Zbigniew Czech Marta Wojciechowicz
* ^ ^ ^Holger Loclair
Self-adhesive tapes for
adhering to EPDM substrates
The technology o f solvent-borne acrylic pressure-sensitive adhesives (PSA) suitable fo r adhering to EPDM substrates has been developed. The resulted acrylic PSAs based on isooctyl acrylate are used fo r manufacturing o f double- sided tapes containing polyester film or non-woven paper carrier.
Key words: acrylic PSA, EPDM, self-adhesive tapes, carrier, adhesion, cohesion
Taśmy samoprzylepne przeznaczone do klejenia wulkanizatów EPDM
Opracowano technologię produkcji rozpuszczalnikowych poliakrylano- wych klejów oraz taśm samoprzylepnych stosowanych do łączenia powierzchni wulkanizatów EPDM. Otrzymane poliakrylanowe kleje samoprzylepne są oparte na akrylanie izooktylu. Produkowane dwustronne taśmy samoprzylepne zawierają jako nośnik folię poliestrową lub włókninę papierową.
Słowa kluczowe: poliakrylanowe kleje samoprzylepne, EPDM, taśmy samoprzylepne, nośnik, adhezja, kohezja
1. Introduction
Since their introduction half a century ago, acrylic pressure-sensitive adhesives have been successfully ap
plied in many fields. They are used in self-adhesive tapes, labels, sign, marking and protective films as well as in dermal dosage systems for pharmaceutical appli
cations, in biomedical electrodes and OP-tapes.
Pressure-sensitive adhesives (PSAs) are non- metallic materials used to bond other materials, mainly on their surfaces through adhesion and cohesion [1].
Three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack, peel (adhesion) and shear (cohesion). The first measures the adhesive’s ability to adhere quickly, the second its abi
lity to resist removal by peeling, and the third its ability to hold in position when shearing forces are exerted.
Generally speaking the first two are directly related to each other but are inversely related to the third. The most important bonding processes are bonding by ad
hesion and bonding with pressure-sensitive adhesives [2].
The performance of acrylic pressure-sensitive ad
hesives such as tack, peel and shear, synthesized by co-polymerization of acrylate monomers and formu
lated in organic solvent mixtures, is to a large extent determined by the molecular weight of acrylic copoly
mer, polymerization method and especially by the type and quantity of the crosslinking agents added to the PSA [3].
Polymer Institute, Szczecin University of Technology Polymer Institute, Szczecin U niversity of Technology
Orafol Europe, Oranienburg, Germany
The acrylic pressure-sensitive adhesives can be ap
plied in different forms as solvent-borne, water-borne (dispersions) and solvent-free systems. Although the solvent-borne pressure-sensitive acrylic adhesives may be dwarfs in terms of quantity, they are giants when considered from the quality point of view. Only by means of these acrylic specialties it was possible to succeed in developing the present surprisingly efficient generation of double-sided pressure-sensitive adhesive tapes for prominent assembly projects at justifiable cost.
Acrylic pressure-sensitive adhesive solutions are nowadays predominantly manufactured by polymeriza
tion of a wide selection of acrylic and methacrylic monomers, often with low levels of monomers having pendant functional groups, in a refluxing organic sol
vent in the presence of an initiator such as organic per
oxides or azo compounds [4-9]. Solvent-borne PSA acrylics offer several advantages such as excellent aging characteristics and resistance to elevated tem
peratures and plasticizers, exceptional optical clarity due to the polymer compatibility and non-yellowing.
They also have the highest balance of adhesion and cohesion and an excellent water resistance.
The versatility of acrylate chemistry is inherently useful in the design of high performance pressure-sen
sitive adhesives. A wide selection of raw material base and a versatility of polymerization processes are suit
able to design of base polymers with unique properties.
For manufacture of polyacrylate pressure-sensi
tive adhesives tackifying common acrylic acid esters with C4 - C]2 carbon atoms in the alkyl moiety are primarily preferred together with other comonomers.
The acrylate polymers that are inherently pressure- sensitive are synthesized by copolymerization of soft (low Tg), hard (high Tg) and functional monomers
SiaatM t& U f, nr 4 lipiec - sierpień 2006 r. TOM 10
taśmy samoprzylepne
[10]. The most important monomers of this class are compiled in Table 1.
