Research on flame retardant properties of HDPE wood-plastic composite materials

Wood-plastic composite material is a green material that complies with national environmental protection and waste recycling policies. It is cheap, has a wide range of raw material sources and has beautiful appearance. It has been widely used in industries such as furniture, building materials, automobiles and transportation. Thermoplastic wood-plastic composites have the advantage of being easy to process and form. Polyolefin/wood flour (WF) composites have been a hot topic in research reports in recent years.

However, general-purpose polyolefin plastics (such as polyethylene, polypropylene, etc.) and polyolefin/WF composites are extremely flammable materials, which greatly limits their application. Therefore, improving the flame retardant properties of wood-plastic composites is an important step. key to broadening its application areas. Phosphorus-based intumescent flame retardants are the main direction for the development of halogen-free flame retardant technology, and phosphorus-containing flame retardants are usually used as acid sources.
Inorganic phosphorus flame retardants (such as ammonium polyphosphate) can play a very good flame retardant role in wood-plastic composite materials. However, such flame retardants have low degree of polymerization, poor thermal stability, and are easy to absorb water and migrate. In most cases, they need Only by compounding with other flame retardants can the ideal flame retardant effect be achieved. Commonly used compound flame retardants have poor compatibility with the resin matrix.
Ammonium polyphosphate
Compared with compound flame retardants, single-molecule intumescent flame retardants are prepared through chemical methods. The acid source, carbon source and gas source are chemically bonded on the same macromolecule. The higher degree of polymerization makes it have higher It has excellent thermal stability and water resistance, and the compatibility of flame retardants and resin matrix can be improved at the same time through the structural design of macromolecules. This is also the development direction of new flame retardants.
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At present, there are few research reports on the flame retardancy of polyolefin wood-plastic composites using phosphate ester copolymers. In this work, 1-oxyphospha-4-hydroxymethyl-2,6,7-trioxabicyclo[2.2.2]octane (PEPA), polyphosphoric acid (PPA), 1,4-butanedi Using alcohol diglycidyl ether, bisphenol A epoxy resin and melamine as raw materials, a multifunctional phosphate ester copolymer was synthesized through a condensation polymerization method. It was used as a flame retardant and a melt blending method was used to prepare flame-retardant high-density polyethylene ( HDPE)/WF wood-plastic composite materials, the effect of phosphate ester copolymer content on the mechanical properties, thermal properties and flame retardant properties of the composite materials was investigated.

It can be seen from Figure 1 that: 3410cm-1 is the hydroxyl stretching vibration peak generated after the ring-opening reaction of the epoxy group, the multi-peak near 2900cm-1 is the stretching vibration peak of C—H, and 1670cm-1 is C= The stretching vibration peak of N, 1610, 1510, 1460cm-1 is the characteristic peak of the benzene ring skeleton vibration, the stretching vibration peak of P=O is at 1309cm-1, the stretching vibration peak of C—N is at 1250cm-1, 1025cm- 1 is the stretching vibration peak of P-O-C, and the strong peak at 855cm-1 is the characteristic absorption peak of caged phosphate. The above peak spectrum data combined with the relative molecular mass data can prove that the phosphate copolymer was successfully synthesized.
Flame retardant properties of HDPE/WF wood plastic composites

It can be seen from Figure 2 and Table 2 that the maximum heat release rate of the composite material without adding flame retardant is as high as 411kW/m2. After adding flame retardant, as the flame retardant content increases, the maximum heat release rate gradually decreases, and the retardant When the fuel content is 20% (w), the maximum heat release rate decreases by about 69%, indicating that the phosphate ester copolymer has a good flame retardant effect and effectively reduces the maximum heat release rate during the combustion process.

As the flame retardant content increases, the ignition time shortens. This may be due to the phosphoric acid generated during the heating process of the phosphate ester copolymer, which catalyzes the dehydration of hemicellulose, lignin and other components in the poplar wood powder. The dehydrated poplar Wood powder is more likely to be ignited; as the flame retardant content increases, the total heat release gradually decreases, and when the flame retardant content is 20% (w), it decreases by about 59%.
At the same time, the ratio of ignition time to the peak heat release rate (FPI) increased from 0.097 to 0.250. FPI is used to measure the ease of combustion of the material after ignition. The higher the FPI, the better the flame retardant performance of the material. Therefore, the significant increase in FPI also further illustrates that the flame retardant performance of composite materials has been significantly improved.
Mechanical properties of HDPE/WF wood-plastic composite materials

It can be seen from Table 3 that as the flame retardant content increases, the tensile strength of the flame-retardant HDPE/WF wood-plastic composite material first increases and then decreases.
The compatibilizer EC-603D is a maleic anhydride-ethylene copolymer. The side chain contains a large number of anhydride groups, which can undergo amidation and esterification reactions with the surface aminated poplar wood powder and the hydroxyl groups on the flame retardant molecular chain. Play the role of reaction compatibilization.
Strong interface interaction can effectively transmit tensile stress and is less likely to cause stress concentration. The rigid epoxy resin component can improve the rigidity and tensile strength of the composite material after reaction and cross-linking.
Although there is a heterogeneous nucleation effect, due to the effect of reaction compatibilization, it becomes difficult for HDPE macromolecular chains to enter the crystal region, which increases the proportion of amorphous regions to a certain extent, and the tensile strain at break increases.
When the flame retardant content exceeds 15% (w), the tensile properties of the composite materials show a downward trend. This is because the reaction compatibilization effect in the system is limited. Excessive flame retardant content makes the compatibility of the system worse and the interface The effect decreases, and interface defects are easily formed inside the system under the action of tensile stress, resulting in a decrease in tensile strength.

