A flame-retardant and toughened polylactic acid composite material with a low dielectric constant and a preparation method thereof

Abstract

The present invention discloses a flame-retardant and toughened polylactic acid composite material with a low dielectric constant and a preparation method thereof. The flame-retardant and toughened polylactic acid composite material has a "sea-island" two-phase condensed state structure with polylactic acid as the "sea" and a flame-retardant and toughening agent encapsulating nano-silica as the "island". The raw materials are composed of the following in parts by mass: 60-100 parts of polylactic acid, 10-40 parts of a flame-retardant and toughening agent, 5-25 parts of nano-silica, and 0.1-1 part of a cross-linking agent. The polylactic acid composite material of the present invention has a low dielectric constant and flame-retardant properties. At the same time, its impact strength and elongation at break are also improved compared with pure polylactic acid. Polylactic acid itself is derived from natural substances and can be biodegradable. The polylactic acid composite material of the present invention is applicable to the electronics and electrical industries, and can solve problems such as current electronic waste pollution and shortage of petrochemical resources.

Description

Technical Field

[0001] The present invention belongs to the technical field of flame retardant of polymer materials, and particularly relates to a flame retardant and toughened polylactic acid composite material with a low dielectric constant and a preparation method thereof. Background Art

[0002] With the rapid development of communication information technology, electrical and electronic products are developing towards the direction of light weight, high performance and multi-function. Therefore, polymer polymer materials with characteristics such as low density and low dielectric constant have also received more and more extensive attention. For example, epoxy resins or polyimides, which have excellent dielectric properties, mechanical properties, etc., have been widely used in the fields of electronics and electrical appliances. However, these materials have the characteristic of being difficult to degrade, and with the continuous development of these industries, the pollution of electronic waste is also increasing. Therefore, developing biodegradable polymer materials with excellent dielectric properties is very important for solving the problem of electronic waste pollution.

[0003] Polylactic acid (PLA), the raw material lactic acid for its synthesis comes from plants and can be biodegradable. It can be considered that in the usage cycle of PLA, its carbon emissions are zero. Moreover, polylactic acid also has a relatively low dielectric constant and dielectric loss, and can replace difficult-to-degrade polymers such as epoxy resins for use in the electronics, electrical and other industries. However, a major drawback of polylactic acid itself is that it is not flame retardant. Therefore, in order to ensure the safety of electronic and electrical appliances, it is necessary to improve the flame retardant performance of polylactic acid. In addition, polylactic acid has strong rigidity and brittleness, and many in the fields of electronics and electrical appliances have requirements for the toughness of materials. Therefore, toughening polylactic acid can obtain better applications.

[0004] To solve the above problems, flame retardants and toughening agents can be added to the polylactic acid matrix; common flame retardants include phosphorus-nitrogen flame retardants and intumescent flame retardants. However, when the existing commercially available flame retardants are added to polylactic acid, while improving the flame retardant performance, they will have a great impact on the dielectric properties and mechanical properties of polylactic acid itself. For example, the most common traditional intumescent flame retardants, such as ammonium polyphosphate (APP), pentaerythritol (PER), melamine (MEL), etc., when added to polylactic acid, due to their poor compatibility, will affect the mechanical properties of polylactic acid itself. More importantly, these flame retardants have a relatively large polarity of surface groups, and when combined with polylactic acid, interfacial polarization will occur, resulting in a significant increase in the dielectric constant of the composite material. At the same time, for toughening polylactic acid, the most common method is to add toughening agents, and common toughening agents include various rubbers, such as cis-butadiene rubber, nitrile rubber, ethylene-propylene-diene monomer rubber, etc. However, rubber itself is non-degradable and will affect the degradability of polylactic acid. Therefore, it is still a great challenge to carry out flame retardant and toughening modification on polylactic acid without affecting the dielectric properties and degradability of polylactic acid itself. Summary of the Invention

[0005] Based on this, the present invention provides a flame retardant toughened polylactic acid composite material with a low dielectric constant and a preparation method thereof, which enhances the flame retardancy and toughness of polylactic acid without affecting the dielectric properties and degradability of polylactic acid itself, and provides an important method for using polylactic acid, a degradable polymer material, to replace non-degradable polymer materials in the production of electronic appliances.

