Influence of Acrylic Acid on Kinetics of UV-Induced Cotelomerization

materials

Article Inﬂuence of Acrylic Acid on Kinetics of UV-Induced Cotelomerization Process and Properties of Obtained Pressure-Sensitive Adhesives

Agnieszka Kowalczyk * , Mateusz Weisbrodt, Beata Schmidt and Konrad Gziut Department of Chemical Organic Technology and Polymeric Materials, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, 70-322 Szczecin, Poland; [email protected] (M.W.); [email protected] (B.S.); [email protected] (K.G.) * Correspondence: [email protected]

Received: 6 November 2020; Accepted: 8 December 2020; Published: 11 December 2020

Abstract: A new environmentally friendly method of photoreactive pressure-sensitive adhesives (PSAs) preparation was demonstrated. PSAs based on n-butyl acrylate (BA), acrylic acid (AA) and 4-acryloyloxy benxophenone (ABP) were prepared via the UV-induced cotelomerization process in the presence of a radical photoinitiator (acylphosphine oxide) and telogen (tetrabromomethane). Hydroxyterminated polybutadiene was used as a crosslinking agent. Influence of AA concentration (0–10 wt %) on kinetics of the cotelomerization process was investigated using a photodifferential scanning calorimetry method, selected physicochemical features of obtained photoreactive BA/AA/ABP cotelomers (molecular masses, polydispersity, monomers conversion and dynamic viscosity) and self-adhesive properties of obtained PSAs (adhesion, tack and cohesion) were studied, as well. It turned out that AA content is the important factor that influences monomers conversion (thereby the volatile parts content in prepolymer) and PSAs’ properties. As the acrylic acid content increases, the reaction rate increases, but the total monomers conversion and the solid content of the prepolymer decreases. Additionally, the adhesion and cohesion of PSAs were grown up, and their tackiness decreased. However, the AA content has no effect on molecular weights (Mw and Mn) and polydispersity (c.a. 1.5) of photoreactive cotelomers. The optimal AA content necessary to obtain a prepolymer with low volatile parts content and good PSA properties was determined.

Keywords: pressure-sensitive adhesives; telomerization; bulk photopolymerization; polyacrylates; adhesion; adhesive joints

1. Introduction Pressure-sensitive adhesives (PSAs) are ubiquitous and exceptional materials. They are used in most consumer applications and many industrial assembly operations. PSAs are unique because they create a strong bond with the surface with only ﬁnger pressure and they do not require activation (e.g., heat or water) [1,2]. For a long time, natural rubber [poly(1,4-cis-isoprene)] was used as base material for PSAs [3]. Since 1929 it is known that alkyl acrylate ester had the characteristic tack of PSAs. However, the ﬁrst formulations based on polybutyl acrylate and polyiso-butyl ether did not have the required tack and cohesive strength. Only in the 1950s, it was found that the performance of PSAs could be substantially improved by adding certain amounts of acrylic acid [4]. Today PSAs are made from a great variety of elastomers, i.e., natural, butyl, nitrile, styrene butadiene rubbers and polyurethanes, polyether, ethylene-vinyl acetate copolymers (EVA) and silicones [5]. However, polyacrylate adhesives are the most widely used [6]. Acrylic PSAs consists mainly of a “soft” monomer with a low glass transition temperature (Tg; i.e., butyl acrylate, iso-octyl acrylate and 2-ethylhexyl

Materials 2020, 13, 5661; doi:10.3390/ma13245661 www.mdpi.com/journal/materials Materials 2020, 13, 5661 2 of 13

