DEVELOPMENT OF NON-SOLVENT-TYPE ACRYLIC PSA WITH UV

DEVELOPMENT OF NON-SOLVENT-TYPE ACRYLIC PSA WITH UV IRRADIATION POLYMERIZATION Mitsuhiko Nakazawa, Researcher, Soken Chemical & Engineering Co.,Ltd., Japan Takayuki Shimada, Researcher, Soken Chemical & Engineering Co.,Ltd., Japan Yoichi Takizawa, Senior Chief Engineer, Soken Chemica 1 & Engineering Co.,Ltd., Japan Dr. Susumu Kawase, General Manager, Soken Chemical & Engineering Co.,Ltd., Japan Introduction Pressure-sensitive adhesives (PSA) are being used for various products represented by adhesive tape, adhesive labels, etc. because of their functional characteristics like instantaneous adhesiveness, repeated adhesiveness, tackiness, etc., as well as their ease of adhesive work, and the applications are spreading in various directions. Together with the expanding applications for PSA, the capabilities required for PSA also are widening, and various types of PSA have been developed. However, as a large part of these PSA uses large quantities of organic solvents, the magnitude of the load onto the environment from discharge of volatile organic compounds (VOC) during the application process is being viewed as a problem. Because of this, hot-melt types, emulsion types, and other technologies for nonsolvent-type PSA are being realized for a part of the applications for PSA. However, technologies for substitute non-solvent-type PSA in sectors requiring high performance in regard to weather resistance or heat resistance have not been completed, and acrylic PSA of the solvent type continue to be used widely. With the large change in social recognition towards giving preference to environment protection, the change to non-solvent-type high-performance acrylic PSA has become a large engineering task. We have engaged in the development of non-solvent-type acrylic PSA which can exhibit the same high performance as the solvent types. 85

Characteristics of Syrup-type PSA Emulsion-type PSA and hot-melt-type psa already have been realized as non-solvent-type acrylic PSA, but we have started the development of syrup types. A syrup-type PSA is composed of acrylic polymers, monomers, cross-linking components, and other components. Acrylic syrup is produced by dissolving polymer resin in acrylic monomer, by partial polymerization of monomers in bulk polymerization, etc. Various methods for the production of acrylic Syrup by bulk polymerization are proposed in patents. ~) 2)3) We have developed a technology for safe bulk. polymerization in a batch reaction vessel, by precise temperature control at the time of polymerization, and we have produced acrylic syrup. Table 1 shows the characteristics of the syrup-type PSA technology. A syrup-type PSA is produced by UV irradiation after application to the base material in order to harden the included t " i i monomer by polymerization. For this reason, there are no limitations in regard to the molecular weight of pressure-sensitive adhesive polymer, and it is a characteristic of this method that high-performance PSA can be obtained easily. Impairment of the polymerization hardening by Oxygen present at the time of polymerization hardening, impairment of the polymerization hardening by addition of a tackifier, reduction of the molecular weight by the formed polymers, etc. can be named as problem points. We have also been engaged in the development of a new tackifier providing strong adhesiveness even in regard to olefin surfaces without impairing the polymerization hardening. Polymer Design for Syrup-type PSA L The molecular weight (Mw) of solvent-type acrylic PSA used for high-performance applications in most cases are designed for a value of 400,000 to 600,000 or higher. For the polymer design for syruptype PSA, the solvent-type make-up design and the molecular weight (distribution) design can be used for reference as they are. However, the polymer design for a syrup-type PSA is completed by two separate polymerizations, the partial polymerization at the time of the manufacture of the acrylic syrup and the UV polymerization after application (Fig. 1). The purpose of the partial polymerization at the time of acrylic syrup manufacture is thickening of the 86

