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Application of selected 2-methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics
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Application of selected 2methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics Z. Czech, J. Kabatc, A. Kowalczyk, D. Sowa, E. Madejska
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S0143-7496(14)00206-1 http://dx.doi.org/10.1016/j.ijadhadh.2014.12.001 JAAD1603
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International Journal of Adhesion & Adhesives
Accepted date: 28 November 2014 Cite this article as: Z. Czech, J. Kabatc, A. Kowalczyk, D. Sowa, E. Madejska, Application of selected 2-methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics, International Journal of Adhesion & Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2014.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 APPLICATION OF SELECTED 2‐METHYLBENZOTHIAZOLES AS CATIONIC PHOTOREACTIVE CROSSLINKERS FOR PRESSURE‐SENSITIVE ADHESIVES BASED ON ACRYLICS Z. Czech1*, J. Kabatc2, A. Kowalczyk1, D. Sowa1, E. Madejska1
1
Institute of Chemical Organic Technology, West Pomeranian University of Technology, Pułaskiego 10, 70‐322 Szczecin, Poland 2
University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
ABSTRACT Since their introduction half a century ago, acrylic pressure‐sensitive adhesives have been successfully applied in many fields. They are used in self‐adhesive tapes, label signs, marking films and protective films as well as in medical pharmaceutical applications for plaster, in dermal dosage systems and in a wide range of biomedical electrodes. In the last 15 years or so, the UV technology, especially UV‐ crosslinking, is well established in the market and allows the production of UV‐crosslinkable pressure‐ sensitive adhesives (PSA) based on acrylics with interesting performance. So much so that the larger manufacturers of pressure‐sensitive adhesive materials and their suppliers now use very expensive equipment to study pressure‐sensitive adhesive behavior: tack, peel adhesion and shear strength. The balance between adhesive and cohesive strength after the crosslinking process is very important and critical for properties of acrylic PSA in form of self‐adhesive films. In this work the cationic UV‐ crosslinking of acrylic PSA containing epoxy groups in their structure and additionally cationic photoinitiators based on 2‐methylbenzothiazoles as photoreactive crosslinkers have been investigated using UV‐lamp as ultraviolet sources. The investigated acrylic PSA were synthesized from 80 wt.% of butyl acrylate, and 20 wt.% of glycidyl methacrylate. The use of selected photoreactive crosslinkers: 1,5‐ bis[N,N’‐(2‐methylbenzothiazolium)]pentane
diiodide
and
1,10‐bis[N,N’‐(2‐
methylbenzothiazolium)]decane diiodide allows manufacturing of high quality PSA materials with interesting properties, such as high tack, high peel adhesion, and excellent shear strength. * Corresponding author E‐mail address: [email protected] (Z. Czech); tel.: +48 91 449 4903; fax: +48 91 449 4365
2 Keywords: photoreactive crosslinkers based on 2‐methylbenzothiazoles, UV‐crosslinkable acrylic PSA, epoxies groups, tack, peel adhesion, shear strength INTRODUTION Modern pressure‐sensitive adhesive (PSA) technologies are growing in so many different directions, such as new applications, new materials, new techniques, new specialties‐that a technology seeking to design new products or to improve a process may sometimes overlook one of the new technologies in photoreactive PSA systems that yield improved bonding and aging properties‐ e.g. initiated crosslinking. The term ‘pressure‐sensitive adhesives’ refers to a permanently tacky composition, which will adhere to a variety of surfaces merely by application of light hand pressure. Ultraviolet (UV) is the most popular of the new crosslinking technologies and is applied using common industrial lamps range from 200 to 400 nm. PSA properties are determined by the nature of the monomers used during synthesis, molecular weight, glass transition temperature Tg, thickness of PSA layer, UV‐crosslinking time and UV‐dose. New types of photoreactive acrylic PSAs are synthesized using typical alkylacrylate monomers and monomers containing oxirane groups. New synthesized cationic photoinitiators based on 2‐methylbenzothiazole have been used as efficient photoreactive crosslinking agents. These photoreactive acrylic PSAs are characterized after UV‐crosslinking by high tack, high peel adhesion and excellent shear strength, especially at high temperatures [1,2]. UV‐RADIATION, UV‐POLYMERIZATION AND UV‐CROSSINKING The interaction of electromagnetic radiation with organic substrates is of widespread interest and has broad commercial applications. The use of UV radiation to alter the physical and chemical nature of materials is sometimes termed UV‐ crosslinking technology. Ultraviolet radiation sources are based on mercury‐vapor lamps. The mercury is enclosed in a quartz tube and a potential is applied to electrodes at either end of the tube. The electrodes can be of mercury, iron, tungsten, or other metals. In making a PSA by this process, a formulated acrylic PSA in the form of a solvent‐based composition or solvent‐free formulation are applied to an appropriate substrate and crosslinked using UV radiation emitted from conventional UV lamps or a UV excimer laser. Acrylic systems that are to be photocrosslinked also require photoinitiator or photosensitizer incorporated directly into polymer chain by polymerization or
3 by modification of side acrylic polymer chain after polymerization. These photoreactive systems that are to be photocrosslinked also require a photoinitiator or photosensitizer to absorb UV radiation and induce UV crosslinking process [3]. The UV radiation spectrum comprises wavelengths in the area between 200 and 400 nm. The UV spectrum is subdivided into UV‐A (320‐400 nm), UV‐B (280‐320 nm) and UV‐C (200‐280 nm) as indicated. UV‐A is transmitted by ordinary glass and plastic. Glass and plastic pass wavelengths above 320 nm and strongly attenuate below 320 nm. If pure quartz is used in the radiation path all wavelengths from 200 to 400 nm are passed. However, quartz is more expensive [4]. Photoinitiated polymerization is typically a process that transforms a monomer into polymer by a chain reaction initiated by reactive species (free radicals or ions), which are generated from photosensitive compounds, namely photoinitiators and/or photosensitizers, by ultraviolet irradiation [5‐7]. It offers high rate of polymerization at ambient temperatures, lower cost energy, and solvent free formulations, thus eliminating issues relating to air and water pollution [8,9]. It also devotes temporal and spatial control of the polymerization when high initiation rate is reached [10]. For photoinitiated polymerization radical or cationic photoinitiators are employed. Cationic polymerization overcomes volatile emissions, limitations due to molecular oxygen inhibition, toxicity, and problems associated with high viscosity. General scheme for photoinduced cationic polymerization is depicted in Figure 1.