acrylate) and a “hard” monomer with high Tg values of its homopolymer (i.e., methyl methacrylate, vinyl acetate and styrene) [7]. Nevertheless, acrylic PSAs must be chemically crosslinking in order to obtain the cohesive strength. For this purpose, polar monomers are incorporated (i.e., acrylic acid, 2-hydroxyl ethyl acrylate or N-vinyl pyrrolidone). However, in its simplest form, a good PSA is made from 95% of a soft monomer and 5% of acrylic acid [8]. Although the vast majority of adhesives are obtained by free radical polymerization (in the presence of the organic solvent), new methods of preparation are under development. Recently, solvent-free methods have been very popular, especially free radical bulk photopolymerization [9]. The process is initiated by the decomposition of the photoinitiator into radicals and occurs in a large volume of reagents (monomers), which are mechanical mixed during UV irradiation. The UV irradiation process is very short and lasts from a few to several dozen minutes. This is its unquestionable advantage compared to the classic radical polymerization in an organic solvent. The final product is the solution of the polymer in unreacted monomers called polymer syrup or prepolymer. An indispensable step in the preparation of PSAs by this method is the modification of the prepolymer with an additional portion of the photoinitiator and crosslinking agent and then UV irradiation of the adhesive composition, often in the presence of an inert gas or protective film. This is usually due to the high presence of unreacted monomers in the prepolymer. Nevertheless, the method is classified as proecological because it does not generate waste (recovered organic solvents or unreacted monomers) and is energy-consuming. This method was first introduced in the context of obtaining optically pure adhesives [10]. Significant is that the authors focused on testing new monomers (tetrahydrofurfuryl acrylate, acryloyl morpholine, menthyl acrylate and derivatives of monosaccharides) with classically used 2-ethylhexyl acrylate, mainly. The processes were carried out in the presence of 1-hydroxy-cyclohexyl-phenyl-ketone as a photoinitiator and 1-dodecanethiol as a chain transfer agent [11–15]. Recent research concerns the preparation of PSAs with controlled hydrophilic properties. The authors also used the chain transfer agent (1-dodecanothiol, 0.01 wt %) [16]. In turn, Zhu and at al. investigated the effect of 1,6-hexanediol diacrylate as a crosslinking agent on the performance of PSAs obtained by bulk photopolymerization of n-butyl acrylate, 2-hydroxyethyl acrylate and acrylic acid [17]. Telomerization (from Greek telos, meaning “end” and mer meaning “part”) is defined as the reactive process where a molecule YZ (named telogen), reacts onto a polymerizable compound M (named taxogen) to form telomeres of general formula Y(M)nZ[18]. The term of telomerization was introduced in 1946 by Peterson and Weber [19,20]. The cleavable bonds of the telogen can be C-H, S-H, P-H, Si-H or C-X, where X = Cl, Br or I [21]. It is noteworthy, that there are some differences between telomerization and polymerization. In telomerization fragments of the initiator mainly induced the cleave bond of the telogen and the number of M units in the final compound is low (n < 100), but is not 1. In contrast to polymerization, chain transfer and kinetic chain termination reactions occur much more frequently in telomerization, thus limiting the molecular mass of the reaction products termed telomeres. It should be also noted that telomerization is not a living polymerization but is a non-living reaction to obtained macromolecules [22]. There are known several initiation methods of telomerization reactions: thermal initiation, UV initiation and γ rays [23]. Nevertheless, the most recognized processes are these initiated by thermal initiators. Interestingly, there are only three publications about photoinduced telomerization (photoinitiated reaction between styrene and bromotrichloromethane [24], telomerization of acrylamide in water in the presence of disulfide telogen [25] and telomerization of tetrafluoroethylene initiated by benzoyl peroxide (as photoinitiator and telogen)) [26]. In the literature, relative numerous papers describe the telomerization process of acrylic acid [27–32]. In contrast, cotelomerization of acrylic monomers is rarely investigated (cotelomerization of acrylic acid and n-octyl acrylate, 2-ethylhexyl acrylate or 2-phenylethyl acrylate in presence of 2-aminoethanethiol hydrochloride [33] and cotelomerization of methyl methacrylate and maleic anhydride with dodecyl mercaptan [34]). In the last decade, the works about telomerization have been directed towards receiving the amphiphilic polymers [35] or block copolymers [36]. The latest scientific reports (2020) concern the preparation of fluorosurfactants [37], preparation of hydrophobic coatings [38] and Materials 2020, 13, 5661 3 of 13 pioneering studies on telomerization of chitosan and hemicellulose with butadiene in water [39]. Additionally, companies indicate that the telomerization process is particularly practical in industrial terms. In 2007, Dow Chemical on an industrial scale began the production of 1-octene from butadiene by palladium catalyzed telomerization, and since 2019 has been working on the process of telomerization of butadiene with methanol [40]. The bulk photopolymerization processes of acrylic monomers described in the literature are generally completed when the desired prepolymer viscosity is reached. Unfortunately, then the obtained prepolymer is characterized by a high content of unreacted monomers, i.e., 60 wt % or more [10–14]. Potentially, a high monomers content in the prepolymer can negatively affect their stability (pot life), lowers its viscosity (which is not always desirable) and determines the use of additional amounts of crosslinkers and using a top protection layer during UV crosslinking. The work presented here demonstrates a new and environmentally friendly method of obtaining photoreactive PSAs, i.e., a UV-induced cotelomerization process of main monomers (BA and AA). The authors’ intention was to obtain a photoreactive prepolymer (with 4-acrylooxy benzophenone) with the lowest possible content of unreacted monomers. Moreover, the influence of AA concentrations on kinetics of the process and self-adhesive properties of prepared PSAs were studied.

2. Materials and Methods

2.1. Materials The following components were used for preparation of BA/AA/ABP cotelomers: n-butyl acrylate (BA), acrylic acid (AA; BASF, Ludwigshafen, Germany) as monomers; 4-acryloylooxy benzophenone (ABP, Chemitec, Scandiccy, Italy) as copolymerizable photoinitiator; ethyl (2, 4, 6-trimethylbenzoyl)-phenyl-phosphinate (Omnirad TPOL, IGM Resins, Waalwijk, The Netherlands) as a radical photoinitiator and tetrabromomethane (TBM; Merck, Warsaw, Poland) as a telogen. The components were applied without puriﬁcation. A hydroxyl terminated polybutadiene resin Hypro 1200 90 HTB (CVC Thermoset Specialties, Emerald Kalama Chemical, Kalama, WA, USA) as a × multifunctional monomer of the UV crosslinking process was used.

2.2. Synthesis and Characterization of BA/AA/ABP Cotelomers The cotelomerization process of BA, AA and ABP were initiated using the radical photoinitiator Omnirad TPOL (0.1 phr; per hundred part of the monomers mixture) and tetrabromomethane (2.5 phr) as telogen was used. Mixtures of monomers and samples’ symbols were showed in Table1. The reaction mechanism was presented in Figure1. The cotelomerization processes were realized at 20 ◦C for 30 min in a glass reactor (250 mL), equipped with a mechanical stirrer and thermocouple, in the presence of argon as inert gas. A mixture of monomers (50 g) was introduced into the reactor and purged with argon for 20 min. The high-intensity UV lamp (UVAHAND 250, Dr. Hönle AG UV Technology, Gräfelting, Germany) as a UV radiation source was used and was placed perpendicularly to the side wall of the reactor. The UV irradiation inside the reactor (15 mW/cm2) was controlled with UV-radiometer SL2W (UV-Design, Brachttal, Germany). The reactor was water-cooled (using water in room temperature).

Table 1. Compositions of regents for the UV-induced cotelomerization process.

Monomers (wt %) Cotelomers Acronym Telogen (phr) Photoinitiator (phr) BA AA ABP BAA-0 99 0 1 2.5 0.1 BAA-2.5 96.5 2.5 1 2.5 0.1 BAA-5 94 5 1 2.5 0.1 BAA-7.5 91.5 7.5 1 2.5 0.1 BAA-10 89 10 1 2.5 0.1 Materials 2020, 13, 5661 4 of 13 Materials 2020, 13, x FOR PEER REVIEW 4 of 14

Figure 1. Schematic graph of the UV-induc UV-induceded cotelomerization process.process.

