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ and cPA resins.

Sample	LDPE			S_xcPA_y			(S₁cPA₁₄)_x rPA_y
Sample	T_m1 (°C)	△H_m1 (J/g)	X_c1 (%)	T_m2 (°C)	△H_m2 (J/g)	X_c2 (%)	T_m2 (°C)	△H_m2 (J/g)	X_c2 (%)
LDPE	109.0	123.9	42.9	—	—	—	—	—	—
(S₁cPA₄)₁LDPE₉	110.5	138.1	52.8	208.5	3.73	19.4	—	—	—
(S₁cPA₁₄)₁LDPE₉	110.5	123.6	47.5	205.6	1.01	4.51	—	—	—
(S₁cPA₂₀)₁LDPE₉	109.3	110.4	42.4	210.3	2.30	10.1	—	—	—
(S₁cPA₂₄)₁LDPE₉	108.3	125.5	48.3	209.9	2.51	10.9	—	—	—
((S₁cPA₁₄)₄rPA₁)₁LDPE₉	110.7	93.80	36.7	—	—	—	—	—	—
((S₁cPA₁₄)₈rPA₁)₁LDPE₉	108.6	82.77	31.8	—	—	—	209.8	2.30	10.4
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉	110.0	89.81	34.5	—	—	—	—	—	—
cPA	—	—	—	219.2	43.5	18.1	—	—	—

Figure 6.

DSC thermograms of the second heating scan for LDPE, (S_xcPA_y)₁LDPE₉, ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ and cPA resins.

The results of thermal analysis showed that the T_g value of (S₁cPA₁₄)₁₂ rPA₁ resin was 30°C higher than that of cPA, which may be attributed to the formation of dense polyimide structure under the optimum rPA content in S₁cPA₁₄, which leads to the decrease of molecular chain fluidity and the increase of glass transition temperature of the resin.^30,31

Mechanical properties

The assessments of mechanical properties of the injected pieces of cPA, S_xcPA_y, (S₁cPA₁₄)_x rPA_y, LDPE,(S_xcPA_y)₁LDPE₉, and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ are summarized in Table 4. The results show that the Young’s modulus (E), Tensile strength (TS), and Elongation at break (ε (%)) of the cPA test pieces were 0.491 GPa, 40.3 MPa, and 8.30%. Among S_xcPA_y resins, those were found that the E values were all greater than that of cPA except S₁cPA₂₄ resin, with S₁cPA₁₄ resin having the highest E, TS, and ε (%) of 0.82 GPa, 42.8 MPa, and 5.59%, respectively. With respect to the mechanical properties of LDPE test pieces, the E, TS, and ε (%) were 0.081 GPa, 7.85 MPa, and 39.3%. For (S_xcPA_y)₁LDPE₉, the E and TS of all samples were higher than those of LDPE. (S₁cPA₁₄)₁LDPE₉ showed the highest E and TS of 0.191 GPa and 11.1 MPa, 1.47 and 1.41 times higher than those of LDPE, respectively. Moreover, (S₁cPA₁₄)₁LDPE₉ resin also showed the highest elongation at break of 48.2%, about 1.23 times higher than that of LDPE. Compared to cPA and (S₁cPA₁₄)_x rPA_y resins showed lower TS and E, but higher ε (%). The better ε (%) was also one of the reasons for improving the blowing properties.³² Therefore, the resins were blended with LDPE before blown film.

Table 4.

Mechanical properties of the blends prepared by injection

Material	E (GPa)	TS (MPa)	ε (%)
cPA	0.491	40.3 ± 0.40	8.80 ± 0.80
S₁cPA₄	0.580	10.1 ± 0.05	3.01 ± 0.80
S₁cPA₁₄	0.820	42.8 ± 1.07	5.59 ± 2.33
S₁cPA₂₀	0.615	30.6 ± 0.04	6.16 ± 0.16
S₁cPA₂₄	0.429	29.2 ± 0.35	6.28 ± 0.45
LDPE	0.081	7.85 ± 0.04	39.3 ± 2.02
cPA₁LDPE₉	0.120	10.9 ± 0.02	33.5 ± 2.90
(S₁cPA₄)₁LDPE₉	0.167	10.1 ± 0.02	38.3 ± 4.20
(S₁cPA₁₄)₁LDPE₉	0.191	11.1 ± 0.06	48.2 ± 0.80
(S₁cPA₂₀)₁LDPE₉	0.134	10.8 ± 0.05	44.8 ± 3.70
(S₁cPA₂₄)₁LDPE₉	0.132	9.69 ± 0.13	37.6 ± 1.00
((S₁cPA₁₄)₄rPA₁)₁LDPE₉	0.185	10.3 ± 0.06	31.8 ± 3.80
((S₁cPA₁₄)₈rPA₁)₁LDPE₉	0.152	10.1 ± 0.03	29.3 ± 2.40
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉	0.247	10.7 ± 0.06	34.3 ± 7.80

E = Young modulus; TS = Tensile strength; ε = Elongation at break.

