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Development of solvent-free acrylic pressure-sensitive adhesives
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 3604–3612
www.elsevier.com/locate/europolj
Development of solvent-free acrylic pressure-sensitive adhesives Zbigniew Czech *, Marta Wesolowska Szczecin University of Technology, Institute of Organic Chemical Technology Pulaskiego 10, 70-322 Szczecin, Poland Received 13 November 2006; received in revised form 25 April 2007; accepted 2 May 2007 Available online 13 May 2007
Abstract The present publication describes a problem to develop solvent-free acrylic pressure-sensitive adhesives (PSA). Solventfree PSA are established materials for the manufacturing of various self-adhesive products. Only by means of these acrylic PSA was it possible to succeed in drafting the present surprisingly efficient generation of double-sided pressure-sensitive adhesive tapes, medical products, protective masking films, films for the graphics market, and various specialty products. New applications and technical specifications stimulate the continuous development of new methods of polymerization of solvent-free acrylates. New syntheses of solvent-free acrylic PSA include polymerization in the reactor with removal of the solvent and polymerization on the carrier. The polymerization process is connected with UV-crosslinking. 2007 Published by Elsevier Ltd. Keywords: Acrylics; PSA; UV; Tack; Peel; Shear
1. Introduction Acrylic pressure-sensitive adhesives are used for various products represented by adhesive mounting tapes, labels, medical pads, protective and decorative films [1]. Their functional characteristics like instantaneous adhesiveness, repeated adhesiveness, tackiness, etc. as well as their ease of adhesive work, leads to the fact that applications are spreading in various directions. Together with the expanding applications of PSA, the capabilities required for PSA also are widening, and various types of PSA as solvent-borne, water-borne and solvent-free PSA have been developed.
*
Corresponding author. E-mail address: [email protected] (Z. Czech).
0014-3057/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2007.05.003
The desire to use PSA without having to deal with organic solvents has been existing since selfadhesive products have been in mass production [2]. The hot-melt types, dispersion types, and other technologies of solvent-free PSA have been realised for a number of the PSA applications. However, technologies to substitute solvent-free types of PSA in sectors requiring high performance in terms of weather resistance or heat resistance have not been completed, and therefore acrylic PSA of the solvent type continue to be widely used [3]. Solvent-free PSA achieved real practical significance in the 70s with the appearance of thermoplastic rubber, the styrene-butadiene (SBS) and styrene-isoprene (SIS) block copolymers [4]. These thermally reversible or physically crosslinking products allow the formulation of solvent-free PSA with good processing performance. For this reason they represent
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
an apparently promising advantage to the substitution of rubber solutions. The dimension of this substitution is surprising because these adhesives satisfy the requirements only in a very limited way. There is no need here to describe in detail the typical disadvantages such as unacceptable temperature resistance, unacceptable oxidation resistance or poor resistance to plasticizers. The adhesives must frequently be subjected to the exigencies of the process in order to achieve the melt characteristics required. It is primarily for this reason that solvent-free PSA based on SBS/SIS block copolymers have not achieved the performance expected of them and are only able to substitute rubber solutions in a limited field of application. According to BASF solvent-free self-adhesives [5] achieve a relatively small, but stable, market share. The ideal would be to link the proven good PSA properties of acrylic PSA produced or formulated in solvents or water with solvent-free acrylic self-adhesive systems. Acrylic pressure-sensitive adhesives which are UV-crosslinked or which are polymerizable on the carrier are promising solutions to reach this ideal. Photoinduced UV-crosslinking or UV-inducted photopolymerization are rapidly expanding technologies on PSA sector resulting from their main advantages such as solvent-free process, efficient and economical energy use, new performances and quality of chemical crosslinking bonding [6–10]. Three different manufacturing processes were tested in the experiment to synthesise solvent-free acrylic pressure-sensitive adhesives: • polymerization in the extruder with subsequent UV-crosslinking, • polymerization in the reactor with removal of the organic solvent and UV-crosslinking of the resulting solvent-free PSA, • polymerization of acrylic-syrup directly on the carrier to create the PSA layer.
2. Polymerization by the use of an extruder The first development work on using an extruder as the chemical reactor for polymerization was done about 63 years ago. A patent publication [11] describes a continuous process for the polymerization of acrylate monomers in a single or double screw extruder. A degree of transformation of 93– 99% is achieved by suitable screw geometry and
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by adjusting the temperature of the various zones of the extruder. The example given, however, does not cover a pressure-sensitive adhesive since the glass transition temperature (Tg) of the extrusion polymer was about 29 C. European patent [12] describes continuous radical polymerization in a special reactor. An isooctyl acrylate/acrylic acid combination is mentioned in the description and in the examples.
3. Experimental The following experiments were conducted in order to study the diverse parameter such as acrylic acid content, radical starter concentration and crew speed during the polymerization in the extruder, the viscosity, molecular mass, tack, peel adhesion, shear strength and other important properties of the polymerisation process such as polymer yield and polymerization conversion. In the case of polymerization in the reactor with removal of the organic solvent the performed trials have shown comparison of various acrylics solventfree PSA, influence on web speed at 100, 500 and 1100 g/m2. The UV-crosslinkable acrylics solventfree PSA were compared with UV-polymerizable acrylic PSA and a conventional acrylic dispersion. During the polymerization process directly on the carrier the conducted experiments have shown the relationship between the UV energy with various irradiation strengths and the conversion ratio for a fixed added amount of photoinitiator, the relation between the number of UV irradiation passes and the conversion ratio under the various conditions, the web speed vs. coat weight and coating costs of UV-crosslinked acrylic PSA based on UVpolymerizable syrup. 2-Ethylhexyl acrylate, methyl acrylate, acrylic acid and 2,2 0 -azo-diisobutyronitrile are available from Tokyo Chemical Industry Co. The unsaturated photoinitiator 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone (ZLI 3331) was synthesised at Szczecin University of Technology. The viscosity of the PSA synthesized in the extruder was measured with a viscometer RM 180 Rheomat, from Rheometric Scientific Company. Their molecular mass was determined by HPLC with Isoctratic Pump & DRI-Detector form HP Company. Tack, adhesion and cohesion of the investigated acrylic pressure-sensitive adhesives were tested
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
according to A.F.E.R.A. 4015 (tack), 4001 (adhesion) and 4012 (cohesion). Conversion [wt.%]
4. Polymerization in the extruder with subsequent UV-inducted crosslinking
22 100
20 18
90
16 Conversion Viscosity
80
14 12
Viscosity (20°C) [Pas]
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The goal of radical polymerization in the extruder is to produce solvent-free crosslinkable acrylic PSA, having a conversation rate above 98–99% (residual monomer content <1–2%), an average molecular mass Mw > 270 000 Dalton and a perfect optical quality (free of gel particles). We had proposed feeding the acrylate monomers into an extruder together with a starter and polymerizing it in situ. This would mean the viscose polymer being delivered from the extruder by way of a gear–wheel pump into a wide-slot nozzle, applied to a web and being cured by UV exposure. An elegant solution at first glance, but, as we soon discovered, also the solution with the most chemical and process-engineering traps.
shortening the real polymerization time. The greatest weakness in the system was the degassing zone which did not permit the most possible removal of the residual monomers at higher screw speeds.