The cotelomerization viscosity of the processes cotelomers were solutions realized (BAAat 20 °C cotelomers for 30 min with in a glass unreacted reactor monomers, (250 mL), i.e.,equipped prepolymers) with a mechanical was measured stirrer at and 25 thermocouple,◦C by means of in thethe DV-IIpresence Pro of Extra argon viscometer as inert gas. (spindle A mixture #6, 50of rpm;monomers Brookﬁeld, (50 g) New was York,introduced NY, USA). into Thethe reac solidtor content and purged in cotelomers with argon solutions for 20 was min. determined The high- usingintensity Moisture UV lamp Analyzer (UVAHAND MA 50 250,/1.X2.IC.A Dr. Hönle (Radwag, AG UV Radom, Technology, Poland). Gräfelting, Samples Germany) (ca. 2 mg) as a were UV heatedradiation in aluminumsource was scaleused pansand was at the placed temperature perpendicu 105 ◦larlyC for to 4 the h. Gelside permeation wall of thechromatography reactor. The UV (GPC)irradiation was inside usedto the determine reactor (15 molecular mW/cm2) masses was controlled (Mw, Mn) with and UV-radiometer polydispersity SL2W (PDI) (UV-Design, of the BAA cotelomersBrachttal, Germany). (post-reaction The reactor mixtures was were water-co driedoled at 140 (using◦C for water 4 h beforein room the temperature). test to remove unreacted monomers);The viscosity the GPC of apparatusthe cotelomers contained solutions the refractive (BAA cotelomers index detector with (Merckunreacted Lachrom monomers, RI L-7490, i.e., Abingdon,prepolymers) UK), was pump measured (Merck at Hitachi 25 °C by Liquid means Chromatography of the DV-II Pro L-7100,Extra viscometer Abingdon, (spindle UK) and #6, interface 50 rpm; (MerckBrookfield, Hitachi New Liquid York, ChromatographyNY, USA). The solid D-7000, content Abingdon, in cotelomers UK) and solutions the Shodex was Ohpak determined SB-806 using MQ columnMoisture with Analyzer Shodex MA Ohpak 50/1.X2.IC.A SB-G precolumn. (Radwag, Radom, The GPC Poland). tests were Samples performed (ca. 2 mg) using were polystyrene heated in standardsaluminum (Flukascale pans and at Polymer the temperature Standards 105 Service °C for GmbH, 4 h. Gel Mainz, permeation Germany) chromatography and tetrahydrofurane. (GPC) was Theused kinetics to determine of the UV-inducedmolecular masses cotelomerization (Mw, Mn) and process polydispersity of BA, AA and(PDI) ABP of the (composition BAA cotelomers as in Table (post-1) werereaction tested mixtures using a were diﬀerential dried scanningat 140 °C calorimeterfor 4 h before with the aUV test attachment to remove (DSCunreacted Q100, monomers); TA Instruments, the NewGPC Castle,apparatus DE, USA;contained UV-light the emitterrefractive Omnicure index S2000;detector Excelitas (Merck Technologies, Lachrom RI Malvern, L-7490, PA,Abingdon, USA) at roomEngland), temperature pump (Merck (isothermal Hitachi measurement). Liquid Chromatogr Samplesaphy (5 mg)L-7100, were Abingdon, irradiated England) with UV and in the interface range 2 320–390(Merck Hitachi nm with Liquid an intensity Chromatography of 15 mW /D-7000,cm in argonAbingdon, atmosphere. England) All and DSC the Shodex photopolymerization Ohpak SB-806 experimentsMQ column with were Shodex conducted Ohpak triplicate. SB-G precolumn. Polymerization The GPC rate tests (Rp, were %/s) performed was calculated using according polystyrene to Equationstandards (1) (Fluka and conversionand Polymer of doubleStandards bonds Service (p, %)—according GmbH, Mainz to, EquationGermany) (2) and [41 ].tetrahydrofurane.

The kinetics of the UV-induced cotelomerization prdHocess of BA, AA and ABP (composition as in dt Table 1) were tested using a differential scanningRp = calorimeter with a UV attachment (DSC Q100, TA(1) Instruments, New Castle, DE, USA; UV-light emHitter0 Omnicure S2000; Excelitas Technologies, Malvern, PA, USA) at room temperature (isothermal measurement). Samples (5 mg) were irradiated ∆Ht with UV in the range 320–390 nm with anp = intensity 100%of 15 mW/cm2 in argon atmosphere. All DSC (2) ∆H0 × photopolymerization experiments were conducted triplicate. Polymerization rate (Rp, %/s) was where: dH/dt- heat ﬂow in the polymerization reaction, H —theoretical heat for the complete degree calculated according to Equation (1) and conversion of double0 bonds (p, %)—according to Equation of conversion (for acrylates: ∆H = 78.0 kJ/mol) and ∆H —the reaction heat evolved at time t. (2) [41]. t

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2.3. Preparation and Characterization of Obtained Pressure-Sensitive Adhesives (PSAs) The adhesive compositions for PSAs preparation were compounded using prepolymer (BAA cotelomers with unreacted parts; 90 wt %), HTB crosslinking agent (7.5 wt %) and TPOL photoinitiator (2.5 wt %). The compositions were applied onto polyester foil (50 µm) and UV-irradiated using the medium pressure mercury lamp (UV-ABC; Hönle UV-Technology, Gräfelfing, Germany). The UV doses were 2, 3, 4 or 5 J/cm2. The UV exposition was controlled with the radiometer (Dynachem 500; Dynachem Corp., Westville, IL, USA). The basis weight of samples was 60 g/m2. The kinetics of the UV crosslinking process of BAA cotelomers with HTB were tested using a differential scanning calorimeter with a UV attachment (DSC Q100, TA Instruments, USA; UV-light emitter Omnicure S2000; Excelitas Technologies, Malvern, PA, USA) at room temperature. Samples (5 mg) were irradiated with UV in the range 280–390 nm with an intensity of 500 mW/cm2. The kinetic curves as the dependence of the heat of reaction on the exposure time have been presented. Self-adhesive tests (adhesion to a steel, tack and cohesion at 20 C) were performed at 23 2 C ◦ ± ◦ and 50% 5% relative humidity. Adhesion to a steel substrate was tested according to AFERA 4001 ± and tack acc. to AFERA 4015. The test were carried out with strength machine Zwick Rolell Z010 (Zwick/Roell, Ulm, Germany). The cohesion at 20 ◦C tests were performed according to FINAT FTM 8. These parameters were evaluated using three samples of each adhesive film. The spectra of adhesive compositions and PSAs after the UV crosslinking process were obtained using the Fourier transform infrared spectroscopy (Nexus FT-IR, Thermo Nicolet, New Castle, DE, 1 USA). Variation of the absorbance value at 810 cm− (C=C double bond in acrylates) and absorbance 1 1 1 values at 975 cm− (1,4-cis), 966 cm− (1,4-trans) and 911 cm− (1,2-vinyl) of double bonds in HTB were monitored.

