Issue 36/2014 - Development of halogen-free flame retardant PA

Development and research of halogen-free flame retardant PA

□ Special and Engineering Plastics (Suzhou) Co.
, Ltd.
Wei Haoran

PA is one of the most used engineering plastics in the electrical and electronic industry, including halogen/antimony flame retardant PA and halogen-free flame retardant PA.

Since entering the 21st century, the development of flame retardant PA has shown the following trends: (1) more halogen-free flame retardant systems are used, while improving flame retardant efficiency and reducing the negative impact
of flame retardants on substrate performance.
(2) Improve the thermal stability
of flame retardants.
In order to meet the needs of future development, flame retardants should be able to withstand higher temperatures (such as processing at 300~310 °C) and stress
.
(3) Give flame retardant PA better processing performance and higher fluidity
.
In order to realize the miniaturization and thinning of components and adapt to the new PA processing technology, the flame retardant used in flame retardant PA should be able to improve the fluidity
of PA.
However, fluidity and heat distortion temperature are often contradictory and need to be comprehensively balanced
.
(4) It is easier to recycle and reuse
.
Obviously, environmentally compatible flame retardant PA has more market advantages
.

Halogen-free flame retardants for PA include metal hydroxides, red phosphorus (mostly microencapsulated products or masterbatches), ammonium polyphosphate (APP), melamine (MA), MA derivatives (including their compounding systems), and reactive phosphorus-based flame retardants
.
In addition, PA inorganic nanocomposites should also be attributed to halogen-free flame retardant PA
.

Melamine derivative flame retardant PA

1.
The corrosive and toxic gas and smoke generation amount
MA of flame retardant PA is weak alkali, PH value is 8.
1, so it can react with most inorganic acids and organic acids to form salts, such as borate (MB), phosphate (MP), cyanurate (MC), polyphosphate (MPP), sulfate (MS), etc.
, of which MC and MPP are especially suitable for flame retardant PA, the latter of which has a particularly good
flame retardant effect on glass fiber reinforced PA66.

PA flame retardant with MC or MPP has a low generation of toxic and corrosive gases and fumes, and is less
corrosive itself.
For example, the glass fiber reinforced PA66 with MC or MPP flame retardant, the corrosion of stainless steel in the melt state (300 °C) is only 3% and 14% of the flame retardant of the bromine/antimony system, respectively, and the amount of smoke and toxic and corrosive gases generated when burned with MPP flame retardant PA6 is much lower than that of the flame retardant PA6
of the bromine/antimony system.

2.
Flame retardant properties and mechanical properties of flame retardant PA The Dutch DSM company found that compared with the flame retardant PA66 of the bromine/antimony system, the mechanical properties and flame retardant properties
of the former are slightly better than the latter, and the smoke density (whether open or smolder) is about 1/4
of the latter.
In addition, the tracking index (CTI) of the former is also higher
than that of the latter.
For glass fiber reinforced PA66, MPP flame retardant and bromine/antimony flame retardant can be compared with the performance of the two, or the former is slightly superior
.
However, MPP has a large impact on the elongation and impact strength of PA66 (enhanced and unenhanced) (as does bromine/antimony systems) and sometimes requires the use of impact modifiers
.

3.
The mechanism of MC flame retardant PA6 and flame retardant PA66
MC is higher than PA66
flame retardancy.
In order to make the material reach U194 V-0 grade, the MC addition amount required for PA66 is 5%~10%, and PA6 requires 10%~15%.

In order to explain this phenomenon, the flame retardant mechanism of MC flame retardant PA6 and PA66 has been studied in detail recently, and it is found that the flame retardant efficiency of MC for PA66 is higher than that of PA6, mainly because the degradation chemical mechanism of PA66 and PA6 is different, that is, the reaction of MC with the degradation products of PA66 and PA6 is different
.
The degradation product of PA6 was caprolactam and the degradation product of PA66 was cyclopentanone
.
At 350~450 °C, caprolactam reacts with MC to form an oligomer with various end groups (such as -C=N); At the same temperature, cyclopentanone is much more active than caprolactam, it can self-condense, and can also interact with melamine decomposition products (such as NH3, NH=C=NH) and cyanuric acid decomposition products, thus forming a very complex reaction product system
.
In PA66, the presence of MC increases insoluble degradation products, but not in PA6
.
Because the degradation product of PA66, cyclopentanone, can be cross-linked with the degradation products of MC (mainly NH3) to form a high molecular weight non-flammable product, while the degradation product of PA has low activity and does not crosslink.


Tehe Engineering Plastics (Suzhou) Co.
, Ltd.
is an independent subsidiary of Heshibi Chemical, with a number of fully imported KraussMaffei Belstorf twin screw extrusion lines and perfect experimental equipment, the company focuses on PA6, PA66, PP, ABS, PBT and other substrates of strengthening, flame retardant, dyeing, wear resistance, heat stability and other direction modification, is committed to the development and production of high-performance modified plastics and alloys
。 In recent years, the company has carried out a lot of research and performance comparison on halogen-free flame retardant PA materials, and finally developed special grades PA6 4500FR2NC and PA66 4800FR2 NC, these two grades are MC flame retardant, the product has high flow (good surface), high heat resistance, high wear resistance, good chemical resistance and other properties, can meet the requirements of electronic products, at present, a number of manufacturers have carried out trials, product appearance is excellent, performance qualified
.

