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samoprzylepne kleje akrylanowe

UV-crosslinkable solvent-free acrylic pressure sensitive

adhesive low viscosity systems and their application

Zbigniew Czech*, Agnieszka Butwin*

The technology of solvent-free acrylic pressure-sensitive adhesives (PSA) with low viscosity at room temperature has been developed. The novel acrylic PSAs contain photoreactive acrylic polymer and photoreaktive diluents can be used after UV-crosslinking for manufacturing of mounting tapes, protective films, sign and marking films and structural adhesive tapes.

Key words: acrylic PSA, solvent-free, low viscosity, mounting tapes, struc- tural tapes

Sieciowane UV bezrozpuszczalnikowe poliakrylanowe kleje samoprzylepne o niskiej lepkoœci i ich zastosowanie

Opracowano technologiê syntezy sieciowanych promieniowaniem UV bez- rozpuszczalnikowych poliakrylanowych klejów samoprzylepnych o niskiej lep- koœci w temperaturze pokojowej. Nowe kleje poliakrylanowe zawieraj¹ce foto- reaktywny polimer oraz fotoreaktywne rozcieñczalniki mog¹ byæ stosowane po usieciowaniu UV do produkcji taœm monta¿owych, folii ochronnych, folii deko- racyjnych oraz adhezyjnych taœm strukturalnych.

S³owa kluczowe: bezrozpuszczalnikowe samoprzylepne kleje poliakryla- nowe o niskiej lepkoœci, taœmy monta¿owe, taœmy strukturalne

1. Introduction

Since their introduction half a century ago, pres- sure-sensitive adhesives (PSA) have been success- fully applied in many areas. They are used to produce self-adhesive tapes, labels, sign and marking films and protective films as well as in biomedical elec- trodes, dermal dosage systems for pharmaceutical and medical applications. The self-adhesive articles mar- ket is dominated by acrylic solvent-borne, water- borne or solvent-free PSA systems (Fig. 1). Here the solvent-free acrylic PSA in form of typical hotmelts, warmmelts and new low viscosity systems (LVS) are still most common [1].

Three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack (ini- tial adhesion), peel adhesion (adhesion) and shear strength (cohesion). The first measures the adhesive’s ability to adhere quickly, the second its ability to resist removal by peeling, and the third its ability to hold in position when shearing forces are exerted. The per- formance of acrylic pressure-sensitive adhesives, such

as tack, peel adhesion and shear strength, synthesized by polymerization of acrylate monomers are to a large degree determined by the molecular weight of PSA co- polymer, polymerization method and especially by the

type and quantity of the crosslinking agents added to the PSA or crosslinking methods [2].

3

UV - crosslinkable PSA systems

solvent-borne solvent-free water-borne

high performances

stabilizer not necessary

water resistance

good cohesion at higher tempe- ratures

hotmelts low viscosity

system

no solvents

easy application at room temperature

Fig. 1. Types of acrylic pressure-sensitive adhesives Rys. 1. Rodzaje poliakrylanowych klejów samoprzylep- nych

* Institute of Chemical Organic Technology, West Pomera- nian University of Technology, Szczecin, Poland

samoprzylepne kleje akrylanowe

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2. Solvent free acrylic pressure sensitive adhe- sives

The application of solvent-free pressure-sensitive adhesives offers incontestable advantages such as re- duction of solvent emission, saving space, energy and processing time so it permits for savings in capital in- vestment and production costs as well [3]. For an easy and homogenous coating without any volatile com- pound while maintaining properties of the final pro- duct, viscosity of solvent-free pressure-sensitive adhe- sives must be lowered. Therefore, the classical hot- melts are coated in a molten stage at high temperatures between about 100oC and 140oC, and low-viscosity systems have reduced viscosity by using of reactive diluents of so they can be perfect coated at the room temperature (Fig. 2).

The production of this new developed class of sol- vent-free acrylic PSA is conducted in first step by sol- vent polymerisation. For some new special acrylic hot melts the polymerisation process is started at higher monomer concentrations. In view of the future degass- ing process, mixture of different solvents is being avoided [4]. At the end of the polymerisation step, the

polymer solution shows a polymer content of more than 50 wt.% (Tab. 1).

