Preparation Of Polyisocyanurate And Improving The Heat Resistance Of Polyester

With the rapid development of electrical,electronic and optical materials,traditional heat resistant materials such as polyimide are finding it difficult to meet the growing demand due to their low optical transmittance and complex synthesis methods.The diisocyanate cyclotrimer reaction produces polyisocyanurates with excellent thermal stability,mechanical properties and yellowing resistance,low isocyanate prices and simple and convenient cyclotrimer reactions,making polyisocyanurates have the potential for a wide range of applications in the direction of electronic and optical materials.In order to meet the application requirements,this paper prepared the polyisocyanurate film material with excellent heat resistance and optical properties,and the isocyanurate structure was grafted on the surface of PET matrix to improve the heat resistance of the matrix.The main studies are as follows:Polyisocyanurate films with different HDI contents were firstly synthesised from aromatic diphenylmethane diisocyanates（MDI）and aliphatic hexamethylene diisocyanates（HDI）by means of the cyclic trimerisation of isocyanates.The rheological behavior of the polymer precursor solution during the curing process was studied.The results showed that the addition of HDI reduced the rate of cyclotrimerization,and the increase of temperature promoted the cyclotrimerization.The time and temperature of the cyclotrimerization reaction were determined based on this.The chemical structure of the polyisocyanurates was characterised by ATR-FTIR.The glass transition temperatures of220～192°C and 5%thermal weight loss temperatures of 476～438°C in a nitrogen atmosphere and carbon residuals of 51.5～39.3%,indicating that the prepared polyisocyanurates had good thermal stability.The tensile strength and elongation at break of polyisocyanurates with different HDI ratios ranged from 58.84 to 70.19 MPa and 5.50to 11.97%,respectively.the addition of HDI improves the optical properties of the film,with a transmission rate of 77.6～86.6%in the visible region and a maximum transmission rate of over 89%in all cases,showing good optical transmission.This research provides a new idea for the preparation of transparent heat resistant films.From the point of view of molecular structure design,fatty ring structures are introduced into the molecular chain of polyisocyanurates to further improve the optical properties of the material.The alicyclic diisocyanate dicyclomethane 4,4?-diisocyanate（HDMI）and isophorone diisocyanate（IPDI）were introduced on the basis of the cyclic trimerization reaction of MDI and HDI,respectively,and the properties of the alicyclic-based polyisocyanurate films were modulated by controlling the feed ratio of HDI.The structures of the two types of alicyclic-based polyisocyanurates were characterised by ATR-FTIR.The thermal stability properties of the two types of alicyclic-based films did not differ significantly and both showed a slight decrease with increasing HDI content,The thermal decomposition temperatures of HDMI-based and IPDI-based polyisocyanurates under nitrogen atmosphere ranged from 441 to 431°C and 443 to426°C,respectively,with residual carbon rates of 6.0 to 15.1%and 6.1 to 14.8%,respectively.The introduction of the alicyclic structure did not reduce the tensile strength,but rather increased the elongation at break.The tensile strengths of HDMI-based and IPDI-based polyisocyanurates were 61.16～72.7 MPa and 52.57～70.79 MPa,respectively,and the elongations at break were 8.98～14.84%and 6.59～16.22%,respectively.The alicyclic-based polyisocyanurate film achieves a maximum transmittance of 88.5%in the visible region and a maximum transmittance of 92.2%.The study provides a feasible technical solution for the development of high transmittance films.Based on the excellent thermal stability of the isocyanurate structure,attempts have been made to graft it onto the PET surface to improve the thermal performance of the polyester.The effect of acrylic acid concentration on the grafting rate of PET was firstly investigated by grafting functional carboxyl groups on the surface of PET by free radical reaction of acrylic acid under UV light,and it was demonstrated that the highest grafting rate was achieved at an acrylic acid concentration of 1.2 mol/L.The surface morphology and elemental composition of the material were analysed using ATR-FILR,XPS,SEM and EDS to verify the successful grafting of isocyanurates on PET.The effect of diisocyanate concentration and reaction time on the thermal properties of the products was also investigated.The results showed that when the concentration of diisocyanate was 60 mmol/L and the reaction time was 24 h,T_g of PET grafting products increased from 70°C（original PET）to 77°C,T_max increased from 428°C to 441°C and the residual carbon rate increased from 20.5%to 28.4%indicating that the heat resistance of PET grafted with isocyanurate was improved.