The document discusses the development of a novel glass adhesive. Previous work achieved strong bonding using a surface additive with both PMMA and silane components. The current work aims to further reduce the polymerization time and increase the adhesive strength. Testing different initiators and cross-linking agents showed that diurethane dimethacrylate dramatically reduces curing time but impacts optical clarity. Future work will focus on finding alternative rate accelerators and optimizing the formulation stability and bonding over time.

1. Polyshell Update – December 18, 2009<br />Purpose:<br />The current purpose of this project is to develop a novel glass adhesive. This work is based of previous work which examined the use of a surface active additive (surfadditive) which was developed in-house. <br />This surfadditive contains both poly(methyl methacrylate) and poly (triethoxy silane) components, allowing it to bond readily to glass while remaining entangled in a PMMA bulk material. This has been previously shown to allow for extremely strong bonding. <br />Previous Work:<br />As mentioned above, previous work in this field by Joe Zhang has shown that this surfadditive allows for the fabrication of a very strong PMMA based glass adhesive. Tensile strengths of over 10MPa have been achieved. These are considerably stronger than those of a comparable product currently on the market (Locktite 349), which has a tensile strength of 5.4MPa. <br />One of the shortfalls of our system, however, is that AIBN was used as the initiator. This system requires up to 4 hours of baking to achieve necessary monomer conversion. In an effort to reduce this time, a UV initiator was substituted in for the AIBN. This allowed for a drastic reduction in time (to about 7 minutes). <br />Although this substitution in initiator allowed for a great reduction in polymerization time, a greater reduction in time was still required. As a benchmark, Locktite 349 (mentioned above) polymerizes in 20 – 60 seconds, depending on adhesive thickness. Furthermore, there was no data indicating whether there was any effect on adhesive strength. <br />Current Work:<br />Training<br />This project was taken over by myself in September of 2009. During the first month my time was spent working with Joe Zhang to develop a strong understanding of the system that had been developed. This included learning the various polymer processing techniques involved in this study (ie. Cast molding, injection molding), as well as learning methods for synthesizing the surfadditive used in this project. This training in polymer synthesis was also helped by Helen Gu, who provided support after Joe had returned to China. <br />Training was also provided by Elizabeth Takacs, who demonstrated the use of the Instron machine for tensile strength testing. Unfortunately due to conflicting scheduling, this training took longer than anticipated. <br />Polymerization Rate Increase<br />After this training was complete, an initial study on the polymerization process was carried out to determine how the rate may be increased. <br />Initiator concentration was first investigated. Here, concentrations of the UV initiator ranging from 1% to 9% were investigated. It was found that at 1% the mixture would polymerize after about 7 minutes of exposure to UV radiation. At 3%, this time decreased to about 6 minutes. However, beyond 3% there was no discernable improvement in polymerization time. Thus, it was noted that additional methods would be required to improve polymerization time.<br />Through a previous study by Joe Zhang, it had been shown that the inclusion of diurethane dimethacrylate in the adhesive acted to significantly increase the polymerization rate. To examine this in greater detail, adhesive solutions of increasing concentration of diurethane dimethacrylate were prepared, and the polymerization times were measured. Table 1 shows these results.<br />Table 1: Effect of diurethane dimethacrylate concentration on polymerization timeDiurethane dimethacrylate ConcenrationPolymerization Time0%7 minutes20%3.5 minutes40%1.5 minutes60%20 seconds<br />As is evident from the table, the addition of diurethane dimethacrylate to the adhesive dramatically reduces the polymerization time. However, it was feared that when this solution was used for thicker applications (ie. Lamellar glass), the urethane in the adhesive would detract from the optical properties. <br />Initially it was thought that this increase in polymerization rate was due to the amine groups in the urethane acting to accelerate the polymerization. However, after a discussion with Dr. Shiping Zhu regarding this, it was speculated that this could be caused by acceleration due to cross linking (and perhaps the gel effect). To test this hypothesis, diurethane dimethacrylate was substituted with ethylene glycol dimethacrylate. These results are shown in Table 2.<br />Table 2: Effect of ethylene glycol dimethacrylate concentration on polymerization timeEthylene glycol dimethacrylate ConcenrationPolymerization Time0%7 minutes20%3.5 minutes40%1.5 minutes60%45 seconds<br />As can be seen in the above table, the polymerization rates are nearly identical to those seen with diurethane dimethacrylate. Thus it can be concluded that the acceleration seen is in fact due to the cross linking. However, when ethylene glycol dimethacrylate was used, the resulting film was white and opaque – hardly useful for a glass adhesive. This may be because of the short chain length of ethylene glycol dimethacrylate causing a drastic decrease in the crystallinity of the PMMA. To attempt to combat this, it was thought that a longer dimethacrylate-type of species should be examined (ie. Hexanediol dimethacrylate, poly(ethylene glycol) dimethacrylate). <br />Unfortunately, when hexanediol dimethacrylate (HDMA) was used, the results were very poor. HDMA is immicible in MMA. Thus when a recipe was formulated and the resulting films polymerized, the result would be a film composed of two phases – one opaque, white and rubbery (HDMA), and the other clear (PMMA). Clearly this is unacceptable for this application.