Failure behavior of nylon products for red phosphorus flame retardant electrical connectors

The failure behavior of red phosphorus flame retardant electrical connectors was investigated by their thermal degradation, changes of surface morphology and elements under an accelerating environmental experiment. The results showed that the nylon electrical connector degraded and cracks were generated on the surface. The temperature of maximum weight loss rate was advanced in the thermogravimetric analysis and the third weight loss peaks appeared after 28 days. The relative atomic content of O and P increased from 0 day to 28 days. The chemical environment of C changed. Partially, C–C bonds broke and became C–O bonds. The release of phosphine increased and this further oxidized to an acid with P 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 O and P–O groups in a warm and humid environment.


Introduction
Electrical connectors are indispensable electronic components of electronic and electrical systems. 1 They act as "bridges" in the circuit to realize the transmission of signals and electrical energy. 2 Their quality has a huge impact on the performance and quality of the products. 3 With maximum output and various types, polyamide is the most widely used engineering plastic. [4][5][6][7] And, PA 66 has good mechanical properties and electrical properties, such as wear resistance, oil resistance, selflubrication and acid and alkali resistance. 8,9 Therefore, PA66 has broad application potentials for electrical and electronic connectors. The limiting oxygen index value of PA66 is 22.5%, 10 reaching V-2 classications in the UL-94 texts. 11 However, the electrical connector has higher requirements for ame retardancy and the addition of glass ber further reduces the ame retardancy of PA with "wick effect". 12 The use of electrical connectors can cause irreversible damage to electronic and electrical products, and even further threaten human's life. Therefore, the use of ame retardants becomes very important.
With low cost, low loaded amount, environmental friendliness and little inuence on mechanical properties, red phosphorus is a kind of excellent ame retardant. [13][14][15] It is widely used as a ame retardant material for electrical and electronic connectors in the industrial production. Red phosphorus would form a liquid lm and a carbon layer, which will affect the kinetics of the thermal decomposition process of PA 66, so that achieve a ame retardant effect. 16,17 The glass ber reinforced PA 66 with 6-8% red phosphorus would reach V-0 level in the UL-94 test. 18 However, phosphine is released during usage for red phosphorus and further produces phosphoric acid derivatives in a warm and humid environment, 19 which causes damage to the performance of the connector and induces combustion. The reliability risks of using red phosphorus as a ame retardant material in encapsulated microcircuits was discussed. The eld failure rate has been studied to drop from approximately 5000 ppm to 500 ppm when the maximum particle diameter is reduced from 180 to 150 mm for epoxy resin. The oxygencontaining phosphorus acids are corrosive and can alter the physical and electrical characteristics of the polymer composites. 20 Michael Pecht considered these acids and ions generated could induce electro-chemical migration, causing short circuits in the electronic device encapsulation. 21 PA 66 can be used as wire and cable, switch socket, relay, connector, etc., with red phosphorus as a common ame retardant. Among them, Panasonic refrigerator and Amway connector caused re accidents. However, the failure mechanism of red phosphorus ame retardant nylon products has not been systematically studied so far.

Experimental
Temperature and humidity environment experiment PA66 electrical connector products (hereinaer referred to as connectors), provided by Amway, was tested for aging at 80 C, 75% RH. A 500 ml jar with the other 50 ml test tube containing distilled water was deposited in a drying oven at 80 C, with a hygrometer showing the humidity inside at 75% RH (Fig. 1). The connector was disassembled aer being cooled by liquid nitrogen. With copper sheets attached by a nylon cable tie, the connector was placed in the above-mentioned jar for 80 C, 75% RH aging test. The products and copper sheets were characterized aer 7 d, 14 d and 28 d respectively.
Phosphine released by red phosphorus would rapidly reacts with air and water to form an acid under high temperature and humidity. In order to determine phosphine release, 2 g product was deposited in a sealed and dried 500 ml aluminum foil gas collection bag (Fig. 2). Aer 7 d, 14 d and 21 d, the concentration of phosphine gas in the bag was determined by the BH-90A phosphine detector.

