Polymers/Laminations/Adhesives/Coatings/Extrusions

Published: July 01, 2001, By David J. Bentley Jr., Editor

Division Announces Keynote Speaker
The keynote speaker for the Polymers, Laminations and Coatings Conference in San Diego will be Professor Edward L. Cussler of the Department of Chemical Engineering and Materials Science at the University of Minnesota. The title of Cussler's presentation is “Fick's Second Law or Diffusion for Dummies.”
Harry Cordatos, the technical program chairperson for the conference, notes the “division is very fortunate to have Professor Cussler as the keynote speaker. He provides a dynamic, enthusiastic, and animated presentation of his subject matter that enthralls his audiences. His topic is especially important with the increased emphasis on diffusion in flexible packaging applications.”
Cussler with his coauthor, Chungfang Yang, won the Best Paper Award at the Polymers, Laminations and Coatings Conference in 2000 for his presentation on “Reactive Barrier Films.” Besides being Distinguished Institute Professor at University of Minnesota, Cussler is a member of the Editorial Board of Journal of Membrane Science and Associate Editor of AIChE Journal. He has published four books and more than 180 publications.
Working in the laboratory of Carl Ludwig in 1854, Adolph Fick suggested that the same equations Fourier had suggested for the conduction of heat could describe the diffusion of solutes. From his scattered experiments, Fick made two more specific suggestions. The first was that diffusion flux is proportional to the concentration gradient of diffusing species. This relationship is now Fick's law. Fick also combined this flux equation with a mass balance to find that solute accumulation is proportional to the second derivative of concentration with respect to position. This second relation is frequently called "the diffusion equation" or Fick's second law.
Cussler's talk will describe applications of Fick's second law to many physical phenomena. The most obvious phenomena described by Fick's second law are those actually involving diffusion. For example, this law can predict psychological reactions such as the relative sweetness of syrups and sauces. These predictions assume that sweetness depends on the transport of sugar from a bulk solution to the surface of the tongue. Apparently totally chaotic data can become a simple representation of Fick's law.
More interestingly, random processes where diffusion itself is not the dominant physical phenomenon may still follow Fick's second law. When a fire ant becomes alarmed, he releases a pheromone to warn his fellows of the danger. Fick's second law can predict the effect of this pheremone on the other ants but with a larger coefficient than expected for diffusion.
Fick's second law has utility far beyond that for diffusion processes alone. For example, Fick's second law accurately predicts the spread of muskrats in Europe following their accidental release in 1905. In the same sense, the concentrations from smokestacks is also predictable although the coefficient can be a million times larger than the diffusion coefficient. Predicting the displacement of hunter-gatherers by farmers in Neolithic times is another application. Curiously, the coefficients for muskrats, smokestacks, and Neolithic farmers are similar. This is a testament to the generality of the relationships suggested by Fick.
The Sheraton San Diego Hotel and Marina is the location for the meeting on August 26-30, 2001. The complete program for the 2001 Polymers, Laminations and Coatings Conference is available at www.tappi.org or from the Tappi Service Line by calling 1-800-332-8686 in the United States, 1-800-446-9431 in Canada, or +1-770-446-1400 from other locations. Send FAX to 1-770-446-6947. Address mail to TAPPI, Box 105113, Atlanta, GA, 30348-5113. Registration information is available from the same sources.

PEER-REVIEWED TECHNICAL PAPERS:
Following are expanded summaries of complete papers that are available on the TAPPI web site at www.tappi.org/public/library.asp

On-Line Gauging as a Process Troubleshooting Tool
by Ted Schnackertz
NDC Infrared Engineering
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Application: On-line gauging is an important source of real time diagnostic information.
An on-line gauging system goes beyond daily control. It provides the base line data reference on current process capability and is a continuous monitor of that performance. Using this data to identify any performance degradation and then analyzing the cyclic variations of the various displays can lead to quick problem diagnosis. Coupled with process knowledge, a rapid resolution is then possible. Since interpretation of the data is important to determining whether a problem exists to solve, understanding the different process variations is a necessary first step.