[0006] The flame retardant and toughened polylactic acid composite material with low dielectric constant of the present invention has a "sea-island" two-phase condensed structure with polylactic acid as the "sea" and a flame retardant and toughening agent wrapped with nano silicon dioxide as the "island". The raw materials thereof are composed of the following by mass: 60-100 parts of polylactic acid, 10-40 parts of flame retardant and toughening agent, 5-25 parts of nano silicon dioxide, and 0.1-1 part of cross-linking agent.

[0007] The polylactic acid is common commercially available polylactic acid.

[0008] The flame retardant toughening agent is a branched biopolyester (HBPE) synthesized from itaconic acid, sebacic acid, ethylene glycol and glycerol modified by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and is prepared by the following method:

[0009] Under the protection of inert gas (N2), 390g of itaconic acid, 404g of sebacic acid, 124g of ethylene glycol and 230g of propylene glycol were added to a reactor equipped with a mechanical stirrer, a thermometer and a water separator, refluxed and stirred for 45min, and then 0.6g of p-hydroxyanisole was added, followed by condensation reflux reaction at 175°C for 4-5h to obtain a branched structure biopolyester HBPE; after the system was cooled to room temperature, 1800mL of a dichloromethane solution containing 648g of DOPO was slowly added to the prepolymer, stirred for 12h under the protection of N2, then heated to 50°C for reaction for 8h, taken out and dried to obtain a flame retardant toughening agent (HBPE-DOPO).

[0010] The cross-linking agent is dicumyl peroxide (DCP).

[0011] The method for preparing the flame-retardant toughened polylactic acid composite material with a low dielectric constant of the present invention comprises the following steps:

[0012] Step 1: Weigh the required raw materials according to the proportion, and dry them at 80°C for more than 4 hours;

[0013] Step 2: adding the pretreated raw materials into an internal mixer in a specific order, melt blending to obtain a polylactic acid composite material, and then crushing and granulating to obtain polylactic acid composite pellets;

[0014] Step 3: Prepare the obtained polylactic acid composite pellets into sheets by a flat vulcanizer, and then cut them into standard test specimens for performance testing.

[0015] Further, the feeding sequence in Step 2 is as follows: First, mix the flame retardant toughening agent and nano-silica in a mixer for a period of time, then add polylactic acid and mix for a period of time, and finally add the cross-linking agent and mix for a certain time to achieve dynamic vulcanization.

[0016] Furthermore, in Step 2, the temperature of the mixer is 180 - 190 °C, the mixing time of the flame retardant toughening agent and nano-silica is 2 - 3 min, the blending time with polylactic acid is 4 - 6 min, and after adding the cross-linking agent, mix for another 2 - 6 min.

[0017] Further, in Step 3, the temperature of the flat vulcanizer is 175 - 185 °C, the pressure is 10 - 15 MPa, and the hot pressing time is 4 - 5 min.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] In the present invention, by chemically bonding DOPO with a biodegradable polyester, the tendency of DOPO molecules to aggregate is weakened, the dipole interaction between DOPO molecules is reduced, and the dielectric constant is lowered. Through molecular design, DOPO with a rigid molecular structure and nano-silica are embedded in flexible biodegradable polyester molecules, which not only increases the phase domain size of the biodegradable polyester, but also can reduce the dosage of the biodegradable polyester, yet can improve the toughening effect of the biodegradable polyester on polylactic acid.

