Nylon Powder Composites with High Leveling Property and Toughness Prepared via Filler-Modified Method

Abstract

Powder coating, as a promising coating material, has attracted widespread attention due to its convenient construction and being a green option, promoting environmental protection. However, the existence of defects such as insufficient leveling and poor mechanical properties of the coating during the coating process limits the further expansion of its application fields. Therefore, for this article, powder coatings with high leveling performance were prepared by composite modification of nylon 12 (PA-12) resin with polyacrylates and ethylene-α-olefin copolymers (POE). The introduction of modified polyacrylates reduces the surface tension of nylon chains, enhancing melt flowability during curing and making the coating surface smooth. Furthermore, by introducing POE, the flexibility of the powder coating was improved, and its fracture elongation increased from 59% for pure PA-12 to a maximum of 234%. This study provides an effective method for the modification of nylon powder coatings and offers new insights into their use in high-performance coating applications.

1. Introduction

Powder coating, as a type of coating that does not contain organic solvents, offers several benefits, including green environmental protection, great effectiveness, low energy consumption, simple process, and superior performance [1,2,3,4]. It has applications in many fields, including machinery [5], building materials [6], and aerospace [7]. Traditional coatings typically undergo a process of formation before painting. However, the coating surface formed often exhibits the Benard vortex phenomenon, which can lead to uneven patterns and orange peel defects on the coating surface, affecting the toughness and fluidity of the coating structure [8,9,10,11]. The powder coating that has been coated at the same time will withstand stamping, shearing, and other forces during processing and forming [12], which requires the coating to have excellent flexibility and adhesion. Therefore, improving the leveling and flexibility of powder coatings is an important measure to expand their application range.

The traditional method of improving fluidity in production is to use surfactants to reduce the surface tension of coatings, making them easy to level under high-temperature conditions. On the coating’s surface, a thin layer of tiny molecules forms that provides uniform surface tension, improves the wettability between the coating and the object being coated, and avoids surface defects such as orange peel and shrinkage caused by uneven surface tension [13,14,15]. In addition, system viscosity is another important factor affecting the leveling performance of coatings, which determines the appearance quality and surface glossiness of coatings. Surface tension can be seen as the driving force in film leveling, while viscosity constrains the fluidity of molten powder coatings [9]. Only when these two factors reach a state of equilibrium can the ideal coating effect be achieved. Wulf et al. [16] studied the effect of additive properties on the surface tension of epoxy resin (DER 664) powder coating adhesive through axisymmetric droplet shape analysis. The research results showed that polysiloxane can significantly reduce the surface tension of DER 664, but the low surface tension value caused by polysiloxane hurts the surface tension. Keddie et al. [8] developed a mathematical description of the surface leveling of thermosetting polymer coatings with time-dependent viscosity. In the acrylic powder formulation used, using a lower baking temperature would slow down the crosslinking rate, allowing more time for leveling. However, the viscosity at lower temperatures was too low to promote particle coalescence at the actual time scale, hindering the wider use of powder coatings. As one of the important additives in powder coatings, leveling agents reduce the possibility of defects and improve the uniformity and smoothness of the coating surface by reducing the viscosity and surface tension of the coating system [9,17,18,19,20]. However, due to the difficulty of effectively dispersing leveling agents within the coating, which affects the overall surface tension of the coating, the preparation of a complete coating structure still faces significant challenges.

Improved resin toughness is achieved by regulating the molecular structure of the resin, introducing flexible chain-like molecules into the molecular structure, increasing the mobility of the resin molecules, and reducing the interaction forces between molecules. By reducing the glass transition temperature of the resin, it becomes more flexible and malleable at low temperatures, thereby achieving a toughening effect [21]. This method can not only improve the physical properties of the resin but also expand its application areas, bringing more possibilities to the field of material engineering. For example, Pulak et al. [22] examined PA2200’s mechanical characteristics and the organically modified nanoclay mixed powder used in the selective laser-sintering process. The research results showed that compared with the original polyamide, the PA2200/clay composite material’s ultimate elongation and tensile strength at fracture were decreased. Through observation via scanning electron microscopy, it was found that the reason was the uneven dispersion of clay in the polymer matrix. Yi et al. [23] prepared a flexible curing agent polyether fatty amide using diethylenetriamine and epoxy fatty acid methyl ester for the thermal curing of epoxy resin E51. The results of the experiment reveal that the cured epoxy resin composition exhibits an increase in fracture elongation, but the tensile strength and hardness decrease. The toughness defects of materials limit the application of coatings on irregular components.

