PASTEURIZABLE, WATER WHITENING RESISTANT EMULSION BASED PRESSURE SENSITIVE ADHESIVE

Transcription

1 PASTEURIZABLE, WATER WHITENING RESISTANT EMULSION BASED PRESSURE SENSITIVE ADHESIVE Tibor Pemecker, Research Specialist, Solutia Inc., Springfield, MA Jong-Shing Guo, Sr. Research Specialist, Solutia Inc., Springfield, MA Augustin T. Chen, Technical Director, Solutia Inc., Springfield, MA I. Introduction Emulsion based pressure sensitive adhesives offer a variety of advantages such as low manufacttldng cost, ease of application, no VOC over solvent based adhesives. Due to their complex, heterogeneous nature their performance very often depends on the properties and behavior of the additives they contain. Some of the additives like tackifiers, thickeners are blended in during formulation to enhance adhesive properties and improve coatability. Other additives like surfactants, buffer, initiator, etc., are used to support the polymerization process during synthesis and promote emulsion stability. Each additive serves its specific purpose but make a strong impact on the overall performance of the adhesive. SurfaCtants are very often called "the necessary evil". Their presence is essential for emulsion stability but have a detrimental effect on the dried adhesive film's long-term performance. During the initial stage of film formation surfactants are located on the particle surface and other ionic species are more or less homogeneously distributed in the film but they tend to segregate in a dried film [ 1, 2, 3]. Depending on their structure, compatibility with the polymer and concentration, the surfactants can either remain on the particle surface [4, 6] or migrate away from the particle toward the adhesive/air or adhesive/substrate interface [2,3,6]. At high concentrations water soluble species (buffer, initiator), and surfactants can form large scale structures such as channels, pockets within the polymer matrix [2, 5] (Scheme 1). The surfactant molecules are oriented with the hydrophilic head facing each other and creating a very hydrophilic environment inside these structures. Many applications where the water whitening resistance is important is the clear label and exterior applications. In this application the adhesive is exposed to high temperature water during the pasteurization cycle. Waterborne acrylic pressure sensitive adhesives were excluded from this market for many year dues to water whitening and rapid loss of adhesive properties under the pasteurization conditions. The objective of the present work is to gain some insight into the role of additives such as crosslinkers, coalescence aid, surfactants and hydrophilic monomer (acrylic acid) in the adhesive's water whitening performance as well as demonstrating the outstanding water whitening resistance of one of Solutia's new adhesive. II. Experimental II.1. Latex and Film Preparation All of the PSA latexes (see Table 1) were synthesized by semi-batch polymerization method. A 2 L jacketed reactor was equipped with a stirrer, condenser and a thermocouple. The reactor was charged with water and of potassium persulfate. Butyl acrylate (BA) and acrylic acid (AA) were premixed in the presence of surfactant in a container. The reactor was heated up to 80 C and the monomer preemulsion 273

2 was added over four hours. When the preemulsion addition was complete the temperature was maintained at 80 C for an additional 60 min. After 60 min the charge was cooled down to 30 C and the ph was increased to 7.00 by adding ammonium hydroxide. One mil (25kt) coatweight PSA films were cast on PET or BOPP (bi-oriented polypropylene) facestocks and dried at at 90 C for 5 minutes. After drying the drowdawns were covered with release liner. Heat aging was conducted at 70 C for 7 days. Texanol (Eastman) and aziridine CX-100 (Zeneca) were used as received. II.2. Water Whitening Test One by four inch strips were cut out from the dried PSA films and the release liner covers were removed. The strips were secured on a plastic panel with the adhesive exposed and immersed into 90 C water. After ten minutes the panel was removed from the water and the films were evaluated for water whitening by a visual rating system between 0 and 5. A level 0 water whitening means that the film stayed transparent and there was no evidence of discoloration of any kind (see Figure 1 A). Level 5- water whitening means that the film turned dull white (see Figure 1 B). A number between 0 and 5 can be used to characterize intermediate water whitening Transmission Electron Microscopy Films were cast onto Mylar at a thickness of approximately 3 mils and faced with a release liner. Prior to microtomy, the release liner was peeled from the laminate and two pieces of the Mylar-backed adhesive were gently pressed together to give a 6 mil adhesive layer. This was trimmed with a razor blade and cooled to -90C in a Reichert Ultracut E microtome equipped with an FC-4 cryochamber. The sections were cut with a dry diamond knife and transferred directly onto a microscope grid. The sections on the grid were stained in the vapors of a 1% osmium tetroxide/water solution or a 2% ruthenium/clorox solution for 5 minutes, depending on the sample composition. Microscopy was performed on a Philips CM-12S TEM at an accelerating voltage of 100 kv. III. Results and Discussion III.1 Model Systems When an adhesive film containing segregated surfactant domains is exposed to water rapid discoloration occurs which process called water whitening (Figure 1). It is believed that water molecules quickly penetrate into the adhesive film and accumulate inside these domains. The water filled pockets and channels swell and start to scatter light. Depending on the actual size of the domains the film's color changes from transparent to blue and white. Temperature has a profound effect on this process. In room temperature water an average commercial waterborne acrylic adhesive turns white within 24 hours while at 90 C water the white color appears after only 10 minutes. As a result of water whitening the adhesive film loses its tackiness. Model latexes 95/5 BA/AA were synthesized to study the effect of ph, inter particle crosslinking, polarity (acrylic acid content) and surfactant on water whitening. The polymer composition and formulation for each model system is listed in Table