Table 5 summarizes the mechanical properties along the X and Y axes of LDPE, (S_xcPA_y)₁LDPE₉, and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ thin films from blowing process. It was shown that all thin films of the same test pieces possessed larger elongation at break along the X-axis than the Y-axis. However, the E and TS values of the Y-axis were generally higher than those of the X-axis. The Young’s modulus (E), Tensile strength (TS), and Elongation at break (ε (%)) of the LDPE thin films along the X and Y axes were 0.122 GPa, 8.271 MPa, 169.9%, 0.260 GPa, 11.08 MPa, and 69.25%, respectively. The data along the X and Y axes of (S_xcPA_y)₁LDPE₉ were all larger than those of LDPE. Among them, the TS and Elongation at break along the X and Y axes of (S₁cPA₂₄)₁LDPE₉ thin film were 16.58 MPa, 182.5%, 17.76 MPa, and 104.6%, respectively. The E and TS values along the X and Y axes of the ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ thin film were also higher than those of LDPE, with the ((S₁cPA₁₄)₁₂ rPA₁)₁LDPE₉ thin film showing the largest TS values along the X and Y axes of 16.80 MPa and 19.39 MPa, respectively.

Table 5.

Mechanical properties of the blends prepared by blown molding (blow-up rate of 3.5 and film thickness of 0.05 mm).

Material	E (GPa)	TS (MPa)	ε (%)
LDPE(X)	0.122	8.271 ± 1.64	169.9 ± 26.0
LDPE(Y)	0.260	11.08 ± 2.45	69.25 ± 21.3
(S₁cPA₄)₁LDPE₉(X)	0.232	14.45 ± 2.35	144.2 ± 28.8
(S₁cPA₄)₁LDPE₉(Y)	0.305	16.78 ± 2.25	48.02 ± 3.00
(S₁cPA₁₄)₁LDPE₉(X)	0.172	12.85 ± 3.33	112.3 ± 54.7
(S₁cPA₁₄)₁LDPE₉(Y)	0.380	13.48 ± 5.28	73.33 ± 10.6
(S₁cPA₂₄)₁LDPE₉(X)	0.213	16.58 ± 2.48	182.5 ± 46.7
(S₁cPA₂₄)₁LDPE₉(Y)	0.195	17.76 ± 2.09	104.6 ± 19.3
((S₁cPA₁₄)₄rPA₁)₁LDPE₉(X)	0.235	11.49 ± 2.64	125.7 ± 44.0
((S₁cPA₁₄)₄rPA₁)₁LDPE₉(Y)	0.325	17.16 ± 1.96	36.30 ± 13.4
((S₁cPA₁₄)₈rPA₁)₁LDPE₉(X)	0.196	11.43 ± 1.54	119.9 ± 52.0
((S₁cPA₁₄)₈rPA₁)₁LDPE₉(Y)	0.289	15.64 ± 5.62	40.36 ± 9.60
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉(X)	0.266	16.80 ± 4.08	158.3 ± 24.4
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉(Y)	0.301	19.39 ± 3.54	78.17 ± 11.1

X-Axis (horizontal); Y-Axis (vertical).

In summary of Tables 4 and 5, the Young’s modulus and tensile strength of resins were increased in the injection or thin film test pieces containing either 10% modified nylon or recycled modified nylon. Blown film with optimal rPA ratio presented the best E and TS values, this may be because cPA could further enhanced the compatibility of S₁cPA₁₄ in LDPE.³³

Morphological properties

The morphological property analysis of the cPA₁LDPE₉, (S₁cPA₁₄)₁LDPE₉ and ((S₁cPA₁₄)₁₂ rPA₁)₁LDPE₉ were summarized in Figure 7. As the results indicated, the cPA without the addition of SMA was blended in the LDPE matrix to show larger cPA streaking dots (0.50–2.00 μm) in the Figure 7(a). The smallest size of S₁cPA₁₄ dots in the (S₁cPA₁₄)₁LDPE₉ test pieces was approximately 0.05–0.1µm (see Figure 7(b)), improved by 10–20 times as compared to the cPA₁LDPE₉ pieces. When rPA was added with (S₁cPA₁₄)₁LDPE₉, the cross section of the film becomes smoother, and its streaking dots almost disappear (see Figure 7(c)). This indicated that the addition of rPA could cooperated with SMA as compatibilizer. These results are consistent with the analysis of mechanical properties, which proved that the compatibility, TS and E of the blown film could be further improved by the combination of rPA and SMA.^34,35

Figure 7.

