Issue 16/2013 - Research on the application of intumescent flame retardants in PP

Intumescent flame retardants in PP application research

■ Cui Xiaoming

Polypropylene (PP) is a general plastic with excellent mechanical properties, good electrical insulation and good chemical resistance, but PP has poor flame resistance and is easy to produce a large amount of molten dripping, which is easy to cause fire
.
Therefore, the research and development of PP flame retardants has become one of
the research hotspots.
PP's flame retardants include hydroxide flame retardants, phosphorus-based flame retardants, silicon-based flame retardants and intumescent flame retardants, among which intumescent flame retardants have become a hot spot
in research and development because of their unique performance and application prospects.

Synergistic and compound flame retardant technology

The compounding of intumescent flame retardants can reduce the amount of flame retardants, improve the flame retardant performance, and minimize the loss of physical and mechanical properties of the material
.
Jiang Shu et al.
of Sichuan University studied the flame retardant properties and microstructure of expanded graphite (EG)-filled PP systems obtained by different processing methods, and the results showed that the flame retardant properties of EG/PP systems not only increased with the increase of EG content, but were also affected
by the EG particle structure.
The strong shear action during processing will destroy the integrity of the graphite particle structure, and its flame retardant effect will be greatly reduced
.
Therefore, in terms of the influence on the combustion performance of EG/PP system, the compression molding method is significantly better than injection molding
.

Li Xu of Shanghai Research Institute of Chemical Industry and others used cyanuric chloride, diethanolamine and ethylenediamine as raw materials to design and synthesize a new type of triazine carbonizing agent (CA), which was compounded with ammonium polyphosphate (APP) and melamine (MA) into an intumescent flame retardant (IFR) to flame retardant
PP.
The results show that the compounded IFR greatly improves the flame retardant performance
of PP.
When IFR is composed of 80.
3% (mass fraction, the same below) APP, 13.
0% MA and 6.
7% CA, IFR has the most effective flame retardancy
for PP systems.
When the IFR addition amount in PP is 30%, the limiting oxygen index (LOI) of the flame retardant PP system reaches 35.
5%; When the IFR addition amount is only 25%, the flame retardant performance of the flame retardant PP system also passes UL-94V-0 grade, and the LOI value reaches 32.
5%.


Bao Wenbo of Northeast Forestry University used molecular sieve as a synergistic agent and compounded it with an intumescent flame retardant (RTB-IFR) and other additives without synergistic agent to make intumescent IFR for flame retardant
PP.
The results show that the flame retardant effect of 4A and H-BETA zeolites is better than that of 13X and ZSM zeolites.
Molecular sieve has little effect on the mechanical properties of the material; The flame retardant added to molecular sieve changed the thermal degradation process of IFR and IFR-PP, improved the amount of carbonization at high temperature, the thermal stability and thermal insulation of the carbon layer, and improved
the flame retardant performance of IFR-PP.

Zhang Haili of South China University of Technology et al.
used resorcinol bis(diphenyl) phosphate (RDP) as a flame retardant synergist and mixed with melamine pyrophosphate (MPP) and pentaerythritol (PER) to prepare halogen-free flame retardant PP
with good flame retardant performance.
The results show that RDP and IFR have obvious synergistic flame retardant effects
.
When the RDP mass fraction is 5.
0%, the LOI of flame retardant PP increases from 28.
5% to 30.
5%, and UL-94 is increased from V-1 to V-0
.
In addition, the notched impact strength of the system has also been greatly improved
.

Surface modification technology

Surface modification technology is one of the key technologies to improve the application performance of
intumescent flame retardants.
The surface state of the intumescent flame retardant can be changed through the chemical reaction and surface coating treatment of various surface modifiers and the surface of the intumescent flame retardant particles, which can improve its surface activity and produce new physical and chemical functions on its surface, thereby improving the affinity between the flame retardant and the polymer matrix, which is conducive to the dispersion of the flame retardant in the matrix, and improves the processing performance and mechanical properties of
the material.
Commonly used surface modifiers mainly include silane coupling agents, titanates, phosphorus-containing ferroates and other compounds with more complex structures, which can also be treated
with saturated or unsaturated fatty acids with simple structure.

Yin Changyu et al.
of School of Materials Science and Engineering, South China University of Technology, used phosphorus-containing titanate coupling agent (PTCA) to surface modify the intumescent IFR composed of MPP and PER compound, and used it to prepare flame retardant PP.

