Brief description of the types of PET nucleating agents and the effect of nucleating agents on the crystalline properties of PET
There are three main types of nucleating agents for PET, namely inorganic nucleating agents, organic nucleating agents and polymeric nucleating agents. This paper briefly describes the impact of these nucleating agents on the crystalline properties of PET.
I. Inorganic nucleating agents.
Inorganic nucleating agents are basically inorganic fillers commonly used in polymers, inorganic nucleating agents in the crystallization process is equivalent to the second phase of small particles in the PET melt, at high temperatures these particles are not molten state, in the process of cooling, PET molecular chain to these particles as the centre, adsorbed to the particles and make an orderly arrangement and the formation of nuclei. These small inorganic molecules, when used as heterogeneous nucleating agents, therefore reduce the activation energy required for the formation of nuclei in PET and have little effect on the subsequent crystallisation process, i.e. the PET molecular chain segments are adsorbed onto the nucleus surface and enter the lattice.
When talc is used as a nucleating agent in PET, the initial crystallisation is formed on the surface of the filler particles and is accompanied by the transfer of crystals from the lamellar structure by a small lateral diffusion on the surface of the particles. The effect of carbonates as nucleating agents on the crystalline properties of PET has been found to be effective as crystalline nucleating agents for PET with Na2CO3, and NaHCO3. With Na2CO3 or NaHCO3 as nucleating agents, PET can be produced with good mechanical properties in a relatively short moulding cycle at 90% of the mould temperature. The nucleation effects of talc, CaCO3 and organic sodium salt nucleating agents were compared and it was found that talc was more favourable to the crystallisation rate of PET than CaCO3. When the mass content of talc is 5%, the contribution to the isothermal crystallisation rate of PET is close to the contribution of 1% organic sodium salt, and the addition of talc significantly improves the tensile strength and bending strength of PET, while the organic sodium salt reduces these properties.
Second, organic nucleating agents.
Organic nucleating agents are mainly Na, Li, Ba, Mg, Ca salts of monocarboxylic acid, Na, K, Ca salts of benzoin acid, aromatic hydroxysulphonates, Mg, Zn salts of organic phosphorus compounds, of which the better effect is the sodium salt of carboxylic acid and potassium salt of carboxylic acid. The nucleation mechanism of organic nucleating agents is mainly related to their chemical structure. PET and sodium carboxylate salts undergo a chemical reaction when extruded at high temperatures, generating PET-COONa substances, which form ionic clusters between the PET melt with ionic end groups.
The effect of adding sodium benzoate derivative (Nu) nucleating agent and polyester polyether copolymer crystallisation promoter (Pro) on the crystallisation rate of PET. Nu/Pro-PET was found to crystallise slightly faster than Nu-PET at crystallisation temperatures of 228 °C and 230 °C; however, at higher crystallisation temperatures, Nu/Pro-PET crystallised slightly slower than Nu-PET, indicating that the crystallisation This accelerator did not further increase the crystallisation rate of PET at higher temperatures. The addition of sodium benzoate nucleating agent resulted in a shorter crystallisation induction period, reduced crystallisation activation energy and an increased overall crystallisation rate for PET; and the variation increased with increasing addition, but reduced crystallinity, which was not conducive to the stability of the properties of the blended material. Therefore, when sodium benzoate is used in PET, attention must be paid to the dosage, and it is also necessary to use with other modifiers.
Third, polymer nucleating agents.
Polymer nucleating agents include polyester zwitterionic alkali metal salts, all aromatic polyester powder, PTFE powder, low molecular weight isotactic PP, high melting point PET, ionomer, liquid crystal polymer (LCP), etc., of which ionomer is a commonly used PET crystalline polymer materials. The glass transition temperature of PET is reduced by blending, which speeds up the crystallisation rate and improves its impact resistance. Ionomers refer to polymers with a small number of ionisable groups on the polymer backbone, with the main components consisting of a non-ionic backbone chain and a small number of ion-containing components. The molar content of ionic groups is generally considered to be no more than 15%. In the binary blending system of ionomers, the ionomer can interact through ion a ion, ion a dipole, hydrogen bonding, acids and bases, charge transfer, transition metal coordination complexes, etc., and the physical cross-linking of multiple ion pairs or ion clusters formed between polymer chains. This special composition and morphological structure of ionomers gives them many unique properties such as excellent toughness, high impact resistance, wear resistance, transparency and high melt viscosity. Surlyn is a widely used ionomer developed by DuPont as Surlyn nucleating agent, which is a sodium salt of ethylene methacrylic acid copolymer, where the weight ratio of ethylene/methacrylic acid is 90/10 and the sodium neutralisation is about 45%. The reaction products can form ion-terminated ionic clusters (PET-COONa), which act as heterogeneous nucleating agents in the cooling crystallisation process.
Compared with small molecule organic nucleating agents, polymer nucleating agent Surlyn with ion end groups in the generation of large molecules at the same time, there is PET-R (R for organic nucleating agent flexible group) generated, because R molecular chain than PET molecular chain is more flexible, in the system and play the role of facilitator, its introduction to promote the molecular movement of PET, reduce the free energy of molecular chain diffusion into the character, and also reduce the PET tile, improve PET. It also reduces the tile of PET and increases the crystallisation rate of PET, which is not the case with small molecule nucleating agents.