Amongst the two main classes of polymer materials, as far as their heating behaviour is concerned, namely thermoplastics and thermosets, the larger publishing interest expressed as number of papers released, was found for the first category (Fig. 1). The same major interest was registered for the flame retardant topic, in connection with the three main classes of thermoplastics based on condensation polymers, i.e. polyamides, polyesters and polyurethanes (Fig. 2). The numbers dynamic was reflected by ISI indexed and impact factor publications, like articles, conferences, letters, data papers, proceedings papers, editorial materials, reviews and books, book reviews, chapters or other similar materials. This large interest reflects on one hand the large application potential and on the other hand the recycling ability [7]. Both directions require more intensive use and less ecological impact. In this context, the future materials are expected from now on, to be researched more and more within this region. Latest published research trends highlighted already the need for replacing several classical solutions.
A New Flame 320 Free Download
Publication trends in sources (ISI indexed sources, reports or books) for flame retardant additives and the main thermoplastics based on condensation polymers (2019 estimated based on existing data and foreseen results)
There are several directions to improve the fire behaviour of polymer materials. A flame retardant (FR), for example, can be used as a comonomer, which, through a chemical reaction, will generate a FR polymer directly from the synthesis [8] or by the chemical modification of the prepolymer [9]. The FR additive can be also incorporated into the polymer matrix, through the melt processing method by generating physical interactions. This later route was proven to be a simple and efficient method that allows the processing of a wide range of polymers by either extrusion or injection moulding in order to achieve a higher fire performance [10, 11] at industrial level. However, some interesting attempts have also revealed the possibility of fire retardation by coating the polymeric surface with an intumescent layer [12, 13].
In order to develop an eligible additive formulation, three required agents must be put forth: the acid source, the carbonizing agent and the blowing agent. The acid donor leads to dehydration of the carbonizing agent that becomes a char, while the blowing agent is important because of the gaseous products it releases. Besides the basic characteristics of a flame retardant agent, an effective additive should also satisfy the following features: it should be inexpensive, affordable and moreover, compatible with the polymer it is loaded in, without drastically affecting other properties (for example, the mechanical properties). The main characteristics of halogen-free organic/inorganic flame retardants are illustrated in Fig. 3 [14].
Polyamide (PA) is a well-known thermoplastic with applications in various areas such as textile, automotive, electrical and construction. These applications were made possible by employing flame retardant polyamide, which untreated, can easily burn into the atmosphere with a melt dripping behaviour. Aliphatic polyamides are less fire resistant, with a LOI value around 21%, while the aromatic polyamides have higher stability.
Polylactic acid (PLA) was often described as a biodegradable and biocompatible polymer obtained from renewable resource known for its medical performance or packaging applications. Although it was proven as a flammable polymer, like most thermoplastics, it has found several application in electrical and electronics fields [30]. This has become possible after improving the fire resistance of the PLA, which, being a biodegradable polymer, required a halogen-free flame retardant.
The flame retardant mechanism involves the chemical and physical interaction of the additive with the polymer matrix where it was loaded in. Under the action of heat, the high temperature of the polymer causes changes in its viscosity, leading to the formation of burning droplets. At the same time, fragmentation of macromolecular chains is followed by the appearance of compounds with low molecular weight. The combustion is maintained by the gases resulting from polymer degradation, which ignites itself in the presence of oxygen. The main role of the additive is to inhibit flame propagation, as burning of the resulting gases increases the level of heat, and the fire becomes uncontrolled. It is important to increase also the time between ignition and flame propagation, as there are seconds in the overall process which very often could save lives.
Aluminium trihydrate (ATH) and magnesium hydroxide (MH) are used in large quantities as halogen free-flame retardants because of their economic accessibility and non-corrosive and environment-friendly character. These inorganic hydroxides act as fire inhibitors by the following mechanisms:
The necessity for a flame-retardant unsaturated polyester resin (UPR) was due to the fact that during combustion, this compound produces a lot of toxic gases, heat and smoke. Therefore, MH was an example of FR studied for delaying the propagation of fire after UPR ignition [54]. The evaluation of combustion performance illustrated that the FR properties of UPR were improved with increasing the additive concentration, the best fire behaviour being achieved at 55 wt% MH content.
