William LaCourse, as well as the other excellent faculty and staff at Alfred University that I have encountered, including my talented professors Drs. Finally, I must especially thank my wife, daughter and extended family for enduring this challenge with me as a go-between. - career student. Between the stress of juggling classes, working full-time and trying to carve out moments for family, as well as the unexpected and multiple effects of COVID-19 on all of our lives, there were many moments where I was not my “best self”.
It was also shown that although the direct-bonded samples had even lower peel strength after initial separation, the bond strength was higher than that of strong tapes and even epoxy prior to edge cracking, indicating the applicability of direct bonding in optical materials and the potential for future development.
INTRODUCTION
Very often a strong or "permanent" bond is required to prevent delamination and a polymeric coating is applied directly to the glass via liquid or vapor phase processes and usually uses a chemical "adhesion promoter" such as an organosilane6. This can also be achieved by using a polymer (or glass) with no visible adhesive, such as when a screen protector is placed on a screen or fastened over a glass dish. Applications also exist where the risk of residue or contamination may limit the use of liquid-phase adhesives, such as packaging films used in shipping windows.
Numerous potential uses include flexible screens for cell phones, tablets, and portable electronic devices, protective lightweight coatings for solar panels, and durable skins for home and office products, such as whiteboards, countertops, and more.
B ACKGROUND ON A DHESION S CIENCE
- Electrostatic
- Mechanical Interlocking
- Chemical Bonding and Diffusion
- Physical Adsorption (Surface Energy)
- EXPERIMENTAL
This leaves us with the consideration of adhesion caused by surface energy such as physical adsorption. The role of surface energy is important in the function of adhesive tapes and films, in that it determines the wettability of a liquid phase adhesive on the glass surface. If the droplet spreads easily, it lowers the surface energy through strong interactions with the substrate surface.
While this may seem obvious and something most scientists “know,” there is actually considerable debate in adhesion science about the true impact of surface energy.
M ATERIALS AND M ETHODS
- Fusion Draw –
- Sheet Redraw –
- Folded Glass Samples –
- Contact Angle –
- Wedge Test –
- Peel Tests –
The viscous glass sheet forms a bead that sags slightly faster as the sheet is driven through the furnace until it is touched by motorized wheels near the edge of the sheet, similar to the fusion process, and the glass is stretched to the desired thickness. One caveat worth noting is that all of the glass samples were harvested from the inside of the traced sheet by diamond scribing and cutting, that's for sure. This was necessary due to the extremely strong wetting of water droplets on all of them.
Schematic representation of Young's equation” by Leon, Boyts and Pustoval is licensed under CC BY 3.0.
E XPERIMENTAL P ROCEDURES
Sample Preparation –
As testing of pellicle samples at 50 - 150 micron thickness tended to have breakage and other problems when 90 and 180 degree tests were attempted, a series of tape and film tests were devised to compare their adhesion strength on differently treated glass surfaces . The tapes and films were selected for applicability on glass and to cover a range of adhesive strengths. The films used were purchased from www.GrafixArts.com, including a PVC film with no noticeable adhesive layer, commonly used for window decals and adhered by static adhesion, and Frisket polypropylene film with an "ultra-low tack" acrylic adhesive layer designed to leave no residue and is used for masking and stenciling on glass.
Both tapes used were manufactured by 3M Corporation, the blue tape used was 3M product 8991, which is a polyester tape with a residue-free silicone adhesive useful for protecting glass displays during shipping and storage, and the red tape used was 3M 850, polyester. tape with strong acrylic adhesive, suitable for splicing, sealing and packing.
Contact angle –
T-Peel Tests –
90o peel tests were performed by adhering a film or tape covered glass sample to a 3" x 12" steel plate placed in a holder set at 45o to the direction of pull and clamped on the gauge the cell was set perpendicular to the mounting plate so that the plate moved away, maintaining a constant peeling angle of 90o (Figure 8). The 180o peel test was performed similarly, but the steel clamping plate was held directly on top of the moving plate, and the clamp was moved to be slightly above the plane of the sample to pull the tape or film without dragging across the surface of the sample to the effects of friction would be avoided. Peeling conditions such as speed and travel time were set using on-board controls, as well as the time delay in data acquisition to avoid unstable results at start-up due to elastic/plastic deformation.
