PROCESSING THERMOFORMABLE LOW-DENSITY FOAM

PROCESSING THERMOFORMABLE LOW-DENSITY FOAM Jim Throne, Consultant Dunedin, FL 34698 Abstract Low-density thermoplastic foams primarily heat by volumetric absorption of incident infrared energy and are primarily formed into functional parts by shearcompression in matched tooling. Introduction Thermoforming is a secondary process that follows sheet extrusion. The way in which low-density foam sheet is extruded is key to understanding the complex technical issues involved in heating and stretching the sheet into its desired product. I begin by summarizing the general desired methodology needed to produce quality lowdensity foam sheet. I follow this with discussion of a heating protocol. And conclude with the rationale behind forming the sheet into useful products. Foam Sheet Extrusion an Overview Here I consider low-density foam as having a density below 100 kg/m 3 (6 lb/ft 3 ). In many foam thermoforming processes, sheet thickness as presented to the forming press is in the range of 5 mm (0.2 inches) or less. To produce a low-density foam sheet, I need a plastic with a relatively high melt viscoelasticity, a foaming agent, and in some instances, bubble growing sites called nucleants [1]. There are two general types of foaming agents: chlorofluorocarbons, or hydrocarbons such as butane. These gases dissolve into the pressurized melt. Foam is produced when the gas-laden melt extrudes through a sheet die. Bubbles grow on solid surfaces called nucleants. I deliberately add nucleants such as talc or calcium carbonate. The solid residue from the decomposition of a chemical foaming agent also acts as a nucleant. Once the melt pressure is released, bubbles form and grow. Three or four characteristic bubble growth phases have been identified. Initial bubble formation occurs on nucleants [2]. Nanobubbles grow by diffusion of gas from the surrounding plastic melt. Melt viscosity controls the rate at which these bubbles grow. Bubble growth rate decreases as the gas is depleted from the surrounding melt. Diffusion of foaming agent gas to the growing bubble site controls this rate. Eventually bubbles stop growing when the pressure within the growing bubble decreases to that of the foam sheet environment. I call the pressure within the bubble as cell gas pressure. As shown in schematic in Figure 1, the cell gas pressure continues to decrease as the formed foam sheet cools. For fresh foam, the only gas within the cell is the foaming agent gas [3]. At room temperature, the cell gas pressure may be 0.3 to 0.6 atm absolute. If the solid plastic has low compression strength, fresh foam may be dimensionally unstable. Low-density foam is usually allowed to age. Usually foam is set aside for hours to days to allow air to diffuse into the cells. This step is critical if the foam is to be thermoformed. Chemical foaming agents are dry powdered chemicals that are metered into the extruder hopper with the plastic. Typical examples are sodium bicarbonate or azodicarbonamide. These chemicals decompose at specific temperatures within the barrel of the extruder. The decomposition products such as water, nitrogen, or carbon dioxide are dissolved into the pressurized plastic melt. Foam is produced when the gas-laden melt is extruded through a sheet die. Physical foaming agents are volatile liquids that are metered under pressure into the extruder. Typical examples are carbon dioxide, certain Figure 1. Changes in various foaming parameters during and after bubble growth. SPE ANTEC Anaheim 2017 / 2324

Ideally, internal cell gas pressure should stabilize before the foam is heated and formed. The only gas in fresh foam is foaming agent gas. Over time, it diffuses from the cells to the environment while air diffuses in. Foaming agent gas diffusion rate depends on the size of the gas molecule [4]. Chlorofluorocarbons diffuse very slowly. Hydrocarbons such as propane diffuse very rapidly. For thin foam sheet, the mixture of cell gases is essentially uniform throughout thin foam sheet after only a few hours. The foam core in thicker sheet may retain high foaming agent gas concentration long after the surface may have very little. There is an additional parameter that is