4.1. Influence of the screw speed of the extruder on polymerization conversion and PSA viscosity
4.2. Influence of the methyl acrylate amount and the starter concentration
The experiments were carried out in a co-rotating double-screw extruder model LSM34GL with 8 zones, available from Leistritz Inc. (Germany) with eight heatable zones including a degassing zone, can adjust the speed of the screws and the temperature in the various zones in a wide range. The polymerization process was performed with a monomers mixture selected of 74.0 wt.% 2-ethylhexyl acrylate (2-EHA), 20.0 wt.% methyl acrylate, 5.0 wt.% acrylic acid (AA), 0.5 wt.% 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone (ZLI 0 3331) and 0.5 wt.% thermal initiator 2,2 -azo-diisobutyronitrile (AIBN) as the basic recipe. The main variable technical parameter is the screw speed of the extruder. As anticipated, the polymerization yield and the viscosity of the PSA produced declines with increasing screw speed as the reaction time actually available decreases by the following temperatures of several zones 1–8: 90/90/90/100/100/110/120/120 C (Fig. 1). The best results of the polymerization yield (99.3 wt.%) and viscosity of the synthesised PSA (20.3 Pa s) were achieved with a the screw speed of 3 rpm. An increase in speed of the extruder from 10 to 70 rpm, can lead to a bigger negatively effect on monomers conversion and on acrylic PSA viscosity. This is a consequence of the effect of
Starting from this basis the methyl acrylate content has been varied and so the quantity of starter, with the temperatures of the various zones and the screw speed being set as follows:
10
70
8 60
6 4
50 0
10
20
30
40
50
60
70
Screw speed [U/min] Fig. 1. Polymerization conversion and polymer viscosity dependence of screw speed.
• Temperature of zones 1–8: 90/90/90/100/100/ 110/120/120 C. • Screw speed: 3 rpm. The influence of the concentration of the starter AIBN and of the methyl acrylate content on the investigated properties such as polymerization conversion and viscosity are presented in Figs. 2 and 3. As expected, the increase of AIBN content affects positively the polymerization conversion. The viscosity shows a maximum for about 0.3 wt% of AIBN. The increase of the methyl acrylat concentration generally has a positive influence on shrinkage. The value of polymerization yield for acrylic PSA with more than 20 wt.% methyl acrylate stayed on the same level. The effect of AIBN and acrylic acid amount on molecular mass of the extruder-polymerized PSA is shown in Fig. 4. The increase of acrylic acid content and increase of initiator AIBN content has a beneficial effect on
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
Conversion [wt.%]
90
90
80
80
70
70
60
60 50
50
Viscosity Conversion
40
40
Viscosity (20°C) [Pas]
100
100
30
30
20
20 0.0
0.1
0.2
0.3
0.4
0.5
AIBN concentration [wt.%] Fig. 2. Effect of AIBN amount on polymerization yield and viscosity.
Conversion [wt.%]
95
25
90 20 85
Conversion Viscosity
80
15
Viscosity [Pas]
30
100
75 10 70 10
15
20
25
30
Methyl acrylate amount [wt.%]
ized with 10 wt.% of acrylic acid and about of 0.3 wt% of AIBN. 4.3. UV-crosslinking of extruder polymers An acrylic PSA determined during the conducted series of experiments has been selected with the highest molecular mass (Mw = 421 000 Dalton) as our model polymer. Although it was possible to crosslink this material 1000 lm thick (about 1 mm thick adhesive layer) coated on the polyester film with UV radiation by the use of a UV lamp U 350-M-I-DL from IST Company with an UV-A wavelength between 315 and 400 nm and a constant UV dose of 1000 mJ/cm2 using a crosslinking time between 30 s and 3 min. The results of tack, peel adhesion and shear strength are presented in Figs. 5 and 6. Tack, peel adhesion [N/2,5 cm]
110
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25
Tack Peeladhesion
20
15
10 5
0
Fig. 3. Effect of methyl acrylate amount on viscosity and polymerization yield.
20
40
60
80
100
120
140
160
180
UV-crosslinking time [s]
Molecular mass [Dalton]
400000 300000 200000 100000 0 0.0
0.1
0.2
0.3
0.4
0.5
AIBN content [wt.%]
Fig. 4. Influence of AIBN and acrylic acid concentration on molecular mass of PSA.
Fig. 5. Tack and peel adhesion of UV-crosslinked extruded acrylics PSA.
90
Shear strength [N/6,25 cm2]
1wt.%AA 3wt.%AA 5 wt.%AA 7wt.%AA 10 wt.%AA
500000
80 70
at 70°C at 20°C
60 50 40 30 20 10 0 20
the molecular mass of the synthesized PSA acrylics. The PSA with high molecular mass were polymer-
40
60
80
100
120
140
160
180
UV-crosslinking time [s] Fig. 6. Shear strength of UV-crosslinked extruded acrylics PSA.
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Dependencies shown in Figs. 5 and 6 point out that the UV exposure time can influence tack and peel adhesion of acrylic PSAs after ultraviolet radiation crosslinking negatively and the shear strength positively. Generally the crosslinked acrylic PSA with high molecular mass and high viscosity gives the self-adhesive layers with high level of cohesion and very good tack and peel adhesion performances after UV-crosslinking. 4.4. Evaluation of the results We were able to obtain a useable, acrylic pressure-sensitive adhesive by mass polymerization in the extruder, which would satisfy the normal requirements for a pressure-sensitive adhesive tape. The minimal observed drawbacks were as follows: 4.5. Gel particle content Because of the significant heat of the reaction in mass polymerization of the acrylate, it is almost impossible to adjust the temperature precisely, or at least this can only be done with difficulty, which leads from time to time to the formation of gels and products of degradation. 4.6. Residual monomer content The holding time, expressed by the speed of the screw, in the extruder can only be adapted to the polymerization process with some difficulty. De facto this means too short a reaction space and hence an unacceptable reaction time. On the other hand, extending the extruder does not guarantee the secondary reaction and thus the reduction of the quantity of residual monomer. The integrated degassing zone also ensures that removal of the residual monomers is incomplete. The best in the praxis accepted results were achieved by the very low screw speed of about 3 rpm. 5. Polymerization in the reactor with removal of the solvent Polymerization takes place in an organic solvent with low boiling point (e.g., ethyl acetate, hexane, acetone,), with an unsaturated photoinitiator being incorporated in the polymer chain during polymerization. The polymerization medium is then removed under a vacuum at high temperature.
