Bonding of Plastic Parts with Dispersion Adhesives Film Formation via Diffusion Processes

Transcription

1 Bonding of Plastic Parts with Dispersion Adhesives Film Formation via Diffusion Processes Elmar Moritzer, Matthias Hopp, Kunststofftechnik Paderborn, Paderborn University, Paderborn, Germany Abstract Bonding of plastic parts poses some challenges e.g. regarding wettability, adhesion or the choice of a suitable adhesive system. Often cost-intensive, chemically setting adhesives such as epoxy resin adhesives are used. In the furniture and paper industries, porous materials such as wood or paper are bonded with dispersion adhesives which are regarded as ecologically friendly and, due to the high water content, are cost-effective. In the context of combining wood and plastic, for example with WPC (Wood-Plastic Composites), hybrid joints of wood/wpc or wood/plastics are becoming more and more important for industrial applications. The investigations in this paper show a possibility to bond non-hygroscopic parts with dispersion adhesives. Through perforation of one bonding partner, the water in the wet adhesive can be removed from the adhesive layer. The process of film formation is analyzed considering diffusion processes, capillary forces and deformation of the particles. Introduction Dispersion adhesives are physically setting adhesives and are mainly used in the wood and furniture processing industry, where they are known as White Glue [1]. They contain polymer particles, ranging in size from 0.05 µm to 10 µm, which are finely dispersed in a liquid phase. Besides a few additives, e.g. for adjustment of the viscosity or for prevention of foaming, water is the main part of the liquid phase. Polymers used in these adhesives are polyvinyl acetate, copolymers from vinyl acetate and ethylene and polyurethane. In order to prevent the agglomeration of the polymer particles emulsifiers or protective colloids are used. Dispersion adhesives are regarded as particularly ecological because they contain hardly any solvents. An additional, significant advantage of this kind of adhesive is the low cost because of the high water content [1, 2]. The film formation is a complex process, which cannot yet be sufficiently explained in its various stages. The principal ones are as follows: Initially, the polymer particles are finely dispersed in the aqueous medium. In the first phase, the water evaporates from the freshly applied dispersion. After joining two parts, the water may also be absorbed by the substrate, depending on the latter s chemical composition. This results in the polymer particles moving closer together. As a result of the ensuing capillary pressure between the converging particles, the latter are deformed. In the following phase, the unification of the polymer particles, known as coalescence, takes place as the polymer particles are destabilized by bursting of their membranes. Lastly, the molecule chains form a transparent, mechanically stable film by interdiffusion with one another [3]. Many investigations in the last sixty years deal with film formation of polymer dispersions. Most of them study the film formation of a dispersion which is applied to a substrate while its surface is exposed to the ambient air. Different effects are discussed regarding the deformation of the particles and the drying: BROWN postulated in 1956 that film formation takes place when the attracting forces like capillary forces or van-der-waals forces are greater than the repulsive forces between the particles [4]. DILLON stated that the drying and the deformation of the particles are independent effects and that the deformation takes place due to the surface tension polymer/air [5]. Recent publications about film formation of colloidal films show under which conditions the different physical effects occur. Describing equations of the particle fraction over time in a film and the prediction of the deformation mechanisms of the particles depending on dimensionless parameters are some of the important scientific findings [6-9]. Bonding with dispersion adhesives requires the removal of the dispersion medium (water) from the joining plane. Principally there are two different routes for the water. Firstly, the water transport out of the adhesive into the substrate is characterized by diffusion processes, which depend on specific parameters of the material, temperature and time. Additionally, this process is hindered by the film formation of the polymer particles which takes place during absorption of the water, because a further barrier for the mass transport occurs. Another possibility for the water transport out of the dispersion is evaporation at the edges of the bonding area into the ambient air. The evaporation process takes place at the interface dispersion/ambient air, which depends among other things on the partial pressure of water in the air and the saturation vapor pressure [3]. The evaporation of the water at the edge of the bonding area results in a concentration gradient within the adhesive. Diffusion processes lead to a mass flow in the wet adhesive layer towards the edges. Since plastics absorb no or only little water in contrast to wood or paper, curing of the adhesive when using a large glued surface is not possible. Only at the places where the dispersion has contact with the ambient air does the evaporation process, described above, take place. The SPE ANTEC Anaheim 2017 / 1671