important. The foaming gas is absorbed in the plastic in the extruder. Some plastics have high affinities for certain foaming gases. This means that even at room temperature, some solid plastics retain small amounts of dissolved gases. Dissolved gases can plasticize plastics, in essence reducing plastic properties such as glass transition temperature, modulus, and compressive strength. Heating Low-density Foam Great care is needed when heating foams. Two physical effects compete as the foam heats [5]. As the plastic heats, its tensile modulus decreases. As the cell gas heats, its pressure increases. At some temperature (range), the cell walls soften to the point where the foam begins to increase in thickness. The foam thickness continues to increase with temperature. Figure 2 illustrates this for butane and polystyrene. The shaded region is measured secondary expansion. If I can adequately control the heating in this region, I can extrude thinner foam at higher density than that required for the final foamed product. Figure 2. Relative sheet thickness for low-density PS foam. Shaded area is measured expansion. Solid lines are upper and lower limits on mathematical model. In Figure 2, the foam thickness abruptly decreases at some temperature (range). At (and above) this temperature, the increasing cell gas pressure causes cell walls to collapse. Overheating can lead to catastrophic loss of the sheet into the thermoforming machine lower heater. If the sheet temperature is too cold, I cannot form it into the desired product. If the sheet is too hot, I experience extensive cell collapse. Typically, the forming temperature range for most low-density foams is only a few degrees. In Figure 2, sheet thickness expansion occurring just above room temperature is the result of dissolved butane depressing PS glass transition temperature. Substantial expansion occurs just before 100C (210F), the traditional glass transition temperature of neat PS. As I said earlier, certain plastics such as polystyrenics have great affinities for certain foaming agent gases such as hydrocarbons and certain chlorofluorocarbons. Other plastics such as polyolefins have very low affinities for most gases. As a result, these plastics exhibit very little secondary expansion during heating. So How do Cellular Plastics Heat? There are three primary modes of heating sheet in thermoforming conduction, convection, and radiation. Conduction is the primary means of moving energy from the sheet surface ot its core. Convection is fluid contact with the sheet surface. Radiation is electromagnetic energy interchange between a remote heater and the sheet. Conduction is solid-solid contact between a heated surface and a cold sheet. Although it is correct that the thermal conductivity of foam decreases as the foam density decreases, it is wrong to assume that that is why conduction is rarely used to heat foam sheet. The key is the rate at which heat is conducted into foam. The rate of conduction is dependent on foam thermal diffusivity. Thermal diffusivity, α, is the ratio of thermal conductivity, k, to the product of density, ρ, and heat capacity, c p : = (1) As apparent from Figure 3, thermal diffusivity actually increases with decreasing foam density. So why isn t conduction used to heat sheet? Thermoformable foam sheet usually is thick. Conduction heat transfer depends on the sheet thickness. I discuss this in some detail elsewhere. In essence, though, doubling the sheet thickness increases the heating time four-fold. SPE ANTEC Anaheim 2017 / 2325

If the forming process is typical, bubbles form isotropically, meaning that they stretch uniformly in all directions. Because the sheet thickness dimension is much smaller than the surface directions, I only consider conduction through the sheet. To further analyze this, I replace the rather random cell structure with a simple cube-in-cube model, Figure 5: Figure 5 Replacing typical cellular structure with cube-incube ideal model [7] Figure 3. Density-dependent thermal diffusivity of rigid PVC and PS. There is relatively little plastic in low-density foam. Expansion is more than ten-fold and can be as high as 40- fold. As I discuss below, extruded low-density foam is usually isotropic, meaning it expands in all directions as it issues the extrusion die. As an example, I squeezed a 0. 