Table 1. Solvent-borne polymerisation processes Tabela 1. Procesy polimeryzacyjne z udzia³em rozpusz- czalnika

Polymerisation process old new Polymer concentration [wt.%] 50 to 60 60 to 80 Solvent system acetone, ethyl

acetate, hexane ethyl acetate

This means that in this first production step a re- duction of the organic solvent is possible. The fact that this beforehand eliminated solvent does not have to be removed during the following degassing process leads to decreased production costs and reduced ecological damage. At the moment, the degassing in the reactor after the polymerisation process seems to be a practic- able way to produce acrylic hotmelts from solvent- -borne systems (Fig. 3). With the evaporation process the production of the acrylic hotmelt PSA is finished.

Solvent recovery permits an almost completely closed solvent circulation [5].

Presently, this method seems to be a useful practice way of solvent-free acrylic PSA hot-melts manufactur- ing from solvent based systems. The organic solvents

4

Photoreactive solvent-free PSA with high viscosity

Photoreactive diluent PSA with low viscosity

Photoinitiator

Low viscosity Photoreactive solvent-free PSA

with low viscosity

UV-lamp

Release liner

Polyester film Self-adhesive product

with UV-crosslinked PSA layer Fig. 2. Solvent-free low viscosity acrylic PSA systems

Rys. 2. Bezrozpuszczalnikowe poliakrylanowe kleje samoprzylepne o niskiej lepkoœci

samoprzylepne kleje akrylanowe

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and free monomers are removed and solvent-free highly viscous material can be processed on a hot-melt coating machine at temperatures between 100 and 150oC. Higher temperature (approx. 150oC) applied over an extended period (24 hours) had no adverse ef- fect on the adhesive properties [6].

3. Photeractivity adjust- ment of acrylic PSA

The idea of replacing the conventional solvent- -borne, water-borne or solvent free conventional cross- -linkable systems with photoreactive UV cross-linkable systems became very attractive to manufacturing indus- tries. However, it is only in the past 20 years that the use in particular of the very reactive mono or multifunc- tional acrylic pressure-sensitive adhesives (PSA) has resulted in a growing market for UV radiation cross- -linkable products, finding uses in the fields of automo- bile, building, labels, protective films, electronics, medical applications, printing and the optics industry [7]. First of all very interesting for industrial applica- tions are acrylic PSA characteristic through by excel- lent adhesive (tack and peel adhesion) and cohesive (shear strength) properties, excellent thermal and age- ing performances.

The basic chemistry of phtoreactive UV cross-link- able adhesives is the conversion of light energy into chemical energy. This energy conversion takes place through the use of chemical species photoinitiators which, upon absorption of a particular wavelength of light, photochemicaly produce reactive radicals or ions capable of initiating a rapid chain reaction. With the use of novel unsaturated photoinitiators (Fig. 4), new de- veloped photoreactive adhesives and commonly avail- able UV sources (UV lamps, UV lasers) it is possible to

manufacturing of high performance UV cross-linkable PSA and high quality self-adhesive articles.

A suitable photoinitiator system must first present high initiation efficiency. This means that it must have a high absorption in the emission range of the UV source and the excited states thus formed must have both a short life time to avoid quenching by oxygen or monomer molecules and split into active radicals with the highest possible quantum yield. Also the photoini- tiator must be soluble in the self-adhesive acrylic poly- mers used and not affect the colour, toxicity and degra- dation of the UV-cross-linked acrylic PSA [8].

All kinds of acrylic PSA offer several advantages such as excellent aging characteristics and resistance to elevated temperatures and plasticizers, exceptional op- tical clarity due to the polymer compatibility and non- -yellowing. They also have the highest balance of adhe- sion and cohesion and an excellent water resistance.