3. Results

3.1. Properties of BAA Cotelomers The results of the dynamic viscosity test and solid content for obtained BAA cotelomers solutions with diﬀerent content of acrylic acid (5; 2.5; 5; 7.5 or 10 wt %) are shown in Table2.

Table 2. Dynamic viscosity, solid content and molecular weights of BAA cotelomers.

Cotelomers Acronym η (Pa s) SC (%) Mn (g/mol) Mw (g/mol) PDI · BAA-0 47.6 91.4 13,470 20,290 1.51 BAA-2.5 29.0 87.2 12,210 18,630 1.53 BAA-5 19.6 82.6 12,870 19,590 1.52 BAA-7.5 14.5 76.3 14,115 21,710 1.54 BAA-10 9.1 70.3 13,810 21,415 1.55

As can been seen, the viscosity and solid content of cotelomers solutions decreased significantly as the acrylic acid content increased (η = 47.6 Pa s and SC = 91.4% for sample without AA and · η = 9.1 Pa s and SC = 70.3% for BAA cotelomer with 10 wt % of AA). Interestingly, the molecular · weights measurements exhibited very similar values of molecular weights (Mn ca. 12,200–14,100 g/mol and Mw ca. 18,600–21,700 g/mol) regardless of the AA content. The obtained values are much lower than in the case of bulk photopolymerization without the use of a classical chain transfer agent [17]. Additionally, the PDI values amounted to ca. 1.52–1.55 a.u. (slightly lower value of PDI was found for the sample without acid, i.e., 1.51 a.u.). That means that obtained cotelomers characterized low polydispersity index as in the case of polymers obtained by the ATRP method and near unimodal molecular weight distribution (1.1–1.2) [42]. It is known that molecular weight distribution of polymers affects the mechanical properties [43]. Preparation of polymers with such a low PDI index and low molecular weights was possible due to the use of a chain transfer agent (CBr4). Additionally CBr4 acts as telogen in the process. Due to this, the obtained products can be called cotelomers. Interestingly, Materials 2020, 13, x FOR PEER REVIEW 6 of 14 in the case of bulk photopolymerization without the use of a classical chain transfer agent [17]. Additionally, the PDI values amounted to ca. 1.52–1.55 a.u. (slightly lower value of PDI was found for the sample without acid, i.e., 1.51 a.u.). That means that obtained cotelomers characterized low polydispersity index as in the case of polymers obtained by the ATRP method and near unimodal molecular weight distribution (1.1–1.2) [42]. It is known that molecular weight distribution of polymers affects the mechanical properties [43]. Preparation of polymers with such a low PDI index andMaterials low molecular2020, 13, 5661 weights was possible due to the use of a chain transfer agent (CBr4). Additionally6 of 13 CBr4 acts as telogen in the process. Due to this, the obtained products can be called cotelomers. Interestingly, the photo-DSC measurements revealed that CBr4 also can act as photoinitiator. Barson andthe photo-DSCcoauthors measurementsshowed that revealedUV-induced that CBr telome4 alsorization can act as of photoinitiator. styrene in Barsonthe presence and coauthors of showed that UV-induced telomerization of styrene in the presence of bromotrichloromethane without bromotrichloromethane without the photoinitiator taking place [24]. They proved that CCl3Br acts the photoinitiator taking place [24]. They proved that CCl Br acts both as a photoinitiator and as a both as a photoinitiator and as a transfer agent. We tested two3 systems: without CBr4 and with CBr4 (bothtransfer contained agent. Wea photoinitiator tested two systems: TPOL). withoutThe photo- CBrDSC4 and thermogram with CBr4 (both was containedshown in aFigure photoinitiator 2. It is knownTPOL). that The heat photo-DSC flow rate thermogramis proportional was with shown the polymerization in Figure2. It rate is known [44]. The that system heat flowcontaining rate is proportional with the polymerization rate [44]. The system containing CBr reached a higher value of CBr4 reached a higher value of heat flow than the system without CBr4, hence4 the conclusion that a heat flow than the system without CBr , hence the conclusion that a sample with a telogen polymerized sample with a telogen polymerized faster.4 On this basis, it was found that the CBr4 molecule also faster. On this basis, it was found that the CBr molecule also acted as a photoinitiator (the same as acted as a photoinitiator (the same as CCl3Br in4 the Barson’s work). The observed increase in the reactionCCl3Br rate in the was Barson’s most likely work). due The to the observed generation increase of additional in the reaction radicals rate from was the most telogen likely molecule. due to the generation of additional radicals from the telogen molecule.