PI
hν
PI *
R
+
R
+
M
Polymer
H
+
M
Polymer
Other products
Figure 1 General scheme of photoinitiated cationic polymerization A photosensitive compound, namely, photoinitiator (PI) absorbs incident light and undergoes decomposition leading to the production of initiating species. Active species, namely, a radical cation (R•+), in turn react with cationic polymerizable monomers (M), and yield polymer. Since the most significant element of photoinitiated cationic polymerization (PCP) is the cationic photoinitiators, their synthesis and initiation mechanism is one of the most important research areas for polymer science. Additionally, the reactivity of the initiating species is an important issue for an efficient photoinitiator.
4 The UV‐crosslinking of various coatings is based on the photoinitiation of radical and cationic crosslinking reactions. The ultraviolet crosslinking technique calls for the use of a photoinitiator to be added to the pressure‐sensitive adhesive system. The photoinitiator is therefore one of the key components in UV‐crosslinking, and the outcome of such a polymerization is critically dependent on the choice of the photoinitiator, including its chemical nature and the amount employed. As it was previously mentioned, a photoinitiator is one of the important and necessary constituents in UV crosslinking of pressure‐sensitive adhesives. For this reason, the activity of a photoinitiator is one of the most important properties that must be considered when choosing a photoinitiator [9,10]. In recent years, there have been many new developments in the synthesis and photochemical studies of novel photoinitiator molecules with more desirable properties such as higher activity coupled with greater reaction velocity coupled with low migration rate to the surface of the cured coating, in order to ameliorate shear strength and minimize toxicity where food contact is important. The concentration depends on the type of photoinitiator, but is typically 1 to 3 % by weight of the monomer. The photoinitiator breaks down under UV light to yield free radicals, which act as the trigger for the crosslinking mechanism. The selection of a particular photoinitiator for use in a composition is generally made on the basis of the solubility, rate of reaction, activating wavelength, and intended use of the photoinitiator [11]. In order to induce the photocrosslinking of an acrylic system, two types of photoinitiators are available. The first one induces a free radical process in which low molecular weight polymers with photoreactive chains are converted by the absorption of UV light into highly crosslinked, pressure‐sensitive adhesive acrylic films. In contrast with free radical‐type photoinitiators the second type of photoinitiator has been developed more recently for ring opening reactions of epoxy‐ and vinyl ether‐based monomers and polymers. The second type of photoinitiator are cationic photoinitiators, which can be characterized in the following ways: •
Onium salts (N‐alkoxypyridinium [12], allylic onium [13,14], trialkyl phenacyl ammonium [15], dialkyl phenacyl sulfonium [16,17], N‐methyl‐2‐alkylthiobenzothiazolium [18], aryldiazonium, diaryliodonium, triarylsulfonium, and tetraalkylphosphonium salts) – the most widely used cationic photoinitiators. They contain chromophoric groups as a light sensitive body with heteroatoms as cationic centers in the structure. Onium salts undergo direct photolysis and generate initiating species upon irradiation at appropriate wavelengths. The cationic polymerization of suitable monomers is initiated by both radical cation and/or protonic acid.
5 •
Iron arene complex‐based photoinitiators or those based upon ferrocenium. Upon irradiation, ferrocenium salts lose arene ligands leading to generation of iron‐based Lewis acids that coordinate with epoxide monomers.
•
Nonsalt photoinitiators (nitrobenzyl esters [19], sulfonyl ketones [20], phenacyl sulfones and phenyl disulfones [21], selenide [22] and organosilanes [23]. This type of initiator does not contain metal atoms or show the types of ionic character that are common with most conventional cationic photoinitiators.
EXPERIMENTAL Synthesis of cationic photoreactive acrylic PSA The cationic photoreactive solvent‐borne acrylic PSAs studied were based on a monomer mixture comprising 80 wt% of butyl acrylate and 20 wt% glycidyl methacrylate. This system was synthesised in ethyl acetate at a combined monomer concentration of 50% with 0.1% wt of free radical initiator (AIBN) at a temperature of 78°C for 2 hours. Following this initial polymerization, the resulting compounds were formulated with two photoreactive initiators i.e. 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐ (methylbenzothiazolium)]decane diiodide (SS10) in concentrations of 0.1 and 2 wt % (Figure 2). Both monomers, ethyl acetate and AIBN were sourced from BASF (Germany). S
S CH3
N
CH3 N
I (CH2)1
I (CH2)6
N
I
N CH3
S
SS5
I CH3
S
SS10
6 Figure 2 Chemical structures of photoreactive crosslinkers 1,5‐bis[N,N’‐(2‐methylbenzo‐ thiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) Preparing of PSA samples in form of self‐adhesive layers The final pressure‐sensitive adhesive properties in the form of self‐adhesive layers with 60 g/m² standard coating weight were coated on polyester film using a specially constructed coating machine from PSAT (Germany) (Fig. 3) and dried for 10 min at 105°C in a drying canal.