5 cm (0.200 in) thick low-density PS foam sheet that had been expanded 32 times in a vise to collapse all the cells. Now 32 1/3 = 3.2 or the compressed plastic should be 0.5/3.2 = 0.16 cm (0.06 in) in thickness. As I show in Figure 4, the dense plastic is about 0.15 cm (0.06 in) thick. Inbound energy moves from the surface in this case a plane parallel to the energy source to the vertical cell walls. It is conducted down the cell walls to a plastic plane again parallel to the sheet surface. And so on. By their very nature, cell walls are long and thin. They are load-bearing supports for foam response to applied load. As expected, energy conduction along a long, thin plastic thermal insulator is poor. I think that convection is the primary means of energy interchange between the cell walls and the cell gas. The vertical cell walls provide little energy. The horizontal cell wall closest to the surface is the primary source of energy to the cell gas. The vertical walls provide some energy and the cell wall farthest from the surface contributes very little. The primary convection parameter is the heat transfer coefficient. Cell gas is quiescent to mildly buoyant. As a result, the heat transfer coefficient is very small. This means of energy transfer is also quite low. I conclude that cell gas convection is negligible. I verified this some time ago by heating side by side - a non-woven polyester fabric of about 60 kg/m 3 (~4 lb/ft 3 ) and a sheet of lowdensity PET foam of about the same density. The heating rates of the two were almost identical, Figure 6. Figure 4. Low-density PS foam compressed to essentially dense plastic. There are three mechanisms to move heat from the sheet surface to its core. One is conduction through the cell walls. The second is conduction/convection in the cell gas. The third is infrared radiation. Consider the cell wall conduction issue first. SPE ANTEC Anaheim 2017 / 2326

(0.090 inch) thick PS foam sheet. The distance from the surface to the centerline of the foam is 1.15 mm (0.045 inches) but technically the plastic thickness to the centerline is only 0.38 mm (0.015 inches). Consider the plastic cell walls parallel to the sheet surface as a series of very thin films (Figure 5). Figure 6. Side-by-side heating profiles for non-woven PP and low-density PS foam Although the thermal diffusivity for low-density foam is equal to or greater than that of the unfoamed plastic, I believe conduction and convection are not the primary modes of heating. That leaves infrared radiation. Radiation Interchange in Thin Films I believe that thin plastic films are semi-radiopaque or more correctly, diathermanous. Diathermanous means transmitting infrared energy. I based this assertion on transmission of infrared energy as measured using Fourier Transform Infrared Spectroscopy. Figure 7 is an FTIR scan for PS. Figure 7. FTIR for PS Imagine that there are 15 cell walls between the sheet surface and its centerline. Consider each cell wall 25 µm (0.001 inch) in thickness 1. From my comments above, about 90% of the inbound energy passes through that cell wall to the next one. Ninety percent of that passes through to the third cell wall and so on. Surprisingly, 20% of the inbound energy has not been absorbed even after passing though 15 cell walls to the centerline. In fact, about 5% of the inbound energy on one surface will pass completely through the entire 2.3 mm (0.090 inch) sheet. Probably what is most important is the seeming uniformity in energy uptake by foams despite their apparent thickness and thermal inertia. In short, radiant energy absorption is the primary means of heating lowdensity foam sheet. Forming Foam Low-density foam cannot be heated to typical unfoamed plastic forming temperature ranges without catastrophic cell collapse. For example, the forming temperature range for unfoamed PS is about 135-160C (280-320F). The best forming temperature for 60 kg/m 3 (4 lb/ft 3 ) PS is not much above 100C (210F). Although dissolved foaming gas may plasticize the PS, softening it, forming forces must overcome internal cell gas pressure that may be in excess