Figure 2. UV-induced cotelomerization curves for monomers compositions (BA—94 wt %, AA—5 wt %, Figure 2. UV-induced cotelomerization curves for monomers compositions (BA—942 wt %, AA—5 wt ABP-1 wt % and TPOL-0.1 phr) with CBr4 (2.5 phr) and without CBr4 (Io = 15 mW/cm and 320–390 nm). %, ABP-1 wt % and TPOL-0.1 phr) with CBr4 (2.5 phr) and without CBr4 (Io = 15 mW/cm2 and 320–390 nm).The heat flow for a sample with CBr4 was higher than for samples without CBr4. Additionally, photo-DSC measurements revealed that the UV-induced cotelomerization process was faster in the presenceThe heat of acrylicflow for acid. a sample In Figure with3a CBr the4 kineticswas higher curves than were for samples shown. Thewithout polymerization CBr4. Additionally, rate (R p ) photo-DSCvalues were measurements higher for systems revealed with that AA the (the UV-ind highestuced values cotelomerization was noticed process in case ofwas 10 faster wt % in of AA).the presenceIt is also of characteristic acrylic acid. that In Figure the maximum 3a the kineti reactioncs curves speed were (maximum shown. peak The on polymerization the curves) was rate reached (Rp) valuesafter ca.were 90 higher s of UV for irradiation. systems with Moreover, AA (the a highest second values peak on was the noticed curve isin discerniblecase of 10 wt (most % of visible AA). It in isthe also case characteristic of sample BAA-7.5),that the maximum which may reaction confirm speed a double (maximum way ofpeak initiating on the acurves) reaction was (by reached radicals aftergenerated ca. 90 s from of UV photoinitiator irradiation. Moreover, and telogen a photolysis).second peak Regardingon the curve the is conversion discernible of (most acrylate visible groups in the(p, case Figure of 3sampleb), it should BAA-7.5), be noted which that may the highestconfirm final a do conversionuble way of was initiating achieved a byreaction the sample (by radicals without generatedAA (90%) from but afterphotoinitiator the longest and UV telogen irradiation photolysis). time (ca. Regarding 840 s). This the is dueconversion to the lower of acrylate polymerization groups (p,rate Figure (Rp) of3b), this it should system be (Figure noted3a). that Samples the highest with 2.5final and conversion 5 wt % of was AA achieved exhibited by a verythe sample similar withoutprofile ofAA conversion (90%) but curves after the and longest the final UV conversion irradiation value time (82% (ca. and 840 83%, s). This respectively). is due to Thethe samplelower polymerizationBAA-7.5 also achievedrate (Rp) of high this final system conversion (Figure 3a). (84%), Samples but in with shorter 2.5 UVand irradiation5 wt % of AA time exhibited (ca. 360 a s). veryThe similar lowest valueprofile of of final conversion conversion curves was and noticed the fi tonal the conversion system with value 10 wt (82% % of and AA 83%, (75%). respectively). In the case of Thea sample sample without BAA-7.5 AA also (BAA-0), achieved it can high be saidfinal thatconversion we are dealing (84%), withbut in a linearshorter photopolymerization UV irradiation time of (ca.a difunctional 360 s). The lowest monomer value (ignoring of final theconversion ABP influence, was noticed which to is the only system 1 wt %with in the 10 system).wt % of AA It is (75%). known Infrom the thecase literature of a sample that such without systems AA are(BAA-0), characterized it can bybe highsaid conversionthat we are values dealing [45 ].with a linear photopolymerizationIn contrast, systems of a withdifunctional AA behaved monomer like multicomponent (ignoring the ABP (multifunctional) influence, which systems, is only which 1 wt were % characterized by high reactivity (Rp) at the beginning of the reaction, there was a phenomenon of autoacceleration and consequently the conversion was lower. The slowdown of the reaction (visible after 180 s of irradiation for systems with AA, Figure3a) the carboxyl group could induce (the possibility of creating interchain hydrogen bonds in the systems, which makes access to double bonds difficult and inhibition of polymerization). It is worth noting that the conversion of acrylate groups’ results (Figure3b) was quite similar to the solids content in the systems after the cotelomerization process in the reactor. The differences in the achieved results were caused by the process conditions. Although the Materials 2020, 13, 5661 7 of 13 Materials 2020, 13, x FOR PEER REVIEW 7 of 14

UVin the dose system). and the It irradiation is known from time werethe literature identical that (in thesuch glass systems reactor are and characterized during the by photo-DSC high conversion test in aluminumvalues [45]. pans), the reaction in the reactor was carried out by mechanical mixing of the components.

0,450.45

0,400.40 BAA-0 BAA-2.5 0,350.35 BAA-5 BAA-7.5 0,300.30 BAA-10

0,250.25

(%/s) 0,200.20 p R 0.150,15

0,100.10

0,050.05

0,000.00 0 60 120 180 240 300 360 UV-irradiation time (s)

(a) (b)

(c)

FigureFigure 3. 3. Cotelomerization rate rate curves curves (a (),a), conversion conversion spectra spectra (b) ( band) and photo-DSC photo-DSC curves curves (c) of (c) UV- of UV-inducedinduced cotelomerization cotelomerization of ofBA BA and and ABP ABP and and vari variousous amount amount of of AA AA in thethe presencepresence ofof TPOLTPOL 22 photoinitiatorphotoinitiator and and CBr CBr4 4(I (I00= = 1515 mWmW/cm/cm ;; 320–390320–390 nm).nm). 3.2. Properties of PSAs Based on BAA Cotelomers

Based on the prepared BAA cotelomers solutions (with unreacted monomers), the hydroxyl terminated polybutadiene resin Hypro 1200 90 HTB and additional dose of photoinitiator TPOL, × the PSAsIn contrast, films were systems prepared. with Figure AA behaved4 shows thelike possible multicomponent cross-linking (multifunctional) reactions that occurredsystems, when which thewere adhesive characterized film was by exposed high reactivity to the UV (Rp) lamp at the (equipped beginning with of the UV-C, reaction, UV-B there and UV-Awas a phenomenon lamps). It is knownof autoacceleration from the literature and consequently that the ABP photoinitiatorthe conversion belongs was lower. to the The type slowdown II photoinitiators of the reaction and its action(visible was after based 180 on s of the irradiation abstraction for of systems hydrogen with from AA, the Figure coinitiator 3a) the molecule. carboxyl For group pressure-sensitive could induce adhesives,(the possibility the coinitiators of creating were interchain considered hydrogen to be thebonds tertiary in the carbon systems, atoms which of the makes monomer access molecules. to double bondsThis difficult process and takes inhibition place under of polymerization). the influence of UV-C It is worth radiation noting [46, 47that]. Anthe illustrative conversion mechanism of acrylate ofgroups’ the reaction results is shown(Figure in Figure3b) was4a. Thequite second similar type to of the reaction solids that content takes placein the during systems theexposure after the ofcotelomerization adhesive films isprocess cross-linking in the reactor. with HTB The and differences unreacted in monomers the achieved (BA results and AA). were The caused reaction by the is shownprocess in conditions. Figure4b. Although In this way the a UV semi-interpenetrating dose and the irradiation polymer time networkwere identical (semi-IPN) (in the isglass to create. reactor Theand course during of the these photo-DSC processes test was in examined aluminum by pans), photo-DSC. the reaction The samples in the (BAA reactor cotelomers was carried solutions, out by HTBmechanical and TPOL) mixing were of irradiated the components. in the range of the UV-A,B,C region with a UV dose of 500 mW/cm2. The photo-DSC curves are shown in the Figure5. As would be expected, the reaction rate increased with3.2. Properties the acrylic of acidPSAs content, Based on which BAA Cotelomers in fact is due to the higher content of unreacted monomers in systemsBased with on higher the prepared AA content. BAA cotelomers solutions (with unreacted monomers), the hydroxyl terminated polybutadiene resin Hypro 1200 × 90 HTB and additional dose of photoinitiator TPOL, the PSAs films were prepared. Figure 4 shows the possible cross-linking reactions that occurred when

Materials 2020, 13, x FOR PEER REVIEW 8 of 14

action was based on the abstraction of hydrogen from the coinitiator molecule. For pressure-sensitive adhesives, the coinitiators were considered to be the tertiary carbon atoms of the monomer molecules.

