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Influence of electron radiation on adhesion of some polymer films to acrylic adhesives
ARTICLE IN PRESS
International Journal of Adhesion & Adhesives 24 (2004) 259–262
Influence of electron radiation on adhesion of some polymer films to acrylic adhesives ’ Marian Zenkiewicz* ! 87-100 Poland Institute for Plastics Processing ‘‘Metalchem’’, ul. M. Sk!odowskiej-Curie 55, Torun, Accepted 27 October 2003
Abstract Influence of the electron radiation on adhesion properties of the films made of low-density polyethylene, biaxially oriented polypropylene, and polyethylene terephthalate was investigated. The films were irradiated with doses of up to 500 kGy, using a highenergy electron beam. The adhesion strength of bonds between the films and acrylic adhesive was measured by means of the 180 peel-off test. For all the films, it was found that this strength increased upon irradiation with the doses of the whole range applied, which suggested that the adhesion properties of the films improved as well. Thus, the radiational modification of polymer films by means of the high-energy electron beam enables one to obtain durable bonds between the films and adhesive without necessity to modify the film surface layer with another method. r 2003 Elsevier Ltd. All rights reserved. Keywords: A. Structural acrylic; B. Plastics; B. Surface modification; C. Peel; Electron–beam technology
1. Introduction Adhesion, being one of the fundamental physical phenomena associated with the structure of matter, is of a great importance for processes including treatment of plastics, like printing and gluing. These processes consist in superficial binding together two different, solid or liquid, bodies as a result of mutual attraction forces [1,2]. As to the processing of plastics, the adhesion is being considered in relation to two system types, solid–liquid and solid–liquid–solid. In the case of the former one, the solid is usually represented by a polymeric material while the liquid, by ink, varnish or coating. This system type manifests itself in the processes of, e.g., printing, varnishing, and applying a protective coating. After such a process has been carried out, the system transforms into the solid–solid one. The other system type relates mainly to the process of gluing and, in this case, the system transforms into the one that consists of three solids. For both system types, a sufficient wettability of a plastic with a liquid being spread on it
*Tel./fax: +48-56-6487887. ’ E-mail address: [email protected] (M. Zenkiewicz). 0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.10.008
is an important factor determining the quality (mostly the strength) of an adhesive bond [3]. Polymeric materials, especially polyolefins, are hydrophobic, i.e., of poor wettability. Thus, modification of their surface layer is necessary, which is mostly being realized by means of corona discharge, flame or lowtemperature plasma treatment [4–8]. Modification of plastics with ionizing radiation is known already for over thirty years and it is still a subject of intensive research. A substantial progress made in the construction of low-energy electron accelerators caused that method to be more and more frequently used in the plastic processing industry, including modification of polymer films [9–13]. During the radiational modification of polyolefinic films, properties of their surface layer change. The layer oxidation is one of the most important processes that occur upon this treatment. This reaction depends significantly on the radiation dose, which was shown earlier [14,15]. Increase in the film wettability was also found [16], which suggested that the adhesion properties improved as well. A positive verification of this statement might imply that, having some plastics modified by means of radiation, a specific modification of the surface layer of these plastics with the purpose of printing or gluing may be waived. The objective of the
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
260
present work was to verify this suggestion while investigating the influence of the electron radiation on the strength of adhesive bonds between acrylic glue and polymer films used in packaging.
2. Experimental
The adhesion investigations were carried out using a double stick tape (25 mm wide and 0.8 mm thick) made of foamed polyethylene that was covered on both sides with a layer of acrylic adhesive. The peel strength was measured with a TIRAtest 2705 tensile testing machine (TIRA Maschinenbau GmbH, Germany), equipped with a gauge head operating in the range of up to 100 N.
