Study Of Multilayer Composite Thermocompression Film Material For Bracketless Invisible Aligners

In this paper,in order to resolve the problems such as high stress relaxation rate,poor corrosion resistance and easy to fracture during orthodontic treatment in oral environment of thermocompression film materials for bracketless invisible aligners,a new multilayer composite film material with crosslinked polyurethane coating was fabricated,both sides of the PETG substrate films were coated with a 0.05 mm thickness thermosetting polyurethane layer.The intermediate PETG substrate film was prepared by extrusion calendering method,and the processing was explored to meet the orthodontic mechanics requirement.The formulation of polyurethane was selected to fit the production in near future.The strategy for polyurethane elastomeric coating was according to the characteristics of the high humidity environment of the oral cavity,the hydrophobicity and stability of the silicone resin were utilized to reduce the water absorption of the composite material.For the problem of low mechanical strength of silicone modified polyurethane coating,the additive of nanoparticles was used to improve the surface hardness of the polyurethane coating.High pressure corona was carried out to strengthen the interface properties between substrate film and coating layer,which could prevent the water penetration to avoid the destruction of PETG film.The mechanical properties of the composites were evaluated,and the results showed that the multilayer composites exhibited excellent properties of low stress relaxation rate and smooth correction force release under simulated oral environment,the curing characteristics of the polyurethane elastomer coating met the production requirements.The main research works of this paper are:1.Preparation of basic multilayer compositesThe substrate layer of PETG material with a thickness of 0.66 mm was prepared by extrusion calendering.It was found that reducing the moisture in the PETG resin particles could improve the elastic modulus of the PETG material.The moisture content in the resin particles was 0.043%when dried at 70°C for 6 hours,and the prepared PETG material had an elastic modulus of 1417.97 MPa and yield elongation of 5.56%.PTMG and MDI-50 were selected as the main raw materials through pre-experiments,mixed and blended at 50°C for 1 hour in a vacuum environment,and held at 70°C for3 hour.The prepared prepolymer had low viscosity and was stored in an inert gas environment for 3 months without decrease in NCO value and precipitation,which met the production requirements.Molecular weight of PTMG,hard segment content,NCO value,chain spreading coefficient and catalyst dosage were considered for the selection of chain extender.The prepolymer with NCO value of 16%was prepared under the situations of PTMG-650 and MDI-50,20%of PDO and 80%of TIPA as mixed chain extender,hard segment content of 60%,chain spreading coefficient of 0.96,0.2%catalyst C-D.The surface drying time of the prepared polyurethane elastomeric coating was 11 min,which met the production requirements.The water absorption rate was about 0.6%in simulated oral environment test.2.Organosilicon modification of composites.The addition of organosilicon was to reduce the hydrophilicity of composite material.The compatibility between organosilicon and polyurethane elastomeric coating as well as micro phase separation structures in the coating layer was studied.176DX was used for the branch modification of polyurethane elastomeric coating,the water contact angle of the modified composite was 107.2°,the light transmission ranged from 10%to 60%.The water absorption rate in simulated oral tests was 0.6%.Silicones with different structures were used for block modification of polyurethane,the water contact angle was more than 100°.The light transmittance of 8526 modified composites was 75%～87%.The addition of 16409 exhibited low stable foam performance,the light transmittance ranged from 38%to 70%.The water absorption rate of 16409 modified composites with 4%content was 0.45%in the simulated oral environment test.3.Nanoparticle of Si O₂,Ti O₂,Zr O₂,Zn O,and Al₂O₃ were dispersed into prepolymer with NCO value of 16%using mechanical dispersion modified low surface energy composites.The conditions of grinding speed,time,and ball mass ratio were investigated,and it was found that the nanoparticles of Si O₂,Ti O₂ and Al₂O₃ had good dispersion and excellent defoaming properties in the polyurethane coating using triple zirconium beads at a temperature of 55°C for 2 hours.The nanoparticles enhanced the surface hardness of the composite.In the simulated oral environment test,the water absorption rates of the composites modified by 1%dosage of Si O₂ and Al₂O₃ were near the same of 0.72%and 0.73%,respectively.Al₂O₃ had stronger ability to promote micro-phase separation of polyurethane coatings.4.High-voltage corona modified the interface properties of composites.The conditions of rod-plate electrode distance,discharge voltage,and treatment speed were examined.The shear strength of the composites prepared by the treatment process with a discharge voltage of 1.2 k V,a rod-plate gap of 1 cm,and a treatment speed of100 cm/min increased from 0.865 MPa（before modification）to 1.844 MPa.No obvious boundary was observed in the interface of modified composites in SEM photographs.5.Biological and mechanical properties of the composites were measured.Biocompatibility testing of the extracts of the composites using MTT method revealed that the composites showed no toxicity to L929 cells.The stress relaxation experiments showed that the stress release was 14.6%in 1 hour and 24.9%in 48 hours in the simulated oral environment.The initial orthodontic force of the composite material was 113.83g by mechanical platform testing,The stress release was 10.7%at 24 h and24.2%at 336 h.The orthodontic force was maintained between 79.54 and 85.5 g during 24 to 72 h of orthodontic treatment,and the orthodontic retention force stability was better than that of pure PETG material during 48 to 336 h of orthodontic retention.