Table 1. Monomers used fo r synthesis o f acrylic PSA Tabela 1. Monomery stosowane do syntezy klejów samoprzylepnych
M o n o m e r Hom opolym er Tg, °C tackifying monomers (soft)
2-ethylhexyl acrylate -70
isooctyl acrylate -70
n-octyl acrylate -65
isononyl acrylate -58
n-butyl acrylate -54
hardening monomers (hard)
ethyl acrylate -24
methyl acrylate -6
N-vinyl pyrrolidon + 5 4
isobornyl acrylate + 9 4
vinyl acetate + 2 8
functional groups containing monomers
2-hydroxyethyl acrylate -15
p-acryloyloxypropionic acid -10
2-hydroxypropyl acrylate -7
acrylic acid + 1 0 6
acrylamide + 1 7 9
Table 1 contains typical soft and hard monomers, as well as the types of functionality that can be incorpo
rated into the polymer. The tack and the peel properties are imparted by the soft that means creating low glass transition temperature segments monomers such as 2-ethylhexyl acrylate, isooctyl acrylate or n-butyl acry
late. The hard monomers, such as methyl acrylate or isobornyl acrylate are included to provide internal strength. The functional groups containing monomers such as acrylic acid or 2-hydroxyethyl acrylate are in
corporated into the balanced soft/hard monomer mix
ture to impart specific adhesion to specified substrates and to provide active sites for crosslinking.
2. Low energy surface substrates
The substrates with low energy surfaces such as polypropylene, polyethylene, silicone rubber, poly- tetrafluoroethylene (teflon) and EPDM are charac
terized by a low critical surface tension between 19 and 31 mN/m. The adhesion of typical conventional sol
vent-borne acrylic PSA systems to above mentioned polymers is insufficient between 0.5 to 6 N/2.5 cm.
The abbreviation EPDM means a terpolymer of ethylene, propylene and a diene monomer with the re
sidual unsaturated portion of the polymerized diene in the side chain (ISO 1629 Rubbers and latices - Nomen
clature). The diene monomers applied to form the EPDM terpolymer are mainly 5-ethylidene-2-norbor- nene, dicyclopentadiene and 1.4-hexadiene. Typical commercial EPDMs are Vistalon 2504 (Exxon Chemi
cal Ltd.), a terpolymer having 55 wt.% of ethylene, 41.2 wt.% of propylene and 3.8 wt.% of 5-ethylidene-2-nor- bornene and Nordel 1070 (DuPont) and ethylene/pro- pylene/l,4-hexadiene terpolymer (exactly content of components is unknown) [11].
3. Experimental section
Monomers
The basic acrylic PSA was synthesized from butyl acrylate and acrylic acid.
The investigated solvent-borne pressure-sensitive adhesives were synthesized from alkyl acrylates and acrylic acid in the organic solvent ethyl acetate at the boiling temperature about 77 °C. Acrylic acid concen
tration used was from 3.0 to 10.0 wt.%. 2,2’-Azo-bis- diisobutyronitrile (AIBN) in the amount of 0.1 wt.%
was applied as the thermal initiator to start radical poly
merization.
Four soft acrylic acid esters: 2-ethylhexyl acrylate (2-EHA), butyl acrylate (BA), hexyl acrylate (HA) and isooctyl acrylate (IA) were purchased from Santomer Co. and acrylic acid (AA) was purchased from BASF.
Crosslinking agents
The properties of pressure-sensitive adhesives such as tack, adhesion and cohesion are determined, to a great extent, by the kind and quantity of a crosslinking agent added. Crosslinking reactions influence the bulk properties of the film and build shear, heat and chemical resistance, while negatively impacting the tack and peel resistance. It is necessary to achieve suitable degree of crosslinking for heat resistance, because the pressure- sensitive adhesive polymers are operating in the region above their glass transition temperature. Therefore, without crosslinks the polymer would readily flow un
der the influence of heat losing its cohesive strength [12]. Acrylic PSAs whose chains are only crosslinked via hydrogen bonds or inter penetrating systems (IPN) can hardly be cohesively loaded. But a few tenths of a percent of a crosslinking agent make them pressure- sensitive adhesives having good mechanical and ther
mal properties.