3 L In Experiments 3-6 and 7 traditional and polymerizable surfactants were used respectively. One way to prevent surfactant migration and segregation is to use a surfactant, which will be part of the polymer chain i.e., a non-migrating or polymerizable surfactant. The use of polymerizable surfactants has been reported in earlier literatures [7]. The polymerizable surfactant should work well in conjunction with other traditional surfactants. The critical micelle concentration (cmc) should be controlled to provide micelles for nucleation during the early stage of polymerization. The surfactant's reactivity ratio should be sufficiently low to provide particle stabilization throughout the polymerization. Surfactants with inappropriate reactivity ratio tend to be buffed inside the growing particle which process can lead to coagulation later on during polymerization and/or insufficient latex stability during storage. High degree of incorporation is also essential to prevent surfactant migration. Just like "traditional" surfactants, unreacted surfactant molecules will tend to segregate in domains later in dried adhesive film. The adhesive film synthesized in Experiment 4. turns matte white after 10 minute in 90 C water. Using polymerizable surfactant in Experiment 7. or the absence of FES-77 in Experiment 1. and 2. combined with the absence of acrylic acid did not improve substantially either the water whitening performance. On the other hand inter-particle crosslinking of the base polymer (Experiment 5) with aziridine greatly improved the water whitening resistance. The results may be rationalized by the following considerations. In the absence of surfactant and acrylic acid (Exp. 1. and 2.) the film formation becomes difficult which results in incomplete coalescence and void formation in the adhesive film. During film formation the highly ionic species (buffer, unreacted initiator)graduauy precipitate from the water phase as the evaporation proceeds and they become trapped between the coalescing particles creating phase separated hydrophilic islands i.e., potential water absorbing reservoirs. In the presence of FES-77 or other traditional surfactant, these islands are presumable surrounded by the surfactant molecules. The presence of a coalescence aid, Texanol (Exp.6) did not prevent phase separation. In hot water these islands are exposed to the incoming water molecules and quickly swell by absorbing more and more water. The absence of surrounding surfactant molecules does not slow down water absorption as it is evidenced by level 4 water whitening in the presence of a polymerizable surfactant. At low ph (Exp. 3) most of the carboxyl groups are embedded inside the particles while upon neutralization (Exp. 4.) most of them located on or near the former particle surface in close contact with the precipitated salts. Due to the limited water solubility of the carboxyl groups they may temporary slow down water accumulation inside the salt/surfactant domains. The only real performance improvement was observed when the film was crosslinked with aziridine. Inter-particle crosslinking does not prevent water absorption but depending on the crosslinking density, it may slow down or even eliminate swelling and therefore light scattering by the swollen domains. III. 2. PSA Film Morphology Solutia has developed a novel water whitening resistant all acrylic PSA-GME As opposed to conventional acrylic PSAs this novel adhesive film can withstand 20 minutes with the unbound adhesive directly exposed to 90 C water. Figure 2. shows the GME-3194 film (left strip on both pictures) before and after hot water exposure. There is no evidence of any discoloration or whitening. 275

