TECHNICAL NOTES
Covering the world. Protecting the Earth.
Rev. 05.15
BLOWN FILM (BF) VS FLAT DIE (FD) EXTRUSION OF POLYETHYLENE (PE) GEOMEMBRANE
SOLMAX.COM
CANADA - MALAYSIA - CHILE - USA - CHINA - SOUTH AFRICA - FRANCE - INDIA
BF VS FD EXTRUSION OF POLYETHYLENE GEOMEMBRANE 1. INTRODUCTION
Geomembranes are impervious sheets of polymeric material primarily used to reduce the hydraulic conductivity of in situ soils. They represent a branch of engineering materials called geosynthetics which are specifically designed to enhance the building properties of natural soils which have been deemed deficient from an engineering point of view.
Geomembranes are thus extensively used to provide additional imperviousness to natural soils in order to protect groundwater tables against contamination from industrial activities such as landfills, mining, and more generally, hazardous material storage. Geomembranes come in a variety of polymers of which polyethylene represents the most significant percentage. Wide width polyethylene geomembranes (e.g. > 6m) are usually manufactured by either blown film or flat die extrusion technologies. Actual world production capacity figures indicate that manufacturing processes are roughly split 2/3 in favor of the Blown Film process. While both technologies may offer high quality products, some particularities exist between them which may or may not influence the useful properties of the finished goods. This paper presents most of those discrepancies while putting them in perspective from both an engineering standpoint and widely accepted industry standards.
2. FOREWORD
It is utterly important to note that all industry standards and federal policies of most industrialized nations endorse both manufacturing technologies in spite of the idiosyncrasies that may exist between them and the differences between their respective yielded products. As a case in point, internationally acclaimed technical references such as the Geosynthetic Research Institute (GRI) GM-13 Standard Specification, “Test Methods, Test Properties and Testing Frequency for High Density Polyethylene (HDPE) Smooth and Textured Geomembranes�, which undoubtedly constitutes the world’s most stringent collection of polyethylene geomembrane minimum technical requirements, do not differentiate between both technologies. And since numerous material properties are used for actual engineering design calculations and dimensioning, manufacturing processes become irrelevant as long as the materials denote acceptable testing values.
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The GRI standard is especially telling since the members of its parent organization, as opposed to some of the other professional associations, include representatives of all industry sectors including regulators (e.g. US Environmental Protection Agency, US Federal Bureau of Reclamation, US Army Corps of Engineers), major owners and operators, engineering consultants and quality assurance services companies, testing laboratories, resin producers, geomembrane manufacturers (both blown film and flat die for that matter), installers and contractors. This basically and clearly illustrates that whatever differences that may exist between competing technologies, they do not impact in any way on the engineering value of the products. And this fact alone should close the issue. But unfortunately the industry is often tainted by misconceptions and faulty perspectives which usually hail from a lack of understanding or flatly, from misrepresentations. Hopefully, the following text will assist in dispelling some of this recurring faulty information.
3. BF AND FD PROCESSES FOR SMOOTH PE SHEET
FIGURE 1
Both Blown Film and Flat Die processes are classified as extrusion processes which entail transforming plastic pellets into a fixed desired cross-section (a flat sheet) through a die opening. In particular, base polymer resin along with anti-oxidant chemical additives are fed to a continuously rotating flight screw, successively passing metering, compressing, and heating sections, emerging as a molten mix which is then directly fed into a form die.
FLAT DIE EXTRUSION PROCESS
Source: M. Koerner, Robert. “Designing with geosynthetics, fifth edition”, p. 51
While the Flat Die process uses a cast sheeting type die (a.k.a. coat-hanger die) which forces the molten mix between two horizontal die lips (see figure 1), the Blown Film process utilizes a vertically oriented annular die (see figure 2). The extruded continuous sheet is then usually air-cooled (longitudinally cut and unfolded in the case of the Blown Film process) and rolled onto a plastic or cardboard core. While different thickness materials are produced by varying the die lip gap, and managing polymer die lip swell, of a Flat Die process, thickness is governed by the rotational speed of the primary nip rolls located at the top of the Blown Film process which draws the extruded polymer cylinder upwards during its cooling period, as the annular die lips are fixed. From this necessary exclusive drawing process, anisotropy of the finished products’ mechanical properties between machine and transverse directions is obviously more pronounced for the Blown Film process, and differences of up to 10% may be observed under uniaxial tension at both Yield and Break points. But this anisotropy is not an engineering issue if minimum resistance is specified for the weakest direction, which is what industry standards always call for as opposed, for instance, to an average for both directions which at that point could seriously endanger the design safety factors.