4.
Effect of MPP on thermal degradation of PA Recently, researchers have used solid-state NMR technology and X-ray diffraction to study the effect of

MPP on thermal degradation of PA66 and PA6.
The mixture of MPP and PA was heated at 350°C and 450°C for different times, and then the X-ray profile of the heated residue and the "PNMR and CNMR profiles
" were determined.
The results show that MPP can cause PA66 to be severely crosslinked and PA6 to depolymerize greatly, which also proves that MPP has flame retardant effect
on PA66 and PA6.
After MPP is blended with PA66 and PA6, MPP is depolymerized above 350 °C to form phosphoric acid and carbon
.
The chemical structure of the carbon formed in the flame retardant performance test is very similar to the residual carbon structure formed after heating the mixture of MPP and PA at 350°C for 90 minutes, and this type of carbon contains moderately degraded PA fragments
.

Reactive phosphorus flame retardant PA

The reactive phosphorus-containing monomer is grafted into the PA chain by copolymerization, which can give the PA a more permanent flame retardancy without the risk
of seepage.
For example, the introduction of triaryl phosphine oxide (TPO) monomer into PA can produce phosphorus-based reactive flame-retardant PA.

As a PA segment, TPO can make the material have excellent flame retardancy, good thermal oxidation stability and hydrolytic stability, and high charring rate
.
At the same time, the glass transition temperature of such PAs is also high
.
Because phenylphosphine oxygen bonds are not coplanar, TPO-containing PAs often form amorphous amorphous structures
.
However, if the content of TPO in PA is controlled, a semi-crystalline copolymer can be formed
.
For example, bis(4-hydroxyphenyl)phenylphosphine oxide (BCPPO) copolymerized with PA66 salt and hexamethylenediamine can be used to obtain PA66 copolymer containing TPO in the main chain, of which the molar fraction of TPO can reach 30%.

The higher the TPO content in the copolymer, the higher the charring rate and the better
the flame retardancy.
In air at 750°C, the charcoal formation rate of ordinary PA66 is 0, while the carbonization rate of PA66 copolymer containing 10%, 20% and 30% TPO is 3.
8%, 7% and 8.
5%,
respectively.
However, TPO slightly reduces the starting weightlessness temperature
of the polymer.
For example, in air, the starting weight loss temperature of a 30% TPO copolymer is 402°C, while a normal PA66 without TPO is 410°C
.

The mass loss rate of the TPO-containing PA66 copolymer measured by the cone calorimeter showed that the mass loss rate vs.
time curve of the copolymer containing 10% TPO was similar to that of PA66, but if the TPO content in the copolymer reached 20% or 30%, the mass loss rate decreased
.
At the same time, the heat release rate and combustion heat of all TPO-containing copolymers are greatly reduced
compared with PA66.
This shows that the flame retardancy of the copolymer is much improved
compared to PA66.
However, the amount of carbon monoxide produced increases
when the copolymer is burned.

XPS analysis proved that PA66 copolymer containing 20% TPO had a significantly higher
phosphorus content on its surface after 5 minutes of exposure to air at 540°C.
This shows that phosphorus mainly plays a flame retardant role
in the condensed phase.
It is hypothesized that most of the phosphorus in the copolymer remains in the condensed phase as a carbon layer component and may be present as phosphate, while only a small amount of phosphorus enters the
gas phase.

PA inorganic nanocomposites

Polymer/inorganic nanocomposites are known as a new generation of flame retardant polymer materials
.
Since the advent of PA6/LS (layered silicate) in the 80s of the 20th century, PA/LS has been favored and has become a typical example
of organic-inorganic molecular composites.
In 1997, Jeflery et al.
used a cone calorimeter to determine the flame retardant performance of PA6/LS, when the LS content in PA6/LS is only 2% and 5%, the material has good thermal stability, and its peak heat release rate (PHRR) is reduced by 32% and 63%, respectively, and does not damage other properties
of the material.
Correspondingly, the mass loss rate (MLR) of the material is also reduced by 45%~65%.

This shows that when a small amount of LS is dispersed in PA6 at the nanoscale, it contributes significantly to the flame retardancy of the material
.
It is believed that the improvement in flame retardancy of such nanocomposites is due to the flame retardancy of the condensed phase rather than the vapor phase flame retardancy
.
However, according to the research results of the author's laboratory, dispersing 3%~5% of inorganic substances (such as montmorillonite) in nanoform in PA cannot significantly increase the OI and UL94 flame retardant levels
of PA.
However, PA/LS can be compounded with conventional flame retardant systems to further improve the flame retardant properties of
the material.

summary

For flame retardant PA, bromine flame retardant products are still the mainstay, but due to the inherent drawbacks of bromine and antimony flame retardant systems and the increasingly strict requirements of human beings for environmental protection, appropriately accelerating the halogen-free process of PA flame retardant is a goal that people have been committed to for a period of time, especially in the field of
electronic and electrical 。 In the future, it is advisable to strengthen research in the following aspects: (1) the mechanism of halogen-free flame retardants, especially phosphorus-nitrogen and intumescent flame retardants flame retardant PA, including the flame retardant mechanism of various PAs; (2) Synergistic effect of a variety of halogen-free flame retardants in PA; (3) Improve the technical ways of flame retardant to enhance the toughness of PA; (4) Charcoal-forming flame retardant of halogen-free flame retardant PA; (5) Synthesis of new high-efficiency halogen-free flame retardants
.
At the same time, the existing laboratory results should be improved and improved, and further promoted to industrialization
.