2.1. Materials
2.3. Measurements
The following polymer films were selected for the investigations:
The film samples were irradiated in the atmospheric air and at the ambient temperature (23 C), i.e., under conditions characteristic of industrial technology. To avoid an excess increase in the film temperature when applying large radiation doses, the irradiation was repeated with relatively small doses (25 kGy) until the expected total doses were reached. Thus, the applied way of film modification was not an adiabatic process: during the successive breaks in the irradiation procedure, the released heat dissipated into the surroundings and the sample temperature increased relatively little. For an irradiation procedure, a film specimen (50 42 cm) was placed in an aluminum container that was put on a conveyor able to move with a controlled speed through the radiation zone. The samples nonirradiated (P0) and irradiated with various doses (P1–P5) are specified in Table 1. The determination of the adhesion strength of bonds between the films and acrylic adhesive was carried out using the 180 peel-off test, according to a standard procedure [17]. A typical joint consisted of an aluminum plate, double stick tape covered with acrylic adhesive, and polymer film (Fig. 1). A piece of the tape was put on the degreased surface of the aluminum plate and pressed with a force of ca. 20 N, using a rubber roller. Then, a protecting silicon paper was removed from the tape to expose a adhesive layer on which a film specimen was subsequently superimposed. Again, the film was pressed with the force of ca. 20 N, using the rubber roller. Next, the plate with the film specimen was placed in a fixture of a tensile tester and the specimen free end, inverted by 180 , was put into a grip. The specimen was then pulled at the speed of 300 mm/min and the peel-off force (F) was continuously recorded as a function of the peel-off path varying up to L=150 mm.A mean peel-off force related to a peel-off length of L0=100 mm was considered as a measurement result (the initial and final portions each equal 25 mm were neglected). Seven measurements were carried out for each of the P0–P5
*
*
*
A low-density polyethylene (LDPE) film, 150 mm thick, produced from parent polyethylene, Malen-E (PKN Orlen SA, P"ock, Poland). The polymer was obtained by a high-pressure polymerization. Its density was 0.918–0.921 g/cm3 (23 C), melt flow rate, 0.2–0.4 g/10 min (21.19 N, 19070.5 C), molecular weight, 6 105, and crystallinity, 40–50%. The film was formed by means of the blowing extrusion with a blowing factor of 2.7. A biaxially oriented polypropylene (BOPP) film, 40 mm thick, produced from parent polypropylene, Malen-PF 401 (PKN Orlen SA, P"ock, Poland). The polymer was obtained by a continuous suspension polymerization with use of catalysts containing titanium salts deposited on aluminum chloride. It contained a high fraction (above 97%) of the isotactic phase, its density was 0.905–0.910 g/cm3 (23 C), melt flow rate, 2.670.2 g/10 min (21.19 N, 23070.5 C), molecular weight, 4 105, and crystallinity, 50–60%. The film was formed by extrusion and then oriented by means of stretching in two mutually perpendicular directions: the machine direction (MD) and the transverse direction (TD). The MD and TD draw ratios were 5:1 and 8:1, respectively. A polyethylene terephthalate (PET) film, 100 mm thick, produced from parent polyethylene terephtha! Poland). late, Elpet-E (Elana Pet Sp. z o.o., Torun, The polymer was synthesized by a continuous homogenization. Its density was ca. 1.4 g/cm3 (23 C), intrinsic viscosity, 0.8270.02 dl/g, molecular weight, 2.5 104, and crystallinity, ca. 35%. The film was formed by extrusion and then oriented by means of stretching in the machine and transverse directions. The MD and TD draw ratios were close to each other (ca. 3:1).
2.2. Equipment An LAE 13/9 linear accelerator (former USSR) was used to irradiate the examined films. The maximum electron energy was 13 MeV, the controllable energy range, 5–13 MeV, and the average power of the electron beam, 9 kW.
Table 1 Radiation doses absorbed by samples of the examined films Sample
P0
P1
P2
P3
P4
P5
Dose (kGy)
0
25
50
100
250
500
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
261
2500
3
LDPE
F
BOPP
PET
2000
W j [J. m-2 ]
2 1500
1000
1
L0 =100 mm
L=150 mm
500
0 0
300
400
500
Fig. 2. Plots of the unit peel-off energy (Wu) versus the radiation dose for the studied polymer films.
Fig. 1. Scheme for an adhesive joint and measurement of the peel-off force (F): 1—polymer film, 2—double stick tape, 3—aluminum plate, L—length of the adhesive bond, L0—peel-off length used to calculate the unit peel-off energy.
samples of the three polymer films. The lowest and the highest results were disregarded and the remaining five were averaged to yield a peel-off force value (Fav) related to a given sample. Then, a unit peel-off energy (Wu) was obtained from the following relation: 2Fav L0 2Fav ; ¼ wL0 w
200 Dose [kGy]
F
Wu ¼
100
ð1Þ
where Fav is the average peel-off force, L0 is the peel-off length, and w is the width of the peeled film specimen.
