1. Introduction
Polyethylene terephthalate (PET) foam is characterized by lightweight, high strength, and good thermal resistance qualities [
1,
2]. It is one of the structural core materials which have been widely used in the wind power field [
3]. In addition, PET foam has low thermal conductivity, low water absorption, no rot, no mildew, easy to process and other characteristics, so it also would have a wide range of applications in automobiles, rail transportation, construction, ships, and other prospects [
4,
5,
6]. Therefore, the flame retardancy of PET foams has become more and more critical, which affects its wide application in construction, automobiles and rail, etc. PET is a flammable material and is prone to dripping due to the random chain-breaking mechanism during combustion [
7,
8], and the limiting oxygen index is only about 21%. The flame retardancy of PET foams is further reduced due to the increased contact area with air [
7,
8,
9]. In order to improve the flame-retardant properties of PET and its foams to make them suitable for more applications, meet national safety standards, and reduce fire hazards, adding flame retardants is a common method to reduce the flammability of PET product.
Before flame-retardant modification of PET, it is necessary to understand its combustion mechanism. The polymer combustion process has five stages: thermal decomposition, ignition, combustion propagation and development, fully stabilized combustion, and combustion decay. The combustion region can be divided into three levels: the polymer surface, condensed phase, and gas phase [
10]. The thermal decomposition temperature of PET in air is 283–306 °C, and the thermal decomposition temperature in nitrogen can reach 420 °C [
11]. At this stage, the PET molecular chain begins to undergo random cleavage, and the relative molecular mass significantly decreases. As the thermal decomposition continues, it begins to produce flammable gases, and in the presence of an ignition source and oxygen, the PET will be ignited. After the ignition, the solid and gas phases will undergo exothermic reactions, releasing a large amount of heat. The heat is transferred to the surface of the PET matrix or even inside and spreads to the surrounding areas, continuing to promote the cracking of PET molecular chains to produce flammable gases; flammable gases escape to the flame zone and are ignited after contacting the air at high temperatures, which makes the flame begin to stabilize the combustion when it reaches the critical point. At the end of PET matrix degradation, the flammable gas generated, and which escapes to the flame area, decreases, and the heat generated by combustion also decreases. At this time, the combustion can no longer be maintained and, thus. attenuated until extinguished. The combustion of the products of PET cracking follows a chain reaction [
12], and to slow or stop the combustion, it is necessary to stop the chain reaction and reduce the heat release.
Flame retardants are characterized by their flame-retardant elements [
13,
14]. For example, the halogen elements fluorine, chlorine, bromine, iodine, etc., are very effective combustion inhibitors of hydrocarbons because they capture the free radicals in the gas phase and, thus, interrupt the chain reaction [
15]. Common halogenated flame retardants include decabromodiphenyl ether [
16], chlorinated paraffin [
17], etc. Due to their highly hazardous effects on the environment and the human body, halogenated flame retardants are constantly being restricted, and the research related to flame retardancy is gradually shifting to non-halogenated flame retardants [
18,
19]. Halogen-free flame-retardant elements include phosphorus, nitrogen, boron, silicon, ammonia, aluminum, magnesium, antimony, etc. [
20], and the flame retardants containing these elements play a flame-retardant role mainly in the gas phase or condensed phase. Common halogen-free flame retardants include red phosphorus [
21], phosphonitrile [
22,
23], phosphonate [
24,
25], polyphosphoric acid [
26], zinc borate [
27], magnesium hydroxide [
28], aluminum hydroxide [
29], antimony trioxide [
30], etc.
In the flame-retardant(FR) modification of PET, a single FR element usually cannot meet the requirements for the FR properties of PET, so it is necessary to select a suitable FR system to jointly play a FR role to achieve a good FR effect. Ordinary FR systems include halogen/antimony synergism [
31], phosphorus/halogen synergism [
32,
33], phosphorus/nitrogen synergism [
34], phosphorus/phosphorus synergism [
35,
36], and nitrogen/phosphorus/silicon synergism [
37], etc. The mechanism of these synergistic systems can be broadly categorized into two types: one is mainly flame retardant in the condensed-phase; the other type mainly plays a flame retardant role in the gas-phase.