2 resulting diffusion flow of the water within the adhesive layer is superimposed by the film formation and, after a short time, nearly comes to a halt. The barrier effect of the dried film at the edge inhibits a sufficiently short setting time. The investigations carried out in this paper show a possibility of how film formation can still take place when bonding non-hygroscopic bonding partners. Materials and Methods Investigations were conducted to analyze the film formation of polymer dispersions for bonding nonhygroscopic bonding partners. The concept was to drill holes through one of the parts to be bonded so that the dispersion medium can travel out of the wet adhesive layer into the ambient air. Within these holes, as described above, the initial film forms (see Fig. 1). The resulting difference in water concentration between the layer of already-set adhesive and still-wet adhesive in contact during stages two, three and four triggers diffusion processes in the interior of the adhesive layer (stage 5). 1) 2) 3) 4) Wet Adhesive Evaporation of Water Deformation of Polymer Particles Formation of Transparent Film 5) Diffusion within Adhesive Layer Bonding Partner A Bonding Partner B Figure 1. Model of Film Formation in the Joining Plane of perforated Bonding Partners To analyze these diffusion processes, experimental investigations were conducted with an experimental set-up described as follows. Holes are drilled through a polypropylene plate, forming an arrangement of equilateral triangles. The plates are then bonded to a glass plate with the polymer dispersion. To set a defined thickness of 50 µm of the adhesive layer, a feeler gauge stock is placed between the two bonding partners. During formation of the film, the specimen is held together with clamps. Glass Plate Adhesive Layer Figure 2. Experimental Set-Up PP Plate with drilled Holes Feeler Gauge Stock The film formation can be observed over time by an optical analysis through the glass plate. Because of the phase boundaries between the polymer particles and water, and the resulting refraction of light waves, polymer dispersions often appear white while they are still wet, and form a transparent film after drying. This allows the time of curing to be determined through the glass. The transparency of the film corresponds to the elimination of most of the inter-particle voids. The requirement for the close packing of the particles is the removal of the water from the film. [3] Photos are taken at intervals of thirty minutes to document the progress of film formation. The brightness of the ambient light is kept constant over the whole period of time. A threshold value correction of the photos allows the degree of drying to be evaluated. Furthermore diffusion parameters can be deduced from the distances of the transparent line to the edge of the holes. As mentioned above, the holes are arranged in an equilateral triangle. The water in the dispersion around the holes diffuses uniformly to the edge of the holes. Thus the circular ring around one hole (shown in Fig. 3) describes the area through which water has to diffuse to the hole. In other words, the water in volume V diffuses through the outlet area A. With a larger distance between the holes and a constant hole diameter, more water has to diffuse to one hole. Conversely, less water has to diffuse to a larger outlet area when using a larger hole diameter. In terms of the arrangement of the holes, a length ratio δ is defined, which is given by the ratio of the lengths c to b (see Fig. 3). At a constant δ, the volume of the dispersion per outlet area formed by the circumference of the hole and the thickness of the adhesive layer is constant - independently of the hole diameter, which enables an extended evaluation as described in the discussion section. SPE ANTEC Anaheim 2017 / 1672

3 PP Dispersion c Figure 3. Geometrical Arrangement of the Holes b The specimens are prepared according to the set-up described above. During film formation, they are placed into a closed box under standard climate (23 C, 296 K, 50 % relative humidity). For the investigations shown here, the hole diameters 3 and 4 mm are used and the length ratio δ is varied from 0.5 to The thickness of the applied dispersion is 100 µm and the gap between the PP plate and the glass plate is set at 50 µm. This ensures a sufficient amount of dispersion between the two parts, even after removal of the water that amounts up to 50 % of the dispersion. Two polymer dispersions are used which are based on the copolymer of vinyl acetate and ethylene. The main difference between the two dispersions is the particle size distribution. The first dispersion, called Dispersion 1 below, contains particles ranging in size from 300 nm to 2,000 nm, Dispersion 2 contains smaller particles from 50 nm to 400 nm in diameter. The difference in size results in different properties of mass transport. With smaller particles, smaller gaps occur when the spheres are closely packed. The capillary pressure is higher and the water molecules can travel faster through the packing. Both dispersions have a water content of about 45 percent by mass. Results and Discussion As seen in Figure 1 the whole process can be divided into two general steps. Firstly, the film is formed in the holes through evaporation into the ambient air. Secondly, the film formation takes place around the holes by diffusion of the water molecules into the holes. A Drill Hole A Glass plate A: Outlet Area V: Volume δ: Length Ratio V V The photos in Figure 4 show the same specimen at different points in time. The first step mentioned above (film formation in the holes) is completed within the first thirty minutes. The dispersion becomes transparent, which indicates that the polymer particles are closely packed. The voids between the particles are smaller than the wavelength of visible light. Complete interdiffusion of the polymer chains has not yet taken place. This conclusion can be drawn from the relatively fast diffusion of the water molecules through this closely packed layer. The diffusion through the polymer itself would take much longer. However, if there were no attracting forces between the particles after the removal of water, cracks would be seen in the dry film. ZOSEL AND LEY describe this with different steps of interdiffusion for un-cross-linked dispersions. There is a fast interdiffusion of the chain ends of the polymer, but the final mechanical strength is developed slowly by complete interdiffusion and entanglement of the molecules [10]. 0 h 0.5 h 5 h 10 h Figure 4. Photos of a Specimen over Time Through the water loss in the holes, a concentration gradient occurs within the dispersion which leads to diffusion processes. Through the diffusion of the water towards the holes, the particles become closely packed and optical transparency is the result. The transparent line is seen around the holes and travels further in a radial direction away from the edge of the hole. After ten hours, the adhesive between the holes appears darker, which indicates that the transparency is higher and the black sample can be seen. Consequently, the inter-particle voids at which the light is refracted are reduced in the dispersion. Because the thickness of the gap between the two samples is constant and the volume of the dispersion decreases, some kind of shrinkage cavities occur and they grow over time. This could distort the threshold value SPE ANTEC Anaheim 2017 / 1673