Acrylics are harder than rubbers. This can be seen in a less aggressive tack and slower build-up of peel

5

CH

CH2 C

O

O (CH2)2 C

O

O C OH

CH3 CH3

4-(2-Acryloyloxy-ethoxy)-phenyl-2-hydroxy-2-propylketone (type I)

CH

CH2 C

O

O C

O

4-Acryloyloxybenzophenone (type II)

monomers mixture + AlBN

solvent

vacuum station

separator

solvent-borne acrylic PSA

compounder

solvent-freePSA

•coating

•radiation cross-linking

•PSA tapes production

Fig. 3. Batch synthesis concept of solvent-free acrylic PSA

Rys. 3. Koncepcja syntezy bezrozpuszczalnikowego kleju poliakrylanowego w reaktorze

Fig. 4. Unsaturated copolymerizable radical photoini- tiators

Rys. 4. Nienasycone zdolne do kopolimeryzacji fotoini- cjatory rodnikowe

samoprzylepne kleje akrylanowe

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strength. Lower adhesion to non-polar polyolefins is caused by the polar chemistry of acrylics. The versa- tility of acrylate chemistry is inherently useful in the design of high performance pressure-sensitive adhe- sives. A broad raw material base and a versatility of polymerization processes lend themselves to design of base polymers with unique properties (Fig. 5). Typical acrylic PSA contain no photoreactive groups and are with it radiation resistant [9].

The glass transition temperature (Tg) and the amor- phous properties are the main issue for self-adhesive properties of various polymers, allowing the selection of raw materials for PSA applications. For permanent adherent pressure-sensitive adhesives Tgranges from about –70oC to –25oC [5].

Photoreactive chemically UV-cross-linkable acrylic PSA consists of polymer backbone molecules that are modified with photoreactive groups. The physicochemical and mechanical properties of photo- reactive pressure-sensitive adhesives are determined to a high degree by type and concentration of the photoreactivity.

Modification of the main poly- mer chain with copolymerizable photoinitiators

It is well known that components of pressure-sen- sitive adhesive systems, which are not chemically linked, can migrate into the PSA surface of the finished coating. These effects usually degrade gloss, adhesion and mechanical properties and can interfere with many applications. Extractability of photoinitiator by-pro- ducts by food in contact with these adhesive coatings excludes their use for food packaging materials. Co- polymerizing the unsaturated photoinitiator, especially based on benzophenones, into the backbone of the acrylic PSA copolymer allows the cross-linking with ultraviolet radiation. Performed polymer structures that crosslink directly under the influence of ultraviolet energy require special photosensitive groups to effect network formation (Fig. 6) [10].

4. Performance adjust- ment of pressure sensitive adhesives

The most important properties of the UV-cross- -linkable acrylics pressure-sensitive adhesives, such as tack, peel adhesion and shear strength, can be control- led by the UV dosage (cross-linking density). The UV-dose can be controlled by adjusting the power of the lamps and/or the speed at which the substrate is passed under the lamps in the production plant. With UV-cross-linkable acrylic PSA, the ratio of tack and peel adhesion to shear strength (cohesion) can be varied within wide limits, by controlling the amounts of UV energy applied to the adhesive film after it has been coated. Films formed by UV-cross linked acrylic adhe- sive become more cohesive as more radiant energy is applied, while their tack and peel adhesion decrease.

This effect is the result of the cross-linking that takes place when the film is exposed to UV radiation. The effect of the UV radiation dose [mJ/cm2], described as density of cross-linking, on the adhesive properties and cohesion is shown schematically in Fig. 7.

6

R

C O R

C R H

UV

R

C HO

R C R C O

O

C O

CH2 CH C4H9

C2H5 C4H9

C O OH

C O O C8H17

C O O CH3

C O O CH2

C O NH2

n-butyl- acrylate (Tg= -54°C)

2-ethylhexyl- acrylate (Tg= -70°C)

acrylic acid (Tg= 106°C)

n-octyl- acrylate (Tg= -65°C)

methyl- acrylate (Tg= -6°C)

2-HEA (Tg= -15°C)

acrylamide (Tg= 179°C) O

CH2 OH

Fig. 5. Architecture of acrylic PSA polymer chain using typical acrylate monomers

Rys. 5. Architektura ³añcucha polimeru akrylanowego zawieraj¹cego typowe monomery akrylanowe

Fig. 6. Crosslinking by use of unsaturated photoinitia- tors incorporated into polymer chain