acrylic monomers, but as this also contributes to the po!ymer design, which widely influences the adhesive performance, it is necessary to perform polymerization under consideration of the final adhesive polymer design from the acrylic syrup manufacturing stage. Hardening Behavior of Syrup-type PSA The UV hardening behavior will be treated from the chang e of the conversion ratio when photoinitiator is added to the syrup-type PSA and actual UV irradiation if performed and the molecular weight of the formed polymer. Fig. 2 shows the relation between the UV energy with various irradiation strengths and the conversion ratio for a fixed addition amount of photoinitiator. Here, the UV irradiation was performed with covering of the irradiation surface by a polyester film separator to shut out oxygen. The polymerization ratio in the UV polymerization process is based on the general radical polymerization reaction theory, and it is influenced widely by the amount of radicals generated per unit of time. As the result, a high UV intensity is effective from the point of view of productivity etc., as it reduces the time required for polymerization hardening, but it means a lowering of the adhesive performance, as the molecular weight of the formed polymers decreases. Fig. 4 shows the relation between the number of passes and the conversion ratio when the photoinitiator addition amount is adjusted so that the molecular weight of the formed polymers (Mw 750,000) with an UV intensity of 40 mw/cm 2 and of 200 mw/cm 2 respectively. The result shows that the polymerization hardening speed becomes constant, independent of the UV irradiation strength, when the molecular weight of the formed polymer is obtained unequivocally. In other words, In order to realize the designed molecular weight (distribution) and to produce the target adhesive performance for syrup-type PSA, it is important to control the molecular weight (distribution) in the UV irradiation process, and this is done by adjustment of the UV irradiation conditions andthe concentration of the photoinitiator. v'.", Next, the hardening behavior will be compared for the case when the UV irradiation surface is covered with a polyester film separator and when the UV irradiation atmosphere is changed to an oxygen "i concentration of 5% and 1% respectively by nitrogen substitution. Fig. 5 shows the relation between the 87

number of UV irradiation passes and the conversion ratio under the various conditions. With an oxygen concentration of 5%, the polymerization hardening speed and the molecular weight of the formed polymer both drop widely in comparison to the method with covering with a polyester film, but with an oxygen concentration of 1%, the drop is small and it is in the range which can be realized. From this it can be said that at the time of adhesive tape manufacture it is required to either to cover the UV irradiation surface with a polyester film Sep~irator or similar.or to perform UV irradiation in an atmosphere with an oxygen concentration of 1% or less by substitution of nitrogen or similar. Formation of a Cross-linked Structure Acrylic PSA in most cases have a mixture of cross-linked components and form a cross-linked structure to improve the shear strength. Syrup-type PSAs have the characteristic that radical polymeric multifunctional monomers are mixed, and at the time of UV hardening, polymerization hardening is done together with the acrylic monomers, so that a cross-linked structure can be formed. Themethod of forming cross-linked structures by mixing of multifunctional monomers is used widely because of it has the advantage that 1 part-system adhesives can be made etc. However, in the case of a method with forming of a cross-linked structure at the time of polymerization hardening, the cross-linked structure is influenced widely by the polymerization hardening conditions, and scatter for the adhesive performance can occur easily. In other words, the adhesive performance can be reproduced only when the UV irradiation conditions are exactly the same. Thus, for our design this time, we have taken the method of mixing epoxy cross-linking agents, isocyanate cross-linking agefits, etc. used for solvent-type acrylic PSA, introducing components with functional groups reacting with them into the adhesive polymer, and forming the cross-linked structure by aging after polymerization hardening. This selection made it possible to suppress the scatter of the adhesive performance of syrup-type PSA and to widen the selection range for UV irradiation lamps, photoinitiator, line speed at the time of tape manufacture, etc. As shown in Table 2, there is a clear correlation between the amount of cross-linking agent added and the shear strength, and it was found that the degree of cross-linkage could be set freely by changing the amount of added cross-linking agent under otherwise the same conditions. BB

Influence of the Tackifier There are various types of performance required for acrylic PSA, and the materials to which adhesion is to be made also show an extreme variety. For this reason, in many cases it is not possible to obtain a sufficient adhesive performance only with acrylic components, and the adhesive performance is improved by addition of tackifiers to realize the desired diverse performance. For example, this is effective to improve the adhesive strength to olefin and other materials with a low-energy surface. However, when a tackifier as used conventionally in solvent-type acrylic PSA exists in a syrup-type PSA, impairment of polymerization hardening and decreased molecular weight of the formed polymers are caused as shown in Fig. 6. By adding newly developed tackifiers to some of the syrup-type PSAs developed this time, they showed good olefin adhesive performance and UV hardening. Adhesive Performance of the Syrup-type PSA "SK Dyne Syrup" Table 3 shows the attributes of the developed SK Dyne Syrup and the adhesion properties when used for double-faced tape. The SK Dyne Syrups A and B have n tackifier added, while the SK Dyne Syrups C and D have a new tackifier added. By adding the new tackifier, a high adhesive strength was obtained in regard to various materials and the target performance could be obtained. Manufacturing Equipment for Adhesive Tape Using Syrup-type PSA Fig. 7 shows an outline of the equipment for manufacture of adhesive tape using syrup-type PSA. Conclusion The syrup-type acrylic PSA "SK Dyne Syrup" developed this time provides an adhesive performance equivalent to that of solvent-type acrylic PSA, and it can be said that this is a technology which can realize solvent-flee adhesives for the high-performance field. From the point of view of the manufacturing characteristics, it is believed that it is still necessary to investigate the hardening characteristics and the manufacturing equipment, but development to a new fimctional adhesive product can be expected. 89