Characterization
The morphology of connectors were observed by means of cold eld-emission scanning electron microscopy (SEM, Hitachi S-4800, 10 kV), with products sprayed with gold to strengthen conductivity. The surface elements were analyzed by Energy Dispersive Spectroscopy (EDS) of the scanning electron microscopy.
A TGA instrument (TG 209 F1, NETZSCH, Germany) provided with an alumina crucible was used. The measurement was conducted in nitrogen (50 ml min À1 ) with a heating rate of 10 C min À1 and samples of 2-3 mg from 40 to 800 C. The experimental error was AE0.1% by weight.
The chemical structure of the connector was analyzed by a Fourier transform infrared spectrometer (Nicolet 6700, Thermo Electron Co., USA) over the wavenumbers range of 400-4000 cm À1 .
X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientic, USA) were performed on the products with a monochromated Al Ka radiation (hn ¼ 1486.6 eV).
Inductively coupled plasma mass spectrometer (Agilent 7500ce, Agilent Technologies, USA) was used to measure the concentration of phosphorus of the substance formed on the surface of the copper sheets, dissolved in 10 ml dilute hydrochloric acid for 2-3 days.

Thermogravimetric analysis
Thermal degradation behaviors of the connectors were investigated by TG and DTG, corresponding curves and data are presented in Fig. 3 and summarized in Table 1. As the environmental test progressing, the initial decomposition temperature (T 5% , temperature based on 5% mass loss), the maximum mass loss rate (R max ), temperature corresponding to the maximum mass loss rate (T max ) and the value of char residues (CR) for connectors at 800 C all reduced, indicating that the thermal stability of connectors exposed to high temperature and humidity environment diminished. The appearance of the mass loss rate peak for 28 days indicates that the connector produced a new component in the high temperature and humidity environment. Reduced CR indicated that the phosphine released from red phosphorus reacted with water to form various oxygen-containing, phosphorus-based acids, such as phosphoric acid, phosphorous acid, and hypophosphorus acid,   and then partially transferred to the copper sheet by corrosion, resulting in a reduced char forming ability. Fig. 4 is an FTIR spectrum of the connectors aer environmental testing. Compared with the initial connectors, the intensity of stretching vibration of -OH for the connectors environmentally tested was enhanced. The peaks representing stretching vibration of P-O and P]O were found at 931 cm À1 and 1143 cm À1 , 22 illustrated that the red phosphorus in the product released PH 3 and then formed acids. At the same time, the generated phosphoric acid would accelerate the degradation of the polyamide. The peak representing -NHshows signicant changes, transferring to -NH 2 , 23 especially for connectors tested by 28 days.