Definitions
The process: The combination of people, equipment, materials, methods, and environment that work together to produce output.
Common cause variation: The many sources of variation within a process that behave like a constant system of chance causes. While individual measured values are all different, as a group they tend to form a stable repeatable distribution pattern.
Special cause variation: Often called assignable cause, this refers to factors causing variation that is intermittent, unpredictable, and creates unstable operation.
Statistical control: A process from which all special causes have been eliminated and only common causes remain. A stable process is in statistical control.
Total process variation: This has three variables.

1. Short term machine direction variation (STMD): Cyclical thickness or weight/area variations that occur in a time period of one minute or less.

2. Long term machine direction variation (LTMD): Cyclical thickness or weight/area variations occurring over time in a period from several minutes to hours or days.

3. Profile variations (PRO): Thickness or weight/area variations across the web also called cross machine or CD variations. Scan to scan PRO variations are affected by STMD and LTMD but primarily STMD.

Discussion

The first step in troubleshooting a process problem is determining whether the variation is common cause (process is stable) or special cause (process is unstable). Since a gauging system displays data for all three components of variation (STMD, LTMD, and PRO), we can build a base-line data history. This will help make a determination whether the variation is common cause or special cause. Note that a process may be in statistical control but not able to meet specifications due to excess common cause variation.

Application of the gauge data

STMD variation cannot be controlled by an on-line gauging system due to the short period. It is always present in displays. A display mode called “in-line” or “single point” establishes STMD levels under normal operating conditions and forms the base for a process in statistical control. STMD is observed by positioning the gauging system at a single point on the web while collecting data for 1-2 minutes. This should be done at three positions across the web and recorded. Observing the STMD cycles again during process problems can give clues or even pinpoint a source of the variation. Note that the magnitude of the STMD will determine how well a process can be controlled. It also influences laboratory test results. These results may lead to false conclusions about controlling the process or whether a product meets specification.

LTMD variation can usually be controlled by an on-line gauge providing the process is in statistical control. Special cause variations prevent tight control. The trending display is a good indicator of LTMD and with reports like shift and roll can form a base line reference for the process. The shift reports provide roll-to-roll average thickness and sigma variations. Comparing these data between product runs, shifts, days, seasons, etc., can identify process performance changes in early stages to allow determination of causes before quality suffers. Some LTMD sources are temperature controllers, resin feed systems, ambient conditions, material changes, regrind or “fluff” ratios, and shift changes.

PRO or cross machine variation is controlled by die bolt adjustments made manually or via an auto-die. The presence of STMD superimposes a cyclic variation on the profile. STMD variation causes the profile to change from scan to scan and limits how tight the PRO cross machine profile can be controlled. Using a single profile or last scan display mode and observing the sigma of successive scans are good indicators of STMD. Changes from scan to scan in the sigma is an indicator of STMD levels in the process.

Analysis tools

On-line gauging systems provide optional software tools to assist process troubleshooting. One tool called Fast Fourier Transform (FFT) is an algorithm used to analyze cyclic data patterns. It breaks them into time and length periods for display. This information may then relate to various process equipment cycles for cause identification.

Another software tool detects narrow web streaks down to 1 mm that would otherwise be lost in the noise of random or periodic product variations. This tool provides early warning of increased streaks or die lines allowing corrective action before production is affected.

Data logging event driven software is also available for real time or historical process analysis.

Summary

An on-line gauging system goes beyond daily control. The system is a universal device that provides benefits whether it is performing a normal process control function or is used as a diagnostic tool. In either situation, economic benefits accrue in time and money. Using the full capability will improve total process performance.

Crosslinking Of Acid-Functional Styrenic Block Polymers With Aluminum Acetylacetonate

by David J. St. Clair Kraton Polymers, Inc. email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Application: Aluminum acetylacetonate as a crosslinker can improve the service temperature of adhesives and sealants formulated from an acid/anhydride functionalized polystyrene-hydrogenated polybutadiene-polystyrene block copolymer.