of 1.3 atm at 100C (210F). I liken this to standing on pillows. As a result, low-density foam is usually formed in matched metal tools, Figure 8. The spectroscope measures wavelength-dependent transmissivity through thin films. Inbound infrared energy is either reflected, absorbed, or transmitted. Reflectivity is small for most plastics and so I usually ignore it. Therefore, whatever energy is not absorbed must be transmitted. For PS, I estimate the average transmissivity in the thinner film (0.001 inch or 25 µm) to be about 90% in the traditional thermoforming wavelength range of 4 μm to 7 μm. This means that only 10% of the inbound energy is absorbed by the film. But foam sheets are thicker than 0.001 inch, right? Suppose the foam expands 27-fold, or 3-fold in every direction. In an egg container, there is only 0.75 mm (0.030 inches) of plastic in the thickness of a 2.3 mm SPE ANTEC Anaheim 2017 / 2327

In Figure 4, I compressed the foam without heat and shear. As a result, the thickness was isotropically reduced by about one-third. In thermoforming the plastic is stretched as well as compressed. As a result, I would expect a draw-down in excess of about 3-to-1. Figure 10 is a comparison of the part thickness in the hinge of the egg carton to that in the undrawn region away from the hinge. The thickness ratio os about 10-to-1. Figure 8. Matched tooling for forming low-density PS foam into egg carton. Essentially the foam is shear-compressed. It does not exhibit excessive stretching. From careful observation of the formed parts, it appears that cell walls that are (near-) perpendicular to draw direction fold and/or rupture. And cell walls that are (near)-parallel to draw direction orient - without stretching - in draw direction. So even though there is cell rupture, the integrity of the formed part is retained, Figure 9 for the draw-down in the egg cup of an egg carton. Figure 9. Draw-down portion of egg carton. Figure 10. Cross-section of PS egg carton hinge area Conclusion I believe that radiant heating of the diathermanous cells dominates conduction and convection. The complex secondary expansion criteria limit foam forming to low sheet temperature. As a result, we form foam products by a combination of shear and compression in matched tooling. Although there is substantial cell rupture, sufficient cell walls parallel to the shearing direction remain unruptured. As a result the final product remains liquid-tight. \ References 1. C. Vachon, Research on Alternative Blowing Agents, Chapter 4, in R. Gendron, Ed., Thermoplastic Foam Processing: Principles and Development, CRC Press, Boca Raton, FL., 2005. 2. N.S. Ramesh, Foam Growth in Polymers, Chapter 5 in S.-T. Lee, Ph.D., Ed., Foam Extrusion: Principles and Practice, Technomic Publishing Co., Inc., Lancaster, 2000. 3. S.P. Levitskiy and Z.P. Shulman, Bubbles in Polymeric Liquids: Dynamics and Heat-Mass Transfer, Chapter 1, Technomic Publishing Co., Lancaster, 1995. SPE ANTEC Anaheim 2017 / 2328

4. A.V. Nawaby and Z. Zhang, Solubility and Diffusivity, Chapter 1, in R. Gendron, Ed., Thermoplastic Foam Processing: Principles and Development, CRC Press, Boca Raton, FL., 2005 5. C.J. Benning, Plastic Foams: The Physics and Chemistry of Product Performance and Process Technology. Volume 1: Chemistry and Physics of Foam Formation, Wiley-Interscience, New Yok, 1969, pg. 90. 6. J.L. Throne, Polystyrene Foam Sheet Expansion During Heating, Jnl. Polym. Eng., 6, Fig. 17, (1986), Fig. 17. 7. L.J. Gibson and M.F. Ashby, Cellular Solids: Structure and Properties, Pergamon Press, Oxford, 1988, Chapter 2. 8. Y. Chen, R. Das, M. Battley, Modelling of Closedcell Foams Incorporating Cell Size and Cell Wall Thickness Variations, 11 th World Congress on Computational Mechanics, 2002. Footnote 1 In a recent paper on closed-cell foam characteristics [8], New Zealand researchers measured sizes of 473 cells and the thickness of 281 cell walls of commercial 150 kg/m 3 (9.4 lb/ft 3 ) styrene-acrylonitrile foam. They found average cell dimensions of 256 µm ± 91.8 µm and average cell wall thickness of 9.2 µm ± 9.3 µm. I used a conservative cell wall thickness of 25 µm in my example File: SPE ANTEC paper Forming Foam SPE ANTEC Anaheim 2017 / 2329