[0020] Through the design of the dynamic vulcanization process, the present invention improves the interfacial bonding force between the biodegradable polyester phase domain and the polylactic acid phase domain, thereby enabling the formation of the required sea-island structure, while ensuring the excellent dielectric properties of polylactic acid, improving the flame retardancy and toughness of polylactic acid. Description of the Drawings

[0021] Figure 1 is a schematic diagram of the molecular structure of DOPO-modified branched biodegradable polyester.

[0022] Figure 2 is a schematic diagram of the sea-island structure of the flame retardant toughening low-dielectric polylactic acid composite material.

[0023] Figure 3 is a scanning electron microscope photograph of the flame retardant toughening low-dielectric polylactic acid composite material in Example 1.

[0024] Figure 4 is a scanning electron microscope photograph of the flame retardant toughening low-dielectric polylactic acid composite material in Comparative Example 3. Detailed Embodiments

[0025] The technical solution of the present invention will be further described below in conjunction with specific embodiments. However, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work belong to the scope of protection of the present invention.

[0026] The flame retardant and toughening agent used in the embodiments of the present invention is a branched biopolyester (HBPE) synthesized from itaconic acid, sebacic acid, ethylene glycol, and glycerol modified by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and is specifically obtained by the following method:

[0027] Under the protection of an inert gas (N2), 390 g of itaconic acid, 404 g of sebacic acid, 124 g of ethylene glycol, and 230 g of glycerol are added to a reaction kettle equipped with a magnetic stirrer, a thermometer, and a water separator, and refluxed and stirred for 45 min. Then, 0.6 g of p-hydroxyanisole is added, and then the reaction is carried out under reflux condensation at 175 °C for 4 - 5 h to obtain a branched structure biopolyester HBPE. After the system is cooled to room temperature, an 80 mL dichloromethane solution containing 648 g of DOPO is slowly added to the prepolymer, stirred for 12 h under N2 protection, then heated to 50 °C and reacted for 8 h, and taken out and dried to obtain the flame retardant and toughening agent (HBPE-DOPO).

[0028] Example 1:

[0029] The flame retardant and toughening polylactic acid composite material with a low dielectric constant is prepared in the following steps in this example:

[0030] 1. First, 10 wt% (6 g) of the flame retardant and toughening agent (HBPE-DOPO) and 10 wt% (6 g) of nano-silica (SiO2) are added to a mixer with a rotation speed of 100 rpm and a temperature of 180 °C, and mixed for 4 min;

[0031] 2. 80 wt% (48 g) of polylactic acid (PLA) is added and mixed for 4 min;

[0032] 3. 0.1 wt% (0.06 g) of DCP is added to the mixture, and dynamically vulcanized in the mixer for 2 min. Subsequently, the obtained PLA / HBPE-DOPO / SiO2 blend is crushed and granulated to obtain PLA composite pellets.

[0033] 4. The obtained PLA composite pellets are heated and pressed into plates in a mold, and then cut into standard specimens. The heating temperature is 180 °C, the pressure is 10 MPa, and the hot pressing time is 5 min to obtain a flame retardant and toughening polylactic acid composite material with a low dielectric constant.

[0034] Example 2:

[0035] In this example, a flame-retardant and toughened polylactic acid composite material with a low dielectric constant was prepared by the same method as in Example 1, except that: in Step 1, 80 wt% (48 g) of PLA, 10 wt% (6 g) of HBPE-DOPO, and 10 wt% (6 g) of SiO₂ were replaced with 70 wt% (42 g) of PLA, 20 wt% (12 g) of HBPE-DOPO, and 10 wt% (6 g) of SiO₂.

[0036] Example 3:

[0037] In this example, a flame-retardant and toughened polylactic acid composite material with a low dielectric constant was prepared by the same method as in Example 1, except that: in Step 1, 80 wt% (48 g) of PLA and 10 wt% (6 g) of HBPE-DOPO were replaced with 60 wt% (36 g) of PLA, 30 wt% (18 g) of HBPE-DOPO, and 10 wt% (6 g) of SiO₂.