Styrenic block copolymers (SBC) are thermoplastic, elastomeric, materials that typically have a polystyrene endblock on both ends of a rubber midblock. The endblocks phase separate from the rubber phase to form polystyrene domains that lock both ends of the rubber block into a physically crosslinked network structure. This network structure gives SBC their high cohesive strength similar to that found for chemically vulcanized rubbers. Since this crosslinking is physical rather than chemical, it is reversible. When the polymer undergoes heating above the glass transition temperature of the polystyrene endblocks, the crosslinks release so the polymer flows like a thermoplastic. Dissolving the polymer in solvent allows processing as a relatively high solids content solution. When the polymer cools after melt processing or the solvent evaporates from a solvent cast film, the polystyrene domains regain their integrity. The polymer again becomes strong and elastic.

SBC materials have wide use in adhesives, sealants, and coatings. They do not have use in high temperature applications or those areas where resistance to solvents or polyvinyl chloride plasticizers is a requirement. For these applications, crosslinking the SBC chemically is necessary so the product won't depend exclusively on the domain structure for cohesive strength.

SBC materials that have a hydrogenated polybutadiene (SEBS) or hydrogenated polyisoprene (SEPS) midblock are difficult to crosslink. SEBS polymers to which a small amount of maleic anhydride (MA) has been grafted are commercially available. This acid/anhydride functionality is useful for crosslinking reactions. For ambient temperature curing reactions, isocyanates in a two-component system can cure SEBS/MA polymers. This paper discusses the use of a metal chelate, aluminum acetylacetonate (AlAcAc), as a crosslinker for ambient temperature cure of SEBS/MA polymers.

Experimental

This work used two SEBS/MA polymers. Polymer A had a polystyrene content of approximately 30% by weight with approximately 1.7% by weight of MA grafting. Polymer B had a slightly higher molecular weight with a polystyrene content of approximately 13% by weight and a grafted MA content of approximately 1.0% by weight.

AlAcAc is a white powder with a melting point of 193°C. Another material, 2,4-pentanedione, is also a “reactant” since it retards the rate of cure in solvent based adhesives. The 2,4-pentanedione is the ligand in the AlAcAc chelate. It is a liquid having a boiling point of 140°C.

The desired effects of crosslinking are an increase in upper service temperature and an improvement in solvent resistance of adhesives. Ring and ball softening point (ASTM D-36) or Shear Adhesion Failure Temperature (SAFT) measured upper service temperature. Solvent resistance is a qualitative test involving soaking a 25 × 25 mm specimen in toluene. If the specimen dissolves, the polymer has not crosslinked. Swelling without complete dissolution indicates crosslinking. Standard industry tests measured rolling ball tack (PSTC-6), Polyken probe tack (ASTM D2979), loop tack (ASTM D6195), 180° peel (PSTC-1), shear (PSTC-7 using 25 × 25 mm with 1 kg), and melt viscosity (ASTM D3236).

Results

Table I shows a typical formulation for a pressure sensitive adhesive (PSA) that would have use as a general-purpose permanent label. The formulation uses a saturated resin and plasticizer that should impart good long term durability and resist degradation by sunlight. The PSA uses Polymer B and crosslinking with AlAcAc at three different concentrations. The adhesives were cast from 40% solids solutions by weight in a 75/25 mixture of toluene/isopropyl acetate onto 25 microns polyester film using 35 microns dry adhesive thickness. They were baked 10 min at 180°C followed by storage at 25°C for one week before testing. Results show an increase in SAFT and an improvement in solvent resistance with increasing concentration of AlAcAc. Crosslinking did not significantly influence tack and peel.