Rys. 6. Sieciowanie z udzia³em nienasyconych fotoini- cjatorów wbudowanych w ³añcuch polimerowy

samoprzylepne kleje akrylanowe

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5. Solvent free photoreac- tive pressure sensitive ad- hesive low viscosity sys- tems (PSA-LVS)

Photoreactive solvent-free pressure-sensitive adhe- sive low viscosity systems (PSA-LVS) contain gene- rally photoreactive self-adhesive polymers, photoreac- tive diluents, radical photoinitiators and multifunc- tional monomers. The presence of photoreactive di- luents is very important for the manufacturing of PSA-

-LVS and their application to produce a wide range of self-adhesive materials. Typical compositions of PSA- -LVS are presented in Fig. 8.

Diverse photoreactive diluents are added to the sol- vent-free photoreactive acrylic hotmelts PSA in order to reduce the high hotmelts viscosity. Commercial avail- able or developed hotmelt PSA are characterized at room temperature by viscosity above 1000 Pa·s, which enables its coating at the temperature lower than 100oC.

Mostly the available on the market photoreactive dilu- ents possess viscosity between 0.3 and 10 Pa·s. There- fore, the addition of photoreactive diluents allows the formulation of solvent-free low viscosity systems for coating the manufactured PSA-LVS even at room tem- perature. Prerequisite for viscosity reducing is compati- bility of photoreactive diluents (Fig. 9) with photoreac- tive hotmelt PSA.

After the selected compatible photoreactive di- luents (Genomer/Rahn, Ebecryl/Cytec) and Laro- mer/BASF) are added to hotmelt acrylic PSA, the vis- cosity at room temperature are measured and in Fig.10 presented.

A small difference between viscosities of solvent- -free PSA-LVS using chosen photoreactive diluents was observed. In addition, the viscosity of PSA-LVS with the weight ratio of PSA to photoreactive diluent of 1:1 is very high and reaches viscosity values between 77 and 100 Pa·s. All three investigated reactive diluents allow on production of typical at room temperature coatable solvent-free pressure-sensitive adhesive low viscosity systems (PSA-LVS) with viscosities between 3 and 35 Pa·s containing photoreactive diluents with the weight ratio to photoreactive hotmelt PSA of between 2:1 and 3:1 (Fig. 11).

7

curing window (balanced

values)

Shear strength Tack

Peel adhesion

Properties[N]

UV dose [mJ/cm2]

Fig. 7. Tack, peel adhesion and shear strength as a function of UV dose

Rys. 7. Tack, adhezja oraz kohezja jako funkcja dawki UV

CH3 NH C

O

O CH(CH3)CH2O C O

NH CH3

n NH C

O

O CH2CH2 C O

CH CH2 NH C

O

O CH2CH2 C O

CH CH2 Photoreactive solvent-free acrylic

pressure-sensitive low viscosity systems

Photoreactive Solvent-free

Acrylic PSA η> 100 Pa·s

photoinitiator (liquid, solid)

•conventional

•unsaturated

•additional

(solid, liquid) photoresistant resin photoresistant plasticizer

(liquid) photoreactive

diluents (η ↓)

multifunctional monomers (η ↓)

Fig. 8. Composition of photoreac- tive solvent-free PSA-LVS

Rys. 8. Kompozycja fotoreaktyw- nych bezrozpuszczalnikowych kle- jów samoprzylepnych o niskiej lepkoœci

Fig. 9. Photoreactive diluent (RV) based on urethane acrylate Rys. 9. Rozcieñczalnik fotoreaktywny (RV) na bazie uretanoakrylanu

samoprzylepne kleje akrylanowe

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The most efficient solvent-free photoreactive PSA- -LVS, chosen owing to the first conducted investiga- tions, consist of photoreacive acrylic hotmelt and pho-

toreactive diluent in the weight ratio of 1:1 containing 2 wt.% of conventional radical photoinitiator was coated at 40oC as warmmelt on polyester film with the