Materials 2020, 13, x FOR PEER REVIEW 8 of 14 action was based on the abstraction of hydrogen from the coinitiator molecule. For pressure-sensitive Materials 2020, 13, 5661 8 of 13 adhesives, the coinitiators were considered to be the tertiary carbon atoms of the monomer molecules.

(a) (b)

Figure 4. Photocrosslinking in pressure-sensitive adhesives (PSAs): (a) via copolymerizable photoinitiator ABP (hydrogen abstractor) and (b) via HTB and unreacted monomers.

solutions, HTB and TPOL) were irradiated in the range of the UV-A,B,C region with a UV dose of 500 mW/cm2. The photo-DSC(a) curves are shown in the Figure 5. As would(b) be expected, the reaction rate increased with the acrylic acid content, which in fact is due to the higher content of unreacted Figure 4.4.Photocrosslinking Photocrosslinking in pressure-sensitivein pressure-sensitive adhesives adhesives (PSAs): (a(PSAs):) via copolymerizable (a) via copolymerizable photoinitiator monomers in systems with higher AA content. photoinitiatorABP (hydrogen ABP abstractor) (hydrogen and abstractor) (b) via HTB and and (b unreacted) via HTB and monomers. unreacted monomers.

This process takes place under the influence of UV-C radiation [46,47]. An illustrative mechanism of the reaction is shown in Figure 4a. The second type of reaction that takes place during the exposure of adhesive films is cross-linking with HTB and unreacted monomers (BA and AA). The reaction is shown in Figure 4b. In this way a semi-interpenetrating polymer network (semi-IPN) is to create. The course of these processes was examined by photo-DSC. The samples (BAA cotelomers solutions, HTB and TPOL) were irradiated in the range of the UV-A,B,C region with a UV dose of 500 mW/cm2. The photo-DSC curves are shown in the Figure 5. As would be expected, the reaction rate increased with the acrylic acid content, which in fact is due to the higher content of unreacted monomers in systems with higher AA content.

2 Figure 5. Photo-DSC curves of adhesive compositions (I0 = 500 mW/cm ; 230–390 nm). Figure 5. Photo-DSC curves of adhesive compositions (I0 = 500 mW/cm2; 230–390 nm). The self-adhesive properties of PSAs ﬁlm based on BAA cotelomers, i.e., adhesion to steel substrate, The self-adhesive properties of PSAs film based on BAA cotelomers, i.e., adhesion to steel tack and cohesion at 20 ◦C, tested according to the UV dose used in the step of the crosslinking process (2;substrate, 3; 4 or 5 tack J/cm 2and) were cohesion presented at in20 Figure °C, tested6. As canacco berding seen to in Figurethe UV6a, dose adhesion used valuesin the for step all PSAsof the 2 ﬁlmscrosslinking UV crosslinking process (2; with 3; 4 doses or 5 J/cm 2 or 3) Jwere/cm2 increased,presented regardlessin Figure 6. of As the can AA be content. seen in In Figure contrast, 6a, 2 foradhesion samples values PSA-0 for and all PSA-2.5PSAs films the adhesionUV crosslinking values increasedwith doses with 2 or an 3 increaseJ/cm increased, of UV dose, regardless while for of PSAthe AA with content. more contentIn contrast, of AA for a samples decrease PSA-0 in adhesion and PSA-2.5 values the was adhesion noted. Itvalues is related increased to the with higher an cross-linking density of the samples PSA-5; PSA-7.5 and PSA-10, which contained more acrylic acid and more unreacted monomers in adhesive composition before the UV crosslinking process. It is known from the literature that an increase in the cross-linking density of adhesives reduces their adhesion [48].

In the case ofFigure samples 5. Photo-DSC with a high curves acid of contentadhesive (PSA-10),compositions no (I damage0 = 500 mW/cm was observed,2; 230–390regardless nm). of the UV dose used. The conducted tests also proved that the adhesion of PSAs increased signiﬁcantly withThe the increaseself-adhesive of acrylic properties acid content. of PSAs For film PSA-10 based the on adhesion BAA cotelomers, values reached i.e., adhesion 9–11.5 N to/25 steel mm substrate,and for reference tack and sample cohesion PSA-0 at only20 °C, 0.5–3 tested N/25 acco mm.rding The to increase the UV in dose adhesion used was in the the step result of of the an crosslinkingincrease in the process content (2; of 3; carboxyl 4 or 5 groupsJ/cm2) were in adhesives presented and in the Figure formation 6. As of can hydrogen be seen bonds in Figure with the6a, adhesionsteel surface. values In for turn, all the PSAs tack films values UV decreasecrosslinking with with an increase doses 2 inor the3 J/cm UV2 doseincreased, (from regardless 3 to 5 J/cm of2), theregardless AA content. of the In AA contrast, content. for In thesamples case of PSA-0 this test, and the PSA-2.5 interaction the adhesion between values surface increased and PSA ﬁlmwith was an less important (the contact time with the test surface was only a few seconds, while in the adhesion test