Figure 3 Coating machine for adhesive with coating weights between 5 and 2000 g/m² The UV‐crosslinking of dried self‐adhesive layers was achieved using UV‐lamp Aktiprint‐mini 18‐2 from Technigraf (Germany) with UV dosage of 800 mJ/cm² at exposure times of 4, 8 and 12 s.
7
Figure 4 UV‐lamp Aktiprint‐mini 18‐2
Investigated properties of cationic UV‐crosslinked acrylic PSA
The investigated solvent-based photoreactive acrylic pressure-sensitive adhesives having epoxy groups in the structure connected to the polymer chain were tested for typical PSA properties, such as tack, peel adhesion and shear strength as determined by standard A.F.E.R.A. (Association des Fabricants Europeens de Rubans Auto-Adhesifs) procedures. Exact details can be found in AFERA 4015 (tack), AFERA 4001 (peel adhesion) and AFERA 4012 (shear strength). Administrative address: 60, rue Auber-94408 Vitry Sur Seine Cedex, France. Those tests were conducted with the use of testing machine Zwick/Roell Z-25.
RESULTS AND DISCUSSION CATIONIC PHOTOINITIATORS BASED ON 2‐METHYLBENZOTHIAZOLES
Two-cationic quaternary ammonium 2-methylbenzothiazole derivatives can be used as a substrate for the synthesis of well-known polymethine dyes, these being popular sensitizers in dyeing photoinitiating systems for free radical polymerization of acrylates. On the other hand, these compounds can be also applied as photoinitiators for cationic polymerization of epoxides.
8 The cationic photoinitiators based on 2-methylbenzothiazole were synthesized by the bisquaternization reaction of heterocyclic compound with α,ω-dihalogenalkane in dioxane as a solvent [24] (Fig. 5). S CH3 N
S 2
CH3 N
X +
X (CH2)n
(CH2)n N X
X
CH3 S
Figure 5 Synthesis of cationic photoinitiators based on 2‐methylbenzotiazole The two‐cationic ammonium salts, 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) studied, absorb light in a range from 300 to 500 nm that thus make them preferable for cationic polymerization. As can be seen form Figure 6, the spectral response of 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) is in the range 350 to 450 nm. Therefore, for practical applications, where polymerization or radiation crosslinking needs to be performed at low energies, since the commercially available high pressure mercury lamps emit light with wavelengths longer than 350 nm (Fig. 6), these compounds are very good candidates as cationic photoinitiators for industrial applications. Moreover, for industrial radiation curing applications, the use of one‐component photoinitiators that have long wavelength absorption characteristics may still be advantageous because of the additional problems associated with co‐initiator systems, such as solubility, compatibility, migration, and cost problems.
9
Absorbance [a.u.]
3
2
1
0 300
400
500
600
700
800
Wavelength [nm]
Figure 6 The absorption spectra of 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS5) in tetrahydrofuran as a solvent at room temperature As shown in Figure 7 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) undergoes a very slow photobleaching process. In other words, they are photochemically stable and may be significantly attractive for photoinduced cationic polymerization.
(A0-At)/A0 [%]
0,08
Absorbance [a.u.]
2
0,04
0,00 0
1
320
1000
1500
2000
Time [min]
0 15 min 10 h 15 h 30 h
0
500
400
480
560
Wavelength [nm]
Figure 7 The photobleaching process of 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) under irradiation with visible light in THF solution at room temperature
10
The irradiation of new photoinitiators with suitable wavelengths leads to the formation of active species, which readily initiate polymerization of appropriate monomers. The photoinitiation of polymerization or crosslinking of epoxies or acrylic pressure-sensitive adhesives containing epoxy groups by new cationic photoinitiators 2-methylbenzothiazole derivatives is presented in Figure 8. S CH3
CH3
CH3 N
*
S
S
N
X hν
(CH2)n
N
X Intramolecular electron transfer
(CH2)n
(CH2)n N X
N X
N X
CH3
CH3
CH3 S
X
S
S R
S
S
CH3
CH3 N
O
N
X
X
O
O
(CH2)n
(CH2)n R
N X
N X
R
CH3
CH3 S
S
Figure 8 Crosslinking mechanisms of acrylic PSA containing oxirane groups using cationic photoinitiators based on 2‐methylbenzothiazole derivatives
Tack and peel adhesion of cationic UV-crosslinked acrylic PSA Important properties of UV‐crosslinked acrylic pressure‐sensitive adhesives as a function of cationic photoinitiator concentration and UV‐crosslinking time are illustrated in Figs. 9‐12. Figure 9 presents tack and Figure 10 peel adhesion of acrylic PSA containing between 0.1 and 2.0 wt.% of cationic photoreactive crosslinkers 1,5‐bis[N,N’‐(2‐methylbenzo‐thiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) after 4, 8 and 12 s UV radiation at 800 mJ/cm² UV dose.