3. Results and discussion The results of determination of the unit peel-off energy (Wu) are shown in Fig. 2. In the case of the LDPE film, it was found that Wu increased almost linearly with the radiation dose. It varied from 80 (sample P0) to 1432 J m–2 (sample P5), which means that the strength of the adhesive bond increased by ca. 18 times as a result of the film irradiation with the dose of 500 kGy. An average rise of Wu in the dose range of up to 100 kGy was 4.5 J m–2 kGy–1, which was approximately twice as high as that in the dose range of 100– 500 kGy (2.3 J m–2 kGy–1). The observed large increase in the adhesion strength of the bond between the LDPE film and acrylic adhesive upon the film irradiation with the high-energy electron beam was caused mainly by oxidation of the film surface layer and formation of
functional groups containing oxygen [15]. Crosslinking occurring in this layer was another factor that produced the increase in the adhesion strength [9,10]. For the BOPP film, variations in the unit peel-off energy with the radiation dose were similar to those for the LDPE film: Wu increased almost linearly from 136 (sample P0) to 1935 J m–2 (sample P5), which is the increase by ca. 14 times. An average rise of Wu in the dose range of below 250 kGy was 4.2 J m–2 kGy–1 and that in the dose range of above 250 kGy, ca 3 J m–2 kGy–1. Like in the case of the LDPE film, the increase in the adhesion strength of the bond between the BOPP film and acrylic adhesive was caused mostly by oxidation of the film surface layer and formation of functional groups containing oxygen. However, the crosslinking process did not affect the adhesion strength since degradation of polypropylene dominated, especially at larger radiation doses [9]. In the case of the PET film, the course of the plot of the unit peel-off energy versus the radiation dose differed clearly from those for the two other films: Wu increased from 1200 to 2000 J m–2 for the P0 and P1 samples, respectively (the increase by 67%). The peel-off force corresponding to Wu=2000 J m–2 was 25 N. At that point, the double stick tape failed, which made impossible to determine the unit peel-off energy at the interface of the film and acrylic adhesive. Therefore, the measurements for samples P1–P5 yielded the energy of breaking the tape. Tapes of several other types were used; however, none of them was of a sufficient strength. Thus, it can be concluded that the adhesion strength of the bond between the acrylic adhesive and PET film irradiated with the dose as small as 25 kGy cannot be measured with the applied method. The unit peel-off energy for either of the P1–P5 samples of the PET film was higher than that for the LDPE and BOPP films irradiated with the dose of 500 kGy. In addition to changes in the adhesion properties, the radiational modification causes variations in the mechanical properties of polymer films, in particular, in the tensile strength at break and tear resistance. For
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
262
some polymers, especially BOPP, these variations may decrease the mechanical strength of the film, which would limit application of this film in packaging [18,19].
modifying the film surface layer is not necessary any more.
Acknowledgements 4. Conclusions The following conclusions may be drawn from the investigations performed: *
*
*
*
The strength of the adhesive bond between the nonirradiated LDPE film and acrylic adhesive is relatively low (the unit peel-off energy is as low as 80 J m–2). It rises almost linearly with the radiation dose. Upon the dose of 500 kGy, this strength increases by ca. 18 times. The strength of the adhesive bond between the nonirradiated BOPP film and acrylic adhesive is higher than that in the case of the LDPE film (the unit peeloff energy is 136 J m–2). The adhesion strength rises rapidly upon the electron radiation and reaches the value of 1935 J m–2 at the dose of 500 kGy (the increase by ca. 14 times). The strength of the adhesive bond between the nonirradiated PET film and acrylic adhesive is many times higher than that in the case of the LDPE and BOPP films (the unit peel-off energy is 1200 J m–2). It rises rapidly with the radiation dose and reaches the value above 2000 J m–2 at the dose of 25 kGy. The adhesion strength for the PET film irradiated with various doses could not be determined because of the failure of the double stick tape. Changes in surface layer properties, being one of the results of the radiational modification of polymer films, improve adhesion properties of the LDPE, BOPP, and PET films. Thus, these films, when irradiated, can successfully be bonded with the acrylic adhesive and application of other methods for
The study was supported by the State Committee for Scientific Research (KBN) with the Grant No. 7 T08E 052 20.