The resultant polymer is then, in a manner similar to a classical solvent-free hot-melt adhesive, coated at high temperature and crosslinked in seconds or minutes with UV radiation. This concept is based on proved and tested techniques and comes with the least risks. However, it is built on a pseudohot melt pressure-sensitive adhesive, since solvents are indeed used. The significant point is that there is the advantage that, by contrast to the mild, slow drying process in adhesive tape coating, large quantities of solvent are rapidly driven from the adhesive material, which can then processed like a hot melt. The solvent removed can, naturally, be used again for the polymerization process [13]. As a standard, a pressure-sensitive adhesive acrylic has been polymerized with the following composition: 2-Ethylhexyl acrylate (2-EHA) Methyl acrylate (MA) Acrylic acid (AA) Thermical starter (AIBN) Unsaturated photoinitiator (ZLI 3331)
74.2 wt.% 20.0 wt.% 5.0 wt.% 0.3 wt.% 0.5 wt.%
in acetone with 50 wt.% solid content. Once the solvent (acetone) is removed, there remains a non-volatile, solvent- free highly viscous material, which can be processed on a hot-melt coating machine at temperatures of 110, . . . ,140 C. Higher temperature (approx. 150 C) over an extended period (24 h) had no deleterious effect on the adhesive properties (‘‘no heat degradation’’). Apart from the typical adhesive ratings such as adhesive power etc., we were interested in the relationship between adhesive application and crosslinking rate. The procedure was as follows in order to obtain useful values on the performance of such adhesives: • single-sided coating of polyester film (36 lm) in the range of 40, . . . ,120 g/m2 with constant UV radiation and varying coating rates. • production of a typical double-sided adhesive tape (with paper substrate) for medium-weight use at varying coating rates and constant radiation dose. Working conditions:
Extruder Wide slot nozzle UV radiation lamp from IST Company.
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
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Table 1 Comparision of various solvent-free acrylic pressure-sensitive adhesives Type
HM HM HM HM HM
Tack [N]
1 2 3 4 5
Peel adhesion [N]
11.5 12.0 6.0 3.5 2.0
70 C
20 C
70 C
12.5 11.5 8.0 7.0 7.0
5.5 5.0 4.0 3.0 2.5
80 60 30 10 7
30 20 10 6 5
An initial screening revealed that apart from the own polymer (HM1) and the solvent-free PSAs from a manufacturer of adhesive raw materials well-known in the market-place (HM 2–HM 5), none of the formulations were worthy of further examination (Table 1). Working process: Backing: Liner: Coat weight: Web speed:
wide slot nozzle 36 pm PETP-film siliconized paper 60 g/m2 5 m/min
The limits of the bond-coat weights were determined and the crosslinking rates using the own synthesized Products HM 1. These were 60 g/m2 and 120 g/m2 and between 5 and 20 m/min (Fig. 7). Peel adhesion at 20 C and shear strength at 70 C for UV-crosslinked solvent-free acrylic PSA were tested. It was determined that, within the various test groups (mass application/coating rate) the adhesive power at 20 C decreases with increasing coating rate and the cohesion decreases with increasing of the web speed on the coating machine. The performance at higher temperatures, however showed typical weaknesses of a solvent-free hot-melt. The UV-crosslinking was not able to make any improvement at it. The adhesion properties of a 35
Properties [N]
Adhesion (20ºC, 120 g/m2)
Cohesion (70ºC, 60 g/m2)
30 25
Adhesion (20ºC, 60 g/m2)
20 15 10 Cohesion (70ºC, 120 g/m2)
5
Shear strength [N]
20 C
double-sided adhesive tape, coated with a UVcrosslinked acrylate hot-melt pressure-sensitive adhesive are respectable and encouraging by comparison with a conventional adhesive tape, as Table 2 shows. The UV-crosslinked acrylics PSA have very good tack, peel adhesion and shear strength at room and at higher temperatures. The increase of temperature decrease a little bit their adhesiveness and cohesive performance. In the case of acrylic PSA dispersions their performance at higher temperatures are on the very low and unacceptable level. 6. Polymerization on the carrier In this case, polymerization is shifted from the extruder to the substrate web, in that acrylate monomers recipe made as a coating compound are polymerized by UV radiation with the exclusion of oxygen. The production of acrylic pressure-sensitive adhesive tapes by mass polymerization under the effects of UV radiation is based on a patent from Novacel Company [14]. As this patent has now expired, UV polymerization may be considered to be a process accessible to all. 3M was the first to pick up the process and constantly enhanced the production process and the compositions of adhesive tapes, as is documented in numerous patents since 1982 [15,16]. Adco (USA) applied for a European patent [17], in which the only claim is a different spectral distribution of the UV radiation to that specified by 3M in various patents. The prospects for winning a grant in Europe must be estimated as under 50%. The production principle is as follows:
0 0
5
10
15
20
25
Web speed [m/min]
Fig. 7. Influence of web speed at 60 g/m2 and 120 g/m2 coat weight on peel adhesion and shear strength.
• mixing the components and thickening to a coatable syrup by incorporating fillers, • coating the syrup in thickness of up to several millimetres onto an UV permeable process film,
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Table 2 Relevant properties of UV-crosslinked solvent-free pressure-sensitive adhesive Tape
Web speed [m/min]
A B
Tack [N]
10 10
Adhesion [N]
12.5 11.5
Cohesion [N]
20 C
70 C
20 C
70 C
26.5 28.0
16.5 14.5 CF
60 45
25 <5
CF – Cohesive failure. A
Experimental product (photoreactive) 2 · 60 g/m2 UV-crosslinked 2 g/m2 paper substrate (carrier) 90 g/m2 silicon paper liner
B
Duplocoll 200 standard adhesive tape (acrylic dispersion) 2 · 60 g/m2 UV-crosslinked 12 g/m2 paper substrate (carrier) 90 g/m2 silicon paper liner.