11
14 12
Tack [N/2.5 cm]
10 8 SS10-800 mJ/cm²- 4 s SS10-800 mJ/cm²- 8 s SS10-800 mJ/cm²- 12 s SS5- 800 mJ/cm²- 4 s SS5- 800 mJ/cm²- 8 s SS5- 800 mJ/cm²- 12 s
6 4 2 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 9 Tack of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
14
Peel adhesion [N/2.5 cm]
12 10 8 6 4
SS10 SS10 SS10 SS5 SS5 SS5
800 mJ/cm²- 4 s 800 mJ/cm²- 8 s 800 mJ/cm²- 12 s 800 mJ/cm²- 4 s 800 mJ/cm²- 8 s 800 mJ/cm²- 12 s
1.0
1.5
2 0 0.0
0.5
2.0
Photoinitiator concentration [wt.%]
Figure 10 Peel adhesion of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
UV-crosslinked acrylic pressure-sensitive adhesives containing photoreactive crosslinkers 1,5bis[N,N’-(2-methylbenzothiazolium)]pentane
diiodide
(SS5)
and
1,10-bis[N,N’-(2-
12 methylbenzothiazolium)]decane diiodide (S10) show similar tack (Fig. 9) and peel adhesion (Fig. 10) profiles depending on both the concentration and the crosslinking time. Figs. 9 and 10 give typical examples with maximum of tack and peel adhesion values for small amount of cationic photoinitiator ranging between 0.8 and 1.3 wt.%. The tack and peel adhesion results reveal that for about 1.0 wt.% photoreactive crosslinkers based on 2-methylbenzothiazole the maximum of tack and peel adhesion was observed. The higher tack and peel adhesion values by applications of 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide (SS10) were noticed. It is also clear that the use of photoreactive crosslinkers with longer organic spacer moieties between the reactive 2-methylbenzothiazole groups promotes increasing bond properties when incorporated into the current photoreactive pressure-sensitive adhesives.
Shear strength of cationic UV-crosslinked acrylic PSA The shear strength of UV‐crosslinked acrylic PSAs containing epoxies groups and the additional cationic photoinitiators SS5 and SS10 as photoreactive crosslinkers measured at 20°C and 70°C as a function of photoinitiator concentration and UV‐crosslinking time are presented in Figs. 11 and 12.
Shear strength at 20°C [N/6,25 cm²]
100 90 80 70 60
SS10-800 mJ/cm²- 4 s SS10-800 mJ/cm²- 8 s SS10-800 mJ/cm²- 12 s SS5- 800 mJ/cm²- 4 s SS5- 800 mJ/cm²- 8 s SS5- 800 mJ/cm²- 12 s
50 40 30 20 10 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 11 Shear strength at 20°C of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
13
Shear strength at 70°C [N/6,25 cm²]
40 35 30 25 SS10-800 mJ/cm²SS10-800 mJ/cm²SS10-800 mJ/cm²SS5- 800 mJ/cm²SS5- 800 mJ/cm²SS5- 800 mJ/cm²-
20 15 10
4s 8s 12 s 4s 8s 12 s
5 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 12 Shear strength at 70°C of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
As indicated the shear strength of the acrylic PSA after UV-crosslinking is directly proportional to the concentration of the cationic photoreactive crosslinkers employed (Figs. 11 and 12). During the UV-crosslinking reaction, the elastomeric acrylic PSA chains react in the presence of photoreactive crosslinkers to form a chemical crosslinked network. At a certain stage in the cure process, after application of 1,5-bis[N,N’-(2-methylbenzothiazolium)]pentane diiodide (SS5) or 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide (SS10) into the base polymer results in a very strong chemical 3-dimensional network. For all of the UV-crosslinking times investigated, the measured temperature resistance shows very high level of 90 N at 20°C and 40 N at 70°C. Using of 1,5-bis[N,N’-(2-methylbenzo-thiazolium)pentane diiodide (SS5) the higher cohesion measured at 20°C and at 70°C are evaluated. The differences in reactivity of both photoinitiators may be due to different redox properties (Fig. 13). From the cyclovoltamperometric measurements it is seen that the length of the polymethylene chain connecting both heterocyclic moieties affects the redox potentials of the co‐initiators. The salt possessing the ten‐carbon atom linkage between two chromophores has lower reduction potential values than that of the co‐initiator with the shorter polymethylene chain. Based on this, the co‐initiator SS10 is more difficult to reduce. Therefore, the nucleophilic attack on the quaternary nitrogen atom occurs more easily in the case of co‐initiators possessing longer polymethylene chains.
14
Ered = -1,29 eV 0
Ered = -1,34 eV E red = -1,23 eV 0
-100
Current [μA]
Current [μA]
-50
-50
E red = -1,29 eV
-150 -100 -2000
-1000
0
1000
2000
Potential [mv]
-2000
-1000
0
1000
2000
Potential [mv]
Figure 13 Cyclic voltammograms of 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide
(SS10) in acetonitrile. Inset: Cyclic voltammograms of 1,5-bis[N,N’-(2methylbenzothiazolium)]pentane diiodide (SS5) in acetonitrile SUMMARY AND OUTLOOK
Cationic 2-methylbenzothiazoles are efficient UV initiated crosslinking agents for pressure – sensitive adhesives containing epoxy groups in the chemical structure.
In the absorption range between 300 and 450nm, both thermal and photochemical stabilities together with simple methods of synthesis, make new bisbenzothiazole-based cationic photoinitiators good candidates for UV-crosslinkable acrylic pressure-sensitive adhesives having high tack together with high levels of peel adhesion and shear strength over a range of temperatures.
The cationic UV-crosslinkable acrylic pressure sensitive adhesives developed, can be crosslinked under mild conditions in an air atmosphere. The presence of an inert gas during the cure process is not necessary to achieve UV-initiated crosslinking.