Xiao et al. [63] synthesized a new flame retardant system made of melamine, cyanuric acid and α-ZrP. This additive was obtained by self-assembly of the three compounds and studied to reduce the flammability of PA6. When the concentration of α-ZrP-modified melamine cyanurate (MCA) was 10 wt%, an improvement in the combustion properties of PA6 was achieved: reduction of the HRR and THR, V-0 rank in UL-94 test and anti-dripping effects. It was found that a content of 30 wt% MCA-α-ZrP influences the crystallinity of PA6 inducing γ phase formation.
Organo-modified or functionalized ZrP (OZrP or F-ZrP) were synthesized to improve fire behaviour of PLA systems [64, 65]. It was reported that PLA composites loaded with a small amount of OZrP (1wt%) showed some improved properties: low HRR and THR and a LOI value increased from 19.0 to 35.5%. In the case of F-ZrP, the initial decomposition temperature of the samples was slightly decreased, a V-0 rating was achieved and the LOI value was increased to 26.5%. In both cases, it was found that the ZrP-based flame retardant contributes to the formation of a compact layer on the surface of char residue. In the same type of polyester, OZrP was used as a synergistic agent associated with polysulfonyldiphenylene phenyl phosphonate [66]. This mixture allowed the PLA fibres to exhibit a better fire performance. The results showed a 20% reduction in HRR, a maximum LOI of 29.3% and a UL-94V-0 classification.
Aluminium hypophosphite (AHP) is representative for phosphorus-containing flame retardants. This inorganic compound was examined in order to improve the thermal stability of condensation polymers.
Although AHP is a halogen free-flame retardant, it can release phosphine during its thermal or impact decomposition which can ignite itself in air. In order to prevent the spontaneous flammability of decomposed AHP, the microencapsulated technique was applied [69]. Through this method AHP was microencapsulated by MCA. The resultant MCA layer adsorbed on the surface of AHP presented the following functions:
Microencapsulated aluminium hypophosphite (MCAHP) was studied as FR for PA6. The thermal degradation study of MCAHP demonstrated MCA efficiency in improving thermal stability of AHP by increasing the decomposition temperature of AHP from 330 to 350 C. Samples supplemented with MCAHP showed an increase in LOI from 21 to 27.5% and a shift in the UL-94 classification from no rating to V-0. A good flame retardancy of PA6 has been proven by a significant decrease in HRR and THR combustion parameters.
Ammonium polyphosphate (APP) is a halogen-free flame retardant suitable for improving the fire performance of polyamides and polyurethane foam, as well as for polyolefins and epoxies. Moreover, it has a low cost and a good processability. The degree of polymerization influences the properties of APP and char formation of the composites that contain it [70]. APP starts to decompose at 240 C releasing ammonia and phosphoric acid with no additional amount of smoke, which makes this additive eco-friendly.
APP proved to be an effective additive for thermosetting materials like glass reinforced epoxy composites [77]. The necessity of flame-retarding this composite is due to the fact that glass fibre acts as a candlewick, generating an accelerated degradation of the polymer. At an APP content of 5 wt%, the samples became self-extinguished, obtaining a V-0 rank in UL-94 tests. In terms of thermal and mechanical properties, APP does not induce significant changes due to its good distribution in the polymer matrix.
Several studies have focused on the development of APP-based systems to reduce the flammability of polyurethane foams. Zhang et al. [32] synthesized hydroxyl-functionalized APP (HAPP) via a cation exchange reaction in order to improve by chemical cross-linking the fire behaviour of a solvent-free two-component polyurethane. The results of LOI, UL-94 and cone calorimeter analyses suggested the effectiveness of the HAPP as FR, highlighted by the increased LOI value to 25.7%, V-1 classification and decreased pHRR, THR and TSP. The advantage of chemical incorporation of HAPP has been demonstrated by improving the tensile strength of the polymer matrix.
Melamine and its derivatives proved excellent flame retardant properties because of the different modes of action. MCA has been used as a halogen-free flame inhibitor for polyamides, polyesters and polyurethanes because of its processing temperature around 300 C. At a higher temperature, the degradation of melamine and cyanuric acid begins with consequent formation of water, ammonia and carbon dioxide. 2ff7e9595c
Commentaires