PETproduces dripping during combustion [
7,
8]. Although the dripping can take away a large amount of heat to slow the matrix combustion down or stop it, the burning droplets will likely ignite other combustibles, leading to the spread of fire. Given the characteristics of PET, which often drips during combustion, many people try to add co-efficacies in phosphorus-containing flame retardants or co-polyesters to achieve the anti-dripping effect. Zhao et al. [
38] synthesized phosphorus-containing and ionomer monomers and phosphorus-containing and high-temperature self-cross-linking monomers, and copolymerized the two monomers into PET molecular chains to obtain anti-dripping PET co-polyesters, respectively. Among them, the ionomer works through the ionic aggregation effect and, thus, forms a reversible physical cross-linking network in the matrix to improve the viscosity of the melt during combustion and inhibits the dripping. At the same time, the high-temperature self-cross-linking has a better effect, which is anti-dripping through the formation of the irreversible cross-linking network by a chemical reaction at high temperatures. Furthermore, Guo et al. [
39] successfully addressed the issue of dripping by incorporating a novel arylene ether-containing monomer (PBPBD) into the PET main chain under elevated temperatures; the PBPBD structural units undergo rearrangement reactions to form a conjugated heterocyclic aromatic structure, which promotes the formation of a char layer during combustion of the PET, and at the same time effectively enhances the viscosity of the PET.
It can be found that the flame-retardant modification of PET generally selects the synergistic system that mainly works in the condensed phase and that also actsin the gas phase, such as phosphorus/halogen synergistic systems, phosphorus/nitrogen synergistic systems, etc. Green [
40] found that the flame-retardant efficiency of phosphorus is much higher than bromine when using brominated phosphonate for flame-retardant PET/PC mixtures. This is further improved when the two are synergistically flame-retardant.
In this study, a phosphorus/bromine synergistic system for flame-retardant modification of PET was used; ZDP has high phosphorus content and a flame-retardant effect in both the condensed and gas phases [
41]. There is no chemical bonding between DBDPE and PET. DBDPE has the advantages of high thermal stability, bromine content, and flame-retardant efficiency [
42], and is a typical representative of additive flame retardants. In this paper, ZDP and DBDPE were used to form a phosphorus–bromine synergistic system to prepare flame-retardant PET, in which ZDP was used as an acid source, and DBDPE was used as a gas source to study the effect of the two complexes on the flame-retardant and foaming properties of PET.
4. Conclusions
To improve the flame-retardant properties of PET foams, a ZDP/DPDPE synergistic system was chosen to modify PET and investigate its impact on the flame-retardant and foaming properties of PET. The main conclusions are as follows.
Adding ZDP and DBDPE alone increased the oxygen index of PET to 29.8% and 28.7%, respectively, but neither improved the dripping phenomenon, resulting in a UL-94 rating of V-2. The compounding of ZDP and DBDPE is a synergistic mechanism for expanding the carbon layer. When the ZDP/DBDPE = 9/3 and 7/5, the residual carbon layer reaches 3–4 cm at the highest level; a porous char layer with a considerable thickness can isolate the heat transfer between the PET matrix and the flame zone and inhibit the thermal decomposition of the PET matrix. The LOI values of the PET/9ZDP/3DBDPE and PET/7ZDP/5DBDPE samples with the most considerable char layer thicknesses reached 32.7% and 33.6%, respectively, and the vertical combustion test also reached the V-0 level.
ZDP and DBDPE have a significant effect on the foaming properties of the PET matrix. Because ZDP is more compatible with PET than DBDPE, increasing the proportion of ZDP can improve the viscoelasticity of the melt at a fixed total addition amount, thus enhancing the foaming performance. In addition, the dispersion of ZDP and DBDPE in PET is critical. An appropriate ratio (PET/7ZDP/5DBDPE) optimizes the dispersibility and reduces the adverse effect on the melt viscoelasticity. This optimization not only enlarges the foaming temperature window but also significantly improves the foaming efficiency, reaching a maximum foaming ratio of 39 times.
The flame retardancy of foamed samples is a function of foam density. A decrease in the density of the foamed samples correlates with a reduction in the LOI, even when an equivalent quantity of flame retardant is incorporated. When the density of the foam is significantly reduced, the samples may exhibit dripping of molten material during combustion, resulting in a vertical burning classification of V-2. Additionally, the ratio of ZDP to DBDPE significantly influences the flammability characteristics of the foam.