4 r/c r/c correction concerning the evaluation of the setting progress. For the evaluation, fifteen circular rings are cut out of the photos for each testing point. With a threshold value correction, a level of setting θ is defined as: θ = A transp. A 0 (1) where A transp. is the area of already set dispersion and A 0 is the whole area of the circular ring. Figure 6 shows the results of the investigations for the two dispersions and hole diameters of 3 and 4 mm. The y- axis describes the distance between the edge of the holes and the transparent line, seen in the photos, in percent (c = r(t n)) , ,8 Dispersion 1 As seen in Figure 4, the transparent line progresses from the edge of the hole in a radial direction away from it , ,4 Hole Diameter 3 mm 4 mm h Hole Distance s HD Radius r H δ = ,2 c b δ = 0.75 r(t) δ = , Time / h Dispersion 2 t n t 3 t 2 t , ,8 Figure 5. Geometric Quantities for Description of the Setting Progress Equation (1) can thus be supplemented for t 0 < t < t n by the area equations according to the geometric quantities shown in Figure 5, namely r H as the hole diameter, s HD as the hole distance and r(t) as the control variable beginning at the edge of the hole: to: r (t) θ(t) = π ((r(t)+r H )2 r 2 H ) π (( 1 2 s HD) 2 r 2 H ) Solving the equation to the control variable r(t) leads r(t) = θ(t) (( 1 2 s HD) 2 r 2 H ) + r 2 H r H (3) (2) 0.6 0, , ,2 δ = 0.5 δ = 0.75 δ = 1.00 r(t) 0.0 0, Time / h Figure 6. Progress of the Transparent Area over Time in percent for Dispersion 1 and Dispersion 2 Generally, the shape of the curves corresponds to a saturation function. At the beginning, the progress of the transparent line is delayed. The reason for this could be that at first the film has to be formed in the holes and the evaluation does not begin in the hole but at the edge of the hole. The reduction of the values in the curve for δ = 0.5 of Dispersion 1 is attributable to growing shrinkage cavities in the dispersion. c b Hole Diameter 3 mm 4 mm SPE ANTEC Anaheim 2017 / 1674