Materials 2020, 13, x FOR PEER REVIEW 9 of 14

increase of UV dose, while for PSA with more content of AA a decrease in adhesion values was noted. It is related to the higher cross-linking density of the samples PSA-5; PSA-7.5 and PSA-10, which contained more acrylic acid and more unreacted monomers in adhesive composition before the UV crosslinking process. It is known from the literature that an increase in the cross-linking density of adhesives reduces their adhesion [48]. In the case of samples with a high acid content (PSA-10), no damage was observed, regardless of the UV dose used. The conducted tests also proved that the adhesion of PSAs increased significantly with the increase of acrylic acid content. For PSA-10 the adhesion values reached 9–11.5 N/25 mm and for reference sample PSA-0 only 0.5–3 N/25 mm. The increase in adhesion was the result of an increase in the content of carboxyl groups in adhesives and Materialsthe2020 formation, 13, 5661 of hydrogen bonds with the steel surface. In turn, the tack values decrease with an9 of 13 increase in the UV dose (from 3 to 5 J/cm2), regardless of the AA content. In the case of this test, the interaction between surface and PSA film was less important (the contact time with the test surface the adhesivewas only film a few adhered seconds, to while the surface in the adhesion for 20 min test before the adhesive tearing film off ).adhered In the caseto the of surface samples for with20 a high acidmin contentbefore tearing (PSA-10), off). noIn damagethe case wasof samples observed, withregardless a high acid of content the UV (PSA-10), dose used. no damage In turn, was the tack valuesobserved, decreased regardless with an of increase the UV dose in the used. UV In dose turn, (from the tack 3 tovalues 5 J/cm decreased2), regardless with an of increase the AA in content. the In theUV case dose of this(from test, 3 to the 5 interactionJ/cm2), regardless between of the surface AA content. and PSA In the film case was of less this importanttest, the interaction (the contact time withbetween the testsurface surface and wasPSA onlyfilm was a few less seconds, important while (the incontact the adhesion time with test the thetest adhesive surface was film only adhered a to thefew surface seconds, for 20 while min in before the adhesion tearing test off). the adhesive film adhered to the surface for 20 min before tearing off).

12 PSA-0 15 PSA-2.5 PSA-0 11 PSA-5 PSA-2.5 10 PSA-7.5 PSA-5 PSA-10 PSA-7.5 9 PSA-10 cf 8 10 7 cf cf 6 cf 5

4 Tack (N) 5 cf af 3 cf

Adhesion (N/25mm) Adhesion cf 2 cf 1 cf 0 0 2345 2345 UV dose (J) UV dose (J)

(a) (b)

PSA-0 60 PSA-2.5 PSA-5 PSA-7.5 PSA-10 50 0,140.14 0,120.12 0,100.10 Cohesion (h) Cohesion 0,080.08 0,060.06 0,040.04 0,020.02 0,000.00 2345 UV dose (J)

(c)

Figure 6. Self-adhesive features of PSAs based on BAA cotelomers: (a) adhesion to steel (cf-cohesive failure),Figure (b) tack6. Self-adhesive and (c) cohesion. features of PSAs based on BAA cotelomers: (a) adhesion to steel (cf-cohesive failure), (b) tack and (c) cohesion. More important is the stiﬀness of the adhesive ﬁlm, which determines its ability to wet/stick to the substrate.More The important sample is PSA-7.5the stiffness was of distinguishedthe adhesive film, among which all determines samples. its Probably ability to awet/stick good balance to betweenthe crosslinksubstrate. densityThe sample and PSA-7.5 polar carboxyl was distinguished groups content amongin all PSA-7.5 samples. gave Probably it higher a good tack. balance It is also knownbetween that adhesion crosslink and density tack and of PSAs polar depend carboxyl on grou glassps content transition in PSA-7.5 temperature gave it of higher the base tack. polymer It is also [49]. Glassknown transition that temperaturesadhesion and tack were of determinedPSAs depend foron glass samples transition after temperature the crosslinking of the processbase polymer with the 2 UV dose of 4 J/cm (PSAs were free from adhesive failures). The results are shown in Figure7 as the dependence of the adhesion, tack and glass transition temperature on the acrylic acid content. The Tg values increased with increasing AA content (from 45 to 27 C, for 0 wt % and 10 wt % − − ◦ of AA, respectively). It is known that the glass transition temperature of poly(acrylic acid) is about 100 ◦C. Hence, its higher content in samples caused an increase in their Tg values. As the Tg values of PSAs increased, the adhesion increased and their tack decreased. The reduction in stickiness is caused by limitations in the mobility of the polymer chains at a higher Tg, thereby worse wetting the surface. The third feature of PSAs adhesives is their cohesion. The results of performed tests are shown in Figure6c. As can be seen, the highest desired result (72 h) was achieved in the case of sample PSA-10, regardless of the applied UV dose. This is most likely due to two factors. Firstly, PSA-10 contained the most carboxyl groups, hence the most hydrogen bonds in the system. Secondly, PSA-10 had the most unreacted monomers before UV crosslinking, hence the densest polymer network was formed at the cocrosslinking stage with HTB (a large proportion of crosslinking polymerization over linear polymerization). For these reasons, the PSA-10 sample had the highest cohesion. In case of sample Materials 2020, 13, 5661 10 of 13

PSA-7.5 the cohesion values after crosslinking using 4 or 5 J/cm2 of UV dose were also excellent. However, the presence of unreacted monomers had a greater negative effect than HTB, since the latter was only 7.5 wt % in the samples and unreacted acrylate monomers from ca. 9–30 wt %. 1 1 1 1 The characteristic peaks (810 cm− ; 911 cm− and 966 cm− and 975 cm− ) of PSAs crosslinked with different UV dose were monitored by FTIR spectroscopy. An exemplary FTIR spectrum (for PSA-7.5) was shown in Figure8. This adhesive was selected for the tests due to the fact that it had the highest tack of all prepared PSAs, high adhesion and the desired cohesion after irradiation with a UV dose 2 1 1 1 of 4 J/cm . With the increasing of UV dose, the intensity of peaks 810 cm− , 911 cm− and 975 cm− 2 Materialsdisappeared 2020, 13, (especiallyx FOR PEER REVIEW after using a high UV dose, i.e., 4 and 5 J/cm ). That means, that all unreacted10 of 14 acrylate monomers and vinyl double bonds in HTB reacted completely. The pendant double bonds [49].(vinyl-) Glass and transition 1,4-cis doubletemperatures bond in were HTB determined were readily for available samples andafter therefore the crosslinking participated process readily with in thecrosslinking. UV dose ofInstead, 4 J/cm2 (PSAs the trans- were double free from bonds adhesive were not failures fully reacted). The results and were are stillshown present in Figure in the 7 final as theproduct dependence (but in of a the low adhesion, concentration). tack and Probably, glass tran thissition is due temperature to the diffi culton the accessibility acrylic acid of content. this type of bonds and steric hindrances in the HTB chain.