Results in Table I also show that the adhesive solutions have limited pot life after adding the AlAcAc. This indicates that the crosslinking reaction proceeds rather rapidly at room temperature. It also shows that addition of the AlAcAc to the adhesive can only occur immediately before coating the adhesive. The crosslinking reaction is probably a ligand exchange in which the AcAc moieties (2,4-pentanedione) leave the aluminum for replacement by the acid/anhydride functional groups on the polymer. The curing reaction can be inhibited for more than two weeks by adding 2,4-pentanedione to the formulation. After coating of the PSA, the 2,4-pentanedione evaporates with the solvent to allow crosslinking to proceed.

This work also examined a typical formulation for a hot melt PSA that would have use as a general-purpose permanent label. Results again showed an improvement in SAFT and solvent resistance with increasing AlAcAc concentration.

TABLE I - CROSSLINKED SOLVENT BASED PSA

Composition	1	2	3	4
Polymer B	30.0	30.0	30.0	30.0
Resin	54.4	54.4	54.4	54.4
Plasticizer	15.5	15.5	15.5	15.5
AlAcAc	—	0.2	0.5	1.0
PSA solution, time to gel, h	—	18	3.5	2
Rolling ball tack, mm	42	24	24	31
Polyken probe tack, kg	0.9	1.1	1.0	1.1
Loop tack, N/m	820	800	670	800
180° peel, N/m	420	420	370	400
Shear, h	>60	>60	>60	>60
SAFT, °C	64	79	83	94
Gel soaked in toluene	No	Yes	Yes	Yes

The hot melt adhesives were mixed in a sigma blade mixer. Polymer, resin, and oil were mixed first. The AlAcAc was added and mixing continued another 30 min. before coating the hot melt adhesives. Concern existed about the pot life of these reactive hot melts. In a separate experiment, adhesives using Polymer A were mixed at 175°C for 3 h after adding AlAcAc. Qualitatively, no adhesive showed a significant increase in melt viscosity with levels of AlAcAc up to 1.0% by weight of the formulation. All adhesives poured readily from the mixer and self-leveled in their containers. Using Polymer B with higher molecular weight, the adhesives changed when held hot. After mixing for 3 h at 175°C, the adhesives still poured readily from the mixer and self-leveled in their containers. When touched with a spatula, the adhesives containing AlAcAc were stringy. The adhesive with no AlAcAc was not stringy. This work did not study factors affecting pot life. Results show that a careful study of pot life is necessary before considering these reactive hot melt adhesives for practical applications.

PSA materials using SBC are not suitable for use in labels and decals that have a polyvinyl chloride (PVC) backing if the PVC contains a typical liquid plasticizer such as dioctyl phthalate (DOP). The reason is that DOP is compatible with the polystyrene endblocks of the SBC and migrates from the backing into the adhesive. This reduces the glass transition temperature of the polystyrene endblocks and destroys the cohesive strength of the PSA. DOP causes a drastic drop in SAFT and shear. The failure mode in the 180∞ peel test switches from a clean peel off the steel to a cohesive failure within the adhesive layer.

Testing of an adhesive containing 10% DOP by weight using crosslinking with 0.2% AlAcAc by weight showed some relief from the negative aspects of the plasticizer. With 20% by weight DOP, crosslinking with 0.5% AlAcAc by weight is necessary to offset some effects of DOP. The most significant effects of crosslinking are restoration of SAFT back to the level of the PSA with no DOP and restoration of sufficient cohesive strength that the PSA can peel cleanly from a steel panel in the 180° peel test.

Since hot melt sealants using SBC are thermoplastic, formulating for good performance at elevated temperatures has always been a challenge. At temperatures around 70°C, the sealants soften and simply flow from a joint. Results in this study show that this problem decreases or disappears by crosslinking the sealant.

SEBS polymers have good compatibility with microcrystalline wax and with blends of paraffin wax and resin. SEBS polymers can be added to wax or wax and resin blends in a range of applications such as improving flexibility of barrier coatings or increasing cohesive strength of non-PSA hot melt adhesives. SEBS/MA polymers are also effective wax modifiers. Crosslinking them with AlAcAc will improve the flow resistance of the wax above its crystalline melting point.

