Understanding of the critical parameters in a vertical burning UL94 test using PA11

• Main Author: Seah, Dean Geng Jie
• Other Authors: Aravind Dasari
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/169643
• Institution: Nanyang Technological University

The burning behaviour of a polymer is one of the key aspects that must be considered in many electronic and electrical applications. In these scenarios, a material scale1 test, Underwriter Laboratories vertical burning test (UL94), is performed to assess the polymer’s response to a candle-like flame. In general, the addition of flame retardants (FRs) to the concerned polymer is a typical approach used in the literature to acquire the desired rating in this fire test. In another approach specifically for PA based polymers, melt-dripping behaviour is exploited. That is, not allowing the polymer to completely degrade and create the necessary concentration of volatiles for the flame to sustain on the polymer. Despite a good number of studies dealing with UL94 testing of polymers in the literature, the governing parameters for achieving a V0 rating are not well established due to the qualitative nature of the test. Therefore, in general, other testing equipment like cone calorimeter, PCFC, LOI, etc., are used to explain/quantify the changes in the flaming/burning behaviour of the polymers. In this thesis, the critical emphasis was on understanding the parameters to obtain some new insights that can be used to build a framework for establishing the methodologies while designing polymer formulations for UL94 tests. The parameters that were considered here include heating rate, melt/flame dripping, frequency and size of drips, and molecular weight distribution. Aside from these, the chemical nature and the rate of generation of volatiles could drive the flammability characteristics of a polymer during flame exposure to the specimen in a UL94 test. This is an important direction of the thesis, that is, quantifying the volatiles and identifying their chemical nature to get an idea of the critical concentration of volatiles required for ignition and sustained combustion. For this purpose, the thermal degradation mechanism of PA11 was studied with a TGIR and GCMS. The volatiles (combustible and non-combustible) in the gas phase are identified, and at the same time, FTIR and XRD were used to study the structural changes in the condensed phase. Afterwards, the concentration of combustible volatiles to ignite and sustained combustion are quantified using a cone calorimeter. Further, the effects of the incorporation of high-loading levels of GF to PA11 and the sample thickness variations on the flame response are studied. Currently, there are limited works in the literature dealing with flame testing of samples between 3.0 mm to 1.0 mm. Understanding the effects of thickness and GF on volatiles (combustible or non-combustible) and the critical concentration of combustible volatiles for ignition and sustained combustion is also important. To address the above mentioned points, different approaches were adopted in the thesis: •A top-down approach (where well-established FR additives for PA6 or PA6,6 are used in PA11). •Melt dripping approach where the melt/flame drips are exploited in UL94 tests to quickly remove the burning part of the polymer from the specimen. This stops further burning of the specimen. Conventionally, MC was used to enhance melt dripping without the ignition of the cotton in PA6 or PA6,6 to achieve a UL94 V0 rating.However, when it is directly adopted into PA11, the V0 rating is not observed. The findings have shown that when a slow heating rate is used, the melt dripping effect as a result of the acceleration of the thermal decomposition of polymer is valid. In other words, cyanuric acid in MC catalyses the degradation of PA11, lowering the viscosity average molecular weight, therefore, increasing dripping. The size and mass of drips produced by PA11 and PA11/MC formulations were also investigated, and there is no correlation between UL94. The usage of Mel as a drip promoter is also explored in this work. Similar to MC, at a slow heating rate, Mel accelerated the thermal decomposition of PA11 because of the disruption of H-bonding within the network. But Mel is a better drip promoter because of the delay in the release of combustible volatiles, which allows sufficient time for it to drip and not ignite the cotton. In another approach, a hybrid combination of phosphorus-based FRs like AlPi coupled with MPP is used in GF-reinforced PA11. This is because when GF is used, an increase in thermal conductivity is noted in the material system. This accelerated the thermal decomposition of PA11. Therefore, FRs that do not form a proper physical thermal barrier to delay the combustible volatiles or delay the release of volatiles by other means (like dilution) have poor UL94 ratings. This was demonstrated by using PA11/Mel and PA11/AlPi-MPP with reinforced GF. •The chemical nature of volatiles and the molecular weight (Mw) are two parameters that will influence combustible volatiles' release rate. To show how these two parameters affect flammability, PA11/MC formulations presented in this work will be compared to PA6/MC formulations obtained from the literature review. It was shown that PA11 releases aliphatic hydrocarbon and mononitrile-based volatile, while PA6 releases caprolactam-dominated volatiles. It was also shown that aliphatic hydrocarbon and mononitrile release more thermal energy as compared to caprolactam. Furthermore, it was reported that the lower Mw polymer volatilise faster than the higher Mw polymer, which means that the release rate of combustible volatiles will be faster. •Lastly, based on all the reported works, a framework is proposed to approach UL94 and future work that needs to be addressed.