One of the goals of our project was to examine the influence of such crosslinkers as tetrabutyl titanate (TBT) obtained from DuPont, aluminum acetylace- tonate (A1ACA) obtained from Wacker, N,N’-bis-pro- pyleneisophthalic acid amide (BPIA) obtained from Arsynco and hexamethoxymethyl melamine resin (Cymel 303) obtained from Dyno Cyanamid on adhe
sion and cohesion of the acrylic PSAs.
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Tackifiers
Tack is the force or energy required to obtain self- -adhesiveness. A high modulus is desirable at the point of rupturing of adhesive bonds. Tackifiers increase low temperature modulus of the PSAs by increasing their glass transition temperature. The self-adhesive mixture gains a high modulus at high strain rates during room temperature performance. The application of tackifiers allows improvement of adhesiveness to various sub
strates, in this case to tested EPDM types.
Three tackifiers: pentaerythritol ester of partially hydrogenated rosin under the trade designation “Penta- lyn H” (Hercules), modified rosin ester “Sylvatac 40N”
obtained from Sylvachem Co. and polytherphenphenol resin “Derthophenone T 100” obtained from DRT were formulated into synthesized acrylic PSAs to control their tack, adhesion and cohesion.
Crosslinking parameters
The synthesized solvent-borne acrylic PSAs con
taining a tackiefier and a crosslinker were cast with a knife coater in the amount of 60 g/m2 on 25 pm poly
ester film Hostaphan RN obtained from Hoechst (Ger
many) and next were dried for 10 min at 105 °C.
taśmy samoprzylepne
horizontal target substrate surface of a EPDM plate at least 12.7 lineal cm in firm contact.
Cohesion
The cohesion is a measure of the shear strength or internal strength of an adhesive. It is based upon the amount of force required to pull an adhesive strip from a standard flat surface in a direction parallel to the sur
face to which it has been affixed with a definite pres
sure according to AFERA 4012, at 20 °C. It is measured in Newtons for 4 h load times. Each test is conducted on two adhesive-coated strips applied to a standard stain
less steel panel in a manner such that a 2.5 cm x 2.5 cm portion of the strip is in fixed contact with the panel with one end of the strip being free.
4. Results and discussion
Influence of the kind of alkyl acrylates and concentration of acrylic acid on viscosity and ad
hesion of acrylic PSAs to EPDM Method of testing of solvent-
-borne acrylic PSAs
Adhesion
The adhesion is the force required to remove a coated flexible pressure-sensitive adhesive sheet ma
terial from a test panel measured at a 180° peel and rate of removal (AFERA 4001). A sample of PSA-coated material 2.5 cm wide and 12.7 cm long is bonded to a
The influence of the acrylic acid concentration on the viscosity of synthesized PSAs based on various monomers is illustrated in Fig. 1.
The viscosity of solvent-borne acrylic PSAs in
creases with the increase of the acrylic acid concentra
tion. All acrylic PSA systems containing alkyl acrylates showed the moderate, acceptable in practice viscosity in the 4 and 8 wt.% range of acrylic acid concentration.
The influence of the applied alkyl acrylates in the form of acrylic PSAs on adhesion to the investigated EPDM types measured at 20 °C is shown in Fig. 2.
Fig. 1. Viscosity o f PSAs based on different monomers versus acrylic acid concentration
Rys. 1. Zależność lepkości klejów samoprzylepnych, bazujących na różnych monomerach, od stężenia kwa
su akrylowego
Fig.. 2. Adhesion o f synthesized PSAs to EPDM sub
strates versus acrylic acid concentration
Rys. 2. Zależność adhezji syntetyzowanych klejów samoprzylepnych do EPDM od stężenia kwasu akrylo
wego
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As can be seen from Figure 2, adhesion to both tested EPDMs at first increased and next considerably decreased with the increase of acrylic acid concentra
tion. The best acrylic PSA according to adhesion to EPDM was the PSA containing 93.0 wt.% isooctyl acrylate and 7.0 wt.% acrylic acid incorporated into polymer backbone. For this PSA a significantly better adhesion to Vistalon 2504 was achieved.