Based on this, this article studies the preparation of nylon 12 (PA-12) powder with polyacrylates and ethylene-α olefin copolymer (POE) composite modification. During the melting process, the alkyl branches of the polyacrylate modifier escape to the surface, making the surface tension more uniform and reducing the possibility of orange peel formation. At the same time, it reduces the melt viscosity of the powder coating system and enhances the fluidity of the melt coating before curing. The introduction of modifier POE effectively reduces the brittleness of nylon materials, thereby significantly improving their tensile properties on the coating, resulting in significant improvements in the leveling and mechanical properties of the modified PA-12 coating. Therefore, this work not only reports on the design of a new type of nylon coating but also has enlightening significance for developing and manufacturing thermoplastic coatings with superior performance.

2. Materials and Methods

2.1. Materials

PA-12 and POE-g-MAH were purchased from Saan Chemical Reagent Co., Ltd. (Shanghai, China). Titanium dioxide, silicon dioxide and EtOH (99% purity) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Polyacrylate and liquid nitrogen were obtained from Huanyu Technology Co., Ltd. (Shandong, China). Unless otherwise specified, chemicals and solvents were used as received.

2.2. Synthesis of PA-12 Modified Particles

A self-made piece of cryogenic comminution equipment (Figure 1) was employed to prepare PA-12 powder. First the PA-12 was thoroughly dried in a vacuum-drying oven at 80 °C for 2 h, and then it was mixed with POE-g-MAH, ethylene-propylene copolymer, leveling agent, and other additives in a certain proportion. Then, it was added to a twin-screw extruder to obtain a molten sample with temperature of the melting section set to 205–215 °C, then it was solidified in a low-temperature water bath, dried by a blow dryer, and cut by a granulator to obtain modified nylon particles. The proportional composition of the mixture is shown in

Table 1. Formulation table of nylon powder coating with different leveling agent and toughener contents.

Sample Designation	Leveling Agent Contents (%)	Toughener Contents (g)
A 1	0	0
A 2	0.2	0
A 3	0.4	0
A 4	0.6	0
A 5	0.8	0
B 1	0.6	10
B 2	0.6	20
B 3	0.6	30
B 4	0.6	40

. The mass fractions of the leveling agent in the PA-12 sample are 0%, 0.2%, 0.4%, 0.6%, and 0.8%, respectively. Correspondingly, the samples are recorded as A 1, A 2, A 3, A 4, and A 5. The masses of toughener in the sample are 10 g, 20 g, 30 g, and 40 g, respectively. Correspondingly, the samples are recorded as B 1, B 2, B 3, and B 4.

2.3. Preparation of PA-12 Powder

PA-12 powder was prepared through the cryogenic comminution method. The preparation process of the deep cold crushing method is as follows: use liquid nitrogen to lower the temperature of PA-12 modified particles to below −70 °C (the embrittlement temperature of PA-12 is −70 °C) and maintain this temperature, then mechanically crush the PA-12 modified particles. Among them, the lower the temperature during crushing, the more favorable the crushing process. Finally, the PA-12 powder was screened using a Whatman’s filter (200 µm) (BioLeaf Biotech Co., Ltd., Shanghai, China) to remove large particles to obtain experimental samples.

2.4. Characterization

The Fourier transform infrared (FT-IR) spectrum of the PA-12 powder was measured with a Bruker TENSOR II FT-IR spectrometer (Bruker Co., Ltd., Bilerica, MA, USA). The spectrum ranging from 500 to 4000 cm⁻¹ was obtained from 120 scans at a 4 cm⁻¹ resolution. The crystallization temperature of the sample was determined by heating it from 30 to 300 °C at a rate of 10 °C/min using a differential scanning calorimeter (DSC). We utilized a Hitachi S-4800 scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan) to inspect the morphology of the PA-12 powder. Concurrently, the distribution and size of the particles in samples of PA-12 powder were examined using a Mastersize 2000 (Spectris Instrument System Co., Ltd., Shanghai, China) laser particle size analyzer (wet method). This analyzer employed laser diffraction to evaluate the particle size distribution of PA-12 powder suspended in water. Using ASTM D 638-10 as the testing standard, the sample is a type II dumbbell strip with a thickness of 4 mm and a width of 10 mm in the middle parallel section. We set the tensile speed to 100 mm/min to obtain the fracture elongation of the specimen. Three valid samples were tested for each formula, and the results are presented as the average. The melt flow rate (MFR) was measured using the SRZ-400E melt flow rate tester according to the technical requirements of GB3682. The test temperature was 190 °C, the test load was 2.16 kg, and the cutting interval time was 5 s.