Surface morphology
Scanning electron microscopy. Fig. 5 is SEM images of connectors and its surface under 80 C, 75% RH environment for 0 d, 7 d, 14 d and 28 d respectively. The surface of the original sample was relatively smooth. However, with the high temperature and humidity environment test progressing, cracks appeared on the surface, resulting in many aky layers and increasing the roughness. Green globular substances appeared on the surface and section area of the sample aer a period of time. It can be observed that the globular substance dispersed on the edge of the cracks aer 14 days. Aer 28 days, the globes increased so much that it is too dense to observe a single sphere and the entire surface becomes shattered. Fig. 6 and Table 2 exhibits the evolution of the elemental dispersion on the surface of connectors. As showed in Fig. 5, the spherical substance was mainly composed of P, O, and Cu elements. The elemental content given in Table 2 indicated that the substance was increasing over time. Phosphorus could not be detected on the surface at the beginning and aer 7 days until the globular substance containing phosphorus was formed. With the time increase, the concentration gradient of phosphorus was produced from inside to surface. At the same time, the content of oxygen increased. It proved that phosphorus reacted with water and air to form phosphoric acid through these cracks in the high temperature and humidity environment and the phosphoric acid corroded the copper sheet attached to the connectors to form copper phosphate species on the surface.
X-ray photoelectron energy spectrum analysis. XPS spectrum of survey scan for connectors is exhibited in Fig. 7 and elemental content of connectors tested for different days is showed in Table 3. The intensity of carbon and oxygen is very strong with traces of N and Si existing. Nevertheless, the signal intensity of phosphorus is very low.  This journal is © The Royal Society of Chemistry 2019 The P 2p high-resolution spectra of connectors are exhibited in Fig. 7. There are two main peaks, 129.4 eV corresponding to red phosphorus; 133.4 eV corresponding to phosphate anion. 24 The presence of these peaks affirms that red phosphorus was oxidized to phosphate. The intensities of peaks increases gradually and reaches a stable state, conrming that phosphate concentration increased with test prolonging, especially in the rst 14 days.
In Table 3, the relative atomic content of ve main elements P, C, N, O and Si, and traces of calcium were conrmed. In addition, due to the close contact between the connector and the copper sheet in the environmental test, a trace of copper was detected. As the experiment prolonged, the relative atomic concentration of phosphorus and oxygen on the surface of the product revealed an increasing tendency. This is consistent with the results of EDS. The red phosphorus ame retardant inside the connector migrated to the surface and was oxidized by air to form an acid. At the same time, the formed acid corroded the copper sheet to form copper phosphate, partially adhering to the product.
The C 1s spectra of tested connectors are tted into three peaks to know the types of functional groups of PA66 from connectors in Fig. 8. The three peaks centered at 284.6 eV corresponding to C-C; 286.0-286.3 eV corresponding to C-O; 287.5-287.7 eV corresponding to C]O. 25 While compared with the C 1s of the initial product, the intensity of the carbonoxygen bond signicantly increased. It is expected that PA66 was degraded and carbon-carbon bonds partially were broken,   7 Wide-scan (a) and P 2p high-resolution (b) XPS spectra for connectors. oxidized to carbon-oxygen bonds. The degradation of PA66 is a complicated process, mainly involving not only the hydrolysis of amide bonds, but also the breakage of molecular chains, accompanied by the formation of various small molecular compounds. The carbon-carbon bond is broken further, accelerating oxidization to cause the rise of the carbon-oxygen bond. This would generate cracks and further reduce the thermal stability of the connector, making it ammability. 26 Phosphate and phosphine analysis In order to roughly determine the amount of the acid oxidized from red phosphorus, copper sheets was attached to connectors so that the acid could corrode them and transferred to the surface of copper sheets. The mass of phosphorus attached to copper was determined by inductively coupled plasma mass spectrometry (ICP-MS). Prior to this analysis, the copper pieces were digested in dilute hydrochloric acid to transfer the phosphorus corroding copper into the HCl solution. The results are shown in Table 4. Aer 80 C/70% RH test for 7 days, the copper sheet was relatively less corrosive with only 8.294 mg of phosphorus detected. The copper sheets tested for 14 days and 21 days were corroded obviously and the weight of phosphorus reached 84.226 and 96.522 mg respectively. Furthermore, 169.66 mg phosphorus was detected aer 28 days. As the test progresses, the phosphoric acid produced by red phosphorus continued to increase. Since the phosphine released by connectors reacted rapidly with water and air to form phosphoric acid, in order to determine the amount of phosphine, 2 gram connectors was placed in a dry gas sampling bag and tested by a phosphine detector. The results are shown in Table 5. Over time, the amount of phosphine released continued to increase.

Conclusions
The mechanism of failure behaviour is revealed in Fig. 9. Aer connectors had undergone high temperature and humidity environmental testing, red phosphorus migrated to the surface and accumulated into several areas. At the same time, signicant degradation occurred, the decomposition temperature was advanced and -OH and phosphorus oxides were formed. The surface morphology of connector became rough with cracks increasing and the copper attached to connector formed copper phosphate. The content of phosphorus and oxygen on the surface increased and the chemical environment of phosphorus changed from elemental phosphorus to phosphoric acid derivatives. The amount of phosphine released increased and it reacted with water and oxygen to form acids. Correspondingly, the formation of phosphoric acid derivatives accelerated the degradation process of nylon. Surface cracks caused by degradation resulted in red phosphorus to be further oxidized to an acid. The mutual promotion caused the failure behavior of the red phosphorus ame retardant PA66 electrical connector.

Conflicts of interest
There are no conicts to declare.