Issue 24, 2012 - Research and application of phosphorus-based flame retardants for epoxy resins

The cured object of epoxy resin has good physical and chemical properties, and can be made into coatings, composite materials, casting materials, adhesives, molding materials and injection molding materials, which are widely used in automobiles, civil engineering, electronics, etc.
, and also put forward higher requirements
for the flame retardant performance of epoxy resins.
Flame retardants for epoxy resin are roughly divided into additive flame retardants and reactive flame retardants, of which reactive is the main
one.
Due to the potential environmental hazards of halogen-containing flame retardants, organophosphorus-based flame retardants have been widely studied and applied
in the flame retardant of epoxy resin because of their high efficiency and low toxicity.
There are two main types of organic phosphorus-containing reactive flame retardants: first, through the reaction of active functional groups with epoxy groups, phosphorus-containing groups are introduced into the epoxy resin chain; Second, a new phosphorus-containing curing agent is synthesized to cure epoxy resin, and phosphorus is introduced into the epoxy matrix
.

Phosphonitrile compounds are compounds with P and N as the basic framework, with excellent properties of inorganic and organometallic polymer compounds, generally with low glass transition temperature, good thermal stability, and can withstand high temperatures
of 250 °C for a long time.
The phosphorus-nitrogen synergistic flame retardant effect gives phosphatonitrile compounds good flame retardant properties, so new epoxy resins and curing agents containing phosphatonitrile structure have been widely studied and applied
in the flame retardancy of epoxy resins.

1.
DGEBA flame retardant resin

In 2009, Moroccan researcher Mustapha El Gouri et al.
synthesized hexa-(epoxypropyl)cyclotriphosphatenitrile (HGCP), HGCP as a reactive flame retardant has good compatibility with the commercialized bisphenol A type glycidyl ether epoxy resin (DGEBA), and then uses 4',4-diphenylaminomethane cured blended epoxy resin
。 The analysis results show that the blending and curing system has good thermal stability, and the residual carbon amount is greatly improved compared with that of non-flame retardant DGEBA.
The resulting carbon layer effectively insulates oxygen and heat, and exhibits excellent flame retardant performance when HGCP content reaches 20%, reaching UL94 V-0 rating
.
HGCP has potential applications in electrical and electronic products
.

In the same year, Liu R et al.
replaced the chlorine atom on hexachlorocyclotriphosphonitrile with 4-hydroxybenzaldehyde, and then reduced the aldehyde group to obtain hexas(4-hydroxymethylphenoxy)cyclotriphosphonic PN-OH.

PN-OH and DGEBA react under the action of Ph3P catalyst to synthesize a new phosphonic nitrile epoxy resin, which is cured with 4,4'-diaminodiphenylmethane (DDM), dicyandiamide, thermoplastic phenolic resin and terephthalic dianhydride as curing
agent, respectively.
The results showed that all four cured compounds exhibited excellent flame retardant properties, LOI values above 30, and UL-94 V-0 ratings
.
This environmentally friendly halogen-free flame retardant epoxy resin has a wide range of application prospects
in the field of electronic and electrical engineering.

Huan Liu et al.
synthesized two Mannich alkaline phosphorus-containing compounds PEDA and PTTA, PEDA and PTTA as a novel phosphorus-containing curing agent, and introduced phosphorus into the
epoxy resin polymer skeleton through a curing reaction 。 The glass transition temperature (Tg) value of DGEBA resin cured by PEDA and PTTA is higher than that of ethylenediamine, triethylenetetramine cured DGEBA resin due to the introduction of phosphorus, and its carbon residue at 700 °C is as high as 23wt%~27wt%, and the high residual carbon generation gives the flame retardant curing system excellent flame retardant performance, with an LOI of 28~30
.