BASIC BLOWN FILM LINE Nip Rolls
Collapsing Frame
Resin Pellets
Lay�lat
Bubble
Roll of Film
Air Ring Die
Hopper
FIGURE 2
Idler Roll
Extruder
Idler Roll
Source: M. Koerner, Robert.“Designing with geosynthetics, fifth edition”, p. 52
The nature itself of the manufacturing process also dictates the required rheology of the raw materials. As opposed to the Flat Die process, the Blown Film process needs to “fight gravity” by way of “structural strength” of the extrudate’s molten phase while being drawned, else the freshly extruded polymer cylinder will not be capable of holding firmly enough while being pulled upwards from the die lips. A “stiffer” mix is thus necessary for the Blown Film process. This property is measured by a melt flow indexer which basically determines the time required for a sample of molten plastic to flow through a small circular opening under a positive displacement piston force of 2 kg. Units of Melt Flow Rates (a.k.a. Melt Indexes) are expressed as grams per 10 minutes; lower numbers indicating “stiffer” mixtures and higher structural strengths.
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As Melt Flow Rates are actually inversely proportional to the average molecular weight (molecular chain length) of the polymer, low melt flow rates will hence correspond to higher average molecular weights (the longer the molecular chain, the longer it will take to go through the melt indexer). Longer molecular chain lengths usually increase the polymer’s Stress Crack Resistance. Property Tensile Strength @ Yield
Stiffness
Low Temp Brittleness
Stress Crack Resistance
Permeability
TABLE 1
Chemical Resistance
Melt Strength
Processability
Effect of Density, Melt Index and Molecular Weight on Polyethylene As Density Increases
INCREASES
INCREASES
WORSENS
DECREASES
DECREASES
INCREASES
NO CHANGE
NO CHANGE
As Melt Index Increases
DECREASES
DECREASES SLIGHTLY
WORSENS
DECREASES
INCREASES SLIGHTLY
DECREASES
DECREASES
INCREASES
As Molecular Weight Distribution Broadens
NO CHANGE
DECREASES SLIGHTLY
WORSENS
INCREASES
NO CHANGE
NO CHANGE
INCREASES
INCREASES
Effect of Density, Melt Index and Molecular Weight on Polyethylene, after Ashland Inc. Source: http://www.ashland.com/pdfs/technical/Cycletime%20Tips%20-%20Volume%2036%20-%20High%20Density%20Polyethylene.pdf
Furthermore, ratios between two melt flow rate values for one material at different piston forces (e.g. 2 kg and 21 kg) may in addition be used as a measure for the broadness of the molecular weight distribution, the higher the ratio, the broader the distribution (75 to 130 for typical Blown Film process, 25 to 100 for typical Flat Die process). Also, the broader the molecular weight distribution, usually the better suited the resin is against stress cracking. So, in theory, low Melt Indexes and high Melt Index Ratios such as the ones typically associated with the Blown Film process should be beneficial when it comes to the polymer’s resistance to stress cracking. However, this potential material property advantage that the Blown Film technology may appear to have over the Flat Die process is easily overshadowed by simply using a lower density resin in the Flat Die process. It could be argued at that point, that the polymer’s chemical resistance will be reduced, as well as its tensile strength at yield, while its low temperature brittleness will improve, as can be seen in Table 1.
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All in all, molecular structure will influence most material properties entailing both benefits and drawbacks. Although, it may be tempting to specify a certain resin type for its apparent benefits should the drawbacks be considered irrelevant for a specific application, very little gains and losses will be made as can be witnessed by close analysis of all manufacturers’ published literature. Bottom line, it is not advisable to specify
Melt Flow Index and/or Ratio required engineering material properties since the exercise may exclusively endorse the acceptable manufacturing process by way of their values, as manufacturing processes call for different molecular structures for processability; 0,06 – 0,25 g/10 min Melt Flow Index for typical Blown Film process, and 0,2 – 0,8 Melt Flow Index for typical Flat Die process. To that effect, the GRI GM-13 Standard Specification simply states that the polyethylene resin will have a Melt Index value per ASTM D1238 of less than 1.0 g/10 minutes, thereby preventing technically unjustifiable manufacturing process endorsement. In theory, the Flat Die extrusion process yields a better sheet thickness control (± 3% thickness variation across the web) than the Blown Film process (± 7%). But most industry standards will endorse both manufacturing processes by allowing a ± 10% thickness variation tolerance for smooth material, above which thickness variations may indeed have an impact on design or installation quality. In practice, individual production units may either fare better or worse, to the point of even reversing the theoretical performance percentages, depending on their wear, maintenance, type of resin and extrusion parameter set-points to mention a few. For instance, although some current Flat Die production units have tested as low as 1.2% thickness variation, some have also reached as high as 8.5%, while Blown Film units have ranged between 2.8% to 10%.