2.5. Leveling Performance Test

For the measurement of the loose mobility of powder materials, a certain quality of powder coating sample was weighed and pressed into a circular sheet with uniform thickness. The powder wafer is placed on the top of the preheated metal plate and marked and then placed back in the oven for a period of time, and then the metal plate is adjusted to a certain angle to continue heating. After heating, it is cooled to room temperature in a horizontal position. The length of the powder coating disk flow is measured using a steel ruler, starting from the mark.

For the powder coating leveling test, we applied a layer of release agent to the metal plate, preheated it in a 220 °C oven for 10 min, and then removed the metal plate. On the premise of ensuring consistent film thickness, we evenly applied the powder coating to the metal plate, then placed it back in the oven and baked for 10 min. After the metal plate cooled to room temperature, we removed the coating and observed its appearance, glossiness, and surface smoothness.

3. Results and Discussion

3.1. Chemical Structural Analysis

The FT-IR spectra of the prepared PA-12 samples are shown in Figure 2. The spectra of PA-12 and granules of PA-12 powder clearly share the same adsorption peaks at 3300 cm⁻¹, 2938 cm⁻¹, and 2867 cm⁻¹ [24,25,26], which could be assigned to the asymmetric stretching of amino group and methylene group stretching vibrations, respectively. The difference is the presence of a new adsorption peak at 1730 cm⁻¹ in the spectrum of PA-12 powder, which could be attributed to the carbonyl C=O in polyacrylate, which demonstrates the successful introduction of polyacrylate leveling agents into powders. Meanwhile, the infrared spectra of nylon-modified coating and POE-g-MAH were compared, and the absorption peaks of carboxylic acid and dissociative carboxylic acid were observed at 1463 cm⁻¹ and 719 cm⁻¹. The successful access of POE-g-MAH was proved [27]. In summary, it is concluded that the target leveling agent and the target plasticizer (POE-g-MAH) were successfully introduced into the nylon-modified powder coating.

3.2. Thermal Analysis

Figure 3 shows the DSC curves of A 1 (without leveling agent) and A 2 (with 0.2% leveling agent). The melting and crystallization peaks of the two materials nearly overlap. Specifically, the crystallization peak temperature of A 1 is 145.7 °C, while that of A 2 is 144.8 °C, with a difference of only 0.9 °C. For the melting peak, A 1 exhibits a peak temperature of 177.4 °C, and A2 shows a peak temperature of 176.1 °C, with a difference of only 1.3 °C. The minimal difference in peak value indicates that the modification of nylon in this experiment had little effect on the thermal characteristics of PA-12.

3.3. Microscopic Morphology and Particle Size Analysis

Using SEM to directly observe the apparent morphology and size of particles, it can be clearly observed from Figure 4 that the size uniformity and distribution uniformity of the modified particles are significantly improved. This morphological optimization provides a structural basis for the smooth flow of powder particles. Figure 5 displays the particle size distribution acquired through wet measurement using a laser particle size analyzer. It indicates that the modification treatment has no significant effect on the average particle size of PA-12 powder coatings, and the particle size stability is well retained. The improvement of particle uniformity and the maintenance of particle size stability ultimately have a positive promoting effect on the leveling performance of PA-12 powder coatings.

3.4. Leveling Performance Testing

The effect of the leveling agent on the loose mobility of powder in the powder coating system is shown in Figure 6. The specific flow length is shown in

Table 2. The effect of different leveling agents on the flow length of PA-12 powder. Data are presented as mean ± standard deviation (n = 5).