2.
New halogen-free flame retardant with phosphaphenanthrene structure

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives have excellent thermal stability
than general uncyclized organophosphates due to the special phosphaphenanthrene structure in the molecule.
A large number of literature reports that DOPO derivatives have the advantages
of high carbon content, halogen-free, low smoke, non-toxicity, non-migration and flame retardant and long-lasting.
Because there is an active hydrogen on the DOPO oxygen ring, it is easy to react with electron-deficient compounds, so it is usually used to prepare phosphorus-containing reactive intermediates, and the structure of phosphaphenanthrene has been widely cited
in the research of new halogen-free flame retardants in the past ten years.

Qian Lijun, Department of Materials Science and Engineering, Beijing Technology and Business University, etc.
synthesized a new type of additive flame retardant HAP-DOPO with phosphonitrile and phosphatifene bifunctional groups, blended with flame retardant epoxy resin DGEBA, and cured
by 4,4'-diaminodiphenyl sulfone (DDS).
When the phosphorus content in the resin is 1.
5% and the nitrogen content is 2.
62%, its oxygen index reaches 35.
2 and the vertical combustion reaches V-0 grade
.

Wang Xiaojian et al.
used DOPO to react with phenolic epoxy resin F-5 l to obtain a new phosphorus-containing epoxy resin, and then used DDS as a curing agent
.
When the phosphorus content in the cured substance is 2wt%, the flame retardant effect can reach UL-94 V-0 and has high mechanical strength; The solidified product Tg is 126 °C and the water absorption rate is only 0.
24%.

This resin is used in the manufacture of electronic products such as copper clad laminate, and is highly operable and low
cost.

Tang Junjie and Tang Anbin reported a phosphorus-nitrogen synergistic flame retardant PN-FR
containing DOPO and s-triazine bifunctional structure.
The flame retardant is initially applied to the flame retardant of epoxy resin insulation laminate, and when the amount of addition is 15wt%, the oxygen index of the insulation laminate reaches 36.
2, which passes the UL-94 V-0 level test
.

Taiwanese researcher Ching Hsuan Lin et al.
used DOPO, 4,4'-dihydroxybenzophenone and phenol (aniline) as raw materials to synthesize two new epoxy resin curing agents 9,10-dihydro-9-oxa-10-oxa-tris(4-hydroxyphenyl)methane and 9,10-dihydro-9-oxa-10-phospha-10-phosphalidene-10-oxo-(4-aminobenzene)-bis(4-hydroxyphenyl)methane.
Used to cure DGEBA, dicyclopentadiene epoxy resin (HP-7200) and linear phenolic epoxy resin (CNE).

At a phosphorus content of about 2.
5 wt%, the LOI can reach 35, and vertical combustion reaches UL-94 V-0 rating
.

Sun Dechao and Yao Youwei of the Institute of Advanced Materials, Shenzhen Graduate School of Tsinghua University generated imine by condensation of aromatic aldehydes and aromatic amines, and then combined by DOPO, and synthesized symmetrical diamine I and diphenol II and III by DOPO addition
.
I.
, II.
, III.
and diaminodiphenylmethane co-curing epoxy resin DGEBA, curing resin has excellent
flame retardant performance and thermal stability.
Due to the phosphorus-nitrogen synergistic flame retardant effect, when the phosphorus content in the cured system is only 1.
0 wt%, the flame retardant performance test reaches UL-94 V-0, and the LOI is higher than 35.
6
.

C S Wang et al.
synthesized bis(3-hydroxyphenyl) phenyl phosphate (BHPP) as a reactive flame retardant flame retardant DGEBA
.
BHPP/DGEBA has better flame retardant properties and thermal stability than conventional bromine-containing flame retardant epoxy
resins.
When the phosphorus content is 1.
5% (compared to 10% bromine), it reaches UL 94 V-0 and there is no smoke and toxic gas release
.