References [1] Kinloch AJ. J. Mater. Sci. 1980;15:2141. [2] Kaelble D. Physical chemistry of adhesion. New York: John Wiley; 1975. ’ [3] Zenkiewicz M. Adhesion and modification of surface layer of macromolecular materials. Warsaw: WNT; 2000. [4] Carley JF, Kitze PT. Polym Eng 1978;18:326. [5] Connell LD. Kunststoffe 1986;76:761. [6] Inagaki N. Plasma surface modification and plasma polymerisation. Lancaster: Technomic; 1996. ’ [7] Zenkiewicz M. Polimery 2000;45:81. ’ [8] Zenkiewicz M. J Adhesion Sci Technol 2001;15:63. [9] Singh A, Silverman J. Radiation processing of polymers. Munich: Hanser; 1992. [10] Singh A. Radiat Phys Chem 1999;56:375. [11] Korenev S. In: Proceedings of the 2001. Particle Accelerator Conference. Chicago, 2001 p. 2509. [12] L.auppi UV. EZCure: Ein Kosteng.unstiger Elektronentrocker. ESI–ESH Seminar, Bielefeld, 30 M.arz 2000. [13] Rangwalla I, Sanders R. Ink World 2000. April. p. 31. ’ ! [14] Zenkiewicz M, Rauchfleisz M, Czuprynska J. Polimery 2003;48:343. ’ ! [15] Zenkiewicz M, Rauchfleisz M, Czuprynska J. Radiat Phys Chem. 2003;68:779. ’ [16] Zenkiewicz M, Czupry!nska J. In: Conference Proceedings of International Conference ‘‘Plastko 2001’’. Zlin, 2001. p. 26. [17] ASTM D903. Standard Test Method for Peel or Stripping Strength of Adhesive Bonds. American Society for Testing and Materials, West Conshohocken, PA, 1999. ’ [18] Zenkiewicz M. Polimery 2003;48:57. ’ [19] Zenkiewicz M. Radiat Phys Chem, in print.
International Journal of Adhesion & Adhesives 24 (2004) 259–262
Influence of electron radiation on adhesion of some polymer films to acrylic adhesives ’ Marian Zenkiewicz* ! 87-100 Poland Institute for Plastics Processing ‘‘Metalchem’’, ul. M. Sk!odowskiej-Curie 55, Torun, Accepted 27 October 2003
Abstract Influence of the electron radiation on adhesion properties of the films made of low-density polyethylene, biaxially oriented polypropylene, and polyethylene terephthalate was investigated. The films were irradiated with doses of up to 500 kGy, using a highenergy electron beam. The adhesion strength of bonds between the films and acrylic adhesive was measured by means of the 180 peel-off test. For all the films, it was found that this strength increased upon irradiation with the doses of the whole range applied, which suggested that the adhesion properties of the films improved as well. Thus, the radiational modification of polymer films by means of the high-energy electron beam enables one to obtain durable bonds between the films and adhesive without necessity to modify the film surface layer with another method. r 2003 Elsevier Ltd. All rights reserved. Keywords: A. Structural acrylic; B. Plastics; B. Surface modification; C. Peel; Electron–beam technology
1. Introduction Adhesion, being one of the fundamental physical phenomena associated with the structure of matter, is of a great importance for processes including treatment of plastics, like printing and gluing. These processes consist in superficial binding together two different, solid or liquid, bodies as a result of mutual attraction forces [1,2]. As to the processing of plastics, the adhesion is being considered in relation to two system types, solid–liquid and solid–liquid–solid. In the case of the former one, the solid is usually represented by a polymeric material while the liquid, by ink, varnish or coating. This system type manifests itself in the processes of, e.g., printing, varnishing, and applying a protective coating. After such a process has been carried out, the system transforms into the solid–solid one. The other system type relates mainly to the process of gluing and, in this case, the system transforms into the one that consists of three solids. For both system types, a sufficient wettability of a plastic with a liquid being spread on it
*Tel./fax: +48-56-6487887. ’ E-mail address: [email protected] (M. Zenkiewicz). 0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.10.008
is an important factor determining the quality (mostly the strength) of an adhesive bond [3]. Polymeric materials, especially polyolefins, are hydrophobic, i.e., of poor wettability. Thus, modification of their surface layer is necessary, which is mostly being realized by means of corona discharge, flame or lowtemperature plasma treatment [4–8]. Modification of plastics with ionizing radiation is known already for over thirty years and it is still a subject of intensive research. A substantial progress made in the construction of low-energy electron accelerators caused that method to be more and more frequently used in the plastic processing industry, including modification of polymer films [9–13]. During the radiational modification of polyolefinic films, properties of their surface layer change. The layer oxidation is one of the most important processes that occur upon this treatment. This reaction depends significantly on the radiation dose, which was shown earlier [14,15]. Increase in the film wettability was also found [16], which suggested that the adhesion properties improved as well. A positive verification of this statement might imply that, having some plastics modified by means of radiation, a specific modification of the surface layer of these plastics with the purpose of printing or gluing may be waived. The objective of the
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
260
present work was to verify this suggestion while investigating the influence of the electron radiation on the strength of adhesive bonds between acrylic glue and polymer films used in packaging.
