Synthesis Of Silicone Adhesion Promoters Containing Phenolic Hydroxyl/Acrylate Groups And Their Application In Addition-Cure Liquid Silicone Rubber

Thanks to the excellent comprehensive properties,addition-cure liquid silicone rubber(ALSR)is widely used in architectural ornament,automotive industry,electronics,aviation,and so on.However,due to the low surface energy of ALSR and its poor adhesion to plastic and metal substrates,the further application of ALSR has been severely limited.Currently,there are three main methods to improve the adhesion performance of ALSR,including surface pretreatment of the bonded substrate,the introduction of functional groups in polysiloxane molecular chain,or adding adhesion promoter.The first two methods have been phased out due to the complex process,low efficiency,and environmental unfriendliness.At present,adding a silicone adhesion promoter is the simplest and the most effective way to improve the adhesion performance of ALSR.The organic coupling of the phenolic hydroxyl group and acrylate group to the same ALSR molecular chain can effectively improve the adhesion of ALSR.Herein,PHMS-g-EE was synthesized by grafting eugenol(EE)onto the polyhydrogenmethylsiloxane(PHMS)via hydrosilylation reaction firstly,and the influence of reaction conditions such as the ratio of raw materials and catalyst content on the adhesion promotion was studied.On this basis,to further improve the adhesive strength of ALSR/PHMS-g-EE and extend its applications,a silicone adhesion promoter containing both phenolic hydroxyl and acrylate groups named PHMS-g-(EE/TD)was synthesized.The effects of PHMS-g-EE and PHMS-g-(EE/TD)on the vulcanization,mechanical and rheological properties of ALSR were investigated,respectively.Moreover,the heat resistance and water resistance of the two adhesives were discussed.Besides,the mechanism of their adhesion promotion was revealed.The main contents and results of this study are mainly as follows:(1)A novel phenolic hydroxysilicone adhesion promoter(PHMS-g-EE)was prepared by hydrosilylation reaction with EE and PHMS.Then it was added to ALSR to improve the adhesion performance of ALSR.The chemical structure and properties of PHMS-g-EE were characterized by Fourier transform infrared spectroscopy(FTIR),nuclear magnetic resonance spectroscopy(¹H-NMR),gel permeation chromatography(GPC),and thermogravimetric analysis(TGA).The effects of the mole ratio of Si-H to C=C groups[n(Si-H)/n(C=C)]in raw materials,catalyst content and type,reaction temperature,and reaction time on the adhesion performance with aluminum(Al),polycarbonate(PC),polyamide(PA)and poly(p-phenyldimethyldecanediamine)(PPA)substrate were investigated.It was found that when n(Si-H)/n(C=C)was 3,Karstedt catalyst content was 4 ppm,the reaction temperature was 80?,the reaction time was 5 h and the hydrogen content of PHMS was 0.8 wt%,the side reactions such as dehydrogenation and self polymerization were the least,and thus the effective adhesion-promoting group content of PHMS-g-EE was the most,which had the best adhesion promotion effect on ALSR.When the PHMS-g-EE content was 4 phr,the shear bond strength between ALSR and PPA substrate reached 3.2 MPa,which was 493%higher than that of ALSR without adhesion promoter,and the failure type was a cohesive failure.(2)The effects of the PHMS-g-EE content on the vulcanization,rheological properties,mechanical properties,thermal stability,and adhesion performance between ALSR and Al,PC,PA,and PPA substrates were detailedly studied.Moreover,the heat resistance and water resistance of ALSR/PHMS-g-EE were investigated.The adhesion promotion mechanism of PHMS-g-EE on ALSR for PA substrate was also revealed.The results showed that PHMS-g-EE promoted the cross-linking of ALSR,and accelerated the vulcanization rate,and enhanced the cross-linking density of ALSR.When the PHMS-g-EE content was 2 phr,the tensile strength of ALSR was 7.3 MPa,the elongation at break was 581%,the tear strength was 31.1k N·m^-1,and the hardness was 38 Shore A.With the increase of the content of PHMS-g-EE,the bonding strength of ALSR/PHMS-g-EE with Al,PC,PA,and PPA substrate increased first followed by a slight decrease.Besides,ALSR/PHMS-g-EE presented good heat resistance and water resistance.At 100?,the bonding strength of ALSR/PHMS-g-EE with PC and PA substrate still maintained 1.0 MPa and 1.6 MPa,respectively,indicating potential applications at high-temperature.After soaking in 25?distilled water for 8 hours,the bonding strength of ALSR/PHMS-g-EE with PC and PA substrate maintained 1.1 MPa and 1.4 MPa,respectively.(3)The adhesion promoter containing phenolic hydroxyl and acrylate groups,denoted as PHMS-g-(EE/TD),was synthesized via the hydrosilylation reaction of eugenol(EE),tripropylene glycol diacrylate(TD),and polyhydrogenmethylsiloxane(PHMS).It was found that PHMS-g-(EE/TD)effectively enhanced the bonding strength of ALSR with PC and PA substrate.When the molar ratio of EE to TD was 2:1,the as-prepared PHMS-g-(EE/TD)showed the best comprehensive adhesion promotion.When the PHMS-g-(EE/TD)content was 2 phr,the bonding strength of ALSR with PC and PA substrate were 3.8 MPa and 3.1 MPa,respectively.After heat treating at 150?for 72 h or soaking in 25?distilled water for 8 h,the bonding strength of ALSR/PHMS-g-(EE/TD)with PC and PA substrate maintained a rather high value.On this foundation,the adhesion promotion mechanism of PHMS-g-(EE/TD)for ALSR was further discussed and revealed.On the one hand,PHMS-g-(EE/TD)participated in the curing process of ALSR through C=C and formed chemical bond connections with the main chain of ALSR;on the other hand,PHMS-g-(EE/TD)was enriched on the ALSR surface to form a boundary layer containing-OH and-COO,forming hydrogen bonds and?-?conjugation with the substrate.PHMS-g-(EE/TD)connected ALSR and substrate tightly just like a bridge,which greatly improved the adhesion performance of ALSR.