SEM photos of (a) cPA₁LDPE₉, (b) (S₁cPA₁₄)₁LDPE₉, (c) ((S₁cPA6₁₄)₁₂ rPA₁)₁LDPE_9.

Analysis of water vapor leakage properties

Table 6 summarizes the water vapor leakage properties of LDPE, cPA₁LDPE_9,(S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films. The results show that the water vapor leakage of cPA₁LDPE₉ film was similar to that of neat LDPE. ((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉ film has the smallest water vapor leakage (0.352g/day), and the improvement rate of water vapor leakage was more than twice that of LDPE. From the results of the thermal properties (Table 2), mechanical properties (Table 5) and morphology analysis (Figure 7) indicated that the presence of optimal rPA content could enhanced the interfacial compatibility of various materials. It might be attributed to rPA plays the role of compatibilizer, which reduced the defects of the resins and improved the water vapor barrier ability.³⁶

Table 6.

Water permeation rate of LDPE, and cPA₁LDPE₉, (S_xcPA_y)₁LDPE₉, and ((S₁cPA6₁₄)_x rPA_y)₁LDPE₉ films at 40 °C.

Sample	Water permeation rate
Sample	g/day	Improvement rate (times)
LDPE	0.777	1
cPA₁LDPE₉	0.757	1.026
(S₁cPA₄)₁LDPE₉	0.748	1.038
(S₁cPA₁₄)₁LDPE₉	0.745	1.043
(S₁cPA₂₀)₁LDPE₉	0.746	1.042
(S₁cPA₂₄)₁LDPE₉	0.743	1.046
((S₁cPA6₁₄)₄ rPA₁)₁LDPE₉	0.569	1.365
((S₁cPA6₁₄)₈ rPA₁)₁LDPE₉	0.459	1.692
((S₁cPA6₁₄)₁₂ rPA₁)₁LDPE₉	0.352	2.207

Optical transparency

Figure 8 illustrates the optical transparency (thickness of 40–60 μm) (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films in the UV-VIS spectrum.³⁷ These optical transparencies include cutoff wavelength (absorption edge, λ₀) and transmittance of various wavelengths as shown in Table 7. The results indicate that the cutoff wavelengths of all films were below 226 nm and transmittance increased with increasing wavelength. The transmittance of LDPE at 500 nm was 59% as a translucent film. The transmittances of films following modification all decreased, with (S₁cPA₂₄)₁LDPE₉ showing the highest transmittance of 51% at 500 nm. The transmittances of films after the addition of recycled nylon were found to decline further.

Table 7.

Optical transparency of LDPE, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films.

Sample	λ₀ (nm)	T₄₀₀	T₄₅₀	T₅₀₀	T₆₀₀	T₇₀₀
LDPE	200	55	57	59	62	65
(S₁cPA₄)₁LDPE₉	202	16	18	21	24	27
(S₁cPA₁₄)₁LDPE₉	211	19	22	23	28	30
(S₁cPA₂₀)₁LDPE₉	213	27	30	33	37	40
(S₁cPA₂₄)₁LDPE₉	216	41	47	51	58	62
((S₁cPA₁₄)₄rPA₁)₁LDPE₉	223	23	27	30	35	38
((S₁cPA₁₄)₈rPA₁)₁LDPE₉	226	19	22	24	29	32
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉	220	13	16	18	21	24

λ₀: UV cutoff wavelength; T400–700: transmittance at 400–700 nm, respectively.

Figure 8.

UV-vis spectra of (a) LDPE, (b) (S₁cPA₂₄)₁LDPE₉, (c) (S₁cPA₂₀)₁LDPE₉, (d) (S₁cPA₁₄)₁LDPE₉, (e) (S₁cPA₄)₁LDPE₉, (f) ((S₁cPA₁₄)₄rPA₁)₁LDPE₉, (g) ((S₁cPA₁₄)₈rPA₁)₁LDPE₉, (h) ((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉ films.