2. Experimental
The adhesion investigations were carried out using a double stick tape (25 mm wide and 0.8 mm thick) made of foamed polyethylene that was covered on both sides with a layer of acrylic adhesive. The peel strength was measured with a TIRAtest 2705 tensile testing machine (TIRA Maschinenbau GmbH, Germany), equipped with a gauge head operating in the range of up to 100 N.
2.1. Materials
2.3. Measurements
The following polymer films were selected for the investigations:
The film samples were irradiated in the atmospheric air and at the ambient temperature (23 C), i.e., under conditions characteristic of industrial technology. To avoid an excess increase in the film temperature when applying large radiation doses, the irradiation was repeated with relatively small doses (25 kGy) until the expected total doses were reached. Thus, the applied way of film modification was not an adiabatic process: during the successive breaks in the irradiation procedure, the released heat dissipated into the surroundings and the sample temperature increased relatively little. For an irradiation procedure, a film specimen (50 42 cm) was placed in an aluminum container that was put on a conveyor able to move with a controlled speed through the radiation zone. The samples nonirradiated (P0) and irradiated with various doses (P1–P5) are specified in Table 1. The determination of the adhesion strength of bonds between the films and acrylic adhesive was carried out using the 180 peel-off test, according to a standard procedure [17]. A typical joint consisted of an aluminum plate, double stick tape covered with acrylic adhesive, and polymer film (Fig. 1). A piece of the tape was put on the degreased surface of the aluminum plate and pressed with a force of ca. 20 N, using a rubber roller. Then, a protecting silicon paper was removed from the tape to expose a adhesive layer on which a film specimen was subsequently superimposed. Again, the film was pressed with the force of ca. 20 N, using the rubber roller. Next, the plate with the film specimen was placed in a fixture of a tensile tester and the specimen free end, inverted by 180 , was put into a grip. The specimen was then pulled at the speed of 300 mm/min and the peel-off force (F) was continuously recorded as a function of the peel-off path varying up to L=150 mm.A mean peel-off force related to a peel-off length of L0=100 mm was considered as a measurement result (the initial and final portions each equal 25 mm were neglected). Seven measurements were carried out for each of the P0–P5
*
*
*
A low-density polyethylene (LDPE) film, 150 mm thick, produced from parent polyethylene, Malen-E (PKN Orlen SA, P"ock, Poland). The polymer was obtained by a high-pressure polymerization. Its density was 0.918–0.921 g/cm3 (23 C), melt flow rate, 0.2–0.4 g/10 min (21.19 N, 19070.5 C), molecular weight, 6 105, and crystallinity, 40–50%. The film was formed by means of the blowing extrusion with a blowing factor of 2.7. A biaxially oriented polypropylene (BOPP) film, 40 mm thick, produced from parent polypropylene, Malen-PF 401 (PKN Orlen SA, P"ock, Poland). The polymer was obtained by a continuous suspension polymerization with use of catalysts containing titanium salts deposited on aluminum chloride. It contained a high fraction (above 97%) of the isotactic phase, its density was 0.905–0.910 g/cm3 (23 C), melt flow rate, 2.670.2 g/10 min (21.19 N, 23070.5 C), molecular weight, 4 105, and crystallinity, 50–60%. The film was formed by extrusion and then oriented by means of stretching in two mutually perpendicular directions: the machine direction (MD) and the transverse direction (TD). The MD and TD draw ratios were 5:1 and 8:1, respectively. A polyethylene terephthalate (PET) film, 100 mm thick, produced from parent polyethylene terephtha! Poland). late, Elpet-E (Elana Pet Sp. z o.o., Torun, The polymer was synthesized by a continuous homogenization. Its density was ca. 1.4 g/cm3 (23 C), intrinsic viscosity, 0.8270.02 dl/g, molecular weight, 2.5 104, and crystallinity, ca. 35%. The film was formed by extrusion and then oriented by means of stretching in the machine and transverse directions. The MD and TD draw ratios were close to each other (ca. 3:1).