Influence of the kind and con
centration of crosslinkers on ad
hesion to EPDM
Further investigations with different kinds and con
centrations of chosen crosslinking agents were carried out with the selected acrylic PSA based on isooctyl
Fig. 3. Adhesion to EPDM versus crosslikner concen
tration
Rys. 3. Zależność adhezji do EPDM od stężenia związ
ku sieciującego
Fig. 4. Cohesion o f the selected PSA versus crosslinker concentration
Rys. 4. Zależność kohezji wybranych klejów samoprzy
lepnych od stężenia zvsiązku sieciującego
acrylate and acrylic acid. The influence of crosslinking by the examined crosslinkers on acrylic PSA adhesion and cohesion is illustrated in Fig. 3-4.
The uncrosslinked solvent-borne acrylic PSA shows low values of adhesion along with cohesive failure. After addition of a small amount of crosslinking agents the acrylic PSA begins to crosslink. The PSA structure is now compact. Adhesion increases with in
crease of crosslinking agent concentration up to maxi
mum different for different crosslinkers. The highest adhesion level to Vistalon 2504 was observed for 0.4 wt.% A1ACA.
The results show (Fig. 4) that the tested crosslin
kers differ in their ability to improve the strength of the acrylic PSA. Permutex 9340, multifunctional pro- pylenimine BPIA and metal chelate A1ACA are the best crosslinking agents. Taking into account the ba
lance between adhesion to EPDM and cohesion, alu
minum acetyl acetonate (A1ACA) has shown the best results.
Influence of the kind and con
centration of tackifiers on adhe
sion to EPDM
Further trials with different kinds and concentra
tions of the selected tackifiers were carried out with the acrylic PSA containing 93.0 wt.% isooctyl acrylate and
Fig. 5. Adhesion o f the selected PSA to EPDM versus tackifiers concentration
Rys. 5. Zależność adhezji wybranych klejów samoprzy
lepnych od stężenia żywicy
7.0 wt.% acrylic acid and crosslinked with 0.4 wt.%
A1ACA. The influence of the applied tackifiers on the acrylic PSA adhesion to EPDM and cohesion is illustra
ted in Fig. 5-6.
It is interesting to note that the adhesion to EPDM is relatively high when the tackifiers Dertophene T 100 and Sylvatac 40 N are used and achieves the highest
TOM 10 lipiec - sierpień 2006 r. S ćaafom & iy nr 4
level for their concentration between 20 and 40 wt.%.
As expected, the increase of tackifiers content affects negatively the cohesion of the investigated acrylic
Fig. 6. Influence o f tackifier kind and concentration on cohesion o f the selected PSA
Rys. 6. Wpływ rodzaju i stężenia żywicy na kohezję wybranych klejów samoprzylepnych
PSAs. Based on the balance adhesion/cohesion the best tackifier in this group was polytherphene phenol resin Dertophene T 100.
taśmy samoprzylepne
5. Summary
From the evaluated solvent-borne acrylic pressure- -sensitive adhesives, modified with different crosslin
kers and tackifiers, the best results according to adhe
sion to EPDM surfaces and cohesion of self-adhesive layers were obtained for acrylic copolymers based on 7.0 wt.% acrylic acid and 93.0 wt.% isooctylacrylate, modified with 20 wt.% resin Dertophene T 100 and crosslinked using 0.4 wt.% metal chelate A1ACA.
Reference
1. DIN 16921
2. Kohler R.: Adhdsion 1970, 3, 90
3. Czech Z : uVernetzung von Haftklebstoffen auf Polyacrylatbasis ”, Szczecin, Szczecin University of Technology (1999)
4. Czech Z : Adhdsion 1994, 38, 26
5. Milker R., Czech Z : Adhdsion 1991, 3JL 13
6. Milker R., Czech Z : Deutsche Papierwirtschaft 1990, 2, 2
7. Milker R., Czech Z : Adhdsion 1990, 34, 32
8. Milker R., Czech Z : Polimery 1990, XXXV. 326 9. Milker R., Czech Z : Adhdsion 1989, 33L 20 10. Milker R., Czech Z : Adhdsion 1985, 2£ 29 11. US Patent 4,450,252 (1984) 3M
12. Ooka M., Ozawa H.: Progr. Org. Coat. 1994, 23, 325
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Nowe monografie
i Uprzejmie informujemy, że w 2006 roku ukażą się dwie nowe monografie:
. ELASTOMERY I PRZEMYSŁ GUMOWY Praca zbiorowa pod redakcją
Wandy Parasiewicz - Instytut Przemysłu Gumowego „Stomil"
i Władysława M. Rzymskiego — Politechnika Łódzka
•
