HIGH THROUGHPUT TESTING OF ADHESIVES

Transcription

1 HIGH THROUGHPUT TESTING OF ADHESIVES Christopher Lester, Sr. Scientist, Rohm and Haas Company, Spring House, PA William B. Griffith, Sr. Scientist, Rohm and Haas Company, Spring House, PA Companies are continually looking to improve productivity and reduce costs. Consequently, most organizations carefully examine their processes, streamline them, and remove bottlenecks wherever possible. Often the result is a better organized version of the original process. Conversely, overstreamlining can also cause less actual data to be collected and cause materials to not be fully optimized. The challenge is how to conduct more developmental work with less effort. In the development or qualification of a pressure sensitive tape, most of us follow a very traditional process. Formulations are blended, draw downs are prepared, and various PSTC test methods are used to decide which adhesive and/or construction will meet a need. In the end, data are usually tabulated in a spread sheet form, and by inspection, a candidate sample is selected. Over the years, this process has been scrutinized repeatedly to improve its optimization. Recently, some have started to consider alternate means that could potentially reduce the effort required to develop a new tape through the use of high throughput methods. High throughput methods are any that permit more experiments to be conducted with a given amount of manpower. Designing a robot to handle repetitive tasks is one example. As subset of high throughput methods is combinatorial techniques. A combinatorial technique is one that varies two or more parameters within one experiment. By examining two or more parameters, a wide range of experiments are performed. By conducting it within one experiment, less manpower is needed. Some areas in science and engineering have particularly benefited from high throughput or combinatorial techniques. In particular, companies that develop pharmaceutical products or agricultural chemicals have adopted approaches that permit them to screen thousands of compounds with relatively little effort. 1 More recently, similar approaches are being adapted to polymer chemistry and adhesive science. 2,3 This paper will provide a brief overview of some of the progress that is being made towards applying high throughput methods in the development of pressure sensitive tapes. Recently, we have been exploring the use of combinatorial techniques in the evaluation of formulating PSA s. The objectives are: To prepare as many formulations as possible with the least possible effort To test as many samples as rapidly of possible under the widest range as possible To rapidly analyze the data as thoroughly as possible Experience from other areas of technology demonstrate that is strongly advisable to miniaturize wherever possible. This not only conserves materials, but also permits more tests to be performed in a given space.

2 Sample preparation The classic method of preparing formulations is to manually blend each formulation one by one. This can be a very time consuming approach. Here is an example of a repetitive task that can be automated. Robotics can readily perform this same task, but also bring along some unique challenges. In particular: Need to handle a wide variety of viscosities (solids to solvents) Some materials such as defoamers need to be premixed An algorithm of how to calibrate masses must be developed Set-up time must be minimal Clean-up time must be minimal If only a single formulation is to be made, using robotics most likely will be more time consuming than simply preparing the sample by hand. The advantage of such a system is not fully realized until a large number of repetitive tasks are involved. In general it is advisable to develop a mini-plant with dedicated stage and dedicated lines thereby greatly minimizing the set-up and clean-up steps. The disadvantage of this method is that it limits to some degree the number of raw materials that can be handled. A lab that spends considerable time formulating would benefit by: Samples could be prepared 24 hours a day/7 days a week Improved reproducibility Applying the same robotics approach to PSTC test methods is difficult. Designing and programming a machine to prepare a drawdown on film or paper, dry it in an oven, and perform peel, shear, and tack measurements would require a substantial effort and may not be practical. Here is a case where the use of combinatorial techniques are beneficial. However, schemes must be devised where several samples can be prepared on one substrate. Once prepared, other schemes that measure the PSA properties within one experiment must be developed. One scheme for preparing formulations is to devise a gradient method that more or less produces a continuum range of compositions. As an example, consider a formulation where the level of tackifier needs to be optimized. An initial set of experiments is designed and targeted to cover 0 to 40% range of tackifier. Traditionally, one would design ladder experiments where, say, samples with 0, 10%, 20%, 30%, and 40 % tackifier are prepared and tested. A gradient method would prepare a single sample where the composition on one side contains 0% tackifier and the composition of the opposite contains 40%. The composition should change gradually and continuously across the sample. For example, the center of the sample should contain 20% tackifier. A prototype composition mixing apparatus has been developed by Meredith et al. for blending polymers in solution to yield composition gradients in thin polymer films. 4 As a test case for water based formulations, consider blends of neat polymers with tackifiers. Suppose that a range of compositions from 0% tackifier to 40% tackifier in a latex adhesive formulation is desired. The mixing vessel would initially contain a 100% latex polymer/0% tackifier mixture. The device is automated in that a syringe pulls up the mixture at the same rate at which the tackifier is added to yield a gradient inside the needle. The rate of tackifier addition is adjusted so that the final mixture in the container reaches 40% tackifier

3 just as the syringe is filled. This is shown pictorially below progressing from left to right with the two polymer solutions being shown as two different liquids. Time The material in the syringe can then automatically be deposited on a substrate and drawn down to yield a blended polymeric film with a continuous concentration gradient across the sample as shown in the picture below. In order to simply test the feasibility of working with water-borne dispersions, an emulsion polymer was dyed red and another sample of the same was dyed blue. Alternately these samples were drawn up such that half the needle was the red dyed sample and half was the blue dyed sample. This was deposited onto a silica substrate and drawn down to yield a film with a fairly sharp interface between the two colors as can be seen below.