15 ACKNOWLEDGEMENT This work was supported by The National Science Centre (NCN) Grant No. 2013/11/B/ST5/01281. REFERENCES [1]
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Application of selected 2methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics Z. Czech, J. Kabatc, A. Kowalczyk, D. Sowa, E. Madejska
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International Journal of Adhesion & Adhesives
Accepted date: 28 November 2014 Cite this article as: Z. Czech, J. Kabatc, A. Kowalczyk, D. Sowa, E. Madejska, Application of selected 2-methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics, International Journal of Adhesion & Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2014.12.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 APPLICATION OF SELECTED 2‐METHYLBENZOTHIAZOLES AS CATIONIC PHOTOREACTIVE CROSSLINKERS FOR PRESSURE‐SENSITIVE ADHESIVES BASED ON ACRYLICS Z. Czech1*, J. Kabatc2, A. Kowalczyk1, D. Sowa1, E. Madejska1
1
Institute of Chemical Organic Technology, West Pomeranian University of Technology, Pułaskiego 10, 70‐322 Szczecin, Poland 2
University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
ABSTRACT Since their introduction half a century ago, acrylic pressure‐sensitive adhesives have been successfully applied in many fields. They are used in self‐adhesive tapes, label signs, marking films and protective films as well as in medical pharmaceutical applications for plaster, in dermal dosage systems and in a wide range of biomedical electrodes. In the last 15 years or so, the UV technology, especially UV‐ crosslinking, is well established in the market and allows the production of UV‐crosslinkable pressure‐ sensitive adhesives (PSA) based on acrylics with interesting performance. So much so that the larger manufacturers of pressure‐sensitive adhesive materials and their suppliers now use very expensive equipment to study pressure‐sensitive adhesive behavior: tack, peel adhesion and shear strength. The balance between adhesive and cohesive strength after the crosslinking process is very important and critical for properties of acrylic PSA in form of self‐adhesive films. In this work the cationic UV‐ crosslinking of acrylic PSA containing epoxy groups in their structure and additionally cationic photoinitiators based on 2‐methylbenzothiazoles as photoreactive crosslinkers have been investigated using UV‐lamp as ultraviolet sources. The investigated acrylic PSA were synthesized from 80 wt.% of butyl acrylate, and 20 wt.% of glycidyl methacrylate. The use of selected photoreactive crosslinkers: 1,5‐ bis[N,N’‐(2‐methylbenzothiazolium)]pentane
diiodide
and
1,10‐bis[N,N’‐(2‐
methylbenzothiazolium)]decane diiodide allows manufacturing of high quality PSA materials with interesting properties, such as high tack, high peel adhesion, and excellent shear strength. * Corresponding author E‐mail address: [email protected] (Z. Czech); tel.: +48 91 449 4903; fax: +48 91 449 4365
2 Keywords: photoreactive crosslinkers based on 2‐methylbenzothiazoles, UV‐crosslinkable acrylic PSA, epoxies groups, tack, peel adhesion, shear strength INTRODUTION Modern pressure‐sensitive adhesive (PSA) technologies are growing in so many different directions, such as new applications, new materials, new techniques, new specialties‐that a technology seeking to design new products or to improve a process may sometimes overlook one of the new technologies in photoreactive PSA systems that yield improved bonding and aging properties‐ e.g. initiated crosslinking. The term ‘pressure‐sensitive adhesives’ refers to a permanently tacky composition, which will adhere to a variety of surfaces merely by application of light hand pressure. Ultraviolet (UV) is the most popular of the new crosslinking technologies and is applied using common industrial lamps range from 200 to 400 nm. PSA properties are determined by the nature of the monomers used during synthesis, molecular weight, glass transition temperature Tg, thickness of PSA layer, UV‐crosslinking time and UV‐dose. New types of photoreactive acrylic PSAs are synthesized using typical alkylacrylate monomers and monomers containing oxirane groups. New synthesized cationic photoinitiators based on 2‐methylbenzothiazole have been used as efficient photoreactive crosslinking agents. These photoreactive acrylic PSAs are characterized after UV‐crosslinking by high tack, high peel adhesion and excellent shear strength, especially at high temperatures [1,2]. UV‐RADIATION, UV‐POLYMERIZATION AND UV‐CROSSINKING The interaction of electromagnetic radiation with organic substrates is of widespread interest and has broad commercial applications. The use of UV radiation to alter the physical and chemical nature of materials is sometimes termed UV‐ crosslinking technology. Ultraviolet radiation sources are based on mercury‐vapor lamps. The mercury is enclosed in a quartz tube and a potential is applied to electrodes at either end of the tube. The electrodes can be of mercury, iron, tungsten, or other metals. In making a PSA by this process, a formulated acrylic PSA in the form of a solvent‐based composition or solvent‐free formulation are applied to an appropriate substrate and crosslinked using UV radiation emitted from conventional UV lamps or a UV excimer laser. Acrylic systems that are to be photocrosslinked also require photoinitiator or photosensitizer incorporated directly into polymer chain by polymerization or
3 by modification of side acrylic polymer chain after polymerization. These photoreactive systems that are to be photocrosslinked also require a photoinitiator or photosensitizer to absorb UV radiation and induce UV crosslinking process [3]. The UV radiation spectrum comprises wavelengths in the area between 200 and 400 nm. The UV spectrum is subdivided into UV‐A (320‐400 nm), UV‐B (280‐320 nm) and UV‐C (200‐280 nm) as indicated. UV‐A is transmitted by ordinary glass and plastic. Glass and plastic pass wavelengths above 320 nm and strongly attenuate below 320 nm. If pure quartz is used in the radiation path all wavelengths from 200 to 400 nm are passed. However, quartz is more expensive [4]. Photoinitiated polymerization is typically a process that transforms a monomer into polymer by a chain reaction initiated by reactive species (free radicals or ions), which are generated from photosensitive compounds, namely photoinitiators and/or photosensitizers, by ultraviolet irradiation [5‐7]. It offers high rate of polymerization at ambient temperatures, lower cost energy, and solvent free formulations, thus eliminating issues relating to air and water pollution [8,9]. It also devotes temporal and spatial control of the polymerization when high initiation rate is reached [10]. For photoinitiated polymerization radical or cationic photoinitiators are employed. Cationic polymerization overcomes volatile emissions, limitations due to molecular oxygen inhibition, toxicity, and problems associated with high viscosity. General scheme for photoinduced cationic polymerization is depicted in Figure 1.