2.2. Equipment An LAE 13/9 linear accelerator (former USSR) was used to irradiate the examined films. The maximum electron energy was 13 MeV, the controllable energy range, 5–13 MeV, and the average power of the electron beam, 9 kW.
Table 1 Radiation doses absorbed by samples of the examined films Sample
P0
P1
P2
P3
P4
P5
Dose (kGy)
0
25
50
100
250
500
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
261
2500
3
LDPE
F
BOPP
PET
2000
W j [J. m-2 ]
2 1500
1000
1
L0 =100 mm
L=150 mm
500
0 0
300
400
500
Fig. 2. Plots of the unit peel-off energy (Wu) versus the radiation dose for the studied polymer films.
Fig. 1. Scheme for an adhesive joint and measurement of the peel-off force (F): 1—polymer film, 2—double stick tape, 3—aluminum plate, L—length of the adhesive bond, L0—peel-off length used to calculate the unit peel-off energy.
samples of the three polymer films. The lowest and the highest results were disregarded and the remaining five were averaged to yield a peel-off force value (Fav) related to a given sample. Then, a unit peel-off energy (Wu) was obtained from the following relation: 2Fav L0 2Fav ; ¼ wL0 w
200 Dose [kGy]
F
Wu ¼
100
ð1Þ
where Fav is the average peel-off force, L0 is the peel-off length, and w is the width of the peeled film specimen.
3. Results and discussion The results of determination of the unit peel-off energy (Wu) are shown in Fig. 2. In the case of the LDPE film, it was found that Wu increased almost linearly with the radiation dose. It varied from 80 (sample P0) to 1432 J m–2 (sample P5), which means that the strength of the adhesive bond increased by ca. 18 times as a result of the film irradiation with the dose of 500 kGy. An average rise of Wu in the dose range of up to 100 kGy was 4.5 J m–2 kGy–1, which was approximately twice as high as that in the dose range of 100– 500 kGy (2.3 J m–2 kGy–1). The observed large increase in the adhesion strength of the bond between the LDPE film and acrylic adhesive upon the film irradiation with the high-energy electron beam was caused mainly by oxidation of the film surface layer and formation of
functional groups containing oxygen [15]. Crosslinking occurring in this layer was another factor that produced the increase in the adhesion strength [9,10]. For the BOPP film, variations in the unit peel-off energy with the radiation dose were similar to those for the LDPE film: Wu increased almost linearly from 136 (sample P0) to 1935 J m–2 (sample P5), which is the increase by ca. 14 times. An average rise of Wu in the dose range of below 250 kGy was 4.2 J m–2 kGy–1 and that in the dose range of above 250 kGy, ca 3 J m–2 kGy–1. Like in the case of the LDPE film, the increase in the adhesion strength of the bond between the BOPP film and acrylic adhesive was caused mostly by oxidation of the film surface layer and formation of functional groups containing oxygen. However, the crosslinking process did not affect the adhesion strength since degradation of polypropylene dominated, especially at larger radiation doses [9]. In the case of the PET film, the course of the plot of the unit peel-off energy versus the radiation dose differed clearly from those for the two other films: Wu increased from 1200 to 2000 J m–2 for the P0 and P1 samples, respectively (the increase by 67%). The peel-off force corresponding to Wu=2000 J m–2 was 25 N. At that point, the double stick tape failed, which made impossible to determine the unit peel-off energy at the interface of the film and acrylic adhesive. Therefore, the measurements for samples P1–P5 yielded the energy of breaking the tape. Tapes of several other types were used; however, none of them was of a sufficient strength. Thus, it can be concluded that the adhesion strength of the bond between the acrylic adhesive and PET film irradiated with the dose as small as 25 kGy cannot be measured with the applied method. The unit peel-off energy for either of the P1–P5 samples of the PET film was higher than that for the LDPE and BOPP films irradiated with the dose of 500 kGy. In addition to changes in the adhesion properties, the radiational modification causes variations in the mechanical properties of polymer films, in particular, in the tensile strength at break and tear resistance. For
ARTICLE IN PRESS ’ M. Zenkiewicz / International Journal of Adhesion & Adhesives 24 (2004) 259–262
262
some polymers, especially BOPP, these variations may decrease the mechanical strength of the film, which would limit application of this film in packaging [18,19].