Sample Designation	Flow Length (mm)
0% leveling agent	13.0 ± 0.7
Leveling agent A	14.0 ± 1.6
Leveling agent B	20.0 ± 1.9
Leveling agent C	20.0 ± 1.2
4% leveling agent A	19.0 ± 0.7

. The leveling agent reduces the attraction between powder coating particles, prevents the powder coating from absorbing moisture, and keeps the powder coating in a dry state. The addition of leveling agents could improve the loose mobility of powder particles. The experimental results show that there are marked differences in the improvement effect of different leveling agents on the loose mobility of powders, among which leveling agent C has the best effect. The leveling effect of PA-12 powder under different leveling agent ratios is shown in Figure 7. It can be seen that the A1 coating surface without leveling agent has serious uneven patterns and orange peel, while the surface defects of A2 to A4 coatings gradually decrease, and the uniformity and flatness of the coating surface gradually improve. The surface quality of the A4 coating is the best, with almost no visible surface defects. When the addition of the leveling agent is too high, thin oily substances easily form on the surface of the A5 coating, which hinders the glossiness of the coating surface.

3.5. Melt Flow Performance Analysis

From

Table 3. The impact of various leveling agent contents on MFR. Data are presented as mean ± standard deviation (n = 5).

Sample Designation	Average Quality (g)	MFR (g/10 min)
A 1	0.230 ± 0.006	27.60 ± 0.72
A 2	0.250 ± 0.006	30.00 ± 0.72
A 3	0.260 ± 0.014	31.20 ± 1.68
A 4	0.260 ± 0.006	31.20 ± 0.72
A 5	0.240 ± 0.012	28.80 ± 1.44

, it can be seen that with the increase in leveling agent content, MFR will increase. This is because the leveling agent reduces the attraction and viscosity between molecules on the surface, promoting better flow and increasing MFR. However, when the content of leveling agent is too high, the MFR will decrease, but it is still larger than when no leveling agent is added. The results further indicate that only by adding an appropriate amount of leveling agent can its flow capacity reach its optimal level.

The function of the leveling agent is to regulate the balance between surface tension and viscosity in the system [28,29], and its mechanism of action is shown in Figure 8. After high-temperature curing, leveling agents enhance the mobility of uncured coatings by reducing their viscosity and surface tension. During the process of coating melting and flowing, the leveling agent will gradually migrate from the molecular interior to the surface, forming a single-molecule layer, thereby eliminating the Benard vortex and making the entire surface flat and smooth.

3.6. Mechanical Performance Testing

In order to study the practicability of the powder coating, its mechanical properties were further evaluated, as shown in Figure 9. The elongation at break and tensile strength of the PA-12 material shows a trend of increasing first and then decreasing. The change trend in tensile strength is relatively gentle, while the change in elongation at break is more significant, increasing from 59% for pure PA-12 to a maximum of 234% for the POE concentration of 30 g. This indicates that POE has a significant toughening effect on PA-12. On the one hand, this is because POE-g-MAH can react with the amino groups in the PA-12 molecular chain to form chemical bonds, enabling POE-g-MAH to be uniformly dispersed in PA-12. On the other hand, according to the silver shear band theory [30,31,32], as seen in Figure 10, the introduction of POE modifier as the stress concentration center can easily induce a number of silver shear bands under the action of external forces. These silver shear bands will affect the development degree of PA-12 silver lines, so that the cracking of PA-12 will stop in time without forming a destructive crack. The use of stress fields to induce shear bands causes the termination of silver lines, while the shear bands formed can also prevent the further development of silver line propagation. By utilizing the energy consumed by the generation and development of a large number of silver lines and shear bands, the crack propagation of nylon is constrained, improving the toughness of the modified PA-12 powder.

4. Conclusions

This study used polyacrylate and POE composite modification of PA-12 to prepare high-leveling and high-toughness powder materials. By utilizing the alkyl branched chains of modified polyacrylate to escape to the coating surface under high-temperature conditions, the surface tension of PA-12 is neutralized, reducing the formation of defects such as orange peel. The use of modifier POE effectively improves the intermolecular forces of nylon, thereby inhibiting the formation of cracks in PA-12 under the action of external forces. The modified PA-12 powder has superior fracture elongation (234%), excellent leveling property, and good particle smoothness, showing great potential for practical applications. Future work will endeavor to explore the applications of composites in functional coating systems, with a focus on conducting systematic tests on the corrosion resistance and wear resistance of the coatings. By simulating demanding environments such as chemical engineering and marine environments, their practical application potential will be quantitatively evaluated to provide data support for engineering applications.