References 1) Patent JP83168610A 2) Patent JP91227305A 3) Patent JP95330815A 90

Table 1. Characteristics of the syrup-type PSA technology Adhesive performance,,',, Advantage Design, of high molecular weights is possible. (high-performance adhesive is Obtained) DiSadvantage, Addition of commercial tackifiers is difficult. (polymerization impairment by the tackifier) Production No solvents are used. Thick-layer application is possible. Polymerization impairment by oxygen. Processing 1 '-part system are possible by mixing of multitimctional monomers., Problems with smell and volatility as monomer are included. Table 2. Amount of added cross-linking agent and shear strength of syrup-type PSA Amount of cross-linking agent 1.0wt% 1.2wt% 1.4wt% 1.6wt% 1.8wt% Shear Streng(la '~ 20mm 20mm/lkg, SUS, at 80 0 drift measured over 1 hour 1.6mm 1. lmm 0.9mm 0.7mm 0.3mm Adhesive Type 2EHA/AA syrup, Heating residue 25% Cross-linking agent : L-75 (isocyanate-type cross-linking agent made by Soken Chemical & Engineering Co., Ltd.) Double-faced tape thickness : 150 ttm The back support of the measuring sample is polyester film. UV irradiation condition' UV lamp : H-bulb made by Fusion UV Systems, Inc. UV irradiation time 90 sec 91

: )!,/ Table 3. Feature Properties Adhesive performance of double faced tapes with SK Dyne Syrup SK Dyne Syrup A Good heat resistance SK Dyne 1604N type (solvent-type) sk Dyne Syrup E B Good heat resistance Low stink... Appearan e... ran par nt!iqu!... transparent Non-Volatile content (%) 19-21 24-26 SK Dyne Syrup C Good olef'm adhesion SK Dyne 1717 type (solvent-type) SK Dyne Syrup D Good plastics adhesion Low stink transparent liquid transparent liquid 34-36 34-37... 10-20 10-20... Vi'sco's']'~"~a':'s'i"2"5"gCi... "i'0'-'20... 8-15 Adhesive performance Shear strength lkg/20x2omm, SUS, at 80 G 1. lnma 1. lmm 14mm 07mm drift measured over 1 hour Peel strength (N/m) 180 peel SUS 880 880 880 880 300mm/min PP 340 340 690 540......... ~,.....~ 23 C/65%R PVC -- -- -- 1100 Z 90 Holding power SUS 20mm 50m...8,.0...,C...:.,2..0,0.g...3..~...c.:..f:... 2~...c..:.f... 7~...c,.:.,f... 5 ~...c.:.f:... PP 40 C-lO0g Slipped down (6min) 40mm 1 lmm 30mm Ball tack (J.Dow method) Z30, at 23 C;/65%RH 32< 32< 26 16 Double faced tape Base material : Rayon nonwoven fabric (14g/~) Tape thickness : 150 ~ m (Back support of evaluation sample is 25//m PET) UV irradiation condition UV Lamp UV irradiation time H-bulb (Fusion UV Systems, Inc. ) 90sec 92

' ' 4 Solvent-type acrylic PS A Svrul~-tyOe acrylic PSA [i!iiiiiiii!iiii!i!!iiiii!ii[ii!!liiiiiiiiiiiiiiiiiiiii!iiii inoi~a~t(~:~oia~i INiiia~6~.i i! i i Batch production (rate of polymerization -- 1 OO%) (Polymerization process!) cryhc syrup[ Partial polymerization of monomers by bulk polymerization i i~~ii!iii~i!!ii~!iiii~iii!iii~!~!i!!i!~i~!iii!i!!~!!~ii!~i~ii!~i~i~i~ii~!i!ii!i!~i~i~i~iii~ii!ii~iii!ni~ Portion to be produced by... [ Reference curve [ UV irradiation polymerization... I curve[partial polymerization ] I.E+08 I.E+O7 1.E+06 I.E+05 1.E+04 1.E+03 I.E+02 molecular weight 1.E+08 1.E+07 I.E+06 1.E+05 '1.E7+04 1.E+03 1.E+02 molecular Weight [e:r6 s. s4!inkihg, i i.. i... ~] i ::i:i ::::::::::::::::::::::::::::::::: /~j~=======================================io~ ~~// Coating process I Coating process (Polymerization process 2) / :: ii :.,.;:" il ":',, N2 atmosphere IUV irradiation polymerization _--_-_-_-_-_-_-_-_-_-_-_-_-_-_- Product (PSA tape) Reference curve After UV irradiati 1.E+08 I.E+07 1.E+06 1.E+05 I.E+04 1.E+03 1.E+02 molecular weight Figure 1. Polymer design of syrup-type pressure-sensitive adhesive 93,?