4. BF AND FD MANUFACTURING PROCESS FOR TEXTURED SHEET
There are a few methods to texturize geomembranes in order to increase their surface roughness, hence their friction angle to secure their stability on steep slopes. Some of these texturizing methods are linked to the manufacturing processes used, whether it be Flat Die or Blown Film extrusion. While irrelevant material property differences do exist between both processes when it comes to smooth sheet, there may be some major discrepancies between their textured versions. However, as it will be seen, from a technical standpoint those differences are inconsequential as they appear beyond the materials’ yield point, therefore clearly beyond their true engineering usefulness.
Impingement texturizing process Resin Silo Weight/ blender
FIGURE 3
Texturizing by impingement consists of randomly projecting hot polymer particles onto a heated finished smooth sheet. This method may independently be used following either Flat Die or Blown Film manufacturing processes as it first requires the production of a smooth sheet (see figure 3). The method is hence described as a secondary process, along with the lamination method of depositing a hot foam layer on a previously manufactured smooth sheet (see figure 4). Also described as a secondary process is the structuring method usually associated with the Flat Die process, which entails running the still hot freshly produced smooth sheet through two counter-rotating knurled rollers, producing a raised surface of repeating patterns (see figure 5).
Carbon black concentrate silo
Spray Equip
Spray Equip
Mixer
The last texturizing method is particular to the Blown Film technology and is performed through co-extrusion with the utilization of a three-layer production unit whereby pressurized Nitrogen (typically at approximately 100 bars) used as a foaming agent, is added to the polymeric formulation and delivered by two small extruders (a.k.a. satellite or skin screws) adjacent to the main extruder (see figure 6). As the three layers coalesce inside the die and exit at atmospheric pressure, the embedded Nitrogen expands violently, creating the necessary texture (akin to decompression sickness associated with scuba diving).
Hot foam texturizing process
Smooth sheet
Spreader
Hopper Finished textured sheet
FIGURE 5
FIGURE 4
Hot PE foam
Structuring texturizing process
Extruder
Counter-rotating patterned rollers
Die
Finished textured sheet
5
FIGURE 6
Air bubble
Die External extruder (N2 gas)
Skin "A"
Finished textured sheet
80%
10%
Extruder A
Circular Die
Extruder B
Main core extruder
Extruder B
N2
Extruder C
N2
Internal extruder (N2 gas)
10%
Skin "B"
Co-extrusion texturizing process
Extruder C
Extruder A
This texturing process along with the sheet extrusion process itself hence take place simultaneously in a single operation and as such, is not considered a secondary process.
It is essential to note that contrary to the other three texturizing methods, the texture of a Blown Film is not being added to a previously manufactured smooth sheet, but is literally part of the sheet itself, deeply embedded in the core material. It is on account of this feature that the textured sheet will behave differently under load than its counterparts due to the presence of high residual stresses at the texturizing points.
Residual stresses of textured sheets
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As can be seen on figure 7, as opposed to trace deformation at the contact points of a secondary process texturized asperity, high residual stresses in the form of both pre-compression and pre-tension locales are found at the base of every asperity of a textured Blown Film sheet. Although undetectable at low elongation under tensile stress, this particularity will seriously impact the material’s behaviour at Break point as can be seen on a typical stress-strain curve (see figure 8). Whereby two-phase texturing processes will offer textured materials with maximum strength and elongation values at break virtually equal to their smooth sheets’ (which they actually are, save for their “added” texture), co-extruded textured sheet will only fare at a fraction (700% vs 100-500% typical elongation).
