Synthesis And Characterization Of UV Curing Waterborne Primer For EVA Foam In Footwear Application

EVA(Ethylene Vinyl Acetate Copolymer)foam is widely used in footwear industry due to good performance and cost advantage.It's difficult for polyurethane adhesive to have good adhesion on its low polar surface,normally primer is used for EVA surface pretreatment to improve adhesion.UV curing EVA primer is widely used due to its high efficiency and energy saving,most of UV curing EVA primers in the market are solvent-borne,for environment and occupational health protection,it's nessesory to develop high performance UV curing waterborne EVA primer.In this paper,multifunctional UV curing waterborne polyurethane acrylates(WPUA)were synthesized by reaction of isophorone diisocyanate(IPDI)and polyols with different structure,dihydroxymethylbutyric acid(DMBA),hydroethyl acrylate(HEA),triethylamine(TEA)by solvent free final-terminated prepolymer method.The structure of the product and structure change after curing were characterized by infrared spectroscopy(FT-IR).Compared different polyols effecting on bonding strength on EVA substrate.Two modification methods were used to improve WPUA adhesion to EVA,one is pre-mixing with PUA prepolymer before dispersing into water and the other is mixing with WPUA emulsion by mechanical stiring.Studies factors which effect the storage stability of WPUA and properties after cured,the factors include polyols species,-NCO/-OH molar ratio R in prepolymer stage,-COOH content.And tested application properties of modified WPUA on EVA substrates with different hardness.It was found that when R=1.1,-COOH content was 2.2%,with 25wt%1,6-hexanediol diacrylate(HDDA)solution of solid chlorinated polypropylene CPP(CPP content 25%)modified PUA prepolymer,the semi-crystalline dimerised fatty acid polyester polyol based WPUA with 30%solid content had the best comprehensive performance.The emulsion was white,storage stability was good.The cured film had 7.7%water absorption rate and tacky feeling was grade 1.When bonding test on EVA60,its max.initial peel strength was 28.4N/cm,and EVA60 tear during final peel strength testing.After hydrolysis resistance test at 70?95%humidity for 7 days,the strength retaintion rate was 100%,which showed good hydrolysis resistance.When wet coating weight was~10g/m~2,added 1.5wt%light initiator 1173,curing at 1000mJ/cm~2 could got the best bonding strength,cured UV-WPUA primer should process bonding in 8 hours.Compared with solvent borne and waterborne UV primers from the market on bonding strength,heat resistance,hydrolysis resistance and yellowing resistance etc.,the best UV-WPUA primer in this paper has comparable hydrolysis resistance and yellowing resistance with solvent borne primer,but still had gap on bonding strength on high hardness EVA substrates.Bonding strength and heat resistance on different EVA substrates was in the same level compared with two waterborne UV primers from market.