Hua Ren et al.
used phenylphosphonyl dichloride to react with resorcinol to synthesize bis(3-hydroxyphenoxy) phenyl phosphate (BPHPPO), and then reacted with epichlorohydrin to obtain epoxy resin monomer (BPHPPO-EP), using DDS as curing agent
.
The experimental results show that the cured substance has better thermal stability than ordinary bromine-containing flame retardant epoxy resin, and its extremely high temperature carbon residual rate and limiting oxygen index of up to 34 prove that BPHPPO-EP is a halogen-free phosphorus-containing epoxy resin
with excellent flame retardant effect.

The Institute of Organic Synthesis, College of Chemistry, Central China Normal University, has been committed to the synthesis and application of new flame retardant compounds, and its screening of compounds with high flame retardant performance for epoxy resin materials shows good flame retardant properties
.
The results showed that 1-oxo-4-hydroxymethyl-1-phosphon-2,6,7-trioxabicyclo(2,2,2) octane derivatives and phosphonitrile derivatives had good thermal stability, and most compounds showed excellent flame retardant properties
when the addition amount was 10%.
The epoxy resin cured to which this flame retardant material is added has a high carbon residue and low smoke generation, showing a good flame retardant effect
.

3.
Conclusions

Epoxy resin is widely used in the life and production sectors, which makes people put forward higher requirements for its own flame retardancy, which greatly promotes the research and development of
phosphorus-based flame retardants.
In recent years, a lot of work has been carried out in the field of organophosphorus flame retardant epoxy resin in China, and a variety of new phosphorus-containing halogen-free high-efficiency flame retardants have been developed, but most of the research results are limited to the research stage and have failed to achieve industrial production and wide application
in materials.
Therefore, the process optimization of high-efficiency, low-toxicity, environmentally friendly organophosphorus flame retardants and their effective application in materials will become the focus
of research.

Background link>>>
In recent years, the application field of polymer materials has gradually expanded with the rapid development of science and technology, and the use of flammable materials is also increasing
at an unprecedented rate.
Due to the flammability of polymer materials, the application and research of flame retardants have received extensive attention
worldwide.
The United States, Europe, Japan and other Asia-Pacific regions are the world's four major flame retardant markets, in 2005~2008 the average annual growth rate of global flame retardants was 4.
1%, while other Asian countries and regions reached an average annual growth rate of 13%, is the fastest growing region
in the world.
It is predicted that the global consumption and sales of flame retardants in 2014 can reach 2.
62 million tons and 6.
1 billion US dollars (about 40 billion yuan)
respectively.
At present, the search for high-efficiency, low-toxicity, halogen-free and environmentally friendly flame retardants has become the focus of
today's flame retardant research.
Organophosphorus flame retardants are favored
by people because of the advantages of low smoke, low toxicity, halogen-free, and low addition.
At the same time, the intumescent flame retardant compounded with ammonium polyphosphate, pentaerythritol or aluminum hydroxide has attracted the attention
of researchers because of the synergistic flame retardant effect of each component.

The flame retardant performance of phosphorus is second only to halogens, and since the 80s of the 20th century, phosphorus-based flame retardants have been used in the field of polymers, triggering a boom
in research and development.
Phosphorus-based flame retardants have the advantages of high flame retardant efficiency, low toxicity, non-corrosiveness and good compatibility with polymer materials, so the research on inorganic phosphorus and organic phosphorus is becoming more and more extensive
.
Inorganic phosphorus-based flame retardants mainly include phosphorus-ammonium flame retardants
such as red phosphorus, phosphate and ammonium polyphosphate.
Organophosphorus-based flame retardants mainly include phosphate esters, phosphonates, phosphite, organophosphates, phosphine oxides and phosphorus-nitrogen compounds, and the most widely used are phosphate esters and phosphonates
.
Because the use of halogen-free flame retardants is gradually limited due to environmental protection standards, according to literature reports, some countries require that bromine-based flame retardants
not be used in 2020.
Under the severe situation faced by bromine-based flame retardants, the demand for organophosphorus flame retardants in the flame retardant market is considerable
.