PI
hν
PI *
R
+
R
+
M
Polymer
H
+
M
Polymer
Other products
Figure 1 General scheme of photoinitiated cationic polymerization A photosensitive compound, namely, photoinitiator (PI) absorbs incident light and undergoes decomposition leading to the production of initiating species. Active species, namely, a radical cation (R•+), in turn react with cationic polymerizable monomers (M), and yield polymer. Since the most significant element of photoinitiated cationic polymerization (PCP) is the cationic photoinitiators, their synthesis and initiation mechanism is one of the most important research areas for polymer science. Additionally, the reactivity of the initiating species is an important issue for an efficient photoinitiator.
4 The UV‐crosslinking of various coatings is based on the photoinitiation of radical and cationic crosslinking reactions. The ultraviolet crosslinking technique calls for the use of a photoinitiator to be added to the pressure‐sensitive adhesive system. The photoinitiator is therefore one of the key components in UV‐crosslinking, and the outcome of such a polymerization is critically dependent on the choice of the photoinitiator, including its chemical nature and the amount employed. As it was previously mentioned, a photoinitiator is one of the important and necessary constituents in UV crosslinking of pressure‐sensitive adhesives. For this reason, the activity of a photoinitiator is one of the most important properties that must be considered when choosing a photoinitiator [9,10]. In recent years, there have been many new developments in the synthesis and photochemical studies of novel photoinitiator molecules with more desirable properties such as higher activity coupled with greater reaction velocity coupled with low migration rate to the surface of the cured coating, in order to ameliorate shear strength and minimize toxicity where food contact is important. The concentration depends on the type of photoinitiator, but is typically 1 to 3 % by weight of the monomer. The photoinitiator breaks down under UV light to yield free radicals, which act as the trigger for the crosslinking mechanism. The selection of a particular photoinitiator for use in a composition is generally made on the basis of the solubility, rate of reaction, activating wavelength, and intended use of the photoinitiator [11]. In order to induce the photocrosslinking of an acrylic system, two types of photoinitiators are available. The first one induces a free radical process in which low molecular weight polymers with photoreactive chains are converted by the absorption of UV light into highly crosslinked, pressure‐sensitive adhesive acrylic films. In contrast with free radical‐type photoinitiators the second type of photoinitiator has been developed more recently for ring opening reactions of epoxy‐ and vinyl ether‐based monomers and polymers. The second type of photoinitiator are cationic photoinitiators, which can be characterized in the following ways: •
Onium salts (N‐alkoxypyridinium [12], allylic onium [13,14], trialkyl phenacyl ammonium [15], dialkyl phenacyl sulfonium [16,17], N‐methyl‐2‐alkylthiobenzothiazolium [18], aryldiazonium, diaryliodonium, triarylsulfonium, and tetraalkylphosphonium salts) – the most widely used cationic photoinitiators. They contain chromophoric groups as a light sensitive body with heteroatoms as cationic centers in the structure. Onium salts undergo direct photolysis and generate initiating species upon irradiation at appropriate wavelengths. The cationic polymerization of suitable monomers is initiated by both radical cation and/or protonic acid.
5 •
Iron arene complex‐based photoinitiators or those based upon ferrocenium. Upon irradiation, ferrocenium salts lose arene ligands leading to generation of iron‐based Lewis acids that coordinate with epoxide monomers.
•
Nonsalt photoinitiators (nitrobenzyl esters [19], sulfonyl ketones [20], phenacyl sulfones and phenyl disulfones [21], selenide [22] and organosilanes [23]. This type of initiator does not contain metal atoms or show the types of ionic character that are common with most conventional cationic photoinitiators.
EXPERIMENTAL Synthesis of cationic photoreactive acrylic PSA The cationic photoreactive solvent‐borne acrylic PSAs studied were based on a monomer mixture comprising 80 wt% of butyl acrylate and 20 wt% glycidyl methacrylate. This system was synthesised in ethyl acetate at a combined monomer concentration of 50% with 0.1% wt of free radical initiator (AIBN) at a temperature of 78°C for 2 hours. Following this initial polymerization, the resulting compounds were formulated with two photoreactive initiators i.e. 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐ (methylbenzothiazolium)]decane diiodide (SS10) in concentrations of 0.1 and 2 wt % (Figure 2). Both monomers, ethyl acetate and AIBN were sourced from BASF (Germany). S
S CH3
N
CH3 N
I (CH2)1
I (CH2)6
N
I
N CH3
S
SS5
I CH3
S
SS10
6 Figure 2 Chemical structures of photoreactive crosslinkers 1,5‐bis[N,N’‐(2‐methylbenzo‐ thiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) Preparing of PSA samples in form of self‐adhesive layers The final pressure‐sensitive adhesive properties in the form of self‐adhesive layers with 60 g/m² standard coating weight were coated on polyester film using a specially constructed coating machine from PSAT (Germany) (Fig. 3) and dried for 10 min at 105°C in a drying canal.
