Synergistic Effect of LDH and Zirconium Phosphate with Aluminum Trihydroxide on Flame Retardancy of EVA Composites

Abstract

Flame-retardant ethylene vinyl acetate (EVA) composite based on aluminum trihydroxide (ATH), layered double hydroxide (LDH) and organo-modified zirconium phosphate (mZrP) were prepared by melt-compounding method.

The synergistic effect of LDH and mZrP with ATH on the fire behavior and thermal stability of EVA composites was studied by limiting oxygen index, UL-94 test, cone calorimeter and thermogravimetric analysis.
 

EVA composite with ATH and LDH passed the V-0 rating while EVA composite with ATH and mZrP exhibited relatively low peak heat release rate.

EVA/ATH composite with 10 mass% LDH exhibited a char yield of 34 % at 700 ℃, while its counterpart with 10 mass% mZrP showed 29 %, indicating LDH possessed superior flame-retardant synergistic efficiency with ATH over mZrP in terms of promoting char formation.
 

Regarding the heat release rate(HRR), EVA/ATH composite with 10 mass% mZrP displayed a 73 % reduction in PHRR, whereas its counterpart with the equivalent loading of LDH showed a lower flameretardant synergistic efficiency (a 58 % reduction in peak HRR). 
 

The results above demonstrate that LDH is mainly functioned as catalyst in char formation, while mZrP is beneficial to
restraining heat release.

Sample preparation




Results and discussion

Tensile properties

Thermal analysis

TG curves of pure EVA and its composites are shown in Fig. 1

The thermal stability was quantified by the initial decomposition temperature that is defined as the temperature where 5 % mass is lost (T5%).and the related data are listed in Table 3.

Flame-retardant properties of EVA composites

1. UL-94 Testing 

Table 4 presents the LOI values and UL-94 testing results of the flame retarded EVA composites.

It can be observed that the LOI value of sample EVA1 increases to 25.5, from 21.3 % of pure EVA. Furthermore, adding mZrP,
LDH and mZrP/LDH shows no significant change in LOI value.however, from UL-94 vertical testing test, only EVA3 could pass the V-0 rating. 

As shown in Fig. 2, in the case of EVA0, fire propagates quickly from igniting with a little char left; for EVA3 sample, no dripping and quick selfextinguishing are observed. the excellent fire resistance of EVA3 could be attributed to the highest char yield as evidenced by TG that significantly reduces heat transfer and air incursion which enhances the flame-retardant performance of EVA composite.

2. Cone calorimeter

It is the most effective method for laboratory evaluation of the fire properties of polymers.

The available parameters of the cone calorimeter include time to ignition (TTI), peak heat release rate (PHRR) and total smoke production (TSP). The heat release rate (HRR) curves measured by cone calorimeter for EVA and its composites are shown in Fig. 3 

Usually, the application of nano-fillers into the fire-retardant polymer systems can improve the char layer’s yield , as shown in Fig. 4

As shown in Table 5, EVA1 shows little change in TTI compared to the virgin EVA. However, after introducing LDH and mZrP, the TTI value of EVA2, EVA3 and EVA4 increases to 36, 42 and 46 s, respectively, referring to the improvement of the difficult-to-ignite ability of the composites.
Moreover, the combustion time of EVA2 and EVA4 is extended to 955 and 1000 s, respectively, from the 465 s of EVA0.

3. The emission of smoke

It  is regarded as another important parameter in the halogen-free flame-retardant materials.
The TSR curve is depicted in Fig. 6


The peak carbon monoxide yield and the peak carbon dioxide yield are summarized in Table 5

It can be observed that both the smoke emission and the carbon oxides yield of EVA composites during combustion process show significant decrease compared to pure EVA, which is attributed to the smoke suppression effect of the nano-fillers.

Moreover, the smoke temperature versus time curves of EVA and its composites during the combustion are  illustrated in Fig. 7

It is interesting to observe that the smoke temperature of all the EVA composites are lowered compared to that of pure EVA, and introduction of nanofillers results in further reduction in the smoke temperature.
 