• polymerization of the monomer material on the carrier to a pressure-sensitive adhesive by UV radiation under the exclusion of oxygen. The UV-polymerization and UV-crosslinking behavior will be treated from the change of the conversion ratio when photoinitiator is added to the syrup-PSA and actual UV light if performed and the molecular mass of the formed polymer. Fig. 8 shows the relation between the UV energy with various irradiation strengths and the conversion ratio for a fixed addition amount of photoinitiator 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]2-methyl-1-propanone (ZLI 3331). Here, the UV irradiation was performed with covering of the irradiation surface by a polyester film separator to eliminate oxygen. The polymerization ratio in the UV-polymerization and UV-crosslinking processes is based on the general radical polymerization reaction theory, and it is influenced widely by the amount of radicals 120
generated per unit of time. As a result, a high UV intensity is effective from the point of view of productivity, as it reduces the time required for polymerization and photocrosslinking, but it means a lowering of the adhesive performance, as the molecular mass of the formed polymers decreases. Next, the crosslinking behavior will be compared for the case when the UV irradiation surface is covered with a polyester film separator and when the UV irradiation atmosphere is changed to on oxygen concentration of 5% and 1% respectively by nitrogen substitution. Fig. 9 shows the relation between the number of UV irradiation passes and the conversion ratio under the various conditions. With an oxygen concentration of 5%, the polymerization and photocrosslinking speed and the molecular mass of the formed polymer both drop widely in comparison to the method with covering with a polyester foil, but with an oxygen concentration of 1%, the drop is small and it is in the range which can be realized. From this it can be said that
20 mW/cm2
Mw = 790 000 Dalton (laminated with polyester film) 100
100
Conversion ratio [%]
Conversion ratio [%]
5 mW/cm2
80
50 mW/cm2 60 40
100 mW/cm2
20
80
Mw = 775 000 Dalton (1 % oxygen amount)
60 40
Mw = 680 000 Dalton (5% oxygen amount)
20 0
0 0
50
100
UV energy
150
200
[mJ/cm2]
Fig. 8. Relation between the estimated UV light amount and the conversion ratio.
0
1
2
3
4
5
6
7
Number of passes Fig. 9. Influence of the oxygen amount onto the crosslinking behavior.
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
Web speed[m/min]
100 80 60 40 20 0
500
100
1100
Coat weight [g/qm]
UV- crosslinking
UV- polymerization
Fig. 10. Web speed vs. coat weight.
1
Coating costs [ /qm]
at the time of adhesive tape manufacture, it is required either to cover the UV irradiation surface with a polyester film separator or similar or to perform UV irradiation in an atmosphere with an oxygen concentration of 1% or less by substitution of nitrogen or similar. Under these conditions, polymerization is completed within approximately one to two minutes (about 1100 g/m2 coat weight) to an end stage characterized by a molecular mass which we are familiar to from solvent polymerization. This is the decisive physical and chemical advantage of this method, that does not depend on the molecular mass of the pressure-sensitive adhesive with respect to the process technology. The complete freedom from solvents, a further plus point, is, however, counteracted by a relatively high residual monomer content and a block colouring that can only be achieved to a limited extent, indispensable for same applications. In contrast to UV-crosslinking which takes only seconds, UV polymerization takes up to two minutes, but at least ten times as long. On the other hand, UV polymerization is more or less independent of the coating thickness of the PSA which means that it is particularly suitable for thick coats. The coating costs are derived from the costs per operating hour. A coating rate of 10 m/min can still be assumed up to 1100 g/m2; under 150 g/m2, the coating rate for UV curing is restricted to 80 m/min. In the case of UV polymerization, the dependence of the coating rate on the coat weight is, as anticipated, considerably less marked, as is shown by the bar chart below (Fig. 10). The coating costs can be derived from this (Fig. 11).
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0.75
0.5
0.25
0
100
500
1100
Coat weight [g/qm] UV-crosslinking
UV polymerization
Fig. 11. Coating costs.
7. Outlook There are numerous examples of competing systems in nature not necessarily implying the extinction of one variant. The candle, for instance, still retains its niche despite the invention of the electric bulb. Neither can it be expected nor would it be feasible or reasonable for acrylate hot-melt pressuresensitive adhesives and their traditional production processes to be regarded as ‘‘old junk’’ because of the established raw material used and/or the established production process. Producers of adhesive tapes are also subject to the second principle of thermodynamics, the law of entropy, and other laws of nature. And they will modify the, initially so promising, progress of new raw materials or innovative methods to a marginal improvement, or sometimes no improvement at all. UV-crosslinkable acrylate adhesives are certainly an alternative worth considering for companies having no machinery for coating solvent-borne adhesives. They would be up-to-date, almost trough the back door, with pseudo- solvent acrylate pressure-sensitive adhesives, as long as the chemistry is correct. UV polymerized, thick acrylate hot-melt pressure-sensitive adhesive coatings created on the substrate are attracting high attention because of their properties, the only problem is that the market for them is characterized by a high entry barrier. In addition, the company implementing this production process would face a risky and expensive development, not least because they would have to design a production plant first. Small- and medium-sized
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adhesive tape producers will not be able to realize such plans. One unrestrictedly positive aspect is that solventfree acrylate adhesives are not only in harmony with the increasing concerns for the environments, but they even make a decisive contribution to reducing the strain on the environment because of their total omission or almost one hundred percent re-use of solvents. References [1] Benedeck I. Development in pressure-sensitive products. New York: Taylor & Francis; 2006. [2] Czech Z. Crosslinking of acrylic pressure-sensitive adhesives, Szczecin: ed. at Szczecin University of Technology, 1999 (ISBN 83-87423-18-1).
[3] Benedek I. Pressure-sensitive design and formulation, application, Leiden Boston, ed. at Martinus Nijhoff Publishers and VSP, 2006. [4] Dupont M, Mayenez C. Adha¨sion 1989;3:22–7. [5] Schumacher KH, Du¨sterwald U, Mayer-Roscher B. Referat 23. Mu¨nchener Klebstoff- und Veredelungsseminar; 1998. p. 112–7. [6] EP 0259 094 (1987) 3M. [7] WO 92/20752 (1992) 3M. [8] DE 42 11 448 (1992) Lohmann. [9] DE 43 03 183 (1993) Lohmann. [10] DE 44 06 977 (1994) Lohmann. [11] DE 33 05 727 (1983) Nitto. [12] EP 0 160 394 (1985) 3M. [13] Czech Z. Kautschuk Gummi Kunststoffe 2002;10:492–501. [14] DE 15 941 93 (1966) Novacel. [15] US 4,181,752 (1980) 3M. [16] US 4,364,972 (1982) 3M. [17] EP 0426 198 (1991) Adco.