-20 T 9 g -25 Adhesion Tack 8 / (N/25mm) Adhesion

-30 7

-35 6 (°C) g T 5

-40 (N)Tack

4 -45

3 -50 0246810 Acrylic Acid (wt %) Materials 2020, 13, x FOR PEER REVIEW 11 of 14 Figure 7. Adhesion, tack and Tg of PSAs as a function of AA content. Figure 7. Adhesion, tack and Tg of PSAs as a function of AA content.

The Tg values increased with increasing AA content (from −45 to −27 °C, for 0 wt % and 10 wt % of AA, respectively). It is known that the glass transition temperature of poly(acrylic acid) is about 100 °C. Hence, its higher content in samples caused an increase in their Tg values. As the Tg values of PSAs increased, the adhesion increased and their tack decreased. The reduction in stickiness is caused by limitations in the mobility of the polymer chains at a higher Tg, thereby worse wetting the surface. The third feature of PSAs adhesives is their cohesion. The results of performed tests are shown in Figure 6c. As can be seen, the highest desired result (72 h) was achieved in the case of sample PSA- 10, regardless of the applied UV dose. This is most likely due to two factors. Firstly, PSA-10 contained the most carboxyl groups, hence the most hydrogen bonds in the system. Secondly, PSA-10 had the most unreacted monomers before UV crosslinking, hence the densest polymer network was formed at the cocrosslinking stage with HTB (a large proportion of crosslinking polymerization over linear polymerization). For these reasons, the PSA-10 sample had the highest cohesion. In case of sample PSA-7.5 the cohesion values after crosslinking using 4 or 5 J/cm2 of UV dose were also excellent. However, the presence of unreacted monomers had a greater negative effect than HTB, since the latter was only 7.5 wt % in the samples and unreacted acrylate monomers from ca. 9–30 wt %. The characteristic peaks (810Figure cm− 8.1; FTIR911 cm spectra−1 and of 966 PSA-7.5 cm−1 afterand the975UV cm crosslinking−1) of PSAs crosslinked process. with different UV dose were monitoredFigure by 8. FTIRFTIR spectra spectroscopy. of PSA-7.5 An after exemplary the UV crosslinking FTIR spectrum process. (for PSA-7.5) was shown in Figure 8. This adhesive was selected for the tests due to the fact that it had the highest tack of4. all Conclusions prepared PSAs, high adhesion and the desired cohesion after irradiation with a UV dose of 4 J/cm2. InWith this thepaper, increasing the UV-induced of UV dose, cotelomerization the intensity process of peaks as a810 new cm and−1, environmentally911 cm−1 and 975 friendly cm−1 disappearedmethod of obtaining (especially PSAs after was using presented. a high UV The dose, effect i.e., of 4 acrylic and 5 J/cm acid2 ).content That means, on kinetics that allof theunreacted reaction acrylateand selected monomers features and of vinyl obtained double prepolymers bonds in HT andB reactedself-adhesive completely. properties The pendantof PSAs weredouble studied. bonds (vinyl-)We stated and that1,4-cis as doublethe acrylic bond acid in contentHTB were increased: readily available and therefore participated readily in crosslinking. Instead, the trans- double bonds were not fully reacted and were still present in the final - productThe (but rate in of a lowthe reactionconcentration). (Rp) increased, Probably, this is due to the difficult accessibility of this type of - bonds Theand stericconversion hindrances of the in themonomers, HTB chain. solid content in prepolymers and its viscosity values decreased, - The adhesion and cohesion values increased and tack values decreased; only in PSA-10 with the highest AA content, the highest values of adhesion and cohesion were obtained (and adhesive film were free from failures), regardless of the applied UV dose. Interestingly, that AA content had practically no influence on the molecular weights of the obtained cotelomers and their polydispersity. The optimal AA content in PSAs based on BA/AA/ABP cotelomers was found to be 7.5 wt %—the highest tack (10 N), adhesion (7.5 N/25 mm) and cohesion (72 h) values after UV crosslinking with 4 J/cm2 of UV dose were reached. Besides, it was found to achieve the highest monomer conversion (84%) in the shortest time (360 s).

Author Contributions: Conceptualization, A.K. and M.W.; methodology, A.K and M.W.; investigation, M.W., A.K., B.S. and K.G; writing—review and editing, A.K. and K.G.; visualization, M.W. and A.K.; supervision, A.K. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest.

References

Materials 2020, 13, 5661 11 of 13

4. Conclusions In this paper, the UV-induced cotelomerization process as a new and environmentally friendly method of obtaining PSAs was presented. The eﬀect of acrylic acid content on kinetics of the reaction and selected features of obtained prepolymers and self-adhesive properties of PSAs were studied. We stated that as the acrylic acid content increased:

- The rate of the reaction (Rp) increased, - The conversion of the monomers, solid content in prepolymers and its viscosity values decreased, - The adhesion and cohesion values increased and tack values decreased; only in PSA-10 with the highest AA content, the highest values of adhesion and cohesion were obtained (and adhesive ﬁlm were free from failures), regardless of the applied UV dose.

Interestingly, that AA content had practically no inﬂuence on the molecular weights of the obtained cotelomers and their polydispersity. The optimal AA content in PSAs based on BA/AA/ABP cotelomers was found to be 7.5 wt %—the highest tack (10 N), adhesion (7.5 N/25 mm) and cohesion (72 h) values after UV crosslinking with 4 J/cm2 of UV dose were reached. Besides, it was found to achieve the highest monomer conversion (84%) in the shortest time (360 s).

Author Contributions: Conceptualization, A.K. and M.W.; methodology, A.K. and M.W.; investigation, M.W., A.K., B.S. and K.G.; writing—review and editing, A.K. and K.G.; visualization, M.W. and A.K.; supervision, A.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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