Adding SBC to asphalt can increase the softening point of the asphalt and reduce its brittleness at low temperature, almost doubling the temperature range over which asphalt has use. The polymers also increase the cohesive strength and give the asphalt elasticity. The blends are still thermoplastic, so their upper service temperature will usually be no higher than about 90°C using a straight run, unblown asphalt. Modifying the asphalt with an SEBS/MA polymer and crosslinking with AlAcAc gives asphalt products with softening points above 150°C.

Oil gels are made by mixing an SBC into a low aromatic content oil at about 130°C-175°C. When the blend cools, the polystyrene endblocks that are insoluble in the low aromatic content oil precipitate to form the network structure that gels the oil. These oil gels find use in many applications ranging from cable filling compounds to clear candles. Because they are thermoplastic, they cannot be used in applications above approximately 80°C because the gels simply melt. Using an SEBS/MA polymer to make the gel allows crosslinking with AlAcAc. This improves the performance at high temperatures.

Conclusion

This work has shown that acid/anhydride functional SEBS polymers can successfully crosslink with AlAcAc to convert the polymer from a thermoplastic to a thermosetting material. Performance in solvent based PSA, hot melt PSA, sealants, modified waxes, modified asphalt, and oil gels demonstrated the feasibility of this technology. Use temperatures well above the glass transition temperature of the polystyrene endblocks of the SEBS polymer are possible. Pot life is long with AlAcAc. Zinc AlAcAc is a good choice for applications requiring nearly instantaneous crosslinking.

Division Chairman Comments

by John Perdikoulias

As current chairperson of the Polymers, Laminations and Coatings Division of TAPPI, I invite and encourage you to attend the division conference in San Diego on August 26-30.

This past year has been difficult for many businesses. Unfortunately, the first compensating restrictions in a company involve travel. This may save some money at the expense of other considerations. I suggest that company management consider the following.

A good business strategy should include plans for a “rainy day.” Business lulls are an excellent time to catch up on new technological developments and investigate ways to improve the processes and products of a company. In good times, people say they are too busy to keep up with research and development. When business slows, people indicate that they cannot afford to attend conferences and other meetings. I believe such an attitude can be damaging.

If a conference attendee can return with a few new ideas or pieces of information, his trip will more than pay for itself in benefits to his company. Other benefits to justify attendance at a conference are the relationships created during the meeting. Industry contacts that can help solve a process problem quickly and efficiently are invaluable. Speaking with someone who can help select a material or piece of equipment using their personal experience rather than a sales pitch is definitely available at a conference.

Before joining the Polymers, Laminations and Coatings Division of TAPPI in 1986, I tried to obtain information from people that had the “misfortune” of having my call routed to their telephone. I was new to my job and looking for technical information. Their responses were little more than the information provided in official company literature. A few months later I attended my first Polymers, Laminations and Coatings Conference. I noticed a big difference in how these people responded to my questions when we discussed issues face to face. I cannot begin to place a value on the assistance that my growing network of acquaintances has provided over the years. I do know that the friends I made from such meetings have definitely helped me with my career. I look forward to attending the Polymers, Laminations and Coatings Conference each year to meet old friends and make new ones while learning valuable information to help my company.

See you in San Diego!

For information about the Polymers, Laminations and Coatings Division of TAPPI, see the web page at www.tappi.org/public/divisions/polymers_laminations_coatings.asp or access the TAPPI web site at www.tappi.org. For the complete papers whose expanded summaries appear in this section, go to the TAPPI web site at www.tappi.org/public/library.asp and click on the logo displayed here.

Telephone inquiries are welcome at the TAPPI Service Line by calling 1-800-332-8686 in the United States, 1-800-446-9431 in Canada, or +1-770-446-1400 in other countries. Send FAX to 1-770-446-6947. Address mail to TAPPI, Box 105113, Atlanta, GA, 30348-5113. Contact “the PLACE” editor using e-mail at This email address is being protected from spambots. You need JavaScript enabled to view it..