[0038] Example 4:

[0039] In this example, a flame-retardant and toughened polylactic acid composite material with a low dielectric constant was prepared by the same method as in Example 1, except that: in Step 1, 80 wt% (48 g) of PLA, 10 wt% (6 g) of HBPE-DOPO, and 10 wt% (6 g) of SiO₂ were replaced with 50 wt% (30 g) of PLA, 40 wt% (24 g) of HBPE-DOPO, and 10 wt% (6 g) of SiO₂.

[0040] Comparative Example 1:

[0041] 1. First, 15.36 g of HBPE without DOPO and 30 g of polylactic acid were added to a mixer with a rotation speed of 100 rpm and a temperature of 180 °C, and mixed for 4 min;

[0042] 2. 14.64 g of nano-silica was added and mixing continued for 4 min;

[0043] 3. 0.1 wt% (0.06 g) of DCP was added to the mixture, and dynamically vulcanized in the mixer for 2 min. Subsequently, the obtained PLA / HBPE / SiO₂ blend was crushed and pelletized to obtain PLA composite pellets.

[0044] 4. The obtained PLA pellets were heated and pressed into sheets in a mold, and then cut into standard test bars. The heating temperature was 180 °C, the pressure was 10 MPa, and the hot pressing time was 5 min to obtain a polylactic acid composite material.

[0045] Comparative Example 2:

[0046] 1. First, add 15.36 g of branched bio-polyester without DOPO and 30 g of polylactic acid into a mixer with a rotation speed of 100 rpm and a temperature of 180 °C, and mix for 4 min;

[0047] 2. Add 8.64 g of DOPO and 6 g of nano-silica, and continue to mix for 4 min;

[0048] 3. Add 0.1 wt% (0.06 g) of DCP into the mixture, dynamically vulcanize in the mixer for 2 min, and then crush and granulate the obtained PLA / HBPE / DOPO / SiO2 blend to obtain PLA composite pellets.

[0049] 4. Heat and press the obtained PLA pellets into a sheet in a mold, and then cut it into standard specimens. The heating temperature is 180 °C, the pressure is 10 MPa, and the hot pressing time is 5 min to obtain a polylactic acid composite material.

[0050] Comparative Example 3:

[0051] 1. Add 10 wt% (6 g) of flame retardant and toughening agent (HBPE-DOPO), 10 wt% (6 g) of nano-silica (SiO2), and 80 wt% (48 g) of polylactic acid (PLA) into a mixer with a rotation speed of 100 rpm and a temperature of 180 °C, and mix for 4 min;

[0052] 2. Crush and granulate the obtained PLA / HBPE-DOPO / SiO2 blend to obtain PLA composite pellets.

[0053] 3. Heat and press the obtained PLA composite pellets into a sheet in a mold, and then cut it into standard specimens. The heating temperature is 180 °C, the pressure is 10 MPa, and the hot pressing time is 5 min to obtain a flame retardant and toughened polylactic acid composite material with a low dielectric constant.

[0054] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Dielectric constant (1 kHz) 3.02 3.02 3.04 3.05 3.08 3.50 3.21 Dielectric loss (1 kHz) 0.0010 0.0009 0.0011 0.0012 0.0013 0.0025 0.0017 Limiting oxygen index (%) 21.4 23.5 24.7 26.2 20.1 21.0 23.0 Elongation at break (%) 90.2 105.2 118.2 138.4 98.1 88.5 106.1 <![CDATA[Impact strength (kJ / m 2 )]]> 11.6 14.2 18.4 23.4 12.0 10.4 15.3

[0055] It can be seen from the data in the above table that in Comparative Example 1, there is no flame retardant DOPO, so the limiting oxygen index is only 20.1%, and there is no flame retardant effect. With the addition of the flame retardant and toughening agent HBPE-DOPO, the flame retardant and toughening effect of the polylactic acid composite material has been significantly improved. For example, in Examples 1 / 2 / 3 / 4, with the increase in the content of the added flame retardant and toughening agent, the limiting oxygen index has increased significantly.

