Preparation Of Plant Oils-based Pressure Sensitive Adhesives And Its Structural Characteristics

The preparation of pressure-sensitive adhesives（PSAs）from renewable resources such as vegetable oils can alleviate the pressure of petroleum resources shortage and address the environmental pollution issue.In the preparation process of plant oil-based PSAs,the double bonds of fatty acid chains in vegetable oils are usually converted into epoxy bonds with high chemical activity,and ester bonds are hydrolyzed to carboxylic acids or methyl esters,finally followed by self-polymerization or copolymerization with other small molecular monomers.However,the low adhesion properties and easily-aging problems of vegetable oils-based PSAs limit their potential applications.Firstly,antioxidants（tea polyphenols palmitate,caffeic acid,ferulic acid,gallic acid,BHT,TBHQ,BHA,PG,tea polyphenols）were grafted into epoxidized soybean oils-based pressure-sensitive adhesives（ESO-PSAs）to study their effects on the adhesion and antiaging properties of PSAs.The results showed that failure did not occur in the ESO-PSAs grafted with caffeic acid,TBHQ,PG,tea polyphenols,tea polyphenol palmitate,and BHA.Comprehensively considering the adhesion and antiaging properties of ESO-PSAs,tea polyphenol palmitate was used as the best antioxidant.The optimal preparation conditions were:0.9%tea polyphenol palmitate,70%rosin ester,7%phosphoric acid,reaction time of 5 min,and reaction temperature of 50°C.Under the optimal conditions,the resulting PSA had a shear strength of more than 99 h,a 180°peel strength of 2.460 N/cm,and a loop tack of 1.66 N,which was 6.0（180°peel strength of 0.407 N/cm）and 3.9 times（loop tack of 0.43 N）that of control sample,respectively.FTIR analysis showed that the peak of epoxy group weakened but the peak of free hydroxyl group strengthened,indicating that phosphoric acid could be used as a catalyst to promote the ring-opening of ESO;on the other hand,the emergence of phosphate ester bonds indicated that phosphoric acid could also participate in the crosslinking polymerization as a crosslinker.¹H NMR results further indicated that tea polyphenol palmitate was partially free and partially crosslinked in the PSA system;and rosin ester was partially embedded in the polymer structure in the covalent or physical form,and partially exposed to the polymer structure in the semi-embedded and free state.DSC and TGA results showed that tea polyphenol palmitate-based ESO-PSA had a lower glass transition temperature than control sample（-2.84°C vs.-2.62°C）,and the thermal decomposition temperatures of both were higher than 300°C.Compared with control sample（1669.99%）,the grafting of tea polyphenol palmitate reduced the peel strength residual rate to 138.09%,which greatly improved the antiaging property of ESO-PSAs.Secondly,highly di-hydroxylated soybean oils（DSO）were prepared by ring-opening of ESO,and then copolymerized as a soft monomer with ESO to further improve the loop tack of PSAs.Therefore,the effects of N-methylpyrrolidinone hydrogen sulfate（[Hnmp]HSO₄）,α-pyrrolidinone hydrogen sulfate（[Hnhp]HSO₄）,and binuclear pyrrolidinone hydrogen sulfate（[[Hnhp]HSO₄]₂）on the ring-opening of ESO were comparatively studied.The results showed that[Hnhp]HSO₄ has a stronger catalytic activity.Under the optimal conditions（6%[Hnhp]HSO₄,water-epoxy group ratio of 9/1,reaction temperature of 45°C,and reaction time of 4 h）,DSO with an epoxy value of 0.87%and hydroxyl group value（OHV）of 213.41 mg KOH/g were prepared,and the OHV conversion rate was 57.98%.In-situ infrared spectroscopy（in-situ IR）and Fourier infrared spectroscopy（FTIR）analysis showed that the rapid ring-opening of ESO and the formation of DSO mainly occurred at 2～3.5 h,and the viscosity of DSO was higher than that of ESO due to the increase of polarity and molecular weight and the formation of hydrogen bonds after ring opening.ESO and DSO still exhibited thermal stability at up to 350°C.[Hnhp]HSO₄ catalyzed the epoxy groups of ESO to be almost completely ring-opened with the lower degree of etherification,thus leading the prepared DSO to have more available hydroxyl groups.Therefore,the resulting DSO can be processed as a chemical intermediate.Finally,ESO and DSO were copolymerized to prepare PSAs.Antioxidants（tea polyphenols palmitate,caffeate acid,ferulic acid,gallic acid,BHT,TBHQ,BHA,PG,tea polyphenols）were grafted into ESO/DSO-based PSAs to study their effects on the adhesion and antiaging properties of PSAs.The optimal preparation conditions were:ESO/DSO of 9/3（mass ratio）,0.8%PG,55%rosin ester,8%phosphoric acid,reaction time of 5 min,and reaction temperature of 50°C.Under the optimal conditions,the resulting PSA had a 180°peel strength of 1.718 N/cm,a loop tack of 4.62 N,and a shear strength of over 99 h,which were 2.0,1.3,and multiple times that（180°peel strength of 0.879 N/cm,loop tack of 3.59 N,and shear strength of 13.88 h）of control sample,respectively.Compared with control sample（484.07%）,the grafting of propyl gallate reduced the peel strength residual rate to 12.16%,which greatly delayed the aging rate of PSAs.The introduction of soft monomer DSO reduced the glass transition temperature of PSA to-18.40°C,enhanced the softness of polymer molecules,and increased the loop tack of PSA.Before and after the addition of PG,the thermal degradation temperatures of PSAs were both higher than 300°C.FTIR and ¹H NMR were used to detect the soaking solutions of PSAs and the PSAs after soaking.The results showed that PG and DSO were partially free and partially crosslinked in the PSA system,and rosin ester was partially embedded in the polymer structure in the covalent or physical form,and partially exposed to the polymer structure in the semi-embedded and free state.During the aging process,the resulting PSAs underwent slight hydrolysis,but still maintained excellent adhesion properties.