8

0 10 20 30 40 50 60 70 80 90 100

3 2 1

1 : 3 1 : 2

1 : 1

Vi sc o si ty [Pa ·s ] a t R T

Ratio: solvent-free PSA : photoreactive diluent

Fig. 10. The viscosity of low vis- cosity PSA (PSA-LVS) containing photoreactive diluents: 1 – Ebe- cryl, 2 – Laromer, 3 – Genomer Rys. 10. Lepkoœæ fotoreaktywnych bezrozpuszczalnikowych klejów samoprzylepnych o niskiej lep- koœci zawieraj¹cych rozcieñczal- niki fotoreaktywne: 1 – Ebecryl, 2 – Laromer, 3 – Genomer

20 30 40 50 60 70 80 90

0 10 20 30 40 50 60 70 80

LVS at 20°C Warmmelts

Warmmelts

PSA-LVS at 20°C

PSA-LVS:RV (1:1) (1:1,5) (1:2) (1:2,5) (1:3)

Viscosity[Pa·s]

Temperature [°C]

Fig. 11. Temperature influence on the visco- sity of solvent-free photoreactive PSA-LVS Rys. 11. Wp³yw temperatury na lepkoœæ bez- rozpuszczalnikowych fotoreaktywnych kle- jów samoprzylepnych o niskiej lepkoœci

PSA : Diluent (1:1) + 2 wt.% photoinitiator

Coath

weig htof

PSA layer [g/m2

]

Crosslinking time [s]

Tack[N]

25

20

15

10

5 0

150 120

90 60

30

60

40 36

32 28

24

Fig. 12. Tack of developed PSA-LVS as function of the coat weight and cross-linking time

Rys. 12. Tack bezrozpuszczalnikowych klejów samoprzylepnych jako funkcja gramatury kleju oraz czasu sie- ciowania

samoprzylepne kleje akrylanowe

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coat weights between 30 g/m2 and 150 g/m2 and UV-crosslink between 24 s and 60 s under UV-lamp with UV dose of 900 mJ/cm2. The developed solvent- -free photoreactive PSA-LVS in form of one-sided self- -adhesive samples was tested with relation to the main typical PSA properties as tack, adhesion (peel adhe- sion) and cohesion (shear strength) at 20oC (Figs.

12-14).

The examined results of tack (Fig. 12) indicate that tack increases together with the coat weight of PSA

layer and decreases with the increasing of cross-linking time under UV-lamp. The adhesion diminishes with the increasing of cross-linking time (Fig. 18). Despite the fact that adhesion achieves the best results for PSA- -LVS with adhesive layers of 90 and 120 g/m2 coat weight, too short cross-linking time causes cohesive failure or partially cohesive failure, which eliminates solvent-free PSA-LVS for potential applications. Better cohesion at 20oC was observed with the cross-linking time increase (Fig. 14). Generally, better cohesion of

9

Coath

weig htof

PSA layer [g/m2

]

Crosslinking time [s]

Cohesionat20°C[h]

70 60 50 40 30 20 10 0

150 120

90 60

30

60 40

36 32

28 24

PSA : Diluent (1:1) + 2 wt.% photoinitiator PSA : Diluent (1:1) + 2 wt.% photoinitiator

Coath

weig htof

PSA layer [g/m2

]

Crosslinking time [s]

Adhesion[N]

150 120

90 60

30

60

40

36 32 28

24 25

20 15 10 5 0 30 30

Fig. 14. Cohesion of developed PSA-LVS as function of the coat weight and cross-linking time

Rys. 14. Kohezja bezrozpuszczalnikowych klejów samoprzylepnych jako funkcja gramatury kleju oraz czasu siecio- wania

Fig. 13. Adhesion of developed PSA-LVS as function of the coat weight and cross-linking time

Rys. 13. Adhezja bezrozpuszczalnikowych klejów samoprzylepnych jako funkcja gramatury kleju oraz czasu siecio- wania

samoprzylepne kleje akrylanowe

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tested PSA-LVS was registered for the coat weights of 60 g/m2and 90 g/m2. The over-curing of PSA-LVS for 60 g/m2thick and thicker adhesive layers for the cross- -linking time from 40 to 60 s causes the decrease of cohesion.