<br />Given the above information, one can conclude that diurethane dimethacrylate is an ideal rate accelerator when only then films are required (such as the adhesive application). However, as film thickness increases, the optical properties quickly deteriorate. Thus, if fast polymerization is required for applications such as lamellar glass, it is necessary to find an alternative rate accelerator which does not have such negative effects on the optical properties. <br />(Note that other rate accelerators were tried and shown to have no positive effect. These were padimate O (2-ethylhexyl 4-dimethylaminobenzoate) and acrylate.)<br />Adhesive Strength<br />Having now developed an adhesive formula which is competitive with commercially available adhesives (ie. Locktite 349), it was then necessary to test the tensile strength of these adhesives.<br />The main goals of this portion of this study were to:<br />Examine the effect of diurethane concentration on the strength of our adhesive.<br />Examine the effect of initiator type of the strength of our adhesive<br />Examine the influence of temperature on the strength of our adhesive<br />Formulations of the adhesive were prepared as shown in Table 3. These formulations used either AIBN or a UV Initiator. For each of these initiators, various concentrations of diurethane dimethacrylate were used. <br />Table 3: Formulation of various additivesRecipeInitiator TypeInitiator ConcentrationAdditive ConcentrationPMMAConcentrationdUdM Concentration1AIBN0.08%3%96.92%0%2AIBN0.08%3%72.69%24.23%3AIBN0.08%3%48.46%48.46%4UV3%3%94%0%5UV3%3%70.5%23.5%6UV3%3%47%47%<br />After formulating the additives, samples were prepared in the following manner:<br />Glass samples were used as substrates. These samples were cut from glass microscope slides, and had dimensions of 1 cm x 2.5 cm.<br />Each piece of glass was soaked in ethanol for 5 minutes, then air dried.<br />One drop of adhesive was placed at the end of a piece of glass<br />A second piece of glass was then placed over the first, such that the ends overlapped by 1 cm<br />This was pressed together, and let to sit for 5 minutes<br />The sample was then placed under UV radiation for 7 minutes to ensure complete conversion<br />After this, the locktite 349 was used to glue metal handles to each side of the sample – these were necessary for measuring tensile strength<br />A schematic of the above process can be seen in Figure 1.<br />Figure 1: Schematic illustrating the testing apparatus for the adhesive<br />For each recipe which used AIBN, four samples were created. For each recipe which used the UV initiator, eight samples were created. Four of these were simply UV cured. The other four were UV cured, then heated in an oven at 120oC for 1 hour. This was done to determine whether heat had a strong effect on the strength of the adhesive. <br />Figure 2 shows the strength of each of the above recipes when tested using the Instron tensiometer.<br />Figure 2: Break strength of adhesives formulated using AIBN and UV initiators and various dUdM : MMA/PMMA Syrup Ratios.<br />As is evident from the above data, the strength of these adhesives was considerably higher than that previously reported by Joe Zhang. This may be due to several factors, including variation in the additive used (the additive used in this study is a different batch than that used previously) or variations in measurement technique. In order to determine where the discrepancy lies, it is necessary to test the strength of the Locktite benchmark that has been used previously. If this is considerably different, then the difference lies in the measurement technique. This will be done at a later date.<br />From the data, we can see that as the concentration of diurethane dimethacrylate increase, the strength of those adhesives which use a UV initiator decreases, whereas the strength of those which use AIBN as an initiator remain constant. This is an interesting find, and needs to be verified with more repeats, as each of these data points only has 2 – 3 samples. Of interest is that even at only 3% surfadditive concentration, the strength of this additive appears to range from about 10 MPa to nearly 20MPa. This is an excellent value, and indicates that this adhesive does indeed provide the strength and polymerization rate necessary to function as a commercial adhesive.<br />Future Work<br />Through the work described above, we have demonstrated a glass-to-glass adhesive which has excellent strength and a high polymerization rate. Moving forward, however, there remain several variables which must be investigated in order to optimize this system. <br />Although we have shown that diurethane dimethacrylate is an excellent rate enhancing additive, this polymer has also been shown to detract from the optical properties of the PMMA. Although this is not an issue when only thin films are used (such as an adhesive application), it will become a large issue when thicker films are used (such as a lamellar glass application). Thus, it would be ideal if a different rate accelerator were used which did not detract from the optical properties of the adhesive. <br />Further, because diurethane dimethacrylate is very viscose, there is a strong probability that it slows the diffusion of our surfadditive. This is important, as it is necessary that the surfadditive make it to the substrate-adhesive interface before polymerization occurs. Previous work by Joe Zhang had shown that when the adhesive was applied and immediately polymerized, the strength of our adhesive was very low. However, in this study the adhesive was applied, then allowed to rest for 5 minutes before polymerization. In this case, the strength of the adhesive was very good. It will be necessary in the future to determine what length of resting time is necessary to obtain sufficient adhesive strength. This will also be strongly dependent on the rate accelerator chosen.<br />Finally, once these issues have been sorted out, it will be necessary to determine lifetime stability. It may be necessary to included compatibilizers or stabilizers to ensure that the adhesive will remain viable and useful for a practical amount of time. <br />