4 As we discussed earlier the primary reason behind water whitening is the presence of segregated surfactant, and salt domains, which provide hydrophilic environment for the incoming water to accumulate. Figures 3. shows the morphology of the film prepared by using formulation # 4 in Table 1. respectively. In this model system instead offes-77, Polystep A18S surfactant was used, which is a stainable linear sodium alpha olefin sulfonate. Since the only component in the film, which could be stained is the surfactant, the dark areas on both pictures mark the location of the surfactant used. According to the film formation mechanism described above, the surfactant molecules detach from the particle surface during film formation and form separate domains, which can be clearly seen on Figure 3 and 4. The size of these domains is about nm. The morphology of the model PSA film does not change after seven days aging at 70 C as it is shown on Figures 4.. In contrast with the model system, no segregated surfactant domains were observed in GME film. After seven days aging at 70 C the film's morphology remained unchanged indicating complete particle coalescence and stable morphology. III.3. Adhesive Performance The basic adhesive test results are listed in Table 2. The 20 minute and 24 hr peel results are very average on both stainless steel and glass substrate. What is remarkable about GME-3194 is the humidity resistance. After 7 days in high humidity environment most of the traditional emulsion based adhesives show no or very little adhesion. GME-3194 not only retains adhesion to both stainless steel and glass but a considerable peel build up occurs. In icy water, GME-3194 also retains good adhesion to glass on bi-oriented polypropylene facestock. This latter result is especially relevant to clear beer bottle application. IV. Conclusions Water whitening of waterbome pressure sensitive adhesives is a very complex phenomenon. To improve the water whitening performance one should prevent either the segregation of ionic species and surfactant during film formation or seriously limit the swelling of the hydrophilic domains by absorbed water. GME-3194 has a unique surfactant morphology compared to traditional pressure sensitive adhesive films. We believe that this morphology is responsible for the excellent water whitening resistance in both hot water and ice/water mixture. GME-3194's morphology is very stable and there is no evidence of surfactant segregation even after a prolonged aging time at 70 C. Acknowledgement The authors thank to Dr. David B. Alward for the excellent TEM work and Michelle Lapointe for her assistance in the experimental work. 276

5 Literature 1. A.D. Chesne, B. Gerharz, G. Lieser, Polymer Intemational, 43, (1997) 2. N. Agarwal, R.Farris, J. Appl. Polym. Sci., ~ (1999) 3. O. Vorobyova, M. A. Winnik, Macromolecules, 34, (2001) 4. A.C. Hellgren, P. Weissenbom, K. Holmberg, Prog. Org. Coatings, 35, (1999) 5. R.J. Roulstone, M. C. Wilkinson, J. Heam, Polymer International, 27, (1992) 6. K.W. Evanson, M. W. Urban, J. Appl. Polym. Sci., 42, (1991) 7. H.A. Schoonbrood, J. M. Asua, Macromolecules, 30, (1997) 277

6 ...! Adhesive/Water Interface Adhesive Film Water 1 5'3 "z. Ib,/ II tl ~ Hydrophilic Environment Adhesive/Substrate Interface Scheme 1. Surfactant segregation in adhesive film A B Figure 1. Water whitening of a typical emulsion pressure sensitive adhesive before (A) and after (B) 10 minutes in 90 C water. 278

7 Table 1. Polymerization and formulation conditions for the synthesis of the BA/AA model systems Reactor Charge DI-Water,g Buffer, g Potassium Persulfate, g Preemulsion DI-Water, g FES-77 (dry), g Polymerizable Surfactant (dry), g Butyl Acrylate, Acrylic Acid, g Formulation PH Aziridine (wet%) Texanol (wet%) Water Whitening (0-5) :-...:.-~+... ~ I -~,.:::. ~ii!i llii~ A B Figure 2. Water whitening performance of Solutia's GME-3194 (left film) versus a model system (fight film, see text for details) before (A) and after (B) 20 minutes in 90 C water. 279

8 Figure 3. TEM picture of the model system #4 in Table 1. using Polystep A18S surfactant.(60k magnification). See Experimental for details. Figure 4. TEM picture of the model system #4 after aging at 70 C for 7 days 280

9 Table 2. Performance of GME-3194 Substrate Stainless Steel Peel 20 min.(lb Peel 24 hr. (lb) 7 days@ 70 C 7 95% relative humidity at 35 C (20 min recovery) 7 95% relative humidity at 35 C (24 hr. recovery) Polyester BOPP Monomeric Vinyl Glass Peel 20 min (lb) Peel 24 hr. (lb) Lo_.qo_p Tack Ice/Water: 20 min (lb) No recovery Ice/Water: 24 hr. (lb) No recove._..~ 7 days@ 70 C ): 7 95% relative humidity at 35 C (20 min recovery) 7 95% relative humidity at 35 C (24 hr. recovery) /5 5 < 281