It can be seen from Figure 3 that the LOI of the composite material without adding flame retardants is 23.5%, which is a flammable material because the flame retardant properties of HDPE and poplar wood powder are poor.
As the flame retardant content increases, the LOI of the flame-retardant HDPE/WF wood-plastic composite material gradually increases. When the flame retardant content is 20% (w), the LOI is 31.5%, which is a flame-retardant material. At the same time, the vertical combustion level reaches It has achieved UL94V-0 level, and its changing pattern is basically consistent with the cone calorimetry analysis results.

It can be seen from Figure 4 that the HDPE/WF composite material without added flame retardant (i.e. sample 1) has very little residue. After adding 20% (w) phosphate copolymer flame retardant, the composite material (i.e. sample 1) has 5) The carbon residue increases significantly after combustion, and the overall appearance is an expanded carbon layer. The formed carbon layer can isolate the internal combustibles from external oxygen, hinder heat transfer, limit the diffusion of small molecule products of combustion and decomposition, and act as a good barrier. combustion effect.
Thermal properties of HDPE/WF wood plastic composites

It can be seen from Figure 5 that the initial decomposition temperature of HDPE is about 420°C, and it is basically completely decomposed at 470°C. The mass loss rate of the entire process is relatively fast.
The slight mass loss of the phosphate ester copolymer before 370°C may be due to the decomposition of residual small organic molecules. The first stage lasts from approximately 380°C to 430°C. The main products are melamine and polyphosphoric acid. At this time, part of the bisphenol A structure is also begin to decompose;
The second stage lasts to 485°C. At this stage, the copolymer main chain decomposes violently, and along with the condensation of melamine and polyphosphoric acid, a high molecular weight P-N-O-containing polymer is formed. At the same time, part of the melamine decomposes and releases non-flammable gases. ;
The third stage is relatively gentle, mainly because the P-N-O-containing polymer catalyzes the carbonization of oxygen-containing segments and releases non-flammable gases such as water vapor and ammonia. In terms of the initial decomposition temperature, its thermal stability is obviously high. For commonly used phosphorus flame retardants.
The initial decomposition temperature of HDPE/WF wood-plastic composites without flame retardants is around 290°C and continues to nearly 400°C, mainly due to the decomposition of hemicellulose, cellulose and lignin of the wood powder itself and the loss of bound water. ;
The second stage lasts to about 435°C, which is the further dehydration and carbonization of the wood powder decomposition products, followed by the violent decomposition of the HDPE matrix. The decomposition rate of the matrix is not much different from that of pure HDPE.
HDPE material
After adding 20% (w) phosphate ester copolymer for flame retardant modification, the initial decomposition temperature of the flame retardant HDPE/WF wood-plastic composite material decreased. This may be caused by the residual organic molecules in the flame retardant. ;
The second stage lasts from 340℃ to 450℃. Compared with the composite material without adding flame retardant, the mass loss process in this stage is relatively gentle. It may be that the flame retardant promotes the dehydration of wood powder into charcoal, forming more charcoal layer;
The third stage is mainly the decomposition of the HDPE matrix, with an initial decomposition temperature of 440°C. Compared with pure HDPE and composite materials without added flame retardants, the mass loss rate at this stage is significantly reduced, and the addition of phosphate ester copolymer effectively delays Reduce the decomposition of HDPE and reduce the decomposition rate.
At the same time, the final carbon residue rate of the flame-retardant HDPE/WF wood-plastic composite material increased from 15.1% (w) to 36.6% (w), an increase of 21.5 (w), further proving the flame-retardant effect of the phosphate ester copolymer.
According to the classic flame retardant mechanism of intumescent flame retardants, combined with TG analysis, it can be seen that during the combustion process, the polyphosphoric acid component in the phosphate ester copolymer can serve as the acid source, melamine as the gas source, and the bisphenol A component as the carbon source. , and then form a single-molecule three-source macromolecular flame retardant.
In addition, the bound water in poplar wood powder and itself can also serve as part of the gas source and carbon source, thereby effectively isolating oxygen and heat during combustion and promoting combustion into char, significantly improving the flame retardant properties of the composite material.

In summary, the following conclusions are drawn:
PEPA, PPA, 1,4-butanediol diglycidyl ether, bisphenol A-type epoxy resin and melamine are used as raw materials to synthesize phosphate ester copolymer, which is used as a flame retardant to perform flame retardant modification of HDPE/WF composite materials. sex.
Compared with conventional phosphorus-based flame retardants, phosphate ester copolymers that integrate acid sources, carbon sources, and gas sources have higher thermal stability. As the content of phosphate ester copolymers increases, the tensile strength of HDPE/WF composites increases. The intensity shows a trend of increasing first and then decreasing.
Phosphate ester copolymer can significantly improve the flame retardant properties of composite materials. Compared with non-flame retardant composite materials, when the phosphate ester copolymer content is 20% (w), the maximum heat release rate and total heat release of the composite material are The reduction was about 69% and 59% respectively, the LOI was 31.5%, the vertical combustion level reached UL94V-0 level, the residual carbon content increased by 21.5% (w), and the flame retardant effect was very good.

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