Study On Synthesis And Properties Of Moisture-curable Hot Melt Polyurethane Adhesive With High Initial Bonding Strength

Moisture-curable hot melt polyurethane adhesive(PUR)has been widely used in fields of bonding due to its environmental friendliness and excellent bonding performance.However,at present,most PUR prepolymers are not suitable for bonding materials with high internal stress because of their low molecular weights which lead to poor initial bonding strength.In this thesis,PUR was prepared by bulk polymerization with polyols as soft segments,4,4’-diphenylmethane diisocyanate(MDI)as hard segments and isocyanate index(R)of 1.8.Moreover,the effects of various resins and their additions(relative to the total mass of polyols and MDI)on the properties of PUR were studied.In this thesis,the effects of the contents of acrylic resin ST 75,polyurethane elastomer TPU8902,petroleum resin A,copolyester S1661 and polyvinyl acetate N60SP on the properties of PUR were studied.The results showed that:(1)The maximum dosage of each kind of resin was 4%,25%,5%,15%and 12.5%,respectively.TPU 8902 and S1661 were both completely compatible with PUR due to terminal hydroxyl groups existed in their molecular structures which could react with-NCO.Resin ST 75,A and N60SP were all thermodynamically compatible with PUR analyzed by differential scanning calorimetry(DSC)when their maximum dosages were not exceeded.The glass transition temperature(T_g)of hard segment phase of PUR and the degree of microphase separation of PUR were both obviously increased by adding resin ST 75,A and N60SP.In addition,the amounts of ST 75 and A had little effects on the T_g of soft segment phase of PUR.With the concentration of N60SP increased,the T_g of soft segment phase of PUR decreased significantly.(2)The optimal dosages of ST 75,TPU 8902,S1611 and N60SP were 3%,15%,10%and 7.5%,respectively.Under the optimal dosage of each resin,the initial bonding strength of PUR used to bond aluminum sheets increased from 39.4 N/25 mm to 151.4 N/25 mm,55.7 N/25mm,298.2 N/25 mm and 185.5 N/25 mm.The initial bonding strength of polycarbonate(PC)sheets increased from 37.5 N/25 mm to 223.1 N/25 mm,55.8 N/25 mm,349.7 N/25 mm and 213.1N/25 mm.The final bonding strength of aluminum sheets increased from 343.3 N/25 mm to 667.4N/25 mm、966.0 N/25 mm、620.3 N/25 mm and 1118.8 N/25 mm,respectively.The final bonding failure type of PC sheets was substrate destruction.Therefore,TPU 8902 had the worst effect on improving the initial bonding performance of PUR.ST 75 as well as S1611 could improve the initial bonding performance of PUR,but the final bonding strength of PUR was much smaller than that of other resins at their optimal dosages.Although N60SP could greatly improve the initial bonding performance and final bonding performance of PUR at the optimal dosage,there was a problem of short opening time which was 510 s,while the opening time of the basic sample was1550 s.So,they were not suitable for the preparation of PUR with high initial bonding performance.(3)The optimal dosage of petroleum resin A was 5%,which was suitable for preparing high initial adhesion PUR compared with other resins.When PUR containing 5%resin A was used to bond aluminum sheets and PC sheets,the initial bonding strength of PUR was 245.7 N/25mm and 280.2N/25mm respectively,both of which were greatly improved about 7 times as much as that of the basic sample.When the PUR containing 5%A was used to bond aluminum sheets,the final bonding strength was 1149.1 N/25 mm,which was more than three times of the basic value.The final bonding failure type for the PC sheets was substrate destruction,and the destruction of the substrates could not be achieved when the basic sample was used.What’s more,the melt viscosity of PUR with 5%A was 4800 m Pa·s at 120℃,while that of basic sample was 4200 m Pa·s at120℃.The two values were similar and small,so it had good wettability.Additionally,opening time of PUR with 5%A was long and 1500 s,which was conducive to large-area coating of PUR.The effect of C80 content on PUR performance was also studied in this thesis.The research results indicated that:(1)When the dosage of C80 was less than 5%,C80 was thermodynamically compatible with the PUR basic sample.C80 could also significantly improve the T_g of hard segment phase of PUR,and the microphase separation degree of PUR was increased as well.(2)C80 was also suitable for the preparation of PUR with high initial adhesion,and its optimal dosage was 3%.In this time,the initial bonding strength of PUR used to bond aluminum sheets and PC sheets was 203.0 N/25mm and 192.2 N/25mm respectively,which was about 5 times of that without adding resin.Although the effect of C80 on the initial bonding performance was not as good as resin A,the initial bonding strength was still very large.When the PUR containing 3%C80 was used to bond aluminum sheets,the final bonding strength was 2543.7 N/25mm,which was more than 7 times of the basic sample.The final bonding failure type of PC sheets was also substrate destruction.The effect of C80 on the final bonding performance was the most obvious among these resins.PUR with 3%C80 had a melt viscosity of 5400 m Pa·s at 120℃,which was similar to the viscosity of PUR containing 5%of A and also showed good wettability.Then its opening time was 1260 s and was close to the opening time of PUR containing with 5%A.(3)UV-visible spectrophotometric analysis showed that when the content of C80 was 3%,the compatibility of C80 with PUR was the best.The water contact angle analysis showed that the addition of C80 improved the water resistance of the cured PUR film.However,according to the wide-angle X-ray diffraction analysis,the addition of C80 reduced the crystallinity of the cured PUR film.