5 r/ mm r/ mm At a greater length ratio δ, the percentage progress of the transparent area is slower. This is understandable because more water has to diffuse to the holes. Only at small length ratios δ is the curve almost linear in the middle part, which indicates that there is no growing barrier effect of the particles moving closer together over time. The other curves show a degressive shape almost from the beginning. For a hole diameter of 4 mm, it takes longer for a transparent film to form when using the same length ratio δ. With higher values of the length ratio, this difference in time increases. The reason for this is a longer diffusion path of the water to the edge of the hole. With a larger diameter the length b (see Fig. 3), which is a sixth of the circumference, is larger, too. Using the same length ratio the length c is also greater. Compared to Dispersion 1, the diffusion within Dispersion 2 is about two times faster. Dispersion 2 has a narrow particle size distribution with small particles. This results in narrower capillaries which form during packing of the particles. With smaller particles, the size of the interparticle voids will be reduced much faster, so that optical clarity is reached earlier, as investigated in detail by VAN TENT AND TE NIJENHUIS [11]. Another reason could be a difference in the mechanical properties of the polymers in the dispersion, which influence the packing by deformation, too. Another scaling of the results is shown in Figure 7 for a hole diameter of 3 mm. For all length ratios the progress of the transparent line is described by the variable r(t). The results show that the progress of the closely packed, transparent area is different with higher length ratio δ. This indicates that not only the water molecules are traveling to the holes but that further flows in the dispersion occur: Through the water loss in the holes a horizontal flow is established and the dispersed particles are carried with the water to the edge of the hole by convection. A contrary effect is the concentration compensation of the particles in the wet dispersion that leads to a horizontal flow in the opposite direction. That is why the close packing of the particles takes longer when the volume of the dispersion is larger. In this case the water diffusion as a function of time is the same as for smaller volumes at the beginning when there no close packing has yet taken place but the concentration compensation of the particles takes longer as there is a larger volume around the hole. This means that close packing occurs earlier when the distance between two holes is smaller. Similar processes have been studied by CIAMPI et al. and DEEGAN et al. who describe the lateral transport of water in emulsion droplets through evaporation and the arrangement of the particles within the droplet [12, 13]. ROUTH and RUSSEL show a modulation of the height profile of a film where evaporation is hindered by a cover with holes [14]. In their experiments, however, the cover has no contact with the film. HARRIS et al. observe the particle migration in dispersions that are covered by a mask with periodic holes, and call it evaporative lithography [15]. They also describe the water and particle flux to regions of higher evaporation below the unmasked film to replace the loss of water. 2, , , , , , , ,6 0.6 Dispersion 1 0,4 0.4 δ=0.75 c b 0,2 0.2 δ=1 r(t) δ=1.25 0, , , , , , , , , ,4 0.4 Time/ h Dispersion 2 δ=0.5 δ=0.5 δ=0.75 δ=1 0,2 0.2 r(t) δ=1.25 0, Time/ h Figure 7. Absolute Progress of the Transparent Area over Time for Dispersion 1 and Dispersion 2 and a Hole Diameter of 3 mm The previously stated results show the adhesive becoming transparent around the holes, which implies a c b SPE ANTEC Anaheim 2017 / 1675

6 close packing of the particles. For this, removal of the water is necessary. The aim of this study is to show that bonding of plastic parts is possible with dispersion adhesives when drilling holes in one of the samples. The removal of the water is the first important step to be analyzed. The next step is to study the progress of the cohesion strength over time during drying of the film because close packing of the particles does not directly relate to coalescence of the particles. The experimental set-up for the investigation of the bond strength is seen in Figure 8. A plate out of polypropylene with seven drilled holes arranged as described before is bonded to a glass plate with the dispersion adhesive. The size of the adhesive area is 1,800 mm² (40 x 45 mm) and the thickness of the wet adhesive layer is 100 µm. As in the previously described experiments, the gap between the two samples is set at 50 µm by a feeler gauge stock. The shear strength of this bond is tested using a special device. The glass plate is fixed and the PP plate is pulled away from it using a standard testing machine. While fixing the sample into the machine, the bond is protected by clamps. The test speed is 5 mm/min. Previously, adhesion tests were carried out so that a principle material compatibility is guaranteed and the cohesion can be analyzed. The aim of these tests is to study the progress of the bond strength over time. It is necessary to use a large adhesive area because the influence of the drilled holes has to be analyzed. As seen from the results, the adhesive becomes transparent first in the holes and then around the holes. The same happens at the edge of the sample. The adhesive is in contact with the ambient air and so evaporation can take place as seen in the holes. That is why reference samples are taken for all points in time without any holes. The bond strength of the samples with the hole pattern is then reduced by the average of the bond strength tested with the reference samples. Before testing, photos are taken of all samples to document the progress of transparency and evaluate the degree of drying with a threshold value correction, as for the investigations shown above. 45 mm Adhesive Layer Glass Plate 40 mm PP Plate with drilled Holes Force F Figure 8. Experimental Set-Up for the Investigation of the Bond Strength For this study, the distance between the holes is 8 mm and the hole diameters are 3 and 5 mm. For the 3 mm hole pattern, the length ratio δ is 0.8 mm and for the 5 mm pattern, it is 0.29 mm. In Figure 9, two photos of bonds are seen with a 5 mm hole pattern compared to the reference. At the edge of the specimens, the dispersion becomes transparent but without holes in the sample no film formation takes place in the middle of the adhesive layer. Around the central hole in the left-hand photo, the film is completely transparent (except for the shrinkage cavities also seen in Fig. 3). Around the hole pattern, there is still wet adhesive which indicates that, for bonding large surface areas, the distance between the drilled holes has to be small enough. The photos show that the use of the holes is necessary so that the film can be formed in the middle of the bond. Figure 9. Photos of Specimens with a 5 mm Hole Pattern compared to a Reference Specimen after 72 h The consequence is an increase of the transmitted force when conducting shear strength tests. Table 1 shows the average increase over time compared to the reference specimens for the two hole patterns and the two dispersions. With Dispersion 1, the average force is five times higher than the reference. The cured adhesive area is larger so that higher forces can be transmitted. Table 1. Average Increase of the Force F compared to the Reference Sample without any Holes 3 mm Hole Pattern 5 mm Hole Pattern Dispersion % 525 % Dispersion % 249 % Figure 10 shows the results of the two kinds of evaluation: the bond strength over time reduced by the reference in the upper diagram, and below the transparency over time for the same specimen. With the used geometric SPE ANTEC Anaheim 2017 / 1676