Furthermore, mZrP is more effective than LDH as far as the smoke temperature is concerned, which is ascribed to the fact that the increased amount of inflammable gases (e.g., CO2) dilutes the combustion heat feedback to smoke.
 

Fire fatalities by the hot smoke are usually reported as one of the main concerning factors during fire accidents.

Therefore, the reduction in the smoke temperature during combustion will be beneficial for fire rescue when an accident

happens.

Generally, the residual chars formed during combustion can give some important information regarding flame-retardant

mechanisms to some extent.

4. The morphology and thestructure of the residual chars collected from the cone calorimeter tests

The digital photographs of residual chars after cone calorimeter test are displayed in Fig. 8

As can be observed, char residue from EVA1 is bitty and cannot form an effective char layer.

In the case of char residue of EVA2, a thermally thick charring residue with lots of cracks and some opening holes is formed.

The quality of char layer of EVA3 is slightly improved compared to that of EVA2, still with some opening holes on the surface.

In the case of EVA4, after introducing LDH/mZrP mixture into the composite, the quality of char layer is obviously improved with a compact and continuous surface.

To further confirm the morphology of the residual chars,SEM was employed to study the exterior and interior chars of the EVA composites, as shown in Fig. 9

Fig. 9 SEM images of the charred residue after cone calorimeter test: a EVA1, b EVA2, c EVA3 and d EVA4

Flame-retardant mechanism

Based on these results, it is worthy to note that only EVA3 passes UL-94 V-0 testing, but its PHRR is higher compared to EVA2 and EVA4. The possible flame-retardant mechanism for different fire-retardant EVA composites is proposed, as illustrated in Fig. 10

In the UL-94 tests, the flame is not stable after ignition so that some inflammable pyrolysis gases (e.g., water vapor) could easily extinguish the fire.

As evidenced by TG, LDH degrades from about 100 ℃, which is much earlier than mZrP. 

Therefore,EVA3 shows V-0 rating since the inflammable gases jetting from the early degradation stage of LDH extinguish the fire. 

For CCTs, the heat flux is much stronger than UL-94 test, and thus, the inflammable gases jetting from the early degradation stage of LDH are not enough to extinguish the fire. 

From the mass loss curve (Fig. 4), mZrP is more effective to suppress the mass loss rate during combustion than LDH. 

The reduced mass loss rate is ascribed to the barrier effect of the char layers. After dissecting, a large cavity in char was observed.

In fact, the cavity under the char layer can function as gasbag to retard the heat and mass transfer, which is much more effective than the solid char layer.

Additionally, EVA2 and EVA4 release much more inflammable gases (e.g., CO2) than EVA3, which dilute the flammable gases during combustion.

Therefore EVA2 and EVA4 show reduced PHRR and THR values compared to EVA3. 
 

Conclusions 

The flame-retardant EVA and its compositeswere prepared by the melt blending method, and their morphology, fire behavior, flame-retardant mechanism as well as mechanical properties were studied. 

The results showed that incorporation of mZrP/LDH mixture into EVA/ATH composite resulted in a reduction in thermal stability (T5%), but improved the char yield at 750 ℃.

With 10 mass% of mZrP and/or LDH with 40 % ATH, EVA composites exhibited improvement in fire retardancy including

the greatly decreased PHRR, THR, TSP and the enhanced char yield. Addition of mZrP/LDH/ATH significantly increased the tensile strength and the Young’s modulus。

For EVA composites, the flame-retardant synergistic efficiency of LDH with ATH was quite different from that of mZrP: LDH was more effective than mZrP in char formation; on the other hand, mZrP showed a 73 % reduction in PHRR, which was more efficient than LDH (58 % reduction in PHRR).

These phenomena could be explained by different roles of LDH and mZrP on the flame retardancy of EVA composites: LDH mainly functioned as catalyst in char formation, while mZrP probably promoted the release of inflammable gases (e.g., CO2) during combustion.

The formulations presented herein of the flame-retardant EVA composites based on mZrP/LDH/ATH have promising potential application in wire and cable industry.

Remark:Article in Journal of Thermal Analysis and Calorimetry · March 2015 DOI: 10.1007/s10973-015-4598-9