Speeds of 3, 6, and 12 in./min were selected as variables, with 6 seconds of total travel time for each, and the first second of data was ignored.
Surface Treatments –
In each case, the film or tape was applied at the earliest possible time (which varied slightly from about 10 to 30 seconds between treatments) and tested about 20 hours later to allow "rest time" for the adhesive to set.
RESULTS AND DISCUSSION
- SFE by Contact Angle –
- SFE by Wedge Test –
- Pellicle T-Peel –
- Film / Tape Tests –
- Film / Tape Tests –
- Elastic / Plastic Effects –
The T-Peel test only applies to very flexible materials, and thicknesses of 50 microns and above seemed to be too stiff to work properly and read artificially high due to the resistance to bending. However, 35-micron samples were extremely ductile and closely matched the peel strength predicted by the wedge test. It was known from literature that the glass-glass direct bond can be in the order of the material's own cohesive strength, yet the pellicle samples peeled apart with very little force.
To examine this, a series of pellicle samples was made by attaching tabs with increasingly stronger adhesives to opposite sides at the very end of the sample, but without initiating separation as was done with the previous samples. This subject is discussed by Abbott29, Lacombe30 and others as "viscoelastic effects", which include plastic and elastic deformation of the strip or film during pulling, as well as stretching and. To distinguish conditional variation from a mere proportional increase in standard deviation, this was normalized as a “coefficient of variance” (CoV) equal to the standard deviation divided by the mean of each data set after it was trimmed to exclude plasticity and elasticity deformation effects seen in the first 1.5 seconds of the peel test.
Finally, it should be noted that the peel strength as measured by the test instrument is many times greater than what experts in the field might consider “true adhesion”31. In short, this is entirely due to elastic-plastic effects on the tape and adhesive during the peel test. Such distortions account for most of the work done by the machine and can make the results very misleading.
This function is quite difficult to calculate from modeling or measured values and somewhat beyond the capabilities of the equipment used here. It is very interesting given the very wide range of input p that the actual work of adhesion, γ', which should equal the peel strength for a 90o peel test, is consistently around 2-2.5 N/m and reminiscent of the comparable small range. of Surface Free Energies obtained from Contact Angle measurements.
SUMMARY AND CONCLUSIONS
This seems to support the argument that all true adhesion is at a comparably low level and that the apparently high experimental values arise from energy absorption by deformation of adhesive and adhesive materials. Over time and with exposure to the atmosphere, surface quality is bound to deteriorate due to the adsorption of water, CO2, hydrocarbons, particulate and fibrous debris and a number of other contaminants that hinder the molecular-scale contact necessary for strong direct interaction. binding. The hypothesis was empirically demonstrated by analyzing freshly prepared samples as immediately as possible from the moment they emerged from the redraw process and then comparing them with aged samples from long-term storage.
Interestingly, the SFE did not decrease in any measurable way over short time frames when calculated from the contact angle using the OWRK method and was only slightly lower for the multi-year samples directly from storage. This was in stark contrast to the inability to produce directly bonded bottle samples from any but the freshest pieces of glass, even when aged for only a few minutes, suggesting that SFE is not telling the whole story. Abbott and many others that the high SFE accompanying very clean surfaces may be necessary to support adhesion initially, either to improve the wettability of the adhesive layer or simply to allow true surface contact at the molecular level.
A perfectly clean, dry, smooth surface lacks contaminant molecules that would physically disrupt "van der Waals" or hydrogen bonding by limiting contact at sub-nanometer (0.2 - 0.5 nm) distances and lower SFE by binding to exposed surface molecules. It can also be seen that the information gathered by various peel tests may not in themselves be useful in determining the true adhesion of a system, as vastly different values can arise from very similar SFE and Work of Adhesion results. However, with a large enough data set and carefully controlled experiments, such studies can be practical for ranking the overall effectiveness of different surface treatments within a given system.
Finally, it was shown that the direct glass-to-glass bond obtained by the immediate formation of pellicle samples with newly formed surfaces was extremely poor by normal peel testing with values even lower than adhesive film or adhesive notes, but also extremely strong before crack initiation. such that even hard, super-adhesive tapes and. This explains the utility of similar techniques in ultra-thin Si wafer finishing, where direct bonding to a rigid substrate resists strong mechanical forces but peels off without damaging the wafer and leaving no residue.
APPENDIX