CIE LAB system

The CIE LAB color coordinates of LDPE, (S_xcPA_y)₁LDPE₉, and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films are illustrated in Figure 9 and Table 8.³⁷ LDPE was a bluish green film with more blue tone. Following modification, (S_xcPA_y)₁LDPE₉ films were mostly bluish green films with more green tone. With the addition of rPA, ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films were converted to yellowish green on appearance. The (S₁cPA₁₄)₁LDPE₉ and ((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉ film were demonstrated as an example (Figure 10).

Table 8.

Color coordinates of LDPE, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films.^a

Sample	Film thickness (mm)	L*	a*	b*	H*
LDPE	0.06	39.6	−0.64	−3.24	259.0
(S₁cPA₄)₁LDPE₉	0.05	47.8	−1.11	1.34	129.7
(S₁cPA₁₄)₁LDPE₉	0.05	47.2	−0.49	−0.29	210.3
(S₁cPA₂₀)₁LDPE₉	0.05	44.9	−0.50	−0.74	236.1
(S₁cPA₂₄)₁LDPE₉	0.06	41.4	−0.66	−0.19	233.4
((S₁cPA₁₄)₄rPA₁)₁LDPE₉	0.05	45.2	−0.59	0.59	135.0
((S₁cPA₁₄)₈rPA₁)₁LDPE₉	0.05	46.9	−0.67	0.40	149.3
((S₁cPA₁₄)₁₂rPA₁)₁LDPE₉	0.06	45.2	−0.67	−0.02	182.1

^a The color parameters were calculated according to a CIE LAB equation. L* refers to lightness; 100 means white, while 0 indicates black. A positive a* means red color, a negative a* indicates green color. A positive b* means yellow color, a negative b* indicates blue color. H* refers to hue angle.²⁹

Figure 9.

CIELAB coordinates for the LDPE, (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films in the a*–b* plane.

Figure 10.

Photographs of (a) (S₁cPA6₁₄)₁LDPE₉ and (b) ((S₁cPA6₁₄)₁₂ rPA₁)₁LDPE₉ films.

Conclusions

The application of reactive extrusion to prepare S_xcPA_y resins showed new characteristic absorption peak of –CO-N-CO- of imides at 1734 cm⁻¹ in ATR-FTIR. The thermal properties indicated that the glass transition temperatures and crystallinity of S_xcPA_y and (S₁cPA₁₄)_xrPA_y increased with increasing SMA and S₁cPA₁₄. The T_g (85.4.0°C) of (S₁cPA₁₄)₁₂rPA₁ resin were enhanced significantly, showing 30°C higher than that of cPA. The tensile mechanical properties revealed that the Young’s modulus and tensile strength both increased but the elongation at break declined with the addition of 10% of modified or recycled nylon in either injection or thin-film test pieces, with the exceptions of (S₁cPA₁₄)₁LDPE₉ and (S₁cPA₂₀)₁LDPE₉ injection pieces and (S₁cPA₂₄)₁LDPE₉ film test pieces, which also showed favorable elongation at break. The morphological properties demonstrated that the S_xcPA_y resins were scattered as smaller spherical particles in the LDPE matrix than unmodified cPA resins. Further blending with rPA, (S₁cPA₁₄)₁LDPE₉ film could hardly be seen in LDPE, which indicated that rPA could assist SMA to improve the dispersion and compatibility of S_xcPA_y in HDPE. The optical transparency test showed that the transmittance of LDPE was 59% at 500 nm as a translucent film. Following modification, the transmittance of (S₁cPA₂₄)₁LDPE₉ thin film at 500 nm was 51%, the highest among all. The color coordinates suggested that LDPE and (S_xcPA_y)₁LDPE₉ films were bluish green thin films while ((S₁cPA₁₄)_xrPA_y)₁LDPE₉ films, with the addition of recycled nylon, were altered to yellowish green. Compared with all the resins, (S₁cPA₁₄)₁₂ rPA₁ resin showed the highest Tg value, and the blown film after blending with LDPE also showed the highest E, TS and barrier performance, indicated that the optimal content of rPA could enhanced the compatibility between S₁cPA₁₄ and LDPE. It is worth noting that, both (S_xcPA_y)₁LDPE₉ and ((S₁cPA₁₄)_x rPA_y)₁LDPE₉ resins were prepared by blow-molding at the processing temperature of 140°C to complete the preparation of films. The reuse of recycled nylon edge trims, improve thermal and tensile mechanical properties, the application of simple and affordable double screw extruder, and the energy-saving blow processing are important indicators for the market applications.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This present research was supported by the Oriental Institute of Technology and Industry Cooperation Support Program (103-5-01-105, 103-5-01-104).

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