7 r/c Bond Strength σ / MPa parameters, the progress of the transparent area is in the same range. The length ratio of the 5 mm hole pattern is small so that rapid removal of the water is possible. Although there are only small differences in the progress of the adhesives transparency, the bond strength differs a lot. Bonding the samples with Dispersion 1 leads to higher bond strength than using Dispersion 2. However the saturation point for the bond strength is reached earlier with Dispersion 2. In the lower diagram it is seen that the packing of the particles of the specimens with a 5 mm hole pattern is faster than that of the specimens with the 3 mm holes. Using the same distance between the holes leads to smaller diffusion path length by an increase in the hole diameter. Consequently, the water is removed much faster from the adhesive layer to the holes. With a larger hole diameter, the absolute evaporation rate is higher because of the larger evaporation area. That is why there is a higher increase of bond strength with the 5 mm holes in the sample , , , , , , , , , , ,2 0.0 Time / h 0, Time / h Dispersion 1 Dispersion 2 3 mm; δ = 0,80 5 mm; δ = 0,29 Figure 10. Progress of the Mechanical Strength over Time in MPa in Comparison to the Progress of the Transparent Area over Time in percent for Dispersion 1 and Dispersion 2 It can be also seen that, with the parameters used here and the two dispersions, the progress of transparency runs parallel to the progress of the cohesion strength of the adhesive. This means that attracting forces between the particles occur when the particles are in contact with each other. As described by ZOSEL AND LEY, there could be a fast interdiffusion of the polymer chain ends [10]. In order to classify the values for the bond strength shown in Figure 10, it can be said that the maximum shear strength of bonds made with dispersion adhesives is generally lower (5 10 MPa) than that of bonds using e.g. epoxy resin adhesives (20 40 MPa) [2]. Here the influence of the hole pattern on the progress of bond strength is studied independently of the specimens edge effects. As seen in Figure 9, the adhesive is still wet around the holes after 72 h. That is why the strength values are so low. If there were more holes drilled into the PP plate, the strength would be higher. Furthermore, polypropylene is a non-polar material and an activation of the surface by e.g. plasma should be applied to improve the properties for bonding. For this study, no plasma treatment is carried out because changes in the surface morphology could also have an influence on the film formation. Conclusions The investigations show that bonding of nonhygroscopic plastic parts is possible by perforation of one bonding partner. Diffusion processes and the packing of the particles determine the film formation of the dispersion in the adhesive layer. With smaller distances between two holes, the setting of the adhesive becomes faster. A larger hole diameter also has a positive effect on the drying time. The transparency of the adhesives used in this study is related to the cohesion strength. However, for an analysis of the degree of interdiffusion of the polymer chains, other special techniques have to be used like Small Angle Neutron Scattering (SANS) or Fluorescence Resonance Energy Transfer (FRET). Both methods are based on a labeling of a fraction of the particles e.g. with dye to detect coalescence of the particles. The techniques used in this study are sufficient to describe the main effects. They help potential users to bond non-hygroscopic parts with dispersion adhesives, for example for hybrid joints of wood and plastics. References 1. W. Brockmann, P.L. Geiß, J. Klingen, B. Schröder: Adhesive Bonding Materials, Applications and Technology. WILEY-VCH GmbH & Co. KGaA, Weinheim, S. Ebnesajjad: Handbook of Adhesives and Surface Preparation /Technology, Applications and Manufacturing. Elsevier Inc., Burlington, USA, C.I. Chung and R.A. Barr, U.S. Patent 4,405,239 (1983). 3. J.L. Keddie, A.F. Routh: Fundamentals of Latex Film Formation Processes and Properties. Canopus Academic Publishing Limited, Bristol, 2010 SPE ANTEC Anaheim 2017 / 1677

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