Small sheet deformation at “welding” points
Generic stress-strain curve STRENGTH
PRE-TENSSION
+
PRE-COMPRESSION
Impingement method
Break Point
TECHNICAL FAILURE!
Yield Point
FIGURE 8
FIGURE 7
Co-extrusion method
Safety Factor Zone
4%
Plastic Deformation
12%
700%
% ELONGATION
It is essential to note that all other properties are equivalent between both single and double phase texturizing methods, especially tensile and elongation up to their Yield Point which, as for all building materials, are two of the most valuable parameters when it comes to designing with geomembranes.
STRENGTH
Stress-strain curve of textured materials
FIGURE 9
As the Yield Point (a.k.a. Elastic Limit) corresponds to the maximum stress (and consequently strain) allowed for any material to return to its original shape once the forces are no longer applied (higher stresses plastically or permanently deforming the materials), designers will usually be wary of encountering deformation stresses higher than 25 or 30% of their Yield Point as illustrated on figure 8. It is important to understand that although materials stressed beyond their Yield Point might not rupture, they are considered technically failed, since their deformation will be permanent. To that effect, one only needs to picture oscillating airplane wings in flight which clearly illustrate the beneficial action of elastic material deformation under stress. The wings absorbing and releasing bending forces by temporary or reversible deformation, while permanently deformed and still wings from excessive stresses would definitely justify white knuckles.
Elongation is reduced at break, and so are tensile strength and puncture resistance.
Yield Point
GRI
2 Step Process Typical Elongation
One Step Process Typical Elongation
100% 150%
400%
% ELONGATION
In essence, as secondary texturing processes do produce materials characterized with both higher elongations and strengths at Break Point, no engineering benefits are truly obtained. Also as seen in figure 9, although double-step processes produce materials with typically higher than 400% uniaxial tensile elongations at Break Point (and corresponding high strengths), compared to about 150% typically for Blown Film process, the GRI GM-13 Standard specifies a minimum acceptable value of 100% only, thereby minimizing the engineering importance of material properties at Break Point, and by the same token endorsing both technologies.
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5. OTHER MISREPRESENTATIONS AND MISUNDERSTANDINGS
So now the question is, why does the GRI GM-13 specify a 700% elongation and corresponding high strength at Break Point for smooth materials if the properties are truly irrelevant engineering-wise? The answer is because material properties at Break Point are considered index properties which simply enable the rapid and revealing identification of the polymer as being polyethylene, as opposed to performance properties (which are helpful in designing). This could potentially lead to another argument of doubting whether the formulation solely contains polyethylene as raw material (actually typically 97% of polyethylene, 2 to 3 % of carbon black and a fraction percentage of anti-oxidants and processing agents) if its’ material properties at Break Point are so low. However, raw material mill certificates and product certifications will rapidly attest to the fact, which is the reason why this quality assurance protocol should be strongly endorsed. On the other hand, it could be argued that Blown Film (e.g. single-pass) textured materials products offer higher abrasion resistance from their strongly embedded and deeply anchored asperities compared to secondary texturizing processes. Although true in most cases, this also becomes a non-issue when comparing abrasion resistances at friction values usually encountered in most earthwork projects. Other misrepresentations that come to mind are issues such as the famous manufacturing “creases” of Blown Film technology (see figure 10) which are often fallaciously claimed as weak material areas, for which there is absolutely no technical basis, as all creased specimens denote exactly the same material properties as samples from the “flatter” portion of the sheets. Nor do they represent alleged preferred thermal expansion locales and ensuing magnified “waves” and reduced intimate contact within the subgrade as polyethylene geomembranes will expand and contract irrelevantly in both directions for both manufacturing processes.
6. CONCLUSION
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FIGURE 10
As for typical roll widths, both Blown Film and Flat Die technologies are available in 5 to 10 meters widths or thereabouts, as very little monetary value is either gained or loss between widths when all procurement and construction sequences are considered such as quantities required, overlaps, installation waste factor, installation efficiency, required welding lengths, etc.
Origins of Blown Film “creases”
In conclusion, both Flat Die and Blown Film technologies are capable of producing excellent engineering materials with comparable material properties. As a result, both technologies are readily endorsed by most if not all federal regulations throughout the world. Although idiosyncrasies between competing technologies and their end products do exist, they usually represent the source of irrelevant engineering argumentations, are simply taken out of perspective, or are flatly deceptive and misleading.