European Polymer Journal 43 (2007) 3604–3612
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Development of solvent-free acrylic pressure-sensitive adhesives Zbigniew Czech *, Marta Wesolowska Szczecin University of Technology, Institute of Organic Chemical Technology Pulaskiego 10, 70-322 Szczecin, Poland Received 13 November 2006; received in revised form 25 April 2007; accepted 2 May 2007 Available online 13 May 2007
Abstract The present publication describes a problem to develop solvent-free acrylic pressure-sensitive adhesives (PSA). Solventfree PSA are established materials for the manufacturing of various self-adhesive products. Only by means of these acrylic PSA was it possible to succeed in drafting the present surprisingly efficient generation of double-sided pressure-sensitive adhesive tapes, medical products, protective masking films, films for the graphics market, and various specialty products. New applications and technical specifications stimulate the continuous development of new methods of polymerization of solvent-free acrylates. New syntheses of solvent-free acrylic PSA include polymerization in the reactor with removal of the solvent and polymerization on the carrier. The polymerization process is connected with UV-crosslinking. 2007 Published by Elsevier Ltd. Keywords: Acrylics; PSA; UV; Tack; Peel; Shear
1. Introduction Acrylic pressure-sensitive adhesives are used for various products represented by adhesive mounting tapes, labels, medical pads, protective and decorative films [1]. Their functional characteristics like instantaneous adhesiveness, repeated adhesiveness, tackiness, etc. as well as their ease of adhesive work, leads to the fact that applications are spreading in various directions. Together with the expanding applications of PSA, the capabilities required for PSA also are widening, and various types of PSA as solvent-borne, water-borne and solvent-free PSA have been developed.
*
Corresponding author. E-mail address: [email protected] (Z. Czech).
0014-3057/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2007.05.003
The desire to use PSA without having to deal with organic solvents has been existing since selfadhesive products have been in mass production [2]. The hot-melt types, dispersion types, and other technologies of solvent-free PSA have been realised for a number of the PSA applications. However, technologies to substitute solvent-free types of PSA in sectors requiring high performance in terms of weather resistance or heat resistance have not been completed, and therefore acrylic PSA of the solvent type continue to be widely used [3]. Solvent-free PSA achieved real practical significance in the 70s with the appearance of thermoplastic rubber, the styrene-butadiene (SBS) and styrene-isoprene (SIS) block copolymers [4]. These thermally reversible or physically crosslinking products allow the formulation of solvent-free PSA with good processing performance. For this reason they represent
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
an apparently promising advantage to the substitution of rubber solutions. The dimension of this substitution is surprising because these adhesives satisfy the requirements only in a very limited way. There is no need here to describe in detail the typical disadvantages such as unacceptable temperature resistance, unacceptable oxidation resistance or poor resistance to plasticizers. The adhesives must frequently be subjected to the exigencies of the process in order to achieve the melt characteristics required. It is primarily for this reason that solvent-free PSA based on SBS/SIS block copolymers have not achieved the performance expected of them and are only able to substitute rubber solutions in a limited field of application. According to BASF solvent-free self-adhesives [5] achieve a relatively small, but stable, market share. The ideal would be to link the proven good PSA properties of acrylic PSA produced or formulated in solvents or water with solvent-free acrylic self-adhesive systems. Acrylic pressure-sensitive adhesives which are UV-crosslinked or which are polymerizable on the carrier are promising solutions to reach this ideal. Photoinduced UV-crosslinking or UV-inducted photopolymerization are rapidly expanding technologies on PSA sector resulting from their main advantages such as solvent-free process, efficient and economical energy use, new performances and quality of chemical crosslinking bonding [6–10]. Three different manufacturing processes were tested in the experiment to synthesise solvent-free acrylic pressure-sensitive adhesives: • polymerization in the extruder with subsequent UV-crosslinking, • polymerization in the reactor with removal of the organic solvent and UV-crosslinking of the resulting solvent-free PSA, • polymerization of acrylic-syrup directly on the carrier to create the PSA layer.
2. Polymerization by the use of an extruder The first development work on using an extruder as the chemical reactor for polymerization was done about 63 years ago. A patent publication [11] describes a continuous process for the polymerization of acrylate monomers in a single or double screw extruder. A degree of transformation of 93– 99% is achieved by suitable screw geometry and
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by adjusting the temperature of the various zones of the extruder. The example given, however, does not cover a pressure-sensitive adhesive since the glass transition temperature (Tg) of the extrusion polymer was about 29 C. European patent [12] describes continuous radical polymerization in a special reactor. An isooctyl acrylate/acrylic acid combination is mentioned in the description and in the examples.
3. Experimental The following experiments were conducted in order to study the diverse parameter such as acrylic acid content, radical starter concentration and crew speed during the polymerization in the extruder, the viscosity, molecular mass, tack, peel adhesion, shear strength and other important properties of the polymerisation process such as polymer yield and polymerization conversion. In the case of polymerization in the reactor with removal of the organic solvent the performed trials have shown comparison of various acrylics solventfree PSA, influence on web speed at 100, 500 and 1100 g/m2. The UV-crosslinkable acrylics solventfree PSA were compared with UV-polymerizable acrylic PSA and a conventional acrylic dispersion. During the polymerization process directly on the carrier the conducted experiments have shown the relationship between the UV energy with various irradiation strengths and the conversion ratio for a fixed added amount of photoinitiator, the relation between the number of UV irradiation passes and the conversion ratio under the various conditions, the web speed vs. coat weight and coating costs of UV-crosslinked acrylic PSA based on UVpolymerizable syrup. 2-Ethylhexyl acrylate, methyl acrylate, acrylic acid and 2,2 0 -azo-diisobutyronitrile are available from Tokyo Chemical Industry Co. The unsaturated photoinitiator 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone (ZLI 3331) was synthesised at Szczecin University of Technology. The viscosity of the PSA synthesized in the extruder was measured with a viscometer RM 180 Rheomat, from Rheometric Scientific Company. Their molecular mass was determined by HPLC with Isoctratic Pump & DRI-Detector form HP Company. Tack, adhesion and cohesion of the investigated acrylic pressure-sensitive adhesives were tested
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
according to A.F.E.R.A. 4015 (tack), 4001 (adhesion) and 4012 (cohesion). Conversion [wt.%]
4. Polymerization in the extruder with subsequent UV-inducted crosslinking
22 100
20 18
90
16 Conversion Viscosity
80
14 12
Viscosity (20°C) [Pas]
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The goal of radical polymerization in the extruder is to produce solvent-free crosslinkable acrylic PSA, having a conversation rate above 98–99% (residual monomer content <1–2%), an average molecular mass Mw > 270 000 Dalton and a perfect optical quality (free of gel particles). We had proposed feeding the acrylate monomers into an extruder together with a starter and polymerizing it in situ. This would mean the viscose polymer being delivered from the extruder by way of a gear–wheel pump into a wide-slot nozzle, applied to a web and being cured by UV exposure. An elegant solution at first glance, but, as we soon discovered, also the solution with the most chemical and process-engineering traps.
shortening the real polymerization time. The greatest weakness in the system was the degassing zone which did not permit the most possible removal of the residual monomers at higher screw speeds.