Figure 3 Coating machine for adhesive with coating weights between 5 and 2000 g/m² The UV‐crosslinking of dried self‐adhesive layers was achieved using UV‐lamp Aktiprint‐mini 18‐2 from Technigraf (Germany) with UV dosage of 800 mJ/cm² at exposure times of 4, 8 and 12 s.
7
Figure 4 UV‐lamp Aktiprint‐mini 18‐2
Investigated properties of cationic UV‐crosslinked acrylic PSA
The investigated solvent-based photoreactive acrylic pressure-sensitive adhesives having epoxy groups in the structure connected to the polymer chain were tested for typical PSA properties, such as tack, peel adhesion and shear strength as determined by standard A.F.E.R.A. (Association des Fabricants Europeens de Rubans Auto-Adhesifs) procedures. Exact details can be found in AFERA 4015 (tack), AFERA 4001 (peel adhesion) and AFERA 4012 (shear strength). Administrative address: 60, rue Auber-94408 Vitry Sur Seine Cedex, France. Those tests were conducted with the use of testing machine Zwick/Roell Z-25.
RESULTS AND DISCUSSION CATIONIC PHOTOINITIATORS BASED ON 2‐METHYLBENZOTHIAZOLES
Two-cationic quaternary ammonium 2-methylbenzothiazole derivatives can be used as a substrate for the synthesis of well-known polymethine dyes, these being popular sensitizers in dyeing photoinitiating systems for free radical polymerization of acrylates. On the other hand, these compounds can be also applied as photoinitiators for cationic polymerization of epoxides.
8 The cationic photoinitiators based on 2-methylbenzothiazole were synthesized by the bisquaternization reaction of heterocyclic compound with α,ω-dihalogenalkane in dioxane as a solvent [24] (Fig. 5). S CH3 N
S 2
CH3 N
X +
X (CH2)n
(CH2)n N X
X
CH3 S
Figure 5 Synthesis of cationic photoinitiators based on 2‐methylbenzotiazole The two‐cationic ammonium salts, 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) studied, absorb light in a range from 300 to 500 nm that thus make them preferable for cationic polymerization. As can be seen form Figure 6, the spectral response of 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]pentane diiodide (SS5) is in the range 350 to 450 nm. Therefore, for practical applications, where polymerization or radiation crosslinking needs to be performed at low energies, since the commercially available high pressure mercury lamps emit light with wavelengths longer than 350 nm (Fig. 6), these compounds are very good candidates as cationic photoinitiators for industrial applications. Moreover, for industrial radiation curing applications, the use of one‐component photoinitiators that have long wavelength absorption characteristics may still be advantageous because of the additional problems associated with co‐initiator systems, such as solubility, compatibility, migration, and cost problems.
9
Absorbance [a.u.]
3
2
1
0 300
400
500
600
700
800
Wavelength [nm]
Figure 6 The absorption spectra of 1,5‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS5) in tetrahydrofuran as a solvent at room temperature As shown in Figure 7 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) undergoes a very slow photobleaching process. In other words, they are photochemically stable and may be significantly attractive for photoinduced cationic polymerization.
(A0-At)/A0 [%]
0,08
Absorbance [a.u.]
2
0,04
0,00 0
1
320
1000
1500
2000
Time [min]
0 15 min 10 h 15 h 30 h
0
500
400
480
560
Wavelength [nm]
Figure 7 The photobleaching process of 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) under irradiation with visible light in THF solution at room temperature
10
The irradiation of new photoinitiators with suitable wavelengths leads to the formation of active species, which readily initiate polymerization of appropriate monomers. The photoinitiation of polymerization or crosslinking of epoxies or acrylic pressure-sensitive adhesives containing epoxy groups by new cationic photoinitiators 2-methylbenzothiazole derivatives is presented in Figure 8. S CH3
CH3
CH3 N
*
S
S
N
X hν
(CH2)n
N
X Intramolecular electron transfer
(CH2)n
(CH2)n N X
N X
N X
CH3
CH3
CH3 S
X
S
S R
S
S
CH3
CH3 N
O
N
X
X
O
O
(CH2)n
(CH2)n R
N X
N X
R
CH3
CH3 S
S
Figure 8 Crosslinking mechanisms of acrylic PSA containing oxirane groups using cationic photoinitiators based on 2‐methylbenzothiazole derivatives
Tack and peel adhesion of cationic UV-crosslinked acrylic PSA Important properties of UV‐crosslinked acrylic pressure‐sensitive adhesives as a function of cationic photoinitiator concentration and UV‐crosslinking time are illustrated in Figs. 9‐12. Figure 9 presents tack and Figure 10 peel adhesion of acrylic PSA containing between 0.1 and 2.0 wt.% of cationic photoreactive crosslinkers 1,5‐bis[N,N’‐(2‐methylbenzo‐thiazolium)]pentane diiodide (SS5) and 1,10‐bis[N,N’‐(2‐methylbenzothiazolium)]decane diiodide (SS10) after 4, 8 and 12 s UV radiation at 800 mJ/cm² UV dose.