[0056] Scanning electron microscope photos of the fracture surface of the composite material sample in Example 1 ( Figure 3 ) can clearly show that dynamic vulcanization can form the required sea-island structure. And the scanning electron microscope photos of the sample in Comparative Example 2 ( Figure 4)It can be seen that the composite material without DCP dynamic vulcanization does not form the required sea-island structure, and PLA, HBPE, and SiO2 are directly mixed together. The flame retardancy and toughness of the obtained composite material are also poorer than those of Example 1, indicating that dynamic vulcanization can improve the interfacial bonding force between the polylactic acid phase region and the bio-polyester phase region, form a sea-island structure, and improve the performance of the composite material.

[0057] Moreover, it can be seen that the dielectric constant and dielectric loss of the composite material in Comparative Example 2 are higher than those in Example 4. When DOPO is incorporated into HBPE through a chemical bond, it will limit the aggregation between its molecules, weaken the dielectric polarization, and thus reduce the constant and dielectric loss.

[0058] Compared with Example 4, DOPO in Comparative Example 3 was introduced later, and the obtained composite material has poor flame retardancy and toughness. It shows that only when DOPO is embedded in the HBPE phase region can it play a better role in flame retardancy and toughening.

Claims

1. A preparation method of a flame-retardant and toughened polylactic acid composite material with a low dielectric constant, characterized in that It includes the following steps: Step 1: Weigh each required raw material according to the proportion, and dry each raw material at 80 °C for more than 4 h; Step 2: Add the pretreated raw materials into a mixer in a specific order. The temperature of the mixer is 180-190 °C, and the feeding order is as follows: First, mix the flame retardant toughening agent and nano-silica in the mixer for 2-3 min, then add polylactic acid and mix for 4-6 min, and finally add the crosslinking agent and mix for 2-6 min to achieve dynamic vulcanization and melt blending to obtain a polylactic acid composite material, and then crush and granulate to obtain polylactic acid composite pellets; Step 3: Prepare the obtained polylactic acid composite pellets into a plate through a flat vulcanizer, and then cut it into standard test specimens for performance testing; The flame retardant toughened polylactic acid composite material has a "sea-island" two-phase condensed structure with polylactic acid as the "sea" and the flame retardant toughening agent encapsulating nano-silica as the "island". Its raw materials are composed of the following by mass: 60-100 parts of polylactic acid, 10-40 parts of flame retardant toughening agent, 5-25 parts of nano-silica, and 0.1-1 part of crosslinking agent; The flame retardant toughening agent is a branched bio-polyester synthesized from itaconic acid, sebacic acid, ethylene glycol, and glycerol modified by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and is obtained through the following method: Under the protection of an inert gas, add 390 g of itaconic acid, 404 g of sebacic acid, 124 g of ethylene glycol, and 230 g of glycerol into a reaction kettle equipped with a mechanical stirrer, a thermometer, and a water separator, reflux and stir for 45 min, then add 0.6 g of p-hydroxyanisole, and then carry out condensation reflux reaction at 175 °C for 4-5 h to obtain a branched structure bio-polyester HBPE; after the system is cooled to room temperature, slowly add 1800 mL of dichloromethane solution containing 648 g of DOPO to the prepolymer, stir for 12 h under N2 protection, then raise the temperature to 50 °C and react for 8 h, take out and dry to obtain the flame retardant toughening agent HBPE-DOPO.

2. The preparation method according to claim 1, characterized in that: The crosslinking agent is dicumyl peroxide.

3. The preparation method according to claim 1, characterized in that: In step 3, the temperature of the flat vulcanizer is 175-185 °C, the pressure is 10-15 MPa, and the hot pressing time is 4-5 min.

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