4.1. Influence of the screw speed of the extruder on polymerization conversion and PSA viscosity
4.2. Influence of the methyl acrylate amount and the starter concentration
The experiments were carried out in a co-rotating double-screw extruder model LSM34GL with 8 zones, available from Leistritz Inc. (Germany) with eight heatable zones including a degassing zone, can adjust the speed of the screws and the temperature in the various zones in a wide range. The polymerization process was performed with a monomers mixture selected of 74.0 wt.% 2-ethylhexyl acrylate (2-EHA), 20.0 wt.% methyl acrylate, 5.0 wt.% acrylic acid (AA), 0.5 wt.% 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone (ZLI 0 3331) and 0.5 wt.% thermal initiator 2,2 -azo-diisobutyronitrile (AIBN) as the basic recipe. The main variable technical parameter is the screw speed of the extruder. As anticipated, the polymerization yield and the viscosity of the PSA produced declines with increasing screw speed as the reaction time actually available decreases by the following temperatures of several zones 1–8: 90/90/90/100/100/110/120/120 C (Fig. 1). The best results of the polymerization yield (99.3 wt.%) and viscosity of the synthesised PSA (20.3 Pa s) were achieved with a the screw speed of 3 rpm. An increase in speed of the extruder from 10 to 70 rpm, can lead to a bigger negatively effect on monomers conversion and on acrylic PSA viscosity. This is a consequence of the effect of
Starting from this basis the methyl acrylate content has been varied and so the quantity of starter, with the temperatures of the various zones and the screw speed being set as follows:
10
70
8 60
6 4
50 0
10
20
30
40
50
60
70
Screw speed [U/min] Fig. 1. Polymerization conversion and polymer viscosity dependence of screw speed.
• Temperature of zones 1–8: 90/90/90/100/100/ 110/120/120 C. • Screw speed: 3 rpm. The influence of the concentration of the starter AIBN and of the methyl acrylate content on the investigated properties such as polymerization conversion and viscosity are presented in Figs. 2 and 3. As expected, the increase of AIBN content affects positively the polymerization conversion. The viscosity shows a maximum for about 0.3 wt% of AIBN. The increase of the methyl acrylat concentration generally has a positive influence on shrinkage. The value of polymerization yield for acrylic PSA with more than 20 wt.% methyl acrylate stayed on the same level. The effect of AIBN and acrylic acid amount on molecular mass of the extruder-polymerized PSA is shown in Fig. 4. The increase of acrylic acid content and increase of initiator AIBN content has a beneficial effect on
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
Conversion [wt.%]
90
90
80
80
70
70
60
60 50
50
Viscosity Conversion
40
40
Viscosity (20°C) [Pas]
100
100
30
30
20
20 0.0
0.1
0.2
0.3
0.4
0.5
AIBN concentration [wt.%] Fig. 2. Effect of AIBN amount on polymerization yield and viscosity.
Conversion [wt.%]
95
25
90 20 85
Conversion Viscosity
80
15
Viscosity [Pas]
30
100
75 10 70 10
15
20
25
30
Methyl acrylate amount [wt.%]
ized with 10 wt.% of acrylic acid and about of 0.3 wt% of AIBN. 4.3. UV-crosslinking of extruder polymers An acrylic PSA determined during the conducted series of experiments has been selected with the highest molecular mass (Mw = 421 000 Dalton) as our model polymer. Although it was possible to crosslink this material 1000 lm thick (about 1 mm thick adhesive layer) coated on the polyester film with UV radiation by the use of a UV lamp U 350-M-I-DL from IST Company with an UV-A wavelength between 315 and 400 nm and a constant UV dose of 1000 mJ/cm2 using a crosslinking time between 30 s and 3 min. The results of tack, peel adhesion and shear strength are presented in Figs. 5 and 6. Tack, peel adhesion [N/2,5 cm]
110
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25
Tack Peeladhesion
20
15
10 5
0
Fig. 3. Effect of methyl acrylate amount on viscosity and polymerization yield.
20
40
60
80
100
120
140
160
180
UV-crosslinking time [s]
Molecular mass [Dalton]
400000 300000 200000 100000 0 0.0
0.1
0.2
0.3
0.4
0.5
AIBN content [wt.%]
Fig. 4. Influence of AIBN and acrylic acid concentration on molecular mass of PSA.
Fig. 5. Tack and peel adhesion of UV-crosslinked extruded acrylics PSA.
90
Shear strength [N/6,25 cm2]
1wt.%AA 3wt.%AA 5 wt.%AA 7wt.%AA 10 wt.%AA
500000
80 70
at 70°C at 20°C
60 50 40 30 20 10 0 20
the molecular mass of the synthesized PSA acrylics. The PSA with high molecular mass were polymer-
40
60
80
100
120
140
160
180
UV-crosslinking time [s] Fig. 6. Shear strength of UV-crosslinked extruded acrylics PSA.
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Dependencies shown in Figs. 5 and 6 point out that the UV exposure time can influence tack and peel adhesion of acrylic PSAs after ultraviolet radiation crosslinking negatively and the shear strength positively. Generally the crosslinked acrylic PSA with high molecular mass and high viscosity gives the self-adhesive layers with high level of cohesion and very good tack and peel adhesion performances after UV-crosslinking. 4.4. Evaluation of the results We were able to obtain a useable, acrylic pressure-sensitive adhesive by mass polymerization in the extruder, which would satisfy the normal requirements for a pressure-sensitive adhesive tape. The minimal observed drawbacks were as follows: 4.5. Gel particle content Because of the significant heat of the reaction in mass polymerization of the acrylate, it is almost impossible to adjust the temperature precisely, or at least this can only be done with difficulty, which leads from time to time to the formation of gels and products of degradation. 4.6. Residual monomer content The holding time, expressed by the speed of the screw, in the extruder can only be adapted to the polymerization process with some difficulty. De facto this means too short a reaction space and hence an unacceptable reaction time. On the other hand, extending the extruder does not guarantee the secondary reaction and thus the reduction of the quantity of residual monomer. The integrated degassing zone also ensures that removal of the residual monomers is incomplete. The best in the praxis accepted results were achieved by the very low screw speed of about 3 rpm. 5. Polymerization in the reactor with removal of the solvent Polymerization takes place in an organic solvent with low boiling point (e.g., ethyl acetate, hexane, acetone,), with an unsaturated photoinitiator being incorporated in the polymer chain during polymerization. The polymerization medium is then removed under a vacuum at high temperature.