11
14 12
Tack [N/2.5 cm]
10 8 SS10-800 mJ/cm²- 4 s SS10-800 mJ/cm²- 8 s SS10-800 mJ/cm²- 12 s SS5- 800 mJ/cm²- 4 s SS5- 800 mJ/cm²- 8 s SS5- 800 mJ/cm²- 12 s
6 4 2 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 9 Tack of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
14
Peel adhesion [N/2.5 cm]
12 10 8 6 4
SS10 SS10 SS10 SS5 SS5 SS5
800 mJ/cm²- 4 s 800 mJ/cm²- 8 s 800 mJ/cm²- 12 s 800 mJ/cm²- 4 s 800 mJ/cm²- 8 s 800 mJ/cm²- 12 s
1.0
1.5
2 0 0.0
0.5
2.0
Photoinitiator concentration [wt.%]
Figure 10 Peel adhesion of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
UV-crosslinked acrylic pressure-sensitive adhesives containing photoreactive crosslinkers 1,5bis[N,N’-(2-methylbenzothiazolium)]pentane
diiodide
(SS5)
and
1,10-bis[N,N’-(2-
12 methylbenzothiazolium)]decane diiodide (S10) show similar tack (Fig. 9) and peel adhesion (Fig. 10) profiles depending on both the concentration and the crosslinking time. Figs. 9 and 10 give typical examples with maximum of tack and peel adhesion values for small amount of cationic photoinitiator ranging between 0.8 and 1.3 wt.%. The tack and peel adhesion results reveal that for about 1.0 wt.% photoreactive crosslinkers based on 2-methylbenzothiazole the maximum of tack and peel adhesion was observed. The higher tack and peel adhesion values by applications of 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide (SS10) were noticed. It is also clear that the use of photoreactive crosslinkers with longer organic spacer moieties between the reactive 2-methylbenzothiazole groups promotes increasing bond properties when incorporated into the current photoreactive pressure-sensitive adhesives.
Shear strength of cationic UV-crosslinked acrylic PSA The shear strength of UV‐crosslinked acrylic PSAs containing epoxies groups and the additional cationic photoinitiators SS5 and SS10 as photoreactive crosslinkers measured at 20°C and 70°C as a function of photoinitiator concentration and UV‐crosslinking time are presented in Figs. 11 and 12.
Shear strength at 20°C [N/6,25 cm²]
100 90 80 70 60
SS10-800 mJ/cm²- 4 s SS10-800 mJ/cm²- 8 s SS10-800 mJ/cm²- 12 s SS5- 800 mJ/cm²- 4 s SS5- 800 mJ/cm²- 8 s SS5- 800 mJ/cm²- 12 s
50 40 30 20 10 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 11 Shear strength at 20°C of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
13
Shear strength at 70°C [N/6,25 cm²]
40 35 30 25 SS10-800 mJ/cm²SS10-800 mJ/cm²SS10-800 mJ/cm²SS5- 800 mJ/cm²SS5- 800 mJ/cm²SS5- 800 mJ/cm²-
20 15 10
4s 8s 12 s 4s 8s 12 s
5 0 0.0
0.5
1.0
1.5
2.0
Photoinitiator concentration [wt.%]
Figure 12 Shear strength at 70°C of acrylic PSA containing photoreactive crosslinkers SS5 and SS10
As indicated the shear strength of the acrylic PSA after UV-crosslinking is directly proportional to the concentration of the cationic photoreactive crosslinkers employed (Figs. 11 and 12). During the UV-crosslinking reaction, the elastomeric acrylic PSA chains react in the presence of photoreactive crosslinkers to form a chemical crosslinked network. At a certain stage in the cure process, after application of 1,5-bis[N,N’-(2-methylbenzothiazolium)]pentane diiodide (SS5) or 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide (SS10) into the base polymer results in a very strong chemical 3-dimensional network. For all of the UV-crosslinking times investigated, the measured temperature resistance shows very high level of 90 N at 20°C and 40 N at 70°C. Using of 1,5-bis[N,N’-(2-methylbenzo-thiazolium)pentane diiodide (SS5) the higher cohesion measured at 20°C and at 70°C are evaluated. The differences in reactivity of both photoinitiators may be due to different redox properties (Fig. 13). From the cyclovoltamperometric measurements it is seen that the length of the polymethylene chain connecting both heterocyclic moieties affects the redox potentials of the co‐initiators. The salt possessing the ten‐carbon atom linkage between two chromophores has lower reduction potential values than that of the co‐initiator with the shorter polymethylene chain. Based on this, the co‐initiator SS10 is more difficult to reduce. Therefore, the nucleophilic attack on the quaternary nitrogen atom occurs more easily in the case of co‐initiators possessing longer polymethylene chains.
14
Ered = -1,29 eV 0
Ered = -1,34 eV E red = -1,23 eV 0
-100
Current [μA]
Current [μA]
-50
-50
E red = -1,29 eV
-150 -100 -2000
-1000
0
1000
2000
Potential [mv]
-2000
-1000
0
1000
2000
Potential [mv]
Figure 13 Cyclic voltammograms of 1,10-bis[N,N’-(2-methylbenzothiazolium)]decane diiodide
(SS10) in acetonitrile. Inset: Cyclic voltammograms of 1,5-bis[N,N’-(2methylbenzothiazolium)]pentane diiodide (SS5) in acetonitrile SUMMARY AND OUTLOOK
Cationic 2-methylbenzothiazoles are efficient UV initiated crosslinking agents for pressure – sensitive adhesives containing epoxy groups in the chemical structure.
In the absorption range between 300 and 450nm, both thermal and photochemical stabilities together with simple methods of synthesis, make new bisbenzothiazole-based cationic photoinitiators good candidates for UV-crosslinkable acrylic pressure-sensitive adhesives having high tack together with high levels of peel adhesion and shear strength over a range of temperatures.
The cationic UV-crosslinkable acrylic pressure sensitive adhesives developed, can be crosslinked under mild conditions in an air atmosphere. The presence of an inert gas during the cure process is not necessary to achieve UV-initiated crosslinking.
15 ACKNOWLEDGEMENT This work was supported by The National Science Centre (NCN) Grant No. 2013/11/B/ST5/01281. REFERENCES [1]
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