Nylon 6 (PA6) is an engineering plastic, which has been widely used in the fields of electronics, construction engineering and other fields. However, PA6 itself is flammable, and in these areas it is required to have good flame retardancy, so it is necessary to modify it. At the same time, due to the shortcomings of PA6, such as dimensional stability and poor processability, it is generally filled with inorganic fillers in practical applications to improve its dimensional stability and processability, and reduce costs to a certain extent. Jiangxi Aote Technology Co., Ltd. studied the effect of inorganic fillers with different morphologies on PA6 / MCA flame retardant composites. The formulations of MCA flame retardant PA6 commonly used in the industry were selected, while wollastonite and calcium acid were introduced at the same time, and their effects on the properties of PA6 / MCA flame retardant composites were investigated.

Morphological characteristics of inorganic fillers

a-wollastonite; b-calcium carbonate TEM image of inorganic filler

The picture shows the TEM image of the inorganic filler. It can be seen from the figure that wollastonite is needle-shaped, the aspect ratio is large, and calcium carbonate is spherical. Due to the different morphologies of the two inorganic fillers, they may have different effects on the properties of PA6 flame retardant composites.

Flame retardant performance

The above table shows the effects of wollastonite and calcium carbonate on the LOI and vertical combustion properties of PA6 / MCA flame retardant composites. As can be seen from the table, compared with the PA6 / MCA flame retardant composite with a LOI of 28% and a vertical combustion level of UL94 V-2, compared with the addition of wollastonite, the PA6 / MCA / wollastonite flame retardant composite The LOI has been improved to 29%, and its vertical combustion rating has reached UL94-0. This is because SiO2 is a component in wollastonite, which bonds to the surface of the wollastonite to form a lipophilic non-polar surface, which makes the wollastonite well dispersed inside the matrix and improves the flame retardancy of the composite Played a favorable role. In addition, silicon can also catalyze the formation of a carbon layer, improve the strength of the carbon layer, and improve the flame retardancy of the PA6 / MCA / wollastonite flame retardant composite. After the addition of calcium carbonate, the LOI of the PA6 / MCA / calcium carbonate flame retardant composite material decreased to 24.3%, while the vertical combustion level was still UL94 V-2, which was consistent with the PA6 / MCA flame retardant composite material. During the burning process of the material, although few droplets are generated, it continues to burn. This may be because the basic calcium carbonate will absorb the cyanuric acid decomposed by MCA, and make the cyanuric acid catalyze the degradation of PA6 into oligomers to form a droplet. The effect of taking away heat is weakened. It can maintain a high melt viscosity The dripping speed is slow, but it does not self-extinguish.

SEM analysis

a-PA6 / MCA / Wollastonite flame-retardant composite material; b-PA6 / MCA / Calcium carbonate composite material; C-PA6 / MCA flame-retardant composite material

SEM image of the surface carbon layer of PA6 flame retardant composite after combustion

MCA, as a nitrogen-based flame retardant, can improve the flame retardancy of PA6, and also meets the requirements of environmental protection. During the combustion of PA6 / MCA flame retardant composites, cyanuric acid catalyzes the degradation of PA6 into oligomers, forming molten droplets, and taking away the heat generated by combustion; while melamine is further decomposed into inert gases such as water, nitrogen, ammonia, etc., and dilutes the air The concentration of oxygen in the atmosphere thereby inhibits combustion. Therefore, the mechanism of MCA flame retardant PA6 is mainly gas phase flame retardant. The generation of flame droplets makes it difficult for the material to reach V-0 level. However, the carbon layer formed on the surface of the material can keep its internal materials away from the flame, and can also block the heat and flammable gases generated by the materials during the combustion process from spreading outside the carbon layer. Therefore, the formation of the carbon layer greatly affects the flame retardancy of the material. Because the added fillers are non-combustible, they remain in the solidified phase and become part of the carbon layer, so the filler has a great influence on the formation and performance of the carbon layer. The figure above shows the micro morphology of the carbon layer formed by the PA6 flame retardant composite after combustion. As can be seen from Figure b, there are many holes on the surface of the PA6 / MCA / calcium carbonate flame-retardant composite material, and the diameter of the holes is relatively large. In this way, the heat and flammable gases generated by the material inside the carbon layer can easily spread out of the carbon layer, making the material Further burning. This is consistent with the analysis results of the poor flame retardancy of the aforementioned PA6 / MCA / calcium carbonate flame retardant composite. As can be seen from Figure c, the surface of the carbon layer of the PA6 / MCA flame-retardant composite material also has some holes, but the number is small and the hole diameter is small. The carbon layer blocks the heat and flammable gases generated by the material inside the carbon layer to the carbon layer. External communication has a role. As can be seen from Figure a, the surface of the carbon layer of the flame-retardant PA6 / MCA / wollastonite flame-retardant composite material is dense and smooth, with almost no holes, which can well isolate the heat and flammable gases generated by the substances in the carbon layer to the carbon. Spreading outside the layer enables the material to self-extinguish and improves the flame retardancy of the material. The difference in the shape of the carbon layer may be caused by the different geometry of the filler. Because calcium carbonate is spherical, the surface is smooth, and the friction coefficient is low. During the formation of the carbon layer, the matrix-filler interface is changed to a carbon layer-filler. Interface, and the interface bonding strength of the latter is not as good as the former. During the expansion of the carbon layer, calcium carbonate easily slips out of the carbon layer. The separation of the filler and the carbon layer makes the surface of the carbon layer not dense and forms many holes, which affects PA6 / MCA. / Calcium carbonate flame retardant composites. Because wollastonite is needle-shaped, it is embedded in the carbon layer during the formation of the carbon layer, forming a dense and smooth carbon layer, which improves the flame retardancy of the PA6 / MCA / wollastonite flame retardant composite.