The resultant polymer is then, in a manner similar to a classical solvent-free hot-melt adhesive, coated at high temperature and crosslinked in seconds or minutes with UV radiation. This concept is based on proved and tested techniques and comes with the least risks. However, it is built on a pseudohot melt pressure-sensitive adhesive, since solvents are indeed used. The significant point is that there is the advantage that, by contrast to the mild, slow drying process in adhesive tape coating, large quantities of solvent are rapidly driven from the adhesive material, which can then processed like a hot melt. The solvent removed can, naturally, be used again for the polymerization process [13]. As a standard, a pressure-sensitive adhesive acrylic has been polymerized with the following composition: 2-Ethylhexyl acrylate (2-EHA) Methyl acrylate (MA) Acrylic acid (AA) Thermical starter (AIBN) Unsaturated photoinitiator (ZLI 3331)
74.2 wt.% 20.0 wt.% 5.0 wt.% 0.3 wt.% 0.5 wt.%
in acetone with 50 wt.% solid content. Once the solvent (acetone) is removed, there remains a non-volatile, solvent- free highly viscous material, which can be processed on a hot-melt coating machine at temperatures of 110, . . . ,140 C. Higher temperature (approx. 150 C) over an extended period (24 h) had no deleterious effect on the adhesive properties (‘‘no heat degradation’’). Apart from the typical adhesive ratings such as adhesive power etc., we were interested in the relationship between adhesive application and crosslinking rate. The procedure was as follows in order to obtain useful values on the performance of such adhesives: • single-sided coating of polyester film (36 lm) in the range of 40, . . . ,120 g/m2 with constant UV radiation and varying coating rates. • production of a typical double-sided adhesive tape (with paper substrate) for medium-weight use at varying coating rates and constant radiation dose. Working conditions:
Extruder Wide slot nozzle UV radiation lamp from IST Company.
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
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Table 1 Comparision of various solvent-free acrylic pressure-sensitive adhesives Type
HM HM HM HM HM
Tack [N]
1 2 3 4 5
Peel adhesion [N]
11.5 12.0 6.0 3.5 2.0
70 C
20 C
70 C
12.5 11.5 8.0 7.0 7.0
5.5 5.0 4.0 3.0 2.5
80 60 30 10 7
30 20 10 6 5
An initial screening revealed that apart from the own polymer (HM1) and the solvent-free PSAs from a manufacturer of adhesive raw materials well-known in the market-place (HM 2–HM 5), none of the formulations were worthy of further examination (Table 1). Working process: Backing: Liner: Coat weight: Web speed:
wide slot nozzle 36 pm PETP-film siliconized paper 60 g/m2 5 m/min
The limits of the bond-coat weights were determined and the crosslinking rates using the own synthesized Products HM 1. These were 60 g/m2 and 120 g/m2 and between 5 and 20 m/min (Fig. 7). Peel adhesion at 20 C and shear strength at 70 C for UV-crosslinked solvent-free acrylic PSA were tested. It was determined that, within the various test groups (mass application/coating rate) the adhesive power at 20 C decreases with increasing coating rate and the cohesion decreases with increasing of the web speed on the coating machine. The performance at higher temperatures, however showed typical weaknesses of a solvent-free hot-melt. The UV-crosslinking was not able to make any improvement at it. The adhesion properties of a 35
Properties [N]
Adhesion (20ºC, 120 g/m2)
Cohesion (70ºC, 60 g/m2)
30 25
Adhesion (20ºC, 60 g/m2)
20 15 10 Cohesion (70ºC, 120 g/m2)
5
Shear strength [N]
20 C
double-sided adhesive tape, coated with a UVcrosslinked acrylate hot-melt pressure-sensitive adhesive are respectable and encouraging by comparison with a conventional adhesive tape, as Table 2 shows. The UV-crosslinked acrylics PSA have very good tack, peel adhesion and shear strength at room and at higher temperatures. The increase of temperature decrease a little bit their adhesiveness and cohesive performance. In the case of acrylic PSA dispersions their performance at higher temperatures are on the very low and unacceptable level. 6. Polymerization on the carrier In this case, polymerization is shifted from the extruder to the substrate web, in that acrylate monomers recipe made as a coating compound are polymerized by UV radiation with the exclusion of oxygen. The production of acrylic pressure-sensitive adhesive tapes by mass polymerization under the effects of UV radiation is based on a patent from Novacel Company [14]. As this patent has now expired, UV polymerization may be considered to be a process accessible to all. 3M was the first to pick up the process and constantly enhanced the production process and the compositions of adhesive tapes, as is documented in numerous patents since 1982 [15,16]. Adco (USA) applied for a European patent [17], in which the only claim is a different spectral distribution of the UV radiation to that specified by 3M in various patents. The prospects for winning a grant in Europe must be estimated as under 50%. The production principle is as follows:
0 0
5
10
15
20
25
Web speed [m/min]
Fig. 7. Influence of web speed at 60 g/m2 and 120 g/m2 coat weight on peel adhesion and shear strength.
• mixing the components and thickening to a coatable syrup by incorporating fillers, • coating the syrup in thickness of up to several millimetres onto an UV permeable process film,
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Table 2 Relevant properties of UV-crosslinked solvent-free pressure-sensitive adhesive Tape
Web speed [m/min]
A B
Tack [N]
10 10
Adhesion [N]
12.5 11.5
Cohesion [N]
20 C
70 C
20 C
70 C
26.5 28.0
16.5 14.5 CF
60 45
25 <5
CF – Cohesive failure. A
Experimental product (photoreactive) 2 · 60 g/m2 UV-crosslinked 2 g/m2 paper substrate (carrier) 90 g/m2 silicon paper liner
B
Duplocoll 200 standard adhesive tape (acrylic dispersion) 2 · 60 g/m2 UV-crosslinked 12 g/m2 paper substrate (carrier) 90 g/m2 silicon paper liner.