6. General conclusion and outlook

The need to replace solvent containing systems, which still have maintained a large market share in vari- ous areas, results in a further push of innovations in the development of solvent-free photoreactive acrylic pres- sure-sensitive adhesive low viscosity systems (PSA- LVS). As oppose to the classical solvent containing or to the aqueous systems, the developed acrylic solvent- free photoreactive pressure-sensitive adhesive low vis- cosity systems (PSA-LVS) provides the following ad- vantages:

l Reduction of environmental impacts to a minimum l Saving of energy costs

l Savings in coasts for space (low investment costs:

no drying channel)

l High production speed at pressure-sensitive adhe- sive coatings

l Production of “thick” self-adhesive carrier-free tapes with PSA layer about 1000 g/m2

l Manufacturing of one-sided or double-sided tapes with PSA layer between 30 and 150 g/m2

The drawbacks of the developed solvent-free pho- toreactive PSA-LVS are the difficulties with UV-cross- -linking process of self-adhesive layers with coat weight below 30 g/m2. This phenomenon is lead back on the inhibition of the oxygen.

The development of novel photoreactive solvent- -free low viscosity acrylic pressure-sensitive adhesives (PSA-LVS) has recently attracted considerable atten- tion, due to their practical and potential applications in various industrial branches within the comprehensive range of self-adhesive technical products. Solvent free LV-PASs with high performance: high tack, very good adhesion and high cohesion can be formed by using suitable commercial available photoreactive diluents, high reactive photoinitiators and UV-cross-linking as a very efficient cross-linking method. The viscosity of the novel LV-PSAs compositions based on acrylic is sufficiently low to manufacture the developed adhesive at room temperature or at temperatures from 40oC to 80oC as warmmelt, in compare with standard hotmelt adhesives coatable with typical hotmelt equipment at

process temperatures of 120oC-140oC. Contrary to con- ventional solvent-borne or water-borne PSA systems, which can be used for production of maximal 140 g/m2 thick self-adhesive layers, newly developed solvent- -free LV-PSAs compositions can be perfectly used and cross-linked within application areas as transfer carrier- -free tapes.

The favorable prognosis for design and develop- ment of tailor-made PSA-LVS composition, which can cope with both the technical and ecological demands, is therefore a continuing challenge for industrial research and development. Progress in the PSA coating techno- logy will open up for new applications and an extended market penetration of solvent-free UV-cross-linkable acrylic adhesive raw materials. New research and de- velopment experiments in the field of novel photoreac- tive solvent-free LV-PSAs include dual-cross-linkable stable low viscosity systems containing thermal cross- -clinking-agents, compositions for removable and repo- sitionable self-adhesive products and precursors for the production of the new generation structural PSA tapes.

References

1. Benedek I., “Developments in Pressure-Sensitive Products”, Edited by Istvan Benedek, Taylor &

Francis a CRC Press Book, New York, USA, 2006.

2. Skeist I., “Pressure-Sensitive Adhesives for Tapes and Labels”, Handbook of adhesive technology, Van Nostrand Reinhold, New York (1990) 641-663 3. Tauber A., T. Scherzer T., Weiss I., Mehnert R., J.

Coatings Technology, 2002,74, 41.

4. Czech Z., Adhäsion, 1996,9, 24.

5. Czech Z., “Vernetzung von Haftklebstoffen auf Polyacrylatbasis”, Ed. Szczecin University of Tech- nology, Szczecin, 1999.

6. Czech Z., Kautschuk Gummi. Kunststofffe, 2002, 10, 492.

7. Czech Z., Butwin A., “UV-crosslinkable solvent- -free acrylic PSA low viscosity systems”, 33. Mün- chener Klebstoff-Veredelungs-Symposium, 20-22.

Oktober, München, Germany, 2008.

8. Schumacher K.-H., Düssterwald U., Fisch R., Ad- hesive Technology, 2000,4, 18.

9. Milker R., Czech Z., “Vernetzung von Copolymeren auf Acrylatbasis”, 9. Münchener Klebstoff- und Veredelungsseminar, München, Germany, 1984, 85.

10. Brockmann W., Meyer-Roscher B., “Relation of Pressure-Sensitive properties and UV crosslinking technology”, TAPPI Hot Melt Symposium, Atlanta, USA, 1998, 174.

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samoprzylepne kleje akrylanowe

References

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