modifying the film surface layer is not necessary any more.
Acknowledgements 4. Conclusions The following conclusions may be drawn from the investigations performed: *
*
*
*
The strength of the adhesive bond between the nonirradiated LDPE film and acrylic adhesive is relatively low (the unit peel-off energy is as low as 80 J m–2). It rises almost linearly with the radiation dose. Upon the dose of 500 kGy, this strength increases by ca. 18 times. The strength of the adhesive bond between the nonirradiated BOPP film and acrylic adhesive is higher than that in the case of the LDPE film (the unit peeloff energy is 136 J m–2). The adhesion strength rises rapidly upon the electron radiation and reaches the value of 1935 J m–2 at the dose of 500 kGy (the increase by ca. 14 times). The strength of the adhesive bond between the nonirradiated PET film and acrylic adhesive is many times higher than that in the case of the LDPE and BOPP films (the unit peel-off energy is 1200 J m–2). It rises rapidly with the radiation dose and reaches the value above 2000 J m–2 at the dose of 25 kGy. The adhesion strength for the PET film irradiated with various doses could not be determined because of the failure of the double stick tape. Changes in surface layer properties, being one of the results of the radiational modification of polymer films, improve adhesion properties of the LDPE, BOPP, and PET films. Thus, these films, when irradiated, can successfully be bonded with the acrylic adhesive and application of other methods for
The study was supported by the State Committee for Scientific Research (KBN) with the Grant No. 7 T08E 052 20.
References [1] Kinloch AJ. J. Mater. Sci. 1980;15:2141. [2] Kaelble D. Physical chemistry of adhesion. New York: John Wiley; 1975. ’ [3] Zenkiewicz M. Adhesion and modification of surface layer of macromolecular materials. Warsaw: WNT; 2000. [4] Carley JF, Kitze PT. Polym Eng 1978;18:326. [5] Connell LD. Kunststoffe 1986;76:761. [6] Inagaki N. Plasma surface modification and plasma polymerisation. Lancaster: Technomic; 1996. ’ [7] Zenkiewicz M. Polimery 2000;45:81. ’ [8] Zenkiewicz M. J Adhesion Sci Technol 2001;15:63. [9] Singh A, Silverman J. Radiation processing of polymers. Munich: Hanser; 1992. [10] Singh A. Radiat Phys Chem 1999;56:375. [11] Korenev S. In: Proceedings of the 2001. Particle Accelerator Conference. Chicago, 2001 p. 2509. [12] L.auppi UV. EZCure: Ein Kosteng.unstiger Elektronentrocker. ESI–ESH Seminar, Bielefeld, 30 M.arz 2000. [13] Rangwalla I, Sanders R. Ink World 2000. April. p. 31. ’ ! [14] Zenkiewicz M, Rauchfleisz M, Czuprynska J. Polimery 2003;48:343. ’ ! [15] Zenkiewicz M, Rauchfleisz M, Czuprynska J. Radiat Phys Chem. 2003;68:779. ’ [16] Zenkiewicz M, Czupry!nska J. In: Conference Proceedings of International Conference ‘‘Plastko 2001’’. Zlin, 2001. p. 26. [17] ASTM D903. Standard Test Method for Peel or Stripping Strength of Adhesive Bonds. American Society for Testing and Materials, West Conshohocken, PA, 1999. ’ [18] Zenkiewicz M. Polimery 2003;48:57. ’ [19] Zenkiewicz M. Radiat Phys Chem, in print.