4 This relatively simple experiment indicates that working with polymer dispersions in this fashion to produce films with composition gradients is possible. Further work is ongoing to feed one dispersion into another and thereby be able to draw up a true concentration gradient. One is certainly not limited to blending just polymer and tackifiers. Other combinations are possible where gradients of polymer dispersions with other formulating additives could be generated. Test Conditions During the development of a tape, it is common to prepare a sample and test it under various conditions. Peels may be done from substrates of various surface energies such as stainless steel, glass, vinyl, and polyethylene. The sample may be evaluated at several temperatures. Imagine the value of a lab where all these tests can be done simultaneously in a single experiment. It is virtually impossible to make a single substrate that contains areas of stainless steel, glass, et cetera. However, this condition may be mimicked by preparing a surface energy gradient on a single substrate. Two approaches of achieving this are by either the controlled acid etching of silica wafers or exposure of chlorosilane treated silica to controlled doses of UV radiation. 5 It is conceivable to produce surface energies over the range of metals (very high surface energy) to fluorocarbon (very low surface energy) by this technique on the surface of a single substrate. Determining the service temperature of a pressure sensitive tape can be time consuming. Often tests are conducted over a wide range of temperatures in 10 o C increments. Hot stages that are modified to generate temperature profiles could have considerable utility in testing pressure sensitive tapes. With some simple modifications, a substrate that contains a temperature gradient can be used to measure peel and tack values. Gradients potentially can be devised for many other parameters. Peel rates can already be ramped over a range within one experiment. Some others that might lend themselves to a gradient method are light exposure, surface roughness, and coat weight. Test Methods If a sample or substrate contains a gradient, some modification of the test method may be required. One can no longer run the PSTC test method for peel and report an average peel value. Suppose a drawdown is prepared where a composition changes linearly in one dimension. If one were to peel along the

5 direction of the gradient, one would expect to observe the peel force vary with the compositional change. Some care must be taken to ensure that the instantaneous peel force can be directly correlated to a particular point in the drawdown. This would permit relating the instantaneous peel to a defined composition. Another test method that lends itself for testing such samples would be any that is based on a probe tack method. By simply coupling an XY positioning transducer to a measuring device, measurements could be made over a range of areas. One concern with this approach would be contamination of the probe by a preceding measurement. To illustrate the utility of this device, a tackifier ladder was prepared and measured using a single apparatus. Analysis of the curves resulted in the following force displacement curves: % 10% 20% 30% 40% Force (mn) Displacement (μ m) These data show the expected trend of increasing force with increasing tackifier level along with some evidence of overtackification at higher levels. Beyond this point, the analysis follows an algorithm similar to traditional methods. Calibration Accurate measurements generally require a calibration algorithm. In cases where a grid and/or gradient are involved, it may be necessary to consider variations that are perpendicular to the direction of the gradient. For example, if a substrate was designed to have a temperature gradient from left to right, then ideally the sample could be characterized by a set of parallel isotherms as illustrated below:

6 Temperature In reality, a situation may arise where temperature varies as: Temperature Even the case where the variation of temperature is more irregular does not prevent the use of high throughput techniques. However, temperature will need to be determined across the grid, and during the data analysis step, the data must be adjusted to accurately reflect the variations in temperature. In summary, some of the issues to consider are: The uniformity of the grid must be assessed Instrumentation may need to be developed to measure the characteristics of the grid The characterization of the grid needs to be automatically folded into the data analysis step Data Analysis The amount of data that a well equipped lab can generate by tradition PSTC test methods can at times be overwhelming. If high throughput methods are instituted, the amount of data that is collected may be orders of magnitude higher. One can no longer analyze data by simply perusing the data. Methods must be developed to store, sort, and sift the data so that only the most relevant data is drawn to one s attention. With recent advances in computer technology, it may seem that it should be relatively easy to devise methods to analyze data. However, the development of artificial intelligence algorithms that will select the just appropriate set and amount of data for one to review is especially challenging.

7 Conclusion Initial feasibility studies have demonstrated that pressure sensitive tapes can be study using high throughput techniques. Based on the specific examples given above, high throughput techniques appear to have great potential. High throughput permits: More testing and data analysis with less effort An improvement in precision due to the automation of techniques and the number of replicates that can be performed High throughput typically requires: A substantial initial investment in technique development Modification of test methods that may differ from the standard PSTC methods Acknowledgement The authors wish to thank Dr. Aaron M. Forster and Dr. Christopher M. Stafford along with the National Institute of Science and Technology for their support. References 1) Lam, K. S.; Renil, M. Curr. Op. Chem. Bio. 2002, 6, 353 2) Amis, E. J.; Xiang, X.; Zhao, J. MRS Bulletin 2002, 27, ) Wicks, D. A.; Back, H. Water-Borne & Higher-Solids and Powder Coatings Symposium 2002, New Orleans LA. 4) Meredith, J. C.; Karim, A.; Amis, E. J. Macromolecules 2000, 33, ) Smith, A. P.; Sehgal, A.; Douglas, J. F.; Karim, A.; Amis, E. J. Macromol. Rapid Commun. 2003, 24, 131.