Flame-retardant Property of TPO Waterproofing Membrane

1. Background

Since its development, PVC waterproofing membrane has been used and Roof waterproofing works for industrial buildings. PVC resin possesses higher ignition temperature and lower heat release, the HCl released during the combustion could capture highly reactive free radicals in the gas phase to resist fire. But for soft PVC, its ignition temperature is decreased and heat release is increased due to the addition of plasticizer, and needs to add flame retardant to improve fire resistance performance.

In recent years, with the increasingly strict requirements of environmental protection and health regulations, especially widespread attention to the safety of phthalate plasticizers, the types of safe and environmentally friendly plasticizers that can be used have decreased. In addition, the plasticizer in the PVC waterproof membrane will gradually migrate and lose during use, and the membrane will turn hard, brittle, and powdering, so the application of PVC membrane was limited to some extent.

The chemical structure of EPDM and TPO are similar, both are olefin copolymers with no unsaturated double bonds in the main chain. EPDM is a copolymer of propylene, propylene, and non-copolymerized propylene, typically with an ethylene content of 40%~70%. TPO is made from the copolymerization of toluene and propylene, and the propylene content is generally greater than 80%.

The processing of EPDM material is complicated, it requires processes such as banburying, open-melting, calendering, and vulcanization. The investment scale is large and the technical difficulty is relatively high. The EPDM waterproofing membrane is a thermoset material, that cannot be recycled, and it needs adhesive to stick when put into use.

TPO waterproofing membrane does not contain plasticizers, has excellent weather resistance, can be welded by hot air, and is easy to process. It has been widely recognized and applied since it was introduced to the market. However, TPO material only contains two elements, carbon, and hydrogen. It has low ignition temperature, is very flammable, and has high heat release. When used in roof waterproofing projects, corresponding flame retardants must be added to improve their flame retardant performance. This paper mainly discusses the choice of flame retardant and flame retardant performance of TPO waterproof membrane.

2. The choice of flame retardant

Brominated flame retardants were selected for early TPO waterproofing membranes. At that time, it was not found that such flame retardants would interact with weathering additives, leading to the failure of some waterproofing projects due to the premature aging of TPO waterproofing membranes.

And mineral flame retardant magnesium hydroxide has the advantages of being environment-friendly, smoke suppression, recyclable, colorable, halogen-free, non-toxic to the processing machine, no toxic, corrosive gas released during combustion, now becomes the first choice for TPO waterproof membrane.

The particle size, particle size distribution, BET, purity, whiteness and color of magnesium hydroxide has a large impact on the flame retardant property of the TPO waterproofing membrane. Particle size and particle size distribution could affect smoke suppression and smoke release. The smaller the particle size and the narrow the distribution, the more effective the char layer formed by MgO during combustion, to isolate the heat, oxygen, and polymer compound. Meanwhile, the finer the magnesium hydroxide, the less impact on the compound mechanical properties such as tensile strength and elongation at break.

Table one shows the impact of MDH particle size on the motor torque and tensile strength of the TPO waterproofing membrane. We can see that the smaller the particle size, the larger the motor torque. This is because small particle size magnesium hydroxide will increase the viscosity of the melting during mixing, making it hard to disperse in the compound and motor torque increases accordingly.

The larger the particle size the smaller the tensile strength, this is because after blending, the magnesium hydroxide particles are dispersed in the matrix resin and encapsulated by the resin. There is an interface between the magnesium hydroxide particles and the matrix resin, and the bonding force at the interface is weak. During the stretching process, the interface is easy to break and forms a cavity, the stress concentration in the cavity causes the specimen to break. The larger the particle size of magnesium hydroxide, the larger the surface area of a single particle, the larger the interface formed, and the more prone to early failure.

3. Processing machine and basic formula

The loading amount of magnesium hydroxide is generally larger than 30%, for a larger loading amount, you can choose suitable machine and processing techniques. Generally speaking, there are two methods to produce TPO waterproof membrane. One is using a twin-screw extruder to modify and pellet then extruded with a single extruder. The other is to use two screw extruders online modifying and pellet. As long as the extruder screw combination is properly matched, the effects of these two processing methods are the same. The former has low investment, but high energy consumption and cost, the latter has large investment, but low energy consumption and cost, and higher formulation flexibility.

The basic formula of this test:

TPO resin 67%, magnesium hydroxide 30%, additive 3%.

Processing temperature: screw temperature 180~190℃, mold temperature 210℃

4. Results and discussion

4.1 The dispersion of MDH in TPO

Table 2&3 is the SEM photo of TPO sample. The white particles are magnesium hydroxide, and we can see from Table 2&3 that the fine magnesium hydroxide is evenly dispersed in the TPO.

4.2 Flame retardant test

4.2.1 Cone Calorimeter Test

The test complies with ASTM E1354, incident heat flux 50 kW/m2.

Table 4 is the comparison between the TPO sample and the foreign sample. We can see that compared with the foreign sample, the test sample's peak heat release rate is 8% higher, total heat release is 13% higher.

Table 5 is the smoke generation rate comparison, and we can see that test sample smoke generation rate is 12% lower, total smoke release 4.3% lesser, and ignition time 10s later. And the fire spreading rate of test sample is 6.95kW/(m2.s), of reference foreign sample is 7.77kW/(m2.s)

From the above information, the tested sample is better in flame retardant performance comprehensively.

4.2.2 ASTM E108 test

The test complies with ASTM E108, continuous combustion is applied, the slope of the roof is 7.1 degree, and the test result is table 6&7.

We can see from table 6 that the tested TPO waterproof membrane ignites by the fire and wind, and flame spread, from table 7 we can see that the combustion route does not have horizontal flame spread and combustion spread length is shorter than 1.8m, could reach level A.

4.3 Weather resistance 

Test complies with GB/T 18244 “ Test method for Aging of building waterproofing materials”. Using xenon arc lamp, irradiance 0.51W/m2@340nm, black standard temperature (65±3)℃, relative humidity (65±5)%, lighting 102min first, and then 18min light and spray. The test sample is a 1.0mm thick TPO piece. Table 8 shows the variation of tensile strength and elongation at break along with the increase in test time.

We can see from Table 9 that after aging 8000h under a xenon arc lamp, the tensile strength and elongation at break do not have an obvious decrease, and there is no sign of a crack, or powdering at the appearance. The reasons are: first, the experiment uses HALS light stabilizer, which can extend the life of TPO membrane. Secondly, the experiment uses high purity, less impurity precipitated magnesium hydroxide, which is halogen-free and variable valence metal ions free, so it will not catalytic TPO resin degradation, and will not generate acid material to affect the HALS light stabilizer.

5. Conclusion

High purity and finer magnesium hydroxide contribute to the flame retardant and high aging resistance of TPO roofing membranes. 

KMT Industrial(HK) Ltd supplies high purity Brucite powder and also precipitated magnesium hydroxide, if you would like to know more detail, please feel free to contact Elena(kmt_elenaniu@163.com), thank you for your time.

This passage is translated from the paper <TPO 防水卷材阻燃性能的初步探讨> by黄鹭鹭，宋冠成 published in China Building Waterproofing, and the purpose of reprinting is to spread knowledge. If there is any infringement, please contact Elena to delete it.