100 --~ 80 O I,,,,,I = O ~,,,t 60 40 (D O =~ 20 0...~.... '..~ f #...... 4mW/cn4 - ~- 20mW/c~ 40mW/c - 100mW/c~ 0 500 1000 1500 2000 UV energy (1TIJ/ClTf) Figure 2. Relation between the estimated light amount and the conversion ratio when the amount of added photoinitiator is the same Adhesive" Type 2EHA/AA syrup, heating residue 25% Photoinitiator 0.3wt% Application quantity 57gm UV lamp H-bulb made by Fusion UV Systems, Inc. (irradiation laminated with polyester film)./ i m i i 60... s~-- ' I Y A...,:, ---;~?;:~:~,,O:::::-... Moleeule-wei~t... 1 ~. ::~... of the formed r~lvmeri 40 " -...'~~... l ~I~... 4mW/crr~ c.) Mw1160:000 t'- *i' t.- w 7o,ooo -~... :::</:~ t *~'~... 40mW/cm Mw 900,000 20 f... 8.~:: -/'::~*~: I loomw/cn~ Mw800000 0... -O...,, I I ' I 0 2 4 6 8 10 Number of passes i 1 Figure 3. Relation between the number of passes and the conversion ratio when the amount of added photoinitiator is the same Adhesive Type 2EHA/AA syrup, heating residue 25% Photoinitiator 0.3wt% Application quantity" 57gm UV lamp" H-bulb made by Fusion UV Systems, Inc. (irradiation laminated with polyester film) 94

100 8o ~. 60 i 40 ~ 2o 8 0 iiiiiiii 0 1 2 3 4 5 6 Number of passes Figure 4. Conversion ratio change when the molecular weight becomes constant Adhesive Type 2EHA/AA syrup, heating residue 25% ApplicatiOn quantity" 57t.tm UV lamp H-bulb made by Fusion UV Systems, Inc. (irradiation laminated with polyester film) 100. _. I - ' Mw 790,q OJ~Y'~m,' ~'~'~''... /.'..:s,.~...!... Mw..780.000.aa..**...4,-" w 700,0013 60... ~ ~ 40! g laminated wuth polyester film I-., 6 => 20 o "~ 'A" 5% 0 i ' '... g... 1% (oxygen concentration) 0 1 2 3 4 Number of passes _.-~.: I Figure 5. Influence of the oxygen concentration onto the hardening behavior Adhesive" Type 2EHA/AA syrup, heating residue 25% UV lamp ' H-bulb made by Fusion UV Systems, Inc. UV intensity 200 mw/cm2 (max. value) 95 i

...,.. 4 1W-w of the formed polymer "i... /~'. ilf...i[:.'[['"i i i:!i... -Mw400,000 1 ~ 60 i/~ ji!... I %i': Rosin resin additive syrup I i j~ili~,.,! = Styrene resin additive syrup I i(initial polymer not included) 20... i M-w 50,000 (Initial polymer not included) 0 ~ r--.,-... 0 1000 2000 3000 UV energy (mj/crd),/ / Figure 6. Tackifire influence at the time of UV polymerization Base syrup Heating residue 20% Tackifier addition" 20 wt% UV irradiation condition" UV lamp High pressure mercury lamp made by EYE GRAPHICS CO., LTD. UV intensity" 40mW/cm 2 (max. value) UV irradiation area Coating head ~:~ii:iii!:~.:ii::~i::~ i~: ::i~:ip01~efiizati0niiiiiiiiiii!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i!iiii~iiiii~ii~i~iiiii~iiiiiii~iiiii~ii~iii~iiii~iiiiiiiiiiiiii~iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii :::N: :::: ':: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :~il zatm~spherez~neiiiiii~i~iii~ii~iiiii~i~iii~iii~i!i!iiiiiiiiiiiiiiiiiiiiiiiiii~iiii Substrate -... Figure 7. PSA tape producing equipment using SK Dyne Syrup Product ; PSA tape 96 -~..