ADVANCEMENTS IN HIGH DENSITY POLYOLEFIN FOAM SUBSTRATES

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1 ADVANCEMENTS IN HIGH DENSITY POLYOLEFIN FOAM SUBSTRATES Anthony Filip, Technical Service Engineer, Tape and Health Care Products Division, Sekisui Voltek Corporation, Lawrence, MA Sekisui Voltek has been manufacturing crosslinked polyolefin (CPO) foam tape substrates for over 35 years. In the mid 1980s, applications for CPO foam tapes began shifting from low-end to more sophisticated, performance-driven mounting. Many industries began to employ CPO foam tape into niche markets. Some specialized examples would be drug delivery patches and window glazing. The foam systems designed to serve these early markets had remained largely unchanged until just the last few years. In the CPO foam world, a Cambrian explosion of sorts has recently unfolded. Foreign competition has forced the North American supplier to focus on cost reductions for commodities and market expansion with specialties. Specialty foams are thinner and stronger; but most importantly, they are more consistent largely due to new processing capability and equipment that can measure critical attributes of the process in real time. This paper will survey these exciting new product offerings and will explain the basic processing challenges and the resultant improvements in product capability. With advancements in PSA technology, the potential uses and markets for foam substrates is expanding. PSA foam constructions are being used for part-to-part assemblies in applications where the total composite can be no more than 20 in thickness. Tape industry engineers are seeking thinner foams for finer applications. Foams are ideal substrates for mounting applications because they are compressible, which allows for improved adherence to irregular surfaces. Foams are expected to provide resiliency in applications that are no longer static in nature. Today s precision CPO foams can offer the design engineer greater dynamic performance made possible only by advancements in manufacturing know-how. In response to new gauge requirements, the foam manufacturing processes needed to be re-engineered. The incorporation of state of the art controls and equipment, and implementation of specialized know how were the corner stones of this effort. In general, the crosslinked foam product offerings had remained unchanged for decades and product quality and consistency has been largely dependant on operator skill. In general, the conventional foam portfolio features density and thickness ranges that are mutually dependant. Low density products require greater thickness, so a product of 2 pounds per cubic foot (pcf) is limited to a minimum foamed thickness of 80 ; whereas a 6 pcf foam can be offered for a minimum thickness of 31. In 2000, a then new high performance foam substrate, termed HPT was introduced. This material had a density of 12 pcf and was 20 thick. At the time, this product represented the thinnest CPO foam available. For this evolutionary step, the focus was on reducing gauge variation. Today, advancements have improved this benchmark material, creating a superior product featuring improved gauge capability and a tremendous improvement in density variation. This capability is only offered at higher densities ranging from 8 pcf and higher, with thickness ranging from 10 to 80. To make a 10 thick foam at +/- 1 pcf; this represents an evolution in process capability. 37

2 The CPO foam process is composed of three discreet steps. To make a high performance foam, you must make a high quality extruded precursor film. Any defects present in the first step will be magnified during the foaming step. Picture 1. Outline of the CPO foam process. The precursor film has been designed and formulated to achieve specific foam characteristics. At this step all the foam components are present and the material remains as a solid and still thermoplastic. Next, the precursor film is irradiation crosslinked, by precisely controlled exposure to electron beam irradiation. Once crosslinked the material is no longer a thermoplastic irradiation crosslinking provokes a permanent chemical change to a specific percentage of the base thermoplastic resin. This step is critical, as even small deviations from target will cause changes in the final foam properties. In the final step, with the addition of heat, the precursor film is continuously foamed. It is here that the final product attributes take shape. It is important to note, the final thickness is naturally achieved; the foam is not nipped or calendared to set the gauge. Proprietary gauge and density measuring systems can continuously scan and present data to the operator in real time. This is the process visibility required to make precision foam. Picture 2 shows a typical operator view during the foaming step. Picture 2. Screen shot of continuous foaming process control. 38