• polymerization of the monomer material on the carrier to a pressure-sensitive adhesive by UV radiation under the exclusion of oxygen. The UV-polymerization and UV-crosslinking behavior will be treated from the change of the conversion ratio when photoinitiator is added to the syrup-PSA and actual UV light if performed and the molecular mass of the formed polymer. Fig. 8 shows the relation between the UV energy with various irradiation strengths and the conversion ratio for a fixed addition amount of photoinitiator 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]2-methyl-1-propanone (ZLI 3331). Here, the UV irradiation was performed with covering of the irradiation surface by a polyester film separator to eliminate oxygen. The polymerization ratio in the UV-polymerization and UV-crosslinking processes is based on the general radical polymerization reaction theory, and it is influenced widely by the amount of radicals 120
generated per unit of time. As a result, a high UV intensity is effective from the point of view of productivity, as it reduces the time required for polymerization and photocrosslinking, but it means a lowering of the adhesive performance, as the molecular mass of the formed polymers decreases. Next, the crosslinking behavior will be compared for the case when the UV irradiation surface is covered with a polyester film separator and when the UV irradiation atmosphere is changed to on oxygen concentration of 5% and 1% respectively by nitrogen substitution. Fig. 9 shows the relation between the number of UV irradiation passes and the conversion ratio under the various conditions. With an oxygen concentration of 5%, the polymerization and photocrosslinking speed and the molecular mass of the formed polymer both drop widely in comparison to the method with covering with a polyester foil, but with an oxygen concentration of 1%, the drop is small and it is in the range which can be realized. From this it can be said that
20 mW/cm2
Mw = 790 000 Dalton (laminated with polyester film) 100
100
Conversion ratio [%]
Conversion ratio [%]
5 mW/cm2
80
50 mW/cm2 60 40
100 mW/cm2
20
80
Mw = 775 000 Dalton (1 % oxygen amount)
60 40
Mw = 680 000 Dalton (5% oxygen amount)
20 0
0 0
50
100
UV energy
150
200
[mJ/cm2]
Fig. 8. Relation between the estimated UV light amount and the conversion ratio.
0
1
2
3
4
5
6
7
Number of passes Fig. 9. Influence of the oxygen amount onto the crosslinking behavior.
Z. Czech, M. Wesolowska / European Polymer Journal 43 (2007) 3604–3612
Web speed[m/min]
100 80 60 40 20 0
500
100
1100
Coat weight [g/qm]
UV- crosslinking
UV- polymerization
Fig. 10. Web speed vs. coat weight.
1
Coating costs [ /qm]
at the time of adhesive tape manufacture, it is required either to cover the UV irradiation surface with a polyester film separator or similar or to perform UV irradiation in an atmosphere with an oxygen concentration of 1% or less by substitution of nitrogen or similar. Under these conditions, polymerization is completed within approximately one to two minutes (about 1100 g/m2 coat weight) to an end stage characterized by a molecular mass which we are familiar to from solvent polymerization. This is the decisive physical and chemical advantage of this method, that does not depend on the molecular mass of the pressure-sensitive adhesive with respect to the process technology. The complete freedom from solvents, a further plus point, is, however, counteracted by a relatively high residual monomer content and a block colouring that can only be achieved to a limited extent, indispensable for same applications. In contrast to UV-crosslinking which takes only seconds, UV polymerization takes up to two minutes, but at least ten times as long. On the other hand, UV polymerization is more or less independent of the coating thickness of the PSA which means that it is particularly suitable for thick coats. The coating costs are derived from the costs per operating hour. A coating rate of 10 m/min can still be assumed up to 1100 g/m2; under 150 g/m2, the coating rate for UV curing is restricted to 80 m/min. In the case of UV polymerization, the dependence of the coating rate on the coat weight is, as anticipated, considerably less marked, as is shown by the bar chart below (Fig. 10). The coating costs can be derived from this (Fig. 11).
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0.75
0.5
0.25
0
100
500
1100
Coat weight [g/qm] UV-crosslinking
UV polymerization
Fig. 11. Coating costs.
7. Outlook There are numerous examples of competing systems in nature not necessarily implying the extinction of one variant. The candle, for instance, still retains its niche despite the invention of the electric bulb. Neither can it be expected nor would it be feasible or reasonable for acrylate hot-melt pressuresensitive adhesives and their traditional production processes to be regarded as ‘‘old junk’’ because of the established raw material used and/or the established production process. Producers of adhesive tapes are also subject to the second principle of thermodynamics, the law of entropy, and other laws of nature. And they will modify the, initially so promising, progress of new raw materials or innovative methods to a marginal improvement, or sometimes no improvement at all. UV-crosslinkable acrylate adhesives are certainly an alternative worth considering for companies having no machinery for coating solvent-borne adhesives. They would be up-to-date, almost trough the back door, with pseudo- solvent acrylate pressure-sensitive adhesives, as long as the chemistry is correct. UV polymerized, thick acrylate hot-melt pressure-sensitive adhesive coatings created on the substrate are attracting high attention because of their properties, the only problem is that the market for them is characterized by a high entry barrier. In addition, the company implementing this production process would face a risky and expensive development, not least because they would have to design a production plant first. Small- and medium-sized
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adhesive tape producers will not be able to realize such plans. One unrestrictedly positive aspect is that solventfree acrylate adhesives are not only in harmony with the increasing concerns for the environments, but they even make a decisive contribution to reducing the strain on the environment because of their total omission or almost one hundred percent re-use of solvents. References [1] Benedeck I. Development in pressure-sensitive products. New York: Taylor & Francis; 2006. [2] Czech Z. Crosslinking of acrylic pressure-sensitive adhesives, Szczecin: ed. at Szczecin University of Technology, 1999 (ISBN 83-87423-18-1).
[3] Benedek I. Pressure-sensitive design and formulation, application, Leiden Boston, ed. at Martinus Nijhoff Publishers and VSP, 2006. [4] Dupont M, Mayenez C. Adha¨sion 1989;3:22–7. [5] Schumacher KH, Du¨sterwald U, Mayer-Roscher B. Referat 23. Mu¨nchener Klebstoff- und Veredelungsseminar; 1998. p. 112–7. [6] EP 0259 094 (1987) 3M. [7] WO 92/20752 (1992) 3M. [8] DE 42 11 448 (1992) Lohmann. [9] DE 43 03 183 (1993) Lohmann. [10] DE 44 06 977 (1994) Lohmann. [11] DE 33 05 727 (1983) Nitto. [12] EP 0 160 394 (1985) 3M. [13] Czech Z. Kautschuk Gummi Kunststoffe 2002;10:492–501. [14] DE 15 941 93 (1966) Novacel. [15] US 4,181,752 (1980) 3M. [16] US 4,364,972 (1982) 3M. [17] EP 0426 198 (1991) Adco.