FTIR analysis

a-PA6 / MCA / calcium carbonate flame-retardant composite material; b-PA6 / MCA / wollastonite composite material

FTIR spectrum of PA6 flame retardant composites after combustion

The figure above is the FTIR spectrum of PA6 flame retardant composites after combustion. From the b spectrum in the figure, it can be seen that the Si-O deformation vibration absorption peak is at 453 cm-1, and the Si-O-C stretching vibration absorption peak is at 1008 cm-1, indicating that after the addition of wollastonite, SiO2 and The surface carbon layer can be well combined to form a dense protective layer and play the flame retardant effect of the condensed phase. l The characteristic absorption peaks of calcium carbonate at 420, 873, 708 cm-1, but there are no such characteristic absorption peaks in the a spectrum in the figure, indicating that calcium carbonate detached from the carbon layer during the formation of the carbon layer, making the surface of the carbon layer The unevenness and many holes make it impossible to isolate the heat and flammable gases inside the carbon layer from spreading out of the carbon layer, thereby reducing the flame retardancy of the composite material.

Mechanical properties

Mechanical properties of PA6 flame retardant composites

As can be seen from the table, the tensile strength and impact strength of PA6 / MCA flame retardant composites are 55.2 MPa and 8.65 kJ / m2, respectively. After the addition of wollastonite, the mechanical properties of PA6 / MCA / wollastonite flame retardant composites have been improved. Its tensile strength is 58.2 MPa, an increase of 5.4%, and its impact strength is 7.78KJ / m2, a decrease of 10.1%. This may be because the acicular wollastonite is oriented along the injection direction, which makes the PA6 / MCA / wollastonite flame retardant composite material better than the PA6 / MCA flame retardant composite material. However, the shape of wollastonite is acicular, and the edges and corners are sharp. When the external force is applied, it shows that the notched impact strength decreases. After the addition of calcium carbonate, the comprehensive mechanical properties of PA6 / MCA / calcium carbonate flame-retardant composites decreased. Its tensile strength was 42.9 MPa, a decrease of 22.3%, and its impact strength was 5.47 KJ / m2, a decrease of 36.8%. This may be due to the low bonding strength between the spherical calcium carbonate and the PA6 matrix, which is easy to debond when subjected to tensile and impact forces, resulting in a decrease in the tensile strength and notched impact strength of the material.

in conclusion

01 The addition of acicular wollastonite improves the flame retardancy of PA6 / MCA / wollastonite flame retardant composites, while the addition of spherical calcium carbonate reduces the flame retardancy of PA6 / MCA / calcium carbonate flame retardant composites. When wollastonite is added, the LOI of PA6 / MCA / wollastonite flame retardant composite material reaches 29%, UL94 reaches V-0 level, and the performance is the best.

02 SEM and FTIR analysis of the surface and residue of the flame-retardant PA6 composite after combustion shows that adding wollastonite to the PA6 / MCA flame-retardant composite, SiO2 is combined with the surface carbon layer, and the carbon layer is dense; Calcium, flame retardant PA6 / MCA / calcium carbonate flame retardant composite material. The calcium carbonate detaches from the carbon layer during combustion, and there are many holes in the carbon layer. PA6 / MCA / Wollastonite flame retardant composites have better flame retardancy.

03 Mechanical properties test shows that compared with PA6 / MCA flame retardant composite, PA6 / MCA / Wollastonite flame retardant composite has a tensile strength of 58.2 MPa, an increase of 5.4%, and an impact strength of 7.78 kJ / m2, a decrease Up 10.1%. After adding calcium carbonate, the tensile strength of nylon 6 / McA / calcium carbonate flame retardant composite was 42.9 MPa, a decrease of 22.3%, and the impact strength was 5.47 kJ / m2, a decrease of 36.8%. After adding wollastonite, the tensile strength of the material is improved to a certain extent.