3 Product improvements have targeted high value, market growth materials. However, when the advanced processing techniques and equipment are applied to lower density material, such as 6 pcf, small measurable improvements can be seen. For example, Figure 1 illustrates the 6 sigma range for gauge on old and new generation materials. From the plot it can be seen that process capability for gauge does improve as the spread narrows slightly. The same can be seen with respect to density distribution, as Figure 2 illustrates a narrowing of the total distribution of measured densities Thickness Control Improvements on 6 PCF Volara '07' / Difference from Target Thickness (mils) Figure 1. Illustrates an improvement in thickness variation of 6 pcf foam. 2.0 Density Control Improvements on 6 PCF Volara Density (PCF) Figure 2. Illustrates an improvement in density variation of 6 pcf foam. The new market demands more consistent products. Consistent foam thickness with minimal variation is critical. Concerning thickness or product gauge, the original high density thin products were designed to improve this key attribute. Referring to Figures 3 and 4, it can be seen that gauge has improved with the precision foam manufacturing process. 39

4 Thickness Control Improvements on 12 PCF Volara Difference from Target Thickness (mils) Figure 3. Gauge improvements of 12 pcf foam Thickness Control on 18 PCF Volara Difference from Target Thickness (mils) Figure 4. Gauge improvements for 18 pcf foam. 3 In general, these plots illustrate 6 sigma gauge measurement distributions for 12 and 18 pcf materials. Note that, although the improvements are marginal, this really illustrates that gauge control has been present in these products since the beginning. Foam tape performance is driven by foam density consistency. Variations in foam density are variations in the product s mass; and this variation is a part of every foam process. With foam, mass is a large part of what dictates the physical properties. With advancements in high density foam manufacturing capabilities, density variation is greatly reduced. Referring to Figures 5 and 6, it can be seen that there is a real measurable improvement in the distribution of recorded densities for a given product. To the tape designer, this means more consistent performance and greater product reliability. 40

5 Density Control Improvements on 12 PCF Volara Density (PCF) 13 Figure 5. Illustrates 6 sigma plot of recorded density distributions and illustrates a large improvement in past versus current capability at 12 pcf Density Control Improvements on 18 PCF Volara Density (PCF) Figure 6. Illustrates 6 sigma plot of recorded density distributions and illustrates a large improvement in past versus current capability at 18 pcf. Traditionally, foam manufacturing required a large specification window for inherent gauge and density variability. Using the best manufacturing techniques available, density specifications can be reduced to +/- 1 pcf. As can be seen in Table 1, the evolution of density capability has progressed from the 2001 target of +/- 20% to today s target of +/- 1 pcf, which translates to +/- 8.3% for a 12pcf product. Correspondingly, Table 2 illustrates the same concept for 18 pcf foams. This illustrates a huge shift in density capability. As the target density increases, the +/- 1 pcf corresponds to a tolerance of +/- 5.6% of target. 41

6 Table 1. Depicting 12 pcf density tolerances by year. 12 PCF Density Tolerance Density Tolerance (%) Series Series Table 2. Depicting 18 pcf density tolerances by year. Density Tolerance (%) 18 PCF Density Tolerance Series Series Foam manufacturers have long studied changes in cell geometry. With lower density foams there is greater foam expansion, which leads to larger cells or cells with greater elongation in the machine direction. Picture 3 show typical high density cells, as viewed in the machine direction. 42

7 Picture 3. Old technology produces long tapered cells. Notice the long elliptical shapes of the cells with ends that taper to points. A better cell would be less tapered and more compact, like the cells in Picture 4. Picture 4. New technology produces stout cells with less elongation and less end taper. These two foams are slightly different density, but the concept is accurate. The more regularly the cell is shaped the greater the product consistency. Considering the current advances in high density foams, cell 43

8 size is much more compact and consistent across the 12 pcf to 22 pcf line of products. Cell geometry is now part of the process design, meaning cell structure is an engineered part of the foam manufacturing process. In conclusion, state of the art manufacturing can create new foam products of unequalled gauge and density consistency. These new high density foams can be combined with the latest advances in PSA technology, to serve new market opportunities. 44

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