The results show that PTCA effectively improves the compatibility between IFR and PP matrix, and improves the mechanical properties and flame retardant properties
of PP/IFR blends 。 When the PTCA dosage was 1.
0%, the tensile strength and notched impact strength of the blend were 27.
3MPa and 3.
2kJ/m2, which were 18.
7% and 6.
7% higher than the unmodified PP/IFR, respectively.
The LOI has been increased from 28.
5% to 31.
5% of unmodified PP/IFR, and has passed UL94 V-0 rating; In addition, the thermal stability of the blend is also significantly improved, and the carbon residue rate at 700°C is increased from 8.
2% to 12.
1%
of unmodified PP/IFR.

Microencapsulation is also an important method of surface modification, which refers to the homogeneous coating of tiny solid particles, droplets or bubbles
with a coated film or shell material.
For filler flame retardants, the essence is a surface modification method
that covers the surface of the particles with a homogeneous film with a certain thickness, so as to increase the dispersion of the filler and improve the flame retardant efficiency.
The encapsulation modification of intumescent flame retardants by microencapsulation technology can improve the moisture absorption of intumescent flame retardants, prevent the migration and drift of effective flame retardant components, and further improve the compatibility between intumescent flame retardants and the matrix, thereby improving the performance
of flame retardant materials.
Liu Yanming of the New Technology Office of Shanghai Research Institute of Chemical Industry synthesized a new halogen-free intumescent flame retardant ANTI-6 in phosphorus-nitrogen system through microencapsulation technology, and used it to modify PP flame retardant
.
The results show that with the increase of the mass fraction of flame retardant ANTI-6, the LOI of flame retardant PP gradually increases, but when the mass fraction of flame retardant is greater than 35%, the LOI remains basically unchanged
.
At the same time, with the increase of the mass fraction of flame retardants, the flame retardant performance is improved, but the tensile strength, impact strength and electrical properties are reduced, and the elongation is slightly improved
.

Nano-intercalation technology

The PP polymer chain intercalation enters the organic montmorillonite (OMMT) layer, and when the composite is subjected to strong heat, the OMMT sheet will form a high-temperature barrier to prevent the transfer
of hot substances.
Li Ying, Department of Materials Science and Engineering, Beijing Technology and Business University, et al.
prepared PP/intumescent flame retardant/organic montmorillonite (PP/IFR/OMMT) flame retardant composites
by fused intercalation method.
The results show that the PP polymer chain intercalation enters the OMMT layer to form an intercalated composite material, and OMMT and IFR have obvious synergistic flame retardancy
.
When the amount of OMMT is 2 parts, the LOI of the composite material reaches 31%, which is 30% higher than that when IFR is added alone.
Compared with pure PP, the carbon residue rate of composite materials is significantly improved
.
With the increase of OMMT content, the tensile strength, flexural strength and impact strength of the composite materials showed a trend of first increasing and then decreasing, and when the OMMT content was 3 parts and the IFR content was 22 parts, the tensile strength, flexural strength and impact strength of the composite materials reached the maximum value
.

Liu Hui of the School of Materials Science and Engineering, Southwest University of Science and Technology, et al.
used nitric acid as acidifying agent, sodium chloride as sodium agent and cetyl trimethylammonium bromide (HDTMA.
Br) as intercalation agent, and prepared organic retort stone (OREC)
by "acidification-sodium-intercalation" process.
PP/intumescent IFR/OREC flame retardant composites
were prepared by fused intercalation method.
The results show that the PP polymer chain intercalation enters the organic reco-retort layer to form an intercalated composite material
.
OREC and IFR have obvious synergistic flame retardancy
.
When the amount of OREC is 2%, the LOI of the composite material reaches 31%, which is 4.
8% higher than that of IFR alone.
Compared with pure PP, the carbon residue rate of the composite is increased from 6% to 22%.

SEM analysis showed that the carbonization of the composite material was good
.

There are problems and suggestions for development

Intumescent flame retardants are in line with the future research and development direction of flame retardants for PP due to their excellent flame retardant performance and low smoke, low toxicity and
non-corrosive gas during combustion 。 However, it also has the following problems: (1) the amount of addition is large, and the compatibility between the three components of IFR and IFR and PP is poor, which reduces the mechanical properties, electrical properties and insulation properties of PP, especially the tensile strength and impact strength decrease significantly; (2) Serious hygroscopicity, such as flame retardant systems with APP, MEL, and PER as the main components, alcohololysis is easy to occur between the components, resulting in a decrease in the water resistance of flame retardant PP; (3) The relative molecular weight is low, so that the thermal stability and migration resistance of flame retardant PP are poor, and eventually the physical and mechanical properties of flame retardant PP are reduced and the appearance is worse
.
Therefore, in the future, the performance of composite materials should be improved by surface modification, ultra-refinement, microencapsulation and other treatment technologies of intumescent flame retardants to improve flame retardancy and improve the thermal stability and moisture absorption of
composite materials.
It is expected that in the near future, intumescent flame retardants will become the main development direction
of PP flame retardants.